Sülfatlanmış dolomit'in geri kazanımı

Abstract

Türk linyitlerinin nem, mineral madde ve kükürt içerikleri dünya Ortalamalarının çok üzerindedir; ısıl değerleri ise oldukça düşüktür. Bu nedenlerle, enerji üretmek amacıyla kullanımları önemli çevre sorunları yaratmaktadır. Fosil yakıtların yanmaları sırasında oluşan kükürt dioksit atmosferdeki en önemli kirleticilerden biridir ve canlı organizmaya oldukça olumsuz etkileri vardır. Ülkemizdeki bazı şehirlerde ve endüstri merkezlerinde atmosferdeki kükürt dioksit oranının zaman zaman sınır değerlerin çok üzerine çıktığı bilinen bir gerçektir. Bu veriler gözönünde bulundurulduğunda, en önemli fosil enerji kaynağımız olan linyitlerimizin yakılmalarından kaynaklanan kükürt dioksit'in yarattığı hava kirliliğini azaltmaya yönelik tedbirlerin vakit geçirilmeden alınması gerektiği ortaya çıkmaktadır. Bu çalışmada, Türkiye'nin çeşitli bölgelerinden toplanmış olan 6 dolomit numunesinin geri kazanım özellikleri incelenmiş ve tekrar kullanılabilirlik sınırları araştırılmıştır.

