Conversion of Carbon Dioxide and Biomass for Fuels and Chemicals Precursor through Gasification: Experimental and Modeling Approach

Abstract

Recycling carbon dioxide is of prime importance due to the harmful effects on the atmosphere. Using CO2 for gasification serves a dual purpose i.e. reducing pollution and generating syngas and is beneficial both environmentally and economically. Therefore, this research work focuses on exploring the application of carbon dioxide as a gasifying agent in gasification of pine. Chapter 1 provides an introduction to the importance of this process, motivation behind the study and a sneak peek into the objectives of the research. Chapter 2 is a detailed review of literature around this topic. Chapter 3 comprises of the work done towards the first objective: understanding the effects of temperature variation and change in CO2/C ratios on the biomass–CO2 gasification process. An extension of the objective was to analyze the syngas component evolution trends and postulate the reactions dominating the process. The process was carried out in a bench scale atmospheric bubbling fluidized bed gasifier. The effect of change in process variables was evaluated in terms of (i) yield of product streams [char, liquid and gas], (ii) syngas composition, (iii) carbon balance and, (iv) syngas heating value. Water gas reaction, Boudouard reaction and methane dry reforming reactions were hypothesized to be the main reason leading to the trends of the components. Change in CO2/C ratio had a significant effect on the yields of the output syngas components. The results obtained from this work were compared in detail to an oxygen gasification study from literature performed on the same experimental setup. The second objective of the research is outlined in Chapter 4 which focuses on the interpretation of the kinetics of char–CO2 gasification step. The process was carried out employing a fixed bed reactor setup with gas analyzer. The conversion–time data were fitted using three empirical solid–gas reaction models viz. volume reaction model (VRM); non-reactive core model (NRC) and random pore model (RPM). The kinetic parameters were calculated for all the three models. Regression analysis and surface area change analysis demonstrated that RPM was the best at predicting experimental data. The values of the kinetic parameters calculated using RPM are: activation energy (Ea) = 219 kJ/mol and natural logarithm of frequency factor (ln A) = 5.13 1/s. The second part of this objective was to understand the effect of alkali and alkaline earth metals (sodium, magnesium, potassium and calcium) on the char-CO2 gasification step. Chars impregnated with four different metal catalysts were gasified and the random pore model was used to calculate the activation energy for each catalyst. The reactivity of the loaded chars was found to be in the order of: K-char > Ca –char > Na –char ≥ Mg –char. Chapter 5 deals with the modeling part of the research. An ASPEN plus model was developed to simulate the steady state performance of the fluidized bed gasification unit used in the experiments. Knowledge accrued from the experimental objectives was used to improve the performance of the model. Different reactors were used to simulate various steps in the gasifier and a FORTRAN subroutine was dynamically linked to include the kinetics. Parameters such as output syngas composition, individual gas yields, syngas higher heating value and carbon conversion are compared to evaluate the validity of the model. The results were then compared to experimental data and a preliminary two stage model using Gibbs free energy minimization. The comparison showed that the model including kinetics predicted the experimental data with much better accuracy as compared to the thermodynamic equilibrium model.