Syngas Production from Gasification of Oil Palm Fronds with an Updraft Gasifier

MATERIALS AND METHODS

The biomass material (OPF) used for these experiments were obtained from a plantation in the state of Perak, Malaysia. OPF (after removing leaves) were cut and dried into the required size and moisture content, respectively. The proximate and ultimate analyses of the feedstock are presented in Table 1.

The experimental apparatus setup that used for gasification of OPF is shown in Fig. 1. It consists of batch type gasifier, air blower, cyclone, cooling unit, oil path filter and gas analyzer. The gasifier unit used in this study was made from mild steel and cement with 25 mm thickness and volume of 0.16 m³. The gasifier is square in cross section. The height from the top of the gasifier to the grate is 1.0 m. The internal length is 0.4 m the grate is in square shape with 0.3 m length. The ash container with square shape is fixed below the grate, has a height of 0.2 m and outside length of 0.45 m. The grate is directly supported to the combustion zone and must be capable of letting the ash fall through, without loss of fuel. The grate is used as an air distributor; however the holes are distributed across the whole section of the grate. The bottom zone under the grate is for collecting and then discharging the ash.

Five Type-K thermocouples are installed along the height of the gasifier in order to measure the temperature inside the gasifier and classify the gasification reactions zones.

Table 1:	Ultimate and proximate analysis of biomass feedstock

Fig. 1:

Schematic diagram of an updraft experiment set-up (1) Air blower (2) Air flow meter (3) Thermocouples (4) Updraft gasifier (5) Ash container (6) Data logger (7) Computer (8) Temperature probe (9) Flare point No. 1 (10) Cyclone (11) Condensate and particulate collector (12) Flare point No. 2 (13) Cooling system (14) Temperature probe (15) Oil path filter (16) X-STREAM Gas analyzer (17) Gas flowmeter (18) Accumulation tank (19) Flare point No. 3 (20) Gas to burner

These thermocouples are placed at 10, 30, 50, 70 and 90 cm above the grate. Two more thermocouples mounted at outlet pipe and after the gas conditioning system (cooling and cleaning units) are used to measure the temperature of the outlet gas and the temperature of the gas leaving the conditioning unit, respectively. All thermocouples are connected to data logger (USB TC-08) and the readings of the temperature are stored in the computer.

RESULTS AND DISCUSSION

Temperature profile: The temperatures profile inside the gasifier was measured every 30 sec by a set of thermocouples located at different heights in the gasifier as shown in Fig. 1. As discussed in our previous study (Elneel et al., 2011), the position of these thermocouples represent the dynamic temperature profile for gasification of OPF in an updraft gasifier, however T1, T2, T3 and T4 represents the oxidation, reduction, pyrolysis and drying zone, respectively. The temperature profile along the gasifier axis for different equivalence ratios during the operation time is presented in Fig. 2a-e. From the same figures, it is observed that, the top part of the gasifier (at 70 cm above the grate) showed a temperature lower than 200°C, this indicate that the feedstock is undergoing drying. While the temperature profiles near the grate (10 cm above the grate) showed a temperature between 700-1000°C, indicating that the fuel in this region is undergoing oxidation process. The regions between those zones showed decomposition and reduction processes.

It is also observed that the temperature profiles of the oxidation zone for all the runs (except at equivalence ratio of 0.4) are relatively stable at temperature above 700°C. This is due to the fact that, higher increase in equivalence ratio may provide more nitrogen which acts as a heat carrier, however lower oxidation zone temperature and higher temperature for the others zones compare to other equivalence ratios could be observed as shown in Fig. 2e.

Syngas composition: The composition of the producer gas was analyzed online at a five-minute interval during the experiment runs and the gas analyzed by using X-STREAM gas analyzer, the gas analyzer was used to analyze four gases of the producer gas; these gases are CO, CO₂, CH₄ and H₂ while N₂ could be calculated by the difference.

Fig. 2(a-e):	Temperature profile during operation time for several ERs (f) Average temperature profile along the gasifier axis for several ERs, (a) ER 0.18, (b) ER 0.22, (c) ER 0.29, (d) ER 0.35 and (e, f) ER 0.40

Fig. 3(a-e):	Syngas composition profiles with the elapsed time, (a) ER 0.18, (b) ER 0.22, (c) ER 0.29, (e) ER 0.35, (f) ER 0.40 and (g) Average value of syngas composition with different ER

Figure 3a-e shows the profile of syngas composition during the gasification operation time for a gasification equivalence ratio of 0.18-0.40 while Fig. 3f presents the average concentration of generated syngas.

The average composition of the syngas at optimum equivalence ratio (0.29) was found to be 20.10% CO, 10.41% CO₂, 1.20% CH₄ and 6.60% H₂. The remaining gases are mostly nitrogen originated from air. It can be seen from the same figure that for equivalence ratio of 0.22 and 0.29, the syngas composition is quite stable during the gasification operation time while it is unstable for the rest employed equivalence ratios.

