Api rp 536 2006 (2013) (american petroleum institute)

Post-Combustion NO x Control for Fired Equipment in General Refinery ServicesAPI RECOMMENDED PRACTICE 536 SECOND EDITION, DECEMBER 2006 REAFFIRMED, SEPTEMBER 2013 Copyright American Petr

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Post-Combustion NO x Control for Fired Equipment in General Refinery Services

API RECOMMENDED PRACTICE 536 SECOND EDITION, DECEMBER 2006 REAFFIRMED, SEPTEMBER 2013

Copyright American Petroleum Institute

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`,,```,,,,````-`-`,,`,,`,`,,` -Copyright American Petroleum Institute

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Post-Combustion NO x Control for Fired Equipment in General Refinery Services

Downstream Segment

API RECOMMENDED PRACTICE 536 SECOND EDITION, DECEMBER 2006 REAFFIRMED, SEPTEMBER 2013

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API publications may be used by anyone desiring to do so Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any authorities having jurisdiction with which this publi-cation may conflict.

API publications are published to facilitate the broad availability of proven, sound ing and operating practices These publications are not intended to obviate the need for applying sound engineering judgment regarding when and where these publications should

engineer-be utilized The formulation and publication of API publications is not intended in any way

to inhibit anyone from using any other practices

Any manufacturer marking equipment or materials in conformance with the marking requirements of an API standard is solely responsible for complying with all the applicable requirements of that standard API does not represent, warrant, or guarantee that such prod-ucts do in fact conform to the applicable API standard

All rights reserved No part of this work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher Contact the Publisher, API Publishing Services, 1220 L Street, N.W., Washington, D.C 20005.

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`,,```,,,,````-`-`,,`,,`,`,,` -Nothing contained in any API publication is to be construed as granting any right, by cation or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent Neither should anything contained in the publication be construed

impli-as insuring anyone against liability for infringement of letters patent

The following definitions apply:

Shall: As used in a standard, “shall” denotes a minimum requirement in order to conform to the specification

Should: As used in a standard, “should” denotes a recommedation or that which is advised but not required in order to conform to the specification

This document was produced under API standardization procedures that ensure appropriate notification and participation in the developmental process and is designated as an API stan-dard Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should

be directed in writing to the Director of Standards, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C 20005 Requests for permission to reproduce or translate all

or any part of the material published herein should also be addressed to the director

Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years A one-time extension of up to two years may be added to this review cycle Status

of the publication can be ascertained from the API Standards Department, telephone (202) 682-8000 A catalog of API publications and materials is published annually and updated quarterly by API, 1220 L Street, N.W., Washington, D.C 20005

Suggested revisions are invited and should be submitted to the Standards and Publications Department, API, 1220 L Street, NW, Washington, DC 20005, standards@api.org

iii Copyright American Petroleum Institute

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1 SCOPE 1

2 APPLICABLE ENVIRONMENTAL REGULATIONS AND REFERENCES 1

2.1 Environmental regulations .1

2.2 U.S Federal Regulations 2

2.3 Environmental Protection Agency (EPA) References 2

2.4 National Standards & Publications 2

3 DEFINITIONS AND ABBREVIATIONS .2

3.1 Definitions 2

3.2 Abbreviations .3

3.3 Units of Emissions Measurements 4

4 PROCESSES DESCRIPTION, CONSIDERATIONS, AND APPLICATIONS 4

4.1 Selective Non-Catalytic Reduction (SNCR) .4

4.2 Selective Catalyctic Reduction 5

4.3 Process Considerations 7

4.4 Process Applications 9

5 DESIGN CONSIDERATIONS 9

5.1 General 9

5.2 SNCR Systems Overview 12

5.3 SCR Systems Overview .13

5.4 Reactant Control and Dilution System Compoents 25

5.5 Reactant Injection System 26

5.6 Catalyst/Reactor .27

5.7 Structures and Appurtenances 27

5.8 Refractories and Insulation 28

5.9 Instrumentation and Electrical Systems 28

5.10 Induced Draft Fan (IDF) 28

5.11 Flue Gas Connections 28

6 OPERATIONAL CONSIDERATIONS .29

6.1 Selective Non-Catalytic Reduction 29

6.2 Selective Catalytic NOx Reduction 30

APPENDIX A POST–COMBUSTION NOX CONTROL DATA SHEETS .35

APPENDIX B CALCULATION METHOD FOR CORRECTING NOX MEASUREMENT 51

APPENDIX C FLOW MODELING CONSIDERATIONS 53

APPENDIX D SCR OPERATIONS AND MAINTENANCE CONSIDERATIONS 55

APPENDIX E DESIGN CONDITIONS AFFECTED BY FLUE GAS QUALITY 57

Figures 1 Temperature Ranges for SCR Catalysts—Typical Examples 6

2 SNCR System Schematics Aqueous Ammonia 15

3 SNCR System Schematics Anhydrous Ammonia 17

4 SNCR System Schematics Urea Injection 19

5 SCR System Schematics Aqueous Ammonia 21

v Copyright American Petroleum Institute

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6 SCR System Schematics Anhdrous Ammonia 23

7 Condensation Temperature of Ammonium Sulfate and Ammonium Bisulfate for Various Conceptrations of NH3 and SO3 30

8 Catalyst Activity Profile versus Time 31

9 Effect of Space Velocity on NOx Reduction Efficiency 32

10 Typical NOx Reduction Efficiency and Ammonia Slip 33

Tables 1 Comparison of Typical SNCR and SCR Systems 1

2 SNCR Design Considerations—New Units and Retrofit Units 10

3 SCR Design Consideration—New Units and Retrofit Units 11

E.1 Catalyst Degradation Sources and Mechanisms 58

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1 Scope

1.1 This recommended practice covers the mechanical description, operation, maintenance, and test procedures of

post-com-bustion NOx control equipment for fired equipment in general refinery service It does not cover reduced NOx formation through combustion design techniques, such as flue gas recirculation (FGR) and staged combustion

