Field test corrosion experiences when co-firing straw and coal: 10 year status within Elsam

Abstract

In Denmark, straw is utilised for the generation of energy and district heating in power plants. Combustion of straw gives rise to high contents of potassium chloride and some sulphur dioxide in the flue gas. These compounds can lead to deposits with high content of potassium chloride and potassium sulphate on superheater tubes resulting in increased corrosion rates. From field experimental results this paper show, that by co-firing straw with coal, corrosion rates can be brought down to an acceptable level.

This paper firstly deals with the results from a demonstration program co-firing coal and straw at the 150MW pulverized coal fired boiler Studstrup unit 1. Two exposure series lasting 3000 hours each were performed for co-firing 10 and 20% of straw (% energy basis) with coal. Using built in test tubes in the hot end of the actual superheaters and air/water cooled corrosion probes, the corrosion during these experiments was monitored. Various ferritic and austenitic materials were investigated at steam temperatures ranging from 520 to 580°C and flue gas temperatures ranging from 925 to 1100°C.

The results obtained in the demonstration program led to the rebuilding of the 350MW pulverized coal fired boiler, Studstrup unit 4, into a co-firing boiler with straw in 2002. During the rebuilding, test tube sections of X20CrMoV12 1 and TP347H FG were built into the superheater and the reheater loops. The temperature ranges during these exposures was for the steam from 470 to 575°C and for the flue gas from 1025 to 1300°C. All these test tubes have been removed during the last three years at one year intervals for corrosion studies.

The corrosion studies performed on all investigated tubes included measurements of the corrosion attack, light optical microscopy and scanning electron microscopy of the corrosion products.     

Waterwall corrosion in pulverized coal burning boilers: root causes and wastage predictions

Abstract

Waterwall corrosion has become a serious problem in the USA since the introduction of combustion systems, designed to lower NO_x emissions. Previous papers have shown that the main cause of the increased corrosion is the deposition of corrodants, iron sulfides and alkali chlorides, which occurs under reducing conditions. In this paper, the contribution of various variables such as the amount of corrodant in the deposit, the flue gas composition and the metal temperature, is further quantified in laboratory tests, using a test furnace allowing thermal gradients across the deposit and the metal tube samples. Approximate deposit compositions were calculated from the coal composition, its associated ash constituents and corrosive impurities. A commercially available thermochemical equilibrium package was used, after modifications to reflect empirical alkali availability data. Predictions from these calculations agreed reasonably well with the alkali chloride and FeS content found in actual boiler deposits. Thus approximate corrosion rates can be predicted from the chemical composition of the coal using corrosion rates from laboratory tests, adjusted to account for the short duration (100 hours) of the laboratory tests. Reasonable agreement was again obtained between actual and predicted results.     

Probes to monitor in-plant material degradation

Abstract

Fireside corrosion in coal fired boilers has been well-investigated. The main causes of water wall fireside corrosion are: (1) impurities in the fuel, such as sulphur alkali metals and chlorine; (2) the lack of control of the combustion process resulting in a reducing gaseous environment at the tube surface; (3) flame impingement; and (4) overtemperature of tube metal.

Co-firing secondary fuels in coal fired boilers is becoming common practice in many power stations in Europe. Secondary fuels like wood, refuse derived fuels, meat and bone meal, straw, poultry litter or mixtures of several secondary fuels are co-fired up to 20-wt%.

Most of these biomass fuels contain high concentrations of alkali chlorides. Considering the composition of these fuels, limitations on the maximum amount of secondary fuels to be co-fired in coal fired boilers are expected.

In addition to the environmental benefits from biomass fired power plants, co-firing can result in “green” power labelling and governmental subsidy. Also savings on fuel costs may be a driving force for an increase of the amount of biomass or secondary fuels to be co-fired.

However, without corrosion monitoring, short-term policies concerning co-firing secondary fuels in large volumes can lead to high costs in the medium or long term. These costs can be due to corrosion damage both in the furnace and superheater sections and penalties due to unplanned outages in a highly competitive electricity market.

This paper summarizes practical experiences from corrosion monitoring programs with KEMA corrosion probes. The first prototype was successfully tested in 1997 at the Hemweg Unit 8 coal fired power plant of Reliant Energy in Amsterdam, the Netherlands. Other corrosion monitoring programs were carried out at coal fired power plants and at a waste incineration plant.

At present a large-scale corrosion monitoring and material testing program is in progress at the Maasvlakte power station Unit 1 near Rotterdam, the Netherlands. In this 520 MWe power plant of E.on Benelux more than 10-wt% of mixtures of secondary fuels are directly co-fired.

