Nötron aktivasyon analizi ile tayin edilen eser elementleri kullanarak kömürlerin hava kalitesine etkisinin değerlendirilmesi

Abstract

Kömür, insan aktiviteleri sonucu çevremize atılmakta olan zararlı maddeler yönünden, geçmişte ve gelecekte önemli bir yere sahiptir. Son yıllarda yapılan çalışmalara göre, gelişmiş kontrol teknolojilerine rağmen, Amerika Bileşik Devletleri'nin, kömürle ilgili kullanımın son derece limitli olduğu doğu bölgesinde, havada bulunan parçacıklarının %50 sinden fazlası kömür kökenlidir. Bunun ana kaynağının 1000 km 'den daha uzakta olan termik santraller olması ise, kömür kökenli atıkların uzun mesafelere taşınmasından sonra bile etkinliğinin sürmesidir. Değişen yakma ve kontrol teknolojilerine ve çevre koruma standartlarına bağlı olarak, tüm dünyada kömür ve kömür atık ürünleri üzerindeki araştırmalar her geçen gün hızla artmaktadır. Bu tez çalışmasında, ülkemizde geniş kullanım alam olan kömürler ve külleri incelenmiş ve ulusal çevre kirliliğine olan etkileri belirlenmeye çalışılmıştır. Temin edilen örneklerdeki element derişimleri, Çekmece Nükleer Araştırma ve Eğitim Merkezi (ÇNAEM) Endüstriyel Uygulama Bölümü' nde nötron aktivasyon analiz tekniği ile hesaplanmıştır. Krom (Cr), çinko (Zn), arsenik (As), selenyum (Se), antimon (Sb), toryum (Th) ve uranyum (U) için zenginleşme faktörü (Z.F.) değerleri hesaplanmış ve bu değerlerden yararlanılarak, her element için ayrı ayrı grafikler çizilmiş ve değerlendirilmesi yapılmıştır. Bu değerlendirmeler sonucunda, Kısrakdere kömüründe ön yıkamanın toksik elementleri arıtması bakımından, çok fazla bir etkiye sahip olmadığı anlaşılmıştır. Türk kömürlerindeki uranyum ve özellikle arsenik miktarının, dış kökenli kömürlerden oldukça yüksek olduğu gözlenmiştir. Elde edilen sonuçlar, genel hava kirliliği çalışmaları ile birlikte değerlendirildiğinde, Türkiye havasındaki arseniğin, tamamen kömür yanması sonucunda doğaya atılan parçacıklardan kaynaklandığı sonucuna ulaşılmıştır. Uranyum ise daha değişik bir şekil göstererek, havada zenginleşmeyip, yerel olarak etkin olabilecek bir düzeyde kalmıştır. Özetle; As ve Se'un yüksek derecede uçucu küllerde zenginleştiği ve gerekli önlemler alınmadığı takdirde doğrudan çevreye karışacağı gözlenmiştir. Ayrıca U'un da uçucu küllerde zenginleştiği görülmüştür. Th ise en fazla yatak küllerinde zenginleşmektedir. Uçucu küllerde zenginleşen elementlerin, öncelikle radyoaktif olanlarının, bu küllerin kullanılması durumunda çevreye vereceği zarar önemlidir ve incelenmesinde yarar vardır. Bu tez çalışması ile, elementlerin kömür ve kömür küllerindeki davranışlarının incelenmesi konusunda, skandiyuma göre de Z.F. değerleri hesaplanmış ve bu elementin de bir standart karşılaştırma elementi olabileceği gösterilmiştir. Yine bu çalışma ile, kömürlerin kontrollü ve kontrolsüz yakılmaları halinde, elementlerin davranışları karşılaştırmalı olarak incelenmiş ve farklılıklar ortaya konmuştur.

