EP0621854A4

EP0621854A4 - Method and apparatus for desulfurization of a gas.

Info

Publication number: EP0621854A4
Application number: EP93904493A
Authority: EP
Inventors: Nobuyasu Hasebe; Nobukatsu Hasebe
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-01-13
Filing date: 1993-01-13
Publication date: 1995-08-30
Also published as: CA2127884A1; JPH07505603A; EP0621854A1; WO1993014026A1

Abstract

An alkaline earth bicarbonate is solubilized in water and contacted by a flue gas. The solubilized alkaline earth bicarbonate reacts with the SO2 in the flue gas to form an alkaline earth sulfite which readily precipitates from the water solution and is efficiently separated therefrom. In accordance with the invention, the slurry exiting the desulfurization step is subjected to a heating step which causes soluble alkaline earth bisulfite formed during the desulfurization step to convert to the insoluble sulfite. The solids are separated from the aqueous phase and subjected to thermal degradation to recover the alkaline earth oxide and SO2. The alkaline earth oxide can be recirculated and reused in the desulfurization process while the SO2, which has practical uses as a precursor in various chemical processes and therefore is of commercial value, is liquified. Thermal degradation of the solids from the desulfurization step is carried out in apparatus which includes a preheater zone, an ignition and heating chamber and a degradation zone. The thermal degradation is carried out in the presence of heated pellets which are themselves inert to the degradation reaction. Preferably, the pellets are heated in the ignition and heating chamber prior to contact with the solids from the desulfurization step.

Description

METHOD AND APPARATUS FOR DESULFURIZATION OF A GAS

Field of the Invention

This invention relates to the treatment of gases for the removal ^' of sulfur therefrom and more particularly to a continuous method for the treatment of flue gases and the like 5 to remove sulphur dioxide.

Background of the Invention

The combustion of fossil fuels, particularly coal and high sulfur petroleum, leads to many environmental problems due to the generation of sulphur oxide products during the 10 combustion. These sulphur compounds, generally referred to as SO_χ are the primary precursors to environmental problems such as acid rain which has recently gained much attention.

Various methods have been utilized in the prior art for the treatment of the effluent from the combustion of such

15 fossil fuels to remove the sulphur combustion products. For example, one method involves the creation of a water solution of sodium sulfite which is then contacted with the flue gas to produce acid sodium sulfite. The acid sodium sulfite is then treated with calcium carbonate or calcium hydroxide to

20 generate calcium sulfite crystals which are subsequently oxidized to gypsum. A similar method involves a slurry of calcium hydroxide which is then contacted with the flue gas to produce calcium sulfite which is subsequently oxidized to gypsum. These methods present a major problem relating to the

25 disposal of the large quantities of gypsum produced. Also, in the sodium sulfite process,disposal of the caustic soda produced by the process presents an additional environmental

^■- problem.

Another method involves contacting the flue gas with a

*30 slurry of magnesium oxide to form magnesium sulfite which may then be thermally decomposed to regenerate S0₂ and MgO. These operations are of low efficiency because the insolubility of the magnesium oxide only permits contact between the flue gas and the surfaces of the magnesium oxide particles contained in the slurry. Once magnesium sulfite is formed on the surface of a particle, the inside of the particle is prevented from contact with the flue gas so that the ultimate reaction efficiency is substantially reduced. Because of the relative inefficiency of the desulphurization process, large amounts of reactant are required for the S0₂ removal and a substantial amount of unreacted MgO plus the MgS0₃ produced by the process results in a large bulk of material for thermal decomposition. It becomes necessary to use large indirect thermal heaters to carry out the decomposition process. These heaters are expensive to purchase and operate yet, without thermal decomposition, the consumption of magnesium oxide and the byproduct magnesium sulfite is large rendering this method for the treatment of flue gasses uneconomical. For instance, desulphurization of gas from a 500 ton steam boiler burning heavy oil would require about 1,310 tons of magnesium oxide per month and would produce about 3,400 tons of magnesium sulfite per month. It is highly desirable to substantially improve both the desulphurization efficiency and the thermal efficiency of desulphurization processes in order to render the treatment of flue gas to remove sulphur combustion products economically feasible.

To avoid the foregoing problems, methods have been developed which take advantage of the solubility of magnesium bicarbonate to more intimately mix with the flue gas to remove the S0₂. Although more efficient than the processes utilizing the less soluble magnesium compounds, recovery of magnesium oxide for subsequent reuse in the process and recovery of S0₂ is reduced by the formation of soluble magnesium sulfate which is not recovered in the process and by inefficient methods for converting magnesium sulfite to magnesium oxide. Summary of the Invention

Accordingly, it is an object of the present invention to provide an improved method for the treatment of combustion gas to remove S0₂.

Another object of the present invention is to provide a method for the treatment of flue gases and the like which substantially reduces the consumption of the reactant products.

Yet another object of the present invention is to provide 5 a method for the treatment of flue gases and the like in which the efficiency of the sulphur removal is substantially enhanced.

Another object of the invention is to provide improved apparatus for carrying out the treatment of sulfur containing 10 gases.

Still another object of the present invention is to provide improved apparatus for the thermal decomposition of sulfite produced in order to efficiently regenerate the reactants and substantially reduce the consumption of the

15 reactants.