In addition to aspects such as emissions, fuel handling and fuel cost savings, co-firing secondary fuels requires corrosion monitoring to check the tolerance to different fuel types of coal fired boilers.     

A discrete element method-based approach to predict the breakage of coal

Pulverization is an essential pre-combustion technique employed for solid fuels, such as coal, to reduce particle sizes. Smaller particles ensure rapid and complete combustion, leading to low carbon emissions. Traditionally, the resulting particle size distributions from pulverizers have been determined by empirical or semi-empirical approaches that rely on extensive data gathered over several decades during operations or experiments, with limited predictive capabilities for new coals and processes. This work presents a Discrete Element Method (DEM)-based computational approach to model coal particle breakage with experimentally characterized coal physical properties. The effect of select operating parameters on the breakage behavior of coal particles is also examined.     

Creep properties and damage behaviour of component-like tubes of Vm12-materials

Abstract

The gap in the energy supply between current availability and the rising demand for electricity worldwide has to be closed primarily by using modern steam and gas power stations with an increased degree of efficiency and decreased CO₂ emissions. Target values for reaching a high degree of efficiency of ≥50% demand increase the steam parameters. The modern creep-resistant steels and their weldments have to have both high creep rupture strength and corrosion resistance.

Within the European research programme COST 536, between 2005 and 2009 research and development work in the field of power plant steels had been carried out for conventional applications. The project was focused on the development of appropriate materials, coatings and surface treatments for components in steam power plants with steam inlet temperatures in the turbine of up to 650°C.

In framework COST 536, Siempelkamp Pruef- und Gutachter-Gesellschaft mbH (SPG) performed component-like creep tests at pressurized tubes made of martensitic steel VM12. This steel was developed by Vallourec & Mannesmann Tubes with the aim of reaching both sufficient creep strength and increased oxidation resistance and is already used for boiler application in new power plants in Germany.

In this paper, the experimental results of uniaxial creep tests, component-like creep tests on tubes with inner pressure and axial loading, metallographic examination and damage characterisation are presented. The tubes are equipped with capacitive high temperature strain gauges for on-line monitoring of strain. All testing data will be implemented as inputs for the numeric FE analysis. The effect of multiaxiality and stress redistribution will be discussed.     

Possible scenarios for the causes of accelerated fireside corrosion of superheater tubes in coal-fired boilers

Recent experience of rapid corrosion of superheater tubes in coal-fired boilers has led to the application of weld-overlay coatings, which have proved effective but are expensive and can degrade mechanical tube life. Lack of understanding of how such corrosion differs from the established mechanism involving low-melting, complex, alkali-iron trisulphates is a major obstacle to devising alternative protective measures. Suggested scenarios for such accelerated corrosion are considered, including (1) increased tube surface temperatures allowing other sulphate mixtures to melt, analogous to Hot Corrosion mechanisms in gas turbines; (2) changes in the corrosivity of tube deposits from carryover of reduced species; and (3) increased release of corrosive species from the coal under substoichiometric combustion conditions. The potential for such scenarios to change the corrosive environment is assessed on the basis of current mechanistic understanding, though the general lack of phase diagrams for relevant ternary and quaternary sulphate systems is a major drawback to specifying the processes involved.     

Origins of tensile stress-induced circumferential cracking of waterwall tubes in boilers

Abstract

The leakage and rupture of boiler tubes in power plants is a serious problem that can lead to unscheduled and costly outages. The predominant failure location of current concern is circumferential cracking on the fireside of waterwall tubes in the furnace waterwall section of a boiler. Although there is basic agreement that cracking results from a combination of thermal fatigue and corrosion, a complete explanation of the basic phenomena needed to establish the root causes of this problem is lacking. The purpose of the present study was to analyse the sources of the tensile stress responsible for initiating circumferential cracking and to identify the key parameters controlling this tensile stress. The results of analytical modelling suggested that a combination of increasing tube wall temperature with increasing thickness of internal oxide layers, and temperature spiking due to deslagging events eventually may result in tensile stresses sufficient to crack the fireside oxide and initiate the development of circumferential cracks. This scenario also led to suggestions for reducing tensile stresses in waterwall tubes which, in turn, would be expected to delay/avoid circumferential cracking and improve the reliability of waterwall tubes.     

