EMISSIONS FROM THE PYROLYSIS OF ALMOND SHELL CONTAMINATED WITH HEAVY ...

37 Pages·2017·2.51 MB·English

1 EMISSIONS FROM THE PYROLYSIS OF ALMOND SHELL
2 CONTAMINATED WITH HEAVY METALS
3 M.A. Martín-Laraa; N. Ortuñob; J.A. Conesab*
4 a Department of Chemical Engineering, University of Granada, Spain
5 b University Institute of Chemical Process Engineering, University of Alicante, Spain
6 *Corresponding author. Email: [email protected]
7 Abstract
8 Heavy metal contaminated almond shell was subjected to pyrolysis to understand the
9 effect of different heavy metals during its thermal degradation. First, physicochemical
10 characterization of native and metal-polluted samples was carried out. Second, pyrolysis
11 behavior of native and metal-polluted samples was studied by thermogravimetric
12 analysis. Similar shapes of thermogravimetric curves indicate that the presence of Cd,
13 Cr, Cu, Ni and Pb did not change the main degradation pathways of almond shell.
14 However, the temperature at which the decomposition in each stage takes place at a
15 higher rate and the char yield was considerably modified by the presence of some
16 metals. Then, pyrolysis tests of native and metal-loaded almond shell samples were
17 performed in a moving tubular reactor at 700 ºC. Gases and volatile organic compounds
18 were collected using Tedlar bags and semivolatile organic compounds were collected
19 using a resin as adsorbent. Significant changes were obtained in the composition of the
20 gaseous fraction as a result of the metal impregnation. The main changes were observed
21 for the nickel-loaded sample, which presented the highest H and CO yields. Also, the
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22 yields of most of the light hydrocarbons decreased in the presence of metal, while others
23 remained quite similar. The total PAH yields reached 103 μg/g Ni-AS, 164 μg/g Cu-AS,
24 172 μg/g Pb-AS, 245 μg/g AS, 248 μg/g Cd-AS and 283 μg/g Cr-AS. Nickel was the
25 most effective in the higher aromatic tar reduction, followed by copper and lead,
26 whereas the presence of cadmium did not affect the total emissions of PAHs, and in the
27 presence of chromium the emissions slightly increased. Finally, the carcinogenic
28 potency of the samples was calculated. Native almond shell sample and the sample
29 contaminated with chromium presented slightly higher values associated to the presence
30 of small amounts of benzo(a)pyrene.
31 Keywords: almond shell, metal, PAHs, pyrolysis, semivolatiles
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33 1. INTRODUCTION
34 Since the beginning of the industrial revolution, with increasing industrialization and
35 economic development, pollution of the water with toxic heavy metals has intensified
36 strongly (Lievens et al., 2008). Consequently, heavy metal contaminated wastewaters
37 have attracted great consideration and, in order to remove heavy metals from
38 wastewater, a lot of techniques have been developed. Sometimes, the treatment of heavy
39 metal contaminated wastewaters can be more problematical than any other
40 manufacturing process. Among the physical and chemical treatment processes,
41 biosorption has been proposed as one of the most promising technologies because it can
42 be highly effective, economical and easy to adapt (Zhao et al., 2011). Biosorption is
43 based on metal-sequestering properties of non-viable biomass to remove heavy metals
44 when they are present in water at low concentrations. Availability is a most important
45 factor to be taken into account to select a biosorbent for clean-up purposes. In this
46 regard, abundant lignocellulosic materials have been principally proposed as potential
47 biosorbents for heavy metals (Vieira and Volesky, 2000).
48 One essential item related to the application of biosorption at the industrial scale is the
49 management of the heavy metal-polluted biomass. Some authors have suggested direct
50 combustion of contaminated biomass in order to decrease the waste amount and to
51 recover the heavy metals (Keller et al., 2005). However, during direct combustion of
52 contaminated biomass, high temperatures are reached, which can produce the
53 volatilization of heavy metals and, also, many other toxic pollutants can be produced as
54 dioxins and furans. Therefore, other mild thermal processes, like pyrolysis, have been
55 explored to reduce the amount of heavy metal-polluted biomass and to obtain less
56 polluting fractions that can be used as fuel products (Al Chami et al., 2014; Collard et
57 al., 2012; Jiu et al., 2015; Lievens et al., 2008).
