Low NOx Formation Research Articles

Effect of vane angle on turbulence-chemistry interactions, temperature, flow field, burnout, and NOx emissions in swirling flames fueled by semicoke mixtures.

To enhance the combustion efficiency and reduce NOx emissions in large-scale semicoke and bituminous coal blends, an extensive numerical study was conducted. The focus of this study was to optimize the quaternary air vane angle (αv) through detailed analysis of the temperature and flow fields, turbulence-chemistry interactions, char burnout, and NOx formation in a carefully scaled 1:5 dual-swirl burner. The results showed that with increasing αv, the high-temperature flame region was narrowed and the peak temperature was reduced along with the broadened inner recirculation zone and the shrunken external recirculation zone due to better pulverized fuel-oxidant blending and reinforced convective heat transfer. The peak turbulent Damköhler number Dat evidently increased from 197.5 to 496 with increasing αv, which implied a strengthened homogeneous combustion. Additionally, the corresponding mixing time scales increased while the chemical kinetics time scales decreased, which denoted that an intense diffusing flame was generated with a strong turbulent intensity. The peak heterogeneous Damköhler number Das-O2 showed a reduction from 2.54 to 2.27, while the peak values of Das-CO2 and Das-H2O decreased from 0.1 to 0.077 and from 0.02 to 0.015, respectively. The char-O2 reaction was controlled by diffusion/kinetics; both char-CO2 and char-H2O reactions were determined by kinetics, and all gas‒solid reactions showed a kinetically controlled regime. With increasing αv, the enlarged inner recirculation region increased the residence time, and a higher dilution level lessened the peak temperature, which led to reductions in fuel-NOx and the thermal-NOx. The αv range of 30-45° (or swirl number Sn = 0.55-0.95) was suggested by taking the high burnout and low-NOx formation into account.

Investigation of the drop-in capabilities of a renewable 1-Octanol based E-fuel for heavy-duty engine applications

This article presents the results of an investigation of neat 1-Octanol and its blends with Diesel, which can be potential drop-in fuels for heavy-duty compression ignition engines. Fuel blends consisting of 10 %, 20 %, and 30 % 1-Octanol by volume in Diesel, including neat 1-Octanol, were investigated for ignition and combustion performances. The ignition behavior was assessed using a constant-volume combustion-chamber-based ignition tester. The experimental investigation was conducted on a heavy-duty single-cylinder engine by varying the combustion center. In addition, injection-rate shaping was performed to understand the effects of various injection-rate shapes, namely square, ramp, and boot rate shapes, on the performance of neat 1-Octanol.The ignition tester results indicated an increased ignition sensitivity of neat 1-Octanol towards the chamber pressure and temperature compared to that of Diesel. At an elevated chamber temperature (650 °C) and pressure (20 bar), the ignition delay of neat 1-Octanol approaches closer to that of Diesel. Engine experiments revealed an improved thermal efficiency with neat 1-Octanol and its blends by up to 1.2 %, owing to their improved mixture homogenization capabilities. Furthermore, neat 1-Octanol and its blend led to a drastic reduction in smoke and carbon monoxide. In addition, it results in the highest peak during the premixed heat release, and with a square injection-rate shape, an additional minor improvement in thermal efficiency was observed. The relative changes in carbon monoxide and hydrocarbon emissions between the injection-rate shapes were negligible. At higher load, the boot-rate shape results in lower NOx formation than the square-rate shape at an almost identical thermal efficiency. Overall, the 10 % 1-Octanol, 20 % 1-Octanol and 30 % 1-Octanol blends, which fulfill the Cetane number requirements for the given fuel norms, can be used as drop-in fuels, as they exhibit a comparable or even better performance than Diesel.

Analysis of low emission characteristics of NH3/H2/air mixtures under low temperature combustion conditions

