Ignition Distance Research Articles

CFD Investigation on Air-Staged Combustion Characteristics of Deep Peak Shaving Wall-Fired Burner Boilers

ABSTRACT In alignment with China’s ambitious goals of reaching peak carbon emissions and achieving carbon neutrality, stringent load peak regulations are being enforced for coal-fired power generation. This underlines the significance of deploying deep load peaking strategies in coal-fired units. The current research focus is on wall-fired boilers operating under ultra-low loads of 600 MWe. It delves into the flow dynamics, combustion characteristics, component concentration field, and NOx emission characteristics under varying air coefficients in the primary combustion zones (α). The findings suggest the formation of a consistently high-temperature region in the primary combustion zone, where temperatures surpass 1500 K. The middle burner exhibits superior combustion stability compared to the bottom burner. As α diminishes, several changes occur. The flow field intensity in the primary combustion zone lessens, the ignition distance contracts, and the O2 concentration in this area drops. This engenders a reducing atmosphere characterized by low O2 and high CO levels, which hampers the formation of NOx. At α = 0.95, the NO concentration peaks, with the primary generation mechanism being temperature-driven. At this juncture, the NO concentration at the furnace flue gas outlet is 287.36 mg/m3.

Investigation into the Suppression Effect of Water Mist on the Self-ignition and Flame Propagation of High-pressure Hydrogen Release

Abstract The self-ignition accident of pressurized hydrogen leakage is regarded as one of the big potential risks in its wide utilization. In this work, a detailed investigation into the suppression effect of water mist on the hydrogen self-ignition and flame propagation is performed. A parametric study of the effect of water concentration and droplet size on flame dynamic is conducted. The results show that the fine water mist effectively reduces the shock-heating temperature, significantly prolonging the ignition time and distance. The water droplets have a direct contact with the fast growing flame, which leads to more intense two-phase interaction that results in the decoupling of shock wave from the leading flame and a more pronounced bimodal flame structures. The droplet size is found to have small effect when the water concentration is low, and the mist with a medium size of 50 μm shows a better inhibitory performance.

Numerical and experimental investigation on the operating characteristics of fan-shaped pulse detonation combustor

The internal flow channel of a turbine engine is typically annular. When circular pulse detonation combustors (PDCs) are installed in this channel, there is a problem with low section utilization rates. To solve this problem, a single-tube fan-shaped PDC test system was developed. The effects of ignition distance on the operating characteristics of the fan-shaped PDC using air and gasoline under different frequencies were studied through both experiment and 3-D numerical simulation. Numerical simulation was also used to investigate the formation and propagation characteristics of detonation waves. The results show that the fan-shaped PDC can operate stably at frequencies ranging from 5 to 25 Hz when the ignition distance is either 2.30 or 2.70 times the equivalent circle diameter from the thrust wall, and it is found to be better to ignite at a distance of 2.30 times the equivalent circle diameter. The blockage ratio of the lower arc surface is larger than that of the other surfaces. This results in more intense collision and reflection. A larger local high pressure area forms near the lower arc surface, which is conducive to the formation of detonation waves. However, the larger blockage ratio makes the flow loss larger, which weakens the intensity of the pressure waves near the lower arc surface. The detonation waves in the fan-shaped PDC travel closer to the upper arc surfaces.

Studies on NOx formation and reduction characteristics in high-ash, low-volatile coal combustion by mechanism experiments and industrial-scale trials

