Environ. Sci. Technol. 1993, 27, 1532-1541
Potassium-Chlorine Interactions in a Coal-Fired Magnetohydrodynamics System Atul C. Sheth,’ Shuylng H. Wang,? and Jeffrey K. Holt The University of Tennessee Space Institute, B. H. Goethert Parkway, Tullahoma, Tennessee 37388
40324.
aggregate of all other air toxics will be considered a major source and will be required to install costly control measures. A combined-cycle MHD system has the potential of increasing the overall power generating plant efficiency to more than 50%. Similarly, because of its higher efficiency, it will release less COZ(a typical greenhouse gas) per unit of electricity generated than a conventional power plant. The MHD system also provides built-in controls for NO, and SO2 emissions and easily satisfies the existing New Source Performance Standards (NSPS). Hence, by also reducing/eliminating the potential HC1 emission problem, the MHD-based combined cycle can provide an attractive option to help solve our nation’s future energy needs in an environmentally acceptable manner. The potassium added as a seed material in the MHD combustor serves two purposes: it provides the necessary electrons by thermal decomposition followedby ionization to make combustion gases electrically conductive, and it also reacts with the sulfur oxides formed from the combustion of sulfur-containing fuels and thereby reduces SO2 emissions. The quantity of potassium added in the MHD combustor is largely determined by the desired plasma conductivity and usually varies from 1 to 1.5 wt % as K in the plasma. The amount of potassium added to the MHD combustor is also routinely reported in terms of potassium/sulfur molar ratio (e.g., KdS). This ratio indicates the total quantities of potassium and sulfur introduced in the MHD system from various sources such as coal (or fuel), make-up seed, and recycled seed. A K d S molar ratio of greater than 1 indicates the excess of potassium, whereas the ratio of less than 1 indicates the potassium deficiency. Any potassium present in excess of sulfur can possibly react with the other species, such as chlorine-containing species. Thus, MHD system can help in reducing or eliminating the HC1 emissions also, if the potassium in the system exceeds the total sulfur level. However, to develop appropriate conditions to control/ minimize the HC1-emissions, the possible interactions taking place between the various potassium- and chlorinecontaining species under typical MHD conditions must be identified. Therefore, a study was initiated at the University of Tennessee Space Institute (UTSI) to determine such possible interactions. In this study, the possible interactions taking place between the potassiumand chlorine-containing species at various locations and under various operating conditions were first calculated from the thermodynamic equilibrium considerations.Some of these calculated results at lower temperatures were than validated using the measurements from the 28 MWI proofof-concept scale MHD coal-fired flow facility (CFFF) that UTSI operates under the US. Department of Energy (DOE) sponsorship. Inconsistencies found between the calculated results and the CFFF measurements were then explained using a limited number of bench-scale experiments. Results from this study are reported here.
1532 Envlron. Scl. Technol., Vol. 27, No. 8, 1993
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From pilot plant test measurements, thermodynamic equilibrium calculations, and bench-scaleexperiments, the interactions between the potassium- and chlorine-containing species in a coal-fired magnetohydrodynamics (MHD) system have been studied. This paper describes the major results and conclusions derived from this study and discusses their potential impacts in controlling the HC1 emissions from a coal-fired MHD system. In particular, based on the results, it is shown that by increasing the Kz/S ratio above 1.2,the HC1 emissions from an MHD stack can be eliminated. However,the resulting chlorides present as NaCl(s) or KCl(s) and collected along with the spent seed material must be removed during the seed reprocessingoperations before the potassium can be reused in the MHD cycle. This will be necessary to avoid potential adverse effects on the entire MHD system.
