Experimental investigation and multi-objective optimization of boiler-recuperator systems to reach MILD combustion characteristics

Abstract

The article aims to achieve the benefits of MILD combustion in a conventional hot water utility boiler by performing a simple revamping and generalizing results to the industrial utility boilers. For this purpose, experimental and numerical investigations were performed on preheating and diluting of combustion air using a recuperator and a flue gas recirculation (FGR) line in a pilot-scale boiler equipped with a conventional premixed burner. Experimental results of maximum preheating and optimal dilution led to increased thermal efficiency and simultaneous NOx reduction in the pilot-scale boiler, which are characteristics of MILD combustion. Thermodynamic modeling and optimization were conducted to determine the optimum air dilution for different industrial utility boilers equipped with various recuperators. Multi-objective optimization provides a set of optimal solutions for fuel flow rate, excess air and FGR to improve system efficiency and reduce NOx emission. It was found that the optimal FGR decreases with increasing boiler efficiency. For instance, in a system equipped with 50 % recuperator effectiveness, the optimal FGR reduces from 0.6 to 0.24 when boiler efficiency increases from 60 % to 90 %, respectively. These optimal FGRs ensure that the system efficiency is maximized while reducing NOx emissions to less than 30 ppm. The novelty of present study is investigating wide range of FGRs for various boiler-recuperator systems and demonstrating that maximum possible preheating together with optimum FGR can provide advantages of MILD combustion to any conventional utility boilers.