John E. Oakey

Gas turbines: gas cleaning requirements for biomass-fired systems

Increased interest in the development of renewable energy technologies has been hencouraged by the introduction of legislative measures in Europe to reduce CO2 emissions from power generation in response to the potential threat of global warming. Of these technologies, biomass-firing represents a high priority because of the modest risk involved and the availability of waste biomass in many countries. Options based on farmed biomass are also under development. This paper reviews the challenges facing these technologies if they are to be cost competitive while delivering the supposed environmental benefits. In particular, it focuses on the use of biomass in gasification-based systems using gas turbines to deliver increased efficiencies. Results from recent studies in a European programme are presented. For these technologies to be successful, an optimal balance has to be achieved between the high cost of cleaning fuel gases, the reliability of the gas turbine and the fuel flexibility of the overall system. Such optimisation is necessary on a case-by-case basis, as local considerations can play a significant part.

Fireside issues in advanced power generation systems

Abstract The requirements to supply increasing quantities of electricity and simultaneously to reduce the environmental impact of its production are currently major issues for the power generation industry. Routes to meeting these challenges include the development and use of power plants with ever increasing efficiencies coupled with the use of both a wider range of fuels and technologies designed to minimise CO2 emissions. For fireside hot gas path components, issues of concern include deposition, erosion and corrosion in novel operating environments and increased operating temperatures. The novel operating environments will be produced both by the use of new fuel mixes and by the development of more complex gas pathways (e.g. in various oxyfired or gasification systems). Higher rates of deposition could significantly reduce heat transfer and increase the need for component cleaning. However, degradation of component surfaces has the potential to be life limiting, and so such effects need to be minimised. Materials and operational issues related to these objectives are reviewed.

Degradation of heat exchanger materials under biomass co-firing conditions

Abstract Co-firing biomass in conventional pulverised coal fired power stations offers a means to rapidly introduce renewable and CO2 neutral biomass fuels into the power generation market. Existing coalfired power stations are both much larger and more efficient than current designs of new biomass combustion systems, so feeding a few percent of biomass feed into an existing large coal fired station will give more biomass derived power than a new dedicated biomass station. Co-firing levels started at ∼2% biomass, but this has increased to ∼5–10% biomass, with higher levels of biomass co-firing being investigated, although supply of biomass becomes an issue with increasing co-firing levels. The lower levels of biomass co-firing (up to ∼5%) can be achieved with relatively minor modifications to existing plants, so avoiding the large capital costs and risks of building new biomass-only fired power systems. However higher levels of co-firing are more difficult to achieve, requiring dedicated biomass supply systems and burners. For existing coal-fired power stations, the co-firing of biomass causes some practical problems, e.g.: the control of co-firing two fuels; changes to bottom/fly ash chemistry; changes to deposition (fouling and slagging) within the boiler; reduced reliability of key high temperature components (e.g. heat exchangers) due to increased corrosion problems relative to those experienced with coal alone. This paper reports the results of assessments carried out to evaluate the potential operating conditions of heat exchangers in combustion systems with biomass (wood or straw) and coal cofiring, as well as laboratory corrosion tests that have been carried out to give an initial assessment of potential effects of biomass-co-firing. The corrosion tests have been carried out using the deposit recoat method in controlled atmosphere furnaces. A series of 1000 hour tests have been carried out at typical superheater and evaporator metal temperatures using simulated deposit compositions and gaseous environments (selected on the basis of plant experience and potential fuel compositions). Five materials were exposed in these tests: 1Cr steel, T22 steel, X20CrMoV121, TP347HFG and alloy 625. In order to produce statistically valid data on the actual metal loss from the materials, the performance of the materials in these tests was determined from dimensional metrology before and after exposure. For each material, these data have been used to determine the sensitivity of the corrosion damage to changes in the exposure conditions (e.g. deposit composition, gas composition) thereby producing initial models of the corrosion performance of the materials. The corrosion data and model outputs have been compared with data available from power plants operating on coal, straw or wood fuels.

