As shown in Figure 1, the flow sheet of the thermodynamic system is established according to the practically running 600 MW thermal power plant with the direct air cooling condenser in Wuxiang, China. From the perspective of research, the whole system is mainly divided into seven subsystems, including the boiler system, coal pulverizing system, fuel air system, steam turbine system, regenerative system, air cooling system and Power generation system, which have different functions respectively.

The main facilities contained in the boiler system are the economizer, superheater, reheater, steam dome, and the cooled water wall. Coal pulverizing system provides the physical preparation of coal satisfying the necessary standards for the combustion such as the physical parameter of particle size and humidity [33] . Before being transported into the boiler, the air absorbs heat from the smoke exhaust in the fuel air system to achieve a certain temperature [34] . Steam turbine system contains high pressure cylinder, medium-pressure cylinder and low pressure cylinder. In the regenerative system, different pressure steam extraction from different cylinder will heat the condensed water from the air cooling system in the corresponding heater respectively. Finally, the heated water whose temperature may be greater than 200℃ as the feed water is pumped into the boiler, starting a new cycle working process, in which the coal burns and convert chemical energy to gas heat energy that realize heat transfer in different heat exchangers in turn.

The method of energy analysis based on the first law of thermodynamics is generally used to analyze power system performance. While the exergy analysis combining the first and the second law of thermodynamics is to probe the essence of the energy loss and the distribution of the available energy from the point of view of energy amount and energy quality, which reveals two things: the destruction of exergy within a system component as well as the exergetic efficiency showing how effectively the exergetic resources supplied to a component have been used.

In this paper, in order to evaluate the performance of the system synthetically, this paper uses the energy analysis method and the analysis method, and evaluates them with different evaluation indexes, such as thermal efficiency, efficiency, loss rate, loss rate and dissipation heat and net coal consumption. The exergy flow is shown in Figure 2, in which the red arrows represent the input and the output exergy of the system, black arrows the exchange exergy among the subsystems, and blue arrows the exergy losses for different process.

In order to investigate the performance of the system by exergy analysis, some hypotheses are made as follows:

a) Ignoring the diffusion exergy loss of working medium in the pipelines and small leakage of working medium at equipment joints or connections.

b) Flow rates of liquid and solid mediums are from the measured data, and that of the gas medium from quality equilibrium calculation of system.

c) The working medium mixes uniformly on the condition of steady flow.

d) To ensure the reliability of the measured parameters, they must be examined and filtered before into model for calculation.

e) The reference temperature is about 15˚C and the reference pressure is about 1atm for the exergy analysis.

In an exergy analysis, the heat rate does not have the same value as the power, and the loss of work equals to the difference between the input exergy and the output exergy of the system. According to energy and exergy balance, the loss can be described as:

where, EX, loss is the exergy loss; ΣEX_in is the input exergy; ΣEX_out is the output exergy.

The exergy in the system contains the exergy of steady flowing medium, the heat exergy, the chemical exergy of coal as fuel, and the mechanical exergy.

where, EX_flow is the exergy of steady flowing medium; H, S are the enthalpy and entropy of the working medium at certain thermal state; while H₀, S₀ are that at the environment state. Generally, it is assumed that the exergy due to kinetic energy and potential energy are negligible.

where, EX_Q is the heat exergy; Q is the heat energy; T₀ is the ambient temperature [35] [36] [37] [38] , K.

where, Q_c,ar is the low heat value of the as-received basis coal, kJ/kg; C_ar, H_ar, O_ar, N_ar are the mass percentage of them in the as-received basis coal, %; ex_c,ch is the chemical exergy per unit mass of coal, kJ/kg.

where, C_c is the specific heat of the dry basis coal, kJ/(kg・K); T_c is the temperature of the coal, K; ex_c,ph is the physical exergy per unit mass of coal, kJ/kg.

where, T_s is the temperature of the smoke exhaust, K; is the average specific heat at constant pressure of the dry smoke from the temperature T₀ to T_s, kJ/(m³・K); V_s is the volume of the dry smoke coming from the combustion of unit mass coal, m³/kg; ex, loss_s is the exergy loss of the smoke exhaust, kJ/kg.

