Physicochemical effects of varying fuel composition on knock characteristics of natural gas mixtures

The physicochemical origins of how changes in fuel composition affect autoignition of the end gas, leading to engine knock, are analyzed for a natural gas engine. Experiments in a lean-burn, high-speed medium-BMEP gas engine are performed using a reference natural gas with systematically varied fractions of admixed ethane, propane and hydrogen. Thermodynamic analysis of the measured non-knocking pressure histories shows that, in addition to the expected changes arising from changes in the heat capacity of the mixture, changes in the combustion duration relative to the compression cycle (the combustion “phasing”) caused by variations in burning velocity dominate the effects of fuel composition on the temperature (and pressure) of the end gas. Thus, despite the increase in the heat capacity of the fuel–air mixture with addition of ethane and propane, the change in combustion phasing is actually seen to increase the maximum end-gas temperature slightly for these fuel components. By the same token, the substantial change in combustion duration upon hydrogen addition strongly increases the end-gas temperature, beyond that caused by the decrease in mixture heat capacity. The impact of these variations in in-cylinder conditions on the knock tendency of the fuel have been assessed using autoignition delay times computed using SENKIN and a detailed chemical mechanism for the end gas under the conditions extant in the engine. The results show that the ignition-promoting effect of hydrogen is mainly the result of the increase in end-gas temperature and pressure, while addition of ethane and propane promotes ignition primarily by changing the chemical autoignition behavior of the fuel itself. Comparison of the computed end-gas autoignition delay time, based on the complete measured pressure history of each gas, with the measured Knock-Limited Spark Timing shows that the computed delay time accurately reflects the measured knock tendency of the fuels.     

Combining experimental and numerical investigations to explore the potential of downsized engines operating with methane/hydrogen blends

Due to the shortening of oil reserves, many research efforts are currently performed to promote alternative fuels for transport. Among them, natural gas, which is mainly composed of methane, offers one of the most promising potential as its large scale production can today be envisaged from biomass or shale. An other advantage of methane is that its high octane rating allows the use of increased compression ratios compared to gasoline, then improving the thermal efficiency of spark ignition engines. This property, combined with its low carbon content makes natural gas one of the best candidates to drastically reduce CO₂ emissions from piston engines. However, methane exhibits a low burning velocity, leading to high cycle-to-cycle variations and, in some cases, to increased CH₄ emissions, these latter having a huge impact in terms of greenhouse effect. One solution then consists in blending natural gas with hydrogen, a component easily available in refineries. H₂ enrichment indeed allows to reach high flame velocities and to limit quenching effects at the combustion chamber walls. Nevertheless, for the specific case of downsized engines, hydrogen may also lead to increase the knock sensitivity. A compromise in terms of blending rate, compression ratio and boost level should then be necessary to reach an optimal configuration. The objective of the present work is to combine experimental and numerical investigations to explore the influence and limits of hydrogen addition in highly downsized engines. The impact of the fuel composition on the combustion velocity and knock occurrence is studied for three compression ratios (9.5, 11.5 and 13). Experiments are conducted with a single-cylinder engine for a wide range of operating conditions in the stoichiometric mode and hydrogen blending rates from 0 to 40%. 3D CFD simulations are then performed using the Extended Coherent Flame Model (Colin et al, Oil & Gas Sci. & Tech., 2003) to describe the turbulent flame propagation, in combination with the Tabulated Kinetics for Ignition model (Colin et al., Proc. Combust. Inst., 2005) for knock prediction. These models require flame velocities as well as auto-ignition delays and reaction rates data, which have to account for the fuel composition. These data are provided using a priori computations of premixed flames and homogenous reactors with the GRI 3.0 and Curran mechanisms. A very good agreement is obtained between engine simulations and experiments, allowing to use CFD to improve the understanding of the observed engine behavior on specific operating points. It is first shown that the effect of hydrogen addition on the combustion velocity is almost linear for the considered blending levels, and that knock can be hardly found even for high load and high compression ratio cases. It is also demonstrated that optimizing an engine for CH₄–H₂ blends combustion is a challenging task and that a dedicated engine design should be chosen.     

