International Journal of Hydrogen Energy 31 (2006) 1384 – 1393 www.elsevier.com/locate/ijhydene Hydrogen enrichment effects on the second law analysis of natural and landfill gas combustion in engine cylinders C.D. Rakopoulos a , ∗ , D.C. Kyritsis b a Internal Combustion Engines Laboratory, School of Mechanical Engineering, National Technical University of Athens, 9 Heroon Polytechniou Street, Zografou Campus, 15780 Athens, Greece b Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, Illinois 61801, USA Available online 15 December 2005 Abstract The availability (exergy) balance during combustion of hydrogen-enriched natural and landfill gas, which are used as fuels in combustion engine cylinders, is studied computationally using a zero-dimensional model of the closed part of the cycle. The main focus is on the demonstration of a fundamental difference in the generation of irreversibility during combustion between hydrogen and hydrocarbons. This difference relates to the mechanisms of entropy generation during the oxidation reaction of the two fuels and yields the particularly attractive characteristic of a monotonic decrease in combustion irreversibility with increasing hydrogen content of the fuel, for mole fractions of hydrogen smaller than 10%. This reduction in combustion irreversibility is reflected in an increase in second law efficiency with increasing proportions of hydrogen. The exhaust gas availability at the end of the closed part of the cycle was found to have a local maximum for a hydrogen mole fraction of the order of 5%. These trends with respect to hydrogen also apply when the fuel is diluted with a significant amount of CO 2 (of the order of 40%, as for example in the case for landfill gas), although the absolute value of each of the terms of the availability balance is affected strongly by the dilution.  2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Second law analysis; Hydrogen enrichment; Natural gas; Landfill gas; Combustion engines 1. Introduction Although analysis of power generating processes in terms of availability (exergy) does not stem from a fun- damental conservation law like its counterpart based on energy, it incorporates the second law restrictions on energy conversion in a way that cannot be captured by analyses based solely on the first law [1,2]. Specifically for combustion and, in general, chemically reacting sys- tems, a second law analysis allows the calculation of ∗ Corresponding author. Tel.: +30 210 7723529; fax: +30 210 7723531. E-mail address: cdrakops@central.ntua.gr (C.D. Rakopoulos). 0360-3199/$30.00  2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.11.002 reaction irreversibility, i.e. the amount of fuel availabil- ity which is destroyed during the irreversible chemical reaction [3–5] and cannot be converted to work due to entropy increase, something that is not evident from a simple energy balance. For this reason, availability calculations have been used in the past to assess the second law efficiency of several novel ideas in power generation, such as the combined use of hydrogen fuel cells and combustion based plans [6], modern internal combustion engines [7], and, more recently, oxygen enrichment [8], solid oxide fuel cells [9], hydrogen–oxygen turbine [10] and spray combustion [11].