coal utilization

Introduction

       combustion of coal or its conversion into useful solid, gaseous, and liquid products. By far the most important use of coal is in combustion, mainly to provide heat to the boilers of electric power plants. Metallurgical coke is the major product of coal conversion. In addition, techniques for gasifying and liquefying coal into fuels or into feedstocks for the chemical industry are well developed, but their commercial viability depends on the availability and price of the competing fossil fuels, petroleum and natural gas.

Properties affecting coal utilization

Coal rank

       Ranks of Coal as Classified by the American Society for Testing and Materials, TableThe formation of coal from a variety of plant materials via biochemical and geochemical processes is called coalification. The nature of the constituents in coal is related to the degree of coalification, the measurement of which is termed rank. Rank is usually assessed by a series of tests, collectively called the proximate analysis, that determine the moisture content, volatile matter content, ash content, fixed-carbon content, and calorific value of a coal. Coal ranks established by the American Society for Testing and Materials (ASTM) using this method of classification are shown in the Table (Ranks of Coal as Classified by the American Society for Testing and Materials, Table).

Moisture content

      Moisture content is determined by heating an air-dried coal sample at 105°–110° C (221°–230° F) under specified conditions until a constant weight is obtained. In general, the moisture content increases with decreasing rank and ranges from 1 to 40 percent for the various ranks of coal. The presence of moisture is an important factor in both the storage and the utilization of coals, as it adds unnecessary weight during transportation, reduces the calorific value, and poses some handling problems.

Volatile matter content

      Volatile matter is material that is driven off when coal is heated to 950° C (1,742° F) in the absence of air under specified conditions. It is measured practically by determining the loss of weight. Consisting of a mixture of gases, low-boiling-point organic compounds that condense into oils upon cooling, and tars, volatile matter increases with decreasing rank. In general, coals with high volatile-matter content ignite easily and are highly reactive in combustion applications.

Mineral (ash) content

      Coal contains a variety of minerals in varying proportions that, when the coal is burned, are transformed into ash. The amount and nature of the ash and its behaviour at high temperatures affect the design and type of ash-handling system employed in coal-utilization plants. At high temperatures, coal ash becomes sticky (i.e., sinters) and eventually forms molten slag. The slag then becomes a hard, crystalline material upon cooling and resolidification. Specific ash-fusion temperatures are determined in the laboratory by observing the temperatures at which successive characteristic stages of fusion occur in a specimen of ash when heated in a furnace under specified conditions. These temperatures are often used as indicators of the clinkering potential of coals during high-temperature processing.

Fixed-carbon content

      Fixed carbon is the solid combustible residue that remains after a coal particle is heated and the volatile matter is expelled. The fixed-carbon content of a coal is determined by subtracting the percentages of moisture, volatile matter, and ash from a sample. Since gas-solid combustion reactions are slower than gas-gas reactions, a high fixed-carbon content indicates that the coal will require a long combustion time.

Calorific value

      Calorific value, measured in British thermal units or megajoules per kilogram, is the amount of chemical energy stored in a coal that is released as thermal energy upon combustion. It is directly related to rank; in fact, the ASTM method uses calorific value to classify coals at or below the rank of high-volatile bituminous (above that rank, coals are classified by fixed-carbon content). The calorific value determines in part the value of a coal as a fuel for combustion applications.

Coal type

      Coal is a complex material composed of microscopically distinguishable, physically distinctive, and chemically different organic substances called macerals. Based on their optical reflectance, mode of occurrence, and physical appearance under the microscope, macerals are grouped into three major classes: (1) Liptinite or exinite macerals, with low reflectance and high hydrogen-to-carbon ratios, are derived from plant spores, cuticles, resins, and algal bodies. (2) Vitrinite macerals, with intermediate reflectance and high oxygen-to-carbon ratios, are derived from woody tissues. (3) Inertinite macerals, with high reflectance and carbon contents, are derived from fossil charcoal or decayed material.

      Although the various macerals in a given group are expected to have similar properties, they often exhibit different behaviour in a particular end use. For example, combustion efficiency is reported to be inversely related to inertinite content, yet micrinite, which is classified as an inertinite maceral, is found to be highly reactive in combustion applications. Correlations between petrographic composition and coal reactivity have not yet been well established.