Kinetics of (reversible) internal reforming of methane in solid oxide fuel cells under stationary and APU conditions

The internal reforming of methane in a solid oxide fuel cell (SOFC) is investigated and modeled for flow conditions relevant to operation. To this end, measurements are performed on anode-supported cells (ASC), thereby varying gas composition (y_CO = 4–15%, y_H2=5−17%, y_CO2=6−18%, y_H2O=2−30%, y_CH4=0.1−20%) and temperature (600–850 °C). In this way, operating conditions for both stationary applications (methane-rich pre-reformate) as well as for auxiliary power unit (APU) applications (diesel-POX reformate) are represented. The reforming reaction is monitored in five different positions alongside the anodic gas channel by means of gas chromatography. It is shown that methane is converted in the flow field for methane-rich gas compositions, whereas under operation with diesel reformate the direction of the reaction is reversed for temperatures below 675 °C, i.e. (exothermic) methanation occurs along the anode. Using a reaction model, a rate equation for reforming could be derived which is also valid in the case of methanation. By introducing this equation into the reaction model the methane conversion along a catalytically active Ni-YSZ cermet SOFC anode can be simulated for the operating conditions specified above.     

Performance study using natural gas,hydrogen-supplemented natural gas and hydrogen in AVL research engine

In the future, hydrogen will be required to supplement and eventually replace a rapidly diminishing natural gas resource for stationary type combustion engines. Combustion properties, knock rating, engine performance and emissions of methane (the chief constituent of natural gas) and hydrogen are different as engine fuels. In the present work, investigations were carried out to obtain data on engine performance, fuel economy and emissions, using natural gas, hydrogen-supplemented natural gas (methane) and hydrogen in AVL² research engine. Investigations were also carried out to suppress flashback and to reduce nitric oxide emissions at different operating conditions, by water induction into the hydrogen-air mixture in the intake manifold for a hydrogen fueled engine.     

Thermodynamic analysis of H2 production from CaO sorption‐enhanced methane steam reforming thermally coupled with chemical looping combustion as a novel technology

In this novel paper, a technique for hydrogen production route of CaO sorption‐enhanced methane steam reforming (SEMSR) thermally coupled with chemical looping combustion (CLC) was presented (CLC‐SEMSR), which perceived as an improvement of previous methane steam reforming (MSR) thermally coupled with CLC technology (CLC‐MSR). The application of CLC instead of furnace achieves the inherent separation of CO₂ from flue gas without extra energy required. The required heat for the reformer is provided by thermally coupling CLC. The addition of CaO sorbents can capture CO₂ as it is formed from the reformer gas to the solid phase, displacing the normal MSR equilibrium restrictions and obtaining higher purity of H₂. The Aspen Plus was used to simulate this novel process on the basis of thermodynamics. The performances of this system examined included the composition of reformer gas, yield of reformer gas (Y_Rg), methane conversion (α_M), the overall energy efficiency (η), and exergy efficiency (φ) of this process. The effects of the molar ratio of CaO to methane for reforming (Ca/M) in the range of 0–1.2, the molar ratio of methane for combustion to methane for reforming (M(fuel)/M) in the range of 0.1–0.3, and the molar ratio of NiO to methane for reforming (Ni/M) in the range of 0.4–1.2 were investigated. It has been found to be favored by operating under the conditions of Ca/M = 1, M(fuel)/M = 0.2, and Ni/M = 0.8. The most excellent advantage of CLC‐SEMSR was that it could obtain higher purity of H₂ (95%) at lower operating temperature (655 °C), as against H₂ purity of 77.1% at higher temperature (900 °C) in previous CLC‐MSR. In addition, the energy efficiency of this process could reach 83.3% at the optimal conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Engine knock and combustion characteristics of a spark ignition engine operating with varying hydrogen and carbon monoxide proportions

