Paul Kilgallon

Comparison of coal/solid recovered fuel (SRF) with coal/refuse derived fuel (RDF) in a fluidised bed reactor

An experimental study was undertaken to compare the differences between municipal solid waste (MSW) derived solid recovered fuel (SRF) (complying with CEN standards) and refuse derived fuel (RDF). Both fuels were co-combusted with coal in a 50 kW fluidized bed combustor and the metal emissions were compared. Synthetic SRF was prepared in the laboratory by grinding major constituents of MSW such as paper, plastic, textile and wood. RDF was obtained from a local mechanical treatment plant. Heavy metal emissions in flue gas and ash samples from the (coal+10% SRF) fuel mixture were found to be within the acceptable range and were generally lower than that obtained for coal+10% RDF fuel mixture. The relative distribution of heavy metals in ash components and the flue gas stream shows the presence of a large fraction (up to 98%) of most of the metals in the ash (except Hg and As). Thermo-gravimetric (TG) analysis of SRF constituents was performed to understand the behaviour of fuel mixtures in the absence and presence of air. The results obtained from the experimental study will enhance the confidence of fuel users towards using MSW-derived SRF as an alternative fuel.

Hydrogen Embrittlement of Cathodically Protected High-Strength, Low-Alloy Steels Exposed to Sulfate-Reducing Bacteria

Abstract Hydrogen embrittlement (HE) of two high-strength, low-alloy steels was studied in conditions typical of the marine environment. Double-cantilever beam specimens, heat-treated to produce th...

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.

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.

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.