Techno-economic and environmental feasibility study of MILD combustion in domestic utility boilers under partial load operation

Abstract

This paper aims to achieve the benefits of MILD combustion, such as enhanced thermal efficiency with simultaneous NOx reduction in existing conventional domestic hot water utility boilers which encounter partial heat loads. For this purpose, a system based on waste heat recovery and flue gas recirculation (FGR) is introduced to perform techno-economic and environmental studies under continuous fluctuations of heat load demand in a typical building at various climate conditions. Technical investigations on several types of utility boilers revealed that MILD combustion could be implemented only in modulation boilers, which can serve combustion stability requirements in turndown conditions due to partial load demands. Up to 6 % fuel saving, which aligns with a commensurate decrease in CO2 emissions and a minimum 75 % reduction in NOx emissions, is foreseeable by manipulating optimal FGR for each specific boiler in various climate conditions. Finally, economic investigation demonstrated that the payback period for the proposed revamped system is less than one year for most climates.