1.2 This document covers two of the methods of post combustion NOx reduction:

a Selective Non-Catalytic Reduction (SNCR)

b Selective Catalytic Reduction (SCR)

1.3 SNCR is a process where the addition of ammonia or urea into the flue gas stream causes the oxides of nitrogen to convert

to nitrogen and water vapor The basis for the selection and limitations of the SNCR systems are described in 4.1

1.4 SCR is a process where the addition of ammonia into the flue gas stream in the presence of a suitable catalyst causes the

oxides of nitrogen to convert to nitrogen and water vapor The basis for the selection of the various catalyst types are described in 4.2

1.5 Table 1 indicates the typical operating performance and limitations of both types of NOx reduction systems The reduction efficiency of SNCR is limited because of the flue gas temperature range and difficulty in achieving proper mixing of the reactants, but is often suitable for retrofitting existing equipment for low or moderate NOx reduction SCR systems operate at high reduction efficiency at a lower temperature window than an SNCR system and are usually selected for lowest NOx emission

Table 1—Comparison of Typical SNCR and SCR Systems

Waste Disposal None Spent catalystThermal Efficiency Debit 0% – 0.3% 0%

Energy Consumption Low Higher if fan is usedCapital Investment Costs Low High

Plot Requirements Low Higher in most cases

3 – 5 years(typical guaranteed catalyst life)Ammonia/NOx (Molar Ratio) 1.0 – 1.5 0.8 – 1.2

Urea/NOx (Molar Ratio) 0.5 – 0.75 Not applicableAmmonia Slip—corrected to appropri-

ate reference O2 value

5 – 20 ppmvd 2 – 5 ppmvdRetrofit Less complicated May be complicatedAdditional Mechanical Draft Not required Required in most cases

2 Applicable Environmental Regulations and References

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`,,```,,,,````-`-`,,`,,`,`,,` -2 API R ECOMMENDED P RACTICE 536

2.2 U.S FEDERAL REGULATIONS

a Clean Air Act

b Code of Federal Regulations (CFR).

2.3 ENVIRONMENTAL PROTECTION AGENCY (EPA) REFERENCES

a New Source Performance Standards 40 CFR 60 Subpart Db, Industrial Commercial—Institutional Steam Generating Units.

b Alternative Control Techniques Documents 1993 NOx Emissions from Process Heaters

c National Ambient Air Quality Standards

2.4 NATIONAL STANDARDS & PUBLICATIONS

API

Std 560 Fired Heaters for General Refinery Service

Publ 534 Heat Recovery Steam Generators

Publ 535 Burners for Fired Heaters in General Refinery Services

ANSI1

K61.1 Safety Requirements for the Storage and Handling of Anhydrous Ammonia

ASME2

Section VIII Boiler & Pressure Vessel Code

B 31.3 Chemical Plant & Petroleum Refinery Piping

3 Definitions and Abbreviations

3.1 DEFINITIONS

3.1.1 ammonia breakthrough: The point at which increasing the NH3/NOx molar ratio does not significantly reduce the amount of NOx

3.1.2 ammonia/NO x ratio: The molar ratio of injected ammonia to the inlet NOx in the flue gas stream

3.1.3 ammonia slip: The amount of unreacted ammonia in the flue gas stream after the reduction of the NOx, measured in ppmvd corrected to the standard oxygen level

3.1.4 ammonium bisulfate and ammonium sulfate: Compounds formed when ammonia or urea injected into a flue gas

stream combine with sulfur trioxide These compounds may foul heat transfer surface and increase particulate emissions

3.1.5 catalyst activity: Measurement of the NOx reduction performance

3.1.6 catalyst handling facilities: Device used for loading and unloading of catalyst modules, usually a monorail and hoist 3.1.7 catalyst matrix or substrate: Substance that is coated or impregnated by the active ingredients of the catalyst The

catalyst matrix can be made from ceramic honeycomb, pellets, metal plates or mesh

3.1.8 catalyst module: Catalyst elements that are shop assembled for installation into an SCR catalyst housing Module

con-sists of a steel frame with suitable lifting lugs for installation and removal of the modules into SCR catalyst housing A catalyst layer consists of catalyst modules that cover the entire cross-section flow area

3.1.9 catalyst poisoning: Degradation of NOx reduction activity when flue gas component is adsorbed on the active faces of the catalyst and renders it inactive

sur-3.1.10 catalyst space velocity: The quantity of flue gas (at standard conditions) flowing per volume of catalyst per hour.

3.1.11 catalyst support: Structure within the reactor housing to support the catalyst modules.

3.1.12 catalyst type: Category of catalyst denoted by the active ingredient, namely vanadium oxide, titanium oxide,

plati-num, or zeolite

1American National Standards Institute, 25 West 43rd Street, 4th floor, New York, New York, 10036, www.ansi.org

2ASME International, 9636 Kinsman Road, Materials Park, Ohio, 44073, www.asminternational.org

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`,,```,,,,````-`-`,,`,,`,`,,` -3.1.13 cell density: Measurement of hole density in a honeycomb catalyst matrix (cells per sq cm [sq in.]).

3.1.14 dilution medium: Fluid (usually air, steam, or water) used to disperse the reactant within the flue gas stream—also

referred to as a “carrier.”

3.1.15 injection grid: a series of distribution pipes and injection nozzles located in the flue gas stream to enable effective

mixing of the reactant and flue gas

3.1.16 masking: Condition when the outer surfaces of the catalyst are covered with foreign material rendering the inner active

surfaces inaccessible for NOx reduction

3.1.17 NO x : General term used to describe all oxides of nitrogen including nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O) For the purpose of emission calculations, NOx is assumed to be nitrogen dioxide MW = 46.01

3.1.18 reactant ammonia: Anhydrous or aqueous ammonia used in the majority of post combustion NOx reduction systems Industrial anhydrous ammonia contains 99.5% minimum by volume ammonia and is injected as a vapor Aqueous contains about 19% – 29% by weight ammonia solution mixed with water and has to be vaporized or atomized before injecting into the gas stream

3.1.19 reactant flow control unit: Contains the equipment and instrumentation necessary for control and injection of the

reactant (ammonia or urea), including but not limited to vaporizer or atomizer, dilution air fan, mixer, and control valves

3.1.20 reactant urea: Used in some SNCR processes Urea is normally used as an aqueous solution containing about 50%

urea by weight

3.1.21 reactor: The housing that contains the catalyst modules and support structure.

