Supercritical Steam Research Articles

Effect of oblique stator on the aerodynamic performance and erosion characteristics of governing stage blades in a supercritical steam turbine

In this article, a new particle tracking model is established through high temperature erosion modeling test. Based on the model, steam-particle flows in the governing stage cascade of a supercritical steam turbine are systematically investigated and the influence of oblique stator on the aerodynamic performance and erosion characteristics of governing stage blades is carefully studied. Results show that with the increase of nozzle oblique angle, the maximum load position gradually moves toward nozzle trailing edge. Consequently, secondary flow loss of nozzle cascade gradually decreases. Compared with the prototype structure of governing stage, stage efficiency increases by 0.8%, 0.35%, and 0.44%, respectively, when nozzle oblique angle increases to 15°, 30°, and 45°. Meanwhile, erosion at the trailing edge of nozzle pressure side and leading edge of rotor suction side gradually decreases, while erosion of rotor pressure surface increases. The maximum erosion position of nozzle pressure surface and rotor suction surface keeps constant, but the maximum erosion position of rotor pressure surface gradually moves forward. Comprehensive analysis shows that when nozzle oblique angle reaches 30°, erosion of nozzle trailing edge reduces by 14%, and stage efficiency increases by 0.35%; erosion resistance and aerodynamic performance of governing stage can be both well considered.

Economic analysis of a 600 mwe ultra supercritical circulating fluidized bed power plant based on coal tax and biomass co-combustion plans

In recent years, circulating fluidized bed combustor (CFBC) has been regarded as a viable alternative to the conventional pulverized coal combustor (PCC) for utility-scale coal power generation owing to its superior technology for fuel flexibility and supercritical (SC)/ultra supercritical (USC) steam circuit adaptability. The objective of this study is to analyze the economic feasibility of a 600MWe USC CFB boiler, in which coal or a mixture of coal and biomass would be used as fuel. After the demonstration and commercialization of SC CFBC units had succeeded up to 600MWe, USC CFBCs have been widely developed throughout the world. Although high capital costs, high auxiliary power use, and technology maturity have hindered the adoption of USC CFBC for utility power generation, the demand of cleaner environments and energy conversion have driven communities to develop and adopt USC CFBC. Its economic feasibility was evaluated in terms of net present value (NPV), benefit/cost ratio (B/C ratio), and internal rate of return (IRR). In particular, the effect of coal tax and domestic biomass co-combustion on economic efficiency was analyzed.

Prospects of Application of Dual-Fuel Combined Cycle Gas Turbine Units

Dual-fuel combined cycle gas turbine units, including power units on the parallel scheme with predominant coal combustion are considered in the paper. The basic equations for determining the energy efficiency of dual-fuel combined-cycle power units are described. The interdependence of the efficiency of the gas turbine and steam turbine parts of the combined-cycle plant for the efficiency of the combined-cycle plant with a variable binary coefficient is presented. It is shown that 55-56% efficiency is achievable for parallel type combined cycle gas turbine units T with predominant solid fuel combustion on the basis of this interdependence between efficiency and binary coefficient. Comparison of competitiveness in the ratio of fuel prices for gas / coal with traditional coal technology and theoretical rejected combined cycle gas turbine units with an efficiency of 60% for dual-fuel combined cycle gas turbine units with the implementation of the Rankine cycle for subcritical (13 MPa) and supercritical (24 MPa) steam parameters is carried out. It is shown that the dual-fuel combined cycle gas turbine units are preferable to traditional coal steam turbine power units in the case when the ratio of the price of fuel does not exceed 5, binary rejected combined cycle gas turbine units, when the ratio of the prices by more 0,5.

Coupled modeling of combustion and hydrodynamics for a coal-fired supercritical boiler

Computational fluid dynamics (CFD) model of coal combustion is coupled with the one-dimensional hydrodynamic model of supercritical steam, and applied in the simulation of the superheater. The convective heat exchangers are neglected. Complex tube arrangements can be modeled with the aid of AutoCAD, and therefore the simulation could offer detailed information on heat exchangers. It is found that the outlet steam temperature of the tube is very sensitive to the variation of ash deposit temperature. Therefore it is not reasonable to use decoupled method for predicting the energy flux transported to the steam. The outlet steam temperature of a row of tubes is compared with the running data from the power plant. The simulation result fits well with running data. Nevertheless, one can impose an average steam temperature plus 50 K as the tube outside surface temperature; then calculates the temperature of the ash deposit according to the thermal resistance of ash deposit and energy flux. The result from this “simplified” coupled method is very close to fully coupled method.

