CET 97 DOI: 10.3303/CET2297014 Paper Received: 28 May 2022; Revised: 10 August 2022; Accepted: 1 September 2022 Please cite this article as: Tan N.T.L., Le N.P.T., Vo T.P., Mai T.P., Nguyen D.Q., 2022, Experiment and Simulation of Rice husk Gasification Process with a Downdraft Gasifier, Chemical Engineering Transactions, 97, 79-84 DOI:10.3303/CET2297014 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 Experiment and Simulation of Rice husk Gasification Process with a Downdraft Gasifier Nhan Tu Le Tana,b, Nguyen Phuc Thien Lea,b, Tan Phat Voa,b, Thanh Phong Maia,b, Dinh Quan Nguyena,b,* aLaboratory of Biofuel and Biomass Research, Faculty of Chemical Engineering, Ho Chi Minh City University of Technology (HCMUT), 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam. bVietnam National University Ho Chi Minh City, Linh Trung Ward, Thu Duc District, Ho Chi Minh City, Vietnam. ndquan@hcmut.edu.vn In this study, the gasification of rice husk in a downdraft process is simulated by Aspen Plus software to predict an appropriate operation condition for designing small-scale equipment. Gibbs models of the reactions occurring in the process are applied for the simulation while tuning the equivalence ratio (ER) to forecast the syngas’ compositions. Based on the analysis, ER value of 0.096 is selected to generate a syngas mixture consisting of CO (43.26 mol%), CH4 (0.09 mol%), and H2 (33.87 mol%), corresponding to the optimal calorific value of 8.92 MJ/kg and efficiency of 86.98 %. A real experiment is performed with the small-scale gasifier to evaluate the results obtained from the simulation. The pilot results show that not only the input air flow rate but other confounding factors also affect the combustion efficiency at some degree, which is compatible with the simulation model. The total energy value acquired from the conversion of 5.0 kg of dry rice husk was 86.82 MJ, while the energy value of the syngas obtained was 44.42 MJ syngas. 1. Introduction The conversion of organic materials and hydrocarbons into syngas is known as gasification. Because of its capacity to generate greater energy sources of syngas and lower environmental effects (smoke, dust, fumes, volatile compounds, and toxic gases), this approach has the potential to replace existing combustion processes. (Mai and Nguyen, 2020). Syngas, including CO, CO2, CH4, and H2, produced from biomass gasification can be optionally purified before being combusted. The combustion of gaseous fuels inherently has higher efficiency than solid matters. That is because the oxidation of a solid object in oxygen/air is gradually happening from its outer surface into the inner layers, which can be described as a heterogeneous process, while a combustive gas like syngas can be burned at a very high mass transfer rate in a homogeneous process (Mai and Nguyen, 2020). There are three main types of equipment, depending on the arrangement of the biomass flow and the gasifier agent that enters the device: updraft, downdraft, and cross draft. This study uses the downdraft device for rice husk gasification, with atmospheric air as the gasification agent. Rice husk is an agricultural by-product of the rice growing process. After the harvest, rice husks are left in the field. Raw materials are abundant and cheap. Rice husk has a small density and low moisture content, which is suitable for combustion. The gasification reaction is generally described in Eq(1) (Mai and Nguyen, 2020): CaHbOcNdSe + O2/air ➔ CO2 + H2O + NxOy + SOz + Heat (1) As a consequence of the aforementioned process, ash and tar are produced. The raw material is rice husk, which is simulated by a non-conventional component consisting of carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S), and ash. Rice husk is fed at the top of the down-draft gasifier crossing reaction zones, including drying (250 °C), pyrolysis (250 – 500 °C), combustion (800 – 1,000 °C), and reduction zone (800 – 600 °C) (Rollinson, 2016) to transform into syngas. The reactions are defined according to the reaction region and the temperature of that region (Saleh and Samad, 2021). 79 This study uses the Aspen plus V.10 software to simulate the gasification process. The Gibbs free energy model is applied to the combustion and gasification regions to simplify the simulation. The equilibrium equations were synthesized and added to the model on Aspen software without adding other kinetic parameters. The reaction was carried out under Gibbs energy conditions suitable for gasification of the feedstock. A tiny gasifier (gasification capacity of 5 kg rice husk/batch) is also set - up in an experiment on a modest production scale. The data is obtained in order to compare the control to the simulation software's result. These reports could provide preliminary assessment of the simulation model's relevance as parameters for the scale-up process and huge production. 