Effects of fuel composition and temperature on fireside corrosion resistance of materials for advanced ultrasupercritical coal fired power plants

Abstract

The fireside corrosion resistance of candidate materials for the waterwalls and superheater/reheater sections of ultrasupercritical coal fired boilers have been evaluated through field testing as part of a programme cosponsored by the US Department of Energy and the Ohio Coal Development Office. The materials tested include high strength ferritic steels (SAVE12, P92, HCM12A), austenitic stainless steels (Super304H, 347HFG, HR3C), and high nickel alloys (Haynes 230, CCA617, INCONEL 740, HR6W). Protective coatings (weld overlays, diffusion coatings, laser claddings) that may be required to mitigate corrosion were also evaluated. The trials were based on previous laboratory evaluations under synthesised coal ash and flue gas conditions typical of three North American coals at temperatures ranging from 455 to 595°C for waterwall materials, while superheat/reheat materials were tested at 650–870°C. Promising materials from the laboratory tests were assembled on air cooled, retractable corrosion probes for testing in utility boilers. The probes were designed to maintain metal temperatures using multiple zones, ranging from 650 to 870°C. Three utility boilers, equipped with low NO_x burners, were identified that have adequate flue gas temperatures and represent each of the three coal types. New fireside corrosion probe results are presented for mid-western and western coal types, after approximately one year of exposure in the field. Visual examination of samples from the mid-western utility site indicated minimal evidence of significant wall loss for any of the tested materials. Samples removed from the western utility site indicated evidence of wall loss for some tested materials. Further evaluation and quantification of total metal wastage through wall thickness measurements and metallographic examination of subsurface penetration is being undertaken.     

Lessons from the past: materials-related issues in the ultrasupercritical boiler,Eddystone Unit 1

Abstract

Major research efforts in both the USA and Europe have made substantial progress toward the development of coal fired ultrasupercritical steam generators that will operate at efficiencies approaching 50% based on higher heating value (HHV). A test facility designed to evaluate operation of major steam generator components at 700°C is now operational in Germany. A focus of these efforts has been the demonstration of reliability of the materials that would be used in an ultrasupercritical steam generator. However, proof of the viability of ultrasupercritical technology can be found at the Eddystone plant in Pennsylvania. The steam generator for unit 1 at that plant has operated successfully for >45 years in steam conditions more advanced than any other central station unit in operation today. The operating history is briefly reviewed, focusing on the materials related problems that forced a modest retreat from the original, unprecedented design conditions, but emphasising the record of many years of reliable operation. Some significant materials related issues involved in operating a steam generator at 'Eddystone' like conditions are used as a basis to argue that there exists today the materials and manufacturing understanding necessary to construct a more advanced ultrasupercritical unit that will operate efficiently and reliably.     

Fireside issues in advanced power generation systems

Abstract

The traditional trend towards the development and use of power plants with ever increasing efficiencies is now being coupled to the use of a wider range of fuels and technologies designed to minimise CO₂ emissions. Alternative solid fuels such as biomass and waste products, which can be classified as CO₂ neutral, are being used alone or cofired with fossil fuels. The cofiring of biomass and coal is currently the most efficient and effective method for using biomass to generate power. CO₂ capture technologies include systems for either precombustion or postcombustion CO₂ removal. Gasification of fuels (using either oxygen or steam as the oxidant) produces a gas that can be conditioned to enable precombustion CO₂ removal. Post-combustion CO₂ capture can be carried out using either solid or aqueous sorbent processes. Oxy firing of fuels is a technology that would enable more efficient post-combustion CO₂ capture. The various combinations of new fuels, novel technologies and higher temperature component operating conditions are producing challenging operating environments for components. Deposition, erosion and corrosion issues for hot gas path components in these advanced power generating systems, which are potentially life limiting, are reviewed. Reduction in heat transfer owing to high rates of deposition can significantly reduce heat transfer and increase the need for component cleaning. Depending on the system, component parts can include various heat exchangers, gas cleaning systems and gas turbines.     

