Catalytic tar cleaning of biomass-derived gas with simultaneous catalyst regeneration

Licentiate thesis, 2011

Tar cleaning of raw gas from biomass gasification is a necessary operation before the gas can be used as feedstock for synthetic fuels. The technology for gas cleaning that is commercially available today (scrubber systems), is associated with problems like heat penalties and loss of the chemically stored energy within the tars. These problems are addressed in this work, and a novel method for secondary tar cleaning is proposed and experimentally validated. The method is applicable to all types of biomass gasifier and regardless of whether primary measures for tar reduction are used or not. The tar cleaning system can be thermally integrated with the gasifier outlet temperature, which leads to minimal heat losses. The method combines tar cleaning with catalyst regeneration in a system of dual fluidized beds, which in this work is experimentally explored in two identically sized reactors systems: a cold Perspex unit and a high temperature steel unit. Fluid dynamics and operation controllability was evaluated in the cold system, where the results indicated that the solid circulation is controllable and sufficient gas tightness of the loop-seals could be achieved. The gained knowledge from operating the cold system was then used to demonstrate the tar cleaning method in the hot reactor system. This was done by using raw gas from Chalmers biomass gasifier, which contained roughly 30 gtar/Nm3tar. Tar reforming activity, catalyst regeneration and the potential for catalytically adjusting the raw gas was investigated with two different catalysts: (1) Ilmenite (Fe2TiO3), a naturally occurring ore with bulk price comparable to hard coal and, (2) Mn4MgZ1150, a synthetic manufactured catalyst consisting of 40 % Manganese oxide (Mn3O4) supported on 60 % Magnesium-Zirconium oxide (MgZrO3). Experiments with 60 % ilmenite in silica sand and 23 % M4MgZ1150 in silica sand were performed. The result showed that the total amounts of tars were reduced by as much as 44.5 % in the M4MgZ1150 case and by 35 % in the ilmenite case. Both catalysts showed activity in Water-Gas Shift reaction and the H2/CO ratio was shifted from 0.7 in the raw gas to 3 downstream of the reactor system for the ilmenite case and 0.6 to 1 in the manganese case. Finally, an application for online monitoring of moisture content in a hot gas streams with rapid response time has been developed and experimentally investigated. The idea is to replace the analogous weighting system for measuring the condensate water downstream of the tar cleaning reactor, which only gives a measurement in the time period of 5 – 10 min. In this work, the method is, however, experimentally tested in the 12 MWth biomass fired CFB-boiler at Chalmers but, it can equally be used downstream of the tar cleaning reactor. The result showed that the application could detect moisture variations with a response time in the order of seconds. No sign of hysteresis was detected and very good precision in moisture content was achieved, with less than 4 % error after calibration. Key words: Tar cleaning, Catalytic gas cleaning, Biomass gasification