# Individual and Catalytic Co-Pyrolysis of Agricultural Outcomes and Polymeric Materials over Nano-HZSM-5 Zeolite: Synergistic Effects and Yield Analysis for Heating Applications

Authors: C. Sowmya Dhanalakshmi; N. Ahalya; P. Vidhyalakshmi; C. Krishnaraj; N. Selvam; Pravin P. Patil; S. Kalippan; S. Prabhakar
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

The catalytic effect of nano-HZSM-5 zeolite on co-pyrolysis of cotton shell (CS) and municipal plastic wastes (MPW) was studied. The influence of reaction temperature during individual pyrolysis, blending ratio, and catalytic effects was studied by applying constant heating rate. The experiments were conducted in a fixed bed batch type reactor. The hindering effect during catalytic decomposition of MPW was carried out and its positive synergistic effect on liquid oil yield was analysed. The reaction temperature for all the experiments are fixed based on the decomposition rate obtained from thermogravimetric study. The experimental outcomes revealed that during co-pyrolysis, the formation of char was reduced to 7.2 wt% with increased liquid oil yield of 66.5 wt%. Furthermore, adding catalyst for co-pyrolysis process improved the reaction by decreasing char formation. During catalytic process, the maximum liquid oil output was 69.3 wt% at 500°C temperature, CS/MPW ratio of 1 : 2. When compared to co-pyrolysis process, the catalytic co-pyrolysis showed 4.21 wt% higher liquid oil yield. The physical analysis of the oil shows maximum hating value of 34.6 MJ/kg. The FTIR study on catalytic co-pyrolysis oil shows the presence of aliphatic and aromatic hydrocarbons.


