FI77511B - FOERFARANDE FOER HOEJNING AV AONGPROCESSENS VERKNINGSGRAD. - Google Patents

Description

1 77511

A method for increasing the efficiency of the steam process

The invention relates to a method for increasing the efficiency of a steam process, in which steam produced in a steam generation system with a hot stream of material is fed to a steam turbine, a cooled steam stream from a steam turbine is condensed and the steam is generated.

In steam turbine processes, which derive their energy from, for example, waste heat from gas turbines, the problem is the smallness of the feed water and the corresponding steam flow relative to the gas flow. Thus, the gases cannot be cooled down very much without lowering the pressure of the steam process, which in turn impairs the efficiency. In practice, the optimum pressure level for the waste gas boiler of a gas turbine is 30-40 bar and the gases can be cooled to slightly below 200 ° C in a single-pressure boiler.

Attempts have been made to solve this problem in a number of slightly different ways. The most common solution has been to form a 2-pressure level process, where the optimum upper pressure level in the gas turbine waste heat processes is 60-80 bar and the gases are then typically above 250 ° C. Some of this heat is recovered by a separate lower pressure evaporator, the steam of which is fed to the turbine intake. In this way, the heat content of the gases can be utilized more precisely and their initial temperature can be reduced by several tens of degrees. An example of the application of a 2-pressure level solution is the Swiss patent in SCH-621 186.

A slightly different solution is disclosed in Swiss patent publication SCH-645 433. The system of this publication has a separate steam boiler arranged next to the gas turbine waste heat boiler, whereby the total amount of feed water becomes reasonably large to cool the gas turbine waste heat gas stream up to almost 100 ° C. Thus, a total of 2,77511 high-pressure steam from both boilers are fed to the steam turbine with only one pressure / temperature level.

Swedish publication SE-416 835 also discloses a multi-level preheating system in which a higher pressure part supplies a steam turbine and a low pressure part provides steam for other general uses such as fuel oil preheating and other heating. The system also has feed water preheating, which takes its heat from the low-pressure steam generation section. Thus, this system also consists of a multi-level preheating system, in which case steam is fed to the steam turbine with only one pressure / 1 temperature level.

Finnish publication FI-58681 discloses a method in which the recirculation of the feed water to the feed water preheater of the waste heat boiler is arranged back to the feed water tank on the suction side of the feed water pump. This achieves controllability in that the evaporation of water in the feed water preheater can be prevented due to the increased amount of water. The presented method, variations of which are also used, mainly results in a deterioration of the flue gas cooling with increasing feed water recirculation, as the temperature of the feed water entering the feed water preheater practically increases. According to the publication, this system makes it possible to increase the power output of the steam turbine, because the amount of exhaust steam removed from the steam turbines is reduced in a situation where the preheater would otherwise evaporate.

From the point of view of thermodynamics, the above is illustrated in Figure 3, in which the temperatures T and the transferred quantities Q of the gas stream or other substance stream K to be cooled and the water V to be cooled are shown.

Figure 3a shows a single pressure level solution in which the evaporation temperature la (I) and the so-called The "pinch point" temperature difference limits the cooling available for evaporation and superheating to 3 77511 and the amount of heat transferred to 0 · The corresponding amount of heat required to preheat the feed water is Q, which is sufficient for relatively low gas cooling in conventional applications with low gas inlet temperature1 a . A considerable amount of heat Q is wasted. The gas stream to be cooled can be cooled to a maximum temperature T.

a

A common way to help with this is to install a second evaporator at a lower pressure level, i.e. the 2-pressure level solution shown in Figure 3b. Usually, the steam (II) thus generated is fed as an intermediate feed to a steam turbine, as in SCH-621 186, or utilized in factory processes or the like, as in SE-416 835. The amount of heat recovered from the cooled gas stream increases by 0 2 compared to a single pressure process. T.

b

If too much feed water is pumped through the feed water preheater and its excess is returned without cooling back to the inlet side e, as in FI -58681, the feed water inlet temperature rises (T -> T -> T, Fig. 3d) and ol2 results in a decrease in the amount of heat taken up by the feed water.

