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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. [ABSTRACT FROM AUTHOR]

A 46 MW pulverized coal industrial boiler with swirl burner had severe slagging and high NOx emission during actual operation. To solve these problems, a full-scale industrial test was carried out. The flue gas temperature and the composition of exhaust gas were obtained under different ratios of secondary and separated secondary air. The results shown that reducing the ratio of secondary and separated secondary air could delay the ignition of pulverized coal and weaken the flame deflection, which was conducive to slowing down the burner slagging problem. However, it will aggravate the erosion of high temperature flue gas on the back wall, and further aggravated the furnace slagging problem. Changing the air ratio couldn't substantially improve the problem of high exhaust temperature. Properly reducing the ratio of secondary and separated secondary air could effectively control NOx emission. When the ratio of secondary and separated secondary air was reduced from 73:27 to 52:48, the NOx emission was reduced by 44.8%. Reducing the ratio of secondary and separated secondary air leaded to the increase of carbon content in fly ash and slag, but the heat loss of exhaust gas was reduced and the thermal efficiency of boiler was increased. [ABSTRACT FROM AUTHOR]

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 (RRL) 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 RRL 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 RRL, the deflection degree of FLCA towards the furnace wall increased, while that of FRCA decreased. At RRL 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. [ABSTRACT FROM AUTHOR]

To effectively control the flow and combustion of pulverised coal in down-fired boilers and improve their thermal efficiency while ensuring low NOx 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 (ROASA) 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 ROASA 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 ROASA 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 ROASA, 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 ROASA 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 ROASA is between 20 and 30%. [ABSTRACT FROM AUTHOR]

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 NOx concentration is lower in the primary combustion zone when burning bituminous coal. Strong reducing atmosphere is formed in the primary combustion zone. The NOx 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 NOx 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 NOx emissions and carbon in fly ash significant reduction by 70% and 90%, respectively, and the boiler thermal efficiency increased when burning bituminous coal. [ABSTRACT FROM AUTHOR]

To 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. [ABSTRACT FROM AUTHOR]

Numerical simulations were conducted to study the effects of the pulverized-coal bias distribution in the primary air on the coal combustion and NOx generation characteristics of a 600-MW down-fired boiler with multiple-injection and multiple-staging combustion technology. The total pulverized-coal in the primary air was kept constant, and the ratio of the pulverized-coal mass flux in the fuel-rich coal/air flow to the total pulverized-coal mass flux (RPR) was set as 60%, 70%, 80%, and 90%. By changing the RPR, the excess air coefficient of the fuel-rich flow was adjusted from 0.700 to 0.467. It was found that numerical simulation results were almost in agreement with cold modeling and in situ experimental results respectively, including the flow fields in the lower furnace at the RPR of 80% and the heating processes for the fuel-rich coal/air flow at the RPR of 90%, which verified the rationality of the numerical model and the grid. The simulation results indicated that the change of RPR has little effect on the symmetry of the flow field in the furnace. With the increase of the RPR from 60 to 90%: (1) the maximum airflow declination angle near the tertiary air slot decreased from 71 to 66°, which indicates that the downward airflow penetration depth gradually decreased; (2) the ignition distance of the fuel-rich coal/air flow decreased from 1.2 to 0.9 m, and the high-temperature area in the furnace hopper decreased and the position gradually moved away from the hopper water walls; (3) the oxygen consumption rate at the initial combustion stage constantly accelerated, and the fuel NOx generation rate under the fuel-rich flow nozzle increased first and then decreased; (4) the NOx emissions at the furnace exit dropped from 778 to 662 mg/m3 at 6% O2, and the carbon in the fly ash decreased from 5.87 to 5.52%. Increasing the RPR reasonably controlled the excess air coefficient of the fuel-rich flow, and realized the high-efficiency combustion in the furnace and the reduction of NOx emissions simultaneously. [ABSTRACT FROM AUTHOR]

• An understanding of the radial air staging method for PC utilization is improved and the key parameter (OFA distribution) is optimized. • Compared to the traditional axial air staging method, for the radial air staging method, the OFA distribution has a greater effect on the heat release from PC particles. • The increased R IO is not conducive to the rapid devolatilization from PC particles, resulting in poorer combustion stability of the boiler. • The increased R IO promotes the oxidation reaction of N released from PC particles, which is detrimental to boiler NO x emission reduction. • The advanced mixture of the OFA with the main airflow facilitates the burnout of the PC particles as the R IO increases. Radial air staging is an innovative method for reducing NO x emissions from coal-fired boilers compared to conventional axial air staging. However, an important issue in using this method is to set the over-fired air (OFA) distribution rationally to achieve good boiler performance. This paper aims to optimize the OFA distribution by evaluating important gas-particle (GP) parameters that influence the pulverized coal (PC) combustion performance. The GP flow experiment is conducted in a laboratory furnace model of a coal-fired boiler with different OFA distributions. The experimental results show that as R IO (Two-level OFA distribution ratio) changes from 0:1 to 1:0, the mixture of the OFA jet and airflow in the main combustion zone is advanced, which is conducive to PC burnout. Meanwhile, the size of the central recirculation zone decreases, resulting in poor PC ignition. It is worth noting that at R IO of 1:0, the central recirculation zone disappears at section y/d = 2.5, indicating that there is no high-temperature flue gas downstream to heat the fresh PC particles, which is extremely unfavorable for the stable combustion of the PC. In addition, at R IO of 0:1, compared to other cases, there are more particles in the central recirculation zone, which promotes the reduction reaction of N element released from PC particles, thus reducing NO x emission. According to the comprehensive evaluation, it is recommended that the R IO of the two-level OFA be set to 1:1 when using radial air staging in a coal-fired boiler. The results provide new insights into the application of the air staging method for efficient and clean PC utilization. [ABSTRACT FROM AUTHOR]

