Kömür Yakılan Kazanda Ve Sobada Ağır Metal Emisyonlarının İncelenmesi

Abstract

Hava kirliliğinin ülkemizde, özellikle bazı büyük şehirlerde giderek arttığı ve tehlikeli sonuçlar meydana getirdiği görülmektedir. Bu noktada dikkatler gerek enerji üretimi ve gerekse endüstriyel ve evsel gereksinmeler için yaygın olarak kullanılan kömür üzerine çekilmelidir. Kömürlerin yaygın olarak kullanılması diğer kirleticilerle birlikte ağır metallerinden atmosferdeki taşınımını arttırmaktadır. Bu çalışmada 80 000 kcal/h kapasiteli, TSE belgeli, elle yüklemeli, ızgaralı, üstten yanmalı ve iki kapaklı sobada çeşitli kömür örnekleriyle yanma deneyleri gerçekleştirilmiştir. Kazan deneylerinde %37 nemli Yeniköy Ağaçlı, nemi %20 civarına getirilmiş Yeniköy Ağaçlı-Güney Afrika harman, nemi %20 civarlarında bulunan Yeniköy Ağaçlı-Sibirya harman kömür örnekleri, soba deneylerinde %37 nemli Yeniköy Ağaçlı, nemi %20 civarına getirilmiş Yeniköy Ağaçlı-Güney Afrika harman, nemi %21 civarlarına getirilmiş Yeniköy Ağaçlı, nemi %15 civarına getirilmiş Yeniköy Ağaçlı kömür örnekleri kullanılmıştır. Yanma deneylerinde gerçekleştirilen emisyon ölçümleri sonucunda her kömür için ağır metallerin toz ve gaz fazdaki atmosferik yayınımlarına yanma sistemlerinin ve kömür cinslerinin etkileri araştırılmış ayrıca elde edilen sonuçlar Türk ve Alman yönetmeliklerinde (Hava Kalitesinin Korunması Yönetmeliği ve TA-Luft) sınır değerlerle de mukayese edilmiştir.

