Appl Biochem Biotechnol (2013) 171:626–642
DOI 10.1007/s12010-013-0375-z
Improving the Mixing Performances of Rice Straw
Anaerobic Digestion for Higher Biogas Production
by Computational Fluid Dynamics (CFD) Simulation
Fei Shen & Libin Tian & Hairong Yuan & Yunzhi Pang &
Shulin Chen & Dexun Zou & Baoning Zhu &
Yanping Liu & Xiujin Li
Received: 26 April 2013 / Accepted: 27 June 2013 /
Published online: 20 July 2013
# Springer Science+Business Media New York 2013
Abstract As a lignocellulose-based substrate for anaerobic digestion, rice straw is characterized by low density, high water absorbability, and poor fluidity. Its mixing performances in
digestion are completely different from traditional substrates such as animal manures. Computational fluid dynamics (CFD) simulation was employed to investigate mixing performances
and determine suitable stirring parameters for efficient biogas production from rice straw. The
results from CFD simulation were applied in the anaerobic digestion tests to further investigate
their reliability. The results indicated that the mixing performances could be improved by triple
impellers with pitched blade, and complete mixing was easily achieved at the stirring rate of
80 rpm, as compared to 20–60 rpm. However, mixing could not be significantly improved when
the stirring rate was further increased from 80 to 160 rpm. The simulation results agreed well
with the experimental results. The determined mixing parameters could achieve the highest
biogas yield of 370 mL (g TS)−1 (729 mL (g TSdigested)−1) and 431 mL (g TS)−1 (632 mL
(g TSdigested)−1) with the shortest technical digestion time (T80) of 46 days. The results obtained
in this work could provide useful guides for the design and operation of biogas plants using rice
straw as substrates.
Keywords CFD simulation . Rice straw . Anaerobic digestion . Mixing performances
F. Shen : L. Tian : H. Yuan (*) : Y. Pang : D. Zou : B. Zhu : Y. Liu : X. Li (*)
Centre for Resource and Environmental Research, Beijing University of Chemical Technology,
15 Beisanhuan East Road, Chaoyang District, Beijing 100029, People’s Republic of China
e-mail: yuanhr@mail.buct.edu.cn
e-mail: xjli@mail.buct.edu.cn
F. Shen
Institute of Ecological and Environmental Sciences, Sichuan Agricultural University—Chengdu Campus,
Chengdu, Sichuan 611130, People’s Republic of China
F. Shen
Provincial Key Laboratory of Agricultural Environmental Engineering, Sichuan Agricultural
University—Chengdu Campus, Chengdu, Sichuan 611130, People’s Republic of China
S. Chen
Department of Biological Systems Engineering, Washington State University, L.J. Smith 213, Pullman,
WA 99163, USA
Appl Biochem Biotechnol (2013) 171:626–642
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Introduction
Anaerobic digestion is a complex biochemical process for the utilization of organic
materials such as animal manures, lignocellulose-based substrates (energy crops and
agricultural residues), food wastes, and sewage and industrial effluents [1]. Biomethane
and biohydrogen can be generated by anaerobic digestion from the previously mentioned organic substrates [2]. Rice straw, as one of the most abundant agricultural
residues, is widely distributed, with an annual total yield of 1.95×108 t in China [3]. In
terms of the chemical components, rice straw has high content of cellulose (25.4–
35.5 %, wt/wt) and hemicellulose (32.3–37.1 %, wt/wt), which can be relatively easily
converted by anaerobic microorganisms into biogas [4, 5]. Moreover, the relatively low
lignin content (6.4–10.4 %, wt/wt) makes bioconversion easier than other agricultural
residues such as corn stove or wheat straw [4, 6]. Therefore, biogasification by anaerobic
digestion has been regarded as one of most important paths to utilize rice straw as a renewable
energy source.
However, a highly efficient anaerobic digestion generally depends on some key factors
such as substrate characteristics, feeding patterns, pH, temperature, redox potential, hydraulic retention time, and mixing behaviors [7]. Recently, the importance of mixing in achieving
efficient substrates conversion has been noted by many studies as it can potentially improve
the distribution of microorganisms to substrates and heat transfer. It can also contribute to the
reduction of substrate size and the improvement of gas separation from substrates [8–10].
