Background of invention

The operation of a steam power plant is based on combustion of various fuels in a steam-producing boiler, i.e. a steam boiler, steam produced by which is guided to a steam turbine. In the steam turbine, steam expands thus making it possible to utilise it in producing mechanical rotary motion, which is further transformed to electric power by means of a generator connected to the steam turbine.

At the end of the steam turbine, steam pressure decreases onto the side of underpressure to a very low level. After the steam turbine, underpressure steam is condensated by means of cooling water or cooling air being at ambient temperature. After the steam has condensated to liquid state, it is pumped back to the boiler after preheating, as so-called feed water. The preheating of feed water occurs by means of bled steams bled from different stages of the steam turbine and/or by means of heat in flue gases generated in the combustion of fuels. The preheating of feed water aims at obtaining the efficiency of the steam turbine process as high as possible. Typically, the temperature of feed water returning to the boiler is about 180-300°C. The optimum temperature of feed water depends, inter alia, on the size of the steam power plant and the number of preheating steps. The combustion of fuels in the boiler releases heat energy, which heat energy the boiler delivers to a steam circuit where the feed water heats up, vaporises and finally superheats. Heat transfers from the combustion to the steam circuit both directly as flame radiation and as convention heat transfer. In convention heat transfer, energy bound to hot flue gases generated in combustion transfers from the flue gases onto heat exchange surfaces of the steam circuit. The heat exchange surfaces of the steam circuit refer to, inter alia, heat exchange surfaces of a vaporiser, superheaters, an economiser (preheating of feed water) and a LUVO (luftvorwarmer; preheating of combustion air). By means of the heat exchange surfaces, flue gases are typically cooled to a temperature of about 120-140°C in order to be able to transfer a part as large as possible of energy released in combustion from the boiler to the steam turbine process.

As a result of the efficiency optimisation of the steam turbine process, feed water returning to the boiler is hotter than the optimal end temperature of flue gases after the boiler. In order to be able to condensate flue gases to a desired temperature, the heat of the flue gases in the boiler have to be recirculated back to the heat balance of the boiler combustion process by preheating combustion air (air used for the combustion of fuels) typically to about 200-350°C in an air/flue gas heat exchanger, i.e. the so-called flue gas LUVO.

The heat capacity flow of combustion air is typically about 5-20% smaller than that of flue gases because of the larger quantity of flue gas and the higher specific heat capacity of components included by it, such as steam generated in combustion. Boiler plants based on pulverised coal firing typically have to convey a small quantity, about 5-10%, of combustion air past the flue gas LUVO to be used as control air in coal pulverisers. The quantity of control air mostly depends on the moisture of coal being combusted.

It is known of prior art to convey flue gas after the superheaters to an economiser, where feed water is preheated by the heat of flue gases, and from the economiser further to the LUVO, where combustion air is preheated by heat contained by flue gases. From patent application FI 20075450 is known a solid fuel boiler where the economiser is two-piece and the flue gas LUVO is replaced by a steam LUVO. Those skilled in the art also know arrangements where a flue gas flow formed in the combustion of fuels in a boiler is divided into two parts such that a part sufficient for heating combustion air is conveyed to the flue gas LUVO and the extra part is conveyed to feed water preheating. From specification DE 4441324 is known a system of the above kind where the flue gas flow is divided into two parts such that one flue gas flow is conveyed to combustion air preheating and the other one to feed water preheating. A weakness in the arrangement in question is the complexity and high costs of its technical implementation due to impurities included by the flue gas flow.

From specification US 5673634 is known a combined power plant process, i.e. a gas turbine and a steam turbine process, which specification describes a problem caused by an additional flue gas flow produced by the gas turbine. As a solution to the problem, the specification suggests transferring heat contained by flue gas flows in a heat exchanger to combustion air without preheating the combustion air before conveying it to the heat exchanger.

Object of invention The object of the invention is to decrease efficiency losses related to the use of a flue gas LUVO and to increase the efficiency of a steam power plant.

Description of invention The method according to the invention is characterised by what is presented in the characterising part of claim 1. The steam power plant according to the invention is characterised by what is presented in the characterising part of claim 8.

The invention relates to a technology by means of which it is possible for a large part to remove an exergy loss related to heat recovery which is caused by a great temperature difference at the cold end of the flue gas LUVO of the boiler, which typically is about 100°C. The temperature difference in the steam preheating of feed water is 3-5°C. In the heat exchange, a typical temperature difference between the steam circuit and combustion air or flue gas is 10-20°C. The exergy loss refers to a loss where the ability of a system to transform heat to electricity is lost. By means of the invention, it is also possible to increase the energy efficiency of the boiler. The end temperature of the boiler can be decreased for e.g. about 20-30°C, in other words the efficiency of the boiler can be improved, when the temperature of the heat exchange material of the flue gas LUVO increases in the preheating system arranged according to the invention.

In the method according to the present invention, air flow through the flue gas LUVO is increased by recirculating heated air, so-called recirculated air, from the hot side of the flue gas LUVO to the cold side of the flue gas LUVO. Recirculated air conveyed from the hot side of the flue gas LUVO is cooled by means of feed water conveyed to the boiler in the heat exchanger before mixing recirculated air with preheated air conveyed to the flue gas LUVO.

The method according to the invention is thus based on the fact that the air flow passing through the flue gas LUVO is increased by recirculating a part, advantageously about 10-20%, of the air flow having passed through the flue gas LUVO from the hot side of the flue gas LUVO to the cold side of the flue gas LUVO. Recirculated air is cooled before it is fed back to the flue gas LUVO in the heat exchanger by feed water conveyed to the boiler. As the air flow passing through the flue gas LUVO increases, air entering the flue gas LUVO can be preheated in a preheater with low-pressure bled steam of the steam turbine up to e.g. about 80°C. As a result of this connection, the preheating capacity transferred to combustion air practically remains the same as when using the flue gas LUVO in the known way, but the use of bled steams of the steam turbine transfers to lower-pressure bleeds. Then, the efficiency of electricity production improves when steam required for preheating expands in the turbine further, thus doing more work before leaving the turbine. With the above method, the quantity of electricity increases for about 5 MWe in a steam power plant utilising coal as fuel and having the capacity of about 560 MWe.

The invention is quite simple to implement, because no drastic changes are required in existing devices and systems. In the arrangement according to the invention, additional investment is directed to the acquisition of a heat exchanger and a blower and to their connections to combustion air and feed water systems.

The arrangement according to the invention is applicable for use in all steam power plants utilising a flue gas LUVO.

Lists of figures

Next, the invention will be described with reference to the attached figures which depict some embodiments of the invention. The examples are not, however, intended to limit the invention solely to the embodiments in question according to the examples.

Fig. 1 shows heat recovery from flue gases by means of a flue gas LUVO.

Fig. 2 shows steam circulation of a steam turbine process.

Fig. 3 shows temperature profiles of the flue gas LUVO in a method according to prior art.

Fig. 4 shows temperature profiles of the flue gas LUVO in the method according to the invention.

Detailed description of invention

Fig. 1 illustrates a process according to the invention for heat recovery from flue gases by means of a combustion air preheater, or a flue gas LUVO 10. Hot flue gas F _H is brought into the hot side of the flue gas LUVO 10 from a boiler, and flue gas Fc cooled to a desired end temperature comes out from the cold side of the flue gas LUVO 10. Air to be heated Ac is conveyed to the flue gas LUVO 10 to its cold side and heated air AH comes out from the hot side of the flue gas LUVO 10.

