Evaluation of Meat and Bone Meal as a Secondary Fuel with Olive

Jun 19, 2017 - ... as a Secondary Fuel with Olive Byproducts in a Fluidized Bed Unit. ... Engineering, Technical University of Crete, Chania 731 00, G...
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Evaluation of Meat and Bone Meal as a Secondary Fuel with Olive Byproducts in a Fluidized Bed Unit. Performance and Environmental Impact of Ashes Despina Vamvuka,* Marcos Papas, N. Alloimonos, and M. Kapenekaki School of Mineral Resources Engineering, Technical University of Crete, Chania 731 00, Greece ABSTRACT: Cocombustion of olive byproducts with meat and bone meal (MBM) was investigated in a fluidized bed system. The performance of the blends in terms of efficiency and emissions as a function of operating conditions was examined. Fly and bottom ashes were characterized and their potential uses and environmental impact were assessed through leaching tests on alkaline and acidic soils. All fuels burned mostly within the bed with a high efficiency. CO emissions were low, and SO2 emissions were negligible, while NOx emissions were below legislation limits, except those of olive kernel/MBM 80:20. The optimum performance for the blends was achieved when the MBM percentage in the mixture was 10%, reactor loading was 0.6 kg/h, and excess air was 30%. Fly ashes were rich in Ca, P, K, and Si minerals and Cu, Zn, and Sr trace elements. MBM ash consisted of high melting point calcium phosphates. Heavy metal values leached through both soils were below legislation limits for ash disposal. Ash materials could be used for soil amelioration.

1. INTRODUCTION MBM is a byproduct of the meat industry obtained after cooking mammal carcasses, eliminating fat, drying, and crushing. MBM has historically been used in animal meals.1,2 However, since the bovine spongiform encephalopathy (BSE) crisis feeding to ruminants has been banned, while import and export from European Union (EU) is not permitted.3 Only the low-risk material from safe animals is allowed to be used as pet food and agricultural fertilizer.2 In the EU more than 3.5 million tons of MBM per year are produced,4 requiring safe management. Even the low-risk material applications can spread pathogens to soil, diseases to animals, and environmental pollution.2 Therefore, thermal degradation treatment with energy recovery seems a highly promising solution for the management of MBM in a safe and economic way, in the light of its classification as biomass fuel5 with significant heating value. Given the chemical composition of MBM, the environmental and technical issues related to its stickiness, ash fouling, and agglomeration, its use as a secondary fuel has been the most evaluated option. Pyrolysis and cogasification have been investigated in lab-scale experiments, such as thermobalance systems,6 fixed bed,7 or fluid bed8−10 reactors, indicating that the processes are technically feasible and the products syngas, bio-oil, and biochar are potentially useful for energy or chemical/agronomic processes.5,11 Cocombustion of MBM, especially with coal and peat, has been extensively studied in thermogravimetric systems12 and in fluidized bed furnaces.5,13 Bed agglomeration tendencies and gaseous emissions were determined under different operating conditions. SO2 emissions were found to decrease with MBM addition to coal, while CO and NOx emissions were found to increase.5,13,14 Thermal valorization of MBM through combustion with high quality woody materials could be particularly advantageous, as opposed to the disposal option, offering technical, environmental, and financial benefits to rural communities. There is © XXXX American Chemical Society

lack of information on the cofiring of MBM with these materials, such as agricultural and agro-industrial wastes, which are available in large quantities in S. European countries. Furthermore, there is limited work on ash properties, related problems in fluidized bed systems, and possible uses. The composition of ashes only from MBM/coal cocombustion has been analyzed,12 to evaluate their suitability for deposition in landfills. Their utilization as potential fertilizers, due to their high phosphorus content, or as pollutants removal of hazardous metals in aqueous effluents,2,4,15 has been suggested. However, leaching experiments to assess risk and select proper management and disposal strategies have not been conducted. Based on the above, the present work aimed at investigating the thermal valorization of MBM through cocombustion with olive byproducts in a fluidized bed unit. The objectives were to study the performance of fuel blends in terms of efficiency and emissions, as a function of operating conditions, such as excess air ratio, fuel feeding rate, and MBM blending ratio and, moreover, to evaluate fly and bottom ashes, produced under different operating conditions via mineralogical, chemical, and fusibility analyses, and assess their environmental impact and potential uses. Finally, the work focused on the leachability of the various elements through alkaline and acidic soils, in order to investigate the environmental aspect of ash disposal or the valorization of ashes as soil ameliorants.

2. EXPERIMENTAL SECTION 2.1. Materials and Characterization. The animal waste under examination was MBM (from swine breeding) provided as received from Creta Farm industry, located in the municipality of Rethymno of the island of Crete. The agricultural residues selected, due to their abundance in the region of Crete, were olive kernel (OK) and pruning Received: April 5, 2017 Revised: June 7, 2017 Published: June 19, 2017 A

DOI: 10.1021/acs.energyfuels.7b00957 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the fluidized bed system. (OP), provided by a local olive oil factory and cultivated field, respectively, in the neighborhood of Creta Farm. Woody materials were air-dried and ground in a cutting mill to a final size of −2.8 + 1 mm, while the MBM sample was sieved to the same particle size. After homogenization and riffling, the fuels were predried overnight in the oven. For cocombustion tests, blends of MBM with each of the woody residues were prepared with ratios 10 and 20% of MBM by weight in the mixture. Representative samples of all three materials were characterized by proximate analysis, ultimate analysis, and calorific value according to the European standard CEN/TC335. The inert material used for fluid bed tests was a Na-feldspar NaAlSi3O8 with an average particle size of 421 μm. This has been found to diminish any bed agglomeration problems.16 Two different soil samples from the municipality of Chania, Crete, were selected for the leaching experiments, one alkaline soil from the area of Agia and one acidic soil from the area of Paleochora. These were subjected to particle size analysis and determination of the sand, silt, and clay proportions, via the hydrometer method,17 after passing a 2 mm sieve. Total organic carbon (TOC) was measured by a Gasometric Carbon analyzer 572-100, pH with an electrode from 1:1 water to solid slurries and cation-exchange capacity (CEC) by applying the ammonium acetate method.18 Mineralogical and chemical analyses were performed using the same techniques as for the biomass ashes described below. 2.2. Fluid Bed Experiments. Combustion tests were carried out in an atmospheric lab-scale fluid bed reactor (Figure 1), with an inner diameter of 70 mm and a total height of ∼2 m, as described in detail in a previous work.16 Inert material was preheated with air to ∼550 °C and when temperature reached steady state, the fuel was fed continuously through the feeders, at the predetermined rate. Temperatures along reactor height were measured by K-type thermocouples and pressure drop was measured by a differential manometer. The product gas was cleaned from particulates in a tangential type cyclone, cooled in a heat exchanger, further cleaned from particles, tars, and moisture in a gas conditioning unit and analyzed online by a multicomponent gas analyzer, model Madur Ga-40plus of Sick-Maihak. All data was displayed and logged on a PC via a data acquisition unit. At the end of each run, bed material and fly ash were drained, weighed and analyzed for unburned carbon, mineral phases, chemical composition, and species leachability through the soils. For each fuel or blend, feed rate and excess air were selected as principal independent variables. Feed rates were 0.6 and 0.72 kg/h, whereas excess air ratios varied between 1.3 and 1.5. Depending on fuel and operating conditions, air flow rates ranged between 3.5 and

