Coal Testing Definition

Calorific Value


Also known as the Heating Value, Energy Value, and Fuel Content, the Calorific Value of a substance, usually a fuel or food items, is the amount of heat released during the complete combustion of a specified amount of it. The Calorific value is characteristic of each substance. It is measured in terms of heat energy liberated per unit quantity of the substance (usually mass), and the common units of it are: kcal/kg, kJ/kg, J/mol, Btu/m³. Calorific value is commonly determined by use of a ‘Bomb Calorimeter’. In the context of Coal testing, Calorific Value can be thought of as a rank-related parameter, but it also depends on the compositions of macerals and minerals.


Rank of Coal


Coal is formed by the slow, natural process of ‘Coalification’, during which buried plant matter in swampy environment changes into a denser, drier, more carbon-rich, and harder material. Coalification is a continuing geological process involving increases in temperature and pressure as a result of burial in the earth. Rank refers to the degree or stage of coalification; increasing rank corresponds to more hardness, lower moisture, higher content of fixed carbon, and hence higher heating value. There are four major ranks of coal (in the order of increasing rank): Lignite, Sub-bituminous, Bituminous, and Anthracite. The applications of the coals of different ranks vary due to difference in the heating values.

Gradation of coals is generally done on the basis of commercial usefulness of coal, like high Calorific value (or Useful Heat Value), fixed carbon content etc. Classifications into grades exist in various countries, and the basis of such classifications is the prevalent industrial needs.


Analysis of Coal


There are basically two methods for analyzing coal, Proximate and Ultimate. Ultimate analysis can be carried out only in a properly equipped laboratory by a trained chemist, while Proximate analysis can be determined with a relatively simple apparatus and laboratory set-up.


Proximate Analysis


Proximate analysis is the most commonly used chemical analysis conducted on coals, and is also the simplest to perform. A typical proximate analysis includes determination of moisture, volatile matter, ash, and fixed carbon content after the coal sample has been ground to pass 0.212 mm sieve. This test is used to ascertain the ‘Rank’ of coals, and also to establish the ratio of combustible to incombustible constituents. These data give substantial indication about the heating/fuel value of coal, which is a very important indicator in the process of selling and buying of coals.

1. Moisture

The importance of determining ‘Moisture’ in coal arises from the fact that all coals are mined in very wet conditions. This moisture is water held within the coal itself, and is also known as ‘inherent moisture’. ‘Adventitious moisture’ refers to Groundwater and other extraneous moisture, which gets readily evaporated. Moisture may occur in four major forms within coal:

  • Surface moisture: water held on the surface of coal particles or macerals.
  • Hydroscopic moisture: water held by capillary action within the micro-fractures of the coal.
  • Decomposition moisture: water held within the coal’s decomposed organic compounds.
  • Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays.

Moisture decreases the heating value of coal, as it decreases the combustible matter of coal.

2. Volatile matter

Volatile matter in coal serves as an index of the inherent gaseous fuels present. Basically, it refers to the components of coal, excluding moisture, which are liberated at high temperatures in the absence of air. This is usually a mixture of aliphatic or aromatic hydrocarbons, like methane, benzene, xylene, and compounds containing sulfur and oxygen. The determination of volatile matter of coal is conducted under rigidly controlled experimental conditions, as mentioned in various national and international standards. For example, in Australian and British laboratories this involves heating the coal sample to 900 ± 5 °C (1650 ±10 °F) for 7 minutes in a cylindrical silica crucible in a muffle furnace. American Standard procedures involve heating to 950 ± 25 °C (1740 ± 45 °F) in a vertical platinum crucible. Volatile matter leads to proportionate increase in flame length, and helps in easier ignition of coal.


3. Ash
Ash content of Coal is the non-combustible powdery residue left after burning of coal. It represents the total inorganic/mineral matter present in coal, after expulsion of all organic compounds, i.e. compounds containing carbon, oxygen, sulfur etc. The analysis involves the complete combustion of coal at a particular temperature, and the ash material is expressed as a percentage of the original mass of the coal sample. The test results are generally expressed in terms of metallic oxides, like Na2O, K2O, SiO2, TiO2 etc.

Below is given an example of Coal analysis, where along-with the composition of the various metallic oxides the composition of the corresponding trace metals are also given. The analysis of the coal ash can be done with the use of instruments like ICP-OES and AAS. Data from such analysis can be used for environmental impact modeling.


