Catalytic Converter Working Principles & Performance: Natural Gas (Methane) vs. Methanol Engines under Stoichiometric & Lean-Burn Conditions
A detailed comparison of Three-Way Catalytic Converters (TWC) for natural gas (primarily methane, CH₄) and methanol (CH₃OH) engines, covering both stoichiometric (λ≈1) and lean-burn (λ>1) combustion modes.
I. Core Differences in Catalytic Requirements Based on Fuel Properties
Property Natural Gas (Methane) Methanol
Chemical Formula CH₄ (saturated alkane) CH₃OH (oxygenated fuel)
H/C Ratio High (4:1) → produces more H₂O vapor Very High (4:1) → even more H₂O vapor
Oxygen Content 0% → relies entirely on air for combustion 50% wt → lower O₂ demand for combustion
Main Emissions CH₄ (unburned HC), NOx Formaldehyde (HCHO), unburned CH₃OH, NOx
Sulfur Content Trace (requires desulfurization) Near-zero (synthetic methanol is sulfur-free)
Ignition Difficulty High CH₄ bond energy (435 kJ/mol) → hard to oxidize Oxygenated molecule → easy to oxidize but prone to aldehyde intermediates
II. Stoichiometric Combustion (λ≈1) Mode Comparison
Reactions rely on high-activity Pd/Rh:
CH₄ + 2O₂ → Pd → CO₂ + 2H₂O
2NO + 2CO → Rh → N₂ + 2CO₂
Oxygen Storage Capacity (OSC): Ce-Zr oxides buffer λ fluctuations to maintain CH₄ oxidation efficiency.
Performance Challenges:
High CH₄ light-off temperature (>350°C) requires high Pd loading (5-10 g/ft³).
Low CH₄ conversion at low temperatures (<200°C, efficiency ≈ 0%).
Formaldehyde (HCHO) is the core issue → must be prioritized for oxidation:
CH₃OH + O₂ → HCHO + H₂O
HCHO + O₂ → Pt → CO₂ + H₂O
NOx reduction path is similar to natural gas, but less CO is generated.
Performance Advantages:
Low light-off temperature (<150°C) due to easy oxidation of CH₃OH.
Lower precious metal loading (Pt/Pd ≈ 20-30 g/ft³).
Example applications:
Substrate 267*152: 25 g/ft³
Substrate 170*152: 40 g/ft³ (front) + 20 g/ft³ (rear)
III. Lean-Burn (λ>1) Mode Comparison
Lean phase: NO → NO₂ → adsorbed as Ba(NO₃)₂.
Rich phase: brief fuel-rich pulses release and reduce NOx.
Performance Limitations:
CH₄ oxidation is difficult: excess O₂ in lean conditions inhibits Pd activity → CH₄ conversion drops sharply.
Sensitive to sulfur poisoning (requires periodic 650°C desulfurization).
Reasons:
Low exhaust temperature (oxygenated fuel has lower flame temperature) reduces LNT adsorption efficiency.
Excess H₂ (from methanol reforming) enhances low-temperature SCR activity.
Fallback (LNT+):
Requires formaldehyde capture layer: zeolite adsorbs HCHO to prevent escape.
Uses H₂ for efficient NOx reduction during rich phases:
2NO + 2H₂ → N₂ + 2H₂O
IV. Key Performance Comparison
Feature Natural Gas TWC (λ≈1) Methanol TWC (λ≈1) Natural Gas LNT (λ>1) Methanol Lean Solution
Main Challenge CH₄ low-temp oxidation Formaldehyde control CH₄ oxidation + S poisoning HCHO control + low-temp NOx reduction
Light-off Temp >350°C (CH₄) <150°C (CH₃OH) 300°C (NOx adsorption) 180°C (SCR) Precious Metal Load High Pd (5-10 g/ft³) Low Pt/Pd (15-20 g/ft³) Very High (Pt-Rh-Ba) Medium (Fe/Cu zeolite SCR) Conversion Efficiency CH₄: >95% (at high temp) CH₃OH/HCHO: >98% NOx: 70-90% (fluctuating) NOx: >90% (SCR)
Sulfur Resistance Requires desulfurization Sulfur-free Frequent desulfurization needed No sulfur impact
Steam Sensitivity Medium (accelerates OSC aging) High (catalyst sintering) High (adsorbent hydrolysis) High (zeolite hydrothermal stability)
System Complexity Simple Medium (HCHO monitoring) Complex (regeneration + desulfurization) Complex (SCR urea injection)
V. Application Summary
Natural Gas/Methane Engines:
Stoichiometric + TWC: Mature technology for buses/heavy trucks → mainstream choice.
Lean-Burn + LNT: Experimental only, limited by CH₄ purification challenges.
Methanol Engines:
Stoichiometric + TWC: Used in ships/static power stations → requires enhanced HCHO oxidation layer.
Lean-Burn + SCR: High-efficiency route (e.g., methanol hybrids) → utilizes H₂ to boost low-temperature SCR activity.
Technology Trend: Methanol is more suitable for low-emission demands due to its sulfur-free nature and easy oxidation. Natural gas requires breakthroughs in low-temperature CH₄ oxidation catalysts (e.g., Pd-CeO₂ nanostructures).