Overview for methanol and formaldehyde unregulated emissions of methanol fueled engines

Methanol can be classified into green methanol, biomass methanol, and fossil fuel derived methanol based on the abundant feedstocks, and it has been widely applied on road and marine transport vehicles. However, the utilization of methanol is favorable for reducing carbon monoxide (CO), hydrocarbon (HC), and particulate matter (PM) engine emissions, its unregulated emissions increase compared to traditional gasoline or diesel engines. The unregulated emissions include formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde, methanol, acetone, ethanol and etc., among which the unburned methanol and formaldehyde are predominant. The unburned methanol is mainly generated by the crevice and quenching effects, and formaldehyde by the incomplete oxidation of methanol in the exhaust system. Factors including exhaust gas recirculation (EGR), engine load and speed, intake and exhaust temperature, methanol substitution rate (MSR), and injection strategy affect the unburned methanol and formaldehyde emissions to a certain extent. Liquid chromatography (LC) and gas chromatography (GC) measure unburned methanol and formaldehyde emissions more accurately than Fourier transform infrared (FTIR), but FTIR can detect data on unburned methanol and formaldehyde emissions in real time under variable operating conditions or online for multiple emissions simultaneously. After-treatment devices such as diesel oxidation catalyst (DOC), particle oxidation catalyst (POC), or three-way catalyst (TWC) can reduce unburned methanol or formaldehyde emissions. The combination of DOC and POC is superior to a single DOC to remove the unburned methanol and formaldehyde emissions. The review provides a systemic analysis on the generation, hazard, measurement, and disposal of methanol and formaldehyde and promotes the clean application of methanol fuel.