DIRECT INJECTION OF AQUEOUS UREA

The present disclosure relates generally to a method and apparatus for reducing emissions of nitrogen oxides (NO_x) from a combustion source. More particularly, to injecting a urea solution directly into the flue gas stream of a coal-fired power plant that utilizes Selective Catalytic Reduction (SCR) to lower NO_xemissions. This invention herein eliminates the vast equipment, risks of using anhydrous ammonia, and costs associated with using aqueous ammonia, or complicated systems to hydrolyze urea, before injection.

One process for lowering NO_xemissions is SCR, which involves chemically converting NO_xto elemental nitrogen by injecting a reagent, often anhydrous ammonia, aqueous ammonia, or aqueous urea.

The following equations describe SCR with urea as the reagent:

(NH₂)₂CO+H₂O→2NH₃+CO₂  Eq. 1:

2NO+2NH₃+½O₂→3H₂O and tm Eq. 2:

3NO₂+4NH₃→3½N₂+6H₂O   Eq. 3:

The prior art contains various methods of SCR in combustion sources such as diesel engines, natural gas power plants, and solid fuel combustion units, for example utility boilers. Prior to the invention herein, a successful direct injection of aqueous urea (DIAU) for SCR has been achieved in diesel engines and natural gas turbines, but not solid fuel combustion units. Diesel engines have relatively small ducts and therefore do not face the same mixing issues as solid fuel combustion units. Diesel engines and natural gas turbines have relatively low NO_xlevels compared to coal-fired units as well.

Reagents include aqueous ammonia (U.S. Pat. No. 3,900,554), a mixture of Na₂CO₃and urea (U.S. Pat. No. 4,844,915), a mixture of aqueous urea and a hydrocarbon above 1,600 degrees F. (U.S. Pat. No. 4,719,092), aqueous urea in a hydroxylic solvent above 1,300 degrees F. (U.S. Pat. No. 4,208,386), aqueous urea, but only into a natural gas power plant (UMICORE Catalyst USA, LLC at Reinhold Environmental Conference, February 2018), a mixture of ammonia and urea in a gas stream of 800 degrees C. to 1,000 degrees C. (U.S. Pat. No. 5,399,326), and aqueous ammonia or urea and a gas (U.S. Pat. No. 5,478.542). Others have used various means such as bypass ducts to convert ammonia to urea (U.S. Pat. No. 7,090,810) or slip streams (U.S. Pat. No. 8,815,197) in which to inject aqueous urea.

Aqueous urea, however, is preferred as a sole reagent for SCR because unlike ammonia it is safe and easier to handle. Nevertheless, the injection of urea has historically required complex and expensive decomposition means to convert the aqueous urea to ammonia gas before injection. The direct injection of urea through an ammonia injection grid (“AIG” or “grid”) has been historically unsuccessful due to formation of deposits in the grid, which plug the grid. In fact, prior art states that the “direct injection of aqueous urea through a grid has generally not been practical due to the formation of deposits in the grid from the incomplete decomposition of urea in the gird” (U.S. Pat. No. 8,815,196). Difficulties also stem from the need to vaporize the urea before it hits the walls of the duct to prevent corrosion of the duct, and from difficulties associated with utilizing a DIAU at low load and low gas temperatures.

This led to attempts to inject at the walls of the duct, which have only been successful on small SCR applications such as diesel engines. This is because of insufficient distribution of the urea, in solid fuel combustion units, into the full flue gas flow and thus insufficient decomposition of the urea to ammonia before reaching the SCR catalyst. The stratification of the NO_xin the flue gas that occurs before injection causes a lack of sufficient mixing when the ammonia (after the urea is converted to ammonia) reacts with it, meaning uniform distribution of ammonia at the SCR catalyst is not established and catalysis is inefficient.

The urea to ammonia conversion processes in solid fuel combustion units in the prior art are also costly to maintain and can use large amounts of high energy steam.

Extensive ammonia vapor piping used also presents safety concerns due to high risk of ammonia leaking from the piping. On the contrary, a DIAU is extremely safe.

For all the aforementioned reasons among others, the prior art lacks a successful DIAU into a solid fuel combustion unit. The disclosure herein contains a method and apparatus for a direct injection of urea into such a unit, the flue gas stream of a coal-fired power plant, avoiding all of the aforementioned expense and safety concerns.

It is an object of the present invention to provide a system and method for a DIAU into a solid fuel combustion unit, such as the duct of a coal-fired power plant comprising a furnace, in order to lower NO_xemissions via SCR. The disclosure herein eliminates the aforementioned plugging, distribution, and decomposition issues experienced, while providing for a system without use of ammonia thus avoiding its intricate handling necessities and safety risks. Injection occurs downstream of the economizer, at a lower temperature range.

