Strategies for Complete Recovery of Carbon in Dual Fluidized Bed Gasifiers

Doktorsavhandling, 2021

To establish a circular economy and curtail our dependency on fossil resources, technologies are needed to extract carbon from biomass and plastic waste. Dual fluidized bed (DFB) gasification is a carbon-extracting technology that offers flexibility in terms of its inputs, outputs, design, and operational conditions. This thesis investigates the carbon distribution produced by DFB gasification and explores the possibilities to achieve total carbon recovery. The various configurations under which DFB gasification can be designed and operated to facilitate the total recovery of carbon are compared on a theoretical basis. Thus, insights into the carbon distribution and energy demands of each configuration are obtained. A method to increase the catalytic activity of the bed in the DFB gasifier so as to enhance the recovery of valuable forms of carbon is experimentally demonstrated, based on the use of a waste generated from the process. However, it is shown that increasing the catalytic activity is not always beneficial for carbon recovery. The development of oxygen transport along with the catalytic activity, a phenomenon that had been reported but never investigated, is here demonstrated. Taking advantage of the oxygen transport properties of certain bed materials to facilitate the total recovery of carbon is the basis for the chemical-looping gasification (CLG) technology, a DFB gasification configuration. The parameters that affect fuel conversion, an essential aspect of CLG, are investigated for a plastic waste that generates its own oxygen-carrying bed material. Oxygen transport is shown to be the most important parameter for the process. Based on these experiments, the numerous challenges associated with CLG are discussed. Finally, a process through which negative-emissions steel is produced, based on the integration of DFB gasification into a CLG configuration with direct reduction (DR) of iron, is proposed and evaluated. Compared with the traditional steelmaking route and alternative DR routes, the proposed process is found to be the most-competitive for carbon prices >60 €/tCO2, which corresponds to both the price for CO2 emissions and the revenue associated with negative emissions. Most of the data used in this work were obtained from experiments conducted at a scale relevant to the industry.