Flowsheet Modelling of Biomass Steam Gasification System with CO 2 Capture for Hydrogen Production

Transcription

1 ISBN Proceedings of International Conference on Advances in Renewable Energy Technologies (ICARET 2010) 6-7 July 2010, Putrajaya, Malaysia ICARET Flowsheet Modelling of Biomass Steam Gasification System with CO 2 Capture for Hydrogen Production Abrar Inayat, Murni M Ahmad*, M I Abdul Mutalib, Suzana Yusup Department of Chemical Engineering Universiti Teknologi PETRONAS, Bandar Seri Iskandar Tronoh, Perak, Malaysia * murnim@petronas.com.my Abstract There exists high potential for hydrogen production in Malaysia from biomass due to abundant agriculture waste. Biomass steam gasification with in situ carbon dioxide capture has good prospects for the production of hydrogen rich gas. This work focuses on the mathematical modeling of the flowsheet design for hydrogen production from biomass via steam gasification with in situ carbon dioxide absorption by CaO, carried out using MATLAB. The effects of temperature, steam/biomass ratio and sorbent on the purity and yield of hydrogen in the product gas stream are predicted using the model. Based on the results, the maximum hydrogen purity predicted is 0.81 mole fraction at 950 K at outlet of the gasifier unit and it can be enhanced to % using a scrubber and a pressure swing adsorption unit. At 950 K with steam/biomass ratio 3.0 and sorbent/biomass ratio, the hydrogen yield obtained g/kg of biomass. Between the temperature range of 800 to 1300 K, hydrogen yield is predicted to increase from 76.5 to 97.3 g/kg of biomass. It is observed that the increase in hydrogen yield is larger when increasing the steam/biomass ratio compared to when increasing temperature, within the selected ranges. The mass conversion efficiency (MCE) showed linear co relation with temperature. The results are compared with the literature and show good agreement. Keywords-hydrogen; biomass; flowsheet; modelling; I. INTRODUCTION Due to the energy crises of the fossil fuel and environmental problems the production of hydrogen as a clean and sustainable fuel is now attractive [1]. Biomass gasification research is recently increasing attention as renewable energy source for the hydrogen production [2]. In 2006 the hydrogen world demand was calculated 50 MT/year with 10% expansion yearly [3]. The potential for hydrogen production from biomass in Malaysia is logical due to the abundance of biomass available estimated at t th -1 y -1 [4-5]. Different gasification agents used for biomass gasification, such as air-steam, oxygen steam and pure steam [6-7]. The use of pure steam as gasification agent is not only in favor of more hydrogen but also economical than other conventional gasifying agents and pyrolysis [8-11]. Furthermore, hydrogen can be increased in the product gas by integrating it with CO 2 capture step using CaO as sorbent [12]. There were several research works have been reported based on experimental and modeling approach applying CO 2 capture using CaO in air-steam and steam gasification process. Initially, Mahishi et al. [13] performed an experimental work using CaO as sorbent with pure steam in a micro reactor. They predicted hydrogen concentration of 66 vol % in the product gas. They argued on the dual role of the CaO as sorbent and catalyst, as the important factor leading to higher hydrogen production. Acharya et al. [14] investigated hydrogen production through steam gasification of biomass in presence of CaO. They reported the hydrogen concentration more than % based on experimental work at steam/biomass of 0.83, CaO/biomass of 2.0 and temperature 670 C. In line with the above findings, Florin et al. [15] developed a thermodynamic equilibrium model for hydrogen production from biomass coupled with CO 2 capture step in a dual fluidized bed gasifier. They investigated the influence of temperature, pressure, steam/biomass and sorbent/biomass ratios on hydrogen concentration. Using the modeling results, they predicted that hydrogen concentration could be increased from 50 to 80 vol% in the product gas by using CaO as sorbent. There was another equilibrium model reported for steam gasification with CO 2 adsorption using CaO as sorbent implemented in the ASPEN PLUS process simulator [16]. Using gasification integrated with absorption system and gas cleaning unit, they predicted that concentration of hydrogen increased by 19% compare to conventional gasification process. Abu-Zahra et al. [17] presented a new concept of integrated process for hydrogen production. Using syngas as a feed stock, simulation result shows 95% hydrogen in product gas. They designed flowsheet with water gas shift reactor, scrubber and membrane separation unit. Emun et al. [18] developed a simplified flowsheet model but for coal gasification using gasifier, gas cleaning and cooling units applied in ASPEN PLUS process simulator. Few authors also proved experimentally and through modeling results that CO 2 capture step is in favor of more hydrogen production [19-20] The objective of the present work is to develop a simplified process for enriched hydrogen production from biomass in Malaysia. The effect of process parameters i.e. temperature, steam/biomass ratio and addition of CaO on hydrogen concentration and yield in the steam gasification process with CO 2 capture was also studied. The flowsheet model incorporates the gasification, adsorption kinetics model and material balance. The flowsheet model

