| Abstract [eng] |
Biomass boiler houses face some typical problems and a task of this work is to recognize these problems, to find out the causes and solutions how to decrease its influence for boiler house operation. Main tasks of the first tests were to perform operating parameters measurements when feeding furnace with average moisture content fuel and dry fuel, to calculate boiler efficiency and to analyze boiler behavior combusting various moisture content fuel. Different grate movements cycles ratio values was noticed. This ration influence in fuel layer thickness on grate, and the higher the ratio, the thicker fuel layer is. Primary air pressure dependence on fuel layer thickness confirms this statement. Different movement of first and second grate part causes fuel layer to fracture. As fuel moisture increases, grate movements cycles ratio should be increased as well. Biomass combustion from bottom to top was noticed. Thin hot coal layer covers the grate and primary air works as catalyst. Hot flue gas dries wet fuel as it flows thru it. Possibilities to improve combustion process were determined. Increasing primary air amount would speed up fuel drying process. Combustion air-preheating acts as catalyst not only for drying, but also for ignition and intensification of combustion process. Recirculation flue gas flow will dry fuel more quickly if it will not be mixed with primary air. Mixing hot flue gas with cold air, mixture temperature drops below water vapor saturation temperature and excess moisture condensates. Increasing grate movements cycle’s ratio improves combustion of wet fuel, however primary air flows has to be redistributed as well. There was no need to change any settings to combust biomass with 54% moisture content, existing regime card was enough. Feeding wet fuel causes boiler water outlet temperature to decrease, i.e. boiler load decreases. In order to increase boiler load, fuel feeding was increased manually. After increasing fuel amount, secondary and tertiary air fans speed increased automatically, causing higher air excess ratio. Tertiary air set value was decreased manually to maintain lower air excess ratio. Combustion process has stabilized after increasing primary air first and second fan load and decreasing third primary air fan load. Wet fuel contained high amount of conifers spikes, which has ~1,8% fuel nitrogen content, therefore higher nitrogen oxides emissions were measured. Fuel moisture content has a highest influence for thermal capacity of flue gas condensing economizer, moreover the higher moisture content is the more significant effect on economizer capacity it has. Flue gas temperature after economizer and air excess ratio also has significant effect. To increase flue gas condensing economizer efficiency, air excess ratio must be kept as low as possible. Continuously CO measurement would help to decrease air excess ratio. Flue gas inlet temperature has low effect to the thermal capacity of condensing economizer. |
