The most economical source of carbon for recycling into fuel is flue-gas emissions from fossil-fuel combustion where it can be obtained for about US$7.50 per ton. However, this is not carbon-neutral, since the carbon is of fossil origin, therefore moving carbon from the geosphere to the atmosphere. Since carbonic acid in seawater is in chemical equilibrium with atmospheric carbon dioxide, extraction of carbon from seawater has been studied. Researchers have estimated that carbon extraction from seawater would cost about $50 per ton. Carbon capture from ambient air is more costly, at between $94 and $232 per ton and is considered impractical for fuel synthesis or carbon sequestration. Direct air capture is less developed than other methods. Proposals for this method involve using a caustic chemical to react with carbon dioxide in the air to produce carbonates. These can then be broken down and hydrated to release pure CO gas and regenerate the caustic chemical. This process requires more energy than other methods because carbon dioxide is at much lower concentrations in the atmosphere than in other sources.
Researchers have also suggested using biomass as a carbon source for fuel production. Adding hydrogen to the biomass would reduce its carbon to produce fuel. This method has the advantage of using plant matter to cheaply capture carbon dioxide. The plants also add some chemical energy to the fuel from biological molecules. This may be a more efficient use of biomass than conventional biofuel because it uses most of the carbon and chemical energy from the biomass instead of releasing as much energy and carbon. Its main disadvantage is, as with conventional ethanol production, it competes with food production.