Process for removing SO2 from gases which contain it, with direct production of elemental sulfur

Abstract: A wet process of regenerative type for removing SO 2 contained in combustion gases is disclosed, which is characterized in that SO 2 is caused to be absorbed by a diluted aqueous solution of BaS, with a mixed precipitate of elemental S, BaSO 3 and BaSO 4 being formed by the consequent reactions. Sulfur is extracted from the precipitate --recovered by filtration -- by using a solution of (NH 4 ) 2 S: the resulting polysulfide is distilled, with pure elemental S and the solution of (NH 4 ) 2 S to be recycled, being obtained. The residual mixture of BaSO 3 and BaSO 4 remaining after sulfur extraction is reduced with coal in a high temperature furnace, in order to obtain BaS to be recycled, as an aqueous solution, to the SO 2 absorption step.

Description: In the flue gases generated by the large facilities which burn coal or fuel oil, in particular power generation plants, the concentration of SO 2 is relatively low, but, on considering the enormous gas volumes discharged to the atmosphere, the amounts of SO 2 , on an absolute basis, reach environmentally unacceptable levels. Installing sulfur removing units in these facilities has been a need for years, and is now also imposed by Law. The flue gases from sulfur containing metal ore roasting facilities are presently sent to H 2 SO 4 productions, but owing to the present difficulty met in selling the latter, an alternative process which converts SO 2 from exhaust gases into elemental sulfur rather than into H 2 SO 4 could contribute to solve the marketing problems. The solutions offered by the prior art for flue gases desulfurization are substantially two: -- the limestone/chalk processes which convert SO 2 into a solid residue comprised of calcium sulfate/sulfite which must be disposed of to purposely fitted areas, -- the regenerative processes which convert SO 2 into marketable sulfur forms, and do not generate residues to be disposed off. These processes are the only ones which are completely acceptable from a strict environmental protection view point, but involve decidedly higher investment and operating costs, even if said operating costs may be partially compensated for by the income deriving from sales of sulfur, liquid SO 2 , H 2 SO 4 , produced by the same processes. At present, on a practical basis the processes of the first type, in which removal of SO 2 is based on the reaction of the latter with a suspension of lime/chalk, are more widely diffused. The exhaust gases from the boilers are caused to flow through a wash tower in which they are contacted, in countercurrent, with a slurry containing lime/limestone, kept continuously recycled and subdivided into extremely fine droplets in order to favour the intimate contact thereof with the gas and consequently SO 2 absorption. Calcium sulfite and sulfate are formed which are usually converted into a homogeneous product by means of an end oxidation. The yields of SO 2 extraction may reach 90%. The reactant consumptions are always largely higher than the stoichiometric amounts, owing to the low solubility of lime and chemical inertness of limestone. Among this type of processes, in particular the Babcock-Hitachi, Combustion Engineering, Costain Deutsche Babcock and Lodge Cottrel processes have been successfully adopted. An analogous process is the double-alkali process, which is based on SO 2 absorption by a sodium carbonate solution. The extraction yield is good (about 90%), the reactant consumption practically corresponds to the stoichiometric requirements, the process management is relatively simple, but for partial alkali regenerations the treatment with lime is used, so also in this case calcium sulfite and calcium sulfate are formed which, together with the excess of lime, must be disposed off to landfill. Among the regeneration processes, the Wellman-Lord process, using the sulfite-bisulfite system, gained a leading position. Due to the effect of absorbed SO 2 , the absorbent sodium sulfite solution is converted into bisulfite and can be thermally regenerated: sodium sulfite is formed again and an SO 2 stream is released which has a high enough concentration for the subsequent alternative treatments: -- transformation into sulfur by the Claus process; -- liquefaction to produce liquid SO 2 ; -- conversion into H 2 SO 4 by the contact method. The absorption step is carried out in an absorption tower, with a removal yield which usually is higher than 90%. The sulfite regeneration is carried out inside a forced-circulation evaporator. During the absorption step, a portion of sulfite is converted into sulfate so from this process also this product is obtained, together with SO 2 . The investment cost is high, facility management is complex. Also the process based on use of Mg oxide developed by Philadelphia Electric/UE&C is a regenerative process of wet type; for the absorption a suspension of MgO in water is used: Mg sulfite is formed which can be decomposed by calcination into MgO and SO 2 (also MgSO 4 , which is also partially formed during the absorption step, can