Investigation of chemical looping combustion by solid fuels. 1. Process analysis

To concentrate CO2 in combustion processes by efficient and energy-saving ways is a first and very important step for its sequestration. Chemical looping combustion (CLC) could easily achieve this goal. However, only limited references are available that use coal as the energy resource in a CLC process even though the development of CLC of solid fuels follows the trend of energy utilization. This paper is the first in a series of two, where we present the concept of a CLC process of solid fuels using a circulating fluidized bed with three loop seals. The riser of this circulating fluidized bed was used as the oxidizer of the oxygen carrier; one of the loop seals was used as the reducer of the oxygen carrier and the separator for ash and oxygen carrier, and the other two loop seals were used for pressure balance in the solid recycle process. Pressure profiles of recycled solids using this process are presented in detail. For the development of an oxygen carrier, we focused on the establishment of a theoretical frame of oxygen transfer capability, reaction enthalpy, a chemical equilibrium, and kinetics. Analysis results indicated that Cu-, Ni-, and Co- based oxygen carriers may be the optimum oxygen carriers for the CLC of solid fuels. Mn- based oxygen carriers have several disadvantages in their lower oxygen transfer capability, thermodynamic limitations of purifying the CO2 stream, or a larger endothermic reduction enthalpy. Fe-based oxygen carriers have the disadvantage of a larger endothermic enthalpy in the reducer and lower reactivity. Thermodynamic analysis indicated that CO2 can be concentrated and purified to at least 99% purity for the gas-solid reaction mode (reduction of the oxygen carrier by gasification products such as CO and H-2) or even higher for the solid-solid reaction mode (reduction of the oxygen carrier directly by solid fuels) on the basis of the selected oxygen carriers. A Cu-based oxygen carrier is the choice that has the potential to make the reducer self-sustaining or autothermal because of its exothermic nature during reduction. This would be beneficial for simplifying the operation of the reducer. The tendency of the Cu- based oxygen carriers to agglomerate can be eliminated by decreasing the operating temperature in the CLC system (600-900 degrees C). In the second part of the series, we will evaluate the reduction kinetics of selected Cu- based oxygen carriers by coal and other opportunity solid fuels using a simultaneous differential scanning calorimetry-thermogravimetric analysis to simulate a microreactor, using an X-ray diffractometer and a scanning electron microscope to characterize the solid residues, and a thermogravimetric analysis coupled with mass spectra to characterize the evolved gas compositions.