Air Quality and Natural Gas

When I started at the National Energy Board in 1980, I learned about the many positive attributes of natural gas, particularly when it came to improving air quality.  With its very low emissions of sulphur dioxide (SO2) and nitrogen oxides (NOx), it was clear that natural gas would be  an important part of efforts to reduce emissions, particularly from industry and power generation, that, at that time, were contributing to acid rain that was greatly impacting North America’s lakes and forests.

Natural gas again proved its benefits in the 1990’s when new emission guidelines were developed for gas turbines, cement kilns and boilers. The new efficiency-based emission guidelines resulted in a move to Dry Low NOx combustion systems used for combined cycle power generation, cogeneration systems, district energy systems and waste heat recovery facilities. As the provinces adopted the new emission rules, these natural gas systems, together with renewable energy systems, took the place of much of Canada’s coal and oil fired power generation. This wasn’t surprising given that when natural gas is compared to coal-based power, output-based emission reductions are over 90 per cent lower for air pollutants, 99 per cent lower for air toxics, and 40 to 80 per cent lower for greenhouse gas emissions.

“When natural gas is compared to coal-based power, output-based emission reductions are over 90 per cent lower for air pollutants, 99 per cent lower for air toxics, and 40 to 80 per cent lower for greenhouse gas emissions.”

The reason natural gas has such overwhelming benefits is simple chemistry and combustion dynamics. Most materials have to be transformed into a vapour to burn, but natural gas is already there. Predominantly methane gas (CH4), natural gas mixes easily with air and burns much cleaner than liquid or solid fuels. This is because the methane fuel molecule’s surface area contacts more of the available oxygen in the air and hence combusts cleaner than a liquid or solid fuel could.

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As a result, natural gas is uniquely beneficial as a combustion fuel.  Smog-forming NOx emissions are produced from the droplets or vapours of all fuels when the air’s nitrogen is broken up by local high combustion temperatures and pressures at peak conventional ‘diffusion flame’ conditions (see graph). However since natural gas is already a vapour it allows for so called “lean pre-mixing” with the air prior to combustion.  This allows for a more optimal fuel/air staging in furnaces and engines, resulting in leaner, cleaner combustion.  This means natural gas combustion has lower NOx emissions, as well as minimal soot particle formation; and this is what gives natural gas combustion that characteristic blue flame.

Natural gas also has considerable air quality benefits in terms of eliminating fine particulate matter (PM). In gas turbine systems (such as those used for power generation) the engine power is produced from the filtered high pressure hot air (a gas) passing through the turbine, and that ambient air filtration results in net zero fine PM emissions from these systems.

Today natural gas satisfies almost one-third of all the energy needs in Canada, coast to coast to coast. As an efficient and clean burning energy choice with fewer emissions than many other fuels, it only makes sense to look at how affordable and abundant natural gas can meet more energy needs.

Manfred Klein is an energy consultant, recently retired after 33 years at the National Energy Board, Environment Canada and the National Research Council. With an Engineering degree from Carleton University, he has been involved in federal policies and training with the natural gas industry on environmental issues, industrial gas turbine and pipeline technologies, and cogeneration systems.

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