WO1999006674A1

WO1999006674A1 - Environment friendly high efficiency power generation method based on gaseous fuels and a combined cycle with a nitrogen free gas turbine and a conventional steam turbine

Info

Publication number: WO1999006674A1
Application number: PCT/SE1997/001564
Authority: WO
Inventors: Per Collin
Original assignee: Nonox Engineering Ab
Priority date: 1997-07-31
Filing date: 1997-09-16
Publication date: 1999-02-11
Also published as: NO20000449D0; AU4796797A; SE9702830D0; NO20000449L

Abstract

Power generation method based on gaseous fuels and a combined nitrogen free thermal cycle with gas (GT) and steam (ST) turbines, the working medium of said GT being CO2 and steam produced by combustion of any gaseous fuel with oxygen in said GT combustor during simultaneously feeding a part flow of compressed flue gas from a steam generator utilising the GT exhaust for steam production, which steam generates power in an ST. The rest of flue gas from said generator is utilised for hot-water production in a heat recovery unit (HRU), the hot-water partly being utilised for moistning said flue gas before compression, for feed water etc. The flue gas from HRU is moist CO2, which can be liquefied and used for EOR, for chemicals and for dumping in the oceans etc. The method's gross efficiency including 02 generation is 60 %; the net efficiency, also including CO2 liquefaction, is 58 %.

Description

ENVIRONMENT FRIENDLY HIGH EFFICIENCY POWER GENERATION METHOD BASED ON GASEOUS FUELS AND A COMBINED CYCLE WITH A NITROGEN FREE GAS TURBINE AND A CONVENTIONAL STEAM TURBINE Power generation baseα on gaseous fuels in a combined cycle with gas as well as steam turbines has become an appreciated method for power generation, aDove all because of the relative simplicity of the method and its high efficiency The only real disadvantage of the method is the unavoidable carbon dioxide emission, a disadvantage the method shares, in lower degree with however, with all other methods based on fossil fuels and which all impair the carbon dioxide balance of the atmosphere and cause the so called green house effect

Several methods have been published concerning elimination of the carbon dioxide emission to the atmospnere from today's power generation methods based on fossil fuels with purpose to eliminate the emission's detrimental effect on the carbon dioxide balance

There are in principal two options for the elimination

1 Capturing of the carbon dioxide from the flue gas of a power generation cycle by chemical absorption followed by stripping from the absorbent and utilisation and/or annilation of produced gaseous or liquefied carbon dioxide by enhanced oil recovery, chemical processing or dumping in extinguished oil wells, aquiferes or in the oceans The absorption and stripping operation is costly and increases the kWhe cost of a combined power station by about 60%

2 Capturing by increase of vegetation areas, suitably by planting of species with large ability of carbon dioxide capturing, especially tropical forests The cost connected with this method has been estimated to between 10% (u-countπes) and 50% (i-countnes) of the cost of primary nuclear power

A substantial reduction of the cost for elimination of the carbon dioxide problem of a power generation cycle based on fossil fuels could be obtained, if a nitrogen free cycle could be created having such a high efficiency, that the production cost of the oxygen necessary in this case could be compensated The present invention, which will be described below, fullfills beyond measure this high goal

The present invention implies a method of power generation based on gaseous fuels with a higher efficiency than that, which can be reached in any conventional combined cycle with a gas turbine operating on air The method has further the advantage of nitrogen free emissions of sequestered moist carbon dioxide and water enabling economic interesting utilisation and/or dumping as above The method implies a combined thermal power generation cycle based on gaseous fuels with gas and steam turbines connected to generators or a common generator, whereby the working medium of the gas turbine is a nitrogen free mixture of steam and carbon dioxide produced in the gas turbine's combustor by combustion of the fuel gas with a somewhat over-stoichiometπc oxygen flow in relation to the fuel gas flow and, after expansion in the gas turbine, being utilised for steam production in a steam generator connected to the turbine's exhaust gas outlet and the flue gas from said steam generator being split into two part flows, one of which, after being compressed under direct or indirect cooling in the gas turbine's compressor, being recirculated to said combustor, whereby the fuel gas flow to said combustor is controlled in such a way as to produce a gas mixture having a temperature adapted for admission to the gas turbine's high pressure turbine, while the other part flow of flue gas from said steam generator is utilised in a heat recovery unit and whereby saiα produced steam has the preferably highest pressure and temperature compatible with said exhaust gas temperature and is utilised for power generation in a conventional condensing steam turoine, possibly adapted for bleeding of process steam

