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 plant1,200 m3 digester, 22,000 tonnes manure)4,100 tonnes straw ,
Grass and maize
Research plant2 x 30m3 CSTR pilot digesters2 x 10m3 CSTR pilot
digesters
Gas upgrading
Fullscale pre-treatmentExtruder Briquetting
>1.7 mill. m3 biogas3,223 MWh el6,200 MWh heat
Solid feeding
Handling and pre-treatment of grass/straw at Foulum
Storage Pre-treatmentDigestion
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?
Pre-treatment Technologies
MechanicalBriquettingMacerationExtrudation
Chemical Acid (acetic acid, Sulfuric acid), Alkali (NH3, KOH, NaOH)
Untreated
Macerated
Extrudation
Briquetting
Gas potentials of straw/grass – effect of briqueting/extrudation
9
Untreated straw
Briqutted straw
Extruded straw
Briqutted grass
0 50 100 150 200 250 300 350 400
15 days
30 days
60 days
90 days
L CH4/kg VS
+36%
+18% +12%
+29%
+12%
+10%
+4,1%
-1,1%
-1,1%
Gas potentials – effect of briqueting and chemicals
10
0.00% 0.50% 1.00% 2.00% 5.00%0
2
4
6
8
10
12
14
16
Increased effect of briqueting by addition of alkalis
30 days (KOH)Polynomial (30 days (KOH))Polynomial (30 days (KOH))60 days (KOH)Polynomial (60 days (KOH))30 days (NaOH)Polynomial (30 days (NaOH))60 days (NaOH)Polynomial (60 days (NaOH))
Addition of alkalis (% of ww)
Incr
ease
d m
etha
ne y
ield
(%)
Pre-treatment – Energy demand
11
20 30 40 50 60 70 80 900
20
40
60
80
100
120
140
160
Extrudation Exponential (Extrudation) Mixing/dosing
Dry-matter (%)
Kwh/
tonn
es fr
eshm
atter
Energy consumption
Mixing/dosing Extrudation
Bale opener Hammermill Briquetting
Energy balances by pre-treatment
12
Liquid manure
Deep litter
Maize silage
Wheat straw
-250 250 750 1250 1750 2250 2750Liquid manure Deep litter Maize silage Wheat straw
Energy feeding unit -3 -10 -15 -30
Energy extrudation -10 -15 -15 -85
Energy production 120 760 855 2000
Adddional yield by extruda-tion
0 228 42.75 400
Energybalances
Kwh/tonnes freshmatter
Economy
Oil changeKnifes
ElectricityOil change
wearingsElectricity
BearingsChain
ElectricityTotal
Biom
ixer
Extr
uder
Redl
er𝛴
0 20 40 60 80 100 120 140 160
Costs for extrudation 2013/2014
(kr/tons)
Economy
No treatment Extruder Briquetting
Medium Capacity Full capacity Full capacity
Biomass Type Maize Straw Straw
Gas yield m3 CH4/ton 100 200 200 200
Electricity kwh/ton 10 120 120 120
Investment Pre-treatment kr 0 4000000 4000000 10000000Capacity ton/år 25000 5000 7000 25920Electricity kr/ton 7 84 84 84
Investment Pre-treatment kr/ton 0 800 571 385
InvestmentInfrastruktuc-ture on biogas kr/ton/år 50 50 50 50
Depreciation 15% p.a kr/ton 7,50 127 93 65Running costs Wearing kr/ton 2 61 61 25
Total costs Excl. manning kr/ton 17 272 238 174Extra gas yield % 0 10 10 10Value extra gas yield kr/ton 0 96 96 96
Total revenue Excl. manning kr/ton 463 784 818 882Buying substrate kr/ton 350 550 550 550
Net revenue Excl. manning kr/ton 113 234 268 332Net revenue Excl. manning kr/ton DM 282 292 335 415
Experiments with co-digestion of straw
Volume: 15 LTemperature: 49±1 oCStirring: 100 rpmHRT: 20 days
30 m3 Temperature: 50 oCHRT: 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
0 5 10 15 20 25 300
500
1000
1500
2000
2500
3000
3500
Manure (90% pig, 10% cattle) Manure+8% straw
Days
Hydr
ogen
sulp
hur (
ppm
)
Co-digestion of straw influence of hydraulic retention time
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 900
10
20
30
40
50
60
70
80
90
CM (mesophilic post digestion) CM (thermophilic post digestion)
CM+straw (mesohilic post digestion) CM+straw (thermophilic post digestion)
Retention time (Days)
Biog
as (m
3/to
nnes
)
+ 30%
Thermophilic Thermophilic/mesophilic
If retention time should be prolonged mesophilic conditions is sufficient
+ 30%
Foulum biogas plant – primary and post digestion
Thermophilic53oC HRT=13 days
Mesophilic25- 30oC HRT=40 days
Psycrophilic10-20oC HRT=40 days
Psycrophilic0-20oC 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
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