Environmental Engineering Reference
In-Depth Information
(EPRI, 1997). Combustion is the burning of biomass in air. This involves the conver-
sion of chemical energy stored in biomass into heat, mechanical power, or electricity
(McKendry, 2002). Although it is possible to use all types of biomass, combustion
is preferable when the biomass is more than 50% dry. High-moisture biomass is bet-
ter suited for biological conversion processes. Net bioenergy conversion efficiencies
for biomass combustion power plants range from 20 to 40%. Higher efficiencies are
obtained with combined heat and power (CHP) facilities and with large size power-
only systems (over 100 megawatt-electrical, MWe), or when the biomass is co-fired
with coal in power plants (McKendry, 2002).
Co-firing biomass with coal is a straightforward and inexpensive way to diversify
the fuel supply, reduce coal plant air emissions (NO
x
, SO
2
, CO
2
), divert biomass from
landfills, and stimulate the biomass power industry (Hughes, 2000). Moreover, bio-
mass is the only renewable energy technology that can directly displace coal. Given
the dominance of coal-based power plants in U.S. electricity production, co-firing
with biomass fuel is the most economical way to reduce greenhouse gas emissions.
Possible biomass fuel for co-firing includes wood waste, short-rotation woody crops,
switchgrass, alfalfa stems, various types of manure, landfill gas, and wastewater
treatment gas (Tillman, 2000). In addition, agricultural residues such as straw can
also be used for co-firing.
A promising technology development currently at the demonstration stage is
biomass integrated gasification/combined cycle (BIG/CC), where a gas turbine con-
verts the gaseous fuel to electricity with a high conversion efficiency, reaching 40
to 50% of the heating value of the incoming gas (McKendry, 2002). An important
advantage of gasification is the ability to work with a wider variety of feedstocks,
such as high-alkali fuels that are problematic with direct combustion. High-alkali
fuels such as switchgrass, straw, and other agricultural residues often cause corro-
sion, but the gasification systems can easily remove the alkali species from the fuel
gas before it is combusted. High silica, also a problem with grasses, can result in
slagging in the reactor. The slagging problem is not unique to one form of biomass
but instead is common among many different types of biomass fuels (Miles et al.,
1993). Slagging deposits can reduce heat transfer, reduce combustion efficiency,
and damage combustion chambers when large particles break off. Research has
focused on two alkali metals, potassium and sodium, and on silica, all elements
commonly found in living plants. In general, it appears that faster growing plants
(or faster growing plant components such as seeds) tend to have higher concentra-
tions of alkali metal and silica. Thus, materials such as straw, nut hulls, fruit pits,
weeds, and grasses tend to create more problems when burned than does wood from
a slow-growing tree.
Potassium and sodium metals, whether in the form of oxides, hydroxides, or
metallo-organic compounds, tend to lower the melting point of ash mixtures con-
taining various other minerals such as silica (SiO
2
). The high alkali content (up to
35%) in the ash from burning annual crop residues lowers the fusion or “sticky tem-
perature” of these ashes from 2200°F for wood ash to as low as 1300°F. This results
in serious slagging on the boiler grate or in the bed and fouling of convection heat
transfer surfaces. Even small percentages (10%) of some of these high-alkali residues
burned with wood in conventional boilers will cause serious slagging and fouling in