Mechanism experiments were performed to investigate NOx formation and reduction during the combustion of high-ash, low-volatile coal. Additionally, industrial-scale trials were conducted to study the effects of air-staging degree on the combustion and NOx emission characteristics of high-ash coal on a 350 MW supercritical down-fired boiler. The results showed that the total BET specific surface area, total pore volume and porosity values of high-ash coal (HAC) and ultra-high-ash coal (UHAC) were all larger than those of normal-ash coal (NAC), whereas NAC contained more N-5 and N-6 than HAC and UHAC. The carbon layer structure of HAC char was less ordered and the content of hydroxyl group was relatively high. The overall NO reduction ratio obtained using the HAC char was higher than the value for the UHAC char. However, in the high temperature range of 1300–1400 °C, NO reduction associated with UHAC char was accelerated because of catalysis by ash. On increasing the excess oxygen ratio λ from 1.0 to 1.2, the char-N conversion shown by the UHAC char slowly increased and remained the lowest among the three chars. When burning UHAC, as the ratio of secondary to tertiary air fluxes, Rst, was decreased from 1.56 to 0.57, heating of the fuel-rich coal/air flow slowed and pulverized coal combustion was inhibited. The situation associated with HAC was different. At the Rst of 1.11, the ignition distance of fuel-rich flow was the shortest at 1.50 m, and the overall temperature in the furnace was the highest. However the NOx emissions at the furnace exit were as high as 715 mg/m3 (6 % O2) in this condition. Decreasing Rst to 0.57 reduced this value to 634 mg/m3 (6 % O2) with carbon levels in the fly ash and slag of 5.17 % and 3.67 %, respectively. Air-staging combustion is evidently a reasonable approach to controlling NOx emissions when burning HAC. While for UHAC, it was necessary to appropriately reduce the air-staging degree to enhance combustion and maintain low NOx emissions.

Experimental and Numerical Study on the Plasma-Laser-Induced Ignition of Strut Stabilizer at Different Locations

The minimum ignition equivalence ratio of the strut stabilizer is an important parameter in the design of integrated afterburners. The ignition location significantly affects the ignition equivalence ratio and flame propagation, and therefore, it should be deeply studied. The ignition equivalence ratio and flame propagation at different axial ignition locations downstream of the strut stabilizer are studied in this paper. When the ignition distance is approximately the bluff body trailing edge width, a lower ignition equivalence ratio is required for ignition, and the flame propagates faster through the entire combustion chamber. For different ignition locations, the generated flame kernel at different locations all first propagates to the shear layer. Subsequently, the unilateral flame rapidly extends, ultimately igniting the entire combustion chamber. The flame propagation trajectory depends on the ignition location controlled by the non-reacting flow field and the distribution of kerosene concentration. The flame propagation trajectory mainly includes three paths: (1) the flame kernel is directly downstream the shear layer when the ignition location is close to the tail edge of the stabilizer, (2) the flame propagates upstream into the shear layer in a U-shape when the ignition location is far from the stabilizer but still in the recirculation zone, and (3) the flame propagates upstream into the recirculation zone and shear layer in a U-shape when the ignition location is outside the recirculation zone. In addition, the time for flame propagation to the shear layer is directly related to the ignition performance when the ignition location is within the recirculation zone. If the flame reaches the shear layer in a longer time, there will be more energy loss during the flame propagation process, and the ignition performance will deteriorate. The speed of the flame-trailing edge extension is directly related to the ignition fuel-air ratio, and the downstream extension of the flame is mainly affected by the turbulence velocity in the shear layer.

Study of the Influence of Heat Flow on the Time to Ignition of Spruce and Beech Wood

Adherence to fire safety regulations for wood is one of the most important tasks in its use in structural and architectural applications. This article deals with determining the influence of heat flux on the ignition process of spruce (Picea abies L. Karst.) and beech wood (Fagus sylvatica L.). The heat flux was generated by an electric radiant panel. The analysed parameters included the ignition time of the spruce and beech wood samples, the influence of wood density, and sample moisture, and the course of sample combustion, both with and without flame, was observed. The heat flux was maintained at constant values, depending on the distance of the examined sample from the panel, along with the specific power of the radiation panel. The power of the radiation panel was set to constant values of 5 kW and 10 kW. The samples were placed at distances of 50, 70, 100, 150, and 200 mm from the heat source, and heat fluxes in the range of 13–92 kW·m−2 were observed. At a power of 5 kW and a heat flux of 64 kW·m−2, neither the sample of beech nor that of spruce wood, placed at the distance of 100 mm from the radiation panel, exhibited flaming combustion. The ignition time for the beech wood was approximately twice that of the spruce wood, likely due to the higher average wood density. It can be stated that wood density, as one of the main factors, significantly influences the ignition phase of burning. The statistical analysis examined variables including wood type, radiant panel output, distance, and heat flux in relation to ignition time. The analysis revealed a significant difference between ignition time and distance (p-value = 0.0000, H = 37.51583) as well as between ignition time and heat flux (p-value = 0.0000, H = 37.69726). Similarly, the time to ignition for all tested beech wood samples was longer than for spruce wood.

