Article pubs.acs.org/EF
Release of Inorganic Trace Elements from Dried Distillers Grains with Solubles during Gasification at 600−1400 °C Marc Blas̈ ing,* Mostafa Zini, Małgorzata Ryś-Matejczuk, and Michael Müller Institute for Energy Research (IEK-2), Forschungszentrum Jülich GmbH, Leo-Brandt-Straße 1, 52425 Jülich, Germany ABSTRACT: This work focused on the release of alkali metals (K and Na) and non-metals (S, Cl, and P) during hightemperature gasification of dried distillers grains with solubles. The release experiments were performed at 600−1400 °C in a flow channel reactor. Hot-gas analysis was performed by a molecular beam mass spectrometer. The underlying release mechanisms are discussed with dependence upon the temperature.
1. INTRODUCTION Fuel ethanol is playing an important role in the transportation sector worldwide; e.g., 7 billion gallons of ethanol has been blended with gasoline and consumed in the U.S.A. in 2006.1 A byproduct of the dry-grind ethanol production process is dried distillers grains with solubles (DDGS). DDGS has different fully established (mainly ingredients for animal feed) and potential (energy production, ingredients for human food, soil fertilizer, and others) applications.2 Recent investigations underlined advantages of using DDGS by biomass integrated gasification combined technology to generate electricity and heat in ethanol production, e.g., reduction of fuel costs, improvement of the carbon footprint and renewable energy balance by replacing fossil fuel, and potential for sale of additional energy to the grid.3−5 An enhanced scientific understanding of the release behavior of inorganic trace elements during gasification of solid carbon-based fuels, especially coal and woody biomass, was generated during the last few decades. However, several issues, especially technically, are still not satisfactorily solved, as recently summarized by Kumar et al.6 In general, the net energy efficiencies for biomass gasification can be increased with optimized operating conditions of the thermochemical conversion process as well as the development of efficient cleaning technologies for the product gas.7 The latter includes among others the topics: particulate matter, tars, nitrogen, sulfur, chlorine, and alkali compounds.7 Here, the focus is on the release of the mentioned inorganics, which can cause severe problems in power plants. Alkali metal compounds can lead to fouling.8 Additionally, alkali species can form eutectics with a significantly lower melting point, which can result in defluidization of bed materials.9−12 Chlorine is notoriously known for corrosion-related problems in power plants. Deactivation of catalysts can be caused by phosphorus and sulfur compounds. It was shown recently that the temperature of the thermochemical conversion process is of high importance for the release of Na, K, Cl, and S compounds from coal.13,14 Correlated studies with DDGS were not found in the literature. The objective of this work was to fill this research gap and to study the influence of the temperature on the release of K, Na, S, Cl, and P species from DDGS under gasification-like conditions. The present experiments were performed at 600, 900, 1200, and 1400 °C. The temperature © 2013 American Chemical Society
range of the experiments covers a broad band of application temperatures, which is important for a broad range of gasifiers (fixed bed, fluidized bed, and entrained flow). The hot gases were analyzed for trace element species by molecular beam mass spectrometry (MBMS). Special emphasis was put on the alkali metals (K and Na) and the non-metals (S, Cl, and P), because of their important role for the release behavior for herbaceous biomass.15−17
2. EXPERIMENTAL SECTION 2.1. Characterization of DDGS. DDGS is a byproduct of the drygrind process, which produces ethanol from cereal. The starch from the cereals is hydrolyzed into monosugars, which are fermented to produce ethanol. This process leads to solid residue, which is rich in proteins, cellulose, hemicellulose, and lignin and contains some residual starch and yeast.18 DDGS is the solid, dry residue. The high heating value (HHV) of the pelletized DDGS samples is 20.6 MJ/kgdry, which is comparable to wood. These pellets were chopped and sieved to obtain powder with a particle size of ≤0.54 mm, which was stored in hermetically sealed bottles at room temperature. Part of the milled and sieved samples was ashed under air on a platinum foil in a muffle furnace at 550 °C for 24 h at a constant temperature. The heating rate of the samples was 2 °C/min. Samples were weighted after ashing and then analyzed by standard methods. The results can be found in Table 1. The chemical analysis and identification of crystalline compounds of the samples were performed by the central division of analytical chemistry of Forschungszentrum Jülich. For X-ray diffraction (XRD), inductively coupled plasma−optical emission spectroscopy (ICP−OES), and chlorine analyses, the ash samples were ground in a mortar to ensure homogeneity of the samples. Inorganic main components were analyzed by ICP−OES. The variance of the data is ±3% for >1 wt %, ±10% for 0.1−1 wt %, and ±20% for +0.90) correlated to the peak area of 58NaCl+, 74KCl+, and 38HCl+. Likely explanations for the release behavior of the sulfur species are given in the following. In general, the oxygen partial pressure dictates the formation of SO2 on one side and H2S on the other side. However, this is not an all or nothing principle; moreover, the partial pressure is shifting the equilibrium between the oxidized and deoxidized sulfur species, and therefore, both oxidized and deoxidized species can coexist at the same time, which has been shown by the release of 34H2S+ and 64SO2+ at the same time. The correlation between 34H2S+ and 60COS+ can be explained by the reaction of H2S with CO, which results in the formation of H2O and COS. Sulfur can be present in DDGS as both inorganic S (metal sulfates) and organically associated S (amino acids, proteins, coenzymes, lipids, and polysaccharides). It was shown recently that the main amount of sulfur (>95 mass % of the total amount of sulfur) is not leachable with water, ammonium acetate, or hydrochloric acid.19 This means that the major part of sulfur is covalently bound to the biomatrix. Opposed to this, the results by Zhang20 showed that there is a
Table 3. Increase/Decrease of the Released Amount of Inorganic Species as a Percentagea 34
H2S+ 38 HCl+ 41 + K 58 NaCl+ 60 COS+ 63 PO2+ 64 SO2+ 74 KCl+ a
600 °C
900 °C
1200 °C
1400 °C
100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
106.4 86.5 841.4 121.3 114.7 50.1 82.2 146.2
129.7 48.9 2088.7 213.4 130.7 47.2 53.2 412.4
223.7 25.7 2574.6 321.3 204.5 37.9 16.8 1638.5
The data for 600 °C were set to the base level.
