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Methane-concentrated oxy-fuel calciner for calcium-looping

University of British Columbia

Greenhouse gas emissions (mostly CO₂) have resulted from massive fuel consumption over recent decades, with devastating effects on humans, climate, and wildlife. Cost-effective environmentally-friendly energy sources and carbon capture are required to diminish the destructive effects of CO₂ emissions. Calcium-looping, a process based on reversible solid-gas carbonation and calcination, utilizing lime-based sorbents to capture CO₂ at elevated temperatures, is an emerging carbon capture technology, also applicable for enhanced hydrogen production. A key challenge in this continuous process is the high temperature needed for cyclical sorbent regeneration (via limestone calcination). This adversely affects the thermal/energy efficiency of the process, while also leading to sorbent deactivation during first calcination-carbonation cycles. Investigations are required to enhance current knowledge on limestone calcination conditions in calcium-looping, while also identifying alternative low-temperature technologies for sorbent regeneration. This thesis proposes a novel methane-concentrated oxy-fuel calciner, combining methane combustion, reforming and limestone calcination in a single reactor. The process is shown to be capable of autothermal syngas-producing sorbent regeneration with in situ CO₂ utilization, reducing the CO₂ concentration within the reactor, thereby decreasing the calcination temperature. The thermodynamic and kinetic performances of the process are evaluated by means of reactor simulations. Appropriate ranges of conditions are determined for autothermal, coke-free and complete limestone calcination. Increasing temperature and nitrogen concentration in air are shown to enhance limestone calcination, whereas elevating pressure and CaCO₃/gas feed ratio hinder sorbent conversion. A design methodology is suggested to determine appropriate operating conditions and/or reactor dimensions for this sorbent regeneration technology. Potential practical constraints of the process (e.g. safe operation and catalyst instability) are also briefly discussed. The thesis examines three potential applications of the process: sorbent-enhanced steam methane reforming, ammonia production without air separation, and Ca(OH)₂/CaCO₃ co-calcination. Thermogravimetric analysis is employed to assess the effect of sorbent regeneration conditions (especially partial calcination) on the cyclic CO2 capture capability of lime-based sorbents. Increasing calcination temperature is shown to reduce sorbent reactivity, while extending calcination duration and exposing limestone to high temperature without reaction did not appreciably change sorbent performance. Partially calcined sorbents are found to offer smoother CO₂ uptake over extended calcination-carbonation cycles.

Text

2020-08-14

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/75512

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/