In the work it is noted that the use of renewable fuels for the production of heat and electricity under conditions of energy shortages is an urgent task today. The numerical studies of the performance of the steam generator E-1-9 in the combustion of wood, coal, peat, straw wastes were carried out. The analysis of literary information on the experimental studies of boilers using alternative fuels has been carried out. Based on the analysis of the literature, it is shown that for wide use of alternative fuels in low power boilers (up to 3 MW), thorough experimental studies of the combustion processes of such fuels and appropriate adjustments of the Normative method are required. To study the performance of the steam generator, a mathematical model of the boiler was created, which was implemented in the MSExcel. The mathematical model consists of four modules: module of calculations of volumes of combustion products, module of calculations of heat exchange in a furnace, module of calculations of heat exchange in a convective beam, module of calculations of heat exchange in a cast iron economizer. The influence of the coefficient of excess air on the adiabatic temperature and the flue gas temperature at the outlet of the furnace is investigated. It is shown that as the coefficient of excess air increases, the adibat temperature decreases, which is explained by the fact that more cold air enters the furnace. For peat and wood, the values of the adiabatic temperature are close. The adiabatic temperature for coal is almost 350 ° C higher than that of wood. The influence of the coefficient of excess air on the efficiency of the steam generator is analyzed. It is shown that the highest value of the efficiency is the steam generator when burning coal, which is explained by the higher heat of combustion and lower moisture content of the fuel. Comparison of pollutant emissions in the case of solid fuel combustion in a steam generator.