Down-fired Boiler Research Articles

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.

A predictive model with time-varying delays employing channel equalization convolutional neural network for NOx emissions in flexible power generation

This study aims to predict NOx (i.e. NO + NO2) emissions at the selective catalytic reduction inlet from coal-fired boilers. It develops a lightweight predictive model that can be adapted to the generation system dominated by renewable energy with fluctuating power. The random forest algorithm was first used to select the input variables from the auxiliary variables. The maximum information coefficient algorithm was then used to estimate the time delay between the input variables and the NOx emissions. Afterwards, to overcome the “channel collapse” issue in convolutional neural networks (CNNs), a Channel Equalization block (CE-Block) was proposed to activate the suppressed channels, and a lightweight network, referred to as CE-CNN, was designed by fusing a CNN with the CE-Block. Finally, a dynamic NOx emissions prediction model was developed employing the CE-CNN, and the model was evaluated using real data from a 600 MW down-fired boiler. The results showed that the dimensionality reduction policy decreased the model training time by 21.74 %, and the analysis of the time delay decreased the RMSE by 7.31 %. Compared to the baseline model 3-layer CNN, the proposed model achieves performance improvements of 20.97 %, 22.11 %, and 3.48 % in terms of RMSE, MAE, and R2, respectively.

Effects of the addition of arch-supplied secondary air on the performance of the Foster Wheeler down-fired boiler: Air/particle flow, combustion and NOx emissions

This study proposes a low NOx combustion method for Foster Wheeler type down-fired boilers. The effectiveness of this technique was validated through laboratory-scale gas-particle two-phase experiments and industrial trials. The flow field characteristics within FW-type boiler furnaces equipped with arch-supplied secondary air injection nozzles were investigated. Furthermore, the application of this technique to a 660 MW subcritical FW-type boiler was examined, focusing on NOx emissions and combustion characteristics. Results demonstrate that optimizing the damper openings for F-layer and arch-supplied secondary air can significantly reduce NOx emissions by over 33 %. Additionally, increasing the mass flow rate of arch-supplied secondary air aids in reducing unburned carbon in fly ash and inducing a downward shift in the flame center within the furnace.

Reducing Persistently High NO x Emissions and the Risk of Hopper’s Thermal-Fatigue in a 600-MWe low-NO x Down-Fired Boiler Furnace with a Staged Arch-Firing Framework: Effect of the Primary-Burner Location

ABSTRACT To address high NO x emissions and the hopper’s overheating environment of a 600-MWe low-NO x down-fired boiler furnace, a staged arch-firing framework (SAF) was implemented. This included an enhanced low-NO x framework, a decreased primary-burner coal/air flux, and a hopper-air arrangement which were regulated for (i) repeated deepening air staging and (ii) decreasing coal/air penetration. To evaluate the primary-burner location effect and confirm the SAF’s viability in resolving these problems, detailed real-furnace measurements and simulations were performed within the original furnace and subsequently within the SAF furnace at primary-burner locations of C b = 0.30, 0.35, 0.40, and 0.45 (C b signifying the dimensionless burner location in the upper-arch depth direction). Increasing C b first improved but then decreased combustion symmetry and the overall low-NO x performance. At C b = 0.40, optimal NO x levels of about 660 mg/m3 at 6% O2 were achieved, along with low carbon levels of about 5% in the fly ash. A comparison before and after the SAF suggests that the SAF effectively shortened the flame penetration and raised both combustion intensity and air utilization for coke combustion downstream. As a result, NO x emissions were reduced by approximately 30% (without affecting burnout). Furthermore, hopper-temperature levels were lowered by about 400 K. These results confirm the viability of the SAF.

