A Surface Ionization Instrument for On-Line Measurements of Alkali

The instrument, its support system and the calibration procedure is described. ... Mohit PushpDan GallKent DavidssonMartin SeemannJan B. C. Pettersson...
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Energy & Fuels 2002, 16, 1369-1377

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A Surface Ionization Instrument for On-Line Measurements of Alkali Metal Components in Combustion: Instrument Description and Applications Kent O. Davidsson, Klas Engvall,† Magnus Hagstro¨m, John G. Korsgren,‡ Benny Lo¨nn, and Jan B. C. Pettersson* Department of Chemistry, Physical Chemistry, Go¨ teborg University, SE-412 96 Go¨ teborg, Sweden Received February 13, 2002. Revised Manuscript Received July 2, 2002

An instrument for on-line measurements of alkali components in hot flue gas streams is presented. The instrument is based on surface ionization technique and is equipped with a hot sampling line used to extract flue gas for continuous alkali measurements at pressures up to 30 bar. The instrument, its support system and the calibration procedure is described. Field campaigns with the instrument have been performed at fluidized-bed combustion facilities operating under pressurized and atmospheric conditions and using several fuels, including coal, biomass, and demolition waste. The instrument performance has been satisfactory both during pressurized circulating fluidized-bed coal combustion with alkali concentrations downstream of a hot gas filter in the ppb range, and in particle-laden conditions with alkali levels up to about 10 ppm. The measured alkali concentration corresponds to the concentration of alkali components present as vapors and fine-mode particles. In systems with high levels of fluidized-bed material or fly ash, a contribution to the signal from alkali bound to coarse particles is also expected. Under these conditions, the instrument is suggested to be operated in a pulse counting mode where the concentration of coarse particles can be estimated. The instrument is concluded to provide a durable, sensitive, and reliable alkali measurement method with a high time resolution and with a lower detection limit of around 1 ppb, which should be suitable for a large range of applications.

1. Introduction The improvement of solid fuel conversion systems is a vital issue driven by environmental concerns and economy. Some of the main obstacles to overcome are maintenance problems caused by inorganic constituents in the fuel. During the thermal conversion of solid fuels, alkali compounds often have a significant impact on the process conditions, and operational problems such as fouling and sintering in fluidized-bed boilers and hightemperature corrosion may arise. In coal-based power generation, the focus of attention with respect to alkali components is generally directed toward their role in high-temperature corrosion.1 Alkali metals are key elements in the build-up of corrosive deposits, which may have detrimental effects on sensitive equipment in advanced combustion systems.2 Even if only a small fraction of alkali-containing fine particles and alkali vapors penetrates a hot gas filtration unit, corrosive deposits may form on gas turbine blades.3 The fuel gas * Corresponding author. Phone: +46 31 772 2828. Fax: +46 31 772 3107. E-mail: [email protected]. † Present address: Siemens-Elema AB, SE-171 95 Solna, Sweden. ‡ Present address: Volvo Car Corporation, SE-405 31 Go ¨ teborg, Sweden. (1) Newby, R. A.; Bannister, R. L. J. Eng. Gas Turbines Power 1994, 116, 338-344. (2) Stringer, J. Annu. Rev. Mater. Sci. 1977, 7, 477-509. (3) Kofstad, P. High-Temperature Corrosion; Elsevier Applied Science: London, 1988.

regulations given by gas turbine manufacturers are therefore very strict with respect to the alkali concentration, with maximum allowed values in the range 0.1-1 ppm(wt) or below.4 During the combustion of renewable fuels the impact of alkali-related operational problems is often more serious compared to coal combustion, and these problems often constitute a limiting factor for the usefulness of the renewable fuels in advanced power production facilities.5 The major part of the alkali content in biofuels is susceptible to volatilization, while part of the alkali components in coal are retained in the ash phase upon heating.6 In addition, the alkali content of renewable fuels is often significantly higher. The presence of alkali in the ash is associated with a lowering of the ash melting temperature, which may result in molten ash phases and sintering problems in fluidized-bed boilers.7 The formation of ash deposits on heat-exchanging surfaces also leads to reduced heat transfer efficiency and may ultimately lead to costly unscheduled (4) Diagnostics of alkali and heavy metal release; Clean technologies for solid fuels (1996-1998); Romey, I. F. W., Garnish, J., Bemtgen, J. M., Eds.; Joule-Thermie Programme, European Commission, 1998. (5) 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. (6) Raask, E. Prog. Energy Combust. Sci. 1985, 11, 97-118. (7) Skrifvars, B.-J.; Backman, R.; Hupa, M. Ash chemistry and behaviour in fluidised bed combustion - an overview. In LIEKKI 2 Technical Review 1993-1998; Hupa, M., Matinlinna, J., Eds.; 1998.

