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Reaction of catalysts with mineral matter during coal gasification
Reaction of catalysts with rniner~~ matter during coal gasification” Lothar
Kiihn and Horst
Bergbau-Forschung 13, GFR
Plogmann
GmbH, Abt. Ghemie-Analytik,
Franz-Fischer-
Weg 61, D-4300
Essen
After a review of mineral matter of German hard coal, its reactions with catalystsduring gasification with steam to form new crystalline phases are dealt with. Products of commercial plants and laboratory experiments were investigated by means of microscopy, XRD, XRF and microprobe analysis. (Keywords: catalysts; coal; coal minerals: instrumental methods of analysis)
unreacted part was dissolved out before analysis. In other experiments mineral matter from coal was used with stoichiometric additions of catalyst and other minerals to adjust certain bulk compositions. In all these experiments gasification conditions were simulated by using an autoclave, applying an atmosphere of 50% product gas and 50% steam, pressure and temperatures corresponding to the gasification runs. Reaction temperatures were reached in 1-2 h and held for 1-5 h. Table I includes ash analyses of the coals, the residues of which were studied, as well as the compositions of the minerals used for the additional experiments without coal. The analyses of these clay minerals and the carbonate are also given for the ashed state without water and CO,. The mineral matter of these coals differs only slightly and is typical of German hard coal. lllite and kaolinite together make up =6&80 wt% of mineral matter. Quartz and dolomite are minor constituents. Siderite and pyrite form the remainder, which is reflected by the Fe,O, content of the ash. Illite is the most abundant mineral in German hard coals. An almost pure illite sample was found in a coal from the Walsum mine. The composition is considered to be fairly representative of German hard coals. The kaolinite and magnesite used are standard samples.
It is well known that potassium is a good catalyst for coal gasification.’ But experiments show clearly that the catalytic activity of potassium is effective only when the element is present in the form of certain salts, of which K2C0, has proved to be the best.2 Evidence for the significance of the bonding form of potassium is also given by Sulimma et a[.,3 who have shown that a considerable part of this catalyst is inactivated by reactions with the mineral matter of coal. In gasification experiments on a technical and laboratory scale various K compounds were generated by such reactions and detected by X-ray analysis. The present study was undertaken to establish conditions for their formation.
EXPERIMENTAL Residues of allothermal steam gasification runs without and with K,CO, catalyst at 4 MPa and 7OO-900°C were studied by X-ray powder diffraction, XRF, microscopy and microanalysis. Additional experiments were carried out with mixtures of K,CO, and mineral matter of coal isolated by low temperature ashing, and with mixtures of catalyst and the clay minerals kaolinite and illite alone. The K,CO, catalyst was added in excess and the
Table 1
Analyses of coal ash and minerals
SiO.2 At203
TiOz Fe203
CaO MgO Na20 K2O so3
Niederberg coal
Leopold char
Pattberg coat
lllite from Walsum
Kaolin from Tirschenreuth
46.4 26. f 1.1 14.5 3.6 1.9 0.9 3.8 1.6
35.6 27.6 0.8 19.5 5.8 2.2 1.1 2.3 4.0
41.7 28.8 1.0 8.8 5.7 3.8 1.5 4.1 4.2
51.2 33.3 1.7 2.4 0.2 1.7
54.3 42.3 0.5 0.6 0.1
1 .o
n.d.
