Good to Know Series - 04 - What is Coal Rank?
Coal rank refers to the degree of coalification—the process that transforms plant material into coal. The further along this process, the higher the rank. But how do we measure it? The two most widely used methods are vitrinite reflectance and volatile matter content. Each has its strengths, so let’s dive in.
Vitrinite Reflectance (Rr%): The Gold Standard
Think of vitrinite reflectance as coal’s built-in maturity meter. By shining light on a vitrinite particle under a calibrated optical microscope, we can measure how much light it reflects. The higher the reflectance, the higher the rank.
How is it measured?
Random reflectance (Rr%): Standard measurement in nonpolarised reflected white light.
Maximum reflectance (Rmax%): Requires a rotating stage and polarised reflected white light—preferred for anthracites due to optical anisotropy.
Why is it so reliable?
It directly measures vitrinite (preferentially in collotelinite), unaffected by other macerals or minerals.
Vitrinite is the best choice as it evolves steadily through coalification.
But is it operator-dependent?
Yes. You need to know how to identify vitrinite to do it right. Some argue this is a weakness, but let’s be real—if you can’t recognise vitrinite, maybe you shouldn’t be doing petrography, you should be learning. (Maybe that’s why you are here 😉)
A note on machine learning
There’s a growing trend to use AI for maceral recognition to avoid human error. Is it a good idea? I’ll cover that in a separate blog, because there’s a lot to unpack. Stay tuned.
Precautions to keep in mind:
Polishing matters: Poorly polished samples lead to inaccurate readings.
Reflectance suppression and oxidation: Hydrogen-rich (perhydrous) or oxidised coals may show lower Rr% values than expected.
Volatile Matter (VM%): The Chemical Approach
If vitrinite reflectance is the petrographic method, volatile matter content is the chemical method. Instead of measuring a single maceral, VM% assesses at the whole coal composition (macerals and minerals). As the rank increases, the VM% decreases.
How is it measured?
Coal is heated to 950°C, and the gases and vapours released are measured.
The result is calculated on a dry, ash-free (daf) basis for rank determination.
When is it most useful?
Works best for medium-volatile bituminous coal to anthracite.
Less reliable for low-rank coals (e.g., lignite, subbituminous) where moisture plays a bigger role.
What can interfere with VM%?
Coal composition: Liptinite-rich coals produce more volatiles than vitrinite or inertinite-rich coals, on the other hand, inertinite-rich coal produce less volatiles.
High ash content (>20-25%): Minerals can release volatile components (e.g., clays release water, carbonates release CO2, sulphides release sulphur).
Other Rank Indicators
Carbon & Hydrogen Content: Measured in ultimate analysis, useful across various ranks.
Moisture & Calorific Value: The best indicators for very low-rank coals (lignite) and peat.
Why Does This Matter?
Knowing coal rank isn’t just a technical detail—it impacts real-world applications:
Power generation: Determines how efficiently coal burns.
Coke production: Crucial for making strong, high-quality coke for steelmaking.
Gasification & liquefaction: Rank affects conversion efficiency and product yield.
The next time you see a lump of coal, remember—it’s not just a rock. Its rank tells a story of millions of years of transformation. And knowing how to measure it makes all the difference.
Coming Up Soon: Can AI Replace Petrographers?
Machine learning is shaking things up in coal analysis. But does it live up to the hype? I’ll tackle this hot topic in an upcoming post. Stay tuned!
