What is a Boiler?
The basic concept of coal based power generation is the conversion of coal energy to electrical energy. This is achieved by the combustion of the coal in a boiler to generate steam which is used to drive turbine alternators. Typical conversion efficiencies of coal energy to electrical energy, for modern coal-fired power plant, are 36-39%, a limitation largely imposed by properties of the steam cycle. The major coal-related components of power generation equipment are as follows:
* Coal handling plant
* Pulveriser plant
* Boiler
* Airheater
* Electrostatic precipitator/Fabric filter
* Atmospheric emissions control
* Ash disposal systems
To understand the operation of a PF fired boiler, the three basic fluid flow processes must be understood, as follows:
* Water and steam path
* Coal and air path
* Flue gas path
Water and Steam Path
In the steam circuit of a boiler, illustrated in Figure 1, water enters the economiser at the correct pressure where it picks up heat to raise its temperature close to saturation temperature. From the economiser, the water enters the boiler steam drum, from which it circulates through the downcomers, outside the boiler, and up through the water wall tubes of the furnace, and returning to the drum. Heat transfer in the furnace is essentially by radiation from the
In the following sections, the different items of plant are discussed, along with the impact of coal properties on individual aspects of behaviour. It must be stressed however that the way in which coal or ash behaves in the various parts of a power plant is governed as much by the features of the plant and how it is operated, as it is coal and ash properties. Some coals may have problematic behaviour in some installations, yet not cause any difficulties in other installations.
Coal and Air Path
The coal and air path is shown schematically in Figure 2. As shown, clean air at ambient temperature is drawn through the forced draft (FD) fans and pumped through the air pre-heater, which recovers heat from the outgoing flue gas and heats the air to a temperature of approximately 300C. On leaving the air heater, the hot air is divided into two streams, viz. primary and secondary air. The secondary (or combustion) air goes to the boiler windbox from where it enters the combustion zone of the furnace.
The primary air goes through the primary air (PA) fans and controlled to give the mill inlet air temperature required to maintain the desired mill outlet air temperature. The pulverised coal produced in the mill is entrained in the primary air and pneumatically conveyed to the burners mounted in the walls of the boiler furnace. Typically, boilers are equipped with a number of mills, each mill supplying pulverised coal to one level of burners.
Flue Gas Path
As shown in Figure 3, the mixture of coal and air issuing from the burners is burnt in the boiler furnace, and the products of combustion pass from the furnace, across the superheaters, reheaters and economisers. A small amount of ash drops out in both the furnace hopper and the economiser hoppers (approximately 5% of the total ash produced, in each). The flue gas enters the air heater at a temperature of approximately 500C and, after heat is transferred to the clean incoming air, leaves the air heater at approximately 130 C, and enters the electrostatic precipitator (ESP). The bulk of the ash produced is removed from the flue gas in the ESP as fine fly ash, and disposed of into ash dams or dry storage silos for subsequent use in other industries.
Boiler Configurations
The essential features of a boiler are:
* Type of burner - swirl, diffusion, slot, multi-fuel (ie. coal/oil/gas), low NOx
* Arrangement of burners - opposed, front/rear wall, corner-fired
* Amount and disposition of heat transfer surfaces
* Number and location of soot-blowers1
* Size of convective (rear) pass - affects velocity of flue gas
Measures of Generating Plant Performance
In general terms, there are several ways in which plant performance can be expressed. Coal properties may impact on any of the following performance aspects. Also contributing to these performance criteria are plant design and the way the plant is operated.
Thermal Efficiency
The overall efficiency of a generating unit is expressed as:
Electrical Energy
Thermal Efficiency (%) = ------------------------ x 100
Coal Energy
It is desirable to maximise thermal efficiency in order to minimise coal costs. It is also a way of minimising the production of greenhouse carbon dioxide.
Availability
Availability is the percentage of time that the plant is able to be operated. Any operating problems that require the plant to be stopped will detract from availability.
