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Thermodynamics Simulation Performance of Rice Husk Combustion with a Realistic Decomposition Approach on the Devolatilization Stage

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Thermodynamics Simulation Performance of Rice Husk Combustion with a Realistic Decomposition Approach on the Devolatilization Stage Thermodynamics simulation performance of rice husk combustion with a realistic decomposition approach on the devolatilization stage Soen Steven Institut Teknologi Bandung Pandit Hernowo Institut Teknologi Bandung Elvi Restiawaty Institut Teknologi Bandung Anton Irawan Universitas Sultan Ageng Tirtayasa Carolus Borromeus Rasrendra Institut Teknologi Bandung Yazid Bindar ( [email protected] ) Institut Teknologi Bandung Research Article Keywords: Biomass, Degree of freedom, Flue gas, Adiabatic ame temperature, Air to fuel ratio Posted Date: April 14th, 2021 DOI: https://doi.org/10.21203/rs.3.rs-359990/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Thermodynamics simulation performance of rice husk combustion with a realistic decomposition approach on the devolatilization stage Soen Steven1, Pandit Hernowo1, Elvi Restiawaty1,2, Anton Irawan5, Carolus Borromeus Rasrendra1,3, and Yazid Bindar1,4* 1Department of Chemical Engineering 2Research Group on Design and Development of Chemical Engineering Processes 3Center for Catalysis and Reaction and Engineering 4Research Group on Biomass and Food Processing Technology Faculty of Industrial Technology, Institut Teknologi Bandung, Jl. Ganesha 10, Bandung 40132, Indonesia 5Chemical Engineering Department, Faculty of Engineering, Universitas Sultan Ageng Tirtayasa, Jl. Jendral Soedirman km 3 Cilegon, Banten, 42135, Indonesia * Corresponding author: [email protected] Abstract Rice husk has a great potential in its calorific value and silica content in ash which makes its valorization through combustion becomes important and interesting. This study presents the thermodynamics simulation performance of rice husk combustion using a realistic decomposition approach. A non-ideal gas approach and fugacity coefficient were also considered in the calculation. From this study, rice husk is devolatilized to form gases (63.37%), tar (8.69%), char (27.94%), and all of these are then oxidized to form flue gas. The realistic decomposition approach calculated that about 2.6 MJ/kg of specific combustion energy is produced, the maximum combustion temperature is up to 1457oC for perfect insulation condition, and up to 1400oC if there is a heat loss. It is found that combustion equipped with larger excess air could quench the heat produced and reduce the combustion efficiency but could maintain the temperature at 700oC. Furthermore, the thermodynamics simulation expressed that NO emission amount from rice husk combustion is negligible and there is still a probability for CO o and H2 to be produced at above 500 C due to Boudouard reaction and homogeneous water gas shift reaction (WGSR). The study showed that a realistic decomposition approach could predict the rice husk combustion performance with a reasonable and logical result. Supplying excess air of about 180-200% is advantageous to keep the combustion temperature at 700oC in order to prevent silica crystalline formation which harms human 1 health, as well as suppressing NO emission and reducing CO emission from the simultaneous Boudouard and WGSR. Keywords: Biomass, Degree of freedom, Flue gas, Adiabatic flame temperature, Air to fuel ratio Statement of Novelty So far, many established simulations have proposed a simple approach to the devolatilization stage. This is unrealistic because assumes that biomass will decompose to C, H, O, N elements before entering oxidation. In the realistic decomposition approach, biomass is devolatilized not to form C, H, O, N elements but light gases, liquid as tar, and solid as char. Interestingly, there is still a lack of study on it and therefore, This study discussed the rice husk combustion simulation performance with a realistic decomposition approach on the devolatilization stage. The results perform that this approach gives a reasonable agreement with the other researchers’ results in terms of specific combustion energy, maximal combustion temperature, and flue gas composition analysis. 1. Introduction Recently, biomass utilization as an alternative fossil fuel and chemicals surrogates has become a major concern. Rice husk is one of the abundant biomass in Indonesia which occupies approximately 20%-wt from paddy, consists of nearly 20%-wt rice husk ash, contains silica as much as 87%-wt from its ash, and has a calorific value of 14-17 MJ/kg [1–3]. The high calorific value, as well as rich silica in ash, make rice husk important and interesting to be valorized. There are several stages occurred in the rice husk combustion, that is drying, devolatilization, oxidation, and ash formation [1, 4, 5]. So far, energy production through rice husk combustion mostly relied on direct experiments and then validate the results with thermodynamics simulation. They have focused on specific combustion energy calculation, rice husk combustion temperature monitoring, and flue gas composition prediction