Experimental Study on Flame Chemical Composition of Coal and Ammonia Gas-Solid Jet in Flat Flame Burner.

Ammonia as a fuel to partially or completely replace fossil fuels is one of the effective ways to reduce carbon dioxide, and the research on ammonia coal cocombustion is of great significance. The combustion characteristics of ammonia are very different from those of pulverized coal, resulting in the ignition and emission characteristics of ammonia and pulverized coal gas flow that is different from traditional pulverized coal flame. In this paper, the effect of pulverized coal concentration in coal and ammonia mixed combustion jet on the ignition distance and gas-phase components at different positions of the jet flame were studied experimentally on the flat flame burner, and the conditions of ignition and ignition stability of coal and ammonia gas-solid fuel were expounded. It was found that the ammonia mixed with pulverized coal changed the temperature field of the flat flame burner and therefore the ignition characteristics of the jet were changed. The ignition delay time at the same jet speed was positively correlated with the pulverized coal concentration, but when the pulverized coal concentration continued to decrease, the influence on the ignition delay time gradually became smaller. The composition of coal ammonia gas-solid fuel changed the heat transfer path and share during combustion, and finally, the flame temperature was negatively correlated with the concentration of pulverized coal. Therefore, the reduction of the pulverized coal concentration was conducive to the stable combustion of coal ammonia mixed fuel. When HAB = 100 mm, the conversion rate of fuel N to NOx per unit mass of coal ammonia mixture increased with the increase of pulverized coal concentration. The NOx production amount first increased and then decreased with the increase of pulverized coal concentration, and the amount of N2O and NO2 decreased rapidly with the increase of HAB. The proportion of NOx in NO exceeded 94%, which was conducive to achieving low nitrogen combustion of coal and ammonia gas-solid fuel. In general, the O2 concentration in the ammonia coal jet flame decreased, the flue gas temperature, and NOx and CO generation increased after mixing ammonia, and the optimal pulverized coal concentration in this experiment was 0.41 kgc/kga (mass ratio of pulverized coal to the sum of N2 and NH3).

Effect of ignition distance on deflagration characteristics of non-uniform oil mist in closed cabins

The deflagration of non-uniform oil mist in closed cabins can easily cause damage to ship equipment and casualties. In this work, the influence of ignition distances on the oil mist deflagration hazard is experimentally investigated, focusing on analyzing the oil mist deflagration propagation process, flame propagation speed, deflagration overpressure, and gas temperature, and exploring its influence mechanism. The results show that as the ignition distance decreases, the oil mist deflagration intensifies, flame propagation speed increases, deflagration overpressure improves, and gas temperature increases. Oil mist deflagration is divided into deflagration stage, turbulent combustion stage, and self-extinguishing stage. In deflagration stage, the center of the flame volume grows, and then gradually shrinks, moving in the direction of the nozzle. Flame propagation speed increases firstly and then decrease, accompanied by oscillatory changes in the process. The deflagration peak overpressure gradually decreases from the nozzle position to the right side of the cabin. With a nozzle diameter of 0.1 mm, the peak oil mist deflagration overpressure is around 0.25 MPa when the ignition distance is 10 cm away from the nozzle. However, when the ignition distance is 30 cm away from the nozzle, the oil mist deflagration peak overpressure is around 0.21 MPa. The high-temperature range of oil mist deflagration is mainly concentrated in the deflagration center area.

