SCR System

David Zhang Product Development 7/22/2013

Introduction to SCR systems in coal fired power plants

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NOx and SCR Basics SCR Design

Common Problems Ammonia Control

Flow Modeling

Table of Contents

Presentation Outline

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NOx Basics

NOx Basics NOx Introduction

• Nitrous Oxides are emitted during combustion processes due to the availability of oxygen and nitrogen in the air and coal

• NOx in the atmosphere reacts in the presence of sunlight to form ozone (O3), which contributes to greenhouse gases

NOx Forms

• Fuel NOx

• Major source of NOx emissions from nitrogen bearing fuels

• 20 to 30 % of N in the fuel is converted to NOx

• Contributes 80% of total NOx emissions from boiler

• Thermal NOx

• NOx formed through high temperature oxidation of nitrogen in combustion air

• Formation rate decreases with decreased temperature and residence time

• Usually formed above 1200C

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SCR Basics

Selective Catalytic Reduction (SCR) Basics

SCR Introduction

• SCR uses a catalyst to convert NOx into separate nitrogen and oxygen molecules

• Reactions occur at temperatures ranging from 340 to 400C

• Higher NOx reduction efficiency (~98%) than SNCR (~20-40%)

SCR Process

• Ammonia reagent is injected in the flue gas through an ammonia injection grid

• Reagent is usually diluted with compressed air or steam to aid in injection

• Reagent and flue gas mix and enter the reactor chamber

• Flue gas and ammonia diffuse through the catalyst and contact active sites to reduce NOx to nitrogen and water

• Heat of the flue gas provides energy for the reaction

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Design Requirements

Design Requirements

Design Parameters

• Required NOx removal efficiency

• Usually reduced to 100 mg/Nm^3 based on government standards

• Maximum amount of unreacted ammonia in the flue gas (ammonia slip)

• Commonly 2-3 ppm/Nm^3

• Maximum oxidation rate of SO2 to SO3

• Desired lifetime of the catalyst

• Design lifetime is usually 16,000 or 24,000 hours

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Chemical Reaction

Chemical Reaction Ammonia

• Aqueous ammonia is a diluted form of ammonia (20-30%)

• Ammonia is vaporized before injection by a vaporizer

• 1 mole of ammonia is required to remove one mole of NOx

• Catalyst lowers activation energy and increases reaction rate

Urea

• Urea has an ammonia equivalence of approximately 53% by weight

• Solid urea is mixed with deionized water and decomposed to ammonia, CO2, and water

• Cheaper to transport than ammonia due to solid phase

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Reactor Location

Reactor Location Upstream (Hot Side)

• SCR reactor placed before the air preheater and ash hopper

• Does not require reheating of flue gas due to sufficiently high temperatures

• Contains greater concentrations of particulate which must be taken into account

Downstream (Cold Side)

• SCR reactor placed after the air preheater and ash hopper

• Requires reheating of flue gas using redirected hot flue gas, which is relatively costly

• Flue gas is virtually free of particulate, and catalyst degradation is decreased

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Reactor Layout

Reactor Layout Catalyst Grouping

• Catalysts are first cut to size and placed into individual blocks

• Blocks are grouped together into easy to transport modules

• Several modules (20-100) form a layer in the reactor

• A reactor typically holds 1 or 2 layers of catalyst

• Larger units include two reactors

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Catalyst Types

Catalyst Structure Types

Block geometry

• Geometries are meant to maximize surface area and minimize pressure drop

• Plate type catalyst are created by paste pressing catalyst onto rolls of stainless steel, plates are then assembled into blocks

• Honeycomb catalysts are formed through an extrusion process with an associated cell pitch

• Corrugated catalyst consists of glass fiber coated with titanium dioxide and impregnated with active components, catalyst is more porous and lighter than plate and honeycomb catalysts

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Catalyst Sizing

Catalyst Sizing Cell Pitch and Wall Thickness

• Choice of cell pitch and wall thickness is determined mainly by the dust content of the fuel

• High dust gases (coal fired boilers) require a cell pitch of 7-10 mm and wall thickness of 0.8 – 1.0 mm

• Low dust gases (gas fired boilers) require a cell pitch of 4-5 mm and wall thickness of 0.4 mm

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Pore Sizing

Pore Sizing 1 Catalyst Composition

• Based on a porous titanium dioxide carrier material on which the active components, vanadium pentaoxide and tungsten trioxide, are dispersed

Pore Creation

•Micro and Meso pores arise from titanium dioxide carrier

•Macro pores are created during a controlled drying and calcination process

Size Distribution

•Micro pores provide a high surface area for catalyst activity

•Meso pores provide branching to micro pores and can handle some poisoning

•Macro pores enhance gas diffusion into the catalyst and ensure access to the interior active sites

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Pore Sizing

Pore Sizing 2

A: Macro pores

200 μm

5 µm

B: Meso pores

200 nm

C: Micro pores

Catalyst Structure

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Catalyst Degredation

Catalyst Degradation Causes for Degradation

• Chemical absorption of ash components (sodium, potassium) onto catalyst active sites

• Fouling of catalyst surface by fine ash particles and other fouling agents (arsenic, chromium, calcium, etc.)

