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### Research Papers

J. Energy Resour. Technol. 2011;133(3):031001-031001-6. doi:10.1115/1.4004151.

Plug-in hybrid electric vehicles (PHEVs) have the potential of substantially reducing petroleum consumption and vehicular CO2 emissions relative to conventional vehicles. The analysis presented in this article first ascertains the cost-effectiveness of PHEVs from the perspective of the consumer. Then, the potential effects of PHEVs to an electric utility are evaluated by analyzing a simplified hypothetical example. When evaluating the cost-effectiveness of a PHEV, the additional required premium is an important financial parameter to the consumer. An acceptable amount for the additional upfront costs will depend on the future costs of gasoline and the on-board battery pack. The need to replace the on-board battery pack during the assumed vehicle lifetime also affects the allowed premium. A simplified unit commitment and dispatch model was used to determine the costs of energy and the CO2 emissions associated with PHEVs for different charging scenarios. The results show that electricity can be used to charge PHEVs during off-peak hours without an increase in peak demand. In addition, the combined CO2 emissions from the vehicles and the electric generation facilities will be reduced, regardless of the charging strategy.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2011;133(3):031002-031002-6. doi:10.1115/1.4004607.

In the currently reported work, three typical mixtures of H2 , CO, CH4 , CO2 , and N2 have been considered as representative of the producer gas (syngas) coming from biomass gasification. Syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. However, there are gaps in the fundamental understand of syngas combustion characteristics, especially at elevated pressures that are relevant to practical combustors. In this work, constant volume spherical expanding flames of three typical syngas compositions resulting from biomass gasification have been employed to measure the laminar burning velocities for pressures ranges between 1.0 and 20 bar tanking into account the stretch effect on burning velocity. Over the ranges studied, the burning velocities are fit by a functional form $Su=Su0(T/T0)α(P/P0)β$; and the dependencies of α and β upon the equivalence ratio of mixture are also given. Conclusion can be drawn that the burning velocity decreases with the increase of pressure. In opposite, an increase in temperature induces an increase of the burning velocity. The higher burning velocity value is obtained for downdraft syngas. This result is endorsed to the higher heat value, lower dilution and higher volume percentage of hydrogen in the downdraft syngas.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2011;133(3):031003-031003-8. doi:10.1115/1.4004809.

Annular pressure reduction during cementing is a major factor causing annular gas flow. It has been widely accepted and proven experimentally that the pressure reduction phenomenon results from the shear stress opposing downward motion of slurry undergoing volume reduction. The models that have been proposed to describe this process are based on the gel strength and shear stress developments in time and ignore system compressibility. They explain the pressure reduction process observed in the lab where compressibility of the system is very small. However, the models cannot explain the pressure reduction patterns observed on the field where compressibility is significant and the time-dependent effects of cement slurry volume loss significantly contributes to the process. The paper presents a mathematical model combining the effects of gel strength, volume reduction, and compressibility of cement slurry to describe pressure loss in the annular cement column. Results from the model, shown in the paper, compare very well with the data from the laboratory and field tests. Also, the simulated results explain discrepancies between the pressure loss patterns observed in the lab and field tests.

Commentary by Dr. Valentin Fuster

### Research Papers: Energy From Biomass

J. Energy Resour. Technol. 2011;133(3):031801-031801-11. doi:10.1115/1.4004788.

The increased use of intermittent renewable power in the United States is forcing utilities to manage increasingly complex supply and demand interactions. This paper evaluates biomass pathways for hydrogen production and how they can be integrated with renewable resources to improve the efficiency, reliability, dispatchability, and cost of other renewable technologies. Two hybrid concepts were analyzed that involve coproduction of gaseous hydrogen and electric power from thermochemical biorefineries. Both of the concepts analyzed share the basic idea of combining intermittent wind-generated electricity with a biomass gasification plant. The systems were studied in detail for process feasibility and economic performance. The best performing system was estimated to produce hydrogen at a cost of \$1.67/kg. The proposed hybrid systems seek to either fill energy shortfalls by supplying hydrogen to a peaking natural gas turbine or to absorb excess renewable power during low-demand hours. Direct leveling of intermittent renewable electricity production was proposed utilizing either an indirectly heated biomass gasifier or a directly heated biomass gasifier. The indirect gasification concepts studied were found to be cost competitive in cases where value is placed on controlling carbon emissions. A carbon tax in the range of \$26–40 per metric ton of CO2 equivalent (CO2 e) emission makes the systems studied cost competitive with steam methane reforming (SMR) to produce hydrogen. The direct gasification concept studied replaces the air separation unit (ASU) with an electrolyzer bank and is unlikely to be cost competitive due to high capital costs. Based on a direct replacement of the ASU with electrolyzers, hydrogen can be produced for \$0.27 premium per kilogram. Additionally, if a nonrenewable, grid-mix electricity is used, the hybrid system is found to be a net CO2 e emitter.

Commentary by Dr. Valentin Fuster

### Research Papers: Deep-Water Petroleum

J. Energy Resour. Technol. 2011;133(3):031501-031501-5. doi:10.1115/1.4004968.

A dual gradient, deepwater drilling system based on dilution of riser mud requires economically separating the riser mud into a low density dilution fluid and a higher density drilling fluid. This study investigated the practicality of accomplishing this separation using hydrocyclones and centrifuges and examined the possible benefits and efficiency of each. The separation experiments were conducted using a laboratory centrifuge and 2 in. hydrocyclones. The laboratory centrifuge was able to separate the riser mud into near ideal densities for dilution and drilling fluid. However, the dense slurry retained in the centrifuge had lower electrical stability than the feed stream. The hydrocyclones achieved much less contrast in density between the low and high density discharges, but their use consistently resulted in a beneficial increase in the stability of the mud emulsion in all of the flow streams and gave more desirable rheological properties. A qualitative comparison indicates that the hydrocyclone separation system may offer a feasible and desirable alternative to a centrifuge separation system.