Ultrasupercritical power plant development and high temperature materials applications in China

Abstract

The rapid development of Chinese economy demands sustainable growth of power generation to meet industrial and domestic demand. The total installed capacity of electricity and annual overall electricity generation are now both the second highest in the world, close to those of the USA. Forecasts of China's electricity demand over the period 2010–20 are presented. Chinese power plants, like those worldwide, are facing demands to increase thermal efficiency and to decrease the emission of CO₂, SO_X and NO_X. In light of the national resource of coal and electricity market requirements in the next 15 years, power generation – especially ultrasupercritical (USC) power plants with the steam temperature over 600°C – will undergo rapid development. The first 1000 MW USC power unit, with steam parameters 600°C, 26·25 MPa, entered service in November 2006. It is estimated that more than 350 USC power units will be installed in China by 2020. USC power plant designs will adopt a variety of qualified high temperature materials for boiler and turbine manufacturing applications. Among these materials, the modified 9–12%Cr ferritic steels, Ni–Cr austenitic steels and certain nickel base superalloys have received special attention in the Chinese materials market.     

Solving corrosion problems in biofuels industry

Abstract

Ethanol is increasingly popular as fuel. It is renewable, cleaner than gasoline, and during the 2008 oil price peak, its costs of production without subsidies actually rivalled those of gasoline. The fuel grade ethanol producers are large consumers of stainless steel, in particular for cookers, fermentation tanks, and distillation columns as well as storage tanks for the finished product. The main reason is that cleanliness is important for the enzymes. However, contaminants play an important role in production as well as storage and distribution. Contaminants are causing a phenomenon called stress corrosion cracking. Biodiesel fuel is a mixture of fatty acid alcohol esters. Modern diesel engines are very sensitive to fuel quality and purity. Biodiesel is unstable, just like butter, which is going rancid in a matter of weeks. Biodiesel degradation is an oxidation process, which among others is speeded up by tiny amounts of metal ions leaching from production and storage vessels. Biodiesel vessels need to be designed in non-degradable materials of construction, as stainless steel. In addition, pretreatment of low cost feedstock and methyl recycling operations use strong acids, which also necessitate stainless steels. Biomass is gaining importance as a source for renewable fuels and hydrogen in addition to heat and electricity. Corrosion issues are numerous and include high temperature corrosion under ash and alkali salt deposits and metal dusting at gasification. Fuel grade ethanol, biodiesel and biomass represent three different corrosion issues, which can be solved using the correct stainless steel.     

An economic and environmental assessment of biomass utilization in lignite‐fired power plants of Greece

The environmental and socio‐economic impacts of biomass utilization by co‐firing with brown coal in an existing thermoelectric unit in Greece or through its pure combustion in a new plant were studied and evaluated in this work. The 125 MW_e lignite‐fired power plant in Ptolemais Power Station (Western Macedonia) was used as reference system. The environmental benefits of the alternative biomass exploitation options were quantified based on the life cycle assessment methodology, as established by SETAC, while the BIOSEM technique was used to carry out socio‐economic calculations. The obtained results showed clear environmental benefits of both biomass utilization alternatives in comparison with the reference system. In addition, co‐firing biomass with lignite in an existing unit outperforms the combustion of biomass exclusively in a new plant, since it exhibits a better environmental performance and it is a low risk investment with immediate benefits. A biomass combustion unit requires a considerably higher capital investment and its benefits are more evident in the long run. Copyright © 2005 John Wiley & Sons, Ltd.     

生物质锅炉输料系统存在的问题及解决方案探讨

秸秆发电作为一种新生事物,是我国近年重点开发建设的可再生能源项目,2006～2008年国家先后投产了一批生物质电厂,生物质电厂为节能减排、资源综合利用、社会主义新农村建设和电源结构调整作出了重要的贡献.但生物质电厂在上料、出灰等技术上仍然存在瓶颈:文章综述了生物质电厂上料系统组成,剖析了上料系统在设计及运行中存在的系统易卡塞,料仓搭桥等问题及原因;结合实际对系统进行了一系列的改造;成功地解决了生物质电厂上料系统存在的技术问题,实现了生物质锅炉长周期连续稳定运行.     

Effects of SnO on growth and physical properties of BSCCO whiskers

Abstract

Superconductor whiskers doped with SnO have been fabricated by annealing a melt quenched (Bi₂Sn₁)-223 precursor using suitable heat treatment cycles. Approximately 5 μm thick, 90 μm wide and 5 mm long whiskers were fabricated, and their physical, electrical and magnetic properties were investigated. Crystallisation activation energies of glass phase fabricated were calculated to be 390 kJ mol^–1 using Kissinger method based on the differential thermal analysis data. The T _c value of the whiskers was found to be 94 K from M–T measurement. The magnetisation of whiskers before superconducting transition increased with decreasing temperature, and after superconducting transition, the magnetisation of whiskers decreased, from positive to negative, due to the diamagnetic nature of superconductivity. The change on magnetisation dependence of applied magnetic fields (M–H) showed that whiskers have paramagnetic–diamagnetic multiphase structure.     

