Q17Engineering Chemistry

Proximate analysis is a standardized, empirical evaluation of coal. Unlike ultimate analysis, which determines the exact elemental chemical makeup (Carbon, Hydrogen, Nitrogen, etc.), proximate analysis simply physically separates the coal into four distinct operational categories by subjecting a sample to rigidly controlled cycles of heating and cooling. The four components determined are Moisture, Volatile Matter, Ash, and Fixed Carbon. The primary significance of proximate analysis lies in its ability to rapidly determine the commercial grade, combustion characteristics, and specific industrial utility of a coal batch.

1. Determination of Moisture Content

Method: A known mass (e.g., $1.00\text{ g}$) of finely powdered, air-dried coal is placed in a silica crucible and heated in an electric oven maintained precisely at $105^{\circ }\text{C}-110^{\circ }\text{C}$ for exactly one hour. The crucible is cooled in a desiccator and re-weighed. The loss in weight corresponds to the moisture driven off.

Significance: High moisture content is highly undesirable. It decreases the Gross Calorific Value of the fuel because a portion of the combustion heat is wasted evaporating this water. Furthermore, it adds useless dead weight, increasing transportation costs, and can cause spontaneous ignition issues during prolonged storage. High-quality coal possesses very low moisture.

2. Determination of Volatile Matter (VM)

Method: The dried coal sample from the previous step is covered with a tight-fitting lid to completely prevent oxidation by air. It is then placed in a specialized muffle furnace heated to an intense $925^{\circ }\text{C}\pm 20^{\circ }\text{C}$ for exactly 7 minutes. It is cooled rapidly and weighed. The further weight loss represents the volatile matter (combustible gases like $C{H}_4,H_2,CO$, and tarry vapors).

Significance: High volatile matter indicates that a large fraction of the coal burns as a gas, producing long, smoky flames. While this makes the coal easy to ignite, it results in lower heating efficiency because the gases often escape the chimney unburnt. High VM coals are entirely unsuitable for manufacturing metallurgical coke. Conversely, coals with low VM burn with a short, clean, smokeless flame and possess superior coking properties.

3. Determination of Ash Content

Method: The residual coal sample in the crucible is now uncovered, completely exposing it to the air. It is placed back in the muffle furnace and heated gradually to $700^{\circ }\text{C}-750^{\circ }\text{C}$ for about half an hour, until the coal is completely burned away, leaving only inorganic residue. Heating is repeated until a constant final weight is achieved.

Significance: Ash is the non-combustible, useless mineral matter (silica, alumina, iron compounds) left behind. It is highly detrimental. It directly reduces the calorific value of the fuel. High ash causes 'clinkering'—the ash melts at high temperatures and fuses into hard lumps that block the air supply through the furnace grate, drastically reducing combustion efficiency. High ash also incurs significant costs for removal and disposal.

4. Determination of Fixed Carbon

Method: Fixed carbon is not determined by direct measurement. It is calculated by difference:

Significance: Fixed carbon is the solid, combustible residue of the coal. It represents the most critical parameter in proximate analysis. A high percentage of fixed carbon directly correlates to a significantly higher calorific value. Coal with high fixed carbon burns with high intensity, provides excellent, sustained heat, and is absolutely essential for the production of high-strength metallurgical coke used in blast furnaces.