Page 159 - New Trends In Coal Conversion

122 New Trends in Coal Conversion

capital cost required because the needed additional installations in an existing coal-
fired power plant are kept to a minimum. Direct cofiring of biomass and coal takes
advantage of the high efficiencies obtainable in large coal-fired power plants and im-
proves combustion due to the higher volatile content of the biomass, although it pre-
sents several limitations such as the tendency of producing ash deposition, the limited
range of cofiring, and the lack of flexibility to use different types of biomass (Roni
et al., 2017). Biomass is directly fed into the furnace with coal. Biomass can be milled
jointly with the coal in the same milling equipment (typically less than 5% in terms of
energy content) or premilled and then fed separately into the same boiler. Common or
separate burners can be used, with the second option enabling more flexibility with re-
gard to biomass type and quantity. As more modifications are introduced, higher cap-
ital investment is required, and a more significant technological risk is involved if the
modifications also affect the boiler itself. However, the degree of control that can be
achieved on combustion conditions is greater, and the risk to boiler operation through
the impact of biomass is minimized. In most cases, the biomass thermal input in direct
cofiring schemes is around 10% owing to technical and economical restraints (Karam-
pinis et al., 2014). This limitation could be minimized by torrefying and densifying
biomass so that it behaves more like coal in terms of energy content and grinding
behavior (Tumuluru et al., 2012).

5.3.2 Indirect cofiring
Indirect cofiring involves gasification of the solid biomass in a separate gasifier, where
it is converted into a fuel gas that is then burned with coal in the same boiler. Although
it is more expensive because of the additional gasifier, this option allows for a greater
variety of biomass types and higher percentages of biomass to be used. Indirect cofir-
ing can reduce slagging because biomass does not directly feed into the coal furnace,
and it allows for a separate collection of residues. As the gas can be injected in many
cases directly into the furnace, the plant can avoid the energy conversion losses and
expensive flue gas cleaning, which are common issues in biomass gasification. How-
ever, depending on the biomass fuel quality and the presence of chlorine and alkalis,
flue gas cleaning could be required (Basu et al., 2011).

5.3.3 Parallel cofiring

In parallel cofiring systems, biomass and coal are fed into separate boilers, and the
steam generation from biomass is then mixed with steam from the conventional boiler.
Biomass preprocessing, feeding, and combustion activities are carried out separately.
This method allows for high biomass percentages and is frequently used in pulp and
paper industrial facilities where dedicated biomass boilers are used for bark and waste
wood. Although the cost of parallel cofiring is significantly higher than the direct op-
tion, it is still lower than a dedicated biomass power plant, and it may assist in opti-
mizing the combustion process and in utilizing difficult fuels with high alkali and
chlorine contents (Dai et al., 2008). The efficiencies that can be achieved are higher
than a stand-alone biomass power plant, whereas the operation of the coal boiler is