Energy Fuels 2010, 24, 6301–6306 Published on Web 11/11/2010
: DOI:10.1021/ef101031u
Performance Evaluation of a Model Electrostatic Precipitator for an Advanced Wood Combustion System Mark Omara, Philip K. Hopke,* Suresh Raja, and Thomas M. Holsen Center for Air Resources Engineering and Science, Clarkson University, P.O. Box 5708, Potsdam, New York 13699, United States Received August 6, 2010. Revised Manuscript Received October 10, 2010
Attempts to reduce greenhouse gas emissions, coupled with the increasingly competitive price of biomass fuels against rising oil prices, have given impetus to the use of renewable biofuels in energy production. However, conventional biomass combustion systems produce significant air pollution making them undesirable particularly for public buildings such as schools. Thus, there has been growing interest in advanced wood combustion (AWC) technologies that have higher energy efficiencies and lower emissions. However, achieving increasingly stringent source emission regulations that serve to protect the public health may necessitate the application of pollution control technology on AWC systems. In the present work, the effectiveness of a model electrostatic precipitator (ESP) in capturing the emissions from a 150 kW wood pellet boiler was assessed. Stack gas sampling was performed using the EPA conditional test method CTM039. Ash resistivity values were estimated using the Bickelhaupt model and measured ash composition, and were found to be consistent with the observed ESP performance. Size-dependent particle collection characteristics of the ESP revealed a slightly reduced collection in the size range of 0.2-1 μm and was independent of the boiler load. The ESP reduced particle mass to approximately 3 mg/m3 (0 °C, 101.325 kPa, dry gas and 13% O2) representing mass collection efficiencies of approximately 96-98%. It was concluded that ash resistivity would not be problematic for an ESP collecting dust from a typical AWC boiler.
advantages. They purport to provide over 90% energy efficiency and emission levels that are an order of magnitude lower than conventional wood-fired boilers.8 Nonetheless, achieving increasingly stringent source emission standards that serve to protect the public health may necessitate the application of appropriate pollution control technologies on AWC systems. Thus, it may be useful to install electrostatic precipitation (ESP) technology on the stack to collect the particles. ESPs are a well-defined technology9 that has been widely used in industrial applications particularly in coal-fired power plants to remove fly ash from the stack effluent. A detailed review of modern electrostatic precipitation technology is provided elsewhere.10 The performance of an ESP in terms of its collection efficiency is significantly influenced by both the ESP design and the process operating conditions.10-15 The most important of the process operating conditions is the resistivity of the ash to be collected.9,16 Ash resistivity describes the
1. Introduction The potential for human induced global warming and the likely contribution of CO2 emissions from fossil fuel combustion to the rising concentrations of greenhouse gases has now been recognized. With the recent variability in oil costs, there is an increased interest in diverse sources of energy including solid biofuels. Particularly in small and medium scale energy production (e10 MW of thermal power), a change from oil or gas to biomass fuels may be economically feasible, especially in areas where biomass fuel is readily available. However, conventional biomass combustion systems produce significant quantities of gaseous and particulate pollutants1-7 that make them undesirable particularly when considering public buildings such as schools. Thus, air quality issues have raised interest in fully automated, low emissions, high efficiency, gasification-type technologies (henceforth referred to as advanced wood combustion (AWC) systems). On the basis of European boiler designs, AWC boilers exhibit a number of
(8) ACT Bioenergy Boiler. Advanced Climate Technologies, LLC. Available online at http://actbioenergy.com/products.html (accessed on March 25, 2010). (9) U.S. Environmental Protection Agency (EPA). Electrostatic Precipitator (ESP) Training Manual, Report No. EPA-600/R-04-072, National Risk Management Research Laboratory: Research Triangle Park, NC, 2004. (10) Jaworek, A.; Krupa, A.; Czech, T. J. Electrostat. 2007, 65, 133– 155. (11) Jedrusik, M.; Swierczok, A.; Teisseyre, R. Powder Technol. 2003, 135 - 136, 295–301. (12) Yang, X.-F.; Kang, Y.-M.; Zhong, K. J. Hazard. Mater. 2009, 169, 941–947. (13) Jedrusik, M.; Swierczok, A. J. Electrostat. 2009, 67, 105–109. (14) Ahn, Y. C.; Lee, J. K. J. Aerosol Sci. 2006, 37, 187–202. (15) Navarrete, B.; Ca~ nadas, L.; Cortes, V.; Salvador, L.; Galindo, J. J. Electrostat. 1997, 39, 65–81. (16) White, H. J. Industrial Electrostatic Precipitation; Addison-Wesley Publishing Co., Inc: Reading, MA, 1963; pp 1-87.
