Thursday, December 31, 2009
Coal Loading Guide
Perform P2H units working in the coal are detailed and documented
Perform cleaning and spraying track excavator that will work in the coal
Perform cleaning and spraying HD OB if the vessel will move into coal
Perform the cleaning process of coal from the OB by using a special excavator (bucket blade) before coal digging
Clean spoil the spoil-front loading area before loading activities
Handpicking do flying rock material above the coal exposure
Prohibited conduct coal exposure is too high (4 meters)
Avoid running water or mud that comes into the front or the loading of coal exposed
1. Make drainage on the toe of the slope of the front loading.2. Make bundwall drainage from coal in situ
1. Do dig coal with a minimum level of 3 meters.
2. Position as shown excavators to dig Seam thick (10 m)
1. Perform excavations in the direction Seam Dip minor.
2. Save the extracted coal from the floor 30cm thick.
Continue digging coal by leaving + / - 10cm and categorize these products as a Hi-Ash Product
Creating channels of each 20-meter excavation progress
Perform excavation in one sequence in the body with inset coal less than 10 cm
Perform the process of coal extraction from the roof to the floor in the coal insert
1. Conduct selective loading of coal in the area difficult (intrusions, inserts and fault).
2. Place a special inspector in this frontloading.
3. Use a small Digger (PC-200 s / d PC 400)
4. Perform excavations on dayshift.
1. Make cleaning the surface of clean coal before digging in the area of intrusion.
2. Put a / ribbon at the intrusion.
3. Use proportional Digger tool for this
4. Place a competent superintendent in the area this frontloading.
Avoid digging coal in the minor dropcut in Seam
Give ribbon blockade on areas prone to contamination
Place the operator and the competent supervisory frontloading prone to contamination, such as: the condition of many inserts, the complex geological structure and prone to landslides
Coal mining activities to do with the maximum size of 50 cm
Position the dig tool (loading path) is right on the front loading of coal.
Excavator position is not accurate because of OB and coal.
Make a space between the roof coal and OB at least 0.5 meters.
1. Do a search to identify materials missing metal at front loading coal or ROM.
2. Make the minutes of the case
1. Conduct fire before it left the front loading.
2. Condition of the above is an example of the left Untidy front loading.
1. Use the wheel support unit dozer or loader in the area of front loading
Function support units wheel dozer / loader:
2. As a cleaner area of front loading
3. Collecting coal is still used as a product of bias
3. Tidying up area of front loading
4. Shortening the time slippery
Use the label on the Seam: loading equipment (excavator, loader) and transport equipment (HD, articulate truck, Dumptruck)
1. Use a special unit to carry coal tailgate
2. Unit on the transport of coal is not recommended because of the potential of the coal spill.
Note :
I. Excavator Unit Classification:
- PC200 - 400 = Small
- PC750 - 1250 = Medium
- PC1600 - 4000 = Great
II. Classification Seam:
- T100, 200.300 = Great
- In addition to the above Seam Seam small / minor.
Coal Handling
Coal is a solid fuel containing ash, therefore the use of coal would involve high costs for equipment required for handling and the combustion of coal. It aims to eliminate all dust and ashes. Coal handling security needs, because there are some problems in the handling of coal, among others:
a. Coal can be burned alone
b. Coal may cause explosion
c. Coal may cause pollution, if there are strong winds dust flying everywhere
Burned OWN
Coal can be burned themselves after experiencing a gradual process that is:
1. The first stage: first coal will absorb oxygen from the air slowly and then the temperature of coal will rise
2. The second stage: as a result of rising temperatures speed coal absorbs oxygen from the air temperature increases and then will reach 100-1400C
3. The third stage: after the temperature reaches 1400C, vapor and CO2 are formed
4. Step four: up to temperatures of 2300c, isolation of CO2 will continue
5. Stage five: when the temperature was above 3500C, this means that coal has reached the point quickly fired and will burn
Cause Burned Own
Coal is an organic fuel, and if touched directly by air in high temperature conditions (eg. prolonged drought) will burn itself. This situation will be accelerated by:
a. Exothermic process (steam and oxygen in the air), this is the most common
b. Bacteria
c. Catalytic action of inorganic objects
While the possibility of burning yourself, especially among other things:
a. Low carbonization
b. Sulfur high levels (> 2%). Threshold levels of 1.2% sulfur should
Fighting The Coal Burned Own
When the accumulation of coal stockpiled a closed place (indoor storage) then the rules must be made so that the warehouse is clean of dust deposits of coal, mainly found in the surface equipment. Thus it is necessary to continue treatment and constant. If the landfill is open (outdoor storage) should be chosen so that the flat and not humid, it is to avoid infiltration impurities (impurities). For coal fly high substance spray should be used (sprinkler). Coal storage too long, too dangerous, we recommend a maximum of 1 month.
High Pile
The high pile of coal piles are difficult to determine because each landfills have their own conditions such as climate, humidity, radiation.
Symptoms Of Early Checking Burned
a. Checking Temperature
To determine the maximum temperature of the pile of coal can be 1-2m below the surface of the pile. Here's how: create a vertical hole assisted by perforated pipes. Utility pipes to the hole no longer being buried in coal use for the temperature inside the perforated holes with temperatures in the pile.
b. Coal may cause explosion
Coal dust explosion caused by:
1. Dust particle size: <20 mesh (= 0.833 mm) 2. There is a relationship between the substance and the degree of explosions fly Volatile (%) Volatile ratio = -------------------------------- Volatile (%) + Fixed carbon (%) If volatile ratio> 0.12 then the possibility of coal dust explosion is always there. When the components of coal ash in the dust> 70-80% will not need to fear the danger of explosion. Conditions for the burst will occur when the particles smooth floating time. Also the burning gases in the air can help the explosion.
c. How to control the explosion
1. Use an inert gas (N2 gas). Gas is expensive enough, but it evaporates too quickly, so always be checked pressure valve. Put this N2 gas cylinders in storage pulverized coal bin, also in part filter (B/F)
2. Cleaning done periodically to avoid the formation of coal dust deposits
3. Eliminates the possibility of achieving a point source of ignition point in the installation
4. Watch, look and find the source of the fire as early as possible
5. In the case of coal deposits is covered with plastic to keep your O2 concentrations less than 12%. In an open heap, use water spray using an automatic sprinkler system will be very helpful in preventing coal fires.
Following way : control panel operator (CPO) in the pipe put in heaps of coal and then fit in a certain temperature. If the temperature increases and the piles of coal-fit temperature exceeds the COP, the automatic sprinkler watering will work itself is a pile of coal.
