論文 / 著書情報
Article / Book Information
題目(和文)

相互反応や反応速度論的解析および反応メカニズムに着目した精油汚
泥または精油汚泥チャーとバイオマスの熱分解および共燃焼挙動の解明

Title(English)

Pyrolysis and combustion of oil sludge/oil sludge char with biomass
focusing on interactions, kinetics, and mechanisms

著者(和文)

XuHao

Author(English)

Hao Xu出典(和文)

学位:博士(工学),学位授与機関:東京工業大学,
報告番号:甲第12225号,
授与年月日:2022年9月22日,
学位の種別:課程博士,
審査員:高橋 史武,中﨑 清彦,CROSS JEFFREY SCOTT,江頭 竜一,時松宏治

Citation(English)

Degree:Doctor (Engineering),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第12225号,
Conferred date:2022/9/22,
Degree Type:Course doctor,
Examiner:,,,,

学位種別(和文)

博士論文

Type(English)

Doctoral Thesis

Powered by T2R2 (Tokyo Institute Research Repository)

Doctoral Dissertation

Pyrolysis and Combustion of Oil Sludge/Oil Sludge Charwith Biomass focusing on Interactions, Kinetics, andMechanisms

Xu HaoSupervisor: Professor Fumitake Takahashi

Department of Transdisciplinary Science and Engineering
School of Environment and Society
Tokyo Institute of Technology

September 2022

Acknowledgments

Acknowledgments
Foremost, I would like to express my sincere gratitude to my supervisor, Assoc. Prof.
Fumitake Takahashi for accepting me as a doctoral student and for his helpful guidanceand support for my research and life. His rigorous attitude towards research, constantpursuit of new things, and balance between work and family, will be the direction of
my future academic path. Also, I would like to express my great gratitude to Assis. Prof
Shuo Cheng for her patient guidance, valuable suggestions for my research and future
career, and for teaching me how to think from the whole perspective. I want to express
my great gratitude to Prof. Kunio Yoshikawa for research opportunities and helpful
guidance. I would appreciate Assoc. Prof. Koji Tokimatsu and Prof. Jeffrey S. Cross fortheir kind recommendations and support for my research and life. Also, I would like toexpress my gratitude to all my referees for valuable suggestions to improve my work.
I would like to thank Mrs. Eriko Ohno for her administrative work and help during
my study. I would appreciate the China Scholarship Council (CSC) for financial support.
I would like to thank all members of Takahashi-Tokimatsu Lab for the scientificdiscussion and non-scientific fun hours.
Last but not least, I would be sincerely grateful to my family for their endless loveand support.

ITable of Contents

Table of Contents
Acknowledgments............................................................................................................................. I
List of Figures .................................................................................................................................IV
List of Tables...................................................................................................................................VI
Chapter 1 Introduction ...................................................................................................................... 1
1.1 Background ......................................................................................................................... 1
1.2 Overview of oil sludge ........................................................................................................ 2
1.2.1 Classification of oil sludge ....................................................................................... 2
1.2.2 Characterization of oil sludge .................................................................................. 4
1.2.3 Hazards of oil sludge................................................................................................ 5
1.3 Overview of oil sludge treatment method ........................................................................... 6
1.4 Integrated pyrolysis and combustion of oil sludge............................................................ 12
1.4.1 Co-pyrolysis of oil sludge with other feedstocks ................................................... 13
1.4.2 Co-combustion of oil sludge char with biomass .................................................... 15
1.5 Research gaps in pyrolysis/combustion of oil sludge/oil sludge char ............................... 16
1.6 Objective of present work ................................................................................................. 17
1.7 Outline of the thesis .......................................................................................................... 18
References ....................................................................................................................................... 20
Chapter 2 Effect of torrefaction on the structure and pyrolysis property of rice straw ................... 28
2.1 Introduction ....................................................................................................................... 28
2.2 Materials and methods ...................................................................................................... 29
2.2.1 Materials and torrefaction pretreatment ................................................................. 29
2.2.2 Characterization of RS and TS ............................................................................... 30
2.2.3 Experimental apparatus and method ...................................................................... 32
2.2.4 Characterization of oil composition ....................................................................... 33
2.3 Results and discussion ...................................................................................................... 34
2.3.1 Characterization of RS and TS ............................................................................... 34
2.3.2 Thermal decomposition characteristics of RS and TS............................................ 40
2.3.3 Effect of torrefaction on pyrolysis of RS ............................................................... 41
2.4 Conclusions ....................................................................................................................... 45
References ....................................................................................................................................... 45
Chapter 3 Effect of torrefaction on synergistic behavior during co-pyrolysis of oil sludge with ricestraw to enhance hydrocarbon production ...................................................................................... 50
3.1 Introduction ....................................................................................................................... 50
3.2 Materials and methods ...................................................................................................... 52
3.2.1 Materials and torrefaction pretreatment ................................................................. 52
3.2.2 Characterization of samples ................................................................................... 52
3.2.3 Experimental apparatus and method ...................................................................... 52
3.2.4 Characterization of oil composition ....................................................................... 53
3.3 Results and discussion ...................................................................................................... 53
IITable of Contents

3.3.1 Characterization of samples ................................................................................... 53
3.3.2 Thermal decomposition characteristics of RS, TS, OS, and blends ....................... 54
3.3.3 Pyrolysis of individual samples.............................................................................. 57
3.3.4 Effect of OS blending on pyrolysis of RS .............................................................. 59
3.3.5 Combined effect of torrefaction and OS blending on pyrolysis of RS ................... 66
3.6 Possible reaction mechanism during co-pyrolysis of RS/OS and TS/OS ......................... 72
3.4 Conclusions ....................................................................................................................... 74
References ....................................................................................................................................... 75
Chapter 4 Co-combustion of oil sludge char with raw/hydrothermally treated biomass: interactions,kinetics, and mechanisms ................................................................................................................ 79
4.1 Introduction ....................................................................................................................... 79
4.2 Materials and methods ...................................................................................................... 81
4.2.1 Sample preparation................................................................................................. 81
4.2.2 Sample characterizations ........................................................................................ 82
4.2.3 Experimental equipment and methods ................................................................... 82
4.2.4 Combustion performance analysis ......................................................................... 83
4.2.5 Interaction analysis................................................................................................. 84
4.2.6 Kinetic analysis ...................................................................................................... 85
4.3 Results and discussion ...................................................................................................... 88
4.3.1 Characterization of samples ................................................................................... 88
4.3.2 Combustion behavior of OSC, RW, and HW ......................................................... 89
4.3.3 Combustion behavior of the blends ........................................................................ 92
4.3.4 Interactions during combustion of the blends ........................................................ 94
4.3.5 Kinetic analysis ...................................................................................................... 97
4.3.6 Possible mechanisms of OSC co-combustion with RW or HW ........................... 102
4.3 Conclusion ...................................................................................................................... 105
References ..................................................................................................................................... 105
Chapter 5 Conclusions and recommendations .............................................................................. 111
5.1 Overall conclusion .......................................................................................................... 111
5.2 Recommendations ........................................................................................................... 113
5.3 Practical applications ...................................................................................................... 113
Appendix ....................................................................................................................................... 115

III

List of Figures

List of Figures
Figure 1.1 Oil supply and consumption in China ..................................................................... 2
Figure 1.2 Typical composition of oil sludge ........................................................................... 4
Figure 1.3 Oil sludge treatment methods ................................................................................. 7
Figure 1.4 Schematic diagram of the integrated pyrolysis and combustion treatment ........... 13
Figure 1.5 The thesis contents and their relationship ............................................................. 20
Figure 2.1 Structure of Chapter 2. .......................................................................................... 29
Figure 2.2 Schematic diagram of the fixed bed reactor for torrefaction. ............................... 30
Figure 2.3 Van Krevelen diagram of RS and TS. ................................................................... 35
Figure 2.4 (a) FTIR spectra of RS and TS, (b) curve fitting of TS200 in 1785-1485 cm-1, and(c) curve fitting of TS200 in 3000-2800 cm-1. ................................................................ 38
Figure 2.5 SEM images of samples: (a) RS, (b) TS200, (c) TS250, and (d) TS300. ............. 39
Figure 2.6 XRD patterns of RS and TS. ................................................................................. 40
Figure 2.7 (a) TG and (b) DTG curves of RS and TS. ........................................................... 41
Figure 2.8 Product yields from individual pyrolysis of RS and TS........................................ 43
Figure 2.9 Relative concentration of (a) hydrocarbon, (b) oxygenate, (a-1) alkane, (a-2)
aromatic, and (a-3) olefin in oil products from pyrolysis of RS and TS. ........................ 45
Figure 3.1 Structure of Chapter 3. .......................................................................................... 51
Figure 3.2 (a) TG and (c) DTG curves individual samples; (b) TG and (d) DTG curves of
mixtures........................................................................................................................... 56
Figure 3.3 Product yields from individual pyrolysis of RS, TS, and OS (modified from Figure2.8). ................................................................................................................................. 58
Figure 3.4 Relative concentration of (a) hydrocarbon, (b) oxygenate, (a-1) alkane, (a-2)
aromatic, and (a-3) olefin in oil products from pyrolysis of OS, RS, and TS (modified
from Figure 2.9). ............................................................................................................ 59
Figure 3.5 Product yields from co-pyrolysis of OS and RS. .................................................. 61
Figure 3.6 Relative concentration of (a) hydrocarbon, (b) oxygenate, (a-1) alkane, (a-2)
aromatic, and (a-3) olefin in oil products from co-pyrolysis of OS and RS.................... 62
Figure 3.7 Deviations between experimental and calculated concentrations of straight chain
alkanes, oxygenates, and aromatics: (a) RS:OS=3:1, (b) RS:OS=1:1, (c) RS:OS=1:3, (d)TS200:OS=3:1, (e) TS250:OS=3:1, and (f) TS300:OS=3:1. .......................................... 63
Figure 3.8 Product yields from co-pyrolysis of RS-OS mixture and TS-OS mixtures. ......... 68
Figure 3.9 XRD patterns of OS and char samples.................................................................. 68
Figure 3.10 Relative concentration of (a) hydrocarbon, (b) oxygenate, (a-1) alkane, (a-2)
aromatic, and (a-3) olefin in oil products from co-pyrolysis of RS-OS mixture and TS-OS
mixtures........................................................................................................................... 71
Figure 3.11 Possible synergistic pathways in co-pyrolysis of RS and OS, TS and OS. ......... 74
Figure 4.1 Structure of Chapter 4. .......................................................................................... 81
Figure 4.2 Schematic diagram of HTT reactor [modified from Ref. 4-12] ............................ 83
Figure 4.3 Van Krevelen diagram of RW and HW. ................................................................ 89
Figure 4.4 TG and DTG curves of OSC, RW, HW, and their respective blends at 40 °C/min (a1) TG and DTG of RW, HW, and OSC; (a-2) TG and DTG of OSC; (b-1) TG of RW andIVList of Figures

OSC/RW; (b-2) DTG of RW and OSC/RW; (c-1) TG of HW and OSC/HW; (c-2) DTG of
HW and OSC/HW. .......................................................................................................... 92
Figure 4.5 The experimental and calculated DTG curves and their deviations of the blends
under different blending ratios. (a-1) and (a-2) OSC/RW; (b-1) and (b-2) OSC/HW; (c-1)
MR values of OSC/RW under different blending ratios; (c-2) MR values of OSC/HW
under different blending ratios. ....................................................................................... 95
Figure 4.6 SEM images of (a) RW, (b) HW, and (c) OSC...................................................... 97
Figure 4.7 The apparent activation energies (E) of OSC, RW, HW, and their respective blends(a) (b) variation of E for OSC, RW, and HW in stage 1 and stage 2; (c) (d) variation of E
for OSC/RW in stage 1 and stage 2; (e) (f) variation of E for OSC/HW in stage 1 and stage
2. S1: Stage 1; S2: Stage 2. ............................................................................................. 99
Figure 4.8 𝑃𝑢𝑃𝑢0.5 versus α in stage 1 at 40 °C/min [(a) OSC, (c) RW and OSC/RW, (e)HW and OSC/HW]; 𝑃𝑢𝑃𝑢0.5 versus α in stage 2 at 40 °C/min [(b) OSC, (d) RW andOSC/RW, (f) HW and OSC/HW]; and the direction of the pink dotted line arrow
represented the variation of the curves with increasing OSC proportion in the blends. 101
Figure 4.9 (co-)combustion mechanism of OSC/RW and OSC/HW ................................... 103
Figure 5.1 Practical application of this work........................................................................ 114
Figure A.1 Iso-conversional plots at various conversion degree for (a) RW, (b) HW, (c) OSC,(d) OSC/RW28, (e) OSC/RW55, (f) OSC/RW82, (g) OSC/HW28, (h) OSC/HW55, (i)OSC/HW82 in stage 1 by FWO method. ...................................................................... 121
Figure A.2 Iso-conversional plots at various conversion degree for (a) RW, (b) HW, (c) OSC,(d) OSC/RW28, (e) OSC/RW55, (f) OSC/RW82, (g) OSC/HW28, (h) OSC/HW55, (i)OSC/HW82 in stage 1 by KAS method. ....................................................................... 122
Figure A.3 Iso-conversional plots at various conversion degree for (a) RW, (b) HW, (c) OSC,(d) OSC/RW28, (e) OSC/RW55, (f) OSC/RW82, (g) OSC/HW28, (h) OSC/HW55, (i)OSC/HW82 in stage 2 by FWO method. ...................................................................... 123
Figure A.4 Iso-conversional plots at various conversion degree for (a) RW, (b) HW, (c) OSC,(d) OSC/RW28, (e) OSC/RW55, (f) OSC/RW82, (g) OSC/HW28, (h) OSC/HW55, (i)OSC/HW82 in stage 2 by KAS method. ....................................................................... 124

VList of Tables

List of Tables
Table 1.1 Summary and comparison of oil sludge resource recovery methods ...................... 10
Table 1.2 A brief review of co-pyrolysis of oil sludge and other feedstocks.......................... 14
Table 2.1 Ultimate analysis, proximate analysis, and composition of samples. ..................... 34
Table 2.2 Ash composition of samples by XRF analysis. ...................................................... 35
Table 2.3 Concentration of typical AAEMs in RS and TS by ICP analysis (mg/kg).............. 36
Table 2.4 Parameters of fitted peaks from TS200’s infrared spectrum. .................................. 37
Table 2.5 FTIR indexes deduced from FTIR spectra of RS and TS (%). ............................... 38
Table 2.6 Pyrolysis characteristics of RS and TS. .................................................................. 41
Table 2.7 Relative concentrations of oxygenates in oil from individual pyrolysis of RS and TS.
......................................................................................................................................... 44
Table 3.1 Ultimate analysis, proximate analysis, and composition of samples. ..................... 54
Table 3.2 Ash composition of samples by XRF analysis. ....................................................... 54
Table 3.3 Pyrolysis characteristics of OS, RS, TS, and the blends. ........................................ 57
Table 3.4 Relative concentrations of oxygenates in oil from OS, RS, and TS (modified from
Table 2.7). ....................................................................................................................... 58
Table 3.5 Mass distribution of oil and hydrocarbons.............................................................. 71
Table 4.1 Most frequently used models of solid-state processes. ........................................... 86
Table 4.2 Proximate and ultimate analyses of samples. ......................................................... 89
Table 4.3 Ash composition of samples by XRF analysis (wt%)............................................. 90
Table 4.4 Combustion characteristics of OSC, RW, HW, and blends at 40 °C/min. .............. 91
Table 4.5 Combustion performance parameters of OSC, RW, HW, and blends at 40 °C/min.
......................................................................................................................................... 93
Table 4.6 Kinetic triplets (E, A, and model) for samples at 40 °C/min by master plot method.
......................................................................................................................................... 98
Table A.1 Relative concentrations of straight-chain alkanes and aromatics in oil fromindividual pyrolysis of samples. .................................................................................... 115
Table A.2 Relative concentrations of straight-chain alkanes, oxygenates, and aromatics in oilfrom co-pyrolysis of samples. ....................................................................................... 116
Table A.3 Combustion kinetic parameters of samples in stage 1 and stage 2 by FWO and KAS
methods. ........................................................................................................................ 118
Table A.4 Apparent activation energies of the combustion of woody biomass from related work.
....................................................................................................................................... 120

VIChapter 1

Introduction

Chapter 1 Introduction
1.1 Background
Rapid global industrialization and economic growth increase demand for petroleumbased products and are estimated to increase by 35% by 2025 [1-1]. At the same time,a large amount of oil-containing waste, known as oil sludge, is inevitably generated
during the exploration, transportation, storage, and refining of oil [1-2, 1-3]. Oil sludge
has a complex composition and contains toxic and harmful substances such as
petroleum hydrocarbons, waste chemicals, and heavy metals. Without or improper
disposal, it will cause severe environmental pollution and threaten human health andecosystems [1-3]. Therefore, oil sludge has been classified as a hazardous waste inmany countries. At present, efficient and clean disposal methods are the mainstream
concerns of oil sludge disposal in terms of its large output and harmfulness. From theperspective of the 3R principle (Recycle, Reuse, and Reduce) in waste treatment, oilsludge contains high content of value-added hydrocarbon components, throughrecycling which can be used as alternative fuel products and mitigate the dilemma offossil energy depletion.
In recent years, various methods have been developed for oil sludge treatment,
among which thermochemical treatments can effectively achieve resource recovery andterminally non-hazardous disposal simultaneously [1-4]. Therefore, a comprehensive
thermochemical treatment, combining pyrolysis and combustion, has been regarded asa promising method to deal with oil sludge [1-5]. However, limited research has been
conducted on this integrated treatment. This system itself is an energy-intensive process,requiring necessary measures to reduce the high energy consumption and operationalcost. Consequently, it is of great research significance to further improve the processing
efficiency and the internal energy recycling in this system. Based on the above
background, this work conducts research on co-pyrolysis and co-combustion of oil
sludge/oil sludge char.
1

Chapter 1

Introduction

1.2 Overview of oil sludge
The yield of oil sludge is dependent on the output of crude oil and the larger
throughput of crude oil results in higher production of oil sludge. According to the BP
Statistical Review of World Energy 2019 [1-6], the United States is still the country
processing the most significant volume of crude oil in the world in 2018, whose refining
volume accounted for 18.8%, followed by China with 15.6%. It is estimated that 1 tonof oil sludge is produced for every 200 tons of crude oil produced or processed [1-7].
Taking China’s crude oil status as an example, according to the 2021 China Statistical
Yearbook [1-8], as shown in Figure 1.1, the production and import of crude oil
gradually increased in the past ten years, reaching approximately 800 million tons in
2019. Therefore, it is certain that the production of oil sludge will continue to growbecause of the ascending global demand for petroleum products.

Figure 1.1 Oil supply and consumption in China

1.2.1 Classification of oil sludge
Oil sludge is a byproduct generated from exploration, transportation, storage, andrefining during crude oil processing. Consequently, it can be classified as oilfield sludge,
oil tank sludge, and refinery sludge based on their sources.
Oilfield sludge is mainly produced during oilfield exploitation, drilling, and other2

Chapter 1

Introduction

operations, including oil-contaminated soil at oil wells (falling sludge), sediments fromthe separation system of union stations, and drilling mud. Falling sludge is generated
due to the accidents, running off, and dripping during the exploitation of oilfield, whereoil penetrates or drops on the ground and is mixed with soil, sand, water, etc., to form
an oil-soil mixture with oil content ranging from 10-30% [1-9]. Meanwhile, fallingsludge contains low moisture, more sediment, high density, high viscosity, and poor
fluidity [1-10]. Falling sludge is often accumulated in a fixed control area. As theaccumulation time increases, the light components are evaporated, and the asphaltene
and colloidal components get enriched, increasing the difficulty of separating oil andsoil. Drilling mud is oil-containing waste mud generated in oilfield drilling, with highwater content, viscous fluid or semi-fluid state, and many chemicals [1-11]. The annual
yield of oilfield sludge in China is around 1 million tons, of which falling sludge
accounts for 10% [1-12]. Oilfield sludge seriously pollutes the surrounding soil andwater environment, accompanied by a large and wide pollution area.
During the storage of crude oil or petroleum products, some impurities like sediments
and metal salts in the oil, as well as heavy components such as colloid and asphaltene,
are prone to be deposited at the bottom due to natural sedimentation to form a blackand thick colloidal material, which is called oil tank sludge [1-13]. The amount of oiltank sludge obtained from oil storage tank cleaning in China can reach about 1 million
tons every year [1-14]. Compared to oilfield sludge, the oil content in oil tank sludge isrelatively high, ranging from 30-70%, entailing higher resource utilization value [1-15].
Nonetheless, due to the addition of cleaning agents during the cleaning process, thewater content and the degree of water-oil emulsification are enhanced, thus increasing
the difficulty of separation [1-16].
Refinery sludge is the grease trap bottom sludge, flotation tank scum, and excess
activated sludge produced by the oil sewage in the refinery after oil separation, flotation,
sedimentation, and other purification processes [1-17]. Refinery sludge contains ahigher amount of water and oil, as well as heavy components [1-18]. The compositionof refining sludge is very complex and varies depending on the types of pollutants in
3

Chapter 1

Introduction

the oil sewage and the used chemicals. It often contains bacteria, microorganisms, etc.,
and it is difficult to undergo advanced treatment, posing serious pollution to the water
environment [1-19].
1.2.2 Characterization of oil sludge
Oil sludge is a highly emulsified viscous semi-solid with three phases of oil, water,and solid, as shown in Figure 1.2. It contains aged crude oil, asphaltenes, bacteria,
suspended solids, heavy metals, chemicals, behaving as stable properties [1-3]. Theasphaltenes, colloids, and fine particles in the water-oil interface play the role ofinterface active materials, promoting the emulsification of water and oil, and forming
water-in-oil (W/O) emulsion. Typically, oil sludge contains about 30-50% water, 3080% oil, and 10-20% solid particles, which vary over a wide range depending on crude
oil source, production, and processing scheme [1-20, 1-21]. Moreover, the equipment
and reagents used in crude oil processing also affect the properties of oil sludge. For
example, the oil content in the oilfield sludge used in this work is around 20%, with a
low water content (~ 10%) and relatively high content of solid particles up to
approximately 70%.

Figure 1.2 Typical composition of oil sludge

The oil phase in oil sludge is derived from crude oil, mainly composed of four
fractions: saturates, aromatics, resins, and asphaltenes, as well as a small amount ofheteroatom organic compounds such as oxygen, sulfur, and nitrogen. Saturates andaromatics are collectively referred as petroleum hydrocarbons. Their content can reachmore than 70% in oil sludge, including straight-chain paraffin, cycloparaffin, benzene,
toluene, xylenes, naphthalene, and various polycyclic aromatic hydrocarbons (PAHs)
4Chapter 1

Introduction[1-3]. The resin fraction contains polar compounds such as carboxylic acids and N or
S-containing compounds (e.g., thiophenes and pyridines). Asphaltenes fraction has acomplex composition compared to other fractions, consisted of PAHs, heteroatoms (O,S, and N), cycloparaffin, and even metal atoms (Ni, V) connected by alkaline bridges
or ether bonds. Resin and asphaltenes are heavy hydrocarbons and are challenging todegrade; consequently, they are prone to accumulate at the bottom during the storageof crude oil, which concentrations increase with the volatilization of light-weightcomponents resulting in an enhancement of density and viscosity of oil sludge [1-22].
The oxygen and nitrogen contents in oil sludge are relatively low, around 0.05-1.5%
and 0.1-2.0%, respectively. Whereas the sulfur content is higher, accounting for 0.056.0% [1-23]. in addition, the enrichment of thiophene in heavy oil increases the sulfur
content, reaching 8% [1-24]. Therefore, the sulfur content in oil-rich oil sludge,
especially oil sludge composed of high fractions of heavy compounds, can be relativelyhigh.
The composition of solid matrices in oil sludge varies significantly depending on thesource of oil sludge. For example, the solid particles of oilfield sludge mainly include
quartz, silicates, and other minerals, as they come from rocks, soil, and sand [1-3]. Oilsludge also contains metal oxides, such as rust and paint in oil tank sludge and refinerysludge, scaling from corrosion of pipes and tank walls [1-13]. A variety of heavy metals
is also found in oil sludge. For example, the metal concentration in refinery sludge wasreported as Zn (1299 mg/kg), Fe (60200 mg/kg), Cu (500 mg/kg), Cr (480 mg/kg), Ni(480 mg/kg), Pb (565 mg/kg), and V (2400 mg/kg), respectively [1-19, 1-25, 1-26].
1.2.3 Hazards of oil sludge
Oil sludge poses severe pollution to soil, water, and air environment and causes harm
to human health due to the presence of high concentrations of toxic components.
Generally, it can be embodied in the following aspects: (1) the petroleum hydrocarbons
in oil sludge will penetrate the soil as oil sludge is directly discarded on the ground,changing the physical and chemical properties of soil [1-27]. On one hand, the organic
5Chapter 1

Introduction

components will combine with nitrogen, phosphorus, and other elements in soil, leading
to the inability of nutrients to be absorbed by plants. On the other hand, oil sludge will
increase soil viscosity, and the heavy fractions are prone to be adsorbed on the soil
surface, forming a hydrophobic coating, and blocking the soil water-conducting
channels, which will reduce soil water permeability, hinder plant water adsorption, andaffect plant and crop growth [1-28]; (2) large amount of petroleum hydrocarbons in oil
sludge will flow into rivers and lakes with natural rainfall during stacking. The oil
fractions float on the water surface, preventing the oxygen in the air from entering thewater body for gas exchange, resulting in the ineffective supplementation of dissolved
oxygen in the water, limiting the survival of aquatic organisms, thereby destroying thebalance of the water ecosystem [1-29]; (3) when oil sludge is directly stacked without
treatment, the light organic components will volatilize into the air with increasing
ambient temperature, polluting the atmosphere and releasing foul odors. In addition,
flammable and explosive gases such as methane and hydrogen are easily generated
during the stacking of oil sludge. Burning and explosion accidents are prone to occur
without effective management [1-30]; (4) oil sludge contains a lot of polycyclicaromatichydrocarbons.

These

substances

havestrongteratogenicity

andcarcinogenicity, and they will irritate the respiratory and skin, and seriously harm
human health after entering the human body. Furthermore, many heavy metals in oil
sludge will be enriched by crops after penetrating the soil and finally go into the humanbody through the food chain, which will cause genotoxicity to humans [1-31].
Considering the harm of oil sludge to the ecological environment and human health,
it is essential to take adequate measures to deal with oil sludge.
1.3 Overview of oil sludge treatment method
A number of methods have been developed for the treatment of oil sludge. The
primary principle of oil sludge treatment is to achieve volume reduction and hazardous
substances removal, and then resource recovery. Therefore, the treating methods can beclassified into two groups: harmless disposal-based methods and resource recovery6

Chapter 1

Introduction

based methods, as listed in Figure 1.3. Harmless disposal methods include oxidation,
stabilization, incineration, and bioremediation. Oil sludge is a kind of solid waste, andthe conventional disposal methods of solid waste are landfill and incineration, which
requires much space and poses a serious threat to the atmosphere and groundwater
environment [1-32]. Bioremediation is a novel and effective method to degrade organic
components in oil sludge into light-weight compounds, reducing its potential hazards[1-12]. However, the degradation of PAHs takes a longer time, and it is difficult todecompose heavy metals; therefore, this technique remains further development [1-13].
Although harmless disposal is relatively thorough, the high-quality oil components inoil sludge cannot be recovered.