5.5 m3/h. The results of two replicates from each test were subjected to statistical analysis via the Statgraphics 5 Plus software package. 2.3. Ash Analyses. Initial deformation temperature (IDT), softening temperature (ST), hemispherical temperature (HT), and fluid temperature (FT) were determined according to European standards DDCEN/TS 15370-1:2006, by an ash fusion determinator type 789-900. Mineralogical analysis of crystalline compounds was conducted with an X-ray diffractometer (XRD), model D8 Advance of Bruker AXS, with application of Cu Kα radiation and nickel filter (U = 35 kV, I = 35 mA). The XRD scans were performed between 2 and 70 2θ°, with increments of 0.02°/s. A software system DIFFRAC plus Evaluation by Bruker AXS and the JCPDS database were used for data processing and identification of crystalline components. Chemical analysis of ashes in major and trace elements was performed by an inductive coupled plasma mass spectrometer type ICP-MS 7500cx, coupled with an Autosampler Series 3000, both by Agilent Technologies (detection limits 0.4−34 ppb, depending on element). The samples were dissolved by a microwave-assisted digestion with HNO3 acid. The microwave digestion was carried out by using Anton Paar Multiwave 3000 oven. Phosphorus and silicon measurements were conducted using a spectrophotometer type UV− vis Hach 4000 V and an atomic absorption spectrometer (AAS) Analyst-100 of PerkinElmer, equipped with a graphite furnace assembly (model HGA 800) and a deuterium arc lamp background correction system. For sample preparation, the procedures of Li2B4O7 fusion or acid digestion (HCl/HF/HNO3) were used, depending on the element under determination. 2.4. Leaching of Fly Ashes through the Soils. Continuous column leaching experiments, simulating the release of components from a soil-ash mixture to a water phase, were adopted in this work, in order to get an estimate of long-term leaching behavior.19 To maintain compatibility with field conditions, soil-ash mixtures were prepared at a ratio 95:5 and no extraction with strong acids was applied. Purified water instead, with an amount corresponding to the average annual quantity of rainfall in the area of Crete (∼620 mm), was percolating through the soil−ash mixture (100g) in a vertical column with an ID of 2.5 cm and a height of 20 cm. The hydraulic head was kept constant during each test and the ratio of solid to water was 1:0.7, in order to simulate the water saturation capacity of the soil. The column effluent was collected in seven equal volume glass flasks, filtered through a micropore membrane filter and the pH of each extract was measured. The filtered leachates were concentrated, transferred to 25 mL plastic vials, and prepared for chemical analysis. B

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3. RESULTS AND DISCUSSION 3.1. Characterization of Raw Materials. A comparison of the proximate and ultimate analyses of the fuels is made in Table 1. As can be seen, MBM had less volatile matter and

Table 2. Physical, Chemical, and Mineralogical Properties of the Soils soil

sand (%) 52.0 clays (%) 3.0 silt (%) 45.0 TOC (%) 0.27 pH 8.2 cation exchange capacity (meq/100 g) 4.0 Metal Content (ppm) Si 318000 Na 180 Mg 1010 Al 1150 K 90 Ca 880 Fe 599.5 Mn 10.6 Sr 8.6 Pb 5.1 Co 1.4 Mineral Phases quartz +++ clinochlore muscovite + anorthite paragonite + hematite

Table 1. Proximate, Ultimate, Fusibility Analyses, and Calorific Value of the Samples (% dry weight) sample volatile matter fixed carbon ash C H N O S Cl GCVa (MJ/kg) initial deformation temperature (IDT) (°C) softening temperature (ST) (°C) hemispherical temperature (HT) (°C) fluid temperature (FT) (°C) a

olive kernel (OK)

olive pruning (OP)

meat and bone meal (MBM)

73.6 19.3 7.1 49.7 6.6 2.0 34.2 0.33 0.06 20.8 1026

79.4 17.1 3.5 48.6 6.3 0.4 41.1 0.03 0.07 19.2 1160

58.7 9.3 32.0 35.0 5.2 8.7 18.6 0.51 0.01 15.6 1240

1230

1360

1440

1530

1520

>1550

1550

1530

>1550

Agia

Gross calorific value.