10.1021/ef020020h CCC: $22.00 © 2002 American Chemical Society Published on Web 08/24/2002

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plant shutdowns for deposit removal, or alternatively tube replacement in the case of excessive corrosion.8 During waste incineration, the corrosion problems are mainly due to high levels of chlorine, but alkali may take part in these processes since chlorine is known to function as an alkali metal carrier in the form of NaCl and KCl.5 The direct measurement of alkali metals and their associated compounds in hot flue gas streams is of great interest for the protection of plant equipment and for the further development of process technology. In the conventional type of alkali concentration measurements, a gas sample is extracted by a condensing probe. After sampling, the probe is rinsed and the obtained solution is analyzed by standard laboratory methods, e.g., atomic absorption spectrometry.4 The disadvantages of this method include the relatively long sampling times and the subsequent laboratory work. Furthermore, since the results are averaged over a certain period of time any variations and transient effects are excluded. Methods for on-line alkali measurements have been developed by different research groups. The excimer laser-induced fluorescence (ELIF) method is based on fragmentation of alkali compounds by a laser beam with simultaneous excitation of the free alkali atoms.9-11 The concentration is subsequently determined by fluorescence measurements. The laser power applied is essentially sensitive only to vapor-phase alkali compounds even though higher power may be suitable for detection of alkali-containing particles.12 A plasma method that has proven reliable for alkali detection is plasma-excited alkali resonance line spectroscopy (PEARLS). It is based on the thermal excitation of alkali atoms in a hot plasma.13 Alkali compounds are decomposed in the plasma, and the concentration of either K or Na compounds is measured by optical absorption or emission spectroscopy. The PEARLS method is used for the detection of alkali compounds both in the vapor phase and bound to aerosol particles. Molecular-beam mass spectrometry (MBMS) has also been applied for on-line alkali vapor measurements.14-16 In this method flue gas is sampled directly into vacuum and the formed molecular beam is analyzed by mass spectrometry, giving a lower detection limit of about 0.1 ppm. (8) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Alkali Deposits found in Biomass Power Plants - A Preliminary Investigation of Their Extent and Nature; Summary report for the National Renewable Energy Laboratory, NREL Subcontract TZ-2-11226-1, Golden, CO, April 1995. (9) Gottwald, U. A.; Monkhouse, P. B. Appl. Phys. B-Lasers O. 1999, 69, 151-154. (10) Hartinger, K. T.; Monkhouse, P. B.; Wolfrum, J.; Baumann, H.; Bonn, B. Determination of flue gas alkali concentrations in fluidizedbed combustion by excimer-laser-induced fragmentation fluorescence. Proceedings of the 25th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; pp 193-199. (11) Helble, J. J.; Srinivasachar, S.; Boni, A. A.; Charon, O.; Modestino, A. Combust. Sci. Technol. 1992, 81, 193-205. (12) Gottwald, U.; Monkhouse, P.; Wulgaris, N.; Bonn, B. Fuel Process. Technol. 2002, 75, 215-226. (13) Ha¨yrinen, V. T.; Hernberg, R. G.; Oikari, R. Plasma assisted spectroscopic monitoring of alkali metals in pressurised combustion and gasification. Proceedings of the 12th International Symposium on Plasma Chemistry, Minneapolis, MN, 1995; pp 705-710. (14) 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, Golden, CO, January 1994. (15) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (16) Dayton, D. C.; Frederick, W. J., Jr. Energy Fuels 1996, 10, 284292.

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In the present paper, an instrument based on surface ionization (SI) technique for on-line measurements of alkali components is presented. The instrument and the calibration procedure are described, and the instrument performance during field measurement campaigns is demonstrated. Surface ionization results in the thermal evaporation of positive alkali ions from hot metal surfaces,17 and the process allows for selective and highly sensitive detection of alkali compounds. The method has previously been used as a measurement technique in fundamental studies within the reaction dynamics research field.18 In the early 1980’s, an alkali measurement device based on SI for hot gas streams was invented by Zarchy.19,20 Related instruments based on SI techniques have also been developed for the chemical analysis of individual aerosol particles.21-25 To our knowledge, the instrument described in the present paper is the only currently developed and operated SI device for alkali measurements in hot gas streams. Preliminary results obtained with the SI instrument have been presented elsewhere,26 including a comparison with results obtained with the ELIF and PEARLS techniques. The paper is organized in the following way. The SI method is described in Section 2 and the developed SI instrument and the calibration procedure are presented in Section 3. Measurement results during pressurized circulating fluidized-bed (PCFB) combustion and atmospheric fluidized-bed (AFB) combustion are presented in Section 4, and the main conclusions are summarized in Section 5. 2. Surface Ionization Technique For the detection of alkali compounds, the SI instrument uses a hot platinum surface for the selective ionization of alkali metals in the gas stream. The theory of surface ionization has been thoroughly described in the literature,17,27,28 and will be briefly summarized here. An atom or molecule which becomes adsorbed at a hot surface may upon its desorption be emitted in (17) Zandberg, E Ä . Ya. Technol. Phys. 1995, 40, 865-884. (18) Atomic and Molecular Beam Methods; Scoles, G., Ed.; Oxford University Press: Oxford, 1988; Vol. 1. (19) Zarchy, A. S. A surface ionization detector for alkali metal level measurements in gas streams. The 1980 Symposium on Instrumentation and Control for Fossil Energy Processes, Virginia Beach, VA, 1980. (20) Zarchy, A. S. An alkali metal detector for use in fluidized-bed combustor effluent streams. The 5th Annual Symposium on Instrumentation and Control for Fossil Energy Processes, San Francisco, CA, 1981. (21) Myers, R. L.; Fite, W. L. Environ. Sci. Technol. 1975, 9, 334336. (22) Davis, W. D. Environ. Sci. Technol. 1977, 11, 587-592. (23) Stoffels, J. J. Int. J. Mass Spectrom. Ion Phys. 1981, 40, 217234. (24) Stoffels, J. J.; Lagergren, C. R. Int. J. Mass Spectrom. Ion Phys. 1981, 40, 243-254. (25) Sinha, M. P.; Friedlander, S. K. Anal. Chem. 1985, 57, 18801883. (26) Ha¨yrinen, V.; Hernberg, R.; Oikari, R.; Gottwald, U. A.; Monkhouse, P. B.; Davidsson, K. O.; Lo¨nn, B.; Engvall, K.; Pettersson, J. B. C.; Lehtonen, P.; Kuivalainen, R. On-line determination of alkali concentrations in pressurized fluidised-bed coal combustion by combined measurement techniques. 6th Conference on Circulating Fluidized Bed Combustion; Wu¨rzburg, Germany, September 1999; DECHEMA: Frankfurt am Mein, 1999. (27) Zandberg, E Ä . Ya.; Ionov, N. I. Surface Ionization (translated from Russian); Israel Program for Scientific Translations: Jerusalem, 1971. (28) Ionov, N. I. Surface ionization and its applications. In Progress in Surface Science; Davison, S. G., Ed.; Pergamon Press: Oxford, 1972; Vol. 1, Part 3, pp 237-354.

