Emissions and wall temperatures for lean prevaporized premixed gas turbine combustor

Experimental studies are carried out for investigating emission and wall temperature for traditional gas turbine combustor converted to lean premixed prevaporized (LPP) combustor. Vortex chamber, air preheating system, flat flame burner and inlet temperature control system are designed. Vortex chamber was maintained at the main air inlet port for controlling secondary air flow rate and wall temperature. Kerosene/air mixture temperature at exit from burner and entering combustion chamber was kept constant at 650 K for all runs. Special considerations were given for measuring NO_X, UHC, CO, local A/F ratio, flame temperature, exhaust gases temperature and wall temperature. For swirl and non swirl cases, secondary air ratio and primary zone air/fuel ratio were varied. The different operating parameters affecting flame temperature through it is affecting on local A/F ratio which is the main parameter for controlling flame temperature, emissions and walls temperatures. Flat flame burner and vortex chamber are useful tools for reducing emission and controlling walls temperatures. The inner liner wall temperatures are more affected by primary zone equivalence ratio while the outer liner wall temperatures are more affected by secondary air flow rate. Semi empirical correlations for NO_X, UHC and CO concentrations, exhaust gases temperature and maximum inner liner wall temperature are carried out. Good agreement between the measured and the calculated results are obtained. The present results are useful for further development of the traditional gas turbine combustor converted to LPP combustor.     

Entrainment effects on combustion and emission characteristics of turbulent non-premixed ammonia/air and methane/air swirl flames through a developed perforated burner

A fuel/oxidizer mixture can be burned using a colourless distributed combustion (CDC) process to obtain low emissions and homogeneous combustion. As an alternative way, a perforated burner can be designed to achieve homogeneous combustion and low emissions without changing the combustion performance by having entrainment effects on the combustion chamber. In this study, computational fluid dynamics (CFD) 3D modelling was performed in a perforated burner for ammonia/methane fuels in order to obtain the distributed regime and focus on the entrainment effects. In numerical analysis, the eddy break-up was used as combustion model, k-Ɛ as turbulence model, and P-1 as radiation model. In this study, 10% and 20% entrainment rates were provided from the flame holder wall of the perforated burner. The effects of entrainment rates on temperature, velocity, and NO_X emission values were examined. According to the results, when the entrainment rate was increased from 10% to 20%, the overall temperature values of ammonia and methane combustion slightly increased by approximately 1.0%, while on the other hand the maximum temperature levels in the near burner zone decreased by about 5.0%. The findings demonstrated that temperature and velocity distributions got more uniform and the flame zones became thinner. This provided a more colourless and invisible flame appearance. In this way, an improvement in the thermal field has been achieved. In conclusion, when the effect of the distributed regime on NO_X emission levels was examined, it has been noted that entrainment effects enable the achievement of low emission levels (approximately 9.0%).     

Combustion characteristics in oil-vaporizing sustained by radiant heat reflux enhanced with higher porous ceramics

Liquid vaporizing combustion in porous ceramic burner has fine flame stability and characteristic of low emission. On the other hand, vaporization control has been seldom mentioned. In this work, kerosene vaporizing type combustor equipped with a porous ceramic plate, which has the porosity of 85%, is developed in order to enhance a rate of vaporization of the liquid fuel. The stability of combustion and NO_x emission characteristics were investigated in fuel vaporizing ceramic combustion. The plate burner is made of Al₂O₃ ceramic which has an optical-thickness of 0.54. The optically thin ceramics improved flame stability and enhances more fuel vaporization rate than optically thick ceramics. The thermal radiation energy from flame and the furnace walls can penetrate easily through the large pore of the ceramic plate. It is found possible to dispense the electric power for the fuel vaporization and the stable combustion is self-sustained by enhancement of vaporization, where the reflux rate of radiant heat was no less than 2% of the heating value.     

Liquid fuels-fired porous combustor-heater

A new concept of a liquid fuels-fired porous combustor-heater (LPCH) without atomization is proposed. Basic experimental study was conducted with liquid kerosene on a down-flow LPCH to prove its concept and evaluate its performance. Effects of dominating parameters, i.e. mass flow rate of the cooling water, equivalence ratio and input thermal power, on temperature profiles within the LPCH, thermal efficiency and pollutant emissions (CO and NO_X) under different conditions have been investigated and compared with those of the conventional free flame combustion system. The LPCH proved very effective in increasing thermal efficiency (up to 28% with respect to the conventional system) with a manageable level of CO emissions but low NO_X emissions. The dominating parameters have a significant effect on flame location, combustion temperature and pollutant emissions. Operating the LPCH at optimized operating conditions is important for the requirement of low CO emissions without compromising thermal efficiency and NO_X emissions. Practical applications of this LPCH are suggested.     

