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Energy & Fuels 2002, 16, 428-435
Hydrogasified Coal Char’s Reactivity Improvement Technique through H2/Coal Feed Ratio, Coal Loading, and Slip Velocity Control Hassan Katalambula† and Shohei Takeda* National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu Higashi 2-17, Toyohira-Ku, Sapporo 062-8517, Japan Received June 21, 2001. Revised Manuscript Received November 8, 2001
A new technique to improve the hydrogasified coal char reactivity through a controlled coal loading at a fixed H2/coal feed ratio is presented. Under this technique, apart from the hydrogasification pressure and temperature, careful manipulation of the H2/coal feed ratio and the concentration of coal particles in the reactor (coal loading) has been found to influence the char reactivity without affecting the coal conversion, whereby the char reactivity increases with increasing concentration of coal particles in the reactor up to a certain point. These results were supplemented by hydrogen gas yields obtained from the steam gasification of the chars, which also showed an increasing trend with increasing coal loading. Between the two coal loadings employed in this study, i.e., 30 g/dm3 and 60 g/dm3, a hydrogen yield increase of between 4 and 20%, depending on the type of coal used, was attained. The findings show that with the manipulation of some hydrogasification parameters, it is possible to establish operating conditions that produce more reactive chars without compromising product gas yields.
Introduction One of the major obstacles in the development of coal hydrogasification technology is the source of hydrogen required for the hydrogasification process. The only possible solution out of this is to produce the hydrogen by the steam gasification of the chars. This however, will not be helpful if the reactivity of the chars is too low to yield any reasonable amount of hydrogen. Therefore, to effectively utilize the hydrogasified coal chars, their properties must be well understood. Further, the effect of various coal hydrogasification parameters on these properties must be made clear. The feed ratio of hydrogen to coal (w/w) is one of the important parameters in hydrogasification process. It is from this parameter where reactor volume and diameter can be decided and the related plant costs and thermal efficiency estimated. Some studies have been undertaken on how this parameter affects the conversion of coal to methane and BTX from which it has been reported that increasing H2/Coal feed ratio (w/w) results into increased coal conversion.1 As for the resulting char, its reactivity has been reported to decrease with increasing H2/coal ratio.2 However, the effect of varying the coal * Corresponding author. Tel: +81-11-857-8959. Fax: +81-11-8578900. E-mail:
[email protected]. † Present address: I.P.I., University of Dar Es Salaam, P.O. Box 35075, Dar Es Salaam, Tanzania. (1) Kaiho, M.; Yamada, O.; Yasuda, H.; Zabat, M.; Makino, M. Investigation of Coal Using Batch Type Reactor (2) Influence of H2/ Coal ratio on Material Balance of Hydrogasification. Proceedings of the 35th Japan Coal Science Conference, JIE 1998; pp 299-302. (2) Katalambula, H.; Takeda. S.; Kumagai, M. Reactivity Assessment of the Hydrogasified Coal Char Generated from the Drop Tube Furnace Reactors. Proceedings of the 36th Japan Coal Science Conference, JIE 1999; pp 185-188.
particle concentration in the reactor at a constant H2/ coal ratio has so far not been investigated. It can be recalled that initial hydrogasification attempts encountered a lot of particle agglomeration problems due to the high temperature and high-pressure hydrogen atmospheres.3 The problems were solved by the introduction of free-fall dilute phase reactors. On the other hand, it is important to decide how dilute should these reactors be in order to avoid agglomeration and at the same time achieve good product yields. Some of the important factors to be looked at include the gas linear velocity and the resulting slip velocity (terminal velocity) between the gas and the solids. In this work, the effect of coal particle concentration on the char reactivity with CO2 and steam has been investigated at a constant H2/coal ratio. This investigation has found that varying the coal particle concentration in the reactor during hydrogasification influences both the product yields (CH4 and other gases) and the byproducts (char) properties. Char bulk density as well as surface area also seem to be influenced. Furthermore, the slip velocity has been found to influence coal conversion. The aim of this work therefore is to elucidate the effects of changing the coal particle concentration on various parameters. Changing coal particle concentration at a constant H2/coal ratio will hereafter be referred to as changing the “coal loading”. Experimental Section Coal was hydrogasified in a drop tube furnace (DTF) reactor at elevated temperatures and pressure (1073 K and 7.1M Pa, (3) Feldmann, H. F.; Mima, J. A.; Yavorsky, P. M. Coal Gasification; Massey, L. G., Ed.; Advances in Chemistry Series 131, 1973; Chapter 8.
