Doğal sorbentlerin rejenerasyon ve aktivasyon özelliklerinin incelenmesi
Investigation of activation and regeneration properties of natural sorbents
- Tez No: 46440
- Danışmanlar: PROF.DR. SADRİYE KÜÇÜKBAYRAK
- Tez Türü: Doktora
- Konular: Kimya Mühendisliği, Chemical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 108
Özet
ÖZET 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. Kireçtaşı ve dolomit'in kalsinasyonu sonucu oluşan kalsiyum oksit'in tamamı kalsiyum sülfat'a dönüşmemekte; bunun sonucu olarak, kullanılan sorbent miktarı artmakta ve çok miktarda katı atık ortaya çıkmaktadır. Kullanılmış kireçtaşı ve dolomit'in geri kazanılması veya kalsiyum oksit'in sülfata dönüşümünün yükseltilmesi, harcanan sorbent miktarını, buna bağlı olarak da katı atık miktarını azaltmaktadır. Bu çalışmada Türkiye'nin değişik yörelerinden toplanmış olan 20 kireçtaşı ve 6 dolomit numunesinin aktivasyon özellikleri incelenmiştir. Farklı yöntemler ile aktiflenen kireçtaşı ve dolomitlerden ısıl bozunma sonucu oluşan sorbentlerin fiziksel özellikleri saptanmıştır. Termogravimetrik analiz sonuçları kullanılarak, beş farklı hesaplama yöntemi ile 20 teorik model uygulanmak suretiyle sorbentlerin bozunma kinetiği araştırılmıştır. Aktiflenmiş 6 kireçtaşı numunesinin sülfatlanma oranları belirlenmiş ve sülfatasyon tepkimelerinin kinetiği araştırılarak modellenmiştir. Seçilen kireçtaşı ve dolomit numunelerinin geri kazanım özellikleri incelenmiş ve tekrar kullanılabilirliklerinin sınırları araştırılmıştır. XI
Özet (Çeviri)
INVESTIGATION OF ACTIVATION AND REGENERATION PROPERTIES OF NATURAL SORBENTS SUMMARY One of the major sources of atmosphere pollution is the coal-fired power plants which produce flue gases containing large amounts of sulfur dioxide. It is important to control potentially harmful sulfur dioxide emitted 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 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 healthy 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 the emission of sulfur dioxide. The available methods for controlling sulfur dioxide emissions from coal combustion sources 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 availability and the low cost of these sorbents 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 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: XIICaC03(s) - CaO (s) + C02 (g) (endothermic) (1) The second step is the sulfation: CaO (s) + S02 (g) + V* 02 (g) ?» CaS04(s) (exothermic) (2) In many cases, utilization of the CaO potentially available is poor. The major rational for enhancing the potential of sulfur capturing lies improving lime stone utilization. Increased utilization of limestone particles may be attained by using small particle size, long exposure time, reaction at optimum temperature, properly selecting the reactive sorbent, and by activation. 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. In coal-fired utility plant, CaO derived from hydrated lime, Ca(OH)2, has been found to be more reactive than that from the respective limestone, CaC03. Three reasons have been suggested for this superiority of Ca(OH)2-derived calcines over their carbonate analogues: 1) The activation rate for production of reactive Ca(OH)2 -derived calcines is much faster than the activation, or calcination rate that forms their carbonate analogues, allowing the sulfation reaction to begin sooner. 2) The inherent particle size of Ca(OH)2-derived calcines is much smaller than of their carbonate analogues, thus lessening mass transfer resistances. 3) Structural differences lend a greater reactivity to Ca(OH)2-derived calcines. Treatment of limestone with certain salts, such as NaCI, has been shown to enhance the sulfur dioxide reactivity of it in coal-fired utility plants. It was found that the addition of enhancement agents affects the pore structure of the sorbent during calcination which increases the extent of sulfation of the sorbent. However, volatilization of the alkali- metal compounds may cause unacceptable corrosion of metal components. Also, the presence of alkali in deposits of bed material on the metal surfaces can result in the formation of molten alkali-rich sulfates. The use of sodium chloride as an enhancement agent for sulfur capture in combustion systems was demonstrated to be a way of decreasing the levels of SO2 in off-gases. The reactivity of sorbents with SO2 was affected by many variables, including the sodium content of natural stone, the method of calcination, the nature of the original sorbent and the types of treatment. Coal-fired utility plant disadvantages are the incomplete utilization of CaO in the sorbent and the resultant increase in cost and environmental impact of (1) extensive quarrying and (2) disposing of solid waste (i.e., spent sorbent). Regeneration and reuse of limestone and dolomite or, alternatively, an increase in utilization of CaO can reduce this cost and environmental impact. In order to reduce limestone or dolomite feed rates to the fluidized bed boiler and to minimize the amount of sulfated material which must be discarded, XIIIsulfated sorbent can be regenerated to CaO and SO2 by reaction with a reducing gas. The principal reactions involved, for CO as the reductant, are: CaS04 + CO CaO + C02 + S02 (3) CaS04 + 4 CO -" CaS + 4 C02 (4) Reaction (3) is endothermic and is favored by high temperature. Reaction (4) is a side reaction which is undesirable because it consumes large quatities of reductant without rejecting sulfur from solids. CaS04 and CaS can react according to: 3 CaS04 + CaS -- 4 CaO + 4 S02 (5) The feasibility of the regeneration process depends on the ability to recycle the sorbent a sufficient number of times without loss of sorbent reactivity for regeneration and without severe decrepitation of the particles. The purpose of this study was to investigate activation and regeneration properties of 23 limestone and 6 dolomite samples from various parts of Türkiye. The samples are from: İSPARTA İZMİR BOLU SAKARYA TRABZON İZMİR MARDİN KONYA YOZGAT KIRKLARELİ ANKARA ÇANAKKALE TRABZON İSTANBUL İSTANBUL BURSA ANKARA KIRKLARELİ Nİ?DE DİYARBAKIR AYDIN ESKİŞEHİR ADIYAMAN ZONGULDAK BALIKESİR HATAY KIRKLARELİ ESKİŞEHİR BALIKESİR Göltaş Naldöken Bürnük Sapanca Araklı Işıkkent Savur Yolu Kumluk Yaylası Şefaatli Sarpdere Beytepe Ezine Meryemana Zeytinburnu Kartal Kestel Güvercinlik Akören Eskigümüş Ergani Söke Gümüşköy Çukurhisar Kayacık Ereğli Marmara Adası-I Yakacık Kapaklı Sivrihisar Marmara Adası-ll Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite xivCalcination reactions of 20 limestone and 6 dolomite samples were conducted in a tube furnace at a constant temperature (1173 K) and in a gaseous mixture consisting of 85 % dry air, 15 % CO2. Three activation methods were applied to the calcined sorbents: 1)Sorbents were hydrated with stoichiometric water (dry hydration), 2) Sorbents were hydrated with five times stoichiometric water (wet hydration), 3) Sorbents were hydrated with water-alcohol mixtures. Physical properties such as, bulk and apparent densities, total pore volume, total surface area and average pore radius of the calcines of activated sorbents were measured by using an AUTOSCAN-33 Mercury Porosimeter. Depending on the activation method, a decrease in total surface area; an increase in average pore radius were observed. In order to determine the effect of sodium chloride and trona addition on the sulphur dioxide sorption capacity of sorbent samples, they were activated with 2 % (wt) NaCI and trona solution. Total sulfur dioxide sorption capacities of the original and activated sorbents were determined and compared. These experiments were conducted in a tube furnace at a constant temperature (1173 K) and in a gaseous mixture consisting of 15 % C02, 0.35 % S02 (vol.) and balance dry air. A number of TG experiments were done to evaluate kinetic parameters for both decomposition and sulphation reactions of activated sorbent samples. Decomposition TG curves were obtained under non-isothermal; whereas sulfation TG curves were obtained under isothermal conditions. 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 decomposition reactions. This program allows the Coats-Redfern, Freeman- Carrol, Horowitz Metzger, Horowitz-Metzger (modified by Dharwadkar and Karkhanavala) and Doyle (modified by Zsako treatments) methods to be performed for up 20 different solid-state rate controlling reactions, including n'th-order, Avrami-Eroffev, phase boundary movement and diffusional models. 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 relationship between conversion and reaction time changes depending on 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. Four series of experiments were performed to evaluate the effects of repeated utilization on the reactivity of four stones, two dolomite and two limestone xvsamples. Each series consisted of five sulphation/regeneration cycles. The regeneration step of each cycle was performed at a tube furnace temperature of 1373 K. A 3:1 volumetric ratio of CO2/CO was maintained in the reducing gas to minimize sulfide formation, initially the capacities of the regenerated stones was higher than that of the fresh stones. This phenomenon is possibly a result of a more favorably crystal lattice, as compared to the fresh stones, formed on eliminating SO3 from the sulfated material. Sorbent activity slowly decreased on repeated regeneration. xvi
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