Varying proportions of hydrogen and carbon monoxide (synthesis gas) have been investigated as a spark ignition (SI) engine fuel in this paper. It is important to understand how various synthesis gas compositions effect important SI combustion fundamentals, such as knock and burn duration, because in synthesis gas production applications, the compositions can vary significantly depending on the feedstock and production method.A single cylinder cooperative fuels research (CFR) engine was used to investigate the knock and combustion characteristics of three blends of synthesis gas (H₂/CO ratio); 1) 100/0, 2) 75/25, and 3) 50/50, by volume. These blends were tested at three compression ratios (6:1, 8:1, and 10:1), and three equivalence ratios (0.6, 0.7, and 0.8).It was revealed that the knock limited compression ratio (KLCR) of a H₂/CO mixture increases with increasing CO fraction, for a given spark timing. For a given equivalence ratio and spark timing, a 50%/50% H₂/CO mixture produced a KLCR of 8:1 compared to a 100% H₂ condition, which produced a KLCR of 6:1. The burn duration and ignition lag is also increased with increasing CO fraction. The results from this work are important for those considering using synthesis gas as a fuel in SI engines. It reveals that although CO is a slow burning fuel, higher CO fractions in synthesis gas can be beneficial, because of its increased resistance to knock, which gives it the potential of producing higher indicated efficiencies through the utilization of an engine with a higher compression ratio.     

A computational study on the combustion of hydrogen/methane blended fuels for a micro gas turbines

To understand the combustion performance of using hydrogen/methane blended fuels for a micro gas turbine that was originally designed as a natural gas fueled engine, the combustion characteristics of a can combustor has been modeled and the effects of hydrogen addition were investigated. The simulations were performed with three-dimensional compressible k-ε turbulent flow model and presumed probability density function for chemical reaction. The combustion and emission characteristics with a variable volumetric fraction of hydrogen from 0% to 90% were studied. As hydrogen is substituted for methane at a fixed fuel injection velocity, the flame temperatures become higher, but lower fuel flow rate and heat input at higher hydrogen substitution percentages cause a power shortage. To apply the blended fuels at a constant fuel flow rate, the flame temperatures are increased with increasing hydrogen percentages. This will benefit the performance of gas turbine, but the cooling and the NO_x emissions are the primary concerns. While fixing a certain heat input to the engine with blended fuels, wider but shorter flames at higher hydrogen percentages are found, but the substantial increase of CO emission indicates a decrease in combustion efficiency. Further modifications including fuel injection and cooling strategies are needed for the micro gas turbine engine with hydrogen/methane blended fuel as an alternative.     

A strategy of mid-temperature natural gas based chemical looping reforming for hydrogen production

Steam methane reforming (SMR) needs the reaction heat at a temperature above 800 °C provided by the combustion of natural gas and suffers from adverse environmental impact and the hydrogen separated from other chemicals needs extra energy penalty. In order to avoid the expensive cost and high power consumption caused by capturing CO₂ after combustion in SMR, natural gas Chemical Looping Reforming (CLR) is proposed, where the chemical looping combustion of metal oxides replaced the direct combustion of NG to convert natural gas to hydrogen and carbon dioxide. Although CO₂ can be separated with less energy penalty when combustion, CLR still require higher temperature heat for the hydrogen production and cause the poor sintering of oxygen carriers (OC). Here, we report a high-rate hydrogen production and low-energy penalty of strategy by natural gas chemical-looping process with both metallic oxide reduction and metal oxidation coupled with steam. Fe₃O₄ is employed as an oxygen carrier. Different from the common chemical looping reforming, the double side reactions of both the reduction and oxidization enable to provide the hydrogen in the range of 500–600 °C under the atmospheric pressure. Furthermore, the CO₂ is absorbed and captured with reduction reaction simultaneously.Through the thermodynamic analysis and irreversibility analysis of hydrogen production by natural gas via chemical looping reforming at atmospheric pressure, we provide a possibility of hydrogen production from methane at moderate temperature. The reported results in this paper should be viewed as optimistic due to several idealized assumptions: Considering that the chemical looping reaction is carried out at the equilibrium temperature of 500 °C, and complete CO₂ capture can be achieved. It is assumed that the unreacted methane and hydrogen are completely separated by physical adsorption. This paper may have the potential of saving the natural gas consumption required to produce 1 m³ H₂ and reducing the cost of hydrogen production.     

