Topic: Amorphous Forms of Carbon
Coal is formed in nature by the carbonization of wood. Conversion of wood to coal under the influence of high temperature, high pressure and in the absence of air is termed carbonization.
Amongst coal varieties, anthracite is the purest form. It contains about 94 – 95% of carbon. The common variety is bituminous coal; it is black, hard and burns with smoky flame.
Types of Coal
We have four types of coal namely:
- Peat – Like coal which is about 60% carbon by mass
- Lignite – Which is a brown soft coal having about 67% carbon by mass
- Bituminous – Which we use every day at home. It contains about 88% by mass of carbon
- Anthracite – This is tough and hard, having about 94% of carbon by mass
Destructive Distillation (Carbonisation) of Coal
Coal is a complex mixture of compounds composed mainly of carbon, hydrogen and oxygen with small amount of nitrogen, sulphur, phosphorus and impurities. A wide variety of substances can be obtained from it by a process known as destructive distillation of coal
The chief and important products of this process are coal gas, ammoniacal liquor, coal tar and solid coke
- Coal Gas: This is the volatile compound which contains about 50% hydrogen, 30% methane, 10% carbon (II) oxide and small amount of other gases e.g. ethane and hydrogen sulphide
- Ammoniacal Liquor: Ammoniacal liquor consists essentially of ammonium compounds and benzene. The ammonium compounds, can be used in the manufacture of nitrogenous fertilizers while the benzene is used for the manufacture of pharmaceutical products and as a solvent
- Coal Tar: Coal tar is a mixture of more than 200 different substances which can be separated by fractional distillation. Most of these e.g. toluene, phenol and naphthalene are used in the synthesis of important commercial products like dyes, paints, insecticides, drugs, plastics and explosives
- Coke: The solid residue, coke with its 98% carbon by mass burns smoothly without smoke and with high calorific value. It is used in the manufacture of gaseous fuels such as producer gas and water gas. It is also used as a reducing agent in metallurgy where it reduces metallic oxides to their respective metals e.g. in the blast furnace for the extraction of iron from its ore
Coal is mainly used:
- As an industrial fuel in steel, power generation plants etc. It is also a domestic fuel to a limited extent.
- For manufacture of producer gas and water gas, which are used as fuel gases.
- For manufacturing coal tar, coke and coal gas.
- Anthracite coal is used for preparing graphite.
- For the manufacture of synthetic petrol by catalytic hydrogenation of coal.
Destructive Distillation of Wood
Wood is a complex substance like coal except that the percentage composition of the elements present in it is different. For example, wood has a higher percentage of carbon than coal. Destructive distillation of wood yields these four fractions:
Wood —> Wood charcoal + Pyroligneous acid + Wood tar + Wood gas
Pyroligneous acid, which in the liquid fraction contains mainly ethanoic acids, propanone, methanol and some other compounds
Charcoal can be made by heating wood, nut shells, bones, sugar and even blood
When wood is heated strongly in a very limited supply of air, wood charcoal is obtained. This is called destructive distillation of wood. The volatile products are allowed to escape.
Charcoal is a black, porous and brittle solid. It is a good adsorbent. Charcoal powder adsorbs colouring matter from solutions and poisonous gases from the air. Charcoal is also a good reducing agent.
- As a fuel.
- As a deodorant and in gas masks to filter pollution.
- As a discoloring agent for decolorizing oils, etc.
- In making gun powder.
Animal charcoal (or Bone charcoal) is obtained by destructive distillation of bones. It contains about 10-12% of amorphous carbon.
It is obtained by heating sugar in the absence of air. Sugar charcoal is the purest form of amorphous carbon.
Sugar charcoal becomes activated charcoal when it is powdered to particle size of about 5 micron and heated at about 1000 K in vacuum. Activated charcoal has an increased adsorption capacity.
Lamp black is manufactured when tar and vegetable oils (rich in carbon) are burnt in an insufficient supply of air and the resulting soot is deposited on wet blankets hung in a room.
Lamp black is a velvety black powder. It is used in the manufacture of Indian ink, printer’s ink, carbon papers, black paint and varnishes.
When natural gas is burnt in limited supply of air, the resulting soot is deposited on the underside of a revolving disc. This is carbon black and it is then scraped off and filled in bags.
It differs from lamp black in being not so greasy. Carbon black is added to the rubber mix used for making automobile tyres, and has replaced the use of lampblack for a number of purposes.
Gas carbon and petroleum coke
Carbon scraped from the walls of the retort used for the destructive distillation of coal is called gas carbon. During refining of crude petroleum, petroleum coke is deposited on the walls of the distillation tower.
Both, gas carbon and petroleum coke are used for making electrodes in dry cells and are good conductors of electricity.
Producer Gas: Producer gas is a mixture of nitrogen and carbon (II) oxide. This can be obtained by passing air over red hot coke in a furnace. The oxygen in the air oxidizes the coke to carbon (II) oxide. The reaction is exothermic and a large amount of heat is given off. Some carbon (IV) oxides may be formed but this is usually reduced by the hot coke to carbon (II) oxide
2C(s) + O2(g) + N2 —–> 2CO(g) + N2
The mixture of the carbon (II) oxide (one third by volume) and the unchanged nitrogen from the air forms the producer gas which is tapped off as a ready gaseous fuel at the top of the furnace
Producer gas has a low heating power because it contains about 67% non-combustible nitrogen and 33% carbon (II) oxide. It is not expensive and it is widely used to heat furnaces. It is also a source of nitrogen for the manufacture of ammonia (Haber process)
Water Gas: Water gas is a mixture of equal volumes of carbon (II) oxide and hydrogen. This is produced by passing steam over coke in the furnace while it is still hot (at about 1000oC)
C(s) + H2O(g) ——> CO(g) + H2(g)
Both the hydrogen and the carbon (II) oxide in water gas burn in air releasing a lot of heat. Water gas has a high calorific value because its component gases are combustible. The demand for it is high and competitive