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either ionic or neutral form. The statistical probability of the ionic and neutral fluxes from the surface is predicted by

R)

[

]

n+ g+ e(φ - IP) ) exp , n0 g 0 k BT

(1)

where R, or n+/n0, denotes the ratio of ionic and neutral fluxes from the surface, g+/g0 is the statistical sum ratio of ions and neutrals (g+/g0 ) 1/2 for alkali metals), and e, φ, IP, kB and T denote elementary charge, surface work function, ionization potential, Boltzmann’s constant, and absolute temperature, respectively. The ionization probability β, i.e. the probability that an adsorbed species will desorb in ionic form, is given by

β ) R/(R + 1)

(2)

For most elements that become adsorbed on a metal surface, IP is larger than the surface work function. The probability β is then close to zero (since R , 1), and the emission of neutrals from the surface is strongly favored over the emission of ions. The ionization potentials of the alkali metals are, however, unusually low and may be lower than φ. The probability β then approaches unity and the emission of ions dominates over the emission of neutrals. The current application developed for alkali metal detection is based on the desorption of Na+ and K+ ions when Na- and K-containing compounds strike a hot Pt surface. The ionization potentials of the alkali metals are IPNa ) 5.14 eV and IPK ) 4.34 eV, and φ = 5.5 eV for Pt.29,30 For Na and K on a Pt surface at T ) 1500 K, the estimated values of β are 0.89 and > 0.99, respectively. These inherent properties of the SI phenomenon open up possibilities for designing selective and very sensitive alkali detection instruments based on SI. An important property of an alkali measurement method is the response toward alkali vapors and particle-bound alkali. The surface ionization efficiencies of different alkali salt compounds on a Pt filament in air have previously been characterized by Ja¨glid et al.,31 and the detailed kinetics of alkali interactions with metal surfaces in air have also been investigated.32,33 Alkali salt molecules and ultra fine particles were concluded to decompose on the hot Pt surface, and the total alkali content of the particles was surface ionized according to eq 1. In the case of a larger alkali salt particle striking the hot metal surface, the surface ionization process becomes more complicated since the particle must first melt and decompose before alkali metals can undergo ionization at the hot surface. The previous work by Ja¨glid et al.31 showed that the surface ionization efficiency decreases with particle size above a particle diameter of 0.01 µm and also depends on the type of salt, indicating incomplete ionization of the alkali content of the particles. Detailed studies of individual alkali salt particles interacting with Pt in air have been (29) Holmlid, L.; Mo¨ller, K. Surf. Sci. 1985, 149, 609-620. (30) Eastman, D. E. Phys. Rev. B 1970, 2, 1. (31) Ja¨glid, U.; Olsson, J. G.; Pettersson, J. B. C. J. Aerosol Sci. 1996, 27, 967-977. (32) Hagstro¨m, M.; Engvall, K.; Pettersson, J. B. C. J. Phys. Chem. B 2000, 104, 4457-4462. (33) Hagstro¨m, M.; Ja¨glid, U.; Pettersson, J. B. C. Appl. Surf. Sci. 2000, 161, 291-299.