Technoeconomic assessment of beetle kill biomass co-firing in existing coal fired power plants in the Western United States

Widespread mortality of forests in the western United States due to a bark beetle epidemic provides a source of biomass for power generation. This study assessed availability and economics of co-firing beetle kill biomass with coal in power plants in the western U.S. Since biomass may be considered carbon neutral under careful management, co-combustion of biomass with coal provides power plants a way to meet emission reduction requirements, such as those in the EPA Clean Power Plan (CPP). Cost has been a barrier to co-firing, but the economics are altered by emission reduction requirements, such as CPP guidelines. The present study assessed beetle kill biomass availability in national forests in Wyoming and Colorado through Geographic Information System (GIS) analysis of U.S. Forest Service (USFS) data. Power plants near beetle kill mortality were identified as candidates for co-firing. An economic assessment of costs to implement co-firing was conducted. Co-firing reduces the need for the USFS to manage beetle kill trees when they are harvested for energy use, and these mitigated treatment costs were considered as an effective subsidy of co-firing. The results of this analysis include beetle kill availability, costs, and annual CO₂ emissions reductions that can be met by co-firing.     

Key experimental parameters in steam oxidation testing and their impact on interpretation of experimental results

Abstract

The acceptance of materials for extended duration, safety critical power generation applications usually requires several stages of testing and data generation. Simple, short term exposures under nominally constant atmospheres and temperatures can eliminate materials that are grossly unsuitable, but do not differentiate between materials that have broadly acceptable properties. To better differentiate between candidate materials it is desirable to tailor laboratory tests such that they more closely replicate in service conditions. In terms of components that are exposed to steam oxidation degradation mechanisms, this means replicating the steam conditions with an aim of producing oxide scale morphologies similar to that seen in service. Key experimental parameters have been identified, including water chemistry, pressure, steam delivery and flowrate, and a series of steam exposure tests on ferritic (P92), austenitic (Esshete 1250) and superalloy (IN740) material conducted to evaluate their effect on degradation rate and oxide scale morphology. The oxidation rate of the austenitic, and to a lesser extent the ferritic, material was found to be sensitive to the level of dissolved oxygen in the feed water, low (10 ppb) dissolved oxygen levels producing an increase in oxidation rate. The propensity to spall was also found to be reduced at low dissolved oxygen concentrations. In addition, the steam pressure and steam delivery method were shown to affect the oxidation rate and scale morphology for these materials.     

Potential of biomass‐fired combined heat and power plants considering the spatial distribution of biomass supply and heat demand

Combined heat and power (CHP) plants fired by forest wood can significantly contribute to attaining the target of increasing the share of renewable energy production. However, the spatial distribution of biomass supply and of heat demand limits the potentials of CHP production. This article assesses CHP potentials using a mixed integer programming model that optimizes locations of bioenergy plants. Investment costs of district heating infrastructure are modeled as a function of heat demand densities, which can differ substantially. Gasification of biomass in a combined cycle process is assumed as production technology. Some model parameters have a broad range according to a literature review. Monte‐Carlo simulations have therefore been performed to account for model parameter uncertainty in our analysis. The model is applied to assess CHP potentials in Austria. Optimal locations of plants are clustered around big cities in the east of the country. At current power prices, biomass‐based CHP production allows producing around 3% of the total energy demand in Austria. Yet, the heat utilization decreases when CHP production increases due to limited heat demand that is suitable for district heating. Production potentials are most sensitive to biomass costs and power prices. Copyright © 2009 John Wiley & Sons, Ltd.     

Microstructure and high temperature tensile properties of wide gap brazed cobalt based superalloy X-40

Abstract

Wide gap brazing (WGB) of X-40 cobalt based superalloy was conducted in this study using BNi-9 braze alloy with X-40 and IN738 additive alloys. A groove was machined into X-40 bars with a nominal width of 6·35 mm before filler application. Following brazing at 1200°C for 15 min, the microstructure of the as brazed joints was examined using SEM, EDS and nanoindentation technique. Both WGB joints with X-40 and IN783 additive alloys contained primary matrix phase in addition to a number of boron containing phases which assumed either eutectic or discrete forms. Nanoindention testing revealed that these boron containing phases exhibited hardness values several times higher than the base alloy and matrix phase contributing to the embrittlement of the braze joint. Porosity was also observed in both types of WGB braze joints, the degree of which was greatest in the braze joints with IN738 additive alloy. Tensile testing at 950°C showed that the yield strength of both WGB joints was higher than that of the baseline specimens while the ultimate tensile strength of the WGB joints was lower than that of the baseline X-40. The ductility of the WGB joints was significantly inferior to that of the baseline X-40, particularly for WGB with IN 738 additive alloy.     

