What is the difference between low NOx natural gas and normal natural gas?

Question

I'm looking to purchase a new pool heater and one says it runs on low NOx natural gas while a different model runs on normal natural gas. Can someone tell me the difference?

Answer 1

I suspect what you are considering buying is a low-NOx *emission* heater, not a heater that runs on low-NOx natural gas. In other words, if the item you are considering buying is a "low-NOx natural gas pool heater", then the "low-NOx" part refers to the heater, not to the natural gas. As far as I know, there's no commodity sold as "low-NOx natural gas".

NOx stands for the various nitrogen oxides (NO, NO2, N2O, N2O3, N2O4, and N2O5) that are unavoidably produced during high-temperature combustion using air. Remember that air is about 80% nitrogen, and 20% oxygen. At high temperatures, these molecules react, forming nitrogen oxides. Higher-temperature combustion produces more NOx, while low temperatures produce less. (Higher temperatures, however, are generally better from the standpoint of complete combustion of a hydrocarbon fuel to CO2 and H2O, so there's a tradeoff here.)

Nitrogen oxides are a a pollutant, and a major component of smog. They can react with water vapor in the air to form nitric acid, and react with atmospheric oxygen to produce ozone (a "good" compound when it's in the stratosphere, where it absorbs UV light from the sun, but bad when it's produced at ground level and we breath it). As a result, some air-pollution control districts have established limits on the NOx emissions of things like water and pool heaters.

By designing the combustion chamber of a heater or furnace to avoid the occurence of "hot spots", and possibly by incorporating a reductive catalytic converter (similar to one part of the catalytic converter in a modern vehicle), one can make a "low-NOx emission" gas-fired heater. I'm 99.9999% sure that's what you are considering buying.

Answer 2

This looks like advertising hype. Natural gas is almost exclusively methane (CH4). Methane combustion does not produce any nitrogen oxides (NOx), and the complete combustion of methane produces only CO2 and H20. An improperly adjusted burner may produce some carbon monoxide (CO), but still no nitrogen oxides. Thus any heater using natural gas could be advertised as using low NOx natural gas, "low NOx" being an unnecessary qualifier, or hyperbole.