Figure 1.3 Oil sludge treatment methods

According to the American Petroleum Institute (API) and Technical Policy for
Pollution Prevention and Control in the Oil and Gas Exploitation Industry issued by
China’s Ministry of Environmental protection, the primary environmental
consideration in handling oil sludge should be the maximization of hydrocarbon
recovery. Therefore, resource recovery method for oil sludge treatment is the current
research hotspot and this work is also focused on this field. Previous studies suggested
that the oil sludge with oil content > 10% deserves oil recovery [1-33]. Resource
recovery method generally includes solvent extraction, centrifugation, froth flotation,
7

Chapter 1

Introduction

pyrolysis, ultrasonic, microwave heating, and freeze. The advantages and disadvantagesof these methods are summarized in Table 1.1. During solvent extraction, the oil phasein oil sludge is dissolved in a specific solvent and recovered by evaporation. The
remaining water and solids are removed by centrifugation [1-34]. This process is indeed
feasible and efficient for oil recovery. However, a large number of solvents are
consumed, and additional energy input is needed for evaporation and centrifugation [135]. Additionally, the volatilization of solvents will cause secondary pollution during
extraction. Although centrifugation and froth flotation have been widely used due totheir high effectiveness and low-cost, high-energy consumption and incomplete
separation of oil and solid make the obtained oil unsatisfactory [1-36]. Ultrasonic andmicrowave heating show relatively high efficiency of oil recovery compared to othermethods, whereas these emerging methods are still limited by their high operation cost
and technical problem of scaling up [1-37, 1-38, 1-39]. The freeze method caneffectively break the emulsified status of oil and water in oil sludge, then oil can berecovered [1-40]. However, this method is restricted by many factors, such as freezing
temperature, heating and cooling rate, and water content. For oil-rich oil sludge,
especially oil tank sludge, the dewatering effect is not obvious [1-41]. The freeze
method is more suitable to be applied in cold areas, where the natural freeze can saveenergy.
Among these methods, pyrolysis is one of the resource recovery technologies worth
in-depth research and industrial application. Pyrolysis of oil sludge is conducted attemperatures ranging from 350 to 600 °C in an inert atmosphere. Through pyrolysis,
the organic substances in oil sludge are degraded, generating low-molecular-weight
condensable oil and small-molecular non-condensable gas, which poses elevatedheating value than raw oil sludge and can be selectively produced by controlling theoperation conditions [1-42]. As mentioned above, resource recovery methods are often
limited by the properties of oil sludge, as the composition of oil sludge varies fromsource to source and with the storing period. The advantage of pyrolysis is that thistechnology poses good material adaptability and can eliminate or weaken the feedstock
8

Chapter 1

Introduction

impact on processing efficiency, especially oil sludge with high viscosity or high ash.
The oil sludge-derived oil can be easily stored and transported, with a quality similar to
low-grade petroleum distillates produced by refineries. From commercialization andpractical application perspective, the mobility of the pyrolysis unit can facilitate theprocess implementation. For example, pyrolysis units can be directly transported to theoilfield sludge producing site, reducing transporting costs of waste to treatment plants[1-43].

9

Chapter 1

Introduction

Table 1.1 Summary and comparison of oil sludge resource recovery methods
MethodCost

Oil recovery

Scale

Advantages

Solvent extractionHigh50-90%

Large

Effective;

Disadvantages
large

processing A large volume of organic solvent consumption;

capacity
Centrifugation

Medium

≤75%

Large

secondary pollution

Effective;

large

processing High energy consumption; noise problems

capacity
Froth flotation

Medium

≤75%

Large

Simple operation; less expensive

Not suitable for high viscosity sludge; incomplete
separation

FreezeHigh50-75%

Small

Simple and cleaning operation

Low efficiency; geographical limitation

UltrasonicHigh

50-90%

Small

Efficient; no secondary pollution

High cost of equipment; low treatment capacity

Microwave heating

High

≥90%

Small

High efficiency

Specificequipment

investment;highenergyconsumption
PyrolysisHigh50-90%

Large

Rawmaterial

adaptability; High operation cost; dewatering pretreatment ismobility; high oil yield10

required; low economic value of oil productChapter 1

Introduction

Many relevant studies have proved that pyrolysis is a promising and ideal choice todeal with oil sludge. The effect of temperature, heating rate, and residence time on
pyrolysis behavior and the properties of pyrolysis products have been evaluated using
thermogravimetric analyzer (TGA) [1-44, 1-45, 1-46] and fixed bed reactor [1-47, 148, 1-49]. Generally, the pyrolysis process of oil sludge can be divided into two steps:
degradation of volatile compounds and decomposition of non-volatile matters. The
degradation of volatile compounds mainly occurs between 150-600 °C with three substages: vaporization of light hydrocarbons and remaining water (150-250 °C),
desorption and thermal cracking of light and medium-molecular-weight hydrocarbons,(250-400 °C), and decomposition and cracking of heavy hydrocarbons (400-600 °C)[1-43, 1-44, 1-45]. The temperature ranges of these three stages fluctuate and overlap
for oil sludge from different sources and under different heating rates. Higher heating
rates and longer pyrolysis time facilitate the degradation of heavy compounds, shifting
forward its corresponding temperature windows [1-46]. Noteworthy, the decompositionof heavy hydrocarbon is accompanied by many secondary reactions, such asaromatization, cyclization, polymerization, re-condensation, and carbonylation [1-45].
At higher temperatures (> 600 °C), degradation of organic fractions terminates, and thedecomposition of minerals in oil sludge dominates, i.e., CaCO3 → CaO + CO2, K2SO4→ K2O + CO2 + 1/2O2 [1-50] [1-51].
Oil sludge is converted to char, oil, and gas through pyrolysis. The recovered oil
contains a higher concentration of hydrocarbons with a boiling point ranging from 150
to 350 °C and can be used as an alternative for refining step or chemical production.
The gas product, including H2, CO, CH4, CO2, and CxHy, has a high possibility forcommercialization and clean utilization [1-43]. During pyrolysis of oil sludge, heavy
components are deposited on the surface of pyrolysis residue, known as coke [1-50];
meanwhile, a small amount of petroleum hydrocarbons remains in the solid residue
after pyrolysis due to the low operation efficiency, posing threat to the environment if
the solid residue is directly discharged [1-5]. Therefore, it is necessary to propose amethod that can simultaneously reuse the oil sludge pyrolysis derived volatiles (oil and
11

Chapter 1

Introductiongas), address the potential hazards of char products, and alleviate the high energy
consumption in the pyrolysis process.
1.4 Integrated pyrolysis and combustion of oil sludge
An integrated pyrolysis and combustion treatment has been put forward to achieve
above mentioned purpose [1-5, 1-51]. As shown in Figure 1.4, oil sludge firstly
undergoes pyrolysis and generate char, oil, and gas; the nascent char and gas products
continuously enter the combustion section, making full use of their residual value andeliminating the potential hazard of residual petroleum hydrocarbons in char (dischargeof char meets environmental standards). Then, the heat generated in the combustion
stage can be recycled for pyrolysis treatment. This scheme has also been applied todispose and utilize oil sand, oil shale, and coal [1-52] [1-53]. However, both pyrolysis
and combustion consume energy, making this integrated thermochemical treatment anenergy-intensive process [1-5]. Consequently, it is inevitable to modify this treatment,
reducing energy consumption and improving its market competitiveness. Since thiscoupled treatment consists of two thermochemical methods, improvements can beseparately implemented for the pyrolysis section and combustion section. For example,(1) the properties of oil sludge can be modified to improve its pyrolysis efficiency, (2)catalysts can be employed to improve the quality of pyrolysis products and thus enhancetheir secondary utilization value, and (3) substances with good combustion properties
can be added to improve the heating value and combustion efficiency of char product.
The former two methods can be referred to as co-pyrolysis and catalytic pyrolysis.
Simplicity and effectiveness are two critical criteria when developing a technique to
produce ideal synthetic liquid fuel. In this regard, co-pyrolysis of oil sludge might be
the better choice for commercial application in the future because of its attractive
performance and cost ratios compared to catalytic pyrolysis. Catalytic pyrolysis indeed
significantly improves the quality of pyrolysis products, especially oil and gas, which,however, consumes many catalysts, causing extra economic losses. Many studies haveshown that co-pyrolysis of oil sludge has significantly improved the oil quantity and
12Chapter 1

Introduction

quality without any improvement during system processing [1-47, 1-49]. Therefore,
this work focuses on improving the integrated pyrolysis and combustion by copyrolysis of oil sludge and co-combustion of oil sludge char.

Figure 1.4 Schematic diagram of the integrated pyrolysis and combustion treatment

1.4.1 Co-pyrolysis of oil sludge with other feedstocks
Table 1.2 summarizes some present oil sludge co-pyrolysis studies. Biomass andplastics are two widely selected feedstocks. Previous work indicated that the key to thesuccess of co-pyrolysis is the synergistic effect that comes from the reaction of different
materials. Hu et al. [1-54] found that oil sludge blending improved the H/C molar ratio
and higher heating value of pyrolysis oil compared to wood waste alone. Lin et al. [149] proposed that the synergistic effect between oil sludge and rice husk increased thearomatic and saturate hydrocarbons in oil by 55-86 % and 15-55%. Zhou et al. [1-55]
conducted co-pyrolysis of oil sludge with a series of agricultural biomass including rice
husk, walnut shell, and sawdust. They noticed that the co-pyrolysis process was
predominantly affected by the biomass composition, such as the proportion ofhemicellulose, cellulose, and lignin. The minerals in reactants were reported tocontribute to the synergistic effect during co-pyrolysis. For example, the ash and alkali
metals (K, Na, and Ca) in biomass can catalytically promote the pyrolysis process, andthe metal oxides in oil sludge such as Fe3O4, CaO, and Al2O3 can also enhance theconversion of char to volatiles [1-47] (Zhu 2020). Milato et al. [1-48, 1-57] observedthat adding polyolefins (HDPE, LDPE, and PP) increased the yields of paraffinic
products and improved the light fractions in oil.

13

Chapter 1

Introduction

Table 1.2 A brief review of co-pyrolysis of oil sludge and other feedstocks
Type

of

Additives

Temperature

Results

References

TGA;

bed

➢

OS derived oil has better properties and has a great potential for fuel source.

[1-54].

tube furnace reactor➢

OS blending improved the H/C ratio and HHV of pyrolysis oil comparing to wood(°C)

oil sludge
Tank

Pyrolysis reactors

Wood waste

400-600

bottomFixedsludgeTankderived bio-oil.
Rice husk

600

bottomHorizontal

fixed-➢

bed reactor[1-49].

content in oil by 55–86% and 15–55%.
➢

sludgeThe synergistic effect between OS and rice husk increased the aromatic and saturate
Catalytic effect of ash and alkali metals derived from biomass, promoting the secondary
reactions of liquid products.

Tank

Agricultural

bottombiomass:

sludgehusk,

500-800
rice

TGA;

Horizontal➢

quartz reactor

shell, sawdust,
Oilfield

Microalgae

sludgeresidue[1-55].

energy conservation.
➢

walnut

The presence of biomass reduced the activation energy during pyrolysis of OS, favoring
Biomass components (hemicellulose, cellulose, and lignin) affected the co-pyrolysis
process.

600

TGA; Tube furnace➢

reactorMicroalgae residue addition increased the oil content by suppressing the secondary[1-56].

cracking of heavy hydrocarbons to form gas products.
➢

Minerals in microalgae residue (Ca, K, and Fe) might play a vital catalytic effect in the
OS pyrolysis, affecting the product distribution.

Refinery

Sawdust500-900

sludgeTG-MS; Fixed bed➢

Sawdust addition promoted the increase of gas yield (H2 and CO).

reactor➢

Sawdust addition increased the carbon content and enhanced the reduction of minerals,[1-47].

for example, the Fe3O4 in solid was reduced by carbon with the generation of CO and
CO2, leading to the increase of gas yields.
Tank

Polyolefins

bottom(HDPE, LDPE,

sludgeand PP)

450

Fixed bed reactor➢

Polyolefins addition increased the paraffinic products in oil.

➢

However, the tertiary carbons in polyolefins inevitably yielded heavier hydrocarbons,requiring catalytic section for oil upgrading.

14[1-48] and [1-57].

Chapter 1

Introduction

It could be noted that previous studies prefer choosing biomass as additives for copyrolysis with oil sludge due to its abundant, renewable, and carbon-neutral
characteristics. For the ash-rich oil sludge, such as the oilfield sludge used in this work,
the addition of biomass can remedy the lack of organic substances in oil sludge topromote the pyrolysis efficiency; lead to enhanced production of volatiles (oil and gas),
and increase the carbon content in char product, thereby improving the recycling valueof pyrolysis products. Plastics and oil sludge are both crude oil-derived matters, andplastic addition can significantly increase the yield of pyrolysis oil. Still, heavy
fractions are accumulated in oil, requiring secondary cracking such as employing
catalysts in the pyrolysis process [1-57].
1.4.2 Co-combustion of oil sludge char with biomass
Biomass, one of the leading renewable energy resources, has attracted worldwide
attention. Through proper utilization, biomass can act as an alternative fuel and play asupplementary role in meeting the world’s energy demand [1-58]. Combustion is themost efficient and sustainable technology among biomass utilization methods due to its
high reactivity and carbon-neutral characteristic [1-59]. Many researchers haveconducted systematic techno-economic analyses on the co-combustion of coal andbiomass [1-60]. These results confirmed the vast potential of biomass in replacing fossil
fuel as an energy source. Given these aspects, co-combustion of biomass and other solid
waste is a promising measurement for waste treatment, especially for the wastes withhigh ash content and cannot be directly landfilled, during which these wastes could be
effectively disposed of, and their additional values could be recovered. Previous studies
pointed out that biomass blending could effectively improve the combustion propertiesof oil sludge char. Gong et al. [1-61] and Wang et al. [1-62] studied co-combustion ofoil sludge char with microalgae residues. Their results demonstrated that the additionof microalgae residues increased the volatile content, promoted the combustion
property of oil sludge char, and reduced heavy metal ecotoxicity in combustion fly ash.
Moreover, SO2 emission from the co-combustion was decreased due to the catalytic
15

Chapter 1

Introduction

cracking effect of metal and metal oxides in OSC.
1.5 Research gaps in pyrolysis/combustion of oil sludge/oil sludge char
According to current related studies, biomass addition can significantly promote thec-processing efficiency and the synergy between oil sludge and biomass improves thequality of products. However, there still exist some research gaps in co-pyrolysis of oil
sludge and biomass, and co-combustion of oil sludge char and biomass:(1) The quality of oil sludge and biomass co-pyrolysis-derived oil needs further
improvement. Biomass addition significantly increases pyrolysis oil yield compared tooil sludge alone. However, the oil still contains a relatively high concentration ofoxygenates, rendering it corrosive and chemically unstable, thus limiting the large-scalecommercial application. Few studies have been carried out to further improve thequality of oil obtained from co-pyrolysis of oil sludge and biomass.
(2) The synergistic behavior during oil sludge and biomass co-pyrolysis isunderstudied. The organic components in oil sludge are mainly petroleum hydrocarbons,
while those in biomass are mainly oxygenates, entailing interactions between oil
sludge-derived hydrocarbon intermediates and biomass-derived oxygen-containing
radicals. Previous studies mainly conducted co-pyrolysis of oil sludge and specificbiomass and speculated the synergistic effect based on the properties of pyrolysis
products. However, biomass composition is complex, and the proportion of basic
components (mainly hemicellulose, cellulose, and lignin) varies among biomass fromdifferent sources. Therefore, it is meant to carry out studies on co-pyrolysis of oil sludgeof biomass with typical characteristics.
(3) The co-combustion mechanism of oil sludge char and biomass remains for further
investigation. Current research on co-combustion of oil sludge char and biomass islimited to raw biomass utilization to improve the combustion characteristics of oil
sludge char. The interactive mechanisms of oil sludge char and biomass co-combustion,such as the nature of synergy/inhibition during the co-combustion of oil sludge char
16

Chapter 1

Introduction

and biomass, are still uncertain. Furthermore, the alkali metals in biomass could also
act as catalysts during co-combustion in addition to causing fouling and slagging
problems. Therefore, the effect of alkali metals on the co-combustion mechanism
should be evaluated. Moreover, kinetic analysis of oil sludge char and biomass cocombustion is still weak and requires in-depth inspection.
1.6 Objective of present work
The main objective of this study is to investigate the interactions, kinetics, andmechanisms during co-pyrolysis/co-combustion of oil sludge/oil sludge char andbiomass. Two widely used thermochemical pretreatments are employed to modify
biomass without the consumption of chemicals. Torrefaction, a mild pyrolysis process
operating between 200-300 °C at a low heating rate, is an effective pretreating method
to alter the physicochemical properties of biomass, after which the content ofholocellulose (hemicellulose and cellulose) decreases while lignin content gets anincrease [1-63]. Moreover, the oxygenated compounds in pyrolysis oil of torrefied
biomass are significantly reduced. Therefore, torrefaction pretreatment is firstly
conducted to (1) further promote hydrocarbon production in pyrolysis oil and (2)support analyzing the synergistic behavior during pyrolysis of oil sludge and typicalbiomass components (hemicellulose, cellulose, and lignin).
Hydrothermal treatment (HTT), which is defined as a treatment of biomass in hotcompressed water at temperatures within 180-260°C (also referred to as hydrothermal
carbonization in this temperature range), has been regarded as a promising andenvironmentally acceptable method to upgrade biomass [1-64]. The hydrochars,
generated by HTT, exhibit ameliorated homogeneity and higher fuel properties than
raw biomass, such as lower oxygen content, higher carbon content, higher calorificvalues, and enhanced reactivity [1-65]. On the other hand, most alkali metals are
removed during HTT [1-66]. The problems (fouling and slagging) can be avoided orsignificantly mitigated, which also assists in evaluating the effect of alkali metals in the
co-combustion of oil sludge char and biomass.
17

Chapter 1

Introduction

Above all, the work of this study includes: (1) investigate the effect of torrefaction
on the morphological structure evolution and pyrolysis reactivity of biomass; (2)conduct co-pyrolysis of oil sludge and raw/torrefied biomass to enhance hydrocarbon
production as well as evaluating the co-pyrolysis synergistic behavior among oil sludge
and model biomass components; and (3) co-combustion of oil sludge char andraw/hydrothermally treated biomass are performed to promote combustion efficiency
and evaluate the effect of alkali metals on co-combustion interactions, kinetics, andmechanisms.
1.7 Outline of the thesis
This thesis consists of 5 chapters and the details of each chapter are as follows:
Chapter 1: Introduction
In this chapter, the background of this thesis is explained. The classification,
characterization, and hazards of oil sludge are listed. Then, the oil sludge treatment
methods are briefly introduced. It could be summarized that the pyrolysis technique has
the advantage of good sample adaptability compared to other methods. Finally, theresearch gaps in existing studies and the research objectives of this work are clarified.
Chapter 2: Effect of torrefaction on the structure and pyrolysis reactivity of ricestraw
In Chapter 2, rice straw, a typical agricultural biomass waste, underwent torrefaction
at 200, 250, and 300 °C. After torrefaction, the proportion of hemicellulose, cellulose,
and lignin was significantly altered, where severe torrefaction (300 °C) resulted inlignin enrichment. Consequently, torrefaction led to enhancement in surface
aromaticity and reduction in crystallinity. Thermogravimetric results indicated thatsevere torrefaction increased rice straw's main decomposition temperature range andweakened the pyrolysis reactivity as regarding the lowered comprehensive pyrolysis
index. Torrefaction altered the pyrolysis pathway of rice straw to enhance charproduction and inhibit volatile (oil and gas) generation. Nonetheless, the hydrocarbon
18

Chapter 1

Introduction

concentration in oil significantly increased.
Chapter 3: Effect of torrefaction on synergistic behavior during co-pyrolysis of oil
sludge with rice straw to enhance hydrocarbon production
The physicochemical properties of rice straw before and after torrefaction werealready characterized in Chapter 2. In Chapter 3, torrefied rice straw was continuously
incorporated into co-pyrolysis with oil sludge to assist evaluating the pyrolysis
synergistic effect between oil sludge and rice straw. Co-pyrolysis of oil sludge and ricestraw yielded a synergistic impact on gas production and promoted char conversion tovolatiles. The co-pyrolysis significantly promoted the formation of alkanes and olefins
while suppressing the generation of oxygenates in oil. The incorporation of torrefied
rice straw into co-pyrolysis yielded a positive synergistic effect on oil generation andfurther enhanced the production of alkanes and olefins. In addition, heavy-weightstraight-chain alkanes were observed from co-pyrolysis of oil sludge and torrefied ricestraw.
Chapter 4: Co-combustion of oil sludge char with raw/hydrothermally treatedbiomass: interactions, kinetics, and mechanisms
Chapter 2 and Chapter 3 demonstrated that biomass addition yielded higher oil
production than oil sludge pyrolysis alone, and a positive synergistic effect was
witnessed in hydrocarbon production, representing improved quality. This chapter aims
at the disposal of oil sludge pyrolysis residue (char), simultaneously recycling its
remaining economic value. Cherry blossom wood incorporation increased the
combustion property and efficiency of oil sludge char regarding the values of several
combustion indexes. Wood blending with oil sludge char resulted in negativeinteractions due to the inhibited diffusion of volatiles. The hydrothermal treatment
developed porous structure of raw wood, promoting the gas diffusion and heat transfer;
thereby its addition into oil sludge char yielded positive interaction. Moreover, diffusion
models were the most efficient kinetic models for oil sludge char/raw wood combustion,
while chemical reactions were found to be the rate-determining steps for oil sludge
19Chapter 1

Introduction

char/hydrothermallytreatedwood

combustion.

The

interactions

betweenraw/hydrothermally treated wood and oil sludge char resulted in opposite variation
trend of apparent activation energies compared to individual biomass. The catalytic
effect of inorganic elements and their physical influence on heat and mass transfer cancontrol the co-combustion performance of oil sludge char and biomass.
Chapter 5: Conclusions and recommendations
This chapter summarizes the important findings on co-pyrolysis/co-combustion ofoil sludge/oil sludge char and biomass. In addition, practical implications of thisresearch and recommendations for future work are provided.
The relationship between each chapter is illustrated in Figure 1.5:

Figure 1.5 The thesis contents and their relationship

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[1-39] Elektorowicz, M., & Muslat, Z. (2008). Removal of heavy metals from oil sludge
24Chapter 1

Introduction

using ion exchange textiles. Environmental technology, 29(4), 393-399.
[1-40] Hu, G., Li, J., & Hou, H. (2015). A combination of solvent extraction and freeze
thaw for oil recovery from petroleum refinery wastewater treatment pond sludge.
Journal of hazardous materials, 283, 832-840.
[1-41] Jean, D. S., Chu, C. P., & Lee, D. J. (2001). Freeze/thaw treatment of oily sludge
from petroleum refinery plant. Separation science and technology, 36(12), 2733-2746.
[1-42] Zhang, S., Yan, Y., Li, T., & Ren, Z. (2005). Upgrading of liquid fuel from thepyrolysis of biomass. Bioresource technology, 96(5), 545-550.
[1-43] Chang, C. Y., Shie, J. L., Lin, J. P., Wu, C. H., Lee, D. J., & Chang, C. F. (2000).
Major products obtained from the pyrolysis of oil sludge. Energy & Fuels, 14(6), 11761183.
[1-44] Cheng, S., Chang, F., Zhang, F., Huang, T., Yoshikawa, K., & Zhang, H. (2018).
Progress in thermal analysis studies on the pyrolysis process of oil sludge.
Thermochimica acta, 663, 125-136.
[1-45] Wang, Z., Guo, Q., Liu, X., & Cao, C. (2007). Low temperature pyrolysis
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[1-47] Zhu, J., Zhu, L., Guo, D., Chen, Y., Wang, X., & Zhu, Y. (2020). Co-pyrolysis ofpetrochemical sludge and sawdust for syngas production by TG-MS and fixed bed
reactor. International Journal of Hydrogen Energy, 45(55), 30232-30243.
[1-48] Milato, J. V., Franca, R. J., & Calderari, M. R. M. (2020). Co-pyrolysis of oil
sludge with polyolefins: Evaluation of different Y zeolites to obtain paraffinic products.
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Introduction[1-49] Lin, B., Huang, Q., & Chi, Y. (2018). Co-pyrolysis of oily sludge and rice husk
for improving pyrolysis oil quality. Fuel processing technology, 177, 275-282.
[1-50] Gong, Z., Liu, C., Wang, M., Wang, Z., & Li, X. (2020). Experimental study on
catalytic pyrolysis of oil sludge under mild temperature. Science of the Total
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[1-51] Wang, Z., Gong, Z., Wang, Z., Fang, P., & Han, D. (2018). A TG-MS study on
the coupled pyrolysis and combustion of oil sludge. Thermochimica acta, 663, 137-144.
[1-52] Jiang, X. M., Han, X. X., & Cui, Z. G. (2007). New technology for thecomprehensive utilization of Chinese oil shale resources. Energy, 32(5), 772-777.
[1-53] Golubev, N. (2003). Solid oil shale heat carrier technology for oil shale retorting.
Oil Shale, 20(3), 324-332.
[1-54] Hu, J., Gan, J., Li, J., Luo, Y., Wang, G., Wu, L., & Gong, Y. (2017). Extractionof crude oil from petrochemical sludge: characterization of products using
thermogravimetric analysis. Fuel, 188, 166-172.
[1-55] Zhou, X., Jia, H., Qu, C., Fan, D., & Wang, C. (2017). Low-temperature copyrolysis behaviours and kinetics of oily sludge: effect of agricultural biomass.
Environmental technology, 38(3), 361-369.
[1-56] Wang, Z., Gong, Z., Wang, W., & Zhang, Z. (2020). Study on combustion
characteristics and the migration of heavy metals during the co-combustion of oil
sludge char and microalgae residue. Renewable Energy, 151, 648-658.
[1-57] Milato, J. V., França, R. J., Rocha, A. S., & Calderari, M. R. M. (2020). Catalytic
co-pyrolysis of oil sludge with HDPE to obtain paraffinic products over HUSY zeolites
prepared by dealumination and desilication. Journal of Analytical and Applied
Pyrolysis, 151, 104928.
[1-58] Saidur, R., BoroumandJazi, G., Mekhilef, S., & Mohammed, H. A. (2012). A
review on exergy analysis of biomass based fuels. Renewable and Sustainable Energy
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Introduction

Reviews, 16(2), 1217-1222.
[1-59] Sahu, S. G., Chakraborty, N., & Sarkar, P. (2014). Coal–biomass co-combustion:
An overview. Renewable and Sustainable Energy Reviews, 39, 575-586.
[1-60] Mandegari, M., Farzad, S., & Görgens, J. F. (2018). A new insight into sugarcanebiorefineries with fossil fuel co-combustion: Techno-economic analysis and life cycle
assessment. Energy Conversion and Management, 165, 76-91.
[1-61] Gong, Z., Wang, Z., Wang, Z., Fang, P., & Meng, F. (2019). Study on the
migration characteristics of nitrogen and sulfur during co-combustion of oil sludge charand microalgae residue. Fuel, 238, 1-9.
[1-62] Wang, Z., Gong, Z., Wang, W., & Zhang, Z. (2020). Study on combustion
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sludge char and microalgae residue. Renewable Energy, 151, 648-658.
[1-63] Dai, L., Wang, Y., Liu, Y., Ruan, R., He, C., Yu, Z., ... & Tian, X. (2019).
Integrated process of lignocellulosic biomass torrefaction and pyrolysis for upgrading
bio-oil production: A state-of-the-art review. Renewable and Sustainable Energy
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[1-64] Bach, Q. V., Tran, K. Q., Khalil, R. A., Skreiberg, Ø., & Seisenbaeva, G. (2013).
Comparative assessment of wet torrefaction. Energy & Fuels, 27(11), 6743-6753.
[1-65] Bach, Q. V., Tran, K. Q., Skreiberg, Ø., Khalil, R. A., & Phan, A. N. (2014).
Effects of wet torrefaction on reactivity and kinetics of wood under air combustion
conditions. Fuel, 137, 375-383.
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investigation of co-combustion characteristics of blends of hydrothermally carbonized
oil palm biomass (EFB) and coal. Fuel processing technology, 118, 228-234.

