Proceedings of the 7th World Congress on Mechanical, Chemical, and Material Engineering (MCM'21)
Prague, Czech Republic Virtual Conference – August, 2021
Paper No. HTFF 135
DOI: 10.11159/htff21.135

Conversion of Mixed Waste of Wood and Plastic to Clean Fuels Using
Pyrolysis in Nigeria – Numerical Study
Andrew N. Aziz 1,3*, Saad Mahmoud1, Raya Al-Dadah1, Surindar Dhesi2, Irina Kuznetsova2, Cyril
Effiong2, Ejikeme Kanu2
1

School of Engineering, University of Birmingham, Edgbaston, Birmingham, B15-2TT
School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston,Birmingham, B15-2TT
3City of Scientific Research and Technological Applications, SRTA-City, New Borg El Arab City, 21934
Alexandria, Egypt
ANM900@student.bham.ac.uk; S.M.MAHMOUD@ bham.ac.uk;
R.K.AL-DADAH@ bham.ac.uk; S.K.DHESI@ bham.ac.uk; I.KUZNETSOVA@ bham.ac.uk;
CXE921@student.bham.ac.uk; EJK899@student.bham.ac.uk
2Abstract - Waste management is a major challenge in Nigeria, where around 32 million tons of waste is generated annually including
13 million tonnes of agricultural waste and 2.5 million tons of plastic waste. Currently, the waste management system is very inefficientwhere almost 70% of the waste ends up in landfills, sewers, beaches and water bodies causing serious environmental and health problems.
This work numerically investigates the conversion of mixed wastes (wood and plastics) to produce clean and affordable solid, liquid andgaseous fuels that can be used for cooking, heating and electricity generation.
Pyrolysis process which involves heating the waste at different rates in the absence of Oxygen has received significant research interest
since it can convert various types of waste to clean fuels thus reducing fossil fuel consumption and CO2 emission. For example, wood
waste can be converted to high-quality syngas, oil, and char while plastic waste can be converted to high-quality syngas, oil, light andheavy waxes. Also, it generates lower emissions compared to other waste conversion processes such as combustion, gasification, andplasma treatment.
In this study, a numerical model is developed to simulate the pyrolysis process of mixed wood and plastic waste materials to predict theoutputs in terms of char, syngas, oil and wax production. The model is based on the kinetics of the reactions associated with wood andplastic when subjected to different heating rates from 320 to 923 Kelvin for 20 minutes. Results showed that for 1kg of mixed waste(30% plastic and 70% wood) and after 20 minutes of heating, the output consists of 50.13% syngas, 8.35% oil, 9.5% char, 10.8% light
Wax and 8.33% heavy wax. This modelling allows for controlling the output composition based on varying the input waste constituents
which can optimize the waste conversion process.
Keywords: Pyrolysis, Mixed Waste, Modelling, Clean energy.

1. Introduction
Nigeria is a developing agricultural country which generates huge amount of agricultural waste reaching 13 million
tonnes per year where significant amounts of this waste are either left to rot or burned in open fields causing significant air
pollution [1]. The most common agricultural wastes are maize cobs, husks, and stalks (62.5 percent) and cassava stalks andpeels 60 percent[2]. Also, in urban cities there is an increasing amount of waste with heavy plastic of around 2.5 tons ofplastic waste per year [3]. Currently, the waste management system is very inefficient where almost 70% of the waste ends
up in landfills, sewers, beaches and water bodies causing serious environmental and health problems[4].
Pyrolysis has been developed since the 1970s [5] which is an advanced thermochemical conversion process of carbonicfeedstock in the absence of oxygen to produce bio-fuel (oil, char, and fuel gases) [6]. This process can reduce greenhousegas emissions and contribute to energy security. There are three types of pyrolysis [7]. Slow pyrolysis: this is the slow heating
rate process between (5–7 K/min) which mainly produce char and low liquid and gaseous products. Fast pyrolysis: in thisprocess, the material heating rate is high 650 K for 5 s and it produces mainly liquid fuel. In the flash process, the material
is briskly heated for few seconds or even less produce pyrolysis oil. The quality and the stability of the generated oil areinfluenced by the char or ash content of bio-oil. Various studies have been conducted to model the slow process of variouswastes. Koufopanos et al. [8] established the kinetic model of the pyrolysis of biomass and its constituents to develop an

HTFF 135-1

efficient pyrolizer. Koufopanos and Papayannakos [9] studied the kinetics coupled with thermal and heat transfer effects
and concluded that the pyrolysis can be applied for a wide range of heating rates 573 K to 873 K and particle sizes from
1mm to 20 mm. Kabir et al. [6] presented a numerical and experimental analysis about the pyrolysis of green waste
process showing that around 19.972 % bio-oil, 40.83 % bio-char, and 29.77 % syn-gas can be generated.. Miljković et.
al [10] built a mathematical model of single-particle biomass under the pyrolysis process and compared the results withexperimental measurements and other different types of wood samples, the ultimate analyses for the composition havebeen reported. Dubdub and Al-Yaari [11] investigated the kinetics of mixed plastic waste. Their results showed thatusing a single thermal degradation stage, the pyrolysis of pure polymer samples happens in the following order: PS, PP,LDPE, and HDPE. Andersen et al. [12] simulated the pyrolysis process of plastic waste to produce environmentallyfriendly fuel. The above literature indicates that most of the research was conducted on wood samples or plastic samples
separately. Therefore, there is a need to develop models to simulate the pyrolysis process for converting mixed wastes
to clean fuels. In this study, a numerical model for simulating the pyrolysis of mixed waste consisting of wood andplastic using the Ordinary Differential Equations (ODEs) solver in MATLAB to predict the outputs in terms of syngas,
char, oil, light and heavy waxes.

