Characterization of Thermal Power Plant Fuel Oil Combustion Residue
In the present study, physical properties, chemical composition and leaching properties of a special waste type; fuel oil combustion residue of thermal a power plant, have been studied. Toxicity Characteristic Leaching Procedure (TCLP), sequential chemical extraction (SCE), alkalinity solubility and release as a function of pH` leach tests and X-ray Diffraction (XRD) analysis have been employed to achieve a more comprehensive understanding of waste characteristics. Elemental V, Ni, Fe, Cr, Pb and Cd concentrations have been traced in leachates from different tests. Results have shown special characteristics such as; extremely low pH level about 2.7, high Sulphur content; 21.50 weight percent and elemental V complex formation in untreated material. Results of TCLP test have shown excessively high concentration of V, Ni, Fe, Cr, Pb and Cd in leachates as 537.75, 472.5, 518.5, 38.54, 19.3 and 5.55 mg L-1, respectively. Results of SCE test show Ni, Fe and Cr, are most likely to be present in residue fraction, Pb in exchangeable fraction, Cd in bound to iron and manganese oxides and V is most likely to be present in bound to organic matter and sulfides but also a considerable fraction of it resulted to be in bound to iron and manganese oxides. Results of alkalinity, solubility and release as a function of pH test show relatively same release change pattern for Ni and Fe in pH range 2 to 5, but release reduction in pH 6 for Fe has steeper pattern than Ni. For other constituents, release changes do not show such abrupt pattern with pH variation. According to TCLP test results, the studied waste should be classified as hazardous and should certainly be stabilized and then disposed of according to hazardous waste disposal considerations. Sequential chemical extraction and release as a function of pH tests have made useful insights of waste intrinsic characteristics that can help for a better decision on best remedial technology that matches the waste characteristics.
Several studies on practical applications of pollution characterization have
been done in different receptor environments and by means of different evaluation
methods and practices such as: characterization of organic pollutants level
in water, sediment and living marine organisms of system of a river delta (Chau,
2006, 2005) and characterization of thermal power plant and combustion residue
waste types (Asokan et al., 2005; Saeedi and Amini, 2006) to ascertain
compliance with national and international risk and hazard regulations. More
pointedly, real importance of waste characterization studies is in their essential
aid to develop a more efficient stabilization method. The characterization of
different stabilized and unstabilized waste materials has been studied by many
researchers from different points of view (Geysen et al., 2004; Luz et
al., 2006; Saika et al., 2006; Sophia and Swaminathan, 2005). Although
the chemical composition and leaching characterization of heavy metal-containing
residue-type waste materials have been studied before elsewhere, most of characterized
waste materials are combustion residues of Municipal Solid Waste (MSW) incinerators
(Bagnoli et al., 2005; Eighmy, 1995; Forestier, 1998; Ibanez et al.,
2000; Jing et al., 2006; Li et al., 2004) or originated from coal-fired
power plants (Armesto, 1999; Asokan et al., 2005; Dincer et al.,
2006; Ilham et al., 2001; Snigdha and Vidya, 2006; Lopes et al.,
2001) or from other sources in thermal power plants (Saeedi, 2006).
Due to the fact that fuel oil is a heavy carbon containing material, commonly called as heavy oils and can be used in many other industries like cosmetic and perfume production, this kind of material is not used for the purposes of heat production internationally, but in Iran, considering the fact that required processing facilities are not available for such intentions and most of electricity is generated by thermal power plants; fuel oil is used for heat production purpose in thermal power plants. Fuel oil is burnt to generate electricity in thermal power plants during late autumn and winter (Anonymous, 2005a). The most commonly fuel oil type used in Iran is Fuel oil-Redwood 2000. In almost all cases, fuel oil production procedure in refinery sites does not include any preplanned facility to completely purify existing impurities imposed to crude, thus sandy-loamy, silica and sulphur containing impurities are often observed in final products of refining towers (Anonymous, 2005c) that may lead to some special characteristics in combustion products. Fuel oil combustion residues in Iran's thermal power plants have not yet been characterized to ensure environmental regulations or to assess any possible alternatives for fuel oil utilization. Even if its usage just be limited to the cold seasons, this type of waste is classified as specific industrial waste, which regardless of quantity needs to be characterized in detail under the waste management act of Iran (Anonymous, 2005a). In the present investigation four composite samples of fuel oil combustion residue of Shahid Rajaei thermal power plant have been subjected to investigation of physical properties, chemical composition and leaching properties to assess its compatibility with Iran's hazard regulations. Shahid Rajaei thermal power plant is located 100 km west of Tehran, Iran's capital, consists four 250 MW, natural gas and fuel-oil-burning units.
