Combustion, Fuel and Energy - Lab Report Example

Combustion, Fuel and Energy s per 1st Affiliation line of Affiliation dept. of organization line 2 of organization, acronyms acceptable line 3-City, Country line 4-e-mail address if desired Abstract-Several factors affect the process of combusting biomass including the kinetics of decomposition, biomass particle size, biomass moisture content, and metal content of the biomass. In this experiment, Pittsburgh coal was combusted, and the kinetics of the reaction analyzed using thermogravimetry. A linear graph was plotted from which the activation energy and the pre-exponential factor were determined. A TGA curve was also plotted and used to explain the losses in the mass of the coal sample during the combustion process. The activation energy of Pittsburgh coal was determined as 4225 and the pre-exponential factor as 1.293. The results obtained deviated from the theoretical values. I. OBJECTIVE To determine the rate constant, activation energy and the pre-exponential factor of coal using thermogravimetry. II. INTRODUCTION Generation of power and sustainable heat from biomass are at the center of scientific and industrial interests because of the increasing awareness of the reduction of fossil fuels and the higher appreciation towards environmental conservation from pollutants produced by energetic systems [1]. Short-cycle carbon that is essential for future energy needs is obtained from biomass [2]. Several factors are known to affect the process of biomass combustion including the kinetics of decomposition, biomass particle size, biomass moisture content and metal content of the biomass. A lot of research has been conducted on kinetic studies of biomass thermal decomposition because it is an important step in the combustion of biomass. Modeling of combustion processes and kinetics of reactors require the knowledge of the kinetics. The composition, quantities and release of volatiles from the biomass affect the stability, temperature profiles and flame ignition, stability, and temperature profiles in the radiant part of furnaces. The factors mentioned above influence the NOx reduction mechanisms and are, therefore, important in designing burners in pulverized fuel power stations. In the study of the kinetics of thermal decomposition, slow pyrolysis and first order reaction kinetics has been made. The basic equation used in all kinetic studies of biomass thermal decomposition is described as [2]: … (1) and … (2) Where α represents the progress of the reaction and its value ranges from 0 to 1, m is the sample current weight, mo is the sample initial weight, and k represents rate constant of the reaction. The rate constant is typically given by the Arrhenius function: … (3) Where E represents the activation energy, A represents the pre-exponential factor, R is the gas constant, and T is the temperature. If we relate equation 3 to equation 1, it yields equation 4: … (4) By setting initial conditions at α = 0, and t = 0 and integrating equation 1, A and E can be obtained and yield equation 5: … (5) Plotting a graph of –ln(1-α) versus t, gives k as the slope in isothermal experiment. A plot of the following relationship can help in obtaining A and E: … (6) Where the slope gives -E/R and the intercept gives ln A. The values obtained from the plot of ln k versus 1/T, and the quality of the data determines the accuracy of the kinetic parameters. The most widely used mathematical method in the derivation of activation energy and pre-exponential factor based on TGA experiments is the reaction rate constant method. Assuming the weight loss with time curve is as a result of one or more first order reactions, then each reaction can be described using the following relation: … (7) From equation 7, the values calculated for k rely on a chosen terminal mass, and can deviate significantly depending on the mass value chosen as terminal (mȹ). Again, evaluation of A and E is straightforward using the relationship expressed in equation 6 [3]. III. EXPERIMENTAL The data from a TGA run on Pittsburgh coal was assigned to us during the laboratory session. An Excel file provided had TGA parameters including the mass of the sample and associated time and temperatures. The given data was then manipulated as per the reaction rate constant method to determine the kinetics for the Pittsburgh coal. IV. RESULTS Fig. 1 to 3 show the various plots that were obtained from the Excel file with TGA parameters. Fig. 1: A graph of –In (1-α) vs. Time Fig. 2: ATG curve of Pittsburgh coal combustion Fig. 3: A graph of In k vs. 1/T A. Calculations Using equation 5, a graph of -ln (1-α) versus time gives k as the slope (fig. 1); therefore the numerical value of rate constant is -0.039. From equation 6: … (6) A graph of ln k vs. 1/T gives ln A as the intercept and –E/R as the slope. The slope = -508.2, therefore, E = -508.2 × 8.314 = 4225.175. Therefore, the activation energy of Pittsburgh coal obtained in this experiment = 4225.175 while the theoretically accepted activation energy is 142.317 KJ/mol [5]. The pre-exponential factor is obtained from the intercept = 1.293/s while the theoretically accepted value is 4.187/s for Pittsburgh coal. V. DISCUSSION From the TGA plot, it is evident that there is one main mass loss stage for combustion of Pittsburgh coal. This