It is known that lowering of peak temperature of flame reduces NOx emission in combustion process. Low oxygenconcentration of diluted combustion air reduces peak flame temperature, but makes flame unstable. So increasing oftemperature of reactants is needed to enhance flame stability. Mixing of high temperature combustion gas with combustionair makes low oxygen concentration and increases air temperature simultaneously. Low oxygen concentration ofcombustion air reduces peak temperature of flame and increased air temperature makes flame stable by enhancement ofcombustion reaction. Special apparatus for recirculation of high temperature combustion gas should be needed, becausegeneral blower cannot be used to return the gas of almost 1,000oC. Air jet type recirculation apparatus has been developedand installed in a commercial scale of 7.2ton/day incinerator and estimated. Oxygen concentration and temperature ofair mixed with inhaled high temperature combustion gas by the apparatus are 16.24~17.78%, 384~512oC, respectively,in a steady state of incineration.

Characteristics of exhaust gas of solid refuse fuel (SRF) burning in a commercial scale of 12ton/day incinerator havebeen investigated. Combustion air for SRF burning is mixed with recirculated high temperature exhaust gas to diluteoxygen concentration and preheat itself. It is called high temperature EGR (Exhaust Gas Recirculation) combustion. Itis known that low oxygen concentration of diluted air reduces flame temperature and NOx emission, but also makes flameunstable. Highly heated air by mixing with high temperature exhaust gas makes flame stable by enhancement ofcombustion reaction. Concentrations of nitric oxide (NOx), carbon monoxide (CO), oxygen (O2) and carbon dioxide (CO2)in flue gas have been measured at stack. High temperature EGR incineration of SRF dramatically reduces nitric oxideemission and residual oxygen. Average concentrations of NOx, and CO are 71.5ppm and 86.6ppm especially at referenceoxygen concentration of 12% without any post treatment of NOx when the average outlet temperature of combustionchamber is 942oC. And average concentrations of O2 and CO2 are 9.59% and 8.3% especially.

Nitrogen oxide (NOx) is one of air pollutants generated from the combustion of fuels, causing serious environmental problems. A novel externally oscillated staged combustion for RPF syngas was proposed in this work. The staged combustion could reduce NOx by the fuel-rich state combustion, while the external oscillation could achieve complete burn-out by stabilizing the flame. It also improved combustibility with an acceleration of the mass and heat momentum transfer. Parametric studies were achieved for the NOx reduction characteristics on the air staging and fuel staging in each case of with or without external oscillation. For the case of without oscillation, NOx reduction rate for the fuel staging had higher value as 75% than air staging as 67%. However, an application of external oscillation for both cases gives higher NOx reduction rate of 79%. The optimal condition for the oscillated fuel staging was that the air ratio in main burning zone, reburning zone and burnout zone were 1.1, 0.6 and 1.15, respectively, having 200 Hz of external oscillation.

Analyzing results of exhaust gas of solid fuel burning are investigated with measuring position in a pilot scale MILD(Moderate and Intense Low oxygen Dilution) combustor using high temperature exhaust gas recirculation. Flue gas hasbeen measured at exit of combustion chamber and stack, especially. Oxygen concentration measured at stack is higherand carbon dioxide concentration is lower than that measured at exit of combustion chamber, because air flows into theflue gas from the post-treatment facilities, such as gas cooler and bag filter, due to negative pressure caused by inducedblower. Low carbon dioxide concentration can cause an error which estimates higher air ratio than actual air flow rateneeded for complete combustion. Average calculated concentration of measured nitric oxide and carbon monoxide forreference concentration of 6% oxygen have no notable difference with measuring position. But, time resolution of thedata measured at exit of combustion chamber is better than that measured at stack. It is confirmed that MILD combustionof solid fuel of pulverized coal using high temperature exhaust gas recirculation can reduce dramatically nitric oxideemission.

The stoichiometric gas from an advanced alkaline electrolysis process as developed by Yull Brown is called as HHO gas or Brown gas. By this process, two moles of H2 and 1mole of O2 gases are generated stoichiometrically in a wellpremixed state. Due to the fact that very clean fuel can be obtained relatively easily by the simple equipment of electrolysis, the research of this gas has been continuously performed, even though the criticism has been made by many researcher of this area. The main controversial argument is in that the use of high quality electrical energy is used again for the generation of another combustible fuel with less than 100% efficiency in its energy transform. In fact, since Brown gas exists in the state of a completely mixed state only with oxygen molecule, there is no time delay due to turbulent mixing occurring in practical combustion process. Therefore, the high reaction rate is likely to have a high chance of backfire. Further, since there is no inert material like nitrogen as in air, the flame temperature rises unnecessarily high. In order to prevent the backfire phenomenon, the increase of injection velocity of fuel nozzle causes the formation of very unstable long flame with good chance of flame lift-off. One of practical application methods, the co-combustion of Brown gas with other fuel like gasoline and LNG, etc has been reported in open literature in order not only to increase the combustion efficiency but also for the reduction of pollutant emission such as NOx. In order to control the negative aspect of flame characteristics of Brown gas, in this study, an novel method is employed by premixing Brown gas with water vapor and the co-combustion performance and characteristics has been studied numerically for a combustor operated for kiln drying method. To this end, a commercial code(STAR-CCM+7.06) has been employed with the program verification against operational data of kiln drying combustor and a parametric numerical calculation has been made with the change of the amount of water vapor in the fuel mixture of Brown gas and water vapor. The calculation results show that the combustion feature looks quite stable without showing any unstable flame feature like long thin flame and backfire. Further temperature and streamline contours with the amount of water vapor content look consistent and physically acceptable. This result suggests that the addition of water vapor in the Brown gas looks one of promising method for the use of Brown gas as clean fuel.

MILD (Moderate and Intense Low oxygen Dilution) combustion using high temperature exhaust gas recirculation is applied to solid fuels of dried sewage sludge and pulverized coal combustion to investigate the effect of reduction of NOx emission in a pilot scale combustor. High temperature exhaust gas recirculation is accomplished by entraining high temperature exhaust gas to air jets at just exit of the combustion chamber without a heat exchanger. High temperature exhaust gas recirculation makes the solid fuel flame stable and extremely uniform color and uniform temperature distribution. NOx concentration at the combustor exit was 62% and 40% less in the high temperature exhaust recirculation MILD combustion compared with the conventional combustion using air jet only for sewage sludge and pulverized coal respectively.

Numerical study is conducted to predict effects of radiative heat loss and fuel composition in synthetic gas diffusion flame diluted with CO₂. The existing reaction models in synthetic gas flames diluted with CO₂ are evaluated. Numerical simulations with and without gas radiation, based on an optical thin model, are also performed to concrete impacts on effects of radiative heat loss in flame characteristics. Importantly contributing reaction steps to heat release rate are compared for synthetic gas flames with and without CO₂ dilution. It is also addressed that the composition of synthetic gas mixtures and their radiative heat losses through the addition of CO₂ modify the reaction pathways of oxidation diluted with CO₂.