HK1183286B - Process of removing nox from a combustion gas stream - Google Patents

Abstract

The present application relates to a process of removing NOx from a combustion gas stream. The process can be achieved using gaseous ammonia by producing ammonia from urea. The process of producing ammonia from urea comprises: (a) heating a liquid phase reaction medium comprising an aqueous solution of urea, or amixture of urea, containing biuret or ammonium carbamate on site, in a hydrolysis reactor, etc., wherein while maintaining the pressure within the reactor in a range between a maximum and a minimum value and the operating pressure is changed within said range to a new set point in relation to the temperature, the heat input is adjusted to maintain the new pressure set point, with the temperature being allowed to adjust to that required to maintain the production rate of the ammonia-containing product gas, and iteratively following this procedure within said range to maintain the desired or predetermined amount of water in said reactor. The reactor can be operated using the above process so as to avoid condensation products which increase general corrosion rates in the system.

Description

Removal of NO from combustion gas streamsxMethod (2)

The present application is a divisional application of an invention patent application having an application date of 28/2008, application number of 200810009369.3 and entitled "improved method for producing ammonia from urea".

Technical Field

This process requires feeding or producing an aqueous urea solution in a hydrolysis column (reactor) in which the aqueous urea solution is heated under pressure to produce an ammonia gas mixture of ammonia, water and carbon dioxide. In the present invention, it is permissible to vary the reactor operating pressure and temperature to match the ammonia demand. The reactor pressure is set to vary with ammonia demand or reactor operating temperature. The operating pressure is varied so that during normal operation the concentration of these gases is in equilibrium with the composition of the solution in the reactor and is equal to the conversion product of the feed composition. In a typical process, the feed solution is in the range of 40 to 50 wt.% urea in water.

Background

Utilities and a/E enterprises have shown strong interest in the technology of urea-ammonia conversion. Utilities are increasingly adopting urea as a preferred alternative to anhydrous and aqueous ammonia for their SCR projects, with several major utilities utilizing urea-ammonia alternatives and many potential users actively assessing the systems of current and future projects. Such as ammonia purge S0₂Is assessing the use of urea-ammonia. Improvements in or relating to the inventionThis is made more feasible by providing a method that significantly reduces the heat requirement for ammonia production from urea. Other applications are also evaluating the use of ammonia production to reduce the on-site storage of ammonia.

Development response of Urea-Ammonia technology to Utility control of NO_XEmissions and implementation of a greatly increased demand for SCR projects that require ammonia as a reductant. Anhydrous ammonia is considered a hazardous and toxic chemical and is regulated by stringent regulations imposed by the EPA and OSHA. Aqueous ammonia, although in relatively small concentrations, poses similar risks and is also increasingly regulated or regulated by local authorities. The use of aqueous ammonia as a substitute for anhydrous ammonia significantly increases the operating costs of chemicals and energy, and raises transportation and storage requirements. These disadvantages are highlighted when relatively dilute aqueous solutions are considered.

Urea-ammonia and other immediate response urea-ammonia systems use urea as a feed chemical and thereby avoid the hazards associated with ammonia transport and storage altogether. This process responds immediately to the conversion of the urea solution into an ammonia mixture to satisfy the NO requirement_XControl the dynamic requirements of the system and other systems using ammonia.

At a urea feed concentration of 40%, the initial system was designed to operate at a temperature of 300 ° f and an operating pressure of 60psig to 80 psig. Higher urea feed concentrations reduce operating costs by reducing the energy required to evaporate the water in the feed solution. As the market matured, higher temperature designs and higher 50% urea feed concentrations may reduce capital costs for the system as well as reduce energy consumption.

Since the rate of hydrolysis of urea increases with excess water, it is essential to maintain the proper amount of water in the reactor liquid for this process. However, feeding 50% and more than 50% urea, the potential for ammonia production rate will be slowed down as the reduction of water used for the reaction must be considered. Spencer et al, article "design Consideration for Generation Ammonia front Ureafror NO_XControlwithSCRs ", AWMA2007Conference shows that the water available for reaction decreases with increasing temperature when the reactor is maintained at constant pressure.

