MXPA00011622A - Process for treating flue gas - Google Patents

Abstract

Process for combustion flue gas conditioning, by injecting ammonia, in a very simple and viable way, in a stream of said combustion flue gas stream, characterized in that dissolved urea is hydrolyzed by heating under pressure to yield ammonia and carbon dioxide the hydrolysis product are stripped from the hydrolized solution by means of hot steam and injected into the flue gas stream while water coming out of the hydrolysed solution is reused to dissolve fresh urea to be fed to the plant.

Description

PROCESS FOR THE TREATMENT OF COMBUSTION GASES FIELD OF THE INVENTION This invention relates to the process for the treatment of combustion gases and more specifically to a process for purifying (conditioning) these combustion gases by the addition of ammonia. Particularly, this invention relates to a process for the treatment of combustion gas by the addition of ammonia, whereby ammonia is derived from the hydrolysis of urea.

Description of the prior art It is well known to add ammonia to the combustion gases to remove, for example, nitrogen oxides, which are formed in the course of their own combustion. The reaction between ammonia and nitrogen oxides, in the presence of oxygen, results in the formation of nitrogen and water according to the following reaction: 2NH3 + NO + 02- »3H20 + N2 The ammonia gas is injected into the combustion gas stream to be treated by steam obtained by the removal of ammonia contained in an aqueous solution of ammonia in which the ammonia concentration is approximately 30% by weight. The aqueous ammonia solution has to be transported from the production plant to the user's plant, stored, sent to the removal plant to produce the desired ammonia, then the ammonia containing waste water has to be neutralized before disposal. Ammonia solutions are currently classified as toxic materials. Therefore, certain specific rules have to be followed for their management, due to the high volatility and the level of ammonia contamination. To avoid the above problems, the use of non-toxic, harmless compounds have been proposed which, under reaction conditions, can produce ammonia. For this purpose, urea has been used to generate ammonia without the formation of any unwanted product. The use of urea as a conditioning agent of combustion gases solves the problem in relation to the handling and storage of highly toxic materials; however, to obtain fine particles of solid urea and inject them uniformly into a flue gas stream still presents substantial problems. In the document US-520688 of Von Harpe, it has also been proposed to employ, as a source of ammonia, instead of a solid urea, the products derived from the hydrolysis of urea. These hydrolysis products have been obtained by heating a urea solution. The hydrolysis products as a whole, called ammonia carbonate, ammonia carbamate, ammonia bicarbonate and ammonia are used for the reduction of N0X. However, the simple heating of a concentrated urea solution does not allow to reach a satisfactory amount of hydrolysis products. The above patent describes the use of an acidic or basic environment or the addition of a catalyst. However, the use of catalysts or basic or acidic products can lead to some problems due to the presence of this material both in the hydrolysis section and in the N0X reduction session. According to US-A-4168299 the removal of urea hydrolysis products can be obtained by the absorption of pressurized C02 in a diluted urea solution then by removing CO 2 absorbed.

This process employs a diluted urea solution and a pressurized C02 resource.

Brief Description of the Invention It is an object of the present invention to provide a process for the conditioning of combustion gas by injection of ammonia, in a very simple and viable way, within a stream of combustion gases that avoid problems associated with the handling and storage of hazardous materials. It is a further object of the present invention to provide a process for the in-situ production of gaseous ammonia, wherein said ammonia can be used without being subject to any particular safety rule and without any responsibility for maintaining, transport and store large volumes of solutions of ammonia and harmful toxic material as required by conventional processes, and have no remaining ammonia or ammonia precursor solution. This and other objects of the present invention are carried out by a process for the conditioning of the combustion gas in which the urea is dissolved in water to obtain a concentrated urea solution and the dissolved urea is hydrolyzed by heating the urea solution Concentrated under pressure, producing products of hydrolysis of ammonia and carbon dioxide. The hydrolysis products are removed from the solution using hot steam and injected into the combustion gas stream while the hydrolysis solution leaves the removal section, still containing some urea re-used in addition with fresh water, to dissolve the new urea to be fed to the plant. The urea solution to be hydrolyzed contains a range of about 10% to 70% urea by weight. The hydrolysis of urea is preferably carried out at a temperature in the range of 100 to 233 ° C and at a pressure in the range of about 0.1 x 103 Kpa to 3.0 x 103 Kpa. The hydrolysis solution, after the removal of the gaseous hydrolysis products, still contains about 1% to 5% by weight of unhydrolyzed urea.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings in which: Figure 1 is a schematic diagram of the process according to an embodiment of this invention.

