Comparing Direct and Indirect Fluidized Bed Gasification: Effect of Redox Cycle on Olivine Activity

摘要

Fluidized bed gasification processes are generally considered a good choice for biomass and waste because of its fuel flexibility. Furthermore, it is a relatively low-temperature highly efficient process operating at 700-900℃ compared with, for example, coal-based entrained flow processes that mostly operate at 1400-1600℃. Indirect fluidized bed gasification is becoming increasingly popular for some applications due to the possibility of producing a N_2-free gas without the need for an air separation unit, as well as complete conversion of the fuel. ECN has developed MILENA indirect gasification, in which gasification and combustion are physically separated, but both reactors are placed in the same vessel. This article aims to compare the performance of olivine as bed material in direct- and indirect (MILENA) gasification. With this purpose, oxidation/reduction cycles have been simulated in a direct gasifier to determine the effect of olivine preoxidation in terms of tar destruction and oxygen transport. Results show that olivine preoxidation mainly enhances the capacity of oxygen transport of the bed material, which adds up to the catalytic effect of iron in olivine towards tar destruction. Oxygen transport capacity of olivine has been quantified as ER = 0.25 at the maximum initial CO_2 peak, and it has been estimated that 20-25% wt of iron in olivine is able to transfer oxygen during the first 10 min operation. On the other hand, it has been found that MILENA operating conditions are equivalent to the point of initial maximum peak of CO_2 in the devolatilization stage. This means that oxygen transport capacity of olivine is kept at its maximum due to the continuous combustion/gasification cycles, and olivine is kept activated by cyclical migration of iron into the surface and subsequent fast reduction. The effect of oxygen transport on the overall heat balance of the MILENA gasifier is equivalent to oxidation of additional fuel in the combustor.