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Application of catalysts to coal gasification processes. Incentives and perspectives
Application of catalysts to coal gasif ication processes. Incentives and perspectives* Harald
Jiintgen
Bergbalr-Forschung
GmbH, 4300 Essen-Kray,
Franz-Fischer-
weg
61, GFR
There are several processes used for coal gasification, but these have three characteristic types of reactors - moving-bed, fluidized-bed and entrained phase. Design and comparison of different processes and reactors is possible if kinetic data relevant to technical performance of processes are available. The state of reaction kinetics of the non-catalysed and catalysed steam-carbon reaction as the basic coal gasification reaction is discussed. Catalysts only have effects on gasification rates in the temperature range of chemical reaction or pore diffusion control. Furthermore the increase in reaction rates by catalysts can also be reached without using catalysts. Therefore the application of catalysts seems not to be attractive in conventional gasification processes because the advantages (less coal and oxygen consumption and lower heat losses) are compensated by their disadvantages (additional costs, side effects). Only such processes in which a temperature increase is limited for different reasons does the application of catalysts have significant advantages. Such processes are the allothermal gasification using nuclear heat and processes leading to synthetic natural gas (SNG). (Keywords:
coal; gasif ication;
catalysts)
The basic application of catalysts to coal gasification processes has been known from patent applications’ for more than 100 years. Systematic investigations and research work on a small scale have also been carried out for more than 60 years. 2 -’ Many catalysts are found to be effective in accelerating the reaction rate of the C-H,O-, the C-CO,-, the C-O,- and the C-H,-reaction. Many publications are concerned with the mechanism of the performance of catalytic gasification of coal or carbon. The aim of thisPaper is not to give a review of the state of the art but an investigation of why no industrial process of catalytic gasification is in operation and what are the incentives and perspectives of catalytic gasification. Key points in the understanding of the technical application of catalysts for coal gasification are first the role of reaction kinetics on plant design and second the change of reaction kinetics with the addition of catalysts. Therefore some general design features of coal gasification processes and current knowledge of reaction kinetics of non-catalytic and catalytic gasification are discussed first. From these considerations the incentives and perspectives of the application of catalysts can be derived.
reduction gas, town gas, SNG). Since the gasification of carbon with steam - the steamcarbon reaction - is an endothermic process the supply of reaction heat also plays a role in the performance of a gasification reactor (Figure 2). In so-called allothermal processes the pyrolysis products of coal are gasified with steam and heat is transferred from a hot gas via a heat exchanger or from a gaseous or solid heat carrier. In so-called autothermal processes an exothermic reaction has to take place in the gasifier parallel to the steamcarbon reaction. This process is suitable for the combustion of part of the carbon with air or oxygen. Therefore in coal gasification
Different
DESIGN OF COAL GASIFICATION
prcpertles - low colorbflc gos - high colorlflc gas
PROCESSES
The fact that the design of processes is dependent on the residence time distribution (RTD) of the reactor type applied and on the kinetics of the reaction performed is well known (Figure 1). The aim is to reach a very high degree of conversion of the carbon in the coal (X,) in a suitable type of reactor. The type of reactor is dependent on the properties of the coal (grain size, caking properties, reactivity) and on the kind of gas desired (synthesis gas, * Presented at International Symposium, ‘Fundamentals of Catalytic Coal and Carbon Gasification’, held at Amsterdam, The Netherlands, 27-29 September 1982 001~2361/83/020234-OSS3.00 @ 1983 Butterworth & Co. (Publishers)
234
FUEL,
1983,
Ltd
Vol 62, February
Figure
1
Significance
of kinetics
for reactor design
Application
t-i. Jiintgen
reaction in which two kinds of process overlap: pyrolysis of coal and reaction of pyrolysis products with the gasifying agent. The gasification itself is also a complex reaction comprising parallel and subsequent reactions of coal with various gasifying agents (H?O, 0,, CO,) or with reaction products (H,, CO), as reactions of products (H,, CO) and gasifying agents (H,O) take place. In Table 2 the most important reactions which take place during autothermal gasification of coal are listed. The reaction rate of the pyrolysis reaction is dependent mainly on the rate of temperature increase in the reactor and on the grain size of the solid particle. For the gasification of carbon the gas-solid reactions with oxygen (for autothermal gasification), steam, carbon dioxide and hydrogen have significance. Under the condition of gasification the rates of these reactions are in the following order:
several reactor types are used with (in principle) wellknown RTD of gases and solids (Figure 3). Moving-bed reactors are used to gasify lump coal in countercurrent flow with the gasifying agent steam and oxygen, with nearly plug flow of solids and gases. Here there are concentration-, and therefore temperatureprofiles resulting in the formation of different zones, in which the individual steps of gasification take place: drying and pyrolysis at low temperature, gasification with steam at elevated temperature and combustion of the residual coke with oxygen at high temperature. In fluidized-bed reactors there is mixed flow of solid particles which gives a uniform temperature in the total volume of the reactor and distinct zones for the individual reaction steps do not exist. Since the heat transfer from a wall into the fluidized-bed is high, this reactor type is also suitable for allothermal processes. Gasification of finely ground coal takes place in entrained-phase reactors at very high temperature in cocurrent flow with the gasifying agent. The RTD of both solids and gases is characterized by a high degree of back mixing, which can be reached by mixing two solid/gasstreams in countercurrent flow. SIGNIFICANCE GASIFICATION
