What is a coal?Know about Composition of coal,Classification of coal and analysis of a coal

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Introduction

Coal, a dark sedimentary rock created from ancient plant remains, has been pivotal in shaping the world's energy scene. It has served as a vital power source for generations, driving the industrial revolution and aiding in global economic growth. Yet, the utilization of coal poses various environmental, social, and health issues, prompting continuous discussions about its place in the constantly changing energy landscape.

The elements carbon, hydrogen, nitrogen, sulfur, and oxygen are the main components of a coal.High levels of expertise are needed for this analysis, and the necessary equipment is pricey. Nonetheless, it provides precise results that are helpful when applying Dulong's formula to determine the calorific value of coal.

Understanding Elemental Composition of a coal

Elemental composition focuses on quantifying the presence of key chemical elements, including carbon (C), hydrogen (H), nitrogen (N), sulfur (S),and oxygen (O). These elements form the building blocks of countless organic and inorganic compounds, and their precise measurement is fundamental to the field of chemical engineering.

1. Carbon (C):

In the realm of chemical engineering, carbon is a linchpin element. Ultimate Analysis enables engineers to determine the carbon content of substances, crucial for optimizing combustion processes,synthesizing organic compounds, and ensuring the efficiency of various chemical reactions.

2. Hydrogen (H):

Hydrogen, with its versatile reactivity, plays a vital role in chemical engineering. Ultimate Analysis allows engineers to quantify hydrogen content, guiding processes involving fuel production, ammonia synthesis, and the development of hydrogen-rich compounds for various industrial applications.

3. Nitrogen (N):

Nitrogen is a key player in chemical processes,particularly in industries involved in the production of ammonia,fertilizers, and nitrogen-rich chemicals. Ultimate Analysis aids chemical engineers in understanding and optimizing nitrogen content for efficient reaction kinetics and product quality.

4. Sulfur (S):

Sulfur, while essential in certain chemical processes,poses environmental challenges. Ultimate Analysis helps chemical engineers manage and control sulfur content, ensuring compliance with environmental regulations and optimizing the quality of end products.

5. Oxygen (O):

Often overlooked, oxygen's role is paramount in combustion processes and overall stoichiometry. Ultimate Analysis quantifies oxygen content, offering insights into reaction efficiency, combustion
characteristics, and the overall chemical balance in various engineering applications.

Applications of Elemental Composition in Chemical Process Optimization

1. Fuel Characterization

Ultimate Analysis significantly contributes to evaluating the fuel’s elemental content. With a careful examination of carbon, hydrogen, and sulfur content, a detailed understanding is achieved which in turn allows the chemical engineers to perfect the posture of combustion and in turn make improvements of efficiency of the energy and also provide further way to find cleaner energy resources.

2. Material Synthesis and Design

In material science, Ultimate Analysis constitutes the various chemical engineers’ efforts to control malleable elemental percentages being polymers, catalysts, and so on. This knowledge therefore is integral to developing the desired material properties and functionality in different industries.

3. Environmental Compliance

On the other hand, environmental management and keeping it at the minimal level become crucial valuess in chemical engineering. Ultimate Analysis measures help to control the proportions of sulfur and nitrogen in emissions; achieving this goal, the company avoids being penalized for violations of the existing regulatory framework and in general contributes to the development of green manufacturing.

4. Process Efficiency and Quality Control

Elemental composition of raw material and the products is of immeasurable importance in optimizing the chemical reaction. In chemical engineering, Ultimate Analysis helps engineers create processes that are optimised and which minimize waste and strongly conform to quality controls.

Classification and rank of coal

The different types of coal in increasing order of alteration are peat, lignite (brown coal immature), sub-bituminous, bituminous and anthracite (mature). Coal starts off as peat. After being subjected to heat and pressure for a considerable amount of time, it is metamorphosed from peat to lignite. Lignite is referred to as brown coal because it is somewhat light in color and it remains soft. As time passes, lignite increases in maturity by becoming darker and harder and is then classified as sub-bituminous coal. As this process of burial and alteration continues, more chemical and physical changes occur and the coal is classified as bituminous. At this point the coal is dark and hard. Anthracite is the last stage of coal formation. Anthracite coal is very hard and shiny.
Fuel Moisture of air dried sample at 40 °C (%) C (%) H (%) N (%) O (%) Calorific value (kcal/kg)
Wood 25 50 6 0.5 43.5 4000-4500
Peat 25 57 5.7 2 35.3 4125-5400
Lignite 20 67 5 1.5 26.5 6500-7100
Sub-bituminous coal 11 77 5 1.8 16.2 7000-7500
Bituminous coal 4 83 5 2 10.0 8000-8500
Semi-bituminous coal 1 90 4.5 1.5 4.0 8350–8500
Anthracite 1.5 93.3 3 0.7 3.0 8650–8700

