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Research Papers: Alternative Energy Sources

J. Energy Resour. Technol. 2018;140(7):071201-071201-9. doi:10.1115/1.4039023.

This study deals with thermodynamic analyses of an integrated wind thermal energy storage (WTES) system. The thermodynamic analyses of the proposed system are performed through energy and exergy approaches, and the energy and exergy efficiencies of the components in the system and overall system are determined and assessed. The magnitudes of irreversibilities are determined, and the impacts of different parameters on the performance of the system are identified. The overall energy and exergy efficiencies of the proposed system and its subsystems are computed as well. The energy and exergy efficiencies of the overall system are defined and obtained as 7.0% and 8.6%, respectively. WTES plants with combined molten salt energy storage application can run continuously, and can provide electrical power for both on-grid and off-grid systems. By converting the wind power into a permanent energy source, the WTES offers a practical solution that can meet the electrical demand of the regions where the climate conditions are feasible for consistent, environmentally benign and cost-effective electric power, and it can be considered as a potential energy solution.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):071202-071202-9. doi:10.1115/1.4039349.

Icing on wind turbines is a major problem in cold regions. To study blade icing, water droplet collection efficiency is calculated on the National Renewable Energy Laboratory (NREL) phase VI blade. First, water droplet conservation equations are embedded into ANSYS Fluent, and the results calculated by the Eulerian method are validated. For the two-dimensional (2D) airfoil, the peak collection efficiency error is 3.7%; for the three-dimensional (3D) blade, the peak collection efficiency error is 2.8%. Second, collection efficiency on the NREL phase VI blade is investigated. The results indicate that water droplets mainly impact on the blade leading edge, and the collection efficiency increases along the radial direction. Finally, the 3D rotating effect on collection efficiency is studied. The results demonstrate that, at a wind speed of 7 m/s, the 3D rotating effect has almost no influence on collection efficiency; however, the effect must be considered in water droplet collection at a wind speed of 10 m/s.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Conversion/Systems

J. Energy Resour. Technol. 2018;140(7):071601-071601-8. doi:10.1115/1.4039446.

For the application of waste heat recovery (WHR), supercritical CO2 (S-CO2) Brayton power cycles offer significant suitable advantages such as compactness, low capital cost, and applicability to a broad range of heat source temperatures. The current study is focused on thermodynamic modeling and optimization of recuperated (RC) and recuperated recompression (RRC) configurations of S-CO2 Brayton cycles for exhaust heat recovery from a next generation heavy duty simple cycle gas turbine using genetic algorithm (GA). This nongradient based algorithm yields a simultaneous optimization of key S-CO2 Brayton cycle decision variables such as turbine inlet temperature, pinch point temperature difference, compressor pressure ratio, and mass flow rate of CO2. The main goal of the optimization is to maximize power out of the exhaust stream which makes it single objective optimization. The optimization is based on thermodynamic analysis with suitable practical assumptions which can be varied according to the need of user. The optimal cycle design points are presented for both RC and RRC configurations and comparison of net power output is established for WHR. For the chosen exhaust gas mass flow rate, RRC cycle yields more power output than RC cycle. The main conclusion drawn from the current study is that the choice of best cycle for WHR actually depends heavily on mass flow rate of the exhaust gas. Further, the economic analysis of the more power producing RRC cycle is performed and cost comparison between the optimized RRC cycle and steam Rankine bottoming cycle is presented.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2018;140(7):072001-072001-6. doi:10.1115/1.4038964.

Mixing of fresh (river) water and salty water (seawater or saline brine) in a controlled environment produces an electrical energy known as salinity gradient energy (SGE). Two main conversion technologies of SGE are membrane-based processes: pressure retarded osmosis (PRO) and reverse electrodialysis (RED). Exergy calculations for a representative river-lake system are investigated using available data in the literature between 2000 and 2008 as a case study. An exergy analysis of an SGE system of sea-river is applied to calculate the maximum potential power for electricity generation. Seawater is taken as reference environment (global dead state) for calculating the exergy of fresh water since the sea is the final reservoir. Aqueous sodium chloride solution model is used to calculate the thermodynamic properties of seawater. This model does not consider seawater as an ideal solution and provides accurate thermodynamics properties of sodium chloride solution. The chemical exergy analysis considers sodium chloride (NaCl) as main salt in the water of this highly saline Lake with concentration of more than 200 g/L. The potential power of this system is between 150 and 329 MW depending on discharge of river and salinity gradient between the Lake and the River based on the exergy results. This result indicates a high potential for constructing power plant for SGE conversion. Semipermeable membranes with lifetime greater than 10 years and power density higher than 5 W/m2 would lead to faster development of this conversion technology.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072002-072002-9. doi:10.1115/1.4039267.