7.2
Magnesite from Oberndorf, Austria
7.8 1.8 3.0 4.2 82.9
1 .5
_ * Presented at International Symposium, ‘Fundamentals of Catalytic Coal and Carbon Gasification‘, held at Amsterdam, The Netherlands,
27-29 September
1982
001s-236~/~3/0Z0205~$3.00 @ 1983 Butterworth & Co. (Publishers)
Ltd
FUEL, 1983,
Vol 62, February
205
Reaction
of catalysts with mineral matter during coal gasification:
Table 1 Crystaiiine~eaction products, with steam up to 900 C and 4 MPa
formed
during gasification ----
Type
Formula
Compound
Spinels
Fe0 * Fez03 Fe0 ’ Al203 2FeO ’ SiOz 2MgO * SiO2 K20 . Als03.6SiO2 Na20 1Al203 ’ 6SiO2 CaO A1203 .2SiO2 3A12Os ’ 2sio2
Magnetite Hercynite Fayalite Forsterite Sanidine Albite Anorthite
Olivines Feldspars
Mullite
-
RESULTS AND DISCUSSION In gasification runs without added catalysts melting of mineral matter does not occur below 900°C. But, apart from quartz, all mineral species decompose, leaving an amorphous mass of oxides. Only illite in the range 80& 850°C may retain some crystalline order detectable by Xray diffraction, but this disappears either at higher temperatures or with time because of subsequent reactions. Pyrite is oxidized to magnetite, which reacts further with other residues. Generally because of these decompositions mineral matter is in a very reactive state, so that residual ash from a fluidized bed with a residence time of hours may be shown by X-ray analysis to contain various new crystalline phases4 which are listed in Table 2. Spinels, olivines and feldspars are typical solid solutions, the end members of which are indicated. While spinels in gas~cation residues generally cover the whole composition range, olivines and feldspars are represented essentially by fayalite and anorthite. They are early formations, while mullite, a typical reaction product of clay minerals at higher temperatures is scarcely formed in this temperature range. The reactions take place between solids in the naturally intergrown dense masses of mineral matter and the new crystals formed are generally too fine to be identified by visual microscopy. But there are always some larger under favourable grown crystallized aggregates, conditions, which permit microscopic and microprobe studies. In such cases at the surfaces of ffuidized-bed particles reaction rims on magnetite aggregates that consisted of fayalite were observed. These structures indicated that reactions between solids and the gas atmosphere had taken place also. It is thought that silicon was transported by the water vapour atmosphere, probably in the form of Si(OH),, to the magnetite aggregates and reacted there to form fayalite. As mineral matter of coal transforms during gasification, catalyst additions meet a very reactive substrate, which promotes reactions with the catalyst (Hedvall effect). But this seems to be valid also for the mineral matter of char, which because of thermal treatment is already altered before gasification. In gasification runs at 700°C of coal and char with K,CO, catalyst no difference in the reactivity of mineral matter was noted. In both cases kaliophilite was formed, which seems to be the stable reaction product for a wide variety of compositions. Kaolinite has the same Si0,/A1,03 ratio as kaliophilite. It is therefore an ideal reaction partner for kaliophilite formation. Pure kaolinite and an excess of K,CO, were found to be completely converted to
206
FUEL, 1983, Vol 62, February
L. Kiihn and H. Plogmann
ka~ophi~ite in 1 h at 700°C. But a parallel experiment with illite, the higher SiO,/Al,O, ratio of which is not so appropriate, also yielded kaliophilite in an amount corresponding to total consumption of the alumina. In corresponding experiments with soda as catalyst nepheline, the analogous Na compound to kaliophilite, was formed. Table 3 shows the composition of various reaction products. Among these reaction products kaliophilite is the compound with the highest potassium concentration. To save potassium it is therefore of interest to know the conditions of their formation. Experiments with stoichiometric compositions were carried out, using mineral matter isolated by lowtemperature ashing from Pattberg coal (Table I), which by addition of other minerals was adjusted to the desired composition. These mixtures were held at reaction temperature for a::5 h. At first a silica/alumina ratio of 70/30 as for leucite was generated by addition of quartz from D~rentrup. Leucite was formed, though the K,O content of the mixture was only 60% of that of leucite. In this experiment because of the low reactivity of quartz, which was added as a powder of 7 pm grain size, there was only partial conversion at 8OO”C,which was complete at 900°C. Kaliophilite and leucite formation may be easily explained by application of the KzO-A&O,-Si03 phase diagram.5 In this diagram the tie-lines from the points, which mark the silica/alumina ratio of kaolinite (56/44) and illite (61/39), to the K,O corner indicate the changes of composition by K additions. They pass into the stability field of kaliophiIite and show further, that the leucite stability field can be reached only by adding SiO, to the clay minerals. For sanidine formation (potash feldspar) the silica/alumina ratio must be 378/22. That means that the SiO, addition to illite must be as high as the illite content itself or higher. As mineral matter of German hard coal is not so rich in SiO,, it is highly improbable that potash feldspar is formed by K additions. However, sanidine was found together with osumilite by X-ray analysis in residues from a pilot plant, in which 800°C was reached, which indicated special conditions of their formation. 0sumilite,6 from which an intermediate composition is given in Table 3, also has a high silica/alumina ratio, comparable to that of feldspar. With quartz, magnesite (Table I) and mineral matter of coal (Table 1) a stoichiometric mixture was prepared to synthesize osumilite. But heating at 800-900°C for 5 h was not successful. Sanidine could be detected microscopically and its Table 3
Reaction
products with catalysts _-
~.~ Component
la
2
3
4
5
6 -“......