Capacity
Capacity is the possible electrical output as a percentage of the rated output. A 500 MWe plant that is able to produce only 400 MWe would have a capacity of 80%. Turn-Down Capability At times of low electricity demand it is necessary for some generating units to run at part-load. Below a certain percentage of full-load the plant may not operate satisfactorily. This load, expressed as a percentage of full-load, is termed the turn-down capability.
* Soot-blowers are devices which are used to clean the ash deposits off the heat transfer surfaces inside the boiler. Typically, soot-blowers consist of lances which, when activated by the operating staff, direct high velocity steam (or in some cases, compressed air) onto the heat transfer surfaces, thus removing the ash deposits.
Operating Costs
These are the costs of keeping the plant operating satisfactorily.
Environmental
Environmental impacts may incur extra operating costs to control them. If they are not controlled the plant owner will be seen as operating irresponsibly, and fines may be imposed.
Figure 1: Schematic Layout of Boiler/Turbo Steam Circuit
Figure 2: Schematic Layout of Coal and Air Circuits in a Boiler
Figure 3: Layout of Flue Gas Path in a Boiler
Processes Occurring During Combustion
Gross Chemical Reactions
The gross combustion of coal involves the chemical reaction of the combustible components of coal, mainly carbon and hydrogen, with oxygen.
C + O2 ---> CO2
(1 mole) + (1 mole) ---> (1 mole)
2H2 + 02 ---> 2H2O
(2 mole) + (1 mole) ---> (2 mole)
S + O2 ---> SO2
(1 mole) + (1 mole) ---> (1 mole)
From these simple chemical equations, the air requirements for combustion can be calculated along with the flue gas quantity and composition. The chemical reactions above are highly exothermic ie. significant heat energy is liberated during the reactions, which is then available for conversion to electricity. The most common
means of achieving this conversion being through the generation of steam in a boiler which is used to drive a steam turbine and electric generator.
Combustion Processes
Coal goes through a number of processes to achieve complete combustion. Some of these are illustrated in Figure 4.
* Release and combustion of volatile matter
* Residual char combustion
* Release of coal mineral matter
* Production of gaseous pollutants.
* Release of trace elements
The first two steps above are necessary to heat production, while the last three are unavoidable side-effects of using coal and these must be managed.
Devolatilisation:
In this process, the coal decomposes as heating takes place, releasing volatile matter. The gaseous volatiles are mixed with the surrounding air and rapidly burned if the prevailing temperature is above the ignition temperature of the volatile species. The rate at which volatiles are released, and the concentration of gaseous species is dependent on:
* The final temperature to which the coal is heated.
* The final temperature to which the coal is heated.
* The nature of the coal (chemical composition, rank etc)
Figure 5 shows the effect of heating rate and final temperature on the total quantity of volatile matter liberated from a high and a low rank coal. As shown, the yield of volatile matter increases with the rate of heating, and the final temperature to which the coal is heated.
The yield of volatile matter during combustion can be significantly higher than the volatile yield as determined by the Proximate Analysis of the coal. Figure 6 shows that some coals have volatile yields up to two times the proximate volatile yield.
Char Burnout:
After devolatilisation, a solid char particle remains which reacts with oxygen on the surface and burns at a much slower rate than the release and combustion of the coal volatile matter.
The process of combustion of the solid char is illustrated in Figure 7 where oxygen diffuses through the boundary layer surrounding the coal particle and reacts with carbon at the exposed coal surface to form carbon dioxide and/or carbon monoxide, which then diffuses back through the boundary layer to the free gas surrounding the particle. The carbon monoxide which is generated is oxidised to carbon dioxide in the free gas stream. There are two competing and limiting processes which determine the rate at which the char burns:
* The rate at which oxygen can diffuse to the char surface,
* The rate at which carbon at the char surface can chemically react with oxygen.
Based on mathematical models of idealised spherical and homogeneous char particles, reaction rates can be calculated over a range of temperatures for representative particles sizes(Figure 8), and the combined effects of diffusion and chemical control calculated.