The foregoing objects as well as other advantages and features are achieved by the present invention in which an alkaline earth bicarbonate is solubilized in water and contacted by a flue gas. The solubilized alkaline earth 20 bicarbonate reacts with the S0₂ in the flue gas to form an alkaline earth sulfite which readily precipitates from the water solution and is efficiently separated therefrom.

During the desulfurization step, a significant portion of the magnesium sulfite formed undergoes further reaction with 25 the S0₂ in the flue gas to form magnesium bisulfite which is soluble in the aqueous phase of the slurry and is normally lost in the aqueous phase. In accordance with the invention, the slurry exiting the desulfurization step is subjected to a heating step which causes the soluble bisulfite to be 30 converted to the insoluble sulfite. Following the heating step, the slurry is then subjected to a separation step to --_, recover the solid phase of the slurry from the aqueous phase. The aqueous phase is then preferably recycled to the ■_Λ desulfurization step or may be disposed of.

35 The solids are subjected to thermal degradation to recover the alkaline earth oxide and S0₂. The alkaline earth oxide can be recirculated and reused in the desulfurization process to reduce the consumption of the alkaline earth reactant while the S0₂, which has practical uses as a precursor in various chemical processes and therefor is of commercial value, is liquified. Thermal degradation of the solids from the desulfurization step is carried out in apparatus which includes a preheater zone, an ignition and heating chamber and a degradation zone. The thermal degradation is carried out in the presence of heated pellets which are themselves inert to the degradation reaction. Preferably, the pellets are heated in the ignition and heating chamber prior to contact with the solids from the desulfurization step.

It is been found that the alkaline earth sulfite which is relatively insoluble in water is more readily precipitated and more easily separated when an alkali metal sulfite is also present. Consequently it is highly preferred that the reactant solution contain an alkali metal bicarbonate which will also be converted into an alkali metal sulfite and will enhance precipitation of the alkaline earth sulfite via the common ion effect.

The solids separated in the process are essentially the sulfites of both the alkaline earth metal or the alkali metal and thus the bulk of material is substantially decreased and less expensive equipment can be used for thermally decomposing the sulfites.

The method of the present invention is advantageously carried out in a continuous counter current fashion in one or more reaction columns. In the preferred embodiment, the columns are divided into a bicarbonation zone and a desulphurization zone and the reactants introduced at the bicarbonation zone travel countercurrent to the flue gas so that the treated gas exits at one end of the column and a liquid phase and sulfite produced by contact with the flue gas exit as a slurry at the opposite end of the column. In a multi-column configuration the zones may be defined in separate columns.

The slurry is conveyed to a heated vessel for conversion of soluble bisulfate to insoluble sulfite. Since the solid byproduct is essentially alkaline earth or alkali metal sulfite, the bulk of the material relative to the volume of -^*» flue gas being treated is less than with conventional methods

5 and can be more efficiently thermally decomposed than is the case for conventional methods in which only the surface area of an oxide particle reacts with the S0₂ to form a sulfite coating on the particle.

It has also been found highly advantageous to provide

10 antioxidants in the reactant in order to prevent the oxidation of sulfite to sulfate which is both water soluble and thus hard to separate and which is not thermally decomposable and thus increases consumption of the reactant products.

These and other objects and features of the present 15 invention will become apparent from the following detailed description of the invention taken in conjunction with the drawings.

Brief Description of the Drawings

Fig. 1 is a block diagram of the steps of the method of 20 the present invention;

Fig. 2 is a schematic diagram of a single column apparatus for carrying out the method of the present invention;

Fig. 3 is a schematic diagram showing an apparatus for 25 thermally decomposing magnesium sulfite;

Fig. 4 is another embodiment illustrating a use of two columns for thermally decomposing magnesium sulfite; and

Detailed Description of the Invention

The present invention provides an improved method for the •30 stripping of S0₂ from gases produced by the combustion of sulphur containing fossil fuels such as high sulphur oil, coal -> and the like. Sulphur in such exhaust gases, hereinafter referred to as flue gas, is oxidized to S0₂ and if released to the atmosphere in will produce a harmful complex of sulfur 35 compounds, commonly referred to as S0_χ, which react with moisture in the air to form the so called acid rain which is so harmful to the environment as well as contributing to smog which is common in urban locations throughout the world. The invention is illustrated hereinafter in connection with the use of magnesium reactants although it will be clearly understood that the other members of the alkaline earth group can be utilized in the present invention.

Referring to Fig. 1, the first step in the method is the bicarbonization step in which an aqueous slurry of magnesium in the form of an oxide, carbonate or hydroxide is reacted with carbon dioxide to produce magnesium bicarbonate which is water soluble in accordance with the following:

MgO + 2 CO- + H₂0 -* Mg(HC0₃)₂ (1)

MgC0₃ + H₂0 + C0₂ → Mg(HC0₃)₂ (2) Mg(OH)₂ + 2C0₂ → Mg(HC0₃)₂ (3)

In preparing the reactant for the bicarbonization step the magnesium oxide, hydroxide or carbonate or mixtures thereof, are added to water to form a slurry of the essentially water insoluble magnesium compounds. The amount of the oxygen containing magnesium compound added in the initial slurry is not critical although it will be seen from

