INTERNATIONAL JOURNAL of RENEWABLE ENERGY RESEARCH

Mahir M. Said et al., Vol.4, No.2, 2014

Thermal Characteristics And Kinetics Of Rice Husk

For Pyrolysis Process
Mahir M. Said*‡, Geoffrey R. John*, Cuthbert F. Mhilu*

*University of Dar es Salaam, College of Engineering and Technology, Department of Mechanical and Industrial Engineering;
P.O.Box 35131, Dar es Salaam, Tanzania
(mahir@udsm.ac.tz)

‡ Corresponding Author; Mahir M. Said, P.O. Box 35131, Dar es Salaam, Tanzania, Tel: +255 717829871, mahir@udsm.ac.tz

Received: 27.01.2014 Accepted: 13.03.2014

Abstract- The trend for material and energy recovery from biomass-waste along with the need to reduce green house gases has
led to an increased interest in the thermal processes applied to biomass. The thermal process applied to biomass produces
either liquid fuel (bio-oil) or gaseous fuel. One of the biomass wastes that are produced in large quantities in Tanzania is rice
husk. The behaviour of this waste is important to any designing of thermal handling equipment when subjected thermal
environment such as burning or thermal degradation. Due to this it is imperative to establish thermal characteristics of the rice
husk pursued in a laboratory to understand its thermal degradation behaviour. The thermal degradation was conducted in a
thermo-gravimetric analyzer from room temperature to 1273 K at different heating rates. The activation energy was 180.075
kJ/mol and suitable heating rate for high degradation of rice husk is 10 K/min, and gives 77.20 wt% of volatile release which is
the suitable heating rate for pyrolysis and energy released was -4437 J/kg, although it has been recommended that the rice to be
used for gasification since it contains high amount of char.
Keywords Gasification; Kinetics; Pyrolysis; Rice husk; Thermal degradation.

1. Introduction Conversion of lignocellulosic material as feedstock faces

problems due to their complex structure and difficulty to
Since the oil crisis in the mid – 1970s, a considerable separate their components in an efficient way. One of the
effort has been directed toward development of renewable simplest technologies to convert lignocellulosic materials to
sources of energy. Conversion of biomass based most efficient fuel is by means of thermal decomposition
lignocellulosic material that consists of cellulose, process of either pyrolysis or gasification process.
hemicelluloses and lignin is a candidate for renewable source
of energy [1]. Unlike carbohydrate material type which The conversion of biomass material by thermal
belongs to food chain that can be digested, lignocellulose in decomposition process yields a range of useful products
nature cannot be digested. In view of this situation there has dependent on the thermal degradation of material [4]. This
been a growing interest looking for alternative way to utilize study deals with the analysis made to investigate thermal
lignocellulosic biomass material in energy production as it degradation behaviour of the rice husk to enable
does not compete with the food supply chain [2]. determination of the energy conversion route.

