Ashish Chaurasia

Modeling for pyrolysis of solid particle: kinetics and heat transfer effects

Abstract In the present study, a mathematical model to describe the pyrolysis of a single solid particle of biomass is developed by incorporating improvements in the existing model reported in literature. It couples the heat transfer equation with the chemical kinetics equations. The pyrolysis rate has been simulated by a kinetic scheme involving three reactions (primary and secondary): two parallel reactions and a third for the secondary interactions between the volatile and gaseous products and the char. The dependence of convective heat transfer coefficient on Reynolds number and Prandtl number is incorporated in the model. A finite difference method using a pure implicit scheme is used for solving the heat transfer equation and the Runge–Kutta 4th order method for the chemical kinetics equations. The model equation is solved for cylindrical pellets, spheres and slab geometries of equivalent radius ranging from 0.00025 to 0.013 m and temperature ranging from 303 to 1000 K. The simulated results obtained using the present model are in excellent agreement with the experimental data, much better than the agreement with the earlier models reported in the literature.

Modeling, simulation and estimation of optimum parameters in pyrolysis of biomass

Abstract Pyrolysis is a process by which a biomass feedstock is thermally degraded in the absence of air/oxygen. It is used for the production of solid (charcoal), liquid (tar and other organics) and gaseous products. The present work involves the estimation of optimum parameters in the pyrolysis of biomass for both non-isothermal and isothermal conditions. The modeling equations are solved numerically using the fourth order Runge–Kutta method over a wide range of heating rates (25–360 K/s) and temperatures (773–1773 K). The simulated results are compared with those reported in the literature and found to be in good agreement qualitatively in the range of operating conditions covered, but some very interesting trends are found, especially with respect to the effect of net heating rate and temperature on final pyrolysis time. The final pyrolysis time first decreases at lower values of net heating rate or temperature and then increases as net heating rate or temperature is further increased, providing an optimum value of net heating rate or temperature at which final pyrolysis time is minimum. This interesting phenomenon, which was not reported by investigators earlier, is well explained by means of the pyrolysis kinetics.

Parametric study of thermal and thermodynamic properties on pyrolysis of biomass in thermally thick regime

Abstract In the present study, a simultaneous chemical kinetics and heat transfer model is used to predict the effects of the most important thermal and thermodynamic properties (thermal conductivity, heat transfer coefficient, emissivity and heat of reaction number) of the feedstock on the convective-radiant pyrolysis of biomass fuels. A finite difference pure implicit scheme utilizing the tri-diagonal matrix algorithm is employed for solving the heat transfer model equation. The Runge–Kutta fourth-order method is used for the chemical kinetics model equations. Simulations are performed considering cylindrical pellets of equivalent radius ranging from 0.003 to 0.011 m and temperatures ranging from 303 to 900 K. For conversion in the thermally thick regime (intra-particle heat transfer control), it is found that variations in the properties mainly affect the activity of the primary reactions. Sensitivity analysis is conducted to find the most dominant properties affecting the pyrolysis and found that the highest sensitivity is associated with the emissivity and thermal conductivity of the biomass. Applications of these findings in reactor design and operation are discussed. The results obtained using the improved models are in excellent agreement with the experimental data, much better than the agreement with the earlier models reported in the literature.

Kinetics of Devolatilization of Black Liquor Droplets in Chemical Recovery Boilers - Pyrolysis of Dry Black Liquor Solids

Black liquor is a by-product of the pulping process in the manufacture of paper. It is a complex mixture of both organic and inorganic chemicals with both chemical value and energy content. To recover these two quantities, the black liquor is concentrated to around 60 - 85 % solids concentration and fired into a recovery boiler. In the boiler, the black liquor is sprayed in the form of droplets which fall through the atmosphere of the furnace to the bottom. During this process, the droplet undergoes drying, devolatilization and char burning in succession before coalescing with the smelt at the bottom. The modeling of these processes during the movement of the black liquor droplet is quite complex and in this paper, we attempt to simulate the weight loss during the devolatilization stage by studying the pyrolysis of dry black liquor solids in a muffle furnace. Experiments were carried out at temperatures varying from 973 to 1273 K and for residence times varying from 30 to 180 s. The kinetic scheme of dry black liquor solids decomposition by a single reaction giving gaseous volatiles and char is used. The proposed single reaction model is simulated and the best values of the kinetic parameters i.e. activation energy and Arrhenius constant are found by using the two-dimensional surface fitting non-linear regression algorithm. The results indicate good agreement between predicted and experimental data.

Gasification of rice husk in two-stage gasifier to produce syngas, silica and activated carbon

ABSTRACT Rice husk was utilized in the production of syngas, silica and activated carbon. Experiments were performed in two-stage gasifier for the production of syngas. The syngas is generated with minimum tar yields due cracking of tar at high temperature. Rice husk char obtained from the pyrolysis stage of the reactor was used in the silica extraction process to obtain silica and activated carbon. Using nitrogen as pyrolysis agent high purity (88.46%) silica was obtained with a good quality of syngas as compared to air as pyrolysis agent. Highest surface area of 276.91 m2/gm of silica was found at 500ᵒC.