Downdraft gasification of biomass

Abstract

The objectives of this research were to investigate the parameters affecting the gasification process within downdraft gasifiers using biomass feedstocks. In addition to investigations with an open-core gasifier, a novel open-topped throated gasifier was designed and used. A sampling system was designed and installed to determine the water, tar and particular content of the raw product gas. This permitted evaluation of the effects of process parameters and reactor design on tar and particular production, although a large variation was found for the particulate measurements due to the capture of large particles. For both gasifiers, the gasification process was studied in order to identify and compare the mechanisms controlling the position and shape of the reaction zones. The stability of the reaction zone was found to be governed by the superficial gas velocity within the reactor. A superficial gas velocity below 0.2 Nms-1 resulted in a rising reaction zone in both gasifiers. Turndown is achieved when the rate of char production by flaming pyrolysis equals the rate of char gasification over a range of throughputs. A turndown ratio of 2:1 was achieved for the hybrid-throated gasifier, compared to 1.3:1 for the open-core. It is hypothesized that pyrolysis is a surface area phenomenon, and that in the hybrid gasifier the pyrolysis front can expand to form a dome-shape. The rate of char gasification is believed to increase as the depth of the gasification zone increases. Vibration of the open-core reactor bed decreased the bed pressure drop, reduced the voidage, aided solids flow and gave a minor improvement in the product gas energy content. Insulation improved the performance of both reactors by reducing heat losses resulting in a reduced air to feed ratio requirement. The hybrid gasifier gave a higher energy conversion efficiency, a higher product gas heating value, and a lower tar content than the open-core gasifier due to efficient gas mixing in a high temperature tar cracking region below the throat and reduced heat losses.