• A novel low-NO x technology is proposed for a down-fired boiler with swirl burners. • Gas/particle flow characteristics are studied under two combustion technologies. • Downward depth of gas/particle flows improves for the improved boiler. • Primary air is effectively ejected by burner secondary air for the novel technology. • Turbulent intensity in the recirculation region increases for the improved boiler. To confirm the validity and progressiveness of the eccentric-swirl-secondary-air combustion technology (ESSACT) in improving the economic and environmental performance of the down-fired boiler (DFB) with swirl burners, the gas/particle (GP) cold-modelling experiments are conducted by using a particle dynamic analyzer (PDA) measurement system in a 1:10 scale model of 300-MW e DFB applying original Babcock & Wilcox combustion technology (BWCT) and novel ESSACT, respectively. The distributions of average velocity, fluctuation velocity and particle size are compared and analyzed under the two combustion technologies in this study, in addition to the decay of particle volume flux and the jet trajectory of downward GP flows. In comparison with the BWCT, the rotation intensity of the burner secondary air is greatly weakened, and the vertical velocities of downward GP flows decay more slowly under the ESSACT. Although the recirculation regions are both formed below the arches under the two combustion technologies, the horizontal velocities and the horizontal fluctuation velocities of GP flows in the recirculation regions are both larger for the ESSACT, which benefits pulverized coal ignition. The particle size distributions under the two combustion technologies reveal that the mixing between the primary air and burner secondary air is obviously delayed and the burner secondary air effectively ejecting the primary air downward is realized by the ESSACT, which is advantageous to improve pulverized coal burnout and inhibit NO x formation. In addition, the maximum particle volume flux in the lower part of the lower furnace under the ESSACT is obviously higher than that under the BWCT, and the downward depth of GP flows and the space utilization ratio in the lower furnace increase for the original BW DFB retrofitted by the ESSACT. [ABSTRACT FROM AUTHOR]

A 300-MW e anthracite and down-fired boiler equipped with swirl burners, which was retrofitted with the previously proposed deep-air-staging combustion technology to reduce particularly high NO x emissions, still suffered from high carbon content in fly ash (despite this content being slightly lower than that before retrofit) on basis of significant NO x reduction. To comprehensively produce a high-burnout and low-NO x setting, a novel combustion technology was proposed in which relative to the axis of the primary air duct, the axes of the inner and outer secondary air ducts of the swirl burner shift away from the furnace center, and this technology is called an eccentric-swirl-secondary-air combustion technology. The coaxial symmetrical arrangements of primary and secondary air ducts for traditional swirl burners are broken. Full-scale industrial measurements under original, deep-air-staging and eccentric-swirl-secondary-air combustion technologies with respect to different loads (i.e., 180, 250, and 300 MW e ) were carried out to compare and analyze the validity of and improvement offered by the eccentric-swirl-secondary-air combustion technology. Under different boiler loads, especially at low and middle loads, compared with original and deep-air-staging combustion technologies, eccentric-swirl-secondary-air combustion technology markedly extends the penetration depth of coal/air flow and recirculation regions below arches. The thermal resistance between primary coal/air flow and high-temperature flue gas at the furnace center is reduced further, and the ignition of pulverized coal is timelier. The heating gradient in the burner outlet zone is maintained throughout, and the flame fullness in the primary combustion zone increases. The above three aspects contribute to pulverized coal burnout. Similar to deep-air-staging combustion technology, a low-oxygen and strong reducing atmosphere was formed in the primary combustion zone to reduce NO x formation due to the introduction of overfire air. Compared with deep-air-staging combustion technology, for the eccentric-swirl-secondary-air combustion technology, the air staged combustion in the furnace is improved further, and pulverized coal combustion in the primary combustion zone is more well distributed, which is beneficial to further reducing NO x emissions. The results of low-NO x and high-efficiency performance show that compared with the original boiler, NO x emissions and carbon content in fly ash for the boiler with the eccentric-swirl-secondary-air combustion technology are significantly reduced by above 42% and 32.5 to 20.7% at loads of 180, 250 and 300 MW e , respectively. [ABSTRACT FROM AUTHOR]

In consideration of increasing the tertiary air damper opening of a 600 MWe down-fired boiler with prior multiple-injection multiple-staging technology facilitated the coal burnout, while largely increasing the NO x emissions. Additionally, the flame kernel was greatly moved downwards, thus causing significant temperature variations in the hopper near-wall region and the water wall in the lower furnace was vulnerable to overheating. This work concentrated on the comprehensively improved multiple-injection multiple-staging technology, both 1:20 scale cold aerodynamic tests and industrial experiments were conducted to examine the effects of tertiary air damper opening on flow, combustion, NO x emissions and especially the hopper near-wall temperatures. The aerodynamic tests indicated that, on increasing the tertiary air damper opening from 40 to 70%, all the flow fields exhibit good symmetry. The tertiary air flows downwards along the hopper near-wall region, with a maximum near-wall dimensionless vertical velocity significantly increasing from 0.48 to 0.66, and accordingly, the dimensionless depth of downward airflow increases from 0.744 to 0.846. The industrial experimental results showed that, upon introducing more tertiary air, the ignition distance of fuel-rich coal/air flow shortens from 1.25 to 0.87 m. The coal burnout is enhanced, carbon in fly ash drops from 6.90 to 6.15%, while the NO x emissions slightly increase from 593 to 641 mg/m 3 at 6% O 2 . On reducing the measuring height of hopper near-wall temperature from 9.1 to 3.3 m, the average heating rate increases from 0.44 to 0.63 °C/mm. The increased tertiary air damper opening presents an increasingly obvious cooling effect on the hopper near-wall region, with the temperature reductions around 50 °C, which is conductive to protect the water wall in the lower furnace from overheating. [ABSTRACT FROM AUTHOR]

