Utilisation Performance of Indonesian and Australian Coals

Introduction
This section examines correlations between coal properties and performance in coal-fired power plants and relates these specifically to the effects of rank and typical differences between Indonesian and Australian coals. This will identify limitations that may occur in some of the power plant processes and may preclude the use of some of these coals in plant not specifically designed for them.

As well as the properties determined by standard laboratory analysis and identified in the previous section, reference will be made to other properties that are more difficult to measure/define such as coal reactivity. Further reference will be made to site-specific variables such as size distribution and moisture content. For convenience, all of these characteristics will be referred to as properties. ACIRL’s experience of testing Indonesian and Australian coals a pilot-scale mill and Boiler Simulation Furnace will also be included.

The Importance of Plant Design
When inherent limitations to coal utilisation are identified, the importance of plant design must be recognised. It is possible to design plant to cope with nearly any shortcoming in coal quality. The job of the coal technologist is to recognise the coal quality issues and to relate these to the requirements of the power plant.

Stockpiling and Handling
This section covers:
· Self-heating and spontaneous combustion
· Bulk handling
· Stockpile slumping
· Fugitive dust emissions

Spontaneous Combustion
Relevant Properties:
· Coal Reactivity
· Size Distribution
· Total Moisture Content

Coal Reactivity. Spontaneous combustion involves reactions between the coal and oxygen or moisture. Reactivity involves characteristics such as molecular structure and porosity that are not normally measured. Generally, low-rank coals tend to have more chemically reactive organic molecular structures and greater porosity to explain their greater propensity to spontaneous combustion.

Laboratory tests have been devised to measure a coal’s inherent propensity to spontaneous combustion without site-specific effects. One such test is the Relative Ignition Temperature test6, also called the Crossing Point Temperature, which has been performed on several hundred coals. Carbon (daf) may correlate well with RIT, however this is not available for the majority of the coals in the database. VM is generally available but does not correlate well with RIT. Air-dried Moisture provides the best correlation from the available data (Figure 17). As the Figure shows, the coals with the lowest RIT (highest propensity to spontaneous combustion) are those with the highest Mad.


Figure 17: Relative Ignition Temperatures versus Air-Dried Moisture

6 This is an ACIRL in-house test.


Figure 18 shows RIT values for a selection of Indonesian and Australian export thermal coals, including some Arutmin and KPC coals.

Figure 18: Relative Ignition Temperatures for Indonesian and Australian Coals
Variations in Total Moisture and Size Distribution. The use of air-dried moisture in the previous section was not intended as a measure of the impact of moisture content but as an indicator of inherent coal reactivity or rank. For completeness it needs to be recognised that in situ variations in Total Moisture content and segregation of coal size distribution are also factors, but these re not obviously connected with the origins or rank of coals.

Bulk Handling
Blockages of chutes and hoppers may be related to:
· Coal size distribution
· Surface Moisture Content
· Types of mineral matter

Coal Size Distribution: The most common cause of bulk handling problems is an excess of fines, such as the percentage -2 mm. A high fines content originates form either:
· Crushing to liberate mineral matter as part of a coal washing process,
· Degradation of coal size due to handling.

Coals that are not washed therefore require less crushing at the mine and their fines content is therefore normally lower. Most Australian coals are washed and may contain up to around 30% -2mm. By comparison, Arutmin and KPC products are understood to contain less than about 20% -2 mm material. Size degradation is more severe with high HGI coals, meaning that some Australian coals would be the most susceptible.

Surface (Free) Moisture Content: There is normally a range of coal moisture contents
within which handling problems are most severe. On the other hand coal that is either airdried or very wet will normally handle satisfactorily. Since the Free Moisture is the criterion, not Total Moisture, there are no inherent differences between high rank and low rank coals.

Types of Mineral Matter: Clays of the bentonite or montmorillinite type become very
sticky when moist, coating the coal lumps and causing them to adhere and interfering with bulk flow. The problem is compounded when a growth forms in the throats of chutes composed of a mixture of the clay and coal fines. The sticky lump grows over time as more clay and fines adhere, then becomes hard and strong with drying.

Fortunately, the majority of clay associated with most coals is the kaolinite type which is not very susceptible to the problem. Bentonitic clays occur in significant quantities in some coals from SE Queensland and may also occur in some Indonesian coals. It is not always possible to remove all bentonite by washing because it may occur as thin bands within the coal lumps. Subsequent weathering on coal stockpiles causes the bentonite to gradually migrate to the coal surface.

Stockpile Slumping
Collapsing of stockpiles may be related to:
· High surface (free) moisture content
· High fines content

High surface moisture content: Slumping is normally a result of high rain-fall at the
stockpile. All coals are susceptible to increases in surface moisture. High Fines Content: It has been suggested that coals with more than 12% of the -0.5 mm fraction are susceptible in extreme rain conditions. As noted above, Indonesian coals are likely to have a lower fines content than Australian coals.