carbon contents, whereas higher ash content than the agroresidues in study, resulting in a lower calorific value. The sulfur and chlorine contents of all samples, being related to emissions, fouling and corrosion in boilers, were low, however the percentage of nitrogen of MBM was high, implying toxic emissions during combustion. The fusibility analysis results in Table 1 demonstrate the differences between the chemical and mineralogical composition of the animal and woody wastes. All fuels presented high fluid temperature (>1500 °C). The high calcium content in phosphate forms with high melting point of MBM, coupled with lower potassium content, increased its fusion temperatures, as compared to olive kernel and pruning. IDT of the woody residues could be low for some combustion processes. However, for fluid bed systems operating below 1000 °C no slagging/fouling problems are anticipated. The properties of the two soils in Table 2 indicate that Agia soil was alkaline, in contrast to Paleochora soil which was acidic, whereas the CEC of both was low, mainly due to the relatively small percentage in clays. The concentrations of Si, Al, and Fe are in accordance with the results obtained by the hydrometer method and the XRD analysis. Heavy metal values were very low, with the exception of Mn and Pb in Paleochora soil. 3.2. Performance of Fuels during Fluidized Bed Combustion. 3.2.1. Axial Temperature Profiles, Flue Gas Emissions, and Combustion Efficiency. Figure 2 illustrates the temperature profiles along the height of the reactor of the fuels at steady state, feed rate of 0.6 kg/h and excess air ratio λ = 1.3. As can be clearly seen, the profiles of the woody agroresidues were very similar. Both fuels attained the maximum temperature (869 °C) just 30 mm above the air distributor plate within the bed, suggesting that as soon as the woody particles were fed into the hot inert material their volatiles, which amounted 74% and 79% for olive kernel and pruning,

Paleochora 44.1 12.5 43.4 1.3 5.2 7.0 282000 170 1050 4760 190 1200 3139 270 17.4 27.0 3.7 +++ ++ + + +

Figure 2. Temperature profiles of the fuels along the reactor height at F = 0.6 kg/h and λ = 1.3.

respectively, were released and burned. The temperature remained quite uniform inside the bed and then decreased toward the end of the expanded bed (>250 mm), due to increased heat transfer rate between the dense phase and the freeboard. Combustion of these fuels continued in the gas phase along the freeboard zone as Figure 2 shows, attaining temperatures of ∼790 °C. Flue gas temperatures measured in the conical section of the furnace (not shown in the graph) were ∼450 °C. When MBM was mixed with either olive kernel or pruning, it can be observed that the axial temperature profiles obtained in both cases presented a similar shape to those of the woody fuels, as the percentage of MBM in the mixtures was up to 20%. However, combustion temperatures inside the furnace were reduced with increasing blending ratio, because MBM material C

DOI: 10.1021/acs.energyfuels.7b00957 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels had lower volatiles and fixed carbon, whereas a much higher ash content than the agroresidues under study. At the lower part of the reactor column, within the bed, the temperatures of the mixtures varied between 831 and 863 °C, while in the freeboard region between 775 and 808 °C. No signs of agglomeration or defluidization were observed during the tests. Flue gas emissions from the combustion of the examined fuels are compared for same operating conditions (feed rate 0.6 kg/h, excess air ratio λ = 1.3) and at steady state (average values ± standard error) in Figure 3. SO2 emissions, being practically

null due to the low sulfur content of the samples, were omitted from the figure. CO levels (Figure 3a) were kept well below legislation limits during all tests and were similar or lower to those reported for other biomass fuels burned in small scale fluid bed units.20 Olive pruning, although burned at nearly the same temperature as olive kernel, presented higher CO emissions, most probably due to its greater amount of volatiles, which boost hydrocarbons concentration in the furnace, inhibiting further oxidation of CO. The increased concentration of CO in the flue gas, with increasing percentage of MBM in the mixtures, is attributed to the lower reactor temperatures attained in this case, as well as the high ash content of MBM, which weakens oxygen penetration to the char particles and therefore their complete combustion. NOx emissions from combustion of the fuels under study were according to the guidelines21,22 of EU countries for small units (200−350 mg/ Nm3), with the exception of those corresponding to olive kernel and MBM blends. Consequently, combustion of this mixture, under the conditions examined, would require several measures, such as air staging, flue gas recirculation, or flue gas treatment. The low NOx values obtained for olive pruning were attributed to its lower content of fuel-N among the materials tested, following the fuel nitrogen mechanism.22 In addition, the large amount of volatile matter released from olive pruning could have created a temporary, reducing environment, favoring NOx decomposition. The combustion efficiencies of the fuels at a feed rate of 0.6 kg/h and excess air 30% in Table 3 are seen to be high, ranging between 98.5 and 99.3%. These values were controlled by the CO levels in the flue gases, which represented the principal heat losses due to incomplete combustion of flue gases in the freeboard zone (LCO). Unburned carbon in fly ashes (Lfa) had the largest portion in the total carbon loss in ash (Lfa and Lba). Olive kernel burned with the highest efficiency. When the share of MBM in the mixtures with the woody fuels was raised, combustion efficiency was dropped, because of reduced reactor temperatures associated with the higher ash content of the mixtures.