*To whom correspondence should be addressed. Telephone: þ315268-3861. Fax: þ1-315-268-4410. E-mail:
[email protected]. (1) Johansson, L. S.; Leckner, B.; Gustavsson, L.; Cooper, D.; Tullin, C.; Potter, A. Atmos. Environ. 2004, 38, 4183–4195. (2) McDonald, J. D.; Zielinska, B.; Fujita, E. M.; Sagebiel, J. C.; Chow, J. C.; Watson, J. G. Environ. Sci. Technol. 2000, 34, 2080–2091. (3) Sippula, O.; Hytonen, K.; Tissari, J.; Raunemaa, T.; Jokiniemi, J. Energy Fuels. 2007, 21, 1151–1160. (4) Tissari, J.; Lyyr€ anen, J.; Hyt€ onen, K.; Sippula, O.; Tapper, U.; Frey, A.; Saarnio, K.; Pennanen, A. S.; Hillamo, R.; Salonen, R. O.; Hirvonen, M.-R.; Jokiniemi, J. Atmos. Environ. 2008, 42, 7862–7873. (5) Wierzbicka, A.; Lillieblad, L.; Pagels, J.; Strand, M.; Gudmundsson, A.; Gharibi, A.; Swietlicki, E.; Sanati, M.; Bohgard, M. Atmos. Environ. 2005, 39, 139–150. (6) Sippula, O.; Hokkinen, J.; Puustinen, H.; Yli-Pirila, P.; Jokiniemi, J. Energy Fuels 2009, 23, 2974–2982. (7) Hellen, H.; Hakola, H.; Haaparanta, S.; Pietarila, H.; Kauhaniemi, M. Sci. Total Environ. 2008, 393, 283–290. r 2010 American Chemical Society
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: DOI:10.1021/ef101031u
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resistance of the collected layer of dust to the flow of electrical current. Jaworek et al.10 observe that although dust of low resistivity (1011 Ω cm) dust forms a tight barrier on the collection electrode, causing an accumulation of electric charge that leads to reverse ionization9,10 and severely inhibits the ESP performance. ESPs operate effectively when ash resistivity is within the range of 107-1011 Ω cm. Ash resistivity could be problematic for ESPs installed on AWC boilers because (1) particles in the stack gas are inorganic salts with essentially no organic material. Dry inorganic salts possess very high resistivity; (2) the wood chips or wood pellets used as fuels in AWC systems are typically low sulfur fuels. Although these fuels provide low SO2 emissions and reduced corrosion of heat exchanger tubes,17 the potential problem of high ash resistivity may be important since ash resistivity generally increases as the ratio of sulfur to ash decreases;9,18 (3) because the particulate matter is essentially all inorganic salt particles, there could be deliquescence of these particles. If the stack relative humidity rises above the deliquescence point, there could potentially be formation of high ionic strength solutions with very low resistivity. The presence of liquid on the collection plates would be problematic causing short circuits and corrosion. Under such operational conditions, the ESP performance can be drastically reduced. This paper explores the ability of a model ESP to charge and collect the particles emitted from a state-of-the-art high-efficiency wood pellet boiler and examines the relative variability in total particulate matter emissions and fly ash chemical composition as functions of the boiler load.
heat input rates of approximately 70-147 kW. The boiler was operated with an automatic “lambda” control to optimize combustion air flows. The lambda control adjusts the combustion air fan speed based on the measured oxygen levels in the flue gas which varied from about 11 to 15%. Estimating Fly Ash Resistivity. The wood pellets were sampled at the beginning of each experiment and taken to the laboratory for fuel and ash analyses. The total sulfur in the fuel, the ash content, and the gross calorific value of the fuel were determined using ASTM methods E775-87, D1102-84, and E711-87, respectively. A sample of the wood pellets was pulverized in a cryomill and ashed at 575 °C in still air for 16-24 h. The elements in the ash were determined through acid digestion and inductively coupled plasmamass spectrometry (ICP-MS) analysis. Fly ash resistivity was estimated from the Bickelhaupt model.21 The correlations are given in terms of volume resistivity,22 surface resistivity,23 and the adsorbed acid resistivity. The correlations were evaluated by Bickelhaupt and Sparks24 and shown to provide reasonable agreement between measured and predicted values. The volume resistivity (Fv) is Fv ¼ expð - 1:8916 ln X - 0:9696 ln Y þ 1:237 ln Z 9980:58 þ 3:628 76 - 0:069 078E þ T
