Near-burner Region Research Articles

Coupling radiative properties with detailed char conversion kinetics

In pulverized solid fuel combustion, radiation is the primary heat transfer mechanism. Especially in the near-burner region with high particle loadings, particle radiation dominates gas radiation. Thus, adequate modeling of particle-radiation interactions in full-scale simulations of combustion chambers may primarily affect the overall accuracy. The radiation behavior depends strongly on the particles? optical properties, characterized by the complex index of refraction (IOR). The IOR depends on particle morphology and material composition which change during pyrolysis and char conversion. For char conversion, the focus of the present paper, the change depends on how the structural transformations take place inside the particle, characterizes by the reaction regimes, boundary layer diffusion limited, pore diffusion limited or kinetically controlled. The formation of an outer ash film layer with progressing burnout can complicate the situation even more.To quantify the effects of the structural changes during the char conversion process on the radiative properties, the output data of the char conversion kinetics (CCK) model (particle size, ash/carbon content, pore fraction) are used to build artificial particles at different conversion degrees and for the different regimes. Finally, the radiation interaction is calculated by applying an extension of the Mie theory for coated particles, and these results are compared to simplified approaches.Simplified approaches were tested to derive the IOR by applying mixing rules based on the available char and ash content with progressing char conversion. The results indicate that only a non-linear mixing rule for the IORs of ash and char lead to appropriate approximations of the radiation behavior of the particle. Furthermore, it is not necessary to model the spatial arrangement of carbon core and ash film for the radiative properties. Modeling a sphere and calculating an effective IOR depending on the carbon, ash, and pore volume fraction results in an average deviation of less than 3% to the exact solution.

Effects of particle size on semicoke and high-volatile bituminous coal cofiring in reducing-to-oxidizing environment

The characteristics of semicoke and high-volatile bituminous coal comprising different-sized particles were experimentally studied during co-firing in a 300-kW pilot-scale down-fired furnace. Ignition was investigated in detail in the near-burner region wherein ignition transitioned from reducing to an oxidizing atmosphere similar to that in pulverized coal (pc) furnaces. Combustion characteristics—including ignition distance and mode, NO emission, and char burnout—were evaluated based on chemiluminescence spectra, flame temperature, and flue gas species. The results showed that the chemiluminescence-spectra measured ignition distance decreased with increasing particle size, which was consistent with the trends determined using gas temperature and flue-gas species analyses. An attempt was made to distinguish the relative contributions of semicoke and bituminous coal to the initiation ignition. The ignition mode transformed from nonhomogeneous to homogeneous at particle size of R90 = 25% because the semicoke ignition share was reduced in the blended fuel. A method was proposed for determining the intensive combustion zone and strong reducing zones (SC and SR, respectively). The SC and SR areas both decreased with increasing particle size. The correlation between SR and the fuel-N/NO conversion ratio showed that a larger SR is the key to low NO emission. Because bituminous coal promoted semicoke ignition, stabler combustion and lower downstream NO emission, R90 ≤ 20% particles may be suitable for cofiring semicoke and bituminous coal.

Investigation on flow dynamics and temperatures of solid fuel particles in a gas-assisted oxy-fuel combustion chamber

Flow dynamics and temperatures of solid fuel particles strongly influence flame stabilization, local heat release and fuel conversion inside pulverized solid fuel combustors. To investigate these phenomena, experiments are carried out under well-controlled inflow and boundary conditions inside a gas-assisted, swirled oxy-fuel combustion chamber. Flow fields of small particles that represent the gas phase velocity are determined in the near-burner region by PIV using a particle separation algorithm. Trajectories of large solid fuel particles are evaluated in a two-dimensional plane using a combined high-speed PIV/PTV approach. Particle temperatures and particles sizes are measured at different levels downstream the burner exit to reveal different stages of combustion. Therefore, a two-color pyrometer is used that dissolve single particles to achieve local particle temperature and particle size distributions. Two oxy-fuel operation conditions with an oxygen fraction of 33%V and a reference operation point in air are investigated within this study. In the flow fields of the gas phase the impact of the atmosphere is clearly visible in the spatial expansion of the internal recirculation area. Regions of high slip velocities and high heat release could be identified by analyzing particle trajectories in terms of direction, velocity and acceleration. Residence times of small and large particles are estimated from the flow fields. Significantly larger residence times are observed for large particles which leads to higher burn out rates in the near-burner region. Furthermore, particle temperature measurements reveal similar particle temperatures for the investigated oxy-fuel and air operation conditions.

