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.

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.

In the present study, MILD (Moderate and Intense Low oxygen Dilution) combustion technology is adopted as one of the most effective tool for reduction of NOx emission in solid fuel combustion. We tried to achieve MILD combustion using the high temperature exhaust recirculation without any heat exchanger for preheating air. High temperature exhaust recirculation is accomplished by entraining the high temperature exhaust gas to air jets at just exit of the combustion chamber. This high temperature exhaust recirculation could recirculate heat and inert exhaust gas simultaneously. MILD combustion using the recirculation of the high temperature exhaust gas is experimented to investigate the effect of low NOx emission for the recycled solid fuel of the dried sewage sludge and pulverized coal. NOx emission could be reduced drastically by using this advanced combustion technique. Maximum 68% and 57% of NOx reduction was achieved for sewage sludge and pulverized coal respectively, in the high temperature exhaust recirculation MILD combustion compared with the conventional combustion using air jet only. This type of MILD combustion makes the apparent flames of both solid fuels extremely uniform without high temperature flamelet.

The Kyoto Protocol, that had been in force from February 16, 2005, requires significant reduction in CO₂emissions for all anthropogenic sources containing transportation, industrial, commercial, and residential fields, etc, and automotive emission standards for air pollutants such as particulate matter (PM) and nitrogen oxides (NOx) become more and more tight for improving ambient air quality. This paper has briefly reviewed homogeneous charge compression ignition (HCCI) combustion technology offering dramatic reduction in CO₂, NOx and PM emissions, compared to conventional gasoline and diesel engine vehicles, in an effort of automotive industries and their related academic activities to comply with future fuel economy legislation, e.g., CO₂emission standards and corporate average fuel economy (CAFE) in the respective European Union (EU) and United States of America (USA), and to meet very stringent future automotive emission standards, e.g., Tier 2 program in USA and EURO V in EU. In addition, major challenges to the widespread use of HCCI engines in road applications are discussed in aspects of new catalytic emissions controls to remove high CO and unburned hydrocarbons from such engine-equipped vehicles.