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Ind. Eng. Chem. Res. 1999, 38, 260-269
The Role of the Filter Cake in Hot Gas Cleaning with Ceramic Filters Wenli Duo, John R. Grace,* C. Jim Lim, Clive M. H. Brereton,† A. Paul Watkinson, and Karin Laursen Department of Chemical and Bio-Resource Engineering, University of British Columbia, 2216 Main Mall, Vancouver, British Columbia, Canada V6T 1Z4
Experiments were carried out in a pilot-scale test rig at temperatures of 600-770 °C to investigate the potential of filter cakes formed in hot gas filtration to remove sulfur dioxide, nitric oxide, and alkalis. The results demonstrate that a filter cake of fly ash particles is capable of contributing to SO2 capture, particularly with injection of a fresh sorbent. A filter cake of mixed limestone and alumina particles formed at 600 °C showed a higher resistance to flow than one formed at 700 °C. The efficiency of SO2 removal increased with the temperature over the range investigated. Both the cake and filter absorbed alkalis by chemical reaction, showing that the filter cake will help to protect the gas turbine, as well as the filter itself, against alkali attack. NOx emissions were not affected by the presence of the filter in an oxidizing atmosphere, while a considerable reduction of NO was obtained in the presence of CO. Introduction Coal resources are abundant. Effective long-term usage of coal is strategically critical to energy security and global economics, particularly for electricity generation. However, production of electricity from coal and other fuels must be accomplished as efficiently and cleanly as possible. Advanced power systems, such as the integrated gasification combined cycle (IGCC) and pressurized fluidized-bed combustion (PFBC), requiring gas turbines, are widely anticipated as future technologies to produce electricity and steam from coal and other hydrocarbon fuels with greater efficiency (and hence reduced-CO2 generation) and reduced emissions of sulfur dioxide and other pollutants. Removal of particulate and other emissions is required for these processes to protect the gas turbines against erosion and corrosion. To maintain a high thermal efficiency, the gas cleaning should be carried out at high temperatures (at least 350 °C for IGCC and 750 °C for PFBC). After rapid development over the past decade, ceramic barrier filters have emerged as the most promising choice for hot gas cleaning1,2 because of their resistance to attack by aggressive gases and their ability to withstand high temperatures up to 1000 °C. A recent survey3 of 34 organizations, including utilities, filter manufacturers, power generation system constructors, government agencies, universities, and research institutes, indicated a 65% preference for high-temperature gas filtration technologies over conventional low-temperature separators. Rigid barrier and ceramic fabric filters were favored for particulate removal at high temperatures and high pressures. Long-term durability, alkali corrosion, cleanability, thermal shock, and particulate penetration into filter media were recognized as major concerns, requiring tests of ceramic filters in pilot-scale facilities at high temperatures. * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: (604) 822 3121. Fax: (604) 822 6003. † Current address: Noram Engineering and Constructors, 400-200 Granville Street, Vancouver, British Columbia, Canada V6C 1S4.
The performance of ceramic filters has been investigated extensively under various conditions.4-6 The work done in our laboratory3,7,8 confirmed excellent particle collection and cake detachment at high temperatures for several types of ceramic filters. In pursuit of energy, space, and cost savings, it is also desirable that, in addition to particulate separation, other pollutants including acid gases (e.g., SO2 and HCl), nitrogenous species (e.g., NOx) and metal fumes be removed in the same unit. In PFBC, a sorbent, usually limestone or dolomite, is added to the coal to capture sulfur in situ. However, complete removal of sulfur and complete conversion of the sorbent is never achieved. The remaining sulfur, mostly in the form of SO2, leaves the combustion chamber together with the nonreacted sorbent and ash particles carried by the flue gas. These particles can be collected in a downstream filter unit forming a sorbent-containing filter cake, while the SO2containing flue gas passes through the cake. With the application of ceramic filters at high temperatures, further sorption of SO2 is possible in the filter cake. In fact, it may be attractive to apply ceramic filters also in conventional (pulverized or fluidized-bed) combustion and incineration units for the combined removal of solid and gaseous emissions, rather than removing them individually in separate units. This paper considers the role of the filter cake in the capture of SO2, NO, and alkalis. A filter cake can contribute to the capture of sulfur dioxide by reaction at high temperatures in the cake with powder sorbents entrained from the combustion chamber or injected upstream of the filter.9-11 This process is sometimes called dry scrubbing.12 With calcium compounds as sorbents for SO2, the following calcination and sorption reactions occur:
CaCO3 S CaO + CO2
(1)
CaCO3 + SO2 + 1/2O2 S CaSO4 + CO2
(2)
CaO + SO2 + 1/2O2 S CaSO4
(3)
10.1021/ie980300d CCC: $18.00 © 1999 American Chemical Society Published on Web 11/25/1998
Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999 261
Figure 1. Schematic of the UBC hot gas filtration rig.
