Coal combustion in a 50 kWth Chemical Looping Combustion unit: Seeking operating conditions to maximize CO2 capture and combustion efficiency

Abstract

In-situ Gasification Chemical-Looping Combustion (iG-CLC) with coal has been proposed as a low-cost process for the capture of CO2 during the energy generation. Previous experimental works have highlighted that high CO2 capture efficiency, close to 100%, can be achieved. However, a certain amount of unburnt gases (mainly CH4, CO and H2) are present in the CO2 stream. These gases can be treated in an oxygen polishing step placed downstream from the CLC unit where they are burnt with pure oxygen, which defines the so-called oxygen demand for the process. The aim of this work was to optimize the performance of the iG-CLC process with coal in order to minimize oxygen demand while maintaining CO2 capture at high levels. Ilmenite was used as the oxygen carrier to burn a bituminous coal in a 50kWth CLC unit, which consisted of a fuel reactor, an air reactor and a carbon stripper unit between them. The fuel reactor temperature, solids inventory in the fuel reactor, solids circulation rate, coal feeding rate and carbon stripper efficiency were varied, and the CO2 capture efficiency and oxygen demand were calculated for each set of operating conditions. The effect of the fuel reactor temperature was higher on the CO2 capture than on the oxygen demand, with a temperature close to 1000°C advisable in order to reach CO2 capture rates higher than 90%. In order to further increase CO2 capture, the efficiency of char separation in the carbon stripper was increased. The solids circulation flow rate and the coal feeding rate -which define the oxygen carrier-to-fuel ratio-, in addition to the solids inventory in the fuel reactor, showed a strong influence on the combustion efficiency. Thus, the total oxygen demand was able to be decreased from 10% to 7% by increasing the oxygen carrier-to-fuel ratio from 1.1 to 1.5 and the solids inventory from 450 to 720kg/MWth. However, an increase in the circulation rate showed a detrimental effect on CO2 capture, which could be offset by increasing the carbon stripper efficiency.