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Alkali retention in hot coal slag under controlled oxidizing gas atmospheres (air–CO2)
Fuel Processing Technology 68 Ž2000. 57–73 www.elsevier.comrlocaterfuproc
Alkali retention in hot coal slag under controlled oxidizing gas atmospheres žair–CO 2 / M. Enders a,) , W. Willenborg a , J. Albrecht b, A. Putnis a a
Institut fur Wilhelms-UniÕersitat, Corrensstr. 24, ¨ Mineralogie, Westfalische ¨ ¨ Munster, ¨ D-48149 Munster, Germany ¨ b Lurgi Umwelt GmbH, Lurgiallee 5, D-60295 Frankfurt am Main, Germany Accepted 20 June 2000
Abstract Many technical high temperature processes including coal conversion face problems of alkali vaporization and subsequent condensation of alkali-rich phases. Most alkali-rich phases cause hazardous reactions with a subsequent corrosion of high temperature refractories or construction materials. A reduction of the alkali emission to flue gases would improve the durability of these materials. The alkali retention of coal slag in varying gas atmospheres was studied under controlled gas atmospheres. Samples of ashed coal were treated with gas atmospheres in the range from air to pure CO 2 for short time intervals of 45–50 min. The residue of the experiments was a homogeneous slag that was quenched. The chemical composition of the experimental slag was quantified using wavelength dispersive electron probe microanalysis. The experiments showed that the concentrations of Si, Ca and Fe in the slag were not affected by varying gas atmospheres and temperature. Alkali concentrations show a strong dependence on temperature. The potassium loss increases by 25% over a temperature range from 13508C to 14508C. The alkali retention is influenced by the gas atmosphere. The highest retention of alkalis is achieved in pure air. This effect can be attributed to an increased polymerization of the siliceous slag by ferrous iron. At pure CO 2 atmospheres the alkali retention displays a second maximum. This effect is most probably due to an accelerated melting already during the heating process of the ashed coal. The rapid melting decreases the time for vaporization and improves the fixing of alkalis into the slag. The results have implications for high temperature processes in coal conversion, ceramics industry and glass industry. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Alkali retention; Hot coal slag; Coal conversion
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Corresponding author. Tel.: q49-251-833-3500; fax: q49-251-833-8397. E-mail address: [email protected] ŽM. Enders..
0378-3820r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 0 0 . 0 0 1 1 0 - 7
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1. Introduction Future power plants combine modern gas turbine techniques and the traditional steam process to reduce the emission of CO 2 and other gaseous species, solid residues and the fuel consumption. Currently these combined cycle power plants are commercially available for clean fuels like gas and oil. The objective of current research is to transfer this highly efficient technology also to coal, which is a rather cheap and easily available fuel w1x. The operation of a gas turbine at high temperatures needs very pure flue gases. Currently in coal-fired combined cycle coal combustion pilot plants the purity requirements of turbine manufacturers have not been met w2,3x. The particulate matter in the flue gas is a complex mixture of incompletely separated slag particles and condensates of mainly alkali and earth alkali sulfates. The latter form from the cooling flue gas at temperatures in the range from 12008C to 6008C w3–6x. The parallel occurrence of particulate matter and volatilized species in the flue gas requires a two-stage purification system with a Žliquid-.particle separator and a getter unit to collect volatile species from the flue gas w7x. The requirements of both units could be less demanding if a reduction of the total generated particulate and volatile matter in the flue gas stream can be achieved. One potential way could be an increased fixation of alkalis into the slag. The mineral fraction of coals is altered in combustion processes. Part of the minerals are dehydrated or fractured, other minerals are fused or partially or completely volatilized during the high-temperature process w7–9x. Several previous studies were concerned with the viscosity development and sintering behavior of coal slag under gasification conditions w10–12x. These workers confirmed the influence of mineral phases crystallizing from the hot melts and the formation of phases during the sintering of the coal ash. Other workers examined the release of alkalis from the slag during coal combustion and their subsequent concentration either in the gas atmosphere or in solid residues collected from the flue gas collected with cascade impactors w13,14x. The available techniques to determine alkalis in flue gases have been compiled by Chadwick et al. w15x. Their work confirmed the importance of certain anions in the slag and the flue gas and the temperature for alkali mobilization during combustion. Finally, gases mobilized during combustion Žalkalis, sulfur dioxide. cause fouling reactions that damage construction materials in the power plant w8,16,17x. The process of alkali emission from coal slag has been analyzed with optical sensors from the flue gas and with the determination of gaseous emissions from ashed coals w13,15x. The state of alkali emission in coal-fired combined cycle processes has been compiled in Hannes w18x. Another way to determine alkali emission from coal slag could be the accurate analysis of the residual after the combustion process. This method allows an accurate mass balance of the emitted fraction and the residual. This study focuses on the alkali vaporization, i.e. the alkali retention in a coal slag as a function of temperature and oxygen fugacity. Two samples of different ashed coal were treated experimentally in a gas-mixing furnace with varying gas atmospheres. The experimental atmospheres were varied from air to CO 2 . This range in gas compositions is considered to be important for combustion processes under oxidizing conditions. After the experiments the recovered slag samples were analyzed with a wavelength dispersive electron microprobe. The purpose of this study is an evaluation of alkali emission from
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coal slag and to define optimum gas atmospheres during combustion to fix alkalis at high temperatures into the slag.
2. Theoretical background Coal slag can be studied as a quenched siliceous melt analogous to technical glasses. The physical parameters of the slag such as viscosity and electrical conductivity and chemical parameters such as the vapor pressure of chemical components in the glass are related to the bulk chemical composition of the slag, the temperature and the overall pressure. Glasses are three-dimensional frameworks of polymerized network-forming elements such as Si and Al and network-modifying cations such as the alkalis, alkali earth elements and certain metal cations. The role of iron in glasses is twofold. Iron can be network-forming or network-modifying at a time depending on its valency w19x. Fe 2q is a network-modifier and causes a de-polymerization of the melt, while Fe 3q substitutes for Si or Al and contributes to the polymerization of the melt. At high concentrations the surplus concentration of Fe 3q causes a de-polymerization of the glass. From the viewpoint of coal slag and technical melts the following questions can be raised. If iron can take part in a silicate melt as a network-former and as a network-modifier, will it then be possible to change the alkali retention of the siliceous melt as a function of the gas atmosphere above the melt and the overall temperature?
3. Methods 3.1. Sample materials The sample material was prepared from two raw coal samples from Spitzbergen, Norway, and Ensdorf, Germany. Each of the coals was ashed for 24 h at 8158C. The chemistry of the ash fraction of the raw coals was quantified using conventional X-ray fluorescence spectroscopy ŽXRF.. The mineral content of the material was characterized in a reconnaissance study with X-ray diffraction experiments and energy dispersive X-ray microanalysis in a scanning electron microscope. 3.2. Experimental setup of the gas-mixing furnace All experiments were conducted in a gas-mixing furnace ŽInstitut fur ¨ Mineralogie, .. The basic construction principle is outlined in Fig. 1. The central part of the Munster ¨ furnace is a zirconia-stabilized alumina ceramic tube, which is gas tight at each end. A gas stream with a pre-mixed gas composition crosses the hot zone of the oven with the sample. An oxygen probe and a thermocouple monitor the oxygen fugacity and the temperature directly at the sample. This setup has advantages to indirect methods where the gas mixture is preset or controlled by buffer systems. The pulverized samples are glued with Mowiol w and fixed to a platinum wire. Finally, the volatile fraction of the
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Fig. 1. Sketch draw of used gas-mixing furnace. The oxygen fugacity and the temperature are measured directly at the location of the sample.
