Chemeca 2012
PYROLYSIS AND GASIFICATION CHARACTERISTICS OF
ALGAL BIOMASS
1,2
Mohd Asyraf Kassim ,Kawnish Kirtania, Ravichanda Potumarthi, Chiranjib Saha,Sankar
1
Bhattacharya
Department of Chemical Engineering, Monash University, Victoria, 3800, Australia
School of Industrial Technology, Universiti Sains Malaysia (USM), Pulau Pinang,
Malaysia
ABSTRACT
Biofuels produced from algae biomass are promising alternatives to partly replace
fossil fuels. Production of biofuel from algae biomass can be carried out using
biochemical and thermochemical routes. Although there are several studies on
biochemical conversion of algal biomass, studies on thermochemical conversion of
algal biomass are few.
This study reports on the pyrolysis and gasification characteristics of one microalgae
species, Chlorella sp. The experiments are carried out in a thermo-gravimetric
analyser (TGA) at temperatures 600-1000C separately in atmospheres of nitrogen,
and different mix of carbon dioxide, steam and nitrogen. Kinetics of pyrolysis has
been determined as a function of the operating variables. The results from this study,
largely exploratory but part of a wider project, shed first insights into the prospects of
thermo-chemical processing of algal biomass for fuels production.
INTRODUCTION
The rising population together with increased demand for fossil fuel has led to the
depletion of traditional energy resources around the world. In addition, utilization of
fossil fuel based energy for transportation, manufacturing and electricity generation
has significantly contributed to the emission of greenhouse gases. This in turn has led
to increased use of renewable and sustainable energy sources such as wind, hydro,
solar and biomass as replacement for energy from fossil fuel. Production of biofuel
from renewable biomass is believed to be one of the most promising approaches for
replacing part of the fossil fuel sources. Several types of biomass such as agroforestry biomass, energy crops, municipal residues and algae biomass can be used for
biofuel production (IEA Bioenergy. 2009). Among these, biofuel production from
algal biomass is considered to have large potential due to the capability of the
different species of algae to grow in varied conditions, produce higher biomass
productivity and lipid yield compared to other feedstock currently being used
(Brennan and Owende, 2009; Verma et al. 2010). In addition, production of biofuel
from algae can be integrated with bioremediation process such as flue gas CO2
capture, waste water treatment and high value chemicals production processes
(Phukan et al. 2011).
Kassim et al.
Generally algae can be classified into two main groups namely macroalgae
and microalgae. Macroalgae is known as seaweed or kelp which can be divided into
three different groups based on their pigmentation i.e brown seaweed
(Phaeophyceae),
red
seaweed
(Rhodophycease)
and
green
seaweed
(Chlorophyceae)(Carlson et al. 2010) In contrast, microalgae are microscopic
organisms that can be categorized into four main classes - diatom, green algae, blue
algae and golden algae (Demirbas 2010). Recently, there have been many efforts put
into production of biofuel from algae biomass. Production of biofuel from algae
biomass can be carried out using three different pathways namely chemical,
biochemical and thermochemical pathway (Naik et al. 2010). Production of biofuel
from chemical (transesterification) and biochemical pathway are well established.
However there are few studies on biofuel production from algae biomass using the
thermochemical pathway. Generally, production of biofuel from algae biomass via
thermochemical process involves liquefaction, pyrolysis, gasification and combustion
(Grierson et al. 2009; Demirbas et al. 2010; Chakihala et al. 2010).
Thermogravimetric analysis (TGA) is extensively used to understand and
investigate the characteristic and the kinetics parameters of the of thermochemical
reactions. Studies on conversion of algal biomass via thermochemical pathway are
limited (Hui et al. 2010; Li et al. 2011; Demirbas et al. 2010, Kirtania and
Bhattacharya, 2012; Shuping et al. 2010; Phukan et al. 2011 and Porphy and Farid et
al. 2012), in particular the kinetic study of algae biomass during the thermochemical
conversion process. In order to design the equipment for thermochemical processing,
it is important to provide models describing the kinetics and mechanism of the
process. (Shuping et al. 2010). Nevertheless, there are a number of studies on thermal
behavior study on algae biomass which have been reported in literature. Based on the
thermolysis study on Chlorella protothecoides and Dunaliella tertiolecta, it was
shown that the dehydration stage for both algae species ranged from 80°C to 150°C,
while the devolatilization stage ranged from 180°C to 480°C, almost 70% of the
weight loss was found to take place at this stage
As part of a larger study, we are investigating the growth, extraction and
subsequent processing of algal biomass. In this paper, we present its pyrolysis and
gasification characteristics from experiments in a thermogravimetric
analyserincluding the results of modeling the pyrolysis weight loss at low heating
rates This study is largely exploratory in nature, assessing the potential of
thermochemical processing of algal biomass.
