Limestone burning

The technical decomposition of limestone (burning of hme to quickhme) can be carried to completion either by raising the temperature until the dissociation pressure is higher than the atmospheric pressure, so that the COg can escape, or by removing the COg as it is formed by means of a current of inert gas, so that the partial pressure of the COg is alTvays less than the dissociation pressure. Thus the burning of lime is facilitated by blowing in steam or carbon monoxide. 

About 95% of all anthropogenic industrial CO2 emission is caused by fossil fuel use 4% is from cement production (limestone burning and CO2 release from past carbonate sediments). China is the world s largest hydraulic cement producer. In 2006 China produced over 1.2 billion metric tons of hydraulic cement, or roughly 47% of the world s production. Emissions from cement production account for 9.8% of China s total industrial CO2 emissions in 2006. 

Let it be supposed that f below its equilibrium value JT, for example, by means of a current of hot air as in limestone burning. This would residt in complete reaction from left to right of the equation... 

There are, however, technological means available to burn incompletely desulfurized fuels at the same time minimizing SO2 emissions. In the auto-desulfurizing AUDE boiler developed by IFF, the effluent is treated in place by an absorbent based on lime and limestone calcium sulfate is obtained. This system enables a gas desulfurization of 80% it requires nevertheless a relatively large amount of solid material, on the order of 200 kg per ton of fuel. 

The term lime also has a broad coimotation and frequently is used in referring to limestone. According to precise definition, lime can only be a burned form quicklime, hydrated lime, or hydraiflic lime. These products are oxides or hydroxides of calcium and magnesium, except hydraiflic types in which the CaO and MgO are chemically combined with impurities. The oxide is converted to a hydroxide by slaking, an exothermic reaction in which the water combines chemically with the lime. These reversible reactions for both high calcium and dolomitic types are Quicklime... 

Many lime plants are able to reduce the impurities in their lime product by careful screening and selecting of stone for burning. Because 9 kg of limestone produce only 5 kg of quicklime, the percentage of impurities in a quicklime is nearly double that in the original stone. Analyses of typical samples of high calcium, magnesian, and dolomitic limestones found in the United States are Hsted in Table 1. 

Hardness. Most limestone is soft enough to be readily scratched with a knife. Pure calcite is standardized on Mohs scale at 3 aragonite is harder, 3.5—4. Dolomitic limestone is generally harder than high calcium. Dead-burned or sintered limes are 3—4 on this scale, whereas most commercial soft-burned quicklimes are 2—3 (see Hardness). 

Quicklime and hydrated lime are reasonably stable compounds but not nearly as stable as their limestone antecedents. Chemically, quicklime is stable at any temperature, but it is extremely vulnerable to moisture. Even moisture in the air produces a destabilizing effect by air-slaking it into a hydrate. As a result, an active high calcium quicklime is a strong desiccant (qv). Probably hydrate is more stable than quicklime. Certainly hydrated lime is less perishable chemically because water does not alter its chemical composition. However, its strong affinity for carbon dioxide causes recarbonation. Dolomitic quicklime is less sensitive to slaking than high calcium quicklime, and dead-burned forms are completely stable under moisture-saturated conditions. 

Except for dead-burned dolomite, all limes are much more reactive with acids than limestone. The high calcium types are the most reactive. 

Theory of Calcination. The reversible reaction involved in the calcination and recarbonation of lime—limestone is one of the simplest and most fundamental of all chemical reactions. In practice, lime burning can be quite complex, however, and many empirical modifications are often necessary for efficient performance. 

In Germany and Japan, pulverized quicklime is used in making self-fluxing sinters, partially replacing limestone. Granular dead-burned dolomite is stiU used to protect the refractory lining of open-hearth and electric furnaces, but not the basic oxygen furnace. Refractory time has declined with the... 

Limestone and marble have been mined as building materials and the oxide of calcium, lime [1305-78-8], has been used ia the manufacture of mortar for centuries (see Building materials, survey Lime and limestone). Lime-burning was one of the first iadustries ia the American colonies, where calcining of limestone was accompHshed ia kilns dug out of the sides of hills. 

The reaction of C2S with CaO to form C S depends on dissolution of the lime Hi the clinker Hquid. When sufficient Hquid is present, the rate of solution is controUed by the size of the CaO particles, which depends Hi turn on the sizes of the particles of ground limestone. Coarse particles of siHca or calcite fail to react completely under commercial burning conditions. The reaction is governed by the rate of solution (10) ... 

Fluidized combustion of coal entails the burning of coal particles in a hot fluidized bed of noncombustible particles, usually a mixture of ash and limestone. Once the coal is fed into the bed it is rapidly dispersed throughout the bed as it bums. The bed temperature is controUed by means of heat exchanger tubes. Elutriation is responsible for the removal of the smallest soHd particles and the larger soHd particles are removed through bed drain pipes. To increase combustion efficiency the particles elutriated from the bed are coUected in a cyclone and are either re-injected into the main bed or burned in a separate bed operated at lower fluidizing velocity and higher temperature. 

Calcium Oxide. Also called lime or quicklime (4,5), calcium oxide [1305-78-8] (Class 1, nonregenerative), is relatively iaexpensive. It is prepared by roasting calcium carbonate (limestone) and is available ia a soft and a hard form according to the way ia which it was burned. For desiccant service, soft-burned lime should always be used. Calcium oxide is most commonly used to dehydrate Hquids and is most efficient when it can be heated to speed the reaction rate. The reaction product is calcium hydroxide, which cmmbles as it picks up moisture. 

Heating zone, where the charge is heated to the reaction temperature, i.e., the decomposition temperature for limestone or burning temperature for cement... 

Fire Hazards - Flash Point Not flammable but may cause fires upon contact with combustible materials Flammable Limits in Air (%) Not pertinent Fire Extinguishing Agents Flood with water, dry powder (e.g., graphite or powdered limestone) Fire Extinguishing Agents Not To Be Used Not pertinent Special Hazards of Combustion Products Not pertinent Behavior in Fire Can increase the intensity of fires Ignition Temperature Not pertinent Electrical Hazard Not pertinent Burning Rate Not pertinent. 

---