QUALITY EVALUATION OF WOOD DUST BIOMASS AS POTENTIAL FEEDSTOCK FOR BIO-COAL PRODUCTION

Abstract

Torrefaction process has been identified as one important factor that enhances energy content of a woody biomass. Woody biomass, being the principal feedstock for solid fuel needs to be subjected to optimum process conditions that will increase its energy content. Therefore, this research is aimed at providing information on the impact of torrefaction process on energy value and chemical compositions of wood dust biomass sample sourced from stool tree (Alstonia congenisis) and viability of other four wood dust biomasses as feedstock for bio-coal production. Preliminary analyses such as proximate, ultimate, fibre and other physico-chemical analyses were conducted on the wood dusts sourced from five different trees namely; Antiaris africana Engl., Milicia excelsa (Welw.) C.C. Berg., Alstonia congensis Engl., Ceiba pentandra (L.) Gaertn., and Albizia lebbek (L.) Benth. Wood dust sample with the highest energy value was investigated as a potential feedstock for bio-coal production. Physico-chemical analyses of the raw biomass samples were first conducted prior the torrefaction process. Torrefaction experiment was performed at varying process temperatures of 200 ºC, 250 ºC and 300 ºC under a residence time of 45 min with an improvised laboratory scale torrefier designed with a steel container. It was observed that wood dust of Alstonia congenisis possessed highest amount of energy value of 19.58 MJ/kg among the selected wood dust biomasses, therefore, wood dust of Alstonia congenisis was torrefied and all the physico-chemical analyses were repeated on the torrefied samples. The results showed that, the torrefied wood dust samples were different in colour from the untorrefied (raw) sample. It was also observed that as the process temperature increased, increase in higher heating value (HHV) was observed with resultant decrease in energy yield. The percentage energy yield decrease with an increased in torrefaction process temperature. From the fiber composition of the torrefied samples, the result of lignin, cellulose and hemicellulose showed a direct connection with their chemical structure, the thermal stability of these components was discovered to be in the following trend: hemicellulose ˂ cellulose ˂ lignin. Moreover, the ash content was observed to increase with increase in torrefaction process temperature while the moisture content and volatile matter of the biomass sample inversely decreased with temperature. With increase in torrefaction process temperature, a significant increase was discovered in the fixed carbon of the biomass sample. The result of elemental composition showed that the carbon content of the biomass was enhanced upon torrefaction which implies good fuel properties of the Wood dust. Grindability and hydrophobicity tests indicated that torrefaction process increased the fragility and water-hating properties of the biomass. The observed decrease in moisture content greatly improved the fuel properties of the biomass as solid fuel. Hence, torrefaction process was confirmed as a value-added thermo chemical conversion process for Alstonia congenisis wood dust biomass.