Are there concerns about sulfur oxides, ammonia, mercury and dioxins in the exhaust of natural gas engines?
Those emissions are not regulated emissions by the EPA in its Heavy Duty Diesel Trucks category.
Diesel trucks are required to use ultra low sulphur diesel (ULSD) which has a sulphur content of less then 15 ppm. Although the emissions recorded during certification testing of diesel trucks do not measure sulphur oxides, a Westport HD equipped truck would have less sulphur dioxide emissions then the equivalent diesel engine due to the fact that liquefied natural gas (LNG) does not contain sulphur.
Mercury is not present in diesel fuel, LNG fuel, nor in any of the exhaust aftertreatment devices and therefore Mercury emissions are not expected from a Westport HD equipped truck or an equivalent diesel truck.
Ammonia is used in the exhaust aftertreatment, specifically the selective catalytic reduction (SCR) catalyst to reduce oxide of nitrogen (NOx) emissions. During this process, unreacted ammonia can be released (referred to as ammonia slip). This ammonia is prevented from leaving the aftertreatment system through an additional oxidation catalyst (slip catalyst). Westport has measured the emissions of the HD engine and found the ammonia levels were 0 ppm (lower than measurable). It is expected the equivalent diesel engine would be the same, with zero ammonia emissions.
There has been public concern regarding the creation of dioxins in diesel engines, specifically in the SCR aftertreatment. Cummins has tested the SCR aftertreatment system on the ISX engine proving to the EPA that the SCR system is not creating or releasing dioxins. The Westport HD engine uses the same aftertreatment as the Cummins ISX, and therefore the same conclusion can be drawn that dioxins emissions are not measurable.