SCRUBBING OF AMMONIA WITH UREA AMMONIUM NITRATE SOLUTION

BACKGROUND

1. Field of the Invention.

The invention is in the field of absorbing ammonia in flue gas scrubbing.

2. Description of the Related Art.

Scrubbing of carbon dioxide with ammonium carbonate solutions has been known for many years. One process for absorbing CO2 uses either an ammonium carbonate solution or a mixed alkali solution, which can be ammonium and either potassium or sodium carbonate. However, due to the volatility of ammonia at the pHs required for carbon dioxide capture, it is necessary to capture any ammonia released from the solution if the process is to be used to remove CO2 from flue gas. What is needed, therefore, is a method and apparatus for scrubbing ammonia vapor in a CO2 scrubbing system.

SUMMARY

The invention is a method and apparatus that satisfies the need for scrubbing ammonia vapor in a CO2 scrubbing system. One method of absorbing the ammonia vapor is to use a urea solution that is pH adjusted with nitric acid to form a urea ammonium nitrate solution (UAN). Urea ammonium nitrate is composed of 30 - 35 wt% urea, 40 - 45 wt% ammonium nitrate, and 20 - 30 % water and is a fertilizer that contains 28 - 32% nitrogen. The specific gravity of the solution ranges from 1.283 to 1.320 and has a pH of 7 - 7.5. When nitric acid and urea are combined in a ratio that produces a solution that is ammonia lean, the solution will have a pH of 4 - 6 and can be used with an appropriate mass transfer device to absorb ammonia. These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings.

DRAWINGS

Fig. 1 is a process flow diagram showing some of the components used to carry out the process of the present invention.

Fig. 2 is a process flow diagram describing the process of the present invention.

DESCRIPTION

The invention is a method and apparatus that satisfies the need for scrubbing ammonia vapor in a CO ₂ scrubbing system. As shown in Fig. 1 , the ammonium carbonate or mixed alkali solution 102 is circulated to absorb CO2 from a flue gas stream containing CO2, H ₂ O, O ₂ , and N ₂ 104. The solution is sent through a regenerator 106 to produce a concentrated CO ₂ stream 108 ready for sequestration. Once regenerated the solution is recycled back to the CO ₂ capture section 110 to absorb more CO ₂ . In the process of absorbing CO ₂ ammonia is released. The amount of ammonia released is determined by the conditions of absorption and the composition of the absorbing solution but will be between 100 - 8000 ppm. After the CO ₂ is captured, the flue gas moves to an ammonia capture mass transfer section 112 where a concentrated urea ammonium nitrate solution is circulating. The solution is pH adjusted to between 4 and 6 with nitric acid 114 to make the solution ammonia lean and forms ammonium nitrate as shown in equation (1 ).

HNO ₃ + NH ₃ ^■ * NH ₄ NO ₃ (1 )

Urea 116 is added to the solution either prior to the NH ₃ capture 112 or after the formation of the ammonium nitrate to maintain a concentrated urea ammonium nitrate solution 118 ready for commercial distribution.

As shown in Fig. 2, a flue gas stream containing CO ₂ 202 is brought into an absorption section 204 that uses an ammonium carbonate solution to absorb CO ₂ . The ammonium carbonate is converted into ammonium bicarbonate 208 through reaction Of CO ₂ with CO ₃ ^2" as shown in (2):

CO ₂ + CO/ 2- ^" + H ₂ O ^■ * 2 HCO ₃ ^" (2)

The ammonium bicarbonate solution is introduced into the regenerator 210, to reverse reaction (2) creating a concentrated CO ₂ stream, which can be processed for sequestration or beneficial use. The regenerated solution is re-introduced into the absorber tower 204 to remove more CO ₂ .

Due to the volatility of the ammonium carbonate solution, the flue gas exits the absorber 204 with >70% CO2 having been removed and with the addition of ammonia vapor. The CO2 lean flue gas 212 now enters an ammonia vapor recovery 214 section to remove the ammonia vapor prior to leaving the stack. In the ammonia vapor recovery section, nitric acid 216 is added to a urea ammonium nitrate (UAN) solution to decrease the pH to <6. The UAN absorbs the ammonia vapor from the flue gas in a gas liquid contactor. When the UAN exits the contactor, part of the solution is removed as product 218 ready to be used as a fertilizer product while the rest is recycled back to the ammonia vapor recovery section 214. Additional urea 220 and HNO3 acid 216 are added to maintain the required ratios of UAN and to maintain the ability to absorb NH ₃ vapor.

The advantages of using UAN to do ammonia absorption as part of a CO2 scrubbing process are:

1. The ability to produce a nitrogen fertilizer, which is the predominant plant nutrient required by crops.

2. The ability to produce a nitrogen fertilizer with minimal capital cost. Since the ammonia capture is required in the CO2 scrubbing process, only incremental increases are required for the nitrogen fertilizer production including pumps and storage allowing one to take advantage of the infrastructure already in place.

Although the preferred embodiments of the present invention have been described herein, the above description is merely illustrative. Further modification of the invention herein disclosed will occur to those skilled in the respective arts and all such modifications are deemed to be within the scope of the invention as defined by the appended claims.