The effects of key parameters on the gas/particle flows characteristics in the furnace of a Foster Wheeler down-fired boiler retrofitted with novel low-load stable combustion technology

Through a gas-particle cold-modeling experiment, the particle dynamic analyzer (PDA) is used to investigate the effects of fuel-lean coal/airflow angle and ratio of oil secondary air to F-layer secondary air on the gas/particle flows characteristics in the furnace of a Foster Wheeler (FW) down-fired boiler with enhanced stable combustion technology. As the fuel-lean coal/airflow angle increases from 10° to 45°, the peak value of the vertical dimensionless velocity of the gas/particle flows gradually increases, and the peak position gradually moves to the near wall region. As the ratio of oil secondary air to F-layer secondary air (the secondary air ratio for short) increased, the peak value of the vertical dimensionless velocity of the gas/particle flows gradually increased and the peak value position of the dimensionless velocity gradually moved towards the near-wall region. In actual operation, for the perspective of performance and safety, the recommended fuel-lean coal/airflow angle and secondary air ratio is 30°and 1:2 respectively. The enhanced stable combustion technology had been applied to two 600 MW FW down-fired boilers, and these boilers can achieve stable operation when burning a wide range of coal quality under the load section from 200 MW to 600 MW.

Retrofit of a 600 MW Down-Fired Pulverized-Coal Furnace for Low NOx Emission

Aiming at solving the problem of high NOx emissions of a down-fired boiler, a new combustion system has been proposed by means of the numerical simulation using Ansys Fluent. The coal-lean stream (tertiary air), which was originally mixed with a separated overfired air (SOFA) stream on the front and rear walls of the upper furnace, was relocated to the lower zone of the furnace after retrofit. The secondary-air slots were transformed into a new annular port type, which was injected into the furnace with a down-tilt angle to increase the residence time of the coal stream. Furthermore, the effect of secondary air distribution and velocity of coal stream on performance was studied. After retrofitting the combustion system, the NO emissions were effectively controlled, decreasing from 906 mg Nm−3 to 576 mg Nm−3, but the carbon content of fly ash increased from 2.46% to 5.78%. Aiming at decreasing the carbon content of fly ash, the effect of coal/primary air velocity on the arch was studied. Less carbon content in fly ash can be observed for the lower arch airflow velocity. The results showed that the NO emissions can be controlled below 595 mg Nm−3, and the carbon content of fly ash was reduced to 3.39% when the velocity was decreased to 18 m s−1.

Numerical Optimization on Char Conversion and NOx Emission under Various Operating Conditions in a Retrofit Biomass Boiler.

Retrofitting retirement or existing fossil boiler with biomass is an important method of curbing electricity shortage and lowering the cost of modern power plants. However, the use of biomass combustion is hampered by operational problems, such as the resulting high unburned carbon, amount of bottom ash, and nitrogen oxide (NOx) release. In this study, we investigated the burning of pulverized biomass in a retrofitting boiler power plant using computational fluid dynamics of commercial software fluent ANSYS to determine the optimal combustion conditions. The objective of this study was to investigate a 125 MWe pulverized biomass boiler that was retrofitted from an anthracite down-fired boiler. The air distribution, including the influence of the secondary air ratio and the location of the burner standby, was evaluated. Key factors such as biomass ash mass at the hopper, char conversion, and high zone temperature relating to NOx formation/reduction were calculated. The adjustment of the secondary air ratio from 30 to 50% of the total air and the mass ash at the hopper significantly decreased to a low value at 247 kg/h and a high value of char conversion at 97.33% in case R (SA40%). The standard deviation temperature was 240 K at the BNR B-A level for case R, which was significantly lower than in other cases. This implies that the best mixing of air and biomass occurs in case R at 40%. Comparative analysis of the burner standby conditions showed that the NOx emission was similar at the boiler outlet (approximately 94-116 ppm). Burner A on standby, with a secondary air ratio of 40%, was used as the optimal case with the highest value of char conversion at 98.43%, the lowest bottom ash release of 204 kg/h, and a low-NOx emission of 106 ppm.