Nitrogen dilution effect on flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The nitrogen dilution effect on flame stability was experimentally investigated in a lifted non-premixed turbulent hydrogen jet with coaxial air. Hydrogen gas was used as the fuel and coaxial air was injected to initiate flame liftoff. Hydrogen was injected into an axisymmetric inner nozzle (d_F = 3.65 mm) and coaxial air jetted from an axisymmetric outer nozzle (d_A = 14.1 mm). The fuel jet and coaxial air velocities were fixed at u_F = 200 m/s and u_A = 16 m/s, while the mole fraction of the nitrogen diluent gas varied from 0.0 to 0.2 with a 0.1 step. For the analysis of the flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF was performed. The stabilization point was in the region of the flame base with the most upstream region and was defined as the point where the turbulent flame propagation velocity was found to be balanced with the axial component of the local flow velocity. The turbulent flame propagation velocity increased as the nitrogen mixture fraction decreased. The nitrogen dilution makes the flame structure more premixed. That is, the stabilization mechanism shifts from edge flame propagation based mechanism toward premixed flame propagation based mechanism. We concluded that the turbulent flame propagation velocity was expressed as a function of the turbulent intensity and the axial strain rate, even though the mole fraction of the nitrogen diluent varied.     

The formation of NOx from nitrogen containing materials in diffusion flames

The conversion of fuel-N species to NO_x in diffusion flames has been studied by adding acetonitrile to the fuel flow for a methane–Oxygen–argon diffusion flame burning in excess oxygen. It is shown that the conversion is significantly lower than that obtained in a ‘corresponding’ pre-mixed flame, although the observed concentrations of NO_x are still much higher than the appropriate thermodynamic equilibrium concentrations. The effect of initial concentration of acetonitrile, flame temperature and amount of excess oxygen on the conversion can all be explained in terms of the basic structure of a diffusion flame. This enables molecular nitrogen to be formed in the reducing atmosphere which exists on the fuel side of the flame through reaction of cyanide radicals with nitric oxide. The latter diffuses back from the oxygen side of the flame where it is formed, but the overall result is that a proportion of the fuel-N is converted to molecular nitrogen before it can be converted to nitric oxide.     

Experimental measurements of the NOx and CO concentrations operating in oscillatory and non-oscillatory burning conditions

The effects of combustion driven acoustic oscillations in carbon monoxide and nitrogen oxides emission rates of a combustor operated with liquefied petroleum gas (LPG) were investigated. Because the fuel does not contain nitrogen, tests were also conducted with ammonia injected in the fuel, in order to study the formation of fuel NO_x. The main conclusions were: (a) the pulsating combustion process is more efficient than the non-pulsating one and (b) the pulsating combustion process generates higher rates of NO_x, with and without ammonia injection, as shown by CO and NO concentrations as function of the O₂ concentration. An increase in the LPG flow rate, keeping constant the air to fuel ratio, increased the acoustic pressure amplitude and the frequency of oscillation. The injection of ammonia had no influence on either pressure amplitude or frequency.     

Effects of injection angles on combustion processes using multiple injection strategies in an HSDI diesel engine

Effects of injection angles and injection pressure on the combustion processes employing multiple injection strategies in a high-speed direct-injection (HSDI) diesel engine are presented in this work. Whole-cycle combustion and liquid spray evolution processes were visualized using a high-speed video camera. NO_x emissions were measured in the exhaust pipe. Different heat release patterns are seen for two different injectors with a 70-degree tip and a 150-degree tip. No evidence of fuel-wall impingement is found for the first injection of the 150-degree tip, but for the 70-degree tip, some fuel impinges on the bowl wall and a fuel film is formed. For the second injection, a large amount of fuel deposition is observed for the 70-degree tip. Weak flame is seen for the first injection of the 150-degree tip while two sorts of flames are seen for the first injection of the 70-degree tip including an early weak flame and a late luminous film combustion flame. Ignition occurs near the spray tip in the vicinity of the bowl wall for the second injection events of the 150-degree tip, however, it is near the injector tip in the central region of the bowl for the 70-degree tip. The flame is more homogeneous for the 150-degree tip with higher injection pressure with little soot formation similar to a premixed-charge-compression-ignition (PCCI) combustion. For other cases, liquid fuel is injected into flames showing diffusion flame combustion. More soot luminosity is seen for the 70-degree tip due to significant fuel film deposition on the piston wall with fuel film combustion for both injection events. Lower NO_x emissions were obtained for the narrow-angle injector due to the rich air–fuel mixture near the bowl wall during the combustion process. Increasing injection pressure leads to increased NO_x emissions for both injection angles because of the relatively leaner and faster combustion process with higher in-cylinder temperature for the increased injection pressure.     

