Energy & Fuels 1997, 11, 779-784
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Alkali Metal Emission during Pyrolysis of Biomass John G. Olsson, Ulf Ja¨glid, and Jan B. C. Pettersson*,† Department of Chemistry, Physical Chemistry, Go¨ teborg University, S-412 96 Go¨ teborg, Sweden
Pia Hald‡ Department of Environmental Science and Technology, Risø National Laboratory, DK-4000 Roskilde, Denmark Received June 20, 1996X
The alkali metal release during pyrolysis of biomass is investigated with a surface ionization method. Wheat straw samples (20 mg) are pyrolyzed in a laboratory unit under N2 atmosphere, and two characteristic temperature intervals for alkali metal emission are identified. A small fraction of the alkali metal content is released in a low-temperature region (180-500 °C) and is attributed to a connection with the decomposition of the organic structure. The two most pronounced emission processes below 500 °C are well described by a first-order rate behavior, and the activation energies are found to be 156 ( 11 and 178 ( 8 kJ/mol. The major part of the alkali metal release takes place in the high-temperature region (>500 °C), and activation energies of alkali metal emission from the ash residues are found in the range 168-238 kJ/mol. A high chlorine content is found to enhance the alkali metal emission from the ash, while the alkali metal release in the low-temperature region cannot be correlated with the chlorine content.
Introduction Environmental concerns have led to a renewed interest in biomass (e.g., agricultural residues, forestry industry residues, short rotation plantations, and energy crops) as an energy source and in the development of new technologies for power generation. Two important advantages of replacing fossil fuels by biomass are that the biomass constitutes a renewable resource and that the net emission of CO2 is reduced. Biomass generally has a high content of volatile matter as well as high reactivity and is thus well suited for gasification processes. However, the heating value of biomass is lower than that of coal because of a higher moisture content and a lower carbon content.1 The efficiency of present biomass-fired plants is relatively low,2 and improved power efficiencies are required to make biomass more economically competitive. A concept that has attracted a great deal of attention is the integrated gasifier-combined cycle (IGCC), where the combination of a steam cycle and a gasification system may lead to a high overall efficiency. Unfortunately, the lifetime of gas turbines used in IGCC systems is limited because of high-temperature corrosion caused by the impaction and condensation of impurities in the product gas.3 The oxidation of the alloy is accelerated by formation of low* Author for correspondence. Fax: +46 31 772 3107. E-mail:
[email protected]. † Also at the School of Environmental Sciences, Go ¨ teborg University. ‡ Present address: Forbrugerstyrelsen, Amagerfælledvej 56, DK2300 København, Denmark. X Abstract published in Advance ACS Abstracts, June 1, 1997. (1) Hallgren, A. Theoretical and Engineering Aspects of the Gasification of Biomass. Doctoral Thesis, Department of Chemical Engineering II, Lund University, Lund, Sweden, 1996. (2) Fjellerup, J.; Gjernes, E.; Hansen, L. K. Energy Fuels 1996, 10, 649-651. (3) Stringer, J. Annu. Rev. Mater. Sci. 1977, 7, 477-509.
