Article pubs.acs.org/EF
Release and Transformation of Sodium in Kitchen Waste during Torrefaction Shuai Liu,† Yu Qiao,*,† Zhaoling Lu,‡ Ben Gui,† Mengmeng Wei,† Yun Yu,§ and Minghou Xu*,† †
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, People’s Republic of China ‡ Analysis and Test Center, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, People’s Republic of China § Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth WA 6845, Australia ABSTRACT: The release and transformation of typical chemical forms of sodium (i.e., H2O-soluble salts and carboxylates) in kitchen waste during torrefaction at 200−300 °C was investigated in this study. It was found that sodium release is negligible (400 °C).20,32−35 The AAEM species in biomass and low-rank coal mainly exist in four different chemical forms: H2O-soluble forms (e.g., soluble salts), CH3COONH4-soluble forms (e.g., carboxylates), acid-soluble © 2014 American Chemical Society
forms (e.g., silicates and aluminates), and stable form (e.g., covered by carbon and silico-aluminates).36,37 Generally, AAEM species contain more-soluble salts and carboxylates than the other two chemical forms. The catalytic effects of AAEM species largely depend on their chemical form. For example, carboxylates were reported to have significant catalytic effects during pyrolysis or combustion of brown coal.38,39 Previous studies also indicated that the transformation of AAEM species largely depends on their chemical form.37,38,40−42 For example, it was reported that sodium carboxylates (−COONa) could transform to Na2CO3 at 850 °C during the pyrolysis and gasification of brown coal41 or into a stable form (e.g., covered by carbon, CM-Na) above 600 °C during the pyrolysis of Loy Yang brown coal.43 H2O-soluble and acid-soluble sodium could also transform to a stable form such as silico-aluminate at 800 °C.37 Generally, the transformation of different sodium chemical forms increases in the following order: CH3COONH4-soluble form < H2O-soluble form < acid-soluble form < stable form.37,41,43 Previous studies on the transformation of AAEM species were mainly undertaken at high temperatures (>600 °C), and the transformation of AAEM species at torrefaction temperatures (i.e., 200−300 °C) remains unstudied, especially for kitchen waste. Knowledge of the release and transformation of AAEM species during torrefaction is important for the utilization of torrefied products as a fuel, because AAEM Received: August 17, 2013 Revised: February 25, 2014 Published: February 26, 2014 1911
dx.doi.org/10.1021/ef500066b | Energy Fuels 2014, 28, 1911−1917
Energy & Fuels
Article
Table 1. Properties of the Noodle Samples Studied Ultimate Analysis (db wt %)
a
Proximate Analysis (db wt %)
sample
C
H
N
S
Oa
ash
volatile matter
fixed carbon
lower heating value, LHV (MJ/kg)
raw noodle acid-washed noodle NaCl-loaded noodle NaHCO3-loaded noodle Na2CO3-loaded noodle Na-exchanged noodle
45.1 44.5 44.8 44.2 44.7 44.8
7.3 7.7 7.8 7.8 7.5 7.7
2.1 2.4 2.5 2.6 2.5 2.7
0.2 0.2 0.2 0.2 0.2 0.2
44.5 45.1 44.1 44.5 44.4 44.3
0.8 0.1 0.6 0.7 0.7 0.3
82.6 83.2 83.6 83.3 82.9 83.5
16.6 16.5 16.6 16.5 16.5 16.6
17.7 17.7 17.8 17.6 17.6 17.7
By difference. extracted solution were termed as “H2O-soluble” sodium species (mainly NaCl, Na2CO3, and NaHCO3). The water-extracted sample was then extracted in a 1 L ammonium acetate solution (0.5 M) for 24 h and dried in nitrogen at 35 °C to prepare the CH3COONH4extracted sample. The extracted sodium forms in this sample were defined as “CH 3 COONH 4 -soluble” sodium species (mainly −COONa). The CH3COONH4-extracted sample was finally extracted in 1 L sulfuric acid (0.1 M) for 16 h and then washed with ultrapure water and dried in nitrogen at 35 °C to obtain the acid-washed sample. Because of the use of NaCl-loaded samples in this work, sulfuric acid was employed as the acid extraction agent, rather than hydrochloric acid, to prevent the possible influence of Cl− ions from hydrochloric acid.46 The chemical forms extracted by sulfuric acid and those remaining in the acid-extracted sample were defined as “acid-soluble” and “stable form” sodium species, respectively. The acid-washed sample was digested in a mixture of hydrofluoric acid and nitric acid by an ETHOS microwave reactor system (Milestone).20 The sodium species contents in the water, ammonium acetate, sulfuric acidextracted solution and the digestion solution were quantitatively analyzed by inductively coupled plasma−mass spectrometry (ICP-MS, Perkin−Elmer, Model ELAN DRC-e). The sodium content in the six samples were calculated as the ratio of the sodium amount of each chemical form (leachates or residues) to the weight of sample, and the results are shown in Table 2. To gain a better understanding of
species may have significant catalytic effects on subsequent thermal processes. Therefore, the main purpose of this study is to investigate the release and transformation of typical sodium chemical forms (i.e., H2O-soluble salts and carboxylates) in kitchen waste at 200−300 °C. A noodle sample was collected as typical kitchen waste44,45 for this study. To gain a fundamental understanding of the chemical transformation of sodium during torrefaction, noodle samples containing different chemical forms of sodium were prepared for torrefaction experiments. The contents of various forms of sodium in the raw and torrefied samples were measured and compared, together with the carboxylic group content, as an indicator of the formation of carboxylate/acids during torrefaction.
