The production of hydrogen as a clean and sustainable fuel is becoming attractive
due to the energy crisis and increasing environmental issues associated with
fossil fuels usage. Under the current scenario, the demand of hydrogen is significant
due its increasing usage in refinery, fertilizer, chemical, food and aerospace
industries (Holladay et al., 2009). Several production
alternatives particularly from renewable sources are receiving increasing attention.
Biomass gasification is considered as one of the potential alternatives for
the production of hydrogen (Momirlan and Veziroglu, 2005).
In 2006, the hydrogen world demand was 50 MT/year with 10% annual increment
predicted (Levin and Chahine, 2010). The potential for
hydrogen production from biomass in Malaysia is appealing due to the abundance
of biomass availability such as from the palm oil industry waste since Malaysia
is the worlds largest producer of palm oil (Sumathi
et al., 2008; Kamarudin et al., 2009;
Khan et al., 2010). Biomass gasification can
be performed using different gasifying agents such as air-steam and oxygen-steam
mixtures or pure steam. It is reported that the use of pure steam is more economical
and in favor of producing more hydrogen yield compared to other conventional
gasifying agents (Franco et al., 2003). The amount
of hydrogen in the product gas from the gasification process can be further
increased by combining the gasification reaction with CO2 adsorption
step using CaO as sorbent (Mahishi and Goswami, 2007a).
The general reaction taking place in a gasifier can be represented by Eq.
There are also other variables that influence the gasification process such
as temperature, pressure, steam/biomass ratio, sorbent/biomass ratio, residence
time biomass feed rate, biomass type, particle size and gasifier design e.g.,
fixed bed, fluidized bed or dual fluidized bed (Corella
et al., 2008).
Several experimental and mathematical model studies have been reported in literature
which looked at the effect of different variables on the concentration and yield
of hydrogen in the product gas. Shen et al. (2008)
simulated the air-steam gasification in an interconnected fluidized bed gasifier
and obtained a hydrogen yield of more than 60 g kg-1 of biomass with
a concentration of 60 mol% in the product gas at 850°C. They also discussed
the effect of the gasifier temperature and steam/biomass ratio on the hydrogen
yield and product gas composition. Meanwhile, Lv et al.
(2007) performed an experimental work on biomass gasification using
conventional air-steam gasifying agent along with the usage of catalyst and
reported a hydrogen yield of 72 g kg-1 of biomass with 56.2 vol%
concentration in the product gas.
There was also research work conducted on integrating CO2 adsorption
using CaO into the gasification process. Mahishi and Goswami
(2007a) conducted an experimental work using CaO as sorbent with pure steam
in a microreactor and reported a hydrogen concentration of 66 vol% in the product
gas. They highlighted the dual role of the CaO as the sorbent and as the catalyst,
as the important factor leading to higher hydrogen production. In line with
the above findings, Florin and Harris (2007) developed
a thermodynamic equilibrium model for hydrogen production from biomass coupled
with CO2 capture step in a dual fluidized bed gasifier. They investigated
the effect of temperature, pressure, steam/biomass and sorbent/biomass ratios
on the hydrogen concentration in the product gas. The model predicted that the
hydrogen concentration could be increased from 50 to 80 vol% in the product
gas by using CaO as sorbent. Florin and Harris (2008)
also presented another equilibrium model for steam gasification of biomass using
CaO as sorbent and investigated the effect of temperature, pressure, steam/biomass
ratio and sorbent/biomass ratio on the hydrogen production. Using the model,
they predicted a hydrogen yield of more than 2 mole H2/mole fuel
is achievable. Guoxin and Hao (2009) proved experimentally
that the CO2 adsorption step strongly favored the forward water gas
shift reaction, thus increasing the hydrogen production. On the contrary, they
found that high reactor temperature will not favor the CO2 adsorption
and thus would have a negative effect on the hydrogen production. Proll
and Hofbauer (2008) presented a model which included the mass and energy
balances for a dual fluidized bed gasification process. Their model results
showed high content of hydrogen in the product gas at lower gasification temperature.
Moreover, the lower operating temperature in the gasifier would lead to higher
efficiency for the energy conversion.
The study on thermodynamic efficiency of hydrogen production is however limited.
Initially, Mahishi and Goswami (2007b) presented a thermodynamic
equilibrium model for the prediction on product gas composition and hydrogen
efficiency in air-steam gasification process using Stanjan (v 3.93 L) software.
They applied the first law analysis on the gasifier and investigated the effect
of temperature, steam/biomass ratio and equivalence ratio on hydrogen efficiency.
