- Email: [email protected]
Design, fabrication and operation of continuous microwave biomass carbonization system
Renewable Energy 66 (2014) 49e55
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Design, fabrication and operation of continuous microwave biomass carbonization system Poomyos Payakkawan*, Suwilai Areejit, Pitikhate Sooraksa Department of Computer Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2013 Accepted 28 October 2013 Available online
This paper presents a new design and the fabrication of biomass carbonization system using microwave heating. The heating system employs multi-feed microwave generators with 8.5 kW maximum power and 2.458 GHz frequency for uniform heating distribution in a 0.847 m3 cylindrical low cement castable reactor. The experiment entails the carbonization of 58,800 kg of coconut shell for 7 days. Afterward, the charcoal and wood vinegar yields are analyzed; meanwhile, uncondensed gases are treated to fuel the engine-generator system. The results show that the proposed approach greatly saves the production time of charcoal yields and allows ease of temperature control. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biomass carbonization Microwave heating Coconut shell charcoal Wood vinegar
1. Introduction The demand for energy by a multitude of industries is rising and will be more so in the future. The increasing consumption of fossil fuels causes numerous negative impacts on the environment, examples of which are the accumulation of carbon dioxide in the atmosphere and thereby rising global temperatures. As a result, attempts have been made to use biomass as an alternative source of energy to reduce carbon dioxide emission in the atmosphere [1]. As an agricultural country with numerous kinds of biomass, Thailand produces approximately 61 million tons of biomass waste annually; however, the country hardly utilizes biomass waste to produce bio-fuel. Most biomass waste of the agriculture industry is typically burned while that from agricultural activities is left in the fields/plantations or burned [2]. It is estimated that Thailand could produce from available biomass over 20 million tons of crude oil, which is more than half of the current oil consumption of the country. Consequently, biomass waste should be greater utilized either by value adding or by translating into fuels or energy. The development of biomass conversion technology is suitable for available biomass resources of different areas thus benefits both the economy of the local community and the whole nation in the long run [3].
* Corresponding author. E-mail addresses: [email protected], (P. Payakkawan).
[email protected]
0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.10.042
Biomass carbonization is a process to decompose biomass with heat in the absence of oxygen and is divided into 4 stages: Firstly, biomass is heated to 180 C at which the water inside the gaps between cells (free water) and at cell boundary (bound water) is completely removed and transformed into non-pungent, nonharmful, pale blue vapor. Secondly, volatiles are removed by heating the biomass at the temperature range of 180e280 C, and once the temperature reaches 280 C, Hemicelluloses are fully decomposed. The temperature is maintained at 280 C for an extended period of time for an optimal carbonization process whereby the heat is evenly distributed to the whole biomass inside the reactor. The yield from this stage is of pale gray color consisting of CO, CO2, acetic acid, and methanol, all of which are nevertheless of low quantity to be of use. Thirdly, the derived biomass is then converted into charcoal. The temperature range at this stage is approximately 280e400 C, the range at which biomass self-decomposes through the exothermic reaction. Celluloses in the biomass rapidly decompose at 280 C and their yield is of white and yellow colors with pungent smell and gives high quality wood vinegar if condensed. Then, lignin decomposes at approximate temperatures of 310e 400 C. Beyond 400 C biomass is entirely converted into charcoal. Finally, charcoal quality is improved at this final stage by removing tar. Although biomass becomes charcoal after approximately 400 C, high quantity of tar remains. Tar causes the charcoal to be of low quality and, once burned, changes into benzopyrene and dibenzanthracene, both of which are carcinogenic. Therefore, charcoal is dried at 500e600 C for a period of time to remove tar [4].
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The carbonization process is normally employed to convert agricultural biomass waste into biofuels, i.e., solid (charcoal), liquid (bio-liquid, wood vinegar, pyroligneous acid), and syngas [5]. Charcoal can be made into charcoal briquettes [6]; or induced to obtain activated carbon [7e9]; wood vinegar can be used as a substitute for the chemical pesticides [10]; and syngas can be used as a fuel in the internal combustion engines or in the electrical production [11]. Therefore, biomass waste and agricultural waste processed to add value would benefit the economy, society, and environment [12]. In addition, the employment of biomass waste materials from agriculture could be an answer to the quest for a sustainable source of renewable energy. The carbonization process of biomass waste from agriculture is a suitable, cost-effective technology for charcoal production in Thailand because the process adds value to the biomass raw materials which are inexpensive and ubiquitous. Furthermore, the process is effective in solving the problem of biomass waste from agriculture and can be suitably and economically employed by villagers in remote areas to produce electricity for local consumption. There have been many developments of new technologies to produce charcoal from biomass. Moreover, several countries, such as European countries, Australia, Brazil and the United States of America [13], not merely promote the utilization of biomass to produce energy but also encourage the use of energy from biomass. The carbonization process (i.e., charcoal production) has evolved since its inception hundreds of years ago to include multiple loads, new designs, etc. The existing carbonization processes of today can be categorized into two types: batch (e.g., pits, earth mound, brick and metal kilns) and continuous (e.g., drum type pyrolysers, screw type pyrolysers, and rotary kilns) [14]. The current charcoal production methods have changed little from the traditional methods. Both production types are nevertheless plagued with many problems, some of which are energy wastage for both types and time loss and environmental unfriendliness for batch process, all of which hinder the industrialization of carbonization [5,14]. In both processes, the conventional heating method is employed whereby the exterior of the reactors is heated and then heat is transferred to biomass inside. The heat transforms the biomass inside the reactors into biofuels, depending upon the size of the biomass and process conditions (i.e., temperature, heating rate and residence time). On the contrary, an alternative efficient heating method, microwave heating, unlike the conventional heating method, causes the heat to originate from inside of materials and migrate outward. Recent research studies on the thermochemical process of biomass with the application of microwave heating have been reported and the research results suggest that this heating method is suited to distributed conversion of large biomass particles [15e21]. The microwave carbonization of biomass is one of the novel thermochemical technologies in which biomass is irradiated with microwave. The microwave carbonization process offers several advantages over the traditional carbonization process, some of which include uniform internal heating of large biomass particles, ease of control, and saving of time and heat energy. As such, the microwave carbonization is therefore adopted by industries. The objective of this research is to build a pilot scale continuous carbonization system of biomass using microwave heating technology to produce charcoal and wood vinegar with uncondensed gases as byproducts of the process. The organization of this paper is as follows: Section 1 deals with the introduction and Section 2 details the design of the continuous microwave biomass carbonization process. The fabrication of the process is addressed in Section 3. Experiment and results are presented in Section 4 while the conclusion is provided in Section 5.
