One of the main goals in the field of transportation is reduction of exhaust gas emissions, and commonly the reduction focuses on CO2 emissions. Natural gas (NG) itself, and, its blend with diesel are good options to reduce CO2 emissions of regular diesel fueled trucks. At the moment, Euro VI emission regulations can be achieved with stoichiometric natural gas engines. In order to reduce CO2 emissions even more, natural gas fueled lean-burn trucks which meet present Euro VI and future regulations need to be developed. However, there is a question of natural gas originated methane emission; methane is over 20-times more potential greenhouse gas than CO2. Other regulated and unregulated emissions need also to be converted to CO2 and H2O, that overall greenhouse potential of heavy-duty vehicles can be minimized. Durability aspect of the emission control catalysts is and will be even more crucial question now and in the future, because Euro VI and post Euro VI legislations for heavy-duty vehicles require a good durability of the ATS.
Exhaust gas cleaning system of lean-burn engine is a complex system, if stringent Euro VI emissions need to be met. The exhaust gas aftertreatment system (ATS) layout depends on the calibration used for the engine and it is always tailor made for the vehicle.
Stoichiometric CH4 oxidation with TWC (Figure 1)
Emission conversion of stoichiometric NG engine can be carried out with a three-way catalyst (TWC). The question will be in the future how ammonia (NH3) and nitrous oxide (N2O) emissions can be minimized, and how improvements in fuel economy changes the raw emissions of the stoichiometric engines. Ammonia is a poisonous and corrosive gas, whereas N2O has about 300-times greater greenhouse gas potential than CO2 has. Based on the laboratory results, NH3 formation occurs on rich side whereas N2O emission formation is probable under lean conditions. NH3 formation in a TWC is time-dependent reaction and its concentration in exhaust gas increases as a function of time spent on rich side. The possible solutions to minimize NH3 and N2O emissions could be (i) strict lambda control of the engine, (ii) shorten the time of rich pulses, and (iii) optimization of catalyst composition with new raw materials and noble metals. NO concentration of exhaust gas affects at least the NH3 emission of the fresh catalyst; the higher NO concentration, the higher NH3 emission. Aging studies showed that the catalyst operation window changed, and, if the same calibration is applied over the vehicle life-time the emissions may change.
Figure 1: Simultaneous emission conversion of stoichiometric natural gas engine can be carried out with a three-way catalyst
Lean-burn CH4 removal with a MOC
Lean-burn conditions in NG combustion requires more complex ATS to clean exhaust gases (Figure 2). Methane oxidation catalyst (MOC) converts CO and CH4 emissions and provides small quantities of NO2 for a selective catalytic reduction (SCR). The SCR converts NOx emissions with NH3 to N2 and H2O. An ammonia slip catalyst (ASC) prevents NH3 emission, which may be caused by the SCR at low temperature and during rapid changes in real operation. The present study clarified challenges of the aftertreatment system of the lean-burn natural gas engine. Sulfur deteriorates the activity of MOC. Methane can be converted with 90% conversion at 450°C with the MOC. However, exhaust gas temperature of the lean-burn engine is not high enough for complete CH4 conversion. To understand why the MOC is deactivating during long-term use, the catalysts were characterized and their activities in CH4 oxidation were studied. According to literature study and laboratory experiments, sulfur poisoning phenomenon is due to (i) formation of surface PdSO4 layers over PdO particles, and (ii) water vapor stabilization of reaction intermediates like Pd(OH)2. Study showed also that high O2 level should be used for lean methane oxidation to avoid harmful N2O formation and to maximize NO2 amount in the exhaust gas stream to promote low temperature NO removal with SCR system.
Figure 2: Exhaust gas after treatment systems of lean-burn natural gas engine
Dual-fuel engine out emission removal
To meet very stringent emissions standards with a dual-fuel engine, a diesel particulate filter (DPF) and possibly a diesel oxidation catalyst (DOC) need to be installed to the ATS as well (Figure 3). Function of the DPF is to collect and burn particulates, formed during diesel fuel combustion. The DOC removes longer hydrocarbon emissions, than methane, and provides NO2 for passive soot removal and for fast-SCR reaction. The DOC may also increase temperature inside the MOC and thus may promote its CH4 conversion level. According to the laboratory gas reactor studies, gaseous diesel originated emissions can be converted simultaneously with the MOC. Diesel originated emissions are easier to oxidize than natural gas originated exhaust gases. Overall, emission control is a joint development process between fuel, applied engine, engine control, calibration and aftertreatment system. All these aspects affect tailpipe out emissions of the heavy-duty vehicle.
View the report: Output from catalyst model by UEF (Public summary)