Effect of KOH, reaction temperature and time, and introduced carbonization step on the amount and composition of syngas as well as porous properties of the carbon products for CO2 gasification of coconut shell at low temperatures (300–700°C) was investigated. Results showed that the presence of potassium hydroxide and gasification temperature had a significant effect on the amount and composition of syngas product and facilitated the rate of hydrogen and carbon monoxide formation. It was also found that carbonization step could promote the generation of hydrogen gas as well as increasing the gas heating value per kg of gas. Furthermore, the porosity development of carbon product was found to be influenced by the chemical ratio and gasification temperature. The optimal conditions for achieving high hydrogen composition and specific surface area were to gasify coconut shell under CO2 at 600°C for 60 min with carbonization step and with chemical weight ratio of 3.0. This condition gave the hydrogen composition as high as 29.70 %weight of produced syngas with heating value of 41.4 MJ/kg of gas and specific surface area of 2650 m2/g of the carbon product.
The instability of price and supply of fossil fuels has necessitated the need for increasing utilization and development of alternative energy. Biomass is an attractive choice to substitute the conventional fossil fuels because of its abundant availability, its net zero emission of CO2, and more importantly being renewable. There are a number of processes to convert biomass into energy such as fermentation, digestion, and thermal decomposition. The fermentation and digestion processes are relatively slow compared with thermal decomposition [
With the purpose to lower energy consumption in a gasification process and to make use of abundantly available coconut shell in Thailand, it was decided in the present work to investigate the catalytic gasification of coconut shell biomass to simultaneously produce synthesis gas and a high porosity activated carbon at gasification temperatures lower than those encountered in normal practice. In this work, potassium hydroxide and carbon dioxide gas were employed as a catalyst and gasification oxidizing agent, respectively. Generally, potassium hydroxide is widely used as an activation agent since it can lead to optimal textural and chemical properties of the carbon [
Coconut shell was crushed by a jaw crusher and screened to obtain an average particle size of 1.55 mm (12 × 14 mesh). The sample obtained was then dried at 110°C for 24 hours in an oven to remove excess moisture contained in the raw material and was kept for further analysis and experimentation. Chemical composition of coconut shell was determined by means of proximate and ultimate analyses. Proximate analysis was determined using a TGA analyzer (TGA 701, LECO) by following the analytical method ASTM D7582-12. Ultimate analysis was determined by using a CHNS analyzer (TruSpec CHN, LECO) to give elemental information on %weight of carbon, nitrogen, hydrogen, sulfur, and oxygen (by difference).
Coconut shell and potassium hydroxide were used, respectively, as a precursor and a catalyst for the gasification experiments. When the carbonization step was introduced prior to the gasification step, coconut shell precursor was heated in a muffle furnace under the atmosphere of flowing argon (100 mL/min) at 450°C for 60 min using the heating rate of 10°C/min and the final char yield was determined. The experimental procedure for the gasification study is as follows. Each of the precursors (raw coconut shell or coconut shell char from the carbonization step) of known weight was thoroughly mixed with potassium hydroxide powder according to the required catalyst to precursor weight ratio by keeping the total weight of mixture constant at 10 g. This solid mixture was then gasified in a fixed bed reactor made from a silica-alumina tube (45 mm ID and 600 