It is well-known that the calcination reaction (1) is promoted by increasing temperature. The kinetics of the sorption reactions ((2) and (3)) have been studied extensively in entrained flow,13,14 in fixed-bed reactors,15,16 and by thermogravimetric analysis.17,18 However, sulfur capture in filter cakes over many filtration cycles has not been extensively investigated. Particulates entrained from fluidized-bed combustors contain alumina, silica, calcium, and carbon, as well as other species. It has been reported that heterogeneous reactions, catalytic and noncatalytic, are involved in the formation and destruction of NO, particularly with char and calcined limestone under fluidized-bed conditions.19-21 In the presence of carbon particles, the following competing reactions can occur:
2NO + C S N2 + CO2
(4)
C + O2 S CO2
(5)
C + 1/2O2 S CO
(6)
CO + NO S 1/2N2 + CO2
(7)
Thermodynamic calculations indicate that for typical flue gas conditions it is also possible for NO to decompose according to
2NO S N2 + O2
(8)
However, the kinetic rates of the above reactions occurring in filter cakes have not been documented in the literature. Flue gases also contain alkali components. A problem with pressurized fluidized-bed combustion and gasification of coal is that the alkali metals in the flue gas corrode the gas turbine. For example, Lee and Carls22 reported an average sodium vapor concentration of 1.31.5 ppm by weight in the PFBC flue gas of Beulah lignite, which is more than 50 times greater than the currently suggested alkali specification limit (0.024 ppm). Emphasis of previous studies has been laid on the prevention of alkali deposition on gas turbines.23 However, it is not well-known whether the exposure of
ceramic filters to alkali vapor affects their properties. Potentially, alkalis can be adsorbed physically or chemically by the filter and cake, both of which contain aluminosilicates. For chemical adsorption, the following reaction occurs:24
2MCl(g) + Al2O3‚xSiO2(s) + H2O(g) S M2O‚Al2O3‚xSiO2(s) + 2HCl(g) (9) where M represents Na or K. It is important to determine the role of filter cakes in capturing alkalis and hence in protecting the filter medium and downstream components. Experimental Section Experiments were carried out in a pilot-scale hot gas filtration unit, shown schematically in Figure 1. Only a brief description is given here, as more details are available elsewhere.3,7,8 Hot flue gases of different flow rates and temperatures were generated by a natural gas burner. The filter housing can hold three ceramic candle filters, each with an outside diameter of 60 mm and length of 600 mm. Constant temperatures were maintained with the help of an electrical furnace surrounding the housing. Ash and sorbent particles were metered into the flue gas stream by a table feeder via a motor-driven rotary valve and ball valve. The filters were cleaned by reverse pulse jets of air. With the aid of a computerized data-logging and process control program, the experimental system operates nearly automatically. The absolute and differential pressures, temperatures at various locations in the systems, gas flow rate, and solid feeding rate (precalibrated) are monitored and recorded continuously during each test. The cleaning action is actuated by three solenoid valves, with the timing and duration of the reverse pulses controlled by a timer. When the pressure drop across the filter reaches a preset value, the solenoid valves are opened for a prescribed duration to clean the filters. Rigid, low-density aluminosilicate fibrous filters with o.d. ) 60 mm, i.d. ) 40 mm, and length ) 600 mm (FIBROSIC 2800, Universal Porosics) were used in the