According to the invention the non-recirculated part flow of flue gas τrom the steam generator, containing carbon dioxide and to the larger part water vapour, is utilised for production of hot-water in a heat recovery unit, e g a scrubber of known type The hot- water produced is preferably utilised, partly, after some heating, for direct cooling in the compressor of recirculated part flow of flue gas from the steam generator through moistning of the flue gas beyond the saturation limit with atomised hot-water before the compression, partly for preheating fuel gas and oxygen to the combustor, partly for preheating condensate from the steam turbine condenser and partly for selective heating purposes Indirect cooling during compression is possible but implies consiαerable interference on today's gas turbine construction

The flue gas from said scrubber (moist carbon dioxide) can with known methods be converted to gaseous or liquid carbon dioxide, which can be utilised as described above The cost for carbon dioxide capturing in connection with the method according to the invention will be low and beyond measure more than outbalanced by the benefit from the method's high efficiency and revenues from sales of gaseous or liquid carbon dioxide for enhanced oil recovery (EOR), production of chemicals etc Even in the case of dumping the carbon dioxide the cost for this operation will be more than outbalanced by the method's high efficiency (see embodiment below) Power generation according to the invention based on gaseous fuels, natural or produced by oxygen gasification of carbonaceous fuels such as fossil fuels, is thus cost effective and does not affect the biosphere negatively and is equally environment friendly and besides more energy effective than power generation based on a conventional combined cycle and biofuels

In order to further elucidate the invention it is described more in detail with reference to FIG 1 , which shows an elementary flow diagram of an embodiment of the invention based on natural gas and data from a commercial industrial gas turbine with shaft in one piece

The method according to FIG 1 works in the following way

Natural gas (16), preheated (18) with hot water from the hot-water scrubber (4), is fed with a controlled flow to the combustor (13) of the gas turbine (1 ) wherein to the gas

1 turbine adapted pressure (14 bar) is maintained Oxygen (17) (02 concentration >

85%, preferably > 92%) produced in a separate conventional unit (2) through distillation of liquid air and preheated in the same way as the natural gas, is fed to said combustor with a somewhat over-stoichiomenc flow in relation to the flow of natural gas A part flow of the flue gas from the steam generator (3) is simultaneously recirculated to said combustor after directly temperature controlled compression in the gas turbine's compressor (11/12) In order to avoid too high local temperatures the oxygen is suitably mixed with the gas flow from said compressor, whereby the flows of natural gas (16) and oxygen (17) are controlled in such a way that the gas leaving said combustor has a temperature adapted to the high pressure turbine (14) (1260°C)

After expansion in the gas turbine's high and low pressure stages (14/15) the exhaust gas therefrom has a pressure somewhat higher than atmospheric pressure and a temperature of 583°C The gas is conducted through the steam generator (3), where its temperature is reduced to 120°C while generating high pressure steam with 530°C temperature and 200 bars pressure, which steam is utilized in the condensing steam turbine (9) for power generation The feed water to the steam generator is partly condensate from the condenser (92), partly hot-water (85°C) produced in the scrubber (4)

The steam generator (3) features modular construction with finned-tube heat transfer surface and natural or forced circulation, for instance of the same type as usually used in connection with a conventional combined cycle with condensing steam cycle

