Kristin Jordal

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.

New Possibilities for Combined Cycles Through Advanced Steam Technology

In order to improve the performance of the combined cycle, much effort has been spent over the past decade on increasing gas turbine performance. As a contrast to this, the present work focuses on possibilities for combined cycle performance enhancement through present and expected future steam cycle and boiler technology. The use of various heat recovery steam generators, (single and dual pressure) with or without supplementary firing are studied, in combination with steam turbine admission temperatures of up till 973 K. Supplementary firing is applied either in the entire gas turbine exhaust duct or in part of it, in a so-called split-stream boiler (SSB). Furthermore, the flashing of pressurized water from an overdimensioned economiser in the SSB, to produce steam for gas turbine vane cooling is studied. Many of the supplementary fired cycles studied are found to have a thermal efficiency superior of the unfired cycles, based on the same gas turbines. Hence, available steam technology and expected future development mean that most of the cycles studied are realistic concepts that merit further attention in the quest for more efficient power production.© 2002 ASME

Comparison of Gas Turbine Cooling With Dry Air, Humidified Air and Steam

In order to study the impact of using a gas turbine coolant other than compressed air, a comparison is made among various gas-turbine based power cycles: the simple-cycle gas turbine, the steam injected gas turbine, the combined cycle and the Humid Air Turbine (HAT) cycle. Depending on the cycle configuration, the coolant, which is compared to the air extracted from the compressor, is either steam or humidified air. The study is based on a modern, medium-sized industrial gas turbine (net power output in base configuration: 40 MW), in order to evaluate the possibilities of different redesign options of the cooling system.It is found that steam cooling is a very efficient method of cooling; whereas, the possible benefits of humid air cooling are mainly obtained through the temperature decrease that is a result of the humidification process. Hence, for the intercooled HAT cycle, the benefit of cooling with humid air is smaller (∼0.2 percentage points increase in thermal efficiency) than for the HAT cycle without intercooler (∼0.5 percentage points increase in thermal efficiency). For the simple cycle, there is an increase in thermal efficiency of 0.8 percentage points when it is cooled with humid air.A parameter variation shows that with more advanced cooling technologies and heat resistant materials, the benefit of cooling with steam or humid air, instead of with compressed air, is reduced for the HAT and combined cycles.Copyright

Optimization with Genetic Algorithms of a Gas Turbine Cycle with H2-Separating Membrane Reactor for CO2 Capture

A gas turbine power process with CO2 capture through precombustion decarbonization, which employs a H2-separating membrane reactor is presented. Optimization with the process thermal efficiency as objective function is made through the use of genetic algorithms. The use of genetic algorithms enabled a division of the optimization parameters into two groups; one group where the values are at their optimum at the limit of the investigated parameter range, and one group where there actually is an optimum within the investigated range. It was found that the process has a severe efficiency penalty caused by the use of heat from hydrogen combustion for the reforming process. The process is a zero CO2 emission power process and also NOx emissions should be low, due to the inherent mixing of hydrogen with steam.

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.

Aspects of Cooled Gas Turbine Modelling for the Semi-Closed O2/CO2 Cycle With CO2 Capture

In order to capture the behaviour of the oxyfuel cycle operating with high combustor-outlet temperature, the impact of blade and vane cooling on cycle performance must be included in the thermodynamic model. As a basis for a future transient model, three thermodynamic models for the cooled gas turbine are described and compared. The first model, known previously from the literature, models expansion as a continuous process with simultaneous heat and work extraction. The second model is a simple stage-by-stage model and the third is a more detailed stage-by-stage model that includes velocity triangles and enables the use of advanced loss correlations. An airbreathing aeroderivative gas turbine is modelled, and the same gas turbine operating in an oxyfuel cycle is studied. The two simple models show very similar performance trends in terms of variation of pressure ratio and turbine inlet temperature in both cases. With the more detailed model, it was found that, without any change of geometry, the turbine rotational speed increases significantly and performance drops for the maintained geometry and pressure ratio. A tentative increase of blade angles or compressor pressure ratio is found to increase turbine performance and decrease rotational speed. This indicates that a turbine will require re-design for operation in the oxyfuel cycle.Copyright

Modelling and Simulation of Transient Performance of the Semi-Closed O

One of the concepts proposed for capture of CO2 in power production from gaseous fossil fuels is the semi-closed O2 /CO2 gas turbine cycle. The semi-closed O2 /CO2 gas turbine cycle has a near to stoichiometric combustion with oxygen, producing CO2 and water vapor as the combustion products. The water vapor is condensed and removed from the process, the remaining gas, primarily CO2 , is mainly recycled to keep turbine inlet temperature at a permissible level. A model for predicting transient behavior of the semi-closed O2 /CO2 gas turbine cycle is presented. The model is implemented in the simulation tool gPROMS (Process System Enterprise Ltd.), and simulations are performed to investigate two different issues. The first issue is to see how different cycle performance variables interact during transient behavior; the second is to investigate how cycle calculations are affected when including the gas constant and the specific heat ratio in compressor characteristics. The simulations show that the near to stoichiometric combustion and the working fluid recycle introduce a high interaction between the different cycle components and variables. This makes it very difficult to analytically predict the cycle performance during a transient event, i.e. simulations are necessary. It is also found that, except for the shaft speed calculation, the introduction of gas constant and specific heat ratio dependence on the compressor performance map will have only a minor influence on the process performance.Copyright