Oxy-fuel Cycles Research Articles

Thermo-environmental analysis of a renewable Allam Cycle for generating higher power and less CO2 emissions

Although CO2 emissions from burning fossil fuels have caused numerous environmental problems, the utilization of oxy-fuel cycles has proven to be an effective method for generating more power while minimizing CO2 emissions. In this study, an innovative power generation system was presented that integrates renewable energies (solar and wind) with a small-scale oxy-fuel cycle (Allam Cycle). The system underwent modeling, simulation, and optimization from energy, exergy, exergoeconomic, and environmental perspectives. Generating higher power and emitting lower CO2 emissions compared to conventional gas turbines was the main goal of this study. Additionally, to present results meaningfully, comparisons were made between two different weather conditions: St. Petersburg in Russia (cold climate) and Tehran in Iran (normal climate). In this regard, dynamic analysis was performed using TRNSYS and Engineering Equation Solver (EES) tools. Furthermore, a multi-objective optimization was conducted using the TOPSIS multi-criteria method on functional parameters. The optimization results revealed that the maximum energy efficiency and minimum total cost rate were achieved at 75 % for Tehran and 1.84 $/h for St. Petersburg. Providing power for internal components, the highest power sold to the grid was reported to be 8365 kWh in December in St. Petersburg and 9646 kWh in May in Tehran. Additionally, the flat plate collector (FPC) demonstrated maximum efficiency and stored heat of 36 % in Tehran and 2.25 kWh in St. Petersburg, respectively. Ultimately, the results of pollutant emissions indicated that the Allam Cycle achieved a significant CO2 reduction compared to a conventional gas turbine, with a reduction ratio of 117 times in St. Petersburg and 119 times in Tehran.

Decarbonisation options of existing thermal power plant burning natural gas

Nowadays power industry faces deepest crises ever with unprecedented prices shocks and climate challenges at the same time. From one hand we realise the need of energy transformation of power industry towards more sustainable future with climate neutral technologies. From the other hand it become obvious that this change could not happen immediately and transition period is needed with some fossil fuel technology still playing an important role as a back-up for renewable energy sources. The biggest question what is the best and cost-efficient way to decarbonise existing thermal power generation. We try to address it on the example of existing combined cycle gas turbine (CCGT) power plant fuelled by natural gas. Clearly the following possible options were identified: 1) replacement of natural gas with alternative gases, such as green hydrogen, bio or synthetic methane, 2) carbon capture and underground storage (CCS) in geological formations, 3) carbon capture, liquefaction and export, 4) carbon capture and utilisation (CCU). US giant General Electric in its publication “Decarbonizing gas turbines through carbon capture” is considering similar options for decarbonising of gas turbines. They divide it into two approaches: 1) pre-combustion by using a zero or carbon neutral fuels, such as hydrogen, synthetic methane, biofuels or ammonia and 2) post-combustion by removing carbon from the plant exhaust, using liquid or solid sorbents or oxy-fuel cycles. In this publication we try to compare these different options, despite they are not clearly comparable. For the analysis we take natural gas fired CCGT plant Riga TPP-2 in Latvia with installed capacity of 881 MW (in condensing mode).

DEVELOPMENT AND RESEARCH OF THE TOPOLOGY OF COOLING BAFFLES FOR BLADES OF THE AXIAL CARBON DIOXIDE TURBINES

Currently, there is an increase in average annual temperature and climate change across the various continents. Carbon dioxide emissions from energy facilities contributed to this condition. Implementation of oxy-fuel cycles is a promising solution for reducing carbon dioxide emissions from the energy sector. To date, the most efficient oxy-fuel cycle is the Allam cycle. In this cycle supercritical carbon dioxide acts as a working fluid of the cycle, wherein СО2’s temperature upstream of the turbine is 1,150 °С and the pressure is 30 MPa. Due to the high temperature of the working fluid, it is necessary to cool first stages of the carbon dioxide turbine. The feature of considered cooling system in this turbine is that carbon dioxide being used as a refrigerant too. This paper investigated two topologies of convective cooling systems in the carbon dioxide turbine’s nozzle blade as well as considers an option for increasing the intensity of heatexchange through the use of helical ribbing in the cylindrical cooling baffle. Numerical simulation involving the ANSYS software package was performed for two topologies of the cooling baffles arrangement in the nozzle blade body: configuration 1 -with 17 baffles of 1 mm diameter, configuration 2 -with three baffles of the blade profile shape. Configuration 1 proved to be more efficient: the Nusselt number has a value of 117, and average value of the heat transfer coefficient on the refrigerant side is 6,413 W/m2∙K. The effect of using helical ribbing in the cooling cylindrical baffle of the blade under study was investigated, which enabled to reduce the metal temperature by 54 °С on average and doubled the heat transfer coefficient.