Experimental investigation of laminar LPG–H2 jet diffusion flame with preheated reactants

This paper presents an experimental investigation of the effect of H₂ addition on flame length, soot free length fraction (SFLF), flame radiant fraction, gas temperature and emission level in LPG–H₂ composite fuel jet diffusion flame for two preheated cases namely, (i) preheated air and (ii) preheated air and fuel. Results show that the H₂ addition leads to a reduction in flame length which may be caused due to an increased gas temperature. Besides this, the flame length is also observed to be reduced with increasing reactants temperature. The soot free length fraction (SFLF) increases as H₂ is added to fuel stream. This might have been caused by decrease in the C/H ratio in the flame and is favorable to attenuate PAH formation rate. Interestingly, the SFLF is observed to be reduced with increasing reactants temperature that may be due to reduction in induction period of soot formation caused by enhanced flame temperature. Moreover, the decreased radiant heat fraction with hydrogen addition is pertinent with the reduction in soot concentration level. The reduction in NO_x emission level with H₂ addition to the fuel stream is also observed. On the contrary, NO_x emission level is found to be enhanced significantly with reactant temperature that can be attributed to the increase in thermal NO_x through Zeldovich mechanism.     

Prediction of a high swirled natural gas diffusion flame using a PDF model

The paper deals with the numerical prediction of a high swirling non-premixed confined natural gas diffusion flame in order to predict the pollutant emissions NO_x using the PDF model coupled with the Reynolds stress model (RSM). A chemical equilibrium model in conjunction with the assumed shape of the PDF is adopted. The chemical combustion reactions are described by nine species and eight reactions [Westbrook CK, Dryer FL. Chemical kinetic modelling of hydrocarbon combustion. Progr Energy Combust Sci 1984;10:1-57]. The PDF of the mixture fraction is described with a β-function. In order to predict the NO_x emissions, a NO_x post-processor of the Fluent code has been performed. The concentration of O and OH radicals are obtained assuming the partial-equilibrium assumption and using a PDF in terms of temperature. The numerical simulation of various factors influencing the combustion process are examined and compared favourably with experimental results.     

Biodiesel engine performance and emissions in low temperature combustion

Engine performance and emission comparisons were made between the use of soy, Canola and yellow grease derived B100 biodiesel fuels and an ultra-low sulphur diesel fuel in the high load engine operating conditions. Compared to the diesel fuel engine-out emissions of nitrogen oxides (NO_x), a high-cetane number (CN) biodiesel fuel produced comparable NO_x while the biodiesel with a CN similar to the diesel fuel produced relatively higher NO_x at a fixed start of injection. The soot, carbon monoxide and un-burnt hydrocarbon emissions were generally lower for the biodiesel-fuelled engine. Exhaust gas recirculation (EGR) was then extensively applied to initiate low temperature combustion (LTC) mode at medium and low load conditions. An intake throttling valve was implemented to increase the differential pressure between the intake and exhaust in order to increase and enhance the EGR. Simultaneous reduction of NO_x and soot was achieved when the ignition delay was prolonged by more than 50% from the case with 0% EGR at low load conditions. Furthermore, a preliminary ignition delay correlation under the influence of EGR at steady-state conditions was developed. The correlation considered the fuel CN and oxygen concentrations in the intake air and fuel. The research intends to achieve simultaneous reductions of NO_x and soot emissions in modern production diesel engines when biodiesel is applied.     