However, this phenomenon is exploited in part-load control when the gas temperature 1a decreases (O-> 1-> 2 in Fig. 3d), whereby the steam production of the boiler decreases, with which the amount of feed water would decrease. Thus, the water would already start to boil in the feed water preheater, which for several reasons is sought to be prevented or limited to a minimum, and one of the means used is this additional recycling of the feed water. On the other hand, for efficiency reasons, it would be necessary for the feed water already in the preheater to heat to the boiling point. FI-58681 discloses a method for this. By controlling the recirculation of the feed water in certain ways, evaporation in the feed water preheater can be prevented on the one hand and almost boiling point can be achieved on the other hand. 1a However, it does not involve the recycling of feedwater.

The above-described systems have the drawback due to the multi-level korkeapainehöyrystyksestä steam generating half the complexity and thus also a high price. In addition, in the solutions discussed above, the problem is usually exacerbated by the partial load11a of the gas turbine1 when the gas flow of the waste heat boiler1a remains unchanged, but the energy obtained decreases, whereby the evaporation of the boiler also decreases. In this case, there are also operational problems when, due to the reduced water flow to be evaporated, the preheaters start to evaporate in the above-mentioned manner, unless changes are made to the process connection, which in turn reduces the efficiency of the part load, or to design the process at nominal point far from economical.

The process according to the invention provides a decisive improvement in the above drawbacks. To achieve this, the method according to the invention is characterized by what is set forth in the characterizing part of claim 1.

The main advantages of the invention are that it enables in a simple way a process which thermodynamically almost corresponds to a multi-pressure process with separate evaporator circuits for each pressure level of the process expansion device of the process presented here, whereby the efficiency becomes high. problems in boilers and other sites where the amount of gas is independent of waste thermal power, 5 77511 economically more advantageous than the conventional processes for achieving the above advantages due to both higher electricity production and lower investment costs.

In the following, the invention will be explained in detail with reference to the accompanying drawings.

Figure 1 shows an embodiment of the process according to the invention as a process diagram. Figure 2 shows an alternative evaporation arrangement according to the invention as a process diagram. Figures 3a-d show 1-temperature-heat quantity diagrams illustrating the invention and the prior art.

The process takes its thermal energy from the boiler arrangement 1, generally indicated by the reference numeral 1. The components of the boiler arrangement 1 are located in the energy supply medium stream 14, which typically consists of a gas turbine waste heat stream, a flue gas stream from another combustion process, or other similar hot gas stream.

The process shown in the figure would operate as a conventional countercurrent system as follows. The feed water VS from the feed water tank 4, which has a temperature of about 55 ° C, enters the steam separation cylinder 2 of the boiler plant via a low-pressure preheater 9, from where it is circulated as a flow VP to the steam generator 7 to obtain steam HP into the cylinder 2. A steam stream HT is taken from the cylinder 2. through the superheater 6 and is then led to a steam turbine at its upper part. The energy consumed exhaust steam HL from the steam turbine 3 passes through the condenser 10, turning into a water stream VL, the temperature of which can be about 40 C. This condensate is usually transferred in different ways through the feed water tank 4 to the system feed water VS. In this process alone, the amount of circulating water VH, which in this case is the same as VS, is so small that it does not allow the flue gas temperature to be lowered very much as previously stated, no matter how cold the feed water VS is. Conventional load control methods, i.e., fixed or sliding pressure, have no significant effect on efficiency in this regard.

In order to solve the efficiency and part-load problems described above, three expansion evaporators 11, 12 and 13 are arranged in this steam process according to the invention (Fig. 1 as an example). Under normal load conditions during operation at both full load and part load 1a, the steam production of the boiler 1 requires, the amount of water used for the primary steam production as evaporated water VH flows into the boiler cylinder 2 and an additional part of the water supplied to the side branch as water VI after the low pressure preheater 9. The water VH to be evaporated continues its circulation through the steam generator 7 as described above.

The water side stream VI enters the first expansion evaporator 11, where it is allowed to expand, whereby it is divided into a steam stream H2 and a residual water stream V2. The steam stream H2 is then superheated by the gas stream 14 in the low-temperature superheater 8 and led to the steam turbine 3 at a point corresponding to this pressure. Typically, the pressure of the superheated steam at this point is 15 bar and the temperature is 250 ° C. The residual water stream V2 is again led to the next expansion evaporator 12, where it is allowed to expand further, whereby it is divided into a steam stream H3 and a residual water stream V3. The pressure of the steam stream H3 is typically 5 bar o and the temperature 153 C and this flow is directed mainly to the zone corresponding to this pressure of the steam turbine 3 as a partial flow of steam H31. Part of this steam stream can be led as a partial flow H32 to the feed water tank 4 for heating the feed water. The corresponding residual water stream V3 is passed on to the next expansion evaporator 13, where it is allowed to expand further to form a steam stream H4 and a water stream Y4. At this point o the pressure of the steam stream H4 can be 1 bar and the temperature 100 C.

