Kalsiyum hidroksitin hidratasyon yoluyla aktivasyonu

Abstract

Ülkemizin enerji ihtiyacının büyük bir kısmı, kül, nem ve kükürt içeriği yüksek, ısıl değeri ise düşük linyit kömüründen karşılanmaktadır. Linyitlerimizin enerji üretmek amacıyla her yıl artan oranlarda yakılması, özellikle büyük ve endüstrinin yoğun olduğu yerleşim bölgelerinde hava kirliliğine neden olmaktadır. Linyitlerin yakılması sonucu oluşan kükürt dioksit atmosferdeki en önemli kirleticidir ve camı organizmaya birçok olumsuz etkisi vardır. Bu nedenle, kükürt oksitlerinin atmosferdeki derişimi kontrol altında tutulmalıdır. Bu amaçla, baca gazlarının atmosfere verilmeden önce temizlenmesi gerekmektedir. Baca gazlarından kükürt dioksitlerin giderilmesi için uygulanan işlemleri; kuru ve yaş yöntemler olmak üzere iki ana grupta toplamak olasıdır. Yaş yöntemlerde sulu çözeltiler kükürt dioksit' i absorplamak amacıyla kullanılırken kuru yöntemlerde kükürt dioksit katı sorbent tanecikleri tarafından tutulmaktadır. Toz tutucu sistemlerde, özellikle torba filtrelerde, baca gazından kükürt dioksit giderilmesi en ekonomik ve en etkili yöntemdir. Son yıllarda, düşük sıcaklıkta uygulanabilen ve yüksek yatırım gerektiren üniteler içermeyen, nemlendirmen' alkali enjeksiyon prosesleri geliştirilmiştir. Bu proseslerde, sorbentler 333-373 K'de nemli gaz içine enjekte edilmektedir. Yaygın olarak kullanılan sorbent Ca(OH)2'dir; ancak, Ca(OH)2'in kullanım veriminin oldukça düşük olması bu sorbentin sülfatasyon öncesi aktiflenmesini gerektirmektedir. Bu çalışmanın ilk aşamasını, yüksek silisyum dioksit içeriğine sahip bentonit ve diyatomit'in Ca.(OH)2 ile farklı koşullarda hidratasyonu sonucu aktif sorbentlerin elde edilmesi; ikinci aşamasını ise, elde edilen bu sorbentlerin kükürt dioksit kirliliğini önlemeye yönelik kullanılabilirliğinin araştırılması oluşturmaktadır. Farklı koşullarda gerçekleştirilen hidratasyon sonucu elde edilen sorbentlerin reaktivitesine hidratasyon koşullarının etkisi incelenmiştir. Ayrıca, farklı özelliklere sahip sorbentlerin kükürt dioksit tutma kapasiteleri belirlenmiş ve sülfatasyon tepkimelerinin kinetiği araştırılarak modellenmiştir.

