I n d . Eng. Chem. Res. 1989, 28, 505-510
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KINETICS AND CATALYSIS Steam Gasification of Brown Coal Using Sodium Chloride and Potassium Chloride Catalysts Takayuki Takarada,t,f Toshihide Nabatame, Yasuo Ohtsuka,**tand Akira Tomita Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Katahira, Sendai 980, Japan
T h e new utilization of NaCl and KC1 as raw materials for catalyst gasification has been studied. An alkali metal can be readily ion-exchanged to brown coal from an aqueous solution of alkali chloride using NH3 or Ca(OH), as a pH-adjusting agent. C1 ions from alkali chloride can be completely removed by a simple water washing. Such a C1-free catalyst markedly promotes the steam gasification of brown coal a t 923 K; the rate for the exchanged coal at the largest loading (5.2 wt % N a or 9.7 w t % K) is 20-30 times that for the original coal. Rate enhancement by a C1-free alkali metal catalyst is the same as t h a t by the impregnated alkali carbonate catalyst. T h e chemical form of the alkali species during gasification and after ashing is alkali carbonate, which can easily be recovered with water. A number of studies have been carried out on the catalytic gasification of coal, as is reflected by the increasing research effort (Figueiredo and Moulijn, 1986; Moulijn and Kapteijn, 1986). In a previous study, we have found that the nickel catalyst markedly promotes the steam gasification of brown coal at a low temperature, around 800 K (Tomita et al., 1983, 1985; Ohtsuka et al., 1986), and that such an extremely high activity makes the direct production of methane possible under pressure (Takarada et al., 1987a). Although nickel can be efficiently recovered from the gasification residue (Tomita et al., 1985), the use of expensive nickel in a commercial scale is not easy. Thus, it is desirable to develop less expensive catalysts than nickel. We have reported that calcium and iron show high catalytic activities in the gasification of brown coal (Nabatame et al., 1986; Ohtsuka and Tomita, 1986; Ohtsuka et al., 1987). Since alkali metal chlorides like NaCl and KC1 are very cheap, they are also attractive as raw materials of catalytic gasification. However, their activities are generally quite low compared with the corresponding carbonates because of the strong affinity between alkali metal ion and chloride ion (Lang, 1986). Very recently we have found that an active and chlorine-free sodium catalyst can be prepared from NaCl solution by using the ion-exchange technique (Takarada et al., 1987b,c). However, there remain many problems to be solved before making practical application of this method. In the present study, the effect of p H on the alkali catalyst loading is clarified, Ca(OH), as well as NH3 being used as a pH-adjusting agent. Then the gasification reactivities of various alkali-loaded samples are determined with a thermobalance. Alkali catalysts are prepared not only from NaCl but also from KC1 and seawater. Furthermore, the chemical form of catalyst at each stage is
* To whom correspondence
should be addressed.
Laboratory. Present address: Department of Chemical Engineering, Gunma University, Tenjin-Cho, Kiryu 376, Japan. t Also Coal Chemistry
investigated with the X-ray diffraction method, and the recovery of spent catalyst components from the ash is attempted.
