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Coal and biomass cofiring: fundamentals and future trends 125

metals react with other inorganic nonmetallic components to produce deposit-forming
compounds. These deposits result in slagging in grate-fired combustors, fouling of heat
transfer surfaces, and bed agglomeration in fluidized bed combustors (FBCs). The
fouling impedes the heat transfer rate, whereas the slagging obstructs fuel feeding,
combustion, and ash removal. The presence of Cl and S also results in the formation
of acidic products on combustion, which causes accelerated corrosion of metal sur-
faces within the combustion system. Leaching or washing of the biomass with water
or an acid has been demonstrated as an effective method to reduce the content of alkali
and alkaline earth metals as well as Cl. Leaching can occur naturally if the biomass is
exposed to rain before harvesting (lower cost but dependent on local weather condi-
tions), or it can be carried out as an artificial process based on soaking the biomass
for a controlled period of time and consisting of a combination of mechanical dewater-
ing and rinsing processes (Madanayake et al., 2017).
Torrefaction. It is desirable to bring the physical properties of biomass as close to
those of coal as possible, which would make it possible to use existing conventional
power plants to fire biomass with minimal modifications of the handling and combus-
tion equipment. Torrefaction is a thermal pretreatment of biomass in two different
ways: dry torrefaction or wet torrefaction. Dry torrefaction is the biomass heating in

the absence of oxygen at temperatures of 200e300 C for nearly an hour, thus creating
a charcoal-like substance with reduced moisture, small particle size, minimal biolog-
ical degradation, and increased energy density. With torrefaction, moisture content is
reduced, which means a reduction of the H and O content of the biomass, and hence a
reduction in the H/C and O/C ratios, increasing the heating value. A further result of
the torrefaction is that the propensity to reabsorb water by the biomass is reduced,
which allows the fuel to be stored stably for extensive periods of time without its mois-
ture levels increasing significantly. Also, this treatment makes the biomass resistant to
microbial colonization, which means that there would be less tendency for biodegra-
dation to occur during storage (Madanayake et al., 2017). The poor grindability of
biomass is another issue that can be alleviated by torrefaction. Raw biomass tends
to be highly fibrous in nature, and the fibers form linkages which result in a resistance
to milling/pulverizing. Torrefied biomass has been observed to lack these fibers, as
well as to reduce the size of the particles at a microscopic level and make them
more spherical, which improves the grindability and flowability of biomass, as well
as their handling characteristics (Arias et al., 2008a; Gil et al., 2015). After torrefac-
tion, biomass can be milled and compressed to very dense pellets (black pellets) or bri-
quettes because torrefied biomass still has a low bulk density, which is challenging in
terms of transportation logistics (Tumuluru et al., 2012).
Wet torrefaction (or hydrothermal carbonization) also results in many of the
mentioned beneficial effects of dry torrefaction, although the two mechanisms differ.
Wet torrefaction is also known as hydrothermal pretreatment because it uses hot com-
pressed H 2 O as a treatment medium (as opposed to a N 2 environment in dry torrefac-
tion). The treatment temperature range is generally slightly lower than that of dry