NOx emission prediction using a lightweight convolutional neural network for cleaner production in a down-fired boiler

The selective catalytic reduction (SCR) denitrification efficiency of coal-fired boilers is used to assess the clean production of power plants. Accurate predictions of NOx emissions at the SCR inlet can help improve the denitrification efficiency. However, the generation and distribution of NOx in a furnace involves complex combustion, thermodynamics, and fluid mechanics processes. The physical-based method for NOx emission models is complex and requires expensive computational resources. To address these issues, deep learning was employed to develop a prediction approach based on the random forest (RF) algorithm and lightweight convolutional neural network (CNN). To avoid curse of dimensionality, the random forest algorithm was utilized to select the importance of the candidate variables. The selected variables were in line with the boiler NOx generation and combustion control trends. The lightweight CNN was developed to build the model that predicts NOx emissions. The key was that a Cross-Channel Communication (C3)-Block was introduced into the lightweight CNN to resolve the issue in which there was no communication between feature maps in the same convolutional layer. This model was named C3–CNN. The predictive effectiveness of the C3–CNN-based model was assessed on the real historical dataset of a 600 MW down-fired boiler. The experimental results indicated that the proposed method reduced the model depth of the CNN (saved 3 convolutional layers, 1.5 times parameters, and 1 times FLOPs than 6-layer CNN) while also ensuring the model prediction accuracy (RMSE = 13.52704 ± 0.21036 mg/m3, MAE = 9.89313 ± 0.18288 mg/m3, R2 = 0.93337 ± 0.00222), which was suitable for the online optimization of industrial pollutant control and may assist cleaner production.

Experimental study on the influence of fuel-rich and fuel-lean coal/airflow ratio on aerodynamic characteristics of a 300MWe foster wheeler down-fired boiler

Abstract To address the problem of late ignition and poor burnout of Foster-Wheeler down-fired boilers, this study proposed a new combustion technology, which combines adding on arch secondary air and alternately arranging the fuel-lean coal/airflow (FLCA) and fuel-rich coal/airflow (FRCA). The coal particle trajectories of FLCA and FRCA and overall aerodynamic characteristics in the furnace were studied at FRCA and FLCA ratio (R RL) of 5:5, 7:3, and 9:1. The results showed that even at a low FRCA ratio, the new combustion technology could still effectively ensure the penetration depth of pulverised coal/airflow. At different R RL values, the main airflow from the arch appeared in the near-wall area and could penetrate downwards into the cold ash hopper area. With an increase in the value of R RL, the deflection degree of FLCA towards the furnace wall increased, while that of FRCA decreased. At R RL of 7:3, both the penetration depth and deflection degree of FRCA and FLCA were at a high level, which was conductive to promoting the burnout and recommended in practical applications.

Influence of the Parallel C-Layer Secondary Air on Flow, Combustion and Nox Generation Characteristics of a 600mwe FW Down-Fired Boiler Retrofitted with a Stable Combustion Organization Mode

ABSTRACT To improve the low-load stable combustion capacity of a low-grade coal-fired boiler, this study proposed to reform C-layer secondary air, which was then used to improve the ignition and combustion stability of coal/airflow of a 600 MWe Foster Wheeler down-fired boiler. Cold airflow experiments and industrial measurements were carried out to investigate the influence of the new C-layer secondary air on the aerodynamic and combustion characteristics at low load under different C-layer secondary air ratios and damper openings, respectively. The flow and mixing characteristics of the C-layer secondary air and coal/airflow, the flue gas temperature and NOx, O2, and CO concentration distributions in the furnace at low load were examined. With increase in the C-layer secondary air ratio, the flow range of coal/airflow gradually expanded. The mixing distance first increased and then decreased. At an air ratio of 25%, the mixing distance was the longest. Industrial measurements showed that compared with that at the C-layer secondary air damper opening of 0%, the coal/airflow ignited earlier at 40% (corresponding to the C-layer secondary air ratio of 23.73%), and the flue gas temperature in the furnace increased substantially. At C-layer secondary air damper opening of 0%, 20%, and 40%, carbon content in the fly ash was 4.62%, 5.32%, and 4.20%, respectively. The respective NOx emissions at the furnace exits were 450 mg/Nm3, 378 mg/Nm3, and 427 mg/Nm3, (O2 = 6%). In actual operation, 40% is recommended as the optimal C-layer secondary air damper opening.

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.

