Nearly 15,000ha of sugar beet being grown this season in Germany is destined for biogas production, equivalent to about one-tenth of the total area in the UK given over to sugar beet.
It is part of the German government’s drive over the past decade to increase renewable energy generation, explained Volker Utesch of KWS SAAT at the recent World Association of Beet and Cane Growers (WABCG) conference held in Cambridge.
At the heart is the feed-in tariff, which gives generators a guaranteed price for their gas/electricity for 20 years. This is the scheme that the UK feed-in tariff introduced earlier this year, was modelled on.
There are 4780 anaerobic digester plants in Germany generating 1600MW, the equivalent of one-and-a-half nuclear power plants, he said.
Of these, there are about 800 on-farm plants producing biogas using energy crops grown on 700,000ha, most of which is maize for maize silage topped up with rye, sorghum and sugar beet.
“Hybrid rye tends to be grown in droughty areas of Germany where maize would struggle,” said Mr Utesch.
However, in the past two years, sugar beet has received more attention, as growers discovered the advantages this crop offered. “Sugar beet yields 400 litres of methane/kg of dry matter – 100 litres more than maize – because of its higher sugar content.
“With beet, 95% of the carbohydrate is converted to gas while for maize and sorghum, it is nearer 70% because of the higher lignin and cellulose content.
Another benefit of sugar beet is the speed of degradation, which three to four times faster than maize.
Trials have so far found that a mixture of 75% maize and 25% sugar beet works best in practice, he said.
To help the biogas sector, KWS has embarked on breeding programmes to develop specialist varieties in the four crops, such as an “energy beet”.
“We are currently looking at digestible dry matter output/ha, but there may be other traits we need to consider as we learn more. We still don’t know what an energy beet will look like. Will it be more like a fodder beet? We are still refining what direction to go in.”
However, Mr Utesch added that growers can already boost yields by 9-14% by tweaking management. “As quality is not as crucial for biogas as for sugar production, growers can apply more nitrogen fertiliser and gain an extra 5-6% yield. If you include crowns, that’s a further 5-8%.”
One practical problem encountered by German growers was preventing soil going into the digester, which impairs its performance. The solution? “We invented a mobile beet washer that can process 50-70t/hour.”
Another challenge was to store the beet, as the digester needs a continuous supply, 365 days a year. “We looked at various options, including storing pulp in silos, which is very expensive.”
Cheaper alternatives include mixing energy beet with maize in clamps, but the problem is that you have to harvest both crops at the same time. This means you don’t get the full yield potential, as sugar beet is harvested too early.
The best solution so far has been ensiling whole beet in maize clamps for several months. “One lesson is that you need plastic sheeting to keep the juice in the clamp or have a means to collect it. That’s because nearly 15% of the energy is in the juice.”
Looking ahead, Mr Utesch believes energy beet could eventually account for 10-15% of the total digester feedstock area grown in Germany. Equally, he believes there is huge potential for energy beet in the UK.
Bioethanol from beet
Bioethanol from sugar beet is more environmentally-friendly in terms of green house gas emissions than that from maize and wheat.
That is according to a major French study, which carried detailed lifecycle calculations for various biofuels.
“Each 1MJ of energy from sugar beet ethanol equates to 30.4g of carbon dioxide equivalents. This compares with 90.1g for petrol, which means switching from petrol to beet ethanol would cut emissions by two-thirds,” said Alain Jeanroy of the French general confederation of beet growers.
Ethanol from maize had a higher greenhouse gas emission of 39.8g and wheat was higher still at 46.2g.
However, sugar cane came out the best with only 25.3g of carbon dioxide equivalents. “This is because the by-product fibre (bagasse) is used as a fuel.”
For a wider global view of how the sugar sector could be at the forefront of biofuel expansion, see earlier FW coverage from WABCG 10.