Influence of outer secondary-air vane angle on combustion characteristics and NOx emissions of a down-fired pulverized-coal 300 MWe utility boiler

Industrial experiments were performed on a down-fired pulverized-coal 300 MWe utility boiler with swirl burners. Gas temperature, concentrations of gas components (O₂, CO, CO₂ and NO_x) in the burning region and carbon content in the fly ash were measured with outer secondary-air vane angles of 25°, 32.5° and 50°. Results indicate that with increasing vane angle, NO_x emission and boiler efficiency decrease. Overall evaluation boiler efficiency and NO_x emission, the vane angle of 32.5° is optimum. Using an IFA300 constant-temperature anemometer system, cold air experiments on a quarter-scaled burner model were also carried out to investigate the influence of various outer secondary-air vane angles on the flow characteristics in the burner nozzle region. No central recirculation zone appeared for vane angles of 25° and 32.5°. Most of the pulverized-coal was ignited in the external recirculation zone. For vane angles of 45° and 55°, a central recirculation zone could be observed, and air flow rigidity and axial velocities decreased rapidly.     

Influence of the adjustable vane position on the flow and combustion characteristics of a down-fired pulverized-coal 300 MWe utility boiler

Experiments with a small-scale cyclone burner used for burner enrichment in a down-fired pulverized-coal 300 MW_e 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.     

Experimental investigation of NOx emissions in oxycoal combustion

This work presents the results of an experimental investigation on NO_x emissions from coal combustion in a pilot scale test facility. Three oxidiser atmospheres have been compared, namely air, CO₂/O₂, and O₂ enriched recirculated flue gas. NO_x emissions from two different combustion modes have been studied, swirl flame and flameless combustion. The influence of the burner oxygen ratio and the oxidiser O₂ concentration on NO_x formation and reduction have been analysed. With increasing burner oxygen ratio, an increase of NO_x emissions has been obtained for air and CO₂/O₂ in both, swirl flame and flameless combustion. In case of the swirl flame, flue gas recirculation leads to a reduction of NO_x emissions up to 50%, whereas in case of flameless combustion this reduction is around 40% compared to CO₂/O₂. No significant impact of the oxidiser O₂ concentration in the CO₂/O₂ mixture on NO_x emissions is observed in the range between 18 and 27 vol.% in swirl flames. An analysis of NO_x formation and reduction mechanisms showed, that the observed reduction of NO_x emissions by flue gas recirculation cannot be attributed to the reduction of recirculated NO_x alone, but also to a reduced conversion of fuel-N to NO.     

Numerical analysis of pulverized coal combustion characteristics using advanced low-NOx burner

A three-dimensional numerical simulation is applied to a pulverized coal combustion field in a test furnace equipped with an advanced low-NO_x burner called CI-α burner, and the detailed combustion characteristics are investigated. In addition, the validities of the existing NO_x formation and reduction models are examined. The results show that a recirculation flow is formed in the high-gas-temperature region near the CI-α burner outlet, and this lengthens the residence time of coal particles in this high-temperature region, promotes the evolution of volatile matter and the progress of char reaction, and produces an extremely low-O₂ region for effective NO reduction. It is also found that, by lessening the effect of NO reduction in Levy et al.'s model and taking the NO formation from char N into account, the accuracy of the NO prediction is improved. The efficiency factor of the conversion of char N to NO affects the total NO concentration downstream after the injection of staged combustion air.     

Influence of vent air valve opening on combustion characteristics of a down-fired pulverized-coal 300 MWe utility boiler

Industrial experiments have been performed on a down-fired pulverized-coal 300 MWe 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 2101 mg/m³ (at 6% O₂ dry) with vent valve opening of 40%.     

Numerical investigation on the flow, combustion and NOx emission characteristics in a 500 MWe tangentially fired pulverized-coal boiler

The characteristics of the flow, combustion, temperature and NO_x emissions in a 500 MW_e tangentially fired pulverized-coal boiler are numerically studied using comprehensive models, with emphasis on fuel and thermal NO_x formations. The comparison between the measured values and predicted results shows good agreement, which implies that the adopted combustion and NO_x formation models are suitable for correctly predicting characteristics of the boiler. The relations among the predicted temperature, O₂ and CO₂ mass fractions are discussed based on the calculated distributions. The predicted results clearly show that NO_x formation within the boiler highly depends on the combustion processes as well as the temperature and species concentrations. The results obtained from this study have shown that overfire air (OFA) operation is an efficient way to reduce the NO_x emissions of the pulverized-coal fired boiler. Air staging combustion technology (OFA operation) adopted in this boiler has helped reduce fuel NO_x formation as well as thermal NO_x formation under the present simulated conditions. The decrease in the formation of fuel NO_x is due to the decreased contact of the nitrogen from the fuel with the oxygen within the combustion air, while the decrease in thermal NO_x formation is caused by a decrease in temperature. The detailed results presented in this paper may enhance the understanding of complex flow patterns, combustion processes and NO_x emissions in tangentially fired pulverized-coal boilers, and may also provide a useful basis for NO_x reduction and control.     