Oxy-fuel Combustion Power Cycles: A Sustainable Way to Reduce Carbon Dioxide Emission

The energy generation from the fossil fuels results to emit a tremendousamount of carbon dioxide into the atmosphere. The rise in the atmosphericcarbon dioxide level is the primary reason for global warming and other cli-mate change problems for which energy generation from renewable sourcesis an alternative solution to overcome this problem. However, the renewablessources are not as reliable for the higher amount of energy production andcannot fulfil the world’s energy demand; fossil fuels will continue to beconsumed heavily for the energy generation requirements in the immediatefuture. The only possible solution to overcome the greenhouse gas emissionfrom the power plant is by capturing and storing the carbon dioxide within thepower plants instead of emitting it into the atmosphere. The oxy-fuel combus-tion power cycle with a carbon capture and storage system is an effective wayto minimize emissions from the energy sectors. The oxy-fuel power cycle canreduce 90–99% of carbon dioxide emissions from the atmosphere. Moreover,the oxy-fuel power cycles have several advantages over the conventionalpower plants, these include high efficiency, lesser plant footprint, much easiercarbon-capturing processes, etc. Because of these advantages, the oxy-fuel combustion power cycles capture more attention. In the last decades, thenumber of studies has risen exponentially, leading to many experimental anddemonstrational projects under development today. This paper reviews theworks related to oxy-fuel combustion power generation technologies withcarbon capture and storage system. The cycle concepts and the advancementsin this technology have been briefly discussed in this paper.

Staged oxy-fuel natural gas combined cycle

Exhaust gas recirculation (EGR) in conventional natural gas-fired oxy-combustion cycles is required to maintain the combustion temperature at an allowable level. However, EGR is not beneficial from the system performance perspective. It is difficult to achieve in oxy-fuel cycles due to the high pressure and increased pressure drop in such systems. Consequently, alternative options to control the combustion temperature need to be considered. In this study, staged oxy-fuel natural gas combined cycle (SOF-NGCC) was proposed, which does not require EGR, and its feasibility was evaluated. A process model was developed in Aspen Plus® in order to evaluate the thermodynamic performance of the proposed system and to benchmark it against the Allam cycle and conventional NGCC. The optimum net efficiency of the proposed cycle (47.63–51.32%) was shown to be lower than that for the Allam cycle (54.58%) and the conventional NGCC without post-combustion capture (PCC) (56.95%). However, the SOF-NGCC is less complex and requires smaller equipment than the Allam cycle. This is mostly because the combined volumetric flow rate into expanders in both topping and bottoming cycles is approximately 25% of that estimated for the Allam cycle. Moreover, with a backpressure of 35 bar, no further compression is required prior to the CO2 purification unit.

Conceptual and basic design of a novel integrated cogeneration power plant energy system

A novel integrated system, including air separation unit (ASU), coal gasification, solid oxide fuel cell (SOFC), carbon dioxide (CO2) transcritical cycle, steam cycle with liquefied natural gas (LNG) vaporization is configured and analyzed. ASU provides the required oxygen for SOFC and oxy-fuel power generation system. Coal gasification provides syngas which is utilized as a part of the essential heat source. Electrical power is generated by SOFC, steam and CO2 oxy-fuel cycles. LNG is vaporized to provide the cold energy and also is utilized as the fuel in the SOFC and CO2 oxy fuel cycles. Sensitivity of the process performance to the major operating parameters is studied. Effect of the LNG flow rate, turbine inlet temperature (TIT), CO2 oxy-fuel cycle pressure ratio and SOFC operating parameters are investigated. The obtained results indicate that the net electrical power is 5.97 × 105 kW, in the condition that TIT = 900 °C, rp,c = 28, Vcell = 0.85 and Uf = 0.8. In this process rate of the utilized LNG is 1.10 × 108 kg.h−1 and rate of the captured CO2 is 1.03 × 104 kg.h−1. Also 1.36 × 107 kg h−1 syngas, 1.38 × 107 kg.h−1 liquid oxygen (LO2), 3.37 × 107 kg.h−1 liquid nitrogen (LN2) and 1.00 × 108 kg.h−1 NG are produced.