- reaction (4) that essentially one mole of the magnesium bicarbonate will react with one mole of the S0₂. Consequently, sufficient magnesium oxide, hydroxide or carbonate must be added to provide sufficient bicarbonate for the reaction with the S0₂ present in the flue gas. Preferably, however, the oxygen containing magnesium compound is added in excess to ensure the production of sufficient bicarbonate in the reactant to strip the SO- in the flue gas. The slurry is contacted with CO. to produce the magnesium bicarbonate for the reaction in accordance with the reactions set out (1), (2) and (3) above. After contact with the C0₂ the solids portion of the reactant slurry becomes substantially solubilized in the aqueous phase as the insoluble oxygen containing magnesium compounds react with the CO- to form the soluble magnesium bicarbonate. Since no such reaction is 100% efficient, it will be understood that after contact with the CO- the reactant still will contain some solids although far less than in the original slurry. These solids will comprise unreacted oxygen containing magnesium compounds as well as magnesium bicarbonate which is not been solubilized in the water bas_e. These solids may be separated from the reactant at this point, although such separation is not required.

The second step in the method is the desulfurization step in which at least the aqueous phase of the slurry containing the solubilized magnesium bicarbonate is contacted with the

S0₂ of the flue gas to form magnesium sulfite in accordance with the following:

Mg(HC0₃)2 + S0₂ → MgS0₃ + 2 CO. + H₂0 (4) As is apparent from reaction 4, there is a mole to mole reaction between the solubilized bicarbonate and the S0₂ and intimate contact between the bicarbonate in solution and the

S0₂ of the gas being treated provides a highly efficient desulfurization operation. The magnesium sulfite produced by the reaction of the bicarbonate and S0₂ is relatively insoluble in water and is thus readily separated for recovery of the magnesium sulfite using conventional liquid/solid separation equipment. The carbon dioxide formed in the desulfurization step is preferably recirculated back to the bicarbonization step to provide the C0₂ for the bicarbonization of the magnesium oxide, carbonate or hydroxide.

It will also be noted that the magnesium sulfite formed during the desulfurization step will react with the S0₂ in the flue gas to form magnesium bisulfite in accordance with the following:

MgS0₃ + S0₂ + H₂0 → Mg(HS0₃)₂ (5)

Although this reaction is not undesirable in that it removes an additional mol of SO-, magnesium bisulfite is soluble in the aqueous phase of the reactant and, unless treated, will be lost in the aqueous phase. It has been found that from between about 0.2 to about 0.6 mols of the bisulfite per mol of sulfite will be formed in the reactant during the desulfurization step.

The third step of the process is to heat the liquid phase of the reactant to a temperature sufficient to convert the bisulfite back to the insoluble sulfite. It has been found that heating the liquid phase-to between about 60°C and about 140*C will effect the conversion in accordance with the following:

Mg(HS0₃)₂ → MgS0₃ + S0₂ (6) The MgS0₃ is then subsequently separated from the aqueous phase and recovered as described below. Although the liquid phase of the reactant slurry can be heated as described to effect the conversion of the soluble bisulfites to insoluble sulfites, it is preferred to subject the entire slurry exiting from the reaction column to the heating step prior to separating solids from the acqueous phase. It will be apparent that, unless treated as described herein, as much as one half of the magnesium compound may be lost in the process as soluble bisulfite. From the reaction it is seen that as a result of heating the slurry to convert the bisulfite to insoluble sulfite, a mole of S0₂ is also formed. By maintaining an excess of MgO in the slurry or by introducing MgO prior to the heating step the

S0₂ thus formed is converted to the insoluble sulfite in accordance with the following:

Mg(HS0₃)₂ + MgO -*» 2MgS0₃ + H-0 (7)

Also during the desulfurization step, some magnesium sulfite may be oxidized to magnesium sulfate (MgS0₄) which is also soluble in water and which represents additional loss of magnesium in the process because it is not easily separated from the liquid phase. This reaction represents a potential loss of magnesium from the process and must be made up by the addition of fresh magnesium oxide, carbonate or hydroxide at the bicarbonization step. The loss of magnesium due to oxidation of the sulfite to sulfate can be reduced by the addition of anti-oxidants to the reactant either at the bicarbonization ste or just prior to desulfurization. The anti-oxidant serves to prevent the oxidation of magnesium sulfite to magnesium sulfate in the presence of oxygen in the flue gas. The anti-oxidant should be soluble in water and have a low vapor pressure so as to maintain its anti-oxidant 5 effect over a substantial period of time in the presence of relatively high temperatures. Among the anti-oxidants which have been found useful in the present invention are hydrazine and hydrazine salts. Also aryl- and alkyl- hydroxylamine containing materials such as p-, o-, or m-amino phenol are

10 also suitable anti-oxidants. Carboxylic acid such as tartaric and citric can serve as anti-oxidants in the method of this invention as well as aromatic polyamines such as ortho-, meta- or para-diaminobenzene. Aromatic hydroxy compounds such as pyrocatechol, pyrogallol and 1,2,4, trioxybenzene have also

15 been found suitable as anti-oxidants in the method of this invention. The anti-oxidant is preferably added in concentrations of between about 50ppm to about 500ppm with the exact amount being a matter of choice depending upon the oxygen content of the flue gas being treated. As previously

20 mentioned, magnesium sulfite is insoluble in water and thus precipitates out of the liquid phase in the desulfurization step.

