Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-02-17
2001-11-27
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S239100, C423S358000
Reexamination Certificate
active
06322762
ABSTRACT:
The present invention relates to the generation of ammonia from urea and/or mixtures also containing biuret in which an aqueous solution of urea is heated under pressure in a reactor from which a gaseous mixture of ammonia, carbon dioxide and water is withdrawn at a controlled pressure and at a controlled rate of flow.
BACKGROUND
Ammonia is a highly volatile noxious material with adverse physiological effects, which becomes intolerable at very low concentrations and presents substantial environmental and operating hazards and risk. It is classified as a hazardous material and many precautions are required in transporting and handling it safely. Urea, on the other hand, is a stable, non-volatile, environmentally benign material that is safely transported, stored and handled without such risk and, accordingly, can serve as a safe source of ammonia. The processes of this invention minimize the risks and hazards associated with the transport, storage and use of anhydrous and aqueous ammonia.
Many industrial plants require the supply of large quantities of ammonia, which frequently must be transported through and stored in populated areas. Important users among these are industrial furnaces, incinerators and the electric power generation industry. All of these are faced with a lowering of the amount of nitrogen oxides being discharged to the atmosphere in the combustion gases being emitted from their operations, as required by environmental regulations. Another important use is for the so-called “conditioning” of flue gas by which an improved collection and removal of particulate matter (fly ash) is obtained.
One of the important methods for removing nitrogen oxides derived from the burning of fossil fuels embodies their conversion to inert nitrogen gas by reaction with amine-type reductant materials, by processes such as Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR). Two main reductant materials have achieved commercial acceptance for this purpose, namely, ammonia and urea. Ammonia is superior to urea as such for SNCR in several important aspects for this application and is required for SCR applications but, as previously related, ammonia presents substantial environmental and operating hazards and risk because of its high volatility and noxious nature. Numerous accidents that have resulted in deaths have occurred in the transport and handling of ammonia and local authorities have placed restrictions on its use in many locations. Urea, on the other hand, is a stable, non-volatile, environmentally benign material that is safely transported, stored and handled without such risk.
This invention is particularly useful for supplying ammonia for the SCR and SNCR processes for removal of nitrogen oxide from combustion gases, the conditioning of flue gas for improving the removal of particulate matter, and other applications in which ammonia is used, by which the environmental hazards of transporting and storing anhydrous and aqueous ammonia solutions may be avoided.
In the SNCR method, ammonia, urea or amine-type materials are injected into the hot flue gas stream, usually in-furnace while in a temperature range of 2000° F.-1800° F. In the SCR method, ammonia is the only reductant used, and the reaction is carried out at a lower temperature level, typically 750° F.-600° F., the specific temperature level being controlled by the catalyst system used.
The chemical reaction taking place in the flue gas by which NO and NO
2
are removed is shown below for ammonia and urea:
Ammonia
4NO+4NH
3
+O
2
→4N
2
+6H
2
O [1]
2NO
2
+4NH
3
+O
2
→3N
2
+6N
2
O [2]
Urea
4NO+2CO(NH
2
)
2
+O
2
→4N
2
+4H
2
O+2CO
2
[3]
2NO
2
+2CO(NH
2
)
2
+O
2
→3N
2
+4H
2
O+2CO
2
[4]
Compared to urea, ammonia is more reactive, is more easily dispersed uniformly into the flue gas stream and is active over a broader temperature range, as well as being more effective in efficiency. Urea, as such, while also an effective reductant, forms unwanted byproducts, such as carbon monoxide (CO) and nitrous oxide (N
2
O), both of which are now under critical scrutiny by environmental authorities.
In the application of ammonia for the “conditioning” of flue gas, ammonia forms ammonium bisulfate with the sulfur oxides also present in the flue gas. These deposit and collect on the fine particles to form larger sticky agglomerates of the fly ash particles which makes their removal easier and more effective by both electrostatic collectors and fabric filters.
In this invention urea is converted to ammonia at the site where the combustion gases are being produced and there is no need to transport and store anhydrous ammonia or its aqueous solutions. Urea is the material that is shipped, stored and safely handled. The concept of this invention is also applicable to many other industrial uses of ammonia, such as pH adjustment, minimization of corrosion problems, heat treating of metal, etc.
The basic chemistry employed in the invention is a reverse of that employed in the industrial production of urea from ammonia and carbon dioxide and employs two reaction steps, as follows:
The first reaction in which urea hydrolyzes to form ammonium carbamate is mildly exothermic, while the second, in which ammonia and carbon dioxide are produced is strongly endothermic, with the result that the reaction to release ammonia and carbon dioxide requires heat and quickly stops when the supply of heat is withdrawn. Excess water promotes the hydrolysis reaction, the overall reaction for which is as follows:
xH
2
O+NH
2
CONH
2
→2NH
3
+CO
2
+x−1H
2
O [7]
In the thermal hydrolysis process of the invention, the liberation of ammonia commences at around 110C and becomes rapid at around 150° C. to 160° C., with or without catalytic assistance.
The generation of ammonia from urea is uniquely applicable to the control of nitrogen oxide emissions and the “conditioning” of combustion gas streams. The products of the hydrolysis are not foreign to those in combustion gas. The composition of combustion gas streams will typically have H
2
O concentrations ranging from 7% to 13% and CO
2
concentrations arranging from 6% to 14% with NO concentrations ranging from 20 ppm up to 2000 ppm. Hydrolysis of a 30% urea solution produces an off gas with a composition of 20.5% NH
3
, 10.2% CO
2
and 69.3% H
2
O (Molar). Ammonia for both SCR and SNCR systems for NO control is injected at NH
3
:NO ratios ranging from 0.5 to 2.0. With urea hydrolysis produced ammonia, the same NH
3
:NO ratios are required and the accompanying CO
2
and H
2
O will add only a small amount of additional material compared to what is already present in the combustion gas stream, since NO concentrations in flue gas are on the order of 100 times less than their CO
2
and H
2
O content. The CO
2
and H
2
O added in the urea hydrolysis products will have no impact on the operation of the combustion process. There is also no major impact on the NO control system. Typically, ammonia is diluted with air, recycled flue gas, or steam prior to injection to insure a larger gas flow so as to provide a uniform distribution of the ammonia being fed to the flue gas for reaction with the nitrogen oxides. There must be a close stoichiometric matching and intimate mixing and molecular contact of the introduced ammonia with all of the nitrogen oxide molecules within a very short time. Otherwise, there will be either a discharge of unreacted ammonia and/or unreacted oxides of nitrogen in the off-gas. The treatment stream is typically introduced by distribution grids covering the entire cross-sectional flow area or by high velocity injection nozzles.
The prior art relating to the hydrolysis of urea has been mainly concerned with two areas of application: (1) a reduction in the amount of urea remaining in the low concentration waste streams produced in urea manufacture by its conversion
Cooper Hal B. H.
Spencer, III Herbert W.
EC & C Technologies
Griffin Steven P.
Medina Maribel
Mueth Joseph E.
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