Some studies reported that the performances of anaerobic digestion could not be efficiently
improved at low mixing [11], whereas higher mixing may potentially disturb digestion [12,
13]. Generally, mixing can be achieved in many ways including gas recirculation, mechanical pumping, and mechanical stirring [14], in which mechanical stirring can easily achieve
homogenization in practice and thereby is widely used in process industry operations
involving various solid–liquid flows [15]. Furthermore, the most efficient mixing device
in terms of power consumption is the mechanical agitator [16]. Therefore, in this work,
mechanical mixing was employed in anaerobic digestion for biogas production from rice
straw.
Current researches on mixing in anaerobic digestion mainly focus on the feedstocks of
animal manures, food wastes, and municipal solid wastes. Rice straw is characterized by low
density, high water absorbability, and poor fluidity, resulting in serious non-homogenization
and poor heat and mass transfer in anaerobic digestion, which thereby negatively affect
digestion performances. Therefore, mixing is more important for rice straw digestion than
for other transitional substrates. Moreover, the optimized mixing parameters for the traditionally investigated substrates could not be completely suitable for agricultural residues
including rice straw. Hence, the mixing performances for anaerobic digestion of rice straw
should be reconsidered.
Computational fluid dynamics (CFD) is a well-established tool for numerical analysis, which offers many advantages over actual experiments. It can give a complete
description for different flow patterns and is widely used to examine different design
scenarios in a short time with a relatively low cost [16, 17]. Currently, considerable
researches have been done on the various bioreactors for biomethane and biohydrogen
production by anaerobic digestion [18]. The manure slurry was mainly investigated for
the CFD simulation as a single-phase flow [7, 19–22]. However, since the characteristics of lignocellulose-based substrates for anaerobic digestion were significantly different from the manure slurry, rice straw digestion should be investigated as a twophase flow for CFD simulation.
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The objective of this study is to improve mixing performance for higher biogas
production from the anaerobic digestion of rice straw by CFD simulation and experimental tests. The CFD technology was firstly applied to simulate fluid dynamics
performances to determine the suitable blade type and number of impellers. The
stirring intensity was also investigated by CFD to determine the suitable parameters.
Then, anaerobic digestion tests were carried out to validate the applicability of the
stirring parameters obtained from CFD.
Materials and Methods
Digester and Operation
A completely stirred tank reactor (CSTR) was used for the CFD simulation and
anaerobic digestion experiment (Fig. 1a). The CSTR was 0.2 m in diameter and
0.3 m in height and had a working volume of 8.0 L. A stirring motor was mounted
on top of the digester. Three typical stirring impellers in practice were investigated,
including the pitched blade (PB), the high efficiency blade (HEB), and the discmounted flat blade (DFB), respectively (Fig. 1b). All impellers had the same blade
diameter of 0.17 m and thickness of 1.0 cm.
To determine a suitable blade type, the single impeller was employed with different blade
types. The distance from the horizontal axis of the blade to the bottom of the reactor was set
as 0.13 m. When the suitable blade for stirring impellers was decided, the impeller number
was investigated to select the suitable one (Fig. 1c). The distance from the horizontal axis of
the blade to the bottom of reactor was 0.13 m for the single impeller. For the double
impellers, the distance was 0.16 and 0.06 m, respectively. For the triple impellers, the
distance from the horizontal axis of the blade to the bottom was designed as 0.225, 0.13,
and 0.035 m, respectively.
Based on the selected blade type and number, the simulation was performed at the stirring
rate of 20 to 160 rpm to investigate flow performances and determine the approximate
stirring intensity for effectively mixing the substrate.
Assumptions
Some theoretical models for describing mechanical mixing performances in anaerobic
digesters have been developed based upon the following assumptions.