The fourth step of the process involves the separation of the insoluble sulfite from the liquid phase of the reactant.

25 The separation of the magnesium sulfite precipitate is carried out using any conventional liquid solid separation apparatus. Good results have been achieved using conventional liquid/solid separation means such as for example, centrifugation, filtration, and screening. Apparatus utilized

30 for carrying out such operations such as for example centrifuges, filters, settling bowls and the like are well

*•> known in the art of solid/liquid separation and do not per se form a part of this invention.

It has been found that the precipitation of magnesium

35 sulfite from the liquid phase of the reactant is greatly enhanced in the presence of an alkali metal sulfite such as for example sodium sulfite. The alkali metal sulphite is prepared by the addition of an alkali bicarbonate to the liquid phase of the reactant slurry or can be added as the carbonate, oxide or hydroxide to the slurry of oxygen containing magnesium compounds for reaction in the bicarbonization step in the same manner as the magnesium compounds. The alkali bicarbonate reacts with^* the S0₂ in the flue gas accordingly to the following:

2NaHC0₃ + S0₂ -*^*• Na-S0₃ 4- H₂0 + 2C0₂ (8)

In addition, one mole of alkali sulfite will further react with a mole of S0₂ in accordance with the following:

Na₂S0₃ + S0₂ + H₂0 → 2NaHS0₃ (9)

Thus, in addition to promoting the precipitation of magnesium sulfite from the liquid phase, the alkali sulfite also helps to remove the S0₂ from the flue gas. As mentioned, it is preferred that the alkali metal be added in the form of an oxide, carbonate or hydroxide for reaction in the bicarbonation step to form the bicarbonate of the alkali metal. In this connection excellent results have been achieved using natural alkali minerals such as nahcolite, trona, and natron. Nahcolite is a mineral whose major component is sodium bicarbonate and which is found in saline mineral deposits. Trona (Na₃H(C0₃)-.2H₂0 is also a natural soda. Nahcolite is a mineral comprising hydrated sodium bicarbonate. The quantity of alkali metal bicarbonate which is present in the reactant should be sufficient to provide between about 0.1 to about 3 moles of alkali sulfite per mole of magnesium sulfite present after desulfurization. A highly preferred range is about 0.4 mole to about 1 mole of alkali sulfite per mole of magnesium sulfite.

The fifth step in the process involves the thermal degradation of the sulfite (both the magnesium and alkali metal) for recovery of MgO, alkali earth oxide, if present, and S0₂. Since magnesium oxide is decomposed at a temperature of between about 800°C and 1200"C which is well above the decomposition of any of the alkaline earth metal sulfites or alkali metal sulfites, the degradation temperature is carried out below the degradation temperature of Mg

Oo and above that of the magnesium sulfite. In this fashion the alkaline earth and alkali metal sulfites are decomposed to

*^** their oxides and recirculated to the bi.carboni.zation for reuse

5 in the process. S0₂ of high purity is formed as a byproduct of the sulfite decomposition and is separated and liquified for subsequent use for the production of sulphur containing products such as, for example, sulfuric acid production.

The method of the present invention is preferably carried

10 out as an essentially continuous process in which the bicarbonate containing reactant is caused to flow countercurrent to the flue, gas being treated. In this manner, intimate contact between the reactant and the flue gas is achieved.

15 Referring to Fig. 2 there is schematically illustrated apparatus for carrying out the method of the present invention. A reaction column 12 is provided with an inlet 14 at its lower end for flue gas and an outlet 16 at its top for treated gas. A spray head 18 is disposed in the top of the

20 reaction column 12 and is connected by a line 20 and a line 22 to a mixing tank 24 in which the alkali metal oxide containing slurry is prepared. The spray head 18 serves to evenly distribute the slurry in the reaction column 12. The reaction column 12 is divided into zones 26 and 28 by a partition 30

25 including an open ended cylinder 32 which defines a passage 34 through the column 12 for communication between the zones, 26 and 28 respectively. The cylinder 32 is partially closed off by a member 36 which permits gaseous communication between the zones but essentially prohibits liquid communication

30 therebetween. Bicarbonation of the alkali and alkaline earth oxide, carbonate and hydroxide occurs in the upper zone or

-. bicarbonation zone 26 of the reaction column 12 while the desulfurization of the gas occurs in the lower or

,, desulfurization zone 28. Liquid containing bicarbonate

35 collects at the partition 30 and is pumped through a line 38 by a pump 40 to a spray head 18' which is disposed in the upper portion of the desulfurization zone 28. The precipitated sulfites and the liquid phase of the reactant are collected in a reservoir 29 in the lower portion of the desulfurization zone 28 of the reaction column 12. These products are pumped out in the form of a slurry by a pump 42 through a line 44 and a line 46 to a heating tank 48 where the slurry is heated to convert soluble bisulfites .in the liquid phase to insoluble sulfites which are precipitated and combined with the slurry solids. S0₂ formed during the heating step is returned to inlet 14 for processing. Preferably, magnesium oxide is added at the heating tank 48 to ensure an excess of the oxide for reaction with S0₂ formed during bisulfite conversion to reduce or eliminate the necessity of processing S0₂ formed during the bisulfite conversion step. From the heating tank 48, the slurry is sent to a liquid/solid separator 50 for separation of the solids from the liquid phase. The solids are moved by a line 52 to a thermal degradation unit 54 as will be described hereinafter as will be described in detail in connection with Fig.3. The liquid phase is conveyed by a pump

55 through a line 56 and line 20 back to the bicarbonation zone 26.

