Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Environmental Engineering (ENAS)

First Advisor

Pfefferle, Lisa

Abstract

In the modern age, human activities depend more and more on energy. Fossil fuels and the related combustion process play important roles in supplying global energy. However, fossil fuels emit CO2 and pollutants into the atmosphere when they combust. CO2 leads to global warming with a single half-life of more than hundreds of years in the atmosphere. In order to contain the rise of the average global temperature, net emissions of CO2 must approach zero. However, given the high energy density of liquid fuels relative to other energy carriers, such as batteries, liquid fuels are likely to remain the dominant power source for many decades. Given that, it is critical to minimize their pollutant emissions including soot. Soot particles are graphitic or amorphous carbonaceous particles that form due to incomplete combustion. They cause millions of global premature deaths annually and also contribute to global warming. Different fuels have different tendencies to form soot under similar conditions. Alternative fuels are promising fuel candidates synthesized from non-petroleum sources. They have great potential to improve engine performance, achieve an overall low carbon footprint, and mitigate soot emission. It is essential to pick alternative fuels with benefits for soot reduction. The objectives of my thesis are: (1) to determine whether certain categories of alternative fuels provide benefits in soot reduction compared to conventional fuels, (2) to determine which specific fuel structures offer the most benefits after accounting for other properties, such as lower heating values, and (3) to provide some insight into the kinetic mechanisms of fuel decompositions and soot formations for selections of optimal fuels. In this work, furans, terpenes, and polyoxymethylene ethers (POMEs) were investigated. Their sooting tendencies were characterized by the Yield Sooting Index (YSI), which was obtained by measuring the line-of-sight spectral radiance (LSSR) of a coflow laminar diffusion flame doped with 1000 ppm of each test fuel. Various methods were used to investigate their soot-fuel relationships and soot formation pathways. Furans are cyclic aromatic compounds whose rings contain four carbon atoms, one oxygen atom, and two carbon-carbon double bonds. They could be produced from lignocellulosic biomass. The YSIs of furans were measured here. The result shows that sooting tendencies of furans are generally lower than most test gasolines and the measured YSIs of alkylfurans increase linearly with the number of side-chain carbon atoms. Reaction pathways were proposed to explain these trends. Terpenes and their derivatives are a diverse family of biological molecules with high degrees of freedom and superior properties. The sooting tendencies of 17 C10 monoterpenes and 7 of their hydrogenated derivatives were quantified with YSI. The YSI follows the trend: terpenes > dihydroterpenes > tetrahydroterpenes. Derived smoke points (DSPs) were estimated from a correlation between previously-measured YSI and smoke point (SP) of hydrocarbons. The DSPs of all the tetrahydroterpenes and some dihydroterpenes are higher than that of a Jet-A fuel, suggesting that they offer soot reduction benefits. Large YSI differences were observed in two pairs of isomers: β-ocimene and myrcene, α-pinene and β-pinene. Kinetic pathways were proposed to explain these interactions. POMEs are oligomerized ethers with favorably high cetane numbers (CN) that can serve as renewable diesel fuels. They could be produced from biomass via methanol and formaldehyde. Here, the YSIs of individual POMEs with methyl, ethyl, propyl, and butyl end-groups and one to five oxymethylene units were quantified. All of the POMEs soot much less than a conventional diesel fuel even accounting for the energy density penalties. The YSI decreases linearly with the number of oxymethylene units and increases linearly with the number of carbon atoms in the end-groups. Reaction pathways were proposed to explain these trends. Moreover, Planck’s-law-based color-ratio pyrometry (CRP) was implemented to quantify the maximum soot volume fraction (fv, max) of a series of dimethoxymethane(M1M)-CH4-N2 flames. The result shows that fv, max drops drastically as the mole fraction of M1M (XM1M) increases. Simplified reaction pathway analysis suggests that the produced CH2O causes the plummet in fv, max.

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