binder is driven off by heating the sample to 1008C. The wire is then attached to a hook, which is inserted into the furnace at 8008C. The sample is heated to maximum temperature Ž13508C, 14008C and 14508C. depending on the experiment. After 45 min Ž14508C samples, 50 min. the samples were released and quenched into a water vessel at the bottom of the furnace. This quenching of the samples excluded any phase reactions during the cooling from the high temperatures that could influence the composition of the glassy phase. Iron-rich samples may react with Pt to form a FePt alloy w20x. Electron microscopy with back-scattered imaging confirmed a possible reaction between Pt and Fe in a range less than 10 mm on each side of the wirerslag interface. These areas were excluded from chemical analysis. The rather small size of the reaction zone effect is probably due to the short duration of the experiment Žmax 50 min.. The small volume affected excludes any influence of the alloy formation on the bulk chemistry of the slag samples. 3.3. WaÕelength dispersiÕe microanalysis The chemical composition of all experimental samples was quantified with a wavelength dispersive fully automated electron microprobe ŽJEOL 8600 MX. Ž15 kV, 15 nA.. The standards for all elements were natural minerals. Standard ZAF procedures
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were used to quantify the results. During electron microprobe analysis of glasses the samples can be damaged by the electron beam due to alkali migration resulting in a low alkali analysis. Therefore, prior to standardization and analysis the stability of the samples under electron bombardment was checked by recording the intensity of the alkali X-ray line count rate as a function of time Ž1-s steps, 40 s total time.. This procedure confirmed that the samples are stable at a beam diameter of ) 5 mm. However, to exclude any influence of the beam energy on the analysis the diameter of the beam was enlarged to 10 mm and measuring times on peak and background were reduced Ž15 and 2 s, respectively. compared to standard mineral analysis. Prior to analysis the homogeneity of each sample was checked with back-scattered electron imaging. The error of electron microprobe analysis is conventionally assumed to be in the range of 1% relative to the reported values in wt.%. For low concentrations low counting statistics increase the error margin to about 10% relative. On each experimental slag sample 50–60 analyses were distributed in a grid in order to improve statistics. The analyses were checked for consistency and homogeneity. Obvious erroneous analyses were excluded from the data set. The analyses commonly summed up to 96–98 wt.%. For the ease of comparison each analysis was normalized to 100. Finally, all analyses on a sample were averaged to obtain a bulk analysis of each sample. 4. Results 4.1. Analysis of raw coal Tables 1 and 2 give the mineralogical and chemical composition of the used raw coals. Both coals contain a considerable fraction of the clay mineral illite, which is
Table 1 Mineralogical analysis of the raw coals from Spitzbergen and Ensdorf Mineral
Spitzbergen
K–Al-silicate Kaolinite Quartz Plagioclase Pyrite Calcite Ca–Mg– ŽFe.-carbonate Fe-oxide Pyroxene Gypsum Ca-silicate Ca-phosphate Chromite Ti–Al-silicate Sum
24 7 13 3 23 7 10 1 1 2 1 1
26 8 14 3 25 8 11 1 1 2 1 1
93
100
counts
Ensdorf percentage
counts
percentage
31 6 25 7 11 12 31 3 11 2 1 1 1 1 143
22 4 17 5 8 8 22 2 8 1 1 1 1 1 100
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Table 2 Chemical analyses and physical parameters of the ashed raw coals from Spitzbergen and Ensdorf
SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2O P2 O5 SO 3 Sum Softening point Deformation point Flow point
Spitzbergen
Ensdorf
37.40 0.90 14.60 16.60 0.06 3.60 9.70 3.20 1.70 0.39 10.50 98.65 1170 1190 1250
42.60 0.84 22.80 12.20 0.17 2.46 9.42 1.03 2.34 0.09 6.01 99.96 1290 1300 1330
considered to be the major K source in coals w11x. The coal from Spitzbergen has a rather high pyrite content, while the Ensdorf coal has significant amount of carbonate minerals. The mineralogical composition is also reflected in the bulk chemical composition of the ash fraction. The Spitzbergen coal has a high iron and sulfur content. The principal alkali species is sodium. In contrast the Ensdorf coal has less Fe and a higher K content. The chemical composition of the non-carbonaceous content of both coals can be compared to silica-deficient volcanic melts. These melts are characterized by a low viscosity at high temperatures. The low flow point of the ash fraction of the raw coal guarantees a liquid removal of the slag in a pressurized pulverized coal combustion plant ŽPPCC. w3x. The physical parameters softening point, deformation point and flow point are reported in Table 2. The data reported in Table 2 give the approximate composition of the ash fraction of the used coals. 4.2. Samples from the gas-mixing furnace The major chemical components of all experimental samples were quantified. The microanalytical data of the slag samples are well reproduced, and the results of samples under varying conditions can easily be compared relative to each other. This information can be used to image the relationship of the concentrations of most of the slag components versus the temperature and the oxygen fugacity. In the following we will focus on elements generally considered to be critical in terms of volatilization such as Si, Ca, Fe and the alkalis w16x. The data for other elements that are considered refractory and as a minor component only is listed in Tables 3 and 4. Fig. 2 displays the temperature–oxygen fugacity Žlog f O 2 . relationship of SiO 2 for both ashed coal samples. As can be seen from Fig. 2A–F and the associated Tables 3 and 4, a Si loss is of minor importance and the Si concentration is independent of temperature and oxygen fugacity. This observation does not exclude that small amounts
Table 3 Experimental parameters and the results of electron probe microanalyses of experimental slag of the Spitzbergen coal Name
log f O 2
Na 2 O
MgO
Al 2 O 3
SiO 2
P2 O5
SO 3
K 2O
CaO
TiO 2
Cr2 O 3
MnO
FeO
NiO
Total
1350 1350 1350 1350 1350 1350 1350 1350 1400 1400 1400 1400 1400 1400 1400 1400 1400 1450 1450 1450 1450 1450 1450 1450
SP35 SP34 SP31 SP36 SP29 SP29 SP30 SP32 SP1neu SP26 SP41 SP17 SP13 SP10 SP18 SP19 SP20 SP43 SP50 SP49 SP45 SP48 SP46 SP44
y0.76 y0.81 y0.9 y1.1 y1.27 y1.27 y2 y5.00 y0.68 y0.73 y0.80 y0.81 y0.92 y1.01 y1.11 y1.34 y1.36 y0.68 y0.8 y1.11 y1.34 y1.34 y2 y3
2.96 2.94 3.01 2.90 2.97 3.00 3.09 3.01 3.05 2.90 2.87 2.89 2.90 2.94 2.95 3.05 2.91 2.90 2.79 2.86 2.95 2.71 2.67 2.90
3.92 3.87 3.88 3.81 3.95 3.84 3.86 3.94 3.78 3.86 3.83 3.84 3.86 3.92 3.86 4.01 3.94 3.95 3.85 3.99 3.95 3.81 3.99 4.00
18.44 18.23 18.45 17.93 17.97 18.12 18.48 18.43 17.76 17.97 17.48 17.87 17.57 17.44 18.11 18.16 18.02 18.25 18.31 18.01 18.19 18.12 18.20 18.16
45.67 45.72 45.53 46.82 45.81 45.71 45.42 46.99 46.96 46.31 46.91 45.89 46.97 46.29 45.19 44.99 45.93 45.28 46.71 45.62 46.13 47.26 46.14 45.79
0.38 0.40 0.39 0.39 0.37 0.37 0.39 0.40 0.38 0.30 0.32 0.33 0.34 0.41 0.38 0.34 0.34 0.21 0.32 0.42 0.30 0.33 0.42 0.32
0.01 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.01 0.01 0.00
1.38 1.34 1.34 1.32 1.35 1.33 1.33 1.30 1.30 1.23 1.32 1.27 1.26 1.25 1.27 1.27 1.25 1.31 1.15 1.18 1.20 1.09 1.02 1.21
12.08 12.21 12.31 11.73 12.25 12.10 12.22 12.34 12.28 11.83 11.96 11.76 11.85 11.90 11.88 12.18 12.15 12.16 12.15 12.12 12.27 11.95 12.27 12.27
1.10 1.05 1.06 1.02 0.98 1.04 1.12 1.12 1.03 1.06 1.02 1.05 1.07 1.04 1.03 1.04 1.00 1.05 1.03 1.08 1.03 1.06 1.12 1.03
0.02 0.01 0.02 0.02 0.02 0.02 0.03 0.03 0.01 0.02 0.01 0.02 0.02 0.02 0.02 0.03 0.02 0.01 0.03 0.03 0.03 0.03 0.03 0.02
0.06 0.05 0.05 0.05 0.02 0.03 0.05 0.06 0.04 0.05 0.05 0.04 0.04 0.04 0.04 0.04 0.06 0.03 0.04 0.05 0.05 0.05 0.05 0.05
13.96 14.15 13.93 13.99 14.28 14.40 13.98 12.36 13.37 14.45 14.19 15.01 14.09 14.73 15.25 14.86 14.32 14.82 13.58 14.61 13.87 13.56 14.05 14.22
0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.03 0.02 0.02 0.02 0.03 0.03 0.03 0.02 0.02 0.02 0.02
100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 99.97 100.00 100.00 100.00 100.00 100.00 100.00 100.00
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T Ž8C.
