Assessment of the influence of thermal characteristics of non-design lignite on their suitability for combustion in the furnace of an energy boiler

The possibility of burning various brown coals (LC) in the furnace of the BKZ-420-140-6 boiler was studied using numerical simulation. To check the compiled numerical model of solid fuel combustion, it was previously validated in relation to the combustion of BU of a certain composition by comparing the calculation results with data obtained during the operation of a real boiler.

A method was developed for averaging the characteristics of coal and dependencies were obtained that determine the composition of coal based on an analysis of the composition of 14 types of coal in the range of values of the lower calorific value $Q_n^r$ from 7.5 to 16 MJ/kg. Based on the obtained dependencies for 4 values of $Q_n^r$, the theoretical average composition (ATC) of coals was determined. For one of the TUSs, the humidity varied by 10% up and down.

As indicators of the efficiency of the combustion chamber of the boiler, the temperature of the gases at the exit from the combustion chamber, mechanical underburning and the concentration of nitrogen oxides in the flue gases are taken. The results of numerical modeling show that with the calorific value of the fuel $Q_n^r$ ≤10 MJ/kg, the mechanical underburning q₄ exceeds the permissible standards. The highest concentration of nitrogen oxides at the level of 800–900 mg/nm³ is observed for brown coals with high $Q_n^r$ and the highest carbon content. It is also shown that the use of direct-flow burners with the organization of staged fuel combustion makes it possible to reduce the formation of nitrogen oxides in the furnace by 3.25 times compared to the original combustion scheme using existing vortex burner devices. The influence of the lower calorific value of fuel on the gas temperature at the exit from the combustion chamber in the range $Q_n^r$ from 11.75 to 16.45 MJ/kg is insignificant. The effect of increasing fuel humidity on the gas temperature at the outlet of the combustion chamber and on mechanical underburning is insignificant up to a working fuel humidity of approximately 45%. In general, studies have shown that the firebox under consideration allows the combustion of various brown coals with changes in the physico-chemical composition and thermal characteristics within a wide range.