In the field of combustion, a special attention was given lately especially to the search for new, greener and more efficient fuels. Among them, hydrogen is intensely studied worldwide as a possible alternative fuel since new ways for producing and transporting it developed lately. Different studies are trying to confirm the possibility of the hydrogen transport using the existing natural gas distribution network, by mixing the two gases. Because the properties of the new mixture influence the combustion parameters, using the existing equipment would face new problems, like the risk of flashback, the effects of higher temperatures, and the modification of the flame front. Hence, new solutions are needed. In this context, this paper presents a newly developed and patented type of injector, designated for the combustion of the premixed hydrogen–methane fuel in various proportions. Based on the characteristics and dimensions of an existing combustion chamber of a gas turbine, different types of injectors were numerically simulated and compared. After the analysis of the results, the preliminary conclusions lead to a first swirl injector made from titanium alloy. The new type of swirled injector was tested on a cheap, simplified low pressure rig, designed to have similar dimensions to the initial combustion chamber, for preliminary validation of the main characteristics and of the stability of the new injector. The experiments indicated good lean blowout characteristics, and the promising results are encouraging for more future tests on a complex experimental setup, for optimizing the final solution.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072003-072003-9. doi:10.1115/1.4039096.

The paper presents a novel concept and method of coal combustion process analysis using flame scanners supervision system. The combustion process analysis and diagnostic has a crucial influence on boiler effectiveness, especially in high variance of load demand, which is nowadays a top challenge for coal-fired power plants. The first indicator of combustion inefficiency is flame stability, which can be observed as variation of flame intensity. Nowadays, there are no validated measuring methods dedicated for industrial usage, which are able to give complete information about flame condition. For this reason, the research activity was launched and focused on usage of commercial flame scanners for fast combustion analysis based on on-line flame parameters measuring. The analysis of combustion process was performed for 650 t/h live steam power boiler, which is supplied by five coal mill units. Each coal mill supplies four pulverized coal burners pulverized fuel ((PF) burners). The boiler start-up installation consists of 12 heavy oil burners placed in PF burners equipped with individual supervisory system based on Paragon 105f-1 flame scanners, which gave the possibility to observe and analyze the PF burner flame and oil burner flame individually. The research included numerous tests in which the combustion conditions inside the combustion chamber were changed. During stable load of selected mills, the primary air flow, secondary air dampers, air–coal mixture temperature, and balance were changed. The results of the changes were observed by flame scanners and the available optical parameters of the flame were analyzed: power spectral density, average amplitude (AA) of flame fluctuation, and flame temperature.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072004-072004-15. doi:10.1115/1.4039266.

The scaling concept is important, effective, and consistent in any application of science and engineering. Scaled physical models have inimitable advantages of finding all physical phenomena occurring in a specific process by transforming parameters into dimensionless numbers. This concept is applicable to thermal enhanced oil recovery (EOR) processes where continuous alteration (i.e., memory) of reservoir properties can be characterized by various dimensionless numbers. Memory is defined as the continuous time function or history dependency which leads to the nonlinearity and multiple solutions during modeling of the process. This study critically analyzed sets of dimensionless numbers proposed by Hossain and Abu-Khamsin in addition to Nusselt and Prandtl numbers. The numbers are also derived using inspectional and dimensional analysis (DA), while memory concept is used to develop some groups. In addition, this article presents relationships between different dimensionless numbers. Results show that proposed numbers are measures of thermal diffusivity and hydraulic diffusivity of a fluid in a porous media. This research confirms that the influence of total absolute thermal conductivities of the fluid and rock on the effective thermal conductivity of the fluid-saturated porous medium diminishes after a certain local Nusselt number of the system. Finally, the result confirms that the convective ability of the fluid-saturated porous medium is apparently more pronounced than its conductive ability. This study will help to better understand the modeling of the EOR process thus improving process design and performance prediction.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072005-072005-13. doi:10.1115/1.4039376.

The Goswami cycle is a cycle that combines an ammonia–water vapor absorption cycle and a Rankine cycle for cooling and mechanical power purposes by using thermal heat sources such as solar energy or geothermal steam. In this paper, a theoretical investigation was conducted to determine the performance outputs of the cycle, namely, net mechanical power, cooling, effective first law efficiency and exergy efficiency, for a boiler and an absorber temperature of 85 °C and 35 °C, respectively, and different boiler pressures and ammonia-water concentrations. In addition, an experimental investigation was carried out to verify the predicted trends of theoretical analysis and evaluate the performance of a modified scroll expander. The theoretical analysis showed that maximum effective first law and exergy efficiencies were 7.2% and 45%, respectively. The experimental tests showed that the scroll expander reached a 30–40% of efficiency when boiler temperature was 85 °C and rectifier temperature was 55 °C. Finally, it was obtained that superheated inlet conditions improved the efficiency of the modified expander.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072006-072006-8. doi:10.1115/1.4039323.