7 77511 This residual water stream V4 is passed through the condenser 10 to the condensate VL, which is typically at a temperature of about 40 ° C.

The arrangement implemented with the surface heat exchanger 5 with respect to the condensate VL and VLK and the feed water VK and VS is directed to the degassing of the feed water and is not per se related to the present invention.

As shown in terms of thermodynamics, the present invention enhances heat recovery as shown in Figure 3c by increasing the water flow through the feed water preheater and the heat surface of the feed water preheater as necessary and profitable. The flue gas can in principle be cooled to the temperature of the feed water from the condenser, in practice to the temperature T, whereby the amount of heat recovered is further increased by the amount Q 1 compared to the 2-pressure level solution. The result is, with the appropriate sizing of the equipment and the regulation of the water flow, more saturated high-pressure water than the amount of heat Q available in the evaporation and superheating section of the boiler can evaporate and superheat. Therefore, excess feed water is discharged from the boiler and used to produce lower pressure steam in the alternative ways presented in this application and, when cooled, is recirculated to the feed water preheater (the water cooling step is not shown in Figure 3c). The resulting low-pressure steam is fed to the steam turbine as an intermediate feed, increasing its electrical power.

In one case according to this example (Figure 1), the effect of the recirculation of the feed water through the evaporator evaporators, when the dimensioning of the equipment is constant, was as follows: 8 77511

Recycling Steam Turbine Power! Father power (net) (net)

0 kg / s 40.73 MW 0 MW

7.38 "41.58" 0.75 "16.08" 42.24 "1.51" 25.91 "42.60" 1.87 "36.72" 42.48 "1.75" In this case, the primary the steam flow to the HT steam turbine 3 has remained constant at 37.4 kg / s.

Figure 1 shows only one example process according to the invention. The principle of the invention can be applied in several different ways by adhering to the principle that the feed water is fed in excess and this excess is evaporated by, for example, the expansion evaporators 11, 12 and 13 shown and this steam one or more than three. The steam from these combustion evaporators can be superheated in one or more superheaters 8 with a gas stream 14 or the steam can be fed to the turbine without superheating. A particularly preferred embodiment with respect to superheating is one (shown in Fig. 2) in which the steam HS currently obtained from the expansion evaporator is superheated in the heat exchanger 23 with the water VI entering this evaporator. Applying to the example of Figure 1 accordingly, the steam stream H3 is superheated with the water stream V2 and the steam stream H4 with the water stream V3, etc. It is also possible to combine the superheat with the gas stream 1a and 14 with the water streams V1, V2 and / or V3. Likewise, the partial steam streams H32 can be taken from different points in the evaporation process and lead to the heating of the feed water or this step can be omitted and all the steam leading to the turbine 3.

9 77511

It is self-evident that the number and structure of the low temperature heaters 9, the superheaters 8, the steam generators 7 and the superheaters 6 are designed according to the flow and type of heat source, applying all structures known per se. Likewise, the superheating of the water stream 11a, V2 and / or V3 can be carried out using any suitable heat exchanger structure known per se between the water stream and the corresponding steam stream HS, H2, H3 and / or H4.

The arrangements related to the feed water tank 4 and the heat exchanger 5 themselves may also be different. The recirculated excess feed water can be returned to the suction side of the feed water pump 16 or using a separate recirculation pump on its pressure side 1e, as required by the process.

In addition to the flash evaporator described above, the low-temperature recirculating water VI can also be evaporated using the surface heat exchanger combination 21 (Fig. 2), whereby as a cooling stream and, if desired, as a superheating agent VI, this excess. At the same time, a smaller stream of water going to evaporation can be taken from this, which generates a corresponding steam stream HS going to the steam turbine. This water can, of course, be taken from elsewhere. The system described above is shown in Figure 2. Otherwise, the modifications of the system described above are available in conjunction with this or such evaporators (when these evaporators have multiple pressure levels).

