Energy & Fuels 2006, 20, 1629-1638
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Assessment of Filter Dust Characteristics that Cause Filter Failure during Hot-Gas Filtration John P. Hurley,* and Biplab Mukherjee Energy & EnVironmental Research Center, UniVersity of North Dakota, Grand Forks, North Dakota 58202-9018
Michael D. Mann Department of Chemical Engineering, UniVersity of North Dakota, Grand Forks, North Dakota 58202-7101 ReceiVed September 21, 2005. ReVised Manuscript ReceiVed April 18, 2006
The high-temperature filtration of particulates from gases is greatly limited because of the development of dust cakes that are difficult to remove and can bridge between candle filters, causing them to break. Understanding the conditions leading to the formation of cohesive dust can prevent costly filter failures and ensure higher efficiency of solid fuel, direct-fired turbine power generation systems. The University of North Dakota Energy & Environmental Research Center is working with the New Energy and Industrial Technology Development Organization and the U.S. Department of Energy to perform research to characterize and determine the factors that cause the development of such dust cakes. Changes in the tensile strength, bridging propensity, and plasticity of filter dust cakes were measured as a function of the temperature and a filter pressure drop for a coal and a biomass filter dust. The biomass filter dust indicated that potential filtering problems can exist at temperatures as low as 400 °C, while the coal filter dust showed good filtering characteristics up to 750 °C. A statistically valid model that can indicate the propensity of filters to fail with system operating conditions was developed. A detailed analysis of the chemical aspect of dusts is also presented in order to explore the causes of such stickiness.
Introduction Various adhesion mechanisms, such as van der Waals attraction, solid and liquid bridging, mechanical forces, and electrostatic forces, have been identified as contributing to the stickiness of filter dust particles at high temperatures. The extent to which these mechanisms will play a role depends on the particle size and shape distribution, cake compactness, aerodynamics of deposition, properties of fuel and bed material, gas composition, temperature, a pressure drop across the cake, and properties of the filter material. Off-line studies to identify the dependence of filter cake tensile strength as functions of filter operating parameters can be used to modify system operations to improve filter performance and prevent costly filter failure during hot-gas filtration. A cake with a tensile strength of less than 50 N/m2 shows a tendency to re-entrain after being backpulsed from a fabric filter, while a cake with a strength of 300 N/m2 or greater shows a strong tendency to bridge between filters, which could lead to the breakage of brittle candle filters.1 Although there is some variation from cake to cake, a tensile strength within this approximate range should enhance operation of the candle filter system. It is also known that the development of strength within a cake changes with operating conditions. By choosing conditions which produce a cake tensile strength in the optimum range * Corresponding author. Phone: (701) 777-5159. Fax: (701) 777-5181. E-mail:
[email protected]. (1) Miller, S. J.; Laudal, D. L.; Heidt, M. K. Cohesive Properties of Fly Ash and How They Affect Particulate Control Optimization. Presented at the 10th Particulate Control Symposium, Washington, DC, April 5-8, 1993.
for that cake, filter operation should be optimized and candle failure reduced, thereby reducing downtime. Of all of the factors that affect filter cake stickiness, the temperature and maximum pressure drop across the cake are two that can easily be controlled by plant operators. As such, a relationship indicating the effects of these two parameters on the tensile strength of a particular filter dust can be used to maximize the power system efficiency while filter effectiveness and availability are maintained. The goal of the work reported here was to determine the effect of temperature and a pressure drop on the tensile strength and plasticity of two filter dusts: one collected from the hot-gas filter of the Electric Power Development Company (Japan) Wakamatsu pressurized fluidized-bed combustion demonstration plant while burning Blair-Athol coal in the summer of 1997 and the other from Karlsruhe University (Germany), which was produced in an industrial combustor while firing biomass. This information is then used to calculate the specific strength or critical thickness index (CTI), which is the maximum thickness that the ash cake would form on the underside of a horizontal surface before breaking loose under its own weight, possibly leading to bridging between filters and filter breakage. In addition, we also present a statistical analysis to formulate equations that can predict the change of tensile strength, elongation, and CTI with temperature and a pressure drop across the two types of cake that could be used in conjunction with in situ observations to indicate appropriate operating conditions to minimize dust re-entrainment and filter bridging.
