Power generation system and method to operate

The invention relates to a power generation system comprising

- an oxy-fuel-burner,

- a first heat exchanger,

- a steam cycle operated with a first exhaust-fluid-stream generated by said oxy-fuel-burner,

- a recirculation line extracting a part of said first ex ^¬ haust-fluid from said steam cycle and feeding said exhaust- fluid-stream into said oxy-fuel-burner,

- wherein said steam cycle comprises at least one first tur- bine expanding at least a part of said first exhaust-fluid- stream, namely a third exhaust-fluid-stream,

- wherein said steam cycle comprises at least one first con ^¬ denser downstream said first steam turbine condensing at least a part of said third exhaust-fluid-stream, namely a seventh exhaust-fluid-stream,

- wherein said steam cycle comprises at least a first feed- water-pump downstream of said first condenser delivering at least a part of said seventh exhaust-fluid-stream, namely a ninth exhaust-fluid-stream.

Power generation systems and respective methods to operate such systems are known for a long time since mechanical power or electrical power is generated especially by burning a fuel with an oxygen containing gas. Recently concerns came up about carbon-dioxide content in air increasing up to an amount where a so called green-house effect might occur.

Since such awareness is rising several projects are initiated to reduce the emission of carbon-dioxide. One of those pro ^¬ jects is burning a fuel with an oxygen containing gas other than air to a avoid the generation of NOx [nitrogen oxides] and to avoid the mixing of essential inert components with the carbon-dioxide generated during combustion to more easily enable the separation of carbon-dioxide from the exhaust gas generated. This easy separation simplifies storage of pure carbon-dioxide in a final storage capacity. Essentially pure carbon-dioxide can further better be used for subsequent chemical processes. The oxygen containing gas is basically pure oxygen with minor impurities generated by for example an air separation unit, which can be of conventional membrane type. In the context of this invention an oxy-fuel-burner is characterized by burning basically a fuel with an oxygen con ^¬ taining gas wherein said oxygen containing gas has signifi- cant higher oxygen content than ambient air and wherein oxy ^¬ gen is its main component and wherein said oxygen containing gas is preferably pure oxygen with some impurities. This oxy ^¬ gen containing gas may contain some further additives but its main component is preferably oxygen.

One known power generation system is disclosed in

US 7,021,063 B2, which deals with an oxy-fuel-burner respectively gas generator comprising a recuperative heat exchanger for reheating of steam that has passed a first expansion ma- chine stage, which heat exchanger is heated by outlet steam respectively exhaust from said gas generator.

The total efficiency of a conventional power generation sys ^¬ tem with an oxy-fuel-burner is significantly below the effi- ciency of an ordinary power generation system if the energy consumption of the air separation unit is considered. The ef ^¬ ficiency is therefore to be improved to make this technology economically feasible and to have a positive effect on the environment .

It is one object of the invention to improve the efficiency of the known power generation system comprising an oxy-fuel- burner . The object of enhancing the efficiency of the incipiently de ^¬ fined power generation system is achieved by a power generation system according to the incipiently mentioned type with the further features of the characterizing portion of claim 1. Further the object is achieved by a method of the incipi- ently mentioned type with the further features of the charac ^¬ terizing portion of the independent method claim. One essential aspect of the proposed improvement of the power generation system respectively the method according to the invention is the addition of heat exchangers for preheating of the re-circulated feed water submitted to the oxy-fuel- burner for mixing into the exhaust stream. According to the invention by preheating with extraction steam the cycle performance is improved.

In the cycle according to the invention the steam respective ^¬ ly exhaust-fluid taken from the steam turbine (s) contains carbon-dioxide in a substantial concentration, typically more than 5%, preferably about 10% by volume, which makes the cy ^¬ cle much different from a conventional steam cycle. The car ^¬ bon-dioxide led to the pre-heaters is preferably separated from the pre-heaters and then collected to be routed to an export carbon-dioxide stream. Preferably a carbon-dioxide compression process for the delivery to a final user - for example enhanced oil recovery or methane synthesis - is inte ^¬ grated in the power generation system respectively method. A further beneficial improvement of the process according to the invention is obtained by providing a recuperator respectively first heat exchanger downstream said oxy-fuel-burner before the exhaust-fluid enters a steam turbine. This heat exchanger respectively recuperator re-heats steam respective- ly exhaust-fluid that has passed a first expansion through said steam turbine, wherein the exhaust-fluid from said oxy- fuel-burner is heating the exhaust-fluid from said steam turbine. This heat exchanger provides a certain protection for the downstream steam turbine as it provides some heat capaci- ty damping thermal gradients from upstream equipment control variations or disturbances. Further this heat exchanger as ^¬ sist in protecting the turbine from possible water droplets carried over from said oxy-fuel-burner . Said oxy-fuel-burner according to the invention is basically a gas generator generating an exhaust gas respectively ex ^¬ haust fluid from a fuel burned with essentially pure oxygen. This exhaust gas is referred to as exhaust-fluid since it might contain liquid components or parts of the fluid might condense to a liquid.

