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Fermentative biohydrogen production: Evaluation of netenergy gain

Karnayakage Rasika J. Perera a, Balachandran Ketheesan a,Venkataramana Gadhamshetty b, Nagamany Nirmalakhandan a,*aCivil Engineering Department, New Mexico State University, Las Cruces, NM 88011, USAbCivil & Environmental Engineering Dept., Rensselaer Polytechnic Institute, Troy, NY 12180, USA

a r t i c l e i n f o

Article history:

Received 30 June 2010

Received in revised form

7 August 2010

Accepted 7 August 2010

Available online 15 September 2010

Keywords:

Biohydrogen

Dark fermentation

Net energy gain

Anaerobic digestion

Microbial fuel cells

a b s t r a c t

Most dark fermentation (DF) studies had resorted to above-ambient temperatures to

maximize hydrogen yield, without due consideration of the net energy gain. In this study,

literature data on fermentative hydrogen production from glucose, sucrose, and organic

wastes were compiled to evaluate the benefit of higher fermentation temperatures in

terms of net energy gain. This evaluation showed that the improvement in hydrogen yield

at higher temperatures is not justified as the net energy gain not only declined with

increase of temperature, but also was mostly negative when the fermentation temperature

exceeded 25 �C. To maximize the net energy gain of DF, the following two options for

recovering additional energy from the end products and to determine the optimal

fermentation temperature were evaluated: methane production via anaerobic digestion

(AD); and direct electricity production via microbial fuel cells (MFC). Based on net energy

gain, it is concluded that DF has to be operated at near-ambient temperatures for the net

energy gain to be positive; and DF þ MFC can result in higher net energy gain at any

temperature than DF or DF þ AD.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Rapidly increasing global demand for the limited fossil fuel

reserves on one hand, and the need to control greenhouse gas

effects due to CO2 emissions from fossil fuel usage on the

other, are driving the search for sustainable and carbon-free

or carbon-neutral energy carriers for the well-being of future

generations. Hydrogen has been identified as a potential

substitute for fossil fuels because it has high calorific value,

which can be converted to electrical energy at higher effi-

ciencies than current fuel-to-energy conversion technolo-

gies. In addition, it is also carbon-free, non-polluting, and

recyclable.

Currently, world hydrogen production is around

5 � 106 N m3, 96% of which is derived from fossil fuels [1] with

net negative energy gain. Production of hydrogen, for

example, by methane-steam reforming at best yields 2.95 mol

H2 per mole of methane, with a negative net energy gain of

�16 MJ/kg of H2; production by electrolysis of water using

electricity generated by a natural gas-fired combined cycle

power plant at best yields 1.37 mol H2 per mole of methane,

with a negative net energy gain of �172 MJ/kg of H2 [2]. If

hydrogen is to be widely accepted as a sustainable substitute

for fossil fuels, it has to be produced from renewable feedstock

other than the fossil fuels it is intended to replace via

processes with a net positive energy gain.

* Corresponding author.E-mail address: nkhandan@nmsu.edu (N. Nirmalakhandan).

Avai lab le at www.sc iencedi rect .com

journa l homepage : www.e lsev ie r . com/ loca te /he

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0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2010.08.037

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Biological processes have been proposed as promising

approaches for cleaner and sustainable production of hydro-

genebiohydrogen. In contrast to current hydrogen-producing

technologies, biological technologies can be engineered to

produce hydrogen utilizing renewable feedstocks under mild

operating conditions without generating any harmful

byproducts [3]. Three different biological processes have been

identified as feasible for biohydrogen production: direct bio-

photolysis, photofermentation, and dark fermentation.

In direct biophotolysis, cyanobacteria decomposes water

to generate hydrogen and oxygen in the presence of light

according to the equation:

2H2O ¼ 2H2(g) þ O2(g) ΔG0 ¼ þ1498 kJ

Since the end product of hydrogen utilization is water, this

process is considered sustainable. However, the rate of this

reaction is low because a large free energy has to be overcome.

As such, its large-scale application has been hindered by high

cost of photo-bioreactors and low solar conversion efficiencies

[4,5]. Separation of hydrogen from oxygen can further add to

the costs.

