Energy & Fuels 2007, 21, 3017-3022
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Qualitative Evaluation of Alkali Release during the Pyrolysis of Biomass Torsten Kowalski,*,† Christian Ludwig,*,†,‡ and Alexander Wokaun† Paul Scherrer Institut (ENE), CH-5232 Villigen PSI, Switzerland, and School of Architecture, CiVil and EnVironmental Engineering (ENAC-ISTE), Swiss Federal Institute of Technology at Lausanne (EPFL), Station 2, CH-1015 Lausanne, Switzerland ReceiVed February 19, 2007. ReVised Manuscript ReceiVed July 10, 2007
The thermochemical emission of alkali metals from biomass samples and two organic salts was investigated for the first time using a novel combination of a thermogravimetric analyzer with a jet flow condenser attached to a surface ionization detector. This equipment allows the identification and comparison of the main alkali release temperatures of various kinds of biomass during pyrolysis. It has also been used to test the release patterns of two organic salts, which are present in various kinds of biomass, with low decomposition temperatures.
Introduction The use of biomass fuels has been increasing steadily during the past few years. Two applications continue to be the focus of research and development worldwide: the replacement of fossil fuels by biomass derived fuels1 and the generation of electrical energy.2 The majority of electrical energy is still produced in traditional cycles combining combustion of biomass, generation of steam, and then generation of electricity. In recent years, the Integrated Gasification Combined Cycle (IGCC) process,3 combining gasification, combustion in a gas turbine, and generation of electrical power has been developed, promising more than twice the efficiency of traditional power generation. However, biomass resources contain a multitude of mineral substances. The alkali metals, which are especially found in fast-growing plants,4 are actively involved in the breakdown, corrosion, and erosion of plant materials5 when released during combustion or gasification. For gas turbines, only very low levels of alkalis are accepted by their manufacturers, and limits of a maximum content of only 0.1 milligram per mN3 of feed gas are reported, e.g., by Turn et al.6 The actual chemical composition of the biomass influences the volatilization of alkalis and the gasification or pyrolysis * To whom correspondence should be addressed. E-mail (T.K.):
[email protected]. E-mail (C.L.):
[email protected]. † Paul Scherrer Institut. ‡ Swiss Federal Institute of Technology at Lausanne. (1) Stucki, S. B.; Vogel, F. Vom Holz zum Methan; Paul Scherrer Institut - Labor fu¨r Energie und Stoffkreisla¨ufe: 2003. (2) EPRI Renewable Energy Technology Characterizations TR-109496; U.S. Department of Energy: Washington and Palo Alto, December 1997. (3) Kurkela, E. S. P.; Laatikainen, J.; Simell, P. Development of simplified IGCC-processes for biofuels: Supporting gasification research at VTT. Bioresour. Technol. 1993, 46 (1-3), 37-47. (4) van der Drift, A.; Olsen, A. ConVersion of Biomass, Prediction and Solution Methods for Ash Agglomeration and related Problems, ECN-Report ECN-C-99-090; Energy Research Center of the Netherlands: November 1999. (5) Monkhouse, P. On-line diagnostic methods for metal species in industrial process gas. Prog. Energy Combust. Sci. 2002, 28, 331-381. (6) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M.; Zhou, J. C. The fate of inorganic constituents of biomass in fluidized bed gasification. Fuel 1998, 77 (3), 135-146.
