Experimental and Modeling Investigation of the Effectof H2S Addition to Methane on the Ignition and Oxidation at High Pressures

The autoignition and oxidation behavior of CH₄/H₂S mixtures has been studied experimentally in a rapid compression machine (RCM) and a high-pressure flow reactor. The RCM measurements show that the addition of 1% H₂S to methane reduces the autoignition delay time by a factor of 2 at pressures ranging from 30 to 80 bar and temperatures from 930 to 1050 K. The flow reactor experiments performed at 50 bar show that, for stoichiometric conditions, a large fraction of H₂S is already consumed at 600 K, while temperatures above 750 K are needed to oxidize 10% methane. A detailed chemical kinetic model has been established, describing the oxidation of CH₄ and H₂S as well as the formation and consumption of organosulfuric species. Computations with the model show good agreement with the ignition measurements, provided that reactions of H₂S and SH with peroxides (HO₂ and CH₃OO) are constrained. A comparison of the flow reactor data to modeling predictions shows satisfactory agreement under stoichiometric conditions, while at very reducing conditions, the model underestimates the consumption of both H₂S and CH₄. Similar to the RCM experiments, the presence of H₂S is predicted to promote oxidation of methane. Analysis of the calculations indicates a significant interaction between the oxidation chemistry of H₂S and CH₄, but this chemistry is not well understood at present. More work is desirable on the reactions of H₂S and SH with peroxides (HO₂ and CH₃OO) and the formation and consumption of organosulfuric compounds

Experimental and numerical analysis of the autoignition behavior of NH3 and NH3/H2 mixtures at high pressure

Measurements of autoignition delay times of NH3 and NH3/H2 mixtures in a rapid compression machine are reported at pressures from 20–75 bar and temperatures in the range 1040–1210 K. The equivalence ratio, using O2/N2/Ar mixtures as oxidizer, varied for pure NH3 from 0.5 to 3.0; NH3/H2 mixtures with H2 fraction between 0 and 10% were examined at equivalence ratios 0.5 and 1.0. In contrast to many hydrocarbon fuels, the results indicate that, for the conditions studied, autoignition of NH3 becomes slower with increasing equivalence ratio. Hydrogen is seen to have a strong ignition-enhancing effect on NH3. The experimental data, which show similar trends to those observed previously by He et al. (2019) [28], were used to evaluate four NH3 oxidation mechanisms: a new version of the mechanism described by Glarborg et al. (2018) [29], with an updated rate constant for the formation of hydrazine, NH2 + NH2 (+M) = N2H4 (+M), and the literature mechanisms from Klippenstein et al. (2011) [30], Mathieu and Petersen (2015) [25], and Shrestha et al. (2018) [31]. In general, the mechanism from this study has the best performance, yielding satisfactory prediction of ignition delay times both of pure NH3 and NH3/H2 mixtures at high pressures (40–60 bar). Kinetic analysis based on present mechanism indicates that the ignition enhancing effect of H2 on NH3 is closely related to the formation and decomposition of H2O2; even modest hydrogen addition changes the identity of the major reactions from those involving NHx radicals to those that dominate the H2/O2 mechanism. Flux analysis shows that the oxidation path of NH3 is not influenced by H2 addition. We also indicate the methodological importance of using a non-reactive mixture having the same heat capacity as the reactive mixture for determining the non-reactive volume trace for simulation purposes, as well as that of limiting the variation in temperature after compression, by limiting the uncertainty in the experimentally determined quantities that characterize the state of the mixture

The Effect of Humidity on the Knock Behavior in a Medium BMEP Lean-Burn High-Speed Gas Engine

The effects of air humidity on the knock characteristics of fuels are investigated in a lean-burn, high-speed medium BMEP engine fueled with a CH4 + 4.7 mole% C3H8 gas mixture. Experiments are carried out with humidity ratios ranging from 4.3 to 11 g H2O/kg dry air. The measured pressure profiles at non-knocking conditions are compared with calculated pressure profiles using a model that predicts the time-dependent in-cylinder conditions (P, T) in the test engine (“combustion phasing”). This model was extended to include the effects of humidity. The results show that the extended model accurately computes the in-cylinder pressure history when varying the water fraction in air. Increasing the water vapor content in air decreases the peak pressure and temperature significantly, which increases the measured Knock Limited Spark Timing (KLST); at 4.3 g H2O/kg dry air the KLST is 19 °CA BTDC while at 11 g H2O/kg dry air the KLST is 21 °CA BTDC for the same fuel. Excellent agreement is observed between the calculated knock resistance (using the Propane Knock Index, PKI) and the measured knock resistance (KLST) for the range in water content in air studied in this work. Since the effect of water on autoignition delay time is negligible, the observed increase in knock resistance of the fuel-air mixture is due a decrease in pressure and temperature of the end gas with increasing water content in as a result of changes in the mass burning rate, and thermophysical properties of the fuel-air mixture

