Combustion processes dominate the entire energy and transport sectors. The different burner types mainly depend on the fuel type (gaseous, liquid, or solid) and the application requirements (operating conditions, size, and emission limits). Although all possible fuel types are presently being used, there are clear signs that natural gas is becoming of increasing importance, especially in fields where lowemission thermal energy is needed. This is the case in domestic appliances such as household heating and hot-water supply systems, where low cost is another important factor that burner must have to be of interest to the market. In industrial processes such as drying, coating, and preheating, more stringent emission requirements and increased process efficiency are the driving forces for further developments in burner technology.Two types of burner emissions are of major importance: 1) emissions due to incomplete combustion such as carbon monoxide and unburned hydrocarbons, and 2) hazardous combustion byproducts such as nitrogen oxides and sulfur oxides (greenhouse gases and/or toxic). Concerning emissions due to incomplete combustion, several measures can be applied, such as premixed combustion instead of diffusion-type flame, longer residence time in the reaction region, better insulation of the combustion chamber, higher combustion chamber temperature, more homogeneous mixing of the reactants, etc. Concerning combustion byproducts, the amount of sulfur oxides is related only to the sulfur content of the applied fuel (which is highest for solid fuels and lowest for natural gas), while the amount of nitrogen oxides, which are the most important nowadays in environmental protection, basically depends on the combustion temperature and the residence time at high temperatures. Thus, all efforts dealing with the abatement of nitrogen oxide emissions focus on reduction of the combustion temperature while taking into account that a spatially homogeneous minimum combustion temperature is compulsory to avoid incomplete combustion.Several concepts for low-NO x burners emerged in the last two decades, especially for burners fired with natural gas. A reduction in combustion temperature is common to all concepts, but it is realized in different ways: exhaust gas recirculation, internal recirculation regions, staged combustion with heat decoupling, and direct 5.5
