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 affordable solutions.
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 sources.
