THERMOCHEMICAL CONVERSION: DIRECT COMBUSTION - Biomass as a Sustainable Energy Source for the Future - page 291

usually very low. This may be explained by their high alkali content, which leads to
the formation of salts (KCl, NaCl) and thus to a lower level of gaseous chlorine for the
formation of dioxins. In general, the PCDD/F emission level from biomass combus-
tion applications using virgin wood as fuel is well below the health risk limit.
Ammonia (NH 3 ) is an intermediate in the conversion of fuel N to oxidized
nitrogen-containing components. The fuel N is mainly released as NH 3 during
devolatilization of biomass. At very low temperatures, this NH 3 is not completely
oxidized and is emitted. Additionally, NO reduction measures utilizing NH 3 injection
may contribute to the NH 3 emission level due to ammonia slippage.

9.5.3 Emission Reduction Measures

Reduction of harmful flue gas emissions can be achieved either by avoiding the for-
mation of such substances (primary measures) or by removing these substances from
the flue gas (secondary measures), which are described in the following sections.

9.5.3.1 Primary Measures for Emission Reduction The goal of primary reduction
measures is to prevent or at least reduce the formation of unwanted substances in the
combustion chamber. Primary measures are mainly used to reduce NO x emissions
and emissions resulting from incomplete combustion. Since the latter emissions are
mainly a result of a lack of oxygen, insufficient mixing of fuel and oxygen, too
low temperatures, and too short residence times, primarymeasures often aim to improve
these conditions. Several possiblemeasures — often interrelated — are discussed below.

Modification of the Fuel Composition : One way to reduce harmful emissions is to
decrease the amount of elements in the fuel that cause these emissions. The possibi-
lities of decreasing the amount of specific elements in biomass are limited. However,
the washing (leaching) of straw has been shown to reduce the amounts of chlorine
and potassium significantly, leading to reduced dioxin and furan emission levels.
Modification of the Moisture Content of the Fuel : Biomass fuels often have a high
moisture content. Fresh wood from forests may contain up to 60 wt% water.
A high moisture content in the fuel reduces the heating value and makes it difficult
to achieve a sufficiently high temperature in the combustion chamber. Drying is
often too costly to make it economically feasible, unless waste heat from another
process can be accessed at a very low cost (see Chapter 8 describing such drying
techniques). Drying biomass in the open air by exposing it to the sun and the wind
is a cheap and simple alternative.
Modification of the Particle Size of the Fuel : The fuel particle size is very relevant for
the burnout time. The fuel size in biomass combustion applications may vary from
whole wood logs to fine sawdust. Due to a large specific surface area, small
particles burn much faster, resulting in more complete combustion. The size of
large fuel particles can be reduced by a shredder or chipper. Special hammer mills
can be used to create millimeter-sized particles for dust firing (see Chapter 8 for
more details on particle size reduction). However, particle size reduction is only
attractive if the benefits outweigh the additional investment and energy costs.