Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 14

Discussions of the origin of coal are typically restricted to geochemical texts or to more theoretical treatises that focus on coal chemistry. However, combustion of coal (as performed in a coal-fired power station) involves knowledge of combustion chemistry and the behavior of different coals in coal-fired power stations. Thus, it is the purpose of this section to focus on the origin of coal as it influences coal chemistry, particularly the combustion chemistry and behavior (Chapter 7).

Coal is a combustible sedimentary organic rock that is formed from decayed plant remains, and other organic detritus. Although coal forms less than 1% of the sedimentary rock record, it is of foremost importance to the energy requirements of many countries and the origin of coal as it influences behavior has received much attention (Speight, 2013, 2020). However, coal is also a compact stratified mass of plant debris which has been modified chemically and physically by natural agencies, interspersed with smaller amounts of inorganic matter. The natural agencies causing the observed chemical and physical changes include the action of bacteria and fungi, oxidation, reduction, hydrolysis, and condensation – the effect of heat and pressure in the presence of water.

Coal has also been considered to be a metamorphic rock, which is the result of heat and pressure on organic sediments such as peat. However, the discussion is in favor of coal as a sedimentary rock because most sedimentary rocks undergo some heat and pressure and the association of coal with typical sedimentary rocks and the mode of formation of coal usually keep low-grade coal in the sedimentary classification system. Anthracite, on the other hand, undergoes more heat and pressure and is associated with low-grade metamorphic rocks such as slate and quartzite. Subducted coal may become graphite in igneous rocks or even the carbonate rich rocks such as carbonatites, which are intrusive or extrusive igneous rocks characterized by mineralogical composition and consisting of greater than 50% w/w carbonate minerals).

Coal is a sedimentary black or dark-brown rock that varies in composition. Some types of coal burn hotter and cleaner, while others contain high moisture content and compounds that, when burned, contribute to acid rain and other pollution. Coals of varying composition are used around the world as a combustible fossil fuel for generating electricity and producing steel. Because peat is not a rock and the unconsolidated plant matter is lacking the metamorphic changes found in coal, it is not typically classified as coal. Thus, coal is classified into four main types, depending on the amount of carbon, oxygen, and hydrogen present (i) lignite, (ii) sub-bituminous coal, (iii) bituminous coal, and (iv) anthracite.

The degree of alteration (or metamorphism) that occurs as a coal matures from lignite to anthracite is referred to as the rank of the coal, which is the classification of a particular coal relative to other coals, according to the degree of metamorphism, or progressive alteration, in the natural series from lignite to anthracite (ASTM D388). However, because of the chemical process involved in the maturation of coal, it is possible to broadly classify into three major types namely (lignite, bituminous coal, and anthracite). However, because of other differences, and the lack of other differences (with overlap between borderline coals) there is no clear demarcation between the different coals and other classifications such as semi-anthracite, semi-bituminous, and subbituminous are also used.

There are two predominant theories that have been proposed to explain the formation of coal: (i) the plant remains which eventually form coal were accumulated in large freshwater swamps or peat bogs during many thousands of years, which supposes that growth-in-place of vegetable material – the autochthonous theory, also often referred to as the swamp theory, and (ii) the coal strata accumulated from plants which had been rapidly transported and deposited under flood conditions – the allochthonous theory, also often referred to as the drift theory.

It is believed that major autochthonous (in situ) coal fields generally appear to have been formed either in brackish or fresh water, from massive plant life growing in swamps, or in swampland interspersed with shallow lakes. The development of substantial in situ coal measures thus requires extensive accumulations of vegetable matter that is subjected to widespread submersion by sedimentary deposits.

However, the types of fossil plants found in coal do not clearly support the autochthonous theory – for example, the fossil lycopod trees (such as Lepidodendron and Sigillaria) and giant ferns (especially Psaronius) that are common in Pennsylvanian coals may have had some ecological tolerance to swampy conditions, yet other Pennsylvanian coal plants (e.g., the conifer Cordaites, the giant scouring rush Calamites, the various extinct seed ferns) by their basic construction may have preferred existence in well-drained soils and not in the proverbial peat swamp. The anatomy of coal-forming plants is considered by many coal geochemists to indicate that initiation of the coalification lay down occurred in a tropical or subtropical climate, a conclusion which can be used to argue against autochthonous theory, for modern swamps are most extensive and have the deepest accumulation of peat in the higher-latitude cooler climates.

By way of explanation, coalification or metamorphosis of coal is defined as gradual changes in the physical and chemical properties of coal in response to temperature and time. The coal changes from peat through lignite and bituminous coal to anthracite. With extreme metamorphism and the changes, with increasing rank, include an increase in carbon content, and decreases in moisture content and volatile matter (Table 1.2). However, the data presented in the table (Table 1.2) are for illustrative purposes only and should not be construed to be precise since other effects (such as the mix of the coal precursors) will also play a role in the coalification process and the process is site specific (Speight, 2013).

In more general terms, the coalification of coal is a consequence of thermal effects and pressure through compaction of the sediment, which depending upon the initial events – including the composition of the coal purposes, will be site specific. However, the coalification processes involved in coal formation are marked by a well-defined progression of increasing rank that does increase with depth, and the combination of depth of burial and geothermal gradient essentially determine the rank of coal. Water, carbon dioxide and methane are generated during the progressive coalification.