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Table 4 Experimental parameters and the results of electron probe microanalyses of experimental slag of the Ensdorf coal Name
log f O 2
Na 2 O
MgO
Al 2 O 3
SiO 2
P2 O5
SO 3
K 2O
CaO
TiO 2
Cr2 O 3
MnO
FeO
NiO
Total
1350 1350 1350 1350 1350 1350 1350 1350 1350 1400 1400 1400 1400 1400 1400 1400 1400 1400 1450 1450 1450 1450 1450 1450
EN25 EN35 EN34 EN31 EN28 EN36 EN29 EN30 EN32 EN1neu EN26 EN33 EN17 EN13 EN10 EN18 EN19 EN20 EN43 EN50 EN49 EN48 EN46 EN44
y0.68 y0.76 y0.80 y0.81 y1.08 y1.10 y1.27 y2.02 y5.00 y0.68 y0.73 y0.80 y0.81 y0.92 y1.01 y1.11 y1.34 y1.36 y0.68 y0.8 y1.11 y1.34 y2 y3
1.21 1.17 1.13 1.14 1.19 1.13 1.13 1.15 1.23 1.21 1.18 1.14 1.15 1.19 1.22 1.10 1.15 1.15 1.33 1.10 1.17 1.13 1.11 1.16
2.57 2.59 2.49 2.77 2.66 2.54 2.75 2.74 2.85 2.73 2.56 2.55 2.55 2.56 2.48 2.53 2.60 2.57 2.51 2.53 2.56 2.63 2.59 2.58
24.71 24.42 25.17 24.37 24.76 25.13 24.08 24.15 24.62 24.54 24.92 24.66 24.56 24.81 24.78 24.71 24.78 24.80 24.83 24.62 25.12 25.08 25.08 25.19
46.33 46.82 46.16 45.84 45.97 45.97 46.82 46.89 46.84 45.99 46.09 46.87 46.09 46.30 46.46 46.47 45.93 46.47 46.26 46.88 46.90 46.73 46.96 46.19
0.07 0.09 0.08 0.08 0.07 0.11 0.07 0.07 0.07 0.07 0.07 0.05 0.09 0.10 0.10 0.08 0.07 0.08 0.08 0.08 0.08 0.07 0.09 0.07
0.01 0.01 0.02 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.01
2.38 2.46 2.43 2.45 2.46 2.34 2.55 2.46 2.51 2.51 2.28 2.29 2.30 2.31 2.33 2.28 2.27 2.26 2.31 2.05 1.97 1.81 1.86 2.21
10.28 10.13 10.52 9.94 10.18 10.54 9.76 9.83 10.30 9.99 10.47 10.23 10.17 10.28 10.26 9.97 10.27 10.25 10.32 10.18 10.40 10.48 10.55 10.42
0.88 0.82 0.88 0.90 0.91 0.88 0.87 0.92 0.83 0.93 0.85 0.82 0.80 0.88 0.85 0.84 0.83 0.81 0.84 0.85 0.87 0.89 0.85 0.85
0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.04 0.03 0.03 0.02 0.01 0.02 0.02 0.03 0.02 0.03 0.02 0.01 0.02 0.03 0.03 0.03 0.02
0.20 0.20 0.19 0.16 0.20 0.20 0.16 0.18 0.17 0.20 0.22 0.15 0.21 0.20 0.20 0.21 0.21 0.20 0.21 0.20 0.20 0.20 0.21 0.20
11.31 11.25 10.87 12.28 11.54 11.10 11.76 11.55 10.51 11.76 11.31 11.17 12.04 11.32 11.25 11.76 11.84 11.38 11.26 11.45 10.67 10.91 10.65 11.08
0.02 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.02 0.02
100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 99.97 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
M. Enders et al.r Fuel Processing Technology 68 (2000) 57–73
T Ž8C.
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Fig. 2. Relationship of SiO 2 concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
of Si can occur as a volatile species, but the stability of the Si data points versus oxygen fugacity clearly confirm that the Si-transfer from the slag to the gas atmosphere is of minor importance. Fig. 3A–F gives a similar relationship for Ca. From the figure it is obvious that no calcium is lost during the experiment to the gas atmosphere. This is expected, as CaO is commonly assumed to be refractory during combustion processes w16x. This observation also excludes the formation of a calcium sulfate phase on surface of the melt droplet during the experiment, as this process would lead to a loss of the water soluble CaSO4 during the quenching.
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Fig. 3. Relationship of CaO concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
The Fe concentration displays a minor variation with oxygen fugacity ŽFig. 4A–F.. In very low oxygen fugacites Žlog f O 2 - y1. a loss of Fe was observed in the Ensdorf sample. Fe is the only element that displays a different behavior in both coal samples. Reconnaissance experiments in CO–CO 2 atmospheres confirmed a loss of Fe from the slag in highly reducing atmospheres ŽTables 3 and 4.. The data points for Fe vary
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Fig. 4. Relationship of FeO concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
randomly about 5–10% relative to the reported number ŽFig. 4, Tables 2 and 3.. The reason for this spread in the data set is not clear. It might be affected by rather low count rates that increase the statistical error on counting statistics during the measurement, or it might be attributed to the Fe 2 O 3 –Fe 2q Fe 3q 2 O4 buffer that is important in the temperature range of the experiments. The scatter of the Fe data points cannot be due to Fe-bearing silicate or oxide phases, as these were not observed in back-scattered imaging. The formation of a Pt–Fe alloy along the platinum wire is — as stated above — negligible. The behavior for alkali elements is different ŽFigs. 5 and 6.. The concentration versus oxygen fugacity relationship for sodium is depicted in Fig. 5A–F. At low temperatures
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Fig. 5. Relationship of Na 2 O concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
ŽFig. 5C and F. only a minor loss of sodium during the experiment is observed. With increasing temperature the absolute sodium loss increases to about 10% compared to the maximum sodium content of the experimental samples Ž13508C.. In high temperature experiments a relation between sodium content and oxygen fugacity is obvious. The maximum loss is detected at oxygen fugacities in the range from log f O 2 Žy1. to Žy2., which corresponds to oxygen concentrations of 1–8% in the gas atmosphere. Within the same temperature range the sodium content varies by 8–10% due to changes in the oxygen fugacity.
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Fig. 6. Relationship of K 2 O concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
At low temperatures we observed only a minor potassium loss rather similarly to sodium ŽFig. 6A–F.. At temperatures of 14508C the absolute loss of potassium can be 25% of the initial values ŽFig. 6A and D.. Potassium displays a pronounced relationship to oxygen fugacity. A minimum K concentration in the slags is observed at oxygen concentrations of about 1–10% in the gas atmosphere. If both temperature and oxygen fugacity are varied, about 1r3 of the potassium can be volatilized from the slag. At very low oxygen concentrations in the gas atmosphere the K concentration rises again. This rise indicates a second process besides a polymerization influencing the alkali retention of the slag.
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5. Discussion The experiments are restricted by the experimental setup to temperatures in the range from 13508C to 14508C. This temperature range rarely equates with temperatures in the actual combustion flame. Under conditions of pressurized pulverized coal combustion maximum temperatures of about 2000 K in the pulverized coal flame are expected w21x. During the residence of the slag on the boiler walls and in the liquid slag separation unit vaporization can become important. The range of oxygen fugacities during the experiments in the gas-mixing furnace were adjusted to the gas atmosphere expected in a PPCC power plant. Most technical high-temperature processes such as steel production, glass production or the processing of ceramics require process temperatures and oxygen fugacities similar to those used in the experiments. In the case of the coal from Ensdorf the lowest temperature applied in the experiments is very close to its flow point ŽTable 2.. The sample temperature difference to the flow point leads to an incomplete homogenization of this coal in the 13508C and 14008C experiments. This effect can easily be seen from the rather varying concentrations for various elements. In the 14508C samples the material had been very well homogenized, as is obvious from Table 4. Further, any dynamic process during the volatilization of alkalis in the actual burner or the different behavior of organic-bounded alkalis versus inorganic-bounded is neglected w14x. This study quantified the concentration of standard slag components in experimental slag samples. From the total of 13 quantified elements we restrict the discussion to those important for volatilization during combustion processes. Si is considered of major importance in combustion processes. Early workers already discovered microsilica spheres that are obviously due a vaporization of Si in the burner flame w16x. Commonly, this process is attributed to the formation of volatile SiO w16x. During our experiments in oxidizing atmospheres we found no indication for a vaporization of Si from the slag. The small amounts of sample material used during the experiments cannot exclude, that in an actual power plant, where tons of coal are converted per hour, even a minimum mobilization of Si can already cause a considerable SiO concentration in the flue gas. In both coals the observed SiO 2 content is higher than the SiO 2 content of the raw coal after normalization to the high temperature composition Žwithout SO 3 .. The microanalyses monitor related changes in composition between experimental samples very accurately. The composition of the ash fraction of raw coal had been determined several times. However, these analyses are inconsistent. This indicates a very problematic analysis of ash fractions of coal depending on the laboratory, the sampling, and the type of instrumental analysis applied. Ca is not mobilized under the conditions applied in the experiments. The Ensdorf coal and less clearly the Spitzbergen coal displayed a partial loss of Fe under reducing conditions Ž- to 1% oxygen. and in CO 2 –CO atmospheres ŽTables 3 and 4.. This effect must be due to a fluxing by Fe 2q. The increased mobility of cations in the slag combined with a low oxygen atmosphere obviously favors a partial volatilization of Fe. In terms of vaporization–condensation reactions alkalis are the most important species in the system. The experiments clearly confirm a temperature-dependent
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volatilization of the alkalis. A reduction in combustion temperature, equivalent to a loss in efficiency of the process could decrease the volatilization of sodium to about 10%. For potassium a decrease of the temperature in the slag separator could decrease the K loss up to 25%. The oxygen fugacity has a considerable influence on the alkali retention of the slag. At oxygen fugacities equivalent to pure air the optimum fixation of alkalis into slag can be achieved. The increased retention of K and Na at high oxygen fugacites is a clear indication of a slag highly polymerized due to a high Fe 3q content. The increased polymerization of the melt inhibits the mobility of alkalis in the slag, and therefore reduces the transfer to the gaseous phase. A way to raise the oxygen fugacity and therewith to reduce the vaporization of alkalis could be the introduction of excess air into the boiler. However, any excess air in the boiler would decrease the overall efficiency of the conversion process. At lower oxygen fugacities when the oxygen concentration in the flue gas is in the range of 1–10% Ži.e. normal power plants conditions., the fixation of alkalis is worst. In conditions close to CO 2 –CO atmosphere a rise of the K concentration in the experimental slag samples indicates an increased fixation of alkalis. The latter effect is a function of oxygen fugacity, and therefore should correlate to a valence change of Fe in the slag. Fe 3q has low concentrations in the slag under low oxygen fugacities. This excludes Fe 3q as the reason for an increased polymerization of the melt. In contrast, this effect must be related to an increased Fe 2q concentration, which is enriched in reducing environments. The action of Fe 2q as a flux is well known from ceramics production especially when alkalis are present w22x. Under reducing atmospheres the increased Fe 2q content of the ash would accelerate the melting of the ashed coal. The more rapid melting would fix the alkalis to the slag rather than vaporize them to the gas atmosphere in the oven. The exchange from the slag to the gas atmosphere is limited by the mobility of ions in the slag. The steady gas stream along the sample material ensures a constant disequilibrium. If the alkalis would be mobilized from the surface of the slag droplet in the furnace one would expect a concentration profile within the slag droplet, with low values at the surface and higher concentrations in the core. This is not observed. Therefore, an increased fixation of alkalis must be attributed completely to changes in the slag sample itself. The bimodal form of the K and Na distribution with oxygen fugacity indicates two processes: at high oxygen fugacities an effective polymerization of the melt, and at low temperatures a considerable shortening of the sintering process of the sample due to the fluxing action of Fe 2q. This confirms the importance of the actual behavior of minerals in the flame for the overall flue gas composition.
