JPH0217763B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a waste heat recovery device comprising a waste heat boiler, a flasher mixed pressure steam turbine, a generator, etc. installed in order to recover electricity as efficiently as possible from a plurality of exhaust gas heat sources having different temperature levels. be. The heat of waste gas discharged from steel, cement, or chemical plants is exchanged with a medium such as water to generate steam, which is then guided to a turbine to drive the turbine and drive driven objects such as generators. Rotating has been practiced since ancient times. In addition, recently, exhaust gas at temperatures around 200℃, which had previously been overlooked, has become a target for heat recovery from the perspective of energy conservation, and there is a need to recover heat by combining exhaust gases at several different temperature levels. has arisen. Conventionally, the heat of exhaust gases with different temperature levels is recovered in a waste heat boiler, and the generated steam drives a turbine and rotates a generator to generate electricity and extract energy as electricity. The method shown in Figure 3 has been used. FIG. 2 shows the state of the temperature change of the exhaust gas and the temperature change of the water side with respect to FIG. 1, and FIG. 4 shows the state of the temperature change of the exhaust gas and the water side with respect to FIG. A conventional heat recovery method will be explained with reference to FIGS. 1 and 2. In FIG. 1, Bl is a waste heat boiler that recovers heat from exhaust gas at a low temperature level, ie, a low-pressure waste heat boiler, and Bh is a waste heat boiler that recovers heat from exhaust gas at a high temperature level, ie, a high-pressure waste heat boiler. In addition, 1l is the economizer of the low pressure waste heat boiler, 2l is the evaporator of the low pressure waste heat boiler, 3l is the brackish water drum of the low pressure waste heat boiler, and 1h, 2h, and 3h are the economizers of the high pressure waste heat boiler, respectively. , evaporator and brackish water drum. The feed water sent to the low-pressure waste heat boiler Bl and the high-pressure waste heat boiler Bh at a temperature of t _w1 by the feed water pump 6 exchanges heat with the exhaust gas in the economizers 1l and 1h of each boiler, and as a result, the water of the low-pressure waste heat boiler Bl Economizer 1l
At the outlet of t _s temperature, also high pressure waste heat boiler Bh
At the outlet of the economizer 1h, the water is heated to a temperature T _s to become saturated water, which enters the brackish water drums 3l and 3h of the respective boilers. In this case, it is natural that the temperature T _s can be higher than the temperature t _s . The hot water stored in the brackish water drums 3l and 3h enters the evaporators 2l and 2h, where they are heated by the exhaust gas and their respective temperatures are reduced.
It evaporated under constant conditions of t _s and T _s , became saturated steam with saturated pressures ps and P _s corresponding to temperatures t _s and T _s12 , and was sent again to the brackish water drums 3l and 3h, where water droplets were separated. Thereafter, the saturated steam at low pressure Ps is guided to the intermediate stage of the mixed pressure steam turbine 4, and the saturated steam at high pressure _Ps is guided to the inlet of the mixed pressure steam turbine 4. On the other hand, the exhaust gas that entered the low-pressure waste heat boiler Bl at a temperature t _g1 loses heat in the evaporator 2L and drops to a temperature t _g2 , then loses heat in the economizer 1L and reaches a temperature t _g3 , and the energy saving device 1l, that is, exits the low-pressure waste heat boiler Bl. The exhaust gas at temperature T _g1 entering the high-pressure waste heat boiler Bh is also transferred to the evaporator 2l, as in the case of the low-pressure waste heat boiler Bl.
While passing through the economizer 1h, the temperature drops to T _g2 and T _g3 and leaves the high pressure waste heat boiler Bh. The steam entering the mixed pressure steam turbine 4 expands and imparts work to the turbine 4, becoming low-pressure steam with a pressure _Pz , entering the condenser 5, where it is cooled and the temperature is lowered.
It becomes condensed water at a low temperature of t _z and is sent to the low-pressure waste heat boiler Bl and the high-pressure waste heat boiler Bh by the condensate pump (here, the feed water pump) 6, and the circulation is repeated. usually,
A bleed ejector cooler and a ground condenser are installed in the water supply system between the condenser 5 and the boiler Bl or Bh, and the feed water is slightly heated here, so the temperature at the boiler inlet is often slightly higher than the temperature t _z . . In addition, for the sake of simplicity, this explanation uses a low-pressure waste heat boiler.