27

Chapter 2

Effect of torrefaction on structure and pyrolysis property

Chapter 2 Effect of torrefaction on the structure andpyrolysis property of rice straw
2.1 Introduction
The shortage of fossil fuels and the issue of global climate change necessitate thedevelopment of renewable and sustainable resources to replace fossil fuels. Among thealternatives, biomass is considered a good alternative due to its abundance, low cost,and eco-friendly characteristics, which have been extensively investigated. Among
multiple utilization approaches of biomass, pyrolysis is an efficient method forconverting biomass into liquid oil, gas, and char [2-1]. The liquid oil can potentially be
employed as a substitute for conventional fuels after deoxygenation. High oxygen
content in biomass pyrolysis oil renders it corrosive and chemically unstable, thus
limiting the direct large-scale commercial application of pyrolysis technology [2-2].
Reducing bio-oil oxygenates via pretreatment is an efficient method to alleviate these
undesirable properties [2-3].
Torrefaction is frequently used for upgrading biomass wastes to obtain solid biofuel.
This moderate thermal process improves biomass's grindability, energy density, andhydrophobicity [2-4, 2-5]. It is well known that deoxygenation of biomass is the crucial
goal of torrefaction pretreatment, and several reactions, like dehydration,
decarboxylation, and decarbonylation, occur during this process [2-6, 2-7].
Consequently, the hydrocarbon content is selectively increased, improving bio-oil
quality. Sabil et al. [2-8] found the oxygen content of palm skin and shell decreased
from 47 to 38% and 50 to 43% after torrefaction at 300 °C, respectively. Chen et al. [24] reported that the oxygen content of cotton stalk decreased from 37.2 to 17.5% as
torrefaction temperature increased from 0 to 280 °C, resulting in the reduction of waterand acid contents in bio-oil. Zheng et al. [2-9] carried out torrefaction of pine wood at
240 – 320 °C and reported improvements in aromaticity, heating value, and bio-oil
density.
28

Chapter 2

Effect of torrefaction on structure and pyrolysis property

In China, rice straw (RS) is widely generated but is usually directly abandoned andburned, causing tremendous environmental concerns. After proper processing, thisagricultural waste can be converted into value-added biofuel. Accordingly, this chapter
characterizes RS's morphological structure evolution and pyrolysis property under
different torrefaction conditions, the structure of Chapter 2 is illustrated in Figure 2.1.
The microstructure of rice straw before and after torrefaction was analyzed via Fouriertransform infrared (FTIR) spectrometer, X-ray diffraction (XRD) pattern, and Scanningelectron microscopy (SEM). The pyrolysis behavior was measured by athermogravimetric (DTG) analyzer. The composition of oil products was determinedby a gas chromatography-mass spectrometer (GC-MS). Finally, the effect of
torrefaction on pyrolysis reactivity and oil properties of rice straw was discussed.

Figure 2.1 Structure of Chapter 2.

2.2 Materials and methods
2.2.1 Materials and torrefaction pretreatment
The RS sample was purchased from a gardening market in Japan. As-received
samples were firstly dried in an oven at 105 °C for 24 h, then milled and sieved smaller
than 180 μm. Torrefied rice straw (TS) samples were obtained by torrefaction of RS ina laboratory-scale fixed bed reactor as described in Figure 2.2. Before torrefaction,around 10 g sample was put in a hanging basket (quartz material) and placed in theheating area; N2 was then fed into the reactor with a flow rate of 300 mL/min for 1029

Chapter 2

Effect of torrefaction on structure and pyrolysis property

min to create an air-free environment; thereafter, the reactor was heated from zoom
temperature to 200, 250, and 300 °C and kept for 30 min, respectively. Torrefaction
experiments at each temperature were repeated four times, and the solid residue
obtained from each run was mixed for subsequent analysis. Torrefied samples were
tagged as TS200, TS250, and TS300 representing TS received at each target
temperature.

Figure 2.2 Schematic diagram of the fixed bed reactor for torrefaction.

2.2.2 Characterization of RS and TS
The proximate analysis of samples was measured by a muffle furnace according to
GB/T212-2008. The ultimate analysis was conducted by an elemental analyzer (Vario
Microcube, Bruker, Germany), and the obtained results were used to calculate thehigher heating value (HHV) via eq 1 [2-10]:
HHV = 357.8C + 1135.6H + 54.9N + 119.5S – 85.4O – 974(1)

Where C, H, N, S, and O represent the weight percentages of carbon, hydrogen,nitrogen, sulfur, and oxygen, respectively. The biochemical analysis of RS and TS(contents of hemicellulose, cellulose, and lignin) were determined according to the Van
Soest method [2-11]. All these above results are listed in Table 2.1. The ash
30

Chapter 2

Effect of torrefaction on structure and pyrolysis property

composition of RS and TS was determined using X-ray fluorescence spectrometry(XRF; S2 Ranger Bruker, Japan) analysis with X-ray tube of palladium anode and theresults are tabulated in Table 2.2. Morphological surface characteristics of RS and TS
were determined by scanning electron microscopy (SEM; JSM-6610LA, JEOL, Japan)
with an acceleration voltage of 5 kV. The alkali metals concentration of RS and TS was
measured with an inductively coupled plasma optical emission spectrometerit speculate(ICP-AES; ICPE-9000, Shimadzu, Japan). Powder X-ray diffraction analysis (XRD;
Bruker D8, Japan) was performed to identify the crystalline structure of RS and TS with
Cu Kα radiation in the scanning range of 2θ = 10 - 80°. To investigate the effect of
torrefaction on the crystallinity of samples, crystallinity index (CrI) was calculated by
eq 2 [2-12]:I

CrI = 002I

- Iam

002

(2)

Where I002 (at 2θ around 22 °) is the maximum peak intensity, representing the crystal
part of the sample, while Iam (at 2θ around 18 ° ) is the minimum intensity,
representing the amorphous part.
The functional groups of RS and TS were measured by an ATR-FTIR analyzer. The
Fourier-transformed infrared (FTIR) spectra were measured from a collection of 32
scans per spectrum at a resolution of 4 cm-1 in the range of 4000 to 400 cm-1. Semiquantitative analysis was performed with the integrated area (IA) from FTIR spectrum
to further measure the macromolecular structure of RS and TS. A-factor, CH2/CH3, and
Aar/Aal were calculated by the peak integrated area. The relevant equations are as
follows:
A - factor = (IA3000 - 2800cm-1 )⁄(IA3000 - 2800cm-1 + IA1650 - 1520cm-1 ) (3)CH2 ⁄CH3 = (IA2920 + IA2860cm-1 )⁄(IA2950 + IA2890cm-1 )(4)

Aar ⁄Aal = (IA1650 - 1520cm-1 )⁄(IA3000 - 2800cm-1 )(5)

Where A-factor represents the hydrocarbon-generating potential, CH2/CH3 represents
31

Chapter 2

Effect of torrefaction on structure and pyrolysis property

the length of aliphatic length, and Aar/Aal represents the aromaticity of samples.
2.2.3 Experimental apparatus and method
Thermogravimetric analysis of samples was conducted by a thermogravimetricanalyzer (TG-DTA; TA-60WS, Shimadzu, Japan). For each run, around 10 mg sample
of RS and TS was heated from 30 to 900 °C under a nitrogen gas flow rate of 100
ml/min. The heating rate was controlled at 20 °C/min.
Three typical temperature parameters, ignition temperature (Ti), burnout temperature(Tb), and peak temperature (Tmax) were obtained from the thermogravimetry anddifferential thermogravimetry (TG-DTG) curves for pyrolysis performance analysis.
The tangent method was employed to determine Ti [2-13]. Tb and Tmax corresponded tothe temperature at 98% conversion rate and the maximum weight loss rate, respectively[2-14]. Three indexes, the ignition index (Di), burnout index (Db), and comprehensive
pyrolysis index (CPI) were calculated to evaluate the pyrolysis properties of samples.
These indexes were determined by eqs 6-8 [2-15]:-Rp

Di = t × ti

Db = ∆t
CPI =

p

-Rp
1/2 × tp × tb(-Rp ) × (-Rmean )T2i × Tb

(6)
(7)
(8)

where Rp denotes the maximum weight loss rate (unit: %/min), Rmean denotes the mean
weight loss rate (unit: %/min), ti, tp, and tb represent the time corresponding to Ti, Tmax,
and Tb, and ∆t1/2 represents the time interval between half value of Rp (unit: min).
Pyrolysis experiments were performed by the same reactor as torrefaction
pretreatment (refer to Figure 2.1). The sample was placed in a quartz basket and hungin the heating area of the heater. Then the reactor was controlled to be air-free by 10
min’s nitrogen gas purging with a flow rate of 300 mL/min. Afterward, the reactor washeated from room temperature to 600 °C at a heating rate of 20 °C/min and maintained32

Chapter 2

Effect of torrefaction on structure and pyrolysis property

for another 20 min. The volatile product was swept out through a condenser tube filledwith ice-cooling water, and the liquid product was finally collected in an Erlenmeyer
flask located in an ice-water bath. After the reaction time was completed, the reactor
was naturally cooled to room temperature with nitrogen gas purging. The residue wasremoved, weighed, and stored in a sample bottle. The yield of liquid product(condensable volatile) was obtained by calculating the weight differences of theErlenmeyer flask and the reactor before and after reaction. The yield of gas product wasdetermined by mass balance. Duplicated experiments were performed to confirm therepeatability validation, and the average results were calculated for analysis.
2.2.4 Characterization of oil composition
Gas chromatography-mass spectrometer (GC/MS; Shimadzu QP 2010, Japan) wasapplied to determine the chemical composition of the oil products. Anhydrous sodium
sulfate was added to the liquid product to remove the aqueous fraction prior to oilanalysis. Then, the filtered oil product was diluted by dichloromethane at a ratio of 1:20
and further filtered through a 0.22 μm filter film; the obtained oil was injected into
GC/MS for analysis. A Shimadzu SH-Rxi-5Sil MS Capacity Column (30 m, 0.25 mm,
and 0.25 μm) was selected for chromatographic separation. The initial oven temperature
was maintained at 40 °C for 10 min, then it was increased to 260 °C at a heating rateof 5 °C/min and then kept for 6 min. Helium (99.999%) was employed as the carrier
gas at a flow rate of 1.0 mL/min, the split ratio was set as 50:1, and the solvent delay
was set at 3 min. The chromatographic peaks were identified by referring to the
National Institute of Standards and Technology (NIST) spectral library and literature
data.

33

Chapter 2

Effect of torrefaction on structure and pyrolysis property

2.3 Results and discussion
2.3.1 Characterization of RS and TS
2.3.1.1 Ultimate analysis, proximate analysis, and composition analysis
The main characteristics of RS and TS are presented in Table 2.1. Ultimate analysis
indicates that the carbon content increased with increasing torrefaction temperature,
whereas the hydrogen and oxygen contents decreased. As a consequence, the HHV of
RS is enhanced after torrefaction, suggesting improved fuel properties. Meanwhile,
H/C and O/C molar ratios of RS and TS both decreased as torrefaction intensified,
implying that dehydration and deoxygenation reactions dominate the devolatilizationof RS during torrefaction, as confirmed by the Van Krevelen diagram in Figure 2.3.
Table 2.1 Ultimate analysis, proximate analysis, and composition of samples.
RS

TS200

TS250

TS300

Ultimate analysis (wt. % dry basis)
H

4.82

4.63

3.87

3.25

C

34.82

37.19

41.56

41.38

N

0.83

0.94

1.18

1.23

S

0.52

0.59

0.68

0.53

1

39.23

34.43

21.1

15.34

H/C

1.66

1.49

1.12

0.94

0.84

0.69

0.38

0.28

13.72

14.77

16.63

16.34

O

O/C
HHV

2

Proximate analysis (wt. % as received basis)
Ash

19.78

22.22

31.61

38.27

Moisture

6.63

2.52

0.96

0.55

Volatile

71.06

67.08

50.25

35.96

Fixed carbon

2.53

8.18

17.18

25.22

Composition of RS and TS (wt. %)Hemicellulose

4.67 ±0.62.36 ±1.100

Cellulose

31.36 ±1.2133.98 ±0.23

11.56 ±0.44

1.98 ±0.87

Lignin33.55 ±1.32

34.54 ±0.93

46.18 ±0.35

58.86 ±1.83

1 O%=100%-C%-H%-N%-S%-Ash%; 2 higher heating value (MJ/mol).

34

Chapter 2

Effect of torrefaction on structure and pyrolysis property

2.4

Dehydration

1.8
H/C atmotic ratio

RS
TS200
TS250
TS300
OS

Demethanation

1.2

Decarboxylation

0.6

0.0
0.8

1.0

1.2

1.4

1.6

1.8

2.0

O/C atmotic ratio

Figure 2.3 Van Krevelen diagram of RS and TS.

According to proximate analysis, enhanced torrefaction reduced the volatile content
of RS from 71.06 to 35.96%, which was attributed to the devolatilization reactions of
RS under torrefaction, accompanied by the removal of light organic compounds andgases [2-9]; hence, in contrast, aggravated torrefaction conditions increased thecontents of fixed carbon and ash. Lignocellulosic analyses of RS and TS show thatsevere torrefaction led to higher lignin content at the expense of hemicellulose andcellulose, which is consistent with previous studies [2-5, 2-9]. The lower proportion ofhemicellulose and cellulose in TS250 and TS300 was ascribed to their chemicaldegradation as torrefaction processed. Furthermore, Table 2.3 shows that torrefaction
enhanced the concentration of alkali metals in TS. For example, Potassium (K), themain component of alkali metals, 300 °C torrefaction increased its concentration almost
two times higher.

Table 2.2 Ash composition of samples by XRF analysis.
Main chemical components (wt. %)
SiO2

Fe2O3

SO3

Al2O3

CaO

K2O

Na2O

Other

RS

76.65

2.54

0.44

1.71

2.14

9.63

0.57

6.32

TS200

76.63

2.56

0.46

1.73

1.96

9.05

0.46

7.15

TS250

77.61

2.38

0.39

1.69

2.12

8.06

0.53

7.22

TS300

75.71

2.37

0.45

1.75

2.03

10.82

0.48

6.39

35

Chapter 2

Effect of torrefaction on structure and pyrolysis property

Table 2.3 Concentration of typical AAEMs in RS and TS by ICP analysis (mg/kg).
K

Ca

Mg

Al

Fe

RS

13932.53

3080.39

1173.14888.541675.92

TS200

15503.52

3628.481331.76

1134.56

1987.63

TS250

20950.75

3934.16

1831.971196.932661.11TS300

26960.654724.642348.161670.273590.37

2.3.1.2 Microstructure of RS and TS
The FTIR spectra of RS before and after torrefaction are displayed in Figure 2.4 (a).
It could be observed that the spectra of RS and TS shared similar trends under different
torrefaction temperatures. Several characteristic peaks in area (1) 3030 to 3643 cm-1,
area (2) 2818 to 3000 cm-1, area (3) 1535 to 1780 cm-1, area (4) 864 to 1280 cm-1, andarea (5) 709 to 848 cm-1 are detected, attributed to the vibration of -OH stretching,vibration of aliphatic carbon C-H stretching, vibration of aromatic C=O stretching,vibration of aliphatic ether C-O-C and alcohol C-O stretching, and vibration of C-H
aromatic out-of-plane deformation, respectively. With increasing torrefaction
temperature, the peak intensity of OH functional groups in area (1) became weaker,
which confirmed the dehydration of RS during torrefaction. However, the peakintensity of aromatic-based functional groups in area (3) and (5) got enhanced,
suggesting the enrichment of lignin. Variation in the deconvoluted peak area of different
functional groups agreed well with the characteristics of RS and TS tabulated in Table
2.1.
Figure 2.4 (a) depicts the FTIR spectra of RS and TS, and Figure 2.4 (b) and (c)displays TS200’s infrared spectrum as examples for semi-quantitative analysis, where
the assignments of the peaks obtained from previous literature are listed in Table 2.4[2-16, 2-17].

36

Chapter 2

Effect of torrefaction on structure and pyrolysis property

Table 2.4 Parameters of fitted peaks from TS200’s infrared spectrum.
Peak

Center (cm-1)

Assignment

Area (%)

1785-1485 cm-1
11513.11

Stretching vibration of C=C in aromatic rings

0.1001

21550.38

Stretching vibration of C=C in aromatic rings

0.17693

1591.65

Stretching vibration of C=C in aromatic rings

0.27484

1639.06

Stretching vibration of highly conjugated C=O (such as

2.3053

quinones)51720.73

Stretching vibration of C=O in carboxylic rings

0.4442

3000-2800 cm-1
62828.49

CH symmetric stretching

0.023472851.24

CH2 symmetric stretching in alkanes

0.084582877.86

CH3 symmetric stretching

0.196792920.97

CH2 symmetric stretching in alkanes

0.4043

10

2966.69

CH3 symmetric stretching

0.0394

Several structural parameters obtained from curve fitting of FTIR spectra of RS and
TS are listed in Table 2.5 to further evaluate the effect of torrefaction temperature on
the evolution of functional groups in RS and TS. The A factor value of RS gradually
decreased from 0.21 to 0.12% as the intensity of torrefaction increased, implying thatsevere torrefaction weakened the hydrocarbon-generating potential of TS, which was
attributed to the enhanced devolatilization of RS during torrefaction. The CH2/CH3
value significantly increased from 0.48 to 2.07% when torrefaction temperature was set
at 200 °C, whereas it decreased as temperature further increased. This can be explained
as follows: under mild torrefaction, the ring structures of polysaccharides inhemicellulose are opened to generate aliphatic intermediates, and these intermediates
will combine with each other to form longer aliphatic chains, therefore behaving in ahigher CH2/CH3 value under 200 °C torrefaction [2-18]. As torrefaction temperature
continuously increases, depolymerization and fragmentation of nascent aliphatic chains
get exacerbated, such as the degradation of the side chains and chain end scissions,leading to shorter aliphatic chains of molecules, thereby resulting in a lower CH2/CH3
value [2-19, 2-20]. Furthermore, higher torrefaction temperature promotes theenrichment of lignin and its dealkylation (side chains) assists in increasing the content
of aromatic compounds. Consequently, the aromaticity index (Aar/Aal) of TS got
37

Chapter 2

Effect of torrefaction on structure and pyrolysis property

enhanced under severe torrefaction.

Figure 2.4 (a) FTIR spectra of RS and TS, (b) curve fitting of TS200 in 1785-1485 cm-1, and(c) curve fitting of TS200 in 3000-2800 cm-1.
Table 2.5 FTIR indexes deduced from FTIR spectra of RS and TS (%).
RS

TS200

TS250

TS300

A factor

0.21

0.20

0.13

0.12

CH2/CH3

0.48

2.07

1.75

0.83

Aar/Aal

3.83

3.68

6.27

7.27

The SEM images are shown in Figure 2.5 for an in-depth investigation of the surface
morphology of RS and TS. The surface of RS is characterized by a flat morphology
with relatively smooth epidermis. TS suggests that torrefaction removes or facilitates
the removal of the epidermis, resulting in a rough and fluffy structure. However, theinner skeletal morphology is not considerably altered, even under severe torrefaction(300 °C). The SEM results demonstrate that the surface microstructure of RS/TS is
associated with the devolatilization intensity during torrefaction.

38

Chapter 2

Effect of torrefaction on structure and pyrolysis property

Figure 2.5 SEM images of samples: (a) RS, (b) TS200, (c) TS250, and (d) TS300.

2.3.1.3 Crystallinity of RS and TS
Figure 2.6 illustrates the XRD patterns of RS and TS, and their calculated
Crystallinity Index (CrI) is also listed. As torrefaction temperature increased, the peakintensity at 2θ = 22° gradually decreased; simultaneously, several sharp peaks occurredat 2θ = 24°, 28°, 41°, and 51° on XRD patterns, representing the appearance ofpotassium chloride, which is in accord with the increasing concentration of potassiumin TS obtained under severe torrefaction. It was reported that the variation of CrI value
was predominantly determined by the competitive decomposition of crystalline
cellulose and amorphous cellulose in biomass [2-12]. As a result, the initial rise of CrI
value at a torrefaction temperature of 200 °C could be due to the recrystallization of
amorphous cellulose before being decomposed under intensified torrefaction. Then,
when the torrefaction temperature was set higher than 200 °C, the CrI value of TS
decreased from 30.03 to 7.41%, implying the constant loss of crystalline components,
especially crystalline cellulose, in TS. Similar results were well documented in previous
literature [2-21].
39

Chapter 2

Effect of torrefaction on structure and pyrolysis property

RS
TS200
TS250
TS300

§

Relative intensity (a.u.)

§

§

10

20

RS
TS200
TS250
TS300

§ Sylvite (KCl)

CrI (%)
27.05
30.03
12.22
7.41

30

§

40

50

§

60

70

80

2θ (°)

Figure 2.6 XRD patterns of RS and TS.

2.3.2 Thermal decomposition characteristics of RS and TS
The thermal decomposition curves of RS and TS are depicted in Figure 2.7 and thecorresponding pyrolysis characteristics are summarized in Table 2.6. TG profiles
represent the weight loss of samples, and DTG profiles reflect its corresponding weightloss rate. As shown in Figure 2.7 (b), the entire thermal decomposition process of RS
and TS mainly exhibited one weight loss peak, representing the overlapped
decomposition of hemicellulose, cellulose, and lignin. Generally, there should be ashoulder peak occurring at 200 °C before the predominant weight loss peak of biomass,representing the decomposition of hemicellulose which has low thermal stability [2-20].
In current work, this shoulder peak was difficult to distinguish, probably due to therelatively low hemicellulose content in RS (refer to Table 2.1) and a relatively highheating rate. Intensified torrefaction led to the degradation of components with low
thermal stability, in the sequence of hemicellulose, cellulose, and lignin, leaving
enrichment of recalcitrant portion in TS and hence enhancing its thermal resistance [22]. As a result of this, the pyrolysis properties of TS deteriorated according to thepyrolysis performance indexes listed in Table 2.6. Meanwhile, the main weight loss
40

Chapter 2

Effect of torrefaction on structure and pyrolysis property

region of TS was moved to higher temperature ranges as torrefaction intensified, as
shown in Figure 2.7 (b). It is worth noting that the maximum weight loss rate of TS200(-11.73 %/min) was slightly higher than that of RS (-11.24 %/min), which might be dueto the following two reasons. Firstly, the maximum decomposition rate of biomass
between 300 and 390 ° C is attributed to the depolymerization of crystalline
components in biomass [2-22]; Secondly, hemicellulose decomposes earlier thancellulose owing to its lower thermal stability, furnished with a liquid film covering thesurface of residues, hindering the subsequent decomposition of cellulose [2-23].
Therefore, the higher crystallinity degree and the weaker inhibition from hemicellulose
in TS200 promoted the maximum decomposition rate.
Table 2.6 Pyrolysis characteristics of RS and TS.
Sample

Ti

Tb

Tp

Rp

Rmean

Di

Db

CPI

RS

269.13850.25324.56

11.24

1.65

58.38

45.54

30.11

TS200

283.02853.17329.98

11.73

1.57

59.46

55.78

26.95

TS250

294.52872.93336.08

4.93

1.17

23.44

17.98

7.62

TS300

342.43881.58463.20

1.72

0.85

4.92

1.23

1.48

Ti: ignition temperature (°C); Tb: burnout temperature (°C); TP: peak temperature corresponding to maximum weightloss rate (°C); Rp: maximum weight loss rate; Rmean: mean weight loss rate; Di: ignition index (10-3%/min-3); Db:
burnout index (10-4%/min-4); CPI: comprehensive pyrolysis index (10^-8min-2°C-3)
110

(a)

RS
TS200
TS250
TS300

100

80

TG (%)(b)-2

DTG (%/min)

90

0

70
60
50
40

-4
-6
-8
-10

30

-12

20
200

300

400

500

600

700

800

200

Temperature (°C)

300

400

500

600

700

800

Temperature (°C)

Figure 2.7 (a) TG and (b) DTG curves of RS and TS.

2.3.3 Effect of torrefaction on pyrolysis of RS
2.3.3.1 Product yields distribution
Figure 2.8 shows the product yield distribution of individual pyrolysis. As
41

Chapter 2

Effect of torrefaction on structure and pyrolysis property

torrefaction temperature increased from 0 to 300 °C, the char yield increased from37.42 to 73.83%, whereas the oil and gas yield decreased from 27.45 to 6.54% and35.13 to 19.63 %, respectively. The pyrolysis product yields of RS and TS align with
their proximate analysis results, as listed in Table 2.1. During torrefaction, RS
underwent devolatilization, cross-linking, and carbonization, leaving TS withaccumulated alkali metals and enhanced surface aromaticity, which altered TS’s
decomposition pathways to promote char formation and reduce volatilization. On one
hand, enriched alkali metals in TS catalyzed the pyrolysis reactions, resulting in an
increase in char and gas yields at the expense of oil compared to RS. On the other hand,severe torrefaction led to TS being covered by thermal stable aromatic structures,
impeding the diffusion of volatiles [2-24]. Similar results on biomass torrefaction arereported elsewhere [2-23, 2-25]. Interestingly, the gas yield of TS200 (36.85%) was
higher than that of RS (35.13%) though an insignificant difference was observed, whichis consistent with the relatively higher maximum decomposition rate of TS200
compared to RS. Firstly, slight torrefaction partially removed hemicellulose andmarginally altered the composition of RS (refer to Table 2.1), thus the hindering effectof hemicellulose decomposition on gas release was undermined [2-23, 2-25, 2-26].
Secondly, TS200 contained more alkali metals, especially K, posing a more substantial
catalytic effect for opening glycosidic rings to produce light compounds and gases [227]. Previous studies also found that slight torrefaction might increase the pyrolysis gas
yield to some extent [2-24].

42

Chapter 2

Effect of torrefaction on structure and pyrolysis property

80
70

Char
Oil
Gas

Product yields (%)

60
50
40
30
20
10
0
RS

TS200

TS250

TS300

Figure 2.8 Product yields from individual pyrolysis of RS and TS.

2.3.3.2 Oil composition
The chemical composition of oil derived from pyrolysis of RS and TS is displayedin Figure 2.9. Detected chemical compounds are mainly divided into two categories:hydrocarbons and oxygenates. It should be noted that neither hydrocarbons nor
oxygenates in this study contain hetero elements other than C, H, and O. Oil generated
from RS pyrolysis was of poor quality as it was dominated by oxygenates, accounting
for 77.79%. The high concentration of oxygenates was resulted from the high oxygen
content and low carbon content in RS. After torrefaction, the property of oil was
significantly improved with a decreasing concentration of oxygenates (from 77.79 to
24.06%). Table 2.7 summarizes the concentrations of typical groups of oxygenatedcompounds. As indicated, torrefaction pronouncedly reduced the concentrations of acid,alcohol, aldehyde, anhydrosugar, ester, furan, and ketone in oil products. These
oxygenated compounds mainly evolved from the dehydration, depolymerization, andrearrangement during pyrolysis of holocellulose (hemicellulose and cellulose) [2-2, 243

Chapter 2

Effect of torrefaction on structure and pyrolysis property19], which concentration therefore extremely decreased as torrefaction consumed theholocellulose content in TS. Besides, accumulated alkali metals in TS were prone topromote carbonization reactions and formation of the lignin-derived char during TS
pyrolysis, suppressing the conversion of levoglucosan into light oxygenates [2-17, 223]. In contrast, the production of phenol was promoted with increasing torrefaction
temperature. This is associated with the increase in relative content of lignin, precursorof phenols, in TS.
Table 2.7 Relative concentrations of oxygenates in oil from individual pyrolysis of RS and TS.
RS

TS200

TS250

TS300

acid

14.72

13.39

7.83

0.78

alcohol

4.8

4.42

3.65

1.65

aldehyde

12.99

10.34

3.52

0

anhydrosugars

2.02

1.8

1.27

0

ester

5.52

4.22

0

0.57

furan

11.99

10.88

4.77

0

ketone

15.35

14.25

10.05

0.61

phenol

10.2

14.75

18.2

20.45

Sum

77.59

74.05

49.29

24.06

Contrary to the reduction of oxygenates, torrefaction enhanced the production ofhydrocarbons from 17.08 to 65.81%. As displayed in Figure 2.9 (a-1), (a-2), and (a-3),
the concentrations of alkane, aromatic, and olefin hydrocarbons all gradually increased
as torrefaction intensified, and the former two contributed to the main increase of totalhydrocarbons. Alkane and olefin hydrocarbons are principally generated from therearrangement of hydrocarbon radicals produced from ring-opening and deoxygenationof holocellulose, and the cleavage of alkyl side chains of lignin [2-19, 2-28]. Aromatichydrocarbons are mainly produced through depolymerization, oligomerization, anddemethylation from the aromatic structure of lignin. Therefore, enriched lignin content
in TS promoted the formation of hydrocarbons.

