Colorless Distributed Combustion Research Articles

Effect of the hydrogen/kerosene blend on the combustion characteristics and pollutant emissions in a mini jet engine under CDC conditions

In this CFD study conducted with the geometry of a mini gas turbine engine using kerosene as fuel, colorless distributed combustion conditions were achieved by increasing the N2 ratio in the air. Then, H2 was added to the kerosene fuel in order to increase the combustion performance and the combustion chamber temperature distributions, NOx, CO, and CO2 values were examined. In the simulations using the simplified GTM-120 geometry, the combustion chamber outlet temperatures at 80 k, 100 k, and 120 k rpm were compared with the experimental results in the literature by changing the ratio of fuel and air for the validation study. Standard k-ε and Discrete Ordinates were chosen for turbulence and radiation models, respectively. In the study conducted with the non-premixed combustion model, simulations were made at O2 rates of 21%, 19%, 17%, and 15% for the formation of colorless distributed combustion conditions. For enrichment with hydrogen, H2 was added from 0% to 50% with 10% increments. HEF (Hydrogen energy fraction) has been used as the hydrogen addition rate. In light of the analyses made, it was seen that the colorless distributed combustion regime reduced the flame temperature from 2292 K to 1912 K and the NOx amount from 2.85 ppm to 0.0086 ppm. On the other hand, it is seen that this regime increased the CO emissions from 6.9 ppm to 38.67 ppm and the mole ratio of CO2 from 6.25% to 6.94%. It was observed that these findings prepared the ground for increasing the amount of hydrogen in the fuel, which increases the flame temperature and, accordingly, NOx emissions. By adding 50% H2 to kerosene under standard atmospheric conditions, the average NOx emission at the combustion chamber outlet increased from 2.85 ppm to 7.97 ppm, while the average CO emission decreased from 6.9 ppm to 1.08 ppm. As a matter of fact, by the addition of 50% H2 to the kerosene with air that contains 15% O2, which is the most extreme combustion situation in this study, the flame temperature decreased to 1912 K, and NOx rose to 0.0713 ppm. However, the CO emissions at the combustion chamber outlet decreased to 1.83 ppm and the CO2 ratio to 3.48%. Thus, with this study, it has become possible to use hydrogen as an alternative fuel in a real engine geometry by using CDC and hydrogen enrichment together, both in terms of pollutant emissions and combustion performance.

Characteristics of Premixed Flames of Hot and Diluted Mixtures

ABSTRACT Internal recirculation of product gases and mixing with fresh reactants, to form a hot and diluted combustible mixture, has been demonstrated to result in lower pollutant emissions. With sufficient amount of product gas recirculation, the mixture can reach above its auto-ignition temperature with low oxygen mass fractions (<4 wt%), resulting in distributed reactions instead of a flame. Some of the technologies with this feature are Moderate or Intense Low oxygen Dilution (MILD) combustion, FLameless OXidation (FLOX) and Colorless Distributed Combustion (CDC). However, if the amount of product gas recirculation is not enough and the auto ignition condition is not reached, premixed reactants of the hot and diluted mixture can be established. In this study, characteristics of such hot and diluted mixtures is studied which include the ignition delay time and the flame attributes such as laminar flame speed, S L and thermal flame thickness, . A perfectly stirred reactor (PSR) using the hot and diluted mixture as the inlet was further examined to simulate the main combustor and combustion efficiency η, and CO and NOx emissions were obtained. The investigations are performed at different product gas recirculation amounts , equivalence ratios ϕ and operating pressures P, using chemical kinetic simulations. The thermo-chemical state of the recirculated product gases was obtained using a PSR with varying residence times (τ) to simulate different “burntness” levels (smaller τ simulates lower “burntness” level). The hot and diluted mixture temperature is higher, while oxygen and fuel mole fraction are lower for higher ξ, ϕ, P, and τ. Radical species’ mole fraction is higher for higher ξ, ϕ, and lower P, τ. Lower is observed for higher ξ, ϕ, P and smaller τ. Higher S L is observed for higher ξ, ϕ and lower P. Lower is observed for higher ξ, ϕ and P. S L and are found to be higher or lower with variation in τ, depending on ξ, ϕ and P, owing to variation in temperature and radicals’ concentration. Higher η is observed for higher ξ, P, lower ϕ, τ, and higher residence time of gases (τ reac ) inside the main combustor. Lower CO is observed with increase in ξ, P and decrease in τ, while exhaust NOx increases with ϕ and mostly remains unchanged with increase in ξ, while inconsistent trends are observed with change in P and τ.