They reported that hydrogen efficiency decreases with the increase in both temperature
and steam/biomass ratio. In another work, Mahishi et
al. (2008) developed an equilibrium model in ASPEN PLUS software for
steam gasification of wood with CO2 adsorption using CaO. They reported
that the concentration of hydrogen in the product gas was increased by more
than 19% due to higher thermodynamic efficiency for the integrated gasification
process compared to the conventional gasification.
The objective of the present research is to study the effect of process parameters, i.e., temperature, steam/biomass ratio and sorbent/biomass ratio on hydrogen concentration, yield and efficiency of the steam gasification process with CO2 adsorption using a model approach in MATLAB. This study has been carried out based on the reaction kinetics for a fluidized bed gasifier.
MATERIALS AND METHODS
The base reaction kinetic models and the preliminary results on the effect
of different variables on the hydrogen concentration in the product gas were
presented in an earlier work (Inayat et al., 2009).
In the modeling framework, biomass (wood) is assumed as char and six major reactions,
given as R1 to R6, are assumed to occur in the gasifier (Shen
et al., 2008; Zhang et al., 2009).
The biomass moisture is assumed to be has 10% moisture content which is an acceptable
assumption used for tropical based biomass sources (Corella
et al., 2008).
Char gasification reaction
C + H2O → CO + H2
|+131.5 kj mol-1
C + 2H2 → CH4
|-74 kj mol-1
C + CO2 → 2CO
|+172 kj mol-1(R3)
Methane steam reforming reaction
CH4 + H2O → CO
|+206 kj mol-1(R4)
Water gas shift reaction
CO + H2O → CO2 + H2
|-41 kj mol-1(R5)
CO2 + CaO → CaCO3
|-178.3 kj mol-1(R6)
The process diagram is illustrated in Fig. 1. The gasifier
is equipped with a steam generator and a biomass dryer.
With the assumption that there is no heat loss and no work done by the system,
the thermodynamic efficiency of the hydrogen can be calculated using Eq.
2-4 (Mahishi and Goswami, 2007b).
The amount of hydrogen (in moles) in product gas and biomass are calculated
using the kinetics model (Inayat et al., 2009)
and the hydrogen yield is calculated using Eq. 5.
LHV represents the lower heating value and is taken from literature for hydrogen
(Kelly-Yong et al., 2007) and biomass, i.e.,
wood (Raveendran and Ganesh, 1996). The values of specific
heat for water and steam are taken from the literature as well (Kelly-Yong
et al., 2007). QEE is the energy supplied to the gasifier
by an external source and is calculated based on the enthalpies of the reactions
mentioned above. As steam gasification process is endothermic and consumes a
lot of energy, QEE value is positive for this case. The zero value
of QEE reflects self-sustained process and can be used as standard
for comparison with actual gasifier (Mahishi and Goswami,
|| Process diagram
Qb is the energy supplied to the dryer to remove the moisture content
from the biomass.
The process is assumed to be steady state and biomass feed rate used in the
modeling work is 0.072 kg h-1. The selected range of the main operating
conditions is: temperature, from 800 to 1150 K for product gas composition and
800 to 1300 K for hydrogen yield; steam/biomass mass ratio, from 1 to 3.5 for
product gas composition profile and from 2 to 5 for hydrogen yield and efficiency
profiles; sorbent/biomass mass ratio, from 0.2 to 1.6 for product gas composition
and 1.0 for hydrogen yield and efficiency profiles. The selection of the ranges
was made so that they represent the operation ranges of many commercial and
research scale gasifiers (Lv et al., 2007; Mahishi
and Goswami, 2007b; Corella et al., 2008;
Kumar et al., 2009; Acharya
et al., 2010).
RESULTS AND DISCUSSION
Among the process parameters that can affect the production of hydrogen from biomass gasification process with in-situ CO2 capture are temperature, steam/biomass fed ratio and amount of sorbent. The effect of each parameter on hydrogen concentration, yield and efficiency is thoroughly studied and discussed in the following sections.
Effect of parameters on hydrogen concentration: Figure 2 shows the effect of temperature on the product gas compostion versus temperature, ranging from 800 to 1300 K.
||Effect of temperature on product gas composition. Steam/biomass
ratio: 3.0; Sorbent/biomass ratio: 1.0. H2 (),
||Effect of Steam/Biomass ratio on product gas composition.
Biomass feed rate: 0.072 kg h-1; Temperature: 800 K; Sorbent/biomass
ratio:1.0. H2 (),
The model predicts that the system produces more than 0.8 mole fraction of hydrogen in the product gas. This is due to usage of pure steam and CO2 adsorption step in the system. The CO amount also increases with the increase in temperature. This observation originates from the exothermic and reversible behavior of the water gas shift reaction and due to the endothermic behavior of the Boudouard, char gasification and methane reforming reactions. On the other hand, CH4 and CO2 amounts are decreasing with the increasing temperature. The decreasing amount of CO2 may be due to the slower rate of the water gas shift reaction at high temperature and the carbonation reaction.
