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Purification of Biogases from Siloxanes by Adsorption: On the Regenerability of Activated Carbon Sorbents Elisabetta Finocchio,† Tania Montanari,† Gilberto Garuti,‡ Chiara Pistarino,§ Flavio Federici,| Michela Cugino,| and Guido Busca*,† Dipartimento di Ingegneria Chimica e di Processo, Facolta` di Ingegneria, UniVersita` di GenoVa, Pl.e Kennedy 1, 16126 GenoVa, Italy, Acqua e Sole S.r.l., Milano, Italy, Ecodeco SpA, Cassinazza di Baselica, 27010 Giussago, PV, Italy, and Ansaldo Fuel Cells, GenoVa, Italy ReceiVed April 22, 2009. ReVised Manuscript ReceiVed June 25, 2009
The effective adsorption capacity of hexamethycyclotrisiloxane (HMCTS), a common siloxane impurity in biogases, by solids, such as activated carbons (ACs), silica, and zeolite, has been evaluated in laboratory experiments using synthetic biogas. The adsorption mode of this molecule has been investigated by infrared (IR) spectroscopy. Similar experiments have been performed with exhausted ACs taken from a commercial adsorption column treating real landfill biogas. It has been found that pure ACs are efficient sorbents for HMCTS, better than alkali-impregnated ACs and inorganic solids. It was found that HMCTS converts in part upon adsorption into silicone and silica. This is likely a reason of the only partial regenerability of ACs after use in purification of real biogases.
1. Introduction Biogases are methane-rich gases produced by anaerobic digestion of several waste materials, such as waste-activated sludge,1 animal sewage,2 and landfill wastes.3 These gases may find energy application:4 they may be burnt to produce heat, used as fuels of spark ignition engines or gas turbines to recover electric power or combined heat and power (CHP/co-generation), used as a fuel of direct biogas fuel cells,5 or used to produce hydrogen or syngas by steam reforming of methane.6 They can also be upgraded as automotive fuels by separation of CO2.7 The application of biogases is hampered by the presence of noxious impurities, among which are polysiloxane compounds.8 These compounds, in fact, which mainly arise from personal * To whom correspondence should be addressed. Telephone: +39-0103536024. Fax: +39-010-3536028. E-mail:
[email protected]. † Universita ` di Genova. ‡ Acqua e Sole S.r.l. § Ecodeco SpA. | Ansaldo Fuel Cells. (1) Appels, L.; Baeyens, J.; Degre`ve, J.; Dewil, R. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 2008, 34, 755–781. (2) Tricase, C.; Lombardi, M. State of the art and prospects of Italian biogas production from animal sewage: Technical-economic considerations. Renewable Energy 2009, 34, 477–485. (3) Themelis, N. J.; Priscilla, A.; Ulloa, P. A. Methane generation in landfills. Renewable Energy 2007, 32, 1243–1257. (4) Jaramillo, P.; Matthews, H. S. Landfill-gas-to-energy projects: Analysis of net private and social benefits. EnViron. Sci. Technol. 2005, 39, 7365–7373. (5) Shiratori, Y.; Oshima, T.; Sasaki, K. Feasibility of direct biogas SOFC. Int. J. Hydrogen Energy 2008, 33, 6316–6321. (6) Ashrafi, M.; Pro¨ll, T.; Pfeifer, C.; Hofbauer, H. Experimental study of model biogas catalytic steam reforming. Energy Fuels 2008, 22, 4182– 4189. (7) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. Upgrade of methane from landfill gas by pressure swing adsorption. Energy Fuels 2005, 19, 2545–2555. (8) Wheless, E.; Pierce, J. Siloxanes in landfill and digester gas update. http://www.scsengineers.com/ Papers/Pierce_ 2004Siloxanes_ Update_ Paper.pdf.
