Gasification inhibition in chemical-looping combustion with solid fuels

Chemical-looping combustion (CLC) is a novel technology that can be used to meet growing demands on energy production without CO2 emissions. The CLC process includes two reactors, an air and a fuel reactor. Between these two reactors oxygen is transported by an oxygen carrier, which most often is a metal oxide. This arrangement prevents mixing of N2 from the air with CO2 from the combustion giving combustion gases that consist almost entirely of CO2 and H2O. The technique reduces the energy penalty that normally arises from the separation of CO2 from other flue gases, hence, CLC could make capture of CO2 cheaper. For the application of CLC to solid fuels, the char remaining after devolatilization will react indirectly with the oxygen carrier via steam gasification. It has been suggested that H2, and possibly CO, has an inhibiting effect on steam gasification in CLC. In this work experiments were conducted to investigate this effect. The experiments were conducted in a laboratory fluidized-bed reactor that was operating cyclically with alternating oxidation and reduction periods. Two different oxygen carriers were used as well as an inert sand bed. During the reducing period varying concentrations of CO or H2 were used together with steam while the oxidation was conducted with 10% O2 in N2. The temperature was constant at 970 °C for all experiments. The results show that CO does not directly inhibit the gasification whereas the partial pressure of H2 had a significant influence on fuel conversion. The results also suggest that dissociative hydrogen adsorption is the predominant hydrogen inhibition mechanism under the laboratory conditions, thus explaining why char conversion is much faster in a bed of oxygen carrying material, compared to an inert sand bed.