The effects of oxygen concentration and gas temperature on coal stream ignition and particle surface temperature in reducing-to-oxidizing environments

In the near-burner region of pulverized coal burners, two zones exist, with very different oxygen concentrations. The first zone is a locally reducing environment, caused by the fast release of volatiles from a region of dense coal particles, and the second zone, which is surrounding the first zone, is a hot oxidizing environment. The transition of coal particles from the reducing zone to the oxidizing zone affects early stage coal combustion characteristics, such as devolatilization, ignition and particle temperature history. In this work, we used a two-stage Hencken flat-flame burner to simulate the conditions that coal particles experience in practical combustors when they transition from a reducing environment to an oxidizing environments. The composition of the reducing environment was chosen to approximate that of a typical coal volatile. Three oxygen concentrations (5, 10 and 15 vol%) in the “ambient” oxidizing environment were tested, corresponding to those at different distances downstream from a commercial burner. The corresponding gas temperatures for the oxidizing environments were adjusted for the different oxygen concentrations such that the “volatile” flame temperatures were the same, as this is what would be expected in a commercial combustor. High speed videography was used to obtain the ignition characteristics, and RGB color pyrometry was used to measure particle surface temperatures. Two different sizes of coal particles were used. It is found that when particles undergo a reducing-to-oxidizing transition at high temperatures, the particles are preheated such that the critical factor for ignition delay is point at which the particle is in the presence of oxygen, not the concentration of oxygen. The ignition delay of large particles is found to be 53% longer than that of small particles due to their higher thermal mass and slower devolatilization. The oxygen concentration in the ambient have a negligible effect on early-stage particle temperatures.

Characteristics of NH3/H2/air flames in a combustor fired by a swirl and bluff-body stabilized burner

Combustion of ammonia (NH3) presents a number of challenges due to its low reactivity, poor flame stability, very low flame speed and high NOx emissions in a range of burning conditions. Therefore, its application in combustion systems requires device modifications, either by adapting existing systems or developing new ones. In this work, a novel combustor fired by a swirl and bluff-body stabilized burner was designed and tested. Initially, flame stability diagrams were established for NH3/H2 fuel mixtures. Then, emissions of NOx and unburnt NH3 were measured for a range of equivalence ratios and NH3 mole fractions in the fuel mixture. In addition, in-flame data for O2 and NOx concentrations and gas temperature were measured for five combustor operating conditions. A parallel mathematical modelling exercise was performed to obtain a qualitative knowledge of the burner aerodynamics and assist the interpretation of the experimental results. The predicted aerodynamic pattern suggests the establishment of an extended central recirculation zone that allows long residence times in the near burner region (NBR) that are critical to ensure stable combustion of the NH3/H2/air mixtures. The measurements revealed that the burner presents a wide range of stable flames for xNH3 between 0.6 and 0.8, showing the positive effects of adding relatively small amounts of H2 to the fuel mixture. The in-flame data show that the flame initiates close to the burner inlet and presents two main reaction zones. NOx is formed in the NBR and is consumed afterwards, through selective non-catalytic reduction reactions. Overall, the present laboratory combustor performs well in terms of flame stability and both NOx and NH3 emissions, and it is foreseen that industrial combustors based on the present design would be able to ensure flame stability and NOx and NH3 emissions control with simple complementary systems.

Industrial-scale investigations on effects of tertiary-air declination angle on combustion and steam temperature characteristics in a 350-MW supercritical down-fired boiler