Figure 4 shows that the total amount of combustible gases (H₂, CO, CH₄) increase from 25.64-33.5% with an increase in equivalence ratio up to 0.29 then decrease to 16.79% with further increase in equivalence ratio up to 0.4, this could be to the fact that at low equivalence ratio the process seems like pyrolysis and produce high amount of char and tar beside the gas while at higher equivalence ratio more oxygen is supplied into the gasifier and hence enhance the degree of combustion reactions that lead to improve char burning to produce CO₂ at the expense of combustible gases such as CO, H₂ and CH₄. Figure 5 shows the change of H₂/CO, CO/CO₂ and CH₄/H₂ molar ratio in the product gas with the employed equivalence ratio. It is observed that the molar ratio of H₂/CO and CH₄/H₂ decrease from 0.3-0.16 and from 0.25-0.12, respectively with an increase of equivalence ratio while the molar ratio of CO/CO₂ increase from 1.34-2.51 as the equivalence ratio increase up to 0.35 then decrease sharply to 0.75 with equivalence ratio of 0.4, this could be attributed to the fact that less equivalence ratio favors incomplete oxidation reaction (C+0.5O₂ = CO) and Boudouard reaction (C+CO₂ = 2CO) while at higher equivalence ratio (more oxygen is available) oxidation reaction (C+O₂ = CO₂) takes place and become stronger due to the higher quantity of oxygen.

Fig. 4:	Effect of ER on amount of combustible gases (H2, CO, CH4)

Fig. 5:	Effect of ER on molar ratios of CO/CO2, H2/CO, CH4/H2

Fig. 6:	Profile of molar ratio of H2/CO with time at different ER

The trend of the H₂/CO of the syngas during the gasification operation time with equivalence ratio is illustrated in Fig. 6. It can be seen that the average value of H₂/CO varied from 0.3-0.16 which is reasonable for air gasification.

Fig. 7:	Effect of ER on syngas calorific value

The figure shows that it is preferable to run the gasifier at equivalence ratio in the range of 0.22-0.29 in order to obtain smooth operation for the equipment that use of the produced synthesis gas.

Calorific value: The heating value of the synthesis gas was evaluated in terms of higher heating value, HHV and lower heating value, LHV, at standard temperature and pressure and can be determined by considering the volumetric percentage of the gas constitutes (CO, H₂ and CH₄), as shown below:

Figure 7 illustrates the effect of equivalence ratio on the LHV and HHV of the syngas produced from the gasification of OPF. Both LHV and HHV increase with equivalence ratio up to a peak value of 0.29. This is due to the increase of CO and H₂ and then starts to decrease and the decrease is due to consume of CO and H₂. The highest calorific value of the syngas was obtained at equivalence ratio of 0.29 with an average value of 4.42 and 4.62 MJ Nm^-3 for LHV and HHV, respectively which corresponds to the highest percentage of combustible gases especially CO and H₂.

Mass balance: An overall efficiency of the gasification system was obtained through mass balance which could be performed for the input and output streams of the gasification process as shown in the equations below. The input stream (M_i) consisted of biomass fuel and air while the output stream (M_o) consisted of dry syngas, tar, char and ash. The total mass balance was found to be in the range of 74.8-94.8% as shown in Fig. 8. The mass balance does not reach 100% due to neglect of water component and experimental losses:

Fig. 8:	Product distribution of OPF

Bridging phenomenon: Bridging which considered as one of the gasification operational problem was occurred during the experiment runs. Once the bridging happen, the fuels stop to flow down towards the gasifier bed and some of the gas generated at the gasifier bed combusted at the gap occurred between fuels. This may lead to increase the temperature of the reduction zone above the oxidation zone as shown in Fig. 2c and e. Moreover, bridging may cause a reduction in CO and H₂ and increasing in CO₂ of the synthetic gas due to combustion of some of the gas generated as shown in Fig. 3c for the period of bridging. Bridging could be avoided by shaking the gasifier in order to allow the fuels to fall downwards the gasifier bed.

CONCLUSION

Production of combustible gas from oil palm from was experimentally investigated in a pilot-scale updraft gasifier. It was found that OPF can successfully converted into combustible gases and the combustible gases was found to be in the range of 16.79-33.9% of the total produced gases with an equivalence ratio between 0.18 and 0.4. Moreover, 0.29 was observed to be an optimum equivalence ratio, in which the average composition of the syngas was found to be 20.10% CO, 10.41% CO₂, 1.20% CH₄ and 6.60% H₂ with an average calorific value of 4.42 and 4.62 MJ Nm^-3 as LHV and HHV. However, OPF can be assumed as one of the feedstock to produce synthetic gas. In the further, more experimental study is required to investigate the effect of other operation conditions such as moisture content and particle size.

ACKNOWLEDGMENT

The authors would like to acknowledge the Universiti Teknologi PETRONAS for providing the financial support and research facilities for this work.