The mixing flow in the digester is considered to be three-dimensional and at steady
state. The fluid is regarded as a two-phase flow including the water phase and solid
phase. The diameter of rice straw particles in the fluid is defined as 1.0 mm with a
density of 950 kg m−3. The volume fraction of the rice straw for the simulation is 0.4,
which was determined by the substrate loading rate of 65 g total solid (TS)L−1for
anaerobic digestion.
The mixture for digestion is regarded as an incompressible and isothermal nonNewtonian pseudoplastic fluid.
The digestion temperature can be maintained at 35 °C constantly for the liquid phase
and the solid phase.
The interaction of the produced biogas (gas phase) and the rice straw (solid phase) is
also negligible.
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a
b
Pitched blade
High efficiency blade
Disc-mounted flat blade
Single impeller
Double impellers
Triple impellers
c
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Appl Biochem Biotechnol (2013) 171:626–642
R Fig. 1 The employed digester, blades, and number of impellers for simulation. a Diagram of the anaerobic
digested (1 digestion tank, 2 blade, 3 motor, 4 jacket for water bath, 5 inlet, 6 outlet, 7 gas flow meter). b
Types of stirring impeller for anaerobic digestion (left pitched blade, middle high efficiency blade, right discmounted flat blade). c Number of impellers (left single impeller, middle double impellers, right triple
impellers)
Governing Equations
The phase volume should be initially defined as:
Z
aq dV
Vq ¼
ð1Þ
V
n
aq=1, in which n is the amount of
where aq is the volume fraction of the q phase and ∑ q=1
phase.
The governing equations, consisting of continuity, momentum, and turbulence transport
under steady state, can be expressed as follows:
The continuity equation of a certain phase can be expressed in Eq. 2 as:
Xn ∂
aq ρq þ ∇⋅ aq ρq !
ð2Þ
vq ¼
mpq −mqp
p¼1
∂t
where t is the time; ρq is the density of the q phase; !
v q is the velocity of the q phase; mpq is
the mass transfer from the p phase to the q phase; and mqp is the mass transfer from the q
phase to the p phase.
The momentum equation can be defined in Eq. 3 as:
Xn ⇀
∂
v q þ ∇⋅ aq ρq !
v q!
v q ¼ −aq ∇p þ ∇⋅ t q þ aq ρq !
g þ
Rpq þ mpq !
v pq −mqp !
v qp
aq ρq !
p¼1
∂t
ð3Þ
where p is the static pressure of all phases; t q is the stress tensor of the q phase; !
g is the
⇀
!
!
gravity constant; v and v are the velocities between the p and q phases; and R is the
pq
qp
pq
force between the p and q phases.
⇀
Equation 3 must be closed with the interphase force Rpq ; FLUENT uses a simple
⇀
interaction term for Rpq as follows:
n
n
X
⇀ X
Rpq ¼
K pq !
v p −!
vq
p¼1
ð4Þ
p¼1
where Kpq (=Kqp) is the interphase momentum exchange coefficient.
The standard k–ε mixture model is employed in this work for describing turbulence. The
turbulent kinetic energy (k) and the dissipation rate of turbulent kinetic energy (ε) in the model
can be expressed with the following equations:
μt;m
∂
∇k þ Gk;m −ρm ε
ðρm k Þ þ ∇⋅ ρm !
v m k ¼ ∇⋅
∂t
σk
ð5Þ
μt;m
∂
ε
ðρm εÞ þ ∇⋅ ρm !
∇ε þ C 1ε Gk;m −C 2ε ρm ε
v m ε ¼ ∇⋅
∂t
k
σε
ð6Þ
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631
In these equations, ρm is the density of the mixture; !
v m is the velocity of the mixture; μt,m is
the eddy or turbulent viscosity; Gk,m represents the generation of turbulent kinetic energy
due to the mean velocity gradient; C1ε and C2ε are constants; and σk and σε are the turbulent
Prandtl numbers for k and ε, respectively.