Colorless distributed combustion characteristics of hydrogen/air mixtures in a micro combustor

Colorless distributed combustion technique enables less pollutants emissions for a fuel/oxidizer mixture to be combusted. In this study, colorless distributed combustion characteristics (CDC) of hydrogen/air mixtures were numerically investigated in a micro combustor to overcome the difficulties associated with micro combustion and to obtain a more uniform temperature profile on the outer wall. To that end, hydrogen/air combustion was simulated by using ANSYS/Fluent CFD code at a constant equivalence ratio (1.0) and thermal input (100 W) and different mixture inlet temperatures of 300 K, 600 K and 1000 K to seek colorless distributed combustion. CDC conditions were tried to be achieved by decreasing the oxygen concentration from 21 % to 9 % at an interval of 3 %. In conclusion, colorless distributed combustion conditions could be achieved at different O2 concentrations, irrespective of the mixture inlet temperature. At 300 and 600 K, such conditions did not make any positive contribution to the power output of the micro combustor. Nevertheless, at 1000 K and 9 % O2 concentration, all combustion and emission performance metrics were improved in a manner that could increase the power output of the micro power generator.

Numerical investigation of combustion and flame characteristics for a model solid oxide fuel cell performance improvement

This paper applies computational fluid dynamics techniques to investigate the flame characteristics in the afterburner of a model solid oxide fuel cell system. The main objective of this research is to analyze the flame characteristics to serve as a reference for performance improvement of the model solid oxide fuel cell. The generated swirl burner was modelled under conventional and colorless distributed combustion conditions. It is because recommended for obtaining a more uniform temperature distributions over the entire of the combustor, resulting in far enough temperature levels for the best performance of the fuel cell. In order to achieve distributed regime during methane combustion, oxygen concentration in the oxidizer was reduced through entrainment of the diluent mixture (90% N2 and 10% CO2). The parameters affecting the performance of a model solid oxide fuel cell were also numerically investigated. The temperature zones obtained from combustion were used to initiate and sustain the solid oxide fuel cell. Cell performance values for different temperatures were analyzed and interpreted. The maximum power value was obtained under distributed regime. While the maximum power points of the cells were gathered in the high performance region at 15% O2, lower power values were obtained at 21% O2 content.

H2 – CH4 blending fuels combustion using a cyclonic burner on colorless distributed combustion

H 2 – CH 4 mixture fuels can be promising for reducing carbon-based emissions. However, because of higher pollutant emission (such as NO X ) problems during hydrogen combustion, a new combustion method can be favorable. Colorless distributed combustion (CDC) is emerging here. CDC enables ultra-low pollutant emissions along with reduced flame instabilities, combustion noise, improved combustion efficiency, etc. Considering those benefits, methane and the hydrogen-enriched methane (60% CH 4 – 40% H 2 , 50% CH 4 – 50% H 2 , 40% CH 4 – 60% H 2 ) fuels have been consumed using a cyclonic burner providing more residence time at an equivalence ratio of 0.83 under distributed regime. For the modelings, Reynolds Stress Model (RSM) turbulence model, the assumed-shape with β-function Probability Density Function combustion model, and P-1 radiation model have been selected. To seek CDC, the oxygen concentration in the oxidizer was reduced with N 2 or CO 2 diluent from 21% O 2 to 13% O 2 at an interval of 2%. The air and the fuel temperatures were kept constant at 300 K. Besides, for seeking high-temperature air combustion (HiTAC) conditions the oxidizer temperature was changed to 600 K to simulate flue gas recirculation. The results showed that the temperature distributions changed to be more uniform considerably with a decrease in oxygen concentration for all cases. CDC also provided a considerable decrease in NO X and a favorable reduction in CO at a certain oxygen concentration. It has been concluded that CO 2 as the diluent was more effective for reducing temperature levels and NO X levels due to its greater heat capacity. • H 2 – CH 4 blending fuels combustion have been examined in a cyclonic burner. • Colorless distributed combustion (CDC) was sought for H 2 – CH 4 blending fuels. • Reduced oxygen concentration affected considerably all combustion characteristics. • Combustion characteristics were changed with increase in hydrogen content. • Ultra-low pollutant emissions were achieved under CDC.

