Modification of Low NOx Burner for Coal-fired Boiler


Modification of Low NOx Burner for Coal-fired Boiler
At present, China is still mainly composed of coal resources, and production and life rely heavily on traditional coal resources, causing great pollution to the atmospheric environment. Coal measures are dominated by coal, which will produce sulfide, nitrogen oxide, and other pollutants during combustion. In order to solve the contradiction between energy supply and environmental protection and reduce the cost of denitrification transformation and operation, the transformation of a low NOx burner is imperative.

First: Introduction to Low NOx Combustion Technology
Reducing NOx emission by changing combustion conditions is a low NOx combustion technology. Low nitrogen combustion technology is the simplest and most economical method widely used to reduce NOx emissions. At present, low NOx combustion technologies mainly include the following:

1. Low excess air coefficient`
Low excess air coefficient is a simple method to optimize combustion and reduce NOx output. The reduction of nitrogen content in flue gas can inhibit the formation of nitrogen oxides. Without changing the structure of the combustion device, NOx emission is reduced and the economy of the device operation is improved. For coal-fired boilers, the limitation is mainly due to the low air excess coefficient, which will lead to the binding slag and corrosion on the heating surface, temperature changes, and the increase of fly ash combustibles, leading to a decline in the economy. Therefore, power station boilers cannot be significantly adjusted in actual operation.

2. Air staged combustion
The basic principle of gas staged combustion is to complete the fuel combustion process in stages. First, reduce the amount of air supplied by the main gas turbine to the furnace to 70-75% of the total amount of combustion air (equivalent to 80% of the theoretical amount of air), so that the fuel can be burned under low oxygen and rich fuel combustion conditions. This method makes up for the simple disadvantage of low excess air combustion. The smaller the air excess coefficient in the first stage combustion area, the better the effect of inhibiting NOx generation. However, the more combustion products, the lower the combustion efficiency, and the greater the possibility of slagging and corrosion. Therefore, in order to ensure NOx emission and the economy and reliability of boiler combustion, the air staged combustion process must be reasonably organized.

3. Fuel classification
When nitrogen oxides and hydrocarbon radicals, incomplete combustion products CO, H2, C, and CnHm are produced during combustion, NO reduction reaction will occur. Using this principle, 80-85% of the fuel will be delivered to the primary combustion area α> At 1 hour, the combustion produces carbon dioxide. The fuel sent to the primary combustion area is called the primary fuel, and the remaining 15-20% of the fuel is sent to the secondary combustion area at the upper part of the main burner α< 1 so that NOx generated in the primary combustion area can be reduced to nitrogen molecules in the secondary combustion area. The secondary combustion area is also called a burning area. The fuel sent into the secondary combustion area is also called secondary fuel, or burning fuel. The burning zone can not only reduce the generated NOx but also inhibit the generation of new NOx and further reduce the emission concentration of NOx.

4. Flue gas recirculation
At present, the flue gas recirculation method is also widely used for low NOx combustion technology. It directly sends a part of low-temperature flue gas into the furnace, or mixes with the primary air or secondary air and sends it to the furnace. This can not only reduce the combustion temperature but also reduce the oxygen concentration, thus reducing the NOx emission concentration.

5. Low NOx burner
A burner is one of the key equipment for boiler combustion systems. In the process of boiler combustion, the performance of the burner directly affects the reliability and economy of the boiler. According to the generation mechanism of NOx, most NOx is generated in the ignition stage of pulverized coal. Therefore, the specially designed burner structure and the adjustment of the air coal ratio of the burner can reduce the oxygen concentration in the ignition zone and the temperature in the ignition zone, so as to achieve the purpose of inhibiting NOx generation.

6. Low NOx combustion system
The low NOx burner and low NOx combustion (air staging, fuel staging, and flue gas recirculation) are combined to form an ultra-low NOx combustion system, so as to better reduce NOx emissions and reduce the carbon content of fly ash.

Second: Problems to be noted in the transformation
(1) In terms of fire safety: the gas-fired boiler room belongs to a Class A plant and its fire resistance rating is not lower than Class II, which is much higher than the coal-fired boiler plant in terms of fire safety. Therefore, when the coal-fired boiler is transformed into a gas-fired boiler, the boiler plant shall be reconstructed according to the specifications, and attention shall be paid to the selection of explosion-proof and fire-resistant grades.

(2) In terms of automatic control: special attention should be paid to the automatic interlock protection of boiler water level, the automatic protection of gas boiler flameout, the automatic interlock protection of low steam pressure, gas pressure, air pressure, etc. In terms of the degree of automatic control, the gas boiler is much higher than the coal-fired boiler.

(3) Two combustible gas alarm devices are set in the boiler room and the surge chamber, and the alarm system shall be interlocked with the emergency shut-off valve and forced ventilation facilities.