The reaction column 12 is further subdivided by fluid permeable packing supports 60, for example movable trays, screens or the like. Each of the packing supports support packing materials 62 which serve to diffuse both the flue gas and the reactant to ensure intimate contact therebetween.

Permeability is provided by openings in the supports 60. The size of the openings is not critical so long as packing material 62 can be retained by the supports 60 The packing material 62 may comprise any of the conventionally used materials such as Raschig rings or Berl saddles. Preferably, however, the packing material 62 comprises a plurality of hollow balls 64 which, as illustrated in FIG.5 are provided with openings 66 and edge portions of which describe inwardly extending contoured baffles 68. The design and function of the hollow balls 64 is set forth in Japanese patent publication 54-37586, dated Nov 15, 1979. The use of the hollow balls 64 as the packing material of choice is preferred since the hollow balls 64 are constantly agitated and moving by the action of the flue gas and the counterflowing reactant in the openings 66 and against the baffles 68 to cause the balls to oscillate and vibrate so that the build up of sulfite 5 on the surface of the ball is avoided. The separated magnesium sulfite is pumped to a heating unit 70 for thermal degradation at a temperature of between about 800°C to about 1200"C. The thermal degradation is achieved using a column heater in which the magnesium sulfite is heated in the presence of alumina 10 pellets. The apparatus may comprise a single unit or multiple units.

Referring to Fig. 3 there is schematically illustrated a single column thermal degradation unit 54 which comprises a hollow column 72 having closed ends defining a top wall 71 and 15 a bottom wall 78. Magnesium sulfite is introduced to the column through an inlet port 74. The alumina pellets are separately added through an inlet port 75 in the top wall 71 opposite the port 74. A depending partition 80 extends across the diameter of the column 72 in the upper portion thereof and

20 terminates intermediate the top and bottom walls 71 and 78 to define in cooperation with the top wall 71 and side wall of the column 72 a drying and pre-heating zone 73 and an ignition and heating zone 76. Fuel and air are introduced to the ignition zone 76 through inlet 82. Alumina pellets are heated

25 to the degradation temperature by the ignited fuel/air mixture in the ignition and heating zone 76. Flue gas generated in the ignition zone 76 is conveyed by a line 73 to the inlet 14 of the reaction column 12 for processing. Magnesium sulfite is pre-heated and dried in the pre-heating zone 73 which is

30 heated by radiation from the ignition and heating zone 76 through the partition 80. Both of the zones 73 and 76 are open

^• at the bottom and the lower portion of the column 72 is undivided so that the magnesium sulfite and the now heated

^'* pellets come into direct contact. A rotating cone shaped mixer

35 86 is located in the column 72 and the mixer 86 is driven through a shaft 88 and motor 90 to mix the pellets and the sulfite solids which are thermally degraded by the heated pellets to produce finely divided magnesium oxide. The finely divided magnesium oxide and the alumina pellets are separated by a vibrating screen 92 located at the bottom of the column 72 and the magnesium oxide is sent to the make up tank 24 for reuse in the process. The alumina pellets are returned by a line 94 to the port 74 and reintroduced in the column 72 for reheating and reuse in the thermal degradation process. S0₂ is separated and exits the column 72 at line 96 for liquification and storage for use in other chemical operations in accordance with known procedures.

Referring now to Fig. 4, there is illustrated a pair of reaction columns 98 and 100 designed for the thermal degradation of larger volumes of magnesium sulfite in large S0₂ stripping operations. The columns are paired with the column 98 serving as the degradation column and the column 100 serving as the ignition and heating column. The ignition column 100 includes a pair of inlets 102 and 103 for receiving fuel and pellets to be heated, respectively. Flue gas generated during the ignition of the fuel is conducted out of the column 100 through an outlet 104. A rotating mixing element 106 of the type described above in connection with Fig.3 is provided to intimately mix the pellets and the fuel which are ignited in the midsection of the column 100 just below the mixing element 106. A vibrating screen 108 is provided at the bottom of the column 100 for separating ash, if any, from the heated pellets and the heated pellets are then transferred by a line 110 to the degradation column 98. The degradation column 98 is provided with inlet ports 112 and 114 for receiving magnesium sulfite and the heated pellets, respectively. An outlet port 116 for S0₂ is provided in the upper portion of the degradation column 98 and a rotating power driven mixing element 118 is provided in the column 98. The magnesium sulfite and the heated pellets are intimately mixed by the mixing element 118 to cause the degradation of the magnesium sulfite to magnesium oxide and SO.,. A vibrating screen 120 is provided at the bottom of the column 98 for separating finely divided magnesium oxide from the alumina pellets. The pellets are then returned by a line 122 to the ignition column 100 for reuse in the process and the magnesium oxide is transferred by a line 133 to the make up tank 24 for reuse in the sulfite stripping process. 5 The following examples are illustrative of specific modes of practicing the invention and are not intended as limiting the scope of the invention as defined by the appended claims.

Example 1 The following example illustrates the S0₂ efficiency of

10 a single reaction column 12 comprising a bicarbonation zone and a desulfurization zone.

