ORNL Researchers Find Thermochemical Exhaust Heat Recuperation In Internal Combustion Engines Could Provide Substantial Boosts in Second-Law Efficiency
Thermochemical exhaust heat recuperation (TCR) in an internal combustion engine could result in substantial boosts in second-law efficiency (as measured in terms of single-stage work output from an ideal IC engine) for a range of fuels, according to a new study by researchers at Oak Ridge National Laboratory (ORNL). A paper on their work was published online 5 February in the ACS journal Energy & Fuels.
The basic concept of TCR involves using exhaust heat to promote on-board reforming of hydrocarbon fuels into syngas (a mixture of carbon monoxide and hydrogen). The syngas is then burned in the engine in place of some or all of the original fuel. Because the reforming reactions are endothermic, the researchers note, they provide a means for recycling exhaust energy in a chemical form.
Methanol can be converted directly to syngas by adding heat alone in the presence of a catalyst; other fuels, such as ethanol and isooctane, require additional oxygen or water for complete conversion to syngas.
One important factor that we include in this study is combustion irreversibility. For most fuels, the entropy generated by unconstrained combustion destroys up to a third of the original fuel exergy, making that portion of the fuel energy unavailable for generating work. As we explain below, carbon monoxide (CO) and hydrogen (H2) fuels have the potential to significantly reduce this exergy loss because of their unique thermodynamic properties. The extremely low flammability limit of hydrogen can also be used to extend the lean combustion limit, which can increase expansion work and reduce emissions. We therefore considered it important to include the expected changes in combustion exergy losses as part of our analysis.
TCR is particularly relevant to alcohol-based engine fuels, because combustion exergy losses for direct combustion of alcohols are typically higher than for alkanes and alkenes. In addition, ethanol produced by fermentation typically contains considerable water. Much of the cost associated with removing the water during ethanol production might be avoided if hydrous ethanol could be directly used by engines. Onboard, precombustion reforming could potentially help make use of hydrous ethanol more practical.—Chakravarthy et al.
To reduce mechanical complexity while considering the fundamental thermodynamics, the team confined its analysis to a frictionless, single-stage IC engine operating over an ideal cycle with the following features: (i) constant pressure or constant volume mixing of gaseous fuel and air in the combustion chamber, (ii) isentropic compression of the fuel and air mixture, (iii) adiabatic constant volume combustion of the mixture at the point of maximum compression, (iv) isentropic expansion of the combustion gases to atmospheric pressure, and (v) operation at steady state, so that the engine state repeats precisely at each point in the cycle.
They also limited the study to work generation by a single-stage expansion of the combustion gases; they did not consider other ways to extract work from the exhaust gases, such as Rankine bottoming cycles.
For an ideal stoichiometric engine fueled with methanol, the researchers found that TCR can increase the estimated second law efficiency by about 3% for constant pressure reforming and over 5% for constant volume reforming. The improvement of constant volume reforming over constant pressure reforming results from the pressure boost caused by themolar expansion. When the engine is operated lean (e.g., at a fuel/air equivalence ratio of 0.4), the expected second law efficiency benefits for methanol could be raised an additional 2%. The estimated second law efficiency increases for constant volume TCR of ethanol and isooctane are 9 and 11%, respectively.
The second law efficiency benefits from TCR in the present study are mainly due to the higher cylinder input exergy for reformate and the pressure boost in the case of constant volume reformation. We note, however, that it will be important in future studies to consider the possibility for using combined cycle work extraction. When additional work can be extracted from the exhaust, the benefits of the reduced combustion irreversibility are likely to be more evident.
In the ideal engine system used here, there is significant potential exergy loss associated with the reformer, where we have made no attempt to minimize the temperature gradient or generate work from the heat transferred between the post-expansion exhaust and reformer. If the proposed engine concept is modified to include a bottoming cycle that uses this heat, one would expect considerable increases in the potential work. Still, even for the relatively simple system considered here, TCR could yield substantial efficiency gains.—Chakravarthy et al.
V. Kalyana Chakravarthy, C. Stuart Daw, Josh A. Pihl and James C. Conklin (2010) Study of the Theoretical Potential of Thermochemical Exhaust Heat Recuperation for Internal Combustion Engines. Energy Fuels, Article ASAP doi: 10.1021/ef901113b