6. Conclusion In a pulverized power plant only a few parameters can be changed to increase to alkali fixation to slag without either decreased efficiency or stopping the combustion process. From the data reported in this study it seems possible to improve the alkali fixation in coal slag with an increased oxygen fugacity due to an increased polymerization of the slag. However, this effect can hardly be used to trap alkalis in the melt as the
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simple introduction of excess air would result in an overall loss in efficiency. Our data confirms an increased alkali fixation also at very low oxygen fugacities. This effect cannot be attributed to an increased polymerization of the melt. The occurrence of an increased amount of Fe 2q has an effect on the sintering behavior of the slag similar to the influence during the firing of ceramics. Under these conditions the alkali emission is low. However, iron might be volatilized under such conditions to flue gas. This effect can take place under high temperatures Ž) 14008C. and low oxygen fugacities close to pure CO 2 atmospheres. These conditions are not achieved in conventional oxidizing combustors. From the viewpoint of combustion under oxidizing conditions vaporization of Si and Fe seems to be negligible. In large-scale combustors even minimum concentrations of Fe and Si in the flue gas can cause slagging and fouling of construction materials. The results of this experimental study can be used to optimize the flue gas composition in a pressurized pulverized coal combustion in order to reduce emission of alkalis not in the flame, but in the slag separation unit. References w1x K. Strauß, Kraftwerkstechnik, Springer-Verlag, Berlin, 1997, p. 494. w2x I. Romey, H. Rode, M. Schuknecht, GuD-Prozesse fur ¨ den Einsatz von Kohle, in: K. Hannes ŽEd.., Erstes Statuseminar Druckflamm, STEAG AG, 17 Nov., Zeche Zollverein, 1998, pp. 163–173. w3x K. Hannes, Pilot plant test results of pressurized pulverized coal combustion, in: BEO ŽEd.., Proceedings of the EU Seminar AStatus of Development and Market Penetration of Clean Coal Technologies ŽCCT. for Power GenerationB, Dusseldorf, Germany, 5r6 Nov. 1998, Eur. CommmisionrBEO, pp. 1–15. ¨ w4x M. Forster, Ergebnisse und Aufgabenstellungen aus dem Versuchsbetrieb in Dorsten, in: K. Hannes ŽEd.., ¨ Erstes Statuseminar Druckflamm, STEAG AG, 17 Nov., Zeche Zollverein, 1998, pp. 4–9 and 163–173. w5x M. Enders, M. Spiegel, J. Albrecht, A. Putnis, Chemical and mineralogical composition of fly ash collected from a hot flue gas in a pressurized pulverized coal combustion, Eur. J. Mineral 12 Ž2000. 639–650. w6x M. Enders, J. Albrecht, A. Putnis, Fractionation of slag and condensates from a hot flue gas of a pressurized pulverized coal combustion ŽPPCC. pilot plant, Energy & Fuels 14 Ž2000. 806–815. w7x A.S. Benson, E.A. Sondreal, J.P. Hurley, Status of coal ash behavior research, Fuel Process. Technol. 44 Ž1995. 1–12. w8x G.E. Vaninetti, H. Busch, Mineral analysis of ash data and utility perspective, J. Coal Qual. 1 Ž1982. 22–31. w9x G.P. Huffman, F.E. Huggins, G.R. Dunmyre, Investigation of the high temperature behavior of coal ash in reducing and oxidizing atmospheres, Fuel 60 Ž1981. 585–597. w10x J.W. Nowok, J.P. Hurley, S.A. Benson, The role of physical factors in mass transport during sintering of coal ashes and deposits deformation near the temperature of glass transformation, Fuel Process. Technol. 56 Ž1998. 89–101. w11x S.K. Gupta, R.P. Gupta, G.W. Brayant, T.F. Wall, The effect of potassium on the fusibility of coal ashes with high silica and alumina levels, Fuel 77 Ž1998. 1195–1201. w12x H.J. Hurst, F. Novak, J.H. Patterson, Viscosity measurements and empirical predictions for some model gasifier slags, Fuel 78 Ž1999. 439–444. w13x D.I. Barnes, D.G Wardle, D. Kean, Sodium and potassium release from UK coals during the early stages of pulverized coal combustion, in: J. Williamson, F. Wigley ŽEds.., The Impact of Ash Deposition on Coal Fired Power Plants, Proc. Eng. Found. Conf., Solihull, England 20–25, June, Taylor & Francis, London, 1993. w14x N.B. Gallagher, L.B. Bool, J.O.L. Wendt, Alkalimetal partitioning in ash from pulverized coal combustion, Combust. Sci. Technol. 74 Ž1990. 211–221.
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w15x B.L. Chadwick, R.A. Ashman, A. Campisi, G.J. Crofts, P.D. Godfrey, P.G. Griffin, A.L. Ottrey, R.J.S. Morrison, Development of techniques for monitoring gas-phase sodium species formed during coal combustion and gasification, Int. J. Coal Geol. 32 Ž1996. 241–253. w16x W. Gumz, H. Kirsch, M-Th. Mackowsky, Schlackenkunde, Springer-Verlag, Berlin, 1958, 422 pp. w17x M. Enders, M. Spiegel, Mineralogical and microchemical study of high-temperature-reactions in fly ash scales from a waste incineration plant, Eur. J. Mineral. 11 Ž1999. 763–774. w18x K. Hannes, Erstes Statusseminar Druckflamm, STEAG AG, 17 Nov., Zeche Zollverein, Essen, Germany, 1998, p. 216. w19x A.K. Varshneya, Fundamentals of Inorganic Glasses, Academic Press ŽHarcourt Brace and Company, Publishers., 1993, p. 570. w20x E. Woermann, Methods of investigation of phase equilibria,Verein Deutscher Eisenhuttenleute ŽVDEh.: ¨ Slag Atlas,1995, pp. 23–25. w21x U. Renz, Experimentelle und theoretische Untersuchungen an der Druckkohlenstaubfeuerung der RWTH Aachen, in: K. Hannes ŽEd.., Erstes Statuseminar Druckflamm, STEAG AG, 17 Nov., Zeche Zollverein, 1998, pp. 16–26. w22x K. Jasmund, G. Lagaly, Tonminerale und Tone, Steinkopff Verlag, Darmstadt, 1993, 490 pp.
