Separated Overfire Air Research Articles

CFD modeling and industry application of a self-preheating pulverized coal burner of high coal concentration and enhanced combustion stability under ultra-low load

Deep participation in peak regulation is a fundamental issue in flexible peak regulation. The ability to operate coal-fired power units under ultra-low load plays a crucial role in China’s pursuit of its energy goals. This study focuses on the performance analysis of a self-preheating pulverized coal burner with high coal concentration under ultra-low load conditions through simulations and experiments. A numerical comparison between the performance of a traditional swirl burner and a self-preheating burner is conducted using a 5 MW combustion test furnace. The results demonstrate that the self-preheating burner exhibits superior combustion stability and can maintain stable combustion even at an ultra-low load of 15 %. Additionally, the air distribution of the self-preheating burner under ultra-low load conditions is investigated numerically. The results reveal that under 15 % load, primary air rate of 12.0 % and internal secondary air rate of 54.2 % enable the self-preheating burner to achieve stable combustion performance with a char burnout rate of 98.6 %. Furthermore, in the absence of separated over fire air (SOFA) conditions, the NOx emission level ranges from 150 to 165 mg/m3. Based on numerical results, the self-preheating burner is implemented in an industrial setting, specifically a 29 MW coal-fired industrial boiler demonstration project. The results indicate that the self-preheating burner can achieve stable combustion at approximately 13 % load, with an average combustion efficiency of 93.24 %. The research validates the excellent performance of the designed self-preheating burner under ultra-low load conditions, which contributes to addressing the challenge of deep participation in peak regulation.

V-shaped effect of primary and secondary airflow momentum ratio on performance of a 660 MW tangentially fired lignite boiler: Identification and verification

To explore how the primary and secondary air ratios affect the overall performance of lignite-fired power generation boilers, a 660 MW wall-tangentially fired lignite boiler was chosen as research subject. 17 combustion scenarios were designed and then numerically simulated through a validated model, and the in-furnace coal combustion behavior, heat transfer process, and NOx conversion characteristics under various conditions were analyzed in detail. Results show that, except for the monotonous increase in NOx emission, the overall boiler performance follows a V-shaped trend with the continuous increase in primary air ratio (PAR). Initially, as PAR is slightly increased, the in-furnace combustion temperature, heat transfer intensity and coal utilization rate notably decrease, followed by a subsequent increase in these parameters as PAR is significantly increased. The separated over fire air (SOFA) ratio doesn’t alter this V-shaped pattern; however, a higher SOFA ratio does reduce the critical PAR at which the overall boiler performance is most compromised. This is attributed to changes in the momentum ratio between primary and secondary airflow caused by varying PAR. Boiler performance deteriorate significantly when their momentum ratios are nearly equal, and a higher SOFA ratio makes it easier. These observations are confirmed under different thermal load conditions, where the close proximity of primary and secondary air momentum is avoided by reducing SOFA ratio. Based on these findings, a substantial increase in PAR should be avoided in practice, if necessary, lowering the SOFA ratio helps mitigate significant deterioration in boiler performance.

Coupled simulation on flue gas and steam deviation of final reheater in a 600 MW tangentially fired boiler with reversed separated overfire air under different loads

Flue gas and steam deviation of the final reheater is an inevitable problem in the tangentially fired boiler due to the remaining gas spinning at the furnace exit. An in-house one-dimensional process simulation code coupled with comprehensive three-dimensional combustion simulation was adopted to accurate investigate the characteristics of the flue gas and steam maldistribution of the final reheater in a 600 MW supercritical tangentially fired boiler. Firstly, the combustion simulation result was validated by the in-house fire-side heat transfer calculation data, then the effect of boiler loads and reversed separated overfire air (SOFA) deflection angles on the gas temperature deviation, velocity deviation and outlet steam temperature of reheater were conducted. The results showed that the deviation of flue gas temperature and velocity increases as the boiler load decreases. The reversed SOFA deflection angle plays a crucial role in improving the gas distribution under low loads, and the deviation of flue gas temperature and velocity is the smallest when the angle is -18? under 100% boiler maximum continuous rating (BMCR), 75% and 50% turbine heat acceptance (THA), but it is the smallest when the angle is -9? under 35% BMCR load. Moreover, the maximum outlet steam temperature difference between parallel heating surfaces of the final reheater increases from 84.68 K to 106.48 K as the boiler load decreases from 100% BMCR to 75% THA, and it is mainly affected by the flue gas temperature deviation rather than the mass flow rate maldistribution when the deflection angle changes from -9? to -18?.

