Some general conclusions from the results of studies on solid fuel steam plasma gasification

Some general conclusions from the results of studies on solid fuel steam plasma gasif ication Ivan B. Georgiev

and Boris I. Mihailov*

Institute ‘MINPROJECT’ Research Department, 14 ‘Kl. Ochridski’ bout., 1756 Sofia, Bulgaria *institute of Thermophysics, Siberian Branch of the Academy of Science USSR, 630090 Novosibirsk, USSR (Received 28 June 7991; revised 22 August 1991)

Peculiarities of the process of plasma treatment of solid fuel are presented. Based on preliminary theoretical estimations and results previously obtained, an installation for investigating coal gasification in a water steam plasma jet or its air mixture is designed. Coals with different ash contents have been gasified. The experimental conditions and the results obtained are given. The influence of single factors on the process are studied. Based on some of the results, it is shown that there is a difference in plasma gasification for low and high ash coals. A preliminary mechanism for the difference is proposed. (Keywords:

gasification;

solid fuel; steam plasma)

Low temperature thermal plasma (usually up to 5000 K ) has wide applications in fine-dispersed powder production, in synthesizing high compounds (nitrides,

melting

and

energy

consuming

oxides, carbides), in ceramics, metallurgy, machinery construction, etc.lp3. Studies on the possibilities for solid fuel plasma treatment were initiated long ago4-‘j. Two main types of study are carried out: plasma pyrolysis and plasma gasification. In the former, a plasma jet fed at a high temperature to a stream of coal dust is effective due to the large amount of thermal energy involved leading to pyrolysis. In the latter, the plasma jet acts as both a chemical reagent and a heat carrier. It is shown that depending on the temperature in the low temperature plasma there is a large fraction of radicals, ions, electrons and excited molecules that together with the high-energy radiation make it a highly reactive plasma. To explain the effect of some of the parameters on the process the thermodynamic equilibrium composition of components in the reaction system has to be known. Based on thermodynamic analysis of the equilibrium composition in the gas phase of the products from coal pyrolysis at low temperature, plasma temperatures could be calculated. Figure I gives the results from thermodynamic estimations of the equilibrium composition for a C-H-O system, typical for solid fuels, during argon pyrolysis and Figure 2 gives the equilibrium composition during steam gasification. For clarity the curves of the concentration changes for different intermediate radicals are not shown. Some genera1 conclusions can be drawn from the figures. Plasma chemical processes are known to be highly selectivedepending on the conditions and the raw material the main products in the coal treatment end products are acetylene (C,H, ), carbon monoxide (CO ) and hydrogen (Hz). Among the hydrocarbons, C,H, is thermo001C%2361,‘92/080895-07 !Q 1992 Butterworth-Heinemann

Ltd.

dynamically the most stable at high temperatures. It is in a high concentration (highest output) in the range of 2500-3200 K while in the range of 1500-800 K it is unstable. Consequently, it has to be cooled very quickly on production. From the figures it can be seen that CO and H, are thermodynamically stable over the whole temperature range and neither their removal from the reacting system nor their fast cooling are required. The behaviour and the type of intermediate and end compounds produced during the coal plasma chemical treatment predetermine the way the two main processes occur and the requirements for the raw materials. Studies on the direct production of C,H, by black coal plasma pyrolysis are at a most advanced stage. According to the general mechanism involved, C2H, is produced on the decomposition in argon or H, (inert medium) of volatiles liberated during the high temperature pyrolysis of coal particles. It is known that H, fed to argon or pyrolysis carried out in H, do not lead to an increased output of C,H, as it does not react directly with H, and probably acts to suppress undesirable side reactions. From Figure 1 and from the studies, it is seen that 0, availability in coal decreases the C,H, output because of liberation of the thermodynamically more stable CO. On the other hand it is shown that the C,H, output is directly related to the quantity of volatiles in the pyrolysed coal. It follows that bituminous black coal appears to be the most favourable raw material for direct C,H, production. This process is now ready for industrial use7-9. The main disadvantages for plasma pyrolysis are : large energy consumption ; certain requirements of the raw material for treatment ; additional dilution of the product may be required and/or cooling very quickly (up to 10’ K s-l ) which may complicate the experimental set-up. It is accepted that the C,H, thus produced is