Us patent 6,761,868 to Brooks et al describes a method of addressing this problem by controlling temperature and pressure. In the present invention, the reactor temperature is not controlled and the pressure is adjusted as a function of ammonia demand or temperature. The Brooks patent does show that pressure and temperature maintain the concentration in the reactor at the feed concentration, but does not show how the concentration in the reactor can be maintained at a desired value independent of the feed concentration, as shown in the present patent application.

U.S. patent No. 6,077,491 to Cooper et al discloses a process in which temperature and pressure are maintained by heat to generate a product gas, but does not show that the liquid concentration in the reactor is maintained at a nearly constant value.

Neither the Cooper patent nor the Brooks patent show the advantage of allowing the pressure in the reactor to vary with demand or with temperature.

The second problem is the additional heat required to maintain a higher urea concentration in the solution. At 40% urea concentration, the heat trace requirement of the urea feed system decreases but the reactor energy consumption increases.

The present invention determines the design factors that need to be considered to maintain water balance in the reactor. This has the advantage that the product gas composition remains nearly constant during changes in the gas production rate. Another advantage of the present invention is that the reactor can be operated such that the product gas temperature is always above the calculated gas dew point for the urea-ammonia production process for all demand conditions. This result occurs in lower corrosion and longer duration reactors.

Disclosure of Invention

Briefly, the present invention comprises a method for producing ammonia from urea, the method comprising:

(a) heating an aqueous solution of urea or a urea mixture containing biuret or ammonium carbamate in situ in a hydrolysis reactor to obtain a gaseous ammonia-containing product substantially free of urea, biuret, or ammonium carbamate, the temperature and pressure being maintained by heat input to the reactor;

(b) separating the gaseous ammonia-containing product from the liquid phase aqueous reaction medium at the operating pressure;

(c) retaining the liquid phase reaction medium in the reactor for further conversion to gaseous ammonia and carbon dioxide, and/or recycling at least a portion of said reaction medium back to the reactor-urea dissolver, or recycling the feed solution to the reactor for further conversion; and

(d) withdrawing the gaseous ammonia and carbon dioxide containing product separated in step (b) at a controlled rate to meet demand;

the improvement is that the temperature in the hydrolysis reactor is not controlled but is allowed to vary to match the demand for ammonia and the pressure is varied as a function of the demand for ammonia or the reactor operating temperature.

The invention further comprises a method of removing nitrogen oxides from a combustion gas stream, the method comprising:

(a) heating an aqueous solution of urea or a urea mixture containing biuret or ammonium carbamate in situ in a hydrolysis reactor to obtain a gaseous ammonia-containing product substantially free of urea, biuret, or ammonium carbamate, the temperature and pressure being maintained by heat input to the reactor;

(b) separating the gaseous ammonia-containing product from the liquid phase aqueous reaction medium at the operating pressure;

(c) retaining the liquid phase reaction medium in the reactor for further conversion to gaseous ammonia and carbon dioxide, and/or recycling at least a portion of said reaction medium back to the reactor-urea dissolver, or recycling the feed solution to the reactor for further conversion;

(d) withdrawing the gaseous ammonia and carbon dioxide containing product separated in step (b) at a controlled rate; and

(e) contacting the gaseous ammonia-containing product with a combustion gas stream at a rate that substantially matches a desired amount of nitrogen oxide removal in the combustion gas stream;

the improvement is that the temperature in the hydrolysis reactor is not controlled but is allowed to vary to match the demand for ammonia and the pressure is varied as a function of the demand for ammonia or the reactor operating temperature.

The temperature is matched to the demand for ammonia by regulating the heat added to the hydrolysis reactor. Using ammonia sources of demand (e.g. containing NO) from chemical reactions such as the consumption of ammonia_XCombustion gas flow) to open and close the reactor branch restrictions to allow product gas flow to match the ammonia demand.

When the demand for ammonia increases, such as when NO is present in the combustion gas stream_XWhen the amount increases, a valve in the ammonia branch line of the hydrolysis reactor is usually opened to increase the gas flow rate. Also, if demand is reduced, the valve is closed. An increase in demand at a given operating temperature will cause a decrease in pressure in the reactor and a decrease in demand will cause an increase in pressure.