Treatment of coal dust
Plastic sheets cover the coal pile is the best, sought not to use dark-colored plastic. Heap is compacted with a bulldozer to reduce the presence of oxygen in the sidelines of coal. On the open surface of coal heap pile should be sprayed with a hardened surface of liquids. This fluid is an additional product from oil refinery.
a. Coal can be burned alone
b. Coal may cause explosion
c. Coal may cause pollution, if there are strong winds dust flying everywhere
Burned OWN
Coal can be burned themselves after experiencing a gradual process that is:
1. The first stage: first coal will absorb oxygen from the air slowly and then the temperature of coal will rise
2. The second stage: as a result of rising temperatures speed coal absorbs oxygen from the air temperature increases and then will reach 100-1400C
3. The third stage: after the temperature reaches 1400C, vapor and CO2 are formed
4. Step four: up to temperatures of 2300c, isolation of CO2 will continue
5. Stage five: when the temperature was above 3500C, this means that coal has reached the point quickly fired and will burn
Cause Burned Own
Coal is an organic fuel, and if touched directly by air in high temperature conditions (eg. prolonged drought) will burn itself. This situation will be accelerated by:
a. Exothermic process (steam and oxygen in the air), this is the most common
b. Bacteria
c. Catalytic action of inorganic objects
While the possibility of burning yourself, especially among other things:
a. Low carbonization
b. Sulfur high levels (> 2%). Threshold levels of 1.2% sulfur should
Fighting The Coal Burned Own
When the accumulation of coal stockpiled a closed place (indoor storage) then the rules must be made so that the warehouse is clean of dust deposits of coal, mainly found in the surface equipment. Thus it is necessary to continue treatment and constant. If the landfill is open (outdoor storage) should be chosen so that the flat and not humid, it is to avoid infiltration impurities (impurities). For coal fly high substance spray should be used (sprinkler). Coal storage too long, too dangerous, we recommend a maximum of 1 month.
High Pile
The high pile of coal piles are difficult to determine because each landfills have their own conditions such as climate, humidity, radiation.
Symptoms Of Early Checking Burned
a. Checking Temperature
To determine the maximum temperature of the pile of coal can be 1-2m below the surface of the pile. Here's how: create a vertical hole assisted by perforated pipes. Utility pipes to the hole no longer being buried in coal use for the temperature inside the perforated holes with temperatures in the pile.
b. Coal may cause explosion
Coal dust explosion caused by:
1. Dust particle size: <20 mesh (= 0.833 mm) 2. There is a relationship between the substance and the degree of explosions fly Volatile (%) Volatile ratio = -------------------------------- Volatile (%) + Fixed carbon (%) If volatile ratio> 0.12 then the possibility of coal dust explosion is always there. When the components of coal ash in the dust> 70-80% will not need to fear the danger of explosion. Conditions for the burst will occur when the particles smooth floating time. Also the burning gases in the air can help the explosion.
c. How to control the explosion
1. Use an inert gas (N2 gas). Gas is expensive enough, but it evaporates too quickly, so always be checked pressure valve. Put this N2 gas cylinders in storage pulverized coal bin, also in part filter (B/F)
2. Cleaning done periodically to avoid the formation of coal dust deposits
3. Eliminates the possibility of achieving a point source of ignition point in the installation
4. Watch, look and find the source of the fire as early as possible
5. In the case of coal deposits is covered with plastic to keep your O2 concentrations less than 12%. In an open heap, use water spray using an automatic sprinkler system will be very helpful in preventing coal fires.
Following way : control panel operator (CPO) in the pipe put in heaps of coal and then fit in a certain temperature. If the temperature increases and the piles of coal-fit temperature exceeds the COP, the automatic sprinkler watering will work itself is a pile of coal.
Treatment of coal dust
Plastic sheets cover the coal pile is the best, sought not to use dark-colored plastic. Heap is compacted with a bulldozer to reduce the presence of oxygen in the sidelines of coal. On the open surface of coal heap pile should be sprayed with a hardened surface of liquids. This fluid is an additional product from oil refinery.
Coal Water Fuel (CWF)
Like known kerosene, obtainable diesel fuel and gasoline with coal liquefaction conversion process. Obtainable gas fuel with coal gasification process. One of process that is simple is modification of coal becomes a coal mixture having the character of liquid that is coal water fuel can replace burning oil which is one of petroleum product.
a. RAW MATERIAL CWF
As component of standard utilized by coal having high caloric value (more or less 7000 kcal/kg) as compensation of usage of water so that caloric value CWF obtained enough heights also. Bitumen type coal raw material with high caloric value and wafting water content (inherent moisture) low is suggested so that constraint the low of caloric value cwf obtained able to be overcome. Actually earns also is utilized bitumen sub and or lignite, But both types of the has high wafting water content so that cwf yielded will have low caloric value. To overcome the thing must be done draining at temperature and high pressure.
Clauses of raw material CWF is ;
1. Low ash content
2. Matter content flies bigger than 20%
3. Number HGI height must
4. Fouling and slagging low index
5. Sulfur content less than 1 %.
Beside doesn't contaminate air, ash content must be low to lessen modification fare of stove at dismissal of base ash (bottom ash). Matter content flies > 20 % to water down en kindling. In making of CWF utilizes smooth coal (- 75 microns) hence required hulling. On that account number HGI height must to lessen grind fare. Height ash melting point must avoid precipitation to of ash which is easy melts at stove interior (boiler). The happening of fouling and slagging can stop operation, on that account fouling and slagging need to be cleaned to return high temperature switching. Index fouling and slagging influenced by alkalies content and brimstone in ash. Side that is sulfur content must be low to prevent environmental contamination and interior corrosion boiler.
b. ADDITIVE
Additive is material added into mixture CWF and functioning to add the stability, mean prilled of the coal doesn't decants during old (2 month or more). There is also functioning additive dispersion to prilled of the coal. Additive addition ranged from 0,1 to 1,5 depends on kinds of the additive. From result of research it is concluded that good additive in the form of surfactant (surface activator reagent item) what can be consisted of surfactant ionik (anionik or kationik) and surfactant non-ionik.
There is also other additive which the function to make mixture having the character of emulsion and stable. Because this surfactant type many variation, so is required research specially suited for coal is being used to raw material CWF. Clauses of good additive is must be effective, be combustible in process of combustion and cheap.
b. Making CWF
Making technology of CWF is including simple especially if using coal raw material having high caloric value (more or less 7000 kcal/kg). Coal having low ash content (< 10%) grind to become 10 mm and then is milled with ballmill. Hulling is done with concentration of high solid (more or less 70% coal). Result of grind is done at one particular size separation (size classifier) at winnow measure 75 microns. Big oversize 75 microns distribute to equipment of reduction of water (dewatering) if required. The biggest particle size of coal doesn't fetch up all standing by 75 microns only, earns also bigger or smooth depended from coal type. Level of concentration of mixture at squealer (mixing) determined when optimization of laboratory scale before all. For coal with quality of height, process of CWF earns more simple.
After hulling earns direct is done squealer where at this additive phase is added. At low level coal with high wafting water content need to be done draining in advance at high temperature. Squealer taken place only during few minutes with high rotation (> 6000) and yields high stability (> 2 month).
Wednesday, December 30, 2009
Performance of Blends
The previous section set out the coal quality parameters which can be calculated for blends and gave an example of blending to improve or optimise the coal specification. The material was relevant because:
· It is often necessary to provide coals to a specification, and
· The coal analysis may be all that is available to estimate the performance of a coal product.
