N2 Dilution Research Articles

Laminar flame speed of CO2 and N2 diluted H2/CO/air flames

The laminar flame speeds of H2/CO/air mixtures with N2 and CO2 dilution were experimentally and numerically studied over a wide range equivalence ratios (0.8–4.0) with N2 (dilution ratio 0–150 vol%) and CO2 (dilution ratio 0–66.7 vol%) dilution in the fuels. The experimentation was conducted using the Bunsen flame method incorporating the Schlieren technique, and the corresponding cases were computed using Chemkin-Pro with four different reactions mechanisms, viz, Li-Mech, Frassoldati-Mech, Davis-Mech and Keromnes-Mech. To eliminate the flame stretch effect on the laminar flame speed measurement based on the Bunsen flame method, a non-linear extrapolation approach was proposed and proved to give comparable accuracy with those from the stretch-corrected spherical flame and counterflow flame methods. A comparison between the experimental and numerical results showed that Davis-Mech and Li-Mech provided better estimates of the laminar flame speed. Keromnes-Mech over-estimated the laminar flame speed especially for fuel-rich mixtures. Meanwhile, Frassoldati-Mech under-estimated the laminar flame speed when the H2 content was high and the disparity decreased as the dilution ratio increased. Detailed analysis of the computation results indicated that the accurate prediction of H and OH mole fractions was crucial to the calculation of laminar flame speeds of H2/CO/air mixtures with N2 and CO2 dilution. CO2 exhibited a stronger dilution effect than N2 due to CO2's stronger thermal and chemical kinetic effects in reducing the flame temperature and radical concentrations. The preferential importance of the thermal, direct reaction and the third body effects caused by the dilution was addressed using numerical computation with two artificially created species. The results revealed that the thermal effect, i.e., the flame temperature reduction by the dilution, dominated the reduction of laminar flame speed, especially under high dilution conditions.

An experimental study of [formula omitted] dilution effects on [formula omitted]–[formula omitted] flame stabilization characteristics in a two-section porous medium

N2 dilution effects on CH4–O2 flame stabilization in a two-section porous medium burner were experimentally investigated for its potential application to the destruction of highly N2-diluted CF4. The burner was made up of two axially-stacked silicon carbide foams having different pore sizes. Various flame behaviors, such as stable flame, flashback, blowout, extinction and transient flames, were observed and, for the stable flame, temperature distributions and exhaust gas emissions were measured. Results showed that the flame was only stable in a belt-like region on the domain of fuel flow rate and oxygen concentration. The region boundaries, termed as O2-enriched and O2-deficient limits, were determined for various equivalence ratios. It was found that our burner configuration was able to sustain the submerged combustion even under the N2 dilution amounting to at least 10.4 times of CH4 flow rate. In addition, temperature measurements showed that the stable flames could have their peak temperatures higher than 1600°C, which is known to be a thermal destruction temperature of CF4. These results may indicate that porous media burners can practically applied in destructing highly N2-diluted CF4 with a reduced fuel consumption.

Experimental investigation on the combustion performance of N2 dilution in syngas non-premix combustion in humid air conditions

Diluted combustion is one of the main methods to reduce NOx emissions in syngas combustion. This work studies the performance of syngas non-premix combustion with N2 dilution under different air humidification conditions. OH-PLIF measurement is used to study the flame structure. Temperature and emission of the exhaust is also investigated in the experiment. The results show that in dry air conditions, as the amount of N2 dilution increases, the NOx emission is decreased obviously and CO emission remains lower than 10ppm. In middle air humidification conditions, NOx emission is lower than all dry air conditions, and CO emission still keeps low. However, the effect of N2 dilution is limited significantly. When in high air humidification conditions, as the amount of N2 dilution increases, CO emission rises significantly, whereas NOx emission changes little. According to the PLIF measurement, the highest flame temperature and the area of reaction zone are reduced to a low level under this condition. The combustion becomes incomplete and results in a significant emission of CO.

