___________________________________________________________________________
Evaluation of bottom ashes usability in road
construction industry based on the example of ashes
from Dolna Odra Power Plant in Nowy Czarnów
(NW Poland)
Krzysztof Chudy
1), Katarzyna Pawlak
2), Szymon Mielcarek
3),
Katarzyna Tupta
21)
KGHM CUPRUM sp. z o.o. – CBR, Wrocław, e-mail: kchudy@cuprum.wroc.pl
2)
University of Wrocław, Institute of Geological Sciences, Wrocław,
3)
PGI „Topaz”, Ostrów Wlkp. Abstract
The study presents the results of physicochemical and mechanical tests of ash-slag mixture occurring in the bedding of north part of bottom ashes landfill - Dolna Odra Power Plant in Nowy Czarnów. Moreover the field tests of shear strength using SLVT rotatory-percussive probe were made. The aim of tests was to determine the strength parameters being the extra requirements listed in PN-S-02205 standard for civil engineering contractors. The results allowed to establish the usability of the materials being tested i.e. wastes from combusting the hard coal in Dolna Odra Power Plant, for strengthening or improving the natural basements or foundations stabilized by binders. The second issue having at least the same importance for the authors, was the attempt to evaluate the factors influencing the geotechnical parameters, basing on laboratory and field tests.
Key words: bottom ashes, by-products of carbon combustion, waste minimization, shear strength, SLVT sounding
Ocena przydatności odpadów paleniskowych w budownictwie
drogowym na przykładzie popiołów z Elektrowni Dolna Odra
w Nowym Czarnowie
Streszczenie
W pracy przedstawiono wyniki badań fizyczno-chemicznych i mechanicznych mieszaniny popiołowo-żużlowej zalegającej w podłożu kwatery 4bb i magazynu A składowiska odpadów paleniskowych Elektrowni Dolna Odra w Nowym Czarnowie. Równolegle wykonano terenowe badania wytrzymałości na ścinanie sondą obrotowo-udarową SLVT. Celem prezentowanych badań było określenie parametrów wytrzymałościowych, które należą do nadprogramowych wymogów w rozumieniu PN-S-02205 dla wykonawstwa drogowych robót ziemnych. W artykule podjęto próbę dyskusji na temat przydatności odpadów paleniskowych w budownictwie drogowym w oparciu o odpowiednie normy branżowe i poradniki. Rezultaty badań materiału pochodzącego ze spalania węgla kamiennego z Elektrowni Dolna Odra, pozwoliły na ustalenie jego przydatności m.in. do wzmacniania lub ulepszania podłoży naturalnych i podbudów stabilizowanych spoiwami. Drugim, nie mniej ważnym dla autorów pracy zagadnieniem, była próba oceny czynników wpływających na parametry geotechniczne wspomnianego materiału, oparta na badaniach laboratoryjnych i polowych. Słowa kluczowe: odpady paleniskowe, uboczne produkty spalania, minimalizacja odpadów,
Introduction
Generation of bottom ashes from solid fuels is the integral element of power energy and heat production in conventional power, power&heat and boiler plants. While the Gross Domestic Product (GDP) had been continuously growing since 2000, 12% reduction of industrial wastes generation was noticed in Poland [3], what may be found as an effect of action taken in the interest of rational waste policy and implementation of applicable legal acts, such as Environment Protection Law from 2001. From the formal point of view, today, the wastes management strategy is regulated by the Act from 14 December 2012 about wastes, supported by Minister of Economy Decree from 8 January 2013 r. concerning the criteria and procedure of admitting the wastes for disposing them on the landfill of given type. Despite of successive decrease of amount of wasted produced in Poland, the disposal of wastes from hard coal combustion on heaps or settling ponds, quarters with “pulp” is a serious problem for the ground and water. In accordance with the above regulations, the sustainable management of power industry wastes consists, first of all, in minimizing their volume, and also in catching, gathering and reusing them, while the last possibility is their neutralization through irrational disposal.
Under so many restrictions and costs related with landfilling of the bottom ashes, a new ways of their re-use was initiated. Due to the progressive urbanization, resources depletion and the modern solutions implemented by leading Polish research institutes, much more often and preferably by-products of carbon combustion has been used as individual materials, hydraulic binders or ground-ash mixtures with good and even better geotechnical parameters than traditional mineral resources, pure lime or cement. From the point of sustainable economy and according to the specialists, e.g. Bolewski [1], Glinicki [4], Neville [8] opinion, in coal processing plants de facto the wastes are not produced any more but only the
potential mineral raw material.
The aim of the presented tests was to determine the strength parameters being the extra requirements listed in PN-S-02205 [12] standard for civil engineering contractors. The results allowed to define the usability of tested material, product of hard coal combustion in Dolna Odra Power Plant, e.g. for strengthening and improving natural basements or foundations stabilized by binders.