process. It is known that alkalis may accelerate the gasification process, while a high chlorine content may inhibit it.7 It was shown that chlorine in turn increases the volatility during thermal treatment. It has been proven that alkali release could be significantly reduced by choosing chlorine-free fertilizers.8,9 However, this seems to affect only the high-temperature release in gasification or pyrolysis. For the origin of low-temperature peaks, no sufficient explanation has been found. These peaks observed at pyrolysis temperature have been attributed to the presence of alkali metals dispersed in the organic phase9 or to organically bound alkalis.10 It was shown that a reduction in the release could be achieved by washing the biomass samples with acetic acid. To tackle the problems associated with alkali release, knowledge of release patterns during gasification and possibly the influence of pretreatment processes of biomass on alkali release is necessary. For our investigation, we have coupled a thermogravimetric analyzer (TGA) with a jet flow condenser (condensation interface, CI). This method of safe transfer of volatile compounds to an analytic device has been used successfully previously for the detection of heavy metals from waste samples.11,12 There, inductively coupled plasma optical spectrometry (ICP-OES) was used for multi-element detection. For the detection of alkalis, this setup has been shown to be insufficiently sensitive. Therefore, a surface ionization detector (SID) was attached to the TGA-CI. This setup was ap(7) Struis, R.; von Scala, C.; Stucki, S.; Prins, R. Gasification reactivity of charcoal with CO2. Part I: Conversion and structural phenomena. Chem. Eng. Sci. 2002, 57 (17), 3581-3592. (8) Davidsson, K. O.; Pettersson, J. B. C.; Nilsson, R. Fertiliser influence on alkali release during straw pyrolysis. Fuel 2002, 81 (3), 259-262. (9) Olsson, J. G.; Ja¨glid, U.; Pettersson, J. B. C.; Hald, P. Alkali metal emission during pyrolysis of biomass. Energy Fuels 1997, 11 (4), 779784. (10) Davidsson, K. O.; Korsgren, J. G.; Pettersson, J. B. C.; Jaglid, U. The effects of fuel washing techniques on alkali release from biomass. Fuel 2002, 81 (2), 137-142. (11) Ludwig, C.; Lutz, H.; Wochele, J.; Stucki, S. Studying the evaporation behavior of heavy metals by thermo-desorption spectrometry. Fresenius’ J. Anal. Chem. 2001, 371 (8), 1057-1062. (12) Ludwig, C.; Wochele, J.; Jorimann, U. Measuring evaporation rates of metal compounds from solid samples. Anal. Chem. 2007, 79 (7), 29922996.
10.1021/ef070094z CCC: $37.00 © 2007 American Chemical Society Published on Web 08/25/2007
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proximately 3 orders of magnitude more sensitive selectively to the alkali metals. Experimental Section TGA-CI. Pyrolysis is the thermal decomposition of biomass in the absence of oxygen. We perform the pyrolysis of samples in an aluminum oxide crucible in the furnace of a commercially available Mettler TGA/SDTA851e thermogravimetric analyzer (TGA) in a flow of typically 80 mL/min inert argon gas. The hot gas from the furnace, carrying the gaseous or particulate alkalis, is quenched rapidly in a jet flow condenser (condensation interface, CI) with 500 mL/min argon. The TGA allows controlling the sample temperature very accurately. As an improvement to previous methods,9,10 in a TGA it is not the crucible containing the sample that is heated; rather, this crucible is placed inside a heated furnace. Thus, condensation of evaporated material at cold parts of the sample chamber, and therefore depletion of the gas, becomes less probable. Furthermore, effects of thermal inertia between the sample and the thermocouple used to measure the sample’s temperature are less distinct when both are heated simultaneously in the furnace. Additionally, a direct observance of the sample mass change is possible which allows identification of specific processes, such as evaporation of constituents of the sample. The samples of biomass were kept small (10-50 mg) to achieve a high surface-to-volume ratio. The samples were heated at a rate of 20 K per minute up to a final temperature of 800 °C. Before each measurement, the crucible was heated in argon to clean it and to record a background signal for the surface ionization alkali detector. SID. For the detection of volatilized alkalis, we rely on the principle of surface ionization as described by Zandberg et al.13 and very successfully applied for similar purposes by Davidsson et al.14 and Olsson et al.9 Positive surface