Essay: Coal – An Analysis

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  • Coal - An Analysis
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Coal:

Coal is usually brown or black, carbon-rich material that often occurs in stratified sedimentary deposits. It is one of the most important of the primary fossil fuels.

A very notable geologist who goes by the name of James Morton Schopf has defined Coal as a substance which contains 50 % or more by weight carbonaceous matter which is produced by the compaction and induration of altered plant remains, which are known as Peat deposits.

Uses of Coal:

Coal is a rich natural resource that can be used for many purposes.

Coal is used as a chemical feedstock from which various synthetic compounds (e.g., dyes, oils, waxes, pharmaceuticals, and pesticides) can be derived. Coal is also used for the production of coke for metallurgical processes.

Electrical energy can be produced using steam generation. In that way, Coal is used as a source of electrical energy as well.

Types of Coal:

Different varieties of coal arise because of variances in the types of plant material (coal type), degree of coalification (coal rank), and range of impurities (coal grade).

Analysis of Coal is used to differentiate and analyze different kind and types of coals. There are two types of analysis of coal; Proximate and Ultimate.

Proximate Analyses:

Proximate analyses of coal is determination of general properties of coal, such as moisture, volatile matter, ash and fixed carbon content as described by a series of prescribed or standard test methods. Proximate Analyses of coal was established as a simple means of determining the distribution of products obtained when the coal sample is heated at a certain temperature under specified conditions.

Definition:

The Proximate Analyses of Coal splits the products into four groups, namely;

1. Moisture

2. Volatile Matter (It consists of gases and different vapors driven off during pyrolysis)

3. Ash (It is the inorganic residue remaining after combustion)

4. Fixed Carbon (It is the non-volatile fraction of coal)

The Proximate Analyses of Coal is presented as a group of test methods that are widely used as the basis for coal characterization in connection with coal utilization. These analyses differ from Ultimate Analyses of Coal which provides information of the elemental composition of Coal.

This analyses of Coal actually mostly deals with four properties of Coal, and is, in fact, a combination of the determination of three of those properties and calculation of the fourth. Moisture content, Volatile Matter content and Ash content of coal is determined by subjecting the coal to prescribed temperature for certain time interval under specified conditions. When the coal is subjected to high temperature, a loss of weight is observed. This loss of weight is due to the loss of moisture, and at even higher temperature, loss of volatile matter. The residue remaining after the ignition at the final temperature is known as Ash. Adding the percentage value of these three readings and subtracting from 100 gives the percentage value of Fixed Carbon content. For Coals with low volatile matter (e.g. Anthracite Coal, Coke etc.) the fixed carbon content is roughly equal to elemental carbon content of the given sample of coal.

The final result of the proximate analyses of coal are reported in 1 decimal figure. The final result of the analyses of coal should contain the results on the basis of air dried coal (i.e. coal in its most stable condition and in which it was analyzed). But for comparison purposes, it is often converted to another basis such as dry coal, dry, ash-free coal, or as-received coal.

1. Moisture:

Water in coal and its quantitative measurement is complicated because the water is present in the coal matrix in different forms. The total moisture in coal can be determined by single stage method or by means of a two-stage method in which the as-received coal sample is first air-dried at approximately room temperature. The residue moisture remaining in the sample is determined. The air-drying step reduces the moisture of the sample to equilibrium conditions with respect to the humidity of the laboratory. This ensures that there is no potential changes in the moisture of the given coal sample when it is prepared for further analyses. This two-stage method is mostly used for extremely accurate analyses of coal.

There are different methods for the determination of moisture in coal and they have been categorized as follows;

1. Thermal Method determines moisture either by the loss of weight in the coal sample when it is subjected to various temperatures (with atmosphere and pressure variable) or by the gain of weight in a vessel containing a desiccant through which the volatile materials which are evolved when the coal is heated escapes.

2. Desiccator Method measures the loss of weight observed by the coal when it is kept in a desiccator in the presence of a desiccant either at atmospheric pressure or reduced pressure but at ambient temperature.

3. Distillation Method for determining the moisture content of the coal requires the collection and then determination of the water evolved from the coal when it is subjected to higher temperatures in a boiling solvent which is immiscible with water.

4. Solution & Extraction Method requires either solvent extraction of the water from the coal (followed by subsequent determination of the water content of the solvent) or by using a standard reagent that will exhibit differences in concentration by virtue of water in the coal.

5. Chemical Method invoke the concept of direct chemical titration of the water or chemical reaction between the water and specific reagents. Due to such reactions, some gases are evolved and the water is determined by measurement of the volume produced.

6. Electrical Method requires the measurement of the value of the dielectric constant of the coal through which water content can be determined.

All these methods have their advantages and disadvantages. The merits and demerits of all these methods should be weighed before their acceptance or application. It must also be noted that the best of these methods is the method which is not only better in accuracy and precision, but also is applicable for the whole range of coal types.