3.1.22 reduction efficiency: The percentage of NOx removed from the flue gas by the reduction process

3.1.23 residence time: The time period the reactant is in contact with the nitrogen oxides and/or catalyst.

3.1.24 sintering: For vanadium and titanium catalysts refers to the irreversible loss of active surface due to the effect of high

temperatures The titania structure change causes catalyst particles to combine eliminating micro and macropores

3.1.25 sonic horn: Acoustic devices to clean surfaces.

3.1.26 sootblower: Mechanical device to clean surfaces that uses steam or air.

3.1.27 standard oxygen concentration: Regulatory baseline oxygen concentration.

3.1.28 temperature window: The flue gas temperature range that is most effective for NOx reduction for a given process

3.2 ABBREVIATIONS

BACT Best Available Control Technology

CEMS Continuous Emissions Monitoring System

DCS Distributed Control System

IGCI Industrial Gas Cleaning Institute

MSDS Material Safety Data Sheet

ppmvd parts per million by volume (dry)

RCDS Reactant Control and Dilution System

RIS Reactant Injection System

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SCR Selective Catalytic Reduction

SIP State Implementation Plans

SNCR Selective Non-Catalytic Reduction

3.3 UNITS OF EMISSIONS MEASUREMENTS

3.3.1 Analysis of NOx and other emissions are normally measured in ppmvd Since some sampling instruments measure the wet sample, care should be taken when analyzing results

3.3.2 The performance of equipment must be compared on a common basis The amount of NOx and ammonia slip should be corrected back to the standard oxygen levels For boilers and fired heaters the U.S.A standard is 3% oxygen (volume dry) in the flue gas, and for gas turbines the standard is 15% oxygen (volume dry) The calculation method for correcting the NOx values is given in Appendix B

3.3.3 In the United States, the most common unit of reporting emissions is lb/MM Btu liberated The heat liberation is based

upon the higher heating value (HHV) of the fuel The API standard basis for heat liberation in process heaters is the lower heating value (LHV) of the fuel Care should be used in expressing the emissions on the correct basis

3.3.4 In other countries, the units of reporting emissions can be milligrams per Normal cubic meter (mg/Nm3) or parts per lion (ppmvd) based on a different flue gas oxygen reference See Appendix B for methods of converting the various units

mil-4 Processes Description, Considerations, and Applications

4.1 SELECTIVE NON-CATALYTIC REDUCTION (SNCR)

4.1.1 General Description

The use of Selective Non-Catalytic Reduction (SNCR) units to reduce NOx emissions was pioneered in the early 1970s These processes use a reactant (ammonia or urea) to react with NOx to form water and inert gas (nitrogen/carbon dioxide) To be effec-tive in reducing NOx emission, the reducing agent must be injected into the fired equipment at a desired temperature point Although the NOx reduction takes place in the 870ºC – 1370ºC (1600ºF – 2500ºF) temperature range, the temperature window in the SNCR process can be extended down to approximately 700ºC (1300ºF) by the injection of hydrogen or enhanced chemicals along with the reducing agents The complex fluid dynamics and chemical reactions involved generally limit the SNCR process

to less than 75% NOx reduction in the fired equipment application

At least two SNCR technologies are commercially available One is an ammonia-based process, the other is a urea-based process Both processes require a series of injection nozzles and a reducing agent distribution and storage system

injec-For initial NOx levels of 200 ppmvd or less, NH3/NOx molar ratios of about 1.5 are commonly used Higher molar ratios will duce NH3 slip

pro-Copyright American Petroleum Institute

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`,,```,,,,````-`-`,,`,,`,`,,` -4.1.3 Urea-Based Process

This SNCR process uses urea, CO (NH2)2 as a reducing agent It injects an aqueous urea solution into the flue gas stream at the appropriate temperature zone The urea thermally decomposes to produce chemical species which react with NOx to form nitro-gen, carbon dioxide, and water

CO (NH2)2 + 2NO + 1/2 O2 → 2N2 + CO2 + 2H2O (3)

From Equation (3) it follows that the stoichiometric molar rate of urea relative to NO in the combustion products is 0.5 since one mole of urea potentially has two moles of NH2 available to react with NO The urea injection process for NOx control is also tem-perature sensitive The urea solution, therefore, must be injected in the temperature range of 870ºC – 1370ºC (1600ºF – 2500ºF) and possibly higher temperature

4.2 SELECTIVE CATALYTIC REDUCTION

4.2.1 Process Description

The use of SCR systems to reduce NOx emissions was pioneered in the early 1970s This process requires the use of a catalyst Selective Catalytic Reduction Process removes nitrogen oxides (NOx) by injecting ammonia (NH3) into the flue gas and passing the well mixed gases through a catalyst bed NOx reacts with NH3 in the presence of the catalyst to produce nitrogen (N2) and water (H2O ) as shown in the following equations

A wide variety of available catalysts can operate at flue gas temperature windows ranging from 125ºC – 580ºC (260ºF – 1075ºF)

as shown in Figure 1 High NOx reduction efficiencies can be achieved if the parameters such as residence time, space velocity, ammonia/NOx distribution, and the correct temperature window are controlled For SCR technology, an NH3/NOx molar ratio of 1.0 is commonly used

4.2.2 Catalyst Types

Listed below are the current catalyst types available The selection of the proper catalyst for the application is dependent on many factors