Air to be heated Ac is conveyed to the flue gas LUVO 10 via a preheater 7, where air Ac is preheated to a desired temperature by bled steam 5 of the steam turbine, advantageously by low-pressure bled steam, or by some other heating medium, such as feed water or circulation water of a closed cooling water circuit. After the flue gas LUVO 10, a part of preheated air AH is recirculated as recirculated air AR along a return line 3 back to the cold side of the flue gas LUVO 10, which return line 3 includes two recirculated-air heat exchangers 8a, 8b and a blower 9. In the recirculated-air heat exchangers 8a, 8b, recirculated air AR is cooled by means of feed water Wi, W ₂ entering the boiler. Recirculated air AR conveyed through the recirculated-air heat exchangers 8a, 8b and the blower 9 is conveyed via a pipeline 1 to be mixed with air Ac that is fed to the flue gas LUVO 10.

The main part, advantageously about 80-90%, of preheated air AH is conveyed after the flue gas LUVO 10 along a pipeline 2 to be used as combustion air in the combustion process of the boiler. As recirculated air AR ends up advantageously about 10-20% of preheated air AH- Optionally, it is possible to convey at least a part of air A _H heated in the flue gas LUVO 10 along a pipeline 4 for some other use, such as e.g. for an industrial process requiring hot air. The arrangement according to Fig. 1 can increase the quantity of air flowing through the flue gas LUVO 10 compared to an arrangement where only the combustion air quantity required by combustion flows through the flue gas LUVO. Then, it is possible to transfer heat quantity greater than before from the flue gas to the air flowing through the flue gas LUVO. When air entering the flue gas LUVO is preheated e.g. to the temperature of about 80°C, the temperature difference between flue gas and air on the cold side of the LUVO can be decreased without increasing the corrosion risk of heat surfaces. Fig. 2 shows steam circulation of a steam turbine process. Fuel is combusted in a boiler 11 , whereby chemical energy contained by the fuel transforms to heat energy, a part of which binds to flue gases and a part transfers to water flowing in a vaporiser of the boiler, heating up, vaporising and finally superheating. Superheated steam 16 is conveyed to a steam turbine 12 where the expansion of steam provides mechanical rotary motion of steam turbine vanes, which is transformed to electric power by means of a generator G connected to the steam turbine. In the steam turbine 12, the pressure of steam decreases. After the steam turbine 12, low-pressure steam Sj is conveyed to a condenser 13 where the steam is condensated to condensation water by cooling it by means of cooling water or cooling air being at ambient temperature. Condensation water is preheated stepwise and finally conveyed as preheated feed water W back to the vaporiser in the boiler 11. Feed water W is heated by bled steams S ₂ -S ₅ bled from the steam turbine 12 in heat exchangers 18a- 18d. The first two heat exchangers 18a, 18b are low-pressure preheaters (bled steams S ₂ -S ₃ are low-pressure), after which feed water W is conveyed to a feedwater container 14. To the feedwater container 14, it is possible to convey bled steam S ₆ from the steam turbine to preheat feed water. From the feedwater container 14, feed water W is still conveyed to two heat exchangers 18c, 18d, which are high-pressure preheaters (bled steams S ₄ -S ₅ are high-pressure), after which the preheated feed water W is fed to the vaporiser of the boiler 11.

From the feed water flow W to be heated is separated at two different points in the feedwater heating circuit a branch flow W _l5 W ₂ . Seen from the flow direction of feed water, the first branch flow W ₂ is conveyed via the recirculated-air heat exchanger 8b which is connected in parallel with the normal low-pressure preheater 18b operating with bled steam S ₃ . The second branch flow Wj is conveyed via the recirculated-air heat exchanger 8a which is connected in parallel with the normal high-pressure preheaters 18c, 18d operating with bled steams S ₄ -S ₅ . The requirement of bled steam decreases, when a part of the feed water flow is heated up with hot recirculated air AR. By means of valves V _1? V ₂ arranged to the branch flows Wj, W ₂ , it is possible to control the quantity of feed water conveyed to the recirculated-air heat exchangers 8a, 8b.