Another beneficial improvement of the invention is given by providing at least one adjustable valve to control the flow through said recirculation line. This control feature allows maintaining the desired exhaust-fluid temperature downstream said oxy-fuel-burner respectively before said exhaust-fluid enters any turbine equipment. Preferably a control unit con- trols the position of said adjustable valve in the recircula ^¬ tion line according to a temperature measurement upstream a turbine of the power generation system. This control unit is designed such that it receives the measurement results from temperature measurement and submits control signals to said control valve. The control method preferably is designed such that the valve opens further when exceeding a temperature limit is recognized. Further the valve control unit can be designed such that upper limits of temperature increases re ^¬ spectively steep temperature transients in a turbine of the power generation system are avoided.

Another preferred embodiment provides a degasification port at said at least one feed water pre-heater to collect gaseous carbon-dioxide from the condensing exhaust-fluid.

Another preferred embodiment of the invention provides an air separation unit upstream of said oxy-fuel-burner to preferably separate oxygen from ambient air to be burned with a fuel in said oxy-fuel-burner . This air separation unit can be of a membrane type.

The above mentioned attributes and other features and advan ^¬ tageous of this invention and the manner of attaining them will become more apparent and the invention itself will be understood by reference to the following description of the currently known best mode of carrying out the invention taken in conjunction with the accompanying drawings, wherein figure 1 shows a schematic flow diagram of an oxy fuel power plant comprising the arrangement accord ^¬ ing to the invention and depicting the method according to the invention.

Figure 1 is a schematic depiction of a simplified flow dia ^¬ gram showing a power generation system and illustrating a method according to the invention. Fuel F and oxygen O ₂ from an air separation unit ASU are both elevated to a higher pressure level by compressors CI, C2, C3, C4, C5 which com ^¬ pressors CI, C2, C3, C4, C5 are respectively provided with intercoolers INT1, INT2, INT3 before both fluids are injected in an oxy-fuel-burner OXB at a pressure of 150bar. In said oxy-fuel-burner OXB - which can also be considered as a gas generator - combustion takes place of said fuel F with said oxygen O ₂ generating exhaust gas hereinafter referred to as exhaust-fluid. The exhaust-fluid - namely a first exhaust- fluid-stream EXHl - exits said oxy-fuel-burner OXB and enters a first heat exchanger HEX1. The temperature of said first exhaust-fluid-stream EXHl is adjusted by controlling a flow of evaporating media to the oxy-fuel-burner OXB to be boiled off and thus cool the exhaust-fluid to the right temperature to subsequently enter a second steam turbine ST2. Downstream said first exchanger HEX1 said first exhaust- fluid-stream EXHl is expanded in said second steam turbine ST2, which is a high pressure steam turbine (high pressure means that this pressure level is higher than the pressure level of the downstream turbine) .

The first exhaust-fluid-stream EXHl exiting said second steam turbine ST2 is divided in a second exhaust-fluid-stream EXH2 and a third exhaust-fluid-stream EXH3, wherein approximately above 90% of said first exhaust-fluid-stream EXH1 becomes said third exhaust-fluid-stream EXH3.

Downstream said second steam turbine ST2 said third exhaust- fluid-stream EXH3 enters said first heat exchanger HEX1 to be reheated taking thermal energy from said first exhaust-fluid- stream EXH1 coming from said oxy-fuel-burner OXB .

Further downstream said third exhaust-fluid-stream EXH3 en- ters a first steam turbine ST1 to be expanded from approxi ^¬ mately 40bar pressure down to a pressure of 0.2bar. Said first turbine ST1 comprises several extractions of exhaust- fluid-streams so that said expanded third exhaust-fluid- stream EXH3 is reduced to a seventh exhaust-fluid-stream EXH7 by extraction of a fourth exhaust-fluid-stream EXH4, extraction of a fifth exhaust-fluid-stream EXH5 and extraction of a sixth exhaust-fluid-stream EXH6. Downstream said first steam turbine ST1 said seventh exhaust-fluid-stream EXH7 is partly liquefied in a first condenser CON1, which is equipped with a degasifier to separate said seventh exhaust-fluid-stream EXH7 into a gaseous eighth exhaust-fluid-stream EXH8 and a liquid ninth exhaust-fluid-stream EXH9 both exiting said first condenser CON1. Said eighth exhaust-fluid-stream EXH8 is basically gaseous carbon-dioxide and compressed in an intercooled multistage compressor consisting of the stages CP1, CP2, CP3 and the intercooling heat exchangers INT3, INT4. Said multi ^¬ stage compressor MCP receives further gaseous streams of car ^¬ bon-dioxide at several intermediate pressure levels of com ^¬ pression to be compressed for subsequent usage, here indicat- ed as storage STO.

