Synthesis Gas Chemistry and Synthetic Fuels
This text is based on the Chapter:
Synthesis Gas to Hydrogen, Methanol and Synthetic Fuels, by J. van de Loosdrecht and J.W. Niemantsverdriet, published in the book “Chemical Energy Storage” (R. Schloegl, Ed.), De Gruyter, Berlin, 2013
Synthesis gas or briefly, syngas, is a mixture of carbon monoxide, carbon dioxide and hydrogen. Syngas can be produced from many sources, including natural gas, coal, biomass, or virtually any hydrocarbon feedstock, by reaction with steam or oxygen. Syngas is a crucial intermediate resource for production of hydrogen, ammonia, methanol, and synthetic hydrocarbon fuels.
The formation of syngas is strongly endothermic and requires high temperatures. Steam reforming of natural gas (or shale gas) proceeds in tubular reactors that are heated externally. The process uses nickel catalyst on a special support that is resistant against the harsh process conditions. Waste heat from the oven section is used to preheat gases and to produce steam. This plant generates syngas with H2/CO ratios in the range of 3-4, and is suitable for hydrogen production.
Partial oxidation of methane (or hydrocarbons) is a non-catalytic, large-scale process to make syngas and yields syngas with H2/CO ratio of about 2 . This is an optimal ratio for gas-to-liquids plants. A catalytic version of partial oxidation (CPO), based on short-contact time conversion of methane, hydrocarbons or biomass on e.g. rhodium catalysts, is suitable for small-scale applications [5,6].
Autothermal reforming (ATR) is a hybrid, which combines methane steam reforming and oxidation in one process . The heat needed for reforming is generated inside the reactor by oxididation of the feed gas. As POX, ATR is also suitable for large-scale production of syngas for gas-to-liquids or large-scale methanol synthesis processes.
Alternative routes to syngas, such as reduction of CO2 from flue gas with H2 from electrolytic splitting of water may become interesting from the viewpoint of storage of wind or solar energy .
Synthesis gas (syngas) can be produced from a variety of sources and is a versatile intermediate for production of chemicals and fuels. Gas-to-Liquids (GTL), Coal-to-Liquids (CTL), Biomass-to-Liquids (BTL) all rely on the catalytic conversion of syngas.
Reactors and process layout for syngas production from natural gas and shale gas.
Hydrogen from syngas
The water-gas-shift reaction
In the water-gas-shift reaction [8,9],
CO + H2O ⇋ CO2 + H2 – 41 kJ/mol (1)
CO is used as a reductor to shift syngas entirely to H2 (and CO2). The high temperature water gas shift uses iron oxide as a catalyst and proceeds at 300-500 dg.C. A low-temperature process (around 200 dg.C) based on a copper-zinc oxide catalyst drives the equilibrium further towards hydrogen, but requires clean feed gas. Pressure swing adsorption purification leads to high-purity hydrogen .
Syngas to Methanol
Methanol is a versatile intermediate for the chemical industry, but can also serve as a fuel. Even better is dimethyl ether, applicable as bottle gas for cooking (like camping gas) or as a substitute for diesel fuel. Methanol is also used in the transesterification of vegetable oils to produce biodiesel. Methanol is produced catalytically from a mixture CO2:CO:H2 = 5:5:90, at 50-100 bar and 225-275 dg.C over Cu/ZnO/Al2O3.
CO2 + 3 H2 ⇋ CH3OH + H2O – 47 kJ/mol (2)
or combined with the water-gas shift reaction (1) above
CO + 2 H2 ⇋ CH3OH – 91 kJ/mol (3)
Copper metal is the catalytically active phase, and ZnO is a chemical and structural promoter, while alumina is only a structural promoter.