CC BY
21Journal of Siberian Federal University. Chemistry 2 (2009 2) 93-101
УДК 661.183.122
Porous Carbons Obtained from Lignite, Anthracite and Graphite in a Bed of Slag and Catalyst Particles
Boris N. Kuznetsova,b*, Maxim L. Shchipkoa and Nikolai V. Chesnokova,b
a Institute of Chemistry and Chemical Technology SB RAS, Akademgorodok, Krasnoyarsk, 660036 Russia b Siberian Federal University, 79 Svobodny, Krasnoyarsk, 660041 Russia 1
Received 20.05.2009, received in revised form 27.05.2009, accepted 04.06.2009
This paper describes the influence of solid fuel nature and conditions of thermal treatment in a flow reactor with a fluidized andfixed bed of slag and catalyst particles on the porosity of obtained carbon products. Lignite, anthracite and natural graphite were used as starting raw materials. Optimal conditions of thermal treatment providing the formation of porous carbons were selected for the each type of a raw material: pyrolysis with high rate of heating (flash pyrolysis) for lignite, the same combined with steam activation for anthracite and chemical modification and flash pyrolysis for natural graphite.
Keywords: porous carbon, lignite, anthracite, graphite, flash pyrolysis, activation, fluidized and fixed beds, slag and catalyst particles.
Introduction
It is known the pyrolysis of carbon-containing materials results in an evaporation of some fragments of organic matter with the obtaining of porous carbon products. Carbons with the developed porosity are usually produced from raw materials with a significant content of volatile matter at the conditions of high rate heating (flash pyrolysis) [1-6].
Different methods were applied for the technological realization of flash pyrolysis processes. Among them are the processes based on the high rate heating of powdery raw material in a flow of a gaseous carrier.
* Corresponding author E-mail address: [email protected]
1 © Siberian Federal University. All rights reserved
In order to increase the efficiency of a mass transfer between solid material and gaseous carrier the special technical decisions are used. Some of them supply the fuel particles moving by curvilinear trajectory, for example in cyclone reactors [5, 7]. As it was shown early, the new possibilities for increasing an efficiency of lignite pyrolysis process gives the technology based on the thermal treatment of lignite particles moving through the heating fluidized bed of more big particles of catalytically active material [1, 8, 9].
The conventional approach in production of porous carbons is the activation of carbon raw material by gaseous reagents: water steam, carbon dioxide and oxygen [10-11]. In this case
the formation of new pores proceeds as a result of gasification of some part of carbon material. These methods are used for activation of raw materials with a high concentration of carbon and with a low content of volatile matter. The good results were obtained using the carbon material activation by steam-air flow in the reactor with a fluidized bed of catalytically active particles [9, 12].
The textural characteristics of produced active carbons are regulated by variation of activation process conditions. When the reactions between gaseous reagents and carbon are limited by the external diffusion they proceed mainly on the external surface of carbon particles. At these conditions the carbons with rather low porosity and surface area are produced. The formation of macro- and mesopores situated close to the surface of carbon particles dominates in carbon products when the internal diffusion limits the activation process [13]. The prevention of diffusion limitations promotes the formation of carbons with the more developed porosity and homogeneous distribution of pores in the particle volume. Besides, the absence of diffusion limitations expand the possibility to produce carbons with the demanded porous texture.
The important direction in the use of a flash pyrolysis technology is the production of thermally expanded graphite and unique carbon materials on its basis [14-16]. The preparation of expanded graphites includes the two main steps - graphite intercalation by chemical reagents and following thermal treatment of intercalated product. The properties of expanded graphites are varied depending on the conditions of intercalated graphite thermal treatment. The use of a flow reactor where powdery graphite particles are moving through a heating bed of the more big particles of thermally resistant material can open the new possibilities for producing of expanded graphites [17].
Aim of the present paper is connected with a selection of optimal operating parameters of thermal treatment of lignite, anthracite and intercalated graphite at flash pyrolysis conditions in fluidized and fixed beds of catalytically active materials.
Experimental
Samples preparation
Some characteristics of initial raw materials are given in Table 1.
Borodinsky lignite of Kansk-Ashinsk basin was taken at Krasnoyarsk Power Station - 2 after its grinding and drying at the industrial conditions. The amount of lignite particles with sizes less 200 mkm was 92 %. The upper size of particles was limited by 315 mkm.