Materials and methods
Testing grounds (Fig. 1) are located in west part of Dolna Odra bottom ashes landfill within the flood terrace of the East Odra river. This area is, as a matter of fact, flat, and the average elevation difference in various parts of the site do not exceed 0.3 m. The subsurface zone consists of Holocene deposits of swampy accumulation (peat with high degree of humification) having maximum thickness of 3 m, totally covered by water. Within sands and gravels, below the peat, shallowly under the landfill bottom, the occurrence of underground water was found [2] having the nature of water table or locally confined. Underground water flows towards north-west, in the direction of Regalica (West Odra), which is the local base of drainage.
Physical and mechanical tests were made on ash-slag materials from different depth of 4bb quarter (ash disposal site) and A depot (Fig. 1), and their diversity is a result of the transport method (hydro transport) and the storage period at the quarter bottom. By-products of carbon combustion are delivered there by drainage system in
the form of „pulp”- wet ash and slag mixture, being a product of hydro-incineration – which is there deposited and drained using depression wells. The wastes transported that way, become, after short time, very similar to sedimentary (clastic) cohesive rocks. Fall down of mixture particles takes place according to the Stokes law – at first the particles having bigger diameters Fż+p, and then finer fπ, fi,, hence
the presence of characteristic bedding/stratification visible on scarp exposures. Within the layers the lithification process, thought colloids hardening and pressing the subjacent deposits, takes place, what contributes to separate the alternating weaker, not lithified - not coherent fly silts and stronger, coherent soft sedimentary rocks (Fig. 2).
Fig. 1. Schematic picture showing location of power plant, position of ash disposals and (marking the) sampling areas
Fig. 2. The alternating zones of very stiff and plastic (clastic) coherent rock (photo. Tupta, 2009)
The initial studies were based on surveying the „ground” foundation through making five test boreholes (two in A depot, three in 4bb quarter) (Fig. 1) with simultaneous taking disturbed and undisturbed samples which were used to determine the basic physical and chemical feature of bottom ashes. On that base the simplified geotechnical sketch of the foundation was made (Fig. 3) with separation of proper geotechnical layers, showing the changes of main strength parameters with the lack ending!. The dynamic sounding using SLVT rotatory-percussive probe was made in 6 places both on scarp (single sounding) and in the bottom of already used and planned quarter (part of depot). For in situ conditions, natural water content and ID compaction degree were taken as leading parameters.
Fig. 3. Average values of shear strength within the industrial wastes landfill of Dolna Odra Power Station
Tests of selected physical parameters were made according to procedures included in PN-88/B-04481 [10] standard. Grain size distribution, dry density ρ, density ρs, index of plasticity Ip, plasticity ratio IL and heave were determined.
Compressibility parameter - oedometer modulus of primary compressibility M0 was
determined for tension 0-200 kPa. The results are presented in Table 1.
Test of shear strength was made in direct shear machine - MATEST S277-01 on samples with modified water content, consolidated during 24 h, close to real field conditions and preliminary thickened to the required dry density of solid particles ρd
(Wroclaw University, Geology Institute Laboratory). Samples were sheared in 6x6 cm square molds at different vertical loads representing the normal stresses for the depth range from 1.2 to 10.0 m below the surface. At known densities there were initial stresses of 27-218 kPa. Two series of shear strength test ware made at the speed of 1.0 mm/min. Material used during the tests were the wastes taken to the cylinders (Ø 70 mm x 97 mm). Samples for shear tests in the direct shearing
apparatus were taken from two boreholes, from the depths 0.4 m, 1.0 m, 1.7 m
(quarter 4bb – sampling area B) and the depth of 0.53 m and 0.9 m (A depot – sampling area E).
