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Content archived on 2024-05-29

Ash and aerosol related problems in biomass combustion and co-firing (BIOASH)

Final Report Summary - BIOASH (Ash and aerosol related problems in biomass combustion and co-firing)

The main aim of the BIOASH project was to investigate still remaining ash related problems in biomass combustion and biomass and coal co-firing systems. Therefore, BIOASH aimed at the investigation of the release behaviour of ash forming compounds from biomass fuels in fixed-bed and in pulverised fuel combustion systems and on the determination of missing thermodynamic data as a basis for investigations concerning aerosol and deposit formation. Furthermore, the project focused on research regarding advanced models for a more precise prediction of aerosol and deposit formation with respect to the release behaviour of ash forming elements from the fuel. As a third main aim of BIOASH, a new technology for cost effective and efficient aerosol precipitation in small-scale biomass combustion units was developed. Moreover, the contribution of biomass combustion and co-firing to concentrations of particulates in the ambient air were studied and a detailed evaluation of the effects of these particulates on the human organism was performed. The BIOASH project focused on fixed-bed combustion and on combustion in pulverised fuel systems, as they are usually applied in co-firing applications.

In detail the objectives of the BIOASH project were:
- investigations concerning the release of ash forming species from selected biomass fuels during fixed-bed and pulverised fuel combustion;
- determination of basic thermodynamic data concerning the melting behaviour of ash mixtures containing Na, Zn, and Pb as well as the determination of the viscosity of molten ash deposits. These data were not available at the beginning of the project but have been identified as crucial within former projects;
- further development of existing computer aided simulation tools regarding the formation of aerosols during biomass combustion, taking special account of element release from the fuel and the effects of transport properties caused by turbulence;
- development of new codes for the simulation of deposit build-up and deposit properties with relation to the combustion of different solid biomasses, mixtures of biomasses, and co-firing of biomass and coal;
- studies of the influence of the co-firing of biomass in coal combustion plants with special emphasis on aerosol and deposit formation as well as deposit melting behaviour. This objective aims at a better understanding of the interactions between ashes from coal and biomass combustion in order to find technical solutions to increase the co-firing ratios of biomass in coal combustion plants;
- development, test and optimisation of a new, efficient and economically sound aerosol precipitation technology concerning the reduction of aerosol emissions from small-scale biomass combustion units;
- investigation of the contribution of typical biomass combustion and biomass co-firing applications to ambient air pollution by particulates. Furthermore, the identification of parameters influencing the effects of particulate emissions on the human organism was an aim. Based on these investigations a better basis for the ecological evaluation of particulate emissions from the combustion of biomass fuels was derived.

The BIOASH fuels have been selected in order to achieve a broad variety of different chemical compositions concerning ash forming elements. In wood fuels, Ca usually is the most relevant ash forming element followed by Si. BM4, however, shows an exception from this tendency which might most probably be due to considerable contents of mineral impurities (sand). Regarding the alkaline metals, which are of special relevance for the BIOASH project, the K content increases from wood chips over sawdust and waste wood to bark. The Na concentrations increase from wood chips over sawdust and bark to waste wood. The same behaviour can be stated for Zn and Pb as well as for S and Cl.

Contrary to wood fuels, straw is dominated by Si. Additionally, the K, P, S and Cl concentrations are significantly higher than in wood fuels while the heavy metals Zn and Pb show lower concentrations.

Olive residues represent an extreme case concerning the K-concentrations. While the Ca and Si contents are comparable with the upper range of these elements in wood fuels, the S and Cl concentrations are significantly higher (in the range of straw). Moreover, olive residues show the highest P contents of all biomass fuels investigated. The same is true for the Fe and Al contents.

Coal significantly differs from biomass. In contrast with biomass considerable amounts of Si, Fe and Al can be found. The Cl contents are in the range of straw and therefore significantly higher than in wood fuels while the S concentrations are significantly higher than in all biomass fuels analysed. Significant differences concerning the concentration levels of almost all ash forming elements between the coal from Poland and the coal from the United Kingdom additionally must be mentioned.

The project was structured into six main work packages (WPs), as follows:

WP 1: Release behaviour of ash forming elements from the fuels investigated
The overall objective of WP 1 was to quantify the transformation and the release of aerosol forming elements such as K, Na, S, Cl, Zn and Pb from biomass fuels as a function of biomass type and local conditions. The investigations performed within WP 1 were based on three different but interlinked approaches:
- fuel characterisation by novel analyses methods (chemical fractionation, SEM/EDX);
- the performance of release tests at lab-scale reactors (fixed-bed combustion, pulverised fuel systems);
- thermodynamic equilibrium modelling.

