Legislation to control NO2 emissions, one cause of acid rain and ozone induced. smog, has created an impetus to control NO2 emissions. Selective Non Catalytic Reduction (SNCR) using urea chemistry is utilized to control NO2 emissions from boilers, municipal waste incinerators, refinery furnaces, recovery boilers, utilities and other stationary combustion sources. Control requires injecting urea-based solutions into the flue gas as specified temperatures. Urea solutions accelerate CaCO3 precipitation in industrial waters used for dilution, and thereby interfere with proper application of the urea solution. The negative effect of urea solutions on hardness stability is discussed as well as how CaCO3 precipitation in urea solution can be controlled by suitable scale inhibitors. Keywords: urea, CaCO3, scale control, CaCO3 inhibitor, SNCR, NO2 emission, boiler, incinerator, refinery, recovery boiler, stationary combustion, utilities, flue gas, acid rain, ozone