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The use of natural-gas-fueled combustion engines at unusual operating conditions to provide electrical and/or chemical energy on demand emphasizes the need for fundamental research on decomposition and formation of base chemicals at these conditions. In this work, the CO formation behind reflected shock waves from the pyrolysis of CO 2 /CH 4 mixtures was investigated for the first time in the context of engine-based dry methane reforming, to understand the interaction of CO 2 and CH 4 at high temperatures and to test the validity of literature reaction mechanisms. Different CO 2 /CH 4 mixtures at atmospheric pressure and temperatures between 1900 K and 2700 K were investigated. Time-resolved CO measurements were performed by laser absorption using a quantum cascade laser. With increasing CO 2 addition later reaction onset was observed, showing a reduction in the overall reactivity. Rate of production and sensitivity analyses highlight competing reactions in the pyrolysis and oxidation pathways and that the number of available H radicals is limited, which is attributed to the reduced reactivity. However, the analysis shows that CO 2 is also a source for OH radicals (via CO 2 + H ⇌ CO + OH), which enhance methane decomposition. The comparison with literature reaction mechanisms showed that none of the tested mechanisms can perfectly predict the time-resolved CO formation, highlighting the need for the validation of detailed kinetics models under nontypical conditions. [ABSTRACT FROM AUTHOR]

The partial oxidation of natural gas in HCCI engines in terms of a polygeneration process could be a promising technology to flexibly produce useful chemicals, heat, and work, depending on demand. Because natural gas is relatively inert, high intake temperatures or compression ratios are required to initiate its conversion which can result in a lower lifetime of the engine. Alternatively, small amounts of more reactive species such as ozone can be added to the initial mixture to provide radicals at more moderate conditions and initiate the main ignition. In this study, plug-flow reactor experiments are performed to assess the influence of ozone on the fuel conversion and product formation during the partial oxidation of methane and natural gas. Experiments are performed in the temperature range of 373 K to 973 K at 4 bar and equivalence ratios of 2. Molecular-beam mass spectrometry coupled with electron ionization is used as analytical technique to detect the mixture composition at the outlet of the reactor. The results show that even very small amounts of ozone can help to shift the conversion onset to much lower temperatures and increase the yields of useful chemicals. The data can further be used to improve reaction mechanisms describing the conversion of hydrocarbons in the presence of ozone. A literature ozone reaction mechanism has been implemented in two recent hydrocarbon mechanisms and the results of simulations using these mechanisms are compared to the experimental data with respect to the low-temperature intermediates which influence ignition. Predictions differ substantially from the experimental results identifying starting points for further investigations. The speciation data provided in this work contribute to extending the reaction kinetics of ozone assisted fuel conversion to higher pressure. [ABSTRACT FROM AUTHOR]

Industrial waste gases from chemical production plants often contain toxic products that usually have to be removed or destroyed by waste gas purification or thermal afterburning. These off-gases are still interesting from an energetic and chemical perspective because they contain products with low-calorific but non-negligible heating values and carbon. This work addresses the conversion of typical ammonia and amine containing off-gases from chemical plants in fuel-rich operated HCCI engines with respect to carbon-recovery and simultaneous output of work and heat. To test this concept and to overcome technical limitations and avoid pollutant formation, the HCCI engine process is simulated. Two off-gas mixtures were chosen exemplarily, containing ammonia, dimethylamine, methanol, and acetaldehyde diluted in nitrogen at equivalence ratios of 9.6 (mixture 1) and 3.2 (mixture 2). The investigation showed that without further oxygen addition, mixture 2 ignites, but the ammonia conversion remains below 42%, while the heat release from mixture 1 is small. Oxygen addition up to equivalence ratios of 2 – 2.5 leads to higher ammonia conversion, entailing maximum yields for H 2 and CO of 60% and 80%, respectively. The NOx and HCN formation are reasonably low at equivalence ratios between 1 and 2 for both mixtures. The equivalence ratio strongly affects the ammonia decomposition and product gas formation, while a variation of the inlet temperature can be used to achieve a stable combustion phasing. Considering all findings, good conditions are inlet temperatures of 348 K (Mixture 1) and 423 K (Mixture 2), and an equivalence ratio of 1.5. High exergetic efficiencies of up to 76% are predicted for these conditions. Since off-gas mixtures may have a varying composition, the sensitivity of the ignition in relation to single species in the mixture was assessed. Methanol and acetaldehyde mainly affect the ignition because they lead to the highest radical concentrations, whereas dimethylamine mainly consumes radicals. The reactions that mostly change the outcome of the ammonia conversion are decomposition reactions of dimethylamine. The theoretical analysis revealed a feasible process awaiting an experimental validation regarding kinetics and engine operation. [ABSTRACT FROM AUTHOR]

