A) HUMIDIFICATION OF SCAVENGE AIR:-
LOW EMISSION OF NOX IN COMBUSTION PROCESS CAN BE ACHIEVED BY HUMIDIFYING SCAVENGE AIR AND THEREBY REDUCING THE PEAK COMBUSTION TEMPERATURE. DEVELOPED BY WARTSILA IN THEIR 4 STROKE CYCLE ENGINE AS “WETPACK HUMIDIFICATION”, THE PRINCIPLE IS TO INTRODUCE SPRAY OF FRESH WATER TO SCAVENGE AIR AFTER COMPRESSION IN TURBO COMPRESSOR. THE HIGH TEMPERATURE OF COMPRESSED AIR EVAPORATES WATER WHICH ENTERS THE CYLINDER AS STEAM WITH AIR. HUMIDIFIED AIR THUS ADMITTED HAS THE CAPACITY TO ABSORB HEAT AND REDUCES THE PEAK TEMPERATURE. THE WATER CONSUMPTION MAYBE TWO TIMES THE FUEL OIL CONSUMPTION. MAN HAS ALSO ADOPTED THE SAME IN 2 STROKE ENGINES AS HUMID AIR MOTOR (HAM) SYSTEM.
AIR AFTER TURBOCHARGER IS SATURATED WITH WATER VAPOUR PRODUCED BY INJECTIONOF SEA WATER. HEATED AIR AFTERTHE COMPRESSOR PASSES THROUGH AN AIR DUCT THAT VAPOURISES WATER AND HUMIDIFIES AIR. THE CONVENTIONAL INTERCOOLING IS REPLACED BY WATER SPRAY COOLING JUST AFTER THE TURBOCHARGER COMPRESSOR. THE LOWER TEMPERATURE OF HOT COMPRESSED OUTLET MAY HAVE A TEMPERATURE OF 70 DEGREE CELCIUS. NO CONVENTIONAL AIR COOLER IS NECESSARY. THE HAM METHOD PARTLY REDUCES OXYGEN CONTENT IN CYLINDER AND INCREASES HEAT ABSORPTION CAPACITY OF WATER VAPOUR.
B) EXHAUST GAS RECIRCULATION:-
IT IS A METHOD OF EXTRACTING AN AMOUNT OF EXHAUST GAS, COOLED AND SCRUBBED BEFORE MIXING WITH SCAVENGE AIR AND RECIRCULATED THROUGH THE ENGINE. IT REDUCES THE O2 CONTENT IN AIR.
IT USES AN ELECTICALLY DRIVEN BLOWER TO FORCE THE GAS THROUGH A SCRUBBER TO REACH THE SCAVENGE AIR RECEIVER.
C) SELECTIVE CATALYTIC REDUCTION (SCR) :-
IN THIS EXHAUST GAS IS MIXED WITH AMMONIA ( OR UREA AS A CARRIER OF AMMONIA). AMMONIA IS SPRAYED IN EXHAUST DUCT IN A SOLUTION OF 40% AMMONIA IN WATER BEFORE THE EXHAUST GAS IS PASSED THROUGH A LAYER OF SPECIAL CATALYST AT A TEMPERATURE WINDOW BETWEEN 300-400 DEGREE CELSIUS.
NOX IS REDUCED TO MOLECULAR N2 AND H2O. THE TEMPERATURE LIMITS ARE VERY IMPORTANT. IF TEMPERATURE IS LOW AMMONIA WILL CONDENSE AND WITH SULPHUR FORM AMMONIUM SULPHATE WHICH WILL DESTROY THE CATALYST. IF TEMPERATURE IS TOO HIGH AMMONIA WILL BURN RATHER THAN REACT WITH NOX. THE CATALYTIC ACTION WILL BE DEGRADED. FOR BETTER REACTION THE FLOW OF GAS SHOULD BE SLOW OTHERWISE THERE WILL BE “AMMONIA SLIP”. THE AMMONIA GAS SLIP PAST THE CATALYST WITHOUT REACTION. AGAIN THE CATALYTICREACTION RATE WITH NOX DEPEND ON CORRECT DOSING OF AMMONIA. THE AMOUNT OF DOSING WILL DEPEND ON SPEED AND LOAD ON ENGINE EXCESSIVE DOSING ALSO CAUSING AMMONIA SLIP.
THE AMOUNT OF AMMONIA IS CONTROLLED BY A PROCESS COMPUTER. THE AMMONIA DOSING IS A FUNCTION OF ENGINE LOAD. THE PROCESS COMPUTER TAKES INPUT OF NOX PRODUCED THROUGH NOX ANALYSER, THE ENGINE LOAD, AND THE TEMPERATURE TO ARRIVE AT A STOICHIOMETRIC DOSE OF AMMONIA/NOX FOR INJECTION TO THE EXHAUST PIPE.