Physical properties

Grindability

      The grindability of a coal is a measure of its resistance to crushing. Two factors affecting grindability are the moisture and ash contents of a coal. In general, lignites (lignite) and anthracites (anthracite) are more resistant to grinding than are bituminous coals. One commonly used method for assessing grindability is the Hardgrove test, which consists of grinding a specially prepared coal sample in a laboratory mill of standard design. The percent by weight of the coal that passes through a 200-mesh sieve (a screen with openings of 74 micrometres, or 0.003 inch) is used to calculate the Hardgrove grindability index (HGI). The index is used as a guideline for sizing the grinding equipment in a coal-preparation plant.

Porosity

      Porosity is the fraction of the volume of an apparent solid that is actually empty space. Owing to porosity, the surface area inside a coal particle is far higher than the external surface area. In any gas-solid or liquid-solid reaction, the rate of reaction depends on the available surface area on which the reaction can occur; therefore, the porosity of a coal affects its rate of reaction in a conversion process. The accessibility of a reactant to the internal surface of a coal particle also depends on the size and shape of the pores and the extent of porosity.

density

      Several types of density measurement are made on coal, depending on the intended end use. The most commonly measured density is the bulk density; this is defined as the weight of coal occupying a unit volume and is expressed in grams per cubic centimetre or pounds per cubic foot. Bulk density depends on the particle size distribution of a coal and is important in the design of storage bins and silos.

Thermoplastic properties

      When many bituminous coals are heated, they soften and form a plastic mass that swells and resolidifies into a porous solid. Coals that exhibit such behaviour are called caking coals. Strongly caking coals, which yield a solid product (coke) with properties suitable for use in a blast furnace, are called coking coals. All coking coals are caking, but not all caking coals are suitable for coke making.

      Thermoplastic properties are dependent on petrographic composition. For example, liptinite macerals exhibit very high fluidities, while inertinite macerals do not. Vitrinites are intermediate between these two groups. Thermoplastic properties are desirable for coke making and liquefaction, but they are undesirable for combustion and gasification because a combustor or gasifier can be choked by the resulting fused mass.

Sarma V.L.N. Pisupati Alan W. Scaroni

Carbonization (coke making)

      Coke is the solid carbonaceous residue that remains after certain types of coal are heated to a high temperature out of contact with air. The process of heating coal in this manner is referred to as carbonization or coke making. High-temperature carbonization, with which this section is concerned, is practiced to produce a coke having the requisite properties for metallurgical use, as in blast furnaces or foundry cupolas. Low-temperature carbonization was once practiced widely to produce a coke suitable for residential heating, but residential furnaces are now fired almost exclusively by oil and natural gas.

High-temperature carbonization reactions

      In high-temperature carbonization, coal is heated to temperatures of 900° to 1,200° C (1,600° to 2,200° F). At these temperatures, practically all the volatile matter is driven off as gases or liquids, leaving behind a residue that consists principally of carbon with minor amounts of hydrogen, nitrogen, sulfur, and oxygen (which together constitute the fixed-carbon content of the coal). Carbonization reactions can be illustrated in the following simplified manner:

      The exact extent and nature of the products depend on the temperature and heating rate of the coal particles during the carbonization process.

Carbonization processes

      For a high-temperature carbonizing process to be commercially satisfactory, it is necessary to (1) pass large quantities of heat into a mass of coal at temperatures up to 900° C or more, (2) extract readily the completed coke from the vessel in which it is carbonized, (3) avoid atmospheric pollution and arduous working conditions for the operators, and (4) carry out the whole operation on a large scale and at a low cost.

Coke ovens

      Modern coke ovens can be as large as 6.5 metres (21 feet) high, 15.5 metres (50 feet) long, and 0.46 metres (1.5 feet) wide, each oven holding up to about 36 tons of coal. The coking time (i.e., between charging and discharging) is about 15 hours. Such ovens are arranged in batteries, containing up to 100 ovens each. A coking plant may consist of several such batteries. Large coking plants in the United States carbonize approximately 5,500,000 to 11,200,000 tons of coal annually, but older coking plants are still operating throughout the world with quite small ovens and annual throughputs of only 112,000 to 336,000 tons. Modern coke ovens are highly mechanized, thus minimizing atmospheric pollution and lessening the labour needed. Massive machines load coal into each oven, push coke sideways away from the oven, and transfer red-hot coke to a quenching station, where it is cooled with water. In some plants the red-hot coke is cooled in circulating inert gases, the heat abstracted being used to generate steam. This is called dry quenching.