10.1021/ef050303k CCC: $33.50 © 2006 American Chemical Society Published on Web 05/20/2006
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Background A worldwide energy demand growth from 13 204 TWh in 1995 to 27 326 TWh in 2020 has seen the emergence of several more efficient types of power generating systems than the typical pulverized coal-fired boiler.2 The solid-fuel direct-fired turbine power generation system is one such technology. A proper utilization of solid-fueled direct-fired technologies requires efficient particulate removal from the gas flow upstream of the turbine while the system temperature is kept at its highest. This arrangement not only ensures the maximum power output but simultaneously minimizes problems of fouling, erosion, and corrosion of turbine parts. This filtration method reduces sulfur and alkali emissions.3 Because of their ability to withstand high temperatures and high pressure drops combined with a large surface area per unit volume, ceramic candle filters are being widely used in such a design. Dust cake that builds up on the ceramic filters during the operation is periodically cleaned by a backpulse operational procedure. The dust cake is basically a mixture of coal ash (formed by the complex interaction of various inorganic elements and compounds present in the fuel source), reacted and unreacted sorbents (in most cases, calcium carbonate that is added to the system for sulfur removal), and unburned fuel. Backpulsing does not always ensure complete removal of the dust cake. If the cake is too sticky, it may hang onto the filter wall. If the stickiness is too low, re-entrainment of the dust back onto the filter occurs once the pulse is over. The magnitude of the cake strength depends on the particulate adhesion mechanism, which is a function of the fuel (coal-biomass) composition, particle-particle interactions, particle-filter surface interaction, face velocity, gas composition, temperature, and pressure drop across the candles.4 An accurate understanding and prediction of the zones of stable filter operation to prevent such problems has been restricted because of the involvement of several adhesive mechanisms interacting in a complex manner.5,6 The properties of the fuel and ash are important for estimating cake buildup and can vary significantly between fuels. Generally, nearly 50% of the inorganics in coal can be termed as clay particles that are basically aluminosilicates.7 Illite, montmorillonite, kaolinite, and complex mixtures of these in various portions are the major form of clay present in the coal.7 Another large fraction of the inorganics is quartz particles (silica). Most of the carbonates are present as calcite, dolomite, ankerite, and a combination thereof. Pyrite, galena, sphalerite, and marcasite are the major sulfides present. Inorganics are also present as cations bound to the organic structure of the fuel. This is especially true for low-rank coals. In the combustor/gasifier, the inorganics undergo fragmentation, coalescence, vaporization, and condensation. The greatest changes occur in the inorganic (2) International Energy Agency. World Energy Outlook 1998, 64, http:// library.iea.org/dbtw-wpd/textbase/nppdf/free/1998/weo98.pdf (accessed June 27, 2004). (3) Al-Otoom, A. Y.; Ninomiya, Y.; Moghtaderi, B.; Wall, T. F. Coal Ash Buildup on Ceramic Filters in a Hot Gas Filtration System. Energy Fuels 2003, 17, 316-320. (4) Hemmer, G.; Hoff, D.; Kasper, G. Thermoanalysis of Fly Ash and Other Particulate Materials for Predicting Stable Filtration of Hot Gases. AdV. Powder Technol. 2003, 14 (6), 631-655. (5) Hurley, J. P.; Dockter, B. A. Factors Affecting the Tensile Strength of Hot-Gas Filter Dust Cake. AdV. Powder Technol. 2003, 14 (6), 695705. (6) Hurley, J. P.; Nowok, J. W.; Bieber, J. A.; Bruce, D. A. Strength Development at Low Temperatures in Coal Ash Deposits. Prog. Energy Combust. Sci. 1998, 24, 513-521. (7) Berkowitz, N. An Introduction to Coal Technology; Academic Press: New York, 1979.
