Performance and emissions of pre-mixed and post-mixed combustion systems with a casting aluminum-silicon alloy (CASA) condensing gas boiler

Abstract

Condensing gas boilers are widely employed for their high heat efficiency, which attributes to their ability to use the recoverable sensible heat and latent heat in flue gas. In this paper, the optimal excess air rate of a novel condensing gas boiler is investigated experimentally. With the change of excess air rate, the pre-mixed and post-mixed systems were used to study the performance, the thermal efficiency and emissions of the boiler. The CO emission reduces firstly then increases regardless of which premixed system, when the excess air rate is from 1.15 to 1.4. The NOx emission decrease from 64 ppm to 10 ppm and the efficiency decreases from 64 ppm to 19 ppm as the excess air rate increases from 1.15 to 1.4. The efficiency decreases from 102.05% to 98.7% and the efficiency decreases from 100.9% to 97.6% as the excess air rate increases from 1.15 to 1.35. Considering both NOx emission and thermal efficiency, the optimum excess air rate is 1.23. With the change of heat load, the emission, the fan speed and the thermal efficiency of the two system were compared. The concentration of CO increases from 7.5 ppm to 65.5 ppm when the pre-mixed system heat load changes from 8 kW to 24 kW. The efficiency of post-mixed system decreases from 106.5% to 97.5% and the efficiency of pre-mixed system decreases from 106.9% to 96.8% as the heat load increases from 9 to 26 kW. The variation of thermal efficiency and condensate volume with various heat load in the post-mixed system with different return water temperature are also studied. The efficiency decreases from 107% to 98.5% as the heat load increases from 9 to 24 kW at the inlet of return water is 30 °C. And the vapor condensation ratio ω decreases from 105% to 97.5% as the heat load increases from 9 to 24 kW at the inlet of return water is 40 °C. In summary, post-mixed system is more suitable for the development of small condensing boilers. The lower the return water temperature and heat load, the higher the thermal efficiency of boiler.