Industrial-scale experiments were conducted to study the effects of tertiary air declination angle (TDA) on the coal combustion and steam temperature characteristics in the first 350-MW supercritical down-fired boiler in China with the multiple-injection and multiple-staging combustion (MIMSC) technology at medium and high loads. The experimental results indicated that as the TDA increased from 0° to 15°, the overall gas temperature in the lower furnace rose and the symmetry of temperature field was enhanced. The ignition distance of the fuel-rich coal/air flow decreased. In near-burner region, the concentration of O2 decreased while the concentrations of CO and NO increased. The concentration of NO decreased in near-tertiary-air region. The carbon in fly ash decreased significantly from 8.40% to 6.45% at a load of 260 MW. At a TDA of 15°, the ignition distances were the shortest (2.07 m and 1.73 m) at a load of 210 MW and 260 MW, respectively. The main and reheat steam temperatures were the highest (557.2°C and 559.4°C at a load of 210 MW, 558.4°C and 560.3°C at a load of 260 MW). The carbon in fly ash was the lowest (4.83%) at a load of 210 MW. On changing the TDA from 15° to 25°, the flame kernel was found to move downward and the main and reheat steam temperatures dropped obviously. The change of TDA has little effect on NOx emissions(660–681 mg/m3 at 6% O2). In comprehensive consideration of the pulverized coal combustion characteristics and the unit economic performance, an optimal TDA of 15° is recommended.

Large Eddy Simulation of a Pressurized, Partially Premixed Swirling Flame With Finite-Rate Chemistry

Finite-rate chemical effects at gas turbine conditions lead to incomplete combustion and well-known emissions issues. Although a thin flame front is preserved on an average, the instantaneous flame location can vary in thickness and location due to heat losses or imperfect mixing. Postflame phenomena (slow CO oxidation or thermal NO production) can be expected to be significantly influenced by turbulent eddy structures. Since typical gas turbine combustor calculations require insight into flame stabilization as well as pollutant formation, combustion models are required to be sensitive to the instantaneous and local flow conditions. Unfortunately, few models that adequately describe turbulence–chemistry interactions are tractable in the industrial context. A widely used model capable of employing finite-rate chemistry is the eddy dissipation concept (EDC) model of Magnussen. Its application in large eddy simulations (LES) is problematic mainly due to a strong sensitivity to the model constants, which were based on an isotropic cascade analysis in the Reynolds-averaged Navier–Stokes (RANS) context. The objectives of this paper are: (i) to formulate the EDC cascade idea in the context of LES; and (ii) to validate the model using experimental data consisting of velocity (particle image velocimetry (PIV) measurements) and major species (1D Raman measurements), at four axial locations in the near-burner region of a Siemens SGT-100 industrial gas turbine combustor.

The effects of particle size and reducing-to-oxidizing environment on coal stream ignition

Coal particles experience a transition from a reducing to oxidizing environment in the near-burner region of pulverized coal (pc) boilers. For the first time, we report a fundamental study of ignition of a coal-particle stream experiencing a flame environment that transitions from a reducing to an oxidizing environment (termed reducing-to-oxidizing environment). High-speed videography is used to observe the particles in situ, and scanning electron microscopy is used to characterize the sampled particles. The effects of particle size on ignition are presented for four size bins (63–74 µm, 75–89 µm, 90–124 µm and 125–149 µm) for PRB subbituminous coal at two nominal gas temperatures (1300 K and 1800 K). An oxidizing environment with 20% molar oxygen composition is used as base-case. In contradistinction to single particle studies where particles are reported to ignite heterogeneously at higher temperatures, this study shows that coal streams ignite homogeneously, irrespective of particle size, in the oxidizing environment. By changing nominal gas temperature from 1300 K to 1800 K, ignition time decreases, on average, by a factor of five for each of the particle size bins. For both gas temperatures, the trend in ignition delays as particle size changes is non-monotonic. However, at 1800 K nominal gas temperature, ignition delays are independent of particle size in the reducing-to-oxidizing environment and ignition delays are doubled on average when compared to those in the oxidizing environment. It is more noticeable at the lower gas temperature of 1300 K that homogeneous ignition of coal streams is oxygen-dependent below 90 µm particle size and temperature-dependent above 90 µm. In general, ignition delay is determined by volatile release rate (controlled by the particle temperature) and the local oxygen concentration. Micrographs of particles also confirm that ignition and char burnout times are longer in the reducing-to-oxidizing environments than those in the oxidizing environments.

