· Flame Stability & Turndown Capability
· Burnout Efficiency & Carbon in Ash
Flame Stability & Turndown Capability
Flame stability is the ability to maintain a strong stable flame at the burners, without pulsations or the threat of extinction. When flame stability is poor at low boiler loads, this gives poor turndown capability.
In order to achieve favourable flame stability it is necessary that the ignition and initial combustion in the boiler be rapid, and most of the heat required to achieve this comes from the volatile matter generated in the coal’s first 50 milliseconds or so in the boiler. Consequently poor flame stability is most often associated with low volatile coals (such as the Australian higher Carbon coals referenced in this chapter).
On a daf basis, the VM content is high at between 45 and 55% for most coals with Carbon content less than 80%, suggesting that flame stability should not be an issue. However, as the rank decreases the heating value of the VM also decreases because the VM contains more oxygen and less hydrogen. The higher moisture content of these coals further reduces the flame temperature which does not favour flame stability. Therefore some of the lower rank Indonesian coals may suffer a deterioration in flame stability and turndown capability, but ACIRL is not aware of this having been an issue.
Burnout Efficiency and Carbon in Ash
Poor burnout efficiency11 arises from the inability to burn all of the coal char remaining after the release of volatiles during combustion. High volatile (low rank) coals benefit from the relatively low yield of char needing to be burnt, and also from the fact that the remaining char tends to be more reactive than those from high rank coals. 11 Burnout Efficiency is defined as the percentage by weight of the coal combustibles that are burnt in the boiler. The balance of combustibles reports to the fly ash and
Indonesian coals typically give very high burnout efficiencies as demonstrated by Figure 25 showing results form ACIRL’s pilot-scale Boiler Simulation Furnace plotted against Carbon (% daf). All of the coals in the Figure were pulverised to the same fineness of 70% passing 75 μm.
Figure 25: Burnout Efficiency for Indonesian and Australian Coals
Carbon in Ash: Though Indonesian coals generally give very high burnout efficiency, this does not guarantee a low Carbon-in-Ash, since the coal ash dilutes the unburnt carbon. That is to say:
· If the ash content of a coal is reduced while the burnout remains the same, the carbon in ash will increase (Figure 26) or,
· To put it another way, the Carbon-in-Ash of very low ash coals is extremely sensitive to small variations in burnout efficiency.
Indonesian coals, which typically give high burnout but have low ash contents, can therefore produce wide ranging levels of Carbon-in-Ash. Australian coals generally have poorer burnout efficiency but their higher ash contents help to keep the Carbon-in-Ash down.
Milling Strategy: Because of their inherently high burnout efficiency, Indonesian coals may be allowed to enter the boiler in a coarser state, than other coals, thus reducing the mill power consumption and increasing the mill capacity, while returning burnout efficiency as good as other coals. However, this approach is not always acceptable because, as just explained, the Carbon-in-Ash may then be too high 12).
Figure 26: Carbon-in-Ash Related to Burnout Efficiency and Coal Ash Content
(indicative ranges for Indonesian and Hunter Valley Coals)
Ash Deposition
Broadly speaking, fouling and slagging are associated with the presence of elevated levels of the fluxing elements iron, calcium, magnesium, sodium and potassium in the coal ash, as well as with direct measurements of the ash fusion characteristics.
It was demonstrated in the Section: Chemical & Physical Properties Related To Coal Rank that many Indonesian coals have higher levels of some of these elements and lower Ash Fusion Temperatures. Many slagging and fouling indices, based on ash analysis, have been devised to predict deposition problems. While these indices are known to lack reliability, there are usually no well-established substitutes.
12) Burnout Efficiency is relevant to overall boiler efficiency, whereas high Carbon-in-Ash may render the fly ash unsuitable for use in cement or concrete.
