Heat Transfer Enhancement And Pollutant Control In Gas-fired Boiler Using Normal Temperature Air Flameless Combustion Technology

Normal Temperature Air Flameless Combustion (NTAFC) is not only different from flame combustion, but also different from flameless combustion in its realization approach。NTAFC employs high speed jet flow of normal temperature reactants and stirred reactor to achieve combustion without a noticeable flame front. NTAFC is able to improve the heat transfer condition in industry boilers, save energy significantly, reduce green house gas and NOx emission, and reduce size of combustion facilities. Using numerical and experimental methods, the characteristics of NTAFC and heat transfer enhancement in NTAFC furnace and control of pollutant production were studied.A gas-burned experimental boiler was built for NTAFC using nonpremixed natural gas and air in normal temperature. Temperature was sampled and thermal efficiency as well as pollutant emission was tested. Using Probability Density Function (PDF) model based on mixture fraction and Eddy Concept (EDC) model simulated the quasi-steady 3D combustion in the furnace. The results indicated that mixture fraction model made a distortion in the simulation of conical jet flow area due to the assumption of "Mixed is burnt" and neglect of chemical reaction rate. This also indicated that the interaction of the turbulent and chemical processes must be considered in NTAFC. While the experimental results agreed well with EDC model results basically. The analysis showed that NTAFC boiler firstly employed high speed jet flow of reactants to increase the momentum, and on the condition that the reactants were in normal temperature, the jet engulfed and entrained more flue gas, and made a relatively low temperature mixing area; secondly, it took advantage of the stirred reactor to increase flue gas circulation and thermal circulation, ensured the concentration reduction and temperature increase of the reactants. Finally, a wild and thick reaction area around the downstream of the jet flow was formed, which is the flameless combustion region.The heat transfer in traditional gas-burned boiler furnace and NTAFC furnace was compared in a simplified analysis. The heat flux of the radiant plate of the stirred reactor as well as the water wall of the boiler was simulated and investigated. Thermal efficiency was tested experimentally. The result illustrated that, compared to traditional gas-burned boiler, the stirred reactor in the NTAFC boiler prolonged the journey of flue gas in the boiler furnace, increased the radiation by the radiant plates of the stirred reactor, and the proportion of radiation to the total heat transfer of the water wall was up to 80%, and the water wall was heated uniformly. The thermal efficiency of NTAFC boiler was higher than traditional gas-burned boiler for the same evaporation capacity, which corroborated the enhancement of heat transfer in NTAFC furnace.Coupling with reduced chemical kinetic reaction mechanisms of methane oxidation, the simulation of pollutants formation in NTAFC furnace was conducted and the exhaust gas at the outlet was tested and analyzed. The simulation results showed that NOx formed in a wide and broad region around the jet downstream, namely the flameless combustion region, and NOx emission at the outlet was below 20x10-6 kg-m-3. Due to the broadened reaction area, the maximum temperature was lower than 1700K and thermal NO formation was significantly suppressed compared with flame combustion. Prompt NO formation was minute, and N20->NO mechanism became the major route of NOx. And the results were agreed well with experiment.