Thermal Engineering最新文献

As conventional exergy analyses do not reveal the exergy destruction rates in a thermal system component caused by inefficiencies of interconnected components, actual potentials for improving the component performances cannot be provided by such analyses. This study analyses a combined-cycle gas turbine power plant using advanced exergy analysis methodologies, which address the shortcomings of conventional exergy analysis by evaluating the exergy destruction rates that are endogenous and exogenous, avoidable and unavoidable. Avoidable exergy destruction rates in the entire plant were found to be 31% of the total exergy destruction rates, indicating a significant potential for improving the plant. Exergy destruction rates for most of the plant components were largely endogenous (95.2%), signifying that contributions of cross-component interactions were limited. Avoidable endogenous exergy destruction rates account for 28.4% of the overall exergy destruction rates in the plant, while avoidable exogenous exergy destruction rates account for 2.1%. A component-level ranking of the plant components ranked the pumps in the plant as first for improvement whereas the highest priority was allocated to the combustion chambers (CC) by a plant-level ranking. A parametric study of the influence of CC operating conditions on the plant’s performance showed that CC operating temperatures more significantly affected plant exergy destruction rates than the CC operating pressures.

The article addresses matters concerned with decreasing the prime cost of thermal energy in the remote and isolated regions of Russia’s Arctic zone (AZ) that have a high wind energy potential by using wind turbines (WTs) jointly with boiler houses operating on expensive imported fossil fuel for heat-supply purposes. The use of wind turbines will make it possible to decrease the participation of boiler houses in the supply of heat to consumers, save fossil fuel, and, thereby, help decrease the prime cost of thermal energy. A procedure for calculating the levelized cost of thermal energy is developed and described in detail. The procedure is adapted to analyzing the efficiency of alternative options of using WTs jointly with boiler houses for heat-supply purposes, among which the alternative ensuring the minimal levelized cost of thermal energy is regarded as the most efficient one. By using the obtained technique, the economic efficiency of applying WTs jointly with boiler houses in the heat-supply systems of remote and isolated regions is evaluated taking the westernmost part of Russia’s Arctic zone as an example. It has been determined that, in such regions, in which the final annual average cost of fossil fuel is more than 1.5 times higher than the fuel cost in the cities and industrial centers of the AZ westernmost part as a consequence of a high transport component, the WTs are most efficient when used jointly with boiler houses operating on diesel fuel. For boiler houses operating on fuel oil and coal, the effect from using WTs is not so high. It is also shown that the cheaper the fuel, the less efficient or even inefficient at all the use of WTs becomes in comparison with the heat-supply option from a boiler house without connecting a WT. For the regions considered, the joint production of thermal energy by WTs and boiler houses operating on diesel fuel, fuel oil, and coal makes it possible to decrease its levelized cost by 7‒55, 5‒20, and 2‒7%, respectively.

Energy efficient and environment friendly power generation is the primary goal for any power generating industries. This paper proposes a thermodynamic approach based on E³ (energy, exergy and environment) analysis for performance improvement of power plant during low main steam and high reheater (RH) temperature conditions through a suitable operation technique. Thermodynamic modeling of a 500 MW Subcritical (SubC) coal based thermal power plant is carried in “Cycle-Tempo” at different conditions. Partial withdrawl of final feed water heater (high pressure heater—HPH-6) from service without RH spray condition during low main steam (MS) temperature and high RH steam temperature condition will help to increase the MS temperature by about 0.85–1.00°C and thereby, the net plant energy and exergy efficiency will be improved by about 0.09 and 0.08% point, respectively. Partial withdrawl of HPH-6 with RH spray condition will deteriorate the plant energetic and exergetic plant performance and this will guide the operation engineer for which extend withdrawl of HPH-6 can be done for getting higher plant performance. The net energy efficiency of turbogenerator (TG) cycle decreases with partial withdrawl of HPH-6 due to decrease in the feed water temperature by about 7°C and more relative energy rejection of the cycle. The net exergy efficiency of TG cycle increases due to less relative exergy destruction rate causing from improvement in steam quality. However, the use of RH spray increases the irreversiblities in the plant and the spray does not expand in high pressure turbine (HPT) which in turn decrease the exergy efficiency. The boiler energy efficiency increases due to decrease in fluegas exit loss as the fluegas exit temperature drops from about 140 to 133°C due to partial withdrawl of HPH-6. The exergy efficiency of boiler also decreases due to increase in exergy destruction in final super heater (FSH), reheater and economizer. For a 500 MW SubC coal power plant, hourly about 930 kg of coal and about 1183 kg of CO₂ emission can be saved and reduced through this operation technique namely, partial withdrawl of HPH-6 without RH spray condition for controlling low MS temperature. Hence, the proposed analysis will help to take proper operational technique for mitigating coal crisis and safeguarding the environment as well.

