Catalytic Hot Gas Cleaning With Monolits In Biomass Gasification In Fluidized Bed

Ind. Eng. Chem. Res. 2006, 45, 1389-1396

1389

Catalytic Hot Gas Cleaning with Monoliths in Biomass Gasification in Fluidized Beds. 4. Performance of an Advanced, Second-Generation, Two-Layers-Based Monolithic Reactor
Jose M. Toledo, Jose Corella,* and Gregorio Molina ´ ´
Department of Chemical Engineering (Faculty of Chemistry), UniVersity Complutense of Madrid (UCM), 28040 Madrid, SpainAfter several years of research with a single-layer-based monolithic reactor, a new, second-generation monolithic reactor was designed, manufactured, set-up, and tested for tar and NH3 elimination from a real gasification gas. This gas was produced in an upstream bubbling fluidized-bed biomass gasifier at small pilot-plant scale (5-10 kg/h) and operated under conditions close to those used inlarge pilot and commercial units. The life of the monolith, which is more important in this application than its activity, is dependent basically on the longitudinal profiles of temperature in the entire monolithic reactor which must be optimal, avoiding very high and very low temperatures at the front and at the exit, respectively, of the monoliths. The longitudinal profile of temperature wasmodified and approached the optimal profile by dividing or partitioning the total air flow to the entire gasification plant into four different flows: two (first and second flows) to the gasifier (bottom and freeboard) and two (the third and fourth flows) to the monolithic reactor, to reheat the gasification gas before it enters the two layers of monoliths. Through the use of an optimal distributionof the air, the performance of the monolithic reactor was good and tar contents as low as 150 ( 50 mg/Nm3 were obtained in the tests reported here.
Introduction Catalytic gasification gas cleaning may be performed currently with two different types of solids: with calcined dolomites or related materials (limestones, olivine, etc.) and with nickelbased steam-reforming catalysts. Nickel-basedcatalysts proved to be very efficient in eliminating tar in biomass gasification gas. This elimination occurs mainly by steam and dry (CO2) reforming mechanisms. For this approach, commercial steam reforming nickel-based catalysts from different manufacturers have been tested in small pilot-scale plants. Corella and coworkers proved the usefulness of these catalysts for tar elimination under verydifferent operating conditions, not only in biomass gasification with air,1,2 but also in gasification with pure steam3 and with steam-oxygen mixtures.4 These commercial catalysts are rings with several shapes and holes that need a gas without particulates. They do not accept particulates in the fuel gas, and, therefore, the gas must be filtered before the catalytic reactor. There is a clear need forthe use of hightemperature (400-550 °C) filters in which some coke may be formed by thermal cracking of the tar present in the raw gasification gas. This coke that is formed in the pores of the filters might plug them, casting doubt on the long-term operation of the hot filters under some gasification gas compositions. For this reason, another solution or approach emerged: to use nickelbasedsteam-reforming catalysts but in the form of monoliths, with honeycomb structures. The feasibility and usefulness of a single-layer monolithic reactor for catalytic cleaning of a real biomass gasification gas, obtained in a fluidized bed biomass gasifier operating under conditions very similar to existing ones in demo and commercial scales, was studied recently. The performance of the nickelbasedmonoliths for tar and NH3 abatement can be found in refs 5 and 6, respectively. Tar and NH3 conversions with these
* To whom correspondence should be addressed. Fax: +34-91-394 4164. E-mail: narvaez@quim.ucm.es.

monoliths are dependent on so many experimental variables that a model was needed to understand and correlate the results obtained with the monoliths. Such a model was presented in the...