CO conversion explained

CO conversion describes how much carbon monoxide is transformed into hydrocarbons during a chemical reaction such as Fischer-Tropsch synthesis.

In sustainable fuel production, CO conversion is a key measure of reactor efficiency because it directly affects fuel yield, recycle requirements, and overall plant economics.

What is CO conversion?

CO conversion is the percentage of carbon monoxide in synthesis gas that reacts during a catalytic process instead of leaving the reactor unconverted.

It is commonly expressed as:

 

CO conversion = carbon monoxide reacted ÷ carbon monoxide entering the reactor

 

Higher conversion means more carbon is captured into useful hydrocarbon products rather than remaining in the gas stream.

visual of molecules to support co conversion explained

Why does CO conversion matter in Fischer-Tropsch synthesis?

In Fischer-Tropsch synthesis, CO conversion determines how efficiently syngas is transformed into liquid hydrocarbons.

Higher conversion can improve:

  • Fuel yield per unit of feedstock
  • Carbon utilization
  • Reactor productivity
  • Downstream process efficiency

 

Lower conversion can raise plant complexity because it typically increases recycle requirements.

 

For reaction chemistry, see Fischer-Tropsch synthesis.

What is the difference between per-pass CO conversion and overall CO conversion?

Per-pass conversion measures how much carbon monoxide reacts in one pass through the reactor, while overall conversion includes recycle loops.

Per-pass conversion

This reflects reactor performance directly.

 

Overall conversion

This includes unreacted gas returned to the reactor system.

 

A plant may achieve:

  • Moderate per-pass conversion
  • Very high overall conversion after recycle

 

Both values are important in evaluating process design.

 

Note that systems with lower per-pass conversion require greater recycling to achieve similar overall conversion, which effects overall system efficiency and total carbon that converts to higher-value hydrocarbons.

What affects CO conversion?

CO conversion depends on catalyst performance, reactor design, temperature control, and syngas composition.

Key factors include:

  • Catalyst activity
  • Hydrogen-to-carbon monoxide ratio
  • Reaction temperature
  • Pressure
  • Residence time
  • Heat removal efficiency

 

Because Fischer-Tropsch synthesis is highly exothermic, temperature stability strongly influences conversion.

How does reactor design affect CO conversion?

Reactor design determines how evenly syngas contacts catalyst and how effectively heat is removed during the reaction.

Poor temperature control can reduce conversion by:

  • Creating hot spots
  • Accelerating catalyst degradation
  • Shifting product distribution toward unwanted, lower-value gases

 

Reactor architecture plays a major role in maintaining high and stable conversion rates.

 

See Fischer-Tropsch reactor for reactor fundamentals.

Photo of top vessel cover from an FT reactor from Fischer Tropsch reactor manufacturer Velocys
CO conversion explained photo showing molecular structure of catalyst

How does catalyst influence CO conversion?

Catalyst activity determines how quickly carbon monoxide reacts on the catalyst surface.

A highly active catalyst can increase conversion by:

  • Improving reaction kinetics
  • Supporting stable hydrocarbon chain growth
  • Maintaining selectivity toward valuable fuel fractions

 

Catalyst design must also resist deactivation, or utilize catalyst regeneration strategies, to sustain conversion over long operating periods.

 

See Fischer-Tropsch catalyst for catalyst details.

Is higher CO conversion always better?

Yes. Higher CO conversion usually improves carbon efficiency, but reactor conditions must remain balanced to preserve selectivity and catalyst life.

If conversion is pushed too aggressively without proper control:

  • Heat release can become difficult to manage
  • Undesired byproducts may increase
  • Catalyst stress can rise

 

Optimal conversion balances efficiency and long-term reactor stability.

What CO conversion levels are typical in Fischer-Tropsch systems?

Commercial Fischer-Tropsch systems vary widely depending on reactor type and process configuration.

Typical ranges include:

  • Moderate per-pass conversion in conventional systems
  • Higher per-pass conversion in advanced reactor designs
  • Very high overall conversion when recycle is used

 

Velocys microchannel systems are designed to support high per-pass conversion while maintaining temperature control in addition to utilizing recycle to maximise yields.

Why is CO conversion important for sustainable aviation fuel?

In SAF production, higher conversion improves how much feedstock carbon becomes finished fuel.

This matters because feedstock cost and carbon efficiency strongly influence SAF economics.

 

Improved conversion can help reduce:

  • Syngas recycle requirements
  • Energy demand
  • Capex
  • Cost per gallon of produced fuel

 

For aviation fuel context, see Syngas to SAF.

Frequently asked questions

What does CO stand for in CO conversion?

CO stands for carbon monoxide.

No. Conversion measures carbon reacted, while fuel yield measures how much desired fuel product is formed.

It reflects how efficiently the reactor performs before recycle is introduced.

Higher conversion can improve carbon utilization and reduce recycle load, which typically improves efficiency and impacts overall economics.