Research Papers: Alternative Energy Sources

J. Energy Resour. Technol. 2017;139(4):041201-041201-12. doi:10.1115/1.4035904.

This study analyzes the feasibility of placing modified exercise equipment in public places to enable human energy harnessing. By assessing the impacts as a system-level synthesis of economic, environmental, productivity, and health benefits, it is shown that introducing human-powered equipment (HPE) in public places would be feasible and beneficial both to society in general and to the specific stakeholders investing in this technology. This study develops a framework to evaluate applications of this technology using benefits to costs analyses. The benefits and challenges for successful implementation of HPE technology are also presented and evaluated in various case studies involving public places at airports and schools.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2017;139(4):042001-042001-5. doi:10.1115/1.4035908.

In this work, the effect of temperature on the pressure loss for Newtonian fluid in fully eccentric annulus with pipe rotation is investigated. Extensive experiments with water are conducted at Izmir Katip Celebi University (IKCU), Civil Engineering Department for various flow velocities ranging between 0.7 m/s and 2.9 m/s, pipe rotation range between 0 rpm and 120 rpm. The effect of temperature on frictional pressure losses is also examined, and the temperature is varied from 20 °C to 65 °C. It was observed that, an increase in the fluid temperature in fully eccentric annulus results in a decrease in the pressure gradient. On the other hand, the influence of temperature on pressure gradient becomes more significant, as the Reynolds number is raised. Variation of Taylor number causes negligible changes on frictional pressure losses for all temperature conditions considered. By using regression analysis of the dataset obtained from the experimental work, a simple empirical frictional pressure losses correlation taking into account of temperature effect is proposed. Results showed that a good agreement between the measured and predicted values is achieved with almost 94% coefficient of determination.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042002-042002-10. doi:10.1115/1.4035909.

In acidizing operations, the acid flows selectively through large pores to create wormholes. Wormhole propagation has been studied by many experts at macroscopic scale. In this paper, the lattice Boltzmann model (LBM), which is a mesoscopic scale method, is adopted to simulate the flow, acid–rock reaction, and rock dissolution in porous media at mesoscopic scale. In this model, a new method based on nonequilibrium extrapolation is proposed to deal with the reactive boundary. On the basis of the model, extensive simulations are conducted on the propagation behavior of wormholes, and the factors influencing wormhole propagation are investigated systematically. The results show that the LBM is a reliable numerical technique to study chemical dissolution in porous media at mesoscopic scale, and that the new method of dealing with the reaction boundary performs well. The breakthrough time decreases with the increase of acid concentration, but acid concentration does not affect the ultimate dissolution pattern. As the reaction rate constant increases, shorter wormholes are created. A higher hydrogen ion diffusion coefficient will result in shorter but wider wormholes. These findings agree well with the previous experimental and theoretical analyses. This study demonstrates the mechanism of wormholing that the unstable growth of pores by the acid rock reaction makes the acid selectively flow through a few large pores which finally form wormholes.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042003-042003-12. doi:10.1115/1.4036045.

Thermocells convert heat energy directly into electrical energy through charge-transfer reactions at the electrode–electrolyte interface. To perform an analytical study on the behavior of thermocells, the Onsager flux relationship was applied to thermocells, which used aqueous copper II sulfate and aqueous potassium ferri/ferrocyanide as the electrolyte. The transport coefficient matrices were calculated for each electrolyte and applied to several simulations, which were subsequently validated through experimental testing and comparison to previous literature results. The simulation is shown to correctly predict the short circuit current, maximum power output, and power conversion efficiency. Validation demonstrates that the simulation model developed, using the Onsager flux equations, works for thermocells with different electrode materials (platinum, copper, charcoal, acetylene black, and carbon nanotube), electrode spacing, and temperature differentials. The power dependence of the thermocell on concentration and electrode spacing, with respect to the Seebeck coefficient, maximum power output, and relative efficiency, is also shown.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042004-042004-7. doi:10.1115/1.4035829.

