Energy & Fuels 1991,5,612-613
612
A New Method for Flue Gas Desulfurization/Coal Deashing for Low-Rank Coals
7000
Sir: The problem of prevention of environmental pollution due to acid gases such as sulfur dioxide or nitrogen oxides which are released during the combustion of fossil fuels remains a major concern of industry and government. A wide variety of technologies for their control or for their removal from the fuel or the combustion products have been developed and have been demonstrated on various scales. One of the most widely used of these technologies is flue gas treatment, either with dry reagents or by wet scrubbing. Wet scrubbing reagents are usually aqueous solutions of chemicals such as calcium carbonate (limestone), calcium hydroxide (lime),magnesium oxide, or carbonate, etc. A very large and extensive literature exists concerning wet scrubbing methods and the use of different reagents and adsorbents. The present Communication is concerned with the use of a different reagent for the removal of sulfur and nitrogen oxides from flue gases or similar gas streams. In addition, a significant deashing of low-rank coals with consequent potential improvement in their performance as a fuel is simultaneously accomplished. Any type of wet scrubber designed to handle slurries would be suitable for this application. Previous Work. Lignites and brown coals contain varying amounts of carboxylic acid functional groups in the organic matrix, the extent of these groups and their character depending on the particular coal. Most of these lowest ranking coals display some ion-exchange character, presumably due to the carboxylic acid functional groups. About 65% of the oxygen content of lignite is found in these groups. While some investigations have been reported of this property of low-rank coals for American or Australian lignites [Morgan et al.' and Q l e r and Schafef], until recently similar information was not available for Canadian lignites. Recently, we have completed a study of the ion-exchange behavior of a Saskatchewan (Estevan) lignite, and of another Saskatchewan lignite ( C ~ r o n a c h ) .Investigations ~ of acid washing of these lignites to remove organically bonded cations showed that any appropriate acid, including sulfurous acid, could be used provided a pH value of 4.5 or less could be achieved. The principal exchangeable cations in Canadian Lignite coals are calcium, magnesium, and sodium, and essentially all calcium and magnesium in the coal are exchangeable; that is, they are organically bonded in the coal. The degree of replacement of these alkaline earth metal cations by hydrogen ions is a function of pH of the treating solution. The magnesium cation is the most readily exchanged followed by calcium and then sodium. At a pH of 3.0-3.5 about 50% of the alkaline earth cations can be replaced by protons, while at lower pH a 90% removal is possible. These results suggested that raw lignite coal could be used as a chemical ion exchanger to remove SO2from flue gases. Accordingly, some preliminary experiments were carried out to assess the performance of the Estevan lignite as a reagent for the removal of SO2.
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Figure 2. Continuous absorption of sulfur dioxide (2000 ppm) in lignite-water slurry.
Experiment. A small bubble column reactor was used (1.2 m high by 0.05 m diameter), initially in a batch mode. The first test used only water, while in the second test a 12 w t % coal-in-waterslurry was used. An S02-air mixture containing 6000 ppm of SO2was bubbled through at a rate of 5.0 L/min. The results of these tests are shown in Figure 1 and indicate the large increase in scrubbing capacity of the solution due to the presence of the lignite. Further, the concentration of SO2in the exit gases was not detectable by the infrared analyzer used (less than 100 PPd. In additional experiments, continuous flow of slurry was used at different coal/S02 ratios. The results for two of these experiments are shown in Figure 2 for an air-S02 mixture containing 2000 ppm SO2 fed at 5 L/min. The test at low coal/S02ratio approached a final effluent pH of 4.5 while at the higher coal/SO2 ratio the exit solution pH was 5.4. In neither test was any SO2 detected in the off-gas (at least 99.5% removal). Sulfur analyses showed that essentially all the sulfur in the gas appeared in the acid extract. The removal of the alkaline earth metal cations from the lignite results in appreciable deashing. Complete removal of exchangeable cations would remove about two-thirds of the ash, so that the approximately 50% removal at a pH of 3.5 would result in about a one-third ash reduction. Although it is not an important factor, the ash replacement by hydrogen improves the lignite fuel value. Particle size used was coarse enough to allow ready filtration and washing of the lignite. As a result, the SO2removal reagent (the lignite) in this case can be used for its full fuel value and is in fact an improved fuel. As opposed to other wet
0887-0624I91-,12505-0612S02.50,I O 0 1991 American Chemical Society -
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Figure 1. Absorption of sulfur dioxide (so00 ppm) in water slurry.