Effect OF CO2/N2 dilution on laminar burning velocity of liquid petroleum gas-air mixtures at elevated temperatures

The present experimental study reports the effect of CO2/N2 dilution on laminar burning velocity of premixed LPG (liquid-petroleum-gas)-air mixtures at elevated temperatures using a preheated mesoscale diverging channel technique. The experiments were carried out for a range of equivalence ratios varying from 0.8 < Φ < 1.3 with percentage dilution of the fuel component by volume (β) for CO2 varying from 10% < β < 30% and N2 varying from 10% < β < 40%. A power–law correlation has been obtained for the present experimental data as a function of percentage dilution, mixture temperature and equivalence ratio. It has been observed that an increase in dilution with CO2/N2 leads to an increase in temperature exponent (α). The increase in temperature exponent due to CO2 dilution is more pronounced as compared to N2 dilution case. A flame structure study has been carried out to understand the effect of mixture temperature and diluent using USC (University of Southern California) Mech II reaction mechanism.

Soot oxidation-induced fragmentation: Part 2: Experimental investigation of the mechanism of fragmentation

Experiments in a two-stage burner, with ethylene as the fuel, were conducted to further understand the mechanisms associated with the fragmentation of soot particles. Experiments were grouped in the following three categories, based on the main objective of the corresponding experiment: (i) the effect of different inert gases, Ar, He, N2 (ii) the effect of an increase in N2 dilution and O2 concentration (iii) and the effect of particle mobility size on the fragmentation. It was shown that the onset of the fragmentation depended on the presence of large aggregates in the flames. In addition, in the presence of aggregates, the rate of fragmentation was found to be inversely proportional to the peak temperature. It was postulated that at lower flame temperature where the soot burnout was delayed, O2 molecules were more likely to diffuse into the bridges between particles and burn them. Also, a role for OH oxidation during fragmentation was postulated by probability analysis, since fragmentation was experimentally shown to continue in the region of the flame where OH was present.

Effects of Buffer Gas Composition on Autoignition of Dimethyl Ether

Experimental and numerical studies are conducted on the thermal, chemical and dilution effects of buffer gas composition on autoignition of dimethyl ether (DME). The buffer gases considered are nitrogen (N2), a mixture of N2 and argon (Ar) at a mole ratio of 50%/50% and a mixture of Ar and carbon dioxide (CO2) at a mole ratio of 61.2%/38.8%. Experiments are performed using a rapid compression machine (RCM) at compressed pressure of 10 bar, equivalence ratio (φ) of 1, and compressed temperature from 670 K to 795 K. The N2 dilution ratio considered ranges from 36.31% to 55.04%. The experimental results show that buffer gas composition has little impact on the first-stage ignition delay. However, significant differences in the total ignition delay as a function of buffer gas composition are observed in the negative temperature coefficient (NTC) region. Compared to N2, N2/Ar (50%/50%) mixture decreases the total ignition delay by 31%. The chemical effects of buffer gas composition on the first-stage and total ignition delays are negligible. With increasing N2 dilution ratio, the first-stage ignition delay slightly increases, while a significant increase in the total ignition delay is observed. Moreover, the NTC behavior of total ignition delay is noted to become more pronounced at high N2 dilution ratio. The heat release during the first-stage ignition decreases as N2 dilution ratio increases. Results of numerical simulations with the Zhao DME mechanism over a wider range of temperature show good agreement with that of experiments. Further numerical simulations are conducted using pure N2, Ar and CO2 as buffer gases. Results indicate that the thermal effects are the dominant factor in low temperature and NTC regions. The chemical effects become pronounced in the NTC region, and the chemical effect of CO2 exceeds the thermal effect at the compressed temperature higher than 880 K.

Pyrolysis and combustion chemistry of tetrahydropyran: Experimental and modeling study

