Formation, composition and particle size distribution of fly-ashes from biomass combustion plants
- Technical Univ. of Graz (Austria). Inst. of Chemical Engineering
Due to the fact that fly-ash particles and aerosols formed during biomass combustion and gasification processes are contaminated with environmentally harmful heavy metals like zinc, cadmium and lead, efficient dust removal from the flue gas is of great importance. In order to characterize biomass fly-ashes and to describe and investigate influencing factors on fly-ash and dust formation, comprehensive particle size measurements were performed with low pressure Berner-type cascade impactors under consideration of different combustion technologies and various types of biomass fuels (bark, wood chips and straw). The results showed that biomass fly-ash can be divided into two major fractions differing in particle size and composition. The first fraction consists of coarse fly-ash particles with an average particle size larger than 5{micro}m results from particles entrained from the fuel bed and is formed mainly by non-volatile minerals (Si, Ca, Mg). The second fraction consists of aerosols (particles smaller than 1{micro}m) which are formed when the flue gas is cooled (in the heat exchanger section) by condensation of volatile ash forming compounds. Chemical analyses of aerosol particles collected by low-pressure impactors revealed that these particles mainly consist of chlorides and sulfates of alkali compounds. These results are in accordance with the outputs of chemical equilibrium calculations performed for the test run conditions. Concerning heavy metal concentrations, the aerosols formed in grate furnaces contain considerable amounts of the environmentally relevant and volatile heavy metals Cd, Zn and Pb due to condensation of metal vapors with decreasing flue gas temperature. The aerosols formed in CFB combustion plants do not contain significant amounts of heavy metals which means that in these systems condensation of gaseous metal compounds does not play a major role. Chemical surface reactions between coarse fly-ash particles and metal vapors seem to be dominant. The particle size distribution of aerosols formed in grate combustion systems is unimodal and has a distinct peak in the range < 1{micro}m without major variations between different biomass fuels. For bark, wood chips and sawdust the major amount of aerosols appears in the range of approximately 0.1{micro}m (Stokes diameter). The dominating particle size of aerosols formed during straw combustion is about 0.5{micro}m. Concerning the combustion of wood and bark, no significant influence of the fuel particle size, the combustion temperature and the boiler load on the size distribution of aerosols was detected for wood and bark in grate furnaces. However, the total amount of aerosols formed by different biomass fuels differs considerably and can be explained by the varying amounts of volatile alkali metals (mainly K and Na), sulfur and chloride in the fuels. That is, the combustion of straw displays total aerosol amounts of up to 900 mg/Nm{sup 3} (related to dry flue gas and 13 Vol.% O{sub 2}) which is about 10 times higher than the emissions of bark- and hardwood-fired grate combustion units. A second influencing parameter revealed is the temperature of combustion. Raising combustion temperatures increase the total amount of aerosols formed.
- OSTI ID:
- 349132
- Report Number(s):
- CONF-980985-; ISBN 1-890977-15-2; TRN: IM9924%%132
- Resource Relation:
- Conference: 15. annual international Pittsburgh coal conference, Pittsburgh, PA (United States), 14-18 Sep 1998; Other Information: PBD: 1998; Related Information: Is Part Of Fifteenth annual international Pittsburgh coal conference: Proceedings; PB: [1500] p.
- Country of Publication:
- United States
- Language:
- English
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