Review of the direct sulfation reaction of limestone

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Abstract

The direct sulfation reaction is defined as the sulfation reaction between SO2 and limestone in an uncalcined state, and is typically relevant for flue gas desulfurization by direct sorbent injection during pressurized fluid-bed combustion (PFBC) and SO2 absorption on limestone in the cyclone preheater used in cement production. In the past decades, this reaction has been extensively studied due to its potential for providing an economical control of SO2 emissions during PFBC and other similar processes. In this paper, a literature review of the direct sulfation reaction is presented. Various subjects, such as the influence of the reaction conditions (gas concentrations, temperature and system pressure), limestone properties and additives to the reaction kinetics, the reaction mechanism and modeling, are discussed.

Introduction

Emissions of SO2 from different industrial activities such as power production, the metallurgical industry and cement production are undesired due to its harmful effects. The world has long been aware of the destructive effects of SO2 emissions. In the past decades, great effort has been made to abate SO2 emissions. Many different processes have been developed for the purpose of cleaning flue gases for sulfur, including wet scrubbing, dry scrubbing, direct dry sorbent injection and regenerable processes. Of these processes, direct dry sorbent injection is a relatively simple and low-cost process. With this method, the sorbent, often limestone, is injected directly into the process at the place where the absorption of sulfur dioxide on the sorbent can readily take place, such as the combustion chamber in power plants.

With limestone as the sorbent, the sulfation reaction can proceed via two different routes depending on whether calcination of the limestone takes place under the given reaction conditions. The dissociation of limestone is normally determined by the CO2 partial pressure in the system. Limestone decomposes to form CaO and CO2 when the partial pressure of CO2 in the system is lower than the equilibrium CO2 pressure over limestone at the same temperature. The equilibrium CO2 pressure over limestone has been investigated by a number of authors (Johnston [1], Mitchell [2], Smyth et al. [3], Hill et al. [4] and Baker [5]).

Fig. 1 shows the dependence of the equilibrium CO2 pressure over limestone on temperature, as measured by Hill et al. [4] and Baker [5].

The curve in the figure can be described by the following equation [5]:log10pCO2e=-8308T+7.079.

Here, pCO2e is given in atmospheres (1 atm.=0.101 MPa), and T is given in Kelvin.

If calcination of the limestone takes place (the CO2 partial pressure in the system is lower than the equilibrium CO2 pressure over limestone), the limestone first decomposes to form CaO. The CaO then reacts with SO2. This process is often called the indirect sulfation reaction and is expressed by the following overall reactions:CaCO3(s)CaO(s)+CO2(g),CaO(s)+SO2(g)+0.5O2(g)CaSO4(s).

If calcination of the limestone does not take place (the CO2 partial pressure in the system is higher than the equilibrium CO2 pressure over limestone), the limestone may react directly with SO2. This process is often called the direct sulfation reaction and is expressed by the following overall reaction:CaCO3(s)+SO2(g)+0.5O2(g)CaSO4(s)+CO2(g).

This reaction is typically relevant in the application of direct dry sorbent injection for the reduction of SO2 emission during pressurized fluid-bed combustion (PFBC) and SO2 absorption in the cyclone preheater used in cement production. In PFBC, due to the high operation pressure, the partial pressure of CO2 in the combustor is normally sufficiently high to prevent the calcination of the limestone. The sulfation reaction in the combustor is thus the direct sulfation reaction. In cement production, the so-called “dry-process” is today the dominant process. In this process, a multi-stage cyclone preheater is used for preheating of the raw meal—powder mixture of the raw materials—by direct countercurrent heat exchange with the hot flue gas from the rotary kiln and the calciner. During the heating process, SO2 is formed mainly by the oxidation of pyrite that is contained in the raw meal. The formed SO2 partly is absorbed by the limestone particles—the major constituent in the raw meal—through sulfation of the limestone. The CO2 partial pressure in the hot flue gas is normally around 30 vol%, which is higher than the equilibrium CO2 pressure over limestone at the highest temperature of about 1073 K in the cyclone preheater. The sulfation reaction in the cyclone preheater is thus the direct sulfation reaction as well. This review is actually part of the project work which aims to get a better understanding of the mechanism and kinetics of the sulfation reaction of limestone in the cyclone preheater for the purpose of reducing the SO2 emission from cement production.