4.2.2.1 Platinum Based Catalysts

Platinum based catalysts can be used for NOx reduction in lower temperature applications Platinum catalysts are also used to dize unburnt hydrocarbons and CO One side effect when using platinum catalysts is that a significant part of the SO2 in the flue gas is converted to SO3 SO3 combines with water vapors, forming acid which is corrosive to the downstream equipment The ideal temperature range for the platinum-based catalyst to effect optimum NOx reduction is 260ºC – 343ºC (500ºF – 650ºF) A platinum-based SCR system will generate more nitrous oxide (N2O) at lower temperatures than other types Commercial applica-tions of this catalyst in SCR applications have become rare

oxi-Copyright American Petroleum Institute

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`,,```,,,,````-`-`,,`,,`,`,,` -Figure 1—Temperature Ranges for SCR Catalysts—Typical Examples

Vanadia-Titania Based Catalyst - Pelletized form

Vanadia-Titania Based Catalyst - Plate form

Vanadia-Titania Based Catalyst - Corrugated form

Vanadia-Titania Based Catalyst - Monolithic form Platinum Based Catalyst Zeolite Catalysts

Flue gas Temperature, °C

Overall Range, °C Ideal Range, °C

4.2.2.2 Vanadium-titania Based Catalyst

Vanadium-based catalysts also convert SO2 to SO3 but to a lesser extent than platinum based catalysts The temperature range for this catalyst is 125ºC – 538ºC (260ºF – 1000ºF) There are four forms of this catalyst: Pelletized Form, Monolithic Form, Plate Form, and Corrugated Form The pelletized form of catalyst can sometimes be referred to as low temperature catalyst Mono-lithic, plate and corrugated forms of catalysts are often referred to as medium temperature catalyst

A vanadia/titania/tungsten catalyst coated on a metal mesh or plate The plates are assembled in baskets which are then installed

in modules The ideal temperature range for this form of catalyst is 315°C – 370°C (600°F – 700°F)

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`,,```,,,,````-`-`,,`,,`,`,,` -The ideal temperature range for this catalyst to effect optimum NOx reduction is 357°C – 580°C (675°F – 1075°F) It is supplied

on ceramic structures in composite honeycomb configurations Commercial applications of Zeolite catalysts are rare

4.3 PROCESS CONSIDERATIONS

4.3.1 Effect of Flue Gas Components on SNCR

The dominant factors for NOx reduction utilizing either of these processes are the flue gas temperature and temperature profile, rather than the fuel type or its products of combustion

The SNCR process is affected by the concentrations of O2, H2O, and CO in the flue gas High CO concentrations are reported to shift the temperature window at the low end, so that NOx removal is effective at relatively lower temperatures, i.e., 800ºC (1470ºF) The SNCR process may retard the oxidation of CO in the flue gas, resulting in slightly higher CO emissions

The presence of HCl or HF in the flue gas in excess of 500 ppmvd may also retard the effectiveness of the SNCR process The other flue gas components, such as CO2, N2, etc., appear to have no effect on the NOx reduction process

4.3.2 Effect of Flue Gas Temperature, Catalyst Poisons, and Catalyst Masking Agents on SCR

There are three common reasons that cause deactivation of catalyst: excess flue gas temperature, catalyst poison, and masking of the catalyst

a Depending on the catalyst substrate material, the catalyst may be quickly damaged due to thermal stresses at temperatures in excess of the design temperature High heat up and cool down rates for the catalyst could negatively impact catalyst life Please refer to the Operations section of this document for commonly used heat up rates

b Catalyst poisoning occurs when a component of the flue gas, such as Sodium or Potassium, gets adsorbed on the active faces of the catalyst and renders it inactive Some other known catalyst poisons are arsenic, chrome, and mercury Arsenic compounds and heavy metal compounds, such as zinc dithiophosphate, tend to accumulate on the periphery of the catalyst They tend to decompose with time, producing free heavy metals, which then react with the catalytic compounds to produce less active material Appendix E discusses catalyst poisons

sur-c Masking is caused by the accumulation of foreign substances on both the carrier and the catalytic component Phosphorous components forming a glaze, dust, soot, and oil mist can all block the pores Catalyst activity can often be regained by removing the material masking the catalyst Ammonium sulfate and/or bisulfate salts could be masking the catalyst as well Sootblowers can sometimes be used to remove some masking agents Please refer to Item 5 of the Appendix D section of this document for additional sootblower information Other design features to remove masking components could be included upstream of catalyst

4.3.3 NO x Reduction from Initial NO x Values

4.3.3.1 SNCR Technology

Whether urea or ammonia based, SNCR is most cost effective in achieving moderate NOx reduction in the 40% – 75% range when the initial NOx values are 100 ppmvd or greater In general, the NOx reduction efficiency decreases as the initial NOx value decreases High NOx reductions become more difficult to achieve when the initial NOx value is below 100 ppmvd

The NOx reduction efficiency of both SNCR processes depends on the following factors:

a Flue gas temperature in reaction zone

b Uniformity of flue gas temperature in the reaction zone

c Normal flue gas temperature variation with load

d Residence time

e Distribution and mixing of ammonia/urea into the flue gases

f Initial NOx concentration

g Ammonia/urea injection rate

h Physical configuration, which affects location and design of injection nozzles

The following are general considerations for selecting a SNCR process for a specific NOx reduction requirement:

a NOx reduction efficiency required

b Allowable NH3 slip to meet requirements

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c Physical configuration of the fired equipment and the flue gas temperature profiles at various loads

d Available potential location for injection

e Performance at various loads and different modes of operation

f Side reactions and corrosion/fouling on the downstream equipment

g Safety hazards on storage, processing, transportation and distribution of the reactants and enhancers

h Operating cost, as well as the initial capital investment

The ammonia slip using either reactant typically ranges between 5 – 20 ppmvd

4.3.3.2 SCR Technology

Applications of SCR NOx reduction are reported for each of the catalyst types listed above for inlet NOx concentrations from 9 to

