Process for the Regeneration of an Additive Used to Control Emissions During the Combustion of High Sulfur Fuel

The FIGURE is a schematic illustration of the process of the present invention.

As previously noted, the present invention relates to a process for regenerating a sulfur capturing additive which is added to a sulfur containing hydrocarbon material which is to be burned as a fuel. The process is particularly useful for fuels in the form of hydrocarbon in water emulsions as disclosed in co-pending applications Ser. Nos. 014,871 and 875,450 which disclosures are incorporated herein by reference. It has been found that the formation and emission of sulfur oxides during the combustion of hydrocarbon fuels including oil in water emulsions can be controlled by adding an additive which captures sulfur during the combustion of the fuel. The preferred additives for use in the process are water soluble and are selected from the group consisting if Na.sup.+, K.sup.+, LI.sup.+, Ca.sup.++, Ba.sup.++, Mg.sup.++, Fe.sup.+++ and mixtures thereof. The additive should be added to the hydrocarbon or emulsion in a molar ratio amount of additive to sulfur in said hydrocarbon so as to obtain SO.sub.2 emissions upon combustion of the emulsion of less than or equal to 1.50 lb/MMBTU. It has been found that in order to obtain the desired emissions level the additive must be present in a molar ratio of additive to sulfur of greater than or equal to 0.050, preferably 0.100, in the hydrocarbon in water emulsion. While the level of additive to obtain the desired result depends on the particular additive or combination of additives employed it has been found that a molar ratio of at least 0.050 of additive to sulfur is required. A complete discussion of the results obtained vis-a-vis sulfur emissions as a result of use of these additives can be found in the aforementioned applications which are incorporated herein. The present invention is drawn to a process for regenerating these sulfur capturing additives.

Referring to the drawing, a fuel 12 to be combusted is delivered via line 14 to a boiler 16. In accordance with the present invention the fuel may be a hydrocarbon residual, a crude or an oil in water emulsion formed from a viscous hydrocarbon or other residual hydrocarbon. In accordance with the present invention, a sulfur capturing additive 18 is admixed with the hydrocarbon fuel, particularly an oil in water emulsion, via line 20 prior to delivering the fuel to boiler 16 for combustion. The preferred sulfur capturing additives for use in the process are water soluble and are selected from the group consisting of Na.sup.+, K.sup.+, Li.sup.+, Ca.sup.++, Ba.sup.++, Mg.sup.++, Fe.sup.+++ and mixtures thereof. After combustion of the fuel in boiler 16 the overhead gases are delivered via line 22 to an electrostatic separator 24 wherein a combustion ash is separated from the exhaust gases which are delivered off via line 26. The combustion ash is delivered via line 28 to a leaching zone 30 wherein the combustion ash is leached with water delivered via line 32 in order to dissolve the water soluble sulfur capturing additive which, after fuel combustion, is in the form of a sulfate.

In accordance with the present invention the combustion ash is leached with water in a water to ash ratio in milliliters to grams of from 1:1 to 30:1, preferably 2:1 to 10:1. The leaching operation is carried out at a temperature of from about 5.degree. to 200.degree. C., preferably 15.degree. to 95.degree. C. It is preferred for operating temperatures less than 100.degree. C. to operate the leaching step at a pressure of 1 atm. The required time for leaching is from about 0.1 to 5 hours, preferably 0.2 to 3 hours. After leaching the solution is withdrawn via line 34 to a separator 36 wherein the pregnant leach liquor is separated from the solid residual and delivered via line 38 to precipitation zone 40. The solid residual is delivered via line 42 for further treatment in leaching zone 44 as will be discussed hereinbelow.

In precipitation zone 40 the pregnant leach liquor is adjusted with a base precipitating agent via line 46 so as to precipitate the sulfur capturing additive as a compound. Suitable base precipitating agents include NH.sub.4 OH, NaOH, Ca(OH).sub.2, NaCO.sub.3 and mixtures thereof. The base precipitating agent is added in an amount sufficient to adjust the pH of the solution to greater than 7 and, preferably to a pH of between 9 and 11. The precipitation operation takes place at a temperature range of from 5.degree. to 95.degree., preferably 25.degree. to 80.degree. C. The precipitated solution is thereafter delivered via line 48 to separation stage 50 wherein the precipitant is separated from the liquid phase which can be discharged via line 52. Depending on the base precipitating agent employed in precipitation zone 40, the precipitant and liquid derived in separation stage 50 will be different. For example, if NaOH is used as the precipitating agent, the precipitate will be in a relatively pure hydroxide form. For example if Mg was used as the sulfur capturing additive, the precipitate is Mg(OH).sub.2. If NH.sub.4 OH is used as the precipitating agent the precipitant will again be a hydroxide in relatively pure form. In this case the liquid discharged via line 52 will contain ammonium sulfate which is useful, for example, as a fertilizer and which can be recovered from the reactor via known crystallization processes. Finally, if NaCO.sub.3 is employed as the precipitating agent, the resulting precipitant is an impure mixture of a carbonate and sulfate which in the case of a Mg additive would be MgCO.sub.3 and Na.sub.2 SO.sub.4. It has been found that either of the above base precipitating agents results in over 90% of the sulfur capturing additive being precipitated with NaOH precipitating 99.9%, NH.sub.4 OH precipitating 93% and NaCO.sub.3 precipitating 96%.