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Steam plasma gasification: I. 8. Georgiev and B. I. Mihailov

A

O-

-I j c -3 .o z z 2 -5g I?4 g 1 -7-

-9IO00

2000

4000 3000 Temperature (K )

5000

Figure 1 An estimated equilibrium composition of the C-H-0-Ar system and dependence on temperature : A, H’ ; B, H, ; C, Ar ; D, CO ;

E, CA

; F, CH,; G, C,H,

cheaper than that produced by the carbide method. Plasma pyrolysis could be increasingly useful with the increasing cost of natural gas and petroleum and increasing quantity of electric energy produced by physical methods. With coal plasma pyrolysis for C,H, production a large amount of fine coal dust containing H2 ( >40%) remains. This dust could be burnt or it could be used in steam gasification for H, production for use in another coal treatment process. Gasification is one of the cheapest methods of producing H, and during steam plasma gasification a gas with a high H, content is produced. Fuel gasification (solid, liquid, gaseous) is widely applied in industry for the production of energy, reduction and for synthesis gases. This process is overall endothermal so autothermal gasification methods are mainly used. The energy required for the process comes from the partial oxidation of the carbon in the raw material. The thermal energy from the allothermal processes is fed outside the system. Generally, heat released from other processes is used for the gasification, e.g. from cooling nuclear power plants. Practically, the allothermal methods are still in the experimental stage. Plasma gasification is an allothermal method. A version of plasma gasification, which is part of a common technological scheme, is ready for industrial application”. However, plasma gasification remains a poorly studied process. The main reasons for this are the need for a large amount of electric power and there is a cheaper alternative available-the simple gasification of fuel. Plasma gasification possesses all the advantages of the high temperature gasification methods-high intensity of the process run, no special requirements for the raw material, ecologically favourable compared with direct burning. It also has specific advantages: 0 high energy concentration in a unit volume makes the plant compact; 0 the use of electric power leads to a more complete and higher extent of automation ; 0 in contrast to plasma pyrolysis, it is constructively simplified with the possibility of fully using the physical heat of the gas ; 0 the H, content in the resulting gas is high;

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0 when necessary, the liquid slag temperature can be increased without affecting the gas composition; 0 no CO2 is discharged into the environment. Plasma gasification has a number of applications. The process can be used to solve ecological problems, using raw material economically, and can be used to improve the burning process in thermal power plants when low quality, low volatile content coals are used. Solid fuel gasification is a complex heterogeneous process involving a large number of physical and chemical processes. Depending on the gasifying agents and the process conditions, various chemical reactions take place. As seen in Figure 2, CO and H, appear to be the main products up to 2000 K. The amount of CO2 produced is < 1% and there is no methane whilst sulphur is present as hydrogen sulphide (H,S). Consequently, there are only two main chemical processes occurring during plasma gasification : C + H,O + CO + H,

(1)

2c + 02 --) 2co

(2)

The most typical condition for steam plasma gasification is the absence of an oxidizing region and heat is directly transferred to the region of the reduction processes. The following stages can be distinguished: plasma which appears to act simultaneously as a gasifying agent according to reaction 1. The 0, used during steam gasification is that available from the starting coal. It is seen from the above equations that by adding some amount of O,, reaction 2 is increased and the total energy consumption in the process is decreased. Consequently, a coal with a high O2 content would require lower energy consumption with the other conditions being the same, i.e. energy consumption should be decreased in the order: anthracite -+ black coal + brown coal + lignite coal.

800

1600

3200 2400 Temperature (K)

Figure 2 An estimated equilibrium composition of the gas phase in coal steam gasification (C:O = 0.51; P = 0.1 MPa): A, H,; B, CO; C,H,O;D,CO,;E,N,;F,H,S;G,S;H,H’;I,CH,;J,COS

Steam plasma gasification: I. B. Georgiev and B. I. Mihailov Table 1 Physico-chemical characteristics of the coal used Coal no. 1 Moisture (wt%) 9.1 Ash (%) 9.0 Volatiles ( % ) 45.3 Elemental composition ( % ) C 65.1 H 4.6 S (total) 0.5 N 0.6 0 (by diff.) 20.2 Heat of combustion 28 300 (kJ kg-‘)