Commentary by Dr. Valentin Fuster

### Research Papers: Petroleum Transport/Pipelines/Multiphase Flow

J. Energy Resour. Technol. 2011;133(3):033001-033001-5. doi:10.1115/1.4004965.

Physics of wax gel formation during shut-in is analyzed and described over a cross-section of a typical subsea pipeline. Two regions are identified during this process: the liquid and gel regions. Phase transition is assumed to occur at the liquid-gel interface. Unsteady-state heat and mass transfer models are proposed for each region. Two diffusion streams are evaluated: the dissolved wax molecules moving from the pipe center toward the wall due to temperature gradient and subsequently concentration gradient and the wax molecules diffusing from the liquid-gel interface into the gel deposit. This model is essentially the modification of the model given by Bhat [1] which considered transient heat transfer and neglected mass transfer of wax molecules through the gel deposit and the model by Singh [2] which considered transient mass transfer of molecules with carbon numbers higher than the` critical carbon number (CCN) necessary for wax diffusion into gel deposit but did not consider transient heat transfer effects during the cooling process. This paper presents a transient-state formulation circumventing the limitations of these previous models and better represents the true cooling and gelation process occurring in a shut-in subsea pipeline filled with waxy crude.

Commentary by Dr. Valentin Fuster

### Research Papers: Alternative Energy Sources

J. Energy Resour. Technol. 2011;133(3):031201-031201-9. doi:10.1115/1.4004360.

This paper presents the first results of a wide experimental investigation on the aerodynamics of a vertical axis wind turbine. Vertical axis wind turbines have recently received particular attention, as interesting alternative for small and micro generation applications. However, the complex fluid dynamic mechanisms occurring in these machines make the aerodynamic optimization of the rotors still an open issue and detailed experimental analyses are now highly recommended to convert improved flow field comprehensions into novel design techniques. The experiments were performed in the large-scale wind tunnel of the Politecnico di Milano (Italy), where real-scale wind turbines for micro generation can be tested in full similarity conditions. Open and closed wind tunnel configurations are considered in such a way to quantify the influence of model blockage for several operational conditions. Integral torque and thrust measurements, as well as detailed aerodynamic measurements were carried out to characterize the 3D flow field downstream of the turbine. The local unsteady flow field and the streamwise turbulent component, both resolved in phase with the rotor position, were derived by hot wire measurements. The paper critically analyses the models and the correlations usually applied to correct the wind tunnel blockage effects. Results highlight that the presently available theoretical correction models do not provide accurate estimates of the blockage effect in the case of vertical axis wind turbines. The tip aerodynamic phenomena, in particular, seem to play a key role for the prediction of the turbine performance; large-scale unsteadiness is observed in that region and a simple flow model is used here to explain the different flow features with respect to horizontal axis wind turbines.

Commentary by Dr. Valentin Fuster

### Research Papers: Energy Extraction From Natural Resources

J. Energy Resour. Technol. 2011;133(3):031701-031701-7. doi:10.1115/1.4004786.

There are much coal with low content of volatile matter (Vad  < 20%), high content of ash (Aad  > 50%), low heating caloric (∼10,000 kJ/kg) in China. It is very important to study pyrolysis performance of the coal to ensure high efficiency of utilization and low pollution emissions. In this paper, we study the pyrolysis reaction details of different types of this coal from different regions of China under different pyrolysis pressures, temperatures, particle sizes, and heating rates by thermo-gravimetry (TG) method. The pyrolysis characteristic temperatures and the characteristic index of volatilization matter released of coal gangue (CG) are obtained in this work. In addition, the detailed process of mechanism and kinetic parameters of pyrolysis are presented. The results show that many factors have an obvious influence on the pyrolysis reaction of the coal. The pyrolysis process of the coal is comprised of two stages. At the primary stage(t < 560 °C), the pyrolysis reaction is dominated by the diffusion rate of volatile matter because of the high ash content, which is the global symmetry diffusion mechanism, and the volatile matter of this stage is more difficult to come out and a high pyrolysis activation energy is observed. With increasing pyrolysis temperature, the pyrolysis reaction is moving into diffusion limitation, the volatile matter is released plentifully, and the low activation energy is found. At the second stage (t > 560 °C), the pyrolysis reaction is governed by the tar-released reaction and the pyrolysis reaction order is 1.5. The high activation energy is also observed for the second stage pyrolysis process.

Commentary by Dr. Valentin Fuster

### Research Papers: Petroleum Wells-Drilling/Production/Construction

J. Energy Resour. Technol. 2011;133(3):033101-033101-8. doi:10.1115/1.4004887.

Horizontal wells have become a popular alternative for the development of hydrocarbon fields around the world because of their high flow efficiency caused by a larger contact area made with the reservoir. Most of the analytical work done in the past on horizontal productivity either assumed that the well is infinitely conductive or the flow is uniform along the entire well length. The infinite conductive assumption is good only when the pressure drop in the wellbore is very small compared to the drawdown in the reservoir otherwise the pressure drop in the wellbore should be taken into account. In this paper, an improved predictive model that takes into account the effect of all possible wellbore pressure losses on productivity index of long horizontal well was developed. Results show that the discrepancies in the predictions of the previous models and experimental results were not only due to effect of friction pressure losses as opined by Cho and Shah but may also be due to all prominent pressure losses such as kinetic change and fluid accumulation experienced by the flowing fluid in a conduit. The effect is most pronounced at the early production time where initial transience at the onset of flow is experienced.

Commentary by Dr. Valentin Fuster