FEATURE: AGRO-ENERGY Bioenergy International No 80, 4-2015 9 And Danish farmers produce a lot of straw. On an annual basis 5.5–6 million tonnes of which roughly one third is used as fuel for heating, one third is used for animal bedding and feeding and the remainder is essentially unused. – The Danish government’s 50 percent manure-based biogas target can only be achieved by using large amounts of straw to increase methane yield and secure plant economies. All other high yield residues such as from the food processing industry are already taken, said Bonde. In theory cereal straw has a potential methane yield of 240–320 Nm³ per tonne but up until now has had limited use in biogas plants. – Under the microscope a strand of cereal straw is a fantastic ligno-cellulosic structure. It is light, strong, hydrophobic and decay resistant with the cellulose and hemi-cellulose locked behind the lignin coated cell walls. The challenge is to make these components readily available for microbial conversion, he said. Quite a challenge as even shredded or macerated straw that absorbs some degree of water is difficult to mix in with cattle or pig slurry. It is voluminous and will inevitably float causing surface crust or dead-zones in a reactor, and blockages in pumps and pipes. Furthermore electrical consumption of pumps, stirrers and mixers goes up in addition to the energy needed for shredding or maceration. Mechanical industrial pre-treatment Given the above mentioned, the technology solution offered by Kinetic Biofuel comes somewhat as a surprise at first as it too is entirely mechanical, a briquetting press line consisting of a bale conveyor, shredder, stone trap, hammermill, cyclone and filter, feed silo and briquette press. Furthermore the press itself is a seemingly standard BP 6510 HD 1.5 tonne per hour briquetting press from Danish manufacturers C.F. Nielsen A/S. As straw briquettes have a much higher bulk density than a big bale, about 550 kg per m³ compared to 150 kg per m³, the immediately obvious benefit is a radical reduction in transport and storage costs. – This is of course an important benefit as it enables cost-effective volume aggregation. Briquetting also preserves straw quality over time with minimum degradation that would otherwise have a negative effect on biogas yield when used, said Torben. There is though more to it than that. The briquetting process itself also has, as Bonde reveals, a remarkable effect on the straw and its properties. – In short it can be described as mechanically induced steam explosion, said Torben adding that the C.F. Nielsen briquette press has been adapted and modified to optimise this internal process, and patented. The shredded and milled particles are compressed under high pressure. The kinetic energy on the moment of impact by the piston and sudden pressure drop on its retraction together with the high temperature caused by dissipation of kinetic energy into the straw causes steam explosions rupturing the fibres, cell walls and pores. – This as you know changes everything. The dry straw particles are able to absorb water at high capacity and the cell structures are opened allowing enzymatic and microbial conversion of the contents, said Torben. The straw briquettes can preferably be fed directly from the briquetting line via a screw conveyor into a biogas reactor or fed into a mixing tank and stirred in liquid manure before pumping the mixture into a reactor. Full-scale research plant The entire briquetting line including both reactor feed-in systems have been installed and tested at Aarhus University’s biogas research plant in Foulum. Built in 2007 the Foulum (Top) Mogen Møller Hansen, Plant Manager Foulum Biogas sharing research results and experiences with visitors from a 70 strong delegation of the European Forage Association. (Above) The complete setup of a 1.5 tonne per straw briquetting line at the Foulum biogas reserach plant. A Linka straw bale feed conveyor and shredder (red) stone trap, hammer mill, cyclone, press feed silo, briquetting press and combined briquette cooling and feed conveyor to the biogas receiving and feedstock mixer bin (photo courtesy Kinetic Biofuel). plant is one of the world’s largest of its kind and the University has its own livestock and crop research and production in the surroundings. Research at the full-scale biogas plant is focused on advanced technologies for improving the biogas production including optimisation of the biogas potential and improvement of methods such as pre-treatment, high technology separation of nutrients, thermal conversion of the residual fibres after digestion, high solid digester technology and upgrading of biogas to vehicle fuel or natural gas quality. The research part of the facility consists of four 15 litre reactors, four 200 litre reactors, two 10 m³ continuously stirred tank reactors (CSTR) and two 30 m³ CSTRs all of which are used for test and piloting research. In addition there is a section that can accommodate up to six independent set-ups for testing equipment and pro
Bioenergy no 4 july 2015
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