Energy Density — Calorific Value — of Boiler Fuels

When purchasing a boiler, there are two decisions to make. Regardless of the purpose of the boiler — be it for an industrial company or a family with a small home, — the two decisions are the same. One decision is in regard to boiler type, “boiler configuration.” The other decision concerns boiler fuel type.

There are five (5) boiler configurations options: cast iron, fire tube, water tube, tubeless, and hot water. Factors that determine which boiler configuration is most appropriate for a particular application include floor space available to house the boiler and heating requirements. Ultimately, the most appropriate boiler type is typically a simple decision. The answer is generally obvious.

Boiler Fuels the Biggest Variable

Choosing a boiler fuel type, on the other hand, is a complex decision. Emissions are a factor. Solid boiler waste disposal is a consideration. Fuel volatility is a variable to consider. It is also critical not to overlook space requirements for fuel staging. However, none of these factors are as big as fuel energy.

The two biggest factors that determine which fuel type is most appropriate are fuel density — the calorific value of a fuel — and fuel cost. Each fuel type has a different set of advantages and disadvantages.

Boiler Fuel Types

There are six boiler fuel types available:

•Fuel Oil

•Coal and Wood

•Natural Gas



•Renewable Energy (Biomass)

Within the six boiler fuel types there are solids, liquids, and gases.

Energy Density of Boiler Fuels

Energy density is the potency of a fuel, how much heat a boiler fuel produces per unit of measure.

Explains, “Energy density is the amount of energy that can be stored in a given mass of a substance or system. The higher the energy density of a system or material, the greater the amount of energy stored in its mass.”

The reason energy density is important when determining which fuel type is most appropriate for an operation is cost. Breaking down the costs of a boiler’s energy consumption can be difficult because the weight or volume measure of a fuel is not a good indication of the energy provided.

Additionally, distributors sell different fuels types using a variety of units of measure.

Coal and wood are sold by weight. Fuel oil, diesel, and natural gas are sold in units of volume. Comparing weight and volume measurements in relation to energy output requires a special formula. That formula is energy density.

Energy density is the measure used to determine which boiler fuel type provides the most energy per unit of measure. The higher the energy density of a fuel — the “fuel density,” — the less fuel that is necessary to burn for a required amount of heat.

Understanding Energy Density and Btu

Energy is measured in calories and kilojoules, though the two are not synonymous. Calories are a measure of energy. A single calorie is the amount of energy required to raise a gram of water one degree. Joules are a measure of work, how much force is required to move one kilogram at a velocity of one meter per second in a vacuum. British Thermal Units (Btu) is the calorie or kilojoule value of a fuel.

Fuel density is the number of Btu per unit of measure. The higher the Btu of a fuel, the greater the fuel energy output (heat produced) per unit of measure. Btu is important, again, because the energy output of a gallon of fuel oil, for example, is much higher than the energy output of, for instance, natural gas.

Because of differences in fuel density, the price per weight or volume of one fuel measured against that of another is not a good indication of the actual price of the fuel. It does not indicate the price being paid for energy. For example, diesel may cost more per gallon than gasoline, but diesel is actually a cheaper fuel for powering a vehicle. The reason being, the energy output of diesel is about 33 percent greater than that of gasoline.

Fuel density is the energy a gallon or liter or pound or ton of fuel puts out. It is generally measured in million Btu per gallon or ton of fuel.

Energy — or “Fuel” — Density of Distillate Diesel and Residual Fuel Oil

Most boiler designs fire one of two kinds of fuel oils: No. 2 distillate diesel and No. 6 fuel (heating) oil. While other types of fuel oil and diesel are not prohibited, No. 2 and No. 6 have the highest energy densities of their respective categories.

No. 2 Distillate Diesel

In addition to firing boilers, No 2 diesel is also used in off-road construction, excavation, and mining. No. 2 diesel powers machinery, generators, and maritime transport vehicles, boats and ships.

No. 2 diesel has the second highest energy density of any boiler fuel with 37,184 megajoules per cubic meter.

The total Btu count of No. 2 diesel is 138,500 per gallon.

No. 6 Residual Fuel (Heating) Oil

The highest density boiler fuel — and the highest energy density of any fossil fuel period — is No. 6 fuel (heating) oil. The moniker for No. 6 heating oil is “Bunker fuel.” No. 6 heating oil produces tremendous amounts of heat, 40.808 megajoules per cubic meter.

The total Btu count of No. 6 heating oil is 152,000 per gallon.

Energy Density of Propane

Propane has the highest energy density of the gas boiler fuels at 29,499 megajoules per cubic meter. That translates to 91,000 Btu per gallon. That means the fuel density of propane is about two-thirds the Btu per gallon of No. 2 distillate and 40 percent less than that of No. 6 fuel oil.

Energy Density of Natural Gas

The energy density of natural gas is considerably less than that of propane and far less than that of diesel, fuel oil, and coal. The energy density of natural gas is, in fact, the lowest of all gas fuels, fuels like butane, ethane, hexane, and octane.

Natural gas is, for all practical purposes, is methane. Consumer side natural gas — from which the propane, trace gases, and contaminants are removed — is between 85 and 95 percent methane.

Methane has an energy density of 23,529 megajoules per cubic meter. That equates to 1,075 Btu per cubic meter.

Energy Density of Coal

Coal comes in a number of types. The coal with the highest energy density is Anthracite. With a fuel density of 36,450 megajoules per cubic meter, Anthracite coal has a comparable fuel density grade to fuel oil. However, on the opposite end of the spectrum is Lignite coal. Lignite has a fuel density of only about 12,000 mj/m3.

Energy Density of Wood and Biomass

The energy density of wood and of biomass — a wood derivative — is the lowest of that of any boiler fuel. Dry wood has an average specific energy (the weight equivalent fuel density) of around 9 megajoules per kilogram. Anthracite coal has a specific energy value of 27 mj/m3.

The megajoules per kilogram fuel density of biomass varies anywhere from between 9.5 (green wood) and 20.4 (dry needles).

Determining Price of Boiler Fuel

In order to determine the true price of a boiler fuel, the first step is to determine the energy density of a fuel. Again, fuel density is measured in meters cubed. By converting the price of a fuel volume or weight into meters cubed, it is possible to compare the prices of boiler fuels.

The energy density of a fuel is far more important than the amount of fuel, particularly when comparing prices and calculating the space required storing the fuel.


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