and may be water- or air-cooled. New grate systems are
water-cooled to avoid slagging and to extend the lifetime
of the materials [51].
Grate-firing systems have some specific advantages: They
are able to handle heterogeneous fuels with large particle
sizes and high moisture contents up to 65 wt% water [17].
The investment costs for plants with a capacity less than
10 MWth are comparatively low and also the operating
costs are low [51]. Further advantages are the good burnout
of fly ash particles and the low dust load in the flue gas.
A good operation is also possible at partial load and the
grate-fired systems are less sensitive to slagging than fluidized
bed combustors. Disadvantages are that efficient NOx
reduction requires special technology and that the combustion
conditions are not as homogenous as in fluidized bed
systems [51]. To reach the same level of mixing of fuel and
air grate-firing units or underfeed stokers need special
equipment [63]. Further, comparatively higher excess air
ratios lower the energy efficiency [51].