44

Chapter 2

Effect of torrefaction on structure and pyrolysis property(a) Hydrocarbon

Relative concentrations (%)

Relative concentrations (%)

100
80
60
40
20

60
40
20
0

0
TS200

TS250

Relative concentrations (%)

80
70 (a-1) Alkane
60
40
30
20
10
0

RS

TS300

24

(a-2) Aromatic

20
16
12
8
4
0

RS

TS200

TS250

TS300

TS200

Relative concentrations (%)

RS

Relative concentrations (%)(b) Oxygenate
80

TS250

TS300

12 (a-3) Olefin
10
8
6
4
2
0

RS

TS200

TS250

TS300

RS

TS200

TS250

TS300

Figure 2.9 Relative concentration of (a) hydrocarbon, (b) oxygenate, (a-1) alkane, (a-2)
aromatic, and (a-3) olefin in oil products from pyrolysis of RS and TS.

2.4 Conclusions
The effect of torrefaction on morphological structure evolution and pyrolysis
properties of rice straw are comprehensively investigated in this chapter.
(1) Intensified torrefaction led to enhanced surface aromaticity, rougher surface
structure, and decreased crystallinity of RS.
(2) The pyrolysis reactivity of RS was weakened after torrefaction, and thedecomposition temperature ranges of TS were shifted to higher temperatureswith increasing torrefaction temperature.
(3) Torrefaction altered the pyrolysis pathway of TS to enhance char production at
the expense of oil and gas.
(4) Torrefaction pretreatment enhanced hydrocarbon production, especially alkanes
and aromatics.
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A review of product properties and effects of pyrolysis parameters. Renewable and45

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sustainable energy reviews, 57, 1126-1140.
[2-2] Dai, L., Wang, Y., Liu, Y., Ruan, R., He, C., Yu, Z., ... & Tian, X. (2019).
Integrated process of lignocellulosic biomass torrefaction and pyrolysis for upgrading
bio-oil production: A state-of-the-art review. Renewable and Sustainable Energy
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[2-3] Saraeian, A., Nolte, M. W., & Shanks, B. H. (2019). Deoxygenation of biomass
pyrolysis vapors: Improving clarity on the fate of carbon. Renewable and Sustainable
Energy Reviews, 104, 262-280.
[2-4] Chen, W. H., Peng, J., & Bi, X. T. (2015). A state-of-the-art review of biomass
torrefaction, densification and applications. Renewable and Sustainable Energy
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[2-5] Chen, D., Li, Y., Deng, M., Wang, J., Chen, M., Yan, B., & Yuan, Q. (2016).
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[2-6] Chen, W. H., Lin, B. J., Lin, Y. Y., Chu, Y. S., Ubando, A. T., Show, P. L., ... &Pétrissans, M. (2021). Progress in biomass torrefaction: Principles, applications andchallenges. Progress in Energy and Combustion Science, 82, 100887.
[2-7] Van der Stelt, M. J. C., Gerhauser, H., Kiel, J. H. A., & Ptasinski, K. J. (2011).
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[2-8] Sabil, K. M., Aziz, M. A., Lal, B., & Uemura, Y. (2013). Effects of torrefaction
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Effect of torrefaction on structure and pyrolysis property[2-10] Cordero, T., Marquez, F., Rodriguez-Mirasol, J., & Rodriguez, J. J. (2001).
Predicting heating values of lignocellulosics and carbonaceous materials fromproximate analysis. Fuel, 80(11), 1567-1571.
[2-11] Van Soest, P. V., Robertson, J. B., & Lewis, B. A. (1991). Methods for dietaryfiber, neutral detergent fiber, and nonstarch polysaccharides in relation to animal
nutrition. Journal of dairy science, 74(10), 3583-3597.
[2-12] Zhang, C., Yang, W., Chen, W. H., Ho, S. H., Pétrissans, A., & Pétrissans, M.
(2022). Effect of torrefaction on the structure and reactivity of rice straw as well as
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[2-13] Li, X. G., Ma, B. G., Xu, L., Hu, Z. W., & Wang, X. G. (2006).
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[2-14] Seggiani, M., Vitolo, S., Pastorelli, M., & Ghetti, P. (2007). Combustion
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[2-15] Wang, C. A., Zhang, X., Liu, Y., & Che, D. (2012). Pyrolysis and combustion
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[2-18] Shen, D. K., Gu, S., & Bridgwater, A. V. (2010). Study on the pyrolytic
behaviour of xylan-based hemicellulose using TG–FTIR and Py–GC–FTIR. Journalof analytical and applied pyrolysis, 87(2), 199-206.

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Effect of torrefaction on structure and pyrolysis property[2-19] Chen, D., Gao, A., Cen, K., Zhang, J., Cao, X., & Ma, Z. (2018). Investigationof biomass torrefaction based on three major components: Hemicellulose, cellulose,
and lignin. Energy conversion and management, 169, 228-237.
[2-20] Collard, F. X., & Blin, J. (2014). A review on pyrolysis of biomass
constituents: Mechanisms and composition of the products obtained from theconversion of cellulose, hemicelluloses and lignin. Renewable and Sustainable
Energy Reviews, 38, 594-608.
[2-21] Neupane, S., Adhikari, S., Wang, Z., Ragauskas, A. J., & Pu, Y. (2015). Effectof torrefaction on biomass structure and hydrocarbon production from fast pyrolysis.
Green Chemistry, 17(4), 2406-2417.
[2-22] Leng, E., Zhang, Y., Peng, Y., Gong, X., Mao, M., Li, X., & Yu, Y. (2018). In
situ structural changes of crystalline and amorphous cellulose during slow pyrolysis at
low temperatures. Fuel, 216, 313-321.
[2-23] Khan, S. R., Zeeshan, M., & Masood, A. (2020). Enhancement ofhydrocarbons production through co-pyrolysis of acid-treated biomass and waste tirein a fixed bed reactor. Waste Management, 106, 21-31.
[2-24] Zheng, A., Zhao, Z., Chang, S., Huang, Z., Wang, X., He, F., & Li, H. (2013).
Effect of torrefaction on structure and fast pyrolysis behavior of corncobs.
Bioresource technology, 128, 370-377.
[2-25] Khan, S. R., & Zeeshan, M. (2022). Catalytic potential of low-cost naturalzeolite and influence of various pretreatments of biomass on pyro-oil up-gradation
during co-pyrolysis with scrap rubber tires. Energy, 238, 121820.
[2-26] Xue, Y., & Bai, X. (2018). Synergistic enhancement of product quality throughfast co-pyrolysis of acid pretreated biomass and waste plastic. Energy Conversion andManagement, 164, 629-638.
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27(10), 1607-1611.

49

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

Chapter 3 Effect of torrefaction on synergistic behavior
during co-pyrolysis of oil sludge with rice straw to enhancehydrocarbon production
3.1 Introduction
Co-pyrolysis is regarded as a simple and effective method to improve the quantityand quality of pyrolysis oil, which has attracted increasing attention [3-1]. Biomass isfrequently selected by researchers for co-pyrolysis with organic wastes due to its
abundant, renewable, and carbon neutral characteristics [3-2]. Wang et al. [3-3] foundthe interactions between sewage sludge and rice straw converted char products intopyrolysis gas and oil, and a 60% blending ratio of rice straw yielded the strongest
interaction. Zhang et al. [3-4] noticed the addition of rice husk into co-pyrolysis withsewage sludge enhanced gas production, and the calorific value of gas products was
significantly increased. Martinez et al. [3-5] obtained a more stable oil product withlower oxygen content and higher calorific value through co-pyrolysis of wood chips
and waste tires.
The results in Chapter 2 indicated that torrefaction pretreatment altered thephysicochemical properties of rice straw, especially increasing the concentration ofhydrocarbons in pyrolysis oil. Recently, combining torrefaction and co-feeding withpetroleum-based feedstock to improve the quality of pyrolysis oil from biomass has
attracted researchers’ attention [3-6]. For example, Bu et al. [3-7] fed low-density
polyethylene (LDPE) as a hydrogen source into microwave pyrolysis of microwave
torrefied lignin (MTL). Their results demonstrated that the hydrocarbon concentrationin the oil obtained from co-pyrolysis increased to around 80%, especially the aromatichydrocarbons rose from 1.9% to 22.8%. Huang et al. [3-8] conducted co-pyrolysis ofChinese herb residues (CHR) and low-density polyethylene (LDPE), and the results
indicated that torrefaction of CHR stimulated synergism between reactants,
simultaneously increased aliphatic hydrocarbons in the oil products. Lee et al. [3-9]50Chapter 3

Effect of torrefaction on co-pyrolysis synergy

also observed that torrefaction of cork oak enhanced the synergistic formation of monoaromatic hydrocarbons in the catalytic co-pyrolysis of Oak and HDPE over HY zeolites.
Nevertheless, to the best of the author’s knowledge, research on co-pyrolysis oftorrefied biomass with oil sludge (OS) is yet to be studied, although sole pyrolysis oftorrefied biomass or co-pyrolysis of biomass and OS are discussed extensively.
Therefore, combining torrefaction of biomass and co-pyrolysis with OS to enhancehydrocarbon production in oil deserves in-depth investigation. On one hand, the alkali
metals in biomass and the ash in OS pose catalytic reactivities, synergisticallypromoting pyrolysis behavior and improving the oil quality. On the other hand,torrefaction increased the content of lignin in biomass at the expense of holocellulose(hemicellulose and cellulose); for example, 300 °C torrefied rice straw (refer to
Chapter 2) was mainly consisted of lignin. As a result, co-pyrolysis of OS and ricestraw/torrefied rice straw can represent the co-pyrolysis of oil sludge and model
biomass compounds, such as oil sludge and holocellulose, oil sludge and lignin.
Selecting rice straw in this study for co-treatment with oil sludge is because agricultural
waste is generated closer to where oil sludge is generated. Their co-processing isrational and economical, saving transportation costs. In this chapter, the fundamental
characteristics of RS and TS in Chapter 2 are included for comparative analysis, andthe structure of Chapter 3 is shown in Figure 3.1.

Figure 3.1 Structure of Chapter 3.

51

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

3.2 Materials and methods
3.2.1 Materials and torrefaction pretreatment
The oil sludge (OS) sample was supplied by Zhejiang Eco-Environmental
Technology Co., Ltd, China. The rice straw (RS) sample was purchased from agardening market in Japan. Details of sample pretreatment and torrefaction experiments
are listed in Chapter 2. It should be noted that the oil sludge sample used in this study
is oilfield sludge, generated during oilfield exploitation, drilling, and other operations,
including oil-contaminated soil at oil wells (falling sludge), sediments from theseparation system of union stations, and drilling mud. The optimal way to deal withoilfield sludge is on-site disposal. Rice is mainly grown in the suburbs, near oil fields
or oil sludge disposal sites, thus rice straw can be easily transported to the processing
site, saving transportation costs. As a large rice-growing country, the annual productionof rice straw is huge in China. Therefore, co-pyrolysis of oilfield sludge and rice straw
can improve the pyrolysis efficiency of oilfield sludge and relieve the pressure resulted
from the urgent need for treatment of rice straw.
3.2.2 Characterization of samples
The proximate analysis and ultimate analysis of OS were characterized by the same
method as RS and TS (refer to Section 2.2.2), and the results are listed in Table 3.1.
The ash composition of OS was measured by an XRF (XRF; S2 Ranger Bruker, Japan)
analyzer, and the result is displayed in Table 3.2. It should be noted that Table 3.1 andTable 3.2 were modified based on Table 2.1 and Table 2.2. XRD (XRD; Bruker D8,
Japan) analysis was performed to identify the crystalline status of OS and char samples
with Cu Kα radiation in the scanning range of 2θ = 10 - 80°.
3.2.3 Experimental apparatus and method
Thermogravimetric analysis was performed on a thermogravimetric analyzer (TGDTA; TA-60WS, Shimadzu, Japan). For each run, around 10 mg sample of OS and its
52

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

mixtures with RS or TS under 1:1 ratio was heated from 30 to 900 °C under a nitrogengas flow rate of 100 ml/min. The heating rate was controlled at 20 °C/min.
The method for evaluating the pyrolysis characteristics of OS and the mixtures were
the same as Chapter 2 in section 2.2.3. Pyrolysis and co-pyrolysis experiments wereconducted in a fixed bed reactor, as shown in Figure 2.2. The experimental procedurefor (co) pyrolysis was the same as that in section 2.2.3.
For co-pyrolysis experiments, OS and RS/TS were mixed with blending ratios of 3:1,
1:1, and 1:3, respectively. For example, OS:RS = 3:1 denotes the blending ratio of OS
and RS is 3:1, and OS:TS = 3:1 denotes the blending ratio of OS and TS is 3:1. Before
adding RS/TS into OS, OS was put in an oven at 50 °C for 1h at first to reduce its
viscosity, and finally, a uniform mixture was obtained.
3.2.4 Characterization of oil composition
Gas chromatography-mass spectrometer (GC/MS; Shimadzu QP 2010, Japan) wasapplied to determine the chemical composition of the oil products. The characterization
method was the same as that in section 2.2.4.
3.3 Results and discussion
3.3.1 Characterization of samples
The main characteristics of OS, RS and TS, and are presented in Table 3.1.
Compared to RS and TS, the carbon, hydrogen, oxygen, and volatile in OS are muchless, whereas OS contains higher ash particles. As seen in Table 3.2, the ash particles
in OS contain more than 30% metal oxides and approximately 12.97% SO3, which has
been reported to act as catalysts during pyrolysis of OS [3-10, 3-11].

53

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

Table 3.1 Ultimate analysis, proximate analysis, and composition of samples.
RS

TS200

TS250

TS300

OS

Ultimate analysis (wt. % dry basis)
H

4.82

4.63

3.87

3.25

2.76

C

34.82

37.19

41.56

41.38

19.73

N

0.83

0.94

1.18

1.23

0.38

S

0.52

0.59

0.68

0.53

0.96

1

39.23

34.43

21.1

15.34

3.67

H/C

1.66

1.49

1.12

0.94

1.68

0.84

0.69

0.38

0.28

0.14

13.72

14.77

16.63

16.34

8.86

O

O/C
HHV

2

Proximate analysis (wt. % as received basis)
Ash

19.78

22.22

31.61

38.27

72.50

Moisture

6.63

2.52

0.96

0.55

1.55

Volatile

71.06

67.08

50.25

35.96

21.74

Fixed carbon

2.53

8.18

17.18

25.22

4.21

Composition of RS and TS (wt. %)Hemicellulose

4.67 ±0.62.36 ±1.100

Cellulose

31.36 ±1.2133.98 ±0.23

11.56 ±0.44

1.98 ±0.87

Lignin33.55 ±1.32

34.54 ±0.93

46.18 ±0.35

58.86 ±1.83

1 O%=100%-C%-H%-N%-S%-Ash%; 2 higher heating value (MJ/mol).

Table 3.2 Ash composition of samples by XRF analysis.
Main chemical components (wt. %)
SiO2

Fe2O3

SO3

Al2O3

CaO

K2O

Na2O

Other

RS

76.65

2.54

0.44

1.71

2.14

9.63

0.57

6.32

TS200

76.63

2.56

0.46

1.73

1.96

9.05

0.46

7.15

TS250

77.61

2.38

0.39

1.69

2.12

8.06

0.53

7.22

TS300

75.71

2.37

0.45

1.75

2.03

10.82

0.48

6.39

OS

49.42

13.55

12.97

8.86

7.68

3.58

1.47

2.47

Metal element analysis (wt. %)Continue of Table 3.2
OS

Fe

Al

Ca

Ni

V

2.56

4.28

0.69

0.00560.00673.3.2 Thermal decomposition characteristics of RS, TS, OS, and blends
The thermal decomposition curves of OS, RS, TS, and blends are depicted in Figure
3.2 and the corresponding pyrolysis characteristics are summarized in Table 3.3(modified by Table 2.6).
The pyrolysis process of OS could be divided into three stages based on weight loss
54

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

peaks, as indicated in Figure 3.2 (c). The weight loss rates of two valleys (404.91 and524.12 °C) among three peaks did not drop to zero, suggesting competitive reactions
among these three stages. The first stage (151.54-404.95 °C) was attributed to theevaporation of remaining moisture and the degradation of light fractions. Then, themiddle and heavy fractions were thermally decomposed into low-molecular-weightsubstances, as displayed in stage 2 (404.95-524.24 °C) and stage 3 (524.24-854.58 °C). In addition, stage 3 was also associated with the decomposition of inorganic matterssuch as carbonate and sulfur minerals (e.g., CaCO3 → CaO + CO2 and K2SO4 → K2O+ CO2 + 1/2O2). The thermal decomposition characteristic of OS in this work is in good
agreement with previous literature [3-12, 3-13]. Table 3.3 indicates the pyrolysis
property of OS was extremely poor compared to RS, while torrefaction narrowed thegap between TS and OS.
The decomposition characteristics of RS and TS are also shown in Figure 3.2, whichwere already discussed in section 2.3.2.
Figure 3.2 (b) and (d) display the decomposition curves of blends. As observed, theTG profiles and DTG profiles (especially 170 – 400 °C) of blends shared similar trends
with those of RS and TS, implying that the primary decomposition characteristics were
dominated by RS and TS because of their relatively higher volatile content. However,OS blending introduced inert ash, which hindered the transfer of external energy andthe release of volatiles, thus leading to poor pyrolysis performance compared to RS/TS
alone, as shown in Table 3.3. When the temperature was higher than 400 °C, multiple
overlapping peaks occurred on the DTG profiles of blends, especially during copyrolysis of OS/TS250 and OS/TS300. For example, a peak could be observed between
400 and 520 °C during co-pyrolysis of OS and TS250, which might come from thecompetitive reactions by OS, as the main decomposition region of OS located in thistemperature range. Furthermore, the decomposition characteristic of OS was more
pronounced when it is blended into TS300, behaving as the interaction interval moved55

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

to a lower temperature range: a weight loss peak between 170 and 360 °C appeared
during co-pyrolysis of OS and TS300. This can be explained by the fact that they have
similar volatile contents and weight loss temperature ranges. Therefore, it can be
speculated that during co-pyrolysis of OS/TS250 and OS/TS300, the contribution of
OS to the interactions gradually increased.
Overall, based on the information obtained from current thermogravimetric analysis,
the interactions during co-pyrolysis of OS and RS/TS can be summarized as: In the firstinterval (170 - 400 °C), RS/TS decomposed at first and dominated the reactions; thenas temperature increases to the second interval (> 400 °C), the degradation of OS
became pronounced and conversely affected the co-pyrolysis process. The result in thiswork about co-pyrolysis of OS and RS is consistent with relevant literature for copyrolysis of RS and other types of petroleum-derived materials [3-14, 3-15], and it
further illustrates that the interactions between OS and TS might be altered due to theincreasingly severe competitive reactions.
(a)

(b)

100

100

90
90

TG (%)TG (%)

80
70
60
50
40
30

RS
TS200
TS250
TS300
OS

80
70
60
50

RS-OS
TS200-OS
TS250-OS
TS300-OS

20
200

300

400

500

600

700

800

200

300

(c)

0

170 °C

400

500

600

700

800

700

800

Temperature (°C)

Temperature (°C)(d)

400 °C0170 °C

400 °C

520 °C

360 °C-1
Stage 2

Stage 3

DTG (%/min)Stage 1

100

0.0

-4
95

-6

90

-8

-1.0
85

80

DTG (%/min)-0.5

TG (%)DTG (%/min)

-2

-3
-4

-1.5

-5

75

-10

-2

OS-TG
OS-DTG

-2.0

70
200

-12

300

400

500

600

700

-6

800

Temperature (°C)

-7
200

300

400

500

600

700

800

200

Temperature (°C)

300

400

500

600

Temperature (°C)

Figure 3.2 (a) TG and (c) DTG curves individual samples; (b) TG and (d) DTG curves of
mixtures.
56

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

Table 3.3 Pyrolysis characteristics of OS, RS, TS, and the blends.
Sample

Ti

Tb

Tp

Rp

Rmean

Di

Db

CPI

RS

269.13850.25324.56

11.24

1.65

58.38

45.54

30.11

TS200

283.02853.17329.98

11.73

1.57

59.46

55.78

26.95

TS250

294.52872.93336.08

4.93

1.17

23.44

17.98

7.62

TS300

342.43881.58463.20

1.72

0.85

4.92

1.23

1.48

OS

330.85

747.64470.12

1.76

0.68

5.23

5.08

1.46

RS/OS

263.76858.53324.73

5.86

1.26

31.74

21.19

12.36

TS200/OS

276.73864.66330.35

6.04

1.06

32.25

25.18

9.67

TS250/OS

277.64875.38335.14

2.98

0.85

15.23

6.25

3.75

TS300/OS

278.98881.76428.79

1.23

0.72

4.82

0.64

1.29

Ti: ignition temperature (°C); Tb: burnout temperature (°C); TP: peak temperature corresponding to maximum weightloss rate (°C); Rp: maximum weight loss rate; Rmean: mean weight loss rate; Di: ignition index (10-3%/min-3); Db:
burnout index (10-4%/min-4); CPI: comprehensive pyrolysis index (10^-8min-2°C-3)

3.3.3 Pyrolysis of individual samples
3.3.3.1 Product yields distribution
Figure 3.3 shows the product yield distribution of individual samples. As discussed
in section 2.3.3.1, intensified torrefaction altered the pyrolysis pathway of TS toenhance char formation, consistent with the enrichment of lignin in TS 300. Whereas,slight torrefaction (200 °C) led to higher yield of gas product than RS due to theweakened interactions between hemicellulose and cellulose, and the catalytic effectfrom accumulated alkali metals. As opposite to RS/TS pyrolysis, char dominated thepyrolysis products distribution of OS, up to 79.91%, because of its high ash content.

57

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

90
Char
Oil
Gas

80

product yields (%)

70
60
50
40
30
20
10
0
RS

TS200

TS250

TS300

OS

Figure 3.3 Product yields from individual pyrolysis of RS, TS, and OS (modified from
Figure 2.8).

3.3.3.2 Oil composition
The chemical composition of oil derived from pyrolysis of OS, RS, and TS isdisplayed in Figure 3.4. As discussed in section 2.3.3.2, torrefaction enhanced theconversion of oxygenates into hydrocarbons, such as alkanes, aromatics, and olefins,due to the removal of holocellulose and the catalytic effect of enriched alkali metals.
Table 3.4 Relative concentrations of oxygenates in oil from OS, RS, and TS (modified from
Table 2.7).
RS

TS200

TS250

TS300

OS

acid

14.72

13.39

7.83

0.78

0.62

alcohol

4.8

4.42

3.65

1.65

11.92

aldehyde

12.99

10.34

3.52

0

0.37

anhydrosugars

2.02

1.8

1.27

0

0

ester

5.52

4.22

0

0.57

0.86

furan

11.99

10.88

4.77

0

0

ketone

15.35

14.25

10.05

0.61

0.33

phenol

10.2

14.75

18.2

20.45

0

Sum

77.59

74.05

49.29

24.06

14.1

For pyrolysis of OS, the obtained oil was mainly compounded of hydrocarbons, up
58

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

to 84.81%, exhibiting a considerably high quality compared to RS-derived oil.
Meanwhile, alkane hydrocarbon was the predominant component in OS pyrolysis oil,accounting for 74.12%. Besides, some typical petroleum compounds were detected
such as Cholestane and 15-Isobutyl-(13. alpha. H)-isocopalane. The concentration ofoxygenated compounds in OS pyrolysis oil was relatively low and most of them werealcohol, as shown in Table 3.4.
(a) Hydrocarbon

Relative concentrations (%)

Relative concentrations (%)

100
80
60
40
20

60
40
20
0

0
TS200

TS250

TS300

Relative concentrations (%)

80
70 (a-1) Alkane
60
40
30
20
10
0

24

RS

OS

(a-2) Aromatic

20
16
12
8
4
0

RS

TS200

TS250

TS300

OS

TS200

TS250

Relative concentrations (%)

RS

Relative concentrations (%)(b) Oxygenate
80

TS300

OS

12 (a-3) Olefin
10
8
6
4
2
0

RS

TS200

TS250

TS300

OS

RS

TS200

TS250

TS300

OS

Figure 3.4 Relative concentration of (a) hydrocarbon, (b) oxygenate, (a-1) alkane, (a-2)
aromatic, and (a-3) olefin in oil products from pyrolysis of OS, RS, and TS (modified from
Figure 2.9).

3.3.4 Effect of OS blending on pyrolysis of RS
3.3.4.1 Product yield distribution
As mentioned in section 3.3.3.2, OS pyrolysis oil contained significantly higher
concentration of hydrocarbons than RS pyrolysis oil. Therefore, during co-pyrolysis of
OS and RS, we focus on the effect of OS blending on pyrolysis of RS (and TS in section
3.3.5), especially the synergistic effect on enhancing hydrocarbon production.
Figure 3.5 shows the product yields from individual pyrolysis and co-pyrolysis of
RS and OS. Meanwhile, the theoretical product yields obtained by linear calculation ofindividual pyrolysis results are also depicted in Figure 3.5. The differences between
experimental yields and calculated yields were employed to investigate the interactions
59

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

between RS and OS. It is observed that with increasing OS proportion in the mixtureof RS and OS, the char yields increased from 47.91 to 68.63%, whereas the oil and gas
yields respectively decreased from 17.82 to 10.95%, and 34.27 to 20.42%. Higher
blending ratio of OS led to higher ash content in the mixture and hence, char production
was enhanced while volatiles (oil and gas) generation was suppressed.
As illustrated in Figure 3.5, the experimental char yields were slightly lower thanthe calculated values, suggesting a synergy in the generation of volatiles. Moreover, theexperimental oil yield was lower than the calculated yield, whereas the experimental
gas yield was greater than the calculated yield, indicating the synergistic effect between
RS and OS promoted the formation of gas at the expense of char and oil. Lin et al. [316] also reported that the synergistic effect during co-pyrolysis of rice husk and oil
sludge resulted in an increase in gas products and a decrease in oil products. The
synergy for enhanced gas production is primarily attributed to the radical reactions
between RS and OS derivatives. RS decomposed earlier than OS (refer to section 3.3.2)
and released a mass of free reactive radicals, such as hydroxyl radical (OH), due to thehigher H/C molar ratio and abundant -OH functional group in RS; these radicals
initiated the devolatilization of OS and promoted the cracking of OS-derived
hydrocarbon intermediates by hydrogen abstraction, thereby the generation of lightweight components or gas was enhanced [3-17, 3-18]. The synergistic promotion of gas
formation could also be ascribed to the catalytic effect from OS ash. The minerals in
OS ash like CaO and MgO, were conducive to separating the organic fractions andsolids and further converting organic fractions into light gases [3-19]. Meanwhile, thesteam reforming reactions, and carbon reduction of metal oxides in OS (e.g., C + H2O→ CO + H2, 2Fe3O4 + 2CO → 6FeO + 2CO2, FeO + CO → Fe + CO2, 2Fe3O4 + 2H2→ 6FeO + 2H2O, and FeO + H2 → Fe + H2O) were enhanced since the presence of RS
increased the carbon and oxygen content in mixtures, thereby gas production wasfavored [3-20]. Furthermore, alkali metals in RS, such as K and calcium (Ca), could act
as catalytic agents to facilitate the secondary cracking of hydrocarbons, increasing theyield of gas products. The catalytic cracking effect of alkali metals has been widely
60Chapter 3

Effect of torrefaction on co-pyrolysis synergy

investigated in related literature reporting co-pyrolysis of biomass and OS [3-16, 3-21].
As the blending ratio of OS increased from 25 to 75%, the synergistic reduction ofchar production got enhanced. This might be because a higher amount of OS introduced
more minerals with catalytic ability, assisting the cracking of coke precursor into lightfractions [3-11]. However, a higher proportion of OS seemed to weaken the synergistic
effect between volatiles, either enhancing gas formation or inhibiting oil formation, as
the differences between experimental and calculated values of oil and gas yields bothdecreased. A higher percentage of OS in the blends produced more hydrocarbon
intermediates, however these intermediates were apt to couple together to form stablecompounds as less RS provided insufficient oxygen-containing radicals to promote
depolymerization of OS derivatives. Additionally, increasing amount of OS introduced
excess inert ash particles, which enhanced the thermal resistance of reactants, covered
the surface of reactants, and blocked the nascent pores during pyrolysis, inhibiting masstransfer and gas release. Yang et al. [3-22] also observed that a higher blending ratio ofhigh ash coal conversely inhibited the gas production during its co-pyrolysis with cotton
stalk.
110
100

Product yields (wt.%)

80
RS
RS:OS=3:1(Exp.)
RS:OS=1:1(Exp.)
RS:OS=1:3(Exp.)

70
60

OS
RS:OS=3:1(Cal.)
RS:OS=1:1(Cal.)
RS:OS=1:3(Cal.)

