Coal is a combustible rock formed from the altered remains of ancient vegetation laid down in peat swamps. It is mainly composed of organic matter (compounds of carbon), moisture and minerals. The transition from peat to coal (coalification) occurs over millions of years as peat is buried and consolidated by the weight of overlying sediments.
Three factors determine the properties of coal:
- type – variation in the original plant material and its subsequent alteration
- rank – difference in the degree of burial and subsequent coalification
- grade – variation in mineral matter. High-grade coal contains little mineral matter, whereas lower-grade coal contains more mineral matter, mainly from mud in the peat swamp.
Changes in coal
As coal becomes buried more deeply, it goes through a sequence of changes in rank: from peat, to lignite, sub-bituminous coal, bituminous coal, semi-anthracite, and anthracite. This process results in irreversible changes in the chemical and physical nature of the coal, and there can be considerable variation within a coalfield. Geological age does not affect rank.
From peat to bituminous coal there is a loss of water but a proportionate increase in carbon. Lignite and sub-bituminous coal are relatively soft, with a brown, earthy appearance – they are often known as brown coals. Higher-rank coals are blacker, shinier, and have a higher proportion of carbon.
As the rank increases, so does the heat (or calorific value) given out by the coal when it burns. It is difficult to make a fire from lignite because the moisture content is so high. Sub-bituminous coal burns satisfactorily, and bituminous coal burns fiercely.
The distinctive nature of New Zealand coal
New Zealand coals have characteristics that distinguish them from coals found in other countries. Most of the world’s coals were formed in great swamps during the Carboniferous period, about 300–350 million years ago. New Zealand coals are much younger, having been formed mainly in the late Cretaceous and Tertiary periods, 30–70 million years ago, from more-evolved vegetation types containing flowering plants. Because the original vegetation is different, New Zealand coals have some unusual properties, giving them high value for specialised purposes.
When coal burns, the ash that remains consists mainly of clay minerals, quartz, and sulfur compounds (mainly pyrite). Most New Zealand coals have an ash content of less than 4%, which is lower than most carboniferous coals. A few seams have an exceptionally low ash content – less than 1%.