Types of Solid Fuels: A Detailed Overview Guide

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If you’ve done enough research and are still not satisfied with the results, your search ends here. I invite you to join Engineers Rail for a journey that will inform you about Solid fuels in a detailed manner.

You will learn about Solid fuel’s definition, types, uses, benefits and much more.

I will start with the basics first to get you familiarised with the topic. So let’s get right to the meat of the post without wasting too much of your time.

Let’s start…

Solid Fuel-

A solid form of fuel which burns itself to produce energy and light with the help of combustion such as wood, dung, Charcoal, and coal, Known as Solid Fuels.

In solid fuel, coal is the most important fuel. Some of the other useful and important solid fuels are tree bark, bushes, bagasse etc.

• Must Read:Sources of Energy- Introductions, Types, Sources Available and Much More

Classification of Solid Fuels-

There are three types of solid fuels given below-

1. Coal
2. Solid Fuels other Coal
3. Artificial or Manmade Solid Fuels

1. Coal-

A fossil or solid fuel, coal is made of vegetation that was compressed between other rocks and changed over millions of years by the combined actions of pressure and heat to form coal.

The energy that coal provides, was once absorbed by plants from the sun millions of years ago.

Here, I am not going deep into this type, Because I have written a detailed guide about Coal. I strongly recommend checking that Article ones.

Here is the link to go-

• Classification of Coal: The Ultimate Guide You Will Never Read

Now, Let’s move to the second one-

2. Solid Fuel Other than Coal-

This type of solid fuel is divided into five categories, Which is-

1. Wood
2. Bush of corn
3. Straw and Bushes
4. Bagasse or Megass
5. Tankbark

I. Wood-

It is a natural fuel obtained from trees. Normally weak, waste, non-structural wood, its chips and dust are used as fuel. Freshly cut wood contains about 50% moisture which is dried before using it as fuel.

The calorific value of dried wood is 18 MJ/kg to 21 MJ/kg, whereas that of wet wood is 10.8 MJ/kg. Its specific density is between 0.3 to 1.3 and One kg of wood gives the energy of about 0.4 kg of coal.

The calorific value of wood depends upon the type of wood, its moisture content, structure, type of furnaces, combustion method etc.

II. Bush of Corn-

Bush of corn obtained from maize, wheat bushes, rice husk etc, are also used as a domestic fuel in villages. Its calorific value is about 12.6 MJ/kg.

When burned, they emit soot and smoke. It is a cheap fuel and very useful in villages.

III. Straw and Bushes-

The straw of different crops like rice, wheat, bushes, leaves etc, can be used as fuels by mixing them in dung and moulding to form blocks.

The moulding process is done on machines to press them and give different shapes and sizes for ease to burn them as fuel.

Wet straw and bushes have a calorific value of 12.6 MJ/kg, whereas after drying it increases fro 14 MJ/kg to 16 MJ/kg.

The composition of Straw and Bushes by weight is:

IV. Bagasse or Megass-

In most sugar mills, Bagasse is left as waste after extracting cane juice. This bagasse is dried and used as fuel for boilers.

The flue gases of the chimney of the boiler are used to dry up the bagasse, thereby saving coal and other fossil fuel to a large extent.

The bagasse can also be used for making other things like boards etc. One kilogram of cane gives about 0.25 kg of bagasse.

Its calorific value depends upon the moisture contained in it. After drying its calorific value is about 1.7 MJ/kg.

The composition of Bagasse by weight is:

V. Tankbark-

The bark of trees, rubber and leather etc. can be used as fuel. It is normally dried up by flue gases of boilers before using as fuel.

The wet bark has a calorific value of 12.8 MJ/kg which after drying, increases up to 21 MJ/kg.

The composition of Tankbark by weight is:

3. Artificial or Manmade Solid Fuels-

Solid Fuels Solid fuels found in nature can be refined and purified to convert them into artificial solid fuels. This fuel is better than natural fuel in quality and is easy to use.

In natural solid fuels, there are several elements like moisture, ash or volatile matter which reduce the calorific value of fuel, thereby making it difficult to burn. Such unwanted elements are controlled by artificial fuels.

In fabricating artificial fuels, their shape and size are made in such a way that they can be used and transported with ease. Their calorific value is more and smoke as well as ash is less.

This type of solid fuel is also divided into Five categories which are-

1. Pulverised Coal
2. Briquetted Coal
3. Caking and Non-caking coal
4. Coke
5. Charcoal

Now, Let’s look at these types one by one-

I. Pulverised Coal-

Low-grade coal containing ash in large quantities is ground into fine particles to form pulverised coal. First, the coal is dried up then it is broken into pieces and put in pulverising machines to grind into fine powder.

Pulverised coal makes better contact with oxygen in a larger area so that its complete combustion takes place with ease. Pulverised coal has a higher calorific value.

Its transportation within the plant can be done by screw conveyors or by compressed air with ease. Pulverised coal has wide applications in metallurgical processes and the cement industry.