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## Body

## 1. Introduction

The increased energy demand and rapid industrialisation due to the population and modern lifestyle force researchers to find alternative sources for conventional fuels. According to the survey conducted in the middle of 2020, the need for global energy is expected to increase by 30% over the next two decades in order to sustain rapid urbanisation [1]. Every living being requires a healthy atmosphere in order to thrive and develop. One of the biggest issues is the degradation of ecosystems caused by the discharge of inorganic and organic pollutants from various industries and automotive engines [2, 3]. There has been considerable demand around the world to investigate the production and use of biofuels for heat and power applications [4]. Fossil fuels have played a major part in satisfying current energy demands, but their widespread usage has aggravated serious environmental challenges, including global warming. Utilising biofuels for power production has increased nowadays due to flexible conversion methods and increased fuel properties. It is an alternative energy source that can replace fossil fuels by up to 30% to 40% in the near future [5]. The augmented solid waste output by the growing population provides carbon-rich energy resources and gives a widespread option for the production of energy. This could also help with the waste disposal problems.Biomass is generally classified as virgin biomass, waste biomass, or agricultural residues. It can be converted into energy-rich fuels through various conversion processes. Lignocellulosic biomass obtained from agricultural wastes is a carbon-free renewable energy source and is an excellent resource for biofuels and chemicals [6]. The basic elements of biomass are categorised as cellulose, hemicelluloses, and lignin (20–30%). Cellulose comprises the major part of the total components (about 40 to 45%). Apart from this, some amounts of extractives, hydrocarbons, and inorganic components are also present. Matured agricultural wastes, commonly called as agro-wastes, are the end product of any agricultural process. The safe disposal of these residues is a challengeable one without affecting environmental sustainability. Due to agricultural activities, the quality of land waster and water bodies is being affected much. Pesticides used to meet global agricultural demand are the primary source of water and soil pollution [7]. These pesticides contaminate water through rain and irrigation systems [8]. In order to remove these contaminants from the body of water, Wabaidur et al. [9] and Alothman et al. [10] utilised absorption techniques through multiwalled nanocomposite and biosorbents. These wastes are typically burned in the open field after harvesting, and this practice is becoming more common, resulting in increased air pollution [11, 12]. The burning of agro-residues on farms is currently a substantial contributor to global warming [13]. As a result, policymakers are now tasked with identifying the best solution to address the aforementioned issues by effectively utilising agricultural wastes to generate clean energy. Converting solid waste into value-added products is a step towards a sustainable world [14]. Pyrolysis has become a popular technology due to its faster reaction time, low energy consumption, and generation of solid carbon-rich fuel as well as value-added liquid products. Khan et al. [15] developed a hydrochar by utilising sesame oil cake through hydrothermal carbonisation conducted at a temperature of 150–250°C. The produced hydrochar was utilised for the adsorption of metal ions from water. Alqadami et al. [16] developed citric anhydride anchored mesoporous metal organic framework to absorb nonessential lead (Pb) trace elements from water. Cotton shell (Gossypium arboretum) often known as tree cotton. It is one of the most important crops that plays a significant role in India’s economic development. It is cultivated on a wide range of soils, such as black clayey, red soil, and black and mixed black soil. Cotton seeds are extracted and used to make high-viscous oil. The pressed oil is used for burning and biofuel production. Nabi et al. [17] produced cotton seed oil methyl ester and utilised for IC engine operation. This study showed lower exhaust gas emissions, except for NOx.Plastics are the materials produced from fossil fuels that have become an integral element of modern life. They are more durable, weigh less, and have a lower cost. Many plastics are made from polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene-terephthalate (PET) [18]. They are nonbiodegradable and can endure for more than a hundred years. The global rate of waste plastics has been increasing rapidly for the past two decades. In India, over 3.4 million tonnes of plastic waste were generated in the financial year 2020, an increase of more than 1.0 million tonnes compared to 2018. In Tamil Nadu, plastic waste generation is around 0.43 million tonnes per annum, of which 0.4 million tonnes are collected and segregated by the local urban bodies. Used plastics are generally recycled to produce new and useful products. When recycling is done properly, it can reduce the dependence on landfills and conserve resources. But at present, only 15–20% of total plastic waste can be recycled using traditional sorting and grinding [19]. The process of energy recovery from used plastics is an alternative way and is gaining more attention nowadays. There are many innovative and promising methods available today that can convert waste plastics into fuels and other valuable resources.Incineration and pyrolysis are the two main processes that are being utilised for energy recovery from plastics in many studies [20, 21]. Incineration is a simple energy recovery process that converts hydrocarbons to combustion products. Incinerating plastic waste creates environmental pollution. Burning in an open field or in a controlled environment emits poisonous gases which is very dangerous to all living organisms [22]. Pyrolysis is an effective technique that converts plastic waste into energy-rich liquid oil. It is a thermal decomposition method that is performed without supplying air. Pyrolysis is a temperature-dependent process that is generally performed at a low, medium, or high temperature to break the structure [23]. Co-pyrolysis is another technique that produces synthetic liquid fuel with improved properties. It is also a very effective method of utilising two or more dissimilar feedstocks. Many reports have reported the maximum liquid oil yield through the co-pyrolysis process. Co-pyrolysis of plastics with agricultural waste is directed towards synergistic effects that possibly improve the favourable properties of the products [24]. Burra and Gupta [25] conducted a co-pyrolysis experiment utilising pinewood and mixed plastic wastes under different mass fractions. The study showed positive synergistic effects compared with individual components. The study carried out by Johansson et al. [26] showed the maximum production of hydrocarbons during co-pyrolysis. In this study, the addition of plastic waste to woody biomass significantly affected the product composition. Jin et al. [27] examined the synergetic effect during co-pyrolysis of wheat straw and plastic waste. The study also showed a positive synergistic effect on char yield. Oyedun et al. [28] demonstrated the quantity and properties of char and oil with respect to the amount of plastic during the co-pyrolysis process. Dewangan et al. [29] conducted co-pyrolysis studies on combined LDPE and sugarcane bagasse. The study produced maximum liquid oil with 40 MJ/kg heating value.In order to improve the quality of liquid oil, a large variety of nanobased catalysts are utilised. Many studies have reported catalytic pyrolysis processes with various catalysts, for instance ZSM-5 [30], HZSM-5 [31], zeolite [32], Cu-Al2O3 [33], and Fe2O3 [34]. These catalysts lower the energy inputs and increase the condensable liquid oil. Zhang et al. [35] conducted pyrolysis experiments on Douglas fir sawdust and HDPE in the presence of ZSM-5 catalysts. From the experimental results, it was understood that adding the HZSM-5 catalyst could decrease the activation energy. Co-pyrolysis of macroalgae and HDPE was done by Xu et al. [36] to analyse the synergistic effect towards enhanced biofuel production. This process showed that the catalytic process lowered the oxygen, nitrogen, and acid levels in the derived bio-oil. Furthermore, adding HZSM-5 with the feedstocks increased the reaction pathways and biofuel production as well as lowered the activation energy.The goal of this work is to investigate the synergistic effects of agricultural residues and plastic during the co-pyrolysis process in the presence of HZSM-5 nanocatalysts. The synergistic effect on liquid oil yield was analysed using a batch type fixed bed reactor. The liquid oil yield and its compositions during thermal, co-pyrolysis, and catalytic co-pyrolysis were analysed based on synergistic effects.


## 2. Materials and Methods

### 2.1. Materials

CS and MPW were used for this study to analyse probable synergistic effects during their co-pyrolysis. The cotton shell used for this study is a typical lignocellulosic biomass and was collected from the local fields in and around Coimbatore. The city is generally called the cotton city of the state. The city has more than 100 spinning mills and produces a variety of cotton fabrics. The feedstocks for these industries are supplied by nearby agricultural farms. The climate in and around the city is more suitable for the cultivation of cotton. After eliminating cotton from the shells, they were thrown away and fired into the field itself. The open burning of these shells causes a severe effect on the environment. In order to avoid this and to find a suitable recycling method, this study focused on the utilisation of CS for the pyrolysis process. The municipal plastic wastes contains majority of PE, HDPE, and PET materials. Both the materials were completely screened and dried in the open sunlight for more than 15 days. They are converted into small particles with an average diameter of 0.5 mm to 1.0 mm. The elemental analysis of both the materials was analysed to find its suitability for the pyrolysis process. The proximate analysis was done to measure the volatile matters within it. All the analysis was performed by following ASTM protocols. Table1 shows the proximate and ultimate analysis of the feedstock materials.Table 1
Feedstock characteristics.
ParametersCSMPWProximate analysis (wt%)Volatile matter61.6089.40Fixed carbon26.060.42Moisture content9.520.35Ash2.819.83Ultimate analysis (wt%) (ash free basis)Carbon46.0282.20Hydrogen4.9115.65Nitrogen0.602.40Oxygen#48.460.30Sulfur0.011.45#By difference.


