After-treatment technologies are required for diesel engines to meet the current and future stringent emissions regulations. Lean NOx traps and SCR catalysts represent the major routes for after-treatment for NOx mitigations under lean exhaust conditions. These technologies require active agents (H2, CO and several hydrocarbons) that either participate in nitrogen oxide reduction reactions or regenerate the NOx storage sites. Nevertheless, these species have to be obtained from either the on-board fuel or additional sources such as urea. Hydrocarbons are seen thus far as the most reliable source for generation of hydrogen. This work focuses then on the generation of hydrogen through Partial Oxidation of heavy hydrocarbons.
The research is first oriented to assess numerically, using a proprietary code, the feasibility of the non-catalytic reforming of hexadecane (C16H34 a heavy hydrocarbon molecule used as a proxy for diesel fuel) under different conditions of equivalence ratio, steam to carbon ratio, and inlet temperatures. The reforming process is then analyzed within the characteristic quantities ranges: 1 ≤ φ ≤ 1.9, 0 ≤ S/C ≤ 2, 1000 ≤ Tini ≤ 1750 K. A novel contribution to the scientific community is the assessment of addition of water through secondary injection to improve the hydrogen production. Of special interest for automotive applications is the use of exhaust as a possible source for oxygen since diesel engines are operated under lean conditions. Such a concept is also studied numerically in this dissertation. A discussion on the kinetic mechanisms producing and consuming hydrogen in the partial oxidation of hexadecane is also included in this dissertation.
Based upon the results obtained in the numerical simulation, a proof of concept of the POx of hexadecane is experimentally performed. Diesel fuel POx reforming is also experimentally studied since this concept is aimed to be utilized for diesel engine after-treatment. Bio-diesel has become very popular due to its chemical configuration containing simple chains and its lower pollution characteristic and its renewability as a bio-fuel. Hence, this research presents a novel study of partial oxidation of bio-diesel (B-100) to generate hydrogen or syngas.
The results obtained using the 0D model strongly indicate that H2, CO and total hydrocarbons concentrations increase with increasing equivalence ratio (φ) for all temperatures and both sources of oxygen (air and lean exhaust). The three fuels tested experimentally showed an increasing H2 concentration with φ as well. Nevertheless, H2 and CO saturate due to the decreasing adiabatic flame temperature (Tad) with increasing equivalence ratio. Steam addition slightly increases the H2 % Vol for low φ ratios, while it caused the H2 concentration to saturate at a faster rate as φ increased. Increasing Tini yields higher H2 concentrations owing to the higher Tad for all φ and S/C ratios. High H2 yields (~ 25-30%) can be obtained for φ ≥ 1.6 for the low temperature case (1000 K), while even higher yields (~ 30-40%) are seen for the high inlet temperature cases (1750 K). Addition of vapor along with the main feeds is beneficial at high inlet temperatures, while secondary injection of water showed a very slight increase in H2 concentration even if injected at high temperatures. However, as S/C is increased, a slower H2 saturation rate is seen for post-injection of vapor than for its equivalent main injection of vapor. Lower product composition are obtained using exhaust gas as O2 provider due to the lower flame temperature caused by the amount of present diluent in the stream.
In summary, this study has demonstrated the feasibility of producing hydrogen-rich syngas from the partial oxidation of diesel fuel in a simple, practically implementable device which could be integrated into an on-board, after-treatment system for diesel engine vehicles. The system is capable of hydrogen yields consistent with the NOx reduction needs of current engines and does not require any additional fluids besides the engine fuel. The detailed 0D kinetics simulations in this study have served to understand the chemical kinetic mechanisms at play and, while not truly predictive, to provide invaluable design guidelines for practical implementations, particularly with respect to operating point equivalence ratio and the dominant role of temperature in maximizing the hydrogen yield.