The complex dynamics of NH3/H2/air mixtures have been numerically investigated in an idealized reactor environment of fast fuel/air mixing represented by perfectly stirred reactors (PSR). The complete steady state “S-curve” solutions are obtained with eigenvalue analysis for quantifying the stable/unstable characteristics of different branches. With four different chemical mechanisms, stable cool flame branches are observed for NH3/H2/air mixtures. The reactions involving species NH2, H2NO and NNH are found important for both fuel consumption and NOx formation on the cool flame branch. The parametric study further shows that with less than 5 % hydrogen addition in ammonia (in volume), stable cool flames exist over a wide range of inlet temperature, equivalence ratio and pressure. For the states in the cool flame branch, although hydrogen is almost consumed completely, only portion of the ammonia is consumed resulting in low NOx emission due to the reduction effects of remaining ammonia and high N2O emission resulting from low temperature. Inspired by the observed stable cool flame characteristics, a multi-staged low-temperature model system is formulated for low NOx emission of NH3/H2/air combustion, in which small amount of hydrogen is sequentially injected to homogeneous reactors of lean NH3/H2/air mixture. With the representative high inlet temperature of 850 K, pressure of 20 atm, fuel lean of ϕ=0.7 and critical fuel efficiency of 99.7 %, results with the 28-species Li reduced mechanism show that less than 75 ppm of NOx and 98 ppm of N2O can be achieved with two-staged homogenous combustion operating at low temperature of 1300 K and 2 % H2 addition in each stage. With three-staged homogenous combustion, less than 70 ppm of NOx and 10 ppm of N2O can be achieved at 1300 K. NOx emission is lower than those of direct NH3/H2/air combustion without aftertreatment, demonstrating the opportunity of utilizing low temperature combustion to achieve high fuel conversion, low NOx and N2O formation for lean NH3/H2/air mixtures.

Impact of fuel chemical function characteristics on spark assisted and kinetically controlled compression ignition performance focused on multi-mode operation

The DOE Co-Optima initiative has a focus on investigating the ability of fuel properties to work in tandem with advanced combustion engines to increase fuel economy. Advanced compression ignition strategies like spark assisted compression ignition (SACI) and partial fuel stratification (PFS) have been shown to achieve better efficiency and emissions performance than traditional combustion processes (i.e., conventional diesel combustion, spark ignited combustion). These strategies rely on a high degree of fuel mixing and a globally dilute environment to achieve lower temperature combustion. The avoidance of fuel rich regions and the reduction in peak flame temperatures result in low soot and NOx formation. Despite their clear benefits, operating range limitations have been identified for all combustion strategies. The limitations stem from the fundamental characteristics of each combustion process, hence they can't be entirely avoided. These limitations are also geometry, fuel type, dilution level and mixture preparation dependent. Metal engine experiments have been conducted on a single-cylinder research engine equipped with variable valve actuation and a 12.5:1 compression ratio, more appropriate for high load boosted spark ignition operation. Five fuels with different chemical class compositions but matched research octane number (RON) were tested under both SACI and PFS at mid load conditions. Combustion phasing was changed over the whole range of operability to show trade-offs between the fuels. Specific chemical, mixture preparation and thermodynamic effects are discussed for performance and emission results. SACI and PFS are then compared to a baseline spark ignition (SI) condition to estimate potential benefits of operating under advanced combustion modes. The results show that there are significant fuel specific effects even at matched RON and octane sensitivity that affect emissions, engine efficiency and range of operability of the different advanced combustion modes.

Large Eddy Simulation on the Effects of Coal Particles Size on Turbulent Combustion Characteristics and NOx Formation Inside a Corner-Fired Furnace

Abstract The effects of pulverized coal particles’ sizes on the coal combustion characteristics are numerically studied in a laboratory-scale tangentially fired furnace. The turbulent gas flow and the coal particle motion are solved by employing the large eddy simulation (LES) and the discrete phase model (DPM). The mixture fraction probability density function (MF-PDF) is coupled to simulate the non-premixed pulverized coal combustion. It is found that the coal combustion efficiency is positively affected by the dispersion of coal powders. The particle dispersion and the coal combustion are augmented by the intensive impingement caused by the corner-injected flow. Large coal particles, with their greater inertia, enhance particle agglomerations, which limit the combustion of volatile and char. Accordingly, the average flame temperature decreases with the growing particle sizes. Also, the O2 concentration increases slightly because of the incomplete coal combustion, and the CO2 concentration decreases gradually. In contrast, the CO concentration increases markedly in the furnace center due to the presence of a reducing atmosphere. The NO concentration exhibits an exponential decline with the increased particle size. A relatively stable combustion and a relatively low NOx formation are acquired inside such a corner-fired furnace when the particle Stokes number is a little greater than 1.