Numerical research on ammonia-coal co-firing in a 1050 MW coal-fired utility boiler under ultra-low load: Effects of ammonia ratio and air staging condition

Co-firing ammonia (NH3) with coal for thermal power generation is a promising strategy for achieving global carbon neutrality. However, the drawbacks of NH3 combustion (e.g., narrow flammability limits, low theoretical flame temperature, high NOX emissions, etc.) can negatively impact the performance of coal-fired boilers, particularly for large-scale ones operating under ultra-low loads. As such, this study was conducted to investigate the ultra-low-load combustion characteristics of a 1050 MW coal-fired boiler when employing NH3-coal co-firing technology. Three-dimensional numerical simulations of ten combustion scenarios were performed to examine the effects of the NH3 co-firing ratio (ranging from 0% to 80%, by calorie) and air staging condition (Separated Over Fire Air (SOFA) ratio ranging from 4% to 24%) on temperature, gas species distributions, and emissions. Results show that as the NH3 co-firing ratio increases to 80%, the in-furnace average gas temperature decreases by 100 ∼ 200 K. The oxidation of NH3 takes precedence over the reduction of NO with NH3, leading to a surge in NO emissions· NH3 slip can be prevented at ultra-low loads due to the rapid consumption of NH3 near coal burners. Meanwhile, CO2 emissions reduce to 20% of that observed in pure coal firing. The increased H2O content in flue gas promotes coal burnout through the water–gas reaction, while the high H2O emissions may potentially cause low-temperature corrosion. At a SOFA ratio of 16%, a relatively large high-temperature region is achieved in the furnace, indicating more stable combustion under ultra-low loads. Excessively small or large SOFA ratios can reduce combustion intensity in the primary combustion zone while contributing to lower NO emissions.

A novel NOx emission prediction model for multimodal operational utility boilers considering local features and prior knowledge

This study proposed a data-driven NOx modelling framework that could capture the multimodal operational characteristics of a utility boiler, improve the training sample quality and strengthen the model interpretability. With this framework, to obtain a high-quality steady-state dataset from operational data, a multivariate F-test algorithm, which enhanced robustness by introducing the minimal number of stable units (MNSU) and the variational mode decomposition (VMD) outlier detection method, was first proposed. Based on the obtained sample, a Gaussian mixture model (GMM) was established to classify the data by their operational modes. In addition, the least absolute shrinkage and selection operator (LASSO) algorithm was employed to select specific features for each mode. Finally, the NOx prediction model was constructed using the light gradient boosting machine (LightGBM) model integrated with prior knowledge. The mechanistic relationships between the selected features (i.e., the oxygen content and separated overfire air damper opening) and target variable (i.e., NOx) were considered as the constraint conditions. By taking a 660 MW utility boiler as the research object, the proposed modelling framework obtained R2, RMSE and MAE values of 0.979, 3.573 mg/m3, and 2.586 mg/m3, respectively, performing better than five comparison models.

Numerical optimization of combustion and NOX emission in a retrofitted 600MWe tangentially-fired boiler using lignite

In order to meet the ultralow emission standard of NOx, the combustion system of a 600 MWe tangentially-fired lignite boiler was retrofitted using air-staged combustion technology. The retrofit scheme is described in detail. Combustion and NOx emission characteristics of the original and retrofitted boiler were numerically studied and compared. Optimization simulations for retrofitted boiler were carried out under different over-fired air ratios. The moisture content in lignite was specially considered in simulations by simplified coal compositions method. The results show that the ignition of lignite particles is delayed due to the high moisture content. After retrofit, the temperature level in furnace rises owing to the reduction of total excess air coefficient in furnace, and the high temperature region moves upward due to the introduction of separated over-fire air. The NOx emission is reduced by approximately 40% through retrofitting, which proves that the low-NOx retrofit for the lignite-fired boiler is successful. With over-fired air ratio increasing from 15% to 25%, the NOx emission for retrofitted design decreases from 394 mg/Nm3 to 317 mg/Nm3 (at 6% O2). OFA ratio of 20% is the most economical case, and OFA ratio of 25% is the most environmentally friendly case. Taking NOx emission and boiler thermal efficiency into account, the practical operating condition with OFA ratio of 20% is optimal for retrofitted boiler. The results can be beneficial for the retrofit and practical operation of similar lignite-fired boilers.