In case the circulating water VH from the preheater already contains steam, i.e. the preheater has evaporated, the excess recycled feed water VI can first be directed to a steam separation device (not shown in the figures), from which the steam is passed to a boiler or boiler cylinder 2 low pressure steam generators ίο 7 751 1 11, 12, 13; 21. Mata-shoulder vapors H2, H3, H4 thus generated; The HS is treated as described above. If it is desired to avoid the cooling water salts entering the low-pressure circuit, the feed water VI is taken from the boiler cylinder 2 from a special feed water chute before the preheated feed water VH is mixed with the boiler 1a water VP.

Irrespective of the above, and in addition, the residual heat coming out of this process, for example a flue gas stream, can still be used by means of the boiler 1 ai device 15 to heat water going for other purposes. In this way, the waste heat energy of the flue gas can be utilized extremely efficiently.

The main application of the process above is the utilization of waste heat from gas turbines. However, the application of the process is not limited to this, but the heat stream 14 may also be another flue gas stream or the exhaust gas stream of any engine, e.g. Stream 14 may also contain solid particles.

Claims (12)

11 7751 1 A method for increasing the efficiency of a steam process, wherein the steam (HT) produced by the hot stream (14) in the steam generation system (1) is fed to a steam turbine, the cooled steam stream (HL) from the steam turbine is condensed, the steam that more feed water (VS) is pumped through the feed water preheater (9) than the steam output of the boiler (1). the low-pressure vapors (H2, H3, H4; HS) are fed to the steam turbine (3) as an intermediate feed, and cooled back to the feed water preheater (9) and the cooled excess feed water is recycled back to the feed water to the feed water preheater (VS). Method according to Claim 1, characterized in that the excess recyclable feed water (VI) is cooled by expanding it to a lower pressure in one or more stages (11, 12, 13) and in that each generated steam fraction (H2, H3, H4) is fed to a steam turbine (3). as an intermediate supply to the point corresponding to that pressure level. Method according to Claim 1 or 2, characterized in that the excess recirculated feed water (VI) is returned to the feed water (VK) to the feed water preheater on the suction side of the feed water pump (16). Method according to Claim 1, characterized in that the recirculated excess feed water (VI) is cooled in surface heat exchangers (21), in which the steam (HS) generated by the cooling of the condenser is fed to the steam turbine (3) as an intermediate feed. Method according to Claim 1 or 4, characterized in that the excess water to be recycled (VI) is returned using a special recirculation pump (22) on the pressure side 1e of the feed water pump (16). 7751 1 Method according to one of the preceding claims, characterized in that the excess feed water (VI) used for cooling is taken after the feed water preheater (9) before the feed water enters the cylinder 1 cylinder (2). Method according to Claim 6, characterized in that the excess recycled feed water (VI) is first fed to a steam separation device of the same pressure, from which the steam is led to the boiler cylinder (2) and the remaining water is cooled in low-pressure steam generating devices (11, 12, 13; 21) and that the low-pressure vapors (H2, H3, H4; HS) thus generated are fed to the steam turbine as an intermediate feed. Method according to one of the preceding claims, characterized in that the excess feed water (VI) taken for cooling is taken from the boiler cylinder (2) from a special feed water chute before the preheated feed water (VH) is mixed with the boiler water (VP). Method according to one of the preceding claims, characterized in that the excess feed water (VI) taken for cooling is taken from the boiler water (VP) in the boiler cylinder (2). Method according to one of the preceding claims, characterized in that the circulating water volume (VS) of the feed water preheater (9) is adjusted so that the feed water (VH) reaches a boiling temperature corresponding to the pressure of the boiler 1 cylinder (2) before separating the excess. feed water section (VI). Method according to one of the preceding claims, characterized in that the steam 13 7751 1 (H2, H3, H4) generated in at least one of the low-pressure steam generating devices (11, 12, 13) is superheated with a corresponding circulating excess of feed water (VI, V2, V3). Method according to one of the preceding claims, characterized in that the steam (H 2, H 3, H 4) generated in at least one of the low-pressure steam generating devices (11, 12, 13) is superheated in a hot stream (14), a preheater (9) and a primary circuit evaporator. (7) in the corresponding temperature zone, with a superheater (8). 14 7 751 1