
Coal Quality Issues
We have learned that coal is very old. Much of the Pennsylvania coal is about 300 million years old, so, much older than the dinosaurs. To put it in perspective, the Jurassic era was about 80 million years ago! But, coal is still being formed today so we have a range of coal with highly variant properties (and quality), from a brown coal, which is relatively young, to a bituminous or anthracite coal, which is relatively old. The material is distinguished by a rank system that is very informative regarding how we expect that coal will behave during the combustion process or other coal applications.
Coal Rank
There are 2 officially accepted methods of determining the rank of a coal in the U.S. (but numerous other approaches). Which approach you use depends on the calorific value of the coal. There are 4 general rank classifications, shown from lowest to highest rank;
Coal | Rank |
---|---|
Lignite | low-rank |
Subbituminous | low-rank |
Bituminous | high-rank (soft coal) |
Anthracite | high-rank (hard coal) |
There are lots of sub-classifications but we only need to consider the groups. The rank of coal is an important feature of the coal, it is used for taxation purposes, contracts, etc. Coals that have a calorific value below 14,000 Btu's are put into a rank classification based on the calorific value. Coals with calorific values above 14,000 Btu's use the proximate analysis to determine rank (specifically volatile matter or fixed carbon values).
Calorific Value
When coal is burned, the exothermic reaction produces heat;
C + O2 → CO Carbon and Oxygen yields Carbon Monoxide
CO + O2 → CO2 Carbon Monoxide and Oxygen yields Carbon Dioxide
2 H + O2 → H2O Hydrogen and Oxygen yields Water (Steam)
This is, after all, why we burn coal! But how much calorific energy we obtain is dependent on the chemical composition of the coal, which tends to change with maturation (age), so bituminous coal will provide more energy than biomass, lignite or a subbituminous coal. This is very important information when buying coal or, as we will see later, very important information when determining pollutant emissions.
Proximate Analysis
This has 4 components, which are useful in determining price and behavior of the coal during combustion, as well as some other quality issues.
1. Moisture
Moisture adds mass to the coal which impacts the transportation costs as well as reducing the useful energy obtained from the combustion of the coal. The steam produced from within the coal (from moisture) is not used to turn turbines but rather goes out of the stack. In low ranks coal, such as lignite, water might be 60% of the total mass! Moisture levels are rank-dependent and bituminous coals may only contain 1-2% moisture by weight (subbituminous coals will have more moisture ~30% unless the coal has been dried). High moisture levels and high oxygen content is why the calorific value is low for lignite and other low-rank coals.
2. Volatile Matter
Most coal is burned, how easily it burns depends on the quantity (and the quality) gasses that are released when the coal is heated. Under prescribed heating rates, under nitrogen (so no combustion losses), and times, the weight loss is determined to be volatile matter. This value is used in rank determination for coals above 14,000 Btu's/lb. Coals with a higher volatile matter yield are easier to burn.
3. Ash Yield
Coal contains inorganic material too, which we call mineral matter. This diluent impacts transportation costs, and after the combustion process, the coal will leave behind the now chemically altered mineral matter (high temperatures, as well as oxygen, will change the composition of the mineral matter) into ash, which will have to be removed influencing the combustor design and operation. Thus, the coal is combusted, leaving behind the ash which is weighed to roughly determine the contribution of mineral matter to the coal mass. Remember coal is purchased on a per ton basis so ash values are important indicators of quality.
4. Fixed Carbon
After the volatile matter determination, the char contains fixed carbon and ash, if we know the ash values the subtraction of the moisture, volatile matter, and ash produces the fixed carbon. Fixed carbon -= 100 - moisture - volatile matter - ash values
Ultimate Analysis
This analysis determines the relative abundance of the organic elements that are contained in the coal. It is influenced by the rank of the coal.
Element | Normal Contribution range (%) in Coal |
---|---|
Carbon | 60-90% ( Higher Rank - Higher % ) |
Hydrogen | 2-6% (Higher Rank - Lower %) |
Oxygen | 1-30% (Higher Rank - Lower %) |
Nitrogen | 1-2% (No real change with Rank) |
Sulfur | 0.5-5% (no significant change with rank) |
We will find out later in the course that nitrogen and sulfur contribute to environmental problems when their oxides are released into the atmosphere following combustion. For those who are impatient, you can find out more information concerning the NOx and SO2 (and sulfate aerosols) emissions here. Nitrogen values for coals are typically around 2%, so nitrogen content does not influence the coal quality when comparing two similar-rank coals. Sulfur, however, can vary dramatically and is certainly an element that impacts coal quality. High S content negatively influencing the price of the coal, making it cheaper as more expense will be incurred in cleaning the coal or reducing the SO2 emissions (acid deposition-Lesson 10). The S content along with the calorific value is useful because you can use it to predict the SO2 emissions per million Btu's of thermal input (which is the measure used in the clean air act of 1990 to ensure pollution controls).
Thus, several measures impact coal quality– how much organic material is actually in the coal (minus ash and moisture values) as this impacts transportation, the quantity of useful energy that will be produced (calorific value and moisture content), removal of the ash from the boiler (ash), and emissions from the combustion process (S content). Of course, the quality of the coal will impact the value of the coal so remember the $ is an important influence on the cost of the electricity, and where the coal is purchased from (transportation).
Coking Coals
Select bituminous coals can be used in coke manufacturing for use in steel production. They need to have low S, low ash yield, and other properties, but they are more valuable. The Pittsburgh coal seam is a classic example and one of the reasons that it is Steel City with the Steelers (not Stealers) football team, and Iron Brew beer.