Minimization of life cycle CO2 emissions in steam and power plants

A methodology is presented to minimize life cycle CO₂ emissions through the selection of the operating conditions of a steam and power generation plant. The battery limits of the utility plant are extended to include CO₂ emissions of: (1) extraction and transport of natural gas burned in its boilers, (2) generation of imported electricity by nuclear, hydroelectric and thermoelectric plants and (3) exploration, extraction and transport of natural gas, oil, coal and uranium consumed by thermoelectric and nuclear plants. The operating conditions of the utility plant are selected optimally to minimize the life cycle CO₂ emissions. There are continuous operating conditions such as temperature and pressure of the high, medium and low pressure steam headers and binary operating conditions to represent discrete decisions to select optional pumps drivers between electrical motors and steam turbines or whether some equipment is on or off. A Mixed Integer Nonlinear Programming problem is formulated and solved in GAMS. Significant reductions in life cycle CO₂ emissions, natural gas consumption and operating cost are achieved simultaneously in the steam and power generation system of an ethylene plant. This is an important tool to support a decision making process to reduce CO₂ emissions in a key industrial sector. An erratum to this article can be found at     

Prediction and real-time monitoring techniques for corrosion characterisation in furnaces

Abstract

Combustion modifications to minimise NO_x emissions have led to the existence of reducing conditions in furnaces. As regulations demand lower NO_x levels, it is possible (to a degree) to continue to address these requirements with increased levels of combustion air staging. However, in most practical situations, a number of adverse impacts prevent the application of deep combustion air staging. One of the more important limitations is the increased corrosion that can occur on wall tubes exposed to fuel rich combustion environments. Current boiler corrosion monitoring techniques rely on ultrasonic tube wall thickness measurements typically conducted over 12 to 24 month intervals during scheduled outages. Corrosion coupons are also sometimes used; typically require considerable exposure time to provide meaningful data. The major drawback of these methods is that corrosion information is obtained after the damage has been done. Management of boiler waterwall loss and system optimisation therefore requires a real-time indication of corrosion rate in susceptible regions of the furnace. This paper describes the results of a program of laboratory trials and field investigations and considers the use of an on-line technology in combination with innovative applications, also modelling and precision metrology to better manage waterwall loss in fossil fuelled boilers while minimising NO_x emissions.     

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.     

Preparation of carbon nanostructures from medium and high ash Indian coals via microwave-assisted pyrolysis

This study is focused on valorizing low value and low quality Indian coals via microwave pyrolysis to produce good quality carbon nanostructures in the heat-treated coal char. The effects of operating conditions such as coal type, coal:susceptor (Fe) mass ratio, and microwave power on product yield and quality are evaluated. The quality of the heat-treated coal char was assessed using different characterization techniques such as electron microscopy, porosimetry, X-ray diffraction, and Raman spectroscopy. The addition of Fe enhanced the heating rates, and led to the formation of carbon nanotubes and nanoparticles. Increasing the proportion of Fe resulted in increase in size of nanotubes and nanoparticles, which is attributed to the fusion of small tubes and particles caused by enhanced localized heating. The yield of carbon nanostructures was more from medium ash (~45%) than from high ash coal (~37%) due to the high fixed carbon and low ash content in the former. In addition to char, coal tar and non-condensable gases were characterized. The major compounds in the coal tar were aromatic hydrocarbons, simple phenols and aliphatic hydrocarbons. Hydrogen and methane were the major gases from medium ash coal, while hydrogen, methane and CO were produced in significant quantities from high ash coal. Microwave-assisted pyrolysis is shown to be a promising process to produce carbon nanostructures in a short time period as compared to conventional thermal processes.     

Development and application of a methodology for the measurement of corrosion and erosion damage in laboratory,burner rig and plant environments

Abstract

Fuel gases derived from solid fuels such as coal, biomass and waste and their mixes have the potential to cause both erosion and corrosion damage to components in gas turbines and diesel engines. To allow the statistically valid assessment of materials performance in short term plant runs, burner rig tests and laboratory simulated environments a methodology has been developed to collect compatible quantitative data on materials degradation. Accurate measurement techniques based on pre-exposure contact metrology and post-exposure optical microscopy/image analysis have been developed. These take into account both the low level of damage required for practical systems and the localised nature of hot corrosion damage. The data produced have been used to derive and test quantitative models for the prediction of the performance of candidate materials in such power systems. For these models to be used with confidence, similar damage morphologies must be produced in both the real and simulated conditions, as well as similar damage rates.     