64.8 .18.3 _
55.0 23.5 _
38.0 32.2 -
42.3
MgG CaC
62.1 23.9 9-Q _
21.9 37.2 -
K+’ NasO
48 -
169 -
21.6 -
298 -
21.8
SiOs A12O3
35.9 -
409 .--
a I, K20. 2, KsO 3, K20. 4, KzO. 5, Nat0 6,ZCaO
4.4MgO * 4.6A1s03. . Al203 .6SiO2 Al203 .4SiO2 A1203.2SiOs . AltO3 ’ 2SiO2 At203 ‘SiO2
20.4SiO2
Osumilite Sanidine Leucite Kaliophilite Nepheiine Gehlenite
Reaction of catalysts with mineral matter during coal gasification: L. Kijhn and H. Plogmann
Ca
Fe
Figure 1
Sanidine formation
in a K-treated gasification
residue from a fluidized
structure indicated special conditions of formation, which are illustrated in Figure I. The photomicrograph (middle, bottom row) shows agglomerated ash particles of the fluidized bed in reflected light. The agglomerate covering the right half of the photomicrograph is cut by a vein, filled with mineral aggregates, which cover the left half. The frame indicates the area of the image below left, from which the element distribution images above have been taken. Vein fillings and the outer rims of the agglomerated particles exhibit the elements of sanidine. The coatings of the ash particles are the result of water vapour transport, which introduced other elements, particularly Si, as already mentioned in the case of olivine formation at the surface of magnetite aggregates.
bed
In the case of sanidine formation it is thought that clayey debris in the veins provided the condensation nuclei for the vapour deposits, the high Si concentration which favoured the formation of sanidine. Osumilite could not be detected microscopically, but it is believed that its formation is similar to that of sanidine, because it also needs a high Si concentration. This example shows that reaction products with low K require which concentrations special conditions, normally are not obtained with German hard coal. Sulimma et al. saved K,CO, catalyst by pretreatment of the coal with Ca(OH),, which causes gehlenite formation. But further reactions with K,CO, catalyst cannot be suppressed completely by this pretreatment.