ionization is a process where an element or molecule deposited on or directed onto a metal surface will evaporate in a positively ionized state.15 The elements to detect are the alkalis sodium and potassium, and the according metal surface is a platinum wire heated to 1200 °C. Alkali salt particles in the probe gas stream impinge on the filament, they melt, molecules dissociate, and the alkali atoms are evaporated as positively charged ions. The detector and its working principle are described in detail elsewhere.16 Though positive surface ionization has been applied in vacuum first, successful application under atmospheric pressure is reported.17 From our own work we conclude that in case of potentially reactive gases carrying the alkalis (such as CO, methane, or hydrogen), a high dilution of the respective probe gas is required to maintain stable detection conditions.16 Therefore, a flow of 600 mL/min argon as inert gas for dilution is used. As a rule of thumb, gasification of wood yields two liters of gas per gram of wood. For the pyrolysis of a 10 mg sample, this yields at most 20 mL of additional gas added to the TG argon during approximately 5 to 10 min, the observed time of pyrolysis. Thus, a dilution of at least 1:100 is ensured, and it can be assumed that the SID alkali signal remains unaltered by the gas composition. (13) Zandberg, E. Y.; Tontegod, A. Surface Ionization of Alkali- Halide Molecules on Platinum. SoV. Phys. Tech. Phys. - USSR 1968, 13 (4), 576. (14) Davidsson, K. O.; Engvall, K.; Hagstro¨m, M.; Korsgren, J. G.; Loenn, B.; Pettersson, J. B. C. A surface ionization instrument for on-line measurement of alkali metal components in combustion: Instrument description and applications. Energy Fuels 2002, 16 (6), 1369-1377. (15) Kawano, H.; Page, F. M. Experimental methods and techniques for negative-ion production by surface ionization. Part I. Fundamental aspects of surface ionization. Int. J. Mass Spectrom. Ion Phys. 1983, 50 (1-2), 1-33. (16) Kowalski, T.; Ludwig, C.; Wokaun, A. Evaluating a Surface Ionisation Detector for the Use in Biomass Gasification. Submitted for publication. (17) Davidsson, K. O.; Engvall, K.; Korsgren, J. G.; Lo¨nn, B.; Pettersson, J. B. C. In On-line Alkali Measurements in Flue Gas from Combustion of Coal and Biofuels, International Conference on Fluidized Bed Combustion, Savannah, Georgia, May 16-19, 1999; American Society of Mechanical Engineers: Savannah, Georgia, U.S.A., 1999.
Kowalski et al. Test Aerosols. If small amounts of samples are investigated, their total alkali content will also be small. Thus, it is necessary to guarantee sufficient sensitivity for the desired application, especially when the probe gas is highly diluted with argon. We have tested the sensitivity of the detector with monodisperse aerosols of KCl and NaCl. Four solutions of KCl (134, 13.4, 1.34, 0.134 mmol/L) and NaCl (171. 17.1, 1.71 mmol/L) in water were nebulized with pressurized air. When the droplets dry, particulate aerosols are generated by crystallization of the respective salt. By means of a differential mobility analyzer DMA (TI Model 3081), monodisperse aerosols can be created. The gas stream containing the aerosols was split, and one part was directed through the SID and the other taken to measure the particle concentration with a condensation particle counter (CPC, TI Model 3022). Samples. The investigated samples were wood saw dust, sewage sludge, cattle manure, two char coke samples, and two organic salts. Wood is frequently suggested as a biomass of primary interest for gasification and IGCC application.3,18 A thermal utilization of the wet biomass sewage sludge has been discussed not only for power generation but also for proper waste disposal of this frequently toxic material.19 Here, pyrolysis is preferred to high-temperature gasification, because due to low temperatures the volatilization of highly toxic heavy metals such as cadmium is prevented.20 Cattle manure has been subject of gasification for several years, both for easing disposal problems and for substitution of fossil fuels, starting in the 1970s.21 A commercially available beech wood pyrolysis coke (Dynamotive Inc., Canada) produced by fast oxygen-free pyrolysis at 400 to 500 °C22 and a straw char coke sample produced in an allothermal pyrolysis at 500 °C (Bioliq process, described by Henrich et al.23) were also subjected to pyrolysis. Acetic acid wash proved especially effective for the removal of alkalis from biomass. We suggest and test a further source of alkali emission by the