One axial fuel injected vortex-tube combustor with high capacity of combustion stabilization for NOx reduction

A novel vortex-tube combustor with axial fuel injection for NOx reduction was proposed to overcome the instability of ultra-lean combustion. The stability limit, flame configuration, NOx and CO emissions, and flame structure were investigated experimentally under various global equivalence ratios and fuel flow rates. The mechanisms of NOx emission reduction were analyzed by numerical simulation. Results show that the limit of global equivalence ratio can be as low as 0.01 and the amplitude of pressure fluctuation is always less than 1300 Pa, indicating fairly good performance in combustion stabilization of the combustor. In the operation range, there is a trade-off region with low NOx and CO simultaneously. The diffusion-like flame structure in this combustor can enhance the local equivalence ratio, whilst the flow field structure can also promote the transport of chemical enthalpy to the flame front, thus facilitating the stabilization. The enhanced stabilization enables the ultra-lean combustion and the ensued small area of the high-temperature zone to conduct, as well as the low NOx formation through reducing thermal NO. The local fuel-rich region and the flow field structure can promote the NOx to be reduced further via the Fenimore mechanism.

Experimental study on NOx formation and burnout characteristics of pulverized coal in oxygen enriched and deep-staging combustion

Deep-staging combustion is an effective technology for reduction of NOx, but is often accompanied by the combustion worsening. Oxygen enriched and deep-staging combustion is a promising technology to realize both low NOx formation and high burnout rate. NOx reduction and burnout characteristics of pulverized coal in oxygen enriched combustion were experimentally investigated in this study with a drop-tube reactor. Results show that the NOx reduction is significantly promoted by elevated O2 concentration under deep-staging condition. The NOx formation of coal under 40% oxygen concentration condition is 40.7% lower than those in air combustion as the stoichiometric ratio in the fuel-rich zone (SR1) is 0.5. The thermal and fuel NOx formations were separately investigated. The fuel NOx reduction with increase of O2 concentration makes a major contribution to the total NOx reduction under deep-staging conditions. The elevated O2 concentration facilitates the release of fuel nitrogen in the fuel-rich zone, avoiding its oxidation in the fuel-lean zone. Moreover, the negative correlation between NOx formation and unburned carbon (UBC) in coal ash is evidently weakened with the increase of O2 concentration. Ultra-low NOx formation and high combustion efficiency could be concurrently achieved in oxygen enriched combustion under deep-staging condition. Furthermore, the comprehensive comparison of the oxygen enriched combustion, air combustion and oxy-fuel combustion was conducted. Low NOx formation can be reached in oxy-fuel combustion under non-staging condition, while oxygen enriched combustion performs much better in NOx reduction under deep-staging conditions.

Formula omitted] enrichment of [formula omitted] blends in lean premixed gas turbine combustion: An experimental study on effects on flame shape and thermoacoustic oscillation dynamics

Thermoacoustic instabilities under lean operation in gas turbine burners hinder the development of lean premixed combustion mode of operation, thus limiting the potential to decrease NOx emissions. A method to improve stability of lean combustion while maintaining low thermal NOx formation is to add hydrogen in typical gas turbine fuels such as natural gas. The present work examines the thermoacoustic dynamic characteristics of hydrogen-enriched methane blends in a swirl stabilized model gas turbine combustor. We blend CH4 with increasing H2 molar content (from 0% to 40%) at a global equivalence ratio of ϕ=0.55 on a constant Reynolds number Re = 19000. The reference case of pure methane is susceptible to blow off at the same equivalence ratio. On increasing the hydrogen content at 10% H2, the flammability limits are extended. However, further increase of the H2 content leads to manifestation of random short bursts of dynamic pressure and heat release. Thermoacoustic dynamics are intermittently injected between a quiescent state and a regime of high amplitude oscillations. On further increasing H2 content, the dynamics are attracted towards a limit cycle; a fully established high amplitude regime, where no requiescence is observed. In this regime, heat release rate and dynamic pressure oscillate in phase and at the same frequency. Based on the observed dynamics, we seek a mixture property to characterize the dynamic state, the combustor operates in. We show that the extinction strain rate associated with each mixture can collapse the dynamic transitions from quiescent to intermittent instabilities and finally to fully established thermoacoustic oscillations. In each dynamic state, we examine the mechanism that affiliates coherent structures of the underlying thermodynamic flow field with the flame through the relation of the spatial distribution of the flow imposed strain rate over the extinction strain rate of each mixture. It is shown that the flame anchoring locations for a given mixture are dictated by the flow imposed strain rate and then that increased extinction resistance couples the flame with naturally excited to swirling flows helical instabilities, that may instigate mechanisms responsible for intermittent heat release rate bursts. Finally, the combustor demonstrates a limit cycle behaviour where the flow-imposed strain rate oscillates about the extinction strain rate along the inner shear layers causing local extinction of the flame at the root close to the centerbody.

