8.3.1 Moisture content

Moisture in coal is an important property (ASTM D1412; ASTM D2961; ASTM D3173; ASTM D3302) – more important than often recognized by the non-industrial coal theorists. Moisture that exists in coal (on the order of 0.5-15% w/w) must be transported, handled, and stored before gasification. Given that the moisture replaces organic volatiles, it (1) decreases the heat content of the coal; (2) increases heat loss, due to evaporation and superheating of vapor; and (3) aids radiation heat transfer. Furthermore, the higher the amount of moisture in coal, the greater the potential for the generation of heat leading to spontaneous ignition and spontaneous combustion (Speight, 2013a). The most dangerous scenario for spontaneous combustion is when wet and dry coals are combined in a stockpile – the interface between wet and dry coal becomes a heat exchanger. If coal is either completely wet or completely dry, the risk is substantially reduced. In general, the moisture content of coal increases with decreasing rank.

8.3.2 Volatile matter

Generally, the original raw coal does not contain much natural volatile matter. The volatile matter in coal refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air (i.e., during pyrolysis or during the initial stages of thermal treatment). The volatile matter obtained during the initial heating stage influences commencement of the gasification process coal, which consists mainly of gases such as hydrogen, carbon monoxide, methane, higher molecular weight hydrocarbons, volatile oil, volatile tar, as well as carbon dioxide and steam. Any coal that can generate substantial amounts of volatile matter can ignite easily, which is a significant factor for coal selected as a feedstock in a coal gasification system.

Just like the moisture content, volatile matter (ASTM D3175; ISO 562) depends on coal rank and ranges from < 5% for anthracites to > 50% w/w for sub-bituminous and lignite. There are large variations in gas content within a single coal at a single location. The gases in coal are located in pores and are retained on the surface of the pores by adsorption forces.

As for all standard test methods, the volatile matter of coal is determined under rigidly controlled standards. In Australian and British standard test methods, the procedure involves heating the coal sample to 900 ± 5 °C (1650 ± 10 °F) for 7 min in a cylindrical silica crucible in a muffle furnace. The standard test method of analysis involves heating coal to 950 ± 25 °C (1740 ± 45 °F) in a vertical platinum crucible (ASTM D3175; ISO 1350). The composition of the volatile matter evolved from coal is substantially different for the different ranks of coal.

8.3.3 Ash

Coal does not contain ash but does contain ash-forming mineral constituents (Speight, 2005, 2013a, 2013b). Ash is further classified into (1) fly ash and (2) bottom ash. Fly ash is the fine particle that rises with the flue gases during gasification (and combustion), whereas bottom ash is the ash that does not rise. The quantity of fly ash generated during gasification and combustion processes is also dependent on the rank of the coal.

The presence of inorganic matter (mineral materials) in coal reduces the heating value of the coal. The mineral matter may also contribute to the volatile matter in coal by virtue of the loss of water from the clay minerals, the loss of carbon dioxide from the carbonate minerals, the loss of sulfur from pyrite (FeS₂), and the generation of hydrogen chloride from chloride minerals. The most commonly found minerals in coal are clay minerals, quartz minerals, sulfide minerals, and carbonate minerals.

Clay minerals, such as montmorillonite, may or may not break down (dissociate) into its constituent parts when coal is heated. If it does dissociate, then, after cooling, it may recombine with other elements or minerals to form mineral deposits on the inside surfaces of furnaces and boilers (slagging or fouling). This produces barriers to heat exchange in the affected equipment, which can substantially reduce its efficiency and require costly repairs. Illite, however, with its simpler composition, does not cause such problems under normal furnace operating conditions.

