Oxidation Catalyst Effect on CNG Transit Bus Emissions

Recently, the California Air Resources Board (CARB) has reported that tailpipe emission samples from a compressed natural gas (CNG)-fueled transit bus without aftertreatment had measurable levels of toxic compounds such as formaldehyde (HCHO) and nanoparticle (=50 nm) and mutagen emissions (Ames assay) that in some cases were greater than that of a similar diesel transit bus equipped with either a diesel oxidation catalyst (DOC) or a diesel particulate filter (DPF) and fueled by ultra-low sulfur diesel (ULSD) [1, 2, 3]. Therefore, CARB has investigated the effectiveness of oxidation catalyst (OC) control for CNG bus applications. This study includes results for regulated gaseous and non-methane hydrocarbon (NMHC) emissions, non-regulated hydrocarbon emissions of toxic risk significance, and total particulate matter (PM). Two driving cycles were investigated: the Central Business District (CBD) cycle and Steady-State (SS) cruise condition at 55 mph.

The catalyst showed statistically significant reduction of total PM, total hydrocarbons (HC), NMHC, and carbon monoxide (CO). HCHO emissions were reduced by the catalyst by over 95% over both CBD and SS cycles. 1,3-butadiene emissions were reduced to levels below detection. Toxic aromatic HC's such as benzene also appeared to be reduced by the catalyst, but a larger data set is required to establish statistical significance. Little effect of the catalyst was found on methane (CH₄) and oxides of nitrogen (NO_x).