Optimised processes for producing wood-based synthetic natural gas

Biomass containing lignin, such as wood and straw, can only be transformed into a combustible product gas via thermochemical processes such as gasification. This project explored the different steps involved in producing synthetic natural gas based on biomass (biomethane or bio-SNG) by means of gas cleaning and methanisation via catalytic fluidised bed. The fluid bed methanation works well at the pilot scale but further research is necessary before it can be implemented in larger production plants. The project focused on investigating the size and movement of bubbles in the reactor, the kinetics of secondary reactions and what conditions allow for the highest methane yield.

The availability of many renewable energy sources is linked to place and time of day. These limitations do not apply to Bio-SNG: plant locations can be chosen freely and the product can be fed into the existing natural gas network anywhere and at any time. Bio-SNG is a local, renewable and CO2-neutral substitute for natural gas and it can be used as fuel for natural gas vehicles.

Existing computer models about fluid bed methanation had not been tested thoroughly. An important aim of the project was therefore to compare the data of the pilot plant with the simulation. On this basis, the researchers hope to upscale the fluid bed methanation to that of commercial plants and to optimise the processes. They experimented with variables such as temperature, pressure, gas flows and gas composition.

The results indicate how to simplify gas cleaning significantly compared with the current state of the art. The project elaborated a methodology to determine the reactions of the important side component ethylene. The same approach can now be adopted to deal with benzene, the second key component.

The techniques elaborated in the project can also be used in power-to-gas applications: a technology that converts renewable electrical power to a gas that can be used as fuel for vehicles or heating systems. By methanising the carbon dioxide share it should be possible to improve biomethane yield.

In the process of methanising the gas obtained in a wood gasifier, bubbles rise up in the reactor and continuously mix up catalyst particles. This phenomenon improves the thermal transfer to the heat exchanger tubes and contributes to a more consistent process. The constant movement in the reactor also means that the catalyst is less affected by unsaturated compounds such as ethylene and benzene. The project showed experimentally what technical conditions allow for the highest methane yield. The results can now be used in a significantly improved reactor model.

Predicting the complex coupling of chemistry and hydrodynamics in fluidised bed methanation reactors for SNG production from wood (bio-SNG – fundamentals of methanation)