• Energy Research

Abstract 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/CH4 fuel combination are available for such conditions while such information for the DMM/CH4 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/CH4 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 H2, 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% (C2 hydrocarbons) and 3.4−8.7% (CH2O), 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/CH4 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.

Dimethyl ether (DME), a widely studied alternative fuel, is known to exhibit complex low- and high-temperature oxidation chemistry. It is also the smallest molecule in the families of symmetric ethers and oxymethylene ethers that receive attention as renewable fuels. Thanks to several studies performed in facilities such as shock tubes, jet-stirred reactors and flames, it can be assumed that the DME oxidation is well understood. However, DME oxidation in flow reactors has been addressed comparatively rarely, although this configuration presents an interesting system with influences of both kinetics and fluid dynamics on the reaction behavior. To examine the interplay of both influences and potential uncertainties resulting from such effects, DME oxidation was experimentally investigated over an extended range of conditions in a flow reactor equipped with mass-spectrometric analysis. Quantitative species profiles were obtained at near-atmospheric pressure in a temperature range of 40 0-110 0 K for three equivalence ratios phi (0.8, 1.0 and 1.2), and three flow rates at each stoichiometry. These nine different cases were first analyzed using a detailed chemical reaction mechanism with a Plug-Flow Reactor (PFR) model. In-depth examination of experimental and reaction model uncertainties led to updates in the reaction mechanism that were performed on the basis of most recent, reliable kinetic information. In spite of the good agreement of the PFR model with the experimental data at selected conditions, especially in the low temperature regime, substantial deviations in the reactivity and associated species profiles were noted in several cases, particularly for lean conditions at low flow rates and intermediate temperatures around and above 700 K. A two-dimensional (2D) computational fluid dynamics (CFD) model was therefore employed to characterize the reactive flow conditions more accurately. Significant contributions of fluid dynamics effects were observed in the cases that presented the most severe deviations, and overall good agreement within experimental uncertainty was obtained for the nine cases with the 2D simulations. With the aid of a mathematical curve matching procedure using a variety of recent, established kinetic mechanisms, it could be convincingly demonstrated under the current conditions that improvements in predictive modeling capability in the sensitive test cases were not a question of improved kinetics but were mainly achieved by considering the two-dimensional reactive flow. As a consequence, the present investigation can serve to alert the community to the potential major influences that might be neglected if standard PFR models are used to predict fuel oxidation without detailed analysis whether the conditions are suited to that approach. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Dimethyl ether (DME) is a widely recognized alternative fuel which can be sustainably produced from different feedstock. Its oxidation mechanism belongs to the most deeply understood within oxygenated fuels. The oxidation of DME in the presence of NO has gained renewed attention in recent experimental and modeling effort s because of a rather complex behavior in accelerating or inhibiting DME consumption, depending on the respective temperature and mixture conditions, similarly to what has been already observed for n- and iso- alkanes. The present investigation focuses on the interaction chemistry of DME-O 2 -NO mixtures in the low- to intermediate-temperature regime. Previously reported flow reactor data from mass spectrometric analysis with and without NO addition are extended by isomer-resolved detection of some key intermediates, including species of formula {HNO 2 } and {CH 3 NO 2 } by using doubleimaging photoelectron photoion coincidence (i 2 PEPICO) spectroscopy. Specifically trans- HONO is highlighted as the most abundant isomer. Starting from the recently published CRECK mechanism for DME oxidation and other models available in the literature, the relevant kinetics of DME/NO x interactions are included and analyzed. The model thus obtained is compared with new experimental data from this study and others from the literature and is used to interpret observed discrepancies. A systematic polynomial chaos expansion (PCE) analysis is also performed to assess the joint uncertainty of key influential reactions under the present conditions. Nevertheless, remaining differences of model and experiment can only be addressed jointly, demonstrating the value of a concurrent experimental-modeling approach. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.