Sulfur continued limestone

A complete circuit for an advanced scrubber is shown in Fig. 4, which includes oxidation of the sludge to form gypsum. In this circuit, limestone is first reduced to a fine particle size by a grinding mill, producing a slurry. The slurry is then added to the absorber tank, and pumped into the scrubber tower. A portion of the descending absorbent is diverted back to the absorber tank, which provides more time for the sulfur dioxide and limestone to react. The remaining absorbent collects in the base of the tower, where it is oxidized by injected air while being recirculated in the lower portion of the scrubber. A portion of the absorbent is continuously drawn off to a hydrocyclone. 

The COt Acceptor Gasification Process is discussed in light of the required properties of the CaO acceptor. Equilibrium data for reactions involving the CO% and sulfur acceptance and for sulfur rejection jit the process requirements. The kinetics of the reactions are also sufficiently rapid. Phase equilibrium data in the binary systems CaO-Ca(OH)t and Ca(OH)jr-CaCOs show the presence of low melting eutectics, which establish operability limits for the process. Data were obtained in a continuous unit which duplicates process conditions which show adequate acceptor life. Physical strength of many acceptors is adequate, and life is limited by chemical deactivation. Contrary to earlier findings both limestones and dolomites are equally usable in the process. Melts in the Ca(OH)2-CaC03 system are used to reactivate spent acceptors. 

The fluidized bed reactor is about 60 years old, but only in recent years has its apphcation to coal combustion taken on commercial significance. The fluidized bed is the dispersion of a solid, usually in powder form, by a gas, under flow conditions such that the solid takes on the properties of a gas. Such reactors can be designed to operate continuously. Thermal conduction (heat transfer) in such systems can be high, and, as a result, in the case of coal and air, the combustion can occur at much lower temperatures than in the fixed bed system. Thus, the addition of limestone (CaCOs) or dolomite (CaCOs MgCOs) to the fluidized bed system can result in the reduction of oxides of sulfur and oxides of nitrogen. 

The humidified gas flows into the JBR where it bubbles through a shallow zone of absorbent. Sulfur dioxide is absorbed, oxidized, and reacted with calcium ions to precipitate calcium sulfate and form a gypsum slurry. Make-up limestone is added as 20% slurry, air is blown into the bottom reactor zones of the JBR to enhance the oxidation reactions, and the product gypsum is continuously withdrawn as a slurry containing about 15% CaS04 2H2O. 

One of the attractions of such a fluidized-bed combustor is that sulfurous gases generated by the fuel combustion are trapped by the lime (which is continuously replenished by the addition of limestone). Since the residence time of the lime is long, in comparison with the 1 or 2 sec of the limestone injected into a conventional furnace, there is hope of absorbing most of the SO2 without using excessive quantities of limestone. 

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