### 2.2. Thermogravimetric Analysis

Thermogravimetric analysis of the selected feedstock was performed to find thermal degradation properties using the TGA701 (LECO Corporation, St. Joe, Michigan, US) thermogravimetric analyser. The test was carried out in a nitrogen environment and its volume flow rate was maintained at 3.5 l/min. The dehydration and devolatilisation were done at the heating rate of 20°C/min. Before heating, the feedstocks were dried at 100°C. The analysis was done by heating the sample from atmospheric conditions to 600°C. For the analysis, 10 mg sample is loaded into the crucibles.


### 2.3. Material Characterisation

The collected feedstock material was subjected to proximate and elemental analysis. The 2400 CHNS Organic Elemental Analyzer (N2410650, Perkin Elmer Ltd, US) is employed to find the elements present in the samples. The same instrumentation is also used for liquid oil analysis. The FTIR analysis of the obtained liquid oil through catalytic co-pyrolysis was done with the aid of a FTIR (BRUKER Optik GmbH, TENSOR 27 Bruker Corporation) spectrophotometer with the support of OPUS version 6.5 in transmission mode. The analysis was done by collecting the spectrum between 400 and 4000 cm-1 with 4 cm-1 resolution. After the sample was disseminated over zinc selenide crystal, the spectrum was obtained in reflectance mode. The physical analyses of the liquid samples were investigated to determine their applicability for various uses. The physical properties of the oil were done in accordance with ASTM standards. Many physical characteristics such as density, kinematic viscosity, flash point, heating value, and pH value were considered for this analysis. The individual and catalytic pyrolysis oils used for physical analysis were collected under experimental conditions that yielded higher liquid oils.


### 2.4. Reactor Facility

Individual and co-pyrolysis experiments were performed using a batch type laboratory scale fixed bed reactor. The internal diameter and height of the reactor are 50 mm and 150 mm, respectively. The set-up consists mainly a reactor with an electrical heater, a condenser, and a liquid collection unit. The reactor is perfectly insulated with Mineral-Chromel and Alumel to avoid energy loss during pyrolysis. The temperature of the reactor is controlled by the PID controller and auto transformer. The temperature inside the reactor was measured by using K type thermocouples fixed at two locations inside the reactor. Once the reactor reached the desired temperature, it was maintained stable by auto cut-off unit. The outlet of the reactor pipe is connected to the series of two condensers. The ice water supplied to the condenser is kept at 5°C. The condensed liquid obtained from the condenser was collected in conical flasks and stored separately for further analysis. At the end of each experiment, the char was collected from the bottom of the reactor and weighed using an electronic weighing machine. The liquid oil obtained from the reaction was also collected and weighed. The weight percentage of uncondensable gas fractions was found by the remaining material balance.


### 2.5. Experimental Procedure

The effect of thermal, co-pyrolysis, and catalytic co-pyrolysis on liquid oil yields was investigated by four different phases. In the first phase, the thermal pyrolysis of CS was examined by changing process temperature from 350°C to 550°C. In the second phase, the thermal pyrolysis of MPW was also conducted by changing the temperature from 350°C to 550°C. The aim of the second phase is to find the optimum temperature for maximum liquid oil yield and the impact of process temperature on biofuel yield. The next phase of experiments was conducted on co-pyrolysis by changing the biomass and plastic blend ratios to 1 : 0, 2 : 1, 1 : 1, 1 : 2, and 0 : 1 at 500°C. The fourth phase of the experiments was carried out on catalytic co-pyrolysis at 1 : 2 blending ratio (biomass to plastic) by utilising HSZM-5 nanocatalysts supplied by Sri Aathi Vinayaga Industries, Tamil Nadu, India, to find the synergistic effect on liquid oil production.


## 2.1. Materials

CS and MPW were used for this study to analyse probable synergistic effects during their co-pyrolysis. The cotton shell used for this study is a typical lignocellulosic biomass and was collected from the local fields in and around Coimbatore. The city is generally called the cotton city of the state. The city has more than 100 spinning mills and produces a variety of cotton fabrics. The feedstocks for these industries are supplied by nearby agricultural farms. The climate in and around the city is more suitable for the cultivation of cotton. After eliminating cotton from the shells, they were thrown away and fired into the field itself. The open burning of these shells causes a severe effect on the environment. In order to avoid this and to find a suitable recycling method, this study focused on the utilisation of CS for the pyrolysis process. The municipal plastic wastes contains majority of PE, HDPE, and PET materials. Both the materials were completely screened and dried in the open sunlight for more than 15 days. They are converted into small particles with an average diameter of 0.5 mm to 1.0 mm. The elemental analysis of both the materials was analysed to find its suitability for the pyrolysis process. The proximate analysis was done to measure the volatile matters within it. All the analysis was performed by following ASTM protocols. Table1 shows the proximate and ultimate analysis of the feedstock materials.Table 1
Feedstock characteristics.
ParametersCSMPWProximate analysis (wt%)Volatile matter61.6089.40Fixed carbon26.060.42Moisture content9.520.35Ash2.819.83Ultimate analysis (wt%) (ash free basis)Carbon46.0282.20Hydrogen4.9115.65Nitrogen0.602.40Oxygen#48.460.30Sulfur0.011.45#By difference.