The theoretical approach can be used to show that the factors which affect the reaction rates of char particles are:
* Char type, density, reactivity, porosity
* Particle diameter, for smaller particles, burning time reduces
* Availability of oxygen, as oxygen concentration increases, burning time reduces.
* Temperature, as temperature increases, burning time reduces.
In real life char particles come in a whole range of sizes, structures and shapes and the distribution of particle types differs for every coal. During devolatilisation, some coal macerals soften and individual particles form various types of chars. Based on shape, wall thickness, and porosity, these chars may be classified into three major groups - "cenosphere", "network/honeycomb", and "solids", as illustrated in Figure 9. The char types have different burning times according to the surface area available for chemical reaction.
The cenosphere type chars are associated with vitrinite, while the network/honeycomb and solid chars are derived from medium to high reflectance inertinite. In general, chars which have thin wall thickness (< 5 m) and are highly porous, more reactive and have shorter burning times than chars with thicker walls and denser textures. Nevertheless, some unfused chars can also be reactive under high temperature and high heating rate conditions. Generally, the ranking of burnout reactivity of these char types, from highest to lowest is network/honeycomb, cenospheres and solids. Release of Mineral Matter: During combustion, the ash contained in the coal is liberated and flies through the boiler with the combustion gas (thus "fly ash"). The fly ash causes problems in boilers by:
* Impinging and sticking on heat transfer surfaces causing ash deposition and subsequent reduction in heat transfer.
* Eroding boiler tubes by impingement.
* Polluting the atmosphere. This requires boilers to be fitted with dust collection equipment such as electrostatic precipitators or fabric collectors.
Fly ash has a wide range of particle sizes, shapes, compositions and structure:
* Larger mineral inclusions in the coal are separated from the coal substance during pulverising of the coal. These large ash particles are of primary importance in wear of pulveriser components and erosion of boiler tubes.
* Finer mineral inclusions, which are initially dispersed through the coal material, enter the boiler in mixed particles of coal and mineral matter. As the particle burns away these inclusions tend to merge to produce a composite residual particle but substantially smaller than the original coal particle. Particles formed this way are often enriched in fluxing elements and are therefore active in the formation of ash deposits.
* Fly ash also contains a proportion of sub-micron particles. These may be due to disintegration of larger ash particles, or from elements in the ash which are vaporised at high temperature and reducing conditions experienced during combustion. These may condense to form fume in the lower temperature parts of the boiler. These sub-micron particles and fumes are difficult to remove from the flue gas steam and pose a pollution problem.
Production of Gaseous Pollutants
Carbon dioxide would once have been considered a harmless consequence of burning a carbonaceous fuel. It is now a high-priority pollutant due to the Greenhouse Effect. Carbon dioxide production is minimised by using fuels that are high in hydrogen as against carbon, and by maximising the thermal efficiency of the power plant process. A more radical approach is the proposed sequestration, whereby the carbon dioxide would be stored to prevent it mixing with the atmosphere.
All coals contain some sulphur and this burns to produce Sulphur dioxide (SO 2). This oxidises in the atmosphere to make sulphur trioxide which causes acid rain. Before the flue gas exits the boiler a small amount of acid is already formed and this impacts on corrosion of duct-work and electrostatic precipitators. Where high sulphur coals are utilised or environmental regulations are stringent, boilers are fitted with flue gas desulphurisation(FGD).
Oxides of nitrogen (NOx) are produced mainly from the nitrogen in the coal. NO x may cause acid rain and brown haze in the atmosphere. NO x emissions may be reduced by a combination of combustion modifications to oxygen availability and flame temperature, and/or a post-combustion Selective Catalytic Reduction (SCR) to remove NO x after it has been produced.
Release of Trace Elements
Given sensitive enough detection equipment, most elements can be found in coal, as they can also be found in garden soil. Some of these trace elements are potentially toxic if they are released in certain forms in high enough concentrations. Of importance is not just the concentrations in the coal, but also whether these elements (i) report to the collected ash and (ii) then to water supplies, or (iii) are emitted from the stack into the atmosphere.
