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peat, soil material consisting of partially decomposed organic matter, found mainly in swamps and bogs in various parts of the northern temperate zone but also in some semitropical and tropical regions. Peat is formed by the slow decay of successive layers of aquatic and semiaquatic plants, e.g., sedges, reeds, rushes, and mosses, and is the earliest stage of transition from compressed plant growth to the formation of coal. One of the principal types of peat in northern regions is moss peat, derived primarily from sphagnum moss; it is used in agriculture as poultry and stable litters as well as a mulch, a soil conditioner, and an acidifying agent; it is also used in industry as an insulating material. Another type of peat is fuel peat, which has been most widely used in regions where coal and wood are scarce, e.g., Scandinavia, Ireland, Scotland, and parts of Russia. The burning of peat, however, produces greater carbon emissions than coal.
The most extensive peat deposits are found in Canada and Russia, with significant peatlands also in N Europe and NE China. A large peatland found in 2014 in N Congo-Brazzaville and neighboring Congo-Kinshasa has since been determined to extend over some 55,000 sq mi (140,000 sq km). Other tropical peatlands are found in Indonesia and parts of the Amazon region. Large deposits of peat in the United States are found in Alaska, Florida, Louisana, Michigan, and California. The formation of peat acts as a carbon sink, removing carbon dioxide from the atmosphere, but that carbon dioxide is released when peatlands dry out naturally or are drained, and when peat is burned.
a combustible mineral product formed naturally by the partial decomposition of dead swamp vegetation under conditions of excess moisture and little oxidation. Peat is generally distinguished from soil by its high content of organic compounds —at least 50 percent of the weight after all moisture is removed.
General information. The organic matter in peat consists of plant residues that have undergone varying degrees of decomposition. Humus imparts a dark color to peat. In the total peat mass, the relative concentration of the products of the decomposition of plant tissues that have lost their cellular structure is called the degree of peat decomposition. A distinction is made between weakly decomposed (approximately 20 percent), moderately decomposed (20–35 percent), and strongly decomposed (more than 35 percent) peat. Depending on the conditions under which it was formed and on its properties, peat is classified as high-moor, transitional, or low-moor peat.
Peat has a complex chemical composition, which is determined by the conditions under which peat-forming plants originated, by the chemical composition of the plants, and by the degree of decomposition. By combustible weight, peat consists of 50–60 percent carbon, 5–6.5 percent hydrogen, 30–40 percent oxygen, 1–3 percent nitrogen, and 0.1–1.5 (sometimes 2.5) percent sulfur. The organic matter is made up of 1–5 percent water-soluble substances, 2–10 percent bitumens, 20–40 percent readily hydrolyz-able compounds, 4–10 percent cellulose, 15–50 percent humic acids, and 5–20 percent lignin.
Peat is a complex, polydisperse, multicomponent system. Its physical properties vary with the properties of its individual components, the relative amounts of individual components, and the degree of decomposition or degree of dispersion of the solid mass (as determined by the specific surface or quantity of fractions less than 250 micrometers in diameter). Peat typically has a high moisture content in the natural state (88–96 percent), high porosity (up to 96–97 percent), and high compressibility. The texture is homogeneous, sometimes stratified, and the structure is usually fibrous or plastic (in strongly decomposed peat). The color varies from yellow or brown to black. When dry, weakly decomposed peat is characterized by low porosity (up to 0.3 g/cm3), a low rate of thermal conduction, and a high gas-absorbing capacity. After mechanical processing, dry, highly dispersed peat forms solid blocks of great strength with a heat-producing capacity of 2,650–3,120 kilocalories per kg for a 40 percent moisture content. Weakly decomposed peat is an excellent filtering material; highly dispersed peat is used as an antifiltration material. Peat absorbs and retains large quantities of moisture, ammonia, and cations (especially of heavy metals). The rate of filtration through peat varies over several orders of magnitude.
