CET 97 DOI: 10.3303/CET2297048 Paper Received: 25 May 2022; Revised: 18 July 2022; Accepted: 29 July 2022 Please cite this article as: Mong G.R., Liew C.S., Chong W.W.F., Idris R., Ng J.-H., Chiong M.C., 2022, Bioenergy Generation Potential of Empty Fruit Bunch and Waste Tire Through Microwave Pyrolysis: An Energy Balance Analysis, Chemical Engineering Transactions, 97, 283- 288 DOI:10.3303/CET2297048 CHEMICAL ENGINEERING TRANSACTIONS VOL. 97, 2022 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš Copyright © 2022, AIDIC Servizi S.r.l. ISBN 978-88-95608-96-9; ISSN 2283-9216 Bioenergy Generation Potential of Empty Fruit Bunch and Waste Tire Through Microwave Pyrolysis: An Energy Balance Analysis Guo Ren Monga,*, Chin Seng Liewb, William Woei Fong Chongc, Rubia Idrisd, Jo- Han Nge, Meng Choung Chiongf aSchool of Energy and Chemical Engineering, Xiamen University Malaysia, Jalan Sunsuria, Bandar Sunsuria, 43900, Sepang, Selangor, Malaysia bDepartment of Fundamental and Applied Sciences, HICoE-Centre for Biofuel and Biochemical Research, Institute of Self- Sustainable Building, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak Darul Ridzuan, Malaysia cSchool of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia dFaculty of Science and Natural Resources, Universiti Malaysia Sabah, Jalan UMS, 88400, Kota Kinabalu, Sabah, Malaysia eFaculty of Engineering and Physical Sciences, University of Southampton Malaysia (UoSM), 79200, Iskandar Puteri, Johor, Malaysia fDepartment of Mechanical Engineering, Faculty of Engineering, Technology & Built Environment, UCSI University, 56000, Kuala Lumpur, Malaysia guoren.mong@xmu.edu.my Energy has been needed more than ever, especially in the 21st century, for daily activities. The generation of energy from fossil sources fuels the industry's evolution but induces irreversible damage to the environment. Focus has been shifting towards bioenergy for a greener and sustainable approach in energy production, especially on biomass waste due to its abundant source that is not limited to geographical and climate changes. Empty fruit bunch (EFB) and waste tire (WT) have been an alarming waste due to industrialization. Unsystematic management of this waste has caused irreversible environmental damage and loss of valuable resources. Pyrolysis has been discovered as an effective process to effectively and safely manage, dispose and valorize waste into higher-value products. Pyrolysis is an energy-intensive process that is speculative to be not sustainable in the long run. This work will study the bioenergy generation potential of EFB and WT through microwave pyrolysis. The result shows that the bioenergy retrieved from WT and EFB is at 93.7 % and 90.99 %, portraying a minor amount of energy lost. Considering the energy consumption during the microwave pyrolysis process, a net energy profit (20.66 %) can be gained from WT, EFB records a loss of 81.17 %. This is attributed to the content of feedstock where EFB contains higher moisture and oxygenated composition, leading to the generation of products with lower heating value. The outcome does not reduce the efficiency of EFB valorization through microwave pyrolysis as the bio-oil contains valuable chemicals like phenols that can be retrieved as a biochemical source. Overall, the bioenergy generation is better when WT is used as feedstock. Bioenergy analysis should be conducted on the co-pyrolysis process to evaluate the feasibility of co-managing the waste, besides improving feedstock flexibility for this technology. 