of catalysts to coal gasification:
rC-O,)
>rC-H,O>kCO,)rH,
For adescription of the kinetics under conditions relevant to reactor design the kinetics of the C-H,O-reaction have Table I
Basic reactions of coal gasification
Reaction5 of pyrolysis (under conditions in gasification reactors)
OF KINETICS FOR PROCESS DESIGN
C,H,Oy=(l
If the contact model of gas and solid and the residence time distribution of both gases and solids are known the performance of the reactor depends only on the kinetics of gasification. Gasification of coal is a very compIex
-yY)C+yCO+
AH
(kJ mol-‘1
XH, 2
C+yCO+;H*
+ 17.4 +;CH4
+
8.1
Heterogeneous gas-solid reactions
Coa I
Cool
Heat ==Z
QXYWW
Heat by c-02Reaction Stwme
0
C+H20hCO+H2
+119
c+co2-,2co
+I62
C+2H2
-
“CH4
87
0
Gas
Gas
c+
Steome
to,
‘CO
-123
c+o23co2
ii
-406
Homogeneous reactions A;h Autothermal
Allothermol
figure 2 Performance of heat supply in allothermal and autothermal gasification
CO+HzO~H2+C02
-
CO + 3H2 -+ CH4 + Hz0
-206
. 1.s
42
Aefl
‘.
:.
Tempe&ure
Drying ond pyrolysis
rising
Gasification
Ccrnbusticwr
1b
: .. ‘._ :, :: ‘. ---,I-
MI Stedm
:
. .
.
.:
..
.
Uniform ternperatun?
_ Steam * G wgcn
Oxygen
PaI
Asn
Steam + Moving-bed
Figure 3
reactor
Fluidized - bed reactor
Entrained-phdse
reactor
Types of gasification reactors
FUEL, 1983, Vof 62, February
235
Application
of catalysts to coal gasification:
Film diffusion
f
Pore
H. Jiintgen
diffusion
-t
Equal
reaction
mte
IiT Figure
4
Arrhenius
diagram
of non-catalysed
and catalysed
greatest significance possibly in the presence of hydrogen, which is present as product gas in gasification reactors in larger concentrations. The kinetics of the C-O,-reaction are so fast that, for first evaluation, the assumption can be made that the oxygen reacts with the corresponding amount of carbon immediately. In contrast the CO, formed reacts so slowly with the carbon that this reaction, which is parallel to the C-H,O-reaction, has no significance. Therefore the following discussion is limited to the kinetics of the C-H,O-reaction. REACTION RATE OF CATALYTIC AND NONCATALYTIC CARBON GASIFICATION For a description of the kinetics of the C-H,O-reaction dependent on temperature, processes of mass transfer have to be taken into account. It is well known that at high temperatures film diffusion, at medium temperatures pore diffusion, and at low temperatures the chemical reaction itself are rate determining. For the discussion here only this last temperature range is interesting. Here the reaction rate is dependent on the active sites in the solid, due to its carbon content, on steam (and hydrogen) concentration in the gas phase, and temperature. The reaction rate for non-catalytic carbon gasification can be formulated as follows: dm, 1 “= -x’m,=(l
1 P.dX, _X,y
dr = k(Ptr,o,PH1,T)
(1)
Where: r, = reaction rate of C-gasification; m, = mass of carbon; X, = conversion of carbon; n = 1, if the pore structure of the char is very open; n = 5, if size of surface determines the reaction rate; and k = reaction rate constant. For the temperature constant there is:
dependence
of the apparent
rate
(2)
k =k,e(-EIRT)
For lignites the apparent activation energy E is in the order of 120 kJmol_‘, for hard coals between 125 and 160kJ mol-‘, and the frequency factor is in the order of lo5 min-‘. The dependence of k on steam and hydrogen
236
FUEL,
1983,
Vol
62, February
gasification,
dependent
partial pressure can Langmuir-Hinshelwood k=
on amount
of catalyst
be described equation:
kiPH,o + k#i,
1+ k&o
basically
by
a
(3)
Summarizing it can be stated that at constant conditions of the gas phase the reaction rate r of the non-catalytic gasification of carbon as defined in equation (1) is constant and n is of an order of 1 or f related to the carbon content of the solid (due to the fact that either the carbon contained in the solid particle is fully available for the reaction or the reaction takes place at the internal surface). In the presence ofcatalysts which are well distributed in the solid the reaction rate of the C-H,O-reaction cannot be related to the C-content of the solid alone but is also a function of the amount of the catalytic sites in the solid:9 %=(kl
+k *CgcatalystM- Xl
(4)
Here cOcatalyst means the initial concentration of the effective catalyst in the solid, n has been found to be 0.67. In general n also can become zero. In this case the reaction rate is only dependent on the active sites of the catalyst. Also the influence of the gas phase concentrations of steam and hydrogen may change. The temperature influence has not yet been investigated for all cases. Recent measurements with alkaline catalysts seem to prove that in the case of potassium compounds activation energy of catalytic gasification changes from non-catalytic to catalytic gasification and stays constant during catalytic gasification dependent on the concentration of the catalysts.” Here only the frequency factor increases with the initial concentration of catalyst in the solid. In the case of sodium compounds there seems to be no change of activation energy between catalytic and non-catalytic gasification. Figure 4 shows an Arrheniusdiagram with a comparison of catalytic and non-catalytic gasification based on this assumption. The temperature range of pore diffusion during catalytic gasification has not been well investigated. In general the transition temperature from chemical reaction to pore diffusion control is dependent on the so-called Thiele modulus:
(5)
@=dJla,& P
P
