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Research Note

Anaerobic digestion of energy crops in batch

Anders M. Nielsen*, Anders Feilberg

Aarhus University, Department of Engineering, Blichers Alle 20, DK-8830 Tjele, Denmark

a r t i c l e i n f o

Article history:

Received 29 November 2011

Received in revised form

19 March 2012

Accepted 22 March 2012

Published online 16 May 2012

* Corresponding author.E-mail addresses: andersm.nielsen@agrsc

1537-5110/$ e see front matter ª 2012 IAgrEdoi:10.1016/j.biosystemseng.2012.03.008

Batch digestion of energy crops at mesophilic temperature indicates that the rate of

anaerobic digestion of energy crops into methane may first reach its maximum rate after

several days for energy crops such as maize, oat, ryegrass, and wheat that are less

degradable than sugars and fats. Experiments show that the cumulative methane

production pattern from energy crops rather follows a sigmoidal, and not exponential,

curve shape. Implications are that optimal usage of energy crops should involve an

evaluation of their degradation kinetics, specifically the rate of which they are converted

into methane, and the optimal digester hydraulic retention time and or reactor

configuration for the specific energy crop.

ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved.

1. Introduction methods, ii) complicate the choice of energy crops for specific

To obtain an economically viable biogas production in

Northern Europe, animal manure is co-digested with energy

crops such as maize, wheat and rye residues. Cumulative CH4

production as a function of time B(t), ultimate methane yield

(B0), degradation constant (k), and a constant (c) describing

how sigmoidal the curve is, follows a pattern described by B(t)

in Eq. (1) for manure and sugars.

BðtÞ ¼ B0

�1� exp�kt

�c; C ¼ 1 (1)

According to this equation, the CH4 production

B0ðtÞ=dt ¼ B0$k$exp�kt reaches it maximum (rmax) at time zero,

although that may not always be the case. In a study

(Hashimoto, 1989) it was shown that B(t) in Eq. (1) could be

applied to the cumulative CH4 production from straw.

An analysis of data from Hashimoto (1989) revealed that

the k-value in (1) increasedwith the inoculum to substrate (I:S)

ratio (Fig. 1) for straw, and reached a plateau I:S ratios higher

than 2e4, meaning that the k-value in full scale is larger than

in most biogas batch experiments.

Currently, energy crops are mainly evaluated in terms of

their B0 which may i) obscure the advantage of pretreatment

i.dk (A.M. Nielsen), ande. Published by Elsevier Lt

hydraulic retention times, and iii) reduce the perceived advan-

tages from new digester configurations. It was the purpose of

this study to investigate if another pattern of CH4 production

would fit experimental data better than (1) and furthermore to

evaluate if this other pattern would have any implications for

the evaluation of pretreatment methods for energy crops and

digester configuration when energy crops are digested.

2. Methods

In this study, 18 biogas batch experiments with six different

energy crops in triplicates were conducted according to the

ISO (ISO, 1995) method and the procedure described in Moller,

Sommer, and Ahring (2004). Experiments were conducted at

35 � 0.5 �C and at an I:S ratio of 1:1.

3. Results

Based on values of the coefficient of determination (R2), the

energy crops cumulative digestion profile fitted better to

rs.feilberg@agrsci.dk (A. Feilberg).d. All rights reserved.

Nomenclature

B(t) cumulative methane production

B0 ultimate methane yield Vfed volume of slurry fed

to the digester each slurry shift

B0(t) differentiated B(t)

k degradation constant

t time in days

c exponent quantifying how sigmoidal a curve is

rmax time at which degradation rate reaches its

maximum

(I:S) The ratio of substrate volatile solids to inoculum

volatile solids

Vfed volume of slurry fed to the digester each slurry

shift

VD1 volume of digester 1 in a two-stage configuration

VD2 volume of digester 2 in a two-stage configuration

ss slurry shifts

Zþ positive integers

x a counter

b i o s y s t em s e ng i n e e r i n g 1 1 2 ( 2 0 1 2 ) 2 4 8e2 5 1 249

a sigmoidal shaped form expressed by B(t) in Eq. (2) than B(t) in

Eq. (1) (Table 1). Eq. (2) predicts lower B0-values after 70 days of

digestion than Eq. (1).