Simulation of emission performance and combustion efficiency in biomass fired circulating fluidized bed combustors

In this study, the combustion efficiency and the emission performance of biomass fired CFBs are tested via a previously published 2D model [Gungor A. Two-dimensional biomass combustion modeling of CFB. Fuel 2008; 87: 1453–1468.] against two published comprehensive data sets. The model efficiently simulates the outcome with respect to the excess air values, which is the main parameter that is verified. The combustion efficiency of OC changes between 82.25 and 98.66% as the excess air increases from 10 to 116% with the maximum error of about 8.59%. The rice husk combustion efficiency changes between 98.05 and 97.56% as the bed operational velocity increases from 1.2 to 1.5 m s^?1 with the maximum error of about 7.60%. CO and NO_x emissions increase with increasing bed operational velocity. Increasing excess air results in slightly higher levels of NO_x emission. A significant amount of combustion occurs in the upper zone due to the high volatile content of the biomass fuels.     

Predicting lifetimes of components in power station engineering plant

Abstract

Today, there is an increasing demand for power station plant to be operated cost effectively and to reliably maintain required electrical supply. This requires minimising the risk of having to shut down the plant for emergency repairs. Of paramount importance is plant operating integrity and well prepared and executed maintenance programmes. The present paper reports on the recent developments to the auto-reference creep management and control system used by E.ON UK. This includes achieving biaxial strain measurement with increased resolution and the employment of digital image correlation.     

Biomass assessment and small scale biomass fired electricity generation in the Green Triangle, Australia

Coal fired electricity is a major factor in Australia’s greenhouse gas emissions (GHG) emissions. The country has adopted a mandatory renewable energy target (MRET) to ensure that 20% of electricity comes from renewable sources by 2020. In order to support the MRET, a market scheme of tradable Renewable Energy Certificates (RECs) has been implemented since 2001. Generators using biomass from eligible sources are able to contribute to GHG emission reduction through the substitution of coal for electricity production and are eligible to create and trade RECs. This paper quantifies the potential biomass resources available for energy generation from forestry and agriculture in the Green Triangle, one of the most promising Australian Regions for biomass production. We analyse the cost of electricity generation using direct firing of biomass, and estimate the required REC prices to make it competitive with coal fired electricity generation. Major findings suggest that more than 2.6 million tonnes of biomass are produced every year within 200 km of the regional hub of Mount Gambier and biomass fired electricity is viable using feedstock with a plant gate cost of 46 Australian Dollars (AUD) per tonne under the current REC price of 34 AUD per MWh. These findings are then discussed in the context of regional energy security and existing targets and incentives for renewable energies.     

A new monitoring system for piping systems in fossil fired power plants

A CEC-funded project has been performed to tackle the problem of producing an advanced Life Monitoring System (LMS) which would calculate the creep and fatigue damage experienced by high temperature pipework components. Four areas were identified where existing Life Monitoring System technology could be improved:

1. 1. the inclusion of creep relaxation

2. 2. the inclusion of external loads on components

3. 3. a more accurate method of calculating thermal stresses due to temperature transients

4. 4. the inclusion of high cycle fatigue terms.

The creep relaxation problem was solved using stress reduction factors in an analytical in-elastic stress calculation. The stress reduction factors were produced for a number of common geometries and materials by means of non-linear finite element analysis. External loads were catered for by producing influence coefficients from in-elastic analysis of the particular piping system and using them to calculate bending moments at critical positions on the pipework from load and displacement measurements made at the convenient points at the pipework. The thermal stress problem was solved by producing a completely new solution based on Green's Function and Fast Fourier transforms. This allowed the thermal stress in a complex component to be calculated from simple non-intrusive thermocouple measurements made on the outside of the component. The high-cycle fatigue problem was dealt with precalculating the fatigue damage associated with standard transients and adding this damage to cumulative total when a transient occurred.

The site testing provided good practical experience and showed up problems which would not otherwise have been detected.     

