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Method for improving efficiency in gas turbine combined plants
[Category : - ELECTRICITY & LIGHTING]
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NATURAL GAS OR HYDROGEN FUELLED 6/2024 PATENTED US 12,000,335 B2 ETC. GAS /STEAM TURBINE COMBINED CYCLE INNOVATION STARTS RACE TOWARDS "IMPOSSIBLE" 70% ELECTRICITY GEN. EFFICIENCY BY UTILIZING COMBUSTION GASES WATER VAPOR CONDENSING ENERGY ENABLING WORLD RECORD THERMAL EFFICIENCY
Innovational idea is to utilisize combustion gas water vapor condensing energy for the first time to increase electricity efficiency. Presently related loss is 9.9% with natural gas and 15.4% with hydrogen. Combined cycle advanced Rankine subatmospheric reheating solves the problem. CCGT power plant would reach about 68% electricity generation efficiency with values (temperatures, pressure losses etc.) used in commercial CCGT power plants. Also according to patent examination efficiency increase!
Groving population and rapid increase in the number of heat pumps, EV´s and e-fuels poses major challenges to electricity generation. Especially because simultaneously we should reduce fossil fuels -especially coal which has presently biggest global share in electricity generation. Second largest energy source natural gas has about 24% share and it will be TOP3 source of energy along with wind and solar energy in the middle of century. 74% of all electricity generated with natural gas is generated in combined cycle power plants. Like professionals know this is power plant where gas turbine combustion gases are used in HRSG (Heat Recovery Steam Generator) to generate steam for steam turbine process. Because wind and solar energy changes much daily hydrogen will be generated with part of renewable energy with electrolysers. Hydrogen can be burned in gas turbines as well as gasified coal. Thus, it is fair to say that the combined cycle will play a key role in future electricity generation.
It may be misconception that converting hydrogen into electricity is most profitable with fuel cells. Firstly fuell cells are very expensive and maximum efficiency of fuell cells is about 65%. Todays combined cycle world record electric efficiency is about 64.4% and so combined cycle is only commercial possibility to generate electricity in industrial scale. Presently 3D cooling and ceramic coatings have alloved 1500 C to 1600 C gas turbine maximum temperatures. This means that with optimal gas turbine parameters combustion gases temperature after gas turbine is about 650 C and optimal steam process (Rankine cycle) maximum pressure is about 180 bar. Because of lower steam temperature and pressure in 70´s and 80´s power plants had no Rankine cycle steam reheating but in todays combined cycle reheating is needed. Otherwise the moisture content of the steam in the end of expansion at condensing pressure 0.03 bar would be too high for blades. With 88% moisture content there is not too much erosion in blades. In single Rankine cycle the higher the average temperature, the higher the efficiency - but only with single Rankine which is explained later. This is according to Carnot efficiency, which is highest theoretical efficiency that can be achieved by a thermal power machine.So efficiency depends on maximum and minimum process temperature. So presently optimal steam reheating pressure in combined cycle is about 20 bar. Reheating pressure according to this CCGT innovation is about 0.7 bar which is kind of contrary to theories cause this means lower average reheating temperature. The reason for discrepancy is that the heat of condensation of water vapor formed in combustion is not included in the Carnot efficiency. So added 10% extra energy means that for example CCGT with 62% electric efficiency would have over 68% efficiency. There are no practical problems cause about 3 bar or 4 bar reheating steam pressure is already used in commercial power plants where is two reheating. So this is also simplier and cheaper reheating pipes can be used because of lower temperatures.
In the current process, the high energy need for water vaporization means that combustion gases temperature is over 100 C when those leave the process and so water vapor condensing energy is lost. Other problem is that there is extra combustion gases heat energy at low temperature area related to same large vaporization energy need. Presently this energy is utilized in dual pressure HRSG and according to this innovation to reheat Rankine process steam at parallel heat exchanger and water preheating take place in other parallel heat exchanger. Present solution means large exergy losses of lower pressure HRSG related to temperature differences in heat excange. On the other hand this reduces also main steam mass flow. Because according to this CCGT innovation reheating is made with extra heat at low temperature area steam mass flow is bigger or alternatively little higher gas turbine pressure ratio can be used. This means directly higher efficiency of pure gas turbine unit. Actually reheating presently steam from 600 K to 873 K has 735 K average temperature where heat is brough into cycle and so under 60% part Carnot efficiency. Efficiency increase only cause evaporation temperature is higher compared to plants without reheating.
EXACT DESCRIPTION OF INNOVATION
Energy related to water vapor (formed in combustion) condensing is lost in present gas turbine combi plants. With this invention reheating of steam cycle will happen with appr. 0.6-0.7 bar level meaning that with 1 atm combustion gases water vapor can be increased Rankine cycle steam enthalpy from appr. 90% to 100%. Because hydrogen has bigger energy loss related to steam condensing (15.4 %) this is very useful with hydrogen giving even better efficiency. Rest of water vapor condensing energy is utilized by vaporizing extra water fed into 0.7 bar pressure. This boost efficiency more than would be theoretically expected cause steam is reheated to much over 100 C temperature - probably to 300 C temperature. Even in warm climate a sufficiently high LP (Low Pressure) steam turbine pressure ratio can be achieved to provide sufficient expansion. And intercooling supercharger with water injection increase efficiency too cause water vapor condensing energy can be utilized.
So in present gas turbine combi plants 2/3 of Heat Recovery Steam Generators exhaust gas flow is enough to preheat process cycle water at low temperature area 300-600 K. Presently this 1/3 extra energy is used in gas turbine combi plants to evaporate steam in double pressure level. This means high exergy losses. In plant according to invention this low temperature energy is used to reheat steam meaning low exergy losses and 1/3 of mass flow is just enoug needed for steam reheating cause water preheating takes double as much energy.
Also GB patent granted in summer 2024. FI patent requirements (novelty etc) accepted and patent may be granted this year. German application exist and patent examination request can be made during 7 years from first application.
Financial information
ECONOMIC AND ECOLOGIC BENEFITS
Like in patent examination is mentioned efficiency increase. So CO2/fuel need emissions are 6-11% lower (estimatedly 10-11% with hydrogen) and compared to present 10-20 years old CCGT with 57% efficiency CO2 emissions may be 15-20% lower. Even 6% lower fuel need means 100.000.000 $ money save in ten years in large 600 MW unit. And taking 95 euros CO2 tax /ton into account total money save may be 170.000.000 $ in ten years. This means that power generation profit may nearly double. Replacing 20 year old 20% less efficient 57% 600 MW CCGT power plant may save money 500.000.000$ in ten years. Old power plant can then be used as free reserve power for example in winter. Other benefits are that cheaper reheating pipe materials can be used because of lower reheating temperatures. Related lower steam turbine maximum temperatures after reheating mean also longer lifetime and less turbine blade erosion. In present combined cycle steam mass flow rate energy is lost after first steam turbine because of reheating. In this innovation first and second steam turbine are cmbined and so isentropic efficiency is very high - maybe 1-2 percent units higher than presently.
Patent publications:
US 12000335
GB 2613679
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