Behavior and effects of ash-forming compounds in fixed-bed biomass furnaces - aerosol formation, aerosol growth, deposition, corrosion and fractionated heavy metal separation

During the last ten years the number of biomass combustion plants in Austria has increased permanently. National and international efforts to increase the share of bioenergy within the European energy production and the additional biomass fuel potential available in Austria guarantee that the trend will be continued. Due to comprehensive research and development work concerning combustion and process control technologies, the gaseous emissions of biomass combustion plants have been reduced to a very low level. But there are still certain problems left to be solved, which mainly depend on the behavior of ash forming elements during combustion, affecting environmental aspects as well as the performance, maintenance and endurance of the combustion plants. In detail these problems are: Aerosols (very small sized particles with mean diameters < 1 pm), formed during the combustion process, containing a considerable amount of heavy metals and toxic organic compounds and causing emissions, which only can be reduced with highly advanced gas cleaning devices like electrostatic and fibrous filters. Even the application of these filters in fixed-bed biomass combustion units sometimes cannot guarantee a permanent operation safety and sufficient aerosol separation efficiency. During the last 2 years 20 hot water boilers in Austrian biomass combustion units with an endurance of less than 10,000 hours had to be exchanged due to corrosion damage. Furthermore, due to ash and aerosol depositions on the surfaces of the boiler tubes the efficiency of energy produc-, tion is lowered due to the decreasing heat transfer. Since the natural circle of elements within the thermal utilization of biomass is disturbed by dry and wet depositions of heavy metals on the forest ecosystem caused by environmental pollution, the guidelines for the utilization of ashes from biomass combustion units on agricultural fields or forest soils sometimes cannot be kept (regarding the heavy metal concentrations in the ashes) With respect to the ash- and aerosol-related problems mentioned above the aims of the project proposed can be defined as following: 1. The basic mechanism of aerosol formation and growth as well the behavior of heavy metals in biomass combustion plants with particular respect to the biomass fuels (wood chips, bark, saw dust) and combustion technologies (grate-fired combustion plants, underfeed stokers) mainly used in Austria will be investigated. 2. Furthermore, basic research on the possibilities of influencing the formation and growth of aerosols in biomass combustion plants will be carried out. The objective of this part of the project is to identify primary measures and technologies which can easily be applied to the existing design of common combustion plants in order to increase the efficiency of downstream dust separation devices. 3. As a third aim the mechanisms and process conditions governing hard deposit formation as well as the relevant reactions and influencing variables for corrosive deposit formation will be investigated. 4. Special sorbents and additives accelerating the growth of aerosol particles, and inerting corrosive substances will be tested and evaluated. 5. Finally the sustainable use of biomass ashes as a secondary raw material with fertilizing and liming effects should be improved by reducing the heavy metal concentrations in the usable ash and forcing a heavy metal enrichment in the filter fly-ash. This goal should be reached by a new combustion design which facilitates the recirculation of parts of the cyclone fly-ash to the furnace. To reach these aims chemical calculations and mathematical modeling of the governing mechanisms aswell as test runs in Austrian large-scale biomass heating plants are intended. Moreover, a pilot-scale biomass combustion unit specially designed for the requirements of the test runs planned within the project, will be available. Aerosol, deposit and fly-ash sampling followed by wet chemical and electron microscopic analyses will provide the information needed to evaluate the mechanisms describing aerosol formation and growth as well as deposition formation, corrosion and the behavior of heavy metals during the combustion process. Furthermore, test-runs with additives in order to influence the formation and growth of aerosols as well as deposition formation and corrosion are planned. Finally, a comprehensive evaluation of the results obtained will form the basis for possible technological improvements. To utilize the available knowledge concerning the central topics of the project (aerosol science, deposition, corrosion, aerosol sampling and measurement technologies), international cooperations with research institutions and industrial companies bearing special knowledge in these fields are intended.

Gaseous emissions from state-of-the-art biomass combustion units have reached such a low level, that they can be compared with emissions from fossil fuel fired systems. However, problems concerning ashes formed during biomass combustion, namely particulate emissions, deposit formation and the distribution of heavy metals within the ash fraction produced are still remaining. Therefore, the project focused on the investigation of aerosol and deposit formation in fixed-bed biomass combustion plants using woody biomass fuels as well as on studies concerning the behaviour of heavy metals in such systems. The methodological approach was based on the performance of test runs at pilot-scale and real-scale applications as well as on mathematical modelling of the different mechanisms and processes concerned. In order to do characterise aerosols and fly ashes and to gain insights into the formation processes, test runs at pilot-scale and large-scale biomass combustion plants were conducted. During the test runs fuel, aerosol, fly ash and deposit samples were taken and subsequently analysed by means of wet chemical analyses and by electron microscopy. The results provided a large amount of totally new high quality data about particle size distributions, shapes and chemical compositions of aerosols and fly ashes from biomass combustion. Furthermore, a mathematical model comprising all relevant mechanisms concerning aerosol formation in biomass furnaces was developed. Based on simulations with this model and on the data gained from the test runs, aerosol formation processes for fixed-bed combustion of woody biofuels were developed and evaluated. The analyses results revealed a distinct difference regarding the formation and chemical composition of aerosols depending on the type of biomass fuel used. The data and experiences from research concerning aerosol characterisation and aerosol formation were then also used to investigate and characterise deposits formed during biomass combustion and to evaluate the influence of aerosols and coarse fly ashes on deposit formation and deposit properties. Moreover, based on the newly gained knowledge about the contribution of heavy metals to aerosol formation, a technology which facilitates a fractionation of heavy metals in the different ash fractions produced in a biomass combustion unit, was developed. By means of this technology the concentrations of environmentally harmful heavy metals in the major part of the ashes can be kept low, while in one small distinct ash streams the major share of these heavy metals is concentrated. This new technology allows the use of biomass ashes as secondary raw material with liming effects and reduces the amount of ashes that has to be disposed. Summing up, the results gained from the project have considerably increased the knowledge about aerosol formation as well as ash and aerosol related problems during fixed-bed biomass combustion and consequently will contribute to a further increase of energy production from biomass.