|Design of a Suspension Burner System for Forestry and Agricultural Residues (NRI, 1991)|
In developing countries it is common to find considerable quantities of particulate forestry and agricultural residues generated by local industries (Atchinson, 1976; Stout, 1979). The residues, a potential source of energy, are often considered to be a waste product and create problems of disposal. Paradoxically, the same industries that generate this potential source of energy are frequently burdened with high energy costs associated with their use of electricity, solid wood and fossil fuels to supply process heat.
Whilst international investigations into uses for particulate agricultural and forestry waste continue, burning probably remains the most important method of disposal. However, difficulties arise when burning these materials is attempted, as their often high moisture content and relatively low calorific values, coupled with difficulties of mechanical handling, make them an unsuitable fuel for standard solid-fired systems. In addition, discharges of black smoke are often associated with burning these materials on open-mounds and in poorly designed burner systems, and there is increasing environmental pressure to reduce these emissions.
The development of a simple and robust system suitable for adoption to burn cleanly - without smoke - a range of agricultural and forestry residues, and which would also lend itself to being easily tailored to supply process heat, would find widespread industrial application in these situations. It would offer a practical means of disposal of particulate forestry and agricultural residues with concomitant savings on other fuels - in particular, fossil fuels and fuelwood.
The Process Development and Storage Engineering Department of the Natural Resources Institute carried out a programme of work to construct and develop two suspension burner systems - a cyclonic furnace and a dual-chamber brick-built furnace - capable of burning a range of particulate forestry and agricultural residues, with a process heat output of approximately 250-1000 MJ/h for use in associated industries in developing countries. The residues tested in the furnaces were sawdust, coir dust, groundnut shells and rice husks.
This bulletin gives an account of the technical aspects of the cyclonic suspension burner which was found to be particularly suitable for burning sawdust. Details of the design and operation of the dual-chamber furnace for burning groundnut shells and rice husks, and which has potential for burning other biomass residues, are to be the subject of a separate report.
The success of the cyclonic suspension burner led to its use for the fuelling of a 16 m3-capacity timber drying system. The system was successfully field tested under commercial operating conditions in Belize. As the importance of kiln-drying timber is becoming more widely recognized in developing countries, there is increasing scope for the application of such systems.