Influence of secondary air blade angle and oxygen-rich combustion characteristics of an improved Babcock swirl burner

Innovations in research methods and stable combustion technologies were urgent to enhance flexibility in faulty coal-fired boiler. Focusing boiler start-stop processes was vital for effective peak shaving in power plants. The integration of enriched oxygen stable combustion with swirling techniques showed promise in improving low-load performance. In the cold single–phase experiment, the influence of secondary blade angle on the flow field of an improved Babcock burner was observed. Industrial experiments were conducted on a 700 MW boiler, exclusively implementing the improved burners in the lower layer of the boiler front wall. Stable annular recirculation zones were observed at the burner outlet with blade angles of 30°, 45°, 60°, and 75° for the secondary air. Increasing the blade angle expanded the recirculation zone length and decreased the gas net flow ratio. The angle, between 45° and 60°, facilitated air mixing and protected against outlet slagging and high-temperature corrosion. During cold start–up process of the boiler, supplying 0 kg·s−1 of pure oxygen to a single improved burner increased the flue gas temperature by about 50 °C within 0.4 m of the primary air duct outlet. Increasing the oxygen flow rate to 0.2456 kg·s−1 steadily raised the flue gas temperature at 405 MW. The gas temperature in the burner central region was generally lower than the secondary air zone, resulting in a longer ignition distance for the pulverized coal. Higher oxygen levels intensified the heating rate and reduced the ignition distance in the central region.

Experimental investigation on initiation mechanism, overpressure, and flame propagation characteristics of methane-air mixtures explosion induced by hexogen in a closed pipeline

Gas explosion accidents under shock wave stimulation cause more severe hazards than electric spark ignition. To explore the initiation mechanism and explosion characteristics of methane induced by shock waves. A series of experimental investigations of methane-air mixtures initiated by hexogen (RDX) was performed based on a self-developed closed pipeline. The effects of incident shock wave intensity and methane concentration on the characteristics of methane explosion were studied. The results indicate that methane participating in the reaction can significantly increase the positive pressure duration compared with RDX explosion. With the increase of incident shock Mach number (Ma), methane deflagration pressure increases and counteracts the attenuation of RDX detonation pressure. When the ignition distance is 0 cm, the overpressure inside the pipeline is directly proportional to the methane concentration from 5 vol% to 13 vol%. When the concentration approaches the upper explosion limit, the flame velocity exceeds the theoretical detonation velocity, resulting in the unstable detonation of methane in the pipeline.

Combustion stability and NOx emission characteristics of three combustion modes of pulverized coal boilers under low or ultra-low loads

Under low or ultra-low loads, poor combustion stability and high NOx emissions hinder the stable operation of pulverized coal boilers. Numerical simulation was used to investigate the impact of boiler load rate and operation oxygen on combustion stability and NOx emission characteristics in tangentially fired, swirl-opposed fired, and W-flame boilers. To quantitatively assess combustion stability, a combustion stability index was introduced using ignition distance and average temperature of the main combustion zone as parameters. The results indicate that as the load decreased, the temperature and combustion stability decreased rapidly below the 50% load; ignition distance and NOx emission concentration (@6% O2) increased. Moreover, under stable low loads, reduced operation oxygen enhanced temperature and combustion stability while reducing ignition distance and NOx emissions (@6% O2), with minimal impact on CO concentration. Under stable low loads in all three boilers, reducing operation oxygen to 3.5% led to an average temperature increase of 56.8 °C. Additionally, the W-flame boiler exhibits the highest average temperature (1371 °C), oxygen concentration, and NOx concentration, and the lowest temperature-decrease rate. Under equal conditions, the average combustion stability index (Ics) decrease rate of the three boilers under low loads was 2.3 times that under mid–high loads. The tangentially fired boiler demonstrates the highest combustion stability capacity (Ics = 0.78), possibly owing to the close distance between each direct-current burner layer, and the mutual support and ignition between each corner of the same layer.

Numerical Study of the Effect of Imaginary Circle Diameter on the Combustion in a Corner-tangentially-fired Boiler

The key factor affecting the combustion and operation of corner tangentially fired boilers (CTFBs) is the diameter of the imaginary circle. In this paper, the temperature and reducing gas distribution in a 350 MW supercritical CTFB were studied numerically, and the influence of imaginary circle diameter on combustion characteristics was analyzed. The results show that: 1) When the diameters of the imaginary circles of the primary and secondary air are decreased, there will be a delay in combustion. This is not profit to the steady-state burning of coal powder. In addition, the temperature near the water wall decreases, and the volume fraction of CO and H2S decreases on the rear side of the main combustion area, but increases significantly on the side of the fire. 2) By reducing only the diameter of the imaginary circle of primary air, the ignition distance of pulverized coal in the lower burner can be advanced, which is profit to improving combustion stability at low loads. Besides, the temperature, CO and H2S volume fraction near the water wall all decrease. 3) When the imaginary circle diameters of primary and secondary air in the upper part of the furnace are reduced, the temperature, volume fraction of CO and H2S in the adjacent layer near the water wall are increased.

