The present invention relates to an improved exhaust gas cleaning system and method for a combustion source comprising a hydrogen generation system, a sulfur oxides trap, and a nitrogen storage reduction (NSR) catalyst trap. The improved exhaust gas cleaning system and method of the present invention also provides for a water-gas-shift catalyst between the sulfur oxides trap and the NSR catalyst trap, and a clean-up catalyst downstream of the NSR catalyst trap. The invention provides also a sulfur trap regenerable at moderate temperatures with rich pulses, rather than at high temperatures. The improved exhaust gas cleaning system of the present invention provides for the sulfur released from the sulfur trap to pass through the nitrogen oxide trap with no or little poisoning of NO.sub.x storage and reduction sites, which significantly improves NSR catalyst trap lifetime and performance to meet future emissions standards. The disclosed exhaust gas cleaning systems are suitable for use in internal combustion engines (e.g., diesel, gasoline, CNG) which operate with lean air/fuel ratios over most of the operating period.