The Coal
The burning of coal for its calorific values is one of the largest and most vital industries in the United States. After all, the electricity required to power all other factories, commercial industries, retail outlets, and private homes totals trillions of kilowatt hours (kWh) annually. The burning of coal is a highly exothermic process which uses energy to break hydrocarbon bonds to release stored energy in the form of heat. It seems simple in theory – but in practice, there can be some challenges with quality control because the quality of coal can vary from batch to batch.
Coal in the United States (anthracite, bituminous, subbituminous, and lignite) are partitioned into different classes, and each class has different chemical and physical properties. For example, bituminous coal is the most abundant with a carbon range of approximately 80% carbon and is suitable for steelmaking and electricity generation. Contrast this with subbituminous coal, which contains only approximately 40% carbon on a non-dry basis with an incredibly high moisture content and is suited for primarily power generation. In terms of physical appearance, bituminous coal also possesses a glossy sheen, but subbituminous coal is dull and black. These two different classes of coal have different carbon content, which greatly affect the calorific values – but the sulfur content present is a problem as well.
When coals are burned for energy, sulfur is released in the form of sulfides. This sulfide can interact with oxygen and water to form acid rain, which will affect water environments, acidify soil, and damage plants or crops. This form of pollution stemming from sulfide release during coal combustion must be carefully controlled due to emissions regulatory bodies. Efficient management of a coal plant therefore must include desulfurization processes to control sulfur content.
Verder Scientific offers multiple brands of equipment tailored to this industry: mills and sieves, drying furnaces, thermogravimetry, and carbon sulfur combustion analysis. For carbon and sulfur analysis, only a couple hundred milligrams of samples are required for a fast and total analysis. However, this small portion of just a few hundred milligrams of coal sample must be reflective of the total homogeneity of the sample. Therefore, prior to analysis of coal using any other technique, coal must be crushed and milled to obtain a representative sample. A sample divider can be used to produce a representative sample with a final grind size of 100 μm. The fineness of the sample becomes critical for analysis of carbon and sulfur in terms of reproducibility and accuracy.
Sample Preparation
The first step in the sample preparation process is pre-crushing the coal with jaw crushers. Retsch ́s jaw crushers are available in four different sizes, accepting feed sizes from 35 mm – 130 mm, as well as a variety of materials. Depending on the jaw crusher model, the sample material and the chosen gap width, a final fineness of 0.5–5mm can be obtained. The BB300 jaw crusher can be used to prepare batches of sample material or run continuously if larger sample amounts are processed.
In a trial, brown coal was pre-crushed and ground before elemental analysis to demonstrate the influence of sample preparation on the analytical results. 4 kg of brown coal (pieces of approximately 130 mm) were pre-crushed with the BB 300. Crushing obtained particle sizes of <8 mm, which were suitable for elemental analysis using for example, Eltra’s CS-580 combustion analyser.
The brown coal sample, obtained by pre-crushing with the BB 300, is divided into representative fractions using the Retsch sample divider PT 100. After that, one fraction is used for fine grinding in the ZM200, which is equipped with a 12-teeth rotor and a 0.2mm ring sieve. The 100 g sample is milled to a final fineness of 100 μm (d90value) in 30 sec. This 100 μm sample is analysed using the Eltraelemental analyser CS-580 and the results are compared to those of the 8mm sample with regard to the standard deviations (Table 1).From the results obtained, it can be clearly seen that fine size reduction significantly lowers the statistical error of the results, because the homogeneity of the sample is improved. Deviations in carbon content were reduced from ±1.21% to only ±0.5% and from ±0.047% to ±0.004%, regarding sulfur content.
The quality control of coal involves a variety of applications, such as producing a representative sample by crushing, dividing and fine grinding before elemental analysis. Correct preparation and homogenisation of the sample is an indispensible prerequisite in obtaining reproducible and reliable analysis results with a minimum standard deviation.
The quality control of coal involves a variety of applications, such as producing a representative sample by crushing, dividing and fine grinding before elemental analysis. Correct preparation and homogenisation of the sample is an indispensible prerequisite in obtaining reproducible and reliable analysis results with a minimum standard deviation.
Sample Drying
Different classes of coals will have different moisture contents – as previously discussed, subbituminous coal can have as much as 25 – 40% moisture, and the carbon content will greatly differ if measured as-is versus on dry-basis. A Carbolite Gero CWF muffle furnace can be used to fully dry a sample to fully remove moisture from the equation for elemental analysis.
Additionally, Carbolite Gero offers equipment for coal ashing, ash fusability, loss on ignition, and more.
Elemental Analysis
Once a sample is homogeneously prepared and divided, an elemental analyzer can be used to determine carbon and sulfur content quickly and accurately through combustion analysis, as shown in Table 1. The Eltra CS-580 is a resistance furnace that is typically held at 1350 °C for this application. Since the sample is homogenous, only a small (i.e., 150 mg) quantity of coal needs to be added onto a tared ceramic boat. This boat is inserted into the resistance furnace for a full burn: carbon and sulfur are released in a pure oxygen stream for conversion into carbon dioxide and sulfur dioxide. Results are delivered within two minutes with readings from an infrared detector.
Loss on Ignition
Thermogravimetric analysis is used to determine the mass loss of a sample as a function of the temperature. Suitable instruments include standard laboratory ovens and muffle furnaces with a fixed temperature and subsequent weighing, as well as TGA analyzers with integrated balance and a variable temperature range.
The Carbolite Gero AAF furnace can be programmed to reached 1000 °C – high enough to measure loss-on-ignition of coal at predetermined temperature ramps. A sample can be safely removed and weighed at each interval to determine moisture, volatiles, and ash content. The Eltra TGA ML achieves the same process but with a carousel that can accommodate upwards of 19 samples simultaneously to determine relevant parameters within five hours.
Verder Scientific offers multiple brands of equipment tailored to this industry: mills and sieves, drying furnaces, thermogravimetry, and carbon sulfur combustion analyzers. The applications and equipment discussed here are for sample preparation and elemental analysis, but there are many applications related to the coal industry for which Verder Scientific offers solutions for. Please reach out to us for a free consultation and demo.