不同过量空气系数下射流点火发动机燃烧、排放和爆震特性的试验研究

基于单缸试验机研究了过量空气系数对射流点火发动机性能的影响。通过分析发动机性能曲线、缸内燃烧情况及爆震特性探究射流点火最佳运行区间,并与火花点火燃烧方式进行对比。结果表明,射流点火可以有效提升瞬时放热率并拓展发动机稀燃极限,缩短缸内混合气滞燃期与燃烧持续期,同时燃油经济性有一定提升。在稀燃条件下氮氧化物排放极低。爆震方面,随着点火提前角增大,射流火焰的多点点火效应会在缸内产生明显压力震荡,继续增大点火提前角会诱导末端混合气自燃。因此射流点火爆震缸压表现为两阶段压力震荡,爆震因子集中性高。提升过量空气系数可以降低射流点火爆震因子幅值,使发动机工作在轻微爆震或无爆震状态。     

Combustion of methane in catalytic honeycomb monolith burners

The heat transfer efficiency, stability, and pollutant emissions characteristics of ultra‐lean methane–air combustion in some precious metal‐based honeycomb monoliths were investigated. The interpretation of the experimental results was assisted using numerical modelling of the gas‐phase combustion process. The thermal radiation output of the monoliths varied between 27 and 30 per cent of the thermal input, and this compared favourably with equivalent porous inert media burners. The minimum fuel concentrations for very‐low emission stable combustion were found to be significantly lower than for conventional gas‐phase combustion and were shown to vary with the nature and loading of the catalyst, as well as with flow rates. The palladium catalyst was found to have a larger window of mixture strengths and flow rates for stable operation than the platinum one. During all the runs under stable combustion conditions, only extremely small amounts of CO, NO_x and unburnt hydrocarbons were detected. Thus, the operating conditions verified ‘near‐zero’ pollutant emissions that only a catalytic combustion process can achieve at present. Temperature profiles inside the monoliths channels proved that the catalyst's role was not only to enable the ignition of fuel mixtures below flammability limits, but also to ensure the complete oxidation of the fuel to CO₂ via surface reactions in the steady state. The reaction zone inside the catalysts was found to end at about 10 mm from the monolith's entrance. The effect of monolith length was investigated and a reduction of 70 per cent in the original length was found possible. Copyright © 2000 John Wiley & Sons, Ltd.     

Decarbonisation potential of dimethyl ether/hydrogen mixtures in a flameless furnace: Reactive structures and pollutant emissions

This article sheds light on the combustion characteristics of dimethyl ether and its mixtures with methane/hydrogen under flameless conditions at different equivalence ratios. It was found that combustion of 100% dimethyl ether in flameless conditions minimises the NO formation, keeping it less than 10 ppm with no CO or unburned hydrocarbons. Progressive addition of methane was found to reduce the NO, reaching up to zero value at 50% methane in molar fraction along with a marginal CO₂ reduction. However, large amounts of CO were found for higher methane levels, greater than 60% CH₄ in molar fraction. Reactive structures based on OH1 chemiluminescence revealed that adding methane results in increased ignition delay times and, consequently, a more distributed reaction zone characterised by reduced temperature gradients. No visible flame was observed for pure dimethyl ether as well as dimethyl ether/methane mixtures. Furthermore, a more intense and narrower reaction zone, characterised by the presence of a visible flame, was formed upon hydrogen addition. Adding hydrogen by 50% in molar fraction did not cause a noticeable rise in NO levels; however, CO₂ was lowered by about 18%. Further addition of hydrogen resulted in increased peak temperatures of about 1700 K and higher NO emissions of about 50 ppm. Additionally, a skeletal Chemical Reactor Network was built and simulated with the commercial software CHEMKIN Pro to investigate the effect of the different mixtures and operating conditions on NO formation from a chemical point of view. N₂O pathway was observed to be the root source of NO emissions for pure DME and DME/CH₄ mixtures, however; the thermal pathway became gradually more important as hydrogen concentration was increased in the mixture.     