Hydrogen production via carbonaceous catalytic methane decomposition is a complex process with simultaneous reaction, catalyst deactivation, and carbon agglomeration. Conventional reaction and deactivation models do not predict the progress of reaction accurately. Thus, statistical modeling using the method of design of experiments (DoEs) was used to design, model, and analyze experiments of methane decomposition to determine the important factors that affect the rates of reaction and deactivation. A variety of statistical models were tested in order to identify the best one agreeing with the experimental data by analysis of variance (ANOVA). Statistical regression models for initial reaction rate, catalyst activity, deactivation rate, and carbon weight gain were developed. The results showed that a quadratic model predicted the experimental findings. The main factors affecting the dynamics of the methane decomposition reaction and the catalyst deactivation rates for this process are partial pressure of methane, reaction temperature, catalytic activity, and residence time.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072007-072007-7. doi:10.1115/1.4039607.

Thermoelectric technology applied in vehicle has become significantly essential due to the global energy crisis and the environmental protection issues. A novelty energy efficient technology called localized air-conditioning (LAC) powered by thermoelectric generator (TEG), i.e., TEG-powered LAC, is proposed in order to better utilize the generated power of TEG, only then will the fuel economy improvement be achieved. This system which has little impact on the original automotive electrical system is basically comprised of LAC, TEG, converter, and battery. The TEG can directly convert thermal energy to electrical energy to power the novelty energy-efficient air-conditioning system called LAC. The submodels of LAC and TEG are built and integrated into a heavy-duty vehicle to quantitatively assess its performance by simulation analysis. The results indicate that the novelty TEG-powered LAC system can work normally with high efficiency and improve the fuel economy by 3.7%. Therefore, this system resolves the problem of proper use of the TEG's power and provides a fully new perspective to substitute the mechanical loads to engine with electrical loads powered by TEG to improve the fuel economy with much more practicality and rationality.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072008-072008-9. doi:10.1115/1.4039615.

Coal fires exist in almost every coal-producing country and generate huge amounts of heat energy every year. In this paper, forced convective heat-extraction is presented as a method to exploit the potential heat in coal fire zones as an energy resource. A geological model of coal fire zones and a combustion model for underground coal in an O2-depleted atmosphere are established. The borehole layouts, the heat transfer medium (HTM) injection rates, and the cooling effect of the HTM on the coal and rock are analyzed using a three-dimensional (3D) simulation software (fluent). The results show that a borehole layout of multihole injection and oriented type proves to be suitable for coal fire zones. The simulation predicts that the temperature of the extracted HTM and the rate of heat extraction decrease as extraction time increases. The simulation further predicts that the temperature of the extracted HTM can be increased by reducing the rate at which the HTM injected. Additionally, the heat-extraction rate is more stable for relatively low HTM injection rates. The temperature of the coal fire zones can be reduced effectively by using forced convective heat-extraction, with the maximum temperature of the coal fire zones and the average temperature in the residual coal zone being cubic and quadratic function relationship of the heat-extraction time, respectively. This research provides a reference for waste-energy exploitation in coal fire areas.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072009-072009-6. doi:10.1115/1.4039271.

The gas-to-liquid (GTL) fuel, a liquid fuel synthesized from natural gas through Fischer–Tropsch process, exhibits better combustion and, in turn, lower emission characteristics than the conventional jet fuels. However, the GTL fuel has different fuel properties than those of regular jet fuels, which could potentially affect its atomization and combustion aspects. The objective of the present work is to investigate the near-nozzle atomization characteristics of GTL fuel and compare them with those of the conventional Jet A-1 fuel. The spray experiments are conducted at different nozzle operating conditions under standard ambient conditions. The near-nozzle macroscopic spray characteristics are determined from the shadowgraph images. Near the nozzle exit, a thorough statistical analysis shows that the liquid sheet dynamics of GTL fuel is different from that of Jet A-1 fuel. However, further downstream, the microscopic spray characteristics of GTL fuel are comparable to those of the Jet A-1 fuel.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072010-072010-6. doi:10.1115/1.4039629.

To evaluate the feasibility of the performance enhancement of a thermophotovoltaic (TPV) converter by using a thermoelectric generator (TEG), a new model of a combined system is established, where the TEG is attached on the backside of the TPV converter to harvest the heat produced in the TPV converter. The effects of the voltage output of the TPV converter, band gap energy of the TPV converter, dimensionless current of the TEG, and emitter temperature on the performance of the combined system are examined numerically. It is found that the performance of the TPV converter can be enhanced by using the TEG. The percentage increment of the maximum power output density is larger than that of the maximum efficiency. There are optimally working regions of the converter voltage, dimensionless current, and band gap energy. The elevated emitter temperature results in the increase of the power output density of the combined system. However, there is an optimal emitter temperature that yields the maximum efficiency of the combined system. Moreover, the TEG is not suitable to harvest the heat produced in the TPV converter when the emitter temperature is sufficiently high.