The mineral matter content of coal, and hence the yield of ash during gasification (usually on the order of 5-40% w/w), can lead to slagging, fouling, and corrosion. Slagging is the deposition of fly ash (ash that does not descend to the bottom of the gasifier) on both heat transfer surfaces and refractory surfaces. Fouling includes deposition of ash and volatiles as well as sulfidation reactions of ash. Fouling results in loss of heat transfer efficiency and blockage of the gas flow path. Corrosion results in thinning of metals walls with the potential for leaks and equipment shutdown.

Determination of the mineral matter content (as the yield of mineral ash) is necessary because it directly affects process efficiency (ASTM D3174; ISO 1171). Several formulae have been proposed for calculating the amount of mineral matter originally in the coal by using the data from ashing techniques as the basis of the calculations. Of these formulae, two have survived and have been used regularly to assess the proportion of mineral matter in coal: the Parr formula and the King-Mavies-Crossley formula.

In the Parr formula, the mineral matter content of coal is derived from the expression:

$\mathit{\%}\mathrm{w}/\mathrm{w}\mathrm{mineral}\mathrm{matter}=1.08\mathrm{A}+0.55\mathrm{S}$

where A is the weight percent of ash produced in the test method and S is the total sulfur in the coal.

The King-Mavies-Crossley formula is a more complex formula:

$\mathit{\%}\mathrm{w}/\mathrm{w}\mathrm{mineral}\mathrm{matter}=1.09\mathrm{A}+0.5{\mathrm{S}}_{\mathrm{pyr}}+0.8{\mathrm{CO}}_2–1.1{\mathrm{SO}}_{3\left(\mathrm{in}\mathrm{ash}\right)}+{\mathrm{SO}}_{3\left(\mathrm{in}\mathrm{coal}\right)}+0.5\mathrm{Cl}$

where A = the weight percent yield of ash, S_pyr = the percentage of pyritic sulfur in the coal, CO₂ = the percentage of mineral (non-organic) carbon dioxide in the coal, SO_{3(in ash)} = the percentage of sulfur trioxide in the ash, SO_{3(in coal)} = the percentage of sulfur trioxide in the coal, and Cl = the percentage of chlorine in the coal.

8.3.4 Fixed carbon

The fixed carbon content (more correctly, the fixed carbon yield or carbonaceous residue yield) (FC) of the coal can be related to the anticipated yield of char produced during the devolatilization process (Chapter 5). It is the carbon found in the material that remains after volatile materials are driven off. Thus:

$\mathrm{FC}=100–\left(\mathit{\%}{\mathrm{H}}_2\mathrm{O}+\mathit{\%}\mathrm{VM}+\mathit{\%}\mathrm{Ash}\right)$

The value for the fixed carbon content of coal differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons in the volatile matter.

In the determination of fixed carbon (ASTM 3172; ISO 1350), the cover from the crucible used in the volatile matter last test is removed and the crucible is heated over the Bunsen burner until all the carbon is burned. The residue is weighed, and the difference in weight from the previous weighing is the fixed carbon.

8.6.1 Density

Density is an important aspect of reactor engineering. It indicates the reactor size and throughput for gasification processes. With the free-swelling index (FSI), the density is also used to estimate the volume of the char produced during the devolatilization process.

The term coal density therefore carries several different connotations. A distinction must be made among bulk densities, which are determined by the average particle (or lump) size, size distribution, and packing density of the coal, because these affect handling, transportation, and storage.

The true density (ASTM D167) is usually determined by displacement of a fluid. Because of the porous nature of coal and physicochemical interactions, the observed density data vary with the particular fluids employed (Agrawal, 1959; Mahajan & Walker, 1978).

The apparent density of coal is determined by immersing a weighed sample of coal in a liquid followed by the accurate measurement of the liquid that is displaced. For this procedure, the liquid should (1) wet the surface of the coal, (2) not absorb strongly to the coal surface, (3) not cause swelling, and (4) penetrate the pores of the coal. Incidentally, the lower the rank of the coal, the greater is the “wettability” with water. On the other hand, the higher the rank, the greater the “wettability” with (coal) tar or the non-volatile pitch.

