The advantages and characteristics of flameless combustion technology

The advantages and characteristics of flameless combustion technology
1、 Comparison between flameless combustion and traditional combustion methods.
Flameless combustion is a mild combustion mode under low oxygen dilution conditions, characterized by low reaction rate, less local heat release, uniform heat flow distribution, small temperature difference in the furnace, low peak combustion temperature, low noise, and low NOX production. In traditional combustion methods, 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 flameless combustion process, high-temperature preheated air and fuel are sprayed into the combustion chamber through different nozzles, forming a strong flue gas reflux, which violently combines with the fuel and air, delaying the diffusion and mixing of air and fuel. Due to the decrease in reactant concentration during flameless combustion, the reaction zone expands, and the fuel burns in a high-temperature and low oxygen atmosphere. The reaction occurs in a broad area, sometimes even filling the entire furnace. The flame volume doubles and expands, and the flame front disappears. The temperature distribution in the entire furnace is uniform, increasing the heat load of the furnace volume and enhancing heat transfer, eliminating the local high-temperature and high oxygen zones that appear in traditional combustion methods, So as to control the NOx concentration at a lower level.
2、 The generation mechanism of NOX in flameless combustion.
For the generation of NOX, it generally refers to NO and NO2, but due to the strong dependence of NO2 generation on NO concentration, which is about two orders of magnitude lower than NO concentration, the discussion mainly focuses on NO, including thermal, rapid, and N2O conversion types.
1. The mechanism of thermal NO reaction is:
N2+0=NO+N (1)
N+O2=NO+O (2)
N+OH=NO+H (3)
Due to the high activation capacity required for the decomposition of nitrogen molecules, equation (1) must be carried out at high temperatures. Therefore, the rate of thermal NO chain reaction depends on the slowest reaction type, as shown in equation (1), whose generation rate is strongly influenced by temperature. In addition, the activation energy of equations (2) and (3) is relatively small and the reaction is faster.
2. Rapid NO is only generated more when hydrocarbons are burned and the excess air coefficient is less than 1. When hydrocarbons burn, they decompose into CH CHJ and other functional groups can break the chemical bonds between N atoms, producing 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 reaction controls 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 traditional flame combustion of nitrogen-free fuels, NOX is mainly generated through thermal combustion due to the presence of high-temperature flame fronts. Due to the expansion of flame volume, dispersion of combustion zone, decrease in peak temperature, and significant reduction in thermal NO fraction, N2O conversion of NO exceeds that of thermal NO, thus becoming the main pathway for NO generation.
3、 Solid fuel flameless combustion characteristics.
Under the same conditions, in the simulation and experiment of flameless combustion of gas fuel, light oil, heavy oil, and coal, it was found that the combustion of heavy oil and coal also exhibited temperature uniformity, with high NO emissions. However, the simulation results of heavy oil and coal showed that due to the N content of heavy oil and coal reaching 0.37% and 1.49%, respectively, fuel type NOx was much higher than thermal type NOx.