Where: dk = grain size; .#&= labrinth factor; f, = pore radius; and D,= pore diffusion coefficient. If CDis greater than 0.3 pore diffusion begins to become effective as a rate determining step. Since k for catalytic gasification at a temperature is very much higher than for non-catalytic gasifications 0 reaches a value >0.3 at lower temperatures. However, it may be that the pore structure and therefore D, and rp also change gasification under compared with non-catalytic formation of greater pores and pore diffusion coefficients. In this case the transition temperature from chemical reaction to pore diffusion control would increase to higher chemical reaction rates compared with noncatalytic gasification. From the Arrhenius diagram shown in Figure 4 it can be concluded that the addition of distinct amounts of catalysts at a low reference temperature has the same effect as a corresponding temperature increase in noncatalytic gasification. However, the effect of catalysts is limited to the temperature region of chemical reaction and pore diffusion control. In the temperature range in which film diffusion controls the reaction rate the catalyst does not become effective. TECHNICAL
APPLICATION
OF CATALYSTS
As has been shown, the advantage of the application of catalysts is, that at low gasification temperature an increase of throughput of a given gasification reactor can be performed, provided it works in the temperature range with chemical reaction or pore diffusion as a rate controlling step, That means that the specific throughput per gasification reactor is increased and therefore the specific investment costs for the gas generator can be decreased. In contrast in non-catalytic gasification processes an increase of throughput is linked with an increase of reaction temperature. Since gasification with steam is an endothermic reaction a higher consumption of oxygen and coal is necessary and furthermore heat losses are greater. The application of catalysts also involves problems, i.e. 1 additional costs for catalysts; 2 possible side effects, e.g. corrosion of materials in the gasifier or in plants downstream from the gasifier; 3 necessity of recovery of catalysts, especially if expensive materials are used; and 4 possible environmental impacts concerning ash disposal. The size of these problems cannot be generally quantified, but depends on the individual system under consideration. The system parameters are: kind of catalysts; costs of catalysts; amount of catalysts necessary for a distinct increase of throughput; corrosion effects; and the question of whether recovery of catalyst is necessary for economic or environmental reasons. These problems can only be studied and finally solved by experimental research and development. UnfortunateIy at the present time insufficient data are available to make a more quantitative balance of advantages and disadvantages of catalyst application. However, as small scale experiments on catalysed
g~if~cation and corresponding patent applications have been known for more than 100 years, and there has been no commercial application, it could be assumed that for conventional gasification processes the advantages of catalytic gasification are not much greater than their disadvantages. Attention should be directed, therefore, at special gasification conditions in which the enhancement of throughput by catalysts cannot be reached by a simple temperature rise. Under this criteria gasificaion processes can be investigated in which the temperature is limited for various reasons.
NEW CATALYTIC GASIFICATION UNDER DEVELOPMENT
PROCESSES
Recently two categories of processes have been under development, in which the gasification temperature is limited. The first process is allothermal steam gasification using high temperature helium coming from an HTR. The reaction heat is transferred into the fluidized-bed gasification reactor by a suitable heat exchanger. The maximum temperature of the helium is limited to 950°C so that the gasification temperature cannot exceed 80% 840°C. In this process two parameters affect economy: (1) the reaction rate in this temperature range and the corresponding throughput of the gasifier; and (2) the temperature difference between the helium input-stream into the heat exchanger (900°C) and the fluidized-bed temperature. The higher this difference is, the more heat can be transferred from the helium stream into the gasilier. A more detailed consideration shows that the economy of the process is improved at lower gasification temperature with higher coal throughput of the gasifier at the low temperature level. In this special situation the application of alkaline catalysts looks very promising. More details will be reported in a seperate paper.’ 1 A different criterion for gasification processes with limited temperature concern processes in which conversion is limited by the thermodynamic equilibrium. Here all processes for the direct production of methane from coal are relevant. In the system carbon/hydrogen the equilibrium leads to high methane concentrations at possibly low temperatures and high overall pressures. However, the rate of gasification is very low at these favourable temperatures from the point of view of thermodynamic equilibrium, and for kinetic reasons it is necessary to increase to a higher temperature with considerably lower methane concentrations. Here the application of a catalyst effective for the C-HZ-reaction would be very promising. Unfortuantely it is not yet possible to tind a catalyst suitable for this reaction under technical conditions. Recently an elegant solution for production of a methane-rich gas by catalytic steam gasification with recycling of the hydrogen and CO of product gas at 690°C has been suggested by Exxon.” Both processes use fluidized-bed reactors - BergbauForschung, a lying, and Exxon, a standing tube - and alkaline cataiysts. Pilot plants have been run successfully, and the construction of larger plants is in preparation. On the whole it can be stated, that the t~hnical application of catalysts has made some important progress, however, its industrial application is still an aim for the future.
