Research of nitrogen oxides generation during low-temperature swirl fuel combustion

The work is devoted to the solution of the issue of environmental protection from harmful emissions of thermal power plants. Mechanisms of nitrogen oxides generation in a low-temperature swirl furnace process are considered. The analysis of the combustion process characteristics influence on the final level of nitrogen oxide concentration in the flue gases of the boilers is presented. Calculations and experimental studies have shown that the method of low-temperature swirl combustion provides a significant reduction in emissions of nitrogen oxides to the atmosphere. 1 Principles and organization of low-temperature swirl furnace process The concept of LTS-method (LTS-technology) was proposed by Professor V.V. Pomerantsev at the Leningrad Polytechnic Institute (now St. Peterburg Polytechnic University of Peter the Great) at the turn of the 1970s [1], and has been further developed now [24]. As an alternative to the pulverized-coal torch (Fig. 1a) [510], the method of lowtemperature swirl combustion (Fig. 1b) has significant advantages, including, in particular, lower emissions of gaseous nitrogen oxides into the atmosphere [11, 12] . The LTS method is based on the principle of organizing multiple circulation of relatively large fuel particles with periodic return to zones with the initial concentration of oxygen. In the LTS furnace, fuel is predominantly burned in the lower swirl zone, where multiple forced circulation of fuel particles is organized, which makes it possible to equalize the temperature field in the combustion chamber and to lower the temperature level in the furnace (on average by 100150 K). The LTS-combustion method opens up great opportunities for reducing the dimensions of boilers and reducing metal costs due to an increase in the intensity of combustion and heat-mass exchange processes [13, 14]. The organization of repeated circulation of particles in the furnace and the creation of a stable fuel ignition zone made it possible to switch to combustion of grinded coal [15], which eliminated the danger of dust preparation system explosion and increased the boiler plant reliability as a whole. * Corresponding author: trinchenko@spbstu.ru


Principles and organization of low-temperature swirl furnace process
The concept of LTS-method (LTS-technology) was proposed by Professor V.V. Pomerantsev at the Leningrad Polytechnic Institute (now St. Peterburg Polytechnic University of Peter the Great) at the turn of the 1970s [1], and has been further developed now [24]. As an alternative to the pulverized-coal torch (Fig. 1a) [510], the method of lowtemperature swirl combustion (Fig. 1b) has significant advantages, including, in particular, lower emissions of gaseous nitrogen oxides into the atmosphere [11,12] .
The LTS method is based on the principle of organizing multiple circulation of relatively large fuel particles with periodic return to zones with the initial concentration of oxygen. In the LTS furnace, fuel is predominantly burned in the lower swirl zone, where multiple forced circulation of fuel particles is organized, which makes it possible to equalize the temperature field in the combustion chamber and to lower the temperature level in the furnace (on average by 100150 K). The LTS-combustion method opens up great opportunities for reducing the dimensions of boilers and reducing metal costs due to an increase in the intensity of combustion and heat-mass exchange processes [13,14].
The organization of repeated circulation of particles in the furnace and the creation of a stable fuel ignition zone made it possible to switch to combustion of grinded coal [15], which eliminated the danger of dust preparation system explosion and increased the boiler plant reliability as a whole. The use of the LTS method made it possible to organize efficient combustion of crushed fuel [15], thus completely eliminating the dust preparation system. Combustion of fuel in the LTS furnace occurs with high efficiency with the complete elimination of the liquid fuel use to illuminate the torch. At the same time, the temperature of the gases in the combustion chamber is reduced, and the slagging problems [16] of the heating surfaces are practically solved, the probability of high-temperature corrosion of the screens is reduced due to a decrease in the temperature level and a decrease in the oxygen concentration in the furnace, and the stepwise supply of the oxidizer to the flare leads to a decrease in the generation of toxic nitrogen oxides, formation of reducing zones, and decomposition of formed nitrogen oxides on the surface of burning coke particles [17].