In photofermentation, anoxygenic photoheterotrophic

bacteria utilize organic feedstock to produce hydrogen in the

presence of light according to the equation:

C6O6H12 (glucose) þ 6H2O ¼ 12H2(g) þ 6 CO2(g)

ΔG0 ¼ þ75.2 kJ

Merits of this process include moderate energy needs, high

theoretical yield of hydrogen (often reported asmoles H2/mole

feedstock) and the ability to stabilize organic waste streams.

Demerits include low solar conversion efficiency (<10%)

resulting from the inefficient nitrogenase enzyme, and low

light intensities at which photosynthesis saturates [4].

In dark fermentation (DF), anaerobic heterotrophic bacteria

utilize organic feedstockwithout any light to produce hydrogen

according to the equation:

C6O6H12 (glucose) þ 2H2O ¼ 2CH3COOH þ 4H2(g) þ 2CO2(g)

ΔG0 ¼ �184 kJ

While dark fermentation is spontaneous and has the high-

est conversion rate among the three bioprocesses, the yield is

low and the conversion is not complete resulting in volatile

fatty acids and alcohols as end products [3]. Thus the feed-

stock-to-energy capture efficiency of DF (based on the energy

contents of glucose and hydrogen of 16 MJ/kg and 120 MJ/kg,

respectively) is about 33%.

Two fundamental factors that have to be evaluated when

comparing biohydrogen production processes are the

number of electrons that can be transferred from the feed-

stock to the end product that can serve as the energy carrier

and the net energy gain of the process. While nearly all the

electron equivalence of the substrate is routed to hydrogen

in biophotolysis and photofermentation, only a fraction of

the electrons in the feedstock is routed to hydrogen in DF as

most of the electrons are routed to organic end products

such as volatile fatty acids and alcohols. In spite of this

disadvantage, DF has been suggested as more practical than

the other two phototrophic processes as it does not require

external energy to drive the process or large surface area to

capture the necessary light [6,7]. It can take advantage of

existing reactor technologies to utilize organic wastes as

feedstock [8], serving dual functions of energy production

and waste stabilization. Additional advantages of the DF

process over the phototrophic processes include its ability to

utilize particulate organic feedstock and to run throughout

the day.

To maximize net energy production from a given feedstock

via DF, suitable cultures have to be employed with required

nutrients; appropriate operating conditions have to be engi-

neered to maximize electron flow towards hydrogen; energy

input to theprocesshas tobeminimized; and additional energy

has to be recovered from the end products. Several reviews of

the extensive literature on the approaches and techniques

proposed to improve hydrogen production by DF have been

published [3,9e12]. Some of the proposed techniques include

metabolic engineering of H2-producing cultures, feedstock

pretreatment (heat-shocking, acid treatment), nutrient

augmentation, pH management (pretreatment, continuous

control), headspace pressure management (intermittent vs.

continuous pressure release, vacuum, gas sparging), and high

temperature operation. Extracting additional energy from the

aqueous end products of dark fermentation via a second step

[13] such as anaerobic digestion [14], photofermentation [15], or

microbial fuel cells [16] can further improve the combined net

energy gain.

This study evaluated whether or not, above-ambient

temperature fermentation is a viable option to improve

productivity of the dark fermentation process in the context

of net energy gain. Previous reports have listed benefits of

high fermentation temperatures that include faster reaction

rates and hence smaller reactor volume; improved hydrogen

yield; and suppression of solvent producers and inhibitory

effects of volatile fatty acids [7,11,17e19]. However, the heat

energy required to maintain higher fermentation tempera-

tures can decrease the net energy gain, and hence, the

viability and sustainability of the process. Goals of this study

were to:

i) compile literature on fermentative biohydrogen studies

at various temperatures to assess the net energy gain as

a function of fermentation temperature;

ii) identify conditions under which the highest net energy

gain can be realized; and

iii) evaluate the feasibility of following upDFwith anaerobic

digestion (AD) andmicrobial fuel cell (MFC) technologies

to identify the optimal fermentation temperature for

maximum net energy gain by the combination.