be reduced to MgO and SO 2 if the calcination is carried out in the presence of coal). Therefore, the only product obtained is a concentrated SO 2 stream, which can be submitted to the usual treatments yielding commercial grades of this product. The investment cost is high and the process management is quite complex. The citrate process uses, for desulfurization, a solution of citric acid and sodium citrate. Said solution is an extremely good absorbant for SO 2 , because it acts as a pH buffering agent. The wash solution enriched with SO 2 is sent to a reactor in which it is reacted with H 2 S (obtained by recycling a portion of produced sulfur and causing it to react with fuel gas in an adjacent facility), so as to form a slurry containing 10% of elemental S. The high-purity sulfur is separated from the solution which is sent back to the absorption step. The presence of sodium sulfate which is formed by oxidation during the absorption makes it necessary a purge to be carried out in order to remove it. Therefore, the process products are sulfur and sodium sulfate. The high investment cost and the complexity of process management have limited the diffusion thereof. For the sake of completeness, the following should be cited as well: -- the method taught by U.S. patent no. 3,475,122, using 3-compartment electrolysis cells for producing NaOH for SO 2 absorption and acid for neutralizing the basic solution saturated with SO 2 , with SO 2 being consequently released; -- the process taught by U.S. patent no. 4,107,015, in which the SO 2 -saturated alkaline solution is charged to an electrodialysis membrane separation unit: from the acidic compartments a solution is obtained which is suitable for SO 2 stripping, and from the basic compartments a solution is obtained which contains the necessary NaOH for absorbing further amounts of SO 2 . DE-A-3523073 (Outokumpu OY) describes the removal of SO2 from a gas stream and the recovery of sulfur in elemental form by contacting the gas stream with a metal sulfide (e.g. BaS) containing solution. After absorption of the SO2 the solution is treated in an autoclave at a temperature above the melting point of sulfur in order to prepare a mixture of elemental sulfur and a metal sulfate containing solution from which the liquid sulfur is separated. The sulfate is further processed with a reducing agent in order to obtain the metal sulfide which is available for recycling the SO2-absorption step. According to USP 4,078,048 (Nadkarni), gases are scrubbed with an alkali metal compound in an aqueous solution to form alkali metal sulfite/sulfate salts which are then reacted with barium sulfide to precipitate barium sulfite/sulfate and form alkali metal sulfide/bisulfide in solution. The alkali metal sulfide/bisulfide is oxidised to form elemental sulfur and regenerated wash liquid; and the barium sulfite/sulfate is reduced by roasting to form the barium sulfide required. All the above regenerative processes aim at obtaining a concentrated enough SO 2 stream to be suitable for being converted, by means of a downstream facility, into a marketable product (liquid SO 2 , H 2 SO 4 or elemental sulfur). The present invention aims at jointly accomplishing the purposes of providing a process for desulfurization of combustion gases which: -- directly produces elemental S, a substance in great demand by the market, easy to be stored; -- reduces the number of steps and of the necessay equipment units; -- is easily operated and controlled, with low operating costs; -- does not produce any substances which may require to be further processed in order to be rendered useable, and -- does not produce wastes to be disposed off. In order to achieve such purposes, the invention proposes a process according to claim 1. More particularly, referring to said step (a), the present invention proposes that as the absorbent means a diluted barium sulfide solution is used which, by reacting with SO 2 , causes elemental sulfur to be directly formed and yields practically insoluble barium salts (i.e., barium sulfite and sulfate). The reactions between SO 2 and the solution take place at a very high reaction rate, SO 2 removal is practically complete, the separation of the precipitate from the solution, carried out by filtering, is easy and fast. As regards said (b) step, the reactant acting as a solvent for sulfur and leaving unchanged any barium salts present, can advantageously be an aqueous solution of ammonium sulfide. The saturated sulfur solution which is obtained from this step can be regenerated without difficulties by distillation, and the resulting product is a particularly pure sulfur. As regards said (c) step, barium sulfite and sulfate, after being separated from sulfur, are mixed with the necessary coal for reducing them into barium sulfide; said reduction step is carried out in a furnace at 1000°C. The product, consisting of fine particles, is easy leached. As solvent solution, a portion is used of the same solution circulating through the absorption tower; before being recycled said solution is submitted to a preliminary filtration. In order to better understand