Numerical simulation of supercritical catalytic steam reforming of aviation kerosene coupling with coking and heat transfer in mini-channel

A novel supercritical catalytic steam reforming reaction model coupling with coking and heat transfer is proposed and validated for aviation kerosene RP-3. The convective heat transfer and reaction characteristics of RP-3 with catalytic steam reforming are then investigated in mini-channel using this model. The effect of key operation parameters including water addition percentage and inlet flow velocity on coking and heat transfer is analyzed in detail. The results demonstrate that water addition can increase chemical heat absorption of RP-3 and enhance heat transfer. However, with increasing inlet flow velocity, there is a trade-off between convective heat transfer and chemical heat absorption. It is difficult to simultaneously achieve the optimal heat transfer performance and maximum chemical heat absorption. The temperature along the flow direction in the mini-channel reactor exhibits stratification phenomenon. In addition, increasing water addition and inlet flow velocity is beneficial to suppress coke formation. The present study provides better insight into the coupling relationship between catalytic steam reforming reaction of aviation kerosene and the coking and heat transfer processes.

Experimental Studies of Honeycomb Seals with Wavy Ridges on the Rotor

During steam turbine operation in transient modes, contact in labyrinth seals is possible. To avoid such contact in the regular stepped labyrinth seals, specialists of the Ural Turbine Works upgraded the end seals during major repairs in several T-250/300-23.5 type turbines of PAO Mosenergo: the regular seals on the rotor were replaced by straight-flow honeycomb seals with wavy ridges. After this upgrading, there were a reduction of air suctions through the end seals of low-pressure cylinder, the normalization of the ejector operation, and oil watering reducing. However, by these indirect indices, it is impossible to evaluate the actual change of leakages through upgraded seals. Therefore, the comparative experimental studies of flow and dynamic characteristics of the straight-flow seals with classic straight and wavy ridges were implemented. According to the investigation results, the flow characteristics of all studied models of straight-flow seals during transition from classical straight ridges to wavy ones deteriorated. The measurements of nonconservative components of the force in the seal models also revealed a deterioration of dynamic characteristics of the models of straight-flow seals during transition from straight ridges to wavy ones. In the end seals, increasing the nonconservative forces capable to cause a self-excited vibration of the shafting of the steam-turbine plant (STP) does not pose a threat owing to their low magnitude. However, in diaphragm seals, especially in the high-pressure section of STP to the supercritical steam parameters, using new honeycomb straight-flow seals with wavy ridges requires special caution, since these forces reach the highest values in the region of high pressures and are capable, in combination with the overbandage forces and forces arising in the oil layer of bearings, to cause low-frequency self-excited vibrations of shaftings. Taking into account that the level of nonconservative forces in the regular stepped seals is by 20% less than in the straight-flow seals, the transition to the straight-flow honeycomb seals with wavy ridges on the rotor in the diaphragm seals appears inappropriate.

Feasibility of using ammonia-based thermochemical energy storage to produce high-temperature steam or sCO2

