The supply of quality coal products to the customer is the culmination of a chain of events that begins in the earliest geological assessments of a lease area, to mine planning, to mine operations and finally, shipment of products to contract specification.
Testing and analysis of exploration samples is critical to the development of new projects and the prediction of product quality and quantity for existing producing mines. It is vitally important that exploration sample programs include appropriate testing and analysis work.
Analysis and Testing
This section describes briefly the most common laboratory tests for coal quality determination and discusses their importance. These tests are shown in the attached table, along with the reference to the relevant standard method. Many of these tests are routinely performed on borecore samples from the exploration phase of a project, on production samples from the process for technical information and finally on shipping samples to establish the qualities of the coal in relation to the specification of a contract.
Total Moisture
Representative samples of a shipment or consignment, generally crushed to minus 6 mm, are heated in special ovens at 105oC for up to five hours. The mass loss on drying as a percentage of the original sample mass is taken as the total moisture content.
Relevance
This is one of the most important contractual parameters in a coal supply contract. The total moisture identifies that part of a consignment or cargo that is dead weight. It does not take long to calculate that for a shipment of 100,000 tonnes of coal at 10% total moisture, transportation costs of 10,000 tonnes of water must be paid by the customer.
Often the price of shipments of high moisture coal are adjusted to a contracted figure of say 8%, and a penalty paid by the coal producer for excess moisture. The "moisture penalty" can be determined by two factors. Firstly the "freight cost" penalty and secondly, the "nuisance" penalty.
High moisture levels can cause operational problems for the producer (hang up in bunkers, bins or chutes that create loading delays in train loading or delays in plant operation) and for the consumer (mill capacities are reduced due to the inability of system to dry the ground product or add to process inefficiencies as heat is wasted in evaporating the water in the coal).
Proximate Analysis
This is the term given to a group of three tests on the air dried sample that are commonly used to provide basic data that can identify the coal type.
The three tests are:
1) Moisture (sometimes still referred to as inherent moisture)
2) Ash
3) Volatile Matter
All these tests are performed on an "air dried" sample and are reported on an "as analysed" basis. The term "proximate" means "first" or "immediate". It does not mean, "approximate".
A brief description of each test is now detailed.
1) Moisture (or "air dried moisture" or “as-analysed moisture”)
This is the moisture remaining in the coal after air drying at room temperature. The moisture level is determined by heating the air dried sample of coal in a nitrogen atmosphere (to eliminate oxidation) at 105oC for 2 hours.
Application
It is essential to know the moisture level at which the Proximate Analysis was determined so that Ash, Volatile Matter and Fixed Carbon can be converted to other bases. It is sometimes called “inherent moisture” but this term dose not strictly apply because the drying conditions are not rigidly controlled.
2) Ash
Rapid testing of ash content has been developed at large mine laboratories over the past 20 years to give access to a result in a shorter time than that spelt out in the Standards. Rapid entry of the coal sample directly into the hot zone of an oven together with injection of nitrogen and then oxygen over a period of 15 minutes gives fairly accurate results.
A one-gram sample of the air-dried coal, after grinding to minus 212 micron is incinerated in a special furnace at 815oC for over one hour. The sample is introduced into the furnace at room temperature and increased to 800 C over 1 hour.
Application
Ash is one the most common parameters used in coal quality evaluation, from early exploratory work, to final shipment certification. It is one of the main parameters of any coal contract and does attract bonus or penalty payments for particular consignments or shipments. Coal ash level is the main control parameter in the coal preparation plant.
Changes to the separation density of the process can increase or decrease the ash content of the product at any time. The trend in the market is towards lower ash coals for particular end uses. From the customer’s point of view, a lower ash level means less waste material that has to be handled or disposed of.
3) Volatile Matter
A one-gram sample of fine coal (minus 0.212mm) is heated, out of contact with air, for seven minutes at 900oC. The mass loss percentage corrected for the moisture % is the volatile matter %. The volatiles, which are driven from the coal during the test, are primarily combustible gases such as hydrogen, carbon monoxide and methane and condensable vapours such as tars.
Application
The volatile matter content is often used as a measure of the rank of the coal by geologists. Volatile matter is also important to combustion engineers. The higher volatile coals give off a greater proportion of their weight as combustible gases allowing easier ignition, superior flame stability, higher flame temperatures and thus faster burnout of carbon residues. Low volatile coals contain a lesser proportion of these combustibles making ignition and flame stability more difficult.
In coking coals, the volatile matter content is important to the estimation of the yields of coke and "by-products". Fixed Carbon is also reported in the Proximate analysis. There is no test for fixed carbon. It is arrived at by subtracting the summation of the ash, moisture and volatile results from 100 percent.
Calorific Value For the test a one gram sample of the air dried coal is burnt in the presence of excess oxygen in a bomb calorimeter. The gross specific energy, or "calorific value", is then calculated in MJ/kg from the measured heat output.