Coal no. 2

Coal no. 3

Coal no. 4

10.8 21.2 41.2

1.2 37.1 37.2

8.2 39.5 36.9

52.6 4.2 4.1 0.4 17.5 23 530

39.5 4.4 5.0 0.3 13.7 17260

36.6 3.9 6.2 0.6 13.2 16470

for heating and melting the mineral fraction (ash). Though the heat accumulated in the molten slag is lost in the technological process, Figure 3 shows that high ash coal plasma gasification is not unattractive as the main energy consumption is for the coal organic matter gasification and for heating the released gases. Figure 4 traces the dependence of energy consumption in plasma gasification on O2 content. The total amount of O2 is taken as the sum of that available from the coal and that gradually added to the plasma stream in place of the water steam up to the stoichiometry needed for only O2 gasification to take place. The lowering of electric power consumption in plasma gasification with increase in O2 is apparent. The power consumption involved in

The most typical condition for steam plasma gasification is the absence of an oxidizing region and heat is directly transferred to the region of the reduction processes. The following stages can be distinguished : a sudden heating of the coal particles as a result of their heat exchange with the plasma jet; 0 an explosive liberation of volatiles from the coal matter pyrolysis in the range of 90& 1200 K. The particle temperature slightly changes due to the endothermal pyrolysis processes and evaporation, irrespective of the plasma jet temperature ; l a very quick (practically instantaneous ) gasification of the discharged volatiles due to the high temperature, the homogeneous phase and the rapid heat and mass exchange ; l semi-coke particle gasification by diffusion, hemosorption on the activated complex, desorption and reverse diffusion”~‘2~‘3 of the products. A peculiarity appears here that during the explosive separation of volatiles, the particle surface and pore volume are considerably enlarged and a large number of the chemical bonds remain activated. Moreover, the gasified particles are absolutely and relatively moving with the plasma stream.

Coal no. I

Coal no. 2 I

I

I

Coal no.3

0

Bearing in mind the processes taking place during plasma gasification, we developed methods for a preliminary estimation of the energy involved. The methods are described in detail elsewhere14. The energy required for plasma gasification of 1 kg of coal is calculated considering the thermal effects involved in reactions 1 and 2, the energy involved in the destruction and removal of volatiles and the quantity of heat needed for heating the plasma, the semi-coke residue containing minerals and the resultant gas up to the final temperature. An overall process efficiency of 0.9 is accepted. The values from these calculations are of an approximate theoretical nature. For technological purposes a precise heat balance of the total process has to be calculated including the processes involving the heat of the released gases. The calculations were carried out for lignite coals with different ash contents (Table 1). The results obtained are presented in Figures 3-5. The results in Figure 3 show that the relative power consumption for gasifying a unit of coal decreases with increase in ash content. The energy consumption for the production of 1 nm3 from the coal increases for coals with higher ash contents. The explanation for this is that the energy consumption for the endothermal processes of organic matter gasification is higher and increases faster than the energy consumption

A

k I I I I I

I I

I I

0

I

IO

B

I

I

I

20

30

Ash content (%)

Figure 3 Relative consumption of electric power W,,, in water plasma jet gasification of lignite coal with different ash content (T = 2000 K; r/ = 0.9) : A, I+‘;,, relative consumption of electric power for gasification of 1 kg of coal; B, W:,,, relative consumption of electric power for production of 1 nms of gas

7

-

1.5

T s

- 1.0

2-

“E c s-

r.7 ‘!

f

r5 &-

-0.5

I-

0 100

I

I

I

I 50 50

I

I

I

I loo 0

H*O(%) O$%)

Figure 4 Change in the relative consumption of electric power Wr., and the volume of the resulting gas Vi during plasma gasification at different steam to oxygen ratios in the jet ( T = 2000 K ; coal no. 1) : A, sum of CO + H, ; B, W,,, ; C, amount of H, ; D, amount of CO