Bl and the high-pressure waste heat boiler Bh generate and evaporate saturated steam, but if necessary, each boiler may be provided with a superheater to superheat the saturated steam. Next, the method shown in FIG. 3 will be explained. The equipment names and corresponding symbols in Figure 3 are the same as in Figure 1, but the equipment configuration is from the brackish water drum 3l of the low pressure waste heat boiler Bl to the high pressure waste heat boiler.
Water supply pump 7 for high pressure waste heat boiler to supply water to Bh
The difference is that there is a The difference between the method in FIG. 3 and the method in FIG. 1 is that in the method in FIG.
In the method shown in Fig. 3, feed water is first sent to the low-pressure waste heat boiler Bl by the feed water pump 6, heated in the economizer 1l from temperature _tw1 to _ts , and then heated from the brackish water drum 3l to the amount of evaporation from the high-pressure boiler Bh. The hot water is extracted and supplied to the high pressure waste heat boiler by the high pressure waste heat boiler feed water pump 7.
This is what I am sending to Bh. That is, the feed water at a temperature t _w1 that enters the low-pressure waste heat boiler Bl is raised to a temperature t _s in the economizer 1l and enters the brackish water drum 3l, but the hot water in the brackish water drum 3l is transferred to the evaporator 2 of the low-pressure waste heat boiler Bl.
The amount is divided into the amount supplied to the high pressure waste heat boiler Bh and the amount supplied to the high pressure waste heat boiler Bh. The hot water that enters the evaporator 2L of the low-pressure waste heat boiler Bl becomes saturated steam at the same temperature as the method shown in Fig. 1, but the hot water that enters the evaporator _2L of the low-pressure waste heat boiler
The hot water that has entered Bh is heated to a temperature of T _s in the energy saver 1h, and then becomes saturated steam in the evaporator 2h while keeping the temperature T _c constant. The exhaust gas entering the high-pressure waste heat boiler Bh at a temperature T _g1 decreases in temperature to T _g2 and T _g ' ₃ while passing through the evaporator 2h and the economizer 1h, and leaves the boiler Bh. On the other hand, the exhaust gas that entered the low-pressure waste heat boiler 1L at a temperature t _g1 decreases to a temperature t _g2 while passing through the evaporator 2L and enters the energy saver 1L, but the exhaust gas enters the energy saver 1L.
Since the amount of water that passes through this is larger than in the method shown in FIG. 1, the temperature of the exhaust gas at the outlet of the economizer 1L is lowered to a temperature t _g ' ₃ lower than in the case of FIG. 1. Before explaining the drawbacks of the conventional method and the advantages of the present invention as described above, the relationship between heat recovery by a waste heat boiler and temperature will be described. FIG. 5 is a cycle diagram of a steam power generation plant when waste heat recovery is performed, and FIG. 6 is a diagram of temperature changes of gas and water corresponding to FIG. 5. Next, the explanation will be made in conjunction with FIGS. 5 and 6. In the case of a waste heat boiler that heats a medium such as water with a high-temperature fluid to generate saturated steam, the pinch point temperature difference Δ where the difference between the temperature of the heating fluid and the temperature of the heated fluid is the minimum is the energy saver 1. It is easy to see that this occurs at the inlet or outlet of the gas, and the pinch point temperature difference Δ when generating saturated steam by exchanging heat with medium-low temperature exhaust gas of about 500°C or less with water of about 60°C or less is the gas side node. This is the difference between the temperature t _g2 at the inlet of the coal burner and the temperature t _s at the outlet of the water economizer. That is, economizer 1
The gas inlet temperature t _g2 of , that is, the gas outlet temperature t _g2 of evaporator 2 is t _s + Δ, and the amount of heat given to water by evaporator 2 is
Q ₁ and the amount of evaporation Gs can be expressed as follows. Q ₁ = CpGg (t _g1 - t _g2 ) = CpGg (t _g1 - t _s - Δ) - (1) Gs = Q ₁ / (is - iw) - (2) where, Cp; specific heat of gas, Gg ;Gas flow rate,
is; saturated steam enthalpy at temperature t _s ; iw; saturated water enthalpy at temperature t _s If the pinch point temperature difference Δ is kept constant and the evaporation pressure of the boiler, that is, the evaporation temperature t _s is increased, the gas outlet temperature of evaporator 2, t _g2 will also rise,