## 2.2. Thermogravimetric Analysis

Thermogravimetric analysis of the selected feedstock was performed to find thermal degradation properties using the TGA701 (LECO Corporation, St. Joe, Michigan, US) thermogravimetric analyser. The test was carried out in a nitrogen environment and its volume flow rate was maintained at 3.5 l/min. The dehydration and devolatilisation were done at the heating rate of 20°C/min. Before heating, the feedstocks were dried at 100°C. The analysis was done by heating the sample from atmospheric conditions to 600°C. For the analysis, 10 mg sample is loaded into the crucibles.


## 2.3. Material Characterisation

The collected feedstock material was subjected to proximate and elemental analysis. The 2400 CHNS Organic Elemental Analyzer (N2410650, Perkin Elmer Ltd, US) is employed to find the elements present in the samples. The same instrumentation is also used for liquid oil analysis. The FTIR analysis of the obtained liquid oil through catalytic co-pyrolysis was done with the aid of a FTIR (BRUKER Optik GmbH, TENSOR 27 Bruker Corporation) spectrophotometer with the support of OPUS version 6.5 in transmission mode. The analysis was done by collecting the spectrum between 400 and 4000 cm-1 with 4 cm-1 resolution. After the sample was disseminated over zinc selenide crystal, the spectrum was obtained in reflectance mode. The physical analyses of the liquid samples were investigated to determine their applicability for various uses. The physical properties of the oil were done in accordance with ASTM standards. Many physical characteristics such as density, kinematic viscosity, flash point, heating value, and pH value were considered for this analysis. The individual and catalytic pyrolysis oils used for physical analysis were collected under experimental conditions that yielded higher liquid oils.


## 2.4. Reactor Facility

Individual and co-pyrolysis experiments were performed using a batch type laboratory scale fixed bed reactor. The internal diameter and height of the reactor are 50 mm and 150 mm, respectively. The set-up consists mainly a reactor with an electrical heater, a condenser, and a liquid collection unit. The reactor is perfectly insulated with Mineral-Chromel and Alumel to avoid energy loss during pyrolysis. The temperature of the reactor is controlled by the PID controller and auto transformer. The temperature inside the reactor was measured by using K type thermocouples fixed at two locations inside the reactor. Once the reactor reached the desired temperature, it was maintained stable by auto cut-off unit. The outlet of the reactor pipe is connected to the series of two condensers. The ice water supplied to the condenser is kept at 5°C. The condensed liquid obtained from the condenser was collected in conical flasks and stored separately for further analysis. At the end of each experiment, the char was collected from the bottom of the reactor and weighed using an electronic weighing machine. The liquid oil obtained from the reaction was also collected and weighed. The weight percentage of uncondensable gas fractions was found by the remaining material balance.


## 2.5. Experimental Procedure

The effect of thermal, co-pyrolysis, and catalytic co-pyrolysis on liquid oil yields was investigated by four different phases. In the first phase, the thermal pyrolysis of CS was examined by changing process temperature from 350°C to 550°C. In the second phase, the thermal pyrolysis of MPW was also conducted by changing the temperature from 350°C to 550°C. The aim of the second phase is to find the optimum temperature for maximum liquid oil yield and the impact of process temperature on biofuel yield. The next phase of experiments was conducted on co-pyrolysis by changing the biomass and plastic blend ratios to 1 : 0, 2 : 1, 1 : 1, 1 : 2, and 0 : 1 at 500°C. The fourth phase of the experiments was carried out on catalytic co-pyrolysis at 1 : 2 blending ratio (biomass to plastic) by utilising HSZM-5 nanocatalysts supplied by Sri Aathi Vinayaga Industries, Tamil Nadu, India, to find the synergistic effect on liquid oil production.


## 3. Results and Discussions

### 3.1. Thermogravimetric Analysis

Figures1 and 2 show the TGA and differential thermogram (DTG) of a CS and MPW. In the case of CS, there were no volatiles released until it reached 200°C. According to d’Almeida et al. [37], the biomass is stable till it reaches 200°C, with negligible weight losses. This stage represents the drying and removal of extractives. The rapid pyrolysis occurred between 290°C and 450°C. The weight loss below 290°C represents the desertion of water molecules from CS. The first and second peaks developed in the experiment due to hemicellulose and cellulose breakdown [38]. Generally, decomposition of hemicelluloses into acetic acid, furfural, and furan occurs at temperatures between 200 and 260°C. The slow degradation rate after 420°C represents the breakdown of lignin. It is the main reason for the formation of phenolic compounds in the pyrolysis products [39]. The differences in thermal behaviour between CS and MPW in TGA analysis occur due to molecular arrangement and degradation rate. The degradation MPW occurs in single step between 340°C and 540°C. Comparing with biomass, the degradation of plastics takes place at elevated temperatures. MPW begins to degrade structurally at around 260°C. The single peak noticed in both the curves represents the weight loss owing to the temperature increase. At 350 to 500°C, the maximum degradation occurred (81.5%). Girija et al. [40] also represented the single stage degradation of PET material. It is also very difficult to explain the thermal disintegration mechanism of plastics due to their aromatic nature.Figure 1
TGA analysis of CS and MPW.Figure 2
DTG analysis of CS and MPW.