2. Design of continuous microwave biomass carbonization system 2.1. Continuous microwave biomass carbonization system Pulverized biomass, as large biomass particles, is conveyed by the bucket elevator to the hoper at the top to dry. A 0.847 m3 cylindrical low cement castable reactor fixed with 10 microwave magnetrons is employed to carbonize biomass. The charcoal yield is conveyed out from the continuous microwave biomass carbonization system by a screw-conveyer and the gas yield is transported by a pipe on top of the reactor to a condensation unit in which wood vinegar is condensed. Meanwhile, uncondensed gases are transferred to a gas treatment unit to purify so that the treated gases are suitable for use to produce electricity. To have a better picture of the carbonization process, the conversion pathway of the continuous microwave biomass carbonization system is presented in Fig. 1. The process begins with biomass waste as input and ends with three forms of product yields, i.e., charcoal, wood vinegar (bio-oils), and fuel gas. Fig. 2 shows the schematic of a full-scale microwave carbonization system. 2.2. Multi-feed microwave cavity The prototype multi-feed microwave cavity, is designed to heat the reactor of the carbonization process as shown in Fig. 3. Typical carbonization temperatures for most types of biomass are 200e600 C. Design of the multi-feed microwave cavity must allow the heating rate to be optimally controlled and thereby improve heating uniformity [22e25]. Therefore, multi-mode microwave radiation is employed in this work to heat the multi-feed microwave cavity as shown in Fig. 3. The 10 rectangular-shaped openings are drilled around the cavity with each opening fixed with a microwave source each consisting of a magnetron and waveguide. The microwave sources are installed in two parallel levels of five sources for each level. At the center of the cavity vertically lies a cement castable reactor through which the biomass to be reacted is passed. The reactor is made of cement to allow penetration by microwave radiation. In this experimental research a heating system using microwave without stirrer mechanism in which the heat can evenly distribute throughout biomass or load is designed. The absence of stirrer mechanism reduces the complexity of the design and construction of the system. The microwave reactor however disallows the installation of a stirrer because the latter would bounce off the electromagnetic wave, causing uneven distribution of heat inside the reactor. 3. Fabrication of continuous microwave biomass carbonization system Fig. 4 shows the snapshot of the continuous microwave biomass carbonization plant used in this study. The plant is 7 m in height and covers an area of 25 square meters. 3.1. Biomass feeding system In the feeding system, pulverized biomass are conveyed by the bucket elevator to the hoper at the top to dry. After drying, dried biomass waste is fed down to the carbonization reactor through the cylindrical steel pipe of 0.30 m internal diameter, which controls via two pneumatic stainless steel trays the amounts of dried biomass fed into the reactor and prevents gas leakage from the reactor.
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Fig. 1. Biomass carbonization conversion pathway.
3.2. Carbonization reactor (low cement castable reactor) The carbonization reactor in this research is made up of a cylindrical low cement castable with organic fiber of alkaline resistance and maximum service temperature of 1400 C. The reactor has 0.30 m in internal diameter, 0.07 m in thickness, 0.9 m in height, 0.85 m3 in volume of reaction. The top part of the reactor is connected to the biomass feeding pipe while the bottom part to a charcoal temperature reduction unit with all the joints sealed by ceramic fiber to prevent a leakage of gases. A hole is bored in the top part of the reactor and a 2-inch steel pipe is fitted to the hole as a passage of carbonization gas yields, the flow rate of which is controlled by a blower.
with microwave. The middle layer is a multi-feed microwave cavity comprising two rows of five magnetrons on each row (totaling 10). The magnetrons to heat the biomass are of OM75S-020 model of Samsung with 2.458 MHz frequency and 0.85 kW each for a combined maximum of 8.5 kW. The outermost layer is installed with 10 cooling fans to transport the heat loss from the rear of the microwave sources to the hopper to pre-heat the biomass prior to passing down through the low cement castable reactor. The shutters on the top and bottom of the castable reactor are to prevent the leakage of microwave radiation. Fig. 5 is the snapshot of the actual heating system of the carbonization reactor. Each microwave source of the multi-feed microwave cavity is controlled for uniform heat distribution by 10 solid state 10 A/220 V relays which are in turn controlled by the Programmable Logic Controller (PLC).
3.3. Heating system of carbonization reactor 3.4. Charcoal temperature reduction unit The heating system of the carbonization reactor as shown in Fig. 3a. consists of: (1) the innermost low cement castable reactor, (2) the microwave sources in the middle, and (3) the outermost layer with cooling fans. The low cement castable reactor is the central cylindrical passage through which the biomass passes and reacts
The unit consists of two stainless steel cylindrical hollow tubes of different sizes with the smaller tube placed inside at the center of the larger one. Cool water from cooling tower 1, entering from the inlets at the bottom and discharged through the outlets at the top, is
Fig. 2. Full-scale continuous microwave biomass carbonization system.
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Fig. 3. Frontal view cross section of a partial of carbonization process (3.a); Isometric view of a partial of carbonization process (3.b).
circulated inside the hollow parts of both tubes. The solid yield from the reactor is then passed through both cylindrical tubes to reduce the temperature. On the top part of the charcoal temperature reduction unit is a thermocouple type K (T2) as shown in Fig. 1 to measure the temperature of the solid yield from the reactor. Meanwhile, at the bottom part of the unit is the same type of thermocouple (T3) as seen in Fig. 1 to measure the solid yield temperature after passing through the charcoal temperature reduction unit. Afterward, the obtained solid yield (i.e., charcoal) is transported by the screw conveyer to the storage container as shown in Fig. 1.
to ensure that the fuel gases are sufficiently clean for use in the engine-generator system of this paper. 3.7. Engine e generator system The engine-generator system which produces 10 kW, 220 V, 50 Hz of electricity to operate the entire carbonization process is fueled by either gasoline in case of the first run of the process or the remaining fuel gas from previous carbonization in case of the subsequent runs. The system is able to generate sufficient electricity to meet the requirement of the whole carbonization process.