Figure 4: Processes Occurring during Combustion
Figure 5: Volatile Matter Yield as a Function of Final Temperature and Heating Rate
Figure 6: Relative Volatile Yield: Actual Yield over Proximate Yield
Figure 7: Ideal Model of the Combustion of Solid Char
Figure 8: Theoretical Reaction Rates of Char with Diffusion and Chemical Control
Figure 9: Types of Chars formed during Combustion
Performance Prediction
The need for testing and evaluation of coals for combustion arises for a number of reasons:
· To provide technical knowledge of a new coal deposit, so that the quality of the resource can be evaluated.
· To demonstrate that a coal satisfies contractual specifications.
· To identify potential problems of coals before boiler designs are produced, or coal supplies are committed.
· To provide design data for boilers and other combustion equipment.
· To evaluate options for coal beneficiation, blending or the use of coal additives.
· To provide technical support for marketing of export coals.
The quality of the coal affects most of the costs associated with coal-fired power plants, and inadequacies in power station design, poor maintenance, and inconsistent coal quality within specified boiler design limits have a large impact on these costs. Proper evaluation of the combustion performance of the coal to ensure optimum utilisation costs is therefore essential.
Scales of Testing
There is a wide range of testing and evaluation procedures used to evaluate the performance of coals for pulverised coal combustion. These procedures range in scale from:
* Standard laboratory analyses of coal,
* Bench-scale testing to simulate some aspect(s) of the coal’s impact on power plant processes,
* Pilot-scale simulation of power plant,
* Testing in full-scale boilers.
The aim of these procedures is to determine the performance of coals in operating plant, and identify and alleviate potential problems in the use of particular coals. When the scale of testing is being chosen, the trade-offs need to be considered:
* Smaller-scale tests tend to be cheaper, require smaller sample masses and are experimentally more precise,
* Larger-scale tests provide a more realistic simulation of the power plant processes.
To illustrate the above points, a laboratory determination of Volatile Matter (VM) content costs a few dollars, requires a about two grams of coal and the test can be performed with excellent repeatability and reproducibility. As such, the VM determination is ideal for borecore programs and for satisfying contractual requirements. However, the intended function of the VM test is as an indicator of the combustibility of a coal; in this respect the test is sometimes found wanting because it sometimes gives erroneous indications. Figure 10 illustrates the stages at which the different coal scales of evaluation may be employed.
Figure 10: Stages and Scales of Coal EvaluationUse of Laboratory Analysis
Standard chemical analyses of coal and ash are often unreliable predictors of power plant performance. Nevertheless this type of data is generally available during borecore exploration programs before there have been opportunities for more sophisticated investigations. The laboratory data may be valuable for the following applications:
* As a pointer to the sort of reaction that potential coal buyers might display to a new product,
* As an indicator of areas of the coal’s performance that may be suspect, requiring investigation by a larger-scale form of testing,
* As a measure of the variability of the coal’s quality over the mine. It is difficult to justify the cost of pilot-scale tests on samples that represent every quality. For these reasons extensive use is made of laboratory analysis is a preliminary predictor.
Their effectiveness can be improved by the use of standard or in-house indices derived from these analytical parameters. Such an index may be useful for specific applications, such as by “calibrating” the index against pilot-scale or full-scale data for a given mine; the variability in the value of the index over that mine area may then have real meaning, whereas an attempt to use the index the same way for coal from a different region may not work.
Pilot Scale Testing
Pilot-scale testing and evaluation is generally adequate to establish the boiler design parameters and operational characteristics of a coal. The level of confidence achieved in the style of evaluation is sufficient to predict the coal's performance, and to select coals for use in specific power plants.