Numerical Study on the Effect of Lambda Value (Oxygen/Fuel Ratio) on Temperature Distribution and Efficiency of a Flameless Oxyfuel Combustion System

The flameless oxyfuel combustion technology has been proven to be a promising new method to reduce the fuel consumption and pollutants in industrial applications [...]

Low NOx Lean Premix Reheat Combustion in Alstom GT24 Gas Turbines

Reducing gas turbine emissions and increasing their operational flexibility are key targets in today's gas turbine market. In order to further reduce emissions and increase the operational flexibility of its GT24 (60 Hz) and GT26 (50 Hz), Alstom has introduced an improved sequential environmental (SEV) burner and fuel lance into its GT24 and GT26 upgrades 2011 sequential reheat combustion system. Sequential combustion is a key differentiator of Alstom GT24/GT26 engines in the F-class gas turbine market. The inlet temperature for the SEV combustor is around 1000 °C and reaction of the fuel/oxidant mixture is initiated through auto-ignition. The recent development of the Alstom sequential combustion system is a perfect example of evolutionary design optimizations and technology transfer between Alstom GT24 and GT26 engines. Better overall performance is achieved through improved SEV burner aerodynamics and fuel injection, while keeping the main features of the sequential burner technology. The improved SEV burner/lance concept has been optimized toward rapid fuel/oxidant mixing for low emissions, improved fuel flexibility with regard to highly reactive fuels (higher C2+ and hydrogen content), and to sustain a wide operation window. The burner front panel features an improved cooling concept based on near-wall cooling as well as integrated acoustics damping devices designed to reduce combustion pulsations, thus extending the SEV combustor's operation window even further. After having been validated extensively in Alstom's high pressure (HP) sector rig test facility, the improved GT24 SEV burner has been retrofitted into a commercial GT24 field engine for full engine validation during long-term operation. This paper presents the obtained HP sector rig and engine validation results for the GT24 (2011) SEV burner/lance hardware with a focus on reduced NOx and CO emissions and improved operational behavior of the SEV combustor. The HP tests demonstrated robust SEV burner/lance operation with up to 50% lower NOx formation and a more than 70 K higher SEV burner inlet temperature compared to the GT24 (2006) hardware. For the GT24 engine with retrofitted upgrade 2011 SEV burner/lance, all validation targets were achieved including an extremely robust operation behavior, up to 40% lower GT NOx emissions, significantly lower CO emissions at partload and baseload, a very broad operation window (up to 100 K width in SEV combustor inlet temperature), and all measured SEV burner/lance temperatures in the expected range. Sector rig and engine validation results have confirmed the expected SEV burner fuel flexibility (up to 18 vol. % C2+ and up to 5 vol. % hydrogen as standard).

A Piston Geometry and Nozzle Spray Angle Investigation in a DI Diesel Engine by Quantifying the Air-Fuel Mixture

Low temperature diesel combustion has been widely investigated over the last few years for reducing in-cylinder emissions of Direct Injection (DI) diesel engines without sacrificing efficiency and fuel consumption. The spatial distribution of the fuel within the combustion chamber and the air-fuel mixing quality are the key factors affecting temperature generation within the cylinder. Avoiding fuel rich areas within the cylinder can significantly reduce the local high temperatures resulting in low NOx formation. This paper investigates the effects of the combustion chamber geometry and spray angle on the air-fuel mixing and emissions formation of a DI diesel engine. A new quantitative factor measuring the air-fuel mixing quality has been adopted in order to analyze and compare air-fuel mixing quality for different piston geometries. The results have shown that pistons with a narrow entrance and a deep combustion re-entrant chamber benefit from increased air-fuel mixtures due to the significantly higher swirl generated within the cylinder. However, the improved air-fuel mixing does not consequently lead to a reduced NOx generation, which is highly affected by the combustion efficiency of the engine.