decomposition of salts with low melting and decomposition temperatures (sodium thiosulfate pentahydrate and sodium acetate, Merck p.A.). To our knowledge, thiosulfate is not generally present in plant-derived biomass, but it is a major constituent of up to 2.7% in dry mass24 in black liquor, a biomass byproduct considered to be an important source of energy in IGCC for the future.5,25 Acetic acid (among others such as formic acid) does not only naturally occur in wood, but is in fact created by chemical reactions during either storage26 or thermal processing such as thermo hydrolysis27 or pyrolysis. Up to 17% (on dry basis) of acetic acid were found in 450 °C pyrolysis products by (18) Helsen, L.; Van den Bulck, E.; Mullens, S.; Mullens, J. Lowtemperature pyrolysis of CCA-treated wood: thermogravimetric analysis. J. Anal. Appl. Pyrolysis 1999, 52 (1), 65-86. (19) Manya`, J. J.; Sa´nchez, J. L.; Gonzalo, A.; Arauzo, J. Air Gasification of Dried Sewage Sludge in a Fluidized Bed: Effect of the Operating Conditions and In-Bed Use of Alumina. Energy Fuels 2005, 19 (2), 629636. (20) Stammbach, M. R.; Kraaz, B.; Hagenbucher, R.; Richarz, W. Pyrolysis of sewage sludge in a fluidized bed. Energy Fuels 1989, 3 (2), 255-259. (21) Engler, C. R.; Walawender, W. P.; Fan, L. T. Synthesis gas from feedlot manure. Conceptual design study and economic analysis. EnViron. Sci. Technol. 1975, 9 (13), 1152-1157. (22) Dynamotive Inc. General BioOil information, www.dynamotive.com. (23) Henrich, E. D., E.; Rumpel, S.; Stahl, R. A two-stage pyrolysis/ gasification process for herbaceous waste biomass from agriculture; Forschungszentrum Karlsruhe, Institut fu¨r Technische Chemie,: 2000. (24) Veverka, P. J.; Nichols, K. M.; Horton, R. R.; Adams, T. N. On the form of nitrogen in wood and its fate during Kraft pulping; 460; Institute of Paper Science and Technology: Atlanta, Georgia, 1993. (25) Dance, M. Hydroxide formation and carbon species distribution during high-temperature Kraft black liquor gasification; Georgia Institute of Technology: Georgia, 2005. (26) Packman, D. F. The Acidity of Wood. Holzforschung 1960, 14, 178-183. (27) Hameed, M.; Behn, C.; Roffael, E.; Dix, B. Water support power of recycling shavings and of shavings extracted directly from wood. Holz Roh-Werkst.2005, 63 (5), 390-391.
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Figure 2. Relative sensitivity of the SID as a function of the aerodynamic particle diameter.
Figure 1. Aerosols from 1.71 mmol/L NaCl solution (A) and from 1.34 mmol/L KCl solution (B), representing the determined minimum sensitivity of the SID. The numbers denote the respective aerodynamic diameter of the incoming aerosols. Table 1. Measured Number Concentration of Particles for the Sensitivity Determination of the SI Alkali Detector aerodynamic particle diameter (nm)
particle concn NaCl (cm-3)
particle concn KCl (cm-3)
50 75 100
50 75 100
963 435 47
Demirbas.28 The emission of acetic acid has been shown to peak sharply at the decomposition temperature of wood.18 During each experiment, a small amount of KCl (Merck, p.A.) was present as a marker for the onset of the evaporation of the inorganic salts.
Results and Discussion Limit of Detection for the SID. Aerosols created by nebulization of Na and K salt solutions were used to determine the lower limit of detection of the SI detector. The SID alkali signal current for monodisperse aerosols was recorded. An example of these measurements is shown in Figure 1. Three plateaus in the SID current are visible, representing the signals of monodisperse aerosols with an aerodynamic diameter of 50, 75, and 125 nm, respectively. The signals are clearly distinguishable from the background. By calculating the total amount of alkali in the gas from the simultaneously measured concentration of particles (see Table 1), assuming cubic shape of the particles,29 we determine the detector’s minimum sensitivity to around 1/10 of a microgram Na and K per m3. This detection limit must be compared with approximated values of alkali concentration expected to be found in the pyrolysis of small biomass samples. We assume a 10 mg sample size of wood with 0.1 wt % alkali.30 Then, the maximum total amount of alkali is 10 µg. The total gas flow from the TGA is (28) Demirbas, A. Mechanisms of liquefaction and pyrolysis reactions of biomass. Energy ConVers. Manage. 1999, 41 (6), 633-646. (29) Scheibel, H. G.; Porstendorfer, J. Generation of monodisperse Agand NaCl-aerosols with particle diameters between 2 and 300 nm. J. Aerosol Sci. 1983, 14 (2), 113-126.