Analysis of Coal

The composition of coal varies widely. It is necessary to analyse and interpret the results for the purpose of price fixation, commercial classification and proper industrial utilisation.
The quality of coal is ascertained by two types of analysis:
  1. Proximate analysis
  2. Ultimate analysis

1.Proximate Analysis

It is an empirical but important analysis dealing with the determination of moisture content, volatile matter, ash content and fixed carbon. It is called proximate analysis because the data vary with the procedure adopted. This analysis provides data for a general assessment of the quality and type of coal. It involves the following determinations:

1.Moisture Content

A known weight of coal (air-dried) is taken in a crucible and heated in an electric hot air oven at about 105 °C–110 °C for about one hour. After 1 hour, it is taken out from the oven and cooled in a dessicator and weighed. Loss in weight of the sample is found out and the percentage of moisture is calculated as follows:

Let the weight of coal sample = W g
Weight after heating for 1 h = `W_1` g
Loss in weight, that is the weight of moisture = (`W` - `W_1`)
% Moisture = `frac{W-W_1}W` × 100

=Loss in weight of the sample or weight of the moistureWeight of the coal sample taken × 100

Significance

  • High moisture content is undesirable because it reduces the calorific value and increases the transportation cost.
  • Presence of excessive moisture quenches fire in the furnace.
  • A considerable amount of heat is wasted in evaporating the moisture during combustion.

2.Volatile Matte

The volatile matter present in the coal may be combustible gases such as `H_2`,CO,`CH_4` and other hydrocarbons or non-combustible gases such as `CO_2` and `N_2` . It does not include moisture of the coal.

It is determined by heating a known weight of moisture-free coal in a silica crucible covered with a vented lid at 950 ± 20 °C for 7 minutes in a muffle furnace. The crucible is then taken out and cooled inside a dessicator and weighed again. Complete removal of volatile matter is judged by bubbling the gas through a water seal. Loss in weight gives the weight of the volatile matter and the percentage of volatile matter is calculated as follows:

Let the weight of sample after removal of volatile matter = `W_2` g

Weight of volatile matter =  (`W_1` - `W_2`) g%

Volatile matter = `frac{W_1-W_2}W` × 100=Loss in weight due to removal of volatile matter × Weight of the coal sample taken × 100

It may be noted here that it is not correct to say coal with 20% volatile matter or coal containing 20% volatile matter because volatile matter is not present as such in coal but it is the product of thermal decomposition of coal; hence, it should be accurately described as coal yielding 20% volatile matter although the former term is widely used in coal analysis.

Significance

  • Coal containing high percentage of volatile matter burns with a long flame and high smoke and has low calorific value. Coals with high percentage of volatile matter ignite easily but burn very quickly.
  • A high percentage of volatile matter indicates that a large proportion of fuel is burnt as gas.
  • Presence of non-combustible gases is undesirable since they do not add to the heat value.
  • For efficient use of fuel, the outgoing combustible gas (volatile matter) has to be burnt by supplying secondary air. This requires a large combustion space.
  • If the furnace volume is small or flame is short, a large proportion of volatile matter will escape unburnt.
  • Coals with higher percentage of volatile matter do not cake well, whereas medium volatile coals containing 20–30% of volatile matter are capable of forming hard and strong coke on carbonisation.
  • Low volatile matter containing coals do not cake at all and are thus unsuitable for coke making.
  • High volatile matter is desirable in coal gas manufacture because volatile matter in a coal denotes the proportion of coal that will be converted into gas and tar products by heat.
  • However, for the manufacture of metallurgical coke, coal with low volatile matter content is preferred.

