HK1151492B - Selective catalytic nox reduction process and control system - Google Patents

Description

Selective catalytic reduction of NOxMethod and control system

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. provisional application SN 60986917, filed on 9.11.2007, the entire disclosure of which is incorporated herein by reference.

Technical Field

The present invention relates generally to the efficient use of urea for NO_xAnd more particularly to feeding the gasification products of urea to a plurality of turbine power plants from a single plant that converts urea to an ammonia-containing SCR reagent while maintaining the ability to fully control the individual SCR plants without excessive use of reagents or loss of pollution control effectiveness.

Background

It is economical to use turbines to generate power in many situations where more conventional power plants cannot be employed. They have the great advantage that they usually produce NO at a minimum_xAnd may utilize fuels such as biogas and landfill produced gases (digesterand landfillgases). However, they do produce significant amounts of NO that can be limited_xTherefore, efforts are being made to reduce emissions to even lower levels.

Biogas and landfill produced gas is a gaseous by-product from sewage treatment or anaerobic decomposition of landfill organic materials, which is composed mainly of methane and carbon dioxide. These are not clean fuels according to the standards generally considered by productive life. Trace amounts of harmful compounds are typically found in these gases and generally include hydrogen sulfide, ammonia, and acid gas forming compounds. In addition, there are certain compounds that may be present in the gas and are known to block NO_xReducing the catalyst and shortening the life of the turbine. The net result is that the use of such low cost fuels can lead to additional costs for the user due to shortened turbine life, corroded piping and contaminated catalyst.

Unless these costs can be recovered by taking advantage of the good energy value of these fuels, the environment will be affected and their energy value is likely to be replaced by imported petroleum. Advantageously, therefore, all the costs are recovered by the user of the device by: combustion in combustion plants, e.g. turbines, to generate electricity or to directly drive equipment, and by means of suitable NO_xAnd (5) carrying out treatment by a reduction technology. NO_xThe best of the reduction technologies is SCR if it can be implemented efficiently in a flexible system like the power grid energy demand without storing dangerous ammonia gas.

SCR has been demonstrated for NO_xReduction is very efficient and the SCR device can usually be scaled up to the size required by the turbine. However, SCR devices typically require the use of ammonia as a reducing agent, and a common problem is that storage of ammonia is difficult and dangerous, particularly in densely populated areas. Thus, for example, as described in U.S. Pat. No. 7,090,810 to Sun et al and U.S. Pat. No. 6,077,491 to Cooper et al, the use of ammonia generators is generally required, but their control of multiple turbine devices is not discussed, and in certain installations, such as turbines used with biogas and landfill generated gases, are more expensive or difficult than economically permissible.

Biogas and landfill gas, which are more commonly produced by flame combustion due to their low quality, can create costs that are difficult to recover. The economic problem is particularly great for these gases. Devices requiring more than one turbine currently do not benefit from a single urea-based ammonia SCR unit. Unfortunately, it has been seen that using a single urea conversion unit for each turbine is the most practical approach.

However, a single ammonia gas generator is not practical due to fluctuations in demand for turbine power generation over time (daily and seasonal fluctuations). SCR devices typically employ an ammonia gas injection grid (AIG), which is essentially an array of distribution tubes arranged with apertures through which ammonia gas is preferably emitted along with a carrier gas to provide sufficient gas momentum at each location to achieve uniform distribution of ammonia gas. When demand is low, ammonia is immediately reduced for one SCR device of one turbine, producing a temporary excess of ammonia for the other device. For the safety of ammonia, the storage tank is not needed, and no good method for regulating fluctuation exists at present. The distribution will be adversely affected or excess ammonia will be supplied and cause ammonia leakage.

There is a need for methods, devices and systems for efficient utilization of urea for NO_xMore specifically for feeding the gasification products of urea from a single urea gasification unit to multiple turbine power units.

There is a particular need for a system that converts urea to ammonia and maintains the ability to fully control individual SCR devices without using excess reagents or losing the effectiveness of pollution control.

Disclosure of Invention

The present invention provides a method of reducing the concentration of nitrogen oxides in combustion gases from multiple turbines or other combustors, wherein each turbine or other combustor has an associated selective catalytic reduction of NO with efficient utilization of gasified urea_xA catalyst, the method comprising: heating aqueous urea under temperature and pressure conditions for a time effective to gasify the urea and water by mixing the aqueous urea with a heated gas stream in an amount relative to the amounts of urea and water to produce a first gasified reductant stream comprising a predetermined mass and concentration of ammonia; monitoring a urea demand at each of a plurality of turbines; feeding the first stream of gasified reductant to each turbine at a rate sufficient to supply gasified reductant to supply the monitored demand to selective catalytic reduction of NO associated with each turbine_xA catalyst; determining an amount of carrier gas required to achieve a predetermined degree of mixing of the gasification agent with combustion gases produced at each turbine; based on the determination, the determination is made,mixing a predetermined amount of carrier gas with the first vaporized reductant stream to provide a catalyst feed stream associated with each turbine; and at each turbine, effectively reducing NO in the effluent from each turbine_xThe associated catalyst feed stream is introduced into the combustion gas upstream of the catalyst by an ammonia gas injection grid under concentration conditions.