Coking coals

      Although chemical composition alone cannot be used to predict whether a coal is suitable for coking, prime coking coals generally have volatile matter contents of 20 to 32 percent—i.e., the low- and medium-volatile bituminous ranks. When heated in the absence of air, these coals first become plastic, then undergo decomposition, and finally form coke when the decomposed material resolidifies into a hard and porous solid. In addition to the coal rank, various maceral groups exhibit strong effects on coking behaviour and the resultant coke properties. In general, the vitrinite and liptinite maceral groups are considered reactive and inertinite macerals unreactive.

      The preparation of the coal charge for coke ovens becomes increasingly important as prime coals become less available. Formerly, single coals were used on their own to yield good strong coke, but today there is rarely enough of such coal to supply large plants. Consequently, less good coking coals have to be used. However, by judicious selection and crushing followed by intimate blending, equally good cokes can be made from a variety of coals. Broadly speaking, suitable blends can be obtained by mixing high-volatile with low-volatile coals, and often small additions of ground, small coke and anthracite are helpful. Drying the coal and even preheating it to 200° C (390° F) may also be helpful and economic. Thus, in modern plants the facilities for preparing the blend may be quite elaborate.

Types and sizes of coke

      During the last hours in the ovens, the coke shrinks and fissures. When it is discharged, it is partly in discrete pieces up to 200 millimetres (8 inches) long or more. After the coke leaves the quencher, it is screened into various sizes and the larger pieces may have to be cut. The bulk of oven coke (sized about 40 to 100 millimetres) is used throughout the world in blast furnaces to make iron. Exceptionally large strong coke, known as foundry coke, is used in foundry cupolas to melt iron. Coke in 10- to 25-millimetre sizes is much used in the manufacture of phosphorus and calcium carbide; from the latter, acetylene, mainly for chemical purposes, is made. Large quantities of the smallest sizes (less than 12 millimetres), known as coke breeze, are suited for sintering small iron ore prior to use in blast furnaces. Any surplus breeze serves as an industrial boiler fuel.

By-products

      Another important and expensive part of the coking plant is the by-product plant. Hot tarry gases leaving the ovens are collected, drawn away, and cooled. Crude tar separates and is removed for refining. The crude coke oven gas is scrubbed free of ammonia, and then usually crude benzol is removed from it. Some of the remaining gas (mainly methane, hydrogen, and carbon monoxide) is used to heat the coke ovens, while the rest is available for use in blast furnaces and in soaking pits for heating steel ingots.

Ronald James Morley Sarma V.L.N. Pisupati Alan W. Scaroni

Coal combustion

      The most common and important use of coal is in combustion, in which heat is generated to produce steam, which in turn powers the turbines that produce electricity. Combustion for electricity generation by utilities is the end use for 86 percent of the coal mined in the United States.

Combustion reactions

       Principal oxidation reactions in the combustion of coalThe main chemical reactions that contribute to heat release are oxidation reactions, which convert the constituent elements of coal into their respective oxides, as shown in the Table (Principal oxidation reactions in the combustion of coal). In the table, the negative signs indicate reactions that release heat (exothermic reactions), whereas the positive sign indicates a reaction that absorbs heat (endothermic reaction).

      The combustion of a coal particle occurs primarily in two stages: (1) evolution of volatile matter during the initial stages of heating, with accompanying physical and chemical changes, and (2) subsequent combustion of the residual char. Following ignition and combustion of the evolving volatile matter, oxygen diffuses to the surface of the particle and ignites the char. In some instances, ignition of volatile matter and char occurs simultaneously. The steps involved in char oxidation are as follows:

      IL 1. Diffusion of oxygen from the bulk gas to the char surface

      IL 2. Reaction between oxygen and the surface of the char particle

      IL 3. Diffusion of reaction products from the surface of the char particle into the bulk gas

      At low combustion temperatures, the rate of the chemical reaction (step 2) determines (or limits) the overall reaction rate. However, since the rate of a chemical reaction increases exponentially with temperature, the carbon-oxygen reaction (step 2) can become so fast at high temperatures that the diffusion of oxygen to the surface (step 1) can no longer keep up. In this case, the overall reaction rate is controlled or limited by the diffusion rate of oxygen to the reacting char surface. The controlling mechanism of the combustion reaction therefore depends on such parameters as particle size, reaction temperature, and inherent reactivity of the coal particle.