20000 ppmvd However, most applications of SCR are for inlet NOx concentrations in the range of 25 – 150 ppmvd In this range,

NOx reductions exceeding 90% are possible with an ammonia slip of no more than 5 ppmvd

While higher NOx reduction is possible, this is likely to be at the expense of increased ammonia slip (above 2 – 5 ppmvd) which adds to the operating costs and may be unacceptable for environmental reasons

It is imperative to achieve uniform distribution of ammonia and NOx at the inlet of the catalyst Catalyst suppliers require a threshold maldistribution ratio of NH3/NOx concentration allowed at the inlet of the catalyst Catalyst suppliers also provide lim-itations on velocity and temperature maldistribution at the inlet of the catalyst Typical maldistribution allowed is as follows:

±10% maldistribution in NH3/NOx ratio, ±20ºF for temperature, ±15% for velocity or flow distribution Design of reactant tion grid will be accomplished to meet these requirements

injec-System supplier guarantees NOx reduction efficiency and ammonia slip within specified maldistributions at the catalyst bed exit This requires proper design of the related duct work, ammonia injection grid (AIG) and confirming the design via either three dimensional computational fluid dynamic (CFD) analysis or cold flow modeling of the SCR system and related ductwork See Appendix C for flow modeling considerations

4.3.4 Excess Reactants

As a result of the complexity of the reactions, ammonia slip must be evaluated for each individual application Few tions can be made Because slip is linked to a certain degree to NOx reduction performance, fired equipment in which the time-temperature relationship is favorable to achieving high NOx reduction will also exhibit low NH3 slip In cases where favorable conditions exist, it has been possible for NH3 slip to be held below 2 – 5 ppmvd The placement of the injectors and injection mix-ing effectiveness are of prime importance in minimizing NH3 slip

generaliza-At temperatures above 455ºC (850ºF), ammonia could decompose and form NOx Other than the decomposition and reactions outlined in equations earlier in this section, there are no significant reactions between ammonia and other compounds in high tem-perature flue gas At low temperatures, ammonia can combine with sulfur or chlorine compounds to form complex salts

Depending on the process, as combustion gases cool, ammonia can react with sulfur trioxide (SO3) and water vapor to form ammonium bisulfate and ammonium sulfate Ammonium bisulfate is a sticky corrosive substance which can foul heat transfer surfaces Ammonium sulfate, on the other hand, is a dry solid, forms as solid particles, and may increase particulate emissions These reactions can be minimized, in many cases to negligible amounts, by limiting the ammonia slip level Please refer to Sec-tion 6 of this document for more information on salt formation

Ammonium nitrate can form in post-combustion systems at low temperatures Those temperatures are a function of NH3 tration Typically, this is not an issue as most systems operate above these temperatures

concen-The amount of excess reactants changes with the life of the catalyst Higher amount of reactant is required as the catalyst progresses to the end of its life which results in higher slip

At flue gas temperatures below 120ºC (250ºF), ammonia can react with hydrochloric acid (HCl) to form ammonium chloride (NH4Cl) NH4Cl is a dry, neutral white salt, which can contribute to a visible plume if present in sufficient quantities

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`,,```,,,,````-`-`,,`,,`,`,,` -4.4 PROCESS APPLICATIONS

4.4.1 SNCR PROCESSES

SNCR processes are effective at relatively high temperatures, so their applications are more numerous with industrial and power boilers SNCR also finds extensive applications in units with relatively high residence times (e.g 2 – 3 seconds), such as inciner-ators These processes are more efficient in reducing NOx from high levels to moderate levels (i.e., 200 to 50 – 75 ppmvd).SNCR processes do not find many applications in gas fired process heaters where modern burner technology offers extremely low

NOx emissions Refer to API RP 535, Burners for Fired Heaters in General Refinery Services.

4.4.2 SCR Processes

The SCR processes find ideal application in gas turbine exhaust heat recovery equipment, heaters, and boilers where the permit requirements require a NOx reduction by 80% – 95% of such values It is quite common to design SCR systems to provide more than 90% NOx reduction

4.4.3 Combination of SNCR, SCR, and Low NO x Burner Technologies

Each of these technologies has its ideal range for achieving NOx reduction There may be industrial applications where all three technologies can be combined to bring the NOx level from initial values in the 200 ppmvd range to final values in the 5 ppmvd range

5 Design Considerations

5.1 GENERAL

5.1.1 Typical Design Considerations

The Selective Catalyst Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) systems should yield an “Industrial Quality” system with the flexibility and reliability to operate continuously between unit turnarounds Designs are usually speci-fied to maintain performance from 3 – 5 years The effective performance is usually longer than the design period Therefore, sys-tem tuning and testing between turnarounds should be taken at every opportunity

Selection of reactant depends upon issues such as permitting, transportation, safety infrastructure and cost The three main sources of reactant are aqueous ammonia (19% or 29%), anhydrous ammonia and urea (liquid or solid) The equipment associ-ated with each of these reactants is slightly different and affects the capital cost

Specific designs for refinery processes such as Fluid Catalytic Cracking Units (FCCUs), heaters, boilers and exhaust gas heat recovery are available Exhaust gas processes with harsh chemicals or particulates must be addressed in the design to account for adverse effects on reactant distribution and acti vity level as well as potential corrosion of catalyst and support materials With regard to additional catalyst layers, maintenance and management plans for the injection system and catalyst should be made

Other considerations for both types of NOx conversion in new and retrofitted applications are noted below in Tables 2 and 3

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`,,```,,,,````-`-`,,`,,`,`,,` -Table 2—SNCR Design Considerations—New Units and Retrofit Units

Design Considerations—SNCR New Unit Retrofit UnitPerformance Expectations Between 40% & 75% reduction Between 30% & 50% reduction

