**3.2 Effect of equivalence ratio (ER)**

The Equivalence Ratio (ER) varied from 0.23 to 0.27 through changing airflow rate at three constant temperatures (900°C, 950°C and 1000°C) at constant feeding rate (0.78 kg/hr) to find the optimum condition for hydrogen production. Table 5 summarize the obtained results and shows that the maximum molar fraction of hydrogen at 1000°C reached to (44.6% at ER: 0.23), (36.65% at ER: 0.23) and (36.38% at ER: 0.22) for palm kernel shell, coconut shell and bagasse, respectively.


Table 5. Summary of results for effect of equivalence ratio on hydrogen yield (gH2/kg biomass

On the contrary, different trend were observed for other produced gaseous. Methane (CH4) increased to 0.7%, 10.8% and 9.83% for palm kernel shell, coconut shell and bagasse, respectively when temperature rises from 750C to 850C but decreased gradually with temperature decreases. This can be explained as contribution of methanation reaction (Eqn. 7) during the gasification process. This was an expected result because as explained above most H2 production reactions are endothermic and content of CH4 decreases because temperature strengthens steam methane reforming reaction (McKendry, 2002, Lucas et al., 2004) and Pengmei et al., 2007). Furthermore, increasing of temperature contributes to decreases in CO2 but increased CO. The content of CO was mainly determined by Bourdouard reaction (Eqn. 5) where the boudouard reaction only produces CO at high temperature around 800-900C (Encinar et al., 2001 and Mathieu and Dubuisson, 2002). Moreover, Tavasoli at el. (2009) reported that decreasing the concentration of CH4 and heavy hydrocarbons with increasing of the rise in temperature in gasification process results in higher conversion of biomass and exhausting of major energy that is the reason for decline in value of LHV, because produced gases contain less quantities of CH4 due to

The Equivalence Ratio (ER) varied from 0.23 to 0.27 through changing airflow rate at three constant temperatures (900°C, 950°C and 1000°C) at constant feeding rate (0.78 kg/hr) to find the optimum condition for hydrogen production. Table 5 summarize the obtained results and shows that the maximum molar fraction of hydrogen at 1000°C reached to (44.6% at ER: 0.23), (36.65% at ER: 0.23) and (36.38% at ER: 0.22) for palm kernel shell,

**Equivalence Ratio (ER)** 0.23 0.24 0.25 0.26 0.27

i) 900°C 20.48 22 26 23.44 20 ii) 950°C 25.28 30.2 27.44 26.7 21.6 iii) 1000°C 35.68 32.24 30.1 28.6 24.65

i) 900°C 23.5 25.4 24.9 22.37 19.4 ii) 950°C 27.8 26.6 25.4 23.7 20.6 iii) 1000°C 29.32 28.9 26.8 25 21.05

i) 900°C 22 23 24.52 22.26 19.86 ii) 950°C 21.9 28.1 26.8 23.72 21 iii) 1000°C 23.7 29.1 27.74 25.22 23.1

Table 5. Summary of results for effect of equivalence ratio on hydrogen yield (gH2/kg

contribution in stem reforming reaction.

**3.2 Effect of equivalence ratio (ER)** 

coconut shell and bagasse, respectively.

**a) palm kernel shell** 

**b) coconut shell** 

**c) Bagasse** 

biomass

Figure 3 shows the gas composition for palm kernel shell gasification (selected sample for optimization study) at different temperature. Hydrogen yield were observed to increase first and decreased as ER increased. The obtained results are in accordance with other researchers where they found that increasing temperature in air gasification contributed to increasing of the hydrogen release (Midilli et al., 2001; Gonzalez et al., 2008; Lucas et al., 2004). In addition, they observed that increasing of the flow rate of air will decrease hydrocarbon contents due to partial combustion which subsequently contributed to decrease in tar and gaseous hydrocarbons. However, high flow rate of air will decrease the lower heating value (LHV) of the gasification gas (Pinto et al., 2003 and Lv et al., 2004). This phenomenon can be discussed by the following explanations.