The flue gas from the steam generator (3), which consists of a mixture of water vapor and C02 (vol% 26.4C02 + 73,6steam) and has a temperature of 120°C, is split into two part flows, one of which (78% of total flow), after being moistened beyond saturation limit, is fed to the suction side of the gas turbine's (1 ) compressor (11/12) The pressurised (14 bar) gas from said compressor is fed to the gas turbine's combustor (13), possibly mixed with the flow of oxygen (17) necessary for the combustion

The balance of the flue gas from the steam generator (3) is fed to the hot-water scrubber (4), suitably of the same type as often used in the cellulose industry for making hot-water from flue gases In such a scrubber the flue gases flow vertically upwards through a series of spray nozzle banks and the wash water counter currently downwards With a flue gas temperature of 120°C and with the current composition of the flue gas from the steam generator (3), the hot-water from the scrubber reaches 85°C and the gas leaving the top of the scrubber to the atmosphere 40°C when the temperature of the , feed- water from the heat sink (5) is 20°C

The flow of hot-water from the scrubber (4) is filtered in a control filter (21 ), whereafter it is split in two part flows, one of which for security reasons is conducted through a deionizmg unit (22) and used, partly for moistning the part flow of flue gas from steam generator (3), which is recirculated to the inlet chamber of the gas turbine's compressor (11/12) and partly as feed water to said steam generator The balance of hot-water flow from said scrubber is exploited for preheating (18) of gas fuel and oxygen, preheating of condensate from the steam turbine condensor (93) and possibly for selective heating purposes (6), whereafter it is fed to the heat sink (5)

The possibility of recycling flue gas (simultaneously the heat of vaporisation of the steam therein) from the steam generator (3) is one of the inventions advantages and bnngs about a considerably higher (about 10%-unιts) efficiency than can be achieved with the embodiment's gas turbine in a conventional combined cycle with condensing steam turbine

The gas turbine's (1) compressor has the pressure ratio 1 ^*14 and in order to minimise superheating of recirculated flue gas from the steam generator (3) during compression and thus reduce the energy consumption, atomised hot-water is added to the flue gas ahead of said compressor in such a controlled flow, that the water content becomes 4 weιght%. During compression the mixture's temperature rises and the added water evaporates thereby The pressurised gas mixture leaves the compressor with a temperature of 315°C and is fed to the combustor (13).

In order to bring about an effective "atomisation" of the hot-water used for moistning the flue gas from the steam generator (3), hot-water from the scrubber (4) after pressunsation and super heating to > 130°C is fed to spray nozzles mounted in the inlet chamber of the gas turbine's compressor (1 1/12). The small drops created by a nozzle explode, when they leave the nozzle, in a large number of micro-drops because the vapour pressure in the inner of a drop is higher than the surrounding pressure, that prevails in said inlet chamber (ca atmospheric pressure) This arrangement brings about that the hot-water forms a mist, which follows the part flow of the recirculated flue gas through said compressor The mist is hereby successively reduced through vaporisation to the point of disappearance in the compressor stage where the temoerature corresponding to the saturation limit is reached.

The heat content of the hot-water from scrubber (4), which is not used as described above, is most economically used for selective heating purposes (6) and the like The water returning from the selective heating (6), the preheating (18), the heat exchanging (93) and the rest of not utilised hot-water from said scrubber is cooled in a heat sink (5) to about 20°C or less, whereafter the water is recirculated to the top of the scrubber (4). Surplus of the circulating water, originating from the hydrogen content of the natural gas, is drawn off (23) in order to keep the inventory of said water constant If supply of sea water or other type of cooling water is lacking, the cooling can be acnieved in cooling towers, whereby the loss of water in case of gas fuel with low hydrogen content may, however, be so high, that fresh water has to be added to the circulation cycle.