FUEL,
1983,
Vol 62, February
207
Reaction
of catalysts with mineral matter during coal gasification:
ACKNOWLEDGEMENT This research was sponsored by Bundesministerium Forschung und Technologie. REFERENCES 1 McKee, D. W. Chem. Phys. Carbon 1981, 16, 1
208
FUEL, 1983, Vol 62, February
fiir
L. Kiihn and H. Plogmann
2 Veraa, M. J. and Bell, A. T. Fuel 1978,57, 194 3 Sulimma, A., van Heek, K. H. and Jiintgen, H. ‘Proc. International Conference on Coal Science’, Dusseldorf 1981, 313 4 Plogmann, H. and Kuhn, L. ‘Proc. International Conference on Coal Science’, Dusseldorf 1981, 241 5 Levitt, E. M., Robbins, C. R. and McMurdie, H. F. ‘Phase Diagrams for Ceramists’, 3rd ed., American Ceramic Society, Columbus, Ohio, 1974 6 Olesch, M. and Seifert, F. Contrib. Mineral Petrol. 1981, 76, 362
Kiihn and Horst
Bergbau-Forschung 13, GFR
Plogmann
GmbH, Abt. Ghemie-Analytik,
Franz-Fischer-
Weg 61, D-4300
Essen
After a review of mineral matter of German hard coal, its reactions with catalystsduring gasification with steam to form new crystalline phases are dealt with. Products of commercial plants and laboratory experiments were investigated by means of microscopy, XRD, XRF and microprobe analysis. (Keywords: catalysts; coal; coal minerals: instrumental methods of analysis)
unreacted part was dissolved out before analysis. In other experiments mineral matter from coal was used with stoichiometric additions of catalyst and other minerals to adjust certain bulk compositions. In all these experiments gasification conditions were simulated by using an autoclave, applying an atmosphere of 50% product gas and 50% steam, pressure and temperatures corresponding to the gasification runs. Reaction temperatures were reached in 1-2 h and held for 1-5 h. Table I includes ash analyses of the coals, the residues of which were studied, as well as the compositions of the minerals used for the additional experiments without coal. The analyses of these clay minerals and the carbonate are also given for the ashed state without water and CO,. The mineral matter of these coals differs only slightly and is typical of German hard coal. lllite and kaolinite together make up =6&80 wt% of mineral matter. Quartz and dolomite are minor constituents. Siderite and pyrite form the remainder, which is reflected by the Fe,O, content of the ash. Illite is the most abundant mineral in German hard coals. An almost pure illite sample was found in a coal from the Walsum mine. The composition is considered to be fairly representative of German hard coals. The kaolinite and magnesite used are standard samples.
It is well known that potassium is a good catalyst for coal gasification.’ But experiments show clearly that the catalytic activity of potassium is effective only when the element is present in the form of certain salts, of which K2C0, has proved to be the best.2 Evidence for the significance of the bonding form of potassium is also given by Sulimma et a[.,3 who have shown that a considerable part of this catalyst is inactivated by reactions with the mineral matter of coal. In gasification experiments on a technical and laboratory scale various K compounds were generated by such reactions and detected by X-ray analysis. The present study was undertaken to establish conditions for their formation.
EXPERIMENTAL Residues of allothermal steam gasification runs without and with K,CO, catalyst at 4 MPa and 7OO-900°C were studied by X-ray powder diffraction, XRF, microscopy and microanalysis. Additional experiments were carried out with mixtures of K,CO, and mineral matter of coal isolated by low temperature ashing, and with mixtures of catalyst and the clay minerals kaolinite and illite alone. The K,CO, catalyst was added in excess and the
Table 1
Analyses of coal ash and minerals
SiO.2 At203
TiOz Fe203
CaO MgO Na20 K2O so3
Niederberg coal
Leopold char
Pattberg coat
lllite from Walsum
Kaolin from Tirschenreuth
46.4 26. f 1.1 14.5 3.6 1.9 0.9 3.8 1.6
35.6 27.6 0.8 19.5 5.8 2.2 1.1 2.3 4.0
41.7 28.8 1.0 8.8 5.7 3.8 1.5 4.1 4.2
51.2 33.3 1.7 2.4 0.2 1.7
54.3 42.3 0.5 0.6 0.1
1 .o
n.d.