The eddy or turbulent viscosity μt,m can be computed by combining k and ε as follows:
μt;m ¼ ρm C μ
k2
ε
ð7Þ
where Cμ is constant, and the model constants of C1ε, C2ε, σk, and σε all followed the
empirical values. Concretely, C1ε=1.44, C2ε=1.92,Cμ=0.09, σk=1.0, and σε=1.3.
In the non-Newtonian flows, the stress tensor ( t ) is controlled by the non-Newtonian
viscosity (η) that is only considered to be a function of the shear rate (γ). The viscosity of the
non-Newtonian power law can be expressed as:
η ¼ K⋅γ n−1
ð8Þ
where K is the consistency coefficient (in pascals per second raised to the nth power), γ is the
shear rate, and n is the power law index. The values of K and n can be obtained by the lower and
upper limits of viscosities and the corresponding shear rate. According to the determination for
the employed mixture, the lower and upper limits of viscosities varied in the range of 0.174–
8.380 Pa s and the shear rate correspondingly varied in the range of 0.106–21.145 s−1.
Consequently, the consistency coefficient (K) and the power law index (n) can be determined
as 1.4 Pa sn and 0.3, respectively.
Since Newtonian and non-Newtonian fluids exhibit different flow behaviors, the definition of
the Reynolds number (Re) for Newtonian fluids is invalid for non-Newtonian fluids. In this work,
the generalized Reynolds number proposed by Metzner and Reed is employed as follows [23]:
Re ¼
nþ1
ρU 1−n
∞ D
K ð0:75 þ 0:25=ðn þ 1ÞÞnþ1 8n
ð9Þ
where U∞ is the average velocity of the fluid at the exit of the moving zone that contains the
impeller and D is the diameter of the zone.
Numerical Approaches and Convergence Criteria
The solution to numerical analysis for mechanical mixing of impellers can be solved by the
multiple reference frame (MRF) approach. In the MRF approach, the equations for the
impeller zone are solved with a rotating reference frame, whereas the equations for the
stationary zone are solved using a stationary reference frame.
Based on the software of MIXSIM (Version 1.7), the digestion reactor was modeled with
a grid size of 405,053 meshes and 73,001 nodes. The CFD simulation was performed with
the software FLUENT (Version 6.3.26) for different types of blades, number of impellers,
and various stirring intensities. Subsequently, the TECPLOT software was used for data
processing to investigate the velocity vectors, distribution of the solid phase fraction, and
turbulence kinetic energy contours.
The method to judge convergence was to monitor the magnitude of scaled residuals. The
residuals are defined as the imbalance in each conservation equation following each
iteration. The solutions were considered to have been converged when the scaled residuals
of two successive iterations were lower than 10−5. A computer integrated with eight processors
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Appl Biochem Biotechnol (2013) 171:626–642
with a clock speed of 3.40 GHz (Intel® Core™ i7-2600) was employed for the related
computation.
Application Experiments
In order to examine the applicability of the obtained results from CFD simulation, the batch
anaerobic digestion tests were conducted using the determined blade and impeller number.
The inoculation of seed sludge, temperature, and pH for digestion were maintained at
15 g L−1, 35±2 °C, and 6.8–7.2, respectively. The rice straw loading rate was controlled at
65 g TS L−1 [24, 25]. The characteristics of rice straw and seed sludge are listed in Table 1.
The biogas production and the methane content were recorded and analyzed according to
reference [26].
Results and Discussion
Mixing Performances with Different Blade Types
Mixing performances of the different blade types were investigated at the stirring rate of
80 rpm. The velocity vectors and the volume fraction of the solid phase on the Y–Z plane are
presented in Fig. 2.
The difference of velocity in the digestion among these blades was not significant. They
varied in the range of 0–0.36 m s−1. In Fig. 2, the black lines represent the streamlines of
flow, which can reflect the footprint of rice straw particles in digestion on Y–Z plane. It was
found that the rice straw particles could move smoothly from the top to the bottom of the
digester, and axial flow could be achieved. Additionally, streamline circles could be formed
at the bottom when the PB and the HEB were employed. This result indicated that the
complete mixing of rice straw in the vertical column could be achieved with these two types
of blade (PB and HEB). However, when the DFB was employed, the streamlines indicated
that the velocity vector was divided into two parts. The straw substrate was well dispersed in
the zone above the blades, but partially dispersed in the zone below the blades. This meant
that the straw substrate could not be mixed well in the whole digester with the DFB at the
stirring rate of 80 rpm.