The difficulties in predicting fouling and slagging are worsened by the extreme sensitivity of deposition to power plant design. Power plants that are more tolerant to coals prone to deposition may include features such as:
· Larger boiler size and greater spacing of the burners to reduce flame temperatures,
· More effective coverage by the soot-blowers,
· Design of burner systems to lessen the contact of ash on the walls,
· Larger spacing of superheater/reheater/economiser tubes to prevent bridging.
When all other factors are the same, it can be expected that coals with high levels of fluxing elements will be more prone to deposition problems. However, Indonesian low rank coals have inherent properties that lessen the impact of ash chemistry to some extent:
· Their higher moisture content makes the flame temperature lower, thus helping to
avoid ash melting,
· The ash content is lower, meaning slower growth of deposits which are more able to
be removed by regular soot-blowing before they grow thick enough to melt.
Many Indonesian coals have been tested for slagging and fouling in ACIRL’s Boiler
Simulation Furnace for periods of typically 8 hours. In this time, the above two factors have often contributed to produce deposits that are relatively soft and easily removed from the boiler surfaces.
The most likely problem to occur with those Indonesian coals that have high sodium levels is fouling, when:
· A boiler of relatively small dimensions may give high furnace exit gas temperatures, due to the relatively low heat removal in the radiant furnace, and in spite of the lower flame temperature at the burners. This gives high gas temperatures in the superheaters reheaters,
· The soot-blower coverage in these convective sections is not adequate. If the deposits are allowed to continue to grow, the surface temperature of the deposits increases, causing them to fuse.
One mechanism that has been suggested for fouling is that sodium vaporises in the flame, then combines with sulphur and condenses to a liquid on the (relatively cool) convective tubes, causing the ash to stick and cementing it into strong deposits.
Sodium in coal occurs in different forms, such as:
· Insoluble minerals,
· Soluble minerals, principally salt,
· Organic sodium, which is part of the coal molecules.
Research studies have suggested that the organic form of sodium, followed by the soluble salts, are the main ones to vaporise. The forms of sodium can be identified in a coal by chemical means, but ACIRL is not aware of much data to place the results in context for Indonesian coals.
Environmental Performance
This section covers the environmental effects of coal firing including:
· Solid particulate emissions,
· Carbon dioxide emissions
· Sulphur dioxide emissions
· Oxides of nitrogen emissions
· Fly ash disposal
· Fly ash utilisation
Solid Particulate Emissions
This section is confined to the use of electrostatic precipitators to collect fly ash. The relevant issues are:
· The coal and fly ash properties that impact on ESP collection efficiency,
· Pilot-scale measurements of ESP performance.
Fly Ash Properties – Ash Resistivity: High electrical resistivity is associated with
limitations to collection efficiency. When the resistivity is greater than 1010 Ohm.metres, there may be some difficulties. Fly Ash Resistivity has been measured for a number of Indonesian and Australian coals (Figure 27). Based on resistivity, a few Indonesian coals may have collection efficiency limitations, though the majority have favourable results.
Figure 27: Fly Ash Resistivity for Indonesian and Australian Coals
Fly Ash Properties – Particle Size Distribution: Fly ash particles of less than about 10 μm are considerably more difficult to collect than larger particles. Comprehensive data on fly ash size is not available.
Fly Ash Properties – Ash Chemistry: The presence of elevated levels of sulphur in the coal and sodium in the ash is associated with favourable collection efficiency. Figure 28 plots the Chubu K Factor which is defined as:
K = 1000*(Sulphur in Coal %adb) x (Na2O in Ash %) / (Coal Ash Content %adb)
Based on their relatively high K Factor, Indonesian coals would be expected to give
favourable collection efficiency. K Factor is probably only significant because it correlates with the physical property resistivity (and possibly with particle size).
Figure 28: K Factor for ESP Performance of Indonesian and Australian Coals
Pilot-Scale ESP Measurements: Figures 29 and 30 show the results of direct measurements of fly ash collection characteristics using pilot-scale test facilities 13). Figure 29 shows that the collection efficiencies of the Indonesian coals tend towards the favourable end of the range of the Australian coals; Figure 30 shows the emissions which, for the Indonesian coals, generally display the additional benefit of lower ash contents.