This paper reports new experimental and numerical data for the pyrolysis and combustion of tetrahydropyran (THP) – a model component for next-generation heterocyclic oxygenated fuels. Pyrolysis experiments were performed using a plug flow reactor at 170 kPa, over the temperature range 913–1133 K at residence times of approximately 0.5 and 0.2 s, with 90% and 96% N2 dilution, respectively. THP combustion was investigated in two premixed flat flame burners and in a shock tube. The first premixed flame burner was operated at 6.7 kPa and was used to study detailed flame structures. Two equivalence ratios (1.0 and 1.3) were investigated with a 78% argon dilution. Ethylene, 1,3-butadiene, formaldehyde, and acrolein were the most important intermediates at both pyrolysis and combustion conditions, while the yield of aromatic species was negligible under flame conditions. Laminar burning velocities of THP-air mixtures using the heat flux method were measured at 298, 358 and 398 K and equivalence ratios from 0.55 to 1.50. Finally, ignition delay times of THP–oxygen–argon mixtures were measured behind reflected shock waves at temperatures from 1350 to 1613 K, pressures from 885 to 914 kPa and mixtures containing 0.15–1% fuel at equivalence ratios between 0.5 and 2.0. A new detailed kinetic model for the THP pyrolysis/combustion was developed by EXGAS, complemented with theoretical calculations for the determining reactions. Good agreements between simulations and acquired experimental data were observed. Reaction path analysis shows that THP is mainly consumed by H-abstractions at both pyrolysis and combustion conditions. The pyrolysis simulations are very sensitive to the unimolecular initiations involving C−C and C−O bond fissions, whereas these reactions play only a very minor role under combustion conditions.

Temperature and emissions characteristics of a micro-mixing injection hydrogen-rich syngas flame diluted with N2

Micro-mixing injection combustion is an effective way to reduce the NOx emissions of gas turbine, but it can also cause high temperature near the burner's nozzle. In this paper, micro-mixing injection combustion and N2 dilution were combined to resolve this problem. The results indicate that this method not only reduces NOx emissions effectively but also protects the nozzles by lowering the temperature of the flame and that near the nozzle. The larger the dilution amount, the lower the temperature and NOx emissions could be. Under various experimental operating conditions, NOx emissions could be reduced to less than 6 ppm (@15% O2), which is lower than the emissions of most conventional gas turbine burners. According to the characteristics of temperature and NOx emissions, it seems the Zeldovich route is not the control route for the flame. With most of the flames, the CO emission hardly changed and remained at a relatively low level within almost 6 ppm (@15% O2).

Effects of argon/nitrogen dilution on explosion and combustion characteristics of dimethyl ether–air mixtures

In this study, an investigation of the explosion parameters, which include maximum explosion pressure (pmax), maximum rate of pressure rise (dp/dt)max, and combustion characteristics, i.e., laminar burning velocity (SL), activation temperature (Ta) and Zel’dovich number (Ze) of dimethyl ether (DME)–air–argon/nitrogen mixtures are systematically carried out. The dilution effects on the explosion and combustion characteristics are also explored through the addition of two diluents, i.e., argon (Ar) and nitrogen (N2), into the DME–air mixture. It is found from experiment that, the explosion parameters of fuel-lean and fuel-rich mixture exhibit different behaviors. For the fuel rich mixture, pmax decreases with increasing amount of Ar and N2 dilution, and a significant drop of (dp/dt)max is observed as the amount of added diluent increases. For the fuel lean mixture, pmax is first increased with increasing diluent, afterwards it decreases as inert gas dilution ratio larger than a critical value, it is also found (dp/dt)max fluctuates with more addition of diluent. The present results also confirm that SL decreases more abruptly in the DME–air mixtures with dilution of N2 than with Ar, which indicates that the increase in density dominates over the retarding effect for laminar burning velocity in the cases of inert gases dilution. The activation temperature of DME–air–Ar is larger than DME–air–N2 at the same condition, which demonstrates N2 dilution is more sensitive than N2 on the suppression of chemical reaction. Results illustrate that Ze goes up with the increase of dilution ratio, and the values of Ze for argon diluted mixtures are larger than that of nitrogen diluted.

Experimental and Modeling Investigation of IT-SOFC for Use with Biogas and Syngas Mixtures