Energy deposition in a telescopic laser filament for the control of fuel ignition.

The efficiency of energy coupled to plasma during femtosecond (fs) laser filamentation plays a decisive role in a variety of filament applications such as remote fabrication and spectroscopy. However, the energy deposition characterization in the fs laser filament formed by a telescope, which provides an efficient way to extend the filament distance, has not yet been revealed. In the present study, we show that when the distance between the two lenses in a telescope changes, i.e., the effective focal length changes, there exists an optimal plateau energy deposition region in which the energy deposited into the filament per unit length called the average lineic energy deposition (ALED) remains at high levels, exhibiting a remarkable difference from the monotonic change in a single-lens focusing system. As a proof of principle, we examined the influence of the energy deposition on the ignition of a lean methane/air mixture, and found that the use of the telescope can efficiently extend the ignition distance when compared with a single-lens focusing system under the same incident laser energy condition. Our results may help understand the energy deposition behaviors in a variety of telescopic filaments and provide more options to manipulating laser ignition at a desired distance.

Study of underwater plasma arc ignition and cutting process for surface nonconductive steel structures for offshore decommissioning

Plasma cutting is expected to play a significant role as a highly efficient thermal cutting method in offshore decommissioning. Offshore steel structures get covered with nonconductive substances such as rust, mud, and barnacles, which affect the ignition and maintenance of the plasma arcs. This study aimed to determine a suitable method for cutting steel structures covered with nonconductive substances. The entire process of the underwater plasma arc, including ignition, piercing, and cutting, was systematically studied experimentally. A multi-arc root state occurred during the ignition of the transferred plasma arc. In this state, the workpiece current is a significant signal for characterizing the piercing of nonconductive substances on the steel structure. The composite plasma arc exhibits better resistance to interference and a longer ignition distance. In lateral water flow, which was used to simulate ocean currents, the composite plasma arc pierced through 12-mm thick mild steel. Based on the experimental results, a novel cutting strategy was proposed, and a double-layered workpiece consisting of 8-mm thick cement and 12-mm thick mild steel was successfully cut. These findings may drive the application of plasma arc cutting in offshore decommissioning.

A visualization investigation on the characteristic and mechanism of spontaneous ignition condition of high-pressure hydrogen during its sudden release into a tube

Hydrogen is regarded as one of the promising fuels in the next decades. However, the spontaneous ignition of high-pressure hydrogen blocks its safe utilization. In this investigation, a visualization investigation is carried out to study on the characteristic and mechanism of spontaneous ignition condition of high-pressure hydrogen during its sudden release into a tube. A rectangular tube with transparent observation window is employed. The high-speed images of spontaneous ignition conditions and light signals emitted from ignition are obtained. Four spontaneous ignition conditions are observed: (1) no ignition; (2) failed ignition with one ignition spot; (3) failed ignition with two ignition spots and (4) successful ignition. The critical burst pressure for four ignition condition is determined. The jet fire (successful ignition) could be generated in the cases with a shorter distance from ignition spot to leading shock wave and contact surface due to the stronger shock wave affection and sufficient flammable mixture maintaining the chemical reaction and flame propagation. The second ignition is the precondition forming the successful ignition. The scaled ignition delay time and distance could be predicted by using the derived dimensionless shock pressure.