BðtÞ ¼ B0$�1� exp�kt

�c; C > 1 (2)

Eq. (2) signifies a digestion rate of B0(t)/dt ¼ B0$k$c

(1 � exp�kt)c�1. The maximum rate of CH4 production (rmax)

occurs at time rmax¼ ln(c)/k for energycrops that followanEq. (2)

cumulative digestion pattern.

Fig. 2 illustrates the magnitude of the difference between

real data and Eqs. (1) and (2) for all energy crops investigated in

this study.

4. Discussion & analysis

4.1. Pretreatment

Pretreatment of energy crops focus on improving B0 by

degrading substances that would otherwise not be degraded

(Hashimoto, 1986) and enhancing degradability. Energy crops

contain some easily degradable structures, protected by ligno-

cellulose, which are most accessible to bacteria and enzymes

when the surrounding plant structures have undergone some

degradation. For biomasses such as manure, Eq. (2) does not

apply, but the increase in biodegradability, and B0, is still be

Fig. 1 e Analysis of data from Hashimoto (1989).

accompanied by an accelerated digestion (Quai et al., 2011).

Pretreatment has the potential to completely destroy plant

structures and should increase k, reduce c, and also reduce

rmax.

If the effect of pretreatment could be quantified in terms of

both B0, k and c in Eq. (2) at different I:S ratios, there would be

a background for extrapolating data and using derivates of Eq.

(1) and Eq. (2), and one ormore adjustment factors, to estimate

the CH4 potential of a crop in e.g. 15 days (B15), 25 days (B25) or

for any hydraulic retention time.

Pretreatment methods aim at destroying protecting plant

structures, decreasing cellulose crystallinity, and increasing

enzyme accessibility to degraded structures. Therefore, the

use of existing pretreatment methods is not given full credit,

as an additional advantage of pretreatment is an improved k-

value and reduced c-value in Eq. (2) signifying a faster CH4

production earlier.

4.2. From batch to full scale digester

Models have been developed that correlates B0 to expected

CH4 production in digesters. However, the developed models

contain flexible dimensionless parameters such as in Chen

and Hashimoto (1978), and e.g. a stress index as in Hill

(1991). Not all such parameters are contained in data from

experiments, and can be chosen rather arbitrarily.

Future models may have to focus on a combination of the

digestion pattern of the energy crop, reactor configuration,

age distribution in the digester and other measurable

parameters.

Two-stage digestion is a configuration with numerous

advantages (Blonskaja, Menert, & Vilu, 2003; Demirer & Chen,

2005; Ghosh, Ombregt, & Pipyn, 1985). first stage

ageðssÞ ¼ Vfed=VD1$��VD1 � Vfed

��VD1

�ss; ss˛Zþ (3)

second stage

ageðssÞ ¼V2fed=VD1$VD2$

Xss�1

x¼1

��VD1 � Vfed

��VD1

�ss�ðxþ1Þ

$��VD2 � Vfed

��VD2

�; ss˛Zþ

(4)

Note. The continuous version of Eq. (3) is (1� Vfed/VD1)t, where

t is time.

Algebraic derivations of age profiles of biomass in one-

stage (Eq. (3)) and second stage digester (Eq. (4)), as a func-

tion of biomass age in slurry shifts (ss), volume of inlet feed

Table 1 e Equations fitted to experimental data.

Crop B(t), Eq. (1) B(t), Eq. (2) rmax Eq. (1) rmax Eq. (2)

Ryegrass 499 (1 � e�0.039 t), R2 ¼ 0.97 440 (1 � e�0.079 t)1.99, R2 ¼ 0.98 0 days 8.7 days

Maize silage 528 (1 � e�0.020 t), R2 ¼ 0.96 385 (1 � e�0.061 t)2.32, R2 ¼ 0.99 0 days 13.8 days

Oil radish 297 (1 � e�0.045 t), R2 ¼ 0.97 274 (1 � e�0.045 t)1.37, R2 ¼ 0.98 0 days 7.0 days

Wheat 515 (1 � e�0.051 t), R2 ¼ 0.98 482 (1 � e�0.083 t)1.63, R2 ¼ 0.99 0 days 5.9 days

Oat 527 (1 � e�0.025 t), R2 ¼ 0.96 413 (1 � e�0.070 t)2.36, R2 ¼ 0.99 0 days 12.3 days

b i o s y s t em s e n g i n e e r i n g 1 1 2 ( 2 0 1 2 ) 2 4 8e2 5 1250

volume (Vfed) and first and second stage reactor volume (VD1

and VD2), was derived for this study. By combining the two

equations, the average age of biomass in different configura-

tions, and individual digesters, can be found. The average age

in slurry shifts is higher in a two-stage configuration, than in

a one-stage digester.

