low-NOx Swirl Burner Research Articles

A novel low-NOx swirl burner with coal-ammonia co-firing: Effect of ammonia ratio in the dual channels on combustion and NOx formation

A novel low-NOx swirl burner with coal-ammonia co-firing: Effect of ammonia ratio in the dual channels on combustion and NOx formation

Numerical investigation of ammonia/coal co-combustion in a low NOx swirl burner

Co-firing of ammonia (NH3) and coal in boilers is a promising technology to reduce CO2 emissions from power plants. However, NH3/coal co-firing in swirl burners can impact the original flame structures and may lead to serious NOx emission issues due to the high concentration of nitrogen in NH3. In this study, a numerical simulation of a low-NOx swirl burner was carried out to investigate the changes in flame structure, carbon emission, and characteristics of NOx generation. The results show that, when the NH3 co-firing ratio is 10 cal%, the flame retains its original swirl structure, and a significant increase in NO formation is observed in situations with different injection positions. When the co-firing ratio increases to 20 cal% and 30 cal% and NH3 is injected through the central air pipe, the flame changes from a swirl flame to an elongated flame. The injected NH3 is wrapped by the low-temperature primary air, separating the NH3 from the high-temperature zone, and pyrolysis becomes the primary reaction pathway for NH3 consumption rather than oxidation. The NO concentrations measured at the chamber outlet can be reduced to 11.2mg/Nm3, which is lower than that from coal combustion. As the co-firing ratio further increases to 50 cal%, NH3 and air are well mixed and burned, forming a “candle-type” flame. This results in a significant rise in NO emissions.

Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging

Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner structures including the pulverized coal concentrator, flame stabilizing ring and baffle plate create an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to in-flame NOX reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers.

Aerodynamically Study by Mathematical model Of Vertical Low NOX Swirl Burner in Natural Draft Furnace

Aerodynamically Study by Mathematical model Of Vertical Low NOX Swirl Burner in Natural Draft Furnace

Optical non-intrusive measurements of internal recirculation zone of pulverized coal swirling flames with secondary swirl intensity

The most favorable condition for NOx reduction in the swirl burner of wall-fired type boilers is to increase the residence time of pulverized coal particles in the internal recirculation zone which should be fuel-rich environments. The purpose of this study is to elucidate fundamental mechanisms for NOx reduction in pulverized coal swirling flames by means of the detailed observation of the flow field of the oxygen lean internal recirculation zone. The structure of pulverized coal swirling flames with secondary swirl intensity is studied experimentally by particle image velocimetry and local flame colors based on OH∗, CH∗, and C2∗ chemiluminescence intensities. The results show that the internal recirculation zone is enlarged with increasing the outer swirl intensity because the location of the stagnation point moves toward downstream. Also, the exhaust tube vortex is observed along the centerline in the flames, and started from near the stagnation point. For the combustion characteristics, the maximum temperature reduces as the outer swirl intensity increases, and the flame then moves downstream. The higher outer swirl intensity would be more effective in the low NOx swirl burner systems because of enhancing a dimension of the internal recirculation zone and reducing a local rate of chemical reaction.

Effect of outer secondary-air vane angle on the flow and combustion characteristics and NOx formation of the swirl burner in a 300-MW low-volatile coal-fired boiler with deep air staging

Small-scale laboratory experiments of airflow through a single burner model and industrial-scale experiments on the centrally fuel-rich, low NOx swirl burner on a 300-MW wall-fired subcritical boiler burning low-volatile coal under deep air staging were performed. The aerodynamic characteristics, flue gas temperature, and gas concentrations were measured for various vane angles of outer secondary air in the burner nozzle region. The results show that a stable, symmetric central reverse-flow zone forms close to the exit of the burner nozzle region under deep air staging. With decreasing vane angle, i) the maximum axial, radial, and tangential velocities and swirl intensity of the airstream increase; ii) the decaying rate of velocity increases between X/D = 0 and X/D = 0.8; iii) the maximum diameter, length, and the jet divergence angle of the central reverse-flow zone increase; and iv) the relative reverse-flow rate increases. While the primary air concentration decreases slightly, the maximum primary air concentration decreases rapidly with decreasing vane angle. In contrast, the maximum axial relative mixing rate increases in the initial stage along the airstream direction. A decrease in vane angle increases the flue gas temperature, the rate of increase indicating a closer ignition position of the anthracite and lean coal along the airstream direction of the burner. The O2 consumption rate and NOx formation rate increase in the initial stage of combustion, whereas in the later stage the CO concentration increases notably and the O2 concentration remains almost constant below 1%. The CO concentration can exceed 20,000 ppm, which restrains the NOx formation and reduces the NOx concentration notably, but beyond X′ = 0.8 m, the NOx concentration remains almost constant. The flue gas temperature varies slightly for different vane angles in the side-wall region. The O2 concentration exceeds 4% near the location of the water-cooled wall. The O2 concentrations are below 2% and the CO concentrations are above 5000 ppm along the radial direction 1.8375 m ≤ R′≤2.3375 m from the centerline of the burner in the side-wall region. With decreasing vane angle, the CO concentration increases while the NOx concentration decreases.