1. Introduction Waste management has always been a problem to tackle. The alarming waste generation rate further increases the difficulty of solving this issue, especially in developing countries. In Malaysia, an estimate of 22-23 Mt of empty fruit bunch (EFB) is being generated annually, with only 10 % of this waste being reused, and others are being disposed of (Anuar et al., 2019). The global rubber consumption has a major attribution towards the tire manufacturing industries and generating about 18 Mt of waste tire annually (Wang et al., 2020). When being disposed of through unsystematic means, these wastes will cause irreversible environmental damage and lead to the degrading of human health. Conventionally, EFB will be left to decompose naturally within palm 283 plantations; some will be collected and burned. WT is usually sent to a landfill if not illegally burned. There is a need to identify new, sustainable and green pathway to properly managed the wastes. Pyrolysis has been discovered as a good way to valorize waste into higher value products (Mong et al., 2021b). Experiments on EFB pyrolysis have reported that the biochar produced can be a good candidate for soil amendment and carbon sequestration. WT pyrolysis is reported to derive diesel-liked liquid fuel (Idris et al., 2021) and activated carbon (Malise et al., 2020) for power production and water treatment purposes. It is undeniable that pyrolysis seems to be the solution to these enormous amounts of waste. The process has a drawback that restricts the wide adoption of such technology. Pyrolysis is a thermal decomposition process that requires a high amount of energy (Liew et al., 2021). Research has been conducted to evaluate the energy-intensiveness and sustainability of pyrolysis (Chico-Proano et al., 2021). The pyrolysis of waste has received reports on energy deficit originating from the formation of non- valuable products (Mong et al., 2020) and inefficiency losses within the conversion system like endothermic and exothermic reactions (Jesus et al., 2018). The bioenergy content of each product derived through pyrolysis contributes to the overall energy profit of the process. It is important to have a thorough energy analysis conducted to evaluate the effectiveness of pyrolysis process in harvesting bioenergy from waste. Despite that there are plenty of research conducted on the pyrolysis of EFB and WT, a complete energy analysis has rarely been reported. Sulaiman and Abdullah (2011) only analyse the bio-oil production from fast pyrolysis of EFB and there is no report on the yield and characteristics of solid and gas products. The bio-oil yield of 36 wt% from EFB fast pyrolysis at 450 °C is retrieved in two states – 60 % organic phase and 40 % aqueous phase. The organic phase contains low moisture content (7.9 %) and high carbon content (69.35 %) with a high viscosity that hardly flows at room temperature. Fang et al. (2018) conducted a detail analysis on the solid, liquid and gas yield from the microwave pyrolysis of WT, but there is no record of energy balance analysis. The microwave pyrolysis of WT at 1,000 W and 40 min produce biochar of 24.83 MJ/kg and 80.25 wt% of carbon, indicating the potential for fuel application. The bio-oil contains high benzene and hydrocarbon compounds but with low oxygenated compounds, showing potential as transportation fuel additives. There is still much research gap to be filled from the bioenergy profitability aspect. This study will analyze the microwave pyrolysis process's energy distribution and profit when EFB and WT are valorized, leading to a thorough insight about the energy profitability of waste valorization through microwave pyrolysis technology. Besides, an additional section discussing the carbon footprint of the process will also be conducted to evaluate the carbon footprint of such a process. 2. Methods 2.1 Experiment and characterization A lab-scale, batch-reactor, microwave pyrolysis rig has been setup to valorize EFB and WT and end products in biochar, bio-oil and gas were retrieved. A 1 kW rated domestic microwave oven with a rated efficiency of 69.5 % was used as the heating source. The temperature within the reactor is kept constant with a temperature controller and a K-type thermocouple. A quartz reactor with a 3-neck lid was used to hold the feedstock of EFB or WT. A constant flow of pure N2 gas was utilized to ensure an inert environment and the hot volatile evolved during pyrolysis is transferred to a set of condensers maintained at 5 °C, aided by a water bath circulator. The experiment was conducted at 500 °C for 1 h. At the end of experiment, the solid leftover within the reactor is collected as biochar, the liquid condensate is collected as bio-oil and the uncondensed vapor is collected as biogas. The detail experimental design and procedure are recorded elsewhere (Idris et al., 2021). The products obtained were sent for thorough characterization to identify their composition and calorific value. The higher heating value (HHV) of biochar is measured using a bomb calorimeter. The HHV of bio-oil is calculated using DIN 51900 standard formula as displayed in Eq(1). 