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Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2018;140(9):092001-092001-9. doi:10.1115/1.4039872.

The use of renewable sources, such as woody biomass waste, for energy purposes helps to reduce the consumption of fossil fuels and therefore the production of associated pollutants and greenhouse gases. Solid oxide fuel cells (SOFCs) are devices that convert the chemical energy of a product gas produced by a gasifier of biomass waste, before being suitably purified, directly into electric energy, with conversion efficiency, which is higher than that of other conventional energy systems. Since they operate at high temperature, they make available also thermal energy, which can be used for co- and tri-generation purposes. This paper aims at studying the arrangement of a complete trigenerative energy system composed of a gasifier of waste biomass; an energy unit represented by a SOFC system; an absorption cooling section for the conversion into cooling energy of the waste heat. In its layout, the SOFC energy unit considers the anode off gas recirculation, a postcombustor to energize the exhaust stream, and a preheater for the fresh gases entering. The integrated plant is completed by means of batteries for electric energy storage and hot water tanks and thermal energy storage. An ad hoc developed numerical modeling is used to choose the working point of the SOFC energy system at which to operate it and to analyze its energy behavior under syngas feeding. Two biomass-derived syngas are analyzed: one from woody biomass and one from urban solid waste gasification. Hence, the entire integrated plant is analyzed for both feeding types. The energy analysis of the integrated SOFC/gasifier is carried out based on a fixed quantity of biomass waste to be processed in an existing gasifier. Then, the design of the SOFC energy section is carried out. The integrated plant is then applied to a case study to satisfy the energy needs of a user of the tertiary sector. Therefore, based on this, the procedure continues with sizing the cooling section for the cooling power delivery in the warm season, the batteries to store the electric energy to be delivered, and the hot water tanks for the thermal energy storage to be delivered as heat when necessary or to feed the absorption cooling plant. The integrated SOFC/Gasifier defined can be considered as a high-efficiency tri-generator capable of accomplishing an energy valorization of high quality waste biomass.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(9):092002-092002-11. doi:10.1115/1.4039873.

This paper deals with an integrated biomass system developed for syngas production with waste heat recovery option and analyzes this system thermodynamically using both energy and exergy approaches. Also, an aspenplus simulation model is developed to demonstrate comparative gasification analyses of wood (Birch) and olive waste using Gibbs reactor for syngas production. Gibbs free energy minimization technique is applied to calculate the equilibrium of chemical reactions. In this newly developed model, the heat of the product syngas and the waste heat from the flue gas are recovered through a unique integration of four heat exchangers to produce steam for the gasification process. The sensitivity analyses are performed to observe the variations in the concentration of the methane, carbon monoxide and carbon dioxide in syngas against various operating conditions. Furthermore, the performance of gasifier is indicated through cold gas energy efficiency (CGE) and cold gas exergy efficiency (CGEX). The overall energy and exergy analyses are also conducted, and the comparisons reveal that the biomass composed of olive waste yields high magnitude of overall and cold gas energy efficiencies, whereas wood (Birch) yields high magnitude of overall and cold gas exergy efficiencies. Moreover, the energy of the product syngas is recovered through an expander which enhances energy and exergy efficiencies of the overall system. The present results show that the CGE, CGEX, and overall energetic and exergetic efficiencies follow a decreasing trend with the increase in combustion temperature. The proposed system has superior and unique features as compared to conventional biomass gasification systems.

Commentary by Dr. Valentin Fuster

Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2018;140(9):092201-092201-11. doi:10.1115/1.4039741.

