All boiler systems employ a fuel mechanism which basically comprises of the apparatus needed to supply fuel for heat generation. The design of the apparatus in use varies according to the kind of fuel employed in the system. Variety of fuels is available for application in boilers, each having different chemical properties. Major chemical characteristics of boiler fuels include:
- C/H2 ratio: It is a unique ratio which mainly determines the quantity of supply air needed for absolute combustion of a particular fuel source. More is the value of carbon in the fuel; excess supply air would be needed for carrying out whole burning process.
- Calorific value: “The calorific value is the quantity of heat obtained per kilogram for solid or liquid fuel or per m3 for gaseous fuel when burnt with an excess of oxygen in a calorimeter.”1
By carrying out a calorific value test, one can distinguish between high and low calorific value fuels. In general, the combustion reaction products containing H2O in the form of liquid indicate Higher Calorific value whereas H2O in vapor form indicate Lower Calorific value.
Each and every fuel whether it is in solid, liquid or gaseous form is explosive and can prove hazardous if not used according to the recommended safety guidelines. “When monitoring the efficiency of a combustion process, it is important to know the fuel being burned since this information will help not only determine a boiler’s optimal working conditions but also maximize the boiler’s efficiency.”2
Commonly Used Fuels
The list of major fuels which are employed in boiler systems is given below:
- Natural Gas
- Solids Fuels like Coal & Wood
- Renewable Energy
Electricity is an alternative source to gaseous fuels employed in traditional boiling systems. It tends to give a heating value around 3.4kBTU per kWh. Use of electricity in boiler systems offers various advantages which are mentioned below:
- Usage of electricity as boiler fuel results in extremely compact design and light weight boiler systems which can be operated at comparatively less cost.
- Besides, it results in entirely noise free boiler operation.
- It can be easily employed in various kinds of heating and cooling devices, for example, basic resistance heating systems, air conditioning systems and heat pumps etc.
LPG (Liquid Petroleum Gas)
In practice, LPG is mainly used in applications where the availability of natural gas is either very limited or costly. LPG works as a boiler fuel in similar manner as natural gas. However, the boiler in use must be capable of conversion features so that it could be made compatible with LPG.
Major solid fuels used for burning in a boiler include coal and wood. They were the only fuel sources available for use in boiler systems prior to the emergence of heating fuel oil. These are the cheapest means of boiler fuels which are getting exhausted day by day due to uncontrolled use. However, boiler systems employing coal or wood as fuel source call for some extra precautions and care due to the reasons mentioned below:
- The use of solid fuels results in more dust and ashes as compared to heating oil and natural gas.
- Since the amount of carbon content present in coal is very high, it produces a substantial amount of carbon dioxide gas upon burning.
- Also, presence of high carbon value needs extra oxygen for combustion because of which relatively high amount of combustion air would be required for burning of coal as compared to other boiler fuels.
- Moreover, burning of coal inside a boiler results in emission of polluted ingredients such as NOx, sulfur dioxide i.e. SO2, sulfur trioxide i.e. SO3 etc. Typically, a chemical reaction takes place between emitted sulfur dioxide and water vapor present in the atmosphere due to which a feeble type of sulfuric acid is created that happens to be one of the major reasons for acid rain.
Different forms of coal are available for burning process, most common among them are listed below:
With the advent of fuel oil, solid fuels such as coal and wood increasingly got replaced in nearly all parts of the world owing to its cleaner and ash free combustion process. Oil fuels generally consist of less carbon content as compared to solid fuels such as coal, which in turn results in less emission of carbon dioxide upon combustion. On the other hand, oils contain higher carbon content as compared to natural gas producing high amounts of carbon dioxide due to burning.
The fuel oil employed for boiler use is mainly manufactured from a mix of extremely heavy hydrocarbons, which tend to contain relatively high amounts of hydrogen content in comparison to coal. Burning of a fuel oil usually produces same kind of pollutants as produced with burning coal. Heating oil is a boiler fuel which is widely employed in the northeastern areas of the United States whereas in other parts of the world, it is facing severe competition with the gaseous boiler fuels available.
Diversity of oil fuels is available for heating such as oil #2, oil # 4, and oil # 6. Fuel oil #2 is popularly referred to as the home heating fuel. It is almost identical to the diesel oil fuel which is largely employed in vehicles and automobiles. Its energy value is found to be approximately 139 kBTU per gallon.
Boiler and heating systems that employ oil for its operation happen to be more expensive than gas powered boiler systems since they need complicated burner mechanism as compared to their gas counterparts for efficient firing. However, at the same time, this difficulty of ignition (or firing) in case of heating fuel turns out to be a great plus point since it results in safer storage of fuel oil in comparison to gas. Otherwise, a leakage in the fuel tank could prove to be very costly and hazardous.
Natural gas is the key fuel source for boilers which is widely employed in United States and Europe for home heating needs. It is largely prepared from methane along with a mixture of few other gases in small proportions. “Natural gas has an energy content of about 100kBTU per therm or 103kBTU per ccf (100 cubic feet). However, anecdotal evidence suggests that the actual heating value of “Natural Gas” coming out of the distribution pipe may vary from as little as 60kBTU up to 160kBTU per ccf.”3
Natural gas can be conveniently put into use for boiler applications since it can be transported easily via gas pipelines when in gaseous sate and trucks or ships when in liquid state. Very less amount of air is needed for burning of natural gas owing to its unique C/H2 ratio. This fuel contains quite low values of carbon and high values of hydrogen because of which the combustion of natural gas results in production of less greenhouse gases which are considered to be highly responsible for global warming. Also, the burning of natural gas is found to be very clean as compared to the burning of oil and solid fuels oil.
In general, an equivalent amount of natural gas burns to generate approximately 30% and 45% less carbon dioxide than heating oil and coal respectively. Besides, carbon dioxide gas, burning of natural gas emits an ingredient called NOx whereas the quantity of sulfur dioxide i.e. SO2 and other emission particles is almost insignificant. However, if the burning of gas takes place facing scarcity of combustion air, there is a possibility of volatile hydrocarbons generation which is very unsafe to human health and surroundings. Hence, care must be exercised to avoid these hazardous possibilities.
Nowadays, natural gas fuel reserves are getting exhausted at a rapid rate. Hence, substitutes for this boiler fuel need to be discovered very soon.
Propane is a boiler fuel which is basically manufactured out of refining process carried out for petroleum goods. It is generally carried and delivered to the usage point with the help of pressurized gas containers. Unlike natural gas, the energy content of propane is measured in gallons instead of cubic feet. It usually consists of 91 kBTU per gallon. In general, propane is available at a fairly higher price as compared to natural gas. Just like natural gas, it offers simple and economic integration into the forced-air heating systems, predominantly in the US market.
Renewable Energy Sources
Variety of renewable energy sources is available for use as boiler fuels. Most common of them includes solar energy and wind energy. The energy from solar radiations can be utilized in many devices such as solar water heating systems and solar cell systems whereas wind energy can be easily used in wind generators. This category of fuel resources proves to be extremely useful for people residing in distant areas where other fuel sources are difficult or costly to achieve. Hence, places which get abundant supply of sun heat are the justified locations for usage of solar energy. Moreover, photovoltaic systems operating via solar energy can be used for production of electricity to meet the power requirements in these areas.
Some other fuels commonly used for consumption in boilers are listed below:
- Diesel oil
- Bio fuels like ethanol
All these fuels have unique combustion characteristics which tend to influence the performance efficiency and emissions of the burning process.