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Part or all of this article has been imported from the 1911 edition of the Encyclopaedia Britannica, which is now free of copyright and hence in the public domain. The imported text may be significantly out of date, and any more recent developments may be completely missing
|Type of Rock:|
|Best Places:||Throughout Shetland in damp, uncultivated areas. Not present on some of the smaller islands.|
|Peat is midway between a form of soil (on its upper layers) and a precursor to coal in its lower levels. It is not generally thought of as a rock.|
Peat'<ref>possibly connected with Med. Lat. pelia, pecia, piece, ultimately of Celtic origin; cf. Old Celtic, pet, Old Irish pit, Welsh peth, meaning a portion</ref>, also known as turf (Norn: Torf) is a product of decayed vegetation found in the form of bogs in Shetland and many other parts of the world.
Peat is rich in hydrocarbons, and as such represents an early stage in the development of coal, oil and natural gas deposits. It has long been used as a fuel in Shetland, where trees are scarce, and the remoteness of the islands, has made the import of other fuels expensive.
The plants which give origin to these deposits are mainly aquatic, including reeds, rushes, sedges and mosses. Sphagnum is present in most peats. It seems that the disintegration of the vegetable tissues is effected partly by moist atmospheric oxidation and partly by anaerobic bacteria, yeasts, moulds and fungi, in depressions containing fairly still but not stagnant water, which is retained by an impervious bed or underlying strata. As decomposition proceeds the products become waterlogged and sink to the bottom of the pool; in the course of time the deposits attain a considerable thickness, and the lower layers, under the superincumbent pressure of the water and later deposits, are gradually compressed and carbonized. The most favourable conditions appear to be a moist atmosphere, and a mean annual temperature of about 45 F. ; no bogs are found between latitudes 45 N. and 45 S.
Peat varies from a pale yellow or brown fibrous substance, resembling turf or compressed hay, containing conspicuous plant remains, to a compact dark brown material, resembling black clay when wet, and some varieties of lignite when dry. Two typical forms may be noticed: " Hill peat " (the mountain or brown bogs of Ireland), found in mountainous districts, and consisting mainly of Sphagnum and Andromeda; and " Bottom peat " (the lowland or red bogs of Ireland), found in lakes, rivers, and brooks, and containing Hypnum. It always contains much water, up to 90%, which it is necessary to remove before the product can be efficiently employed as a fuel, and for most other purposes. A specimen dried at 100 C. had the composition: carbon = 60-48%, hydrogen = 6- 10%, oxygen = 32-55%, nitrogen = 0-88%, ash = 3~3O%; the ash is very variable from i to 65 % and consists principally of clay and sand, with lesser amounts of ferric oxide, lime, magnesia, &c. The specific gravity has been variously given, owing to the variable water content and air spaces; when dried and compressed, however, it is denser than water.
Peat-winning presents certain special features. The general practice is to cut a trench about a foot deep with a peculiarly shaped spade, termed in Ireland a " slane," and remove sods from 3 to 4 ft. long. When one layer has been removed, the next is attacked, and so on. If the deposit be more solid stepworking may be adopted, and should water be reached recourse may be had to long-handled slanes. The sods are allowed to drain, and then stacked for drying in the air, being occasionally turned so as to dry equally; this process may require about six weeks. The dried sods are known as " dug peat." Excavators and dredges are now extensively used, and the drying is effected in heated chambers, both fixed and revolving.
The low value of ordinary dug peat as a fuel has led to processes for obtaining a more useful product. In M. Ekenberg's process the wet peat is pulped and milled so as to make it of uniform composition, and the pulp passed into an oven maintained at l80-200 F., where it is carbonized by superheated water. The pressed product, which resembles lignite, still contains 8 to 14% of water; this is driven off by heat, and the residue briquetted. The final product is nearly equal to coal in calorific value, and has the additional advantage of a lower sulphur content 0-2 to 0-4 % against about 2 % in ordinary coal.
Peat has also been exploited as a source of commercial alcohol, to be employed in motors. Of other applications we may notice C. E. Nelson's process for making a paper, said to be better than ordinary wrapping; the first factory to exploit this idea was opened at Capac, Michigan, in 1906.
Peat has been employed as a manure for many years, and recently attempts have been made to convert artificially its nitrogen into assimilable nitrates. However, environmentalists have been busily campaigning against this, saying that it damages the ecosystem.
According to Sustainable Shetland:
- "Peat is a carbon sink. Peat takes thousands of years to form, and actively stores and absorbs climate damaging CO2. Damaging peat on this scale releases large quantities of CO2. It is madness to damage ancient peat deposits for a so-called environmental project."<ref>http://www.sustainableshetland.org/</ref>
This article contains text from the article peat in the 1911 Encyclopædia Britannica
See P. R. Bjorling and F. T. Gissing, Peat and its Manufacture (1907); F. T. Gissing, Commercial Peat (1909); E. Nystrom, Peat and Lignite (1908), published by Department of Mines of Canada.