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Online Measurements of Individual Alkali-Containing Particles Formed in Biomass and Coal Combustion: Demonstration of an Instrument Based on Surface Ionization Technique Maria Svane, Magnus Hagstro¨m, and Jan B. C. Pettersson* Department of Chemistry, Atmospheric Science, Go¨ teborg University, SE-412 96 Go¨ teborg, Sweden Received March 22, 2004. Revised Manuscript Received October 13, 2004
Online measurements of individual alkali-containing submicrometer particles in a circulating fluidized bed (CFB) combustion facility are presented. A recently developed particle beam mass spectrometer based on surface ionization technique is demonstrated and its performance during a field campaign in a 12 MW CFB reactor operating under atmospheric conditions is discussed. The boiler was fired with a biomass fuel consisting of a mixture of wood chips and pellets. During part of the campaign K2CO3, HCl, and SO2 were also added to the fuel to investigate their influence on alkali-related processes. Size-selected particles with diameters of 50-300 nm in general had high potassium content, while sodium, rubidium, and cesium were present as minor constituents. Particles formed by recondensation of compounds evaporated during the combustion process could be distinguished from fly ash particles on the basis of their potassium and sodium contents. The number of alkali-containing particles increased when the K, Cl, and S fluxes into the boiler increased, and the trend was confirmed by independent measurements of the total particle concentrations in the flue gas. The concentration of potassium bound to submicrometer particles was found to be in the range 0.8-13 mg/m3 under the studied operating conditions. When the facility was instead fired with bituminous coal as a fuel, the relative concentration of sodium compared with potassium was higher, and a larger fraction of the alkali compounds were found in fly ash particles. The particle beam mass spectrometer is concluded to accurately provide online information on particle-bound alkali concentrations under the conditions typically prevailing at a commercial-size boiler, and the possibilities for future research and technical development are discussed.
1. Introduction The direct measurement of alkali metal compounds in hot flue gas streams is of interest for the protection of plant equipment and for the further development of process technology. During combustion of biofuels, the major part of the alkali content is volatilized, while in the case of coal combustion a fraction of the alkali components is also retained in the ash phase.1 The released alkali compounds may give rise to a number of operational problems. This is in particular true for renewable fuels that are generally rich in alkali, and the problems often constitute a limiting factor for the usefulness of the fuels in advanced power production facilities.2 The formation of ash deposits on heat-exchanging surfaces leads to reduced heat transfer efficiency and may ultimately lead to costly unscheduled plant shutdowns for deposit removal, or alternatively tube replacement in case of excessive corrosion.3 The presence of alkali in the ash is associated with a lowering of the ash * Corresponding author: phone +46 31 772 2828; fax +46 31 772 3107; e-mail
[email protected]. (1) Raask, E. Prog. Energy Combust. Sci. 1985, 11, 97-118. (2) 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.
melting temperature, which may result in molten ash phases and sintering problems in fluidized-bed boilers.4 The volatilized alkali compounds may also have detrimental effects on sensitive equipment in advanced combustion systems.5 Even if only a small fraction of alkalicontaining fine particles and alkali vapors penetrate a hot gas filtration unit, corrosive deposits may form on gas turbine blades.6 In conventional 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.7 As an alternative method, online measurement techniques have been developed based on several different principles; see Monkhouse8 (3) 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; National Renewable Energy Laboratory, 1995. (4) Skrifvars, B.-J.; Backman, R.; Hupa, M. In LIEKKI2 Technical Review; 1993-1998; pp 609-690. (5) Stringer, J. Annu. Rev. Mater. Sci 1977, 7, 477-509. (6) Kofstad, P. High-Temperature Corrosion; Elsevier Applied Science: London, 1988. (7) Romey, I. F. W.; Garnish, J.; Bemtgen, J. M. Diagnostics of Alkali and Heavy Metal Release; Clean Technologies for Solid Fuels (19961998); Joule-Thermie Programme, European Commission, 1998.