Effects of adjusting the inner-secondary-air damper opening among 100%, 70%, and 50% on the flow, combustion, and NO x emissions of a 600-MWe down-fired boiler incorporating multiple-injection and multiple-staging technology were investigated. Both 1 : 20-scale aerodynamic tests and industrial experiments were conducted. The small-scale tests showed that symmetric flow patterns formed in all cases, and the velocities of boundary and outer secondary air decayed more quickly than the velocity of inner secondary air. As the inner-secondary-air damper opening decreased from 100% to 50%, the average dimensionless depth of downward airflow reduced from 0.830 to 0.788 while the horizontal velocity in the recirculation zone increased. Industrial experiments demonstrated that, as the inner-secondary-air damper opening decreased from 100% to 50%, the heating rate of fuel-rich coal/air flow increased from 572 to 687°C/m, the corresponding ignition distance shortened from 1.36 to 1.11 m, and the hopper near-wall temperature decreased by approximately 50°C. Carbon in fly ash decreased from 7.31% to 6.93%, and NO x emissions rose from 343 to 371 ppm at 6% O2. The boiler efficiency improved from 89.78% to 89.92%. An inner-secondary-air damper opening of 70% is more reasonable than 50% and 100% considering the ignition distance, carbon in fly ash, and NO x emissions. Copyright © 2017 Curtin University of Technology and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Semi-coke is the solid pyrolysis product of low-rank coal, with high carbon content and difficult to ignite and burn out. The blending of semi-coke with bituminous coal can realize a large amount of utilization of semi-coke. In this paper, the combustion, thermal conversion, and kinetic analyses of semi-coke blending with bituminous coal are investigated under oxygen-enriched conditions (30% O 2 atmosphere). Compared with 20% O 2 atmosphere, the ignition and burnout temperatures are reduced by 10, 7, 8, 12, 7 °C and 129, 76, 85, 115, and 152 °C for the semi-coke blending ratios of 0, 30, 50, 70 and 100% under 30% O 2 atmosphere. Oxygen-enriched combustion can effectively decrease the ignition temperature and improve the combustion performance of the mixture. In 30% O 2 and 20% O 2 atmospheres, the synergy effect of the co-combustion process is different. Under the 30% O 2 atmosphere, semi-coke, and bituminous coal have a strong synergistic effect, which can reduce burnout temperature. However, under 20% O 2 atmosphere, the burnout temperature of the mixture will increase. At a higher blending ratio of semi-coke, the effect of oxygen-enriched conditions on co-combustion is greater, and high oxygen concentration will make the interaction of the mixture combustion process more intense. Compared with bituminous coal, the activation energy of the mixture gradually increases with the increase of the blending ratio of semi-coke, and different combustion stages correspond to different mechanism functions, respectively. The research results can provide ideas for the utilization of semi-coke under oxygen-enriched conditions. [Display omitted] • The activation energy of mixture combustion is reduced in 30% O 2 atmosphere. • The co-combustion synergistic effect of mixture is enhanced in 30% O 2 atmosphere. • The increase of semi-coke blending ratio will result in higher activation energy of mixture. • The influence of oxygen-enriched co-combustion is greater under higher semi-coke blending ratio. [ABSTRACT FROM AUTHOR]

CGFA recycling process using NaOH-HCl hydrothermal treatment. [Display omitted] • Successful harmless treatment and resource utilization of residual carbon in CGFA. • The physicochemical evolution of residual carbon improves its combustion behavior. • Hydrothermal treatment improves the residual carbon's combustion characteristics. • DA-N and DA-NC properties are similar to lean coal and anthracite, respectively. Coal gasification fine ash (CGFA) is difficult to handle, which makes its effective utilization difficult. This study modified a typical CGFA sample, referred to as dry ash (DA), by using a NaOH-HCl hydrothermal process and analyzed the physicochemical properties of the carbon residue after treatment. The evolution of the physicochemical properties and the effect of this evolution on the combustion characteristics of the sample were investigated in detail. It was determined that the loss on ignition (LOI) significantly increased after DA was subjected to NaOH-HCl hydrothermal treatment. The carbon skeleton of the residual carbon was also modified, with a higher degree of looseness and a lower degree of cross-linking. At the same time, the specific surface area of the carbon particles and the content of surface C C bonds increased. This was beneficial for improving the combustion reaction activity of the treated sample. It is worth noting that compared with DA, DA-NC (samples obtained after DA was hydrothermally treated with NaOH) exhibited an LOI increase of 27.98 %, an ash removal rate of 48.32 %, and an S BET as high as 354.09 m2/g. Proximate analysis and thermogravimetric analysis further showed that NaOH-HCl hydrothermal treatment effectively increased the residual carbon content of the sample and reduced the activation energy required for combustion. The comparable combustion characteristics of DA-N and DA-NC (samples obtained after DA was hydrothermally treated with NaOH-HCl) with coal demonstrated their value as fuels. This work demonstrates that DA can be successfully modified into high calorific value solid fuel by NaOH-HCl hydrothermal treatment, which is beneficial for realizing the large-scale resource processing of CGFA. Moreover, the evolution of the physical and chemical properties of the residual carbon during the treatment process leads to a significant improvement in combustion characteristics. [ABSTRACT FROM AUTHOR]