As can be seen from equation (1) above, the amount of heat Q ₁ transferred to the gas in the evaporator 2 decreases. As a result, it can be easily seen from the above equation (2) that the evaporation amount Gs also decreases. However, the adiabatic heat drop inside the turbine naturally increases as the evaporation pressure increases. Therefore, the turbine output, which is expressed as the product of the flow rate passing through the turbine, that is, the amount of evaporation and the adiabatic heat drop inside the turbine, becomes maximum at a certain evaporation pressure. On the other hand, the exhaust gas outlet temperature t _g3 of the economizer 1 is expressed by the following equation based on the thermal balance in the economizer 1. t _g3 = t _g2 - Gw (iw - t _w1 ) / CpGg - (3) where, Gw: Flow rate passing through the economizer (amount of water supplied to the boiler) Flow rate passing through the economizer 1 and evaporation amount in the evaporator 2 If they are equal, Gw in equation (3) can be written as Gs. Therefore, if the evaporation amount Gs decreases, the second term in equation (3) decreases, and the exhaust gas outlet temperature t _g3 increases. Also, when the water supply temperature t _w1 to the energy saver 1 is increased, the second term of equation (3) becomes smaller and the temperature t _g3 becomes larger. When the evaporation amount Gs and the water supply amount Gw are equal, only one exhaust gas outlet temperature is determined for the evaporation pressure Pso that maximizes the turbine output, but the evaporation amount Gs and the water supply amount
When the values of Gw are different, as the water supply amount Gw is increased, the second term of equation (3) becomes larger and the economizer outlet temperature t _g3
decreases. That is, it becomes t _g ' ₃ in FIG. In this way, the flow rate Gw passing through the economizer 1 is made larger than the amount of evaporation Gs, and after heating to the temperature t _s , the brackish water is discharged from the drum 3.
Extract the amount of hot water Gw−Gs and guide it to the flasher 9 to flash it to generate flash steam,
If this generated steam is mixed in the middle stage of the steam turbine 4, the turbine output increases by an amount commensurate with the flow rate of the mixed air. Furthermore, the unflushed hot water in the flusher 9 is put into the mixing tank 10 and the condenser 5
If the feed water flowing from to the waste heat boiler B is directly mixed, the temperature of the feed water entering the waste heat boiler B will rise from _{tw1 to tw'1 .} However, it is a matter of course that this feed water temperature cannot be made higher than the value t _g ' ₃ −Δ obtained by subtracting the pinch point temperature difference Δ from the gas outlet temperature t _g ' ₃ of the waste heat boiler B. In Figure 6, the state of the water side temperature change shown by the solid line is when hot water is extracted from the brackish water drum 3 and the water supplied to the waste heat boiler B is heated, and the broken line is when hot water is extracted from the brackish water drum 3. In the case where the supply water is not heated either, the dotted line shows the case where hot water is extracted from the brackish water drum 3 but the supply water is not heated. FIGS. 7 and 8 show the states of change in the turbine output explained above. That is, FIG. 7 shows the relationship between the turbine output and the boiler outlet gas temperature using the boiler feed water temperature as a parameter, and the feed water temperature t _w1 ,
For t _w2 and t _w3 , there are boiler outlet temperatures T _g31 , T _g32 , and T _g33 that provide the maximum turbine output. FIG. 8 shows the relationship between turbine output and evaporation temperature with respect to boiler outlet gas temperature. As already explained, when the water supply amount Gw and the evaporation amount Gs are equal, the turbine output is as shown by the solid line in Fig. 8, and when the boiler outlet gas temperature t _g31 is the evaporation temperature t _s1 , the maximum turbine output L ₂₁ can be obtained. Can be done. Furthermore, hot water at temperature t _s1 is extracted from the brackish water drum at the evaporation temperature t _s1 , which is the maximum turbine output, and flash steam is generated in the flasher, mixed with the turbine, and the unflushed hot water heats the feed water to the boiler. In this case, as described above, the turbine output increases and the boiler outlet gas temperature decreases, and the maximum turbine output is obtained when the difference between the boiler outlet gas temperature and the boiler feed water temperature reaches the pinch point temperature difference. This state is shown by the broken line in FIG. That is, when the boiler outlet gas temperature t _g32 , the turbine output L ₂₂ is obtained. Next, instead of directing the entire amount of hot water extracted from the brackish water drum to the flasher, a portion of it is transferred to other equipment, such as a third
When water is fed to a high-pressure waste heat boiler Bh or the like shown in the figure, the boiler outlet gas temperature naturally decreases by that amount, as shown by the dotted line in Figure 8. The disadvantages of adopting the conventional method shown in FIGS. 1 and 3 will be explained with reference to FIGS. 7 and 8.
