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Boiler Fuels & their property-Mechanical Engineering Notes

Boiler Fuels & their property-Mechanical Engineering Notes

Boiler is a contrivance which converts water to steam by utilizing heat energy. The production of this heat energy which is utilized for the production of steam, can happen via nuclear energy & solar radiation, but combustion of a fuel in the presence of oxygen is the most preferred way. 

Combustion can be defined as a rapid chemical reaction between oxygen & a fuel. This fuel can be in solid, liquid or gaseous form. Atmospheric air is the source of oxygen required in the process of combustion. 

Boiler is a highly popular mechanical device which finds its application in a range of residential & industrial setups. All type of boilers irrespective of their application & size, have a boiler specification and use a specific boiler fuel type.

Boiler configuration is related to the boiler’s “firing practice”. Boiler firing practice means the type of heating element which is used as the boiler’s source for converting water to steam or it can be thermal oil which is circulated through the piping of the facility where the boiler is being used.

There are 5 types of boiler configurations:

  1. Cast iron
  2. Fire tube
  3. Water tube
  4. Tubeless
  5. Hot water

Boiler configuration & boiler fuel type influence the efficiency, costs, maintenance requirements etc. 

Irrespective of the type of boiler configuration, all boilers use one of the following fuel types:

  1. Diesel/Fuel Oil
  2. Solid Fuel
  3. Natural Gas or Propane
  4. Electric
  5. Renewable Energy (Biomass)

Each of the above fuel types has a boiler fuel value, which is a function of:

  1. Fuel type efficiency (Btu)
  2. Fuel cost
  3. Emissions

Some fuels are very economical but they produce low boiler thermal efficiency. On the other hand some fuels have high Joule-per-unit-of-measure outputs or Calorific Value but they are very expensive. We can also have some boiler fuel types which are highly economical and have high Btu outputs, but they have very high emissions.

So when we are determining the appropriate boiler fuel type, we have to balance the efficiency, cost & emissions. 

Fuel Oil Boiler

Fuel Oil is divided into 6 types: distillate types of diesel and residual oils. Both distillate diesel & residual oils are divided into numbered sub-categories, Number 1-6.

boiler fuel

Fuel oil boilers can work in all boiler configurations & serve well for both residential & industrial applications. The most commonly used fuel oils to fire boilers are number 2 distillate diesel & number 6 residual oil, also known as “bunker fuel”. 

The most important value of fuel oil boilers is energy density, which is measured in Btu. The fuel oil energy density is very high. Due to the high energy density of diesel & residual oils, fuel oil boilers are considered to be highly efficient. If we add a pre-combustion fuel catalyst, the efficiency can be further increased. 

Propane boiler

Fuel oils are fossil fuels, and amid a high demand to save fuel oils, propane comes as the next best alternative as a boiler fuel which provides high efficiency and is easy to resource. 

Because propane is a by product of Natural Gas, doubts are raised on its negative impact on the environment. Apart from this, propane is also comparatively expensive with respect to other boiler fuels. 

Propane boilers can work for 3 out of 5 boiler configurations, namely water tube, fire tube & cast iron. In addition to being expensive in comparison to fuel oil & diesel, propane generates less amount of energy that a comparable unit of number 2 diesel oil. 

The fuel density of propane is approximately 65% of the fuel density of fuel oil. 

Natural gas boiler

Natural gas boiler finds its most common application in residential installations. The main reason for this is the easy availability of natural gas and it is economical as well.

Like propane the energy density of natural gas is low in comparison to number 2 diesel, only 62%. In general, the energy density of natural gas is less than 50% of that of a comparable unit of fuel oil. 

One of the biggest (almost 80%-90%) components of natural gas is methane. Methane is a greenhouse gas which is highly dangerous for the environment. 

Electric boiler

These boilers are highly efficient, as they don’t make use of flue gases and there is no heat loss due to flue gas leakage. 

In addition to being highly efficient, electric boilers are also highly economical & readily available. Due to the cost of electricity, the application of electric boilers is only limited to residential use. 

Although, an electric boiler does not give out harmful emissions, but electricity is produced by burning coal or from nuclear energy. So indirectly, electric boilers also contribute to the environmental pollution. 

Biomass boiler

Biofuel is a renewable source of fuel. Essentially, biofuel boilers are wood burning boilers, but the wood comes in the form of pellets, wood chips, or logs. Biomass boilers use discarded and recycled wood.  

Due to use of recycled product, biofuel is expensive when compared to other fuels. 

Also, biomass has the lowest energy density among all boiler fuel types. Its energy density is 38% less than that of coal. This indicates a low return on Btu. 

Coal boiler

This falls into the category of solid fuel boilers, which finds its application more attractive for residential installations due to their low cost & economical fuel. 

But, coal boilers produce high emissions and a considerable amount of waste products like fly ash & bottom ash. In addition to this, the energy density for coal is also very low. 

The energy density of coal depends upon the type of coal:

  1. Anthracite
  2. Bituminous
  3. Sub-bituminous
  4. Lignite coal

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