Alkali retention in hot coal slag under controlled oxidizing gas atmospheres žair–CO 2 / M. Enders a,) , W. Willenborg a , J. Albrecht b, A. Putnis a a
Institut fur Wilhelms-UniÕersitat, Corrensstr. 24, ¨ Mineralogie, Westfalische ¨ ¨ Munster, ¨ D-48149 Munster, Germany ¨ b Lurgi Umwelt GmbH, Lurgiallee 5, D-60295 Frankfurt am Main, Germany Accepted 20 June 2000
Abstract Many technical high temperature processes including coal conversion face problems of alkali vaporization and subsequent condensation of alkali-rich phases. Most alkali-rich phases cause hazardous reactions with a subsequent corrosion of high temperature refractories or construction materials. A reduction of the alkali emission to flue gases would improve the durability of these materials. The alkali retention of coal slag in varying gas atmospheres was studied under controlled gas atmospheres. Samples of ashed coal were treated with gas atmospheres in the range from air to pure CO 2 for short time intervals of 45–50 min. The residue of the experiments was a homogeneous slag that was quenched. The chemical composition of the experimental slag was quantified using wavelength dispersive electron probe microanalysis. The experiments showed that the concentrations of Si, Ca and Fe in the slag were not affected by varying gas atmospheres and temperature. Alkali concentrations show a strong dependence on temperature. The potassium loss increases by 25% over a temperature range from 13508C to 14508C. The alkali retention is influenced by the gas atmosphere. The highest retention of alkalis is achieved in pure air. This effect can be attributed to an increased polymerization of the siliceous slag by ferrous iron. At pure CO 2 atmospheres the alkali retention displays a second maximum. This effect is most probably due to an accelerated melting already during the heating process of the ashed coal. The rapid melting decreases the time for vaporization and improves the fixing of alkalis into the slag. The results have implications for high temperature processes in coal conversion, ceramics industry and glass industry. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Alkali retention; Hot coal slag; Coal conversion
)
Corresponding author. Tel.: q49-251-833-3500; fax: q49-251-833-8397. E-mail address: [email protected] ŽM. Enders..
0378-3820r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 0 0 . 0 0 1 1 0 - 7
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M. Enders et al.r Fuel Processing Technology 68 (2000) 57–73
1. Introduction Future power plants combine modern gas turbine techniques and the traditional steam process to reduce the emission of CO 2 and other gaseous species, solid residues and the fuel consumption. Currently these combined cycle power plants are commercially available for clean fuels like gas and oil. The objective of current research is to transfer this highly efficient technology also to coal, which is a rather cheap and easily available fuel w1x. The operation of a gas turbine at high temperatures needs very pure flue gases. Currently in coal-fired combined cycle coal combustion pilot plants the purity requirements of turbine manufacturers have not been met w2,3x. The particulate matter in the flue gas is a complex mixture of incompletely separated slag particles and condensates of mainly alkali and earth alkali sulfates. The latter form from the cooling flue gas at temperatures in the range from 12008C to 6008C w3–6x. The parallel occurrence of particulate matter and volatilized species in the flue gas requires a two-stage purification system with a Žliquid-.particle separator and a getter unit to collect volatile species from the flue gas w7x. The requirements of both units could be less demanding if a reduction of the total generated particulate and volatile matter in the flue gas stream can be achieved. One potential way could be an increased fixation of alkalis into the slag. The mineral fraction of coals is altered in combustion processes. Part of the minerals are dehydrated or fractured, other minerals are fused or partially or completely volatilized during the high-temperature process w7–9x. Several previous studies were concerned with the viscosity development and sintering behavior of coal slag under gasification conditions w10–12x. These workers confirmed the influence of mineral phases crystallizing from the hot melts and the formation of phases during the sintering of the coal ash. Other workers examined the release of alkalis from the slag during coal combustion and their subsequent concentration either in the gas atmosphere or in solid residues collected from the flue gas collected with cascade impactors w13,14x. The available techniques to determine alkalis in flue gases have been compiled by Chadwick et al. w15x. Their work confirmed the importance of certain anions in the slag and the flue gas and the temperature for alkali mobilization during combustion. Finally, gases mobilized during combustion Žalkalis, sulfur dioxide. cause fouling reactions that damage construction materials in the power plant w8,16,17x. The process of alkali emission from coal slag has been analyzed with optical sensors from the flue gas and with the determination of gaseous emissions from ashed coals w13,15x. The state of alkali emission in coal-fired combined cycle processes has been compiled in Hannes w18x. Another way to determine alkali emission from coal slag could be the accurate analysis of the residual after the combustion process. This method allows an accurate mass balance of the emitted fraction and the residual. This study focuses on the alkali vaporization, i.e. the alkali retention in a coal slag as a function of temperature and oxygen fugacity. Two samples of different ashed coal were treated experimentally in a gas-mixing furnace with varying gas atmospheres. The experimental atmospheres were varied from air to CO 2 . This range in gas compositions is considered to be important for combustion processes under oxidizing conditions. After the experiments the recovered slag samples were analyzed with a wavelength dispersive electron microprobe. The purpose of this study is an evaluation of alkali emission from
M. Enders et al.r Fuel Processing Technology 68 (2000) 57–73
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coal slag and to define optimum gas atmospheres during combustion to fix alkalis at high temperatures into the slag.
2. Theoretical background Coal slag can be studied as a quenched siliceous melt analogous to technical glasses. The physical parameters of the slag such as viscosity and electrical conductivity and chemical parameters such as the vapor pressure of chemical components in the glass are related to the bulk chemical composition of the slag, the temperature and the overall pressure. Glasses are three-dimensional frameworks of polymerized network-forming elements such as Si and Al and network-modifying cations such as the alkalis, alkali earth elements and certain metal cations. The role of iron in glasses is twofold. Iron can be network-forming or network-modifying at a time depending on its valency w19x. Fe 2q is a network-modifier and causes a de-polymerization of the melt, while Fe 3q substitutes for Si or Al and contributes to the polymerization of the melt. At high concentrations the surplus concentration of Fe 3q causes a de-polymerization of the glass. From the viewpoint of coal slag and technical melts the following questions can be raised. If iron can take part in a silicate melt as a network-former and as a network-modifier, will it then be possible to change the alkali retention of the siliceous melt as a function of the gas atmosphere above the melt and the overall temperature?
3. Methods 3.1. Sample materials The sample material was prepared from two raw coal samples from Spitzbergen, Norway, and Ensdorf, Germany. Each of the coals was ashed for 24 h at 8158C. The chemistry of the ash fraction of the raw coals was quantified using conventional X-ray fluorescence spectroscopy ŽXRF.. The mineral content of the material was characterized in a reconnaissance study with X-ray diffraction experiments and energy dispersive X-ray microanalysis in a scanning electron microscope. 3.2. Experimental setup of the gas-mixing furnace All experiments were conducted in a gas-mixing furnace ŽInstitut fur ¨ Mineralogie, .. The basic construction principle is outlined in Fig. 1. The central part of the Munster ¨ furnace is a zirconia-stabilized alumina ceramic tube, which is gas tight at each end. A gas stream with a pre-mixed gas composition crosses the hot zone of the oven with the sample. An oxygen probe and a thermocouple monitor the oxygen fugacity and the temperature directly at the sample. This setup has advantages to indirect methods where the gas mixture is preset or controlled by buffer systems. The pulverized samples are glued with Mowiol w and fixed to a platinum wire. Finally, the volatile fraction of the
60
M. Enders et al.r Fuel Processing Technology 68 (2000) 57–73
Fig. 1. Sketch draw of used gas-mixing furnace. The oxygen fugacity and the temperature are measured directly at the location of the sample.