(1) Morgan, M. E.; Jenkins, R. G.; Walker, P. L. Fuel 1981, 60, 189-194. (2) Tyler, R. J.; Schafer, H. N. S. Fuel 1980,59, 487-491. ( 3 ) Royce, A. J.; Miyawaki, S.; Piskon, J.; Scott, D. S.; Fouda, S. Fuel Process. Technol. 1990,25, 201-214. (4) Scott, D. S. Method of Removing Acid Gas Pollutants from Flue Gases. US Patent 4824 647, April 1989. I
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Energy & Fuels 1991,5,613-614
scrubbing agents, in particular lime or limestone, the alkaline earth metal cations in the lignite are readily reactive in solution and cost nothing. The product solution from the scrubber would contain a mixture of calcium, magnesium, and sodium sulfites and sulfates, similar to the product from conventional alkaline scrubbing. Removal of sodium, in particular, from the coal might considerably alleviate the formation of serious deposita on fire tubes of boilers which have been related to sodium content. Registry No. Na, 7440-23-5; SOz, 7446-09-5. Donald 5.Scott,* Alan J. Royce Department of Chemical Engineering University of Waterloo Waterloo, Ontario, Canada Received December 21, 1990 Revised Manuscript Received March 22, 1991
Catalyzed Conversion of Acetylene to Higher Hydrocarbons Sir: The continuous catalyzed conversion of acetylene to higher hydrocarbons has been the subject of numerous Interest in this process reflects the fact that a succesful conversion of this type could serve as a possible alternative source of synthetic fuel.'" The synthetic fuel possibility is centered on the fact that acetylene is obtainable in industrial quantities from coal and methane.5 However, as noted explicitly by previous workers, the unavailability of an effective catalyst for continuous C2H2 conversion has prevented development of this alternative fuel route.'-3 The difficulty encountered with C2H2conversions is the rapid deactivation of the catalysts employed. This loss in catalytic activity is believed to arise primarily from the rapid polymerization of CzH2 to polycyclic aromatics. These polycyclic aromatics serve as precursors to coke formation and eventual catalytic deactivation. For example, Tsai and Anderson,' using a ZSM-5 zeolite catalyst, report an approximate 70% decrease in C2H2conversion after only 190 min of on-stream conversion at a C2H2space velocity of only 460 h-' and reaction temperature of 573 K. Similarly,Allenger et al.? in a study of C2H2conversion over amorphous fluorinated alumina catalysts, report significant decreases in C2H2 conversion with time on stream, with this rate of catalytic deactivation increasing rapidly with increasing C2H2concentration in the reactant stream. Details of the catalyst deactivation in this system have been r e p ~ r t e d . ~ In sharp contrast with previous studies, the present report communicates a dramatically improved continuous flow catalyzed conversion of C2H2to higher hydrocarbons. This conversion is achieved by using a modified H-ZSM5 zeolite catalyst and a reactant gas feed consisting solely of C2H2 plus water. Using this combination, we have demonstrated efficient continuous 100% conversion of C2H2to higher hydrocarbons for over 24 h at a C2H2space velocity of 2.1 X 103 h-' and a reaction temperature of only 623 K. (1)Teai, P.;Anderson, J. R. J. Catal. 1989,80,207-214. (2) Ailenaer, V. M.: Fairbridae, - C.: Mchan, D. D.; Ternan, M. J. Catal. 1987;105,71-80. (3)AUenger, V. M.; McLean, D. D.; Teman, M. Fuel 1987,66,43+43f3. (4) Allenner. V. M.: Brown. J. R.: Clunston.. D.:. Teman, M. McLean, D.D:Appi.-c&ai. isie,39,igi-2ii. (5) Cf. Tedeschi, R. J. Acetylene-Based Chemicals from Coal and Other Natural Resources; Marcel Dekker: New York, 1982.
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0887-0624/91/2505-0613$02.50/0
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Figure 1. Comparison of catalytic lifetimes for H-ZSM5 unmodified (curve A) and modified (curve B) in C2Hzconversion. Curve A reactant gas, CzHz;space velocity, 315 h-l; temperature, 673 K; catalyst, H-ZSM5. Curve B: reactant gas, CzHz/H20= 2.3; total gaseous flow space velocity, 2100 h-l; temperature, 623 K; catalyst, Ni/H-ZSM5/A1209.
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Figure 2. Hydrocarbon product distribution obtained in C2Hz/H20conversion as a function of time on-stream under experimental conditions shown for curve B, Figure 1. The shape-selective ZSM-5 zeolite catalyst employed has a Si/Al ratio of 30. This catalyst was converted to a more active form by using the ZSM-5 activation procedure described by Rajadhyaksha and Anderson! The resultant H-ZSM5 was then mixed with Al(OH)3 (weight ratio HZSM5/A1203= 2/3) which was followed by addition of a solution of Ni(N03)2. This mixture was dried at 110 "C and then calcined a t 550 "C for 2 h. The resulting solid was ground and then screened to grains of 20-40 mesh. Finally, this solid was subjected to temperature-programmed reduction by H2by using the recommended procedure for NiO reduction.' The resulting Ni/H-ZSM5/A1203 catalyst was employed in this study. Reactions were carried out in a fixed-bed continuous flow microreactor at 1 atm pressure. A reactant feed gas of C2H2+ H20 was obtained by bubbling C2H2through a thermostated vessel containing water. The effluent gas mixture of C2H2and H 2 0 was fed directly to the catalyst, using a heated transfer line to prevent condensation. Preliminary control experiments using only the H-ZSM5 without added Ni and pure C2H2reactant feed resulted (6) Rajadhyakaha, R. A.; Anderson, J. R. J . Catal. 1980,63,510. (7)Bartholomew, C. H.;Farruato, R. J. J. Catal. 1976,45,41-53. 0 - 1991 _. . American Chemical Societv ~