Introduction Magnetohydrodynamics (MHD) involves interactions among electromagnetic fields and electrically conducting gases and liquids. Faraday’s law of induction underlies MHD’s electric power generation capability. The conductor in an MHD system is an ionized fluid (gas or liquid) that passes through a fixed magnetic field. In a typical coal-fired, open-cycle power generation system, an MHD topping cycle for direct energy conversion is combined with a steam-bottoming cycle for generating electricity using a steam turbine. The high-t.emperaturecombustion gas in the MHD system is made electrically conductive by adding potassium salt, primarily in a sulfur-free form, as seed material. Chlorine generally occurs in fuels in a form that is easily decomposed or vaporized during combustion. According to a CONOCO technical bulletin (I), some Illinois coals contain chlorine concentrations of up to 0.65 9% ,and many US. coals exhibit chlorine contents varying from 0.01 % to about 0.5%. Depending upon the grade of the seed material, its chlorine content as KC1 or NaCl can vary from 0.05% to 1.5% (2). Potassium salts used as seed material in the MHD system are recovered and recycled for economic and environmental reasons. However, multiple recycling results in the potential problem of building up the chlorinecontaining species such as KC1 in the MHD system. If the buildup of such chlorine-containing species is not controlled below an acceptable level, potentially serious problems may result in the entire MHD system. Under the Clean Air Act Amendment of 1990, the emission of HC1 gas/aerosol from a continuous point source must be less than or equal to 10ton/year. The plant emitting more than 10 ton/year of one air toxic and 25 tontyear of + Toyota Motor Manufacturing, U.S.A., Inc., Inspection Engineering Raw Materials, 1001Cherry Blossom Way, Georgetown, KY
0 1993 American Chemlcal Soclety
Table I. Typical CFFF Test Conditions (LMFI-U Test) aarameter value before secondary after secondary combustion combustion
Table 111. Coal and Ash Analysis for Illinois No. 6 Coal Used During LMFI-U Test proximate and ultimate analysis
~~~
parameter coal fuel oil seed temp (K) pressure (atm) K2/S molar ratio primary stoichiometry (SR1) secondary stoichiometry (SR2) oil/coal (wt ratio)
Illinois no. 6 fuel oil no. 2 KzC03 1500-2400
Illinois no. 6 fuel oil no. 2 KzC03 350-1500
1
1
0.75-2.00 0.80.95
0.75-2.00
0.564
1.05-1.30 0.564
carbon (wt % ) hydrogen (wt % ) nitrogen (wt %) sulfur (wt %) moisture (wt % ) ash (wt %) heating value (BTU/LB)
66.60 4.43 1.67 2.39 3.30 10.11 11032
chlorine (Darn)
2569
wt%
50.15 20.67 12.85 1.03 3.27 0.89 0.51 6.67 4.13
Table IV. Fuel Oil Analysis for Test LME'I-U Table 11. Compounds Considered in Equilibrium Calculations Which Contain Potassium, Sodium, or
component
wt
carbon hydrogen nitrogen sulfur heating value (BTU/LB)
(%)
86.89 12.92 0.15 0.17 19 900
Temperature, ' F
-m s
;20 s o 1200
1400
1600
1600 Temperalure, K
2000
2200
2400
Figure 1. Atomic percentage of chlorine-containing species before secondary combustion (K*/S = 1.0, SR1 = 0.85).
Thermodynamic Equilibrium Calculations Equilibrium chloride distribution in the CFFF was calculated using the NASA SP-273 Code (3) and typical CFFF test conditions (see Table I). From this list, the temperature and K& ratio were considered as major variables in the calculations. A total of 70% of the raw coal ash was assumed to be removed from the radiant furnace as slag, and 10% of the potassium seed was considered to be lost as K2O in the slag (4). Increasing the potassium level in the system may lead to slightly higher potassium levels in the insoluble slag material. However, this potassium, present mostly as insoluble KAlSiOl-type compounds, is recoverable in a sulfur-free form by a process developed at UTSI (5). The NASA SP-273Code considers over 100 species containing either potassium, sodium, or chlorine in either gaseous, liquid, or solid phases. These results only include those species which comprise a significant fraction of the total, since listing every species regardless of concentration would quickly become tedious. Species not listed here were either not formed or the concentration was too low (i.e., molar concentration less than 1X to be included. Chemical species considered in the calculations which contain either chlorine, potassium, or sodium are listed in Table 11. The required thermodynamic properties such as heat capacity, enthalpy, and free energy of formation as a function of temperature for these species are available in ref 3 and other MHD publications. The typical compositions of the Illinois No. 6 coal and fuel oil used in these calculations are given in Tables I11 and IV, respectively.