Traditionally coal is known to be the "dirtiest" fossil fuel and inspite of the advanced emission control technologies its a combustion products continuos to have a major negative impact on the environment. In addition to green house gasses such as CO2 and acid precursors, (SO2, NOx), main portion of the toxic metals found in the atmosphere is result of emissions from coal fired power plants, industries using coal, home heating, etc. Chemical and physical characteristics of particulate material are mainly controlled by the kinds of pollution sources influencing in that area. Effect of particulate material on health is also a function of the particle size. Rather than how heavily the atmosphere contains particulates, whether and to what extend they will reach to lungs is more important. Scavenging mechanisms such as, rain-out and wash-out and dry precipitation due to the gravity are the factors controlling the amount and size of particles in the air. Small particles are much more persistent in the atmosphere and reach into the lungs more easily. Finer particles of fly ash have been shown to be weakly mutagenic. Certain compounds containing Cr, As and Se have been reported to be human carcinogens. Carcinogeniticity in animals used for experiments has also been reported for Be, Cr, Co and Zn. For this reason, chemical properties of the trace elements during coal combustion are important in the study of their toxicity. As it is very well known, our environment is mostly polluted in winter times. The main reason for this is the use of low quality lignite for heating during winter season. So, coal is the most important source for pollution. In countries such as Turkey, extensive usage of low quality lignites, increases the environmental and health impacts of coal related emissions. Also current coal cleaning combustion and emission control technologies may not be applicable to these kind of coals and national strategies should be established based on the type XII and quality on the coal consumed. Having this in mind, in this study five different coals representing the most common lignites used in Turkey, and their ashes were characterized with respect to their face element concentrations to established a base line for understanding their combustion properties as environmental impacts. All of these samples were analyzed by instrumental neutron activation analysis (INAA), using TR-2 Reactor at Çekmece Nuclear Research and Training Center and TRİGA MARK-II Research Reactor at Istanbul Technical University-Nuclear Energy Institute. Coal samples which came from the different regions (Çanakkale-Çan, Ankara-Beypazan and Kütahya Soma), from the same region (Kütahya-Soma) but different mines (Kısrakdere, Eynez, Soma) and from the same mine Kısrakdere- Lavvar(washed) and unwashed, first were combusted in İTÜ Metallurgy and Chemical Faculty fluidised bed reactor and men the same coals were burned in an open oven at Çekmece Nuclear Research and Training Center, Industrial Applications Department's Chemistry Laboratory. All coal samples were ground in shatter box and then about 5 g each put in a crucible and heated at 200° C and then combusted at 650° C. About 0.2 g homogenized coal, bottom ash, and fly ash samples were put in a I cm3 polyethylene tubes and irradiated for two hours at a thermal neutron flux at 2xl013 n. cm"2. s"1 in TR-2 Reactor and at about 4xl012 n. cm"2. s"1 TRIGA MARK II Research Reactor. U.S. Department of Commerce National Bureau of Standards (NBS)'s 1632b and 1633a Coal and Coal Fly Ash samples were used as the standards. A well equipped neutron activation analysis laboratory at Çekmece Nuclear Research and Training Center of the Turkish Atomic Energy Authority was used in this work. Gamma-spectrometry system was consisted of a sample holder, a HPGe detector, a high voltage supplier, a preamplifier, an amplification device, an oscilloscope, an analogue to digital converter, a multichannel analyzer with its computer and printer. The gamma spectra of all samples and standards were taken using a PC/486 based gamma-ray spectrometer equipped with Canberra ACCUSPEC/B type 4K MCA card, relevant electronics and two HPGe detectors (ORTEC, GMX and CANBERRA Coaxial Ge) with energy resolutions 1.9 keV and 1.8 keV for the 1333 keV line of 60Co respectively. At present methodologies such as gas chromatography, mass spectrometry, atomic absorption and atomic emission spectrometry coupled with inductively coupled plasma and nuclear-physical methods are routinely used to fulfill the requisites and requirements of environmental analysis. Neutron activation analysis (NAA) is one of the most sensitive methods currently available. It is also nondestructive and does not require any pre-irradiation, sample preparation steps other than weighting. More than 40 elements are been routinely determined by this method in most of the environmental samples. Samples weighting as small as micrograms can be analyzed by instrumental neutron activation analysis without any preconcentration. One of the drawbacks of neutron activation analysis with respect to general purpose usage is the limited availability of high thermal neutron sources, such as nuclear research reactors. XIII The method of activation analysis in principle depends on the facts that, whenever a sample is irradiated with particles produced from reactors, generators or other suitable sources, some of the nuclei precent in the medium interacts with those particles and produce radioactive isotopes. The radioisotope so formed, would decay with a characteristic half life by emission of alpha, beta and/or gamma rays of definite energies. In this study, selected nine elements