Ultimately it is important to know:
· How well the specification of the blend will predict the performance, and
· If the performance of the component coals is already known, whether the performance measurements are additive in a blend.
If the component coals behave independently during combustion processes, it may be
expected that the performance of the blend can be predicted by summation. However in some cases there are interactions between the coals that may produce unexpected results, favourable or otherwise. It is generally believed that the likelihood of these interactions is greater when the properties of the component coals are more dissimilar.
Milling
Moisture content, CV, HGI and Abrasion Index have been identified as coal quality
parameters associated with mill performance. The effect of moisture on mill heating requirements, and of CV on mill throughput, are straightforward and additivity therefore applies.
HGI is known to be an imprecise predictor of mill capacity, power consumption and product fineness for single coals and the same applies for blends. If predictions are to be made it is better to base them on a weighted HGI rather than on a measurement.
If the performance of the individual coals is known, experience suggests that most blends will perform approximately as the weighted average of the components. This is illustrated in Figure 1 for mill power consumption and seems to work for pairs of very different coals.
Figure 1: Mill Power Consumption for Blends of Coal Pairs
Mill wear rates for blends generally lie somewhere between those of the component coals, but do not appear to be strictly additive5 (Figure 2).
Figure 2: Mill Wear Rates for Blends of Coal Pairs
5) Most of the coals shown had low abrasivity, where the precision of the measurement was poor. One coal was very abrasive and appeared to have a strong influence on the blends.
Flame Stability
This performance characteristic is difficult to measure quantitatively. Flame stability is mainly aided by the rapid release of heat with the volatiles, and is hindered by high moisture levels. The calorific value of the volatiles is difficult to predict because the quantity of volatiles released in a boiler flame is different (normally higher) than is measured in the proximate VM analysis. It is not known whether this process is additive.
Burnout Efficiency
High burnout efficiency is normally expected of high VM coals, that is low Fuel Ratio coals. However it is well known that FR or VM provide only very rough estimates of burnout. For blends the burnout efficiency is mostly not additive; it is not uncommon for a blend to perform worse or better than either of the component coals (Figure 4). It is reasonable to suggest that two coals in a blend do not burn independently because each one influences the environment of temperature and oxygen availability which affects the other one.
Another factor contributing to the erratic burnout behaviour of blends is the fact that they are milled together. Though the gross behaviour of mill power consumption and PF fineness appears to be approximately additive, the particles from the softer coal in a blend will be finer than those of the harder coal; the single coals in Figure 4, and the blended coals overall, were all at the same fineness (70% passing 75 μm).
Figure 3: Burnout Efficiency for Blends of Coal Pairs
Deposit Formation
Slagging and fouling are very difficult to predict for single coals based on coal properties because so many mechanisms are involved. For an ash deposit made up of finely mixed elements of known proportions, the fusibility behaviour is complex because of eutectic behaviour6, but can be predicted using complex mathematical models. However in a real case this is complicated by factors such as:
· The deposit is not homogeneous and includes large particles of different compositions, so that eutectic equilibrium may never be reached,
· The bulk composition of the deposit is not the same as the coal ash composition
because of differences in the stickiness and size of different particles that impinge on the boiler surfaces, giving preferential deposition. It may therefore not be possible to predict this composition.
6) A eutectic is a critical mixture of substances that melt at a lower temperature than other mixtures of slightly different composition. Given the great number of elements in coal ash there are many eutectic combinations.
For a blend, the complexity of the ash composition and distribution is magnified and the possibilities for eutectic interactions are great. Even if the above interactions were not present, the temperature environment is normally modified by adding a second coal. Therefore it is possible to combine two relatively harmless coals and to make a blend that fouls or slags.
However it would be wrong to suggest that all blends cause unexpected deposition problems. In an average case it makes sense to use a trouble-free coal in a blend to upgrade a troublesome one. In spite of the risk of trusting the Ash Fusion Test to predict deposition problems, it should be used to test laboratory samples of blends to try to avoid poor coal combinations.
Electrostatic Precipitation
The most important ash property for ESP collection efficiency is its electrical resistivity. Over a relatively narrow range of resistivity there tends to a marked change in collection efficiency; above this range the efficiency is uniformly poor, while below the range it is generally favourable.
When two coals of different resistivity are blended the resistivity of the blend ash tends to lie between those of the component coals; when the results are plotted on a log scale (Figure 4) the appearance is of approximate additivity. In the Figure the resistivity of the blend is in the middle of the range between those of the parent coals (based on the log scale), but the slippage (ie, emission for a constant ash loading) is closer to that of coal A.
The example given demonstrates a bonus to be obtained by blending these two coals. In another case (Figure 5) where the two blended coals have higher resistivities the blend may turn out worse than anticipated.
These two examples seem to explain most results obtained for blends in ACIRL’s pilot-scale Boiler Simulation Furnace. The collection efficiency is often not additive but the result for the blend lies somewhere in the range between those of the parent coals (not better or worse than both parent coals).
The above does not take into account the effects of coals moisture and ash content. For high resistivity ash coals, moisture in the flue gas tends to lower the ash resistivity and therefore improve collection efficiency, and moisture in coal is additive. Ash content does not impact appreciably on collection efficiency but impacts on emissions. Ash is additive and needs to be considered when predicting emissions.
Figure 4: Impact on ESP Efficiency of Blending two Coals with Low to Moderate
Ash Resistivity
Figure 5: Impact on ESP Efficiency of Blending two Coals with Moderate to High
Ash Resistivity
Sulphur Dioxide Emissions
Sulphur Dioxide emissions are almost proportional to the sulphur content of the coal (% daf) because only a small proportion of the sulphur is absorbed by the ash. Therefore a good estimate of SO2 emissions can be calculated for a blend by calculating the daf sulphur content (Figure 6). It is not clear whether the small proportion absorbed by the ash is additive.
Figure 6: SO2 Emission from Two Blends from Coal Nos. 299 and 240
Emission of Oxides of Nitrogen
The emission of NOx does not correlate with the coal nitrogen content, and other reliable methods have not been developed.
A number of sets of blends have been tested in ACIRL’s pilot-scale Boiler Simulation
Furnace with the results shown in Figure 7. The NOx level of blends certainly cannot be predicted based on that of two parent coals that have very different levels. In these cases the blend seems to behave approximately like one or other of the parent coals. However when the two parent coals have fairly similar emissions, blends made from them do not appear to give any great surprises.
Figure 7: NOx Emissions for Blends of Coal Pairs
CONCLUSIONS
Enormous possibilities for satisfying coal specifications by blending, including the use of optimisation.
There are pitfalls and risks because satisfying a specification does not guarantee satisfactory utilisation performance. Nevertheless blending to a specification is a necessary precursor to combustion trials of blends that look promising. When blends are planned between two coals with very different properties, it is advisable to allow for trials at the pilot-scale or full-scale.
· It is often necessary to provide coals to a specification, and
· The coal analysis may be all that is available to estimate the performance of a coal product.