In this regard, if the pressure is reduced, the heat input to the reactor is increased, and if the pressure is increased, the heat input is reduced. Because of the endothermic nature of the reaction process and the heat of water evaporation, a decrease in heat input will decrease the reactor temperature and an increase in heat input will increase the reactor temperature. Prior to the present invention, the pressure was maintained at a constant value as a function of operating temperature or demand by controlling the heat input to the reactor. In this improved process of the present patent application, instead of maintaining the pressure constant, the pressure is adjusted according to the ammonia demand or reactor temperature. In practice, this may be achieved by establishing a pressure set point. In the prior art, the pressure is maintained constant. However, in the improved process of the present invention, the pressure set point is varied with temperature or ammonia demand to maintain a desired or predetermined amount of water in the hydrolysis reactor. The heat input is adjusted to match the reactor pressure to the new set point, which in turn causes a temperature change. In practice, an iterative procedure is found that produces a nearly constant water balance in the reactor.

The following discussion is intended to better illustrate the significance of the improvements provided by the present invention.

Drawings

FIG. 1 shows a graph of ammonia production rate as a function of temperature and increasing exponentially with temperature.

Figures 2 and 3 show the equilibrium concentrations of urea-carbamate for 40% and 50% urea feed solutions.

FIG. 4 shows dew points at 40% and 50% feed solutions with and without fugacity. The dew point line is shown for reference.

Figure 5 shows a cross section of a typical reactor.

Fig. 6 shows the method control state for performing the present invention.

Figures 7 and 8 show the pressures required to maintain constant 20%, 30%, 40% and 50% concentrations in the reactor as a function of temperature at 40% and 50% feed solutions.

FIG. 9 shows an example of an operating line at a pressure set point.

Detailed Description

Water balance

The concentration of ammonia and carbon dioxide maintained in the reactor liquid solution is relatively low at operating temperatures of 250F to 400F and operating pressures of 30psig to 180 psig. Assuming ideal gas properties, the equilibrium of the solution can be understood using Raoult's law (Raoult's law) and Dalton's law (Dalton's law).

Raoult's law states that the vapor pressure of each component in an ideal solution is related to the vapor pressure of the individual components and the mole fraction of the components present in the solution. When the solution is in chemical equilibrium, the total vapor pressure of the solution is:

P_{solutions of}＝(P₁)_PureX₁+(P₂)_PureX₂.......

Wherein (P)_i)_PureIs the vapor pressure of the pure component and X_iIs the mole fraction of the components in the solution.

To calculate the water balance during ammonia formation, urea and ammonium carbamate dissolved in the reactor solution are considered to have had zero vapor pressure.

Dalton's law states that the total pressure of a gas mixture is the sum of the partial pressures of the gases in the mixture. Dalton's law can be expressed as P ═ P_A+P_B+ … wherein P_JIs the partial pressure of gas J and P is the total pressure of the mixture.

The partial pressure is defined as:

P_J＝y_JP

wherein y is_JIs the mole fraction of gas J, the molar amount of which is the ratio of the total number of moles of gas molecules present in the mixture. For purposes of this definition, the total pressure of a mixture of gases of any kind is the sum of its partial pressures.

P_{General assembly}＝P₁+P₂+P₃…＝y₁P_{General assembly}+y₂P_{General assembly}+y₂P_{General assembly}+…

Based on the above, the equilibrium concentration of urea-carbamate in the reactor solution can be estimated over a range of temperatures. Figures 2 and 3 show the estimated equilibrium urea-carbamate concentration in a urea-ammonia reactor at typical operating pressures and temperatures for 40% and 50% urea feed solutions.

These figures show that as the operating temperature of the reactor increases, the nominal operating pressure must be increased to maintain excess water in the reactor to continue to promote the hydrolysis reaction.

For a urea-ammonia reactor designed to operate at a maximum temperature of 315 ° f at 40% urea feed concentration and an operating pressure of 60psig, fig. 2 shows that the urea-carbamate concentration in the reactor will be about 50% of the solution. At minimum load (10%) and lower reactor temperature, the urea-carbamate concentration is reduced to about 20% at 60psig operating pressure.