mm in length) by heating at 10°C/min from room temperature to the desired gasification temperature under a constant flow (100 mL/min) of oxidizing gas (50% Ar + 50% CO2). The temperature and holding time for gasification were varied from 300 to 700°C and from 30 to 180 min, respectively. During gasification, the produced gas products were passed through a water condenser to remove the liquid biooil and the remaining gas was continuously collected every 10 minutes in a 10-liter gas sampling bag until no gas products were generated which normally took about 240 min of gasification time. The main gas composition including CO, CO2, H2, CH4, and C3H8 in the product gases was determined by a gas chromatograph (Shimadzu, GC-2014) using a thermal conductivity detector (TCD) and flame ionization detector (FID), with helium gas being introduced as the carrier gas. The final solid product obtained was mixed with HCl solution (5% by volume concentration) and stirred for 1 hour, followed by rinsing with deionized water to remove any remaining potassium hydroxide and potassium hydroxide derivatives until the pH of the rinsing water was constant at proximately 7.0. Then, the carbon product was dried at 110°C for 10–12 hours and weighed. Previous works have shown that HCl solution in the concentration range from 0.85 to 43% by volume has been employed to effectively remove contaminants from activated carbons produced by chemical activation with KOH [
The specific surface area of the activated carbon sample was determined from the N2 adsorption isotherm at 77 K with an automatic sorption meter (BELSORP-max). The sample was placed into the sample tube, heated to 300°C, and evacuated until the pressure became less than 6.7 × 10−7 Pa prior to the surface area measurement. The Brunauer-Emmet-Teller (BET) theory [
X-ray diffraction (XRD) analysis of the activated carbon product was carried out by using a Shimadzu EDX-700 energy dispersive X-ray spectrometer with Cu K
The percent yields of solid and liquid products were determined from the known weight of the collected products and the initial weight of the coconut shell. The gas yield was then determined from the mass balance. Table
Experimental conditions for studying low temperature catalytic gasification of coconut shell.
Sample code | Step I (carbonization) | Step II (gasification) | ||||||
---|---|---|---|---|---|---|---|---|
Temp. (°C) | HT (°C/min) | Time (min) | CR (g/g) | Temp. (°C) | HT (°C/min) | Time (min) | CO2 (%) | |
G01 | — | — | — | 0 | 600 | 10 | 180 | 50 |
G02 | — | — | — | 0.5 | 600 | 10 | 180 | 50 |
G03 | — | — | — | 0.75 | 600 | 10 | 180 | 50 |
G04 | — | — | — | 1.0 | 600 | 10 | 180 | 50 |
G05 | — | — | — | 1.5 | 600 | 10 | 180 | 50 |
G06 | — | — | — | 2.0 | 600 | 10 | 180 | 50 |
G07 | — | — | — | 0.75 | 300 | 10 | 180 | 50 |
G08 | — | — | — | 0.75 | 400 | 10 | 180 | 50 |
G09 | — | — | — | 0.75 | 500 | 10 | 180 | 50 |
G10 | — | — | — | 0.75 | 600 | 10 | 180 | 0 |
G11 | 450 | 10 | 60 | 2.0 | 600 | 10 | 180 | 50 |
G12 | 450 | 10 | 60 | 2.0 | 600 | 10 | 60 | 50 |
G13 | 450 | 10 | 60 | 2.0 | 600 | 10 | 30 | 50 |
G14 | 450 | 10 | 60 | 3.0 | 600 | 10 | 60 | 50 |
G15 | 450 | 10 | 60 | 4.0 | 600 | 10 | 60 | 50 |
G16 | 450 | 10 | 60 | 4.0 | 700 | 10 | 60 | 50 |
HT: heating rate.
CR: chemical-to-biomass weight ratio.
Time: holding time.
To test the reproducibility of the experimental data, all the gasification experiments were repeated three times (triplicate tests) for each gasification condition. The standard deviation of error (
The standard deviations of error were found to be 6.43 and 8.47 for the solid yield and gas yield, respectively. This indicates that the reproducibility of the gas yield is somewhat less than that of the solid yield. The average errors (percent deviation from mean) were estimated to be 4.95% and 6.40% for the solid yield and gas yield, respectively.
Table
Proximate and ultimate analysis of coconut shell precursor.
