The exhaust from a rotary kiln hazardous waste incinerator must be cooled from 1200 degrees C to 300 degrees C before entering a spray dryer for acid gas removal. The engineer in charge of this project investigated severla alternatives to cool the exhaust gas and has concluded that the two most practical solutions would be either the installation of a waste heat boiler or the installation of a spray injection system in which water is sprayed directly into the exhaust duct upstream of the spray dryer. Based on the following data, which option should be selected?
Waste Heat Boiler
Cost installed $2 million
Service life 10 years
Operating cost excluding depreciation $0.4 million/year
Generated steam value $0.4 million/year
Water injection system
Cost installed $0.5 million
Service life 5 years
Operating cost excluding depreciation $0.15 million/year
Note: The additional water vapor added to the exhaust gas will increase the size of the spray dryer by 50%. The original spray dryer cost was estimated to be $1.0 million with a service life of 10 years.