Biomedical Engineering Reference
In-Depth Information
4. Biomass is less brittle and more fibrous than coal which results in signifi-
cantly different grinding characteristics.
The ash in biomass is much richer in compounds like K, Ca, and Si
(
Table 10.1
). Waste biomass could additionally pick up chlorine, potassium,
and heavy metals. All of these greatly increase the fouling, slagging, and cor-
rosion potential in a coal-fired boiler.
Table 10.1
shows the differences in
ash constituents between biomass and coal that affect fouling, corrosion, and
some results in operating problems in the cofired boiler.
10.6.1 Combustion Issues
Biomass particles are generally more reactive due to their higher volatile
content and more porous structure compared to those of coal. So a biomass
particle, dried to the same extent and ground to the same size as a coal parti-
cle, might burn faster. In this respect, it is not necessary to grind biomass as
finely as PC, but its size should not be so large that it will drop into the fur-
nace hopper unburnt when injected instead of being conveyed up the furnace
in flames.
There is another issue related to the ignition of fuels. The relatively
high-moisture content of biomass might delay the ignition of such parti-
cles. If the delay is significant, the flame could move further and further
away from the burner extinguishing itself. This is countered to some
extent by the fact that ignition temperature of dry biomass is much lower
than that of coal.
Experience (Van Loo and Koppejan, 2008, p. 235) suggested that there is
only negligible effect on the combustion efficiency for a modest level
(3
5%) cofiring of relatively dry (
,
10% moisture) biomass. The efficiency,
however, could be low for higher moisture in biomass. Batista et al. (1998)
developed an empirical correlation to assess the impact of the extent of cofir-
ing on the overall thermal efficiency of a coal-fired boiler:
Boiler efficiency
ð
%
Þ
5
TE
A
U
X
B
U
Y
C
U
Z
(10.3)
2
2
2
where
TE is the theoretical thermal efficiency (%)
X is the oxygen concentration in flue gas at boiler economizer exit (%)
Y is the sunburnt carbon in fly ash (%)
Z is the biomass cofired as mass percent of coal fired
A, B, and C are empirical constants
10.6.2 Fuel Preparation
The typical size of coal fed into a pulverized coal (PC) furnace is about 75
m
for suspension firing in burners. Biomass needs to be ground to a little larger
μ
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