Content – Other fuels
Syngas describes various mixtures of carbon monoxide and hydrogen with minor amounts of carbon dioxide and possibly methane.
Syngas is produced from natural gas (methane), biogas (methane), naptha, coal, biomass, or any type of hydrocarbons, by reaction with steam or oxygen. Syngas is an intermediate resource in the GTL process for production of hydrogen, ammonia, methanol, and synthetic hydrocarbon fuels.
Since syngas is used as feedstock for different processes, composition and in particular the H2: CO ratios are different, depending on which process it will take part.
In the methanol synthesis, CO2 and CO are both reactants and the composition is determined by a module M = (H2-CO2): (CO+CO2) that preferably should be close to 2.0.
For Fischer – Tropsch synthesis, Gas to Liquid (GTL) applications, in which CO2 is not a reactant, the required synthesis gas compositions have a H2: CO ratio of about 2.0.
For aldehydes production the optimal H2: CO ratio needs to 1.0.
For synthesis of Ammonia/Urea and oil refining maximizing the hydrogen (H2) production is desired.
Production of synthesis gas from methane (natural gas or biomethane) occurs by steam methane reforming (SMR), partial oxidation (POX) or autothermal reforming (ATR), a combination of the first two.
Steam methane reforming
Steam methane reforming is the most commonly used method of reforming natural gas into hydrogen. A catalytic reaction takes place within tubes heated by an external source of hot gas. Steam and lighter hydrocarbons such as methane are converted into hydrogen and carbon monoxide (syngas).
Steam methane reforming uses a nickel-based catalyst. It proceeds in parallel with the water-gas shift reaction.
CH4+ H2O = 3H2+ CO
If utilised for H2 production the SMR step is followed by a Water Gas Shift step for CO conversion:
H2O + CO = H2 + CO2
Partial oxidation (POX) occurs when a substoichiometric fuel-air mixture is partially combusted in a reformer, creating a hydrogen rich syngas. Partial oxidation may be done with or without a catalyst.
Partial oxidation has traditionally been used in large-scale production of hydrogen from heavier hydrocarbons such as oil and coal including deasphalter pitch and petroleum coke but this production method also works for natural gas.
The reaction may be represented by the equation:
CH4 + ½ O2 = CO + 2H2
The hydrocarbons are pre-heated before being mixed with Oxygen within a high temperature burner. The syngas exit temeperatures are 1200 – 1400 oC. Cooling (heat recovery) and cleaning of the syngas is necessary.
The main advantage of using POX compared to steam reforming is the ability of utilizing a broad specter of feedstock qualities containing compounds that would be harmful to the SR catalyst
Neither steam reforming nor partial oxidation are ideal processes for producing synthetic fuel by use of the Fischer – Tropsch synthesis, as partial oxidation resulting in a H2: CO ratio ranging 1,4 – 1,9 (too low) and steam reforming results in a H2: CO ratio ranging 3 – 4 (too high).
A combination of steam reforming and partial oxidation, called Autothermal reforming (ATR), provides a H2: CO ratio ranging 1,9 – 3,2 which is better suited to the Fischer – Tropsch synthesis that require a ratio of around 2.
Autothermal reforming, combines catalytic steam and CO2 reforming with non-catalytic partial oxidation methane (natural gas and biogas) in a single reactor, by supply of steam and oxygen. The heat demand for the steam reforming is covered by combustion inside the reforming reactor.
The ATR reactor consists of a burner, combustion chamber and catalyst in a refractory steel container.
Autothermal reforming may use air, as for the production of ammonia (NH3) because of the need for nitrogen. For other processes such as the production of synthetic diesel, nitrogen is not desirable since it will provide large quantities of inert gas, which will reduce the yield and increase costs; oxygen is therefore preferable to air.
The reactions is represented by the following equations:
CH4 + 3/2O2 → CO + 2H2O
Follwed by catalytic steam and CO2 reforming
4CH4 + CO2 → 2CO + 2H2
The exit temperature of the syngas is between 950-1100 oC with an outlet pressure up to 100 bar.
An ATR reactor consists of a burner, combustion chamber and catalyst in a refractory steel container.
The reforming of natural gas is sometimes referred to as steam-methane reforming as natural gas is predominately composed of methane. Reforming uses a nickel-based catalyst to promote the reaction. It proceeds in parallel with the water-gas shift reaction according to the following reactions.