Process Considerations Utilizing the least cost heating medium for

vaporization or direct injection can be quately designed for in new units

ade-Either direct injection or vaporization of tant either by steam, electricity, or air is a design consideration, influenced by variable costs and the effect on system performance.SNCR System of Control Control logic and planning done with plant

reac-DCS

Can be controlled by its own PLC or loop controller as well as signals tied back to existing DCS if available

Physical Space Duct space needed to insert nozzles can be

included in design

Duct space needed to insert nozzles may be a constraint

Turnaround Schedule Not usually the critical path for new projects Careful consideration is needed for up front

work and timing for retrofit during turnaround.Terminal Points Clearly defined in overall project plan Special attention should be given to minimize

issues with matching up insulation, platforms and ladders

Reactant Selection Based on reactant cost, availability and

permitting restrictions

Consideration should be given to storage and routing of reactant piping Physical space needed for reasonable storage duration and distribution Ambient temperature is a design factor at the extremes with each reactant.Flue Gas Temperature Narrow band of temperature for non-catalytic

reduction Could be affected by load

Narrow band of temperature for non-catalytic reduction Could be affected by load

Pressure Drop No effect on exhaust gas flow No effect on exhaust gas flow

Flow Profile Velocity, NOx concentration and temperature

distribution are known parameters

Velocity, NOx concentration and temperature may be completely different than upon initial start up due to modifications or age

Computational Fluid Dynamics

(CFD Analysis)

Flow model and thermal mass model over complete operating envelope to ensure the proper design of reagent injection versus existing NOx distribution See Appendix C for further details

It is important to flow model and thermal mass model the existing process gas flow over the complete operating envelope to ensure proper design of reagent injection and NOx distribution See Appendix C for further details

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`,,```,,,,````-`-`,,`,,`,`,,` -Table 3—SCR Design Considerations—New Units and Retrofit Units

Design Considerations—SCR New Unit Retrofit UnitPerformance Expectations > 90% reduction > 90% reduction

Process Considerations Utilizing the least cost heating medium for

vaporization can be adequately designed for in new units

Vaporization of reactant by steam, electricity, gas recirculation etc is decided on design, cost and performance considerations

SCR System of Control Control logic and planning done with plant

DCS

Can be controlled by its own PLC or loop controller Signals may be tied back to existing DCS if available

Physical Space Incorporated with plant layout Availability of space or the affect of moving

existing equipment determines design Turnaround Schedule Not usually the critical path for new projects Planning and execution must meet turnaround

schedule

The design should consider hoists and monorails or crane lifts for catalyst change outs

Terminal Points Clearly defined in overall project plan Special attention should be given to minimize

issues with matching up ducting, insulation, platforms and expansion joints

Reactant Selection Based on reactant safety, injector plugging

considerations, cost, availability, and permitting restrictions

Physical space is needed for reasonable storage duration and distribution Ambient temperature is a design factor at the extremes with each reactant

Flue Gas Temperature System to be designed for optimum

performance and cost

This design consideration combined with space limitations dictates the performance and size of the retrofit unit

Flue Gas Quality For both new installations and retrofit applications, design considerations should be given to

the flue gas constituents Constituents such as particulates create abrasive atmospheres that may reduce the life of the catalyst Particulates have to be handled in such a way that soot blowing and reactor orientation prevents accumulation Accumulation of ash would act as a masking agent and effectively reduce the activity of the catalyst If not designed properly the ash also may cause overall system pressure to rise and therefore affect efficiency SO2 which oxidizes to SO3 and reacts with ammonia generates either an ammonia salt or sticky bisulfate

at its dewpoint temp Further discussion on these topics are outlined in Appendix E

Pressure Drop All equipment can be initially designed to

overcome system pressure

New equipment to existing unit requires design to evaluate effects on process efficiency and equipment sizing

Flow Profile Flue gas rate, NOx concentration and

temperature distribution are known parameters based on design

Flue gas rate, NOx concentration and ture may be completely different than initial start up due to earlier equipment

tempera-modifications, fouled or damaged combustion equipment, or changes in process conditions Collect actual operating data

Computational Fluid Dynamics

(CFD Analysis)

Not as critical on new installations It is important to model the existing process

gas flow to ensure the proper design of reactant injection, sealing and NOx distribu-tion See Appendix C for further details.Vertical or Horizontal Orientation of Reactor Determined by process Space consideration determines orientation in

most retrofits Particulates when present can also influence choice of orientation

Mechanical Sealing New units can be designed and installed with

tight tolerances

SCR system providers can design the reactor system such that no gas bypass emerges downstream without having been processed Key to designing this sealing mechanism is whether internal insulation or external insulation is used Expansion joints must be employed if the insulation is not uniform through the duct

Test and Verification Test port locations designed with ductwork Test ports to be included in the reactor housing

as well as accessible areas of upstream and downstream ductwork

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5.1.2 Project Definition

During planning, design and implementation phases, the following items should be addressed:

a Reactant unloading, storage and handling

b Reactant vaporization, conveyance, distribution and mixing

c Reactor housing, catalyst and controls (SCR)

d Adequate system definition and scope delineation

e System commissioning advisor from equipment system supplier

f Spares for commissioning and two years operation

g Performance guarantees for NOx reduction, pressure drop and catalyst life

5.1.3 Typical Purchasing Considerations

In the development of a “Job Scope,” the following items should be addressed:

a Adequate system definition and scope delineation

b Drawings, diagrams, and documentation

c Handling, storage, shipping, and preservation of equipment

d Material testing and inspection

e Shop testing and inspection of individual components

f Installation, operation, and maintenance manuals complete with MSDS

g System commissioning advisor from equipment supplier (if needed)

h Spares for commissioning and two-year operation

i Performance Guarantee(s) for NOx reduction efficiency over potential operating conditions

a RCDS

i Aqueous ammonia: Aqueous ammonia is pumped from the storage tanks and is commonly mixed with a heated carrier air stream in an ammonia vaporizer/mixer Alternatively, the ammonia can be vaporized in a once-through vaporizer before mixing with either an air or steam carrier The primary components in this system are as follows, and Figure 2 illustrates a typical sche-matic representation of the system

