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Catalysis and Kinetics
Catalytic Gasification of Biomass in Dual Bed Gasifier for producing Tar free Syngas Sukumar Mandal, Sateesh Daggupati, Sachchit Majhi, Sayani Thakur, RAJIB BANDYOPADHYAY, and Asit Kumar Das Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04305 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019
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Energy & Fuels
Catalytic Gasification of Biomass in Dual Bed Gasifier for producing Tar free Syngas Sukumar Mandal1, Sateesh Daggupati1, Sachchit Majhi1, Sayani Thakur1, Rajib Bandyopadhyay2, Asit Kumar Das1 1Refining 2Pandit
Research and Development, Reliance Industries Limited, India Deendayal Petroleum University, Knowledge Corridor, Raisan, Gandhinagar, Gujarat -382007
Correspondence: Dr. Asit Kumar Das, Refining Research and Development, Reliance Industries Limited, India, Tel +91-2884012314, Email asit.das @ril.com
Abstract Process development for the production of syngas by gasification of biomass, a carbon neutral source of energy, has been researched very extensively in the recent past. One of the technical challenges in biomass gasification is to produce clean syngas with no tar. Tar removal by employing secondary bed is a solution, but faces higher cost and reliability issues. Hence, extensive work is directed to produce syn gas with no tar formation in the gasifier bed itself, by employing suitable catalyst or other options. In the present work, a dual fluidized bed process for catalytic gasification of biomass is proposed for the production of high quality syngas. This process employs an alkali impregnated alumina or silica-alumina catalyst that eliminates tar formation completely, while increasing the H2 to CO molar ratio to more than 10. The catalyst is able to completely gasify the volatiles and solid char generated from biomass in the temperature range of 500 to 750 ℃, at near atmospheric pressure, with catalyst to biomass ratio of 10-20 and steam to biomass ratio of 1.0. The present work established for the first time, a new solid–solid catalytic reaction mechanism based on the novel concept of “migratory catalysis”, which is supported by in-depth micro-reactor studies, phase identification by in-situ XRD and conventional XRD. It has been established in this work that potassium from the catalyst particles becomes mobile and migrates to feed carbon particles through mass transfer of volatile potassium intermediate, which has low vapor pressure 10 mass % of feed, depending on the type of feed used and the operating conditions. Tar is a complex mixture of heavy hydrocarbons and oxygenates. As per IEA Gasification Task meeting (Brussels, March 1998), it was stated that all organics boiling at temperatures above that of benzene should be considered as tar. If tar is not removed from syngas, it would condense on cold surface of downstream equipment (especially heat exchangers and filters), leading to operational problems (blockage, coke formation etc.).12 The formation of tar and char can be decreased by increasing the temperature >800 ℃.13 However, to avoid ash agglomeration in the unit, it is necessary to restrict the gasification below 750 ℃.14 It is well established15 that carbon conversion increases mainly by increasing temperature and co-feeding water or carbon dioxide or by lowering particle size or by increasing residence time. However, char generated from pyrolysis of biomass is very reactive in comparison with coal or petroleum coke due to its porous morphology16.
Hence, gasification temperature of 700-750 ℃ is sufficient for complete
gasification of biomass in presence of suitable catalyst, adequate steam to biomass ratio and sufficient residence time. From thermodynamic equilibrium calculations, the steam gasification reactions of carbon/char and hydrocarbons are dominates at high temperature and low pressure whereas Boudouard reactions, (reaction between carbon dioxide with carbon), are slower. Both these reaction type are endothermic. Water gas shift reactions are moderately exothermic, favorable at low temperature and independent of pressure. Another reaction, i.e., methanation reaction favors at high pressure and moderate temperature (
