This literature review examines the problems associated with ash deposition and deposit formation in low-power heating devices and identifies possible solutions. The combustion of herbaceous biomass, especially that from waste from the agri-food industry, causes a large increase in the amount of mineral deposits in heating devices at a low melting point of the compounds they are composed of (particularly due to the presence of potassium (K), sodium (Na), and sulfur (S) compounds). They affect not only the formation of deposits and sediments but also the emission of fly ash, its handling, and the possibilities for its use and disposal. Problems related to ash depend on the content of alkaline oxides, which vary in proportion across different types of biomass. A dominant percentage of silicon dioxide (SiO₂) is characteristic of ash from grass, rice husks, miscanthus, and straw pellets. Grass ash also contains a significant amount of potassium oxide (K₂O), while ash from poplar wood, willow, and soy pellets is notable for its high calcium oxide (CaO) content. The deposition of ash volatiles and aerosols on boiler surfaces reduces heat exchange efficiency. The solution to the ash problems during combustion is the use of mineral additives, with kaolin being the most promising because of its very good ability to capture problematic alkali-containing compounds during combustion and convert them into potassium aluminium silicates with a high melting point. In addition, kaolin exhibits certain catalytic properties and can act as a carrier for active catalytic components that can be used in the processes of reducing nitrogen oxides (NO_x) and particulate matter (PM). Natural carrier materials, such as kaolin clays, are of considerable practical importance as acidic catalysts. Methods for measuring the surface acidity of solids - including Brønsted and Lewis acid centres - which play a key role in catalysis are therefore of significant interest. The introduction of such additives increases the heating efficiency of the plant and at the same time reduces carbon monoxide (CO) and NO_x emissions (which is inconsistently confirmed in the literature).

This study analyzes the combustion of pellets and briquettes made of plant biomass in low-power heating devices powered periodically with fuel being placed on the grate, as well as after modification using an automatic fuel feeding system in the gutter burner. The use of herbaceous biomass in the form of pellets in low-power heating devices with automatic fuel feeding and combustion in a gutter burner is not widely promoted and popular. Therefore, this study used four types of herbaceous waste biomass (wheat straw, rye straw, oat straw and hay) and one type of woody waste biomass (birch sawdust) for testing. The basic chemical characteristics were determined for the raw materials. After appropriate preparation, the selected starting materials were subjected to briquetting and pelleting processes. Selected physical properties were also determined for the obtained biofuels. Biofuels made from birch sawdust had the lowest heat value (16.34 MJ·kg⁻¹), although biofuels made from wheat, rye and hay straw had a slightly lower calorific value, respectively: 16.29; 16.28 and 16.26 MJ·kg⁻¹. However, the calorific value of oat straw biofuels was only 15.47 MJ kg⁻¹. Moreover, the ash content for herbaceous biomass was 2–4 times higher than for woody biomass. Similar differences between herbaceous and woody biomass were also observed for the nitrogen and sulfur content. To burn the prepared biofuels, a domestic grate-fired biomass boiler was used, periodically fed with portions of fuel in the form of pellets or briquettes (type A tests), which was then modified with a gutter burner enabling the automatic feeding of fuel in the form of pellets (type B tests). During the combustion tests with simultaneous timing, the concentration of CO₂, CO, NO and SO₂ in the exhaust gases was examined and the temperature of the supplied air and exhaust gases was measured. The stack loss (qA), combustion efficiency index (CEI) and toxicity index (TI) were also calculated. The research shows that the use of automatic fuel feeding stabilizes the combustion process. The combustion process is balanced between herbaceous and woody biomass biofuels. Disparities in CO₂, CO and T_gas emissions are decreasing. However, during type B tests, an increase in NO emissions is observed. At the same time, the research conducted indicates that the combustion of herbaceous biomass pellets with their automatic feeding into the combustion chamber is characterized by an increase in combustion efficiency, indicating that when the combustion process is automated, they are a good replacement for wood biofuels—both pellets and briquettes.

Combustion is the most advanced and proven method on the market for using agricultural by-product residues and waste from the agri-food industry. Currently, a wide range of combustion technologies is used to produce heat and electricity in low-power heating devices (>50 kW) using various types of biofuels from biomass (woody biomass, herbaceous biomass, waste and residues from the agri-food industry). Combustion of biomass fuels, especially those of wood origin, causes lower carbon dioxide (CO₂) and sulfur oxides (SO_x) emissions into the atmosphere compared to coal combustion. The growing interest in solid biofuels has contributed to intensive activities on improving the combustion process and energy devices enabling effective and economic conversion of chemical energy contained in biomass into other usable forms such as heat, electricity. Having good quality fuel, it is necessary to ensure an appropriate, clean combustion technique, which allows to achieve the highest thermal efficiency of the heating device and at the same time the lowest emission of pollutants. The article presents issues related to the theory, characteristics of the combustion process and problems related to the formation of harmful chemical compounds nitrogen oxides (NO_x), SO_x, carbon monoxide (CO), particulate matter (PM) emitted to the atmosphere during the combustion process in low-power heating devices. The analysis indicates the possibility of minimizing undesirable phenomena during the combustion of these biofuels related to ash sintering, the formation of deposits, corrosion and improving the amount of condensable solid particles formed and therefore reducing the emission of gaseous products to the environment.

The aim of the study was to assess the selected physical characteristics of the briquettes of English ryegrass waste biomass and its mixtures with waste components from the agri-food industry: pea husks and oat middlings. The raw materials used for the tests are characterized by high calorific value and low ash content. Among the tested raw materials, the most favorable values were recorded for oat middlings in this respect. The produced briquettes were characterized by high volumetric density and very diverse mechanical durability. At the same time, the results of the tests on the mechanical durability of briquettes indicated that the use of components of ryegrass mixtures selected for testing brought unsatisfactory results, as compared to other analyzed physical and energy features.

In this study, an analysis was carried out of the combustion of pellets made from chamomile and English ryegrass biomass, including those with the addition of kaolin and urea, in terms of their physical and chemical properties. During combustion tests with synchronized timing, the concentrations of CO₂, CO, NO, and SO₂ in the flue gases were measured, along with the temperatures of the supplied air and the flue gases. The addition of kaolin improved combustion parameters, reduced CO emissions, and stabilized the combustion process, despite the deterioration of the mechanical durability of the pellets. Combustion in the drop-in burner (type B tests) showed higher energy efficiency (CEI) and lower flue gas toxicity (TI) than in the grate system (type A tests). The SiO₂ content in the chamomile ash explained its higher resistance to slagging, confirmed by characteristic ash temperatures. Comparison with other biofuels (straw, hay, sawdust) showed similarities or advantages in terms of reducing CO, NO, and SO₂ emissions. NO emissions were lower for pellets with urea and kaolin added, although in the case of biomass with high nitrogen content these relationships require further improvement. The research results indicate the potential of herbaceous biomass as a fuel in local heating systems. However, modification of such fuels is also associated with the need for further research on reducing emissions during unstabilized combustion phases, with particular emphasis on the ignition phase.