7.2
Magnesite from Oberndorf, Austria
7.8 1.8 3.0 4.2 82.9
1 .5
_ * Presented at International Symposium, ‘Fundamentals of Catalytic Coal and Carbon Gasification‘, held at Amsterdam, The Netherlands,
27-29 September
1982
001s-236~/~3/0Z0205~$3.00 @ 1983 Butterworth & Co. (Publishers)
Ltd
FUEL, 1983,
Vol 62, February
205
Reaction
of catalysts with mineral matter during coal gasification:
Table 1 Crystaiiine~eaction products, with steam up to 900 C and 4 MPa
formed
during gasification ----
Type
Formula
Compound
Spinels
Fe0 * Fez03 Fe0 ’ Al203 2FeO ’ SiOz 2MgO * SiO2 K20 . Als03.6SiO2 Na20 1Al203 ’ 6SiO2 CaO A1203 .2SiO2 3A12Os ’ 2sio2
Magnetite Hercynite Fayalite Forsterite Sanidine Albite Anorthite
Olivines Feldspars
Mullite
-
RESULTS AND DISCUSSION In gasification runs without added catalysts melting of mineral matter does not occur below 900°C. But, apart from quartz, all mineral species decompose, leaving an amorphous mass of oxides. Only illite in the range 80& 850°C may retain some crystalline order detectable by Xray diffraction, but this disappears either at higher temperatures or with time because of subsequent reactions. Pyrite is oxidized to magnetite, which reacts further with other residues. Generally because of these decompositions mineral matter is in a very reactive state, so that residual ash from a fluidized bed with a residence time of hours may be shown by X-ray analysis to contain various new crystalline phases4 which are listed in Table 2. Spinels, olivines and feldspars are typical solid solutions, the end members of which are indicated. While spinels in gas~cation residues generally cover the whole composition range, olivines and feldspars are represented essentially by fayalite and anorthite. They are early formations, while mullite, a typical reaction product of clay minerals at higher temperatures is scarcely formed in this temperature range. The reactions take place between solids in the naturally intergrown dense masses of mineral matter and the new crystals formed are generally too fine to be identified by visual microscopy. But there are always some larger under favourable grown crystallized aggregates, conditions, which permit microscopic and microprobe studies. In such cases at the surfaces of ffuidized-bed particles reaction rims on magnetite aggregates that consisted of fayalite were observed. These structures indicated that reactions between solids and the gas atmosphere had taken place also. It is thought that silicon was transported by the water vapour atmosphere, probably in the form of Si(OH),, to the magnetite aggregates and reacted there to form fayalite. As mineral matter of coal transforms during gasification, catalyst additions meet a very reactive substrate, which promotes reactions with the catalyst (Hedvall effect). But this seems to be valid also for the mineral matter of char, which because of thermal treatment is already altered before gasification. In gasification runs at 700°C of coal and char with K,CO, catalyst no difference in the reactivity of mineral matter was noted. In both cases kaliophilite was formed, which seems to be the stable reaction product for a wide variety of compositions. Kaolinite has the same Si0,/A1,03 ratio as kaliophilite. It is therefore an ideal reaction partner for kaliophilite formation. Pure kaolinite and an excess of K,CO, were found to be completely converted to
206
FUEL, 1983, Vol 62, February
L. Kiihn and H. Plogmann
ka~ophi~ite in 1 h at 700°C. But a parallel experiment with illite, the higher SiO,/Al,O, ratio of which is not so appropriate, also yielded kaliophilite in an amount corresponding to total consumption of the alumina. In corresponding experiments with soda as catalyst nepheline, the analogous Na compound to kaliophilite, was formed. Table 3 shows the composition of various reaction products. Among these reaction products kaliophilite is the compound with the highest potassium concentration. To save potassium it is therefore of interest to know the conditions of their formation. Experiments with stoichiometric compositions were carried out, using mineral matter isolated by lowtemperature ashing from Pattberg coal (Table I), which by addition of other minerals was adjusted to the desired composition. These mixtures were held at reaction temperature for a::5 h. At first a silica/alumina ratio of 70/30 as for leucite was generated by addition of quartz from D~rentrup. Leucite was formed, though the K,O content of the mixture was only 60% of that of leucite. In this experiment because of the low reactivity of quartz, which was added as a powder of 7 pm grain size, there was only partial conversion at 8OO”C,which was complete at 900°C. Kaliophilite and leucite formation may be easily explained by application of the KzO-A&O,-Si03 phase diagram.5 In this diagram the tie-lines from the points, which mark the silica/alumina ratio of kaolinite (56/44) and illite (61/39), to the K,O corner indicate the changes of composition by K additions. They pass into the stability field of kaliophiIite and show further, that the leucite stability field can be reached only by adding SiO, to the clay minerals. For sanidine formation (potash feldspar) the silica/alumina ratio must be 378/22. That means that the SiO, addition to illite must be as high as the illite content itself or higher. As mineral matter of German hard coal is not so rich in SiO,, it is highly improbable that potash feldspar is formed by K additions. However, sanidine was found together with osumilite by X-ray analysis in residues from a pilot plant, in which 800°C was reached, which indicated special conditions of their formation. 0sumilite,6 from which an intermediate composition is given in Table 3, also has a high silica/alumina ratio, comparable to that of feldspar. With quartz, magnesite (Table I) and mineral matter of coal (Table 1) a stoichiometric mixture was prepared to synthesize osumilite. But heating at 800-900°C for 5 h was not successful. Sanidine could be detected microscopically and its Table 3
Reaction
products with catalysts _-
~.~ Component
la
2
3
4
5
6 -“......