2 incorporates the gasification and adsorption kinetics models and material balances. The developed model is used as a platform to investigate the feasible operating conditions for the production of hydrogen rich gas from biomass using a single-pass fluidized bed gasifier. This study has been carried out for single pass fluidized bed gasifier using MATLAB. II. TECHNICAL APPROACH A. Process Devalopment A simplified process has been developed for enriched hydrogen gas production from biomass using pure steam as gasification agent and CaO as CO 2 sorbent. The block diagram of the process is shown in Fig 1. The whole process is consists of four sections, feed treatment, steam generation, gasification and gas cleaning section. The detail of each section is described in next headings. The process flow diagram (PFD) is shown in Fig 2. Figure 1. Block diagram of the process. The operating conditions and process parameters for the flowsheet modeling are assumed, which are also close to many commercial and research scale biomass gasification processes [12, 16, 21-23]. The assumptions are as follows: Biomass feed rate: 72 g/hr. Temperature range: 800 to 1300 K Steam/biomass ratio range: 1 to 3.5 for hydrogen purity and from 2 to 5 for hydrogen yield Sorbent/biomass ratio: 1.0 for both hydrogen purity and hydrogen yield profiles. B. Feed Treatment Pretreatment of biomass for gasifier is generally consisting of drying and size reduction. Drying used to remove the moisture from the biomass either from flue gases or by steam but steam drying is preferred due to very low emissions and safer [24]. Usually drying removes the moisture contents from % in the biomass [25]. The best condition of biomass for fluidized bed gasifier is that the biomass must well grind as well [26]. So to achieve such best condition for biomass feed to gasifier a dryer and ball mill used to remove moisture from the biomass and fine grinding respectively shown in Fig 2. C. Steam Generation The process design includes a steam generation system that produced steam by a general steam generator. Furthermore steam is super heated until 523 K by super steam heaters. The steam is supplied to the gasifier at atmospheric pressure. The steam generation system is also shown in fig 2. D. Gasification The conversion of biomass to hydrogen takes place in single pass fluidized bed gasifier through steam gasification process integrated with CO 2 capture. There are few assumptions were considered in flowsheet development modeling for gasification process are as follows. The gasifier operates under steady state conditions and atmospheric pressure. The reactions proceed adiabatically and at constant volume. There is no tar formation in this process. In the modeling framework, biomass is assumed as char and six major reactions [6-7, 21], given in Table I, are assumed to occur in the gasifier. The base reaction kinetic models along with validation and the preliminary results on the effect of different variables on the product gas compositions are presented in an earlier work [27]. The total moles and the moles of hydrogen in product gas are calculated using the kinetics model [27]. The mole fraction of hydrogen in product gas calculated from equation (1). Mass and energy balances calculated by the equations (2) and (3) respectively [28]. (1) Figure 2. Block diagram of the process.