50

-10

-20

40
30
-30

20
10
0

-40
Char

Oil

Gas

Figure 3.5 Product yields from co-pyrolysis of OS and RS.

61

Exp. yield - Cal. yield (%)

090Chapter 3

Effect of torrefaction on co-pyrolysis synergy

3.3.4.2 Oil composition
When OS was co-pyrolyzed with RS, an improved oil was obtained which contained
higher concentration of hydrocarbons compared to RS alone, as shown in Figure 3.6.
The incorporation of RS resulted in an increased concentration of total hydrocarbons
from 53.25 to 82.27%, accompanied by a declining concentration of oxygenates from45.99 to 17.71%. OS addition also yielded interactions that favored the production ofhydrocarbons and suppressed the formation of oxygenates, as can be seen from thecomparison of experimental and calculated values in Figure 3.6 (a) and (b). The
concentrations of specific oxygen-containing components obtained from co-pyrolysis
of RS and OS are listed in Table A.1 in Appendix. It is observed that the concentrationsof acid, aldehyde, anhydrosugar, ester, furan, ketone, and phenol got reduced withincreasing blending ratio of OS; concurrently, their experimental values were also
basically lower than the calculated values, as depicted in Figure 3.7. The metal oxides
such as Al2O3, Fe2O3, and CaO in OS ash could act as catalysts during pyrolysis,
promoting the reduction of oxygenates in pyrolysis oil via decarboxylation, anddecarbonylation reactions [3-11]. Additionally, hydrocarbon radicals evolved from OS
degradation would also interact with oxygen-containing radicals (such as OH radical)
from RS decomposition to yield oxygenated compounds [3-18], which could explain
the enhanced alcohol formation (refer to Figure 3.7).
Experimental

Calculated

Relative concentrations (%)

30

25

60
40

20

20

DW (%)

15

-16

60
50

-20

40
30

-24

20
10

-28

0

0
3:1

1:1

1:3

RS

OS

60

40
16
30
20

12

10
0

8
RS

3:1

1:1

1:3

OS

DW (%)

20

50

1:1

1:3

OS

-1.2

5

(a-2) Aromatic

24

Relative concentrations (%)

Relative concentrations (%)10(a-1) Alkane

3:1

12 (a-3) Olefin

8
-1.0
6
4
-0.8
2
0

-0.6
RS

3:1

1:1

1:3

OS

10
4

8

DW (%)

RS

70

-12

70

DW (%)
Relative concentrations (%)

Relative concentrations (%)(a) Hydrocarbon
80

90 (b) Oxygenate80

DW (%)

100

6
3

4
2
0

2
RS

3:1

1:1

1:3

OS

Figure 3.6 Relative concentration of (a) hydrocarbon, (b) oxygenate, (a-1) alkane, (a-2)
aromatic, and (a-3) olefin in oil products from co-pyrolysis of OS and RS.
62

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

Exp. - Cal. (%)

Exp. - Cal. (%)

Exp. - Cal. (%)

Exp. - Cal. (%)

Straight chain alkanes (C10-C24)
6
4
2
0
-2
-4
-6
-8
-10
4
2

Oxygenates

Aromatics(a) RS:OS=3:1(b) RS:OS=1:1

0
-2
-4
-6
-83
2
1
0
-1
-2
-3
-4
4
2
0
-2
-4
-6
-8
15

Exp. - Cal. (%)

C25-C27(c) RS:OS=1:3(d) TS200:OS=3:1(e) TS250:OS=3:1

10
50

Exp. - Cal. (%)

-5
15
10

(f) TS300:OS=3:1

5
0
-5
-10

A Aci
l
A
nh ld coh dyd eh ol
ro yd
su e
ga
Es rs
F ter
K uranet
Ph one
en
M
on ol
oc
Bi ycli
T cy cTe ric clic
tra ycl
cy ic
cli
c
A

C2
C25
C26
7

C1
C10
C11
C12
C13
C14
C15
C16
C17
C18
C29
C20
C21
C22
C23
4

-15

Chemical compounds

Figure 3.7 Deviations between experimental and calculated concentrations of straight chain
alkanes, oxygenates, and aromatics: (a) RS:OS=3:1, (b) RS:OS=1:1, (c) RS:OS=1:3, (d)TS200:OS=3:1, (e) TS250:OS=3:1, and (f) TS300:OS=3:1.

Among hydrocarbons, alkane dominated, and its concentration increased from 41.01
63

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

to 68.32% with an increasing proportion of OS. The addition of OS also enhanced olefin
production, whereas the concentration of aromatic hydrocarbon gradually decreased
from 5.05 to 2.26% when more OS particles were introduced. Overall, the compositionof co-pyrolysis oil resembled that of OS oil as the blending ratio of OS increased.
However, although OS addition promoted the generation of total hydrocarbons, ityielded different synergistic effects on the production of specific hydrocarbon
components. As can be seen in Figure 3.6 (a-1), (a-2), and (a-3), the experimental
concentrations of alkane and olefin were higher than their calculated values, whereas
the observed concentration of aromatic was lower than the calculated value under
different mixing ratios, entailing that OS blending promoted the production of alkane
and olefin, while inhibited the formation of aromatic. The fundamental reason was that
the RS-derived oxygen-containing radicals facilitated the cracking and chain scissionof OS primary pyrolysis vapors via dehydrogenation reaction, leading to enhanced
formation of alkane and olefin [3-18]. Conversely, the hydrogen atoms from OS
decomposition promoted the deoxidation reactions of RS-derived oxygenatedcompounds and enhanced hydrocarbon production [3-23]. As shown in Table A.1 andA.2 in Appendix, the experimental concentrations of light-weight straight-chain
alkanes (Carbon number: C10 – C24) was lower than their calculated concentrations;in contrast, the production of heavy-weight straight-chain alkanes (Carbon number:
C25 – C27) was enhanced (refer to Figure 3.7), implying that co-pyrolysis of RS andOS favored heavy fractions formation in oil. This could be attributed to the repolymerization of OS-derived hydrocarbon intermediates and RS-derived hydrocarbon
fragments.
Table A.1 and A.2 in Appendix also tabulate the concentration of specific aromatichydrocarbons based on the number of aromatic rings. As observed, feeding OS into RS
suppressed the formation of aromatics, either monocyclic aromatic hydrocarbons(MAHs) or polycyclic aromatic hydrocarbons (PAHs) (refer to Figure 3.7). It should
be noted that the result in this work, OS addition inhibits aromatics production, is notin line with some relevant literature conducting co-pyrolysis of biomass and petroleum64

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

based products, for example, cotton stalk and waste tires (WT) [3-14], and corn stalkand high-density polyethylene (HDPE) [3-23]. Their work pointed out that lignin
derivatives (from biomass site) were provided with hydrogen from WT or HDPE, which
suppressed their coupling and promoted the depolymerization of lignin, thereby
enhancing aromatic production. Although OS blending was found to reduce aromatic
production, it did not mean that OS did not supply hydrogen to accelerate thedecomposition of lignin. Because these nascent aromatics may continue to undergohydrogenation reaction with OS-derived hydrogen radicals, and then these aromatics
were converted to naphthenes or olefins. Besides, OS ash contains high content of Ni
element, which could catalytically promote hydrogenation of aromatics, as Ni/SiO2
catalysts have been widely used for hydrogenation of heavy oil and vacuum residuum[3-24]. Previous work seldom mentioned the inherent catalytic effect of additives;
therefore, the result in this work does not contradict their work.
It should be noted that in addition to Ni/SiO2, some metal oxides in oil sludge ash,especially Fe2O3 (the main component of red mud), also entail outstandinghydrogenation characteristics. For example, Serrano et al. [3-25] noticed that thepresence of Fe2O3 and TiO2 in red mud resulted in the formation of ethylbenzene fromstyrene. Klopries et al. [3-26] evaluated the catalytic property of red mud for the hydroliquefaction of rye straw via hydrogenation experiment. It was found that red mud could
catalyze the liquefaction of biomass in the presence of sulfur, yielding a moresignificant number of C1-C4 hydrocarbons, gasoline, and heavy oils.
The RS-to-OS mass ratio plays a vital role in the degree and nature of interactions
during co-pyrolysis process. The synergistically promoted generation of totalhydrocarbons, alkane, and olefin increased at first and then decreased at 50% OS
blending. Likewise, one to one blending ratio of RS and OS yielded the most prominentsynergy facilitating the consumption of oxygenates and aromatic hydrocarbon. These
results demonstrated that a 50% addition of OS might be the optimal blending ratio forhigher synergy in enhanced hydrocarbon production. A higher blending ratio of OS
entailed fewer oxygen-containing radicals from RS, and hence, the radical reactions
65

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

were suppressed, and hydrocarbon formation cannot be promoted. On the other hand,excess OS inevitably introduced a large number of inert ash particles other thanhydrocarbon intermediates and catalytic metal oxides, which inhibited the mass andenergy transfer; this result was consistent with the severe hindering of gas production
at 75% OS addition.
3.3.5 Combined effect of torrefaction and OS blending on pyrolysis of RS
3.3.5.1 Product yield distribution
The combined effect of torrefaction and OS blending (RS/TS:OS = 3:1) on productyield distribution is illustrated in Figure 3.8. As torrefaction temperature increased
from 0 to 300 °C, the char yield of the blends proportionally increased from 47.91 to
75.93%, while the oil yield and gas yield decreased from 17.82 to 6.23% and 34.27 to
17.85%, respectively. Obviously, OS addition did not change the effect of torrefaction
temperature on the product yield distribution (refer to Figure 3.3). It is noticed that theexperimental char yields under different torrefaction temperatures were close to thecalculated values, demonstrating weak interaction. However, the deviations between
experimental values and calculated values of oil and gas products were apparent, andthe synergistic effect can be classified into two groups: (1) the interactions between
RS/OS and TS200/OS inhibited oil production while promoted gas formation, andconversely (2) the interactions between TS250/OS and TS300/OS promoted oil
production while inhibited gas formation. It is reasonably inferred that torrefaction
conditions determined the synergistic effect in product yields, and severe torrefaction
turned the synergistic effect from favoring gas products to promoting oil generation.
As discussed in section 3.3.4.1, the radical reactions between RS-derived oxygenates
and OS-derived hydrocarbon intermediates, the catalytic effect from metal oxides in
OS ash and alkali metals in RS promoted gas production. Torrefaction under 200 °C
marginally altered the properties of RS, resulting in similar fundamental properties in
TS200 and RS, thus, the interactions remained the same as that between RS and OS.
66

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

Nonetheless, 200 °C torrefaction slightly lowered the degree of synergy since the partial
removal of holocellulose undermined the radical reactions enhancing gas formation.
When OS was blended into co-pyrolysis with TS250 or TS300, contrary synergy was
obtained. Severe torrefaction significantly altered the properties of biomass, lowering
the volatile content, reducing H/C molar ratio, and decreasing the intensity of -OH
functional group. Herein, the radical reactions favoring gas production were weakened.
Intensified torrefaction led to accumulation of lignin in TS250 and TS300; accordingly,
the decomposition of lignin was predominant during pyrolysis of TS250 and TS300,
and a lot of phenolic radicals were produced due to aryl ether linkage cleavage.
Thereafter, these radicals were prone to combine with each other via rearrangement
reactions and form stable C-C bonds if insufficient hydrogen radicals were provided
nearby, leading to a higher degree of reticulation and to a higher thermal residence ofthe residue, and thereby the further degradation of lignin was hindered [3-14]. The
thermogravimetric analysis in section 3.3.2 revealed that the main decomposition
temperature ranges of TS250 and TS300 were close to that of OS compared to RS and
TS200, implying intense radical reactions between TS250/OS and TS300/OS. The
decomposition of OS provided free hydrogen radicals due to its relatively higher H/Cmolar ratio than TS250 and TS300. These free hydrogen radicals stabilized phenolicradicals, preventing their mutual combination, which promoted depolymerization oflignin and enhanced oil production. As for the promoting effect of minerals (alkali
metals in RS) on gas generation, it might be weakened due to their reactions with SiO2
to form aluminosilicates, which conversely suppressed the synergistic effect. Qin et al.
[3-27] observed that alkali metals in corn straw reacted with SiO2 and Al2O3 in coal ash
under 600 °C, producing K(AlSi3O8), Ca(Al2Si2O8), and KAl(SO4)2(H2O)12, inhibiting
gas formation. Figure 3.9 shows the XRD patterns of OS and char samples. The peakintensity of SiO2 in OS got weakened after its co-pyrolysis with biomass, especiallywith TS under severe torrefaction, which confirmed the reactions between alkali metals
and SiO2.
In general, higher torrefaction temperature led to more minor differences between
67

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

experimental and calculated oil and gas yields, entailing weaker interactions.
Intensified torrefaction resulted in increased ash and reduced volatiles, which further
inhibited mass and gas transfer, thereby suppressing the interactions.
100
90
0
70

RS:OS=3:1 (Exp.)
TS200:OS=3:1 (Exp.)
TS250:OS=3:1 (Exp.)
TS300:OS=3:1 (Exp.)60

RS:OS=3:1 (Cal.)
TS200:OS=3:1 (Cal.)
TS250:OS=3:1 (Cal.)
TS300:OS=3:1 (Cal.)

-10

50
-20

40
30

Exp. yield - Cal. yield (%)

Product yields (wt.%)

80

-30

20
10
0

-40
Char

Oil

Gas

Figure 3.8 Product yields from co-pyrolysis of RS-OS mixture and TS-OS mixtures.
§§ SiO2§§§§

Relative intensity (a.u.)§

TS300:OS=3:1
Char

TS250:OS=3:1
Char

TS200:OS=3:1
Char

RS:OS=3:1
Char

OS
Char

OS

10

20

30

40

50

60

2θ (°)

Figure 3.9 XRD patterns of OS and char samples.

68

70

80

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

3.3.5.2 Oil composition
Figure 3.10 indicates that enhanced torrefaction led to an increase in theconcentration of total hydrocarbons and to a decline in the concentration of oxygenatescompared to co-pyrolysis of RS and OS, which followed the effect of torrefaction on
RS pyrolysis oil composition. The experimental concentration of total hydrocarbons
was superior to the calculated values, whereas the experimental concentration ofoxygenates was inferior to the calculated values, indicating that torrefaction did notalter the promoting effect of OS blending on the conversion of oxygenates tohydrocarbons. However, higher torrefaction temperature weakened the synergy inhydrocarbon production, as the differences between experimental and calculated values
became smaller. This is because severe torrefaction significantly reduced the amount ofholocellulose, leading to insufficient oxygen-containing radical sinks, and hence, fewerhydrocarbons were generated via radical reactions or deoxygenation reactions. As listed
in Table A.2 in Appendix, the reduction of acid, aldehyde, anhydrosugars, ester, furan,ketone, and phenol were witnessed with increasing torrefaction temperature due to thepromotion of deoxygenation from torrefaction and OS blending (refer to section 2.3.3.2
and 3.3.4.2).
As shown in Figure 3.10 (a-1), Compared to RS/OS co-pyrolysis, oil derived from
TS/OS co-pyrolysis contained a higher concentration of alkanes while with a gradually
declined synergy in alkane production as torrefaction intensified, following the same
variation trend as the concentration of total hydrocarbons. Moreover, Figure 3.7 andTable A.2 in Appendix indicate the deviations between experimental and calculated
concentrations of heavy-weight straight-chain alkanes in TS/OS-derived oil were more
prominent than in RS/OS-derived oil, implying enhanced radical reactions resulted
from torrefaction. Figure 3.10 (a-2) and (a-3) demonstrate that OS blending altered thevariation trend of aromatic and olefin concentration in TS under the studied torrefaction
temperature range. As shown in Figure 3.10 (a-3), the oil obtained from co-pyrolysis
of TS200/OS and TS250/OS had slightly lower concentrations of olefins than that from
RS/OS-derived co-pyrolysis oil; concurrently, the corresponding synergy for olefins69Chapter 3

Effect of torrefaction on co-pyrolysis synergy

production decreased from 2.32 to -3.71%, suggesting hindering effects. This might bebecause TS-derived olefin radicals were consumed via Diels-Alder reactions with OSderived hydrocarbon intermediates to form alkanes [3-23]. As indicated in Figure 3.10(a-2), inhibition in aromatic production was observed, and higher torrefaction
temperature yielded stronger inhibition as their experimental concentrations were
exceedingly lower than the calculated values. Therefore, it could be reasoned thatsevere torrefaction (300 ° C) enhanced the hydrogenation and ring-opening ofaromatics to form alkanes and olefins during co-pyrolysis of RS and OS.
This work aims to recover high-quality oil products (hydrocarbon-rich) from oil
sludge and biomass using thermochemical treatments. To achieve this, we measured thecontent of oil yields and the concentration of typical components in the oil. As shownin Table 3.5, the specific masses of hydrocarbons were obtained according to the massof raw samples, oil yields, and hydrocarbon concentrations. It could be noticed that
torrefaction promoted the hydrocarbon recovery rate, whereas severe torrefaction(300 °C) limited the recovery rate due to the meager oil yield. Similarly, co-pyrolysis
of TS300 and OS under a blending ratio of 3:1 resulted in a relatively low hydrocarbon
recovery rate compared to other cases of co-pyrolysis. The addition of OS significantly
increased the concentration of hydrocarbons in oil products, thereby enhancing thehydrocarbon recovery rate at least two times higher. Under the same blending ratio of
3:1, torrefaction (200 and 250 °C) led to a higher concentration of hydrocarbons with alower oil yield; thus, the hydrocarbon recovery rate marginally altered. Overall, copyrolysis of RS and OS under a 1:1 blending ratio yielded the highest hydrocarbon
recovery rate, which can be determined as the optimal parameter for hydrocarbon
recovery.

70

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

Experimental

Calculated
024

70 (b) Oxygenate

18

60
50
40

12

30
20
6

10

60
-6

50
40

-12

30
20

-18

10
0

RS/OS

TS200/OS

TS250/OS

RS/OS

TS300/OS

TS200/OS

20

18

30
16

20

0

10

-2

12

-4
-6

8

-8
4

-10

DW (%)

40

TS300/OS3(a-3) Olefin16

DW (%)

50

Relative concentrations (%)

20

TS250/OS

10(a-2) Aromatic

Relative concentrations (%)

60 (a-1) Alkane

2

8

1
6

0
-1

4

-2
2

-3
-4

14
0

0
RS/OS

TS200/OS

TS250/OS

DW (%)

0

Relative concentrations (%)

DW (%)

Relative concentrations (%)

70

DW (%)

Relative concentrations (%)

80 (a) Hydrocarbon

TS300/OS

-12
RS/OS

TS200/OS

TS250/OS

0TS300/OS

RS/OS

TS200/OS

TS250/OS

TS300/OS

Figure 3.10 Relative concentration of (a) hydrocarbon, (b) oxygenate, (a-1) alkane, (a-2)
aromatic, and (a-3) olefin in oil products from co-pyrolysis of RS-OS mixture and TS-OS
mixtures.
Table 3.5 Mass distribution of oil and hydrocarbons
Massof Oil yield Hydrocarbon

raw

(%)

sample (g)Massof Hydrocarbon

concentrationhydrocarbons

recovery rate

(%)

(g) a

(%) b

RS

5.00

27.43

17.03

0.234

1

TS200

5.00

21.33

22.61

0.241

102.93

TS250

5.01

12.89

47.20

0.305

130.13

TS300

4.99

6.52

65.75

0.214

91.32

OS

4.99

5.21

84.80

0.220

94.22

RS:OS=3:1

5.01

17.76

53.20

0.473

202.33

RS:OS=1:1

5.00

15.50

78.20

0.605

258.70

RS:OS=1:3

5.00

10.90

82.20

0.448

191.25TS200:OS=3:1

5.02

15.63

59.08

0.464

198.15

TS250:OS=3:1

4.97

14.19

64.09

0.452

193.14

TS300:OS=3:1

5.00

6.21

76.13

0.236

100.87a Mass of hydrocarbons = Mass of raw sample × Oil yield × Hydrocarbon concentration × 10-4
b Hydrocarbon recovery rate = Mass of hydrocarbons (exclude RS) / mass of hydrocarbons (RS)

*100

71

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

3.6 Possible reaction mechanism during co-pyrolysis of RS/OS and TS/OS
Based on the above analyses and the results of previous work, the possible synergistic
mechanism and reaction pathways during co-pyrolysis of RS/TS and OS weresummarized in Figure 3.11. Low thermal stability led to earlier decomposition of RS
during co-pyrolysis of RS and OS, releasing adequate oxygen-containing radicals (suchas OH radicals), which stimulated the degradation of OS; moreover, these radicals could
accelerate the cracking of OS-derived primary vapors via dehydrogenation reaction(Pathway 1), thereby enhancing the production of light gases, alkanes, and olefins.
Thereafter, the hydrogen atoms from OS degradation conversely facilitated thedeoxidation of RS-derived oxygenates; the metal oxides such as Al2O3, Fe2O3, and CaO
in OS ash could also catalytically enhance decarboxylation, and decarboxylation
reactions of RS-evolved oxygenates, converting oxygenates to hydrocarbons (Pathway
2). Simultaneously, the hydrocarbon intermediates evolved from OS would interact
with RS-derived oxygen-containing radicals or hydrocarbon fragments to form
oxygenates or heavy-weight straight-chain alkanes (Pathway 3). When the temperature
increased above 400 °C, decomposition of OS dominated, and multiple hydrocarbon
radicals and hydrogen atoms were produced. These radicals interacted with RS
derivatives, suppressing their coupling to form oxygenates, MAHs, or PAHs. For
example, the hydrogen atoms supplied by OS stabilized the phenolic radicals from RS,
promoting the depolymerization of lignin (Pathway 4). Moreover, these hydrogenatoms significantly strengthened the hydrogenation and ring-opening reactions ofaromatic structures (Pathway 5), leading to enhanced production of alkanes and olefinsat the expense of aromatics. The synergistic Pathway 5 was also catalytically enhancedby Ni/SiO2 particles in OS ash. Besides, OS-derived hydrocarbon intermediates andlignin-derived alkyl radicals were re-polymerized to form heavy-weight straight-chain
alkanes (Pathway 6). A higher blending ratio of OS yielded weaker synergy for gas
production and hydrocarbon formation due to the introduced excessive inert ash
particles acting as physical barriers to mass and heat transfer during co-pyrolysis.

72

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

Torrefaction partially removed hemicellulose and cellulose in RS, accompanied by
the enrichment of lignin and alkali metals. Consequently, TS pyrolysis yielded a higher
concentration of hydrocarbons, and their co-pyrolysis with OS further enhancedhydrocarbon production. However, fewer organics in TS provided insufficient oxygencontaining radicals (mainly derived from the decomposition of holocellulose),weakening the radical reactions between TS and OS. Hence, the gas formation and thesynergy for hydrocarbon production were suppressed. Severe torrefaction resulted inclose decomposition temperature ranges between TS300 and OS, entailing an intensive
interaction. As a consequence, the depolymerization of lignin was promoted, thehydrogenation and ring-opening of lignin-derived aromatic structure got enhanced, andthe re-polymerization of OS-evolved hydrocarbon intermediates and lignin-derived
alkyl radicals were strengthened, leading to the increase in production of alkanes andolefins and the decrease in generation of aromatics.

73

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

Figure 3.11 Possible synergistic pathways in co-pyrolysis of RS and OS, TS and OS.