3.Ash

Ash is the non-combustible, useless matter that is left behind when all the combustible substances have burnt off from coal. Ash usually consists of silica, alumina, iron oxide and small quantities of lime, magnesia, etc.

Ash content is determined by heating the residue obtained after removal of moisture and volatile matter at 700 ± 50 °C for half an hour without a lid in muffle furnace. The residue left is cooled in a dessicator and weighed. From the weight of residue, the percentage of ash is calculated as

Let the weight of ash formed = `W_3` g

% ash = `frac{W_3}W` × 100 =Weight of ash formed × Weight of the coal sample taken × 100

Significance

  • High percentage of ash is undesirable as it reduces the calorific value of the fuel.
  • Presence of ash increases the transporting, handling and storage cost.
  • It also involves additional cost of ash disposal.
  • Fused ash lumps (clinkers) block the interspaces of the grate on which coal is being burnt. This causes obstruction in air supply. Hence, burning of coal becomes irregular. Hence, lower the ash content better the quality of the coal.

4.Fixed Carbon

Fixed carbon is the material remaining after determination of moisture, volatile matter and ash content. It is determined indirectly by the formula:

Percentage of fixed carbon = 100 – percentage of (moisture + volatile matter + ash).

Significance

1. A higher calorific value corresponds with a higher percentage of fixed carbon.
2. Although they burn slowly, coals with a high fixed carbon content are challenging to ignite.
3. Because fixed carbon burns in its solid state, the percentage of fixed carbon aids in furnace design.

2.Ultimate analysis

1.Carbon and Hydrogen

Carbon in coal sample occurs in combined form in complex organic matter and can also be present as mineral carbonates.Hydrogen on the other hand is found in organic matter in coal and is also found associated with the moisture present in coal. To estimate carbon and hydrogen accurately weighed coal sample is burnt in a combustion tube in a current of `O_2` (free from `CO_2` and moisture).

The carbon and hydrogen present in the fuel is converted to `CO_2` and `H_2O`,respectively.These are then absorbed by previously weighed tubes containing KOH and anhydrous `C_aCl_2` The increase in weights of these tubes gives the amount of `CO_2` and `H_2O`formed.The percentage of C and H is then calculated as follows.

C + `O_2` `rightarrow` C`O_2`
2KOH + C`O_2` `rightarrow` `K_2CO_3` + `H_2O`
`H_2`+ ½ `O_2` `rightarrow` `H_2O`
`C_aCl_2+7H_2Orightarrow C_aCl_2. 7H_2O`
Let the weight of coal sample taken = x g
Increase in weight of KOH tube = y g
Increase in weight of `C_aCl_2` tube = z g
Now,
44 g of  `CO_2` contains 12 g of C
y g of `CO_2` contains `frac{12}{44}` ×y g of C
Percentage of carbon = `frac{12}{44}` × `frac{y}{x}`× 100
or percentage of carbon
= `frac{12}{44}` × Increase in weight of KOH tubeWeight of coal sample taken× 100
Similarly,
18 g of `H_2O` contains 2 g of H
z g of `H_2O` contains `frac{2z}{18}`g of H
Percentage of hydrogen = `frac{2}{18}` × `frac{z}{x}` × 100
Percentage of hydrogen

= `frac{2}{18}` × Increase in weight of CaCl2 tubeWeight of coal sample taken× 100

Significance

  • Calorific value of a fuel is directly related to the carbon content; hence, greater the percentage of carbon, greater is the calorific value of the fuel.
  • Percentage of carbon increases from lignite to anthracite;thus, the percentage of carbon forms the basis of classification of coal.
  • High percentage of hydrogen also increases the calorific value of coal. However, hydrogen is mostly associated with volatile matter and affects the use to which coal is put.
  • In carbonisation and gasification industries, hydrogen of coal is responsible for the production of many useful materials such as gaseous hydrogen, methane, etc.