3 = (2) = K the hydrogen starts decreases. This observation can be explained due to the exothermic and reversible behavior of water gas shift reaction. Along with water gas shift reaction the carbonation reaction also becomes slower due to highly exothermic behavior. (3) Where mi is the inlet mass (g), mo is the out mass (g) and E is the energy flowrate (kj/h). The variation in the hydrogen yield can be used to investigate the effect of temperature and steam/biomass ratio on the hydrogen production from biomass steam gasification. The definition of hydrogen yield is defined using equation (4) [21]. = ℎ ( ) (4) ℎ ( )/ The mass conversion efficiency (MCE) is one of the mass performance parameter. The MCE of gasifier is calculated by using equation (5) [29]. (%) = TABLE I. No ) ) 100 ( ( (5) RECTIONS INVOLVED IN PROCESS [6-7, 21] Name Char Gasification Methanation Boudouard Methane Reforming Water Gas Shift Carbonation Reaction C + H2O CO + H2 C + 2H2 CH4 C+ CO2 2C CH4 + H2O CO + 3H2 CO + H2O CO2 + H2 CO2 + CaO CaCO3 H (kj/mol) E. Gas Cleaning The product gas produced by the gasification process contained hydrogen, carbon monoxide, carbon dioxide, methane, steam and fly ash. To get pure hydrogen as end product, there were several steps involved in product gas cleaning with different units like filter, scrubber and pressure swing adsorption as shown in Fig 2. Fly ash was removed from the system by filter. It is assumed that the product gas contained 13% fly ash of biomass feed rate [30]. Furthermore the steam was removed by passing through scrubber with fresh water [31]. Along with the steam there are also some others product gases will be also absorbs in water which was calculated by chart of solubility of gases in water at atmospheric pressure and different temperature [32]. The scrubber is also used to cool down the product gas. Finally pressure swing adsorption (PSA) unit applies to get pure hydrogen (99.99%). As the advantages of PSA that it remove the impurities at any level and produced high purity hydrogen as product [33]. III. RESULTS AND DISCUSSION A. Effect of Variabales on Hydrogen Purity Temperature is one of the important variables in biomass fluidized bed gasifier. The effect of temperature on the hydrogen mole fraction versus temperature change from 800 to 1300 K is shown in Fig 3. The figure shows that the hydrogen mole fraction is more than This is might be due to pure steam gasification process along with CO2 capture step in the system. These results can be explained by the Le Chatelier s principle on the endothermic reforming reactions of char and CH4 that are promoted by the increasing temperature. The figure also shows that the maximum hydrogen mole fraction obtained at 950 K. It is also observed that after 950 Figure 3. Effect of temperature on hydrogen mole fraction. Steam/biomass ratio: 3.0, Sorbent/biomass ratio: 1.0. There is another very important variable in steam gasification process i.e. steam/biomass ratio. To study the effect of steam/biomass ratio on hydrogen concentration a three-dimensional surface plot predicted along with effect of temperature shown in Fig 4. The figure shows that with increasing steam/biomass ratio the mole fraction of H2 increases. As steam is the only gasification agent being used, so the reactions involving steam i.e. methane reforming and water gas shift, are highly dependent on steam feed rate. Figure 4. Effect of temperature and steam/biomass ratio on hydrogen mole fraction. Sorbent/biomass ratio: 1.0. It is observed that at 800 K with lower steam/biomass ratio, i.e. 1.0, the hydrogen mole fraction is 0.73, and at high temperature 1300 K with same steam/biomass ratio (1.0), the hydrogen amount is almost 0.80 mole fraction. In addition, the surface plot shows that the highest hydrogen mole fraction achieved is 0.81 mole fraction that occurs at 950 K and at steam/biomass ratio of 3.0. The presence of sorbent (CaO) in system increased the hydrogen mole fraction in product gas by absorbing the CO2 present in the system. The difference of H2 and CO2 mole fraction in product gas by using CaO as sorbent and without CaO is show in Fig 5. Also Fig 5 shows that hydrogen can be increased from 0.65 to 0.83 and CO2 can decreased from 0.31 to 0.09 by using CaO as sorbent. The amount of sorbent influenced a lot on the production of hydrogen, as sorbent

4 used to increase H 2 and decrease CO 2 in product gas composition. Figure 6. Mass balance at gasifier. Temperature: 950 K, Steam/biomass ratio: 3.0, Sorbent/biomass ratio: 1.0. Figure 5. Effect of CaO on hydrogen and carbon dioxide. Temperature: 950 K, Steam/biomass ratio: 3.5, Sorbent/biomass ratio: 1.5. B. Mass Balance of the Process Mass balance calculated to evaluate the process performance. The operating condition for mass balance are selected base on the discussion in previous section i.e. 950 K temperature, 3.0 steam/biomass ratio and 1.0 sorbent/biomass ratio. Fig 6 shows the calculation result of mass balance on gasifier. It is observed that hydrogen yield obtained g/kg of biomass. It is also observed that the feed rate of steam is 216 g/hr and at the out let of gasifier the steam flowrate is 182 g/hr, which shows that only 15 % steam consumed in the gasification reactions. Which also showed that more than 80 % steam used to fluidize the biomass inside the gasifier. Several authors already has been reported that the product gas of gasifier contains more than 60 % of unreacted steam [34]. Fiorenza et al. [35] also reported less than 20 % steam conversion in the fluidized bed gasifier. Corella et al. [36] reported unreacted steam from the outlet of the fluidized bed gasifier as a weakness of the steam gasification process and there is need of more attention to solve this problem. It is also observed that g/hr of CaCO 3 obtained from the gasifier, which can be regenerated. The overall mass balance of the flowsheet is shown in Fig 7. It is assumed that the biomass is pretreated and fed to the gasifier. The figure shows that after the gasifier 9.7 g/hr fly ash removed through filter. Furthermore the steam in the product gas removed through scrubber with fresh water. Mean while very little amount of the H 2, CO and CH 4 also absorbs in the water and exit through scrubber. It is also observed that high amount of CO 2 i.e g/hr also Figure 7. Overall mass balance of flowsheet. Temperature: 950 K, Steam/biomass ratio: 3.0, Sorbent/biomass ratio: 1.0.