Ignition delay times of NH3 /DME blends at high pressure and low DME fraction:RCM experiments and simulations

Autoignition delay times of ammonia/dimethyl ether (NH3/DME) mixtures were measured in a rapid compression machine with DME fractions of 0, 2 and 5 and 100% in the fuel. The measurements were performed at equivalence ratios phi=0.5, 1.0 and 2.0 and pressures in the range 10-70 bar; depending on the fuel composition, the temperatures after compression varied from 610 K to 1180 K. Admixture of DME is seen to have a dramatic effect on the ignition delay time, effectively shifting the curves of ignition delay vs. temperature to lower temperatures, up to similar to 250 K compared to pure ammonia. Two-stage ignition is observed at phi=1.0 and 2.0 with 2% and 5% DME in the fuel, despite the pressure being higher than that at which pure DME shows two-stage ignition. At phi= 0.5, a reproducible pre-ignition pressure rise is observed for both DME fractions, which is not observed in the pure fuel components. A novel NH3/DME mechanism was developed, including modifications in the NH3 subset and addition of the NH2+CH3OCH3 reaction, with rate coefficients calculated from ab initio theory. Simulations faithfully reproduce the observed preignition pressure rise. While the mechanism also exhibits two-stage ignition for NH3/DME mixtures, both qualitative and quantitative improvement is recommended. The overall ignition delay times for ammonia/DME mixtures are predicted well, generally being within 50% of the experimental values, although reduced performance is observed for pure ammonia at phi= 2.0. Simulating the ignition process, we observe that the DME is oxidized much more rapidly than ammonia. Analysis of the mechanism indicates that this 'early DME oxidation' generates reactive species that initiate the oxidation of ammonia, which in turn begins heat release that raises the temperature and accelerates the oxidation process towards ignition. The reaction path analysis shows that the low-temperature chain-branching reactions of DME are important in the early oxidation of the fuel, while the sensitivity analysis indicates that several reactions in the oxidation of DME, including cross reactions between DME and NH3 species presented here, are critical to ignition, even at fractions of 2% DME in the fuel. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Autoignition enhancement of methane by admixture of low fraction of acetaldehyde:Simulations and RCM experiments in stoichiometric and rich mixtures

The effect of small fractions of acetaldehyde (CH3CHO) on the ignition delay time of methane (CH4) was examined at high pressure. Measurements are reported for the ignition delay time obtained in a rapid compression machine (RCM) at a compression pressure (Pc) of ∼60 bar and temperatures after compression (Tc) in the range 750–900 K for fuel-air equivalence ratios ϕ in the range 1–4. The results show that mixtures of 2%–5% CH3CHO in CH4 ignite under conditions at which pure methane does not ignite experimentally. The efficiency of acetaldehyde as a promoter seems to be comparable to that of other oxygenated fuels like alcohols and ethers. For comparison with the experimental results, ignition delay times are computed using an updated reaction mechanism and two mechanisms from the literature for CH3CHO oxidation. For most conditions, the simulations using the current mechanism agree with the measurements to within a factor of two. The ignition profile shows a pre-ignition temperature rise and two-stage ignition similar to that previously observed in low fractions of dimethyl ether in ammonia; both phenomena are captured by the simulations. Analysis of simulations at constant volume indicates that CH3CHO is oxidized much more rapidly than CH4, producing reactive species that initiate the oxidation of CH4 and generates heat that accelerates oxidation toward ignition. The low-temperature chain-branching reactions of CH3CHO are important in the early oxidation of the fuel mixture. Additional simulations were performed for equivalence ratios of ϕ = 1 and 4, at a compression pressure (Pc) of 100 bar and Tc = 750–1000 K. The simulations indicate that CH3CHO has a strong ignition-enhancing effect on CH4, with small fractions reducing the ignition delay time by up to a factor of 100, depending on the temperature, as compared to pure CH4.</p

Experimental study of the combustion properties of methane/hydrogen mixtures

In this thesis the combustion properties of methane / hydrogen mixtures are investigated by measering autoignition delay times in methane/hydrogen mixtures under conditions relevant for gasengines. Moreover HCN and C2H2 measurements have been performed in fuel-rich one dimensional laminar CH4/H2/air flames. All measurements have been compared with numerical calculations.