Ultimately it is important to know:
· How well the specification of the blend will predict the performance, and
· If the performance of the component coals is already known, whether the performance measurements are additive in a blend.
If the component coals behave independently during combustion processes, it may be
expected that the performance of the blend can be predicted by summation. However in some cases there are interactions between the coals that may produce unexpected results, favourable or otherwise. It is generally believed that the likelihood of these interactions is greater when the properties of the component coals are more dissimilar.
Milling
Moisture content, CV, HGI and Abrasion Index have been identified as coal quality
parameters associated with mill performance. The effect of moisture on mill heating requirements, and of CV on mill throughput, are straightforward and additivity therefore applies.
HGI is known to be an imprecise predictor of mill capacity, power consumption and product fineness for single coals and the same applies for blends. If predictions are to be made it is better to base them on a weighted HGI rather than on a measurement.
If the performance of the individual coals is known, experience suggests that most blends will perform approximately as the weighted average of the components. This is illustrated in Figure 1 for mill power consumption and seems to work for pairs of very different coals.
Figure 1: Mill Power Consumption for Blends of Coal Pairs
Mill wear rates for blends generally lie somewhere between those of the component coals, but do not appear to be strictly additive5 (Figure 2).
Figure 2: Mill Wear Rates for Blends of Coal Pairs
5) Most of the coals shown had low abrasivity, where the precision of the measurement was poor. One coal was very abrasive and appeared to have a strong influence on the blends.
Flame Stability
This performance characteristic is difficult to measure quantitatively. Flame stability is mainly aided by the rapid release of heat with the volatiles, and is hindered by high moisture levels. The calorific value of the volatiles is difficult to predict because the quantity of volatiles released in a boiler flame is different (normally higher) than is measured in the proximate VM analysis. It is not known whether this process is additive.
Burnout Efficiency
High burnout efficiency is normally expected of high VM coals, that is low Fuel Ratio coals. However it is well known that FR or VM provide only very rough estimates of burnout. For blends the burnout efficiency is mostly not additive; it is not uncommon for a blend to perform worse or better than either of the component coals (Figure 4). It is reasonable to suggest that two coals in a blend do not burn independently because each one influences the environment of temperature and oxygen availability which affects the other one.
Another factor contributing to the erratic burnout behaviour of blends is the fact that they are milled together. Though the gross behaviour of mill power consumption and PF fineness appears to be approximately additive, the particles from the softer coal in a blend will be finer than those of the harder coal; the single coals in Figure 4, and the blended coals overall, were all at the same fineness (70% passing 75 μm).
Figure 3: Burnout Efficiency for Blends of Coal Pairs
Deposit Formation
Slagging and fouling are very difficult to predict for single coals based on coal properties because so many mechanisms are involved. For an ash deposit made up of finely mixed elements of known proportions, the fusibility behaviour is complex because of eutectic behaviour6, but can be predicted using complex mathematical models. However in a real case this is complicated by factors such as:
· The deposit is not homogeneous and includes large particles of different compositions, so that eutectic equilibrium may never be reached,
· The bulk composition of the deposit is not the same as the coal ash composition
because of differences in the stickiness and size of different particles that impinge on the boiler surfaces, giving preferential deposition. It may therefore not be possible to predict this composition.
6) A eutectic is a critical mixture of substances that melt at a lower temperature than other mixtures of slightly different composition. Given the great number of elements in coal ash there are many eutectic combinations.
For a blend, the complexity of the ash composition and distribution is magnified and the possibilities for eutectic interactions are great. Even if the above interactions were not present, the temperature environment is normally modified by adding a second coal. Therefore it is possible to combine two relatively harmless coals and to make a blend that fouls or slags.
However it would be wrong to suggest that all blends cause unexpected deposition problems. In an average case it makes sense to use a trouble-free coal in a blend to upgrade a troublesome one. In spite of the risk of trusting the Ash Fusion Test to predict deposition problems, it should be used to test laboratory samples of blends to try to avoid poor coal combinations.
Electrostatic Precipitation
The most important ash property for ESP collection efficiency is its electrical resistivity. Over a relatively narrow range of resistivity there tends to a marked change in collection efficiency; above this range the efficiency is uniformly poor, while below the range it is generally favourable.
When two coals of different resistivity are blended the resistivity of the blend ash tends to lie between those of the component coals; when the results are plotted on a log scale (Figure 4) the appearance is of approximate additivity. In the Figure the resistivity of the blend is in the middle of the range between those of the parent coals (based on the log scale), but the slippage (ie, emission for a constant ash loading) is closer to that of coal A.
The example given demonstrates a bonus to be obtained by blending these two coals. In another case (Figure 5) where the two blended coals have higher resistivities the blend may turn out worse than anticipated.
These two examples seem to explain most results obtained for blends in ACIRL’s pilot-scale Boiler Simulation Furnace. The collection efficiency is often not additive but the result for the blend lies somewhere in the range between those of the parent coals (not better or worse than both parent coals).
The above does not take into account the effects of coals moisture and ash content. For high resistivity ash coals, moisture in the flue gas tends to lower the ash resistivity and therefore improve collection efficiency, and moisture in coal is additive. Ash content does not impact appreciably on collection efficiency but impacts on emissions. Ash is additive and needs to be considered when predicting emissions.
Figure 4: Impact on ESP Efficiency of Blending two Coals with Low to Moderate
Ash Resistivity
Figure 5: Impact on ESP Efficiency of Blending two Coals with Moderate to High
Ash Resistivity
Sulphur Dioxide Emissions
Sulphur Dioxide emissions are almost proportional to the sulphur content of the coal (% daf) because only a small proportion of the sulphur is absorbed by the ash. Therefore a good estimate of SO2 emissions can be calculated for a blend by calculating the daf sulphur content (Figure 6). It is not clear whether the small proportion absorbed by the ash is additive.
Figure 6: SO2 Emission from Two Blends from Coal Nos. 299 and 240
Emission of Oxides of Nitrogen
The emission of NOx does not correlate with the coal nitrogen content, and other reliable methods have not been developed.
A number of sets of blends have been tested in ACIRL’s pilot-scale Boiler Simulation
Furnace with the results shown in Figure 7. The NOx level of blends certainly cannot be predicted based on that of two parent coals that have very different levels. In these cases the blend seems to behave approximately like one or other of the parent coals. However when the two parent coals have fairly similar emissions, blends made from them do not appear to give any great surprises.
Figure 7: NOx Emissions for Blends of Coal Pairs
CONCLUSIONS
Enormous possibilities for satisfying coal specifications by blending, including the use of optimisation.
There are pitfalls and risks because satisfying a specification does not guarantee satisfactory utilisation performance. Nevertheless blending to a specification is a necessary precursor to combustion trials of blends that look promising. When blends are planned between two coals with very different properties, it is advisable to allow for trials at the pilot-scale or full-scale.