Modelling of the gas-turbine colorless distributed combustion: An application to hydrogen enriched – kerosene fuel

This study examines hydrogen-enriched kerosene combustion under distributed regime in a gas turbine combustion chamber. With hydrogen enrichment, it is aimed at increasing combustion performance of those fuels. However, in this circumstance, it is obvious to increase the flame temperature with taking place hydrogen enrichment. Thus colorless distributed combustion (CDC), which is one of the advanced combustion techniques, can be suggested to control flame temperature with slowing down the reaction rate, resulting in ultra-low NOX emissions and more uniform temperature distribution with a broadened flame. For this purpose, the hydrogen-enriched kerosene fuels were examined by modeling a CFD code using the eddy dissipation concept, the radiation model (P-1) and the turbulence model (standard k-ε). In this way, the thermal fields and the NOX distributions have been obtained. The results showed that hydrogen enrichment increased the flame temperatures from about 2490 K to 2605 K at air-combustion conditions until 30% H2, resulting in the NOX levels predicted increased in the combustor. With reducing oxygen percentage the flame started to be the broadened one. The flame temperatures decreased, for instance, from about 2605 K to 2230 K at 15% O2 for the 30% H2 containing fuel. As a result of this, the NOX levels reduced from about 30 ppm to the values lower than 1 ppm in the combustor. It is concluded that increments in temperature and NOX levels with hydrogen can be suppressed with distributed regime, which enables that gas turbines can be operated at wider flammability limits with hydrogen enrichment.

Investigation of colorless distributed combustion regime using a high internal recirculative combustor

This study aims at investigating the combustion characteristics of methane and a hydrogen-rich fuel on distributed regime in a combustor with high internal recirculation as distributed regime can achieve with highly internal entrainment. The model validation was first taken place with the existing experimental results under non-reacting conditions, and it is demonstrated that the mean velocity profiles predicted are in satisfactorily good agreement with the existing data in the combustor. Then, the methane and the hydrogen-rich fuel were consumed at an equivalence ratio of 0.8 and a thermal power of 10 kW under distributed conditions which means that oxygen concentration in the oxidizer is reduced from 21% to 15% along with internal entrainment for each fuel used. The results showed that the velocity magnitudes increased in the combustor with decrease in oxygen concentration because of diluent introduction. Moreover, distributed regime enabled uniformly temperature field inside the combustor together with the ultra-low NO X values with introducing the diluent. In addition, one can say that the maximum temperature and the outlet NO X values (1800 K and 27.60 ppm at 21% O 2 – 1450 K and 0.05 ppm at 15% O 2 for methane, 2000 K and 32.47 ppm at 21% O 2 – 1700 K and 0.19 ppm at 15% O 2 for the hydrogen-rich fuel) were relatively higher while combusting the hydrogen-rich fuel compared to those of methane due to the presence of hydrogen. However, It is concluded that distributed regime provided uniformly temperature fields and ultra-low NO X levels even the hydrogen-rich fuel is used. • Colorless distributed combustion was investigated using a recirculative combustor. • External/internal recirculation zones were obtained in the combustor. • Velocity profiles were highly affected with highly internal recirculation. • More uniform thermal fields and ultra-low NO X emissions were achieved.

