Radiological characteristics and investigation of the radioactive equilibrium in the ashes produced in lignite-fired power plants

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Abstract

Coal- and lignite-fired power plants produce significant amounts of ashes, which are quite often being used as additives in cement and other building materials. In many cases, coal and lignite present high concentrations of naturally occurring radionuclides, such as 238U, 226Ra, 210Pb, 232Th and 40K. During the combustion process, the produced ashes are enriched in the above radionuclides. The different enrichment of the various radionuclides within a radioactive series, such as that of 238U, results in the disturbance of radioactive secular equilibrium. An extensive research project for the determination of the natural radioactivity of lignite and ashes from Greek lignite-fired power plants is in progress in the Nuclear Engineering Department of the National Technical University of Athens (NED-NTUA) since 1983. This paper presents detailed results for the natural radioactivity, the secular radioactive equilibrium disturbance and the radon exhalation rate of the fly-ash collected at the different stages along the emission control system of a lignite-fired power plant as well as of the bottom-ash. From the results obtained so far, it may be concluded that 226Ra radioactivity of fly-ash in some cases exceeds 1 kBq kg−1, which is much higher than the mean 226Ra radioactivity of surface soils in Greece (25 Bq kg−1). Furthermore, the radioactivity of 210Pb in fly-ash may reach 4 kBq kg−1. These results are interpreted in relation to the physical properties of the investigated nuclides, the temperature in the flue-gas pathway, as well as the fly-ash grain size distribution. It is concluded that towards the coldest parts of the emission control system of the power plant, the radioactivity of some natural nuclides is gradually enhanced, secular radioactive equilibrium is significantly disturbed and the radon exhalation rate tends to increase.

Introduction

Fossil fuels such as coal and lignite play an important role in electric power generation worldwide. Coal, lignite and their combustion residues (fly-ash and bottom-ash or slag) contain trace elements, including the naturally occurring radionuclides 238U, 226Ra, 210Pb, 232Th and 40K. The fractions of the combustion residues collected at different points following their pathway inside the power plant have different radiological characteristics, such as natural radioactivity content and radon exhalation rate. Since the produced ashes may be either disposed off, or further utilized in other applications such as the building materials industry, it is very important to study in detail the radiological characteristics of their various fractions. Furthermore, a detailed knowledge of the radiological characteristics will allow for a better determination of the radiation exposure, both occupational and of the public, due to the produced ashes.

About 65% of electricity in Greece is generated in lignite-fired power plants (44.5% installed capacity). Two major lignite field basins are under exploitation today:

  • 1.

    Ptolemais lignite field, with estimated reserves of about 2700 Mt lignite, where five power plants with 17 Units and a total capacity of 4000 MW is installed. The total lignite consumption is about 51 Mt year−1, and the ash production reaches 8 Mt year−1.

  • 2.

    Megalopolis lignite field, with estimated reserves of about 370 Mt lignite, where two power plants, with 4 Units and a total capacity of 850 MW is installed. The total lignite consumption is about 12.5 Mt year−1, and the ash production reaches 2.5 Mt year−1.

Greek lignite and especially Megalopolis lignite is classified as low-rank coal with high water content (up to 60%), high ash content (up to ~20%) and low calorific value of ~4.2 MJ kg−1. Furthermore, in both deposits, and especially in that of Megalopolis, the natural radioactivity of lignite is much higher than that of the surrounding soil (Simopoulos and Angelopoulos, 1987), due, presumably, to the leaching of radionuclides from uraniferous rocks in the vicinity. From this point of view this lignite has to be considered of high radiological importance. Silica (SiO2) content is also relatively high (~40% in the produced ashes), especially in the case of the Megalopolis lignite (Sakorafa et al., 1996).