3.5. Condensing unit 3.8. Control unit As shown in Fig. 1, the condensing unit in this study is of the vertical shell and tube condenser type with cooling water as heat exchanger. The unit consists of 120 tubes, each with 0.038 m in diameter and 1.15 m in height. Cool water from cooling tower 2, entering from the upper inlets and released through the lower outlets, is circulated inside the hollow parts of all 25 tubes of the condensing unit. The gas yields from the reactor, passing through the condensing unit, are condensed to wood vinegar.
All operating system is controlled automatically by a PLC. The process temperature can be monitored with the temperature controller in the control panel and seven K-type thermocouples are used. Temperature readings in the reactor and at various points in the system are assigned by T1eT5 in Fig. 1, in which T1eT3 denote the temperatures inside the reactor, T4 condensation system, and T5 the gas system. The functioning and status of the system are shown on a touch screen display.
3.6. Gas treatment unit 4. Experiment and results The uncondensed gases leaving the condensing unit are treated with water spray and then rock bed filter inside the gas treatment unit to remove moisture and tar. The two-stage filtering is required
Coconut shells, as large biomass particles, cut into small chips of 2e4 cm in size as shown in Fig. 6 were used in this experiment as
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Fig. 6. Coconut shell chips of 2e4 cm in size used as the raw material.
Fig. 4. A snapshot of the continuous microwave biomass carbonization plant.
the raw material. Coconut shells of 350 kg/h at 50% mc were fed into this system. The carbonization process starts with filling the reactor with the dried coconut shells and then the coconut shells were heated up by all ten microwave radiation sources until the reactor temperature (T1) reached 500 C. This period is called the startup period and the temperatures vs time characteristics of this period are presented in Fig. 7. After the temperature reached 500 C, it entered the operating period during which the charcoal output yields of the microwaved and then cooled down coconut shells were transported by the screw conveyer to the storage containers. The temperature was maintained at 500 C until the end of
Fig. 5. A snapshot of the heating system of the carbonization reactor.
the operating period by automatic on/off cycle of each microwave source. This cycle was controlled by the PLC which switched on (off) the microwave sources if the actual temperature dropped below (reached) the reference temperature of 500 C. The operating time vs temperature characteristics are presented in Fig. 8. Table 1 shows the optimal configuration of process parameters with the duration of continuous actual startup and operation for 7 days. To prolong system lifetime, the operation needs 1 day for maintenance before starting the new running. The electrical energy consumptions of process in the startup and operation period are 8.8 kW and 9.6 kW respectively. In case of the system far from a power distribution grid, the startup period is used electrical power from the gasoline engine e generator system which can be provided the electrical power to start the system. After running for several hours, the system can also generate fuel gases which can feedback them to the generator. The system can be run by alternative choices for providing inputs to the generators. The analysis results by Oxygen Bomb Calorimeter KikaÒ Labortechnik C5000 of the charcoal product yields compared with the
Fig. 7. The temperature vs time characteristics at startup period (temperature T vs heating time t, (recorded every 5 min)), T1 in the center of reactor; T2 at the bottom of reactor; T3 at the top of cooling zone; T4 at the bottom of cooling zone; and T5 in the purified gas pipe.
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550
Table 4 The pH of the wood vinegar from coconut shell sample.
500 T1 T2 T3 T4 T5
450
Temperature (Celsius)
400 350
pH values Wood Vinegar:distilled water Wood vinegar
100:0
1:100
1:200
1:300
1:400
1:500
3.75
4.15
4.19
4.22
4.23
4.23
300 Table 5 The product yield proportions from carbonization process using 58,800 kg of coconut shell sample.
250 200 150 100 50 0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 Time (Minute)
Fig. 8. The temperature vs time characteristics of operating time (temperature T vs heating time t (recorded every 5 min)), T1 in the center of reactor; T2 in the bottom of reactor; T3 in the top of cooling zone; T4 in the bottom of cooling zone; and T5 in the purified gas pipe.
Table 1 The optimal configuration of process parameters at the reference temperature of 500 C. Item
Rate
Screw conveyer speed (rpm) Blower speed (rpm) Water flow rate at cooling tower 1 (cc/s) Water flow rate at cooling tower 2 (cc/s) Pneumatic stainless steel trays (time/h) Average microwave power (kW)
Startup period
Operation period
0 25 5 50 0 8.50
9 25 280 220 12 5
Table 2 Proximate analysis results of coconut shell sample.
Total moisture (wt.%) Volatile mater (wt.%) Fixed carbon content (wt.%) Ash (wt.%)
Coconut shell raw material
Charcoal product yield
10.53 78.30 20.96 0.74
3.18 14.32 81.20 4.48
Product yields
%wt
Yields
Coconut charcoal Wood vinegar, bio-liquid Gases
31 35 34
w10,900 kg w3300 l w11,700 m3
Table 6 The comparison of product yields of this system with other research works. Product yields
Coconut charcoal (%wt)
Wood vinegar, bio-liquid (%wt)
Gases (%wt)
Our experiment Beget et al. [26] Sundaram et al. [27]
31 45 26
35 28 42
34 27 32
raw material (i.e., coconut shells) are presented in Table 2. The heating value of output charcoal yields is 7260 kcal/kg with low value electrical resistance. The chemical components and pH values found in wood vinegar tested by Gas Chromatography-Mass Spectrometry are respectively presented in Tables 3 and 4. From the results, the proportions of the product yields from the carbonization process using 58,800 kg of coconut shells are presented in Table 5. Meanwhile, Table 6 shows a comparison of the product yields produced by this system with those of other research works [26,27]. The photograph of charcoal output yields is shown in Fig. 9, which appears shiny when broken and is of low value electrical resistance. The brown-black color and burned smell wood vinegar output yields are illustrated in Fig. 10. Fuel gas yields from the carbonization were supplied to the engine-generator system. The electrical energy of approximately 10 kW was produced by the engine-generator system, which is sufficient to run the electrical system of the carbonization system.
Table 3 The percentage of the chemical components found in wood vinegar of coconut shell sample. No.
RT/min
ID
Area%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
4.21 4.23 5.89 6.34 7.34 7.41 8.46 9.21 9.68 9.08 10.37 13.05 14.92 15.97 17.02 17.78
Toluene Propanoic acid Furfural 2-furanmethanol 2-cyclopenten-1-one Ethanone Phenol 2-cyclopenten-1-one Phenol, 2-methyl Phenol, 4-methyl Phenol, 2-methoxy 2-methoxy-4-methylphenol Phenol, 4-ethyl-2-methoxy Phenol, 2, 6-dimethoxy Acetic acid 2-butanone
n 1.40 4.80 2.59 0.52 1.13 48.97 2.00 2.15 2.76 8.40 2.72 1.21 7.62 1.39 0.57
Fig. 9. Coconut shell charcoal yields from the carbonization process.