Fig. 3. Comparison gas composition at temperature (a) 900°C (b) 950°C and (c) 1000°C in different ER at optimized condition of palm kernel shell gasification

At highest temperature 1000°C, low ER was suitable with compare to 900°C and 850°C. At low ER the combustion reactions in Eqn. 2 was dominated when compared to the combustion reaction in Eqn. 3 because of lack of oxygen. This is further verified by Wan Ab karim Ghani et. al. (2009) and Pengmei et. al. (2004) that explained that ER not only represents the oxygen quantity introduced to the reactor but also affects the gasification

Air Gasification of Malaysia Agricultural Waste in a

gasification

(Eqn. 9).

Where:

**3.5 Carbon conversion efficiency** 

a = Total reacted carbon in the system (kg) b = Total carbon fed to the system (kg).

Fluidized Bed Gasifier: Hydrogen Production Performance 239

increase, the production gas resultant inside the particles is more difficult to diffuse out and process is mainly controlled by gas diffusion. On other hand, larger particles are not only difficult to be entrained by fluidizing gas, but also produce fewer smaller particles after gasification reaction. This results in a reduction in fine particle entrainment, and hence

Fig. 5. Effect of particle size on gas composition at optimized condition of palm kernel shell

The carbon conversion efficiency in this study were calculated based on the below equation

In this study, the maximum carbon conversion efficiency reached up to (89%), (88.6%) and (94.5%) for palm kernel shell, coconut shell and bagasse, respectively at 1100°C under the air/biomass ratio (1.12 Nm3/Kg). These variations were observed as resulted from the biomass properties (see Table 2). As expected bagasse with the lowest carbon content and lowest density should be burned completely under given fluidizing velocity. As for other samples, the unburned carbon out of the gasifier might attributed by the sort residence time of biomass particles to further react either with O2 and CO2 and H2O at the same fluidizing conditions. This phenomenon is explained by Cao et al. (2006) that the carbon conversion also has relation with air/biomass ratio where they founded the maximum carbon conversion occurs at air/biomass ratio about 2.5 Nm3/kg. They reported that carbon conversion increased rapidly with increasing of the air/biomass ratio and decreased gradually with further air/biomass ratio increased. This is due to the fact that higher gas

Carbon conversion efficiency = (a/b) x 100 %(9)

decreases the amount of volatile matter and unburned char (Leung et al., 2003).

temperature under the condition of auto thermal operation. Higher equivalence ratio caused gas quality to degrade because of more oxidization reaction. In addition, the usage of air as oxidants contributed to higher ER which introduced large percentage of nitrogen into the system and diluted the combustible constituents in fuel gas (Pengmei et. al., 2007). On other hand, small ER will cause of lower oxygen be available for complete the gasification reactions which is not favourable for process. Therefore the gas composition is affected by the two contradictory factors of ER.

### **3.3 Effect of feeding rate**

Various feeding rate ranging from 0.20 to 1.21 kg/hr were tested for palm kernel shell to determine the time required for complete reactions of gasification of biomass and suitable feeding rate of reactor by considering of value of reactor and minimum fluidization velocity of biomass particles. Figure 4 shows that with increasing of the feeding rate, the hydrogen yield increased and reached to the maximum value of 29.1%. It was found that higher feeding rates did not have great influence neither on net gas production nor on the hydrogen yield. This is explained by the fact that the higher feeding rate attributed to less residence time per volume air which will caused less oxygen be in contact with the biomass particles (W.A.W.A.K. Ghani et. al., 2009). Thus, decreasing of temperature at pyrolysis and consequently gasification process will be occurred and hence the biomass samples will remain raw or partially gasified.