The use of fuels containing a significant amount of sulfur causes the emission of flue gases rich in sulfur dioxide, which are responsible for dangerous atmospheric pollution, damage to human health and environmental degradation such as acid rain. The tolerance level of plants and animals to sulfur dioxide depends on a number of factors: the concentration 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 healty man to work for eight hours is 5 ppm. Lignite, is the primary source of energy in Turkey. 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 their sulfur dioxide emission levels which is relatively high. The quantity of pollutants such as, sulfur dioxide, nitrogen oxides and particulate solids, permited to be released in the flue gas from coal-fired boilers is regulated in Turkey. Control of sulfur dioxide is the most diffucult since nitrogen oxide release can be kept within limits by modification of boiler equipment and particulates can be controlled by proper choice of equipment. The available methods for controlling sulfur dioxide emissions from combustion of coal fall into three main categories: 1) Physical or chemical removal of sulfur from coal before combustion, 2) Removal of sulfur oxides during combustion, 3) Flue gas desulphurization. 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 availably and the low cost of these sorbents together with the case of handling underline the practical and economical aspects of their usage for sulfur dioxide removal. VI Natural limestone and dolomite sorbents have been proposed for use in commercial fluidized bed coal combustors. This technique is economically and technically preferable to processes in which sulfur dioxide is washed from flue gas. If coal bums in a fluidized bed of limestone, 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) + C02 (g) (endothermic) (1) The second step is the sulfation: CaO (s) + S02 (g) + Vi 02 (g) CaS04(s) (exothermic) (2) 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. The limestone bed is sulfated by the S02 produced by the coal combustion to form an unreactive, spent material, i.e., limestone has a finite capacity for the sorption of S02. The spent stone must be removed and fresh limestone added to the bed continuously to maintain the capacity of the bed to react with S02. The designer of a fluidized bed combustor must determine the limestone feed rate needed to achieve the S02 emmision control required by environmental standards. Although the release of sulfur compounds to the atmosphere is effectively controlled by this method.the impact of quarrying large quantities of the sorbent and of handling the CaS04 -containing solids generated in the process must be examined more thoroughly. In the case of a typical bituminous coal containing 4% sulfur, the quantity of rejected sulfated sorbent can be greater than the quantity of ash that must be discarted. The rejected sulfated sorbent and ash can probably be used as landfill or disposed of in open pits in an environmentally safe manner. Methods for commercially utilizing the material may also be developed. Studies are Underway to determine if the available lime and sulfate in the reject sorbent can be used for agricultural purposes or as a component in the manufacture of cement blocks. The quantity of reject that can be used in these applications is uncertain and since a large quantity may have to be disposed of, options for reducing the quantity of sorbent used in the process should be examined. VII Five options are available, as follows : 1) Since sorbents vary in reactivity, use only the reactive sorbents. However, reactive sorbents may be unavailable within an economic distance of the combustion plant, 2) Modify nonreactive sorbents by changing the porosity, thereby increasing the quantity of sulfur that can be captured. Two methods have been demonstrated as a) a slow calcination of the sorbents and b) salt addition. In the slow calcination method, the sorbent would probably be precalcinate in a separate vessel specially constructed for this purpose. In the second method, additives NaCI, Na2C03, Na2 S04, KCI and CaCI2> have effectively increased the utilization of the calcium present in the sorbent, 3) Using a synthetic sorbent that has the porosity characteristics for good sulfur oxide sorption and which can be re-used. Various synthetic sorbents can be developed, but preparation of synthetics is expensive and unless a cheaper preparation method can be developed, this option may not be feasible. 4) Adjust boiler operating conditions using higher bed temperatures, lower gas velosities and longer particle residence times, which will help utilize more of the calcium in the particle, 5) A fifth method is to regenerate the sulfated sorbent for reuse in the combustor. The objective of sorbent regeneration is to convert calcium sulfate back to calcium oxide so it may be re-used and to liberate sulfur in a form where it can be recovered. In the one step -process, calcium sulfate is converted to calcium oxide at about 1373 K, and sulfure is rejected as S02 at a concentration high enough to permit it to be utilized. The reactions which can occure during the regeneration of calcium sulfate with a reducing gas such as CO and H2 is given below: 1- CaS04+CO ^ CaO + S02 + C02 2- CaS04 + H2 CaO + S02 + H20 3- CaS04 + 4CO -« CaS + 4C02 VIII 4- CaS04 + 4H2 CaS + 4H20 5- 3CaS04 + Ca 4CaO + 4S02 The first two reactions are the principal reactions; whereas reactions 3 and 4 are the side reactions that produce calcium sulfide. Since these reactions use up large amounts of reductant do not produce sulfur dioxside they are undesirable. The reduction of calcium sulfate to calcium oxide is favored by high temperatures (> 1313 K) and mildly reducing conditions. At lower temperatures (~ 1173 K) and under more highly reducing conditions, the formation of CaS is favored. The feasibility of the regeneration process depends on the ability of recycle the sorbent a sufficent number of times without loss of sorbent reactivity for regeneration and without several decrepitation of the particles. In this investigation regeneration properties of sulfated dolomite samples originating from the various localities of Turkey were evaluated. The origin of the samples are given below: ZONGULDAK Ereğli BALIKESİR Marmara Adası- HATAY Yakacık KIRKLARELİ Kapaklı ESKİŞEHİR Sivrihisar BALIKESİR Marmara Adası- The chemical analyses of samples were performed according to ASTM methods. Physical properties such as, bulk and apparent densities, total pore volume.totai surface area and average pore radius of the calcines were measured by using an AUTOSCAN 33 Mercury porosimeter. A number of TG experiments were done to evaluate the calcination sulfation reactions of samples. Calcination and sulfation TG experiments were performed under non-isothermal and isothermal conditions, respectively. Initial and final calcination temperatures of the dolomite samples ranged from 858 to 978 K and 1 158 to 1 177 K, respectively. Regeneration experiments were carried out in a tube furnace. In the experiments original dolomite samples were fully calcined at 1223 K in a gaseous mixture consisting of 85% dry air and 15% C02 for 30 minutes. Calcined sorbents were then sulfated at 1223 K with gaseous mixture of 15% C02, 0.50 %S02 and balance dry air for 60 minutes. Sulphated sorbents were reacted with reducing gas at 1373 K for 20 minutes. A 3:1 volumetric ratio of C02/CO was maintained in the IX reducing gas to minimize sulfide formation. In order to prevent the possible formation of CaS regenerated sorbent was oxidized with dry air at 1373 K for 20 minutes and the regeneration step was repeated again. Since the repeated sulfation-regeneration steps caused a strong change on crystal lattice, as compared to the fresh stones sorbent activity was also changed. Sulfation and regeneration properties of samples were affected from the physical properties of original calcines. It was found that the calcines which have small and large pore radius show different sulphation and regeneration efficiencies.