In most utilities of the recycle facility, typical control ranges are about 33% to 100% load, in which case the reactor mother liquor urea-carbamate concentration varies from 38% to 50%, and excess water (62% to 50%) is maintained. For the commercial exemplary unit of allegheny energy, operating at a constant pressure set point of 80psig, 36% urea feed, and a typical maximum operating temperature of 295 ° f, the urea-carbamate concentration is in the range of 15% to 25%. During the first year of operation of another commercial plant, operating at a temperature of up to 305 ° f of a 40% feed solution and operating at a constant pressure set point of 60psi, the measured concentrations of urea, carbamate, and urea-formaldehyde compounds ranged from 44% to 57%. These two units are typical daily cycle boilers and the field measurements have good agreement with the estimated water balance.

At reduced operating pressures, the urea-carbamate concentration will increase and the water will decrease to the limit where water is insufficient. In this case, the rate of the hydrolysis reaction will also decrease and higher temperature operation is required to maintain ammonia production. For this case, it is desirable to increase the reactor operating pressure while taking into account the effect of pressure on the product gas dew point temperature as discussed in the next section.

Urea-ammonia reactor: dew point

Dew point factor

The urea-ammonia reactor should be set up to operate at a pressure with a dew point less than the normal minimum operating temperature to avoid condensation products that increase the overall corrosion rate in the system. The corrosion margin (corrosionallowance) included in normal designs allows for extended operation below the dew point, but it is recommended that operation be maintained above the dew point. The operating procedure of the present invention allows this to achieve NH over a larger operating range than previously available₃-CO₂-H₂The phase equilibrium factors of the O gas system allow the dew point temperature to be determined as a function of pressure and concentration as explained below.

Estimating the dew point of a gas mixture

The algorithm given below provides a suitable calculation for the NH of a gas mixture₃-CO₂-H₂Dew point temperature of O condensation.

At NH₃-CO₂-H₂The equation for property estimation can be seen in the technical literature of O and in articles specifically describing urea production and is valid over a wide range of temperatures and pressures, including those used in urea-ammonia systems.

The determination of the liquid phase concentration for the equilibrium of product gas and reactor liquid assumes that the presence of urea, carbamate and urea-formaldehyde species in the liquid does not incorporate other non-idealities, i.e., that the solution of urea in water interacts with ammonia in the same way as if only water were present in the liquid. To further refine these calculated values, the activity coefficients of urea and other ionic species in the liquid phase would have to be incorporated.

This algorithm is given by:

t and y are known and P and x must be found. Starting with the following equation:

x_iγ_iP_i ^sat＝y_iΦ_iP

wherein:

y_iis the concentration of a component in the gas phase

Φ_iIs the fugacity coefficient (function of y, P and T) of component i in the gas phase

P is the total pressure

x_iIs the concentration of component i in the liquid phase

γ_iIs the activity coefficient (function of x and T) of component i in the liquid phase

P_i ^satIs the saturated vapor pressure of component i at temperature T.

P^satEffective only when the component is below its critical state. Since ammonia exceeds its critical temperature and pressure, incorporation of Φ must be used_iEase equation of (c). An iterative solution was found using MATLAB6.5 to generate the data shown in fig. 4, fig. 4 containing a plot of dew point with and without fugacity (MATLAB6.5 with fugacity).

Dew point without fugacity

The dew point is also estimated using raoult's law simpler points without fugacity factors. The results of both are compared to the water dew point in fig. 4 below. As would be expected, the dew point of the urea-ammonia product branched off gas is less than pure water. When considering the fugacity to account for the interaction between water, ammonia and carbon dioxide, a slightly higher dew point is estimated.

Operating environment for urea-ammonia reactor

Most urea-ammonia reactors operate under a controlled constant liquid level, as shown in figure 5 below, resulting in a fixed liquid space and vapor space. The pressure in the reactor is controlled at nominally 40psig to 120psig while the temperature is varied with production rate within 250F to 315F.

The reactor liquid typically contains 15-50% urea, 0-18% higher urea derivatives and 3-6% ammonia. At temperatures in excess of 250 ° f, any ammonium carbamate formed immediately in the liquid decomposes to ammonia and carbon dioxide and thus very small concentrations (1-2%) of ammonium carbamate will be present in the reactor mother liquor. The balance of water.

From the plot in FIG. 2, it can be deduced that if the product gas is at 80psig (40% urea solution feed), the gas will not condense as long as 296F is exceeded. On the other hand, for a 50% urea solution feed, the gas composition was different and as long as the temperature exceeded 275 ° f, this product (at 80 psig) did not start to condense.