### 3.2. Thermal Pyrolysis of Cotton Shell

Figure3 depicts the product distributions of the thermal pyrolysis of CS samples under different operating temperatures. The results reveal that when the pyrolysis temperature is increased, the char yield is reduced. This is owing to increased primary breakdown at higher temperatures [41]. The abrupt increase in gas fractions and the reduction in liquid oil products have been noticed as the temperature increased above 450°C. This could be due to secondary reactions of the pyrolysis liquid into noncondensable gases at elevated temperatures [42]. The results are also in good agreement with the previous literature conducted on different biomass materials such as rapeseed and soybean cake [43, 44]. The yield of liquid oil is lowered at lower temperatures and increased with increasing temperatures. The increased decreased pattern of the liquid oil yield with increased temperature is due to the higher decomposition of condensable volatiles at 450°C. The liquid oil production reached maximum at 450°C (36.6 wt%). With increasing temperatures, the yield of gas fractions also increases. At 350°C, the gas yield is 33.2 wt% and at 550°C, it reaches the maximum value up to 44.9 wt%. In most cases, the lower pyrolysis temperature favours a higher char yield and the higher temperature promotes gas yields. The same trend is also observed in the flash pyrolysis of CS [45].Figure 3
Product distributions during pyrolysis of CS.


### 3.3. Thermal Pyrolysis of Municipal Plastic Wastes

Figure4 demonstrates product distributions during the thermal pyrolysis of MPW at various temperatures. In this study, the yield of char products decreased with increased temperature. At 550°C, the char production is only 3.3 wt%. The reduction in char yield is attributed to secondary repolymerisation reactions [46]. When the temperature is changed from 350°C to 550°C, the yield of liquid oil changes from 62.4 wt% to 69.4 wt% and the maximum yield is recorded as 70.6 wt% at 500°C. The higher temperature of the reactor increases the gas yield and reduces the char production. When compared to biomass, the pyrolysis of plastics yields more liquid oil due to its structure. In plastic pyrolysis process, the stability of the benzylic radical favoured unzip reaction and restricted further cracking process. As a result, the main product of plastic pyrolysis was liquid oil.Figure 4
Product distributions during pyrolysis of MPW.


### 3.4. Co-pyrolysis at Different Blend Ratio

Figure5 demonstrates product distributions during co-pyrolysis of CS and MPW under different blend ratios. The figure shows increased liquid oil production with respect to the increased amount of plastics in the feedstock. The aim of the experiments in this phase is to produce high-quality and large-quantity liquid oil. The liquid oil yield is increased to 66.5 wt% at 1 : 2 ratio. The increased yield at higher percentage of plastics in the feedstock is attributed to the presence of higher levels of hydrogen in the feedstock [47, 48]. During the co-pyrolysis process, polyolefin polymers in the plastics can act as perfect hydrogen donors, resulting in higher oil production [49]. While biomass is reacting with plastics, a number of reaction radicals can be created. These radicals interact and favour the production of liquid oil without phase separation. A considerable increase in pyrolysis liquid output has been reported by many researchers, indicating synergistic impact [50, 51]. Furthermore, the reduction of char was found as the plastic ratio increased, indicating a higher degree of conversion.Figure 5
Product yield at different blend ratio.


### 3.5. Catalytic Co-Pyrolysis Process

For the catalytic process, 30 grams of blended feedstock (1 : 2-CS:MPW) was mixed thoroughly with the catalyst (10% wt%/wt% of feedstock) and kept in the reactor. The temperature throughout this phase of experiments is kept at 500°C. Figure6 evaluates the yield of liquid oil, char, and gas obtained from co-pyrolysis and catalytic co-pyrolysis processes. From the figure, it can be understood that the catalytic co-pyrolysis process yields more liquid products than the co-pyrolysis process. At co-pyrolysis, the maximum liquid oil production was 66.5 w%. But through catalytic co-pyrolysis, the production of liquid was enhanced to 69.3 wt%. The catalytic process increased the yield by up to 4.21%. The increase in liquid oil production and the reduction in char residue showed a beneficial synergy between CS and MPW. The reaction between the catalyst and feed materials is quite complicated due to the complex structure of the materials [52]. But the main reason for the positive synergy effect on liquid yields is the relationship between biomass and plastic-derived olefins [53]. In the presence of the catalysts, it was found that the oxygenated components could react with olefins. Furthermore, it has also been shown that plastic-derived hydrocarbons can perform as hydrogen donors for biomass-derived oxygenates, reducing the formation of char in catalytic conversions [54].Figure 6
Comparison between catalytic and noncatalytic co-pyrolysis process.


