While exergy analysis is by now commonly used on the system level to identify losses and recommend ways for reducing them, its use on the “intrinsic”, field, level where the exergy of a process is calculated as a function of location and time, is still developing. Intrinsic exergy analysis is a most useful method for identifying and understanding the specific reasons for exergy losses in a process, and in devising methods for their reduction. A good example, which is the sample case of this paper, is the analysis of exergy losses in combustion processes, which are known to be responsible for around 30 % of the fuel potential to produce power. In this paper we develop a methodology for intrinsic exergy analysis and for its use for process improvement, using the case of combustion of a n-heptane droplet as example. The time-dependent continuity, energy and species conservation equations together with the reaction kinetics, state equations, and temperature and concentration dependent transport properties, are solved numerically to determine the temperature and concentrations fields. These results are then used to calculate the rates of local entropy generation to determine the spatial and temporal irreversibilities produced during the combustion process, as well as the exergy efficiency. The results obtained indicate, among other things, that after ignition has taken place, the exergy loss (or entropy generation) component most responsible for the overall exergy loss is the chemical entropy, having the same order of magnitude as the rest of the entropy generation terms combined for all the cases evaluated. The computed exergy efficiency for the base case is 68.4%, in agreement with previous droplet combustion exergy studies. To develop guidelines for the process improvement, the sensitivity of the second law efficiency to the initial gas temperature (Tgi), reaction rate (ω), and combustion duration were analyzed. The results generated several promising improvement avenues.

Volume Subject Area:
Advanced Energy Systems
Keywords:
Combustion, exergy, entropy, efficiency
Topics:
Combustion, Drops, Entropy, Exergy, Exergy analysis, Fuels
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