Description of the embodiments presented This invention will now be described in greater detail with reference to Figure 1 as well as the following experimental example. The drawing and example are provided to better illustrate the invention and are not constructed as limiting the scope thereof. As shown in Figure 1, the solid urea, in the form of pellets or crystals, is introduced into the feed tank 1 which has a sufficient capacity for at least one day of continuous hydrolyzate operations. The urea, through the roll type feeder 3, is introduced into the solvent 4 where a urea solution of a concentration of about 40% is formed using condensed water from the boundaries of the power plant and / or from the condensate discharge discharged , which is also introduced into the solvent. The urea dissolving operations are facilitated by the action of the agitator 5. The slidable valve 2 if necessary can be provided at the outlet of the supply tank 1.

The urea / water solution thus obtained is compressed by the pump 6 to the hydrolyzate operating pressure and preheated to hydrolysis temperature by means of the preheater 7. According to one embodiment of this invention, the operating conditions of the hydrolyzer are as follows. indicate below: Pressure 1.9 x 103 Kpa Temperature 195 ° C Residence Time 40 minutes Under these conditions, approximately 30% - 40% by weight of the urea concentration in the feeder storage decreases to approximately 1% - 5% by weight in the hydrolyzed solution. The ammonia generated during the hydrolysis process is removed from the water solution by means of a defined amount of steam coming from the limits of the battery and injected to the base of the hydrolyzer 8. The hydrolysis products that comprise ammonia saturated with water and carbon dioxide are brought to the reaction zone through line 10. The water is expanded through the valve 11 and sent to the separator 9, operating at atmospheric pressure, and then recycled for it. The vapors derived mainly from ammonia and the expansion of water through line 12 are added to the hydrolysis vapors to avoid any environmental contamination. Water from separator 9 is recycled through line 13 to solvent 4 at a temperature of about 100 ° C so that, after having the urea supplied with the heat of the solution, a solution is obtained at a temperature of about 40 ° C.

EXAMPLE To better illustrate the process of this invention, a quantitative example is given below based on an ammonia capacity of 100 kg / h. The amount of urea feed to the plant is 177 kg / h and the condensed amount that reaches from the limits of the battery plant is 87 kg / h. The amount of recycled water is 150 kg / h including approximately 4.5 kg / h of residual urea in the solution. The solution from the solvent 4 is at a temperature of about 40 ° C and a urea concentration of about 44% by weight is sent to the pump 6, increasing the pressure to about 1.6 x 103 KPa and, through the preheater 7, increased the temperature of approximately 195 ° C. The solution enters the hydrolyzer 8 resulting in the formation of hydrolysis products having the following characteristics: Temperature 195 ° C Pressure 1.6 x 103KPa NH composition. 100 kg / h 24.12% by weight CO, 129 kg / h 31.24 'by weight H20 185 kg / h 44.64% by weight Total 414 kg / h 100.00 The direct flow of steam to hydrolyzer 8 required to maintain a constant hydrolysis temperature and to move the reaction products is approximately 155 kg / h. The result of the process of this invention is that the plant requires only facilities to store urea, the most used nitrogen fertilizer, which is not subject to any strict regulation and therefore does not require any special handling. Other major advantages of the present invention are: Safe plant operations, absence of any storage by operating personnel of hazardous substances, absence of any contaminating material, and reduction of investment costs compared to the use of ammonia solutions in accordance with the prior art. It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects to which it relates.

Claims (5)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property. 1. In a process for the conditioning of the combustion gas in which the ammonia is injected into a stream of the combustion gas stream, the improvement characterized in that it comprises: heating and pressurizing the dissolved urea solution, forming a solution of dissolved urea heated and pressurized; hydrolyzing the urea dissolved in the heated and pressurized dissolved urea solution, forming a hydrolyzed solution comprising ammonia and carbon dioxide; remove the aforesaid ammonia and carbon dioxide from the hydrolysed solution by contacting the hydrolysed solution with steam, leaving behind a hydrolysed solution removed; injecting the ammonia and carbon dioxide into the flue gas stream; and recycle the hydrolysed solution removed to dissolve the urea.
2. 2. A process according to claim 1, characterized in that the solution of dissolved urea is comprised of a range value of about 10 to about 70% by weight of urea.
3. 3. A process according to claim 1, characterized in that the dissolved urea is hydrolyzed at a temperature in a temperature range from about 100 ° C to about 233 ° C.
4. 4. A process according to claim 1, characterized in that the dissolved urea is hydrolyzed at a pressure in a pressure range from about 0.1 x 103 KPa to about 3.0 x 103 KPa.
5. 5. A process according to claim 1, characterized in that the hydrolyzed solution removed is expanded, forming hydrolysis vapors that combine with the ammonia and carbon dioxide and injected into the flue gas stream.