On-line monitoring of fuel moisture-content in biomass-fired furnaces by measuring relative humidity of the flue gases

Combustion of biomass for heat and power production is continuously growing in importance, because of incentivesfor replacing fossil energy resources with renewable ones. In biomass combustion, the moisture content of the fuel isan essential operation parameter, which often fluctuates for biomass fuels. Variation in moisture content complicatesthe operation of the furnaces and results in an uncertainty in the energy content of the fuel delivered to a plant.The fuel moisture-content in a furnace may be determined either by direct measurement on the entering fuel or bymeasuring the moisture and oxygen contents of the flue gases deriving the moisture content of the fuel. However,reliable methods of a motivated cost for the small to medium-scale furnaces are today not available. An exception isif the furnace is equipped with flue-gas condenser, which can be used to estimate the moisture content of the fluegases. A limitation of this method is, though, that not all furnaces have flue-gas condensers and that the measuredsignal has an inherent time delay.In thiswork, measurement of the relative humidity (RH) of the flue gases froma furnace is investigated as the centralcomponent in the on-line monitoring of the moisture content of the fuel in a furnace. The method was analysedwith humid air in a laboratory environment and tested for accuracy and dynamical behaviour in two biomass-firedheat-production units, one circulating fluidised-bed boiler (CFB) and one grate furnace. The results show that themethod, which is easy to calibrate on site, can be used to predict the moisture content of the biomass fuel in thegrate furnace with very good precision