Turbine blades are cooled by a jet flow from expanded exit holes (EEH) forming a low-temperature film over the blade surface. Subsequent to our report on the suction-side (low-pressure, high-speed region), computational analyses are performed to examine the cooling effectiveness of the flow from EEH located at the leading edge as well as at the pressure-side (high-pressure, low-speed region). Unlike the case of the suction-side, the flow through EEH on the pressure-side is either subsonic or transonic with a weak shock front. The cooling effectiveness, η (defined as the temperature difference between the hot gas and the blade surface as a fraction of that between the hot gas and the cooling jet), is higher than the suction-side along the surface near the exit of EEH. However, its magnitude declines sharply with an increase in the distance from EEH. Significant effects on the magnitude of η are observed and discussed in detail of (1) the coolant mass flow rate (0.001, 0.002, and 0.004 (kg/s)), (2) EEH configurations at the leading edge (vertical EEH at the stagnation point, 50 deg into the leading-edge suction-side, and 50 deg into the leading-edge pressure-side), (3) EEH configurations in the midregion of the pressure-side (90 deg (perpendicular to the mainstream flow), 30 deg EEH tilt toward upstream, and 30 deg tilt toward downstream), and (4) the inclination angle of EEH.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042004-042004-8. doi:10.1115/1.4036053.

Growing evidence suggests that research must be done to develop energy efficient systems and clean energy conversion technologies to combat the limited sources of fossil fuel, its high price, and its adverse effects on environment. Thermoacoustic is a clean energy conversion technology that uses the conversion of sound to thermal energy and vice versa for the design of heat engines and refrigerators. However, the efficient conversion of sound to thermal energy demands research on altering fluid, operational, and geometric parameters. The present study is a contribution to improve the efficiency of thermoacoustic devices by introducing a novel stack design. This novel stack consists of alternative conducting and insulating materials or heterogeneous materials. The author examined the performance of eight different types of heterogeneous stacks (combination 1–8) that are only a fraction of the displacement amplitude long and consisted of alternating aluminum (AL) and Corning Celcor or reticulated vitreous carbon (RVC) foam materials. From the thermal field measurements, the author found that combination eight performs better (12% more temperature difference at the stack ends) than all the other combinations. One interesting feature obtained from these experiments is that combination 7 produces the minimum temperature at the cold end (17% less than other combinations). The thermal performance of the heterogeneous stack is compared to that of the traditional homogeneous stack. Based on the study, the newly proposed stack design provides better cooling performance than a traditionally designed stack.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042005-042005-10. doi:10.1115/1.4036383.

In this work, parametric analysis of ejector expansion refrigeration cycles (EERC) with two different types of ejectors (constant area (CA) ejector and constant pressure (CP) ejector) is performed, and comparison of the results is presented. Effects of variation in operational parameters (condenser temperature, evaporator temperature, and cooling capacity) on coefficient of performance (COP), entrainment ratio (w), and pressure lift factor (Plf) are investigated. The range of variation for evaporator temperature, condenser temperature, and cooling capacity are −5 to 15 °C, 50–70 °C, and 10–80 kW, respectively. The ejector refrigeration cycle is simulated by ees software. The obtained results are validated by the experimental data available in the literature. The refrigerant R134a is selected based on the merit of its environmental and performance characteristics. The results show that the effect of evaporator temperature is much higher than that of condenser temperature on Plf. In contrast, the influence of condenser temperature on COP is much stronger than that of evaporator temperature. It is seen that COP and Plf of ejector expansion refrigeration cycle with constant pressure ejector (CP-EERC) are higher than those of refrigeration cycle with constant area ejector.

Commentary by Dr. Valentin Fuster

Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2017;139(4):042201-042201-10. doi:10.1115/1.4035886.

The present study explores the impact of ethanol on the performance and emission characteristics of a single cylinder indirect injection (IDI) Diesel engine fueled with Diesel–kerosene blends. Five percent ethanol is added to Diesel–kerosene blends in volumetric proportion. Ethanol addition to Diesel–kerosene blends significantly improved the brake thermal efficiency (BTE), brake specific energy consumption (BSEC), oxides of nitrogen (NOx), total hydrocarbon (THC), and carbon monoxide (CO) emission of the engine. Based on engine experimental data, an artificial neural network (ANN) model is formulated to accurately map the input (load, kerosene volume percentage, ethanol volume percentage) and output (BTE, BSEC, NOx, THC, CO) relationships. A (3-6-5) topology with Levenberg–Marquardt feed-forward back propagation (trainlm) is found to be optimal network than other training algorithms for predicting input and output relationship with acceptable error. The mean square error (MSE) of 0.000225, mean absolute percentage error (MAPE) of 2.88%, and regression coefficient (R) of 0.99893 are obtained from the developed model. The study also attempts to make clear the application of fuzzy-based analysis to optimize the network topology of ANN model.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042202-042202-8. doi:10.1115/1.4036046.

