EP0086504B1

EP0086504B1 - A process for generating mechanical power

Info

Publication number: EP0086504B1
Application number: EP83200018A
Authority: EP
Inventors: Leonard Willem Ter Haar
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1982-02-16
Filing date: 1983-01-05
Publication date: 1988-03-09
Also published as: AU1138383A; EP0086504A2; JPH0475372B2; NL8200585A; AU555824B2; EP0086504A3; DE3375936D1; CA1222382A; NL191444B; NL191444C; ZA83985B; JPS58150030A

Description

The invention relates to a process for generating mechanical power by burning a gaseous fuel in the combustion chamber of a gas turbine and expanding the hot combustion gas in the gas turbine, comprising the steps of premixing the gaseous fuel with steam by vaporizing preheated water in the gaseous fuel stream and conducting the mixture thus formed into the combustion chamber, wherein the combustion gas expanded in the turbine is passed to a steam boiler arranged downstream, subsequently used to heat water which is at least partly used for the said premixing of gaseous fuel and steam, to a temperature in the range from 130 to 200°C by indirect heat exchange and subsequently leaves the steam boiler.
Such a process is known from DE-A-2,005,656.
In this known process the expanded combustion gas from the gas turbine is fed directly to a stack (exhaust) in which several heat exchange operations are carried. Further, the water to be vaporized in the fuel stream is preheated in a heater preceding the gas turbine and in an exhaust heat exchanger.
However, in this process the heat of the turbine off-gas can not be used in the most efficient manner.
The present invention, therefore, provides a process for generating mechanical power by means of a gas turbine, wherein the turbine off- gas is used in a very efficient manner.
The process of the invention is characterized in that the mixture of gaseous fuel and steam is heated to a temperature in the range from 250 to 450°C by indirect heat exchange with a first branch stream of hot air originating from a compressor, wherein the heat exchange is at the end pressure of the compressor, the cooled first stream of compressed air being used to prepare the gaseous fuel, and wherein a second branch stream of hot air from the compressor is conducted to the combustion chamber of the turbine. In this manner waste heat of a relatively low temperature level is put to good use.
By thoroughly premixing the gaseous fuel with steam it is ensured that in the combustion of said fuel in the combustion chamber of a gas turbine less nitrogen oxides are formed. The same object can also be aimed at by injecting water or steam into the flame in the combustion chamber, but by thoroughly premixing steam and gaseous fuel up to 35% of steam be saved compared with the method in which steam is injected into the flame. Preferably a quantity of 0.1-1.0 kg of steam per kg of gaseous fuel is mixed with said fuel and the mixture is passed to the combustion chamber of the turbine. Within the limits set the quantity of steam depends on the type of gaseous fuel. For example, in the event of a fuel producing a very hot flame during combustion, such as hydrogen or synthesis gas generated by means of pure oxygen, a larger quantity of steam is advantageously mixed with the fuel than in the event of a fuel producing a less hot flame during combustion, such as carbon monoxide or synthesis gas generated by means of air.
The gaseous fuel can be mixed with the steam in any conceivable manner. However, gaseous fuel having a temperature in the range from 40 to 100°C is preferably contacted with water having a temperature in the range from 80 to 200°C, at a pressure in the range from 10 to 30 bar. Thus, at least part of the water vaporizes the same time thoroughly mixed with the fuel. The water and fuel are advantageously contacted with each other by spraying the water in the top of a column and allowing the gaseous fuel to rise from the bottom of the column so that fine droplets of water are vaporized in the rising gas stream when they drop down in the column. The fuel/steam mixture leaves the column at the top. It has then a temperature in the range from 130 to 160°C.
The fuel/steam mixture is subsequently preferably further heated to a temperature in the range from 250 to 450°C by indirect heat exchange.
It is subsequently burnt with air in the combustion chamber of a gas turbine and the hot combustion gas is expanded in the turbine. On leaving the turbine the offgas has a temperature in the range from 500 to 550°C at substantially atmospheric pressure. The offgas is now advantageously first introduced into a steam boiler in which it is used for generating steam at a temperature in the range from 450 to 500°C and a pressure in the range from 40 to 60 bar. The off- gas leaves the steam boiler at a temperature in the range from 140 to 250°C and is subsequently preferably used for heating water to a temperature in the range from 130 to 200°C by indirect heat exchange. Said water is advantageously at least partly used for vaporization in the gaseous fuel as described hereinbefore. By said process low-temperature heat from the flue gas between 125°C and 200°C is effectively used to save compressed air.
Although any gaseous fuel, for example methane, ethane and propane, can be used for the process according to the invention, preference is given to a fuel obtained by partial oxidation of a fossil fuel, for example hard coal, brown coal, petroleum or a petroleum fraction, with oxygen, air or oxygen-enriched air at a pressure of 10-100 bar.
Since in the present process steam is mixed with the gaseous fuel less air is required in the combustion chamber of the turbine than in the case where no steam is added to the fuel.
Now, any gas turbine is generally equipped with an air compressor designed for supplying a sufficient quantity of air at adequately high pressure (15-25 bar) in order to keep the outlet temperature of the combustion chamber within the temperatures permitted for the gas turbine, namely 900-1100°C, even without steam having been supplied to the fuel.
Consequently, when using a gasification product as fuel, there is mostly already an excess of air, since the fuel has a calorific value of far below 10,000 kcal/ton. Said air is advantageously used to gasify the fossil fuel, but in the event of O2 being used for the gasifier, it can be used, for example, as plant process air or instrument air.