A gas turbine's compressor and turbine stages are designed, so that the speed of the working medium in the individual stages of stator as well as rotor are near the sonic speed. The parameters, that control the possible maximum capacity of a compressor or a turbine stage (specific mass flow of medium kg/m².sec) is, besides the sonic speed, the density of the gas there. The sonic speed is a function of the local pressure, density and adiabatic exponent, which latter in turn is a function of the molecular weight of the gas and its specific heat. In the first stage of the embodiment's compressor the actual medium (weιght%: 44.4C02 + 51 ,6steam + 4,Owater) has atmospheric pressure and ca 90°C temperature. A conventional gas turbine operates with the same inlet pressure but at 20°C. Inspite of said higher temperatur the maximum capacity of a compressor will with said actual medium, however, be the same as when operating at standard conditions because of the medium's much higher sonic speed and specific heat. A turbine's maximum capacity will of the same reason be somewhat higher with the actual medium (vol%: 26.4C02 + 73,6steam) than when the unit operates under conventional conditions.

Start of power generation according to the invention can be achieved through feeding, besides controlled flow of fuel gas (16) and oxygen (17), atomised water of such a controlled flow to the gas turbine's combustor (13), that the gas from said combustor to the turbines has adapted temperature. Said flow of atomised water is successively phased out simultaneously with the pressurised, increasing part flow of flue gas from the steam generator (3) being fed to said combustor.

The moist carbon dioxide (20), which leaves the scrubber's (4) top, is preferably utilised for production of (liquid) carbon dioxide with known methods, suitably a method through which the carbon dioxide is freed from inerts such as residual oxygen etc. Carbon dioxide, gaseous as well as liquid, can be transported in pipelines and as liquid in ships. Both liquid and gaseous carbon dioxide has extensive use for enhanced oil recovery or so called "flooding" of oil wells and in average the net consumption hereby is ca 1 kg/kg oil. About 40% more oil can usually be reclaimed from a well through "flooding".

The embodiment according to FIG 1 implies a total efficiency of 60% when combusting natural gas with oxygen in the shown combination of equipment units including the separate oxygen production unit A second important benefit on behalf of the method according to the invention is the moist NOx free carbon dioxide in the flue gas from the scrubber (4), which is useful for production of (liquid) carbon dioxide, while the surplus of hot-water (85°C) from said scrubber is useful for selective heating purposes. The net power generation efficiency of the invention method amounts to 58% after production in a separate unit of liquid carbon dioxide from said scrubber flue gas. The method according to the invention implies advantageous use of, besides natural gas, all clean gaseous fuels produced by oxygen gasification of carbonaceous material, for instance fossil fuels such as coal and oil but biomass as well.

Liquid carbon dioxide can be dumped in the depth of the oceans, in extinguished oil and gas wells, aquiferes etc. Dumping in combination with the method according to the invention enables thus power generation based on gaseous fuels without increasing the carbon dioxide content of the atmosphere, which implies that the method enables power supply based on fossil fuels, which is as environment friendly as power generation based on biofuels but considerably more energy effective

An important environmental advantage of power generation according to the method of the invention is the total absence of nitrogen oxides in the flue gas put into the atmosphere from the scrubber (4), if the carbon dioxide therein is not utilised for the various purposes descπoed above Another advantage is the absence of the comprehensive cleaning devices ahead of the gas turbine compressor, which the conventional combined cycle requires

Claims

1 An environment friendly high efficiency power generation method based on gaseous fuels in a combined cycle with gas (1) and steam (9) turbines connected to power generators (8/91 ) or a common generator, characterised by the gas turbine's working-medium being a nitrogen free mixture of steam and carbon dioxide produced in the gas turbine's combustor (13) by combustion of the fuel gas (16) with a somewhat over-stoichiometnc oxygen (17) flow in relation to said fuel gas flow and after expansion in the gas turbine being utilised for steam production in a steam generator (3) connected to the turbine's (1 ) exhaust gas outlet and the exhaust gas being split into two part flows, one of which after being compressed under direct or indirect cooling in the gas turbine's compressor ( 11/12) and recirculated to said combustor, whereby the fuel gas flow (16) to said combustor (13) is controlled in such a way as to produce a gas mixture having a temperature adapted for admission to the gas turbine's high pressure turbine stage (14), while the other part flow is utilised in a heat recovery unit (4) and the steam from said steam generator (3) is produced at the preferably highest pressure and temperature compatible with the exhaust gas temperature and utilised for power generation in a conventional condensing steam turbine (9), possibly adapted for bleeding of process steam