In ammonia-based solar thermochemical energy storage systems (TCES), solar energy is stored via endothermic ammonia dissociation reaction and released when the ammonia synthesis reaction is utilized to heat the working fluid for a power block. It has been shown that a working fluid i.e., supercritical steam, can be heated in an ammonia synthesis reactor to 650 °C. In this paper, steam and supercritical carbon dioxide (sCO2) are proposed to be heated to a higher temperature, i.e., 800 °C, utilizing the ammonia synthesis in the context of ammonia-based TCES. Steam at 800 °C has the potential to be used for high-temperature electrolysis with a relatively high electrical-to-hydrogen efficiency. Supercritical carbon dioxide (sCO2) at 800 °C can be used for a high temperature Brayton cycle, which has the potential to achieve a more compact power cycle with higher efficiency than steam Rankine cycle. Two different synthesis systems, i.e., System I and System II, are designed in this paper for achieving heating the working fluid i.e., steam or sCO2. System I consists of a heat recovery reactor to heat the working fluid with ammonia synthesis, a preconditioning system and an autothermal synthesis reactor to preheat the synthesis gas. System II is similar to System I except the heat recovery reactor is replaced by a heat exchanger, which eliminates the need for another catalyst-filled reactor. A two-dimensional pseudo-homogeneous model is developed to simulate heating of the working fluid in reactors of the systems. Another two-dimensional model is developed to simulate the heat transfer process in heat exchangers. The results demonstrated the feasibility of heating steam or sCO2 to 800 °C in the two systems. But the autothermal synthesis reactors for preheating the synthesis gas are required to be relatively large for both systems. The autothermal synthesis reactor in System II is required to be even larger than that in System I since the synthesis gas without reaction in a heat exchanger need be preheated to a higher temperature. The result shows both the required catalyst volume and reactor wall volume of System I are lower than System II. This study provides a baseline for further design refinement and economic analysis.

Influences of tube wall on the heat transfer and flow instability of various supercritical pressure fluids in a vertical tube

The subcritical steam generators in the current high temperature gas-cooled reactor pebble-bed module, HTR-PM, in China can be replaced by supercritical steam generators to better coordinate the reactors and the supercritical steam turbine unit to improve the thermal efficiency with no fluid phase change at supercritical pressures. This study then numerically analyzes the heat transfer and flow instabilities of supercritical pressure fluid flow in a vertical tube with zero or finite-thickness walls. The wall thickness has negligible effect on the heat transfer at supercritical pressure water at steady state. However, for transient calculations, the flow rate oscillates intensely beyond a critical heat flux with the zero-thickness wall, while the flows in tubes with finite-thickness walls are very different. The wall properties including density, specific heat and thermal conductivity are varied to analyze why the oscillations are suppressed by the finite-thickness wall. The flow and heat transfer characteristics and the heat stored in the wall compared with the total power are presented during the heating process at various moments. The flow instabilities are also analyzed for water at various pressures and CO2 to analyze the effects of the fluid properties.

Supercritical steam outflow through a thin nozzle: forming a hollow jet

The process of hollow jet formation during steam outflow through a thin nozzle was studied; the water steam was initially at high pressure and in the supercritical state. The numerical solution for this problem was obtained with the sonicFoam solver library from the open-source CFD software package OpenFOAM in 2D axisymmetric formula-tion. The reliability of results is estimated by comparing two approaches for simulation of the dynamics of unloading wave propagating in the high-pressure nozzle using the OpenFOAM package with Peng−Robinson equation of state and numerical solution of a similar problem by method of through computation in the case of 1D planar approximation for perfect gas equation of state.

CHALLENGES AND OPPORTUNITIES OF UTILIZATION OF ASH AND SLAG WASTE OF TPP (THERMAL POWER PLANT). PART 2

For existing and already constructed coal TPP plants, known methods of utilization of fly ash and slag wastes (FASW) may be in de­mand when all emerging environmental and economic risks are taken into account. But for the new power generating source when choosing coal combustion technology, it is necessary to increase the significance of the environmental component of the project more essentially. It is known that the most promising technologies for coal combustion, which increase environmental safety exactly by burning, are technolo­gies based on a circulating fluidized bed. These technologies can sig­nificantly reduce sulfur and nitrogen oxide emissions behind the boiler, but the solution to the problem of flay ash and slag waste remains at the same level. It is proposed to solve the problem of FASW utilization during the implementation of new energy projects or when replacing the decommissioning capacities of coal generation by replacing the method of coal combustion in a stream or fluidized bed with meth­ods of burning solid fuel in a bubbling slag melt. The descriptions and schemes of these methods are given. The comparison of the main qual­itative technical and ecological parameters of pulverized coal combus­tion and technologies of coal combustion in slag melt is presented. The development of coal generation is expected in two main areas: coal combustion with increasing steam parameters and gas generation with a combined cycle of electricity generation: steam and gas, based on the gasification of solid fuels. These directions will allow achieving electric efficiency of steam-power plants from 30 – 36 %, up to 44 – 45 % on supercritical steam parameters, and using a combined steam-gas cycle up to 50 – 55 %. A technological scheme of gasification of coal in a slag melt is proposed, which increases the electrical efficiency of the installation. The ecological and economic efficiency of the gasifica­tion method for solid fuel and the simplicity of the production of slag products by casting are shown. The quality of cast slagstone products is much higher than similar cement-sand products with the addition of fly ash, and the ease of transition from one casting mold to another allows quickly responding to market demands.