trend for the released amount of 64SO2+ was negative. The amount of 64SO2+ decreased −17.8% at 900 °C, −46.8% at 1200 °C, and −83.2% at 1400 °C. Correlation analysis showed that the release of 34H2S+ is highly positively correlated to 60 COS+ and both 34H2S+ and 60COS+ are highly negatively correlated to 64SO2+. Furthermore, the release of all three sulfur 5984
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The main part of the chlorine species 38HCl+ was released during the devolatilization phase at 600−1400 °C. There was no release found for the char reaction phase. A likely explanation is given by the mode of occurrence of chlorine in DDGS. Chlorine can be found nearly exclusively in volatile form, as shown by leaching with water.19 This is one likely reason for the positive, high correlation of the amount of 38 HCl+ in the gas phase with the amount of chlorine of DDGS. In principal, chlorine is highly volatile at the temperatures of the present experiments. However, the released amount of 38 HCl+ strongly decreased with an increasing temperature. The decrease was −13.5% at 900 °C, −51.1% at 1200 °C, and −74.3% at 1400 °C. A likely explanation is the shift of the gas phase reaction of HCl, as shown by eq 3. The reaction balance shifts to the side of KCl and H2O with an increasing temperature. However, this can be only part of the explanation, because there are several other reactions that can modify the release of HCl. An important example is the competitive reaction of alkaline earth metals with chlorine and sulfur, which is indicated by the moderate, negative correlation of the released amount of 38HCl+ with the ratio (Ca + Mg)/S. An example for a competitive reaction of calcium with chlorine and sulfur is given in eq 4. The averaged, normalized peak areas of the alkali species 41 + 41 K / NaO+, 74KCl+, and 58NaCl+ are depicted in Figure 2. DDGS showed an usual high release of 58NaCl+ in comparison to 74KCl+. This observation is unlikely for common biomass. However, it can be explained by the high amount of Na in DDGS and the low K/Na ratio, as shown in Table 1. Furthermore, the main alkali species is 41K+/41NaO+ (the latter is a fragment of NaOH), which can be explained by the low Cl/ (K + Na) ratio. All three alkali metal species under investigation showed a very high increase of the released amount with an increasing temperature. The increase for 58NaCl+ was +21.3% at 900 °C, +113.4% at 1200 °C, and +221.3% at 1400 °C. The increase for 74KCl+ was +46.2% at 900 °C, 312.4% at 1200 °C, and 1538.5% at 1400 °C. Furthermore, a positive, high correlation of the release of 74KCl+ and 58NaCl+ with the amount of 34H2S+ and a negative, moderate correlation with the amount of 63PO2+ were found. A likely explanation is given in the following. The main part of K and Na (>92.5 mass %) is easily removed by leaching with water, and therefore, the two alkali metals are highly volatile under the given experimental conditions. This can explain the positive release trend of 58 NaCl+, 41K+/41NaO+, and 74KCl+ with an increasing temperature, which is in line with the negative release trend of 38HCl+ and the temperature-related shift of the alkali metal chloride and hydrogen chloride (eq 4). Further, both alkali metals can be effectively captured by aluminosilicates as recently shown (eq 5).16 Nevertheless, DDGS lacks Al, as shown by the low Al/Si in Table 1, and therefore, the capture of alkali metals by aluminosilicates plays a minor role for DDGS.