2. Volatile Matter:

The volatile matter obtained by the pyrolysis of coal consists mainly of combustible gases such as hydrogen, carbon monoxide and methane along with other hydrocarbons. Tar and incombustible gases such as carbon dioxide and steam are evolved as well.

The composition of the volatile matter evolved from coal is considerably different for the different ranks of coal, and the proportion of incombustible gases increases as the coal rank decreases. In macerals isolated from any one particular coal, the volatile matter content decreases in a specific order.

It is very important to determine the volatile matter content of coal because the data of volatile matter is an integral part of coal classification systems. It also forms the basis of evaluating coals for their ability of combustion and carbonization.

The method for determining the volatile matter content of coal consists of heating a weighed sample of coal in a covered crucible to a predetermined temperature. The loss observed in weight (excluding the moisture content) is the volatile matter content which is expressed in weight percent.

There are many modifications and differences in the methods for determining the volatile matter content. All of these variances in the method are capable of affecting the result of the tests. These differences are

1. Variations in size, weight, and materials of the crucibles used

2. The rate in which the temperature rises

3. The final temperature

4. The duration of heating the sample

5. Any modifications that are absolutely required for coals that may lose particles on sudden release of moisture or other volatile materials.

It is therefore necessary that the standard procedures which are already defined should be followed strictly.

3. Ash:

Ash is the residue derived from mineral matter during complete burning of the coal. The mineral portion of coal is an integral part of the structure of coal, hence the ash and its contents are also considered when a new use is proposed for coal.

Ash is quantitatively and qualitatively different from the mineral matter originally present in the coal. The chemical composition of ash of coal is a significant factor in fouling and slagging problems. Thus determination of the ash content of coal is very important for coal analysis.

The amount of ash in coal is usually determined by heating (burning) a sample of the coal in a sufficiently ventilated muffle furnace at temperature in the range of 700-9500C. Other standards may differ and need slightly higher temperatures for the determination of the ash in coal. There are some other methods as well, predominantly thermal, which can be used for the analysis of the ash in coal.

4. Fixed Carbon:

Fixed Carbon is defined as the material remaining after the determination of moisture, volatile matter and ash. It is the measure of the solid combustible material in coal after the expulsion of volatile matter. Also fixed carbon plus ash gives the approximate yield of coke from coal.

The value of Fixed Carbon content is given by subtracting the sum of the values of moisture, volatile matter and ash from 100. All percentages must be taken on the same moisture reference base.

Ultimate Analysis:

Unlike proximate analysis which is essentially an examination of the suitability of coal for combustion or for coking purposes, ultimate analysis is an absolute measure of the elemental composition of coal. For example, all of the carbon content in coal is determined by Ultimate Analysis. Just like Proximate Analysis, Ultimate Analysis must also follow certain standard methods.

Ultimate Analysis of coal is used to express the composition of coal in percentages of carbon, hydrogen, oxygen, nitrogen, sulfur, and ash, regardless of their origin. The carbon which is determined by the ultimate analysis includes both forms of carbon; that present in the organic coal substance as well as that originally present as mineral carbonates. Similar is the case with the determination of Hydrogen content. All of the Nitrogen is present as part of the organic coal substance. Sulfur on the other hand is present in three different forms; 1) organic sulfur compounds within the coal structure, 2) Pyrite or Marcasite 3) Inorganic Sulfates.

1) Carbon & Hydrogen

Carbon accounts for 70-95% whereas Hydrogen accounts for 2-6% of the organic substance of coal and are believed to be the most important constituent of coal. The method for the determination of these two elements in the ultimate analysis involves combustion of a certain exact amount of coal sample in a closed system and determining the products of the combustion (CO2 for Carbon, H2O for Hydrogen) by absorption. This combustion is typically accomplished by placing the coal sample in a stream of dry oxygen at temperatures which are in the order of 850-900oC. Complete conversion of Carbon to Carbon Dioxide and Hydrogen to Water is needed and this can be achieved by passing the said gases through heated cupric oxide. The Carbon and Hydrogen are calculated from the increase in weight of the absorbents used to collect the water and carbon dioxide.

2) Nitrogen

Nitrogen is present in coal in ring systems, particularly in pyridines and pyrroles. The determination of nitrogen in coal is based on the principles of decomposition, oxidation and reduction.

There are various different methods for the determination of nitrogen in coal for the ultimate analysis but the standard method followed in most laboratories is the Kjeldahl’s method which was introduced in 1883. This method consists of boiling the pulverized coal with concentrated sulfuric acid containing potassium sulfate and a suitable catalyst (Selenium or a Selenium compound) to reduce the time for digestion. The solution is then distilled with a small amount of Sodium Hydroxide which converts the ammonium salt to ammonia. The amount of ammonia present which means the amount of nitrogen present is determined by back titration.

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