262 Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999 Table 1. Chemical Composition and Physical Properties of the Dust and Filter chemical composition
properties
material
CaO (%)
Fe2O3 (%)
K2O (%)
Na2O (%)
Al2O3 (%)
SiO2 (%)
P (%)
alumina limestone Pennsylvania coal ash Syncrude coke ash filter, FIBROSIC 2800
0.01 56.15 6.81 9.21 0.03
0.04 0.13 5.01 9.66 0.07
0.04 0.04 1.12 1.55 0.04
0.50 0.12 0.49 1.30 0.24
96.50 0.23 17.41 18.98 34.35
0.19 1.40 44.29 42.27 62.48
0.0 0.0 0.15 0.11 0.0
C
(%)a
0.0 12.5 18.4 3.4 0.1
S (%)
dP,50 (µm)
FB (kg/m3)
0.00 0.09 1.01 2.60 0.03
8.0 21 18.9 6.2 1-5b
597 906 302 523 270
a The carbon is present in carbonate form in limestone but in organic form in the ashes. b Diameters of filter fibers estimated using SEM images.
Figure 2. Cumulative particle size distributions of the dust particles.
experiments. Three types of dust were tested under oxidizing conditions. One consisted of mixtures of 62% alumina and 38% limestone particles, used as a sorbent for SO2, while the other two dusts consisted of fly ash particles generated in the UBC circulating fluidizedbed combustion (CFBC) pilot plant from the combustion of Pennsylvania coal and Syncrude coke, respectively. The chemical composition, mass median particle diameter (dP,50), and bulk density (FB) of the solids appear in Table 1. As indicated in Table 1, the Pennsylvania coal ash and Syncrude coke ash have high contents of calcium, iron, and sulfur. Both calcium and iron oxides can adsorb sulfur under the present conditions. However, the reactivity of the calcium and iron species contained in the ashes was not known. For quantitative experiments, alumina powder, instead of real fly ash, was therefore used as the diluent of the sorbent for SO2 capture in filter cakes. To prepare each sorbent, coarse particles of alumina and Texlime (a British Columbia limestone from Texada Island) were ground separately in a jar mill into fine powders, with particle size distributions as shown in Figure 2. The alumina powder and the Texlime powder were then mixed in the jar mill according to the desired composition. The table feeder was calibrated for individual powders. SO2 was introduced from pressurized gas cylinders at a constant flow rate to the flue gas in the postcombustion zone before the ash injection point to give 5001000 ppm of SO2 by volume. To enhance the mixing of SO2 with the flue gas, the SO2 stream was premixed with air, with the latter having a flow rate 30-50 times larger than that of SO2. The main stream of filtered flue gas was cooled in a heat exchanger before entering a wet scrubber containing a Na2CO3 solution to ensure the complete removal of SO2 emissions from the stack
gas. The sampling gas was kept hot by heating the sampling line using electrical heating tape to prevent vapor condensation until the sample gas reached a specially designed vapor condenser and water drainer immersed in an ice bath. The sampling gas was further dried by passing it through a packed bed of magnesium perchloride [Mg(ClO4)2] particles before being sent to the SO2 analyzer for continuous measurement of SO2 concentrations. The measured concentrations were also logged by a computer. The experimental setup and test procedures for NO removal were similar. A combustion gas analyzer with fuel cells as the sensors was used to measure the concentrations of NO, NO2, O2, CO, and CO2 in the flue gas. To test for alkali capture, NaCl and KCl were added to the flue gas in the form of aqueous solutions of 0.6 and 1.7 wt %, respectively. They were injected in the postcombustion zone, where the temperature was higher than that in the burner and the filter housing, to ensure