A brief history of syngas: from town gas to Fischer-Tropsch

Syngas (short for "synthesis gas" or "synthetic gas") has a history that predates the oil era by decades. In the second half of the 19th century, European and North American cities were lit and heated by "town gas", obtained from coal carbonisation. This mixture of H₂, CO and CH₄ was distributed through iron pipe networks, supplying street lamps, domestic stoves and the first gas engines. Berlin, Paris, London, Milan, all had their gasworks, with cylindrical gasometers topped by mobile domes that dominated the industrial skylines of the era.

In the 20th century, the Fischer-Tropsch process, developed in Germany in the 1920s, brought the synthesis of syngas (produced from coal gasification) to an extraordinary industrial scale: synthetic fuels for aircraft and military vehicles during the Second World War. Today, the same chemical reaction is being intensively studied as a vector for the production of renewable synthetic fuels (e-fuels) from green hydrogen and captured CO₂.

In the BioGS-1.0, syngas is the intermediate energy carrier that transforms solid biomass into electrical energy and heat. It is not synthesised to produce fuels, but used directly as the fuel for the external burner of the Stirling engine.

Syngas vs biogas: two gases, two processes, two uses

The terms "syngas" and "biogas" are often confused, but they describe profoundly different products and processes:

  • Biogas: produced by anaerobic digestion of organic biomasses (livestock effluents, OFMSW, sludge). It is a mixture of CH₄ (50–70%) and CO₂ (30–50%), with traces of H₂S. The process is biological, at low temperature (35–55 °C), and takes weeks. Biogas contains less energy per Nm³ than natural gas.
  • Syngas: produced by thermochemical gasification of dry biomass. It is a mixture of CO, H₂, CH₄ and N₂, with a lower heating value of 4–6 MJ/Nm³. The process is thermochemical, at high temperature (700–900 °C), and takes only seconds. It does not contain CO₂ in significant quantities (it is converted to CO in the reduction zone).

Syngas burns with a hot, stable flame, suitable for feeding industrial burners and gas engines. Biogas, due to its more dilute composition, generally requires upgrading (CO₂ removal) to reach biomethane quality for grid injection.

Composition of syngas produced by the BioGS-1.0

The composition of syngas at the outlet of the BioGS-1.0 gasifier, measured by continuous infrared spectroscopy, varies within a typical range (dry basis, after filtration):

  • CO (carbon monoxide): 15–19% vol., primary energy carrier, produced in the reduction reactions
  • H₂ (hydrogen): 13–16% vol., high calorific value (285 kJ/mol), produced by the water-gas reaction and hemicellulose decomposition
  • CH₄ (methane): 4–6% vol., still present after the reduction zone, contributes to total calorific value
  • CO₂ (carbon dioxide): 14–17% vol., product of partial oxidation reactions, dilutes the syngas without contributing to calorific value
  • N₂ (nitrogen): balance, derived from process air; it is the main diluent in air-gasification syngas (vs. pure oxygen gasification, which produces syngas with a higher calorific value but requires far more expensive plant)

The lower heating value (LHV) of the dry syngas produced is typically 4.5–5.5 MJ/Nm³, compared to 36 MJ/Nm³ for natural gas.

The tar problem and the downdraft solution

The main technical problem in biomass gasification for micro-cogeneration applications is not the syngas composition in terms of CO, H₂ and CH₄, the real problem is tar: a complex mixture of condensable polycyclic aromatic hydrocarbons (naphthalene, anthracene, benzene, phenanthrene and dozens of other compounds) that form during biomass pyrolysis and accompany the syngas as high-temperature vapours.

Tar is not simply a "gas impurity": it is an energy carrier that, if not managed correctly, condenses on cold surfaces forming tarry deposits that block pipework, coat metal walls and jam moving parts. The tar problem is historically the primary cause of failure in small-scale gasification systems, which can operate in the laboratory but cannot withstand the operating conditions of a real-environment plant.

The solution adopted in the BioGS-1.0 is multi-layered:

  • Downdraft geometry: syngas passes through the oxidation zone (700–1,000 °C) where tar is thermally cracked. The thermal cracking efficiency in downdraft geometry significantly reduces tar concentration, from 50–150 g/Nm³ (updraft) to 0.5–5 g/Nm³.
  • Extended gas residence time in the reactor: syngas remains at high temperature (700–1,000 °C) for an extended period (several seconds). This allows greater effectiveness of thermal tar cracking as well as complete development of the reduction reactions (Boudouard: C + CO₂ → 2CO and water-gas: C + H₂O → CO + H₂).
  • External combustion: even with good cracking, small quantities of tar and particulate may remain in the syngas. By adopting the Stirling engine, syngas combustion occurs continuously and externally to the engine itself, turning a technological limitation into a useful energy contribution. The burner can thus be optimised to handle low-LHV gas, ensure optimal air-fuel mixing and maintain a stable, continuous, well-anchored flame.

Syngas as the energy carrier of the future

In the broader context of the energy transition, syngas from biomass occupies a unique strategic position: it is the only renewable gaseous energy carrier producible locally, without hydrogen storage infrastructure and without dependence on distribution networks. It can be produced from agricultural and forestry residues that would otherwise decompose or be burned in the field, with uncontrolled emissions.

Current research at power scales above 1–10 MWe is exploring the use of biomass syngas for the synthesis of methanol, ammonia and synthetic fuels (power-to-X) via the Fischer-Tropsch process. At these scales, syngas becomes a key element of the renewable chemical industry.

The BioGS-1.0 sits within this scenario as the micro-scale, distributed, local, zero-infrastructure building block that makes it possible to valorise diffuse residual biomasses across the territory.