### 3.6. Characterisation Study

#### 3.6.1. Physical Analysis of Liquid Oils

Table2 represents the physical characterisation of liquid oils obtained from this study. The table shows some of the basic fuel properties. When compared with CS pyrolysis oil, the co-pyrolysis oil offers a higher heating value. The density of MPW pyrolysis oil was found to be lower than other pyrolysis oils. The heating value of the individual MPW oil is higher than other oils due to the absence of oxygenated elements. The comparison of MPW pyrolysis oil, co-pyrolysis oil, and bio-oil obtained from other samples is provided in Table 3. From the table, it can be understood that the MPW pyrolysis oil has a higher HHV than co-pyrolysis oil. The presence of lower oxygen molecules in the bio-oil is the main reason for the increased heating value. The addition of wood-based lignocellulosic material with plastic waste reduced the heating value to 34.6 MJ/kg. Furthermore, individual biomass pyrolysis oils showed lower heating values than co-pyrolysis oils due to a higher fraction of oxygenated elements in the feedstock materiel. The pH value of the co-pyrolysis and catalytic pyrolysis oils is moderate in the range of 4.6-4.8. It is a representation of the corrosiveness of the oil. The value of pH indicates that the bio-oil has less corrosive properties than other bio-oils produced from other biomass materials [55–57]. Acids in the pyrolysis oils are mostly caused by the breakdown of hemicellulose present in the biomass material. The pyrolysis oil typically has a pH value of between 3 and 6 [58]. The lower pH value is naturally corrosive to steel, aluminum, and nickel-based materials.Table 2
Physical characterisation of liquid oils.
Type of oilDensity in kg/m3Viscosity in cStFlash point in °CpHHHV in MJ/kgCS pyrolysis oil10057.71603.417.65MPW pyrolysis oil9352.6956.241.6Co-pyrolysis oil9805.71104.834.1Catalytic co-pyrolysis oil9805.61054.634.6Table 3
Comparison of heating value of the liquid oils.
SourceValue in MJ/kgReferenceMPW41.6This studyCS and MPW34.6This studyNapier grass19.79[55]Pine seed20.00[56]Ficus religiosa18.30[57]Spirulina platensis25.70[59]Hard wood18.63[60]Palmyra fruit bunch13.92[61]


#### 3.6.2. FTIR Spectra

The FTIR spectrum of liquid oil obtained from the catalytic process derived at maximum yield conditions is shown in Figure7. From the obtained spectra, it can be confirmed that the liquid oil is a combination of water, phenolic compounds, aromatic compounds, acids, alkenes, and nitrogenated elements. The existence of water and phenolic compounds in the oil is identified by O-H stretching vibration at 3200-3600 cm-1. The same compounds were discovered and reported in the literature [62, 63]. The existence of aliphatic hydrocarbons is specified by C-H stretching and bending vibrations at 2900-3000 cm-1 and 1300-1450 cm-1. The existence of alkenes is identified by the peak obtained at 675-900 cm-1.Figure 7
FTIR spectra of catalytic co-pyrolysis oil.


## 3.1. Thermogravimetric Analysis

Figures1 and 2 show the TGA and differential thermogram (DTG) of a CS and MPW. In the case of CS, there were no volatiles released until it reached 200°C. According to d’Almeida et al. [37], the biomass is stable till it reaches 200°C, with negligible weight losses. This stage represents the drying and removal of extractives. The rapid pyrolysis occurred between 290°C and 450°C. The weight loss below 290°C represents the desertion of water molecules from CS. The first and second peaks developed in the experiment due to hemicellulose and cellulose breakdown [38]. Generally, decomposition of hemicelluloses into acetic acid, furfural, and furan occurs at temperatures between 200 and 260°C. The slow degradation rate after 420°C represents the breakdown of lignin. It is the main reason for the formation of phenolic compounds in the pyrolysis products [39]. The differences in thermal behaviour between CS and MPW in TGA analysis occur due to molecular arrangement and degradation rate. The degradation MPW occurs in single step between 340°C and 540°C. Comparing with biomass, the degradation of plastics takes place at elevated temperatures. MPW begins to degrade structurally at around 260°C. The single peak noticed in both the curves represents the weight loss owing to the temperature increase. At 350 to 500°C, the maximum degradation occurred (81.5%). Girija et al. [40] also represented the single stage degradation of PET material. It is also very difficult to explain the thermal disintegration mechanism of plastics due to their aromatic nature.Figure 1
TGA analysis of CS and MPW.Figure 2
DTG analysis of CS and MPW.


## 3.2. Thermal Pyrolysis of Cotton Shell

Figure3 depicts the product distributions of the thermal pyrolysis of CS samples under different operating temperatures. The results reveal that when the pyrolysis temperature is increased, the char yield is reduced. This is owing to increased primary breakdown at higher temperatures [41]. The abrupt increase in gas fractions and the reduction in liquid oil products have been noticed as the temperature increased above 450°C. This could be due to secondary reactions of the pyrolysis liquid into noncondensable gases at elevated temperatures [42]. The results are also in good agreement with the previous literature conducted on different biomass materials such as rapeseed and soybean cake [43, 44]. The yield of liquid oil is lowered at lower temperatures and increased with increasing temperatures. The increased decreased pattern of the liquid oil yield with increased temperature is due to the higher decomposition of condensable volatiles at 450°C. The liquid oil production reached maximum at 450°C (36.6 wt%). With increasing temperatures, the yield of gas fractions also increases. At 350°C, the gas yield is 33.2 wt% and at 550°C, it reaches the maximum value up to 44.9 wt%. In most cases, the lower pyrolysis temperature favours a higher char yield and the higher temperature promotes gas yields. The same trend is also observed in the flash pyrolysis of CS [45].Figure 3
Product distributions during pyrolysis of CS.


