Ind. Eng. Chem. Res. 2006, 45, 1389-1396
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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, Spain
After 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 in large 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 was modified 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 distribution of 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-based catalysts 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 very different 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 for the 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 nickelbased steam-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 nickelbased monoliths 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:
[email protected].
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 second paper of this series7 and enabled an analysis and understanding of the results for tar and NH3 eliminations with monoliths. From previous research on nickel-based monoliths for gasification gas cleaning, it was concluded (from previous work5,6) that the performance of a single-layer monolithic reactor for this application was not good enough. For example, the overall, effective, or apparent kinetic constants for the overall tar and NH3 eliminations were of the same order of magnitude5,6 as those obtained with the competitive and much cheaper calcined dolomites. After a detailed analysis of the results, it was concluded that the low effectiveness of a single-layer monolithic reactor for this application was the relatively high ∆T (drop of temperature) value across the monolith. For the case of a tar content at the inlet of the monolith of 5.2 g/Nm3 and for total tar conversion (Xtar ) 1), the value of ∆T under adiabatic operation (∆Tadiab) was calculated to be 100-120 °C.7 Nevertheless, in the small pilot plant that was used, the quantity Texit - Tinlet () ∆Texp) was 150-200 °C,7 because of the loss of heat to the surroundings. The temperature measured at the monolith exit was then much lower than that if the monolith had been really adiabatic. For this reason, the real ∆T decrease (∆Texp) was always higher than the value of ∆Tadiab. Because of the relatively low temperature (∼700 °C) at the exit of the monolith, some filamentous whisker-type coke was formed there. This type of coking is very well-documented in the literature on the steam reforming of natural gas and naphthas,8,9 and it must be avoided in a gasification gas cleaning process. To avoid this coking, a temperature higher than 750-800 °C must be obtained within (and always maintained at the exit of) a nickel-based monolith. In previous research, another important type of deactivation of the monolith was also noticed: the plugging or clogging of
10.1021/ie051171l CCC: $33.50 © 2006 American Chemical Society Published on Web 01/19/2006
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the channels of the monolith at its face and at the inlet zone. The steam-reforming nickel-based catalysts require temperatures of at least 900 °C to be effective; thus, there was a reheating zone before the monolith face. Reheating of the gasification fuel gas was made using a third (or reheating) air flow to burn a portion of the fuel gas, heating it to 1050 °C, if needed. This heated gas then enters directly into the monolith; however, these very high temperatures can be higher than the softening points of the char and ash coming from the upstream biomass gasifier and present in the gasification gas. Remember that there is no filtering between the gasifier and the monolithic reactor. Therefore, some char and ash particles can become sticky under these conditions,10-12 stick on the front/face of the monolith, and deactivate it.5 From previous research with these monoliths, it was concluded that the main problem and bottleneck for their further use on a commercial scale was their deactivation, which was relatively fast. The problem with the monoliths proved to be not their activity, but their life. To achieve a long life for the monolith, the temperature at the monolith front/face should not be very high (not above 900 °C), to prevent the ash from sticking on it, and the temperature at the monolith exit should not be very low (not
