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SPHEROIDAL PARTICLES AS PASSIVE TRACERS OF SPREADING AND DEPOSITION OF FLY ASH FROM HIGH-TEMPERATURE COMBUSTION

Marko Kaasik, Institute of Environmental Physics, University of Tartu, Estonia, e-mail: marko.kaasik@ut.ee
Tiiu Alliksaar, Institute of Geology, Tallinn University of Technology, Estonia, e-mail: tiiu.alliksaar@gi.ee


What are the spheroidal fly ash particles?

In the case of high temperature during the combustion process ashes from the furnaces of thermoelectric power plants, central heating systems and several industrial processes (e.g. metallurgy) contain certain particles of spheroidal form. There exist two principal types of spheroidal particles (see Fig.1):

The first type is dominating in the emissions of liquid fuel (mainly oil products) combustion. The second type is more frequent in the ashes of solid fuels (coal, oil shale, brown coal, peat), although the carbonaceous particles exist in these ashes, too. Scanning electron microscopy and energy-dispersive x-ray spectroscopy analysis of individual fly ash particles have shown that particle morphology and composition is in some extent indicative to the fuel burnt and could be used to estimate the deposition of atmospheric pollution from combustion of different fuels (Rose et al., 1999).

Spheroidal fly ash particles are not harmful to the ecosystems but contain high concentrations of chemical elements on their surface and if inhaled cause damage of respiratory system of mammals and stomata of plants, especially when their concentration in the air is exceptionally high (like all types of solid particles). In opposite, these particles are chemically rather inert and therefore can be applied as passive tracers to follow fly ash pollution in present and past. Spheroidal particles are removed from the atmosphere with precipitation and by the way of dry deposition and afterwards deposited in the environment, including aquatic sediments and peat. If we have "calibration" of emissions from different technologies by means of spheroidal particles, then microscopic studies of these accumulative media give us a quantitative imagination of deposition fluxes of airborne industrial wastes in the past. Most of natural processes (except volcanic eruptions), as well as e.g. domestic heating do not produce temperatures high enough to generate the spheroidal particles. Thus, this tracer is highly selective in respect to large industrial objects, which is its advantage compared to several chemical tracers.

Estonian experience: spheroidal particles in the oil shale fly ash

Nowadays the Estonian economy is based on the electric energy produced from local Kukersite oil shale. Consumption of this relatively uneconomic (low caloric value) and "non-ecological" (thin layers - large mined areas, up to 60% of mineral matter - problems with ash disposal) fuel is a direct consequence of Soviet time "planned economy". Nevertheless, it is not realistic to abandon the oil shale energy in close future, but only diminish its consumption gradually, substituting it with renewable sources of energy. Two largest oil-shale-fired power plants in the world, the Estonian and the Baltic power plant (EPP and BPP), have total maximum capacity about 3 GW, but due to new structure of economy and diminished demand at Russian energy market no more than a half of that capacity is applied nowadays.

The chemical composition of the mineral part of oil shale is quite similar to limestone and actually the deposits of oil shale alternate with limestone layers. Therefore about 30% of fly ash from oil shale combustion constitutes calcium oxide. There are several other alkaline oxides and various trace metals in addition (Pets et al., 1985). Therefore the deposition of airborne wastes from oil shale combustion causes alkalisation of the environment, just opposite to the acidification under the influence of coal-fired power plants and metallurgy factories. Alkaline deposition has severely damaged bog plant communities in the influence zone of oil-shale-fired power plants (Kaasik et al., 2003). Deposition of emissions from oil shale combustion has been studied since 1985 in Estonia. Since the beginning of 1990's the atmospheric dispersion modelling is also applied besides the field measurements. Although the deposition fluxes of main alkalising components (calcium ion in first order) and sulphate ion in the oil shale processing region are well above the background levels at remote areas and their effects to the ecosystem are obvious, these components are generated in the cause of several natural and anthropogenic processes and therefore are not good tracers for fly ash when dealing with model validation and source-reception relationships. Spheroidal particles are expected to be more appropriate for that purpose.

Until 2003 the only technology applied in Estonian oil-shale-based energy production was the combustion of pulverised oil shale. During 1990's all oil-shale-fired power plants in Estonia had relatively inefficient flue gas purification systems (cyclon plus electrostatic precipitators) of Soviet production. Microscopic counting of the spheroidal particles in the ash samples of the precipitators revealed their concentration being about 3ˇ105 particles (diameter 5 mm and more) per one gram of ash and this amount did not vary a lot in samples from different energy production units. Most of these analysed particles belonged to the "glassy" (mineral) type. Dominant mass of the ash constituted of yellow particles of irregular form, probably mainly dispersed (and not melted) mineral matter. Ash samples from the tract after electrostatic precipitators were not available at that time. As a result of reconstruction, entire EPP and one energy block in the BPP have modern and more efficient electrostatic precipitators since 2002. As recent studies show, the fly ash from the smoke tract of EPP contain 1.4-2.0ˇ106 spheroidal particles per gram and they are still predominantly from "glassy" type.

