Research On Surface Functional Groups And Oxidation Characteristics Of Biodiesel Combustion Particulates

Carbon dioxide emissions can be effectively reduced,and"carbon neutrality"can be achieved,when diesel engine using clean and renewable biodiesel.This paper was based on an electronically controlled high-pressure common rail diesel engine for engine bench test,experiments with different raw materials of biodiesel and diesel were carried out,and particulate matter was collected.The relationship between the combustion characteristics of different raw materials of biodiesel,the surface functional groups of the particulate matter and the oxidation characteristics were systematically studied by using fuel consumption meter,combustion analyzer,Fourier transform infrared spectrometer（FTIR）,X-ray photoelectron spectrometer（XPS）and Thermogravimetric analyzers（TGA）and other instruments.The main contents are as follows:（1）The engine bench test was carried out,and the fuel economy and combustion characteristics of different fuels was analyzed.Compared with diesel,the equivalent fuel consumption rates of soybean oil biodiesel,palm oil biodiesel and waste cooking oil biodiesel increased by 1.7%,2.9%and 4.7%respectively,and the effective thermal efficiency was reduced by 1.7%,2.8%and 4.5%respectively,the peak pressure in the cylinder was reduced by 7.4%,7.3%and 1.5%,the peak temperature in the cylinder increased by 0.6%,0.8%and 3.4%respectively.The difference between the peak pressure rise rates was less than 0.1MPa·（°CA）^-1,the combustion duration of biodiesel was lower than diesel.Among all raw material biodiesel,waste oil biodiesel show the highest in-cylinder pressure,pressure rise rate and in-cylinder temperature peak.（2）The composition of the surface functional groups,the relative content of aliphatic C-H functional groups,and the chemical stability of the four fuel particles were analyzed by using FTIR.Compared with diesel particulate matter,the relative contents of aliphatic C-H functional groups on the surface of soybean oil biodiesel,palm oil biodiesel and waste cooking oil biodiesel particles increased by 6.8%,11.4%and 18.2%respectively,and the degree of branching increased by 12%,24.1%and 42.2%respectively.The relative content of aliphatic C-H functional groups on the surface of the biodiesel particulate matter and the degree of surface branching were significantly higher than diesel particulate matter,while the chemical stability shows the worst performance.The relative content of aliphatic C-H functional groups in the waste cooking oil biodiesel particles and the degree of branching are both the highest.（3）The molar ratio of oxygen to carbon on the surface of the four fuel particles,the relative content of oxygen-containing functional groups and the form of surface hybridization was analyzed by using XPS.The surface of the particulate matter produced by diesel engine combustion was mainly C and O elements.Compared with diesel particles,the O/C values of soybean oil biodiesel,palm oil biodiesel and waste cooking oil biodiesel particles increased by67.7%,82.3%and 95.6%respectively,sp³/sp²values increased by 54.5%,59.1%and 68.2%respectively,the relative content of C-OH functional groups on the surface was between 2.1%and 6.8%,and the relative content of C=O functional groups was between 0.7%and 1.3%.The O/C value,C-OH functional group,C=O functional group relative content and disorder degree of the waste cooking oil biodiesel particles were the highest.（4）The oxidation characteristics of the four fuel particles was analyzed by using TGA,and the apparent activation energy was calculated by the Coats-Redfern integral method.Compared with diesel particulate matter,the ignition temperature of soybean oil biodiesel,palm oil biodiesel and waste cooking oil biodiesel particles decreased by 43.6°C,58.3°C and 72.1°C respectively,the burnout temperature decreased by 41.6°C,56.6°C and 69.3°C respectively,the peak temperature of oxidation rate decreased by 65.1℃,69.9℃and 79.8℃,and the apparent activation energy decreased by 22.4%,23.2%and 26.6%respectively.Biodiesel particulate matter shows higher oxidation activity.It is worth noting that the waste cooking oil biodiesel particles shows the lowest oxidation characteristic temperature and the highest oxidation activity.（5）The relationship between the oxidation activity of biodiesel particulate matter and the physical and chemical properties and surface functional groups of biodiesel was analyzed.With the increase of oxygen content of biodiesel and the decrease of iodine value,the degree of disorder of the particulate matter increased.As a result,the relative content of the aliphatic hydrocarbon functional groups,oxygen-containing functional groups on the surface of the particulate matter,and the oxidation activity was increased.However,the apparent activation energy was decreased.