For small power plants, using Diesel engine generators to generate electricity is most convenient and cost effective. The Diesel engine exhaust emissions, however, is a concern needs to be dealt with when contemplating for a new plant.

    The Diesel engine exhausts typically contain large percentages of submicron smoke particles in the order of 20% to 70% of total particulates depending on the metallic contaminant content of the fuel oils. High metallic contaminant content oils produce more submicron particles that are difficult to capture. During initial engine startup, heavy black smoke is generated for a brief duration due to cold engine. The appearance of dark black smoke for even just for a brief duration, however, could be objectionable in a populated residential areas. SO2 is also released from the Diesel engines but is of a lesser degree of problem as low sulfur content fuel oils are used by most operators. The SO2 emission from low sulfur oil fired engines are not high enough to cause a problem.

    The case presented is to solve the stack gas emission problem of a Diesel engine power generation station located in a 28 story office building right in the center of mid-town New York City. For economy, the building owner had installed eight Diesel engines to supply the electricity needed for the building. High grade fuel oil is used to fire these engines in order to minimize the stack emission. The engines are housed in the basement and the hot exhaust gases from the eight engines are piped to a common main to a heat recovery economizer on the 25th floor to heat the hot water for the building uses. After the economizer, the total exhaust gas flow of 50,000 cfm at about 450 degree F. was discharged to the atmosphere through a 15' stack on the partial roof of the 24th floor. The stack effluent had been within the emission limits set by New York City environmental agency until more stringent standards were put into effect. The new standards not only limit the particulates discharge but also impose a very tough stack gas opacity standard of not exceeding No. 1 density on the standard smoke chart based on observation at the point of emission or at an observable point nearest to the point of emission. Comparing with other opacity standards, New York City's standard is much more stringent because it specifies that the observation is made at point of emission instead of from the ground level. The effect of the distance on observed opacity could be as much as four folds. To meet this opacity standard, most of the submicron smoke particles would have to be removed as the submicron particles are largely responsible for the opacity resulting from diffraction of light by particles.

    The most effective means of removing submicron particles in stack discharge gases is by wet scrubbing using the high energy Venturi type gas scrubbers. This type of gas scrubbers are specifically developed for capturing submicron particles by generating millions of fine liquid  droplets to capture the particles.  The working mechanics of a high energy Venturi scrubber is illustrated in Fig. 1 with an artist rendering. Upon entering the scrubber inlet, the contaminated gases are forced through a Venturi throat causing the gases to accelerate to high speed and atomize the scrubbing liquid to millions of fine droplets. These droplets serve as the collecting targets onto them the submicron particles impact and become attached to. The gas-liquid mixture subsequently enters tangentially into a cyclonic separator wherein the duct laden liquid droplets are spun off and drained off through the bottom drain while the clean gases spiral up to exit the top gas outlet. For other performance and operating characteristics of the high energy Venturi gas scrubber, please refer to web page www.advantechnology.com/index.html. The operation mode of the Diesel engine system, however, causes the total exhaust gas flow to fluctuate as the engines are monitored and controlled by a power distribution control system which determines how many engines are to be put into operation at any given time and when to start or stop any given engine based on the building electricity demand.  The exhaust gas flow can change abruptly at times especially in the morning as the electricity demand increases from minimal to 100% within a short time. This fluctuating gas flow condition can not be handled with the commonly available fixed throat Venturi scrubbers or even with the variable throat version using instrumentation to