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897

Steam plasma gasification: I. B. Georgiev and B. I. Mihailov

L 0

I 2000

I

I

I

2500

3000 3500 Temperature ( K ) Figure 5 Change in the relative consumption of electric power W,,, and dependence on temperature during gasification with a plasma jet of water steam (coal no. 1): A, WE,, relative consumption of electric power for production of 1 kg of gas; B, W&,, relative consumption of electric power for production of 1 nm3 of gas

the production of O2 is not included as it is only a small amount. Figure 4 also shows the change in volume of the resulting gas, H, and CO, respectively, according to reactions 1 and 2 as well as the carbon consumption per unit of gas produced. Figure 5 shows the dependence of electric power consumption during steam plasma gasification on temperature. This dependence is not linear. It can be seen that it is not economical to carry out the process above 2500 K. Different authors suggest different regions where plasma gasification takes place. The particular region involved is not only of scientific importance but also of practical importance. As it was shown in Figure 5, energy consumption for the process sharply rises above a certain temperature. Thus, it would not be profitable to alter the gasification speed and extent only by increasing temperature. In the initial studies on plasma gasification it has been assumed that the process takes place in the kinetic region, i.e. the higher the plasma jet temperature, the faster and more complete the gasification would be. This assumption has been affected by the results of plasma pyrolysis where the time of heating (pyrolysis) the particles is almost equal to the time of C,H, formation. Consequently, during the gasification tests the aim was for the flow of coal particles to be fed into the centre of the plasma stream where the temperature is highest15*16. In some test units the coal dust is fed through a special opening in the anode of the plasma torch17. Nevertheless, the gasification level (the level of converting the coal carbon into gas) was not high. One of the first mathematical models describing steam plasma gasification was proposed by Kalinenko et al.‘*. It was developed on the basis of the kinetic data for the probable chemical reactions taking place (about 63 reactions including intermediate compounds). Some experimental data for a single stream plasma reactor,

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with the plasma stream rate being up to 50 m s-l, were also used. Coal dust with a coarseness of co.063 mm was fed into the centre of the reactor and after applying three different rate constants, it was determined that the process takes place in the kinetic region. By a series of tests on plasma gasification with steam, it is shown that after the initial fast separation and gasification of volatiles, the semi-coke particles that remain are incompletely gasified irrespective of the plasma jet temperature. A main reason for this is that after the first fast destruction and gasification, the particle carried away by the high speed plasma jet has the same speed as the plasma jet. On leaving the jet nozzle, the plasma jet temperature drops which generally leads to a sharp decrease in the mass transfer processes and the gasification practically stops. The results show that the level of plasma gasification is not directly proportional to temperature and the concentration of the reagent (which is in excess) but on the time that the particle stays in the reactor and on the construction of the reactor’5*16,‘9*20. For traditional gasification it is accepted that above 1500 K the reactivity of all types of coal is equal and the process takes place in the diffusional region above that temperature. Certain differences are found in plasma gasification : these include the high speed of the reagent flow, the blowing over and trapping of the gasified particles ( > 0.1 mm) and the large inner and outer surfaces of the particles. The processes involved are complex and are the subject of macrokinetics studies which consider not only the main chemical equations but also the diffusion and adsorption kinetics. Gasification with steam is complicated by the processes13 of hemosorption and the desorption of the resulting H,. Our aim was to examine the process of plasma gasification using coals with different ash contents and particle sizes of 0.1-0.4 mm. The rate of the plasma stream in the reactor was varied from 10 to 15 m s-‘, and the time spent in the reactor was >O.Ol s. Temperatures up to 2200 K were used and the gasifying agents were used in excess ( z 1.5 times). EXPERIMENTAL The plant design is shown in Figure 6. It includes a plasma chemical reactor, a steam generator, dust collectors, a

Figure 6 Scheme of the installation for coal plasma gasification: 1, compressors; 2, 4, reservoirs; 3, steam generators; 5, air heater; 6, plasma torch ; 7, plasma chemical reactor ; 8, hopper ; 9, dosimeter ; 10, vessel containing argon; 11, cyclone; 12, refrigerator; 13, welding transformer; 14, electric supply of the plasma torch ; 15, control system; 16, instrumentation block ; I, steam line ; II, air line ; III, coal line ; IV, gas carrier (if necessary); V, resulting gas; VI, water for steam production ; VIII, slag