13) For Specific Collection Area of 120 m2/(m3/s)
Figure 29: ESP Slippage (100-Efficiency) for Indonesian and Australian Coals
Figure 30: Solid Particulate Emissions for Indonesian and Australian Coals
Sulphur Dioxide
The relevant coal properties for SO2 emissions are:
· Sulphur content
· Ash content and chemistry
Figure 31, showing the SO2 emissions measured in ACIRL’s Boiler Simulation Furnace
plotted against coal sulphur content, demonstrates the strong influence of sulphur content and shows that Indonesian coals are therefore wide-ranging in their SO2 production.
A small proportion (typically 5-20%) of the sulphur is absorbed by the ash, thus lowering the SO2 emission below the theoretical maximum. The available calcium in the fly ash is thought to provide the mechanism for this absorption, but the correlation between total calcium in the coal and sulphur absorption is not strong enough to provide a confident prediction.
Figure 31: SO2 Emissions for Indonesian and Australian Coals
Emission of Oxides of Nitrogen
NOx emission levels are heavily dependent on plant operating conditions and design features.Nevertheless coal properties must be relevant because some coals inherently produce low NOx levels.
ACIRL has measured NOx emission levels for several hundred coals using standardised
conditions in the pilot-scale Boiler Simulation Furnace. Figure 32 presents these results plotted against nitrogen content, showing that:
· NOx has very little correlation with coal nitrogen content,
· Indonesian coals generally produce low NOx levels compared with Australian and the
other coals shown. This may be partly explained by the generally higher moisture
contents of the Indonesian coals resulting in lower flame temperatures.
In certain situations, high volatile coals tend to produce lower NOx levels than other coals. This applies especially where boilers are fitted with low-NOx burners (the ACIRL furnace is not fitted with a low-NOx burner). This may be a further advantage to Indonesian coals because they all have high volatile coals contents.
Figure 32: NOx Emissions for Indonesian and Australian Coals
Ash Utilisation
The major application of fly ash is as a component increment or concrete. There are a number of physical and chemical requirements for suitable fly ashes. The ones covered here are chemical requirements relating to the content of carbon, silicon, aluminium, iron and sulphur.
The following is a guide 14):
Loss on Ignition: This is equivalent to carbon-in-ash which, as indicated in the section on Burnout (see Figure 26), is lowest when the burnout efficiency is high and the ash content of the coal is high. Coals that satisfy only one of these requirements may suffer from high carbon-in-ash. Low ash Indonesian coals are very sensitive to small variations in burnout efficiency, which in turn may vary from one boiler to another depending on design.
Therefore Indonesian coal fly ashes sometimes fail this requirement because of their low ash content, whereas when Australian coals fail it is more likely a result of lower burnout efficiency. SiO2 + Al2O3 + Fe2O3 Content: Most Indonesian and all Australian coals satisfy the requirement for Class F fly ash, whereas the remainder, which generally contain high levels of calcium, would satisfy the requirement for Class C fly ash. SO3 Content: The SO3 in the fly ash will normally be lower than the SO3 determined from the Ash Analysis of the coal. Nevertheless it is possible that fly ashes from coals high in both calcium and sulphur may exceed this limit.
Ash Disposal
The main issue for ash disposal is the contamination of surface-water and ground-water by trace elements leaching out of the ash. Laboratory leaching tests of fly ash indicate that most fly ashes would not be classified under the definition of hazardous wastes, based on concentrations limits given in drinking water standards. Nevertheless safeguards may be required at disposal sights.
Compared with the fly ash from Australian coals, most trace elements are leached in similar levels from Indonesian coal fly ashes. Elements for which the median levels are relatively low for Indonesian fly ashes are cadmium, fluorine and nickel. However boron levels from Indonesian fly ashes are relatively high.
14) ASTM C618-1996: Coal fly ash and raw or calcined natural Pozzolan for use as a mineral admixture in concrete. Local requirements may vary.