64.8 .18.3 _
55.0 23.5 _
38.0 32.2 -
42.3
MgG CaC
62.1 23.9 9-Q _
21.9 37.2 -
K+’ NasO
48 -
169 -
21.6 -
298 -
21.8
SiOs A12O3
35.9 -
409 .--
a I, K20. 2, KsO 3, K20. 4, KzO. 5, Nat0 6,ZCaO
4.4MgO * 4.6A1s03. . Al203 .6SiO2 Al203 .4SiO2 A1203.2SiOs . AltO3 ’ 2SiO2 At203 ‘SiO2
20.4SiO2
Osumilite Sanidine Leucite Kaliophilite Nepheiine Gehlenite
Reaction of catalysts with mineral matter during coal gasification: L. Kijhn and H. Plogmann
Ca
Fe
Figure 1
Sanidine formation
in a K-treated gasification
residue from a fluidized
structure indicated special conditions of formation, which are illustrated in Figure I. The photomicrograph (middle, bottom row) shows agglomerated ash particles of the fluidized bed in reflected light. The agglomerate covering the right half of the photomicrograph is cut by a vein, filled with mineral aggregates, which cover the left half. The frame indicates the area of the image below left, from which the element distribution images above have been taken. Vein fillings and the outer rims of the agglomerated particles exhibit the elements of sanidine. The coatings of the ash particles are the result of water vapour transport, which introduced other elements, particularly Si, as already mentioned in the case of olivine formation at the surface of magnetite aggregates.
bed
In the case of sanidine formation it is thought that clayey debris in the veins provided the condensation nuclei for the vapour deposits, the high Si concentration which favoured the formation of sanidine. Osumilite could not be detected microscopically, but it is believed that its formation is similar to that of sanidine, because it also needs a high Si concentration. This example shows that reaction products with low K require which concentrations special conditions, normally are not obtained with German hard coal. Sulimma et al. saved K,CO, catalyst by pretreatment of the coal with Ca(OH),, which causes gehlenite formation. But further reactions with K,CO, catalyst cannot be suppressed completely by this pretreatment.
FUEL,
1983,
Vol 62, February
207
Reaction
of catalysts with mineral matter during coal gasification:
ACKNOWLEDGEMENT This research was sponsored by Bundesministerium Forschung und Technologie. REFERENCES 1 McKee, D. W. Chem. Phys. Carbon 1981, 16, 1
208
FUEL, 1983, Vol 62, February
fiir
L. Kiihn and H. Plogmann
2 Veraa, M. J. and Bell, A. T. Fuel 1978,57, 194 3 Sulimma, A., van Heek, K. H. and Jiintgen, H. ‘Proc. International Conference on Coal Science’, Dusseldorf 1981, 313 4 Plogmann, H. and Kuhn, L. ‘Proc. International Conference on Coal Science’, Dusseldorf 1981, 241 5 Levitt, E. M., Robbins, C. R. and McMurdie, H. F. ‘Phase Diagrams for Ceramists’, 3rd ed., American Ceramic Society, Columbus, Ohio, 1974 6 Olesch, M. and Seifert, F. Contrib. Mineral Petrol. 1981, 76, 362