Calcination of limestone

The term calcination of limestone refers to the process of its thermal decomposition into quicklime and carbon dioxide. It is frequently abbreviated by lime producers to calcination . 

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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 main stages of coal combustion have different characteristic times in fluidized beds than in pulverized coal combustion. Approximate times are a few seconds for coal devolatilization, a few minutes for char burnout, several minutes for the calcination of limestone, and a few hours for the reaction of the calcined limestone with SO2. Hence, the carbon content of the bed is very low (up to 1% by weight) and the bed is 90% CaO in various stages of reaction to CaSO. About 10% of the bed s weight is made up of coal ash (91). This distribution of 90/10 limestone/coal ash is not a fixed ratio and is dependent on the ash content of the coal and its sulfur content. 

Irregular-shaped particles exhibit greater surface area than regular-shapea cubes and spheres, the amount of this increase being possibly 25 percent. The effect of particle size and size distribution on effective surface, in a shaft employed for calcination of limestone, is shown in... 

Thermal decrepitation occurs frequently when crystals are rearranged because of transition from one form to another or when new compounds are formed (i.e., calcination of limestone). Sometimes the strains in cases such as this are sufficient to reduce the particle to the basic ciystal size. 

CO2 is also recovered economically from the flue gases resulting from combustion of carbonaceous fuels, from fermentation of sugars and from the calcination of limestone recovery is by reversible absorption either in aqueous Na2COi or aqueous ethanolamine (Girbotol process). 

Let the points A and B in Figure 4.1 be considered for a description of calcination of limestone which occurs as shown below ... 

The two other main types of human-made building cements, lime cement and gypsum cement, have been and still are used in many areas of the world. Both these cements require quite elaborate thermal procedures for producing their main components, which are slaked lime in lime cement and plaster of Paris in gypsum cement. Making them involves the calcination of an appropriate type of stone, a process that has been practiced since prehistoric times. Slaked lime is made by the calcination of limestone plaster of Paris, by the calcination of gypsum (see Textbox 33) (Cobum et al. 1990 Lea 1962). 

In the end, both approaches require the calcination of limestone to lime. The energy penalty in this process is about 4.5-5 GJ/ton of C02, which amounts to a 30-40% energy penalty of a transportation sector that uses air extraction for managing its own C02 emissions. This is comparable to the energy penalty incurred in the conversion of fossil fuels into hydrogen as a transportation fuel (Zeman and Lackner, 2004). 

The sulfation reaction, (D), in Section 9.1.1, is the second step of a process to remove SO, from the flue gas of a coal-fired utility furnace. The first step is the calcination of limestone (CaC03) particles, injected either dry or in an aqueous slurry, to produce CaO ... 

Calcination of limestone has been chosen as a model reaction and pore size distributions of the limestone particles are determined at different extents of calcination at different temperatures. Although the calcination reactions have been investigated for ages there are still questions about the actual mechanism of such reactions. The literature does not involve the structural variations. 

Table IA Structural Variations During Calcination of Limestone at 860 °C...

This process is directly coupled to the manufacture of quicklime. Milk of lime is obtained by slaking and then carbon dioxide, produced by the calcining of limestone, is passed into the suspension forming calcium carbonate (see Sections 5.3.2.3 and 5.3.2.4). 

In the 1700s Joseph Black gave the first sound technical explanation of the calcination of limestone including the evolution of carbon dioxide. Lavoisier confirmed and developed Black s explanation. In 1766 De Ramecourt published a detailed account of the art of the lime burner , which described the design, operation and economic aspects of limestone quarrying and lime burning. 

Stages (a), (b) and (e) involve direct heat transfer between a gaseous medium and the particles. As a unit process, the heat transfer mechanism is well understood and not specific to the calcination of limestone. Stages (c) and (d), however, are specific to limestone, are influenced by the design of lime kiln, and influence the properties of the quicklime. They are described in detail below. 

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