Date of Award

8-16-2024

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Kevin Huang

Abstract

Greenhouse gases, especially CO2 emissions, are responsible for global warming and the reduction of greenhouse gases is vitally important for the sustainability of our modern society. Currently, the most mature CO2 capture, utilization and storage (CCUS) technology relies on flue gas “amine washing”, a costly and energy intense process that can negatively impact on the efficiency of existing power plants and the cost of electricity produced. Therefore, developing more efficient and cost-effective CCUS technologies is highly imperative. With the increasing production of natural gas in recent years, direct natural gas conversion into valuable chemicals also becomes more attractive than simple combustion for heat and power, which adds burdens to the current CCUS effort. By converting natural gas to valuable products will make a full utilization of natural gas and delay the CO2 emission. Hence, how to convert abundant and inexpensive natural gas into value-added products has attracted significant interest from both academia and industry. Motivated by the concerns over climate change, two types of high-temperature dual- phase membranes that can capture CO2 at point sources and/or convert fossil fuels (e.g. CH4 and C3H8) into valuable chemicals with captured CO2/O2 as a mild oxidizer were proposed. Additionally, CaO-based sorbents have also been explored for permanent CO2 capture and storage purposes, offering an alternative solution for mitigating greenhouse gas emissions.

In the first part of this work, a new dual-phase CO2 capture membrane comprising of a porous proton conducting BaZr0.8Y0.2O,sub>3-d,/sub> (BZY) matrix and molten carbonate (MC) was firstly synthesized. Through the microstructure optimization, the permeated CO2 flux of BZY-MC membrane of 0.8 mm thick reaches 0.34 mL×cm-2×min-1 at 650 oC. The high flux is thought to arise from the synergistic effect of modified microstructure, MC loading and high bulk oxide-ion conductivity of porous BZY matrix. The activation energy of CO2 permeation of BZY-MC membranes is close to that of oxide-ion migration in the BZY bulk, suggesting the rate-limiting nature of oxide-ion conduction in BZY bulk for CO2 transport in the BZY-MC membrane. It is also demonstrated that introducing steam into the sweeping gas can significantly enhance CO2 permeation, which can be ascribed to the formation of new charge balance species (OH-) in both of BZY and MC phases. With 3%H2O addition into sweeping gas, the membrane exhibits 30% CO2 flux density enhancement and good stability over 250 h at 650 oC.

In the second part of this work, a disk-type CO2/O2 co-transport membrane based on Sm-doped CeO2 (SDC)-NiO-MC is demonstrated using a mockup flue gas (75% N2 + 15%CO2+ 10% O2) as the feed gas and Ar as the sweep gas. At 850 oC, the membrane exhibits a high CO2 and O2 flux density of 1.16 and 0.48 mL×cm-2×min-1 , respectively. By switching the sweep gas to CH4-Ar and incorporating an OCM-specific catalyst (2%Mn–5%Na2WO4/SiO2), the same membrane reactor produces C2 products (C2H6 and C2H4). The results show that the co-captured CO2/O2 mixture converts CH4 into C2H6 in the presence of catalyst, followed by thermal cracking of C2H6 into C2H4 and H2. The presence of CO2 decreases the local partial pressure of O2, thus reducing the propensity of C2 products re-oxidation and leading to higher C2 selectivity. At 2.5% CH4, the reactor achieves >20% CH4 conversion, ~57% C2 selectivity, resulting ~12% C2 yield. The long-term test of the membrane reactor shows a stable performance for ~100 h at 825 oC.

In the third part of this work, a tubular plug-flow membrane reactor based on silver (Ag)-MC for CO2/O2 capture from flue gas and instant conversion of propane to propylene over CrOx/SiO2 catalyst bed was developed. The tubular plug-flow design of the membrane reactor enables a gradual, continuous, and controlled addition of CO2/O2 (in 2:1 mole ratio) into C3H8 stream along the axial length of the membrane, thus striking an excellent balance between conversion and selectivity, while avoiding overoxidation and suppressing coking. A parallel investigation into direct dehydrogenation of propane (DDHP) and catalytic oxidative dehydrogenation of propane with CO2 (CODHP-CO2) reveals that coking is a major catalytic fouling mechanism for DDHP and CODHP-CO2. CO2 in the latter participates in reverse water shift reverse reaction (RWGS) to promote C3H8 conversion and regenerate active Cr6+ species in the CrOx/SiO2 catalyst. It is worth noting that the positive effect of CO2 is not as profound as CO2/O2. The plug-flow membrane reactor exhibits a C3H6 yield of ~30%, C3H8 conversion of ~35% and C3H6 selectivity of 85% at 600 oC for an impressive 173 hours of operation with minimal degradation.

In the fourth part of this work, a series of CaO-SDC sorbents were synthesized via sol-gel method. The SDC additives were uniformly distributed on the surface of CaO particles and chemically compatible with CaO sorbents. Due to the high Tammann temperature (1063 oC) of SDC, SDC additives can serve as a barrier, which prevents CaO particles from sintering and agglomeration and thus improves the stability of CaO-based sorbents over the cyclic testing at high temperature. With 5wt% SDC added into CaO sorbents, a high initial COo uptake of 0.643 gCO2 / gsorbent can be achieved, which slightly decreased to 0.626 gCO2 / gsorbent after undergoing 19 carbonation-calcination cycles. In comparison, the CO2 uptake of pure CaO drastically decreased from 0.656 to 0.469 gCO2 / gsorbent highlighting the effectiveness of SDC addition in enhancing the durability and performance of the CaO-based sorbents.

Rights

© 2024, Kangkang Zhang

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