The sulfur capturing additive compound separated in separator 50 may be delivered as desired via line 54 back to line 20 for admixing with the hydrocarbon fuel in line 14. In accordance with the present invention, it is desirable that the sulfur capturing additive be in the form of a carbonate. In order to achieve the foregoing, the sulfur capturing additive compound is delivered from separation stage 50 via line 56 to a closed vessel 58 where the additive hydroxide compound is treated with water and CO.sub.2 gas via lines 60 and 62. The CO.sub.2 is added to vessel 58 and the sulfur capturing additive hydroxide compound at a CO.sub.2 partial pressure of up to 1000 psi, preferably between 1 and 500 psi at a temperature of from 0.degree. to 150.degree. C. The mixture is agitated in the closed vessel 58 and a sulfur capturing additive carbonate is removed from the vessel in solution and can be recycled via line 64 for addition to the hydrocarbon fuel.

As noted above, the solid residual from separation zone 36 can be treated in leaching zone 48 so as to recover vanadium and nickel. A solid residual is dissolved with an acid solution delivered via line 66. A suitable acid solution is, for example, a 20% solution of H.sub.2 SO.sub.4. After the acid leaching wherein the solid mixture is dissolved, the solution is delivered via line 68 to separation zone 70 wherein the solid waste is carried off via line 72 and the pregnant liquor is delivered via line 74 to a metal precipitating zone 76 where the vanadium is precipitated by adjusting the pH of the solution to about 2. Thereafter, nickel can be precipitated by adjusting the pH of the liquor to 5 to 6.

It should be appreciated that the following example is given for purposes of illustrating the process of the present invention and it is to be understood that this example is not intended to limit the generally broad scope of the present invention as claimed.

An oil in water emulsion was prepared by admixing a sulfur containing hydrocarbon and water with an emulsifier. A water soluble additive in the form of a magnesium salt (MgCl.sub.2) was added to the emulsion in an amount with respect to the hydrocarbon such that the molar ratio of magnesium to sulfur was equal to 0.100. The resultant emulsion was burned leaving behind a combustion ash containing magnesium sulfate. 1000 grams of ash were leached with 5000 ml of water (a ratio of ash to water of 5:1) for two hours at a temperature of 90.degree. C. The solution was then separated by filtration leaving a solid residue of 300 grams indicating that 70% of the magnesium originally present in the ash went into solution.

Three 100 ml samples of the pregnant leach liquor recovered above were reserved and labelled Sample I, Sample II and Sample III, respectively. Sample I was mixed with 61 ml of a base precipitating agent in the form of a 10% solution of N.sub.2 OH at a temperature of 80.degree. C. at 1 atm pressure. The precipitation of Mg was almost instantaneous. 99.9% of the Mg present in the pregnant leach liquor was precipitated and identified by X-ray diffraction as relative pure Mg(OH).sub.2. Sample II was mixed with 61 ml of a base precipitating agent in the form of a 10% solution of NH.sub.4 OH at a temperature of 80.degree. C. at 1 atm pressure. 93% of the Mg present in the liquor was precipitated and identified by X-ray diffraction as relative pure Mg(OH).sub.2. In addition, ammonium sulfate was recovered from the liquor via crystallization. Finally, Sample III was mixed with 61 ml of NaCO.sub.3 as precipitating agent at the same pressure and temperature as Sample I and II. 96% of the Mg was precipitated. The precipitate was identified by X-ray diffraction and was found to be an impure mixture of MgCO.sub.3 and Na.sub.2 SO.sub.4. The regenerate and recycle the Mg(OH).sub.2 precipitate obtained above, the precipitate was dissolved in a closed vessel with mechanical agitation and CO.sub.2 gas was injected at a partial pressure of 30 psi at 25.degree. C. to yield MgCO.sub.3.

100 g of the solid residue from the water leaching step of the original combustion ash was leached with 500 ml of 20% solution of H.sub.2 SO.sub.4 at 80.degree. C. for 2 hours. After leaching 88% of the residue was dissolved and all vanadium and nickel in the residue was solubilized. The resulting liquid was separated and vanadium was precipitated by adjusting the pH of the solution to 2 with HCL. Thereafter the pH was adjusted to 6 to precipitate nickel.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.