scandium (Sc), chromium (Cr), iron (Fe), zinc (Zn), arsenic (As), selenium (Se), antimony (Sb), thorium (Th), and uranium (U), and were determined in coal, bottom ash and fly ash samples by using this technique. Element concentrations were calculated by International Atomic Energy Agency's (IAEA) Gamma Spectrum Analysis, Activity Calculations and Neutron Activation Analysis Software program ( GANAAS). Analytical results were evaluated by using elemental patterns, ratio and simple "receptor modeling" techniques such as enrichment factor (E.F.) calculations. Enrichment factor (E.F.) of element x is defined by, (Cx1/Cn1) E.F. = (1) (Cx2/Cn2) in which C's are the concentrations of element x and a normalizing element n in medium 1 and in the reference medium 2. The major purpose of this double normalization is to eliminate the sample variationing contributions from a specific single source and understanding if there are additional sources influencing the sample other than the normalizing matrix. Enrichment factors of individual elements were determined in ashes and atmospheric particulate material representing Turkish air masses, using average crustal pattern and coal compositions determined in this study. Scandium and Fe were chosen as normalizing elements, based on their common properties such as even distribution in soils from different regions, minimum volatility and solubility and presence of both in all environmental samples. However, in this study Sc is found to be more appropriate, due to the variations in Fe concentrations with the amount of pyrite in coal matrix. A second reason is the much lower detection limits are achievable for Sc by NAA. When enrichment factor results compared, although in most cases they were in good agreement with each other, there were some discrepancies based on the normalizing element chosen. After the calculation of enrichment factors, all values were given in a table and then for every element their graphics were obtained. These graphics were examined and the some conclusions are reached through this normalization. The major findings of this thesis are outlined below: Arsenic, which is known to be associated with pyrite, is expected to be high in Turkish lignites containing percent amounts of S. In this study most commonly used lignite's from different regions of the country and from different mines from the same region were studied with respect to their trace element contents. Only two elements, As and U were found to be more than 30 and 10 times higher than the coals of foreign origin respectively. Significance of these findings were examined with respect to their possible adverse health effects. Arsenic and selenium, among other possible sources, are mainly emitted from coal-fired power plants, and other processes utilizing coal. In ambient air their ratios (As/Se) is found to be less than 1, except areas influenced by specific point sources, such as Cu smelters. However, in long term atmospheric particulate studies in Turkey, the annual average of this ratio is found to be much higher than unity, reaching hundreds in certain days. This finding is rather unique and provides a robust marker for air masses influenced by the coal-related emissions in Turkey. Since atmospheric pollution is not limited within national boundaries, it is rather critical to identify regional markers in order to assess the impact of trans-boundary long-range transport. Another finding of this study is the fact that traditional coal cleaning (washing) methods applied to Turkish lignites do not reduce the concentration of toxic metals. Although there is a limited data available to reach to a firm conclusion, it is suggested that this may be due to the large particle size of the coals processed and/or the chemical forms and associations of toxic metals in the coal matrix are not suitable for the washing procedure applied. This point requires further investigation. Among the elements, determined in coal and corresponding fly ash samples, only As and Se have shown significant enrichment in fly ash compared to natural crustal composition and coal. This is a clear indication that these two elements would have regional influence due to longer atmospheric residence times because of their particle sizes. Other elements are either evenly distributed themselves in all phases (bottom ash, first and second cyclones) such as U or remained in mostly in the bottom ash such as Th. Coal fly ash is extensively used as a raw material in cement, ceramic products, road making, brickmaking, etc. all over the world. However due to this high concentrations of U found in Turkish lignites, usage of coal fly ash especially in construction materials should be carefully evaluated. Calculations based on the findings of this study has shown that, in cases where coal fly ash has been used in building materials in percentages within the industry specifications, inhabitants would be subject to an additional radioactivity of about 33 Bq/kg(concrete) due to the natural uranium. Activity due to the Th is found to be negligible. xv One of the most economical and efficient way to reduce negative effects of coal related emissions is the use of clean coal. However, it is a fact that due to the economical constrains, local lignites would continue to be major source for power generation industry and heating in Turkey. Therefore it is critical to understand the compositions of coals, their combustion and cleaning characteristics. Commonly applied cleaning (e.g. washing) methodologies should be modified and regionally specific methods should be adopted. Alternative techniques, such as density based physical cleaning methods need to be examined.