Combustion systems

Fixed bed

      In fixed-bed systems, lumps of coal, usually size-graded between 3 and 50 millimetres, are heaped onto a grate, and preheated primary air (called underfire air) is blown from under the bed to burn the fixed carbon. Some secondary air (overfire air) is introduced over the coal bed to burn the volatiles released from the bed. Based on the method of feeding the coal, these systems can be further classified into underfeed, overfeed, spreader, and traveling-grate stoker methods.

      The coking characteristics of a coal can influence its combustion behaviour by forming clinkers of coke and ash and thus resisting proper air distribution through the bed. Fines in the coal feed can also cause uneven distribution of air, but this problem can be reduced by adding some water to the feed coal. This procedure, known as tempering, reduces resistance to airflow by agglomerating the fines.

      The relatively large coal feed size used in fixed-bed systems limits the rate of heating of the particles to about 1° C per second, thereby establishing the time required for combustion of the particles at about 45 to 60 minutes. In addition, the sizes of the grates in these systems impose an upper limit on a fixed-bed combustor of about 100,000 megajoules (10⁸ British thermal units) per hour. Therefore, this type of system is limited to industrial and small-scale power plants.

Fluidized bed

      In fluidized-bed combustion, a bed of crushed solid particles (usually six millimetres or less) is made to behave like a fluid by an airstream passing from the bottom of the bed at sufficient velocity to suspend the material in it. The bed material—usually a mixture of coal and sand, ash, or limestone—possesses many of the properties of, and behaves like, a fluid. Crushed coal is introduced into the bubbling bed, which is usually preheated to about 850°–950° C (1,562°–1,742° F). Coal particles are heated at approximately 1,000° C (1,800° F) per second and are devolatilized, and the residual char is burned in about 20 minutes. Coal concentration in the bed is maintained between 1 and 5 percent by weight. Since the bed is continuously bubbling and mixing like a boiling liquid, transfer of heat to and from the bed is very efficient and, hence, uniform temperatures can be achieved throughout the bed. Because of this efficient heat transfer, less surface area is required to remove heat from the bed (and raise steam); therefore, there are lower total capital costs associated with a given heat load. Also, lower combustion temperatures reduce the fouling and corrosion of heat-transfer surfaces. (Fouling is the phenomenon of coal ash sticking to surfaces.) Ash from a fluidized-bed combustion system is amorphous—that is, it has not undergone melting and resolidification.

      Fluidized-bed combustion systems are particularly suitable for coals of low quality and high sulfur content because of their capacity to retain sulfur dioxide (SO₂; a pollutant gas) within the bed and their ability to burn coals of high or variable ash content. When limestone (calcium carbonate; CaCO₃) or dolomite (a mixture of calcium and magnesium carbonates; CaMg(CO₃)₂) is introduced into the bed along with the coal, the limestone decomposes to calcium oxide (CaO), which then reacts in the bed with most of the SO₂ released from the burning coal to produce calcium sulfate (CaSO₄). The CaSO₄ can be removed as a solid by-product for use in a variety of applications. In addition, partially spent calcium or magnesium can be regenerated and recycled by a variety of techniques. The formation and emission of nitrogen oxides (NO_x; another pollutant gas) are inhibited by low operating temperatures. Fluidized-bed combustors, in general, need additional equipment (such as cyclone separators) to separate fines containing a high amount of combustibles and recycle them back into the system.

Pulverized coal

      Pulverized-coal combustion is widely used in large power stations because it offers flexible control. In this method, coal is finely ground so that 70 to 80 percent by weight passes through a 200-mesh screen. The powder is burned in a combustion chamber by entraining the particles in combustion air. Because finely ground coal has more surface area per unit weight than larger particles, the combustion reactions occur at a faster rate, thus reducing the time required for complete combustion to about 1 to 2 seconds. The high heating rates associated with fine particles (10⁵°–10⁶° C per second), coupled with the high combustion temperatures (about 1,700° C, or 3,092° F) and short burning times, lead to high throughputs.

      By carefully designing the combustion chamber, a wide variety of coals—ranging from lignites to anthracites and including high-ash coals—can be burned at high combustion efficiencies. Depending on the characteristics of the mineral content, combustion furnaces are designed to remove ash as either a dry powder or a liquid slag. Furnaces used for pulverized coal are classified according to the firing method as vertical, horizontal, or tangential.