2. Theory/calculation

The theoretical net energy gain, En [kJ/g COD in feedstock] in

this study is defined as the total energy produced equivalent

to the hydrogen volume generated by DF, Edf [kJ], minus any

heat energy required, Ef [kJ] to raise the reactor contents from

ambient temperature [Ta] to the fermentation temperature

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[Tf]. The following equations form the basis of our analysis in

batch reactors:

Edf ¼ GrH2ðLHVÞ (1)

Ef ¼ Vrwcr�Tf � Ta

�(2)

where, G is the volume of hydrogen generated [L]; rH2is the

density of gaseous hydrogen [8.9� 10�5 kg/L]; rw is the density

of water [1 kg/L]; LHV is the lower heating value of hydrogen

[120,000 kJ/kg]; V is the liquid volume in the reactor [L]; and cpis the specific heat of water [4.2 kJ/kg K]. Assuming negligible

heat loss,

EN ¼ Edf � Ef

VC(3)

where, C is the COD concentration of the feedstock [g COD/L].

Most dark fermentation studies have reported hydrogen

yield, Y, in terms of moles hydrogen produced per mole

feedstock. The net energy gain defined above can be estimated

from the reported yields from the following equation:

EN ¼ YCkVðLHVÞ�MWH2=MWf

�10�3 � Vrwcr

�Tf � Ta

�

VC(4)

where, k is the COD equivalent of the feedstock (g feedstock/g

COD), and MW is the molecular weight. In this study, Ta was

set equal to the Standard Ambient Temperature of 25 �C.

Nomographs illustrating the dependence of the net energy

gain per unit volume of the reactor on C, Y, and Tf as described

by Equation (4) are presented in Fig. 1(a) with sucrose as the

feedstock and in Fig. 1(b) with glucose as the feedstock. These

plots show that the net energy gain of the process can be

negative at higher temperatures for a given feedstock

concentration, if the yield is not high enough. For example,

from Fig. 1(a), at a sucrose concentration Cf ¼ 10 g/L, and

a typical experimental yield Y¼ 3mol H2/mole sucrose, the net

energy yield at 30 �C is 0 kJ/L; at this concentration and yield,

the fermentation temperature has to be less than 30 �C for the

net energy gain to be positive. The actual net energy gain will

be less than that indicated in this plot because, it does not

include the conversion efficiencies of hydrogen-to-energy and

a Sucrose

b Glucose

-80 -60 -40 -20 0 20 40 60 80 100

Net energy gain [kJ/L]

5 6 7 8 9 10 11 12 13 14

Sucrose concentration [g/L]

8.07.06.05.04.03.0

2.0

1.0

Hydrogen yield

[moles H2/mole sucrose]Temperature [ºC] 40 35 30 25

+venet energy-ve

net energy

-80 -60 -40 -20 0 20 40 60 80 100

Net energy gain [kJ/L]

5 6 7 8 9 10 11 12 13 14

Glucose concentration [g/L]

4.03.53.02.52.0

1.5

1.0

0.5

Temperature, Tf [ºC]

-ve

net energy+ve

net energy

Hydrogen yield, Y

[moles H2/moles glucose]

40 35 30 25

Fig. 1 e Dependence of net energy gain per unit volume by dark fermentation on feedstock concentration, hydrogen yield,

and fermentation temperature: (a) sucrose (b) glucose. To read these plots, pick feedstock Cf on x-axis of the left quadrant, go

vertically to any Y, go horizontally to any Tf in the right quadrant, and go vertically down to read the net energy gain.

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fuel-to-heat. This analysis, albeit a theoretical one, provides

a strong motivation to identify the conditions for biohydrogen

production with the highest yield at the lowest temperature

and at the lowest feedstock concentration possible for the net

energy gain to be positive. Even though the above analysis is

based on ideal conditions and simplifying assumptions,

consideration of the net energy gain estimated from experi-

mental results, rather than hydrogen yield, can be seen as

a more appropriate measure to evaluate and compare the

different test conditions to discern the optimal ones for scale

up or further experimental investigations.

When liquid organic wastes are used as the feedstock, the

net energy gain is estimated from Equations (1) to (3), using

the reported data on cumulative hydrogen production, G;

culture volume, V; fermentation temperature, Tf; and the

feedstock concentration, Cf. When particulate organic wastes

are used as feedstock, the heat required to heat the solids is

added to Equation (2), which can be estimated from the solids

content of the waste, M [kg] and the specific heat of solids cp,s[¼4.2 kJ/kg K] as ¼M cp,s (Tf e Ta).