the characteristics and advantages of the invention, the process of the invention, as substantially defined hereinabove, is disclosed in the following in greater detail by referring to the Flow Scheme shown in the figure of the accompanying drawing. The gases to be purified are fed to the bottom of an absorption tower, in which they come into intimate contact with an aqueous BaS solution sprayed from tower top-side, in the form of extremely fine droplets. Such a solution is an active desulfurizing agent at both high and low concentrations. Inside the tower, both the following reactions take place rapidly after each other: 2BaS + 2SO 2 + 2H 2 O → 2BaSO 3 + 2H 2 S 2H 2 S + SO 2 → 3S + 2H 2 O leading to the global reaction: 2BaS + 3SO 2 → 2BaSO 3 + 3S Together with BaSO 3 and S in the precipitate also an amount of 1-20% of BaSO 4 is found, which is formed from sulfite owing to the oxidizing action of air contained in the gas stream. In the process, the presence of BaSO 4 together with BaSO 3 does not cause any difficulties, because it is reduced to BaS simultaneously to sulfite reduction. In order to separate the mixed precipitate from BaS-depleted solution, said solution is submitted to filtration. The filtration step is fast and easy, thanks to the crystal character of the precipitated substances and thanks to their amounts participating in the process, which are the minimal compatible with the level of SO 2 in the treated gases. By means of this separation, the solution which contains unreacted BaS can be returned back to the absorbent solution recycle tank, whilst the mixed precipitate is sent to the dissolver for sulfur extraction. For that purpose, a solution containing 10% (NH 4 ) 2 S is preferably used in a solid:solution ratio which is preferably of 1:4, according to the reaction: (NH 4 ) 2 S + nS → (NH 4 ) 2 S n+1 The dissolution of sulfur, with polysulfide formation, is complete at room temperature within approximately 30 minutes. The slurry is filtered with a cake of barium sulfur salts (which are sent to the regeneration) and a clear polysulfide (NH 4 ) 2 S n+1 solution (which is sent to distillation), being obtained. From the distillation, the following products are obtained: a solution of (NH 4 ) 2 S to be recycled to dissolve further sulfur amounts; elemental sulfur, which can be processed to a marketable form. The distillation of polysulfide solution restores, with no losses, the initial sulfide solution, and makes it possible sulfur to be obtained at a rather high purity level. The regeneration of the absorbent means is the end process step. In this step, barium sulfite and sulfate are reduced to sulfide. The reactions which can take place inside the reduction furnace respectively yield CO and CO 2 . For sulfite : 2BaSO 3 + 3C → 2BaS + 3CO 2 BaSO 3 + 3C → BaS + 3CO For sulfate : BaSO 3 + 2C → BaS + 2CO 2 BaSO 4 + 4C → BaS + 4CO Actually, the representative reaction is half-way between both above results, so, by combining the latter reactions with the overall reaction which takes place in the absorption step, the overall process according to the present invention can be summarized by means of the following reactions: SO 2 + C → S + CO 2 SO 2 + 2C → S + CO 2 Summing-up, the overall process consists in converting SO 2 into sulfur and carbon oxides. In order to reduce barium sulfur compounds to barium sulfide, barium sulfite and sulfate, admixed with 20% of coal powder, are charged to a furnace, in which they are kept 1 hour at 1100°C. The product is in powder (granular) from and can be directly leached with the same solution from the recycle tank. The BaS enriched solution is filtered in order to retain any coal and possibly unreacted sulfite. The filter cake is sent back to the furnace and the clear solution is fed back to the recycle tank in order to get a homogeneous composition solution, and, from there, to the absorption tower. A non-limitative example of practical implementation of the process of the invention is disclosed now. After some process tests, the following raw materials were available: -- exhausted absorption solution containing 3.55 g of Ba 2+ /litre, in sulfite form, correspondingly to its solubility, -- a powder material coming from the furnace reduction of barium sulfur salts, containing: 72.66% of BaS 24.68% of BaSO 3 2.66% of BaSO 4 and inerts An amount of 15 g of the above powder was dissolved in 250 ml of exhausted absorption solution and after a 30-minute stirring the mixture was filtered: 4.1 g and 250 ml of undissolved residue and, respectively, of solution containing 43.6 g of BaS/l, were obtained. Through this solution SO 2 (produced by means of a Kipp device, in which Na 2 S 2 O 5 with H 2 SO 4 were reacted), was bubbled. The solution turned immediately into hazy: the precipitate turned into pale yellow and during the test, this colour turned first into white and then, again, into pale yellow. SO 2 was fed until no BaS was present any longer in solution. Now, the feed was discontinued and the precipitate was filtered, washed and dried. Against a theoretical weight of 17.08 g, calculated according to the overall reaction of the