Figure 1. High-temperature tensile tester. Table 1. Experimental Matrix Different Ashes Karlsruhe filter dust
Wakamatsu filter dust
temp., °C
∆P, cm of H2O/cm of cake thickness
temp., °C
∆P, cm of H2O/cm of cake thickness
500 500 500 400 400 400 300 300 300
35 17 9 35 17 9 35 17 9
800 800 800 715 715 715 550 550 550
107 85 65 107 85 65 107 85 65
matter associated with the fuel because of the much higher temperatures those particles reach because of combustion of the organic matrix in which much of the mineral matter resides. The degree of chemical and physical change experienced by the particles is dependent on the chemical composition of the mineral particles, whether they are included within the organic material, the temperature they reach, and the local gas composition. It is the combination of these minerals during hightemperature combustion or gasification that results in the formation of the complex ash compounds that cause problems during hot-gas filtration. At temperatures over 1000 °C, glassy silicate-based compounds are usually responsible for the stickiness of ash deposits, but at the lower temperatures of hot-gas filters, sulfates play a significant role and are a common occurrence in high-calcium ashes.6 Although the actual mechanism is still unclear, the decomposition of calcium carbonate has been associated with the increase of cohesive forces in powder cakes.5 In most cases, low-temperature strength development can be explained by the formation of eutectics.6 Shear tests show a noticeable increase of adhesion at temperatures higher than 850 °C, while a 3-fold increase was seen in the presence of limestone with CO2 in the gas phase.8 The importance of liquid in the enhancement of tensile strength has been shown by Hurley and Dockter.5 The same also presents the concept of CTI as a proper indicator to characterize the bridging propensity of a particular filter dust.4 A split cell-type test apparatus was used by Tsukada et al. to report the change in tensile strength of biomass ash.9 They found the tensile strength development in biomass powder to be biphasic. While the presence of a liquid surface was observed (8) Kanaoka, C.; Hata, M.; Makino, H. Measurement of Adhesive Force of Coal Flyash Particles at High Temperatures and Different Gas Compositions. Powder Technol. 2001, 118 (1-2), 107-112. (9) Tsukada, M.; Yamada, H.; Kamiya, H. Analysis of Biomass Combustion Ash Behavior at Elevated Temperatures. AdV. Powder Technol. 2003, 14 (6), 707-717.
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Figure 2. Tensile stress changes for Karlsruhe filter dust.
Figure 3. Tensile stress changes for Wakamatsu filter dust.
within a temperature range of 400-700 °C, it was absent above 700 °C, probably because of evaporation or decomposition during which the cake turns brittle.9 Pure fine silica and two kinds of fly ash were tested by Kamiya et al. while investigating the development of cohesive forces between fine particles under elevated temperatures.10 A linear relation in the development of cohesive force with temperature was observed up to 727 °C, after which a sudden increase in strength was noticed, believed to be controlled by viscous flow sintering.10 Studies on filtration cycles using a ceramic filter with various ashes and pure powders at different temperatures have shown the possibility of metastable filter operation zones.11 Models have also been developed for a better understanding of the flow and deposition process in the filter system. On the
basis of computational fluid dynamics, the effect of the ratio of approach cross-flow velocity to face velocity on deposition in relation to particle size has been presented by Al-Hajeri et al.12 Probabilistic models presented by Duo et al. attempt to describe the area distribution of different-aged cake patches as well as velocity, pressure drop, and thickness distribution variation with time.13 Studies have indicated patchy cleaning as one of the major causes hindering the cleaning efficiencies of the filters.11 Probabilistic models have been successfully implemented in the simulation of conditioning curves to determine filtration efficiencies.14 To predict the mechanism in which this ash forms a filter cake, it is not just a matter of understanding all of the ways in which particles can attract one another but also the way in which
(10) Kamiya, H.; Kimura, A.; Yokoyama, T.; Naito, M.; Jimbo, G. Development of a Split-Type Tensile-Strength Tester and Analysis of Mechanism of Increase of Adhesion Behavior of Inorganic Fine Powder Bed at High-Temperature Conditions. Powder Technol. 2002, 127 (3), 239245. (11) Hemmer, G.; Hoff, D.; Kasper, G. Thermoanalysis of Fly Ash and Other Particulate Materials for Predicting Stable Filtration of Hot Gases. AdV. Powder Technol. 2003, 14 (6), 631-655.