Table 1. Basic physical and mechanical properties of tested material
Property Parameter Ash Depot A Ash disposal 4bb
D-E sampling area A-C sampling area
Depth m. below surface
0.53 m 0.9 m 0.4 m 1.0 m. 1.7 m
Grain size distribution
Gravel fraction content [%] (< 2.0 mm) fż
0.87 0.4 0.1 Sandy fraction content [%]
(0.05-2.0 mm) fp 13.7 8.73 0.97 61.11 72.3
Silt fraction content [%]
(0.002-0.05) mm fπ 79.3 80.74 90.53 32.6 22.9
Clay fraction content [%]
(> 0.002 mm) fi 7.0 9.66 8.5 5.9 4.7
Soil symbol after
PN-86-B/02480 π π π πp Pg Soil symbol after
PN-EN ISO 14688-2:2006 Si Si Si saSi clSa Coefficient of graining
non-uniformity Cu
15 10 8 9 6
Water content Natural wn [%]
51.39-54.14 76.53-79.32 47.63-48.27 24.76-26.06 22.13-23.21 Frost heave
Frost heave group High frost heave
Content [%] of de < 0.075 particles 90.3 85.4 98.6 52.8 42.6 Content [%] of de < 0.02 particles 57.6 75.0 76.5 22.7 16.0
Density
bulk density of soil ρ [g/cm3] 1.10-1.21 1.3 1.44-1.71 1.17-1.34 0.79-0.93 of solid particles ρs [g/cm3] 2.27 2.19
dry of solid particles ρd [g/cm
3 ] 0.72-0.79 0.72 0.97-1.15 0.94-1.07 0.64-0.76 Consistency Liquid limit wPlasticity index wL[%] Tixotropy under mechanical impact
p[%] - - - - - Water permeability Coefficient of permeability „k” [m/d] 2.99-4.06 E 4.41-6.02 D 0.60-1.35 27.36-39.27 Swelling Swelling index Vp [%] 2.6-2.7 2.7-2.8
pH 8.43 8.89 8.45 8.,38
Strength
Angle of shearing resistance φ [○] 32.05-56.83 49.43-51.34 36.65-45.68 34.02-41.25 43.01-45.00 Cohesion c [kPa] 77.7-89.63 40.39-77.05 5.31-15.57 6.15-15.70 7.73-13.64 R2 0.6808 0.8539 0.9983 0.9645 0.8109 Compressibility Oedometer modulus M0 [MPa]vσ’
0-200 kPa 17.82-36.05 5.05-27.68
Results
Bottom ashes from wet disposal has different properties but many aspects indicate that they are very similar to coherent natural soils (Fig 3). They may be used in many industries, e.g. in construction, special civil engineering, agriculture, sewage neutralization or for reclamation of polluted areas.
Granulation of bottom ashes is, taking in the consideration the natural soils classification, similar to silts or flour sands, since the most particles are smaller than 0.05 mm, i.e. are similar to fine fly ashes [17]. The content of particles having de<0.075 mm is from 42.6% to 98.6% (Tab. 1) and their specific surface is from 0.76
to 6.61 m2/g. Such big specific surface (for ashes is, generally <0.3 m2/g) results from relatively high content of silt fraction. Percentage of individual fractions indicate
that the material with high frost heave inclination - C group [15]. Big content of fine particles (de<0,05 mm) is accompanied by the presence of more than 10% particles
of de<0.002 mm at low share of sand fraction. Density of solid particles is lower than
the density of solid particles of natural soils. In comparison with natural soils, which density of solid particles is averagely 2.65 g/cm3, the values of 2.19-2.27 g/cm3 were found in ashes being tested. This property is very advantageous when the wastes are used as a material for construction of roads embankments, especially on weak basements.
Discussion
Due to the necessity of modeling the expected work conditions of basement and possibility of alternative description of shear strength, the strength tests are much more complex then determination of other physical and mechanical properties of soils [9]. Before starting the tests it is necessary to analyze carefully the program assumptions e.g. type and purpose of the strength parameters being determined, range of loads, condition of consolidation, rate of stress changes and method of results interpretation. The procedure of shear strength testing in the shear box apparatus allows, for example, to determine quickly the angle of shearing resistance at different loads and material thickening, which indicates the potential load capacity or frost heave of material.
Additional difficulty in model studies may be the fact that tests are carried out on transformed area, where both the current and past geological processes overlap. Constant delivery of fresh pulp, which afterwards dries, causes the continuous change of stresses/relaxations. The fluctuations of volume and shear strength within the individual soil complexes (variability of physical parameter under own load) takes place. The consequence of such long term changes may be the disturbance of soil-water conditions, e.g. forming new, favored paths of filtration and changes of the geotechnical parameters in ashes vicinity. Only careful separation, within them, of soil complexes and series (Fig. 3) allows for full evaluation not only of geotechnical parameters, but also for examining the most problematic and the weakest elements of the layers’ geological structure, what is of crucial importance to understand, among other things, the landslip processes [16].
In situ techniques often regarded as minor ones, are not limited only to establish
the stratigraphy of basement, drainage conditions or load history, but first of all to evaluate the geotechnical parameters under the existing stress conditions sounding were made using the SLVT rotatory-percussive probe with cross end. Results of shear strength (Fig. 4) were correlated with the compaction degree (ID), basing on
comparing the load and number of half-turns of the cross end.
Compiled results of sounding suggest that the resistance of drilling rods do not result only from the compaction degree but also a lithological-genetic type, water content and conditions of sedimentation. Lower shear strength occurs in 4bb quarter, the weakest are silts, especially layer between 1.4 and 1.8 m. below the surface, in the zones of water seepage within those silts (Fig. 4). Much higher values were obtained in the depot (in 2009), within well consolidated fine sands (100-370 kPa). On the scarp, in turn, the was the values of 100-170 kPa. Authors made soundings and took samples for laboratory tests outside the zones where structural surfaces occur.
