ISBN 978-983-2271-21-5Proceedings of International Conference on Process Engineering and Advanced Materials (ICPEAM 2010)

(Paper No. PEAP1_GTR_3) Kula Lumpur, Malaysia, 15-17 June, 2010

Simulation of Hydrogen Production from Biomass via Pressurized Gasification using iCON

Chai Kian Chiew, Abrar Inayat, Murni M Ahmad*Department of Chemical Engineering,

Universiti Teknologi PETRONAS,Bandar Seri Iskandar, 31750 Tronoh, Malaysia.

*murnim@petronas.com.my

Renewable energy in the form of biomass has been used to produce heat, electricity, steam and petrochemicals due to the zero net carbon emission. With regards to the environmental concerns related to fossil fuel usage, hydrogen has the potential as an alternative clean energy. Currently, production of hydrogen from biomass using a pressurized system is not widely being analyzed and developed yet. Thus, process and flowsheet development of pressurized gasification process of biomass coupled with carbon dioxide adsorption for hydrogen production were investigated using a PETRONAS iCON simulation model. The effect of parameters such as pressure, temperature and steam-to-biomass ratio on the hydrogen yieldwas investigated. Hydrogen yield is predicted to be increasing with pressure, temperature, and steam-to-biomass ratio in this high pressure gasification system.

I. INTRODUCTION

With the current energy crisis and moreover, the fluctuating price of fossil fuel price, biomass is one of thepotential solution to the problem. Production of energy from biomass is sustainable and environment-friendly featured by its low emissions of SO2 while producing zero net carbon dioxide due to photosynthesis [1]. Biomass is reported to contribute in green industries with associated growth in rural economies [2, 3]. Energy from biomass on short rotation forestry and other energy crops can contribute significantly towards the objective of the Kyoto Agreement in reducing the green house gases emissions and to the problems related to climate change [4]. Besides this, it is abundantly available in Malaysia, a palm oil producer at low costs and could also be supplied easily compared to other natural resources.

Biomass could be readily gasified to produce high purity hydrogen. Hydrogen produced from biomass is a type of clean energy where zero net carbon dioxide is being produced during its utilization and could be readily used in most of the present natural gas derived hydrogen energy conversion systems as well as advanced power generation devices such as fuel cells.

Through gasification technology, biomass can be easily gasified into hydrogen and other gases such as methane, carbon monoxide and carbon dioxide. Gasification includes both bio-chemical and thermo-chemical process gasification. Biochemical gasification refers to the gasification by microorganism at normal temperature and pressure while thermo-chemical gasification requires the use of air, oxygen or steam at temperature more than 800ºC [5]. Thermo-chemical gasification produces a product gas, which could be used to produce hydrogen or co-produce value-added by-products such as methane, and so on. Gasification technology is being updated from time to time in order tomeet the demand of lowest cost of production and higher

production rate. One of the improvements available is to incorporate carbon dioxide removal step in the gasification process or to conduct the gasification at high pressure.

Fermoso et al. [6] performed an experimental work on the gasification of a mixture of coal, biomass and petroleum coke with the feed particle size of 75-150 µm at 1 MPa. They reported that hydrogen and methane compositions in the product gas are almost constant when the reaction temperature increases. Hydrogen and carbon monoxide production was reported to be decreasing with increasing pressure while methane and carbon dioxide depict increasing trends.

Mahishi et al. [7] conducted a thermodynamic analysis of hydrogen production from biomass using equilibrium modeling. However, they reported that pressure does not have significant effects on the increment of hydrogen in the product gas near to atmospheric pressure.

Meanwhile, Hanaoka et al. [8] conducted a gasification experiment at high pressure using woody biomass and steam together, incorporated with the carbon dioxide removal unit. They reported that at pressure of 0.6-0.7 MPa, hydrogen yield increases with temperature. The highest hydrogen yield is predicted at 0.6 MPa.

In addition, Florin et al. [9] investigated on a system of gasification combined with carbon dioxide removal. Based on their results through modelling, they reported that hydrogen yield is increasing with temperature. Hydrogen yield also poses the same trend of increment corresponding to the steam-to-biomass ratio. Meanwhile, overall hydrogen production is observed to increase with its pressure.

Since the hydrogen production from biomass via pressurized gasification that is coupled with carbon dioxide adsorption has not been widely investigated and there are limited models to represent the case, this paper hence focus to develop such system and predict its performance via a simulation approach. In this work, a simulation model is developed in PETRONAS iCON process simulator and is used to investigate the technical feasibility of the biomass pressurized gasification system based on the effect of parameters such as pressure, temperature and steam-to-biomass ratio on the hydrogen yield.

II. METHODOLOGY

A. Reactions Schemes

The reactions assumed to occur in the gasification unit is listed in Table I, along with the stoichiometric equations and heat of reactions.