Anthracite was taken from Donetsk basin (Ukraine). The amount of particles with sizes less 200 mkm was 86 % and their upper size -315 mkm.
The initial pieces of Kureisky graphite with the size 100 - 500 mkm have fine granular structure which includs admixtures of lamellas (mica, chlorite) and rounded (pyrite and calcite) structures. These admixtures have the length 20 - 40 mkm and the width 2 - 4 mkm. Graphite samples after grinding to particles with sizes less 200 mkm were used as starting raw materials.
The same procedure was used for the preparation of Noginsky and Zavalevsky graphites samples with particles sizes less 200 mkm.
The preparation of modified graphite samples included the next steps. Powdery graphite was subjected to magnetic separation. The yield of final fraction was near 97 - 98 % from the weight of the initial one. The graphite samples were demineralizated by boiling during 12 h in HCl solution at ratio graphite:HCl:H2O=1:3:3, then treated by HF during 24 h at ratio graphite:HF=1:2 and by
Table 1. Characteristics of starting carbon-containing materials
Parameters Borodinsky lignite Donetsky anthracite Kureisky graphite Noginsky graphite Zavalevsky graphite*
Moisture, % wt 7.2 3.4 1.3 2.7 1.3
Ash content (Ad), % 9.5 18.6 11.7 16.3 0.1
Yield of volatiles (Vdaf), % 50.5 7.2 2.1 2.8 1.9
Total volume of pores, cm3/g 0.2 0.08 0.05 0.08 0.06
*This sample was demineralized at industrial conditions on Zavalevsky Plant.
boiling water during 40 min. After filtration the samples were dried at 108 oC until the constant weight. The content of ash in treated samples was varied between 1.2 - 2.4 % wt.
Demineralized samples were intercalated with chloric acid at graphite/HClO4 ratio 1:5. Step by step procedure of graphite intercalation included the treatment by HClO4 at 120 oC during 0.5 h, than at 130 oC, 0.5 h at 140 oC, 0.5 h and at 150 oC, 0.5 h. Finally the samples were treated by boiling HCIO 4 at temperature 160 oC during 1.5 h with the addition of HNO3 (5-7 % from HClO4 content). Intercalated graphite was separated on a glass filter, than it was washed by distilled water until the neutral pH and dried at 105 °C until the constant weight.
The other method of intercalation was based on demineralized graphite treatment by nitric and acetic acids. Concentrated nitric acid (10 cm3) with density 1.5 gcm-3 was added to 25 g of graphite and mixed during 10 min at conditions which supply removing the nitrogen oxides. Then the 100 cm3 of concentrated acetic acid was added and the reaction mixture was mixed during 10 min. Treated graphite was separated on a glass filter, washed by distilled water and dried at 105 oC until the constant weight.
Thermal treatment
Lignite and anthracite samples were thermally treated in a reactor with a fluidized bed [9]. The mixture of powdery raw material and air (or air and water-steam mixture) was blown
through a heated at 700-900 oC fluidized bed of catalyst particles.
The activation of natural graphite was carried out by water steam in a fixed bed reactor at temperature 850 - 900 oC, steam concentration in nitrogen - steam mixture 80 % vol, time of treatment 1.5 h.
Two following procedures of thermal treatment of intercalated graphite were applied. According to the first one the metallic reactor with volume 90 cm3 was heated in the electric furnace up to demanded temperature (400-900 oC). The sample of intercalated graphite with weight 0.1-0.3 g was quickly placed into heated reactor and kept during 60-180 s. Then the reactor with expanded graphite was taken out the furnace and cooled down to room temperature.
The another procedure included the thermal treatment of intercalated graphite in a flow stainless reactor with inner diameter of 2 cm and with high of 20 cm. This reactor was filled by Marting slag particles with sizes 0.5-0.8 mm, which act as a heat accumulators. Owing to the big mass of slag bed a temperature in the reactor was changed only slightly after feeding graphite particles. Graphite particles are moving by curvilinear trajectory between the more big slag particles. The special construction of sprayer allows to organize graphite particles feeding into reactor volume from the center to the wall of reactor. It makes possible to increase the time of particles thermal treatment and to improve the heat transfer conditions in the reactor.