The emission of sulphur oxides resulting from coal combustion is one of the great problems that leads to undesirable changes in the environment. This type of pollution can cause serious damage not only public health but also on soil, water and other natural resources. It is important to control potantially harmful sulphur dioxide emmited into atmosphere by the combustion of fossil fuels. The tolerance level of plants and animals to sulphur 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 sulphur dioxide in which it is considered possible for a healthy man to work for eight hours is 5 ppm. As known, the major part of energy requirement of our country has been supplied from lignites. Due to the low calorific values, high ash and sulphur contents and readily changing characteristics, of Turkish lignites, their increase utilization presents potential enviromental problems in great industrialized cities. Therefore, flue gases must be desulphurized before leaving the combustion system to control sulphur oxides emissions into the atmosphere. The available methods for controlling sulphur oxide emissions from combustion sources fall into three main categories: 1) Physical and chemical removal of sulphur before combustion, 2) Removal of sulphur oxides during combustion and 3) Removal of sulphur oxides from the combustion flue gas by scrubbing Besides desulphurization before combustion, flue gas desulphurization is considered the most promising method that could also be applicable to all other fossil fuels. Based on the mechanism of sulphur dioxide removal, flue gas desulphurization processes can generally be classified into two main groups, namely, wet processes and dry processes. In wet processes, the removal of sulphur dioxide is achieved by a chemical absorption using a limestone or slaked lime slurry as sorbents in spray towers. In dry processes, sulphur dioxide is removed via physical or chemical sorption on a sorbent material in a typical fixed bed. The major wet processes are; 1) Lime or limestone/sludge processes, 2) Magnesium oxide-magnesium carbonate/sludge processes, 3) Dual alkali processes, 4) Ammonia based scrubber processes, 5) Sodium based scrubber processes 6) Organic based scrubber processes and 7) Seawater process Dry processes are; 1) Spray dry processes, 2) Alkali Injection processes, a) Calcium based alkali injection process b) Sodium based alkali injection process c) Humidified duct injection process 3) Active carbon adsorption process, 4) Catalytic oxidation process and 5) Electron beam treatment Among these processes, low temperature humidified duct injection flue gas desulphurization processes are probably the best ones not only from the technical point of view but also from economical ones. These processes do not require costly atomizers, slurry preparation systems, or solid dewatering equipment which are necessary for the other flue gas desulphurization processes. Depending on the way at which sorbent and water are introduced into the duct, two different humidified duct injection processes are exist. Both of these processes are generally carried out at the temperature range of 333-373 K. In the first process, dry sorbent particles are injected into the flue gas between the air preheater and the particulate control device, which may be a fabric filter or an electrostatic precipitator. The flue gas humidified by spraying water either upstream or downstream of the sorbent injection port. Many companies namely, Dravo Halt, Epri and Consol Coolside have been using this process. In the second process, hydrated lime or another sorbent slurry is injected into the duct and the water evaporated from the slurry used for the flue gas humidification. Sulphur dioxide present in flue gas is removed not only by wet sorbent particles but also when they have been dried and the gas humidified, which can take place in the last part of the duct and in the particulate collection system. This process have been applied by the companies of Bechtel Czd, General Electric IDS and Epa & Sox. In both of these processes, the total sulphur dioxide removal is equal to the sum of removal in the duct section and the removal in the particulate collection device, especially if a bag filter is used. The major advantages of these processes over xi conventional wet methods are that a dry solid waste is produced and the equipment can be easily set up in an existing power plant. If the desulphurization of flue gases is carried out by hydrated sorbents in the presence of humidity, high sulphur dioxide removal efficiency can be obtained. The total sulphur dioxide capture at a given temperature increases as the adiabatic saturation temperature is approached. Since the relative humidity reaches its maximum at this temperature, the total sorption capacity of a sorbent is improved and maximum sulphur dioxide removal is obtained. Beside the humidity of the environment, surface area of the sorbent and alkalinity also play very important role in sulphur dioxide removal. The sorbent material which used widely in the humidified duct injection processes is hydrated lime (Ca(OH)2). The