Commentary by Dr. Valentin Fuster

Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2018;140(7):072201-072201-7. doi:10.1115/1.4039268.

In this paper, the effects of hydrothermal modification on sulfur-containing pollutants, such as sulfur dioxide (SO2) and carbonyl sulfide (COS), during coal pyrolysis and combustion, have been investigated. Three typical Chinese low-quality coals, Zhundong, Yimin, and Zhaotong coal (ZT), have been treated by hydrothermal modification at final modification temperatures of 200 °C, 250 °C, and 300 °C. Coal pyrolysis and combustion experiments using raw coal and modified coals were performed using a tube furnace. Results showed that SO2 and COS emission were suppressed after hydrothermal modification in the pyrolysis process. Lower emission of both SO2 and COS were also achieved when final hydrothermal modification was increased, this was attributed to the loss of aliphatic sulfur, e.g., sulfoxide, sulfone, and thiother, during the modification process. For ZT, hydrothermal modification also caused a delay in the release of sulfur-containing gases. In combustion experiments, hydrothermal modification reduced the SO2 emission for Yimin coal, but for ZT, the SO2 release amount almost doubled, and for Zhundong coal (ZD), it also increased, after hydrothermal modification. Hydrothermal modification also caused a delay in peak SO2 emission during the combustion of ZT; this is attributed to conversion of sulfur containing structures to stable aromatic compounds through hydrothermal modification.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072202-072202-7. doi:10.1115/1.4039322.

In spark ignition engines, gasoline direct injection (GDI) is surely the most attractive technology to achieve the demand of high energy efficiency by directly injecting fuel into combustion chamber. This work, as a preliminary study, investigates the effect of retarded injection timing on knock resistance and cycle-to-cycle variation in gasoline engine by experimental method. The retarded injection timing during compression stroke coupled with increased intake air temperature was employed to concentrate on suppressing knock occurrence with stable combustion. Based on the great advantage of injection timing retard on knock suppression, intake temperature was used in this work to reduce cycle-to-cycle variation. In addition, piezo-electrically actuated injector was employed. The results show that injection timing retard during compression stroke can significantly suppress the knock tendency, but combustion becomes unstable and cycle-to-cycle variation is larger than 10%. Thus, increasing intake temperature decreased the cycle-to-cycle variation but increased significantly the knock tendency, as expect. Meanwhile, rich fuel–air mixture in this work also had the same effect as intake temperature did. It can be concluded that retarded injection timing is of significant potential to suppress the knock in GDI engine, although the high intake temperature causes high probability of large knock occurrence. The percentages of knock at the spark timings of 24 °CA before top dead center (BTDC) and 26 °CA BTDC were significantly reduced from approximately 40% to 7% and from approximately 60% to 10%, respectively. Furthermore, the retarded injection timing not only reduced the probability of knock occurrence, but also decreased the knock intensity obviously.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072203-072203-10. doi:10.1115/1.4039445.

Two waste biomass materials, pine needle (PN) and corn stalk (CS), were pyrolyzed at different temperatures (200–900 °C). The organic functional groups and carbonaceous structure of the biomass chars were characterized by Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, respectively. The combustion characteristics and kinetics of biomass chars were investigated by thermogravimetric analysis (TGA). The content of carbon-, hydrogen-, and oxygen-containing functional groups in the biomass samples decreases with an increase in preparation temperature, leading to more aromatic macromolecular structure at elevated pyrolysis temperatures. With increasing pyrolysis temperature, the comprehensive combustibility index (S) of both chars related to combustion reactivity generally decreases especially for CS char because of the loss of active groups. However, the Raman spectra show that the degree of order decreases with increasing pyrolysis temperature from 400 to 700 °C because of the generation of isolated sp2 carbon.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072204-072204-10. doi:10.1115/1.4039611.