The bulk density (ASTM D29l) is not an intrinsic property of coal and varies depending on how the coal is handled. Bulk density is the mass of many particles of coal divided by the total volume occupied by the particles. The total volume includes particle volume, interparticle void volume, and internal pore volume. This variable composition allows the density of coal to be expressed in terms of the cubic foot weight of crushed coal, which varies with particle size of the coal and packing in a container.

8.6.2 Porosity and surface area

Coal is a porous material, thus the porosity and surface area of coal (Mahajan & Walker, 1978) have a large influence on coal behavior during gasification because the reactivity of coal increases as the porosity and surface area of the coal increases. Porosity dictates the rate at which volatile matter can diffuse out of the coal (in the gasifier) and the rate at which oxygen or other gasification agents can interact with the coal.

As already noted with respect to coal density, the porosity of coal decreases with carbon content, reaching a minimum at coal containing ~ 89% carbon followed by a marked increase in porosity. There are also differences in the pore size that make up the porosity of coal. For example, macropores are usually predominant in the lower carbon (rank) coals, whereas higher carbon (rank) coals contain predominantly micropores. Thus, pore volume can be calculated from the relationship:

${\mathit{V}}_{\mathrm{p}}=1/{\mathit{\rho }}_{\mathrm{Hg}}-1{\mathit{\rho }}_{\mathrm{He}}$

In this equation, ρ_Hg is the mercury density, and ρ_He is the helium density; both decrease with carbon content. In addition, the surface area of coal varies over the range of 10-200 m²/g and also tends to decrease with the carbon content of the coal. The porosity of coal is calculated from the relationship:

$\mathit{\rho }=100{\mathit{\rho }}_{\mathrm{Hg}}\left(1/{\mathit{\rho }}_{\mathrm{Hg}}-1{\mathit{\rho }}_{\mathrm{He}}\right)$

By determining the apparent density of coal in fluids of different, but known, dimensions, it is possible to calculate the pore size (pore volume) distribution. The open pore volume (V), (i.e., the pore volume accessible to a particular fluid) can be calculated from the relationship:

$\mathit{V}=\left(1{\mathit{\rho }}_{\mathrm{Hg}}-1{\mathit{\rho }}_{\mathrm{a}}\right)$

where ρ_a is the apparent density in the fluid.

The size distribution of the pores within the coal can be determined by immersing the coal in mercury and progressively increasing the pressure. Surface tension effects prevent the mercury from entering the pores with a diameter smaller than a given value d for any particular pressure P such that

$\mathit{P}=4\mathit{\sigma }.cos\mathit{\theta }/\mathit{d}$

In this equation, σ is the surface tension and θ is the angle of contact (Van Krevelen, 1957). However, the total pore volume accounted for by this method is substantially less than that derived from the helium density, thereby giving rise to the concept that coal contains two pore systems: (1) a macropore system accessible to mercury under pressure and (2) a micropore system that is inaccessible to mercury but accessible to helium.

8.7.1 Strength

There are different methods for estimating coal strength and hardness: compressive strength, fracture toughness, and grindability, all of which show a trend relative to rank, type, and grade of the coal. The measurement of coal strength is affected by the size of the test specimen, the orientation of stress relative to banding, and the confining pressure of the test (Hobbs, 1964; Medhurst & Brown, 1998; Zipf & Bieniawski, 1988).

Thus, the strength of a bituminous coal specimen is influenced also by its lateral dimension, the smaller specimens showing greater strength than the larger, which can be attributed to the presence in the larger specimen of fracture planes or cleats. In fact, it is the smaller samples that present a more accurate indication of the strength of the coal. The variation of strength with rank of coals has been noted and a plot of strength against volatile matter shows the customary minimum to be 20-25% dry, ash-free volatile matter for compression both perpendicular and parallel to the bedding plane (Speight, 2013a).

