“How to understand a coal sample analysis? What is the difference between proximate and ultimate analysis? What is AR (as-received) basis? Is is the same with DAF (dry, ash free) basis? How about AD (air-dried) basis? And what coal ash analysis is all about?”
Well, it is indeed a long list of questions to answer but the explanation is actually not as twisted as it seems. The main purpose of coal sample analysis is to determine the rank of the coal along with its intrinsic characteristics. Furthermore, these data will be used as the fundamental consideration for future concerns, for instance: coal trading and its utilizations.
Coal Properties
Coal comes in four main types or ranks: lignite or brown coal, bituminous coal or black coal, anthracite and graphite. Each type of coal has a certain set of physical parameters which are mostly controlled by moisture, volatile content (in terms of aliphatic or aromatic hydrocarbons) and carbon content.
1. Moisture
Moisture is an important property of coal, as all coals are mined wet. Groundwater and other extraneous moisture is known as adventitious moisture and is readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analyzed. Moisture may occur in four possible forms within coal:
* Surface moisture: water held on the surface of coal particles or macerals
* Hydroscopic moisture: water held by capillary action within the microfractures of the coal
* Decomposition moisture: water held within the coal’s decomposed organic compounds
* Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays.
2. Volatile matter
Volatile matter in coal refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulfur. The volatile matter of coal is determined under rigidly controlled standards. In Australian and British laboratories this involves heating the coal sample to 900 ± 5 °C (1650 ±10 °F) for 7 minutes in a cylindrical silica crucible in a muffle furnace. American Standard procedures involve heating to 950 ± 25 °C (1740 ± 45 °F) in a vertical platinum crucible.
3. Ash
Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk mineral matter after carbon, oxygen, sulfur and water (including from clays) has been driven off during combustion. Analysis is fairly straightforward, with the coal thoroughly burnt and the ash material expressed as a percentage of the original weight.
4. Fixed carbon
The fixed carbon content of the coal is the carbon found in the material which is left after volatile materials are driven off. This differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an estimate of the amount of coke that will be yielded from a sample of coal. Fixed carbon is determined by removing the mass of volatiles determined by the volatility test, above, from the original mass of the coal sample.
Coal Proximate Analysis
The objective of coal ultimate analysis is to determine the amount of fixed carbon (FC), volatile matters (VM), moisture, and ash within the coal sample. The variables are measured in weight percent (wt. %) and are calculated in several different bases. AR (as-received) basis is the most widely used basis in industrial applications. AR basis puts all variables into consideration and uses the total weight as the basis of measurement. AD (air-dried) basis neglect the presence of moistures other than inherent moisture while DB (dry-basis) leaves out all moistures, including surface moisture, inherent moisture, and other moistures. DAF (dry, ash free) basis neglect all moisture and ash constituent in coal while DMMF (dry, mineral-matter-free) basis leaves out the presence of moisture and mineral matters in coal, for example: quartz, pyrite, calcite, etc. Mineral matter is not directly measured but may be obtained by one of a number of empirical formula based on the ultimate and proximate analysis.
.
| Proximate Analysis | unit | (ar) | (ad) | (db) | (daf) |
|---|---|---|---|---|---|
| Moisture | (wt. %) | 3.3 | 2.7 | ||
| Ash | (wt. %) | 22.1 | 22.2 | 22.8 | |
| Volatile Matter | (wt. %) | 27.3 | 27.5 | 28.3 | 36.6 |
| Fixed Carbon | (wt. %) | 47.3 | 47.6 | 48.9 | 63.4 |
| Gross Calorific Value | (MJ/kg) | 24.73 | 24.88 | 25.57 | 33.13 |
A table is shown above containing an example of proximate analysis data of coal. Conversion from one basis to another can be performed using mass balance equations. The standard practice for proximate analysis of coal may be referred to ASTM D3172-07a or ISO 17246:2005.
Coal Ultimate Analysis
Similar to coal proximate analysis, the objective of coal ultimate analysis is to determine the constituent of coal, but rather in a form of its basic chemical elements. The ultimate analysis determines the amount of carbon (C), hydrogen (H), oxygen (O), sulfur (S), and other elements within the coal sample. These variables are also measured in weight percent (wt. %) and are calculated in the bases explained above.
| Ultimate Analysis | unit | (ar) | (ad) | (db) | (daf) |
|---|---|---|---|---|---|
| Carbon (C) | (wt. %) | 61.1 | 61.5 | 63.2 | 81.9 |
| Hydrogen (H) | (wt. %) | 3.00 | 3.02 | 3.10 | 4.02 |
| Nitrogen (N) | (wt. %) | 1.35 | 1.36 | 1.40 | 1.81 |
| Total Sulfur (S) | (wt. %) | 0.4 | 0.39 | 0.39 | |
| Oxygen (O) | (wt. %) | 8.8 | 8.8 | 9.1 |
A table is shown above containing an example of coal ultimate analysis data and showing significant elements only. Conversion from one basis to another can be performed using mass balance equations. The standard practice for ultimate analysis of coal may be referred to ASTM D3176-89(2002) or ISO 17247:2005.
Ash Analysis
| Oxides | wt.% of ash (Calculated) | Elements | wt.% of ash (Measured) |
|---|---|---|---|
| Na2O | 0.35 | Na | 0.26 |
| MgO | 0.48 | Mg | 0.29 |
| Al2O3 | 20.0 | Al | 10.6 |
| SiO | 74.1 | Si | 34.6 |
| P2O5 | 0.05 | P | 0.05 |
| K2O | 1.1 | K | 0.92 |
| CaO | 0.68 | Ca | 0.49 |
| TiO2 | 0.80 | Ti | 0.48 |
| Mn3O4 | 0.06 | Mn | 0.05 |
| Fe2O3 | 3.25 | Fe | 2.28 |
An analysis of coal ash may also be carried out to determine not only the composition of coal ash, but also to determine the levels at which trace elements occur in ash. These data are useful for environmental impact modelling, and may be obtained by spectroscopic methods such as ICP-OES or AAS. An example of coal ash composition is shown on the right.
Beside composition of coal ash, ash fusion point is also one significant parameter in ash analysis. The optimum operating temperature of coal processing will depend on the gas temperature and also the ash fusion point. Melting of the ashes may cause them to stick to the walls of the reactor and result in a build-up.
(sourced MajariMagazine.com)
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