complete evaporation. The solutions were injected continuously at a constant rate such that the partial pressure of the injected alkalis was equal to the equilibrium partial pressure of that species at the given temperature. The contents of Na, K, and Cl were measured before and after the experiments in the samples of ash (chemically) and filters (both chemically and by EDX). Since gas-solid reactions are usually sensitive to temperature, it is necessary to know the temperature distribution in the filter chamber. In the present work, axial temperature profiles were measured in the filters at different positions under steady-state conditions. Although some temperature variation was observed, particularly near the open (top) end of the filters, the difference was generally within (10 °C. Temperatures reported below are average values over the length of the filter candles. Results and Discussion Experiments were carried out over the temperature range of 600-770 °C. The flue gas from burning natural gas typically contained 9% O2, 7% CO2, and 20 ppm NO by volume with negligible CO. For complete combustion, the water vapor content was estimated to be 13 vol % with injection of the alkali solution and 11% otherwise. SO2 Removal. Tests for SO2 removal lasted typically 6-10 h, sufficient to cover a number of filtration cycles (8-20 cycles). It was observed that the SO2 concentration downstream of the filters decreased gradually during each filtration cycle as the sorbent-containing cake built up, until the end of the cycle when a backpulse of air caused the cake to detach. The removal of the cake caused the SO2 breakthrough to increase sharply before it started to decrease again as the filter
Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999 263
Figure 3. Effect of temperature on SO2 removal. Face velocity ) 5 cm/s; Ca:S molar ratio ) 4.
Figure 4. Effect of temperature on SO2 removal. Face velocity ) 5 cm/s; initial concentration of SO2 ) 1000 ppm; Ca:S molar ratio ) 2.
cake began to accumulate on the filter surface in the new cycle. This process resulted in a cyclical variation of SO2 emissions with time, as shown in Figs 3-5. The average SO2 removal efficiency was found to increase significantly with increasing temperature and Ca:S molar ratio for the range of conditions studied. Effect of Temperature. The effect of temperature on SO2 removal is shown in Figures 3 and 4 for Ca:S molar ratios of 4 and 2, respectively. The same face velocity and approximately the same initial concentrations of SO2 and O2 were used in both cases. The effectiveness of SO2 removal increases dramatically with increasing temperature from 600 to 705 °C. Further elevation of the temperature to 770 °C resulted in a slightly better
performance. The substantially lower efficiency at 600 °C is attributed to a combination of a slower chemical reaction, slower product layer diffusion of SO2 gas, and, particularly, lesser calcination of limestone. Thermodynamic calculations indicate that limestone does not calcine at 600 °C under flue gas conditions (e.g., 10% CO2). Uncalcined limestone is usually nonporous and has only a small surface area available for gases to contact, and hence the sorption of SO2 by reaction 2 is slow. The surface area of the calcined sorbent may be 10-30 times larger.16 At 705 °C, calcination occurs and SO2 can be adsorbed rapidly by calcined limestone (eq 3). A removal efficiency of 85% was obtained at 705 °C for a Ca:S molar ratio of 4, filtration face velocity of 5
264 Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999
Figure 5. Effect of Ca:S molar ratio on SO2 removal. Temperature ) 705 °C; initial concentration of SO2 ) 1000 ppm.