Experimental investigation on controlling of airflow trajectories and flow-field of down-fired boiler by adding on arch secondary air

Abstract To effectively control the flow and combustion of pulverised coal in down-fired boilers and improve their thermal efficiency while ensuring low NO x emissions, on arch secondary air (OASA) combustion technology and a smoke-tracing analysis were proposed. The aerodynamic characteristics of a 300-MWe Foster Wheeler (FW) down-fired boiler with the OASA ratios (R OASA) of 0%, 10%, 20%, and 30%, the flow trajectories and mixing characteristics of OASA, and fuel-rich coal/air flow (FRA) and fuel-lean coal/air flow (FLA) were studied experimentally. For the R OASA of <10%, FRA and FLA flow to furnace centre soon after they are injected into the furnace. OASA and FLA may mix with FRA before ignition. For the R OASA of 20% or higher, FRA and FLA flow to the near-wall area soon after being injected into the furnace. FRA mixes with OASA after ignition. With the increase in R OASA, the mixing distance between OASA and FRA increases first and then decreases. The penetration depth of coal/air flow and furnace airflow fullness increases. For the R OASA of 30%, the dimensionless penetration depth of coal/air flow can exceed 0.7. Furnace dimensionless airflow fullness at all the dimensionless heights can exceed 0.6. In real operation, the recommended R OASA is between 20 and 30%.

Industrial measurement of combustion and NOx formation characteristics on a low-grade coal-fired 600MWe FW down-fired boiler retrofitted with novel low-load stable combustion technology

The randomness and intermittence of solar and wind power generation require coal-fired thermal power boilers that have a low-load stable combustion capacity. This study proposed a novel low-load stable combustion technology in which the oil secondary air flow is arranged parallel to the fuel-rich coal/air flow and participates in pulverised coal combustion during boiler operation. The method was applied to a 600 MWe down-fired boiler manufactured with the technology of Foster Wheeler Corp. Full-scale industrial measurements of furnace flue gas temperature, fuel-rich coal/air flow ignition distance, and distribution characteristics of NOx, O2, and CO in the furnace at loads of 200, 240, and 540 MWe were performed. The results showed that the boiler can stably operate without oil support at 240 MWe (40% rated load) and 200 MWe (33% rated load) respectively burning a blended coal of anthracite and lean coal, and blending bituminous coal. At loads of 200, 240, and 540 MWe, the fuel-rich coal/air flow ignition distances were, respectively, approximately 1.4, 1.7, and 1.3 m. NOx emissions at the furnace exit were 406, 407, and 730 mg/Nm3 (O2 = 6%), and the boiler thermal efficiencies were 92.42%, 92.45%, and 90.58%, respectively. This novel technology presents the characteristics of high efficiency and low NOx emission while realizing low-load stable combustion of down-fired boilers.

Evaluation of wide-range coal combustion performance of a novel down-fired combustion technology based on gas–solid two-phase flow characteristics

In order to achieve peaking carbon dioxide emissions and carbon neutrality, a variety of down-fired combustion technologies has been proposed. The existing central air supplied combustion technology (CASCT) used for burning blended bituminous coal faces some serious problems. In order to solve the problem of the wide-range coal combustion in down-fired boiler, this study proposes a novel primary air biased combustion technology (PABCT). The coal gas–solid two-phase flow characteristics and numerical simulation of combustion characteristics under the PABCT and CASCT were compared. The results indicated that compared with the CASCT, under the PABCT, the velocity attenuation in the vertical direction is slower and the airflow diffusion range is smaller below the arch. The attenuation rate of particle volume flux with the downward flow of pulverized coal is slow, and the space utilization rate of the lower furnace is high under the PABCT. The thermal numerical simulation confirms the conclusion obtained from the cold gas–solid two-phase experiment. The PABCT exhibits the characteristics of large undershoot depth of coal/air flow, low temperature in the burner nozzle area, high burnout, low NOx emissions, and slag prevention, thus making it more suitable for wide-range coal combustion from the perspective of furnace flow and combustion.

