With the ever escalating demand for power generation, CO2 emission from coal-fired power plants is becoming a growing environmental concern. In order to reduce the CO2 emissions from the energy conversion processes, carbon capture techniques have been developed, such as a chemical absorption process for CO2 capture using MEA solvent, Integrated Gasification Combined Cycle (IGCC) and oxy-combustion capture. However, the parasitic energy required for these methods to produce a concentrated, sequesterable CO2 stream significantly mitigates their economic feasibility.
Currently, most chemical looping processes being developed have focused on the converting gaseous fuels, such as syngas and natural gas. However, due to the unstable natural gas prices and the capital intensive gasifier required for the syngas generation, a chemical looping scheme that utilizes the direct conversion of solid fuels such as coal and biomass can potentially be economically attractive. The Sold Fuel Direct Chemical Looping (SDCL) technology developed at the Ohio State University is a potentially attractive alternative to the conventional carbon separation techniques. In the SDCL process, an iron-based composite particle is used to serve as a chemical intermediate to produce heat for power generation from solid fuels via an iron reduction/oxidation reaction scheme. A sequestration-ready stream of CO2 is produced in this system without an elaborate separation step.
The iron-based oxygen carrier in the SDCL process directly converts the coal and biomass with the effective CO2 capture. The process consists of two main reactors, i.e. a reducer, and combustor. In the reducer, the solid fuels are converted to CO2 and H2O using the oxygen liberated in the conversion of Fe2O3 to a mixture of Fe and FeO. However, the reaction between the oxygen carrier and char is fundamentally slow solid state reaction. In order to overcome the slow kinetics, a small portion of the CO2¬/H2O exhaust stream from the reducer is recycled to enhance the solid-solid reaction. The reduced iron particles are then transferred to the combustor, where they are fully oxidized to Fe2O3 and pneumatically conveyed to the reducer completing the cycle.
The experimental study of the SDCL process in a thermogravimetric analyzer (TGA) and fixed bed apparatus show that the oxygen carrier particle remains highly reactive after several reduction/oxidation cycles at 900°C. Both the volatiles and chars from coal and woody biomass are effectively converted to CO2 and H2O with minimal side reactions using the oxygen carrier particle. Additionally, the char conversion rate is enhanced when the oxygen carrier is coupled with reducer exhaust stream. During the oxidation of the particle, an exothermic behavior is observed and their reactivity is maintained.