Coal Analysis
Chemical analyses and testing of a coal sample are generally done off site in a laboratory. The main purpose of coal sample analysis is to determine the quality or rank of the coal along with its intrinsic characteristics. Furthermore this data will be used as the fundamental consideration for coal trading and its utilization. General coal analysis and testing include the following.
1. Proximate analysis: Total moisture, Inherent moisture, ash content, volatile matter, fixed carbon; Gross Calorific Value
2. Ultimate analysis: carbon, hydrogen, oxygen, nitrogen, sulphur;
In addition, special coal analyses may also be performed on trace elements, coal hardness, ash fusion temperature, chlorine, fluorine, boron, arsenic, mercury, selenium, phosphorus, size analysis, and so on. Standardization of the procedures and conditions is essential for obtaining results that are comparable within any one laboratory and between different laboratories. The standard test methods used in routine laboratory analyses of coal will be presented in the following sections. The variables are expressed as a percentage fraction (weight percent wt. %) and are calculated in several different bases. As received (ARB) basis puts all variables into consideration and uses the total weight as the basis of measurement. Air dried (ADB) basis neglects the presence of moisture other than inherent moisture while dry basis (DB) leaves out all moisture, including surface moisture, inherent moisture, and other moisture. Dry ash free (DAFB) basis neglects all moisture and ash constituents in coal while dry mineral-matter-free (DMMFB) basis leaves out the presence of moisture and mineral matters in coal, for example: quartz, pyrite, and calcite.
Proximate Analysis:
The proximate analysis of a coal determines the moisture content, volatile matter (VM), ash and, by difference, the fixed carbon within the coal sample. It is the simplest and most common form of coal evaluation, and constitutes the basis of many coal purchasing and performance prediction indices used by utility operators.
Moisture:
Moisture may occur in various forms within a coal:
- Surface or free moisture: water held on the surface of coal particles or macerals;
- Inherent (residual) moisture: water held by capillary action within the pores/capillaries of coal;
- Decomposition moisture: water produced from the thermal decomposition of organic constituents of coal
- Mineral moisture (water of hydration of mineral matter): water which comprises part of the crystal structure of hydrous silicates such as clays and inorganic minerals in coal.
There are various standardized tests to determine the moisture content of coal and they are classified depending upon content of coal and they are classified depending upon the type of moisture content tested:
Total Moisture:
Take 1kg of coal sample of 12.5 mm size dry it at lab atmospheric environment for 24 hr up to constant weight. Take final weight after constant weight this is called as a surface moisture (X). After that sieve it by using 2.8mm sieve and take a 10 gm. of sample and kept it in the oven for drying for 1.5 hr to 3 hr at 108 deg c.. this drying is called as oven drying method (Y)
Calculation:
TM % = X+Y (1-X/100)
Inherent Moisture:
Take 1.5 gm. of 212 mic coal sample in petri dish dry it in oven for 1.5 hr to 3 hrs. At 108 deg c.
Calculation:
IM% = (SW-FW/SW-IW)x100
Equilibrium Moisture:
Take a 5 gm. of 212 mic sample in beaker and boil it for 10 min and filter it with normal filter paper kept it in humidity chamber for 72 hrs at 40 deg c. temp. And 60 % humidity. After that analyzed same as like inherent moisture.
Ash:
Ash is the residue remaining after the combustion of coal in an air and is derived from inorganic complexes present in an original coal substance and from associated mineral matter. Therefore the result of the determination is ash and not ash content as coal does not contain any ash.
Ash is determined by weighing the residue remaining after burning approximately 1gm of coal under rigidly controlled conditions of sample weight, temperature, time, atmosphere, and equipment specifications. In general analysis test sample of coal (ground to pass a sieve of 212 μm aperture), accurately weighed, is heated in air in a furnace from room temperature to 500°C over a period of 60 minutes at a uniform heating rate and is held at this temperature for 30 minutes The sample is then heated to 815°C ± 10°C and maintained at this temperature for a minimum of 60 minutes until the sample is constant in mass. When the incineration period is complete, the sample is allowed to cool and then weighed. The results are reported as a percentage mass fraction on an air dried basis.
Calculation:
Ash% = (FW-IW/SW-IW) x100
Volatile matter:
Volatile matter (VM) in coal refers to the thermal decomposition products liberated when coal is heated at high temperature in the absence of air. The VM obtained during the analysis of coal consists mainly of combustible gases such as hydrogen, carbon monoxide, hydrocarbons, tar, ammonia as well as incombustible gases like carbon dioxide and steam. VM content may be used to establish the rank of coals, to provide the basis for purchasing and selling, or to establish burning characteristics such as combustibility (reactivity) of a coal, and ease of ignition and hence flame stability. VM is determined by establishing the loss in weight resulting from heating a coal sample under rigidly controlled conditions. The measured weight loss, corrected for moisture as determined using standard methods, establishes the volatile matter content. Take a 1 gm. of coal sample in VM crucible and ignite in muffle furnace for 7 min at 900 deg c in absence of oxygen.