Figure 3. Average (± standard error) CO (a) and NOx (b) emissions of the fuels at F = 0.6 kg/h and λ = 1.3.

Table 3. Fluidized Bed Combustion Performance of Fuels at Different Excess Air Ratios and a Feed Rate of 0.6 kg/h flue gas emissions (ppmv) sample OK

OK/MBM 90:10

OK/MBM 80:20

OP

OP/MBM 90:10

OP/MBM 80:20

heat losses (%)

excess air ratio λ

bed temperature (°C)

freeboard temperature (°C)

CO

SO2

NOx

LCO

Lba

Lfa

efficiency η (%)

1.3 1.4 1.5 1.3 1.4 1.5 1.3 1.4 1.5 1.3 1.4 1.5 1.3 1.4 1.5 1.3 1.4 1.5

858−869 846−860 841−855 847−863 838−855 835−850 831−841 827−838 824−834 850−869 850−868 849−863 847−858 840−852 836−847 832−839 827−835 823−831

453−806 423−803 417−792 430−784 431−778 423−770 385−796 370−787 363−778 472−800 470−795 470−799 431−801 410−797 380−799 390−808 375−804 360−800

475 538 592 637 468 510 659 714 768 1166 1175 1189 529 980 1172 718 1101 1203

7.6 13.8 14.1

272 325 380 345 354 386 420 429 455 91 93 97 131 150 185 159 178 215

0.52 0.59 0.65 0.70 0.52 0.56 0.72 0.78 0.84 1.26 1.27 1.28 0.57 1.07 1.28 0.79 1.20 1.30

0.10 0.11 0.11 0.10 0.43 0.41 0.25 0.30 0.27 0.15 0.20 0.25 0.45 0.30 0.33 0.30 0.34 0.40

0.10 0.10 0.10 0.10 0.17 0.17 0.20 0.22 0.21 0.10 0.10 0.12 0.18 0.14 0.15 0.15 0.15 0.20

99.28 99.20 99.14 99.10 98.88 98.86 98.83 98.70 98.68 98.49 98.43 98.35 98.80 98.49 98.24 98.76 98.31 98.10

D

14.9 15.2 15.5 15.9

0.9

1.0

DOI: 10.1021/acs.energyfuels.7b00957 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels 3.2.2. Effect of Excess Air. The effect of the λ coefficient on the axial temperature distribution within the furnace is shown in Table 3. As can be observed, an increase in excess air from 30−50% caused a reduction of temperature along the reactor height, due to the dilution effect of excessive air, which resulted in flue gas cooling. The drop in temperature was up to about 30 °C for agroresidues/MBM mixtures. It is interesting to note that the temperature at the end of the expanded bed (higher value in fourth column) was not lowered with excess air, but remained nearly constant, revealing combustion of increased elutriated particles in this case. Concerning flue gas emissions, Figure 4a indicates that when excess air ratio was increased CO levels were higher, following

the declining temperature trend along the furnace. Nevertheless, all CO values were well below legislation limits.20,23 It must be mentioned though that, in large scale applications the injection of secondary air is recommended for minimizing CO emissions, especially for fuels with high volatiles content. Furthermore, Figure 4b shows that when λ coefficient was raised from 1.3 to 1.5 NOx emissions were increased, despite the lower temperatures due to dilution effects of excessive air. This behavior is typical of the fuel−NOx formation mechanism. In the case of blends, NOx concentrations in flue gases were higher, due to the elevated fuel-N of MBM with respect to olive kernel and pruning. For olive kernel/MBM mixtures NOx values exceeded emission guidelines. Finally, as already discussed, SO2 emissions from all fuels were negligible. The higher temperatures attained in the reactor, at diminishing excess air, resulted in lower heat losses due to incomplete combustion or unburned carbon in fly and bottom ashes, thus improving combustion efficiency of individual fuels or blends (Table 3). 3.2.3. Effect of Fuel Feed Rate. A comparison between Tables 3 and 4 shows that when the feeding rate of the fuels or the mixtures was increased, at constant excess air percentage, combustion temperature was raised along the reactor height, due to higher heat release. However, for olive pruning, a higher reactor loading (at λ > 1.3) created a fuel-rich zone within the inert bed, which lowered oxygen concentration resulting, in conjunction with lower residence time, in reduced temperatures. As can be observed, in the freeboard area burnout of the fuel continued and the temperature exceeded the one attained at lower feeding. The effect of reactor loading on pollutant emissions is presented in Figure 5. As seen, CO concentration in flue gas was very sensitive to combustion temperatures at all locations inside the furnace. In principle, CO levels were lower when temperature was higher and burnout of the fuels was improved. All values were well below legislation limits.20,23 With respect to NOx emissions, the effect of feed rate was small. The drop in NOx production at the higher reactor loading with the woody residues can be explained by the oxygen-lean zone formed, due to the higher amount of volatiles fed in this case, which suppressed conversion of fuel-N to NOx. On the other hand, when MBM was mixed with olive kernel, NOx values at the higher feed rate exceeded those allowed for small combustion