Targeting a Coal Specification
In order to formulate a blend to satisfy a coal specification it is necessary to understand whether the coal properties are additive. Additive properties are those that can be calculated as a weighted average of the properties of the component coals. For practical purposes these properties include:
Moisture
Ash
Volatile Matter
Carbon
Hydrogen
Nitrogen
Sulphur
Forms of sulphur
Chlorine
Phosphorus
Ash analysis2
Calorific value
Trace elements
Properties that are not additive include:
Ash Fusion Temperature:
It is well established from phase equilibrium theory that the melting behaviour of mixtures of ash oxides are not additive. Occasionally blends can have AFTs that are higher than or lower than those of either of the component coals.
Hardgrove Grindability Index:
The HGI of a blend is normally lower than the weighted average. Figure 1 shows HGI measured for blends between many pairs of coals as well as of the component coals. The connecting lines are not straight but generally curved downward.The measured HGI therefore provides an unfavourable indication of the pulverising characteristics of the blend which is often not justified in practice. It is not recommended that the HGI of a blend be reported if it can be avoided.
2) This may not apply to sulphur (as SO3) because not all of the coal sulphur remains in the ash. In such a case the other elements would need to be normalised to a constant sulphur level.
Figure 1: Measured HGI for Blends of Coal Pairs
Abrasion Index: Experience suggests that the Abrasion Index may not be additive, though this may be a result of poor precision of the test.
Calculating the Coal Analysis for Additive Properties
In principal it is necessary to convert all analytical results to the as-received basis before determining the weighted average. Once this has been done the results can be converted back to any other basis if required.
The Ash Analysis of a blend is an important example of the above. In this case it is necessary to firstly calculate the percentages in the coal rather in the ash. The required steps for calculating the CaO in a 50/50 blend of two coals A and B are:
(1) CaOA (arb) = CaOA (% in Ash) x AshA (arb) / 100
(2) CaOB (arb) = CaOB (% in Ash) x AshB (arb) / 100
(3) CaOblend (arb) = 0.5 x CaOA (arb) + 0.5 x CaOB (arb)
(4) Ashblend (arb) = 0.5 x AshA (arb) + 0.5 x AshB (arb)
(5) CaOblend (% in Ash) = CaOblend (arb) / Ashblend (arb) x 100
A spreadsheet can easily be made to perform these steps without risk of errors. Alternatively a more compact spreadsheet can be made to do the calculations more directly and for blends of several coals 3).
Optimising the Composition of a Blend
Blends may be designed to satisfy a number of criteria such as top or bottom-limits on coal composition parameters using an optimising program4. At the same time it is possible to optimise the solution, for example:
Two coals are to be blended to achieve less than 15% total moisture, less than 0.9%
sulphur, a Fuel Ratio less than 2, and Na2O in ash between 0.5 and 2%. While
achieving this it is desired to minimise the cost of the coal, that is to maximise the proportion of the cheaper component coal. The required solution is the proportions of the two coals that satisfy all of these criteria at minimum cost.
The above example includes the Fuel Ratio which is derived from two parameters (Fixed Carbon/Volatile Matter). It is possible to include more complex parameters such as a Slagging Index. Occasionally there is no possible solution to the problem as defined, that is all of the requirements cannot be satisfied simultaneously. In such a case it is necessary to either relax one of the requirements or look at different component coal(s).
3) An Excel spreadsheet will do this using the SUMPRODUCT function. It is wise to check the calculations once using the long-hand method described above.
4) For example the Add-in Solver operation in Excel.
A Blending Scenario
Table 1 gives the analysis of two coals of very different properties, and the analysis of a 70/30 blend calculated as described above.
Table 1: Calculated Composition of a 70/30 Blend
In this case each of the coals has properties that may make them unattractive to coal customers. Because they are not the same properties for the two coals, the blend has properties that are generally more attractive, demonstrating that there may be mutual benefits with blending. Some of the features that are improved by blending are listed in Table 2.
Table 2: Properties that may be improved by blending Coals A and B
Worth noting is that in the case of sulphur, Fe2O3 and Na2O, intermediate values may be preferable to low or high values, so that blending improves these values for both coals. Table 1 includes estimates of the HGI and Abrasion Index of the blends event though these may not be strictly additive. As mentioned above, a measured HGI on the blend would probably be lower (perhaps around 48) than the value in Table 1, however experience suggests that the calculated value is a better indicator of mill performance than a measured value.
Ash Fusion Temperature. As indicated above, the AFT of a blend cannot be predicted by weighting the numbers. Nevertheless is reasonable to calculate a hypothetical value by this means (taking into account the different ash contents of the component coals), then confirming by direct measurement on a laboratory sample of the blend.
Blending of Thermal Coals
Introduction
Coal has many diverse properties, each one of which impacts on some aspects of power plant performance. The value of coal to the user is a maximum when the set of coal properties is matched to the particular requirements of the user’s power plant. Because there are so many diverse properties, there may be few coals that give a good match for all properties.
This shortage of matching coals is favourable to neither the coal producer nor the coal buyer, hence the need for blending - this increases the scope tremendously for presenting coals that fully satisfy a strict specification.
A normal coal specification is based on standard chemical and physical laboratory tests. The ability to satisfy a specification by blending is limited by:
· The properties of the available coals, and
· The additivity of properties, that is whether they are a weighted average of the properties of the component coals.
The next major issue with blending is how a coal blend performs. Achieving a coal specification does not infallibly predict the coal’s performance in power plant, though it is often all we have to make these predictions. Satisfying a specification does not guarantee satisfactory performance, even in the case of unblended coals, which raises the question as to whether standard laboratory tests are less reliable predictors for blends than for single coals.
Benefits of Blending
Both coal producers can benefit from blending and blends are produced anywhere between the mine and the power plant. Coal producers may blend for a combination of the following reasons:
· To enable them to offer a product that satisfies customers’ specifications,
· To enable them to sell coals that may be otherwise unsaleable,
· To increase their tonnage sales,
· To offer products that can be produced more cheaply and are more competitive on the market,
· To formulate new supplies to match an established product that may come from a mine approaching the end of its life,
· To win favour with customers by supplying more consistent quality,
The above benefits can also accrue from blending with a competitor’s coal and there may be more of this in the future. Power station operators may blend for a combination of the following reasons:
· To design acceptable feedstocks from coals that may be unsuitable in the unblended state,
· The coals used can be cheaper, thus saving fuel costs,
· To make boiler plants more easy to operate by improving the consistency of coal quality,
· To diversify the sources of the coal supply, thereby increasing competition and securing the supply,
· To enable poor quality domestic coals to be used.
Blending Logistics & Methods
Coal blends may be produced at:
· The mine, from different seams or areas,
· The outgoing seaport, where competitors may make blends for mutual advantage,
· The receival/distribution port, where customers may specify blend proportions,
· The power station stockyard, where two coals may be reclaimed simultaneously and
combined on the conveyors,
· In the power plant, where different coals may be sent to separate mills1.
Methods of blending include:
· Stacking the coal in different layers. Reclaiming is performed in a different direction, across the layers, in order to achieve a degree of mixing.