A glass column 380cm in height and 10 cm in diameter was partitioned into an upper zone 160cm in length and a lower zone 220cm in length. The upper zone was further divided into

15 two 15cm coaxial sections by permeable supports consisting of screen having 7.25mm openings and the lower zone was divided into three 15cm sections by the screens. Each section contained a plurality of hollow polypropylene packing balls of 10 mm diameter. Each ball had a surface wall thickness of 0 0.03 mm and was provided with five through running openings, each 2mm in diameter. Each section contained sufficient number of packing balls to fill it to a depth of 30 cm.

A gas comprising 0.2% SO-, 15% CO., 84.8% N- and no oxygen was introduced into the column at a flow rate of 1.18 nm³/min. 5 A reaction slurry comprising 4.5 gr of magnesium bicarbonate and 5.7 gr of the mineral trona in water was charged to the column at an initial flow rate of about 8.4 liters per minute. 0.6 gr of magnesium carbonate and 0.7gr of trona were added on an hourly basis to the reactant to 0 compensate for any lost magnesium or sodium bicarbonate.

» The influent and effluent gases were analyzed every two hours over a ten hour period and the results are set forth in

» Table 1.

The exiting material from the reaction column was filtered to separate magnesium sulfite from the liquid phase and the liquid phase analyzed for its bisulfite content. The ratio of magnesium bisulfite in the liquid phase to magnesium sulfite solids was found to be in the mole ratio of between 0.2:1 to 0.6:1. The liquid phase was heated to about 65°C to convert the magnesium bisulfite salt to insoluble magnesium sulfite and refiltered to recover the magnesium sulfite.

The magnesium sulfite was thermally decomposed in the presence of alumina pellets heated to approximately 800°C and the magnesium oxide in finely divided form was separated from the alumina pellets. Recovery of the magnesium oxide was on the order of 99.5% of the magnesium oxide introduced to the thermal degradation.

Example 2

A series of operations were performed to determine recovery efficiency of MgO in the thermal degradation operation using a pellet heater of the type described herein. The heater was 1 meter in height and had an inside diameter of 2 meters. The inner surface of the heater was lined with magnesium chrome bricks and the outside was lined with insulating brick, 25 mm in thickness. A pellet hopper was provided at the top of the apparatus for feeding pellets to the interior. The inner wall of the hopper was lined with a mortar consisting of a mixture of magnesium oxide and alumina to avoid contaminating the thermal degradation unit with metal particles from the sidewalls of the hopper. An electrically powered vibrating screen (3.9mm openings) was installed at the outlet of the apparatus for separating the pellets and the magnesium oxide.

A test material comprising magnesium sulphite (58.14% moisture) was supplied to the apparatus through the feeding hopper at the rate of 10.37 kilograms an hour. Alumina pellets (about 5mm in diameter and length) were fed to the thermal degradation unit at the rate of about 50kg/hr. Temperatures inside the apparatus were measured at 700°C. at the top, 990°C. at the midsection, and 850°C. at the bottom of the apparatus. The fuel used was propane which was fed at the rate of 4 kilograms per hour. The amount of magnesium oxide recovered was 1.59 kg. per hour which represented a mol percent yield of 94.84% of the magnesium sulfite introduced. In a second operation magnesium sulfite (68.28% moisture) was introduced at the rate of 10.08 kg/hour while the pellets were introduced at 50kg/hr . Temperatures inside the apparatus were measured as at 788°C. at the top, 998°C. in the midsection and 845°C at the bottom of the apparatus. Propane fuel was fed at the rate of 4 kil. per hour. The amount of magnesium oxide recovered was 1.5 kil. per hour with a mol yield efficiency of 92.99%.

A third run was made in the same manner as described above using magnesium sulfite test material (66.57% moisture) fed at the rate of 13.87 kg/hr. The pellets were fed at the rate of about 70 kg/hr. Temperatures measured inside the apparatus were 775°C. at the top, 998"C. at the midsection and 827°C at the bottom. Propane was utilized at the rate of 4kg/hr as the fuel. The amount of magnesium recovered was 1.76 kil. representing a mol yield efficiency of 98.28%.

Example 3

The desulfurization process of the present invention was utilized on a flue gas from a boiler utilizing the apparatus illustrated in Fig. 1. The reaction column was 12 meters in height and had an inside diameter of 1.5 meters. The column was divided into a carbonization zone and a desulfurization zone with each zone being further divided in sections by screens having 38mm openings, each section included hollow plastic balls to a depth of 50 centimeters. Each plastic ball was 45mm in diameter, had a surface wall thickness of 0.1mm, was provided with fourteen through running holes of 4 mm in diameter and provided with inwardly extending contoured baffles as illustrated in Fig. 5.

Fuel gas from a boiler having an average composition of 2000ppm SO., 14%-15% C02 and 3%-5% 0₂ with the remainder being nitrogen was introduced into the reaction column at a rate of 17,000 nm³/hr. A reactant slurry comprising 3.2 kg/1 of magnesium carbonate and 3.6 kg/1 of the mineral trona (C0₃ 23%, HC0₃ 17%) was prepared in water to a salt concentration of 0.0759 kmols/kl. The slurry was heated to a temperature of about 50"C. and introduced through a spray head into the bicarbonation zone of the reaction column. Following bicarbonization, the reactant, in the form of a thin slurry was led into the desulfurization zone of the reaction column through a spray head as shown in FIG. 2. The slurry was introduced at a rate of about 2 kl/min which resulted in a magnesium compound supply of about 1.6 mols per hour and a trona supply of about 0.8 mols per hour.