At low loads, the product gas temperature (250-. In this operating range, the weakly ammoniated solution in the water condenses out of the gas stream whenever it comes into contact with a colder surface. Inspection of the reactor interior has shown that the liquid condenses contaminating the gas-side reactor surfaces. These condensed vapors on the cold surface also cause a slightly higher corrosion rate on the gas side and should therefore be minimized, as is possible by adjusting the operating pressure. By using the procedure of the present invention, condensation is avoided.

By designing urea-ammonia reactors to operate at gas side conditions that avoid dew points and at temperatures less than 400 ° f, satisfactory corrosion rates of less than 3 mils/year have been achieved for the 316LSS material of the reactor vessel and piping and more exotic materials of some instruments and valves due to heat dissipation factors. For the present invention, the operating range for avoiding corrosion is increased.

Heat is provided to the reaction process to maintain the pressure, with the set pressure varying with temperature or ammonia demand. The pressure can also be set using the demand signal by using the relationship of reactor temperature as a function of ammonia demand, i.e. the temperature T will follow the relationship:

t ═ b/k (ln (G/a)), where G is the generation rate (number per unit time) and B, k and a are constants.

The pressure set point is adjusted according to the relationship P ═ f (t), where the function is selected to maintain a relatively constant concentration of dissolved solids in the reactor. Fig. 7 and 8 show the theoretical relationship of pressure and temperature to maintain a constant solution concentration. The function f (t) can be found from these curves or from the measured concentration as a function of temperature and pressure. Bench scale experimental data shows that operating around higher solution concentrations increases the given reactor liquid volume to a greater extent until the reaction becomes water deficient. Any solution above 78% will not have enough water to complete the hydrolysis.

Feeding a urea solution in or producing a urea solution in a hydrolysis reactor and quantitatively converting the urea into ammonia and carbon dioxide to produce ammonia, carbon dioxide and water vapor product gases stoichiometrically equivalent to the converted feed solution. Heat is supplied to maintain the temperature and pressure in the reactor. The improvement of the present invention is to adjust the pressure in the reactor as a function of the rate of formation or temperature in the reactor. The pressure is adjusted in such a way that the excess water in the reactor maintains a relatively uniform value as the rate of ammonia production changes, wherein the temperature is allowed to adjust to meet the ammonia demand.

The following examples are for illustrative purposes only.

Examples of the invention

Example 1

In example 1, a 40% feed solution was fed to the reactor or a 40% feed solution was produced in the reactor by feeding urea, concentrated urea, and a separate stream of water or steam. (anhydrous urea is typically pelletized and coated with formaldehyde.) a heater is provided to heat the solution in the reactor. The heat input is adjusted to maintain the pressure in the reactor at the set pressure set point. Initially, the pressure set point is set to begin at a value above 30 psia. When the reactor was heated to produce 30psia gas, the pressure set point was set at a pressure corresponding to the reactor temperature according to the data shown in figure 7. This can be done in a number of ways, such as by looking up a table or by calculating from an equation fitting the data in FIG. 7. The desired dissolution content, such as 30%, is initially selected for the process. This will provide a large excess of water to the urea and give the solution in the reactor a low salt-out temperature. The reactor branch valve is opened or closed to match the desired ammonia production rate. The heat input to the reactor is increased or decreased as needed to maintain the reactor at the pressure set point. Because higher temperatures are required to generate gas at higher loads, more heat will need to be input at higher demands and the temperature of the reactor will automatically adjust to maintain the pressure at the pressure set point. As the rate at which gas is withdrawn from the reactor changes, the pressure set point is adjusted to a new value, either according to the rate required or as the reactor temperature changes. As the temperature increases, the pressure set point increases, which will cause more heat to be input into the reactor, which will help quickly bring the system to the new higher production rate. When the demand decreases, the pressure will start to rise causing a decrease in heat input. This process uses heat because the reaction of this process is endothermic and because of the evaporation of water. When heat is used, the reactor temperature decreases. This will now result in a decrease in the pressure set point. Since the set pressure set point is lowered, control of the heat input will further reduce the heat input to the reactor, which will further allow the temperature to reduce the generation of reducing gas to match the new demand.