The distribution of the solid fraction can directly reflect the mixing efficiency in the
digester. As shown in Fig. 2b, the volume fraction of the solid phase varied from 0 to 1 as the
color changed from blue to red. The higher volume fraction of the solid phase meant better
mixing performance. The substrate accumulation zone at the top and the no-substrate zone at
the bottom were observed in the digester with three types of blade. As a solid–liquid flow,
the solid suspension in the digester strongly depends on the velocity profiles and turbulence
intensity [27]. In this work, the impellers ran at a lower stirring rate with only a single-layer
blade, which may cause incomplete mixing in the digester. However, the areas of such two
Table 1 Characteristics of raw rice straw and the seed sludge (in percent, dry basis)
TS
VS
Ash
TC
TKN
Cellulose
Hemicellulose
Lignin
Rice straw
95.5
82.0
13.4
40.7
0.8
34.0
27.7
7.1
Seed sludge
5.6
3.2
2.3
31.2
2.4
N/A
N/A
N/A
TS total solid, VS volatile solid, TC total carbon, TKN total Kjeldahl nitrogen, N/A not available
Appl Biochem Biotechnol (2013) 171:626–642
633
a
m·s-1
Pitched blade
High efficiency blade
Disc-mounted flat blade
Pitched blade
High efficiency blade
Disc-mounted flat blade
b
Fig. 2 a Velocity vectors and b volume fraction of the solid phase at different types of blade on the Y–Z plane.
Left pitched blade, middle high efficiency blade, right disc-mounted flat blade
zones with the PB were obviously smaller than those of the other two blades, implying better
mixing performances with the PB. Furthermore, the PBs are often used in a variety of
process operations because of the strong axial flow component, especially for the mixing of
suspension substrates [27]. Therefore, the PB was selected for the subsequent work.
Mixing Performances with Different Numbers of Impellers
As stated previously, the solid phase accumulation zone appeared in all digesters with a
single impeller. Increasing the number of impellers could change the flow pattern, generating
an even stronger axial recirculation loop and consequently improving the mixing performances [27, 28]. The velocity vectors and the distributions of the volume fraction of the
solid phase on the Y–Z plane are presented in Fig. 3.
Based on the velocity vectors on the Y–Z plane, it was found that the flow velocity varied
in range of 0–0.36 m s−1 with the single and double impellers at 80 rpm. When triple
impellers were employed at 80 rpm with the same speed, the flow velocity varied from 0 to
0.44 m s−1. Moreover, it was observed that substrate particles moved from the top to the
bottom of the digester and the axial flow was easily achieved with three types of impellers.
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Appl Biochem Biotechnol (2013) 171:626–642
a
m·s-1
Single impeller
Double impellers
Triple impellers
b
Single impeller
Double impellers
Triple impellers
Fig. 3 a Velocity vectors and b volume fraction of the solid phase at different numbers of impellers (Y–Z
plane). Left single impeller, middle double impellers, right triple impellers
The substrate circle flow (see the black lines in Fig. 3a) in the digester with the double
impellers was significantly smaller than that of the single impeller, and the flow circle even
disappeared when triple impellers were employed. As a result, the presence of the upper
impellers had a significant impact on the axial flow throughout the digester [27]. As
presented in Fig. 3b, the substrate accumulation zone at the top and the no-substrate zone
at the bottom can be obviously observed with the single impeller, while their distribution in
the digester was significantly reduced when double impellers were employed. The volume
fraction of the solid phase in the digester with triple impellers was well distributed, which
meant that better mixing performance was achieved.
These results indicated that increasing the number of impellers could significantly
improve the mixing performances, and triple impellers could be recommended for the
mixing of rice straw in the anaerobic digester.