care products released in landfills, create a number of problems to energy production devices. They decompose in spark-ignition engines, producing silica powders, which rapidly damage the pistons, and depose on steam reforming catalysts, causing their silication and consequent deactivation. The same phenomenon occurs on the anodes of direct biogas fuel cells. The removal of siloxanes from biogases may be attempted in different ways, such as condensation, decomposition, absorption in liquids, and adsorption in solids.9,10 Although hightemperature treatment with solids may be efficient for siloxane removal,11,12 room-temperature adsorption in solids is the most practical technique today. The most used solids to remove siloxanes are activated carbons (ACs), although other solids, such as inorganics (silica and zeolites) or polymeric resins, may also give interesting results. To make adsorption practical from an economic point of view, regeneration of the adsorbent by desorption is needed. This is, however, a major problem with the adsorption of siloxanes on solids,1 in particular ACs. In this paper, we summarize results concerning the adsorption of hexamethylcyclotrisiloxane (HMCTS), which is usually one of the most abundant siloxanes in landfill biogases, on some ACs and the exhaustion of them in laboratory and industrial conditions. 2. Experimental Section Data on the materials used as adsorbents are summarized in Table 1. HMCTS was purchased from Aldrich. Flow tests (Figure 1) have (9) Schweigkofler, M.; Niessner, R. Removal of siloxanes in biogases. J. Hazard. Mater. 2001, B83, 183–196. (10) Dewil, R.; Appels, L.; Baeyens, J. Energy use of biogas hampered by the presence of siloxanes. Energy ConVers. Manage. 2006, 47, 1711– 1722. (11) Finocchio, E.; Garuti, G.; Baldi, M.; Busca, G. Decomposition of hexamethyl-cyclotrisiloxane over solid oxides. Chemosphere 2008, 72, 1659–1663. (12) Urban, W.; Lohmann, H.; Salazar Go´mez, J. I. Catalytically upgraded landfill gas as a cost-effective alternative for fuel cells. J. Power Sources 2009, 193, 359–366.
10.1021/ef900356n CCC: $40.75 2009 American Chemical Society Published on Web 07/13/2009
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Table 1. Adsorption Capacities for HMCTS over Solidsa adsorbent silica gel Faujasite NaX AC AC AC impregnated AC impregnated AC impregnated AC impregnated AC impregnated AC impregnated spent AC spent AC a
with with with with
KI CuII and CrVI salts KOH < 15% alkali
with KOH 8%
origin
apparent density (g/mL)
effective adsorption capacity (gH/gCAT)
Grace Sylobead MS 544 Grace Picacarb NORIT RB4 NORIT ROZ NORIT RGM1 NORIT RBAA1 Carbonfilter Multisorb DS SH4 Chemviron carbon Solcarb KS3 Sicav Si 30K NORIT RB4 spent end column NORIT RB4 spent half column
0.72 0.69
0.230 0.276 0.561 0.580 0.483 0.878 0 0.06 0.285 0.332 0.27 0.07
0.41 0.47 0.49 0.53
Most of these commercial ACs are not sold as specific for biogas treatment.
Figure 1. Schematics of the laboratory flow adsorption apparatus.
been performed at room temperature and atmospheric pressure in a tubular quartz reactor (30 cm long and 0.6 cm internal diameter), where a fixed bed of 1.3 g of adsorbent material in pellets was fixed with glass wool. The adsorbents were exposed to a synthetic biogas [44:56% CO2/CH4 (v/v), purchased from SIAD] saturated at room temperature with HMCTS (corresponding to about 5 mg/ mL) and water vapor by a bubbler (25 °C). The total flow rate is 60 mL/min, corresponding to a contact time of around 2 s calculated using the apparent bed volume of powdered pellets. Analysis of the reactants and reaction gaseous products have been performed by an on-line Fourier transform infrared spectroscopy (FTIR) instrument (ThermoFisher Nicolet 6700) equipped with a gas cell connected to the adsorption apparatus and operating with OMNIC Series acquisition software. This analysis allows us to obtain a complete infrared (IR) spectrum in the mid-IR range every 4 s, thus monitoring almost continuously the decrease of the diagnostic siloxane IR bands and the formation of other volatile reaction products. The siloxane effective adsorption capacity of the material has been determined by evaluating the time-on-stream at 100% siloxane removal before breakthrough. The fresh and spent adsorbents have also been studied by skeletal FTIR: the powders have been finely ground and diluted with KBr in air. The partial desorption of the adsorbed species from spent adsorbents arising from real biogas purification plant has been performed under static vacuum, at 10-1 Torr. The analysis of the desorbed species was performed by gas chromatography-mass spectrometry (GC-MS) analysis (HP G1800 B GCD plus system, He carrier, HP VOC MS, 70 m × 0.32 mm) of the gases evolved under vacuum. Laboratory data have been compared to those arising from an industrial landfill biogas caption and utilization plant in Corteolona (Pavia, Italy). A column 6 m high, filled with a bed containing 10 tons of the RB4 adsorbent, treats 1500 N m3/h of real landfill biogas. At the inlet, the temperature is 55 °C and the relative pressure is 120 mbar (1.12 atm absolute).
Figure 2. FTIR spectrum of the HMCTS-containing synthetic biogas. H ) main bands of HMCTS.
After use, regeneration of ACs is performed by the producers. However, effective adsorption capacity of the regenerated ACs is strongly reduced.