Pulverized coal particle ignition in a combustion environment with a reducing-to-oxidizing transition

A reducing-to-oxidizing (RO) environment is characteristic of what a coal particle experiences in the near-burner region of pulverized coal (pc) furnaces. The RO environment can influence early-stage coal combustion processes such as ignition, aerosol formation, and char burnout. However, fundamental studies have focused on either oxidizing conditions (mimicking the post-flame region) or reducing conditions (mimicking the devolatilization region). The effect of this RO environment on early-stage coal combustion has, until now, not been considered. Here, the role of this reducing-to-oxidizing environment on single-particle ignition is evaluated. Powder River Basin (PRB) sub-bituminous coal was used, with a particle size of 125–149 μm and two nominal gas temperatures of 1300 K and 1800 K. The experimental findings for purely-oxidizing conditions with 20 vol% oxygen are compared with those of reducing-to-oxidizing environment. Single particles were tracked using high speed, high resolution videography. Emission intensities of the particles were used to evaluate the prevailing ignition modes, and to determine the characteristic ignition and induction times in both oxidizing and reducing-to-oxidizing environments. Experimental findings show that homogeneous-to-heterogeneous mode of ignition is prevalent for purely oxidizing conditions for both nominal gas temperatures of 1300 K and 1800 K. However, hetero-homogeneous ignition is favored in reducing-to-oxidizing environment at 1800 K and heterogeneous ignition at 1300 K gas flame temperature. The reducing-to-oxidizing environment leads to longer ignition delay times of about 20% and 40% on average for 1300 K and 1800 K nominal gas temperatures respectively but shorter induction times than those of oxidizing condition. The results show that ignition behavior in a reducing-to-oxidizing post-flame environments can be quite different from those in oxidizing environments.

Characterization of a new Hencken burner with a transition from a reducing-to-oxidizing environment for fundamental coal studies.

In pulverized coal burners, coal particles usually transition from a locally reducing environment to an oxidizing environment. The locally reducing environment in the near-burner region is due to a dense region of coal particles undergoing devolatilization. Following this region, the particles move into an oxidizing environment. This "reducing-to-oxidizing" transition can influence combustion processes such as ignition, particulate formation, and char burnout. To understand these processes at a fundamental level, a system is required that mimics such a transition. Hence, we have developed and characterized a two-stage Hencken burner to evaluate the effect of the reducing-to-oxidizing transition and particle-to-particle interaction (which characterizes dense region of coal particles) on ignition and ultrafine aerosol formation. The two-stage Hencken burner allows coal particles to experience a reducing environment followed by a transition to an oxidizing environment. This work presents the results of the design and characterization of the new two-stage Hencken burner and its new coal feeder. In a unique approach to the operation of the flat-flame of the Hencken burner, the flame configurations are operated as either a normal flame or inverse flame. Gas temperatures and oxygen concentrations for the Hencken burner are measured in reducing-to-oxidizing and oxidizing environments. The results show that stable flames with well-controlled conditions, relatively uniform temperatures, and species concentrations can be achieved in both flame configurations. This new Hencken burner provides an effective system for evaluating the effect of the reducing-to-oxidizing transition and particle-to-particle interaction on early-stage processes of coal combustion such as ignition and ultrafine particle formation.

Phase behaviors, fuel properties, and combustion characteristics of alcohol-vegetable oil-diesel microemulsion fuels

ABSTRACTBiofuels are being considered as alternatives to fossil-based fuels due to depletion of petroleum-based reserves and pollutant emission concerns. Vegetable oils and bioalcohols have proven to be viable alternative fuels both with and without engine modification. However, high viscosity and low energy content are long-term operational problems with vegetable oils and bioalcohols, respectively. Therefore, vegetable oil-based microemulsification is being evaluated as a method to reduce the high viscosity of vegetable oils and enhance the miscibility of alcohol and oil phases. Studies have shown that microemulsification with different alcohols lead to varying fuel properties depending on their structure. The overall goal of this study was to formulate microemulsion fuels with single and mixed alcohol systems by determining the effects of water content, alcohol branching structure and carbon chain length on phase behaviors, fuel properties, and emission characteristics. It was found that microemulsion fuels using certain alcohols displayed favorable stability, properties, and emission characteristics. Flames of fuels with linear short-chain-length alcohols had larger near-burner blue regions and lower CO and soot emissions indicating the occurrence of more complete combustion. In addition to alcohol effects, the effect of vegetable oils, surfactants, and additives on emission characteristics were insightful in pursuit of appropriate microemulsion fuels as cleaner burning alternatives to both No.2 diesel and canola biodiesel.

