CURRENT RESEARCH
Reaction Rate of Sulfur Dioxide with Particles Containing Calcium Oxide C. Y. Wen' and M. lshida
Department of Chemical Engineering, West Virginia University, Morgantown, W . Va. W The rates of reaction of sulfur dioxide with pelletized calcium oxide particles and calcined limestones were investigated. Reaction rates of a pellitized calcium oxide particle and sulfur dioxide in air stream were measured a t temperatures between 590" and 860°C. At low temperatures, the overall rate was controlled by the rate of reaction taking place on each of the calcium oxide grains embedded in the agglomerated mass. At high temperatures, the effect of intraparticle diffusion on the overall reaction rate became significant. The effect of particle size on the overall reaction rate can be explained based on the grain model. The rates of reaction with various types of limestone were correlated in terms of the size of the crystalline grain.
cases more than an order of magnitude (Borgwardt, 1970a, 1972; Coutant et al., 1970). The deviation may be attributed to the different magnitude of the diffusion effect and the different grain sizes-Le., the crystalline size of calcium oxide in the limestones used in the various studies. However, how the differences in physical properties of calcined limestones affect the reaction rate of CaO-S02-02 system is not clearly understood. The present study develops a simple reaction model of this system based on laboratory data by taking into consideration the internal structure of calcined limestone. An attempt is made to elucidate the inter-relationship between the chemical reaction rate and the diffusion rate within the solid particle.
Although limestone is a relatively inexpensive and convenient means of removing sulfur oxide from flue gases in existing boilers, both coal and oil fired, it is unfortunately accompanied by inefficiency due primarily to two major difficulties: very limited residence time of injected particles resulting in poor utilization of limestone and scale formation over boiler tubes which considerably reduces heat transfer. These problems are largely overcome by use of other contacting processes such as fluidized bed combustor operating with a limestone bed. The fluidized bed is an excellent gas-solid contactor and is therefore not only an ideal combustion system but also provides attractive possibilities for removing sulfur oxides from combustion gases. Therefore, a working knowledge of the reaction mechanism between sulfur dioxide and particles containing CaO is needed to develop fully the potential of various limestonebased processes. This paper deals with analysis of the reaction rate of sulfur dioxide on calcium oxide and calcined limestones. The sulfur dioxide sorption apparently occurs mainly through the pores with absorption capacity generally increasing with decreasing mean grain size (Harvey, 1970). The variations in reactivity are due to the differences in physical properties of calcined limestones, such as, the size of calcium oxide grains and the pore structure (Harrington et al., 1968; Potter, 1969; Falkenberry and Slack, 1969; Attig and Sedor, 1970; Davidson and Small, 1970; Borgwardt and Harvey, 1972). Various kinetic studies have indicated that the rate of reaction differs considerably depending on the type of stones. Reported rates differ in some
Solid-Gas Reaction Model The general characteristics of the grain model (Ishida and Wen, 1971) for the reaction between porous solid and gas of the type represented by:
'To whom correspondence should be addressed.
A(g)
+
S(s)
-
product
(1)
is summarized below so that analysis of the experimental results can be made based on this model. When the solid particle is porous, the gaseous reactants can diffuse easily into the interior of the porous particles and the reaction zone may spread within the particle, which is made up of small individual grains as shown schematically in Figure 1.
kh7! I------
1 1
1 1
I
I 1
1 1
0
'0
Groin Model
- _--Figure 1 . Schematic diagram of concentration profile and solid structure in the particle Volume 7 , Number 8, August 1973
703
Petrographic examination of calcined carbonate rocks (Figure 2) shows spongy structure composed of small but highly dense grains (Harvey, 1970; Borgwardt and Harvey, 1972). If each of these grains is assumed to react in such a manner as to form an unreacted core covered by the product layer at a rate proportional to the initial solid reactant concentration in the grain, Cs,,', and to the gaseous reactant concentration a t the unreacted core of the grain, Cnc, we have, - aCsJat = (reaction rate per unit surface area) x (reaction surface area per unit volume) =
When the rate of reaction a t the surface of unreacted core in the grain is controlling, Cac is equal to the concentration of A within the pore of the particle and Equation 2 reduces to
acs
--=
~~,'C,J,'~C,/R'~
at
(3)
The effect of temperature on the reaction rate and on the concentration profiles of solid and gas reactants in the grain model is shown schematically in Figure 3. In the lowesttemperature region, V, the reaction zone spreads throughout the particle so that the concentration profiles of gas and .. . . . _ _. . .. solids are both tlat. 'The reaction rate in this temperature region is controlled by chemical reaction rate a t the individual grain. On the 1ither hand, in the highest temperature region, I, rata .y k rnntrnllnrl hlr rliffn&nn nf 1.0rrtant the reactir,..m .yll ",thn y...ul lv.l gas through the product layer. In this region the concentration of the reactant gas a t the boundary between the reaction zone and the product layer is practically zero so that most of the reaction takes place within a narrow hand adjacent to this biIundary, resulting in a formation of a clear reaction front. r tho ana that nf In Region IL, ".._rrtn nf rhnmical r-aetinn l__l._.. -.."..-" diffusion are of the same order of magnitude. The reaction takes place within a relatively narrow zone which moves inI
".."
I~.yIIyL.I
____ _.-..-.-.--.
ward as the reaction proceeds. This region is related to the chemical-reaction-controlling region in the unreacted coreshrinking model so that the kinetic behavior in Region III can be characterized by an apparent surface reaction rate (Ishida and Wen, 1971). Borgwardt and Harvey (1972) presented a semiempirical model for sulfation reaction of calcined rocks. The reaction rates a t initial condition (at zero conversion) were obtained by extrapolation and were then related to the BET surface area and effectiveness factor. Howard et al. (1971) presented a comprehensive analysis of mathematical models of reaction between sulfur dioxide and calcine particles. They analyzed the data of Borgwardt (1970a) based on two versions of the model. One is characterized by effective calcium oxide grain size and effective specific surface area but constant pore structure. The other is based on varying pore structure as reaction proceeds. They reported that only the first version was successful in matching experimental data. To match the experimental data based on the first version of the model, it was necessary that the size of grains in a given type of calcine be increased with an increase in particle diameti31, The grain model presented in ttlis paper is characterized by an average grain size uniquely related to the given type ,. . of calcine and is not affected by the parcicie size. I ne moaei is believed to be a simple geometric model realistic enough to represent closely the observed behavior of sulfation ph enomena.
.
-.
..
Experimental Data . .. . .. . " . . .. 'lo elucidate the ettect of gas aittusion on the overall i
^^
single reaction rate of CaO-SOz-02 system, the following ~. 1particle experiment was performed. A spherical particle was prepared by pressing calcium
_.
~
-
0
z. Limeslone (uDUA reTerence lO00"C (from McClellan, 1970) Figure
704
5P sample
Environmental Science 8 Technology
MYJ Camneu
m
tiharacteristic behavior Of solid-gas reaction systems under various temperature regions
Figure 3.
carhonate powder, which had been mixed with a sm all amount of water, into a cylindrical pellet of 1.2 cm diamelter hy a pelletizer. The pellet was then polished into a spheri
