Breaking the straw to biogas conundrum
With around 13 percent of the global cereals production 2013, the European Union (EU) is also one of the largest straw producers. Most of this is unused something that Netherlands-based biogas technology developers HoSt seeks to address. Recently it commissioned a 1.5 MWe manure-based biogas plant in Bulgaria with a novel dedicated straw input line.
According to 2013 figures from Eurostat, some 305.7 million tonnes of cereals including rice was harvested in the EU-28. This represents about 13 percent of global cereal production based on estimates by the United Nations’ Food and Agriculture Organisation (UN FAO). Of the EU-28 total land area, about 40 percent or 175 million ha is utilised agricultural area of which around 105 million ha was used for to grow cereals 2013.
– This annual cereal production results in enormous amounts of straw as each ha of cereal generates between 2 and 3 tonnes of straw yet maybe 5 percent is currently used for energy purposes. In fact most of the straw has no direct application but is directly ploughed under, said Herman Klein Teeselink, Director for HoSt.
According to Klein Teeselink, one tonne of digested straw can yield 300 – 400 Nm3 of biogas which, if upgraded is about 180 – 250 Nm3 biomethane or fossil gas equivalent.
– Based on a EUR 40 per tonne cost for straw, the feedstock cost works out at EUR 0.16 – 0.22 per m3 biomethane whereas the fossil gas price fluctuates EUR 0.16 – 0.30 per m3, said Klein Teeselink.
Essentially there are four energy conversion pathways for straw; fermentation for liquid biofuels, anaerobic digestion for biogas, combustion for heat and/or power and gasification for syngas.
– We provide technologies for the three latter straw conversion pathways, each have their complexities, advantages and disadvantages, remarked Klein Teeselink.
Why is straw not more widely used in biogas plants and how can this be addressed? According to Klein Teeselink the early feed-in tariff (FIT) regimes put the focus on electricity production resulting in ever-larger fuel, energy and cost inefficient power only plants. In countries where biogas production is stimulated, digesting manure with an energy crop such as maize is the most common as an input combination.
– It is not widely known that you can build anaerobic digestion plants using only agricultural residues as input streams, without the need for dedicated energy crops, said Klein Teaseling. Part of the problem is that straw is a “difficult” material. As a light, dry material, using straw will also increase the solid content in the digester, which calls for special mixers that can handle this.
– If you want to digest large amounts of straw, you need special feeding systems, because of the large volumes. You need special pre-treatment and the conversion process is relatively slow, especially in the beginning, he remarked.
HoSt has conducted an extensive research programme in association with the Technical University of Münster in Germany on the preparation of straw to increase conversion. Some of the research topics included making powder, hammering, thermal treatment at medium temperatures together with acid or high temperature treatment and enzymes. The high thermal treatment gave the highest gas production per tonne of straw, but needs a large amount of steam, more than a gas engine can supply. It is also an expensive system which is difficult to make economical feasible.
– In nearly all the biogas projects we’re currently involved in the biogas is converted in biomethane for grid injection or compressed. This means that there is no heat available for the high thermal treatment option, explained Klein Teeselink.
First straw to biogas
Therefore HoSt is focusing on low temperature pre-treatment, where hammering is one of the best options. In Chernozemen, Bulgaria the company has commissioned its first biogas plant in which large amounts of straw are fed. The 1.5 MWe capacity plant will use around 50 000 tonnes per annum of cow slurry co-digested with maize silage and straw. The Chernozemen facility consists of a dedicated solid feeding system, a liquid input system consisting of a pump and cutter, two 2 174 m3 digester tanks, a single 2 174 m3 post-digester tank, a separator system, after-storage tanks and a building with a control and heating room, a 1.5 MWe gen-set and a room for the operator.
One of the striking features is the dedicated straw input system. Straw bales are placed on a large straw conveyer that can handle straw bales and feeds the bales to a bale breaker. Here the first initial size reduction takes place before a hammer mill. The hammer mill is sufficient to mill the straw enough to break its tubular structure to prevent floatation. Tests were conducted with different sieve sizes to find the optimal particle size for the digestion process.
– Apart from the feeding system, the digester tanks are optimized for the digestion of straw. A special paddle mixing technology combined with propeller mixers ensures that no floating layers are formed. Furthermore, recirculation ensures that enzymes and bacteria are retained, which improves the degradability, explained Klein Teeselink.
According to him another advantage is that nutrients, such as nitrogen (N), potassium (K) and phosphorus (P) are made available for use as fertiliser along with the organic matter in the digestate.
– In many countries with a cool climate, a second commercial grain harvest is not possible, though a fast growing cover crop such as grass, winter rye or tubers is. These can be used for the biogas plant to increase the overall production while the same amount of cereal can be produced, he suggested.
The additional biomass not only leads to an increased biogas production, also the available organics on the land increases. Moreover, the winter cover crop reduces the risk of mineral and nutrient leaching. The concept of using straw seems to be catching on. No sooner than Chernozemen came on line, HoSt revealed it has started building a second plant, this time in France. More are likely to follow.