Steam plasma gasification: I. B. Georgiev and B. I. Mihailov

condensation cooling system and a system for measuring the process parameters. A plasma torch working on steam and power up to 50 kW was used. Single tests were carried out in the following way. The plasma torch was turned on until the lines and the system were heated. Steam was supplied with the air being completely or partially turned off (depending on the test). After the temperature required was reached, the coal dust with or without a carrier was fed to the reactor. The plasma chemical reactor was made of graphite with an inner coating of glass carbon (less reactive). During the tests, the reactor walls were covered with a liquid layer of slag from coal ash. The outside of the reactor walls were cooled by water. Direct measurement of the reaction medium temperature cannot be done and it was estimated on the basis of the enthalpies of the substances mixed. Consequently, the heat loss from the reacting mixture to the reactor walls was not taken into account and so the estimated temperature used was higher than the optimal value. The temperatures of the reactor walls and of the fluid flows were measured directly. The quantities of the steam, air and gas carrier, as well as the resulting products (gas, drift matter and slag), were also measured. A mean gas sample and samples from intermediate stages were analysed by gas chromatography. RESULTS

AND DISCUSSION

The initial tests and results from other studies showed a low level of coal gasification. This led us to examine the effect of a large number of parameters on the process run. We took note of the fact that steam acts not only as a chemical reagent but also as a heat carrier. At a definite steam to coal ratio it may be that as a chemical reagent the amount of steam is sufficient but the enthalpy it supplies is insufficient for the process run. Moreover, heating the same amount of steam to different temperatures changes its volume to different extents and consequently the speed of its flow and the hydrodynamic conditions in the reactor, respectively. This influences the particle stay time in the region of high temperatures. With all the other conditions being the same, the hydrodynamic conditions change when coals with different ash contents are used-the quantity of plasma required as well as the volume of gases resulting from low ash coal per unit time are higher. The volume of gases from the same coal during steam or steam-air gasification also changes. Also, a given plasma torch has an optimum, not very wide, working range. From the macrokinetics requirements, we can make limited changes to the process parameters. Above certain conditions, changes in temperature, the speed of the plasma stream and reagent concentration could not be made. In fact, mass transfer can be more easily changed by altering the turbulence of the streams and increasing the stay time of the coal particle. Bearing in mind the thermodynamic stability of the end products produced (CO and H,) at high temperature the time in the reactor is not limited from this point of view to the complete conversion of carbon. After solving the problems encountered, the gasification level in our studies exceeded 90%. The changes gave opportunities during the single tests to alter within definite limits the conditions in the reactor, namely the stream rate, temperature, mass and

heat exchange, stay time of the particles and to take into consideration the type of gasified coal used. The preliminary tests showed that with the gasification conditions used, coal particles up to 0.25 mm are successfully gasified and consequently were used for all our further examinations21-25. After specifying the limits for changes in the conditions (the parameters) we carried out similar tests on steam and steam-air plasma gasification of lignite coal with mean and high ash contents. The physico-chemical characteristics of the coals are listed in Table I. The hydrodynamic conditions could be influenced by the temperature of the plasma jet, by the excess amount of plasma needed for reacting with carbon and by changing the quantity of coal in the reactor per unit time. Using these factors we aimed to provide relatively similar conditions for the process. To clarify the influence of 0, during steam plasma gasification we used air---not only is it cheaper and practically more applicable but it enabled us to examine the influence and behaviour of nitrogen (N,) during the process. Generally, the quantity of supplied air is selected so that the sum of the 0, available in it and in the raw coal is approximately half that of the stoichiometrically required amount for steam-air plasma gasification. The rest of the carbon is gasified by the water plasma. Results are summarized in Tables 2 and 3. As the temperature of the reactor walls does not exceed 2400 K, the temperatures indicated for the reacting mixture may be used for comparison. Because of the high heat loss and lack of use of the physical heat of the gases produced, the energy efficiency of this plant is low (-0.3). Consequently, calculating the energy efficiency of the single tests and commenting on the electric power consumption are pointless. From the data in Tables 2 and 3 it is seen that the degree of gasification (conversion of coal carbon into gas) is high (between 94% and 96%). We do not expect a higher level of conversion because of the mineral fraction being reduced to carbides. Reactions of the mineral fraction with coal carbon at high temperatures Table 2