𝐻𝐻𝐻𝐻𝐻𝐻 = 34 𝐶𝐶+124.3 𝐻𝐻+6.3 𝑁𝑁+19.3 𝑆𝑆− 9.8 𝑂𝑂 100 (1) The C, H, S, A, O and N denote the mass fractions (wt%) of carbon, hydrogen, sulphur, ash, oxygen, and nitrogen contents. The LHV of the gaseous product is calculated using Eq(2). 𝐿𝐿𝐻𝐻𝐻𝐻 = (𝐻𝐻2∗107.98)+(𝐶𝐶𝑂𝑂∗126.36)+(𝐶𝐶𝐻𝐻4∗358.18)+(𝐶𝐶2𝐻𝐻2∗56) 1000 (2) The input and output of the microwave pyrolysis process is shown in Table 1. 2.2 Energy analysis and balance The bioenergy content of all three types of products obtained from the microwave pyrolysis process is calculated using Eq(3). 284 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑌𝑌𝐵𝐵𝐵𝐵𝑌𝑌𝑌𝑌 (𝑀𝑀𝑀𝑀) = 𝐹𝐹𝑀𝑀 ∗ 𝑌𝑌𝑃𝑃 ∗ 𝐶𝐶𝐻𝐻𝑃𝑃 (3) Where 𝐹𝐹𝑀𝑀 is the mass of feedstock used during experiment, 𝑌𝑌𝑃𝑃 is the yield of product from the microwave pyrolysis process and CV is the calorific value and the subscript of P and F represents the product and feedstock. The bioenergy distribution is evaluated from the feedstock initial HHV, according to Eq(4). 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝐷𝐷𝐵𝐵𝐷𝐷𝐷𝐷𝐵𝐵𝐵𝐵𝐷𝐷𝐷𝐷𝐷𝐷𝐵𝐵𝐵𝐵𝐵𝐵 (%) = 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑌𝑌𝐵𝐵𝐵𝐵𝑌𝑌𝑌𝑌 𝐹𝐹𝑀𝑀∗𝐶𝐶𝐶𝐶𝐹𝐹 ∗ 100 (4) The energy balance analysis is conducted considering the energy input during the microwave pyrolysis process and the energy output, as retrieved from the products in Eq(3). The outcome of attaining an energy profit or energy deficit is measured using Eq(5) 𝐸𝐸𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑃𝑃𝐵𝐵𝐵𝐵𝑃𝑃𝐵𝐵𝐷𝐷/𝐷𝐷𝐵𝐵𝑃𝑃𝐵𝐵𝐷𝐷𝐵𝐵𝐷𝐷 (%) = ∑𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑌𝑌𝐵𝐵𝐵𝐵𝑌𝑌𝑌𝑌 𝐸𝐸𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝐶𝐶𝐵𝐵𝐵𝐵𝐶𝐶𝐶𝐶𝐶𝐶𝐵𝐵𝑌𝑌 ∗ 100 (5) Table 1: The inputs and outputs of the microwave pyrolysis process of EFB and WT Input Description Unit Microwave Power 0.7 kWh Processing Time 1 h Temperature 500 °C Feedstock 0.1 kg EFB 17.9 MJ/kg WT 39.9 MJ/kg Catalyst Activated Carbon Output EFB WT Yield (%) Heating Value (MJ/kg) Yield (%) Heating Value (MJ/kg) Biochar 35.11 25 39.79 42.5 Bio-oil 38.26 11.76 39.12 43.05 Gas 26.63 14.63 22.09 18.52 2.3 Carbon footprint analysis The carbon sequestration credits (CSC) for the products derived were measured through Eq(6). In this work, the product with carbon sequestration potential is biochar as it contains stable carbons and can withstand degradation when buried underground. The bio-oil and gas products are assumed to be utilize as biofuel with no carbon sequestration potential. 𝐶𝐶𝐶𝐶𝐶𝐶 = 𝑌𝑌𝑃𝑃 ∗ 𝐶𝐶𝐶𝐶𝑃𝑃 ∗ 𝑅𝑅𝐶𝐶/𝐶𝐶𝑂𝑂2 ∗ 𝐶𝐶𝐶𝐶𝑃𝑃 (6) The CCP refers to the carbon content of product, 𝑅𝑅𝐶𝐶/𝐶𝐶𝑂𝑂2refers to the carbon to CO2 equivalent ratio (using a value of 12/44) and CSP is the carbon stability of product, where CS for biochar is reported at 0.7 (Luo et al., 2021). The carbon footprint of the process is analyzed by evaluating the CO2-eq emission of the process (mainly from electrical consumption) and the carbon storage potential of the products, according to Eq(7). The carbon emission from the consumption of energy supplied by the power grid is taken as 373.48 g CO2-eq/kWh, considering key global warming potential gases like CO, CO2, CH4 and NOx (Spath and Mann, 2000). The microwave pyrolysis process is calculated to consume a power of 0.7 kWh and the emission is at 261.4 g CO2- eq. The products used for energy production are assumed to release biogenic CO2 during combustion and will not be accounted for its carbon footprint. 𝐶𝐶𝐶𝐶𝐵𝐵𝐷𝐷𝐵𝐵𝐵𝐵 𝑃𝑃𝐵𝐵𝐵𝐵𝐷𝐷𝑓𝑓𝐵𝐵𝐵𝐵𝐵𝐵𝐷𝐷 = (𝐸𝐸𝐴𝐴 + 𝐶𝐶𝐶𝐶𝐶𝐶) − �𝐶𝐶𝐶𝐶2 − 𝐵𝐵𝑒𝑒𝑃𝑃𝐵𝐵𝐵𝐵𝑃𝑃𝐵𝐵𝐶𝐶𝐶𝐶 (7) 𝐸𝐸𝐴𝐴 refers to the avoided emission from energy production through fossil fuels like coal (replaced by biochar), fuel oil (replaced by bio-oil) and natural gas (replaced by gas product). The avoided emission is calculated by evaluating the net emission of CO2-eq with reference of unit energy produced (kWh). According to the EPA (2014), coal coke, residue fuel oil No.6 and natural gas will have an emission of 113.67 kg CO2/mmBTu, 75.1 kg CO2/mmBTu and 53.06 kg CO2/mmBTu. The products derived with acceptable fuel properties will be 285 assumed to be used for bioenergy production. The amount of bioenergy produced will be assumed to directly substitute fossil fuels, calculating the avoided emissions. 