Page 164 - New Trends In Coal Conversion

Coal and biomass cofiring: fundamentals and future trends 127

35
Coal Coal (anthracite)
30
Solid wood
Energy density (GJ/m 3 ) 20 Raw biomass Processed biomass Sawdust
Wood chips
25
Black liquor
Wood pellets
15
Torrefied wood pellets
10
Straw (baled)
5 Pyrolysis oil
Organic waste
0
0 500 1000 1500
3
Bulk density (kg/m )
Figure 5.2 Energy density of biomass and coal.
Adapted from IEA-ETSAP and IRENA (2015).

5.5.3 Biomass burning
5.5.3.1 Fuel conversion
Biomass combustion differs from coal combustion due to differences in their physical
properties and chemical composition. The higher volatiles content and low activation
energy of biomass make the pyrolysis and subsequent volatile oxidation start earlier
than for coal and at lower temperatures. Thus, a substantial fraction of the energy
from the biomass combustion comes from the oxidation of the volatiles, whereas
almost all the energy for coal combustion comes from char oxidation (Chao et al.,
2008). The devolatilization stage of the combustion process of blends of coal and
biomass (5e20 wt.% of biomass) has been studied by thermogravimetry, and higher
thermochemical reactivity was found for the biomass fuel compared with lignite
(Vamvuka et al., 2003b).
Biomass can burn more intensively and may give rise to higher local peak temper-
atures due to its higher reactivity than coal. The earlier release of volatiles from
biomass reduces the ignition temperature compared with coal and promotes flame sta-
bility. Both coal and biomass have similar ignition processes, but biomass fuels may
experience more homogenous and flaming combustion due to higher volatile materials
(Agbor et al., 2014). Flame stability has been found to be little affected by the amount
of biomass added when the addition is less than 20 wt.%. Moreover, premixing
biomass and coal can enhance the combustion of the two fuels, whereas poorly mixed
biomass and coal tend to burn independently at different rates (Lu et al., 2008).
In addition, biomass chars tend to be highly porous and very reactive under the con-
ditions found in pulverized fuel furnaces, while the shape of biomass particles, large and
physically complex, promotes more rapid combustion than the typically spherical shape