Power generation based on gaseous fuels in a combined cycle with gas as well as steam turbines has become an appreciated method for power generation, above all because of the relative simplicity and high efficiency of the method. The only real disadvantage of the method is the unavoidable carbon dioxide emission, a disadvantage the method shares, in lower degree 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 to eliminate the carbon dioxide emission from power generation based on fossil fuels have been published. Among these methods specially two have been much discussed: Increase of forest area and absorption of carbon dioxide from the flue gases followed by dumping, respectively.

Increase of forest areas implies change of an area's type of vegetation by planting of species with larger ability of carbon dioxide capturing. Tropical forests have proved to be specially effective for this purpose. Thus, power generation and carbon dioxide capturing are not locally tied up to each other in this case. The cost for carbon dioxide capturing with this method has been estimated to between 10% (u-countries) and 50% (l-countries) of the cost of primary nuclear power

Carbon dioxide absorption implies that the flue gases from the combined cycle are scrubbed with a water solution holding substances like ethanol amme, which binds carbon dioxide. By heating the solution the carbon dioxide is stripped off and can through compression and cooling be condensed to a liquor, which can be dumped (as chlathrate) in the depth of the oceans, in exhausted oil and gas wells, aquviferes etc. The cost for carbon dioxide capturing by this method are high and have been estimated to increase the kWhe cost of a combined power station by about 60%.

The present invention implies a method of power generation based on gaseous fuels with a higher efficiency than that, which can be reached in a conventional combined cycle and which further has the advantage of nitrogen free emissions of moist carbon dioxide and water, respectively, enabling dumping as above. The method implies a thermal power generating cycle with a gas turbine as the principal machine, whereby the working medium of the gas turbine is the gas mixture of steam and carbon dioxide, which is formed by combustion in the gas turbine's combuster of fuel gas with oxygen in a somewhat over-stoichiometric flow in relation to the flow of the fuel gas, and with a simultaneous flow to the combuster of the steam, which is produced with adapted pressure in a steam generator connected to the exhaust gas outlet of the gas turbine and with a simultaneous recirculation to said combuster of a part flow of the flue gas from said steam generator after compression in the gas turbine compressor, temperature controlled by direct or indirect cooling, whereby the feed of fuel gas to said combuster is controlled in such a way that produced hot gas mixture has a inlet temperature adapted to the gas turbine.

According to the invention the non-recirculated part of the flue gas flow from the steam generator, containing carbon dioxide and to the larger part water vapour, is utilized for production of hot-water in a scrubber of known type. The hot-water is utilized, partly as feed water to the steam generator, partly for direct cooling in the compressor of recirculated part flow of the flue gas from the steam genera- tor through moistening of the part flow beyond the saturation limit with atomized hot-water before the compression. Indirect cooling is possible in a twin-speed compressor but implies considerable interference on today's gas turbine construction.

The flue gas from said scrubber (moist carbon dioxide) can with known methods be converted to liquid carbon dioxide, which can be dumped as above . The cost for carbon dioxide capturing in connection with the invention will be very low and is together with the cost for the dumping more than out-

balanced by the benefit from the higher thermal efficiency, which the method according to the invention brings about. The method enables thus a cost effective power generation based on gaseous fuels, natural or produced by oxygen gasification of carbonaceous fuels such as fossil fuels. Power generation according to the method of the invention 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 ele¬ mentary flow diagram of an embodiment of the invention based on natural gas and a commercial aero-derived gas turbine with shaft in one piece (GE/LM 6000) and with reference to fig. 2, a variant of the method shown in fig. 1, showing the method based on a gas turbine of industrial type.

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 combuster (13) of the gas turbine (1) wherein to the gas turbine adapted pressure (30 bar) is maintained. Oxygen (17) (0 ₂ 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 the combuster with a somewhat over-stoichiometric flow in relation to the flow of natural gas. The flow of steam pro- duced in the steam generator (3) connected to the gas tur¬ bine's (1) exhaust outlet together with a part flow of the flue gas from said steam generator (3) after directly or indirectly temperature controlled compression in the gas turbine's compressor(s) (11/12) , are simultaneously fed to the combuster (13) . In order to avoid too high local combustion temperatures the oxygen is suitably mixed with the gas flow from the high pressure compressor (12) , whereby the flows of natural gas and oxygen are controlled in such a way, that the gas leaving the combuster has a temperature adapted to the gas

turbine ( 123 7 ° ) .

