scholarly journals Localization of exergy losses in the exhaust gases heat-recovery exchanger of gas-fired heat plants

2020 ◽  
pp. 5-17
Author(s):  
N. Fialko ◽  
◽  
A. Stepanova ◽  
R. Navrodska ◽  
S. Shevchuk ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Conduction ◽  
Heat Recovery ◽  
Heat Engine ◽  
Flue Gases ◽  
Complex Method ◽  
Exhaust Gases ◽  
Relative Contribution ◽  
Exergy Losses ◽  
Transfer Agents

The problem of increasing the thermodynamic efficiency of power plants can be solved only by using a complex approach using methods based on modern methods of exergy analysis in combination with methods of heat transfer theory, theory of linear systems, structural-variant methods, multi-level optimization methods, etc. The analysis of the possibility of applying the discrete-modular principle and the corresponding complex method for analyzing the efficiency of the exhaust gases heat-recovery exchanger of a cogeneration unit heat engine is performed in the paper. The aim of the work is to analyze the localization of exergy losses, their differentiation, and the establishment of the relative contribution of various types of losses to the general exergy losses in the exhaust gases heat-recovery exchanger of a cogeneration unit heat engine. The structural features of the heat-recovery exchanger and the exergy properties that reflect the essence of exergy methods: universality and additivity, made it possible to use the discrete-modular principle and a complex method based on exergy-dissipative functions for efficiency analysis. The advantage of this method is the ability to analyze the localization of exergy losses in separate modules of the heat-recovery exchanger and to differentiate the exergy losses associated with nonequilibrium heat transfer between the heat-transfer agents and the wall, heat conduction and the movement of heat-transfer agents. Using the chosen complex method, the analysis of the localization of exergy losses in the heat-recovery exchanger was carried out and the exergy-dissipative functions of each of the eight modules of the heat-recovery exchanger were calculated. Differentiation of exergy losses was carried out and the relative contribution of exergy losses associated with the processes of heat transfer from flue gases to the wall, from wall to water, in heat conduction processes, as well as exergy losses associated with the movement of heat-transfer agents, in the general exergy losses was analyzed. To determine the exergy losses due to nonequilibrium heat transfer between the heat-transfer agents and the motion of the heat-transfer agents, the differential exergy equations, the equations for the heat flow densities between the heat-transfer agents and the wall, the equation for the heat flow density due to heat conduction through the wall and the equations of motion are used. It has been established that the localization of maximum exergy losses in all modules of the heat-recovery exchanger is associated with losses due to heat transfer from flue gases to the wall.

scholarly journals EXERGIC EFFICIENCY OF THE HEAT RECOVERY UNIT FOR WASTE GASES OF A HEAT ENGINE OF A COGENERATION PLANT

2020 ◽  
Vol 42 (3) ◽  
pp. 56-60
Author(s):  
N. Fialko ◽  
A. Stepanova ◽  
R. Navrodskaya ◽  
S. Shevchuk
Keyword(s):  
Heat Exchanger ◽  
Heat Recovery ◽  
Exergy Analysis ◽  
Heat Engine ◽  
Cogeneration Plant ◽  
Advantages And Disadvantages ◽  
Recovery Unit ◽  
Recovery Schemes ◽  
Exergy Losses ◽  
Complex Methods

The paper presents the results of a study of the efficiency of a heat recovery unit for waste gases of a heat engine of a cogeneration plant. The possibilities of using for this purpose the discrete-modular principle and complex methods of analyzing the efficiency of heat recovery systems, which are based on the methods of exergo-dissipative functions and exergic balances, are analyzed. The design features of the heat exchanger are considered and a conclusion is made about the possibility of presenting it as a system of eight discrete modules. The results of calculating the exergy characteristics for each of the eight heat exchanger modules, performed within the framework of the indicated methods, are presented. A regular decrease in exergy losses and heat-exergy criterion of efficiency is observed during the transition from the first to the eighth module of the heat recovery unit. However, exergy characteristics for the third and fourth modules of the heat exchanger are somewhat higher than the indicated dependence suggests. This indicates the thermodynamic imperfection of these modules. The main exergy losses in all heat exchanger modules are associated with losses due to heat transfer from flue gases to the wall. An insignificant discrepancy between the values ​​of the total exergy losses calculated within the framework of the methods used indicates that both methods can be used in various heat recovery schemes. However, in each specific case, it is necessary to choose a methodology with which it is possible to identify individual elements that need optimization or constructive improvement. Particular attention is paid to the comparative analysis of the selected techniques and consideration of the advantages and disadvantages of their use in various cases. It is noted that the technique based on the integral balance method of exergy analysis can be considered effective due to the small number of initial parameters and the simplicity of the analytical and calculation methods. The advantage of the technique using exergo-dissipative functions is that it allows one to differentiate exergy losses in a heat exchanger and establish the causes and areas of their localization.