Dustiness
Dust emissions during handling and stockpiling may be related to:
· Low surface (free) moisture content
· High fines content

Low surface moisture content can equally be an issue for any coals. As indicated above, Indonesian coals are likely to contain less coal fines, in which case fugitive dust emissions are likely to be lower.

Ranking of Indonesian and Australian Thermal Coals

Figure 1 ranks a selection of Indonesian and Australian thermal coals (generally export coals) according to the ASTM ranking and also according to the Carbon Content (% daf). Generally the Indonesian coals are of lower rank than Australian, with very little overlap.

Most Indonesian coals are either Sub-bituminous (A, B or C) or High Volatile Bituminous (Bor C). In contrast the majority of Australian coals are High Volatile Bituminous A with lesser numbers at higher and lower ranks than this. Regarding Carbon content (daf), the Indonesian coals range from about 72 to 82%, whereas the Australian coals range from about 78 to 89%.


Figure 1: Ranks of Arutmin, KPC, other Indonesian and Australian Coals

Chemical and Physical Properties Related to Coal Rank

Apart from carbon content, many coal properties follow trends with rank. Figures 2 to 16 show variations in Moisture content, Volatile Matter, Ash content, Calorific Value, Hydrogen, Nitrogen, Sulphur, HGI, Ash Composition and Ash Fusion Temperature with rank for Indonesian coals (with Arutmin and KPC coals identified) and Australian coals. Notable observations include:

Moisture (Figures 2 and 3): There is a sharp increase in Moisture (arb4 or adb5) as the Carbon content decreases from 80 70 70%.

Volatile Matter Content (% daf, Figure 4): Decreases sharply for higher rank coals in the carbon range 80-90%. Reasonably constant at around 50% for Carbon in the range 70-80%.

Ash Content (% db, Figures 5): A slight trend for increasing Ash with Carbon in the range 70-80%.

Calorific Value (Figures 6 and 7): A sharp increase in CV (daf or arb) in the Carbon range 70-80%. Continuing increase in the Carbon range 80-90%.

Hydrogen Content (% daf, Figure 8): Generally reaches a maximum at Carbon about 79%,
with considerable scatter.

Nitrogen Content (% daf, Figure 9): Rough trend for nitrogen to increase with Carbon.

Sulphur Content (% daf, Figure 10): Not much of a trend. The range of Sulphur is wider for Indonesian coals than for Australian.

3 Data sources: KPC and Arutmin coal data was provided by the Bumi Group. Other
Indonesian and Australian coal data came from the Barlow Jonker database.
4 Total Moisture is strictly not a property since it can vary as a result of weather conditions and coal preparation processes. The values used here are considered typical.
5 Air-dried Moisture is not a property to the extent that the laboratory conditions of temperature and humidity are not rigidly controlled. Nevertheless it is a useful indicator of the inherent attraction of moisture to a coal. The sometimes used term inherent moisture is not completely justified.

HGI (Figure 11): Pronounced trend for HGI to increase for Carbon in the range 80-90%. For lower Carbon the HGI is generally low with some exceptions.

SiO2 in Ash (Figure 12): There is a general increase for Carbon in the range 70-80%. Above that, the SiO2 is very wide ranging.

Fe2O3, CaO and Na2O in Ash (Figures 13, 14 and 15): Generally higher for lower rank coals in the Carbon range 70-80%.

Ash Fusion Temperature (Figure 16): The Initial Deformation Temperature is generally lower for lower rank coals in the Carbon range 70-80%.


Figure 2: Total Moisture of Indonesian and Australian Coals

Figure 3: Moisture (adb) of Indonesian and Australian Coals


Figure 4: Volatile Matter Content (% daf) of Indonesian and Australian Coals


Figure 5: Ash Content (% db) of Indonesian and Australian Coals


Figure 6: Calorific Value (daf) of Indonesian and Australian Coals


Figure 7: Calorific Value (arb) of Indonesian and Australian Coals


Figure 8: Hydrogen Content (daf) of Indonesian and Australian Coals


Figure 9: Nitrogen Content (daf) of Indonesian and Australian Coals


Figure 10: Sulphur Content (daf) of Indonesian and Australian Coals


Figure 11: HGI of Indonesian and Australian Coals


Figure 12: SiO2 in Ash of Indonesian and Australian Coals


Figure 13: Fe2O3 in Ash of Indonesian and Australian Coals


Figure 14: CaO in Ash of Indonesian and Australian Coals


Figure 15: Na2O in Ash of Indonesian and Australian Coals


Figure 16: Ash Initial Deformation Temperature of Indonesian and Australian Coals

Summary: The above observations demonstrate that Indonesian coals typically differ in
many ways from Australian coals. Many of the noted trends can be related to differences in rank and are also observed over a much wider range of coals than just Indonesian and Australian. A summary of typical differences in given in Table 2.