· Reclaiming simultaneously from more than one stockpile and feeding onto a common
belt.
· Feeding from separate coal bins onto a common belt.
1) Though this is not really blending, many of the impacts of plant performance are like blending. After the milling of the separate coals and their ignition at the burners, the remaining processes occur with “blended” products of combustion.
It is clear that a degree of homogeneity, that is adequate mixing, is needed after blending to avoid slugs of unblended coals entering the boiler. Nevertheless it is recognised that some mixing occurs as a result of materials handling after the point of blending.
Targeting Coal Blend Characteristics
Coal Analysis:
The initial approach in formulating a blend is to aim for a particular set of coal properties, based on the standard analysis of blends. This is logical since these results are the normal basis for coal specifications that satisfy coal supply contracts. Additionally, it is standard coal analysis that provides the first (and sometimes only) indication of how a coal will perform in a power plant.
Coal Performance:
Ultimately it is the actual performance of a coal in the power plant that matters. It is difficult enough to rely on coal analysis to predict the performance of single coals, and it may be expected that it would be more difficult for coal blends, particularly when the properties of the component coals are very different.
Coal has many diverse properties, each one of which impacts on some aspects of power plant performance. The value of coal to the user is a maximum when the set of coal properties is matched to the particular requirements of the user’s power plant. Because there are so many diverse properties, there may be few coals that give a good match for all properties.
This shortage of matching coals is favourable to neither the coal producer nor the coal buyer, hence the need for blending - this increases the scope tremendously for presenting coals that fully satisfy a strict specification.
A normal coal specification is based on standard chemical and physical laboratory tests. The ability to satisfy a specification by blending is limited by:
· The properties of the available coals, and
· The additivity of properties, that is whether they are a weighted average of the properties of the component coals.
The next major issue with blending is how a coal blend performs. Achieving a coal specification does not infallibly predict the coal’s performance in power plant, though it is often all we have to make these predictions. Satisfying a specification does not guarantee satisfactory performance, even in the case of unblended coals, which raises the question as to whether standard laboratory tests are less reliable predictors for blends than for single coals.
Benefits of Blending
Both coal producers can benefit from blending and blends are produced anywhere between the mine and the power plant. Coal producers may blend for a combination of the following reasons:
· To enable them to offer a product that satisfies customers’ specifications,
· To enable them to sell coals that may be otherwise unsaleable,
· To increase their tonnage sales,
· To offer products that can be produced more cheaply and are more competitive on the market,
· To formulate new supplies to match an established product that may come from a mine approaching the end of its life,
· To win favour with customers by supplying more consistent quality,
The above benefits can also accrue from blending with a competitor’s coal and there may be more of this in the future. Power station operators may blend for a combination of the following reasons:
· To design acceptable feedstocks from coals that may be unsuitable in the unblended state,
· The coals used can be cheaper, thus saving fuel costs,
· To make boiler plants more easy to operate by improving the consistency of coal quality,
· To diversify the sources of the coal supply, thereby increasing competition and securing the supply,
· To enable poor quality domestic coals to be used.
Blending Logistics & Methods
Coal blends may be produced at:
· The mine, from different seams or areas,
· The outgoing seaport, where competitors may make blends for mutual advantage,
· The receival/distribution port, where customers may specify blend proportions,
· The power station stockyard, where two coals may be reclaimed simultaneously and
combined on the conveyors,
· In the power plant, where different coals may be sent to separate mills1.
Methods of blending include:
· Stacking the coal in different layers. Reclaiming is performed in a different direction, across the layers, in order to achieve a degree of mixing.
· Reclaiming simultaneously from more than one stockpile and feeding onto a common
belt.
· Feeding from separate coal bins onto a common belt.
1) Though this is not really blending, many of the impacts of plant performance are like blending. After the milling of the separate coals and their ignition at the burners, the remaining processes occur with “blended” products of combustion.
It is clear that a degree of homogeneity, that is adequate mixing, is needed after blending to avoid slugs of unblended coals entering the boiler. Nevertheless it is recognised that some mixing occurs as a result of materials handling after the point of blending.
Targeting Coal Blend Characteristics
Coal Analysis:
The initial approach in formulating a blend is to aim for a particular set of coal properties, based on the standard analysis of blends. This is logical since these results are the normal basis for coal specifications that satisfy coal supply contracts. Additionally, it is standard coal analysis that provides the first (and sometimes only) indication of how a coal will perform in a power plant.
Coal Performance:
Ultimately it is the actual performance of a coal in the power plant that matters. It is difficult enough to rely on coal analysis to predict the performance of single coals, and it may be expected that it would be more difficult for coal blends, particularly when the properties of the component coals are very different.
Upgrading Low Rank Coals
Processes to upgrade low rank coals normally attempt to overcome their main shortcomings, namely high moisture content and, as a direct result, low calorific value. Merely drying these coals is not a solution because (a) they will reabsorb moisture during handling and stockpiling and (b) it will create dust problems. The answers therefore lie in modifying the pore structure of the coals as part of the drying process. Some solutions that are being tried include:
· Hot briquetting,
· The Upgraded Brown Coal process (UBC) whereby the coal is immersed in recycled
oil and heated to dry. Most of the oil is recycled, but the coal pores are sealed in the process, preventing moisture from entering,
· Sequential processes of drying, heating to generate tar from the coal, followed by
absorption of the tar to seal the pores.
These measures will reduce transport costs and reduce the required handling and milling capacities at the power plant. A valuable side-effect may be a reduction in the coal’s propensity to spontaneous combustion.
BLENDING OF LOW RANK COALS
Blending with higher rank coals may enable what are sometimes cheaper coals to be utilised in power plants not designed specifically for the unblended coals, thereby reducing the capital costs of the plant as well as the fuel costs.
However, blending also offers greater opportunities to overcome inherent problems of some higher rank coals, which may include:
· High nitrogen content which may exceed legal limits
· High ash content, resulting in difficult stack cleanup and high ash disposal costs,
· Abrasive minerals causing pulveriser wear and boiler tube erosion,
· Reflective ash limiting radiative heat transfer in the furnace,
· High resistivity ash lowering electrostatic precipitator efficiency.
Based on these considerations there are many scenarios for blending low rank and high rank coals to mutual advantage.
CONCLUSION
Many Indonesian coals have negative properties, some of which are related to their rank being lower than many Australian and other overseas coals. Depending on power plant design features, these properties will limit the plant performance and reduce the value of the coals.
Some of these limitations include:
· Low CV giving high freight costs,
· Self-heating and spontaneous combustion,
· Low CV needing high coal handling capacity,
· Low HGI together with low CV requiring higher mill capacity ratings,
· High moisture requiring greater air–heater capacity and causing mill fires,
· Slagging and fouling associated with their ash chemistry,
· Larger more expensive boilers.