Effects of the air-staging degree on performances of a supercritical down-fired boiler at low loads: Air/particle flow, combustion, water wall temperature, energy conversion and NOx emissions

Low load operation of down-fired boilers has become the norm such that the comprehensive performances of such boilers at low loads need to be investigated to increase the energy conversion efficiency and reduce carbon and NOx emissions. In this work, cold-model air/particle flow experiments and industrial-scale trials were performed to study the effects of the air-staging degree (ASD) on the air/particle flow, combustion, water wall temperature, energy conversion and NOx emission characteristics of a 350 MW supercritical down-fired boiler at low loads. The experimental conditions are four different combinations of damper openings associated with the secondary-air, tertiary-air and OFA (i.e. 60%/40%/15%, 50%/50%/15%, 35%/70%/15% and 30%/55%/55%), and are referred to as ASD = I, II, III and IV, respectively. The results showed that, as the ASD was increased from I to IV, the injection effect of the secondary-air on the fuel-rich flow was weakened. In addition, the downward diffusion of the air/particle flow toward the furnace center increased, while the particle concentration near water wall decreased and the velocity decay was accelerated. Decreasing the ASD shortened the ignition distance of the fuel-rich flow, reduced the carbon in fly ash and slag and improved the boiler efficiency. At an ASD setting of I, a higher thermal load and increased temperature near the water wall were obtained in the lower furnace. These conditions gave the highest superheated and reheated steam temperatures of 568.1 and 570.7 °C, respectively. As the ASD was increased from II to III, the cooling effect of the tertiary-air reduced the maximum water wall temperature. The reduction, in turn, increased the regulating margin of superheating degree, which raised the steam temperature. Increasing the ASD from I to IV increased the specific coal consumption from 337.36 to 348.12 g·(kW·h)−1, indicating that the energy conversion efficiency of the unit was decreased. Additionally, the NOx emissions at the furnace exit were reduced from 770 to 528 mg/m3 at 6% O2. On the basis of the present results, an ASD setting of II is recommended to ensure the safe and economical operation of such units while minimizing environmental impact.

Effects of the gas/particle flow and combustion characteristics on water-wall temperature and energy conversion in a supercritical down-fired boiler at different secondary-air distributions

To alleviate the over-temperature of water-wall and increase the energy conversion efficiency of supercritical down-fired boilers, cold-model air/particle flow experiments and in-situ trials were performed to investigate the effects of the secondary-air distribution wall-side deflection coefficient (ws) on air/particle flow and combustion characteristics. As ws increased from 0 to 0.33, the amount of air/particle flow diffused toward water-wall and the corresponding particle concentration both decreased, while the velocity decay was accelerated. At a ws of 0.16, due to prior ignition of fuel-rich flow and a longer flame stroke in lower furnace, the carbon in fly ash and slag were lowest. As ws increased, the thermal load was gradually focused at the furnace center, and the maximum value and deviation of water-wall temperature in lower furnace decreased. Consequently, the temperatures of superheated and reheated steam respectively increased from 542.7 °C to 543.9 °C–560.2 °C and 560.1 °C, respectively. Furthermore, increasing ws from 0 to 0.28 decreased the specific coal consumption from 346.29 to 341.09 g (kW h)−1, indicating the energy conversion efficiency was improved. However, adjusting ws had only a minimal effect on NOx emissions. A ws of 0.28 is recommended to optimize the water-wall temperature, pulverized-coal burnout, and economic performance.

Industrial-scale Investigations on Combustion Characteristics and NO x Emissions of a 300-MWe Down-fired Boiler: Bituminous Coal Combustion and Coal Varieties Comparison

ABSTRACT Number of down-fired boilers burning bituminous coal or anthracite blended with bituminous coal has exceeded 10% of the total down-fired boilers in China. However, lacking of the industrial-scale research on the performance of down-fired boiler burning bituminous coal or blended coal, which limits the actual operation adjustment. Industrial-scale experiments on a 300-MWe down-fired boiler fueled with bituminous coal and blended coal were performed, which boiler was originally designed to burn low-volatile coal. Without the over-fire air, air-staging combustion can still be realized when the down-fired boiler burning bituminous coal. The combustion performance in the down-fired boiler under different coal varieties was compared. Compared with the anthracite case, flue gas temperature is higher, coal flame fullness is greater, O2 consumption occurs faster and NO x concentration is lower in the primary combustion zone when burning bituminous coal. Strong reducing atmosphere is formed in the primary combustion zone. The NO x emissions are low at a level of 410 mg/m3 at 6% O2 at furnace exit and boiler thermal efficiency reaches 92.590% when burning bituminous coal. The NO x emissions cannot be further reduced by burning blended coal. In addition, the boiler thermal efficiency reduced by 1.108%. Without the over-fire air, compared with burning anthracite or lean coal, the NO x emissions and carbon in fly ash significant reduction by 70% and 90%, respectively, and the boiler thermal efficiency increased when burning bituminous coal.

