After-Treatment Technologies for Emissions of Low-Carbon Fuel Internal Combustion Engines: Current Status and Prospects

In response to increasingly stringent emission regulations, low-carbon fuels have received significant attention as sustainable energy sources for internal combustion engines. This study investigates four representative low-carbon fuels—natural gas, methanol, hydrogen, and ammonia—by systematically summarizing their combustion characteristics and emission profiles, along with a review of existing after-treatment technologies tailored to each fuel type. For natural gas engines, unburned hydrocarbon (UHC) produced during low-temperature combustion exhibit poor oxidation reactivity, necessitating integration of oxidation strategies such as diesel oxidation catalyst (DOC), particulate oxidation catalyst (POC), ozone-assisted oxidation, and zoned catalyst coatings to improve purification efficiency. Methanol combustion under low-temperature conditions tends to produce formaldehyde and other UHCs. Due to the lack of dedicated after-treatment systems, pollutant control currently relies on general-purpose catalysts such as DOC, Three-way catalyst (TWC), and POC. Although hydrogen combustion is carbon-free, its high combustion temperature often leads to elevated NOx emissions, requiring a combination of optimized hydrogen supply strategies and selective catalytic reduction (SCR)-based denitrification systems. Similarly, while ammonia offers carbon-free combustion and benefits from easier storage and transportation, its practical application is hindered by several challenges including low ignitability, high toxicity, and notably NOx emissions compared to conventional fuels. Current exhaust treatment for ammonia-fueled engines primarily depends on SCR, SCR-coated diesel particulate filters (SDPF). Emerging NOx purification technologies such as integrated NOx reduction via hydrogen or ammonia fuel utilization still face challenges of stability and narrow effective temperature.