Extracto del Horno de Cal del Libro de Celulosa “Alkaline Pulping Volume 5” p ag. 590.

La Fig 526 muestra el recuperador de calor, que es tá en la página 592 del capítulo XII "LIME REBURNING apartado B. THE ROTARY KILN sec ción 1.-Physical equipment".

Videos para el entendimiento de un horno rotatorio de cal:

https://www.youtube.com/watch?v=GU4KNV1hRiQ

https://www.youtube.com/watch?v=K_Ah3QPwA_I

The calcining reaction for lime is: CaCO3 + delta H ==> CaO + CO2 Where : Delta H = heat of reaction, 1736 kJ./ kg of CaCO3 @ NTP

An often overlooked feature of this reaction is that it is reversible, i-e., at sufficiently high gas phase CO2 concentrations the reaction is driven to the left. The calcining process requires heat and in all industrial calciners this heat is supplied by fuels which contain carbon such as fuel oil, natural gas, coal, petroleum coke, wood, etc. As a result, during calcination the lime is exposed to a gas environment which contains both CO2 from combustion as well as CO2 from calcination.

Typically, the C02 concentration in the calcining zone is 25% (by volume). This sets a lower limit on the lime temperature required to drive the calcining reaction to the right. A temperature of about 300°C is required for calcination to proceed. At these temperatures the reaction itself is quite fast so that the rate of calcination is limited only by the rate that the heat of reaction can be transferred to the lime.

The entire process of producing reburned lime from lime mud involves first drying the lime mud, followed by heating the dry mud (barro seco) to the calcining temperature near 800°C. The heat of calcination is then transferred to the heated material in order to complete the reaction-

Finally, it is necessary to heat the calcined powder just a little further to temperatures between 932 and l093°C to achieve agglomeration and slight sintering of the resulting nodules- This process requires a great deal of heat so it is not surprising that industrial calcining units are good heat exchangers. Rotary lime kilns are widely used to reburn lime and more recently fluidized bed calciners have seen application in the pulp and paper industry. Both of these units will be discussed in detail below. Because of the very fine powdery nature of precipitated lime mud found in the industry, neither shaft calciners nor flash calciners have proven effective despite their common use in other calcining applications.

Due to the very high energy requirement for calcination, it is the fuel cost and energy efficiency of these units which is most important to their economic operation. However, the production capacity and service life of the refractory bricks that are always used in their construction will also be important in the overall impact of the calciner on mill production and profitability- All three of these issues will be dealt with below for both the rotary lime kiln and the fluidized bed calciner-

1. -Physical equipment

Rotary lime kilns are basically large steel tubes which are lined on the inside with refractory bricks. They are slightly inclined from the horizontal and are slowly rotated on a set of riding rings. Lime mud is introduced at the uphill end and slowly makes its way to the discharge due to the inclination and rotation- A burner is installed at the downhill or discharge end of the kiln where the fuel is burned to form a roughly cylindrical flame. Heat transfer from this flame and the hot combustion gases that flow up the kiln dries, heats, and calcines the counter-flowing lime solids. Schematic diagrams of the outside and inside of a rotary lime kiln are shown in Figs. 521 and 522 and some of the features and nomenclature are indicated.

Rotary lime kilns in the pulp and paper industry range in size from 2,1 m in diameter by 53 m long to 4 m in diameter by 122 m long. The steel shells range in thickness from 2.5 cm to 6.4 cm and the refractory lining is from 15.2 cm to 25,4 cm thick. Production capacities for these units range from 45 to 400 metric t/d of CaO.

There are several different types of refractory materials available for application in lime reburning, and usually two or three of these are used at different locations along the length of the

kiln. A very common refractory system consists of bricks which are either shaped to fit the curvature of the shell or are in thin wedges which can be laid in an arch pattern in order to produce a complete shell lining. This is shown in Fig. 523.

The refractory bricks are composed of special heat and chemical-attack resistant materials which are most often alumina and silica compounds. Traditionally, the bricks in the hot zone of the kiln near the flame are composed of TU% alumina in order to resist the high temperatures and chemical attack in this region.

The burner and flame play an important role in kiln production capacity, efficiency, and product quality, as well as the service life of the refractory. As with any combustion-fired heat exchange equipment, the higher the flame temperature the higher the production capacity and efficiency. However, too high a temperature causes refractory damage and overburned, slow-reacting lime product. This tradeoff in performance results in a compromise in flame length. A comparison of several flame shapes is shown in Fig. 524.

Shorter flames are too hot and cause refractory damage and overburned lime, while longer flames cause loss in production, loss in efficiency,

and loss of control of the product quality. A compact, medium-length flame approximately three times the kiln diameter in length is the best tradeoff between efficiency and refractory service life. in any case the flame must not touch the refractory or serious refractory washing will occur.

Proper flame shaping at the hot end improves heat transfer there, but at the cold end heat transfer is hampered by the relatively low gas temperatures. To improve this, a section of chain is hung from the shell in this part of the kiln. This chain is made up of links which are typically 1.9 cm x 7.6 cm. Lengths of this chain are attached by hangers directly to the kiln shell either from one end or both ends. When hung from one end it is referred to as curtain chain, and when hung from both ends it is most often called a garland system- Schematics for three types of chain hanging systems are given in Fig. 525.

The method of hanging the chain makes little difference in its effectiveness as a regenerative heat exchanger surface as long the chain alternately contacts the combustion gases and the lime mud as the kiln rotates.

The rotary lime kiln is slightly inclined and rotates slowly (1–2 rpm). The moist lime mud enters the lime kiln at the highest end of the kiln. It is transported successively to the lower end as a result of both the inclination and rotation of the kiln. It is, in actual of fact, a bed of solid material that is moved towards the lower end of the kiln. The flue gas is generated at the lower end of the kiln and is moved counter-current with respect to the lime. The flue gas is cooled and heat is transferred to the lime. The rotary lime kiln is often divided into drying, heating, calcinations and sintering zones,