FUEL,
1983,
Vol 62, February
237
Application
of catalysts to coal gasification:
PERSPECTIVES
H. Jiintgen
OF CATALYST APPLICATION
It has been shown that by catalytic gasification an increase of the specific coal throughput in a gasifier without more oxygen- and coal-consumption can be achieved. At present there are not enough data available to evaluate the economic chances of an application of catalytic gasification for conventional processes. Due to the fact that catalysts are only effective in the temperature range of chemical reaction and pore diffusion as rate controlling steps, catalytic gasification can only be applied to moving-bed or fluidized-bed gasification at temperatures lower than =lOOO”C. In all cases of processes with limited temperature - allothermal processes or processes for methane production -catalytic processes will have great chances in the future. However, a realization of such processes can take place only after the following problems have been solved. Objective comparison gasification
of catalytic
and
non-catalytic
For an objective comparison of catalytic with noncatalytic gasification engineering studies based on reliable data of kinetics are necessary. Therefore kinetic investigations under process-relevant conditions should have the highest priority. Process-relevant conditions mean: similar pressure and temperature range as the technical process has, studies under conditions in which the influence of high product gas concentrations can be investigated also. According to the LangmuirHinshelwood kinetics it is well known that high concentrations of hydrogen inhibit reaction rate of steam gasification of carbon. First results show that the degree of inhibition is also dependent on the kind of catalyst used. Also studies at different temperatures to evaluate reliable values of activation energies are extremely important, since only in this way is a precise comparison with noncatalytic processes possible. Selection of the optimal catalyst for a given system
A question which is still open despite the immense amount of work done in the area is which catalyst has an optimal effect on gasification processes? This question has to be investigated under very different aspects. First kinetic studies are eminently important under processrelevant conditions with different coals, as experience has shown that it is difficult to transfer results from the
238
FUEL,
1983,
Vol 62, February
gasification of pure carbon to the gasification of coals. Here some additional problems arise in relation to coal pyrolysis, changing of chemical reactivity according to different pyrolysis conditions, and different sizes of, pore structure and internal surfaces. Also reactions of the mineral substance of coal with the catalyst can lead to some deactivation. Favourable properties of catalyst
Furthermore an optimal catalyst must have additional favourable properties, such as no corrosion effects, as cheap as possible, no damage for the environment if disposed of, possibility to separate the catalyst from coal ash with the aim of recovery. Distribution of catalyst
Another problem which is still open is the best possible feeding method for the catalyst. There is evidence that the effect of catalyst on reaction rate depends on the kind of feeding. In this context it is important to ask, what is the state of distribution of catalyst in the grains of the solid. It has been found that some catalysts, such as potassium carbonate or potassium oxide, are freely moveable within the pore structure of coal grains. Other catalysts, such as Ni or Fe compounds, seem to be immoveable. Here a good distribution is especially important. Different methods for an optimal mixing of catalyst and coal are suggested, e.g. feeding in the coal mill or feeding with steam.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
Tessie du Motay, C. M. and Marechal, C. R. British Patent No. 2548, 1867 Taylor, H. S. and Neville, H. A. J. Am. Chem. Sot. 1921,43,2055 Kriiger, C. Angew. Chem. 1939, 52 (6), 129 Veraa, M. J. and Bell, A. T. Fuel 1978,57, 194 McKee, D. W. and Chatterji, D. Carbon 1978, 16, 53 Harker, H. ‘Proc. of the 4th Conf. on Carb.‘, Pergamon Press, New York, 1960, pp. 125-139 Otto, K., Bartosiewicz, L. and Shelev, M. Carbon 1979, 17, 351 Wigmans, T., Elfring, M., Hoogland, A. and Moulijn, J. A. ‘Proc. at the Int. Conf. on Coal Science’, Diisseldorf, 1981, pp. 301-306 Leonhardt, P., Sulimma, A., van Heek, K. H. and Jiintgen, H. Fuel 1983, 62, 200 Huhn, F., Klein, J. and Jiintgen, H. Fuel 1983, 62, 196 Kubiak, H., Schriiter, H. J., Sulimma, A. and van Heek, K. H. Fuel 1983,62, 239 Nahas, N. C. Fuel 1983,62, 239
Jiintgen
Bergbalr-Forschung
GmbH, 4300 Essen-Kray,
Franz-Fischer-
weg
61, GFR
There are several processes used for coal gasification, but these have three characteristic types of reactors - moving-bed, fluidized-bed and entrained phase. Design and comparison of different processes and reactors is possible if kinetic data relevant to technical performance of processes are available. The state of reaction kinetics of the non-catalysed and catalysed steam-carbon reaction as the basic coal gasification reaction is discussed. Catalysts only have effects on gasification rates in the temperature range of chemical reaction or pore diffusion control. Furthermore the increase in reaction rates by catalysts can also be reached without using catalysts. Therefore the application of catalysts seems not to be attractive in conventional gasification processes because the advantages (less coal and oxygen consumption and lower heat losses) are compensated by their disadvantages (additional costs, side effects). Only such processes in which a temperature increase is limited for different reasons does the application of catalysts have significant advantages. Such processes are the allothermal gasification using nuclear heat and processes leading to synthetic natural gas (SNG). (Keywords:
