Carbonization and briquetting of sawdust for use in domestic cookers

Abstract

Sawdust represents a considerable amount of energy that is currently under-utilized. With an appropriate thermodynamic process this by-product can be converted into charcoal briquettes suitable for domestic use. In this study sawdust was carbonized in a traditional brick-lined metal kiln. Pyrolysis conditions were studied with particular reference to temperature, air flow rate and moisture content. Charcoal yields of 28% (dry weight basis) was obtained from this system at pyrolysis temperatures of 450-550·C. Lower yields were obtained at temperatures below and above this range. Sawdust and wood containing moisture content of 40% and 35% respectively gave charcoal yields of 19.8% compared wi th 28% for input wi th moi sture content of 30% (sawdust) and 26% (fuelwood) on dry weight basis. The energy content of sawdust, wood and their carbonized products was determined using an adiabatic bomb calorimeter. Fully carbonized charcoal had calorific value between 28MJ/kg and 30 MJ/kg against 19 MJ/kg for uncarbonized sawdust. Partially carbonized or torrid sawdust had calorific value of 23-24 MJ/kg. Proximate analysis revealed that the percentage fixed carbon of pyrolysis products increases with the degree of carbonization. The fixed carbon content was 56% and 70% for fully carbonized sawdust charcoal and wood charcoal respectively. Torrefied sawdust contained more volatiles but less fixed carbon (50%). Densification of sawdust charcoal was done using a hand operated hydraulic press and mechanical briquetter. The bulk density of briquettes could be increased from 160kg/rrf of loose material to about 540 kg/rrf using a compression pressure of 875N/cm2 in conjunction with either 5% molasses or 3% starch 'binder. With a constant compression pressure of 350N/crrf the bulk density increased from 330 kg/rrfto 450kg/rrf on increasing the ratio of molasses from 0 to 20%, respectively. Fully carbonized sawdust charcoal was more friable and had better briquetting properties than torrefied sawdust. Charcoal briquettes and ordinary lump charcoal were burned in ordinary cookers and their combustion characteristics compared. Charcoal briquettes caught fire more ready than ordinary charcoal when lighting and on average took 9 and 11 minutes respectively. Charcoal briquettes reached' their peak glowing combustion 30 minutes after ignition but their fire died out within one and a half hours compared with 40 minutes and two hours respectively for ordinary charcoal.