|Table 1. Classification of peat|
|Type||Woody subtype||Woody-swamp subtype||Swamp subtype|
|Woody group of species||Woody-grass group of species||Woody-moss group of species||Grass group of species||Grass-moss group of species||Moss group of species|
|Transitional||Woody transitional||Woody-sedge transitional||Woody-sphagnum transitional||Sedge transitional|
|Sedge-sphagnum transitional||Hypnoid transitional|
|High-moor||Pine high-moor||Pine-cotton grass||Pine-sphagnum||Cotton grass Scheuchzeria high-moor||Cotton grasssphagnum|
Historical outline. The first reference to peat is Pliny the Elder’s description of “combustible earth” for heating food (AD. 46). In the 12th and 13th centuries peat was known as a fuel in Holland and Scotland. The world’s first book on peat, A Treatise on Peat by Martin Schoock, was written in Latin and published in Groningen in 1658. Many erroneous ideas on the origin of peat were refuted in 1729 by J. Degner, who studied peat with a microscope and demonstrated its plant origin. In Russia the first references to peat and its use appeared in the 18th century in the works of M. V. Lomonosov, I. G. Leman, V. F. Zuev, and V. M. Sever-gin. In the 19th century peat was studied by V. V. Dokuchaev, S. G. Navashin, and G.I. Tanfil’ev. In Russia it was the botanical aspect of peat that came under study.
After the Great October Socialist Revolution, research, industrial, and academic organizations were created to make a complete study of peat and its use in the economy; they included the Institute of Peat (Instorf) and the Moscow Peat Institute.
Soviet researchers identified the geographic patterns of the distribution of peat bogs, classified the types of peat and peat bogs, compiled cadastres and maps of peat bogs, and studied the chemical composition and physical properties of peat. Prominent researchers in this field included I. D. Bogdanovskaia-Gienef, E. A. Galkina, D. A. Gerasimov, V. S. Dokturovskii, E. K. Iva-nov, N. Ia. Kats, M. I. Neishtadt, N. I. P’iavchenko, V. E. Ra-kovskii, V. N. Sukachev, and S. N. Tiuremnov. Research on the uses of peat in the USSR is conducted at the All-Union Scientific Research Institute of the Peat Industry in Leningrad, with its branches in Moscow and the village of Radchenko in Kalinin Oblast, the Peat Institute of the Academy of Sciences of the Byelorussian SSR, and special laboratories of the Kalinin, Kaunas, and Tomsk polytechnical institutes.
Formation of peat. In the view of some scientists, peat is a precursor of a genetic series of coals. The places where peat is formed, peat bogs, are found in river valleys (floodplains and terraces) and at water divides.
Peat is formed from the residues of dead plants whose aerial organs have become humified and mineralized in the aerated surface stratum of a bog (the peat-producing horizon) by soil invertebrates, bacteria, and fungi. The underground organs in an anaerobic medium are preserved, and they form the structural, or fibrous, part of peat. The rate of decomposition of peat-forming plants in the peat-producing horizon varies with the plant species, the water content, acidity, and temperature of the medium, and the composition of the mineral constituents. Despite the annual growth of the dead organic mass, the peat-producing horizon maintains its own independent existence as a natural “factory” for the production of peat. Since many plant species grow on peat bogs (bog phytocoenoses) and since the conditions under which these species grow vary in mineral content, water content, and reaction of the soil solution, the properties of peat vary with the parts of the bogs on which the peat is formed.
Some peat deposits were formed during periods between glaci-ations or became superimposed by loose deposits of varying thickness as a result of change in the level of erosion. The age of these buried deposits is measured in tens of millennia. The water content of buried peat is less than that of recent peat.
Classification. The three types of peat—high-moor, transitional, and low-moor—are classified according to the composition of the original plant material, conditions under which the peat was formed, and the peat’s physical and chemical properties. Depending on the percent of woody residues present, each type is subdivided into three subtypes—woody, woody-swamp, and swamp. These subtypes differ in degree of decomposition. The woody subtype is highly decomposed (sometimes as much as 80 percent), the swamp subtype is minimally decomposed, and the woody-swamp subtype occupies an intermediate position.
|Table 2. Peat reserves and extraction in the USSR and abroad (1975)|
|Reserves (billion tons; 40% moisture content)||Extraction (million tons per year)|
|Federal Republic of Germany||6.0||1.5|
The three subtypes are divided into groups consisting of four to eight species each (Table 1). A species is the primary taxonomic unit of peat classification; it reflects the original plant association and the primary conditions of peat formation. The species is characterized by a specific combination of dominant residues of individual plant species and of other, typical residues. Several primary peat species, differing little from each other in properties and forming large, homogeneous, horizontal beds, are called layer-forming species. Deposits of layer-forming species of varying extent and thickness follow one another in a particular succession to form a peat bog. Geomorphological, geological, hydro-geological, and hydrological conditions prevailing in each part of a bog influence the structure of a deposit in a given climatic zone.