binder is driven off by heating the sample to 1008C. The wire is then attached to a hook, which is inserted into the furnace at 8008C. The sample is heated to maximum temperature Ž13508C, 14008C and 14508C. depending on the experiment. After 45 min Ž14508C samples, 50 min. the samples were released and quenched into a water vessel at the bottom of the furnace. This quenching of the samples excluded any phase reactions during the cooling from the high temperatures that could influence the composition of the glassy phase. Iron-rich samples may react with Pt to form a FePt alloy w20x. Electron microscopy with back-scattered imaging confirmed a possible reaction between Pt and Fe in a range less than 10 mm on each side of the wirerslag interface. These areas were excluded from chemical analysis. The rather small size of the reaction zone effect is probably due to the short duration of the experiment Žmax 50 min.. The small volume affected excludes any influence of the alloy formation on the bulk chemistry of the slag samples. 3.3. WaÕelength dispersiÕe microanalysis The chemical composition of all experimental samples was quantified with a wavelength dispersive fully automated electron microprobe ŽJEOL 8600 MX. Ž15 kV, 15 nA.. The standards for all elements were natural minerals. Standard ZAF procedures
M. Enders et al.r Fuel Processing Technology 68 (2000) 57–73
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were used to quantify the results. During electron microprobe analysis of glasses the samples can be damaged by the electron beam due to alkali migration resulting in a low alkali analysis. Therefore, prior to standardization and analysis the stability of the samples under electron bombardment was checked by recording the intensity of the alkali X-ray line count rate as a function of time Ž1-s steps, 40 s total time.. This procedure confirmed that the samples are stable at a beam diameter of ) 5 mm. However, to exclude any influence of the beam energy on the analysis the diameter of the beam was enlarged to 10 mm and measuring times on peak and background were reduced Ž15 and 2 s, respectively. compared to standard mineral analysis. Prior to analysis the homogeneity of each sample was checked with back-scattered electron imaging. The error of electron microprobe analysis is conventionally assumed to be in the range of 1% relative to the reported values in wt.%. For low concentrations low counting statistics increase the error margin to about 10% relative. On each experimental slag sample 50–60 analyses were distributed in a grid in order to improve statistics. The analyses were checked for consistency and homogeneity. Obvious erroneous analyses were excluded from the data set. The analyses commonly summed up to 96–98 wt.%. For the ease of comparison each analysis was normalized to 100. Finally, all analyses on a sample were averaged to obtain a bulk analysis of each sample. 4. Results 4.1. Analysis of raw coal Tables 1 and 2 give the mineralogical and chemical composition of the used raw coals. Both coals contain a considerable fraction of the clay mineral illite, which is
Table 1 Mineralogical analysis of the raw coals from Spitzbergen and Ensdorf Mineral
Spitzbergen
K–Al-silicate Kaolinite Quartz Plagioclase Pyrite Calcite Ca–Mg– ŽFe.-carbonate Fe-oxide Pyroxene Gypsum Ca-silicate Ca-phosphate Chromite Ti–Al-silicate Sum
24 7 13 3 23 7 10 1 1 2 1 1
26 8 14 3 25 8 11 1 1 2 1 1
93
100
counts
Ensdorf percentage
counts
percentage
31 6 25 7 11 12 31 3 11 2 1 1 1 1 143
22 4 17 5 8 8 22 2 8 1 1 1 1 1 100
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Table 2 Chemical analyses and physical parameters of the ashed raw coals from Spitzbergen and Ensdorf
SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2O P2 O5 SO 3 Sum Softening point Deformation point Flow point
Spitzbergen
Ensdorf
37.40 0.90 14.60 16.60 0.06 3.60 9.70 3.20 1.70 0.39 10.50 98.65 1170 1190 1250
42.60 0.84 22.80 12.20 0.17 2.46 9.42 1.03 2.34 0.09 6.01 99.96 1290 1300 1330
considered to be the major K source in coals w11x. The coal from Spitzbergen has a rather high pyrite content, while the Ensdorf coal has significant amount of carbonate minerals. The mineralogical composition is also reflected in the bulk chemical composition of the ash fraction. The Spitzbergen coal has a high iron and sulfur content. The principal alkali species is sodium. In contrast the Ensdorf coal has less Fe and a higher K content. The chemical composition of the non-carbonaceous content of both coals can be compared to silica-deficient volcanic melts. These melts are characterized by a low viscosity at high temperatures. The low flow point of the ash fraction of the raw coal guarantees a liquid removal of the slag in a pressurized pulverized coal combustion plant ŽPPCC. w3x. The physical parameters softening point, deformation point and flow point are reported in Table 2. The data reported in Table 2 give the approximate composition of the ash fraction of the used coals. 4.2. Samples from the gas-mixing furnace The major chemical components of all experimental samples were quantified. The microanalytical data of the slag samples are well reproduced, and the results of samples under varying conditions can easily be compared relative to each other. This information can be used to image the relationship of the concentrations of most of the slag components versus the temperature and the oxygen fugacity. In the following we will focus on elements generally considered to be critical in terms of volatilization such as Si, Ca, Fe and the alkalis w16x. The data for other elements that are considered refractory and as a minor component only is listed in Tables 3 and 4. Fig. 2 displays the temperature–oxygen fugacity Žlog f O 2 . relationship of SiO 2 for both ashed coal samples. As can be seen from Fig. 2A–F and the associated Tables 3 and 4, a Si loss is of minor importance and the Si concentration is independent of temperature and oxygen fugacity. This observation does not exclude that small amounts
Table 3 Experimental parameters and the results of electron probe microanalyses of experimental slag of the Spitzbergen coal Name
log f O 2
Na 2 O
MgO
Al 2 O 3
SiO 2
P2 O5
SO 3
K 2O
CaO
TiO 2
Cr2 O 3
MnO
FeO
NiO
Total
1350 1350 1350 1350 1350 1350 1350 1350 1400 1400 1400 1400 1400 1400 1400 1400 1400 1450 1450 1450 1450 1450 1450 1450
SP35 SP34 SP31 SP36 SP29 SP29 SP30 SP32 SP1neu SP26 SP41 SP17 SP13 SP10 SP18 SP19 SP20 SP43 SP50 SP49 SP45 SP48 SP46 SP44
y0.76 y0.81 y0.9 y1.1 y1.27 y1.27 y2 y5.00 y0.68 y0.73 y0.80 y0.81 y0.92 y1.01 y1.11 y1.34 y1.36 y0.68 y0.8 y1.11 y1.34 y1.34 y2 y3
2.96 2.94 3.01 2.90 2.97 3.00 3.09 3.01 3.05 2.90 2.87 2.89 2.90 2.94 2.95 3.05 2.91 2.90 2.79 2.86 2.95 2.71 2.67 2.90
3.92 3.87 3.88 3.81 3.95 3.84 3.86 3.94 3.78 3.86 3.83 3.84 3.86 3.92 3.86 4.01 3.94 3.95 3.85 3.99 3.95 3.81 3.99 4.00
18.44 18.23 18.45 17.93 17.97 18.12 18.48 18.43 17.76 17.97 17.48 17.87 17.57 17.44 18.11 18.16 18.02 18.25 18.31 18.01 18.19 18.12 18.20 18.16
45.67 45.72 45.53 46.82 45.81 45.71 45.42 46.99 46.96 46.31 46.91 45.89 46.97 46.29 45.19 44.99 45.93 45.28 46.71 45.62 46.13 47.26 46.14 45.79
0.38 0.40 0.39 0.39 0.37 0.37 0.39 0.40 0.38 0.30 0.32 0.33 0.34 0.41 0.38 0.34 0.34 0.21 0.32 0.42 0.30 0.33 0.42 0.32
0.01 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.01 0.01 0.00
1.38 1.34 1.34 1.32 1.35 1.33 1.33 1.30 1.30 1.23 1.32 1.27 1.26 1.25 1.27 1.27 1.25 1.31 1.15 1.18 1.20 1.09 1.02 1.21
12.08 12.21 12.31 11.73 12.25 12.10 12.22 12.34 12.28 11.83 11.96 11.76 11.85 11.90 11.88 12.18 12.15 12.16 12.15 12.12 12.27 11.95 12.27 12.27
1.10 1.05 1.06 1.02 0.98 1.04 1.12 1.12 1.03 1.06 1.02 1.05 1.07 1.04 1.03 1.04 1.00 1.05 1.03 1.08 1.03 1.06 1.12 1.03
0.02 0.01 0.02 0.02 0.02 0.02 0.03 0.03 0.01 0.02 0.01 0.02 0.02 0.02 0.02 0.03 0.02 0.01 0.03 0.03 0.03 0.03 0.03 0.02
0.06 0.05 0.05 0.05 0.02 0.03 0.05 0.06 0.04 0.05 0.05 0.04 0.04 0.04 0.04 0.04 0.06 0.03 0.04 0.05 0.05 0.05 0.05 0.05
13.96 14.15 13.93 13.99 14.28 14.40 13.98 12.36 13.37 14.45 14.19 15.01 14.09 14.73 15.25 14.86 14.32 14.82 13.58 14.61 13.87 13.56 14.05 14.22
0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.03 0.02 0.02 0.02 0.03 0.03 0.03 0.02 0.02 0.02 0.02
100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 99.97 100.00 100.00 100.00 100.00 100.00 100.00 100.00
M. Enders et al.r Fuel Processing Technology 68 (2000) 57–73
T Ž8C.