Erosion-corrosion modelling of gas turbine materials for coal-fired combined cycle power generation

Abstract The development of coal-fired combined cycle power generation systems is receiving considerable worldwide interest. The successful development and commercialisation of these new systems require that all the component parts are manufactured from appropriate materials and that these materials give predictable in-service performance. Corrosion and erosion-corrosion, resulting from coal derived particulates, deposition and gaseous species, have been identified as potential life limiting factors for these systems. Models to predict these modes of materials degradation are under active development. This paper outlines the development and testing of models suitable for use in gas turbine environments. The complexity of the corrosion processes means that an empirical approach to model development is required whereas a more mechanistic approach can be applied to erosion processes. For hot corrosion conditions, statistically based corrosion models have been produced using laboratory tests for two coatings and a base alloy at typical type I and type II hot corrosion temperatures (900 and 700°C). These models use the parameters of alkali sulphate deposition flux and SO x partial pressure (at each temperature and for set HCl partial pressures), to predict the rate of the most likely localised damage associated with hot corrosion reactions. For erosion-corrosion modelling, a series of laboratory tests have been carried out to investigate erosion behaviour in corrosive conditions appropriate to coal-fired gas turbines. Materials performance data have been obtained from samples located in the hot gas path of the Grimethorpe PFBC pilot plant, under well characterised conditions, for testing the corrosion and erosion-corrosion models. The models successfully predict the materials damage observed in the pilot plant environments.

Hot gas cleaning—materials and performance

Hot gas cleaning systems for the removal of contaminants from combustion and gasification gas streams are critical components in the next generation of advanced coal/biomass/waste-fired combined cycle plants. The contaminants of concern are those environmental pollutants which would otherwise exceed emissions legislation and those which would directly impact on the life or operability of the gas turbine used in the cycle. It is essential that these ‘solid fuel’ plants can provide gases suitable for the advanced, high inlet temperature gas turbines being developed currently for natural gas combined cycle plants. Regardless of the gas turbine, the application of hot gas cleaning in solid fuel combined cycle plants leads to increased cycle efficiencies over those achievable with conventional wet scrubbing techniques and greatly reduces any waste water disposal problems. The introduction of systems for particulate removal, alkali reduction, H2S/SOx, reduction, NH3/NOx(reduction, HCI removal etc. into the adva...

Development of oxides at TBC - bond coat interfaces in burner rig exposures

Abstract There is a desire to use gases derived from increasingly ‘dirty’ fuels (e.g. coal and biomass) in industrial gas turbines. The contaminants in these fuels have the potential to cause significant damage to the gas turbine hot gas path materials, many of which were developed and selected for natural gas fired conditions. This paper reports results of a study investigating the performance of thermal barrier coatings (TBCs) and bond coatings, applied to current industrial gas turbine materials, within clean and ‘dirty’ gas environments generated within a burner rig. The materials covered by this study included: • TBCs based on 8%Y2O3–ZrO2 applied by both air plasma sprayed (APS) and electron beam – physical vapour deposition (EB–PVD) routes.• Bond coats of the overlay and diffusion classes, applied by vacuum plasma spraying (VPS), electroplating (EP), chemical vapour deposition (CVD) and high velocity oxy-fuel (HVOF) spraying• Base alloys of IN6203, CMSX-4 and Haynes 230 The required TBC/bond coat combinations were applied by commercial coating processes to cylindrical samples of base alloys manufactured for use in a burner rig. The burner rig used in this study is designed to enable air-cooled probes of cylindrical samples to be exposed to a natural gas combustion environment. In this study, this enabled specific metal temperatures (~800 and ~900°C) to be targeted within a much higher temperature combustion gas stream (~1150°C). ‘Dirty’ fuel gas environments were simulated by introducing gaseous (SO2 and HCl) and vapour phase (Na, K, Pb, Zn) contaminants into the burner rig just upstream of the edge of the gas flame. These conditions enabled continuous tests to be performed for 1,000 hours in both natural gas and ‘dirty’ fuel environments. The relative performance of the materials was determined from cross-sections prepared after the 1000 hour exposures. These cross-sections were examined by optical and SEM/EDX to determine the thicknesses of the oxides at the TBC – bond coat interfaces, the morphologies of these interfaces and to characterise the elemental distributions in these regions.