Table 2: Typical differences between Indonesian and Australian coals based on
Standard Laboratory Analysis

COAL RANK

The Path from Plant Matter to Anthracite
As long ago as 600 million years, lush growing plants deposited thick layers of dead material in shallow swamps. The initial transformation was due to biochemical decomposition to form peat bogs.

Due to local movements of the earth’s crust the peat bogs sank, increasing the depth of water over the deposit and allowing mineral sedimentation to cover the layer, which became the beginning of a coal seam. These processes of plant deposition, further sinking and covering with sediment may be repeated several times, forming a multi-seam coal deposit.

Continuing movements of the earth’s crust with deposition of mineral matter caused the new coal seams to sink to depths of several kilometres, where elevated temperatures and pressures progressively changed the chemical and physical properties of the coal. High temperatures from volcanic activity sometimes played a part also. The most pronounced change was an increase in the carbon content and this process is known as coalification. Due to coalification the coal progresses from peat, to lignite, to coal, and finally to anthracite.

The coalification path may be followed relatively quickly or slowly depending on the severity of the conditions to which the coal is exposed. The path is not identical for all coals, as there may be differences in:
· The raw materials, that is types of plant matter – species, foliage, branches, spores,algae,
· The chemical environment for the initial plant decomposition – availability of oxygen,
· The temperature and pressure history.

Though the paths differ, coal geologists like to define the progress a coal has made on its journey from peat to anthracite. This leads to definitions of coal rank. Coal rank is the term used to describe the extent of the coalification process, going from low rank to high rank.

Any discussion of coal formation should recognise that dirt was also deposited in the swamps simultaneously with the plant matter, so that all coals contain intimately mixed mineral matter in varying amounts. Many utilisation properties of coal tend to change with coal rank, making it a useful concept.

However, it must be reiterated that different coals of the same rank may differ markedly in the chemical composition and heterogeneity of the organic matter, as well as of the mineral matter.

Definition of Coal Rank
The simplest definitions of coal rank are based on a single coal quality parameter that changes progressively during coalification. There are two main examples:
Carbon Content: It is necessary to exclude the mineral matter in coal so as to characterise the organic component, consequently organic carbon content on a dry mineral matter free basis is the most relevant. This differs slightly from the organic carbon on a dry ash free basis, but the latter is more easily measured and is therefore often used. Carbon (daf) varies from approximately 65% for lignite to 95% for anthracite.

Reflectance of Vitrinite: The metallurgical coal trade makes extensive use of the Reflectance of the Vitrinite maceral. Reflectance of vitrinite generally increases as coalification proceeds1, typical values being 0.3% for lignite, against 3% for anthracite.

Descriptive ranking makes use of traditional terms used for coal types. The ASTM Ranking System2 is defined in Table 1 and classifies coals according to their Volatile Matter content

1. Coal is composed of microscopically identifiable components termed the maceral groups vitrinite, inertinite and liptinite. The relative proportions of these in a coal relate principally to the types of plant matter that were present and the biochemical conditions that applied during the initial transformations, and not so much to the degree of coalification. Both the reflectance and the carbon content differ significantly between the maceral groups within a single sample, therefore the measurement of a property (such as reflectance) of a single maceral group gives technically a more precise measure of the degree of coalification than does a value averaged over the three maceral groups.

2. ASTM D388-95 (1997), Coal by Rank and/or Calorific Value. The system is widely used but the definitions depend on unusual bases: dry mineral matter free (dmmf) basis requires either a measurement or estimate of the mineral matter content of a coal, while moist mineral matter free (mmmf) basis requires a measurement or estimate of the Equilibrium Moisture content.

Table 1: The ASTM Ranking of Coals


* Dry mineral matter free basis
# Moist mineral matter free basis. This refers to the moisture content determined by the ASTM Moisture Holding Capacity Test

Furnace Ash Deposition: Fouling

Fouling is the deposition that occurs when certain volatile constituents (usually sodium compounds) condense on fly ash particles which stick to surfaces in areas where the temperature regime is such that the constituents remain liquid (i.e. do not evaporate). These constituents react with fly ash, other deposits and the flue gas to form bonded deposits.

Fouling is usually restricted to the lower temperature parts of the boiler including the convective section, and the air heaters. Though the temperature regime and chemistry of the deposits are different than those for slagging, the same considerations apply regarding stagewise increase in strength as the deposits grow and the effects of soot-blowing.

The effects of fouling may include:
· High boiler exit temperature leading to poor boiler efficiency
· Low steam temperature
· Excessive soot-blowing (steam wastage, erosion)
· Poor gas flow distribution leading to erosion of tubes
· Poor air-heater performance (blockages, heat transfer).

Relevant Coal Properties
Ash Fusion Temperatures
The Initial Deformation Temperature is sometimes taken as an indication of the onset of the sintering that occurs with fouling. Consequently boiler manufacturers may try to design a boiler so that the temperature of the gas entering the convective passes is lower than the IDT. This approach may be unreliable because it does not account for the selective deposition of the more troublesome components of the ash.