10.1021/ef049925g CCC: $30.25 © 2005 American Chemical Society Published on Web 12/16/2004
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for a recent review of alkali measurement techniques. Online measurement of alkali metals using the plasma excited atomic resonance line spectroscopy method in a 20 kW bubbling fluidized bed was recently reported by Ha¨yrinen et al.9 Another technique is based on surface ionization and a new instrument for online measurements of alkali components in hot flue gas streams was demonstrated by Davidsson et al.10 The surface ionization phenomenon results in the thermal evaporation of positive alkali ions from hot metal surfaces,11 and the process allows for selective and highly sensitive detection of alkali compounds. The alkali concentration measured with the instrument described by Davidsson et al. 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 was suggested to be operated in pulsecounting mode where the concentration of coarse particles can be estimated. The instrument was concluded to provide a sensitive alkali measurement method with a high time resolution and with a lower detection limit of around 1 ppb. A new type of instrument for online measurements of particle-bound alkali compounds has recently been described by Svane et al.12 The instrument relies on the combination of surface ionization technique with vacuum and mass spectrometric techniques, and is capable of online quantitative chemical analysis of individual alkali salt particles with diameters down to 0.014 µm. The instrument could work as a complement to the instrument previously described by Davidsson et al.10 since it only measures particle-bound alkali compounds and also distinguishes between sodium and potassium. The robustness of the instrument makes it suitable for field measurement applications, and in the present paper its performance is demonstrated during biomass and coal combustion in a 12 MW circulating fluidized bed (CFB) facility. The alkali content in individual particles of selected sizes is measured during combustion of biomass with different additives and during coal combustion. The number concentration of alkali-containing particles is measured and compared to the total particle concentrations in the flue gas, and the total concentration of particle-bound potassium in submicrometer particles is determined. 2. Experimental Section Experimental Setup. Measurements were carried out during the winter season 2002/2003 at a 12 MW CFB reactor at Chalmers University of Technology in Go¨teborg, Sweden.13 The instrumental setup is shown in Figure 1. A sampling probe was used to extract a product gas flow from the center of the hot flue gas stream in the reactor. The sampling probe inlet was oriented perpendicularly to the flue gas flow, extracting (8) Monkhouse, P. B. Prog. Energy Combust. Sci. 2002, 28, 331381. (9) Ha¨yrinen, V.; Hernberg, R.; Aho, M. Fuel 2004, 83, 791-797. (10) Davidsson, K. O.; Engvall, K.; Hagstrom, M.; Korsgren, J. G.; Lo¨nn, B.; Pettersson, J. B. C. Energy Fuels 2002, 16, 1369-1377. (11) Zandberg, E. Y. Technol. Phys. 1995, 40, 865-884. (12) Svane, M.; Hagstro¨m, M.; Pettersson, J. B. C. Aerosol Sci. Technol. 2004, 38, 655-663. (13) Miettinen Westberg, H.; Bystro¨m, M.; Leckner, B. Energy Fuels 2002, 17, 18-28.
