Combustion of solid biomass offers the highest short-term potential for significant CO2-reduction in energy production of all renewable energy technologies in a cost-effectve manner. Comprehensive R&D work carried out during the last decades has already led to a state-of-the-art which allows a highly efficient combustion of a wide range of solid biomass fuels at high plant availabilities as well as low gaseous emissions.
However, several problems related to the formation of fine particulates during combustion, so called aerosols, are still unsolved. These problems affect both, the plant operation itself due to the formation of deposits and ash mixtures with comparably low melting temperatures as well as particulate emissions. Therefore, a considerable number of research projects has been initiated to find solutions for these technological problems. Aerosol immissions as well as their impact on the human health are nowadays intensively investigated and discussed.
Against this background and to present and discuss on-going research, IEA Bioenergy, Task 32, "Biomass Combustion and Co-firing" in co-operation with the Austrian Bioenergy Centre and the Graz University of Technology have organised an international seminar "Aerosols in Biomass Combustion". The seminar presented, discussed and assessed the actual state-of-science in this field and provided information about non-technical issues especially related to legislative regulations, aerosol immissions and health effects of fine particulate immissions. Approximately 70 researchers, furnace and boiler manufacturers, filter manufacturers as well as utilities, plant operators, energy agencies and public authorities participated in the workshop.
SUMMARY AND CONCLUSIONS
One should clearly distinguish between old and new biomass combustion devices. This is of special importance if small-scale units are considered because new biomass furnaces achieve an almost complete burn-out, which substantially reduces the amount of organic aerosols formed.
Due to the fact that old small-scale biomass combustion plants emit significantly larger amounts of aerosols (due to incomplete combustion) than new systems, aerosol emissions from biomass combustion could be substantially reduced by replacing old with new automatically operated small-scale biomass combustion devices or at least by improving the operation of old systems (e.g. by the installation of storage tanks).
Inventories concerning aerosol emissions and immissions need actual and secured data. New data concerning aerosol emissions from biomass combustion (based on measurements performed within ongoing R&D projects) should therefore be supplied for new as well as old combustion devices as well as concerning the actual ratio between old and new systems installed in a certain country. These data are of great relevance in order to achieve correct calculations and evaluations regarding the influence of aerosols from biomass combustion on the overall emission and immission situation.
Regarding health effects of aerosols from biomass combustion, first studies are ongoing but there is still a long way to go in order to understand which influencing variables are of relevance (e.g. particle size, chemical composition). Besides cell tests also inhalation tests will be necessary to evaluate health effects properly.
First sets of release data, describing the amount of aerosol forming species released to the gas phase during combustion, are available for pulverised biomass fuels as well as for grate combustion systems and will also be further investigated. These data are of great relevance for aerosol formation as well as for subsequent deposit formation modelling.
Aerosol formation models for biomass combustion systems have already achieved a high level of development and will be further improved within the next years. These models are of great relevance to understand aerosol formation pathways as well as relevant influencing variables.
Moreover, relevant investigations regarding the influence of aerosols on deposit formation have been performed and have shown that aerosols can strongly influence the ash melting behaviour of deposits due to the formation of salt mixtures with low melting points.
New small-scale aerosol precipitation devices are under development. Tests with prototypes have started but these technologies are not yet commercially available.
Medium and large-scale biomass combustion plants (nominal boiler capacity > 500 kWth) can precipitate aerosols well if appropriate filters are installed. An efficient aerosol precipitation can be achieved by electrostatic precipitators as well as by baghouse filters.
Most of the presentations held at this workshop can be downloaded below.
This workshop was organised by Prof. Dr. Ingwald Obernberger on behalf of