Figure 1. The SI instrument. The heated sampling line is used for extracting flue gas to the SI detector consisting of a Pt filament and an ion collector.

carried out by Hagstro¨m,34 and related work on particlesurface interactions under vacuum conditions has been performed by Korsgren and Pettersson.35 In the normal alkali measurement mode used in the applications presented in this study, we do not distinguish between alkali vapors and particles. However, the surface ionization technique can be employed for the counting of alkali-containing particles, and this application is discussed in Section 4.3. 3. Instrument Description and Calibration The developed SI instrument is illustrated in Figure 1. It consists of two separate parts; a heated sampling line for extraction of flue gas and a surface ionization detector. The sampling line shown on the left-hand side of Figure 1 is equipped with feedthroughs for thermocouples and two separate furnaces. The actual surface ionization detector, which consists of a Pt filament, an ion collector, and feedthroughs, is shown on the righthand side of Figure 1. The measurement should ideally take place in situ to avoid sampling errors and reactions of alkali compounds with the inner walls of the sampling line. However, considering the harsh and in many cases particle-laden hot gas stream environment, ex situ measurements are preferred to prolong the lifetime of the detector. Furthermore, the use of a sampling line gives a better control of the gas load toward the filament, enabling a larger dynamic concentration range to be covered in the measurements. The sampling line is used in order to extract a small product gas flow from the main gas stream to the detector. The inner tube (length ) 0.45 m, i.d. ) 6 mm) is made of hightemperature stainless steel (253MA, Avesta-Sheffield Inc.). The tube is surrounded by two separately thyristor-heated furnaces, enabling a homogeneous temperature profile to be maintained throughout the tube. For temperature measurements, three thermocouples (type K) are positioned along the outer surface of the inner tube. During operation, the sampling line is kept at temperatures similar to the flue gas temperature (>800 °C) to avoid condensation of alkali compounds within the inner tube. The sampling line is mounted upon a (34) Hagstro¨m, M. Dissertation thesis, Go¨teborg University, Sweden, 2000. (35) Korsgren, J. G.; Pettersson, J. B. C. J. Phys. Chem. B 1999, 103, 10425-10432.

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Figure 3. The SI instrument support system. The instrument is mounted onto the flue gas pipe. Abbreviations: FU-filter unit; MFC-mass flow controller; E-electrometer; and PCcomputer. Figure 2. A close-up view of the SI detector, as seen from the end of the sampling line. The Pt filament is spun into a spiral and bent around the ion collector (IC). The gas outlet (GO) and a thermocouple (TC) are positioned beside the filament. In addition to the two feedthroughs connected to the filament ends, a third feedthrough is connected to the center of the filament for additional support and stability. Purge gas is added from behind the bottom plate for protection of the sensitive ion collector and filament feedthroughs.

DN150/PN40 connection flange, and the instrument is certified for operating pressures up to 30 bar. The product gas flow rate through the sampling line is typically varied in the range 0.5-2.5 lpm, depending on the alkali concentration. Downstream of the detector, a flow of N2 purge gas (0.5 lpm) is applied during all measurements to protect sensitive detector parts. The surface ionization detector shown in Figure 2 is positioned inside a small cavity at the end of the sampling line. An ionizing filament is constructed from Pt wire (diameter 0.35 mm, purity 99.997%) which is spun into a tight spiral. The spiral-shaped filament is then bent almost one full turn around the ion collector electrode and mounted perpendicularly to the flow direction of the sampled product gas. The filament is resistively heated to 1500 K by a current of around 5 A. To maintain a constant filament temperature regardless of flow rate, flue gas temperature, and pressure, the resistance across the filament is kept constant. For calibration, the temperature is measured at the central part of the filament by optical pyrometry corrected for spectral emissivity. A positive potential, generally +400 V, is applied to the filament during all measurements. The alkali components that strike the filament are adsorbed, melted, and dissociated. Sodium and potassium are thermally evaporated as Na+ and K+ ions that are repelled by the positive potential of the filament and drift to the nearby ion collector. The instrument thus measures the sum of the alkali compounds in the gas stream led to the detector. The ion current is generally measured with a sensitive electrometer (Model 602 Electrometer, Keithley Instruments) connected to ground. In the normal measurement mode, one value of the signal produced by the electrometer is registered each second. The measured ion currents during operation are in the range 10-11-10-6 A, and the resistance across the

insulative feedthroughs of the ion collector must thus be maintained very high (R > 1012 Ω). In addition, the filament feedthroughs must be kept clean in order to avoid leak currents and maintain the high positive voltage. To protect these sensitive surfaces from contamination by condensable material, a number of precautions have been taken. As shown in Figure 2, the sampled flue gas exits through a gas outlet tube directly downstream of the filament. In addition, a N2 purge gas flow passes the detector opposite the sampled gas flow. A shield plate, through which the purge gas flows, covers the feedthroughs for protection. Due to the presence of alkali metals in the sampling line as well as the filament itself, a background signal is measured at all times. During measurements, the background level is estimated regularly by turning off the product gas flow through the sampling line. To obtain the actual signal, the background level is subtracted from the signal measured with the product gas flow turned on. It is not possible to measure the potential alkali contribution from the sampling line tube during field measurements in a pressurized system. This has to be done before or after the actual measurement, so that a clean gas flow can be applied through the sampling line for correct background level determination. During measurements in an atmospheric system this can be done at any time, since the instrument can easily be dismounted. The flow of sampled gas is adjusted so that the background constitutes less than 10 percent of the total signal. Fortunately, this contribution to the background signal varies only slightly with time when the sampling line is kept at high temperature. The supporting system for the SI instrument is schematically outlined in Figure 3. Box 2 is connected to the main power supply and provides the power for the sampling line furnaces. Box 1 is used for the acquisition of electrometer signal data and four temperature readings (three thermocouples in the sampling line and one thermocouple in the detector). In addition, Box 1 is used for controlling the filament current, the two thyristors for the sampling line furnaces and the mass flow controllers (Model El-flow, Bronkhorst HiTec) used for regulating the purge gas and product gas flows. The product gas is passed by a filter unit before reaching the mass flow controller. Two rotameters are

