Batch Testing of Solid Fuels with Ilmenite in a 10 kWth Chemical-Looping Combustor

Artikel i vetenskaplig tidskrift, 2010

Batch experiments were conducted in a 10 kWth chemical-looping combustor for solid fuels using ilmenite, an iron titanium oxide, as the oxygen carrier with two solid fuels: a petroleum coke from Mexico and a bituminous coal from South Africa. The purpose of these batch tests was to attain detailed information on fuel conversion, complementary to previous continuous operation of the unit. At steady-state, a fuel batch of typically 25 g was introduced in the fuel reactor and gas concentrations were measured at the outlet of both air and fuel reactors. The fuel reactor was fluidized with steam and the amount of bed material was typically 5 kg. The fuel introduced devolatilizes rapidly while the remaining char is gasified and the resulting syngases H2 and CO react with the oxygen carrier. Operation involved testing at different fuel reactor temperatures from 950 to 1030°C, and investigation of the influence of particle circulation between air and fuel reactors. The fuel conversion rate was increased at higher temperature: at 950°C the instantaneous rate of conversion for petroleum coke averaged at 17.4 %/min while at 1030°C, the value was 40 %/min. For the much more reactive South African coal, the averaged rate at 970°C was 47 %/min and increased to 101 %/min at 1000°C. For petroleum coke testing with particle circulation, the oxygen demand - defined as oxygen lacking to fully convert the gases leaving the fuel reactor - was typically 12-14% for the gasified char including H2S, in line with previous experiments with the same unit and fuel. If only syngases are considered, the oxygen demand for char conversion was 8.4-11%. Similar or even lower values were seen for the char of South African coal. This is in line with expectations, i.e. that it is possible to reach fairly high conversion, although difficult to reach complete gas conversion with solid fuel. It was also seen that the volatiles pass through the system essentially unconverted, an effect of feeding the fuel from above. Moreover, the oxygen demand for char conversion decreased with increasing temperature. Finally, the CO2 capture - defined as the proportion of gaseous carbon leaving the fuel reactor to total gaseous carbon leaving the system - decreased at higher particle circulation and a correlation between capture and circulation index was obtained.