Hydrogen production from an on-board reformer for a natural gas engine: A thermodynamics study

Abstract

In the present study, for an insight into the optimizing windows of comprehensive energy efficiency for open-loop system of marine NG engine and reformer, a peculiar thermodynamic equilibrium model for simulating the exhaust gas-fuel reforming process is established using equilibrium constant and experimental results. The accuracy of the model is evaluated by comparing numerical simulations with data derived from experiment of the open-loop operation system. Accordingly, the effect of engine loads and excess air (λ) as well as reformer feedstocks on maximum theoretical reforming characteristics are investigated. In addition, the energetic distribution in open-loop operation system can be clarified by the first law of thermodynamic. The results shown that fuel conversion toward H2 is highly selective due to the enhancement of reaction temperature with increasing engine loads. Especially, under 75% engine load, the maximum theoretical H2 yield reaches up to approximately 42.3% when λ = 1.4, CH4/O2 (M/O) = 1.5 and H2O/CH4 (S/M) = 2; The high λ will dilute the feed concentration of the reformer, thereby inhibiting the reforming reaction moving toward H2 production. Hence, an appropriate λ (1.4) can maximize the hydrogen production capacity of the reformer. In addition, according to the energy distribution of the open-loop system, the theoretical reforming energy efficiency of the reformer reaches a peak of 93.5% at S/M = 2 and M/O = 5. However, it can be found that M/O should be maintained above 1.5 to acquire optimal reforming energy utilization efficiency of system. Summarily, the optimal operation conditions for the reformer in open-loop operation system are confirmed: M/O = 1.5–2.5 and S/M = 1.5–2 under 75% engine load, which could fulfill efficient reforming performance.