A system-level computational model of a recently patented and prototyped novel steam engine technology was developed from first principles for the express purpose of performing design optimization studies for the engine's inventors. The developed system model consists of numerous submodels including a flow model of the intake process, a dynamic model of the intake valve response, a pressure model of the engine cylinder, a kinematic model of the engine piston, and an output model that determines engine performance parameters. A crank-angle discretization strategy was employed to capture the performance of engine throughout a full cycle of operation, thus requiring all engine design submodels to be evaluated at each crank angle of interest. To produce a system model with sufficient computational speed to be useful within optimization algorithms, which must exercise the system level model repeatedly, various simplifying assumptions and modeling approximations were utilized. The model was tested by performing a series of multi-objective design optimization case studies using the geometry and operating conditions of the prototype engine as a baseline. The results produced were determined to properly capture the fundamental behavior of the engine as observed in the operation of the prototype and demonstrated that the design of engine technology could be improved over the baseline using the developed computational model. Furthermore, the results of this study demonstrate the applicability of using a multi-objective optimization-driven approach to conduct conceptual design efforts for various engine system technologies.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072205-072205-10. doi:10.1115/1.4039612.

A numerical investigation was conducted to explore the kinetic effects of methanol addition on the formation and consumption of formaldehyde and benzene in premixed stoichiometric n-heptane/air flames at atmospheric pressure. The flame modeling was performed by solving the premixed flame model with a comprehensive kinetic scheme of hydrocarbon fuels. We studied the species distributions, formation temperatures, temperature sensitivities, reaction contributions, and the rates of production and consumption for formaldehyde and benzene. Results showed that formaldehyde and benzene were produced in two temperature zones and the accumulation effect in the low-temperature zone was the most important factor for the peak concentrations of them in flames. When methanol was added into n-heptane/air flames, cross-reactions were hardly found in the formation routes of formaldehyde and benzene. Both the increased peak concentration and the decreased formation temperature of formaldehyde were primarily attributed to the fact that CH3O (+M) <=>CH2O + H (+M) and CH2OH + O2<=>CH2O + HO2 were promoted in low-temperature zone. Methanol addition decreased the rates of production and consumption of benzene proportionally, and served as a diluent fuel in benzene formation and consumption. CH3, CH3O, CH2OH, C3H3, and A-C3H5 were the most important precursors for the formation of formaldehyde and benzene. The conversion rates of these species into formaldehyde and benzene were explored as well. Results showed that methanol addition suppressed the conversion of C3 species into benzene, but it hardly showed obvious effect on the conversion of CH3, CH3O, and CH2OH into formaldehyde.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072206-072206-11. doi:10.1115/1.4039546.

A turbocharged three cylinder automotive common rail diesel engine was modified to operate in the n-butanol diesel dual fuel mode. The quantity of butanol injected by the port fuel injectors and the rail pressure, injection timing, and number of injection pulses of diesel were varied using open engine controllers. Experiments were performed in the dual fuel mode at a constant speed of 1800 rpm at varying brake mean effective pressure (BMEPs). Butanol to diesel energy share was varied, and the injection timing of diesel was always set for highest brake thermal efficiency (BTE). Single pulse injection (SPI) and two pulse injection (TPI) of diesel were evaluated. In SPI, with increase in the butanol to diesel energy share, the BTE remained unchanged. At high loads and high amounts of butanol, the heat release rate (HRR) variation indicated that butanol auto ignited before diesel with both SPI and TPI of diesel. NO emission always decreased because of reduced temperatures due to evaporation of butanol. Butanol also reduced the smoke levels except at high loads. HC levels were always higher. With optimized injection parameters, TPI of diesel resulted in lower NO, similar smoke, and BTE with lesser rate of pressure rise as compared to SPI of diesel in the dual fuel mode at high loads. On the whole, the SPI mode is suitable for low to medium outputs and the TPI mode is suitable for high outputs.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072207-072207-5. doi:10.1115/1.4039326.

The effects of adding N2 or CO2 as diluents to a premixed methane–air flames under strain conditions (associated with a stagnation plate) were examined for flame stand-off distance, stability, intensity, and global flame behavior at various equivalence ratios. A stagnation plate was used to simulate the flame behavior near a combustor wall that can help provide some insights into reducing thermal stresses and enhance combustor lifetime. Decrease in equivalence ratio at the same thermal intensity provided larger strain rates while maintaining a stable flame. At stoichiometric condition, a balance was provided between high strain rates and low oxygen concentration flames to mitigate the peak (maximum) flame temperatures, and the associated temperature-dependent pollutants emission, such as NOx, CO, and unburnt hydrocarbons. Higher thermal intensities provided higher strain rates; however, the addition of diluents impacted in destabilization of flame. The flame stand-off behavior occurred at lower strain rates, low thermal intensity, and increased equivalence ratios. CO2 dilution reduced flame intensity, increased flame stand-off distance and overall flame destabilization than that with N2 dilution.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072208-072208-11. doi:10.1115/1.4039547.

