Optical Study of Soot Characteristics of Biofuel Spray Combustion

Abstract

Liquid biofuels for internal combustion engines will likely be part of the future solution for mitigating CO2 emissions to the atmosphere. Several types of biofuels for compression ignition (CI) engines are being developed at the moment, made from sustainable feedstock such as waste animal fat, forest residue and algae. These fuels will only reach the consumer market if they become economically viable and meet the requirements set by authorities for fuel quality and hardware compatibility. Additionally, issues related to pollutant emissions of particulate matter and nitric oxides from CI engines must be considered, where advanced aftertreatment systems have been developed in order to comply to regulations. Particle matter emissions from CI engines poses a considerable challenge since expensive aftertreatment systems are needed and can over time reduce engine performance. By introducing biofuels, an opportunity to reduce the particulate emissions is offered. Biofuels can be designed to have low sooting tendencies and favorable combustion properties, such that clean combustion can be achieved. Biofuels can also be blended to diesel fuel gradually, where both well-to-wheel CO2 emissions and pollutant emissions can be reduced. In order to achieve this, research needs to be conducted on combustion and emission characteristics of existing and novel biofuels.

An experimental suite has been developed, enabling detailed investigations on combustion and soot processes in CI engines fueled with biofuels. A redesigned engine with optical access to the combustion chamber has been commissioned, i.e. the optically accessible compression ignition chamber (OACIC), which enables direct optical measurements of a single CI spray. The OACIC is a reciprocating rapid compression machine designed to perform fuel comparison studies, allowing for fast fuel switching and high speed data acquisition. The intake air is heated and compressed, offering a range of engine-like thermodynamic conditions.

For the current work, the optical technique diffuse back-illuminated extinction imaging (DBIEI) of soot has been applied to the OACIC. The technique measures the optical density of in-flame soot, which can be related to the soot mass and concentration. Further development of the technique has been performed, including an improved method for correcting the flame luminosity interference on the measurement. Uncertainties related to beam steering have also been assessed in detail, which led to minimization of these effects in the current setup. In addition, a framework for dealing with non-ideal camera characteristics which occurs when subjecting an image sensor for rapid high and low light intensities has been developed and adapted to the DBIEI measurement. By combining DBIEI and high speed OH* chemiluminescence imaging, in-flame soot and flame lift-off length were simultaneously measured, providing information about in-flame soot mass and jet air entrainment. These measurements together offer detailed information on soot production, which can be used to validate numerical simulations and to help elucidate the complex processes behind soot formation and oxidation.

The experimental suite was applied to the investigation of combustion of biofuels:

An experimental study on in-flame soot in CI sprays of biodiesel surrogate fuels was conducted. The surrogate fuels in subject were methyl oleate, methyl decanoate, n-heptane and a 50/50% molar blend of n-heptane and methyl decanoate. One of the objectives of the study was to compare the surrogates to a commercially available rapeseed methyl ester (RME) biodiesel. In this way, a direct evaluation of the combustion and sooting characteristics was enabled. The sooting tendency and combustion characteristics of methyl oleate are similar to that of RME, since the major component in RME is methyl oleate. Methyl decanoate was observed to have a very low sooting tendency, which is explained by its high oxygen content. Methyl oleate proved to be a fitting surrogate fuel for biodiesel. However, the kinetic reaction mechanism for methyl oleate is very complex and computationally expensive. This makes methyl decanoate based surrogate fuels more fitting, since they have similar combustion chemistry to that of RME, due to the ester moiety.

Soot and combustion characteristics of hydrotreated vegetable oil (HVO), RME and diesel fuel were measured in the OACIC. The study showed that diesel fuel produced the highest level of in-flame soot, while HVO and RME produced similar levels. The higher in-flame soot concentration observed for diesel fuel was explained by its aromatics content. HVO produced slightly more in-flame soot than RME, which was likely due to the lack of fuel-bound oxygen in HVO.

An investigation of the possible utilization of glycerol as a soot-reducing additive in diesel fuel has been conducted, including experimental and numerical work. Glycerol is a byproduct of the biodiesel production process, where a possible value stream would be to sell it directly as a fuel additive. The main advantage of using glycerol as a fuel is its high oxygen content, potentially reducing particulate emissions. However, the disadvantages of having a low reactivity, high viscosity, and being immiscible with diesel fuel were uncovered. Adding glycerol to diesel resulted in an observed increase of the carbon monoxide concentration in the exhaust, which was due to glycerol's low reactivity. In addition, a decrease in particulate matter emissions and in-flame soot was observed. Numerical simulations showed a reduction of soot precursors and an increase in CO concentration after adding glycerol to n-heptane. The reduction in soot precursors is explained by the added oxygen to the fuel.