Reducing cycling costs in coal fired power plants through power to hydrogen

The increase of renewable share in the energy generation mix makes necessary to increase the flexibility of the electricity market. Thus, fossil fuel thermal power plants have to adapt their electricity production to compensate these fluctuations. Operation at partial load means a significant loss of efficiency and important reduction of incomes from electricity sales in the fossil power plant. Among the energy storage technologies proposed to overcome these problems, Power to Gas (PtG) allows for the massive storage of surplus electricity in form of hydrogen or synthetic natural gas. In this work, the integration of a Power to Gas system (50 MWe) with fossil fuel thermal power plants (500 MWe) is proposed to reduce the minimum complaint load and avoid shutdowns. This concept allows a continuous operation of power plants during periods with low demand, avoiding the penalty cost of shutdown. The operation of the hybrid system has been modelled to calculate efficiencies, hydrogen and electricity production as a function of the load of the fossil fuel power plant. Results show that the utilisation of PtG diminishes the specific cost of producing electricity between a 20% and 50%, depending on the framework considered (hot, warm and cold start-up). The main contribution is the reduction of the shutdown penalties rather than the incomes from the sale of the hydrogen. At the light of the obtained results, the hybrid system may be implemented to increase the cost-effectiveness of existing fossil fuel power plants while adapting the energy mix to high shares of variable renewable electricity sources.     

Evaluation of oxygen as oxidant in coal fired open cycle MHD-steam power plants

This study compares the thermal efficiency and economics of using oxygen rather than air as the oxidant in large coal-fired MHD-steam energy conversion plants, using a computer model to calculate thermal efficiency. The systems compared are a coal-air system with a thermal input of 2000 MW_th and two coal-oxygen systems, one with an input of 2000 MW_th and one with 6600 MW_th. The paper describes the process; compares flame temperature, electrical conductivity, and specific enthalpy; and presents Mollier diagrams for the two systems. At an oxidant preheat temperature of 1644 K, the net thermal efficiency of the coal-oxygen system is 8 to 9 percentage points lower than that for the coal-air system, if the power required to produce the oxygen is taken into account; however, despite its lower thermal efficiency, the coal-oxygen system has a lower cost of electricity. At a preheat temperature of 1644 K, the cost advantage is small, but at temperatures below 1200 K, the cost advantage is significant.     

Application of modified Kalina cycle in biomass chp plants

This paper presents alternatives to Kalina cycles typically used in place of the organic Rankine cycle in biomass power plants. Overviews of both Rankine and Kalina cycles are given alongside the possibilities of using biomass as a viable energy source and recommended guidelines from the engineering practice for selection and management of these cycles. Benefits of Kalina novel bottoming cycle (and the alternative cycles presented herewith) over the Rankine cycle are the higher thermodynamic cycle efficiency and lower capital expenditures combined with the possibility of using low-grade heat sources, such as biomass or waste heat from exhaust gases. Analysis of ammonia-water binary system under various operating conditions has been performed for all the proposed cycles based on the published references and it has been shown that the proposed alternative models prove to be simpler and to have similar or even greater thermodynamic efficiency compared with the Kalina novel bottoming cycle.     

Gas phase embrittlement and time dependent cracking of nickel based superalloys

Abstract

Nickel based superalloys are critical to the safe operation of many energy conversion systems operating at high temperatures. Time dependent intergranular cracking of these alloys, under both sustained and cyclic loads, is dominated by environmental interactions at the crack tip. This review is concerned mainly with the interaction of oxygen in alloys used for combustion turbine discs, although interactions with other more aggressive species are considered. The phenomenology of this cracking is shown to be consistent with the same mechanism as that associated with oxygen embrittlement resulting from pre-exposure of uncracked material, and also with environmentally induced reduction in creep rupture life. Gas phase embrittlement (GPE), resulting from intergranular oxygen penetration, is shown to be responsible for all four streams of experimental observations. Three distinct processes of intergranular embrittlement involving oxidation reactions have been confirmed experimentally. One of these, the oxidation of intergranular sulphides, results in elemental sulphur embrittlement and subsequent local decohesion under stress. The other two, oxidation of carbon or carbides to form carbon dioxide gas bubbles and oxidation of strong oxide formers to form intergranular internal oxides, result in a reduction of the local ability to accommodate stress concentrations associated with sliding grain boundaries in an intermediate temperature range. This in turn leads to a temperature dependent minimum in ductility and maximum in crack propagation rate. Attempts to reduce the sensitivity to time dependent cracking based on chemistry (chromium level or trace element addition), microstructure control (using thermal–mechanical treatment or controlled cooling), or reversal of environmental embrittlement, are all considered. The conclusions form a basis for the development of life prediction methods for energy materials operating in diverse environments.