The ammonia-ethanolamine water chemistry used at NPPs with VVER-1200 ensures low rates of corrosion, mass transfer, and growth of corrosion product deposits. The content of corrosion products in the feedwater of the steam generator is less than 1 μg/dm³. This significantly increases the period between flushing the steam generator to remove deposits. However, ethanolamine and ammonia are absorbed by the cation exchange resin in the ion-exchange filters of the secondary circuit purification systems, which leads to the need to regenerate the cation exchange resin and continuously dose reagents to maintain the required pH value in the feedwater. Waste solutions from regeneration containing ethanolamine and large amounts of ammonia must be treated to ensure that the concentrations of these substances do not exceed maximum permissible values when discharged into the environment. To remove ethanolamine and ammonia from regeneration solutions, special installations are created, the operation of which is based on various principles. A pilot plant for cleaning regeneration solutions was manufactured and installed at the Belarusian NPP with VVER-1200. An analysis of the pilot plant’s operation showed that it successfully fulfills its function of protecting the aquatic environment but, at the same time, it is forced to release a significant amount of ammonia into the surrounding air. Removing ammonia is energy-consuming, environmentally unsafe, and requires the additional use of chemical reagents. In this regard, the water-chemical regime of the secondary circuit without ammonia is very promising. Possible options could be either switching from ammonia to dimethylamine or using ethanolamine as the only corrective reagent with the replacement of some of the structural materials of the secondary circuit with steels with a high chromium content, which have higher corrosion resistance compared to those currently used. Both options will simplify the wastewater treatment technology and reduce the environmental impact while maintaining the low corrosion rates achieved by using the ammonia–ethanolamine water chemistry.

The prospects are examined for application of ammonia-fired gas turbine units (GTUs) with thermochemical recuperation of the exhaust gas heat. Examples of operating ammonia-fired gas turbine units are given, and the main operating restrictions for the use of existing gas turbine units are specified. A thermodynamic analysis of a simple gas turbine unit with thermochemical heat recuperation (TCR) was performed in a wide range of operating conditions: the gas temperature at the turbine inlet varied from 700 to 1300°C and the compressor pressure ratio from 5 to 20. It has been established that the thermochemical heat recuperation can increase the GTU efficiency by as much as 9%. The effectiveness of TCR application has been demonstrated to depend on such operating parameters as pressure and temperature. At a temperature above 500°C, the enthalpy of the ammonia decomposition reaction reaches a value close to the maximum of approximately 3.0 MJ/kg NH₃. Thermochemical recuperation leads to the decomposition of ammonia with production of a hydrogen-rich gas (up to 75% (by volume)), which is burned in the combustion chamber, thereby changing the combustion process characteristics. The flame propagation velocity in a gas mixture consisting of hydrogen, nitrogen, and ammonia in different proportions was calculated on the basis of the GRI-Mech 3.0 list of elementary reactions in the Chemkin-Pro module. It has been found that the products of complete thermochemical decomposition of ammonia have a flame propagation velocity that is approximately two times higher than that for methane and more than ten times higher than that for ammonia. Thus, the implementation of the thermochemical heat recuperation in ammonia-fired gas turbine units is expected to increase the energy efficiency and improve the combustion process stability.

Ejectors are devices designed to suck fluid, steam or gas (primary fluid) from a closed space using a powerful jet of steam (secondary fluid), usually operated under specified boundary conditions using specific working fluids. If ejectors are to be used under new boundary conditions, predicting their performance requires either numerical or experimental studies. This paper presents a simple theoretical model capable of accurately predicting the performance of an ejector, given its geometry and boundary conditions, under different operating conditions. The model can predict the entrainment ratio, critical back pressure, and break-up back pressure using a given simple performance curve. The accuracy of the model is validated using computational fluid dynamics (CFD) simulations. Two ejectors with different geometries, dimensions, and boundary conditions are studied using ANSYS Fluent 19.2, and the results are compared with those from two other studies. The model successfully predicts the performance of all four ejectors across a wide range of operating conditions. Finally, the model is extended to any working fluid and temperature and validated numerically using air as the working fluid instead of water vapor. The results show that the model has an entrainment ratio error of less than 2%. It’s worth noting that this model’s applicability is contingent upon simultaneous changes to both the primary and suction streams by the same factor. Under these conditions, the model aligns closely with CFD-simulations.