Composition of Ash
The presence of significant quantities of sodium and possibly potassium in the ash elemental analysis may suggest that ash deposition problems would be expected.

Organic Composition and Moisture Content of Coal
As is the case for slagging, some low rank/high moisture coals produce lower flue gas temperatures and, in spite of unfavourable ash composition, they do not cause serious fouling.

Predictive Indices
As with slagging, predictive indices have been derived from the ash composition and may be used successfully within narrow bounds of coal type.


Electrostatic Precipitation
The majority (typically 80%) of the coal ash is carried beyond the furnace and economiser hoppers as fly ash and would be emitted to the atmosphere unless control measures are in place.

Electrostatic precipitators (ESP, Figure 24) are the most common method of removing fly ash from the flue gas steam. In an ESP, a high voltage (40-50kV) is applied between an emitting electrode wire and collecting plates. The flue gas passes through the electric field that is generated and the fly ash particles are electrostatically charged. The particles are then attracted to the collecting plates where they are collected. Periodically the plates are mechanically rapped to cause the ash layer to fall off the collection plates into the hoppers.

The size of an electrostatic precipitator is usually defined in terms of this Specific Collecting Area (SCA), defined as the total collecting area per volume flow of gas. ESP performance can be reasonably expressed in terms of the modified Deutsch equation as follows:


Where :
η = collection efficiency
SCA = specific collecting area
Wk = effective particle migration velocity

The effective migration velocity of the particles is a parameter related to the notional "velocity" at which the particles move transversely to the flow of gas towards the collecting plates, and which can be related to the ESP performance of particular coals. Coals with high wk will achieve high collection efficiencies in ESPs.



Relevant Coal Properties
Electrical resistivity of the ash.
If it has a high resistivity, the ash residing on the collection plates hinders the electriccu current flow between the emitting electrodes and the collecting plates. Trying to compensate by applying a larger potential difference causes the gas within the ash layer to ionise, generating positive ions that migrate towards the emitting electrodes and interfere with the normal process of applying a negative charge to the entrained ash particles.

Moisture Content
Increased moisture levels in flue gas tends to lower the resistivity of the ash and has a beneficial effect on ESP collection efficiency.

Ash Content
Utilities are governed by statutory regulations as to the maximum allowable level of particulate emissions being discharged from their plant. One method of reducing the dust loading in flue gas discharging from an ESP is obviously to reduce the dust loading of the flue gas entering the ESP. The coal ash content directly affects the dust loading of the flue gas entering the ESP.

Chemical composition of Ash
The electrical resistivity can be correlated to some extent with the ash composition. Sodium and potassium in the ash tend to give a lower resistivity and are therefore conducive to better collection efficiency.

Sulphur Content of Coal
A small concentration of sulphur trioxide (formed from the coal sulphur) always occurs in the flue gas. This tends to condense on the ash particle surfaces, lowering the resistivity. High sulphur coals sometimes display superior ESP performance, but the effectiveness of SO3 depends on other elements present in the ash.

Fly Ash Particle Size
Smaller ash particles are more difficult to collect because their drift velocity towards the collection plates is lower.

Fly Ash Cohesivity
When the collection plates are rapped it is desirable that the ash layer remains cohesive and falls en masse into the hopper. If the ash disperses it may be re-entrained by the flue gas passing through the ESP and, unless successfully collected further downstream in the ESP, it passes out the stack.

Pilot-Scale Results versus Coal Properties
Figure 25 shows a correlation between ash collection performance and resistivity. It demonstrates the importance of resistivity, but shows that other factors are also very significant.

The trend line shown in the Figure is of a form found in the literature. It shows that resistivity has the most impact within a relatively narrow band. Below this band performance is uniformly favourable; above the band the performance is uniformly unfavourable.

Presumably this trend was developed for ashes that had relatively constant properties other than resistivity.


Figure 25: Correlation between ESP Losses and Ash Electrical Resistivity

          Total Sulphur in Coal (%) x Na2O in Ash (%)
K = ------------------------------------------------------ x 1000
                               Ash in Coal (%)

As is the case with resistivity, the K Factor predictor has some use but can be unreliable.


Figure 26: Correlation between ESP Losses and Coal Composition K Factor

Gaseous Emissions: Sulphur Dioxide
Oxides of sulphur are formed by combustion of the sulphur in the coal, as shown in Equation(1). Most of the sulphur in the coal is emitted as SO2, however some SO3 (about 1% of SO2) is also produced in the boiler. Fly ash absorbs some of the sulphur oxides, particularly if it is rich in calcium. For bituminous coals, the level of sulphur absorption in the fly ash may be typically 5 - 15%, and may be higher for lower rank coals.

