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The aim of this study was the modification of agricultural biofuels to allow for low emission combustion in domestic biomass boilers without slagging or other ash related problems. Four agricultural biomasses were additivated with kaolin. Fuel blends were produced from wheat straw and woody biomass. All fuels were combusted in a 30 kW biomass boiler while emissions and slag formation were measured. Due to additivation or fuel blending, total particle and CO emissions were reduced by 53–77% and 69–95%, respectively. At the same time, slag formation (determined as the mass of agglomerates > 3.15 mm) strongly declined. For fuels containing high amounts of sulphur or chlorine, kaolin additivation may lead to increased HCl and SOx emissions. Moreover, NOx-formation may be higher during combustion of additivated fuels (up to 21%). Overall, additivation with kaolin or blending of fuels might be an interesting option for the utilization of agricultural biomasses. The selection of the optimal upgrading method should consider the chemical and physical biomass properties and address the minimum amount of additive or blending ratio to achieve a sufficient reduction of emissions and slagging.

Wood chip quality is essential for failure-free and low-emission combustion in automatically stoked small-scale biomass boilers (

Within six case studies, different drying and sieving process steps were employed for the removal of adhering soil and other extraneous impurities from wood chips. Consequently, it was systematically investigated to which extent this strategy can be used to jointly mitigate the risk of bottom ash slagging and high pollutant emission levels during combustion in an automatically stoked small-scale boiler. Throughout all combustion tests, slag and emission formation were recorded. Formation of agglomerates in the bottom ash was not observed in the fuel bed. However, fuel processing resulted in an increase of the ash shrinking–softening range up to 230 K indicating a lower slagging risk in the bottom ash. An asymptotic trend for the ash melting temperatures was observed as a function of the molar (Si + P + K)/(Ca + Mg) ratio. It was also found that potassium is less efficiently retained in the bottom ash with lower Si content in the fuel. Lower moisture contents in the wood chips typically resulted in lower CO emissions and higher boiler efficiencies for the investigated range of moisture content. The sieving of the unprocessed wood chips reduced NOx emission levels up to 28%. However, fuel processing did not necessarily reduce the level of particulate matter emissions.

Ashes from biomass heat (and power) plants that apply untreated woody biofuels may be suitable for use as fertilizers if certain requirements regarding pollutant and nutrient contents are met. The aim of this study was to examine if both bottom and cyclone ashes from 17 Bavarian heating plants and one ash collection depot are suitable as fertilizers (n = 50). The range and average values of relevant nutrients and pollutants in the ashes were analyzed and evaluated for conformity with the German Fertilizer Ordinance (DüMV). Approximately 30% of the bottom ashes directly complied with the heavy metal limits of the Fertilizer Ordinance. The limits were exceeded for chromium(VI) (62%), cadmium (12%) and lead (4%). If chromium(VI) could be reduced by suitable treatment, 85% of the bottom ashes would comply with the required limit values. Cyclone ashes were high in cadmium, lead, and zinc. The analysis of the main nutrients showed high values for potassium and calcium in bottom ashes, but also relevant amounts of phosphorus, making them suitable as fertilizers if pollutant limits are met. Quality assurance systems should be applied at biomass heating plants to improve ash quality if wood ashes are used as fertilizers in agriculture.

The storage of wood chips is important for the biomass supply chain as it compensates for temporal differences in production and consumption. Typical storage-related problems are dry matter and energy losses due to microbial activity. In extensive field trials, we investigated the storage of spruce wood chips from forest residues (FRC) and from energy roundwood (ERC) with and without rain protection under Central European conditions. Additionally, we examined the storage of unchipped piles. The results indicate that the investigated factors, i. e. storage duration, season, assortment and rain protection, have a statistically significant influence on moisture content and dry matter loss of wood chips. During five months of storage, the highest decline in moisture content was 22.6 %-points, the highest dry matter loss 11.1 %. In winter, energy losses reached up to 11.3 %. In summer, energy contents did not change or even increased slightly (max. 4.7 %). Pile temperature and dry matter losses were significantly positively correlated in FRC. Formation of different layers within the piles could be detected. Storage performance was better in unchipped than in chipped energy roundwood. Storage of unchipped forest residues was not beneficial concerning energy content, but fuel quality increased due to reduced ash and fine particle content. Clear best practice recommendations could be drawn regarding wood chip storage under Central European conditions. During winter, FRC should be stored with rain protection or as short as possible while during a dry and warm summer, wood chips can be stored with only few restrictions.

