Effects of secondary combustion on efficiencies and emission reduction in the diesel engine exhaust heat recovery system

An experimental study on the effects of secondary combustion on efficiencies and emission reduction in the diesel engine exhaust heat recovery system has been undertaken. The co-generation concept is utilized in that the electric power is produced by the generator connected to the diesel engine, and heat is recovered from both combustion exhaust gases and the engine by the fin-and-tube and shell-and-tube heat exchangers, respectively. A specially designed secondary combustor is installed at the engine outlet in order to reburn the unburned fuel from the diesel engine, thereby improving the system’s efficiency as well as reducing air pollution caused by exhaust gases. The main components of the secondary combustor are coiled Nichrome wires heated by the electric current and diesel oxidation catalyst (DOC) housed inside a well insulated stainless steel shell. The performance tests were conducted at four water flow rates of 5, 10, 15 and 20 L/min and five electric power outputs of 3, 5, 7, 9 and 11 kW. The results show that at a water flow of 20 L/min and a power generation of 9 kW, the total efficiency (thermal efficiency plus electric power generation efficiency) of this system reaches a maximum 94.4% which is approximately 15–20% higher than that of the typical diesel engine exhaust heat recovery system. Besides, the use of the secondary combustor and heat exchangers results in 80%, 35% and 90% reduction of carbon monoxide (CO), nitrogen oxide (NO_x) and particulate matter (PM), respectively.