10.1021/ef0101356 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/08/2002
Hydrogasified Coal Char’s Reactivity Improvement
Energy & Fuels, Vol. 16, No. 2, 2002 429 Table 1. Properties of Raw Coal Samples Used proximate analyses, wt % coala
moist.
ash
V.M F.C
Taiheiyo Asam Asam Adalo Cerrejon Shimboku Mae Moh
4.9 17.5 12.0 6.6 10.9 20.5
9.3 3.6 1.3 6.2 5.4 14.6
46.1 42.9 42.4 36.8 33.9 36.2
39.8 36.0 44.3 50.4 49.8 28.8
ultimate analyses, d.b. wt % C
H
O
N
S
68.8 65.7 69.0 74.0 72.4 54.3
6.0 5.2 5.3 5.3 4.9 4.1
15.5 24.2 23.5 12.0 15.4 25.3
1.2 0.9 1.1 1.4 0.9 1.9
0.2 0.2 0.2 0.6 0.3 4.8
a Taiheiyo: Japan; Asam Asam and Adalo: Indonesia; Cerrejon: Colombia; Shimboku: China; Mae Moh: Thailand.
Figure 1. Schematic diagram of the drop tube reactor. respectively). The schematic diagram of the reactor is shown in Figure 1. It consists of three main parts, namely the coal feeder, furnace for pyrolysis or hydrogasification, and the char collector. The reactor tube which is made of incroi has an internal diameter of 20 mm and is 850 mm long. Mantle heaters are provided on the char collector as well as on the gas exhaust line in order to avoid the condensation of tar. Cooling is done on the tar trap so that the tar can condense at this point. Finally the filter is installed on the exhaust gas line in order to trap any remaining tar before the gases are allowed to go through the mass flow controller. Coal was fed into the reactor at a rate of 0.2 to 1.2 g/min through a screw feeder and the H2 was passed at 0.2 to 10.0 NL/min. This enabled the setting of different H2/coal ratios as well as different coal loadings. Essentially, the loading was varied by changing both the gas and solid feed rates by the same factor, thus maintaining the same H2/coal ratio. Char reactivity was determined using a thermo-gravimetric analyzer (TGA) under a CO2 atmosphere. The determination method involved placing a char sample of about 20 mg in a ceramic cell. The pressure in the TGA was then raised to 1.6 MPa and the temperature increased at 10 K/min to 1273 K while an inert gas was passed at a rate of 1.1 NL/min to give a linear velocity of 2.1 cm/s. After the required temperature was attained, the inert gas was replaced by CO2 which was passed at the same rate until the reaction was completed. During the reaction, weight loss as a function of time was recorded. Carbon conversion, X, and the time derivative of the extent of conversion, dX/dt, were then evaluated. The reaction rate constant, k, was then obtained graphically from the equation below:
kt ) -ln(1 - X)
(1)
Steam gasification of the char was undertaken using a highpressure tubular reactor at 0.8 MPa, 1073 K, and 50% steam partial pressure with argon as carrier gas. Each sample was gasified for a total of 30 min. Char surface area was determined using Brunauer-Emmett-Teller (BET) method under CO2 gas. In addition, the char bulk density was also measured. The coal sample used was mainly Taiheiyo (Japan) coal with
Figure 2. Effect of H2/coal feed ratio on coal conversion and char reactivity. a particle size range of 0.090-0.106 mm. Five other coals were also used. Properties of these coals are shown in Table 1.
