The development of low nitrogen combustion technology abroad has experienced three generations. The first generation technology does not change the combustion system greatly; the second generation technology is characterized by air staged burner; the third generation technology is a three-stage combustion mode (or burner) in which air and fuel are staged simultaneously in the furnace.
The first generation of low NOx Combustion Technology
This generation of technology does not require major changes to the combustion system, but only to adjust or improve the operation mode or part of the operation mode of the combustion device. Therefore, it is simple and easy to operate, and can be used in the active device conveniently, but the reduction range of NOx is very limited, mainly through the following ways to reduce the NOx emission concentration.
(1) Low excess air factor operation.
This is a simple method to optimize combustion and reduce NO x production. It doesn't need to modify the structure of the combustion device. The range of NO x reduction is related to the fuel type, combustion mode and slag removal mode. The excess air coefficient of utility boiler can't be adjusted greatly. For coal-fired boilers, reducing excess air coefficient will result in fouling, slagging and corrosion of heating surface, change of steam temperature characteristics, and decrease of economy due to increase of fly ash combustibles. For gas and oil fired boilers, the main limitation is that CO concentration exceeds the standard.
(2) Reduce the preheating temperature of combustion air.
Reducing the preheating temperature of combustion supporting air can reduce the peak temperature in the flame zone, thus reducing the generation of thermal NOx. This measure is not suitable for coal-fired and oil fired boilers. For gas-fired boilers, no is reduced. The obvious effect of emission.
(3) Thick and thin combustion technology.
This method is to let part of the fuel burn under the condition of insufficient air, i.e. the fuel is too rich; the other part of the fuel burn under the condition of excess air, i.e. the fuel is too lean. The coefficient of excess air α is not equal to 1 whether it is over concentrated combustion or over diluted combustion. The former is α & lt; 1, and the latter is α & gt; 1, so it is also called non chemical equivalent combustion or deviation combustion. When the fuel is rich or lean, because the oxygen is insufficient and the combustion temperature is not high, both fuel type NOx and thermal type NOx will be reduced. Because of the large amount of air and the low combustion temperature, the generation of thermal NOx is also reduced. The total result is that NO x production is lower than that of conventional combustion.
(4) Flue gas recirculation in the furnace.
It is a way to reduce the NO x emission of coal-fired liquid slag extractor, especially gas and oil fired boiler by mixing flue gas with combustion supporting air and reducing the oxygen concentration of combustion supporting air. The usual method is to take out the flue gas from the outlet of the economizer and add it into the secondary air or primary air. When the secondary air is added, the flame center will not be affected, its only function is to reduce the flame temperature, which is conducive to reduce the formation of thermal NOx. For the solid-state slag removal boiler, about 80% of NO x is generated by fuel nitrogen, so the effect of this method is very limited.
For the non classified burner, it is better to mix human flue gas in the primary air, but the combustion conditions near the burner will change, so the combustion process should be adjusted.
(5) Some burners are out of operation.
This method is suitable for utility boilers with multi-layer burners. The specific method is to stop the fuel supply of the top layer or several layers of burners and only send air. In this way, all the fuel is fed into the furnace from the lower burner, the lower burner area is rich in fuel, and the air from the upper layer forms a staged air supply. This method is especially suitable for gas and oil fired boilers without major modification of the fuel delivery system. This method has been used in large lignite units in Germany with good results.
The second generation of low NOx Combustion Technology
The feature of this generation technology is that the combustion supporting air is delivered to the combustion device in stages, so as to reduce the oxygen concentration in the initial combustion area (also known as the primary area), and correspondingly reduce the peak temperature of the flame. Among the measures of this generation are various low NO x Air Staged burners which are widely used in utility boilers at present. For example, abb-ce's integrated furnace air staged DC burner, coaxial combustion system (CFS
1. CFS 11), low NO x co axial combustion system (lncfs) and its various variations, tfs2000 combustion system, double adjustable swirl burner (DRB, drb-xcl) of B & W company, various swirl burners of steinmuller company and Babcock company of Germany, etc.
——Author：ebico