Due to current and future exhaust emissions regulations, oxidation catalysts are increasingly being added to the exhaust streams of large-bore, two-stroke, natural gas engines. Such catalysts have a limited operational lifetime, primarily due to chemical (i.e., catalyst poisoning) and mechanical fouling resulting from the carry-over of lubrication oil from the cylinders. It is critical for users and catalyst developers to understand the nature and rate of catalyst deactivation under these circumstances. This study examines the degradation of an exhaust oxidation catalyst on a large-bore, two-stroke, lean-burn, natural gas field engine over the course of 2 years. Specifically, this work examines the process by which the catalyst was aged and tested and presents a timeline of catalyst degradation under commercially relevant circumstances. The catalyst was aged in the field for 2-month intervals in the exhaust slipstream of a GMVH-12 engine and intermittently brought back to Colorado State University for both engine testing and catalyst surface analysis. Engine testing consisted of measuring catalyst reduction efficiency as a function of temperature as well as the determination of the light-off temperature for several exhaust components. The catalyst surface was analyzed via scanning electron microscope (SEM)/energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) techniques to examine the location and rate of poison deposition. After 2 years online, the catalyst light-off temperature had increased ∼55 °F (31 °C) and ∼34 wt % poisons (S, P, Zn) were built up on the catalyst surface, both of which represent significant catalyst deactivation.

Commentary by Dr. Valentin Fuster

Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2018;140(7):072901-072901-8. doi:10.1115/1.4039269.

In order to assess the critical sand deposition condition, a unique 4-in ID test facility was designed and constructed, which enables the pipe to be inclined 1.5 deg upward. Experiments were conducted with air–water-glass beads at low sand concentrations (< 10,000 ppm), and the air and water flow rates were selected to ensure stratified flow regime along the pipe. At constant superficial liquid velocity, the gas velocity was reduced to find the critical sand deposition velocity. Six sand flow regimes are identified, namely, fully dispersed solid flow, dilute solids at the wall, concentrated solids at the wall, moving dunes, stationary dunes, and stationary bed. The experimental results reveal that sand flow regimes under air–water stratified flow are strong functions of phase velocities, particle size, and particle concentration. Also, the results show that air–water flow regime plays an important role in particle transport; slug flow has high capability to transport particles at the pipe bottom, while the stratified flow has high risk of sand deposition. As long as the sand dunes are observed at the pipe bottom, the critical sand deposition velocities slightly increase with concentrations, while for stationary bed, the critical velocity increases exponentially with concentration.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072902-072902-6. doi:10.1115/1.4039319.

The application of water flooding is not successful for the development of low permeability reservoirs due to the fine pore sizes and the difficulty of water injection operation. CO2 can dissolve readily in crude oil and highly improve the mobility of crude oil, which makes CO2 flooding an effective way to the development of the ultralow-permeability reservoirs. The regularities of various CO2 displacement methods were studied via experiments implemented on cores from Chang 8 Formation of Honghe Oilfield. The results show that CO2 miscible displacement has the minimum displacement differential pressure and the maximum oil recovery; CO2-alternating-water miscible flooding has lower oil recovery, higher drive pressure, and relatively lower gas-oil ratio; water flooding has the minimum oil recovery and the maximum driving pressure. A large amount of oil still can be produced under a high gas-oil ratio condition through CO2 displacement method. This fact proves that the increase of gas-oil ratio is caused by the production of dissolved CO2 in oil rather than the free gas breakthrough. At the initial stage of CO2 injection, CO2 does not improve the oil recovery immediately. As the injection continues, the oil recovery can be improved rapidly. This phenomenon suggests that when CO2 displacement is performed at high water cut period, the water cut does not decrease immediately and will remain high for a period of time, then a rapid decline of water cut and increase of oil production can be observed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072903-072903-8. doi:10.1115/1.4039270.

Permeability is a key parameter related to any hydrocarbon reservoir characterization. Moreover, many petroleum engineering problems cannot be precisely answered without having accurate permeability value. Core analysis and well test techniques are the conventional methods to determine permeability. These methods are time-consuming and very expensive. Therefore, many researches have been introduced to identify the relationship between core permeability and well log data using artificial neural network (ANN). The objective of this research is to develop a new empirical correlation that can be used to determine the reservoir permeability of oil wells from well log data, namely, deep resistivity (RT), bulk density (RHOB), microspherical focused resistivity (RSFL), neutron porosity (NPHI), and gamma ray (GR). A self-adaptive differential evolution integrated with artificial neural network (SaDE-ANN) approach and evolutionary algorithm-based symbolic regression (EASR) techniques were used to develop the correlations based on 743 actual core permeability measurements and well log data. The obtained results showed that the developed correlations using SaDE-ANN models can be used to predict the reservoir permeability from well log data with a high accuracy (the mean square error (MSE) was 0.0638 and the correlation coefficient (CC) was 0.98). SaDE-ANN approach is more accurate than the EASR. The introduced technique and empirical correlations will assist the petroleum engineers to calculate the reservoir permeability as a function of the well log data. This is the first time to implement and apply SaDE-ANN approaches to estimate reservoir permeability from well log data (RSFL, RT, NPHI, RHOB, and GR). Therefore, it is a step forward to eliminate the required lab measurements for core permeability and discover the capabilities of optimization and artificial intelligence models as well as their application in permeability determination. Outcomes of this study could help petroleum engineers to have better understanding of reservoir performance when lab data are not available.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072904-072904-7. doi:10.1115/1.4039316.