Predicting the performance and exhaust NOX emissions of a spark-ignition engine generator fueled with methane based biogases containing various amounts of CO2

The effect of methane–CO2 blends with varying CO2 concentrations (corresponding to various biogas compositions) on combustion and emission characteristics were investigated numerically for a gas engine generator. CO2 variation in a methane-based biogas fuel produced effects similar to the CO2 effects of exhaust gas recirculation (EGR), decreasing the cylinder temperature, pressure, heat release rate, and flame speed. By design of experiment (DOE) analysis, the maximum brake torque (MBT) timing was calculated for various values of the spark timing and excess air ratio (EAR). As the EAR increased from 1.2 to 1.8 and the CO2 content decreased, the MBT timing was far advanced from top dead center (TDC). At 50% CO2 content, incomplete combustion phenomena were detected in the leaner region at EAR values above 1.6. In spite of the performance degradation of the engine itself, the addition of CO2 up to 50% in volumetric content improved the electrical generating efficiency by an average of 6.4%, with lower NOX formation.

Experimental Study on the Effect of Hydrogen Enrichment of Methane on the Stability and Emission of Nonpremixed Swirl Stabilized Combustor

An ultra lean mixture (ϕ ≤ 0.5) of methane–hydrogen–air was experimentally investigated to explore the effect of fuel flexibility on the flame stability and emission of a nonpremixed swirl stabilized combustor. In order to isolate the effect of hydrogen addition to methane, experiments were carried out at fixed fuel energy input to the combustor while increasing the hydrogen content from 0% up to 50% in the methane–hydrogen mixture on volume basis. The combustor fuel energy was then increased up to the range of typical gas turbine combustors. Equivalence ratio sweep was carried out to determine the lean stability limit of the combustor. Results show that the hydrogen content in the fuel mixture and fuel energy input have a coupled effect on the combustor lean blow off velocity (LBV), temperature and emissions. The LBV increases by ∼103% with the addition of 30% H2. On the other hand, the LBV increases by ∼20% as the fuel energy increases from 1.83 MW/m3 to 2.75 MW/m3. Burning under ultra lean condition serves two purposes. (1) The excess air supplied reduces the overall combustor temperature with its ensuing effect on low NOx formation. (2) It increases the overall combustor volume flow rate which reduces the residence time for NOx formation. The axial temperature profile presented along with the emission data can serve as basis for the validation of numerical models. This would give more insight onto the effect of hydrogen on the turbulence level and how it would improve the localized extinction of methane in a cost-effective way.

Numerical simulation of a hydrocarbon fuelled valveless pulsejet

Jet propulsion technology has been limited to being identified largely with turbojets and turbo-machinery driven jet engines. Of late there has been a renewed interest in pulsejet technology and it has once again caught the imagination of academia. The specific characteristics of the pulsejet, such as easy scalability, absence of moving parts, reduced combustion temperatures; lower NOX formation and the like make it possess varied capabilities for use in the field of jet propulsion and the most viable option for small-scale jet propulsion.In the current work, a numerical analysis encompassing feasibility and validation of a valveless pulsejet engine was attempted using CD-adapco׳s STAR-CCM+ CFD package. Due to lack of comprehensive established mathematical laws to govern the working of a pulsejet, most experimental work being performed is done by trial and error. This necessitates in-depth computational studies in order to shed more light on the understanding of valveless pulsejets. The results have been encouraging and in agreement with observed experimental conclusions such as, i) changes in dimensions affect the working of a pulsejet, ii) presence of a flare enhances the working of a pulsejet, and the close agreement in the frequency of operation. Through continuous study, an optimum initial condition was achieved which enabled the pulsejet to begin operation even before 0.05s, thereby greatly reducing computational costs if a higher time-scale were to be used.

Combustion characteristics of diesel HCCI engine: An experimental investigation using external mixture formation technique

In compression ignition engines, soot–NOx paradox is an extremely challenging unresolved issue. Homogeneous charge compression ignition (HCCI) is one of the most promising solution that combines the advantages of both SI and CI combustion modes. It gives high thermal efficiency similar to compression ignition engines and resolve the associated issues of high levels of NOx and PM simultaneously. In HCCI combustion, homogeneous mixture of air and fuel burns spontaneously throughout the combustion chamber, which reduces the total combustion duration due to very high rate of heat release. Determination of precise control parameters for controlling the ‘rate of heat release’ and ‘start of combustion’ are major research challenges in the development and deployment of this technology. In the present research, experiments were performed in a two cylinder engine, in which one cylinder is modified to operate in HCCI mode, while other cylinder operate in conventional CI mode. Homogeneous mixture preparation is the most challenging part for achieving diesel HCCI combustion. Low diesel volatility remains the main obstacle in preparing the homogenous fuel–air mixture therefore a dedicated device called ‘diesel vaporizer’ was developed. Exhaust gas recirculation (0%, 10% and 20%) was used for controlling the rate of heat release. To study the combustion behavior, experiments were performed at three different relative air–fuel ratios (λ=4.95, 3.70 and 2.56). Enrichment of fuel–air mixture enhances the rate of heat release and the location of peak of in-cylinder pressure shift towards BTDC side due to earlier start of combustion. This was effectively controlled by EGR for leaner HCCI combustion conditions. Exhaust gases diluted the homogeneous charge and presence of non-reactive species reduce the rate of combustion. It controls the peak in-cylinder temperature, which is a responsible for extremely low NOx formation. For richer fuel–air mixtures, EGR was relatively less effective due to dominance of ‘rate of heat release’, which was significantly high.