Colorless distributed combustion (CDC) effects on a converted spark-ignition natural gas engine

This study examines combustion and emission characteristics of a converted-engine, which suggests that a diesel engine with high compression ratio is transformed to a spark-ignition engine through natural gas addition coming from the intake port. The pollutant emissions such as NOX for the converted-engine pollutant is generally higher than that of a diesel-engine along with higher in-cylinder temperature. Those challenges can be overcome through new and novel combustion methods such as colorless distributed combustion (CDC). Colorless distributed combustion is a method that provides ultra-low pollutant emissions, a more uniform thermal field and reduced combustion noise along with mitigated flame instability. CDC is basically achieved by reducing oxygen concentration in the oxidizer. That decrease can be provided with recirculation of hot combustion products into the oxidizer. In order to achieve CDC, N2 as the diluent was entrained into the fuel-oxidizer mixture from 23 % to 13 % O2 concentrations by mass. The converted-engine representing a tractor engine operated at 2300 rpm and full load was investigated numerically to obtain its performance, combustion and emission characteristics under CDC conditions through ANSYS Forte 3-Dimensional analysis program. G-equation combustion model with methane chemical kinetic mechanism consisting of 29 species and 171 equations, and RANS k-e turbulence model were selected for all modelings. According to the results predicted, it is demonstrated that for the converted engine there has been an improvement of up to 29% in terms of performance. In addition, it is concluded that a 97.66% decrease in NOX emission has been achieved without reducing the combustion performance of the converted engine under CDC.

Effect of thermal input, excess air coefficient and combustion mode on natural gas MILD combustion in industrial-scale furnace

To promote the application of MILD scheme in furnace, the present work was performed to evaluate the effect of thermal input, excess air coefficient and combustion mode on the MILD combustion at high thermal level (thermal input varies from 600 to 786 kW); meanwhile, the heat from the utilized furnace was extracted by an annular water jacket and a corundum tube was installed in the furnace tube to create the high temperature atmosphere. The results were presented on flow field using numerical simulations and on global flame signatures, temperature/species distribution and exhaust emissions using experiments. Better combustion performance of MILD mode than conventional swirling diffusion combustion mode was verified in the present work. The positive effect of increased thermal input and excess air coefficient on gas recirculation, thermal uniformity and exhaust emissions reduction was observed. In the MILD combustion regime, the switching from nonpremixed to premixed combustion resulted in lower oxygen level and reduced reaction temperature in the combustion region, also much lower NOx emissions and slightly higher CO emission were observed. At 786 kW thermal input, the reaction zone covers the entire combustion chamber; meanwhile, low NOx emission level of 32 ppm@3%O2 are achieved for premixed MILD combustion mode. The MILD regime was established for natural gas fueled furnace with colorless distributed combustion and suppressed NOx and CO emissions.

Large-eddy simulation/filtered mass density function of non-premixed and premixed colorless distributed combustion

Turbulent mixing and combustion in non-premixed and premixed colorless distributed combustion (CDC) systems are simulated with a hybrid large-eddy simulation/filtered mass density function (LES/FMDF) model. The CDC systems have low (NOx and hydrocarbon) emissions, stable combustion, and low-pressure drop and noise. They are also characterized by distributed combustion as opposed to thin flamelets, seen in ordinary combustion systems. The two parts of hybrid LES/FMDF model, i.e., the Eulerian gas dynamics finite-difference solver and the Lagrangian stochastic FMDF solver are shown to be fully consistent and computationally robust and accurate for both nonreacting and reacting flows. The LES/FMDF results are also shown to be in good agreement with the available experimental data. The numerical results show that the variation in inflow air temperature or the air to fuel jet momentum ratio has a significant effect on the turbulent flow, mixing, and combustion. They also indicate the importance of the flow configuration in the CDC combustors.