[Display omitted] • The combustion performance of CGFA in two oxygen-riched atmospheres is compared. • The emperical optimization of the CGFA oxygen-rich combustion process is analyzed. • 30%O 2 concentration combustion performance of DA is comparable to that of DA blended with 60% bituminous coal in air atmosphere. Coal gasification fine ash (CGFA) has the characteristics of high carbon content, fine-grained, and sizeable annual output, causing environmental pollution and energy waste. Oxygen-enriched combustion is an effective way to solve the CGFA problem. This paper used thermogravimetric to study the effect of thermal conversion, kinetic analysis, and empirical optimization on the combustion process of CGFA in oxygen-enriched atmosphere. The results show that oxygen-enriched combustion of raw coal and CGFA improve ignition, burnout performance, combustion stability, and reduced activation energy. As the O 2 concentration increased from 30 % to 70 %, the activation energy of CGFA increased, but the improvement in combustion performance was not significant. A two-factor analysis of variance was carried out using a generalized linear model to obtain the effects of oxygen concentration, heating rate, and temperature variations on the combustion properties of CGFA, showing that atmosphere, heating rate and temperature variations all had significant effects on the combustion performance of CGFA, but atmosphere and heating rate had a more substantial impact on the combustion performance of CGFA. The combustion performance of raw coal under 30 %O 2 /70 %Ar atmosphere was better than that of CGFA, and the oxygen-enriched combustion (30 %O 2 /70 %Ar) performance of CGFA was comparable to that of CGFA blended with 60 % bituminous coal in air atmosphere. A kinetic model was developed to describe the oxygen-enriched combustion process (R2 > 0.92). The oxygen-enriched conditions of CGFA allow for efficient and clean combustion. [ABSTRACT FROM AUTHOR]

• Flow and combustion characteristics of down-fired boiler with central secondary air supply are studied. • Central secondary air ratio that is beneficial to the flow field symmetry and penetration depth is given. • Reasons for the burnout of burner nozzle and the blending proportion limitation of bituminous coal are obtained. • Technical improvement ideas for increasing the blending proportion of bituminous coal are proposed. 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 NO x 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 NO x emissions at furnace outlet are 333, 329, and 355 mg m−3 @ 6% O 2 , 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. [ABSTRACT FROM AUTHOR]

To ensure good combustion stability, industrial pulverized coal-fired boilers normally employ a swirl burner with pre-combustion chamber (PCC). Owing to the high cross-sectional heat load of the PCC, there is potential for slagging, which can affect the safe and stable operation of the boiler, but can be avoided or reduced through the selection of suitable operating parameter values. Here, the effects on the slagging characteristics of PCC of varying the ratio of the inner to the outer secondary air mass flow rates in a swirl burner, R SA , from 0.00 to 0.83 were investigated using a computational fluid dynamics (CFD) simulation approach. The rates of collision, deposition, and sticking were analyzed. The results showed that as the R SA increases, the total collision rate of the pulverized coal particles in the PCC increases, and when R SA = 0.83, the total collision rate in the PCC is about 7 times of that when R SA = 0.00. Further, slagging tends to appear on the wall near the outlet of the PCC, and the degree of slagging in the PCC first decreases and then increases with increasing R SA. The optimum R SA range was found to be 0.22–0.38, which is conducive to the safe and stable operation of industrial pulverized coal-fired boilers. [Display omitted] • The slagging characteristics in the pre-combustion chamber were studied. • The rates of collision, deposition, and sticking were analyzed. • Slagging tends to appear near the exit area of the pre-combustion chamber. • The degree of slagging first decreases and then increases as R SA increases. [ABSTRACT FROM AUTHOR]

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 NO x emissions, and slag prevention, thus making it more suitable for wide-range coal combustion from the perspective of furnace flow and combustion. • Primary air biased combustion technology is proposed. • Gas–solid flow characteristics of primary air biased and central air supplied technology are compared. • Space utilization of lower furnace under primary air biased technology primary is high. • Separated secondary air under primary air biased technology has a strong ejection effect. • Primary air biased technology is suitable for wide-range coal combustion of down-fired boiler. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to show the influence of the primary air velocity on lean coal combustion in a lateral-ignition tiny-oil burner. Using the lateral ignition scheme, a lean-coal ignition experiment was conducted at a coal feed rate of and primary air velocities of 15, 20, 25, and . During firing up with three oil guns in the presence of coal, the gas temperature along the center line increased along the flow of primary air. The gas temperatures on the side of the oil guns () were higher than those on the opposite side (). As the primary air velocity increased, the gas temperature along the center line at the same measurement point decreased while that on the side with the oil guns rose. However, the gas temperature on the opposite side changed only a little. concentrations at the exit of the burner were 1-3%. The burner resistance resulting from the flow and combustion of pulverized coal was 1,300-1,800 Pa. [ABSTRACT FROM AUTHOR]