ISBN 978-983-2271-21-5Proceedings of International Conference on Process Engineering and Advanced Materials (ICPEAM 2010)

(Paper No. PEAP1_GTR_3) Kula Lumpur, Malaysia, 15-17 June, 2010

TABLE IREACTIONS IN THE GASIFICATION UNIT

Reaction Scheme ∆H (kJ/mol) Reference

Carbon Gasification

(R1)

C + H2O → CO + H2 131.5 [12, 13]

Methanation

(R2)

C + 2H2 → CH4 -74.8 [12]

Methane Reforming

(R3)

CH4 + H2O → CO + 3H2 206 [12, 14]

Water Gas Shift

(R4)

CO + H2O CO2 + H2 -41 [12, 13, 14]

Carbonation

(R6)

CO2 + CaO → CaCO3 -178 [13, 14]

Boudouard

(R5)

C + 2CO2 → 2CO 172 [12]

B. Gasifying Agent

In this process, steam is used as the gasifying agent in order to have higher hydrogen yield and lower solid residues[10]. Gonzalez et al [10] observed that solid amount produced from steam gasification is significantly lower with higher impact of temperature variations i.e. 28 to 6%, compared to 23 to 18% from air gasification for the temperature range between 700 and 900oC. The hydrogen yield for steam gasification increases considerably from 8 to 33% compared to decrement observed for air gasification from 9 to 5% for temperature range of 700 to 900oC.

C. Reaction Kinetics

Due the specifically pressurized biomass gasification system coupled with carbon dioxide adsorption, relevant kinetics is adapted from various literatures. The compilation and the basis of selection are given in Table II.

For reactions R2 (methanation), R3 (methane reforming) and R6 (Boudouard), kinetics was adapted from Lv et al. [11] who investigated gasification of biomass. As the same kinetics is applied by Biba et al. for a gasification system at a high operating pressure i.e. 2.648 MPa [12] even though for coal, the same kinetics is used in this work.

Hence, for reactions R1 i.e. carbon gasification and R4 i.e. water gas shift, the kinetics model proposed by Lv et al. [11] is also adapted due to the same reasons.

Carbonation kinetics was adapted from Lee et al. [13] in which the operating pressure reported are 3, 7, and 15 bar.

TABLE IIKINETICS OF REACTIONS IN THE GASIFICATION UNIT

ReactionKinetics

Constant (m/h)Reference Basis

Carbon Gasification

(R1)

Lv et al. (2008)Wood

Powder

Methanation(R2)

Biba et al. (2002) Coal

Raman et al. (2002)Feedlot Manure

Lv et al. (2008)Wood

Powder

Methane Reforming

(R3)

Biba et al. (2002) Coal

Raman et al. (2002)Feedlot Manure

Water Gas Shift(R4)*

Lv et al. (2008)Wood

Powder

Boudouard(R5)

Biba et al. (2002) Coal

Raman et al. (2002)Feedlot Manure

Lv et al. (2008)Wood

Powder

Carbonation(R6)

Lee et al. (2003)Methane + Steam

*m3/kmol.h

D. Process Assumptions

Assumptions made in the simulation are :

1. Biomass is represented as carbon, C2. The reactions occur sequentially and are in

isothermal condition3. Carbonation is assumed to be a forward reaction4. Ash is assumed to be inert hence does not

participate in the reaction.Feed rate of biomass is set to be 1kg/hr.

E. Process Development

The biomass pressurized gasification process is assumed to consist of reactions R1 to R6 happening in sequence. The limiting factor of this sequence is that some gases are required to be produced first before it can be used as a reactant for the next reactions. For example, methane needs to be produced in R2 before it can be used in R3 as a reactant. Without R2 occurring first, R3 would not be able to proceed, hence affecting the subsequent reactions. The block and process flow diagrams of the gasification system are illustrated in Figures 1 and 2 respectively.

Figure 1: Block diagram representation of gasification process for hydrogen production

ISBN 978-983-2271-21-5Proceedings of International Conference on Process Engineering and Advanced Materials (ICPEAM 2010)

(Paper No. PEAP1_GTR_3) Kula Lumpur, Malaysia, 15-17 June, 2010

Figure 2: Process flow diagram of enhanced biomass gasification system

III. RESULTS AND DISCUSSION

A. Process Simulation in iCON

Biomass is dried and pelletized before it is being fed into the gasifier. Since this is the high pressure unit, compression of steam needs to be done before being fed into the gasification unit. The biomass gasification is assumed to occur in the gasification unit along with the carbon dioxide capture. Reactions assumed to occur in the gasification unit include char gasification, methanation, methane reforming, water gas shift, boudouard, and carbonation reaction. The product gas is then being fed into the pressure swing adsorption (PSA) unit to get pure hydrogen gas.

The iCON process simulation package is employed to develop the bio-oil steam reforming process. iCON is a commercial process simulator developed by PETRONAS with Virtual Materials Group (VMG) Inc. The simulation engine is based on Sim42, an open source process simulator, and it runs on VMGThermo as the plug-in thermodynamics property package database standard. The Gasification property package is used in the simulation. This package enables iCON to caters gasification properties with solid support. Figure 3 shows the iCON simulation snapshot of the gasification process incorporating the carbon dioxide removal step while Table III shows the mass balances and the operating conditions for the gasification process.