Appl Biochem Biotechnol (2013) 171:626–642
635
a
m·s-1
(A) 20 rev.·min-1
(B) 40 rev.·min-1
(C) 60 rev.·min-1
(D) 80 rev.·min-1
(E) 100 rev.·min-1
(F) 120 rev.·min-1
(G) 140 rev.·min-1
(H) 160 rev.·min-1
(A) 20 rev.·min-1
(B) 40 rev.·min-1
(C) 60 rev.·min-1
(D) 80 rev.·min-1
(E) 100 rev.·min-1
(F) 120 rev.·min-1
(G) 140 rev.·min-1
(H) 160 rev.·min-1
b
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Appl Biochem Biotechnol (2013) 171:626–642
c
m2·s-2
(A) 20 rev.·min-1
(B) 40 rev.·min-1
(C) 60 rev.·min-1
(D) 80 rev.·min-1
(E) 100 rev.·min-1
(F) 120 rev.·min-1
(G) 140 rev.·min-1
(H) rev.·min-1
Fig. 4 (continued)
Mixing Performances with the Different Stirring Rates
Based on the results from the blade type and simulation of impeller number, triple impellers
with PBs were selected for the anaerobic digester of rice straw. CFD computation was
completed at different stirring rates from 20 to 160 rpm to determine a suitable string
intensity. Figure 4 presents the velocity vectors and the distributions of the solid phase
fraction at various stirring rates on the Y–Z plane.
It was found that the flow pattern in the digester showed significant difference at different
stirring rates. The flow velocities were maintained at the lower levels (<0.3 m s−1) when lower
stirring rates (<60 rpm) were applied. The flow velocity was improved to the range of 0.3–
0.5 m s−1 as the stirring rate was increased to 80 rpm. However, the flow velocity could reach
over 0.5 m s−1 in some zones of the digesters when the stirring rate was higher than 100 rpm. As
reported in the reference, the flow velocity should be lower than 0.5 m s−1 as the municipal solid
waste was employed for anaerobic digestion. Otherwise, the sludge structure and the microorganisms would be potentially destroyed, resulting in lower digestion efficiency and biogas
production [29].
The obvious substrate accumulation zone at the top and the no-substrate zone at the bottom
of digester were found with the stirring rates of 20 and 40 rpm, although triple impellers were
used. However, the volume fraction of the solid phase reached 0.3–0.6 and these two zones
were significantly reduced as the stirring rate increased to 60 rpm and disappeared completely
as stirring rate increased to higher than 80 rpm (Fig. 4b).
ƒ Fig. 4 a Velocity vectors, b volume fraction of the solid phase (Y–Z plane), and c turbulence kinetic energy
contour (X–Z plane) at different stirring rates (A 20 rpm, B 40 rpm, C 60 rpm, D 80 rpm, E 100 rpm, F
120 rpm, G 140 rpm, H 160 rpm)
Appl Biochem Biotechnol (2013) 171:626–642
637
Fig. 5 Standard deviation of the solid phase volume fraction at various stirring rates
Figure 4c presents the turbulence kinetic energy at different stirring rates, which can
reflect the mixing efficiency directly. Generally, the higher the turbulence kinetic
energy obtained, the more efficient the mixing achieved. Overall, the turbulence
kinetic energy was increased when a higher stirring rate was employed. Moreover,
the increase appeared more rapidly in the zones near the stirring impellers than the
wall or bottom of the digester. For lower levels of the stirring rate (20–60 rpm), the
highest turbulence kinetic energy in the zones near the impellers could be lower than
0.006 m2 s−2. The result implied that the mixing of the upper flow and the bottom
flow were not well constructed, and the substrate particles could not be dragged to the
bottom with lower stirring rates. For the higher stirring rate of 80 rpm, the turbulence
kinetic energy in the digester rose to 0.004–0.012 m2 s−2 and the distribution of the
turbulence kinetic energy tended to be more uniform than that of the lower stirring
rates. For even higher stirring rates of 120 and 160 rpm, the turbulence kinetic energy
was further increased to more than 0.014 and 0.02 m2 s−2, respectively.