The only standard test method that is available is actually a test method for determining coke reactivity and coke strength after reaction (ASTM D5341). This test method describes the equipment and techniques used for determining lump coke reactivity in carbon dioxide (CO₂) gas at elevated temperatures and its strength after reaction in carbon dioxide gas by tumbling in a cylindrical chamber.

8.7.2 Hardness

Although the resistance of coal to abrasion may have little apparent commercial significance, the abrasiveness of coal is, on the other hand, a factor of considerable importance when coal is used in a gasifier. The wear of grinding elements due to the abrasive action of coal results in maintenance charges that constitute one of the major items in the cost of grinding coal for use as pulverized fuel. Moreover, as coals vary widely in abrasiveness, this factor must be considered when coals are selected for plants that employ pulverized coal (Speight, 2005, 2013a).

The abrasiveness of coal may be determined more by the nature of its associated impurities than by the nature of the coal substance. For example, pyrite is 20 times harder than coal, and the individual grains of sandstone, another common impurity in coal are hard and abrasive.

8.7.3 Friability

Friability is of interest primarily because friable coals yield smaller proportions of the coarse sizes, which may (depending on use) be more desirable. There may also be an increased amount of surface in the friable coals. This surface allows more rapid oxidation; hence conditions are more favorable for spontaneous ignition leading to loss in coking quality in coking coals, and other changes that accompany oxidation.

The tumbler test for measuring coal friability (ASTM D441) employs a cylindrical porcelain jar mill (7.25 in., 18.4 cm in size) fitted with three lifters that assist in tumbling the coal. A 1000-g sample of coal sized between 1.5 and 1.05-in. square-hole screens is tumbled in the mill (without grinding medium) for 1 h at 40 rpm. The coal is then removed and screened on square-hole sieves with openings of 1.05, 0.742, 0.525, 0.371, 0.0369, and 0.0117 in.

A drop shatter test is also used for determining the friability of coal (ASTM D440), which is similar to the standard method used as a shatter test for coke (ASTM D3038). In this method, a 50-lb sample of 2- to 3-in. pieces of coal is dropped twice from a drop-bottom box onto a steel plate 6 ft below the box. The materials shattered by the two drops are then screened over round-hole screens with 3.0 in. (76.2 mm), 2.0 in. (50.8 mm), 1.5 in. (38.1 mm), 1.0 in. (25.4 mm). 0.75 in. (19.05 mm) and 0.5 in. (12.7 mm) openings and the average particle size is determined.

8.7.4 Grindability

The grindability of coal (i.e., the ease with which coal may be ground fine enough for use as pulverized fuel) is a composite physical property embracing other specific properties such as hardness, strength, tenacity, and fracture. Several methods of estimating relative grindability utilize a porcelain jar mill in which each coal may be ground for, say, 400 revolutions and the amount of new surface is estimated from screen analyses of the feed and of the ground product. Coals are then rated in grindability by comparing the amount of new surface found in the test with that obtained for a standard coal.

A particularly important mechanical test designed to provide a measure of the ease of pulverization of a coal in comparison with other standard reference coals is the Hardgrove grindability index (HGI). Grindability changes with coal rank; that is, coals of very low and very high rank are more difficult to grind than middle-rank coking coals. The test for grindability (ASTM D409; ISO 5074) utilizes a ball-and-ring-type mill in which a 50-g sample of closely sized coal is ground for 60 revolutions after which the ground product is screened through a 200-mesh sieve.

The results are converted into the equivalent HGI. The HGI numbers indicate easy-to-grind coals. There is an approximate relationship between volatile matter yield and grindability in the low-volatile, medium-volatile, and high-volatile bituminous coals. Among these, the low-volatile coals exhibit the highest values for the HGI, often in excess of 100. The high-volatile bituminous coals range in the HGI from ~ 54 to 56 and as low as 36 to 39. Soft, easily fractured coals generally exhibit relatively high grindability index (GI) values. There are two standard test methods for measuring friability (ASTM D440: the drop shatter test, and ASTM D441: the tumbler test, D441) that should be used where a more accurate estimation of friability is required.