Figure 6. Pressure drop curves corresponding to SO2 breakthrough curves in Figure 4. Face velocity ) 5 cm/s; feeding rate of Texlime + alumina ) 12.4 g/min. Curve a: 600 °C. Curve b: 705 °C.
cm/s, and cycle duration of 35 min. This confirms excellent performance of ceramic filters for the combined removal of particles and acid gases. The effect of temperature on the cycle duration is shown in Figures 3 and 4 for SO2 breakthrough curves and in Figure 6 for pressure drop curves. Approximately the same cycle duration was observed in the tests at 705 and 770 °C. However, the cycle duration at 600 °C was clearly shorter (ca. halved) than those at
the higher temperatures. This is apparently due to the fact that both the baseline pressure drop and the rate of pressure drop increase are significantly higher for filtration at 600 °C, as shown in Figure 6. Poor cleaning is usually the reason for a high baseline pressure drop. These observations suggest that a denser filter cake was formed at 600 °C, with a higher resistance to flow and a larger bonding force to the filter surface. The fundamental difference is that the filter cake formed at 600 °C contained uncalcined limestone particles with a lower degree of sulfation, whereas those formed at 705 and 770 °C contained calcined limestone particles with a higher degree of sulfation. This suggests that calcined limestone is a much better sorbent than uncalcined limestone in terms of both SO2 removal and filtration performance. Inefficient filter cleaning may be due to “uniform cleaning” with a residual dust layer or “patchy cleaning”.25,26 A shorter cycle duration means a shorter reaction time available for sorbent particles deposited on the filter in the earlier stages of the cycle. However, inefficient “uniform cleaning” results in a thicker layer of particles on the filter surface after cleaning. An extended reaction time would be available for these residual sorbent particles. If patchy cleaning occurs, the removal of SO2 should be enhanced.27 Because of these contradicting factors, the effect of cycle duration on SO2 removal cannot be determined from the limited experimental results obtained in the present study. More work is needed to investigate the mechanism of cake cleaning at high temperatures. Effect of Ca:S Ratio. Figure 5 shows the effect of the Ca:S molar ratio on SO2 emissions. As expected, the higher the ratio, the more SO2 that is removed. For a very high molar ratio of 16, almost no SO2 breakthrough occurred. Also, the amplitude of the SO2 concentration variation became smaller at higher Ca:S ratios. The different cycle durations shown in Figure 5 were caused by different feeding rates of particles and different face velocities.
Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999 265
Figure 7. Effect of the ceramic filter and filter cake of Texlime + alumina on NO emissions. Temperature ) 703 °C; face velocity ) 5.05 cm/s; oxygen concentration ) 9 vol %.
Figure 8. Effect of Pennsylvania coal ash filter cake on NO emissions. Temperature ) 702-719 °C (increasing over time), face velocity ) 3.1 cm/s; oxygen concentration ) 5-2 vol % (decreasing over time).
It should be noted that the Ca:S molar ratios in Figure 5 were calculated from the solid feeding rates and the initial concentration of SO2. However, not all particles injected were carried onto the filter, with a portion of those injected dropping directly to the bottom of the filter housing or depositing on the housing wall. The estimated average residence time of gas in the volume between the solid injection point and the filter surface is about 3 s for a face velocity of 5 cm/s. If it is assumed that the solid particles entrained by the gas had similar residence times in this volume, the gas-solid reaction in the entrained flow would be minor. Thus, those particles that dropped to the bottom were virtually excluded from participating in the SO2 capture. The part of the dust deposited on the filter is defined as the percent deposition, η, which is a function of equipment design and operating conditions. Duo et al.26 reported values of η for limestone particles with a mean diameter of 5 µm of 26.5, 41.4, and 46% at face velocities of 4, 7, and 10 cm/s, respectively, in a single candle filter