Application
This is the test that quantifies the amount of heat released by the complete combustion of coal. It represents the true value of the coal as a fuel, so it follows that this test quantifies a very important contractual requirement for a customer. Selection of a coal with a high specific energy for a power station can allow reduction in the design capacity of coal receiving, handling, grinding and ash disposal facilities with savings to the customer in capital and operating costs.
Total Sulfur
Sulfur occurs in coal in three forms. As organic sulfur, as pyritic sulfur, and as sulfate sulfur. The usual test is to determine the total of the three types i.e. total sulfur. A small sample (0.5 gram) of fine air-dried coal is combusted in the presence of excess oxygen in a tube furnace operating of 1350oC. The acid gases given off are absorbed or measured to give the sulfur content of the coal.
Application
Total sulfur is a parameter used in all coal contracts because of the environmental importance of acidic sulfur oxide emissions when coal is burnt. Australia is fortunately endowed with low sulfur coals in comparison with some other countries. This fact encourages the utilisation of Australian coals to meet emission standards for sulfur oxides in flue gases. Evaluation of the sulfur levels of raw coal in an exploration program gives important information as to the eventual saleability of the coal.
A knowledge of the forms of sulfur gives an indication whether total sulfur content can be reduced because the pyritic sulfur can be removed by special operations in the preparation plant. Pyritic sulfur in coal may signify the presence of iron pyrite which is associated with spontaneous combustion, acid drainage from stockpiles and slagging.
Hardgrove Grindability Index
The Hardgrove Grindability Index (HGI) is a widely used test for measuring the ease of size reduction of a particular coal. The HGI test involves crushing 50 gram of sized coal (1.18 mm x 0.6 mm) in a small ball in track mill for 60 revolutions at 20 rpm. The grindability index is derived from a sieve analysis of the ground product and by reference to a calibration prepared from Australian certified reference coals.
Application
The HGI value is used to estimate the grinding capacity of mills and to select the type of mill best suited to the coal. It is a fairly common contractual parameter because grinding capacity is of major concern to a power station customer. The higher the HGI figure the more easily crushed is the coal. HGI is not suitable for predicting pulveriser wear rates.
Abrasion Index
The test involves milling 2 kg of -6.7 mm coal in a small hammer mill for a fixed number of revolutions. The mill has removable blades that are weighed before and after. The Abrasion Index is defined as the milligrams of metal removed per kilogram of coal.
Application
The Abrasion Index is used to determine the abrasiveness of coals. This is particularly relevant to abrasive wear of pulverising mill components, but also applies also to the wear of materials handling equipment. It is commonly used for high ash coals or those that have high levels of hard minerals.
Ultimate Analysis
This is the term given to the group of tests that determine the elemental composition of the organic part of the coal. These tests determine the levels of Carbon, Hydrogen, Sulfur, Nitrogen and Oxygen in a coal. Generally C, H, S and N are derived directly and O by difference. This test does not appear as a contractual parameter, but does play an important role in determining the suitability of coals for various uses.
Application
Carbon content is another means of determining the rank of the coal. C, H, N, S and O are used by engineers to calculate the products of combustion Hydrogen and oxygen are necessary for calculating nett specific energy of a coal. Nitrogen and sulfur contents may be limited by coal contracts for environmental reasons related to the emission of the gaseous oxides.
Ash Fusion Properties
Slipcase cones of coal ash are heated to high temperatures in either a reducing or oxidising atmosphere. Photographic or video records are kept of the changing shape of the ash cone as the minerals in the ash begin to soften, fuse and melt.
Temperatures at which specific states occur are noted:
1) Initial deformation temperature oC
2) Spherical temperature oC
3) Hemispherical temperature oC
4) Flow temperature oC
Temperatures of up to 1600 C are reached in this test. Reducing atmospheres in the furnace are achieved by introduction of a hydrogen/carbon dioxide gas mixture during the test. Samples of ash subjected to oxidising conditions tend to produce higher ash fusion temperatures.
Application
The test was designed to characterise the fusion properties of the ash to predict the formation of sticky ash deposits in combustion processes. The deformation temperature is usually included in tender or contract specifications, even though it is the least reproducible of all the coal laboratory tests.
Ash Analysis
In this test the elemental composition of the ash from the combustion of the coal is determined by x-ray fluorescence or atomic absorption spectrometry or inductively coupled plasma. The elements usually reported are Silicon, Aluminium and Iron, Calcium, Magnesium, Sodium, Potassium, Titanium, Manganese, Sodium and Phosphorus. The above elements are reported as the most commonly occurring oxides.
Application
Coal contracts often specify limits for silicon, iron, calcium and sodium. In cement manufacture, the coal ash ends up in the cement product and therefore must be known to control the cement composition.
Trace Elements
Coal samples have been analysed for more than 50 elements using, predominantly, atomic absorption spectrometry and neutron activation analysis. The elements usually tested for are barium, beryllium, cadmium, chromium, copper, lithium, nickel, lead, strontium, vanadium, fluorine, boron, zinc, mercury and selenium. A standard method for determination of trace elements is given in AS 1038.10.1. Methods for determination of boron and fluorine are given in AS 1038.10.3 and AS 1038.10.4 respectively.
