Semi-closed Oxy-fuel Combustion Combined Cycle Research Articles

Thermodynamic Analysis of a Semi-Closed Oxy-fuel Combustion Combined Cycle

Semi-closed oxy-fuel combustion combined cycle (SCOC-CC) is a strong concept of carbon capture and storage (CCS) in gas-fired power plants. This technology is similar to a conventional combined cycle, however oxygen instead of air is used in fuel combustion. In the oxy-fuel combined cycle, the gas turbine flue gases consist mainly of CO2 and H2O. One of the problems to implement this technology is the necessity of an air separation unit (ASU) to separate the oxygen from the air, which increases the energy consumption of the power plant. Thus, a comparative thermodynamic analysis was performed between a conventional combined cycle (base case) and an oxy-fuel combined cycle. The objective is to identify each technology's pros and cons, the influence of oxygen purity in the oxy-fuel combine cycle, and the main irreversibilities of each case. The SCOC-CC optimal operating point (maximum energy efficiency) was found utilizing particle swarm optimization (PSO), which lead to the optimal ASU oxygen purity of 95.99%. It was noticed that the oxy-fuel combined cycle first law efficiency is 6.9% lower than the base case, and the second law efficiency is 6.5% lower. Despite the efficiency loss the SCOC-CC is more environmentally friendly than the conventional combined cycle since it can theoretically capture all CO2 produced in the combustion chamber.

Research and Development of the Oxy-Fuel Combustion Power Cycles with CO2 Recirculation

The transition to oxy-fuel combustion power cycles is a prospective way to decrease carbon dioxide emissions into the atmosphere from the energy sector. To identify which technology has the highest efficiency and the lowest emission level, a thermodynamic analysis of the semiclosed oxy-fuel combustion combined cycle (SCOC-CC), the E-MATIANT cycle, and the Allam cycle was carried out. The modeling methodology has been described in detail, including the approaches to defining the working fluid properties, the mathematical models of the air separation unit, and the cooled gas turbine cycles’ calculation algorithms. The gas turbine inlet parameters were optimized using the developed modeling methodology for the three oxy-fuel combustion power cycles with CO2 recirculation in the inlet temperature at a range of 1000 to 1700 °C. The effect of the coolant flow precooling was evaluated. It was found that a decrease in the coolant temperature could lead to an increase of the net efficiency up to 3.2% for the SCOC-CC cycle and up to 0.8% for the E-MATIANT cycle. The final comparison showed that the Allam cycle’s net efficiency is 5.6% higher compared to the SCOC-CC cycle, and 11.5% higher compared with the E-MATIANT cycle.

Parametric optimization of the semi-closed oxy-fuel combustion combined cycle

The report discloses thermodynamic studies of the semi-closed oxy-fuel combustion combined cycle. The computer simulation and parametric optimization approaches are described in details. The oxy-fuel cycle net efficiency in relationship to the carbon dioxide turbine exhaust pressure and the steam turbine inlet pressure is shown. The power production efficiency reduction is related to the turbine cooling losses is described. It is shown that the semi-closed oxy-fuel combustion cycle maximal net efficiency of 52.5% occurs at the initial temperature and pressure 1400°C and 60 bar at the carbon dioxide turbine exhaust pressure 0.5 bar and the steam turbine inlet pressure 90 bar. The cooling losses consideration leads to the net efficiency of 47.76% that is reached at the carbon dioxide turbine exhaust pressure 1 bar and the steam turbine inlet pressure 90 bar.

Methods for efficiency improvement of the semi-closed oxy-fuel combustion combined cycle

The paper presents thermal schemes of the semi-closed oxy-fuel combustion combined cycle with preliminary cooling of the coolant flow. The first scheme involves the coolant flow preliminary cooling in a surface heat exchanger. The heated agent is a part of condensate taken from the steam turbine circuit. The net efficiency improvement against the base cycle is 3.2% for the working fluid initial temperature of 1700°C. The coolant flow temperature is reduced down to 150°C. The second scheme involves injection of the water taken from the cooler-separator. In this case the power facility net efficiency improvement may be up to 1.5%.