MATERIAL AND METHODS
Algae species and medium cultivation
A microalgae species namely Chlorella sp. was used in this study.
Modified MLA medium with 0.494 g/L MgSO4.7H2O, 1.7 g/L NaNO3, 0.14 g/L
K2HPO4, 0.029 g/L CaCl2.2H2O was used as the seed culture and biomass production
medium. The medium was initially sterilized using 0.22 µ m Millipore filter.
Microalgae seed cultivation was conducted in 1 L Scott bottle contained 500 mL of
Kassim et al.
modified MLA. The bottle was incubated in illumination incubator chamber with 0.1
L/min of compressed air sparging, a photon intensity of 150
µmol/m2s. The
cultivation temperature was 20.0±0.5°C.
Chemical composition
The lipid, carbohydrate and protein content of the algae biomass were determined by
using soxhlet extraction followed with gravimetric method, phenol-sulpuric acid
method and lowry method respectively.
Pyrolysis and gasification
Thermogravimetric analysis was carried out in order to study the pyrolysis and
gasification behavior of algae biomass. Algae cell from late log phase were harvested
and centrifuged at 4500 rpm for 15 minutes. The pellet were rinsed twice with
distilled water to remove salt and then dried at 70°C for 24 hours. Pyrolysis and
gasifcation of algae biomass were determined by using thermal analyzer (Model STA
449 F3 Jupiter, NETZSCH). In this experiment, 50 mg of dried Chlorella sp. sample
was spread uniformly on the bottom of alumina crucible. The pyrolysis process was
performed at temperature from ambient temperature to 600°C at three different slow
heating rates 5, 10 and 20°C/min. High purity nitrogen gas (99.99%) was fed at a
constant flow rate at 100 ml/min as an inert purge gas to displace air in the pyrolysis
site, thus avoiding unwanted oxidation in the sample. In addition, for steam
gasification process, the reaction was performed 900°C. Carbon dioxide (CO2) or
steam was injected after 600°C. Weight loss in response to temperature changes were
recorded on-line and used for analysis.
Kinetic Analysis and modelling
Kinetic analysis is a technique to understand the thermal processing characteristics of
a solid fuel. In case of the algal biomass, the activation energy and pre-exponential
factor have been determined by an nth order distributed activation energy model
(DAEM) developed by Kirtania and Bhattacharya (2012). The nth order DAEM has
greater flexibility to estimate pyrolysis kinetics of any solid fuel. This has already
been applied this model on the pyrolysis of one type of algae. The differential
equation for the nth order distributed activation energy model can be represented as
Where, W is the weight of volatile content remaining and W* is the total volatile
content. ko is the pre-exponential factor, n is the order and f(E)dE is the distribution
that characterizes the activation energy.This differential equation can be integrated
into the following form
Kassim et al.
1For this algal biomass, Gaussian distribution of activation energy was assumed.
Thermogravimetric experiments were performed at constant heating rates to feed into
this equation. The energy equation can be transformed to the temperature domain by
putting t = (T-To)/a, where a is the heating rate. After few rearrangements, Equation
(2) can be written as
1-
Where, mE is the mean activation energy and σ is the distribution. An objective
function has been formulated to find out the best match with the experimental data
and thereby finding out the kinetic parameters. The details of this procedure can be
found elsewhere.
RESULTS AND DISCUSSION
Chlorella sp. is a single cell, spherical non-motile green algae that can be found either
both fresh and marine water. Table 1 shows the chemical composition of Chlorella sp.
which consisting of 10% lipid, 30% of protein and 60% of carbohydrate. Generally
Chlorella sp. contains approximately of 2-9% lipid, 51-58% protein and 12-23%
carbohydrate depend on the species and cultivation conditions (Kay and Barton
1991;Peng et al. 2001).