8.8.1 Heat capacity

The heat capacity of coal is the heat required to raise the temperature of one unit weight of a substance 1° and the ratio of the heat capacity of one substance to the heat capacity of water at 15 °C (60 °F) in the specific heat. The heat capacity of coal can be measured by standard calorimetric methods for mixtures (e.g., see ASTM C351).

The units for heat capacity are Btu per pound per degree Fahrenheit (Btu/lb/°F) or calories per gram per degree centigrade (cal/gm/°C), but the specific heat is the ratio of two heat capacities and is therefore dimensionless. The heat capacity of water is 1.0 Btu/lb/°F (= 4.2 × 103 J/kg/°K); thus, the heat capacity of any material will always be numerically equal to the specific heat. Consequently, there is a tendency to use the terms heat capacity and specific heat synonymously.

From the data for various coals, it has been possible to derive a formula that indicates the relationship between the specific heat and the elemental analysis of coal (mmf basis):

${\mathit{C}}_{\mathit{p}}=0.189\mathrm{C}+0.874\mathrm{H}+0.491\mathrm{N}+0.3600+0.215\mathrm{S}$

C, H, N, O, and S are the respective amounts (% w/w) of the elements in the coal.

8.8.2 Thermal conductivity

Thermal conductivity is the rate of transfer of heat by conduction through a unit area across a unit thickness for a unit difference in temperature:

$\mathit{Q}=\mathit{kA}\left({\mathit{t}}_2-{\mathit{t}}_1\right)/\mathit{d}$

where Q = heat, expressed as kcal/sec cm °C or as Btu/ft h °F (1 Btu/ft h °F = 1.7J/s m °K), A = area, t₂ − t₁ = temperature differential for the distance (d), and k = thermal conductivity (Carslaw & Jaeger, 1959). However, the banding and bedding planes in coal (Speight, 2013a) can complicate the matter to such an extent that it is difficult, if not almost impossible, to determine a single value for the thermal conductivity of a particular coal. Nevertheless, it has been possible to draw relevant conclusions from the data.

8.8.3 Plastic and agglutinating properties

Plastic and agglutinating properties, as well as phenomena such as the agglomerating index, give indications of how coal will behave in a gasification reactor. For example, when coal is heated, it passes through a transient stage called a plastic state (caking). If a particular coal does not pass through a plastic state, it is called sintered mass (non-coking). Although the plastic properties of coal are more definitive in terms of the production of metallurgical coke from coal blends, such properties can also influence coal gasification and whether or not the stickiness or fluidity of the coal will influence coal behavior in a gasifier as used on a coal-fired power plant (Speight, 2013a, 2013b).

All coals undergo chemical changes when heated, but there are certain types of coal that also exhibit physical changes when subjected to the influence of heat. These particular types of coals are generally known as caking coals, whereas the remaining coals are referred to as non-caking coals.

Caking coals pass through a series of physical changes during the heating process insofar as they soften, melt, fuse, swell, and resolidify within a specific temperature range. This temperature has been called the “plastic range” of coal and thus the physical changes that occur within this range have been termed the plastic properties (plasticity) of coal. On the other hand, when non-caking coal (non-plastic coal) is heated, the residue is pulverent and non-coherent. Furthermore, caking coals produce residues that are coherent and have varying degrees of friability and swelling. In the plastic range, caking coal particles have a tendency to form agglomerates (cakes) and may even adhere to surfaces of process equipment, thereby giving rise to reactor plugging problems. Thus, the plastic properties of coal are an important means of projecting and predicting how coal will behave under various process conditions as well as assisting in the selection of process equipment.