Results and discussion
Lignite and anthracite thermal treatment
The influence of temperature and time of lignite pyrolysis at isothermal conditions on the yield of volatile substances was studied (Fig. 1).
40
Time, min
Fig. 1. Influence of temperature and time of lignite thermal treatment at isothermal conditions on the yield of volatiles
The parameters of lignite pyrolysis process were varied in the next range: temperature between 500 - 750 °C, time within 7 - 90 min. The BET surface area of carbonized samples goes up to 90 - 120 m3/g with the increase of temperature and time of lignite pyrolysis. But a total volume of pores was practically the same for the initial and thermally treated lignite samples (0.2-0.25 cm3/g).
It was found that the shrinkage of lignite samples after isothermal treatment reached to 35 % from the initial volume. This fact can indicate that the main part of macro and mesopores which are formed owing to the emission of volatiles from the organic matter of lignite gets closed at the final steps of lignite isothermal treatment. The observed increase of BET surface area is probably connected with opening of the closed micropores of starting lignite.
The pyrolysis experiments with a high heating rate were accomplished in a pilot reactor by means of powdery lignite blowing by air
flow through the fluidized bed of the more big particles of quartz sand, Martin slag and oxidation catalysts. The material of a fluidized bed had the temperature between 400 - 900 oC.
Fig. 2 illustrates the significant increase of semicoke porosity at the rates of lignite particles heating 103-104 oC per second. The rate of heating in the range 102-104 oC per second was regulated by variation of lignite particles, according to [5].
0,6 0,5 -
"e
o
}£■ 0,3
0,2 -
0,1
0,1
10
100
1000
10000
Rate of heating, oC/s
Fig. 2. Influence of the rate of lignite heating to final temperature 750 oC on the total volume of pores in carbon products
No distinct dependence between the heating rate and the specific surface area of obtained semicokes were observed. The BET surface area of all studied semicokes was varied near 120 m3/g (±30 m3/g). But the significant influence of activating agent composition on the BET surface area of activated semicokes was established. Textural characteristics of the selected samples of activated semicokes were compared. The first sample was activated in the flow of exhaust gases produced at combustion of diesel fuel and the second one - in the flow of exhaust gases with the addition of water-steam. The total pore volume of the both activated carbons was 0.36 - 0.40 cm3/g. But these two samples have the different surface areas: 146 96 -
0
Pore size, A
Fig. 3. Distribution of pores in carbon products from lignite activated by exhausted gases at final temperature 700 °C, time of treatment 1.2 s. and rate of heating 103 °C per second.
Composition of activating gases: 1 - 20±2 % of water-steam, 10±0.5 % CO2, N2 - the rest; 2 - 40±2 % of water-steam, 7.5±0.5 % of CO2, N2 - the rest
m3/g - for the first sample and 390 m2/g - for the second one.
Fig. 3 demonstrates the different distribution of pores in these activated semicokes. Obtained data show that even at the low time of treatment (less than 2 s) the increase of water-steam concentration in activating mixture promotes the intensive formation of micropores, which are responsible for the higher surface area of the second sample.
Oxidation catalyst with composition (% wt.): 55,9 AI2O3, 14,4 CuO, 29,7 &2O3 was by 2.5 times more active in reactions of combustible gases oxidation than catalyst containing 94,1 Al2O3, 2,2 CuO, 3,7 Cr2O3. Martin slag has the most low catalytic activity in oxidation reactions at elevated temperatures. As it was shown [12] at the used conditions of lignite thermal treatment the catalyst particles promote the reactions of volatile products oxidation which supply by heat the pyrolysis process. Therefore in the case of lignite, containing the high amount of volatiles
the pyrolysis process goes at autothermal conditions.
Experimental results of anthracite and lignite thermal activation with a high heating rate in catalytic fluidized bed are compared in Table 2. At the comparable temperature of catalyst bed the flow of gaseous products of lignite pyrolysis has the lower temperature on the reactor outlet as compared to that of anthracite pyrolysis. Obviously the two main factors can be responsible for the lower temperature of the flow of gaseous products of lignite pyrolysis. The first one is connected with the higher consumption of heat for evaporating and heating of water, which content in lignite by 2 times large then that in anthracite. The another factor is connected with a heat expenses on the endothermic reactions of char products gasification by steam. At the pyrolysis temperatures used (785-840 oC) anthracite has the lower reaction ability in gasification than that of lignite.
