november 24, 2014 · • silica, phosphorus – form glassy coating on the catalyst surface, not...
TRANSCRIPT
November 24, 2014
Rita Aiello – Johnson Matthey, Catalyst Development and
Applications Scientist
Marc Rost - Johnson Matthey, Regional Manager
Michael Baran – Johnson Matthey, SCR Product Manager
Review of technology:
◦ Selective Catalytic Reduction of NOx (SCR)
Chemistry, NOx conversion and NH3 Slip
◦ Ammonia slip catalyst (ASC) technology
◦ Catalyst poisoning mechanisms
Case studies using biogas
◦ Bio Energy Washington (BEW) landfill-gas dual-fuel power plant
◦ Orange County Sanitation District (OCSD) Technology Demonstration
Project
Relevant chemical reactions:
4 NH3 + 4 NO + O2 4 N2 + 6 H2O standard SCR reaction (fast)
4 NH3 + 2 NO2 + 2 NO 4 N2 + 6 H2O fast SCR (very fast)
4 NH3 + 5 O2 4 NOx + 6 H2O undesired reaction (above 425°C)
note: there are other reaction pathways but the above reactions are dominant in
lean exhaust
Reaction stoichiometry: one molecule NH3 reacts with one molecule of NOx
Urea sometimes used as NH3 source because it is easier to handle/store than NH3
One molecule of urea decomposes into two moles of NH3:
(NH2)2CO + H2O 2 NH3 + CO2
Lab reactor data: NOx conversion (solid) and corresponding NH3 slip (dashed) At ANR 0.8 the max NOx
conversion is 80% because there is insufficient NH3 to fully react with the NOx although NH3 slip is low At ANR 1.0 the max NOx conversion achieved is 90% with NH3 slip ≈ 12 ppm At ANR 1.2 excess NH3 enables slightly higher NOx conversion, but with much higher NH3 slip
Even the best SCR catalyst cannot achieve maximum NOx conversion with
non-uniform NH3 distribution
Non-uniform NH3 distribution can be a result of:
o Control system
o Gas flow characteristics
o Fluctuating load
Non-uniform NH3 distribution can result in localized ANRs:
◦ ANR < 1 results in incomplete NOx conversion
◦ ANR >1 results in NH3 slip
At high flow rates it is necessary to decrease ANR or increase catalyst volume
in order to maintain low NH3 slip
Challenge: High NOx conversion at low NH3 slip with reasonable
catalyst volume
Advances in ammonia slip catalyst (ASC) technology
significantly improve overall SCR performance
Characteristics of previous generation ASCs
• Excellent activity (high NH3 conversion)
• Poor selectivity (NH3 was oxidized to NOx)
Advanced ASC performs both oxidation function and SCR function:
• Some of the NH3 is stored in SCR component
• Some of the NH3 is oxidized to NOx by the oxidation component
• SCR component selectively converts stored NH3 + NOx to N2
• Oxidation component also converts CO to CO2 and enhances HC conversion
ASC can compensate for non-uniform NH3 distribution
ASC allows operation at higher ANR boosting NOx conversion while
maintaining low NH3 slip
(selectivity = fraction of specific product)
Advanced dual-function ASC:
• 50 ppm NH3 + 50 ppm HC fed to reactor
• Very active for NH3 conversion
• Highly selective to N2
Previous generation ASC:
• 20 ppm NH3 fed to reactor
• Very active for NH3 conversion
• Not selective to N2
NOx formed from NH3
N2O formed from NH3
Allows operation at higher ANR increasing NOx conversion at low NH3 slip
ASC improves HC conversion
Incomplete combustion of HC over V-SCR results in the formation of CO
ASC provides CO conversion
In some applications, use of ASC
can eliminate the need for an
oxidation catalyst
Benefits of adding ASC to SCR catalyst system
Engine data: equal volume SCR and SCR+ASC+DOC
• More compact housing results in
lower material costs
• High NOx, HC and CO conversions
achievable with SCR+ASC+DOC
• Low NH3 slip
Addition of DOC to emission control system may be necessary when very
high VOC and CO conversions are required
Engine data: equal volume SCR and SCR+ASC+DOC
Active materials are dispersed on high surface area oxide materials