## 3.3. Thermal Pyrolysis of Municipal Plastic Wastes

Figure4 demonstrates product distributions during the thermal pyrolysis of MPW at various temperatures. In this study, the yield of char products decreased with increased temperature. At 550°C, the char production is only 3.3 wt%. The reduction in char yield is attributed to secondary repolymerisation reactions [46]. When the temperature is changed from 350°C to 550°C, the yield of liquid oil changes from 62.4 wt% to 69.4 wt% and the maximum yield is recorded as 70.6 wt% at 500°C. The higher temperature of the reactor increases the gas yield and reduces the char production. When compared to biomass, the pyrolysis of plastics yields more liquid oil due to its structure. In plastic pyrolysis process, the stability of the benzylic radical favoured unzip reaction and restricted further cracking process. As a result, the main product of plastic pyrolysis was liquid oil.Figure 4
Product distributions during pyrolysis of MPW.


## 3.4. Co-pyrolysis at Different Blend Ratio

Figure5 demonstrates product distributions during co-pyrolysis of CS and MPW under different blend ratios. The figure shows increased liquid oil production with respect to the increased amount of plastics in the feedstock. The aim of the experiments in this phase is to produce high-quality and large-quantity liquid oil. The liquid oil yield is increased to 66.5 wt% at 1 : 2 ratio. The increased yield at higher percentage of plastics in the feedstock is attributed to the presence of higher levels of hydrogen in the feedstock [47, 48]. During the co-pyrolysis process, polyolefin polymers in the plastics can act as perfect hydrogen donors, resulting in higher oil production [49]. While biomass is reacting with plastics, a number of reaction radicals can be created. These radicals interact and favour the production of liquid oil without phase separation. A considerable increase in pyrolysis liquid output has been reported by many researchers, indicating synergistic impact [50, 51]. Furthermore, the reduction of char was found as the plastic ratio increased, indicating a higher degree of conversion.Figure 5
Product yield at different blend ratio.


## 3.5. Catalytic Co-Pyrolysis Process

For the catalytic process, 30 grams of blended feedstock (1 : 2-CS:MPW) was mixed thoroughly with the catalyst (10% wt%/wt% of feedstock) and kept in the reactor. The temperature throughout this phase of experiments is kept at 500°C. Figure6 evaluates the yield of liquid oil, char, and gas obtained from co-pyrolysis and catalytic co-pyrolysis processes. From the figure, it can be understood that the catalytic co-pyrolysis process yields more liquid products than the co-pyrolysis process. At co-pyrolysis, the maximum liquid oil production was 66.5 w%. But through catalytic co-pyrolysis, the production of liquid was enhanced to 69.3 wt%. The catalytic process increased the yield by up to 4.21%. The increase in liquid oil production and the reduction in char residue showed a beneficial synergy between CS and MPW. The reaction between the catalyst and feed materials is quite complicated due to the complex structure of the materials [52]. But the main reason for the positive synergy effect on liquid yields is the relationship between biomass and plastic-derived olefins [53]. In the presence of the catalysts, it was found that the oxygenated components could react with olefins. Furthermore, it has also been shown that plastic-derived hydrocarbons can perform as hydrogen donors for biomass-derived oxygenates, reducing the formation of char in catalytic conversions [54].Figure 6
Comparison between catalytic and noncatalytic co-pyrolysis process.


## 3.6. Characterisation Study

### 3.6.1. Physical Analysis of Liquid Oils

Table2 represents the physical characterisation of liquid oils obtained from this study. The table shows some of the basic fuel properties. When compared with CS pyrolysis oil, the co-pyrolysis oil offers a higher heating value. The density of MPW pyrolysis oil was found to be lower than other pyrolysis oils. The heating value of the individual MPW oil is higher than other oils due to the absence of oxygenated elements. The comparison of MPW pyrolysis oil, co-pyrolysis oil, and bio-oil obtained from other samples is provided in Table 3. From the table, it can be understood that the MPW pyrolysis oil has a higher HHV than co-pyrolysis oil. The presence of lower oxygen molecules in the bio-oil is the main reason for the increased heating value. The addition of wood-based lignocellulosic material with plastic waste reduced the heating value to 34.6 MJ/kg. Furthermore, individual biomass pyrolysis oils showed lower heating values than co-pyrolysis oils due to a higher fraction of oxygenated elements in the feedstock materiel. The pH value of the co-pyrolysis and catalytic pyrolysis oils is moderate in the range of 4.6-4.8. It is a representation of the corrosiveness of the oil. The value of pH indicates that the bio-oil has less corrosive properties than other bio-oils produced from other biomass materials [55–57]. Acids in the pyrolysis oils are mostly caused by the breakdown of hemicellulose present in the biomass material. The pyrolysis oil typically has a pH value of between 3 and 6 [58]. The lower pH value is naturally corrosive to steel, aluminum, and nickel-based materials.Table 2
Physical characterisation of liquid oils.
Type of oilDensity in kg/m3Viscosity in cStFlash point in °CpHHHV in MJ/kgCS pyrolysis oil10057.71603.417.65MPW pyrolysis oil9352.6956.241.6Co-pyrolysis oil9805.71104.834.1Catalytic co-pyrolysis oil9805.61054.634.6Table 3
Comparison of heating value of the liquid oils.
SourceValue in MJ/kgReferenceMPW41.6This studyCS and MPW34.6This studyNapier grass19.79[55]Pine seed20.00[56]Ficus religiosa18.30[57]Spirulina platensis25.70[59]Hard wood18.63[60]Palmyra fruit bunch13.92[61]


