Tar formation and destruction in a fixed bed reactor simulating downdraft gasification: Effect of reaction conditions on tar cracking products

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 O₂ 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 O₂. 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.