The influent and effluent flue gas was measured every two hours and the results are set forth in Table 4 below.

Example 4 Utilizing the apparatus of the foregoing example, the flue gas was modified by the addition of oxygen to provide an oxygen content of 10%. The desulfurization operation was run as described in Example 4 above and the process was operated or two hours . The sulphate content of the liquid phase after separation of the magnesium sulphite was measured and reported as a percent of magnesium sulphite recovered. After the first two hours of operation antioxidant (100 ppm) was added prior to introduction of the slurry to the bicarbonization zone and after 2 hours of operation the sulphate ion concentration of the liquid phase was determined as a percent of the magnesium sulphite recovered. A second 2 hour run was then conducted using a different antioxidant. Runs were repeated as described above for a total of six different antioxidant compositions. The antioxidants tested were hydrazine, hydroxylamine, sodium polythionate and diaminodiphenyl, p-phenylenediamine and o- phenylenediamine. At the end of each run of each of the antioxidants the sulphate ion content was of the liquid phase was measured and reported as a percentage of the magnesium sulphite recovered. The results- are set forth in Table 3 below.

SUBS_TITUTESHEET

The foregoing results show the effectiveness of the use of antioxidant in preventing the formation of the sulphate of magnesium sulphate and sodium sulphate which are soluble in the liquid phase and which represent a loss of magnesium and sodium since the sulphate salts are not recovered.

While the foregoing invention has been described in connection with certain preferred embodiments thereof, various embodiments other than those described in detail in this specification will occur to those persons skilled in the art, which arrangements lie within the spirit and scope of the invention. It is therefore to be understood that the invention is to be limited only to the claims appended hereto. Having described the invention, we claim:

SUBSTITUTE SHEET

Claims

1. A method for removing SO- from a gas comprising the steps of: a. forming a reactant comprising a water solution of an alkaline earth bicarbonate; b. contacting said gas with said reactant to form an essentially water insoluble alkaline earth sulfite by reaction between said S0₂ and said bicarbonate thereby desulfurizing said gas; c. heating said reactant after contact with said gas to a temperature of above about 60°C thereby to convert soluble alkaline earth bisulfite to the insoluble alkaline earth sulfite; and d. separating said alkaline earth sulfite from said liquid phase.

2. The method of claim 1 further including the step of forming a slurry comprising water based liquid phase and an alkaline earth compound selected from the group consisting of an alkaline earth oxide, hydroxide, carbonate and combinations thereof, contacting said slurry with carbon dioxide to react at least a portion of said alkaline earth compound therewith to form said alkaline earth bicarbonate, a substantial portion of said bicarbonate thus formed being solubilized in said liquid phase.

3. The method of claim 1 wherein said alkaline earth sulfite removed from said liquid phase is heated to at least the decomposition temperature thereof to form an alkaline earth oxide and S0₂.

4. The method of claim 3 wherein said alkaline earth oxide is reacted with carbon dioxide to form an alkaline earth bicarbonate and said bicarbonate thus formed is returned to said reactant.

5. The method of claim 2 wherein said carbon dioxide formed during said desulfurization of said gas is recirculated to contact said slurry.

6. The method of claim 1 wherein said alkaline earth bicarbonate is selected from the group consisting of magnesium bicarbonate and calcium bicarbonate.

5 7. The method of claim 1 wherein an antioxidant is added to said reactant, said antioxidant comprising a water soluble compound selected from the group consisting of hydrazine, hydroxylamine, carboxylic acid, aromatic polyamines and aromatic hydroxy compounds.

10 8. The method of claim 7 where in said antioxidant is selected from the group consisting of p-, o-, m-phenol and mixtures thereof.

9. The method of claim 7 wherein said antioxidant is selected from the group consisting of pyrocatechol,

15 pyrogallol, 1,2,4-trioxybenzene and mixtures thereof.

10. The method of claim 7 wherein said antioxidant is selected from the group consisting of tartaric acid, citric acid and oxalic acid and mixtures thereof.

11. The method of claim 7 wherein said antioxidant comprises 20 from about 50ppm to about 500ppm in said reactant.

12. The method of claim 1 wherein said reactant contains an alkali metal sulfite during the separation of said alkaline earth sulfite from said liquid phase.

* 13. The method of claim 12 wherein said alkali metal sulfite 5 is selected from the group consisting of sodium sulfite, potassium sulfite and mixtures thereof.

14. The method of claim 12 wherein said alkali metal sulfite comprises between about 0.1 to about 3 mols of alkali metal sulfite per mol of alkaline earth sulfite present in said reactant after contacting said gas.

15. The method of claim 12 wherein said alkali metal sulfite comprises between about 0.14 mol and 1 mol of alkali sulfite per mol of alkaline earth sulfite present in said reactant after contacting said gas.

16. The method of claim 12 comprising the step of introducing an alkali metal sulfite precursor to said water based slurry, said precursor being selected from the group consisting of an alkali metal oxide, alkali metal carbonate, alkali metal hydroxide or combinations thereof.