      The disadvantages of this mode of combustion are the relatively high costs associated with drying and grinding coal, the fouling and slagging of heat-transfer surfaces, and the need for expensive fine-particle-collection equipment.

Cyclone

      In a cyclone furnace, small coal particles (less than six millimetres) are burned while entrained in air. The stream of coal particles in the primary combustion air enters tangentially into a cylindrical chamber, where it meets a high-speed tangential stream of secondary air. Owing to the intense mixing of fuel and air, the temperatures developed inside the furnace are high (up to 2,150° C, or 3,900° F). At such high temperatures, the rate of the overall reaction is governed by the rate of transfer of oxygen to the particle surface, and the availability of oxygen is increased by the high turbulence induced in the combustion chamber. Combustion intensities and efficiencies are therefore high in cyclone combustors. As a result of the high temperatures, ash melts and flows along the inclined wall of the furnace and is removed as a liquid slag.

Coal-water slurry fuel

      Pulverized coal can be mixed with water and made into a slurry, which can be handled like a liquid fuel and burned in a boiler designed to burn oil. Coal-water slurry fuel (CWSF) normally consists of 50–70 percent pulverized or micronized coal, 29–49 percent water, and less than 1 percent chemicals to disperse the coal particles in the water and prevent settling of the coal. The slurry is finely sprayed (atomized) into a combustion chamber in a manner similar to that used for fuel oil. However, the challenge in combustion of CWSF is to achieve quick evaporation of the water from the droplets in order to facilitate ignition and combustion of the coal particles within the available residence time. This can be achieved by ensuring very fine atomization of the CWSF, using preheated combustion air, and providing good recirculation of hot combustion-product gases in the flame zone. Heat loss owing to evaporation of water imposes some penalty on the thermal efficiency of the boiler, but this may represent less of a cost than the dewatering of wet coal or the capital costs involved in converting an oil-fired combustor into a dry-coal-fired unit. The commercial viability of CWSF depends on the price and availability of naturally occurring liquid fuels.

Advanced combustion technologies

      The burning of coal can produce combustion gases as hot as 2,500° C (4,500° F), but the lack of materials that can withstand such heat forces even modern power plants to limit steam temperatures to about 540° C (1,000° F)—even though the thermal efficiency of a power plant increases with increasing operating fluid (steam) temperature. An advanced combustion system called magnetohydrodynamics (MHD) uses coal to generate a high-temperature combustion gas at about 2,480° C (4,500° F). At this temperature, gas molecules are ionized (electrically charged). A part of the energy in the product stream is converted directly into electrical energy by passing the charged gases through a magnetic field, and the partially cooled gases are then passed through a conventional steam generator. This process enhances the overall thermal efficiency of energy conversion to about 50 percent—as opposed to conventional processes, which have an efficiency of about 36 to 38 percent.

      Another advanced method of utilizing coal, known as the Integrated Gasification Combined Cycle, involves gasifying the coal (described below) and burning the gas to produce hot products of combustion at 1,600° C (2,900° F). These gaseous products in turn run a gas turbine, and the exhaust gases from the gas turbine can then be used to generate steam to run a conventional steam turbine. Such a combined-cycle operation involving both gas and steam turbines can improve the overall efficiency of energy conversion to about 42 percent.

Gasification

      While the goal of combustion is to produce the maximum amount of heat possible by oxidizing all the combustible material, the goal of gasification is to convert most of the combustible solids into combustible gases such as carbon monoxide, hydrogen, and methane.

Gasification reactions

       Coal gasification reactionsDuring gasification, coal initially undergoes devolatilization, and the residual char undergoes some or all of the reactions listed in the Table (Coal gasification reactions). The table also shows qualitatively the thermodynamic, kinetic, and equilibrium considerations of the reactions. As indicated by the heats of reaction, the combustion reactions are exothermic (and fast), whereas some of the gasification reactions are endothermic (and slower). Usually, the heat required to induce the endothermic gasification reactions is provided by combustion or partial combustion of some of the coal. Gasification reactions are particularly sensitive to the temperature and pressure in the system. As is shown in the table, high temperature and low pressure are suitable for the formation of most of the gasification products, except methane; (methane) methane formation if favoured by low temperatures and high pressures.