Fig. 4. Shear strength of bottom ashes determined using SLVT rotatory-percussive probe. A - ash depot A, B - ash disposal 4bb
Non-uniform changes of angle of shearing resistance were stated during standard shearing tests. In most cases the change of shear strength was the result of cohesion changes (different water content), at small changes of internal angle of friction. The increase of water content was accompanied by cohesion growth (Tab.1, Fig. 5) and the shear strength (Fig. 3). Such condition was noticed in almost all samples (Fig. 6).
Fig. 5. Comparison of tests results for square box: angles of shearing resistance (A) and cohesion (B) vs. water content
Fig. 6. Strength parameters of chosen soils: angles of shearing resistance vs. cohesion The decrease of cohesion resulted mainly from lithology, type of „soil”, its consistence, which, due to the authors’ independent reasons was not determined (tixotropy even after small tremors). Angle of shearing resistance is high both in low coherent sandy silts, till sands and in silts which behave, due to consolidation and high water content, not as a „soil” but as coherent (clastic) sedimentary rock. As a result of shear both materials, despite of different structure, give the same results, especially in case of angle of shearing resistance, and different in case of cohesion (Fig. 5)
Summary and conclusions
Bottom ashes, as composite materials, may be used as construction materials having advantageous or even better geotechnical parameters than natural mineral resources. Especially favorable are their characteristic granulation, dry density of solid particles and shear strength.
Disposed fly ashes meet the requirements concerning the soil material for making foundations and layers of soil material stabilized by cement, included in PN-S-96012:1997 [13] standard. They cannot be used as separate material for communication and hydrotechnics embankments construction, because of too high content of particles smaller than 0.075 mm, a and too small content of sand-gravel fraction. Upon preliminary tests it was found that upper parts of disposed by-products of carbon combustion (only fly ashes) as properly dried “pulp” could be used as a hydraulic additive to other binding materials and to improve the grain size composition of non-cohesive soils according to PN-S-96035 [14] standard. This standard does not refer to the ashes from fluidized combustion or from installations for exhaust gases desulfurization. In accordance with PN-B-06050:1999 [11] standard, the ash-slang mixtures under investigation, qualified as having high frost
heave inclination, disposed on the bottom of storage quarter cannot be used for embankments construction even in dry places, isolated from ground and surface water, below the freezing line.
From the environment protection point, significant meaning has the sulfur content in ashes. Its content determines the quality of over-sediment water of wet landfills (ponds), methods of their reclamation and economic usefulness of ashes [18]. High SO4
content in wastes results from the process of their production i.e. combusting the hard coal combined with the process of exhaust gases desulfurization. Therefore they have also high Ca2+ content because it is a sulfur sorbent in that process [5]. Basing on analyze of the results the tested ashes may be qualified as sulfate-calcium ones, which are characterized by high hydraulic activity resulting from CaO content, what confirms that they are the wastes from desulfurization. Through this property hydraulically active ashes may become a valuable material for hydraulic binders and injection grouts production (improving foundations made of mechanically stabilized aggregates). Hydraulic activity similarly as pozzolana activity enables to reach low water-permeability, what confirms the field test of „k” - the coefficient of permeability (2.99-6.02 m/d). In may be supposed that silicate and aluminosilicate compounds are present there, because of high load capacity (estimated on the base of low compressibility), shear strength, chemical resistance (PE and PP) and reduced heave. Pozzolana activity like hydraulic activity causes low water-permeability of material. Both values cause that the pulp being disposed, after a short time resemble the clastic, coherent rock [7], with substantial cohesion, which gives the improvement of strength parameters of combustion products. Electrolytic conductivity of water extracts of ashes (ECW) measured in July 2009 was 137-456 μS/cm [6]. That value indicates on relatively low leaching and solubility of particles such as biogenic matter, calcium sulfates, chlorides, calcium, sodium, potassium, magnesium hydroxides and metals: barium and chromium, which dissolve in water very well. It is confirmed by hydraulic activity resulting from CaO content. In the ashes composition anhydrous and poorly soluble oxygens should dominate, which at the presence of water make the binding process difficult.
While presenting the study results, they were interpreted and analyzed where it was necessary. The results authorize to draw the following conclusions.
The most important advantages distinguishing the by-products of coal combustion from Dolna Odra landfill are: high shear strength, little compressibility, heave reduction, high Ca2+ content, small water-permeability, chemical resistance for dissolution and leaching as well as absence of naturally radioactive elements.
Acknowledgments
Authors are grateful to the Institute of Geological Sciences, University of Wrocław for financial support and “PGE Górnictwo i Energetyka Konwencjonalna, Oddział Zespół
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