Water Solubility in CO2 Mixtures: Experimental and Modelling Investigation

AbstractThe capture of CO2 from power plants and other large industrial sources is offering a main solution to reduce CO2 emissions. The captured mixture will contain impurities like nitrogen, argon, oxygen, water and some toxic elements like sulfur and nitrogen oxides, the types and quantities of which depend on the type of fuel and the capture process. The presence of free water formation in the transportation pipeline causes severe corrosion problems, flow assurance failure and might damage valves and instrumentations. In the presence of free water, CO2 dissolves in the aqueous phase and will partly ionize to form a weak acid. Thus, free water formation should be avoided.This work aims to investigate the solubility of water in CO2 mixtures under pipeline operation conditions in the temperature range of (5–35°C) and the pressure range of (90–150bar). A test set up was constructed, which consists of a high pressure reactor in which a CO2 mixture containing water at initial soluble conditions was prepared. The purpose of this study is to identify the maximum water content level which could be allowed in CO2 transportation pipelines. The experimental data generated were then compared to the calculations of two mixture models: the GERG-2008 model and the EOS-CG model

Requirements for gas quality and gas appliances

Introduction The gas transmission network in the Netherlands transports two different qualities of gas, low-calorific gas known as G-gas or L-gas and, high calorific gas (H-gas). These two gas qualities are transported in separate networks, and are connected by means of five blending and conversion stations where high-calorific gas can either be blended with low-calorific gas or ballasted with nitrogen to produce gas that can be introduced into the low-calorific, G-gas, network. The network was originally developed following the discovery of the large Groningen gas field. The Groningen field is low calorific gas. The low-calorific gas from the Groningen field became the standard for the consumers in the Netherlands. Later on, H-gas gas was produced from the so-called small fields in the Netherlands. High-calorific gas is also imported from Norway, Russia and through the LNG terminal in Rotterdam. The H-gas is supplied to industrial end users via approximately 80 connections to the high-calorific network and is also exported to other countries. The standard Wobbe Index bandwidth currently specified for exit points in the gas transmission network for H-gas is 47-55.7 MJ/m3 (25/0), with a number of regional variations as described in the Ministerial Ruling (MR) for gas quality. The supply of low-calorific gas from the Groningen field is in decline, and ultimately the future supply to end users will be high-calorific gas. In March 2012, the Minister of Economic affairs declared that all appliances falling under the Gas Appliance Directive (the GAD) should be able to switch to high-calorific gas to prepare for a smooth transition at the end of the lifetime of the Groningen field. For this purpose, a formal notification has been given to the European Commission. However, at the moment it is unclear what the future range of high-calorific gas quality should be; that is, the range of gas quality that appliances falling under the GAD can accept is at present uncertain. The question posed by the Dutch government is whether or not the government should change the existing notification of the requirements that GAD gas appliances should be able to meet to cope with high-calorific gas, and if so, how and when. Analysis The study comprised three major areas of investigation. In the first step, the limitations in appliance performance with varying gas quality were assessed. This assessment is based on a critical analysis of existing data from laboratory experiments on appliances from well-defined field tests and also from a critical analysis of non-experimental information (theoretical studies and evidence-based experience). In the second step, the future ranges of gas quality expected to be supplied in the Netherlands was inventoried, and the possibilities for, and costs of, treating the gases to limit the range of gas quality was assessed. In the third step, the potential of innovation for widening the range of gases acceptable for appliances was assessed. The recommendations summarized below are based on the synthesis of these three elements. Limitations in appliances We assessed the existing experimental, theoretical and practical/experiential evidence regarding which bands of Wobbe Index maintain the safety and reliability of the population of H-/E-band appliances installed in the field. In our opinion, the distribution practices in the UK, France, Denmark and Belgium give the best reflection of a practical range for Wobbe Index: the years-long practice in these countries shows that H-/E-band appliance performance with distribution limits in the range 4-5 MJ/m3 satisfies the national requirements and/or customs for safety and reliability in these countries. We also note that DNV GL – Report No. 74106553 .01b – www.dnvgl.com Page 2 these countries have some form of active maintenance regime. Provided adjustable appliances are properly adjusted, the laboratory experiments assessed support a range of 4-5 MJ/m3, although these experiments require extra interpretation before being applied to the situation in practice. The theoretical analyses show that the approval regime does not safeguard the intended appliance performance in a number of situations. However, these