5 absorbs in water. So the scrubber not only help to remove steam from the system and to cool down the temperature, it also helps to decrease the more amount of CO 2 in the product stream. Finally, the PSA unit separates the rest amount of CO, CO 2 and CH 4 from H 2. The result showed g/hr of pure H2 (99.99 %) at the end of process. Furthermore the effect of Temperature and steam/biomass ratio are also on hydrogen yield is shown in Fig 8. The figure shows that both variables are in favor for hydrogen yield. The Fig 8 shows that at 800 K and steam/biomass ratio of 2.0, hydrogen yield is 78.5 g/kg of biomass. Taking same temperature but with higher amount of steam/biomass ratio i.e. 5.0, hydrogen yield obtained 96 g/kg of biomass. It is observed that the difference due to increase of steam/biomass ratio at same temperature is On the other hand, at high temperature 1300 K and low value of steam/biomass ratio i.e. 2.0, hydrogen yield is 88.5 g/kg of biomass. But at the same temperature (1300 K) with high steam/biomass ratio i.e. 5.0, hydrogen yield is obtained 97 g/kg of biomass. The difference observed in hydrogen yield is 8.5. Figure 9. Effect of temperature on mass conversion efficiency. Steam/biomass ratio: 3.0, Sorbent/biomass ratio: 1.0. C. Comparison with Literature The results of current study are compared with literature. The results on hydrogen purity from the current flowsheet modeling are compared with Mahishi et al. [13] experimental and Florin et al. [12] modelling results on biomass steam gasification with CO 2 capture. The comparison along with operating condition and basis is shown in Table II. It has been observed that the results of this study showed good agreement with the literature. TABLE II. COMPARISON FOR HYDROGEN PURITY Basis This Study Mahishi et al. [13] Florin et al. [12] Approach Modelling Experimental Modelling Gasification Steam Steam Steam Temperature (K) Pressure (atm) Steam/biomass ratio Sorbent/biomass ratio H 2 mole fraction Deviation error with current study Figure 8. Effect of temperature and steam/biomass ratio on hydrogen yield. Sorbent/biomass ratio: 1.0. So the values of differences for both cases shows that the influence of steam feed rate at lower temperature is more significant than at high temperature for the steam gasification process. This is because the endothermic forward water gas shift reaction is favored at low temperature as mentioned earlier. Fig 8 also shows that the influence of steam/biomass ratio is more than temperature influence on hydrogen yield. This is might be due to the pure steam gasification process. The effect of temperature on MCE is shown in Fig 8. It has been observed linear co relation between temperature and MCE, which means as temperature increase the MCE increases as well. The figure shows 76 % MCE at 950 K and more than 95 % at 1300 K. This is might be due to endothermic behavior of overall process, as mentioned before that steam gasification is endothermic process, so according to Le Chatelier s principle by increasing temperature the endothermic process moves in forward direction. For hydrogen yield, the comparison has been done with Lv et al. [37] experimental work carried out conventional gasification with catalyst and Shen et al. [20] modeling results on air steam gasification. The comparison results with operating conditions are shown in Table III. The results showed that current study predicts more hydrogen compare to others conventional methods. The comparison also proved that the hydrogen yield is higher in steam gasification system with CO 2 capture step rather than other conventional gasification processes even with usage of catalyst. TABLE III. COMPARISON FOR HYDROGEN YIELD Basis This Study Lv et al. [36] Shen et al. [20] Approach Modelling Experimental Modelling Gasification Steam Air-Steam + Catalyst Air-Steam Temperature (K) Pressure (atm) Steam/biomass ratio Sorbent/biomass ratio H 2 (g/kg of biomass) IV. CONCLUSION A simplified flowsheet has been designed for the enriched hydrogen gas production from biomass steam

6 gasification integrated with CO 2 capture. The flowsheet mainly consist of four sections i.e. pretreatment, steam generation, gasification, and gas cleaning. The effect of process parameters temperature and steam/biomass ratio was studied on hydrogen production. Both temperature and steam/biomass ratio is the important variables, as the hydrogen production increased by increasing both. Initially hydrogen increases with increasing of temperature but at very high temperature, hydrogen purity decreases due to the exothermic and reversible behavior of water gas shift reaction. By capturing CO 2, the hydrogen purity increased as CO 2 is removed from the system. In addition CO 2 can also be removed from the product gas in scrubber through fresh water. It is observed that 950 K and 3.0 steam/biomass ratio provides the maximum hydrogen mole fraction in the product gas i.e and hydrogen yield obtained g/kg of biomass at same conditions. 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