Results and Discussion Influence of H2/Coal Ratio On Conversion. From our previous investigation,4 it has been reported that coal conversion increases with H2/coal ratio as shown in Figure 2. However, a striking observation was that at low H2/coal values, the conversion under hydrogen atmosphere was low, much lower than even that under N2 atmosphere, something indicating that even pyrolysis could not be completed under these conditions. This suggested that hydrogasification is not feasible at such low H2/coal ratios since the rapid carbon, which is defined as the difference between conversions under H2 to that under N2, was negative. The low coal conversion under H2 atmosphere at low H2/coal ratio necessitated an investigation into a possible cause to be undertaken. Results from the investigation suggest that the relative velocities between the gas and the particle (terminal velocity) as they flow and fall down the reactor, respectively, might be the cause for this. Theoretically, the gas linear velocity can easily be determined from the flow rate and the geometry of the reactor, whereas the terminal velocity (Ut) can be estimated from the Haider and Levenspiel’s5 equation below: (4) Katalambula, H.; Takeda. S. Hydrogasified Coal Chars’ Property Assessment for Hydrogen Synthesis Suitability. J. Inst. Energy, Japan 2001, 80, 933-943. (5) Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth-Heinemann: Boston, 1991; Chapter 3.
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Figure 3. Theoretical representation of the effect of gasification pressure on slip velocity as well as the slip velocity transition point.
Ut )
x
4dp(Fs - Fg)g 3FgCD
(2)
However, since eq 2 is generally used when either gas or particle is stationary, some modifications were done in the particle Reynolds number equation by replacing the terminal velocity term with the difference between gas and particle velocity (Us) as shown below:
U s ) Ug - Up
(3)
The particle Reynolds number was then calculated from
Rep )
UsdpFg µg
(4)
and the drag coefficient CD was estimated using the equation5
CD )
24 [1 + (8.171(e-4.0655φ))Rep(0.0965 + 0.5565φ)] + Rep 73.69(e-5.0748φ)Rep Rep + 5.378(e6.2122φ)
(5)
where Ug is the gas velocity, Up is the particle velocity, dp is the particle size, Fg is the H2 density, and µg is the H2 viscosity. The values for H2 density and viscosity at
different temperature and pressure were obtained from literature.6 In this case, the hydrogasification temperature was 1073 K and pressure was varied from 0.4 to 7.1 MPa. Calculation was done iteratively until Ut obtained from eq 1 was equal to the value of Us used in eq 3. As the hydrogasification pressure as well as the hydrogen feed rate were varied, Ug and Up also changed thus making Us assume negative values at some instances. However, in the calculation of Rep, only the absolute value of Us was used. A negative value for Us means that the particle is falling down faster than the gas velocity and we will refer to this as negative slip velocity and vice versa. Results from the calculation show that at atmospheric pressure, the slip velocity is negative for the whole range of H2/coal ratio investigated as can be seen in Figure 3a. In the figure, the x-axis is given in H2 flow rate corresponding to the H2/coal ratio in Figure 2. At the gasification pressure of 0.4 MPa (Figure 3b), the slip velocity is negative until the gas flow reaches 5.3 NL/ min, after which the slip velocity suddenly changes to positive values, indicating that from this point, the trend has reversed and the gas velocity has become larger than the particle velocity and thus the particle is being (6) Lide, D. R. Handbook of Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, FL, 1995; Chapter 6.