Abstract: A phase Doppler anemometer system was used to study gas-particle flow characteristics of a conical rings concentrator for a centrally fuel-rich burner. The influences of distance between adjacent rings on the distribution of mean axial velocity, particle volume flux, particle number concentration, rich/lean air ratio, concentration ratio and resistance coefficient were obtained. In each cross-section, within the radius range from 0 to 35 mm, the particle axial volume flux for four distances was always far larger than at other radial positions. When x/D was larger than 1.5 and L was larger than 48 mm there was only a slight influence of distance on the concentration ratio. The resistance coefficient decreased with increasing distance and the larger the distance was, the more slowly the resistance coefficient decayed. In the five cross-sections from x/D = 0.1 to 1.5, RCR for the four distances were always greater than 2. This indicates that the centrally fuel-rich burner with a conical rings concentrator for four distances could achieve a stable flame. Although the length of the primary air duct is long enough and the primary air fan total head is limited, it is best to choose the larger distance. [Copyright &y& Elsevier]

To overcome the problem of high carbon content in the fly ash of down-fired utility boilers using low-volatility coals, the combustion system of a 660 MWe full-scale down-fired boiler was retrofitted, with the direction of the secondary air under the arch being changed from horizontal to an angle of declination of 20°. Industrial experiments were performed using the boiler before and after the retrofit to determine the reconstruction effect. Data are reported for the gas temperature distribution along the primary air and coal mixture flow, furnace temperature, gas compositions, such as O2, CO, CO2, and NOx, and gas temperatures in the near-wall region. Comparisons between the two cases were made, and the results show that with the angled secondary air under the arches, ignition of the primary air and pulverized coal mixture was brought forward in the boiler. Gas temperatures rose in the fuel-burning zone, and the residence time of pulverized coal in the fuel-burning zone was extended. Thus, the quantity of unburned carbon in fly ash and the gas temperature at the furnace outlet decreased, and the boiler efficiency increased. [ABSTRACT FROM AUTHOR]

Abstract: Due to the limits of reserves and price for the high rank coal, the low rank coal has been employed as fuel for power generation in China and will be eventually employed in the world. To burn low rank coal, centrally fuel-rich swirl coal combustion burner has been studied in Harbin Institute of Technology. This paper reviews and analyzes the major research results. The work has included both experiments and numerical simulation. The experiments were conducted using small-scale single-phase experimental equipment, a gas/particle two-phase test facility and 200- and 300-MWe wall-fired utility boilers. For the burner, the primary air and glass beads partially penetrate the central recirculation zone and are then deflected radially. At the center of the central recirculation zone, there is high particle volume flux and large particle size. For the burners the local mean CO concentrations, gas temperatures and temperature gradient are higher, and the mean concentrations of O2 and NO x in the jet flow direction in the burner region are lower. Moreover, the mean O2 concentration is higher and the gas temperature and mean CO concentration are lower in the side wall region. Centrally fuel-rich burners have been successfully used in 200- and 300-MWe wall-fired pulverized coal utility boilers. [Copyright &y& Elsevier]

A three-component particle-dynamics anemometer is used to measure, in the near-burner region, the characteristics of gas/particle two-phase flows with a centrally fuel rich swirl coal combustion burner and enhanced ignition–dual register burner, on a gas/particle two-phase test facility. Velocities, RMS velocities, particle mean diameters and particle volume flux profiles were obtained. For the centrally fuel rich burner, particles penetrate the central recirculation zone partially, and are then deflected radially. For the enhanced ignition–dual register burner, particles completely penetrate the central recirculation zone. Compared with the enhanced ignition–dual register burner, in the same cross-section, the particle volume flux peak value for the centrally fuel rich burner is much closer to the chamber axis and much larger near the chamber axis. In six cross-sections from x/d =0.3 to 2.5, the particle volume flux in the central recirculation zone for the centrally fuel rich burner is much larger than that for the enhanced ignition–dual register burner. For the centrally fuel rich burner, most of bigger particles are resident in the region near the chamber axis and the residence time is prolonged. The influence of gas/particle flow characteristics on combustion has been analyzed. [Copyright &y& Elsevier]

This study proposes changing the inner and outer secondary air ratios (R SA) under sub-stoichiometric (sub- SR) conditions to optimize the burn-out performance and reduce the NO x emissions of a swirl burner. The ignition and combustion characteristics, NO x formation, and the effect of R SA on the carbon and nitrogen conversion processes under sub- SR (SR = 0.85) were investigated via thermal tests carried out in a 0.5 MWe pulverized coal-fired furnace. The fuel-rich flow in the center of the furnace was further rearward than the fuel-lean flow. The pulverized coal burnt rapidly after entering the furnace. The average percentage of unburned carbon was less than 15% in the x/d = 3.5 (x is the axial direction distance, and d is the diameter of the burner) section away from the burner. The N to NO conversion at the beginning of combustion (x/d < 3.5) was effectively suppressed in the swirl burner under sub- SR conditions. The NO reduction reaction almost completely stopped after x/d > 11.5. R SA mainly affected the combustion characteristics and NO x generation characteristics of pulverized coal by changing the degree of air staging at different positions in the furnace. Larger R SA values meant a greater amount of oxygen involved in the early combustion. This accelerated the early burn-out of pulverized coal and the release of elements, increasing NO x emissions. This study recommends R SA values between 0.7 and 1, that the over-fire air (OFA) nozzle be positioned behind the near burner region (x/d > 3.5) for the combustion of bituminous coal to minimize NO x emissions and unburned carbon. [Display omitted] • The fuel-lean gas ignited in advance of the fuel-rich gas. • The conversion process of the percentage of energy loss was obtained. • The conversion process of fuel-N were analyzed. • The combustion effectivity and conversion rate of N to NO with the increase of R SA the under sub- SR. • The recommended R SA range and OFA position were given. [ABSTRACT FROM AUTHOR]

