Emissions Control Technologies for Natural Gas Engines

Abstract

In recent years, there has been a rising interest in alternative cleaner low-carbon fuels as they have a significant potential in decreasing the harmful exhaust emissions and contribute in decarbonising transportation. Natural gas (NG) is one of the most promising alternative fossil fuel that has been widely investigated in internal combustion (IC) engines. It is expected that global consumption of NG from 2015 to 2040 will rise 1.4% annually, accounting for the largest increase in world primary energy consumption. In this chapter, a review of the performance of NG-fuelled internal combustion engines, exhaust emissions produced from the combustion of natural gas engines and aftertreatment systems used to control those emissions is performed. In addition to the reduction of carbon dioxide (CO2) from NG fuelling, lower levels of unburnt hydrocarbons (HC) and particulate matter emissions (PM) than conventional petrol and diesel engines have also been reported. However, they tend to produce higher nitrogen oxide (NOx) and methane (CH4) emissions which are difficult to oxidise, particularly at engine operation at stoichiometric conditions. On the other hand, the slow flame speed of NG is a major problem under lean-burn operation as it increases cycle-to-cycle variations, significantly compromising engine efficiency. The addition of hydrogen enhances the combustion of NG in addition to improving engine stability and reducing exhaust emissions. The difference in combustion, emission characteristics and aftertreatment systems of stoichiometric, lean-burn and hydrogen-enriched natural gas engines is outlined.