fundamentals of scr -...
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Fundamentals of SCR
2018 HRSG User’s Group Conference & Expo Houston, TX
February 12, 2018
Bill Gretta, P.E. Senior Vice President
CONCORD ENVIRONMENTAL 520 SOUTH BURNT MILL ROAD VOORHEES, NEW JERSEY 08043
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Outline
NOx
SCR System Basics
Catalyst Designs
Ammonia Systems
Ammonia Injection Grid (AIG)
Flow Modeling
Testing & Troubleshooting
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What is NOx?
Nitrogen, one of the earth’s most abundant elements, is essential for life on earth. It is an important plant nutrient and a key building block of proteins.
The burning of fossil fuels is one of the major atmospheric sources of nitrogen. Combustion of fuels at high temperatures converts the elemental nitrogen in the air and fuel to nitrogen oxides (NOx).
Emissions from power plants and mobile sources are the largest source of NOx in the US.
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Why Do We Need to Reduce NOx?
Nitrogen oxides are dangerous to breathe. When released from power plants or cars into nearby neighborhoods, they are associated with respiratory side effects such as asthma attacks.
Ozone pollution (ground level ozone or smog) forms when NOx reacts with hydrocarbons in the presence of heat and sunlight. Ozone at ground level is linked with asthma attacks and even birth defects and also retards growth of trees and crops.
Nitrogen oxides form small nitrate particles that are associated with serious health impacts like heart attacks and result in hazy skylines in our cities and national parks.
Nitrogen oxides lead to pollution of soils, groundwater and estuaries.
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SCR Basics
Selective Catalytic Reduction (SCR) is the process of removing NOx (NO and NO2) out of the flue gas stream by injecting ammonia (NH3) into the flue gas as a reagent.
The flue gas passes over a fixed bed of catalyst installed in a reactor.
Ammonia reacts with nitrous oxides on the catalyst surface to form safe and clean nitrogen and water.
4NO + 4NH3 + O2 4N2 + 6H2O Catalyst 2NO + 2NO2 + 4NH3 4N2 + 6H2O
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Typical SCR System Design
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SCR Catalyst – Combined & Simple cycle
Designed to reduce NOx from engine exhaust
>90% Reduction typical
Ammonia (NH3) Slip typically 5-10 ppm
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SCR Catalyst Types
Plate Honeycomb Corrugated
Metal carrier, surface coated with active ingredients
Extruded homogeneous Glass fiber carrier, surface coated with active ingredients
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SCR Catalyst Location in HRSG
Catalyst typically located in HRSG where temperature is 550 - 750°F
501F GT with N-E HRSG
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Ammonia Systems
Three Types of Reagents for SCR
Anhydrous Ammonia Aqueous Ammonia Urea
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Types of Reagents
Anhydrous • NH3
• Pres. Liquid • BP = -28°F • MP = -108°F
Aqueous • NH4OH • Liquid • BP = 97°F • MP = -98°F
Urea • (NH2)2CO • Solid • BP = NA • MP = 270°F
1
3 0
1
2 0 0
1 0
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Anhydrous Ammonia
• Pure (99.5%) NH3
• Lowest Capital and Operating Cost • Unloading and Storage Facility Requires RMP (Risk Management Plan)
which can be costly to initiate and maintain • Vaporization
– Requires the least amount of energy for vaporization – Electric is commonly used
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Anhydrous Ammonia – Storage & Forwarding
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Anhydrous Ammonia – Storage & Forwarding
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Anhydrous Ammonia – Storage & Forwarding
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Aqueous Ammonia
• 19% to 29% (by weight) NH3…the rest is water • Higher capital and operating costs
– Storing, forwarding and vaporizing large amounts of water • Less stringent safety requirements • Conventional Vaporization
– Flue Gas Vaporization or Electric Vaporizers
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Aqueous Ammonia Vaporizer – Typical Flow Diagram Ammonia Flow Control Unit (AFCU)
From HRSG or Ambient Air
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Urea to Ammonia Conversion
Urea – a very safe material – is converted to NH3, H2O and CO2 by a hydrolysis process
Urea is usually delivered as a liquid solution • DEF…Diesel Exhaust Fluid (32.5% Urea)
Urea Solution (NH2CONH2) Clear, colorless or slightly hazy liquid Light ammonia (pungent) odor Non Flammable Stable Material
No shipping, handling, storage or transfer of ammonia Used where SCR is in close proximity to
residential areas Easier public acceptance
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Urea to Ammonia Conversion
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Urea to Ammonia Conversion
To AIG
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Direct Injection
Direct Aqueous or Urea Injection
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Direct Injection
Direct aqueous ammonia or urea injection utilizes dual fluid nozzles to introduce liquid and atomizing air into the turbine exhaust