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melting eutectics, of which alkali metal sulfates and chlorides are believed to be important constituents. The maximum tolerable alkali metal concentration has been specified as approximately 24 ppb (wt),4 and efficient hot gas cleaning methods are required if the product gas from biomass gasification is to comply with this regulation.5-7 The alkali metal concentration may be suppressed either by gas cooling or by the use of suitable sorbent materials. However, if the sulfur content of the fuel is very low (∼0.01%), as is often the case in biomass, the high-temperature corrosion resistance of the turbine blades may be sufficient even though the concentration of alkali metals should exceed the specified level.8 The presence of alkali metals in combustion and gasification may also cause other problems, having a profoundly negative effect on the overall efficiency. The function of fluidized beds may be deteriorated by agglomeration of the bed material particles due to lowmelting eutectic alkali salt mixtures.1 Condensation of alkali metal compounds on heat-exchanging surfaces may require costly and unscheduled plant shutdowns for the removal of deposits.9 The problems related to alkali metals often become more serious when biomass is used.10,11 In coal, part of the alkali metal content is (4) Rubow, L.; Zaharchuk, R. Proceedings of Second Annual Contractors’ Meeting on Contaminant Control in Hot Coal-Derived Gas Streams; U.S. Department of Energy: Morgantown, WV, 1994. (5) Mojtahedi, W. Release of alkali metals in pressurized fluidizedbed combustion and gasification of peat. Publication 53; Technical Research Centre of Finland: Espoo, Finland, 1989. (6) Kurkela, E.; Ståhlberg, P.; Laatikainen, J.; Nieminen, M. Removal of particulates and alkali metals from the product gas of a pressurized fluidized-bed gasifier. International Filtration & Separation Conference FILTECH 1991, Karlsruhe, Germany, October 1517, 1991. (7) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (8) Agblevor, F. A.; Besler, S. Energy Fuels 1996, 10, 293-298. (9) Turnbull, J. H. Biomass Bioenergy 1993, 4, 75-84.
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dispersed in the mineral phase, limiting the vaporization of alkali material.12,13 In biomass, however, over 90% of the alkali metals (i.e., potassium) are found in water soluble or ion exchangeable forms and are available for release during combustion.14 The alkali metal content of biomass varies depending on agricultural factors, i.e., the type of plant, the soil properties, the extent of fertilization, and the amount of rain to which the plant has been exposed.11 Previous studies of biomass combustion and gasification with an on-line alkali metal measurement method have been performed by Dayton et al.,7,15,16 where the release of alkali metals from biomass was studied with molecular beam sampling/mass spectrometry. This technique utilizes electron-impact ionization, which enables identification of the released sodium and potassium compounds. In the present work, the temperature dependence of alkali metal release from some different types of wheat straws is determined using relatively slow heating rates (1-250 °C/min). The emission of alkali metals is measured continuously by surface ionization during pyrolysis of a ground fuel sample in the temperature range 180-960 °C. A small fraction of the alkali metal content is found to be released in the low-temperature region of 180-500 °C. At higher temperatures (>500 °C), the release of alkali metals from the ash is governed by first-order kinetics. The dependence of the heating rate is studied and activation energies are obtained for the low-temperature processes. The paper is organized as follows. The detection technique is presented shortly, after which the experimental setup and the course of the analyses are described. The results from pyrolysis of biomass samples and the effect of different heating rates are then given followed by a discussion of the results. Experimental Section Principle for the Detection Method. The on-line detection of alkali metals is performed with the surface ionization (SI) method, which is well described in the literature.17-21 The basis of this technique is that alkali metals are effectively ionized on a hot metal (e.g., platinum) surface, and the emitted ions may be collected and measured. In the experimental (10) Jensen, P. A.; Stenholm, M.; Hald, P.; Christensen, K. A. Deposition investigation in straw fired boilers. American Chemical Society Meeting, Chicago, Illinois, 1995. (11) Hald, P. Alkali Metals at Combustion and Gasification Equilibrium Calculations and Gas-Phase Measurings. Doctoral Thesis, Department of Chemical Engineering, Technical University of Denmark, Lyngby, Denmark, 1994. (12) Raask, E. Prog. Energy Combust. Sci. 1985, 11, 97-118. (13) Raask, E. Mineral impurities in coal combustion, behaviour, problems and remedial measures; Springer-Verlag: Berlin, 1985. (14) Miles, T. R. Alkali Deposits found in Biomass Power Plants. NREL/TP-433-8142; National Renewable Energy Laboratory: Golden, CO, 1995; Vol. 1. (15) Dayton, D. C.; French, R. J.; Milne, T. A. The Direct Observation of Alkali Vapor Species in Biomass Combustion and Gasification. NREL/TP-430-5597; National Renewable Energy Laboratory: Golden, CO, 1994. (16) Dayton, D. C.; Frederick, W. J., Jr. Energy Fuels 1996, 10, 284292. (17) Kingdon, K. H.; Langmuir, I. Phys Rev. 1923, 21, 380-384. (18) Zandberg, E. Ya.; Ionov, N. I. Surface Ionization; Israel Program for Scientific Translations: Jerusalem, 1971 (translated from Russian). (19) Ionov, N. I. Progress in surface ionization: Surface ionization and its applications; Pergamon Press: Oxford, 1972; Vol. 1, pp 237354. (20) Olsson, J. G.; Pettersson, J. B. C. J. Aerosol Sci., submitted. (21) Ja¨glid, U.; Olsson, J. G.; Pettersson, J. B. C. J. Aerosol Sci. 1996, 27, 967-977.
