Scrubbing calcium based

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. 

Wet Scrubbing in Packed Towers. The packed absorption tower has been considered and used in lime and limestone scrubbing of SO2. It has demonstrated its capabilities in most of the previously listed characteristics. However, commercially available packings have difficulty maintaining a reliable operation with calcium-based wet scrubbing because of scale deposition. 

Qince the first large sulfur dioxide control system was installed at the Battersea plant in London, it has taken almost 50 yrs for calcium-based scrubbing technology to become commercially acceptable. In 1926, the 125 MW coal-fired Battersea power plant was equipped with a spray packed tower and final alkaline wash section which removed more than 90% of the sulfur dioxide and particulate (I). Thames River water provided most of the alkali for absorption, and about 20% was made up from lime addition. The process operated in an open-loop manner, returning spent reagent to the Thames. 

Since the utility industry represents the major market for sulfur dioxide control systems, it was necessary to develop a simple system which would not require a lot of attention, be inexpensive to operate, have moderate capital requirements, and not take effort away from their power producing function. Calcium-based scrubbing processes meet all of these requirements. In addition, the calcium reagents are inexpensive and form relatively insoluble reaction products which can be disposed of in sanitary landfills and slurry ponds. 

By-product processes have the advantages of soluble-based systems but have problems associated with more complex operation, increased capital and operating costs, and the need to develop by-product markets. Therefore, Research-Cottrell initially chose to develop calcium-based scrubbing technology to best meet the existing needs of its customers. 

Figure 1 illustrates the components of a typical calcium-based sulfur dioxide scrubbing system. Flue gases enter a quenching section where... 

Figure 1. Typical calcium-based sulfur dioxide scrubbing system...

Over two hundred dry or wet processes have been proposed for the removal of SO2 gases. Limestone and dolomite find extensive use in many of the desulfurization processes. Activated soda is considered as an attractive and more reactive alternate to limestone. In addition to calcium and sodium based processes, ammonia based, magnesia based, organic based scrubbing systems and catalytic processes have also been proposed for SQ2 removal. Calcium based dry processes have found wide application especially in fluidized bed combustors and in... 

PVC in the feedstock stream in limited amounts can thus be tolerated if a HCl removal process based on calcium hydroxide (slaked lime) injection is used. This is essentially a form of in scrubbing in the pyrolysis reactor itself. This method is used by a number of pyrolysis processes since the fuel oil end products need to be low in chlorine to minimize engine corrosion problems. In the pyrolysis of commingled plastics, lime can be added to the input material and the PVC content reduced down to 2-3% in order to avoid deposits of CaCl2 in pipework. The chlorine content of the final diesel should not exceed 10 ppm. 

Environmental concerns of lime kiln operation chiefly relate to exhaust gas dust control and are usually solved by water scrubbing. The slaked lime (calcium hydroxide) produced by the scrubber can be employed as a base in the operations of a chemical complex, or sold. Electrostatic precipitation of precooled gases is also used, sometimes in series with water scrubbing. The dissociated carbon dioxide discharged is not ordinarily regarded as a pollutant. The amount discharged from this source is far less than the carbon dioxide produced by fossil fuel combustion. Lime kilns associated with Solvay ammonia-soda plants may be able to recycle a part of the dissociated carbon dioxide, particularly if the concentrations are raised by the use of oxygen-enriched combustion air. Recently, interest has been shown... 

Dewatered flue gas desulfurization (FGD) scrubber material is most frequently generated as calcium sulfite, although some power plant scrubbing systems have the forced oxidation design, resulting in a calcium sulfate (or by-product gypsum) material. Calcium sulfite FGD scrubber material is oxidized to sulfate and used for road base, while the calcium sulfate FGD scrubber material is frequently used for wallboard or as a cement additive [66-80]. 

Miscellcineous Processes - Numerots processes have been proposed or developed for recov ering sulfur fi om flue gas. These processes are based on scrubbing wrath ammonia or injection of ammonia into the flue gas ammonium sulfite, bisulfite, sulfate, or mixtures of these compounds result from the processes. Ammonium sulfate can be produced as a final product, and some commercial use has been reported in Japan. However, the demand for ammonium sulfate is small in comparison wrath the potential supply from flue-gas sulfur therefore, most processes involve disposal of the sulfur as calcium sulfate or sulfite and recycBi of ammonia. 

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