Abstract
Flue gas recirculation (FGR) is an effective approach for simultaneously reducing nitrogen oxide emissions and improving the energy performance of coal-fired boilers by controlling flame temperature and the composition of the working mixture. This paper presents a computational and analytical study of FGR application to the E-90-3.9/440 boiler at TPP-1 under firing of D-grade coal from the Karazhyra open-pit mine. The aim is to determine a rational FGR ratio that ensures NOx reduction while maximizing gross boiler efficiency within operational constraints. The methodology is based on a mathematical boiler heat-balance model and processing of operational data before and after FGR activation; changes in flue gas composition and heat losses were assessed as the recirculation fraction was varied. The results show that, within the considered FGR range, increasing recirculation is associated with a rise in calculated gross efficiency and a decrease in NOx concentration: a 1% increase in FGR raises gross efficiency by 0.25%, while NOx decreases by 9 mg/m 3 . Rational performance is achieved at FGR ≈ 20%, corresponding to a calculated gross efficiency of 95.5% and a 30% reduction in NOx relative to the baseline mode. The findings confirm the feasibility of FGR as a technological solution for enhancing the environmental and operational performance of coal-fired thermal power plants in Kazakhstan.