For an energy crop following a CH4 production pattern,

such as Eq. (2), Eq. (5) describes how much CH4 can be

produced in a single stage digester:

CH4 producedXNt¼0

0@�

1�Vfed=VD1

�t$

Ztþ1

t

B0$k$C$�1�exp�kt

�C�1

1A

(5)

For a two-stage configuration, it is obvious that CH4

production in a digester with an age profile skewed towards

more older biomass reaching itsmaximum rate of digestion at

rmax > 0, would benefit from a rmax being reached after

a number of days.

For single stage digesters, the data presented in this study

is only relevant for tuning the HRT to best fit the specific

energy crop e e.g. a shorter HRT may be suitable for some

energy crops, but not for others.

Fig. 2 e Experimental data, Eq. (1), and Eq. (2). Note that the

origins of the equations are located at five different values

on the ordinate.

4.3. I:S ratio

A study by Rapose, Banks, Siegert, Heaven, and Borja (2006)

emphasised the importance of I:S ratios when studying

batch experiments as their study showed higher B0-values at

I:S values up to 2. Indeed, the effect of I:S ratio need further

investigation. As the I:S ratio is a metric for the bacteria to

substrate ratio, the c-value may also be affected by the I:S

ratio, as more bacteria may be faster at degrading otherwise

difficult degradable structures. That is, a higher I:S ratios there

will be a tendency for lower c-values.

5. Conclusion

The degradation pattern of some biomasses is better

described by Eq. (2) than Eq. (1). The exact effect of I:S ratio

on the k-value in Eq. (2) remains to be established

for specific energy crops, but it likely follows a pattern

like the one that can be derived from data by Hashimoto

(1989).

Future progress in pretreatment of energy crops should

focus on not only increasing B0, but increasing B0 and k, and

reducing rmax and c in Eq. (2) when it applies, and imple-

menting the knowledge in equations such as Eq. (5). Other-

wise, sufficient credit may not be given to existing

pretreatment methods.

r e f e r e n c e s

Blonskaja, V., Menert, A., & Vilu, R. (2003). Use of two-stageanaerobic treatment for distillery waste. Advances inEnvironmental Research, 7, 671e678.

Chen, Y. R., & Hashimoto, A. G. (1978). Kinetics of methanefermentation. Biotechnology and Bioengineering Symposium, 8,269e282.

Demirer, G. N., & Chen, S. (2005). Two-phase anaerobic digestionof unscreened dairy manure. Process Biochemistry, 40,3542e3549.

Ghosh, S., Ombregt, J. P., & Pipyn, P. (1985). Methane productionfrom industrial wastes by two-phase anaerobic digestion.Water Research, 19(9), 1083e1088.

Hashimoto, A. G. (1986). Ammonia inhibition ofmethanogenesis from cattle wastes. Agricultural Wastes, 17,241e261.

Hashimoto, A. G. (1989). Effect of inoculum/substrate ratio onmethane yield and production rate from straw. BiologicalWastes, 28, 247e255.

Hill, D. T. (1991). Steady-state mesophilic design equations formethane production from livestock. Transactions of the ASAE,34(5), 2157e2163.

b i o s y s t em s e ng i n e e r i n g 1 1 2 ( 2 0 1 2 ) 2 4 8e2 5 1 251

ISO. (1995). Water quality: Evaluation of the ‘ultimate’ anaerobicbiodegradability of organic compounds in digested sludge - methodby measurements of the biogas production. InternationalStandard. ISO/DIS 11734.

Moller, H. B., Sommer, S. G., & Ahring, B. K. (2004). Methaneproductivity of manure, straw and solid fractions of manure.Biomass and Bioenergy, 26, 485e495.

Quai, W., Yan, X., Ye, J., Sun, Y., Wang, W., & Zhang, Z. (2011).Evaluation of biogas production from different biomasswastes with/without hydrothermal pretreatment. RenewableEnergy, 36(12), 3313e3318.

Rapose,F.,Banks,C. J., Siegert, I.,Heaven,S.,&Borja,R. (2006). Influenceof inoculum to substrate ratio on the biochemical methanepotentialofmaize inbatchtests.ProcessBiochemistry,41, 1444e1450.