Premixed charge compression ignition (PCCI) combustion is a novel combustion concept, which reduces oxides of nitrogen (NOx) and particulate matter (PM) emissions simultaneously. However, PCCI combustion cannot be implemented in commercial engines due to its handicap in operating at high engine loads. This study is focused on the development of hybrid combustion engine in which engine can be operated in both combustion modes, namely, PCCI and compression ignition (CI). Up to medium loads, engine was operated in PCCI combustion and at higher loads, the engine control unit (ECU) automatically switched the engine operation to CI combustion mode. These combustion modes can be automatically switched by varying the fuel injection parameters and exhaust gas recirculation (EGR) by an open ECU. The experiments were carried out at constant engine speed (1500 rpm) and the load was varied from idling to full load (5.5 bar brake mean effective pressure (BMEP)). To investigate the emission and particulate characteristics during different combustion modes and mode switching, continuous sampling of the exhaust gas was done for a 300 s cycle, which was specifically designed for this study. Results showed that PCCI combustion resulted in significantly lower NOx and PM emissions compared to the CI combustion. Lower exhaust gas temperature (EGT) in the PCCI combustion mode resulted in slightly inferior engine performance. Slightly higher concentration of unregulated emission species such as sulfur dioxide (SO2) and formaldehyde (HCHO) in PCCI combustion mode was another important observation from this study. Lower concentration of aromatic compounds in PCCI combustion compared to CI combustion reflected relatively lower toxicity of the exhaust gas. Particulate number-size distribution showed that most particulates emitted in PCCI combustion mode were in the accumulation mode particle (AMP) size range, however, CI combustion emitted relatively smaller sized particles, which were more harmful to the human health. Overall, this study indicated that mode switching has significant potential for application of PCCI combustion mode in production grade engines for automotive sector, which would result in relatively cleaner engine exhaust compared to CI combustion mode engines.

Commentary by Dr. Valentin Fuster

Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2018;140(9):092901-092901-11. doi:10.1115/1.4039744.

Horizontal well drilling technology is widely used in the exploitation of petroleum and natural gas, shale gas, and geothermal resources. The temperature distribution of wellbore and surrounding formation has a significant influence on safe and fast drilling. This study aims to investigate the temperature distribution of horizontal wellbores during circulation by using transient temperature model. The transient temperature prediction model was established by the energy conservation law and solved by the relaxation iterative method. The validity of the model has been verified by the field data from the Tarim Oilfield. The calculation results showed that the highest temperature of the drilling fluid inside the drill string was at the bottomhole and the highest temperature of annulus drilling fluid was at some depth away from the bottomhole. Sensitivity analysis of various factors that affect the temperature distribution of annulus drilling fluid were carried out, including the circulation time, the flow rate, the density of drilling fluid, the inlet temperature, the vertical depth, the horizontal section length, and the geothermal gradient. It can be found that the vertical depth and the geothermal gradient have a significant influence on the bottomhole temperature, and inlet temperature plays a decisive influence on the outlet temperature. These findings can supply theoretical bases for the horizontal wellbore temperature distribution during drilling.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(9):092902-092902-10. doi:10.1115/1.4039871.

In down-hole interventions, the thin elastic coiled tubing (CT) extended for thousands of meters underground would typically undergo helical buckling as a result of axial compressive force. This paper builds an analytical model to describe the unbuckling behavior of a helically buckled CT with a new view to the stretching process in the plug milling operations. The new dynamic unbuckling equation is built on the basis of the general bending and twisting theory of rods. Under the continuous contact assumption, the helical angle is only subject to time; thus, the dynamic equations can be simplified and the analytical solutions can be obtained. By using the new governing equations, the angular velocity, axial force, and contact force relative to CT are analyzed in the unbuckling process. The calculation results indicate that the parameters including CT diameters and wellbore diameters have a strong influence on the variation of axial force and wellbore contact force. Moreover, the wellbore contact force is greater than zero during the whole unbuckling process which confirms the continuous contact assumption. These new results provide important guidance for accurate job design for the plug milling operations during the well completion stage.

Commentary by Dr. Valentin Fuster

Research Papers: Petroleum Wells-Drilling/Production/Construction

J. Energy Resour. Technol. 2018;140(9):093101-093101-9. doi:10.1115/1.4039875.

Field data indicate production profile along horizontal wells is nonuniform. This paper develops an analytical model of multisegment horizontal wells (MSHWs) to estimate rate distribution along horizontal wellbore, interpret the effective producing length (EPL), and identify underperforming horizontal sections using bottom-hole pressure (BHP) data. Pressure solutions enable to model an MSHW with nonuniform distribution of length, spacing, rate, and skin factor. The solution is verified with the analytical solution in commercial software. Type curves are generated to analyze the pressure-transient behavior. The second radial-flow (SRF) occurs for the MSHWs, and the duration of SRF depends on interference between segments. The pressure-derivative curve during SRF equals to 0.5/Np (Np denotes the number of mainly producing segments (PS)) under weak interference between segments. The calculated average permeability may be Np times lower than accurate value when the SRF is misinterpreted as pseudoradial-flow regime. The point (0, 0, h/2) are selected as the reference point, and symmetrical cases will generate different results, enabling us to distinguish them. Finally, field application indicates the potential practical application to identify the underperforming horizontal segments.

Commentary by Dr. Valentin Fuster

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