2. Materials and methods 2.1 Material Rice husk is collected from Thai My commune, Cu Chi district, Ho Chi Minh City, Vietnam. The material is dried before being brought to the lab and stored in a dry environment. The National Renewable Energy Laboratory methodology is used to analyse the fiber material component (NREL). In Aspen Plus, Rice husk is described by non-conventional composition. In solids process modeling, the elemental components in Table 1 are entered in the software to determine the material. 2.2 Simulation description Aspen plus software is being used to model the gasification process. As shown in Figure 1, this process is split into four processes corresponding to four reaction zones: drying, pyrolysis, combustion, and reduction. Figure 1: Aspen plus flowsheet of the downdraft gasification model Drying zone: the moisture in the raw material is removed by heating it with temperature of 250 °C. Pyrolysis zone: The material is pyrolyzed to diverse low molecular compounds and BIOCHAR - Eq(2) under the influence of high temperature (setting value of 500 °C) (Puig-Gamero et al., 2021). This biochar is split into smaller, non-equivalent components, produced in a mixture of H2, O2, N2, C, and ash. Biomass → aC + bCH4 + cCO + dCO2 + eH2 + fC2H4 + gC2H6 +hC6H6 +iC7H8 + jC6H6O+ kC10H8 + lH2O + BIOCHAR (2) with a, b, c, d, e, f,..,k, l are the numbers of moles C, CH4, CO, CO2, H2, C2H4, C2H6, C6H6, C7H8, C6H6O, C10H8, and H2O, and these are calculated according to the adjustment of moles mass balance (Neves et al., 2011). Combustion zone: the Gibbs free energy model is used to calculate the behavior of the mixture in this zone based on the reaction equilibrium coefficient (Jarungthammachote and Dutta, 2008). Under temperature of 1 000 °C, Oxygen gas in the air will react with intense combustion according to Eq(3), Eq(4) and (Eq5) at the reaction center. The product gases will react with carbon according to Eq(6) and Eq(7) but the reaction is an equilibrium reaction so it is slower. C + 0.5 O2 → CO (partial oxidation of carbon) (3) H2 + 0.5 O2 → H2O (H2 ∙ oxidation) (4) CO + 0.5 O2 → CO2 (CO oxidation) (5) C + CO2 ↔ 2CO (Boudouard reaction) (6) C + 2H2 ↔ CH4 (Hydrogen combustion) (7) 80 Reduction zone: this zone occurs when carbon and carbon dioxide are reduced according to Eq(8) and Eq(9). For this reactions, the Gibbs free energy model is used to describe the process (Barba et al., 2011). C + H2O ↔ CO + H2 (Steam gasification) (8) CO + H2O ↔ CO2 + H2 (Water - gas shift reaction) (9) In this reduction zone, CH4 and H2O (steam) can react with each other according to a reaction called steam methane reforming (SMR) - Eq(10). With the downdraft gasifier when the agent is air, the steam in reduction zone generated consists of the moisture of the raw material and a smaller amount of steam generated in the combustion zone according to reaction 4. Because the Gibbs model will give preference to reactions 8 and 9, the amount of steam supplied is insufficient for SMR reaction. The temperature of the combustion zone and gasification zone is very large, in range 600 – 1,000 °C, the water in the feedstock has been rapidly vaporized and passed rapidly through the device. So in the above gasification model, it will be based on the SMR reaction theory to occur little and can be ignored when simulating the reduction zone. CH4 + H2O ↔ CO + 3H2 (Steam methane reforming reaction) (10) In the SYNGAS flow, the product gas is produced at the end of the conversion process. 2.3 Experiment setup for rice husk gasification by air Figure 2a shows the process flow diagram. Rice husk gasifier in the form of downdraft equipment with a batch yield of 5 kg. Figure 2: (a) Process flow diagram and (b) gasifier of the downdraft pilot-scale fixed-bed gasification. The real gasifier is filled with rice husk (Figure 2b), and the gasification agent (air). The air is controlled by flow through the flowmeter. The syngas gas is cooled to room temperature (about 30 °C) before being analysed for components. In a charcoal drum, ash and burning material are not entirely contained at the bottom of the device. When gas flow is controlled in a flow meter with values of 0.5, 1, 1.5, and 2 Nm3.h-1, the device is run on equivalence ratio (ER) parameters of 0.032, 0.063, 0.095, and 0.127. The gasifying agent being air at a temperature of approximately 30 °C. The syngas composition in this experiment is analyzed with a TESTO 350XL gas meter. 