Modified Rankine HRSG Beats Triple-Pressure System

1996 ◽  
Author(s):  
Peter Eisenkolb ◽  
Martin Pogoreutz ◽  
Hermann Halozan
Keyword(s):  
Heat Transfer ◽  
Gas Turbine ◽  
Gas Turbines ◽  
Heat Recovery ◽  
Power Plants ◽  
Fossil Fuels ◽  
Combined Cycle ◽  
Exhaust Gas ◽  
Pressure System ◽  
Exergy Losses

Gas-fired combined cycle power plants (CCP) are presently the most efficient systems for producing electricity with fossil fuels. Gas turbines have been and are being improved remarkably during the last years; presently they achieve efficiencies of more than 38% and gas turbine outlet temperatures of up to 610°C. These high outlet temperatures require modifications and improvements of heat recovery steam generators (HRSG). Presently dual pressure HRSGs are most commonly used in combined cycle power stations. The next step seems to be the triple-pressure HRSG to be able to utilise the high gas turbine outlet temperatures efficiently and to reduce exergy losses caused by the heat transfer between exhaust gas and the steam cycle. However, such triple-pressure systems are complicated considering parallel tube bundles as well as start up operation and load changes. For that reason an attempt has been made to replace such multiple pressure systems by a modified Rankine cycle with only a single-pressure level. In the case of the same total heat transfer surfaces this innovative single-pressure system achieves approximately the same efficiency as the triple-pressure system. By optimising the heat recovery from the exhaust gas to the steam/water cycle, i.e. minimising exergy losses, the stack temperature is much higher. Increasing the heat transfer surfaces means a decrease of the stack temperature and a further improvement of the overall CCP-efficiency. Therefore one has to be aware that the proposed system offers advantages not only in the case of a foreseeable increase of gas turbine outlet temperatures but also for presently available gas turbines. Using existing highly efficient gas turbines and subcritical steam conditions, power plants with this proposed Eisenkolb Single Pressure (ESP_CCP) heat recovery steam generator achieve thermal efficiencies of about 58.7% (LHV).

USE OF MATRIX BALANCES FOR ANALYSIS OF EXERGY LOSSES IN A COMBINED HEAT RECOVERY SYSTEM OF A BOILER PLANT

2017 ◽  
Author(s):  
Nataliia Fialko ◽  
◽  
Alla Stepanova ◽  
Raisa Navrodskaia ◽  
Nataliia Meranova ◽  
...  
Keyword(s):  
Heat Recovery ◽  
Matrix Form ◽  
Boiler Plant ◽  
Operating Modes ◽  
Relative Contribution ◽  
Energy And Exergy ◽  
Recovery System ◽  
Heat Recovery System ◽  
Installed Capacity ◽  
Exergy Losses

The results of the analysis of exergy losses in separate elements of the installation containing a gas-fired boiler and the combined heat recovery system for heating water and blast air are resulted. It is noted that a complex technique combining the methods of exergy analysis with one of the methods of the theory linear systems is used to determine exergy losses, namely, the method of RP-representation thermodynamic balances in matrix form. When implementing this technique for separate elements of the heat recovery system, the balances of mass, energy and exergy are recorded in matrix form. On the basis of matrix balances the input matrix by means of which total exergy losses in heat recovery system at various operating modes of a boiler are calculated is defined. The results of calculating the relative contribution of exergy losses in each element of the heat recovery system to the total irreversibility of processes in the system at different operating modes of the boiler are analyzed. It is noted that the smallest losses in the main elements of the heat recovery system occur when the boiler capacity is up to 55 % of the installed capacity.