example: Al2O3 , TiO2, CeO2, SiO2 (100-150 m2/g)
Catalytic material can be extruded into honeycomb structure or coated onto metallic and ceramic substrates to make flow-through catalysts
Catalyzed reactions occur on the surfaces of the catalyst particles
Microscopic catalyst structure
Catalyst particles dispersed on high surface area oxide support
Coated metallic and ceramic catalysts
Extruded SCR catalysts
SCR catalyst module
Major modes of SCR catalyst deactivation
mechanism description
poisoning strong chemisorption of species on or near catalytic sites, blocking or altering sites and inhibiting catalytic reaction
fouling, masking physical deposition of species on the catalytic sites and in pores of catalyst, physically blocking the sites
thermal loss of catalytic surface area, support area and catalyst-support interactions
attrition loss of catalytic material via abrasion, mechanical disruption of the catalyst structure
Catalyst deactivation: Poisoning
Poisoning is a result of the strong chemisorption of materials on catalytic sites:
• Active sites can be blocked
• Chemisorbed materials can interfere with reactants reacting with one another
• Strongly adsorbed materials can negatively modify the catalyst surface
Typical catalyst poisons:
• Lube oil components: P, Zn, Na, Ca, K
• Heavy metals: As, Pb, Hg
Many catalyst poisons adsorb irreversibly and cannot be easily removed
Materials physically block the pores and surface of the catalyst preventing exhaust
gas from contacting the catalyst particles:
• Soot, coke – can be removed by heat treatment
• Ash and dust – Soot blowers, sonic horns can prevent deposition and
accumulation, can also be removed by vacuuming or blowing
• Silica, phosphorus – form glassy coating on the catalyst surface, not easily
removed
Advanced gas cleaning technology removes silanes, siloxanes, etc., and results in
pipeline quality gas
Masking can be reversible or irreversible
Catalyst deactivation: Fouling and masking
Catalyst deactivation: ABS formation
• Unlike most other catalysts, vanadia-titania SCR is very resistant to sulfur
• At high S and low temperatures ammonium bisulfate (ABS) can form, plugging the
pores of the catalyst and fouling downstream equipment
270
280
290
300
310
320
330
340
350
NH3 * SO3 (ppm)
Tem
pe
ratu
re °
C
Minimum operating temperature f (NH3, SO3, H20, catalyst type)
increasing H2O
Catalyst deactivation – thermal: sintering of the active metal
crystallites
Catalyst particles are highly dispersed on oxide support
At high temperatures, particles become mobile can agglomerate into larger particles (sintering).
Because larger particles have lower surface area than smaller particles the overall surface area of the catalyst is reduced
Reducing the surface area of the catalyst decreases the activity of the catalyst Sintering of the metallic crystallites is irreversible
Catalyst deactivation – thermal: sintering of the oxide support
Reducing the surface area of the catalyst decreases the activity of the catalyst Sintering of the oxide support is irreversible
High surface area Anatase TiO2 is a commonly used SCR catalyst support material. Exposure to high temperatures causes changes in the crystalline structure of the resulting in decreased surface area
Anatase TiO2
high surface area
Rutile TiO2
low surface area
Example - Anatase to Rutile TiO2 phase transformation:
Catalyst deactivation: Attrition
Attrition is a common problem for SCR catalysts in coal-fired power plants
Abrasive SiO2-based ash moves through the catalyst channels at a high linear
velocity
Catalyst is rapidly worn away
Plate-based SCR catalyst was developed to be much more resistant to attrition
than extruded or coated ceramic SCR catalyst
Comparison of plate SCR catalyst (L) and extruded SCR catalyst (R) after operating under the same conditions in the same plant.