Hydrogen sulfide formation characteristics of pulverized coal combustion – Evaluation of blended combustion of two bituminous coals

Abstract This paper presents the characteristics of sulfur release during pyrolysis and hydrogen sulfide (H2S) formation for the pulverized coal combustion of blends of two bituminous coals in a pulverized coal combustion test furnace. The amount of sulfur released during pyrolysis is affected by the total sulfur content and the form of sulfur. Organic sulfur is more readily released to the gaseous phase than pyritic sulfur and sulfate sulfur. When blends of two bituminous coals releasing different amounts of sulfur are fired, the H2S concentrations are between those for the combustion of the single coals. The peak H2S concentration in the near-burner region in the blended combustion coincides with the value estimated from the blending ratio, which means that there is no interaction between the sulfur releases of the two coals during pyrolysis. The reaction behavior of H2S and SO2 after the release of sulfur during the blended combustion is the same as that in the combustion of each single coal.

Effects of particle concentration variation in the primary air duct on combustion characteristics and NOx emissions in a 0.5-MW test facility with pulverized coal swirl burners

The effects on combustion characteristics and NOx formation of the particle concentration variation in the primary air duct were investigated on a 0.5-MW pulverized-coal combustion test facility with swirl burners. The gas temperature, components and fly ash in the near-burner region and furnace center were sampled and measured. It shows that the pulverized-coal concentration variation in the primary air duct is of great importance as for defining the flame structure, gas temperature profile and gas components. With the burner changing from general type to inner-lean-outer-rich to inner-rich-outer-lean types I and II, the flame diffusion zone shrinks, its color less bright, the O2 concentration in the flame zone decreases and the CO concentration increases significantly. Additionally, the NOx concentration decreases, with NOx emissions at the furnace exit dropping from 1182 to 861 mg/m3 (a reduction of 27.15%). Finally, the release rates of carbon, hydrogen and nitrogen and the burnout rate of carbon all decrease. The gas temperatures observed in the inner-rich-outer-lean types I and II burners are lower than those of the general and the inner-lean-outer-rich type burners in the near-burner region. The release rates of elements and the burnout rate of carbon are close to 100% for all four burners.

The Impact of Biomass Cofiring on Volatile Flame Length

The physical characteristics and composition of biomass can vary significantly from coal and from one fuel to another, and these differences can impact the structure of the resulting pulverized fuel flame, particularly the volatile flame. The length and location of the volatile flame, which is the zone dominated by the combustion of volatiles, is important not only to flame stability and length but also to the formation of pollutants such as NOx. In this work, the effects of biomass volatile fraction and particle size on the length of the volatile flame were investigated for cofired flames of pulverized coal and wood waste. The volatile flame length was measured experimentally in a 35 kW combustion facility via axial measurements of CO and CO2 made along the centerline. Numerical simulations were employed to aid the interpretation of the impacts of wood waste cofiring on the volatile flame. The length of the volatile flame zone was found to be sensitive to the location of volatile matter release and the amount of volatile matter released within the volatile flame zone. Large biomass particles, with high axial momentum and long heating times, break through the fuel-rich volatile flame zone, releasing volatiles downstream into an oxygen-rich environment. The delayed release of volatiles reduces the amount of volatile fuel in the fuel-rich region, leading to shorter volatile flames, which in turn augments breakthrough of particles before complete release of volatiles. On the basis of CO and CO2 concentration measurements, cofired flames with 20 wt % wood waste and 80 wt % coal were found to have 21% shorter volatile flames compared with coal-only flames. Numerical simulations verified that volatile reactions take place past the end of the volatile flame. To reduce the amount of particles that pass through the volatile flame before complete devolatilization, the wood waste particle size was reduced. The resulting volatile flame was 27% longer for the 20 wt % cofired case compared with coal-only volatile flames. Increased volatile matter content, characteristic of biomass fuels, leads to an increased volatile flame length when all of the volatiles are released in the near-burner region.

