Natural gas is a fossil fuel composed predominantly of methane (\(\text{CH}_4\)), often making up 95% or more of its volume. Burning natural gas, or combustion, is a chemical reaction where methane rapidly reacts with oxygen (\(\text{O}_2\)) from the air. This exothermic process releases heat energy, which is harnessed for heating, cooking, and generating electricity. The compounds released are known as combustion products, and they vary depending on the efficiency of the burn.
The Primary Emissions of Burning Natural Gas
When natural gas burns efficiently in a plentiful supply of oxygen, the carbon and hydrogen atoms in the methane fuel are fully oxidized, resulting in two main products. The carbon atom combines with oxygen to form carbon dioxide (\(\text{CO}_2\)), and the hydrogen atoms combine with oxygen to form water vapor (\(\text{H}_2\text{O}\)). This complete conversion is the ideal chemical outcome of the combustion process.
Carbon dioxide is the most significant greenhouse gas released by natural gas combustion, contributing directly to atmospheric warming. In a properly tuned appliance, nearly all of the carbon in the fuel is converted to \(\text{CO}_2\). Water vapor is also released in large quantities, as the combustion of just one methane molecule produces two molecules of \(\text{H}_2\text{O}\).
While water vapor is technically a powerful greenhouse gas, it is short-lived in the atmosphere and its climate impact is generally treated differently than the long-lasting \(\text{CO}_2\). The sheer volume of \(\text{CO}_2\) released makes it the primary climate concern from ideal natural gas burning.
Products of Incomplete Combustion
If the combustion process is not completely efficient, typically due to insufficient oxygen supply or poor mixing of the gas and air, the reaction yields additional, partially oxidized carbon products. One of the most concerning of these is carbon monoxide (\(\text{CO}\)), a gas formed when carbon atoms only bond with one oxygen atom instead of two. Carbon monoxide is extremely hazardous because it is colorless, odorless, and tasteless, earning it the nickname “the silent killer” due to its toxicity.
When inhaled, \(\text{CO}\) binds to hemoglobin in the blood, preventing oxygen from reaching the body’s tissues and organs. The formation of \(\text{CO}\) is a direct indicator of a faulty or poorly ventilated appliance, which poses an immediate public health threat. Another product of inefficient burning is uncombusted methane, often referred to as “methane slip,” where \(\text{CH}_4\) passes through the burner without reacting.
Although methane slip is a small fraction of the total emissions, uncombusted methane is a highly potent greenhouse gas, with a warming potential over 80 times greater than \(\text{CO}_2\) over a 20-year period. This means that even small leaks or inefficient combustion processes along the natural gas supply chain and at the point of use can significantly increase the fuel’s overall climate impact. Incomplete combustion can also produce trace amounts of other compounds, such as volatile organic compounds and black carbon.
Nitrogen and Sulfur Oxide Pollutants
Beyond the carbon-based emissions, the high temperatures inherent to the combustion process cause the nitrogen and oxygen present in the surrounding air to combine chemically. This reaction forms nitrogen oxides (\(\text{NO}_x\)). Nitrogen oxides are air pollutants that contribute to the formation of ground-level ozone, commonly known as smog, and can also lead to acid rain.
Natural gas is considered a relatively clean fuel concerning sulfur content, which is typically removed during processing. Trace amounts of sulfur remain, however, and these combine with oxygen during combustion to form sulfur dioxide (\(\text{SO}_2\)). Sulfur dioxide is a pollutant linked to respiratory problems and is a major precursor to acid rain formation.
Comparing Natural Gas to Other Fuels
The emissions profile of natural gas is significantly different from that of other fossil fuels like coal and oil. When used to generate electricity in an efficient power plant, natural gas releases 50 to 60 percent less carbon dioxide than a typical coal plant.
The burning of natural gas also produces virtually no particulate matter, which is a major component of air pollution from coal and oil combustion. Furthermore, because natural gas has very low sulfur content, it releases only trace amounts of \(\text{SO}_2\), whereas coal and oil are significant sources of this acid rain precursor. Natural gas also releases negligible quantities of heavy metals like mercury, which are typically released during coal combustion.