Flue gas washing

The Effect of Condensation Upon Transfer Rates with Application to Flue-Gas Washing Plants and Cooling Towers... 

The effect of condensation upon transfer rates with application to flue-gas washing plants and cooling towers are discussed. Theoretical models were developed for determining the rate of heat and mass transfer under conditions where fog formation prevails. Derived relationships are functions of the vapor and liquid equilibria and local heat and mass transfer of driving forces. They were used for a numerical study of the amount of fog formation as a function of the operational variables of a flue-gas washing plant in which the inlet gas temperature is typically... 

C with a water content of 0.075 kg/kg. Although heat and mass transfer rates were relatively insensitive to the choice of the model, the amount of fog formation was not. The models neglect the effects of condensation within the boundary layer, thus underestimating fog formation by a factor of up to three. The amount of fog formed in flue-gas washing plants increased up to a maximum value with decreasing feed-water temperature over a narrow band of liquid-to-gas ratios. 

Lessing, R., The Development of a Process of Flue Gas Washing Without... 

The evaluation procedures presently employed at NBS will be briefly reviewed and the use of these procedures to provide updated values applicable to flue gas washing processes will be discussed in this chapter. 

Tests using loop analysis combined with least sums-least squares analysis can also be used to see whether a large-scale revision of data on compounds of a given element is needed or justified. The complexities of such networks can be great, as is the case for the lithium network. This network approach will assist us in evaluating data for flue gas washing processes. 

A subset of chemical species have been selected to form the basis of an evaluation for flue gas washing processes. The selection is based on discussions with persons of DOE-Morgantown and from the 1979 FGD workshop proceedings (3). The chemical species include S02 and C02, of course, and all combinations of the aqueous cations and anions listed in Table I. Other sulfur containing species will be added in future work. 

Properties for processes can be calculated from thermodynamic quantities for individual species. A sample (Table II) from the NBS Interim Report (4), A Report on Some Thermodynamic Data For Desulfurization Processes, shows typical values for selected quantities of some chemical species extracted from the NBS Technical Note 270-series (5). A sample (Table III) from the NBS Interim Report illustrates a set of processes for a few reactions related to the flue gas washing process. This reaction table can be constructed from the data on individual species. 

In a two-step flue gas wash , the gaseous inorganic pollutants, such as hydrogen chloride and sulfur dioxide, are removed and neutralized. 

The water from the flue gas wash flows through a wastewater treatment plant in which heavy metals and dioxins are precipitated and filtered out. In this process, about 1001 of sludge arise per year. The sludge is then drained in a filter press. One percent of the remaining sludge still contains heavy metals and must be disposed of in an underground deposit. 

The heavy metal content, which is released into the river along with the water from the flue gas wash columns, has been reduced to a few kilograms per year (Fig. 114). 

Figure 30.6 Flue gas losses - coal (Glasshoughton washed singles)...

Cleaning procedures include an internal washdown with a 0.5 to 2% soda ash solution, using a high-pressure jet. All wash water must be drained and deposits removed from the boiler. Usually, all external surfaces of the boiler are cleaned as well, in addition to the flue gas side of the economizer, air heater, and ID fans. 

Battersea A pioneering flue-gas desulfurization process, operated at Battersea power station, London, from 1931 until the station was closed. The flue-gases were washed with water from the River Thames whose natural alkalinity was augmented by chalk slurry. One of the problems of this process was cooling of the stack gases, which caused the plume to descend on the neighborhood. 

In order to remove very fine particulate, flue gas leaving the spray tower is distributed to a bank of parallel filtering modules. Within each module, the flue gas first accelerates (compresses) and then decelerates (expands). This action causes water to condense from the flue gas. The water uniformly washes the module s walls. More importantly, water condenses on the fine particulate and acid mist (mostly H2SO4 from condensation of SO3 in the saturated flue gas) present in the flue gas, increasing both their size and mass. Some agglomeration also takes place. 

The SCR catalyst designed for FCCU regenerator flue gas service is a homogenous monolith, typically made from 1 mm thick material. Some catalysts are extruded clays that receive a wash coat of titanium dioxide before impregnation of the vanadium and tungsten metals. Another type involves painted plates of expanded metal... 

The elemental sulfur is removed by conventional technology. The gases are purified by the Lurgi Rectisol process which uses a low temperature methanol wash to remove H2S, COS and CO2. The acid gas stream is then passed to a Stretford unit which is preferred to the Claus unit because of the high percentage of carbon dioxide in the stream. Sulfur in the stack gas would be removed by conventional flue gas desulfurization techniques and the sulfur would then remain as sulphite sludge and not be recovered as elemental sulfur. 

Satriana (2) provides a summary of the development of flue gas treatment technology. The first commercial application of flue gas scrubbing for sulfur dioxide control was at the Battersea-A Power Station [228 MW(e)] in London, England, in 1933. The process used a packed spray tower with a tail-end alkaline wash to remove 90 percent of the sulfur dioxide and particulates. Alkaline water from the Thames River provided most of the alkali for absorption. The scrubber effluent was discharged back into the Thames River after oxidation and settling. A similar process was also operated at the Battersea-B Power Station [245 MW(e)] beginning in 1949. The Battersea-B system operated successfully until 1969, when desulfurization efforts were suspended due to adverse effects on Thames River water quality. The Battersea-A system continued until 1975, when the station was closed. 

When the catalyst is exposed to the flue gas in a biofuel-fu ed boiler, it starts to deactivate immediately. The conversion (figure 3) declines exponentially from 80% down to about 40 %, where it seems to levels out, after about 17 combustion cycles. The reason for the deactivation is a loss of specific surface area. Washing in a water solution of citric acid can restore this area and the activity (ftgure 3 and 4). The catalyst cannot be restored by thermal treatment, at least not by heating to 700-800°C. This means that the poison is non-organic and easily soluble in slightly acidic solution compound, probably a salt. 

Similarly, sodium chloride is formed in the absorber by the reaction of chloride, present in the flue gas as HC1 vapor, with the alkaline sodium solutions. The level of sodium chloride in the system builds up to a steady state concentration, such that the rate at which sodium chloride leaves the system with the washed filter cake is equivalent to the rate at which it is absorbed by the process liquor in the absorber. 

A buildup of sodium salts took place at the top of the scrubber, near the wet/dry interface where the flue gas enters the scrubber. It is at this location that the process liquor first contacted the flue gas. A substantial evaporation of water from the scrubbing liquor took place leaving the sodium salts behind. These sodium salts are rather soluble in water and can, therefore, be easily cleaned by a periodic spray wash. 

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