Tanzania economy as is the case for other African 2. Methodology

countries depends on agriculture, employing over 80% of the
population. Experience gained on the agricultural activities The rice husk used in this experiment was collected from
show that most agricultural crops produced generate Morogoro Region, which is located in southern part of
considerable amount of waste providing a large potential Tanzania. Another material that was mainly used in the
source for lignocellulosic material. One among the experiment was pure nitrogen as inert gas.
lignocellulosic biomass species that are obtained in Tanzania
in large quantities is derived from the rice husk. An The experiments were divided into two steps; the first
evaluation made on the total amount of rice husk that step was to study the characteristics of rice husk by using
originates from agricultural activities shows that over 4.1 proximate, ultimate analysis and higher heating value by
million tonnes are produced annually and is mostly burned in using bomb calorimeter. The second step was to study the
open heaps as waste [3].
INTERNATIONAL JOURNAL of RENEWABLE ENERGY RESEARCH
Mahir M. Said et al., Vol.4, No.2, 2014
pyrolysis kinetics of rice husk by using Thermo-gravimetric 2.2 Heat for biomass pyrolysis
analyzer.
Analysis of Heat of biomass degradation can increase
The proximate analysis of the rice husk sample was understanding of combustion, allowing traditional
carried out according to ASTMD 3172 method and ultimate combustion techniques to be improved and burning methods
analysis was done according to ASTMD 3176. The thermal- with high efficiency and environmental effects to be
degradation was carried out by using TGA. The sample of a developed. The heat of biomass degradation in the absence of
ground rice husk was obtained by grinding the rice husk to oxidation agent resembles pyrolysis process and is needed
an average particle size of 100 µm. A 20 mg of the rice husk for modelling the reactivity of fuel particles in reactors. The
sample was used in the experiment. release of products decrease the density of the char residue
and change its thermal conductivity and specific heat, and the
Experiments were conducted at heating rate of 5, 10, 20 generation of condensable tars and vapors may disturb the
and 40 K/min in the atmosphere of pure Nitrogen. The measurement. The more accurate measurement of heat of
residual weight as a percentage of the initial weight of the degradation/pyrolysis is obtained by using Differential
sample as a function of temperature was recorded. Scanning Calorimetry (DSC) [10]. The DSC has been used
to characterize physical transformations as well as chemical
2.1 Thermal Degradation Kinetics reactions. In most common form, this technique consists of
measuring the temperature difference between a sample and
Understanding the thermal degradation kinetics of the a reference, while temperature of the environment in which
fuel pyrolysis is critical in development of technologies to they both sit is increased linearly with time. This temperature
utilize fuels in the most efficient manner. Kinetic models are difference is related to the difference in heat flow into the
useful tool in energy conversion technologies by enabling sample compared to the usually inert reference material [11].
researchers to predict and understand the depolarization
properties of fuels. Most of these kinetic models focus on the 3. Methodology
chemical reactions simulated by a pyrolysis process and
represented by the reaction scheme in Equation (1). The characterizations of rice husk, which include
proximate, ultimate analyses together with higher heating
(1) value, are shown in Table1.

The pyrolysis rate is expressed by the following nth Table 1. Characterization of rice husk
order as shown in Equation (2), where x is fractional mass of Proximate Analysis Ultimate analysis Higher
biomass at time t, and k is the reaction rate constant. (%) (%) Heating
Moisture 9.00 C 49.63 Value
dx Volatile 56.20 H 5.78 (MJ/kg)
 k (1  x) n (2) matter
dt
Fixed 12.60 N 0.24
There are different methods for determination of carbon
pyrolysis kinetics from Thermo-gravimetric analysis. These Ash 22.20 O (by 44.25 13.24
are Coats and Redfern [5], Agrawal sivasubramanian [6], difference)
Freeman and Carroll [7], Kissinger’s method [8] among Cl 0.10
others. This study will consider Kissinger’s method, since
the process employed for thermal-gravimetric analysis of rice The carbon and hydrogen contents are the indicative of
husk was non-isothermal. hydrocarbons that can be released during thermal
degradation process also the high oxygen content, it means
The Kissinger’s method does not depend on reaction the fuels has low energy content. This has been shown in van
mechanism for determination of activation energy, although Krevelen diagram in [2].
other parameters assume first order reaction mechanism [9].
The peak temperature (Tmax) is used to determine the Figure 1 shows typical weight loss against temperature
activation energy (Ea). Thermal decomposition rate are curve for the degradation/pyrolysis of rice husk in nitrogen
measured at different heating rate, through sequence of atmosphere under non isothermal conditions at heating rate
experiments. The pyrolysis rate is expressed by using of 10 K/min, this heating rate was chosen, since it gave
Arrhenius Equation (3) and k is the rate constant, which highest degradation of rice husk. The curve is divided into
depends on temperature. Ea is the activation energy, R is the three regions. The first region is drying zone, the second is
gas constant and Tmax is the absolute temperature. devolatilization zone and the third one is char degradation
zone.