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Figure 1. Schematic picture of the instrumental setup. Abbreviations: PBMS, particle beam mass spectrometer; DMA, differential mobility analyzer; CPC, condensation particle counter; LPI, low-pressure impactor; T1 and T2, thermocouples; P, pressure gauge. particles directly from the main gas stream through the sampling line toward the instruments. The sampling took place downstream from the primary cyclone in the facility, and water cooling of the surfaces of the probe was required to avoid thermal degradation of the probe. The product gas flow rate through the sampling line was typically 1.8 m3/h. As the hot flue gas entered the probe it was directly diluted with pressurized and clean air with a flue gas:clean air ratio of 1:1.5. The dilution factor was varied for some studied cases to ensure that the measured particle size distributions were not affected by it, but a limited influence from condensation of gaseous compounds on existing particles in the sampling probe cannot be completely ruled out under all conditions investigated here. A cyclone connected to the probe removed the coarse fraction of the particles. Because of the high particle number concentration, further dilution of the flue gas was required during aerosol particle measurements. This was accomplished by adding three ejector diluters connected in series prior to the instruments, resulting in a total dilution factor of 350. Furthermore, the first one of these diluters was heated to 150° C to avoid problems due to condensation of water vapor. Particle Beam Mass Spectrometer. Single alkali-containing particles were detected and analyzed with a particle beam mass spectrometer (PBMS). The instrument has been described in detail elsewhere12 and is only briefly presented here. The PBMS combines surface ionization technique with mass spectrometry for online chemical characterization of the alkali metal content in individual aerosol particles. The instrument illustrated in Figure 2 consists of a differentially pumped vacuum system that contains a particle inlet, a surface ionization unit, a mass spectrometer, and an ion detection unit. The instrument employs an aerodynamic inlet system for efficient sampling of particles into vacuum. The inlet system consists of a critical orifice, controlling the pressure drop and flow rate, and an aerodynamic lens system that produces a sharply focused particle beam with a high transmission of particles into the detection unit.14-16 The formed particle beam is focused onto a heated platinum surface in the detection vacuum chamber and is highly collimated when it hits the surface situated 90 mm from the exit nozzle of the lens system. The surface is constructed as a box (3 mm × 3 mm × 5 mm) (14) Liu, P.; Ziemann, P. J.; Kittelson, D. B.; McMurry, P. H. Aerosol Sci. Technol. 1995, 22, 314-324. (15) Liu, P.; Ziemann, P. J.; Kittelson, D. B.; McMurry, P. H. Aerosol Sci. Technol. 1995, 22, 292-313. (16) Jayne, J. T.; Leard, D. C.; Zhang, X.; Davidovits, P.; Smith, K. A.; Kolb, C. A.; Worsnop, D. R. Aerosol Sci. Technol. 2000, 33, 49-70.
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Energy & Fuels, Vol. 19, No. 2, 2005 413 Table 1. Ionization Potentials, IP, and Ionization Probabilities, β, for the Alkali Metals Used in This Study and the Work Function, O, for Platinum alkali-metal surface material Na K Rb Cs Pt
IP (eV)
φ (eV)
β (Pt surface at 1500 K)
5.14 4.34 4.18 3.89
0.96 1 1 1 5.5
ref 18 18 18 18 19
Cs+ ions, respectively. The phenomenon of surface ionization is characterized by the fact that an atom or molecule being adsorbed on a hot metal surface is thermally desorbed in ionic form. The statistical probability of ionic and neutral fluxes from a surface is described by the Saha-Langmuir equation:
R)
[
]
n+ g + e(φ - IP) ) exp n0 g 0 kBT
(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 the elementary charge, surface work function, ionization potential, Boltzmann’s constant, and absolute temperature, respectively. The probability that an adsorbed species will desorb in ionic form is described by the ionization probability β:
β)
Figure 2. Mobile particle beam mass spectrometer (PBMS). (a) Cross-section view of the vacuum system: (1) aerosol inlet with a focusing aerodynamic lens system; (2) resistively heated Pt surface for volatilization of aerosol particles and surface ionization of the alkali content; (3) quadrupole mass spectrometer; (4) continuous dynode electron multiplier used for ion detection. (b) Photograph of the mobile PBMS mounted onto a rack together with electronic devices and pumps. from a 0.025 mm thick Pt foil and is resistively heated to 1500 K. The surface temperature was measured by optical pyrometry and corrected for spectral emissivity. A positive potential of +100 V was applied to the surface during all measurements. The alkali components, i.e., sodium, potassium, rubidium, and cesium compounds, in each particle that strikes the surface are adsorbed, dissociated, and further transformed into alkali ions that are thermally evaporated as Na+, K+, Rb+, and
R R+1
(2)