main drawback of these processes is that the sorbent utilization is lower than from conventional wet flue gas desulphurization processes due to the short residence time of solids in the ductwork. In order to increase desulphurization efficiency, hydrated lime is activated toward sulphur dioxide by reacting with different additives such as siliceous materials (fly ash, silica fume, bentonite, diatomaceous earths, etc.) and inorganic deliquescent salts (CaCl2, Ca(N03)2, NaCl, FeCl3 etc.). Thus the economics of humidified duct sorbent injection processes can be significantly improved. The production of reactive sorbents using these siliceous materials can be achieved by hydration of siliceous materials with Ca(OH)2 slurry. The reaction between the silicious material and Ca(OH)2 is called as a pozzolanic reaction (Eqs. 1-4) and gives a reactive product with a large surface area. In pozzolonic reaction, activation starts with the digestion of vitreous-phase silica and/or alumina by alkaline water and this stage is considered to be rate limiting. The dissolution can be enhanced by increasing the temperature, the reaction time, or/and by addition of (NH)4HP04, H3P04, NaOH reagents. The major pozzolanic reactions are Ca(OH)2 + Si02 + H20 > (CaO), (Si02)y (H20)z (1) Ca(OH)2 + AI2O3 + H20 > (CaO)x (Al203)y (H20)z (2) Ca(OH)2 + A1203 + Si02 + H20 -» (CaO)x (Al203)y (Si02)z (H20)w (3) Ca(OH)2 + A1203 + S03 + H20 > (CaO)x (Al203)y (CaS03)z (H20)w (4) where the coefficients w, x, y and z can assume a big number of values. xn The deliquescent inorganic salts (NaCl, KC1, LiCl, CaCl2, BaCl2, Na2SC>4, NaS03, Ca(N03)2, NaN03, NaN02, Na2S203, NaBr, etc.) are also used to increase the Ca(OH)2 reactivity. These deliquescent substances improve the Ca(OH)2 reactivity by absorbing a great amount of water on the sorbent surface. As mentioned earlier, the relative humidity and the amount of water adsorbed on the surface of sorbent have strong effect on sulphur dioxide capture. Sulphation reactions that take place in these processes for different sorbents are given below: Ca(OH)2 + S02 > CaS03. V2 H20 + V2 H20 (5) 2 CaOSi02 3/2 H20 + 2 S02 > 2 CaS03 14 H20 + 2 Si02 + H20 (6) Ca(OH)2. Salt + S02 > CaS03. V* H20 + Salt + V4 H20 (7) In this study, reactivation of Ca(OH)2 with bentonite and diatomaceous earths was achieved by hydration. Hydration experiments were performed at different conditions and conducted in two different systems namely; 1) In a water bath at atmospheric pressure and 2) In an autoclave under pressure. Also, the sulphation properties of activated sorbents were investigated. The effect of hydration conditions such as temperature, pressure, time and pozzolan /Ca(OH)2 weight ratio on the physical properties of activated sorbents were determined. A statistical design technique was applied by using of two-level factorial design matrix to interprate experimental results. The physical properties, which involve pore volume (cc/g), surface area (m2/g) and average pore radius (um) of the activated sorbents prepared under different conditions were determined using a mercury porosimeter (AUTOSCAN-33). Our experimental results showed a close link between the physical properties of the sorbents and the hydration conditions and chemical composition of puzzolan material. Amprical equations were obtained for the surface area values of the activated sorbents related to their hydration conditions. The correlation coefficients for these equations ranged from 0.83 to 0.99. The surface area values of the sorbents produced by the hydration of Ca^ETh with bentonite increased with increasing hydration temperature and decreased with increasing bentonite/ Ca(OH)2 weight ratio. The effect of hydration time was negligable. The surface area values of the sorbents produced by the hydration of Ca(OH)2 with diatomaceous earth increased with increasing hydration time and temperature. These values were not effected from the diatomaceous/ Ca^H^ weight ratio. xm Sulphation experiments were performed under bag-filter conditions, at constant temperature (338 K) and in a gaseous mixture consisting of 5 % 02, 10 % C02, 85 % N2 with the 55 % relative humidity and conducted in a thermogravimetric analysis system. Ca(OH)2 conversions and total sulphur dioxide sorption capacities of the activated sorbents were determined. It was found that these values increased with increasing surface area of the sorbents. The measured Ca(OH)2 conversions of the activated sorbents ranged from 56.80 % to 98.24 %; and total sulphation capacities ranged between 1.49-4.03 (mmol S02/g sorbent). However, the conversion of the non activated Ca(OH)2were determined as 17.96 %. The unreacted shrinking core model was used to modeling sulphation reactions. According to this model, for spherical particles, the analytical relationship between conversion and reaction time depends upon the rate controlling step. A comparison between the experimentally determined conversion-time data and the theoretical values calculated by using the equations of this model were made. The experimental results were found to be correlated successfully by this model.