Coal is likely to become an increasingly important fuel for electrical energy production during the next two decades. This trend appears inevitable due to the decreased emphasis on the construction of nuclear plants and relatively minor short-term impact usually projected for alternate energy sources (solar and geothermai). The emissions of environmental concern from coal fired plants may be divided into four categories: (1) SO2 and SO3 (2) NO and NO2 (3) organic compounds and (4) inorganic compounds. The organic and inorganic compounds include both gas phase emissions (such PAH emissions and mercury vapor) and particulate emissions (e.g. soot and fly ash). While the chemistry associated with the formation and ultimate fate of coal sulphur and nitrogen has been fairly well-defined, until recently the chemical nature and fates of the remaining trace elements during and following combustion have attracted considerably less interest. The control of particulate emissions has been of concern for many years, but with emphasis being placed primarily on the visible stack emissions from the combustion facilities. Recent research into the nature of the inorganic emissions from coal-fired power plants, however, has given reason for renewed concern. These results indicate that particulate emissions may be greatly enriched in certain trace elements, and that these trace elements may be in chemical for physical forms, which have an enhanced impact upon man. To understand the complex chemistry involved in the trace element enrichment process during coal combustion one must know something of the chemical/or physical nature of these elements in coal. The majority of trace elements in coal are associated with the inorganic mineral matter present in all coals. This mineral matter consists primarily of clays (aluminosilicates), quartz (SİO2), carbonates, sulphides, sulphates and oxides. The trace elements may also be associated with the coal macerals, having been present in the original vegetation from which the coal was formed. While many trace elements, have primarily either organic or inorganic associations some trace elements show an affinity for both fractions. During combustion the mineral matter undergoes both decomposition and transformation reactions which may result in the release of the more volatile elements. The ultimate fate of the trace elements will largely depend on content and initial concentration of the trace elements in the coal combustion temperature of the facilities particle size of the ashes, operation temperature of the control systems. In recent years, attention has been directed rather more to the elemental composition of the dust rather than to its nuisance value with some stress on the trace elements likely to be present-particularly the heavy metals. Whereas the coal before combustion has on elemental composition broadly similar to soils and crustal rocks-and hence similar to the natural dust content of the atmosphere the combustion process acts to concentrate a number of elements into the ash and dust by a concentration factor of five or six. Beyond this, a number of the more volatile elements re-condense after combustion preferentially on to the finer particles-because of their greater specific surface area enhancing the concentration of these elements by on even greater factor. Elements may be divided into two groups on the basis of their concentration dependence upon particle size: those, which show no enrichment in the smallest particles, and those, which are enriched. The primary interest is with the enriched elements, since they are most likely to have a significant environmental impact. Results of analyses of fly ash as a function of particle size at laboratory indicate that the elements Mn, Ba, V, Cr, Co, Ni, Cu, Ga, Nd, As, Sb, Sn, Br, Zn, Se, Pb, Hg and S are volatilized to a significant extent in the combustion process. The elements Mg, Ti, Na, K, Mo, Ce, Rb, Cs and Nb appear to have a smaller fraction volatilized during coal combustion, or have significant variations in behavior between plants. The remaining elements, Si, Al, Fe, Ca, Sr, La, Sm, Eu, Tb, Py, Yb, Y, Sc, Zr, Ta, Na, Th, Ag and In, are either not volatilized, or may show minor trends which might be related to the geochemistry of the mineral matter. The most important phenomenon of the trace element distribution is that of the vaporisation-condensation, which is present in all stages of combustion process. That is why the combustion temperature has the most relevant role to play in the distribution of trace elements in combustion products, the ideal situation is to have an exact knowledge of the chemical form of the elements and the operating temperature of the boiler and of the control systems. Thus, it should be possible to determine the fate of the trace elements fairly exactly. The analytical results provide firm evidence that a volatilization-condensation process account for the trace element enrichment observed in the fly ash emitted from coal-fired power plants. The enrichment process results from condensation of volatilized material preferentially upon the smaller fly ash particles. A relationship in which the concentration is proportional to D"2 usually applies for particles larger than 1-15 um in diameter. For smaller particles, in situations where other particle formation mechanisms become, important, or where the thickness of the condensed material becomes appreciable, a more detailed approach appears to more correctly describe the concentration dependence upon particle size. In some cases, the concentration of volatilized elements becomes independent of particle size for particles as large as several microns in diameter. Some mechanisms have been postulated to explain these observations. xi If the combustion conditions are always maintained the same and the coals used come from the same coal basin, which mean similar properties and rank of coal, a prediction of the trace elements destination in the final products, through correlations and the mathematical models, will be possible The volatilized elements, which condense upon, fly ash before particulate collection devices are often emitted into the atmosphere in greater abundance by a factor of up to 10 or more than elements not volatilized. These elements include As, Sb, Pb, Cd, V, Mo, Zn, Ga, Cr and U. These elements would typically have a 10-10 greater probability of pulmonary deposition upon respiration. Elements which are totally volatilized during combustion and which do not condense on particulate matter before the pollution control devices will often be emitted to the atmosphere in approximately 10 or greater abundance than elements not volatilized during combustion. These elements include the hologens, Hg, significant partions of the Se, B and perhaps-other elements such as Pb and Sb. These elements have 10 -10, or even greater, probability of pulmonary deposition upon respiration respiration than elements not volatilized during combustion. Further, these elements may be enriched by two to three orders of magnitute in the low temperature coal ash compared to their crustal abundance, leading to enrichment factors of 10 or greater for pulmonary deposition relative to the crustal abundance. The situation for the elements which are primarily in the gas phase at stack temperatures is similar to organic compounds, which include numerous mutagenic polycyclic aromatic hydrocarbons, which also rapidly become associated with the fly ash after leaving the stack by either condensation or adsorption processes. There are still several major gaps in existing knowledge of the trace element chemistry during coal combustion processes. When these gaps are filled, it should be possible to predict (at least semiquantatively) the extent of trace element emissions for a certain coal in a given coal-fired power plant. As noted above, the major affecting the trace element emissions from coal-fired power plants is the volatility, of the element during the combustion process. This necessarily involves a more complete understanding of volatilization from complex mineral phases and the fate of "organically-associated" species during combustion. In addition to the volatility of trace elements, the particle size distribution plays a major role in determining the emission rates for elements, which condense before the particle collection devices. Shifting the size distribution to smaller sizes will increase the emission rates due to a drop in collection efficiency for nearly all devices for 0.1-1.0 jim diameter particles. The size distribution may be altered by the combustion conditions. Research is necessary to determine the size distribution of particles resulting from the bursting or fracturing process and the dependence on combustion conditions and coal composition. Since the major parameters are likely to be the heating rate and composition of the particle, this process may be amenable to quantative treatment Regardless, it is important to determine if increased combustion xu temperatures necessarily increase the abundance of submicron particles if so, this factor would have to be considered in evaluating the advantages of increased combustion temperatures (e.g. increased plant efficiency, lower emission rates for other pollutants, etc.). Research must also address questions concerning the rate particle growth during combustion. Other problems involve the nature of the diffusion and crystal growth of trace species in ply ash particles after formation. Increased efforts should also be applied to the development of techniques for actual sampling of the high temperature combustion region. Ideally, these techniques should analyze major, minor and trace species in the gas phase and the particle size distribution well in to the condensation nuclei range, as well as elemental concentrations in the particulate matter as a function of particle size. Knowledge of the size distributions and compositions of the particulate phase through a combustion facility will be vital 10 a complete understanding of the combustion process and fly ash formation. The impact of new combustion and pollution control technologies must be care fully evaluated. And, there is an obvious need for more extensive and careful measurements of trace element emissions and particle size distributions from the various types of coal- fired plants. For example, particulate sampling methods need to be developed which avoid the loss of components with high vapour pressures. To increase the usefulness of these measurements, the coal should be analyzed, and the affinities determined for important trace element. Attempts should also be mode to determine the particle size distribution before the pollution control devices, and in the plume after most species emitted in the gas phase have become associated with the particles. There is also a need to understand the chemical and physical processes which the rates and temperatures at which the volatile species become associated with fly ash. A drop in the operating temperatures of pollution control devices may significantly reduce the emissions of these species. The correlation of these data with plant design and combustion conditions con provide both valuable emprical data on other factors affecting trace element emission rates and the means of greatly limiting the atmospheric discharge of trace element. To emphasize the effect of domesting heating to the air pollution in Turkey, concentrations of heavy metals in gas phase and in particular phase emitted from some kinds of lignite which are combusted extensively using the boiler and the stove have been studied. Gas sampling was carried out by passing the flue gas, sampled by a pump, through a thimble to remove the solid particles and extracting the trace elements in impingers including 0.1 N nitric acid. The stack particulates were sampled isokinetically using Andersen Universal Stack Sampler for the boiler and the small system for the stove and collected in the thimble for both stove and boiler. xm All samples were chemically analyzed using a number of techniques including atomic absorption spectrophotometry. Datas of the coal combustion have been compared with each other and limit values of "Air Quality Assurance Regulation".