The NO_x concentration in the exhaust of an automotive gasoline engine with spark ignition can be as high as 4000 ppm. However, the exhaust of the lean-burn engines contains excess oxygen and so the three-way catalytic converter for the stoichiometric-burn engine cannot function to reduce NO_x. Urea-based ammonia selective catalytic reduction (SCR) is one of the most promising methods for NO_x removal from diesel engine exhaust. Nevertheless, the urea-SCR after-treatment system is quite complex and raises concerns regarding urea distribution infrastructure, potential freezing of the urea solution, ammonia slip, and the inconvenience and cost of refilling urea. Therefore, every possible means of achieving a low NO_x concentration has been used, including the extensive use of exhaust gas recirculation (EGR), which reduces the fuel efficiency. Therefore, a technology for controlling high-concentration NO_x emission is also required to realize highly efficient diesel engines, which requires the deletion of EGR. Notably, EGR deletion can lower the cost of the diesel-fueled automobile by simplifying its engine and the associated after-treatment system.

...Simultaneous NO_x reduction and power generation using solid oxide fuel cells (SOFCs) has been shown to be feasible; this method is denoted SOFC-DeNO_x. Nevertheless, although SOFCs can generate an electrical current during the reduction of NO_x, operation at the temperature of the engine exhaust, currently below 450 °C, would generate very little electricity and is therefore inefficient for power generation; otherwise, the operating temperature must be increased by extra heating. In addition, the consumption of the anode fuel increases cost and imposes the inconvenience of refilling the anode fuel if it differs from the automotive fuel. Therefore, an alternative SOFC operation at the exhaust temperature and open circuit is sought. Such an SOFC operation should require no extra heating and consume no anode fuel; it should support applications onboard automobiles without refilling the anode fuel; in this setup, the anode fuel acts merely as a reductant to generate the open-circuit voltage and can be enclosed in the anode side with or without circulation.

This work primarily studies such operation to control high- concentration NO_x emission. Since hydrocarbons (HCs) are usually formed in automotive gasoline engines owing to cylinder wall quenching and other effects, HCs must be treated in an exhaust after-treatment system. Therefore, simultaneous HCs emission control is also studied.

—Huang et al.

Schematic diagrams of the experimental electrochemical reactor. Credit: ACS, Huang et al. Click to enlarge.

The team constructed an SOFC unit with Ni–YSZ as the anode, YSZ as the electrolyte, and La_0.6Sr_0.4CoO₃ (LSC)–Ce_0.9Gd_0.1O_1.95 as the cathode, with or without adding vanadium to LSC. LSC is a well-known cathode material and GDC is a well-known electrode material for intermediate-temperature SOFCs.

They performed an activity test at 450 °C with the SOFC operating at open circuit. The anode gas was pure hydrogen. The inlet cathode gas consisted of 10% H₂O and 10% CO₂ always, 14% O₂ (except where noted otherwise), various concentrations of NO_x, and the balance helium. This composition of the cathode gas was similar to that of lean-burn engine exhausts, the team said.

For designated tests, 300 ppm C₃H₆ and/or 35 ppm SO₂ were added to the cathode gas. The overall flow rate of either the anode or the cathode gas was always 150 mL min⁻¹.

Experimental results indicated:

• Very high concentrations of NO_x can be treated using the SOFCs operated at open circuit and 450 °C, which is the operating temperature of low-temperature SOFCs.

• The SOFC-based NO_x emission control system can function without consuming any anode fuel (a reductant) and hence can be “care free”. It can also function at near the exhaust temperature and hence requires no extra heating.

• Complete oxidation of HCs, in terms of propylene conversion, can be achieved by adding silver to the LSC current collecting layer. Therefore, high-concentration NOx and HCs can be removed from the exhaust simultaneously by the low-temperature SOFCs at open circuit.

The simultaneous control of NO_x and HCs emissions has been demonstrated to be effective using SOFCs at open circuit. Since existing SOFC technologies on both materials development and devices manufacturing are well-established and commercialized, they can be fully utilized for emissions control of automobiles. A care-free exhaust after-treatment converter that is based on SOFCs can be immediately implemented onboard automobiles.

...It is noted that the current device (SOFC) for this novel technology is more suitable for stationary power generation applications. However, the application of this technology can be extended from stationary emissions sources to “lean-burn engine” or even “lean-burn gasoline engine” by designing a more suitable device for the latter. A low-temperature SOFC operating at open circuit, as reported in this article, is a way to use the current device for the latter and also to show the general idea. Notably, one example of a technology extended successfully from stationary emissions sources to “lean-burn engine” (diesel engine) is selective catalytic reduction using ammonia.

—Huang et al.

Resources

• Ta-Jen Huang, Sheng-Hsiang Hsu, and Chung-Ying Wu (2012) Simultaneous NO_x and Hydrocarbon Emissions Control for Lean-Burn Engines Using Low-Temperature Solid Oxide Fuel Cell at Open Circuit. Environmental Science & Technology doi: 10.1021/es2033058