Uses the heat of the flue gas to vaporize the ammonia (or decompose urea to ammonia)
Relies on turbulence at GT exhaust to blend NH3 with NOx
Pros • Does not require AIG or vaporizer • Can be retrofit into compact HRSG designs
Cons • Tuning of nozzles can be difficult or impossible • Nozzles subject to extremely turbulent conditions…longevity and pluggage an
issue • Decomposition of urea to ammonia unpredictable. Lack of experience
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Ammonia Injection Grid
AIG used to distribute ammonia/air mixture into exhaust flue, upstream of SCR catalyst
Located downstream of CO catalyst • Otherwise CO catalyst would oxidize
ammonia and convert to NOx
AIG designed so that it can be tuned • Tuning should not be necessary if the
system is properly design and modeled
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Ammonia Injection Grid - Tuning
AIG tuning done via branches or zones, with control valves
# of Zones dependent on size of flue and desired controllability
Valves can be in centralized balancing header, or along side of HRSG
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Ammonia Injection Grid - Lances
Lances are typically SS or CS pipes with drilled holes
Carbon steel piping not recommended…rust or scale can plug holes
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Fluid Flow & CFD Modeling
Poor flow distribution can have huge effect on system performance
CFD or Physical modeling is an effective economical means to identify AND fix flow problems
Poor flow, ammonia and temperature distribution can affect:
1. CO performance
2. SCR performance
1. NOx reduction
2. Ammonia Slip
3. Ammonia Usage
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CFD modeling should be used to design all new systems
Can be used to optimize existing systems to improve performance
Example below shows how significant improvements can be made to ammonia distribution, with a minimal increase in pressure drop
CFD Modeling
Parameter Units Baseline AIG Final (New) AIG
Ammonia Distribution %RMS 70.2 5.8
Maximum Deviation from Avg. % +439 +13
Minimum Deviation from Avg.
% -44 -21
Pressure Drop in. w.c. ~0 0.2
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Testing & Troubleshooting
Signs of Poor Performance
Ammonia Flow Increase. Trend ammonia flow to understand performance.
Increased ammonia flow will continue to the point where stack ammonia slip is exceeded or NOx becomes difficult to control.
Typical Causes
• NH3/NOx Distribution,
• Bypass
• Catalyst Deactivation
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AIG Tuning
AIG should be designed with ability to tune as much as possible
Systems designed with no tuning capability have very limited flexibility to improve distribution
Adjustments to AIG can have significant benefit to performance
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SCR Sealing Systems
Inspections are a great method for identifying performance problems
Check for Leaks - Gas Bypass is major contributor to performance problems
Seals between modules and along perimeter of reactor should be checked
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SCR Sealing Systems
Test Date Configuration Ammonia Slip, ppmvd @ 15% O2
5-16-12 Original Sealing System
6.8
9-6-12 With Modified Sealing/Gas Reinforcement System
2.0
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SCR Catalyst Testing – Bench Reactor
Bench testing utilizes full-size elements
• Not “core” sample or micro reactor tests
Bench testing more accurately represents gas conditions
Full-Size Elements
Core Samples
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SCR Catalyst Testing – Bench Reactor
Bench testing utilizes full-size elements
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SCR Catalyst Testing
Testing will provide determination of the catalyst activity…
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 10000 20000 30000
Operating Time (h)
DeN
Ox
Act
ivity
Rat
io (-
)
Unused Unit-7 Unit-8 Unit-9 Required
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SCR Catalyst Testing
Testing will show when it’s time to replace the catalyst…
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CO Catalyst
Designed to reduce CO from engine
Simple reaction (no reagent required):
O2 + 2CO 2CO2
Made from stainless steel substrate with precious metals added (platinum)
>90% Reduction typical
Deactivation over time can dramatically reduce performance
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CO Catalyst Sampling
Should be tested when performance problems develop
Annual testing can identify problems before problems develop
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CO Catalyst Deactivation
Catalyst deactivation (aging) causes performance degradation
There are fewer active sites on the catalyst and the “lightoff” temperatures will increase, with a slower ramp up
Maximum conversions will also be lower
TEMP, °F
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SCR Catalyst Management
“Cost effective planning for end of catalyst life”
Testing is a significant aid to optimal catalyst management
Sample testing
Performance testing
Operations
Options:
• Replace catalyst with new
• Regenerate used catalyst
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QUESTIONS ??