1 Aqueous ammonia storage tank

2 Carrier air supply: two air blowers (or compressors) or one blower and a backup air source; or a source of carrier steam

3 Ammonia supply pump(s)

4 Two cartridge filters and/or strainers

5 Ammonia vaporizer utilizing either an air heater or an external source of heat

6 Instrumentation and interconnecting piping for a fully functional system

7 The recommended components and features noted in 5.4

ii Anhydrous Ammonia: Vaporized anhydrous ammonia is mixed directly with either an air or steam carrier A vaporizer usually supplies heat to the storage tank to maintain pressure, and the ammonia vapor is drawn from the vapor space in the tank The pri-mary components in this system are as follows, and Figure 3 illustrates a typical schematic representation of the system

1 Pressurized anhydrous ammonia storage tank

2 Carrier gas supply: two air blowers, or one blower and a backup air source; or a source of carrier steam

3 Ammonia vaporizer

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`,,```,,,,````-`-`,,`,,`,`,,` -4 Ammonia—air static mixer (if required).

b Reactant Injection System (RIS), consisting of:

1 Flow distribution manifold with a series of injector nozzles that introduce the vaporized ammonia/carrier gas mixture to the flue gas stream

2 The recommended features noted in 5.5 and 5.7

c System controls, provided by either a local control panel or the plant’s DCS

5.2.2 Urea-Based SNCR Systems

The primary components of a urea-based SNCR system are as follows, and Figure 4 illustrates a typical schematic representation

of the system

a Urea Reactant Control and Dilutions System (RCDS), consisting of:

1 A reactant storage tank

3 A reactant metering pump

4 A water pump, or dilution water source connection

5 An in-line static mixer as required

6 A series of flow indicators and regulating valves, as required, balancing the flow of reactant to each reactant injector

7 All pertinent instrumentation and interconnecting piping for a fully functional system

1 An atomizing chamber

2 Interconnecting tubing

3 Injector nozzles that introduce the atomized reactant/carrier air mixture into the flue gas stream

c System controls, provided by either a local control panel and/or the plant’s DCS

5.3 SCR SYSTEMS OVERVIEW

There are two major types of SCR systems: aqueous ammonia and anhydrous ammonia The difference lies in the process used in the Reactant Control and Dilution System (RCDS) to vaporize the ammonia and mix it with the carrier air stream to obtain the reactant charge SCR systems have one of these two RCDS systems, a Reactant Injection System (RIS), a Selective Catalytic Reactor, system controls, and an Induced Draft Fan (if required):

a RCDS

i Aqueous Ammonia: Aqueous ammonia is pumped from the storage tanks and is commonly mixed with a heated carrier air stream in an ammonia vaporizer/mixer Alternatively, the ammonia can be vaporized in a once-through vaporizer before mixing with either an air or steam carrier The primary components in this system are as follows, and Figure 5 illustrates a typical sche-matic representation of the system

1 Aqueous ammonia storage tank

2 Carrier air supply: two air blowers, or one blower and a backup air source; or a source of carrier steam

3 Ammonia supply pump(s)

5 Air heater and ammonia—air vaporizer or ammonia vaporizer

6 All instrumentation and interconnecting piping for a fully functional system

ii Anhydrous Ammonia: Vaporized anhydrous ammonia is mixed directly with carrier air or steam A vaporizer usually supplies heat to the storage tank to maintain pressure, and the ammonia vapor is drawn from the vapor space in the tank The primary com-ponents in this system are as follows, and Figure 6 illustrates a typical schematic representation of the system

1 Pressurized anhydrous ammonia storage tank

2 Carrier air supply: two air blowers or one blower and a backup air source, or a source of carrier steam

3 Ammonia vaporizer

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Figure 2—SNCR System Schematics Aqueous Ammonia

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Figure 3—SNCR System Schematics Anhydrous Ammonia

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Figure 4—SNCR System Schematics Urea Injection

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Figure 5—SCR System Schematics Aqueous Ammonia

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Figure 6—SCR System Schematics Anhdrous Ammonia

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4 Ammonia—air static mixer (if required).

1 A flow distribution manifold with a series of flow indicators and regulating valves

2 A set of internal elements, with injection nozzles, capable of accommodating the entire range of temperatures

c A Selective Catalytic Reactor, consisting of:

1 Insulated housing, with at least one catalyst access door

2 Catalyst support structure

3 Catalyst module(s)

4 Appropriate ladders and platforms

5 Catalyst module loading and unloading facilities such as hoists, monorail etc

6 Recommended components and features noted in 5.6 through 5.8

d Induced Draft Fan, if required, which should be designed and purchased in accordance with 5.10

e System Controls, provided by either a local control panel or the plant’s DCS

5.4 REACTANT CONTROL AND DILUTION SYSTEM COMPONENTS

5.4.1 Dilution Air Blower System

Dilution air blowers should be industrial quality and of a design suitable for the intended application The following features should be included in the system:

a An isolation valve downstream of each blower, to provide the means to positively isolate either blower from the operating tem, which permits the safe maintenance of the idle blower

sys-b A check valve, to prevent the flow of air through the idle blower

c An inlet air filter—silencer

d A ducting design that allows the removal of either blower without the removal of adjacent ducting

5.4.2 Air Heater

a Electric Air Heater—Since the heating elements require periodic replacement, the air heater should be constructed for ease of maintenance and long term reliability, and contain the following features:

1 Elements should provide a minimum mean-time between-failure of 60 months

2 Heating elements should be designed for easy replacement

3 The heater control should be a silicon controlled rectifier type, with a 4 – 20 mA control input

b Other air heater types, such as a once-through exchanger heated by a fluid such as flue gas or steam, may be used