The success of the surface ionization method depends on the difference between the IP of the adsorbed atom and the work function of the metal surface.11,17 For most elements IP is larger than φ of the surface and desorption in neutral form dominates. However, the IPs of the alkali metals are unusually low and lower than φ when a platinum surface is used; see Table 1. The use of a platinum surface thus offers a selective detection of low IP alkali atoms and excludes the ionization of most other elements. Ions produced at the Pt surface are subsequently accelerated toward a quadrupole mass spectrometer. The mass filter only allows ions of a selected mass-to-charge ratio to reach the detector. The ion current is amplified with a continuous dynode electron multiplier (Model 206.10-SL, De Tech Inc.) and a current amplifier (Model 427, Keithley Instruments, Inc.). The output signal is sampled and logged by computer at a frequency of 10 kHz. Calibration of the PBMS. The calibration procedure has been described in detail elsewhere.12 The calibration was carried out with laboratory-generated submicrometer alkali salt particles of known size.12 An aerosol was generated from a salt solution with a constant-output atomizer. The aerosol was then dried and neutralized by a Kr85 radioactive source and fed to a differential mobility analyzer (DMA), and the formed monodisperse aerosol was led to the inlet of the PBMS. The measured signal resulting from each particle was integrated to give the particle size, and the particle count rate was also determined. Simultaneously, the total particle number concentration of the aerosol was measured with a condensation particle counter. Calibration curves were obtained for different sodium and potassium salts by this method, and the detection efficiency was found to be close to unity for Na as well as K originating from salt particles with diameters larger than 40 nm. For smaller particles, the detection efficiency for particles (17) Ionov, N. I. Progress in Surface Science; Pergamon Press: Oxford, U.K., 1972. (18) Handbook of Chemistry and Physics; 56th ed.; Weast, R. C., Ed.; CRC Press Inc: Cleveland, OH, 1975. (19) Eastman, D. E. Phys. Rev. B 1970, 2, 1-2.
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reaching the detector is still high, but the transmission of particles through the inlet of the PBMS decreases rapidly and approaches a few percent for a particle with a diameter of 10 nm. Complementary Aerosol Measurement Techniques. In addition to the PBMS measurements number size distributions in the size range 0.01-0.4 µm were obtained with a scanning mobility particle sizer (SMPS) system (model 3936, TSI, Inc.), which consists of a differential mobility analyzer (DMA) and a condensation particle counter (CPC).
3. Results 3.1. Biomass Combustion. The main parts of the experimental results are based on measurements of alkali-containing particles performed with the PBMS under different conditions. In addition, particle number size distributions in the size range 10-370 nm were continuously monitored with a SMPS system with a time resolution of 7.5 min. This system was operating around the clock, with interruptions in the data collection only for sampling periods when size-selected measurements were performed with the PBMS. As each size distribution scan from the SMPS system was monitored in real time, it was possible to determine when periods of stable combustion conditions arose. During the main part of the measurement campaign the CFB facility was fired with a biomass fuel consisting of a 50:50 mixture of wood chips and pellets. This mixture constituted the base case fuel and typically contained 8 × 10-2 % (wt) potassium and 4 × 10-3 % (wt) sodium. For the base case, the fluxes of K, Cl, and S into the boiler were 33, 3, and 4 mol/h, respectively. During part of the campaign K2CO3, HCl, and SO2 were also added to the fuel to investigate their influence on alkali-related processes in the boiler. When K2CO3 and HCl were added to the fuel, the total fluxes of K, Cl, and S became 85, 57, and 4 mol/h, respectively. When K2CO3, HCl, and SO2 were all added to the fuel, the corresponding fluxes became 85, 85, and 43 mol/h. A short sequence of a typical K+ signal obtained with the PBMS during biomass combustion is shown in Figure 3a. Combustion aerosol particles with a diameter of 100 nm were preselected with a DMA in this measurement. The decomposition of individual particles on the hot platinum surface in the detector is observed as short bursts of alkali ions with duration on the millisecond time scale. The detailed pulse duration depends on the particle size, the type of salt, and the surface temperature. The previous study by Svane et al.12 showed that the alkali atoms in a particle are efficiently transformed into ions on the Pt surface. The number of alkali atoms in a single particle can therefore be determined by integrating over the total current of each ion peak. This is exemplified in Figure 3b, where the number of potassium atoms in each of the recorded pulses has been summarized for measurements carried out under the same conditions as in Figure 3a. The sampled 100 nm particles contained about 107 alkali atoms. This number agrees well with the calculated number of alkali atoms in a spherical KCl particle with a diameter of 100 nm, and we conclude that a major fraction of the particles in this case consisted of potassium salt. The number of potassium and sodium atoms in submicron particles is displayed in Figures 4-6 for
Figure 3. Analysis of the signal for potassium obtained from particles with a diameter of 100 nm. (a) Example of the recorded output signal obtained with PBMS during biomass combustion. Individual aerosol particles give rise to clearly time-resolved peaks. (b) Calculated number of K atoms in 100 nm combustion particles from the data obtained with the PBMS.