Experiments 1 and 4 (Table 2) demonstrate the differences in the behavior of lignite and anthracite at the same parameters of a thermal treatment. The minimum temperatures, corresponding to the stable conditions of lignite pyrolysis process are 615 °C for active catalyst 1 and 720 oC - for less active catalyst 2. In the case of anthracite the corresponding temperatures are 785 oC for catalyst 1 and 840 oC for catalyst 2.
The reaction ability to activating reagents is very important parameter defining the possibility of coal application for the preparation of carbon sorbents. It is known that the low reactivity of anthracite in activation by such gaseous reagents as water-steam and carbon dioxide limits its wide application as a sorption material.
The reaction ability of thermally treated anthracite was measured by DTA technique under the next conditions: sample weight 35-40 mg, rate of heating 2 °C/min, final temperature 800 °C, air medium. Obtained data are given in Table 3.
Table 2. Parameters of lignite and anthracite thermal treatment in a fluidized bed of catalyst and some properties of obtained carbon products
Number of experiment
1 2 3 4 5 6 7
Material of fluidized bed Active catalyst 1 Less active catalyst 2 Martin slag
Fuel type* L A L L A L A
Air expenditure, m3/h 76.3 72.6 98.6 72.3 71.0 79.3 68.2
Raw material expenditure, kg/h 64.9 59.1 116.1 38.1 36.0 56.7 41.6
Temperature of bed, oC 770 785 615 825 840 720 810
Temperature of product flow on reactor outlet, oC 560 650 440 650 715 530 685
Composition of dry gas, % vol: H2 7.9 0.2 0.1 11.7 0.2 3.4 0.9
CO 5.5 0.7 0.2 7.4 1.0 2.2 1.3
CH4 2.9 absent absent 2.4 absent 1.4 0.2
CO2 13.1 13.8 13.1 11.4 12.1 10.3 10.8
N2 67.9 81.2 83.8 62.4 81.2 76.6 80.1
O2 2.6 4.2 2.9 4.7 4.8 6.1 6.6
Carbon product composition: Moisture (Wr), % 2.1 2.1 4.4 0.3 0.2 2.6 0.3
Ash (Ad), % 15.6 20.9 11.2 25.9 46.6 13.4 32.1
Cdaf 80.2 94.2 72.8 89.1 93.6 77.2 92.8
Hdaf 3.2 0.9 4.4 2.1 1.4 3.5 1.4
Heat of combustion (Qi), MJ/kg 24.3 25.1 22.0 23.1 17.4 23.0 14.7
* L - lignite, A - anthracite
Catalyst 1 - 55.9 % Al2O3, 14.4 % CuO, 29.7 % Cr2O3 Catalyst 2 - 94.1 % Al2O3, 2.2 % CuO, 3.7 % Cr2O3
Table 3. Characteristics of texture and reaction ability of the initial coals and obtained carbon products
Samples Porosity* cm3/g BET surface area m2/g Ignition temperature °C Temperature of maximum loss of mass °C Temperature of complete combustion °C
Lignite 0.19 6.1 240 410 470
Anthracite 0.02 0.4 437 566 617
Carbon products**:
experiment 3 (L) 0.31 27.0 252 380 460
experiment 4 (L) 0.49 38.9 280 345 425
experiment 5 (L) 0.21 26.0 402 550 600
experiment 7 (L) 0.19 14.1 404 549 605
* measured by kerosene absorption
** numbers of experiments are the same as in Table 2
Curves of the mass loss show that the thermally treated anthracite reacts with air with a much higher rate then non-treated anthracite. The higher reactivity of anthracite and lignite after their thermal treatment is connected with the increase of their porosity. According to the data
of Table 3, the thermal treatment of anthracite in a catalytic fluidized bed increases by 10 times its total volume of pores and by 65 times its BET surface area. In the case of lignite the effect of thermal treatment on porosity and surface area of carbon product was less pronounced.
