Hanne M. Kvamsdal

Gas Turbine Combined Cycle With CO2-Capture Using Auto-Thermal Reforming of Natural Gas

A concept for capturing and sequestering CO 2 from a natural-gas fired combined-cycle power plant is presented. Previously, a number of methods for capturing CO 2 from power plants have been suggested, among other including chemical absorption of CO 2 from exhaust gas and stoichiometric combustion with pure oxygen. The present approach is to de-carbonise the fuel prior to combustion by reforming natural gas, producing a hydrogen-rich fuel. The reforming process consists of an air-blown pressurised auto-thermal reformer that produces a gas containing H 2 , CO and a small fraction of CH4 as the combustible components. The gas is then led through a water-shift reactor, where the equilibrium of CO and H 2 O is shifted towards CO 2 and H 2 . The CO 2 is then captured from the resulting gas by chemical absorption. The gas turbine of this system is then fed with a fuel gas containing approximately 50% H 2 . A very important aspect of this type of process is the integration between the combined cycle and the reforming process. The pressurised air for the reforming is taken from a gas turbine compressor bleed, and there is an exchange of MP- and HP-steam between the steam cycle and the reforming process. This integration is necessary in order to achieve acceptable level of fuel-to-electricity conversion efficiency.

SOFC and gas turbine power systems—Evaluation of configurations for CO2 capture

Publisher Summary This chapter highlights that pressurized solid oxide fuel cells (SOFC) integrated in a gas turbine cycle is a promising power generation concept. The benefit of such combined systems is the potential for high electrical efficiency at small scale. By including an afterburner for the fuel cell, the remaining fuel in the anode exit gas is fully converted to water and CO2 while the anode and cathode streams from the fuel cell are kept separated. This enables the CO2 capture from an exhaust stream consisting of only CO2 and water. This chapter evaluates, three afterburner technologies based on different membrane conductors from the perspective of thermodynamic cycle analysis and materials technology. The total SOFC and gas turbine system with the different afterburners has been modeled in a general purpose flow sheet simulator, and mass and energy balances have been calculated. The electrical efficiency has been determined and compared for each of the three afterburners. The potential of the three technologies for future use as afterburners is evaluated.

BENCHMARKING OF GAS-TURBINE CYCLES WITH CO2 CAPTURE

Publisher Summary This chapter explores that cycle performance studies are carried out with different models and computational assumptions. Consequently, results from various sources are difficult to compare. The intention of is to make a comparison of various natural gas-fired power cycle concepts with CO2 capture. Nine different concepts for natural gas fired power plants with CO2 capture have been investigated, and a comparison is made based on cycle performance. These cycles constitute one post-combustion, six oxy-fuel and 2 pre-combustion concepts. A common basis for the comparison of all concepts is defined and employed in heat- and mass-balance simulations of the various concepts. As turbine cooling impacts the performance at high turbine inlet temperatures, a simplified model has been applied in the simulations. It is shown that the concepts, in which emerging technology is employed, exhibit the best performance with respect to efficiency.

Chapter 29 – A Comparison of the Efficiencies of the Oxy-Fuel Power Cycles Water-Cycle, Graz-Cycle and Matiant-Cycle

Publisher Summary This chapter evaluates three oxy-fuel power generation concepts—Water-cycle, Graz-cycle and Matiant-cycle, based on direct stoichiometric combustion with oxygen. Considering cycle efficiency and given similar computational assumptions, the Graz-cycle and the latest versions of the Matiant-cycle seem to give rather similar efficiencies, while the Water-cycle is 3-5%points behind. The Water-cycle is a Rankine-type cycle, while the Graz-cycle is a mixed Brayton/Rankine cycle, and the more recent Matiant-cycle is a combined topping/bottoming Brayton/Rankine cycle. Generally, Brayton cycles, in combination with Rankine cycles, exhibit higher efficiencies than Rankine cycles alone. The thermodynamic explanation for this is that Brayton cycles combined with Rankine cycles have a higher ratio of the temperatures at which heat is supplied to, and rejected from, the cycle, compared to that of a Rankine cycle. According to the Carnot cycle efficiency definition, the efficiency is improved when this temperature ratio increases.