0.6 L/min argon. Thus, if 10% of the total alkali in the wood were released continuously during the experiment (duration 40 min), a concentration of around 40 µg per cubic meter argon would be achieved. This is high above the detection limit. Thus we conclude that the SI detector’s sensitivity is sufficient even when the evaporated alkalis are highly diluted. Influence of the Particle Size. Measurements with monodisperse aerosols also allow comparison of the measured background-corrected SID current with the total mass of incoming particles. In Figure 2, the ratio of SID current to particle mass as a function of particle diameter is given in arbitrary units, normalized to the number of potassium or sodium atoms in a particle, respectively. A higher ratio means a higher sensitivity, and vice versa. In this way, we aim to determine the SID signal’s dependence on the aerosol particle size. The highest current per atom was observed for particles of an aerodynamic diameter between 75 and 200 nm. For both bigger and smaller particles, the current per mass is smaller. The variance even at the same particle size is a factor of 2 (given by the shaded area in Figure 2). This observed decrease of current per atom toward large particles of 200 nm diameter and above is in good agreement with former predictions and experiments31 which show that large particles do not melt completely and are therefore only partially detected. However, the same authors suggest that the sensitivity increases toward smaller particle diameters. This is in contradiction with our observations. We believe that the phenomena of mass transport of aerosol particles are responsible for this effect. The mass transfer namely of small aerosol particles toward the hot platinum filament may be greatly reduced by the phenomena of gas convection inside the SI detector.16 Particle size distributions in biomass combustion have been measured and shown to have maxima at around 100-200 nm.32 The majority of total alkali mass was found for particle diameters between 50 and 200 nm.16 This is also the range of highest sensitivity of the SI detector. Therefore, the SID’s decline of sensitivity toward both bigger and smaller particles can be regarded as insignificant for further pyrolysis measurements. (30) Obernberger, I. B.; Friedrich; Widmann, W.; Riedl, R. Concentrations of inorganic elements in biomass fuels and recovery in the different ash fractions. Biomass Bioenergy 1997, 12 (3), 211-224. (31) Ja¨glid, U.; Olsson, J. G.; Pettersson, J. B. C. Detection of sodium and potassium salt particles using surface ionization at atmospheric pressure. J. Aerosol Sci. 1996, 27 (6), 967-977. (32) Jimenez, S. B.; Javier, Particulate matter formation and emission in the combustion of different pulverized biomass fuels. Combust. Sci. Technol. 2006, 178, 655-683.
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Figure 3. Pyrolysis of three types of biomass: wood saw dust (A), sewage sludge (B), and cattle manure (C). The solid line (left axis) gives the background-corrected SID alkali signal (normalized to the initial mass), and the dashed line gives the mass loss of the sample in percent. Note that in images B and C, logarithmic scales were used to show all features.
Pyrolysis of Wood, Sewage Sludge, and Cattle Manure. Both aforementioned tests show that the SID is applicable to measure the alkali emissions during the pyrolysis of small biomass samples. The TGA mass signals and SID alkali currents, derived during the pyrolysis of wood, sewage sludge, and cattle manure, are shown in Figure 3. For a wood sawdust sample, image A in Figure 3, we observe alkali emission starting at around 300 °C. Simultaneously, the mass declines rapidly. This is the region where pyrolysis of the organic matrix of the wood is observed. The position of the pyrolysis peak in the alkali signal is in good agreement with the results by Davidsson et al.10 The large mass loss of nearly 60% is not due to the alkali loss but a result of the decomposition of the wood and development of pyrolysis gas. Many commercial pyrolysis processes work in this temperature range around 300 °C.33 The release of alkalis is believed to be a result of the volatilization of alkalis from organic salts. With further heating, at around 600 °C the alkali signal increases again. This is the start of the evaporation of inorganic salts, namely KCl.16 The nearly constant mass signal proves that no pyrolysis, i.e., no decomposition of the organic content to gaseous substances, occurs at this temperature. (33) Reed, T. B.; Gaur, S. A SurVey of Biomass Gasification 2000; National Renewable Energy Laboratory and Biomass Energy Foundation: 1999.