On-Line Measurements of Alkali Metals in Combustion

used to check the flow rates of the product gas and purge gas. The entire measurement process is controlled by an external computer. Both boxes are water-cooled for protection of the electronic equipment, and the entire apparatus can be remotely controlled through the PC if the measurement site provides a working environment that is too difficult for manual supervision. The ion current monitored in the detector is a measure of the concentration of alkali components in the product gas flow. For the transformation between these quantities, the SI instrument has been independently calibrated by two separate methods: (1) evaporation of a known amount of crystalline alkali salt, and (2) detection of an aerosol of monodisperse alkali salt particles. During the evaporation experiments, the purge gas and product gas flows were 0.5 lpm and 1.0 lpm, respectively. When a steady background level had been reached, a small amount (0.5-1.0 mg) of NaCl or KCl was placed in the sampling line maintained at 850°C. The sample was allowed to evaporate until the ion signal had again stabilized at the background level. The total number of detected alkali ions was determined from the observed ion current and compared to the total number of evaporated alkali atoms. At atmospheric pressure, the detection probability for alkali atoms entering the instrument was found to be (1.1 ( 0.3) × 10-3, and at 11 bar absolute pressure the detection probability was (4 ( 1) × 10-4. The results for NaCl and KCl were similar and within the relatively large error limits. The second type of calibration was performed using a scanning mobility particle sizer (SMPS) system (Model 3936, TSI Inc.). A submicron NaCl aerosol was generated by a constant output atomizer. The particles were dried, neutralized by a Kr85 radioactive source, and fed to a differential mobility analyzer. A monodisperse aerosol of 100 nm particles was formed and led to the sampling line. The particle concentration was measured by a condensation particle counter (CPC), and the total amount of alkali passing the SI detector could be correlated with the measured signal. The NaCl particles are expected to evaporate completely during their transport through the hot sampling line. To obtain the detection probability, the actual signal measured by the SI instrument was compared to the concentration of NaCl particles monitored by the CPC. Seven calibration sessions were performed using this method, and the detection probability was found to be (1.2 ( 0.3) × 10-3. The response is thus in good agreement with the results from the evaporation method. At a net gas flow rate of 1 lpm, 1 ppbv of alkali corresponds to a detector signal of 78 pA. Since the lower signal detection limit during field measurements is approximately 10 pA, the calibrations confirm that the instrument can measure sub-ppb alkali levels. 4. Applications This section presents field measurement results obtained during pressurized circulating fluidized-bed (PCFB) combustion and atmospheric fluidized-bed (AFB) combustion. In addition, the use of the surface ionization technique for measuring the concentration of alkalicontaining particles is discussed. Pressurized circulating fluidized-bed combustion usually employ an efficient gas filtration unit since the

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Figure 4. Measured alkali concentration (solid) and reactor temperature (dashed) with the SI instrument operated downstream of a hot gas filtration unit at a coal-fired 10 MW PCFB power plant in Karhula, Finland. The data correspond to a startup period where the operation of the plant was shifted from oil to coal between 15:00 and 16:00. The displayed alkali concentration data are averaged over 60 s.

limits for tolerable alkali (Na + K) concentrations specified by gas turbine manufacturers are very low.4 The alkali concentrations in PCFB combustion are therefore expected to be low, setting high demands on on-line measurement devices. Atmospheric fluidized-bed combustion facilities, on the other hand, do not normally employ efficient filtration units in the hot zones of the facilities, and the alkali concentrations are expected to be considerably higher. The following examples of field campaign applications of the SI instrument thus cover a wide range of measurement conditions. 4.1. Alkali Detection in PCFB Combustion. More than 500 measurement hours have been performed at a 10 MW PCFB power plant operated by Foster Wheeler Energia Oy in Karhula, Finland. The facility was fired with a bituminous coal containing 0.048%(wt) sodium and 0.19%(wt) potassium. The SI instrument was mounted on a DN150 flange in the main duct, approximately 2 m downstream of a hot gas ceramic filtration unit. The sampling line inlet was oriented perpendicularly to the flue gas flow, sampling gas directly from the main gas stream. The instrument feedthroughs were protected by a purge gas flow of 1.01.3 lpm (NTP), and a flue gas flow of 2.2-2.8 lpm was led through the sampling line toward the detector. The background level of the detector was checked every half hour by shutting off the sampled gas flow for one minute. The background has been subtracted from the measured signal, and the resulting signal has been converted to an alkali concentration in ppb by volume (ppbv) using the vapor calibration for a pressure of 11 bar. Examples of the results obtained with the SI probe have been published elsewhere,26 including comparisons with results from the PEARLS and ELIF on-line instruments operated simultaneously. Figure 4 shows alkali concentration and reactor temperature during start-up of the plant on October 24, 1997. The plant was started up using oil in the morning, and was shifted over to coal combustion between 15:00 and 16:00. The plant was operated at 60% load with a pressure of 11 bar. The reactor temperature was 800850 °C and the flue gas temperature at the measurement point downstream of the filter was 700-720 °C.