The invention also includes systems and apparatus for practicing the methods as described and illustrated, as well as reasonable variations thereof.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention and, together with a general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. As shown throughout the drawings, like reference numerals designate like or corresponding parts.

FIG. 1 is a schematic flow diagram of a preferred embodiment of the method and system of the present invention.

FIG. 2 is a schematic illustration of a single turbine having an SCR device supplied with vaporized reductant according to the present disclosure.

FIG. 3 is a schematic illustration of one form of ammonia injection grid whose operation in an SCR system may be improved in accordance with the present invention.

Detailed Description

In describing the invention, reference is made to the accompanying drawings, in which a preferred embodiment is shown by way of example in fig. 1. The drawings and the method represented therein will be briefly described, wherein the sensors, pumps, indicators, transmitters, valves, pumps, etc. known to those skilled in the art of engineering systems will not be described in detail. The various reference symbols used in the drawings have the following meanings: TE temperature element/sensor, LT level transmitter/sensor, VFD variable frequency drive, SC speed control, PI pressure indicator/sensor, TI temperature indicator/sensor, FT flow transmitter, I/P current to pressure converter, FIT flow indicating transmitter, M flow meter, PLC programmable logic controller.

Referring to FIG. 1, a feed line 12 is connected to a urea tank 14 to maintain an adequate supply of an aqueous solution of a chemical such as urea, as described in U.S. Pat. No. 7,090,810, the disclosure of which is incorporated herein by reference. The method is effective for urea, but may utilize other NO that can generate an ammonia-containing reactant gas upon heating_xAnd (3) reducing the reagent. As will be clear from the following, when some of these reagents are gasified, the reactant gas also contains HNCO which reacts with water to convert to ammonia and carbon dioxide. The advantage of the invention is that such a situation can be easily achieved without carrying out NO with the attendant risk of clogging nozzles and other equipment_xPre-hydrolysis of the reducing agent. The term "gasification" refers to the conversion of substantially all of the urea to a gas such that no significant amount of dissolved or free solids or liquids contact and contaminate the SCR catalyst.

The term "urea" is meant to include agents equivalent to urea in the sense that they form ammonia and HNCO upon heating, whether or not they contain significant amounts of pure chemical urea in the form introduced into the combustion gases; however, agents equivalent to urea often contain measurable amounts of urea in their commercial forms and thus comprise urea. In the presence of NO which can be gasified_xAmong the reducing agents are those comprising a member selected from the group consisting of: melamine monoamide; melamine diamide; ammonium carbonate; ammonium bicarbonate; ammonium carbamate; ammonium cyanate; ammonium salts of inorganic acids including sulfuric acid and phosphoric acid; ammonium salts of organic acids including formic acid and acetic acid; a biuret; a triurea; cyanuric acid; isocyanic acid; urea formaldehyde; melamine; tricyanourea, and mixtures of any of these. But also can be used in different shapesOther NO to HNCO but decomposed to a gas mixture containing hydrocarbons_xAnd (3) reducing the reagent. Among such amines are various amines and their salts (especially their carbonates), including guanidine, guanidine carbonate, methyl amine carbonate, ethyl amine carbonate, dimethyl amine carbonate, hexamethylene amine; hexamethylene carbonate amine; and urea-containing by-product waste from chemical processes. The amine having a higher alkyl group may be such that the released hydrocarbon component does not interfere with NO_xThe degree of reduction was used. Thus, the term "urea" is meant to include all commercial and equivalent forms of urea. Typically, commercial forms of urea consist primarily of urea, containing 95% by weight or more urea. This relatively pure form of urea is preferred and has many advantages in the process of the present invention. It is preferably provided to the tank 14 at a concentration of about 10% to about 50%, about 30% to about 35%.