According to the invention the extra excess air of the gas turbine compressor resulting from the addition of steam is preferably used for the partial oxidation of extra fossil fuel. A larger quantity of gaseous fuel is then generated than is required for generating the maximum quantity of mechanical power for which the turbine has been designed.
This extra quantity of gaseous fuel is advantageously used for supplying heat to the inlet side of the steam boiler described hereinbefore in the course of complete combustion.
The offgas from the gas turbine is then advantageously heated by burning part of the gaseous fuel therein. In this manner the offgas is preferably heated to a temperature that is 50 to 75°C higher than the desired temperature of the steam to be generated in the steam boiler arranged downstream by indirect heat exchange between the boiler feed water and the heated turbine offgas.
By mixing the gaseous fuel with steam before burning it in the turbine combustion chamber according to the invention, a quantity of 10 to 30% of the gaseous fuel can suitably be used for heating the turbine offgas. This method makes it possible to produce steam at 80 bar and 550°C. The mechanical power generated in the gas turbine is advantageously converted into electric power by means of a dynamo. The steam produced in the steam boiler can also be used for electrical power generation by means of a steam turbine and a dynamo.
The invention will now be further illustrated with reference to the Figure giving a diagrammatic representation of the apparatus in which the process according to the invention is carried out. The auxiliary equipment to be used therein, such as pumps, compressors, valves, cleaning devices and control instruments, have been omitted for ease of review.
However, the invention is by no means limited to this description of the Figure.
A fuel, for example heavy oil, is passed through a line 1 to a gasification reactor 2, where said fuel is partially burnt by reaction with air supplied through a line 3, to form a raw gas mixture substantially consisting of H₂, CO and N₂. The air stream from the line 3 originates from an air compressor 6 via lines 4 and 40, which compressor is an integral part of the apparatus. The raw gas mixture leaves the reactor 2 via a line 7 at a temperature in the range from 1200 to 1400°C. It is cooled to a temperature in the range from 250 to 400°C in a waste heat boiler 8 by heat exchange with boiler feed water that is supplied via a line 9 at a temperature in the range from 150 to 300°C and is vaporized to steam in the boiler 8, which steam leaves the boiler 8 through a line 10 at a temperature in the range from 250 to 325°C. The raw gas mixture leaves the boiler 8 via a line 11 and is further cooled in a heat exchanger 13 to a temperature in the range from 150 to 200°C by means of cold boiler feed water that is introduced via a line 14. The raw gas mixture is subsequently passed via a line 15 to a soot-removing unit 16 where it is scrubbed with an aqueous stream that is supplied through a line 17. This results in a substantially clean gas mixture that is discharged via a line 18 and an aqueous soot slurry that is drained from the apparatus through a line 19. The substantially clean gas mixture is freed of the remaining solid impurities, mainly soot, in a scrubber 20. This is effected by washing the mixture countercurrently to fresh water that is supplied via a line 21 and an aqueous recycle stream reaching the column 20 via a line 22. The latter stream 22 is a branch stream of a stream 23 that is drawn off at the bottom of the column 20 and is split into the stream 17 and a recycle stream that is recycled to the column 20 via a line 24 and a cooler 25. The gas mixture now substantially purified from solid impurities is discharged from the column 20 via a line 26 to a gas purification unit 27 where the gas mixture is freed of gaseous impurities, mainly H₂S, at a temperature in the range from 40 to 150°C. It is discharged from the unit 27 via a line 28 and subsequently split into two streams by means of the lines 29 and 30. The gas mixture stream in the line 30 is passed to a column 31 where said stream is sprinkled with droplets of water from a sprinkler 37 from which water vaporizes at low temperatures of 80-180°C. Said water is introduced into the apparatus through a line 32 and subsequently combined with a recycle water stream leaving the column 31 via a line 33. The combined water stream is passed via a line 34 to a boiler 35 in which it is heated from a temperature in the range from 80 to 130°C to a temperature in the range from 120 to 180°C. The stream leaves the boiler 35 through a line 36 in which it is passed to the sprinkler 37. In the column 31 a quantity of the water sprinkled by the sprinkler 37 is vaporized and entrained by the rising gas mixture. The gas mixture thus treated has a water vapour content in the range from 10 to 20% by volume and a temperature in the range from 120 to 140°C. It is conducted via a line 38 to a heat exchanger 39 in which it is heated to a temperature in the range from 250 to 450° by heat exchange with hot air originating from the compressor 6 from which it is discharged via a line 40. The compressed air from the line 40 is split into two branch streams. The first stream is passed to the reactor 2 via the line 4 and the line 3. The second stream is conducted to a combustion chamber 43 of a turbine 44 through a line 48. In the combustion chamber 43 the mixture of gaseous fuel and steam with compressed air from the air compressor 6 is ignited and the combustion gas thus formed, which has a temperature in the range from 900 to 1100°C and a pressure in the range from 10 to 20 bar, is expanded in the turbine 44 by which mechanical power is generated. The expanded combustion gas is passed via line 45 to a boiler 35 at a temperature in the range from 500 to 550°C and substantially atmospheric pressure, in which boiler it is cooled by heat exchange with water that is supplied through a line 46, is vaporized and discharged as steam via a line 47. To increase the gas inlet temperature in the boiler 35 a branch stream of the gas mixture is passed to the boiler 35 via the line 29 and completely burnt with the excess air in the gas turbine exhaust gas (45). The offgas from the boiler 35 leaves the latter via line 49 at a temperature in the range from 125 to 150°C after heat exchange with water in the line 34. It leaves the apparatus via a stack 50.

