A comprehensive fireside corrosion study was undertaken to better understand the corrosion mechanisms operating on the superheaters and lower furnace walls of advanced coal- fired utility boilers. The study intended to evaluate the fireside conditions generated from burning eight U.S. coals individually in a pilot-scale combustion facility. These coals consisted of a wide range of compositions that are of interest to the utility industry. The combustion facility was capable of producing the realistic conditions of staged combustion existing in coal-fired utility boilers. During each of the combustion tests, gas and deposit samples were collected and analyzed via in-furnace probing at selected locations corresponding to the waterwalls and superheaters. Testing of five of the eight coal groups has been completed to date. Results of these online measurements helped reveal the dynamic nature of the combustion environments produced in coal-fired boilers. Coexistence of reducing and oxidizing species in the gas phase was evident in both combustion zones, indicating that thermodynamic equilibrium of the overall combustion gases was generally unattainable. However, the amount of sulfur released from coal to form sulfur-bearing gaseous species in both the reducing and oxidizing zones was in a linear relationship with the amount of the total sulfur in coal, independent of the original sulfur forms. Such a linear relationship was also observed for the measured HCl gas relative to the coal chlorine content. However, the release of sulfur from coal to the gas phase appeared to be slightly faster and more complete than that of chlorine in the combustion zone, while both sulfur and chlorine were completely released and reacted to form respective gaseous species in the oxidizing zone. The information of sulfur and chlorine release processes in coal combustion generated from this study is considered new to the industry and provides valuable insight to the understanding of fireside corrosion mechanisms.

A combined pilot-scale combustion test and long-term laboratory study investigated the impact of oxy-firing on corrosion in coal-fired boilers. Four coals were burned under both air and oxy-firing conditions with identical heat input, with oxy-firing using flue gas recirculation unlike air-firing. Despite higher SO 2 and HCl concentrations in oxy-firing, laboratory tests showed no increase in corrosion rates compared to air-firing. This is attributed to several factors: (1) Reduced diffusion: High CO 2 in oxy-firing densified the gas phase, leading to slower diffusion of corrosive species within the deposit. (2) Lower initial sulfate: Oxy-fired deposits initially contained less sulfate, a key hot corrosion culprit, due to the presence of carbonate. (3) Reduced basicity: CO 2 and HCl reduced the basicity of sulfate melts, leading to decreased dissolution of metal oxides and mitigating hot corrosion. (4) Limited carbonate/chloride formation: The formation of less corrosive carbonate and chloride solutes was restricted by low O 2 and SO 3 near the metal surface. These findings suggest that oxy-firing may not pose a greater corrosion risk than air-firing for boiler materials.

The combustion of coal and biomass fuels in power plants generates deposits on the surfaces of superheater / reheater tubes that can lead to fireside corrosion. This type of materials degradation can limit the lives of such tubes in the long term, and better methods are needed to produce predictive models for such damage. This paper reports on four different approaches that are being investigated to tackle the challenge of modelling fireside corrosion damage on superheaters / reheaters: (a) CFD models to predict deposition onto tube surfaces; (b) generation of a database of available fireside corrosion data; (c) development of mechanistic and statistically based models of fireside corrosion from laboratory exposures and dimensional metrology; (d) statistical analysis of plant derived fireside corrosion datasets using multi-variable statistical techniques, such as Partial Least Squares Regression (PLSR). An improved understanding of the factors that influence fireside corrosion is resulting from the use of a combination of these different approaches to develop a suite of models for fireside corrosion damage.

Oxyfuel combustion is considered as one of the most promising technologies to facilitate CO 2 capture from flue gases. In oxyfuel combustion, the fuel is burned in a mixture of oxygen and recirculated flue gas. Flue gas recirculation increases the levels of fireside CO 2 , SO 2 , Cl and moisture, and thus promotes fouling and corrosion. In this paper the corrosion performance of two superheater austenitic stainless steels (UNS S34710 and S31035) and one Ni base alloy (UNS N06617) has been determined in laboratory tests under simulated oxyfuel conditions with and without a synthetic carbonate based deposits (CaCO 3 - 15 wt% CaSO 4 , CaCO 3 - 14wt% CaSO 4 - 1 KCl) at 650 and 720°C up to 1000 hours. No carburization of the metal substrate was observed after exposure to simulated oxyfuel gas atmospheres without deposit, although some carbon enrichment was detected near the oxide metal interface. At 720°C a very thin oxide formed on all alloy surfaces while the weight changes were negative. This negative weight change observed is due to chromium evaporation in the moist testing condition. At the presence of deposits, corrosion accelerated and considerable metal loss of austenitic alloys was observed at 720°C. In addition, clear carburization of austenitic steel UNS S34710 occurred.