58 Spain is today an important consumer of almond and the second producer of almond in
59 the world, with the U.S. being the first producer. Production in Spain is concentrated in
60 the Mediterranean communities: Catalonia (Lleida, Tarragona), Valencia (Alicante,
61 Valencia), Balearic Islands (Mallorca) and Andalusia (Almería, Granada, Málaga).
62 World production of almonds reached 2.9 million tonnes in 2013, with the United States
63 as the largest producer with 1.8 million tonnes. The Spanish production, in second
64 place, accounts only for 149,000 tonnes, with a total harvested area of 534,100 Ha in
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65 2013 (FAO data). The almond industry in Spain generates large quantities of waste
66 products that need to be recycled or processed. In general, almond shell is a material
67 used as fuel in boilers and sometimes in domestic heating systems and barbecues. To
68 that respect, there are few studies that have evaluated the use of almond shells as heavy
69 metal biosorbent (Calero et al., 2013; Khan et al., 2015; Pehlivan and Altun, 2008;
70 Pehlivan et al., 2009; Ronda et al., 2013) and others that have studied the use of almond
71 shell as biomass fuel in pyrolysis processes (Caballero et al., 1997; González et al.,
72 2005; Grioui et al., 2014).
73 In this work, heavy metal-polluted almond shell samples were pyrolyzed at 700 ºC in
74 order to study the influence of the different heavy metals on the pyrolysis behavior of
75 almond shell. Special attention has been brought to the characterization of the emissions
76 and its toxicity, covering the analyses of the main gases (H , CO, CO , N , O ), light
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77 hydrocarbons (from methane to xylenes), semivolatiles and polycyclic aromatic
78 hydrocarbons (PAHs).
79 2. MATERIAL AND METHODS
80 2.1. Sample preparation and characterization
81 Almond shell (AS) was supplied by a factory in Granada (Spain), Carsan
82 Biocombustibles S.L. Prior to use, almond shell was ground to a particle size smaller
83 than 1 mm.
84 The impregnation was carried out by biosorption as follows: 40 g of almond shells were
85 well mixed with heavy metals solutions of a concentration of 200 mg/L in different
86 batch reactors at a constant temperature of 25 ºC until equilibrium was reached. The
87 metallic salts used for the impregnation were nitrate salts of different heavy metals
88 (Cd(II), Cr(III), Cu(II), Ni(II) and Pb(II)) purchased to Panreac, Spain. Then, the solid
89 samples were filtered, washed with deionized water to remove any of the metal ions not
90 bound to the material and finally, the metal-polluted samples were dried in an oven at
91 40 ºC for 24 h. The amount of metal biosorbed or metal uptake (mg/g) was calculated
92 according to the mass balance of the biosorption system after determining the residual
93 or final metal concentration in the solutions by atomic absorption spectrometry
94 (AAnalyst 200, Perkin-Elmer).
95 The moisture, volatile matter and ash content were determined following ISO 18134-
96 1:2015, ISO 18123:2015 and ISO 18122:2015 applicable standards, respectively. The
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97 fixed carbon content was determined by subtracting the percentages of moisture,
98 volatile matter and ash from the original mass of the sample. The higher heating value
99 (HHV) was determined by a Phywe LEC-02 calorimeter according to the procedure
100 described in the standard EN 14918:2011.
101 An infrared analysis was performed in the range of 4000–400 cm-1 using a Fourier
102 Transform Spectrophotometer of Perkin-Elmer (model Spectrum-65) for detecting
103 functional groups and characterizing covalent bonding information in native and metal-
104 polluted samples.