3.4 Conclusions
In this work, pyrolysis of OS and RS/TS in a thermogravimetric analyzer and a fixed
bed reactor was investigated.
(1) Co-pyrolysis of OS and RS promoted char conversion to volatiles, especially gas
products.
(2) The incorporation of OS into either RS or TS promoted hydrocarbon production
at the expense of oxygenates. A significant synergistic effect was witnessed in74

Chapter 3

Effect of torrefaction on co-pyrolysis synergy

enhancing hydrocarbon production after OS addition.
(3) RS/OS mass ratio of 1:1 was a critical point considering the degree of promoting
the formation of hydrocarbons. A higher blending ratio of OS hindered mass andheat transfer, thereby suppressing the gas formation and lowering the synergy forhydrocarbon generation.
(4) Severe torrefaction increased TS's main decomposition temperature range close
to that of OS, and positive synergistic effects for oil generation were observed
when OS was blended into TS obtained at 250 and 300 °C.
(5) A notable result in this work is that OS addition supplied ample hydrogen atoms
and Ni/SiO2 minerals, which further promoted the hydrogenation and ringopening of aromatic structures, enhancing alkane and olefin formation at theconsumption of aromatics.
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Co-pyrolysis of lignocellulosic biomass and microalgae: products characteristics andinteraction effect. Bioresource technology, 245, 860-868.
[3-2] Kan, T., Strezov, V., & Evans, T. J. (2016). Lignocellulosic biomass pyrolysis:
A review of product properties and effects of pyrolysis parameters. Renewable andsustainable energy reviews, 57, 1126-1140.
[3-3] Wang, X., Deng, S., Tan, H., Adeosun, A., Vujanović, M., Yang, F., & Duić, N.
(2016). Synergetic effect of sewage sludge and biomass co-pyrolysis: a combined
study in thermogravimetric analyzer and a fixed bed reactor. Energy conversion andmanagement, 118, 399-405.
[3-4] Zhang, W., Yuan, C., Xu, J., & Yang, X. (2015). Beneficial synergetic effect on
gas production during co-pyrolysis of sewage sludge and biomass in a vacuum
reactor. Bioresource Technology, 183, 255-258.
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Effect of torrefaction on co-pyrolysis synergy[3-5] Martínez, J. D., Veses, A., Mastral, A. M., Murillo, R., Navarro, M. V., Puy,N., ... & García, T. (2014). Co-pyrolysis of biomass with waste tyres: Upgrading of
liquid bio-fuel. Fuel Processing Technology, 119, 263-271.
[3-6] Lee, H. W., Kim, Y. M., Jae, J., Jeon, J. K., Jung, S. C., Kim, S. C., & Park, Y.
K. (2016). Production of aromatic hydrocarbons via catalytic co-pyrolysis of torrefied
cellulose and polypropylene. Energy Conversion and Management, 129, 81-88.
[3-7] Bu, Q., Chen, K., Xie, W., Liu, Y., Cao, M., Kong, X., ... & Mao, H. (2019).
Hydrocarbon rich bio-oil production, thermal behavior analysis and kinetic study of
microwave-assisted co-pyrolysis of microwave-torrefied lignin with low density
polyethylene. Bioresource technology, 291, 121860.
[3-8] Huang, S., Qin, J., Chen, T., Yi, C., Zhang, S., Zhou, Z., & Zhou, N. (2022).
Co-pyrolysis of different torrefied Chinese herb residues and low-density
polyethylene: Kinetic and products distribution. Science of The Total Environment,
802, 149752.
[3-9] Lee, H. W., Kim, Y. M., Lee, B., Kim, S., Jae, J., Jung, S. C., ... & Park, Y. K.
(2018). Catalytic copyrolysis of torrefied cork oak and high density polyethylene overa mesoporous HY catalyst. Catalysis Today, 307, 301-307.
[3-10] Cheng, S., Takahashi, F., Gao, N., Yoshikawa, K., & Li, A. (2016). Evaluationof oil sludge ash as a solid heat carrier in the pyrolysis process of oil sludge for oil
production. Energy & Fuels, 30(7), 5970-5979.
[3-11] Cheng, S., Wang, Y., Gao, N., Takahashi, F., Li, A., & Yoshikawa, K. (2016).
Pyrolysis of oil sludge with oil sludge ash additive employing a stirred tank reactor.
Journal of Analytical and Applied Pyrolysis, 120, 511-520.
[3-12] Liu, H., Hong, R., Xiang, C., Wang, H., Li, Y., Xu, G., ... & Zhu, K. (2020).
Thermal decomposition kinetics analysis of the oil sludge using model-based method
and model-free method. Process Safety and Environmental Protection, 141, 167-177.
[3-13] Singh, B., Singh, S., & Kumar, P. (2021). In-depth analyses of kinetics,
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thermodynamics and solid reaction mechanism for pyrolysis of hazardous petroleum
sludge based on isoconversional models for its energy potential. Process Safety andEnvironmental Protection, 146, 85-94.
[3-14] Khan, S. R., Zeeshan, M., & Masood, A. (2020). Enhancement ofhydrocarbons production through co-pyrolysis of acid-treated biomass and waste tirein a fixed bed reactor. Waste Management, 106, 21-31.
[3-15] Khan, S. R., & Zeeshan, M. (2022). Catalytic potential of low-cost naturalzeolite and influence of various pretreatments of biomass on pyro-oil up-gradation
during co-pyrolysis with scrap rubber tires. Energy, 238, 121820.
[3-16] Lin, B., Huang, Q., & Chi, Y. (2018). Co-pyrolysis of oily sludge and rice husk
for improving pyrolysis oil quality. Fuel processing technology, 177, 275-282.
[3-17] Song, Y., Tahmasebi, A., & Yu, J. (2014). Co-pyrolysis of pine sawdust andlignite in a thermogravimetric analyzer and a fixed-bed reactor. Bioresource
technology, 174, 204-211.
[3-18] Liu, X., Burra, K. R. G., Wang, Z., Li, J., Che, D., & Gupta, A. K. (2021).
Towards enhanced understanding of synergistic effects in co-pyrolysis of pinewood
and polycarbonate. Applied Energy, 289, 116662.
[3-19] Li, J., Zheng, F., Li, Q., Farooq, M. Z., Lin, F., Yuan, D., ... & Chen, G.
(2022). Effects of inherent minerals on oily sludge pyrolysis: Kinetics, products, andsecondary pollutants. Chemical Engineering Journal, 431, 133218.
[3-20] Zhu, J., Zhu, L., Guo, D., Chen, Y., Wang, X., & Zhu, Y. (2020). Co-pyrolysis
of petrochemical sludge and sawdust for syngas production by TG-MS and fixed bed
reactor. International Journal of Hydrogen Energy, 45(55), 30232-30243.
[3-21] Hu, G., Li, J., Zhang, X., & Li, Y. (2017). Investigation of waste biomass copyrolysis with petroleum sludge using a response surface methodology. Journal ofenvironmental management, 192, 234-242.

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Effect of torrefaction on co-pyrolysis synergy[3-22] Yang, P., Zhao, S., Zhang, Q., Hu, J., Liu, R., Huang, Z., & Gao, Y. (2021).
Synergistic effect of the cotton stalk and high-ash coal on gas production during copyrolysis/gasification. Bioresource Technology, 336, 125336.
[3-23] He, T., Zhong, S., Liu, C., Shujaa, A., & Zhang, B. (2021). Enhancing
hydrocarbon production via ex-situ catalytic co-pyrolysis of biomass and high-density
polyethylene: Study of synergistic effect and aromatics selectivity. Waste
Management, 128, 189-199.
[3-24] Cheng, S., Li, A., & Yoshikawa, K. (2015). High quality oil recovery from oil
sludge employing a pyrolysis process with oil sludge ash catalyst. International
Journal of Waste Resources, 5(2), 656-660.
[3-25] Serrano, D. P., Aguado, J., & Escola, J. M. (2000). Catalytic conversion ofpolystyrene over HMCM-41, HZSM-5 and amorphous SiO2–Al2O3: comparison
with thermal cracking. Applied Catalysis B: Environmental, 25(2-3), 181-189.
[3-26] Klopries, B., Hodek, W., & Bandermann, F. (1990). Catalytichydroliquefaction of biomass with red mud and CoO

MoO3 catalysts. Fuel, 69(4),448-455.
[3-27] Qin, Q., Zhou, J., Lin, B., Xie, C., & Zhou, L. (2020). Influence of coal ash on
the characteristics of corn straw pyrolysis products. Bioresource Technology, 318,
124055.

78

Chapter 4

Effect of hydrothermal treatment on co-combustion

Chapter 4 Co-combustion of oil sludge char with
raw/hydrothermally treated biomass: interactions, kinetics,
and mechanisms
4.1 Introduction
The results in Chapter 2 and Chapter 3 demonstrated that adding biomass/torrefied
biomass into co-pyrolysis with oil sludge (OS) could enhance the oil production andthe hydrocarbon concentration in oil was synergistically promoted. Consequently, theeconomic value of oil was improved, and its utilization could compensate for the energy
consumption of the integrated pyrolysis and combustion system. Moreover, the second
section of this approach, combustion of oil sludge char (OSC), is also an inevitable andeffective means of harmless disposal and energy production. Previous studies pointed
out that recycling oil sludge ash as a catalyst for oil sludge pyrolysis and incorporating
steam injection could potentially allow the integrated process to be a self-sufficientsystem, as well as realizing efficient energy recovery [4-1]. However, OSC itself posesa low calorific value, and a large amount of additional energy is consumed for
combustion treatment, which reduces this approach's economic benefit. To address thelimitation, co-combustion with biomass is cost-effective and promising.
Nonetheless, to the best of the author’s knowledge, current research on the cocombustion of OSC and biomass is limited to raw biomass utilization to improve the
combustion characteristics of OSC [4-2, 4-3]. The interactive mechanisms of
OSC/biomass co-combustion, such as the nature of synergy/inhibition that occurs
during co-combustion of OSC/biomass, are still uncertain. Furthermore, the alkali
metals in biomass could also act as catalysts during co-combustion in addition tocausing fouling and slagging problems [4-4]. Hydrothermal treatment (HTT) canpartially remove alkali metals and convert materials into hydrochars with ameliorated
homogeneity and higher fuel properties [4-5]. Therefore, the incorporation ofhydrochars into co-combustion with OSC could improve the combustion efficiency,
79

Chapter 4

Effect of hydrothermal treatment on co-combustion

and the removal of alkali metals after HTT might help identify the interactive
mechanisms of OSC co-combustion with biomass when compared with OSC solocombustion and OSC/rawbiomass co-combustion. On the other hand, kinetic analysis
of OSC/biomass co-combustion is a prerequisite to understanding reaction-based
mechanisms and scaling them up for industrial applications, which still needs more indepth investigation.
Thermogravimetric analysis (TGA) is one of the most used techniques to rapidly
investigate and provide a quantitative method for detailed observation of thermal eventsand kinetics during solid fuel combustion, such as coal and biomass [4-6, 4-7]. The
information obtained from TGA combustion profiles could be used to predict industrialscale combustion. Although TGA cannot be directly extrapolated to other equipment at
a larger scale, it is beneficial to offer fundamental referential opinions for implementing
and optimizing the co-combustion field [4-8]. In this Chapter, the TGA analysis wasused to analyze the combustion behaviors of OSC, biomass, hydrochar, and theirrespective blends (OSC/biomass and OSC/hydrochar). As illustrated in Figure 4.1, themorphological structure and alkali metals concentration of biomass before and afterhydrothermal treatment was evaluated to investigate the effect of alkali metal on cocombustion. The interactions between two blends were studied under different blending
ratios. The interactive mechanisms of OSC co-combustion were analyzed via kinetics
and discussed to further comprehend biomass-supported combustion of poorly
combustible materials like OSC.

80

Chapter 4

Effect of hydrothermal treatment on co-combustion

Figure 4.1 Structure of Chapter 4.

4.2 Materials and methods
4.2.1 Sample preparation
OS samples were supplied by Zhejiang Eco-Environmental Technology Co., Ltd,
China. OSC samples were obtained by pyrolysis of OS. Pyrolysis experiments were
performed in a vertical tube furnace and details were outlined in Chapter 2 and 3. Ineach experiment, 20 g of OS was first placed in the pyrolysis reactor and then nitrogengas was purged for 20 min to create an oxygen-free environment. Thereafter, the reactor
was heated from room temperature to 450 °C at 10 °C/min and maintained for 20 min.
When the heating process was finished, the reactor was naturally cooled to room
temperature with the continuous nitrogen purge, and finally the OSC was taken out andstored in a desiccator. Our previous research indicated that 450 °C was an optimal
temperature for OS pyrolysis to obtain high-quality oil and save energy [4-9]. Cherry
blossom wood chips, purchased from a gardening market in Japan, was selected for its
low moisture, low ash, and high volatile content. The addition of woody biomass cansignificantly improve the combustion efficiency of oil sludge char, simultaneously
mitigating the ash-related problems.

81

Chapter 4

Effect of hydrothermal treatment on co-combustion

4.2.2 Sample characterizations
All OSC, RW, and hydrothermally treated wood (HW) samples were crushed andsieved smaller than 200 μm for subsequent experiments. The proximate analysis was
performed on an elemental analyzer (Vario Microcube, Bruker, Germany), and theultimate analysis was measured by a muffle furnace according to GB/T212-2008. The
higher heating value of samples were calculated using eq 1 which was developed by
Francis and Lloyd [4-10]:
HHV = 357.8C + 1135.6H + 54.9N + 119.5S – 85.4O – 974(1)

Where C, H, N, O, and S denoted the weight percentages of carbon, hydrogen, nitrogen,sulfur, and oxygen, respectively.
The ash composition of samples was measured by XRF (S2 Ranger Bruker, Japan)
analysis with X-ray tube of Palladium. The morphological surface characteristics ofsamples were observed via a scanning electron microscopy (JSM-6510LA, JEOL Ltd.,
Japan) with an acceleration voltage of 6 kV.
For convenience, the blend of OSC and RW were named OSC/RW. OSC/HW
represents the blend of OSC and HW. According to blending ratio, OSC/RW blends
were tagged as OSC/RW28 (20% OSC and 80% RW), OSC/RW55 (50% OSC and 50%
RW), and OSC/RW82 (80% OSC and 20% RW), respectively. OSC/HW blends werealso tagged as OSC/HW28, OSC/HW55, and OSC/HW82, respectively. Pyrolysis led
to the evaporation of most organic components and accumulation of ash in OS,
therefore, the combustion efficiency of OSC was extremely low. In this study, theanalyses of OSC/RW82 and OSC/HW82 were still performed for a comprehensive
understanding of the interactive mechanisms during co-combustion, though the
combustion efficiency was lower due to the high proportion of OSC.
4.2.3 Experimental equipment and methods
As shown in Figure 4.2, HTT experiments were conducted in a 400 ml stainless steel
82

Chapter 4

Effect of hydrothermal treatment on co-combustion

reactor, and HTT steps were reported in detail previously [4-11, 4-12]. 6 g of RW and
60 g of pure water were mixed as raw samples. The hydrolysis of lignocellulose
interfered with the carbonization process when the hydrothermal temperature was lower
than 200 °C, whereas the hydrochar yield was reduced at a temperature above 250 °C[4-13]. Therefore, the reaction temperature was set at 220 °C in this study.

Figure 4.2 Schematic diagram of HTT reactor [modified from Ref. 4-12]

Non-isothermal experiments were performed using a differential thermogravimetricanalyzer (TA-60WS, Shimadzu, Japan). 10 ± 0.5 mg of a sample was put on the bottomof an Al2O3 crucible and then set in the TGA analyzer with an empty Al2O3 reference
for baseline calibration. The sample was heated from ambient temperature to 1000 °C
under an air atmosphere at 60 ml/min flow rate and different heating rates (5, 10, 40,
and 50 °C/min). Each experiment was conducted twice or more for repeatability
validation.
4.2.4 Combustion performance analysis
For combustion performance analysis, ignition temperature (Ti), burnout temperature(Tb), and peak temperature (Tmax) were obtained from the thermogravimetry anddifferential thermogravimetry (TG-DTG) curves. Ti was determined by the tangent
method [4-14]. Tb and Tmax represented the temperature at the conversion rate of 98%
and the maximum weight loss rate, respectively [4-15]. Moreover, the ignition index
83

Chapter 4

Effect of hydrothermal treatment on co-combustion(Di), burnout index (Db), and comprehensive combustion index (CCI) were determined
to evaluate the combustion performance of tested samples. These indexes were
calculated by following formulas (eqs 2-4) [4-16]:-Rp

Di = t × ti

Db = ∆t
CCI =

p

-Rp
1/2 × tp × tb(-Rp ) × (-Rmean )T2i × Tb

(2)
(3)
(4)

where Rp was the maximum weight loss rate (unit: %/min), Rmean was the average
weight loss rate (unit: %/min), and t i , tp , tb , and ∆t1/2 represented ignition time,
peak time, burnout time, and time interval between half value of Rp , respectively (unit:
min).
4.2.5 Interaction analysis
To evaluate the interactions between two blends during the co-combustion, thetheoretical DTG curves were calculated based on their decomposition data andcompared with experimental curves. The theoretical DTG curves were calculated by
the average weight of the individual samples, according to eq 5:
DTGcal = Xrw∙DTGRW (Xhw∙DTGHW ) + Xosc∙DTGOSC(5)∆W = DTGexp – DTGcal

(6)

MR =

(∑ni=1(xi,exp -xi,cal ))n∙xmean, cal(7)where DTGRW , DTGHW , and DTGOSC were the weight loss rate of RW, HW, andOSC, respectively (unit: %/min). The Xrw, Xhw, and Xosc were the proportion of
RW, HW, and OSC in the blends, respectively (unit: %). The interaction index, ∆W,
was the difference between experimental and theoretical weight loss rate (unit: %/min).
If ∆W < 0, it indicates a positive interaction for combustion at a specific temperature.
84

Chapter 4

Effect of hydrothermal treatment on co-combustion

The interaction index MR (mean of the absolute error between experimental andcalculated value divided by the mean of the calculated value) was used to analyze thatthe interaction during the whole process is positive or not. If MR > 0, representing a
positive interaction, otherwise, a negative interaction.
4.2.6 Kinetic analysis
4.2.6.1 Introduction of conventional kinetic analysis
The reaction rate of samples was described as [4-17]:dαdt= k(T) f(α)(8)

where dα⁄dt was the conversion rate; α was the conversion degree; t was thereaction time; T was the reaction temperature; f(α) was the differential form ofreaction model; k(T) was the rate constant relating to temperature and was determinedby the Arrhenius equation [4-17]:
Ek(T) = Aexp(- RT )(9)where E, A, and R were the apparent activation energy, the pre-exponential factor,
and the universal gas constant (8.314 J/(K·mol)), respectively.
The conversion degree of each stage was described as [4-18]:
αs =ms -mtms -ms+1(s = 1, 2)(10)where ms , mt , and ms+1 were the initial weight, the weight at time t, and the final
weight in a certain stage, respectively.
Combining eqs 8 and 9, the reaction rate could be transformed into:dαE

β dT = Aexp(- RT )f(α)
where β represented the heating rate and β = dT/dt.

85

(11)

Chapter 4

Effect of hydrothermal treatment on co-combustion

Thereafter, eq 11 could be transferred into:
α dα

A

T

E

G(α) = ∫0 f (α) = β ∫T exp(- RT ) dT

(12)

0

where G(α) represented the integral form of the reaction function f(α), as exhibited inTable 4.1.
Table 4.1 Most frequently used models of solid-state processes.
f ( )
ModelsSymbolG ( )

Order of reaction
First order

F1

1-α

-ln(1-α)2(1-α)-1-1

Second order

F2

(1-α)

Third order

F3(1-α)3[ (1-α)-2-1]/2

One-way transport

D1

0.5α

α2

Two-way transport

D2[-ln(1-α)]-1

Three-way transport

D3

1. 5(1-α)

2/3

Ginstling-Brounshtein equation

D4

1.5 [ (1-α)-1/3-1]-1(1-2α/3)- (1-α)2/3

R1

1

α

Diffusion(1-α) ln(1-α)+α
1/3 -1[1-(1-α) ][1-(1-α)1/3]2

Limiting surface reaction between
both phase
One dimension

1/2

1-(1-α)1/2

Two dimensions

R22(1-α)

Three dimensions

R3

3(1-α)2/3

1-(1-α)1/3

Power law, n=1/2

P2

2α1/2

α1/2

Power law, n=1/3

P3

3α2/3

α1/3

Power law, n=1/4

P4

4α3/4

α1/4

Two-dimensionalA22(1-α) [-ln(1-α)]1/2[-ln(1-α)]1/2

Three-dimensionalA33(1-α) [-ln(1-α)]2/3[-ln(1-α)]1/3

Exponential nucleation

Random nucleation and nuclei growth

4.2.6.2 Model-free methods
The iso-conversional methods had been frequently used to estimate the reaction
kinetics of solid fuels without referring to the reaction mechanism. In this study, FlynnWall-Ozawa (FWO) method and Kissinger-Akahira-Sunose (KAS) method wereapplied to calculate the apparent activation energy for combustion.
The FWO method was expressed as:

86

Chapter 4

Effect of hydrothermal treatment on co-combustionAEE

ln (β) = ln[RG(α)] – 5.331 – 1.052 RT(13)

The KAS method was described as:
β

AR

E

ln( 2 ) = ln[EG(α)] – RTT

(14)

For a certain α, the apparent activation energy, E, was calculated depending on theslopes of the fitted straight line by plotting ln (β) versus 1⁄T for the FWO method
and ln( β⁄T2 ) versus 1⁄T for the KAS method, respectively.
4.2.6.3 Master plot methods
In this study, the integral master plot method was employed to determine the kinetic
model of various decomposition stages for samples. The starting decomposition rate ofsamples was slow, and thus the conversion degree 𝛼 was close to zero at the initial
temperature 𝑇0 , so eq 12 could be transferred as follows:αd(α) A T
E
A T
EAE= ∫ exp (- ) dT ≈ ∫ exp (- ) dT =
P(u)
β T0
RT
β 0
RT
βR
0 f(α)

G(α) = ∫(15)

where u = E⁄RT and 𝑃(u) was determined by the empirical equation, P(u) =exp(-u)⁄u × (1.00198882u + 1.87391198) [4-19]. From iso-conversional methods, the𝐸 value was estimated and used to determine a proper reaction model by simulating
TG data. Taking α = 0.5 as a reference, eq 15 could be transformed into:G(0.5) =AE
P(u0.5 )βR(16)Where G(0.5) denoted the integral of reaction model at α=0.5, u0.5 = E⁄RT0.5 , T0.5
was the temperature at α=0.5. Dividing eq 15 by eq 16, the integral master plots canbe further converted to:
G(α)

P(u)

G(0.5)

= P(u

0.5 )

(17)

On the left side of eq 17, the theoretical master plots, G(α)⁄G(0.5) versus 𝛼, wereobtained from various G(α) functions. The experimental plots, P(u)⁄P(u0.5 ) versus
87Chapter 4

Effect of hydrothermal treatment on co-combustion

α, were calculated from TG data and located on the right side of eq 17. By calculating
the deviation between experimental plots and theoretical plots, the best fitted model
was identified to determine the dominant combustion mechanisms of different samples.
4.3 Results and discussion
4.3.1 Characterization of samples
The proximate and ultimate analysis results are shown in Table 4.2. HW had higher
fixed carbon and lower volatile content compared to RW due to the hydrolysis ofhemicellulose, which enriched energy-dense cellulose and lignin during HTT [4-20].
The ultimate analysis indicated that the C and H contents increased at the expense of O
after HTT. Correspondingly, the O/C and H/C ratios of HW both decreased, whichindicated that HW had a higher energy density due to the lower bonding energies of CH and C-O bonds than that of C-C bonds [4-21]. The Van Krevelen’s diagram further
illustrated that the dehydration reaction was the governing reaction pathway, as shownin Figure 4.3. Moreover, it could be observed that the S content in HW was significantly
reduced, consequently the reduction of SOx emission during HW combustion was
expected. These results were consistent with previous studies [4-20, 4-22]. Comparedto raw OS samples, OSC contained much lower C, H, O, volatiles, and fixed carbon
due to the expulsion of petroleum hydrocarbons and oxygenates during pyrolysis
process. However, the S content in OSC was still higher because some of S-containing
substances remain in the form of sulfides in the char products [4-23].

88

Chapter 4

Effect of hydrothermal treatment on co-combustion

Table 4.2 Proximate and ultimate analyses of samples.
Analysis

OS

OSC

RW

HW

Ultimate analysis (wt%, dry basis)
C

19.67

4.90

47.38

52.78

H

2.66

0.67

5.46

5.57

N

0.29

0.10

0.06

0.14

0.85

0.78

0.10

0.02

4.13

2.15

46.70

41.29

72.40

91.40

0.30

0.20

0.16

0.33

0.74

0.59

1.62

1.64

1.38

1.27

8.85

1.46

18.21

20.72

S
O

1

Ash

2

O/C
H/C
HHV

3

Proximate analysis (wt%, as received basis)
Moisture content

1.51

0

8.59

1.97

Volatiles

21.73

6.77

86.07

81.61

Fixed carbon

4.27

0.85

5.17

15.88

Ash content

72.49

92.38

0.17

0.54

1 O%=100%-C%-H%-N%-S%-Ash%; 2 obtained from elemental analyzer; 3 higher heating value (MJ/mol).

2.00
RW
HW
Dehydration

H/C atmotic ratio

1.75

Demethanation

1.50

Decarboxylation

1.25

1.00
0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

O/C atmotic ratio

Figure 4.3 Van Krevelen diagram of RW and HW.

4.3.2 Combustion behavior of OSC, RW, and HW
The combustion experiments were performed at a heating rate of 40 °C/min, and their89

Chapter 4

Effect of hydrothermal treatment on co-combustion

TG and DTG curves were presented in Figure 4.4 (a-1). It should be noted that the
combustion profiles obtained from TGA referred to the volatile release. Figure 4.4 (a2) shows the combustion process of OSC could be divided into three stages. The firststage (280-600 °C) represented the evaporation of volatiles mainly consisting of heavyorganic components. Light compounds were already decomposed and evaporated fromoil sludge in the previous pyrolysis process [4-24]. The second stage (600-750°C) was
ascribed to the combustion of non-volatile compounds (high-boiling point) and fixed
carbons. Moreover, calcium carbonate (CaCO3) undergoes thermal decomposition
between 650 and 800 °C (eq 18), forming calcium oxide (CaO) and carbon dioxide(CO2), which was consistent with the CaO content in Table 4.4 [4-25]. The last stage(750-940°C) was associated with the decomposition of inorganic matters like carbonate
or sulfate minerals, such as potassium carbonate and potassium sulfate, as shown inbelow reactions [4-25, 4-26].
CaCO3 (s) → CaO (s) +CO2 (s)(18)

K2 CO3 (s) → K2 O (s) + CO2 (g)

(19)

1

(20)

K2 SO4 (s, l) → K2 O (s) + CO2 (g) + 2 O2 (g)Table 4.3 Ash composition of samples by XRF analysis (wt%).
OSC

SiO2

Fe2O3

SO3

Al2O3

CaO

K2O

Na2O

Other

50.41

12.50

11.93

9.84

7.62

3.51

1.37

2.82

K

Fe

S

Cu

Other

RW

23.60

6.43

5.35

6.22

58.40

HW

7.71

5.12

5.80

1.11

80.26

Figure 4.4 (a-1) indicated that the combustion of RW and HW proceeded along two
main stages, identified by two prominent DTG peaks, where the first stage and thesecond stage (stage 1 and stage 2) were associated to the devolatilization and the
combustion of fixed carbon, respectively. The slight slope of the decomposition peakat around 300 °C for RW combustion was disappeared for HW due to the degradationof hemicellulose during HTT [4-20]. Correspondingly, HW had slightly higher ignition
temperature, lower peak temperature, and lower weight loss rate in stage 1, as listed in90

Chapter 4

Effect of hydrothermal treatment on co-combustion

Table 4.4. However, the second stage of HW combustion was extended to higher
temperature region with an increasing burnout temperature compared to RW, eventhough the corresponding weight loss rates of the two showed little difference. It was
resulted from the formation of hard solid substances containing strongly bonded
cellulose, lignin, and insoluble particles through recombination reactions during HTT[4-20]. It should be noted that the temperature range of stage 1 and stage 2 of RW or
HW were within that of OSC. Therefore, HW could continuously release heat during
co-combustion, representing a more adequate synergistic behavior, which wasbeneficial for the co-combustion. Compared with RW and HW, OSC exhibited inferior
combustion properties with a lower degradation rate and longer reaction time.
Therefore, the addition of RW and HW can effectively address the drawbacks of OSC
combustion.
Table 4.4 Combustion characteristics of OSC, RW, HW, and blends at 40 °C/min.
Sample

Ti

Tb

S1

S2

(dw/dt)mean

T1(dw/dt)1

T2(dw/dt)2

RW

307.14536.82343.9449.71

490.749.70

4.10

OSC/RW28

309.84542.03343.8941.19

486.747.66

3.38

OSC/RW55

312.22651.77344.4323.70

510.274.08

1.90

OSC/RW82

315.22704.65347.22

12.87

498.62

2.70

1.12

HW

308.54620.60326.5149.20

563.369.44

4.10

OSC/HW28

308.32593.87325.9740.46

545.16

8.63

3.28

OSC/HW55

308.08606.95325.9426.08

553.64

6.82

2.31

OSC/HW82

314.43724.32333.5410.16

513.14

3.38

1.10

OSC

420.34916.64

485.46

1.73

/

/

0.31

S1: stage 1; S2: stage 2; Ti: ignition temperature (°C); Tb: burnout temperature (°C); T1 and T2: peak temperature instage 1 and stage 2 (°C); (dw/dt)1, (dw/dt)2, and (dw/dt)mean: the maximum weight loss rate in stage 1, the maximum
weight loss rate in stage 2, and the mean weight loss rate during the whole combustion process, respectively.

91

0

80

-10

60

TG-RW
TG-HW
TG-OSC40

DTG-RW
DTG-HW
DTG-OSC20(a-1) stage 1 stage 2

-30

0.0

98

-0.4

-0.8

96
TG-OSCDTG-OSC

-1.2

94
-40
92

-50

-1.6

(a-2)

stage 3200 300 400 500 600 700 800 900

Temperature (°C)
0

80

-10

60
40

-20
-30

20

-40

0

-50

stage 1(b-1)

stage 2RW
OSC/RW28
OSC/RW55
OSC/RW82(b-2)

200 300 400 500 600 700 800 900

200 300 400 500 600 700 800 900

Temperature (°C)

Temperature (°C)

100

0

80

-10

DTG (%/min)

TG (%)

stage 2Temperature (°C)
100

60
40
20

-20
-30
-40
stage 10stage 1200 300 400 500 600 700 800 900

DTG (%/min)

TG (%)

0

-20

100

DTG (%/min)100

TG (%)

Effect of hydrothermal treatment on co-combustion

DTG (%/min)

TG (%)

Chapter 4

-50

(c-1)

stage 2HW
OSC/HW28
OSC/HW55
OSC/HW55(c-2)

200 300 400 500 600 700 800 900

200 300 400 500 600 700 800 900

Temperature (°C)

Temperature (°C)

Figure 4.4 TG and DTG curves of OSC, RW, HW, and their respective blends at 40 °C/min(a-1) TG and DTG of RW, HW, and OSC; (a-2) TG and DTG of OSC; (b-1) TG of RW andOSC/RW; (b-2) DTG of RW and OSC/RW; (c-1) TG of HW and OSC/HW; (c-2) DTG of HW
and OSC/HW.