### 3.6.2. FTIR Spectra

The FTIR spectrum of liquid oil obtained from the catalytic process derived at maximum yield conditions is shown in Figure7. From the obtained spectra, it can be confirmed that the liquid oil is a combination of water, phenolic compounds, aromatic compounds, acids, alkenes, and nitrogenated elements. The existence of water and phenolic compounds in the oil is identified by O-H stretching vibration at 3200-3600 cm-1. The same compounds were discovered and reported in the literature [62, 63]. The existence of aliphatic hydrocarbons is specified by C-H stretching and bending vibrations at 2900-3000 cm-1 and 1300-1450 cm-1. The existence of alkenes is identified by the peak obtained at 675-900 cm-1.Figure 7
FTIR spectra of catalytic co-pyrolysis oil.


## 3.6.1. Physical Analysis of Liquid Oils

Table2 represents the physical characterisation of liquid oils obtained from this study. The table shows some of the basic fuel properties. When compared with CS pyrolysis oil, the co-pyrolysis oil offers a higher heating value. The density of MPW pyrolysis oil was found to be lower than other pyrolysis oils. The heating value of the individual MPW oil is higher than other oils due to the absence of oxygenated elements. The comparison of MPW pyrolysis oil, co-pyrolysis oil, and bio-oil obtained from other samples is provided in Table 3. From the table, it can be understood that the MPW pyrolysis oil has a higher HHV than co-pyrolysis oil. The presence of lower oxygen molecules in the bio-oil is the main reason for the increased heating value. The addition of wood-based lignocellulosic material with plastic waste reduced the heating value to 34.6 MJ/kg. Furthermore, individual biomass pyrolysis oils showed lower heating values than co-pyrolysis oils due to a higher fraction of oxygenated elements in the feedstock materiel. The pH value of the co-pyrolysis and catalytic pyrolysis oils is moderate in the range of 4.6-4.8. It is a representation of the corrosiveness of the oil. The value of pH indicates that the bio-oil has less corrosive properties than other bio-oils produced from other biomass materials [55–57]. Acids in the pyrolysis oils are mostly caused by the breakdown of hemicellulose present in the biomass material. The pyrolysis oil typically has a pH value of between 3 and 6 [58]. The lower pH value is naturally corrosive to steel, aluminum, and nickel-based materials.Table 2
Physical characterisation of liquid oils.
Type of oilDensity in kg/m3Viscosity in cStFlash point in °CpHHHV in MJ/kgCS pyrolysis oil10057.71603.417.65MPW pyrolysis oil9352.6956.241.6Co-pyrolysis oil9805.71104.834.1Catalytic co-pyrolysis oil9805.61054.634.6Table 3
Comparison of heating value of the liquid oils.
SourceValue in MJ/kgReferenceMPW41.6This studyCS and MPW34.6This studyNapier grass19.79[55]Pine seed20.00[56]Ficus religiosa18.30[57]Spirulina platensis25.70[59]Hard wood18.63[60]Palmyra fruit bunch13.92[61]


## 3.6.2. FTIR Spectra

The FTIR spectrum of liquid oil obtained from the catalytic process derived at maximum yield conditions is shown in Figure7. From the obtained spectra, it can be confirmed that the liquid oil is a combination of water, phenolic compounds, aromatic compounds, acids, alkenes, and nitrogenated elements. The existence of water and phenolic compounds in the oil is identified by O-H stretching vibration at 3200-3600 cm-1. The same compounds were discovered and reported in the literature [62, 63]. The existence of aliphatic hydrocarbons is specified by C-H stretching and bending vibrations at 2900-3000 cm-1 and 1300-1450 cm-1. The existence of alkenes is identified by the peak obtained at 675-900 cm-1.Figure 7
FTIR spectra of catalytic co-pyrolysis oil.


## 4. Conclusion

In this work, the synergistic effect of catalytic co-pyrolysis of cotton shell and municipal plastic wastes on liquid oil yield was analysed. The four-phase experimental study showed higher liquid oil yield during the catalytic co-pyrolysis process. During catalytic pyrolysis, a positive synergistic effect on liquid oil production was identified with reduced char production. During the co-pyrolysis process, polyolefin polymers in the MPW act as perfect hydrogen donors, resulting in higher liquid oil products than biomass pyrolysis oil. On the other hand, the catalytic co-pyrolysis process increased the yield by up to 4.21% more than the co-pyrolysis process because of its positive synergistic effect. The relationships between biomass-derived oxygenated compounds and plastic-derived olefins were the main reason for the positive synergy effect. By the catalytic co-pyrolysis process, the maximum liquid oil yield of 69.3 wt% was acquired at 500°C with a CS/MPW ratio of 1 : 2. The physical analysis of the oil obtained at the maximum yield condition shows 34.6 MJ/kg. The obtained liquid oil can be tested as an alternative fuel for boilers and IC engines. Further research is also possible on kinetic studies of noncatalytic and catalytic pyrolysis reactions.



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