Case Studies
Bio Energy Washington (BEW) landfill-gas dual-fuel power plant
Location - Cedar Hills gas processing facility (WA)
Largest landfill gas-to-pipeline quality gas facility in the USA
Approximately 4.5 million cubic feet of gas produced per day
Clean methane gas sold to Puget Sound pipeline
Bio Energy Washington (BEW) power plant
18 Detroit Diesel 350 kW engines
Electrical power generating capacity of 6 MW
Generates up to 95% of the power required by gas processing
facility
Schematic of BEW Cedar Hills facility
Catalyst housing containing SCR and oxidation catalysts
Exhaust gas
compressed air urea
Cleaned exhaust gas
• 18 dual-fuel 350 kW engine generators fueled by blended landfill gas and diesel
• Each SCR + oxidation catalyst
system treats the exhaust from 6 engines
• Emission control systems installed 2008
• Feed forward urea control based on engine load and real time fuel blending
Emission Raw Engine Emissions Achieved Emissions
NOx 10 g/bhp-h < 1.0 g/bhp-h (90%+)
formaldehyde (CH2O) 0.30 g/bhp-h < 0.031 g/bhp-h (90%+)
ammonia NA < 7 ppmv
Case Studies
Orange County Sanitation District (OCSD) Technology Demonstration Project
OCSD operates two wastewater treatment plants and one central power generating system to provide power for plant operations
◦ Plant 1: Fountain Valley – 3 engines, 7.5 MW total
◦ Plant 2: Huntington Beach – 5 engines, 16 MW total
◦ Central Power Generating Station – Located in Plant 1
Digester gas (by-product of anaerobic digestion of wastewater solids) is used to fuel eight 2.5 - 3 MW engines (95% digester gas + 5% NG)
SCAQMD Rule 1110.2 requires lower NOx, VOC and CO emissions
Pilot study was conducted on Engine 1 in Plant 1 from October 2009 to March 2011 to determine if Rule 1110.2 emissions could be attained
Schematic of OCSD pilot test facility
OCSD Plant 1 digester gas composition gas cleaning system - inlet:
Component Average concentration
CO2 33.9%
CH4 58.7%
N2 2.2%
O2 0.6%
H2S 26.4 ppmv
*other sulfur compounds
2 ppmv
Total siloxanes 5.5 ppmv
OCSD Digester Gas Cleaning System
Activated carbon bed with 9,900 lbs capacity
*sulfides, mercaptans, thiols
• Both hydrogen sulfide (H2S) and siloxanes were monitored
• H2S was used as indicator of contaminant breakthrough
• The carbon media was replaced 3 times during the demonstration period
• The system outlet concentrations were:
146 million ft3 gas treated (2 ppm H2S, 0.248 ppm siloxanes)
169 million ft3 gas treated (2.5 ppm H2S, siloxanes <MDL)
157 miillion ft3 gas treated (1.76 ppm H2S, siloxanes <MDL)
Gas Cleaning System - Outlet Composition
OCSD Demonstration Engine – Plant 1
Cooper Bessemer LSVB-12-SGC:
• 3471 hp
• lean-burn
• engine drives 2.5 MW
generator
• heat recovery steam generator
Urea Injection
Oxicat
SCR
• Oxidation catalyst followed by SCR
• NOx and CO CEMS • Feed forward urea control
based on engine load and fuel composition
Emission SCAQMD 1110.2 Limit* Achieved* (% Reduction)
NOx 11 6.6 (78-86%)
CO 250 7.9 (96%)
VOC 30 3.6 (96%)
NH3 slip 10 < 0.5
*concentrations are dry and corrected to 15% O2
OCSD demonstration project - summary
• Significant reductions in CO, VOCs and NOx emissions were attained with demonstration system
• Installation of emission control systems on remaining engines is in progress
• It was not necessary to replace either the oxidation catalyst or the SCR catalyst during approximately 4.5 years of operation
• During operation, there was a failure of the gas cleaning system • H2S and siloxanes deposited on the oxidation catalyst reduced efficiency • Oxidation catalyst was washed and placed back in service- activity was
restored
References
http://jmsec.com/Library/Fact-Sheets/Application_Fact_Sheet_1306-Ingenco.pdf
http://jmsec.com/Library/Fact-Sheets/Application_Fact_Sheet_1304-Orange_County_Sanitation_District.pdf
http://www.wcsawma.org; Orange County Sanitation District Technology Demonstration Project; A&WMA West Coast Section: Biogas Engine Catalyst & Gas Pretreatment Workshop – May 16th, 2013
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