Biomedical Engineering Reference
In-Depth Information
variable, the actual variation in quality experienced by the burner is
much less. Torrefaction helps reduce the difference in combustion
characteristics and heating value of the biomass feed.
10.7.1 Effect of Cofiring on Plant Output
The capacity or thermal output of a boiler could reduce for two reasons: the
boiler furnace is not able to generate the designed energy input and the avail-
able heating surfaces of the boiler are not able to absorb the required amount
of heat. The volumetric energy density of biomass is much lower than that
of coal. For example, volume energy density of raw wood is 5
8 MJ/m 3
is 30-40 MJ/m 3
while that
for
typical coal
because of
lower density
1350 kg/m 3 ) and lower heating value (
(350
680 vs. 1100
17
21 vs.
B
24
33 MJ/kg dry basis) of the biomass (Table 4.3). When coal is replaced
by an equivalent (by energy content) amount of biomass in a boiler plant, a
significantly larger volume of biomass is to be handled by the existing feed
preparation, feeder, and the burner system. In most cases, these components
of a PC-fired boiler lack adequate spare capacity to handle such a large
increase in volume throughput. Among these, the capacity of the pulveriza-
tion mills is the major limitation.
There is another reason why capacity of a coal-fired boiler could reduce
when cofired with biomass. For a given energy input, the amount of flue gas
increases when one replaces coal with biomass. In Example 10.1, one can
see that though biomass contains a significantly larger amount of oxygen,
the air requirement per unit MJ heat input is about the same as that for coal,
but the mass of flue gas produced by biomass is higher. So, biomass cofiring
could place an extra load on the existing induced draft fan and downstream
units of the boiler plant. This necessitates reductions in boiler output. For
heat absorption, the only limitation may be on the flame emissivity due to
higher H 2 O fraction in the flue gas. Since biomass may constitute only a
small part of the total feed in normal cofiring, flue gas emissivity may not
bring about major change in heat absorption. The only limitation could be
the flame temperature if it is reduced due to lower heating value of the bio-
mass fuel.
10.7.2 Feed Preparation
In a pulverized coal (PC)-fired boiler, pulverizing mills grind coal to about
75
m size, and transport it pneumatically through pipes to burners for com-
bustion in a flame. A fluidized-bed boiler, on the other hand, would require
the fuel to be crushed to only less than 10 mm size and dropped under grav-
ity into the furnace. Thus, cofiring of biomass in a fluidized-bed combustion
μ
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