The two options (DF þ AD and DF þ MFC) for recovering

additional energy fromtheaqueousendproductsarecompared

on the basis of net electrical energy, EE, that can be potentially

generated. Electrical energy that can be generated from

hydrogen fromDF via fuel cells, EEdf, is estimated assuming an

efficiency of 65% for the fuel cell: EEdf ¼ 65%Edf. Potential for

energy production by AD, EEad is found by estimating the stoi-

chiometricmethane volume that canbeproduced fromtheend

products of DF (VFAs and alcohols) and multiplying it with the

LHV of methane (¼50,000 kJ/kg). An efficiency of 30% was

assumed for methane-to-electricity conversion via ICE/alter-

nator combination. The digestion temperaturewas assumed to

be 35 �C. If the fermentation temperature is less than 35 �C,additional heat energy required, Ed, to raise the temperature to

the digestion temperature of 35 �C was deducted from EEad; if

the fermentation temperature is >35 �C, it was assumed that

the effluent fromDF could be digestedwithout any heating and

Edwas set to zero. Potential for energy production byMFC, Emfc,

is deduced from the free energies of the aqueous end products

of DF. An efficiency of 60% was assumed for electricity

production by MFC, EEmfc. It has to be noted that these theo-

retical estimates of energy outputs by AD and MFC are

conservative ones and ignore the effect of temperature.

3. Results and discussion

3.1. Net energy gain via DF of pure organic substrates

Literature reports that had tested pure organic substrates

were compiled and separated into two subgroups: those using

glucose and those using sucrose. Within each subgroup,

reports that had reported the hydrogen yield, Y, the feedstock

concentration, Cf, the working volume, V, and the fermenta-

tion temperature, Tf, were selected for evaluating the net

energy gain, En. A total of 18 reports were included in the

glucose subgroup and 20 reports, in the sucrose subgroup. A

summary of the results of this analysis is presented in Fig. 2,

ranked by fermentation temperature. Hydrogen yield, Y, was

normalized relative to the theoreticalmaximumyield possible

with the corresponding feedstock: 8mol H2/mole in the case of

sucrose and 4 mol H2/mole in the case of glucose. It can be

seen from this summary that all the studies in the glucose

subgroup had been done at temperatures >30 �C and had

negative net energy gain. In the sucrose subgroup, all studies

conducted at temperatures<30 �C (4 out of the 20 studies) had

positive net energy gain. This evaluation shows that the

hydrogen yield, Y, that has often been reported as the

measure of productivity may not be the appropriate one to

judge the viability and sustainability of the process, because of

the net negative energy gain.

3.2. Net energy gain via DF of organic wastes

Summaries of the analysis of 10 reports on organic wastes in

liquid form and 15 reports on organic wastes in particulate

form are presented in Fig. 3(a) and (b), respectively. Even

though higher temperatures have been viewed as favorable

for solubilization of particulate wastes, the net energy gain is

still negative as illustrated in Fig. 3(b). In the case of liquid

organic wastes, the net energy gain is positive only when the

fermentation temperature is <30 �C except in the case of

Skonieczny and Yargeau [50], where DF of glucose in

a synthetic wastewater solution using a pure culture e Clos-

tridium beijerinckii, at 30 �C yielded a net energy gain of �4.8 kJ/

g COD. In that study, relatively large amounts of ethanol were

produced (w25%) resulting in lower hydrogen volume and

hence negative net energy gain. Another exception was the

study by Venkata Mohan et al. [52] where DF of synthetic

wastewater using mixed microflora at 29 �C yielded a net

energy gain of �3.3 kJ/g COD. Even though the fermentation

temperature was low, the hydrogen generation in their study

was also low, which was attributed to pH decline and meta-

bolic shift towards solventogenesis [52].