Numerical investigation on H2S formation characteristics in air-staging combustion of a tangentially coal-fired boiler

ABSTRACT Existing H2S generation models are mainly applied in laboratory-scale models and thoughtless about the fuel sulfur release characteristics in fuel-rich zone. So, a novel H2S generation model is proposed for describing the H2S formation characteristics of real-scale boilers in this work. It consists of two parts: fuel sulfur release and chemical reactions. The ratio of the gasification rate to the consumption rate for each char particle is calculated in coal combustion model through the user-defined function (UDF) approach, which is delivered to the H2S generation model for determining the proportions of char sulfur converted to COS and SO2. The numerical results show that the proportions of char sulfur converted to H2S are about 0.545 over the excess air coefficient range from 0.6 to 1.2. The strongly reducing atmosphere could just accelerate the H2S formation to a certain extent in the devolatilization stage. For a 600 MW tangentially coal-fired boiler, the H2S concentration over 400 mg∙kg−1 appears mainly in the zone below burner A and the zone between burner E and the separated over-fire air nozzle. This study is intended to guide for predicting possible corrosion zone and delivering targeted solutions for inhibiting metal corrosion.

Effects of Ammonia Solution and Pyrolysis Gas on NOx Emission from a 75 t/h Pulverized Coal Boiler

To explore methods of reducing NOx emission from pulverized coal boilers, the effects of injecting ammonia solution and pyrolysis gas into the furnace on NOx emission were experimentally investigated on a 75 t/h pulverized coal boiler. Results show that the deep air staging with 30% separated over fire air (SOFA) creates a high temperature and strong reducing atmosphere in the reducing zone, providing the prerequisites for NOx reduction by ammonia solution and pyrolysis gas. Compared with deep air staging itself, NOx emission can be reduced by 16.7% when ammonia solution is injected from the reducing zone with a normalized stoichiometric ratio of 2.0. However, NOx reduction efficiency is largely affected by its injection position. Similarly, NOx emission is decreased by 28.2% through injecting pyrolysis gas with its calorific value of 10% into the furnace, while a further increase of pyrolysis gas input will not increase NOx reduction efficiency. When ammonia solution and pyrolysis gas are simultaneously injected into the furnace under deep air staging conditions, the overall NOx reduction efficiency reaches 92.0% and NOx emission is decreased to 39.1 mg/m3. Considering the increasingly strict NOx emission standard, these findings can provide theoretical and practical guides to the future NOx reduction in pulverized coal boilers.

Methane Gas Cofiring Effects on Combustion and NO x Emission in 550 MW Tangentially Fired Pulverized-Coal Boiler.

A shift from coal to liquefied natural gas for electricity generation can mitigate CO2 emissions and respond to the intermittent and variable characteristics of renewable energy. With this objective, numerical simulation was performed in this study to determine the optimal position of the methane injector and evaluate the achievable reduction in NOx emissions before applying methane cofiring to an existing 550 MW tangentially fired pulverized-coal boiler (Boryeong Unit 3). The combustion and NOx reduction in the furnace were intensively analyzed based on the methane cofiring rate (up to 40%). The optimal position of the methane injector was found to be inside the oil port based on the spatial distribution of NOx and the stoichiometric ratio along the furnace height. The NOx reduction rate was logarithmically proportional to the methane cofiring rate, and compared to the base case, a 69.8% reduction was achieved at the 40% cofiring rate. In addition, the fraction of unburned char at the boiler outlet was equivalent to that of the existing boiler as the increase in the flow rates of the close-coupled and separated overfire air improved fuel and air mixing. Simultaneously, methane cofiring led to a reduction in the total fuel loss and CO emissions. Finally, this study showed that the recommended optimum cofiring rate was 20% based on the furnace exit gas temperature. Under the 20% methane cofiring condition, the boiler achieved a 57.3% reduction in NOx emissions and a 7.4% improvement in fuel loss.