Technical reequipment of the research object to the LTS combustion method and its results
The advantages of the low-temperature swirl method made it possible to realize it at many energy facilities, including the BKZ-210 boiler (station No. 9) of the Kirov TPP-4 ( Fig. 2).
The boiler is vertical-water-tube, one-drum, with natural circulation, U-shaped configuration, with solid slag removal, with balanced traction, has the following design parameters: steam capacity Dpp = 210 t/h, superheated steam pressure ppp = 13.8 MPa, superheated steam temperature Tpp = 813 K (540 °C). The nominal heat output of the boiler is Q = 143 MW (123 Gcal/h). One of the project fuels of the boiler is Kuznetsk coal of grades G and D, the characteristics of which are given in Table 1.  Commissioning tests on the boiler BKZ-210-140f st. No. 9 after its technical reequipment for the LTS-combustion method was carried out according to the ORGRES method [18] according to the approved programs. The coefficient of efficiency of the boiler was determined by the inverse balance method with the introduction of corrections for the operating conditions for the selection of ash and was, depending on the boiler load, 89. from the boiler is within 350500 mg / Nm 3 (Fig. 4), which on average meets the requirements of regulatory documents [19] (Table 2).

Generation of nitrogen oxides in a low-temperature swirl furnace
The degree of transition of fuel nitrogen (Nr) to nitrogen oxides (NOx), in the experiments carried out on the BKZ-210-140f boiler before and after technical reequipment on the LTS combustion method, was determined as the ratio of the measured nitrogen oxide concentration at the furnace outlet (NO2), to the maximum possible (under given conditions of the combustion process) concentration of nitrogen oxides, provided that all nitrogen of the fuel passes into nitrogen oxides NO2-"fuel": (1) Concentration of NO2 in terms of the volume of wet combustion products of fuel, (mg/nm 3 ): where V dg , V wgthe volume of dry and wet combustion products per 1 kg of burned fuel, with the excess air factor corresponding to this experiment, nm3 / kg; NO2-meas  measured in the experiment concentration of NO2, mg/nm 3 . The maximum possible (theoretical) concentration of nitrogen oxides (mg/nm 3 ): where N rnitrogen content in fuel for the working mass, %; V g  is the actual volume of combustion products, nm 3 /kg. Substituting NO2  and NO2-"fuel" in (1), we get: The where: R=8.31 Dj/(molK)universal gas constant. After technical re-equipment for low-temperature swirl combustion, the value of the nitrogen transition of the fuel in NOx (NLTS) is 0.050.08, which is, on average, 34 times lower than when burning coal dust in a direct-flow torch ( Table 2). Calculations to determine the degree of nitrogen fuel transition in NOx, carried out in [20] for LTS incineration and incineration in direct-flow pulverized coal torch (DPCT), show that the degree of transition of nitrogen-containing fuel compounds to NO depends on the method of fuel combustion, the temperature level in the combustion chamber, the excess air ratio and the nitrogen content of the fuel (Fig. 5). These data correspond to the results obtained on the boiler BKZ-210 of Kirov CHPP-4 with LTS-combustion technology [21,22]. The value of N = f(N r ), described by curve 1 of Fig. 5, tends to 0,070,09 with increasing nitrogen content in fuel (N r ) to 1,92,0 %.