Surfactants have the potential to reduce the interfacial tension between oil and water and mobilize the residual oil. An important process which makes the surfactant injection to be less effective is loss of surfactant to porous medium during surfactant flooding. This study highlights the results of a laboratory study on dynamic adsorption and desorption of Trigoonella foenum-graceum (TFG) as a new nonionic surfactant. The experiments were carried out at confining pressure of 3000 psi and temperature of 50 °C. Surfactant solutions were continuously injected into the core plug at an injection rate of 0.5 mL/min until the effluent concentration was the same as initial surfactant concentration. The surfactant injection was followed by distilled water injection until the effluent surfactant concentration was reduced to zero. The effluent concentrations of surfactant were measured by conductivity technique. Results showed that the adsorption of surfactant is characterized by a short period of rapid adsorption, followed by a long period of slower adsorption, and also, desorption process is characterized by a short, rapid desorption period followed by a longer, slow desorption period. The experimental adsorption and desorption data were modeled by four well-known models (pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Elovich models). The correlation coefficient of models revealed that the pseudo-second-order model predicted the experimental data with an acceptable accuracy.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042203-042203-7. doi:10.1115/1.4036049.

The structural parameters of combustion chamber have great impacts on the process of air–fuel mixing, combustion, and emissions of diesel engine. The dynamic characteristics and emission performances could be improved by means of optimizing the parameters of the combustion chamber. In this paper, the key structure of a diesel engine combustion chamber is parameterized, and the influence of individual structural parameter on dynamic characteristics and emissions of the engine is simulated and analyzed by computational fluid dynamics (CFD) software avl-fire. The results show that under constant compression ratio, the in-cylinder peak pressure decreases with increasing inclination angle of the combustion chamber (α), while the height (Tm) and bowl radius (R) have little influence on the in-cylinder peak pressure. With increasing α, NO emissions decrease, and soot emissions first increase and then decrease. With increasing R, both NO and soot emissions decrease first and then increase. Therefore, the combustion chamber parameters could be optimized by comprehensive consideration of cylinder pressure, NO and soot emissions.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042204-042204-7. doi:10.1115/1.4035752.

This manuscript presents experimental results on the reduction of sulfur oxide emissions from combustion of a high-sulfur content pulverized bituminous coal (Illinois #6 Macoupin) using a dry sorbent injection method. The coal particles were in the size range of 90–125 μm and were blended with calcium-, sodium-, potassium-, and magnesium-containing powdered sorbents at different proportions. The alkali/sulfur molar ratios were chosen to correspond to stoichiometric proportions (Ca/S = 1, Mg/S = 1, Na2/S = 1, and K2/S = 1) and the effectiveness of each alkali or alkali earth based sorbent was evaluated separately. Combustion of coal took place in a drop-tube furnace, electrically heated to 1400 K under fuel-lean conditions. The evolution of combustion effluent gases, such as NOx, SO2, and CO2 was monitored and compared among the different sorbent cases. The use of these sorbents helps to resolve the potential of different alkali metals for effective in-furnace sulfur oxide capture and possible NOx reduction. It also assesses the effectiveness of various chemical compounds of the alkalis, such as oxides, carbonates, peroxides, and acetates. Reductions in SO2 emissions were in the range of 5–72%, with sodium being the most effective metal followed by potassium, calcium, and then magnesium. Acetates were effective as dual SO2 and NOx reduction agents.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042205-042205-8. doi:10.1115/1.4036044.

This paper explores the feasibility of using Syngas with low methane number as fuel for commercial turbocharged internal combustion engines. The effect of methane number (MN), compression ratio (CR), and intake pressure on auto-ignition tendency in spark ignition internal combustion engines was determined. A nondimensional model of the engine was performed by using kinetics mechanisms of 98 chemical species in order to simulate the combustion of the gaseous fuels produced from different thermochemical processes. An error function, which combines the Livengood–Wu with ignition delay time correlation, to estimate the knock occurrence crank angle (KOCA) was proposed. The results showed that the KOCA decreases significantly as the MN increases. Results also showed that Syngas obtained from coal gasification is not a suitable fuel for engines because auto-ignition takes place near the beginning of the combustion phase, but it could be used in internal combustion engines with reactivity controlled compression ignition (RCCI) technology. For the case of high compression ratio and a high inlet pressure at the engine's manifold, fuels with high MN are suitable for the operating conditions proposed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042206-042206-9. doi:10.1115/1.4036252.