scholarly journals The combined heat and mass transit processes of water droplets in biofuel technologies

Energetika ◽  
2017 ◽  
Vol 63 (2) ◽  
Author(s):  
Virginijus Ramanauskas ◽  
Monika Maziukienė ◽  
Gintautas Miliauskas
Keyword(s):  
Heat Transfer ◽  
Heat Recovery ◽  
Droplet Surface ◽  
Heat Fluxes ◽  
Flue Gases ◽  
Heating Process ◽  
Water Droplets ◽  
Transfer Processes ◽  
Mass Transfer Processes ◽  
Instantaneous Temperature

The number of thermal technologies is based on water droplets heat and mass transfer processes. In this paper droplets heat transfer and phase transformations were modelled in the context of water spraying in humid flue gases application for biofuel combustion technologies. Droplets warming is defined when its surface instantaneous temperature is calculated according to an iterative numerical scheme, which is based on heat fluxes balance that flows in and from the droplet surface. The transfer processes interaction is taken into account. The influence of spraying water temperature, biofuel flue gases temperature and its humidity as well as droplets heating process impact on spraying water droplets transit phase transformation regimes were highlighted. Technological cases of water spraying in a biofuel furnace and heat recovery from removed flue gases were highlighted. In condensing economizers at recovered flue gases sprayed droplets heat transfer conditions are defined by their slipping factor in flue gases, while in the case of biofuel combustion in the furnace, droplets slipping and its surrounding radiation factors are equivalent.

METHODS FOR PROTECTING BOILER CHIMNEYS AGAINST CORROSION DUE TO FALL-OUT CONDENSATE FROM FLUE GASES

2017 ◽  
Author(s):  
Nataliia Fialko ◽  
◽  
Raisa Navrodska ◽  
Georgii Gnedash ◽  
Georgii Presich ◽  
...  
Keyword(s):  
Heat Recovery ◽  
High Efficiency ◽  
Flue Gases ◽  
Thermal Methods ◽  
Exhaust Gases ◽  
Boiler Units

The application of thermal methods for protection of chimneys of boiler units with deep heat-recovery systems of exhaust-gases was proposed. The results of the research have confirmed the high efficiency of these methods.

Prevention of condensation in chimneys of boiler plants with heat-recovery systems

2021 ◽  
pp. 5-17
Author(s):  
N. Fialko ◽  
◽  
R. Navrodska ◽  
S. Shevchuk ◽  
G. Presich ◽  
...  
Keyword(s):  
Heat Recovery ◽  
Dew Point ◽  
Flue Gases ◽  
Boiler Plant ◽  
Exhaust Gases ◽  
Operating Modes ◽  
Wide Range ◽  
Recovery System ◽  
Cooling Air ◽  
Heating Boiler

The results of studies of the effectiveness of using the air method of preventing condensation formation in the gas-exhaust ducts for anticorrosive protection of chimneys of gas-fired heating boiler plants are presented. This method is used in heat-recovery systems of boiler plants, characterized by deep cooling of gases (below the dew point temperature of water vapor contained in exhaust-gases). The essence of this method is to change the thermal and humidity characteristics of exhaust-gases after heat-recovery by mixing dry and heated air in front of the chimney. Schematic solutions of heat-recovery systems using two options for using the air method are presented. The first option corresponds to the use of the air method when mixing air from the heater of boiler plant. In the second option, for the implementation of the air method, air heated in the heat-recovery system itself is used. To assess the efficiency of the air method, computational studies were carried out to determine the thermal and moisture characteristics of flue gases at the mouth of different types of chimneys under different operating modes of the boiler during the heating period. The studies were carried out for two proposed options for using the air method when using air with a change in its temperature over a wide range. The values of the dew point of the flue gases at the mouth of the chimney and the temperature of its inner surface were calculated at various proportions of the mixed air. The parameters of flue gases and mixed air were determined, ensuring the absence of condensation in the chimneys. Based on the values of the obtained parameters, a comparative analysis of the effectiveness of the considered options for using the air method was carried out. It is shown that for heating boilers the use of this method is the most effective in complex heat-recovery systems when using recovered heat for heating return heat-network water and combustion air. Key words: gas-fired boilers, exhaust-gases, deep cooling, air method, thermal and humidity condition, chimney, anticorrosive protection

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