Methane is the predominant gas generated in the bituminous coal and anthracite stages of coalification, and the carbon dioxide produced at lower ranks is typically flushed out of the coal by methane. The sorption capacity of coal increases with rank. Typically, high-rank coal can absorb more gas and the adsorptive capacity of coal for methane increases with coal rank. The sorption capacity of coal can be influenced by different intrusions and by the tectonic events such as folding and faulting. Coals near igneous intrusions, such as dykes, may contain calcites and pyrites which are likely to influence the ability of gases to drain.

To follow on from above, it was the difference in coal properties of Gondwana (Indian) coals that led to the formation of the drift theory. The mode of deposition of coal forming can be explained as follows: (i) coal is formed largely from terrestrial plant material growing on dry land and not in swamps or bogs, (ii) the original plant debris was transported by water and deposited under water in lakes or in the sea, (iii) the transported plant debris, by its relative low density even when water logged, was sorted from inorganic sediment and drifted to a greater distance in open water – the sediments, inorganic and organic, settled down in regular succession, (iv) the process of sedimentation of the organic and inorganic materials continues until the currents can deposit the transported vegetation in the locations, (v) these deposits are covered subsequently by mineral matter, sand, and results in coal seams, (vi) the depositions can also stop for a particular period and again begin to occur when tidal and current conditions are correct, and (vii) even within coal rank, coal properties vary widely due to the varied types of vegetation deposited.

Table 1.2 Illustration of the effects that can contribute to the coalification process.*

Coal rank	DoB	MT	C	VM	CV	M
Lignite	650-4,900	25-45	60	49-53	23,000	30-50
Subbituminous	4,900-8,200	45-75	71-77	42-49	29,300	10-30
Bituminous	8,200-19,500	75-180	77-87	29-42	36,250	5-10
Anthracite	>19,500	>180	87-92	8-29	>38,000	<5

Key:

DoB: approximate depth of burial, feet.

MT: approximate maximum temperature during burial, °C.

C: approximate carbon content, % w/w dry ash-free basis.

VM: approximate volatile matter, % w/w dry, ash-free basis.

CV: approximate calorific value (heat content), ash-free basis.

M: approximate moisture content, % w/w (in situ).

*The data are for illustrative purposes only; the actual conditions may vary somewhat from the data presented here.

It is also factual that marine fossils such as fish, mollusks, and brachiopods occur in coal. Coal balls, which are rounded masses of matted and exceptionally well-preserved plant and animal fossils (including marine creatures) are found within coal strata and associated with coal strata (Mamay and Yochelson, 1962). Since there is little anatomical evidence suggesting that coal plants were adapted to marine swamps, the occurrence of marine animals with non-marine plants suggests mixing during transport, thus favoring the allochthonous model (Rupke, 1969; Cohen, 1970).

Many factors determine the composition of coal: (i) the mode of accumulation and burial of the plant debris forming the deposits, (ii) the age of the deposits and the geographical distribution, (iii) the structure of the coal-forming plants, particularly details of structure that affect chemical composition or resistance to decay, (iv) the chemical composition of the coal-forming debris and its resistance to decay, (v) the nature and intensity of the peat-decaying agencies, and (vi) the subsequent geological history of the residual products of decay of the plant debris forming the deposits. In short, coal composition is subject to site-specific effects and is difficult to generalize on a global basis (Speight, 2013).

In summary, there are advantages and disadvantages of both theories. While the coal purist may favor one or the other, there are the pragmatists who will recognize the merits of both theories. Whichever theory is correct (if that is possible) and whatever the origin of coal, there are expected to be differences in properties and behavior.

Finally, Hilt’s law is a geological term that states the deeper the coal seam, the deeper the rank (grade) of the coal – i.e., anthracite would be expected to lie in deeper buried seams than lignite (Figure 1.1) (Elphick and Suggate, 1964; Suggate, 1974; Ward, 2008). The law holds true if the thermal gradient is entirely vertical, but metamorphism may cause lateral changes of rank, irrespective of depth. Furthermore, increasing depth of burial results in a decrease in the oxygen content of the coal.

Chemically, coal is a hydrogen-deficient hydrocarbon with an atomic hydrogen-to-carbon ratio near 0.8, as compared to crude oil hydrocarbon derivatives, which have an atomic hydrogen-to-carbon ratio approximately equal to 2, and methane (CH₄) that has an atomic carbon-to-hydrogen ratio equal to 4. For this reason, any process used to convert coal to alternative fuels must add hydrogen or redistribute the hydrogen in the original coal to generate hydrogen-rich products and coke (Speight, 2013).

The chemical composition of the coal is defined in terms of its proximate and ultimate (elemental) analyses (Chapter 5) (Speight, 2013, 2020). The parameters of proximate analysis are moisture, volatile matter, ash, and fixed carbon while the ultimate analysis (also referred to as the elemental analysis) encompasses the quantitative determination of carbon, hydrogen, nitrogen, sulfur, and oxygen within the coal. Additionally, specific physical and mechanical properties of coal and particular carbonization properties are also determined.

Figure 1.1 Schematic showing tendency of coal rank to increase with depth of burial*.

*Numbers are approximate and used for illustration only; peat is included only for comparison and it should not be construed for this diagram that peat is a type of coal.