Aftermarket dual-fuel injection systems using a variety of different fumigants have been proposed as alternatives to expensive after-treatment to control NOx emissions from legacy diesel engines. However, our previous work has shown that available add-on systems using hydrous ethanol as the fumigant achieve only minor benefits in emissions without recalibration of the diesel fuel injection strategy. This study experimentally re-evaluates a novel aftermarket dual-fuel port fuel injection (PFI) system used in our previous work, with the addition of higher flow injectors to increase the fumigant energy fraction (FEF), defined as the ratio of energy provided by the hydrous ethanol on a lower heating value (LHV) basis to overall fuel energy. Results of this study confirm our earlier findings that as FEF increases, NO emissions decrease, while NO2 and unburned ethanol emissions increase, leading to no change in overall NOx. Peak cylinder pressure and apparent rates of heat release are not strongly dependent on FEF, indicating that in-cylinder NO formation rates by the Zel'dovich mechanism remain the same. Through single zone modeling, we show the feasibility of in-cylinder NO conversion to NO2 aided by unburned ethanol. The modeling results indicate that NO to NO2 conversion occurs during the early expansion stroke where bulk gases have temperature in the range of 1150–1250 K. This work conclusively proves that aftermarket dual fuel systems for fixed calibration diesel engines cannot reduce NOx emissions without lowering peak temperature during diffusive combustion responsible for forming NO in the first place.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042207-042207-5. doi:10.1115/1.4036253.

This work evaluated the genotoxic potential of the soluble organic material (SOM) extracted from the particulate matter (PM) emitted by an automotive diesel engine. The engine was modified to operate with a home-made multipoint-port injection system to substitute 10% of ultralow-sulfur diesel (ULSD) fuel in energy basis by hydrous ethanol (h-Et) or n-butanol (n-Bu) injected into the manifold during the intake stroke. A low engine load mode named M4 (43 N·m at 2410 min−1) and a medium-load mode M2 (95 N·m at 2410 min−1) were selected from the vehicle homologation cycle. PM was collected with a stainless steel filter located 1.5 m downstream the exhaust manifold. The SOM of the PM was extracted to evaluate the genotoxic activity on human lymphocytes using the comet assay. Results indicated that independently of the mode, the SOM coming from alcohols led more genotoxicity than ULSD, following the order h-Et > n-Bu > ULSD. The low engine load operation exhibited much more deoxyribonucleic acid (DNA) damage than mode M2, especially the PM produced by hydrous ethanol port-injection. Although further research is still necessary, these findings suggest that the biology activity of the SOM coming from alcohols PM could be a barrier for the implementation of alcohol port-injection technology.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042208-042208-6. doi:10.1115/1.4036254.

Ignition delay times and methane species time-histories were measured for methane/O2 mixtures in a high CO2 diluted environment using shock tube and laser absorption spectroscopy. The experiments were performed between 1300 K and 2000 K at pressures between 6 and 31 atm. The test mixtures were at an equivalence ratio of 1 with CH4 mole fractions ranging from 3.5% to 5% and up to 85% CO2 with a bath of argon gas as necessary. The ignition delay times and methane time histories were measured using pressure, emission, and laser diagnostics. Predictive ability of two literature kinetic mechanisms (gri 3.0 and aramco mech 1.3) was tested against current data. In general, both mechanisms performed reasonably well against measured ignition delay time data. The methane time-histories showed good agreement with the mechanisms for most of the conditions measured. A correlation for ignition delay time was created taking into account the different parameters showing the ignition activation energy for the fuel to be 49.64 kcal/mol. Through a sensitivity analysis, CO2 is shown to slow the overall reaction rate and increase the ignition delay time. To the best of our knowledge, we present the first shock tube data during ignition of methane/CO2/O2 under these conditions. Current data provides crucial validation data needed for the development of future kinetic mechanisms.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042209-042209-12. doi:10.1115/1.4036323.