Thermodynamic analysis of a modified oxy-fuel cycle, high steam content Graz cycle with a dual-pressure heat recovery steam generator

Energy and exergy analyses are reported for a modified oxy-fuel cycle, which includes the Graz cycle with an atmospheric condensation and a dual-pressure heat recovery steam generator (HRSG). Oxy-fuel cycles are proposed to capture all the produced CO2. Due to limited fossil fuel resources, it is necessary to optimise the use of energy in the power plant cycles as much as possible. The presented configuration improves the energy and exergy efficiency of the cycle with usage of dual-pressure HRSG instead of single pressure. In this study, thermodynamic analysis for the performance of modified Graz cycle has been done which shows the effect of different parameters on the performance of cycle. Results show that the thermal and exergy efficiencies increase up to 54.5% and 47.8%, respectively, by adding of dual-pressure heat recovery steam generator unit.

Techno-economic assessment of sour gas oxy-combustion water cycles for CO2 capture

Growing energy demand coupled with the threat of global warming call for investigating alternative and unconventional energy sources while reducing CO2 emissions. One of these unconventional fuels is sour gas, which consists of methane, hydrogen sulfide and carbon dioxide. Using this fuel poses many challenges because of the toxic and corrosive nature of its combustion products. A promising technology for utilizing it is oxy-fuel combustion with carbon capture and storage, including the potential of enhanced oil recovery for added economic benefits. Although methane oxy-fuel cycles have been studied in the literature, using sour gas as the fuel has not been investigated or considered. In this paper, water is used as the diluent to control the flame temperature in the combustion process, and the associated cycle type is modeled to examine its performance. As the working fluid condenses, sulfuric acid forms which causes corrosion. Therefore, either expensive acid resistant materials should be used, or a redesign of the cycle is required. These different options are explored. A cost analysis of the proposed systems is also conducted to provide preliminary estimates for the levelized cost of electricity (LCOE). The results show the acid resistance cycle with a 4.5% points increase in net efficiency over the cycle with SOx removal. However there is nearly a 9% decrease in the cycle's LCOE for the latter case.

Natural gas oxy-fuel cycles—Part 1: Conceptual aerodynamic design of turbo-machinery components

Several oxy-fuel cycles have been considered in the ENCAP project. These cycles require compressors and turbines that operate at different conditions and with different gas compositions from conventional heavy-duty gas turbines. Conceptual designs of selected turbo-machinery have therefore been carried out to assess the feasibility of the proposed components. A compressor and turbine from the SCOC-CC and S-Graz cycles have been selected for this preliminary design study. The unusual boundary conditions would ideally require new hardware. As entirely new designs might rule out the commercial realization of these cycles possible ways of using existing components for the SCOC-CC compressor have been investigated.

Natural gas oxy-fuel cycles—Part 3: Economic evaluation

As part of the sixth European framework project on enhanced capture of CO2 (ENCAP), several novel power generation cycles with CO2 pre-capture methods are identified and the promising technologies are selected for a techno-economic assessment. For this analysis, the chemical process simulation package ECLIPSE is utilised. Following a detailed mass and energy balance calculation, the economic assessments of the Semi-close Oxygen Combustion (SCOC), Water Cycle and Graz as well as S-Graz Cycles is performed in reference to year 2004. The total capital cost estimation of the studied cycles is implemented in a bottom-up approach. Subsequently, the breakeven electricity selling price (BESP) is determined according to the net present value. Through the modification of parameters such as fuel price or capacity factor, the sensitivity analysis is carried out to assess the effect of endogenous or exogenous changes on the economic viability of the cycles. Among the systems, the S-Graz Cycle is the most cost intensive process. Yet, due to its high plant efficiency, it delivers the lowest electricity price and the lowest CO2-avoidance costs. The Water Cycle is the least capital intensive technology in this study. Due to its poorer plant efficiency however, the economics of this cycle scored third on the list.

Natural gas oxy-fuel cycles—Part 2: Heat transfer analysis of a gas turbine

Abstract The influence of the cycle conditions of Semi-Closed Oxy-Combustion Combined Cycles (SCOC-CC) onto the heat transfer to the 1st row turbine vane in gas turbines was investigated using empirical correlations for a cylinder in a cross flow and for a flat plate in a parallel flow being representative for the leading edge and for the vane span, respectively. Radiative heat transfer was modelled using absorptivities/emissivities describing the reduction of the thermal radiation absorption/emission by the hot combustion flue gas as compared to a black-body. The contribution of chemiluminescence was evaluated by means of numerical simulation using a detailed kinetic model of the combustion process extended for radiation processes. SCOC-CC with flue gas recirculation before condensation results in a 20–30% increase of heat transfer due to convection and thermal radiation as compared to SCOC-CC with flue gas recirculation after condensation. Further the increase of the pressure ratio from π=17 to π=40 at fixed thermal power and turbine inlet temperature results in an increase of both the convective and the radiative heat transfer of the order of 10%. Chemiluminescence did not have any influence on the heat transfer, because the powers emitted by excited OH- and CH-radicals were smaller than 10−8% of the thermal power.

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.