ABSTRACT
The term solid is normally applied to fuels that are in a solid state at ambient temperature and pressure� A wide range of fuels exist within this category, both organic and inorganic� Inorganic solid fuels involve mainly high-energy metallic fuels, for example, Al, Fe, Zr, and Mg, and some nonmetallic fuels, such as sulfur� Organic solid fuels can be
• Naturally occurring fuels such as coal, wood, and peat • Manufactured or processed fuels from natural sources such as char-
coal and coke
Natural solid fuels derived from living sources can be either fossil or nonfossil in origin such as vegetation and animal refuse and remains�
As shown schematically in Figure 14�1, depending on the type of solid fuel considered, not all its fractions can be initially combustible� The part that is combustible will be mostly volatile components of a hydrocarbon nature with some nonvolatile parts that are carbonaceous and polymeric� The noncombustible part will be associated with the presence of moisture and inorganic components that eventually appear following combustion as solid ash�
Coal is the most abundant fossil fuel; it is relatively evenly distributed worldwide� Its global reserves are enormous and far exceed those of oil or natural gas� Coal is considered to have been formed from accumulated vegetable matter that was buried and subjected under high pressure and temperature to anaerobic and aerobic decay over geological times� The decomposition of the organic matter proceeded at different rates depending on the raw materials and local conditions and their changes over time�
Coal is not a simple, homogenous carbonaceous material but a complex substance with a varying chemical composition� The quality of a coal is critical for its exploitation and varies widely� This makes it difficult to generalize the design and operation of coal-burning equipment� There are a number of broad categories that describe the quality of coal� These relate primarily to the extent of volatiles a coal contains, which is also indicative indirectly of the geological age and origin of the coal�
Coal extraction is usually done through either surface mining or deep underground mining with associated increased environmental and safety concerns� Much of the current usage of coal globally is in the production of steam in plants that operate on variations of the Rankine cycle type for the generation of electric power, usually in relatively very large stationary units� However, much effort and resources are increasingly being expended following coal combustion on removing the undesirable compounds in exhaust gases such as oxides of sulfur and nitrogen, ash, and particulates, which contribute to increased environmental pollution, depletion of the ozone layer, and acid rain� More recently, concern is being increasingly focused on dealing effectively with emissions of carbon dioxide, the main component of the products of coal combustion, which contributes very significantly to increased greenhouse gas emissions and associated global warming� This is imposing serious concerns on and increasingly limiting the prospects of wider continued exploitation of coal as an energy resource�
Specially processed coal is also a major raw material that is required industrially in the making of various chemicals and coke, which is needed in the manufacture of steel� However, the use of coal domestically such as for home heating, where its combustion cannot be well optimized as in large power plant applications is being increasingly restricted worldwide, mainly for environmental reasons�
The energy and combustion characteristics of coal depend significantly on its composition, which in turn depends on many factors that include its original source of vegetation, the associated inorganic matter, and the history of the sequence of events and processes that led to its eventual formation� Generally, coals burn as either “caking” or “free-burning” solid fuels�
Caking coals tend to fuse together when heated, forming a semi-coke pasty material that is nonporous, whereas free-burning coals tend to crumble when heated�
Carbon, with its high heating value, is the dominant component in coals� It represents on a dry mass basis typically from more than 60% for lignite to more than 80% for anthracite� Hydrogen accounts for only around 5% or less� Some varying amounts of oxygen are present in coals� The higher the oxygen content the lower the heating value of a coal� The corresponding sulfur content also varies widely depending on the type of coal, but it averages typically from 1% to 2% by mass�
There are different approaches for the preparation, firing, and combustion of coal depending on key factors such as its grade, rank, composition, heating value, porosity, and the tendency to cake and on whether an auxiliary fuel such as natural gas is burned with it or not� Coal is usually fired after being broken into sufficiently small pieces or pulverized into fine particles that are mixed with some air and introduced as a jet through burners into coal-fired boilers and furnaces�
Coal-fired power plants of a very large capacity are usually built not very far from where a coal resource is located for improved efficiency, economics, reduced transportation costs, and better environmental control� These plants operate usually as heat engines of the modified Rankine cycle type or, increasingly more recently, plants that operate on combined gas turbine and Rankine cycles, because of their very superior work production capacity and associated thermal efficiencies�
Coal is burned mainly as a solid, but it can be converted through chemical processing to fuel gas or a liquid fuel� The products of gasification of coals are primarily carbon monoxide and hydrogen� Such gas mixtures can be synthesized to a variety of liquid fuels, usually with the aid of catalysts at appropriate temperatures and pressures� The products after further processing may be made into fuels, including both diesel and gasoline types� However, such approaches do represent a relatively costly way of manufacturing liquid fuels in comparison with the direct processing of petroleum�
The main approach employed for the combustion of coal in large-scale industrial installations involves having the coal pulverized into small particles and blown into the furnace with air and sometimes with the addition of natural gas� Another combustion method that is being increasingly employed uses fluidized beds� There are advantages and limitations, as described earlier in this section, associated with the employment of such methods for coal combustion�
The transportation of coal through pipelines in the form of slurry with water or other liquids so far has not been generally successful� This is because of the amount of energy required, water contamination, cost of separation at destination, abrasion and wear of the carrying pipes and associated equipment, as well as a multitude of environmental constraints�
In steelmaking, both coke, a product of coal processing, and thermal coal for the production of heat energy are used� These are used to heat the iron ore in blast furnaces� Coke is made from coal that is softened and liquefied when heated sufficiently in an atmosphere devoid of air� Coke, which is solidified on cooling into large, porous, lightweight lumps, is used to provide carbon as a reducing agent for the iron ore, which comprises mostly iron oxides� Thermal coal provides heat and also acts as a permeable load-bearing layer that supports the iron ore while permitting the flow of reducing gases within the furnace�
The production of fuel gas from coal involves partial oxidation and reforming processes during which the coal is reacted with steam at high temperatures and pressures to produce a product gas made up of largely hydrogen and carbon monoxide� Such “coal gas” was in wide use for a very long time as the main town gas used for a wide range of domestic and industrial applications� The development of relatively cheap natural gas resources and providing the appropriate supplies, transport, and distribution infrastructure reduced very significantly the manufacture and use of the coal gas�
There are huge coal reserves in many parts of the world, yet increasingly more coal combustion facilities are being denied or the burning of coal is being phased out altogether� Some of the contributory factors to this trend could be the following:
• Coals by virtue of their containing large amounts of carbon produce CO2 on combustion, which is a greenhouse gas that must be curtailed� They also contain relatively high amounts of sulfur, nitrogen, and impurities, which produce as pollutants oxides of sulfur and nitrogen, ash, and particulates� Many of them need to be removed before the products of coal combustion can be discharged into the open atmosphere� Moreover, coal-mining operations tend to release both methane and carbon dioxide in large quantities, which are greenhouse gases that undermine the quality of local water and its resources� The increasing reliance on surface mining of coal in recent years is contributing to water, soil, and air pollution�
• Coal varies widely in quality, composition, and heating value� It has its combustion problems, since as a solid fuel its burning is more difficult to control� The gas emissions of coal are also not easy to deal
with or clean� The mining, transport, cleaning, milling, and pulverizing of coal are highly energy-intensive processes with products and leftovers to dispose of� High-quality coals are becoming harder to get and increasingly more expensive�
• Large plants are needed for the effective processing and exploitation of coal, which is usually found far away from where it is needed� Accordingly, electric power is often generated near the coal mines and has to be transported over long distances, incurring great losses and requiring extensive infrastructure that has to be financed and maintained�
• Coal-fired plants suffer from poor-turndown-ratio combustion; take much time to start up and stop; are not easy to control; and are associated with excessive radiative heat transfer, ash disposal, corrosion, and erosion problems�
• There are still significant safety and long-term health problems associated with the mining, transport, handling, and storage of coal� Very serious fire and explosion hazards are associated with the uncontrolled releases of coal bed methane (CBM) and coal powder into the mine atmosphere, often very deep underground�
• Coal cannot be used directly by the current transport sector except via electric power or possibly via its products of gasification or liquefaction� Fluidized bed combustion for the exploitation of coal as fuel can still benefit from further developments and improvements� Moreover, there are alternative fuels that have fully developed infrastructures and are more convenient and attractive to use at present relative energy pricing�
It has been shown in Chapter 10 that the combustion of a solid fuel takes place through a complex interaction between physically controlled transport processes, with the chemical composition of the solid playing a role of somewhat lesser significance� The emissions from coal combustion vary according to the rank of coal (carbon-to-volatiles ratio) and composition of the fuel, the combustion equipment used and its maintenance, and a range of other operational factors� Figure 14�2 shows a schematic representation of the physical and chemical processes that occur during the combustion of a solid fuel such as coal�
It can be seen that flaming and combustion of the fuel proceed via the release of volatiles through heat transfer that contain combustible components, which diffuse outward and mix with the necessary air for oxidation reactions to proceed� The energy released promotes further volatilization and continued burning� Accordingly, the volatile contents of a solid fuel are of prime importance to the ease of its combustion� Moreover, by pulverizing the solid fuel, ignition and other combustion processes are sped up enormously�
Figure 14�3 shows a schematic representation of the fractional components that make up a typical coal� These components govern its quality and combustion characteristics� They can vary relatively widely depending on the class of coal� “Proximate analysis” is the composition by mass of the major constituents of coal, which, according to industrial standards, is as follows:
• Inherent moisture, %: Inherent moisture is the concentration of moisture in the coal as received� Increased moisture reduces the heating value of coal proportionally�
• Volatile matter, %: Volatile matter is the mass loss from a coal sample when heated in an inert atmosphere� Volatiles consist of vapors of various hydrocarbons, tar, and so on� Coals of high volatile content (e�g�, low-rank bituminous) are easy to ignite, burn quickly with a long orange flame, and require an adequate supply of secondary air to ensure complete combustion of the released volatiles within a relatively short time� On the other hand, coals of low volatile content (e�g�, anthracite) are difficult to ignite and slow to burn� They produce a short flame and require an ample supply of primary air through the coal bed�
• Ash, %: Ash in coals consists mainly of mineral matter such as oxides of Si, Fe, Al, and Ca� It is the leftover material following the heating of a known coal sample in a stream of air heated to high temperatures (e�g�, 700°C-750°C)� The presence of ash reduces the heating value and the caking of coal and adds to the difficulties of effective utilization of coal� Following burning, relatively very large quantities of leftover ash are produced, which must be removed�
• Fixed carbon, % (Figure 14.3): Fixed carbon is a term given to the remaining mass after subtracting the moisture, volatile, and ash contents of a coal�
• Combustible, %: A combustible is usually reckoned on the basis of volatiles and fixed carbon components�
• Rank of coal: This is a way of classifying the quality of coal by comparing the fixed mass of carbon to the volatiles of the fuel, that is, d/b in Figure 14�3� On this basis, anthracite has a higher rank than bituminous coals�
• Ultimate analysis (chemical): This is the percentage on a mass basis of the various individual elements and ash making up dry coal�
• Heating value: The heating value of coal can be quoted in a variety of ways, such as a higher heating value, when the water in the products has been condensed and its energy counted, and a lower heating value, when all the water in the products remains as vapor and is considered not to have been removed from the products of combustion� Care is needed to find out on what mass basis the heating value of a coal has been determined� The combustible part of the mass is usually used when comparing the heating value of different coals�
There are also a number of other properties associated with coal such as
1� Caking and swelling tendency 2� Ash fusion temperature 3� Size of pellets and ease of grindability
One of the big issues of cold weather operational problems in the power industry includes dealing with frozen coal stockpiles and condensers, pipe breaking, and frozen instruments and controls� Once a plant had to stop because of excessive cold it would be very difficult and would take much time to bring it back to steady high load operation�
Conventional coal classification or ranking is based arbitrarily on the specific coal from its initial state toward complete carbon� Coals are classified broadly into the following major categories (Figure 14�4):
1� Anthracite is hard, brittle, and black in color� Its moisture and volatile matter contents are low and carbon content is high�
2� Bituminous is dense, compact, and brittle coal and has a dark black color� It is more resistant to disintegration in air than a lower grade of quality coals� Its moisture and volatile matter contents are variable from high to medium, and its heating value is high� Different varieties of coal can be identified within this category�
3� Subbituminous is somewhat difficult to distinguish from bituminous, and it is dull in color and shows some woody material� It has lost some moisture but it is still of a relatively low heating value�
4� Lignite is the lowest rank of coal, which was formed from peat, compacted and changed� Its color is dark brown to black� Sometimes it is described as brown coal� It comprises woody materials embedded into pulverized and partially decomposed ancient vegetable matter� Lignite has a high moisture content and a low heating value compared to coals of higher quality�
5� Peat is the initial product in the long chain of conversion of vegetable matter into coal�
Table 14�1 lists the range of compositions of different classes of coal on mineral matter-free basis together with their corresponding gross heating values�
Coal has two kinds of sulfur, that is, organic and inorganic� Inorganic sulfur can be reduced mainly by washing the coal� Organic sulfur is more difficult to remove since it is bound chemically within the carbon molecules� One approach to reduce organic sulfur in coal is to heat it with caustic soda in relatively high proportions and wash it afterward to remove soluble salts; it is treated with dilute acid to remove any insoluble compounds� Fluidized bed combustion is being increasingly used to remove the sulfur in coal by mixing it with limestone�
1� A coal when received at the point of its usage contains 30% moisture and 10% ash by mass� The composition on a moisture-and ash-free basis by mass is 10% hydrogen and 6% sulfur, and the remainder is carbon� If the dry and ash-free lower heating value of the coal is 17�09 MJ/kg, calculate the higher heating value on a mass basis as received�
Answer: Given: (LHV)d,af = 17,090 kJ/kg� It is noted that 1�0 kg of (coal)d,af contains
10% hydrogen by mass, which produces
0 0 18 2
0 0� �1 9 kg of H O2× =
( ) � �, ,HHV 17, 9 9 17, 9 9d af fg water= + × = +0 0 0 0 0 0 0 0h ×
=
2,442
19,288 kJ kgd af/ ,
But 1�0 kg of dry-and ash-free coal is 1/0�60 kg of coal as received�
(HHV) as received = 19,288 × 0�6 = 11,572 kJ/kg
Note: The moisture as received is liquid and does not contribute to the heating value�
2� The ultimate analysis on a mass basis of a certain coal is given as follows: 0�80 carbon, 0�055 hydrogen, 0�065 oxygen, and 0�08 ash� Its lower heating value is 21�2 MJ/kg� The coal was burned with natural gas that can be assumed to be methane, which has a lower heating value of 33�95 MJ/m3 at ambient conditions of 1�0 bar and 288 K in the proportion of 10:1 on a thermal basis, respectively� Estimate the ideal dry volumetric analysis of exhaust products when 15% excess air was used�
Answer: Using subscripts “c” and “m” to refer to coal and methane respectively, the
given composition is as follows:
C 8 H 55 O 65, and ash 8 withx C: � , : � , : � : �0 0 0 0 0 0 0 0 Q /Qm 1= 0
(LHV)c = 21�2 MJ/kg, (LHV)m = 33�95 MJ/m3, P = 1 atm, T = 288 K, 15% excess air� Consider 1 kg of coal; then QC = 21�2 MJ/kg and Qm = 2�12 MJ/kgc� The volume of methane per kilogram of coal is as follows:
cLHV 2 12 33 95 621 m kg/( ) � / � � /= = 0 0