ð1Þ
The surface resistivity (Fs) is Fs ¼ expð - 2:233 348 ln X - 0:001 76W 2303:3 - 0:069 078E þ 27:597 74 - 0:00073895W exp T ð2Þ The adsorbed acid resistivity (Fa) (for Z > 3.5% or K < 1%) is 13049:47 - 0:069 078E þ 59:0677 Fa ¼ exp - 0:854 721CSO3 T ð3Þ
2. Experimental Methods
where X, Y, K, and Z are the percent atomic concentrations of Li þ Na, Fe, K, and Mg þ Ca, respectively. E is the applied electric field (kilovolt/centimeter), W is the stack gas moisture fraction (% H2O by volume), CSO3 is the concentration of SO3 (parts per million, dry basis), and T is the absolute temperature in Kelvin. The combined resistivity (Fvsa) is calculated using the equation for parallel resistances expressed in terms of resistivity: Fa Fvs Fvsa ¼ ð4Þ Fa þ Fvs
Description of the ESP and AWC Boiler. The ESP used in this study is a two-stage device originally designed for the differential mass measurement of ambient particulate matter.19 A detailed description and design specifications of the ESP is provided by Yi et al.20 Although the ESP is not a standard industrial style unit, its design specifications and performance allow for the exploration of the resistivity issue on a real AWC system. The ESP was set to operate at a corona current of approximately 80 nA when energized. The AWC system was a 150 kW Hamont CATfire (Hamont Consulting and Engineering, Austria) wood pellet boiler. The wood pellets are fed by an automated screw-auger up through the center of the burn plate in the combustion chamber. The fuel is heated to about 400 °C and undergoes gasification. The primary air necessary for this process is fed through holes on the burn plate. Above the burn plate, secondary and tertiary air are tangentially delivered to create intense turbulence and facilitate complete combustion at temperatures up to 1100 °C. The resulting hot gases are then sent through a heat exchanger section of the boiler where water is heated to supply heat. The boiler has a large heat exchanger surface area equipped with turbulators that ensure optimal heat transfer. In this work, the AWC boiler was configured to gasify wood pellets at nominal
where Fvs ¼
Fv Fs Fv þ Fs
ð5Þ
Sampling Procedure. Stack gas sampling was performed using the EPA conditional test method CTM-039 train.25 The dilution sampling train was leak-tested before each experiment. A maximum of 2% of the total flow was allowed for leakage at both the diluted and undiluted portions of the sampler. Before sampling, the probe and the sample venturi compartment were heated to at least 5.6 °C higher than the stack gas temperature to reduce particle loss from thermophoresis or condensation on the walls of the sampler. The average stack gas temperature was 126 ( 6.0 °C, (21) Bickelhaupt, R. E. A Technique for Predicting Fly Ash Resistivity. U.S. EPA Report No. EPA-600/7-79-204, 1979. (22) Bickelhaupt, R. E. Environ. Sci. Technol. 1975, 9 (4), 336–342. (23) Bickelhaupt, R. E. J. Air Pollut. Control Assoc. 1975, 25, 148– 152. (24) Bickelhaupt, R. E.; Sparks, L. E. Environ. Int. 1981, 6, 211–218. (25) U.S. Environmental Protection Agency (EPA). Conditional Test Method (CTM)-039, available online at http://www.epa.gov/ttnemc01/ ctm/ctm-039.pdf (accessed on March 25, 2010).
(17) Obernberger, I.; Brunner, T.; Barnthaler, G. Biomass Bioenergy 2006, 30, 973–982. (18) Harrison, W. A.; Nicholson, J. K.; DuBard, J. L.; Carlton, J. D.; Sparks, L. E. J. Air Pollut. Control Assoc. 1988, 38, 209–216. (19) Patashnick, H.; Rupprecht, G.; Ambs, J. L.; Meyer, M. B. Aerosol Sci. Technol. 2001, 34, 42–45. (20) Yi, S.-M.; Ambs, J. L.; Jeffrey, L.; Patashnick, H.; Rupprecht, G.; Hopke, P. K. Aerosol Sci. Technol. 2004, 38 (S2), 46–51.