According to energy and exergy balance, the flow rate of the steam extraction can be calculated through the energy equilibrium matrix Equation (7), and then the exergy loss in the regenerative system can be got.

where, i is the stage serial number, q_i is the enthalpy drop at extraction stage i, kJ/kg; γ_i is the enthalpy drop of the drainage from the exheater in the current heater (No.i), kJ/kg; τ_i is the enthalpy rise of the feed-water in the current heater (No.i), kJ/kg; D_i is the flow rate of the extraction stage i, kg/s; D_fw is the flow rate of the feed-water, kg/s.

The calculation of the exergy efficiency can be expressed in different forms for the different facilities in the energy system. However, the description of them can be unified as the same form from the point of view of the principle of the exergy flow and the exergy efficiency that is the ratio of the exergy revenue and the exergy consumption in the thermodynamic process.

or

where, ΣEX_in and ΣEX_out are the input exergy and the output exergy respectively.

where, ΔEX_h and ΔEX_c are separately the exergy difference of the high and low temperature heat source in the process of heat transfer.

Based on the parameters of each stream in the Figure 1, as shown in Table 1, the values of the exergy loss and its proportion in the total exergy loss, as well as the exergy efficiency at 470 MW are shown in Table 2. We get that the exergy loss of the total system is about 1111.41 MW, and the exergy efficiency is 30.75% or so.

Figure 3 shows the specific values of the exergy loss and its ratio for the main facilities in the whole system. We can see that the main parts of the exergy loss focus on the boiler, air cooling system and the steam turbine. Meanwhile, the

exergy loss of the regenerative system is quite little comparatively which just about 15.29 MW, and the ratio of the exergy loss about 1.38%, among which the exergy loss of the low pressure heater No. 7 is relatively large, and that of the other heaters is very little and the ratio of the exergy loss of them is less than 0.5% of the whole system.

① smoke exhaust, ② chemistry factor, ③ machinery factor, ④ heat dissipation, ⑤ boiler ash, ⑥ combustion, ⑦ heat transfer.

The distribution of the exergy loss of the main four subsystems is shown in Figure 4. The exergy loss of the boiler system is the largest which is about 866.60 MW and the exergy efficiency is about 42.83% with the ratio of the exergy loss 77.97% or so. The exergy losses of the steam turbine and the air cooling system are almost the same, which are about 118.39 MW and 111.13 MW respectively with the ratio of the exergy loss 10.65% and 10.00% or so. Nevertheless, the exergy efficiencies of them differ greatly, which are about 81.22% and 5.66% respectively. The exergy efficiency of the steam turbine is second to that of the regenerative system which is about 91.32%, meanwhile the exergy efficiency of

the air cooling system is the lowest in the whole system. Therefore, we can see that the main energy saving potential is in boiler as well as the air cooling system.

Especially, the detailed exergy loss of the boiler is shown in Figure 5. We can see that the combustion exergy loss which is about 592.68 MW occupies the great proportion in the boiler system, about 68.39%; the heat transfer exergy loss takes the second place which shows 217.62 MW and 25.11% approximately. The every other exergy loss including machinery factor, the chemistry factor, the smoke exhaust, the heat dissipation, and the fly ash is less than 3% in the boiler system. Therefore, the key for the energy saving of the boiler is to decrease the exergy loss of the combustion and the heat transfer.

In order to investigate the comprehensive performance of the system, it’s necessary to precede the exergy analysis of the system at various loads, which is always changing all the time in actual operation. According to measured data of the power plant in practical running, we have gotten the variation laws of exergy, exergy loss as well as exergy efficiency.

Figure 6 shows the exergy variation of the live steam, the cold reheat steam, the reheated steam, the exhaust steam of the Intermediate pressure turbine (IPT) and the exhaust steam of the steam turbine. We can see that all the exergy mean value of them will increase with the load increment, nearly linear increasing, and that of the live steam is the largest increase, the exhaust steam of the steam turbine the minimum. As the load increases, the required main steam flow increases, resulting in an increase in the corresponding flow, so that the exergy of each steam increases. In addition, apart from the 5# and 6# Low pressure heater

(LPH), the average exergy of every extraction increases with load increase as shown in Figure 7. To analyze the cause of this variation, we know that with the load increases, the amount of condensed water that needs to be heated increases, and the corresponding low-temperature heat increases. Therefore, we need to increase the exergy of 7# LPT, while to reduce the 5# and 6# extraction heat appropriately.

