Kinetic Study of Cotton Stalk and Rice Husk Samples under an Inert and Oxy Combustion Atmospheres

Biomass materials such as CS (Cotton Stalks) and RH (Rice Husk) are a renewable source of energy. As biomass resources have potential to offer a uninterpted supply of biofuels through thermal decomposition processes. Nevertheless, an appropriate understanding of reaction kinetics and thermal properties of biomasses play a vital role in designing of the commercial plants using biomass as a raw material for energy generation. The kinetic characteristics of the CS and RH samples under pure nitrogen 99.9% and pure oxygen 99.9% was performed. The temperature was raised from ambient to 900 o C maintaining the heating rate of 10 o C/min. CS and RH decomposition was noticed in three dissimilar regions. The kinetic characteristics such as (pre-exponential factor, the order of reaction and activation energy) were calculated for both selected materials. The activation energies calculated under nitrogen environment for CS and RH was 68.77 and 72.31 kJ/mole, whereas the regression coefficient (R 2 ) was 0.9877 and 0.9731 respectively. The activation energies under oxygen environment were higher, it was 106 and 118 kJ/mole. The regression coefficient (R 2 ) under oxygen environment was 0.9987 and 0.99883 for above sample sequence.


INTRODUCTION
F ossil fuels consumption and their ever growing demand cause the increase in GHG (Greenhouse Gas) emissions [1]. The application of biomass for clean and sustainable energy in substantial quantities from agricultural residues is considered as one of the best substitute for fossil fuels worldwide. In addition, fossil fuel deposits take millions of years to accumulate, makers aim to get energy from biomass including agricultural wastes. Agricultural residues include CS, RH, corn straw, wheat straw, and wheat husks, among these sources, CS and RH are least used resources in developing countries [5][6][7][8]. Agricultural residues such as CS, RH corn cobs, sugarcane bagasse and wheat straw are a viable source for bioenergy production. The copious quantities of agricultural wastes are underutilized resources in developing countries. These resources of renewable energy are used either as an animal feed or as a raw material for paper industry. The excessive agricultural residues are burned in the fields or left untouched as a waste material, thereby causing substantial environmental hazards. The conversion methods for biomass may be physical, chemical, biological or thermal to give a solid liquid or gaseous products. Among all conversion methods, the thermochemical conversion method seems to be one of the most promising conversion methods for bioenergy production.
Nevertheless, the development of economically and technically viable method for the conversion of the CS and RH residues into higher value fuels through thermochemical conversion processes involves an understanding of its thermal characteristics and reaction kinetics. The thermochemical conversion of agricultural residues is strongly influenced by their chemical composition such as cellulose, hemicellulose and lignin contents [9]. The above agricultural residues can be used to generate energy through direct combustion, which is a less efficient method of energy recovery from residues.
The most efficient method to obtain energy from biomass residues is pyrolysis, as through pyrolysis process various energy products such as solid, liquid and gaseous are obtained. Hence for valid kinetic data regarding the thermochemical conversion of RH and CS, it is important to perform comprehensive TGA (Thermogravimetric Analysis) investigation on both selected materials.
Therefore, in the present study, TGA techniques were applied to investigate the thermal degradation behavior and kinetic characteristics of RH and CS residues in aninert (N 2 ) and oxidative (O 2 ) atmospheres.
This analysis was carried out in order to understand the thermal degradation parameters and reaction kinetics of CS and RH samples. The aims of this study include: To perform TGAon the CS and RH samples at the heating rates of 10 o C/min in an inert (N 2 ) and oxidative (O 2 ) environments.
To determine the initial degradation temperatures, thermal degradation rates and residual weight of RH and CS samples at 900 o C.
To estimate the kinetic constraints (such as preexponential factor, apparent activation energy and order of reaction) by means of TGA data.

Elemental Analysis of Waste Agricultural Biomass
The analysis of carbon, hydrogen, nitrogen and sulfur elements in WAB was conducted through the standard procedure according to ASTM D3176 method [9]. The percentages of Sulphur, nitrogen, hydrogen and carbon were calculated by using vario MAX elementary analyzer.
The percentage of oxygen content was determined by subtracting the percentages of hydrogen, sulphur, nitrogen, carbon and ash from hundred percent. The results of CHNS analysis are given in Table 1.

Kinetic Study under (N 2 ) Environment
The thermal behavior of CS and RH samples at a heating   Fig. 3. During the pyrolysis of samples, the two different areas of DTG curve were observed. The first region appeared with a more or less prominent shoulder at nearly 197 and 220 o C for the sample sequence.
Whereas the subsequent second region was followed by the peaks maxima at 260 and 285 o C for above sample sequence. The appearance of the shoulder in DTG curve may be due to the degradation of the hemicellulose and lignin. Whereas the generated DTG peak relates to the degradation of cellulose and lignin. However, when cellulose and hemicellulose degradation overlap, at that point the shoulder generated marks the peak of the hemicellulose degradation [11]. Next, to the end of the second degradation stage, a continuous slow degradation of selected biomasses was observed that may correspond to the slow decomposition of lignin. proportional to the value of its corresponding temperature TDTG max .The reactivity of samples was calculated by the method adopted in previous studied performed by [13,17,27].

Kinetic Study of Cotton Stalk and Rice Husk Samples under an Inert and Oxy Combustion Atmospheres
The RM (Reactivity Values) of CS is higher when compared with RH samples under oxy combustion atmosphere. Table 3 shows reactivity of CS and RH along with the reactivity values calculated by different authors.
Combustion characteristics such as initial ignition temperatures, burn out temperatures, TGA and DTG curve are generally used to measure relative combustion characteristics of the biomasses [28]. As CS sample showed lower ignition temperature when compared with RH sample ignition temperature. This study is in agreement with the study performed on biomass and coal samples by [29]. Moreover, an ignition index (D) and combustion index (S) of the samples were calculated from the following equations [29].

Kinetic Study of Cotton Stalk and Rice Husk Samples under an Inert and Oxy Combustion Atmospheres
Here, t max shows a maximum rate of combustion t i represents ignition temperature. R max is the maximum reduction in mass. R a represents the maximum reduction in mass under oxygen environment. The results of both samples are given in Table 4. It is obvious from the results that with the rise in ignition index (D) value, the ignition performance of biomass materials increases.
Besides that, an enhanced burning reactivity may be given to the material showing the higher value of combustion index (S). The CS is more reactive material than RH. The apparent activation energy and order of reaction is shown in Table 5.

Kinetics of Waste Agricultural Biomass
The kinetic investigation has been done by using thermal analysis techniques in order to calculate the kinetic characteristics of biomass [30]. Numerous methods and approaches have been adopted to investigate the thermal behavior of different biomass materials using TGA. In    (6) Where dm/dt displays the ratio of change in mass with respect to change in time, R represents the universal gas constant, A is the pre-exponential factor. Where Equation (7) lead to linear form written as:

CONCLUSION
The kinetic study of CS and RH revealed that in the first zone, reduction in mass was because of the moisture evaporation and volatile release observed in the temperature 22