Numerical optimization of separated overfire air distribution for air staged combustion in a 1000 MW coal-fired boiler considering the corrosion hazard to water walls

Multigroup separated overfire air (SOFA) has been widely applied in large-scale coal-fired boilers for air staged combustion to reduce NOX emissions. However, unreasonable flow distribution in multigroup SOFA nozzles would cause boiler performance degradation, including decreased coal burnout rate, severe gas temperature inhomogeneity, and risks of high-temperature corrosion. In this study, six SOFA distribution modes including the reference case based on the field test were numerically investigated for operation optimization of a 1000 MW double-tangential circular coal-fired boiler. The combustion characteristics including flow field, temperature distribution, residual char particles, and gas species were comprehensively analyzed. Meanwhile, particular attention was paid to the reducing atmosphere near water walls of the upper furnace for ameliorating the problem of high-temperature corrosion. The numerical results show that SOFA distribution significantly determines the gas flow field in the burnout zone, which subsequently influences coal combustion behavior and boiler performance. The pagoda mode is suggested to be optimum for sufficient penetration of SOFA in the upper furnace, which features gradually decreasing air induced from SOFA nozzles along with the furnace height. Compared with the baseline scenario, the burnout rate is increased by about 0.2% due to the enhanced mixing of SOFA and combustibles, and the NOX emission is slightly decreased by about 8 mg/Nm3 in the pagoda mode. Besides, better temperature homogeneity is obtained at the furnace exit, which helps improve the heat transfer performance of downstream heat exchangers. Moreover, the reducing atmosphere near the water walls of the upper furnace is significantly weakened in this mode, contributing to the alleviation of the corrosion hazard to water walls.

Insights into the causes and controlling strategies of gas temperature deviation in a 660 MW tangentially fired tower-type boiler

• Experiment & simulation on thermal deviation are performed in a tower-type boiler. • Four upper furnace structures are studied to fully reveal the deviation causes. • Properly SOFA horizontal angle of 34–45° is recommended. • SOFA velocity offset with 1.2 of left/right ratio is recommended. • Offsetting SOFA velocity is successfully applied without adverse effects. Gas temperature deviation is a common problem for tangentially fired boilers, affecting the boiler operation safety and the boiler generation efficiency. The gas temperature deviation in the tangentially fired π-type boilers has been well studied. However, the gas temperature deviation in the tangentially fired tower-type boilers has yet to be fully understood. This paper develops insights into the causes and controlling strategies of gas temperature deviation in a 660 MW tangentially fired tower-type boiler with obvious steam temperature deviation by experiment and simulation. Firstly, four upper furnace structures are established to fully reveal the causes of gas temperature deviation. The gas temperature deviation is mainly caused by the flue gas rotation, the heating tubes division, and the suction effect of the induced draft fan. More evident recirculation is observed near the right side than the left side in the platen heater region, resulting in that the flue gas deflects to the left side, and the left side region presents higher temperature. Furthermore, the controlling strategies of gas temperature deviation are studied comprehensively by adjusting SOFA (Separated Over-fire Air) horizontal angle and setting SOFA left–right-side velocity offset. Properly SOFA horizontal angle or enhancing the left-side SOFA velocity may effectively reduce the gas temperature deviation. 34–45° for the SOFA horizontal angle and 1.2 for the SOFA left–right-side velocity ratio are recommended. In the actual operation, offsetting the SOFA left–right-side velocity ( i. e. SOFA air damper opening degree: left side—80% and right side—50%) is successfully applied to reduce the re-heated steam temperature deviation from 18 K to 1 K without bringing adverse effects under the low load. These deep insights may provide useful information to control the steam temperature deviation in the platen heater zone.

