**3.2 Experimental setup**

A 10 kg/hr lab-scale downdraft gasifier which is belonging to the Waste Incineration Research Center (WIRC) and located at Department of Mechanical and Aerospace Engineering, King Mongkut's University of Technology North Bangkok, Thailand is a vertical reactor with fuel feeding system. The reactor is 2,000 mm height and has a diameter of 600 mm. It can be separated into 4 parts as followed: fuel hopper, pyrolysis chamber, reaction chamber and ash chamber, as shown in Figure 10. The temperature in pyrolysis chamber ranges between 200-500 °C, whereas the reaction chamber has the temperature of 500-1200 °C. The temperature in ash chamber is about 300-1000 °C. There are totally 11 type-K thermocouples installed over the height of gasifier: 5 thermocouples in pyrolysis chamber, 4 thermocouples in reaction chamber and 2 thermocouples in ash chamber.

Fig. 10. A 10 kg/hr lab-scale downdraft gasifier

In additional to a downdraft gasifier which is the core component, a lab-scale gasifier system consists of air blower, air pre-heater, gas cleaning unit, weighing apparatus and data logger. Figure 11 shows the process diagram of a lab-scale gasifier system.

Fig. 11. Process diagram of a lab-scale downdraft gasifier system

#### **3.3 Experiment procedure**

138 Renewable Energy – Trends and Applications

In additional to a downdraft gasifier which is the core component, a lab-scale gasifier system consists of air blower, air pre-heater, gas cleaning unit, weighing apparatus and data

logger. Figure 11 shows the process diagram of a lab-scale gasifier system.

Fig. 11. Process diagram of a lab-scale downdraft gasifier system

Fig. 10. A 10 kg/hr lab-scale downdraft gasifier

In case of as-received PEFB, 2 to 3 kg of PEFB was fed into a 10 kg/hr downdraft gasifier per batch. After feedstock feeding, the lid at the top of gasifier was closed and the feedstock inside the gasifier was ignited by a burner. When feedstock started to be ignited, the gasification air was introduced into gasifier and the gasification process was taken place. When the first batch of feedstock was almost completely gasified, 2-3 kg of feedstock was then introduced again into the gasifier. This step was repeated until the total amount of approximately 25 kg PEFB was fed into gasifier in the whole period of time. In case of pelletized PEFB, 25 kg of it was fed into hopper in one time. During the gasification process, the temperatures inside the gasifier at each position were continuously recorded. If the gasification temperature reached the constant value, the volume flow rate of producer gas were measured. A little amount of producer gas was taken out as gas sample in order to further be investigated, while the remaining producer gas was flared at the stack outlet. The gasification process terminated, when the fuel was completely burnt and the reactor was naturally cooled down. The ash remaining in the reactor was then taken from the reactor and measured the weight in order to determine the percentage of ash production. The air flow rates of 6, 9, 12, 15 and 18 Nm3/hr for as-received PEFB gasification and the air flow rate of 15, 18, 21, 27 and 33 Nm3/hr for pelletized PEFB gasification were varied for each experiment.

After each experiment, producer gas composition, in which only H2, CO, CO2, CH4, N2 and O2 were taken into account, was investigated by gas chromatography according to ASTM. Lower heating value of producer gas was also calculated according to Equation 1 and cold gas efficiency was determined by Equation 2.

$$\mathbf{LHV\_{G}} = \Sigma \mathbf{v\_{i}} \cdot \mathbf{LHV\_{i}} \tag{1}$$

$$\eta = \frac{LHV\_{\rm G} \times \dot{V}\_{\rm G}}{LHV\_{\rm F} \times \dot{m}\_{\rm F}} \tag{2}$$

LHV is the lower heating value. The subscript G, i and F refers to the producer gas, each combustible gas component and PEFB, respectively. Vi is the fraction of each combustible gas component in producer gas by volume. VG and mF are producer gas yield by volume and PEFB consumption rate by mass, respectively.

#### **3.4 Results and discussions**

#### **3.4.1 Producer gas composition and its lower heating value**

The composition of producer gas obtained from air gasification of both as received PEFB and pelletized PEFB is shown in Figure 12 and Figure 13, respectively.

From Figure 12, it can be seen that the concentration of CO increases with increasing air flow rate and its increasing rate is slow down at the higher air flow rate. The concentration of CO2 decreases until the air flow rate of 9 Nm3/hr and further increases with the air flow rate. H2-concentration is very fluctuated and cannot predict its tendency from Figure 12, whereas there is no significant change in the concentration of CH4 for all air flow rates.

Compared Figure 13 to Figure 12, it can be noticed that the concentration of each gas composition is not fluctuated. The tendency of each gas can be predicted from Figure 13. Due to the high density of pelletized PEFB, the fuel is more homogenous and the fuel flow is more stable. Consequently, the reactions between air and fuel during gasification process are more stable and can reach their equilibriums. In case of pelletized PEFB (Figure 13), the concentration of CO and H2 increases with increasing air flow rate. The increasing rate is more rapid at the air flow rate until 21 Nm3/hr and for further increase in air flow rate from 21 Nm3/hr, the concentration of both gases increases slowly or almost remains constant. In contrast to H2 and CO, the concentration of CO2 decreases with the air flow rate until its minimum point at the air flow rate of 21 Nm3/hr. With further increase in air flow rate, CO2-concentration increases. The concentration of CH4 in case of pelletized PEFB is almost constant.

