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Coal and biomass cofiring: fundamentals and future trends 123

not affected by the addition of a biomass fuel. Combustion residues are also produced
separately and can be handled independently.

5.4 Cofiring ratios

The optimum cofiring ratios for selected combustion blends of biomass and coal must
be determined considering costs and performance of the plants. Cofiring more than
20% of biomass in terms of energy content is technically feasible. Depending on
the plant setup and the cofiring technology, substitution of more than 50% of coal
could also be achieved (IEA-ETSAP and IRENA, 2013). However, in most cases,
cofiring level in most commercial applications is up to 5%e10% (as energy content).
To control the furnace efficiency and production, the effect of biomass particle size,
biomass injection rate, thermal and fluid-dynamic behavior, as well as the design of the
burner are very important. Usually, the boiler types used for biomass cofiring record
little or no loss in total boiler efficiency after adjusting combustion output for the
new fuel mixture. Therefore, the efficiency of biomass feedstock combustion to elec-
tricity may range from 33% to 37% when biomass feedstock is cofired with coal. The
net electric efficiency of a typical biomass cofiring plant usually ranges from 35% to
44% (Agbor et al., 2014). R€ udiger et al. (1996) suggested that comparatively higher
burnout was observed with up to 20% thermal input of biomass. In addition, it has
been found that the physical and chemical properties of the biomass fuels had a direct
impact on the characteristics of the flame with the variation of cofiring proportions.
The main differences were observed in the flame ignition points and brightness,
although the flame stability was also affected by the addition of biomass (no more
than 20% by weight) (Lu et al., 2008).

5.5 Technical and logistical issues of cofiring

The biomass cofiring technology presents certain difficulties and drawbacks. These is-
sues are primarily caused by the difference in properties between coal and biomass.
The main drawbacks associated to the biomass cofiring may be related to the inherent
characteristics of the biomass as a fuel, i.e., quality and availability, biomass logistics
issues (handling, storage, transportation, preprocessing as drying or milling), price of
the biomass, or size of the biomass particles for burning and possibilities for injecting
biomass into the boiler. Other important groups of drawbacks of cofiring are related to
the biomass combustion process (performance), to the deposit formation (slagging and
fouling) and corrosion, to the flue gas from the burning process (cleaning), and to the
ash generated during combustion (landfill costs, applications).

5.5.1 Biomass type and availability

Biomass feedstock includes forestry and agriculture residues, residues from pulp and
paper industries, animal manure, wastes such as sawdust or bark from the timber