Figure 4. CFB combustion of a 50:50 mixture of wood chips and pellets: number of K (s) and Na (---) atoms in individual combustion aerosol particles measured with the PBMS. A DMA was used to select a specific particle size for each measurement and these preselected sizes are indicated in the panels. The vertical line indicates the calculated number of alkali atoms in a pure and spherical alkali chloride particle with the preselected size.
several preselected particle diameters. The studied cases include biomass combustion without additives (Figure 4), biomass combustion with addition of K2CO3 and HCl (Figure 5), and biomass combustion with addition of K2CO3, HCl, and SO2 (Figure 6). The flue gas was fed through a DMA for particle size selection prior to the inlet of the PBMS and the measurement series were performed with the same experimental setup during all
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Figure 5. CFB combustion of a 50:50 mixture of wood chips and pellets with addition of K2CO3 and HCl: number of K (s) and Na (---) atoms in individual combustion aerosol particles measured with the PBMS. A DMA was used to select a specific particle size for each measurement and these preselected sizes are indicated in the panels. The vertical line indicates the calculated number of alkali atoms in a pure and spherical alkali chloride particle with the preselected size.
Figure 6. CFB combustion of a 50:50 mixture of wood chips and pellets with addition of K2CO3, HCl and SO2: Number of K (s) and Na (---) atoms in individual combustion aerosol particles measured with the PBMS. A DMA was used to select a specific particle size for each measurement and these preselected sizes are indicated in the panels. The vertical line indicates the calculated number of alkali atoms in a pure and spherical alkali chloride particle with the preselected size.
combustion situations. Figure 4 shows that the number of potassium atoms increases with particle size. As indicated in the panels, the number of atoms agrees well with the calculated potassium content in pure potassium salt particles of the same size. These particles are concluded to be formed by recondensation of alkali compounds that have evaporated during the combustion process. The observed width and shape of the potassium size distributions is determined by the transmission function of the DMA used to preselect particles and by the variation in shape and composition between individual particles in the flue gas. When the particle size is increased above 100 nm, a large fraction of the particles have a lower potassium content of roughly 1-10% of the total particle mass. These results are interpreted as being due to fly ash particles with a minor fraction of condensed alkali compounds on the surface of the particles. The particles correspond to the lower tail of the size distribution for fly ash particles, and the distribution is also observed as a minor fraction in the data for particles with diameters e100 nm. The sodium content in the particles is about 1-10% of the potassium content, which is consistent with the lower sodium concentration in the biomass fuel. The number of sodium atoms in the particles also follows the same trends with particle size as observed for potassium, including a fraction attributed to fly ash particles for 150 nm particles. On the basis of the results, we conclude that the PBMS is capable of distinguishing between different types of particle fractions in the flue gas and to quantitatively determine the alkali content in single particles during online measurements.