2. Model Description
This model aims to simulate the pyrolysis process for converting heavy plastics and wood to clean fuels including
syngas, oil and charcoal. It uses the kinetic equations of heavy plastic and wood subjected to different heating rates from
320 K to 923K. Eq. 1 to Eq. 8 represents the kinetic reactions for wood where Eq. 1 describes the initial concentrationof wood. After applying the heat, the gas starts to be produced as shown in Eq. 2. At the same heating rate Eq. 3 reveals
the amount of char produced from the same sample. Then during the second phase reaction, the same outputs occur asin Eq. 4 and Eq. 5. The auxiliary equations 6-8 are used to determine the values of the reaction constants related to thewood sample with Table 1 listing the required input parameters.

∂𝐶𝑤= −k 𝑤1 ∙ (𝐶𝑤 )n1 − k 𝑤2 ∙ (𝐶𝑤 )n1∂𝑡∂𝐶𝑔𝑎𝑠1n2= k 𝑤1 ∙ (𝐶𝑤 )n1 − k 𝑤3 ∙ (𝐶𝑔𝑎𝑠1 ) ∙ (𝐶𝑐ℎ𝑎𝑟1 )n3∂𝑡∂𝐶𝑐ℎ𝑎𝑟1n2= k 𝑤2 ∙ (𝐶𝑤 )n1 − k 𝑤3 ∙ (𝐶𝑔𝑎𝑠1 ) ∙ (𝐶𝑐ℎ𝑎𝑟1 )n3∂𝑡∂𝐶𝑔𝑎𝑠2n2= k 𝑤3 ∙ (𝐶𝑔𝑎𝑠1 ) ∙ (𝐶𝑐ℎ𝑎𝑟1 )n3∂𝑡∂𝐶𝑐ℎ𝑎𝑟1
n2= k 𝑤3 ∙ (𝐶𝑔𝑎𝑠1 ) ∙ (𝐶𝑐ℎ𝑎𝑟1 )n3∂𝑡𝐷𝐿( 1+ 1 )k 𝑤1 =A𝑤1 ∙ 𝑒 𝑇 𝑇2
k 𝑤2 =A𝑤2 ∙ 𝑒

(1)
(2)
(3)
(4)
(5)
(6)

𝐷𝐿
( 2 + 22 )
𝑇 𝑇

(7)

−𝐸
( 3)
𝑇.𝑅𝑐

(8)

k 𝑤3 =A𝑤3 ∙ 𝑒Where 𝐶𝑤 , 𝐶𝑔𝑎𝑠 , 𝐶𝑐ℎ𝑎𝑟 are concentrations for Wood, gas, char respectively.
Eq. 9 to Eq. 13 represents the kinetic reactions for heavy plastics, where Eq. 9 represents the initial plasticconcentration. After applying the heat, the light wax starts to form first which is represented in Eq. 10. At the same
heating rate, Eq. 11 determines the amount of heavy wax produced from the same sample. Then, Eq.12 gives the oil
production while Eq.13 determines the amount of the produced gas. The values of the reaction constants related to theplastic sample at 713 Kelvin plastic are shown in table 2.

∂𝐶𝑝= −k 𝑝1 ∙ 𝐶𝑝 − k 𝑝2 ∙ 𝐶𝑝 − k 𝑝3 ∙ 𝐶𝑝 − k 𝑝4 ∙ 𝐶𝑝∂𝑡
HTFF 135-2(9)∂𝐶𝐿𝑊= k 𝑝1 ∙ 𝐶𝑝 − k 𝑝6 ∙ 𝐶𝐿𝑊 − k 𝑝5 ∙ 𝐶𝐿𝑊∂𝑡∂𝐶𝐻𝑊= k 𝑝4 ∙ 𝐶𝑝 − k 𝑝8 ∙ 𝐶𝐻𝑊 − k 𝑝9 ∙ 𝐶𝐻𝑊∂𝑡∂𝐶𝑜𝑖𝑙= k 𝑝2 ∙ 𝐶𝑝 − k 𝑝5 ∙ 𝐶𝐻𝑊 − k 𝑝9 ∙ 𝐶𝐻𝑊 − k 𝑝7 ∙ 𝐶𝑜𝑖𝑙∂𝑡
∂𝐶𝑝𝑔𝑎𝑠= k 𝑝3 ∙ 𝐶𝑝 − k 𝑝6 ∙ 𝐶𝐿𝑊 − k 𝑝7 ∙ 𝐶𝑜𝑖𝑙 − k 𝑝8 ∙ 𝐶𝐻𝑊
∂𝑡

(10)
(11)
(12)
(13)

Table 1: Model inputs for wood.

