The basic test that almost all coal quality reports and shipments analyses utilize is the proximate test. This test is where the main non coal components, moisture and ash, are estimated. I use estimated rather than measured due to the word proximate meaning approximate. The moisture content in sub-bituminous and other lower rank coals is high (20-40+) percent. The sampling process and local conditions can influence moisture content of all coal samples and the high moisture can be impacted. Moisture leaves no evidence like high ash coals can ("there's a lot of rock in this coal") and any drying of the sample raises the heating value and usually the price. This helps explain why plants burn lower heating value coal than what was purchased. The boiler burns all the coal so there is no "sample drying bias" in boiler derived heating values.
The original test includes volatile matter estimates as many combustion and carbon uses can be estimated from this values. Generally high rank coals like anthracite and low volatile coals have low volatile and slow burning rates. Sub-bituminous coals (high volatile) are so reactive and fast burning they like to light off in storage, called spontaneous combustion. If you burn the same rank of coal, the volatile can consistently be in the same range and the volatile test is sometimes dropped, hence the common test called "Short Prox" which includes Moisture, Ash, Sulfur, CV, but not Volatile.
The ash levels of a coal are influenced by the ash (inorganic material) in the coal, (typically low) and the amount of out of seam dilution, OSD (rock above and below the coal seam) that gets mixed in during the mining process. The photo above shows how this might happen with potential ash contamination coming from OSD above the coal falling in the coal product, or scrapping the bottom layers to recover all the coal. Sales and shipment specifications typically include both these sources of ash, with only fully washed coals having it removed.
The moisture measured using the proximate test also includes the bed or moisture in the coal as mined along with any additional moisture gathered on the surface of the coal as it is broken up during mining and crushing. Any rain, snow, mist, and even humidity can increase this surface moisture. Moisture likes moisture, so low rank sub-bituminous, lignites, or fine wet coals tend to gain moisture once mined if exposed to additional water. Higher rank bituminous coals are more oily and tend to shed moisture better than low rank coals. Moisture and ash both dilute the coal so their levels impact the "As Received" heating value. Mechanical sampling systems and the sampling standards are designed to obtain a coal sample that is close to the actual "dry" ash levels of the sampled product. Sampling systems tend to dry the coal sample due to air flow and heat generated during the sample crushing. Expect as mined and as shipped moisture levels to be lower than the coal used at the plant for these reasons.
Generally sulfur and heating values are reported in the "Short Proximate" analyses, with the short meaning the volatile matter was dropped.
The sulfur levels are usually determined using a combustion tube method that measures the total sulfur content of the fuel. It does not differentiate between sulfur organically attached to the coal, the iron pyrite that can be in the coal or part or the OSD, or any sulfur in a sulfate form like gypsum (CaSO4). Actual power plant emissions can be slightly different than the total sulfur in coal due to pyrite removal in the pulverizers, combination with ash, and the calibration of the emission monitor.
The heating value of the coal is typically determined using the temperature rise in a known weight of water due to the combustion of a known weight of coal. The definition of a Btu is that energy needed to raise a pound of water one degree F.; a KCal is that amount of energy needed to raise a Kg (2.2 Lbs) of water one degree C.
This definition of energy units using water indicates how important heating water is to mankind.
As mentioned before, the heating value of a coal is diluted by the moisture and ash levels of the sample provided to the lab. Whereas the heating content of a particular seam (MAF CV) can be relatively constant, the As Received value shows the impact of the moisture and ash levels on the product itself. The use of Air Dry or As Determined, ADB is not useful in commercial terms as it has been partially dried in the lab. If the sampling procedure alters the levels of these non-combustibles, the heating value reported is impacted. (it is wrong)
This article provides a short summary of the Proximate test. The next in this series will explore what calculations and uses there are for this important, but approximate test.