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References
Fig. 10. The vinegar yields from the wood vinegar condensation unit.
5. Conclusion The results of the pilot scale experiment of continuous carbonization system of biomass using microwave heating technology have shown the potential of the proposed system which can reduce the production time and run smoothly. The internal temperature of the proposed reactor can be not only fully controlled but also easy to adjust for various process conditions. Of the obtained product yields, charcoal can substitute fossil fuels, wood vinegar can be used in household and agricultural sectors as insecticides, and fuel gas is to run the engine-generator system to generate the electrical energy adequate to power the electrical system without the need for electric grid, thus much suitable for remote areas without the power grid. The application of this new continuous microwave biomass carbonization system to produce renewable energy would considerably benefit the country economically and socially in general and environmentally in particular due mainly to its lower CO2 emission. Notwithstanding, if the scale of the system were enlarged the excess electrical energy from the larger-scale carbonization system could be resold, thereby generating incomes for the community. Acknowledgments This work was materialized with the financial support from National Innovation Agency, Ministry of Science and Technology, Thailand; and Thailand Research Fund under grant number RGJ e PHD/0131/2551. The authors would also like to express deep appreciation to Assoc. Prof. Dr. Sombat Teekasap for his encouragement throughout this work.
[1] Berndes Goran, Hoogwijk Monique, van den Broek Richard. The contribution of biomass in the future global energy supply e a review of 17 studies. Biomass Bioenergy 2003;25:1e28. [2] Prasertsan S, Sajjakulnukit B. Biomass and biogas energy in Thailand: potential, opportunity and barriers. Renew Energy 2006;31:599e610. [3] Yokoyama Shin-ya, Ogi Tomoko, Nalampoon Anan. Biomass energy potential in Thailand. Biomass Bioenergy 2000;18:405e10. [4] Jeng-Chyan, Lin Muti. Development of a high yield and low cycle time biomass char production system. Fuel ProcessTechnol 2006;87:487e95. [5] Bridgwater AV. Renewable fuels and chemicals by thermal processing of biomass. Chem Eng J 2003;91:87e102. [6] Blesa MJ, Miranda JL, Moliner R, Izquierdo MT, Palacios JM. Low-temperature co-pyrolysis of a low-rank coal and biomass to prepare smokeless fuel briquettes. J Anal Appl Pyrolysis 2003;70:665e77. [7] Ioannidou O, Zabaniotou A. Agricultural residues as precursors for activated carbon production e a review. Renew Sustain Energy Rev 2007;11:1966e2005. [8] Karagoz Selhan, Tay Turgay, Ucar Suat, Erdem Murat. Activated carbons from waste biomass by sulfuric acid activation and their use on methylene blue adsorption. Bioresour Technol 2008;99:6214e22. [9] Encinar JM, Gonzalez JF, Roman S, Martnez G. Pyrolysis of various biomass residues and char utilization for the production of activated carbons. J Anal Appl Pyrolysis 2009;85:134e41. [10] Nami Kartal S, Terzi Evren, Kose Coskun, Hofmeyr John, Imamura Yuji. Efficacy of tar oil recovered during slow pyrolysis of macadamia nut shells. Int Biodeterior Biodegradation 2011;65:369e73. [11] Bain Richard L, Overend Ralph P, Craig Kevin R. Biomass-fired power generation. Fuel Process Technol 1998;54:1e16. [12] Ludwig C, Hellweg S, Stucki S. Municipal solid waste management. Germany: Springer; 2003. [13] Pelaez-Samaniego MR, Garcia Perez M, Cortez LB, Rosillo Calle F, Mesa J. Improvements of Brazilian carbonization industry as part of the creation of a global biomass economy. Renew Sustain Energy Rev 2008;12:1063e86. [14] Hensley Duku Moses, Gu Sai, Ben Hagan Essel. Biochar production potential in Ghana e a review. Renew Sustain Energy Rev 2011;15:3539e51. [15] Appleton TJ, Colder RI, Kingman SW, Lowndes IS, Read AG. Microwave technology for energy-efficient processing of waste. Appl Energy 2005;81:85e113. [16] Menéndez JA, Domínguez A, Inguanzo M, Pis JJ. Microwave-induced pyrolysis of sewage sludge. Water Res 2002;36:3261e4. [17] Menéndez JA, Domínguez A, Inguanzo M, Pis JJ. Microwave pyrolysis of sewage sludge: analysis of the gas fraction. J Anal Appl Pyrolysis 2004;71: 657e67. [18] Oloyede A, Groombridge P. The influence of microwave heating on the mechanical properties of wood. J Mater Process Technol 2000;100:67e73. [19] Salema AA, Ani FN. Microwave induced pyrolysis of oil Palm biomass. Bioresour Technol 2011;102:3388e95. [20] Du Zhenyi, Li Yecong, Wang Xiaoquan, Wan Yiqin, Chen Qin, Wang Chenguang, et al. Microwave-assisted pyrolysis of microalgae for biofuel production. Bioresour Technol 2011;102:4890e6. [21] Zhao Xiqiang, Wang Min, Liu Hongzhen, Li Longzhi, Ma Chunyuan, Song Zhanlong. A microwave reactor for characterization of pyrolyzed biomass. Bioresour Technol 2012;104:673e8. [22] Metaxas AC, Meredith RJ. Industrial microwave heating. London: IEE Power Engineering Series; 1993. [23] Chan TVCT, Reader HC. Understanding microwave heating cavities. Artech House Publishers; 2000. [24] Gupta M, Wong E, Leong W. Microwaves and metals. John Wiley & Sons; 2010. [25] Mehdizadeh M. Microwave/RF applicators and probes for material heating, sensing, and plasma generation, a design guide. Elsevier; 2010. [26] Beget MRA, Mohammad RI, Mohammad UHJ. Pyrolysis of coconut shell for bio-oil. In: Proceedings of the international conference on mechanical engineering 2011. Dhaka: Bangladesh; December 2011. pp. 18e20. [27] Sundaram EG, Natarajan E. Pyrolysis of coconut shell: an experimental investigation. J Eng Res 2009;6(2):33e9.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Design, fabrication and operation of continuous microwave biomass carbonization system Poomyos Payakkawan*, Suwilai Areejit, Pitikhate Sooraksa Department of Computer Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2013 Accepted 28 October 2013 Available online
This paper presents a new design and the fabrication of biomass carbonization system using microwave heating. The heating system employs multi-feed microwave generators with 8.5 kW maximum power and 2.458 GHz frequency for uniform heating distribution in a 0.847 m3 cylindrical low cement castable reactor. The experiment entails the carbonization of 58,800 kg of coconut shell for 7 days. Afterward, the charcoal and wood vinegar yields are analyzed; meanwhile, uncondensed gases are treated to fuel the engine-generator system. The results show that the proposed approach greatly saves the production time of charcoal yields and allows ease of temperature control. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biomass carbonization Microwave heating Coconut shell charcoal Wood vinegar