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Figure 5. Alkali concentration data taken at the PCFB plant during a 12 h period on October 25, 1997. The signal peaks coincide with the pulsing of the hot gas filter, and different pulsing frequencies were applied during the run. The displayed alkali concentration data are averaged over 60 s.

The alkali concentration was initially below 10 ppbv when the facility was operated on oil and then increased when operation was shifted over to coal. After the process had stabilized at about 17:00, the alkali base level remained fairly steady at 20-30 ppbv. Recurrent peaks started to appear after 20:00, synchronously with the cleaning pulses of the hot gas filter. Alkali concentration data for a 10-hour period on October 25, 1997, are shown in Figure 5. The alkali concentration is similar to the data from the day before with values in the 20-40 ppbv range. The sharp peaks in alkali concentration observed in connection with the pulsing of the hot gas filter are now more pronounced. The hot gas filter pulsing interval was varied during the period to assess the influence on the downstream alkali concentration. At 13:00, the pulsing interval was shortened from 20 to 10 minutes, which resulted in an increase in alkali concentration. When the filter pulsing interval was set to 30 minutes at 18:00, the alkali concentration returned to a lower level. In normal operation an ash deposit is built up on the hot gas ceramic filtration unit. The ash deposit has a strong effect on the alkali capture efficiency of the filter, and leads to a decrease in the alkali concentration downstream of the filter. The ash deposit is rapidly removed as the filter is periodically cleaned by a rapid gas pulse. This results in a lowered filter efficiency and a transient increase in flue gas alkali concentration until a new deposit is being built up. The higher alkali base level observed with a shorter pulsing interval is likely due to a simultaneous increase in fuel load from 60 to 64%. The results show that the SI instrument can be used to follow rapid concentration changes under the conditions prevailing in a PCFB facility, and is capable of covering the low alkali concentration range of interest in these types of applications. More specifically, the results show that use of on-line alkali detectors makes it possible to optimize plant operation, as exemplified in case of hot gas filter operation above. 4.2. Alkali Detection in AFB Combustion. In this section, results from measurement campaigns at two different AFB combustion facilities are presented. One campaign was carried out at the Idba¨cken bubbling

Davidsson et al.

Figure 6. An overview of the average alkali concentrations (and standard deviations) measured during periods of stable operation at the Idba¨cken AFB combustion power plant in Nyko¨ping, Sweden. The fuel blends are indicated in the graph. Abbreviations: WR-wood residues; DW-demolition waste.

fluidized-bed (BFB) heat and power plant in Nyko¨ping, Sweden, during the period January 28 to February 7, 1998. The Idba¨cken facility may be fired with a variety of solid fuels, e.g., biomass, coal, and wastes, and delivers 35 MW of electricity and 80 MW of heat at full load. The SI instrument was mounted directly in conjunction with the upper part of the boiler, level with the superheater tubes, where the temperature was around 800 °C and the particle load a few gm-3. The detector itself was protected with a purge gas flow of 0.5 lpm, and the net gas flow rate to the detector was kept low (0.15-0.25 lpm) due to the high concentration levels. An overview of the averaged alkali concentrations measured during the combustion of blends of demolition waste (DW), wood residues (WR), and coal is shown in Figure 6. The averages were obtained from periods with stable conditions except for the case of WR at half load. The alkali level for demolition waste lies at 1-2 ppmv but progressively rises to higher levels when wood residues are included in the fuel. On the time scale of these measurements, the effect of lowering the plant load from full to half load is not significant although the measurements as for WR at half load may have been obscured because the conditions were not fully stable at this occasion, and the addition of up to 40% coal to the WR fuel does not have a clear effect on the alkali concentration. A decrease appears on January 30 upon switching fuel from demolition waste to oil. The alkali content of the oil is not known, but the total ash content is very low compared to solid fuels. To illustrate the time scale on which such a change takes place, a detailed view of this measurement is shown in Figure 7. The transition time with respect to the change in the alkali level takes place during a few hours. A summary of the composition of ash components in the different fuels is given in Table 1. Both demolition waste and wood residues are highly heterogeneous fuels, and these values have been averaged over five and six analyzed samples, respectively. Based merely on the amounts of potassium and sodium in the fuel (Table 1), one would not expect wood residues to yield significantly higher alkali levels than demolition waste. The higher levels of vaporized alkali components in wood residues

On-Line Measurements of Alkali Metals in Combustion

Figure 7. Alkali concentration data taken at Idba¨cken power plant on January 30, 1998. A fuel transition from demolition waste to oil is indicated in the graph.