105 Thermogravimetric experiments were performed using a thermobalance (Perkin-Elmer
106 model 6000 STA) as part of the characterization of the sample properties. Experiments
107 were carried out for the native and metal-polluted samples at a heating rate of 15 ºC/min
108 in an atmosphere of nitrogen (flow rate of 20 mL/min). The weight of the used sample
109 was approximately 40 mg and the temperature range studied from 30 ºC to 800 ºC.
110 Experiments were performed in duplicate to verify the reproducibility of the data.
111 2.2. Pyrolysis tests
112 Pyrolysis tests of native and metal-loaded almond shell samples were performed in a
113 moving tubular reactor at 700 ºC. The reactor has a quartz tube with an internal
114 diameter of 10 mm that was continuously flushed with a nitrogen flow of 500 mL/min,
115 in parallel to the sample. Approximately one gram of sample was uniformly placed in
116 the quartz tube (distributed in three quartz boats of 70 mm of length) and introduced, at
117 a feeding rate of 1 mm/s, in a horizontal furnace while the constant flow of nitrogen was
118 passing through. This permits to simulate the continuous pyrolysis during several
119 minutes. A detailed description of the pyrolysis set-up has been reported earlier by
120 Conesa et al. (2013). The reproducibility of this kind of tests has been recently
121 addressed in our laboratory, using a very similar laboratory scale horizontal reactor
122 (Garrido et al., 2016), showing a good reproducibility for all kind of compounds
123 analyzed in the emissions from pyrolysis and combustion of wastes.
124 2.3. Sampling of pyrolysis products and analytical procedure
125 Gases and volatile organic compounds were collected using Tedlar bags (Restek, USA)
126 and then analyzed by gas chromatography with flame ionization detector (GC-FID)
127 (Shimadzu GC-17A) with an Alumina KCl Plot capillary column, and with thermal
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128 conductivity detector (GC-TCD) (Agilent 7820A GC) using two packed columns (Haye
129 Sep Q 80/100 and Molecular Sieve 5A 80/100) coupled with a pneumatic valve.
130 Semivolatile organic compounds were collected using a polyaromatic Amberlite XAD-2
131 resin as adsorbent (Supelco, Bellefonte, USA), extracted with a mixture of
132 dichloromethane/acetone (50% vol.), concentrated in a rotary evaporator and with a
133 gentle N stream, and analyzed by high resolution gas chromatography coupled to a
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134 mass spectrometer (HRGC/MS) following the isotope dilution method, according to the
135 U.S. EPA 8270D method (US EPA, 2014). Details of the analytical procedure are
136 reported elsewhere (Ortuño et al., 2014). The analysis of semivolatile compounds has
137 focused on the group of 16 polyaromatic hydrocarbons established by the United States
138 Environmental Protection Agency (US EPA) as priority pollutants and potential
139 carcinogens (US EPA, 1998) but also, in order to investigate the effect of the heavy
140 metal action on pyrolysis, many sugar-derived compounds from the depolymerization of
141 holocellulose or lignin-derived compounds were quantified according to information
142 reported by Collard et al. (2012).
143 The char was cooled to room temperature and weighed. Then, the remaining heavy
144 metals and other minor elements were analyzed by acid digestion and later
145 determination by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-
146 OES) following ISO 16967:2015 and ISO 16968:2015 standards.
147 3. RESULTS AND DISCUSSION
148 3.1. Sample characterization
149 Figure 1 shows the amount of metal biosorbed by weight of solid (mg/g) in metal-
150 polluted almond shell samples. A biosorption process includes a solid phase
151 (biosorbent, almond shell) and a liquid phase (contaminated discharged effluent, heavy
152 metal solutions) containing the species to be biosorbed or removed from the
153 contaminated liquid (sorbate, heavy metal ions). The sorbate is bound to the solid by
154 different mechanisms (ion-exchange, adsorption, precipitation, etc.) due to the affinity
155 of the biosorbent for the sorbate. This process is produced until equilibrium is
156 established between the amount of sorbate species bound to the solid and its remaining
157 concentration in the solution (residual, final or equilibrium concentration). As Figure 1
158 shows, the affinity of the almond shell for the lead ions is higher than for the other
159 heavy metals studied. The opposite result is obtained for copper ions, which are the
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160 least biosorbed metals by almond shell. However, in order to carry out a more complete
161 comparison, other initial concentrations of metals should be analyzed and the respective
162 q values, which are calculated for example from Langmuir isotherm model, should
max
163 be compared.