Another important variable in steam gasification process is the steam/biomass ratio. It is observed that with increasing steam/biomass ratio, H2 amount increases, CO and CH4 amount decrease. Figure 3 shows the profile for product gas composition when increasing steam/biomass ratio. Steam is the only gasification agent being used, so the reactions involving steam, i.e., methane reforming and water gas shift, are highly dependent on steam feed rate. Therefore, at higher steam/biomass ratio, H2 yield is increased while the amount of the reactants, i.e., CH4 and CO, is decreased.
Figure 4 shows the three-dimensional surface plot to capture
the effect of both temperature and steam/biomass ratio on the hydrogen mole
fraction in the product gas. The figure shows that hydrogen concentration increases
by increasing both temperature and steam/biomass ratio. These results can be
explained by the Le Chateliers principle: the endothermic reforming reactions
of char and CH4 are promoted by the increasing temperature.
||Surface plot of hydrogen mole fraction at different temperatures
and steam/biomass ratios. Sorbent/biomass ratio: 1.0
||Effect of sorbent/biomass ratio on product gas mole fraction.
Biomass feed rate: 0.072 kg h-1; Temperature: 1000 K; Steam/biomass
ratio: 3.0. H2 (),
At 800 K with lower steam/biomass ratio of 1.0, the hydrogen amount is 0.73
mole fraction and at 1150 K with high steam/biomass ratio, the hydrogen amount
is almost 0.80 mole fraction. Furthermore, the surface plot shows that the highest
hydrogen mole fraction achieved is 0.81 mole fraction that occurs at 950 K and
at steam/biomass ratio of 3.0.
The presence of CaO as sorbent in the system increases the hydrogen mole fraction
in the product gas by absorbing the CO2 present in the system. Figure
5 shows the effect of sorbent/biomass ratio on the product gas composition
in the integrated gasification process.
||Surface plot of hydrogen yield at different temperatures and
steam/biomass ratios. Sorbent/biomass ratio: 1.0
It shows clearly that by increasing sorbent/biomass ratio, the H2
amount increases and CO2 amount decreases. At sorbent/biomass ratio
of 1.56, it is predicted that all CO2 is absorbed by the sorbent
and no CO2 exists in the product gas. The maximum hydrogen observed
is 0.99 mole fractions.
Effect of parameters on hydrogen yield: Temperature and steam/biomass
ratio are both in favor for higher hydrogen yield. Figure 6
shows the surface plot of hydrogen yield with respect to temperature and steam/biomass
ratio. Hydrogen yield is predicted to increase with the increase in both temperature
and steam/biomass ratio. Figure 6 shows that in case of 800
K and lower steam/biomass ratio of 2.0, hydrogen yield is 78.5 g kg-1
of biomass. At the same temperature but with higher steam/biomass ratio of 5.0,
hydrogen yield is 96 g kg-1 of biomass. The difference due to the
increase of steam/biomass ratio at same temperature is 17.5. On the other hand,
at a high temperature of 1300 K and low value of steam/biomass ratio of 2.0,
hydrogen yield is 88.5 g kg-1 of biomass. At the same temperature
of 1300 K with high steam/biomass ratio of 5.0, hydrogen yield is increased
to 97 g kg-1 of biomass. In this case, the difference in hydrogen
yield at high temperature is 8.5. By a crude comparison between the yield differences
in both cases, it seems that the influence of steam feed rate at lower temperature
is more significant than at high temperature for the steam gasification process.
This is because the endothermic forward water gas shift reaction is favored
at low temperature. Furthermore, water gas shift reaction is highly dependent
on steam feed rate.
||Effect of temperature on hydrogen efficiency. Sorbent/biomass
ratio: 1.0, Steam/biomass = 2.0 (),
Steam-/biomass = 3.0 (),
Steam/biomass = 4.0 (),
Steam-/biomass = 5.0 ()
Previously it has been observed that there was insignificant change in the
hydrogen yield and efficiency when varying the sorbent/biomass ratio.
Moreover, Fig. 6 shows that the hydrogen yield is significantly more affected by steam/biomass ratio compared to by temperature. This might be due to the usage of pure steam as the gasifying agent.
Effect of parameters on hydrogen efficiency: As discussed in the previous
section, it is concluded that more hydrogen can be produced by increasing the
steam feed rate to the gasifier. However, this means more energy is required
to generate more steam and more energy is lost via the steam loss in the product
gas. Figure 7 shows the thermodynamic efficiency of hydrogen
at different steam/biomass ratios and temperature. The maximum efficiency of
hydrogen, i.e., 87% is predicted at steam/biomass ratio of 2.0 and at 800 K.