Hydrogasified Coal Char’s Reactivity Improvement
entrained by the gas. The point at which the gas velocity becomes larger than the particle velocity will be referred to as a slip velocity transition point and the area to the left of this will be referred to as a negative slip velocity zone whereas that to the right will be positive slip velocity zone. The particle velocity in the figure refers to the velocity with respect to the reactor, at which a particle attains the terminal velocity. At the transition point, the particle attains terminal velocity while its velocity is still zero because at this time the gas is already flowing at the same speed as the particle terminal velocity. Also from the calculation it was revealed that, at a given pressure, the particle terminal velocity is constant, having the same magnitude as slip velocity and is independent of the gas flow rate. For calculations undertaken at the hydrogasification pressure of 3.1 MPa (Figure 3c), the transition point is seen to shift to the left and occur at the gas flow rate of about 0.7 NL/min. This trend continues and at 7.1 MPa (Figure 3d), the slip velocity transition occurs very early, at about 0.25 NL/min. As to how the particle slip velocity affects the coal conversion, consider a small cross section volume of gas in the reactor in which at a given instant t, a particular particle is in that section and releasing either volatiles or product gases. Since the particle is falling faster than the gas, after some time interval ∆t, it will have left the previous gas volume and entered a new one. The previous gas volume’s composition will have changed because of the volatiles or product gases released by the particle. As the particle enters the next gas volume, the particle that was in that volume before will have already changed the gas composition in that volume by making it richer with product gases. So, as the particle falls down the reactor, it continuously enters into the gases which have a higher concentration of product gases, i.e., CH4, CO, CO2, etc. At the same time, the amount of reactive elements in the particle will continuously decrease. This will reduce the concentration gradient of reactants since the particle surroundings become laden with product gases, hence the reaction rate is reduced with a possibility of reverse reaction,7 leading to less coal conversion. In the case of pyrolysis under N2 atmosphere, a similar but less serious phenomenon can be considered to take place because no chemical reaction is involved, leading to a smaller loss in coal conversion as seen in Figure 2. The situation reverses after the transition point, in which case the gas now travels downward faster than the particle. In such a situation, the gas sweeps away either partially or completely the product gases or volatiles released from the particle and at the same time providing the particle with a fresh supply of gas, making the gas surrounding the particle relatively clean. This enhances chemical reactions between particle and gas, resulting in higher coal conversions. The illustration in Figure 4 shows how the product gas composition will be in both the negative and positive slip velocity zones as the particle travels down the reactor. In the negative slip velocity zone, the gas is product rich toward the bottom of the reactor whereas for the positive slip velocity zone, it is relatively lean. In general, it can be said that, the larger the gas (7) Muhlen, H.-J.; van Heek, K. H.; Juntgen, H. Kinetic Studies of Steam Gasification of Char in the Presence of H2, CO2 and CO. Fuel 1985, 64, 944-949.
Energy & Fuels, Vol. 16, No. 2, 2002 431
Figure 4. Schematic representation of the product gas composition in negative and positive slip zone.
flow, the cleaner the particle surroundings, leading to higher concentration gradients, faster reactions, and more coal conversion. The sweep away effect can be experienced in both zones and the product gases or volatiles are swept upward (with respect to particle) in the negative slip velocity zone and downward in the positive velocity zone. The slip velocity transition point moves left as the hydrogasification pressure is increased as can be seen from Figure 3, thus making the negative slip velocity zone very small at high pressures. To confirm experimentally what was found from the theoretical work above, hydrogasification runs were undertaken at 0.4 MPa pressure. This pressure was chosen because it gives enough room in both the negative as well as positive slip velocity zones. Theoretically, under this pressure, the transition is supposed to occur at the H2 flow rate of about 5.3 NL/min. So, experiments were undertaken at flow rates of between 2.0 and 10.0 NL/ min. Results obtained are presented in Figure 5. A sharp rise in coal conversion can clearly be seen at H2 flow rates of 5.3-5.6, which are very close to the value obtained theoretically. This shows that there is a very good agreement between theoretical and experimental results, solidifying the above reasons for low coal conversions at low H2/coal ratios. Influence of Coal Loading on Conversion and Char Reactivity. To study the effect of coal loading, the investigation was undertaken at two loadings, namely 30 and 60 g/dm3. Increasing the coal loading from 30 to 60 g/dm3 at various H2/coal ratios did not result in much change as far as the coal conversion is concerned as can be seen in Figure 6. The conversion curves between the two loadings are quite close to each other. However, there is a remarkable difference in the rate constants of the two loadings. Chars produced under higher loading conditions appear to be more reactive than those from lower loading. As for the
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Figure 5. Variation of coal conversion with H2 flow rate. A sudden increase in coal conversion is seen as the slip velocity changes from negative to positive. The vertical dotted line shows the slip velocity transition point.
