Sulfur dioxide limestone sorbent

Scale formation in the scrubber can lead to sodium carbonate as an additional dry sorbent in the scrubber. Alternatively, limestone is also introduced into combustion chambers to treat sulfur dioxide emissions. Decomposition of CaC03 into CaO and CO2 occurs in the combustion chamber, and the resulting CaO combines with S02 to produce calcium sulfite. Notice that this process produced another potentially environmentally harmful pollutant (CO2) as it gets rid of a definite environmentally harmful pollutant (SO2). 

As to sulfur dioxide, limestone as a S02 sorbent, when heated, is decomposed into CaO, which then reacts with S02 ... 

Spray-dry scrubbers are an alternative to conventional wet scrubbers. In this type of scrubber, an alkaline slurry or solution is sprayed in fine droplets into a reaction vessel, along with the flue gas. The droplefs rapidly react with the sulfur dioxide while drying to a fine powder of sulfite salts. This powder is entrained in the gas stream, and is carried to a dust precipitator where it is collected, as shown in Fig. 7. Most of the sulfur dioxide is collected in liquid-phase reactions while the droplets are drying, but 10-15 /o additional sulfur dioxide can be absorbed in gas/solid reactions, as the absorbent powder is swept through the ductwork and particulate collector. These are cocurrent devices, and so the limestone utilization and sulfur removal efficiency are inherently lower than those of countercurrent devices such as wet scrubbers. Partial recycle of the sorbent is often used to improve the sorbent utilization. 

The reactivity of limestones with respect to the reaction with sulfur dioxide varies markedly. For example, for a given fluidised bed combustor, the Ca S stoichiometric ratio required to achieve a 90 % reduction in sulfur emission at atmospheric pressure, varies from 2 to 5. The reasons for such a variation are not understood, but are likely to include decrepitation, catalytic effects of minor components such as iron, and the structure of the limestone and lime [12.12]. Laboratory test methods have been developed for predicting the performance of sorbents [12.13,12.14]. 

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]. 

Both the atmospheric and pressurized fluid bed combustors bum coal with limestone or dolomite in a fluid bed that, with recent modifications to the system, allows the limestone sorbent to take up about 90% of the sulfur that would normally be emitted as sulfur dioxide. In addition, combustion can achieved at a lower temperatnre than in a conventional combustor thereby reducing the formation of nitrogen oxide(s). 

The temperature in the fluidized-bed combustor is on the order of 800°C-900°C (1470°F-1650°F) compared with 1300°C-1500°C (2370°F-2730°F) in pulverized coal combustion systems (PCC). Low temperature helps minimize the production of nitrogen oxides and, with the addition of a sorbent (typically limestone) into the fluidized bed, much of the sulfur dioxide formed can be captured. The other advantages of fluidized-bed combustors are compactness, ability to bum low calorific values (as low as 1800 kcal/kg), and production of ash which is less erosive. 

Titanium oxide based sorbents were prepared with which an absorption and regeneration cycle could be performed at a same temperature. The conversion was nearly complete but the sulfur dioxide transport per cycle small due to the low content of active material thusfar. The mechanical strength at high temperatures of these sorbents, as tested, can be compared with that of the hardest natural limestones. 

The limestone calcination reaction proceeds best at tenqieratures near 2300°F. Reaction between sulfur dioxide and calcined limestone particles occurs primarily in tbe tenqioatuie range from about 1,000 to 2,600°F. Temperatures in the vicinity of 3,000 F occur near the bottom of typical boiler furnaces and are high enough to render the limestone inactive if the sorbent is injected at this elevation. As a result, the boilra injection point must be caiefiilly selected. Injection directly with the fuel has resulted in low SO removal efficiencies presumably because of the excessive temperature encountered by the sorbent. 

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