The effect of O2 enrichment on NOx formation in biomass co-fired pulverised coal combustion

Oxygen enrichment of the combustion air in pulverised coal combustion for power plant is seen as a possible retrofit measure to improve CO₂ scrubbing and capture. This technique produces a reduced volume of flue gas with higher CO₂ concentration than normal air combustion that will contributes to the enhancement of amine scrubbing plant efficiencies. We report in this article the results of a study at the small pilot scale into the effect of these combustion modifications on the formation of NO_x and associated carbon burnout changes. Experiments were performed using a Russian coal, typical of that used in some UK power stations with shea meal and Pakistani cotton stalk as biomass fuels co-fired at a fraction of 15%th. The down-fired pulverised coal combustor was operated at 20 kWth under air-staged conditions for NO_x control and the secondary and over-fire air flows were both enriched by up to 79% (100% O₂) for a range of splits giving a 35% overall O₂ concentration for full enrichment. When the same enrichment process was applied to biomass/coal combustion different behaviour was observed with respect to NO_x formation. We have shown that oxygen enrichment can achieve benefits of improved carbon burnout with a positive impact on NO_x emissions over and above the primary aim of increasing CO₂ concentration in the flue gas for enhanced capture efficiencies. With all other conditions of overall stoichiometry, OFA levels and O₂ enrichment levels remaining the same, NO_x levels at 22% OFA initially increased over the range of secondary air enrichment, particularly for shea meal/coal co-firing. At 31% OFA the trends were to lower NO_x at high enrichment levels. However, co-firing with shea meal initially showed an increase in NO_x emission at lower levels of enrichment (up to 40% O₂) followed by overall lower NOx emissions at 100% O₂ in the secondary air. The results show that NO_x emissions can either increase or decrease depending on the operating conditions. The differences in behaviour are attributed, not only to the effects of enrichment on the stoichiometry of the near-burner zone, but also on the flame dynamics and intensity of combustion related to the associated reductions in gas velocity and swirl intensity by the transition from air to pure O₂ in the secondary oxidant stream.     

Fractal and turbulence characteristics of aerodynamic fields of swirl burners

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.     

Impact of air staging along furnace height on NOx emissions from pulverized coal combustion

Experiments were carried out on an electrically heated multi-path air inlet one-dimensional furnace to assess NO_x emission characteristics of an overall air-staged (also termed air staging along furnace height) combustion of bituminous coal. The impact of main parameters of overall air-staged combustion technology, including burnout air position, air stoichiometric ratio, levels of burnout air (the number of burnout air arranged at different heights of the furnace), and the ratios of the burnout air flow rates and pulverized coal fineness of industrial interest, on NO_x emission were simulated to study in the experimental furnace, as well as the impact of air staging on the carbon content of the fly ash produced. These results suggest that air-staged combustion affects a pronounced reduction in NO_x emissions from the combustion of bituminous coal. The more deeply the air is staged, the further the NO_x emission is reduced. Two-level air staging yields a greater reduction in NO_x emission than single-level air staging. For pulverized coal of differing fineness, the best ratio between the burnout air rates in the two-level staging ranges from 0.6 to 0.3. In middle air-staged degree combustion with f_M = 0.75, pulverized coal fineness, R₉₀ (%), has a greater influence on NO_x emission, whereas R₉₀ has little influence on NO_x emission for deep air-staged degree with f_M = 0.61. Air-staged combustion with proper burnout air position has little effect on the burnout. For overall air-staged combustion, proper burnout air position and air-staged rate should be considered together in order to achieve high combustion efficiency.     

A reduced NOx reaction model for pulverized coal combustion under fuel-rich conditions

A reduced NO_x reaction model was developed for analysis of industrial pulverized coal firing boilers. The model was developed from experiments of laminar premixed combustion under a variety of stoichiometric ratios, burning temperatures, coal ranks (from sub-bituminous coal to anthracite) and particle diameters. Calculations agreed with experimental results for NO_x and nitrogen species (NH₃ and HCN), if the model assumed that the hydrocarbon radicals were formed not only from pyrolysis of volatile matter, but also from char oxidation and gasification. The presence of hydrogen in char at the final burnout stage supported this assumption. NO_x reduction by hydrocarbon radicals was the most important reaction in high temperature (>1500 K), fuel-rich, char combustion regions. NO_x reduction from nitrogen species was sensitive to peak NO_x concentration in volatile combustion regions, but NO_x emission downstream had little influence from the peak NO_x concentration. The heterogeneous reaction between char and NO_x was important for fuel-lean or low-temperature conditions.     