Figures 5 and 6 show the number of alkali atoms in particles measured during biomass combustion with addition of K2CO3 and HCl (Figure 5) and biomass combustion with addition of K2CO3, HCl, and SO2 (Figure 6). The observed trends for potassium and sodium are similar for the two cases and also agree well with the results shown for the base case in Figure 4. Regardless of fuel composition, the majority of the particles with selected diameters e150 nm mainly consisted of potassium salt. As larger particle sizes were selected with the DMA, the fraction of particles with a low potassium content increased. This indicates that the fraction of fly ash particles increases with increasing particle size. Approximately 20% of the recorded particles with a diameter of 150 nm contained a low number of potassium atoms, and for the largest selected particle sizes, e.g., 250 nm in Figure 5 and 300 nm in Figure 6, as many as 65% of the total number of detected particles had a low potassium content. When the data for a preselected diameter of 150 nm are compared for the three cases, the relative importance of fly ash particles compared to newly formed particles is larger in the base case. The likely reason for this is the addition of potassium in the form of K2CO3 in the two other cases that should increase the concentration of alkali compounds in the flue gas and thereby allow newly formed particles to grow to larger size. Signals attributed to the sodium content were found to correspond to minor fractions of the original particle size irrespective of fuel blends or selected size. The results in Figures 5 and 6 are comparable and the extra addition of SO2 in the case
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Figure 7. Number of alkali atoms in individual combustion aerosol particles with a selected diameter of 150 nm measured with the PBMS. The signal corresponding to potassium, sodium, cesium, and rubidium, respectively, was obtained during four consecutive measurements. The dotted line corresponds to the calculated number of K atoms in a 150 nm pure and spherical KCl particle.
shown in Figure 6 does not have a strong effect on the alkali content in individual particles. In addition to potassium and sodium, the particles also have low concentration of rubidium and cesium. Size distributions derived from K, Na, Rb, and Cs signals are plotted in Figure 7 as a function of the number of alkali atoms detected in single preselected 150 nm particles. The results where obtained during addition of K2CO3 and HCl to the fuel. The contents of rubidium and cesium in the sampled particles are 3-4 orders of magnitude lower than the potassium content, which further illustrates the high sensitivity of the PBMS instrument. In parallel with the PBMS measurements of alkalicontaining particles, the number concentration of particles was obtained with a SMPS system. The SMPS system measured the number size distribution in the size range 10-370 nm for all particles in the flue gas. To compare the data obtained with the two instruments, the PBMS results were transformed into apparent size distributions. The determined number of potassium atoms in each detected particle was recalculated into an apparent particle diameter by assuming that the particle was a spherical KCl particle with the density of the salt. The apparent diameter is smaller than the true diameter since other components in the particle are neglected. Deviations from spherical shape can also influence the calculated diameter by typically 5-15%.12 However, as shown in Figures 4-6, a large fraction of the particles mainly consists of potassium salts, which support the use of the calculation procedure. Apparent size distributions from the PBMS measurements are displayed in Figure 8 together with total particle number size distributions recorded with the SMPS system. The size distributions measured with the PBMS system are relatively broad with high particle concentrations in general observed for apparent diameters of 30-100 nm. The distributions drop rapidly for particle diameters smaller than approximately 30 nm due to a decreased transmission through the inlet system of the PBMS for these sizes. The results resemble the distributions obtained with the SMPS, although they may be
Figure 8. Number concentrations of total particle concentration (obtained with SMPS) and potassium-containing particles (obtained with PBMS) measured simultaneously during CFB combustion of a 50:50 mixture of wood chips and pellets: (a) combustion without additives; (b) with addition of K2CO3 and HCl; (c) with addition of K2CO3, HCl, and SO2. See text for further details.
shifted in size and the absolute number concentrations in general are lower than the number concentrations obtained with the SMPS. When K2CO3 and HCl are added to the fuel (Figure 8b), the number concentrations measured with both instruments increase compared to the base case (Figure 8a), and the concentrations increase further when SO2 is also added (Figure 8c). The additives also make the distributions measured with the SMPS shift to large size, a change that is not clearly observed in the data for alkali-containing particles. An obvious reason for the different appearance of the two types of distributions shown in Figure 8 is the fact that the particles do not consist only of potassium salt. This makes the distribution shift since the alkali-containing particles appear at a size that is smaller than the true
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diameters above 200 nm may indicate that part of the alkali may have remained in the solid phase during the conversion process. 4. Concluding Remarks
Figure 9. CFB combustion of a bituminous coal: number of K (s) and Na (---) atoms in individual combustion aerosol particles measured with the PBMS. A DMA was used to select a specific particle size for each measurement and these preselected sizes are indicated in the panels. The vertical line indicates the calculated number of alkali atoms in a pure and spherical alkali chloride particle with the preselected size.
