Bioenergy Potential from Waste Potatoes

Interface Food and Drink funded an industry–academia partnership project led by Agrico in partnership with Celtic Renewables, Hatton Farms, Edinburgh Napier University and the Scottish Biofuel Programme to address the question what is the ‘Bioenergy Potential from Waste Potatoes’. The project looked at the bioenergy potential using the ABE Fermentation process.

The project consisted of a mapping study aimed to quantify the level of potato loss and waste before reaching the consumer, a lab based study to determine the feedstock suitability for conversion followed on by testing for suitability for commercial scale up.

The mapping study focused on the Tayside area as 49% are grown. The loss via the supply chain is mostly
• Loss – Potatoes that did not reach their target market, although fit for human consumption do not reach the consumer so are of little value.
• Waste – potatoes unfit for human or animal consumption (diseased, rotten or mechanically damaged)

The estimated overall loss for Scotland is approximately 91,000 (t)/annum for Scotland, 44, t/annum for Tayside and 22,000 t/annum from respondents of survey conducted during the waste mapping study. 22,000 t/annum volume of waste presents a significant opportunity for the waste to bioenergy/biofuel project, for example,  Branston’s Anaerobic Digestion plant, in Lincoln uses 10,000 t/annum of ‘out of spec’ potatoes to produce 400 kWe of power.

There are three dominating markets in potato industry; fresh potato, seed potato and processed potato, each with its own supply chain. There is no major potato processing plant in Scotland. The high haulage prices and suitability of Scottish potatoes for processing (low in sugars and dry matter content due to colder, wet weather conditions) results in most of the ware potato being supplied to the fresh potato market.

There is little in terms of alternative markets for rejected potatoes in Scotland, other than animal feed or in a few cases, composting.

However, there is potential to convert these rejected potatoes into high value commodity chemicals and biofuels and potentially a higher grade animal feed, using the Acetone Butanol Ethanol (ABE) fermentation process.  This gives the farmer an extra source of income during poor growth seasons, providing business security and sustainability without impacting on the food market.

The study found that one strain, C. saccharobutylicum, was able to produce from nearly 430g potatoes a total of just over 18 grams per Litre (gL-1) of ABE (9.5 gL-1 acetone, 8.6 gL-1 butanol, and 0.23 gL-1 ethanol.

Further work was carried out with alternative bacterial strains, but this did not result in an improvement in yields. Later work involved fermentation without the use of enzymes. This proved successful, although the fermentations of this type took longer.

Potatoes are a carbon rich substrate, the project identified the need for an additional nitrogen rich feedstock to optimise the process and investigated the use of pot ale.

There are as number of alternative nitrogen rich sources that can be used, such as;
• Agricultural slurry
• Sewage
• Fish waste
• Chicken manure

However, potatoes and pot ale are indigenous, sustainable and available in abundance in Scotland.  So have great potential as commercial opportunity for Scotland, dealing with a disposal problem and generation of valuable and sustainable end products. 

Solid residues from successful fermentations were analysed for their suitability as animal feed by the Scottish Agricultural College (SAC Consulting). The material leftover from the process had a high protein content and other favourable characteristics that could potentially provide a high value livestock feed for young piglets in particular.  

The project research the potential for fermentation commercial scale-up. A 500 litre fermentation was carried out. This worked well, giving solvent yields similar to that obtained at laboratory scale. This indicates that the process can be scaled up successfully.

The ABE fermentation process and the supporting solvent recovery technology has been proven worldwide at industrial scale to produce biosolvents and only went into decline when the availability of cheap oil made the process uneconomical.  The end products could be used to generate an income, off-set the use of petrochemicals in vehicles and in heat and power generation.  This allows for economic savings in the business and energy is delivered from sources which are sustainable, renewable and essentially carbon neutral. 
This project has the potential benefit nationally because this process be applied to a number of regions across Scotland, allowing all these benefits to reach nationwide.

Alternative value added disposal routes: Micro Anaerobic Digestion

Although this project researched the potential for potato waste conversion through the ABE fermentation process this is only feasible if the feedstock is being converted in large quantities in a central industrial plant. A potential solution for smaller quantities and an on-site facility is micro-anaerobic digestion. Using potato waste as a feedstock for anaerobic digestion is proven and is currently in use. Brandston’s has an anaerobic plant installed on their Lincoln site . However, this option has not yet been looked at in a Scottish context.

The Scottish Biofuel Programme is keen to explore this option in partnership with a Scottish business. If you have a supply of reject potatoes and would like to explore anaerobic digestion as a value added disposal route please contact the Scottish Biofuel Programme on 0131 455 2217 or email biofuels@napier.ac.uk.
 

We can help you get started with Biofuels

Call us on 0131 455 2217 or email us at biofuels@napier.ac.uk