      For thermodynamic and kinetic considerations, char is taken to be graphite, or pure carbon. In reality, however, coal char is a mixture of pure carbon and impurities with structural defects. Because impurities and defects can be catalytic in nature, the absolute reaction rate depends on their amount and nature—and also on such physical characteristics as surface area and pore structure, which control the accessibility of reactants to the surface. These characteristics in turn depend on the nature of the parent coal and on the devolatilization conditions.

Gasification systems

      The operating temperature of a gasifier usually dictates the nature of the ash-removal system. Operating temperatures below 1,000° C (1,800° F) allow dry ash removal, whereas temperatures between 1,000° and 1,200° C (1,800° and 2,200° F) cause the ash to melt partially and form agglomerates. Temperatures above 1,200° C result in melting of the ash, which is removed mostly in the form of liquid slag. Gasifiers may operate at either atmospheric or elevated pressure; both temperature and pressure affect the composition of the final product gases.

      Gasification processes use one or a combination of three reactant gases: oxygen (O₂), steam (H₂O), and hydrogen (H₂). The heat required for the endothermic gasification reactions is suppled by the exothermic combustion reactions between the coal and oxygen. Air can be used to produce a gaseous mixture of nitrogen (N₂), carbon monoxide (CO), and carbon dioxide (CO₂), with low calorific value (about 6 to 12 megajoules per cubic metre, or 150–300 British thermal units per cubic foot). oxygen can be used to produce a mixture of carbon monoxide, hydrogen, and some noncombustible gases, with medium calorific value (12 to 16 megajoules per cubic metre, or 300 to 400 British thermal units per cubic foot). Hydrogasification processes use hydrogen to produce a gas (mainly methane, CH₄) of high calorific value (37 to 41 megajoules per cubic metre, or 980 to 1,080 British thermal units per cubic foot).

      Methods of contacting the solid feed and the gaseous reactants in a gasifier are of four main types: fixed bed, fluidized bed, entrained flow, and molten bath. The operating principles of the first three systems are similar to those discussed above for combustion systems. The molten-bath approach is similar to the fluidized-bed concept in that reactions take place in a molten medium (either slag or salt) that disperses the coal and acts as a heat sink for distributing the heat of combustion.

The Lurgi system

      The most important fixed-bed gasifier available commercially is the Lurgi gasifier, developed by the Lurgi Company in Germany in the 1930s. It is a dry-bottom, fixed-bed system usually operated at pressures between 30 and 35 atmospheres. Since it is a pressurized system, coarse-sized coal (25 to 45 millimetres) is fed into the gasifier through a lock hopper from the top. The gasifying medium (a steam-oxygen mixture) is introduced through a grate located in the bottom of the gasifier. The coal charge and the gasifying medium move in opposite directions, or countercurrently. At the operating temperature of about 980° C (1,800° F), the oxygen reacts with coal to form carbon dioxide, thereby producing heat to sustain the endothermic steam-carbon and carbon dioxide-carbon reactions. The raw product gas, consisting mainly of carbon monoxide, hydrogen, and methane, leaves the gasifier for further clean-up.

      Besides participating in the gasification reactions, steam prevents high temperatures at the bottom of the gasifier so as not to sinter or melt the ash. Thus, the Lurgi system is most suitable for highly reactive coals. Large commercial gasifiers are capable of gasifying about 50 tons of coal per hour.

The Winkler system

      The Winkler gasifier is a fluidized-bed gasification system that operates at atmospheric pressure. In this gasifier, coal (usually crushed to less than 12 millimetres) is fed by a screw feeder and is fluidized by the gasifying medium (steam-air or steam-oxygen, depending on the declared calorific value of the product gas) entering through a grate at the bottom. The coal charge and the gasification medium move cocurrently (in the same direction). In addition to the main gasification reactions taking place in the bed, some may also take place in the freeboard above the bed. The temperature of the bed is usually maintained at 980° C (1,800° F), and the product gas consists primarily of carbon monoxide and hydrogen.

      The low operating temperature and pressure of the Winkler system limits the throughput of the gasifier. Because of the low operating temperatures, lignites (lignite) and subbituminous coals, which have high ash-fusion temperatures, are ideal feedstocks. Units capable of gasifying 40 to 45 tons per hour are commercially available.