164 Figure 1
165 Table 1 gives the proximate analysis as well as the Higher Heating Values (HHV) of the
166 native and metal-polluted samples. The data show a higher volatile matter content for all
167 metal-polluted samples (excluding Pb-AS) than the native one; being this fact especially
168 significant for Ni-AS. Also, a decrease in fixed carbon is obtained for the samples
169 impregnated with metal. With respect to the results obtained for the HHV, data show
170 that the presence of heavy metals did not influence the heating value of native AS.
171 Higher heating values are close to the values of 16-18 MJ/kg reported by other
172 researchers as Önal et al. (2017) or Atkas et al. (2015).
173 Table 1
174 The FTIR spectra of the native and metal-polluted samples are shown in Figure 2. All
175 samples showed a large number of peaks, associated to the functional groups of
176 cellulose, hemicellulose and lignin (Lalhruaitluanga et al., 2010; Lu et al., 2009;
177 Saygideger et al., 2005). Table 2 reports the vibration positions, ʋ (cm-1), of the most
178 representative peaks found in the infrared analysis of native and metal-polluted samples,
179 together with its identification. For example, groups as O-H, C=O, COO- in
180 carboxylate, C-O-R, aromatic organic compounds, etc. can be detected in the spectra.
181 Figure 2
182 Table 2
183 Although the metal-polluted samples showed a similar spectrum to native AS, less
184 pronounced peak intensities were found. This can be due to the strong interaction of the
185 heavy metals bound to functional groups. Also, two different mechanisms can be
186 observed for the different heavy metals. As during biosorption a combination of
187 mechanisms is produced for the retention of the metal, for each metal the relative
188 relevance of these mechanisms can vary. For Ni-AS and Pb-AS samples, no vibration
189 band disappears. The peaks which significantly changed their position were the peaks at
190 3365.5 and 1729.7 cm-1, assigned to OH groups (probably phenolic) and carboxyl
191 groups, respectively, whereas the majority of the organic functional groups showed
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192 small changes. This suggests an intense role of ligands of these groups during
193 biosorption of nickel and lead by almond shell. As for the drop in intensity, it should
194 also be mentioned the decrease observed at 1029.2 cm-1, assigned to alcohol groups. For
195 Cd-AS, Cr-AS and Cu-AS more peaks were involved in the observed changes (several
196 vibrational bands disappeared after metal biosorption). In the case of chromium the
197 intervention in biosorption of phenolic and carboxyl groups is observed. In the case of
198 cadmium and copper, only phenolic groups seem to be involved in the process, although
199 in the case of copper the difference in the vibration band is smaller. Finally, for these
200 three cations (cadmium, chromium and copper) the intervention of the peak at 1234.0
201 cm-1 was observed, corresponding to ester groups.
202 3.2. Thermogravimetric tests
203 Figure 3 shows the pyrolysis behavior of native and metal-polluted samples. Similar
204 shapes of thermogravimetric curves (TG) indicate that the presence of Cd, Cr, Cu, Ni
205 and Pb did not change the main degradation pathways of almond shell from a point of
206 view of mass loss. All curves present three different stages of mass loss, identified by
207 peaks in the derivative curves (DTG). The first stage (first peak in DTG curves) can be
208 attributed to the water vapor release and takes place at a temperature below
209 approximately 100 ºC. The decomposition of the main components of almond shell start
210 around 175-200 ºC and the main mass loss occurs between 250 and 400 ºC due to the
211 devolatilization of hemicellulose and cellulose (second and third peaks in DTG curves).