The increasing price of conventional diesel fuel, its deficiency, and the injurious outcome of combustion produced contaminants seem to make different sources more fascinating. Leucas zeylanica plant is noncomestible in nature and available abundantly. Leucas zeylanica methyl ester is renewable and least polluting fuel, which can supplement fossil fuels with unmodified engine condition. The existing experimentation assesses the performance and emission analysis by using various blends of leucas zeylanica methyl ester, diesel, and diesel additives like 2-ethylhexyl nitrate. This experimental investigation gives less engine emission and better performance as compared with mineral diesel. In the radical portion of this investigation, fuzzy-based Taguchi optimization for predicting the optimum input blends results in the optimum combination of performance and emissions parameter.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042210-042210-13. doi:10.1115/1.4036330.

Oxygenated fuels have beneficial effects for leaner lifted-flame combustion (LLFC), a nonsooting mode of mixing-controlled combustion associated with lift-off length equivalence ratios below approximately 2. A single-cylinder heavy-duty optical compression-ignition engine was used to compare neat methyl decanoate (MD) and T50, a 50/50 blend by volume of tripropylene glycol monomethyl ether (TPGME) and #2 ultralow sulfur emissions-certification diesel fuel (CF). High-speed, simultaneous imaging of natural luminosity (NL) and chemiluminescence (CL) were employed to investigate the ignition, combustion, and soot formation/oxidation processes at two injection pressures and three dilution levels. Additional Mie scattering measurements observed fuel-property effects on the liquid length of the injected spray. Results indicate that both MD and T50 effectively eliminated engine-out smoke emissions by decreasing soot formation and increasing soot oxidation during and after the end of fuel injection. MD further reduced soot emissions by 50–90% compared with T50, because TPGME could not completely compensate for the aromatics in the CF. Despite the low engine-out soot emissions, both fuels produced in-cylinder soot because the equivalence ratio at the lift-off length never reached the nonsooting limit. With respect to the other engine-out emissions, T50 had up to 16% higher nitrogen oxides (NOx) emissions compared with MD, but neither fuel showed the traditional soot-NOx trade-off associated with conventional mixing-controlled combustion. In addition, T50 had up to 15% and 26% lower unburned hydrocarbons (HC) and CO emissions, respectively, compared with MD.

Commentary by Dr. Valentin Fuster

Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2017;139(4):042901-042901-8. doi:10.1115/1.4036043.

At the end of 2015 the U.S. held 5.6% or approximately 369 Tcf of worldwide conventional natural gas proved reserves (British Petroleum Company, 2016, “BP Statistical Review of World Energy June 2016,” British Petroleum Co., London). If unconventional gas sources are considered, natural gas reserves rise steeply to 2276 Tcf. Shale gas alone accounts for approximately 750 Tcf of the technically recoverable gas reserves in the U.S. (U.S. Energy Information Administration, 2011, “Review of Emerging Resources: U.S. Shale Gas and Shale Oil plays,” U.S. Department of Energy, Washington, DC). However, this represents only a very small fraction of the gas associated with shale formations and is indicative of current technological limits. This manuscript addresses the question of recovery efficiency/recovery factor (RF) in fractured gas shales. Predictions of gas RF in fractured shale gas reservoirs are presented as a function of operating conditions, non-Darcy flow, gas slippage, proppant crushing, and proppant diagenesis. Recovery factors are simulated using a fully implicit, three-dimensional, two-phase, dual-porosity finite difference model that was developed specifically for this purpose. The results presented in this article provide clear insight into the range of recovery factors one can expect from a fractured shale gas formation, the impact that operation procedures and other phenomena have on these recovery factors, and the efficiency or inefficiency of contemporary shale gas production technology.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042902-042902-11. doi:10.1115/1.4036251.