Hydrogen as a clean fuel is increasingly being used to propel gas turbines and to power combustion engines. Metallic materials including Ni-based alloys are commonly used in conventional gas turbines and combustion engines. However, hydrogen may cause embrittlement in these materials, depending on their chemical composition. In this work, the hydrogen embrittlement behavior of Ni-based alloys containing up to 50 wt.% Fe has been investigated using slow strain rate tensile testing, under cathodic hydrogen charging at room temperature. It was found that the larger the Ni equivalent concentration in an alloy, the more severe the hydrogen embrittlement. It was also found that solid solution alloys have less severe hydrogen embrittlement than precipitation alloys of the same Ni equivalent concentration. In solid solution alloys, hydrogen embrittlement led to cleavage type fracture, which is in line with literature where hydrogen enhanced planar deformation. In precipitation alloys, hydrogen embrittlement resulted in a typical intergranular fracture mode.

Recently, boiler waterwall tube damage such as fireside corrosion and circumferential cracking in low NOx environments has become a serious issue in Japan, despite the typical use of relatively lower sulfur content coal is typically being used than in US. Thermal spray coating has been the most popular method for tube protection in Japan, and thermal spray coated tubes have been used for this purpose. However, extensive damage to thermal spray coating tubes from cracking and exfoliation has been recently experienced. It has been reported that the thermal fluctuations occurring due to operational changes create alternating stress, leading to cracking and exfoliation of the thermal sprayed thin coating. Corrosion-resistant weld overlays, such as Type 309 stainless steel (in sub-critical boilers) and Alloy 622 (in sub-critical and super-critical boilers), are commonly used to protect boiler tubes from corrosion in low NOx coal fired boilers in U.S. In order to develop a fundamental understanding of the high temperature corrosive behavior of Alloy 622 weld overlay, gaseous corrosion testing and certain mechanical tests for consideration of long-term aging were undertaken. After four years of service in the low NOx combustion environment of a coal fired supercritical boiler, field tests on Alloy 622 weld overlay panels are in continuation. This paper describes the field test behavior of Alloy 622 weld overlay panels installed in a Japanese supercritical boiler, the laboratory results of weight loss corrosion testing, and the results of cyclic bend tests with overlay welded tubes related to aging.

While the water vapor content of the combustion gas in natural gas-fired land based turbines is ~10%, it can be 20-85% with coal-derived (syngas or H 2 ) fuels or innovative turbine concepts for more efficient carbon capture. Additional concepts envisage working fluids with high CO 2 contents to facilitate carbon capture and sequestration. To investigate the effects of changes in the gas composition on thermal barrier coating (TBC) lifetime, furnace cycling tests (1h cycles) were performed in air with 10, 50 and 90 vol.% water vapor and in CO 2 -10%H 2 O and compared to prior results in dry air or O 2 . Two types of TBCs were investigated: (1) diffusion bond coatings (Pt diffusion or simple or Pt-modified aluminide) with commercially vapor-deposited yttria-stabilized zirconia (YSZ) top coatings on second-generation superalloy N5 and N515 substrates and (2) high velocity oxygen fuel (HVOF) sprayed MCrAlYHfSi bond coatings with air-plasma sprayed YSZ top coatings on superalloy X4 or 1483 substrates. In both cases, a 20-50% decrease in coating lifetime was observed with the addition of water vapor for all but the Pt diffusion coatings which were unaffected by the environment. However, the higher water vapor contents in air did not further decrease the coating lifetime. Initial results for similar diffusion bond coatings in CO 2 -10%H 2 O do not show a significant decrease in lifetime due to the addition of CO 2 . Characterization of the failed coating microstructures showed only minor effects of water vapor and CO 2 additions that do not appear to account for the observed changes in lifetime. The current 50°-100°C de-rating of syngas-fired turbines is unlikely to be related to the presence of higher water vapor in the exhaust.

Nitridation is a high-temperature material degradation issue that can occur in air and in environments containing nitrogen, ammonia, etc., and in a variety of industrial processes. The nitridation behavior of several commercial nickel- and cobalt-based alloys is reviewed in this paper. The alloys include Haynes 230, Haynes 188, Haynes 625, Haynes 617, Haynes 214, Hastelloy X, and Haynes 233. The environments discussed are high-purity nitrogen gas between 871°C and 1250°C, 100% ammonia gas at 982°C and 1092°C, and a simulated combustion atmosphere at 982°C. The results showed that nitridation occurred in all the environments containing nitrogen. The nitridation attack was strongly influenced by the alloy compositions and the type of oxide formed (i.e., chromia or alumina), as some degree of oxidation was expected in the environments in which residual oxygen was present. Thermal cycling is briefly discussed because the integrity of protective oxides is also an important factor in resisting high-temperature oxidation and nitridation attack.