MATERIALS AND METHODS
In January 2006, 20 waste samples were taken from bottom ash of combustion tower of Shahid Rajaei power plant. The collected samples were stored in a cool place in sealed bags until analysis. Four composite samples were prepared by homogenizing and combining every fifth sample. The samples were air dried at room temperature (<40°C) to constant mass before being divided and screened. The water content of samples used for leaching was determined on a parallel sample by drying at 110°C overnight. Moisture content then was determined using ASTM 4254 (Anonymous, 2000) method. The natural pH of samples was determined using Cyber Scan PC510 pH meter. The chemical composition of dried samples was determined by X-ray Fluorescence (XRF) method and X-ray Diffraction (XRD) measurement was used to identify possible mineral phases in the waste sample.
Different leaching tests utilized for a comprehensive coverage of waste leaching characteristics:
||Toxicity Characteristic Leaching Procedure (TCLP) (Anonymous,
1992); a commonly used leaching procedure to identify waste characteristic
under a specific supposed worst case field condition; Acetic acid as leachant
in pH 2.88±0.05 over liquid to solid (L/S) ratio 20/1 and tumbling
in an end-over-end fashion for period of 18 h.
||Sequential chemical extraction; according to Tessier et
al. (1979); elements can be classified into the following five operationally
defined fractions: 1) Exchangeable: the sample was extracted for 5 h with
0.5 M MgCl2 at pH 7.0 and a L/S ratio of 8; 2) Bound to carbonates:
the solid residual from step (1) was extracted with 1M NaAc at pH 5 and
a L/S ratio of 8 for 5 h; 3) Bound to iron and manganese oxides: the residual
from step (2) was extracted with 0.04 M NH2OH.HCl in 25% (v/v)
HAc for 6 h at 96°C. The L/S ratio was 20; 4) Bound to organic matter
and sulfides: the residual from step (3) was extracted with 30% H2O2
for 6 h at 85°C, then extracted with 3.2 M NH4Ac in 20% (v/v)
HNO3 with continuous agitation for 30 min and (5) Residual: this
fraction was calculated from digestion of remnant of step (4)
||Alkalinity solubility and release as a function of pH (Kosson
et al., 2002); The equivalents of acid or base are added to a combination
of de-ionized (DI) water and the particle size reduced material (<2 mm),
with Nitric acid solution 2N in situation where natural pH of samples is
to be reduced or Potassium Hydroxide solution 2N where natural pH is to
be raised, to cover desirable range of pH values which may occur under different
field conditions, with final L/S ratio 10 in an end-over-end fashion for
24 h to identify leaching characteristics of samples over a broad range
of possible pH value (2 to 12) and in fact more realistic conditions over
different plausible management scenarios.
Performing the tests, elemental V, Ni, Fe, Cr, Pb and Cd concentrations of eluates from different tests were determined using atomic absorption spectrometry (Buck Scientific 210 VP model).
RESULTS AND DISCUSSION
This study is the first waste characterization practice of this type in Iran which looks forward to assess the quality of a special thermal power plant waste product; fuel oil combustion residue. The results show mean moisture content, wet density, dry density and natural pH of the waste, these parameters amount to 2.02%, 1929.5, 1859.5 kg m-3 and 2.71, respectively (Table 1). The waste obviously shows low pH level and high acidic condition that stands notably higher than many other thermal power plant originated wastes (Ilham et al., 2001; Saeedi, 2006) which are usually nearly among 5 to 7 in the worst case. It must be noted that, this matter has caused some ephemeral harsh irritation effect on inspiratory system of researcher because of tiny particle smog arising during mixing and homogenizing procedure. From another point of view, water content of the waste is considerably low and its density is not excessively high. These factors may be considered as matters of notice if the waste type is to be used as secondary light material production, which may be a scope of research throughout future remediation and stabilization studies. Low water content of waste type is considered to be as an obstacle or at least a limiting factor in selection of possible recycling techniques for constituents of interest, because many of physical or chemical recycling technique are based on separation, precipitation or absorption of elements from aqueous phase to solid phase which may necessitate a pretreatment operation on waste in present relatively dry form.
