The development and industrial implementation of circulating
fluidized bed (CFB) combustion technology was a successful
response to the needs to reduce the emission of SO2 and NOx, and to
implement efficient co-combustion of coal with biomass or alternative
fuels in order to decrease the consumption of fossil energy,
generate electricity from renewable fuels, and reduce the anthropogenic
emission of CO2 [1e3].
However, in numerous publications [e.g. Refs. [4e6]] it is reported
that combustion processes (particularly co-combustion of
coal and biomass) are often associated with accumulation of solids
and formation of troublesome ash deposits on heat transfer surfaces,
particularly those located in the boiler convective section.
The deposits are in many cases considered to be brought about by
biomass ash alkali [7e13] and are often responsible for boiler
emergency shutdown, decrease of the heat transfer rate, or damage
of heat transfer elements and shortening of their lifetimes.
There are numerous reasons responsible for solids accumulation
and formation of deposits on heat transfer surfaces. However, due
to high flue gas temperature, the majority of operational ashrelated
problems usually occur on superheater tubes [14e17]. To
large extend, the range of problems and their significance also
depend on boiler particular design and hydrodynamics, as well as
on the composition of fuels particularly the presence of alkali and
chlorine that may form low temperature eutectic mixtures on heat
exchanger surfaces and thus increase the corrosion and tube
damage [14,18,19].