Numerical study of flow and heat transfer of supercritical water in rifled tubes heated by one side

Deep understanding of flow and heat transfer of supercritical fluids in rifled tubes with non-uniform heating is of great importance for the safety of modern supercritical steam power equipment. In this paper, the heat transfer of supercritical water flowing in a vertical one-side heated rifled tube was numerically investigated. The differences of flow and heat transfer between the circumferentially non-uniform heated rifled tube (NRT) and uniform heated rifled tube (URT) were investigated. The distributions of temperature field, thermophysical properties, axial velocity and turbulent kinetic energy were analyzed. The mechanism of heat transfer improvement of NRT was comprehensively revealed. Results showed that the maximum inner wall temperature in NRT (Tiw,N,max) was 10 K lower than that in URT for the simulation conditions. Both radial and circumferential heat transfers were improved in NRT due to the swirling flow. Under high q/G (ratio of heat flux to mass flux) conditions, the buoyancy effect was weakened in NRT comparing to that in URT. Flow turbulence in boundary layer of y+ > 30 was intensified in NRT, which was crucial for supercritical fluid heat transfer. The positions of Tiw,N,max were not always at the midpoint of the heated side. And a prediction method for Tiw,N,max was proposed.

Processing Ash and Slag Wastes from Thermal Power Stations. Part 2

For existing coal-fired power plants, current methods of utilizing ash and slag waste may be considered in addressing new environmental and economic risks. However, for new power sources, environmental considerations are much more important in selecting the coal-combustion technology. Technology based on a circulating fluidized bed is sometimes cited as the most promising approach to environmentally sound coal combustion. It permits significant decrease in emissions of sulfur and nitrogen oxides beyond the boiler, but is of no help in processing ash and slag waste. For waste disposal, a different approach is recommended for new plants or the upgrading of coal-fired plants: instead of coal combustion in a gas flux or a fluidized bed, combustion in bubbling slag melt. Such methods are described. The basic characteristics of pulverized-coal combustion and combustion in slag melt are compared. Two basic approaches to the development of coal-based power generation are proposed: coal combustion with supercritical steam parameters; and gas generation with a hybrid (steam + gas) power-generating cycle based on gasification of solid fuels. The electrical efficiency of steam-based plants may be increased from 30–36 to 44–45% with supercritical steam parameters; and to 50–55% when using a hybrid steam–gas cycle. The proposed industrial system for coal gasification in slag melt increases the overall electrical efficiency. The environmental and economic efficiency of carbon gasification is demonstrated. It is simple to produce components from slag by casting. The cast slag–coal components are of considerably higher quality than analogous cement–sand components with added fly ash. The ease of switching from one type of casting to another permits rapid response to market demand.

Allowable Rates of Fluid Temperature Variations and Thermal Stress Monitoring in Pressure Elements of Supercritical Boilers

ABSTRACTAllowable rates of fluid temperature changes in critical elements of supercritical steam boilers were determined assuming a quasi-steady state of temperature distribution in pressure components. A method was proposed for monitoring the thermal stresses in the critical pressure components.