Introduction Applications based on Solid Oxide Fuel Cells (SOFC) for the distributed production of energy require the cell to operate with both traditional hydrocarbon fuels (CH4 and LPG) and biomass-derived fuels (biogas, ethanol). Benefits in terms of lifetime and thermo-mechanical resistance are achieved by decreasing the operating temperature to 500-700°C (IT-SOFC). On the one hand, the use of hydrocarbons leads the traditional Ni-based anodes to deactivate due to coke formation; on the other hand, lower temperatures ask for electrolytes different from YSZ, for instance based on Ce oxides. Novel cells and materials are required, whose development must be closely accompanied by numerical model analysis, to rationalize chemical and physical phenomena. In this work, IT-SOFCs based on Samarium doped Ceria (SDC, Ce0.8Sm0.2O1.91) electrolytes and Cu/Pd/CZ80 anodes are investigated for applications with biogas mixtures (CO2/CH4) and syngas mixtures (H2/CO/H2O). Polarization curves and EIS experiments were performed between 600 and 700°C, at varying concentration of the reactants. A numerical model of the SOFC was used to analyze the experimental results and kinetic equations were derived for both the catalytic and the electrocatalytic reactions of the process. Materials and Methods SDC-electrolyte supported cells with Cu/Pd/CZ80 (20 wt% CeO2, 80 wt% ZrO2) composite anodes and LSCF cathodes were prepared by die-pressing. SDC powders containing graphite as pore former were uniaxially pressed with pure SDC powder. This bilayer was calcined in air (1400°C for 3 h) producing a porous 150 μm thick SDC scaffold supported on a dense SDC electrolyte (300 – 380 μm thick). The porous SDC scaffold was impregnated with Cu/Pd/CZ80-based solutions [1]. The LSCF cathode (20 μm) was brushed on the opposite side of the cell. The cell was calcined at 700°C for 3 h. Polarization and EIS experiments were performed between 600 and 700°C with H2/H2O, CO/CO2, H2/CO, and CH4/CO2 mixtures, at different concentration of the reactants and N2 dilution. The EIS tests were performed at the OCV. A model was applied to analyze both the EIS spectra and the polarization curves. The model is one-dimensional, dynamic, isothermal and heterogeneous and allows to predict the cell voltage as a function of current density, temperature, pressure and composition of the anodic and cathodic gas streams. The model includes molecular kinetic schemes for both the catalytic reactions occurring in the anode (CH4 Steam Reforming and Dry reforming, WGS) and the electrocatalytic reactions (anodic H2 and CO electro-oxidation, cathodic O2 reduction). Appropriate correlations are included to account for the presence of electronic current leakage in the electrolyte. Results and Discussion The IT-SOFCs were tested in H2/N2 mixtures at varying H2 partial pressure to derive a power-law kinetic equation for the rate of H2 electro-oxidation. The polarization curves (Fig. 1A) show that the cell voltage decreases at decreasing H2 partial pressure. Coherently, an increase of the polarization resistance at decreasing H2 content is observed in the EIS tests (Fig. 1B). The model predictions well agree with the experiments, assuming that the rate is first order in H2. Under all the operating conditions, the measured OCV values were lower than those expected from the Nernst equation. This result is coherent with the use of Ce-based electrolytes, which at 600 and 700°C show a ionic and electronic mixed conductivity, causing current leakage and decrease of the OCV. The effect was observed also in the EIS tests, wherein the ohmic resistance increased at decreasing the H2 partial pressure. Along with the kinetic investigation, the CO/CO2 experiments allowed to derive a power-law rate for the CO electro-oxidation reaction. The experiments with syngas mixtures (H2/CO and H2/CO2/CO) allowed to verify the kinetic rate of the electro-oxidation reactions, and pointed out the simultaneous occurrence of H2-oxidation and CO-oxidation, which was numerically verified by taking into account also specific WGS kinetic rates. Decrease of the OCV to 0.6 V was observed with the CH4/CO2 feed, although the cell proved resistant to coke formation. Ageing tests under humidified H2/air conditions at 700°C (100% H2, Fig. 1C) reveled loss of performance due to segregation of the Cu phase with time on stream. Conclusions The development of IT-SOFCs requires the experimental efforts to be accompanied by numerical tools, able to quantify the contributions of the different chemical, electrochemical and physical phenomena occurring in the cells. In this work, a comprehensive model is applied to rationalize the polarization behavior and the EIS results of a novel, Ni-free, SDC-based IT-SOFC, tested over a wide range on conditions, spanning from biogas to syngas mixtures.