Experimental analysis of the effects of hydrogen addition on methane combustion

This paper presents gas emissions from turbulent chemical flow inside a model combustor, for different blending ratios of hydrogen–methane composite fuels. Gas emissions such as CO and O₂ from the combustion reaction were obtained using a gas analyzer. NOx emissions were measured with a NOx analyzer. The previously obtained flame temperature distributions were also presented. As the amount of hydrogen in the mixture increases, more hydrogen is involved in the combustion reaction, and more heat is released, and the higher temperature levels are resulted. The results have shown that the combustion efficiency increases and CO emission decreases when the hydrogen content is increased in blending fuel. It is also shown that the hydrogen–methane blending fuels are efficiently used without any important modification in the natural gas burner. Copyright © 2011 John Wiley & Sons, Ltd.     

Flow Field Calculations for Afterburner

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Numerical simulation of laminar premixed combustion in a porous burner

Premixed combustion in porous media differs substantially from combustion in free space. The interphase heat transfer between a gas mixture and a porous medium becomes dominant in the premixed combustion process. In this paper, the premixed combustion of CH₄/air mixture in a porous medium is numerically simulated with a laminar combustion model. Radiative heat transfer in solids and convective heat transfer between the gas and the solid is especially studied. A smaller detailed reaction mechanism is also used and the results can show good prediction for many combustion phenomena. Translated from Journal of Combustion Science and Technology, 2006, 12(1): 46–50 [译自: 燃烧科学与技术]     

不同燃烧模式的爆震特性及爆震强度评价方法

基于一台配备了全可变气门机构的单缸四冲程发动机开展了不同燃烧模式的燃烧和压力震荡特性研究,包括均质充量压燃(homogeneous charge compression ignition,HCCI)、汽油压燃(gasoline compression ignition,GCI)、湍流射流点火(turbulent jet ignition,TJI)和火花点火(spark ignition,SI)。研究结果表明:不同燃烧模式具有不同的统计学特征,其中HCCI、GCI和TJI的爆震强度分布较为集中,不易出现偶发的高爆震强度的燃烧循环;SI爆震的分布较为离散,通常具有较高的最大值和99%分位数,高爆震强度燃烧循环的偶发性较强;而低速早燃工况则是具有极高的爆震强度最大值和很低的99%分位数。此外,对于爆震工况的评价方面,对传统的算数平均值法和爆震循环占有率法进行了改进,提出了加权平均值法和破坏性循环均值法两种改进的爆震评价方法。二者在HCCI、GCI、TJI和SI爆震判定的准确性和适应性上相比改进前有了很大的提升;但对于低速早燃工况,破坏性循环均值法无法准确识别出其破坏性,加权平均值法具有非常好的准确性。     

Probability density function approach coupled with detailed chemical kinetics for the prediction of knock in turbocharged direct injection spark ignition engines

In this work a new knock model is derived which accounts for the inherent feature of knocking combustion, namely that it is a stochastic phenomenon. It provides the probability of autoignition and distinct criteria to determine the mean knock onset as well as the relative number of knocking cycles. For modeling purposes an ignition progress variable is proposed to determine the reactive state of the unburnt fuel–air mixture and the occurrence of autoignition. Statistical information of this quantity is introduced by presuming a clipped Gaussian probability density function (PDF). Its shape is defined by the Favre mean and variance of the ignition progress variable for which transport equations are derived. The chemical source terms that appear in these equations are closed by employing a presumed PDF approach to account for turbulence chemistry interaction. A clipped Gaussian PDF distribution for temperature and a β$\beta$-PDF for mixture fraction are employed. Hence, the impact of temperature and mixture fraction fluctuations on the ignition progress variable is accounted for. The chemical source terms are evaluated based on tabulated chemistry incorporating detailed chemical kinetics. For the assessment of the knock model a spark timing sweep was performed on the engine test bench for a full-load operating point at n=2000$n=2000$ rpm. In-cylinder flow simulations including gas exchange, mixture formation, combustion, and knock were carried out and the results are compared with experimental data. It is shown that the knock model is able to predict the mean knock onset with reasonable accuracy and that the impact of a spark timing sweep on the number of knocking cycles is well captured.     

An experimental study of combustion and emissions in a spark-ignition engine fueled with coal-bed gas

Coal-bed gas has been considered an attractive alternative fuel for internal combustion engines due to its abundant source and low emissions. In the present study, a combustion system with a swirl chamber has been developed for a spark-ignition engine using coal-bed gas. Detailed experiments have been carried out to investigate the combustion and emission characteristics of the engine operating with three different grades of coal-bed gas. The results have shown that this combustion system allows satisfactory operation of the engine with a wide range of methane content in the supplied coal-bed gas. For all tested conditions, the CO emission has a maximum value of 0.062%, and the HC emission is less than 380 ppm. The NO emission increases with the engine load but is less than 1800 ppm, demonstrating a great advantage of coal-bed gas as a relatively clean engine fuel.     