To find the corresponding moles of methane, assume it to be an ideal gas at 1 atm and 288 K:
PV nRT=
1 1 325 (kPa) 1 m 8 314 (kJ kmol K) 288 30 0� � � /× = × ⋅ ×n K
1 m of CH is 423 621 263 kmol kg3 4 c0 0 0 0 0 00� � � /× =
The stoichiometric equation per kilogram of coal is as follows:
C H O 0�00263 CH (O0� 0� �0 0� 4 28012 0551 06516 +( ) + +λ 3�76 N ) CO H O O N
→
+ + +
a
b d e
Find by considering the stoichiometric oxygen balance� The actual amount used was with 15% excess air, which makes the actual reaction equation the following:
C H O CH0 0 0 0 48012 0551 06516 0 00263 1 15 0� � � � � � �+ + × 0838 3 762 2 2
( � )O N CO+ →
+ + +
a
b H O d O e N
C balance: a = 0�80/12 + 0�00263 = 0�0693 H balance: 2b = 0�055 + 0�00263 × 4 = 0�0655 b = 0�0327 Ox balance: 2a + b + 2d = 0�065/16 + 1�15 × 2 × 0�0838 = 0�1386 +
0�0327 + 2d d = 0�0255 N2 balance: e = 1�15 × 0�0838 × 3�76 = 0�3626 The dry exhaust composition becomes: CO2% = a/(a + d + e) = 15,15% O2% = 5�54% N2% = 79�27%
As indicated in Chapter 11, much research and development efforts went into the effective utilization of fluidized beds for the generation of thermal energy while using pulverized or granular solid fuels such as coal� There are still benefits to be gained by investing further development efforts to make these furnaces more attractive and widely used�
The employment of fluidized bed combustion to burn pulverized coal has some notable advantages� They include the following:
• Most types of coal can be burned; sulfur removal may be performed by including calcium or magnesium oxides in the bed�
• Oxides of nitrogen emission problems are not serious since low combustion temperatures are involved� Ash in the coal remains in the bed and is then removed�
• There is good heat transfer in the beds for indirect firing, and the beds can be pressurized�
There are, at the same time, limitations to the wider employment of fluidized beds� These may include the following:
• The exhaust gas needs treatment to remove fly ash due to frittering� • Poor furnace turndown ratio and much time needed to start up and
shut down� • Must abstract heat effectively from the hot exhaust gases� • Attrition of the granular bed material through escaping into the
exhaust products contributing to emissions, operational problems, and increased costs�
Generally, coal gasification at high temperatures and pressures refers to the reaction of coal with air, oxygen, steam, carbon dioxide, or a mixture of these gases to yield a combustible gaseous product for use as a fuel� The principal gases produced in coal gasification with oxygen, steam, and/or hydrogen are mainly CO, H2, CO2, steam, and some methane, for example, through the water-gas shift reaction, that is,
CO H O CO H2 2 2+ ↔ + This reaction shifts the composition of the gas to favor hydrogen and car-
bon dioxide production since carbon monoxide reacts with steam to produce carbon dioxide and hydrogen� Catalysts are usually used, such as zinc oxide or copper oxide�
The gasification of coal involves sequential processes (Figure 14�5)� First, heating and drying of coal occurs to remove moisture, and further heating results in pyrolysis in which the coal’s weakest bonds are disrupted with the evolution of volatile combustibles and polymerization of carbon into char� The combustion of volatiles and char then occurs, followed by an endothermic reduction to carbon monoxide and yield due to a combination of the effects of the endothermic reaction of coal with steam and mildly exothermic reaction of carbon monoxide with steam� Further, the reaction of hydrogen with carbon produces some methane exothermically� The term “synthesis gas” is given to the hydrogen-carbon monoxide mixture produced�
A number of different gasifiers are used for coal gasification� Figure 14�6 shows a schematic representation of a coal gasifier and the processes and overall reactions involved in gasification� Generally, the choice of a gasifier depends on the following requirements and constraints:
• Production rate of energy • Turndown ratio requirements • Heating value of the product gas
• Pressure and temperature • Allowed gas purity (e�g�, contents of sulfur and carbon dioxide tolerated) • Allowed gas cleanliness (tars, soot, ash, etc�) • Coal availability, type, and cost • Gasifier end-use location and size constraints
It was considered highly desirable over the years to be able to gasify a coal seam underground and produce combustible gas mixtures in situ� This way there can be a very significant benefit from the environmental and safety points while exploiting coal deposits mostly unmineable by conventional methods� The gas is produced with the underground reaction of injected oxygen and steam with coal� Typically, two boreholes are drilled into a coal seam; steam and oxygen are pumped down one borehole, whereas the product gas is extracted from the other borehole� The product gas is made up of complex mixtures of carbon monoxide, hydrogen, and methane, with some carbon dioxide and water vapor, tars, and hydrogen sulfide�
Within the gasifier, a burn cavity is formed once gasification takes place� Its behavior and growth are important as they dictate the quality of the gas produced over the years� Various testing programs have been made over the years, but no large-scale underground production of fuel gas has been commercially successful� This is mainly due to the difficulties in properly controlling the combustion process, with the product gas typically tending to be of low to medium heating value, which is substantially lower than that of natural gas� Also, the quality of coal and the geological structure of the coal seam have a significant impact on the feasibility and commercial success of fuel gas� The environmental consequences, such as increased water pollution and subsidence, and the low energy recovery efficiency of this approach are often sufficiently serious that they have impeded its widespread application�
Peat: Peat is a soft organic material of much recent age in comparison with coal� Peat contains a very high concentration of water and consists of partly decayed vegetable matter of woody plants, reeds, and mosses that accumulated in anaerobic water-saturated conditions and were subjected to bacterial actions� Peat is used in horticulture and when dried through its smoky burning for the production of thermal energy� It may be compressed at high temperatures to form briquettes that are used occasionally as a domestic fuel� Only a limited number of countries, such as Finland and Ireland, use peat as an energy resource� Peat has a low heating value even when dried (20-22 MJ/kg) compared to other conventional fossil fuels� Often, peat fields emit in significant quantities the greenhouse gas methane, which is much more potent than carbon dioxide in its contribution to global warming�
Wood: Wood as a fuel still holds an important place not only in the developing world but also in the developed world� Leftovers from the production of lumber, pulp, and paper are often used to produce biogases, process steam, and heat�
The various types of wood comprise broadly three types of material: the largest fraction by mass of various types of wood is cellulose� Wood combustion similar to the combustion of other solids takes place in consecutive, initially endothermic stages that begin with preheating, followed by drying near the combustion surface, volatilization, pyrolysis, and then exothermic oxidation (Figure 14�2)� The more moisture present, the more energy is required to drive the water out and initiate combustion�
Charcoal and coke: Charcoal and coke are fuels manufactured or processed from naturally occurring solid fuels and raw materials� When carbonaceous materials are burned in the closed environment of a retort with insufficient air, the volatiles are driven off and a residue of coke or charcoal is left� The term coke is reserved for residues from coal and petroleum products, whereas charcoal comes from a wide variety of woody, agricultural, and animal products, for example, wood, coconut shells, and bones� Charcoal is highly porous and has an enormously large surface area of contact per unit mass� Hence, it is widely used as an absorber of various gases and vapors in air-purifying installations, gas masks, and so on�
Biomass: Biomass is an important source of sustainable energy� It includes industrial, agricultural, livestock, and forestry residues� Some are grown specifically for conversion into energy resources� Most biofuels have the potential to produce energy that is, on occasion, greater than the energy required for their production� Biomass can be burned directly or converted into solids (e�g�, charcoal), liquids (e�g�, methyl alcohol, also known as wood alcohol), or gaseous fuels (e�g�, biogases) through the processes of gasification, liquefaction, fermentation, or bacterial digestion� Bacterial degradation of biomaterials produces mainly methane in association with carbon dioxide� It may be loosely considered that in principle there is no net release of carbon dioxide into the atmosphere from the combustion of biofuels� The carbon dioxide released by combustion can be viewed to be similar or less in quantity to that taken in by plants through photosynthesis� However, there are some who consider the generation of biofuels may release in total more carbon dioxide than burning fossil fuels when the total emissions released during their production are accounted for� These may include emissions associated with the preparation of land, fertilization, harvesting of crops, transport, and processing�
Municipal refuse: Using unprocessed municipal refuse as a fuel is becoming relatively widespread� It tends to be unattractive mainly because of its heterogeneous nature, which varies very widely depending on its origin� The refuse usually contains low-heating-value material along with other materials containing high ash and moisture, which makes it difficult to have efficient, trouble-free combustion with only low levels of undesirable emissions� Organic waste material is broken into simpler, less-complex molecules
through anaerobic digestion combined with the action of bacteria� These biochemical processes, in the absence of oxygen, produce mainly methane, moisture, and carbon dioxide, with very small amounts of other gases including hydrogen sulfide and ammonia� These product gases, which are described as biogases, are combustible and can be harnessed as potential energy sources� The anaerobic processes involving large quantities of refuse are carried out in large vessels, described as “digesters,” in the absence of air� Domestic waste is often processed first before being incinerated� Table 14�2 lists the typical composition and some properties of some examples of common solid fuels�
Supplementing with natural gas: Natural gas is burned simultaneously often with less-attractive solid fuels such as municipal refuse or brown coal� Such an approach requires minimal installation time and small-to-moderate capital expenditure� In addition, it provides flexibility in meeting environmental efficiency and supply concerns� The gas is usually injected at a point above where the bulk fuel is burned� This increases the amount of energy released, whereas
the amounts of NOx and SOx emitted are reduced� This is often described as a “dual-fuel reburns” procedure, whereas the term “cofiring” usually refers to when the supplementary gas is injected into the primary burning region�
The methane emitted from coal beds, sometimes known as “firedamp,” is a significant energy resource� It is also considered a serious source of greenhouse emissions and fire and explosion hazards associated with underground coal mining� The rates of CBM production are the outcome of several major factors that vary from one formation to another and from well to well� Most of the methane is stored within the molecular structure of the coal and held by the overlying rock or water in the structure of the coal seam� Generally, the higher the pressure of a formation, the higher the gas content of the coal� The gas is released through desorption by the lowering of pressure� Fractures in the coal provide pathways for the methane to travel to the well bore and be ultimately guided to the surface� In general, the older the coal, the higher its gas content� CBM is generally classified as “sweet gas” since it often contains hardly any hydrogen sulfide; otherwise, it would be described as “sour gas�”
The exploitation of CBM has improved considerably in recent years because of advances made in drilling, including a much better control on directional and horizontal drilling� “Fracture stimulation” of the coal bed takes place through forcing a fluid under high pressure into a fracture in the coal seam to cause it to widen and create new passages for the migration and release of gas; it improves gas productivity�
Fast-burning solid propellants are relatively easy to handle and store� Their high density makes the rockets that use them more compact and often lighter than rockets using liquid propellants� However, solid propellants tend to produce a lower specific impulse� A general drawback to solid propellants is that they cannot normally be throttled back or shut down once ignited� The amount of thrust generated in a solid propellant is directly related to the surface area of the fuel that is exposed to combustion�
There are two categories of solid propellants: Solid propellants may be of the homogeneous type, which are produced as a single phase made up of a single compound that has both oxidation and reduction properties, such as nitrocellulose, or they may comprise two compounds, such as nitroglycerin or nitrocellulose�
Propellants can also be subdivided as monopropellants, bipropellants, and tripropellants� Solid fuels are mixed in a single solid grain and described as monopropellants� Many liquid propellants are composed of a separate fuel and oxidizer and are called bipropellants because they do not mix until introduced into the combustion chamber�
The early development of the diesel engine saw a wide variety of fuels tried that included the potential use of powdered coal� However, such attempts were soon abandoned� What do you consider some of the negative features associated with the employment of coal as a fuel in compression ignition type engines?
