# Anaerobic digestion of energy crops in batch

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the optimal digester hydraulic retention time and or reactor

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B0t=dt B0$k$expkt reaches it maximum (rmax) at time zero,

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

In this study, 18 biogas batch experiments with six different

3. Results

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

energy crops cumulative digestion profile fitted better to

* Corresponding author.ders.feilberg@agrsci.dk (A. Feilberg).

Available online at www.sciencedirect.com

vi

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 1E-mail addresses: andersm.nielsen@agrsci.dk (A.M. Nielsen), analthough 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

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

in Eq. (1) for manure and sugars.

Bt B01 expktc; C 1 (1)

According to this equation, the CH4 production

digester configuration when energy crops are digested.

2. Methods1. Introduction

To obtain an economically viab

Northern Europe, animal manure i

crops such as maize, wheat and rye

production as a function of time B(

(B0), degradation constant (k), and1537-5110/$ e see front matter 2012 IAgrEdoi:10.1016/j.biosystemseng.2012.03.008 2012 IAgrE. Published by Elsevier Ltd. All rights reserved.

gas production in

gested with energy

es. Cumulative CH4ate methane yield

stant (c) describing

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

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 andconfiguration for the specific energy crop.into methane, andAccepted 22 March 2012

Published online 16 May 2012 evaluation of their degradation kinetics, specifically the rate of which they are convertedResearch Note

Anaerobic digestion of energy

Anders M. Nielsen*, Anders Feilberg

Aarhus University, Department of Engineering, Blichers Alle 20, DK

a r t i c l e i n f o

Article history:

Received 29 November 2011

Received in revised form

19 March 2012

Batch digestion of e

anaerobic digestion o

several days for ene

degradable than sug

production pattern fr

curve shape. Implica

journal homepage: www.else. Published by Elsevier Ltgy crops at mesophilic temperature indicates that the rate of

ergy crops into methane may first reach its maximum rate after

crops such as maize, oat, ryegrass, and wheat that are less

and fats. Experiments show that the cumulative methane

energy crops rather follows a sigmoidal, and not exponential,

ns are that optimal usage of energy crops should involve anrops in batch

30 Tjele, Denmarker.com/locate/ issn/15375110d. All rights reserved.

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).

Bt B0$1 expktc; C > 1 (2)

use of existing pretreatment methods is not given full credit,

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

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 249Pretreatment 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 beEq. (2) signifies a digestion rate of B0(t)/dt B0$k$c(1 expkt)c1. The maximum rate of CH4 production (rmax)occurs at time rmax 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. PretreatmentFig. 1 e Analysis of data from Hashimoto (1989).as an additional advantage of pretreatment is an improved k-

value and reduced c-value in Eq. (2) signifying a faster CH4production 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 numerousaccompanied 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

(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

Z positive integersx a counteradvantages (Blonskaja, Menert, & Vilu, 2003; Demirer & Chen,

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

agess Vfed=VD1$VD1 Vfed

VD1

ss; ssZ (3)

second stage

agess V2fed=VD1$VD2$Xss1x1

VD1 Vfed

VD1

ssx1

$VD2 Vfed

VD2

; ssZ

(4)

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

volume (Vfed) and first and second stage reactor volume (VD1and 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 producedXNt0

0@1Vfed=VD1t$

Zt1

t

B0$k$C$1expktC1

1A

(5)

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

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

will be a tendency for lower c-values.

pretreatment methods.

Table 1 e Equations fitted to experimental data.

Crop B(t), Eq. (1)

Ryegrass 499 (1 e0.039 t), R2 0.97 440 (1 Maize silage 528 (1 e0.020 t), R2 0.96 385 (1 Oil radish 297 (1 e0.045 t), R2 0.97 274 (1 Wheat 515 (1 e0.051 t), R2 0.98 482 (1 Oat 527 (1 e0.025 t), R2 0.96 413 (1

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 1250production 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.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, 40,5. 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 existing4.3. 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

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

e0.079 t)1.99, R2 0.98 0 days 8.7 dayse0.061 t)2.32, R2 0.99 0 days 13.8 dayse0.045 t)1.37, R2 0.98 0 days 7.0 dayse0.083 t)1.63, R2 0.99 0 days 5.9 dayse0.070 t)2.36, R2 0.99 0 days 12.3 days3542e3549.Ghosh, S., Ombregt, J. P., & Pipyn, P. (1985). Methane production

from 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.

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.

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

Anaerobic digestion of energy crops in batch1. Introduction2. Methods3. Results4. Discussion & analysis4.1. Pretreatment4.2. From batch to full scale digester4.3. I:S ratio

5. ConclusionReferences