Once emitted to the atmosphere, SO2 is oxidised to SO3 and may form acid rain, consequently power stations in highly industrialised and/or highly populated areas may be required by law to either use low sulphur coals or to equip the boilers with flue gas desulphurisation (FGD), a scrubbing device fitted to the flue gas ducting. FGDs use either wet or dry scrubbers with limestone or quicklime to absorb the SO2. They produce waste CaSO4 (gypsum), which may find industrial applications.

Relevant Coal Properties
Sulphur Content
The sole source of sulphur is the coal.

Ash Content and Composition
Reactive forms of calcium, magnesium, sodium or potassium can absorb some of SO2. Pilot-Scale Results versus Coal Properties Figure 27, showing the measured SO2 emissions versus coal sulphur content, demonstrates that coal sulphur content is a moderately reliable predictor of SO2 emissions.

Figure 27: Sulphur Dioxide Emissions from Pilot-Scale ESP versus Coal Sulphur Content

Gaseous Emissions: Oxides of Nitrogen (NOx)
NOx is formed principally as NO, with minor quantities of N2O and NO2. NOx emitted from the stack forms NO2 in the atmosphere, which may contribute to acid rain or brown haze.

The formation of this pollutant will occur regardless of the fuel being used in the combustion process. However, of the three major fuel types, coal is the most significant producer of NOx due to the nitrogen contained in the fuel itself.
When coal is burned in air, nitrogen oxides are formed by two distinct processes:
· By the combination of atmospheric nitrogen and oxygen at high temperatures, known
as thermal NOx; and
· By the oxidation of chemically bound nitrogen in the fuel, known as fuel NOx.

The formation of thermal NOx occurs when molecular nitrogen reacts with free oxygen atoms in the combustion air. The nitrogen molecules can also react with oxygen or hydroxide ions, formed from the decomposition of water, to form NO. These reactions are very dependent on temperature and residence time. Hence, thermal NOx production may be limited by lowering the flame temperature or by limiting the oxygen available for reaction. In coal combustion, thermal NOx contributes about 10% of the NOx emissions.

The formation of fuel NOx during coal combustion, contributing up to 90% of total NOx emissions, can be simplified into three processes as shown in Figure 28. As the coal particle is heated, it decomposes and the volatiles are evolved. The nitrogen in the coal is split between the solid and volatile fractions of the coal, determined by the time-temperature history of the particle, as well as its chemical nature.

The gas-phase nitrogen that is produced can react to form NOx, NH3, HCN or N2. The formation of NOx is favoured under fuel lean conditions where oxygen molecules are freely available; while under fuel rich conditions, the formation of molecular N2 is preferred in the gaseous reactions.

The nitrogen in the solid material (char) can also be oxidised to form NOx during burnout of the char. However, char nitrogen conversion efficiency is low, and between 60 and 80% of the NOx evolved during combustion is due to the volatile nitrogen.

From an understanding of NOx formation mechanisms, the following conclusions may be drawn to formulate NOx control strategies:
· At least 80% of the total NOx emitted from coal combustion is derived form fuel nitrogen, and of this, between 60 and 80% evolves from the volatile nitrogen compounds.
· The amount of volatile compounds evolved depends on temperature, and the reaction of those compounds to form NOx is dependent on the early mixing history between the coal and the combustion air.
· Mixing conditions in the early stages of pulverised fuel combustion do not significantly affect char nitrogen oxidation.

Many countries impose legal limits on NOx emissions. Control measures for NOx include:
· Modifications within the combustion chamber including low-NOx burners, air-staging and fuel staging. These techniques work principally by delaying the mixing of some of the combustion air with the coal. This provides oxygen-depleted conditions that favour the formation of molecular nitrogen instead of NOx.
· Post-combustion devices, principally Selective Catalytic Reactors that reduce the NOx after it has formed.


Figure 28: Fuel NOx Reaction Scheme

Relevant Coal Properties
Coal Nitrogen Content
As the majority of NOx is formed from the coal nitrogen, this is often seen as a critical ash property. It will be shown later that this is not necessarily so.

Coal Organic Composition
The organic component of coal is composed of very complex molecules, principally containing carbon, hydrogen and oxygen, but also some nitrogen. The form of occurrence of nitrogen influences the mode of release, whether as volatile HCN or NH3 or as char nitrogen. This in turn influences the NOx forming reactions.

Coal Volatile Matter Content
The volatile matter in high volatile coals is released and burned rapidly at the burners and consumes a considerable proportion of the oxygen in the combustion air. Boilers fitted with low-NOx burners utilise this effect to provide oxygen-limited conditions in the early stages of combustion. This favours the conversion of reactive coal nitrogen into molecular nitrogen rather than into NOx. Therefore low NOx burners are potentially more effective with high volatile coals.

