Evaluation of different oxygen carriers for biomass tar reforming (I): Carbon deposition in experiments with toluene

In this work, an innovative method for gas conditioning in biomass gasification is analyzed. The objective is to remove tar by selectively reforming the unwanted hydrocarbons in the product gas with a chemical looping reformer (CLR), while minimizing the carbon formation during the process. Toluene, in a concentration of 600-2000 ppmv, was chosen as a tar model compound. Experiments were performed in a TGA apparatus and a fixed bed reactor. Four oxygen carriers (60% NiO/MgAl2O4 (Ni60), 40% NiO/NiAl2O4 (Ni40), 40% Mn3O4/Mg-ZrO2 (Mn40) and FeTiO3 (Fe)) were tested under alternating reducing/oxidizing cycles. Several variables affecting the reducing cycle were analyzed: temperature, time for the reduction step and H2O/C7H8 molar ratio. Ni40 and Mn40 presented interesting characteristics for CLR of biomass tar. Both showed stable reactivity to C7H8 after a few cycles. Ni40 showed a high tendency to carbon deposition compared to Mn40, specially at high temperatures. Carbon deposition could be controlled by decreasing the temperature and the time for the reduction step. The addition of water also reduced the amount of carbon deposited, which was completely avoided working with a H2O/C7H8 molar ratio of 26.4.