Straw to biogas

Straw contains a lignin carcass, which itself is not fermented and doesn’t allow to ferment cellulose. In order for straw to be fermented in biogas reactors, the carcass must be removed.

We have tried a wide variety of straw pre-treatment methods:

• steam explosion
• millig
• pelletizing or briqueting 

The biogas plant in Teofipol-4, made by Zorg Biogas, is equipped with 6 steam explosion units x 25 tonnes straw/day each. The total capacity of all modules is 54,000 tons of straw per year. The heat for the steam explosion is taken from the exhaust gases of the co-generators using a thermo-oil system. The prepared straw is fermented in high-loaded reactors with the inclined agitators. Biogas output is 450 m3 per tonne of straw. Methane content 59%. The generated electric power is 6 MW. The cost of the straw pre-treatment is 8 million Euro. The disadvantages are the high project cost and the high energy demand 600 kW electric power + 6MW heat. The advantage is high biogas output, high methane content.

At another biogas plant, in which our team member took part in the past, a different method of the straw pre-treatment is used. Straw is cut in a shredder to 5 mm. After shredding, straw is milled in a hammer mill to a wool condition. This increases the surface area for anaerobic bacteria. In addition to cutting and milling, the secret lies in a special inexpensive additive, without which the straw can't be fermented. The biogas output biogas from the milled straw exceeds the output from the maize silage and amounts to 350 m³ of biogas per tonne of straw. Methane content is 52%. In the case described here, the annual volume of straw is 50,000 tonnes. At the time of this writing, the biogas plant has been operating for 5 years. The cost of the equipment for the preparation of straw together with the conveyor is 3 million EURs, which is 2,5 times cheaper than the steam explosion. The disadvantages at the particular plant are (1) The mill is very sensitive to moisture in the straw. The mills works well when the straw absolutely dry. If the straw is wet, the mill stops working; (2) lower biogas output;  (3) high maintenance expences for the mill and the cutter.

There is pelletizing or briqueting of straw. Biogas output from 1 tonne of straw pellets with 10% moisture makes 500-530 m³ that is equivalent of 450-480 m3 fro the initial straw with 20% moisture. Methane content 52%. We have carried out laboratory tests of the pelleted straw in two independent, certified laboratories in Germany and France. An industrial demonstration also was carried out at a biogas plant with a 650 kW electric power with the high-load reactor during 3 months. After being fed into the biogas reactor and in contact with the liquid, the straw pellets instantly disintegrate, well mixed and do not form a crust. The results are super promising. The expected CapEx for 50 000 tonnes pellet /year with a conveyor, a dryer, a cutter, a press-extruder, a building is 5 million EUR.

Of all the methods described, we like pelltetizing in a combination with a high-load reactor. High biogas output, medium budget, moisture is not critical. Teh expences for tear & wear are justified with the higher yields.

There is also a spin-off of the pelleting method. Straw pellets and briqietes can be first used as a bedding for livestock. Having passed though the bedding stage, straw pellets, due to the reaction with manure and urine, improve their properties as a feedstock for biogas. The methane content rises to 60%. The biogas yield is 550 m3/tonne pellets. An additional advantage is reduced consumption of straw for bedding. Pellets consume 8 times more water than the untreated straw. This method is well suitable for poultry and beef farms, where straw bedding is used. But this method requires changes in the husbandry technology.

The methods described here are aplicable not only for wheat straw but also for rice straw.