Pilot-Scale Results versus Coal Properties
Figure 29, showing the measured NOx emissions versus coal nitrogen content obtained under standardised combustion conditions in the pilot-scale BSF. The Figure demonstrates that coal nitrogen content is a very poor predictor of NOx emissions. In spite of the fact that the majority of the NOx is formed form coal nitrogen, it remains a fact only a small fraction of the nitrogen in the coal forms NOx. The conversion rate depends on the complex organic chemistry of the coal as explained above but, even more, depends on operating conditions and boiler design.


Ash Utilisation
Ideally, ash is utilised to make cement or concrete or for more novel applications to avoid the need for disposal. When used as a component of cement or added to the concrete mix, fly ash must possess pozzolanic properties, that is it must be capable of reacting with free lime in the cement to form a cementitious material. In this way the use of fly ash reduces the proportion of cement(manufactured in a cement kiln) needing to be included. It is possible to substitute up to about 25% of cement with fly ash in concrete, depending on specific application. At even high rates of addition, fly ash also replaces some of the aggregate.

Concrete containing fly ash has many desirable properties:
· Improved chemical resistance against sulphates and chlorides
· Reduces permeability of concrete
· Improved workability with less water
· Improved surface finish
· Reduced temperature due to lower heat of hydration; useful for large concrete structures

Impact of coal properties:
Reactivity
The reactivity of the coal affects the carbon-in-ash level of the fly ash. In overseas plant, carbon-in-ash levels above approximately 5% make disposal of fly ash to the cement industry difficult.

Ash Content
It is easier to satisfy the top-limit on carbon-in-ash for higher ash coals because the ash dilutes any unburnt carbon from combustion.

Ash Composition
There are various standard requirements including an ASTM Standard7 that specifies a minimum on silica, alumina plus iron oxide and maxima on sulphur trioxide, moisture, carbon-in-ash and available alkalis.

Ash Disposal
Not all power stations find markets for their fly ash, even when it may be suitable for utilisation, consequently it is necessary to dispose of some of it in ash dams or as fill for open cut mine excavations. Environmental concerns arise from leaching of trace elements into the water, which end up in streams or in the sea if the water is discharged, while seepage of water from the pond may affect ground water quality. Fly ash and bottom ash may be disposed of using wet or dry sites.

In wet systems the ash is pumped as a slurry from the power station to a pond which may be formed by a dam or by excavation. The ash settles leaving water that may be treated and discharged, recycled, evaporated or impounded. Environmental concerns arise from leaching of trace elements into the water, which end up in streams or in the sea if the water is discharged, while seepage of water from the pond may affect ground water quality. In dry systems, ash is conditioned with a small amount of water and transported by truck to be used as a land-fill. Consideration must still be given to avoiding contamination of groundwater by leachates.

*)ASTM C 618: Coal fly ash and raw or calcined natural pozzolan for use as a mineral admixture in concrete. (1996)

Variations include pumping to dewatering ponds, then trucking to dry sites, or pumping of thick slurries to the final site.

Factors that Determine Levels of Trace Elements Leached
Typically, about two to four percent of the weight of coal ash is water soluble but the majority of this consists of elements which exist in major proportions (sodium, potassium, calcium, magnesium, sulphate, chloride) rather than trace elements.

Factors that affect leaching of trace elements at a disposal site include:
· Properties of ash – composition, particle size, particle morphology, porosity and permeability),
· Properties of the leaching fluid – pH (acidity or alkalinity), degree of aeration,
· Duration of leaching – age of disposal area.

The pH of the leaching fluid may vary because of the background chemistry of the local water, but it may also be markedly affected by the solution of (relatively) major elements from the ash itself.

Prediction of Field Behaviour of Ash
Laboratory leaching tests may be designed to model conditions at specific sites, or they may be for general predictions. The principal classes of test are column-leaching tests, whereby a continuous stream of water is passed through a bed of the ash, and shake extraction tests in which a batch of the ash together with water is placed in a flask and agitated for a fixed period of time.

ACIRL uses a generalised shake extraction test8, with demineralised water added to the ash in the ratio 20:1 and agitated for 18 hours, after which the leachate is filtered and analysed by normal water analysis procedures.

To provide a relative measure of the impact of elements leaching into the ash water, the absolute concentrations are compared with U.S. Drinking Water standards and/or guidelines (US EPA, 2004) for each element. A "Concentration Index" (CI) is defined as:

*) ASTM D3987: Shake extraction of solid wastes with water. (1985)
*) US Environmental Protection Agency, (2004), 2004 Edition of the Drinking Water
Standards & Health Advisories.

              Measured Concentration
CI = -----------------------------------
          US Drinking Water Standard

A Concentration Index in excess of unity does not necessarily signify an unacceptable result, since it would be expected that ash-dam leachates would be diluted in the process of mixing with any drinking water supplies. The U.S. Environmental Protection Agency stipulates a a criterion for hazardous waste whereby the concentration in laboratory leachates should not exceed 100 times the concentration allowed in drinking water. Figure 30 shows a typical set of results for an Australian export coal compared with overseas export coals.