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Image B in Figure 3 shows the pyrolysis of the wet biomass sewage sludge. The biggest mass loss for sewage sludge occurs at around 100 °C, when the water content evaporates. A small alkali signal is observed there due to salts taken away by the rapid evaporation. As in wood, an alkali signal can be observed in the pyrolysis region at around 300 °C. However, the signal is smaller than that of wood, due to the comparatively small content of organic material (since most of the sample is water). Starting at around 600 °C, evaporation of the inorganic salt is observed. Image C in Figure 3 shows the alkali signals derived from the pyrolysis of cattle manure. The total mass loss due to evaporation of water is highest here (shortly after 100 °C), and a considerable amount of alkalis is torn away by the evaporating water, as the SID signal peak at 100 °C shows. In the pyrolysis region at around 300 °C, no peaks either in mass loss or in the SID signal are observed, but rather a constant small background of alkalis emission is visible. Evaporation of the inorganic salts is appearing already at around 500 °C. This is due to the very high peak signal which is (at the same temperature) 2 orders of magnitude higher than the one for wood: although the vapor pressure of inorganic salts is lower at lower temperatures (e.g., the one of KCl is 2 orders or magnitude lower at 500 °C than at 600 °C34), this effect is compensated by the far greater concentration of these salts in the manure. Therefore, the amounts evaporated even at lower temperatures are sufficiently high above the detection limit. For both wet biomass samples, cattle manure and sewage sludge, the mass loss is greatest for temperatures around 100 °C, where the water content rapidly evaporates. An increase in the SID signal was observed there, which is especially evident for the cattle manure. Apparently, with the evaporating water, alkali atoms are also transferred from the sample to the gas. A similar effect was observed by Olsson et al.35 for heating at atmospheric pressure, but not for heating under vacuum conditions, and was explained there by the loss of unspecified volatile alkali species in the vacuum. However, in our measurements, the simultaneous measurement of the true sample temperature, the sample mass, and the SID alkali signal allow the assignment of this alkali signal to the evaporation of the water of the sample. It seems likely that for the results of Olsson et al., the sample may have been vacuum-dried rapidly during the time elapsed between the introduction of the sample to the vacuum chamber and the start of the measurement. Thus, no evaporation of water occurred, and no alkali release was observed. Pyrolysis of Straw Coke and Beech Wood Coke. As an addition to the natural biomass samples discussed above, pyrolysis products were also analyzed by the same method. In Figure 4, the SID alkali signal and the TGA mass change are shown for two pyrolysis char coke samples. Image A shows results of Dynamotive Inc. coke, and image B shows the results of pyrolysis straw coke from the Bioliq process. In both samples, a considerable alkali release occurs only at temperatures of 400 °C and above. For the straw coke, a very high signal of alkali emission in the high-temperature region was observed. This is a result of the high alkali content of the raw material straw (up to 1% of (34) Talonen, T.; Eskelinen, J.; Syva¨jaa¨rvi, T.; Roine, A. HSC Calculation of Equilibrium Composition V. 5.1; Outokumpu Research Oy: Pori, Finland 2002. (35) Olsson, J. G.; Pettersson, J. B. C.; Padban, N.; Bjerle, I. Alkali metal emission from filter ash and fluidized bed material from PFB gasification of biomass. Energy Fuels 1998, 12 (3), 626-630.
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Figure 4. SID alkali signal (solid line) and mass change (dashed line) during the heating of Dynamotive beech wood pyrolysis coke (A) and a pilot scale pyrolysis straw coke sample (B).