1. Introduction The demand for energy by a multitude of industries is rising and will be more so in the future. The increasing consumption of fossil fuels causes numerous negative impacts on the environment, examples of which are the accumulation of carbon dioxide in the atmosphere and thereby rising global temperatures. As a result, attempts have been made to use biomass as an alternative source of energy to reduce carbon dioxide emission in the atmosphere [1]. As an agricultural country with numerous kinds of biomass, Thailand produces approximately 61 million tons of biomass waste annually; however, the country hardly utilizes biomass waste to produce bio-fuel. Most biomass waste of the agriculture industry is typically burned while that from agricultural activities is left in the fields/plantations or burned [2]. It is estimated that Thailand could produce from available biomass over 20 million tons of crude oil, which is more than half of the current oil consumption of the country. Consequently, biomass waste should be greater utilized either by value adding or by translating into fuels or energy. The development of biomass conversion technology is suitable for available biomass resources of different areas thus benefits both the economy of the local community and the whole nation in the long run [3].
* Corresponding author. E-mail addresses: [email protected], (P. Payakkawan).
[email protected]
0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.10.042
Biomass carbonization is a process to decompose biomass with heat in the absence of oxygen and is divided into 4 stages: Firstly, biomass is heated to 180 C at which the water inside the gaps between cells (free water) and at cell boundary (bound water) is completely removed and transformed into non-pungent, nonharmful, pale blue vapor. Secondly, volatiles are removed by heating the biomass at the temperature range of 180e280 C, and once the temperature reaches 280 C, Hemicelluloses are fully decomposed. The temperature is maintained at 280 C for an extended period of time for an optimal carbonization process whereby the heat is evenly distributed to the whole biomass inside the reactor. The yield from this stage is of pale gray color consisting of CO, CO2, acetic acid, and methanol, all of which are nevertheless of low quantity to be of use. Thirdly, the derived biomass is then converted into charcoal. The temperature range at this stage is approximately 280e400 C, the range at which biomass self-decomposes through the exothermic reaction. Celluloses in the biomass rapidly decompose at 280 C and their yield is of white and yellow colors with pungent smell and gives high quality wood vinegar if condensed. Then, lignin decomposes at approximate temperatures of 310e 400 C. Beyond 400 C biomass is entirely converted into charcoal. Finally, charcoal quality is improved at this final stage by removing tar. Although biomass becomes charcoal after approximately 400 C, high quantity of tar remains. Tar causes the charcoal to be of low quality and, once burned, changes into benzopyrene and dibenzanthracene, both of which are carcinogenic. Therefore, charcoal is dried at 500e600 C for a period of time to remove tar [4].
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The carbonization process is normally employed to convert agricultural biomass waste into biofuels, i.e., solid (charcoal), liquid (bio-liquid, wood vinegar, pyroligneous acid), and syngas [5]. Charcoal can be made into charcoal briquettes [6]; or induced to obtain activated carbon [7e9]; wood vinegar can be used as a substitute for the chemical pesticides [10]; and syngas can be used as a fuel in the internal combustion engines or in the electrical production [11]. Therefore, biomass waste and agricultural waste processed to add value would benefit the economy, society, and environment [12]. In addition, the employment of biomass waste materials from agriculture could be an answer to the quest for a sustainable source of renewable energy. The carbonization process of biomass waste from agriculture is a suitable, cost-effective technology for charcoal production in Thailand because the process adds value to the biomass raw materials which are inexpensive and ubiquitous. Furthermore, the process is effective in solving the problem of biomass waste from agriculture and can be suitably and economically employed by villagers in remote areas to produce electricity for local consumption. There have been many developments of new technologies to produce charcoal from biomass. Moreover, several countries, such as European countries, Australia, Brazil and the United States of America [13], not merely promote the utilization of biomass to produce energy but also encourage the use of energy from biomass. The carbonization process (i.e., charcoal production) has evolved since its inception hundreds of years ago to include multiple loads, new designs, etc. The existing carbonization processes of today can be categorized into two types: batch (e.g., pits, earth mound, brick and metal kilns) and continuous (e.g., drum type pyrolysers, screw type pyrolysers, and rotary kilns) [14]. The current charcoal production methods have changed little from the traditional methods. Both production types are nevertheless plagued with many problems, some of which are energy wastage for both types and time loss and environmental unfriendliness for batch process, all of which hinder the industrialization of carbonization [5,14]. In both processes, the conventional heating method is employed whereby the exterior of the reactors is heated and then heat is transferred to biomass inside. The heat transforms the biomass inside the reactors into biofuels, depending upon the size of the biomass and process conditions (i.e., temperature, heating rate and residence time). On the contrary, an alternative efficient heating method, microwave heating, unlike the conventional heating method, causes the heat to originate from inside of materials and migrate outward. Recent research studies on the thermochemical process of biomass with the application of microwave heating have been reported and the research results suggest that this heating method is suited to distributed conversion of large biomass particles [15e21]. The microwave carbonization of biomass is one of the novel thermochemical technologies in which biomass is irradiated with microwave. The microwave carbonization process offers several advantages over the traditional carbonization process, some of which include uniform internal heating of large biomass particles, ease of control, and saving of time and heat energy. As such, the microwave carbonization is therefore adopted by industries. The objective of this research is to build a pilot scale continuous carbonization system of biomass using microwave heating technology to produce charcoal and wood vinegar with uncondensed gases as byproducts of the process. The organization of this paper is as follows: Section 1 deals with the introduction and Section 2 details the design of the continuous microwave biomass carbonization process. The fabrication of the process is addressed in Section 3. Experiment and results are presented in Section 4 while the conclusion is provided in Section 5.
