FEATURE: AGRO-ENERGY If the results at Foulum biogas plant are anything to go by then Kinetic Biofuel duo, Dr Torben A. Bonde (top) and Mogens Slot Knutsen have good reason to be pleased. Bioenergy Int Internat ional No 80, 4-2015 11 Under the microscope a strand of cereal straw is a fantastic lignocellulosic structure. The challenge is to make these components available for enzymatic and microbial conversion. DR TORBEN A. BONDE Kinetic Biofuel. cesses along with a fully-equipped laboratory and conference facilities. Furthermore there is also a separate unit for testing biomass combustion up to 1 MW thermal. The main biogas plant, supplied by Danish biogas technology designers and builders Xergi A/S, consists of a 1200 m³ primary digester, a 1500 m3 secondary and two 1500 m³ gas storage tanks. The daily gas production is around 4800 m³ with a 15-day average retention time in the main digester. A biological gas scrubber is used to reduce hydrogen sulfide (H2S) to below 100 ppm and the biogas is cooled to condense water vapour before pressurisation to around 200 mbar. The high-pressure is needed to ensure 150 mbar entry pressure for the 625 kWe 12-cylinder Jenbacher 312 gas engine which is located around 1.5 km from the biogas plant. The normal daily output for the engine is around 1 MWh power and 1.4 MWh heat. The plant also has several feedstock reception and pre-treatment options. This includes two 75 m³ tanks for fatty residues, one of which is heated and a 600 m³ slurry tank. A decanter centrifuge enables slurry separation into a solid fraction with a high dry matter content of 25–30 percent and a liquid fraction. For solid feedstock a 50 m³ stationary feeder is installed and, since 2011, a Kinetic Biofuels straw pre-treatment line. There is also a mixer/dosage module installed Background Kinetic Biofuel where fats, raw slurry and solids are mixed and pre-heated to around 52°C before being pumped into the reactor. The plant facilitates testing at all temperature levels and this makes it possible to test for instance process efficiencies related to changes in temperature. The results It is reasonable to assume that there have been ample testing and demonstration opportunities. Results show that briquetted straw has up to 19 percent higher gas yield than shredded or macerated straw. The addition of 1 percent lye to briquetted straw gives another 10 to 20 percent higher gas yield. The resulting digestate is also a better fertilizer with soil improvement properties. – Briquetting allows for the addition of large quantities of straw and, at the same time increases the biogas yield. This means that production can be doubled in biogas plants that currently use manure as the key feedstock, said Mogens Møller Hansen, Plant Manager at Foulum. The overall increased yield depends of course on the technology used and retention time in the biogas plant. Achieving a biogas yield of around 250 Nm³ methane per tonne of briquetted straw is possible provided it is good quality straw, uses a thermophyilic digestion of 30-40 days ideally in serial reactors, and not least has a welladapted micro-flora for ligno-cellulosic In late 2011 a consortium consisting of C.F. Nielsen, Biofuels Technologies and Aarhus University were granted project co-funding by the Danish Energy Agency, EUDP to build, demonstrate and further develop a full-scale mechanical straw pre-treatment technology at Aarhus University’s biogas research plant in Foulum. Up to 10 000 tonnes of straw was to be pre-treated and co-digested with 50 000 tonnes of manure and other biomass over the two-year project. In addition a partial stream of the pre-treated material was to undergo enzymatic liquefaction to demonstrate the viability of the concept for cellulosic ethanol. Aarhus University has conducted and documented a wide range of pilot and full-scale measurements of biogas yield under various conditions as well as experiments with the addition of chemical additives such as sodium or potassium hydroxide, lye. The results of the development project have been patented and transferred to Kinetic Biofuels A/S, a company formed in 2014 by C.F. Nielsen and Biofuels Technology to market the technology. biomass. – Another important result is that the addition of straw briquettes to manure reduces H2S in the raw biogas significantly, as much as 75 percent compared to manure only, said Møller Hansen. Very encouraging technical and biological results, but do the numbers stack up? They do according to a presentation on the subject comparing different pre-treatment options by Henrik B. Möller, Senior Scientist, Aarhus University at the 5th Nordic Biogas Conference in Rekavijk, Iceland last August. A benchmark dimensioning figure is a 100 000 tonne per annum slurrybased biogas plant can increase its biogas production by up to 160 percent, from around 2.5 million Nm3 to up to 6.5 million Nm3, by adding 10 000 tonnes straw briquettes, 10 percent, as co-substrate. According to Mogen Slot Knudsen, CEO for C.F. Nielsen and also CEO for Kinetic Biofuel, the investment cost for a complete 1.5 tonne per hour straw pre-treatment briquetting line is in the region of EUR 400 000. Factoring in operating costs such as power consumption of about 75 kW per tonne, spares and maintenance of about EUR 3-4 per tonne, and labour at 25-50 percent of full-time per shift, the total pre-treatment cost is around EUR 30-40 per tonne, the cost being further reduced for larger plants treating for instance up to 50 000 tonnes per annum. Gas production: 2.08 million m³/yr Power production: 4.4 million kWh/yr, 625 kWe gas engine Digesters: 1 200 m³ primary & 1 500 m³ secondary Final storage: 2 × 1 500 m³ Process temperature: 52°C Biomass boiler: 895 kWh Biomass feedstock used 25 890 t/yr Cattle manure: 16 000 t/yr Pig manure: 6 200 t/yr Mink manure: 1 000 t/yr Chicken litter: 135 t/yr Sheep manure: 80 t/yr Fodder residuals: 1 000 t/yr Non-food crops: 360 t/yr Cover crops: 15 t/yr Fats: 500 t/yr Foulum Biogas – Such a 1.5 tonne per hour unit is built semi-mobile meaning it can be dismantled, and moved to another location within one or two days which makes it attractive from a financing perspective, remarked Slot Knudsen adding that Kinetic Biofuel is offering capacities of up to 10 tonnes per hour. If the technology, biology and numbers add up for a manurebased biogas plant, as they seem to do, one cannot help but wonder how it could work for say a straw based cellulosic biofuels plant. Or with other agricultural residues for that matter. With that in mind it’s not a stretch to suggest that we may learn more from Messrs Bonde and Slot Knudsen in the not too distant future. Text & photos: Alan Sherrard BI80/4853/AS
Bioenergy no 4 july 2015
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