Life Cycle of Boiler Fuels And Much More

Combustion Emissions Just One Chapter of the Carbon Foot Print Story

The labels clean and dirty, when applied to boiler fuels, are — as often as not — misleading. Almost without exception, emissions are the only standard determining what a “clean” boiler fuel is. But, contrary to what marketers of “clean” fuels would have you believe, emissions are just one component of a carbon footprint.

Some of the boiler fuels that leave the largest carbon footprints are those fuels given the clean label. And, some dirty boiler fuels leave the smallest carbon footprints.

There are seven (7) types of boiler fuel: No. 2 distillate fuel, No. 6 residual fuel (heating) oil, wood, coal, natural gas, propane, and biomass (renewables). The lifecycle of each boiler fuel is unique. From extraction, refinement, transport, and storage to combustion, every boiler fuel undergoes a different process.

Some boiler fuels — like natural gas and propane — do the majority of their damage to the atmosphere at beginning of their fuel life cycles. Fuel lifecycle processes like fracking, open-pit mining and refining impact the environment heavily. The lifecycle of boiler fuels can impact water quality, lead to erosion through deforestation, and scar the surface of the Earth for centuries.

Only an account of each boiler fuel’s impact on the atmosphere throughout its lifecycle, however, is required to debunk the misnomer that low post-combustion emissions make a fuel “clean.” If the production or combustion of a boiler fuel increases global warming to a greater degree than the emissions of a “dirty” boiler fuel, it is disingenuous to label that fuel “clean.”

Life Cycles of Natural Gas and Propane

Over the last two decades, proponents of natural gas have given it the label “the cleanest fossil fuel.” However, many consumers aren’t aware of what natural gas really is. Natural gas contains a number of impurities as well as propane and other trace gases. The valueless impurities in natural-state natural gas include carbon dioxide (CO2) and water.

Natural gas is not an element on the periodic table. Natural gas is a marketing term.

Natural gas is methane, for all practical purposes, once the propane, trace gases and some of the contaminants are removed; consumer natural gas is 85 and 95 percent methane.

And, therein lays the problem.

Methane (CH4) has Greater Global Warming Potential than CO2, much, MUCH Greater

Methane releases 50% less CO2 into the air than coal upon combustion. And, methane releases 20-30% less CO2 into the atmosphere than oil upon combustion. But, methane itself is far more destructive to the atmosphere than carbon dioxide.

The global warming potential of methane exceeds that of CO2 in every respect. During the lifecycle of “natural gas,” tremendous amounts of methane are lost into the atmosphere as “fugitive methane.”

During the drilling and extraction process of natural gas alone, up to 10% of the total methane that reaches the wellhead is lost, as fugitive gas. “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,” according to the World Resources Institute.

There is an argument that oil production also releases methane.

However, there is roughly an equal number of oil wells in the United States as natural gas wells. But, natural gas wells leak more than seven (7) times the fugitive methane, 172.6 MMT CO2. Oil wells in the U.S. release 23.3 MMT CO2.

Quite simply, “clean” is an inappropriate adjective to use when describing natural gas and propane in relation to other boiler fuels.

Life Cycle of Coal

During the lifecycle of coal, until combustion, there is virtually no release of greenhouse gases. Coal is inert in its natural state. Unlike natural gas and propane which are greenhouse gases, coal does not release greenhouse gases during storage.

Fugitive Methane Gas from Coal Mining

But, during the mining of coal — like the extraction of natural gas and petroleum, — coal mining releases trapped methane into the atmosphere. Coal mining methane (CMM) is by far the largest production emission of coal mining. But, not all types of coal mining produce substantial amounts of methane.

According to the World Coal Organization, “underground mines account for the overwhelming majority (up to 90%) of all methane emissions from the coal sector.”

Coal mines do not leak near the sum of fugitive methane as gas wells. Coal mines leak a third of that released during the production of gas. However, the sum is still incredibly high. The EPA reports that “U.S. coal mines emitted nearly four billion cubic meters or 61 million metric tons of carbon dioxide equivalents (MMTC02E) in 2015.”

And the U.S. isn’t even the biggest culprit, not by a long shot in fact. In the same article, the EPA shows the numbers for China. By 2020, China is expected to be releasing 420 million metric tons of carbon dioxide equivalents in the form of methane.

Fugitive Methane from Coal Mining versus Gas Production

The methane sums released in the form of fugitive gas are not equal to those of gas production. But, combustion emissions, in the form of CO2, exceed that of gas. Different types of coal release different sums of CO2 into the air upon combustion. Anthracite and coke release more CO2 into the atmosphere upon combustion than any other type of coal. Anthracite coal releases 103 kg of CO2 per million Btu and coke releases 114 kg.

But, even the dirtiest coal types are cleaner than natural gas and propane.

Researchers from Cornell University found that “Compared to coal, the [greenhouse gas carbon] footprint of shale gas is at least 20% greater and perhaps more than twice as great on the 20-year horizon and is comparable when compared over 100 years.”

According to Dr. Robert Howarth of Cornell, “On an equal mass basis, the global warming potential of methane has been calculated to be about 100 times that of carbon dioxide over a 20-year period, and about 33 times that of CO2 on a 100-yr period.”

While the emissions from coal may appear dirtier than methane, when measured for global warming potential, it’s simply not the case.

Life Cycle of BioFuel

Boiler fuel biomass is a wood derivative. Wood pellets have the highest energy density while wood chip biomass is the least expensive. A mechanical chipper produces wood chip biomass while the wood for pellets is pulverized in a hammer mill. Next, the pulverized material is pressed through a die with holes of a required size.

The friction between the press and moving through the die holes heats the biomass. When heated, the lignin in the wood biomass creates natural cement. This cement adheres the biomass together in pellets the diameter of the die holes.

Production of wood pellet biomass consumes, “15% of the input biomass,” per kilowatt generated. However, even including production, the grams of CO2 equivalent per kilowatt-hour (g CO2 equiv/kWh) is still 20% less than that of coal. The g CO2 equiv/kWh of coal is 1001. The g CO2 equiv/kWh of wood-derivative biomass is 821.

In other words, unlike natural gas and propane, biomass is truly a cleaner boiler fuel than coal.

Life Cycle of Diesel and Fuel Oil

Like the production of natural gas — which is, again, almost entirely methane — and coal mining, during the drilling and extraction of oil, methane fugitive gas leaks occur. However, petroleum wells in the U.S. release 23.3 MMT of CO2  annually. While significant, it is seven times less than the 172.6 MMT of CO2 released into the air as fugitive methane during the production of gas.

Additionally, unlike wood and coal, the kilograms of CO2 released per Btu by diesel and fuel oil during combustion is low. The CO2 emissions from diesel and fuel oil are much closer to that of natural gas and propane than coal. Natural gas releases 53 kg of CO2 per million Btu, propane releases 63 kg, distillate diesel 73, and Residual Heating Fuel releases 78 kg of CO2 per million Btu.

Cornell researchers are certain natural gas and propane have a greater impact on global warming than coal. Therefore, as coal has a much greater global warming potential than diesel and fuel oil, diesel and fuel oil are the two cleanest boiler fuel oils.

“Clean” Fuel a Relative Term

Consideration of the entire lifecycle of a fossil fuel is a requisite to determining the cleanliness of a fuel. By taking the entire lifecycle of boiler fuels into account, it becomes clear that diesel and fuel oil are clean and gas fuels are anything but.


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