3.3. Cases with positive net energy gain

Of the cases evaluated in this study, positive net energy gain

was realized only with sucrose. The study by Wu et al. [20]

achieved positive net energy gain of 0.8 kJ/g COD in spite of

the high fermentation temperature of 40 �C. They utilized high

sucrose concentration of 30 g/L and high-hydrogen-producing

cultures, identified as Clostridium pasteurianum. Lin and Jo [21]

and Lee et al. [22] conducted their study at 35 �C and achieved

positive net energy yields of 0.02 and 0.37 kJ/g COD, respec-

tively. Kim et al. [23] achieved positive net energy gain of

0.6 kJ/g COD at a temperature of 35 �C. They had resorted to

gas sparging (with biogases, nitrogen, and carbon dioxide)

to lower headspace hydrogen partial pressure and hence to

improve hydrogen yield. From the results of sparging with the

three gases, they concluded that sparging with carbon dioxide

yielded the greatest benefit with little effect on hydrogen-

producers. They attributed the increased production to

inhibitory effects of carbon dioxide on competitors such as

acetogens and lactic acid bacteria. Under their test conditions,

they found the dominant organisms to be Clostridium tyrobu-

tyricum, C. proteolyticum and Clostridium acidisoli. Chen and Lin

[24] achieved net energy gain of 1.6 kJ/g COD at 35 �C with

sucrose at 20 g/L, using sludge from the aeration tank of

a wastewater treatment plant as the seed.

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All the studies at near-ambient temperatures had a positive

net energy gain. The study by Lo et al. [25] was able to achieve

a high positive net energy gain of 2.6 kJ/g COD at ambient

temperatures, albeit with pure cultures e C. pasteurianum CH4.

They concluded that mixing efficiency could be important in

hydrogen production. Logan et al. [26] were able to achieve

positive net energy gain of 0.4 kJ/g COD at near-ambient

temperature of 26 �C using mixed cultures originating from

soils used for tomato plants. Comparing hydrogen production

at continuous vs. intermittent headspace pressure release

methods, they concluded that the continuous pressure release

method yielded higher hydrogen volumes. Even though they

presumed that lower production under the intermittent pres-

sure was due to the higher hydrogen partial pressures, they

could not confirm it thermodynamically. In contrast, in our

studies at 22 �C [27],wehaveobtainednet energygainof 2.6 kJ/g

COD under continuous pressure release method and 3.5 kJ/g

COD under the intermittent pressure release method. Our

thermodynamic analysis showed that there is no negative

effect of hydrogen partial pressure and intermittent pressure

release conditions on sucrose-to-hydrogen fermentation.

Culturesobtained fromcompost andenriched inour laboratory

were able to achieve relatively higher yields and positive net

energy gains. Microbial analysis of these cultures revealed

Gram positive anaerobic consortium consisting of 18 bacterial

community members, of which the following two members

comprised 98% of the consortia: C. pasteurianum (71e84%) and

Sporolactobacillus laevolacticus (28e14%). The remaining 2%

comprised of 16 other members from Firmicutes.

Whenwe compared these cultures at 22 �C and 37 �C under

otherwise identical conditions [27], we found the productivity

to be higher at 22 �C as summarized in Table 1. Based on the

Fig. 2 e Fermentation temperature, relative hydrogen yield, and net energy gain reported in the literature for glucose and

sucrose.

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Van Ginkel et al., [53]

Van Ginkel et al., [53]

Van Ginkel et al.,[53]

Venkatamohan et al., [52]

Ferchichi et al., [51]

Skonieczny et al., [50]

Tang et al., [49]

Liu et al., [48]

Zhang et al., [47]

Ueno et al., [46]

0 10 20 30 40 50 60 70Temperature [C]

23

23

23

29

30

30

45

55

55

60

-50 -40 -30 -20 -10 0 10

Net energy gain [kJ/g COD]

0.4

1.1

1.4

-3.3

1.2

-4.8

-34.2

-20.4

-22.3

-10.7

Cui et al., [67]

Kim et al., [66]Datar et al., [65]

Xing et al., [64]

Fan et al., [63]Cai et al., [62]

Danko et al., [61]Fang et al., [60]

Pattra et al., [59]

Lay et al., [58]Kim et al., [57]

Fan et al., [56]Pan et al., [55]

Gilroyed et al., [19]

Yokoyama et al., [54]

0 10 20 30 40 50 60 70

Temperature [C]

35

3535

36

3636

3737

37

3740

4550

52

60

-15 -10 -5 0 5

Net energy gain [kJ/g feedstock]

-2.3

-0.5-0.4

-0.0

-0.4-9.4

-3.4-0.3

-0.5

-0.0-0.1

-0.7-2.8

-1.6

-8.2

a Feedstock: Liquid organic wastes

b Feedstock: Particulate organic wastes

Fig. 3 e Fermentation temperature and net energy gain reported in the literature for liquid organic wastes (a) and particulate

organic wastes (b).

