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Content archived on 2024-05-15

Aerosols in fixed-bed biomass combustion - formation, growth, chemical composition, deposition, precipitation and separation from flue gas (BIO-AEROSOLS)


Objectives and problems to be solved: The project focuses on the solution of problems related to aerosols and fly-ashes in fixed-bed biomass combustion systems namely particulate emissions and deposit for-motion. Characteristics and the behaviour of aerosols should be investigated considering different biomass fuels. Mechanisms governing deposit formation in furnaces and boil-errs and the corrosive potential of these deposits should be explored. Based on this knowledge, technologies reducing the problems mentioned should be developed. Efficient aerosol precipitators should be enhanced by setting up an aerosol data base for filter manufacturers accompanied by techno-economic recommendations as well as by op-itemising the RPS, which is an innovative dust precipitator for small-scale applications. Additives to influence aerosol formation and growth will be investigated. Finally, environmental and health risks of aerosol emissions from biomass combustion should be evaluated as a basis for the recommendation of emission limits. Description of the work: At first, aerosols formed during the combustion of different biomass fuels (bark, wood chips, waste wood) should be characterised. Test runs at a pilot plant and a large-scale CHP-plant including aerosol, fly-ash and deposit sampling with subsequent chemical analyses and analyses by electron microscopy should provide information about particle size distribution, shape and chemical composition of aerosols and fly-ashes. Based on these data, the mechanisms of aerosol and ash formation and behaviour should be in-vest gated with computer aided mathematical models. Additionally, mathematical flow simulation will be used to describe the motion and deposit formation of aerosols and fly-ashes in furnaces and boilers. The knowledge gained from this basic research should be used as a basis for the following investigations concerning aerosol precipitation and reduction of corrosive deposits. To optimise ESP and bughouse filters, which are mainly used in large-scale biomass combustion units, an aerosol data base as well as relevant design data for filter manufacturers should be worked out and techno-economically evaluated. Concerning small-scale combustion units, the rotational particle separator (RPS), an innovative dust separator, should be optimised. Moreover, additives injected into the furnace in order to influence formation, growth and chemical composition of aerosols should be developed and tested. The effectiveness and techno-economic evaluation of additive injection will be performed based on the results from test runs. Finally, the ecological and health risks of aerosol emissions from biomass combustion will be evaluated and compared to aerosol emissions from other sources as a basis for the recommendation of appropriate emission limits. Expected Results and Exploitation Plans: Based on an increase of the knowledge about aerosol and fly-ash formation and behaviour during fixed-bed biomass combustion by identifying the most important influencing variables, primary and secondary measures should be found to reduce aerosol emissions from biomass combustion units. Guidelines for the design of more advanced dust precipitators should be one major result from the project. Furthermore, additives, which influence the formation and growth of aerosols should be developed to reduce deposit formation in furnaces and boilers as well as to reduce the potential for corrosion of these deposits. Finally, an ecological evaluation of the data about aerosol emissions collected during the project, should lead to recommendations for future emission limits.
Within the project in total 5 testing campaigns at a pilot-scale combustion unit (440 kWth) and a large-scale CHP plant (40 MWth) were performed. A huge amount of high quality measurement data concerning characteristics of aerosols and fly ashes formed during fixed-bed combustion of woody biofuels was gained. These data (particle size distributions, concentrations in the flue gas, shapes and chemical compositions of aerosols and fly ashes) were summarised in an aerosol and fly ash database. Furthermore, a huge amount of new data concerning the characterisation of furnace and boiler tube deposits (build-up rates, structure and chemical compositions) was gained. Based on the results from the test runs, existing models to predict aerosol formation and deposit melting behaviour were improved and new models for the prediction of the behaviour of aerosols and fly ashes in fixed-bed biomass combustion units were developed. Based on the modelling results and the evaluation of the test runs, new insights into aerosol and fly ash formation processes as well as in deposit build up were achieved.

Consequently, the knowledge about these processes was substantially increased. In future, the furnace and boiler designs as well as process control strategies of the industrial partners, engaged in the project, will be adjusted according to the results gained from the project in order to reduce deposit formation as well as particulate emissions. Another important results of the project is the development of an aerosol and fly ash data base, which comprises all measurement data from the test runs performed. Based on these data, also recommendations for filter manufacturers concerning the application of different dust separation technologies in biomass combustion units with respect to the plant capacity and the biomass fuel used were worked out. Regarding particle precipitation in small-scale biomass combustion units, a new technology, the rotational particle separator (RPS) was tested and potentials for optimisation were identified. The RPS has in general proven its applicability for aerosol precipitation in small-scale biomass combustion units. However, in order to realise a market introduction some problems with the manufacturing of the filter element, used in this device, have to be solved. Another interesting result gained from the project was a new approach for aerosol precipitation based on a totally new heat exchanger concept, which facilitates surface condensation of aerosols at distinct surfaces in the heat exchanger and therefore, decreases aerosol emissions.

This new approach shall be further developed within follow-up projects, which have already been initiated. In order to reduce deposit build-up in furnaces and boilers, an additive injected into the furnace was developed. Tests with this additive have shown, that during long-term operation, a reduction of deposit build up and deposit hardness was achieved. Short-term tests including deposit sampling and aerosol and fly ash sampling as well as subsequent analyses of the samples did show comparable results. However, it was not possible to identify the exact mechanism on which the effect of the additive was based, and therefore, additional research on this topic will be needed. Finally, investigations concerning the health risks caused by particulate emissions from biomass combustion units were performed. Summing up the most relevant results of the project are:
- Comprehensive data concerning the characteristics of aerosols and fly ashes formed during fixed-bed combustion of woody biofuels.
- Advanced, new data of fireside deposit samples from 2 grate fired units.
- Direct practical conclusions concerning deposit build up important for boiler operators and designers and for deposition model development.
- Modelling approaches describing aerosol formation in fixed-bed combustion units using woody biomass fuels.
- Modelling approaches describing the behaviour of aerosols and fly ashes in biomass combustion units.
- A newly developed additive to reduce deposit formation in waste wood fired combustion units.
- A newly developed cheap and simple additive injection system for small-scale combustion units.
- A new approach for aerosol precipitation in small-scale biomass combustion units.
- Data concerning the effects of aerosol and fly ash emissions on the human organism.

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