2.4 Calculation formula The equivalence ratio (ER) effect on gasification (Figueroa et al., 2014) is calculated using Eq(11). Super charger Air Flow meter Rotary valve 1 Rotary valve 2 Feed conveyer Syngas Charcoal Charcoal reactor PG T1 FI FI Carry Fan Air cyclone Rice husk BA-V1 Charcoal drum 81 𝐸𝐸𝐸𝐸 = � 𝐴𝐴 𝑌𝑌 � 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 / � 𝐴𝐴 𝑌𝑌 � 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 (11) where: �𝐴𝐴 𝑌𝑌 � 𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 and �𝐴𝐴 𝑌𝑌 � 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 is the amount of air supplied to the actual gasifier and according to the analytical method Shown in Eq(12). �𝐴𝐴 𝑌𝑌 � 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 = �1 + 𝑠𝑠 4 − 𝑧𝑧 2 �� 𝑀𝑀𝑂𝑂2+3.76 𝑀𝑀𝑁𝑁2 𝑀𝑀𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 � (12) where M is the molecular mass (g/mol), the biomass has the molecular formula CxHyOz. The high heating value (HHVbiomass) and the low heating value (LHVbiomass) of biomass are calculated based on a dry ingredient according to Eq(13) and Eq(14) (Kirsanovs and Žandeckis, 2015). HHVbiomass = 339.1Cdr + 1178.3Hdr + 100.5Sdr – 103.4Odr – 15.1Ndr – 21.1 Adr (MJ/kg) (13) LHVbiomass = HHVbiomass (1 - M) - 2.44M (MJ/kg) (14) where Cdr, Hdr, Odr, Sdr, Ndr: the Fuel chemical composition on dry basis, %; Adr: ash content in the Fuel on dry basis, %; hfG: specific heat of vapor; M: the wet basis moisture content, %. The low heating value (LHV) of syngas is the percentage of gas constitutes from our equilibrium model as shown in Eq(15). LHVsyngas = 10.7H2 % + 12.6CO % + 35.8CH4 % (MJ/kg) (15) Cold gas efficiency (CGE) is the percentage of the energy of syngas gas at standard temperature conditions compared to the energy of biomass in theory shown in Eq(16). CGE = (𝜔𝜔.LHVsyngas/.LHVbiomass).100 (%) (16) where 𝜔𝜔 is the weight of syngas production (including the weights of the gases CO, CO2, and H2) per the dry biomass. 3. Results and discussion 3.1 Components of rice husk The elements C, H, N, O, and S are found in the rice husk and are listed in Table 1 below. The analysis indicated that rice husk represents a source of energy-seeking raw materials for the gasification process, containing up to 40.94 wt% carbon content. The water component for reactions in the reduction zone to generate H2 and CO is provided by the moisture content of the material, which is 10.6 wt% (Pellegrini and de Oliveira, 2007). Table 1: The characteristic properties of rice husk Item Value Item Value Proximate analysis (wt%) Elemental analysis (wt%) Moisture 10.6 C 40.94 Volatile matter 56.8 H 4.6 Fixed carbon 20.1 N 0.7 Ash 12.5 O 30.6 S 0.06 HHVbiomass (MJ/kg) 14.94 LHVbiomass (MJ/kg) 13.02 3.2 The composition change of syngas according to the ERs ratio on Aspen plus simulation When the ER changes, the syngas components in the product line differ significantly, as shown in Figure 3a. When the ER goes from 0 to 0.127, the CO content increases, indicating that the ER is rising and the CO content is declining. The initial CO2 level is shallow (less than 1.0 mol%), then rapidly increases when the ER is more than 0.127. In the combustion zone, when starting with a low rate of ERs, the amount of O2 gas provided is moderate for reactions (3) and (4) to occur, further raising the ER, implying that more air volume, the amount of oxygen will be more than enough to fulfill all three reactions (3), (4), and (5), leading to a decline in H2 levels and the creation of more CO and CO2. At this point, the reactions will occur in the backward direction to rebalance the amounts 82 of H2, resulting in additional CO2 in reaction (8), and this CO continues to convert into CO2 in reaction (9). The CH4 gas content is low (less than 0.2 mol). In the reaction (7), just a tiny CH4 is released. When the ER rate increases from 0 to 0.096, LHV slowly declines from 9.85 to 8.93 MJ/kg. As the ER increases, LHV decreases quickly, and at 0.253, LHV is only 4.97 MJ/kg (Figure 3b). The change is based on a percentage of syngas gas moles (Including CO, CH4, and H2). When ER is lower than 0.096, the amount of CO rises, and CH4 and H2 decrease slowly, resulting in a subdued reduction in LHV. As the ER rises, the amount of CO2 generated rises, and the amount of