absorption step, an amount of 16.1 g was obtained (yield 94.3%). In the solution obtained from this test, a further 18.1 g of available powder was dissolved. After a 30-minute stirring, the solution was filtered, with 4.9 g of undissolved residue and 250 ml of solution with 52.8 g of BaS/l, being obtained. Like in the preceding test, through the solution SO 2 , produced by means of the Kipp apparatus, was bubbled, until the solution did not contain BaS any longer. Now, the feed of SO 2 was discontinued, the solution was filtered and the precipitate was washed, dried and weighed. Against a theoretical weight of 20.69 g, as computed above, 19.9 g was obtained (yield 96.2%). In the 250 ml of exhausted solution, 3.37 g of Ba 2+ /litre was still present, as sulfite. Both precipitates from the preceding tests, for the total weight of which was of 36 g, were treated at room temperature during 30 minutes, with 150 ml of a solution containing 100 g of (NH 4 ) 2 S/litre. The weight of the undissolved white residue was of 29.5 after being washed and dried and the material was ready for being submitted to the reduction in furnace (the so-said "absorbant regeneration"). The extraction solution was distilled and from it 130 ml of (NH 4 ) 2 S solution and 6.35 g of pure sulfur were recovered. 29.5 g of a white precipitate obtained after sulfur extraction, blended with 5.9 g of coal powder, were charged to a graphite crucible, and were covered with 1 g of coal powder. The crucible was kept at 1100°C for 1 hour in a muffle furnace. The product from the reduction step weighed 24.5 g and had the following composition: BaS 19.3 g 78.77% BaSO 3 4.7 g 19.19% BaSO 4 and inerts 0.5 g 2.04% 24.5 g 100.00% The reduction yield was low (83.9%), but this is of no moment as for the purposes of the process, because any unreacted sulfite is returned back to the reduction furnace with the subsequent batch. From the above, the advantages of the process according to the present invention over the techniques known from the prior art will be evident: the process yields one single product, i.e., pure sulfur, always marketable, easy stored, of well defined value, the production of said sulfur takes place directly during the first process step, i.e., during SO 2 absorption. Neither further products to be reprocessed, nor waste materials to be disposed off, are generated, the operations are few and straight. The investment costs can be contained, the high gas desulfurization rate is secured by the strong reactivity of the selected absorbant, the dilution of the used solutions, the type and speed of the reactions implied in the absorption.

Claims: Process for removing sulfur dioxide from gases which contain it, characterized in that it comprises the steps of (a) reacting said SO 2 containing gases with an aqueous solution of BaS according to the reactions: 2BaS + 2SO 2 + 2H 2 O → 2BaSO 3 + 2H 2 S 2H 2 S + SO 2 → 3S + 2H 2 O with a precipitate being thus formed which comprises BaSO 3 and S, which is then filtered off, (b) treating said precipitate with a suitable reactant which dissolves elemental sulfur leaving BaSO 3 unchanged, with the resulting solution being separated from the solid residue, (c) treating said solid residue from (b) with elemental carbon, such as, e.g., coal, at high temperatures, according to the reactions 2BaSO 3 + 3C → 2BaS + 3CO 2 BaSO 3 + 3C → BaS + 3CO with the thus formed BaS solution being recovered to be recycled to said (a) step; (d) distilling the solution obtained from the above (b) step in order to recover elemental sulfur in pure form. Process according to claim 1, characterized in that in said (b) step as the sulfur dissolving reactant an aqueous solution of ammonium sulfide (NH 4 ) 2 S is used according to the reaction: (NH 4 ) 2 S + nS → (NH 4 ) 2 S n+1 with a solution of polysulfide (NH 4 ) 2 S n+1 being thus obtained which is sent to the distillation of said (d) step, from which a solution of (NH 4 ) 2 S to be recycled to (b) step and elemental sulfur in pure form, are obtained. Process according to claim 1, characterized in that in said (a) step also barium sulfate BaSO 4 is formed which in said (b) step remains in said solid residue together with barium sulfide BaSO 3 . Process according to claim 3, characterized in that in said (c) step, also the following reactions occur: BaSO 3 + 2C → BaS + 2CO 2 BaSO 4 + 4C → BaS + 4CO Process according to claim 1, characterized in that said (c) step is carried out in a furnace at approximately 1100°C, using coal powder. Process according to claim 1, characterized in that said BaS solution formed in said (c) step is filtered in order to remove unreacted barium sulfide and coal and is then recycled to said (a) step. Process according to claim 1, characterized in that the solution obtained after said filtration of the precipitate formed in said (a) step, i.e., a solution containing unreacted BaS, is fed back to said (a) step through a suitable recycle tank.

Inventors: MACCAGNI MASSIMO, OLPER MARCO, MACCAGNI, MASSIMO, OLPER, MARCO
Applicants: ENGITEC SPA, ENGITEC S.P.A.
Application year: 1996
Main classification: C01B17
IPC codes: B01D53, C01B17