Proximate analysis (%weight) | Ultimate analysis (%weight) | ||
---|---|---|---|
Moisture | 2.06 | Hydrogen | 6.52 |
Volatile matter | 78.95 (80.61 |
Carbon | 54.67 |
Ash | 0.31 (0.32 |
Nitrogen | 0.57 |
Fix carbon | 18.68 (19.07 |
Sulphur | 0.02 |
Oxygen (by diff.) | 38.22 |
The effect of time/temperature history on the yield and composition of produced gas during the heat-up period from room temperature to the final gasification temperature of 600°C is shown in Figure
The effect of chemical weight ratio on the composition of gas products generated during the CO2 gasification at 600°C for 180 min.
CR = 0
CR = 0.5
CR = 2.0
The results of product gas yields, composition, and heating values under various gasification conditions are shown in Table
Yields and composition of produced gas under various gasification conditions (see Table
Sample |
Gas yield (%) | Gas composition (mmol/g) | Gas composition (%w) | Heating value | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
H2 | CO | CO2 | C3H8 | CH4 | H2 | CO | CO2 | C3H8 | CH4 | Other | MJ/kg gas | MJ/kg of coconut shell | ||
G01 | 26.22 | 0.53 | 1.54 | 2.62 | 0.00 | 0.35 | 0.40 | 16.40 | 43.97 | 0.00 | 2.16 | 37.07 | 3.2 | 0.84 |
G02 | 26.97 | 1.58 | 2.19 | 2.81 | 0.38 | 1.12 | 1.17 | 22.74 | 45.85 | 6.16 | 6.67 | 17.41 | 10.0 | 2.68 |
G03 | 27.22 | 1.23 | 2.98 | 3.56 | 0.38 | 0.76 | 0.91 | 30.68 | 57.57 | 6.12 | 4.44 | 0.28 | 9.3 | 2.53 |
G04 | 28.57 | 1.25 | 2.43 | 4.38 | 0.57 | 0.49 | 0.81 | 22.06 | 62.42 | 8.15 | 2.56 | 4.00 | 8.3 | 2.38 |
G05 | 29.25 | 3.28 | 6.24 | 2.14 | 0.44 | 1.22 | 1.83 | 48.74 | 26.26 | 5.38 | 5.43 | 12.36 | 12.4 | 3.62 |
G06 | 16.15 | 8.23 | 0.89 | 1.18 | 0.44 | 0.64 | 10.20 | 15.45 | 32.15 | 11.85 | 6.37 | 23.98 | 22.6 | 3.64 |
G07 | 9.10 | 0.88 | 0.71 | 1.51 | 0.00 | 0.07 | 1.92 | 21.76 | 72.82 | 0.15 | 1.15 | 2.20 | 5.1 | 0.47 |
G08 | 12.55 | 1.03 | 2.43 | 0.78 | 0.01 | 0.03 | 1.65 | 54.24 | 27.36 | 0.18 | 0.38 | 16.19 | 7.7 | 0.97 |
G09 | 17.17 | 0.85 | 4.13 | 0.69 | 0.02 | 0.98 | 0.99 | 67.35 | 17.68 | 0.46 | 9.17 | 4.35 | 12.8 | 2.20 |
G10 | 17.86 | 1.23 | 2.98 | 3.10 | 0.38 | 0.76 | 0.91 | 30.70 | 50.16 | 6.12 | 4.44 | 7.67 | 9.3 | 1.66 |
G11 | 7.95 | 7.16 | 0.10 | 0.75 | 0.03 | 0.23 | 18.18 | 3.63 | 42.04 | 1.60 | 4.71 | 29.84 | 25.2 | 2.01 |
G12 | 7.46 | 7.49 | 0.10 | 0.96 | 0.03 | 0.20 | 20.29 | 3.80 | 57.44 | 1.48 | 4.25 | 12.74 | 27.5 | 2.05 |
G13 | 7.42 | 7.97 | 0.11 | 1.14 | 0.02 | 0.17 | 21.58 | 4.19 | 67.78 | 1.34 | 3.69 | 1.42 | 28.7 | 2.13 |
G14 | 7.90 | 8.67 | 0.05 | 0.82 | 0.07 | 0.18 | 22.22 | 1.82 | 45.97 | 3.80 | 3.73 | 22.46 | 30.4 | 2.40 |
G15 | 7.99 | 11.11 | 0.05 | 0.04 | 0.30 | 0.00 | 27.90 | 1.81 | 2.31 | 16.50 | 0.00 | 51.48 | 41.4 | 3.31 |
G16 | 7.82 | 11.73 | 0.06 | 0.00 | 0.10 | 0.29 | 29.23 | 2.03 | 0.00 | 5.39 | 5.75 | 57.60 | 40.6 | 3.17 |
Effect of chemical weight ratio on (a) production yield, (b) composition, and (c) heating value of gas products from gasification of coconut shell with CO2 and KOH at 600°C for 180 min.