Table 1 e Comparison of experimental results at 22 �C and 37 �C (Gadhamshetty et al. [27]).

Headspace pressure release Temperature [�C]

22 37

Intermittent Continuous Intermittent Continuous

Sucrose concentration, g COD/L 10 10 10 10

H2 produced, mL 472 356 196 266

H2 content of gas phase, % 41e61 41e56 43e66 54e58

Sucrose consumption, % 98 96 45 47

Sucrose-to-H2 conversion, % 53 40 21 29

H2/sucrose yield mol/mol 4.3 3.2 1.7 2.3

Relative yield, % 54 40 21 29

Net energy gain, kJ/g COD 3.5 2.6 �3.66 �3.17

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data from that study, the net energy gain at 37 �C is estimated

as negative while that at 22 �C as positive (Fig. 2 and Table 1).

High temperatures may reduce the performance of DF

systems due to inactivation of hydrogenase enzyme in fer-

menting bacteria. In batch reactors with insufficient or poor

pH control, faster fermentation kinetics at higher temperature

may result in faster volatile fatty acid accumulation and

abrupt pH decline (<4.5) to cause shift of fermentation mode

from acid production to solvent production, and thereby

losing gaseous hydrogen to solvents; and rapid VFA accumu-

lation leading to pH inhibition. Effects of high temperature in

batch DF systems have been presented elsewhere [27].

Based on our theoretical considerations as well as experi-

mental data compiled from the literature, it can be concluded

that, to maximize net energy gain via dark fermentation,

appropriate cultures capable of high-hydrogen yield have to

be employed and the process has to be operated at near-

ambient temperatures with the lowest feedstock concentra-

tion as possible. Any additional energy that can be harvested

from the resulting aqueous end products can also improve the

net energy gain as discussed in the following sections.

3.4. Net energy gain by DF as a function of temperature

Wang and Wan [28] have reported an internally consistent

dataset on hydrogen evolution and product mix from dark

fermentation of 10-g/L glucose over a temperature range of

20e55 �C in 100-mL batch reactors. Using the measured

hydrogen evolution vs. temperature data, they had derived

empirical equations relating the two. We have used those

equations to calculate hydrogen evolution and net energy gain

as a function of fermentation temperature to develop Fig. 4.

Even though hydrogen production more than doubled from

0.1 L to 0.26 L as the fermentation temperature is increased

from 20 �C to 40 �C, that increase is not justified as the net

electrical energy declined significantly from 0.74 kJ to�6.58 kJ.

The product mix data reported by Wang and Wan [28] in

the temperature range of 20e55 �C was used to estimate the

net energy gain that could be expected when dark fermenta-

tion is followed by the two options mentioned earlier. Table 2

shows the data used here as reported by Wang and Wan [28]

and our energy estimates for the two options at the different

temperatures. Results of this analysis are summarized in Fig. 5

and discussed in the next sections.

-20

-15

-10

-5

0

5

10

15

20

0.00

0.05

0.10

0.15

0.20

0.25

0.30

20 25 30 35 40 45 50 55 60

Ene

rgy

[kJ]

Hyd

roge

n pr

oduc

ed [

mL

]

Temperature [C]

1. Hydrogen produced [mL]2. Energy for heating [kJ]3. Energy generated [kJ]4. Net energy gain [kJ]

1

2

3

4

Fig. 4 e Hydrogen produced, energy generated, energy

required for heating, and net energy gain as a function of

fermentation temperature, estimated from data reported

by Wang and Wan [28].

Table 2 e Data used and energy estimates at different temperatures.

