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### Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2019;141(12):122001-122001-10. doi:10.1115/1.4043746.

This study demonstrates comparative applications of the standard pinch and exergy analysis and the combined pinch-exergy analysis methodologies to a gas-fired steam power plant’s heat exchanger network. The extent to which each methodology could be used for pin-pointing the location of performance deteriorations in the network and their relative criticality were shown. Using a 12 °C minimum temperature difference, the network minimum hot utility requirement in current operation was determined by a pinch analysis as 539,491 kW, at a supply temperature of 549 °C. This represented a 6% (30,618 kW) increase in the utility requirement when compared with the design minimum requirement (508,873.7 kW). The combined exergy pinch analysis showed the severity of performance deteriorations more clearly, determining a 25% increase in global plant exergy losses with respect to design conditions. With a standard exergy analysis, additional information on the actual network components responsible for the changes was obtained—there were general declines in component performances except for two heaters and the deaerator, whose operation performances improved slightly. Furthermore, avoidable and inevitable exergy losses ($Ξ˙d,AVO$ and $Ξ˙d,INE$, respectively) were determined for network components. Whereas both were highest for the boiler, the values of the ratio $Ξ˙d,AVO/Ξ˙d,INE$ showed that higher potentials for performance improvement existed in the other network components. This indicates the ratio $Ξ˙d,AVO/Ξ˙d,INE$ as an appropriate measure for deciding equipment in the heat exchanger network that are in need critical attention.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(12):122002-122002-12. doi:10.1115/1.4043823.

Oxy-moderate or intense low-oxygen dilution (MILD) combustion, which is a novel combination of oxy-fuel technology and MILD regime, is numerically studied in the present work. The effects of external preheating and CO2 dilution level on the combustion field, emission, and CO formation mechanisms are investigated in a recuperative laboratory-scale furnace with a recirculating cross-flow. Reynolds-averaged Navier–Stokes (RANS) equations with eddy dissipation concept (EDC) model are employed to perform a 3-D simulation of the combustion field and the turbulence–chemistry interactions. In addition, a well-stirred reactor (WSR) analysis is conducted to further examine the chemical kinetics of this combination when varying the target parameters. The simulations used the skeletal USC-Mech II, which has been shown to perform well in the oxy-fuel combustion modeling. Results show that with more preheating, the uniformity of temperature distribution is noticeably enhanced at the cost of higher CO emission. Also as inlet temperature increases, the concentration of minor species rises and CO formation through the main path (CH4→CH3→CH2O→HCO→CO→CO2) is strengthened, while heavier hydrocarbons path (C2H2→CO) is suppressed. Meanwhile, greater CO2 addition notably closes the gap between maximum and exhaust temperatures. In a highly CO2-diluted mixture, chain-branching reactions releasing CH2O are strengthened, while chain-terminating reactions are weakened. CH2O production through CH3O is accelerated compared with the straight conversion of methyl to formaldehyde. When diluting the oxidant, methylene CH2(s) plays a more influential role in CO formation than when pure oxygen is used, contributing to higher CO emission.

Commentary by Dr. Valentin Fuster

### Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2019;141(12):122201-122201-6. doi:10.1115/1.4043825.

The Achates Power Inc. (API) opposed-piston (OP) engine architecture provides fundamental advantages that increase thermal efficiency over current poppet valve 4 stroke engines. In this paper, the combustion performance of diesel and gasoline compression ignition (GCI) combustion in a medium-duty, OP engine are shown. By using GCI, NOx and/or soot reductions can be seen compared with diesel combustion at similar or increased thermal efficiencies. The results also show that high combustion efficiency can be achieved with GCI combustion with acceptable noise and stability over the same load range as diesel combustion in an OP engine.