These results suggested that an appropriate stirring rate is extremely critical; both too low
and too high levels would not be beneficial to the efficient digestion and biogas production
from rice straw.
To further clarify the effect of stirring rate, mixing performances can be characterized by the
standard deviation (S) of volume fraction of the solid phase (Eq. 10) in the digester [15, 30]:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
uX u
vi −v
t
ð10Þ
S¼
n
Fig. 6 Biogas production at different stirring rates during 70 days digestion. a Daily biogas production, b b
cumulative biogas production, c methane content in the biogas. Open squares 40 rpm, open circles 80 rpm,
open triangles 120 rpm, open inverted triangles 160 rpm
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639
Table 2 Degradation rate of main composition in rice straw after digestion
Stirring rate (rpm)
TS (%)
VS (%)
Cellulose (%)
Hemicellulose (%)
Lignin (%)
40
35.4
39.6
71.9
65.2
6.9
80
41.2
47.5
73.9
70.5
7.3
120
38.2
43.5
72.3
68.3
7.2
160
37.0
44.5
72.4
69.5
7.1
where n is the number of sampling locations used for measuring the solid volume fraction, vi is
the volume fraction of the solid phase at point i in the digester, and v is the average volume
fraction of the solid phase in the digester (0.4 in the current work).
The increase in the degree of homogenization (better suspension quality) is generally
manifested as the reduction of the standard deviation value. On the basis of the quality of the
suspension, the range of the standard deviation is broadly divided into three ranges. For
uniform (homogeneous) suspensions, the value of the standard deviation is found to be
smaller than 0.2 (S<0.2). For a marginally suspended condition, the value of the standard
deviation lies between 0.2 and 0.8 (0.2<S<0.8), and for an incomplete suspension, S>0.8 [31].
As plotted in Fig. 5, the standard deviations were higher than 0.2 as stirring rate was lower
than 60 rpm. When stirring rate was enhanced from 80 to 160 rpm, the standard deviations were
lower than 0.2, showing no significant decrease. Thus, the stirring rate of 80 rpm can be
recommended for the anaerobic digestion of rice straw with loading rate of 65 g TS L−1 when
triple impellers were employed with the PB.
Anaerobic Digestion Tests
According to the results previously discussed, anaerobic digestion with rice straw at different
stirring rates (40, 80, 120, and 160 rpm) was performed in the CSTR (Fig. 1a). The stirring
frequency was set as 12 times per day, and stirring run lasted 5 min for each time. The total
digestion process was maintained for 70 days. The daily biogas production at different
stirring rates is presented in Fig. 6a.
Three to four peaks of biogas production were observed during the digestion process, which
was similar to other agricultural residues such as corn stover [25]. Two days later after
inoculation, the first biogas production peaks appeared for all the four employed stirring rates.
At this point, the daily biogas production reached 0.6, 8.0, 7.4, and 5.64 L at 40, 80, 120, and
160 rpm, respectively. Biogas production then tended to decrease during the following 3–
4 days, and restarted to increase and finally reached the highest production. The highest biogas
production was observed at the 24th, 31st, 31st, and 37th days with the stirring rates of 120, 80,
160, and 40 rpm, respectively. The highest peak of biogas production was 13.6 L day−1 at the
Table 3 Comparison of biogas yield and technical digestion time at various stirring rates
Stirring
TS biogas yield VS biogas yield Digested TS biogas yield Digested VS biogas yield T80
(mL (g VSdigested)−1)
(days)
rate (rpm) (mL (g TS)−1) (mL (g VS)−1) (mL (g TSdigested)−1)
40
299
348
688
615
66
80
120
370
332
431
386
729
706
632
620
46
49
160
327
381
719
597
46
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Appl Biochem Biotechnol (2013) 171:626–642
stirring rate of 80 rpm, which was higher than that of other stirring rates by 32.9 % (40 rpm),
22.6 % (120 rpm), and 11.0 % (160 rpm), respectively.
