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Advantages and characteristics of flameless combustion technology

Advantages and characteristics of flameless combustion technology
1、 Comparison between flameless combustion and traditional combustion methods.
Flameless combustion is a mild combustion mode under the condition of low oxygen dilution, which is characterized by low reaction rate, low local heat release, uniform heat flow distribution, small temperature difference in the furnace, low combustion peak temperature, low noise and low NOX output. In the traditional combustion method, the concentration of reactants before entering the reaction zone is high and the temperature is low, while the concentration of reactants before entering the reaction zone is low and the temperature is high. In the process of flameless combustion, the high-temperature preheated air and fuel are injected into the combustion chamber through different nozzles to form strong flue gas backflow, which is strongly combined with fuel and air, delaying the diffusion and mixing of air and fuel. As the concentration of reactants in flameless combustion decreases, the reaction zone expands, and the fuel burns in a high temperature and low oxygen atmosphere, the reaction occurs in a wide area, sometimes even fills the entire furnace, the flame volume doubles and expands, the flame front disappears, and the temperature distribution of the entire furnace is uniform, which increases the heat load of the furnace volume, enhances the heat transfer, and no longer appears the local high temperature and high oxygen zone that appears in the traditional combustion mode, Thus, the NOx concentration is controlled at a lower level.
2、 The formation mechanism of NOX in flameless combustion.
For the generation of NOX, it generally refers to NO and NO2, but since the generation of NO2 strongly depends on the concentration of NO and is about two orders of magnitude lower than the concentration of NO, it mainly discusses NO, including thermal type, rapid type and N2O conversion type.
1. The thermal NO reaction mechanism is:
N2+0=NO+N(1)
N+O2=NO+O(2)
N+OH=NO+H(3)
Due to the large activation capacity required for the decomposition of nitrogen molecules, formula (1) must be carried out at high temperature, so the reaction rate of the thermal NO chain depends on the slowest reaction type, as shown in formula (1), and its formation rate is strongly affected by temperature. In addition, the reaction activation energy of formula (2) and formula (3) is relatively small and the reaction is fast.
2. Rapid NO is only generated when hydrocarbons are burned and the excess air coefficient is less than 1. When hydrocarbons burn, they decompose into CH.CHJ and other groups, which will destroy the chemical bond between N atoms and produce free radicals such as HCN and NH. At the same time, a large number of atomic groups such as 0 and 0H in the flame react with the intermediate products of the above reactions to produce NO. The following reactions control the overall reaction rate: CH+N2=HCN+N
3. The main reaction mechanism of N2O conversion type NO is:
N2+O+M=N2O+M 
N2O+O=2NO
For the traditional flame combustion of nitrogen-free fuel, NOX is mainly generated by thermal type due to the existence of high temperature flame front. Due to the expansion of flame volume, dispersion of combustion zone, reduction of peak temperature, and significant reduction of thermal NO share, N2O conversion of NO over thermal NO becomes the main way to generate NO.
3、 Flameless combustion characteristics of solid fuel.
Under the same conditions, in the simulation and test of the flameless combustion of gaseous fuel, light oil, heavy oil and coal, it was found that the combustion of heavy oil and coal also showed the characteristics of uniform temperature and high NO emission. However, the simulation results of heavy oil and coal showed that the fuel type NOx was far greater than the thermal type NOx because the N content of heavy oil and coal reached 0.37% and 1.49% respectively.