In this example, a constant feed concentration is fed into or produced in the reactor and the operating temperature of the reactor is allowed to vary to meet the ammonia demand.

There are several advantages to operating through the modes presented in this example. One advantage is that the branch gas composition can remain nearly constant as the ammonia demand increases and decreases. When the reactor is operated at constant pressure and constant feed concentration, there will typically be excess water in the product gas during load increase and insufficient water in the product gas during load decrease until a new water balance is achieved in the reactor. Only in a gradual change does the water remain in equilibrium with the fixed pressure set point. Because the pressure is allowed to adjust to match the operating temperature, the gas composition exiting the reactor can be maintained at a composition that is an equilibrium value for the feed concentration being fed or produced in the reactor. For this process, a more constant water balance is maintained in the reactor to produce a more constant branched gas composition without the need to change the feed concentration as described by Brooks et al. Changing the feed concentration to match the demand is difficult due to the time and heat required to mix and dissolve the urea with the water.

Another advantage is that a greater dynamic control range is obtained for a given branch valve. The basic valve flow equation isWhere Q is the flow rate and ap is the pressure drop across the valve and K is a constant related to the valve coefficient Cr, such that K varies with valve opening. For example, for a branch control valve, where K can vary by a factor of 10, the flow rate can only be precisely controlled within a range of a factor of 10. For example, with a usable reactor size, a temperature of 311 ° f is required for 100% flow to generate ammonia, and according to figure 7, a pressure of 80psig is required to maintain 30% water balance. It is now shown that the required amount is reduced by a factor of 15.7. The flow is now below the control range of the valve. At the new flow rate, the temperature and pressure will decrease, so the pressure will decrease to 30psig and the temperature will decrease to 270 ° f. At 100% flow, the desired K value at 80psig was 11.2. With the flow reduced, if the pressure is maintained at 80psig, a K value of 0.71 would be required and a different valve would be required. By allowing the pressure to decrease to 30psig in the operating mode as described in this example, a K value of 1.2 is required, which would still be within the dynamic range of the valve of factor 10 in this example. In this example, assume that the valve bank amplifies the gas pressure and ignores the temperature effect.

If the demand signal is a feed forward signal, such as may be the case for mobile applications, the pressure in the reactor may be measured and the valve opening required for the desired flow calculated.

Another advantage of this process is the ability to keep the product gas temperature above the dew point of the gas. For example, the dew point of the product gas is 296F at 80psig for a 40% feed solution. When demand decreases, in this example, the gas temperature will decrease to 270 ° f, which is below the dew point. This can lead to corrosion of the gas side of the reactor. But if the pressure is reduced to 30psig with demand or reactor temperature as in this example, the product gas, now at 270F temperature, remains above the dew point because the dew point of the product gas is now 256F due to the pressure reduction.

Example 2

Example 2 is similar to example 1 with the exception that the pressure ammonia demand load curve in this case is found from the temperature demand relationship in figure 1 and the temperature pressure relationship for constant water balance in the reactor shown in figures 7 and 8. Note that using the procedure described in this patent, similar curves and relationships can be found for other feed concentrations or one skilled in the art can determine the relationship for setting the pressure set point as a function of temperature and ammonia production rate from measurements of reactor solution concentration at various pressures and temperatures operating at fixed feed concentrations. In practice, the operating pressure set point for each fixed load can be manually adjusted and the reactor solution concentration measured. A measure of solution concentration can be obtained using density measurements. In this example, the pressure set point is set by replacing the reactor temperature with the demand signal. This is advantageous over temperature due to the mass of the reactor and the possible lag in demand changes of the liquid in the reactor. It should be noted that because the reaction process uses heat to hydrolyze the urea and water evaporates, the reactor temperature varies closely with the amount needed.