63
64
Table 4 Experimental parameters and the results of electron probe microanalyses of experimental slag of the Ensdorf coal Name
log f O 2
Na 2 O
MgO
Al 2 O 3
SiO 2
P2 O5
SO 3
K 2O
CaO
TiO 2
Cr2 O 3
MnO
FeO
NiO
Total
1350 1350 1350 1350 1350 1350 1350 1350 1350 1400 1400 1400 1400 1400 1400 1400 1400 1400 1450 1450 1450 1450 1450 1450
EN25 EN35 EN34 EN31 EN28 EN36 EN29 EN30 EN32 EN1neu EN26 EN33 EN17 EN13 EN10 EN18 EN19 EN20 EN43 EN50 EN49 EN48 EN46 EN44
y0.68 y0.76 y0.80 y0.81 y1.08 y1.10 y1.27 y2.02 y5.00 y0.68 y0.73 y0.80 y0.81 y0.92 y1.01 y1.11 y1.34 y1.36 y0.68 y0.8 y1.11 y1.34 y2 y3
1.21 1.17 1.13 1.14 1.19 1.13 1.13 1.15 1.23 1.21 1.18 1.14 1.15 1.19 1.22 1.10 1.15 1.15 1.33 1.10 1.17 1.13 1.11 1.16
2.57 2.59 2.49 2.77 2.66 2.54 2.75 2.74 2.85 2.73 2.56 2.55 2.55 2.56 2.48 2.53 2.60 2.57 2.51 2.53 2.56 2.63 2.59 2.58
24.71 24.42 25.17 24.37 24.76 25.13 24.08 24.15 24.62 24.54 24.92 24.66 24.56 24.81 24.78 24.71 24.78 24.80 24.83 24.62 25.12 25.08 25.08 25.19
46.33 46.82 46.16 45.84 45.97 45.97 46.82 46.89 46.84 45.99 46.09 46.87 46.09 46.30 46.46 46.47 45.93 46.47 46.26 46.88 46.90 46.73 46.96 46.19
0.07 0.09 0.08 0.08 0.07 0.11 0.07 0.07 0.07 0.07 0.07 0.05 0.09 0.10 0.10 0.08 0.07 0.08 0.08 0.08 0.08 0.07 0.09 0.07
0.01 0.01 0.02 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.01
2.38 2.46 2.43 2.45 2.46 2.34 2.55 2.46 2.51 2.51 2.28 2.29 2.30 2.31 2.33 2.28 2.27 2.26 2.31 2.05 1.97 1.81 1.86 2.21
10.28 10.13 10.52 9.94 10.18 10.54 9.76 9.83 10.30 9.99 10.47 10.23 10.17 10.28 10.26 9.97 10.27 10.25 10.32 10.18 10.40 10.48 10.55 10.42
0.88 0.82 0.88 0.90 0.91 0.88 0.87 0.92 0.83 0.93 0.85 0.82 0.80 0.88 0.85 0.84 0.83 0.81 0.84 0.85 0.87 0.89 0.85 0.85
0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.04 0.03 0.03 0.02 0.01 0.02 0.02 0.03 0.02 0.03 0.02 0.01 0.02 0.03 0.03 0.03 0.02
0.20 0.20 0.19 0.16 0.20 0.20 0.16 0.18 0.17 0.20 0.22 0.15 0.21 0.20 0.20 0.21 0.21 0.20 0.21 0.20 0.20 0.20 0.21 0.20
11.31 11.25 10.87 12.28 11.54 11.10 11.76 11.55 10.51 11.76 11.31 11.17 12.04 11.32 11.25 11.76 11.84 11.38 11.26 11.45 10.67 10.91 10.65 11.08
0.02 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.02 0.02
100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 99.97 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
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T Ž8C.
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Fig. 2. Relationship of SiO 2 concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
of Si can occur as a volatile species, but the stability of the Si data points versus oxygen fugacity clearly confirm that the Si-transfer from the slag to the gas atmosphere is of minor importance. Fig. 3A–F gives a similar relationship for Ca. From the figure it is obvious that no calcium is lost during the experiment to the gas atmosphere. This is expected, as CaO is commonly assumed to be refractory during combustion processes w16x. This observation also excludes the formation of a calcium sulfate phase on surface of the melt droplet during the experiment, as this process would lead to a loss of the water soluble CaSO4 during the quenching.
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Fig. 3. Relationship of CaO concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
The Fe concentration displays a minor variation with oxygen fugacity ŽFig. 4A–F.. In very low oxygen fugacites Žlog f O 2 - y1. a loss of Fe was observed in the Ensdorf sample. Fe is the only element that displays a different behavior in both coal samples. Reconnaissance experiments in CO–CO 2 atmospheres confirmed a loss of Fe from the slag in highly reducing atmospheres ŽTables 3 and 4.. The data points for Fe vary
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Fig. 4. Relationship of FeO concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
randomly about 5–10% relative to the reported number ŽFig. 4, Tables 2 and 3.. The reason for this spread in the data set is not clear. It might be affected by rather low count rates that increase the statistical error on counting statistics during the measurement, or it might be attributed to the Fe 2 O 3 –Fe 2q Fe 3q 2 O4 buffer that is important in the temperature range of the experiments. The scatter of the Fe data points cannot be due to Fe-bearing silicate or oxide phases, as these were not observed in back-scattered imaging. The formation of a Pt–Fe alloy along the platinum wire is — as stated above — negligible. The behavior for alkali elements is different ŽFigs. 5 and 6.. The concentration versus oxygen fugacity relationship for sodium is depicted in Fig. 5A–F. At low temperatures
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Fig. 5. Relationship of Na 2 O concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
ŽFig. 5C and F. only a minor loss of sodium during the experiment is observed. With increasing temperature the absolute sodium loss increases to about 10% compared to the maximum sodium content of the experimental samples Ž13508C.. In high temperature experiments a relation between sodium content and oxygen fugacity is obvious. The maximum loss is detected at oxygen fugacities in the range from log f O 2 Žy1. to Žy2., which corresponds to oxygen concentrations of 1–8% in the gas atmosphere. Within the same temperature range the sodium content varies by 8–10% due to changes in the oxygen fugacity.
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Fig. 6. Relationship of K 2 O concentration versus temperature and oxygen fugacity. ŽA–C. Spitzbergen coal, ŽD–F. Ensdorf coal. The errors of analysis and the oxygen fugacity are approximately in the size of the symbols.
At low temperatures we observed only a minor potassium loss rather similarly to sodium ŽFig. 6A–F.. At temperatures of 14508C the absolute loss of potassium can be 25% of the initial values ŽFig. 6A and D.. Potassium displays a pronounced relationship to oxygen fugacity. A minimum K concentration in the slags is observed at oxygen concentrations of about 1–10% in the gas atmosphere. If both temperature and oxygen fugacity are varied, about 1r3 of the potassium can be volatilized from the slag. At very low oxygen concentrations in the gas atmosphere the K concentration rises again. This rise indicates a second process besides a polymerization influencing the alkali retention of the slag.
