Thermodynamic analysis of a biomass anaerobic gasification process for hydrogen production with sufficient CaO
Introduction
H2 is a kind of new and ideal energy sources with the features of high combustion heat value, no pollutants and wide applicability. Biomass is a renewable resource which is abundant. Compared with fossil fuels, biomass is characterized by high volatile matter, high carbon reactivity, low sulfur and nitrogen content and low ash content, which is considered as a timely utilized green coal. It can achieve zero emission of CO2 in H2 production by using biomass, which is helpful for solving the green house problem caused by fossil fuels utilization. From any point of view, either energy sources or environment, it is very important to develop technology for H2 production from biomass.
At present, the methods of H2 production from biomass mainly included microorganism and thermo-chemical conversion. The secondary pollution of microorganism conversion method is lower, while the microorganism culture is difficult and H2 generation efficiency is low. The thermo-chemical conversion methods mainly contain gasification, pyrolysis, supercritical water and biomass pyrolysis oil reforming. All these methods are under investigation and developing now. With the development of gasification technology, many novel H2 production schemes that gasification and purification process are carried out in a same reactor are presented. Especially the schemes based on CO2 acceptor gasification technology [1], [2] are typical, such as coal anaerobic gasification zero emission system [3], [4] presented by Zero Emission Coal Alliance (ZECA, America), advanced gasification–combustion (AGC) system [5] presented by GE energy and environment research corporation (GE EER ), and hydrogen production by reaction integrated novel gasification (HyPr-Ring) system [6], [7], [8], [9] presented by New Energy and Industry Technology Development Organization (NEDO, Japan), etc. At present, relevant theory and experimental investigation are proceeding. In China, relevant research is done by Zhejiang University [10], [11] and Institute of Engineering Thermophysics of Chinese Academy of Science which presents carbonaceous energy directly producing hydrogen near zero emission system [12].
This paper utilizes thermodynamic equilibrium calculation software FactSage 5.2 which is based on system Gibbs free energy minimization principle [13] to investigate a new kind of biomass anaerobic gasification system for H2 production which is presented by us. The influence of CaO on C/H2O reaction system is analyzed. According to the system fueled with corn stalk, the influences of reaction pressure, temperature, mole ratio of H2O to carbon in the biomass ([H2O]/[C]) on H2 production process were investigated. The H2 production differences between biomass and coal are compared. The conditions of CaO regeneration under different pressure and atmosphere in the combustor are studied. This paper will be helpful for further development and operation optimization of this H2 production system. Meanwhile, it has reference value for other H2 production systems based on CO2 acceptor gasification technology.
Section snippets
System introduction
Fig. 1 shows the biomass anaerobic gasification system for H2 production based on CO2 acceptor gasification technology presented by us. Biomass with steam was sent to pressure circulating fluidized bed (PCFB) gasifier for gasification. The main reactions are as follows: water gas reaction (1) and water gas shift reaction (2):
In general, reaction (1) can achieve higher reaction velocity only if the temperature is above 1000 °C, while
Influence of CaO on C/H2O reaction system
Fig. 2 shows the influence of the mole ratio of CaO to carbon in the biomass ([Ca]/[C]) on product gas mole fraction of the C/H2O reaction system. The reaction temperature is 800 °C, reaction pressure is 2 MPa and the mole ratio of H2O to carbon in biomass ([C]/[H2O]) is 2. As shown in Fig. 2, H2 mole fraction increases obviously when [Ca]/[C] increases from zero to 0.5 in the system. With further increasing of [Ca]/[C], the H2 mole fraction increases gradually and H2O, CO, CO2 mole fraction
Conclusions
Based on CO2 acceptor gasification technology, this paper presents a biomass anaerobic gasification system for H2 production in which water gas reaction and water gas shift reaction are effectively carried out in a same reactor. From the above analysis, the following conclusions can be drawn.
- 1
The increase of CaO can obviously increase H2 mole fraction in C//H2O reaction products. When the mole ratio of CaO to carbon ([Ca]/[C]) is 1, H2 concentration achieves the maximum value.
- 2
It can be seen from
Acknowledgments
We would like to thank National Natural Science Foundation of China (50306022) for the financial support to this work.
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