Results from water steam plasma Coal no. 2

Plasma torch power (kW) Mean mass temperature (K) of plasma torch of reaction mixture Coal consumption (kg h- ’ ) Water steam consumption (kgh-‘) Process products (kg kg- ’ coal) gas slag” drift off” Gasification degree (% ) Resulting gas composition

‘Approximate bMean values

Coal no. 3

Coal no. 4

32.5 3200

33.0 3170

35.2 3370

2900 2.5 3.5

2800 2.8 3.6

2890 3.0 3.6

1.37

0.94

0.94

0.22 0.03 94.0

0.38 0.03 94.0

0.40 0.04 93.0

54.85 39.60 4.20 1.35 0.65 12 140

61.10 34.90 2.25 1 .I5 0.75 12730

59.40 37.15 1.12 2.33 0.54 12350

2.10

1.66

1 .I4

(~01% )b

HZ co CO, H,S Density of gas (kg nm Heat of combustion (kJ nme3) Gas volume kg-’ coal (nm3 kg-‘)

gasification

3)

values

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Steam plasma gasification: I. B. Georgiev and B. I. Mihailov Table 3

Results from steam-air plasma gasification Coal no. 2

Plasma torch power (kW) 35.2 Mean mass temperature (K) 3090 of plasma jet of reacting mixture 2900 Coal consumption (kg h-r) 3.0 Water steam consumption 3.5 (kgh-‘) Air consumption (kg h - ’ ) 3.0 Process products (kg kg-’ coal) 2.15 gas slag” 0.22 drift off 0.03 Gasification ( % ) 95.0 Composition of resulting gas (~01%)~ 40.66 H, co 33.70 1.83 CO* H,S 1.10 22.71 Density of gas (kg nme3) 0.79 Heat of combustion (kJ nm-‘) 9630 Volume of gas kg-’ coal 2.72 (nm3 kg-‘)

6

Coal no. 3

Coal no. 4

30.8 3700

36.8 3800

3090 3.0 2.5

3025 3.6 2.5

1.9 1.55

1.55 1.34

0.38 0.04 96.2

0.40 0.05 95.0

46.28 32.94 1.93 1so 17.34 0.72 10380 2.14

43.46 31.33 3.46 2.13 19.64 0.71 9970 1.73

“Approximate values *Mean values

and in other chemical reactions are not excluded. The net efficiency of the process depends, of course, on the efficiency of the electricity generation, and will hence be much lower. Whether the gasification is by steam or steam-air, the degree of converting the coal sulphur to H,S is >90%, which is ecologically very favourable. We draw special attention to the fact that with the air to water steam ratios used in the low temperature plasma and in the end gas, the presence of nitrogen oxides is not noticed. The composition of the gas produced depends on the type of gasification and for steam plasma gasification the H, content in the gas exceeds 61 ~01% for the coal studied. It is also possible that part of the H, in the gas is produced in the thermal decomposition of the water steam. The gas produced in steam plasma gasification is of higher caloricity because the ballast N, is missing and the H, to CO ratio changes. The low thermal efficiency does not allow there to be a precise quantitative control of the influence of O2 on the electric power consumption. From Tables 2 and 3 it can be seen that a decrease in consumption is observed when air is used. From calorimetric tests the influence of 0, from different types of coal on energy consumption was found to be higher than theoretical estimations. Black, glass-like solid slag, as well as gas and drift matter, is produced in plasma gasification. In cases when cooling was fast and time in the reaction zone was short, the structure of slag is porous especially when obtained from the gasification of high ash coal. The results of our investigations are presented elsewhere23-25. Additional studies have been carried out to examine aspects of the process run. It has been established that during the first stage of gasification-during the explosive separation of volatiles-a decrease in particle size’ is observed for low ash coal while the reverse is true for high ash coal. In the latter case, as can be seen from Table 4, the particle size increases. In that case, it may be supposed that the liberated volatiles do not fragment