Identification of coherent structures in distributed swirl-stabilized wet combustion

Towards achieving sustainable and decarbonized power, biomass to electricity is an attractive pathway. To that end, the humidified gas turbine cycle is a promising technology. Recirculated steam which contains low-grade heat can be used to replace part of the air flow. This immediately reduces power loss in the compressor and increases specific power output, benefiting higher electrical efficiency compared to dry cycles. With high steam content, wet combustion leads to the so-called flameless combustion (FC) or colorless distributed combustion (CDC) which is presently investigated using large eddy simulation and a detailed finite rate chemistry method. Further insight regarding the coherent structures is obtained. Proper orthogonal decomposition method is applied on both the velocity and the heat release field aiming to explore the in-depth dynamic of flow-flame interaction in a swirl burner. Our results are the first reporting two distinct sets of helical coherent structures. A higher frequency mode or structure at Strouhal number St ~ 0.7 is caused by the vortex shedding, and a lower frequency mode at St ~ 0.1 corresponds to the off-central motion of an intermittently occurring precessing vortex core (PVC). With high steam content (hence very distributed reaction regime), more frequent occurrence of the marginally stable PVC is observed. The wet flame local extinction is evidenced to be an important driver towards the promotion and suppression of the PVC structure. Compared to the very energetic flow-flame interaction in conventional flames, FC or CDC flames undergo less fluctuations.

Design and investigation of a central air jet flameless combustor

Due to strict regulations on harmful emissions such as NOx and CO, many novel combustion techniques have been investigated in order to decrease emissions from gas turbines. One of these novel techniques is colorless distributed combustion (CDC) or flameless combustion (FC). CDC has been examined under reacting conditions using ANSYS Fluent at 27 different operating cases for the aim of developing design equations and a 3D flameless combustion map that includes the effect of air diameter, confinement size and excess air ratio on recirculation ratio to allocate flameless combustion regions. A simple lab scale combustor with central air jet arrangement is employed for the analysis. Eddy dissipation concept (EDC) either global or with detailed mechanism (DRM19) fails to predict the effect of excess air ratio in flameless combustion. A numerical approach for calculating the recirculation ratio is presented. As the air jet diameter decrease 40% there is about 54% increase in the recirculation ratio and about 90% decrease in the NOx emissions. It is found that above a recirculation ratio of 3 there is an insignificant effect on NOx emissions. Reducing the combustor diameter by a constant value for a given air diameter is found to result in a decrease in the recirculation ratio and the percentage of decrease is higher at high thermal intensities combustors. Moreover, it is observed that as the combustor diameter decrease to half of its value at constant air jet diameter and constant excess air ratio, there is an 80–90% increase in the NOx emissions inside the combustor. Furthermore, design equation is obtained to predict the appropriate air diameter that if combined with the available combustor diameter achieves a minimum recirculation ratio suitable to sustain flameless combustion.

Performance and emissions of camelina oil derived jet fuel blends under distributed combustion condition

Renewable biofuels offer good performance characteristics as fossil liquid fuels while reducing the emission of pollutants and greenhouse gases helping promote sustainability. Colorless distributed combustion (CDC) for gas turbine applications offers robust means of providing understanding of performance and emissions characteristics using liquid biofuels. In this study, performance and emission characteristics are reported from the combustion of a liquid biofuel, hydroprocessed esters and fatty acids synthetic paraffinic kerosene (HEFA-SPK), derived from camelina oil. Currently HEFA-SPK is approved in up to a 50% blend with jet fuel (JP-8) for commercial and military applications. Experiments conducted on various blended ratios of HEFA-SPK to JP-8 are reported, ranging from 100% JP-8 to 100% HEFA-SPK. Experiments were carried out on a swirl-assisted burner at thermal intensity of 5.7 MW/m-atm, with oxidizer preheated and fuel pre-vaporized to 600 K. Distributed combustion conditions were achieved by reducing the oxygen concentration in the oxidizer through adding diluent gases of N2 and CO2 simulating internal entrainment of hot reactive gases. The effect of blended fuel properties on flame structure (through OH* chemiluminescence) and emissions are reported here. HEFA-SPK and JP-8 showed ultra-low NO emission under CDC, ranging between 2 and 2.5 ppm. HEFA-SPK showed significant reduction in CO of up to 50% compared to JP-8 fuel. This study revealed that camelina oil derived fuel produced from biomass can be used as a mixture component with JP-8, maintaining high combustion performance while providing ultra-low CO emission. The biofuels also help support reduction of lifecycle CO2 emission.