Peat bogs are classified according to the combination of individual species of peat at successive depths. The peat industry distinguishes four types of peat bogs: low-moor, transitional, high-moor, and mixed. The type of peat bog is the primary unit of classification (Figure 1). Twenty-five primary types of peat bogs are distinguished in the European USSR and 32 in Western Siberia.
Peat deposits. Peat deposits of commercial value are accumulations of peat clearly demarcated geographically and not associated with other accumulations. The area occupied by peat deposits and bogs throughout the world is approximately 350 million hectares (ha), of which approximately 100 million ha are commercially important. There are 51 million ha in Western Europe, more than 100 million ha in Asia, and more than 18 million ha in North America. Data on peat reserves and extraction in the USSR and abroad are given in Table 2. Explored peat reserves in the USSR are given by region in Table 3.
Peat reserves have not been investigated equally in all economic regions of the country. For example, more than 70 percent of the reserves in the Central Zone of the RSFSR has been thoroughly surveyed, compared to 0.6 percent in Western Siberia, where 82.8 percent of the reserves has undergone only preliminary investigation.
|Table 3. Distribution of explored peat reserves in the USSR (1975)|
|Republic or economic region||Total area of peat beds in commercial deposits (million ha)||Peat reserves (billion tons; 40% moisture content)|
|Central Chernozem Zone||0.04||0.1|
|Soviet Far East||5.7||5.2|
Prospecting for peat deposits entails an analysis of maps and aerial photographs; the exploration and prospecting stage is supplemented by field work. Preliminary prospecting is conducted on deposits covering an area of more than 1,000 ha in order to determine the feasibility of exploitation. Thorough prospecting is done in order to obtain data for plans for the exploitation and utilization of a peat deposit.
Exploitation of peat deposits. Peat deposits must be drained and the surface specially prepared before the deposit can be worked. The surface is prepared after a drainage network has been set up and preliminary drainage has been finished. Trees and sometimes moss are removed, regardless of the purpose for which the peat is to be used. Woody matter in the layer to be worked is removed to a depth of 25–40 cm, or it may be ground into fractions less than 8–25 mm in diameter. The surface of the deposit is divided into individual sections by sighting paths and drainage canals. The site is leveled in a horizontal direction perpendicular to the drainage channels, and a screw-type excavator is used to profile the site with a transverse slope in the direction of the sighting paths. This preparation helps lower the groundwater level and reduce the moisture content of the peat bog to 86–89 percent, thereby increasing the efficiency of the machines used to extract, dry, and harvest the peat. All the operations involved in preparing the surface are mechanized. Woody vegetation is removed by felling trees and shrubs and then stacking the trees into piles with a special machine. The piles are then loaded onto tractor-drawn dump trucks and transported to intermediate railroad warehouses. Tree stumps and woody inclusions are extracted by stump pullers or crushed by deep-working milling machines; they are then separated and removed.
Peat with average properties is harvested by machines that intermix the beds or drainage-enrichment machines. The latter extract the peat mass from a layer by means of milling devices that process and spread a layer of peat on the surface of the field. Small wood fragments and chips are collected from the surface of the plots by machines with harrows or tractor-drawn drums.
Peat is extracted in the USSR by milling (more than 95 percent of the total commercial extraction), excavation, and the deep-pit method. The prototype of the excavation method was the elevator method, by which approximately 1.3 million tons of lump peat were extracted in 1913. Before the October 1917 Revolution peat was extracted by hand. Elevators transported raw peat from an open pit, then mixed and pressed it into bricks. The drying, collecting, and loading operations were performed manually. A hydraulic (hydro-peat) method of extracting peat with complete mechanization of the production processes was developed in the 1920’s and used from 1922 to 1962. In excavation with integrated mechanization, the peat is removed from the deposit with a scoop, processed, and pressed into bricks, which are laid on top of the deposit and later collected and stored.
