Lime/limestone scrubber slurry

If coal or oil is the fuel source, the FGD control technologies result in the generation of solid wastes. Wet lime/limestone scrubbers produce a slurry of ash, unreacted lime, calcium sulfate, and calcium sulfite. Dry scrubber systems produce a mixture of unreacted sorbent (e.g., lime, limestone, sodium carbonates, and calcium carbonates), sulfur salts, and fly ash. 

Chemical scrubbing systems for SO/ absorption fall into two broad categories (a) Disposable systems and (b) regenerative systems. Typical of systems in use for a number of years are those that use an aqueous slurry of an insoluble caldum compound, which can be discarded after use. Disposable 02-removal systems use aqueous slurries of finely ground materials, such as lime, limestone or dolomite, to produce a mixture of insoluble sulfites and sulfates. On passing through the scrubber, S02 from the waste gas dissolves to form sulfurous acid S02 ... 

Originally, bubble cap plates had been used for absorption of pollutant gases such as sulfur dioxide. However, the solids in the slurries used as absorbents can more readily plug bubble caps. Typical absorbents used in current processes include, for example, conventional lime slurry lime-limestone slurries mixed sodium sulfite/ lime slurries and magnesium sulfite/bisulfite mixed with lime slurries. Conventional lime slurry towers may consist of a multilevel spray tower combined with a venturi scrubber. Venturi scrubbers will be discussed briefly below. Mixed sodium sulfite/lrme slurries may be contacted in a plate tower. Sieve plates might be used with larger than normal holes to help prevent plugging due to the solids in the slurries. 

This paper summarizes the results of tests conducted from July 1978 through March 1981 at the EPA, 10-MW equivalent, lime/limestone wet-scrubbing FGD test facility, during which adipic acid as an additive was tested and shown to be a powerful scrubber additive for improving SO2 removal. The optimum concentration of adipic acid is only 700 to 1500 ppm at a scrubber inlet pH of 5.2 or higher. SO2 removal efficiencies in excess of 90 percent and reliable operation were demonstrated in four long term, limestone/adipic acid runs. Factorial tests were also conducted to characterize SO2 removal as a function of gas and slurry flow rates, pH, and adipic acid concentration. Intermediate duration optimization runs and favorable economics are also reported. 

The concentration of absorbing species in the scrubber solution is obviously an important consideration in scrubbing stack gas with lime-limestone slurry. Thus, the results of this study are useful both in understanding solution absorptivity and in pointing the way to possible improvement. 

There are hundreds of desulfurization processes proposed in the literature. These processes may be classified as wet and dry (Patkar, 1981). Spray drier absorption is a semi-wet system in which an alkali slurry is fed to the unit and the reaction product is collected dry. This process is generally applied to power stations. It is reported by Felsvang et al (1988) that spray drier scrubbers are in operation in power plants of over 10,000 MWe in different parts of Ae world. These absorbers are generally coupled with fabric filters or electrostatic precipitators to collect particulates. Spray drier absorption is experienced as a wet-dry system with different reagents such as lime, limestone, trona (natural soda) and ammonia. 

Current practice with lime and limestone scrubber systems often injects slurry into the scrubber loop beyond the boilers. These scrubbers can remove well over 90% of both SO2 and fly ash when operating properly. In addition to corrosion and scaling problems, disposal of lime sludge poses formidable obstacles. The quantity of this sludge may be appreciated by considering that approximately 1 ton of limestone is required for each 5 tons of coal. 

In the lime or limestone FGD process, SO2 is removed from the flue gas by wet scrubbing with a slurry of calcium oxide or calcium carbonate [3]. The waste solid product is disposed by ponding or landfill. The clear hquid product can be recycled. Many of the lime or limestone systems discharge scrubber waters to control dissolved solids levels. 

C02. Unfortunately, S02 is less soluble (and hence less easily removed by scrubbing) in slightly acid solutions, so that it is extremely difficult in practice to operate a calcium-bascd system in such a manner that SO2 removal is maximized while the quantities of calcium chemicals are minimized in order to approach stoichiometric conditions. As calcium-based slurry systems are usually operated at pH 6-10, disposal of the very large masses of used slurry presents a major problem. A typical power station using a caldnm-based SO2-removal slurry system will produce several hundred tons of spent slurry per day. A further disadvantage of lime or limestone systems is their marked tendency to precipitate insoluble calcium salts inside the scrubber. Unless the scale is removed, the scrubber shortly becomes inoperable. 

