Fuel Types for Boilers And Its Affect

There are Seven Fuel Types for Boilers, Each with Advantages and Disadvantages

There is a range of residential and industrial boiler classifications. And, each classification falls into different categories, sub, and sub-categories. But, there are two principal variables that determine the appropriateness of a boiler for a given purpose. All boilers — irrespective of size and application — have a specific boiler configuration and use a specific boiler fuel type. 

Boiler configuration is a function of the boiler’s “firing practice.” Boiler firing practice is the heating element that serves as the boiler’s means of heating the water or thermal oil that circulates throughout the piping of a house, building or facility.

There are five boiler-firing configurations: cast iron, fire tube, water tube, tubeless, and hot water. Each boiler configuration plays a role in determining heating efficiency, heating operation costs, maintenance requirements, and emissions.

Fuel type also influences efficiency, costs, maintenance requirements and emissions. And, to an even greater degree, fuel impacts emissions.

As such, the fuel type used to fire a boiler is often more important than boiler configuration.

Boiler Fuel Types – The Value of Each

Whether the configuration is cast iron, fire tube, water tube, tubeless, and hot water, all boilers use of one five possible fuel types:

•Diesel/Fuel Oil

•Solid Fuel

•Natural Gas or Propane


•Renewable Energy (Biomass)

The value of a boiler fuel type is a function of three variables: fuel type efficiency (measured in Btu), fuel cost, and emissions.

Some fuels are inexpensive but produce low boiler thermal productivity. Some fuels have high joule-per-unit-of-measure outputs but are expensive. And, some boiler fuels are inexpensive and have high boiler Btu outputs, but extremely high emissions.

Determining the appropriate boiler fuel type is a calculation of the balance of efficiency, cost and emissions.

Fuel Oil Boilers

There are six types of fuel oil: distillate types of diesel and residual oils. Both types of fuel oil are divided into numbered subcategories, No. 1 – 6. Fuel oil No. 1 and fuel oil No. 2 are distillate types of diesel. Fuel oil No. 5 and fuel oil No. 6 fuel oils are residual fuel oils. No. 3 fuel oil and No. 4 fuel oils are combinations of residual oils and distillate diesel.

The two most common categories of fuel oil used to fire boilers are No. 2 distillate diesel and No. 6 residual oil, aka, “bunker fuel.” No. 2 distillate fuel oil emits less greenhouse gases upon combustion but has a lower energy density.

One benefit of fuel oil boilers that separates this category from others is diversity. Fuel oil boilers are available in five of the six boiler configurations. As such, fuel oil boilers serve well for both residential and industrial applications.

However, the greatest value of fuel oil boilers is energy density. Measured in Btu (British Thermal Unit), fuel oil energy density is extremely high. Of fuels used to fire boilers, no other fuel type — solid, liquid, or gas — has a higher energy density than distillate diesel and residual oil. Because of the energy density of diesel and residual oil, fuel oil boilers are extremely efficient.

And, a pre-combustión fuel catalyst increases that efficiency to an even greater degree.

Propane Boilers

There is a large surplus of propane in the United States and there has been since the mid-90s. And, propane has a higher energy density than all other types of boilers — save fuel oil. That means propane boilers are very efficient and fuel is easy to come by.

But, there are questions about the eco-friendliness of propane as it is a byproduct of natural gas. And, propane is relatively expensive in relation to other boiler fuels.

Propane boilers are only available in three of the five boiler configurations: water tube, fire tube, and cast iron. Again, propane has a high energy density in relation to most other boiler fuels. But, its energy density is about 20% less than that of diesel. And, diesel produces 15% less Btu per gallon than fuel oil.

So, in addition to being more expensive than fuel oil and diesel, propane produces less energy than a comparable unit of No. 2 diesel. The fuel density of propane is only about 65% of the fuel density of fuel oil.

Natural Gas Boilers

Natural gas boilers are the most common type of residential boilers in the U.S. The reasons being, the availability of natural gas and the fact it is inexpensive. Considerably less expensive than propane, natural gas is one of the least expensive means of firing a boiler. At least, that is, on a unit of measure scale.

But again, like propane, the energy density of natural gas is very low in relation to No. 2 diesel, 62%. The energy density of natural gas is less than 50% of that of fuel oil. However, given that natural gas is easy to transport and burns cleanly, natural gas is popular as a residential (area) boiler fuel.

The one big concern about natural gas is that is between 80 and 90 percent methane. Methane is a greenhouse gas 34 times more potent than carbon dioxide. While methane gas is inert when burned, if it escapes into the atmosphere, “methane gas wreaks havoc.”

So, though cheap, there is a price to pay with natural gas. 

Electric Boilers

Electric boilers are extremely efficient because, as electric boilers require no flue, no heat is lost out the flue. And, electric boilers are small and inexpensive to purchase. But, the cost of electricity means electric boilers are for residential purposes only. Even in homes and small facilities, the price of electricity makes electric boilers expensive in relation to fuel-fired boilers according to the EPA.

With respect to emissions, 85 percent of the electricity generated in the U.S comes from coal or nuclear-powered plants. So, the vast majority of electric boilers are — indirectly — solid fuel furnaces or powered by nuclear.

Wood and Renewable Energy (Biomass) Boilers

Though the source of fuel comes from different sources, it is appropriate to categorize wood and biomass boilers together.

Biomass Boilers

The biggest benefit of biomass boilers is the fact that biofuel is a renewable resource. While biofuel boilers are essentially wood-burning boilers, the wood comes in the form of pellets, wood chips, or logs. But, unlike traditional wood-burning boilers, biomass boilers use discarded and recycled wood.

But, there are drawbacks to biomass boilers. For one, biomass — though a recycled or repurposed product — can be expensive. Additionally, biomass boilers are generally more expensive than boilers that use other fuels.

Most importantly, however, biomass is the least energy dense of all boiler fuels. Biomass is about 38 percent less fuel dense than coal. That means for the cost and the emissions produced; there is not a high return in the form of Btu.

Wood boilers

The least expensive boiler types in every respect, wood boilers are only used for residential purposes. With low Btu production, large sums of emissions and large amounts of waste material, wood boilers are inefficient and dirty.

But, wood is relatively inexpensive and available in almost every part of the country.

Coal Boilers

Solid fuel boilers fired with coal — like wood — are attractive for residential uses because they are inexpensive to purchase and install and fuel is cheap. But again, like wood, coal boilers produce high emissions and large amounts of waste: fly and bottom ash. And also like wood, the energy density of most coal is very low.

But, the energy density of coal does depend on the type: Anthracite, Bituminous, Sub-bituminous, and Lignite. While Bituminous, Sub-bituminous, and Lignite coal have some of the lowest fuel densities of all boiler fuel types, Anthracite coal is different.

Anthracite coal has a higher energy density than natural gas (methane), propane, and even gasoline. Only fuel oil, kerosene, and diesel fuel have higher megajoule per meter cubed outputs than Anthracite coal.

And, Anthracite coal has much lower CO2 emissions than other types of coal.

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