Sources of NOx during combustion process
During the combustion process, there are two sources of NOx:
a. Fuel based NOx, which is derived from the decomposition reaction of substances such as nitrates contained in the fuel;
b. Thermal NOx refers to the NOx produced by the reaction of nitrogen and oxygen in the combustion air during the combustion process in a high-temperature environment;
When using natural gas as fuel, the nitrogen oxides produced are all thermal NOx;
The main factors affecting the generation of thermal NOx
Flame temperature;
The flame temperature is the first influencing factor, and experiments have shown that thermal NOx is generally produced above 1300 ℃. As the temperature increases, especially above 1500 ℃, the rate of NOx production increases exponentially.
The temperature field in the flame zone is generally characterized by the highest external flame temperature and the lower internal flame temperature;
When the ratio of air and gas is closer to the chemical equivalence ratio, the flame temperature is higher;
b. Reaction time
The residence time of smoke in high-temperature areas will greatly affect the production of NOx, and the thickness of the high-temperature layer perpendicular to the diffusion direction is the key influencing factor of this indicator;
c. Reaction surface area;
When the rate of NOx generation per unit volume remains constant, the larger the reaction area, the higher the total amount of NOx generation;
d. Oxygen and nitrogen concentration;
The closer the chemical equivalence ratio of oxygen and nitrogen is, the faster the reaction rate;
e. Target flue gas temperature
When the target flue gas temperature is higher, it may be affected by factors such as radiation heat, and the flame temperature will also be higher, resulting in higher NOx emissions;
Differences in design concepts
Ordinary burners generally have fewer factors to consider, with a focus on factors such as mixing efficiency and material temperature resistance;
Low nitrogen burners essentially use technology to control the combustion process. In addition to considering factors such as power output and material temperature resistance during design, various methods will be used to reduce the influence of various generation factors in response to low nitrogen requirements, thereby achieving lower NOx emissions;
Methods such as grading, segmentation, and concentration dilution coordination can effectively control the local flame temperature and reduce the generation of thermal NOx by deviating from the chemical equivalence ratio of air and gas during combustion in a local space and at a certain stage;
Improving the mixing effect, such as premixed combustion, when the mixing effect of air and gas is better, it can minimize the flame thickness in the high-temperature zone and reduce the travel distance and residence time of flue gas in the higher temperature zone, thereby reducing the generation of thermal NOx;
Gas recirculation, commonly used in boiler engines, utilizes combustion flue gas for reflux mixing to reduce the oxygen content in the flue gas. Excessive N2 and combustion products can also lower the maximum flame temperature, thereby suppressing the production of NOx;
Divide the flame area, improve the avoidance of excessive flame concentration, and reduce the reaction residence time;
A good low nitrogen burner generally strives to balance multiple aspects as much as possible;
Emission, regulation ratio, and energy consumption comparison
The emissions of low nitrogen burners are better than those of ordinary burners. Taking the application of automotive coating drying and heating as an example, low nitrogen burners can achieve lower NOx emissions, ranging from 50mg, 60mg to 80mg. The above data are all requirements converted to 3.5% O2;
When the maximum power of the burner is a constant value, the higher the output power, the lower the NOx emission value; The lower the output power, the higher the NOx emission value;
The increase in excess air coefficient is beneficial for improving mixing efficiency, but it may lead to incomplete combustion and excessive CO emissions;
Power regulation ratio
The adjustment ratio of low nitrogen type burners is generally 6:1-10:1. When used beyond the range, the low nitrogen effect cannot be guaranteed;
Compared to conventional burners, the regulation ratio of ordinary burners only considers power factors and can achieve higher regulation ratios, generally ranging from 30:1 to 50:1;
energy consumption
Low nitrogen type burners generally consume more energy than ordinary type burners to achieve lower emissions; The main reasons are as follows:
a. The complex structure will lead to increased pressure loss of gas and air, and higher power consumption of combustion fan;
b. Overall, the air excess coefficient of low nitrogen burners is generally high, and the excess air will take away some of the heat in the exhaust process, resulting in a slight increase in air consumption (the increase is influenced by the combined effect of excess air volume and exhaust temperature);
Important factors for achieving low nitrogen
To achieve low nitrogen emissions, the following factors need to be given special consideration:
a. It is necessary to carefully calculate the power demand range based on actual operating conditions and achieve precise positioning of the demand;
b. Correct selection to achieve accurate power coverage for conventional operating conditions;
c. Providing professional and meticulous debugging services is a necessary condition for achieving low nitrogen emissions;
d. Regular maintenance, cleaning or replacing air and gas filters.