are likely due to differences in the ash composition, leading to a lower retention of alkali in the ash phase. The same is true for the coal used where the potassium concentration of the fuel is higher than for the two other fuels, but also the total ash content is considerably higher. In a second set of experiments, the SI instrument was operated in a circulating fluidized-bed (CFB) heat and power plant in Na¨ssjo¨, Sweden. The facility is exclusively fired with wood residues from local sawmills and delivers 9 MW of electricity and 26 MW of heat at full load. The measurements were performed in the ”return leg” where fluidized-bed material separated at the hot cyclone is returned to the boiler. The particle load was around 10 gm-3. The measurement conditions are unusually demanding, and the work was performed as part of an erosion/corrosion research project, where the durability of heat exchanging surfaces located in the socalled particle seal was evaluated. The SI instrument was mounted at the particle seal, which is located below the hot cyclone and connected to the same by a vertical duct. An example of a typical untreated detector signal and the net gas flow during a field measurement is shown in Figure 8. In this case, the net gas flow was kept quite low (0.2 lpm) due to the relatively high signal levels. The gas flow through the sampling line was periodically turned off to check the background level, to which the signal decreased within a few minutes. The time delay shows that during operation with a low net gas flow, brief memory effects may occur in the sampling line. Although the measurements exemplified in Figure 8

Energy & Fuels, Vol. 16, No. 6, 2002 1375

Figure 8. Example of the alkali signal during measurement in an AFB combustion power plant in Na¨ssjo¨, Sweden. The gas flow through the sampling line is varied, and the signal decreases to the background level within a few minutes when the gas flow is turned off. The signal has been averaged over 15 s intervals.

were performed in heavily particle-laden flue gas, only minor maintenance problems were encountered, e.g., loading of the detector cavity with sand. The fluctuations displayed by the signal may be attributed mainly to two factors. First, variations in pressure and gas flow were caused by the fact that the measurement site was located directly above fluidization gas distributors. Second, the particle loading was occasionally very high, resulting in signal peaks due to the detection of particlebound alkali components. To produce accurate readings in this type of environment, the signal should be averaged over a certain period of time, typically 15-60 s. An overview of the alkali concentrations measured during the campaign at the Na¨ssjo¨ heat and power plant on March 28-31, 2000, is displayed in Figure 9. The values lie in the range 0.5-1.5 ppmv. A general trend, which is especially clear on March 28 and 30, is that the concentration decreases after the morning hours. This trend is probably closely connected to the plant load, which was changed every day since the weather was clear and cold during the entire week. The plant was always operated at full load at night, but the load was lowered to around 75% as the demand for district heating decreased rapidly after the early morning hours. Part of the reason for the sharp decrease in alkali concentration upon lowering the plant load to 75% is likely to be found in the reduced flow in the particle return leg. Fluid dynamic modeling work has shown a significant reduction in the solid flux through this

Table 1. Composition of Ash-forming Elements, Given as Oxides, in the Fuels Used at the Idba1 cken AFB Combustion Facility in Nyko1 ping, Sweden fuel

demolition waste

wood residues

coal

dry sample, wt %

fuel

filter ash

fuel

filter ash

fuel

ash content SiO2 Al2O3 CaO Fe2O3 K2O MgO MnO2 Na2O P2O5

11 ( 5 2.7 ( 1.8 0.7 ( 0.5 0.8 ( 0.3 0.6 ( 0.6 0.19 ( 0.10 0.20 ( 0.15 0.06 ( 0.05 0.27 ( 0.15 0.12 ( 0.10

97 ( 3 38 ( 2 13 ( 2 20 ( 2 6.4 ( 1.8 3.5 ( 0.2 3.2 ( 0.4 0.6 ( 0.2 1.9 ( 0.2 0.8 ( 0.2

2.5 ( 0.2 1.2 ( 0.5 0.2 ( 0.1 0.7 ( 0.3 0.2 ( 0.2 0.16 ( 0.08 0.14 ( 0.04 0.08 ( 0.05 0.06 ( 0.03 0.07 ( 0.06

98 ( 2 29 ( 2 7(2 21 ( 4 3.7 ( 1.0 4.6 ( 0.2 4.1 ( 0.3 1.2 ( 0.2 1.5 ( 0.2 2.9 ( 0.3

14.7 9.0 3.0 0.63 0.89 0.42 0.33 0.01 N/A 0.19

1376

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Davidsson et al.

Figure 9. Overview of the alkali concentration measurements carried out in the AFB combustion power plant in Na¨ssjo¨, Sweden, during the period March 28-31, 2000.