Figure 30: Trace Elements in Leachate – Test Sample Compared with Overseas Coals

Furnace Ash Deposition: Slagging

Slagging is defined as ash deposition occurring as a result of the physical transport of molten or partially fused ash particles from the high temperature (radiant) combustion zone to the furnace wall or tube surface. The occurrence of slagging, if left unchecked, may cause severe interference to the heat transfer distribution in the boiler, and may ultimately disrupt the operation of the plant.

Slagging deposits form on the water-wall tubes, or on superheaters when the platen type is used. When the ash layer depositing on these surfaces is soft and friable they may self-shed or be easily removed by soot-blowing. Sticky sintered or molten deposits are the troublesome ones. These may accumulate on the tubes in a stage-wise manner (Figure 22). The ash that sticks to the clean tube may be powdered because the tube metal is relatively cool (Tw) compared with the combustion gases (Tf). Because this layer insulates the tube, the ash that deposits on top of it gets progressively hotter (Td) as the layer thickness grows. Ultimately the temperature may rise high enough for the surface of the deposit to become molten.



Figure 22: Progressive accumulation of ash deposits: (a) thin layer of adhering powder, (b) sintered deposit, (c) molten deposit

If the deposit layers are soft while they are still thin, they may be prevented from growing to the troublesome stage by frequent soot-blowing, but only if the soot-blowers are situated in the correct locations.

The effects of slagging may include:
· High furnace exit temperature. This may lead to damage from high superheater metal temperature
· High boiler exit temperature, associated with poor boiler efficiency
· Large lumps falling to the bottom of the furnace, causing damage or blockages
· Interference with burner flow patterns
· Excessive soot-blowing (steam wastage, erosion).

As is the case with most adverse coal-related behaviour, slagging occurs as a result of the combination of coal properties, boiler design and boiler operation. Boilers that are physically small relative to their output, or those with the burners situated in a relatively small zone tend to operate at higher flame temperatures and are prone to slagging problems. Boilers that are operated as base-load stations are more prone than those with cycling loads.

Relevant Coal Properties
Ash Fusion Temperatures
The AFT test was designed specifically to predict the formation of sintered or fused deposits. It is often unreliable because it does not account for the selective deposition of the more troublesome components of the ash, not does it account for the stage wise growth of deposits.

Composition of Ash
The presence of significant quantities of Fe2O3 and CaO in the ash elemental analysis may suggest that ash deposition problems would be expected, although this is not always the case, and the influence of other properties especially flame temperature, boiler design and mode of boiler operation often dominate the influence of ash composition.

Organic Composition and Moisture Content of Coal
These parameters influence flame temperature. Some coals produce ash that is fusible at high temperatures but do not cause slagging difficulties because flame temperatures are relatively low. This applies particularly to low rank/high moisture coals.

Predictive Indices
Numerous predictive indices based on ash composition have been derived by several authors (Tables 2 and 3) to better describe ash deposition behaviour, however, the fact that there are so many indices demonstrates their limited application. Some of these are successfully used for a particular type of coal (eg. eastern USA coals) but, because they are not based on sound physical principles, may fail for other coals.

*) Even when soot-blowing is effective, too frequent application is not desirable because (a) it uses steam which is not therefore used to drive turbines and (b) it may cause the removed ash to erode the boiler tubes.

Table 2: Slagging Indices
Table 3: Fouling Indices

Pilot-Scale Results versus Coal Properties
Figure 23 shows a correlation related to ash deposition, and shows the relationship between coal ash constituents (in this case Fe2O3 + CaO) and the flue gas temperature at which the deposited ash will form sintered and/or molten deposits, which will cause problems in operating boilers. This correlation was developed from the pilot-scale testing of a wide range of coals, and includes both coal property data and a plant design parameter in terms of flue gas temperature.

The figure shows a direct correlation of actual boiler performance (flue gas temperature at which deposits become troublesome) with coal properties. The temperature at which the formation of sintered and/or molten deposits occur can be predicted and the relevant temperatures in a full-scale boiler can be designed for a particular coal. The scatter of results in the Figure demonstrates the limited reliability of predictions based on calculated indices.

Figure 23: Nature of Slagging Deposits (Powder or Molten versus (Fe2O3 + CaO) in Ash for Variable Furnace Temperature

Furnace Ash Deposition: Fouling
Fouling is the deposition that occurs when certain volatile constituents (usually sodium compounds) condense on fly ash particles which stick to surfaces in areas where the temperature regime is such that the constituents remain liquid (i.e. do not evaporate). These constituents react with fly ash, other deposits and the flue gas to form bonded deposits.

Fouling is usually restricted to the lower temperature parts of the boiler including the convective section, and the air heaters. Though the temperature regime and chemistry of the deposits are different than those for slagging, the same considerations apply regarding stagewise increase in strength as the deposits grow and the effects of soot-blowing.