Fig. 3 shows temperature profiles for an arrangement according to prior art in a flue gas LUVO of a large boiler using coal as fuel. Hot flue gases entering the hot side of the flue gas LUVO are cooled to about 380°C by means of boiler feedwater. Combustion air enters the cold side of the flue gas LUVO at about 30°C. Combustion air heats up in the flue gas LUVO due to the degree difference required by heat exchange to 10-30°C from the entering temperature of flue gas, i.e. in this case to about 350°C. The flue gases again cool down in the flue gas LUVO to about 140°C.

In the embodiment according to Fig. 3, fouling and corrosion of the flue gas LUVO limit the decrease of the end temperature of flue gases below the normal end temperature, which is typically about 140°C. At a temperature lower than this, acids and moisture contained by flue gases start to condensate on the heat exchange surface. The temperature of the heat exchange surface on the cold side of the flue gas LUVO is the average between the temperatures of entering air and exiting flue gas, i.e., typically about 85°C. Fig. 4 shows temperature profiles for a flue gas LUVO of a large boiler using coal as fuel when the operation is based on the arrangement according to the invention. Combustion air is preheated before the flue gas LUVO to about 80°C e.g. by low- pressure bled steam of a steam turbine process in a preheater, whereby no condensation of flue gas acids and moisture occurs in the flue gas LUVO. Then, the end temperature of flue gases can be decreased e.g. to 110°C. As a result of this, the boiler efficiency increases for about 0.5%, which equals the improvement of the efficiency of a steam power plant, having a capacity of about 560 MWe, for the additional electric power of about 2.5 MWe. The total benefit from the improvement according to the invention in the steam power plant in question using coal as fuel is 5-7.5 MWe, whereby the efficiency of electricity production thus improves from 43% to 43.5%. In an embodiment according to the invention, the end temperature of flue gases after the flue gas LUVO can be adjusted to a desired temperature by adjusting the preheating temperature of combustion air to be fed to the flue gas LUVO. The preheating temperature of combustion air can be adjusted e.g. by means of steam bled from the steam turbine.

The end temperature of flue gases can also be adjusted by adjusting the quantity of recirculated air or by adjusting the quantity of feed water flow used for cooling recirculated air.

In an embodiment according to the invention, combustion air is preheated before conveying it to the flue gas LUVO by means of heat brought from outside the steam turbine. Alternatively, combustion air can be preheated by means of condensation heat of steam contained by flue gases.

An advantageous embodiment of the present invention relates to steam power plants operating in connection with industrial processes. Industrial processes typically require steam having a pressure of 2-10 bar, the so-called back-pressure steam, from the steam power plant and several industrial processes again produce plenty of waste heat of about 40-80°C in the form of condensation water and moist air. This waste heat cannot be utilised in totality in the preheating operations of the industrial process or the steam power plant.

By means of the present invention, the utilisation possibilities of waste heat can be improved by employing waste heat for preheating combustion air before the flue gas LUVO. Hot recirculated air conveyed from the hot side of the flue gas LUVO preheats feed water (condensation and additional water returning from the steam turbine) in the heat exchanger. Combustion air is preheated with waste heat in the heat exchanger. Then, the method according to the invention provides savings in the use of back-pressure steam required for preheating of feed water, whereby the saved steam is thus released to steam production for the industrial process. As a result of this, the production site of the steam power plant producing back-pressure steam and electricity can be decreased. Then, it is possible to benefit from both savings in fuel and, in new plants, savings in investment due to the smaller steam power plant dimensioning. Often, the industrial process requires hot drying gas of over 300°C (e.g. in the impingement drying of paper), the production of which is impossible by means of back-pressure steam because the condensation temperature of steam is below 200°C. Now, the production of drying air in question often has to utilise natural gas, light fuel oil or electricity. Air heated in the flue gas LUVO according to the invention can also be conveyed as such to an industrial process, whereby the savings will be high- quality fuel or electricity required in the industrial process.

Many different variations of the invention are possible within the scope defined by the claims to be presented next.