Peach and apricot stone combustion in a bubbling fluidized bed

In this study, a bubbling fluidized bed combustor (BFBC) of 102 mm inside diameter and 900 mm height was used to investigate the combustion characteristics of peach and apricot stones produced as a waste from the fruit juice industry. A lignite coal was also burned in the same combustor. The combustion characteristics of the wastes were compared with that of a lignite coal that is most widely used in Turkey. On-line concentrations of O₂, CO, CO₂, SO₂, NO_X and total hydrocarbons (C_mH_n) were measured in the flue gas during combustion experiments. By changing the operating parameters (excess air ratio, fluidization velocity, and fuel feed rate), the variation of emissions of various pollutants was studied. Temperature distribution along the bed was measured with thermocouples.During the combustion tests, it was observed that the volatile matter from peach and apricot stones quickly volatilizes and mostly burn in the freeboard. The temperature profiles along the bed and the freeboard also confirmed this phenomenon. It was found that as the volatile matter of fruit stones increases, the combustion takes place more in the freeboard region.The results of this study have shown that the combustion efficiencies ranged between 98.8% and 99.1% for coal, 96.0% and 97.5% for peach stone and 93.4% and 96.3% for apricot stones. The coal has zero CO emission, but biomass fuels have very high CO emission which indicates that a secondary air addition is required for the system. SO₂ emission of the coal is around 2400–2800 mg/Nm³, whereas the biomass fuels have zero SO₂ emission. NO_X emissions are all below the limits set by the Turkish Air Quality Control Regulation of 1986 (TAQCR) for all tests. As the results of combustion of two biomass fuels are compared with each other, peach stones gave lower CO and NO_X emissions but the SO₂ emissions are a little higher than for apricot stones. These results suggest that peach and apricot stones are potential fuels that can be utilized for clean energy production in small-scale fruit juice industries by using BFBC.     

Effect of nozzle orifice geometry on spray, combustion, and emission characteristics under diesel engine conditions

Diesel engine performance and emissions are strongly coupled with fuel atomization and spray processes, which in turn are strongly influenced by injector flow dynamics. Modern engines employ micro-orifices with different orifice designs. It is critical to characterize the effects of various designs on engine performance and emissions. In this study, a recently developed primary breakup model (KH-ACT), which accounts for the effects of cavitation and turbulence generated inside the injector nozzle is incorporated into a CFD software CONVERGE for comprehensive engine simulations. The effects of orifice geometry on inner nozzle flow, spray, and combustion processes are examined by coupling the injector flow and spray simulations. Results indicate that conicity and hydrogrinding reduce cavitation and turbulence inside the nozzle orifice, which slows down primary breakup, increasing spray penetration, and reducing dispersion. Consequently, with conical and hydroground nozzles, the vaporization rate and fuel air mixing are reduced, and ignition occurs further downstream. The flame lift-off lengths are the highest and lowest for the hydroground and conical nozzles, respectively. This can be related to the rate of fuel injection, which is higher for the hydroground nozzle, leading to richer mixtures and lower flame base speeds. A modified flame index is employed to resolve the flame structure, which indicates a dual combustion mode. For the conical nozzle, the relative role of rich premixed combustion is enhanced and that of diffusion combustion reduced compared to the other two nozzles. In contrast, for the hydroground nozzle, the role of rich premixed combustion is reduced and that of non-premixed combustion is enhanced. Consequently, the amount of soot produced is the highest for the conical nozzle, while the amount of NO_x produced is the highest for the hydroground nozzle, indicating the classical tradeoff between them.     

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

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

Effect of HHO gas on combustion emissions in gasoline engines

Reducing the emission pollution associated with oil combustion is gaining an increasing interest worldwide. Recently, Brown’s gas (HHO gas) has been introduced as an alternative clean source of energy. A system to generate HHO gas has been built and integrated with Honda G 200 (197 cc single cylinder engine). The results show that a mixture of HHO, air, and gasoline cause a reduction in the concentration of emission pollutant constituents and an enhancement in engine efficiency. The emission tests have been done with varying the engine speed. The results show that nitrogen monoxide (NO) and nitrogen oxides (NO_X) have been reduced to about 50% when a mixture of HHO, air, and fuel was used. Moreover, the carbon monoxide concentration has been reduced to about 20%. Also a reduction in fuel consumption has been noticed and it ranges between 20% and 30%.     

Predictions of CO and NOx emissions from steam cracking furnaces using GRI2.11 detailed reaction mechanism – A CFD investigation

This investigation develops a three-dimensional Computational Fluid Dynamics (CFD) model to simulate the turbulent diffusion flame on the fire-side of the radiation section of a thermal cracking test furnace coupled with a non-premixed low NO_x floor burner. When this type of burners which uses the internal Flue Gas Recirculation (FGR) technique is coupled with large scale furnaces, both the turbulent mixing and chemical reaction rates are comparable and hence this should be considered in the model. Different combustion models are used to simulate the turbulence–chemistry interactions for this flame. The CFD model, based on the Eddy Dissipation Concept (EDC) combustion model coupled with the detailed GRI2.11 reaction mechanism, gives the most reasonable predictions compared with the available experimental data or empirical correlations for the diffusion flame in the thermal cracking test furnace, especially for the flame length and the CO and NO_x emissions.     