Figure 6. The influence of coal loading on coal conversion and char reactivity.
variation of the reaction rate constant with the H2/coal ratio, both loadings indicate a decreasing rate constant with increasing ratio. It is worth noting that the coal conversion falls rapidly at H2/coal values less than 0.25 due to the reasons explained in the previous section. In this work, therefore, although all results are presented, focus will be on H2/coal above 0.25. From the figure, it can be said that at a given H2/coal ratio, increasing coal loading increases the reactivity of the produced chars. To explain the above phenomenon, an investigation on what was happening inside the coal particles during the hydrogasification process was undertaken. This was done by analyzing changes in surface area and bulk density of the char particles. Figure 7 shows the effect of H2/coal ratio on the char surface area and bulk density. It can be seen that surface area increases with increasing H2/coal ratio. This is in agreement with the fact that since the coal conversion increases with H2/ coal, then more micropores are made following the hydrogenation reaction, the presence of many micropores in turn causes the increase in surface area. Similar results have been reported by Ono8 when studying the development of pores during gasification and Lu9 in char activation study. On the other hand,
Katalambula and Takeda
Figure 7. Variation of surface area and bulk density with the H2/Coal feed ratio.
the bulk density decreases with increasing H2/coal, presumably due to the increased hydrogenation reaction as mentioned above, thus leaving the inside of the particle with a lot of pores. The trends of surface area and bulk density nicely complement each other. Figure 7 also presents the effect of loading on both the surface area and bulk density. The figure shows that higher coal loading produces chars with lower bulk density and larger surface area at all values of H2/coal ratio. The decrease in bulk density, in addition to hydrogenation reaction mentioned above, could be caused by either or both of the following facts: (i) particles have swelled during the reaction, or (ii) particles have agglomerated to form coarser and irregular shapes hence leading to poor packing and hence lower bulk density. The first fact has been observed and reported from our previous works.10 The second fact can be considered to occur and its effect may keep increasing with loading since as the concentration of particles increases inside the reactor, the distance between particles decreases, hence making it easier for one particle to collide and stick on the other. The above facts, however, cannot explain why there is a corresponding increase in surface area when the bulk density decreases and at the same time the char reactivity increases. Since volatiles have been found to contribute significantly toward the char’s reactivity, volatile matter content of the chars was also analyzed. Results show that the volatile matter content was almost the same for both loading conditions as can be seen in Figure 8. This, together with coal conversion results presented in Figure 6, suggests that the increase in char reactivity due to increased loading is not caused by the presence of some unreacted materials in it but rather due to a possible physical modification of char itself. Considering what happens inside the reactor, for example, when the coal loading is increased, the gas linear velocity will increase because of the increased gas (8) Ono, T.; Haga, T.; Nishiyama, Y. Studies on Catalytic Gasification of Coal Chars. Part 1. Development of Pores During Gasification. Fuel Process. Technol. 1984, 9, 265-278. (9) Lu, G. Q. Evolution of Pore Structure of High-Ash Char During Activation. Fuel 1994, 73 (1), 145-147. (10) Katalambula, H.; Takeda. S.; Kumagai, M. Effects of Pressure, H2/Coal feed ratio and coal loading on the Hydrogasified Char’s Reactivity. Proceedings of the 65th Annual Conference of the Society of Chemical Engineering, Japan 2000; p 277.