Figure 4. Effect of excess air ratio on the average (± standard error) CO (a) and NOx (b) emissions of OP and OP/MBM 90:10 at F = 0.6 kg/h.

Table 4. Fluidized Bed Combustion Performance of Fuels at Different Excess Air Ratios and a Feed Rate of 0.72 kg/h flue gas emissions (ppmv) sample OK OK/MBM 90:10 OK/MBM 80:20 OP OP/MBM 90:10 OP/MBM 80:20

excess air ratio λ

bed temperature (°C)

freeboard temperature (°C)

CO

1.3 1.4 1.3 1.4 1.3 1.4 1.3 1.4 1.3 1.4 1.3 1.4

866−875 854−861 849−861 845−854 844−850 839−848 859−873 853−858 860−868 854−863 845−850 840−845

434−825 414−840 487−802 453−807 418−810 403−798 445−832 438−828 428−816 420−818 417−825 402−821

371 580 457 817 443 488 1141 1295 520 840 701 1038

E

SO2 1.8

1.1

heat losses (%)

NOx

LCO

Lba

Lfa

efficiency η (%)

147 196 363 423 441 467 93 89 135 154 165 182

0.38 0.75 0.50 0.89 0.48 0.53 1.25 1.41 0.56 0.92 0.77 1.10

0.03 0.09 0.15 0.15 0.20 0.27 0.15 0.23 0.50 0.35 0.33 0.35

0.07 0.11 0.15 0.10 0.20 0.20 0.10 0.15 0.34 0.20 0.30 0.35

99.52 99.05 99.20 98.86 99.10 99.0 98.50 98.21 98.60 98.53 98.60 98.20

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Energy & Fuels

most probably formed by dehydration of gypsum (CaSO4· 2H2O → CaSO4 + H2O), as the SO2 content in flue gases was extremely low. The presence of hydroxyapatite is associated with the use of fertilizers in agriculture, such as NH4H2PO4.24 Olive kernel ashes contained small amounts of periclase and hematite, while siderite identified in olive pruning ashes remained in the bottom of the reactor during the combustion process. Among silicates, quartz was abundant in both woody ashes, whereas microcline, dawsonite, and ferropargasite were detected in smaller amounts. Microcline was possibly formed by reactions of arcanite or sylvite found in olive kernel ash with quartz and alumina (K2SO4 + Al2O3·SiO2 + 5SiO2 → 2KAlSi3O8 + SO3 or 2KCl + Al2O3·SiO2 + 5SiO2 + H2O → 2KAlSi3O8 + 2HCl). Potassium in olive pruning ashes was incorporated in fairchildite and microcline. In olive kernel ashes, additionally to these phases, potassium was incorporated in arcanite and sylvite. These sulfate and chloride species partly vaporized during combustion and condensed on cyclone ash. On the other hand, the characteristic of fly and bottom ashes, obtained by mixing MBM with either of the woody residues studied, is the presence of calcium compounds bound in phosphates, such as hydroxyapatite, calcium sodium phosphate, and whitlockite magnesian. These mineral phases are attributed to the composition of MBM in bones. Furthermore, a minor amount of sylvite appeared in both ashes of olive kernel and pruning mixed with MBM, pointing to MBM ash origination. As concerns the effect of excess air or fuel loading on the mineral phases of fly and bottom ashes, the results herein have shown that, for the range studied, no differentiation occurred in the mineralogical composition of the ashes with respect to these operating parameters. The small increases in some elements (Ca, Mg, P) with feeding rate, observed from the chemical analyses, were also reflected in the XRD spectra. 3.3.2. Chemical Analysis. The chemical composition of fly ashes in major elements, expressed in the usual way as oxides, obtained at a fuel loading of 0.6 kg/h and excess air of 40%, are compared in Figure 6a. These data are consistent with the

