Until now making biogas from straw and other similar grain crop residues has been considered uneconomic by the AD industry. But, that was before the advent of bioextrusion technology.
Use of any food crops for energy production often attracts public criticism, although huge amounts of grain go to alcoholic drink production without that raising similar public fears of rising food prices.
Bioextrusion is likely to be in the news again soon because the UK government ministry, which is the administrator of the UK’s Renewable Transport Fuels Obligation, said in June that it would soon be announcing the successful applications for “development technology” status. This status, if given by the minister to the applicant companies under the Renewable Transport Fuel Credits (RTFCs) for development fuels, means additional financial help. See the full explanation of this, below the video.
Let’s Make Biogas from Straw Not Field Crops
Around 30 million tonnes of cereal straw are produced in Germany annually.
It has been estimated that 8 to 13 million tonnes of this could be used sustainably for different energetic paths of utilization.
Large quantities of straw are also produced in the UK, and throughout temperate climate regions globally.
Straw is one of the agricultural residues with the largest untapped potential for use as a biomass feed for biogas.
But, so far here has been only limited use of the energy in straw and what has been used has been based on thermal recovery, such as by pelletizing straw for domestic heating.
The disadvantages of this are the extremely large storage capacity needed for the dry material, as well as the high CO2 emissions from transport and processing.
The use of straw in anaerobic digestion
In contrast, the use of straw in anaerobic digestion seems sensible.
The nutrients and organic matter, which was not converted into biogas in the fermentation process, are available again as a high quality digestate after fermentation, with the resulting digestate available to sustain this as a cycle by, its use as a crop fertilizer.
But there is a problem with this. Straw has a very high content of lignocelluloses and a low portion of readily fermentable materials.
During the fermentation process this causes very long digestion times and a low biogas yield.
Also, straw tends to float in the digester, even after being shredded.
Unwanted floating layers can then easily become a mixing problem, again reducing biogas production.
While some digester mixers might be able to cope, the mixing energy used reduces the remaining energy which can be sold.
In Germany a company has devised a solution which they call bioextrusion.
The treatment (extrusion) of the straw has 3 beneficial effects:
1 – It reduces the particle size (fibre length) for reduced viscosity and easier mixing.
2 – The lignocellulosic structure is partially destroyed and,
3 – At the same time, the absorptive capacity of the straw increases, and the floating behavior of straw fibers inside the fermenter is much reduced.
After bioextrusion the straw is described as spreading almost perfectly in the operating volume of the fermenter.
Straw substrate which is modified by bioextrusion is then suitable for wet-fermentation in the standard CSTR process.
On arable farms bioextrusion may also be used to raise biogas output from other crop residues such as maize.
Want to know more?
Visit Rika Biogas Technologies at: www.bioextruder.co.uk
Phone: +44 (0)1746 714 704
Text based upon pdfs at: www.bioextruder.co.uk/about
More About the Biogas from Straw Concept
To make biogas from straw usually requires more than just a bio-extrusion device to crush and shear the straw so that it becomes more readily digestible to the AD process. A full system is often requires including the ancillary plant which includes the following equipment:
1. Straw bale opener
2. De-stoning module
3. Feed hopper
4. Metal removal
5. The mechanical bio-extrusion device.
Adopting such a technology can create totally new feedstock options. It will do this through the digestion of new biomass sources, such as straw and desiccated grasses.
Nevertheless, there is also scope to use bioextrusion to increase the biogas yield or reduce digestion time for traditional substrates such as maize or grass silage.
A fully integrated feeding and feedstock processing solution is described as ultimately reducing running costs by the main supplier. However, there is no assessment given as to the power that will need to be sacrificed in running the additional bioextrusion and related substrate pre-treatment plant itself. How significant those running costs in energy use and maintenance would need to be carefully assessed for any specific project.
Rika Biogas Biotechnology Rumoured to be One of the First to Be Granted New UK Government Assistance
First development fuel credits could be issued next month: UK govt
The UK’s Department for Transport could next month issue its first credits to projects that are classified as development fuels, attaining an important milestone for projects that qualify for multiple bonus crediting under government rules, project developers and traders have said.
The UK government ministry, which is the administrator of the UK’s Renewable Transport Fuels Obligation, on June 20 told market participants that it was assessing applications for Renewable Transport Fuel Credits (RTFCs) for development fuels.
“We have received applications, and if they are approved, it is feasible that these could be issued by next month,”
Tom Reid, an official with the DfT, told a quarterly stakeholder meeting. …
In December last year the DfT said four projects had been shortlisted to stage two of the dRTFC application process, including a straw-to-biomethane project operated by:
- Rika Biogas;
- a Velocys waste-to-jet fuel project;
- LanzaTech’s sustainable aviation fuel from waste-based ethanol and
- Advanced Thermal Conversion operated by Kew Projects Limited.
Source: EnergyCensus [RE]fuel Report 24 June 2019 Issue