2. Design of continuous microwave biomass carbonization system 2.1. Continuous microwave biomass carbonization system Pulverized biomass, as large biomass particles, is conveyed by the bucket elevator to the hoper at the top to dry. A 0.847 m3 cylindrical low cement castable reactor fixed with 10 microwave magnetrons is employed to carbonize biomass. The charcoal yield is conveyed out from the continuous microwave biomass carbonization system by a screw-conveyer and the gas yield is transported by a pipe on top of the reactor to a condensation unit in which wood vinegar is condensed. Meanwhile, uncondensed gases are transferred to a gas treatment unit to purify so that the treated gases are suitable for use to produce electricity. To have a better picture of the carbonization process, the conversion pathway of the continuous microwave biomass carbonization system is presented in Fig. 1. The process begins with biomass waste as input and ends with three forms of product yields, i.e., charcoal, wood vinegar (bio-oils), and fuel gas. Fig. 2 shows the schematic of a full-scale microwave carbonization system. 2.2. Multi-feed microwave cavity The prototype multi-feed microwave cavity, is designed to heat the reactor of the carbonization process as shown in Fig. 3. Typical carbonization temperatures for most types of biomass are 200e600 C. Design of the multi-feed microwave cavity must allow the heating rate to be optimally controlled and thereby improve heating uniformity [22e25]. Therefore, multi-mode microwave radiation is employed in this work to heat the multi-feed microwave cavity as shown in Fig. 3. The 10 rectangular-shaped openings are drilled around the cavity with each opening fixed with a microwave source each consisting of a magnetron and waveguide. The microwave sources are installed in two parallel levels of five sources for each level. At the center of the cavity vertically lies a cement castable reactor through which the biomass to be reacted is passed. The reactor is made of cement to allow penetration by microwave radiation. In this experimental research a heating system using microwave without stirrer mechanism in which the heat can evenly distribute throughout biomass or load is designed. The absence of stirrer mechanism reduces the complexity of the design and construction of the system. The microwave reactor however disallows the installation of a stirrer because the latter would bounce off the electromagnetic wave, causing uneven distribution of heat inside the reactor. 3. Fabrication of continuous microwave biomass carbonization system Fig. 4 shows the snapshot of the continuous microwave biomass carbonization plant used in this study. The plant is 7 m in height and covers an area of 25 square meters. 3.1. Biomass feeding system In the feeding system, pulverized biomass are conveyed by the bucket elevator to the hoper at the top to dry. After drying, dried biomass waste is fed down to the carbonization reactor through the cylindrical steel pipe of 0.30 m internal diameter, which controls via two pneumatic stainless steel trays the amounts of dried biomass fed into the reactor and prevents gas leakage from the reactor.
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Fig. 1. Biomass carbonization conversion pathway.
3.2. Carbonization reactor (low cement castable reactor) The carbonization reactor in this research is made up of a cylindrical low cement castable with organic fiber of alkaline resistance and maximum service temperature of 1400 C. The reactor has 0.30 m in internal diameter, 0.07 m in thickness, 0.9 m in height, 0.85 m3 in volume of reaction. The top part of the reactor is connected to the biomass feeding pipe while the bottom part to a charcoal temperature reduction unit with all the joints sealed by ceramic fiber to prevent a leakage of gases. A hole is bored in the top part of the reactor and a 2-inch steel pipe is fitted to the hole as a passage of carbonization gas yields, the flow rate of which is controlled by a blower.
with microwave. The middle layer is a multi-feed microwave cavity comprising two rows of five magnetrons on each row (totaling 10). The magnetrons to heat the biomass are of OM75S-020 model of Samsung with 2.458 MHz frequency and 0.85 kW each for a combined maximum of 8.5 kW. The outermost layer is installed with 10 cooling fans to transport the heat loss from the rear of the microwave sources to the hopper to pre-heat the biomass prior to passing down through the low cement castable reactor. The shutters on the top and bottom of the castable reactor are to prevent the leakage of microwave radiation. Fig. 5 is the snapshot of the actual heating system of the carbonization reactor. Each microwave source of the multi-feed microwave cavity is controlled for uniform heat distribution by 10 solid state 10 A/220 V relays which are in turn controlled by the Programmable Logic Controller (PLC).
3.3. Heating system of carbonization reactor 3.4. Charcoal temperature reduction unit The heating system of the carbonization reactor as shown in Fig. 3a. consists of: (1) the innermost low cement castable reactor, (2) the microwave sources in the middle, and (3) the outermost layer with cooling fans. The low cement castable reactor is the central cylindrical passage through which the biomass passes and reacts
The unit consists of two stainless steel cylindrical hollow tubes of different sizes with the smaller tube placed inside at the center of the larger one. Cool water from cooling tower 1, entering from the inlets at the bottom and discharged through the outlets at the top, is
Fig. 2. Full-scale continuous microwave biomass carbonization system.
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Fig. 3. Frontal view cross section of a partial of carbonization process (3.a); Isometric view of a partial of carbonization process (3.b).
circulated inside the hollow parts of both tubes. The solid yield from the reactor is then passed through both cylindrical tubes to reduce the temperature. On the top part of the charcoal temperature reduction unit is a thermocouple type K (T2) as shown in Fig. 1 to measure the temperature of the solid yield from the reactor. Meanwhile, at the bottom part of the unit is the same type of thermocouple (T3) as seen in Fig. 1 to measure the solid yield temperature after passing through the charcoal temperature reduction unit. Afterward, the obtained solid yield (i.e., charcoal) is transported by the screw conveyer to the storage container as shown in Fig. 1.
to ensure that the fuel gases are sufficiently clean for use in the engine-generator system of this paper. 3.7. Engine e generator system The engine-generator system which produces 10 kW, 220 V, 50 Hz of electricity to operate the entire carbonization process is fueled by either gasoline in case of the first run of the process or the remaining fuel gas from previous carbonization in case of the subsequent runs. The system is able to generate sufficient electricity to meet the requirement of the whole carbonization process.