The configuration of the level sensor and the feed pump ensures that there is always sufficient urea solution in the tank 14 to meet the programmed requirements. The urea solution flows from tank 14, which may be heated for cryogenic operation, via line 16 assisted by metering pump 18 and flow monitor 20 to line 22 for introduction through injector nozzle 24, which injector nozzle 24 vaporizes the aqueous urea solution by spraying it in vaporization chamber 28 with air from line 26. The gasification of urea is promoted by heated air from line 34, line 34 sending air to heater 36 and to line 38 and auxiliary heater 39 to be supplied into chamber 28. In chamber 28, the aqueous urea solution is heated at a temperature (e.g., from about 175 deg.C to about 650 deg.C) and a pressure (e.g., near atmospheric pressure, i.e., from about 0.5 to about 1.5atm) for a time effective to vaporize the urea and water by mixing the aqueous urea with a heated gas stream from line 38 in an amount relative to the amounts of urea and water to produce a first vaporized reductant stream comprising a predetermined concentration of ammonia. Typical gas streams contain from about 0.5 wt% to about 5 wt% ammonia gas. In certain embodiments, an air line 30 may be used to blow air into the nozzle 24 during cleaning. The vaporized urea solution (also referred to as vaporized reductant) is drawn from the chamber 28 via line 40 for distribution as a first vaporized reductant stream to a supply line 42 (described in detail below) serving an SCR device associated with each turbine.

Indicating NO in combustion gases from a plurality of turbines by sensing at least one of the combustion gases from the turbines by sensors on each of the turbines_xAnd the demand is determined by a control programmable logic controller 43 or other similar device to monitor the demand for urea. The controller may feed forward with or without feedback. The first stream of vaporized reductant is fed to each turbine via a separate line 42 utilizing control valves and flow monitors, shown generally at 44 and 44', respectively, sufficient to provide vaporized reductant for selective catalytic reduction of NO associated with each turbine_xThe catalyst provides the rate at which the demand is monitored. Since turbines have different NO due to their load, fuel supplied or other factors_xReduction requirements, and since the vaporized reductant must be uniformly dispersed within the combustion gas, the typical Ammonia Injection Grid (AIG) does not provide the desired distribution with efficient reagent utilization.

FIG. 2 is a schematic illustration of a single turbine with an SCR device supplied with vaporized reductant according to the present disclosure. The turbine 70 is fed fuel from line 72 and air from line 74 and produces combustion gases 76 which gases 76 pass through an SCR unit 78 in which they are treated according to the invention and then are discharged to an exhaust pipe 80.

FIG. 3 shows one form of ammonia gas injection grid (AIG)62, the operation of which in an SCR system may be improved in accordance with the present invention. Such a cascade plate typically has an array of tubes 63 with an array of apertures 64 through which the gasification reagent is distributed from the associated turbine to the combustion gases. The SCR unit includes an AIG ammonia injection grid 62 which is fed with gasification reagent via line 60.

An in-line mixing device 65 is typically provided to ensure good mixing of the gasification reagent with the combustion gas. Then, at that point, the system is setPassing the gas at temperature through a series of stages effective for NO_xA selective catalytic reduction catalyst of (1).

Referring again to fig. 1, it can be seen that the vaporized reagent is passed via line 40 to separate line 42 to mix with a sufficient amount of carrier gas so that the correct metered amount of vaporized reagent can be utilized and mixed with sufficient carrier gas to achieve the correct mass flow rate and velocity profile of the reducing gas when introduced into the SCR device 78 by way of the injection grid 62.

A separate supply system is provided to supply carrier gas to each individual turbine. Fig. 1 shows air being introduced to heater 48 and line 50 via line 45, blower 46. The rate of supply and the degree of heating can be controlled by suitable sensors via a controller 43 using meter air 43' or equivalent. Because the load on the one or more turbines may not be high enough to require that sufficient ammonia gas be maintained at the appropriate temperature through the flow feed line 42, it is important that the heater 48 be employed to maintain the temperature of the gasification agent in line 60.

To achieve the advantages of the present invention, controller 43 or other logic device will determine the amount of carrier gas needed to achieve a predetermined degree of mixing of the gasification reagent with the combustion gases produced by each turbine. Based on such a determination, then, predetermined amounts of carrier gas from supply line 50 and separate line 52 are mixed with the first vaporized reductant stream from lines 40, 42 to provide a catalyst feed stream associated with each turbine and which may be fed to the turbine via line 60. Then, in each turbine 70, the NO in each turbine effluent is effectively reduced_xThe associated catalyst feed stream 60 is introduced to the combustion gas upstream of the catalyst 66 through the ammonia injection grid 62 at concentration conditions. Similarly, as with the supply of the first stream of vaporized reductant via the single line 42, the line 52 may utilize control valves and flow monitors, shown generally at 54 and 54', respectively, to control the rate sufficient to supply the catalyst feed stream having the mass and flow rate required by each turbine.