Olsson et al. setup, gaseous compounds emitted from a pyrolyzed sample are transported by carrier gas to a hot Pt filament where a fraction of the particles and molecules melt and dissociate. Alkali metals are ionized, and the ions emitted from the filament are subsequently measured at a closely situated collector. The process of SI is described by the SahaLangmuir equation:18
R)
[ ] [
]
n+ g+ 1 - r+ e(φ - IP) ) exp n0 g0 1 - r0 kBT
(1)
In eq 1, R denotes the ratio of ions and neutrals leaving the surface, g+/g0 is the statistical sum ratio of ions and neutrals (g+/g0 ) 1/2 for alkali metals), r+ and r0 are the reflection coefficients of ions and neutrals, and e, φ, IP, kB, and T denote the elementary charge, the work function of the metal surface, the ionization potential, the Boltzmann constant, and the absolute temperature, respectively. If the work function is larger than the ionization potential of the adsorbed species, R is large and the greater part of the emitted species will be ions. The probability that an adsorbed species will desorb from the surface in ionic form is called the ionization probability β, defined as
β ) (1 + 1/R)-1
(2)
For most elements, R , 1 and desorption of neutrals is strongly favored, since β is close to zero. However, the ionization potentials of alkali metals are extremely low. Although the work function of Pt is 531 kJ/mol,22 the ionization potentials of Na and K are 496 and 419 kJ/mol, respectively. For these elements, R . 1 and β is close to unity. Ionic desorption is thus strongly favored. All other elements present in significant amounts in biomass have ionization potentials substantially higher than the work function of Pt, and for these species β becomes negligibly low. Therefore, the SI technique offers a way of selectively detecting alkali metals. We have previously performed SI studies of alkali salt aerosol particles with diameters in the range 1-100 nm.20,21 The obtained signal is proportional to the particle size for diameters below 5 nm.21 Tests on various sodium and potassium salt (chloride, hydroxide, nitrate, carbonate, and sulfate) particles with the same alkali metal content show that the variation in acquired signal is less than 5% for these sizes. For larger particles, however, substantial differences in detection efficiency are observed for the various salts, which are probably due to differences in the melting and dissociation processes of the large particles on the filament surface. The SI technique is well suited for in situ alkali metal measurements in combustion and gasification systems. Condensation methods using extractive sampling provide only time-averaged alkali concentrations, whereas SI is an on-line method for monitoring swift changes in the alkali metal concentration. By using a voltage modulation technique that rapidly changes the accelerating electric field outside the filament, we have found that the time response of the SI technique is on the order of milliseconds.23 The detection range covers 6 orders of magnitude with a lower detection limit below 1 ppb (wt).20 The major drawbacks are that an SI instrument must be calibrated and that it is not possible to identify the molecular form (i.e., anions) of the detected alkali metal compounds. In the present state, the total alkali metal concentration is measured without differentiation of Na and K compounds. Experimental Setup. The analyses were carried out in the experimental setup shown in Figure 1. A cylindrically shaped Al2O3 crucible (height of 14 mm, inner diameter of 7 mm) was placed in a sample heater (heating element: 1 mm Kanthal wire). The crucible holding the sample was resting (22) Holmlid, L.; Mo¨ller, K. Surf. Sci. 1985, 149, 609-620. (23) Hagstro¨m, M.; Engvall, K.; Pettersson, J. B. C. To be submitted.