Figure 5. Effect of fuel feed rate on the average (± standard error) flue gas emissions of OK and OK/MBM 90:10 at λ = 1.4.

units.22 Finally, SO2 emissions for all fuel combinations were of no concern. Combustion efficiencies presented in Table 4 reflect the domination of heat losses due to incomplete combustion, as compared to unburned carbon in fly and bottom ashes. Thus, the highest efficiencies were obtained at the increased fuel feeding (0.72 kg/h) and at excess air ratio 1.3. For agroresidues/MBM blends all values were still high, varying between 98.2 and 99.2%. 3.3. Characterization of Fly and Bottom Ashes. 3.3.1. Mineralogical Analysis. The crystalline mineral species of fly and bottom ashes of olive kernel and pruning and their mixtures with MBM, burned at a feed rate of 0.6 kg/h and excess air ratio 1.4, are represented in Table 5. Both woody residues apart from the albite, muscovite, and partly quartz, which were the constituents of bed material elutriated in the cyclone, were dominated by Ca-based minerals, due to the high content of calcium naturally occurring in wood, such as calcite, anhydrite, fairchildite, hydroxyapatite, and ferropargasite. Dolomite was concentrated in bottom ashes. Anhydrite was

Table 5. Mineralogical Analysis of Fly and Bottom Ashes Obtained at a Feed Rate of 0.6 kg/h and λ = 1.4a sample OK mineral phases quartz SiO2 calcite CaCO3 anhydrite CaSO4 calcium sodium phosphate NaCaPO4 albite (Na,Ca)Al(Al,Si)3O8 muscovite KAl2(Si3AlO10)(OH)2 microcline KAlSi3O8 whitlockite magnesian Ca18Mg2H2(PO4)14 fairchildite K2Ca(CO3)2 hydroxyapatite Ca5(PO4)3(OH) dolomite CaMg(CO3)2 hematite Fe2O3 arcanite K2SO4 periclase MgO siderite FeCO3 dawsonite NaAl(CO3)(OH)2 sylvite KCl ferropargasite NaCa2Fe4AlSi6Al2O22(OH)2 a