Ash Analysis

Oxides wt.% of ash(Calculated)
(Desirable Limits)
Elements wt.% of ash(Measured)
Na2O 0.35 Na 0.26
MgO 0.48 Mg 0.29
Al2O3 20.0 Al 10.6
SiO2 74.1 Si 34.6
P2O5 0.05 P 0.05
K2O 1.1 K 0.92
CaO 0.68 Ca 0.49
TiO2 0.80 Ti 0.48
Mn3O4 0.06 Mn 0.05
Fe2O3 3.25 Fe 2.28

4. Fixed carbon
The ‘Fixed Carbon’ content of the coal refers to the solid combustible mass left after almost all volatile materials are driven or distilled off. The solid mass is mainly ‘Coke’, which is pure carbon, but it may also contain other elements like hydrogen, nitrogen, oxygen, and sulphur which are not distilled off. Hence knowledge of ‘Fixed Carbon’ is useful in the production of coke. ‘Fixed Carbon’ is always less than the Ultimate Carbon in a coal sample, since the latter takes into account the carbon from the volatile hydrocarbons. Fixed carbon is determined indirectly by subtracting the mass of volatile matter, ash, and moisture from the original mass of the coal sample, and expressed as percentage. This parameter gives an indication about the heating value of coal.


Ultimate Analysis


The main objective of Ultimate analysis of coal is to determine the elemental constituents of coal. This analysis determines the amount of carbon (C), hydrogen (H), nitrogen (N) oxygen (O), sulfur (S), and other elements within the coal sample. These elements are reported in terms of percentage by mass of the coal sample. Knowledge of such elements enables determination of the quantity of air required for coal combustion and the volume and composition of the combustion gases. All these information, in turn facilitates calculation of flame temperature, flue duct design, etc.


Coke Reactivity Test


The quality of coal and the subsequent coke produced, used in blast furnaces is very important for improving efficiency of the iron-making equipment. The coke used in the furnace serve three key roles: (1) Thermal role: the coke is used as a source of fuel providing heat required for melting of iron, formation of slag, and other endothermic reactions that take place inside the blast furnace. (2) Chemical role: Reduction of iron oxides is effected by the chemical action of coke and regeneration of reducing gases. ((3) Physical role: The coke maintains the permeability of the cohesive zone, which is very important for the operational stability, fuel efficiency, and the productivity of blast furnace. Coke Reactivity test is an extremely useful and accurate measure of the quality of coke used. It is related to the strength of the coke or its resistance to degradation. This test was initiated by a Japanese steelmaker, Nippon Steel Corp in the 1970’s, with the objective of evaluating the performance of coke in a blast furnace. This test has two parts; the Coke Reactivity Index (CRI) and the Coke Strength after Reaction (CSR). The CRI component measures the reactivity of coke, in terms of loss of mass, under the reducing environment presented by CO2, and the CSR component measures the strength of the coke after exposure to the reducing environment and high temperature. A coke with a low CRI value and a high CSR value is desirable.


Hard-grove Grindability Index of Coal


Hard-grove Grindability Index (HGI) is a standardized physical test conducted on Coal to determine the ease of its pulverization, or it is a measure of its resistance to crushing. The test is done in comparison to standard coals. High values indicate a coal easy to pulverize, and vice versa. There are mainly two factors that affect grindability of coals, moisture and ash content. The HGI test was developed in the 1930s from an experimental work initiated by R. Hardgrove to determine the relative difficulty or ease of grinding various coals to a particle size required for efficient combustion in pulverized coal boiler furnaces. This index serves as a guideline for devising the grinding equipments used in Coal-preparation plants. The HGI is now frequently used in specifications for coal, to decide suitability of its use in the iron making, cement and chemical processes.

Since this parameter of coal depends on the method of its determination, HGI is an empirical measure and not an exact physical property of coal. Hence, the results of any standard test have relatively poor repeatability for a given coal sample. There are several different "standard" test methods for the determination of HGI, e.g. - ASTM D-5003 and ASTM D409-2006.

There is an empirical relationship between HGI value and the rank of the tested Coal; higher the rank of the coal, higher is the HGI value. HGI value is also influenced by the petrographic nature (types of macerals present) of the coal. HGI of bright coals, containing 89-90% Carbon, can reach a maximum value of 105. But for anthracite, the HGI value has a low value of 35.


POROSITY


Porosity is the fraction of the volume of voids/pores of a solid substance over the total volume. Porosity is the reason for which the internal surface area of a coal sample is far higher than the external surface area. The creation of pores in coal takes place throughout the process of coalification. The importance of pore spaces in coal can be seen in the processes of production of coke, gasification, liquefaction, research and development, catalyst evaluation, and the generation of high surface-area carbon for purifying water and gases. The knowledge of the presence of adsorbed gases in pores can help prevent accidents in mining operations.


DENSITY


Density is simply mass per unit volume. But for Coal, the density depends on the method of measurement, and also varies with rank. This is due to the intricately structured void volume of coal. Several types of density measurement are made on coal, depending on the intended end use. The most commonly measured density is the bulk density, which is defined as the mass of coal occupying a unit volume and is expressed in grams per cubic centimeter or pounds per cubic foot. Bulk density depends on the particle size distribution of a coal and is important from the perspective of handling, storage, and transportation.


