Amorphous Allotropes of Carbon

Amorphous Forms of Carbon

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Allotropy is defined the existence of an element in two or more different physical forms in the same physical state, with the different forms (allotropes) having different physical properties but the same chemical properties.

Carbon exist as allotropes. These are grouped into:

1. Crystalline allptropes of carbon: Examples are diamond, graphite and fullerenes. Please read more on the crystalline allptropes of carbon here.

2. Amorphous allotropes of carbon.

 

Amorphous forms (or allotropes) of carbon look shapeless and lack a macro-crystalline structure. They do not have a regular crystal lattice; but rather, consist mainly of graphite in a very fine, impure and disordered form.

Below are examples of amorphous forms of carbon:

• Coal: They are formed from fossilized plant remains over millions of years.

• Coke: They are obtained from the destructive distillation of coal (that is, heating coal in the absence of air)

• Charcoal: They are obtained from the destructive distillation (or incomplete combustion) of wood.

• Animal charcoal (or bone charcoal): They are obtained by heating bones in the absence of air. They serve as good adsorbent, are and used in sugar refining.

• Soot / Lamp black / Carbon black: They are fine carbon powder formed from incomplete combustion of carbon compounds (e.g., burning oil/kerosene with insufficient air). They are used in making printing ink, shoe polish, and tyres (as rubber reinforcement).

 

The table below shows the differences between crystalline and amorphous allotropes of carbon.

Feature	Crystalline Allotropes of Carbon	Amorphous Allotropes of Carbon
Atomic Arrangement	Regular, definite, repeating 3D pattern with long-range order	Irregular, random arrangement with only short-range order
Hardness and Rigidity	They are highly rigid and often very hard (e.g, diamond)	They are usually softer and less rigid
Purity	They are pure forms of carbon	They often contain impurities
Melting Point	They have high and more precise (definite melting point)	They have lower melting points, and melts gradually over a wide temperature range.
Shape and Geometry	They have well-defined geometric shapes and sharp edges	They lack a definite geometric shape (powdery or irregular)
Conductivity	Graphite conducts heat and electricity.	Generally poor conductors of heat and electricity
Examples	Diamond, Graphite, Fullerene and Graphene	Coal, Charcoal, Coke, Lampblack (soot)

 

Please read the introduction to carbon and crystalline allotropes of carbon here.

 

Below are explanations on the amorphous forms of carbon.

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Coal

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Coal is a hard, black, combustible sedimentary rock formed from the compressed, fossilized remains of plants that lived and died millions of years ago, subjected to heat and pressure underground in the absence of air. This process of coal formation is termed carbonization.

The image below shows the physical structure of coal.

 

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Types of Coal (in increasing order of carbon content/age)

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1. Peat: This is the first stage of coal type. It is a soft, brown, spongy, partially decayed plant material containing the lowest carbon content (~50–60%). It has a low heat value.

2. Lignite. It is often termed "brown coal". It is soft and brownish-black; with carbon content about 60–70%. Lignite still shows plant texture.

3. Bituminous coal: It is a dense and black; with carbon content about 70–85%. It is the most commonly mined and used type. It burns with a smoky flame.

4. Anthracite: This is the hardest, shiniest, and oldest type of coal. It has the highest carbon content (over 90–95%); and burns with little smoke. It gives the most heat.

You can read on relative acidity and alkalinity (pH scale) here.

 

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Properties of Coal

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• Coal has a color that varies from brown (in lignite) to dull or shiny black (in bituminous coal and anthracite).

• It has variable hardness. It is soft and crumbly in lower ranks like lignite, becoming tough and hard in higher ranks like anthracite.

• Its density ranges from 1.1 to 1.5 g/cm³, making it slightly denser than water and lighter than common rock minerals.

• It is made primarily of carbon (~ 50% to 90% by weight, serving as the primary heat source. Other elements comprising coal include hydrogen (2% to 40%), Oxygen & Nitrogen (~2% and 12%), and sulfur (0.5% to 3%).

• Coal contains a network of macropores, mesopores, and micropores, giving it high internal surface areas which can be useful for gasification and filtration.

You can read on mixtures and characteristics of mixtures here.

 

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Location of Coal Mines in Nigeria

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Nigeria's coal deposits are found mainly in the eastern and central parts of the country: Below are the states where they are found:

• Enugu State: This is the major historical coal-producing area of Nigeria: It include places like Iva Valley, Onyeama, Okpara, and Ribadu mines. Enugu is often called the "Coal City" because of this.

• Kogi State: It include places like Okaba and Lafia-Obi coalfields.

• Benue State: Include places like Orukpa/Owukpa coal deposits.

• Gombe State: Include the Dadin-Kowa coal deposit.

• Plateau State: Coal deposits are found around the Plateau area.

• Anambra State: Coal deposits in areas around the Anambra basin.

• Smaller deposits of coal are also found in Abia and Adamawa states.

Note: Coal mining in Nigeria began in Enugu in 1909 under the colonial government; and was historically managed by the Nigerian Coal Corporation (NCC).

You can read on bases and alkalis, its classification, physical.and chemical properties here.

 

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Destructive Distillation of Coal and Wood

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Destructive distillation is defined as the strong heating of an organic substance in the absence of air (oxygen), so that it decomposes into simpler, more useful products, instead of burning completely to ash.

