synthesis gas

Synthesis gas, or syngas, is a mixture of carbon monoxide and hydrogen produced by passing steam and air (or oxygen) over hot coal, coke, or biomass. The primary reaction, called the water-gas reaction, occurs when steam contacts carbon above 700°C, yielding CO and H₂. A secondary shift reaction then adjusts the H₂/CO ratio depending on downstream use. The result is a combustible gas that serves both as fuel and as chemical feedstock for fertilizers, methanol, diesel, and other products.

The composition and yield depend heavily on feedstock and process conditions. Coal-derived syngas typically contains 40 to 50 percent CO, 30 to 40 percent H₂, with the remainder being CO₂, N₂, and water vapor. If air is the oxidant rather than pure oxygen, nitrogen dilution reduces the energy content significantly. Biomass gasification and natural gas reforming produce syngas with different CO/H₂ ratios; the Fischer-Tropsch process, for example, requires a ratio near 2:1, while methanol synthesis works best around 2 to 2.05.

Production and equipment

Industrial synthesis gas is made in fixed-bed, fluidized-bed, or entrained-flow gasifiers. Coal feed sizes range from fine powder to lumps depending on reactor type. Temperature, pressure, steam/air ratio, and residence time all affect product gas composition and tar content. Tar contamination is a persistent problem, as are alkali metal compounds that foul downstream catalytic converters. Raw syngas requires cooling, scrubbing, and often shift conversion to reach purity and composition specs for chemical synthesis.

The term "coal gas" historically referred to syngas made in town gas plants that supplied early urban heating and lighting before electricity dominated; those plants are largely obsolete in developed economies. Modern syngas production is capital-intensive and economically viable mainly in regions with cheap coal or biomass, or where integration into a larger chemical plant justifies the infrastructure. South Africa's Sasol plants and several Chinese coal-to-chemicals facilities operate large gasification units to produce syngas for downstream conversion.

Syngas is dangerous to handle: CO is colorless, odorless, and lethal at low concentrations, and the mixture is explosive within a wide fuel/air envelope. Safe operation demands robust sensing, inert blanketing during shutdowns, and strict isolation procedures. The shift toward renewable hydrogen and circular carbon sources is driving renewed interest in biomass gasification and in syngas derived from waste plastic, though economic and technical hurdles remain significant.