This result could stem from the improvement of the water-gas shift reaction (
Water-gas shift reaction is as follows:
KOH decomposition is as follows:
Furthermore, the increase in hydrogen content could possibly result from the dehydrogenation reaction of KOH that could occur during gasification as suggested by Viswanathan et al. (2009) [
The effect of gasification temperature on the yield, composition, and heating value of produced gas for the gasification time of 180 min and the chemical ratio of 0.75 is shown in Figure
Effect of temperature on (a) gas yield, (b) composition, and (c) heating value of gas products from CO2 gasification of coconut shell for 180 min and chemical ratio of 0.75.
From these obtained results, the gasification schemes for the low temperature gasification process without a prior carbonization step could be proposed as follows:
(
(
XRD patterns of activated carbons from raw coconut shell by one step gasification method (without the carbonization step) under the same conditions by using different oxidizing agents.
The effect of the carbonization step on the yield, composition, and heating value of the gas products was investigated at 600°C for 180 min and the chemical weight ratio of 2.0 and the results are shown in Figure
Effect of carbonization step on (a) gas yield, (b) gas composition, and (c) heating value of gas products from gasification of coconut shell with 50% CO2 at 600°C for 180 min and chemical ratio of 2.0.
Effect of chemical ratio on gas yield and gas composition from gasification of coconut shell char with carbonization step using 50% CO2 at 600°C for 60 min.
Figures
Effect of temperature on (a) gas yield, (b) gas composition, and (c) gas heating value from gasification of coconut shell with carbonization step using 50% CO2, chemical ratio of 4.0 for 60 min.
The effect of the chemical ratio on the yields and porous properties of the activated carbons produced from the coconut shell by CO2 gasification under varying conditions is shown in Table
Yields and porous properties of the activated carbons from various gasification conditions (see Table
Sample |
Yield (%) | Surface area (m2/g) | Micropore volume (cm3/g) | Meso- and macropore volume (cm3/g) | Total pore volume (cm3/g) | Micropore volume (%) | Meso- and macropore volume (%) | Average pore diameter (nm) |
---|---|---|---|---|---|---|---|---|
G01 | 28.13 | 119 | 0.0633 | 0.0064 | 0.0697 | 90.82 | 9.18 | 2.34 |
G02 | 25.51 | 569 | 0.2544 | 0.0082 | 0.2626 | 96.88 | 3.12 | 1.85 |
G03 | 25.09 | 640 | 0.2950 | 0.0094 | 0.3044 | 96.91 | 3.09 | 2.75 |
G04 | 23.02 | 619 | 0.2839 | 0.0123 | 0.2962 | 95.85 | 4.15 | 1.91 |
G05 | 18.59 | 670 | 0.3267 | 0.0115 | 0.3382 | 96.60 | 3.40 | 2.02 |
G06 | 14.18 | 1000 | 0.4613 | 0.0351 | 0.4964 | 92.93 | 7.07 | 1.98 |
G07 | 31.10 |
|
|
|
|
— | — | — |
G08 | 33.86 | 63.0 | 0.0519 | 0.0109 | 0.0628 | 82.64 | 17.36 | 3.37 |
G09 | 32.46 | 88.7 | 0.0493 | 0.0087 | 0.0580 | 85.00 | 15.00 | 3.25 |
G10 | 15.38 | 591 | 0.3191 | 0.0524 | 0.3715 | 85.90 | 14.10 | 3.12 |
G11 | 23.89 | 921 | 0.4246 | 0.0057 | 0.4303 | 98.68 | 1.32 | 1.87 |
G12 | 24.09 | 1030 | 0.4626 | 0.0105 | 0.4731 | 97.78 | 2.22 | 1.84 |
G13 | 24.97 | 1330 | 0.6014 | 0.0169 | 0.6083 | 98.86 | 1.14 | 1.83 |
G14 | 20.96 | 1710 | 0.7929 | 0.0123 | 0.8052 | 98.47 | 1.53 | 1.88 |
G15 | 20.12 | 2650 | 1.3128 | 0.0452 | 1.3580 | 96.67 | 3.33 | 2.05 |