During the combustion process, the burn-out of all combustible matter results in a significant enrichment of the ashes in incombustibles, which are partitioned between the bottom-ash (or slag), that falls inside the boiler, and the fly-ash that is suspended in the flue-gas together with vapours of volatile elements and compounds. Among the incombustible elements are all the natural radionuclides in the lignite. In the case of the nuclides of the uranium series, the three most important nuclides—238U, 226Ra and 210Pb—have different physicochemical properties, resulting in a different behavior and enrichment at the different stages of the combustion process. For example, according to Coles et al. (1978), the enrichment of 238U in the ashes depends upon the chemical and physical characteristics of the input fuel, as well as the conditions inside the furnace; since uranium may exist in the fuel both as uraninite as well as coffinite. Volatile and nonvolatile 238U species may be formed during combustion. 226Ra behaves in a similar way; a portion of 226Ra will reside with the uraninite fraction of its 238U parent, allowing for more mobile species than the silica associated 226Ra. 210Pb which is more volatile, leaves the boiler in gaseous form with the flue-gas and condenses as the temperature of the flue-gas drops. The difference in the enrichment of 238U, 226Ra and 210Pb in the ashes results in secular radioactive disequilibrium in the ashes (Coles et al., 1978). Furthermore, there is noticeable fly-ash partitioning along the emission control system, because most of the heavier and higher temperature fly-ash particles are removed from the flue-gas in the first stages of the system; the rest being detained at the electrostatic precipitators (ESP). A small portion of finer fly-ash particles may escape into the stack.

This work examines the relation of the radiological characteristics, and specifically natural radioactivity, secular radioactive equilibrium disturbance and radon exhalation rate of the ashes collected at certain stages of the emission control system of the 300 MW Unit-IV, Megalopolis-B Power Plant, to the temperature in the flue-gas pathway, as well as to the ash grain size distribution in the case of fly-ash.

Section snippets

Fly-ash and bottom-ash sampling inside the Power Plant

The investigation of the natural radioactivity of Greek lignite and ashes at the Nuclear Engineering Department of the National Technical University of Athens (NED-NTUA) commenced in 1983. Among the samplings conducted and the analyses presented so far, are those of representative lignite, fly-ash and bottom-ash samples from Units I and III of the Megalopolis-A Power Plant (Simopoulos and Angelopoulos, 1987). In the present work two sampling schedules were organised in the Unit-IV of

Gamma spectroscopic determination

The collected samples were analyzed at NED-NTUA using γ-spectroscopic techniques for the determination of the natural radionuclides: 238U, 226Ra, 210Pb, 232Th and 40K. For this purpose high-resolution high-efficiency Ge detector set-ups, including a LEGe and an XtRa detector were used. Details about the techniques used may be found in Simopoulos and Angelopoulos (1987). The 226Ra radioactivity was determined both indirectly from its decay products in equilibrium and directly from the 186.25 keV

Radon exhalation measurements of lignite and ashes

In the course of this research, several samples of ashes as well as feed lignite were analyzed for radon exhalation. The technique used is that of a closed-chamber method (Petropoulos et al., 2001) based on the enclosure of the sample (~300 g) in an airtight container and the monitoring of the initial radon growth inside the container for up to 10 h. In order to investigate the radon exhalation rate in the various fractions of the ashes collected inside the Megalopolis-B Power Plant, the

Conclusions

Ashes produced in thermal Power Plants may contain high levels of natural radioactivity and therefore they constitute a potential health hazard to the Power Plant personnel, and to the population living in the vicinity, due to fly-ash releases, fly-ash depositions and fly-ash industrial utilization. A detailed investigation inside a lignite-fired Power Plant was undertaken in order to determine the radiological characteristics of the produced ashes. From the results obtained the following

Acknowledgements

The authors would like to thank the Public Power Corporation of Greece and the Megalopolis-B Power Plant personnel, especially Mr P. Tsiampas and Mr P. Scordias, for collecting the lignite and ash samples. The authors would like to extend their thanks to the Hellenic Cement Research Center and especially Dr C. Malami for the laser granulometry analysis of the samples.

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