The average chemical composition of four waste samples determined by XRF analysis
shows LOI, SO3, V2O5, Fe, Ni, Na2O,
Cr2O3 , SiO2, CaO, Al2O3,
MgO, K2O, PbO, MnO and TiO2 are the main constitutes of
the waste on weight percentages basis in order of 36.76, 21.59, 17.12, 7.01,
6.02, 4.15, 2.07, 1.79, 1.53, 0.87, 0.3, 019, 014, 011 and 0.08, respectively
and with detection limit of 0.05 weight percent.
|| Physical properties of four analytical samples of fuel oil
combustion residue and average values
||Result of XRD analysis for final composite sample of fuel
oil combustion residue waste
File: 1764a.raw-Type: 2TH/ locked-Start:4.000-End 70.000-Step: 0.02 -Steptime:
1 35-0615(1)-Munirite-NaVO3.2H2O- Y:50%-d x by:1 Operations:
26-1165(1)- Calcium Vavadium Oxide-beta-Ca0.17V205-Y: 50%-d x by::1- WL:
39-1425(*)- Cristobalite, Syn-SiO2- Y 50%- d x by:1-WL: 1.5436-Tetragonal-a
37-1496(*)- Anhydrite, Syn- CaSO4- Y:50%- d x by:1-WL: 15406-Orthorhombic-a
27-0950(1)- Vanady1 Sulfate Hydrate- VOSO4.3H2O- Y:50%-
d x by:1-WL: 1.5406-Mon.
03-1209(D)- Iron Nickel- FeNi- Y:50%- d x by:-WL: 1.5460- Cubic-a 3.59100-
40-1115-Hydrogen Vanadium Oxide- H1.87V205- Y:50%- d x by:1WL:1.5406-1/lc
As it could be predictable due to formerly discussed possible impurities in
fuel oil production process, high percentage of sulphur and silicium content
is observed. This may strongly be contributed to probable sand, silica and sulphur
impurities inserted due to oil extraction procedure (Anonymous, 2005c) and the
accumulation process that may occurs in oil refineries and within the combustion
unit. High percentage fraction of vanadium, nickel, chromium and lead in the
waste material is also obviously notable. In a large majority of earlier studied
wastes, which heavy metal concentrations have been in a considerable amount,
elemental V has not even exceeded 1 weight percent fraction, similarly for the
case of Ni and Cr, concentrations have not been reported more than about 2-3
weight percent in the literature and more often in concentrations below 1% (Asokan
et al., 2005; Dinceral et al., 2006; Jing et al., 2006;
Saeedi, 2006). This may lead to future investigations on recycling potential
of elemental V, Ni and Cr through different plausible and applicable techniques,
specially for vanadium which its high concentration level interestingly provokes
comprehensive studies on recycling opportunities. It seems that compared to
many other studied residual waste forms such as: coal-fired thermal power plant
bottom and fly ashes and MSW incineration residues, present investigated type
of fuel oil combustion residue comparatively owns different characteristics,
Four homogenized samples were mixed together again to produce new composite
sample for XRD analysis (Fig. 1). Result of XRD analysis which
affirms metal complex formation in waste material. This must almost certainly
be contributed to high temperature condition, about 1100-1500°C, in power
plant combustion furnace that has made complex formation phenomenon possible.
Regarding dominating phases, based on qualitative percentage present in the
sample, major controlling complex phase is concluded to be Calcium Vanadium
Oxide (CaO.17V2O5) and minor one, Vanadyl Sulphate Hydrate
(VOSO4.3H2O) based on XRD analysis result.
||Concentrations of constituents of interest in leachats of
TCLP leach test for four samples in mg L-1
||Results of SCE test, depicting percent of constituents of
interest present in different predefined fractions in Tessier method
|Si: sample code (I: 1 to 4)
Results of TCLP leaching test are presented in Table 2. Concentrations of constituents of interest in leachats are too much higher than standard limits for TCLP test (Anonymous, 1992).