Abstract: A three-component particle-dynamics anemometer is used to measure, in the near-burner region, the influence of the particle bias in the primary air duct on the gas/particle flow characteristics for a centrally fuel rich swirl coal combustion burner, in conjunction with a gas/particle two-phase test facility. Velocities, particle volume flux profiles and normalized particle number concentrations were obtained. Compared with a common burner (a centrally fuel rich burner without a particle concentrator), the degree of penetration for the centrally fuel rich burner is higher, the residence time of particles in the central recirculation zone is longer and the central recirculation zone is larger. The particle volume flux and normalized particle number concentration for the centrally fuel rich burner are much larger near the chamber axis. The influence of gas/particle flow characteristics on combustion has been analyzed. [Copyright &y& Elsevier]

Abstract: A three-component particle-dynamics anemometer is used to measure the characteristics of two-phase gas/particle flows in the near-burner region for a centrally fuel rich swirl coal combustion burner using a gas/particle two-phase test facility. Velocities, mean particle diameters and particle-volume flux profiles were obtained. The primary air and glass beads partially penetrate the central recirculation zone and are then deflected radially. At the center of the central recirculation zone, the mean radial velocities and tangential velocities are low and a zone of high particle-volume flux and large particle size is formed. The influence of gas/particle flow characteristics on combustion is analyzed. [Copyright &y& Elsevier]

• Industrial experiments of down-fired boiler with bituminous coal under various loads are taken. • The ignition heat of coal/air flow under different coal varieties and loads is given. • Burning bituminous coal has a large furnace flame fullness under the low-load. • Strong combustion stability is determined by the low ignition heat and high furnace temperature under the low-load. • The combustion performances of bituminous coal and anthracite under the low-load are analyzed and compared. 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 NO x 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 NO x emissions at the furnace exit are 410, 397 and 288 mg/m3 @ 6% O 2 , respectively. [ABSTRACT FROM AUTHOR]

Deep peak shaving requires extremely high requirements for low-load combustion stability of boilers. In this study, a novel swirl burner (NSB) with an eccentric secondary air arrangement was proposed, and the validity and the progressiveness of the NSB in achieving ultra-low-load combustion stability for down-fired boilers (DFBs) were confirmed from laboratory experiments to industrial applications. Firstly, the cold-modelling experiments of gas/particle (GP) two-phase flow characteristics involving two combustion systems (i.e., the DFB with traditional swirl burners (TSBs) and the DFB with NSBs) were performed at an ultra-low load of 90 MW e. Compared with the original boiler with TSBs, the maximum horizontal recirculation velocity and the area of recirculation zone below arches significantly increase for the improved DFB with NSBs. The particle number concentration near furnace center for the improved DFB with NSBs is much higher than that for the original DFB with TSBs. The above two aspects will effectively guarantee timely ignition of anthracite. The downward depth of GP flows and the space utilization ratio of the lower furnace increase, which will be beneficial to promoting volume heat load and heat flux density in primary combustion zone. In addition, full-scale industrial-sized measurements aiming at a 300-MW e DFB improved by NSBs were carried out at ultra-low loads of 100 and 90 MW e. For the original DFB after improved by NSBs, the minimum load for stable combustion without oil support is reduced from 150 to 90 MW e , and the ultra-low-load combustion stability is achieved. For the improved DFB with NSBs, pulverized coal ignition distances for operating burners are about 2 and 2.6 m, respectively, and the signal strength of flame detectors for all operating burners is above 95% at 100 and 90 MW e. The flame fullness and combustion stability are good at the initial combustion stage. At ultra-low loads, furnace negative pressure, superheat steam pressure and oxygen concentration at furnace outlet fluctuate slightly. The temperature at air preheater inlet meets the needs of normal operation of denitrification system, and there is no problem of low temperature corrosion on the surface at flue gas side of air preheater. The unburned carbon in fly ash is about 4%. The maximum concentrations of NO x emission at furnace outlet are 714 and 687 mg/m3 (O 2 = 6%) at 100 and 90 MW e , respectively, and ultra-low emission of NO x after denitrification system is achieved. • A novel swirl burner with an eccentric secondary air arrangement was proposed. • Laboratory and industrial experiments were performed. • Performances of flow, mixture, ignition and combustion are all improved. • Ultra-low-load combustion stability is achieved for the improved down-fired boiler. [ABSTRACT FROM AUTHOR]

• Proposed effective low NO x combustion technology with arch-supplied secondary air. • The flow characteristics of the two-phase flows in the furnace were investigated. • Arch-supplied secondary air increases the downward impulse of the two-phase flow. • Increasing arch-supplied secondary air rate can improved the combustion performance. This study proposes a low NO x 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 NO x emissions and combustion characteristics. Results demonstrate that optimizing the damper openings for F-layer and arch-supplied secondary air can significantly reduce NO x 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. [ABSTRACT FROM AUTHOR]

• Carbon disorder degree of bituminous coal is the highest. • CO 2 aids blend reactivity only at heating rates >5 K min−1. • Double parallel random pore model (DRPM) is better in oxygen-rich conditions. • Based on DRPM, volatiles and partial carbon initiate synchronous reactions. This study used various analytical techniques to explore the structural characteristics of anthracite, lean coal, and bituminous coal. Thermogravimetric analysis assessed the combustion performance of individual coals and their blends. Results revealed distinct structural differences among the three coals. Although lean coal's functional groups distribution resembled bituminous coal, its carbon ordering aligned with anthracite. Increasing the proportion of bituminous coal minimally improved combustion in air. Under O 2 + N 2 , rising oxygen levels lowered ignition and burnout temperatures, enhancing the comprehensive combustion index. At high heating rates, 30 %O 2 + 70 %CO 2 outperformed 30 %O 2 + 70 %N 2. The fitting performance of the double parallel random pore model (DRPM) was superior to that of the random pore model (RPM). Kinetic analysis suggested a DRPM for lean coal and bituminous coal co-combustion, unfolding in two stages. Activation energy increased with O 2 concentration in O 2 + N 2 but remained lower than in O 2 + CO 2. [Display omitted] [ABSTRACT FROM AUTHOR]