Calculations:
VM% = (SW-FW/SW-IW) x100 – IM%
Fixed Carbon:
Fixed carbon in coal is the carbon that remains in the coal sample after volatile matter is driven off. This differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with volatiles. The ratio of fixed carbon to proximate VM, the fuel ratio, is used as a measure of combustion reactivity and burnout. Fixed carbon is not determined directly, it is calculated by subtracting from 100 the resultant summation of moisture, VM and ash, all as a percentage mass fraction on the same moisture reference basis.
Calculation
FC% = 100 – (IM%+VM%+ASH %)
Gross Caloric Value:
Calorific value (CV) is a direct indication of the heat content (energy value) of the coal. It is one of the most important parameters by which the coal is mined for combustion applications, and is the most commonly used benchmark of coal quality and hence its economic value. The CV is usually expressed as the gross calorific value (the higher heating value) or the net calorific value (the lower heating value). The difference between the gross calorific value (GCV) and the net calorific value (NCV) is the latent heat of condensation of the water vapors produced during the combustion process. The GCV assumes that all of the vapors produced from coal moisture during the combustion process is fully condensed. The NCV assumes that the water is removed with the combustion products without being fully condensed. Standard methods for determining the CV of coal employ calorimeters and burning coal in oxygen under pressure in a closed system. The bomb calorimeter provides the most suitable and accurate apparatus for determination of the CVs of solid and liquid fuels, and is adopted in ASTM D5865 (Standard test method for gross calorific value of coal and coke) and ISO 1928:2009 (Solid mineral fuels – Determination of gross calorific value by the bomb calorimetric method and calculation of net calorific value).
The procedures specified in the standards involve burning a weighed portion of the analysis sample of coal in high pressure oxygen in a bomb calorimeter under specified conditions. The effective heat capacity of the calorimeter is determined in calibration tests by combustion of certified benzoic acid under similar conditions, accounted for in the certificate. The temperature measurements required for the evaluation of the corrected temperature rise are made during a fore period, a main period (reaction period) and an after period. For the adiabatic type calorimeter (ASTM, ISO), the CV is calculated from the corrected temperature rises during the fore and after periods. For the isoperibol (isothermal jacket) (ASTM, ISO) and the static jacket type calorimeters (ISO), the CV is computed from the corrected temperature rise observed during the fore, main and after period. Allowances are made for contributions from ignition energy, combustion of the fuse(s), for thermochemical corrections and, if appropriate, for heat losses from the calorimeter vessel to the water jacket. Water is added to the bomb initially to give a saturated vapour phase prior to combustion, thereby allowing all the water formed from the hydrogen and moisture in the sample to be regarded as liquid water. The result obtained is the GCV of the analysis sample at constant volume with all the water of the combustion products as liquid water, and is usually expressed in J/g (or kJ/g, MJ/kg), cal/g (or kcal/g, kcal/kg). Btu/lb is also used in some parts of the world such as the USA. In practice, fuel is burned at constant (atmospheric) pressure and the water is not condensed but is removed as vapour with the flue gases. Under these conditions, the operative heat of combustion is the NCV of the fuel at constant pressure. The standards provide equations for converting GCV to NCV of the fuel at constant pressure and at consent volume.
Ultimate analysis:
Ultimate analysis determines the percentage mass fraction of the major constituents of coal such as carbon, hydrogen, sulphur, nitrogen, and, usually by difference, oxygen. The ultimate analysis is used with the calorific value of the coal to perform combustion calculations such as the determination of coal feed rates, boiler performance and sulphur emissions. All ultimate analysis done by CHNS analyzer.
ADB to ARB Conversion of GCV
ARB GCV % = (100-TM%/100-IM %) X ADB GCV
ADB to Dry Basis conversion of GCV
DB GCV % = (100/100-IM%)x ADB GCV
Conversions – Units
From kcal/kg to MJ/kg multiply kcal/kg by 0.004187
From kcal/kg to Btu/lb multiply kcal/kg by 1.8
From MJ/kg to kcal/kg multiply MJ/kg by 238.8
From MJ/kg to Btu/lb multiply MJ/kg by 429.9
From Btu/lb to kcal/kg multiply Btu/lb by 0.5556
From Btu/lb to MJ/kg multiply Btu/lb by 0.002326