Construction of erosion–corrosion maps for erosion in aqueous slurries

Abstract

Although much research has been carried out on the erosion of materials in aqueous slurries, little attention has been given to defining transitions between erosion regimes in such environments. This is despite the large amount of work that has been carried out in this area in ‘dry’ high temperature corrosion environments. Defining transitions between regimes permits distinction between erosion and corrosion dominated behaviour, which can be an aid to materials selection and to process monitoring in such conditions. This paper describes the basis of a relatively simple theoretical method for evaluating the transitions between erosion and aqueous corrosion regimes. Models for solid particle erosion at normal impact are combined with those for aqueous corrosion to define regimes of damage in corrosion conditions varying from dissolution to passivation. This permits the construction of the erosion–aqueous corrosion map where the transitions between the regimes are shown as a function of erosion and aqueous corrosion variables. The rationale for the regimes suggested is discussed on the basis of research on erosion maps in oxidising conditions. The boundaries on the maps are compared with experimental data. Other issues addressed in this study include using the erosion–corrosion map to identify favourable and adverse operating conditions in addition to identifying the mechanism of damage.

MST/3543     

Research and Improvement on structure stability and corrosion resistance of nickel-base superalloy INCONEL alloy 740

INCONEL alloy 740 is a newly developed Ni–Cr–Co–Mo–Nb–Ti–Al superalloy in the application to ultra-supercritical boilers with steam temperatures up to 700 °C. By means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), micro-chemical phase analyses, and corrosion-resisting test, this paper investigates the structure stability of the alloy at elevated temperature and concentrates on coal ash corrosion performance of the alloy under the simulated coal ash/flue gas condition. Experimental results show that the most important structure instabilities of the alloy during prolonged aging are γ′ coarsening, γ′ to η transformation and G phase formation at grain boundary. The performance of corrosion resistance of the alloy would meet the requirement of ultra-supercritical boiler tubes. The phase computation by means of Thermo-Calc has been adopted in chemical composition modification for structure stability improvement. Two suggested new modified alloys in adjustment of the Al and Ti contents and in control of Si level, and also in maintenance of Cr content of the alloy were designed and melted for experimental investigation. These two modified alloys exhibit more stable microstructure during 760 °C long time aging.     

Application of salt engineering to reduce/mask bitter taste of clindamycin

Abstract

Palatability of a formulation is one of the primary requirements for therapeutic compliance in children. Clindamycin (CLN) often prescribed to children to treat various infections. However, it has a bitter taste and bad smell. The focus of the present investigation was to develop salt of CLN with a commonly used sweetener such as cyclamic acid (CYA) to improve the palatability. The salt forms were prepared by solubilization crystallization method and characterized by Fourier transformed infrared (FTIR), Near infrared (NIR), Raman, X-ray powder diffraction, scanning electron microscopy, solubility, dissolution, and solid-state physical and chemical stability at 25?°C/60% RH and 40?°C/75% RH for 1 month and 60?°C for 2 weeks. Spectroscopic and diffraction data indicated the formation of a new solid phase, which was different from hydrochloride salt of CLN. Shape of crystal was rectangular prism. Stoichiometric ratio between CLN and CYA in the new salt CLN-CYA was 1:1 and its melting point was 85.6?°C. There was a 2.4-fold reduction in solubility of CLN-CYA at pH 4 compared with CLN-HCl. Moreover, the dissolution rate and extent were similar between the two salts and meeting USP requirement of 85% dissolution in 30?min. Salt was physically and chemically stable at 60?°C, 25?°C/60% RH, and 40?°C/75% RH conditions but hygroscopic at high humidity condition. In conclusion, new salt will provide a new avenue for the development of a palatable formulation of CLN.     

Evidence for fullerene in a coal of Yunnan, Southwestern China

 In two types of coal from a coal mine in Yunnan Province, Southwestern China, the presence of fullerene is confirmed. The fullerene had been suggested earlier by its characteristic infrared absorption spectrum. The present work reports verification by a high performance liquid chromatograph. A critical step leading to the confirmation is in the process of preparation of the liquid solution from the coal for chromatography and this is described. Possible conditions for the search of natural fullerenes are suggested. Received: 2 May 1997/Accepted: 21 May 1997     

Performance and energy calculation on a green cementitious material composed of coal refuse

Coal refuse as industrial solid waste has become great threats to the environment. To activate coal refuse is one practical solution to recycle this huge amount of solid waste as substitute for ordinary Portland cement (OPC). Compared with conventional cement production, successful development of this new material could potentially save energy and reduce greenhouse gas emissions, recycle vast amount of coal wastes, and significantly reduce production cost. Coal refuse was confirmed as a pozzolanic material, which enhances its durability performance. In this experiment, 60 % of the OPC was substituted with the pozzolana mixture (30 % coal refuse + 25 % slag + 5 % FGD gypsum), which is an optimal solution for the creation of good-performance cementitious material. Compared with OPC, the 60 % pozzolana blended sample has a much higher resistance to the alkali-silica reaction and Cl ion penetration. In addition, microanalyses of the activated coal refuse by XRD demonstrated that some of the mineral phase changes in coal refuse were related to the performance of the cementitious material. For example, the transformation of kaolinite into metakaolin and the dehydroxylation of muscovite enhance the resistance of the cementitious material to the alkali-silica reaction and Cl penetration, respectively. Compared with conventional cement production by calculation, successful development of a new thermal activation process (800 °C) to convert coal refuse into desirable pozzolanic material for producing the new material would potentially save energy around by about 50 %, reduce greenhouse gas emissions by about 67 %.     