• Thermal degradation of pores due to vanadium vaporization into VOCl3

Erosion

• Erosion by abrasive components in the fly ash (quartz)

• Erosion is proportional to gas velocity to the 3rd power

• Gas velocity should be kept around 4-5 m/s for high particulate gases (~40 g/Nm^3)

• Minimized by keeping flow angle vertical

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SO3 Formation

SO3 Formation Formation of SO3

• Vanadium pentaoxide, an active component of the catalyst, contributes to the oxidation of SO2 to SO3

• SO3 will react with residual ammonia to form ammonia bisulphate (ABS)

• ABS can cause fouling in the catalyst, reducing reduction efficiency

• ABS also causes fouling and corrosion in the air preheater

Rate of formation

• Oxidation rate increases with temperature and pressure

• Oxidation is not limited by diffusion, and can diffuse into the titania clusters

• Lowering vanadium decreases oxidation rate but also decreases NOx efficiency

• High ratio between active surface area and catalyst bulk density minimizes oxidation rate

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Ammonia Mizing

Ammonia Mixing Ammonia Distribution

•Uneven ammonia distribution can create ammonia rich and ammonia lean zones in the gas

•Ammonia rich zones cause greater ammonia slip

•Ammonia lean zones create areas of decreased NOx reduction efficiency

Static Mixers

•Static mixers are often placed after the ammonia injection grid to increase ammonia-flue gas mixing at short distances

•Mixers should minimize disc area in order to have low pressure loss

•Mixing efficiency calculated through CFD models

•Increasing number of nozzles and changing nozzle angle also increases mixing

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Ammonia Flow Control

Ammonia Flow Control Rapid Load Change

• Rapid load chances can cause non uniform outlet NOx concentrations and ammonia slip

• SCR control system should quickly regulate ammonia injection

Analyzing System

• Two NOx analyzers are placed ahead of the ammonia injection grid and at the exit of the reactor

• NOx output can be controlled by setting a constant outlet NOx concentration (i.e. 100 mg/Nm3) or constant NOx reduction (i.e. 80%)

• Ammonia concentration is calculated proportionally to the flue gas flow rate, inlet and outlet concentrations, and NOx reduction point

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Computer Simulation

Flow Modeling 1 CFD Models

• Ensure uniform velocity in the flue gas throughout the process

• Verify proper mixing of ammonia into flue gas

• Optimize layout of ducts and decrease dust deposits

Splitter Plates

• Splitter plates should be installed to achieve uniform velocity distribution

• Plates should be installed at turns, especially before the ammonia injection grid and catalyst layers to minimize ammonia slip

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Flow Modeling

Flow Modeling 2 Popcorn Ash

• Large particle ash (LPA) can cause plugging in catalyst layers and reduce NOx reduction

• Hoppers should be installed to capture a portion of the ash

• A wire mesh should be installed before the reactor along with soot blowers to separate large particles

SCR Bypass

• Possible to conduct maintenance on the reactor while keeping the boiler under operation

• Installed if NOx reduction is only required during certain times of year

• Increased duct work increases pressure drop and overall costs

• Ash deposits can accumulate at dampers at the entrance of the bypass, thus requiring additional soot blowers or ash hoppers

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NOx Basics

References

• “Implementation of SCR DeNOx technology on coal-fired boilers”, Hans Jensen-Holm, Haldor Topsoe, Published at 12th SO2 and NOx Pullution Control Technology International Seminar, Chongqing, P.R. China, 2008

• “Implementation of SCR techology on coal-fired boilers in P.R. China”, Hans Jensen-Holm, Nan-Yu Topsøe and Jim Jianhua Cui, Haldor Topsoe, Presented at PowerGen Asia/China Power, Hong Kong, P.R. China, 2006

• “SCR Design Issues in Thermal Power Plants, Hans Jensen-Holm”, Peter Lindenhoff, Sergey Safronov, Haldor Topsoe, Presented at Russia Power, Moscow, Russia, 2008

• “Steam, It’s Generation and Use”, Babcock and Wilcox Company, 41st Edition