Downstream said first condensers CON1 said ninth exhaust- fluid-stream EXH9 it delivered to a higher pressure level by a first feed water pump FWP1. At a downstream division point DIV1 said ninth exhaust-fluid-stream EXH9 is split into a tenth exhaust-fluid-stream EXH10 - which basically consists of liquid water H20 - and an eleventh exhaust-fluid-stream EXH11, which enters a downstream mixing pre-heater and degasifier MPD. In said mixing pre-heater and degasifier MPD said eleventh exhaust-fluid-stream EXH11 mixes with said sixth exhaust-fluid-stream EXH6 extracted from said first steam turbine ST1 to increase the temperature and further mixes with a 22nd exhaust-fluid-stream EXH22, which is throttled by a valve TH3 into said mixing pre-heater and

degasifier MPD. The gaseous amount generated in said mixing pre-heater and degasifier MPD is directed to said multistage compressor MCP as a twelfth exhaust-fluid-stream EXH12. The liquid amount from said mixing pre-heater and degasifier MPD is delivered to a downstream second feed-water-pump FWP2 as a thirteenth exhaust-fluid-stream EXH13. Further downstream said thirteenth exhaust-fluid-stream EXH13 is heated-up in a second sub-cooler SC02 exchanging heat with said 22th ex- haust-fluid-stream EXH22 before it enters said mixing pre- heater and degasifier MPD. Further downstream said thirteenth exhaust-fluid-stream EXH13 enters a first feed water pre- heater WPH1, a first sub-cooler SCOl, a second feed water pre-heater WPH2, a third heat exchanger HEX3 and a third feed water pre-heater WPH3 and a second heat exchanger HEX2. Downstream this pre-heating sequence said thirteenth exhaust- fluid-stream EXH13 joins into said recirculation line RCL through an adjustable valve WSV to be injected into said oxy- fuel-burner OXB to adjust the temperature of said first ex- haust-fluid-stream EXH1 as said above mentioned cooling media .

Said third feed water pre-heater WPH3 is heated by said se ^¬ cond exhaust-fluid-stream EXH2 extracted from said second steam turbine ST2 downstream passing said second heat ex ^¬ changer HEX2 transferring thermal energy to said thirteenth exhaust-fluid-stream EXH13. Said third feed water pre-heater WPH3 splits the hot side of this heat exchange into a gaseous component supplied to the multistage compressor MCP as an eighteenth exhaust-fluid-stream EXH18. The liquid component of the hot side of the third feed water pre-heater WPH3 is provided as a heating fluid through a first throttle TH1 into said second feed water pre-heater WPH2. Said second feed water pre-heater WPH2 subsequently receives said fourth exhaust-fluid-stream EXH4 from said first steam turbine ST1 to heat-up said thirteenth exhaust-fluid-stream EXH13.

Said second feed water pre-heater WPH2 discharges a gaseous sixteenth exhaust-fluid-stream EXH16 - consisting basically of carbon-dioxide - and a liquid 21st exhaust-fluid-stream EXH21 both resulting from said incoming fourth exhaust-fluid- stream EXH4 and 20 ^th exhaust-fluid-stream EXH20. Said 21th exhaust-fluid-stream EXH21 enters the heating side of said first sub-cooler SCOl and further downstream enters said first feed water pre-heater WPH1 through a second throttle valve TH2 on the heating side.

Said first steam turbine ST1 and said second steam turbine ST2 both drive at least one generator GEN to produce electrical power. As an alternative a direct drive can be provided for example for a compressor or any other unit to be driven.

Said first condenser CON1 can be cooled by ambient air, ambi ^¬ ent water from a sea or a river or it can be a water spray condenser cooling the fluid to be condensed by water jet. Wa ^¬ ter can be provided by for example water extracted from said power generation system cooled and re-injected.

To prevent accumulation of undesired products in the cycle a water treatment WT can be inserted for example in said recir ^¬ culation line RCL . Alternatively or in addition said water treatment WT can be inserted upstream of the extraction of water H20 as the tenth exhaust-fluid-stream EXH10. This location would also improve the quality of water to be extracted for any potential subsequent usage.