Catalytic reduction of nitrogen oxides in methane-fueled engine exhaust by controlled methane injections

A method for controlling the reduction of nitrogen oxides in an exhaust gas from a methane-fueled internal combustion engine, wherein said exhaust gas contains oxygen, methane, and said nitrogen oxides, said method comprising:

• (a) selecting a catalyst capable of reducing nitrogen oxides with methane in the presence of oxygen in said exhaust gas;
• (b) determining the reaction characteristics of said catalyst including the degree of nitrogen oxide reduction as a function of temperature and as a function of the molar ratio of methane to nitrogen oxides in said exhaust gas;
• (c) determining the operating characteristics of said methane-fueled internal combustion engine including the concentration of nitrogen oxides in said exhaust gas and the volumetric flow rate of said exhaust gas as functions of selected operating variables comprising one or more engine variables selected from the group consisting of inlet air mass flow, manifold pressure, engine speed, and throttle position;
• (d) selecting a required degree of nitrogen oxides reduction in said exhaust gas, and determining from the information of step (b) a required temperature range and a required molar ratio of methane to nitrogen oxides in said exhaust gas to achieve said required degree of nitrogen oxides reduction;
• (e) providing a methane-containing fuel and operating said methane-fueled internal combustion engine utilizing at least a portion of said methane-containing fuel;
• (f) controlling the temperature of said exhaust gas within said required temperature range and measuring said one or more engine variables selected from the group consisting of inlet air mass flow, manifold pressure, engine speed, and throttle position;
• (g) utilizing values of said one or more engine variables measured in step (f) with said operating characteristics determined in step (c) to predict the molar flow rate of nitrogen oxides in said exhaust gas;
• (h) utilizing the molar flow rate of nitrogen oxides determined in step (g) with said required molar ratio of methane to nitrogen oxides in said exhaust gas to determine the minimum required molar flow rate of methane in said exhaust to achieve said required degree of nitrogen oxides reduction;
• (i) measuring the concentration of unburned methane in said exhaust gas, determining the actual molar flow rate of said unburned methane, and
• (1) if said actual molar flow rate of methane is less than said minimum required molar flow rate of methane, introducing an additional flow of methane into said exhaust gas sufficient to achieve said minimum required molar flow rate of methane, or
• (2) if said actual molar flow rate is equal to or greater than said minimum required molar flow rate,
• then passing the exhaust gas through a reactor containing said catalyst to reduce said nitrogen oxides with methane in the presence of oxygen in said exhaust gas;
• and
• (j) repeating steps (f) through (i) while operating said methane-fueled internal combustion engine according to step (e).