The effects of different intake charge diluents on the combustion and emission characteristics of a spark ignition natural gas engine

Exhaust gas recirculation (EGR) is the most common method to control NOx emission of internal combustion engine. The major components of EGR are CO2 and N2, which have different influences on engine combustion and pollutants formation through thermal, dilution and chemical effects. The main objective of this work is to investigate the different influences of CO2 and N2 on engine combustion and emission on a four-cylinder, turbo charged, spark ignition natural gas engine with electronically control unit, simultaneously to separate the thermal effect with the comparison with Ar. It was found that the peak in-cylinder pressure and heat release rate both decreased along with the increase of intake dilution extent regardless of the diluent's type. For each diluent gas, NOx emission decreases while HC emission increases with the increased dilution ratio. However, CO emission firstly decreased and then increased. Results also revealed that NOx and CO emission could be simultaneously reduced by intake charge dilution at a little sacrifice of HC emission. The effects of three diluents are different compared with each other. Among these three diluents, it can be found that CO2 is the most effective on reducing NOx and CO emission followed by N2. However, both CO2 and N2 dilution deteriorates the thermal efficiency while Ar dilution improved it. Besides, when NOx emission was reduced to the same level, the thermal efficiency is the highest and CO emission is the lowest for Ar dilution.

Effects of inert dilution on the propagation and extinction of lean premixed syngas/air flames

The dilution effects of inert components N2 and CO2 on the propagation and extinction of lean premixed H2/CO/air syngas flames were experimentally and numerically investigated. Extinction stretch rates were measured using the counterflow technique while laminar flame speed data were obtained from literatures. Numerical simulations were conducted at 1-D freely propagating configuration and opposed-jet configuration with detailed chemistry and molecular transport description. The numerical results well predicted the experimental measurements. Both results revealed that CO2 dilution had more profound effect on flame propagation and extinction than N2 dilution. In addition, numerical simulation assessed the preferential importance in a rather quantitative manner among the three effects, i.e., the thermal effect, the diffusivity change effect and the chemical effect with artificial manipulation of mass diffusivities and chemical reactions of CO2 and N2. The results showed that the thermal effect dominated the reduction of laminar flame speed and extinction strain rate. The chemical effect caused by CO2 dilution was slightly stronger to the reduction of extinction limit than to that of laminar flame speed. The diffusivity change effect is negligible for both CO2 and N2 dilutions. N2 only acts as a thermal inert in the propagation and extinction of the H2/CO/air flames.

Numerical and experimental study of the influence of CO2 and N2 dilution on soot formation in laminar coflow C2H4/air diffusion flames at pressures between 5 and 20 atm

The effects of fuel dilution by CO2 and N2 on soot formation and the flame structure in laminar coflow C2H4/air diffusion flames at pressures between 5 and 20atm were investigated both experimentally and numerically. Experimentally a constant ethylene flow rate and a constant dilution rate of 1:2 (fuel:diluent by mass) were maintained throughout the experiments. The flames were stable and non-smoking over the pressure range investigated. The radially-resolved soot volume fraction and temperature distributions were measured by the spectral soot emission (SSE) technique. Numerical calculations were conducted using two C2 chemistry models with formation of PAHs up to pyrene and a soot model incorporating pyrene collision as the soot inception step and hydrogen-abstraction acetylene addition mechanism and PAH condensation as the surface growth processes. The two C2 chemistry models were the ABF mechanism [Appel et al. (2000)] and the DLR mechanism [Slavinskaya and Frank (2009)]. The DLR mechanism predicted little or no chemical effect of CO2 dilution, depending on the pressure, in the present context. Numerical results are in qualitative agreement with experimental measurements. Soot volume fractions and carbon conversion are lower in the CO2-diluted flames due to the additional chemical effect of CO2. CO2 is still more effective than N2 as a diluent to suppress soot formation at elevated pressures. The primary pathway for the chemical effect of CO2 dilution is through the reverse reaction of CO+OH↔CO2+H. The chemical effect of CO2 lowers the rates of soot inception, C2H2 addition, and PAH condensation. The effectiveness of the CO2 chemical effect on soot formation suppression diminishes with increasing pressure. The diminishing effectiveness of the chemical effect of CO2 dilution with increasing pressure is due to the significant decrease in the H radical mole fraction.

