Experiments On The Key Processes Of Coal Chemical Looping Gasification Based On CaO Sorbent

Coal chemical looping gasification is a newly emerging, promising, effective, clean and low emission gasification technology. Coal or char is continuously introduced into the gasifier and reacts with steam, CO2generated during gasification is captured via the carbonation of CaO sorbent and leaves the gasifier in the form of solid CaCO3; CaCO3is calcined to CaO in the high temperature air-blown regenerator and then circulated back to the gasifier. Due to the selective removal of CO2during the gasification, hydrogen production reactions, such as water-gas, WGS and SMR, are enhanced according to the Le Chatelier’s principle. H2-rich product gas and a concentrated stream of CO2are available in the outlet of gasifier and regenerator, respectively Currently, the development of chemical looping gasification is in its initial stage, and there are still several technical issues need to be solved urgently. Mechanism and experimental studies on the key processes of coal chemical loping gasification were conducted in this doctoral thesis in order to improve the knowledge about chemical looping technology and provide technical reference.Experimental study on one of the key processes during chemical looping gasification, namely cyclic carbonation-calcination reactions (CCR) of reagent grade CaO sorbent for CO2capture was performed on a pressurized TGA. Experimental results indicated that the carbonation conversion of CCR under carbonation temperature of650℃was the highest in all runs with various carbonation temperatures (550℃-750℃). Higher or lower carbonation temperatures weren’t beneficial to the CCR process. The influence of CO2concentration in the carbonation atmosphere was not remarkable to the final carbonation conversion. On the contrary, with the increasing CO2concentration and calcination temperature, the deactivation of CaO sorbent was enhanced, and this deactivation resulted in the decrease of carbonation conversion during CCR. The increase of pressure during CCR accelerated the loss of CO2capture capacity of CaO sorbent as well.To resist the remarkable deactivation of CaO-based sorbent during cyclic carbonation/calcination, steam hydration reactivation of spent CaO-based sorbent after calcination was investigated on a modified TGA system. The influences of hydration temperature, steam concentration, hydration frequency and various steam reactivation strategies on the cyclic carbonation conversion were examined. Results suggested that300℃was the optimal temperature for steam hydration reactivation. With increasing steam concentration during hydration and hydration frequency, the reactivation performances were enhanced, namely the carbonation conversion was far better than the CCR without reactivation treatment. By comparing three various steam hydration methods, i.e. steam addition during carbonation, steam addition during calcination and separate steam hydration after calcination, the third method was superior to the other two. The morphology changes indicated that the steam hydration could maintained and regenerated the pore structure of the spent sorbent. Based on the excellent reactivation performance of separate steam hydration after calcination, the precursors of CaO-based sorbent were expanded form reagent grade CaO to two natural limestone and one dolomite sorbents. Experimental studies on the influences of hydration temperature, steam concentration, frequency, duration and steam reactivation strategies were carried out. Results suggested that the effects of hydration temperature, steam concentration and frequency on the cyclic CO2capture process of natural CaO-based sorbent were similar to the reagent grade CaO sorbent. Influence of hydration duration was mainly on the extent of hydration and carbonation reaction, carbonation conversion of CCR with longer hydration duration was higher than the process with short duration. By comparing the steam consumption and CO2capture amount among various steam reactivation strategies, the cyclic with short duration and high frequency steam hydration achieved the best CO2capture performance.During chemical looping gasification process, the mole ratio of calcium to carbon was on a relatively high level. Therefore, in the very beginning stage of gasification, coal was pyrolyzed with excess addition of CaO sorbent. In this work, the influence of CaO addition on the coal pyrolysis was performed on TG-FTIR and horizontal tube furnace system. With the increasing of addition amount of CaO, char and liquid products decreased, but total gas yield was enhanced, especially the yield of H2, CO and CH4. It was speculated that the presence of CaO stimulated the deoxygenation of phenols, dealkylation of aromatic compound and dehydrogenation of aliphatic side chains, thus the CO, CH4and H2yield was enhanced respectively.A lab-scale fluidized bed gasification testing facility was build to investigate the steam gasification of coal with in-situ CO2capture under pressurized conditions. The influences of gasification temperature, pressure, steam to carbon mole ratio and calcium to carbon mole ratio on the gasification product gas composition were examined, the morphology, surface elemental distribution and pore characteristics of residual solid product was measured by SEM-EDX, XRD and nitrogen adsorption as well. Experimental results showed that H2concentration in the product gas increases remarkably with the increasing temperature and pressure. On the contrary, CO concentration deceased due to the enhanced WGS reaction. With the increase of steam to carbon mole ratio, H2content was enhanced and CO, CH4was lowered simultaneously. The gasification was significantly promoted by the addition of CaO sorbent. The highest H2content was achieved at calcium to carbon mole ratio of one. Excess CaO in the gasifier was unfavorable for the gasification. The optimal product gas composition was:H2,77.98%; CO,7.23%; CO2,2.71%; CH4,12.06%, and it was obtained under the gasification condition of750℃,4bar pressure, S/C=2and Ca/C=1.Based on the experiments of gasification with in-situ CO2capture, chemical looping gasification for hydrogen production on the dual circulating fluidized bed system was conducted. Under the condition of gasifier temperature:650-750℃, combustor temperature:900℃, S/C mole ratio:1-2, Ca/C mole ratio:0-2, the influences of gasification temperature, S/C and Ca/C on the product gas composition from exit of gasifier were examined. Results suggested that increasing temperature and S/C enhance the H2concentration in the syngas. Meanwhile the concentration of CO and CH4was lowered, CO2concentration was lower than3%. With the increase of Ca/C mole ratio, H2content was improved, CO, CO2, CH4and C2-C3content was lowered at the same time. The optimal gas composition was achieved at temperature of750℃, S/C=2and Ca/C=2, the content of product gas was:H2,73.7%; CO,14.6%;CO2,0.3%; CH4,8.9%. With the cycle number of CaO carbonation-calcination increased, its CO2capture capacity dropped dramatically. Continuous chemical looping gasification indicated that H2concentration deceased and CO, CO2, CH4increased with the experiment duration. The carbon conversion of gasification also slightly decreased.