THERMOCHEMICAL CONVERSION: DIRECT COMBUSTION - Biomass as a Sustainable Energy Source for the Future - page 290

Carbon monoxide (CO) is the final intermediate in the conversion of fuel
carbon to CO 2 . Oxidation of CO is the last elementary reaction step in this process.
CO emissions can, therefore, be regarded as a good indicator of how complete the
combustion is. If oxygen is available, the rate at which CO is oxidized to CO 2 depends
primarily on temperature. Therefore, CO emission levels are minimal at an optimum
air ratio: lower air ratios will result in fuel-rich regions, while higher air ratios will
lead to lower combustion temperatures and shorter residence times, all resulting in
increased CO emissions.
Methane (CH 4 ) is an important intermediate in the conversion of fuel carbon to
CO 2 and fuel hydrogen to H 2 O in biomass combustion. Hydrocarbons, in general,
are earlier intermediates than CO and are, therefore, emitted at lower levels. Methane
is usually mentioned separately from the other hydrocarbons since it is a direct green-
house gas.
Volatile organic compounds (VOCs) are a group of species that include all
hydrocarbons except methane, polycyclic aromatic hydrocarbons (PAHs), and other
heavy hydrocarbons that condense and form particle emissions. They are all inter-
mediates in the conversion of fuel carbon to CO 2 and fuel hydrogen to H 2 O. VOCs
have a negative effect on the human respiratory system. Furthermore, VOCs are indi-
rect greenhouse gases because they are precursors of ozone in the atmosphere, which
is a greenhouse gas.
PAHs are often mentioned separately from the other hydrocarbons due to their car-
cinogenic effects. PAHs that condense at relatively low temperatures (<200 C) are
called tar (Milne et al., 1998). Tars evolve from primary tar, i.e., the tar that is released
from the solid fuel by breaking of molecular bonds during pyrolysis. After release,
these primary tars can grow larger in size by different kinds of reactions. Alternatively,
when the residence time at elevated temperatures is sufficiently long, tars can also be
cracked into smaller molecules, mainly gases. The fact that the tars condense is a
major problem in biomass installations since the condensed tars foul downstream
equipment and cause engine wear (by corrosion and erosion). The fouled installation
parts need to be cleaned regularly, thus increasing operating costs.
Particle emissions from incomplete combustion consist of soot, char, and con-
densed heavy hydrocarbons (tar). Soot particles mainly consist of carbon and are
formed in fuel-rich regions of the flame. Char particles are entrained in the flue
gas due to their very low density, especially at high gas flow rates. Tar particles sig-
nificantly contribute to the total particle emission level in small-scale biomass com-
bustion applications such as woodstoves and fireplaces. Particle emissions have
carcinogenic and negative respiratory effects.
Polychlorinated dibenzodioxins and furans (PCDD/F) are a group of highly
toxic compounds. Dioxins are formed during combustion of fuels containing chlorine
(Chagger et al., 1998). Ideally, a combustion process converts all carbon to CO 2 and
all chlorine to HCl; however, in a temperature window between 400 and 700 C, car-
bon, chlorine, and oxygen react in the presence of a catalyst (Cu, present in fly ash
generated) to form PCDD/F. The emissions of PCDD/F are highly dependent on
the conditions under which combustion and flue gas cooling take place. Although her-
baceous biomass fuels have high chlorine contents, their PCDD/F emissions are