k  A exp( Ea / RTmax ) (3)

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Mahir M. Said et al., Vol.4, No.2, 2014
experiments under non-isothermal conditions were obtained
from Figure 2 and the decomposition reaction was assumed
to be first order. The results revealed that the activation
energy was 180.075 kJ/mol, and pre exponential factor was
2.401E+27 s-1. The reaction constant obtained is expressed
as Arrhenius equation (Equation 4).

k  2.401x1027 e  (180.075/ RT ) (4)

The mass of biomass decomposed during thermal

analysis can be estimated by solving Equation 2, since the
reaction was first order, Equation 5 was developed. At every
time t and certain temperature T the biomass decomposed is
Fig. 1. Thermo-gravimetric curve of rice husk determined using Equation 5. Therefore the rate of
decomposition of biomass can be easily estimated.
The curves show moisture released was 7.56%, the
volatile was 77.20% and the char yield is 13.82%. The
greater mass loss was observed between 400 K and 1000 K, x  1  e kt (5)
due to volatile release at devolatilization zone.
The heat for rice husk degradation or pyrolysis was
obtained by using DSC. The DSC and TG curves are shown
in Figure 3. the TG curve is represented by smooth line and
DSC curve is represented by dotted line. The DSC curve
form a hump at the drying zone, since heat is absorbed for
moisture released this process is known as endothermic
process and the estimated energy used was 161.5 J/g. The
devolatilization zone is exothermic due to the formation due
to several chemical reactions such as fragmentation,
reforming, cracking, polymerization and dehydration, most
of these chemical reactions are exothermic processes and the
energy released was -4437 J/g. The char degradation zone is
endothermic process, the estimated energy absorbed was 313
J/g.

Fig. 2. The DTG of rice husk at different heating rates

Figure 2 shows the derivative thermo-gravimetry (DTG)

for rice husk sample at four different heating rates. The peaks
of the DTG curve tend to move to the right hand side as the
heating rate increases. The peaks were formed at 571.5 ,
581.6 , 585.9 and 589.2 K at heating rate of 5, 10, 20 and 40
K/min respectively.

Two distinctive DTG peaks are found at all heating rates.

The peak at low temperature corresponds to hemicellulose
decomposition, while the peak at high temperature represents
decomposition of cellulose [9]. It is believed that lignin Fig. 3. The DSC and TG of rice husk
decomposition is slow and distributed along a wide range of
temperature interval and its peak cannot be distinguishable [ 4. Conclusion
]. The peaks could be used to determine the kinetic
parameters of the cellulose, hemicellulose and lignin, but in
The high degradation of rice husk was observed at
this paper the global kinetic parameter will be determined.
heating rate of 10 K/min and the overall pyrolysis behaviour
of rice husk was determined, it was concluded that the rate of
3.1 Chemical Kinetics rice husk pyrolysis can be estimated by using first order
reaction which gives Equation 5.
Determination of kinetics parameters was considered at
the peak regions, because it is where, the pyrolysis takes The energy for devolatilization and char degradation
place. The method mentioned earlier was deployed in the processes were -4437 J/g and 313 J/g, respectively. The
analysis. The kinetic parameters calculated for the pyrolysis

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Mahir M. Said et al., Vol.4, No.2, 2014
devolatilization and char degradation were exothermic and [12] C. Di Blasi, Modeling Chemical and Physical Processes
endothermic processes respectively. of Wood and Biomass Pyrolysis. Progress in Energy and
Combustion Science, 34, 47-90, 2008.
The rice husk has high ash and oxygen content, which
cause to have low HHV. Although oxygen is not suitable
during pyrolysis for bio-oil production, but it makes the rice
husk to be suitable for gasification process since it can be
used for partial oxidation during the process.

Acknowledgements

The Authors would like to thank the SIDA SAREC

program and the University of Dar es Salaam for financing
the project.

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