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A comparative study on pyrolysis characteristic Indonesia biomassa and low grade coal

A comparative study on pyrolysis of biomass and low grade coal was conducted using a thermogravimetric analyzer. Each kind of biomass and coal has a characteristic pyrolysis behavior which is explained based on its individual component characteristics. All fuels experienced a small weight loss as temperatures approached 450K because of moisture evaporation. The coal had smallest total weight loss compared to biomass due to its high content of fixed carbon, suggesting that coal would produce high amounts of char and small amounts of volatile matter (e.g., tar and gas). The biomass exhibits similar tendency regarding the decomposition process which is the hemicelluloses break down first at temperatures of 470 to 530K, cellulose follows in the temperature range 510 to 620K, and lignin is the last component to pyrolyzer at temperatures of 550 to 770K. The thermal decomposition of biomass consisted of two predominant peaks corresponding first to the decomposition of cellulose and, second, to the decomposition of lignin. Meanwhile, the coal exhibited only single peak because these fuels were predominantly composed of carbon. Based on the kinetic analysis, coal have the smaller activation energy (55.32kJ/mol) compared to biomass (range from 89.80–172.86 kJ/mol). Pyrolysis process also created more pore material in the solid product. These results were important for the optimization of energy conversion from those solid fuels. Biomass resulted lower solid product and higher tar product, thus would be suitable for liquid and gas energy production.


Introduction
Indonesia produce 147.7 million tons of biomass per year, equivalent to about 470 GJ/year. Biomass is a renewable energy source [1]. Indonesian biomass spread all over the country such as rice residue, wood, and oil palm residue. Biomass is a solution to substitute the excessive demand of nonrenewable energy like fossil fuel. Burning fossil fuels release large amount of carbon dioxide into atmosphere causing environmental problem. Therefore, if biomass is used as an alternative fuel it will reduce the consumption of fossil fuel and also reduce the environmental pollution of greenhouse [2,3].
The energy source within the biomass can be obtained either through biological or thermochemical conversion. Biological conversion involves the usage of microorganisms such as anaerobic digestion and fermentation. Thermochemical conversion involves the application of heat and chemicals in the production of energy. Thermochemical conversion can be divided into combustion, gasification and pyrolysis. Among the thermochemical conversion, pyrolysis is considered the most efficient process due to its high feed to fuel ratio in comparison to combustion and gasification process [4].
Biomass pyrolysis can be converted into bio-fuels, adsorbent bio-char, and other useful chemicals. Kinetic is very important to understand reaction mechanism, predict the conversion and design reactor TGA measure the amount and rate of change in the weight of tested material as a function of temperature or time in atmospheric pressure. Various weight loss processes determined during the TGA reflect the physical and chemical structure changes during the conversion. Differential thermogravimetry (DTG) shows thermal behavior of material and also the optimum operating condition of the material pyrolysis. The kinetics of these thermal events has been determined by the application of the Arrhenius equation corresponding to the separate slopes of constant mass degradation [5,6].

Material
The biomass residues of rice huck and ulin wood were obtained from agricultural residue in Bantul Yogyakarta while low grade coal was obtained from Kalimantan. The proximate analysis of the samples was shown in table 1. To avoid the effect of particle size, the sample were crushed and sieved to a granular size.

Thermogravimetric analysis (TGA)
The pyrolysis characteristics of the samples were examined using a thermogravimetric analyzer (TG\DTA). Sample was placed inside the TG where the weight was constantly measured. The functions of the TG were to measure and record the dynamics of sample weight loss with increasing temperature or time. Sample use for TG analysis is 11.788 mg for palm kernel shell, 8 to completely remove moisture and provide a basis for analysis. Nitrogen was used as the carrier gas in the TG, so that the sample was paralyzed in an inert environment without oxygen.

Pyrolysis Kinetics
The non-isothermal kinetics for solid decomposition is usually written as follows: where X is fuel conversion, and is given In the above equation, W, Wi , and Wf represent the instantaneous, initial (at 105 o C), and final (at 800 o C) weights of the sample. The reaction rate constant k is expressed in terms of the Arrhenius equation as and the function f(X) can be written as Substituting Eqs. (3) and (4) into Eq. (1) gives For a constant heating rate = , Eq.(5) can be rearranged to the following equation The integral method based on Coats and Redfern (CR) equation [7,8,9] is used in this work, and the approximate integration of Eq. (6) gives or [ For most reactions, the value of   2 ] versus 1 is also a linear line for ≠ 1. Accordingly, the apparent activation energy (Ea) and the apparent frequency factor (A) can be determined from the slope and intercept of the regression line, respectively  Table 2 show the result of kinetic parameter obtained from the Arrhenius method for rice husk, palm kernel shell, ulin wood, and coal. All the data obtained at different value of n are fitted and the best-fit regression line that has highest value of correlation R 2 was shown in each figure. Coal has pyrolysis range from 594 -779 o C activation energy 55.32 kJ/mol A = 610 R and the best-fitted regression n = 2.6 with correlation coefficient R= 0.99. Palm kernel shell the decomposition divided into two stage with activation energy value for the first region is 172.86 kJ/mol second stage 164.23 A for first stage and second stage is 5275810276075090.00 and 141694773043700.00 best-fitted regression n = 7 for first stage and n = 4 for the second stage which give coefficient correlation (R) 0.99 and 0.97. Rice husk activation energy for first and second stage is 89.80 and 108.5226 A for first and second stage 11963597.96 and 688935605.9 and the best-fitted regression n = 2.2 and 2.5 which give coefficient correlation 0.99 and 0.97. Ulin wood activation energy for first and second stage is 85.42635 and 135.0692 A for first and second stage 3745831.323 and 88315997175 and the best-fitted regression n = 3 and 4 which give coefficient correlation 0.98 and 0.97. Coal give least energy activation and ulin wood give most value of activation energy. It mean that coal is still a material which easiest to burn. Calculation using Arrhenius method shown that value of activation energy for ulin wood in first stage is the least among the other biomass.    Figure 5 shows the comparison of SEM (Scanning Electron Microscopic) image between raw ulin wood residues before pyrolysis process and char products after pyrolysis process. It is seen that raw ulin wood there are no visible pores, while char products has visible pores. (a) (b) Figure 5. The effect of the pyrolysis process on the SEM image of the ulin wood.

Conclusion
Based on TG-DTG analyses found that the biomass pyrolysis process occurs in two stages, the decomposition of hemicellulose and lignin decomposition while coal pyrolysis occurs only in one stage. This happens due to differences between the composition of biomass and coal. As a whole, coal have the smaller activation energy (55.32kJ/mol) compared to biomass (range from 89.80-172.86 kJ/mol).