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Results and bottle necks of energy crop digestion plants- Required process technology innovations -

P. Weiland

Federal Agricultural Research Centre (FAL)Braunschweig / Germany

Workshop “ENERGY CROPS & BIOGAS”Utrecht, 22 September 2005

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Outline

� Introduction

� Actual situation in Germany

� Results from evaluation of 60 biogas plants

� Bottle necks of crop digestion plants

� Technologies for process optimization

� Summary and outlook

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Renewable Energy Sources Act (2005)

Technology-Bonus: 2 Cent/kWhel (e.g. dry-fermentation)

CHP-Bonus: 2 Cent/kWhel for external heat utilization

4.08.77500-5,000

6.09.75150 – 500

6.011.33150

Bonus Paid forBiomass

[Cent/kWhel]

Compensation Paidfor Electricity[Cent/kWhel]

Electrical Capacity[kW]

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Biogas plants in Germany

100 120 139 159 186274

370450

617

8501050

1600

1800

2000

2400

3000 ?

0

500

1000

1500

2000

2500

3000

Num

ber

of p

lant

s

1990 1992 1994 1996 1998 2000 2002 2004

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Application frequency of substrates

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"��� ��������

#���$�� ����

%��

%�� ����

&�

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Manure content of fermenter input

0

10

20

30

40

50

0-25 25-50 50-75 75-100

Manure content of fermenter input [%]

Rel

. fre

quen

cy [%

]

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Direct-feeding systems for solids

Feed screw Flushing systemFeed piston

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Substrate feed per day

0

5

10

15

20

25

30

35

1-2 3-4 5-10 11-15 16-20 21-24

Substrate feed per day

Rel

. Fre

quen

cy [%

]

NWSESW

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Digester with double membrane roof

Output

Mixer

Membrane roof

Gasfoil

Blower

Input

Wall heating

Floor heating

Biogas

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Upright large-scale digester

Output

max: 5000 m³

Externalheat exchanger

Input

Centralmixer

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Number of process stages

0

10

20

30

40

50

60

70

1 2 3 4 Process stages

Rel

. fre

quen

cy[%

]

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Organic loading rate

0

10

20

30

40

50

< 1 1-2 2-3 3-4 4-5 > 5

Total loading rate [kgoDM /(m³d)]

Rel

. fre

qu

ency

[%

]

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Typical retention time of biogas plants

0

5

10

15

20

25

30

35

< 30 30-60 60-90 90-120 120-150 > 150

Total hydraulic retention time [d]

Rel

. Fre

quen

cy [%

]

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Methane content of biogas

0

510

1520

2530

3540

45

<50 50-55 55-60 60-65 > 65

CH4-Content [Vol-%]

Rel

. fre

quen

cy [%

]

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H2S-content after desulphurization

0

200

400

600

800

1000

AT

B 0

1

AT

B 0

3

AT

B 0

5

AT

B 0

7

AT

B 0

9

AT

B 1

1

AT

B 1

3

AT

B 1

5

FA

L 02

FA

L 04

FA

L 06

FA

L 08

FA

L 10

FA

L 12

FA

L 13

FA

L 14

ILT

01

ILT

03

ILT

05

ILT

07

ILT

09

ILT

11

ILT

13

UH

01

UH

03

UH

05

UH

07

UH

09

UH

11

UH

13

UH

15

H2S

-co

nte

nt [

pp

m]

1645 1099 1879

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Residual methane potential of digesterresidues

0

5

10

15

20

25

30

35

40

0-5 5-10 10-15 15-20 20-25 >25

% of residual methane formation on methane production (bei 20 oC)

Rel

. Fre

quen

cy [%

]

N = 11

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Process steps with typical bottle necks

Cropsstorage Feeding

Fermen- tation

Residuestorage

Biogasupgrading

Biogas

Crops Fertilizer

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Storage and substrate pre-treatment

�Reduced anaerobic degradationrate�Risk for scum formation infermenter�Bad handling properties of thesubstrate

Insufficient disintegration ofenergy crops

�Inhibit the methanogenicactivity

Mold formation during ensilingand storage of energy crops

�Energy losses during ensilingand storage�Increased risk for inhibition ofthe methanogenic process

Non optimized formation oforganic acids by ensiling

EffectBottle neck

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Storage and substrate pre-treatment

�Reduced anaerobic degradationrate�Risk for scum formation infermenter�Bad handling properties of thesubstrate

Insufficient disintegration ofenergy crops

�Inhibit the methanogenicactivity

Mold formation during ensilingand storage of energy crops

�Energy losses during ensilingand storage�Increased risk for inhibition ofthe methanogenic process

Non optimized formation oforganic acids by ensiling

EffectBottle neck

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Solids feeding

�Risk for blockage for screwconveyor diameter < 300 mm�Piston systems compacts longfiber crops�Flushing systems cannot beapplied for crops of low density

Direct solids feeding by screwconveyor, piston and flushingsystems

�Energy losses by methaneemissions�High energy demand for mixing

Mixing of silage and processwater in an external open tank

�Reduced process stability�Reduced biogas yield�H2S-peaks in biogas

Discontinuously feeding of fewcharges per day

EffectBottle neck

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Fermenter and storage tank (1)

�Reduced biogas yield�Incomplete degradation of thesubstrate

Short circuit flow of substrate

�Reduction of the gas storagecapacity in the top of fermenter�Fermenter can be operated onlyat reduced loading�Risk for clogging of the gas pipe

Accumulation of biogas in thefermenter digestate

�Reduced biogas yield�Clogging of the overflow pipe�Danger for the function of thewhole process

Scum formation

EffectBottle neck

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Fermenter and storage tank (2)

�Uncontrolled methaneemissions (climate effect)Open digestate storage tanks

�Stable mesophilic temperatureconditions cannot be achieved�Process failure due to thereduced microbial activity above42 °C

Formation of biogenic heat bymono-fermentation of energycrops

�Large reactor volumes�Low specific methaneproductivity�High energy input per ton ofsubstrate for heating and mixing

Long hydraulic retention time

EffectBottle neck

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Biogas upgrading

Formation of condensate candisturb:�The transportation of biogas�All measuring devices in the gasmain�The function of the CHP

Incomplete drying of biogas

�Reduction of the ignitability ofthe gas due to the low CH4-content of biogas

Feeding of surplus air to thefermenter for biologicaldesulphurization

� Reduced lifespan of the CHPInsufficient biologicaldesulphurization

EffectBottle neck

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Sizing of Equipment

�The efficiency of H2S reductioncannot be calculated exactlyresulting in over- or undersizedinstallations.

Reliable dates concerning thedegradation capacity of the H2S-oxidizing bacteria

Insufficient adaptation offermenter and CHP-capacitywhich result in:�Reduced electrical efficiency ofCHP�Increased pollutant emissionfrom CHP�Intermittent operation of CHP

Reliable dates of the biogas yieldof energy crops

EffectBottle neck

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Summary and outlook

� The results have shown that all importantagricultural crops can be used for biogasproduction.

� For increasing the process efficiency andreliability the whole process chain has to beoptimized.

� All process units must be adapted on thespecific properties of energy crops.

� Few experiences are available from plantswith mono-fermentation of energy crops.

� The missing stabilizing effect of manuremakes a better process control necessary.

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Many thanks for yourattention!

Workshop “ENERGY CROPS & BIOGAS”Utrecht, 22 September 2005