The NASA SP-273 computer program developed by NASA-Lewis calculates the equilibrium composition of the given system by minimizing the total Gibbs free energy of the system while maintaining the conservation of mass/ material. Input to the NASA Code consists of the number of atoms or moles of C, H, 0, N, S, C1, etc., type elements introduced in the form of coal, fuel oil, seed, oxidant, etc. following the conditions given in Table I. Temperature, pressure, convergence conditions, etc. are also mentioned in the input file. Typical output from this NASA Code combines all the species present in any of the gaseous, solid,or liquid phases whose calculated fractions are greater Species whose concentrations than the set limit (1X are less than this limit are excluded from the output file. Lower limits can be used to identify other possible species, but then computational time becomes excessive, and the convergence of the results is not possible all the times. Modifications were made to collect all the potassium, chlorine, and sodium species in separate groups, and then the percent distribution of the desired element (e.g., potassium, chlorine, etc.) was calculated by collecting the appropriate species. More about the typical inputs and outputs from the various computer runs is available in refs 4 and 6. From equilibrium calculations, prior to secondary combustion, KCl(g) and HCl(g) are found to be the major chlorine-containing species while NaCl(gj, Cl(g), and K2Clz(g) are found in trace quantities (see Figure 1). This means that other chlorine-containing species are either not formed or if formed are most likely present in Envlron. Sci. Technol., Vol. 27, No. 8, 1993
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Temperature, 'F
500
1000
1500
2000
100
-; m
80
Ul
.-g)
60
5-
40
8
400
600
800 io00 Temperature, K
1200
1400 Temperature, K
Flgure 2. Atomic percentage of chlorine-containing species after secondary combustion (K& = 1.0, S R l = 0.85, SR2 = 1.1). Temperature, 'F
1000
500
2000
1500
100 I
tK2iSd.80
3
3
80
= 0 5.
60 60
-M- KdS11.10
40
20
0
8
0 400
600
800 1000 Temperature, K
1200
1400
Flgure 3. Effects of K2/S ratio and temperature on atomic percentage of chlorine as HCI(g) after secondary combustion (SR1 = 0.85, SR2 = 1.1).
concentrations less than the set limit (e.g., 1 X At Kz/S values exceeding 1.0, the concentration of HCl(g) prior to secondary combustion is found to approach zero. The calculated results after the secondary combustion are shown in Figures 2-4. Figure 2 shows that the combustion gases after the secondary combustion will include more chlorine-containing species and that the chlorine-containing species will exist in the gas phase as well as the solid phase. Interactions between the chlorinecontaining species and other species, such as sulfur- and carbon-containingspecies,may also take place at this stage. As shown in Figure 2, the atomic percentage of chlorine as KCl(g) drops quickly from about 95% at 1500 K to 0 at 1000 K, indicating a major chloride redistribution or a phase transition taking place at about this temperature. As the temperature drops further, the major gas-phase chlorine-containing species is HCl(g), and the major condensed-phase chlorine-containing species is NaCl(s). Figure 2 represents the results for Kz/S equal to 1. This means that an excess of potassium is not available to form KCl(s), and, therefore, NaCl(s) is formed by reaction between sodium introduced via coal ash and chlorinecontaining species. As temperature drops from 1400 to 1000K, the fraction of chlorine as HCl(g) increases rapidly from zero to its maximum value. Between 1000 and 600 K it remains unchanged, and as the temperature drops further from 600 to 350 K, the fraction of chlorine as HC1(g) decreases sharply as it is converted to Clz. As shown in Figure 3, the fraction of chlorine as HCl(g) decreases dramatically as the Kz/S ratio increases above 1.06, and this fraction approaches zero at a Kz/S = 1.2. As shown in Figure 4, the atomic percentage of chlorine as KCl(s) increases with the K d S ratio and is very sensitive to this ratio. KCl(s) exists only at Kz/S I1.06 and becomes the exclusive form of chlorine-containing species at Kz/S 1 1.15. Under this temperature range, a phase transition of 1534
Flgure 4. Effectsof Kz/S ratio and temperature on atomic percentage of chlorine as KCl(s) after secondary combustion (SR1 = 0.85, SR2 = 1.1).
KCl(g) to KCl(s) seems to be more important than the other possible chemical reactions under certain KdS ratios.