3. Results and Discussion In Figure 2, the IR spectrum of the synthetic biogas is shown. The quantitative analysis of HMCTS in the biogas can be performed by following the intensity of the bands at 1035 and/ or 818 cm-1, which may be ascribed to asymmetric and symmetric stretchings of the Si-O-Si siloxane bridge bonds of HMCTS. The synthetic biogas was admitted through the bypass line until the intensity of the recorded spectrum was constant. Time zero in the adsorption analysis corresponds to the switch of the flow into the adsorber reactor line. Since time zero, the intensity of the spectrum of gaseous HMCTS falls down, depending upon the effective adsorption capacity of the
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Figure 3. Behavior of the concentration of HMCTS in the synthetic biogas flowing out of the adsorption column evaluated using the absorbance of the IR band at 818 cm-1, with fresh and spent Norit RB4 ACs. Time 0 is the switch time on the column.
adsorbent, while no adsorption of methane and CO2 has been observed. In fact, their IR spectrum retains the same intensity. With the best adsorbents, the IR absorption of gaseous HMCTS falls until zero in nearly 10 min. The sensitivity of this method has been evaluated to be of few parts per million. This time is needed to allow the gas that was already in the cell and in the last section of the line to flow out of the cell. However, the data show that counter-diffusion of HMCTS vapor also probably occurs. In fact, for some less active adsorbents, the time needed to measure zero concentration of HMCTS in the flow is longer than for the most active adsorbents. This is interpreted as being due to counter-diffusion of part of HMCTS to the adsorbent bed. The curve relative to the adsorption of HMCTS over the fresh NORIT RB4 AC is reported in Figure 3. After nearly 130 min, the HMCTS spectrum starts to be observed again in the flowing gas, showing that the effective adsorption capacity of the AC is nearly fulfilled. The adsorption capacities of the adsorbents reported in Table 1 have been calculated as the amount of HMCTS adsorbed on the solid when the HMCTS vapor starts to be observed in the gas (i.e., when its abatements are no longer complete). In Table 1, these amounts, expressed as the weight of adsorbed HMCTS per weight of adsorbent are reported for several commercial solids, i.e., some ACs and some inorganic adsorbents, such as silica and zeolite. It is clear that the adsoption capacities per unit weight of ACs are higher than those of silica and zeolites. Even taking into account the significantly higher apparent density of such inorganic materials, their effective adsorption capacity is lower than that of pure ACs also per unit volume. Among ACs, alkali-impregnated ACs find poorer adsorption capacities than pure ACs. The best effective adsorption capacity is observed for the sample Norit RGM1, an extruded steam AC impregnated with CrVI and CuII salts.13 The presence of such transition-metal ions makes this material more difficult to handle and dispose (when spent) with respect to pure ACs. For this reason, the practical application of such a material might find some limit, with the use of pure ACs being preferred. The exhausted adsorbents after the above laboratory adsorption experiments have been analyzed by IR spectroscopy in comparison to the fresh ACs. In Figure 4, the IR spectra of fresh and spent Norit RB4 and RGM1 adsorbents are reported. As the result of the adsorption of HMCTS, the spectrum is modified during the use. New bands appear that are reported (as subtraction spectra) in the same Figure 4. It is evident that the spectra in both cases do not correspond to that of HMCTS. In the case of Norit RGM1 adsorbent, the spectrum corresponds (13) www.norit-americas.com/norit-rgm1.html.
Figure 4. FTIR spectra of KBr-pressed disks of Norit RB4 and RGM1 ACs fresh and after a complete laboratory adsorption run. (Down spectra) Subtraction spectra (spent-fresh) compared to spectra of polydimethylsiloxane and HMCTS solid.