Tab 1: The chemical content analysis of Chlorella sp. (on dry weight basis)
Crude protein (%)
62±1
Crude lipid (%)
10±1.12
Crude carbohydrate (%)
25.46±2.5
Tab. 2: The ultimate analysis of Chlorella sp. (on dry weight basis)
C (%)
43.92
H(%)
6.1
O(%)
42.59
N(%)
7.39
S(%)
-
Themogravimetric study of Chlorella sp.
Pyrolysis of Chlorella sp.
Figure 1 shows the weight loss curve obtained from pyrolysis of Chlorella sp. at a
heating rate of 10°C/min under nitrogen atmosphere. Thermogravimetric study of
Kassim et al.
Chlorella sp. algae indicated that there are three different stages of thermal
degradation process during the pyrolysis reaction (Figure 1). A similar observation
has been reported by others on pyrolysis of algae biomass (Grierson et al. 2011;
Phukan et al. 2011; Porphy and Farid 2012). As shown in Figure 2, the first stage of
slow weight loss occurred from room temperature up to 160°C This could be due to
the dehydration process during which cellular water and the external water bound by
surface tension was eliminated (Li et al. 2011).
The second stage (stage II) of Chlorella sp. thermal degradation was observed
from 160°C to 520°C. This second stage is characterized by the major weight loss,
approximately 60% of the weight loss observed at this stage. The first derivative of
the thermogravimetric (DTG) curve indicated that the maximum mass loss was
obtained at temperature of 264 to 289°C. It was found that initial decomposition
temperature for Chlorella sp. used in current study is slightly higher compared to C.
protothecoides where the initial decomposition temperature started at 150°C (Peng et
al. 2001). On the other hand, a study by Phukan et al. (2011) reported that the initial
decomposition temperature of Chlorella sp. was observed at 130°C and 160°C for 10
and 30°C/min heating rates.
In our experiments, only one major peak was observed as the temperature
increased from 210 to 250°C. A similar observation has been reported during
thermolysis of Chlorella sp., Dunaliella tertiolecta and Tetraselmis chui which show
only one strong peak during the pyrolysis process (Shuping et al. 2010). However,
different observation has been reported on pyrolysis of Chlorella protothecoides and
Nannochloropsis sp. which indicated that there were two different fractions this stage
(Porphy and Farid 2012; Peng et al. 2001). This study also showed that the pyrolysis
behavior of Chlorella sp. is different from pyrolysis behavior of macroalgae and
lignocellulosic material (Moeszaros et al. 2004; Li et al. 2010; Abdullah et al, 2010).
Pyrolysis study (Li et al. 2011) on red macroalgae, Plocamium telfairiae indicated
two different major peaks when it was pyrolysed at heating rate 10°C/min under
nitrogen atmosphere Similar observation has also been reported on pyrolysis of two
brown algae, Laminaria japonica and Saegassum pallidum which have been
pyrolysed at similar condition (Li et al. 2010). According to Shuping et al. (2010),
thermal degradation process of biomass is influenced directly by the composition of
raw materials. Generally, the algae biomass mainly consists of lipid, carbohydrate and
protein whereas lignocellulosic materials mainly consist of cellulose, hemicellulose
and lignin.
The last stage of thermal degradation of Chlorella sp. biomass was observed at
temperature 520°C. It was found that after 520°C, the rate of weight loss became
eventually stagnant.
Kassim et al.
(a)
(b)
Fig.1: Thermogravimetry for Chlorella sp. at heating rate of 10°C/min under nitrogen
atmosphere (a) weight loss curve, (b) DTG curve.
Effect of heating rate
Effects of different heating rates on reactivity and devolatilization of algae biomass
has been carried out by using three different heating rates (5, 10 and 20°C/min).
Figure 2 shows that the decomposition of algae biomass started at 164, 161 and
165°C for heating rates of 5, 10 and 20°C/min. The study also shows that the
devolatilization process of the three heating rate was complete at 520°C. Table 3
shows the total mass losses of volatile matter during the second stage of pyrolysis
process were 56.60%, 56.63% and 56.75% for 5, 10 and 20°C/min respectively.