Pilot-scale study on ozone-enhanced catalytic oxidation of waste gas emissions from the pulp and paper industry

Field studies were conducted at Domtar's Kraft pulp and paper mill (Hawesville, KY) for 2 weeks to investigate the treatment of high volume, low concentration (HVLC) waste gas streams using ozone-enhanced catalytic oxidation technology. The contaminants in the HVLC waste gas from the pulping area of the mill contained mainly methanol (360 ± 80 ppm) and total reduced sulfur compounds [dimethyl sulfide (DMS) (4400 ± 670 ppm) and dimethyl disulfide (150 ± 25 ppm). The catalysts used in the field studies included ViO2/TiO2 and CuO/MoO3/a-Al2O3. The results of the field study showed ozone-to-sulfur ratios >2, space velocities 90% destruction of DMS in the HVLC waste gas stream. Ozone-enhanced catalytic oxidation has key environmental advantages over incineration, including mild operating temperatures and thus, lower energy costs and lower NOx formation. Although the technology appeared to be feasible at the laboratory scale, the field study data revealed that several obstacles must be overcome prior to this technology being implemented at large scale. © 2010 American Institute of Chemical Engineers Environ Prog, 2011

Characteristics of Low NOx Formation of Over-Rich-Mixture Plume Combustion in High Pressure Direct Injection SI Hydrogen Engines

Characteristics of Low NOx Formation of Over-Rich-Mixture Plume Combustion in High Pressure Direct Injection SI Hydrogen Engines

Low NOx Emission From an Ambient Pressure Diffusion Flame Fired Gas Turbine Cycle (APGC)

The prospects of reduced NOx emission, improved efficiency, stable, and oscillation-free combustion, and reduced construction costs achieved by an “Inverted Brayton Cycle” applied to midsize (0.5 to 5.0 MWe) power plants are discussed. In this cycle, the combustion products of an atmospheric pressure combustor are expanded in the gas turbine to subatmospheric pressure and following heat extraction are compressed back to slightly above the atmospheric, sufficient to enable a controlled fraction of the exhaust gas to be recirculated to the combustor. Due to the larger volume flow rate of the gas, the polytropic efficiency of both the turbine and compressor of this small machine is increased. Because of the low operating pressure and flue gas recirculation, both of which are instrumental to low NOx formation, the combustor can be operated in the diffusion flame mode; this, on the other hand, assures good flame stability and oscillation-free combustion over wide ranges of the operating variables. For the task of obtaining very low NOx formation, the well-tested multi annular swirl burner (MASB) is chosen. Recent computational and experimental development of the MASB by Siemens-Westinghouse as a topping combustor is discussed. It is shown that the MASB operated in rich-quench-lean mode is capable of single-digit NOx emission. The emissions are further lowered in the APGC by ambient pressure combustion, and by the injection of the recirculated gas in the quench zone of the combustor. Results of a computational optimization study of the ambient pressure gas turbine cycle (APGC) are presented.

Structure of Opposed Jet Flame and NOx Formation.

High-intensity premixed combustion [0.71-1.12×109kJ/(m3·h)] was achieved using an opposed jet burner. The NOx emission was measured by an NO/NOx chemiluminescence analyzer. Maximum NOx formation measured was extremely low and varied from 20 to 40ppm, depending on the combustion intensity. Such a low NOx formation is attributed to the short residence time and the structure of the distributed reaction zone. The contribution of the Zeldovich mechanism is suppressed and the NOx originating from prompt NO is predominant in the distributed reaction zone. The equivalence ratio at which NOx maximizes was found to be 1.3. The temperature dependence of NOx concentration can be given empirically by an Arrhenius expression.