Table 4. Characteristics of Noginsky graphite before and after activation by water-steam at 900 °C
Parameters Initial graphite Activated graphite
Solid product yield, % wt - 76.50
Ash content (Ad), % 16.30 23.20
Density, g/cm3 0.87 0.79
Activity in I2 adsorption, % 11.10 15.40
Attrition strength, % 96.00 98.00
Total volume of pores*, cm3/g:
measured by water absorption 0.075 0.217
measured by mercury intrusion 0.070 0.183
measured by N2 adsorption 0.098 0.227
Volume of mesopores*, cm 3/g 0.028 0.044
*from data of N2 adsorption
Summarizing the obtained results the next conclusions should be made. The pyrolysis of lignite and anthracite at high heating rates promotes the formation of carboneous materials with the increased porosity and surface area. Semicokes obtained from lignite have the higher ignition temperature and lower temperature of maximum loss of a mass as compared to the initial lignite.
Only slightly variations in chemical composition were observed after the thermal treatment of anthracite despite on a significant growth of its specific surface area. For the thermally treated anthracite the ignition temperature and temperature of maximum loss of a mass is lower in comparison with the initial anthracite.
Graphite thermal treatment
Some characteristics of carbon materials obtained by water-steam activation of natural Noginsky graphite at 900 °C, steam concentration 80 % vol. and time of treatment 1.5 h are given in Table 4.
Obtained results show that activation at 900oC increases by 2.5 times the porosity of graphite. Taking into account the extremely low content of volatile matter in graphite sample, the
increased porosity of activated graphite can be explained by gasification of amorphous fragments and another imperfect places on graphite surface with the formation of the additional pores.
It is known [18] that the porosity of carbon materials with a graphite structure is increased after its chemical modification or intercalation. The following fast heating of intercalated graphite results in some cases in a huge expansion of the carbon network giving the low-density porous carbons.
The treatment of demineralized Kureisky and Nogiusky natural graphites by HClO4 increases the content of volatile substances to 7-9 % wt. (for initial graphites 2.1-2.8 % wt). The thermal treatment of chemically modified graphites results in a significant increase of their volume. The microscopic analysis of thermally treated samples show that only some part of graphite particles were expanded. The content of expended particles both in Kureisky and Noginsky graphites was near 10-12 %. The main part of graphite particles keeps the initial acute-angled shape.
Zavalevsky natural graphite treated by concentrated nitric acid and then by concentrated acetic acid was expanded in a flow reactor, described in Experimental section.
Specific volumes of thermally expanded samples were calculated from the equation K = V/m, were V - sample volume (cm3) and m -sample mass (g). Three parallel measurements were made for the each sample. The difference in K value for parallel experiments was near 5%. Zavalevsky graphite samples thermally expanded in a stationary reactor have the K value near 200 cm3/g. But the K value of expanded graphite samples obtained in a flow reactor was only about 5-10 cm3/g. Expanded graphite samples produced in a stationary reactor have low mechanical strength and they easily decrease their volume at slight press or even at shaking.
It is worth to note that the specific volume of a sample (K) is defined by few factors including the internal porosity of particles and the porosity between particles and their agglomerates. The observed decrease of specific volume of expanded graphite after its mechanical pressing is connected obviously with the reduce of porosity between particles and between particles agglomerates.
The thermal expansion of intercalated graphite in a flow reactor is accompanied by a significant mechanical stress owing to the collision of expanded graphite particles with particles of fixed bed filler (Martin slag) and with walls of a reactor. This results in a destruction of not strong particles of expanded graphite or their agglomerates. Therefore the rather low values of specific volume of expanded graphite samples produced in a flow reactor can be conditioned by destruction and packing processes.
The thermal treatment of intercalated graphite in flow reactor has some advantages as compared to stationary reactor. The expanded graphite produced in a flow reactor has the higher density in comparison with that obtained by heating at stationary conditions. Owing to that the expanded graphite of this type has the more stable properties. Expanded graphite with higher
density is more suitable for transportation and its using decreases the number of steps in mould articles producing under the pressure.
Conclusion
The processes of thermal treatment based on powdery carbon raw material blowing by air flow through a fluidized or fixed bed of more big slag or catalyst particles have their own features for each type of raw material used.
More then 40 % wt of organic mass is removing with volatile compounds in the case of lignite pyrolysis at low rates of heating. But the formation of new pores is accompanied by the decrease of lignite volume which reaches to 35 %. This shrinkage prevents the significant growth of obtained semicoke porosity. The semicokes with an increased porosity were obtained by lignite pyrolysis in a flow reactor with fluidized bed of catalytically active material at high rate of heating (103-104 oC/s) and short time of treatment (few seconds).
