Biomass gasification in a 100 kWth steam-oxygen blown circulating fluidized bed gasifier: Effects of operational conditions on product gas distribution and tar formation

Biomass gasification is one of the most promising technologies for converting biomass, a renewable source, into an easily transportable and usable fuel. Two woody biomass fuels Agrol and willow, and one agriculture residue Dry Distiller's Grains with Solubles (DDGS), have been tested using an atmospheric pressure 100 kWth steam-oxygen blown circulating fluidized bed gasifier (CFB). The effects of operational conditions (e.g. steam to biomass ratio (SBR), oxygen to biomass stoichiometric ratio (ER) and gasification temperature) and bed materials on the composition distribution of the product gas and tar formation from these fuels were investigated. Experimental results show that there is a significant variation in the composition of the product gas produced. Among all the experiments, the averaged concentration of H(2) obtained from Agrol, willow and DDGS over the temperature range from 800 to 820 degrees C was around 24 vol.%, 28 vol.% and 20 vol.% on a N(2) free basis, respectively. A fairly high amount of H(2)S (similar to 2300 ppmv), COS (similar to 200 ppmv) and trace amounts of methyl mercaptan (<3 ppmv) on a N(2) free basis were obtained from DDGS. Due to a relatively high content of K and Cl in DDGS fuel, an alkali-getter (e.g. kaolin) was added to avoid agglomeration during gasification. Higher temperatures and SBR values were favorable for increasing the mole ratio of H(2) to CO and the tar decomposition but less advantageous for the formation of CH(4). Meanwhile, higher temperatures and SBR values also led to higher gas yields, whereas a higher SBR caused a lower carbon conversion efficiency (CCE%), cold gas efficiency (CGE%) and heating values of the product gas due to a high steam content in the product gas. From solid phase adsorption (SPA) results, the total tar content obtained from Agrol was the highest at around 12.4 g/Nm(3), followed by that from DDGS and willow gasification. The lowest tar content produced from Agrol, willow and DDGS using Austrian olivine (Bed 1) as bed materials was 5.7, 4.4 and 7.3 g/Nm(3), values which were obtained at a temperature of 730, 820 and 730 degrees C, SBR of 1.52, 1.14 and 1.10, and ER of 0.36, 0.39 and 0.37, respectively. (C) 2011 Elsevier Ltd. All rights reserved.