passage at partial load, and this tendency is closely coupled to the lowered gas velocities in the main boiler.36,37 The alkali concentrations shown in Figure 9 may be regarded as rather low, considering that the alkali content of wood residues is generally quite high. Some process circumstances may, at least partly, influence the alkali level. The gas temperature in the particle seal was in the range 850-900 °C during all measurements, and these temperatures should be high enough to avoid significant losses of alkali due to condensation on particles. However, the gas exiting the hot cyclone toward the measurement site passes a duct section with lower temperatures that could cause condensation of alkali on particles or relatively cold spots on surfaces. Furthermore, the concentration of bed material particles is naturally very high in the particle seal where the measurements are performed. Alkali components, which otherwise may have condensed upon particles in the fine fraction (d < 1 µm), may then predominantly end up on coarse particles for which the SI detection efficiency is low. The results from the two AFB campaigns show that the SI instrument can be successfully applied also under these conditions, including higher alkali concentrations and sometimes high particle loading. The alkali concentration measured provides a useful estimation of the potentially harmful alkali components present. As previously mentioned, the detection probability is equivalent with respect to alkali vapors and ultra fine alkali particles, and significantly lower for alkali components bound to coarse particles. At high particle loading the contribution to the signal from large particles can, however, be significant. The next section describes an alternative measurement mode that can help to distinguish between the different components contributing to the signal. 4.3. Detection of Alkali-Containing Particles. In the normal alkali measurement mode used in the applications presented above, we do not distinguish between alkali vapors and particles. However, there are (36) Pallare`s, D.; Johnsson, F. Fluid dynamic modeling of large CFB units. In Proceedings of the 7th International Conference on Circulating Fluidized Beds; Grace, J. R., Zhu, J., de Lasa, H., Eds.; Canadian Society for Chemical Engineering: Ottawa, Canada, 2002; pp 387394. (37) Johnsson, F.; Leckner, B. Vertical distribution of solids in a CFB-furnace. In Proceedings of the 13th International Conference on FBC; Heinchel, K. J., Ed.; ASME: New York, 1995; pp 671-679.

Figure 10. The ion current observed with the SI instrument on the millisecond time-scale during operation in laboratory air containing pure NaCl particles. The peaks in the data correspond to individual NaCl particles with a diameter of about 0.1 µm colliding with the hot Pt filament. The aerosol particles decompose on the filament and the alkali content of the particles is partially surface ionized, giving rise to an ion pulse at the ion collector.

ways of employing the surface ionization technique for the monitoring of individual alkali-containing particles. In the preceding section, one signal value was sampled from the electrometer each second. The ion collector may instead be connected to a fast current amplifier enabling high sampling frequencies. In this measurement mode the signal pulses corresponding to the detection of individual particles can be resolved. Detailed studies of individual alkali salt particles interacting with a Pt filament in laboratory air have previously been carried out by Hagstro¨m.34 The melting and dissociation of an alkali-containing particle at a hot Pt filament takes place on the microsecond to millisecond time scale, and the emission of Na+ and K+ ions from an individual aerosol particle on the filament gives rise to a sharp ion pulse at the ion collector. The pulse duration depends on the particle size, the type of salt and the surface temperature. Figure 10 shows examples of the results obtained when individual NaCl particles with a diameter of about 0.1 µm collide with a Pt filament. The observed ion peaks cannot be directly related to the size of the aerosol particle colliding with the filament due to incomplete ionization of the alkali content of the particle. However, the SI instrument run in this mode can be used to measure the particle concentration of alkali-containing particles, and thereby help to evaluate

On-Line Measurements of Alkali Metals in Combustion

the effect of large particles on the alkali signal observed in the normal instrument operation mode. We expect that a simplified version of the SI instrument described in this paper can be used as a rapid and efficient particle concentration monitor for combustion applications. One possible application would be to continuously monitor the particle loading after a hot gas filter. The instrument would detect a failure in the filtration unit within seconds, and could therefore function as an early warning system for protection of a sensitive gas turbine downstream. Tests of the SI instrument in particle-counting mode under realistic combustion conditions are currently underway. 5. Conclusions An instrument for on-line measurements of alkali components in hot flue gas streams has been presented. The instrument is based on surface ionization technique and uses a heated sampling line to extract flue gas for continuous measurements at pressures up to 30 bar. Calibration has been performed in two independent ways using alkali vapors and alkali-containing particles giving consistent results. Field campaigns were performed at fluidized-bed combustion facilities operating under both pressurized and atmospheric conditions. Several fuels have been used, including coal, biomass, and demolition waste. The instrument has performed well both during PCFB coal combustion with alkali concentrations in the ppb range and in particle-laden measurement sites with alkali levels above 1 ppm. We conclude that the SI detector

Energy & Fuels, Vol. 16, No. 6, 2002 1377

constitutes a durable, sensitive, and reliable alkali measurement method with a high time resolution and with a lower detection limit of around 1 ppb. The instrument has an equivalent response toward alkali vapors and fine-mode alkali-containing particles. In systems with high levels of fluidized-bed material or fly ash, the measurement results should be interpreted in a different manner. Part of the total alkali content is bound to coarse particles for which the detection probability is low. In these circumstances, operation of the instrument in a pulse-counting mode could provide further information to evaluate the effect of coarse particles on the detected alkali signal. The SI measurement principle makes the instrument design simple and the manufacturing cost relatively low, which should make the instrument attractive for several applications. The instrument presented here has been prepared for pressurized conditions, and a less complex construction could also be produced for atmospheric conditions. The method should be helpful during the development of new processes and during changes in fuel composition. The alkali content of the flue gas could also be continuously monitored at one or several positions in advanced combustion facilities. Acknowledgment. This work has been supported by the European Commission under Grant JOF3-CT950057, the Swedish Research Council, and the Swedish Energy Administration. EF020020H