Sorbent kalsinasyon ve sülfatasyon kinetiğinin incelenmesi

Abstract

Kömürün sabit veya akışkan yataklı bir yakıcıda yakılması sırasında sisteme kireçtaşı veya dolomit ilavesi, kükürtlü bileşiklerin yarattığı hava kirliliğini önlemek amacıyla gerçekleştirilen en yaygın uygulamadır. Yurdumuzun hemen her yöresinde bulunan zengin kireçtaşı ve dolomit yatakları, yüksek kükürt içerikli linyitlerimizin yakılması sonucu oluşan kükürt dioksit'in yarattığı hava kirliliğini önleme konusunda önemli bir potansiyel oluşturmaktadır. Bu çalışmada, Türkiye'nin değişik yörelerinden toplanmış olan 36 kireçtaşı ve 6 dolomit numunesinin kalsinasyon ve sülfatasyon özellikleri incelenmiştir. Farklı gaz atmosferlerinde ve çeşitli sıcaklıklarda gerçekleştirilen kalsinasyon tepkimeleri sonucu oluşan kalsine kireçtaşı ve dolomitlerin fiziksel özellikleri incelenmiştir. Termogravimetrik analiz sonuçları kullanılarak, bes farklı hesaplama yöntemi ve 14 değişik teorik model uygulanmak suretiyle kalsinasyon tepkimelerinin kinetiği incelenmiştir. Kireçtaşı ve dolomit numunelerinin kükürt trioksit tutma kapasiteleri belirlenmiş ve sülfatasyon tepkimelerinin kinetiği araştırılarak modellenmiştir.