Indonesian coals are typically superior to many Australian and other overseas coals in the following areas:
· Their low ash content has environmental advantages for stack cleanup and ash
disposal,
· They are highly reactive and give high burnout efficiency,
· The mineral matter has low abrasivity, giving low mill wear and boiler tube erosion,
· Some have very low sulphur contents,
· The ash has favourable electrostatic precipitation characteristics,
· They have medium to low nitrogen contents, satisfying any legal nitrogen limits,
· They produce low NOx levels.
Blending will enable many of these characteristics to complement the different characteristics of competitor coals.
Combustion Performance
This section covers the combustion reaction and the effective generation of heat form the coal, specifically:
· 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.
· 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.
Milling
Areas where coal characteristics can cause limitations to mill performance include:
· Power consumption & product fineness
· Primary air heating requirements
· Mill fires
· Mill wear
Power Consumption and Product Fineness
Both of these performance aspects relate to the grindability of the coal, which is related to the amount of energy required to reduce the coal particle size. Low rank coals are not normally hard in the sense that they are relatively easy to deform, however much of the deformation is plastic rather than elastic, that is they are not brittle. Therefore the amount of deformation required to break the particles is higher than for many higher rank coals, therefore requiring a greater expenditure of energy.
The Hardgrove Grindability Index is essentially a measure of this energy requirement, though, as a small-scale simulation of the milling process, it has some shortcomings as a test. Figure 11 showed that there was a clear trend for HGI to reduce (more difficult to mill) as coal Carbon content dropped from 90 down to about 80. With a further reduction in Carbon content, there was less of a trend, with HGI values lying between approximately 30 (for an Australian coal) and 60. The HGI test is known to lack precision for low rank, high moisture, coals because the test result is highly sensitive to the moisture content for a
particular determination7.
Thus, the HGI has limited reliability in predicting the fineness and power consumption when pulverising a low rank coal at a given tonnage per hour. A second important factor not covered by the HGI is that low CV coals need to be milled at a higher tonnage rate for a given boiler duty, thus compounding the problem.
When a new coal is introduced to a power station, there are two possible courses of action for the operators regarding the running of the mills:
· They may retain all the previous mill settings and hope that the performance is
satisfactory,
· They may adjust the mill settings (including roll pressure, classifier setting) to suit the new coal.
ACIRL adopts both these approaches when testing coals in the pilot-scale vertical spindle mill:
· The mill is run at standard settings; power consumption and fineness are reported,
· The mill settings are adjusted to give a standard fineness of PF of 70% passing 75 μm.
Standard Mill Settings: Figures 19 and 208 show mill power consumption and fineness
respectively for standard mill settings. Figure 19 shows the typical trend for the power consumption to increase with lower HGI. However, it shows that most Indonesian coals perform better than expected, ie, the mill power consumption is lower than the trend for their HGI range. On the other hand, Figure 20 shows that the Indonesian coals produced coarser PF than the trend. This represents an enforced trade-off between power consumption and fineness when coals are milled in this way.
7 HGI of low rank coals may also depend on the history of wetting and drying before the test.
8 Mill performance data and coal quality data in this section came from projects
commissioned by coal companies and industry funding.
Figure 19: Mill Power Consumption (kW.h/t) at Standard Mill Settings for Indonesian
and Australian Coals
Figure 20: PF Fineness at Standard Mill Settings for Indonesian and Australian Coals
Milling to a Standard Fineness: Figure 21 shows the trend of power consumption versus
HGI when the mill is adjusted for standard fineness. Under these conditions, Indonesian coals perform close to the overall trend, though the majority are still slightly better than the trend.
Figure 21: Mill Power Consumption (kW.h/t) to Produce Standard PF for Indonesian
and Australian Coals
The significance of these results is as follows:
· If an Australian coal is replaced by an Indonesian coal without mill adjustments, thepower consumption will probably be similar to that for Australian coal with HGI
about 10 points higher, so it is not likely to be excessive,
· The PF of the Indonesian coal is likely to be coarser than that of the Australian coal. This may not become obvious because not all power station operators sample and
analyse the PF,
· Coarser PF can lead to a deterioration in burnout efficiency. However, as will be
covered later, Indonesian coals inherently produce very favourable burnout efficiency, so this may not be a problem,
· If it is found necessary to adjust the mills to produce standard PF, then the mill power consumption is likely to be only slightly better than that of an Australian coal of the same HGI.
Calorific Value: The above comments are based on the same coal feed rate for all coals. Taking into account the need for a greater tonnage for lower CV coals, Figure 22 allows for this by plotting mill power consumption (kW) per unit coal energy input (GJ/h). The data spread is much wider than that of Figure 19, indicating that lower CV coals may be subject to limitations based on mill power consumption.
Figure 22: Mill Power Consumption per Unit Coal Energy at Standard Mill Settings for
Indonesian and Australian Coals
Primary Air Heating Requirements
Coal is dried in the mills as well as pulverised, requiring higher primary air temperatures for higher moisture coals. Based on the as-fired moisture content of the coal entering the mills, and assuming the PF exiting the mills contains a fraction (approximately half) of the air-dried moisture, the quantity of moisture removed can be estimated. The problem is compounded for low CV coals (which are often the high moisture coals) because a greater tonnage throughput of coal is required. Consequently a ranking of the drying requirements may be calculated as:
Drying Requirement = (Moisture Removed)/CVNAR (kg/GJ) (1)
Figure 23 (9) shows the Drying Requirement versus Carbon (%daf) for Arutmin, KPC, other Indonesian and Australian export coals. The Drying Requirement rises steeply as Carbon content decreases below about 78%.
Figure 23: Moisture Removed per Unit Coal Energy during Milling for Indonesian and
Australian Coals
In a given power plant there will be a top limit on the acceptable value of Drying
Requirement; the limit will depend on the mill and burner design. The specific limitations may arise from:
· The air-heater capacity may be insufficient to attain the required primary air
temperature,
· The operators may place a top-limit on the allowable mill inlet air temperature to
avoid the possibility of mill fires.
It is clear that, because of high as-fired moisture content, some Indonesian coals will not suit some plants that are not designed for them.
9) Based on data supplied by the Bumi Group (KPC and Arutmin coals) and the Barlow Jonker database (other coals).
Mill Fires
As indicated under the previous heading, high moisture coals may require higher mill inlet temperatures which increase the hazard of mill fires. Unfortunately, as well as having high moisture contents, low rank coals tend to be more prone to spontaneous combustion because of the reactivity of their organic matter. The two characteristics compound the mill-fire problem. For very low rank coals, special plant design features may include:
· Higher than normal primary air volume to reduce the temperature requirement
· The use of attrition mills and recycling some flue gas into the primary air10.
Mill Wear
It is a misconception that low HGI coals are inherently abrasive. Abrasive wear of mill components is normally caused by hard mineral matter in the coal, typically free silica (quartz) or iron pyrite. The Abrasion Index (or Yancey Geer Price Index) is a standard laboratory test to provide an indication of abrasiveness.
ACIRL’s pilot-scale vertical spindle mill has the capability of measuring mill wear, which tends to correlate, though not perfectly, with Abrasion Index. Indonesian coals typically have lower Abrasion Index than Australian coals and this is reflected in lower mill wear rates in the vertical spindle mill (Figure 24).