The Gieseler test is a standard test method (ASTM D2639) that attempts to measure the actual extent of the plasticity of fluidity. The Gieseler test is used to characterize coals with regard to thermo-plasticity and is an important method used for coal blending for commercial coke manufacture. The maximum fluidity determined by the Gieseler is very sensitive to weathering (oxidation) of the coal.

8.8.4 Agglomerating index

The agglomerating index is a grading index based on the nature of the residue from a one-gram sample of coal when heated at 950 °C (1740 °F) in the volatile matter determination (ASTM D3175).

The agglomerating index has been adopted as a requisite physical property to differentiate semi-anthracite from low-C volatile bituminous coal and also high-volatile C bituminous coal from sub-bituminous coal (Speight, 2013a, 2013b). From the standpoint of the caking action of coal in a gasifier, the agglomerating index has some interest. For example, coals having indexes NAa or NAb, such as anthracite or semi-anthracite, certainly do not give any problems from caking, whereas those coals having a Cg index are, in fact, the high-caking coals.

The agglomerating (or agglutinating) tendency of coal may also be determined by the Roga test (ISO 335). The Roga index (calculated from the abrasion properties when a mixture of a specific coal and anthracite is heated) is used as an indicator of the agglomerating tendencies of coal.

8.8.5 Free-swelling index

The FSI of coal is a measure of the increase in volume of a coal when it is heated (without restriction) under prescribed conditions (ASTM D720; ISO 335). The ISO test (ISO 335) and the Roga test measure mechanical strength rather than size profiles of coke buttons; another ISO test (ISO 501) gives a crucible swelling number of coal.

The nature of the volume increase is associated with the plastic properties of coal. As might be anticipated, coals that do not exhibit plastic properties when heated do not exhibit free swelling. Although this relationship between free swelling and plastic properties may be quite complex, it is presumed that when the coal is in a plastic (or semi-fluid) condition, the gas bubbles formed as a part of the thermal decomposition process within the fluid material cause the swelling phenomenon. This, in turn, is influenced by the thickness of the bubble walls, the fluidity of the coal, and the interfacial tension between the fluid material and the solid particles that are presumed to be present under the test conditions.

The test for the FSI of coal requires that several 1-g samples of coal be heated to 820 °C (1508 °F) within a specified time to produce buttons of coke. The shape, or profile, of the buttons determines the FSI of the coal (BSI, 2011). Anthracites do not usually fuse or exhibit a FSI, whereas the FSI of bituminous coals will increase as the rank increases from the high-volatile C bituminous coal to the low-volatile bituminous coal.

Other effects which can influence the FSI of coal include the weathering (oxidation) of the coal. Hence, it is advisable to test coal as soon as possible after collection and preparation. There is also evidence that the size of the sample can influence the outcome of the free-swelling test; an excess of fine (100 mesh) coal in a sample has reputedly been responsible for excessive swelling to the extent that the FSI numbers can be up to two numbers higher than is the true case.

8.8.6 Ash fusion temperature

The behavior of the coal ash residue at high temperature is a critical factor in selecting coals for gasification. Coal that has ash that fuses into a hard glassy slag (clinker) is usually unsatisfactory in gasifiers, but gasification equipment can be designed to handle the clinker, generally by removing it as a molten liquid.

Ash fusion temperatures are determined by viewing a molded specimen of the coal ash through an observation window in a high-temperature furnace (ASTM D1857). The ash, in the form of a cone, pyramid, or cube, is heated steadily past 1000 °C (1832°°F) to as high a temperature as possible, preferably 1600°°C (2910°°F).

The fusibility of ash is important in understanding the process of slagging and fouling in a gasifier. Ash fusion temperatures give an indication of the softening and melting behavior of fuel ash and therefore an estimation of the variability in fusibility characteristics among different coals. Ash fusion temperatures are also able to provide an indication of the progressive melting of coal ash to slag.