The exergy loss as well as its ratio of the main four parts with load variation is shown in Figure 8. We can see that the exergy loss of the boiler system is always the maximal for the value, and the second the steam turbine and the air cooling system, and the minimum the regenerative system, and we find that the ratio of the exergy loss of the steam turbine and the regenerative system rises with the increase of the load, while that of the boiler system slowly rises first, then reduces. With the load increases, the exergy loss of combustion and heat transfer decreases, which accounts for most of the exergy loss in the boiler, resulting in a gradual decrease in energy loss at the boiler. In addition, the variation of the ratio of the exergy loss for the air cooling system is inconspicuous and is affected by multifarious factors such as environmental parameters, fan operation parameters, and so on. The exergy efficiency of the steam turbine as well as its three cylinders is shown in Figure 9. Due to the measured parameters fluctuate at low load, so we can see that the exergy efficiency of the three cylinders fluctuates relatively obviously at low load, generally less than 350 MW - 400 MW. Because the use of the nozzle adjustment method, it is obvious that the exergy efficiency of the high pressure cylinder averagely appears decreasing with the load increase and when the load rises approximately from 400 MW to 560 MW, its efficiency increases from about 70% to 78%. The exergy efficiency of the whole steam turbine with load changing, between about 80% and 85%, is closely related with every cylinder, and the minimum value exists around 300MW to 350MW. Therefore, the unit should be kept away from low load from the point of view of economy or safety.

As shown in Figure 10, the exergy efficiency of the boiler increase obviously with the load increment, e.g., when the load increases from 350 MW to 550 MW, the exergy efficiency changes from 33.8% to 46.4%, about 6 percent points increment per 100 MW increment. In addition, we can see that the major exergy

losses in the boiler are from the combustion and the heat transfer, whose average values decrease with the load increment, while the heat transfer exergy loss decrease gently, and the combustion exergy loss drops obviously when the load is more than 500 MW, with exergy loss about 150 MW from 500 MW to 550 MW or so. Because the coal combustion is more adequate at high load and the average temperature difference is reduced, the exergy loss of combustion and heat transfer decreases with increasing load. Additionally, we also see that even at the high load, the efficiency of the boiler is not better than 55%, so that the im-

provement of combustion and maintain the boiler running at high load as far as possible are crucial for boiler performance enhancement.

Figure 11 shows the variation of the total exergy loss and the exergy efficiency of the whole system with the load changing. The total exergy loss decreases slowly at the load less than about 500 MW and decreases sharply at the load more than 500 MW or so with load rising, e.g., the former, dropping about 20 MW with the load increment about 300 MW, and the latter, decreasing from 1100 MW to 900 MW with the load increasing from 510 MW to 560 MW, which is similar to that of the boiler aforesaid. The increase of the total exergy efficiency is even uniform with the load rising. The total exergy efficiency increases from about 20% to 40% when the load varies from 250 MW to 550 MW. From above, we can see that reduction of the boiler combustion exergy loss is of importance for the energy saving and the rational energy utilization, meanwhile, the operation at high load is an effective way to improve the performance of the thermal power system because the system parameters’ matching is more reasonable with the increase of load.

In addition to the exergy analysis, according to the common indexes generally used in power plant, we have also obtained the heat and net coal consumption change with the load increment, as shown in Figure 12. Because the system exergy efficiency increases with increasing load, the amount of coal consumed by the unit power will be reduced, we can see that the heat and net coal consumption have similar decreasing trend with load increment. When the load is less than 330MW or so, the heat and net coal consumption increases fast with the load decreasing, e.g., with the load decreasing from 320 MW to 260 MW, the heat and net coal consumption increases from 10098.2 kJ·kW⁻¹·h⁻¹ to 11226.1 kJ·kW⁻¹·h⁻¹, and from 376.1g·kW⁻¹·h⁻¹ to 428.2 g·kW⁻¹·h⁻¹, respectively. Adversely, with the load

decreasing from 550 MW to 350 MW, the heat and net coal consumption increases from 8965.3 kJ·kW⁻¹·h⁻¹ to 9893.4 kJ·kW⁻¹·h⁻¹, and from 327.3 g·kW⁻¹·h⁻¹ to 368.2 g·kW⁻¹·h⁻¹, respectively. Therefore, in order to improve the performance of the whole system, it is advised that the unit should run at load more than 350 MW.