Biomass Cofiring and Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 354

of biomass cofiring is relatively small ( 10% by mass), much of the existing

infrastructure of an existing power plant could be utilized for cofiring. For
the same reason unlike CCS, cofiring would have a minimum performance
penalty on the existing power plant.
Several excellent references (Tumuluru et al., 2011a,b; Van Loo and
Koppejan, 2008) are available for direct combustion of biomass for cofiring.
So, this chapter would not discuss much on direct combustion of biomass in
boilers. It will instead discuss cofiring in general and an improved option of
biomass cofiring, where biomass is torrefied before it is fired in the boiler.

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10.2 BENEFITS AND SHORTCOMINGS OF BIOMASS
COFIRING
The option of cofiring a small amount of biomass in an existing fossil fuel
fired power plant has several advantages over complete switch over to
biomass:

1. It is one of the most cost-effective practical means for GHG reduction.
2. Modern fossil fuel fired steam plants, because of its high-temperature and
pressure, are much more efficient than smaller conventional biomass
energy conversion systems. Cofiring that rides on such plants naturally
offers much higher energy conversion efficiency for biomass.
3. Cofiring biomass in existing coal-fired boilers is among the lowest gener-
ation unit ($/kWh) cost among biomass-based power production options.
4. Combustion technology being conventional, cofiring has the lowest tech-
nical risk and is ready for immediate implementation in large scale.
5. The carbon dioxide abatement cost ($/ton CO 2 ) of cofiring is much lower
than that in CCS ( Figure 10.1 ).
6. Biomass cofiring may have some synergistic effect on corrosion. For
example, sulfur and aluminum silicate in coal/peat could combine with
the alkali in biomass forming alkali silicate/sulfate preventing the forma-
tion of corrosive alkali chloride compound in biomass-fired plant
(Kasman and Berg, 2006).

Since biomass cofiring rides on a highly efficient steam power plant
equipped with advanced pollution control systems, the biomass energy con-
version efficiency is very high and cleanest. Figure 10.1 shows that cofiring
could reduce the CO 2 abatement cost considerably. One should, however, be
careful about this comparison as it depends much on the price of biomass. For
example, the cost of CO 2 abatement by cofiring may even exceed that by CCS
if the biomass price exceeds $120/dry ton (Ortiz et al., 2011, p. xvii).
One shortcoming of biomass cofiring is a modest reduction in the thermal
efficiency of the plant. Using data from three commercial plants, Tillman
(2000) correlated empirically the loss in plant efficiency with the amount of
biomass fired on percentage mass basis (Z). This equation may not be