Results of SCE test, presented in Table 3, show that Ni, Fe and Cr, are most likely to be present in residue fraction, whereas fraction percent for step 5 is 57.74, 40.93 and 55.03% of total, respectively which stands for the biggest portion of fractions for mentioned elements; Cr has shown a considerable percent of about 22% for two first fractions that still stands for a large leachable potion of Cr that can readily leach to ambient environment (Table 3). Pb is most likely to be present in exchangeable fraction due to its highest percent contributed to first step, 57.41% and Cd in bound to iron and manganese oxides due to its highest percent in fraction 3, 43.72%, but obviously there exist a large potion of these two hazardous elements in first two fractions; 64 and 34%, which should stand as a dramatic point of notice in future supposed stabilization processes. V is most likely to be present in bound to organic matter and sulfides due its higher percent in this fraction; 36.14% but also a considerable percent of 25.84% is resulted to be in bound to iron and manganese oxides, high percent of Fe reported in XRF analysis affirms high concentration of Fe present in samples and possible bounds to Fe oxides, while XRD result shows vanadium complex formation with Sulpure and Calcium in oxide form as well.
Results of alkalinity, solubility and release as a function of pH (Table
4). Amount of constituent's concentration in leachate for pH ranges between
2 to 12 are presented. According to observed pattern of release changes, generally,
it is obvious that all constituents show excessively high release amounts on
a wide range of pH values and release is reduced with increase in pH for all
constituents of interest, but there are some different change rates at different
pH values. For Ni and Fe, leaching in range pH 2 to 5 has relatively same change
pattern but release reduction in pH 6 for Fe has steeper pattern than Ni. For
other constituents, release changes do not show such abrupt pattern with pH
||Concentration of constituents of interest in alkalinity, solubility
and release as a function of pH test for different pH values in mgL-1
Relatively high changes in released amounts in different pH levels must be
considered as a good representative of real field conditions which may lead
to a more efficient stabilization method with less operational costs. For example
in a supposed site land filling of raw waste material or even stabilized waste,
real alkaline condition of surrounding soil must be evaluated to assess real
potential of acid attack to the dumped materials and leaching potential of metallic
elements as long term environmental condition of final dump site. This may lead
to a not over-designed stabilization scheme, with fair consideration of real
site-specific conditions and finally in lower operational costs in field. As
stated in Kosson investigation (Kosson et al., 2002) on evaluation of
waste materials from the view point of real site-specific condition of management
scenario, just considering TCLP leach test standard criteria as adequate characteristic
of violation from hazard standards, is one of the most critical criticisms have
been done to TCLP leach test.
In the present investigation, first of this type in Iran, four composite samples of fuel oil combustion residue of thermal power plant, as a special waste, have been subjected to investigation of physical properties, chemical composition and leaching properties. Use of fuel oil for heat production purposes in thermal power plants is by itself a unique feature of this study.
Results show that this waste material owns special characteristics resorted as follows; high acidic condition, high percentage of S, V, Ni, Cr and Si in untreated material and dramatically high release amounts of constituents of interest in different test methods upper than health and hazard limits. According to TCLP test results, the studied waste should be classified as hazardous and should certainly be stabilized and then disposed of according to hazardous waste disposal considerations. Sequential chemical extraction and alkalinity, solubility and release as a function of pH tests have made useful insights of waste intrinsic characteristics that can help for a better decision on best remedial technology that matches the waste characteristics.
More pointedly, with reasonable attention to specific characteristics of waste material, remedial options with final dump destinations may be the last options. In fact, comparison of different plausible waste management scenarios and on the other hand; assessment of potential recycling methods with regard to volume of waste generated each year must be done in parallel. Different possible physical and chemical recycling methods, waste management scenarios and their operational costs for elements with considerable concentration levels have not been investigated in the present study; this may be an open area of study for future investigations.
This study is funded by Iran National Science Foundation (INSF) which is acknowledge.
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