Tar Formation and Destruction in a Fixed Bed Reactor Simulating Downdraft Gasification: Effect of Reaction Conditions on Tar Cracking Products

Abstract

A lab scale, two stage fixed bed reactor has been used to study the cracking of tars formed during the pyrolysis of biomass, under conditions that are intended to simulate those in the downdraft gasifier. Experiments were done at near atmospheric pressure, with tars being produced at 500 °C under pyrolysis conditions in the first stage and then cracked in the second stage under a range of conditions. These conditions simulated those in the gasification zone of a downdraft gasifier. Both condensable and gaseous products were collected, quantified, and characterized. Cracking in an empty second stage reduced the tar from an inlet value of 33% (of input biomass) to 0.7% under the optimum conditions (temperature 1000 °C, velocity 0.4 ms −1 ). The presence of char in the second stage decreased the tar to 0.2%. The extent of cracking was not influenced by the source of the second stage char or particle size, within the ranges studied. The addition of a low percentage of O 2 to the inlet of the second stage had variable effects depending on the conditions in that stage, particularly the presence of char which scavenged the O 2 . A selection of biomasses was also studied, and in general, less than half of the volatile matter in the parent biomass was emitted as primary tar. Cracking of this tars in the presence of char decreased the tar by over 98% for all feedstocks. The effects observed in this work can be explained in terms of changes in the extents of cracking, reforming, gasification, and water−gas shift as a function of temperature, time, and presence of char.

On the suitability of thermogravimetric balances for the study of biomass pyrolysis

“…This reactivity of tar in the presence of char has been demonstrated by numerous researchers. [11][12][13][14][15] In this way, the more time gas-phase pyrolysis products spend in close proximity to char particles (the sample bed), the greater char yield will be, and any experimental parameter that affects this residence time, will thus affect char yield. [16][17][18][19] To minimise the effect of repolymerisation on char yield, inert gas is often flowed through the sample bed to sweep away gas-phase tars, but in fixed-bed reactors the effects of particle stacking are unavoidable, as gas-phase pyrolysis products do not move as quickly through the sample bed as does sweep gas.…”

Influence of Reactor Design on Product Distributions from Biomass Pyrolysis

“…A second stage may be added, when catalytic or thermal post-treatment of the product stream is required. Nunes et al, Dabai et al, and Volpe et al have studied the cracking reactions of biomass pyrolysis tars/oils, by injecting tar/oil vapors generated in the first stage into a solids-packed second-stage fixed-bed reactor.…”

Gasification of torrefied kitchen waste: release of sodium and its influence on the formation of gasification products

“…However, volatile matter of torrefied noodles is much higher than that of torrefied biomasses, and fixed carbon is lower. Higher volatile matter of torrefied noodles can cause higher tar concentration downstream of gasifier compared with torrefied biomass, which can block the gasifier and decrease gasifier efficiency …”