3.5. Condensing unit 3.8. Control unit As shown in Fig. 1, the condensing unit in this study is of the vertical shell and tube condenser type with cooling water as heat exchanger. The unit consists of 120 tubes, each with 0.038 m in diameter and 1.15 m in height. Cool water from cooling tower 2, entering from the upper inlets and released through the lower outlets, is circulated inside the hollow parts of all 25 tubes of the condensing unit. The gas yields from the reactor, passing through the condensing unit, are condensed to wood vinegar.
All operating system is controlled automatically by a PLC. The process temperature can be monitored with the temperature controller in the control panel and seven K-type thermocouples are used. Temperature readings in the reactor and at various points in the system are assigned by T1eT5 in Fig. 1, in which T1eT3 denote the temperatures inside the reactor, T4 condensation system, and T5 the gas system. The functioning and status of the system are shown on a touch screen display.
3.6. Gas treatment unit 4. Experiment and results The uncondensed gases leaving the condensing unit are treated with water spray and then rock bed filter inside the gas treatment unit to remove moisture and tar. The two-stage filtering is required
Coconut shells, as large biomass particles, cut into small chips of 2e4 cm in size as shown in Fig. 6 were used in this experiment as
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Fig. 6. Coconut shell chips of 2e4 cm in size used as the raw material.
Fig. 4. A snapshot of the continuous microwave biomass carbonization plant.
the raw material. Coconut shells of 350 kg/h at 50% mc were fed into this system. The carbonization process starts with filling the reactor with the dried coconut shells and then the coconut shells were heated up by all ten microwave radiation sources until the reactor temperature (T1) reached 500 C. This period is called the startup period and the temperatures vs time characteristics of this period are presented in Fig. 7. After the temperature reached 500 C, it entered the operating period during which the charcoal output yields of the microwaved and then cooled down coconut shells were transported by the screw conveyer to the storage containers. The temperature was maintained at 500 C until the end of
Fig. 5. A snapshot of the heating system of the carbonization reactor.
the operating period by automatic on/off cycle of each microwave source. This cycle was controlled by the PLC which switched on (off) the microwave sources if the actual temperature dropped below (reached) the reference temperature of 500 C. The operating time vs temperature characteristics are presented in Fig. 8. Table 1 shows the optimal configuration of process parameters with the duration of continuous actual startup and operation for 7 days. To prolong system lifetime, the operation needs 1 day for maintenance before starting the new running. The electrical energy consumptions of process in the startup and operation period are 8.8 kW and 9.6 kW respectively. In case of the system far from a power distribution grid, the startup period is used electrical power from the gasoline engine e generator system which can be provided the electrical power to start the system. After running for several hours, the system can also generate fuel gases which can feedback them to the generator. The system can be run by alternative choices for providing inputs to the generators. The analysis results by Oxygen Bomb Calorimeter KikaÒ Labortechnik C5000 of the charcoal product yields compared with the
Fig. 7. The temperature vs time characteristics at startup period (temperature T vs heating time t, (recorded every 5 min)), T1 in the center of reactor; T2 at the bottom of reactor; T3 at the top of cooling zone; T4 at the bottom of cooling zone; and T5 in the purified gas pipe.
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550
Table 4 The pH of the wood vinegar from coconut shell sample.
500 T1 T2 T3 T4 T5
450
Temperature (Celsius)
400 350
pH values Wood Vinegar:distilled water Wood vinegar
100:0
1:100
1:200
1:300
1:400
1:500
3.75
4.15
4.19
4.22
4.23
4.23
300 Table 5 The product yield proportions from carbonization process using 58,800 kg of coconut shell sample.
250 200 150 100 50 0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 Time (Minute)
Fig. 8. The temperature vs time characteristics of operating time (temperature T vs heating time t (recorded every 5 min)), T1 in the center of reactor; T2 in the bottom of reactor; T3 in the top of cooling zone; T4 in the bottom of cooling zone; and T5 in the purified gas pipe.
Table 1 The optimal configuration of process parameters at the reference temperature of 500 C. Item
Rate
Screw conveyer speed (rpm) Blower speed (rpm) Water flow rate at cooling tower 1 (cc/s) Water flow rate at cooling tower 2 (cc/s) Pneumatic stainless steel trays (time/h) Average microwave power (kW)
Startup period
Operation period
0 25 5 50 0 8.50
9 25 280 220 12 5
Table 2 Proximate analysis results of coconut shell sample.
Total moisture (wt.%) Volatile mater (wt.%) Fixed carbon content (wt.%) Ash (wt.%)
Coconut shell raw material
Charcoal product yield
10.53 78.30 20.96 0.74
3.18 14.32 81.20 4.48
Product yields
%wt
Yields
Coconut charcoal Wood vinegar, bio-liquid Gases
31 35 34
w10,900 kg w3300 l w11,700 m3
Table 6 The comparison of product yields of this system with other research works. Product yields
Coconut charcoal (%wt)
Wood vinegar, bio-liquid (%wt)
Gases (%wt)
Our experiment Beget et al. [26] Sundaram et al. [27]
31 45 26
35 28 42
34 27 32
raw material (i.e., coconut shells) are presented in Table 2. The heating value of output charcoal yields is 7260 kcal/kg with low value electrical resistance. The chemical components and pH values found in wood vinegar tested by Gas Chromatography-Mass Spectrometry are respectively presented in Tables 3 and 4. From the results, the proportions of the product yields from the carbonization process using 58,800 kg of coconut shells are presented in Table 5. Meanwhile, Table 6 shows a comparison of the product yields produced by this system with those of other research works [26,27]. The photograph of charcoal output yields is shown in Fig. 9, which appears shiny when broken and is of low value electrical resistance. The brown-black color and burned smell wood vinegar output yields are illustrated in Fig. 10. Fuel gas yields from the carbonization were supplied to the engine-generator system. The electrical energy of approximately 10 kW was produced by the engine-generator system, which is sufficient to run the electrical system of the carbonization system.
Table 3 The percentage of the chemical components found in wood vinegar of coconut shell sample. No.
RT/min
ID
Area%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
4.21 4.23 5.89 6.34 7.34 7.41 8.46 9.21 9.68 9.08 10.37 13.05 14.92 15.97 17.02 17.78
Toluene Propanoic acid Furfural 2-furanmethanol 2-cyclopenten-1-one Ethanone Phenol 2-cyclopenten-1-one Phenol, 2-methyl Phenol, 4-methyl Phenol, 2-methoxy 2-methoxy-4-methylphenol Phenol, 4-ethyl-2-methoxy Phenol, 2, 6-dimethoxy Acetic acid 2-butanone
n 1.40 4.80 2.59 0.52 1.13 48.97 2.00 2.15 2.76 8.40 2.72 1.21 7.62 1.39 0.57
Fig. 9. Coconut shell charcoal yields from the carbonization process.