Commentary by Dr. Valentin Fuster

### Research Papers: Geothermal Energy

J. Energy Resour. Technol. 2019;141(12):122301-122301-11. doi:10.1115/1.4043748.
FREE TO VIEW

Geothermal power plants are considered important renewable energy resources for clean energy production. Flash steam type plants constitute a significant portion of worldwide geothermal power. In this study, single, double, triple, and quadruple flash steam geothermal power plants are investigated with reinjection options. The optimal operating points are determined specifically through optimal flashing pressures. The turbine power outputs, energy efficiencies, and exergy efficiencies are further studied. A rise in the flashing stages from single to double is found to increase the power outputs considerably. However, when the flashing stages are increased from double to triple and triple to quadruple, the increase in turbine power outputs is found to drop significantly. Also, both exergy efficiency and energy efficiency are found to reduce with increasing number of flash stages. The energy efficiencies are obtained as 28%, 25.5%, 24.2%, and 23.5% for single, double, triple, and quadruple plants, respectively. Furthermore, the exergy efficiencies are found to be 72.6%, 70.9%, 70.2%, and 69.8% for these plants, respectively.

Commentary by Dr. Valentin Fuster

### Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2019;141(12):122901-122901-12. doi:10.1115/1.4043824.

In this study, new and pragmatic interfacial tension (IFT) correlations for n-alkane–water and n-alkane–CO2 systems are developed based on the mutual solubility of the corresponding binary systems and/or density in a pressure range of 0.1–140.0 MPa and temperature range of 283.2–473.2 K. In addition to being more accurate (i.e., the absolute average relative deviation (AARD) is 1.96% for alkane–water systems, while the AARDs for alkane–CO2 systems are 8.52% and 25.40% in the IFT range of >5.0 mN/m and 0.1–5.0 mN/m, respectively) than either the existing correlations or the parachor model (the AARDs for alkane–CO2 systems are 12.78% and 35.15% in the IFT range of >5.0 mN/m and 0.1–5.0 mN/m, respectively), such correlations can be applied to the corresponding ternary systems for an accurate IFT prediction without any mixing rule. Both a higher mutual solubility and a lower density difference between two phases involved can lead to a lower IFT, while pressure and temperature exert effects on IFT mainly through regulating the mutual solubility/density. Without taking effects of mutual solubility into account, the widely used parachor model in chemical and petroleum engineering fails to predict the IFT for CO2/methane–water pair and n-alkane–water pairs, though it yields a rough estimate for the CO2–water and methane–water pair below the CO2 and methane critical pressures of 7.38 and 4.59 MPa, respectively. However, the parachor model at least considers the effects of solubility in the alkane-rich phase to make it much accurate for n-alkane–CO2 systems. For n-alkane–CO2 pairs, the correlations developed in this work are found to be much less sensitive to the liquid density than the parachor model, being more convenient for practical use. In addition, all the IFTs for the CO2–water pair, methane–water pair, and alkane–CO2 pair can be regressed as a function of density difference of a gas–liquid system with a high accuracy at pressures lower than the critical pressures of either CO2 or methane.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Energy Resour. Technol. 2019;141(12):124501-124501-7. doi:10.1115/1.4043880.

Thermodynamic analysis of double effect parallel and series flow direct fired absorption systems with lithium bromide–water has been carried out for different operating conditions. Temperatures in primary generator (Tg) and secondary generator (Tgs)/secondary condenser (Tcs) are optimized analytically using an iterative technique for maximum coefficient of performance (COP) and minimum energy required. A solution distribution ratio for a parallel flow cycle is also optimized. Source of energy used to drive the cycles is considered as compressed natural gas (CNG) and liquefied petroleum gas (LPG). Exergy destruction rate (EDR) in individual components as well as in the whole cycle along with volume flow rate of LPG and CNG is presented and compared. Results show that maximum COP for the parallel flow cycle is 3–6% higher than the series flow cycle. Also, minimum EDR of the parallel flow cycle is around 4% less while energy consumption is 2–3% low as compared to the series flow cycle.

Commentary by Dr. Valentin Fuster