WP 2: Data acquisition and development of high-temperature particle sampling devices
The overall goal of WP 2 was to develop new high temperature particle measurement and sampling equipment as well as to gain new data concerning the melting behaviour of ashes, the viscosity of deposits and the risk of corrosion caused by deposits. The tasks of WP 2 were structured accordingly to the four main objectives:
- development and construction of high temperature particle sampling devices to be applied during test runs at real-scale combustion plants as well as special deposit probes for longterm measurements and for deposit build-up measurements in large-scale boilers;
- determination of viscosity data for typical deposits in biomass furnaces as a basis for deposit formation modelling;
- determination of new data for thermodynamic melting sub-models that cover compositions within the system (K, Na, Ca, Mg, Pb, Zn)( SO4, Cl, CO3) to gain more reasonable predictions of the melting behaviour of ashes containing Pb, Zn and Na;
- determination of data concerning the risks of corrosion caused by deposits.

WP 3: Advanced aerosol formation and deposit formation modelling
One goal of WP 3 was to develop a practical tool for the prediction of the timescales involved in aerosol formation processes based on new fundamental insights in turbulent flows. The improvement of a basic aerosol formation model, developed by TUG by using and implementing the data gained from WP 1 and WP 2 to gain a better quantitative prediction of aerosol formation during combustion was another main objective.

Moreover, the development of a user friendly deposit formation prediction tool for boiler manufacturers, engineering companies and plant operators was a main goal which is strongly connected to the important objective to implement the aerosol formation models and the deposit prediction tool developed into CFD-calculation routines, especially developed to simulate the reacting flow in biomass furnaces and boilers, in order to make them directly applicable for furnace and boiler design calculations.

WP 4: Test runs at real-scale application
The objective of WP 4 was to perform test runs at pilot-scale and real-scale applications to gather data concerning aerosol and deposit formation as well as to gain real-scale data concerning the release of ash forming elements from the fuel. Furthermore, test runs at biomass and coal co-firing applications aim at the identification of potential problems related to fuel selection, increased deposit formation and boiler tube corrosion. In addition, it was an important objective to use the results and experiences from the test runs to verify the models developed and to apply them for the further development and optimisation of furnace and boiler designs as well as process control strategies.

The test runs focused on the following main objectives:
- fuel sampling and subsequent analyses of the fuels;
- aerosol and fly ash sampling with low-pressure cascade impactors and total dust sampling in the flue gas downstream the boiler;
- deposit sampling with conventional deposit probes as well as with long-term deposit probes developed within WP2;
- aerosol sampling in the hot furnace applying the newly developed high temperature low pressure impactor (developed and manufactured within WP 2);
- flue gas measurements concerning CO, CO2, O2, SO2 and HCl;
- sample taking from all relevant ash fractions (bottom ash, furnace fly ash, cyclone fly ash, filter fly ash) and subsequent analyses of the samples taken;
- recording of all relevant operation parameters (furnace temperatures, load, combustion air supply etc.);
- calculation of mass and element balances over the respective plant.

WP 5 Aerosol precipitation in small-scale biomass combustion units
The aim of this WP was to investigate the possibility to reduce aerosol emissions in small-scale biomass combustion plants by stimulating condensation of aerosol forming vapours on heat exchanger surfaces (which is contrary to the goal of current heat exchanger designs) and thus to combine heat exchange and aerosol condensation in one unit, the so called aerosol condenser heat exchanger.

An analytical approach which incorporates heterogeneous- and wall condensation was derived to investigate whether there is a physical basis for condensation of aerosol forming elements on heat exchanger walls by preference. Consequently, stimulating wall condensation results in fine particulate emission reductions to the atmosphere. Continuum diffusion of molecules towards already present nuclei in the flue gas in combination with a correction factor for diffusion outside the continuum limit is used in the approach. This approach indicates a significant reduction of aerosol emissions by preference of wall condensation when sufficient heat exchanging area is provided, the distance to the wall in each cross section is in the millimetre range and the flow regime is laminar.

WP 6: Health effects of aerosols
The determination of the effect of particulate emissions from biomass firing and co-firing sources on the particulate emissions by performing exhaust gas distribution calculations and the evaluation of the main influencing parameters on the short-term toxicity of particulate emissions from biomass combustion and co-firing were the main objectives of WP 6.

Moreover, the estimation of effects of particulate emissions from biomass combustion and cofiring on the human health as well as the comparison with the risks caused by other particulate emission sources (e.g. coal combustion, traffic) was another important goal.

Summarising, it can be stated that all milestones and deliverables defined have been successfully reached. The work performed and the results achieved represent an important step forward in the research field of ash and aerosol related problems in biomass combustion and co-firing.

Comprehensive test series (in vitro as well as in vivo) were carried out concerning the assessment of health effects by PM emissions from biomass firing and co-firing. Results of invivo tests e.g. showed that for different parameters indicating inflammatory effects in the lung, a significant increase was observed for groups exposed to carbon black (positive control sample), fly ash sample C1 (coal) and fly ash sample C1+BM4 (co-firing of coal and sawdust) which all contain poorly soluble particles. In contrast, the biomass fly ash samples predominantly consist of salts which are characterised by high water solubility. The long-term in-vivo exposure tests have shown that especially particle fractions with low water solubility as well as particle fractions with high organic or elemental carbon contents can cause health effects.