212 The second peak corresponds to the decomposition of the hemicellulose, with a
213 maximum decomposition rate at 300-315 ºC. The third peak can be mainly attributed to
214 the degradation of cellulose. In this stage, the maximum rate of decomposition occurs
215 (at a temperature around 360-380 ºC). Furthermore, a fourth stage of decomposition is
216 found although it appears as a broad peak with smaller intensity. It can correspond to
217 lignin degradation, one of the major components in almond shell, and more
218 thermostable, whose decomposition takes place slowly over a greater temperature range
219 (between 400-800 ºC).
220 Figure 3
221 Table 3 includes the information obtained from the TG and DTG curves for each stage
222 of decomposition, where T is the onset temperature of decomposition, T is the
i f
223 temperature at which the stage of decomposition ends, T is the temperature at which
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224 the decomposition in each stage takes place at a higher rate (peak on the DTG curve),
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225 w is the percentage of mass loss corresponding to each stage, and (dw/dT) is
volatilized máx
226 the value of maximum loss rate corresponding to each stage, which takes place a T .
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227 In general, metal-polluted samples showed higher decomposition temperatures and
228 decomposed at higher rates. In particular, Ni-AS and Cr-AS samples presented the
229 major effect on thermal decomposition of almond shell in nitrogen atmosphere, showing
230 some substantial changes on characteristic data. Both samples significantly improved
231 the maximum volatilization rate of hemicellulose and cellulose. Also, for Ni-AS, the
232 percentages of mass loss corresponding to most stages were higher than the values for
233 AS and, for Pb-AS, T was higher than the values of the native sample in all stages.
max
234 Table 3
235 Regarding the almond shell char yield, it was considerably modified by the presence of
236 some metals. Excluding the Pb-AS sample, all metal-polluted samples showed an extra
237 mass loss during the thermal degradation in nitrogen atmosphere. For example, char
238 yield decreased to 19% for Ni-AS (a decrease of 24% with respect to the native sample).
239 This enchance of char yield by nickel impregnation was also reported by other authors,
240 as Shen et al. (2014).
241 Different results were reported by Liu et al. (2012) in their study about the influence of
242 different additives (ZnO, Fe O , CuO and Al O ) on pyrolysis of municipal solid waste
2 3 2 3
243 (MSW). In general, the additives caused a slight shift of TG and DTG curves toward
244 lower temperature due to the prevention of the formation of stable chemical structures,
245 accelerating the decomposition of MSW. But an exception was found to CuO that had
246 negligible effects on the breakdown of long molecular chain hydrocarbons of MSW and
247 shifted to higher temperatures.
248 It is also important to indicate that there was a weight loss peak detected only in the
249 native sample at temperatures between 450 and 500 ºC. According to Caballero et al.
250 (1996) lignin decomposes across a very wide range of temperatures and can be
251 reproduced assuming two independent fractions which decompose simultaneously. The
252 second fraction coincides with the weight loss peak detected in the native sample in this
253 work and therefore, a different behavior of lignin was found when metals form part of
254 the almond shell, proving the importance of the interactions between metals and this
255 component.
256 3.3. Formation and release of pollutants
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257 3.3.1. Composition of the gases and volatiles collected using Tedlar bags
258 In this sub-section the composition and the evolution of the collected gas during
259 pyrolysis of almond shell samples is discussed. Table 4 presents the gas yields
260 (µg/g sample) of the pyrolysis at 700 ºC of the native and metal-polluted samples.