coal; gasif ication;
catalysts)
The basic application of catalysts to coal gasification processes has been known from patent applications’ for more than 100 years. Systematic investigations and research work on a small scale have also been carried out for more than 60 years. 2 -’ Many catalysts are found to be effective in accelerating the reaction rate of the C-H,O-, the C-CO,-, the C-O,- and the C-H,-reaction. Many publications are concerned with the mechanism of the performance of catalytic gasification of coal or carbon. The aim of thisPaper is not to give a review of the state of the art but an investigation of why no industrial process of catalytic gasification is in operation and what are the incentives and perspectives of catalytic gasification. Key points in the understanding of the technical application of catalysts for coal gasification are first the role of reaction kinetics on plant design and second the change of reaction kinetics with the addition of catalysts. Therefore some general design features of coal gasification processes and current knowledge of reaction kinetics of non-catalytic and catalytic gasification are discussed first. From these considerations the incentives and perspectives of the application of catalysts can be derived.
reduction gas, town gas, SNG). Since the gasification of carbon with steam - the steamcarbon reaction - is an endothermic process the supply of reaction heat also plays a role in the performance of a gasification reactor (Figure 2). In so-called allothermal processes the pyrolysis products of coal are gasified with steam and heat is transferred from a hot gas via a heat exchanger or from a gaseous or solid heat carrier. In so-called autothermal processes an exothermic reaction has to take place in the gasifier parallel to the steamcarbon reaction. This process is suitable for the combustion of part of the carbon with air or oxygen. Therefore in coal gasification
Different
DESIGN OF COAL GASIFICATION
prcpertles - low colorbflc gos - high colorlflc gas
PROCESSES
The fact that the design of processes is dependent on the residence time distribution (RTD) of the reactor type applied and on the kinetics of the reaction performed is well known (Figure 1). The aim is to reach a very high degree of conversion of the carbon in the coal (X,) in a suitable type of reactor. The type of reactor is dependent on the properties of the coal (grain size, caking properties, reactivity) and on the kind of gas desired (synthesis gas, * Presented at International Symposium, ‘Fundamentals of Catalytic Coal and Carbon Gasification’, held at Amsterdam, The Netherlands, 27-29 September 1982 001~2361/83/020234-OSS3.00 @ 1983 Butterworth & Co. (Publishers)
234
FUEL,
1983,
Ltd
Vol 62, February
Figure
1
Significance
of kinetics
for reactor design
Application
t-i. Jiintgen
reaction in which two kinds of process overlap: pyrolysis of coal and reaction of pyrolysis products with the gasifying agent. The gasification itself is also a complex reaction comprising parallel and subsequent reactions of coal with various gasifying agents (H?O, 0,, CO,) or with reaction products (H,, CO), as reactions of products (H,, CO) and gasifying agents (H,O) take place. In Table 2 the most important reactions which take place during autothermal gasification of coal are listed. The reaction rate of the pyrolysis reaction is dependent mainly on the rate of temperature increase in the reactor and on the grain size of the solid particle. For the gasification of carbon the gas-solid reactions with oxygen (for autothermal gasification), steam, carbon dioxide and hydrogen have significance. Under the condition of gasification the rates of these reactions are in the following order:
several reactor types are used with (in principle) wellknown RTD of gases and solids (Figure 3). Moving-bed reactors are used to gasify lump coal in countercurrent flow with the gasifying agent steam and oxygen, with nearly plug flow of solids and gases. Here there are concentration-, and therefore temperatureprofiles resulting in the formation of different zones, in which the individual steps of gasification take place: drying and pyrolysis at low temperature, gasification with steam at elevated temperature and combustion of the residual coke with oxygen at high temperature. In fluidized-bed reactors there is mixed flow of solid particles which gives a uniform temperature in the total volume of the reactor and distinct zones for the individual reaction steps do not exist. Since the heat transfer from a wall into the fluidized-bed is high, this reactor type is also suitable for allothermal processes. Gasification of finely ground coal takes place in entrained-phase reactors at very high temperature in cocurrent flow with the gasifying agent. The RTD of both solids and gases is characterized by a high degree of back mixing, which can be reached by mixing two solid/gasstreams in countercurrent flow. SIGNIFICANCE GASIFICATION
of catalysts to coal gasification:
rC-O,)
>rC-H,O>kCO,)rH,
For adescription of the kinetics under conditions relevant to reactor design the kinetics of the C-H,O-reaction have Table I
Basic reactions of coal gasification
Reaction5 of pyrolysis (under conditions in gasification reactors)
OF KINETICS FOR PROCESS DESIGN
C,H,Oy=(l