2.Nitrogen

All the nitrogen present in coal is assumed to occur within the organic matter of coal. Nitrogen present in coal sample is estimated by Kjehldahl’s method.
  • Accurately weighed coal sample is heated with concentrated `H_2SO_4` in the presence of `K_2SO_4` in a long-necked flask known as Kjehldahl’s flask. Nitrogen present in the coal gets converted to `{left(NH_4right)}_2SO_4`
  • When a clear solution is obtained (i.e., whole nitrogen is converted into ammonium sulphate), the contents are transferred into a round bottom flask and the solution is heated with excess of NaOH to liberate `NH_3`.
  • The `NH_3` thus liberated is absorbed in a known volume of a standard `H_2SO_4` solution `left(frac N{10}right)`.
  • The volume of unused `H_2SO_4` is determined by titrating against standard NaOH solution `left(frac N{10}right)`.Difference gives the volume of acid used. From the volume of acid used by `NH_3`,the percentage of nitrogen can be calculated.
`{left(NH_4right)}_2SO_4+2NaOHrightarrow2NH_3+Na_2SO_4+2H_2O`
`NH_3+H_2SO_4``rightarrow` `{left(NH_4right)}_2SO_4`
Let the weight of coal taken = W g
Volume of acid used = `V_1` mL
Normality of acid = `N_1`
As we know,
Equivalents of `H_2SO_4` used = Equivalent of
`NH_3` formed
∵ Equivalents of `H_2SO_4` used = `N_1V_1`
Hence, equivalents of `NH_3` formed = `N_1V_1`
Now,
Normal (1N solution of `NH_3` means 1 g equivalent
`NH_3` dissolved in 1 liter water)
∴ 1000 mL contains 17
g  `NH_3` = 14 g nitrogen.
`N_1V_1` of `NH_3` will contain
= `frac{14}{1000}` × `N_1V_1` nitrogen
% Nitrogen = `frac{14}{1000}` ×  `frac{N_1V_1}W` × 100
% Nitrogen = 1.4×Normality of `H_2SO_4`×Volume of `H_2SO_4` used Weight of coal sample taken× 100

Significance

  • Since nitrogen is an inert and incombustible gas with no calorific value, its presence in fuel is undesirable.

3.Sulphur

Sulphur is found in coal in three forms, as organic sulphur compounds, as inorganic sulphides and also as inorganic sulphates.To estimate the amount of sulphur in the coal sample a known amount of coal is burnt completely in a bomb calorimeter in a current of oxygen. Sulphur in the coal is oxidised to sulphates.

The ash from the bomb calorimeter is extracted with dilute hydrochloric acid. The acid extract is then treated with `B_aCl_2` solution to precipitate sulphate as `B_aSO_4`.The precipitate of `B_aSO_4` is filtered, washed, dried and heated to a constant weight.From the weight of `B_aSO_4` formed, S is estimated as follows

S→`O_2\left(SO_4\right)_2`→`Ba\left(Cl_2\right)Ba\left(SO_4\right)`

Let the weight of coal sample=Wg

Weight of

`Ba\left(SO_4\right)`=xg

∵ 233g of `Ba\left(SO_4\right)`=32g of S

∴ xg of `Ba\left(SO_4\right)`=32233 × [x]

%S=32233 × xw × 100 or %S=32233×Weight of `Ba\left(SO_4\right)`× Weight of coal taken×100

Significance

  • Although sulphur increases the calorific value of fuel, yet its presence is undesirable because it gets oxidised to `SO_2` and `SO_3` which causes environmental pollution.
  • Sulphur-containing coal is not suitable for preparing metallurgical coke. Presence of sulphur in coke used in the iron industry affects the quality and properties of steel.

4.Ash

  • It is determined in the same way as in proximate analysis

5.Oxygen

It occurs both in organic and inorganic portions of coal and is determined indirectly by deducting the % of C, H, N, S and ash from 100% of oxygen = 100 − % of ( C + H + N + S + ash).

Significance

  • Oxygen is present in coal in combined form. It is present in association with hydrogen; hence, it reduces the hydrogen available for combustion.
  • Moreover, high oxygen-containing coals have high inherent moisture and hence low calorific values.
  • Calorific value decreases about 1.7% for every 1% increase in oxygen.Thus, a good-quality coal should have low percentage of oxygen.

Conclusion

Ultimate Analysis stands as a cornerstone in the field of chemical engineering, providing engineers with a lens to scrutinize and optimize the elemental composition of substances. Through this meticulous exploration of the chemical landscape, chemical engineers are empowered to drive innovation, improve efficiency, and contribute to a sustainable future. Ultimate Analysis remains an invaluable compass, guiding the way to a world where the manipulation of matter is both precise and purposeful.

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