GASIFICATION OF WOOD WASTE IN A DOWN DRAFT GASIFIER FOR THE GENERATION OF GASEOUS FUEL

Abstract

Gasification of lignocelluloses wood waste in a downdraft gasifier for the generation of gaseous fuel was investigated. Wood wastes (sawdust) from Pterygota macrocarpa; Cetis africana; Cetis zenkeri; Cordia millenii; Antiaris africana; Terminalia superba; Cieba pentandra and mixture of species were obtained from sawmills in Akure metropolis, Nigeria. The seven (7) sawdust samples and one unclassified mixed sawdust sample were dried and sieved into two particle sizes (600 and 850 microns retained) before characterisation. The various analyses conducted to determine the properties of these samples include proximate, ultimate (CHN), bulk density, heat capacity, mineral and compositional analysis , thermal behaviour using thermogravimetric analyser, and structural using X-ray diffractometer (XRD) and Raman spectrometer. The results of the analyses helped to select three samples (Cetis Africana, Cordia millenii and an unclassifief sawdust mixture) for further study. Densification of the three selected samples into pellets, involved moisture content determination, introduction of binder and pellet formation. The binder used for the pellet formation was starch sourced from cassava. The moisture content, ash content and solubility of the binder were assessed using standard methods. The pellets were formed at different binder concentrations of 20%, 25%, 30%, 35% and 40% w/v. An analytical study of these concentration was carried out on the selected samples using an in-house pelletiser of shape factor 2.78 and 35% w/v were selected as best concentration. The moisture content, pellet dimension (length (L), diameter (D) and L/D ratio), unit density and bulk density were determined using standard methods. The compressive strength was determined v using a universal testing machine (Instron, 3369). Combustion behaviour of the pellets was assessed using an eco-friendly domestic stove which burns with less carbon (iv) oxide emission. Surface analysis of the pellets was carried out using scanning electron microscope, and the calorific value was determined using a bomb calorimeter. Particle size x > 600μm and 30% w/v binder/water concentration gave the best strength for the samples. Cetis africana was adjudged as the best, it generated the least smoke. A throated downdraft fixed bed gasifier was fabricated in-house with different process units. The test running, leak test, and calibration of the process unit temperature were done to confirm the functionality and optimise the gasifier operational conditions. The gasification was achieved at a temperature of 780oC, an equivalent ratio ER of 0.3 and a flow rate of 2.08 x 10-3 m3/h and efficiency of 70%. The biochar was used as a modifier in the process. The gas produced was collected in a gas tube of internal diameter 0.067m, and quantified by water displacement to determine production of the syngas (CO and H2). The absorption of CO in the catalytic converter was approximately 190g and the H2 absorbed using a porous clay gave approximately 0.061g, both in 45minutes of gas absorption. A flame test was done to confirm the presence of syngas. This relatively clean gas supported burning with a blue flame and then a characteristic pop sound with blue flame was observed as it passes through the catalytic converter confirming the presence hydrogen gas. The data obtained from the study were analysed statistically using SPSS version 23.