A Novel Low Cost Aftertreatment Solution for Lean Burn Gas Engines
Jacques Nicole, Ricardo, Inc.
Lean burn methane fueled combustion engines give low tailpipe CO2 emissions compared to a diesel and stoichiometric natural gas applications. Optimized engine efficiency by operating lean combined with a low carbon containing fuel lead to the lowest possible tailpipe CO2. However, due to the lean combustion mode, both NOx and CH4 become a challenge for exhaust emissions control. Traditionally, urea-based SCR is used to control NOx and a highly loaded PGM based methane oxidation catalyst (MOC) is used to achieve low tailpipe CH4 emissions. Therefore, due to the number of catalyst and the associated control requirements, the cost of the combined aftertreatment solution becomes a significant proportion of the total engine cost. In addition to the high cost of the methane control catalyst, the MOC only becomes highly efficiency in the region of 500°C, which for lean operating engines is rarely reached under normal engine operation.
Due to the high temperature stability of methane, a novel approach has been taken to develop a catalyst system which is able to oxidize methane at low temperatures, via the use of alternative oxidizing agents. Dioxygen (O2) is relatively stable, whereas ozone (O3) is highly reactive and is a significantly stronger oxidizing agent compared to O2. Synthetic gas reactor experiments were performed using O3 as the oxidizing agent and methane as the hydrocarbon feed. A current production iron-based SCR catalyst was used and was found to oxidize methane at 220°C with a conversion of 60%. Experiments are continuing with a multi staged fixed bed reactor with the aim of demonstrating >95% methane conversion, leading to the potential to eliminate the expensive PGM based MOC. These results will be presented at the conference. There is a cost associated with the on-board O3 generator and the associated power consumption needs to be included in the engine efficiency. However, the solution is anticipated to deliver a significant GHG (CO2 + CH4 equivalent) reduction compared with diesel and stoichiometric natural gas solutions but at an aftertreatment cost that is lower than diesel and equivalent to stoichiometric natural gas. This paper will discuss the novel approach to emissions control utilizing an alternative oxidizing agent for low temperature and low-cost emissions control, including the advantages and disadvantages of the system for different applications.