The sandstone rocks' integrity and consolidation may be highly affected by the type and the strength of the stimulation fluids. Strong acids such as HF/HCl impair the rock consolidation. The reduction in the sandstone rock consolidation will trigger the sand production. Sand causes erosion of downhole and surface equipment especially when it is produced with high gas flow rates. In this study, gentle stimulation fluids for sandstone that consists of chelating agents and catalyst were proposed. The chelating agents are diethylene triamine penta acetic acid (DTPA) and ethylene diamine tetra acetic acid (EDTA). This is the first time to introduce a catalyst (potassium carbonate) in sandstone acidizing. Potassium carbonate was found to work as a clay stabilizer and catalyst that enhances the dissolution of chlorite clay mineral in the sandstone rock. The objective of introducing the catalyst is to enhance the solubility of the insoluble minerals such as chlorite clay minerals. The change in the mechanical properties of sandstone rocks (Bandera and Berea) was evaluated. The possibility of the formation damage after using seawater-based chelating agents was investigated and compared to HF/HCl mud acid. Coreflooding experiments were conducted to evaluate the effect of these fluids on the rock integrity. Computed tomography (CT) scanner was used to assess the formation damage. Different models were used to predict the sand production possibility after the stimulation with chelating agent/catalyst, and this was compared to the HF/HCl mud acid. The results showed that the permeability of sandstone core increased after acidizing. The reduction in CT-number after acidizing confirmed that no formation damage occurred. Rock mechanics evaluation showed no major changes occurred in the rock moduli and no sand production was observed. The model results showed that using chelating gents to stimulate Berea (BR) and Bandera (BN) sandstone cores did not cause sand production. Applying the same models for cores stimulated by HF/HCl acids indicated high possibility of sand production. The addition of potassium carbonate to DTPA chelating agents enhanced the chlorite clay mineral dissolution based on the inductively coupled plasma (ICP) analysis. Potassium carbonate as a catalyst did not affect the sandstone integrity because it only enhanced the dissolution of chlorite clay minerals (selective dissolution) and did not affect the solubility of carbonate minerals which are the primary cementing materials in the sandstone cores. A new dimensionless number was developed that describes the relation between the number of pore volumes (PVs) contacted the rock and the radial distance from the wellbore.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(4):042903-042903-11. doi:10.1115/1.4036382.

Reservoir characterization is needed for estimating reservoir properties and forecasting production rates in a reliable manner. However, it is challenging to figure out reservoir properties of interest due to limited information. Therefore, well-designed reservoir models, which reflect characteristics of a true field, should be selected and fine-tuned. We propose a novel scheme of generating initial reservoir models by using static data and production history data available. We select representative reservoir models by projecting reservoir models onto a two-dimensional (2D) plane using principal component analysis (PCA) and calculating errors of production rates against observed data. These selected models, which will have similar geological properties with the reference, are used to regenerate models by perturbing along the boundary of the different facies. These regenerated models have all the different facies distributions but share principal characteristics based on the selected models. We compare cases using 400 ensemble members, 100 models with unbiased uniform sampling, and 100 regenerated models by the proposed method. We analyze two synthetic reservoirs with different permeability distributions: one is a typical heterogeneous reservoir and the other is a channel reservoir with a bimodal permeability distribution. Compared to the cases using all the 400 models with ensemble Kalman filter (EnKF), the simulation time is dramatically reduced to 4.7%, while the prediction quality on oil and water productions is improved. Even in the more complex reservoir case, the proposed method shows great improvements with reduced uncertainties against the other cases.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Energy Resour. Technol. 2017;139(4):044501-044501-5. doi:10.1115/1.4035427.

Experimental research has been carried out on a single cylinder naturally aspirated spark ignition engine which was modified to operate with coal-bed gas fuel to investigate the method of improving operation stability and lean burn limit. Varied fuel composition with methane concentration 30–100% and CO2 volumetric fraction 0–0.7 was employed to simulate coal-bed methane (CBM) and coal mined methane (CMM), respectively. Hydrogen was then employed to improve operational stability and lean burn limit. The results show that a stable operation range of the engine was obtained under most of the fuel compositions even if up to CO2 volumetric fraction = 0.6 was employed. Besides lean burn limit, the unstable operation with COVIMEP > 10% only appears at lean burn limit as well as CO2 volumetric fraction = 0.7 at each equivalence ratio. The lean burn limit of coal-bed gas has been significantly enlarged from the equivalence ratio equals to 0.6–0.4 by hydrogen addition. Stable operation with COVIMEP < 5% at the equivalence ratio equals to 0.4 has also been obtained at some high hydrogen concentration conditions. Hydrogen addition induced the reduction of both carbon monoxide (CO) and total hydrocarbon (THC) emissions at all equivalence ratio conditions, especially at the equivalence ratio equals to 0.4 and 0.6. CO2 addition improves NOx emission significantly; however, high CO2 volumetric fraction will lead to unstable operation, which results in deteriorated CO and THC emissions. Hydrogen addition has benefits of improving operation stability and enlarging lean burn limit of coal-bed gas engine, which has practical significance to improve the application of coal-bed gas engine technology.

Commentary by Dr. Valentin Fuster

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