**3. Results and discussion**

The gasification performance mainly will be evaluated based on the gas production quality (hydrogen yield and carbon conversion efficiency) and quantity (gas composition). Furthermore, the ash and oil yield will also be determined and quantified.

### **3.1 Effect of gasification temperature**

The product yields (hydrogen, ash and oil) and detail gas composition of studied biomass at different gasification temperature are summarized in Table 4 and Figure 2, respectively. In this study, reactor temperature is increase from 700 to 1100 °C in 50°C and at constant feeding rate (0.78 kg/h) and equivalence ratio(ER)(0.26).

Air Gasification of Malaysia Agricultural Waste in a

bagasse at different temperature

Fluidized Bed Gasifier: Hydrogen Production Performance 235

(a) (b)

(c)

Figure 2 illustrates that hydrogen mol fraction significantly increased while the content of other produced gas particularly methane (CH4) showed an opposite trend for all studied samples. This is in accordance with Le Chatelier's principle; higher temperatures favour the reactants in exothermic reactions and favour the products in endothermic reactions. The H2 formation is favoured by increasing of the gasification temperature, which could be due to the combination effect of exothermal character of water-gas shift reaction (Eqn. 8) which occur and predominate between 500-600°C and the water-gas reaction (Eqn. 6) which becomes significant at the temperature from 1000 to 1100°C and upward (Midilli et. al., 2001). The water shift reaction occurred in any gasification process due to the presence of water inside of fuel and water vapour in side of air. Water vapour and carbon dioxide promote hydrogen production in biomass gasification process (Cao et. al, 2006). Furthermore, increasing of gasification temperature also increases thermal cracking of tar and heavy hydrocarbons into gaseous components (Babu, 1995). At the same time, the gas production also increased due to cracking of liquid fraction developed in this range of temperature (300-500°C). These observations are in accordance with Encinar et al. (1996), Fagbemi et al. (2001), Zanzi et al. (2002) and Chen et al. (2003) where they found that the pyrolysis temperature below 600°C should be favoured for overall hydrogen production.

Fig. 2. Comparison gas composition for (a) palm kernel shell, (b) coconut shell and (c)


Table 4. Summary of results for effect of gasification temperature on hydrogen production

In general, higher temperature favoured production gas as compared to ash and oil. Hydrogen yield increased as the temperature increased from 750 to 1000°C with the value of 14 to 31 mol%, 18 to 25.44 mol% and 11 to 23 mol% for palm kernel shell, coconut shell and bagasse, respectively. Palm kernel shell gave the highest H2 compared to other samples due to the highest lignin content in their structure (Worasuwanarak et. al., 2007 and Dawson and Boopathy, 2008). Meanwhile, the product gas low heating value (LHV) showed a maximum value, 30, 23, 23 and 27 MJ/KgNm3 for palm kernel, coconut shell and bagasse, respectively. Ash and oil products yield ranging 0.10-0.29 % and 0.02-0.29%, respectively. These phenomena would be due to various reasons namely (i) higher production of gases in initial pyrolysis step whose rate is faster at higher temperature (Franco et al., 2003); (ii) higher gas production caused by endothermic char gasification reactions, which are favored at high temperature in pyrolysis zone, (iii) elevated temperature in gasification zone is favourable for tar and heavy hydrocarbons cracking that result to higher gas production (Tavasoli et al., 2009).