Pilot scale testing and evaluation is generally undertaken on combustion test facilities which closely simulate the important aspects of the combustion processes occurring in full-scale boilers. Pilot-scale testing requires coal samples of about three tons for a coal input rate of 20-40 kg/h (150 to 300 kW thermal heat input), and is therefore an economical means of testing for new coal deposits.
A typical pilot scale evaluation furnace, the ACIRL Boiler Simulation Furnace, is illustrated in Figure 11. At this scale, successful evaluation of the coal depends upon comparisons between the sample coal and known reference coals tested in the same facilities, the experience of the testing personnel, and the methods used in the evaluations to extrapolate the test results to full-scale performance.
Pilot Scale Evaluation Procedures
The Australian Combustion Technology Centre, operated by ACIRL, has a range of pilot scale combustion test facilities with which to evaluate the pulverised coal combustion performance of coals. These facilities simulate all the coal-related processes which occur in a power station and have the capability to investigate aspects of combustion, including:
· Pulverising performance, including mill wear.
· Combustion characteristics and performance in power station burners.
· Ash deposition (fouling and slagging).
· Electrostatic precipitation and fabric filter fly ash collection.
· Gaseous emissions of NOx and SOx.
· Fly ash disposal and utilisation.
Pulverising Performance:
Investigations are carried out in a Raymond bowl type vertical spindle mill with a nominal capacity of 500 kg/h, to assess the performance of particular coals with respect to pulverising.
Measurements are carried out to assess:
· Mill roll pressure versus grinding performance.
· Feedrate versus grinding performance.
· Mill power consumption.
· Pressure drop through the mill.
· Mill wear, including the performance of different mill materials.
Combustion Performance:
Testing for combustion performance is carried out in the Boiler Simulation Furnace (BSF). The coal is fired through a variable swirl burner mounted at the top of the down-fired combustion chamber, at a nominal thermal input of 150 kW, equivalent to approximately 20-25 kg/h of bituminous coal.
Testing for combustion performance is carried out over a range of burner swirl settings and primary to secondary air ratios to determine the regimes of flame stability and turn-down ratios. Relative ignitability is determined by measuring the flame stand-off distance from the burner under conditions of zero swirl. At a standard firing rate, samples of solid particulate are sampled along the flame axis and subjected to proximate and ultimate analyses to determine volatile release and carbon burnout along the flame axis.
Evaluations based on the combustion performance procedures provide positive conclusions about a particular coal's performance with regard to burner stability, ignitability and completeness of combustion. The evaluations are related to the known performance of reference coals in power plants.
Ash Deposition:
Test procedures to evaluate a coal's propensity to cause ash deposition problems are carried out in the BSF. The combustion chamber is fitted with three slagging panels that simulate the water walls of a utility boiler. Also, hot gas and suspended ash particles enter a tunnel with incorporates fouling probes which simulate super heater tubes in the convective pass of a utility boiler.
Measurements are made on the rate of build up and strength of deposits, and variations in heat transfer are monitored over time. Operating conditions such as temperature and gas velocity are chosen to simulate real operating conditions in utility boilers, and testing is carried out over a range of gas temperatures to identify the gas temperature at which troublesome deposits may form.
Electrostatic Precipitation:
The time/temperature history of ash particles in the BSF closely simulate that in a utility boiler, and hence the fly ash structure closely resembles that produced in full-scale plant.
The gas leaving the Boiler Simulation Furnace is fed to a pilot scale electrostatic precipitator(ESP), and testing is carried out to measure the collection efficiency of the fly ash over a range of specific collecting areas. The ESP can be operated over a wide range of operating conditions including intermittent energisation and flue gas conditioning.
The evaluation procedure enables the performance of the fly ash to be predicted in existing plant where the size of precipitators is known, as well as the size of precipitator necessary to meet clean air requirements for plants which are being designed for the test coals.
Gaseous Emissions:
Measurement of NOx and SOx generation during combustion are determined under standard combustion conditions in the BSF using on-line flue gas sampling and analysis. Testing is also carried out in the NOx Evaluation Furnace, which has a nominal thermal input of 20kW. NOx levels generated under conditions of excess air and staged combustion are measured.