2. EXPERIMENTAL SECTION 2.1. Sample Preparations. The raw noodles (mainly starch) used in this work were collected at a waste treatment station in Wuhan, China. The noodles contain significant amounts of sodium as soluble salts (mainly NaCl, Na2CO3, and NaHCO3) and carboxylates (mainly −COONa), because of the prevalence of sodium salts in food additives. The methods used to prepare the NaCl-loaded, NaHCO3loaded, Na2CO3-loaded, and Na-exchanged noodle samples have been described elsewhere.31,32,39,46−49 Briefly, the raw noodles were washed with sulfuric acid to remove all AAEM species, producing an acidwashed sample. NaCl (0.04 M), NaHCO3 (0.04 M), and Na2CO3 (0.02 M) solutions were then loaded into the acid-washed sample by physical impregnation. A dilute sulfuric acid solution (0.1 M) was employed as the washing agent to prevent significant changes in the noodle substrate structure.31,39 Meanwhile, the Na-exchanged noodle sample was prepared by ion-exchanging the acid-washed particles with a CH3COONa solution (0.5 M).31,39 Excess CH3COONa was removed by washing with ultrapure water. The filtered slurries of noodle samples were dried at 35 °C and sieved to a particle size of 106−150 μm for this study. The ultimate and proximate analysis results are listed in Table 1. With the exception of ash content, no significant changes in the sample properties were found for various noodle samples loaded with different forms of sodium. 2.2. Torrefaction of Various Noodle Samples. Torrefaction experiments were carried out in a fixed-bed reactor, which has been detailed elsewhere.50 Approximately 1 g of each noodle sample was preloaded into the fixed bed and heated at 200−300 °C for 15 min and then cooled to room temperature in an N2 atmosphere. Nitrogen gas at a flow rate of 0.5 L min−1 (measured under ambient conditions) was used as the carrier gas. The char yield was determined by the weight change of the sample holder before and after the torrefaction experiment, taking the moisture contents of the raw noodle and torrefied noodle residue into consideration. Note that the char yield shown in this work represents the average of 3−5 replicates under identical conditions. 2.3. Quantification of Sodium Species in Raw and Torrefied Samples. The chemical form of sodium in raw and torrefied samples was determined by a sequential chemical extraction method.36,37,39 Briefly, ∼2 g of solid sample was extracted in 1 L ultrapure water for 24 h and then filtered and dried in nitrogen at 35 °C to obtain the water-extracted sample. The fractions of sodium forms in the water-
Table 2. Content of Various Forms of Sodium in the Noodle Samples Studied Soidum Content (wt %)
a
H2Osoluble form
CH3COONH4soluble form
acidsoluble form
stable form
sample
total
raw noodle acid-washed noodle NaCl-loaded noodle NaHCO3loaded noodle Na2CO3loaded noodle Na-exchanged noodle
0.219 ND
0.200 ND
0.003 ND
0.002 ND
NDb ND
0.409
0.379
0.003
0.001
ND
0.465
0.430
0.004
ND
ND
0.538
0.500
0.005
ND
ND
0.133
0.001
0.118
ND
ND
a
Total detected from the digested solution of raw noodle and saltloaded noodles. bNot detected.
sequential chemical extractions, the total sodium content of raw noodles and each solid residue was also determined. Briefly, the raw noodles, the water-washed samples, and the CH3COONH4-washed samples were digested, and the sodium content in each digestion solution was also analyzed by ICP-MS.39 The contents of other AAEM species (K, Mg, and Ca) were not given, because of their relatively low contents in the noodle samples. 1912
dx.doi.org/10.1021/ef500066b | Energy Fuels 2014, 28, 1911−1917
Energy & Fuels
Article
It should be noted that part of the organic matter in biomass can be extracted by water to generate organic acids, resulting in the leaching of some water-insoluble inorganic species (e.g., organically bound).48 This leaching leads to an overestimation of the water-soluble species content in biomass.48 Therefore, a relatively high ratio of solution to solid (1 L water to 2 g sample) was employed in this work. Meanwhile, the pH value of the water-washed solutions in this work was in the range of 6.3−6.6, preventing an overestimation of the water-soluble sodium content. 2.4. Determination of Carboxylic Groups in Raw and Torrefied Samples. The carboxylic group content in each noodle samples was measured by a Methylene Blue (MB) sorption method.17,51,52 Briefly, ∼50 mg of dry noodles were weighed into eight tubes and mixed with 3 mL of deionized water for 1 h. Then, gradually increasing volumes of a mixture of 0.4 mM MB and 0.6 mM barbital (pH 8) were added into each of eight tubes.17 After 20 min of stirring at room temperature, the suspensions were filtered, and the filtrate was diluted 25-fold using 0.6 mM barbital solution. The diluted solution was then analyzed by ultraviolet−visible (UV-vis) spectrophotometry (LengGuang Tech Spectrumlab, Model 725s) at a wavelength of 664 nm. The amount of carboxylic groups in each sample can be calculated according to the amount of consumed MB.