High-efficiency combustion of natural gas with 21-30% oxygen-enriched air

This investigation was aimed at studying the influence of 21-30% oxygen concentration on the heating rate, emissions, temperature distributions, and fuel (natural gas) consumption in the heating and furnace-temperature fixing tests. Increase in the oxygen concentration led to a more rapid heating rate and lesser fuel consumption due to lower levels of the inert gas (N₂). When the oxygen concentration was increased from 21% to 30%, the heating rate was increased by 53.6% in the heating test and the fuel consumption was reduced by 26.1% in the furnace-temperature fixing test. Higher oxygen concentrations yielded higher flame temperature; hence, the NO_x emission increased with increasing oxygen concentration. However, the increase of NO_x emission in the furnace-temperature fixing test was less than that in the heating test. Moreover, the NO_x emission was more sensitive to the excess oxygen at higher oxygen levels. The CO₂ concentration in the flue gas increased linearly with the oxygen concentration. Additionally, the temperature distributions became progressively nonuniform with increasing oxygen concentration because the convective heat transfer coefficient was altered.     

Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR

This work investigates partial HCCI (homogeneous charge compression ignition) combustion as a control mechanism for HCCI combustion. The premixed fuel is supplied via a port fuel injection system located in the intake port of DI diesel engine. Cooled EGR is introduced for the suppression of advanced autoignition of the premixed fuel. The premixed fuels used in this experiment are gasoline, diesel, and n-heptane. The results show that with diesel premixed fuel, a simultaneous decrease of NO_x and soot can be obtained by increasing the premixed ratio. However, when the inlet charge is heated for the improved vaporization of diesel fuel, higher inlet temperature limits the operational range of HCCI combustion due to severe knocking and high NO_x emission at high premixed ratios. Gasoline premixing shows the most significant effects in the reductions of NO_x and soot emissions, compared to other kinds of premixed fuels.     

Gas concentration and temperature in acoustically excited Delft turbulent jet flames

This paper shows the experimental results for changes in the flame structure when acoustic fields are applied in natural gas Delft turbulent diffusion flames. The acoustic field (pulsating combustion) generates zones of intense mixture of reactants in the flame region, promoting a more complete combustion and, consequently, lower pollutant emissions, increase in convective heat transfer rates, and lower fuel consumption. The results show that the presence of the acoustic field changes drastically the flame structure, mainly in the burner natural frequencies. However, for higher acoustic amplitudes, or acoustic pressures, a hydrogen pilot flame is necessary in order to keep the main flame anchored. In the flame regions where the acoustic field is more intense, premixed flame characteristics were observed. Besides, the pulsating regime modifies the axial and radial combustion structure, which could be verified by the radial distribution of concentrations of O₂, CO, CO₂, and NO_x, and by the temperature profile. The experiments also presented the reduction of flame length with the increase of acoustic amplitude.     

A numerical investigation into the anomalous slight NOx increase when burning biodiesel; A new (old) theory

Biodiesel is a notable alternative to petroleum derived diesel fuel because it comes from natural domestic sources and thus reduces dependence on diminishing petroleum fuel from foreign sources, it likely lowers lifecycle greenhouse gas emissions, and it lowers an engine's emission of most pollutants as compared to petroleum derived diesel. However, the use of biodiesel often slightly increases a diesel engine's emission of smog forming nitrogen oxides (NO_x) relative to petroleum diesel. In this paper, previously proposed theories for this slight NO_x increase are reviewed, including theories based on biodiesel's cetane number, which leads to differing amounts of charge preheating, and theories based on the fuel's bulk modulus, which affects injection timing. This paper proposes an additional theory for the slight NO_x increase of biodiesel. Biodiesel typically contains more double bonded molecules than petroleum derived diesel. These double bonded molecules have a slightly higher adiabatic flame temperature, which leads to the increase in NO_x production for biodiesel. Our theory was verified using numerical simulations to show a NO_x increase, due to the double bonded molecules, that is consistent with observation. Further, the details of these numerical simulations show that NO_x is predominantly due to the Zeldovich mechanism.