Carbon Recovery in A Dual Fluidized Bed Gasifier

Licentiatavhandling, 2020

As the concept of a circular economy gains acceptance and awareness of climate change and its disastrous consequences increases, the ways in which we produce the carbon-based goods upon which so much of our economy depends need to change. Society must end its reliance on fossil carbon resources and shift to renewable sources that enable the establishment of a carbon cycle. Plastic and biogenic wastes should become our main sources of carbon for new materials. This would simultaneously address the critical issues related to the current deficit of recycling and robust collection systems, which currently entails immense loss of value and represents a threat to the environment. The biomass waste would also compensate for inevitable leakage from the carbon cycle and enable overall-negative CO2 emissions.

Technologies are needed that are capable of extracting efficiently the carbon from these new sources. This will be challenging given the expected heterogeneous nature of such sources. In this context, the Dual Fluidized Bed (DFB) gasification process is an attractive technology owing to its flexibility. This thesis investigates the possibility for carbon recovery in DFB gasification processes. The various configurations in which a DFB gasification unit can be designed and operated are discussed, with respect to their carbon balances. In addition, as the valorization of waste materials is a critical aspect of the circular economy, the use of such materials in DFB processes, beyond the consideration of carbon extraction, is considered. These materials may possess properties that can enhance the carbon recovery of the existing process or around which an entire process can be devised.

To illustrate the challenges related to optimizing carbon recovery in DFB gasification processes, and the possible uses and impacts of waste materials in those processes, experiments were carried out in the Chalmers University of Technology DFB gasifier. The first set of experiments was aimed at increasing carbon recovery in the form of valuable products in a regular DFB gasifier by increasing the catalytic activity of the bed towards the reforming of non-valuable compounds, in this case tar. This was achieved by circulating the flue gas ash, which is a waste material produced by the process, back into the system. The second set of experiments was designed to investigate the feasibility and carbon balance of Chemical-Looping Gasification (CLG), which is a DFB gasification technology whereby an oxygen carrier produces the heat needed for the gasification reaction. The CLG technology, which potentially enables carbon recovery into a single, undiluted stream, was explored for two cases. The first case involved CLG of biomass using a waste from the steel-making industry as the oxygen carrier. The second case entailed CLG of Automotive Shredder Residue (ASR), a plastic-containing waste the high ash content of which leads to the formation of an oxygen-carrying bed material.

The results presented in this thesis reveal that the fate of the CO2 in the raw gas is a critical issue, both in regular DFB gasification with an active bed and in CLG. As the activity of the bed material increases, as achieved by circulating the flue gas ash, the carbon is transferred from both non-valuable and valuable products to CO2 in the raw gas, owing to the action of the water-gas shift. In the case of CLG, the oxygen transport from the bed material results in significant oxidation of H2 and CO, predominantly, thereby transferring carbon to CO2, which becomes the main carbon output from the process. However, the oxygen transport is also identified as the key parameter for solving a crucial issue in CLG, i.e., the achievement of complete conversion of the fuel in the fuel reactor. The gasification of ASR in the Chalmers gasifier led to an oxygen-carrying bed, which may facilitate operation in CLG mode, provided that the oxygen transport level is increased. The possibilities to achieve this are discussed in this thesis.

In terms of the selection and operation of DFB gasification processes, the results of this work have the following implications: (i) for regular DFB gasification, the catalytic activity and temperature levels should be carefully selected, based on the comparison of the energy cost of separation of the CO2 from the raw gas on the one hand, and the destruction or valorization of non-valuable products on the other hand; and (ii) for CLG, the process should be designed to maximize the conversion of the fuel in the fuel reactor, while minimizing the oxidation of the raw gas. The viability of CLG in terms of producing simultaneously chemical precursors and CO2 that is ready for sequestration should be assessed by comparing with the alternative process, i.e., DFB gasification with oxyfuel combustion.