Combined effects of flow/spray interactions and EGR on combustion variability for a stratified DISI engine

This study investigates combustion variability of a stratified-charge direct-injection spark ignited (DISI) engine, operated with near-TDC injection of E70 fuel and a spark timing that occurs during the early part of the fuel injection. Using EGR, low engine-out NOx can be achieved, but at the expense of increased combustion variability at higher engine speeds. Initial motored tests at different speeds reveal that the in-cylinder gas flow becomes sufficiently strong at 2000rpm to cause significant cycle-to-cycle variations of the spray penetration. Hence, the fired tests focus on operation at 2000rpm with N2 dilution ([O2]=19% and 21%) to simulate EGR. In-cylinder flow, spray, and early-flame measurements are correlated to reveal their effect on the combustion variability.Results reveal two types of flow/spray-interactions that predict the likelihood of a partial burn. (1) Proper flow direction before injection with a more collapsed spray leads to high kinetic energy of the flow during injection, thus generating a rapid early burn, which ensures complete combustion, regardless of the EGR level. (2) Improper flow direction and less collapsed spray generate low flow energy during the early phase of combustion. For this second type of flow/spray-interaction, application of EGR results in a partial-burn frequency of 30%, whereas without EGR, early combustion is shown to be insensitive to flow variations. Flame-probability maps illustrate that the partial-burn cycles for operation with EGR have a weak flame development in that the flame does not develop uniformly and reliably from the spark plug. Without EGR, the flame development is more repeatable regardless of the type of flow/spray-interaction, at the expense of higher NOx emissions.

Impact of N2 dilution on combustion and emissions in a spark ignition CNG engine

In order to reduce NOx (nitrogen oxides) emissions, N2 (nitrogen) was introduced as dilution gas to dilute mixture with a specially-designed injection device. The impacts of varying N2 DR (dilution ratio) on the combustion and the exhaust emissions were investigated, including engine heat release rate, indicator diagram, NOx, CO (carbon monoxide), THC (total hydrocarbon) emissions and so on. For this study, a modified 6.6L CNG (compressed natural gas) engine was tested and N2 was injected into the end of intake manifold by a specially-designed device. The results showed that N2 dilution has a significant influence on the combustion and the exhaust emissions. With the rise of N2 DR, the maximum of pressure in cylinder and the maximum of heat release rate exhibited decrease trends, the centre of heat release curve showed a moving backward tendency. Higher N2 DR exhibited lower NOx (17–81%) emissions, but higher emissions of THC (3–78%) and CO (1–28%). The change of BSFC (brake specific fuel consumption) can be ignored with N2 DR no more than 167%. Satisfactory results can be obtained, with lower NOx (31%) emissions, lower BSFC (0.5%), and relatively higher THC (6%) and CO (1%) emissions, when N2 DR is 67%.

Short contact time catalytic partial oxidation of biogas – A comprehensive study on CO2 and N2 dilution

The short contact time catalytic partial oxidation of methane diluted with nitrogen and/or carbon dioxide to act as a surrogate for biogas conversion was carried out. Experiments were carried out under varying operating conditions to determine the possible use of the products in pyrolysis.Carbon dioxide has a larger effect on the product selectivities and back face temperature of the catalyst compared to nitrogen for equal dilutions. Carbon dioxide is consumed in the reactor whereas nitrogen is not. Since carbon dioxide likely takes part in endothermic reactions, the temperature of the catalyst is lower as is the conversion of methane and selectivity to carbon monoxide and hydrogen.The product stream is at an appropriate composition and temperature for subsequent use in a pyrolysis reactor. The presence of hydrogen and carbon monoxide will result in the removal of oxygen from bio-oils that are produced in the pyrolysis reactor.

