Emission reduction measures for biomass combustion are available for
all harmful emission components; whether the emission reduction measures
are implemented or not is merely a question of emission limits and
cost-effectiveness. Though scale-effects ensure that large installations (such
as coal power plants) can be equipped with flue gas cleaning more
economically, local availability of the biomass fuel and transportation
costs will usually be a limiting factor for size.
NOX and SOX emissions from biomass combustion applications are in
general low compared to those from coal and oil combustion, and secondary
reduction measures are usually not required to meet emission limits.
Emissions of NOX from biomass combustion applications originate mainly
from the nitrogen content in the fuel, in contrast to fossil fuel
combustion applications where nitrogen in the air to some extent also
contributes to the NOX emission level. In most cases the NOX emission
level can be significantly lowered by the use of primary emission
reduction measures, and can be further decreased by implementing secondary
emission reduction measures.
The main disadvantage of small-scale applications that are based on
natural draft and operated batchwise (such as wood stoves, fireplaces, and
wood log boilers) are their high levels of emissions from incomplete
combustion. For such small units, combustion process control systems are
usually not cost-effective.
Limiting values for gaseous (especially NOx) and particulate emissions
are continuously reduced by the authorities, and this raises the need for
major R&D efforts. This is particularly the case for biomass fuels
rich in N and ash, such as waste wood and energy crops. Small-scale
combustion units are of special concern, as they need simple and
Solid ash and soot particles, emitted from biomass combustion
installations, are important sources of aerosols. Therefore, mitigation of
aerosols that result from biomass combustion deserves increased attention
from research organizations, manufacturers of boilers, and particle
removal technologies as well as policy makers. Equipment manufacturers
therefore need to be encouraged to develop novel, low?cost, combustion
installations and filtration techniques that result in low particulate
emissions in small-scale applications as well as large ones.
Finally, questions remain regarding the most environmentally sound and
affordable manner for processing ashes from clean and contaminated biomass