The principle of low nitrogen burner

Taishan Boiler

From the perspective of NOx generation mechanism, the NOx control technology of oil/gas boilers also focuses on two directions: lowering the combustion flame temperature and lowering the oxygen content. Therefore, an important measure to suppress the generation of NOx is to start with the burner. So what is a low-nitrogen burner? What is its principle?

Low-nitrogen burner refers to a burner that emits low NOx during fuel combustion. Conventional natural gas boiler burners usually emit NOx around 120~150mg/m³. The usual NOx emissions of low-nitrogen burners are around 30~80mg/m³. NOx emissions below 30mg/m³ are usually called ultra-low nitrogen burners. At present, low-nitrogen burners can be roughly divided into the following categories according to their principles:

1) Stage burner

The stage burner designed according to the principle of staged combustion makes the fuel and air mixed and combusted in stages. Since the combustion deviates from the theoretical equivalent ratio, the production of nitrogen can be reduced.

2) Self-recirculating burner

One is to use the pressure head of the combustion air to suck back part of the combustion flue gas, enter the burner, and mix and burn with the air. Due to the recirculation of the flue gas, the heat capacity of the combustion flue gas is large, the combustion temperature is reduced, and the NOx is reduced.

Another type of self-recirculating burner is to recirculate part of the flue gas directly in the burner and add it to the combustion process. This kind of burner has the dual effects of suppressing nitrogen oxides and energy saving.

3) Concentration burner

The principle is to make part of the fuel burned richly, and the other part burned lightly, but the air volume remains the same as a whole. Since both parts are combusted under deviating stoichiometric ratio, NOx is very low. This kind of combustion is also called deviated combustion or non-stoichiometric combustion.

4) Split flame type burner

The principle is to divide a flame into several small flames. Due to the large heat dissipation area of ​​the small flames, the flame temperature is lower, which reduces the "thermal reaction NO". In addition, the small flame shortens the residence time of oxygen, nitrogen and other gases in the flame, and has a significant inhibitory effect on "thermal reaction NO" and "fuel NO".

5) Mixing promotion type burner

The residence time of the flue gas in the high temperature zone is one of the main factors that affect the amount of NOx generated. Improve the mixing of combustion and air, which can reduce the thickness of the flame surface. Under the condition of the combustion load unchanged, the flue gas is at a high temperature on the flame surface. The staying time in the zone is shortened, thus reducing the amount of NOx generated. The mixing-promoting burner is designed according to this principle.

6) Low nitrogen pre-combustion chamber burner

The pre-combustion chamber is a high-efficiency, low-nitrogen staged combustion technology developed and researched in my country in the past 10 years. The pre-combustion chamber is generally composed of primary air (or secondary air) and a fuel injection system. The fuel and primary air are rapidly mixed. A fuel-rich mixture is formed in the primary combustion zone of the pre-combustion chamber. Due to the lack of oxygen, only part of the fuel is burned. The fuel releases volatiles in the primary flame zone where the oxygen is lean and the flame temperature is lower, thus reducing the generation of NOx.

Any low-nitrogen combustion technology is essentially a technology to control the combustion process. According to the 20 years of experience in the use of low-nitrogen burners in European and American countries, if industrial boilers want to achieve truly continuous and reliable low-nitrogen combustion, it is not enough to replace or modify the burner. It is also necessary to upgrade the combustion control device, such as the installation of "timely control". "Combustion control device.