The introduction of wood chips into the market for small-scale heating appliances of below 100 kW has become more challenging due to stricter regulations on emission levels for carbon monoxide and particle emission, e.g. in Germany. Therefore, it is important to identify high wood chip qualities that are suitable for failure-free and low-emission combustion. In this investigation, several wood chip assortments produced from stem wood and forest residues were analysed and combusted in up to two different wood chip boilers with a nominal heat capacity of 50 and 99 kW at full load. Some combustion tests were performed at part load (i.e. 30% heat output). Throughout experiments, the emission behaviour was measured (CO, NOX and total particulate matter (TPM)). In total, seven different wood chip assortments were tested in the large boiler, whereas ten wood chip assortments were combusted in the smaller furnace. Wood chips from stem wood are characterized by lower ash contents (0.27–1.05 w%, d.b.), lower nitrogen contents (0.04–0.14 w%, d.b.) and lower aerosol forming elements (379–1075 mg/kg, d.b.) compared to forest residues (A = 0.58–2.05 w%¸ N = 0.13–0.32 w%, aerosols = 728–2199 mg/kg, d.b.). Combustion of stem wood caused lower emissions compared to forest residues at full-load operation. Overall, part-load operation caused higher CO and particle emissions. In addition, increases of fine woody particles lead to increases in CO emissions. The influence of moisture content was more pronounced especially for CO emissions.

Air pressure resistance during ventilation was measured experimentally for a high variety of conventional wood chip samples that had been produced using various raw materials, chippers and chipper settings (n = 95). Physical fuel properties such as bulk density or particle size distribution were determined according to international standards for solid biofuels. In addition, particle length and particle shape were determined using a continuously measuring image analysis device. Pressure resistance (in Pa m−1) was determined using a custom-built flow cylinder (2 m height, 0.4 m3 filling volume) and related to air flow rate. Pressure resistance increased exponentially with air flow rate for all samples. Thereby, pressure resistance was highly variable among wood chip samples, ranging from 10 to 107 Pa m−1 at low air flow rates (0.1 m3 s−1 m−2) and from 197 to 1240 Pa m−1 at higher air flow rates (0.5 m3 s−1 m−2). Four common equations to describe pressure resistance (Ramsin, Shedd, Hukill & Ives, Ergun) were compared. Multiple linear regression analysis was applied to express pressure resistance as a function of selected fuel quality parameters. Sensitivity analyses of the resulting equations were performed. Results indicate that pressure resistance strongly depends on bulk density, particle size and particle shape as these parameters determine the number, size and shape of free air voids within wood chip fillings.

Aerobic and anaerobic storage of wood chips from coniferous forest residues, coniferous energy roundwood and short rotation coppice were investigated regarding dry matter losses, energy losses and changes in fuel properties using several approaches, i.e. a field trial with 90 m3 wood chip piles, small-scale storage containers (0.5 m3) incl. greenhouse gas measurements and miniature bunker silos (1.5–2 m3). After storage, selected fuels were combusted in a 30 kW wood chip boiler and emissions (CO, NOX, TPM) were measured. Dry matter losses in fleece-covered piles were 7%–9% after 153 days of storage. Drying compensated for energy losses. Container trials achieved full anaerobic conditions in the lab reducing dry matter losses to 0.4–2% while preserving fuel properties. Methane emissions during anaerobic storage in containers were low. Anaerobic conditions could not be achieved in outdoor bunker silos due to insufficient airtightness. The mass fraction of H2O in fuels increased in bunker silos by 4.2–10.4% due to precipitation and dry matter losses up to 22% were recorded. During combustion, CO and TPM emissions were often increased significantly with materials that were stored in silos or piles compared to technically dried fuels, most likely due to changes in their woody or physical-mechanical structure and their mass fraction of H2O. Anaerobic storage might in theory be an interesting option to reduce dry matter losses but could not sufficiently be realized during this study using miniature bunker silos. For many applications, aerobic storage in fleece-covered piles seems preferable to anaerobic storage.