Natural Gas Engines for Cogeneration: Highest Efficiency and Near-Zero-Emissions through Turbocharging, EGR and 3-Way Catalytic Converter

Combustion engines for decentralized power generation or cogeneration in general, are subject to increasingly stringent pollutant emissions regulations. Motivated by the Europe-;wide lowest allowable NO_x levels in Switzerland - particularly in the Zurich metropolitan area with 50 mg/Nm³ at 5% O₂ - and in close cooperation with industry, the I.C. Engines and Combustion Laboratory (LVV) of the Swiss Federal Institute of Technology Zurich (ETHZ) has investigated some new operating concepts and engine processes in order to overcome the dilemma between low emissions and high efficiency, which is usually encountered in engine optimization. Our final approach thereby involves the Exhaust Gas Recirculation (EGR) combined with stoichiometric mixture (λ = 1) and a 3-way catalytic converter. The engine is supercharged and the intake mixture aftercooled for high power density and thermal efficiency.

The results obtained so far demonstrate the benefits of EGR against the conventional lean-burn process. Since knock is no more a limiting factor, highest BMEP of up to 23 bar (corresponding to an engine power of 290 kW from a displacement of 10 litres at 1′500 rpm) together with an engine efficiency of over 42 % have been demonstrated. The latter number is unrivalled by state-of-the-art gas engines of this size. In addition NO_x, CO and THC emission levels have been achieved, which correspond to roughly 1/10 of the German Air Act limits “TA-Luft” [1] and are at least comparable with the most advanced gas turbines in lean premixed operating mode. Finally, we have evaluated the thermal and mechanical engine components loading and found this comparable or less than the corresponding one for diesel engine operation. Additional studies have been carried out to quantify the influence of the natural gas composition (in terms of both inert components and higher hydrocarbons) on the engine performance parameters and the sensitivity of the obtained pollutant emission levels on specific catalytic converter operating parameters. The results suggest that this new concept is overall very robust with concern to both natural gas composition and catalyst specification within quite a wide range.