This section examines correlations between coal properties and performance in coal-fired power plants and relates these specifically to the effects of rank and typical differences between Indonesian and Australian coals. This will identify limitations that may occur in some of the power plant processes and may preclude the use of some of these coals in plant not specifically designed for them.
As well as the properties determined by standard laboratory analysis and identified in the previous section, reference will be made to other properties that are more difficult to measure/define such as coal reactivity. Further reference will be made to site-specific variables such as size distribution and moisture content. For convenience, all of these characteristics will be referred to as properties. ACIRL’s experience of testing Indonesian and Australian coals a pilot-scale mill and Boiler Simulation Furnace will also be included.
The Importance of Plant Design
When inherent limitations to coal utilisation are identified, the importance of plant design must be recognised. It is possible to design plant to cope with nearly any shortcoming in coal quality. The job of the coal technologist is to recognise the coal quality issues and to relate these to the requirements of the power plant.
Stockpiling and Handling
This section covers:
· Self-heating and spontaneous combustion
· Bulk handling
· Stockpile slumping
· Fugitive dust emissions
Spontaneous Combustion
Relevant Properties:
· Coal Reactivity
· Size Distribution
· Total Moisture Content
Coal Reactivity. Spontaneous combustion involves reactions between the coal and oxygen or moisture. Reactivity involves characteristics such as molecular structure and porosity that are not normally measured. Generally, low-rank coals tend to have more chemically reactive organic molecular structures and greater porosity to explain their greater propensity to spontaneous combustion.
Laboratory tests have been devised to measure a coal’s inherent propensity to spontaneous combustion without site-specific effects. One such test is the Relative Ignition Temperature test6, also called the Crossing Point Temperature, which has been performed on several hundred coals. Carbon (daf) may correlate well with RIT, however this is not available for the majority of the coals in the database. VM is generally available but does not correlate well with RIT. Air-dried Moisture provides the best correlation from the available data (Figure 17). As the Figure shows, the coals with the lowest RIT (highest propensity to spontaneous combustion) are those with the highest Mad.
Figure 17: Relative Ignition Temperatures versus Air-Dried Moisture
6 This is an ACIRL in-house test.
Figure 18 shows RIT values for a selection of Indonesian and Australian export thermal coals, including some Arutmin and KPC coals.
Figure 18: Relative Ignition Temperatures for Indonesian and Australian Coals
Variations in Total Moisture and Size Distribution. The use of air-dried moisture in the previous section was not intended as a measure of the impact of moisture content but as an indicator of inherent coal reactivity or rank. For completeness it needs to be recognised that in situ variations in Total Moisture content and segregation of coal size distribution are also factors, but these re not obviously connected with the origins or rank of coals.
Bulk Handling
Blockages of chutes and hoppers may be related to:
· Coal size distribution
· Surface Moisture Content
· Types of mineral matter
Coal Size Distribution: The most common cause of bulk handling problems is an excess of fines, such as the percentage -2 mm. A high fines content originates form either:
· Crushing to liberate mineral matter as part of a coal washing process,
· Degradation of coal size due to handling.
Coals that are not washed therefore require less crushing at the mine and their fines content is therefore normally lower. Most Australian coals are washed and may contain up to around 30% -2mm. By comparison, Arutmin and KPC products are understood to contain less than about 20% -2 mm material. Size degradation is more severe with high HGI coals, meaning that some Australian coals would be the most susceptible.
Surface (Free) Moisture Content: There is normally a range of coal moisture contents
within which handling problems are most severe. On the other hand coal that is either airdried or very wet will normally handle satisfactorily. Since the Free Moisture is the criterion, not Total Moisture, there are no inherent differences between high rank and low rank coals.
Types of Mineral Matter: Clays of the bentonite or montmorillinite type become very
sticky when moist, coating the coal lumps and causing them to adhere and interfering with bulk flow. The problem is compounded when a growth forms in the throats of chutes composed of a mixture of the clay and coal fines. The sticky lump grows over time as more clay and fines adhere, then becomes hard and strong with drying.
Fortunately, the majority of clay associated with most coals is the kaolinite type which is not very susceptible to the problem. Bentonitic clays occur in significant quantities in some coals from SE Queensland and may also occur in some Indonesian coals. It is not always possible to remove all bentonite by washing because it may occur as thin bands within the coal lumps. Subsequent weathering on coal stockpiles causes the bentonite to gradually migrate to the coal surface.
Stockpile Slumping
Collapsing of stockpiles may be related to:
· High surface (free) moisture content
· High fines content
High surface moisture content: Slumping is normally a result of high rain-fall at the
stockpile. All coals are susceptible to increases in surface moisture. High Fines Content: It has been suggested that coals with more than 12% of the -0.5 mm fraction are susceptible in extreme rain conditions. As noted above, Indonesian coals are likely to have a lower fines content than Australian coals.
Dustiness
Dust emissions during handling and stockpiling may be related to:
· Low surface (free) moisture content
· High fines content
Low surface moisture content can equally be an issue for any coals. As indicated above, Indonesian coals are likely to contain less coal fines, in which case fugitive dust emissions are likely to be lower.