The effects of fouling may include:
· High boiler exit temperature leading to poor boiler efficiency
· Low steam temperature
· Excessive soot-blowing (steam wastage, erosion)
· Poor gas flow distribution leading to erosion of tubes

Garmin Approach G5 GPS-enabled Golf Handheld

If you want to track it, Garmin seems like an option. Garmin is one of the original GPS companies on the market, one of the big boys, if you will. But they branched out into more than just giving directions to the average driver.

In an age where everything is getting smaller, including our bank accounts, Garmin has a line of handheld, personal GPS units that come standard with their proprietary software and have such added features such as two-way voice, yes a walkie talkie.

For around $599 (USD) you can buy a dog collar for your dog (or teenage daughter) and pin point the location of the pooch (or daughter) using a hand held tracker. You do not have to be within line of color to do it either. All the tracking is done via satellite.



Garmin's handheld GPSMAP series is handy for the out of doors type and will keep you on the trail and near where you want to be on the lake. Yes, on the lake. These handheld are marine friendly. That doesn't mean they take military men to diner, but they are mostly waterproof. Some of the models will actually float if dropped into the water- but should not be used as a flotation device for really small people. In addition to floating, these handheld come with topographical map downloads, so you get a bird's eye view of where you are at, fresh water and salt water maps, electronic compass and cameras. These units will not set up your tent or guide you to where the big ones are biting though, that is up to you.

Garmin has even gotten into sports somewhat. Their Forerunner series will monitor your heart rate, time your run and keep you from getting lost in the park; all of this in something about the size of a wristwatch. Prices on these units range from $299 to $399 (USD) which is a bit pricey for people who don't run or sports train much. For those who do it simplifies your training regiment and makes the time your training more productive.

In addition to helping people run faster, Garmin took into account the cyclist and developed a smaller version of the GPS units and made them bike mountable. These have a color screen, mapping capability, heart rate monitor and a barometric altimeter (I have no idea why).

Not to be left out, the golfer can have a GPS for the golf course. Which considering the number of people who disappear on a golf course on a daily basis, this is probably not a bad thing. These units come preloaded with golf course maps to easily find your way from hole to hole. It's an advantage to have a 3D representation of the hole you are playing. But realistically, if you backswing was bad before, the Garmin Approach 5G won't make it any better.

By crossing applications and other electronics, such as the walkie talkie function and Bluetooth technology, Garmin has been able to keep a competitive edge and produce useful and life changing technology. While the practice uses for GPS tracking are obvious, Garmin has attempted to take everything to the next level and cut down on the amount of things we have to carry while camping or maps we might have to fold while boating.

Zeeman Haus enjoys writing articles online on a variety of subjects. You can check out his latest website on Garmin GPS Accessories which showcases reviews of some of the top accessories for your Garmin GPS system.

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GolfLogix GPS

The Garmin Golflogix GPS 8 is one device that can actually help you lower your golf score without even going to the driving range, looking at another video, or hiring that golf coach you were thinking about hiring. If that statement alone is not enough to make you run to the store and buy it, then I'm not quite sure you're a real golfer. All kidding aside, the Garmin Golflogix GPS 8 is one amazing device, and in this article, we're going to talk about all of its major features so you can make an informed buying decision.

The Garmin Golflogix GPS 8 actually calculates the distance from wherever you are to the front, back and center of the green. I can also figure out the distances to hazards and bunkers. Even better, all of this information is presented to you on a large, easy to read screen. And...you don't have to press one button to get that information.



You probably know that Garmin is the world's top manufacturer of GPS devices of all kinds. They have dominated the US market for automotive GPS systems for the past several years. With the Golflogix GPS 8, Garmin brings all of their technical expertise to bear on the problem of helping duffers like you and me improve our games.

Of all the rangefinders and golf GPS devices, the Garmin Golflogix GPS 8 might be the easiest to use. All you do when you buy it is hook it up to your computer and download the data on the courses you're interested in, and then you're good to go. It will take no more than a few minutes to download 20 courses' worth of information.

The Garmin Golflogix GPS 8 is very lightweight and small. It fits right into the palm of your hand. As a matter of fact, it weighs only 5.3 ounces and that's with batteries. It's got a LCD screen that you can adjust for bright sunlight. The screen is 1.25" x 2.25", which might seem small, but Garmin's made very good use of that small screen. So, it actually looks larger than it is.

Besides being a really accurate golf GPS device, the Garmin Golflogix GPS 8 has other features you need to be aware of. It's got anti-theft protection for one thing, you can put your personal name on the screen, and it has a clip so you can secure it to your golf bag.

For the money, Garmin's Golflogix GPS 8 is an excellent device.

Next, if you want to know more about the Garmin Golflogix GPS 8, you need to take a look at my website, BestGolfGPSReviews.

Rick Cole is a sports and technology enthusiast. He writes for a number of sports and technology blogs. You can learn more about which Golf GPS to buy from his website, BestGolfGPSReviews.

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