After expansion m the gas turbine's high- and low pressure turbine (HPT/LPT) stages (14/15) the exhaust gas from the latter turbine has a pressure somewhat higher than atmospheric pressure and a temperature of 440°C. The gas is conducted through the steam generator (3) , where its temperature is reduced to 120°C while generating high-pressure steam with 400°C temperature and 30 bar pressure (alternatively 550°C and 200 bar for a gas turbine of industrial type according to FIG. 2) . The feed water to the steam generator (3) is branched off from the hot-water produced in the scrubber (4) and has a temperature of 94°C.

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

The flue gas from the steam generator (3) , which consists of water vapour with 10,4% C0 ₂ and has 120°C temperature, is split into two part flows, one of which (54,3%) , after being moist¬ ened beyond the saturation limit, is fed to the suction side of the gas turbine's (1) low pressure compressor (LPC) (11) . The compressor (s) (11/12) increase the gas/steam mixture's pressure to 30 bar and the pressurized gas flow is fed to the gas turbine's combuster (13) , possibly mixed with the flow of oxygen (17) necessary for the combustion.

The rest of the flue gas from the steam generator (3) is fed to the hot-water scrubber (4) , suitably of the same type as often is used in the cellulose industry for making hot-water from flue gases. In such a scrubber the flue gases flow verti¬ cally upwards trough a series of spray nozzle banks and the wash water flows counter-currently downwards. With a flue gas temperature of 120°C and with the composition of the gas from the steam generator (3) , the hot-water from the scrubber reaches 94°C temperature and the gas leaving to the atmosphere 40°C when the feed-water temperature 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 into two part flows, one of which of security reasons is conducted through a deionizing unit (22) , and used, partly as feed water to the steam generator (3) , partly for moistening the part of the flue gas from the steam generator, which is recirculated to the gas turbine's LPC (11) . The balance of the hot-water flow from the scrubber is exploited for preheating (18) of gas fuel and oxygen and possibly for selective heating purposes (6) whereafter it is fed to the heat sink (5) .

Direct recirculation of the heat of vaporization of steam in the exhaust gas from a gas turbine is not possible in a con¬ ventional combined cycle, where the gas turbine uses air as oxygen source for the combustion. The possibility of recycling heat of vaporization according to the invention in the way described above through direct recirculation of flue gas from the steam generator (3) is one of the invention's advantages and brings about a considerably higher (about 8%-units) total efficiency than can be achieved with the embodiment's gas turbine in a combined cycle with condensing steam turbine

The gas turbines' (1) compressors (11/12) have the pressure ratios 1:2,5 and 1:12. In order to minimize superheating of the gas/steam mixture during the compression and thus reduce the energy consumption, atomized hot-water from scrubber (4) is added to the gas/steam mixture from the steam generator (3) in such a controlled flow, that the mixture's moisture per¬ centage becomes 15% beyond the saturation limit, whereafter the moist mixture is fed to the LPC (11) , where it is com¬ pressed to 2.5 bar and gets the temperature 128°C and a mois¬ ture content of 10% beyond the saturation limit. This moist gas/steam mixture from the LPC (11) is further compressed in the gas turbine's HPC (12) to 30 bar, turns thereby dry and gets the temperature 332°C whereafter the gas/steam mixture, possibly after mixing with all or part of the oxygen flow (17) necessary for the combustion, is fed to the gas turbine's

combuster ( 13 ) .

In order to bring about an effective "atomization" of the 94°C hot-water from the scrubber (4) when moistening the recircu- lated flue gas flow from the steam generator (3) to the LPC

(11) , the hot-water is according to the invention heated to _> 130°C before it is pumped to spray nozzles in the inlet of the LPC (11) . The small drops created by a nozzle explode, when they leave the nozzle, m a large number of micro-drops be- cause the vapour pressure in the inner of a drop is higher than the surrounding pressure, near the atmospheric pressure, that prevails m the suction chamber of the LPC (11) . This arrangement brings about that the hot-water forms a mist, which follows the part flow of flue gas from the steam genera- tor (3) through the LPC (11) and which mist successively is reduced through vaporization to the point of disappearance m the HPC (12) when the temperature there exceeds 180°C. The most simple way to heat the hot-water is brought about by pumping the water through a separate pipe coil in the steam generator (3) .