OK/MBM 90:10

OP

OP/MBM 90:10

fly ash

bottom ash

fly ash

bottom ash

fly ash

bottom ash

fly ash

bottom ash

+

++ ++ ++

++ ++ +

++ ++

+++ ++ +

+ +++ +

++ +++

+++ +

+ + +

++ + +

++ + +

+++ + +

++ +

+ +

+ + +

+ +++ + + + + +++

+ ++ + + +++ + + + + ++

+ ++ +

+++ ++ ++ + ++ +

+ ++ ++ + + + + + ++ ++ + ++ +

+ ++ +

+ +

+ +

+

+

inert bed material

+ ++ + + + ++ +++

+ + +

+ +

+

+++ high intensity; ++ medium intensity; + low intensity. F

DOI: 10.1021/acs.energyfuels.7b00957 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Olive kernel ash was rich in Cu and Mn, olive pruning ash in Zn, while both ashes contained elevated amounts of Sr, possibly due to contamination of these materials with soil.25 The rather high concentration of Cr was attributed to the operating parameters and furnace configuration, as also confirmed by other studies.26 Blending of olive kernel or pruning with MBM at percentages up to 20% resulted in reduction of Cu, Ni, Sr, Mn, and Ni, respectively, in fly ashes. However, the concentration of Zn, which is also known27 to have a moderate volatility, was increased. Generally, all heavy metal values measured in present samples were low and below the upper limit for disposal in landfills, according to EU directives.28 The environmental impact of these ashes upon land recycling is examined in the following section. The accumulation of trace metals in fly ashes was not found to be affected by the λ coefficient in the present work. On the other hand, the influence of fuel feed rate on elements partitioning was somehow greater. Thus, when a higher amount of fuel was burned into the reactor and consequently air velocity was higher (for constant excess air ratio) more trace elements were condensed on fly ash particles. 3.3.3. Analysis of the Leachates through the Soils. Table 6 represents the cumulative concentration of the elements leached through the soil/fly ash mixtures, according to the column leaching tests previously described. All values were within the limits stipulated by the European Community directives.3 The highest concentrations extracted were observed at the beginning of the experiments, tending to decrease rapidly with time. Ti, Cd, and Hg were not quantified in the leachates, because their concentration was below the instrument’s detection limit (1.96−10.76 ppb depending on element). By comparing the levels of major elements released from Agia soil/fly ash mixtures, it can be seen that these were higher for Na, K, and Ca. However, the leachability of P was dependent on the mineral phase in which it was incorporated. Thus, hydroxyapatite was weakly extracted and kept in solution, while calcium sodium phosphate and whitlockite magnesian, identified in MBM ash, were insoluble. As concerns trace elements, Cr presented the highest mobility from soil/woody ash samples, followed by Sr, in the case of olive pruning. Cu and Zn were extracted in moderate amounts, while Co, Mn, and Ni levels were the lowest in the leachates (between 0.3 and 14.6 ppb). When MBM was blended with either olive kernel or pruning fuels, the relative mass leached of heavy metals from Agia soil/fly ash was generally increased, with As showing the greatest mobility. Yet, the amount extracted was 5 ppb. On the other hand, when the acidic soil of Paleochora was used for the tests, the extractability of almost all elements was enhanced up to 10 times in case of trace metals, while that of K and P was doubled. This behavior can be basically explained by the pH of solid and liquid phases, as well as the mineralogical and chemical composition of the solid materials involved. This is supported by the fact that both soils presented a very low cation exchange capacity for adsorbing toxic substances, mainly due to their small percentage in clays. Fly ashes studied were alkaline (pHMBM = 10.4, pHolive kernel = 10.3, pHolive pruning = 8.2). When leached through the alkaline soil of Agia (pH = 8.2), the pH of the water extracts increased up to 11.3 for olive pruning ash. These high values are mainly attributed to the basic Ca-bearing minerals of the ashes, such as carbonates and hydroxides, which were soluble in water. However, when fly ash of olive kernel/ MBM mixture was leached through the acidic soil of Paleochora

Figure 6. Chemical analysis of fly ashes in major oxides (a) and trace elements (b) at F = 0.6 kg/h and λ = 1.4.

previous analyses derived from XRD. All ashes were rich in Ca, P, and Si, while woody agroresidue ashes were also rich in K oxides. The elevated amounts of Si, Al, and Na in the fly ashes are attributed to some elutriation of bed material in the cyclone. The higher proportion of K2O in olive kernel fly ash is in accordance to the higher content of K originally found in this sample, as well as the high combustion temperature, which increased the vaporization of arcanite and sylvite identified through XRD and their consequent condensation on fly ash particles. Furthermore, Figure 6a clearly shows that, when MBM was mixed with olive kernel or pruning, the concentrations of Ca and P were significantly increased in the fly ashes, as compared to those produced from combustion of the woody fuels, whereas the concentration of K was decreased, confirming the presence of the calcium phosphates found in the XRD spectra of the mixtures and the minor amount of sylvite, respectively. Hydroxyapatite, the principal phosphate of all samples, was volatilized to a great extent under the conditions of the tests and captured in the cyclone. The effects of excess air ratio or fuel feed rate on the composition of fly ashes were so small, that the differences cannot be clearly seen in a graph. As a general observation, a lower amount of fuel and consequently air, a lower temperature and an increased residence time favored the retention of calcium, potassium and phosphorus compounds in the bed. Trace element analysis of fly ashes is represented in Figure 6b. Toxic elements As, Hg, Cd, Co, and Pb were omitted from the graphs, as their values varied only from