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 ContentGaseous 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