Influence of a recuperator on the performance of the semi-closed oxy-fuel combustion combined cycle

A recuperator was applied to the semi-closed oxy-fuel combustion combined cycle (SCOC-CC) to reduce its very high optimal design pressure ratio to a feasible level. The recuperator was installed in different locations in the heat recovery system: (1) before the entire heat recovery steam generator (HRSG) of the bottoming steam cycle and (2) before the HP evaporator of the HRSG. Component design parameters from state-of-the-art gas turbines (F and H classes) were used. A thermodynamic cooling model was used to estimate the reasonable amount of turbine coolant needed to maintain the turbine blade temperature of each stage at a feasible level. The turbine stage efficiency was corrected while considering the effect of turbine blade loading. The optimal pressure ratio ranges in terms of net system efficiency were found, and the influence of the recuperator effectiveness on the cycle performance was investigated. The optimal pressure ratio of the recuperated SCOC-CC was about 40–50, which was considerably lower than that of the simple SCOC-CC. The net efficiency of the recuperated SCOC-CC increased by as much as 0.6 and 1.3 percentage points in the F- and H-class gas turbines cases, respectively. The benefits of the recuperated cycle were discussed.

A technical evaluation, performance analysis and risk assessment of multiple novel oxy-turbine power cycles with complete CO2 capture

In recent years there has been growing concern about greenhouse gas emissions (particularly CO2 emissions) and global warming. Oxyfuel combustion is one of the key technologies for tackling CO2 emissions in the power industry and reducing their contribution to global warming. The technology involves burning fuel with high-purity oxygen to generate mainly CO2 and steam, enabling easy CO2 separation from the flue gases by steam condensation. In fact, 100% CO2 capture and near-zero NOx emissions can be achieved with this technology.This study examines nineteen different oxy-turbine cycles, identifying the main parameters regarding their operation and development. It also analyses the use of advanced natural gas (NG) combustion cycles from the point of view of the carbon capture and storage (CCS) and considering political, legislative and social aspects of deploying this technology. Six oxy-turbine cycles which are at the most advanced stages of development (NetPower, Clean Energy Systems CES), Modified Graz, E-MATIANT, Advanced Zero Emission Power AZEP 100% and Semi-Closed Oxy-fuel Combustion Combined Cycle (SCOC-CC)), were chosen to conduct a Political, Environmental, Social, Technological, Legislative and Economic (PESTLE) risk analysis. This compares each technology with a conventional combined cycle gas turbine (CCGT) power plant without carbon capture as the base-case scenario. Overall, the net efficiency of the different oxy-turbine cycles ranges between 43.6% and 65%, comparable to a CCGT power plant, while providing the extra benefits of CO2 capture and lower emissions.A multi-criteria analysis carried out using DECERNS (Decision Evaluation in Complex Risk Network Systems) software determined that, depending on the specific criterion considered, one can draw different conclusions. However, in terms of technology, environment and social opinion, the most promising cycles are the NetPower and CES cycles, whereas from an economic point of view, E-MATIANT is more competitive in the energy market. Giving each factor equal importance, the NetPower cycle must be considered to be the best oxy-turbine cycle based on our analysis.Most of the oxy-turbine cycles are still under development and only a few cycles (e.g., CES and NetPower) are progressing to the demonstration phase. In consequence, political measures such as CO2 tax and emission allowances need to be implemented for oxy-turbine technologies to become the preferred option for fossil fuel power plants burning natural gas.

Comparison of Natural Gas Combined Cycle Power Plants with Post Combustion and Oxyfuel Technology at Different CO2 Capture Rates