Oxy-fuel combustion characteristics of pulverized-coal in a drop tube furnace

Experiments were conducted in a drop tube furnace (DTF) for a bituminous coal, two coal particle size distributions (unsieved coal with a mass mean diameter of 76μm and sieved coal with a mass mean diameter of 29μm) and three oxy-fuel atmospheres (21% O2/79% CO2, 26% O2/74% CO2 and 31% O2/69% CO2) at a furnace temperature of 1100°C. For comparison purposes, tests were also performed under air firing conditions. The data reported includes gas temperatures, major gas species concentration and particle burnout measured along the DTF. In addition, a number of selected final char samples were also morphologically and chemically characterized. Under oxy-fuel conditions: (i) the near burner region (NBR) temperature increases and the HC and CO concentrations decrease as the O2 concentration in the oxidizer increases, regardless of the coal particle size; (ii) the NOx concentration levels along the DTF increase as the O2 concentration in the oxidizer increases, regardless of the coal particle size, but, compared with air combustion, the results show that oxy-coal combustion lowers the conversion of fuel-N to NO; and (iii) the NBR burnout levels increase as the O2 concentration in the oxidizer increases, regardless of the coal particle size. The combustion of the sieved coal yields higher NBR temperatures, lower NOx concentration and higher burnout levels along the DTF than the unsieved coal, regardless of the oxidizer composition. Carbon and nitrogen are released at about the same rate as total coal mass release, while hydrogen is released more rapidly regardless of the oxidizer composition and coal particle size. In the combustion of both unsieved and sieved coals, under oxy-fuel conditions, the final char structure becomes more porous as the O2 concentration in the oxidizer increases. The final char elemental composition is essentially independent of the oxidizer composition, but the combustion of the unsieved coal leads to chars with higher percentages of S and lower percentages of Cl than those from the sieved coal combustion.

A comparison of sulfur and chlorine gas species in pulverized-coal, air- and oxy-combustion

Gas phase measurements of sulfur species (SO2, H2S, SO3, and COS) and HCl were collected in the staged near-burner region and the post tertiary injection oxidizing region of a pulverized-coal flame under air- and oxy-fired conditions. The near burner region was characterized by a high CO region near the center of the reactor directly below the burner. This reducing region as indicated by high CO concentrations produced high H2S and COS and lower SO2 concentrations in comparison to the post tertiary injection oxidizing region for both air and oxy-fired conditions. The concentrations of HCl were in most cases insensitive to radial location. Concentrations of all measured sulfur gases were 2.4–2.9 times higher in oxy-fired conditions than air-fired conditions consistent with the increase expected by the removal of N2 from the oxidizer. The concentration of H2S were on average 5.8 times higher in oxy-fired reducing conditions than air-fired reducing conditions indicating that the oxy-fired near burner regions were higher in H2S and lower in SO2 than would be predicted by removal of N2 alone from an air-fired system. The concentrations of SO3 were less than 2% of the total sulfur for both air and oxy-fired conditions but the uncertainty of the measurement did not allow for a comparison between the two firing modes. The concentrations of HCl were 2.4–2.8 times higher in oxy-fired conditions consistent with the removal of N2 from the oxidizer. The total measured sulfur, HCl, SO3, and reducing zone H2S were all found to increase linearly with increasing sulfur and chlorine concentrations in the coal.

Influence of mass-flow ratio of inner to outer secondary air on gas–particle flow near a swirl burner

The influence of mass-flow-rate ratio of inner to outer secondary air on gas–particle flow characteristics was determined in the near-burner region of a centrally fuel-rich swirl coal combustion burner. Velocity and particle volume flux profiles and normalized particle number concentrations were obtained. Peaks in tangential mean velocity and three-dimensional root-mean-square fluctuation velocities were found to decrease as the mass-flow-rate ratio increased. Moreover, the peaks in the mean axial velocities and particle volume flux near the wall increased, whereas those near the chamber axis decreased. Simultaneously, both recirculation zone and swirl number decreased as the mass-flow-rate ratio increased.