Hydrogasified Coal Char’s Reactivity Improvement
Energy & Fuels, Vol. 16, No. 2, 2002 433 Table 2. Particle Velocity and Residence Times at Various Coal Loadings and H2/Coal Ratiosa flow slip H2 coal particle res. rate, velocity, velocity, velocity, time, H2/coal loading, 3 NL/min m/s ratio g/dm m/s m/s s 0.045 0.1125 0.225 1.125 a
Figure 8. Variation of volatile matter content with H2/coal under different coal loadings.
Figure 9. Effect of coal feed rate on coal conversion at a fixed H2 feed rate.
feed rate since the geometry of the reactor remains the same. The increase in gas velocity will reduce its residence time in the reactor. As for the solids, their velocity is limited by parameters determining the terminal velocity. So, if these parameters are kept constant, increasing coal feed rate to keep the H2/coal ratio will simply increase the concentration of particles in the reactor and not the terminal velocity. However, since the terminal velocity is tied to the gas velocity, then the particle residence time will also be affected. Consider a case where the gas flow rate (gas velocity) is increased in a given setup. It is clear from the previous discussion that the concentration of products in the gas will be lean, hence more extensive reactions leading to higher coal conversion as has been shown in Figure 2. If in the same setup, the gas flow rate is held constant and instead the coal feed rate increased, the gas in the reactor will be rich in the product gases because of the increased coal feed. This will hamper the reaction from proceeding, resulting in lower coal conversion. The higher the coal feed rate, the lower will the coal conversion become as shown in Figure 9. It is therefore evident that the simultaneous proportional increase of both H2 and coal (constant H2/coal ratio) will prevent the coal conversion from decreasing because the increased H2 flow will reduce the products’ concentra-
30 60 30 60 30 60 30 60
0.2 0.4 0.5 1 1 2 5 10
-0.24 0.24 0.24 0.24 0.24 0.24 0.24 0.24
0.192 0.384 0.479 0.959 0.959 1.918 4.794 9.588
0.432 0.143 0.239 0.719 0.719 1.677 4.554 9.348
1.969 5.931 3.554 1.183 1.183 0.507 0.187 0.091
Reaction pressure ) 7.1 MPa.
tion and enhance the reaction, hence maintaining almost the same coal conversion value as that shown in Figure 6. From the preceding paragraph, it is clear that the increased gas velocity and hence particle velocity reduces the residence time of the particle in the reactor. Taking an example from the theoretical results presented in Figure 3, increasing gas flow rate from 1 NL/ min to 2 NL/min while maintaining the H2/coal ratio of 0.225 would cause the particle velocity to increase from 0.72 to 1.7 m/s hence reducing the particle residency time from 1.2 s to 0.5 s. The reactor length in this work was 85 cm. Chisotra et al. 11 when studying the effect of soak time on the reactivity of char in steam reported that the longer the soak time the less reactive the char became. They attributed this to the thermal annealing which promotes deactivation of char due to the realignment of the coal layer planes, thus reducing the accessibility of micropores to the reactant gas. Their experimental results from a drop tube reactor with a soak time of 0.2 and 0.7 s showed a difference, though small, in reactivity with a longer soaked char being less reactive. The preceding argument can be applied in this case since residence times involved, as shown above, are much longer than those used by Chisotra et al., hence the soaking time effect becomes more pronounced. The calculated residence times for experimental conditions applied in this work at 7.1 MPa are shown in Table 2 for some selected points. It can be seen that at all H2/ coal ratio values, the residence time is longer in the 30 g/dm3 loading. At the H2/coal ratio of 1.125, the residence times for both coal loadings are too short to cause any change due to the thermal annealing, hence leading to almost the same reactivity. It is worth noting that in this case, there is only one case in which the reaction is occurring in the negative slip velocity region, and this is under the H2/coal ratio of 0.045 and gas flow rate of 0.2 NL/min. The very same argument can also explain why the surface area increased and bulk density decreased with increasing coal loading in Figure 7. In general terms, however, increasing the particle feed rate increases the particle-to-particle interaction which may give rise to particle agglomeration on one hand and a possibility of increasing particle collisions on the other. Agglomeration may also contribute to the drop in bulk density because of the resulting loose packing of particles. In addition, agglomeration may lead to the creation of new pores or cavities because of (11) Chisotra, C. T.; Muhlen, H.-J.; van Heek, K. H.; Juntgen, H. The Influence of Pyrolysis Conditions on the Reactivity of Char in H2O. Fuel Process. Technol. 1978, 15, 17-29.
