NOx Reduction Techniques

NOx Reduction Techniques

Nitrogen oxide (NOx) emissions from combustion equipment must be minimized to meet increasingly stringent air quality regulations (9-60 ppm depending on jurisdiction and source size). NOx reduction strategies include combustion modification techniques achieving 40-80% reduction through flue gas recirculation (FGR), staged combustion, and ultra-low NOx burner designs, and post-combustion treatment via selective catalytic reduction (SCR) achieving 80-95% removal for ultra-low emission requirements (<9 ppm). Selection among NOx control methods requires balancing capital cost ($100-500/kW for combustion controls, $200-1000/kW for SCR), operating cost (auxiliary power, catalyst replacement, chemical consumption), emission reduction effectiveness, and equipment retrofit feasibility.

NOx Formation Mechanisms

Thermal NOx (Zeldovich Mechanism)

Primary NOx formation route in most combustion:

Occurs at high temperatures (>2800°F) via atmospheric nitrogen oxidation:

$$\text{O} + \text{N}_2 \leftrightarrow \text{NO} + \text{N}$$ $$\text{N} + \text{O}_2 \leftrightarrow \text{NO} + \text{O}$$ $$\text{N} + \text{OH} \leftrightarrow \text{NO} + \text{H}$$

Rate equation:

$$\frac{d[NO]}{dt} = k_0 e^{-E_a/RT} \times [O_2]^{0.5} \times [N_2]$$

Where:

• $k_0$ = Pre-exponential factor
• $E_a$ = Activation energy ≈ 70,000 cal/mol
• $R$ = Universal gas constant
• $T$ = Absolute temperature (°R)

Temperature sensitivity:

NOx formation rate approximately doubles every 90°F above 2800°F.

Exponential temperature dependence:

At 3000°F vs. 2800°F (200°F increase):

$$\frac{Rate_{3000}}{Rate_{2800}} \approx e^{-70000/1.987 \times (1/3460 - 1/3260)} \approx 5.7 \times$$

Primary control strategy: Reduce peak flame temperature.

Fuel NOx

Formation from nitrogen compounds in fuel:

Organic nitrogen in fuel (primarily residual oils, coal) oxidized during combustion:

$$\text{Fuel-N} + \text{O}_2 \rightarrow \text{NO}$$

Fuel nitrogen content:

• Natural gas: <1 ppm (negligible fuel NOx)
• No. 2 oil: 50-500 ppm (10-30% of total NOx)
• No. 6 oil: 0.2-0.5% by weight (50-80% of total NOx)
• Coal: 0.5-2.0% (60-90% of total NOx)

Control strategy: Fuel staging (create fuel-rich primary zone to reduce fuel-N to N₂)

Prompt NOx

Formation in fuel-rich flame zones:

Hydrocarbon radicals react with atmospheric nitrogen:

$$\text{CH} + \text{N}_2 \rightarrow \text{HCN} + \text{N}$$ $$\text{HCN} + \text{O}_2 \rightarrow \text{NO} + \ldots$$

Significance:

• Minor contributor for natural gas (<10% of total)
• Occurs in fuel-rich zones regardless of temperature
• Difficult to control by temperature reduction alone

Combustion Modification Techniques

Flue Gas Recirculation (FGR)

Operating principle:

Recirculate portion of flue gas (10-30%) back to combustion air or fuel stream.

FGR ratio:

$$FGR% = \frac{\dot{m}{flue,recirculated}}{\dot{m}{air}} \times 100$$

Effects on combustion:

1. Reduces oxygen concentration:

   • Dilutes combustion air with inert gases (CO₂, H₂O, N₂)
   • Lowers O₂ from 21% to 16-18% in mixture
   • Reduces oxidizer availability

2. Increases heat capacity of mixture:

   • Flue gas has higher $C_p$ than air
   • More energy required to heat mixture to flame temperature
   • Reduces peak temperature

3. Reduces peak flame temperature:

$$\Delta T_{reduction} = \frac{FGR \times C_{p,flue} \times (T_{flue} - T_{air})}{C_{p,mix} \times (1 + FGR)}$$

Example: 20% FGR, flue gas 450°F, combustion air 70°F

$$\Delta T \approx \frac{0.20 \times 0.26 \times 380}{0.25 \times 1.20} \approx 66°F$$

Adiabatic flame temperature reduced approximately 200-400°F with 20-30% FGR.