Advantages of Pulverised Coal-

The following advantages are encountered in using pulverised coal-

• Complete combustion of coal is possible due to increased contact area.
• Low-grade coal can be used for combustion.
• The combustion rate of coal can be regulated.
• Less expenditure on transportation and transfer of coal
• Calorific value and efficiency increase.
• Heat losses are reduced.
• High temperature can be achieved.

II. Briquetted Coal-

This type of coal is prepared from fine ground coal by mixing it with binding materials or without such materials in moulds under pressure.

The binding materials generally used are pitch coal tar, lime, crude oil, starch, clay, water etc.

To prepare the briquets, the fine ground coal is mixed with wood powder, rubber, leather bark Bagasse paper, pieces of cardboard, straw and bushes etc.

These materials are first dried to reduce the quantity of moisture in them. These are grounded and mixed with binding materials. Then briquets are prepared in specially constructed presses by moulding and pressing.

These briquets are then heated and dried so that the moisture level is reduced to a minimum. The briquets are made in different sizes and shapes as per the facilities of their burning in the furnace.

These shapes may be rectangular, cylindrical, cubical, elliptical, triangular etc.

Briquets are classified by their shape and the fuel used. Generally, briquets are made in small sizes. Its maximum mass should not be more than 3 kg.

Different briquet shapes are shown in Fig-

Advantages of briquet Coal-

The main advantages of making briquets out of coal are:

• Equal temperature and higher calorific values are obtained.
• The combustion rate is high.
• Low-grade coal and other types of fuel like wooden chips, dust etc, can be used.
• Transportation is easy.
• Briquet coal is easy to use and the wastage of fuel in the form of chips/powder is less.
• Clinkers are not formed by fuel combustion.
• Due to the rounded shape, airflow is easy during combustion.
• Due to the similar shape of briquets, their combustion is uniform.
• The emission of dust particles passing through chimneys is less so pollution is controlled.
• Due to the same-size of briquettes, it is easy to feed them in the furnace.

III. Caking and Non-caking Coal-

During the combustion of coal, the tendency to formation of clinkers is called caking. 

Caking coal becomes soft when hot and fuses to form coke in the form of large pieces. If the residue of the above process, which is coke, is porous, strengthful and reusable than the primary coal is called caking coal.

Non-caking coal does not fuse, so large pieces and clinkers are not formed. Caking properties increase with the increase in the quantity of solid carbon.

Caking coal contains volatile matter and hydrocarbons, so it is used for gas production. Non-caking coal burns with blue flame without melting.

It is mostly used in steam boilers. Its calorific value is 28 to 35 MJ/kg.

A good quality caking coal contains 83 to 90% carbon and traces of ash, sulphur and phosphorus.

IV. Coke-

Coke is prepared from coal by its carbonization. It is obtained by burning bituminous coal at a very high temperature in the absence of air.

In this process, gases, tar, water vapours, acids, and bases are emitted and coke remains at the end as residue. Coke has different properties than the original coal. This process is also known as dry distillation or fractional distillation.

It is of the following two types:

• Low-temperature carbonisation or semi-coking
• High-temperature carbonisation or coking

Low-temperature carbonisation or semi-coking-

In the low-temperature carbonisation, process coal is heated in an oven up to 550°C. Coke is obtained from bituminous, lignite and peat varieties of coal.

In this process, semi-coke is obtained as a byproduct which contains 10 to 15% volatile material.

Being a brittle material it cannot be used for metallurgical processes. Its transportation is also difficult to carry out.

High-temperature carbonisation or coking-

High-temperature carbonisation consists of heating coal in an oven up to 1100°C in the absence of air. In this process, coke is also obtained as a byproduct.

During the carbonisation process, combustible coke gas and coal-tar are also produced. These combustible gases have a calorific value of about 19 MJ/m’ and they are used in the manufacture of plastic and rubber.

From coal-tar, liquid-fuel oil is produced which is an important ingredient in the chemical industry. Coke is brownish-black or light grey in colour.

It is hard and porous with a rough surface. It contains about 90% carbon and 1.3% volatile material. Its calorific value is 27 to 30 MJ/kg.

About 90% of coke is obtained from normal coal. It burns with a short flame.

Advantages of Artificial Solid Fuel-

The benefits of using artificial or manmade coal are as follows:

• Utility of fuel increases.
• The combustion process becomes easy. 
• Higher calorific value is achieved.
• Less smoke is produced during combustion, thereby reducing pollution.
• Less ash is produced.
• The effect of atmospheric changes is reduced.
• Transportation becomes easy and wear and tear reduce.
• Suitable to be burnt in furnaces.

• Must Read: Liquid Fuels: A Detailed Guide All you Need

Wrapping Up-

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Abhishek Tiwary

Abhishek Tiwary is a blogger by passion and a Quality Engineer by profession. He completed his B.Tech degree in the year 2017. Now working in a reputed firm. He loves to share his knowledge with others.