Adequate mixing of the flue gas stream, and of the urea injected within the flue gas stream, is critical to achieving efficient conversion of decomposition of the urea to ammonia before reaching the SCR catalyst. The stratification of the NO_xin the flue gas that occurs before injection causes a lack of sufficient mixing when the ammonia (after the urea is converted to ammonia) reacts with it, meaning uniform distribution of ammonia at the SCR catalyst is not established and catalysis is inefficient.

A preferred embodiment disclosed herein allows for the achievement of adequate mixing due to the arrangement of turbulence producing devices in the duct. However, other arrangements could also achieve adequate mixing and thus have adequate distribution of ammonia at the SCR catalyst.

Another essential component of the preferred embodiment is an atomizing urea solution injection nozzle at the end of a lance, the lance comprising an inner pipe within a larger outer pipe. Blanketing air is supplied in between the two pipes of the lance. This maintains the exterior cleanliness of the attached nozzle and maintains the temperature of the urea solution before it is injected, thereby preventing the urea solution from scaling (precipitating out of solution) but still allowing the urea to vaporize before reaching the duct walls. The minimum required pressure of the blanketing air is above the pressure of the flue gas. The preferred embodiment has significantly fewer nozzles than AIG systems, also decreasing opportunities for plugging.

These features allow for a safe DIAU using a much simpler system lacking an AIG, involved piping, and bypass ducts. The following detailed description of the preferred embodiments in conjunction with the drawings and claims elucidates these and other features of the present application.

FIG. 1 shows urea solution system (100), comprising a urea solution mix tank (3), urea prill (1) and condensate (2) from condensate tank (9). This preferred embodiment does not necessitate a mixer but the use of mixers would suffice. A mix tank pump (4) pumps the resulting urea solution (11) to a urea solution tank (6) and ultimately to direct injection system (200). The concentration of the urea solution (11) in this preferred embodiment is 32.5%.

A recirculation line (5) and mix tank pump (4) aid with mixing of the urea solution (11) in urea solution mix tank (3). A flushing pump (10) is preferred to flush out system (100). A urea solution pump (7) supplies urea solution (11) to system (200). A urea solution recirculation line (8) protects the urea solution pump (7).

FIG. 2 shows the urea solution (11) injected into flue gas (12) in flue gas duct (13) of direct injection system (200). Direct injection system (200) comprises a solid fuel combustion unit, which in the preferred embodiment is a coal-fired power plant.

This injection occurs after flue gas (12) has been passed over first turbulence producing device, (14) and (15), to generate adequate mixing of the flue gas (12). Other arrangements of turbulence producing devices can suffice, as long as adequate mixing results.

The urea solution (11) is injected via a urea solution injection nozzle with a lance further comprising an outer larger pipe and a smaller inner pipe at the end (17). Blanketing air (22) having a velocity equal to that of urea solution (11) to be injected travels through lance (17), between the two pipes of lance (17), to maintain the cleanliness of the tip of the lance's nozzle. This also prevents urea solution (11) from scaling (precipitating) on the inner pipe of lance (17), thereby preventing plugging. Blanketing air (22) must be above the pressure of flue gas (12). Once urea solution (11) enters flue gas duct (13), the thermal energy of flue gas duct (13) heats the urea solution to a temperature adequate for the catalysis reaction using a SCR (20).

The preferred embodiment contains one nozzle per 50-100 megawatts (MW) of generation, as well as flow rate of 0.25 gallons per minute to 1.5 gallons per minute, which also prevents plugging due to the increased flow through fewer nozzles. An opening of 0.05 inches to 0.1 inches per nozzle is preferred for minimal plugging.

Proximate to lance (17) is a second turbulence producing device, (16) and (18), which provides for adequate mixing of urea solution (11) in flue gas (12). Turning vanes (19) also help reduce pressure drop and help provide even distribution before the SCR (20). Other arrangements of turbulence producing devices can suffice, as long as adequate mixing results. The urea solution (11) is converted to ammonia using only the heat in the flue gas duct (13). Then the flue gas, comprising ammonia, reaches SCR (20), where NO_xis reduced, and the flue gas (12) and its components are ultimately discharged to the atmosphere (21).

Experimental Data

Data from a continuous emissions monitoring system (CEMS) supports the conclusion that there is a 3% increase in process efficiency, resulting in the same NO, removal rate using less urea as the prior ammonia system. The 3% increase in efficiency may be due to blow down that occurred in the prior ammonia system, as the DIAU disclosed herein has no blow down.