Effects of biomass particle size during cofiring under air-fired and oxyfuel conditions

Carbon capture and storage (CCS), when applied to biomass cofiring systems, can remove atmospheric CO2 since the CO2 that is consumed by the biomass during growth is not released back into the atmosphere. Biomass cofiring can also potentially contribute to meeting renewable portfolio standards (RPS), and result in reduced pollutant emissions, including sulfur oxides (SOx) and mercury. However, biomass fuels are widely variable in composition, particle size, and nitrogen content, which can make utilization of these fuels challenging. In this work, a numerical study was conducted for cofiring of pulverized coal and sawdust under air-fired and oxyfuel conditions to investigate the effects of cofiring on flame length and nitric oxide (NO) formation. Previous experiments have shown an increase in nitrogen conversion to NO when cofiring under both air-fired and oxyfuel combustion, despite the fact that the sawdust cofired had less fuel-bound nitrogen. Computational fluid dynamics (CFD) is used to determine the cause of the increased NO conversion and to identify differences between air-fired and oxyfuel cofired flames. The simulations reveal that cofired flames have longer volatile-flame regions (the flame envelope), and this length is influenced by the increased volatile fraction and particle size associated with the biomass. Flame length theory for turbulent, non-premixed gaseous diffusion flames was found to be useful in interpreting the observed results in both air-fired and oxyfuel combustion. Large biomass particles that are not entrained in the near-burner region breakthrough the flame envelope, and this was shown to be detrimental to controlling NO formation. During oxy-cofiring combustion, particle breakthrough occurs at smaller diameter, leading to increased nitrogen conversion to NO when compared to air-fired conditions. This is a direct result of a decreased flame envelope length and elevated oxygen concentrations.

Cold experimental investigations into gas/particle flow characteristics of a low-NO x axial swirl burner in a 600-MWe wall-fired pulverized-coal utility boiler

With the use of a three-dimensional particle-dynamics anemometer in conjunction with a gas/particle two-phase test facility, we measured the flow characteristics in the near-burner region of a low-NO x axial swirl burner (LNASB). Mean velocities, root mean square (RMS) velocities and particle volume flux profiles were obtained. The data shows that the central recirculation zone exists only in the region x/ d = 0.1–0.3 between r/d = 0.25–0.4. The central recirculation zone has maximum size at cross-section x/ d = 0.1 located between the primary and secondary air flow zones. Moreover, with a maximum recirculating velocity of 4 m/s, the recirculation zone has a high particle volume flux that can easily result in the slagging and deformation in the burner nozzle. Oxygen concentrations are also high in this zone which can easily lead to NO x formation. In the region x/ d = 0.1–0.5, the primary and secondary air flow zones exhibit two peaks in both axial and radial velocity profiles while a single peak appears in the tangential velocity profiles in the secondary air flow zone.

Measurement of nitrogen evolution in a staged oxy-combustion coal flame

Oxy-combustion is a technology attracting a significant amount of attention because it enables CO2 capture from the products of coal combustion. Understanding the potential for NOX reduction is a key factor in the economic analysis and design of oxy-combustion systems. In order to better understand NOX formation and destruction in oxy-combustion, measurements of major gas species and nitrogen containing species, NH3 and HCN, were obtained in a down-fired, premixed, staged, laboratory reactor. The near-burner region was fuel-rich and reducing followed by the addition of burnout oxidizer in order to simulate the staged combustion process of a full-scale boiler. In the vicinity of the burner, gas species measurements were obtained at a resolution of 25mm. The data show that nitrogen from the coal is rapidly converted to NOX in the first 50–100mm of the reactor followed by a reduction of NOX in the fuel-rich, near-burner region. NOX increases at the point where burnout oxidizer is added and then remains constant. A slight increase in NOX which is potentially the result of the thermal mechanism in air combustion does not occur in oxy-combustion. In the fuel-rich, near-burner region, stoichiometric ratio (S.R.)=0.75, oxy-fuel combustion produces more hydrocarbons, NH3, and HCN suggesting reburning reactions are more prevalent. Air combustion can be made to produce a similar NOX profile to oxy-combustion by lowering the S.R. (deeper staging) of the fuel-rich region. The minimum NOX achievable is approximately the same for air and oxy-combustion, but the oxy-combustion minimum NOX occurs at a higher S.R. in the fuel-rich zone which allows oxy-combustion to achieve higher carbon conversion for the same NOX reduction. The higher concentrations of NH3 and HCN seen at a given fuel-rich zone equivalence ratio also suggest recycled NOX may be reduced more efficiently in oxy-combustion than would be expected for air combustion.