Flowfield impact on distributed combustion in a swirl assisted burner

Colorless distributed combustion (CDC) is a novel method to enhance flame stability and thermal field uniformity, increase combustion efficiency and reduce pollutants emission, including noise. CDC is achieved through the use of a carefully prepared oxidizer mixture of reduced oxygen concentration through added high temperature reactive species. In this study, a partially premixed, swirl assisted cylindrical combustor utilized a propane-air flame with either nitrogen or carbon dioxide gas in order to reduce the oxygen concentration of the oxidizer. OH* chemiluminescence signatures were used to determine transition to distributed combustion condition. The results showed transition to CDC at approximately 15% using N2, and 17% using CO2 dilution. Emission of NO and CO were determined under conditions approaching CDC. NO levels of only 2 or 1 ppm were achieved using N2 or CO2 dilution, respectively under CDC condition. In order to determine how the flow velocity structure and eddy size effect the stability and emissions a high speed (3 kHz) particle image velocimetry (PIV) system was used. Increase in dilution enhanced both the radial and axial mean and fluctuating velocities under CDC that foster mixing. Additionally, the Kolmogorov length decreased with increase in dilution resulting in smaller eddy size particularly in the swirl lobe region, which enhanced turbulent dissipation that resulted in lower peak temperatures and reduced thermal NOx emission. Investigation of Reynolds stress showed that dilution with CO2 provided stronger impact on stress than N2 due to the increased density and reduced viscosity of CO2.

Flame structure and emission signature in distributed combustion

Colorless distributed combustion (CDC) combustion technology offers significant advantages of ultra-low pollutants emission, stable operation and improved pattern factor for high intensity stationary gas turbines applications. Detailed knowledge on distributed combustion behavior is required to further deploy this technology. This paper reports the evolution of swirl flame shape, flame expansion and pollutants emission characteristics using propane, methane, and 20% and 40% hydrogen enriched methane fuels. The entrainment of hot reactive gases was simulated by diluting the inlet air stream with inert nitrogen or carbon dioxide. OH* chemiluminescence signatures, captured at different dilution levels, manifested gradual reduction of flame luminosity when CDC was approached. Flame boundaries derived using image threshold technique helped to visualize the broadened reaction zone under CDC conditions, which had much reduced chemiluminescence signal intensity. The rms to mean OH* signal variation at different dilution levels were analyzed to detect the initiation of CDC. The higher flame lift-off observed with CO2 dilution was due to higher heat capacity of CO2, resulting in greater flame speed reduction. Flame expansion, evaluated from the area encompassed within the flame boundary at different dilution levels showed a power-law behavior with both the diluents. Expansion of 4 to 5 times of initial flame volumes was observed under CDC using CO2 and N2 dilution, respectively. Significant reduction of NO and CO emission achieved under CDC was due to reduction of overall flame temperature, hotspot mitigation, and widened reaction zone occupying large flame volume, which makes it favorable for many practical combustor applications.