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Figure 11. Variation of char bulk density and reactivity with coal loading for the Adaro coal.
Figure 10. Comparison of H2 yields obtained from different coal loadings at different H2/coal ratios.
two or more particles sticking to each other, with a possibility of increasing the pore surface area. On the other side, there is a possibility of the surface structure being modified as a result of increased particle-particle interaction and collisions, hence exposing some additional active sites. This will help improve the char reactivity since the rates of reactions involving carbon depend strongly on the availability of active sites leading to the formation of surface complex intermediates as shown below:12
Cf + CO2 f C(O) + CO (Bourdouard reaction)
(6)
Cf + H2O f C(O) + H2 (steam reforming)
(7)
where Cf is a carbon-free active site and C(O) is a fleeting oxygen complex. It should be noted, however, that increasing loading will not necessarily result in increased char reactivity. Due to the limitations in the current experimental equipment, higher loading tests could not be conducted for this coal type, but it is envisaged that higher loading will increase the agglomeration, which may then lead to the equipment blockage and hence have negative effects. This indicates that there is a suitable range of coal loading at which char reactivity can be improved without affecting the coal conversion. Operating within this range will not only be beneficial in terms of increased char reactivity but also will increase the capacity utilization of the reactor leading to economical operation. H2 Gas Yields. Since the objective of this work was to improve char reactivity for H2 production, the chars produced under different conditions were steam gasified in order to see if the H2 yield corresponded to the char reactivity observed. The steam gasification results are shown in Figure 10 for the two different loadings. From the figure, it can be seen that there is a reasonable agreement between the char reactivity from Figure 6 and the hydrogen yield, where the yield is seen to decrease with increasing H2/coal ratio, especially in higher loading char. In the lower loading char, the yield (12) Linares-Solano, A.; Mahajan, O. P.; Walker, P. L., Jr. Reactivity of Heat-treated Coals in steam. Fuel 1979, 58, 327-332.
