Temperature limestone particle

Further work on the absorption of sulphur dioxide by Uchida et aln5> has shown that the absorption rate changes with the surface area of the limestone particles which in turn varies with the size and the number of particles, and that the rate of dissolution plays a very important role on the absorption. It was further found the absorption rate does not vary significantly with temperature and that the reactions involved may be considered as being instantaneous. 

It is shown that the mechanism of gas-solid noncatalytic reactions can be understood better by following the variations in pore structure of the solid during the reaction. By the investigation of the pore structures of the limestone particles at different extents of calcination, it has been shown that the mechanism of this particular system can be successfully represented by a two stage zone reaction model below 1000 °C. It has also been observed that the mechanism changes from zone reaction to unreacted core model at higher temperatures. 

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. 

Single Pellet One Reaction. The sulfation reaction which is considered here for calcium carbonate is given by Eq. 3, and the temperature and concentration profiles of a typical growing limestone particle are shown in Figure 2. The rate of disappearance of sulfur dioxide is assumed to be the first order and is given by... 

Figure 2. Gas-solid reaction of a growing limestone particle at height H in the fluidized bed concentration and temperature profiles...

The above calculation is quite tedious and gets complicated by the fact that the properties which ultimately control the magnitude of these fourteen unknown quantities further depend on the physical and chemical parameters of the system such as reaction rate constants, initial size distribution of the feed, bed temperature, elutriation constants, heat and mass transfer coefficients, particle growth factors for char and limestone particles, flow rates of solid and gaseous reactants. In a complete analysis of a fluidized bed combustor with sulfur absorption by limestone, the influence of all the above parameters must be evaluated to enable us to optimize the system. In the present report we have limited the scope of our calculations by considering only the initial size of the limestone particles and the reaction rate constant for the sulfation reaction. 

Saraiva et al [121] presented an extended model for a circulating atmospheric fluidized bed combustor (CAFBC) which included hydrodynamics for the fast section at the top of the bed as well a bubbling bed section at the bottom of the CAFBC. For the fast section of the bed, one dimensional momentum and energy balances were used to predict the temperature and velocity profiles for gas and particles throughout the reactor. The model contain species mass balances for five gas species including SO2, as well as a model of SO2 retention by limestone particles. A bubbling bed model was considered to simulate the chemical process at the bottom of the combustor. 

With this the main object of the clinkering process, i.e., the formation of the valuable compound C3S, has been achieved, and it is this that requires and justifies the effort and cost of heating the raw materials to the high clinkering temperature, in addition, the liquid phase promotes other reactions, e.g., involving relatively coarse quartz or limestone particles. 

Fig. 8 shows the effect of the limestone particle size on the content of free CaO at various temperatures, bearing in mind that these are values obtained in the laboratory and give only a tentative indication of conditions in actual industrial practice. 

Fig. 8 Effect of limestone particle size on free CaO content at various burning temperatures (from Lehman/Locher/Thormann, 1964) dp — average particle size of a fraction lime standard KStI = 96 silica modulus = 3.0 alumina modulus = 2.2 At/At = 5to K/min. t = 30min. clay component illite...

Hartman, M. and Tranka, 0. (1980) "Influence of temperature on the reactivity of limestone particles with sulfur dioxide", Chem. Eng. Sci., 35, 1189-94. 

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. 

Bjerle, I., Xu, F., Karlsson, A., and Gustafsson, L., 1990, Studies of the SO2 High Temperature Reactivity of Super Fine Limestone Particles in Laboratory and Pilot Plant Scale, paper presented at the EPRI/EPA 1990 SO2 Control Symposium. New Orleans. LA, May 8-11. 

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. 

The lower explosive limit and minimum explosive concentrations of flax, wool, cotton, jute, hemp and sisal fibres are of the same order of magnitude as those of highly explosive dusts [15], The explosibility of pyrites dusts with sulfur contents above 20% was evaluated experimentally. Dusts of 30% sulfur content gave explosion pressures of 3 bar at pressure rise rates of 16 bar/sec. Mixtures of 60% pyrites and 40% powdered limestone still showed significant pressure effects, and the proportion of limestone actually needed to suppress explosions was considerably above the values currently accepted by mining industries [16], Effects of mixtures of particle sizes in combustible dusts upon minimum ignition temperature (T ") and upon presence or absence of explosion were studied. Presence of 30% of fines in a coarse dust lowers Tf significantly [17], Experimental explosions of polyethylene,... 

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