The Koppers-Totzek system

      The Koppers-Totzek gasifier has been the most successful entrained-flow gasifier. This process uses pulverized coal (usually less than 74 micrometres) blown into the gasifier by a mixture of steam and oxygen. The gasifier is operated at atmospheric pressure and at high temperatures of about 1,600°–1,900° C (2,900°–3,450° F). The coal dust and gasification medium flow cocurrently in the gasifier, and, because of the small coal-particle size, the residence time of the particle in the gasifier is approximately one second. Although this residence time is relatively short, high temperatures enhance the reaction rates, and therefore almost any coal can be gasified in the Koppers-Totzek system. Tars and oils are evolved at moderate temperatures but crack at higher temperatures, so that there is no condensible tarry material in the products. The ash melts and flows as slag. The product gas, known as synthesis gas (a mixture of carbon monoxide and hydrogen), is primarily used for ammonia manufacture.

Advanced gasification systems

      Many attempts have been made to improve the first-generation commercial gasifiers described above. The improvements are primarily aimed at increasing operating pressures in order to increase the throughput or at increasing operating temperatures in order to accommodate a variety of coal feeds. For example, British Gas Corporation has converted the Lurgi gasifier from a dry-bottom to a slagging type by increasing the operating temperature. This allows the system to accommodate higher-rank coals that require higher temperatures for complete gasification. Another version of the Lurgi gasifier is the Ruhr-100 process, with operating pressures about three times those of the basic Lurgi process. Developmental work on the Winkler process has led to the pressurized Winkler process, which is aimed at increasing the yield of methane in order to produce synthetic natural gas (SNG).

      The Texaco gasifier appears to be the most promising new entrained-bed gasification system that has been developed. In this system, coal is fed into the gasifier in the form of coal-water slurry; the water in the slurry serves as both a transport medium (in liquid form) and a gasification medium (as steam). This system operates at 1,500° C (2,700° F), so that the ash is removed as molten slag.

Gas-cleanup systems

      The product gas leaving a gasifier sometimes needs to be cleaned of particulate matter, liquid by-products, sulfur compounds, and oxides of carbon. Particulate matter is conventionally removed from the raw gas with cyclones, scrubbers, baghouses, or electrostatic precipitators. Acidic gases such as hydrogen sulfide (H₂S) and carbon dioxide are absorbed by various solvents such as amines and carbonates. Since most gas-cleanup systems operate at only moderate temperatures, the raw gases from a gasifier have to be cooled before processing and then reheated if necessary before end use. This reduces the overall thermal efficiency of the process. For this reason, there is considerable interest in the development of hot gas-cleanup systems capable of cleaning raw gas at high temperatures with high efficiencies.

Liquefaction (thermal fusion)

      Liquefaction is the process of converting solid coal into liquid fuels. The main difference between naturally occurring petroleum fuels and coal is the deficiency of hydrogen in the latter: coal contains only about half the amount found in petroleum. Therefore, conversion of coal into liquid fuels involves the addition of hydrogen.

Liquefaction reactions

      Hydrogenation of coal can be done directly, either from gaseous hydrogen or by a liquid hydrogen-donor solvent, or it can be done indirectly, through an intermediate series of compounds. In direct liquefaction, the macromolecular (macromolecule) structure of the coal is broken down in such a manner that the yield of the correct size of molecules is maximized and the production of the very small molecules that constitute fuel gases is minimized. By contrast, indirect liquefaction methods break down the coal structure all the way to a synthesis gas mixture (carbon monoxide and hydrogen), and these molecules are used to rebuild the desired liquid hydrocarbon molecules.

      Since coal is a complex substance, it is often represented in chemical symbols by an average composition. Given this simplification, direct liquefaction can be illustrated as follows:

      Direct liquefaction of coal can be achieved with and without catalysts (represented by R), using high pressures (200 to 700 atmospheres) and temperatures ranging between 425° and 480° C (800° and 900° F).

      In the indirect liquefaction process, coal is first gasified to produce synthesis gas and then converted to liquid fuels:

      The principal variables that affect the yield and distribution of products in direct liquefaction are the solvent properties (such as stability and hydrogen-transfer capability), coal rank and maceral composition, reaction conditions, and the presence or absence of catalytic effects. Although most coals (except anthracites) can be converted into liquid products, bituminous coals are the most suitable feedstock for direct liquefaction since they produce the highest yields of desirable liquids. Medium-rank coals are the most reactive under liquefaction conditions. Among the various petrographic components, the sum of the vitrinite and liptinite maceral contents correlates well with the total yield of liquid products.