4.3.3 Combustion behavior of the blends
Combustion of the blends was performed at the heating rate of 40 °C/min, and thedecomposition curves were shown in Figure 4.4. According to Figures 4.4 (b-1) and92

Chapter 4

Effect of hydrothermal treatment on co-combustion(b-2), OSC/RW blends showed similar degradation curves with RW. The
decomposition rates of OSC/RW blends decreased with the increase of OSC proportion
both in stage 1 and stage 2. Meanwhile, the peak temperature in stage 1 remained
constant within 343.89 to 347.22 °C. Moreover, Figure 4.4 (b-2) indicated that the OSC
addition prolonged the reaction time in stage 2 and resulted in a higher burnout
temperature (see Table 4.4). As presented in Figures 4.4 (c-1) and (c-2), the OSC
addition gave a similar effect on HW’s thermal decomposition with that on RW
combustion, lowering the decomposition rate and reducing the combustion reactivity.
The reduction of combustion efficiency for OSC/RW and OSC/HW might be ascribedto the ash originally contained in OSC, which accounted for extra energy consumption.
It is noteworthy that the reaction time of OSC/HW was shortened compared with HW,
and the burnout temperature of OSC/HW28 (593.87°C) and OSC/HW55 (606.95°C)
was lower than that of HW (620.6°C).
Table 4.5 Combustion performance parameters of OSC, RW, HW, and blends at 40 °C/min.
Sample

Di

Db

CCI

RW

9.54

62.58

40.25

OSC/RW28

7.83

56.12

26.76

OSC/RW55

4.46

29.18

7.09

OSC/RW82

2.37

14.89

2.06

HW

9.97

66.77

34.14

OSC/HW28

8.23

61.32

23.51

OSC/HW55

5.31

33.45

10.46

OSC/HW82

1.96

11.25

1.56

0.25

0.03

0.16

OSC
-1

3

-2

4

Di: ignition index (10 %/min ); Db: burnout index (10 %/min ); CCI: comprehensive combustion index (10 -7%2/min2°C3).

Table 4.5 presented several combustion performance parameters to comprehensively
illustrate combustion behaviors of blended samples. The ignition index (Di), burnout
index (Db), and comprehensive combustion index (CCI) values of OSC were increased
after blending with RW or HW, suggesting a promoting effect of RW and HW on OSC
combustion performance. On the other hand, no significant differences in the
combustion performance parameters were found between OSC/RW and OSC/HW,
which implied that hemicellulose decomposition and partial removal of alkali metals
by HTT gave an insignificant impact on the co-combustion performance evaluated by93

Chapter 4

Effect of hydrothermal treatment on co-combustion

these indices. This proposed that similar interactions in OSC co-combustion between
RW and HW were expected. It would be discussed in the next section.
4.3.4 Interactions during combustion of the blends
4.3.4.1 The interaction between OSC and RW
According to Figure 4.5 (a-1), experimental curves of weight loss rate in stage 1
were mostly above the calculated curves. It means that experimental weight loss rate
was smaller than the predicted rate at specified temperature range. Figure 4.5 (a-2)
further shows the ∆W (∆W = DTGexp – DTGcal ) values of OSC/RW with different
OSC proportions were mainly positive in stage 1 and stage 2, suggesting negativeinteractions to the co-combustion. Zhang et al. [4-27] reported similar results for cocombustion between coal gangue and pine sawdust. It was noteworthy that the ∆Wvalues at the temperature range of 530-575 °C were exceptionally negative,
representing a positive interaction at this temperature range. According to Figure 4.5(a-1), however, the reaction time was prolonged in this temperature interval, whichindicated more energy consumption. Therefore, it could be concluded that interactions
between OSC/RW co-combustion were mostly negative. Alkali metals in RW wereregarded as positive for co-combustion owing to their catalytic properties [4-28, 4-29].
However, this study demonstrated that the promoting effect of alkali metals during cocombustion might be weakened by ash-rich materials like OSC. The alkali metals easily
reacted with the ash and formed amorphous matrices such as aluminosilicate. It would
decrease the catalytic performance of alkali metals, block pore structure of reactants,
and subsequently decrease heat transfer and gas penetration [4-28]. Yao et al. [4-30]
observed the similar phenomenon during co-combustion of oil shale and its
hydrothermally treated hydrochar. High contents of alkali metals could react with ash
to produce eutectic substances leading to melt-inducing slagging, which reduced theweight loss rate and lowered the corresponding temperature. Furthermore, Figure 4.5(c-1) indicates that OSC/RW55 posed the lowest MR value in stage 1 and stage 2,
94

Chapter 4

Effect of hydrothermal treatment on co-combustion

exhibiting the strongest inhibiting interaction behaviors. This was resulted from the
maximum formation of non-catalytic aluminosilicate in OSC/RW55. It consumed
catalytic alkali metals in RW and catalytic metal oxides in the ash of OSC, thus finallyreduced co-combustion support from alkali metals and metal oxides. When RW or OSC
blending ratio is higher (OSC/RW28 or OSC/RW82), alkali metals or metal oxides,
which were not consumed after aluminosilicate formation, could support cocombustion. It would mitigate negative interaction in co-combustion. Negativeinteraction between OSC and RW is discussed further in section 4.3.6.
Exp.OSC/RW28
Cal.OSC/RW55
OSC/RW28-TG

Cal.OSC/RW28
Exp.OSC/RW82
OSC/RW55-TG

Exp.OSC/HW28
Cal.OSC/HW55
OSC/HW28-TG

Exp.OSC/RW55
Cal.OSC/RW82
OSC/RW82-TG

-20
-30

(a-1)

60
-2
40

∆W (%/min)800TG (%)∆W (%/min)

-30

-4

stage 1
300

400

500

600

100
80

2
60
0
40
-2

20

stage 2

(b-1)

4

2

200

-20

-40
100

(a-2)

-10

(b-2) stage 1
200

700

20

stage 2300

Temperature (°C)

400

500

600

700

Temperature (°C)

0.00
0.09
0.06

-0.10

0.03

MR

MR

-0.05

-0.15

0.00

-0.20
-0.25

(c-1)

OSC/RW28
OSC/RW55
OSC/RW82

stage 1

-0.03

(c-2)

OSC/HW28
OSC/HW55
OSC/HW82

stage 1stage 2stage 2

Figure 4.5 The experimental and calculated DTG curves and their deviations of the blends
under different blending ratios. (a-1) and (a-2) OSC/RW; (b-1) and (b-2) OSC/HW; (c-1) MR
values of OSC/RW under different blending ratios; (c-2) MR values of OSC/HW under different
blending ratios.

4.3.4.2 The interaction between OSC and HWAs shown in Figure 4.5 (b-1), experimental curves of weight loss rate were mostly
below the calculated curves. It means that experimental weight loss rate was larger than95

TG (%)DTG (%/min)

DTG (%/min)

-10

-6

Exp.OSC/HW55
Cal.OSC/HW82
OSC/HW82-TG

0

0

-40
4

Cal.OSC/HW28
Exp.OSC/HW82
OSC/HW55-TG

Chapter 4

Effect of hydrothermal treatment on co-combustion

the predicted rate. Correspondingly, ∆W values were almost negative in stage 1 andstage 2, suggesting positive interaction for co-combustion (see Figure 4.5 (b-2)).
Notably, Figure 4.5 (b-2) shows that the ∆W values were positive between 570 and
660 °C under different blending ratios, indicating a significant negative interaction.
However, it should be noted that this largely negative interaction occurred at the end
stage of co-combustion. The TG curve of OSC/HW blends, shown in Figure 4.5 (b-2),
also indicated that the combustion process was almost completed when the temperature
increased above 600 °C. It was consistent with the lower burnout temperatures of
OSC/HW than that of HW (see Table 4.4 and section 4.3.3). Consequently, it could besummarized that the co-combustion interaction between OSC and HW was mainly
positive.
The contrast interaction between OSC/RW and OSC/HW was attributed to themodified properties of samples and partial removal of alkali metals after HTT (seeTable 4.3). On one hand, HW surface became rougher than RW after HTT, as displayedin Figure 4.6. It accelerated the pore formation via devolatilization process, which
provided more gas diffusion channels to accelerate the decomposition rate, positively
affecting the interactions between OSC and HW. Similar result was also obtained by
Külaots et al. [4-31] They found that the removing of AAEMs could open more poresand thereby provide active sites to the gaseous products. On the other hand, the lower
contents of alkali metals in HW should be a disadvantage to co-combustion owing toless catalytic support. Positive interaction between OSC and HW would be discussed
further in section 4.3.6 according to kinetic analysis results.

96

Chapter 4

Effect of hydrothermal treatment on co-combustion

(a)

(b)

(c)
Figure 4.6 SEM images of (a) RW, (b) HW, and (c) OSC.

4.3.5 Kinetic analysis
4.3.5.1 Model-free methods
Apparent activation energy (E) at conversion ratio (α) from 0.2 to 0.8 with an interval
step of 0.05 in stage 1 and stage 2 was determined. It is noted that the conversion ratio
is not the overall conversion degree but the local conversion degree in each stage. The
Arrhenius plots were depicted for stage 1 and stage 2 by FWO and KAS method (seeFigures A.1 – A.4 and Table A.3 in Appendix). Table 4.6 listed the kinetic parameters
based on conversion ratio for each sample. The E values as a function of α were shownin Figure 4.7. The results indicated that the variation of E calculated by the FWO
method had good agreement with those of KAS methods, verifying the determined E
97Chapter 4

Effect of hydrothermal treatment on co-combustion

were consistent.
Table 4.6 Kinetic triplets (E, A, and model) for samples at 40 °C/min by master plot method.
SampleStage 1*

Stage 2
2

Eave

A

Model

R

Eave*

A

Model

R2

OSC

128.65

1.08E12F2

0.999

172.44

3.03E10

R3

0.67

RW

211.33

1.80E20D4

0.998

169.284.12E13D3

0.999

OSC/RW28

212.268.00E20D2

0.999

177.34

1.30E14D3

0.996

OSC/RW55

207.903.43E20D2

0.997

163.61

8.86E12D3

0.996

OSC/RW82

206.51

2.45E20D2

0.996

157.882.97E12

D3

0.983

HW

208.402.47E21F1

0.983

126.188.09E9D3

0.998

OSC/HW28

204.44

1.23E21F1

0.983

145.402.57E12D3

0.997

OSC/HW55

186.96

3.26E19F1

0.981

151.16

6.01E11D3

0.998

OSC/HW82

202.83

5.54E20F1

0.990

150.93

6.54E11D3

0.991* avergae E values

As shown in Figures 4.7 (a) and (b), the E value of OSC gradually decreased in stage
1 whereas the E value reached a peak at α = 0.5 and increased again when α > 0.75 instage 2. The variation of E value was consistent with the thermal decomposition of OSC(see section 4.3.2). For RW, one peak of the E value was observed at α = 0.3, and thenthe E value decreased until α = 0.75 in stage 1. This stage could be attributed to the
combustion of hemicellulose and cellulose. Hemicellulose was reported to have higher
E value than cellulose during thermal degradation, thus leading to such a change in the
E value [4-32, 4-33]. After HTT, less amount of hemicellulose was remained in HW
comparing to RW. Therefore, the initial E value of HW was lower than that of RW andcontinuously increased with α. When α > 0.6, the E value of HW was eventually larger
than that of RW in stage 1, which was consistent with the composition profiles of theirvolatile and fixed carbon contents with increasing temperatures. The E values of RW
and HW decreased with α in stage 2. In addition, HW had lower E than RW at any
conversion ratio. This result agreed with previous studies [4-20, 4-27]. HTT degraded
the highly cross-linked cell wall composed of cellulose, hemicellulose, and lignin, andthereby the E value was decreased. The results of apparent activation energies of RW
and HW are close to previous related work, as listed in Table A.4. Moreover, it can benoted that the activation energies of biomass got a reduction after torrefaction due tothe removal of hemicellulose, which agreed with the lower E value of HW in this work.
98Chapter 4350

S1-RW-FWO
S1-HW-FWO
S1-OSC-FWO(a)

S1-RW-KAS
S1-HW-KAS
S1-OSC-KAS

(b)

200
180

Eα (kJ/mol)

300

Eα (kJ/mol)

Effect of hydrothermal treatment on co-combustion

250
200
150

160
140
120
S2-RW-FWO
S2-HW-FWO
S2-OSC-FWO

100

100

80
0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.2

α

0.4

0.5

0.6

0.7

0.8

α

220

(c)

230

0.3

S2-RW-KAS
S2-HW-KAS
S2-OSC-KAS

(d)

200

Eα (kJ/mol)

Eα (kJ/mol)

220
210
200
190
S1-RW-FWO
S1-OSC/RW28-FWO
S1-OSC/RW55-FWO
S1-OSC/RW82-FWO

180
170
0.2

0.3

0.4

S1-RW-KAS
S1-OSC/RW28-KAS
S1-OSC/RW55-KAS
S1-OSC/RW82-KAS

0.5

0.6

0.7

180
160
140
S2-RW-FWO
S2-OSC/RW28-FWO
S2-OSC/RW55-FWO
S2-OSC/RW82-FWO

120
100
0.8

0.2

0.3

0.4

α

S2-RW-KAS
S2-OSC/RW28-KAS
S2-OSC/RW55-KAS
S2-OSC/RW82-KAS

0.5

0.6

0.7

0.8

α
200

S1-HW-FWO
S1-OSC/HW28-FWO
S1-OSC/HW55-FWO
S1-OSC/HW82-FWO

300

S1-HW-KAS
S1-OSC/HW28-KAS
S1-OSC/HW55-KAS
S1-OSC/HW82-KAS

(e)

(f)
180
160

Eα (kJ/mol)

Eα (kJ/mol)

350

250
200

140
120
100

S2-HW-FWO
S2-OSC/HW28-FWO
S2-OSC/HW55-FWO
S2-OSC/HW82-FWO

80

150

60
0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.2

α

0.3

0.4

S2-HW-KAS
S2-OSC/HW28-KAS
S2-OSC/HW55-KAS
S2-OSC/HW82-KAS

0.5

0.6

0.7

0.8

α

Figure 4.7 The apparent activation energies (E) of OSC, RW, HW, and their respective blends(a) (b) variation of E for OSC, RW, and HW in stage 1 and stage 2; (c) (d) variation of E for
OSC/RW in stage 1 and stage 2; (e) (f) variation of E for OSC/HW in stage 1 and stage 2. S1:
Stage 1; S2: Stage 2.

As discussed in section 4.3.2, the combustion stages (stage 1 and stage 2) of RW and
HW had different temperature ranges compared with OSC but similar with OSC/RW
and OSC/HW. It means that combustion stages of the blend co-combustion were mainly
dominated by RW or HW. Therefore, we just analyzed the effect of OSC addition on
the E values of OSC/RW and OSC/HW. The E values of the blends in stage 1 and stage
2 displayed similar trends with RW or HW. On the other hand, Figures 4.7 (c) and (e)shows that the E values of OSC/RW and OSC/HW were lower than those of RW and
HW in stage 1. Furthermore, the OSC addition significantly lowered the E values of
OSC/HW, especially OSC/HW55. It agrees with the result in the previous section, in
99

Chapter 4

Effect of hydrothermal treatment on co-combustion

which OSC/HW55 displayed the strongest positive interaction in stage 1 (see Figure
4.5 (c-2)). Figure 4.7 (d) indicates that the E values of OSC/RW in stage 2 became
lower with increase of OSC proportion. However, the E values of OSC/RW shifted
higher than RW when α > 0.75. Figure 4.7 (f) indicates that the E values of OSC/HW
gradually increased in stage 2 with OSC blending, implying that more energy input was
required during co-combustion. It proposes that the OSC addition probably inhibited
initializing the fixed carbon combustion of HW due to the accumulated ash layer when
the combustion process continued. This result was not consistent with positive
interactions to OSC/HW co-combustion (see section 4.3.4.2). A similar phenomenon
was also observed during the co-combustion of dyeing sludge and rice husk by Wanget al. [4-34] They found that the E values of the blends were higher than individual
samples regardless of positive interactions. Despite the inconsistence, either in stage 1or stage 2, the E values of OSC/HW were significantly lower than OSC/RW under
different OSC blending ratios.
4.3.5.2 Model-based methods
As shown in Figure 4.8, the average apparent activation energy, calculated from
FWO and KAS methods, were used to establish experimental master plot curves of RW,
HW, OSC, and their respective blends at 40 °C/min of heating rate. The curves of G(α)
versus EP(u)⁄𝛽R were plotted, and Table 4.6 listed the most feasible models ofsamples determined based on the linear least square. For RW, the α-dependent trendsof P(u)⁄P(u0.5 ) had a good agreement with the D4 and D3 models in stage 1 and stage
2, respectively. It was consistent with previous studies [4-35]. D3 and D4 are diffusionbased models which assume that diffusive transfer of gaseous products controls overall
reaction rate including numerous chemical reactions or micro-structure changes inparticles [4-35]. For HW, the degradation curves in stage 1 and stage 2 were fitted toF1 and D3, respectively.

100Effect of hydrothermal treatment on co-combustion

4

4

2

4

4

OSC
F2

3

2

F1
F3
D2
D4
R2
A2
P2
P4

3

G(α)/G(0.5)G(α)/G(0.5)

3

F2
D1
D3
R1
R3
A3
P3

P(u)/P(u0.5)

F1
F3
D2
D4
R2
A2
P2
P4

2

F2
D1
D3
R1
R3
A3
P3

OSC

3

2

F2

P(u)/P(u0.5)

Chapter 4

R3
1

(a)

0
0.4

0.5

0.6

0.7

0

0

0.8

1

(b) 0
0.2

0.3

0.4

α
4
S1-RW
S1-OSC/RW28
S1-OSC/RW55
S1-OSC/RW82

3

2

(c)

0
0.3

3

D4

1

0.2

0.4

0.5

0.6

0.7

2

1

1

0

0

0.8

F2
D1
D3
R1
R3
A3
P3

S2-RW
S2-OSC/RW28
S2-OSC/RW55
S2-OSC/RW82

(d) 0
0.3

0.4

0.5

0.6

0.7

0.8

α

3

3

2

1

4
F1
F3
D2
D4
R2
A2
P2
P4

G(α)/G(0.5)

S1-HW
S1-OSC/HW28
S1-OSC/HW55
S1-OSC/HW82

P(u)/P(u0.5)

G(α)/G(0.5)

2

3

4

4

3

D3

2

α
F2
D1
D3
R1
R3
A3
P3

F3

1

0.2

4
F1
F3
D2
D4
R2
A2
P2
P4

0.8
4

F1
F3
D2
D4
R2
A2
P2
P4

G(α)/G(0.5)

2

0.7

4

P(u)/P(u0.5)

G(α)/G(0.5)

3

F2
D1
D3
R1
R3
A3
P3

0.6

α

4
F1
F3
D2
D4
R2
A2
P2
P4

0.5

2

1

1

0

0

F2
D1
D3
R1
R3
A3
P3

S2-HW
S2-OSC/HW28
S2-OSC/HW55
S2-OSC/HW82

F3
D3

3

2

1

F1

(e)

0
0.2

0.3

0.4

0.5

P(u)/P(u0.5)

0.3

1

0.6

0.7

(f)
0.2

0.8

0.3

0.4

0.5

0.6

0.7

0.8

α

α

Figure 4.8 𝑃(𝑢)⁄𝑃(𝑢0.5 ) versus α in stage 1 at 40 °C/min [(a) OSC, (c) RW and OSC/RW,(e) HW and OSC/HW]; 𝑃(𝑢)⁄𝑃(𝑢0.5 ) versus α in stage 2 at 40 °C/min [(b) OSC, (d) RW
and OSC/RW, (f) HW and OSC/HW]; and the direction of the pink dotted line arrow
represented the variation of the curves with increasing OSC proportion in the blends.

In stage 1 of OSC, the heavy components wrapped on the OSC surface were
thermally evaporated and oxidated into sticky oils or tars with increase of temperature[4-36]. Hence, the rate-determining reaction was chemical reaction (F2) (see Figure
4.8 (a)). However, Figure 4.8 (b) indicates that the reaction models of OSC varied with
the increase of α in stage 2. As α ranging from 0.2 to 0.4, the degradation profile of
OSC showed the best agreement with R3 theoretical plots, a three-dimensional phaseboundary reaction, which was regarded as the governing model in the combustion ofcarbonaceous materials [4-37]. For OSC, heterogeneous reactions between fixed
carbon and nascent ash layer took place, and the surface area of ash became the limiting
factor [4-18, 4-38]. When α > 0.4, the degradation curves of OSC sharply increased and1010

P(u)/P(u0.5)

0.2

1Chapter 4

Effect of hydrothermal treatment on co-combustion

could not locate into a specific reaction model.
As shown in Figures 4.8 (c) and (e), the fittest models of OSC/RW (D2) andOSC/HW (F1) were similar with those of RW and HW in stage 1 under different OSC
blending ratios. In stage 2, the degradation profiles of OSC/RW and OSC/HW still
followed the D3 model, same as RW and HW (see Figures 4.8 (d) and (f)). Notably,
the degradation profiles of both OSC/RW and OSC/HW located between D3 and F3
and gradually approached F3 with the increasing proportion of OSC. It is consideredthat OSC ash increased the surface area of the reactants and enlarged gas diffusion gaps.
It might enhance gas diffusion during the fixed carbon combustion [4-18].
4.3.6 Possible mechanisms of OSC co-combustion with RW or HW
Previous research confirmed that the intrinsic solids in oil sludge had positive effects
on the pyrolysis of oil sludge [4-18, 4-39], but the effect of solids on combustion andco-combustion with other feedstocks was still uncertain. The above sections revealed
that the combustion of the blends contained complex reaction mechanisms. Therefore,
in this section, these possible combustion mechanisms during the main (co-)combustion
process (stage 1 and stage 2) were proposed based on the combustion characteristics,
interactions, and kinetic analysis, with a schematic diagram depicting in Figure 4.9.

102

Chapter 4

Effect of hydrothermal treatment on co-combustion

Figure 4.9 (co-)combustion mechanism of OSC/RW and OSC/HW

The devolatilization process at low conversion stages of RW enhanced the formationof fixed carbon and heavy hydrocarbon layers. These layers conversely hindered the
diffusion of gaseous products [4-37]. Besides, highly ordered cellulose regions in RW
acted as barriers and obstructed heat transfer from the external source [4-40]. Therefore,
the rate-limiting step in stage 1 was the diffusion-reaction (D4). After HTT, theinorganic substances were partially leached out, and many strongly bonded compounds
were cracked and removed, giving HW improved homogeneity and well-developed
pore structures. Heat transfer and gas diffusion inside HW particles became easiercompared to RW, which enhanced gas diffusion. Thus, the rate-determination step was
eventually shifted to F1 [4-18]. After blending with OSC, RW or HW ignited at first,
and the released heat was consumed for OSC combustion. Meanwhile, the volatiles
might be partially adsorbed by OSC due to its developed porosity (see Figure 4.6) andsurface functional groups, leading to the reduction of the E values [4-41]. These
interactions should positively promote the decomposition rate of the blends. However,heavy components on the surface of OSC were thermally stimulated and accelerated
the generation of sticky oils, which were inclined to coat the reactants (RW in this case)103

Chapter 4

Effect of hydrothermal treatment on co-combustionand conversely inhibited the diffusion [4-37]. As a result, a negative interaction
appeared between OSC and RW. On the other hand, the positive interaction between
OSC and HW could be due to faster gas diffusion in HW according to the kinetic models(Diffusion rate: F1 > D4). The volatiles from HW easily passed through owing to larger
porosity of HW even when HW was coated by heavy components. Therefore, thepositive interaction eventually remained.
In stage 2, the reaction model of RW was shifted from D2 in stage 1 to D3,
representing a higher diffusion rate. This could be attributed to enhanced pore structureduring devolatilization process, which provided channels for heat and gas transfer.
However, with the removal of weakly bonded components, strongly bonded compounds
in RW were recombined into hard solid polymers, impeding the diffusion reaction.
Therefore, the dominant reaction model for HW became D3 in stage 2, signifying alower diffusion rate than F1 in stage 1. For the blend co-combustion in stage 2, most oforganic compounds in OSC were already evaporated and decomposed, eventuallyleaving a large amount of ash. The accumulated ash particles contained inert materials
and could block the pore structure, which increased the thermal resistance of reactants.
Hence, it made the E values of OSC/HW increase apparently with increase of OSC
blending ratio. Besides, some metal oxides such as Fe2O3 and CaO in OSC ash werereported to pose catalytic effect during combustion [4-2]. Cyclic deoxidation/oxidation
reactions of these metal oxides assisted oxygen transfer to carbon surface of HW andaccelerated the diffusion rate [4-42]. It promoted the decomposition and burnout of
fixed carbon, which was consistent with decreased burnout temperature of OSC/HW.
The catalytic support from metal oxides contained in OSC contributed into the positive
interaction in OSC/HW co-combustion. For OSC/RW, low-melting-point substances,formed from reactions between alkali metals in RW and ash in OSC, were more
susceptible to thermal decomposition than the inert ash particles, even though theyaggravated the restriction on the decomposition rate. Therefore, OSC/RW exhibiteddecreasing E values with an increase of OSC addition, whereas with negativeinteractions due to inhibited diffusion.
104

Chapter 4

Effect of hydrothermal treatment on co-combustion

4.3 Conclusion
This chapter conducted co-combustion of OSC and RW/HW.
(1) RW or HW addition significantly improved combustion property and efficiencyof OSC regarding the conventional combustion parameters.
(2) The RW blending caused negative interactions in co-combustion with OSC,
although it decreased apparent activation energy. It mainly resulted from
inhibited diffusion of volatile matters.
(3) HW addition yielded positive interactions owing to the developed porosity of HW,
which effectively promotes volatiles diffusion, coupled with catalytic support bymetal oxides in OSC.
(4) Not only the catalytic effect of inorganic elements on co-combustion but also
their physical influence on heat and volatiles transfers can contribute toimproving co-combustion performance.
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110

Chapter 5

Conclusions and recommendations

Chapter 5 Conclusions and recommendations
5.1 Overall conclusion
Integrated pyrolysis and combustion treatment can efficiently recover resources inoil sludge and remove the hazards of oil sludge. However, for ash-rich oil sludge,
especially oilfield sludge, this integrated treatment is still limited by some shortcomings,
such as low pyrolysis efficiency and low energy recovery efficiency. Aiming at thisproblem, this study introduces biomass into this treatment system, investigating theinteractions and kinetics of co-pyrolysis/co-combustion of oil sludge/oil sludge charand biomass, and deducing the reaction mechanism.
Chapter 1 introduced the background of this research, including the classification,
characterization, and hazards of oil sludge, the overview of treatment methods of oil
sludge, the integrated pyrolysis and combustion of oil sludge, and two typical biomass
pretreatment methods to give a comprehensive understanding about the research
objective and significance of this work.
To obtain hydrocarbon-rich pyrolysis oil, torrefaction is an effective method topretreat biomass. In Chapter 2, the effect of torrefaction on the morphological structure
evolution and pyrolysis reactivity of biomass was investigated. The results indicated:(1) Intensified torrefaction led to enhanced surface aromaticity, rougher surface
structure, and decreased crystallinity of rice straw. (2) The pyrolysis reactivity of ricestraw was weakened after torrefaction, and the decomposition temperature ranges of
TS were shifted to higher temperatures with increasing torrefaction temperature. (3)Torrefaction altered the pyrolysis pathway of torrefied rice straw to enhance charproduction at the expense of oil and gas. (4) Torrefaction pretreatment enhancedhydrocarbon production, especially alkanes and aromatics.
The results in Chapter 2 indicated that torrefaction of rice straw enhancedhydrocarbon production in pyrolysis oil and enriched lignin content in torrefied ricestraw. Therefore, in Chapter 3, co-pyrolysis of oil sludge and raw/torrefied biomass to111