3. Results and discussion 3.1 Product yield and energy content Table 2 display the bioenergy yield within each product derived from the microwave pyrolysis of EFB and WT. It can be observed that the solid product (biochar) contains the highest yield with 52.75 % and 42.38 % of bioenergy being transferred from the original feedstock. The HHV of WT-biochar (42.5 MJ/kg) is higher than EFB-biochar (25 MJ/kg), the bioenergy transferred is higher for EFB-biochar. This can be attributed to the initial calorific value of the waste where WT (39.9 MJ/kg) is much higher than EFB (17.9 MJ/kg). The bio-oil within WT has a higher bioenergy yield with up to 41.13 % being distributed within the liquid product, while liquid derived from EFB only contains 25.05 %. This showcase the superiority of WT for bio-oil yield with high energy content. The lower bioenergy distributed within the liquid yield for EFB is due to the presence of oxygenated compounds and moisture content, lowering the energy content. Biomass-derived bio-oil generally contain high oxygenated compounds giving it a lower calorific value (Miskolczi et al., 2020). Nonetheless, the bio-oil from biomass like EFB contains valuable chemicals (mainly from oxygenated compounds) like pyridine, phenols and ketones that can be extracted as bio-chemical source. Gas products from EFB and WT contain the least bioenergy distributed with 21.68 % and 10.25 %. This is due to the non-condensable vapor generated during microwave pyrolysis containing small gaseous molecules like CO, CO2, H2 and a small portion of CH4 with lower heat capacity than larger gaseous compounds C2H6 and C3H8. Gaseous product yield is lower than solid and liquid products, having a lower amount of bioenergy. Microwave pyrolysis aided by activated carbon has been reported to produce a higher yield of CO and H2. Summarizing the total bioenergy content within the solid, liquid and gas products, it has been found that the microwave pyrolysis process can retrieved up to 93 % of energy contained within the raw feedstock and only a minor amount of energy is lost. The bioenergy lost during microwave pyrolysis may be attributed to the formation of products like water and coke (Mong et al., 2020). Microwave pyrolysis can retrieve a higher energy content for WT, mainly due to the nature of feedstock (fossil-derived product with low moisture content of 0.92 %). EFB contains 3.96 % of moisture after being pre-treated before pyrolysis. Water may also be formed from the reaction between oxygenated compounds during the experiment. It is to note that the oxygen composition within EFB (50.44 %) is higher than WT (10.81 %). The energy profit of the microwave pyrolysis when WT is used as feedstock is better than EFB. WT achieves an overall 20.66 % energy profit after considering the energy consumed during the valorization process (estimated to be 0.7 kWh). On the other hand, EFB attains an energy deficit of 81.17 %. The large discrepancy is due to the initial energy content of raw feedstock. WT has a higher energy content (39.9 MJ/kg) due to its fossil-based properties, biomass sources like EFB have a lower energy content (17.97 MJ/kg). As both feedstocks are processed using the same experimental setup and consume the same amount of energy, the feedstock with an initial lower energy content will have a lower energy yield. In addition, the bio-liquid with high oxygenated compounds and low heating value due to the generation of water during the microwave pyrolysis of EFB contributes to the process's energy deficit. The outcome of this study brings to view the possibility of co-pyrolysing different types of waste simultaneously to improve the overall energy profit. Table 2: Energy yield, distribution and profit for EFB and WT for microwave pyrolysis