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Simulation on Hydrogen Production from Biomass with CaO-based Chemical Looping Gasification

Hongtao Fan, Qinhui Wang*, Abdul Rahim Shaikh, Mengxiang Fang, Leming Cheng, Zhongyang

Luo

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38, Hangzhou 310027, Zhejiang, People’s Republic of China

Abstract The concept of biomass CaO-sorbent based Chemical Looping Gasification (CLG) have gained increasing attention for its ability of reducing CO2 emission by in situ CO2 capture, allowing for the production of a CO2-rich stream for sequestration and a high-hydrogen-content product gas. The core unit of CLG system is comprised of an interconnected dual-circulating fluidised bed reactors: the gasifier and the combustor (regenerator). Biomass feedstock is given into the gasifier, in which steam is gasifying agent and fluidizing wind, CaO sorbent could absorb CO2 generated from biomass gasification through carbonation, which changes the chemical equilibrium and strongly stimulates H2 production. In combustor, the remain char from gasifier burns with air/O2, generates the necessary heat for the CaCO3 decomposition and sensible heat delivery through bed material circulation. In this research, parametric analyses on the dual-fluidized bed system were carried out based on ASPEN Plus simulation. Gibbs free energy minimization approach was used to build the reactors model, the PB-RM is chosen as the property method. Native pine sawdust was chosen as feedstock, at a feed rate of 10 t/hr. Mass and energy coupling of two reactors are calculated by setting the combustor temperature at 900°C constantly. The compositions of the product gases, hydrogen yield & hydrogen production efficiency (HPE), cold gas efficiency (CGE) and gasifier carbon conversion ratio were correlated to the gasification operating variables, such as gasifier temperature (T), pressure (P), minimum mole ratio of CaO to carbon ([Ca]/[C]), and mole ratio of steam to carbon ([H2O]/[C]). The appropriate temperature should consider two aspects: the thermodynamics and the chemical kinetics in gasifier. The model shows that higher temperature than 700°C may not fit for H2 production because of the dramatic decrease of H2 and increase of CO2 concentration in product gas, while the model also overlooked the backward chemical kinetics under lower temperatures than 700°C, which means 700°C maybe a favourable one. Pressure study demonstrate that higher pressure than atmosphere is obviously beneficial for H2 concentration, which increased from 84.5% to 93.9% when system pressure increased from 1 bar to 3 bar. 3 bar is likely the best value for the hydrogen production, at which the hydrogen yield and HPE was up to 43.7 mol·kg-1 and 78.5%. Higher pressure than 3 bar is in favour of CGE and H2 concentration in product gas, but not observably. Thermodynamically there is a minimum value of [Ca]/[C], which is 0.296 in this biomass case under standard operating condition, to insure the absorption of CO2 in gasifier. But in reality, researchers must consider the sorbent maximum conversion, kinetics of CO2–sorbent reaction and sorbent capacity decay during cyclic carbonation–regeneration reaction, so that higher [Ca]/[C] than the minimum is needed, e. g. unity or larger. Optimal [H2O]/[C] can be discerned around 1.25, 76.14%

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of HPE was reached under 700°C, 1 bar and [Ca]/[C]=1. But lower [H2O]/[C] is better choices for CGE and gasifier carbon conversion. In a word, calculation results showed that 700°C, 3 bar, [H2O]/[C] of 1.25 and reasonable [Ca]/[C] favour the H2 production most. This work revealed an idealized, pure-thermodynamical instruction on the CaO based CLG technology, for our later experimental researches and development. More works on simulation, e.g. model establishment based on kinetics or semi-kinetics is needed in the future, to meet the data of certain experiment. Plant techno-economics modeling including peripheral facilities is also in the plan.