THERMOCHEMICAL CONVERSION: DIRECT COMBUSTION - Biomass as a Sustainable Energy Source for the Future - page 288

turbine power cycle. Temperatures are controlled by recycling water (or CO 2 ) in com-
plete power systems. In both pulverized coal and power cycle applications, the current
state of the art is such that oxyfuel combustion plants can be built but also existing
plants can be retrofitted using existing technologies. However, such plants are not
optimized due to a lack of data or proven computer models of oxyfuel combustors,
boiler systems, and CO 2 recovery systems. Technology is in a continuously develop-
ing stage to model these systems with increasing accuracy. At present, several oxyfuel
combustion facilities at various scales of power generation are in operation or being
constructed around the world.
9.5 EMISSIONS
Combustion of biomass results in the emission of unwanted by-products. In this section,
the emissions from biomass combustion and measures to reduce them are discussed.
The emissions can be a result of complete and incomplete combustion.
9.5.1 Emissions from Complete Combustion
Carbon dioxide (CO 2 ) is the main combustion product, originating from the carbon
content in the biomass, and anthropogenic CO 2 emissions are believed to be the main
cause of global warming. However, combustion of biomass can be regarded as being
highly CO 2 neutral with respect to the greenhouse effect as is explained in Chapter 1.
This is the main environmental benefit of using biomass as a fuel.
Nitric oxides (NO x ) emissions from combustion applications are caused by
oxidation of nitrogen originating from the fuel or the air. During the combustion proc-
ess, mainly NO is formed (>90%), which is converted to NO 2 in the atmosphere. Nitric
oxides react to form smog and acid rain. The three main NO formation mechanisms in
combustion applications are (Glarborg et al., 2003; Hill and Douglas Smoot, 2000):
1. Thermal NO mechanism : At high temperatures (T > 1600 K), nitrogen in the air
reacts with O radicals present in the postflame gases. This leads to the formation
of NO and N radicals, which can further react with O 2 and OH radicals formed
in the flame to NO. The initiating reaction step has a very high activation
energy, which makes this formation mechanism very sensitive to temperature.
The temperature in biomass combustion applications is often too low to form
large quantities of thermal NO.
2. Prompt NO mechanism : Nitrogen in the air may also react with CH radicals in
the flame. This reaction forms NCN, which is quickly converted to HCN and
NH 3 . If sufficient oxygen is available, NH 3 and HCN are mainly converted
to NO via different routes. However, at fuel-rich conditions, NO will react
with NH 3 and HCN, forming N 2 . In applications of biomass combustion,
the prompt NO formation mechanism has not been found to be of major
importance.