Photo: Anni Norddahl

Energy and fertilizer from straw

Tuesday 12 Nov 13


Jesper Ahrenfeldt
Senior Scientist
DTU Chemical Engineering
+45 21 32 53 44
Biomass DTU, DONG Energy and Bregentved Gods estate intend to show that agricultural biomass can be used both as a renewable energy source and as a substitute for costly fertilizer.

Renewable energy technologies linked to agricultural food production are the way forward. Working together with DONG Energy and Bregentved Gods estate, a research team at DTU Chemical Engineering has set out to prove this very theory—and the initial findings are very positive.

Biomass is a limited resource and the energy sector needs more straw and other biomass for energy production. On the other hand, farmers do not feel they get enough for their straw, which they therefore put to good use in their fields in the form of fertilizer.
A possible solution to this dilemma could be to process the straw through a thermal gasification facility linked to a coal-fired power plant instead of spreading it directly onto the fields. Thermal gasification converts biomass into a combustible gas in an oxygen-poor environment, the bi-product being bio-ash. The gas can then be used to produce electricity and heating, as thermal gasification exploits 90-95 per cent of biomass energy.
“The bio-ash derived from gasification isn’t mixed with coal ash and can therefore be returned directly to the farmer’s fields as a substitute for phosphate and potassium fertilizers—and as a stable carbon layer in the soil,” explains Senior Scientist Jesper Ahrenfeldt.

Returning carbon to the soil

In 2012, researchers began a series of practical thermal gasification and bio-ash tests together with Bregentved Gods estate and DONG Energy. Bregentved Gods estate is one of Denmark’s biggest farms with 3,284 hectares of agricultural land. The estate uses the straw for soil improvement and is therefore ideally suited for the alternative straw treatment.
DONG previously acquired a thermal gasification unit developed by DTU and has now installed a 6 MW demonstration facility in Kalundborg called Pyroneer. The plant currently produces large volumes of pure bio-ash from straw.
“We spread about 20 tons of bio-ash onto Bregentved’s fields and set up simultaneous experiments in the lab to determine the effect of bio-ash on harvest yields, carbon levels, microorganisms and soil fertility in general. The initial results from the fields and the lab clearly indicate that bio-ash can be used to boost carbon levels and replace fertilizers,” explains PhD student Veronika Hansen.

Lots of energy in straw

In 2012, Denmark produced 5.5 million tons of straw, 1.6 million tons of which is used for energy—leaving an estimated unexploited resource of 2.1 million tons. So it makes perfect sense to take a closer look at how best to exploit this resource.
“We’re pleased that both thermal gasification and carbon return are highlighted in the Ministry of Climate and Energy’s recently published catalogue of ideas as a proposal for political negotiations on a future climate agreement,” says Senior Scientist Henrik Hauggaard-Nielsen.

The project is supported by the Villum Foundation.