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FUEL, 1992, Vol 71, August

but only increase particle size as the separated amount of steam gas mixture is relatively smaller and the mineral skeleton is larger. Attention is drawn to the fact that in an inert medium, depending on the mineral fraction, coals soften and melt at different temperatures (from 1300 to 1800 K). The coal used by us has the following melting temperatures: coal no. 4, 1600 K; coal no. 2, 1750 K; and coal no. 1, softening is not observed up to 2300 K. It may be assumed that after removal of the volatiles, the low ash coal behaves as a graphite-like product and practically does not melt before being gasified. The gasification of a high ash coal with a low melting temperature must not be carried out near to the softening temperature as the particle pores are closed and there is no access for the gasifying agent. Diffusion into the molten particles does not take place because of the short time spent in the reactor. In such a case, for complete gasification the time in the reactor does not change over very wide limits and is not increased, and the mass transfer processes with the low temperature plasma are intensified. The latter is also related to intensification of the process of heat exchange (the temperature increase). Change in these two processes, however, leads to a reduction in the time spent in the reactor and a faster increase in the particle temperature. These contradictions lead us to assume that during high ash coal plasma gasification an additional stage takes place. The gasification process is also active after the melting. At the moment of melting there are water molecules diffusing into the pore volume and along the whole inner surface. Melting leads to an increase in pressure (and concentration) on the active centres which leads to an increase in the absorption rate and further reactions with the remaining ungasified carbon. The reaction products together with the unreacted steam are then removed from the molten particles and the porous structure is formed. The calculation of the number of water molecules in the pores volume and the number of remaining carbon atoms at a given inner surface shows the possibility of the process occurring even when carbon atoms remain after the first stage of pyrolysis and separation of volatiles in the case of high ash coal. CONCLUSIONS The results obtained and the subsequent discussion show that coal plasma gasification shows certain peculiarities. It is clear that the sharp temperature ‘attack’ and the high temperature of the plasma jet do not necessarily lead to a successful process run. There is also a temperature limit over which the process does not proceed. There are also differences in the behaviour of low and high ash coals as well as for different amounts of 0, in the system. Practically the process of plasma Table 4 Results from sieve analysis of thermally treated dust of coal no. 3 in an inert plasma jet

Fraction (mm)

Starting coal (%)

Semi-coke (%)

Drift off (%)

0.2 0.2-0.16 0.16-0.10 0.1 Losses

1.97 16.26 26.15 54.12 1.50

18.58 21.61 24.14 34.14 1.54

11.50 17.50 31.56 38.07 1.37

Steam plasma gasification: I. B. Georgiev and B. I. Mihailov

gasification could be intensified according to the macrokinetics requirement (technical kinetics) only by turbulizing the plasma and coal streams. The degree of gasification depends to a large extent on the time the gasified particles remain in the reaction region. The latter is provided not only by constructive changes but also by the hydrodynamic conditions in the reactor. Even though the results from this study were obtained using a single-flow reactor, all the above also refers to multi-flow reactors. The conditions of flow turbulization and particle stay time are easier to resolve in multi-flow reactors. To increase the efficiency and the economics of the process, the physical heat of the gases must be used and optimum heating conditions, and optimum mass and heat transfers are required for all the raw materials. Plasma gasification may be realized as a technical method and can already be used for small-scale practical applications.

6 7 8

Nicholson, R. and Littlewood, K. Nature 1972, 236, 397 Polak, L. S. and Kalinenko, R. A. ‘Plasmennaya gasif. i pirolys niskosortn. uglei’, 1987, p, 21 Bittner, D., Bauman, H., Penckert. C. and Klein, J. Erdol Kohke Erdgas Petrochem.

9 10 11 12 13 14

198 1,

34, 237

Peuckaert, C. and Muller, R. in ‘Proc. Int. Symp. on Plasma Chemistry’, 1985, p. 232 Herlitz, H. and Santen, S. Chem. Sros. 1984, 28, 49 Hoffman, E. I. Coal Conversion’, 1980 Fedoseev, S. D. and Tchernishev, A. B. ‘Semi-coking and Gasification of Fossil Fuels’, Moscow, 1960, p, 325 Blyholder, G. and Eyring, H. J. Phys. Chem. 1959, 63, 693 Georgiev, I. B. and Michailov, B. I.~l:wsriu Siberian Branch of Science Akad. USSR. Technol. Sci. 1987. 4. 83

15 16 17 18 19

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