Production of synthetic natural gas (SNG)
by hydrothermal gasification of wet biomass






Synthetic natural gas (SNG) is a viable option for a renewable energy carrier when produced from biomass. It is also an option for making countries less dependent from imported fossil fuels. It contains largely methane, but also hydrogen may be present to some extent (about 10 vol%). Biomass wastes such as liquid manure contain large energy potentials. The economically expensive need to dry the biomass prior to gasification becomes obsolete with this process.

“Hydrothermal” designates an aqueous system at elevated pressures and temperatures, especially near the critical point of water (374°C, 22.1 MPa) or above it. Near-critical and supercritical water provides an interesting environment for carrying out chemical reactions.

Whereas salts are highly soluble in subcritical water, they precipitate out in supercritical water. Supercritical water is more like an organic solvent. With a suitable device, the salts can be separated in a continuous way from the biomass stream prior to gasification. This has several advantages. Not only could salts poison the catalyst, but once separated in a concentrated form, they can also be used as nutrients.

See also: Ion pair formation in solution of inorganic biomass constituents at and near the critical point of water

The process being studied in this project has several advantages over existing technologies that convert biomass waste into SNG:
  • Process efficiency is possibly higher (eta = 0.6 ÷ 0.7)
  • No need for drying first – the moisture is the reaction media itself
  • Products are clean water and SNG only – all possible hormones and bioactive proteins (e.g. prions) are destroyed. There is no solid residue that needs to be dried and burnt as hazardous waste.
  • Reaction side-product (carbon dioxide) can be easily and economically separated from the SNG as most of the CO2 is dissolved in water at system pressure that carries the unburnable gas out of the system.
The ability of supercritical water to destroy potentially hazardous substances is also used in the so-called SCWO (supercritical water oxidation) processes. The difference to our technology is that with SCWO, the organic material is oxidized ultimately up to CO2 and therefore made chemically inactive, whereas we focus on the conversion to SNG.

Projects

The aim of this PhD Study is to develop a process capable of producing synthetic natural gas (SNG) from wet biomass (waste wood slurries, manure and sewage sludge) by hydrothermal gasification. For the case of manure, the dissolved salts are collected as mineral nutriments for further applications. The SNG may be used for heating purposes or for the use as clean fuel in natural gas vehicles (Vision Ecogas).

Approach

Several pathways are combined within this PhD project:
  • Development of analytical methods to characterize the biomass in a complete way with respect to organics, inorganics and elemental composition
  • Co-Development and testing of several gasification catalysts
  • Development of tools to study the gasification visually at process conditions (400°C, 30 MPa)
  • Design of a continuously operating plant capable of processing real biomass up to 10 wt%
  • Modelling of several system components, e.g. salt separation with FEMLAB.
  • Experiments with real biomass and biomass model substances in batch wise and continuously operating plants
  • System- and economic analyses to justify the process not only in an ecologic way (it is CO2- neutral)

Exemplary Results

In the batch reactor, wood and manure slurries were gasified in presence of a gasification catalyst on nickel basis. See the impressive conversion in the following picture:


The products gained with catalytic gasification in our batch reactor corresponded to the thermodynamic equilibrium composition (the chemically reachable composition), as can be seen in the following graph: