GB2069615A - Method of producing power - Google Patents

Abstract

In a counterpressure steam system, high-pressure steam is expanded in turbine 2 and fed to an intermediate pressure line 15, from which a portion of the steam is withdrawn and heated at constant pressure in a recuperator 3 and then in a fuel-fired heater 4 before being expanded in a turbine 5, which drives an electrical generator. The steam exhausted from turbine 5 is cooled at constant pressure in indirect heat exchanger 3 and then further cooled in feed-water heater 6 before being supplied through low pressure line 16 to load 10. Condensate is returned to boiler 1 by pump 12 through heater 6. <IMAGE>

Description

SPECIFICATION Method of producing power The present invention relates to a method for the production of power using a counterpressure steam system.

Counterpressure steam systems are power units by means of which combustion heat, i.e. the heat generated by combustion of fuel, or heat from some other external source, can be used to cover the power requirements and the thermal requirements of the system.

In such a system, steam at high pressure and high temperature is generated in a boiler and is used to drive a high-pressure turbine in which the steam is expanded to a required temperature level or to a required pressure. In conventional systems of this kind, moreover, steam can be recovered at the output for expansion in a second stage in another turbine and/or to cover thermal requirements of the plant. A system of this type is described, for example, in Linde Berichte aus Technik und Wissenschaft, (Linde Reports of Technology and Science), 38, 1976, pages 3-8.

In power-intensive industries, the requirements for electrical and mechanical energy cannot always be covered by systems of this kind. This is the case, for example, in olefin and ammonia plants as described in the above-mentioned article. In order to satisfy the energy requirements of such plants, it is usually necessary either to introduce additional electrical energy, in the form of current supplied from outside the plant, or to generate mechanical energy outside the plant using further fuel. With the current high cost of energy, this need for the use of external energy sources poses a significant problem, especially since the internal energy of the plant is frequently not fully utilized or economically exploited.

It is an object of the present invention to provide a method of producing power, particularly in the form of electrical energy using a counterpressure steam system which avoids the disadvantages of earlier systems and, in particular, at least reduces the extent to which external energy sources are required to satisfy the power requirements of a plant.

It is a further object of the invention to provide such a method in which the counterpressure steam system is operated with an increased power/heat ratio.

According to the invention, there is provided a method of producing power using a counterpressure steam system in which a first supply of steam is available at a relatively high pressure and a second supplyof steam is available at a relative low pressure, said method comprising the steps of: (a) substantially isobarically heating steam withdrawn from said relatively high pressure supply; (b) expanding the heated steam to substantially the pressure of said relatively low pressure supply in a turbine with the performance of mechanical work; (c) substantially isobarically cooling the expanded steam; and (d) feeding the cooled expanded steam to said second steam supply.

The Term "substantially isobarically" as used herein means at a substantially constant pressure The invention is particularly applicable to a system in which steam is available at a relatively high pressure level, e.g. from a first steam line, and at a relatively low pressure level, e.g. in a second steam line, in a plant or installation in which the high pressure steam is used at least in part to feed thermal loads and at least in part to drive a turbine, e.g. in a main power generator.

In such a system, moreover, a portion of the steam of the high pressure line can be produced by evaporating condensate (i.e. boiling feed water produced by condensation of steam) which at a still higher pressure is fed to a further turbine and thereafter is returned to the high pressure line previously referred to. Thus, in such a system, the higher pressure line carries steam under an intermediate pressure.

According to the invention, the steam at this intermediate pressure may be subjected to isobaric heating in two stages before it is expanded in operating a turbine by means of which the energy of the steam is converted into electrical energy, the low pressure steam from this turbine being used to recuperatively heat the intermediate-pressure steam in a recuperative heat exchanger, this heating constituting the first of the two isobaric heating stages.

In previously known systems utilizing two steam pressure lines, the steam from the higherpressure line is expanded simply in a turbine to the pressure prevalent in the lower-pressure line and is fed to this line.

The steam used to operate the expansion turbine of the invention is withdrawn from the higher-pressure line and is initially heated recuperatively by the expanded steam and thereafter further heated isobarically by means of heat supplied from an external source, for example, in a combustion-type or fuel-fired heater. Only then is it expanded, usually in a plurality of stages, to produce the expanded steam, the thermal energy of which is recovered in the recuperator. This expanded and isobarically cooled steam is thereafter returned to the lower-pressure steam supply at the pressure level thereof.

This technique has been found to increase the conversion of thermal to mechanical energy and hence raise the power output/heat ratio of the plant. Only a small amount of externally supplied heat. derived from fuel is necessary and a major part of the heat supplied to the steam in the two-stage heating process can be transferred to it in the recuperator which not only allows an optimum utilization of the steam as a heat carrier, but also enables an optimum utilization of the fuel to produce a high temperature in the steam prior to its expansion.

Since the energy output of the system increases with increasing temperature of the steam prior to expansion and the efficiency likewise increases, the highest possible temperature of the steam prior to expansion is desirable. With such a method according to the pesent invention, this temperature is limited only by the materials of the second stage heating.

A further advantage of using recuperative heating in the first stage is that the expanded steam from the turbine is already at a relatively high temperature because of the contribution of thermal energy derived from the second heating stage using a fuel-fired heater for example. This of course results in a higher temperature at the second heating stage for the steam to be expanded and provides optimum temperature conditions for all phases of the auxiliary generating system.

For ideal gases and even for steam with relatively small temperature differentials across the recuperative heat expansion stage. the turbine output is the mechanical energy equivalent of the externally supplied thermal energy The use of the externally supplied energy markedly increases the overall efficiency in terms of electrical power output by comparison with that obtainable using conventional techniques.

After the isobaric cooling e.g., in the recuperator, the expanded steam frequently still has a relatively high heat content and. indeed, a higher heat content than the steam in the lowpressure line coming from a conventional turbine. Preferably, therefore, further heat is withdrawn from the expanded steam before it is fed to the lower-pressure steam supply. For example, condensate may be circulated through a heat exchanger traversed by the partially cooled expanded steam before it is introduced into the low-pressure steam line. This additional heat withdrawn can be used to satisfy further heating rquirements, e.g. to heat a fluid medium of an additional power generating process or for some other purpose in conjunction with the installation or system.It is most practical, however, to use this additional quantity of heat to preheat the feed water of the counterpressure steam-generating system and thus make up in whole or in part the thermal losses in the evaporator (boiler) used which customarily operates with an efficiency of only about 90%.

Alternatively, operating with the same efficiency of the auxiliary generating system, the steam throughput of the counterpressure steam system can be reduced because of the recovery of the additional heat.

Finally, in yet a third alternative, the excess of the expanded and substantially isobarically cooled steam can be introduced directly into the low-pressure steam line without further cooling and without satisfying other heating requirements.

The method of the present invention thus increases the power/heat ratio of the counterpressure system operation in comparison With conventional processes and thereby provides a higher efficiency in the method of the present invention. As has already been described, the higher efficiency is a function of the higher temperature level of the steam before expansion in the turbine of the auxiliary generating arrangement which corresponds to a higher specific energy output, lower fuel consumption, and so on.

If in the method of the present invention, condensation turbines are used instead of simple expansion turbines, it has been found that the cooling water demand is significantly reduced by comparison with otherwise analogous previously known methods using condensation turbines and that the quantity of steam required is less because of the higher power/heat ratio.

The invention will now be further described with reference to the drawing, which is a schematic flow sheet of one method of power production according to the invention.

Referring to the drawing, a system is illustrated having an intermediate-pressure steam line 1 5 and a low-pressure steam line 1 6 which are merely representative of two steam lines in any system, e.g. one of those described in the aforementioned publication, of an industrial process or power plant. Naturally, the number of steam lines can vary, i.e. can be more than two, as long as at least two of the steam lines are at different pressures.

The steam required for operation of a counterpressure steam system is generated in an evaporator or boiler 1 and is fed through a line 2a at a pressure higher than that prevailing in the intermediate pressure line, to a high pressure turbine 2 in which the steam is expanded to a pressure of about 39.2 X 105 N/m2 and at a temperature of 642"K is fed to the intermediatepressure line 15.

In conventional processes, the steam of this line, insofar as it is not required to satisfy thermal requirements indicated schematically at 11, is expanded in a turbine T and fed directly to the low-pressure steam line 1 6 as illustrated in dot-dash lines in the drawing. Naturally, such a turbine can also be provided in the system of the present invention in combination with the counterpressure arrangements illustrated in solid lines.

According to the invention, however, a portion of the steam at least equivalent to that supplied by the high pressure turbine 2 to the intermediate pressure steam line 1 5 is fed through a line 7 to a recuperative heat exchanger 3, hereinafter referred to as a recuperator, in which the steam is substantially isobarically heated to a temperature of 770"K. The steam is then passed into a fuel-fired heater 4 in which its temperature is substantially isobarically raised to 993"K.

