Factors affecting combustion efficiency in wood-fired boilers

The combustion efficiency of wood-fired spreader-stoker boilers is limited primarily by carry-over of unburned char from the furnace. This carry-over is caused by incomplete combustion of entrained fuel particles. Data from four experimental studies and one computer study of wood-particle entrainment and combustion are brought together to investigate the factors that cause char carry-over in these units. Single-particle data on entrainment and combustion are first presented, followed by data for multiple-particle combustion. This information is then used in a model of the processes occurring in a wood-fired furnace. It is shown that for typical fuel size distribution, the fraction of fuel entrained can be as little as 10% but is typically greater than 50%. The state of the largest particle entrained is that of dry wood. Under typical furnace conditions, the volatile fraction of the wood is approximately 90%, so that char will constitute only about 10% of the original dry wood mass. Devolatilization is very rapid for the entrained particles, so that the char combustion rate will be the most important factor in determining carry-over. Because particles smaller than about 1 mm usually burn out in the furnace, char carry-over is attributable to the fraction of the fuel thatmore » is initially larger than 1mm but smaller than the largest particle entrained. (Refs. 4).« less