Kassim et al.
(a)
(b)
Fig. 2: TGA-DTG curve for Chlorella sp. at different heating rate 5, 10 and 20°C/min.
at nitrogen atmosphere. (a) TGA curve, (b) DTG curve
Tab. 3: Characteristic of devolatilization temperature and total volatile matter of
Chlorella sp. at different heating rate
Heating rate
(°C/min)
5
10
20
Ti
Tmax
164
161
165
272
283
294
(dw/dt)max(%C-1)
2.97
6.57
12.89
VM(%)
56.60
56.63
56.75
Ti: Initial decomposition temperature, Tmax: maximum devolatilation temperature, VM: total volatile
matter evolved in the second stage
Kinetic Analysis and modelling
Kassim et al.
Thermogravimetric experiments performed at 5, 10 and 20 oC/min were analysed with
the DAEM and the averaged kinetic parameters were determined. The mean
activation energy, distribution, pre-exponential factor and order were determined to
be 196.75 kJ/mol, 14.31, 6.9577 × 1016 s-1 and 5.34 respectively. Figure 3 shows the
comparison between modelled data generated with the kinetic parameters stated
above and experimental data at 10 oC/min. The modelled data shows close match with
the experimental data. The following parameter was defined to check the fit with the
experimental data.
The parameter calculated based on Equation (4) was found to be 0.9991;of Figure 3
confirms the fit with the experimental data.
1.2
Experimental at 10C/min
Weight Fraction
1
Estimated at 10C/min
0.8
0.6
0.4
0.2
0
161
200
300
400
500
Temperature ( C)
Fig. 3: Comparison of experimental and estimated data from proposed algorithm
CO2 and Steam Gasification of Chlorella sp.
Gasification of Chlorella sp. was carried out by using CO2 and steam as gasifying
agent. As pyrolysis of Chlorella sp. biomass was completed after 600°C, steam and
CO2 was injected into the system at that temperature. Figure 4 shows the thermogram
of Chlorella sp. gasification using CO2 and steam as gasifying agent. The experiments
indicate that the major weight loss of char from pyrolysis of Chlorella sp. took place
between 750°C to 900°C (Figure 4a and 4b). . Based on the DTG curve, the
maximum weight loss was obtained at 840°C and 780°C for co2 and steam
gasification respectively. The incremental weight loss from the CO2 gasification was
Kassim et al.
about 50% of the weight at the conclusion of pyrolysis. On the other hand, study
indicated that the weight loss from the steam gasification was about 13% of the
weight of char from pyrolysis process. The weight loss of char from pyrolysis of algae
biomass may be due to the formation of hydrogen during the gasification process.
Based on DTG curve, the char from steam gasification began to give peaks at the
beginning of the interaction with the steam and the mass loss started went on as long
as temperature increases (Figure 4d). This is in similar agreement with study by
Haykiri-Acma et al. (2006), who mention that the conversion of char is increase with
increasing of temperature and time. Comparison on gasification reactivity of char
using CO2 and steam indicates that instantaneous reactivity of CO2 gasification was
higher compared to reactivity of steam gasification (Figure 5).
(a)
(b)
(c)
(d)
Fig. 4: Thermogravimetry curve of CO2 gasification of Chlorella sp. at heating rate of
10°C/min (a) TGA curve, (b) DTG curve
Kassim et al.
Fig. 5: Comparison of CO2 and steam gasification of Chlorella sp. biomass.
CONCLUDING COMMENTS
Pyrolysis and gasification characteristics of Chlorella sp. was assessed using two
different gasifying agents in a thermogravimetric analyser.. The analysis shows three
different stages during the pyrolysis. In order to estimate related kinetic parameters,
an nth order algorithm based on Gaussian distribution has been applied. The model
was further tested on other heating rates, and showed a good agreement with the
experimental data. During gasification, the reactivity of Chlorella sp. is observed
increase with temperature. In addition, this study shows that CO2 gasification results
in higher reactivity compared to steam gasification. Further experiments are in
progress to ascertain the detailed kinetic parameters under both pyrolysis and
gasification conditions over a wider range of conditions.
Acknowledgement
This work has been supported by the Department of Chemical Engineering, Monash
University, Australia and the Ministry of Higher Education, Malaysia.
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