17. The method of claim 16 wherein said alkali metal precursor is a mineral selected from the group consisting of nahcolite, trona, natron and mixtures thereof.

18. The method of claim 1 further including the step of heating said liquid phase after separation of said magnesium sulfite therefrom to a temperature below the boiling point thereof and above 60°C. thereby to convert water soluble alkaline earth metal bisulfite to said alkaline earth sulfite thereof and separating said sulfite from said liquid phase.

19. A method for desulfurizing an effluent gas by the removal of S0₂ therefrom comprising the steps of a. forming a reactant slurry having a liquid phase comprising water and a solids phase comprising an oxygen containing magnesium compound selected from the group consisting of magnesium oxide, magnesium carbonate, magnesium hydroxide and mixtures thereof; b. contacting said slurry with carbon dioxide thereby to convert said oxygen containing magnesium compound to the water soluble bicarbonate and to solubilize a substantial portion of said bicarbonate in said liquid phase thereof; c. contacting said liquid phase with said gas thereby to cause the reaction of the SO- therein with said bicarbonate to form essentially water insoluble magnesium

, 5 sulfite and CO-; and d. separating said magnesium sulfite from said liquid phase; whereby a substantial portion of said S0₂ in said gas is stripped therefrom.

10 20. The method of claim 19 further including the step of heating said magnesium sulfite to a temperature of between about 800^βC. and about 1200°C. thereby to decompose said magnesium sulfite and to form magnesium oxide and S0₂ and said magnesium oxide is recirculated to said slurry for reuse in

15 said process.

21. The method of claim 19 wherein said liquid phase is heated to a temperature of between 60°C. and it's boiling point to decompose water soluble magnesium bisulfite to insoluble magnesium sulfite, separating said magnesium sulfite

20 from said liquid phase and thereafter recirculating said ^• ' liquid phase to form said slurry.

22. The method of claim 19 further including the step of adding an antioxidant selected from the group consisting of hydroxylamine, hydrazine, carboxylic acid, aromatic

25 polyamines, aromatic hydroxide compounds and mixtures thereof, thereby to substantially reduce the formation of water soluble sulfates caused by the reaction between magnesium sulfite and

^"* oxygen.

23. The method of claim 19 further including the step of 30 maintaining an alkali metal sulfite in said liquid phase of said reactant during contact with said gas to promote precipitation and separation of magnesium sulfite. 24. The method of claim 23 wherein said alkali metal sulfite comprises between about 0.1 mols and about 3 mols of alkali sulfite per mol of magnesium sulfite.

25. The method of claim 24 wherein an alkali metal bicarbonate is added to said^"liquid phase and said bicarbonate reacts with S0₂ in said gas to form said alkali metal sulfite.

26. The method of claim 25 wherein an effective amount of an alkali metal compound selected from the group consisting of an alkali metal oxide, carbonate, hydroxide and mixtures thereof is added to said slurry thereby to form said alkali metal bicarbonate when contacted with C0₂.

27. The method of claim 23 wherein said alkali metal sulfite is selected from the group of minerals consisting of nahcolite, trona and natron and said mineral is added to said reactant prior to contact with the S0₂ of said gas being treated.

28. The method of claim 2 wherein a reaction column defines a bicarbonatization zone and a desulfurization zone, said bicarbonization zone comprising the top portion of said reactant column and said desulfurization zone comprising the lower portion of said column, introducing said reactant into the top of said column and contacting said reactant with carbon dioxide thereby to form a water soluble alkaline earth bicarbonate and thereafter conveying said alkaline bicarbonate in solution in said liquid phase into said desulfurization zone for contact with said gas being treated thereby to react the S0₂ of said gas with said bicarbonate to form a water insoluble alkali earth sulfite and separating said sulfite from said liquid phase whereby said gas expelled from said column is essentially free of S0₂.

29. The method of claim 28 wherein said reaction column contains packing material which is agitated by the flow of said gas and said reactant through said column thereby to avoid an agglomeration of said magnesium sulfite formed therein and to promote formation of said magnesium sulfite in a finely divided form,^* "said packing material comprising a plurality of hollow spheres, each sphere having at least a pair of openings communicating from the interior to the exterior thereof and edge portions of each of said openings defining a baffle which is acted upon by said gas and said reactant as it flows through said reaction column thereby to cause said sphere to be fluidized and to rotate.

30. A device for converting magnesium sulfite to magnesium oxide by the thermal decomposition of magnesium sulfite, said device comprising a heat resistent container for receiving said magnesium sulfite and a heating chamber for receiving alumina pellets and fuel, said heating chamber including means for igniting said fuel thereby to heat said alumina pellets, means for mixing said heated pellets and said magnesium sulfite in said heat resistant container whereby said magnesium sulfite is thermally decomposed to magnesium oxide and SO^ , means for separating said magnesium oxide and said alumina pellets and for recovering said magnesium oxide and means for returning said alumina pellets to said heating zone.

31. The device of claim 30 further including means for partitioning said heating zone from said heat resistant container means whereby said alumina pellets are heated prior to contact with said magnesium sulfite in said heat resistant container.

32. The device of claim 30 wherein said heating zone is separate from said heat resistant container and further includes means for transferring said heated alumina pellets from said heating zone to said heat resistant container.

SUBSTITUTE SHEET