Flow characterization and dilution effects of N2 and CO2 on premixed CH4/air flames in a swirl-stabilized combustor

Numerically-aided experimental studies are conducted on a swirl-stabilized combustor to investigate the dilution effects on flame stability, flame structure, and pollutant emissions of premixed CH4/air flames. Our goal is to provide a systematic assessment on combustion characteristics in diluted regimes for its application to environmentally-friendly approaches such as biogas combustion and exhaust-gas recirculation technology. Two main diluting species, N2 and CO2, are tested at various dilution rates. The results obtained by means of optical diagnostics show that five main flame regimes can be observed for N2-diluted flames by changing excess air and dilution rate. CO2-diluted flames follow the same pattern evolution except that all the domains are shifted to lower excess air. Both N2 and CO2 dilution affect the lean blowout (LBO) limits negatively. This behavior can be counter-balanced by reactant preheating which is able to broaden the flammability domain of the diluted flames. Flame reactivity is degraded by increasing dilution rate. Meanwhile, flames are thickened in the presence of both diluting species. NOx emissions are significantly reduced with dilution and proved to be relevant to flame stability diagrams: slight augmentation in NOx emission profiles is related to transitional flame states where instability occurs. Although dilution results in increase in CO emissions at certain levels, optimal dilution rates can still be proposed to achieve an ideal compromise.

GHGT-12 Performance of the IGCC with distributed feeding of H2 in the gas turbine burner

Abstract A main hurdle for realizing the IGCC with CO 2 capture is the lack of gas turbines capable of burning hydrogen in an environmental-friendly, yet energy-efficient manner. With currently proposed combustion schemes an efficiency penalty is caused by the preparation (separation and compression) of the inert diluent, i.e. nitrogen. Distributed Fuel Injection (DFI) , where hydrogen is provided to the combustion air through a H2-separating membrane or a porous wall, could be a means of avoiding both concentrated fuel point sources and N2 dilution of the fuel. The paper presents two potential schemes for DFI: Scheme 1 with upstream H2 separation and scheme 2 with integrated H2 separation from the shifted syngas. First process simulation results for an IGCC with a reheat gas turbine yielded an IGCC efficiency increase of 0.7%-points with Scheme 1 and of 1.3%-points with scheme 2, compared to a reference case with nitrogen fuel dilution. This is mainly due to savings in N2 compression work but for scheme 2 also because the partial pressure difference of H2 over the Pd membrane can be employed as driving force. The results should however be regarded as indicative only one of the reasons being that fuel injection pressure drop for DFI is yet unknown, and is presumably a trade-off between membrane/porous wall size.

Effect of N2/CO2 dilution on laminar burning velocity of H2–air mixtures at high temperatures

The laminar burning velocities of H2–air mixtures diluted with N2 or CO2 gas at high temperatures were obtained from planar flames observed in externally heated diverging channels. Experiments were conducted for an equivalence ratio range of 0.8–1.3 and temperature range of 350–600 K with various dilution rates. In addition, computational predictions for burning velocities and their comparison with experimental results and detailed flame structures have been presented. Sensitivity analysis was carried out to identify important reactions and their contribution to the laminar burning velocity. The computational predictions are in reasonably good agreement with the present experimental data (especially for N2 dilution case). The burning velocity maxima was observed for slightly rich mixtures and this maxima was found to shift to higher equivalence ratios (Ф) with a decrease in the dilution. The effect of CO2 dilution was more profound than N2 dilution in reducing the burning velocity of mixtures at higher temperatures.

Thermoneutral conditions in dry reforming of ethanol

ABSTRACTThe reaction enthalpy (ΔH) of reforming processes like steam and dry (CO2) reforming is of great importance for scale‐up and process development. Generally, the reforming processes are considered endothermic in nature. However, a detailed study of the reaction enthalpy of reforming process, taking the example of dry reforming of ethanol considered in this study, reveals the existence of exothermic reaction enthalpy at low temperatures. A study of reaction enthalpy of ethanol dry reforming within pressure (1–10 bar), temperature range (300–900 °C), and CO2 to carbon in ethanol ratio (CCER 1–5) was initiated to determine the existence of thermoneutral temperatures for the overall reaction. The variation of thermoneutral conditions and product yields at the thermoneutral temperatures was studied. The utilization potential of the products generated at thermoneutral conditions was also evaluated. The low‐pressure thermoneutral operation favored higher hydrogen production, lower methane and water formation, whereas the high‐pressure thermoneutral operation favored product gas of lower syngas ratio with higher CO2 conversion (utilization) in the process. The study can be extended to steam and dry reforming of other fuels to generate valuable products at thermoneutral conditions avoiding use of air in the process and subsequent N2 dilution of the product gas. © 2013 Curtin University of Technology and John Wiley &amp; Sons, Ltd.