One of the major sources of atmospheric pollution is the coal-fired power plants which produce flue gases containing large amounts of sulfur dioxide. It is im portant to control potantially harmful sulfur dioxide emmited into atmosphere by the combustion of fossil fuels. The tolerance level of plants and animals to sulfur dioxide depends on a number of factors: the concentrat ion of the pollutant and the exposure time, the type of plant or animal, and its condition and age. The maximum allowable concentration of sulfur dioxide in which it is considered possible for a healthy man to work for eigth hours is 5 ppm. Lignite, is the primary source of energy in Türkiye. Due to the low calorific values, high ash, sulfur and nitrogen contents and readily changing characteristics, of Turkish lignites, their increased utilization presents potential environmental problems. The main difficulty in the utilization of Turkish lignites is the emission of sulfur oxides. The available methods for controlling sulfur oxide emissions from combustion sources fall into three main categories: 1) Physical and chemical removal of sulfur before combustion, 2) Removal of sulfur oxides during combustion, 3) Removal of sulfur oxides from the combustion flue gas by scrubbing. xvi i Limestone and dolomite are the two natural sorbents which find extensive utilization for the removal of sulfur oxides resulting from the combustion of coal. The widespread availability and the low cost of these sor- bents together with the case of handling underline the practical and economical aspects of their usage for sulfur dioxide removal. Natural limestone and dolomite sorbents have been proposed for use in commercial fluidized bed coal com- bustors. This technique is economically and technically preferable to processes in which sulfur dioxide is washed from flue gas. If coal burns in a fluidized bed of lime stone, the limestone bed has a dual purpose; it acts as the fluidized medium for heat transfer, and it reacts with the sulfur dioxide produced by oxidation of sulfur present in the coal. The reaction between sulfur dioxide and limestone or dolomite is assumed to occur in two steps. The first step is the calcination, i.e. carbon dioxide is emitted from the carbonate according to the formula CaC03 (s) <=====> CaO (s) + C0=» (g) (endothermic) The second step is the sulfation: CaO (s) + S0= (g) + h 0* (g) > CaSCU (s) (exothermic) The major rationale for enhancing the potential of sulfur capturing lies in improving limestone utilization. Increased utilization of limestone particles may be attained by using small particle size, long exposure time, reaction at optimum temperature, and by properly selecting the reactive sorbents. Each of these individual approaches towards increasing limestone utilization is somewhat limited and, therefore, their simultaneous and combined use should be considered in any effort for improving limestone-sulfur dioxide sorption processes. This study deals with calcination and sulfation reactions of 36 limestone and 6 dolomite samples from various parts of Türkiye. Both, calcination and sulfation reactions of samples were conducted in two different systems; XVII i 1) In a tube furnace, and 2) In a thermogravimetric analysis system. In order to determine the effect of calcination conditions on the physical properties of the calcines, a number of experiments were carried out in a tube furnace at 5 different calcination temperatures (1073 K, 1123 K, 1173 K. 1273 K, 1373 K) and in two different gaseous atmospheres (100 % dry air and 15 % by volume COs in dry air). Physical properties such as, bulk and apparent densities, total pore volume, total surface area and average pore radius of the calcines of limestone and dolomite samples were measured by using a AUTOSCAN-33 Mercury Porosimeter. Because of sintering and shrinkage effects, a decrease in porosity and surface area; an increase in bulk density and average pore radius were observed at high temperatures. A high partial pressure of carbon dioxide during calcination of limestones and dolomites typically produced calcines with pores of larger dia- - meters than those of the calcines prepared in dry air. Depending on the calcination atmosphere, total pore volume of limestone and dolomite samples reaches its maximum value at different temperatures. These tempera tures are; 1123 K and 1173 K for limestones; 1073 K and 1123-1173 K for dolomites, in dry air and dry air + carbon dioxide calcination atmospheres, respectively. Total sulfur dioxide sorption capacities of lime stone and dolomite samples were determined. These exper iments were conducted in a tube furnace at constant temperature (1173 K) and in a gaseous mixture consisting of 15 % C0=, 0.35 % S0= (vol.) and balance dry air. Measured sulfur dioxide capacities of limestones samples range from 50 % to 90 % and of dolomites range between 40-60 %. The chemical composition of limestone and dolomite is of only minor importance in sulfation, whereas wide variations in reactivity can be related to differences in the physical properties of calcines of the stones. The xtx conditions in which calcination is carried out can great ly influence the physical structure and the reactivity of calcined limestones and dolomites. A relationship between sulfur dioxide sorption capacities and physical properties of calcines were found. A number of TG experiments were done to evaluate kinetic parameters for both calcination and sulfation reactions. Calcination TG curves were obtained under non-isothermal; whereas sulfation TG curves were obtained under isothermal conditions. Initial and final temperatures of the calcination reactions of the sorbent samples were determined from their TG curves. These temperatures are effected by calcination gaseous atmosphere. Initial calcination temperature of limestones ranges from 893 to 965 K in nitrogen and from 1073 to 1109 K in dry air + carbon dioxide atmosphere; on the other hand final calcination temperature ranges from 1148 to 1169 K in nitrogen and 1184 to 1216 K in dry air + carbon dioxide atmosphere. For dolomite samples, initial and final calcination temperature ranges were determined as; 813-922 K and 1125-1160 K in nitrogen; 858-978 K and 1158-1177 K in dry air + carbon dioxide atmospheres, respectively. A computer program in BASIC which enables regression analysis and determination of kinetic parameters from experimental non-isothermal thermogravimetric data was used to calculate kinetic parameters of the calcination reactions. This program allows the Coats-Redfern, Freeman- Carroll, Horowitz-Metzger, Horowitz-Met zger (modified by Dharwadkar and Karkhanavala) and Doyle (modified by Zsako treatments) methods to be performed for up to 14 differ ent solid-state rate controlling reactions, including n'th-order, Avrami-Erofeev, phase boundary movement and diffusional models. Kinetic parameters for the calcination reactions of the limestone and dolomite samples which were used in this study were computed. These parameters are depend ing on the calculation method and calcination atmosphere. xx The unreacted shrinking core model was used to modelling sulfation reactions which were performed in a thermogravimetric analysis system. According to this model, for spherical particles, the analytical relation ship between conversion and reaction time depends upon the rate controlling step. A comparison between the experimentally determined conversion-time data and the theoretical values calculat ed by using the equations of this model were made.