The present invention has the advantage of being able to feed the gasification products of urea and has the ability to fully control individual SCR devices without over-dosing the reagents or losing pollution control effectiveness. Controller 43 may determine the amount of reagent required for each turbine to control NO_xThe vaporized urea is discharged and then directly mixed with the correct amount of carrier gas to effectively operate each individual SCR device despite variations in demand between turbines. In this way the gasification unit can be suitably controlled to provide the required urea without the need to store large inventories of ammonia-containing gas to correct for fluctuations in demand. Although the description has been given for the sake of specific effectiveness and importance of the invention in the context of a turbine, it will be clear to a person skilled in the art that the advantages thereof may be extended to other types of burners, including furnaces, digesters, engines, incinerators, etc. Another advantage of the present invention is that the temperature of the gasification agent in line 60 can be maintained at a sufficient temperature to avoid condensation or chemical reactions even when the load of the one or more turbines may not be high enough for adequate mass flow through the feed line 42.

The above description is for the purpose of teaching the person skilled in the art how to practice the invention. It is not intended to detail all of those obvious modifications and variations which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the invention which is defined by the following claims. The claims are intended to cover the claimed components and steps which are effectively satisfied in any order, unless the context clearly indicates otherwise.

Claims (4)

1. Method for reducing the concentration of nitrogen oxides in combustion gases from a plurality of burners, wherein each burner has an associated selective catalytic reduction of NO with efficient use of gasified urea_xA catalyst and the method is capable of being controlled to provide urea on demand without the need to store large inventories of ammonia-containing gas to accommodate seasonal and daily and inter-combustor demand fluctuations, the method comprising:

a. heating aqueous urea under time, temperature and pressure conditions effective to vaporize the urea and water by mixing the aqueous urea with a heated gas stream in an amount relative to the amounts of urea and water to produce a first vaporized reductant stream comprising a predetermined mass and concentration of ammonia;

b. monitoring a demand for gasified urea at each of a plurality of combustors;

c. feeding the first stream of gasified reductant to each burner at a rate sufficient to supply gasified urea to be supplied to the selective catalytic reduction of NO associated with each burner at the monitored demand_xA catalyst;

d. determining a required amount of carrier gas to achieve a predetermined degree of mixing of the gasified urea with combustion gases produced by each burner;

e. mixing a predetermined amount of carrier gas with the first vaporized reductant stream to provide a catalyst feed stream associated with each burner based on the determination; and

f. at each burner, in the effective reduction of NO in the combustion gases from each burner_xThe associated catalyst feed stream is introduced into the effluent upstream of the catalyst through an ammonia injection grid under concentration conditions, thereby accommodating seasonal and daily and inter-burner demand fluctuations without the need to store large inventoried amounts of ammonia-containing gas.

2. Method for reducing the concentration of nitrogen oxides in combustion gases from a plurality of turbines, wherein each turbine has an associated selective catalytic reduction of NO with efficient use of gasified urea_xAnd the method is capable of being controlled to provide urea on demand without the need to store large inventories of ammonia-containing gas to accommodate seasonal and daily and inter-turbine demand fluctuations, the method comprising:

a. heating aqueous urea under time, temperature and pressure conditions effective to vaporize the urea and water by mixing the aqueous urea with a heated gas stream in an amount relative to the amounts of urea and water to produce a first vaporized reductant stream comprising a predetermined mass and concentration of ammonia;

b. monitoring a demand for gasified urea at each of a plurality of turbines;

c. feeding the first stream of gasified reductant to each turbine at a rate sufficient to supply gasified urea to be supplied to the selective catalytic reduction of NO associated with each turbine at the monitored demand_xA catalyst;

d. determining an amount of carrier gas required to achieve a predetermined degree of mixing of the gasified urea with combustion gases produced by each turbine;

e. mixing a predetermined amount of carrier gas with the first vaporized reductant stream to provide a catalyst feed stream associated with each turbine based on the determination; and

f. at each turbine, in the effective reduction of NO in the combustion gases from each turbine_xThe associated catalyst feed stream is introduced into the effluent upstream of the catalyst through an ammonia injection grid under concentration conditions, thereby accommodating seasonal and daily and inter-turbine demand fluctuations without the need to store large inventories of ammonia-containing gas.

3. The method according to claim 2, wherein the urea is mixed with heated air in an amount relative to the amount of urea and water to produce a first stream of a gasified reductant comprising 0.5 wt.% to 5 wt.% ammonia.

4. The method according to claim 1, wherein the urea is mixed with heated air in an amount relative to the amount of urea and water to produce a first stream of a gasified reductant comprising 0.5 wt.% to 5 wt.% ammonia.