Liquefaction processes

The Bergius process

      The first commercially available liquefaction process was the Bergius process, developed in Germany during World War I. This involves dissolving coal in a recycled solvent oil and reacting with hydrogen under high pressures ranging from 200 to 700 atmospheres. An iron oxide catalyst is also employed. Temperatures in the reactor are in the range of 425°–480° C (800°–900° F). Light and heavy liquid fractions are separated from the ash to produce gasoline and recycle oil, respectively. In general, one ton of coal produces about 150 to 170 litres (40 to 44 gallons) of gasoline, 190 litres of diesel fuel, and 130 litres of fuel oil. The separation of ash and heavy liquids, along with erosion from cyclic pressurization, pose difficulties that have caused this process to be kept out of use since World War II.

The Fischer-Tropsch (Fischer-Tropsch reaction) process

      In the first-generation, indirect liquefaction process called Fischer-Tropsch synthesis, coal is gasified first in a high-pressure Lurgi gasifier, and the resulting synthesis gas is reacted over an iron-based catalyst either in a fixed-bed or fluidized-bed reactor. Depending on reaction conditions, the products obtained consist of a wide range of hydrocarbons. Although this process was developed and used widely in Germany during World War II, owing to poor economics it was discontinued afterward. It has been in operation in South Africa (the SASOL process) as part of a state-supported effort to maintain self-sufficiency in energy.

Advanced processes

      Lower operating temperatures are desirable in direct liquefaction processes, since higher temperatures tend to promote cracking of molecules and produce more gaseous and solid products at the expense of liquids. Similarly, lower pressures are desirable for ease and cost of operation. Research efforts in the areas of direct liquefaction have concentrated on reducing the operating pressure, improving the separation process by using a hydrogen donor solvent, operating without catalysts, and using a solvent without catalysts but using external catalytic rehydrogenation of the solvent. Research has also focused on multistage liquefaction in an effort to minimize hydrogen consumption and maximize overall process yields.

      In the area of indirect liquefaction, later versions of the SASOL process have employed only fluidized-bed reactors in order to increase the yield of gasoline and have reacted excess methane with steam in order to produce more carbon monoxide and hydrogen. Other developments include producing liquid fuels from synthesis gas through an intermediate step of converting the gas into methanol at relatively low operating pressures (5 to 10 atmospheres) and temperatures (205°–300° C, or 400°–575° F). The methanol is then converted into a range of liquid hydrocarbons. The use of zeolite catalysts has enabled the direct production of gasoline from methanol with high efficiency.

Sarma V.L.N. Pisupati Alan W. Scaroni

Additional Reading

Harold H. Schobert, Coal, the Energy Source of the Past and the Future (1987), provides a good nontechnical introduction to coal for a reader with little or no chemical background; it also provides an understanding of the role that coal has played in society from ancient times to the present. N. Berkowitz, An Introduction to Coal Technology, 2nd ed. (1993), is useful for a reader learning about coal for the first time. James G. Speight, The Chemistry and Technology of Coal (1983), an introductory textbook, deals with geology, petrography, chemistry, and utilization technology. D.W. van Krevelen, Coal: Typology, Chemistry, Physics, Constitution (1961, reprinted 1981), although older, is still a good reference book on coal structure and science. D. Merrick, Coal Combustion and Conversion Technology (1984), offers a comprehensive guide to coal-utilization technology for engineers, planners, and policy makers and is especially good on fluidized-bed combustion. Other works discussing coal utilization include Martin A. Elliot (ed.), Chemistry of Coal Utilization: Second Supplementary Volume (1981), a monumental and exhaustive reference book with a good review of fundamentals and utilization processes up to the late 1970s; H.-D. Schilling, B. Bonn, and U. Krauss, Coal Gasification: Existing Processes and New Developments, 2nd rev. ed. (1981; originally published in German, 2nd rev. ed., 1979), a comprehensive collection; G.J. Pitt and G.R. Millward (eds.), Coal and Modern Coal Processing: An Introduction (1979); Perry Nowacki, Coal Liquefaction Processes (1979), a systematic discussion; and C.y. Wen and E. Stanley Lee (eds.), Coal Conversion Technology (1979), a good graduate-level textbook, with descriptions of processes.Alan W. Scaroni Sarma V.L.N. Pisupati

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