CO2 CAPTURE AND HYDROGEN PRODUCTION IN SORBENT ENHANCED WATER-GAS SHIFT (SEWGS) PROCESS WITH REGENERABLE SOLID SORBENT
ZARGHAMI KHANEHSAR, SHAHIN
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Carbon dioxide emission from fossil fuel combustion and its impact on global warming is one of the most critical environmental issues nowadays. Coal as a main source of produce energy is the most CO2-intensive fossil fuel. Advanced power generation processes that use gasification technology, such as Integrated Gasification Combined Cycle (IGCC), which offer higher efficiency, are among the leading contenders for power generation in the 21st century. In an IGCC process, because of high pressure, carbon dioxide in the fuel gas is at higher concentration, which can be captured and sequestered at lower costs. Utilization of regenerable MgO-based sorbents has been shown to be an effective method for capturing CO2 from gasification-based processes at elevated temperatures and pressures (i.e. p > 20 atm and 330° < T < 450°C). Low cost MgO based sorbent can be prepared through modification of natural dolomite. The reactivity of the sorbent in carbonation/regeneration cycles has a significant impact on the economics of the proposed regenerable process. Although the sorbent can be regenerated in successive cycles, the sorbent reactivity and capacity gradually decline during the cyclic process. Therefore, it is crucial to develop a better understanding on the role of the key parameters affecting the reactivity of the sorbent going through the cyclic carbonation/regeneration process. In this work, a systematic study on the sorbent preparation parameters (i.e., calcination temperature, calcination duration, calcination temperature ramp, potassium concentration, impregnation duration, drying temperature, re-calcination temperature, and re-calcination duration) was conducted to understand the effect of each parameter on the overall capacity and reactivity of the sorbent. The concentration of potassium additive (as carbonation reaction promoter) has the most significant effect on the reactivity of the sorbent and the optimum K/Mg molar ratio appears to be in the range of 0.1-0.16. The reactivity of the sorbent toward carbon dioxide at various operating conditions (i.e. temperature, CO2 concentration and steam concentration) was experimentally evaluated. The presence of steam significantly improves the reactivity of the sorbent which is attributed to formation of more favorable pore structure as well as the existence of a parallel carbonation reaction pathway involving the formation of a transient MgO.H2O* compound. The optimum carbonation reaction temperature in the presence of steam is around 380˚C. The effect of cycling on CO2 capture capacity of MgO-based sorbent was also experimentally investigated in this work. Series of carbonation/regeneration cycles (up to 25) have been carried out in a dispersed bed reactor to determine the effect of various variables on long term durability of the sorbent. The gradual loss of CO2 sorption capacity appears to be mainly due to loss potassium (a carbonation reaction promoter) in the cyclic process. Durability of the sorbents improves in the presence of steam, which is likely due to the favorable changes in the pore structure of the sorbents. A kinetic model was developed to fit the reactivity curves obtained from the dispersed bed tests at different operating conditions which was needed to predict the sorbent/catalyst performance in the regenerative process. Model parameters were defined and discussed for each of the operating conditions, as well as dispersed bed cyclic tests. Furthermore, the thermal behavior and the kinetics of partial decomposition of dolomite were studied in a dispersed-bed reactor to improve the reactivity of the sorbent. The microstructure and the nature of the solid products were found to be strongly dependent on the CO2 partial pressure near the reacting interface and on the decomposition temperature. A significant increase in the rate of the dolomite decomposition reaction was found in the presence of steam. Steam improves the kinetics of decomposition, modifies the radial distribution of the pores; and improves the connectivity of the pores inside the dolomite particles, which decreases the diffusion resistance of produced carbon dioxide inside the particle. A shrinking core model with variable product layer diffusivity was used to fit the experimental data and determine the kinetic parameters of the dolomite decomposition reaction. The results indicate that transport of CO2 across the reacting interface in the porous particle was the main limiting factor in the decomposition reaction at the experimental conditions investigated. A lab-scaled high-pressure/high-temperature packed-bed reactor was utilized to evaluate the performance of the sorbent in simultaneous water-gas shift reaction and sorbent carbonation environment. It was shown that the CO2 in the coal gas can be removed by regenerable MgO-based sorbents at temperatures around 350°C, and the CO2 removal can shift WGS reaction to enhance hydrogen production. Therefore, Sorbent Enhanced Water-Gas-Shift (SEWGS) can result in much higher hydrogen production without lowering the temperature, leading to higher overall process efficiency.