Fermentation temperature [�C] Note

20 25 30 35 40 45 50 55

Ethanol [mg/L] 32.2 64.4 133.4 372.6 271.4 174.8 69.0 41.4 a

Acetic acid [mg/L] 186.0 270.0 384.0 978.0 942.0 486.0 324.0 414.0 a

Propionic acid [mg/L] 44.4 59.2 59.2 0.0 0.0 88.8 51.8 37.0 a

Butyric acid [mg/L] 0.0 17.6 0.0 0.0 79.2 0.0 0.0 0.0 a

Hydrogen volume [L] 0.11 0.16 0.21 0.25 0.26 0.24 0.18 0.09 b

Hydrogen yield [M/M] 0.84 1.29 1.70 1.99 2.08 1.90 1.41 0.72 c

i) Energy calculations for dark fermentation:

Edf [kJ] 1.14 1.73 2.28 2.68 2.80 2.55 1.90 0.96 d

EEdf ¼ 65%Edf [kJ] 0.74 1.12 1.48 1.74 1.82 1.66 1.24 0.63

Ef for heating [kJ] 0.00 0.00 2.10 4.20 6.30 8.40 10.50 12.60 e

NEEdf ¼ EEdf � Ef [kJ] 0.74 1.12 �0.62 �2.46 �4.48 �6.74 �9.26 �11.97

ii) Energy calculations for dark fermentation followed by anaerobic digestion:

Ead [kJ] 0.42 0.68 0.97 2.28 2.14 1.27 0.71 0.73 f

EEad ¼ 30%Ead [kJ] 0.12 0.20 0.29 0.68 0.64 0.38 0.21 0.22

Ed for heating [kJ] 6.30 4.20 2.10 0.00 0.00 0.00 0.00 0.00 f

EEdfþad ¼ NEEdf þ EEad-Ed [kJ] �5.44 �2.87 �2.43 �1.78 �3.84 �6.36 �9.05 �11.75

iii) Energy calculations for dark fermentation followed by microbial fuel cell:

Emfc [kJ] 0.44 0.73 1.05 2.46 2.31 1.37 0.76 0.78 f

EEmfc ¼ 60%Emfc [kJ] 0.27 0.44 0.63 1.47 1.39 0.82 0.46 0.47

EEdf þ mfc ¼ NEEdf þ EEmfc [kJ] 1.01 1.56 0.01 �0.99 �3.10 �5.92 �8.81 �11.51

*Note: a e Table 2 in Wang and Wan [28]; b e Eq. 1 to 4 in Wang and Wan [28] ;c e calculated from b; d e Eq. 1; e e Eq. 2; f e see text.

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3.5. Electricity production by DF followed by AD

Wehave used the productmix data reported byWang andWan

[28] over the temperature range of 20e55 �C to assess the net

electrical energy gain of DF þ AD as a function of fermentation

temperature. The aqueous products observed by them included

ethanol and acetic, propionic, and butyric acids. Our energy

estimates are based on the following stoichiometric methane

potentials of these end products: 1.5, 1.0, 1.75, and 2.5 mol

methane/mole of the respective end product. Since the diges-

tion temperature in these estimates is taken as 35 �C, additionalheat energy has to be supplied when the fermentation

temperature is less than 35 �C resulting in negative net energy.

Even though additional heat energy is not supplied to the reac-

tors at fermentation temperatures greater than 35 �C, the

methane volume generated is not adequate to offset the heat

energy required to maintain the fermentation temperature. As

such, the net electrical energy gain of DF þ AD is negative over

the range of temperatures considered here (Fig. 5).

3.6. Electricity production by DF followed by MFC

The same product mix data reported by Wang and Wan [28]

over the temperature range of 20e55 �C was used to assess

the net electrical energy gain of DF þ MFC as a function of

fermentation temperature. Our estimates are based on the

following free energies for the electrochemical reactions of

the products: 1324 kJ/mol ethanol, 848 kJ/mol acetic acid,

1485 kJ/mol propionic acid, and 2206 kJ/mol butyric acid.

Based on the results presented in Table 2 and in Fig. 5,

DF þ MFC can be seen to be a better option than DF þ AD to

recover additional energy from the end products of DF.

However, for the net electrical energy gain to be positive, DF

should be operated at ambient temperatures.

