The thermochemical recuperation process uses endothermic reformation reactions to upgrade a portion of an engine’s primary fuel into a hydrogen-rich gas, thereby converting part of the exhaust heat from an internal combustion engine into chemical potential energy. Enriching the primary fuel air mixture of the internal combustion engine with this hydrogen-rich gas potentially enables combustion with very lean or dilute mixtures, resulting in higher efficiency and lower emissions as compared to standard combustion regimes. It may be possible to simplify thermochemical recuperation system architecture by directly mixing exhaust gases with the fuel in the reformation process to supply a significant portion of the heat and water required. To evaluate the effect of direct exhaust gas mixing on ethanol autothermal reformation, this work experimentally and theoretically investigated dilution with a mixture of nitrogen and carbon dioxide to simulate an exhaust composition, in combination with a range of inlet temperatures, to simulate exhaust gas temperatures, at a constant steam to carbon ratio.
Parameters such as the chemical coefficient of performance, chemical energy output divided by chemical energy input, are introduced to better enable quantification of thermochemical recuperation. Trends in yield and performance metrics for ethanol autothermal reformation were observed under operating conditions across a range of oxygen to carbon ratio, a range of dilution amount, and a range of inlet temperature. For high inlet temperature cases, dilution increases hydrogen yield and chemical coefficient of performance suggesting that direct exhaust mixing would be beneficial. However, for low inlet temperatures, dilution decreased hydrogen yield and other performance metrics suggesting that direct exhaust mixing would not be beneficial. Dilution decreased methane production for many conditions.
High inlet temperature conditions were found to cause homogeneous oxidation and homogenous conversion of ethanol upstream of the catalyst leading to high conversions of ethanol and high methane yields before reaching the catalyst.
Coke formation rates varied over two orders of magnitude, with high coke formation rates for the high inlet temperature cases and low coke formation rates for the low inlet temperature cases. Dilution decreased the rate of coke formation.
Models of intrinsic rate phenomenon were constructed in this study. The models predict that mass transport rates will be faster than the rate of chemical reaction kinetics over the range of ethanol concentrations and temperatures measured in the catalyst monolith both with and without dilution. Bounding cases for heat generation and transfer rates indicate that these phenomena could be the rate limiting mechanism or could be faster than both chemical kinetics and mass transport rates depending upon the distribution of oxidation heat between the catalyst and gas stream.
Based on these results direct exhaust gas mixing is expected to be a practical method for supplying heat and water vapor for ethanol autothermal reformation in thermochemical recuperation systems when exhaust temperatures are above a certain threshold. For low exhaust temperatures direct exhaust gas mixing can supply water vapor but reduces other performance metrics.
|Advisor:||Erickson, Paul A.|
|Commitee:||Jenkins, Bryan M., Ogden, Joan M.|
|School:||University of California, Davis|
|Department:||Mechanical and Aeronautical Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 72/05, Dissertation Abstracts International|
|Subjects:||Alternative Energy, Mechanical engineering, Energy|
|Keywords:||Autothermal reformation, Ethanol, Hydrogen enrichment, Internal combustion engine, Thermochemical recuperation|
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