G16 | 18.85 | 2760 | 1.4528 | 0.0321 | 1.4849 | 97.84 | 2.16 | 2.15 |
Effect of chemical ratio on (a) yield, (b) the pore volume, and (c) specific surface area (
Table
Figure
Effect of chemical ratio on nitrogen isotherm at 77 K of activated carbon samples produced by CO2 gasification of coconut shell at 600°C for 180 min.
Figure
Effect of gasification temperature on (a) yield, (b) pore volume, and (c) specific surface area (
When incorporating the carbonization step by heating the biomass in argon prior to gasification, the results showed a significant increase in the solid yield and a slight decrease in the porous properties of the activated carbon from the gasification process by using potassium hydroxide and carbon dioxide (see G06 and G11 in Table
Effect of carbonization step on (a) pore volume and (b) specific surface area (
The highest surface area of activated carbons (2760 m2/g) was achievable from the gasification at a temperature of 700°C for 60 min and a chemical ratio of 4.0 with the introduced carbonization step (sample G16). This surface area is higher than that of commercial activated carbons (normally ranging from 800 to 1500 m2/g) and that reported by Geng et al. (2013) [
Figure
Effect of gasification conditions on (a) specific surface area, (b) hydrogen gas yield, (c) carbon monoxide gas yield, (d) heating value of gas, and (e) heating value of gas base on weight of coconut shell precursor.
As shown in Figure
An attempt was made to investigate the CO2 gasification of coconut shell biomass at low temperatures from 300 to 700°C using KOH as a catalyst. Increasing catalyst loading and temperature had a favorable effect on promoting the porous properties of activated carbon product. The incorporation of the carbonization step prior to gasification slightly decreases surface area and pore volume of resulting carbon products. Catalyst loading and gasifying temperature showed a significant influence on the yield, composition, and heating value of the syngas product. Carbon dioxide was found to be the main gas component for CO2 gasification without KOH. However, gasifying the coconut shell with both CO2 and KOH could enhance the generation of hydrogen and carbon monoxide and retard the formation of carbon dioxide. The heating value of gas products tended to increase with the amount of KOH catalyst and to increase with temperature in the range from 300 to 500°C. Gas yield decreased substantially when the carbonization step was included prior to gasification but the gas heating value was not greatly affected. The optimal condition for achieving high hydrogen composition (27.90 %weight), high heating value (41.4 MJ/kg of gas) of produced gas, and high specific surface area (2650 m2/g) of activated carbon product was to first carbonize coconut shell and gasify the resultant char at 600°C for 60 min using chemical weight ratio of 3.0. Finally, it might be worthwhile to continue this work on analyzing the economic feasibility of this low temperature catalytic gasification of biomass for large-scale applications.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was financially supported by The Ministry of Science and Technology of Thailand and The Advanced Low Carbon Technology Research and Development Program (ALCA) funded by Japan Science and Technology Agency.