3.7. DF vs. DF þ AD vs. DF þ MFC

Of the three options considered here to generate electrical

energy, on the basis of net energy gain, the DF þ MFC

combination appears to be better than DF and DF þ AD over

the entire temperature range considered. Based on the simple

analysis presented here considering only the heat energy

inputs and the theoretical energy outputs, DF þ AD does not

appear to be energy-efficient due to heat addition. However,

AD technology is well established at full scale and could be

cost-effective compared to MFC technology, which is still in

early stages of development. Practical application of the MFC

technology is limited because of the low voltages that they can

currently generate. Improvement in net energy gain by DF

may be possible if the waste heat generated in the hydrogen-

to-electrical energy conversion via fuel cells could be utilized

to raise the fermentation temperature. Even though thiswaste

heat is of low grade (w80 �C), it would be adequate to raise the

temperature of the feed to about 40 �C to take advantage of the

higher hydrogen yields.

4. Conclusions

Literature data on fermentative hydrogen production was

used to evaluate the viability of above-ambient temperatures

in the context of net energy gain. Results of this study showed

that the net energy gain declined with fermentation temper-

ature and was negative when the fermentation temperature

exceeded 30 �C. Feasibility of improving the net energy gain by

recovering additional energy from the end products of dark

fermentation using anaerobic and microbial fuel cell tech-

nologies was also evaluated. Based on our simple but

conservative analysis, it is concluded that DF followed by

microbial fuel cell could be a better alternative than the DF

followed by anaerobic digestion.

Acknowledgement

This studywas funded inpart by theOfficeofVice President for

Research at New Mexico State University and by the National

Science Foundation’s CBET Division, under Grant N� 0607175.

Nomenclature

cp specific heat of water, kJ/kg K

cp,s specific heat of solids, kJ/kg K

-12

-10

-8

-6

-4

-2

0

2

15 20 25 30 35 40 45 50 55 60

Net

ene

rgy

gain

[kJ

]

Fermentation temperature [C]

1

2

3

Net energy gain: 1- DF 2- DF + AD 3- DF + MFC

0

200

400

600

800

1000

1200

0

0.1

0.2

0.3

0.4

15 20 25 30 35 40 45 50 55 60

Eth

anol

and

VFA

s [m

g/L

]

Hyd

roge

n [m

L]

Fermentation temperature [C]

12

3

45

Product mix in DF: 1- Hydrogen 2- Acetic acid 3- Ethanol 4- Butyric acid 5- Propionic acid

b

a

Fig. 5 e Experimentally measured product mix in DF (a) and

estimated net energy gain (b) as a function of fermentation

temperature. Experimental data from Wang and Wan [28].

DF e dark fermentation; AD e anaerobic digestion; MFC e

microbial fuel cell.

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C COD concentration of the feedstock, g COD/L

Cf concentration of feedstock, g/L

Ead energy production by anaerobic digestion, kJ

Ed heat energy requirement in anaerobic digestion, kJ

Edf energy production by dark fermentation, kJ

Ef heat energy required at fermentation temperature, kJ

Emfc energy production by microbial fuel cells, kJ

En net energy gain by dark fermentation, kJ/g COD in

feedstock

Enþad net energy gain from dark fermentation followed by

anaerobic digestion, kJ

Enþmfc net energy gain from dark fermentation followed by

microbial fuel cell, kJ

EEad electrical energy generation from anaerobic

digestion, kJ

EEdf electrical energy generation from hydrogen via fuel

cells, kJ

EEmfc electrical energy generation bymicrobial fuel cells, kJ

G volume of hydrogen generated, L

k COD equivalent of the feedstock, g feedstock/g COD

LHV lower heating value, kJ/kg

M mass of solids in the reactor, kg

MWf molecular weight of feedstock, g

MWH2 molecular weight of hydrogen, g

NEEdf net electrical energy generation from hydrogen via

fuel cells, kJ

Ta ambient temperature, K

Tf fermentation temperature, K

V working volume of liquid in the reactor, L

Y hydrogen yield, moles hydrogen produced/mole

feedstock

rH2density of gaseous hydrogen, kg/L

rw density of water, kg/L

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