Catalytic Gasification of Biomass in Dual-Bed Gasifier for Producing Tar-Free Syngas

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 it faces higher cost and reliability issues. Hence, extensive work is directed to produce syngas with no tar formation in the gasifier bed itself, by employing a 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 500–750 °C, 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 microreactor studies and phase identification by in situ X-ray diffraction (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, <2.4 × 10–2 Pa, at temperature of 600–910 °C, but sufficient enough for required mass transfer at gasification conditions. However, potassium migrates back to the catalyst particles by the reaction of KOH with carbon dioxide in the reaction environment. Thus, there is no effective loss of potassium (K) from the bed. Such migration of intermediate species between solid catalyst and solid feed particles demonstrates a new pathway of catalysis among solid particles.

Catalyst Deactivation and Its Mitigation during Catalytic Conversions of Biomass

“…Dolomite, a magnesium mineral with the general formula MgCO 3 ·CaCO 3 and its calcined form (MgO-CaO) have been widely explored as cheap disposable catalysts for biomass gasification to reduce tar formation or inhibit selectivity to C 2+ . ,− The dolomite catalyst deactivates due to carbon deposition and attrition, but its low cost makes it easily replaceable. Alkali metal salts (usually carbonate) have also been investigated for eliminating tar and upgrading the product gas. ,,,, Alkali catalysts are often directly added to biomass, by either wet impregnation or dry mixing, to increase the rate of gasification, and recovery of the catalyst is difficult and costly. K 2 CO 3 and CH 3 COOK were found to enhance the H 2 yield and carbon conversion in steam gasification of biomass .…”

“…K 2 CO 3 and CH 3 COOK were found to enhance the H 2 yield and carbon conversion in steam gasification of biomass . K 2 CO 3 addition to Al 2 O 3 and SiO 2 –Al 2 O 3 showed complete tar removal and a high yield of H 2 -rich gas (H 2 /CO > 10) in the gasification of wood sawdust . However, these catalysts deactivate by particle agglomeration.…”

“…Alkali metal salts (usually carbonate) have also been investigated for eliminating tar and upgrading the product gas. 44,46,48,56,57 Alkali catalysts are often directly added to biomass, by either wet impregnation or dry mixing, to increase the rate of gasification, and recovery of the catalyst is difficult and costly. K 2 CO 3 and CH 3 COOK were found to enhance the H 2 yield and carbon conversion in steam gasification of biomass.…”

“…58 K 2 CO 3 addition to Al 2 O 3 and SiO 2 −Al 2 O 3 showed complete tar removal and a high yield of H 2 -rich gas (H 2 /CO > 10) in the gasification of wood sawdust. 56 However, these catalysts deactivate by particle agglomeration.…”

Review on Catalytic Biomass Gasification for Hydrogen Production as a Sustainable Energy Form and Social, Technological, Economic, Environmental, and Political Analysis of Catalysts

“…Apart from the advantages of biomass gasification, tar formation is a serious problem that faces the adoption of this process. , Due to the condensation of tar, equipment becomes blocked, and engines and turbines are damaged, resulting in costly maintenance and gas cleaning. − Additionally, tar condensation causes cracking in filter pores and adverse effects on the cold gas efficiency and heating value of the produced syngas. Besides tar formation, pollutants such as NH 3 , H 2 S, HCl, SO 2 , dust, ash, etc.…”