Example 3

Several users of the current technology use a nominal 70% urea solution obtained before the granulation or prilling process in urea production. Typically 70% is diluted to 40% to 50% feed concentration. In this example 3, 70% of the feed was fed to the reactor. The reactor was initially charged with water, so initial filling with 70% solution would produce an initial solution in the reactor showing 40% urea. The product vent gas will be equivalent to 70% of the feed composition. In this example the reactor pressure was adjusted so that the water balance in the reactor was maintained at a concentration of less than 40% of the feed concentration. The advantage of this method is that the amount of heat to evaporate the water is significantly reduced. In this case, the heat consumption was reduced from 4526 btus/lb ammonia to 2193 btus/lb ammonia. The disadvantage is that the salting out temperature of the 70% feed solution is 133 ° f compared to the 30.5 ° f salting out temperature of the 40% feed solution. If the feed solution is maintained at an elevated temperature, such as 133 ° f, part of the urea will be converted to biuret. The biuret in the reactor feed increases the operating temperature required for the ammonia production reactor. The relationships shown in fig. 7 and 8 show that higher operating pressures are required to maintain water balance. According to example operations, operating pressures may be automatically modified to account for higher biuret contents.

Example 4

In this example 4, a simple operating line is shown in fig. 9. For example, a straight line is shown. A power function may also be used. 4 set points were established, low pressure (p1) setting and low temperature (t1) and high pressure (p2) and high temperature (t2), (below t1), pressure was maintained at (p1), and above (t2), pressure was maintained at (p 2). In between, the pressure set point pressure is the calculated variable between the low and high points. P_{Setting up}P1+ (p2-p1) × (t-t1)/(t2-t 1). Heat is applied to this process to maintain the reactor at the pressure set point. The operating line in this example was chosen to closely follow the pressure-temperature relationship found by raoult's law to maintain a 20% solution in the reactor. In practice, the operating data indicate that higher concentrations should be maintained to increase the reaction rate. Theoretically, the solution concentration must be maintained below 76.9% to have enough water to complete the hydrolysis of urea in the reactor. In practice, lower concentrations must be maintained. The operating data show that the optimum is in the range of 30% to 50% depending to some extent on the urea purity. The procedure described in this patent allows the operator to maximize the concentration of the solution in the reactor and maintain a nearly constant concentration within the desired ammonia production range. This allows fluid balance and water balance to be maintained with better control than previously available.

The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications within the scope of the invention may be apparent to those having ordinary skill in the art.

Claims (6)

1. Removal of NO from a combustion gas stream with gaseous ammonia by production of ammonia from urea_xThe process for producing ammonia from urea, comprising:

(a) heating in situ in a hydrolysis reactor an aqueous urea solution or an aqueous solution of a urea mixture comprising biuret or ammonium carbamate to obtain a product containing pressurized gaseous ammonia and carbon dioxide substantially free of urea, biuret or ammonium carbamate;

(b) separating the pressurized gaseous ammonia and carbon dioxide-containing product from the liquid-phase aqueous reaction medium at the operating pressure;

(c) retaining the liquid-phase aqueous reaction medium in the reactor for further conversion to gaseous ammonia and carbon dioxide, and/or recycling at least a portion of the liquid-phase aqueous reaction medium back to the reactor-urea dissolver, or recycling feed solution to the reactor for further conversion;

(d) withdrawing the pressurized gaseous ammonia and carbon dioxide-containing product separated in step (b) at a controlled rate; and

(e) contacting the gaseous ammonia and carbon dioxide-containing product with the combustion gas stream at a rate that substantially matches nitrogen oxides in the combustion gas stream that are desired to be removed;

wherein when the pressure within the reactor is controlled in the range of 40psig to 120psig while varying the temperature with production rate in the range of 250 ° F to 315 ° F and the operating pressure is changed within the range to a new set point associated with the temperature, the heat input is adjusted to maintain the pressure at the new set point as the temperature is allowed to adjust to the temperature needed to maintain the production rate of ammonia-containing product gas, and this procedure is iteratively followed within the range to maintain a desired or predetermined amount of water in the reactor.

2. The method of claim 1, wherein the feed solution is in the range of 20% to 72% urea.

3. The method according to claim 1, wherein the solution is produced in the reactor by feeding urea or concentrated urea and a separate water source.

4. The process of claim 1, wherein solution is withdrawn from the reactor for backup or mixed to make a urea feed solution for the reactor.

5. The method of claim 1, wherein the aqueous solution of urea or urea mixture does not have formaldehyde or formaldehyde compounds.

6. The method of claim 1, wherein the aqueous solution of urea or urea mixture has formaldehyde or a formaldehyde compound.