An important technology for removal of S02 is Flue Gas Desulfurization (FGD), carried out in units known as scrubbers. Most scrubbers contact the flue gas with a slurry of lime or limestone to capture the sulfur oxides and produce a sludge containing calcium sulfite and calcium sulfate. However, disposal of sludge is another environmental problem, and some scrubbers include oxidation to convert all the calcium sulfite to sulfate (gypsum), which can be used for wallboard manufacture. Fluidized-bed combustion units add a sulfur... 

In the case of a solid alkaline feed, such as hydrated lime and limestone, the raw material is fed from a bin (7) by a screw conveyor (8) through a mixer (9) into a dissolver (10). From the dissolver the reagent slurry is pumped to the second-stage impingement zone (11) where the liquid height is controlled by a level tank (12). The liquid overflow is recirculated to the mixer. The lower scrubber stage is provided with par-... 

A finely powdered limestone or lime slurry in water used in a suitably designed scrubber is an effective and relatively low-cost sulfur dioxide containment method (Eqs. 3.34-3.37). 

The most common absorbents are lime (calcium hydroxide) and limestone (calcium carbonate) slurries. Limestone is the preferred absorbent in many modern scrubbers, because of its low cost compared with lime and other absorbents.However, lime is also used because of its higher reactivity, which allows it to absorb sulfur more rapidly. This makes it possible to use smaller scrubbers to treat a given quantity of gas when lime is the absorbent. When lime or limestone... 

Fig. 2 Basic schematic of a wet scrubber column. Absorbent slurry percolates down through the packing, while the flue gases flow upward. The most common absorbents for sulfur oxides are limestone (calcium carbonate), lime (calcium hydroxide), and magnesium-enhanced lime made from dolomite. The sulfur-bearing sludge for some scrubbers is market-grade gypsum, but for other scrubbers it is a waste product that must be landfilled.

The reaction systems of interest in this study include both research samples containing reagent grade solution, which are carefully purged of oxygen, as well as samples drawn from pilot scale sulfur dioxide scrubbers using commercial grade lime and limestone slurries exposed to air. 

Bleed stream oxidation of unenhanced lime or limestone slurry is usually not feasible because the pH rise caused by the residual alkali in the oxidation tank makes it difficult to redissolve the solid calcium sulfite. With adipic acid-enhanced limestone scrubbing, however, this constraint is removed because of the low operating pH and low residual alkali in the bleed slurry. Thus, the oxidation tank can be maintained at a low pH for good sulfite oxidation, while achieving high SO2 removal efficiency with a sufficiently high concentration of adipic acid in the scrubber liquor. 

Lime scrubbing is quite similar to limestone scrubbing except that the pH in the delay tank is higher, about 8-10 as compared with 5.8-6.0 in limestone scrubbing. The pH apparently drops quickly when the slurry returns to the scrubber, caused by absorption of carbon dioxide and sulfur dioxide, so that the scrubber exit pH is about 5.4-5.8 no matter which absorbent is used. 

The burning of pulverized coal in electric power plants produces sulfur dioxide (SO2) gas emissions. The 1990 Clean Air Act and its subsequent amendments mandated the reduction of power plant SOj emissions [66-70]. The Best Demonstrated Available Technology (BDAT) for reducing SOj emissions is wet scrubber flue gas desulfurization (FGD) systems. These systems are designed to introduce an aUcahne sorbent consisting of lime or limestone in a spray form into the exhaust gas system of a coal-fired boiler. The aUcaU reacts with the SOj gas and is collected in a liquid form as calcium sulfite or calcium sulfate slurry. The calcium sulfite or sulfate is allowed to settle out as most of the water is recycled [66-80]. 

Slurries of either lime (Ca(OH)2) or limestone can be injected into stack gas scrubbers downstream from the boilers. With lime, the reaction is... 

Flue-gas desulfurization (FGD) is the best way to remove sulfur dioxide from fossil fuel power plants. FGD commonly uses either wet or dry scrubbers. In wet scrubbers, a slurry of limestone, CaC03, or hydrated lime Ca(OH)2 is sprayed... 

Materials of construction for double alkali process plants are quite similar to those used for limestone/lime systems—stairdess steel (316L) venturis and scrubbers, rubbo -lined pumps and slurry lines, Hastelloy G tubes for direct steam tube gas rdieat, and fiberglass reinforced plastic or plastic-lined carbon steel tanks. 

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