Chapter 5

Conclusions and recommendations

enhance hydrocarbon production and evaluate the co-pyrolysis synergistic behavior
among oil sludge and model biomass components were conducted. According to theresults: (1) Co-pyrolysis of OS and RS promoted char conversion to volatiles, especiallygas products. (2) The incorporation of OS into either RS or TS promoted hydrocarbon
production at the expense of oxygenates. A significant synergistic effect was witnessedin enhancing hydrocarbon production after OS addition. (3) RS/OS mass ratio of 1:1
was a critical point considering the degree of promoting the formation of hydrocarbons.
A higher blending ratio of OS hindered mass and heat transfer, thereby suppressing thegas formation and lowering the synergy for hydrocarbon generation. (4) Severe
torrefaction increased TS's main decomposition temperature range close to that of OS,
and positive synergistic effects for oil generation were observed when OS was blended
into TS obtained at 250 and 300 °C. (5) A notable result in this work is that OS addition
supplied ample hydrogen atoms and Ni/SiO2 minerals, which further promoted thehydrogenation and ring-opening of aromatic structures, enhancing alkane and olefin
formation at the consumption of aromatics.
Chapter 2 and Chapter 3 focused on incorporating biomass into co-pyrolysis withoil sludge. Additionally, adding biomass to combustion with oil sludge char is also aneffective method to enhance energy recovery. In Chapter 4, co-combustion of oil
sludge char and raw/hydrothermally treated biomass is performed to promote
combustion efficiency and evaluate the effect of alkali metals on co-combustion
interactions, kinetics, and mechanisms. The results showed: (1) RW or HW addition
significantly improved combustion property and efficiency of OSC regarding theconventional combustion parameters. (2) The RW blending caused negativeinteractions in co-combustion with OSC, although it decreased apparent activationenergy. It mainly resulted from inhibited diffusion of volatile matters. (3) On thecontrary, HW blending yielded positive interactions owing to the developed porosity of
HW, which effectively promotes volatiles diffusion, coupled with catalytic support bymetal oxides in OSC. (4) Not only the catalytic effect of inorganic elements on cocombustion but also their physical influence on heat and volatiles transfers can
112

Chapter 5

Conclusions and recommendations

contribute to improving co-combustion performance.
5.2 Recommendations(1) This study employed torrefied rice straw to enhance hydrocarbon production andinvestigated the interactions between oil sludge and model biomass components sinceintensified torrefaction gradually increased the content of lignin in rice straw. It is
recommended to perform co-pyrolysis of oil sludge with typical model biomass
components (hemicellulose, cellulose, and lignin) to observe a concrete interaction andmechanism.
(2) This study proposed that the radical reactions between samples led to the synergistic
effect which promoted hydrocarbon production. Future work should evaluate thegeneration and migration of radicals, thereby illustrating the concrete synergistic
pathway.
(3) This study investigated the synergistic behavior during co-pyrolysis of ricestraw/torrefied rice straw. Furthermore, a deconvoluted understanding of synergistic
effects should be conducted. For example, the contributions of volatile interactions,volatile-char interactions, and biomass-oil sludge ash (to the synergy) could be detailed
by different experimental configurations such as non-contact pyrolysis of samples.
(4) Biomass addition increased the pyrolysis oil yields compared to oil sludge alone.
Future work can add catalysts to further improve oil quality, such as increasing theconcentration of light-weight compounds in the oil.
5.3 Practical applications
The practical application of this work is illustrated in Figure 5.1. In the same
pyrolysis reactor, torrefaction (200 – 300 °C) of biomass is conducted at first to modify
raw biomass. Then the temperature of the reactor is increased around 600 °C and oil
sludge is added to co-pyrolysis with torrefied biomass. The steam generated from113

Chapter 5

Conclusions and recommendations

torrefaction can be used for pyrolysis, since pyrolysis with steam addition is an efficientmethod to increase the conversion of solid fuels and biomass into liquid products.
Therefore, the energy input of the integrated pyrolysis and combustion system can be
fully utilized, and the quality of co-pyrolysis oil can be promoted, with higher
concentration of hydrocarbons and lower concentration of oxygenates.

Figure 5.1 Practical application of this work.

114

Appendix

Appendix
Table A.1 Relative concentrations of straight-chain alkanes and aromatics in oil from individual
pyrolysis of samples.
RS

TS200

TS250

TS300

OS

Straight-chain alkanes (C10-C24)
C10

0

0

0

0

0

C11

0

0

0

0

0.54

C12

0.54

0

1.49

1.07

0.76

C13

0.63

1.02

1.52

0.46

1.61

C14

0

0

0

0

1.77

C15

0.33

0.26

0.64

2.05

2.36

C16

0.62

0.73

2.93

5.45

2.86

C17

0

0

0

0

3.52

C18

0.23

0.29

0.5

1.43

4.42

C19

0

0

0

0

3.15

C20

0.26

0.48

3.03

2.77

2.34

C21

0.41

0.12

0.87

0

2.64

C22

0.38

0

0

0

2.58

C23

0

0

0

0

2.2

C24

2.98

3.64

4.68

23.61

4.37

Sum

6.38

6.54

15.66

36.84

35.12

Straight-chain alkanes (C25-C27)
C25

0

0

0

0

0

C26

0

0

0

0

0.87

C27

0

0

0

0

0

Sum

0

0

0

0

0.87

Monocyclic

0.59

0.71

0.92

0.74

0.52

Bicyclic

6.73

10.26

17.1

22.23

0.2

Tricyclic

0

0

1.59

0

0.8

Tetracyclic

0

0

1.25

0

0

Sum

7.32

10.97

20.86

22.97

1.52

Aromatics

115

Appendix

Table A.2 Relative concentrations of straight-chain alkanes, oxygenates, and aromatics in oil from co-pyrolysis of samples.
RS:OS=3:1
Exp.

Cal.

RS:OS=1:1
Exp.Cal.

Exp.

Cal.

RS:OS=1:3
Exp.Cal.

Exp.

Cal.

TS200:OS=3:1
Exp.Cal.

Exp.

Cal.

Exp.Cal.

TS250:OS=3:1
Exp.

Cal.

Exp.Cal.

TS300:OS=3:1
Exp.

Cal.

Exp.Cal.

Straight chain alkanes (C10-C24)
C10

0

0

0

0

0

0

0

0

0

0.09

0

0.09

0

0

0

0

0

0

C11

0

0.14

-0.14

0.3

0.27

0.03

0.45

0.41

0.04

0

0.14

-0.14

0

0.14

-0.14

0

0.14

-0.14

C12

0.44

0.6

-0.16

0.45

0.65

-0.2

0.62

0.71

-0.09

0.47

0.19

0.28

0.57

1.31

-0.74

0

0.99

-0.99

C13

0.29

0.88

-0.59

0.97

1.12

-0.15

1.49

1.37

0.12

0.38

1.17

-0.79

0.49

1.54

-1.05

0.53

0.75

-0.22

C14

0.34

0.44

-0.1

0.7

0.89

-0.19

0.98

1.33

-0.35

0.93

0.44

0.49

0.69

0.44

0.25

0.71

0.44

0.27

C15

0.71

0.84

-0.13

1.09

1.35

-0.26

1.21

1.85

-0.64

0.73

0.79

-0.06

1

1.07

-0.07

1.07

2.13

-1.06

C16

0.68

1.18

-0.5

0.94

1.74

-0.8

1.69

2.3

-0.61

0.62

1.26

-0.64

0.88

2.91

-2.03

1.06

4.8

-3.74

C17

2.05

0.88

1.17

1.85

1.76

0.09

2.26

2.64

-0.38

1.55

0.88

0.67

1.04

0.88

0.16

2.51

0.88

1.63

C18

0.69

1.28

-0.59

1.3

2.33

-1.03

2.23

3.37

-1.14

1.16

1.32

-0.16

1.56

1.48

0.08

1.12

2.18

-1.06

C19

0.68

0.79

-0.11

1.41

1.58

-0.17

1.95

2.36

-0.41

0.68

0.79

-0.11

0.89

0.79

0.1

1.08

0.79

0.29

C20

1.54

0.78

0.76

2.35

1.3

1.05

3.91

1.82

2.09

0.72

0.95

-0.23

1.09

2.86

-1.77

1.09

2.66

-1.57

C21

0.71

0.97

-0.26

1.64

1.53

0.11

1.42

2.08

-0.66

0.92

0.75

0.17

1.05

1.31

-0.26

1.37

0.66

0.71

C22

0.79

0.93

-0.14

1.19

1.48

-0.29

2.22

2.03

0.19

0.82

0.65

0.17

1.18

0.65

0.53

1.61

0.65

0.96

C23

1.58

0.55

1.03

1.73

1.1

0.63

1.46

1.65

-0.19

0.9

0.55

0.35

1.88

0.55

1.33

2.5

0.55

1.95

C24

2.18

3.33

-1.15

2.84

3.68

-0.84

4.16

4.02

0.14

1.18

3.82

-2.64

5.66

4.6

1.06

6.2

18.8

-12.6

Sum

12.68

13.57

-0.89

18.76

20.75

-1.99

26.05

27.94

-1.89

11.15

13.69

-2.54

17.98

20.53

-2.55

20.85

36.41

-15.56

Straight chain alkanes (C15-C27)
C25

0

0

0

0.84

0

0.84

1.21

0

1.21

0.43

0

0.43

0.75

0

0.75

1.25

0

1.25

C26

1.05

0.22

0.83

1.12

0.44

0.68

1.32

0.65

0.67

0.99

0.22

0.77

1.49

0.22

1.27

1.94

0.22

1.72

C27

2.79

0

2.79

3.18

0

3.18

0

0

0

3.52

0

3.52

4.83

0

4.83

3.31

0

3.31

116

Appendix

Sum

3.84

0.22

3.62

5.14

0.44

4.7

2.53

0.65

1.88

4.94

0.22

4.72

7.07

0.22

6.85

6.5

0.22

6.28

Acid

3.14

11.20

-8.06

1.15

7.67

-6.52

0.88

4.15

-3.27

4.30

10.20

-5.90

3.23

6.03

-2.80

2.14

0.74

1.40

Alcohol

11.42

6.58

4.84

9.88

8.36

1.52

10.16

10.14

0.02

7.40

6.30

1.11

20.56

5.72

14.84

15.78

4.22

11.56

Aldehyde

7.42

9.84

-2.42

0.52

6.68

-6.16

0.48

3.53

-3.05

7.38

7.85

-0.47

0

2.73

-2.73

0

0.09

-0.09

0.75

1.52

-0.77

0

1.01

-1.01

0

0.51

-0.51

0.60

1.35

-0.75

0

0.95

-0.95

0

0

0

Ester

3.12

4.36

-1.24

1.87

3.19

-1.32

1.98

2.03

-0.04

2.04

3.38

-1.34

0

0.22

-0.22

1.77

0.64

1.13

Furan

0

8.99

-8.99

0

6.00

-6.00

0

3.00

-3.00

0.83

8.16

-7.33

0

3.58

-3.58

0

0

0

Ketone

10.81

11.60

-0.78

4.05

7.84

-3.79

2.23

4.09

-1.86

7.43

10.77

-3.34

3.1

7.62

-4.52

2.65

0.54

2.11

Phenol

9.33

7.65

1.68

2.37

5.10

-2.73

1.98

2.55

-0.57

10.94

11.06

-0.12

9.02

13.65

-4.63

1.23

15.34

-14.11

Sum

45.99

61.72

-15.73

19.84

45.85

-26.01

17.71

29.97

-12.26

40.92

59.06

-18.14

35.91

40.49

-4.58

23.57

21.57

2.00

Monocyclic

0

0.57

-0.57

0.64

0.56

0.08

0.53

0.54

-0.01

0.06

0.66

-0.6

0.94

0.82

0.12

0

0.69

-0.69

Bicyclic

5.05

5.1

-0.05

2.18

3.47

-1.29

1.73

1.83

-0.1

7.7

7.75

-0.05

8.48

12.88

-4.4

6.07

16.72

-10.65Tricyclic

0

0.2

-0.2

0.5

0.4

0.1

0

0.6

-0.6

0.49

0.2

0.29

0

1.39

-1.39

0.54

0.2

0.34

Tetracyclic

0

0

0

0

0

0

0

0

0

0

0

0

0.3

0.94

-0.64

0.39

0

0.39

Sum

5.05

5.87

-0.82

3.32

4.42

-1.1

2.26

2.97

-0.71

8.25

8.61

-0.36

9.72

16.03

-6.31

7

17.61

-10.61Oxygenates

AnhydrosugarsAromatics*

ΔW denotes the difference between experimental concentrations and calculated concentrations; the background color scale: from green to red, represents Δ value increases from the smallest

negative number to the largest positive number, implying the synergistic effect from inhibition to promotion.

117

Appendix

Table A.3 Combustion kinetic parameters of samples in stage 1 and stage 2 by FWO and KAS
methods.
Sample

RW

MR28

MR55

α

Stage 1

Stage 2

SampleαStage 1

Stage 2

FWO

KAS

FWO

KAS

FWO

KAS

FWO

KAS

E kJmol-1

E kJmol-1

E kJmol-1

E kJmol-1

E kJmol-1

E kJmol-1

E kJmol-1

E kJmol-1

0.2

218.91220.99202.24201.200.2

174.10173.68171.65168.940.25

222.94225.06200.03198.790.25

173.95173.43160.35156.800.3

228.79231.13

195.21

193.550.3

172.76172.10150.47146.240.35

227.37229.55190.94189.060.35

172.29171.60142.89138.100.4

220.81222.57186.43184.160.4

173.55172.93136.17130.860.45

216.31217.83177.98175.260.45

175.53174.93130.72124.960.5

209.59210.68170.47167.190.5

182.01181.74124.79118.640.55

203.11203.78164.86161.290.55

190.94191.06119.81113.24

0.6

197.42

197.79158.22

154.140.6

202.55203.19115.70108.750.65

196.07196.21152.77148.320.65

220.57222.07110.72103.43

0.7

192.44

192.39144.15139.180.7

248.31251.25106.53

98.94

0.75

195.84195.96135.70130.280.75

284.83289.58102.1194.11

0.8

210.14210.93127.48121.550.8

331.22338.21

98.08

89.79

Eave

210.75211.91

169.73166.46Eave

207.89208.91128.46122.520.2

222.63224.89206.90206.190.2

175.84175.51186.99185.150.25

225.32227.64201.76200.620.25

172.68172.10179.00176.51

0.3

227.29229.55198.05

196.630.3

171.34170.60169.44166.360.35

227.77229.97

192.36190.560.35

170.94170.19160.75156.970.4

220.57222.32185.17182.910.4

171.42

170.69155.53151.400.45

216.23217.66178.06175.260.45

176.63176.09150.16145.580.5

209.27210.26

173.63170.520.5

179.56179.08145.18140.260.55

203.35204.03169.99166.530.55

188.88188.89139.88134.520.6

199.00199.37168.41164.870.6

200.97201.53134.90129.120.65

197.58

197.79164.38160.460.65

217.97219.41130.08123.960.7

196.86

197.12160.19156.050.7

238.36240.77125.90119.470.75

196.86196.96157.27152.890.75

272.50276.52

122.34115.560.8

208.80209.51153.08148.400.8

315.33321.59117.76110.66Eave

211.66212.85

177.64174.76Eave

204.03204.84147.53142.730.2

216.94218.91

213.15212.760.2

164.70163.79194.65193.22

0.25

218.12220.07205.40204.440.25

162.09160.96183.27181.000.3

221.68223.65

195.60

194.050.3

161.22

159.96172.44169.440.35

219.39221.07185.25182.990.35

159.01

157.63164.94161.460.4

213.46214.83177.19174.590.4

160.27158.88159.17155.220.45

210.14211.26172.29169.110.45

164.62163.45154.43150.070.5

203.42204.11

166.52

162.950.5

168.97168.03149.92145.250.55

200.66201.20162.64158.800.55

174.97174.26146.21141.17

0.6

197.58

197.79159.48155.31

0.6

184.30183.99142.57137.180.65

193.62

193.72155.61151.150.65

199.39199.79139.49133.86

118

HW

MH28

MH55

Appendix

MR82

OSC

0.7

194.65194.71

154.19149.57

0.7

216.70217.91135.54129.530.75

197.26

197.37151.42146.490.75

242.15244.60132.30126.120.8

209.51210.26148.50143.330.8

272.58276.52128.98122.47Eave

207.42208.38172.86

169.66Eave

187.00186.91

154.15149.690.2

221.52223.65209.75209.100.2

185.01185.07191.17189.480.25

227.29229.63198.92

197.540.25

176.87176.42179.32176.840.3

227.37229.55187.86185.820.3

176.63176.09172.44169.520.35

222.47224.31179.08176.51

0.35

175.21

174.59165.33161.870.4

218.04219.57173.47170.440.4

177.50176.92160.35156.550.45

210.14211.26168.33164.950.45

180.11179.58157.03152.890.5

203.42204.11

165.49161.870.5

186.83186.73153.87149.400.55

201.13201.61161.85157.880.55

190.23190.22152.45147.740.6

198.05198.37160.98156.890.6

202.32202.86149.45144.580.65

197.66

197.87160.43156.140.65

215.12

216.33146.76141.590.7

195.76

195.79159.88155.390.7

229.66231.46144.78139.340.75

200.66200.87160.12155.550.75

258.75261.97141.31135.520.8

221.60222.90160.43155.720.8

278.35282.51136.09129.95Eave

211.16212.27172.82169.52Eave

202.51

203.14

157.72

153.480.2

140.44136.43180.27174.590.25

136.72132.36185.25179.670.3

134.04129.37184.22

178.420.35

132.61127.87184.46178.670.4

130.80125.79189.75184.070.45

130.40125.21

191.65185.900.5

129.61124.38193.78188.060.55

128.27122.88189.75183.740.6

127.79122.22179.64172.930.65

127.40121.72167.15159.380.7

128.11

122.30146.76

137.600.75

127.56

121.63138.94128.620.8

132.38126.54155.14145.08Eave

131.24126.05175.90168.98119

MH82

Appendix

Table A.4 Apparent activation energies of the combustion of woody biomass from related work.
SampleE kJ/mol (ave)Kinetic method

References

RW

190.31

FWO, KAS

Present work

HW

167.29

FWO, KAS

Present work

Acacia190.45FWO[A-1]

Torrefied acacia

163.85

FWO

[A-1]

Pine

191.14

FWO[A-1]

Torrefied pine

143.12FWO[A-1]

Fir wood

197.33Coat Redfern[A-2]

Poplar wood chip181.22

FWO, KAS[A-3]

Pine wood

186.92FWO[A-4]

wood sawdust

178.43FWO[A-5][A-1] Wilk, M., Magdziarz, A., Gajek, M., Zajemska, M., Jayaraman, K., & Gokalp,
I. (2017). Combustion and kinetic parameters estimation of torrefied pine, acacia andMiscanthus giganteus using experimental and modelling techniques. Bioresource
Technology, 243, 304-314.
[A-2] López-González, D., Fernandez-Lopez, M., Valverde, J. L., & Sanchez-Silva,
L. (2013). Thermogravimetric-mass spectrometric analysis on combustion of
lignocellulosic biomass. Bioresource technology, 143, 562-574.
[A-3] Liu, L., Pang, Y., Lv, D., Wang, K., & Wang, Y. (2021). Thermal and kinetic
analyzing of pyrolysis and combustion of self-heating biomass particles. Process
Safety and Environmental Protection, 151, 39-50.
[A-4] Xu, X., Pan, R., & Chen, R. (2021). Combustion characteristics, kinetics, andthermodynamics of pine wood through thermogravimetric analysis. Applied
Biochemistry and Biotechnology, 193(5), 1427-1446.
[A-5] Wang, T., Hou, H., Ye, Y., Rong, H., Li, J., & Xue, Y. (2019). Combustion
behavior of refuse-derived fuel produced from sewage sludge and rice husk/wood
sawdust using thermogravimetric and mass spectrometric analyses. Journal of Cleaner
Production, 222, 1-11.
120

Appendix

ln(β)

3.0

2.5

3.5

3.0

2.5

2.0

2.5

1.5

(a)
0.001700.001750.001800.001851.5(b)0.00160

0.001650.001700.001750.001800.00185

3.5

ln(β)

3.0

2.5

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

3.5

3.0

2.5

2.0

1.5

(d)
0.001750.001800.00185

3.5

3.0

2.5

1.5

(e)
0.00160

0.001650.001700.001750.001800.00185

ln(β)

3.0

2.5

2.0

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

3.5

3.0

2.5

2.0

1.5

(g)
0.001650.00170
1/T (K)(f)
0.00160

0.001650.001700.001750.001800.001750.001800.00185

1/T (K)

ln(β)

3.5

0.00160

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

1/T (K)
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

0.0016

2.0

1/T (K)

4.0

0.00154.0

2.0

0.001700.0014
1/T (K)

ln(β)

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

0.001650.0013

1/T (K)

4.0

0.00160(c)0.0012ln(β)

0.00165

2.0

1/T (K)

1.5

3.0

2.0

0.00160

1.5

3.5

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

3.5

3.0

ln(β)

1.5

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

ln(β)

3.5

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

ln(β)

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

2.5

2.0

1.5

(h)
0.00160

0.001650.00170
1/T (K)

0.001750.00180(i)0.00160

0.001650.001700.001750.00180

1/T (K)

Figure A.1 Iso-conversional plots at various conversion degree for (a) RW, (b) HW, (c) OSC,(d) OSC/RW28, (e) OSC/RW55, (f) OSC/RW82, (g) OSC/HW28, (h) OSC/HW55, (i)OSC/HW82 in stage 1 by FWO method.

121

Appendix

-8.5

-8.5

ln(β/T2)

-9.5

-10.0

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-9.0

-9.5

-10.0

-10.5

-10.5

-11.0

-11.0

-9.0

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-9.5

-10.0

ln(β/T2)-9.0ln(β/T2)

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-10.5

-11.0

(a)
0.00160

-11.5

(b)

-11.5
0.001650.001700.001750.00180-11.50.00160

0.001850.001650.00170

1/T (K)

-9.5

-10.5

-11.0

-10.0

-11.5
0.001750.001800.001850.001700.001750.001800.001850.00160

0.00170

-10.5

-9.5

-10.0

-10.5

-11.0

0.00180

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-9.0

-9.5

-10.0

(i)

-11.5
0.001750.00185

-11.0

(h)

-11.5

0.00180

-10.5

-11.0

(g)

0.00175

-8.5
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

ln(β/T2)-10.0

1/T (K)

0.00165

1/T (K)-9.0

ln(β/T2)-9.5

0.00170(f)

1/T (K)-9.0

0.00165

-10.0

-11.5
0.00165-8.5

0.00160

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-11.0

(e)

1/T (K)

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

0.0016-10.5

0.00160-8.5

0.0015

-9.5

-11.0

(d)
0.001700.0014

-9.0

-10.5

0.001650.0013

-8.5
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

ln(β/T2)-10.0

0.00160(c)0.0012

1/T (K)-9.0

ln(β/T2)-9.5ln(β/T2)

0.00185

-8.5
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-9.0

ln(β/T2)

0.00180

1/T (K)

-8.5

-11.5

0.001750.00160

0.001650.00170
1/T (K)

0.001750.00180-11.50.00155

0.00160

0.001650.001700.001750.00180

1/T (K)

Figure A.2 Iso-conversional plots at various conversion degree for (a) RW, (b) HW, (c) OSC,(d) OSC/RW28, (e) OSC/RW55, (f) OSC/RW82, (g) OSC/HW28, (h) OSC/HW55, (i)OSC/HW82 in stage 1 by KAS method.

122

Appendix

ln(β)

3.0

2.5

3.5

3.0

2.5

2.0

2.5

1.5

(a)

2.0

1.5

(b)
0.00120.0013

1/T (K)

0.00140.0009

ln(β)

3.0

2.5

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

3.5

3.0

2.5

2.0

1.5

0.00125 0.00130 0.00135 0.00140 0.00145 0.00150 0.00155

3.5

3.0

2.5

2.0

1.5

(e)

0.00120.00130.0014

1/T (K)

0.00150.0013

3.5

ln(β)

3.0

2.5

2.0

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

3.5

3.0

2.5

2.0

1.5

(g)
0.00140.00150.00140.0015

1/T (K)

ln(β)

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

1/T (K)(f)
0.0012

1/T (K)

4.0

0.0012

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

2.0

(d)

0.0011ln(β)

3.50.00130.00101/T (K)

ln(β)

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

0.0012(c)0.0015

1/T (K)

4.0

1.5

3.0

2.0

0.00125 0.00130 0.00135 0.00140 0.00145 0.00150 0.00155

1.5

3.5

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

3.5

3.0
ln(β)

1.5

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

ln(β)

3.5

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

ln(β)

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

4.0

2.5

2.0

1.5

(h)
0.00120.00130.0014
1/T (K)

0.0015(i)0.00120.00130.00140.0015

1/T (K)

Figure A.3 Iso-conversional plots at various conversion degree for (a) RW, (b) HW, (c) OSC,(d) OSC/RW28, (e) OSC/RW55, (f) OSC/RW82, (g) OSC/HW28, (h) OSC/HW55, (i)OSC/HW82 in stage 2 by FWO method.

123

Appendix

-9.0

-9.5

-10.5

-9.5

-10.0

-10.5

-11.5

-11.5

-11.5

-12.0

(b)

(c)

-12.0

-12.5
0.00120.0013

1/T (K)

0.00140.00150.0009

-10.5

-10.0

-10.5

-10.0

-11.0

-11.0

(d)

-10.5

-11.0

-11.5

-11.5

-11.5

(e)

-12.0

-12.0
0.00120.00125 0.00130 0.00135 0.00140 0.00145 0.00150 0.00155(f)0.00130.0014

1/T (K)-12.0
0.00120.0015

-10.0

-10.5

-10.0

-10.5

-10.0

-10.5

-11.0

-11.0

-11.5

-11.5

-11.5

(g)

(h)

(i)

-12.0
0.00140.0015

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-9.5

-11.0

-12.0

0.0015

-9.0
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-9.5

ln(β/T2)-9.5

0.0014
1/T (K)

-9.0
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

1/T (K)

0.0013

1/T (K)-9.0

0.0013

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-9.5

ln(β/T2)-10.0

0.0012

-9.0
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-9.5

ln(β/T2)-9.5

0.0011

1/T (K)

-9.0
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

0.00120.00101/T (K)-9.0

ln(β/T2)

-11.0

-11.0

(a)

ln(β/T2)

-10.5

-11.0

-12.0
0.00125 0.00130 0.00135 0.00140 0.00145 0.00150 0.00155

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

-10.0

ln(β/T2)ln(β/T2)

-10.0

-9.5
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

ln(β/T2)

0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8

ln(β/T2)

-9.0

-12.0
0.00120.00130.0014
1/T (K)

0.00150.00120.00130.00140.0015

1/T (K)

Figure A.4 Iso-conversional plots at various conversion degree for (a) RW, (b) HW, (c) OSC,(d) OSC/RW28, (e) OSC/RW55, (f) OSC/RW82, (g) OSC/HW28, (h) OSC/HW55, (i)OSC/HW82 in stage 2 by KAS method.

124