CFD modeling of pollutant generation during the combustion of pellets, common on the Ukrainian market, in a SET-AP 25 boiler

Abstract

The presented work is devoted to the analysis of energy and environmental characteristics of fuel pellets available on the Ukrainian market when they are burned in a domestic hot water boiler of the SET type with a capacity of 25 kW. Boilers of this series are intended for heat supply of residential buildings, municipal and industrial facilities equipped with heating systems with natural or forced circulation of water as a coolant. Sketch drawings and part of the thermal hydraulic characteristics of SET boilers are in the public domain. Modeling of solid biofuel combustion processes was carried out using the finite element approach in the ANSYS Student software package. This software product, which has been available for academic use free of charge since 2015, is widely used to solve introductory and educational tasks in technical higher educational institutions. In order to confirm the reliability of the model, verification of CFD modeling results was carried out based on the passport data of the boiler prototype "SET-25" for the case of burning pellets from hardwood. The results showed that the discrepancy between the passport characteristics and the results of numerical modeling does not exceed 2%. This indicates the correctness of the constructed model and allows it to be used to simulate the combustion of other types of biofuels: pellets from coniferous wood, rapeseed, bird droppings, straw, as well as bottom sludge. The data obtained allowed us to evaluate the thermal and environmental characteristics of the boiler when using the specified types of pellets. These results can be useful both for designing new designs of water-heating boilers and for improving existing models through calibration calculations. During the study, emission levels of harmful substances, in particular nitrogen oxides (NOₓ), were established during the combustion of different types of pellets. The highest level of NOₓ emission (up to 550 ppm) was observed when using pellets from bottom sludge, while the lowest (about 180 ppm) was observed when burning pellets from hardwood. It was shown that regardless of the type of biofuel used, the boiler is able to provide the necessary operating parameters. However, for pellets with a lower calorific value, an increase in the daily fuel consumption is required compared to the basic option - hardwood pellets