Within this work a natural gas-fired combined cycle plant is investigated with post-combustion CO2 capture (PCC) and as a semi-closed oxyfuel combustion combined cycle (SCOC-CC) using process simulation tools. In most studies the CO2 capture rate of PCC units is set to 90% while the SCOC-CC is often considered for zero emission plants. To allow a comparison between the processes the capture rate needs to be taken into account. If a certain CO2 emission value is required a different capture rate may be economically attractive. The effect of the capture rate is investigated from 60% to 97.5% for PCC and from 75% to 100% for the SCOC-CC. For PCC the capture rate has a strong influence on the efficiency penalty. For a broad range of capture rates the specific heat duty is found to be approx. constant. However, for very high capture rates the heat duty strongly increases. The efficiency penalty of the SCOC-CC is much less affected by the chosen capture rate, because the duty of the air separation unit (ASU) is independent on the capture rate. However to achieve a sufficient CO2 purity a reduced capture rate can be necessary.The processes are compared regarding the loss of electrical energy per kg CO2 captured. PCC has a significantly lower specific loss for low capture rates of approx. 0.38 kWhel/kg CO2 compared to the SCOC-CC with more than 0.48 kWhel/kg CO2. For capture rates of > 90% the losses of both processes approach each other. With high purity oxygen the SCOC-CC can achieve a capture rate of 100% as well as a CO2 purity of >96 vol.-% at a specific loss of 0.49 kWhel/kg CO2.

A Thermodynamic Analysis of Two Competing Mid-Sized Oxyfuel Combustion Combined Cycles

A comparative analysis of two mid-sized oxyfuel combustion combined cycles is performed. The two cycles are the semiclosed oxyfuel combustion combined cycle (SCOC-CC) and the Graz cycle. In addition, a reference cycle was established as the basis for the analysis of the oxyfuel combustion cycles. A parametric study was conducted where the pressure ratio and the turbine entry temperature were varied. The layout and the design of the SCOC-CC are considerably simpler than the Graz cycle while it achieves the same net efficiency as the Graz cycle. The fact that the efficiencies for the two cycles are close to identical differs from previously reported work. Earlier studies have reported around a 3% points advantage in efficiency for the Graz cycle, which is attributed to the use of a second bottoming cycle. This additional feature is omitted to make the two cycles more comparable in terms of complexity. The Graz cycle has substantially lower pressure ratio at the optimum efficiency and has much higher power density for the gas turbine than both the reference cycle and the SCOC-CC.

Optimisation Potentials for the Heat Recovery in a Semi-closed Oxyfuel-combustion Combined Cycle with a Reheat Gas Turbine

Abstract Semi-closed oxyfuel-combustion combined cycle plants are one possibility to realise CCS technologies with natural gas-fired power plants. Due to the recirculation of exhaust gas the working fluid in the gas turbine consists mainly of CO2. This leads to the necessity for an increased pressure ratio to achieve the efficiency and the exhaust gas temperatures of conventional gas turbines. In this work a conventional natural gas-fired combined cycle plant with a reheat gas turbine modified for oxyfuel operation will be presented. As it is assumed that the necessary pressure ratio is not realisable with axial compressors two different scenarios are considered. On the one hand the pressure ratio is the same as in the conventional process and on the other hand the pressure ratio is set to an assumed maximum of 60. The exhaust gas temperature lies significantly above the values of the conventional plant. Generally the higher pressure ratio results in a higher net efficiency of the plant. An unchanged heat recovery process results in a loss of efficiency of at least 9.3 to 10.8%-pts. depending on the pressure ratio. Increasing the steam parameters can achieve a reduction of this loss ranging from 0.5%-pts for the high pressure ratio to 1.0%-pts for the low pressure ratio. An alternative approach for the heat recovery is the application of an internal recuperator in the gas turbine. When the achievable pressure ratio is far from the optimum value, as in the low pressure ratio case, this approach can lead to significantly lower losses of 9.1%-pts. With a higher pressure ratio the effects of the optimisation options are similar.

Conceptual Mean-Line Design of Single and Twin-Shaft Oxy-Fuel Gas Turbine in a Semiclosed Oxy-Fuel Combustion Combined Cycle.