Well cements are an important aspect of wellbore integrity and recent investigations focus on describing the cement lifetime using, when possible, nondestructive tests like ultrasonic measurements. However, the original API and ASTM testing standards were based on destructive mechanical testing of cements, leading to the decision to investigate the backward and forward compatibility between ultrasonic measurements and mechanical testing, which makes the subject of this work. Ultrasonic cement measurement became a very popular method to assess the mechanical properties of the cement in a nondestructive manner. Since various measurement systems exist on the market, the development of an accurate reference data base that can be used to calibrate such measurements becomes very important. Two major systems have therefore been compared: the ultrasonic compressive strength, using the ultrasonic pulse velocity (UPV) principle, and the unconfined compressive strength (UCS), using the standard testing frame according to API and ASTM standards. The tests have been performed at different curing times, using both devices, on API Class G cements with bentonite and other additives. This paper presents the results of over 200 experiments that have displayed a different UPV response as a function of the additive content. Cement specific UPV versus UCS correlations were established. Thereby, a new level of accuracy was reached. Moreover, it was observed that after a given curing time, depending on the additive and its concentration, the UPV response is not as sensitive as the results yielded by the UCS method. The outcomes are an important step forward to improve and understand the wellbore integrity.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072905-072905-8. doi:10.1115/1.4039613.

Static Poisson's ratio (νstatic) is a key factor in determine the in-situ stresses in the reservoir section. νstatic is used to calculate the minimum horizontal stress which will affect the design of the optimum mud widow and the density of cement slurry while drilling. In addition, it also affects the design of the casing setting depth. νstatic is very important for field development and the incorrect estimation of it may lead to heavy investment decisions. νstatic can be measured in the lab using a real reservoir cores. The laboratory measurements of νstatic will take long time and also will increase the overall cost. The goal of this study is to develop accurate models for predicting νstatic for carbonate reservoirs based on wireline log data using artificial intelligence (AI) techniques. More than 610 core and log data points from carbonate reservoirs were used to train and validate the AI models. The more accurate AI model will be used to generate a new correlation for calculating the νstatic. The developed artificial neural network (ANN) model yielded more accurate results for estimating νstatic based on log data; sonic travel times and bulk density compared to adaptive neuro fuzzy inference system (ANFIS) and support vector machine (SVM) methods. The developed empirical equation for νstatic gave a coefficient of determination (R2) of 0.97 and an average absolute percentage error (AAPE) of 1.13%. The developed technique will help geomechanical engineers to estimate a complete trend of νstatic without the need for coring and laboratory work and hence will reduce the overall cost of the well.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072906-072906-11. doi:10.1115/1.4039324.

The damage mechanism of fracturing fluids has always been the hot research topic in the development of low-permeability reservoir with hydraulic fracturing. At present, the research in this area is conducted mostly by the conventional core fluid flow test designed with industrial standards, less in the experiment operated from a microperspective. Against the reservoir cores with different permeability, and based on the results of SEM, mercury injection experiment, and core fluid flow test, this paper uses the technology of nuclear magnetic resonance (NMR) to systematically analyze the degree and rule of water-sensitivity, water-block, and solid-phase adsorption damage resulted from hydroxypropyl guar gum (HPG) and carboxymethyl guar gum (CMG) fracturing fluids, and proposes a comprehensive test method for evaluating the fracturing fluids damage to the reservoir. The test results show that fracturing fluid infiltrating into the core causes the increase of bound water, mobile water retention, and solid-phase macromolecule substance absorption inside the core in varying degrees, decreasing the reservoir permeability. The extent of reservoir water-sensitivity damage is positively correlated with the increment of bound water, and the extent of water-block damage is positively correlated with mobile water retention volume. The adsorption and retention of solid-phase macromolecule substance causes largest loss of core permeability, averaging about 20%, and it is main damage factor of fracturing fluids, the water-sensitivity damage causes 11% of core permeability loss, and the water-block damage causes 7% of loss. As the reservoir permeability doubles, the comprehensive damage resulted from guar gum fracturing fluid decreases by 14%. The comprehensive damage of CMG fracturing fluid to reservoir is 6.6% lower than that of HPG fracturing fluid, and the lower the reservoir permeability, the larger the gap between damage of CMG and HPG fracturing fluids. With the technology of NMR, the objective and accurate evaluation of various damages to reservoir resulted from fracturing fluids is realized, and the corresponding relation between damage mechanism and damage extent is established, which provides reference for research on improvement of fracturing fluid properties and reservoir protection measures.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(7):072907-072907-10. doi:10.1115/1.4039325.