Claims

1. A process for generating mechanical power by burning a gaseous fuel in the combustion chamber of a gas turbine and expanding the hot combustion gas in the gas turbine, comprising the steps of premixing the gaseous fuel with steam by vaporizing preheated water in the gaseous fuel stream and conducting the mixture thus formed into the combustion chamber, wherein the combustion gas expanded in the turbine is passed to a steam boiler arranged downstream, subsequently used to heat water which is at least partly used for the said premixing of gaseous fuel and steam, to a temperature in the range from'130 to 200°C by indirect heat exchange and subsequently leaves the steam boiler, characterized in that the mixture of gaseous fuel and steam is heated to a temperature in the range from 250 to 450°C by indirect heat exchange with a first branch stream of hot air originating from a compressor, wherein the heat exchange is at the end pressure of the compressor, the cooled first stream of compressed air being used to prepare the gaseous fuel, and wherein a second branch stream of hot air from the compressor is conducted to the combustion chamber of the turbine.

2. The process as claimed in claim 1, characterized in that the gas turbine off-gas is heated by burning therein a quantity of the gaseous fuel.

3. The process as claimed in claim 2, characterized in that the turbine off-gas is heated to a temperature in the range from 50 to 75°C above the desired temperature of the steam to be generated in the steam boiler by indirect heat exchange between boiler feed water and the heated turbine off-gas.

4. The process as claimed in claims 2 or 3, characterized in that 10 to 30% of the gaseous fuel is used for heating the off-gas.