The conversion of mechanical to chemical energy offers an option for long-term and versatile energy storage. It was already proven that piston engines can be used as flexible reactors for energy conversion. Here, a novel method for energy conversion in piston engines is investigated, the pyrolysis of natural gas/hydrogen mixtures for energy storage. The supplied energy is stored by chemical conversion into hydrogen and higher energy hydrocarbons. The storage efficiency and the product composition are addressed here. To reach sufficiently high temperatures after compression, a dilution with 85–99% argon is used. The main products are hydrogen, acetylene, ethylene and benzene but also soot precursors are formed. The piston engine is simulated as a time-dependent four-stroke single-zone model with detailed chemical kinetics. The intake pressure is kept constant at 2 bar, while intake temperature, intake argon mole fraction and the hydrogen/natural gas ratio is varied. The hydrogen addition allows a reduction of the intake temperature and argon dilution but also reduces the storage power and efficiency. Yields of acetylene or ethylene are increased and the formation of soot precursors is suppressed. A storage power of 1.59 kW is reached with an efficiency of 52%. • Chemical energy storage enables the long-term storage of renewable energies. • Flexible energy conversion in a piston engine is investigated theoretically. • The thermodynamics and kinetics within the piston engine are modelled. • High energy products formed by natural gas pyrolysis using excess renewable energy. • Hydrogen addition reduces the production of PAH's. [ABSTRACT FROM AUTHOR]

The potential of dimethyl ether (DME) and dimethoxymethane (DMM), representatives of the attractive oxymethylene ether (OME) alternative fuel family, are explored here as reactivity enhancers for methane-fueled polygeneration processes. Typically, such processes that can flexibly generate power, heat, or chemicals, operate under fuel-rich conditions in gas turbines or internal combustion engines. To provide a consistent basis for the underlying reaction mechanisms, it is recognized that speciation data for the DME/CH 4 fuel combination are available for such conditions while such information for the DMM/CH 4 system is largely lacking. In addition, it should be noted that a detailed speciation study in flames, i.e. , combustion systems involving chemistry and transport processes over a large temperature range, is still missing in spite of the potential of such systems to provide extended species information. In a systematic approach using speciation with electron ionization molecular-beam mass spectrometry (EI-MBMS), we thus report, as a first step, investigation of six fuel-rich premixed flames of DME and DMM and their blends with methane with special attention on interesting chemicals. Secondly, a comprehensive but compact DME/DMM/CH 4 model (PolyMech2.1) is developed based on these data. This model is then examined against available experimental data under conditions from various facilities, focusing preferentially on elevated pressure and fuel-rich conditions. Comparison with existing literature models is also included in this evaluation. Thirdly, an analysis is given on this basis, via the extensively tested PolyMech2.1 model, for assumed polygeneration conditions in a homogeneous charge compression ignition (HCCI) engine environment. The main interest of this model-assisted exploration is to evaluate whether addition of DME or DMM in a polygeneration process can lead to potentially useful conditions for the production of syngas or other chemicals, along with work and heat. The flame results show that high syngas yields, i.e. , up to ∼78% for CO and ∼35% for H 2 , can be obtained in their burnt gases. From the large number of intermediates detected, predominantly acetylene, ethylene, ethane, and formaldehyde show yields of 2.1−4.4% (C 2 hydrocarbons) and 3.4−8.7% (CH 2 O), respectively. Also, methanol and methyl formate show comparably high yields of up to 0.6−6.7% in the flames with DMM, which is 1–2 orders of magnitude higher than in those with DME as the additive. In the modeling-assisted exploration of the engine process, the PolyMech2.1 model is seen to perform at significantly reduced computational costs compared to a recently validated model without sacrificing the prediction performance. Promising conditions for the assumed polygeneration process using fuel combinations in the DME/DMM/CH 4 system are identified with attractive syngas yields of up to 77% together with work and heat output at exergetic efficiencies of up to 89% with DME. [ABSTRACT FROM AUTHOR]

Piston engines are typically considered devices converting chemical energy into mechanical power via internal combustion. But more generally, their ability to provide high-pressure and high-temperature conditions for a limited time means they can be used as chemical reactors where reactions are initiated by compression heating and subsequently quenched by gas expansion. Thus, piston engines could be "polygeneration" reactors that can flexibly change from power generation to chemical synthesis, and even to chemical-energy storage. This may help mitigating one of the main challenges of future energy systems – accommodating fluctuations in electricity supply and demand. Investments in devices for grid stabilization could be more economical if they have a second use. This paper presents a systematic approach to polygeneration in piston engines, combining thermodynamics, kinetics, numerical optimization, engineering, and thermo-economics. A focus is on the fuel-rich conversion of methane as a fuel that is considered important for the foreseeable future. Starting from thermodynamic theory and kinetic modeling, promising systems are selected. Mathematical optimization and an array of experimental kinetic investigations are used for model improvement and development. To evaluate technical feasibility, experiments are then performed in both a single-stroke rapid compression machine and a reciprocating engine. In both cases, chemical conversion is initiated by homogeneous-charge compression-ignition. A thermodynamic and thermo-economic assessment of the results is positive. Examples that illustrate how the piston engine can be used in polygeneration processes to convert methane to higher-value chemicals or to take up carbon dioxide are presented. Open issues for future research are addressed. • Piston engines can be efficiently used for polygeneration of chemicals, work, and heat. • Methods of combustion science were used to address new targets. • Combining kinetics, thermodynamics, optimization, and thermo-economics yields new synergies. • Chemical energy storage and reforming processes are feasible with piston engines. • The low reactivity of methane as a base fuel can be increased with additives. [ABSTRACT FROM AUTHOR]