NOx reduction effectiveness:

$$\text{NOx}{FGR} = \text{NOx}{base} \times e^{-k \times FGR}$$

Where $k \approx 0.04-0.06$ depending on burner design.

For 20% FGR, $k = 0.05$:

$$\text{NOx}{FGR} = \text{NOx}{base} \times e^{-0.05 \times 20} = 0.368 \times \text{NOx}_{base}$$

NOx reduction: 63%

FGR system configurations:

External FGR:

Components:

• FGR fan (induced draft)
• Flue gas ductwork from stack to burner
• FGR damper (modulating control)
• Temperature control (cool flue gas if needed)

FGR fan sizing:

$$\dot{V}{FGR} = \dot{V}{air} \times FGR%$$

For 10 MMBtu/h burner, 20% FGR:

• Combustion air: 1825 scfm
• FGR required: $1825 \times 0.20 = 365$ scfm (at flue gas temperature)

At 450°F flue gas:

$$\dot{V}_{FGR,actual} = 365 \times \frac{910}{520} = 639 \text{ acfm}$$

Fan static pressure: 4-10 in w.c. typical (ductwork + burner back pressure)

FGR control:

• Modulate FGR damper to maintain target FGR ratio
• Interlock with burner firing rate (proportional FGR)
• Temperature limit to prevent overheating burner

Internal FGR (induced FGR):

Operating principle:

• High-velocity combustion air creates low-pressure zone
• Induces furnace flue gases into burner without external fan
• Burner design feature (no external ductwork)

Advantages:

• No external FGR fan or ductwork
• Lower capital cost
• Simpler installation
• Lower maintenance

Limitations:

• FGR ratio lower (15-25% vs. up to 30% for external)
• Dependent on furnace pressure (requires slightly negative furnace)
• NOx reduction limited to 40-60% vs. up to 70% for external FGR

Staged Combustion

Air staging:

Primary combustion zone:

• Sub-stoichiometric combustion (fuel-rich)
• Equivalence ratio $\phi = 1.3-1.8$
• Insufficient oxygen limits NOx formation
• Temperature moderate due to incomplete combustion

Secondary combustion zone:

• Downstream air injection (20-40% of total air)
• Completes combustion
• Lower temperature (already partially cooled products)
• Minimal NOx formation in secondary zone

Overall equivalence ratio:

$$\phi_{overall} = \frac{(\text{F/A}){actual}}{(\text{F/A}){stoichiometric}}$$

For lean overall combustion: $\phi_{overall} = 0.9-0.95$ (5-10% excess air)

NOx reduction mechanism:

• Primary zone: Low O₂ limits thermal NOx despite moderate temperature
• Secondary zone: Lower temperature limits thermal NOx despite O₂ available

Reduction effectiveness: 50-75% vs. conventional single-stage burner

Fuel staging:

Primary combustion zone:

• Majority of fuel (70-80%) burned with excess air
• Lean combustion ($\phi = 0.7-0.9$)
• Lower flame temperature

Secondary combustion zone:

• Remaining fuel (20-30%) injected downstream
• Reburning zone
• Reduces NOx formed in primary zone

Reburning chemistry: Hydrocarbon radicals from secondary fuel reduce NO to N₂:

$$\text{NO} + \text{CH}_i \rightarrow \text{HCN} \rightarrow \text{N}_2$$

Tertiary air:

• Final air injection completes combustion
• Oxidizes CO and unburned hydrocarbons

Reduction effectiveness: 50-70%

Ultra-Low NOx Burner Design

Design features for ultra-low NOx (<30 ppm, <9 ppm for ultra-low):

1. Lean premix combustion:

   • Fuel and air thoroughly premixed upstream
   • Operate lean ($\phi = 0.6-0.9$)
   • Uniform stoichiometry prevents local hot spots
   • Peak temperature reduced 200-400°F

2. Surface stabilized combustion:

   • Flame stabilized on porous ceramic or metal fiber matrix
   • Heat loss to surface further reduces gas temperature
   • Radiant emission from surface increases efficiency

