Biomass Cofiring and Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 366

A preferred feedstock for a cofired boiler is biomass pellet. The uniform size
of pellets makes it convenient to handle the fuel and grind it in mills. It is,
however, difficult to produce biomass pellets from a wide range of biomass
feedstock (Bergman et al., 2005, p. 11). This is an important barrier in the
wide-scale use of biomass cofiring, but this can be done using torrefaction
pretreatment.

10.6.6 Capacity Reduction
As mentioned earlier, the output of a coal-fired power plant could reduce
when biomass replaces coal in an existing boiler in spite of maintaining the
energy input unchanged. Additionally, if the existing coal mills are used for
biomass pulverization, the output of fuel would reduce due to higher mois-
ture in biomass, low-energy density of biomass, and other factors.

10.6.7 Safety Issue
Cofiring biomass with coal raises some safety issues that involve potential
for fire and explosion. Several incidents of fire and explosion in biomass
underscore the need for this issue. There are three major hazards of handling
coal in a cofired power plant. They are briefly described below:

1. Combustible dust: Coal, when moved through a belt conveyor or
loaded into a bunker, produces dusts. Such dusts are an explosion haz-
ard. The explosion potential depends on several factors including def-
lagration index. Higher the deflagration index larger is the explosion.
Table 10.4 compares this index of several fuels.
2. Spontaneous combustion: Self-ignition causes spontaneous combustion of
a fuel left in storage. Although very little data is available in published

TABLE 10.4 Deflagration Index of Coal and Biomass

Bituminous
coal

Powder river basin
coal Fuel Petcoke Cellulose Paper

Deflagration
index

47 150 225 229 200

Fuel Barley

straw

Corn Wood pellet Wood

bark

Sawdust

Deflagration
index

72 75 105 132 149

Source: From Power (2012), p. 22.