Optimization of operating conditions to achieve combustion stability and reduce NOx emission at half-load for a 550-MW tangentially fired pulverized coal boiler

The off-peak period of the grid load (i.e., deep peak load) prevents a power generation boiler from operating at full load. To achieve stable combustion of 550-MW tangentially fired pulverized coal boiler and ultra-low pollutant emissions (nitrogen oxides and unburned carbon) under half-load conditions, different operating parameters have been analyzed and optimized. In this study, under half-load conditions, the numerical method was used to simulate the flow field characteristics, combustion stability, and pollutant emissions of the boiler under various operating conditions. The operating conditions included various burner group arrangements, close-coupled overfire air (CCOFA)/separated overfire air (SOFA) distributions, and excess air ratios. The simulation prediction results showed that the middle burner group (BCDE) arrangement has a good flow field distribution. Compared with the upper burner group (CDEF) arrangement, this reduces NOX by approximately 62 ppm and also maintains a higher pulverized coal burnout rate than the lower burner group (ABCD) arrangement. Considering the stability of the combustion and lowest emissions, the ratio of CCOFA of 5% and SOFA of 15% were preferred as the operating conditions for the air-staging distribution. The high excess air caused an increase in NOX while the combustion temperature significantly reduced in the furnace, made the furnace exit gas temperature (FEGT) too low, and affected the steam temperature. In addition, the simulation results of the optimized scheme were in good agreement with the field test results.

Combined effects of yaw and tilt angles of separated overfire air on the combustion characteristics in a 1,000 MW coal-fired boiler: A numerical study

Separated overfire air (SOFA) is typically employed in coal-fired boilers with air staged technology to sustain lower NOX emission, and SOFA nozzle angles are crucial adjustment parameters. In this work, the combined effects of SOFA yaw and tilt angles on combustion characteristics were numerically investigated for a 1,000 MW dual circle tangentially coal-fired boiler. Numerical results show that the forward increase of the SOFA yaw angle from 0° brings about the enlarged SOFA tangential circle and the gradual appearance of bimodal high-temperature zones at furnace exit. With further tuning SOFA tilt angles vertically, the bimodal high-temperature zones would separate to the two halves of the furnace, inducing a more severe deviation of gas temperature. Besides, the gas residual rotating momentum is strengthened as SOFA yaw angles forward increase, resulting in the enhancement of traction effect in the upper furnace as well as the rise of flame. However, the gas velocity deviation is somewhat eliminated as SOFA rotates reversely. No matter that the SOFA yaw angle increases forward or reversely, the coal burnout would deteriorate with the overly enlarged SOFA tangential circles. Tuning SOFA yaw and tilt angles, respectively, at 5° and 0° can simultaneously guarantee lower CO and NOX emissions.

Study on NO emissions during the coupling process of preheating-combustion of pulverized coal with multi-air staging

Controlling the NOx formation during the combustion process is the most direct and efficient way to minimize NOx emissions, so low NOx combustion technology is essential. In this work, in order to develop multi-air staging pulverized coal (PC) preheating-combustion technology and further optimize the preheating-combustion burner, a 35 kW PC preheating-combustion test system was built, and three types of burners with different outer secondary air (OSA) injection structures were designed. Using Huangling bituminous coal (HL coal) and Wuhai bituminous coal (WH coal), the coupling process of PC preheating-combustion with multi-air staging on NOx emissions was studied systematically. The results showed that the depth furnace air staging could further reduce NOx emissions. When close-coupled over-fire air (CCOFA) and separated over-fire air (SOFA) were applied simultaneously, a significant NOx reduction of 70.2% could be reached 217 mg m−3. Compared with the NOx emissions, the unburned carbon in fly ash presented opposite tendency. Compared to burner II (high-speed swirl jet) and III (low-speed axial jet), burner I (high-speed axial jet) showed the lowest NOx emissions under the multi-air staging preheating-combustion conditions. Furthermore, the depth furnace air staging had greater denitrification potential for high-quality bituminous coal.

