Co-digestion of manure with straw and perennial grasses Henrik Bjarne Møller, Department of Engineering, Aarhus University, Denmark

Straw and perennial grass for biogas Why use straw and perennial grass?  Biogas plants miss high value substrates for biogas production as co-substrates for livestock manure - crops should be phased out.  Straw is the agricultural by-product with the highest energy potential.  Recycle nutrients from uncultivated meadows to organic arable land.  Harvest of meadows has a number of positive side effects What conditions must be met?  There must be a positive economy and in the short term the economy must be in line with corn.  Biogas technology should be optimized to handled and convert the biomass

Biogas research plant Fullscale plant 1,200 m3 digester, 22,000 tonnes manure) 4,100 tonnes straw, Grass and maize Research plant 2 x 30m 3 CSTR pilot digesters 2 x 10m 3 CSTR pilot digesters Gas upgrading Fullscale pre-treatment Extruder Briquetting >1.7 mill. m3 biogas 3,223 MWh el 6,200 MWh heat Solid feeding

Handling and pre-treatment of grass/straw at Foulum Storage Pre-treatment Digestion Feeding Dry-line (>80% DM) Wet-line (20-80% DM)

Transition from maize to fiber rich material In warm periods dosing of solid material is reduced due to lack of gas sale

Biomass – Foulum biogas plant

cellulose Lignin hemicellulose Pre-treatment of ligno-cellulose - why? Improvement  Lowering of viscosity and mixing ability  Higher methane yield Drawbacks  Higher energy demand  Investment and running costs

Types of pre-treatment of ligno- cellulose - why? Untreated Macerated Extrudation Briquetting

Gas potentials of straw/grass – effect of briqueting/extrudation 9

Gas potentials – effect of briqueting and chemicals 10

Pre-treatment – Energy demand 11 Energy consumption Mixing/dosingExtrudation Bale openerHammermillBriquetting

Energy balances by pre-treatment 12

Economy

Experiments with co-digestion of straw Volume: 15 L Temperature: 49±1 o C Stirring: 100 rpm HRT: 20 days 30 m 3 Temperature : 50 o C HRT: 25 days Briqueting of straw

Thermophilic co-digestion of straw influence of sulfur in gas Addition of 8,3% wheat straw reduce sulphur to 25% of original

Co-digestion of straw influence of hydraulic retention time + 30% ThermophilicThermophilic/mesophilic If retention time should be prolonged mesophilic conditions is sufficient + 30%

Foulum biogas plant – primary and post digestion Thermophilic 53 o C HRT=13 days Mesophilic o C HRT=40 days Psycrophilic o C HRT=40 days Psycrophilic 0-20 o C HRT=100 days 0,5-5 m3/ton + 37% Thermophilic Thermophilic/mesophilic

Conclusion Straw can succesfully be used for co-digestion of straw and manure, pre-treatment can enhance the yield. The increased yield depends on technology and retention time in the biogasplant. With short retention time, post digestion is recomended Mechanical and chemical pre-treatment can be combined with briquetting technology Foulum biogas plant has been through a succesfull transition from high value energy crops to straw Straw reduces sulphur in the gas significantly

20 Thanks for your attention