2 A method according to claim 1 , characterised in that the temperature control through direct cooling in the gas turbine's compressor (11/12) is effected by moistuπng the recirculated part flow of flue gas from the steam generator (3) with an adapted flow from the heat recovery unit (4) of hot-water pressurised and heated to > 130°C and atomised through spray nozzles in the suction chamber of said compressor

3 A method according to claims 1-2, characterised in that the non-recirculated part flow of the flue gas from the steam generator (3) is utilised for hot-water production through direct contact with cold water in a heat recovery unit (4) of known type, e g a scrubber, whereby part flows of produced hot-water are utilised for temperature control in the gas turbine's compressor (11/12) and as feed water to said steam generator

4 A method according to claim 3, characterised in that the part flow of the hot- water from the heat recovery unit (4) , which is not used for temperature control in the compressor (11/12) respectively for feed water to the steam generator (3), is utilised partly for preheating of fuel gas (16) respectively oxygen (17), partly for preheating (93) the condensate from the steam turbine condenser (92) before feeding to said steam generator and partly for other purposes such as selective heating (6), whereafter the water is recirculated to the heat recovery unit (4) after adapted cooling in a heat sink (5), which can be of type heat exchanger with for instance sea water as cooling medium or cooling towers, whereby the inventory of in the system circulating water is kept constant through tapping the surplus water (23) originating from the fuel gas' hydrogen content

5 A method according to claim 3, characterised in that the total flow of hot-water from the heat recovery (4) unit is filtered in a control filter (21) and the part flows, which are distributed as feed water to the steam generator (3) respectively for moistuπng part of the flue gas flow from said steam generator, are in addition deionised in an lon- exchanging unit (22)

6 A method according to claim 1 , characterised in that for starting power generation, besides fuel gas (16) and oxygen (17), atomised water is fed to the gas turbine's combustor (13) in such a controlled flow that the turbines (14/15) of the gas turbine receive combustion gas of adapted temperature and pressure, whereby said water flow successively is phased out and replaced by compressed flue gas from the steam generator (3) when the compressed flow from same builds up

7 A method according to claim 1 , characterised in that the moist carbon dioxide (20) leaving the heat recovery unit (4) is utilised for production of gaseous or liquid carbon dioxide in a known way

8 A plant for power generation based on combustion of gaseous fuels, characterised by a combination of a gas turbine (1 ) of type derived from jet motors or of industrial type, with passing or divided shaft, including compressor (11/12), combustor (13) and turbine (14/15), a condensing, possibly for bleeding adapted, steam turbine (9) with condenser (92), a unit for production of oxygen through distillation of liquid air in a conventional way (2) , a steam generator (3), connected to the exhaust outlet of the gas turbine's low pressure stage (15), producing steam with high pressure and high temperature, a heat recovery unit (4), on the gas side connected to said steam generator's flue gas outlet respectively to the atmosphere, producing hot-water from part of the flue gas flow from said steam generator, a control filter unit (21 ) for the total hot- water flow from the heat recovery unit (4) and a deionizing unit for the part flow of the hot-water used as feed water respectively for moistuπng a part flow of flue gas, a heat sink (5) in form of a heat exchanger or cooling tower, cooling circulating water before its reuse, and electric generator(s) (8/91) connected to the axles of the gas turbine's turbines or power turbine and of the condensing steam turbine.