size. The factor of 2-10 lower counting rate observed with the PBMS compared to the SMPS could be explained by a relatively large population of particles with an alkali content below the detection limit of the PBMS. Further studies with alternative measurement techniques will be required to confirm this conclusion. On the basis of the PBMS data in Figure 8, we have also calculated the concentration of potassium in submicrometer particles. The concentrations range from 0.8 mg/m3 for the base case fuel (Figure 8a) to 5 and 13 mg/m3 when additives were injected with the fuel corresponding to the cases shown in Figure 8, panels b and c, respectively. These calculated values are lower than the total particle mass concentrations (5, 24, and 149 mg/m3 for Figure 8, panels a, b, and c, respectively) obtained from the SMPS data but show the same trend for the three cases. 3.2. Coal Combustion. In addition to the studies with biofuels described above, the CFB facility was fired with a bituminous coal during part of the measurement campaign. The coal contained 0.048% (wt) sodium and 0.19% (wt) potassium. The numbers of sodium and potassium atoms in particles with selected sizes are displayed in Figure 9. The alkali emissions from coal combustion exhibit a somewhat different pattern compared to that observed during combustion of biomass. Particles with diameters e 100 nm mainly consist of potassium and sodium compounds. These particles are concluded to be formed by recondensation of evaporated alkali compounds, similar to the results for biomass combustion. Particles with diameters larger than 100 nm in general have a relatively low alkali concentration, and the observed distributions are interpreted as being due to fly ash particles. The alkali content of these particles is likely to be partially due to alkali compounds that have condensed on the surface of the fly ash particles. In addition, the fact that the potassium and sodium concentrations are also similar in particles with
A particle beam mass spectrometer based on surface ionization technique has been demonstrated to provide online measurements of alkali metal components in a hot flue gas stream. The high sensitivity of the instrument allows quantitative chemical analysis of the alkali content in individual submicrometer aerosol particles with diameters down to 50 nm. During biomass combustion, size-selected particles with diameters of 50-300 nm are in general found to have high potassium content, but also sodium, rubidium, and cesium are observed in low concentrations. The concentrations of alkali-containing particles are observed to be influenced by an increase in the total fluxes of K, Cl, and S added to the boiler. The results observed during biomass and coal combustion are also distinctly different, both concerning the relative concentrations of potassium and sodium and the importance of fly ash particles compared to particles formed by nucleation in the combustion process. We conclude that the surface ionization technique implemented in the PBMS is a sensitive and reliable alkali measurement method with a high time resolution and with a low detection limit. The instrument design is relatively robust and its design makes it suitable for measurements in the rough environment met in a large combustion facility. Nevertheless, the present instrument is a prototype and there is ample room for improvements. The earlier studies12 already showed that detection of particles with diameters down to about 10 nm can be achieved in the laboratory. The size of the instrument can also be significantly reduced. Another interesting possibility is the replacement of the hot filament by a material with a higher work function than platinum, which would increase the range of elements that can be detected by surface ionization beyond the alkali metals. It may also be possible to replace the quadrupole mass spectrometer used in the present study with a time-of-flight mass spectrometer that allows simultaneous quantitative measurements of multiple elements in single particles. We are currently attempting to improve the instrument along these lines. The high sensitivity of the instrument should make it attractive for several research applications and in the development of advanced combustion processes. As one example, it is currently used to follow the behavior of trace concentrations of cesium in order to characterize the transformations and fate of alkali compounds in commercial-scale boilers. Simplified designs based on the instrument could also be developed for continuous monitoring in advanced combustion facilities, e.g., upstream of a gas turbine that is sensitive to alkali-induced corrosion. Acknowledgment. We thank the personnel at Energy Conversion, Chalmers University of Technology, for support during the field study. This work was supported by the Swedish Energy Agency. EF049925G