In this paper, experimental and numerical techniques have been utilized to quantify heavy oil properties in CO2 huff-n-puff processes under reservoir conditions. Experimentally, fluid properties together with viscosity reduction of heavy oil and interfacial properties between CO2 and heavy oil have been quantified, while five cycles of CO2 huff-n-puff processes have been conducted to determine oil recovery together with component variation of produced and residual oils. Theoretically, numerical simulation has been conducted to analyze the underlying recovery mechanisms associated with the CO2 huff-n-puff processes. CO2 huff-n-puff processes are only effective in the first two cycles under the existing experimental conditions, while the effective sweep range is limited near the wellbore region, resulting in poor oil recovery in the subsequent cycles. As for produced oil, its viscosity, density, resin and asphaltene contents, and molecular weight of asphaltene are reduced, whereas, for the residual oil, they are increased. The asphaltene component in the residual oil shows weak stability compared to that of the produced oil, while the ultimate oil recovery after the fifth CO2 cycle of huff-n-huff processes is measured to be 31.56%.

Commentary by Dr. Valentin Fuster

Research Papers: Petroleum Transport/Pipelines/Multiphase Flow

J. Energy Resour. Technol. 2018;140(7):073001-073001-9. doi:10.1115/1.4039019.

The relieving system using the choke valve is applied to control the pressure in CO2 pipeline. However, the temperature of fluid would drop rapidly because of Joule–Thomson cooling (JTC), which may cause solid CO2 form and block the pipe. A three-dimensional (3D) computational fluid dynamic (CFD) model considering the phase transition and turbulence was developed to predict the fluid-particle flow and deposition characteristics. The Lagrangian method, Reynold's stress transport model (RSM) for turbulence, and stochastic tracking model (STM) were used. The results show that the model predictions were in good agreement with the experimental data published. The effects of particle size, flow velocity, and pipeline diameter were analyzed. It was found that the increase of the flow velocity would cause the decrease of particle deposition ratio and there existed the critical particle size that causes the deposition ratio maximum. It also presents the four types of particle motions corresponding to the four deposition regions. Moreover, the sudden expansion region is the easiest to be blocked by the particles. In addition, the Stokes number had an effect on the deposition ratio and it was recommended for Stokes number to avoid 3–8 St.

Commentary by Dr. Valentin Fuster

Research Papers: Petroleum Wells-Drilling/Production/Construction

J. Energy Resour. Technol. 2018;140(7):073101-073101-11. doi:10.1115/1.4039327.

Fluid flow in fractured porous media has always been important in different engineering applications especially in hydrology and reservoir engineering. However, by the onset of the hydraulic fracturing revolution, massive fracturing jobs have been implemented in unconventional hydrocarbon resources such as tight gas and shale gas reservoirs that make understanding fluid flow in fractured media more significant. Considering ultralow permeability of these reservoirs, induced complex fracture networks play a significant role in economic production of these resources. Hence, having a robust and fast numerical technique to evaluate flow through complex fracture networks can play a crucial role in the progress of inversion methods to determine fracture geometries in the subsurface. Current methods for tight gas flow in fractured reservoirs, despite their advantages, still have several shortcomings that make their application for real field problems limited. For instance, the dual permeability theory assumes an ideal uniform orthogonal distribution of fractures, which is quite different from field observation; on the other hand, numerical methods like discrete fracture network (DFN) models can portray the irregular distribution of fractures, but requires massive mesh refinements to have the fractures aligned with the grid/element edges, which can greatly increase the computational cost and simulation time. This paper combines the extended finite element methods (XFEM) and the gas pseudo-pressure to simulate gas flow in fractured tight gas reservoirs by incorporating the strong-discontinuity enrichment scheme to capture the weak-discontinuity feature induced by highly permeable fractures. Utilizing pseudo-pressure formulations simplifies the governing equations and reduces the nonlinearity of the problem significantly. This technique can consider multiple fracture sets and their intersection to mimic real fracture networks on a plain structured mesh. Here, we utilize the unified Hagen–Poiseuille-type equation to compute the permeability of tight gas, and finally adopt Newton–Raphson iteration method to solve the highly nonlinear equations. Numerical results illustrate that XFEM is considerably effective in fast calculation of gas flow in fractured porous media.

Commentary by Dr. Valentin Fuster

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