Investigation On Fe-based Oxygen Carriers In Chemical Looping Combustion With Coal As Fuel

Chemical looping combustion splits the conventional combustion intotwo-step oxidation and reduction with an intermediate oxygen carrier; therefore,it can separate CO2inherently, which provides a benefit for reducing the cost ofCO2capture. This dissertation investigated the Fe-based oxygen carrier inchemical looping combustion with coal as fuel from three aspects, that is,gas-solid reaction mechanism, the development and test of oxygen carriers, andinteraction between coal and oxygen carriers.In the aspect of gas-solid reaction mechanism, multi-crystal Fe withpolished smooth surfaces was used for the study on gas-solid reaction as asubstitution for conventional porous particles. The growth of solid productduring the oxidation of Fe in chemical looping combustion was observed undera scanning electron microscope. The results represent that the oxidation productgrows in a hybrid island-layer mode. The solid products appear as dispersed,three-dimensional shaped islands, indicating the continued ionic diffusion. Atthe same time, a thin product layer (≤100) is formed on the base surface of thereactant. Both the reaction temperature and the gas concentration are critical factors forsolid product morphologies. The microscopic morphology of the product directly affectschemical reaction kinetics. A rate equation theory was developed, which linked themicroscopic product growth with the macroscopic reaction kinetics.According to the ionic diffusion mechanism in gas-solid reaction, thereactivity of natural ilmenite was enhanced by introducing foreign ions (K+、Na+、Ca2+). The reactivity of15%K promoted ilmentie was improved by7times. Thebottleneck problem for the application of ilmenite was solved. A low-cost andeasily available oxygen carrier with high reactivity was developed for chemicallooping combustion with coal as fuel. The characterization analysis showed thatthe enhanced reactivity was attributed to the pore development effect from K+、Na+and the catalytic effect of the compound formed between alkali metals andilmenite. In order to investigate the long-term performance of the newdeveloped ilmenite carrier, a three fluidized beds reactor with double loops wasdesigned and built in either cold or hot mode. The hydrodynamics study in the cold mode proved the feasibility of the structure design of the reactor. In thedual fluidized beds in loop1of the hot mode, long-term (140h) operations ofboth raw and10%K promoted ilmenite were performed. The advantage of thepromoted ilmenite was confirmed. Furthermore, the chemical loopingcombustion process with lignite semi-char as fuel was achieved. The carboncapture efficiency was over95.2%. In the bed containing10%K promotedilmenite, the CO conversion ratio reached as high as93%.Based on the problems appeared in hot mode operation, the effects of coalexistence on Fe-based oxygen carriers and reaction process were furtherlystudied in a fluidized bed reactor. The experimental results showed that thevolatile hydrocarbon from coal pyrolysis could reduce the oxygen carrier,inducing sintering and carbon deposition. The active components, Fe2O3andCaSO4, from coal ash functioned as oxygen carriers. Except the ash rich inCaSO4, all the other ashes had detrimental effect on oxygen carriers’ reactivitydue to ash deposition and chemical combination between the ash and the oxygencarrier. The existence of ash also caused sintering and agglomeration. Thesegregation of coal particles along with its fragmentation and attrition hadinfluence on the fuel gas conversion. In order to obtain a higher gas conversion,a fast fluidized gas velocity or the oxygen carrier with high reactivity wasrequired.