The aim of this study was to compare single- and twin-shaft oxy-fuel gas turbines in a semiclosed oxy-fuel combustion combined cycle (SCOC-CC). This paper discussed the turbomachinery preliminary mean-line design of oxy-fuel compressor and turbine. The conceptual turbine design was performed using the axial through-flow code luax-t, developed at Lund University. A tool for conceptual design of axial compressors developed at Chalmers University was used for the design of the compressor. The modeled SCOC-CC gave a net electrical efficiency of 46% and a net power of 106 MW. The production of 95% pure oxygen and the compression of CO2 reduced the gross efficiency of the SCOC-CC by 10 and 2 percentage points, respectively. The designed oxy-fuel gas turbine had a power of 86 MW. The rotational speed of the single-shaft gas turbine was set to 5200 rpm. The designed turbine had four stages, while the compressor had 18 stages. The turbine exit Mach number was calculated to be 0.6 and the calculated value of AN2 was 40 · 106 rpm2m2. The total calculated cooling mass flow was 25% of the compressor mass flow, or 47 kg/s. The relative tip Mach number of the compressor at the first rotor stage was 1.15. The rotational speed of the twin-shaft gas generator was set to 7200 rpm, while that of the power turbine was set to 4800 rpm. A twin-shaft turbine was designed with five turbine stages to maintain the exit Mach number around 0.5. The twin-shaft turbine required a lower exit Mach number to maintain reasonable diffuser performance. The compressor turbine was designed with two stages while the power turbine had three stages. The study showed that a four-stage twin-shaft turbine produced a high exit Mach number. The calculated value of AN2 was 38 · 106 rpm2m2. The total calculated cooling mass flow was 23% of the compressor mass flow, or 44 kg/s. The compressor was designed with 14 stages. The preliminary design parameters of the turbine and compressor were within established industrial ranges. From the results of this study, it was concluded that both single- and twin-shaft oxy-fuel gas turbines have advantages. The choice of a twin-shaft gas turbine can be motivated by the smaller compressor size and the advantage of greater flexibility in operation, mainly in the off-design mode. However, the advantages of a twin-shaft design must be weighed against the inherent simplicity and low cost of the simple single-shaft design.

Evaluation of Design Performance of the Semi-Closed Oxy-Fuel Combustion Combined Cycle

This study aims to present various design aspects and realizable performance of the natural gas fired semi-closed oxy-fuel combustion combined cycle (SCOC-CC). The design parameters of the cycle are set up on the basis of the component technologies of today’s state-of-the-art gas turbines with a turbine inlet temperature between 1400 °C and 1600 °C. The most important part of the cycle analysis is the turbine cooling, which considerably affects the cycle performance. A thermodynamic cooling model is introduced in order to predict the reasonable amount of turbine coolant needed to maintain the turbine blade temperature of the SCOC-CC at the levels of those of conventional gas turbines. The optimal pressure ratio ranges of the SCOC-CC for two different turbine inlet temperature levels are researched. The performance penalty due to the CO2 capture is examined. The influences of the purity of the oxygen provided by the air separation unit on the cycle performance are also investigated. A comparison with the conventional combined cycle, adopting a postcombustion CO2 capture, is carried out, taking into account the relationship between the performance and the CO2 capture rate.

Qualitative and Quantitative Comparison of Two Promising Oxy-Fuel Power Cycles for CO2 Capture

Since the Kyoto conference, there is a broad consensus that the human emission of greenhouse gases, mainly CO2, has to be reduced. In the power generation sector, there are three main alternatives that are currently studied worldwide. Among them oxy-fuel cycles with internal combustion with pure oxygen are a very promising technology. Within the European project ENCAP (enhanced CO2 capture) the benchmarking of a number of novel power cycles with CO2 capture was carried out. Within the category oxy-fuel cycles, the Graz Cycle and the semiclosed oxy-fuel combustion combined cycle (SCOC-CC) both achieved a net efficiency of nearly 50%. In a second step, a qualitative comparison of the critical components was performed according to their technical maturity. In contrast to the Graz Cycle, the study authors claimed that no major technical barriers would exist for the SCOC-CC. In this work, the ENCAP study is repeated for the SCOC-CC and for a modified Graz Cycle variant as presented at the ASME IGTI Conference 2006. Both oxy-fuel cycles are thermodynamically investigated based on common assumptions agreed upon with the industry in previous work. The calculations showed that the high-temperature turbine of the SCOC-CC plant needs a much higher cooling flow supply due to the less favorable properties of the working fluid. A layout of the main components of both cycles is further presented, which shows that both cycles rely on the new designs of the high-temperature turbine and the compressors. The SCOC-CC compressor needs more stages due to a lower rotational speed but has a more favorable operating temperature. In general, all turbomachines of both cycles show similar technical challenges and are regarded as feasible.