The milling method of obtaining peat was developed in the USSR in the late 1940’s. The method is completely mechanized and requires less labor and consumption of metal and energy than other methods. The principal operations include milling the top peat layers of a deposit to a depth of 25 mm, drying the milled peat, and gathering the finished product in the form of crumbs. A layer takes one to two days to dry. There are 20–28 such cycles in a season and approximately 40–50 cycles when the pneumatic method of collecting the peat is used. The milling may be done with harrows, cultivators, or drum-type milling machines hooked to tractors. Peat extracted by peat machines is kept in piles in the field for an average of approximately six months. The most effective way to store peat and prevent spontaneous combustion is to insulate the piles from the air with a layer of wet peat. The use of a polymer film for this purpose was introduced in 1975.
The deep-pit method of extracting lump peat is used to supply nonindustrial needs. In this method the peat is dug from narrow trenches, processed, and pressed into bricks. The peat bricks are laid out on the field to dry, and the trenches are packed down by the extracting machine.
The useful properties of peat are improved in the course of processing as a result of the increase in the specific surface of the dispersed material. The dispersion of raw peat increases the rate of shrinkage, a prerequisite for obtaining a compact and strong product. Processing decreases the moisture capacity of fuel peat. Peat is mechanically processed by various working elements: augers, augers and blades, spiral cones, cones, slits, and crushing and grinding devices.
Complex use of peat. In the 16th and 17th centuries, coke and tar were obtained from peat by roasting. Peat was used in both agriculture and medicine. The commercial production of peat semicoke and tar began in the late 19th and early 20th centuries. From the 1930’s to the 1950’s, peat was used in power engineering, in the production of gas, and as a nonindustrial fuel. Research on the use of peat in power technology was carried out in the 1950’s. The possibility of using peat from a single deposit simultaneously for agricultural and industrial purposes fostered a new trend in peat utilization—the combination use of peat—made possible by the varied properties of the different species types of peat. For example, weakly decomposed high-moor peat
|Table 4. Agricultural and chemical characteristics of peat (in percent of dried matter)|
|Peat type||Ash content||Organic matter content||pH (in KCI extract)||Chemical composition|
|Up to 12|
|More than 88|
contains 40–50 percent carbohydrates; humic acids constitute 50 percent or more of strongly decomposed peat. Some peat types are rich in bitumens (2–10 percent). Weakly decomposed high-moor peat has a high water- and gas-absorbing capacity and a low coefficient of thermal conductivity.
Strongly decomposed peat finds varied applications in agriculture (see Table 4). It is used to prepare composts, mixtures containing mineral fertilizers and lime, and peat-mineral and peat-mineral-ammonia fertilizers. Peat containing vivianite is used as a phosphorus fertilizer, and peat with lime as a lime fertilizer. Low-moor peat added in large quantities (500 tons per ha or more) helps condition sod-podzolic soils and improve their physical and chemical properties.
Peat-humus bricks and greenhouse soil mixtures are used with other constituents, including manure and mineral fertilizers, to grow vegetables and flowers; undecomposed peat serves as a biological fuel. Well-aerated and thoroughly decomposed peat is used to mulch seeded fields. High-moor peat provides good bedding for cattle and poultry. Some types of strongly decomposed peat contain substantial quantities of bitumens and are used to produce waxes. The world’s only plant for the production of alcohol and furfural from weakly decomposed peat is located in Leningrad Oblast. Heat- and sound-insulating peat slabs and garden pots made from peat are also produced. Activated charcoal is produced from peat in the Federal Republic of Germany, the Netherlands, and the USSR. Pressed peat briquettes are used in the USSR and Ireland for nonindustrial purposes.
Advances in the technology of processing peat are being made in two directions. The first is based on isolating individual constituents—bitumens, humic acids, carbohydrates, and so on. These constituents are extracted with insignificant changes in the original material in the form of ready products or as raw material for further processing. The second trend involves the thorough decomposition of peat and the conversion of peat into completely new substances. These substances are the products of thermal and oxidative degradation, hydrogenation, and other processes.
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N. A. KOPENKINA (formation and classification of peat),
M. I. NEISHTADT (historical outline), and
V. I. CHISTIAKOV