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Figure 1. Dilution sampling schematics (dotted lines indicate the APS was not operated simultaneously with the SMPS and that sampling on 47 mm PTFE filters were performed upstream of the ESP separately from the number size distribution measurements).
medium, and low thermal inputs were tested in this study. The thermal inputs were obtained as functions of the fuel feeding rate and the fuel quality. The thermal inputs at high, medium, and low loads were 146.5, 103.3, and 75.6 kW, respectively. Samples for trace elemental composition of the particles were collected upstream of the ESP on 47 mm polytetrafluoroethylene (PTFE)-membrane filters. Trace elements were analyzed by X-ray fluorescence spectroscopy (XRF). Half of the prebaked 142 mm quartz fiber filters were analyzed for organic and elemental carbon (OC/EC) by the NIOSH 5040 protocol,26 while the other half were analyzed for water-soluble ions by ion chromatography (IC). The number and mass concentration results were normalized to unit volume of dry gas at 101.325 kPa, 0 °C, and 13% O2.
and the moisture fraction of the stack gas was 7.4% (% H2O by volume). Stack gas was isokinetically extracted at flow rates of 7-12 L/min from the center of a 25.4 cm duct through a PM2.5 cyclone. The flow was subsequently diluted and cooled with HEPA-filtered ambient air at flow rates of 300-350 L/min to give dilution ratios of 30-40. The average temperature and relative humidity of the exhaust at the end of the dilution train were 29.3 ( 1.6 °C and 7.1 ( 3.2%, respectively. Sample and dilution flow rates, temperature, and relative humidity (RH) of the dilution air and the diluted sample were recorded in real time using a computer-based data acquisition system. All flows were monitored during the test and adjusted as required. The ESP was installed downstream of the dilution tunnel as shown in Figure 1. While the bulk of the flow was drawn through a 142 mm prebaked quartz fiber filter to permit PM2.5 mass measurements, a subsample flow was drawn through sampling ports at the end of the dilution train. Particle number size distributions were determined using a scanning mobility particle sizer (SMPS) consisting of a differential mobility analyzer (DMA) (model 3071, TSI Inc., St. Paul, MN) and a condensation particle counter (CPC) (model 3775, TSI Inc., St. Paul, MN). The SMPS was operated in overpressure mode to sample particles of mobility diameter (dm) of size 0.01 < dm < 0.64 μm at 0.3 L/min. The particle aerodynamic (da) size distribution in the size range of 0.7 < da < 10 μm were measured using an aerodynamic particle sizer (APS) (model 3321, TSI Inc., St. Paul, MN). The APS and the SMPS instruments were connected downstream of the ESP using a switching valve but were not operated simultaneously. For each measurement with the APS or the SMPS, the ESP was alternately switched on (energized) and off (de-energized) in 5 min intervals. The collection efficiency (η) of the ESP was calculated as Con η ¼ 100 1 ð6Þ Coff
3. Results and Discussion Ash Resistivity. Table 1 shows the results of the fuel and ash analyses. The total sulfur content of the wood pellets was about 0.7%. The low sulfur content indicates that emissions of SO2 would not be significant for this particular boiler and fuel. The fuel ash was subjected to chemical analyses and the constituents are reported as oxides in weight percent. The weight percent values were totaled and normalized to 100%. The weight percentages were then converted to molecular fractions from which the molecular percent oxides and atomic percent cations were determined. The chemical constituents of the wood pellet ash showed very low concentrations of acidic compounds (Al2O3, Fe2O3) and high concentrations of MgO, K2O, and CaO. With the use of the equations presented above, resistivitytemperature calculations were made and plotted in Figure 2. At typical stack gas temperatures of approximately 120 °C, stack moisture fraction of 7.4%, and SO3 concentration of
where Coff and Con are the number (mass) concentrations at a given particle size measured with the ESP de-energized and energized, respectively. Different boiler load conditions representing high,
(26) Birch, M. E.; Cary, R. A. Aerosol Sci. Technol. 1996, 25, 221–241.
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Table 1. Fuel and Ash Characterization parameter Fuel Analysis moisture (%) ash (%) sulfur (%) gross calorific value (MJ/kg) Li2O (%) Na2O (%) K2O (%) MgO (%) CaO (%) Fe2O3 (%) Al2O3 (%) sum
Ash Analysis
5.04 0.64 0.72 18.8 0.06 0.47 30.70 20.61 43.77 1.00 0.77 97.36
Figure 3. Particle number size distribution measured by the SMPS (a) and the APS (b). Exhaust RH upstream of the ESP was ∼7%.
of the ash, and the analysis of the flue gas emitted from the AWC boiler. Particle Collection Characteristics of the ESP. Time-averaged, particle number size distributions as measured using the SMPS and APS are shown in Figure 3 (ESP de-energized periods). The number size distributions at the different boiler loads were unimodal with mode diameters of around 0.1 μm. The total number concentrations, the geometric mean diameters, and the total collection efficiencies based on the number concentrations are presented in Table 2. The total number concentration was about 3 107 number/N cm3 at low load and increased by about 40% when the boiler load was increased from low to high. The increase in number emissions was consistent with a slight decrease in the geometric mean diameter from about 0.12 to 0.13 μm in the low load conditions to approximately 0.1 μm in the high load conditions for particles of size 0.01