(12) Al-Hajeri, M. H.; Aroussi, A.; Simmons, K.; Pickering, S. J. A Parametric Study of Filtration Through a Ceramic Candle Filter. Proc. Inst. Mech. Eng., Part A 2005, 219 (A1), 77-90. (13) Duo, W.; Kirkby, N. F.; Seville, J. P. K.; Clift, R. Patchy Cleaning of Rigid Gas Filters. 1. A Probabilistic Model.Chem. Eng. Sci. 1997, 52 (1), 141-151. (14) Duo, W.; Seville, J. P. K.; Kirkby, N. F.; Buchele, H.; Cheung, C. K. Patchy Cleaning of Rigid Gas Filters. 2. Experiments and Model Validation. Chem. Eng. Sci. 1997, 52 (1), 153-164.
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Figure 4. CTI changes for Karlsruhe filter dust.
Figure 5. CTI changes for Wakamatsu filter dust.
the particles fit together in the bulk. Any attempt to sum the microscopic contributions made by the particles themselves must take into account three factors, all associated with the nature of the particle surface: the interparticle forces, the threedimensional shape of the particle, and the way in which the particle interacts geometrically to form a packed structure. Most ashes are not easily characterized in terms of microscopic quantities. Consider that, if the size of a single particle is given as its equivalent spherical diameter, the definition of particle size by a single parameter becomes somewhat arbitrary, and in any theoretical study, it will be necessary to use a description of the particle that will define its shape equivalencies. The situation is no less complicated with the interparticle forces which are subject to changes in the environment and the effect of their previous history. In real ashes, these forces can be a variety of types, including the following:15mechanical forces caused by interlocking particles; liquid bridging caused by either capillary forces or viscous flow sintering; electrostatic forces, particularly for surfaces which become easily charged; solid (15) Berbner, S.; Loffler, F. Influence of High Temperatures on Particulate Adhesion. Powder Technol. 1994, 78, 273-280.
bridge forces, where crystallization at contact points causes joining of the particles; and molecular (or van der Waals) forces, particularly significant for particles of small diameters, typically less than 10 µm. High combined interparticle forces in filter dusts can cause a cake buildup that results in poor cleaning. This increases the pressure drop across the filters and concurrently adds to several other problems, notably, the problem due to bridging between adjacent filter elements. Bridges often form when thick cakes build up on surfaces near the top of the filter and break free to fall and lodge between filter elements. Bridges between the candles can build and force the elements apart to the point of failure, allowing dusty gas to pass through to the main turbine chamber, causing erosion and fouling of the turbine blades and, ultimately, resulting in downtime. It is believed that the region near the top end of the filter that is supported by the plenum is prone to cake buildup owing to the negative pressure drop during backpulsing. Whether the cake forms bridges depends on the strength developed within it and the extent to which it can support itself against gravitational pull. When its weight exceeds
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Figure 6. EL changes for Wakamatsu filter dust. Table 2. XRF Analysis of Wakamatsu and Karlsruhe Filter Dust dust composition, normalized wt % as oxides
Wakamatsu coal dust
Karlsruhe biomass dusta
CaO MgO Na2O SiO2 Al2O3 Fe2O3 TiO2 P2O5 K2O SO3
28.6 0.8