Numerical investigation of 700 °C boiler flue gas thermal deviation based on orthogonal experiment

In the present work, computational fluid dynamic (CFD) models have been established to investigate a newly presented flue gas temperature deviation solution on the basis of ANSYS FLUENT16.0 considering a 700 °C tangentially fired pulverized-coal boiler, which confronts with severe flue gas temperature deviation. Orthogonal test method was used to complete the optimization parameters with deflected secondary air swing angle (A), separated over-fire air reverse tangent angle (B), burner upward swing angle (C); and the thermal deviation at the outlet of the furnace as the performance index. The orthogonal test table L9(34) with three levels for each factor was designed. Using the range analysis method and the weight-based matrix analysis method respectively, the significance degree of the influence of different factors and levels on the deviation of the flue gas temperature at the furnace outlet was discussed, the optimal factor and level combination was determined, and the corresponding influence weights were given. It is found (1) Residual swirling flow in the upper furnace causes the flue gas velocity and temperature deviations in crossover pass. (2) The influence degree of various factors on the deviation of the flue gas at the furnace outlet is as follows: separated over-fire air reverse tangent angle > deflected secondary air swing angle > burner upward swing angle, the influence weights of different factors are A = 0.2184, B = 0.5962, C = 0.1560, and the best factor and level combination is A1B3C2. (3) In this test, when the deflection secondary air angle is 0°, separated over-fire air reverse tangent angle is 25°, and the burner upward swing angle is 11°, the flue gas temperature deviation of the furnace outlet section is the minimum, which is 29.297 K, compared with the maximum, the decrease is up to 86.22 K.

Thermal deviation analysis of high-temperature reheater for single-tangential π type boiler

• The thermal deviation adjusting experiments of a 660 MW ultra-supercritical boiler were carried out. • The grid distribution of radiation intensity for particles at the high-temperature reheater area was measured by radiation spectroscopy. • Thermal deviation analysis of high-temperature reheater were presented. • A new type principle of reforming combustion system was proposed. For some π type boilers with four-corner tangential and deep air staged low nitrogen combustion technology, under full load condition, the steam temperature deviation for both sides of reheater outlet can reach as high as 15 °C. The left and right cross arrangement of steam between two stage reheaters increases the steam temperature deviation at the outlet, and the large amount input of desuperheating water makes the economic efficiency of the unit decrease. The thermal deviation adjusting experiments of a 660 MW ultra-supercritical boiler were carried out by adjusting the level and up-and-down swing angle of the separated over-fire air nozzle, and the grid distribution of radiation intensity for particles at the high-temperature reheater area was measured by radiation spectroscopy. The results show that there exists strong visible radiation in this area. Under the 600 MW-load condition, the radiation heat transfer of particle accounts for 25.95% of the total heat absorption of the high-temperature superheater and particle combustion still exists, whose combustion share accounts for about 1.83% of the received carbon content of coal. The particle temperature and emittance data fitted from the Planck's law indicate that the high-temperature particles are enriched at single side due to the inertial effect of residual rotation of flue gas. The residual rotation momentum of the separated over-fire air is insufficient to balance the temperature rise of both sides of the low-temperature reheater. Therefore, a new type principle of reforming combustion system was proposed, which changes a five-layer Separated Over-fire Air (SOFA) nozzle to two-section of six-layer SOFA nozzle to mitigate the combustion delay of pulverized coal particles and sets the wall-type tangent circle arrangement of the upper separated over-fire air nozzle to improve the residual rotation momentum of flue gas.

Optimization of separated overfire air to reduce NOX emissions under combustion stability for the retrofit of a 500 MW tangentially pulverized coal boiler

Retrofitting of an aging 500 MW tangential coal-fired boiler with high pollutant emissions in South Korea was investigated to achieve low nitrogen oxide (NOX) emissions and high combustion efficiency. This study evaluated and analyzed the combustion and emission characteristics of five arrangements to optimize the position and direction of a separated overfire air (SOFA) installation using a computational fluid dynamics (CFD) simulation. The results show that the air injection of SOFA at the center of the boiler wall causes substantial obstruction of the rising airflow below, causing unstable combustion behavior in the boiler, and increasing the amount of unburned carbon (UBC). However, combining the wall injection with corner SOFA helps reduce the formation of NOX and has the effect of promoting the burnout of UBC. For the direction of the wall injection under combined conditions, a clockwise airflow helps to reduce the formation of NOX with increasing UBC. Otherwise, the counter-clockwise airflow weakens the flow rate of the ascending airflow, increasing the residence time of unburned particles, increasing the coal burnout rate, and increasing the furnace exit gas temperature (FEGT). Among the five SOFA arrangements studies, this study recommends Case 4, which has a clockwise wall SOFA (30%) with corner SOFA (70%). This case has the best performance in terms of combustibility, NOX emission, UBC, and FEGT. The proposed condition was verified through a field test, and the results indicate that NOX emissions decreased from 169 ppm to 57 ppm, the UBC content decreased from 3.64% to 0.7%, and the boiler efficiency increased by 2%.