Swirl assisted distributed combustion behavior using hydrogen-rich gaseous fuels

Controlled entrainment of hot reactive gases into the fresh oxidizer mixture prior to ignition is critical for achieving colorless distributed combustion (CDC) condition that results in a uniform thermal field, ultra-low emissions, enhanced flame stability, low noise and mitigation of combustion instability. This paper examines the fuel flexibility of colorless distributed combustion in a swirl-stabilized burner using hydrogen enriched methane gas. Distributed combustion condition was achieved by reducing the oxygen concentration in the oxidizer through addition of either N2 or CO2. Three different gas fuel compositions were investigated. Hydrogen enriched methane in various concentrations in the presence of CO2 and N2 represented coke oven gas. Results on the effect of fuel properties on global flame structure and emissions are reported here. The effect of equivalence ratio on flame stability and emissions was also investigated to determine the impact of air dilution with no change in oxygen concentration. The results showed that for the different gas compositions investigated, transition to distributed combustion conditions occurred at oxygen concentrations of 10–12% with N2 and 13–15% with CO2 as the diluent gas. Increase in hydrogen concentration (or decrease in methane concentration) in the gas mixture resulted in transition to distributed condition at reduced oxygen concentration. The NO emission decreased considerably (to less than 1 ppm) as compared to normal air combustion under the distributed combustion condition for all the gas compositions examined using the different diluents. The emission of CO decreased gradually when approaching distributed conditions, and then increased slightly when the condition moved toward the lower flammability limit. Single digit ppm CO levels were achieved with CDC under nitrogen dilution while carbon dioxide dilution resulted in slightly higher CO emission that was attributed to the dissociation of CO2 at high temperatures. The propensity for flashback with high hydrogen content gas was eliminated under distributed condition and provide wider flame stability limits. These results demonstrate fuel flexibility of diluted hydrogen rich methane gas (akin to coke oven gas) under distributed combustion that provided enhanced stability and ultra-low emissions.

Hydrogen concentration effects on swirl-stabilized oxy-colorless distributed combustion

Oxy-colorless distributed combustion is a novel combustion technique to achieve more uniform and controlled thermal field, enhance flame stability, and reduce emissions including combustion noise. Colorless distributed combustion (CDC) conditions was achieved through controlled entrainment of hot reactive gaseous species. This allowed for reduction of oxygen concentration to provide improved mixture preparation for distributed combustion with the reaction zone distributed across the entire volume of the combustor. In this study, oxy-colorless distributed combustion was examined for fuel flexibility using different mixtures of methane and hydrogen diluted with 10% each of N2 and CO2 to represent low heating value fuels. The oxidizer used was an O2-CO2 mixture that eliminated the formation of NOx from the combustor. The evolved OH* chemiluminescence signatures were recorded for various hydrogen concentrations in the fuel at equivalence ratios in the range of 0.6–0.9 under distributed combustion condition. At Φ = 0.9, transition to CDC initiated at oxygen concentrations of 17%, 19% and 21% for fuels having a hydrogen concentration of 60%, 50% and 40% (on volume basis), respectively. Differences in transition point were attributed to higher flame speed associated with more hydrogen in the fuel. Gaseous fuel having high hydrogen concentration provided more distributed OH* flame structures at lower oxygen concentration while the flame stability enhanced at higher oxygen concentration. Oxy-colorless conditions provided only 10–30 ppm CO even at high oxygen concentration and high (0.9) equivalence ratio. The results showed fuel flexibility from different heating value gaseous fuels having high hydrogen content. The results provided very low CO emission and enhanced flame stability under oxy-colorless distributed combustion for the low heating value fuels using O2-CO2 mixture as the oxidant.