This substantially isobarically heated steam is then passed to a turbine 5 which is arranged to drive an electrical generator 5a whose lines 5b supply power to the plant or otherwise operate various electrical loads. In the turbine 5, the steam is expanded substantially to the pressure prevailing in the low-pressure steam line 16, for example, 9.8 X 105 N/m2. However, prior to feeding the steam to this low-pressure line, the steam is substantially isobarically cooled by passing it via lines 8 through the recuperator 3 in indirect heat exchange with the steam which is recuperatively and substantially isobarically heated therein.

The steam, which leaves the turbine 5 at a temperature of about 791 'K, leaves the recuperator 3 at a temperature of about 653"K and, because it still has a relatively high residual heat content in comparison with known conventional systems, is used as the heat carrier for a heat exchanger 6 which serves to preheat feed water supplied for the boiler 1. The steam leaves the heat exchanger 6 at a temperature of about 494"K and is supplied at this temperature to the low-pressure line 16.The feed water, e.g. condensate from a low-pressure process-steam consumer or similar load indicated at 10, is displaced by a pump 1 2 via a line 1 4 through the heat exchanger 6 and is returned to a line 1 7 from which it is fed by a pump 1 3 to the boiler 1 at an elevated pressure. All of the turbines drive respective electrical generators or are connected to pumps or compressors (not shown).

In the accompanying Table 1 the temperature, pressure, specific enthalpy and specific entropy of the steam at the points a to f of the drawing are given. Table 2 provides the details of an example of the invention and a comparative known process.

With a known process using only a turbine T between the steam lines 1 5 and 16, the energy recovered is only 266 kJ per kg of steam, whilst with the method of the present invention, 514 kJ per kg of steam is supplied in heater 4, and 345 kJ per kg of steam is recovered in the recuperator 3 and the heat exchanger 6, so that the total added energy is 1 69 kJ per kg of steam. With the method of the present invention, a minimum of 1 69 kJ of mechanical energy per kg of steam is recovered at the turbine in excess of that recovered in the known system and because of the higher operating temperatures, the efficiency is likewise increased.

TABLE 1 Enthalpy i Entropy s J J Tempera- Pressure 106 - 103 Position ture 'K 105 N/m2 kg kg a 642 39.2 3.14 6.68 b 770 39.2 3.44 7.11 c 993 39.2 3.96 7.68 d 791 9.8 3.52 7.83 e 653 9.8 3.22 7.42 f 494 9.8 2.87 6.81 TABLE 2 Invention Known Fuel heat in heater (net) 514 kJ qB - kg Heat recovered in recuperator 345 3 and heat exchanger 6 kJ qR kg Additional heat consumption 1 69 Aq X qS kJ kg Output of turbine 432 266 kJ (0T 0.8) kg Output increase using invention 169 AL kJ kg AL Efficiency q= = - Substantially Aq unitY overall efficiency 0.9 for a heater 4 and boiler 1 of this efficiency

Claims (8)

1. A method of producing power using a counterpressure steam system in which a first supply of steam is available at a relatively high pressure and a second supply of steam is available at a relatively low pressure, said method comprising the steps of: (a) substantially isobarically heating steam withdrawn from said relatively high pressure supply (b) expanding the heated steam to substantially the pressure of said relatively low pressure supply in a turbine with the performance of mechanical work; (c) substantially isobarically cooling the expanded steam; and (d) feeding the cooled expanded steam to said second steam supply.

2. A method as claimed in Claim 1, wherein the substantially isobaric heating of the steam in step (a) is effected in two stages, a first stage in which steam withdrawn from said first supply is heated in indirect recuperative heat exchange with steam expanded in step (b), this expanded steam being simultaneously isobarically cooled, and a second stage in which the recuperatively heated steam is heated by an external heating source.

3. A method as claimed in Claim 2, wherein said external heating source is a fuel combustion heater.

4. A method as claimed in Claim 2 or Claim 3 wherein the expanded steam after isobaric cooling in heat exchange with high pressure steam, is further isobarically cooled by the removal of heat therefrom.

5. A method as claimed in Claim 4, wherein heat is removed from said expanded steam to effect said isobaric cooling, by indirect heat exchanger with feed water to a boiler serving to produce said relatively high pressure steam supply, said feed water being thereby preheated.

6 A method as claimed in Claim 5, wherein said feed water is converted in said boiler to steam at a pressure greater than that of said relatively high pressure supply, and the steam so produced is expanded in a high pressure turbine to produce at least a part of said relatively high pressure supply.

7. A method as claimed in any one of the preceding Claims, wherein the mechanical work produced by the expansion of said substantially isobarically heated steam in said turbine is used for the generation of electrical power.

8. A method of producing power as claimed in Claim 1 substantially as hereinbefore described with reference to the drawing.