well to that of polydimethylsiloxane, which is the result of ringopening polymerization of HMCTS. On the adsorbent Norit RB4, the observed spectrum is likely a mixture of that of HMCTS and the polymer. These spectra show that HMCTS polymerizes, at least partially, during the adsorption experiment. The pure steam AC Norit RB4 has been tested in the field; i.e., it has been used to purify by adsorption in the real landfill biogas caption and utilization plant in Corteolona (Pavia, Italy; see the Experimental Section). Adsorption of siloxanes and sulfur compounds with fresh AC is complete. After AC saturation, sulfur compounds and siloxanes start to be released. At this moment, the biogas flow was switched to another column and the adsorbent was discharged. Two samples were taken at two different height levels of the column and tested in our laboratory plant (Figure 3). The data, reported in Table 1, show that indeed, after working on stream, the effective adsorption capacity of the adsorbent is progressively reduced and that, as expected, this reduction is faster at half the column while lower at its end. Actually, the sample at the end of the column was still able, in our conditions, to adsorb HMCTS completely but only for 60 min (Figure 3), i.e., half time with respect to the fresh adsorbent. The sample taken from the middle of the column, instead, was never able to abate completely the siloxane from the synthetic biogas. To try to identify the more important reasons for the exhaustion of the effective adsorption capacity of such a solid, we put the sample under static vacuum (10-1 Torr) at different temperatures and analyzed the released vapors by GC-MS. The compounds identified by our instruments are reported in Table 2. In Figure 5, the gas chromatogram relative to the vapor desorbed in the range of 150-200 °C is reported, as an example. Below 100 °C, only the components of air were found to desorb. Between 100 and 200 °C, we can find desorbed alkylaromatic hydrocarbons, hexamethyl disiloxane, hexamethylcyclotrisiloxane, and octamethylcyclooctasiloxane, H2S, and organic sulphides. In the range of 150-200 °C, mercaptans (methanthiol, ethanthiol, and propanthiol) are mostly desorbed. To study further the deactivation phenomena, we analyzed by IR spectroscopy the same fresh and exhausted Norit RB4 ACs after working in real biogas purification. The spectra observed for the spent samples are reported in Figure 6. The difference spectrum is shown in Figure 7, where it is compared
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Table 2. Compounds, Identified by GC-MS, Observed upon Desorption from Exhausted ACs after the Run in a Real Landfill Biogas Treating Planta 50-100 °C air
100 °C octamethylcyclotetrasiloxane pinene alkylbenzene limonene 100-150 °C -3.93 carbon anhydride -4.27 propane -5.00 1-propene -9.06 2-butanone -12.50 toluene -12.71 hexamethylcyclotrisiloxane -14.20 ethylbenzene -14.40 xylene -15.52 octamethycyclotetrasiloxane -15.68 pinene -17.49 alkylbenzene -17.58 limonene -15.53 -15.58 -17.48 -17.58
a
150-200 °C -3.94 carbon anhydride -4.17 H2S -4.3 propene/COS -5.01 2-methyl-propene -5.33 methylsilane -5.54 mercaptomethane -6.90 acetone -7.10 ethanthiol -7.96 thiourea -8.20 propanthiol -8.97 2-propanthiol -9.06 2-butanone -9.17 furan -9.37 1-propanthiol -10.31 hexamethyldisiloxane -10.51 2-butanthiol -10.85 2-pentanone -12.20 alkyldisulfide -12.50 toluene -12.70 hexamethylcyclotrisiloxane -14.27 ethylbenzene -14.40 xylene -15.52 octamethycyclotetrasiloxane -17.49 alkylbenzene -17.58 limonene
Figure 6. FTIR spectra of KBr-pressed disks of Norit RB4 ACs fresh and after the field run in real landfill biogas.
Retention times in minutes.
Figure 7. Subtraction FTIR spectra of Norit RB4 ACs fresh and after the field run in real landfill biogas (spent half column - fresh) compared to spectra of single siloxanes and silica.
Figure 5. GC response of the GC-MS instrument: gas evolved from the Norit RB4 sample used in the field run in real landfill biogas, half column, evolution at 150-200 °C in static vacuum (initial pressure of 10-1 Torr).
to those of the most relevant siloxanes in the condensed state and that of amorphous silica. It is evident that the spectrum of amorphous silica grows progressively upon contact of the AC on stream. 4. Conclusions The conclusions from the present work are the following: (1) The effective adsorption capacity of pure ACs for HMCTS in synthetic biogas is higher than those of inorganic solids (silica gel and zeolite) and alkali-impregnated ACs. (2) The effective adsorption capacity of transition-metal-containing ACs may be even higher. However, the presence of such metals may cause
more difficulties in the handling and disposal of spent adsorbents. (3) HMCTS polymerizes at least partially upon adsorption experiments at room temperature in our laboratory plant, giving rise to polydimethylsiloxane. (4) Spent pure ACs used in an industrial biogas purification plant show the accumulation of silica. (5) The formation of non-volatile compounds, such as silicone polymer and silica, on ACs upon biogas streams explains why regeneration of ACs after use in biogas purification is usually only partial if at all. Acknowledgment. This work has been supported in part by MIUR-PNR-FIRB [Risparmio energetico con valorizzazione dei Bacini Secondari di Energia quale fonte energetica distribuita, Unita` Natural Energy from Waste (NEW)] and EU (6FP IP Biogas Integrated ConceptsA European Program for Sustainability, BICEPS). Giovanni Marchitelli, Davide Bocciardo, Gabriella Garbarino, and Irene Bozzano are aknowledged for performing part of the experimental work. EF900356N