Combustion dynamics of high Mach number scramjet under different inflow thermal nonequilibrium conditions

To characterize the impact of thermal nonequilibrium conditions in shock-induced combustion in high Mach number scramjet engines, a comparative numerical investigation has been performed. The combustor configuration was based on the HyShot II scramjet while the total enthalpy is 3.3 MJ/kg which resembles Mach 8 flight condition. The improved delay detached eddy simulation with Park two-temperature approach in combination with the vibration-chemical coupling model was employed in the nonequilibrium simulations. It is shown that under thermal nonequilibrium inflow conditions, noticeable stratification in the thermal state exists in the circumferential direction both in the combustor section and exhaust nozzle. In the inner core where the fuel mixes with the mainstream, flow deceleration caused by the heat release leads to a sufficient transformation of energy between the vibrational and translational-rotational modes approaching the thermal equilibrium state. However, the τv,mix is quite large in the supersonic mainstream, whereby the thermal nonequilibrium dominates. For the case with higher Tv in the inflow, the fuel jet's penetration height is lower while the local viscosity is higher which results in poor mixing between the fuel and air stream. Nevertheless, a 400 K increase in inflow Tv gives a higher effective temperature, which promotes the dissociation reactions, shortens the ignition distance and yields an 8% increase in the overall combustion efficiency. With better combustion performance, an additional 6% deceleration in the supersonic mainstream and the energy exchange rate between vibrational and translational-rotational modes becomes almost four times faster at the combustor outlet, which promotes the restoration of the thermal equilibrium state. Furthermore, under higher inflow vibrational temperature, the shock-induced combustion in supersonic crossflow becomes less stable with noticeable fluctuations in combustion heat release and local static pressure.

Increased Plasma Ignition Distance Practice may Prevent the Obturator Reflex Occurrences and Compare of its Effectiveness Versus Obturator Block: A Prospective, Randomized, Controlled Study

ObjectiveTo compare the ability of the obturator nerve block (ONB) and increased plasma ignition distance practice (IPDP) techniques to inhibit obturator nerve reflex (ONR) occurring with bipolar transurethral resection of the bladder. MethodsSixty patients who had a tumor placed at the lateral sidewall or had a tumor in another part of the bladder along with the lateral wall were randomly enrolled. Cystoscopic and ultrasonographic examinations and a computerized tomography scanning of the urinary bladder were used to determine the ONB side. Group 1 consisted of patients who had the ONB procedure. Group 2 consisted of patients who had IPIDP. The severity of the ONR was classified as severe, mild, and very mild. The study's primary endpoint was ONR occurrences and successful completion of the surgery. The secondary endpoints were bleeding and bladder perforation. ResultsThere was a significant difference in the occurrence of ONR between the two groups (P = 0.0011). However, there was no significant difference between the two groups in the ability to resect the tumor and complete the surgery (P = .764). There was no correlation between the ONR and the tumor size (P = 0.478). ConclusionOur study concluded that both ONB and IPIDP have comparable results, especially in resecting tumors and completing the operation. IPIDP has some advantages over ONB, such as shorter operative time, lower total costs, and less trained personnel requirements.

Combustion stability and NOX emission characteristics of a 300 MWe tangentially fired boiler under ultra-low loads with deep-air staging

To accomplish the goals of carbon peaking and carbon neutrality in China, coal-fired units are required to implement real-time responses to the requirements of intermittent renewable energy sources. Thus, they typically operate under ultra-low loads, resulting in unstable combustion and high NOX emissions. In this study, the combustion stability and NOX emission characteristics of a 300 MWe tangentially fired boiler operated under ultra-low loads of 90 MWe and 60 MWe and with two pulverized coal (PC)-staging strategies at 90 MWe were investigated by performing experiments and numerical simulations. The results indicate that as the boiler load decreases to 60 MWe, the ignition of the PC flow is delayed and the imaginary circle gradually disappears. At 90 MWe, the shorter ignition distance, lower ignition heat, and higher furnace fullness degree during three-mill operation compared to those during two-mill operation suggest that the combustion is more stable with wider PC staging. As the load decreases from 90 MWe to 60 MWe during two-mill operation, the ignition distance and flame boundary temperature change negligibly and the ignition heat decreases, indicating stable combustion at 60 MWe in the case of easily ignitable coal. The upper burner exhibits greater combustion stability than the lower burner under ultra-low loads. Furthermore, the local mean stoichiometric ratio (LMSR) can be employed to predict corrected NOX concentrations at stable ultra-low loads. The vast majority of NOX is fuel NOX, and the average corrected NOX concentration at 90 MWe and 60 MWe is 795 mg/m3 (O2 = 6%), which indicates that the deep-air-staging effect of the low-NOX combustion system is weakened, particularly when the LMSR is higher than 0.92.