significant variance in the content and mode of occurrence of sulfur from DDGS from different origins. Furthermore, the relatively high sulfur levels in some sources of DDGS are caused by the ethanol production process. Additionally, it was shown that organic sulfur in DDGS contributes mainly from cysteine and methionine in grain, and also, it was highlighted that the bulk of total sulfur in some sources of DDGS contributes from inorganic sulfur, which results from the addition of inorganic sulfur during ethanol production.20 The release of S during the devolatilization phase in our experiments is most likely mainly related to the decomposition of organic, aliphatic associated sulfur in DDGS, because this kind of sulfur shows less thermal stability. Additionally, the decreasing thermal stability of Sorganic with an increasing temperature is part of the explanation of the increased release of 34H2S+ from 600 to 1400 °C. However, the spectra of the sulfur species 64 SO2+ shows that the release of sulfur took longer than the devolatilization phase. This can be partly explained by the release of trapped sulfur with ongoing char consumption, which is shown by the decreasing signal intensity of 64SO2+ spectra over the char reaction phase. Additionally, the decomposition of sulfates occurs at higher temperatures than decomposition of aliphatic sulfur compounds. Therefore, it can be concluded that at least part of 64SO2+ released after the devolatilization phase originates from the decomposition of inorganic sulfur compounds, mainly sulfates as mentioned by Zhang,20 with an increasing temperature. The released amount of the phosphorus species 63PO2+ decreased strongly with an increasing temperature. The decrease was −49.9% at 900 °C, −52.8% at 1200 °C, and −62.1% at 1400 °C. The release of 63PO2+ is in negative, moderate correlation to 58NaCl+ (−0.72) and 74KCl+ (−0.60) but positive, moderate correlation to 38HCl+ (+0.79). Explanations for the observed release behavior and temperature dependence of the sulfur and phosphorus species under investigation are given in the following. The negative correlation to the alkaline earth metal sulfur and phosphorus ratio can be explained by the formation of stable, non-volatile alkaline earth metal sulfides and phosphates.15,16 DDGS has high (Ca + Mg)/S and (Ca + Mg)/P ratios (Table 1), which are an important precursor for a high sulfur and phosphorus capture capability. Additionally, the mode of occurrence plays an important role. Piotrowski et al.19 showed that main parts of alkaline earth metals Ca and Mg as well as the alkali metals Na and K are easily removed by leaching experiments with water and ammonium acetate, which indicate that this amount is not bound to silicate or aluminosilicate. Furthermore, the water and acetate leachable part is easily available for reaction with other compounds, e.g., S and P. This is proven by the results of the ash analysis in Tables 1 and 2, which showed that several P- and S-based alkaline earth metal and alkaline compounds were formed. Equations 1 and 2 show as an example of a capture reaction. Porbatzki et al.16 showed that phosphates from biomass ash are stable even at temperatures up to 1150 °C. Additionally, the alkali metals sodium and potassium can form sulfides and phosphates, which can effectively reduce the amount of released sulfur and phosphorus species under gasification conditions. However, the thermodynamic stability of alkaline earth as well as alkali metal sulfides and phosphates decreases with an increasing temperature, which can partly explain the increasing released amount of 58NaCl+, 74KCl+, 63 PO2+, 34H2S+, and 60COS+ with an increasing temperature.
3CaO + P2O5 ↔ Ca3(PO4 )2
(1)
CaO + H 2S ↔ CaS + H 2O
(2)
KCl + H 2O ↔ KOH + HCl (shifts to the left side with an increasing temperature) CaS + 2HCl ↔ CaCl 2 + H 2S 5985
(3) (4)
dx.doi.org/10.1021/ef401713f | Energy Fuels 2013, 27, 5982−5986
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2KCl + H 2O + x AlSi yOz ↔ K−aluminosilicate + 2HCl
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(5)
4. CONCLUSION The influence of the temperature on the release of inorganic trace elements during gasification of DDGS was investigated online by MBMS. The main species of interest were 34H2S+, 60 COS+, 64SO2+, 38HCl+, 41K+/41NaO+, 74KCl+, 58NaCl+, and 63 PO2+. The release of these species occurred mainly during the devolatilization phase. The amount of the determined sulfur and phosphorus species depends upon the amount of sulfur and phosphorus of DDGS and the ratios (Ca + Mg)/S and (Ca + Mg)/P; the release of 38HCl+ mainly depends upon the chlorine content; and the alkali release mainly depends upon the amount of alkali metals and the ratio (K + Na)/Cl. Further, it was shown that the mode of occurrence of the species under investigation is one of the most important factors of influence for the release. Additionally, it was shown that the release trends are significantly affected by the temperature, especially the shifted gas-phase equilibrium of alkali chloride.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +49-2461-61-1574. Fax: +49-2461-61-3699. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work described in this paper has been performed in the framework of the Green Syngas Project, funded by the European Commission (EC) in the Seventh Framework Program.
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REFERENCES
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