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. • A Foster Wheeler down-fired boiler retrofitted with novel low-load stable combustion technology is studied by PDA. • Effects of fuel-lean coal/airflow angle and secondary air ratio on the gas/particle flows characteristics were investigated. • The fuel-lean coal/airflow angle of 30° and the secondary air ratio of 1:2 is recommended. • This novel low-load stable down-fired boiler combustion technology is suitable for a wide range of coal quality combustion. [ABSTRACT FROM AUTHOR]

In this paper, based on the MIMSC (multi-injection and multi-stage combustion) technology, new burner arrangement and air distribution parameter settings were proposed for a 300-MW subcritical down-fired boiler originally using MBEL (Mitsui Babcock Energy Limited) combustion technology. Numerical simulations were conducted to study the influences of the secondary-tertiary air proportion on the airflow mixing effect in the furnace, the ignition and pulverized coal combustion characteristics. The airflow mixing effect in the furnace is characterized by the size of the dimensionless vertical velocity decay area (V da) and the fluctuation of the maximum vertical velocity decay curve (V dc). During the research, the sum of the secondary and tertiary air rate remained constant, and the secondary air rate was set to 30.54%, 33.54%, 36.54%, 39.54%, and 42.54%. It was found that under the condition of using new type burner, with the secondary air rate increased from 30.54% to 42.54%, the ignition distance of pulverized coal decreased from 1.30m to 0.84m, the dimensionless penetration depth decreased from 1.46 to 1.27, and the NO x emission and carbon in the fly ash decreased first and then increased. The variation of V dc showed the following regulation. At the secondary air rate of 30.54% to 33.54%, increasing the secondary air rate decreased the fluctuation amplitude. While at the secondary air rate of 39.54% to 42.54%, increasing the secondary air rate increased the fluctuation amplitude. Only at the secondary air rate of 36.54%, V dc in the tertiary air mixing area was smooth. When V dc was smooth, V da was small at 0.043, the carbon in fly ash at the furnace outlet was the lowest at 4.19%, and the NO x emission was low at 675.9 mg/m3 at 6% O 2. An optimal secondary air rate of 36.54% is recommended. For the subsequent design of burner structure and parameters, it is suggested that V dc in the tertiary air mixing area should be smooth. [ABSTRACT FROM AUTHOR]

In order to achieve peak carbon dioxide emissions and carbon neutrality goals, a higher requirement is put forward for coal type adaptability medium, load stable combustion performance, and pollutant emission for wall-fired boiler with swirl burners. In this work, through industry experiments, the combustion characteristics of a retrofitted wall-firing mode 330 MWe power unit feed with anthracite were investigated. The combustion system was retrofitted employing the centrally fuel-rich (CFR) swirl burner and deeply-staged combustion technology. The experiment data were measured, including flue gas temperature and composition, furnace temperature, and the main operational parameter under high-load (300 MWe), medium-load (230 MWe), and low-load (150 MWe) conditions. The stable combustion of anthracite is achieved at various loads. With the excessive air coefficients of the main combustion region increase, the corresponding second air damper opening increase under high-load condition or over-fire-air damper opening decrease, both the flue gas temperature and heating rate of pulverized coal increase advance the ignition and promote the stable combustion and burn-out of anthracite coal. In the initial combustion stage, the O 2 given into the chamber increase, both O 2 consumption rate and CO concentration decrease, but NO x concentration increases. While in the later stage, both the O 2 and CO concentration tend to become flat. Meanwhile, the O 2 concentration is lower than 2.1%, the CO concentration maintains a relatively high level, and NO x production is inhibited effectively and becomes flat gradually. With the excess air coefficient increase, the temperature in the main combustion region increases, and the difference in furnace temperature between the burn-out region and the main combustion region diminishes. The unburned carbon content in fly ash and exhaust gas temperature decreases, while NO x emission increases. There is little difference for the flue gas temperature distribution in the side-wall region, and the measured O 2 concentration is always higher than 3.5% with the different mass flux of secondary air and over-air-fire. With decreasing load, the NO x emission decreases from the highest 1392.66 to the lowest 958.38 mg/m3 (O 2 6%), and the boiler efficiency improves. Compared with high-load and medium-load, the slagging tendency of the water wall is lower under low-load condition. • Stable combustion of anthracite was achieved under various loads in retrofitted boiler. • The combustion characteristics under deeply-staged technology were studied. • With boiler load decreased, NO x emission decreased, boiler efficiency increased. [ABSTRACT FROM AUTHOR]

• A novel low-load stable combustion technology is applied on a 600MWe FW down-fired boiler. • 33% rated load stable combustion without oil support is achieved. • Full-scale industrial measurements are carried out at 200, 240 and 540MWe. • Combustion and NOx formation characteristics at high and low load are contrastively studied. 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, O 2 , 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 (O 2 = 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. [ABSTRACT FROM AUTHOR]