 

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Destructive Distillation of Coal

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When bituminous (soft) coal is strongly heated in a closed retort/oven without air, it breaks down into the following:

1. Coke: This is the solid residue left behind (mainly carbon). It remains in the retort.

2. Coal tar: This is a thick, black liquid mixture of many organic compounds (benzene, naphthalene, phenol, etc), which are condensed from the vapours.

3. Ammoniacal liquor: This is a liquid containing ammonia and its compounds, also condensed out of the vapours.

4. Coal gas: This is a mixture of gases that do not condense. They largle include hydrogen gas (H₂), methane (CH₄), carbon monoxide (CO), and small amounts of other hydrocarbons; used as fuel gas.

Below is a simple summary equation of the destructive distillation of coal in word form:

Coal (heated, no air) → Coke + Coal tar + Ammoniacal liquor + Coal gas

Please read on the physical and chemical properties of acids, test and uses of acids here.

 

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Destructive Distillation of Wood

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When wood is heated strongly in the absence of air, it decomposes to give the following:

1. Charcoal: This is the solid carbon residue.

2. Wood tar: This is a liquid mixture of organic compounds.

3. Pyroligneous acid (wood vinegar): This is an aqueous liquid containing acetic acid (CH₃COOH), methanol (CH₃OH), wood spirit/wood alcohol, and acetone (C₃H₆O).

4. Wood gas: This is a mixture of combustible gases such as methane, carbon monoxide, hydrogen, etc. They are used as fuel.

Below is a simple summary equation of the destructive distillation of wood in word form:

Wood (heated, no air) → Charcoal + Wood tar + Pyroligneous acid + Wood gas

You can read on salts in chemistry, their classification, solubility and colors here.

 

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Coke and Gasification of Coke

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Coke is the solid, porous, greyish-black residue left after the destructive distillation of coal. It is almost pure carbon (with some ash and small amounts of sulphur and other impurities). It can serve as a good, smokeless fuel.

Coke shows the following properties:

• It is a hard, porous, and black or greyish-black solid.

• It is smokeless when burnt.

• It has a high carbon content, giving high calorific (heat) value.

• It does not soften or melt when heated further (unlike the original coal).

• It has a high crushing strength (that is, high impact resistance). This implies that coke can resist fracturing from heavy impact forces (and the weight of the blast furnace burden, that is; iron ore and limestone).

• Its bulk specific gravity is typically around 0.77.

You can attempt various questions in chemistry here.

 

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Gasification of Coke (Manufacture of Water Gas and Producer Gas)

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Gasification refers to the process of converting solid coke into combustible gaseous fuels by reacting it with steam and/or air at high temperature.

Gasification of coke is applied in the manufacture of producer gas and water gas.

 

1. Water gas

Water gas is manufactured by passing steam over red-hot coke (at about 1000 °C). This is shown via the equation below:

• C_(s) + H₂O_(g) → CO_(g) + H₂_(g)

Water gas is a mixture mainly of carbon monoxide (CO) and hydrogen (H₂)

The above  reaction is endothermic (meaning that it absorbs heat from the surroundings). As a result, the carbon must be kept red-hot by an external heat source or alternating air blasts. Since the coke cools down as the reaction proceeds, the process must be interrupted periodically to reheat the coke (usually by blowing hot air through it, which is how producer gas is made in the same cycle).

Water gas is a mixture mainly of carbon monoxide (CO) and hydrogen (H₂). It is a valuable industrial fuel and source of hydrogen (e.g., for the Haber process and for hydrogenation reactions).

Please read on rate of chemical reaction here.

 

2.  Producer gas

Producer gas is manufactured by passing a limited (insufficient) supply of air over red-hot coke. This is shown via the equation below:

• 2C_(s) + O₂_(g) → 2CO_(g) (with some nitrogen from the air remaining unreacted in the gas mixture)

Producer gas is mainly a mixture of  carbon monoxide (CO) and nitrogen (N₂).

This reaction is exothermic (meaning that heat is given off to the surrounding), and it reheats the coke bed, allowing the process to alternate with water-gas production.

Producer gas is mainly a mixture of carbon monoxide and nitrogen, (with the nitrogen acting as a diluent, giving it a lower heat value than water gas).

You can read on chemical (dynamic) equilibrium in chemistry here

 

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Uses of Coke

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• It is used as a smokeless domestic fuel.

• It is used as a reducing agent in the extraction of metals, especially in the blast furnace for smelting iron ore. In this reaction, coke reduces iron(III) oxide to iron and also burns to provide heat. This is shown via the equation below

Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)

• It is utilized in the manufacture of water gas and producer gas (industrial fuel gases).

• It is used in the manufacture of calcium carbide (CaC₂), by heating coke with quicklime (CaO) in an electric furnace . Calcium carbide is used to produce ethyne (acetylene) gas.

• It is used as a source of carbon in various metallurgical and chemical industries.

• It is used in electrodes for some electrochemical processes.

You can read on ionic theory and electrolysis here.

 

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Uses of Coal

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• It is used as a domestic and industrial fuel for heating and generating steam.

• It is used in thermal power stations for the generation of electricity.

• It is used as a raw material for the destructive distillation industry, (to obtain coke, coal tar, ammoniacal liquor and coal gas).

• Coal tar is a derivative of coal used in making dyes, drugs, perfumes, explosives (e.g., TNT), disinfectants (e.g., phenol), and synthetic fibres.

• It is used in the iron and steel industry (as coke) for smelting.

• It is used in the manufacture of synthetic petrol and other chemicals.

Please read the next topic (oxides of carbon CO and CO₂) here.