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