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5. Discussion The experiments are restricted by the experimental setup to temperatures in the range from 13508C to 14508C. This temperature range rarely equates with temperatures in the actual combustion flame. Under conditions of pressurized pulverized coal combustion maximum temperatures of about 2000 K in the pulverized coal flame are expected w21x. During the residence of the slag on the boiler walls and in the liquid slag separation unit vaporization can become important. The range of oxygen fugacities during the experiments in the gas-mixing furnace were adjusted to the gas atmosphere expected in a PPCC power plant. Most technical high-temperature processes such as steel production, glass production or the processing of ceramics require process temperatures and oxygen fugacities similar to those used in the experiments. In the case of the coal from Ensdorf the lowest temperature applied in the experiments is very close to its flow point ŽTable 2.. The sample temperature difference to the flow point leads to an incomplete homogenization of this coal in the 13508C and 14008C experiments. This effect can easily be seen from the rather varying concentrations for various elements. In the 14508C samples the material had been very well homogenized, as is obvious from Table 4. Further, any dynamic process during the volatilization of alkalis in the actual burner or the different behavior of organic-bounded alkalis versus inorganic-bounded is neglected w14x. This study quantified the concentration of standard slag components in experimental slag samples. From the total of 13 quantified elements we restrict the discussion to those important for volatilization during combustion processes. Si is considered of major importance in combustion processes. Early workers already discovered microsilica spheres that are obviously due a vaporization of Si in the burner flame w16x. Commonly, this process is attributed to the formation of volatile SiO w16x. During our experiments in oxidizing atmospheres we found no indication for a vaporization of Si from the slag. The small amounts of sample material used during the experiments cannot exclude, that in an actual power plant, where tons of coal are converted per hour, even a minimum mobilization of Si can already cause a considerable SiO concentration in the flue gas. In both coals the observed SiO 2 content is higher than the SiO 2 content of the raw coal after normalization to the high temperature composition Žwithout SO 3 .. The microanalyses monitor related changes in composition between experimental samples very accurately. The composition of the ash fraction of raw coal had been determined several times. However, these analyses are inconsistent. This indicates a very problematic analysis of ash fractions of coal depending on the laboratory, the sampling, and the type of instrumental analysis applied. Ca is not mobilized under the conditions applied in the experiments. The Ensdorf coal and less clearly the Spitzbergen coal displayed a partial loss of Fe under reducing conditions Ž- to 1% oxygen. and in CO 2 –CO atmospheres ŽTables 3 and 4.. This effect must be due to a fluxing by Fe 2q. The increased mobility of cations in the slag combined with a low oxygen atmosphere obviously favors a partial volatilization of Fe. In terms of vaporization–condensation reactions alkalis are the most important species in the system. The experiments clearly confirm a temperature-dependent
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volatilization of the alkalis. A reduction in combustion temperature, equivalent to a loss in efficiency of the process could decrease the volatilization of sodium to about 10%. For potassium a decrease of the temperature in the slag separator could decrease the K loss up to 25%. The oxygen fugacity has a considerable influence on the alkali retention of the slag. At oxygen fugacities equivalent to pure air the optimum fixation of alkalis into slag can be achieved. The increased retention of K and Na at high oxygen fugacites is a clear indication of a slag highly polymerized due to a high Fe 3q content. The increased polymerization of the melt inhibits the mobility of alkalis in the slag, and therefore reduces the transfer to the gaseous phase. A way to raise the oxygen fugacity and therewith to reduce the vaporization of alkalis could be the introduction of excess air into the boiler. However, any excess air in the boiler would decrease the overall efficiency of the conversion process. At lower oxygen fugacities when the oxygen concentration in the flue gas is in the range of 1–10% Ži.e. normal power plants conditions., the fixation of alkalis is worst. In conditions close to CO 2 –CO atmosphere a rise of the K concentration in the experimental slag samples indicates an increased fixation of alkalis. The latter effect is a function of oxygen fugacity, and therefore should correlate to a valence change of Fe in the slag. Fe 3q has low concentrations in the slag under low oxygen fugacities. This excludes Fe 3q as the reason for an increased polymerization of the melt. In contrast, this effect must be related to an increased Fe 2q concentration, which is enriched in reducing environments. The action of Fe 2q as a flux is well known from ceramics production especially when alkalis are present w22x. Under reducing atmospheres the increased Fe 2q content of the ash would accelerate the melting of the ashed coal. The more rapid melting would fix the alkalis to the slag rather than vaporize them to the gas atmosphere in the oven. The exchange from the slag to the gas atmosphere is limited by the mobility of ions in the slag. The steady gas stream along the sample material ensures a constant disequilibrium. If the alkalis would be mobilized from the surface of the slag droplet in the furnace one would expect a concentration profile within the slag droplet, with low values at the surface and higher concentrations in the core. This is not observed. Therefore, an increased fixation of alkalis must be attributed completely to changes in the slag sample itself. The bimodal form of the K and Na distribution with oxygen fugacity indicates two processes: at high oxygen fugacities an effective polymerization of the melt, and at low temperatures a considerable shortening of the sintering process of the sample due to the fluxing action of Fe 2q. This confirms the importance of the actual behavior of minerals in the flame for the overall flue gas composition.
6. Conclusion In a pulverized power plant only a few parameters can be changed to increase to alkali fixation to slag without either decreased efficiency or stopping the combustion process. From the data reported in this study it seems possible to improve the alkali fixation in coal slag with an increased oxygen fugacity due to an increased polymerization of the slag. However, this effect can hardly be used to trap alkalis in the melt as the
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simple introduction of excess air would result in an overall loss in efficiency. Our data confirms an increased alkali fixation also at very low oxygen fugacities. This effect cannot be attributed to an increased polymerization of the melt. The occurrence of an increased amount of Fe 2q has an effect on the sintering behavior of the slag similar to the influence during the firing of ceramics. Under these conditions the alkali emission is low. However, iron might be volatilized under such conditions to flue gas. This effect can take place under high temperatures Ž) 14008C. and low oxygen fugacities close to pure CO 2 atmospheres. These conditions are not achieved in conventional oxidizing combustors. From the viewpoint of combustion under oxidizing conditions vaporization of Si and Fe seems to be negligible. In large-scale combustors even minimum concentrations of Fe and Si in the flue gas can cause slagging and fouling of construction materials. The results of this experimental study can be used to optimize the flue gas composition in a pressurized pulverized coal combustion in order to reduce emission of alkalis not in the flame, but in the slag separation unit. References w1x K. Strauß, Kraftwerkstechnik, Springer-Verlag, Berlin, 1997, p. 494. w2x I. Romey, H. Rode, M. Schuknecht, GuD-Prozesse fur ¨ den Einsatz von Kohle, in: K. Hannes ŽEd.., Erstes Statuseminar Druckflamm, STEAG AG, 17 Nov., Zeche Zollverein, 1998, pp. 163–173. w3x K. Hannes, Pilot plant test results of pressurized pulverized coal combustion, in: BEO ŽEd.., Proceedings of the EU Seminar AStatus of Development and Market Penetration of Clean Coal Technologies ŽCCT. for Power GenerationB, Dusseldorf, Germany, 5r6 Nov. 1998, Eur. CommmisionrBEO, pp. 1–15. ¨ w4x M. Forster, Ergebnisse und Aufgabenstellungen aus dem Versuchsbetrieb in Dorsten, in: K. Hannes ŽEd.., ¨ Erstes Statuseminar Druckflamm, STEAG AG, 17 Nov., Zeche Zollverein, 1998, pp. 4–9 and 163–173. w5x M. Enders, M. Spiegel, J. Albrecht, A. Putnis, Chemical and mineralogical composition of fly ash collected from a hot flue gas in a pressurized pulverized coal combustion, Eur. J. Mineral 12 Ž2000. 639–650. w6x M. Enders, J. Albrecht, A. Putnis, Fractionation of slag and condensates from a hot flue gas of a pressurized pulverized coal combustion ŽPPCC. pilot plant, Energy & Fuels 14 Ž2000. 806–815. w7x A.S. Benson, E.A. Sondreal, J.P. Hurley, Status of coal ash behavior research, Fuel Process. Technol. 44 Ž1995. 1–12. w8x G.E. Vaninetti, H. Busch, Mineral analysis of ash data and utility perspective, J. Coal Qual. 1 Ž1982. 22–31. w9x G.P. Huffman, F.E. Huggins, G.R. Dunmyre, Investigation of the high temperature behavior of coal ash in reducing and oxidizing atmospheres, Fuel 60 Ž1981. 585–597. w10x J.W. Nowok, J.P. Hurley, S.A. Benson, The role of physical factors in mass transport during sintering of coal ashes and deposits deformation near the temperature of glass transformation, Fuel Process. Technol. 56 Ž1998. 89–101. w11x S.K. Gupta, R.P. Gupta, G.W. Brayant, T.F. Wall, The effect of potassium on the fusibility of coal ashes with high silica and alumina levels, Fuel 77 Ž1998. 1195–1201. w12x H.J. Hurst, F. Novak, J.H. Patterson, Viscosity measurements and empirical predictions for some model gasifier slags, Fuel 78 Ž1999. 439–444. w13x D.I. Barnes, D.G Wardle, D. Kean, Sodium and potassium release from UK coals during the early stages of pulverized coal combustion, in: J. Williamson, F. Wigley ŽEds.., The Impact of Ash Deposition on Coal Fired Power Plants, Proc. Eng. Found. Conf., Solihull, England 20–25, June, Taylor & Francis, London, 1993. w14x N.B. Gallagher, L.B. Bool, J.O.L. Wendt, Alkalimetal partitioning in ash from pulverized coal combustion, Combust. Sci. Technol. 74 Ž1990. 211–221.
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w15x B.L. Chadwick, R.A. Ashman, A. Campisi, G.J. Crofts, P.D. Godfrey, P.G. Griffin, A.L. Ottrey, R.J.S. Morrison, Development of techniques for monitoring gas-phase sodium species formed during coal combustion and gasification, Int. J. Coal Geol. 32 Ž1996. 241–253. w16x W. Gumz, H. Kirsch, M-Th. Mackowsky, Schlackenkunde, Springer-Verlag, Berlin, 1958, 422 pp. w17x M. Enders, M. Spiegel, Mineralogical and microchemical study of high-temperature-reactions in fly ash scales from a waste incineration plant, Eur. J. Mineral. 11 Ž1999. 763–774. w18x K. Hannes, Erstes Statusseminar Druckflamm, STEAG AG, 17 Nov., Zeche Zollverein, Essen, Germany, 1998, p. 216. w19x A.K. Varshneya, Fundamentals of Inorganic Glasses, Academic Press ŽHarcourt Brace and Company, Publishers., 1993, p. 570. w20x E. Woermann, Methods of investigation of phase equilibria,Verein Deutscher Eisenhuttenleute ŽVDEh.: ¨ Slag Atlas,1995, pp. 23–25. w21x U. Renz, Experimentelle und theoretische Untersuchungen an der Druckkohlenstaubfeuerung der RWTH Aachen, in: K. Hannes ŽEd.., Erstes Statuseminar Druckflamm, STEAG AG, 17 Nov., Zeche Zollverein, 1998, pp. 16–26. w22x K. Jasmund, G. Lagaly, Tonminerale und Tone, Steinkopff Verlag, Darmstadt, 1993, 490 pp.