Application
Trace element content is becoming an increasingly important environmental consideration in many countries.
Moisture Holding Capacity
In this test a sample of coal is allowed to come to equilibrium with an atmosphere saturated with moisture vapour (in practice 96% humidity) at 30C. The equilibrium moisture content is determined.
Application
This gives a good indication of the moisture that the coal can carry without being visibly wet (i.e.without any surface moisture), sometimes equated to the "in-situ" or "bed" moisture content of the coal. A similar test is covered by ASTM standards where it is sometimes referred to as “inherent” moisture (not to be confused with air-dried moisture). The Moisture Holding Capacity is sometimes used as a basis for trying to estimate a reasonable “as-received” moisture for coal sales.
Maceral Analyses
Maceral analysis involves the microscopic examination of the coal to determine its constituents. Macerals are the microscopically recognisable organic constituents of coal, analogous to the minerals of inorganic rocks, but differing from the latter in that macerals have no characteristic crystal form.
The macerals are named with words ending with "-inite" and are distinguished from one another microscopically on the basis of their differences in such properties as reflectance, colour, morphology, anisotropy and hardness. They originate from the remains of different organs and tissues of plants and have acquired their present properties during peat accretion and coalification.
For reporting, the macerals are arranged into the three groups vitrinite, exinite (also called liptinite) and inertinite. Minerals, form the fourth group reported. The reflectance (i.e. brightness) of the vitrinite maceral is an accurate indicator of the rank of the coal.
Application
The main application is to coking coals, where vitrinite content and vitrinite reflectance are key components of a specification. Maceral analysis has been the subject of many research projects to try to predict the combustion performance of thermal coals.
Standard Methods
Consistency is an important attribute of a commodity like coal. A prerequisite for consistency of a coal product is that the analysis which defines the quality should be repeatable and reproducible within close tolerances.
For any test, there must be a specified procedure to follow in order to minimise differences between personnel in one laboratory, or between two different laboratories. In Australia, all the tests associated with coal sampling and analysis are documented in the following Australian Standard Methods; published by the Standards Association of Australia:
a. AS 4264.1, covers all aspects of sampling and sample preparation.
b. AS 1038, covers the general analysis and testing methods.
These Australian standards spell out in detail:
a. The type of equipment to be used for the test
b. The operating conditions for the test, and
c. The procedure required to complete the test.
Other countries have similar organisations to the Standards Association of Australia, for example:
a. USA has the ASTM - American Society for Testing Materials
b. Japan has the JIS - Japanese Industrial Standards.
c. U.K. has the BS - British Standards
d. European Countries have the ISO - International Standards Organisation.
Close examination of the various standard methods from each organisation shows that the procedures and methods are very similar. Generally, robust standard methods developed in one country are adopted by other countries with little modification.
There are however a few standard methods where there are major differences in equipment that can produce different results when comparing with other standards. The most significant of these are the ASTM tests for Volatile Matter and Ash Fusion Temperatures.
In Australia the Standards Association of Australia has permanent committees established that continually review and update Standard Methods as and when necessary. These committees are made up of (in the case of coal) specialist coal technologists, or engineers from major organisations that participate in the Australian Coal Industry. These people meet on a regular basis to discuss any changes or reviews of the Standard Methods. A list showing a cross-reference to the most commonly used coal standards is provided in Table 1.
Reporting of Results
Precision and Reproducibility of a Laboratory Test Generally all analyses in a laboratory are performed in duplicate. The average of the two results is reported as the best estimate of the true value of the test result for the sample.
When the difference between the duplicate results of a test falls outside a specified range, called the "tolerance", the test will be repeated. Even in the most carefully conducted test, differences occur between the results of two tests on two sub-samples of the same coal.
The standard methods spell out precisely the equipment and procedures so that the differences between tests within a laboratory or between laboratories are as small as possible. The repeatability to be expected for each of the common coal tests performed according to Australian Standard methods, is shown below:
Repeatability
Ash Content
< 10 percent 0.10% 10-20 percent 0.20% 20-40 percent 0.25% Volatile Matter % 0.20% Specific Energy (MJ/kg) 0.12 Total Sulfur £ 1.5% 0.03% > 1.5% 2% of Mean
The reproducibility to be expected between different laboratories on the same coal sample for tests conducted according to Australian Standards, are shown in Table 2.
Reporting Basis
All analyses in the laboratory are determined on an "air-dried" basis. However, parameters are reported to different bases for contractual requirements e.g. thermal coal consignments sometimes report the specific energy (Kcal/Kg) to an 'as received' basis.
This takes into account the moisture that is present in the consignment and thus its actual energy value. Sometimes results are reported to "dry", "dry and ash-free" or "dry, mineral matter-free" bases so that true comparisons between different coals can be made.
Australian Standard AS 1038 Part 16 "Reporting of Test Results" gives all the information on reporting terms, abbreviations, changes of bases, interpretation of results and the repeatability and reproducibility tolerances for all coal analyses.