5.4.3 Ammonia Vaporizer/Mixer

This vessel should be designed in accordance with ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 Vaporizer

shall provide an even distribution of aqueous ammonia within the outlet carrier stream A reliable heat/mass transfer method to insure ammonia absorption into the dilution stream is required The ammonia feed shall be filtered and metered Ammonia filters with instrumentation for monitoring pressure drop shall be provided Permanent lifting lugs shall be provided to facilitate installa-tion and maintenance

5.4.4 Filters/Strainers

Filters and/or strainers should be industrial quality and of a design suitable for the intended application It is recommended that two in-line cartridge filters and/or strainers be piped in parallel, and valved to permit quick and safe switching (on-line vs off-line)

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b The skid equipment layout should provide easy access for operation and maintenance

c The design and placement of piping, conduit and mechanical members should permit the removal of skid components without the removal of piping, conduit or mechanical members

d The skid structural design should be sufficient for a four-point lift without temporary bracing

5.4.6 Reactant Control and Dilution System Piping

Piping between the system inlet and the RIS should be in accordance with the following minimum design practices:

a Piping design, fabrication, inspection and testing should be in accordance with the job specifications and ASME/ANSI B31.3 and K61.1

b Skid piping should be properly supported and terminated with flanged connections at the skid edge

c Skid terminals should be designed to accept reasonable forces, moments and movements from the interconnecting piping per API Std 560

d Piping should not obstruct any access openings

e Piping should be properly supported and protected to prevent damage from vibration, operation and maintenance

f Piping should be designed to minimize the use of flanges and fittings

g All piping should be of seamless construction

h Corrosion allowances for carbon steel materials should not be less than 3.0 mm (0.125 in.) Stainless steel materials may be designed without a corrosion allowance

i Flange bolt holes should straddle vertical centerlines

j Connections 1.50 in NPS and smaller should be socket welded

k Connections 2 in NPS and larger should be butt-welded or flanged

l Instrument connections and stubs, including root valves, should not be less than 3/4 in NPS

m Vents and drains should be provided to completely vent and drain pressure parts High or low point pockets should be avoided

n Components made of, or containing, copper, brass, and cast iron should be avoided

o Pressure part flange faces should be raised face with 125 – 250 AARH concentric or spiral serrated finish suitable for metal gaskets

p Pressure part gaskets should be spiral wound metal gaskets with retaining rings

5.5 REACTANT INJECTION SYSTEM

The RIS design should be in accordance with the following:

5.5.1 Piping design, fabrication, inspection, and testing should be in accordance with the RCDS piping requirements, as noted

in 5.4.6, above

5.5.2 RIS piping connections inside the flue gas path (i.e., internal elements) should be socket or butt-welded.

5.5.3 Internal elements should be fed from a common external header.

5.5.4 Each spray nozzle assembly should be removable for maintenance

5.5.5 Spray nozzles and other internal elements should be 300 series stainless steel material.

5.5.6 Internal supports and guides for the injection grid components should be provided as required to prevent deformation of

the components

5.5.7 Reactant injection grid design should accommodate the entire range of possible operating temperatures RIS design

should be based on “no flow” conditions

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`,,```,,,,````-`-`,,`,,`,`,,` -5.6 CATALYST/REACTOR

5.6.1 Catalyst

a The catalyst should be suitable for treating the flue gases specified on the data sheets

b Catalyst modules in a reactor should be identical in size and interchangeable if practical This concept should not prevent the use of test modules with coupons, or other form of monitoring device

c It is recommended that IGCI standard catalyst module sizes be used whenever practical

d Catalyst shall have sufficient strength so that permanent deformation or damage to the catalyst shall not occur due to stresses from design seismic, pressure, thermal, chemical conditions, or combinations thereof

addi-c Sufficient maintenance access should be provided for both the initial and future catalyst layers, and associated equipment

d Provisions for thermal expansion should take into consideration the operating conditions specified on the data sheets

e Placement of access lanes and cleaning facilities should anticipate future catalyst installation

f A catalyst module support structure should be provided in the reactor housing It should be designed to support and retain both initial and future catalyst modules

g Internal seals should be provided to prevent bypass of flue gas around the catalyst modules

h Provisions for catalyst module removal should be included in the reactor housing design

i The catalyst reactor housing should have a bolted access door sized to allow catalyst removal

5.7 STRUCTURES AND APPURTENANCES

5.7.1 General

Structural steel, reactor casing, and ducting should generally be designed in accordance with API Std 560

a All loads from the catalyst modules and connecting ducting should be supported by the structural steel and should not be mitted by the insulation system External steel frames should carry the structural loading

trans-b Metal casing may be used to provide lateral bracing between the structural columns and to support the insulation system

c Structural steel should be designed to permit lateral and vertical expansion of all SCR parts

d Structural supports should be designed to support ladders, stairs and platforms in existing and future locations

5.7.2 Ladders, Platforms, and Stairways

Ladders, Platforms, and Stairways should generally be designed in accordance with API Std 560

a Platforms, with handrails and ladders or stairs, should provide access to catalyst removal doors Such platforms should not inhibit the removal of catalyst

b Platforms and/or ladders should provide access to all instrumentation and controls, valves, dampers, actuators, and access doors not accessible from grade

c Catalyst access platform should be of sufficient size and strength to accommodate at least one module

5.7.3 Casing and Ducts

Ducts and reactor casing should generally be designed in accordance with API Std 560 and Appendix E

a Casing plate should be seal-welded to prevent air and water infiltration

b Lifting lugs should be provided on all ducting and reactor housing components

c Bolt spacing on all duct flanges should be 150 mm (6 in.) maximum

d The transition angle from the upstream ducting to the SCR reactor housing should not be greater than a 30º included angle, unless flow distribution devices are provided

e Expansion joints should be provided as required to accommodate system and/or component expansion and contraction

Tiêu đề	Post-combustion NOx Control For Fired Equipment In General Refinery Services
Trường học	American Petroleum Institute
Chuyên ngành	Petroleum Engineering
Thể loại	Recommended Practice
Năm xuất bản	2013
Thành phố	Washington, D.C.

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Số trang	70
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