and analysis. A study on combined heat power fuelled by rice husk with a maximal temperature of about 1100oC gave a specific rice husk consumption of 1.2-1.7 kg/kWh [6] or recalculated as specific energy produced in the range of 2.12- 3.00 MJ/kg. Apart from that, the case study from Pakistan stated that as much as 4947.28 MWh of energy could be generated from 6432 tonnes of rice husk combustion [7] or equal to specific combustion energy of about 2.77 MJ/kg. The energy balance evaluation on 2.8 g/s rice husk combustion for steam and hot air generation resulted 2 in 21.09 kJ/s of utilized energy [8] and it is equal to 7.53 MJ/kg of specific energy. Meanwhile, the rice husk combustion modelling study in a grate bed furnace had a maximum combustion temperature of 1406.85oC and o flue gas exhaust temperature of 693.85 C with the composition of N2, CO, CO2, H2O, and remaining O2 [9]. The thermodynamics study from the research papers and well-established textbooks mostly proposed that at the devolatilization stage, biomass will decompose to C, H, O, N elements prior to being oxidized to form flue gas [10–12]. This approach is simple but quite unrealistic so that it should be developed with a more realistic one. Realistic decomposition assumes that rice husk devolatilization products are not C, H, O, N elements but gases, tar, and char [10]. Nevertheless, there is still a lack of study on it and therefore, this study intends to discuss the thermodynamics simulation performance of rice husk combustion with a realistic decomposition approach. The discussion comprises of problem preliminary analysis, devolatilized products result, specific energy produced from rice husk combustion, adiabatic flame temperature, air supply strategy for maintaining rice husk combustion temperature, and the flue gas composition analysis due to the attendance of Boudouard reaction and homogeneous water gas shift reaction (WGSR). 2. Methods 2.1. Devolatilized products prediction calculation In this study, rice husk has a calorific value of 14.95 MJ/kg. The ultimate and proximate analysis results are given in Table 1. The proximate analysis procedure is performed at 900oC follows ASTM 3175-07 procedure, whereas the combustion occurred at = 700oC. 푇Table 1. Ultimate and proximate analyses of rice husk Ultimate Analysis (Dry Basis) Proximate Analysis (As-received) C 37.95% Moisture 8.98% H 5.41% Volatile Matter 58.13% O 36.56% Fixed Carbon 15.49% N 0.96% Ash 17.40% A volatile enhancement factor , which is commonly found in 0.8-1.3, plays a role to correct the devolatilized product at a temperature beyond푉퐸 900oC [10, 11]. To determine , the biomass type number ( ) should be calculated first as in equation 1a. The value is then applied to푉 퐸determine the predicted volatile푁퐶푇 matter value ( ), equation 1b. Subsequently, equation푁퐶푇 1c is a temperature correction function, in oC unit, which is later ′ employed푌푉푀 in equation 1d to obtain the predicted devolatilized products mass fraction푓5(푇) ( ). Afterward, is ′ calculated following equation 1e [10, 11]. Rice husk devolatilization produces gases, tar,푌 푉푌and char. Gases 푉were퐸 3 represented with CO, H2, CO2, CH4, HCN, and H2O. Tar was assumed to consist of C16H18, C10H14O2, C7H8O, and C9H7N. Char was presumed to contain only C. Each product amount is determined from equation 2 and should consistent with the value from dry-ash-free basis of proximate and ultimate analysis [10, 11]. (1a) 2 퐶,푢푙푡 퐶푇 푌 푁 = 퐻,푢푙푡 푂,푢푙푡 (1b) ′ 푌 . 푌 2 푉푀 퐶푇 퐶푇 (1c) 푌 = 푒푥푝[0.031 −5 0.029 푙푛 푁 − 0.4038(푙푛 푁 ) ] 3 2 푇 푇 푇 푇 푇 푓5(푇) = 3.87 ( ) − 28.28 ( ) + 80.87 ( ) − 112.8 ( ) + 77.07 ( ) − 20.26 (1d) ′ ′ 900 900 900 900 900 푉푌 푉푀 5 (1e) 푌 = 푌′ . 푒푥푝[푓 (푇)] 푌푉푌 푌푉푌 푉퐸 = ′ = 푌푉푀 푌푉푀 (2a) 푐ℎ푎푟 푉푌 푖 푌 = 1 − 푌 = 1 − ∑ 푌 (2b) 푌퐶,푖. 퐴푟퐶 푌퐶,푢푙푡 = 푌푐ℎ푎푟 + ∑ ( ) 푀푟푖 (2c) 푌퐻,푖. 퐴푟퐻 푌퐻,푢푙푡 = ∑ ( ) 푀푟푖 (2d) 푌푂,푖. 퐴푟푂 푌푂,푢푙푡 = ∑ ( ) 푀푟푖 (2e) 푌푁,푖. 퐴푟푁 푁,푢푙푡 푌 = ∑ ( 푖 ) where is the푀푟 actual devolatilized products mass fraction, is the volatile matter value from proximate analysis푌 푉푌on dry-ash-free basis, is the ith component of actual 푌devolatilized푉푀 products mass fraction, is the char fraction, is the number푌 푖of C atoms in ith component, is the number of H atoms in ith component,푌푐ℎ푎푟 퐶,푖 퐻,푖 푂,푖 is the number 푌of O atoms in ith component, is the number푌 of N atoms in ith component, is the C-H-O-푌N 푁,푖 푢푙푡 content ultimate analysis in dry-ash-free basis,푌 is the atomic weight for C-H-O-N, and 푌 is the molecular weight of ith component. 퐴푟 푀푟푖 In the rice husk combustor, all devolatilized products react with oxygen from the air. The air amount was varied under stoichiometric and excess conditions. The flue gas temperature was also varied from 200-700oC. The excess air ( ) is calculated from equation 3 and the O2 and CO2 percentages in the flue gas are acquired from equation 4, respectively.퐸퐴 (3) 푛푎 − 푛푠 퐸퐴 = 푥 100% 푛푠 (4a) 푛푂2,푎 − 푛푂2,푠 푂2 푖푛 푓푙푢푒 푔푎푠 = 푥 100% 푛푓푔 (4b) 푛퐶푂2 퐶푂2 푖푛 푓푙푢푒 푔푎푠 = 푥 100% 푛푓푔 4 where is the actual air amount; is the stoichiometric air amount; is the actual oxygen amount; is 푎 푠 푂2,푎 푂2,푠 the stoichiometric푛 oxygen amount;푛 and is the carbon dioxide amount;푛 and is the flue gas amount.푛 퐶푂2 푓푔 푛 푛 2.2.
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