Measurements are also carried out in a nitrogen free environment to determine the proportion of NOx generated from the nitrogen in the fuel.
Other Testing:
Other pilot scale evaluation procedures include:
· Flame radiation characteristics.
· Fly ash disposal and utilisation.
The pilot scale test procedures, along with a range of bench scale testing can be performed at ACTC to demonstrate the suitability of a coal properties on the performance of power plant, and the range of power plant parameters under which it will perform satisfactorily
Figure 11: ACIRL Boiler Simulation FurnaceImpact on Power Plant Processes
Introduction
This Section covers those power plant processes for which coal quality may impact on the performance. For each process will be detailed:
* The principal of operation,
* Coal quality parameters and indices that may be associated with the process,
* A demonstration of the correlation between these parameters/indices and performance measured in pilot-scale plant.
Self-Heating & Spontaneous Combustion at the Stockpile
Description of the Phenomenon
Self-heating is caused by the adsorption of water (vapour or liquid) and oxygen onto the coal surfaces. These are exothermic reactions and, if the rate of heat generation is greater than the rate at which it is removed (by conduction, ventilation and evaporation) the coal will progressively increase in temperature and may eventually ignite. If an equilibrium condition can be maintained by limiting the chemical reaction rates and/or removing heat at a sufficient rate, then ignition may never occur. In such cases it is possible that the coal’s reactivity to these low temperature reactions may decline over time and the temperature may in fact drop. In these cases the term “self-heating” is more appropriate than “spontaneous combustion”.
Spontaneous combustion occurs because the rates of the reactions increase with higher temperature. A gradual temperature increase reaches a stage where the reaction rate accelerates, thus upsetting the approximate equilibrium between heat generation and heat removal. After the stockpile temperature reaches about 75˚C, acceleration of heating is likely to occur, leading eventually to ignition, though this temperature may be lower for low-rank coals.
Physical Factors
The physical factors associated with spontaneous combustion provide the clues to optimising stockpile management practices for a given coal. These factors include:
(a) Rate of heat generation
· Smaller particle size allows a larger surface area for reaction
· More humid air increases the water/coal reaction
· Flow rate of air into the coal bed increases the reaction rate. This is increased by:
· High permeability of coal bed (uniformly large lumps of coal)
· High stockpiles assisting the chimney effect
· High prevailing winds
(b) Rate of heat dissipation
· Flow rate of air into the coal bed (same factors as under (a))
· Thermal conductivity of coal
(c) Rate of heat accumulation
· Balance of (a) and (b)
· Heat capacity of coal (including moisture)
Clearly, some of the above physical factors are associated with competing mechanisms (eg, air flow into the bed helps to increase the rates of both heat generation and heat dissipation). On the balance, the following factors are generally considered undesirable:
· Segregation causing localised areas of uniformly coarse coal and other areas of fine coal,
· Loosely packed coal,
· Exposure to high prevailing winds,
· High ambient temperature and humidity,
· Exposure to heavy rain ,
· Long residence time on stockpile
Factors Relating to Coal Quality
All coals are capable of self-heating and spontaneous combustion under certain conditions. It is not always possible to say why one coal has a greater propensity for self-heating than another. The most general observation that can be made is that lower rank coals have a greater propensity. Definitions of lower rank include:
· High volatile matter content (% daf),
· Low carbon content (% daf) reported with the Ultimate Analysis,
· Low Fuel Ratio (defined as the ratio of Fixed Carbon to Volatile Matter content),
· Low Calorific Value (basis defined by the ASTM)
· Low reflectance of vitrinite.
ACIRL performs two laboratory-scale tests to determine a coal’s propensity to spontaneous combustion. When these test results are not available, the coal property that seems to predict the results is the Air-Dried Moisture.
* These tests, the Relative Ignition Temperature and the Adiabatic Oxidation Test (R70), are small-scale tests of coal properties and do not account for site factors.