If the contact model of gas and solid and the residence time distribution of both gases and solids are known the performance of the reactor depends only on the kinetics of gasification. Gasification of coal is a very compIex
-yY)C+yCO+
AH
(kJ mol-‘1
XH, 2
C+yCO+;H*
+ 17.4 +;CH4
+
8.1
Heterogeneous gas-solid reactions
Coa I
Cool
Heat ==Z
QXYWW
Heat by c-02Reaction Stwme
0
C+H20hCO+H2
+119
c+co2-,2co
+I62
C+2H2
-
“CH4
87
0
Gas
Gas
c+
Steome
to,
‘CO
-123
c+o23co2
ii
-406
Homogeneous reactions A;h Autothermal
Allothermol
figure 2 Performance of heat supply in allothermal and autothermal gasification
CO+HzO~H2+C02
-
CO + 3H2 -+ CH4 + Hz0
-206
. 1.s
42
Aefl
‘.
:.
Tempe&ure
Drying ond pyrolysis
rising
Gasification
Ccrnbusticwr
1b
: .. ‘._ :, :: ‘. ---,I-
MI Stedm
:
. .
.
.:
..
.
Uniform ternperatun?
_ Steam * G wgcn
Oxygen
PaI
Asn
Steam + Moving-bed
Figure 3
reactor
Fluidized - bed reactor
Entrained-phdse
reactor
Types of gasification reactors
FUEL, 1983, Vof 62, February
235
Application
of catalysts to coal gasification:
Film diffusion
f
Pore
H. Jiintgen
diffusion
-t
Equal
reaction
mte
IiT Figure
4
Arrhenius
diagram
of non-catalysed
and catalysed
greatest significance possibly in the presence of hydrogen, which is present as product gas in gasification reactors in larger concentrations. The kinetics of the C-O,-reaction are so fast that, for first evaluation, the assumption can be made that the oxygen reacts with the corresponding amount of carbon immediately. In contrast the CO, formed reacts so slowly with the carbon that this reaction, which is parallel to the C-H,O-reaction, has no significance. Therefore the following discussion is limited to the kinetics of the C-H,O-reaction. REACTION RATE OF CATALYTIC AND NONCATALYTIC CARBON GASIFICATION For a description of the kinetics of the C-H,O-reaction dependent on temperature, processes of mass transfer have to be taken into account. It is well known that at high temperatures film diffusion, at medium temperatures pore diffusion, and at low temperatures the chemical reaction itself are rate determining. For the discussion here only this last temperature range is interesting. Here the reaction rate is dependent on the active sites in the solid, due to its carbon content, on steam (and hydrogen) concentration in the gas phase, and temperature. The reaction rate for non-catalytic carbon gasification can be formulated as follows: dm, 1 “= -x’m,=(l
1 P.dX, _X,y
dr = k(Ptr,o,PH1,T)
(1)
Where: r, = reaction rate of C-gasification; m, = mass of carbon; X, = conversion of carbon; n = 1, if the pore structure of the char is very open; n = 5, if size of surface determines the reaction rate; and k = reaction rate constant. For the temperature constant there is:
dependence
of the apparent
rate
(2)
k =k,e(-EIRT)
For lignites the apparent activation energy E is in the order of 120 kJmol_‘, for hard coals between 125 and 160kJ mol-‘, and the frequency factor is in the order of lo5 min-‘. The dependence of k on steam and hydrogen
236
FUEL,
1983,
Vol
62, February
gasification,
dependent
partial pressure can Langmuir-Hinshelwood k=
on amount
of catalyst
be described equation:
kiPH,o + k#i,
1+ k&o
basically
by
a
(3)
Summarizing it can be stated that at constant conditions of the gas phase the reaction rate r of the non-catalytic gasification of carbon as defined in equation (1) is constant and n is of an order of 1 or f related to the carbon content of the solid (due to the fact that either the carbon contained in the solid particle is fully available for the reaction or the reaction takes place at the internal surface). In the presence ofcatalysts which are well distributed in the solid the reaction rate of the C-H,O-reaction cannot be related to the C-content of the solid alone but is also a function of the amount of the catalytic sites in the solid:9 %=(kl
+k *CgcatalystM- Xl
(4)
Here cOcatalyst means the initial concentration of the effective catalyst in the solid, n has been found to be 0.67. In general n also can become zero. In this case the reaction rate is only dependent on the active sites of the catalyst. Also the influence of the gas phase concentrations of steam and hydrogen may change. The temperature influence has not yet been investigated for all cases. Recent measurements with alkaline catalysts seem to prove that in the case of potassium compounds activation energy of catalytic gasification changes from non-catalytic to catalytic gasification and stays constant during catalytic gasification dependent on the concentration of the catalysts.” Here only the frequency factor increases with the initial concentration of catalyst in the solid. In the case of sodium compounds there seems to be no change of activation energy between catalytic and non-catalytic gasification. Figure 4 shows an Arrheniusdiagram with a comparison of catalytic and non-catalytic gasification based on this assumption. The temperature range of pore diffusion during catalytic gasification has not been well investigated. In general the transition temperature from chemical reaction to pore diffusion control is dependent on the so-called Thiele modulus:
(5)
@=dJla,& P
P