Experimental Section Coal Sample. Briquettes of Yallourn brown coal from Australia were used. They were crushed and sieved to 74-150 pm. The elemental analysis was C, 67.1; H, 4.8; N, 0.8; S, 0.3; C1,0.09; Od3, 26.9 w t 70 (daf). Carboxyl and hydroxyl groups in the coal were determined as 1.7 and 6.0 mequiv/g (daf) of coal according to a standard method (Zhou et al., 1984). Ion Exchange. The ion exchange was carried out by stirring the mixture of coal and an alkaline aqueous solution of NaCl or KC1 until no further change in the pH was observed. NH, or Ca(OH), (mainly NH3) was used as an additive to control the initial pH of catalyst salt solution. The solution was filtered off, and the coal was washed with deionized water to remove the C1 ions. The details of the ion-exchange procedure have been described elsewhere (Takarada et al., 1987~).An experiment using seawater in place of pure NaCl or KC1 solution was also carried out in the same manner as above. The contents of Na, K, Mg, and Ca were 10.5, 0.40, 1.2, and 0.40 mg/g of seawater, respectively. For comparison with the ion-exchange method, NaC1, KC1, Na2C03,and K2C03 were loaded on brown coal by the impregnation method, the coal was soaked in the aqueous solution for 30 min, and then water was evaporated under a N2 stream. Determination of Metal and Chlorine. Metal contents in original and catalyst-loaded coals were determined by flame spectroscopy or by atomic absorption spectroscopy (Japan Jarrell Ash Co., AA-855) after extraction of metals from the coal with hot HCl. The C1 content in the coal was determined by a standard Eschka method (IS0 587-1981(E)). Steam Gasification. The gasification experiments were carried out with a thermobalance (Shinku-Riko, TGD3000). The coals (about 20 mg) mounted onto a quartz cell were heated in a H20 (66 kPa)/Np stream at a rate of
0888-5885/89/2628-0505$01.50/0 0 1989 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989
Table I. Metal and Chlorine Contents and Gasification Reactivities at 923 K for the Exchanged Coal pH-admetal content, mequiv/g of coal justing catalyst precursor agent final pH Na K Mg Ca total C1, wt 9'0 6.2 1.2 1.2 0.10 NaCl 1.7 9.2 1.7 NaCl 0.09 2.4 11.1 n.d." 2.4 NaCl n.d." 6.3 0.4 1.2 0.8 NaCl 9.4 1.5 2.4 NaCl 0.9 0.08 10.6 2.1 KC1 2.7 0.10 1.3 2.2 0.10 9.3 KC1 0.9 11.2 1.4 0.3 2.5 0.10 0.8 0.01 seawater a
specific rate, h-' 1.7 2.1 2.6 1.8 2.9 4.7 2.9 1.8
Not determined.
about 300 K/min up to a predetermined temperature and soaked for 2 h. The reaction consisted of the devolatilization and subsequent char gasification steps. The reactivity of char in the latter stage will be discussed throughout this paper. Characterization of the Catalyst. X-ray diffraction analysis (XRD) of ion-exchanged samples was conducted by using Cu Ka radiation (45 kV X 35 mA) to characterize the catalyst at the devolatilization and gasification stages. The samples were prepared in a thermobalance in the same manner as the gasification runs, and then they were cooled quickly to room temperature for XRD. Catalyst Recovery. The experiment was performed to recover Na catalyst from the ash, which was obtained by burning the residue after the steam gasification of Na-exchanged coal. In this experiment, a Pt cell was used instead of a quartz cell to avoid the Na loss which may be caused by the reaction between Na compounds and SiOl during burning the gasification residue. The cell, containing the ash, was drenched in a boiling water and the amount of Na ion in the leachate after filtration was determined by flame spectroscopy.
Results Extent of Exchange. As the ion exchange between Hf in coal and alkali metal ion progressed, the pH of the mixture of coal and catalyst solution rapidly decreased with time, and it finally became constant, indicating the completion of ion exchange. The time required for the completion depended on the concentration of NH, or Ca(OH), as a pH-adjusting additive. The time was shorter at a higher concentration of additive. In most cases, the ion exchange was completed within 30 min. Figure 1 and Table I show the relationship between the final pH and the amount of exchanged alkali ion. With a higher concentration of NH,, the final pH was higher; for example, the pH was 2-3 without N H , whereas the pH was about 11with 5 N NH,. When a larger amount of NH3 was added, the final pH became higher, and the extent of cation exchange increased. A t a pH of around 11, the amount of exchanged alkali metal reached around 2.5 mequiv/g of coal, which was more than 10 times that (0.2 mequiv/g) obtained without NH,. The exchanged amount hardly depended on the concentration of alkali chloride, as is seen in the case of NaCl (Figure 1). The pH dependence of the extent of KC1 exchange was similar to NaC1; the amount of exchanged alkali metal ion at the same level of final pH was almost the same between the two. When seawater was used instead of pure alkali chloride, the total amount of exchanged Na, K, Mg, and Ca at a pH of 11.2 was 2.5 mequiv/g (Table I), which was roughly equal to that obtained with pure alkali chloride. Since Ca(OH), is cheaper than NH,, the use of Ca(OH)* as a pH-adjusting agent is more desirable. Table I also shows the exchange results with Ca(OH), in place of NH,.
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