The purpose of the review is to find the best currently available correlations for calculating heat transfer and pressure drop in the main heat-transfer equipment items in organic Rankine cycle (ORC) units. The search is limited to the designs of apparatuses, which are the best ones in the opinion of the authors of this paper, for a conventional two-circuit ORC-unit, where thermal oil cools a heat source in the first circuit and transfers heat to refrigerant in the vapor generator (hereinafter referred to as the evaporator). Besides the evaporator, the second circuit of the unit includes a “refrigerant–water” or “refrigerant–air” condenser and a regenerative heat exchanger which heats up liquid refrigerant upstream of the evaporator with the exhaust vapor of the turbine (or expander). The criteria are presented for selecting working fluids for such units depending on the heat source temperature. The working fluids that have found the widest application at each temperature level (such as cyclopentane, benzene, toluene, MM, MDM, R1233zd, R245fa, R601, R601a, RC318, R134a) are listed, and their characteristics and thermodynamic properties are presented at specified condensation (25°C) and boiling (200, 120, and 70°C) points. The analysis of these data, including information on the proposed working fluids, has yielded nominal parameters of ORC-units. Thousands of fundamental and engineering works are devoted to the study of boiling and condensation processes, the interest in which has been growing over the past 10–15 years. The development of new energy conversion technologies and the appearance of new working fluids, materials, and methods of surface treatment has given a second wind. This paper reviews correlations for heat-transfer coefficients and hydraulic resistance factors in apparatuses with refrigerant boiling in round tubes, condensation in tubes and channels and in the shell side (on tube bundles), and heating and cooling of single-phase refrigerant in tubes and channels. The correlations for engineering calculation of the main heat-transfer equipment of ORC-units, which are the most convenient ones in the authors’ opinion, are presented.

A mathematical model of a high-temperature cylindrical reactor for heterogeneous pyrolysis of methane during its filtration through a moving layer formed by granules of carbonized wood is presented. The carbon matrix was modeled by spheres of the same diameter with a simple cubic packing. The carbon matrix was heated through the reactor wall. Preheated methane was fed into the lower part of the reactor. The process of pyrocarbon formation as a result of heterogeneous pyrolysis of methane was described by one gross reaction taking into account hydrogen inhibition and changes in the reaction surface. It was assumed that the rate of pyrocarbon deposition is directly proportional to the partial pressure of methane. The system of two-dimensional, nonstationary differential equations describing the operation of the reactor in a cyclic mode with periodic unloading of a portion of carbon–carbon composite and synchronous loading of carbonized wood granules was solved numerically using the DIFSUB algorithm. The reactor radius and operating parameters (specific mass flow rate of methane, carbon composite unloading frequency) were varied in calculations. Based on the obtained results, the dependences of the quality of the carbon–carbon composite (average density and maximum density spread), the composition of the hydrogen-containing gas mixture at the reactor outlet, the degree of methane conversion, the reactor productivity for carbon composite and hydrogen on the operating parameters, and the reactor radius were analyzed. Data are provided on energy consumption for heating methane and carbonized granules loaded into the reactor as well as for compensation of the endothermic effect accompanying methane pyrolysis.

The prospects for achieving carbon neutrality in economically developed countries that are members of the Organization for Economic Cooperation and Development (OECD) and other countries are examined. An analysis of the energy and land use structure in these countries was carried out. Scenario assessments of the dynamics of changes in carbon indicators of the study economies have been developed, and a comparison has been made with forecasts from leading global energy agencies. It has been shown that, at the current rate of decarbonization and development of the carbon capture and storage (CCS) industry, it is impossible for countries in both groups to fulfill their commitments to achieve climate neutrality in 2050–2070; this goal cannot be achieved before the end of this century. The central challenge in achieving climate neutrality is the rapid and large-scale implementation of CCS technologies in all their possible manifestations. Using a set of global climate system models, calculations of the global average temperature (GAT) were performed for the proposed scenarios, and their results were compared with other works. Despite the fact that climate change occupies almost a leading place on the global agenda, the actual results of efforts in this area are far from those declared, and it is now impossible to cap warming to within 1.5°C. The key task is to minimize the time the global climate system remains in the dangerous extreme zone (above 1.5°C), which will require the emergence of a global economy with negative greenhouse gas (GHG) emissions.

Heat exchange during condensation of freons has been studied quite well; however, various flow regimes of the steam-condensate mixture may arise during condensation inside heat-exchange pipes. There is a large amount of experimental data on the condensation of freons inside pipes with different internal diameters. However, the results obtained by different authors are contradictory, and experimental dependencies can give a high error in the event of a discrepancy between the calculated and actual flow regimes of the steam-condensate mixture. Due to the difficulty of identifying these modes for each such case, reliable recommendations for the calculation and design of heat exchangers must be based on experimental data. In order to obtain such materials, an experimental stand was developed and manufactured, allowing the study of condensation processes of various working fluids in a horizontal cooled tube. The working section of the stand was a copper pipe with an external diameter of 32 mm and a wall thickness of 2 mm, built into an external steel pipe with a diameter of 45 × 3 mm with an annular gap of 3.5 mm. Five chromel-copel thermocouples were installed in the gap to measure the water temperature; they were led to the measuring instruments through the wall of the outer pipe. Thermocouples were also installed in the copper pipe wall. The stand’s thermocouples were precalibrated, and the freon and cooling water consumption was determined by the differences on the flow diaphragms with an error not exceeding 1.5%. The temperatures of cooling water and condensing freon R142b along the length of the heat-exchange pipe were obtained for some flow regimes with different parameters of the working fluid at the pipe inlet. A sharp decrease in the local heat-transfer coefficient along the length of the heat-exchange pipe during complete condensation is shown and is especially significant at its inlet section. The obtained data will be used in the design of heat exchangers with condensation of R142b freon in horizontal pipes.