Characterization of the coal fly ash for the purpose of improvement of industrial on-line measurement of unburned carbon content
Introduction
Fly ash is the principal by-product of coal combustion. The main component of fly ash is silicon dioxide (SiO2). However, fly ash also contains several other oxides (e.g. Al2O3, Fe2O3, CaO, MgO) and unburned carbon. Particulate carbon lost with fly ash is the major determinant of combustion efficiency in coal fired boilers. The carbon content will normally be in the range 2–5 wt% although the fly ash may contain up to 20 wt% carbon. Excessive amount of residual carbon dispersed in the fly ash means a significant loss of energy. The disposal of fly ash containing excessive amounts of residual carbon is both difficult and expensive. The physical and chemical properties of coal fly ashes, their large quantity and environmental reasons enable to use them in many industrial branches [1], [2], [3], [4], [5], [6], [7], [8], [9]. If the content of unburned material (carbon) is below 5 wt% [9] fly ash may be a valuable raw material for the construction industry (e.g. for the production of concrete). The application of low NOx combustion systems causes the high residual carbon levels in ash. On-line monitoring enables the carbon concentration in fly ash to be kept at a reasonable level. The loss-on-ignition (LOI) [10] test is now the standard method for determination of the carbon content of fly ash from coal fired boilers. Standard method data have only historical meaning, because they do not make possible to control the work of boiler in real time. Fly ash sampled from the exhaust of a combustor is dried and weighed before being placed in a muffle furnace for several hours at 850 °C. The sample is then reweighed and the loss in weight is assumed to be due to the carbon initially present in the sample. On-line monitoring systems for determination of unburned carbon in coal fly ash use several techniques:
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microwaves absorption [11], [12]
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capacity measurement [13]
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photoacoustic effect [14], [15], [16]
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infrared emission [17], [18]
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optical dispersion–reflection technique [19], [20], [21], [22], [23]
The last method is mostly used in Polish power plants. The optoelectronic system for on-line determination and monitoring of the unburned carbon content of ash samples operates on the principle that the reflectance of infrared light is proportional to carbon content (Kubelka–Munk law) [24]. Ash samples are collected isokinetically from the flue gas duct and placed in sample tube with a flat glass bottom. The sample is exposed to the monochromatic light of The reflectance intensity is used by the system computer to determine residual carbon content from correlation curves [22], [23]. The sample is then air purged back to the duct or to the attached sample container to enable laboratory check analysis. The total cycle takes between 5 and 10 min. Real time results of carbon content with accuracy 0.3–0.7% are reported and can be used for boiler controlling. At the present Polish power plants use discriminated coal from different sources. This variation in the kind of coal used must be taken into account during the control of fired coal boiler. Proper control of combustion process maybe effectively improved if current information on the amount of unburned carbon is provided in real time. The goal of this research study carried out during the production process was to systematically follow up the results of measurements by an industrial analyzer and link them to physical and chemical properties of ashes. The study should facilitate the improvement and verification of data obtained from industrial analyzer as well as its practical utility.
Section snippets
Experimental
The fly ash used in the experiment originated from a one of Polish power plants—‘Kraków’ Combined Heat and Power Plant S.A. (EDF Group)—that uses the technique of pulverized fired boiler. On the output duct, before the electrofilter an on-line industrial optic analyzer (AWP model, Kwant Instruments, Poland) was installed to determine the content of non-combusted carbon in ash by using the technique of optical dispersion reflection. A study was performed for 3 subsequent months. Twelve sets
Chemical composition
The chemical compositions of coal fly ashes is shown in Table 2. In order to analyze their chemical composition two single portions were selected from the day series, varying significantly in terms of the content of combustible fractions from the optic analyzer of four following sets (days of measurements) from the first to the fourth and samples of first set collected from electrofilter. One may assume that the higher content of iron oxide may subsequently lead to too high results of unburned
Conclusions
The results obtained in long period of power plant work can be stated as follows:
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The content of unburned carbon is closely linked to the particle size distribution in fly ashes. The content of unburned carbon diminishes with the smaller size of particles. The ash from the optic analyzer is characterized by the highest content of grains ranging from 30 to 70 μm (25%) and in the ranges 10–30 and 70–100 μm (30% in total). Analyzing the grain size distribution for subsequent fields of the
Acknowledgements
This work was supported by the Polish State Committee for Scientific Research (KBN) under university grant no 10.10.150.549 and by the Kościuszko Foundation, American Center for the Polish Culture with the funds provided by Alfred Jurzykowski Foundation. We appreciate the help of Barbara Trybalska and Jacek Kordek in performing analyses on the fly ash samples.
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