Abstract: Using an IFA300 constant temperature anemometer system, cold air experiments on a scale-model of a down-fired pulverized-coal 300MWe utility boiler with swirl burners were performed to investigate the influence of the staged-air flow ratio on flow characteristics in the furnace. This ratio was increased in incremental steps over the range of 7%–20%. The flame reach of the downward airflow was decreased with the ratio increased. A shallower reach is accompanied by a weaker mixing of vent and staged-air. At lower ratios, mixing is good and thus favors coal combustion but weak mixing at intermediate ratios produces a delay in the mixing point. This is compensated by an increasing recirculation zone under the arch that benefits ignition and the appearance of an upward airflow below the mixing point due to an increasing influence of vent air. At higher ratios still, this upward flowing disturbance is centered higher up in the furnace generated primarily by mixing with staged-air as vent-air mixing weakens. [Copyright &y& Elsevier]

Abstract: Cold airflow experiments on a small-scale burner model, as well as in situ experiments on a centrally fuel-rich swirl coal combustion burner were conducted. Measurements were taken from within a 300 MWe wall-fired pulverized-coal utility boiler installed with eight of centrally fuel-rich swirl coal combustion burners in the bottom row of the furnace during experiments. Various primary air ratios, flow characteristics, gas temperature and gas species concentrations in the burner region were measured. The results of these analyses show that with decreasing primary air ratio, the swirl intensity of air, divergence angles and maximum length and diameter of the central recirculation zone all increased, and the turbulence intensity of the jet flow peaked but decayed quickly. In the burner nozzle region, gas temperature, temperature gradient and CO concentration increased with decreasing primary air ratio, while O2 and NOx concentration decreased. Different primary air ratios, the gas temperatures and gas species concentrations in the side-wall region varied slightly. [Copyright &y& Elsevier]

Abstract: A start-up ignition burner has been proposed to reduce oil fuel consumption during the firing-up process and partial-load operation. To investigate the influence of different excess air ratios on bituminous coal combustion in the start-up ignition burner, full-scale reacting-flow experiments were performed for an experiment setup. The ignition burner was identical to that normally used in an 800 MWe utility boiler. Gas temperature distributions in the burner were obtained for excess air ratios of 0.56, 0.75, 0.98 and 1.14 (corresponding to primary air velocities of 17, 23, 30 and 35 m/s). Coal burnout and the release of C and H were observed at the exit of the burner nozzle. Gases such as O2 and CO were measured at the center of the burner. A change in resistance was obtained within the burner. [ABSTRACT FROM AUTHOR]

Abstract: Turbulence intensities at the exit of a centrally fuel rich (CFR) burner were measured employing a probe with hot-film sensors. In addition, using glycol as a smog tracer, optical images of the highlighted primary airflow were taken with a CCD camera and contrast-enhanced by computer. Profiles of the interface between primary and secondary air were obtained from which fractal dimensions (FDs) of the primary air boundary were estimated. Results indicate that FDs of this air boundary are in the range of 1.10–1.30 depending on burner conditions. As the FD and mean turbulence intensities of the primary air boundary have uniform distributions, they can be used to analyze coal combustion and NO x formation characteristics of the burner. When either decreasing the outer secondary air vane angle or increasing either the inner secondary air vane angle or total mass flow rate of the secondary air, the burner swirl number increases which subsequently shows up as an increase in the FD and mean turbulence intensities of the primary air boundary. Increasing the ratio of the mass flow rate of inner to outer secondary air decreases the burner swirl number, but increases the FD and mean turbulence intensities of primary air boundary. [Copyright &y& Elsevier]

Abstract: Experiments with a small-scale cyclone burner used for burner enrichment in a down-fired pulverized-coal 300 MWe utility have been conducted on an air/particle test facility. Particle separating efficiency was obtained with different positions of an adjustable vane. Industrial experiments were performed on a full-scale boiler. The gas temperature distribution along the primary air and coal mixture flow, gas temperature distribution of the furnace, and gas components such as O2, CO, CO2 and NO X in the near-wall region were measured for the first time. The influence of the adjustable vane position on coal combustion in the furnace was determined. With the adjustable vanes at the nozzle, ignition of the primary air and pulverized-coal mixture was delayed and the gas temperature peak was above the burner arch, with high NO X emission. Raising the vanes can bring forward the ignition point but results in the fuel-rich flow being up ahead of time, leading to a rise in carbon content in fly ash and NO X emission. [Copyright &y& Elsevier]

Abstract: Industrial experiments have been performed on a down-fired pulverized-coal 300MWe utility boiler with vent air valve opening of 100% and 40%. The gas temperature distribution along the primary air and coal mixture flow, gas temperature distribution in the furnace, and gas components such as O2, CO, CO2 and NO x in the near-wall region were measured for the first time. The influence of vent air valve opening on coal combustion in the furnace was determined. The results indicate that ignition of the primary air and pulverized-coal mixture is delayed. The position of the gas temperature peak is above the arches. Emission of NO x is up to 2101mg/m3 (at 6% O2 dry) with vent valve opening of 40%. [Copyright &y& Elsevier]

[Display omitted] • Effect of fuel-lean flow damper opening (FLD) in a down-fired boiler is investigated. • Relation of water-wall and steam temperature in a supercritical boiler is discussed. • On decreasing the FLD, heating of the fuel-rich coal/air flows gradually accelerates. • Specific coal consumption is lowered and energy conversion efficiency is improved. • The optimum NO x emissions and carbon in fly ash are as low as 672 mg/m3 and 4.51%. 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 NO x emissions, which remained in the range of 654–672 mg/m3 at 6% O 2. Based on optimizing coal combustion characteristic and economic performance of the unit, an optimal FLD of 25% was recommended. [ABSTRACT FROM AUTHOR]