Session: 09-06-01: Decarbonization with Hybrid Energy Systems

Paper Number: 166055

CFD Analysis of Steam and CO2 Gasification of Biochar in a Downdraft Reactor 

The consumption of fossil fuels is increasing exponentially due to rapid population growth and industrialization. This economic growth has led to the emission of carbon dioxide, which is the primary constituent of greenhouse gas emissions. To mitigate greenhouse gas emissions, carbon capture has been recommended to implement in different applications. The captured CO₂ could be employed in the gasification process as a feedstock to generate fuels, chemicals, and energy. CO₂ gasification of biomass can be an alternative to fossil fuels and aid in minimizing CO₂ emissions. However, utilizing biomass as a feedstock for the gasification process generates tar in syngas, and its removal through a cleaning system is expensive. We recommend biochar for the CO₂ gasification process due to its low volatile content, which could reduce the tar generation and its associated cleaning cost. Thermochemical conversion processes such as pyrolysis, hydrothermal carbonization, and gasification typically produce biochar as a carbon-dense byproduct. This study utilized commercial Ansys Fluent software to model CO₂ gasification of biochar. The model utilized a three-dimensional downdraft gasifier. It was a cylindrical-shaped reactor with a diameter of 2.54 cm and a length of 107 cm. The CO₂ was supplied from the top of the reactor, and biochar was injected from the reactor zone, which is located 66 cm from the top of the reactor. The inlet and outlet ports were one-fourth the diameter of the reactor. Species transport was used for the chemical kinetics. The k-epsilon model and p-1 model were used to model the turbulence and radiation, respectively. To feed the biochar in the gasifier, discrete phase model (DPM) was implemented. The errors between experiment and simulation work range from 1% to 15%. A mesh sensitivity test was also performed to ensure optimized mesh selection. The concentration of CO₂ through the inlet varied from 10% to 90% (N₂ balance) and two flow rates of CO₂ were used: 3 L/min and 5 L/min. CO₂ gasification is dependent upon the Boudouard reaction which is favorable at elevated temperatures. The temperatures were changed from 700ºC to 900ºC. For the constant CO₂ concentration, as the temperature increased, the mole fraction of CO increased in the outlet because the higher temperatures favored the Boudouard reaction. The conversion of CO₂ to CO reduces as the CO₂ concentration increases. The optimal conditions for maximum CO production are examined through this model. This paper will present the CFD modeling results of CO₂ gasification of biochar and its validation using experimental data.

Presenting Author: Md Mahmudul Hasan Georgia Southern University

Presenting Author Biography: Currently, employed as a Graduate Research Assistant on a project at Georgia Southern University which is
funded by the National Science Foundation (NSF). Served as an Assistant Engineer for two years at
the Bangladesh Power Development Board, the largest entity in the country's power sector. Proficient
with applications such as Ansys and SolidWorks. Possess robust oral and written communication
skills. Concentrated on objectives and proficient in time management.

Authors:

Md Mahmudul Hasan Georgia Southern University
Prakash Bhoi Georgia Southern University