Parameter namevalue
650 [kg/m3]
9.973 × 10-5 [1/s]1.068 × 10-3 [1/s]
5.7 × 105 [1/s]−9061227 K2−6123081 K2
81000 J / mol
1
1.5
1.5
[320:923] [Kalvin]

Wood density (ρ0 )Reaction constant(A1 )Reaction constant(A2 )Reaction constant(A3 )Reaction constant(L1 )Reaction constant(L2 )Activation energy(E3 )Reaction order (n1 )Reaction order(n2 )Reaction order(n3 )Applied Temperature range(T)Table 2: Estimated rate constants at 713 Kelvin for plastic [13].

Parameter name (The rate constants of the reactions)
k 𝑝1
k 𝑝2
k 𝑝3
k 𝑝4
k 𝑝5
k 𝑝6
k 𝑝7
k 𝑝8
k 𝑝9

value
0.170488
2.43E-08
0.03012690.206132
0.0146288
0.0103907
2.25E-14
0.0204982
3.48E-10

Figure 1 illustrates the structure of the pyrolysis for the mixed waste of heavy plastic and wood, while figure 2 shows
the applied heat rates for this process.

Fig. 1: Model structure.

HTFF 135-3

Fig. 2: Applied heating rates.

The total volume of gas generated from the pyrolysis of wood and heavy plastics by combining Eq. 2, Eq. 4, andEq. 13 as shown in Eq. 14:∂𝐶𝑊𝑝𝑔𝑎𝑠= k 𝑤1 ∙ (𝐶𝑤 )n1 + k 𝑝3 ∙ 𝐶𝑝 − k 𝑝6 ∙ 𝐶𝐿𝑊 − k 𝑝7 ∙ 𝐶𝑜𝑖𝑙 − k 𝑝8 ∙ 𝐶𝐻𝑊∂𝑡(14)

Fig. 3 shows the model output in terms of the wood sample mass change with time as subjected the heating rate
shown in Fig. 2 compared to the results of Miljkovi et. al. [10]. It can be seen that the model predicted the publishedexperimental results with maximum error of 13.5%.

Fig. 3: Model validation with published experimental data.

3.

Results And Discussion

Fig. 4 shows the percentage yield of the pyrolysis process of a mixed waste consisting of 300 grams of plastic and700 grams wood using the heating rates shown in Fig. 2 for a period of 20 minutes. It can be seen that during the first 11minutes, the wood showed no change in its behaviour, while the plastic started to degrade. Table 4 shows that using one
kg of mixed waste composed of 300 grams plastic and 700 grams wood will produce 50.13% syngas, 8.35% oil, 9.5%char, 10.8% light Wax, and 8.33% heavy wax. Although light and heavy waxes are not fuels but they have other useful
applications like their significance stems from the distinct properties that they impart to these formulations. Waxes, forexample, can minimize the viscosity of a hot melt adhesive, add lustre to an ink or fruit coating emulsion, serve as alubricant through the fabrication of PVC pipes, or impart desirable buff ability, gloss, and toughness properties to emulsion
paste polish or a solvent., G. Webber[14].

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Fig. 4: Model output Time vs percentage yield.

Plastic 300 grams

Plastic outputs
grams

LW=36.15%
HW=27.75%
Oil =27.82%
Gas=8.25%

10883.25
83.46
24

Table 4: Output analysis for mixed waste.
Wood 700 grams
Wood outputs ingrams

Gas=68.19%
Char=13.55%

477.3394.85

1000 grams (Mixed waste)
10.8%
8.33%
8.35%
50.13%
9.5 %Conclusion
Nigeria is an agricultural developing country with huge amount of waste including 13 million tonnes per year of
agricultural waste where significant amounts of this waste are either left to rot or burned in open fields causing significant
environmental damage. Also, in urban cities there is an increasing amount of waste where heavy plastic represents asignificant amount. In this work, a numerical model for the pyrolysis process of mixed waste consisting of heavy plastic andwood was developed using MATLAB software and the built-in solver for ODEs to predict the amount of clean fuel produced
in terms of syngas, oil and char. The results of the model using wood were compared to published results showing goodagreement with maximum deviation of 13.5% Using one kg of mixed waste composed of 300 grams plastic and 700 grams
wood, the model shows that 50.13% syngas, 8.35% oil, 9.5% char, 10.8% light Wax, and 8.33% heavy wax can be produced.
Results also shows that mixed waste pyrolysis has the potential to manage the wood and plastic waste in Nigeria, improvethe environment and generate clean fuels like syngas, oil and charcoal.
4.

Acknowledgements

The authors would like to acknowledge the financial support from the Institute for Global innovation (IGI) and theInstitute of Advanced Studies (IAS) of the University of Birmingham as part of the IGI/IAS Clean Air pump prime funding,
project ID 4037.

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HTFF 135-6