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Coal and biomass cofiring: fundamentals and future trends 133

to reduce their GHG emissions significantly by substituting a portion of their base fuel
with a “carbon-free” fuel such as biomass. Biomass cofiring helps to reduce carbon
dioxide emissions, other airborne emissions such as oxides of sulfur and nitrogen,
as well as trace metals.

5.6.1 CO 2 emissions

One of the primary drivers behind adoption of cofiring is the reduction of net CO 2
emissions, as biomass is considered to be a carbon-neutral fuel. Biomass cofiring
has the potential to reduce the GHG emissions due to the net CO 2 released from the
combustion of biomass being reduced to zero. Although different studies have reported
a wide variation in the CO 2 avoidance cost when adopting cofiring, the current
consensus is that biomass cofiring is one of the most cost-effective and easily
deployed, with short implementation times and CO 2 -mitigation options from the
coal power sector. In any case, the cost of CO 2 avoidance by biomass cofiring seems
comparable with that of CO 2 mitigation by carbon capture methods, and cofiring has
the advantage of being a more proven and less risky technology.

5.6.2 NO x and SO x emissions
In general, it has been demonstrated that SO x and NO x emissions decrease uniformly
when biomass is cofired with coal (Badour et al., 2012; Leckner and Karlsson, 1993;
Pedersen et al., 1997; Skodras et al., 2002). This is because the biomass contains
comparatively less S than coal, although a greater reduction is predicted during sulfur
retention from coal by alkali and alkaline earth compounds (Ca, K) in the biomass
fuels (Armesto et al., 2003). In biomass cofiring, the main sources of NO x are thermal
NO from the N 2 in the air and fuel NO from coal, while NO x emissions from biomass
fuel are minimum. The fuel nitrogen content of biomass is mainly converted to NH 3
during combustion, which has a lower conversion to NO than the HCN usually formed
from coal combustion (Gayan et al., 2004). Ammonia may also contribute to the cat-
alytic reduction of NO x under reducing conditions (Sami et al., 2001). In an experi-
mental study of cofiring, where the amount of biomass mixed with coal was in the
range of 0%e16% on the basis of energy input, at a high feed rate of biomass of about
24 t/h in a large-scale (300 MW) coal-fired power plant, there was a drastic reduction
of NO x emissions, about 10% (Wang et al., 2011). Biomass fuels typically produce
higher volatile yields than coals, creating larger fuel-rich regions compared with
coal in the near-burner regions, which promote NO x reduction reactions.
Generally, the flue gas passes to an electrostatic precipitator (ESP) or bag filter to
remove particulate matter. The emissions of particulate matter that occur during
biomass combustion are usually higher than those of natural gas or gasified coal.
The effectiveness of the flue gas treatment systems, such as ESPs, might be affected
owing to the increase in flue gas volume during cofiring. Sulfur can be removed using
flue gas desulfurization, whereas oxides of nitrogen can be controlled by modifications
to the burners. Cleanup systems for NO x such as selective catalytic reduction (SCR)
and selective noncatalytic reduction can also be adopted. Each of these technologies