Alkali Metal Emission
Figure 1. Experimental setup for monitoring alkali metal emission during biomass pyrolysis: (1) carrier gas outlet; (2) ion collector; (3) Pt filament; (4) sample heater with crucible; (5) thermocouple; (6) carrier gas inlet. The inset picture shows a schematic view of the filament from beneath. upon the thermocouple (Pt-10%Rh/Pt), ensuring direct contact between the crucible and thermocouple. Carrier N2 gas (100 °C/min) and the slowest (500 °C), we have estimated the activation energies for alkali metal release from the ash residues. The matter is complicated by the fact that the emission curves are incomplete, i.e., the rate does not go back to zero. Arrhenius plots of the data in the range 700-900 °C were used to determine activation energies in the range 168-238 kJ/ mol. These values are listed in Table 1. The most abundant potassium species released in the hightemperature region, or the “char combustion phase”, is reported to be KCl in the case of combustion at 800 °C.7 Although the mechanism of KCl emission from the ash may be different compared to sublimation of the crystalline salt, the activation energies in Table 1 are in fairly good agreement with the sublimation energy of 211 kJ/ mol for KCl.30 The correlation between the presence of Cl and alkali metal release may be illustrated by comparing the Cl content of the three samples, Type 100, APAS, and Type 77 (Table 1), with the signal levels in Figure 3b. The samples contain similar amounts of K, but their Cl contents are 0.1%, 0.2%, and 0.3%, respectively. The signal levels for the three samples rise in this order at high temperatures (Figure 3b), which indicates that the presence of Cl enhances the alkali metal emission from the ash residue.
(4)
where m is the heating rate and Tm is the signal peak temperature. The obtained values of Tm for the different peaks are listed in Table 2. The values of A and E were obtained for the second and third signal peaks using eq 4, and a least sum-of-squares fit is shown in Figure 6. We conclude that these alkali metal emission processes are well described by a first-order rate behavior. For the second peak, E ) 156 ( 11 kJ/mol and A ) 1012.9(1.6 s-1, while for the third peak, E ) 178 ( 8 kJ/mol and A ) 1013.3(0.9 s-1. It is beyond our present scope to fully explain the processes responsible for the signal peaks in the lowtemperature region. However, the obtained results may be compared to kinetic parameters of cellulose decomposition in small (1 mg) wheat straw samples given in a review by Antal and Varhegyi,29 where activation energies were reported in the range 187-219 kJ/mol. The activation energy obtained for the second peak is fairly low, and values in this range are generally considered to be underestimations due to heat transfer limitations in large samples.29 The activation energy obtained for the third peak is also lower than the values
Conclusions A novel analysis technique based on surface ionization has been applied in studies of alkali metal emission during pyrolysis of biomass samples. A small but significant fraction of the alkali metal content has been found to be released below 500 °C, and the emission from the ash commences around 500 °C. The alkalireleasing processes are well described by first-order kinetics. A high chlorine content has been found to enhance the alkali metal emission from the ash above 500 °C, whereas the alkali metal release below 500 °C cannot be correlated with the content of chlorine. Acknowledgment. We express our gratitude to Benny Lo¨nn for the design and construction of the experimental apparatus and to Kent Davidsson for helpful discussions and assistance during the experimental work. Jens Jørgen Nielsen, Biotechnological Institute, and Bo Sander, ELSAM are thanked for supplying the Type 77 and Type 100 samples. Lars K. Hansen and Aksel Olsen at Risø National Laboratory are thanked for supplying the APAS samples. EF960096B
(28) Kissinger, H. E. J. Res. Natl. Bur. Stand. 1956, 57, 217-221. (29) Antal, M. J.; Varhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703717.
(30) Milne, T. A.; Klein, H. M. J. Chem. Phys. 1960, 33, 1628-1637.