Combustion stability, burnout and NOx emissions of the 300-MW down-fired boiler with bituminous coal: Load variation and low-load comparison with anthracite

The new energy generating capacity is increasing year by year in the electric industry structure, but it is greatly influenced by the natural conditions. Coal-fired power generation unit boilers connected to the grid have the ability of flexible peak-load regulation and long-term low-load operation is required. However, the combustion stability is weak under the low-load when the coal-fired boiler burning anthracite. Down-fired boiler fueled with bituminous coal gradually attract the attention. Ignition and combustion stability, burnout, and NOx emissions are experimentally studied under the various loads on a 300 MW down-fired boiler fueled with bituminous coal, and the results compared with anthracite under the low-load. The ignition distance of the coal/air flow burning bituminous coal is gradually advanced with the reduction of load. And under the corresponding load, the ignition heat of the coal/air flow when burning bituminous coal is more than 37% lower than that burning anthracite. With the reduction of load, the decreasing rate of the overall furnace temperature and the cross-sectional heat load in the primary combustion zone decreases, and the furnace temperature in the primary combustion zone is high under the low-load when bituminous coal is burned. Short ignition distance, low ignition heat and high furnace temperature ensures the strong combustion stability of burning bituminous coal under the low-load. Moreover, under three various loads with burning bituminous coal, the carbon in the fly ash are all less than 1.1%, and the NOx emissions at the furnace exit are 410, 397 and 288 mg/m3 @ 6% O2, respectively.

Effects of the fuel-lean coal/air flow damper opening on combustion, energy conversion and emissions in a supercritical down-fired boiler

In-situ trials were carried out to investigate the effects of the fuel-lean coal/air flow damper opening (FLD) on the pulverized-coal combustion and energy conversion efficiency in a 350-MW supercritical down-fired boiler. This boiler was operated with inferior coal, having a high ash content and low calorific value. The influence of combustion in furnace on steam temperature and coal consumption of a supercritical down-fired boiler were discussed innovatively. The experimental results indicate that, as the FLD was decreased from 100% to 25%, heating of the fuel-rich coal/air flow accelerated such that the overall furnace temperature increased and the thermal load gradually focused at the furnace center area. As a result, the maximum temperature of the water-wall in the lower furnace was reduced and the superheating degree rose significantly. Thus the superheated and reheated steam temperatures increased from 550.4 and 545.8 °C to 558.2 and 555.0 °C. Reducing the FLD also lowered the carbon in the fly ash and slag from 8.17% and 7.70% to 4.51% and 3.43%, respectively. Furthermore, the increase of boiler efficiency and the decrease of specific heat consumption both contributed to the reduction of the specific coal consumption, from 354.45 to 340.43 g·(kW·h)−1, thus the energy conversion efficiency of the unit was improved. However, adjustment of the FLD had only a minimal effect on NOx emissions, which remained in the range of 654–672 mg/m3 at 6% O2. Based on optimizing coal combustion characteristic and economic performance of the unit, an optimal FLD of 25% was recommended.

Industrial Measurement of Combustion and NOx Formation Characteristics on a Low-Grade Coal-Fired 600MWe FW Down-Fired Boiler Retrofitted with Novel Low-Load Stable Combustion Technology

Industrial Measurement of Combustion and NOx Formation Characteristics on a Low-Grade Coal-Fired 600MWe FW Down-Fired Boiler Retrofitted with Novel Low-Load Stable Combustion Technology