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].
Since the fuel ash and its composition directly affect the gassolids
flow and the conditions in the vicinity of heat exchanger
surfaces and since almost 50% of heat transfer surface failures in
power boilers are associated with malfunctioning of superheaters it
is important for each individual boiler to investigate the potential
effect of the addition of any new fuel on possible increase of solids
accumulation and decrease of the heat transfer rate and tube
damage. Those aspects have to be particularly carefully investigated
when co-combustion of various fuels with coal is performed since
in such case the probability to form any eutectic mixture on the
steel surfaces is significantly increased [12,14]. Accordingly, in this
paper the effect of co-combustion of coal and some chosen
renewable fuels of biomass and agromass origin is investigated
with particular attention put to the assessment of the effect of
biomass or agromass type on solids accumulation on P91 steel tube
samples. The P91 or X10CrMoVNb9-1 steel ([17]) is martensitic
quenched and tempered alloy steel used in modern power boilers
to manufacture the tubes of steam final superheaters. Due to special