mass36). The alkali signal at around 400 °C can be explained by the fact that not all organic material had been converted during the pyrolysis process. A decrease of 15% in the mass signal at the same temperature supports this hypothesis, as organic residues may be decomposed there. The Dynamotive beech wood coke sample shows alkali release only at around 580 °C. At high temperatures above 800 °C, the overall signal remains comparatively weak. This is because the raw material, wood, is known to have only a small alkali content, by a factor of around 10 smaller than the alkali concentration in straw.37 We also believe that in the commercially produced coke, the pyrolysis is nearly complete, thus preventing alkali release during the decomposition of organic substances in the low-temperature region. Summary. For all samples, we conclude that in cases where a high content of degradable organic substances is present (as in wood and sewage sludge), a considerable alkali release was observed in the low-temperature pyrolysis region around 300 °C. In biomass gasification, low-temperature pyrolysis is preferred to high-temperature processes because of an inherently better energy balance compared to high-temperature processes and less wear-and-tear problems of the equipment. However, even at moderate gasification temperatures of around 400 °C, alkalI release must be taken into account when alkali-sensitive equipment downstream of the gasification process has to be protected. Organic Salts. Each of the two salts, sodium thiosulfate pentahydrate and sodium acetate, and a mixture of both were heated in the TGA in the same manner as the biomass samples above. The results are found in Figure 5. For both salts, there is a clear alkali signal peak at the respective decomposition temperature of the salt, i.e., at (36) Fachagentur Nachwachsende Rohstoffe e. V. Energetische Nutzung Von Stroh, Ganzpflanzengetreide und weiterer halmgutartiger Biomasse; Band 17; Fachagentur Nachwachsende Rohstoffe e. V.: Tautenhein, 8./9. Mai 2001, 2001. (37) Obernberger, I.; Brunner, T.; Ba¨rnthaler, G. Chemical properties of solid biofuels-significance and impact. Stand. Solid Biofuels Eur. 2005, 30 (11), 973-982.
Figure 5. Heating of sodium acetate (A), sodium thiosulfate (B), and a 1-to-1 mixture of both (C). The solid line is the SID current signal normalized to the sample mass, and the dotted line is the mass change of the sample.
500 °C for the acetate, and at 475 °C for the thiosulfate. Both peaks have their maxima when the mass loss due to decomposition and evaporation is highest. The release of crystal water from the thiosulfate at around 130 °C does not result in a measurable release of sodium. In both cases, the onset of the peak due to KCl evaporation is at 550 °C. For a mix of both salts, the observation changes. Decomposition starts earlier, and so does the alkali release, which shows a first peak at 350 °C. Then, a plateau is observed, with a second peak at 415 °C. Afterward, the alkali signal declines (but not to the ground line) and the KCl peak onset begins. After the experiment, a black residue is seen in the crucible believed to be carbon. As the TGA mass signal indicates, the decomposition of the mixture accompanied by a mass loss of 25% starts at around 330 °C. This is the melting point of sodium acetate.38 At this temperature, the thiosulfate is already molten,37 and both salts react in the melt. The spectrum of the mixture in Figure 5C is in good qualitative agreement with the bimodal spectra observed for biomass in our measurements in Figure 3. Similar release patterns at even lower temperature have been reported by Olsson et al.9 We conclude that the decomposition of organic alkali salts must be considered as a source of alkalis in the gasification gas. Namely the salts of organic acids as the acetate should be subject of further investigations. (38) CRC, Handbook of Chemistry and Physics, 74 ed.; CRC Press, Inc.: Boca Raton, 1993-1994.
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Conclusions We have been monitoring the alkali release from various types of biomass using a thermogravimetric analyzer with a condensation interface and a surface ionization detector. This is the first study to investigate the thermochemical behavior of biomass samples using a TGA-SID combination. Generally, a bimodal release spectrum is observed when high organic contents are present in the sample. The bimodal spectrum has one peak of alkali emission between 300 °C and 400 °C, and a second peak at high temperatures above 600 °C. We attribute the latter to the evaporation of inorganic salts. We suggest that the low-temperature peak may be a result of the decomposition of organic alkali salts, and believe acetates to be a potential candidate. In samples with low organic content, alkali emission is restricted to the higher temperatures above 500 °C, where the inorganic salts are volatilized.
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Chemical reactions of an acetate and a thiosulfate yield alkali release at temperatures 100 K below the decomposition temperatures of the respective salts. Since acetates are created by damp storage conditions of biomass, appropriate storage under dry conditions may lower the alkali release during gasification at low temperatures. Further investigations are necessary to evaluate these results. Acknowledgment. We gratefully acknowledge the support of Mr. Albert Schuler in the TGA pyrolysis experiments. We thank Silke Weimer for assistance help with the operation of the nebulizer and the DMA. Financial support was obtained from the Gebert Ru¨f Foundation as GRS-058/00. The TGA-CI has been developed in a joint project together with Mettler-Toledo AG Switzerland. Investigations on the pyrolysis straw coke were partly supported within the frame of the EU project RENEW (project number 502705). EF070094Z
