Energy & Fuels 2002, 16, 1033-1039
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Alkali Emission from Birchwood Particles during Rapid Pyrolysis K. O. Davidsson, B. J. Stojkova, and J. B. C. Pettersson* Department of Chemistry, Physical Chemistry, Go¨ teborg University, SE-412 96 Go¨ teborg, Sweden Received October 29, 2001. Revised Manuscript Received April 30, 2002
The emission of potassium- and sodium-containing compounds during rapid birchwood pyrolysis was studied. Birchwood particles (2-130 mg) were inserted into a preheated furnace at constant temperature (350-850 °C) and the alkali emission was measured. Particle mass, furnace temperature, and moisture were varied. At temperatures e 500 °C, the alkali emission from birchwood particles took place solely during the pyrolysis phase. At temperatures g 600 °C, alkali evaporation from the ash increased. The total alkali release increased with temperature in the range studied and the release during the pyrolysis was larger or equal to the release from the ash phase. Small particles (2-10 mg) emitted more alkali per unit mass than large ones (60130 mg) and this tendency increased with temperature. At 800 °C the emission per unit mass from small particles was 10 times the one for large particles. Wet particles went through a drying phase, which delays the heating, and thereby the alkali emission. The present findings are of importance for actions aimed at minimizing alkali related problems during large-scale biomass conversion.
Introduction Biofuels are becoming increasingly important for large-scale energy production due to the fact that they are renewable. The combustion of biofuels, of course, produces CO2 but since biofuels grow by absorbing CO2 from the atmosphere their utilization during energy production leads to low net emission of CO2 compared with fossil fuels. Unfortunately the possibility to replace, e.g., coal with biofuels is limited, partly because biofuels generally contain more alkali. The term alkali will henceforth denote alkali metal compounds. Alkali, present in combustion facilities cause severe problems such as agglomeration of fluidized bed material, deposition on heater surfaces, and corrosion.1 Agglomeration is caused by condensation of alkali on particles and the forming of a liquid layer, which makes the particles more easily stick together. If large enough aggregates are formed, defluidization eventually occurs. Deposition of alkali may take place on metal surfaces such as heat exchanger tubes. Such depositions may be formed very quickly and deteriorate the heat transfer and enhance corrosion. Both agglomeration and depositions lower the efficiency and may lead to costly shutdowns and repairs. Biofuels have a high content of volatile matter, which makes them suitable for gasification processes where volatiles are combusted in a gas turbine. However, gas turbines are very sensitive to alkali, since they are involved in corrosion of the turbine blades. Manufacturers specify maximum tolerable alkali levels in the range * Corresponding author. Tel.: +46(0)31 772 2828. Fax: +46(0)31 772 3107. E-mail:
[email protected] (1) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47-78.
70-600 ppb 2 and this calls for advanced filtering of the gas. Biofuels constitute a heterogeneous group of fuels and the alkali content varies widely. Wood generally contains less alkali than straw; e.g., cereal straw and wood chips have been found to have potassium levels in the range 0.2-1.9 and 0.05-0.4% (dry mass), respectively.3 To lower the alkali content of the fuel and thereby the alkali emission, different leaching techniques have been studied. About 35% of the alkali in wood waste was found to be water-soluble,4 while about 90% of the alkali in straw is water-soluble.4-6 Another strategy is to hinder evaporation of alkali by increasing the ash melting temperature. Laboratory tests on straw fuels show that kaolin added to the fuel has this effect and also reacts with potassium to form more stable compounds.7 The alkali release depends not only on alkali concentration but also on other ions present. Cl has been found to enhance alkali release from straw during pyrolysis.8-10 In a recent study, chloride was replaced by sulfate in the fertilizer of wheat and oat (2) Romey, I. F. W., Garnisch, J., Bemtgen, J. M., Eds. JouleThermie Programme, Diagnostics of Alkali and Heavy Metal Release, European Commission, Clean Technologies for Solid Fuels (1996-1998) 1998, EUR 18291 EN. (3) Sander, B. Biomass Bioenergy 1997, 12, 177-183. (4) Davidsson, K. O.; Korsgren, J. G.; Pettersson, J. B. C.; Ja¨glid, U. Fuel 2001, 81, 137-142. (5) Jenkins, B. M.; Bakker, R. R.; Wei, J. B. Biomass Bioenergy 1996, 10, 177-200. (6) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Energy Fuels 1999, 13, 860-870. (7) Steenari, B.-M.; Lindqvist, O. Biomass Bioenergy 1998, 14, 6776. (8) French, R. J.; Milne, T. A. Biomass Bioenergy 1994, 7, 315-325. (9) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (10) Olsson, J. G.; Ja¨glid, U.; Hald, P.; Pettersson, J. B. C. Energy Fuels 1997, 11, 779-784.