Fig. 4. Effect of feeding rate on gas composition at optimized condition of palm kernel shell gasification

#### **3.4 Effect of biomass particle size**

Figure 5 illustrates the hydrogen production performance for palm kernel shell at difference particle size (0.1, 2 and 5 mm). It was observed that with decreasing the particles size, the produced hydrogen and hydrogen yield decreased with the maximum value of 22.2% which belong to the smallest particle size. Lv et al. (2004) reported that pyrolysis process of small particles mainly controlled by reaction kinetics. Thus, as the size of biomass particles increase, the production gas resultant inside the particles is more difficult to diffuse out and process is mainly controlled by gas diffusion. On other hand, larger particles are not only difficult to be entrained by fluidizing gas, but also produce fewer smaller particles after gasification reaction. This results in a reduction in fine particle entrainment, and hence decreases the amount of volatile matter and unburned char (Leung et al., 2003).

Fig. 5. Effect of particle size on gas composition at optimized condition of palm kernel shell gasification

### **3.5 Carbon conversion efficiency**

The carbon conversion efficiency in this study were calculated based on the below equation (Eqn. 9).

$$\text{Carbon conversion efficiency} = \text{(a/b)} \times 100 \text{ \%} \tag{9}$$

Where:

238 Sustainable Growth and Applications in Renewable Energy Sources

temperature under the condition of auto thermal operation. Higher equivalence ratio caused gas quality to degrade because of more oxidization reaction. In addition, the usage of air as oxidants contributed to higher ER which introduced large percentage of nitrogen into the system and diluted the combustible constituents in fuel gas (Pengmei et. al., 2007). On other hand, small ER will cause of lower oxygen be available for complete the gasification reactions which is not favourable for process. Therefore the gas composition

Various feeding rate ranging from 0.20 to 1.21 kg/hr were tested for palm kernel shell to determine the time required for complete reactions of gasification of biomass and suitable feeding rate of reactor by considering of value of reactor and minimum fluidization velocity of biomass particles. Figure 4 shows that with increasing of the feeding rate, the hydrogen yield increased and reached to the maximum value of 29.1%. It was found that higher feeding rates did not have great influence neither on net gas production nor on the hydrogen yield. This is explained by the fact that the higher feeding rate attributed to less residence time per volume air which will caused less oxygen be in contact with the biomass particles (W.A.W.A.K. Ghani et. al., 2009). Thus, decreasing of temperature at pyrolysis and consequently gasification process will be occurred and hence the biomass samples will

Fig. 4. Effect of feeding rate on gas composition at optimized condition of palm kernel shell

Figure 5 illustrates the hydrogen production performance for palm kernel shell at difference particle size (0.1, 2 and 5 mm). It was observed that with decreasing the particles size, the produced hydrogen and hydrogen yield decreased with the maximum value of 22.2% which belong to the smallest particle size. Lv et al. (2004) reported that pyrolysis process of small particles mainly controlled by reaction kinetics. Thus, as the size of biomass particles

is affected by the two contradictory factors of ER.

**3.3 Effect of feeding rate** 

remain raw or partially gasified.

gasification

**3.4 Effect of biomass particle size** 

a = Total reacted carbon in the system (kg)

b = Total carbon fed to the system (kg).

In this study, the maximum carbon conversion efficiency reached up to (89%), (88.6%) and (94.5%) for palm kernel shell, coconut shell and bagasse, respectively at 1100°C under the air/biomass ratio (1.12 Nm3/Kg). These variations were observed as resulted from the biomass properties (see Table 2). As expected bagasse with the lowest carbon content and lowest density should be burned completely under given fluidizing velocity. As for other samples, the unburned carbon out of the gasifier might attributed by the sort residence time of biomass particles to further react either with O2 and CO2 and H2O at the same fluidizing conditions. This phenomenon is explained by Cao et al. (2006) that the carbon conversion also has relation with air/biomass ratio where they founded the maximum carbon conversion occurs at air/biomass ratio about 2.5 Nm3/kg. They reported that carbon conversion increased rapidly with increasing of the air/biomass ratio and decreased gradually with further air/biomass ratio increased. This is due to the fact that higher gas

Air Gasification of Malaysia Agricultural Waste in a

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