If the gas temperature at the boiler outlet can be selected arbitrarily, apply the method shown in Figure 1, supply water to the high-pressure waste heat boiler and the low-pressure waste heat boiler at a low feed water temperature t _w1 , and reduce the gas temperature at the outlet of each boiler. Temperature T _g31 and T _g31
By lowering the power up to 100 degrees, steam can be generated from the respective boilers to obtain outputs L ₁₁ and L ₂₁ .
Also, sometimes the outlet gas temperature of a waste heat boiler is determined, in particular the outlet gas temperature of a high pressure waste heat boiler is provided to other systems of the main process. For example, it is often regulated at a fairly high temperature for heating raw materials in cement plants. When this determined boiler outlet gas temperature is T _g32 , if the water supply temperature to the boiler is set to a low temperature.
If t _w1 is maintained, the turbine output obtained from the steam generated by the boiler will be significantly reduced to L ₁₂ [kw], as shown in Figure 7. Therefore, if the feed water temperature t _w2 can be set to obtain the maximum output corresponding to the temperature T _g32 , the turbine output obtained from the steam generated by the boiler can be maintained at L ₁₁ [kw]. As a way to deal with this problem, as shown in Figure 3, the water supply to the high pressure waste heat boiler is controlled by the economizer of the low pressure waste heat boiler at a temperature of t _w2.
It is possible to heat it up to However, the optimum feed water temperature for high pressure waste heat boilers
t _w2 is not necessarily the optimum evaporation temperature of the low-pressure waste heat boiler t _s1
However, depending on the case, if the evaporation temperature of the low-pressure waste heat boiler is selected to be t _w2 , the turbine output from the steam generated by the boiler will be L ₂₄ [kw], and the maximum output will be L ₂₁
[kw] may be significantly reduced. moreover,
It is clear from FIG. 8 that the conventional method shown in FIGS. 1 and 2 does not use flash steam, so the output is reduced accordingly. The present invention was devised in order to solve the drawbacks of the conventional methods, and to utilize the given exhaust gas conditions to the maximum extent possible. FIG. 9 shows an embodiment of the system of the present invention. The present invention will be explained using FIG. 9, FIG. 7, and FIG. 8. Assuming that the gas temperature entering the high-pressure waste heat boiler Bh is T _g1 and the outlet temperature is determined by T _g32 , the optimum water supply temperature to the economizer 1h of the boiler is t _w2 , as shown in Figure 7. be. On the other hand, if the gas temperature entering the low-pressure waste heat boiler Bl is t _g1 , the optimum evaporation temperature of the boiler is t _s1 . If the temperatures t _s1 and t _w2 are equal, the feed water of the high pressure waste heat boiler Bh is the same as that of the low pressure waste heat boiler Bl.
The hot water extracted from the brackish water drum 3 liters may be directly supplied, and if t _s1 < t _w2 , a heater such as a deaerator may be used while the hot water extracted from the brackish water drum 3 liters is being supplied to the high-pressure waste heat boiler Bh. There is no problem as long as the temperature is set up and heated to a temperature t _w2 . Conversely, when t _s1 > t _w2 , in order to obtain the maximum output from the steam generated by the high-pressure waste heat boiler Bh, the hot water extracted from the brackish water drum 3l of the low-pressure waste heat boiler Bl at a temperature t _s1 and a flow rate Gw must be heated to a temperature t _w2 need to be lowered to. Therefore, the temperature from 3 liters of brackish water drum
If the hot water at t _s is led to the flasher 9 and the internal pressure of the flasher is maintained at the saturated pressure Pf corresponding to the temperature t _w2 , it will be separated into saturated steam Gf at the pressure Pf and hot water at the temperature t _w2 .
This hot water is introduced into the economizer 1h of the high-pressure waste heat boiler Bh to generate high-pressure steam with a pressure Ph, temperature Th, and flow rate Gh, which is sent to the inlet of the mixed pressure steam turbine 4 to drive the turbine. An output of L ₁₁ [kw] can be obtained. On the other hand, the flow rate Gw−Gh obtained by subtracting the flow rate Gh supplied to the high-pressure waste heat boiler Bh from the unflushed hot water Gw−Gf, which is the temperature t _w2 of the flusher 9
-Gf, the condensate sent from the condenser 6 is heated in the mixing tank 10, and the water is supplied to the economizer 1l of the low-pressure waste heat boiler Bl at a temperature of t _w ' ₁ . The amount of water supplied is the sum Gl + Gw of the evaporation amount Gl from the boiler Bl and the amount Gw extracted from the brackish water drum 3L, and the temperature t _w ′ ₁ is the gas outlet temperature from the boiler Bl.