-A- K2/S=1 .06
Ul
g 8
20
0
/
Envlron. Scl. Technol., Vol. 27, NO. 8, 1993
Disposition of Chlorine-Containing Species i n Coal-Fired MHD System Some of the results calculated from the NASA Code were then verified using measurements from the various LMF4 test series carried out using Illinois No. 6 coal in the CFFF facility. Because our immediate interest was in determining the HCl/Clz emissions from the MHD stack as well as characterizing the amount of chlorides in the spent seed material, the emphasis was placed on the validation of the low-temperature results. In the future, if interest is shown,we can extend the CFFFmeasurernenta to higher temperature components also. The schematic of the CFFF is shown in Figure 5. This facility consists of upstream and downstream components and closely simulates a typical steam-bottoming plant. The upstream section includes an oxidant preheater (vitiation heater), a primary combustor, a supersonic nozzle, an aerodynamic duct, and a diffuser to simulate the time-temperature history for the topping cycle. The downstream section includes the radiant furnace, secondary combustor, superheater test module (SHTM), air heater, baghouse (BH), dry electrostatic precipitator (ESP), venturi scrubber/cyclone system, and stack. The near-future plans include replacing the venturi scrubber/ cyclone system with a wet ESPhotary vacuum drum filter system. The BH and ESP, being the possible candidates for the commercialsystem, are operated in parallel to allow for simultaneous evaluations of their particulate capture efficiencies under identical feed conditions. Additional information about the CFFF as well as its operating conditions are available in ref 7. Several investigators have measured the chloride content in various streams in and out of conventional combustion systems, but no one has studied in great detail the MHD system. Also, there appears to be considerable difficulty in arriving at a good material balance. For example, Meserole et al. (8)studied emissions from coal-fired power plants. They reported C1 output flows at 131-362% of input flows for three boilers. Klein et al. (9)accounted for only 2% of the coal chlorine in slag and fly ash streams but did not measure C1 in the flue gas stream. They assumed that the remaining chlorine was discharged into the atmosphere. Battelle (IO) reported on an Electric Power Research Institute study which indicated HC1 emissions 2 orders of magnitude higher than calculated if all C1 was emitted as HCl. These studies suggested that
8. Cornbustor C. SupersonicNozzle D. Circular Aerodynamic Duct
G. Secondaly Combustor H. SuperheaterTest Module I. Air Heaters
J. Venturi Scrubber M. Elecbostaticl'recioitator N. Stack
Flgure 5. Coal-fired low-mass flow faclllty at UTSI.
there is a problem either in chlorine analyses or in obtaining representative samples for analysis. There is general agreement that the major, common form is HCl(g), which often passes through the equipment to the atmosphere. Halstead and Raask (11)found that no NaCl enters the superheater section of a utility boiler if fuel and combustion air are well mixed, and the furnace residence times are on the order of 2 s. Only NaZS04 is available for deposition (not NaCl), and HC1 gas passes through the unit. However furnace walls close to the flame may have initial deposits containing NaC1, FeS, and carbon residues. Based on thermodynamic data and equilibrium calculations, Blackburn et al. (12)reported that in the spent seed material derived from a coal-fired MHD system, chlorine will be present as chlorides of sodium, potassium, and zinc. Samples of seeded coal, slag from radiant boiler, spent seed/ash deposits (after secondary combustion), cyclone slurry, and particulates in the stack were analyzed by James and co-workers (13). These samples were obtained from a high-temperature test in the CFFF, designated as LMFlD-2b. During this test period, the low mass flow (LMF) test train at UTSI did not represent prototypical configuration. Hence, the samples obtained and analyzed by Texas A&M University from that test are considered nontypical. This conclusion is based on the reasoning that instead of a slow cooling that is possible when all the rest of the downstream components (superheater, air heater, baghouse, and electrostatic precipitator) are in place (as shown in Figure 5 ) , the gas in the LMFl configuration was quenched quickly by water. Such sudden cooling of gas and entrained particulate material is expected to change the chemistry and distribution of certain species. Hence, the analysis performed by Texas A&M University should be considered preliminary. Chloride concentrations measured by James and co-workers in various MHD samples are given in Table V (12). They did not measure the chloride present possibly as gaseous HC1 in the stack effluent. However, assuming that no chlorides were present as gaseous HC1, they showed nearly 300% recovery of chloride. Such a high recovery of
Table V. Chloride Concentrations in MHD Samples. (LMFlD9b Test) sample description
chloride concn (pg/g)
seeded coal radiant boiler slag spent seed/ash deposit cyclone slurry, solids cyclone slurry, liquid stack solids
240