rig at ambient temperature. Few data are available in the literature for high-temperature operation because of practical difficulties. In the present study, one test was performed to measure the deposition percentage for Texlime particles at 700 °C and a face velocity of 5 cm/ s. The result was η ) 56.5% with the particles depositing on the housing wall excluded (η ) 66.2% if included). After correction using the η value of 56.5%, the effective Ca:S molar ratios in Figure 5 should be 1.13 and 2.26, respectively. The η value corresponding to the highest Ca:S ratio is expected to be smaller since the face velocity was lower. Effect of Initial Concentration. Little effect was observed on an overall SO2 removal of the initial concentration of SO2 in the 500-1000 ppm range for the same Ca:S molar ratio. NO Reduction. The possibility of NO reduction by filter cakes was examined using slightly different experimental procedures. Two different dusts were tested. One, a mixture of alumina and Texlime, con-
266 Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999
tained no carbon, whereas the other, Pennsylvania coal ash, contained 18% carbon. Only single cycle tests were performed. NO emissions in the absence and presence of a cake are reported for comparison. Cake of Alumina/Texlime. Although reaction 8 leading to the decomposition of NO is thermodynamically favored under the present conditions, it is well-known that gas-phase decomposition is kinetically negligible. The decomposition may only be possible in the presence of a suitable catalyst. Possible catalysis of the ceramic filter and filter cakes of alumina/Texlime on NO decomposition was examined in the present study. The experiments were carried out at 703 °C with an inlet concentration of NO of 510 ppm. Emissions of NO, NO2 CO, CO2, and O2 were measured continuously. Initially, the filters were clean, as shown in Figure 7 by the relatively low-pressure drops between times 9:18 and 9:38. Solids injection was then started and continued at a constant rate of 6.2 g/min until just before 11:55, when the pressure drop reached the preset value of 400 mmH2O. This pressure drop was maintained until the end of the experiment, as shown by the pressure drop bars at 11:55 and 12:18 in the figure, indicating that a stable filter cake was formed. However, the NO emission level remained approximately the same throughout the entire run. Constant concentrations of NO2, CO, CO2, and O2 were observed also, with values of 20 and 0 ppm, 7 and 9%, respectively. These results suggest that neither the filter nor the cakes have any appreciable catalytic effect on NO decomposition under oxidizing conditions. Cake of Pennsylvania Coal Ash. The effect of a cake of Pennsylvania coal ash on NO emissions is shown in Figure 8. Doped with NO from a gas cylinder, the inlet flue gas contained 550 ppm NO, 14 ppm NO2, 2 ppm CO, 9% CO2, and 5% O2. Initially, the filters were clean, as shown by the lower pressure drops between times 16:28 and 16:33. The filter temperature was initially maintained at 702 °C before solids injection. However, with continuous injection of Pennsylvania coal ash at a constant rate of 8.4 g/min, the temperature increased to a maximum value of 719 °C as a result of burning the residual carbon in the system and the oxygen concentration dropped to 2%. Solids injection stopped when the pressure drop reached the preset maximum of 400 mmH2O. The five pressure drop bars in the figure indicate that a filter cake was being built between 16:33 and 18:21. The cake remained on the filter surface between 18:21 and 18:56 with a constant cake thickness. A 20% variation of NO emissions was observed during the cycle, with up to 500 ppm CO being formed. As shown in Figure 8, however, the variation of NO emissions was apparently associated with the variation of CO concentration, rather than with the cake thickness indicated by the pressure drop across the filter. The higher the CO concentration, the lower the NO emission. A substantial increase of CO2 concentration (by 1%) was observed, suggesting that the char contained in the ash was primarily burned to CO2 by oxygen (eq 4) in the flue gas before or in the filter cake, with a very limited portion available for NO reduction by eq 5 or by eq 7 via the formation of CO (eq 6). It was unexpected that the highest concentration of CO was formed at the beginning of the solids injection. This may be due to the increase of the filter temperature
Table 2. Capture of Alkali and Chlorine after Exposure for 15 h at 700 °C (Face Velocity: 5 cm/s) sample
Na
original dust cake fresh filter filter with cake naked filter
0.96 2.32 0.18 0.25 0.93
content (wt %) K Cl 1.29 7.57 0.03 0.27 4.02
Cl:(Na+K) molar ratio
0.220
0.028
0.002 0.091
0.007 0.019
Figure 9. Potassium distribution along thickness of naked and cake-protected ceramic filters. Filter: FIBROSIC 2800 low-density fibrous. Cake: Syncrude coke ash.
later in the run as a result of combustion of the carbon in the injected ash (measured carbon content in the used ash was