decreases and is seen to increase as H2/coal ratio increases further. This can be due to the fact that at higher H2/coal ratios, gas velocities are quite high thus making the particle residence time quite short, thereby making the soaking time effect explained earlier less pronounced. Generally, the hydrogen yield per gram of carbon is higher at high coal loading, nicely agreeing with the char reactivity results. However, as the H2/ coal ratio increases, the difference in yield between the two loadings decreases. This infers that at very high H2/coal ratios, the effect of loading may become less distinct. On average, the hydrogen yield per gram of carbon increased by about 10% when coal loading was increased from 30 to 60 g/dm3. It can therefore be said that controlled increasing of coal loading can increase char reactivity and consequently the hydrogen yield obtained from the hydrogasified chars. Other Types of Coal. The above results were obtained from Taiheiyo coal only. Given the fact that coal properties vary widely from one coal type to another, more coal samples were tested. Figure 11 presents results from Adaro, a brown coal from Indonesia. In this case, three coal loading levels were tested at the H2/coal ratio of 0.225. It can be seen that the char reactivity increased with the coal loading, while the bulk density decreased, agreeing closely with results obtained from the Taiheiyo coal. The coal conversions and the char volatile matter content remained almost constant under the three loadings. As for the hydrogen yield, it can be seen from Figure 12 that the yield increases with coal loading up to about 70 g/dm3, soon after which it starts to decrease. This shows that increased hydrogen yield can be achieved only up to a certain limit, beyond which increased interparticle interaction starts to be a disadvantage to the process. The hydrogen yields from other coals that were also gasified in this work are shown in Figure 13. The H2/ coal ratio for all char preparation runs was 0.225. The chars were gasified at the same two loadings (30 and 60 g/dm3) and it can be seen that all coals, except for Cerrejon, had higher hydrogen yields when the they were hydrogasified at higher coal loadings. The increase in hydrogen yield was 20% for Shimboku coal, 8.1% for Taiheiyo, 7.9 for Adaro, 7.7% for Asam Asam, and 3.6% for Mae Moh coal. Cerrejeon, on the other hand showed a different behavior even during the gasification process since it tended to stick onto the reactor walls and its overall coal conversion was low when compared to other
Hydrogasified Coal Char’s Reactivity Improvement
Energy & Fuels, Vol. 16, No. 2, 2002 435
Conclusion
Figure 12. Variation of hydrogen yield with coal loading for Adaro coal.
Figure 13. The H2 yield from gasification of different coals at two different coal loadings.
coals, indicating that Cerrejon coal is not suitable for hydrogasification. Also, during the steam gasification of its char, the 30 min reaction time was not sufficient to gasify all the Cerrejon char sample, whereas for other coals, the reaction was over within 10-15 min. From these results, it can therefore be said that the effect of coal loading on char reactivity and hence hydrogen yield holds for a wide range of coal types, from lignites to bituminous. In short, the above findings show that controlled coal loading can be one way of physically modifying hydrogasified char properties, hence making it possible to improve char reactivity and consequently increase hydrogen yield without affecting the CH4 gas product yields.
The effect of coal particle concentration on char reactivity and char-steam gasification hydrogen yield has been investigated. Within the limits of this work, the following can be concluded: • The influence of slip velocity on coal conversion has been elucidated. Theoretical and experimental results agree quite well, indicating the presence of product rich and lean gases when the slip velocity changes from negative to positive. • Char reactivity as well as hydrogen yield has been found to increase with coal loading in the range investigated. This is accompanied with an increase in surface area and a decrease in bulk density. Generally, doubling the loading increased the hydrogen yield by 4-20%, depending on the type of coal used. • The increase in coal loading increases the particleto-particle interaction. The interaction influences the bulk density and surface area because of the occurrence of some agglomeration, hence influencing the char reactivity. • Longer particle residence time in the case of low coal loading decreases the char reactivity. This is due to the thermal annealing which promotes deactivation of char due to the realignment of the coal layer planes, thus reducing the accessibility of micropores to the reactant gas. • Most of the coal types used in this investigation showed similar results as far as coal loading is concerned, suggesting that the technique can be used on a wide range of coal types. Acknowledgment. The Authors thank the Institute of Research and Innovation (IRI) and the New Energy and Industrial Technology Development Organization (NEDO) who supported this work as part of the longterm Advanced Rapid Coal Hydrogenation (ARCH) project. Authors are also grateful to Profs. T. Chiba and J-i. Hayashi of CARET, Hokkaido University, for their valuable suggestions and advice. Last but not least is Anuwat Luxsanayotin, a JICA trainee for his participation in the experimental work. Notations CD ) drag coefficient dp ) particle diameter [m] g ) gravitational constant [m s-2] Rep ) particle Reynolds number Up ) particle velocity [m s-1] Us ) slip velocity [m s-1] Ut ) particle terminal velocity [m s-1] µg ) gas viscosity [Pa s] Fg ) gas density [kg m-3] Fs ) particle density [kg m-3] φ ) particle sphericity EF0101356