Development of can-type low NOx combustor for micro gas turbine (fundamental characteristics in a primary combustion zone with upward swirl)

A low NO_x combustor for kerosene-fueled micro gas turbine based on a new concept was proposed, and the combustion characteristics of the prototype combustor were investigated. The new concept combustor consisted of primary and secondary combustion zones, and they were connected by a throat. A swirler was set between the primary and secondary combustion zones. In order to enhance the recirculation of burned gas in the primary combustion zone, the combustion air was introduced through the swirler and forced to flow upward to the combustor bottom, from where fuel spray was supplied through a nozzle. An optimum configuration of the primary combustion zone such as length of primary zone, swirler vane angle, diameter of throat, etc. were investigated to achieve high combustion stability and low emission in wide ranges of fuel flow rate and excess air ratio. The optimum value of each part in the primary combustion zone was found out by measuring fundamental combustion characteristics such as lean combustion limit, flame luminosity, exhaust gas composition and combustion gas temperature.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

Detailed measurements in a pulverized-coal-fired large-scale laboratory furnace with air staging

The aim of the present work was to evaluate the performance of a pulverized-coal-fired large-scale laboratory furnace with air staging. New data are reported for gas phase species concentration, temperature and particle burnout for two primary zone stoichiometric ratios, 1.15 (unstaged flame) and 0.95 (staged flame), other operating conditions being fixed. The results revealed that the reduction in primary zone stoichiometric ratio caused a decrease in NO_x emissions from 421 to 180 mg/N m³@6%O₂, an increase in CO emissions from 51 to 168 mg/N m³@6%O₂ and a reduction in particle burnout from 81.8% to 76.5%. It was concluded that the reduction of the O₂ availability in the primary zone inhibits the NO formation, mainly via the fuel mechanism, but it affects negatively both the CO and the char oxidation processes because, under staging conditions, both processes tend to occur in the vicinity of the over fire air injection region, where the temperatures are relatively low.     

Modeling of oxy-fuel combustion for a western Canadian sub-bituminous coal

The motivation of this research is to develop practical oxy-coal combustion techniques in order to facilitate the conversion of coal-fired utility power plants so as to recover a CO₂ rich flue gas stream for use and/or sequestration. The objective of this study is to ascertain the applicability and accuracy of a modeling tool to assist with future pilot scale oxy-fuel combustion experiments and burner scale-up studies. Two modes of oxy-coal combustion, O₂ enriched air (OEA) and recycled flue gas (RFG), were experimentally tested in a 0.3 MW_th pilot-scale combustor using a western Canadian sub-bituminous coal. The computational fluid dynamic tool was utilized to model the combustion, heat transfer and pollutant formation characteristics of these test cases and to examine the impact due to changes in the combustion medium, burner swirl and burner configuration. The model provided insights for the observed variation in NO_x production among the test cases: the dramatic increase in the OEA mode, the drop at higher burner swirl settings and the surprisingly small reduction in the RFG mode. Overall the model results compared well with measured data in all test cases and established confidence in using the model to explore new design concepts for oxy-coal combustion.     

Numerical simulation of the influence of over fire air position on the combustion in a single furnace boiler with dual circle firing

The Computational fluid dynamics (CFD) code PHOENICS is applied to simulate and evaluate the combustion process within the furnace of a 1,000 MW dual circle tangential firing single furnace lignite-fired ultra supercritical (USC) boiler. The dependence on overfire air (OFA) positioning on the combustion process is studied. The results show that the highest temperature appears on the upside of the burner zone close to the front wall, and the high temperature zone rises with elevated OFA positions. However, the temperature field distributions are similar despite differing OFA positions. The char content near the rear wall is higher than that near the front wall, and below the furnace arch, coal particles concentrate towards the front wall. Also with elevated OFA positions, nitrogen oxide (NO _x ) concentrations at the outlet fall, but char content increases. In regard to NO _x emission and char burnout, the suggested optimal distance from the OFA center to the center of the uppermost primary air nozzle should be 6 meters. This work was presented at the 7 ^th China -Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Influence of burner-port geometry in hydrocarbon oxidation and NOx formation mechanisms in methane/air flames

The influence of burner-port geometry in the mechanisms of hydrocarbon oxidation and NO_x formation from a 50 kW industrial-type methane-fired burner was investigated experimentally. Imaging and tomographic reconstruction techniques were used to assess the effects of port geometry upon flame visible length and C₂ chemiluminescence distribution in the recirculation zone. C₂ emission of methane flames depicts that low fuel jet velocities allow very rich conditions at recirculation zone and lead methane oxidation through O₂-scarcity mechanism. Higher velocities imply that methane oxidises via a path including dissociation into free radicals. In-furnace measurements were performed from a refractory-lined vertical furnace. NO_x concentration results revealed that NO formation is closely connected with the dissociation process, suggesting that prompt-NO_x mechanism is more important than hitherto supposed.     