Investigations on combustion optimization and NOX reduction of a 600-MWe down-fired boiler: Influence of rearrangement of tertiary air and jet angle of secondary air and separated over-fire air

Low-NOX combustion is the main technology for reducing NOX emission in coal-fired plants. In this work, numerical simulation was conducted on a 600-MWe down-fired lean coal boiler to study the effects of important factors on NOX reduction, namely, the arrangements of tertiary air (TA), the jet angle of secondary air (SA), the separated over-fire air (SOFA) ratio, and the SOFA jet angle. The optimal arrangement was obtained with the original TA moved down; the jet angle of SA (A layer) at 20° (downdip); the SA (F layer) and the TA at 20° and 30° (downdip), respectively; and the SOFA set in the upper furnace, with a 20% ratio and a 30° jet angle. The NOX emissions were reduced from 1527 mg/m3 to 773 mg/m3 in the optimized system. Meanwhile, the exhaust gas temperature decreased from 1387 K to 1369 K, and the unburned carbon content increased slightly by 0.24%, with the overall boiler efficiency kept nearly constant. The in situ measurement results are consistent with the simulation predictions.

Numerical simulation of the heat transfer and NOX emissions in a 660 MW tangentially fired pulverised-coal supercritical boiler

Numerical simulation of the influence of operating conditions of the furnace on its performance characteristics like heat flux distribution, Furnace Exit Gas Temperature (FEGT) and NOx emission levels was performed for a tangentially fired pulverised coal boiler. The results revealed that the FEGT and NOx emission are slightly higher when the top seven coal nozzles are in operation compared to the bottom seven coal nozzles. While the increase in burner tilt led to an increased FEGT, the least NOx emission was obtained for the burner tilt with 0°. The NOx emission started to increase with both increase and decrease in tilt from 0°. In addition, the FEGT and NOx emission were directly and inversely proportional to the percentage of SOFA (Separated OverFire Air) opening respectively. NOx emission was lower by around 7.5% when the air staging was done with the top-most SOFA nozzle instead of the bottom-most SOFA nozzle. However, the FEGT was higher in the former case.

Effect of separated over-fire air angle on combustion and NOx emissions in a down-fired utility boiler with a novel combustion system

A novel combustion system, consisting of moving fuel-lean nozzles from the arches to the front/rear walls, rearranging staged air, and introducing separated over-fire air (SOFA), has been proposed and successfully applied in a 600 MWe Foster Wheeler down-fired boilers to reduce high NOx emissions while limiting the carbon content in the fly ash. This study comprehensively evaluates the effect of SOFA angles on flow, combustion characteristics and NOx emissions for the novel combustion system by numerical simulation and in-situ experiment. The numerical simulation shows in acceptable consistence with the in-situ experiment. The simulated results show that significant NOx reduction is obtained for all five SOFA angles compared to the original combustion system. O2 and CO concentrations at the furnace outlet, as well as the carbon content in the fly ash decrease significantly with increasing SOFA angle from 0° to 40°. NOx emissions increase slightly and the average temperature at the re-heater entrance changes slightly with the increase of SOFA angle from 0° to 30°. However, with further increasing SOFA angle from 30° to 40°, NOx emissions increase substantially due to the smaller reducing region below the SOFA and the average temperature at the re-heater entrance falls significantly. Taking combustion, NOx and steam parameters into account, a SOFA angle of 30° is advisable. The actual industrial measurements also show that the steam can achieve the designed values when the SOFA angle is set at 30° instead of 40°. Significant NOx reduction (over 50 %) and good performance are attained after adopting the novel combustion technology of 30 °SOFA angle.