Effect of the flue gas recirculation supply location on the heavy oil combustion and NOx emission characteristics within a pilot furnace fired by a swirl burner

To reduce NOx emissions within industrial furnaces fuelled with heavy oil, a combustion configuration consisting of a low-NOx swirl burner, overfire air (OFA) and flue gas recirculation (FGR) was developed and then demonstrated in a pilot-scale furnace. In considering that FGR supply location always affects greatly combustion and NOx reduction performance, four combustion experiments with varying FGR were performed in turn: (i) under the circumstances without FGR and (ii) at three different FGR supply locations, i.e., FGR supplied through the burner periphery (BP-FGR), the main air pipe of the burner (MAP-FGR), and the duct between primary air and inner secondary air (DPS-FGR). Comparisons of gas temperatures and species concentrations downstream of the oil/air flame as well as NOx and CO emission levels were performed among the four settings to evaluate the effect of varying the FGR supply location. In comparison with the circumstances without FGR, supplying FGR in the furnace generally resulted in the change trends consisting of (i) decreasing the oxygen content and peak gas temperature in the primary combustion zone, (ii) prolonging the entire combustion process, and (iii) restraining the NOx generation while slightly increasing the CO emission. However, among the three settings with FGR, BP-FGR exhibited the least contribution in the aforementioned change trends, whereas DPS-FGR attained the optimal FGR performance. Consequently, the swirl burner plus an OFA ratio of approximately 25% and a DPS-FGR pattern with a 10% FGR ratio achieved low NOx emissions of approximately 200 mg/m3 at 3% O2 and an acceptable CO emission of approximately 50 ppm within the furnace.

Numerical investigation of gas-particle flow in the primary air pipe of a low NOx swirl burner – The DEM-CFD method

The gas-particle flow in the primary air pipe (PAP) of a low NOx swirl burner was investigated using the computational fluid dynamics (CFD) coupled with the discrete element method (DEM). The mathematical models were validated using the measured values obtained at the outlet of the primary pipe through a phase Doppler anemometer (PDA) system. Particles of different Stokes numbers in the primary air pipe (PAP) were investigated, and the effects of the structure of the primary air pipe and the particle–particle interaction on particle dispersion were analyzed. The results indicate that particles under the combined effects of the Venturi pipe and the spindle body are concentrated into a narrow band area and that the PAP structure can more efficiently concentrate particles with large Stokes numbers. The formed fuel rich/lean jet persists for a long distance out of the burner, thereby favoring of air-staged combustion and NOx reduction. The particle collision frequency and its fluctuation range increase as the particle Stokes number increases. The collisions among particles result in an increase of the spanwise dispersion of particles. Experimental results indicate that the models that take particle–particle collision into consideration are more able to predict particle concentration.

Influence of the gas particle flow characteristics of a low-NOx swirl burner on the formation of high temperature corrosion

In order to figure out the reasons for high temperature corrosion on side water walls, experimental investigations have been conducted on a 600MWe boiler. Samples collected from the side water walls were analyzed by XRD and SEM–EDS. The results revealed that the deposition of FeS and unoxidized fuel was responsible for the high temperature corrosion. Then the gas particle flow characteristic downstream of the HT-NR3 swirl burner was investigated using a three-component particle-dynamics anemometry (PDA) system. Velocities, particle concentrations and particle diameters were obtained. Results showed that there was an annular recirculation zone existing between the primary air and the secondary air. The mixing of the secondary air with the primary air downstream of the recirculation zone was very weak, so as the entrainment effect of the swirling outer secondary air on particles. Large particles tended to reach the water wall more easily. These gas particle flow characteristics strengthened the oxygen-deficient combustion for pulverized coal particles and increased the possibility of the deposition of FeS and unoxidized fuel.

Numerical simulation of the combustion characteristics of a low NOx swirl burner: Influence of the primary air pipe

Numerical investigations of the combustion process and NOx emission were conducted for two swirl burners (a prototype swirl burner and an optimised swirl burner) and a 600MWe wall-fired boiler. The predicted results and the measured information from the full-scale boiler were found to be in good agreement. The flow field, particle distribution, temperature pattern, and species concentration of two burners were discussed and analysed in detail. The species distributions in a furnace equipped with the prototype swirl burners were compared with those for optimised swirl burners. Simulation results showed that particles ejected from the prototype swirl burner diffused to the secondary air quickly, whereas particles of the optimised swirl burner recirculated for a long distance and then diffused into the secondary air passing through the recirculation zone. The gas particle field of the optimised swirl burner favours the formation of a large reducing atmosphere zone, contributing to NOx reduction. When the prototype swirl burners were replaced by optimised swirl burners, NOx emission decreased from 440 to 265mg/Nm3 at 6% O2, but the unburnt carbon content of the fly ash increased from 1.02% to 1.33%. The detailed results of this work promotes a full understanding of NOx formation characteristics, and helps to reduce NOx emission in pulverised-coal furnaces.

MODELLING PULVERISED COAL COMBUSTION USING A DETAILED COAL COMBUSTION MODEL

ABSTRACT The ability to assess the combustion behaviour of internationally traded coals and accurately predict flame characteristics, stable species concentration, unburned carbon and pollutant emissions is of importance to the power generating industry. Despite recent advances in coal combustion modelling detailed understanding is still lacking on the exact role of the coal maceral content on the combustion process. Here, a CFD-based coal combustion model that includes these effects has been developed to try to improve the predictive capability. A computational simulation of a 1 MW (thermal), pulverised fuel combustion test furnace, which was designed to replicate the time-temperature history of a full-scale furnace, was performed. This furnace also contained a triple-staged low-NOx swirl burner. A number of simulations were made using a number of coals in order to calculate NOx and the unburned carbon-in-ash, the latter being a sensitive test for the accuracy of the char combustion model.