11.3 Flameless and Catalytic Combustion

Flameless and catalytic combustion are game-changers in clean energy systems. They burn fuel without visible flames, using diluted reactants or catalysts to lower temperatures and spread out reactions. This leads to more uniform combustion and fewer emissions.

These techniques are all about efficiency and cutting pollution. By recirculating heat and using catalysts, they can burn fuel more completely at lower temps. This means less NOx, CO, and other nasty stuff coming out the exhaust pipe.

Flameless and MILD Combustion

Principles of Flameless Combustion

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• Flameless combustion occurs without visible flame due to highly diluted reactants
• Involves distributed reaction zones throughout combustion chamber
• Utilizes high temperature air preheating (above fuel auto-ignition temperature)
• Achieves uniform temperature distribution in combustion zone
• Reduces peak flame temperatures compared to conventional combustion

MILD Combustion and HiTAC

• MILD (Moderate or Intense Low-oxygen Dilution) combustion characterized by reactant dilution with combustion products
• High temperature air combustion (HiTAC) employs preheated air above 1000°C
• Both techniques reduce oxygen concentration in reaction zone
• Result in more uniform temperature distribution and extended reaction zones
• Achieve stable combustion at lower equivalence ratios

Heat Recirculation and Emissions Reduction

• Heat recirculation essential for maintaining high combustion temperatures
• Exhaust gases used to preheat incoming air through heat exchangers
• Flue gas recirculation dilutes reactants and lowers oxygen concentration
• Low NOx emissions achieved through reduced peak temperatures and oxygen availability
• CO and unburned hydrocarbon emissions also minimized due to uniform temperature distribution

Catalytic Combustion

Fundamentals of Catalytic Combustion

• Catalytic combustion occurs on catalyst surface without visible flame
• Utilizes catalysts to lower activation energy of combustion reactions
• Enables combustion at lower temperatures compared to conventional flames
• Catalyst materials typically include noble metals (platinum, palladium) or metal oxides
• Catalyst support structures provide high surface area for reactions (honeycomb monoliths, pellets)

Reaction Kinetics and Catalyst Performance

• Reaction kinetics in catalytic combustion governed by surface reactions
• Adsorption of reactants onto catalyst surface initiates reaction process
• Surface reactions proceed through series of elementary steps
• Desorption of products completes catalytic cycle
• Catalyst activity influenced by temperature, pressure, and reactant concentrations
• Catalyst deactivation occurs over time due to poisoning, sintering, or fouling

Combustion Efficiency and Applications

• Catalytic combustion achieves high combustion efficiency at lower temperatures
• Complete oxidation of fuel possible even at low equivalence ratios
• Reduced emissions of CO, unburned hydrocarbons, and NOx
• Applications include gas turbines, industrial burners, and catalytic converters
• Challenges include catalyst durability, pressure drop, and cost

Combustion Characteristics

Combustion Stability and Control

• Combustion stability refers to maintaining consistent and reliable combustion process
• Factors affecting stability include fuel-air mixing, residence time, and heat release rate
• Flameless and catalytic combustion offer improved stability over conventional flames
• Wider operating range achieved through extended flammability limits
• Control strategies involve adjusting air preheat temperature, fuel injection, and catalyst properties

Emissions Reduction Mechanisms

• Low NOx emissions in flameless combustion result from reduced peak temperatures
• Catalytic combustion minimizes NOx formation through lower reaction temperatures
• Both techniques reduce thermal NO formation pathway
• CO emissions reduced through complete oxidation at uniform temperatures
• Particulate matter emissions minimized in flameless and catalytic systems

Efficiency Improvements and Energy Savings

• Combustion efficiency enhanced through complete fuel utilization
• Heat recirculation in flameless combustion improves overall thermal efficiency
• Catalytic combustion allows for efficient low-temperature operation
• Reduced heat losses due to lower peak temperatures and more uniform heat distribution
• Energy savings achieved through fuel flexibility and extended turndown ratios