High-Temperature Oxidation of Commercial Alloys in Supercritical CO2 and Related Power Cycle Environments

The oxidation behavior of several Fe and Ni commercial alloys was evaluated in high-temperature environments expected in next-generation supercritical carbon dioxide power cycles. The alloys were exposed to pure CO2 at 0.1 and 20 MPa and to CO2 containing H2O, O2, with and without SO2 at 0.1 MPa, at temperatures of 550°C (Fe alloys) and 700–750°C (Ni alloys). For comparison, the alloys were also exposed to air and supercritical steam. Low-Cr ferritic Fe alloys formed Fe-rich oxides leading to high oxidation rates in pure CO2, while high-Cr austenitic Fe alloys formed Cr-rich oxides leading to low oxidation rates. H2O and O2 impurities had negligible effect on the oxidation behavior of all Fe alloys. The addition of SO2 had negligible effect on the oxidation of ferritic Fe alloys, but significantly enhanced the rate for austenitic Fe alloys, where S in the oxide and underlying alloy contributed to the transition from Cr-rich to Fe-rich oxide growth. Ni alloys formed Cr-rich oxides and showed low oxidation rates in all CO2-containing environments which were very similar to those obtained in air and supercritical steam. Little or no effect of pressure was observed for any of the alloys exposed to pure CO2.

Integration of the 660 MW supercritical steam cycle with the NH3-based CO2 capture process: System integration mechanism and general correlation of energy penalty

Aqueous ammonia based post-combustion CO2 capture (PCC) is an attractive alternative to amine based technologies for reduction of CO2 emission from coal fired power stations. However its integration with power stations is not well studied. This research examined the system integration mechanism of the 660 MW supercritical steam cycle with a NH3-based PCC process. Three integration methods were proposed:Method 1 (steam extraction from the IP/LP crossover), Method: 2 (steam extraction from the IP/LP crossover and the addition of a new letdown turbine while the condensate is returned to the deaerator), and Method: 3 (steam extraction from the IP/LP crossover and addition of a new turbine while the condensate is returned to the condenser). Three cases were presented using the above three methods, and were designated as Case 1, Case 2, and Case 3, respectively. The original power plant was taken as the base case. Among three cases studied, the thermal efficiency of case 2 is the highest and at 36.08% with a decrease of thermal efficiency by 11.61 percentage points compared to the base case. A sensitivity analysis was performed to investigate the effect of important parameters including the concentration of NH3, CO2 loading, and stripper pressure on the regeneration duty of the capture process and the energy output of the integrated process. The minimum energy requirements of the capture process does not lead to the minimum energy penalty introduced on the power plant. The findings indicate that the energy penalty in PCC integrated power plant depends on the temperature of the extracted steam, the heat duty of the reboiler, and the stripper pressure. A general correlation of the energy penalty of ammonia based PCC onto a supercritical power plant was obtained from the simulation results, which can enable a reliable evaluation of the PCC energy penalty imposed on the power plant.

Supercritical cyclic steam stimulation of wellbore temperature and pressure distribution in Lukeqin oilfield

Supercritical cyclic steam stimulation of wellbore temperature and pressure distribution in Lukeqin oilfield

Development of a technological approach to the control of turbine casings resource for supercritical steam parameters

A comprehensive model for evaluation of the resource of HPC of the turbine K-800-240-2, which includes calculation of thermal, stressed-strained state, cyclic and static damageability, is presented here. The numerical studies conducted with the use of modern methods of mathematical modeling showed a high impact of forces of pins’ tightening on the stressed-strained state of the casing elements (the stress level increased by 17.7 %). A technological approach to resource control, aimed at a change in pins’ tightening efforts, was proposed. It was established that this method decreases static damageability of basic metal of casings (by 9.7 %), improving its long-term strength. When taking into account tightening forces, the maximum stress intensity decreased by 9.3 %, while the stress level in the flange joint decreased by 11–41 %. These positive moments are accompanied by an increase in individual resource of the casing by 10 %. The developed concept and recommendations have significant importance for ensuring long-term operation of steam turbines with the initial pressure of hot steam at 24 MPa.