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References
Fig. 10. The vinegar yields from the wood vinegar condensation unit.
5. Conclusion The results of the pilot scale experiment of continuous carbonization system of biomass using microwave heating technology have shown the potential of the proposed system which can reduce the production time and run smoothly. The internal temperature of the proposed reactor can be not only fully controlled but also easy to adjust for various process conditions. Of the obtained product yields, charcoal can substitute fossil fuels, wood vinegar can be used in household and agricultural sectors as insecticides, and fuel gas is to run the engine-generator system to generate the electrical energy adequate to power the electrical system without the need for electric grid, thus much suitable for remote areas without the power grid. The application of this new continuous microwave biomass carbonization system to produce renewable energy would considerably benefit the country economically and socially in general and environmentally in particular due mainly to its lower CO2 emission. Notwithstanding, if the scale of the system were enlarged the excess electrical energy from the larger-scale carbonization system could be resold, thereby generating incomes for the community. Acknowledgments This work was materialized with the financial support from National Innovation Agency, Ministry of Science and Technology, Thailand; and Thailand Research Fund under grant number RGJ e PHD/0131/2551. The authors would also like to express deep appreciation to Assoc. Prof. Dr. Sombat Teekasap for his encouragement throughout this work.
[1] Berndes Goran, Hoogwijk Monique, van den Broek Richard. The contribution of biomass in the future global energy supply e a review of 17 studies. Biomass Bioenergy 2003;25:1e28. [2] Prasertsan S, Sajjakulnukit B. Biomass and biogas energy in Thailand: potential, opportunity and barriers. Renew Energy 2006;31:599e610. [3] Yokoyama Shin-ya, Ogi Tomoko, Nalampoon Anan. Biomass energy potential in Thailand. Biomass Bioenergy 2000;18:405e10. [4] Jeng-Chyan, Lin Muti. Development of a high yield and low cycle time biomass char production system. Fuel ProcessTechnol 2006;87:487e95. [5] Bridgwater AV. Renewable fuels and chemicals by thermal processing of biomass. Chem Eng J 2003;91:87e102. [6] Blesa MJ, Miranda JL, Moliner R, Izquierdo MT, Palacios JM. Low-temperature co-pyrolysis of a low-rank coal and biomass to prepare smokeless fuel briquettes. J Anal Appl Pyrolysis 2003;70:665e77. [7] Ioannidou O, Zabaniotou A. Agricultural residues as precursors for activated carbon production e a review. Renew Sustain Energy Rev 2007;11:1966e2005. [8] Karagoz Selhan, Tay Turgay, Ucar Suat, Erdem Murat. Activated carbons from waste biomass by sulfuric acid activation and their use on methylene blue adsorption. Bioresour Technol 2008;99:6214e22. [9] Encinar JM, Gonzalez JF, Roman S, Martnez G. Pyrolysis of various biomass residues and char utilization for the production of activated carbons. J Anal Appl Pyrolysis 2009;85:134e41. [10] Nami Kartal S, Terzi Evren, Kose Coskun, Hofmeyr John, Imamura Yuji. Efficacy of tar oil recovered during slow pyrolysis of macadamia nut shells. Int Biodeterior Biodegradation 2011;65:369e73. [11] Bain Richard L, Overend Ralph P, Craig Kevin R. Biomass-fired power generation. Fuel Process Technol 1998;54:1e16. [12] Ludwig C, Hellweg S, Stucki S. Municipal solid waste management. Germany: Springer; 2003. [13] Pelaez-Samaniego MR, Garcia Perez M, Cortez LB, Rosillo Calle F, Mesa J. Improvements of Brazilian carbonization industry as part of the creation of a global biomass economy. Renew Sustain Energy Rev 2008;12:1063e86. [14] Hensley Duku Moses, Gu Sai, Ben Hagan Essel. Biochar production potential in Ghana e a review. Renew Sustain Energy Rev 2011;15:3539e51. [15] Appleton TJ, Colder RI, Kingman SW, Lowndes IS, Read AG. Microwave technology for energy-efficient processing of waste. Appl Energy 2005;81:85e113. [16] Menéndez JA, Domínguez A, Inguanzo M, Pis JJ. Microwave-induced pyrolysis of sewage sludge. Water Res 2002;36:3261e4. [17] Menéndez JA, Domínguez A, Inguanzo M, Pis JJ. Microwave pyrolysis of sewage sludge: analysis of the gas fraction. J Anal Appl Pyrolysis 2004;71: 657e67. [18] Oloyede A, Groombridge P. The influence of microwave heating on the mechanical properties of wood. J Mater Process Technol 2000;100:67e73. [19] Salema AA, Ani FN. Microwave induced pyrolysis of oil Palm biomass. Bioresour Technol 2011;102:3388e95. [20] Du Zhenyi, Li Yecong, Wang Xiaoquan, Wan Yiqin, Chen Qin, Wang Chenguang, et al. Microwave-assisted pyrolysis of microalgae for biofuel production. Bioresour Technol 2011;102:4890e6. [21] Zhao Xiqiang, Wang Min, Liu Hongzhen, Li Longzhi, Ma Chunyuan, Song Zhanlong. A microwave reactor for characterization of pyrolyzed biomass. Bioresour Technol 2012;104:673e8. [22] Metaxas AC, Meredith RJ. Industrial microwave heating. London: IEE Power Engineering Series; 1993. [23] Chan TVCT, Reader HC. Understanding microwave heating cavities. Artech House Publishers; 2000. [24] Gupta M, Wong E, Leong W. Microwaves and metals. John Wiley & Sons; 2010. [25] Mehdizadeh M. Microwave/RF applicators and probes for material heating, sensing, and plasma generation, a design guide. Elsevier; 2010. [26] Beget MRA, Mohammad RI, Mohammad UHJ. Pyrolysis of coconut shell for bio-oil. In: Proceedings of the international conference on mechanical engineering 2011. Dhaka: Bangladesh; December 2011. pp. 18e20. [27] Sundaram EG, Natarajan E. Pyrolysis of coconut shell: an experimental investigation. J Eng Res 2009;6(2):33e9.