Currently, biomass accounts for the largest share of renewable energy sources that participate in total primary energy consumption in the EU. Direct combustion of biomass in the form of wood is the oldest energy source known to man. Solid fuel combustion, including biomass, in domestic combustion appliances represents a significant source of environmental pollution. This is especially valid for volatile organic compound (VOC), polycyclic aromatic hydrocarbon (PAH) and dust (PM) emissions. Thus, we are very often faced with increased content of fine particulate matter in the atmosphere (especially during the winter months). Numerous studies have shown that there is a close relationship between the exposure to increased proportions of these compounds and heart, circulatory and pulmonary diseases, with traffic (especially diesel engines) and solid fuel combustion being the main sources. In addition, biomass combustion systems are the cause of a significant amount of carbon monoxide (CO) and nitrogen oxide (NOx) emissions, and we can assume the same for dioxins and furans. Today, because of the progress made in technology and understanding of the biomass combustion process, its exploitation through direct combustion is experiencing a new take-off. The lowest emissions are achieved when pelleted biomass is used. Pellets consist of compressed wood fibers with very little water content. Increasingly stringent environmental requirements of the EU, particularly regarded to pollution emissions, require the continuous development of new hot water boiler designs. EcoDesign Directive has in the case of hot water boilers that burn solid biomass reduced the maximum permissible carbon monoxide emissions six times and organic gaseous compounds (OGC) and dust emissions four times. In addition, the nitrogen oxide emission limit value is also introduced. The quality of pellets in the EU is expressed by the ENplus Certificate, which classifies pellets into different classes A1, A2, and B (for woody pellets) and A and B (for non-woody pellets). An obstacle to the wider use of quality pellets (class A1) is in its price, which is higher than the gas price on the Croatian market. It should also be noted that the certification of pellets is voluntary, so much of the pellets available on the market, as well as those home-made, do not even meet the requirements set for A2 class. Emissions are becoming an even bigger problem if, instead of wooden biomass, various residues from agricultural production are used. This is very important as agro-fuels are becoming more and more demanded fuel type due to its low cost (26% cheaper compared to the natural gas). The term residues refers primarily to the residue of arable crops, which is generally considered as a by-product of the agricultural production. The low energy density (calorific value/volume) of straw material primary form aggravates their applicability in the household combustion appliances. Therefore, straw material in the form of pellets has a greater potential for becoming a fuel used in household appliances. Unlike conventional wood biomass combustion, the combustion of agricultural residues has several related challenges. The main challenges are associated with significantly higher pollutant emissions (this applies especially to CO, NOx, dust and same can be presumed for dioxins and furans), lower efficiency for the same power, and ash related problems. The most significant problem, along with the much greater fuel ash content, is its low melting point. This can result in serious operating problems, which include deposition and agglomeration of ash on the grid (thus affecting the supply of combustion air) and heat exchange surfaces (reducing heat output and corrosion). This research is focused on solving the above mentioned problems in the case of hot water boilers that are used in the residential sector. Research can be divided into three parts. In the first part, the physical and chemical properties of agricultural biomass and the combustion characteristics have been investigated experimentally. The results were compared with those obtained for wood pellets and significant differences were found. Consequently, rapeseed straw pellets were selected as, from the technical point of view, the most suitable fuel for the combustion in domestic hot water boilers. In the second part, in order to to determine places with more intense chemical combustion reactions inside the boiler, numerical modeling of the combustion process in the hot water boiler was conducted in the Ansys Fluent software. Unlike other models available in the open literature, the developed model in not based on user-defined functions (UDFs) and, as such due to the its availability and simplicity, is more applicable in the industry. The numerical model has been developed and validated for the combustion of pelleted wood biomass. Based on the developed model, temperature fields and flue gas flows were determined, as well as the areas with more intense rates of combustion reactions. Also, important knowledge about the mechanisms to form these fields and parameters that influence the acceleration of combustion rate (turbulence, flue gas temperatures, fraction of unburned gases) was gained. It was assumed that the conclusions obtained for wood biomass can also be applied to the pelleted residues from agricultural production. In the end, following the results obtained from the first two parts, a total of nine design guidelines were given for the production of a prototype of the commercial biomass boiler burning agricultural residues. Two of the proposed design solutions were analysed numerically and experimentally. Numerical simulations of the proposed design solutions have been validated experimentally for the case of wood biomass combustion, while improvements in the case of agricultural residues have been experimentally quantified. In both cases, the reductions in carbon monoxide, nitrogen oxide and dust emissions were found. Objective and hypothesis of the research This research is based on the hypothesis that by proper selection of low quality biomass or their mixture and new design solutions, it is possible to, in compliance with all environmental requirements of European standards, directly burn low quality biomass (e.g. residues from agricultural production) in hot water boilers at the conventional modes of operation. The objectives of this research are: • Analytically and experimentally investigate the characteristics of certain biomass types, available in the Republic of Croatia, during combustion in the hot water boilers in order to select the most appropriate fuel from the ecological and technical point of view (e.g. emissions of pollutants, ash slagging and deposition of unburned substances). • Conduct numerical modeling of the combustion process in the hot water boiler, with model validation based on the obtained experimental data. The aim is to estimate temperature fields, fields where intense chemical reactions take place and flue gas composition in certain parts of the analysed hot water boiler. • Numerically and experimentally investigate the effect of the proposed design solutions of the hot water boiler combustion chamber on the reduction of flue gas emissions. Scientific contribution The scientific contribution includes: • determining the applicability of the particular residue from agricultural production (with chemical analysis) for the combustion in residential hot water boilers • experimentally validated results of the solid biomass (pellets) combustion process numerical modeling in the hot water boiler and burner • experimentally validated results of the flue gas combustion process numerical modeling in the insert unit for subsequent combustion (honeycomb and plate structure) • extension of knowledge on the mechanisms of pollutant emission reduction depending on the working and design parameters • guidelines for the development of a commercial boiler prototype fueled by agricultural residues that meet all the requirements of EN standards with regard to pollutant emissions, energy efficiency, construction and usage • calculation model for determining the hot water boiler thermal efficiency by the indirect method Conclusion Numerical and experimental investigations conducted in this study indicate that by implementing the proposed design solutions it is possible to burn pellets from agricultural residues in hot water boilers with all requirements of the EN standards (pollutant emissions, energy efficiency, construction and usage) being met. The advantage of the proposed solution is that it can be easily installed as a “complete solution” in almost all residential combustion appliances.