Where: dk = grain size; .#&= labrinth factor; f, = pore radius; and D,= pore diffusion coefficient. If CDis greater than 0.3 pore diffusion begins to become effective as a rate determining step. Since k for catalytic gasification at a temperature is very much higher than for non-catalytic gasifications 0 reaches a value >0.3 at lower temperatures. However, it may be that the pore structure and therefore D, and rp also change gasification under compared with non-catalytic formation of greater pores and pore diffusion coefficients. In this case the transition temperature from chemical reaction to pore diffusion control would increase to higher chemical reaction rates compared with noncatalytic gasification. From the Arrhenius diagram shown in Figure 4 it can be concluded that the addition of distinct amounts of catalysts at a low reference temperature has the same effect as a corresponding temperature increase in noncatalytic gasification. However, the effect of catalysts is limited to the temperature region of chemical reaction and pore diffusion control. In the temperature range in which film diffusion controls the reaction rate the catalyst does not become effective. TECHNICAL
APPLICATION
OF CATALYSTS
As has been shown, the advantage of the application of catalysts is, that at low gasification temperature an increase of throughput of a given gasification reactor can be performed, provided it works in the temperature range with chemical reaction or pore diffusion as a rate controlling step, That means that the specific throughput per gasification reactor is increased and therefore the specific investment costs for the gas generator can be decreased. In contrast in non-catalytic gasification processes an increase of throughput is linked with an increase of reaction temperature. Since gasification with steam is an endothermic reaction a higher consumption of oxygen and coal is necessary and furthermore heat losses are greater. The application of catalysts also involves problems, i.e. 1 additional costs for catalysts; 2 possible side effects, e.g. corrosion of materials in the gasifier or in plants downstream from the gasifier; 3 necessity of recovery of catalysts, especially if expensive materials are used; and 4 possible environmental impacts concerning ash disposal. The size of these problems cannot be generally quantified, but depends on the individual system under consideration. The system parameters are: kind of catalysts; costs of catalysts; amount of catalysts necessary for a distinct increase of throughput; corrosion effects; and the question of whether recovery of catalyst is necessary for economic or environmental reasons. These problems can only be studied and finally solved by experimental research and development. UnfortunateIy at the present time insufficient data are available to make a more quantitative balance of advantages and disadvantages of catalyst application. However, as small scale experiments on catalysed
g~if~cation and corresponding patent applications have been known for more than 100 years, and there has been no commercial application, it could be assumed that for conventional gasification processes the advantages of catalytic gasification are not much greater than their disadvantages. Attention should be directed, therefore, at special gasification conditions in which the enhancement of throughput by catalysts cannot be reached by a simple temperature rise. Under this criteria gasificaion processes can be investigated in which the temperature is limited for various reasons.
NEW CATALYTIC GASIFICATION UNDER DEVELOPMENT
PROCESSES
Recently two categories of processes have been under development, in which the gasification temperature is limited. The first process is allothermal steam gasification using high temperature helium coming from an HTR. The reaction heat is transferred into the fluidized-bed gasification reactor by a suitable heat exchanger. The maximum temperature of the helium is limited to 950°C so that the gasification temperature cannot exceed 80% 840°C. In this process two parameters affect economy: (1) the reaction rate in this temperature range and the corresponding throughput of the gasifier; and (2) the temperature difference between the helium input-stream into the heat exchanger (900°C) and the fluidized-bed temperature. The higher this difference is, the more heat can be transferred from the helium stream into the gasilier. A more detailed consideration shows that the economy of the process is improved at lower gasification temperature with higher coal throughput of the gasifier at the low temperature level. In this special situation the application of alkaline catalysts looks very promising. More details will be reported in a seperate paper.’ 1 A different criterion for gasification processes with limited temperature concern processes in which conversion is limited by the thermodynamic equilibrium. Here all processes for the direct production of methane from coal are relevant. In the system carbon/hydrogen the equilibrium leads to high methane concentrations at possibly low temperatures and high overall pressures. However, the rate of gasification is very low at these favourable temperatures from the point of view of thermodynamic equilibrium, and for kinetic reasons it is necessary to increase to a higher temperature with considerably lower methane concentrations. Here the application of a catalyst effective for the C-HZ-reaction would be very promising. Unfortuantely it is not yet possible to tind a catalyst suitable for this reaction under technical conditions. Recently an elegant solution for production of a methane-rich gas by catalytic steam gasification with recycling of the hydrogen and CO of product gas at 690°C has been suggested by Exxon.” Both processes use fluidized-bed reactors - BergbauForschung, a lying, and Exxon, a standing tube - and alkaline cataiysts. Pilot plants have been run successfully, and the construction of larger plants is in preparation. On the whole it can be stated, that the t~hnical application of catalysts has made some important progress, however, its industrial application is still an aim for the future.