The high-temperature oxidation of Fe-9Cr-1Mo steel in a CO 2 environment, with varying oxygen content (0.6-3%), was investigated at 700°C. While the steel heavily oxidized in pure CO 2 , the oxidation mass gain decreased significantly with increasing oxygen content. Microscopic analysis revealed the formation of Fe-rich nodules with an internal Cr-carbide layer beneath them. Notably, the number of nodules decreased with increasing oxygen content but remained independent of the oxidation time. To explain these observations, the authors propose that “intrinsic” defects within the initially formed protective Cr 2 O 3 scale facilitated gas permeation. This mechanism is believed to be responsible for the observed dependence of nodule formation on the oxygen content in the gas mixtures.

This paper outlines a comprehensive UK-based research project (2007-2010) focused on developing fireside corrosion models for heat exchangers in ultra-supercritical plants. The study evaluates both conventional materials like T22 and advanced materials such as Super 304H, examining their behavior under various test environments with metal skin temperatures ranging from 425°C to 680°C. The research aims to generate high-quality data on corrosion behavior for materials used in both furnace and convection sections, ultimately producing reliable corrosion prediction models for boiler tube materials operating under demanding conditions. The project addresses some limitations of existing models for these new service conditions and provides a brief review of the fuels and test environments used in the program. Although modeling is still limited, preliminary results have been presented, focusing on predicting fireside corrosion rates for furnace walls, superheaters, and reheaters under various service environments. These environments include those created by oxyfuel operation, coal-biomass co-firing, and more traditional coal firing.

For future carbon neutral society, a novel thermal power generation system with no CO 2 emission and with extremely high thermal efficiency (~ 70 %) composed of the oxygen/hydrogen combustion gas turbine combined with steam turbine with the steam temperature of 700°C is needed. The key to realize the thermal power plant is in the developments of new wrought alloys applicable to both gas turbine and steam turbine components under higher temperature operation conditions. In the national project of JST-Mirai program, we have constructed an innovative Integrated Materials Design System , consisting of a series of mechanical property prediction modules (MPM) and microstructure design modules (MDM). Based on the design system, novel austenitic steels strengthened by Laves phase with an allowable stress higher than 100 MPa for 10 5 h at 700°C was developed for the stream turbine components. In addition, for gas turbine components, novel solid-solution type Ni-Cr-W superalloys were designed and found to exhibit superior creep life longer than 10 5 h under 10 MPa at 1000°C. The superior long-term creep strengths of these alloys are attributed to the “grain-boundary precipitation strengthening (GBPS)” effect due to C14 Fe 2 Nb Laves phase and bcc α 2 -W phase precipitated at the grain boundaries, respectively.

A recent engineering design study conducted by the Electric Power Research Institute (EPRI) has compared the cost and performance of an advanced ultra-supercritical (A-USC) pulverized coal (PC) power plant with main steam temperature of 700°C to that of conventional coal-fired power plant designs: sub-critical, supercritical, and current USC PC plants with main steam temperatures of 541°, 582°, and 605°C, respectively. The study revealed that for a US location in the absence of any cost being imposed for CO 2 emissions the A-USC design was a slightly more expensive choice for electricity production. However, when the marginal cost of the A-USC design is compared to the reduction in CO 2 emissions, it was shown that the cost of the avoided CO 2 emissions was less than $25 per metric ton of CO 2 . This is significantly lower than any technology currently being considered for CO 2 capture and storage (CCS). Additionally by lowering CO 2 /MWh, the A-USC plant also lowers the cost of CCS once integrated with the power plant. It is therefore concluded that A-USC technology should be considered as one of the primary options for minimizing the cost of reducing CO 2 emissions from future coal power plants.

Key components within gas turbines, such as the blades, can be susceptible to a range of degradation mechanisms, including hot corrosion. Hot corrosion type mechanisms describe a sequence of events that include the growth and fluxing of protective oxide scales followed by the degradation of the underlying coating/alloy; this can significantly reduce component lifetimes. To better understand the progress of this type of damage mechanism, a model of hot corrosion progression with both time and corrosive deposit flux is presented for IN738LC and compared to experimental test data collected at 700 °C for four different deposit fluxes. One approach to the interpolation of model parameters between these four fluxes is illustrated. Of particular importance is that the model accounts for the statistical variation in metal loss though the use of Weibull statistics.

