Dirtiest Boiler Fuels: May Not be Coal and Heating Oil

Combustion Emissions of “Dirty” Fossil Fuels Damage Atmosphere No More than “Cleanest” Fossil Fuel

A shift within the boiler industry occurred after the turn of the century. Boilermakers moved away from coal and fuel oils toward natural gas and propane. The biggest reason why, emissions. Proponents of natural gas and propane claim solid fuels — coal and wood — are “dirty” fuels. The same advocates for natural gas and propane labeled diesel and heating oil “dirty” as well.

But, scientists are beginning to ask if these claims are valid. Researchers are questioning whether or not the gas-fuel industry’s generalizations about natural gas and propane are legitimate. Some claim the gas fuel industry’s myopic focus on emissions have led to rather misleading claims.

Is natural gas the world’s cleanest fossil fuel?

CO2 Emissions Do not tell the Whole Story

The gas-fuel industry uses Btu-to-CO2 ratios to propagate phrases like eco-friendly, minimal carbon footprint, and clean fossil fuel. To back claims to these labels, supporters of propane and natural gas boiler-fuels cite the low CO2 emissions of natural gas and propane. In fact, post-emissions combustion emission are used almost exclusively as the standard of measure.

But, emissions do not tell the whole story.

With respect to global warming and changes to the atmosphere, combustion emissions don’t even tell the majority of the story. The gas industry packages propane and natural gas as “clean” and safe for the environment. However, that might not be the case.

Determining the carbon footprint of boiler fuels requires more than a post-combustion analysis of emissions. The carbon footprint of a fossil fuel begins at extraction — from the mine, pit, or well. The refining process also plays a role in determining the carbon footprint. And, the carbon footprint consequences of leakage/spillage during transportation and storage are another consideration to take into account.

Again, post-combustion emissions are just one measure of the environmental friendliness of boiler fuels, not the measure. When the standard of environmental friendliness is pre-combustion emissions, natural gas and propane are not so clean.

Boiler Fuel Types

There are five boiler fuel types:

•Fuel Oil




•Natural Gas



•Biomass (Renewables)

Boiler fuels generally labeled “clean” include natural gas, propane, biomass, and electric. The so-called “dirty” boiler fuels are fuel oil, diesel, coal, and wood.

“Clean” Boiler Fuels

Again, the gauge by which boiler fuels are typically measured — with respect to cleanliness — is emissions. However, emissions are just a cursory look at the total impact boiler fuels have on the environment. By considering the entire lifecycle of boiler fuels, it becomes apparent the word “clean” can be a misnomer. By only using combustion emissions as the measure, “clean” becomes a misleading label.

Take natural gas for example.

Is Natural Gas the Cleanest Fuel?

Yes, but also, and emphatic no. Natural gas produces less carbon dioxide than any other boiler fuel upon combustion. Per million Btu, natural gas only produces 117 pounds of CO2. As such, proponents of natural gas like the International Gas Union market, it is the, cleanest fossil fuel. “When burned, natural gas releases up to 50% less CO2 than coal and 20-30% less than oil,” explains the IGU.

However, low CO2 emissions do not make natural gas clean.

Natural gas is not an element on the periodic table. The label “natural gas” is little more than a moniker. “Natural gas” is a marketing label for methane. Because there are contaminants mixed in, distributors can repackage it as “natural gas.” But, natural gas is between 80 and 95 percent methane.

Carbon Footprint of Methane “Natural Gas” 

Methane is one of the most potent greenhouse gases with respect to global warming potential. The standard for global warming potential is carbon dioxide. As such, the global warming value of CO2 is one (1). Methane has a global warming potential of between 28 and 36 over a 100-year span according to the EPA.

“Emitting methane into the atmosphere is something like throwing kerosene on a fire. It warms the atmosphere a lot over a very short period of time,” according to the Scientific American. “over the span of 20 years is 86 times more potent as carbon dioxide to warm the atmosphere, and 35 times as potent as carbon dioxide over the span of a century.”

But again, proponents of methane fall back on the CO2 combustion emissions of natural gas.

Methane Production — Extracting and Processing Methane — Contributes More to Global Warming than All U.S. Power Plants Combined

The problem with natural gas is methane leakage, “fugitive gas.” Even though natural gas emissions from combustion are less than that of coal, “if fugitive methane emissions exceed 3 percent of total gas production, natural gas’s climate advantage over coal disappears over a 20-year time horizon,” according to the World Resources Institute.

And, leakage, “[occurs] at every stage of the natural gas life cycle, from pre-production through production, processing, transmission, and distribution.” Physicians for Social Responsibility believe the EPA’s estimation of a 2.5% leakage rate across the board is conservative.

“One study by Petron, based on air sampling in the natural gas fields of Colorado, showed a leakage rate in the field of 4.1% (range of 2.3-7.7%).[iii] This is only in the field, at the well sites and in storage tanks on- site, and doesn’t count pipeline and end storage leaks. Another analysis by Howarth estimates fracturing in shale to have a leakage rate of 3.6% to 7.9% from venting at the time of drilling, backflow, and leaks over the lifetime of the well”

World Resources Institute’s James Bradley claims the number of natural gas wells in the United States exceeds half a million (500,000) gas wells. And according to Bradley, they all leak. Plus, there are thousands upon thousands of miles of pipeline. All the pipelines leak as well. And, fugitive gas leaks occur during the storage of natural gas.

Again, there are fugitive gas leaks at every stage of the natural gas lifecycle.

Impact of Fugitive Gas Leaks

Convert the potency of methane to a CO2 equivalent. Then, assume the EPA’s estimates are correct: 6 million tons of fugitive gas is escaping from well alone annually. The consequences are staggering explains Bradley. “Over a 100-year time horizon, that’s more greenhouse gases that were emitted by all U.S. iron and steel, cement, and aluminum manufacturing facilities combined.”

Methane is Invisible, Unfortunately

Methane is invisible to the naked eye, so methane leaks do not garner near the attention of, say, an oil spill. As The Economist explains in a 2016 issue,

“METHANE is invisible to the naked eye and does not make for good television. So when about 100,000 tonnes billowed out of a natural-gas system in Aliso Canyon, Los Angeles, over 112 days last winter (pictured in infra-red above), it drew relatively little media attention—even though it forced the evacuation of thousands of homes and the plume was big enough to be detectable from space. Compare that with coverage of the Deepwater Horizon oil spill in 2010, which was the top item of news for weeks in America, much of it focused on the environmental impact on the Gulf coast.”

Unfortunately, since people can not see methane, they do not consider it a serious threat.

And Natural Gas Combustion Produces Carbon Dioxide

In its natural state, the carbon footprint of methane — natural gas — is exceptionally large. Then methane is converted into, among other chemical compounds, CO2. The amount of CO2 emitted from natural gas combustion is about half that of coal. But, the gap between natural gas CO2 emissions and diesel and fuel oil is considerably smaller.

Natural gas emits 53.07 kilograms of CO2 per million Btu. The CO2 emissions per million Btu of distillate diesel is 73.16 kilograms. That of residual oil is 78.79. But, fugitive methane gas leaks from natural gas wells are responsible for seven times that of petroleum wells. Petroleum wells in the U.S. release 23.3 MMT CO2  annually.

Natural gas wells release 172.6 MMT CO2  of fugitive methane into the atmosphere annually.

Diesel and Fuel Oils May be Answer After All

With pre-combustion emissions reducing devices like the Rentar Fuel Catalyst that lower CO2 emissions by up to 20% in some cases, diesel and heating oil may, in fact, be the cleanest boiler fuels available.


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