Influence of the parallel oil‐secondary air and F‐layer secondary air distribution on the flow, combustion, and NOx generation characteristics of FW down‐fired boilers retrofitted with a stable combustion technology

AbstractTo further reduce the NOx emissions of the down‐fired boiler while improving its low‐load stable combustion capacity, a new low NOx combustion organization mode that utilizes the high velocity parallel oil‐secondary air to improve the pulverized coal combustion process was proposed based on a novel stable combustion technology. We carried out cold airflow experiments under different oil‐secondary air and F‐layer secondary air distributions (mass flow rate ratios of 1:9, 1:3, 2:3) based on a 300 MWe Foster Wheeler down‐fired boiler to study its influence on the flow and mixing characteristics of the coal/airflow. Results show that the mixing between the oil‐secondary air and the fuel‐rich coal/airflow is gradually delayed with the increase of the secondary air distribution. When the secondary air distribution reaches more than 2:3, the oil‐secondary air starts to become the dominant airflow that influences the whole flow field in the furnace. Based on the result of the cold airflow experiment, we carried out industrial measurements under different parallel oil‐secondary air and F‐layer secondary air distributions (4:6, 5:5, 6:4) on a 600 MWe FW down‐fired boiler under the condition of using the high velocity oil‐secondary air. The ignition distance of the fuel‐rich coal/airflow, the flue gas temperature, the O2, CO, and NOx concentrations in the furnace were measured. At the secondary air distribution of 4:6, the ignition distance of the fuel‐rich coal/airflow was about 1.31 m. When the secondary air distribution was increased to 5:5 and 6:4, the ignition was put off, but the pulverized coal/airflow could still catch on fire in time. With the increase of the parallel oil‐secondary air proportion, the flame center in the furnace did not move downward substantially. In three cases, the lowest O2 concentrations in the inspection port area under the burner were all below 1% (peak CO concentration values were all above 28,000 ppm). The pulverized coal all burned in a strong reducing atmosphere. At secondary air distributions of 4:6, 5:5, and 6:4, NOx emissions of the boiler were, respectively, 825.83, 819.48, and 853.86 mg/Nm3 (O2 = 6%). The thermal efficiency was, respectively, 90.09%, 89.16%, and 90.98%. In actual operation, the recommended secondary air distribution is 6:4.

Effect of central secondary air on flow and combustion characteristics of 600-MWe down-fired boiler: From laboratory to industrial site

The existing central secondary air swirl pulverized coal burner and separated secondary air interval arrangement down-fired boiler combustion technology used for the combustion of blended bituminous coal encounter several problems such as burner nozzle burnout, limited proportion of bituminous coal blending under high load, etc. In this study, the effects of central secondary air on the flow, pulverized coal ignition, burnout, and NOx emissions characteristics of a 600-MWe down-fired boiler with the central secondary air combustion technology were studied by the combination of cold-modeling experiments and industrial-scale experiments. Moreover, the reasons for the burnout of the burner nozzle and limited blending proportion of bituminous coal were analyzed and revealed. Cold-modeling experiments showed that with the increase of the central secondary air ratio, the peak value of the dimensionless vertical velocity increases; however, the flow field asymmetry occurs in the furnace when the air ratio increases to 6%. Considering the symmetry of the flow field and the characteristics of the downdraft, the central secondary air ratio is advised to be less than 3.52%. The industrial-scale experiments of burning bituminous coal under 360-MWe load show that the ignition distance of coal/air flow is about 0.10 m for all the cases at 30, 50, and 75% of the central secondary air damper opening, respectively, which reveal that the serious burner nozzle burnout is caused by the premature ignition of the coal/air flow. Under three types of central secondary air damper openings, the NOx emissions at furnace outlet are 333, 329, and 355 mg m−3 @ 6% O2, respectively. By considering the flow, combustion, and temperature field characteristics in the furnace, reduction in the central secondary air supply air volume can delay the ignition of the coal/air flow, which provides a new idea and method to achieve a high proportion of bituminous coal blending at rated load.