The pyrolysis with high rate ofheating promotes mainly the formation of meso- and macropores, therefore the obtained carbon materials have rather low BET surface area. The fast pyrolysis of lignite in catalytic fluidized bed in the presence of water-steam makes it possible to increase the BET surface area of semicoke to 400 m2/g at short time of the process (less then 2 s).
Since anthracite has low content of volatile matter (near 7.2 % wt.) the carbon materials with the increased porosity and BET surface area are produced only after its activation by water-steam at 900 °C.
The treatment of natural graphites by water-steam makes low influence on their porosity. But the chemical intercalation of graphite with following thermal treatment at conditions of a high rate heating has prospects for producing of carbons with low density and high specific volume.
References
1. Wiktorsson L.P., Wanzl W. Kinetic parameters for coal pyrolysis at low and high heating rates-a comparison of data from different laboratory equipment. Fuel. 2000. V. 79. P. 701-716.
2. Bonnamy S. Carbonization ofvarious precursors. Effect of heating rate - Part I: Optical microscopy studies. Carbon. 1999. V. 37. P. 1691-1705.
3. Bonnamy S. Carbonization of various precursors. Effect of heating rate - Part II: Transmission electron microscopy and physicochemical studies. Carbon 1999, 37, 1707-1724.
4. Chemistry of Coal Utilisation /Elliot M.A. Ed. New York: Wiley, 1981. 2nd suppl. vol.
5. Reynolds III R.A., Greinke R.A. Influence of expansion volume of intercalated graphite on tensile properties of flexible graphite. Carbon. 2001. V. 39. P. 479-481.
6. Henning B., Kellerhoff Th., Scherer V. and Kremer H. Experimental Investigations of Coal Pyrolysis in an Inert Atmosphere under high Heating Rates. Proc. of the 11th International Conference on Coal Science. S. Francisco, 2001. Pr. 153.
7. Fomin A.P., Rotapenko O.G. Production of Environmentally Safe Solid Fuel from Low Rank Coals. Proc. of the 11th International Conference on Coal Science. S. Francisco, 2001. Pr. ABS108.
8. Kuznetsov, B. N., Shchipko, M. L., Azhishev, N.A. and Gritsko, S. L. Thermal conversion of lignite in a fluidized bed of catalyst. Fuel. 1987. V. 66. P. 412-414.
9. Shchipko M.L., Kuznetsov B.N., Catalytic pyrolysis of Kansk-Achinsk lignite for production of porous carbon materials. Fuel. 1995. V. 74. P. 751-755.
10. Martin-Gullon I, Asensio M, Font R and Marcilla A. Steam-activated carbons from a bituminous coal in a continuous multistage fluidized bed pilot plant. Carbon. 1996. V.34. P. 1515-1520.
11. Rodryguez-Reinoso F, Pastor AC, Marsh H and Huidobro A. Preparation of activated carbon cloths from viscous rayon - Part III. Effect of carbonization on CO2 activation. Carbon. 2000. V. 38. P. 397-406.
12. Kuznetsov B.N. and Shchipko M.L. The application of apparatus with fluidized catalytic bed for Kansk-Achinsk lignite thermal processing. Proc. of the Fourth International Symposium "Catalytic and Thermochemical Conversions of Natural Organic Polymers". Krasnoyarsk, 2000. P. 279-291.
13. Frank-Kamenetsky D. A. Diffusion and heat transfer in chemical kinetics. Moscow: Nauka. 1987.
14. Yang L, Gu J, Cao W, Zhang TY. Influences of density and flake size on the mechanical properties of flexible graphite. Carbon. 1998, V. 36. P. 875-881.
15. Reynolds III R.A., Greinke R.A. Influence of expansion volume of intercalated graphite on tensile properties of flexible graphite. Carbon. 2001. V. 39. P. 479-481.
16 Tanaike O. and Inagaki M. Degradation of carbon materials by intercalation. Carbon. 1999. V. 37. P. 1759-1769.
17. Shchipko M.L., Rudkovsky A.V., Kuznetsov B.N. Method and reactor for expanded graphite obtaining. Russian Patent 2294894. 2007.
18. Furdin G. Exfoliation process and elaboration of new carbonaceous materials. Fue.l 1998. V. 77. P. 479-485.
