Gas separation: processes – Electric or electrostatic field – With addition of liquid to gaseous fluid mixture
Reexamination Certificate
2001-01-08
2003-03-04
Chiesa, Richard L. (Department: 1724)
Gas separation: processes
Electric or electrostatic field
With addition of liquid to gaseous fluid mixture
C055SDIG003, C095S073000, C095S079000, C096S053000, C096S066000, C096S070000, C096S073000, C096S077000, C096S097000
Reexamination Certificate
active
06527829
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and apparatus for purifying the intake air of a gas turbine of solid and liquid particles.
2. Description of the Related Art
The intake air of a gas turbine compressor, i.e. the combustion air of the turbine, must be purified before is taken to the compressor. Most commonly, the intake air is purified by means of mechanical fiber filters. The purpose of such filtration is to prevent abrasive and fouling particles form entering the compressor and the turbine and thus to prevent equipment wear and tear, and to reduce the need for cleaning and maintenance. Big gas turbines used in energy production require considerable amounts of combustion air, wherefore even minute contents of impurity will lead to intensive deposition during use, and thus, the best possible combustion air filtration is desirable. Pressure losses in the intake air passage, on the other hand, reduce the efficiency of the equipment, wherefore filtration efficiency is always a compromise selected with regard to the prevailing conditions. Small water droplets and humidity are always conveyed into the compressor with the intake air. At low temperatures, the water droplets will freeze onto the surface they encounter forming greater layers of ice, which may damage the compressor blades when coming off. In the worst possible case this may break the entire compressor.
Today, different kinds of mechanical filters are almost exclusively used as the intake air filters in compressors. In these filters, the filtering matter comprises a fibrous layer which forms a labyrinth to which the impurities adhere. The filter capacity is dependent on the filter material and on the packing density of the material. In the case of a given filter material, the filtration capacity can be enhanced by increasing filter thickness or packing density by pressing the filter material into a denser form. Both ways will result in a rapid increase in the pressure losses of the filter, and the filter capacity of mechanical filters can only be increased to a given limit, and it is safe to say that in practice present fibre filters achieve complete separation of particles having a size of 1 to 5 &mgr;m. In known filters, the separation of sufficiently small particles is always incomplete. Furthermore, mechanical filters are susceptible to wetting caused by the small air-borne water droplets and the pressure losses from the air over the filter. A mechanical filter is then clogged and, at a sufficiently low temperature, it freezes. In such a case the filter must be dried by heating the intake air such that its relative humidity and temperature will allow the reevaporation of the water and ice formed back to vapour phase, and the convection thereof into the compressor along with the intake air.
The use of a greater number of efficient filtering systems has not been considered necessary but recent efficiency measurements at turbine installations reveal that even the smallest particles play a role in efficiency and service life. It is the smallest particles that cause most of the deposition in the gas turbine and the compressor, wherefore they should be removed. The proportion of minute particles in the total quantity of impurities is only fractional, but will in the long run and at large air masses in the gas turbines accumulate to a substantial total mass which may add up to even tens of kilograms annually. The deposition reduces the operating efficiency and electric output of the turbine. The reduced power is the sum of a number of factors, such as the wear and tear caused by the particles, inefficient cleaning methods resulting in a failure to remove all deposit material, the abrasive effect of cleaning, and the increased giving at the insulants and the increased leakage resulting therefrom. Even the slightest reduction in output capacity during the entire service life incurs considerable losses. The operating efficiency of the turbine is also reduced by fouling, which is why the compressor and the turbine itself must be cleaned at regular intervals. Water and different kinds of coarse powders are used for the cleaning. Washing with water, in particular, is hampered by freezing in cold conditions. The costs incurred by fouling are due to the increased need for fuel, the reduced efficiency and the washing costs of the compressor. The deposition intensity and the effects thereof are naturally dependent on the using environment and the impurity content of the intake air. Even with washes, not all impurities can be removed, and it remains a factor contributing to power losses in the range of tens of percents in just a few years. During the cold periods, preheating of intake air must be used in combination with mechanical filters to minimize the clogging caused by the wetting thereof as well as the risk of freezing caused by water droplets. The use of intake air preheating causes considerable production losses annually and results in increased operational costs due to the reduced operational efficiency.
Filter fouling leads to pressure losses when the filters are clogged by contamination, and the foul filters must be replaced periodically. The more closely meshed the filter, the greater the need for filter replacement, and filter replacement incurs costs due to the price of the filters and the down time. Conventionally, filtration has been boosted by developing the material of coarse and fine filters, and not by combining different filtration methods. In addition, electric filters have been considered so unreliable that the use thereof as an only filter has been considered too great a risk. One downside of electric filtration methods has involved humidity and the short circuits it causes. As deposit accumulates on the insulation material used in electric filters, the layer of impurities may form a conducting bridge which may conduct the current. One problematic type of deposit comprises different kinds of fibres, which due to their longitudinal shape may connect live parts to each other, thus giving rise to short-circuiting in the equipment. The occurrence of different kinds of fibres and other impurities in the intake air is affected by the conditions in the using environment of the gas turbine such as the amount of traffic, industrial establishments, and vegetation and soil type. Similarly, the capacity for damp separation varies considerably according to climate conditions. Particularly under conditions with considerable air humidity and simultaneous low temperature, ice formation on the filter plates may occur. The biggest problem related to ice formation is the detaching in big pieces of the ice formed on the plates of an electric filter, as such pieces may cause serious damage when ending up in the compressor. The freezing risk is further increased by the fact that air may cool down in the suction passages due to the pressure drop, whereby freezing may occur even with the ambient temperature above the freezing point. This is particularly problematic because warmer air may have a higher humidity content. Water reaching the plates of an electric precipitator is also harmful because it may wash off some of the deposit accumulated on the plates, which then has unprevented access into the compressor and turbine in particles of considerable size.
SUMMARY OF THE INVENTION
The aim of the present invention is to achieve a method for purifying the intake air of a gas turbine and for eliminating the need for intake air preheating without using any mechanical filter.
The invention is based on the notion of charging the intake air particles collected by an electric precipitator by means of electrodes separated from the collector plates, said electrodes having a voltage which by far exceeds the collection voltage.
According to a preferred embodiment of the invention, the intake air is charged with electrodes equipped with needle-like discharge tips, whereby the direction of the field may be selected, thus obtaining a more uniform field over the entire charge region.
More specific
Lehtimäki Matti
Malkamäki Matti
Chiesa Richard L.
Fortum Oyj
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