Feedstock/ Products EFB WT Biochar Bio-oil Gas Biochar Bio-oil Gas Bioenergy Yield (MJ) 0.95 0.45 0.39 1.69 1.64 0.41 Bioenergy Distribution (%) 52.75 16.56 21.68 42.38 40.94 10.25 Energy Profit/Deficit (%) -81.17 20.66 3.2 Carbon footprint analysis The carbon footprint was evaluated based on emission generated from the microwave pyrolysis process, carbon sequestration potential of the product and avoided emission from fossil sources. Figure 1 display the net carbon emission of the process where the microwave pyrolysis of EFB records a positive carbon emission of 0.09 kg CO2 and WT records a negative carbon footprint of -0.14 kg CO2. EFB has a net positive carbon emission due to several factors. Firstly, the bioproducts derived from EFB is lower in energy content, capable of replacing a lower amount of fossil fuel. Secondly, the carbon content of biochar-EFB is lower, reducing the CSS potential. The scenario is different for WT as it has a higher energy content and carbon content, which can balance out 286 the emitted CO2 during microwave pyrolysis. Overall, the work portrays the possibility of microwave pyrolysis to be a negative carbon emission technology in waste valorization and management. WT has a better environmental footprint as the derived products contains higher energy 0content that can replace more fossil sources to generate the same amount of energy. Despite the less attractive findings on the EFB feedstock with energy deficit and positive carbon emission, microwave pyrolysis technology has been proven to be an effective waste-to-energy technology that is both sustainable and environmentally friendly. The drawbacks can be overcome by integrating both feedstock of EFB and WT as blends, to be co-pyrolyzed under the same operating condition. Synergism has been reported to exist between the co-pyrolysis of EFB and WT (Mong et al., 2021a), which might positively affect the energy yield. These analyses can be proposed in the future study. Figure 1: The net carbon emission from the microwave pyrolysis of (a) EFB and (b) WT 4. Conclusions This study demonstrates a bioenergy and carbon footprint analysis between EFB and WT, which has undergone valorization through microwave pyrolysis. WT is able to attain a higher bioenergy content of 93.59% and EFB has a lower value of 90.99%. By accounting the energy consumption, WT valorization through microwave pyrolysis achieved an overall energy profit of 20.66 %, mainly due to the raw feedstock having a high calorific value. EFB records an overall energy deficit (-81.17 %) through microwave pyrolysis, mainly due to its higher oxygen composition causing the products generated to be high in oxygenated compounds, lowering the calorific value. Evaluation on the carbon emission of the process demonstrate WT as feedstock successfully achieved a net negative carbon emission of -0.14 kg CO2. The results demonstrated the possibility of managing and valorizing wastes such as EFB and WT through microwave pyrolysis. The positive results from WT for example the negative carbon emission and positive bioenergy production can serve as a trade-off to overcome the less desirable results when EFB is processed. This calls for the possibility in co-managing different types of waste, in this context it will be WT and EFB. The present of synergism when different wastes is being process will be of interest where the target is to further increase the bioenergy generation and lower the environmental footprint of such process. -0.5 -0.3 -0.1 0.1 0.3 0.5 Carbon Footprint (kg CO₂) Consumption CSC Avoided NG Avoided Fuel Oil Avoided Coal Emitted Net = 0.09 kg CO₂ Stored -0.5 -0.3 -0.1 0.1 0.3 0.5 Carbon Footprint (kg CO₂) Emitted Net = -0.14 kg CO₂ Stored a) b) 287 Acknowledgments This research was supported by Xiamen University Malaysia Research Fund (Grant No: XMUMRF/2022- C9/IENG/0045) References Anuar N.I.S., Zakaria S., Gan S., Chia C.H., Wang C., Harun J., 2019, Comparison of the morphological and mechanical properties of oil palm efb fibres and kenaf fibres in nonwoven reinforced composites, Industrial Crops and Products (127), 55-65. 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