It is equal to t _g ′ ₃ minus the pinch point temperature difference Δ. Gl+Gw entered the low pressure waste heat boiler Bl
The feed water is heated in the energy saver 1l to the optimum evaporation temperature _ts1 of the boiler Bl concerned and enters the brackish water drum 3l, from which hot water in an amount Gw is extracted and sent to the flasher 9.
At the same time, the remaining saturated water in an amount of Gl is heated in an evaporator 2L to reach a saturation pressure Ps ₁ corresponding to a temperature t _s1.
Therefore, it is mixed into the middle stage of the mixed pressure steam turbine 4. In addition, the pressure generated by the above-mentioned flash
Pf flash steam is also mixed into the intermediate stage of the mixed pressure steam turbine 4. In this way, a low pressure waste heat boiler
By generating steam at pressure Ps ₁ and temperature t _s1 from Bl, generating steam at pressure Pf from the flasher 9, and guiding these steams to the turbine to drive the turbine, the turbine of L ₂₃ [kw] in Fig. 8 is created. You can get the output. As can be seen from FIG. 8, L ₂₃ [kw] is greater than L ₂₁ [kw] by ΔL [kw]. As explained above, by applying the present invention, a total turbine output of L ₁₁ +L ₂₃ [kw] can be obtained, which is at least ΔL [kw] more than the conventional method, and in some cases, ΔL′ = [(L ₁₁ +L ₂₃ )−
(L ₁₂ + L ₂₁ )] will result in more output. The results of an example calculation are as follows.

【table】

[Table] However, turbine efficiency x generator efficiency x mechanical efficiency = 75%.
In this result, the recovered power is 6176 [kw],
This yields 371 [kw] more power than the 5805 [kw] recovered using the conventional method shown in Fig. 3. The present invention is applicable not only to two exhaust gas heat sources but also to a larger number of exhaust gas heat sources, and the temperature of the gas entering the high pressure waste heat boiler, the low pressure waste heat boiler, and the outlet gas of each or any boiler can be A method of extracting hot water from the brackish water drum of a high pressure waste heat boiler and guiding it to the flasher instead of extracting hot water from the brackish water drum of the low pressure waste heat boiler and guiding it to the flasher depending on the temperature and gas flow rate values to each boiler. Needless to say, the same can be said about the method of extracting hot water from the brackish water drum of each waste heat boiler and guiding it to the flusher.

[Brief explanation of drawings]

Figures 1 and 3 are cycle diagrams showing conventional examples of waste heat recovery equipment for power generation plants, and Figures 2 and 4 are temperature change states on the gas side and water side corresponding to Figures 1 and 3. (t _g , t _w ), Fig. 5 is a cycle diagram of a general waste heat recovery device, and Fig. 6 shows the state of temperature change on the gas side t _g and water side t _w corresponding to Fig. 5. Fig. 7 is a chart showing the change in turbine output when the high pressure boiler outlet gas temperature is changed using the boiler feed water temperature _tw as a parameter, and Fig. 8 is a chart showing the change in the turbine output when the low pressure boiler outlet temperature is changed. FIG. 9 is a cycle diagram showing an embodiment of the waste heat recovery device according to the present invention. B...waste heat boiler, Bh...high pressure waste heat boiler,
Bl...Low pressure waste heat boiler, 1, 1h, 1l...Economy device, 2, 2h, 2l...Evaporator, 3, 3h, 3
l... Brackish water drum, 4... Mixed pressure steam turbine,
5... Condenser, 6, 7... Water pump, 8... Generator, 9... Flushsha, 10... Mixing tank.

Claims (1)

[Claims] 1 A plurality of waste heat boilers with different temperature levels, a flusher that extracts hot water from the brackish water drum of the waste heat boiler with a lower temperature level among the waste heat boilers to generate steam, and each of the waste heat boilers and It consists of a mixed-pressure steam turbine powered by steam from the flasher, and part of the unflushed hot water from the flasher is used to feed the waste heat boiler at a higher temperature level, and the rest is used to feed the waste heat boiler at a lower temperature level. A waste heat recovery device characterized in that it is used for heating water supply.