Thermokinetic interactions leading to knock during homogeneous charge compression ignition

Experiments have been performed in a rapid compression machine to investigate the conditions for and the origins of “knock” in controlled autoignition (CAI), or homogeneous charge compression ignition (HCCI). The combustion of n-pentane in air at the composition φ = 0.5 and a gas density of 217 mol m⁻³ was studied in the compressed gas temperature range 720 to 820 K. This corresponds to the region in which a transition from non-knocking to knocking reaction occurred in the two-stage ignition regime, close to the minimum of the ignition delay before the negative temperature dependence is encountered. High-resolution pressure records, combined with image intensified, natural light output (with spectral resolving filters in some experiments) were used to characterize the reaction and to identify the behavior in terms of chemical activity associated with chemiluminescence and spatial variations in temperature, respectively. It appears that the knock observed in a rapid compression machine (and hence during CAI) originates from the localized development of the hot stage of ignition, often from near the combustion chamber walls. Exceedingly rapid development of ignition centers may be attributed to the onset of vigorous chain branching via O atoms. In conditions where knock does not occur, there is a much more spatially uniform and slower overall development of ignition, which may be restricted by the persistence of reactions involving HO₂ radicals to a very late stage of the combustion. The distinctions of these modes of behavior are traced to the way in which the early stages of two-stage ignition interact with the temperature field set up by the compression stroke.     

Methane membrane steam reforming: Heat duty assessment

Membrane reactors are an innovative technology with huge application potentialities for equilibrium limited endothermic reactions. Assembling a membrane selective to a reaction product avoids the equilibrium conditions to be achieved, supporting the reactions at lower operating temperatures. Taking as an example the natural gas steam reforming, a methane conversion around 98% can be reached imposing an operating temperature of 823 K, much lower than that of the traditional process. In the present paper, a stringent analysis of heat power requirement needed to carry out the natural gas steam reforming process by applying a membrane reactor is made. The simulations allows to understand how the main operating parameters (inlet temperature, inlet methane flow-rate, steam to carbon ratio, ratio between sweeping steam and inlet methane, operating reaction pressure) influence the total heat power required by the process, divided among power contributions for the reaction heat duty, reactant steam and permeation steam generation and preheating. Moreover, the specific thermal energy per mole of pure H₂ is computed and assessed. Optimizing the operating conditions set, a specific thermal energy per mole of pure hydrogen of 92.3 kWh kmol⁻¹ is obtained corresponding to a total thermal power of 687.4 kW required to convert, in a single membrane reactor, a methane flow-rate of 2 kmol h⁻¹ (GHSV = 9.590 h⁻¹) with a conversion around 98%.     

Comparisons of hydrogen and gasoline combustion knock in a spark ignition engine

Combustion knock is one of the primary constraints limiting the performance of spark-ignition hydrogen fuelled internal combustion engines (H2-ICE) as it limits the torque output and efficiency, particularly as the equivalence ratio nears stoichiometric operation. Understanding the characteristic of combustion knock in a H2-ICE will provide better techniques for its detection, prevention and control while enabling operation at conditions of improved efficiency.

Engine studies examining combustion knock characteristics were conducted with hydrogen and gasoline fuels in a port-injected, spark-ignited, single cylinder cooperative fuel research (CFR) engine. Characterization of the signals at varying levels of combustion knock from cylinder pressure and a block mounted piezoelectric accelerometer were conducted including frequency, signal intensity, and statistical attributes. Further, through the comparisons with gasoline combustion knock, it was found that knock detection techniques used for gasoline engines, can be applied to a H2-ICE with appropriate modifications. This work provides insight for further development in real time knock detection. This would help in improving reliability of hydrogen engines while allowing the engine to be operated closer to combustion knock limits to increase engine performance and reducing possibility of engine damage due to knock.