This paper discusses the design of a prototype for accurately inspecting the degree of wall thinning in boiler tubes, which plays a critical role in power plants. The environment in power plants is characterized by extreme conditions such as high temperatures, high pressure, and ultrafine dust (carbides), making the maintenance and inspection of boiler tubes highly complex. As boiler tubes are key components that deliver high-temperature steam, their condition critically affects the efficiency and safety of the power plant. Therefore, it is essential to accurately measure and manage the wall thinning of boiler tubes.

The manufacture of large, complex components for ultra-supercritical and oxy-combustion applications will be extremely costly for industry over the next few decades as many of these components will be manufactured from expensive, high strength, nickel-based alloys casting and forgings. The current feasibility study investigates the use of an alternative manufacturing method, powder metallurgy and hot isostatic processing (PM/HIP), to produce high quality, and potentially less expensive components for power generation applications. Benefits of the process include manufacture of components to near-net shapes, precise chemistry control, a homogeneous microstructure, increased material utilization, good weldability, and improved inspectability.

The INCONEL filler metals 72 and 72M have been utilized significantly for weld overlay protection of superheaters and reheaters, offering enhanced corrosion and erosion resistance in this service. Laboratory data conducted under simulated low-NOx combustion conditions, field exposure experience, and laboratory analysis (microstructure, chemical composition, overlay thickness measurements, micro-hardness) of field-exposed samples indicate that these overlay materials are also attractive options as protective overlays for water wall tubes in low-NOx boilers. Data and field observations will be compared for INCONEL filler metals 72, 72M, 625 and 622.

Along with rapid development of thermal power industry in mainland China, problems in metal materials of fossil power units also change quickly. Through efforts, problems such as bursting due to steam side oxide scale exfoliation and blocking of boiler tubes, and finned tube weld cracking of low alloy steel water wall have been solved basically or greatly alleviated. However, with rapid promotion of capacity and parameters of fossil power units, some problems still occur occasionally or have not been properly solved, such as weld cracks of larger-dimension thick-wall components, and water wall high temperature corrosion after low-nitrogen combustion retrofitting.

Preface for the 2013 Advances in Materials Technology for Fossil Power Plants conference.

The Advanced Ultrasupercritical (A-USC) power plants are aimed to operate at steam inlet temperatures greater than 700°C; consequently, a complete materials overhaul is needed for the next-generation power plants. HAYNES 282, a gamma-prime strengthened alloy, is among the leading candidates because of its unique combination of properties, superior creep and LCF strength, fabricability and thermal stability. It is currently being evaluated in wrought and cast forms for A-USC turbine rotors, casings, boiler tubings, header, and valves. The candidate materials for A-USC applications not only require oxidation resistance for steam cycles but fireside corrosion resistance to coal ash is also of an extreme importance. In order to study the effect of both environments on the performance of 282 alloy, the alloy was exposed for extended periods in various oxidizing environments, such as air, air plus water vapor (10%), and 17bar steam up to 900°C. The fireside corrosion resistance of 282 alloy was evaluated at 700°C in synthetic coal ash and at 843°C in alkali salt deposits in a controlled gaseous environment.

The EU NextGenPower-project aims at demonstrating Ni-alloys and coatings for application in high-efficiency power plants. Fireside corrosion lab and plants trials show that A263 and A617 perform similar while A740H outperforms them. Lab tests showed promising results for NiCr, Diamalloy3006 and SHS9172 coatings. Probe trials in six plants are ongoing. A617, A740H and A263 performed equally in steamside oxidation lab test ≤750°C while A617 and A740H outperformed A263 at 800°C; high pressure tests are planned. Slow strain rate testing confirmed relaxation cracking of A263. A creep-fatigue interaction test program for A263 includes LCF tests. Negative creep of A263 is researched with gleeble tests. A263 Ø80 - 500mm trial rotors are forged with optimized composition. Studies for designing and optimizing the forging process were done. Segregation free Ø300 and 1,000mm rotors have been forged. A263 – A263 and A293 – COST F rotor welding show promising results (A263 in precipitation hardened condition). Cast step blocks of A282, A263 and A740H showed volumetric cracking after heat treatment. New ‘as cast’ blocks of optimized composition are without cracks. A 750°C steam cycle has been designed with integrated CO 2 capture at 45% efficiency (LHV). Superheater life at ≤750°C and co-firing is modeled.