3. Delayed mixing:

   • Fuel and air streams segregated initially
   • Gradual mixing along flame length
   • Avoids stoichiometric combustion at peak temperature
   • Creates distributed reaction zone

4. Multiple flame zones:

   • Burner divided into multiple small flames
   • Each zone operates lean
   • Distributed combustion reduces peak temperatures

5. Internal flue gas recirculation:

   • Burner induces FGR without external system
   • 15-25% effective FGR
   • Reduces O₂ concentration and temperature

Control requirements:

Ultra-low NOx burners require:

• Precise air-fuel ratio control (±2% accuracy)
• Oxygen trim mandatory
• Electronic control system
• High-quality fuel pressure regulation
• Wobbe index variation <±3%

Performance:

Water or Steam Injection

Operating principle:

Inject water or steam into combustion zone:

• Water evaporates, absorbing heat
• Steam increases heat capacity of mixture
• Peak flame temperature reduced

Injection rate:

$$\frac{\dot{m}{water}}{\dot{m}{fuel}} = 0.5 - 2.0 \text{ (mass ratio)}$$

Higher ratios achieve greater NOx reduction but increase efficiency penalty.

Temperature reduction:

Heat of vaporization of water:

$$Q_{evap} = \dot{m}{water} \times h{fg}$$

For water at 60°F injected into 3500°F flame:

$$Q = \dot{m}{water} \times [h{fg} + C_p(T_{flame} - T_{water})]$$

Approximately 1000 Btu/lb water:

$$\Delta T \approx \frac{\dot{m}{water} \times 1000}{\dot{m}{products} \times C_p}$$

NOx reduction:

Approximately 10-20% NOx reduction per water/fuel mass ratio of 1.0.

Efficiency penalty:

Water/steam carries away sensible heat in flue gas:

$$\eta_{loss} = \frac{\dot{m}{water} \times C_p \times (T{stack} - T_{ambient})}{Q_{input}}$$

Typical penalty: 1-3% efficiency loss

Applications:

• Emergency NOx control (temporary measure)
• Gas turbines (common)
• Boilers (less common due to efficiency penalty)
• Deprecated for modern installations (better methods available)

Post-Combustion Treatment

Selective Catalytic Reduction (SCR)

Operating principle:

Inject ammonia (NH₃) or urea upstream of catalyst bed. Catalyst promotes selective reaction of NH₃ with NOx to form N₂ and H₂O:

$$4\text{NH}_3 + 4\text{NO} + \text{O}_2 \rightarrow 4\text{N}_2 + 6\text{H}_2\text{O}$$ $$8\text{NH}_3 + 6\text{NO}_2 \rightarrow 7\text{N}_2 + 12\text{H}_2\text{O}$$

Catalyst types:

Vanadium-titanium (V₂O₅/TiO₂):

• Operating temperature: 450-850°F
• Most common for boilers and furnaces
• Tolerates some SO₂ and particulate
• Deactivation by ammonium bisulfate below 450°F

Zeolite:

• Operating temperature: 500-900°F
• Higher temperature capability
• Less SO₂ tolerance
• Used for gas turbines and engines

Precious metal:

• Operating temperature: 350-600°F
• Very active but expensive
• Low dust tolerance
• Specialty applications

System components:

1. Ammonia/urea injection:

   • Ammonia storage tank (anhydrous or aqueous)
   • Injection grid (uniform distribution critical)
   • Dilution air system
   • Flow control (proportional to NOx load)

2. Static mixer:

   • Ensures uniform NH₃ distribution
   • Located upstream of catalyst
   • Duct length: 10-20 duct diameters for complete mixing

3. Catalyst reactor:

   • Honeycomb or plate-type catalyst elements
   • Parallel flow channels minimize pressure drop
   • Multiple layers (2-4 typical) for high removal efficiency
   • Face velocity: 5-15 ft/s

4. Catalyst support structure:

   • Pressure vessel housing
   • Access doors for inspection/replacement
   • Insulation (maintain operating temperature)

Ammonia-to-NOx molar ratio (NSR):

$$NSR = \frac{\text{NH}_3 \text{ injected (mol)}}{\text{NOx} \text{ inlet (mol)}}$$