At 800 K, hydrogen efficiency at steam/biomass ratios of 3.0, 4.0 and 5.0 are
60, 50 and 46%, respectively. This shows that by increasing steam/biomass ratio,
the efficiency of hydrogen decreases as more energy is required to produce more
steam. On the other hand, by increasing steam/biomass ratio, hydrogen yield
and concentration increase, as reported in the previous sections. The figure
also shows that at lower temperature, hydrogen efficiency is higher and with
the increase in temperature, the efficiency decreases due to the increased external
heat requirement to maintain the temperature inside the gasifier.
||Surface plot of hydrogen efficiency at different temperatures
and steam/biomass ratios. Sorbent/Biomass ratio: 1.0
In Fig. 7, it also observed that all trends for different
steam/biomass ratio ends at the same point, i.e., 30% at 1300 K. This is due
to fact that the same amount of biomass used for all profiles hence the LHV
of biomass is limited to the same value.
A three-dimensional surface plot for hydrogen efficiency with respect to temperature and steam/biomass ratio is shown in Fig. 8.
The surface plot shows that the hydrogen efficiency decreases with the increase in temperature and steam/biomass ratio. The curves show that the steam/biomass ratio has higher impact than temperature to decrease the efficiency of hydrogen. Hence, steam/biomass ratio has more significant impact on the hydrogen yield and the hydrogen efficiency, compared to temperature.
Comparison with experimental and modelling data: A comparison of results from the current study for hydrogen concentration, yield and efficiency has been done with published data on modeling and experimental work and is shown in Fig. 9.
The comparison has been done on the respective results at 900 K. The results
on hydrogen concentration are compared with that of experimental results by
Mahishi and Goswami (2007a) and simulation results by
Florin and Harris (2008) on biomass steam gasification
with CO2 adsorption. The comparison shows good agreement between
For the hydrogen yield, the comparison has been done with that of Lv
et al. (2007) on experimental work carried out with catalyst in the
presence of air and steam and with that of Shen et al.
(2008) on modeling results for air steam gasification.
|| Comparison with literature
The results show that this model predicts higher hydrogen production. The comparison
also indicates that the hydrogen yield is higher in steam gasification system
with CO2 capture step than in other conventional gasification and
even with catalyst. This is also due to the presence of CaO that acts as sorbent
Due to limited data available in literature on hydrogen efficiency, our results
are compared with the modeling results by Mahishi and Goswami
(2007b) that is based on air-steam gasification system. The result shows
that the hydrogen efficiency is higher in pure steam gasification system than
air-steam gasification system. The steam gasification system is an endothermic
reaction and more energy is required for steam generation, but in air-steam
gasification system, there is some energy available from air-gasification which
is an exothermic reaction. Nevertheless, the efficiency of hydrogen in steam
gasification system is still high than that in conventional gasification process,
due to the higher hydrogen yield in steam gasification process.
The effect of temperature, steam/biomass ratio and sorbent/biomass ratio is studied for a biomass steam gasification process with CO2 adsorption using a modeling approach. The effect is captured in terms of hydrogen concentration, yield and thermodynamic efficiency. Initially, temperature is the important variable, as the hydrogen yield increases and efficiency decreases by increasing temperature. Meanwhile, CO amount increases due to the net effect from the exothermic and reversible behavior of the water gas shift reaction and the endothermic behavior of Boudouard, char gasification and methane reforming reactions. On the other hand, CH4 and CO2 amounts decrease due to the slower rate of the water gas shift reaction at high temperature and the carbonation reaction.
In addition, steam/biomass ratio is a very important variable in steam gasification process because hydrogen concentration and yield increase when increasing steam/biomass ratio. Meanwhile, CO and CH4 amounts decrease in the product gas. These observations are based on the reason that both methane reforming reaction and water gas shift reaction are highly dependent on the steam feed. It is predicted that the maximum hydrogen mole fraction in the product gas of 0.81 is achieved at 950 K and the steam/biomass of 3.0. However, by increasing the steam/biomass ratio, the hydrogen efficiency decreases. Maximum hydrogen efficiency of 87% occurs at low temperature and low steam/biomass ratio, i.e., at 800 K and steam/biomass ratio of 2.0, due to the minimum consumption of energy.
Furthermore, the sorbent presence in the gasification process also affects the process performance. At sorbent/biomass of 1.52, it is predicted that all CO2 present in system is absorbed. By capturing CO2, the hydrogen yield increases as the water gas shift reaction is shifted forward when CO2 is removed from the system. In conclusion, it was observed that steam/biomass ratio has more significant impact on hydrogen yield and efficiency among the process parameters.
The authors gratefully acknowledge the financial support from Universiti Teknologi PETRONAS and Petroleum Research Fund of PETRONAS to carry out this research.