torrefied samples. The data indicate that the carbon content gradually increases with increasing torrefaction temperature while the oxygen content decreases significantly as the temperature increases, i.e., from ∼44.5% in the raw noodle sample, to ∼41.7%, ∼39.5%, and ∼25.1% at 200, 250, and 300 °C, respectively. The decrease in oxygen content and the increase in carbon content appear to be more significant at 300 °C. In addition, the char yield decreases substantially due to the release of more volatiles at 300 °C. Accordingly, an increase in the heating value of the torrefied samples with increasing torrefaction temperature can be observed, from 17.7 MJ/kg for the raw noodle sample to 22.0 MJ/kg for the torrefied sample at 300 °C. These results clearly show the strong effect of temperature on the torrefaction of noodle samples.53,54 To gain a clearer understanding of the fuel properties of torrefied noodles, the H/C and O/C atomic ratios of the torrefied samples at various temperatures were calculated and compared in Figure 2. When the torrefaction temperature was
3. RESULTS AND DISCUSSION 3.1. Properties and Yields of Solid Products from the Torrefaction of the Raw Noodle Sample. Torrefaction experiments of various noodle samples were carried out at 200, 250, and 300 °C, respectively. Figure 1 shows the char yields of
Figure 2. The H/C and O/C atomic ratios of the raw noodle and torrefied samples in a van Krevelen diagram. Brown coal data are taken from ref 46.
increased to 200, 250, and 300 °C, the O/C value of the torrefied sample decreased from 0.74 to 0.65, 0.59, and 0.30, while the H/C value decreased from 1.95 to 1.69, 1.57, and 1.02. This indicates that the torrefaction of noodles is similar to the coalification process, decreasing the O/C value to a level close to that of brown coal.46 The O/C and H/C values also show that the noodle sample torrefied at 300 °C has better fuel properties than samples torrefied at lower temperatures.12 3.2. Release and Transformation of Sodium during the Torrefaction of Raw, Sodium-Loaded, and NaExchanged Noodle Samples. Figure 3 shows the retention of Na after torrefaction at 200−300 °C and its distribution in both the residue and leachate after each sequential chemical
Figure 1. Char yields of raw, sodium-loaded, and Na-exchanged noodle samples after torrefaction at 200−300 °C.
the raw, sodium-loaded, and Na-exchanged noodle samples at 200−300 °C. No significant differences were found for various noodle samples at each identical temperature. The char yields were ∼97.5 wt % and ∼93.0 wt % at 200 and 250 °C, respectively, but the yields sharply decreased to ∼48.5 wt % at 300 °C. Table 3 shows the ultimate and proximate analysis results and the lower heating value (LHV) for the raw and
Table 3. Properties of Torrefied Samples from the Raw Noodle Sample at Various Temperatures Ultimate Analysis (db. wt %)
Proximate Analysis (db. wt %)
sample
C
H
N
S
O*
ash
volatile matter
fixed carbon
lower heating value, LHV (MJ/kg)
raw noodle torrefied noodle at 200 °C torrefied noodle at 250 °C torrefied noodle at 300 °C
45.1 48.3 50.6 63.7
7.3 6.8 6.6 5.4
2.1 2.2 2.2 3.8
0.2 0.2 0.2 0.3
44.5 41.7 39.5 25.1
0.8 0.8 0.9 1.7
82.6 81.2 78.4 61.8
16.6 17.9 20.8 36.5
17.7 18.3 18.9 22.0
*
By difference. 1913
dx.doi.org/10.1021/ef500066b | Energy Fuels 2014, 28, 1911−1917
Energy & Fuels
Article
Figure 4. Retention of sodium after torrefaction at 200−300 °C and its distribution in the residue and leachate after each chemical extraction step of the NaCl-loaded noodle and its torrefied samples: (A) before extraction, (B) after H 2O extraction, (C) after CH3COONH4 extraction, and (D) after acid extraction.
Figure 3. Retention of Na after torrefaction at 200−300 °C and its distribution in the residue and leachate after each chemical extraction step of the raw noodle and its torrefied samples: (A) before extraction, (B) after H2O extraction, (C) after CH3COONH4 extraction, and (D) after acid extraction.
extraction step. The sodium retention in the residue or leachate after extraction was defined as sodium fraction (%) Na amt in residues (leachates) of extraction from torrefied sample = Na amt in sample before torrefaction × 100
The release and transformation of sodium during torrefaction can be clearly observed in Figure 3. Sodium release was negligible (