FUEL,
1983,
Vol 62, February
237
Application
of catalysts to coal gasification:
PERSPECTIVES
H. Jiintgen
OF CATALYST APPLICATION
It has been shown that by catalytic gasification an increase of the specific coal throughput in a gasifier without more oxygen- and coal-consumption can be achieved. At present there are not enough data available to evaluate the economic chances of an application of catalytic gasification for conventional processes. Due to the fact that catalysts are only effective in the temperature range of chemical reaction and pore diffusion as rate controlling steps, catalytic gasification can only be applied to moving-bed or fluidized-bed gasification at temperatures lower than =lOOO”C. In all cases of processes with limited temperature - allothermal processes or processes for methane production -catalytic processes will have great chances in the future. However, a realization of such processes can take place only after the following problems have been solved. Objective comparison gasification
of catalytic
and
non-catalytic
For an objective comparison of catalytic with noncatalytic gasification engineering studies based on reliable data of kinetics are necessary. Therefore kinetic investigations under process-relevant conditions should have the highest priority. Process-relevant conditions mean: similar pressure and temperature range as the technical process has, studies under conditions in which the influence of high product gas concentrations can be investigated also. According to the LangmuirHinshelwood kinetics it is well known that high concentrations of hydrogen inhibit reaction rate of steam gasification of carbon. First results show that the degree of inhibition is also dependent on the kind of catalyst used. Also studies at different temperatures to evaluate reliable values of activation energies are extremely important, since only in this way is a precise comparison with noncatalytic processes possible. Selection of the optimal catalyst for a given system
A question which is still open despite the immense amount of work done in the area is which catalyst has an optimal effect on gasification processes? This question has to be investigated under very different aspects. First kinetic studies are eminently important under processrelevant conditions with different coals, as experience has shown that it is difficult to transfer results from the
238
FUEL,
1983,
Vol 62, February
gasification of pure carbon to the gasification of coals. Here some additional problems arise in relation to coal pyrolysis, changing of chemical reactivity according to different pyrolysis conditions, and different sizes of, pore structure and internal surfaces. Also reactions of the mineral substance of coal with the catalyst can lead to some deactivation. Favourable properties of catalyst
Furthermore an optimal catalyst must have additional favourable properties, such as no corrosion effects, as cheap as possible, no damage for the environment if disposed of, possibility to separate the catalyst from coal ash with the aim of recovery. Distribution of catalyst
Another problem which is still open is the best possible feeding method for the catalyst. There is evidence that the effect of catalyst on reaction rate depends on the kind of feeding. In this context it is important to ask, what is the state of distribution of catalyst in the grains of the solid. It has been found that some catalysts, such as potassium carbonate or potassium oxide, are freely moveable within the pore structure of coal grains. Other catalysts, such as Ni or Fe compounds, seem to be immoveable. Here a good distribution is especially important. Different methods for an optimal mixing of catalyst and coal are suggested, e.g. feeding in the coal mill or feeding with steam.
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Tessie du Motay, C. M. and Marechal, C. R. British Patent No. 2548, 1867 Taylor, H. S. and Neville, H. A. J. Am. Chem. Sot. 1921,43,2055 Kriiger, C. Angew. Chem. 1939, 52 (6), 129 Veraa, M. J. and Bell, A. T. Fuel 1978,57, 194 McKee, D. W. and Chatterji, D. Carbon 1978, 16, 53 Harker, H. ‘Proc. of the 4th Conf. on Carb.‘, Pergamon Press, New York, 1960, pp. 125-139 Otto, K., Bartosiewicz, L. and Shelev, M. Carbon 1979, 17, 351 Wigmans, T., Elfring, M., Hoogland, A. and Moulijn, J. A. ‘Proc. at the Int. Conf. on Coal Science’, Diisseldorf, 1981, pp. 301-306 Leonhardt, P., Sulimma, A., van Heek, K. H. and Jiintgen, H. Fuel 1983, 62, 200 Huhn, F., Klein, J. and Jiintgen, H. Fuel 1983, 62, 196 Kubiak, H., Schriiter, H. J., Sulimma, A. and van Heek, K. H. Fuel 1983,62, 239 Nahas, N. C. Fuel 1983,62, 239