MILD Combustion of Prevaporised Liquid Fuels

Abstract

Combustion of liquid fuels is the dominant source in the global energy supply, and its crucial importance is expected to remain well into the foreseeable future. Whilst providing human beings with energy, combustion of liquid fuels also produces undesired byproducts, including pollutants and greenhouse gases. In response to the mounting concern for energy sustainability and the environment, concerted efforts have been invested in the development of advanced combustion technologies. Moderate or Intense Low-oxygen Dilution (MILD) combustion technology has a great potential to abate pollutant and greenhouse emissions while maintaining a high thermal efficiency. In practical applications of MILD combustion, hot exhaust gases are recirculated inside the combustion chamber, simultaneously preheating and diluting reactants. A combination of hot reactants' temperature and low local oxygen concentration across the entire combustion chamber lead to volumetric reactions, resulting in a more uniform temperature and heat distribution. As a consequence, the peak flame temperature is reduced, thereby suppressing the formation of pollutants, such as nitrogen oxides. Most of the previous studies on MILD combustion have been focused on simple gaseous fuels. There is a paucity of information concerning liquid fuels burning under MILD combustion conditions, despite their critical role in the world energy supply. This thesis aims to advance the understanding of MILD combustion of liquid fuels through a combined experimental and computational investigation. In this investigation, liquid fuels are prevaporised in order to avoid the complexity of spray dynamics. Thus the focus is on the fundamental aspects of chemical kinetics of these fuels under MILD combustion. This thesis consists of a compilation of four journal articles, presenting results and findings from a combination of experimental and numerical studies. The first part of the experimental studies were conducted in a pressurised reverse- flow MILD combustor burning prevaporised ethanol, acetone, and n-heptane. These fuels are chosen to represent different classes of hydrocarbons, namely, an alcohol, a ketone, and a long-chain alkane. The pollutant emissions and the combustion stability under a wide range of operating conditions are examined. This investigation identifies several key operating parameters, namely, fuel type, equivalence ratio, carrier gas, air jet velocity, and operating pressure inside the combustion chamber. In order to further investigate the stabilisation of MILD flames and assess the impact of important parameters independently, parametric studies of prevaporised ethanol, acetone, and n-heptane are performed in a well-controlled environment, namely in a Jet in Hot Coflow (JHC) burner. Turbulent jet flames of dimethyl ether (an isomer of ethanol) are also investigated and compared to ethanol flames. Simultaneous imaging of OH, CH2O, and temperature, together with digital photography and imaging of OH* chemiluminescence, are performed to reveal the flame structure. Reaction flux analyses of various fuels are conducted to complement the experimental results. These results reveal that the local oxygen concentration plays a significant role in the flame structure. A transitional flame structure (a strong OH layer connected with a weaker "tail") is observed in the ethanol and the DME flames in a 9% O2 coflow instead of a 3% O2 coflow. This occurrence of the transitional flame structure is considered as an indicator of flames deviating from the MILD combustion regime. Simulations of ethanol and DME flames reveal that the importance of H2/O2 pathways in their oxidation processes decreases and intermediate species pool changes as the oxygen level increases from 3% to 9%. This suggests that a three-fold increase in the oxygen concentration leads to fundamental changes in the chemical kinetics of ethanol and DME. It is also found that n-heptane flames do not have the characteristics of a typical MILD combustion flame as observed in the ethanol and the DME flames. A transitional flame structure is seen in the n-heptane flames even at the 3% O2 coflow. In the reverse-flow combustor, stable combustion of ethanol is established under all tested conditions. However, n-heptane flames become more unstable than ethanol and acetone flames at high equivalence ratios and pressures. Calculations suggest that n-heptane flames burn faster than acetone and ethanol flames under elevated pressures. This indicates that n-heptane flames may ignite prior to a thorough mixing with hot combustion products. Furthermore, the jet velocity also decreases linearly with the increasing operating pressure inside the combustor. This is suspected to weaken the mixing of fresh reactants and exhaust gases, thus contributing to the unsuccessful establishment of MILD combustion. One criteria of MILD combustion, based on heat release profiles, is adopted to investigate the distinctive behaviour of n-heptane. This numerical investigation is focused on two unique features identified in flames in the MILD combustion regime: the mismatch between the location of the peak net heat release rate (Zhmax) and the location of stoichiometric mixture fraction (Zst); the absence of a net negative heat release region. For ethanol flames, Zhmax and Zst are uncorrelated under all the oxygen levels and strain rates investigated, while the absence of a net negative heat release region is dependent on the strain rate. These results indicate that the transition boundary between the conventional combustion regime and the MILD combustion regime cannot be determined by the oxygen level alone. For n-heptane flames, a net negative heat release region exists despite a low O2 level and a high strain rate. This is attributed to changes between alternative pyrolytic channels of n-heptane under different conditions due to its complex chemistry. The fundamental aspects revealed by this study shed more light on the MILD combustion of more complex fuels. An improved understanding on the role of fuel structure in the establishment of MILD combustion is achieved by this work. The findings of this study are relevant to the implementation of MILD combustion technology in a variety of combustion devices.