Page 165 - New Trends In Coal Conversion

128 New Trends in Coal Conversion

of coal particles (Karampinis et al., 2014). Particle shape and size affect char burnout
because biomass does not melt, and irregular shapes are maintained during combustion
(Backreedy et al., 2005). Overall, despite the unfavorable sizing, fuel conversion can be
maintained in high levels, especially when the biomass thermal share is low.

5.5.3.2 Deposits and corrosion
Most biomass cofiring projects use existing coal-fired combustion technologies
because a coal-designed power plant can be suitable for blending biomass with coal
after minimal modifications, making it an attractive proposition economically. A major
problem of biomass firing comes from the deposit formation, corrosion, and erosion
within the boiler system, which are related to the ash formed.
Two modes of deposit formation can occur, i.e., slagging and fouling. Slagging is
dominant in the high-temperature radiative sections of the boiler, such as furnace
walls, and occurs due to molten ashes, whereas fouling is primarily found in the
low-temperature convective sections, such as the heat exchanger tubes, and is related
to ash deposits during cooling (Teixeira et al., 2012). The slag forms when ash gets
fused or partially fused, whereas fouling occurs when alkali compounds condense
on the metal surfaces and also when fly ash is quenched below its melting temperature
and gets deposited (Madanayake et al., 2017). Slag and fouling deposits are primarily
composed of the chlorides, sulfates, hydroxides, and silicates of alkali and alkaline
earth metals. Deposits tend to reduce the heat transfer to tubes, decreasing the combus-
tion efficiency.
Some biomass fuels, such as herbaceous crops, have high alkali (principally potas-
sium) or chlorine content, or both. This can lead to unmanageable ash deposition prob-
lems on heat exchange and ash handling surfaces. Chlorine in combustion gases,
especially at high temperatures, can cause accelerated corrosion of combustion system
and flue gas cleanup components. Woody biomass seems to be less likely to contribute
to corrosion and deposition because it is lower in alkaline and chlorine. Normally high
alkaliecontaining biomasses settle down on the heat transfer surfaces causing Cl
condensation. There is a solution to reduce the alkali chlorides (primarily from the
biomass) while reacting with the sulfur (primarily from the coal) to form alkali sulfates
(Bhuiyan et al., 2018). On the other hand, as fouling, slagging, and corrosion of boiler
surfaces is influenced mostly by alkali and alkaline earth metals (such as K) and Cl,
leaching these elements from the fuel using H 2 O dramatically improves these ash-
related problems.
A further related problem which is associated to FBCs is theagglomeration of bed me-
dia dueto thepresence of thebiomass ash,which would resultin a reductionof heat trans-
fer. This issue is considered noteworthy, as FBC can otherwise be regarded as one of the
more desirable biomass combustor technologies due to the low and stable operating tem-
€
peratures that it uses, as well as the fuel flexibility it affords (Ohman et al., 2000).
5.5.3.3 Combustion modeling

Combustion modeling for coal/biomass blends is complex due to gas and two partic-
ulate phases, as well as the effect of turbulence on chemical reactions. Two chemically