Fig. 12. Producer gas composition with different air flow rates for as-received PEFB

Fig. 13. Producer gas composition with different air flow rates for pelletized PEFB

more stable. Consequently, the reactions between air and fuel during gasification process are more stable and can reach their equilibriums. In case of pelletized PEFB (Figure 13), the concentration of CO and H2 increases with increasing air flow rate. The increasing rate is more rapid at the air flow rate until 21 Nm3/hr and for further increase in air flow rate from 21 Nm3/hr, the concentration of both gases increases slowly or almost remains constant. In contrast to H2 and CO, the concentration of CO2 decreases with the air flow rate until its minimum point at the air flow rate of 21 Nm3/hr. With further increase in air flow rate, CO2-concentration increases. The concentration of CH4 in case of pelletized PEFB is almost

3 6 9 12 15 18 21

Fig. 12. Producer gas composition with different air flow rates for as-received PEFB

12 15 18 21 24 27 30 33 36

Fig. 13. Producer gas composition with different air flow rates for pelletized PEFB

Air flow rate (Nm3/hr)

/hr)

Air flow rate (Nm3

0

0

1

2

3

LHV (MJ/Nm3)

4

CO CH4 CO2 H2 LHV

5

1

2

3

LHV (MJ/Nm3

)

4

CO CH4 CO2 H2 LHV

5

constant.

Concentration (% Vol.)

Concentration (% Vol.)

Considered the reactions occurred in a downdraft gasifier, PEFB is firstly dried and the moisture containing in PEFB is driven off as steam. During pyrolysis process, PEFB is thermally decomposed into gaseous products, tars and chars, as written in Equation 3. Tar which is heavy hydrocarbon compound is also thermally cracked into light hydrocarbons and other gases, as written in Equation 4 (Rui et al., 2007).

$$\text{PEFB} \blacktriangleright \text{gas} \text{+ } \text{tar} \text{+ } \text{char} \tag{3}$$

$$\text{Tars } \rightharpoonup \text{ light and heavy hydrogen bonds + CO} + \text{CO}\_2 + \text{H}\_2 \tag{4}$$

Gases and the remaining PEFB pass through the oxidation zone where oxidation process occurs. In this zone, combustible gas and combustible material are oxidized to be steam and CO2 by oxygen containing in gasification air. Equation 5 to Equation 7 shows the examples of oxidation process (Kaltschmitt & Hartmann, 2001; Schmitz, 2001). As the air flow rate increases, the oxidation process is accelerated by increasing amount of O2 in gasification air and results in the higher reaction temperature (exothermic reactions).

$$\mathrm{H\_2 + \frac{1}{2}O\_2 \leftrightarrow H\_2O} \quad \Delta H = -241.8 \text{ kJ/mol} \tag{5}$$

$$\text{CO} + \frac{1}{2}\text{O}\_2 \leftrightarrow \text{CO}\_2 \quad \Delta \text{H} = -283.0 \text{ kJ/mol} \tag{6}$$

$$\rm C\_mH\_n + \left(m + \frac{n}{2}\right)O\_2 \leftrightarrow mCO\_2 + \frac{n}{2}H\_2O \tag{7}$$

With further increase in air flow rate, the reactions almost approach their equilibriums. therefore the concentration of each gas composition remains constant. The products of oxidation process react further with other gases and un-reacted fuel in reduction zone. The increase or decrease in composition of producer gas is resulted from reactions in this zone. The increase in CO and H2 from the experiments is resulted from endothermal Boudouard reaction (Equation 8) and endothermal heterogeneous water gas shift reaction (Equation 9) (Kaltschmitt & Hartmann, 2001; Laohalidanond, 2008; Schmitz, 2001).

$$\text{C} + \text{CO}\_2 \leftrightarrow \text{2CO} \quad \Delta H = 159.9 \text{ kJ/mol} \tag{8}$$

$$\text{C} + \text{H}\_2\text{O} \leftrightarrow \text{CO} + \text{H}\_2 \quad \Delta\text{H} = 118.5 \text{ kJ/mol} \tag{9}$$

With increasing air flow rate, the gasification temperature raises as a result of exothermal oxidation. The endothermal Boudouard reaction and endothermal heterogeneous water gas shift reaction are then shifted to the right hand side, consequently, CO and H2 in producer gas increase. The above mentioned reactions take also the responsibility for the decrease in CO2 concentration in producer gas.

With respect to the heating value of producer gas, the lower heating value of producer gas yields from as received PEFB is fluctuated and the tendency cannot be predicted because of the non-equilibrium reactions. Taken the results from gasification process of pelletized PEFB into account, it can be remarkably seen that the heating value of producer gas varies with the air flow rate. At the air flow rate of 15 Nm3/hr, the producer gas has the lower heating value of 4.20±0.31 MJ/Nm3 and the lower heating value increases to 4.77±0.29 MJ/Nm3 at the air flow rate of 33 Nm3/hr. The increase in the lower heating value is resulted from the increase in combustible gases, e.g. H2 and CO with increasing air flow rate.

From the experiments with both as received PEFB and pelletized PEFB, it can be concluded that using pelletized PEFB can provide more stable gasification process than using asreceived PEFB and the relevant reactions can approach their equilibriums; hence, pelletized PEFB is more proper to be used as fuel in gasification process than as-received PEFB. Since the producer gas will further be used as fuel in a combustion engine generator for electricity production, the heating value of producer is the major parameter to be concerned. The maximum heating value of 4.77±0.29 MJ/Nm3 is achieved from gasification of pelletized PEFB at the air flow rate of 33 Nm3/hr.