Based on the cumulative biogas production at different stirring rates in Fig. 6b, it can be seen
that biogas production was accelerated with the increase of stirring rate below 120 rpm, but an
obvious retardation was observed as the stirring rate was at 160 rpm. The highest cumulative
biogas production of 192.3 L was obtained at the stirring rate of 80 rpm, which was higher than
that of other stirring rates by 23.6 % (40 rpm), 11.4 % (120 rpm), and 18.3 % (160 rpm),
respectively. The finding clearly indicated an obvious effect of stirring rate on biogas production and also agreed with the results from CFD simulation previously discussed. These results
could be explained by the improved mass transfer and interaction of substrate and anaerobic
microorganisms, when a suitable mixing state was employed. It was seen clearly that part of the
rice straw floated on the surface when mixing was not well performed at the lower stirring rate
of 40 rpm, but the rice straw can be evenly dispersed from the surface to the bottom of the
digester when higher stirring rates of 80 to 160 rpm were used. Therefore, the stirring rate of
80 rpm was considered to be the best one.
Figure 6c shows the methane content profile during the 70-day digestion. The methane
content can be used to indicate whether the methanogens dominate the anaerobic digestion, and
methane content in biogas is normally higher than 50 % [32]. For four stirring rates, the methane
contents all started to increase after inoculation and reached normal levels later. The difference
was the time to reach the normal level (more than 50 %). For the stirring rate of 80–160 rpm, it
took 15–16 days for methane content to reach above 50 % and maintain in the range of 55–60 %
in the following time, but the stirring rate of 40 rpm took 20 days for methane content to reach
above 50 %. When the impellers ran at a low rate, the produced organic acid from acidification
could not be mixed completely, resulting in acid accumulation. This could inhibit the activity of
anaerobic microorganisms, subsequently decreasing the biogas production and methane content. However, higher stirring intensity might potentially disrupt the granule structure, destroy
the anaerobic microorganisms by shear force, and shorten the interactions between substrate and
microorganisms [13, 33]. These could also adversely affect biogas production. Gómez et al. also
stated that moderate stirring could obtain a better biogas production and methane content [12].
To better understand the effect of stirring rate on the biological conversion of substances, the
degradation rates of the main composition in rice straw at different stirring rates were investigated (Table 2). When the stirring rate of 80 rpm was applied, the degradation of TS, volatile
solid (VS), cellulose, and hemicellulose were higher than the others. It was also found in Table 3
that the biogas yield based on TS (digested TS) and VS (digested VS) reached 370 mL (g TS)−1
(729 mL (g TSdigested)−1) and 431 mL (g VS)−1 (632 mL (g VSdigested)−1), respectively, with
80 rpm, which were all higher than those of other stirring rates. These indicated that better
stirring rate can help to achieve a higher composition degradation rate and can result in higher
biogas production. This finding also corresponded well to the highest biogas production as
mentioned previously. In addition, the technical digestion time (T80) for 80 rpm was 46 days,
which was also shorter than that of others. Shorter T80 could potentially reduce the cost of
biogas production from the agriculture residues [26, 34]. These data again proved that the
simulated results from the CFD method were reliable for the application.
Conclusions
The results from the CFD simulation indicated that triple impellers with PB at 80 rpm were
the best mixing option for efficient anaerobic digestion of rice straw for biogas production.
The simulation results agreed well with the experimental results. The highest biogas yield of
Appl Biochem Biotechnol (2013) 171:626–642
641
370 mL (g TS)−1 (729 mL (g TSdigested)−1) and 431 mL (g VS)−1 (632 mL (g VSdigested)−1)
and the shortest T80 of 46 days could be achieved by employing the CFD simulation results.
Acknowledgments We would like to acknowledge the financial support from the National Eleventh FiveYear Research Program of China (2008BADC4B13 and 2008BADC4B14) and the International Cooperation
Project between China and USA (2011DFA90800) from the Ministry of Science and Technology of the
People’s Republic of China. We would also like to acknowledge the China Postdoctoral Science Foundation
(2012M520145).
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