N2 (inert component loaded in the air) increases rapidly, reducing the amount of CO, CH4, and H2. As a result, the LHV lowers significantly. The initial ω value is 1.07 kg/kg (ER = 0), which comes to a peak value of 1.26 kg/kg (ER = 0.096), then dropped (Figure 3b). Cold gas efficiency (CGE) reached a peak of 86.98 % at the same ω value. So the optimal ER ratio design would be 0.096, with CO (43.26 mol%), CH4 (0.09 mol%), and H2 as syngas components (33.87 mol%). Figure 3: (a) Effect of ER on syngas composition in the Aspen Plus model, (b) Low heat value (LHV) and syngas production per weight biomass (𝜔𝜔) and (c) Cold gas efficiency of Aspen model and experimental model 3.3 Experimental results for rice husk gasification The simulation model has described the change in gas composition of the real gasifier. In the process of gasification by down-draft gasifier, CO and H2 accounted for the largest amount with CO accounted more than 40 mol% and H2 is more than 30 mol%. Figures 4a and 4c show similarity in CO and H2 concentrations in different ERs of the simulation model and the experiment gasifier. The differences are mostly less than 10 %. The CO2 and CH4 concentrations of the two models have a relatively large difference (probably more than 50 %), and the concentrations of CH4 and CO2 tend to change in the same way with increasing ER (Figure 4b, 4d). Since the concentration in the gas mixture is very small, with CO2 concentrations less than 2 mol%, and CH4 less than 0.5 mol%. The difference in measurement methods is the main cause. The very low concentration of CH4 also supports the original assumption when simulating the gasification process, in the reduction region ignoring the SMR reaction. The highest concentration of CH4 obtained by the real gasifier is 0.49 mol%, so the SMR reaction is not significantly affected until the syngas composition can be ignored to simplify the simulation. The LHV value of the experimental model corresponds to the Aspen model. The difference is less than 7 % at the ERs of 0.032 to 0.095 (Figure 3b). At ER of 0.127, more air supply leads to a rapid increase in N2 content, which reduces the LHV value, which is similar as predicted by the Aspen model. The ω values of the two models differ by about 30 %. Part of the syngas is not collected in the product stream from the real gasifier, so the real gasification efficiency is lower than the simulated model. At ER of 0.096, ω value is 0.89 kg/kg, the highest efficiency is 64.88 % (Aspen Model is 86.98 %) for experiment model (Figure 3c). Figure 4: Comparison of (a) CO, (b) CO2, (c) H2, and (d) CH4 concentrations between ASPEN PLUS and experiment model 0.032 0.063 0.095 0.127 0 20 40 60 C O (m ol % ) Equivalence ratio (ER) (b) (c) 0.032 0.063 0.095 0.127 0 1 2 Exp Aspen C O 2 (m ol % ) Equivalence ratio (ER) Exp Aspen Exp Aspen Exp Aspen 0.032 0.063 0.095 0.127 0 20 40 60 H 2 (m ol % ) Equivalence ratio (ER) 0.032 0.063 0.095 0.127 0.0 0.4 0.8 C H 4 (m ol % ) Equivalence ratio (ER) (a) (d) 0.00 0.05 0.10 0.15 0.20 0.25 2 4 6 8 10 LHVAspen LHVExp ω Aspen ω Exp Equivalence ratio (ER) Lo w h ea t v al ue ( M J/ kg ) (b) 0.5 1.0 1.5 2.0 ω ( kg /k g) 0.00 0.05 0.10 0.15 0.20 0.25 20 40 60 80 100 C ol d ga s ef fic ie nc y (% ) Equivalence ratio (ER) Aspen Exp (c) 0.00 0.05 0.10 0.15 0.20 0.25 0 10 20 30 40 50 CO CO2 H2 CH4 Equivalence ratio (ER) C O , C O 2, H 2 (m ol % ) (a) 0.00 0.05 0.10 0.15 0.20 C H 4 (m ol % ) 83 Energy balance is also a part of checking energy flow (Table 2). Calculating total balanced energy based on the law of energy conservation, described according to Eq(17). Input energy [Erice husk(66 MJ) + Efan(1.11 MJ)+Econveyer(8.7 MJ) + Esuper-Charger(4 MJ) + Echarcoal(6.97 MJ) + Eair(0.04 MJ)] = Output energy [(Esyngas(41.42 MJ) + Eash(0.71 MJ) + Ebiomass loss(0.73 MJ) + EHeat loss(40.92 MJ)]= 86.82 MJ (17) Table 2: Material and energy balance for the experimental model. Stream/ Device Input Stream/ Device Output Weight (kg) Energy (MJ) Weight (kg) Energy (MJ) Rice husk 5.53 66.01 Syngas 4.63 44.42 Air 3.78 0.04 Ash 0.84 0.71 Fan - 1.11 Inter gas 3.40 0.03 Coveyer - 8.70 Biomass loss 0.06 0.74 Super-charger - 4.00 Heat loss - 40.92 Charcoal - 6.97 - - - 4. Conclusions The component analysis study indicated that rice husks are a potential biomaterial for syngas production. Down- draft gasifier simulation on Aspen plus, with ER of 0.096, syngas gas in the product line with the LHV of 8.92 MJ/kg, the ω value of 1.27 kg/kg, CGE peaking at 86.98 %. 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