The heat content of the hot-water with 94°C temperature, which is not used as above, is most economically exploited for selective heating purposes (6) and the like. The water return- ing from the selective heating (6) , the preheating (18) and the rest of not utilized hot-water from the scrubber (4) is cooled m the heat sink (5) to about 20°C or less, preferably a sink m the form of a heat exchanger operating with e.g. sea water, 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 circulating water constant. If supply of sea water or other type of cooling water is lacking, the cooling can be achieved 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.

Fig. 2 shows a variant of the embodiment according to fig. 1,

which is favourable for industrial type of gas turbines with normally an exhaust gas temperature about 600°C, where the high pressure steam from the steam generator (3) is fed to a back pressure turbine (7) generating power down to the pres- sure of the gas turbine's (1) combuster (13) through feeding the back pressure steam to said combuster. The back pressure ζurbine's (7) shaft can be connected to the LPC shaft, the gas turbine's LPT shaft or a separate generator. The specific investment cost and the total efficiency for this variant will be roughly the same as for the method according to the embodi¬ ment .

Gas turbines of industrial type usually operates at lower pressure and temperature than aero derived gas turbines, for instance 14 bar and 1190°C, but can be made with higher capac¬ ity. The exhaust gas temperature is higher, for instance 590°C, which implies that the variant according to fig. 2 is favourable because the back pressure turbine (7) then can operate with sharper steam data, for instance 200 bar and 550°C.

A compressor stage and turbine stage, respectively of a gas turbine is designed so that the speed of the working medium in the individual stages both for the stator and the rotor blades, is near the sonic speed, which can be computed from the formula

where a = sonic speed at the conditions in the stage in ques¬ tion, P = local gas pressure, r = local gas density and K = the isentropic exponent, which is dependent on the medium's molecular weight according to the relation

(κ-l)/κ = Ro/M*cp

where Ro = gas constant, cp = specific heat at constant pres¬ sure and M = the medium's molecular weight . For the open area of the first turbine stage of the embodiment's gas turbine the following data are valid for the respective medium:

r = relative energy flow (relative capacity) shows that the gas turbine would have 14% higher capacity with carbon dioxide/steam mixture provided that its mechanical strength so allows. When the gas turbine is of aero-derived type this is probably not the case, and thus the capacity will be the same in both cases .

Start of power generation according to the method of the invention can be achieved through feeding, besides fuel gas

(16) and oxygen (17), atomized water instead of steam to the gas turbine's combuster (13) , whereby the water flow is con¬ trolled in such a way that the combustion gas from the combuster (13) to the gas turbine"s HPT (14) has adapted temperature. The water to the combuster is successively phased out simultaneously with the gas flow from the steam generator

(3) and the compressors (11/12) build up.

The moist carbon dioxide (20) , which leaves the scrubber's (4) top, is preferably utilized 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 has extensive use for "flooding" of oil wells. The carbon dioxide is hereby pumped into the well where it partly dissolves in the oil and makes it more fluid. In this way further oil quantities can be reclaimed.

The embodiment according to fig. 1 implies a total efficiency of 55% when combusting natural gas with oxygen in the combina¬ tion of equipment units according to the embodiment including a separate oxygen unit. The produced flue gas from the hot-

water scrubber is moist carbon dioxide gas, which is useful for production of (liquid) carbon dioxide, while the surplus of hot-water from the scrubber (4) is useful for selective heating purposes. The net efficiency amounts to 50% after deduction of power consumption for production in a separate unit of liquid carbon dioxide (0,2 kWh/kg carbon dioxide) . The method according to the invention implies advantageous use of, besides natural gas, all clean gaseous fuels produced by oxygen gasification of carbonaceous fuels, for instance fossil fuels as coal and oil but biomass as well.

Liquid carbon dioxide can be dumped in the depths of the oceans, in extinguished oil and gas wells, aquiferes etc. Dumping in combination with the method according to the inven- tion enables thus power generation based on gaseous fuels without increasing the carbon dioxide content of the atmo¬ sphere, which implies that the method enables power supply based on fossil fuels, which is as environment friendly as power generation based on biofuels but more energy efficient.

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, which is put into the atmosphere. Another advantage is the absence of the comprehen- sive cleaning devices for the air intake, which the conven¬ tional combined cycle requires.