(99d) Clean Gas from Biomass by Catalytic Filtration

Biomass gasification is regarded as a good candidate for localized, small scale renewable energy production, but will only be broadly implemented if simple and efficient technology is available for removing undesired impurities such as particles and tar from the producer gas in order to avoid serious operating problems. The same holds for larger scale gasification for the production of synthesis gas for conversion into fuels, chemicals or hydrogen. This work focuses on the further development and improvement of a novel one-step high temperature gas cleaning catalytic based on a candle filter for the simultaneous removal of tars and other impurities and particles. After a brief review of gasification gas cleaning issues, synthesis of the catalytic filter (several recipes), results on a laboratory setup, long term testing and results on a biomass gasifier are given.

An optimized nickel-based alumina ceramic catalytic filter material for the use in integrated high temperature removal of tars and particles from biomass gasification gas has been extensively tested in a broad range of parameters allowing the identification of the operational window of such a filter. The optimal composition of the material determined from screening a large number of combinations is a porous alfa-alumina filter material with pores coated by 2.5 wt% Al2O3, 1 wt% Ni and 0.5wt% MgO. At a typical face velocity of 2.5cm/s, in the presence of H2S and at 900°C, the conversion of naphthalene in very high 20g/m3 tar loading is almost complete and a 1000 fold reduction in tar content is obtained. Technically, it would be better to run the filter close to the exit temperature of the gasifier around 800 - 850°C. At 830°C, conversions of 99.0% could be achieved in good gasifier operation conditions with typically only 2g/m3 tars, but further enhancement of the performance at low temperatures is desirable.

Results of tests of several types of catalytic filter (Ni based with different supports, modifiers and dopants) were collected with a particle free synthetic gasification gas with 50 vol% N2, 12 vol% CO, 10 vol% H2, 11 vol% CO2, 12 vol% H2O, 5 vol% CH4, 0-200 ppm H2S, and the selected model tar compounds: naphthalene and benzene. Both performance data in a wide range of temperatures, gas composition, tar loading, flow rate were obtained and a simple kinetic model for the conversion derived.

Furthermore, a method was developed to deposit a new support ZrO2 material to increase the surface area of the alumina candle filter and further enhance the catalytic activity, especially at lower temperature. A series of filter discs were prepared by this urea method in order to compare the catalytic performances of catalysts containing different types of supports: ZrO2, Al2O3 and a mixed ZrO2-Al2O3 over a broad range of parameters. It indicated that the tar conversion achieved with the three types of supports: ZrO2, Al2O3 and a mixed ZrO2-Al2O3 are similar, but at high velocity and low temperature, the best is the mixed support, which can probably be further optimized. The mixed material also exhibited a better stability (tests over hundreds of hours) compared to the pure alumina based recipe. Although the developed candle preparation method has been proven to be scalable, alternative preparation techniques for large scale production will also be discussed.