New Method for Controlling Combustion in a Grate Boiler

The control of disturbances due to fuel quality variation is important development area in the grate combustion technology. Main challenges in the control of the grate combustion are inhomogeneous fuel, variation in fuel moisture content, non-uniform distribution of fuel on the grate area and high percentage of combustibles in ash. Information on events which occur in the furnace plays an important role in controlling of the grate combustion process. As the direct indicators of the combustion efficiency are not usually available, efficiency estimation reveals an important and challenging problem. In this article we propose a new measurement method to estimate combustion on the grate and consider aspects of its application. The purpose was to find regularities in the behaviour of the measurements and to analyse the reliability and the utilization of the measurements. The method is based on combining information obtained from the fuel bed pressure signals and the grate temperature measurement signals. Furthermore, besides information of the new measurements, the normal combustion measurements at the power plant were also taken into account. Online aspect of the new measurements receives special emphasis as the estimates of combustion efficiency should be available in real-time for the control purposes. The share of biomass (e.g forest residue and agro biomass) increases in the power plants due to their environmentally friendly and domestic character. It means that there will be a high importance to develop new effective methods to control the combustion process in the future. The method was tested at a combined heat and power plant 17 MWth combusting spruce bark. The new measurements give useful information on combustion on the grate. Especially by studying the pressure difference it was possible to notice variation in fuel quality. Combustion and ignition front location could be estimated by analysing simultaneously both pressure difference and temperature measurements. Changes in fuel bed height and combustion rate in the outermost circles of the grate were easy to spot.