Angle of Repose


It is the maximum angle formed between the slope of the heaped pile of coal and the horizontal plane on a given surface, maintaining no sliding or rolling of the substance. It bears its significance in storage of coal, and in its flow in the conveyors and feed hoppers. In general the higher the sizes of coal, the higher the angle of repose. There are mainly three methods of determining ‘Angle of Repose’, Tilting Box method, Fixed Funnel method, and the Revolving Cylinder method. All these methods produce different results for a particular substance, and hence the test method should be specified while reporting the test result.


Reflectivity


Reflectivity or Vitrinite reflectance is the ability of the Coal to reflect light. It is measured by shining a beam of monochromatic light (with a wavelength of 546 nm) on a polished surface of the vitrinite macerals (identified using microscope) in a coal sample, and then measuring the percentage of light reflected with a photometer. The reflectivity can also be measured by making use of light reflectance. The measurement is made by comparisons against readings of high-index glass standards.

This property is an important index for determining rank of coal, and also gives an idea about how well a coal will form coke. The value of light reflected increases with the rank of coal.


Crucible Swelling Index/ Free Swelling Index


The caking behaviour of Coal is critical to the manufacturing of coke, which is a key ingredient for steel plants. Crucible swelling index or Free swelling index is the simplest qualitative test to evaluate the caking capacity of coal. The analysis involves heating a particular small sample of coal in a standardized crucible to around 800 0C for a specified period of time. After the expulsion of volatile matter, a small coke button remains in the crucible. Finally, the free swelling index is determined by comparison of the cross-sectional profile of this coke button to a set of standardized profiles. Coal for which free swelling index lies between 2 to 5 is considered ideal for manufacturing coke.

Ash Fusion Temperature

The behaviour of the coal's ash residue at high temperature is a critical factor in selecting coals in boilers, for the generation of steam in power plants and other industries. Ash fusion temperature does not refer to a single temperature, but involves monitoring four different temperatures: Initial Deformation temperature, Softening temperature, Hemispherical temperature, and Flow temperature. These temperatures are determined by viewing a moulded specimen of the coal ash through an observation window in a high-temperature furnace. The ash, in the form of a cone, pyramid or cube, is heated steadily past 1000 °C to as high a temperature as possible, preferably 1600 °C (2900 °F), as mentioned in standard methods.


Bomb Calorimeter

A Bomb Calorimeter or an Oxygen Bomb Calorimeter is a standard instrument used to measure the Calorific Value of a combustible substance or fuel like coal, coke, food items etc. There are two modes of operation of a bomb calorimeter (with a temperature-controlled jacket system in place): Isoperibol (no temperature change) and adiabatic (no heat exchange). The general steps involved in the use of this instrument are as follows:

  • A known/weighed amount of sample is placed in an explosion-proof (able to withstand high pressure) steel container, called the bomb whose volume does not change during a reaction.
  • The bomb is pressurized with oxygen to ensure complete combustion, and sealed to prevent escape of the combustion products.
  • The sample is then ignited by passing a current through a fuse wire within the bomb.
  • The bomb is then submerged in water that absorbs the heat of reaction, and the rise in temperature of water is noted.

The calculation of the Calorific Value of a fuel is done by multiplying the temperature rise with the energy equivalent or heat capacity of the calorimeter, which is calculated by using a standard substance like benzoic acid (which has a constant calorific value) and following the same procedure as with the sample. Other corrections need to be incorporated as well. All these steps can be automated with the use of proper electronics and data acquisition system. Spectro has developed a Bomb Calorimeter which gives accurate and rapid test results.


Carbon & Hydrogen determination apparatus


Carbon and hydrogen determination is very important for characterization of coal. The ratio of Carbon to hydrogen in a coal sample gives vital insight into the organic nature of the coal sample, and also reveals information about the rank of the sample. The analysis involves combustion of a small but accurately weighed quantity of the coal sample. Carbon is determined by gravimetric process (using CO2 absorbent) and hydrogen by coulometric method. The credibility of test results is established by use of standard samples for calibrating the instrument.


Proximate Analyzer


Proximate analysis is a vital quality control parameter for coal and coke. This analysis covers the estimation of moisture, volatile matter, fixed carbon, and ash. Proximate analysis is extremely crucial from the point of view of buying/selling of coals. A ‘Proximate Analyzer’ makes use of a thermo-gravimetric method which involves measurement of changes in mass (weighing is done on an analytical balance) of the test material in response to a programmed temperature increase/gradient. This instrument can also determine the Calorific value of the coal sample. For proper working, the analyzer has to be regularly calibrated with standard coal samples. The use of this instrument allows rapid and accurate analysis of a number of coal samples. Spectro’s ‘Proximate Analyzer’ has been manufactured according to the accepted specifications.


Ash Fusion Tester


The melting behavior or the fusibility of the coal’s ash is an important characteristic of the ash in respect to the conditions that exist in boilers. Study of Coal Ash analysis is used to assess erosion, slagging, and fouling potentials of Coal inside boilers. Spectro has indigenously innovated to manufacture its own ‘Ash Fusion Tester’. The instrument has been manufactured as per ASTM standard.