Biomass combustion is technologically difficult. It is also problematic because of the necessity to manage the ash that is generated in the process. The combustion of biomass pellets is optimum when their moisture is 6-8%. The calorific value of pellets made from straw and willow wood (4:1) was 17.3-20.1 MJ·kg −1 . There were serious problems with burning this material caused by the accumulation and melting of bottom ash on the grate, which damaged the furnace. These problems with optimizing the biomass combustion process resulted in increased CO emissions into the atmosphere. It was shown that pelletization could also be used to consolidate the ash generated during the combustion process, which would eliminate secondary dust during transport to the utilization site. For this purpose, it was suggested to add binding substances such as bentonite and bran. The analysis showed that an optimum material for pelletization should contain, on average, 880 g of ash, 120 g of bentonite, 108 g of bran, and 130 g of water.

Biomass can be used for the production of energy from renewable sources. Because of social resistance to burning crop plants, mixtures and pellets made from or including waste materials are a good alternative. The mixtures analyzed, prepared from wood and municipal waste, were characterized for their calorific values, 7.4–18.2 MJ·kg−1. A result, over 15 MJ·kg−1 was obtained for 47% of the quantities of mixtures being composed. It has been demonstrated that wood shavings and sewage sludge have a stabilizing effect on the durability of pellets. The emissions of acidic anhydrides into the atmosphere from the combustion of pellets from waste biomass were lower for NO, NO2, NOx and H2S than emissions from the combustion of willow pellets. Obtained emission results suggest the need to further optimize the combustion process parameters.

As a step towards the diversification of electricity and heat sources, the EU countries suggest the use of biomass. The combustion of biomass poses the problem of the use of ash produced in the process. There are fluctuations in the properties of energetic biomass, which results in high variability of ash obtained by combustion, especially in terms of specific conductivity (EC) (8.1–9.7 mS·cm−1), the total content of components and their bioavailability. The combustion of biomass leads to large fluctuations in the total content of carbon in the ash (13.6%–28.6%). In this way, waste material with very different biological properties and cation exchange capacity is obtained. Ash from the combustion of biomass is an alkalizing material, rich in Ca, K and Mg carbonates (4.5%). The high average bioavailability of Pb (87.4%), Cd (63.1%) and Zn (46.9%) present in the ash is an environmental problem. The mobility of these heavy metals was reduced by half by the addition of bentonite during the process of ash granulation. With high doses of ash (4.4% of the mass of substrate), there is a significant bioaccumulation of Cd in the roots of Begonia semperflorens and Thuja occidentalis (1.0–3.8 mg·kg−1). Another disturbing issue is that during the cultivation of these plants, Cr is bioaccumulated in the roots (0.5–3.8 mg·kg−1).