261 Table 4
262 For all samples, the gas fraction consisted in 88 – 91% of CO and CO . However,
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263 significant changes were obtained in the composition of the gaseous fraction as a result
264 of the metal impregnation. The main changes in the composition of the gas were
265 observed for Ni-AS, that presented the highest H and CO yields. Collard et al. (2012)
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266 evaluated the effect of metal impregnation on the pyrolysis products of biomass samples
267 (lignin, cellulose, xylan and beech wood) and observed a significant increase in H
2
268 yield, due to the rearrangement of the aromatic rings in the matrix promoted by the
269 presence of Ni in the samples, with a 472% increase in hydrogen production for nickel-
270 impregnated lignin (relatively similar to the 329% increase observed in the present
271 study). Liu et al. (2012) have already reported that Ni plays a critical role in the
272 composition of gaseous products, showing that the yield of carbon monoxide increased
273 with an increase in Ni loading. According to these researchers, the decrease of carbon
274 deposit, due to the presence of NiO nanoparticles (NiO is resistant to carbon deposition
275 according to Ruckenstein and Hu, 1995), could be the reason for this increase in CO
276 yield. This result agrees with the decrease in char yield presented previously in section
277 3.2. Even though the amounts of sample employed in the present work were rather
278 small, and experiments with larger samples would be advisable to confirm these results,
279 the observed behavior agrees with the reported by other authors on pyrolysis of Ni-
280 polluted biomass. For example, Shen et al. (2014) found an increase in the
281 concentration of hydrogen from 17.3% to 38.6% in pyrolysis at 750 ºC of nickel-
282 impregnated rice husk. Richarson et al. (2010) obtained additional H and CO for nickel
2
283 impregnated wood samples. These authors reported that at about 400 ºC nickel is
284 present as an amorphous Ni O H phase dispersed into the char matrix that is reduced
x y z
285 by carbon atoms to Ni0. This reduction step results in a formation of CO and H .
2
286 It is also important to explain the significant quantities of carbon oxides in the
287 emissions from the pyrolysis process (nitrogen atmosphere). These oxides are present
288 on the released gases due to the content of oxygen from the polymeric compounds of
289 almond shell and hardly due to the presence of atmospheric oxygen accessible in the
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290 reaction atmosphere, because a test of air-tightness was carried out before each run in
291 the pyrolysis reactor. Moreover, it was confirmed for every sample that the amount of
292 oxygen released in the form of CO and CO was much lower than the oxygen content of
2
293 the almond shell samples. Richardson et al. (2010) proposed an associative mechanism
294 to explain CO and also H formation from adsorbed CO, OH and H on a wood surface.
2 2
295 Jiu et al. (2015) used a fixed bed reactor to study the influence of Pb on the pyrolysis
296 products of water hyacinth (WH) between 275 ºC and 550 ºC, and found out that the
297 presence of Pb led to a decrease in CO and an increase in H . They proposed that Pb2+
2 2
298 stabilized the carboxyl or carbonyl groups and changed the position of chemical bond
299 scission of aliphatic components formed via depolymerization of hemicellulose and
300 neutral detergent solute (one of the main components of water hyacinth, which includes
301 protein, fat, sugar, etc.) rather than decarboxylation.
302 The rest of the metal-polluted samples show similar trends, although the increase in the
303 yields of CO and H and the decrease in CO are not so marked as for the Ni-polluted
2 2
304 sample.
305 With respect to the light hydrocarbons obtained in the pyrolysis of native and metal-
306 polluted samples, Table 5 shows that methane and ethylene are the main light
307 hydrocarbons for all samples, followed in order by 1,3-butadiene, ethane, n-hexane and
308 acetylene. Comparing the results from the several metal-polluted samples with that of
309 almond shell, it can be observed that the yields of most of the light hydrocarbons
310 decrease in the presence of metal, while others remain quite similar. These compounds
311 are found as final products of cracking reactions (Conesa et al., 2009), so it seems that
312 the presence of the metals catalyzes such reactions, that are carried out to a greater
313 extent, favoring the obtention of lower molecular weight compounds, i.e. CO and H .,
2
314 and thus decreasing the percentage of C -C hydrocarbons. Eibner et al. (2015) also
2 6
315 observed a decrease in the production of methane while studying the effect of nitrate
316 salts from different metals (Ce, Mn, Fe, Co, Ni, Cu and Zn) on the pyrolysis products of
317 Eucalyptus at 500 ºC.
318 The only compound that showed an increase for the metal-polluted samples is
319 acetylene. Pohořelý et al. (2006) also observed an increase in acetylene formation, while
320 all other minor hydrocarbons decreased, when increasing the temperature of gasification
321 of coal and PET in a fluidized bed reactor. The authors attributed this behavior to the
322 fact that acetylene is one of the final products of recombination of free radicals presents
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