B. Effects of Temperature

For high pressure gasification system, as shown in Figure 4, hydrogen amount in product gas increases with increasing temperature. This observation matches the findings published by Hanaoka et al. [8] and Fermoso et al. [6]. Figure 4 plots the composition of each component in the product gas with respect to change in temperature while Figure 5 shows the effects of temperature on hydrogen yield. Based on Figure 4, hydrogen increases with temperature from 1200°C to 1500°C while water is in the decreasing trend. Changes in the amount of methane, carbon dioxide and carbon monoxide produced are negligible with the effects of temperature increase. The reason behind these observations is high temperature promotes the endothermic reactions of char gasification, methane reforming, and boudouard.

With these three reactions being enhanced, more hydrogen is produced. Meanwhile, carbon gasification and methane reforming

lead to lesser water in the system. High temperature also induces the reverse of water gas shift reactions, however the carbon dioxide removal reaction promotes the water gas shift reaction forward.

C. Effects of Pressure

It is observed that hydrogen yield for high pressure system is found to be in the increasing trend with pressure. This profile matches thefindings reported by Florin et al. [9] and Mahishi et al. [7]. Figure 6shows hydrogen increases with pressure while carbon monoxide decreases. The result also shows zero percent of carbon dioxideand methane while water is in the increasing trend with increasing pressure. Similarly, Figure 7 shows hydrogen yield increase with pressure. When pressure is increased, more heat is being supplied into the system.

Figure 6: Effects of pressure on product gas compositions (T = 850°C, SBR = 2.45 [17])

Figure 4: Effects of temperature on product gas compositions (P = 800kPa, SBR = 2.45, [17])

Figure 5: Effects of temperature on hydrogen yield (P = 800kPa, SBR = 2.45, [17])

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ISBN 978-983-2271-21-5Proceedings of International Conference on Process Engineering and Advanced Materials (ICPEAM 2010)

(Paper No. PEAP1_GTR_3) Kula Lumpur, Malaysia, 15-17 June, 2010

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ISBN 978-983-2271-21-5Proceedings of International Conference on Process Engineering and Advanced Materials (ICPEAM 2010)

(Paper No. PEAP1_GTR_3) Kula Lumpur, Malaysia, 15-17 June, 2010

Figure 7: Effects of pressure on hydrogen yield (T = 850°C, SBR = 2.45 [17])

D. Effects of Steam-to-Biomass ratio

It is observed that hydrogen yield for high pressure system is in the increasing trend with increase of steam-to-biomass ratio. This trend is similar to that observed by Florin et al. [9]. Figure 8 shows that the hydrogen yield is increasing with steam-to-biomass ratio but the effect is not obvious. However in Figure 9, the hydrogen yield is predicted to increase from 9% to 13% with the increase in steam-to-biomass ratio. When more steam is being fed into the system, char gasification, methane reforming, and water gas shift reactions are promoted. With these reactions being pushed forward, it leads to an increasing production of hydrogen. The carbon dioxide removal step further promotes the water gas shift reaction forward, based on Le Chatelier’s principle.

Figure 8: Effects of steam to biomass ratio on product gas compositions (P = 800kPa, T = 850°C [17])

Figure 9: Effects of steam to biomass ratio on hydrogen yield (P = 800kPa, T = 850°C [17])

IV. CONCLUSION

The prediction model of enhanced biomass gasification in steam-assisted gasifier is successfully developed and simulated in PETRONAS iCON software. The results obtained show good agreement and follow empirical trends in referred literatures. The model predicts that for the biomass pressurized gasification process incorporating the carbon dioxide removal step, the higher the temperature, pressure and steam-to-biomass ratio, the higher hydrogen is being yield. This work produces a good fundamental model to further develop the gasification process for commercial purposes. Nevertheless, the flowsheet of this hydrogen production from biomass gasification can be further improved and optimized for better production yield with competitive cost. Experiments can also be performed in order to generate more applicable reaction kinetics that can further assist in the simulation work. Exact formula for biomass should be used to produce more accurate predictions.

ACKNOWLEDGMENT

The authors would like to thank Universiti Teknologi PETRONAS for the financial and facilities support.

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ISBN 978-983-2271-21-5Proceedings of International Conference on Process Engineering and Advanced Materials (ICPEAM 2010)

(Paper No. PEAP1_GTR_3) Kula Lumpur, Malaysia, 15-17 June, 2010[14] S. Lin, M. Harada, Y. Suzuki, H. Hatano, “Hydrogen production from

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[17] M.K. Yunus, M. M. Ahmad, A.Inayat, S. Yusup, “Simulation of Enhanced Biomass Gasification for Hydrogen Production using iCON”, World Academy of Science, Engineering and Technology, Vol.62, 2010, pp. 753-759.