On-line monitoring of fuel moisture-content in biomass-fired furnaces by measuring relative humidity of the flue gases
Journal article, 2011
Combustion of biomass for heat and power production is continuously growing in importance, because of incentives
for replacing fossil energy resources with renewable ones. In biomass combustion, the moisture content of the fuel is
an essential operation parameter, which often fluctuates for biomass fuels. Variation in moisture content complicates
the 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 by
measuring 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 is
if the furnace is equipped with flue-gas condenser, which can be used to estimate the moisture content of the flue
gases. A limitation of this method is, though, that not all furnaces have flue-gas condensers and that the measured
signal has an inherent time delay.
In thiswork, measurement of the relative humidity (RH) of the flue gases froma furnace is investigated as the central
component in the on-line monitoring of the moisture content of the fuel in a furnace. The method was analysed
with humid air in a laboratory environment and tested for accuracy and dynamical behaviour in two biomass-fired
heat-production units, one circulating fluidised-bed boiler (CFB) and one grate furnace. The results show that the
method, which is easy to calibrate on site, can be used to predict the moisture content of the biomass fuel in the
grate furnace with very good precision (<4% error). Furthermore, the method detects variations in moisture content
of the furnace flue gases due to changes in the moisture content of the combusted fuel within the order of seconds.
Since the transport time of the flue gases from the furnace to the measurement position is of the same order of
magnitude, the total time for detection of a change in the moisture content of the fuel is small enough for the signal
to be used to control both the fuel feed and the combustion air in a grate furnace.