Exergy Analysis of Gas-Turbine Combined Cycle With CO2 Capture Using Pre-Combustion Decarbonization of Natural Gas

A concept for natural-gas fired power plants with CO2 capture has been investigated using exergy analysis. The present approach involves decarbonization of the natural gas by authothermal reforming prior to combustion, producing a hydrogen-rich fuel. An important aspect of this type of process is the integration between the combined cycle and the reforming process. The net electric power production was 47.7% of the Lower Heating Value (LHV) or 45.8% of the chemical exergy of the supplied natural-gas. In addition, the chemical exergy of the captured CO2 and the compression of this CO2 to 80 bar represented 2.1% and 2.7%, respectively, of the natural-gas chemical exergy. For a corresponding conventional combined cycle without CO2 capture, the net electric power production was 58.4% of the LHV or 56.1% of the fuel chemical exergy. A detailed breakdown of irreversibility is presented. In the decarbonized natural-gas power plant, the effect of varying supplementary firing (SF) for reformer-feed preheating was investigated. This showed that SF increased the total irreversibility and decreased the net output of the plant. Next, the effects of increased gas-turbine inlet temperature and of gas-turbine pressure ratio were studied. For the conventional plant, higher pressure led to increased efficiency for some cases. In the decarbonized natural-gas process, however, higher pressure ratio led to higher irreversibility and reduced thermal-plant efficiency.Copyright

Integration of H2-Separating Membrane Technology in Gas Turbine Processes for CO2 Sequestration

Publisher Summary The chapter describes the possibility of capturing CO2 in natural gas fired power cycles through the integration of a H2-separating membrane in a component for steam reforming of natural gas, a so-called membrane reactor. Two types of membranes are investigated: Pd membranes, which could allow for zero-emission power cycles, and microporous membranes, the use of which in the present work means that 20% of the generated CO2 is emitted to the atmosphere. to reduce the CO2 emission from natural-gas based power-generation plants, three different main types of concepts have emerged as the most promising: (1) separation of CO2 from exhaust gas coming from a standard gas-turbine (GT) combined cycle (CC), using chemical absorption by amine solutions; (2) oxyfuel CC with a close-to-stoichiometric combustion using as oxidizing agent with CO2 and water vapor as the combustion products; and (3) fuel decarbonization in which the carbon of the natural gas (NG) is removed prior to combustion and the fuel heating value is transferred to hydrogen. Three non-optimized power cycles with a membrane reactor are studied, the best of which (a combined cycle with a directly fired gas turbine) has a thermal efficiency of only 41.6%. Through refined process layout and systematic process optimization, it should be possible to design a process with significantly higher thermal efficiency.

Natural gas fired power plants with CO2-capture-process integration for high fuel-to-electricity conversion efficiency

A concept for capturing and sequenstering CO 2 from a natural gas fired combined cycle power plant is presented. The present approach is to decarbonise the fuel prior to combustion by reforming natural gas, producing a hydrogen-rich fuel. The reforming process consists of an air-blown pressurised auto-thermal reformer that produces a gas containing H 2 , CO and a small fraction of CH 4 as combustible components. The gas is then led through a water gas shift reactor, where the equilibrium of CO and H 2 O is shifted towards CO 2 and H 2 . The CO 2 is then captured from the resulting gas by chemical absorption. The gas turbine of this system is then fed with a fuel gas containing approximately 50% H 2 . In order to achieve acceptable level of fuel-to-electricity conversion efficiecy, this kind of process is attractive because of the possibility of process integration between the combined cycle and the reforming process. A comparison is made between a “standard” combined cycle and the current process with CO 2 -removal. This study also comprise an investigation of using a lower pressure level in the reforming section than in the gas turbine combustor and the impact of reduced steam/carbon ratio in the main reformer.