Corrosion in Biomass Combustion Systems

There is growing concern over the effects of global warning. In response the power generation sector is having to consider a wider range of systems and fuels for use in generating heat and power. One of the classes of solid fuels that is being increasingly developed is biomass, which is regarded a both sustainable and carbon neutral. In fact, the term biomass covers a wide range of fuels from waste products, such as straw, forestry wastes and sawdust, through to purpose grown energy crops, such as coppiced willow and miscanthus. To maximise combustion plant efficiency it is necessary to use high temperature/pressure steam turbines. However, to generate such steam conditions, the high heat exchanger surface temperatures can interaction with the various potential products of biomass combustion to cause excessive deposition and corrosion of these surfaces. This paper considers the range of heat exchanger operating environments that can be produced by the combustion of different potential biomass fuels, especially the effects of the higher K and Cl contents of the faster growing biomass fuels. This paper reports the results of a series of laboratory corrosion tests that have been carried out to assess the effects of various types of biomass on the corrosion of high temperature heat exchanger materials in combustion plants. The corrosion tests have been carried out using the deposit recoat method in controlled atmosphere furnaces. Six 1000 hour tests have been carried out at typical superheater / reheater and evaporator conditions (450-600°C) using simulated deposit and gas compositions, which have been selected on the basis of potential biomass fuel compositions. The five metals exposed in this study are widely used in power plant heat exchangers: 1% Cr steel, 2.25% Cr steel (T23), 9% Cr steel (T91), X20CrMoV121, TP347HFG and alloy 625. During the course of the tests, the material degradation was monitored using traditional mass change measurements. In order to produce statistically valid data on the actual metal loss from the materials, the performance of the materials in these tests was determined from dimensional metrology before and after exposure: pre-exposure measurements were made using a micrometer; post-exposure measurements were made using an image analyser system. SEM/EDX and XRD analyses have been used to confirm corrosion mechanisms and their association with corrosion damage levels. For each material, the dimensional metrology data have been used to determine the sensitivity of the corrosion damage to changes in the exposure conditions (e.g. deposit composition, gas composition) to generate models of the corrosion performance of the materials. The corrosion data and model outputs have been compared with data available from power plants operating on coal, straw or wood fuels.

Continuous analysis of elemental emissions from a biofuel gasifier

In recent years there has been significant and high-profile interest in the use of biofuels as possible alternatives to fossil fuels, as part of a move to reduce carbon dioxide emissions. Although combustion accounts for most biofuel use, there has also been significant research into biofuel gasification. However, the behaviour of trace elements during gasification can be problematic, with environmental concerns over toxic components, and process problems caused by alkali metal corrosion and fouling. Experiments have been conducted to continuously monitor the concentration of various trace elements in the raw gasification gas from an experimental reactor, in an effort to determine which elements are volatilised, using ICP-OES. Results of initial tests indicate that the concentration of some elements in the gas phase are extremely high, far higher than in combustion processes, and therefore are of significant concern. Owing to to problems with tar formation in the gasification process, the analysis proved extremely challenging, and further development of the sampling and pre-treatment procedure would be required to obtain more accurate, reliable, and long-term continuous monitoring results.

Hot corrosion resistance of gas turbine materials in combusted syngas environments

Abstract To reduce CO2 emissions, there is interest in a new generation of industrial gas turbines operating in advanced integrated gasification combined cycles (IGCC) with the option for pre-combustion CO2 removal systems. These gas turbines may be fired on syngas, cleaned H2 rich syngas, or natural gas, and this will alter the hot corrosion experienced by components in the hot gas path. Deposit recoat laboratory tests have looked at the response of 11 state-of-the-art materials systems under the expected IGCC conditions. MTData has been used to calculate the optimal deposits under the temperature and gas conditions for these tests. Studies of metal loss have helped assess quantitatively the resistance of the different materials systems, while microscopy techniques have given information on the degradation mechanisms experienced. Rene 80 has been selected to demonstrate the data from these tests and shows the most significant metal loss under partially cleaned syngas conditions.

High temperature oxidation and corrosion of gas turbine materials in burner rig exposures

Abstract With the introduction of novel fuels, which may contain high levels of trace impurities including sulphur and alkali metals, industrial gas turbines are operating with increasingly corrosive combustion environments. To investigate the effect that this, coupled with the higher combustion gas temperatures needed to increase power plant efficiency, has on current state-of-the-art gas turbine component materials, three burner rig exposure tests have been run. The tests evaluated the effects of fly ash, gas moisture and gas temperatures on the alkali sulphate induced hot corrosion of CM247LC, Haynes 230, IN939 and IN728LC. Type I (sulphidation and internal damage), type II (pitting) and mixed mode hot corrosion were observed under different test conditions; however, the presence of fly ash appeared to reduce the levels of hot corrosion. CM247LC, with its high Al content improving oxidation resistance, showed less resistance to hot corrosion than the other, higher Cr content, alloys.