analyses say nothing about the actual safe and reliable performance of appliances in the field. Therefore, we conclude that a bandwidth of in the range of 4-5 MJ/m3 can be realized for appliances approved under the GAD. Future supply Ultimately, when Dutch indigenous gas production becomes small, gas is expected to be supplied by pipeline imports from Norway and Russia, and as LNG from the worldwide market. The Netherlands is expected to become a net importer from 2025 onwards. The expectation is further that the imported gas will have a Wobbe Index between 51 and 55.7 MJ/m3 (25/0). The upper limit of 55.7 MJ/m3 is set by the Dutch government. This gas quality bandwidth is significantly smaller than the current standard bandwidth permitted in the H-network, from 47 to 55.7 MJ/m3. The ‘small fields’ contribute predominantly to the lower half of this range. At the moment an annual volume of 25 bcm is produced from the small fields. By 2032 the total capacity including so-called futures is expected to be almost three times lower than the 2014 capacities. These futures are however uncertain. When the futures are not taken into account, the total capacity will be 15 times lower compared to 2014. In 2032 supply from the small fields will then only be 3% of demand (20% if the futures fully materialize). Excluding futures, the production volume of the small fields decreases to 1 bcm in 2030. Three cases with different Wobbe Index bandwidths were evaluated: a band of 2 MJ/m3, significantly narrower than the expected range of import qualities, a wide band of 8 MJ/m3 and a band coincident with the expected future import band of 4.7 MJ/m3. The results show that the narrow band option requires the most gas treatment, particularly nitrogen ballasting, which given the expected import will be required indefinitely. The costs for gas treatment (especially nitrogen ballasting) will thus recur every year. The widest band allows the widest accommodation of both import gases and residual small-field gases. Referring to the appliance limitations analysis above, not all appliances can handle a band of 8 MJ/m3 without further gas treatment measures. We note that the potential measures that support the intake of ‘off-spec’ gas with a Wobbe Index lower than the minimum Wobbe Index of H-gas imports are expected to be temporary. The volume of gas from the small fields is expected to become very small after 2030 as mentioned above. The use of the gas quality management options described here is seen more as a transition measure and not as permanent. In light of the expected gas supply, the widest band gives therefore only a temporary advantage. For the intermediate band of 4.7 MJ/m3, the import gases can be easily accepted, but relatively more small-field gases become ‘off spec’, requiring blending with the H-gas import. The forecast of H-gas imports volumes indicate that sufficient H-gas is available for blending the ‘off spec’ gas in this case. Ballasting with nitrogen will not be required if the upper Wobbe limit is set at 55.7 MJ/m3. Concluding, choosing the intermediate band of 4.7 MJ/m3 corresponds to the bandwidth of the expected import of future H-gases of 51-55.7 MJ/m3, and is also within the range of 4-5 MJ/m3 to which the GAD appliances have been exposed in practice. In this option there are no extra costs for nitrogen ballasting. This range may provide an optimum between appliance performance and expected gas supplies.Innovation An inventory has been made to determine the status of the development of innovative products aimed at extending the fuel flexibility of GAD end-use equipment, e.g. by means of active control systems. The inventory is based on interviews with different stakeholders and collecting existing information available, including progress made in existing innovation programs such as SBIR. It became clear that commercially available (premixed) appliances having active control systems are suited to operate across the entire E-band and for handling abrupt Wobbe fluctuations. We also observed promising developments for (inexpensive) control systems for premixed appliances to extend the fuel flexibility for both new domestic appliances and suitable appliances already installed in the field. Also, a sensor-based hob burner (cooker) is under development to guarantee high performance and capacity while using variable gas quality (L+H band). DNV GL is not aware of any existing developments or innovations to make other type of partially premixed domestic appliances, such as flow-through hot water heaters, suitable for a wide range of gas compositions. To our knowledge, no innovation regarding the development of fuel-adaptive control systems for non-domestic burners is currently being undertaken. However, several control strategies are possible and economically feasible. Recommendation Based on the analysis, DNV GL recommends setting the long term quality bandwidth for H-gas at 51 – 55.7 MJ/m3 (25/0). In this choice, the bandwidth (4.7 MJ/m3) is within the 4-5 MJ/m3 with which millions of H-/E-band appliances function in other EU countries, aligning the distribution practice with those in other countries, and complex and costly gas quality management measures are limited. A consistent policy for appliance adjustment using known gas quality is essential for maintaining this band. We recommend updating the existing notification and suggest modifications to the notification as formulated in Appendix A. We further note that in terms of Wobbe Index this band allows room for the accommodation of a reasonable bandwidth for renewable gases