Seismic imaging in the Krafla high-temperature geothermal field, NE Iceland, using zero- and far-offset vertical seismic profiling (VSP) data

Among geothermal exploration methods, active surface seismic methods have played only a minor role to date. Especially in high-temperature volcanic systems, reflection seismic data often reveal poor delineation of volcanic features, due to the internal heterogeneity of volcanic sequences. To enhance the vertical resolution, one possibility is the application of downhole seismic methods like vertical seismic profiling (VSP). A test experiment was carried out in the Krafla high-temperature geothermal field, NE-Iceland, to assess the ability of VSP to image subsurface structures, such as fractures, zones of high permeability, magmatic bodies, and zones of supercritical fluids and steam. Logging in such hostile environments is technical challenging in many aspects, but mainly due to the high temperature impact on the downhole electronic components of the measuring equipment. This requires a thorough pre-examination and implementation of the measurement, especially to avoid delays and tool failures. This paper presents results of zero- and far-offset VSP data from the K-18 borehole from within the Krafla caldera, which reveal good correlation with the surrounding lithology. The raw three-component seismic data display a good signal-to-noise ratio and dominant signal frequencies between 20 and 40 Hz, down to c. 2200 m depth, for air gun and explosive sources, respectively. A zero-offset source comparison was also conducted to assess the use of different impulsive sources for future VSP surveys in similar settings. By applying a standard VSP processing, we identified stratigraphic boundaries between lavas, hyaloclastites, and intrusions, which are in good agreement with existing well data. For the zero-offset VSP, both P- and S-wave velocity models were calculated and a depth-converted corridor stack was determined. In addition, multicomponent Kirchhoff depth migration and Fresnel volume migration were tested around the borehole. The 3D results are promising, but the specific shape and lateral extent of the reflectors could not be determined due to the restriction to only two sources and the insufficient spatial coverage (aperture). Our study demonstrates that vertical seismic profiles can clearly detect variations in the subsurface volcanic stratigraphy in high-temperature geothermal fields. A more detailed reservoir characterization can be achieved by further data integration, enhanced survey design including more source positions, and improved processing and imaging techniques, such as full-waveform inversion.

Potential of molten lead oxide for liquid chemical looping gasification (LCLG): A thermochemical analysis

Molten lead oxide is revealed to have favourable thermodynamic performance for gasification in a new process employing chemical looping of a molten liquid metal oxide. In this process, the feedstock is partially oxidized with molten lead oxide in the fuel reactor, while the reduced molten lead is oxidized in the air reactor. As with other chemical looping processes, this avoids direct contact between air and fuel, which prevents the undesirable dilution of the gaseous product with nitrogen. The Gibbs minimization method was employed together with thermo-chemical equilibrium analysis to assess the feasibility of the gasification process using graphite as a surrogate for more realistic, but complex carbonaceous fuels, together with steam and/or carbon dioxide as the gasifying agent. It was found that both the reduction and oxidation reactions of molten lead oxide with carbonaceous fuel are spontaneous. Likewise, the ratio of H2:CO can be as high as 2.5, while the carbon conversion can reach 94% based on the thermochemical analysis. An energetic performance analysis was also employed for the case of a supercritical steam turbine cycle to extract work from the hot gaseous co-products. On this basis, the first law efficiency of the power cycle was estimated to be up to 33.8%, while the syngas co-product stream for applications such as Fischer-Tropsch synthesis has a chemical exergy efficiency of 41%.

Novel online simulation-ready models of conjugate heat transfer in combustion chamber waterwall tubes of supercritical power boilers

This paper presents two models of fluid heating in waterwall tubes of supercritical steam boilers. The models are named 1D/2D and 1D/3D. The models are formulated and discussed in detail with an industrial application example. For the 1D/2D model, a two-dimensional (2D) transient heat conduction equation is solved for the tube wall with the fin domain, while one-dimensional (1D) mass, momentum, and energy equations are solved for the fluid domain. The 1D/3D model considers a three-dimensional tube wall domain (3D) and one-dimensional fluid domain. At the fluid-solid interface, a conjugate heat transfer model is applied. The model is based on convective flux between the fluid and solid domains. Nonlinear governing balance equations of mass, momentum, and energy for fluid are solved using the forward time backward space (FTBS) scheme. The proposed models allow incorporation of the effect of heat flux nonuniformities along the waterwall tube and on its outer circumference. Transient simulations are carried out to determine the temperature histories for both the fluid and the tube wall in the selected cross sections. The computations are performed using the finite volume method formulation. The results obtained from the 1D/2D model are nearly the same as those from the 1D/3D model; however, the computation time is more than five times shorter. Therefore, an efficient 1D/2D model can be used in power unit simulators.