Coal characteristics affect burning performance

Press, 8 May 1985, Page 41

Coal characteristics affect burning performance

Every coal deposit presents its own set of mining difficulties, and each coal has its own characteristics. Coal’s properties differ from deposit to deposit, and even within the same seam, variations occur.

To determine how a particular coal will burn in a furnace, to establish how much ash it will leave in the grate, coal samples are periodically analysed and tested.

From the data gathered it is possible to determine which coal is best suited for which purpose, and this may have a substantial bearing on whether a new mine is viable.

The usual form of coal analysis, “proximate analysis,” involves taking a sample, weighing it immediately, and weighing it again after allowing it to air dry, then heating it to high temperatures in the absence of air. Everything that can

evaporate is driven off and measured, as is the remnant.

Vapourised substances are called volatile matter.

The dry remnant is separated into carbon, ash and mineral trace elements.

CARBON: Most carbon in coal is said to be fixed; this is the solid combustible part which combines with oxygen as coal burns to form carbon dioxide and carbon monoxide gases.

VOLATILE MATTER: Volatile matter includes a wide range of gaseous and vapourised hydrocarbons. As the name suggests these are comprised of carbon and hydrogen locked together in various proportions. Other elements in various combinations may also be locked in with them.

Hydrocarbons contribute greatly to coal’s heat output. They are the source of manufactured gas, which most of New Zealand’s

cities relied on until recently, and they can provide a range of chemicals when collected in quantity. WATER: Many coals have a considerable moisture or water content. A significant proportion of a coal’s heat may be expended in driving it off. ASH: Ash is the rock material embedded in coal while it was forming. Other inert substances contribute. The amount of ash in coal will dictate how frequently grates need cleaning, a factor which could become a significant cost element. SULPHUR: Fortunately, New Zealand coals are generally low in sulphur content, at less than 1 per cent, which compares favourably with some fuel oil. However a number of coals have more than this.

Where large quantities of high sulphur coal (more than 5 per cent) are used, toxic gases may present pollution problems.

The problem can be reduced by blending low and high sulphur coal, and sulphur gases can be removed in boiler chimneys through the installation of “scrubbers” to clean emissions. New fluidised bed furnaces can use high sulphur coal without emitting dangerous gases.

OTHER COAL USE FACTORS: The temperatures at which coal softens, and when coal ash fuses with carbon, need to be known. At the ash-fusion temperature, ash liquefies, flowing down the grate to form clinker. Air channels block, and burning rates slow down.

Bituminous coal may swell to as much as three times its original size as it is heated. The extent of the swelling is noted on an index of one to nine for each coal with this property. although some coals exceed the index.