In-bed corrosion of alloys in atmospheric fluidized bed combustors

A series of tests, each of 250 h duration, which is designed to examine the effect of changes in operational parameters on in-bed corrosion, is in progress using a 0.3 m-square atmospheric fluidized bed combustor (AFBC). A 2000-h test has been conducted on a 1.1 mdiameter AFBC, partly to determine the comparability between the results of the tests in the small unit and those in more realistically sized beds, and partly to assess the performance of candidate materials of construction in a longer-term test. The results indicatethat the smaller unit is indeed a satisfactory representation of large AFBCs. Variables such as coal type, coal sulphur content, acceptor type, excess combustion air and bed temperature appear to have relatively little effect on in-bed oxidation/ sulphidation corrosion, certainly within the limits likely in normal operation. Several materials which appear to be suitable for construction of a fluidized bed boiler are identified, although it is emphasized that extrapolation of short-term tests is particularly dangerous for this form of corrosion.     

Hot corrosion: a modification of reactants causing degradation

Abstract

It has been conjectured that if sulfur in the fuel is removed, engine materials will cease to experience attack from hot corrosion since the fuel sulfur has been viewed as the primary source that has caused hot corrosion and sulfidation. Hot corrosion has been defined as an accelerated degradation process that is generally considered to involve deposition of corrosive species (e.g. sulfates) from the surrounding environment (e.g. combustion gas) to the surface of hot components, followed by subsequent destruction of the protective oxide scale. Most papers in the literature since the 1970s consider sodium sulfate as the single salt causing hot corrosion. There has been a push to remove fuel sulfur content to less than 15 ppm. However, sulfur species may still enter the combustion chamber via air intake or with seawater entrained in the air through the air intake of the ship. Seawater contains, in addition to sodium sulfate, magnesium, calcium, and potassium salts. Additionally, sulfate speciation and the content of atmospheric particulate matter (PM) varies considerably around the world and in some places, such as China and India, high PM seems to cause an increased risk of deposit-induced hot corrosion due to atmospheric pollutants rather than a combustion process (i.e. sulfur impurity in the fuel). The increasing operating temperatures of turbine engines and activities within regions of the world that have relatively high pollutant (PM, SO₂, C, Ca, etc.) levels are working conjointly to cause previously unobserved forms of high-temperature corrosion. This paper will cover some of our revised understanding of hot corrosion and consider other possible contaminants that could further complicate our understanding of hot corrosion.     

Adsorbents for capturing mercury in coal-fired boiler flue gas

This paper reviews recent advances in the research and development of sorbents used to capture mercury from coal-fired utility boiler flue gas. Mercury emissions are the source of serious health concerns. Worldwide mercury emissions from human activities are estimated to be 1000 to 6000 t/annum. Mercury emissions from coal-fired power plants are believed to be the largest source of anthropogenic mercury emissions. Mercury emissions from coal-fired utility boilers vary in total amount and speciation, depending on coal types, boiler operating conditions, and configurations of air pollution control devices (APCDs). The APCDs, such as fabric filter (FF) bag house, electrostatic precipitator (ESP), and wet flue gas desulfurization (FGD), can remove some particulate-bound and oxidized forms of mercury. Elemental mercury often escapes from these devices. Activated carbon injection upstream of a particulate control device has been shown to have the best potential to remove both elemental and oxidized mercury from the flue gas. For this paper, NORIT FGD activated carbon was extensively studied for its mercury adsorption behavior. Results from bench-, pilot- and field-scale studies, mercury adsorption by coal chars, and a case of lignite-burned mercury control were reviewed. Studies of brominated carbon, sulfur-impregnated carbon and chloride-impregnated carbon were also reviewed. Carbon substitutes, such as calcium sorbents, petroleum coke, zeolites and fly ash were analyzed for their mercury-adsorption performance. At this time, brominated activated carbon appears to be the best-performing mercury sorbent. A non-injection regenerable sorbent technology is briefly introduced herein, and the issue of mercury leachability is briefly covered. Future research directions are suggested.