Model of low-temperature swirl process and combustion of fuel of polyfraction composition
Reduction of the degree of fuel nitrogen transfer to nitrogen oxides during the transfer of the boiler to the LTS combustion method and a significant increase in its environmental performance is explained by the peculiarities of the aerodynamic organization of the movement of gas-air flows in the LTS furnace and the use of fuel of coarse grinding. To study this process, with reference to a BKZ-210 boiler with an LTS furnace (Fig. 6), a mathematical model of fuel combustion was developed that takes into account the generation and conversion of nitrogen oxides. Consideration of the combustion process from the diffusion-kinetic positions made it possible to compile a system of nonlinear differential equations of diffusion and kinetics of the type [23]: taking into account the oxidation and reduction reactions of particles coming on to the surface, and homogeneous reactions occurring within the boundary layer, and obtain an expression for determining the carbon flow from the surface of burning coke particles (kmol/(m 2 s)): To solve the problem, the concepts of "reduced film" (/=1/(NuD2)); are applied; dimensionless coordinate (=x/); criterion of Semenov (Se=(k4/D) 0,5 ); diffusionchemical criterion (Ni=ki/D); the Arrhenius relation for reaction rate constants (ki=k0iexp(Ei/RT)); "Poles" with coordinates k* = 100 m/s, T* = 2600 K; activation energy.
As a result of the calculations, the burnup curves of particles of Kuznetsk coal of various sizes (Fig. 7), trajectories of their movement to complete combustion (Fig. 8a) and the concentrations of nitrogen oxides in the sections along the height of the combustion chamber of the LTS furnace are determined (Fig. 8b). Applied to the explored scattering characteristics of Kuznetskiy coal (R100 = 30 %, R500 = 1 %), the burning time of the smallest particle (dmin = 40.6 μm) is ~ 0.7 s, and the time of complete combustion of the particle of maximum size (dmax = 780 μm) is within 19 seconds.
The amount of nitrogen oxides reacted with the carbon surface of burning coke particles was from the balance of the reaction: taking into account the carbon consumption of the coke reacted according to this reaction: The calculated results (Fig. 8b) are in good agreement with the experimental data obtained on the BKZ-210 boiler with low-temperature swirl combustion technology  (Table 2). To the exit from the furnace, the concentration of nitrogen oxides in the flue gases is at the level of 350500 mg/nm 3 ; the same NO concentrations are fixed during the operation of the boiler.

The results of the study, their analysis and discussion
The high ecological indexes of the LTS combustion method have been confirmed by calculation and experimental methods. With regard to the boiler BKZ-210 Kirovsk CHPP-4 with LTS-furnace, a three-fold reduction in emissions of nitrogen oxides was recorded in comparison with the technology of the direct-flow charcoal flare. Swirl aerodynamics LTSfurnaces and step-by-step supply of oxidizer to fuel affect the generation of nitrogen oxides by the following mechanisms: 1. The combustion process in the lower swirl zone, where the bulk of the fuel burns (Fig. 8a) occurs in the absence of an oxidizer. A decreased concentration of oxygen (Fig. 9a) entails a decrease in the generation of "fuel" nitrogen oxides formed at the initial part of the flare (at the stage of escape and combustion of the volatile substances of the fuel).
2. The formed nitric oxides under the conditions of the reducing medium (Fig. 9b, 9c) react intensively by the reaction (8) with the carbon of the coke, which also reduces the total concentration of NO. The two-nozzle scheme of the bottom blast ensures the separation of fuel particles in the flow along the front slope of the furnace funnel. Small (for the considered fineness of grinding) particles (particles < 250280 μm) pass along the front ramp without touching the furnace shields (Fig. 10a) quickly enough, but considering their large amount, they make a significant contribution to the decomposition of nitrogen oxides on its surface. In turn, the large particles (Fig. 10b) stay near the front edge of the furnace funnel for a considerable time (while being in a reducing medium (Fig. 9b, 9c)), which increases the amount of NOx reacted with the coke carbon by reaction (8). In general, calculations show that burnout of particles of any size in the lower swirl zone of the LTS furnace contributes to a decrease in the concentration of nitrogen oxides in flue gases.

Final provisions and conclusions
As a result of the calculation study, the main parameters that influence the process of nitrogen oxide generation during low-temperature swirl combustion of fuel are revealed. The parameters studied include the particle size distribution of the fuel, the aerodynamics of the combustion chamber created by combustion air and lower blast air streams, the concentrations of the main reactive components, and the temperature level in the lowtemperature swirl furnace The organization of a stepped supply of oxidant to the fuel (burnt air, bottom and tertiary blasting) tightens the ignition process, reduces local maximum temperatures in the furnace, inhibits the generation of nitrogen oxides and significantly contributes to their reduction. At the same time, the presence of a stepped air supply has a positive effect on the aerodynamic structure of gas-fuel flows and the operation of the combustion chamber as a whole. The most intensive decomposition of the formed nitrogen oxides occurs in the lower swirl zone of the furnace, where the bulk of the fuel burns out.
In general, the proposed model makes it possible to reliably predict the generation of nitrogen oxides in the introduction of the low-temperature swirl combustion method, as well as enable analysis of the effect on the burning process of the main regime and constructive factors.