Process for separating nitrogen oxides and soot particles from a lean exhaust gas from a combustion engine and exhaust gas cleaning system therefor

Removing nitrogen oxides from the lean exhaust gas of an internal combustion engine by passing the gas through a nitrogen oxide storage catalyst and periodically denitrogenating the catalyst by enriching the exhaust gas comprises using an engine control unit to initiate denitrogenation in response to signals from a sensor. Independent claims are also included for: (1) removing nitrogen oxides (NO x) and particulates from the lean exhaust gas of an internal combustion engine, where the gas also contains small amounts of sulfur oxides, by passing the gas through a NO x storage catalyst and a particulate filter, periodically denitrogenating the catalyst by enriching the exhaust gas, regenerating the filter by increasing the temperature of the lean exhaust gas to a value at which soot burnoff is initiated and then desulfating the catalyst by enriching the exhaust gas; (2) exhaust gas purification apparatus for carrying out a process as above, comprising a NO x storage catalyst and optionally a particulate filter.