CO2 capture using oxygen enhanced combustion strategies for natural gas power plants

This paper describes a series of experiments conducted with natural gas in air and in mixtures of oxygen and recycled flue gas, termed O₂/CO₂ recycle combustion. The objective is to enrich the flue gas with CO₂ to facilitate its capture and sequestration. Detailed measurements of gas composition, flame temperature and heat flux profiles were taken inside CANMET's 0.3 MWth down-fired vertical combustor fitted with a proprietary pilot scale burner. Flue gas composition was continuously monitored. The effects of burner operation, including swirling of secondary stream and air staging, on flame characteristics and NO_x emissions were also studied. The results of this work indicate that oxy-gas combustion techniques based on O₂/CO₂ combustion with flue gas recycle offer excellent potential for retrofit to conventional boilers for CO₂ emission abatement. Other benefits of the technology include considerable reduction and even elimination of NO_x emissions, improved plant efficiency due to lower gas volume and better operational flexibility.     

Detailed measurements in a laboratory furnace with reburning

This article presents a detailed experimental characterization of the reburning process in a large-scale laboratory furnace. Natural gas, pine sawdust and pulverized coal were used as reburn fuels. Initially, the study involved the collection of in-flame combustion data, without reburning, in order to define appropriate locations for the injection of the reburn fuels. Next, flue-gas data were obtained for a wide range of experimental conditions using the three reburn fuels and, subsequently, detailed measurements of local mean O₂, CO, CO₂, HC and NO_x concentrations, and gas temperatures have been obtained in the reburn zone for three representative furnace operating conditions, one for each reburn fuel studied. The flue-gas data revealed that the sawdust reburning leads to NO_x reductions comparable or even higher than those attained with natural gas reburning, while coal reburning yields much lower NO_x reductions. The detailed data obtained in the reburn zone indicates that the reburning process remains active throughout all the reburn zone in the cases of natural gas and sawdust reburning, while in the case of coal reburning its relatively low volatile matter content is insufficient to establish an effective reburn zone. In the cases of the sawdust and coal reburning the burnout levels remain approximately constant, regardless of the NO_x emissions reduction, with the sawdust reburning leading to higher particle burnout performance than the coal reburning.     

Coal characterisation for NOx prediction in air-staged combustion of pulverised coals

A series of world-traded coal samples has been tested using the Imperial College high temperature wire mesh apparatus (HTWM) in order to assess the relationship between high temperature (1600°C) char nitrogen content and NO_x formation in Hemweg Power Station (in the Netherlands) using deep furnace air staging. A linear relationship between high temperature char nitrogen and NO_x formation has been confirmed. These results suggest that high temperature char N content is the main factor limiting NO_x emissions with deep air-staged combustion.Char N and (hence apparently deep air-staged NO_x) can be predicted with an accuracy of approximately ±20% for most coals from the coal proximate and ultimate analysis—but this might not be sufficient for stations operating close to their emission limits. Measuring high temperature char N directly reduces the likely uncertainty in deep air-staged NO_x emissions for coals (and most blends) to approximately ±10%. Its use should be considered on a routine basis for coal selection on plants employing this technology.     

Parametric study of combined premixed and non-premixed flame coal burner

A combined gas/air mixture–coal burner was developed to include heat recirculation by utilizing a radiative solid material with premixed flame jets impinging onto the downstream side to preheat the fuel/air jet on the upstream side. Providing the heat recirculation mechanism at different air staging degrees enhanced the destruction rates of the fuel nitrogen oxides. Concentric elliptical premixed gas/air and coal/air jets had a stronger preheating effect and a consequent increased NO_x reduction effectiveness as compared to concentric circular jets, where the inner elliptical jets enlarged the contact diffusion area and entrainment thus increasing the preheating time. The parametric variation in the feeding ports to the coal combustor affected the exhaust emissions, wherein the use of an inclined or shifted injection from the centre-line contributed to the NO_x reduction. Increasing the jet angle in the upstream direction reduced the CO concentrations, while the NO_x emissions varied depending on the degree of staging. The inverse/normal flame configuration was found more effective than the normal flame configuration with respect to NO_x reduction that was enhanced at higher heat input ratios. Utilizing inverse triple flames led to a further NO_x reduction since higher temperatures prevailed in the initial flame region with a five reaction zone structure. Finer particles produced less NO_x, which was further reduced by blending the coal with biomass.