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To optimize wood-fired heat (and power) plants, it is essential to rapidly determine the chemical composition of solid biofuels on-site shortly before combustion. However, the standard procedures for chemical analysis [inductively coupled plasma-optical emission spectrometry (ICP-OES), inductively coupled plasma-mass spectrometry] are time-consuming, expensive and require highly trained personnel. Thus, they are unsuitable for online fuel analysis. Newly developed, rapid, on-site analysis methods might enhance analysis at the plant. Thereby, X-ray fluorescence (XRF) analysis is one of the most promising methods. So far, XRF has been evaluated in various fields like geology, coal, ash analysis, as well as biomass in general, but this method is still insufficiently investigated for the sector of solid biofuels. The aim of this work is to evaluate and optimize an XRF analyzer for the rapid determination of chemical elements in wood fuels. The XRF analyzer was calibrated using several wood chip samples. Measurements before and after calibration were compared with the reference method (ICP-OES). Results show that XRF can be recommended for analyzing the elements Mg, P, K, Ca, Si, Al, Cr, Mn, Fe, Zn and Pb, while S, Cl, Ti and Ni can potentially be determined with element-specific calibrations. Cu, As, and Cd could not be measured satisfactorily, as many measurements were below the limit of detection. Still, XRF might be used for threshold value monitoring (e.g., in the scope of the German Waste Wood Ordinance).

The established practice of wood ash disposal in landfills removes valuable elements, such as metals and plant nutrients, from the utilization cycle. In order to use the residual material wood ash as a secondary raw material and to conserve important natural resources (landfill space, mineral, and metallic raw materials), a treatment process must be developed. As a basis for such a process, fundamental knowledge of element solubility is required. Therefore, a sequential extraction process describing the mobility of the ash-forming elements was carried out for three ash fractions from a wood-fired heat and power plant. This work describes the extraction of 24 elements from ash by four sequentially applied extractants. As an aqueous solvent, bidest. water was used, acetic acid was used as the acidic solvent, hydroxylamine hydrochloride was used as the reducing solvent, and ammonium acetate with hydrogen peroxide was used as the oxidizing solvent. Element concentrations in the individual extractants were determined by ICP–OES. We found that the extraction is influenced by the ash fraction, the particle size, and the element-specific behavior during ash formation. Extractability is higher from filter and cyclone ash fractions compared to grate ash as well as from smaller particle size fractions within grate ash compared to coarse grate ash particles. The majority of the metals were acid-soluble. In parameter studies, we found that extractability can be increased by using stronger solvents, grinding the ash, and a longer extraction time. The results provide information on (I) the environmental mobility of the ash-forming elements and (II) suitable solvents and process parameters for the processing of ashes, with the aim of a consistent recycling of valuable substances and nutrients.

To design and optimize furnaces, modeling is a key tool. Modeling of solid fuel conversion requires the availability of valid kinetic data for the parametrization of combustion models. To provide kinetic data for low temperature devolatilization, 1 g of wood dust (oak and spruce) was devolatilized under isothermal conditions at 250–400 °C in ambient air. Between 250 and 300 °C, the reaction mechanism changes; thus only the data of 300–400 °C can be used for a kinetic analysis. For spruce, a pre-exponential factor of 237 s–1and an activation energy of 0.222 × 105J/mol were measured, and for oak 89.6 s–1and 0.181 × 105J/mol. As compared to literature data, the reaction is less temperature sensitive, indicating a diffusion limitation caused by the large sample size and experimental conditions. However, the results might be closer to reality as the conditions are similar to a furnace during start-up. Additionally, the release of the main elements, carbon, hydrogen, and oxygen, was measured. At low temperatures, carbon is enriched in the sample, as the sample is partially carbonized. With increasing temperature, more carbon is liberated as gases and less solid carbon remains.

The combustion of natural untreated wood in firewood stoves generates ashes with high contents of minor and trace elements. Those ashes can be disposed via garbage collection. Those ashes can also be used as a fertilizer. This study investigates the elemental composition of four natural untreated wood species (beech, oak, spruce, and fir) and their ashes produced in a firewood stove. In total, 30 minor and trace elements were analyzed using inductively coupled plasma-optical emission spectroscopy. With a complete set of fuel and ash data, possible effects of ashes from untreated wood on the environment are discussed. They contain nutrients in concentrations so that ashes are comparable to commercial fertilizers, but they also contain toxic trace elements in concentrations at or above limits given by relevant German legislation. However, the trace elements are not only potentially harmful but also valuable. Therefore, ashes are also valuable in terms of an urban mining prospectus because they do contain certain elements in comparable concentrations as commercially used ores. It is therefore necessary to separate the trace elements from the ash matrix before it is put on soil, and the separated trace elements need to be processed to crude materials for demanded rare earth elements.

Firewood combustion is the main renewable heating source for households in Europe and is responsible for a certain share of harmful emissions such as particle matter. Common wood combustion appliances in households are firewood stoves. Forced by stringent limits of European emission control legislations, the stove industry developed a wide variety of new pollution control technologies which can be integrated into firewood stoves. The aim of this study was to evaluate the performance of three emission control systems to be applied in firewood stoves which were a foam ceramic element, a catalytic active coated foam ceramic element, and a honeycomb catalyst. Combustion tests with these devices and dummies under real life operation conditions were conducted which included starting phases and stove operation in nominal and partial load. Particulate and gaseous emissions were measured, and emission conversion rates were calculated. Results showed no significant emission reduction rates for the foam ceramic element. The catalytic active coated foam ceramic element reduced the emissions considerably in nominal and partial load operation up to 32% for carbon monoxide, 61% for organic gaseous carbon, and up to 41% for particulate matter. However, emission reduction rates were rather low in the starting phase. The honeycomb catalyst showed the highest emission reduction potential of all systems in the study. The reduction rates were significant in all combustion phases and were up to 73% for carbon monoxide, 58% for organic gaseous carbon, and up to 33% for particulate matter.

The crucial point in inorganic particle formation from biomass combustion is the temperature-dependent release of inorganic compounds, especially potassium (K). Currently, common wood fuels comprise of a comparatively low amount of K, but the increased usage of wood energy requires new feedstocks in the future. Potentially new feedstocks, such as short rotation coppice (SRC), fuels from agriculture (e.g., straw), or wood from broad-leafed trees of low rotation, contain usually high ash contents and/or high K concentrations. Apparently, these feedstocks will cause increased inorganic particle emissions from biomass combustion processes. The principle of a decreased firebed temperature as a primary measure aiming at a retention of K in the ashes of the firebed is a common approach for particle emission reduction and was investigated in several previous studies. The present study describes the usage of an ash-rich fuel from SRC pellets made from willow in a residential pellet boiler modified with an unique prototype of direct water-based firebed cooling. This test setup enables the study of the isolated impact of decreased firebed temperatures and its influence on the combustion process and emissions as well. A statistically significant effect of the firebed cooling on temperatures below the burner plate as on gaseous HCl and SO2was found. The high ash content of the used fuel limited the effectiveness of the applied direct firebed cooling in residential biomass combustion. The accumulation of a thick and thermal insulating ash layer above the burner plate decreased the heat transfer, limited the cooling efficiency, and revealed deviations from the expected particle formation process.