Date of Award
Doctor of Philosophy (PhD)
Michael D. Mann
Countries across the world have different expectations on carbon dioxide (CO2) capturing and the willingness to commit to international agreements continually change. At present, the CO2 capture market is weak as industries are reluctant to take up the costs and risks associated with implementing the capture technologies. Globally, and in the United States of America (U.S.A.) in particular, the perception is that emerging energy technologies with carbon capture are too expensive or inefficient to attract investors without government backing and subsidies. Coal usage has accordingly declined. By expanding the coal value chain into more than just electricity generation, it can possibly attract new investments and improve confidence in novel carbon capture technologies.
Chemical looping combustion is a technology that can utilize coal and benefit both the electricity and valuable chemicals market. This flexibility of chemical looping combustion represents a promising technology to integrate the required time flexibility so urgently needed within the U.S.A. electricity generation sector. Fostering the development and scalability of chemical looping combustion related technologies, especially using coal, rather than focusing purely on the expected cost reduction and usefulness of chemical looping combustion as a CO2 capture technology, can ensure stability within the electricity generation and coal industry of the U.S.A.
Chemical looping combustion is an induced fuel combustion process that uses recyclable redox materials as oxygen carriers to transfer oxygen selectively from an air stream to a fuel reactor, thus eliminating the requirement for end-of-pipe CO2 gas separation processes. To date, no oxygen carriers have been identified or developed that exhibit adequate long-term performance. There is also a lack of sufficient experience related to the design and operation of full-scale chemical looping combustion systems.
Oxygen carriers serve as oxygen sorbents that release or adsorb oxygen, depending on the temperature, pressure and gas composition within the chemical looping combustion system. Oxygen carrier performance is mainly characterized by its affinity to react under both oxidizing and reducing conditions and its resistance to attrition.
Based on the research opportunities, two primary hypotheses have been developed:
i) A laboratory-scale evaluation system, operating under high temperature and reacting conditions, can be used to assess oxygen carrier performance. The experimental results can be used to develop correlations for determining oxygen carrier lifetime in scaled-up processes.
ii) A spouted fluid bed reactor can improve carbon conversion efficiencies as compared to a bubbling fluidized bed reactor. Computational fluid dynamic simulations can be used to model the movement of oxygen carriers in such a spouted fluid bed reactor to gain a better understanding of the transport phenomena involved deep within the reactor.
To prove or disprove the research hypotheses, the research scope was broken down into three main efforts:
i) Evaluate several materials being considered by the chemical looping combustion development community to ascertain whether a single test procedure is adequate for oxygen carrier performance characterization
ii) Further develop the oxygen carrier performance evaluation methodology (based on jet attrition testing) to include a second attrition source (cyclonic attrition) critical in chemical looping combustion systems involving circulation of oxygen carriers
iii) Assess whether a spouted fluid bed can be used for chemical looping combustion and if it is scalable using a modular approach based on experimental and computational fluid dynamic tools. This effort will target the development of a computational modeling tool for the design of a multi-zone spouted fluid bed.
Parts i) and ii) of the research scope pertained to testing different oxygen carriers in a jet attrition unit and a cyclonic attrition unit. An attrition unit can be defined as a device that is used to attain information concerning the ability of material to resist particle size reduction. The ASTM D5757 test method is typically used to determine the relative attrition characteristics of fluid catalytic cracking (FCC) catalysts under ambient conditions.
In contrast to the ASTM D5757 test method, the jet and cyclonic attrition units were set up to expose the oxygen carriers to various operating conditions that could typically be encountered in actual chemical looping combustion systems. The operating principle of the jet- and cyclonic-induced attrition systems provides a vast improvement over previous methods that neglect chemical and thermal stresses.
The cyclonic attrition unit ultimately represents a more favorable test method for assessing the attrition of oxygen carriers compared to the jet attrition unit. The cyclonic attrition test method merely speeds up the particle impact frequency compared to large-scale cyclones. However, the particle impact velocity within the cyclonic attrition unit is similar to large-scale cyclones (9.0 – 27 m/s). The cyclonic attrition unit can therefore provide relevant attrition data on an oxygen carrier within 9 hours, using as little as 70 grams of material.
Two attrition models (cyclonic and jet) were identified that could be used to investigate attrition rates at operational chemical looping combustion conditions. The models were based on the concept of efficiency within a comminution process. The models related particle attrition to the kinetic energy used to produce fines. The cyclonic attrition model provided the best fit for the attrition data with coefficients of determination ≥ 0.94.
Part iii) of the research scope related to exploring the use of a spouted fluid bed as a reactor configuration for chemical looping combustion. The spouted fluid bed was identified as a suitable configuration to improve fuel conversion and operational flexibility over the typically employed bubbling fluidized bed designs. This part of the study had two objectives: i) to assess the viability of a single-spouted fluid bed as an efficient chemical looping combustion reactor, and ii) to assess if computational fluid dynamic based simulations can be employed to show the hydrodynamic behavior of both a single- and multi-spouted fluid bed reactor.
A modeling and experimental approach were followed to accomplish the objectives. Firstly, Multiphase Flow with Interphase eXchanges (MFiX) software was used to establish a spouted fluid bed reactor design using the two-fluid model. An experimental setup was built to supplement the model. The experimental setup was modified for testing under high temperature, reacting conditions (1073 - 1273 K). The setup was operated in either a spouted fluid bed or a bubbling bed regime, to compare the performance attributes of each using a mixture of carbon monoxide and hydrogen as fuel.
For the single-spouted fluid bed investigation, the cold flow model results provided key information for rapid experimental design and operating envelope determination. The single-spouted fluid bed modeling and experimental results illustrated the potential of the configuration to improve gas/solid contact, lower energy requirements and increase operational robustness in comparison to a bubbling fluidized bed reactor. The cold flow models proved adequate in depicting the intermittent spouting regime as well as providing valuable information pertaining to material circulation rate.
The modeling and experimental work on the single-spouted fluid bed reactor were used as the starting point to investigate the scalability of the system into a multi-spouted fluid bed reactor. MFiX software was again used to design a multi-spouted fluid bed and compare the hydrodynamic aspects of the system to that of a bubbling fluidized bed. A reactor comprising nine spout/draft tubes, arranged in a 3x3 setup, was modeled in 2-D using the two-fluid model. The model incorporated both inlet and outlet regions to study to bulk movement of solids within the reactor design. The focus of this work was on capturing the hydrodynamic trends associated with a multi-spouted fluid bed. The modeling results indicated that the solids in a multi-spout system has a slightly narrower residence time distribution compared to that in a bubbling fluidized bed. The narrower residence time distribution could potentially improve fuel conversion in chemical looping combustion systems. Ultimately, a baseline model was configured that can be used to investigate alternative layouts of modular spouted fluid bed reactors for various applications.
Van Der Watt, Johannes George, "Modeling And Improving Oxygen Carrier Performance In Chemical Looping Combustion Systems" (2019). Theses and Dissertations. 2871.