Figure 24: Mill Wear Rate for Indonesian and Australian Coals
10) These measures are used for Australian brown coals (typical Carbon content 68% daf), which are not exported.
· Power consumption & product fineness
· Primary air heating requirements
· Mill fires
· Mill wear
Power Consumption and Product Fineness
Both of these performance aspects relate to the grindability of the coal, which is related to the amount of energy required to reduce the coal particle size. Low rank coals are not normally hard in the sense that they are relatively easy to deform, however much of the deformation is plastic rather than elastic, that is they are not brittle. Therefore the amount of deformation required to break the particles is higher than for many higher rank coals, therefore requiring a greater expenditure of energy.
The Hardgrove Grindability Index is essentially a measure of this energy requirement, though, as a small-scale simulation of the milling process, it has some shortcomings as a test. Figure 11 showed that there was a clear trend for HGI to reduce (more difficult to mill) as coal Carbon content dropped from 90 down to about 80. With a further reduction in Carbon content, there was less of a trend, with HGI values lying between approximately 30 (for an Australian coal) and 60. The HGI test is known to lack precision for low rank, high moisture, coals because the test result is highly sensitive to the moisture content for a
particular determination7.
Thus, the HGI has limited reliability in predicting the fineness and power consumption when pulverising a low rank coal at a given tonnage per hour. A second important factor not covered by the HGI is that low CV coals need to be milled at a higher tonnage rate for a given boiler duty, thus compounding the problem.
When a new coal is introduced to a power station, there are two possible courses of action for the operators regarding the running of the mills:
· They may retain all the previous mill settings and hope that the performance is
satisfactory,
· They may adjust the mill settings (including roll pressure, classifier setting) to suit the new coal.
ACIRL adopts both these approaches when testing coals in the pilot-scale vertical spindle mill:
· The mill is run at standard settings; power consumption and fineness are reported,
· The mill settings are adjusted to give a standard fineness of PF of 70% passing 75 μm.
Standard Mill Settings: Figures 19 and 208 show mill power consumption and fineness
respectively for standard mill settings. Figure 19 shows the typical trend for the power consumption to increase with lower HGI. However, it shows that most Indonesian coals perform better than expected, ie, the mill power consumption is lower than the trend for their HGI range. On the other hand, Figure 20 shows that the Indonesian coals produced coarser PF than the trend. This represents an enforced trade-off between power consumption and fineness when coals are milled in this way.
7 HGI of low rank coals may also depend on the history of wetting and drying before the test.
8 Mill performance data and coal quality data in this section came from projects
commissioned by coal companies and industry funding.
Figure 19: Mill Power Consumption (kW.h/t) at Standard Mill Settings for Indonesianand Australian Coals
Figure 20: PF Fineness at Standard Mill Settings for Indonesian and Australian Coals
Milling to a Standard Fineness: Figure 21 shows the trend of power consumption versus
HGI when the mill is adjusted for standard fineness. Under these conditions, Indonesian coals perform close to the overall trend, though the majority are still slightly better than the trend.
Figure 21: Mill Power Consumption (kW.h/t) to Produce Standard PF for Indonesian
and Australian Coals
The significance of these results is as follows:
· If an Australian coal is replaced by an Indonesian coal without mill adjustments, thepower consumption will probably be similar to that for Australian coal with HGI
about 10 points higher, so it is not likely to be excessive,
· The PF of the Indonesian coal is likely to be coarser than that of the Australian coal. This may not become obvious because not all power station operators sample and
analyse the PF,
· Coarser PF can lead to a deterioration in burnout efficiency. However, as will be
covered later, Indonesian coals inherently produce very favourable burnout efficiency, so this may not be a problem,
· If it is found necessary to adjust the mills to produce standard PF, then the mill power consumption is likely to be only slightly better than that of an Australian coal of the same HGI.
Calorific Value: The above comments are based on the same coal feed rate for all coals. Taking into account the need for a greater tonnage for lower CV coals, Figure 22 allows for this by plotting mill power consumption (kW) per unit coal energy input (GJ/h). The data spread is much wider than that of Figure 19, indicating that lower CV coals may be subject to limitations based on mill power consumption.
Figure 22: Mill Power Consumption per Unit Coal Energy at Standard Mill Settings for
Indonesian and Australian Coals
Primary Air Heating Requirements
Coal is dried in the mills as well as pulverised, requiring higher primary air temperatures for higher moisture coals. Based on the as-fired moisture content of the coal entering the mills, and assuming the PF exiting the mills contains a fraction (approximately half) of the air-dried moisture, the quantity of moisture removed can be estimated. The problem is compounded for low CV coals (which are often the high moisture coals) because a greater tonnage throughput of coal is required. Consequently a ranking of the drying requirements may be calculated as:
Drying Requirement = (Moisture Removed)/CVNAR (kg/GJ) (1)
Figure 23 (9) shows the Drying Requirement versus Carbon (%daf) for Arutmin, KPC, other Indonesian and Australian export coals. The Drying Requirement rises steeply as Carbon content decreases below about 78%.
Figure 23: Moisture Removed per Unit Coal Energy during Milling for Indonesian and
Australian Coals
In a given power plant there will be a top limit on the acceptable value of Drying
Requirement; the limit will depend on the mill and burner design. The specific limitations may arise from:
· The air-heater capacity may be insufficient to attain the required primary air
temperature,
· The operators may place a top-limit on the allowable mill inlet air temperature to
avoid the possibility of mill fires.
It is clear that, because of high as-fired moisture content, some Indonesian coals will not suit some plants that are not designed for them.
9) Based on data supplied by the Bumi Group (KPC and Arutmin coals) and the Barlow Jonker database (other coals).
Mill Fires
As indicated under the previous heading, high moisture coals may require higher mill inlet temperatures which increase the hazard of mill fires. Unfortunately, as well as having high moisture contents, low rank coals tend to be more prone to spontaneous combustion because of the reactivity of their organic matter. The two characteristics compound the mill-fire problem. For very low rank coals, special plant design features may include:
· Higher than normal primary air volume to reduce the temperature requirement
· The use of attrition mills and recycling some flue gas into the primary air10.
Mill Wear
It is a misconception that low HGI coals are inherently abrasive. Abrasive wear of mill components is normally caused by hard mineral matter in the coal, typically free silica (quartz) or iron pyrite. The Abrasion Index (or Yancey Geer Price Index) is a standard laboratory test to provide an indication of abrasiveness.
ACIRL’s pilot-scale vertical spindle mill has the capability of measuring mill wear, which tends to correlate, though not perfectly, with Abrasion Index. Indonesian coals typically have lower Abrasion Index than Australian coals and this is reflected in lower mill wear rates in the vertical spindle mill (Figure 24).
Figure 24: Mill Wear Rate for Indonesian and Australian Coals10) These measures are used for Australian brown coals (typical Carbon content 68% daf), which are not exported.
Subscribe to:
Posts (Atom)