Effect of naphthalene addition to ethanol in distributed combustion

Naphthalene as a fuel additive to ethanol was examined in a swirl combustor with the objective to obtain efficient burning and ultra-low emissions using different heating value fuels. Naphthalene is a polyaromatic compound often regarded as a waste fuel that results in high levels of pollutants emission. The effectiveness of naphthalene as a fuel additive to ethanol on NO and CO emissions and stability was determined. The naphthalene concentration was varied from 0 to 0.4 mol/L in ethanol, corresponding to a heating value increase of 8.8% on a volumetric basis (or 5.7% on mass basis). Emissions data were reported for Colorless Distributed Combustion (CDC) using N2/CO2 dilution, at equivalence ratios (Φ) of 0.9 and 0.7. The data under normal fuel–air combustion conditions also reported, clearly showing the benefits of CDC. NO and CO emission below 1 and 6 ppm, respectively, were achieved under CDC conditions for each of the naphthalene-ethanol fuels examined. The results at both the equivalence ratios showed lower NO emission with increase in naphthalene concentration, which provided higher heating value fuel mixture. The CO emission was also low and remained negligibly unchanged with change in naphthalene concentration and equivalence ratio. The results show the use of naphthalene addition to ethanol for increased heating value fuel with ultra-low emissions and higher stability under distributed combustion conditions.

Fostering distributed combustion in a swirl burner using prevaporized liquid fuels

Colorless Distributed Combustion (CDC) has presented itself as an environmentally friendly combustion method with significant benefits on ultra-low emissions, uniform thermal field (pattern factor), reduced noise, mitigation of instability and enhanced flame stability. CDC requisites include controlled entrainment of hot reactive species from within the combustor and their subsequent mixing with fresh reactants to form a low-oxygen concentration high-temperature oxidizer prior to ignition. This mixture results in distributed reaction over the entire combustion volume. To date, most of the CDC research efforts have focused on gaseous fuels only. In this paper, conditions fostering distributed combustion using JP-8 and alternative ethanol fuels. Data revealed that transition to CDC can be achieved at oxygen concertation of approximately 9.5% for both the fuels. This oxygen concentration was determined based on reaction field uniformity as identified from OH∗ chemiluminescence signatures. Under distributed combustion, emissions were substantially reduced by some 95% to result in NOx emissions of less than 2 PPM with minimal impact on CO emission from both JP-8 and ethanol fuels at an equivalence ratio of 0.9, with even lower emissions at lower equivalence ratios. Combining the data obtained with liquid fuels with those from gaseous fuels revealed that, regardless of the fuel used, the oxygen concentration at which CDC prevailed can be predicted based on mixture temperature within a range of 0.75%. This knowledge enables designing a combustor to achieve CDC regardless of the fuel used. The data are useful in the design and development of fuel flexible CDC for high combustion intensity gas turbine applications.

Flame fluctuations in Oxy-CO2-methane mixtures in swirl assisted distributed combustion

Swirl assisted oxy-methane combustion investigated here has focused on the impact of carbon dioxide dilution. Carbon dioxide dilution is essential as it lowers the flame adiabatic temperature and flame speed of oxy-fuel combustion leading to a more stable operation. However, carbon dioxide dilution may result in flame fluctuations, under fuel lean conditions. Experiments using a swirl burner examined the flame fluctuations with different amounts of CO2 dilution, with the goal to achieve stable oxy-CO2-methane combustion via colorless distributed combustion (CDC). This novel CDC technique has shown significant performance gains in gaseous fuel-air flames on emission reduction, enhanced thermal field uniformity, noise reduction, and improved flame stability. Results have shown that for oxy-CO2-fuel combustion, most of the heat release fluctuations had frequencies below 60Hz. The results obtained revealed that increase in CO2 dilution increased the magnitude of flame fluctuation till a maximum of 28% O2 concentration in the oxidizer. Further increase in dilution gases and reduction in O2 concentration led to a more stable flame, along with more favorable conditions for colorless distributed combustion. Data analysis of flame images revealed the source of oscillations under different conditions. At maximum fluctuations with 28% O2, the flame oscillated between two modes leading to unstable operation and large fluctuation in the heat release signal. Decrease in oxygen concentration to below 27%, promoted a more stable flame behavior, occupying a larger flame volume, which is characteristic of distributed combustion. Further reduction in O2 led to flame blow off at around 21% O2. This work outlines the importance CO2 dilution in oxy-fuel flames for achieving stable distributed combustion and mitigate the increased oscillations normally encountered in higher oxygen concentration flames.