Typical operating range: NSR = 0.9-1.1

NOx removal efficiency:

$$\eta_{NOx} = \frac{NOx_{inlet} - NOx_{outlet}}{NOx_{inlet}} \times 100%$$

Typical performance: 80-95% removal efficiency

Example:

• Inlet NOx: 100 ppm
• Target outlet: 9 ppm
• Required efficiency: $(100-9)/100 = 91%$
• NSR required: Approximately 1.0

Ammonia slip:

Excess NH₃ not consumed (“slip”) causes:

• Visible plume (white NH₃ vapor)
• Ammonium salt formation (fouling downstream equipment)
• Odor (detectable at 5-10 ppm)

Typical slip target: <5 ppm NH₃

Trade-off: Higher NSR → better NOx removal but higher slip

Operating temperature control:

Temperature too low (<450°F typical):

• Low catalyst activity
• Ammonium bisulfate formation: $\text{NH}_3 + \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4\text{HSO}_4$
• Catalyst fouling and corrosion

Temperature too high (>850°F typical):

• Catalyst sintering (deactivation)
• NH₃ oxidation to NOx (counterproductive)

Solution:

• Locate SCR at proper point in flue gas path
• Or use auxiliary heating/cooling

Pressure drop:

Catalyst adds static pressure drop:

$$\Delta P_{catalyst} = 1-6 \text{ in w.c.}$$

Affects ID fan sizing and auxiliary power.

Catalyst life and replacement:

Deactivation mechanisms:

1. Thermal aging (sintering at high temperature)
2. Chemical poisoning (alkaline metals, phosphorus, arsenic)
3. Physical fouling (particulate blockage)
4. Ammonium salt formation (acid dew point operation)

Typical catalyst life: 3-5 years (boilers), 5-10 years (gas turbines with clean fuel)

Replacement cost: $100-300 per ft² of catalyst

Performance monitoring:

• NOx inlet/outlet (continuous)
• NH₃ slip (continuous or periodic)
• Pressure drop across catalyst (indicates fouling)
• Catalyst activity testing (annually)

Selective Non-Catalytic Reduction (SNCR)

Operating principle:

Inject ammonia or urea into furnace at 1600-2100°F. Thermal reactions (no catalyst) reduce NOx:

$$4\text{NH}_3 + 4\text{NO} + \text{O}_2 \rightarrow 4\text{N}_2 + 6\text{H}_2\text{O}$$ $$4\text{NO} + 2\text{(NH}_2)_2\text{CO} + \text{O}_2 \rightarrow 4\text{N}_2 + 4\text{H}_2\text{O} + 2\text{CO}_2$$

(Urea reaction shown in second equation)

Critical temperature window:

Optimal range: 1600-2100°F

• Below 1600°F: Insufficient thermal energy for reactions, high NH₃ slip
• Above 2100°F: NH₃ oxidizes to NOx (counterproductive)

Temperature sensitivity: ±100°F significantly affects performance.

System components:

1. Reagent storage and preparation:

   • Aqueous ammonia (19-29%) or urea solution (20-50%)
   • Storage tank with level monitoring
   • Pump and flow control

2. Injection system:

   • Multiple injection lances at different elevations
   • Atomizing nozzles (compressed air or steam atomization)
   • Temperature-based injection control

3. Control system:

   • Flue gas temperature measurement (multiple points)
   • Select active injection level based on temperature
   • Modulate reagent flow proportional to NOx load

NOx removal efficiency:

Typical: 30-70% (lower than SCR)

Example:

• Inlet NOx: 150 ppm
• SNCR efficiency: 50%
• Outlet NOx: 75 ppm

Normalized stoichiometric ratio (NSR):

$$NSR = 1.5-2.5 \text{ (higher than SCR due to incomplete mixing)}$$

Ammonia slip: Typically 5-20 ppm (higher than SCR)

Advantages vs. SCR:

• Lower capital cost ($50-150/kW vs. $200-1000/kW for SCR)
• Retrofit easier (inject into existing furnace)
• No catalyst replacement
• Lower pressure drop (no catalyst pressure drop)

Disadvantages vs. SCR:

• Lower NOx removal efficiency
• Higher NH₃ slip
• Temperature window critical (difficult in cycling boilers)
• Not suitable for <50 ppm NOx targets

Applications:

• Utility boilers (coal, oil, gas)
• Industrial boilers where 40-60% reduction adequate
• Combined with combustion controls for overall compliance
• Sources without space for SCR catalyst

Hybrid Systems (SNCR + SCR)

Configuration:

• SNCR injection in furnace (1600-2100°F zone)
• SCR catalyst in convection pass (450-850°F)

Benefits:

• SNCR provides 40-60% reduction
• SCR polishes to final target (<20 ppm)
• Smaller SCR catalyst than standalone
• Lower ammonia consumption than standalone SCR

Economics:

• Capital cost between SNCR-only and SCR-only
• Operating cost lower than standalone SCR (less catalyst replacement)

Applications:

• Utility boilers with stringent NOx limits
• Retrofit situations with space constraints
• Optimize capital vs. operating cost

NOx Control Strategy Selection

Regulatory Compliance Requirements

U.S. EPA NSPS (New Source Performance Standards):

• Subpart Dc (utility boilers >250 MMBtu/h): NOx limits 0.10-0.20 lb/MMBtu
• Subpart Db (industrial boilers): 0.10-0.30 lb/MMBtu depending on fuel
• RACT/BACT requirements vary by state

California SCAQMD (South Coast Air Quality Management District):

• Rule 1146.2 (boilers >2 MMBtu/h): 20 ppm @ 3% O₂ (after 2025: 9 ppm)
• Rule 1147 (process heaters): 20-40 ppm depending on category

Bay Area AQMD:

• Regulation 9-7: 30 ppm @ 3% O₂ for most sources

Northeast states (RACT requirements):

• Varies 30-60 ppm depending on size and fuel

Selection Matrix

Economic Analysis

Capital cost estimates:

Burner replacement:

• Low-NOx burner: $20,000-100,000 depending on size
• Ultra-low NOx burner: $50,000-200,000

FGR system:

• External FGR: $50,000-200,000 (fan, ductwork, controls)
• Internal FGR (burner feature): Included in burner cost

SCR system:

• Small boiler (5 MMBtu/h): $200,000-500,000
• Medium boiler (50 MMBtu/h): $1,000,000-3,000,000
• Large boiler (500 MMBtu/h): $5,000,000-20,000,000

Operating costs:

FGR:

• Fan power: 5-20 kW depending on size
• Annual cost: $2,000-10,000 at $0.10/kWh
• Maintenance: Minimal

SCR:

• Catalyst replacement: $50,000-500,000 every 3-5 years
• Ammonia consumption: $5,000-50,000/year depending on size
• Auxiliary power: $10,000-50,000/year
• Maintenance: $20,000-100,000/year

Efficiency impact:

• Low-NOx burner: Neutral to +1% (better mixing)
• FGR: -0.5 to -2% (higher stack losses)
• ULN burner: +1 to +3% (lean premix, high efficiency)
• SCR: -0.2 to -1% (pressure drop, heat loss to catalyst)

Decision Flowchart

Step 1: Determine required NOx limit (ppm @ 3% O₂)

Step 2: Evaluate combustion controls:

• If limit >60 ppm: Standard burner may suffice
• If limit 30-60 ppm: Low-NOx burner or FGR
• If limit 9-30 ppm: Ultra-low NOx burner required
• If limit <9 ppm: SCR likely required

Step 3: Assess retrofit constraints:

• Space available for SCR catalyst reactor?
• Flue gas temperature in SCR range (450-850°F)?
• Budget for SCR capital and operating costs?

Step 4: Consider fuel characteristics:

• Natural gas: Thermal NOx dominant, combustion controls very effective
• Residual oil: Fuel NOx significant, staged combustion or SCR needed

Step 5: Evaluate lifecycle cost:

• Calculate total cost of ownership (capital + operating over 20 years)
• Consider emission reduction cost-effectiveness ($/ton NOx removed)

Step 6: Select optimal strategy

Common outcomes:

• <30 ppm, natural gas: Ultra-low NOx burner
• <20 ppm, natural gas: ULN burner + O₂ trim
• <9 ppm, natural gas: ULN burner + small SCR
• <30 ppm, residual oil: Low-NOx burner + FGR + fuel switching
• <20 ppm, residual oil: SCR required

Best practice: Maximize combustion controls effectiveness before adding post-combustion treatment. SCR should be final polishing step, not sole control method.