To enhance the energetic efficiency and diminish the emissions to the atmosphere, one energy block in both EPP and BPP was transferred to a new combustion technology - circulating fluidised bed combustion - in 2004. Microscopic studies of ashes from the new block of EPP show dramatic changes in comparison with old furnaces - no "glassy" particles and very few carbonaceous particles remain. This result was expected, as the temperature in fluidised bed furnace (about 800 °C instead of 1400 °C in pulverised oil shale furnace) is not high enough to melt the alumosilicate matter. Thus, deposited spheroidal particles in the oil shale processing region indicate only pollution from the old type of furnaces. Nevertheless, as the renewed energy production capacity constitutes a minor part of the total one and fly ash concentration in the flue gas is three times lower than before, the spread of spheroidal particles must be still rather informative in respect to entire fly ash amount.

Influx of fly ash and spheroidal particles from the air

Spheroidal particles as all types of coarse solid particles are removed from the atmosphere by two principal ways: (1) washed out with precipitations and (2) deposited to the underlying surface due to gravitational fall and turbulent transfer. How representative are the measured deposition fluxes of spheroidal particles compared with the entire mass flux of fly ash?

In Table 1 are given some results of a case study carried out in December 2002 in North-Eastern Estonia. The traditional method to estimate the fluxes of fly ash in the oil-shale processing region is based on the calcium ion (Kaasik & Soukand, 2000), which is its major chemically active component (nearly 22% in ash, Pets, 1985). Another possibility is to calculate the total mass of all analysed components (cations, anions and suspended mineral matter), assuming that most of the deposited matter originates from fly ash. Concentration of spheroidal particles is the third possibility to estimate the total mass. The assumption that there are 1.6ˇ106 particles per one gram of ash, as found in the emissions of EPP, was used.

The fluxes estimated by these three methods in the oil-shale region are rather similar by magnitude. Total mass gives lower values than other two methods (presumably not all deposited components were included into analysis) and spheroidal particles give the largest values, but all discrepancies are not big compared to the standard deviations. Modelled deposition fluxes are lower, but difference is within factor two in all cases. At background area (140 km away) the deposition fluxes are much smaller and discrepancies between different methods much larger. That is a proof that both calcium and total mineral matter originate mainly from different sources than oil shale combustion. Spheroidal particles give presumably the best estimation of oil-shale fly ash flux at remote areas, but even these figures can be considerably overestimated due to different combustion sources at various distances.

Site type
(number of samples)
Measured deposition flux of fly ash, mg/m2 per day Modelled
deposition flux
of fly ash, mg/m2
per day (AEROPOL)
based on Ca2+ based on sphere. part. based on total mass
Forest (6,15-20km NNW) 29.0 ± 4.5 30.2 ± 5.4 25.6 ± 2.2 21.7 ± 3.2
Open land (5,15-20km NNW) 28.3 ± 3.6 38.5 ± 4.1 26.5 ± 1.7 19.9 ± 3.7
Forest (2,140km SSW) 1.01 ± 0.11 0.35 ± 0.13 1.68 ± 0.05 -
Open land (3,140km SSW) 1.89 ± 0.04 0.54 ± 0.19 2.32 ± 0.64 -

Table 1. Deposition fluxes (average and standard deviation) of fly ash estimated from measurements and computed, December 2 - 14, 2002. Number of samples and approximate distance and direction from nearest of two main power plants is indicated in brackets.

Spheroidal particles deposited in the environment

As already told, spheroidal particles are resistant to chemical degradation and when deposited preserve well in terrestrial or aquatic environments for centuries. This makes them a good tool for studying historical trends of industrially derived atmospheric pollutants. For that purposes accumulating environments acting like natural historical archives such as lake sediments, peat sequences, ice sheets etc. are used most often. In this way contemporary distribution of airborne particles is recorded in surface layer of sediments, while the temporal aspect of deposition can be studied from sediment cores.

Oil-shale-region in Estonia with precisely determined point sources and with well-documented combustion history of local fossil fuel offer good possibilities for studying spatio-temporal deposition of atmospheric pollutants. It has been noticed that the spheroidal fly-ash particle concentration profiles in the sediment cores follow very well the known fuel combustion history of the region (Alliksaar et al., 1998). Therefore sediment record of these particles not only gives us information about historical deposition of industrial particulates, but it also serves as a useful dating tool of sediments for the industrial era (Renberg & Wik, 1984). Lake surface sediments have been used to follow contemporary spatial deposition of spheroidal particles over the study area showing quite good relationship with real deposition data and respect to the location of power plants (Alliksaar & Punning, 1998). But among the natural archives peat deposits are better for this purposes as for lake sediments there often rise problems concerning the between-lake sedimentation rate variability and the in-lake processes. Studies have shown that Sphagnum peat sequences preserve well atmospheric fallout of particles due to trapping them in or between the hyalocysts of these mosses (Fig. 2) (Punning & Alliksaar, 1997).

Recently the identification of potential sources (fossil-fuel types) for particles accumulated in environment using their surface chemical composition have become one of the promising methods in order to study deposition of fuel combustion products. For Estonia another indicator of the distribution of locally derived oil-shale-combustion pollution is the ratio of glassy and carbonaceous particles in environmental samples being well above 1 in this region (Alliksaar & Punning, 1998).

Acknowledgements

Studies of atmospheric deposition of fly-ash components and analysis of spheroidal particles in the smoke tract ash were funded by the Estonian Science Foundation, grant No. 5002.

Fig. 1. Spheroidal carbonaceous particle (A) from oil combustion and alumosilicate glassy particle from oil shale fly-ash (B)

Fig. 2. Spheroidal particle trapped on the surface of Sphagnum moss.


References:

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