This paper discusses the direct sulfation reaction based on studies of the relevant literature. Subjects such as the influence of reaction conditions, the influence of additives, reaction mechanism, kinetics and modeling are discussed. Besides references on the direct sulfation reaction, a number of references related to the indirect sulfation reaction are also included; to the extent they are relevant to the subjects and beneficial for the discussions.

Section snippets

Influence of reaction conditions

The direct sulfation reaction can be significantly influenced by various parameters, such as temperature, system pressure and gas concentrations. The degree of influence of each of these parameters on the direct sulfation reaction varies with the reaction conditions and is often difficult to describe by using a simple rate law. The following sections provide an overview of literature findings concerning the influence of the above-mentioned parameters on the direct sulfation reaction, which

Reactivity of limestones

Different limestones often show different reactivities as shown by the studies of Zevenhoven et al. [29] and Alvarez et al. [15]. Zevenhoven et al. [29] studied the kinetics of the direct sulfation reaction of five different limestones at 1123 and 1223 K. The measured rate constants of these limestones varied from 0.00071 to 0.0013 m/s, an approximately 2-fold variation. Alvarez et al. [15] performed similar studies on five different limestones at 1123 K. The measured sulfation rates of these

Influence of additives

Both the direct and indirect sulfation reactions can be enhanced by various additives. Most of the results published in the literature are related to the indirect sulfation reaction. Considering that both the direct and indirect sulfation reactions may be enhanced by the additives by the same mechanisms, the results related to the indirect sulfation are presented and discussed here together with the results related to the direct sulfation reaction. Table 4 lists the additives that are found to

Porosity of the product layer

The direct sulfation reaction (Reaction (4)) is a gas–solid reaction with the formation of a solid product. Calcium sulfate (CaSO4) is normally the final product in an oxygen-containing atmosphere [91], [92]. The formed CaSO4 is of the type anhydrate II [93] and has a molar volume of 46 cm3, which is 24.7% higher than the molar volume of limestone (calculated as calcite with a molar volume of 36.9 cm3). The percentage of the volume increase is much higher than the normal porosity of a natural

Reaction mechanism

The detailed mechanism of the direct sulfation reaction is presently not well known. There are only few suggestions presented in the literature. Van Houte et al. [49], [95] suggested that the direct sulfation reaction takes place according to the following reaction steps at low temperatures (in the range of 573–900 K):CaCO3(s)+SO2(g)CaSO3(s)+CO2(g),2CaSO3(s)+O2(g)2CaSO4(s),2CaSO3(s)+SO2(g)2CaSO4(s)+S(g),S(g)+O2(g)SO2(g).

In this mechanism, Reactions (6) and (7) were assumed to be the main

Kinetics

The studies of the kinetics of the direct sulfation reaction are generally in a quite empirical stage, probably due to the lack of detailed knowledge of the reaction mechanism. In the following sections, the results of the intrinsic reaction rate of the direct sulfation reaction and the different opinions on diffusion in the product layer published in the literature are presented and discussed.

Conclusions

The direct sulfation reaction can be significantly affected by all the gaseous reactants and products (SO2, O2 and CO2) as well as water. The degree of influence of each of these gases varies with reaction conditions. The apparent reaction order of oxygen usually goes to zero at high concentrations. Higher CO2 concentrations can significantly hinder the direct sulfation reaction, most likely via its influence on the solid-state mobility.

Higher temperatures and higher system pressures may

Acknowledgements

This work was carried out in the Combustion and Harmful Emission Control (CHEC) research centre at the Department of Chemical Engineering, Technical University of Denmark, and financially supported by the Technical University of Denmark and FLSmidth A/S.

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