Thermochemical Recuperation and Catalytic Strategies for Anhydrous Ammonia in Combustion Systems
2021-06
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Thermochemical Recuperation and Catalytic Strategies for Anhydrous Ammonia in Combustion Systems
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2021-06
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The carbon intensity of combustion engines poses a major challenge to worldwide efforts to minimize climate change. Anthropogenic carbon dioxide (CO2) is greatest source of atmospheric warming and its emission must be curtailed to affect climate forcing in a meaningful way. Carbon-neutral alternatives such as ethanol and biodiesel recycle atmospheric carbon under ideal conditions yet result in net carbon emissions due to process inefficiencies. Fuels decoupled from chemical carbon are necessary to reduce carbon intensity and halt climate change. Anhydrous ammonia is one such fuel as it can be produced entirely by renewable means and contains no carbon. This body of work investigates combustion applications of anhydrous ammonia in compression ignition (CI) engines, methods of catalytically enhancing ammonia for more efficient combustion and use of the endothermic ammonia decomposition reaction for waste-heat recovery.This thesis presents the applications of catalytic ammonia decomposition, specifically pertaining to, but not limited to internal combustion engines. Ammonia has proved a suitable replacement fuel in spark-ignition (SI) engines and as a secondary fumigated fuel for CI engines. Stability of the ammonia molecule results in poor flame propagation and low ignition reactivity. Using ammonia in a dual-fuel arrangement overcomes these issues in existing engines and combustors designed for hydrocarbon fuels. Alternatively, ammonia can be converted to hydrogen using catalysis, which in turn enhances ammonia combustion without the need for a secondary high reactivity fuel. This work explores hydrogen-enhanced ammonia and diesel combustion in CI engines equipped with catalytic waste-heat recovery. Engines were operated over their full range under various levels of ammonia fuel replacement to determine the effects on engine and combustion efficiency as well as emissions and stability. Thermochemical recuperation and thermal recovery were analyzed across the operability range towards identifying optimal system parameters. The primary finding in the first part of this work is that ammonia effectively recovers waste energy using low-temperature high-active catalysts. Activity is demonstrated as low as 200 °C for Ruthenium-based catalysts, and full conversion to hydrogen results in a net lower heating value increase of 15%. Heat transfer to sustain the decomposing ammonia proved difficult however, as the experimental catalyst unit was undersized for engine operation. Despite low conversion, sufficient hydrogen was generated to enhance flame speed, combustion efficiency and engine thermal efficiency as compared to pure ammonia fumigation. Fuel-bound nitrogen in ammonia generated high oxides of nitrogen (NOx) and N2O emissions upon combustion. However, unburned ammonia present in exhaust was measured to be ideal for passive elimination of these species using selective catalytic reduction (SCR). Emissions and efficiencies measured suggest that future implementation of ammonia dual fuel requires higher rates of heat recovery and higher ammonia replacement rates than those demonstrated in this study. Both conditions can be met using a modified catalyst design and higher flow ammonia fueling system, respectively. Ammonia decomposition catalysis is thoroughly described in literature but heat transfer inside a supporting monolith structure is not. This work presents a computationally efficiency quasi-2D modeling procedure for understanding heat and mass transfer in metal monoliths. A finite difference model was developed to simulate thermal behavior of the decomposition catalyst used in experimental studies. The heat transfer model was calibrated against inert gas experiments and showed excellent agreement with convective and conductive values from literature. Argon, air and CO2 were used under identical thermal conditions to demonstrate robustness in simulating generic flows through the reactor. Agreement of the model against the entire experimental dataset demonstrated robustness in predicting metallic support thermal behavior while the simplicity of the approach presented a computationally inexpensive alternative to CFD or physical prototype design screening. Design screening was then conducted using varied input conditions and showed the relative importance of each parameter on heat exchange effectiveness and wall-average heat transfer coefficient. Optimal performance was quantified, and the effects of design parameters on heat exchange was discussed. High catalyst activity and reaction residence time are needed to achieve high hydrogen yield, promoting efficient combustion of ammonia-hydrogen mixtures. To overcome thermal limitations posed by waste-heat driven decomposition, ammonia partial oxidation can be used to create an endogenous heat source and increase yield. Oxidation and decomposition were combined in an autothermal process and were shown to increase both hydrogen fraction and the hydrogen-to-ammonia ratio of the reformate stream. Autothermal ammonia decomposition (ATD) resulted in fuel heating value decrease, which was comparable to heating value losses expected from poor combustion efficiency in engines. A comprehensive reactor model was developed using two global reaction rates and the previous monolith heat transfer model. Rates were determined through non-linear regression and showed excellent fit across thermal and autothermal regimes. Deficiencies in experimental reactor design were identified using the model, and potential design changes were discussed. The model and experiment both suggest that ATD is a promising alternative to waste-heat recovery approaches alone when a high reactivity reformate mixture is needed. The research shows that ammonia ATD reformate is of sufficient reactivity to enable drop-in replacement of hydrocarbon fuels in unmodified engines and combustors.
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University of Minnesota Ph.D. dissertation. June 2021. Major: Mechanical Engineering. Advisor: William Northrop. 1 computer file (PDF); xvi, 195 pages.
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Kane, Seamus. (2021). Thermochemical Recuperation and Catalytic Strategies for Anhydrous Ammonia in Combustion Systems. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/260645.
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