However, despite the shortcomings, fusion temperatures are valuable guides to the high-temperature behavior of the fuel inorganic material. The ash fusion temperature has been correlated with the mineral and chemical composition of coal ash (Vassilev, Kitano, Takeda, & Tsurue, 1995).

Real-time analysis of coal, once considered difficult because of the nature of the solid heterogeneous nature of coal, is now the wave of the future. Real-time analysis affords immediate knowledge of any unanticipated, unknown, and unmonitored changes in coal quality. Coal has a varying composition and its properties vary considerably from coal type to coal type and even from sample to sample within a coal seam. For decades, reliable property data could be obtained only by application of a series of standard test methods (Speight, 2005, 2013a, 2013b; Zimmerman, 1979), provided that the standard sampling methods are adhered to strictly. However, the operator of the gasification process may need to change equipment parameters to prevent loss of efficiency. An informed decision based on real-time data can change an outcome based on guesswork while analytical data are produced by the standard time-consuming methods.

To be profitable and sustainable the coal industry needs to achieve (1) greater energy efficiency, (2) improved use of coal in existing plants, (3) improved product quality and safety margins, and (4) reduced waste material and pollution levels. To do this improved control systems across the full range of industry applications, from mining to processing and use, are needed. These systems rely heavily on the availability of suitable on-line process instrumentation to provide the data and feedback necessary for implementation.

In the past, measurement of coal properties has involved (and still does involve) manual sampling followed by sample preparation (drying, mixing, crushing, and dividing) and off-line laboratory analysis. However, this procedure is often too slow for control purposes. By contrast, on-line analysis can provide rapid and accurate measurement in real time, opening up new possibilities for improved process control. On-line analysis can also lead to a reduction in the cost of sampling and analysis, and a reduced reliance on sampling equipment. As a result, there has been a rapid increase in the industrial application of on-line analysis instrumentation over the past few decades (Snider, 2004; Woodward, Empey, & Evans, 2003). Although it is not the purpose of this section to promote any one particular method, it is the purpose of this section to outline the methods that are available or under further development for real-time analysis of coal.

A current method for on-line analysis of coal ore is prompt gamma neutron activation analysis. With this technique, the sample is irradiated with a continuous neutron beam. The neutrons are absorbed by each of the elements within the sample, which then emit gamma rays at characteristic energies. The gamma rays are then directed toward a gamma ray spectrometer where the peaks are identified. The energies where the peaks are found signify the constituent elements within the sample, and the magnitudes of the peaks reveal the concentrations of each component. The response time for measurements is on the order of 1 min (Gaft, Nagli, Fasaki, Kompitsas, & Wilsch, 2007; Gozani, 1985; Romero et al., 2010). This analysis tool can be installed on a conveyor belt to continuously analyze coal samples. The biggest drawback is the requirement of maintaining a nuclear isotope source to provide the neutrons and maintaining a safe environment for employees.

A pulsed laser technique that is promising for many real-time applications in coal gasification operations is light-induced breakdown spectroscopy (LIBS). All elements radiate characteristic frequencies of light when excited to high enough temperatures. LIBS exploits this by focusing an energetic laser pulse into the sample to be investigated. For solid targets, the laser pulse ablates a small amount of material from the target surface. The ablated material is heated to high enough temperatures to ionize and form localized plasma from the target constituents. Immediately following the plasma formation, a continuum of light frequencies is radiated from the plasma. Shortly after this phase, the plasma begins to cool and the characteristic emission lines from the target's constituent elements become visible. This light is collected and analyzed with a spectrometer to reveal the chemical makeup of the target (Cremers & Radziemski, 2006; Gaft et al., 2007; Gaft et al., 2008).

On-line analysis of coal ore at mines allows process engineers to determine the proper direction to take in mining operations. Coal compositions can vary with mine location and depth. Prompt analysis of compositions reveals whether chosen mining directions are maintaining steady quality or are moving toward unfavorable composition (Yin, Zhang, Dong, Ma, & Jia, 2009).