Temperature (C) 750 800 850 900 950 1000 1100

Hydrogen yield 14.08 16.8 22.88 23.44 26.7 28.56 31.04

LHV (MJ/kgNm3) 25.776 29.964 25.451 24.954 24.439 21.3 18.3 Ash (w/w) 0.174 0.158 0.142 0.136 0.12 0.114 0.1 Oil (w/w) 0.1 0.13 0.164 0.144 0.29 0.16 0.1

Hydrogen yield 18.93 19.8 20.64 22.37 23.7 25 25.44

LHV (MJ/kgNm3) 24.68 26.328 25.872 25.489 23.274 20.936 20.247 Ash (w/w) 0.183 0.167 0.156 0.132 0.128 0.122 0.114 Oil (w/w) 0.06 0.078 0.11 0.11 0.12 0.07 0.05

Hydrogen yield 11.6 13.1 13.47 17.44 19 21.4 23

LHV (MJ/kgNm3) 23.245 26.74 26.224 25.674 25.152 24.53 21.653 Ash (w/w) 0.178 0.143 0.122 0.1 0.092 0.088 0.083 Oil (w/w) 0.052 0.052 0.064 0.084 0.072 0.052 0.03 Table 4. Summary of results for effect of gasification temperature on hydrogen production In general, higher temperature favoured production gas as compared to ash and oil. Hydrogen yield increased as the temperature increased from 750 to 1000°C with the value of 14 to 31 mol%, 18 to 25.44 mol% and 11 to 23 mol% for palm kernel shell, coconut shell and bagasse, respectively. Palm kernel shell gave the highest H2 compared to other samples due to the highest lignin content in their structure (Worasuwanarak et. al., 2007 and Dawson and Boopathy, 2008). Meanwhile, the product gas low heating value (LHV) showed a maximum value, 30, 23, 23 and 27 MJ/KgNm3 for palm kernel, coconut shell and bagasse, respectively. Ash and oil products yield ranging 0.10-0.29 % and 0.02-0.29%, respectively. These phenomena would be due to various reasons namely (i) higher production of gases in initial pyrolysis step whose rate is faster at higher temperature (Franco et al., 2003); (ii) higher gas production caused by endothermic char gasification reactions, which are favored at high temperature in pyrolysis zone, (iii) elevated temperature in gasification zone is favourable for tar and heavy hydrocarbons cracking that result to higher gas production (Tavasoli et al.,

Reactor

wet basis)

**a) Palm kernel shell** 

(g H2/kg biomass,

**b) Coconut shell** 

(g H2/kg biomass,

(g H2/kg biomass,

wet basis)

**c) Bagasse** 

wet basis)

2009).

Fig. 2. Comparison gas composition for (a) palm kernel shell, (b) coconut shell and (c) bagasse at different temperature

Figure 2 illustrates that hydrogen mol fraction significantly increased while the content of other produced gas particularly methane (CH4) showed an opposite trend for all studied samples. This is in accordance with Le Chatelier's principle; higher temperatures favour the reactants in exothermic reactions and favour the products in endothermic reactions. The H2 formation is favoured by increasing of the gasification temperature, which could be due to the combination effect of exothermal character of water-gas shift reaction (Eqn. 8) which occur and predominate between 500-600°C and the water-gas reaction (Eqn. 6) which becomes significant at the temperature from 1000 to 1100°C and upward (Midilli et. al., 2001). The water shift reaction occurred in any gasification process due to the presence of water inside of fuel and water vapour in side of air. Water vapour and carbon dioxide promote hydrogen production in biomass gasification process (Cao et. al, 2006). Furthermore, increasing of gasification temperature also increases thermal cracking of tar and heavy hydrocarbons into gaseous components (Babu, 1995). At the same time, the gas production also increased due to cracking of liquid fraction developed in this range of temperature (300-500°C). These observations are in accordance with Encinar et al. (1996), Fagbemi et al. (2001), Zanzi et al. (2002) and Chen et al. (2003) where they found that the pyrolysis temperature below 600°C should be favoured for overall hydrogen production.

Air Gasification of Malaysia Agricultural Waste in a

phenomenon can be discussed by the following explanations.

Fluidized Bed Gasifier: Hydrogen Production Performance 237

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

(a) (b)

(c) Fig. 3. Comparison gas composition at temperature (a) 900°C (b) 950°C and (c) 1000°C in

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

different ER at optimized condition of palm kernel shell gasification

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 contribution in stem reforming reaction.
