Over the past several decades the control of air pollution has become a priority concern of society. The United States and other countries have developed elaborate regulatory programs aimed at requiring factories and other major sources of air pollution to install the best available control technology (BACT) for removing contaminants from gaseous effluent streams released into the atmosphere. The standards for air pollution control are becoming increasingly stringent, so that there is a constant demand for ever more effective pollution control technologies. In addition the operating costs of running pollution control equipment can be substantial, so there is also a constant demand for more energy efficient technologies.
Concerns about pollution control are directed to more than air pollution, and removing contaminants from one medium frequently results in their introduction into another. For example, the treatment of municipal wastewater under the Clean Water Act has resulted in an enormous increase in the amount of sewage sludge that must be disposed of. Many communities lack adequate disposal sites to discard sludge that is generated by their municipal wastewater treatment plants in landfills, and are turning to incineration as an alternative method of disposal. Incineration of sludge, or other waste products, while greatly reducing the volume of material that must be disposed of on land, may result in the release of contaminants in the sludge into the atmosphere. In this regard, it is noted that the sludge generated by many municipalities is contaminated by highly toxic heavy metals and organic compounds, as well as acidic compounds such as chlorides and sulfates. The release of such compounds into the atmosphere is highly regulated, and sludge incineration systems are required to use BACT for controlling the release of contaminants into the atmosphere.
Air pollution control systems often use venturi scrubbers to remove particulates and other contaminants from the effluent streams of incinerators. Venturi scrubbers are generally recognized as having the highest fine particle collection efficiency of available scrubbing devices. As the name implies, in a venturi scrubber the effluent gas is forced or drawn through a venturi tube having a narrow “throat” portion. As the gas moves through the throat it is accelerated to a high velocity. A scrubbing liquid in the form of droplets, typically of water, is added to the venturi, usually at the throat, and enters the gas flow. The water droplets used are generally many orders of magnitude larger than the contaminant particles to be collected and, as a consequence, accelerate at a different rate through the venturi. The differential acceleration causes interactions between the water droplets and the contaminant particles, such that the contaminant particles are collected by the water droplets. The collection mechanisms involve, primarily, collisions between the particles and the droplets and diffusion of particles to the surface of the droplets. In either case, the particles are captured by the droplets. Depending on the size of the contaminant particles, one or the other of these mechanisms may predominate, with diffusion being the predominant collection mechanism for very small particles, and collision or interception being the predominant mechanism for larger particles. A venturi scrubber can also be efficient at collecting highly soluble gaseous compounds by diffusion. A detailed description of these scrubbing mechanisms is discussed in Chapter 9 of Air Pollution Control Theory, M. Crawford, (McGraw-Hill 1976).
After the particulate contaminants are collected by the water droplets, the droplets are removed from the effluent stream, which is thereby cleansed. Removal of the water droplets may be accomplished by a number of known means. For example, several removal methods rely on the fact that the droplets are relatively large and, due to inertia, cannot change direction rapidly, such as when the gas flow is directed toward a surface of an impingement plate. While the gas moves around the surface, the inertia of the relatively large water droplets causes them to strike the surface where they are captured. Likewise, if the droplets are subjected to a circular flow, as in a cyclonic separator, the large droplets are captured as a result of collisions with the wall of the separator due to centripetal force.
Although venturi scrubbers are highly effective in removing entrained fine particulates, several problems have been encountered in their operation. First, effluents may contain hard particles, such as silica, that are abrasive, particularly in the high velocity flows of the venturi throat. These particles can damage portions of the scrubber, especially when they are larger than about a micrometer in diameter. Prior art venturi scrubbers have venturi throat velocities of 200 feet per second or greater that result from pressure drops of greater than 10″ H2O. Entrained particles in these high-speed gas flows can be extremely abrasive, and can rapidly erode exposed surfaces within the venturi. Contraction sections and parts that intersect the flow, such as throat dampers used for pressure drop control, are particularly vulnerable to this type of erosion. Thus, for example, entrained silica particles having aerodynamic diameters greater than about 2 micrometers are removed in high energy venturi scrubbers, but can cause unacceptable erosion rates within the venturi. The aerodynamic diameter of a particle account for the size, shape, and density of particles, and is defined as the theoretical diameter of a spherical particle with unit density and the same settling velocity as an actual particle.
Second, the amount of water consumed in prior art scrubbers can be large, and it is usually advantageous to recycle the water, either as scrubber water or as cooling water. Large particles in the scrubber water can damage nozzles, pumps, and the like, and so the larger particles must be removed from the water prior to recycling. Therefore, it would be desirable to have a system and method for removing entrained abrasive particles from an effluent gas within an air pollution control system so that the scrubbing water can be more economically dealt with.