10.1021/ef010257y CCC: $22.00 © 2002 American Chemical Society Published on Web 07/03/2002
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Energy & Fuels, Vol. 16, No. 5, 2002
Davidsson et al.
Figure 1. The single particle pyrolysis reactor (not entirely to scale): (1) surface ionization detector, (2) heater, (3) sample plate, (4) mass spectrometer capillary, (5) Ar flow, (6) sample insertion tube.
straw and this was found to lower the alkali release during pyrolysis.11 Wood contains less Cl than straw6 but the effect of the Cl should be the same. Until now, the interest has been focused on straw fuels since they can be expected to induce more severe problems. Wood fuels are, however, more abundant in Sweden and many other areas. The problems associated with large-scale wood combustion are well-known. These problems are often dealt with by co-firing with coal, which of course contributes to the CO2-emission. Alkali is released from straw during the devolatilization9,10 and we may therefore suspect that alkali bonded to the organic structure is released as the organic structure is thermally degraded. Wood contains less ash than straw but, apart from that, their compositions are similar. The main difference between straw and wood, with respect to fuel conversion, should be found in physical properties, e.g., particle size, heat transfer coefficients, and porosity. Therefore it is of interest to describe the alkali emission during pyrolysis of wood, in connection with the devolatilization processes of the organic structure. The surface ionization (SI) technique has proven to be very suitable for alkali measurements. The alkali emission from wheat straw has been studied, using the SI-technique10 and the effect of washing on alkali release has been investigated for wood waste and wheat straw.4 We have also constructed a single particle reactor to study wood pyrolysis kinetics.12 In the present work the reactor and the surface ionization technique are combined. The purpose of the present study is to describe the alkali emission from wood. To resemble the process where a wood chip is fed into a large-scale reactor we use rapid insertion of a single particle into a preheated furnace. The temperature of the furnace, the size of the particle and the water content of the particle are varied. The results are compared with alkali emission during constant heating rate. Experimental Section The experimental setup is shown in Figure 1. It consists of a furnace, a balance, an alkali detector, and a mass spectrometer (MS). The furnace consists of a sample plate, an insertion tube, and entrances for the MS-capillary and the alkali detector. Five u-shaped heaters provide heating and a ther(11) Davidsson, K. O.; Pettersson, J. B. C.; Nilsson, R. Fuel 2001, 81, 259-262. (12) Davidsson, K. O.; Pettersson, J. B. C.; Bellais, M.; Liliedahl, T.; Sjo¨stro¨m, K. In Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science Ltd.: Oxford, 2001; Vol. 2, pp 1129-1142.
Figure 2. The surface ionization detector (not entirely to scale): (a) side view, (b) frontal view. (1) Pt-filament, (2) ion collector, (3) current feeder, (4) contours of a wood particle, (5) sample plate, (6) pin connected to the balance. mocouple situated near the sample plate measures the temperature of the furnace. Through the furnace is a flow of Ar of 1 L min-1. The alkali detector is based on surface ionization and its principle is described elsewhere.10 Figure 2 gives a detailed picture of the detector, which consists of a hot (1500 K) Pt filament, biased at 400 V, and a grounded collector. Alkali (K + Na)-containing compounds that hit the filament dissociate on the hot Pt surface. Due to their uniquely low ionization energy alkali metal atoms are ionized when leaving the Pt surface. The positive ions are repelled by the filament toward the grounded collector where they give rise to a current proportional to the flow of alkali atoms onto the filament. During wood pyrolysis, gaseous hydrocarbons are emitted. To avoid a buildup of a carbon layer on the Pt surface the O level was kept at 2-4% during the experiments. This O2 level was sufficiently low to avoid detectable char oxidation. The MS (Balzers QMG 421C) is connected to the furnace by a capillary at 200 °C. The retention time for CO2 in the capillary is 1-2 s. The samples consisted of wood from birch (Betula verrucosa) and can be divided into two mass ranges. Large particles were cubical (5 mm) with a mass in the range 60-130 mg and small particles had a less well-defined shape and a mass in the range 2-10 mg. The birchwood was analyzed with respect to alkali, and the concentrations were found to be 0.079 wt % K and