mass loss stage occurs at 380oC to 550oC (fig. 2). The rapid loss of mass between temperatures 380oC to 550oC is as a result of various physical and chemical processes. During combustion of coal, there is evaporation of moisture due to a rise in temperature that causes the mass of coal to decrease due to the low stability of most organic compounds. As the temperature increases, the organic components of coal decompose and generate volatiles that are swept by the flowing gas leading to a further loss in mass [4]. In the temperature range mentioned before, there is a 100% percentage loss of mass. Comparing the results obtained in this experiment and the theoretical values, the pre-exponential value obtained in this experiment is much smaller than the theoretical value and the activation energy value obtained is much higher than the theoretical one. There is a standard deviation of 2.046 between the pre-exponential factor value obtained in this experiment and the theoretical one. A number of factors related to the instrument and sample could have contributed to the experimental errors and influenced the form of the TG curve. The main factors that can contribute to experimental errors in TG curves are heating rate and sample size [6]. An increase in heating rate or sample size tends to increase the temperature at which the decomposition of the sample occurs, thereby decreasing the resolution between consecutive mass losses. The size of the sample material particles, the manner in which they are packed, the shape of the crucible, and the flow rate of the gas are other factors that can affect the reaction progress of coal combustion. The decrease in the concentration of oxygen concentration can lower the frequency of collision between the coal particle interface and oxygen reducing the combustion rate. Moreover, a further increase in oxygen concentration beyond 50-volume %, the combustion rate and the rate constant become limited due to finiteness of active sites on the surface of coal particle and the dissociation of the products of combustion. The activation energy and the concentration of oxygen concentration show a similar trend because a sufficient combustion reaction requires overcoming a higher energy barrier. A small coal particle size increases the number of active sites spread throughout the surface of coal particle. The distribution then decreases the process devolatilization resistance resulting in the increase of combustion rate and rate constant value, and the reduction of the activation energy value. Although, a small coal particle size (< 74 mm), has an insignificant contribution to the combustion efficiency of the fuel [6]. A decreased heating rate tends to decrease the temperature at which the decomposition process takes place thus increasing the resolution between consecutive mass losses. On the other hand, an increase in heating rate tends to increase the temperature at which the decomposition of the sample occurs thereby decreasing the resolution between consecutive mass losses. As the particle size decrease, the pore volume, and the specific area increases leading to the formation of more active sites to enhance the rate of combustion. Small pore volume generates ash bed and polycondensation structures that have high thermal stability on the coal surface after injecting the coal sample into the reactor. VI. CONCLUSION This experiment was conducted to study the mechanism of the reaction and kinetic characteristics of Pittsburgh coal combustion based on the emission of CO and CO2. Based on the data obtained from this experiment, the rate constant of Pittsburgh coal combustion was determined to be 0.039, the pre-exponential factor as 1.293 and the activation energy as 4225. The temperature of the reaction significantly affects the combustion reaction rate and the coal combustion mechanism. The mass losses occurred greatly as the temperature increased from 380oC to 550oC, indicating that the change in the combustion reaction mechanism occured in this temperature range [6]. A decrease in the particle size of a coal sample can increase the number of available active sites spread throughout the surface of the coal particle thereby decreasing the resistance of devolatilization process. The result of this is an increase in the rate of combustion and the rate constant as well as a decrease in the activation energy. REFERENCES [1] D. Colomba, "Modelling chemical and physical processes of wood and biomass pyrolysis," Progress in Energy and Combustion Science, Vol. 34, no. 1, pp. 47-90, 2008. [2] P. Matheus, A. J. Zattera, and R. M. C. Santana "Thermal decomposition of wood: kinetics and degradation mechanisms," Bioresource technology, Vol. 126, pp. 7-12, 2012. [3] Saddawi, Abha. The role of alkali metals in biomass thermochemical conversion. Diss. University of Leeds, 2011. [4] C. Haixiang, W. Zhao and N. Liu, "Thermal analysis and decomposition kinetics of Chinese forest peat under nitrogen and air atmospheres," Energy & Fuels, Vol. 25, no.2, pp. 797-803, 2011. [5] S. IWi, "The combustion rates of coal chars: a review." Symposium (International) on combustion, Vol. 19. no. 1, 1982. [6] L. Zhengang, Z. Guo, X. Gong and H. Tang, "Kinetic characteristics of pulverized coal combustion in the two-phase flow," Energy, Vol. 55, pp. 585-593, 2013. Read More