the autothermal evaporation of liquid fuels by using cool flames
THE AUTOTHERMAL EVAPORATION OF LIQUID FUELS BY USING COOL FLAMES Klaus Lucka, Heinrich Koehne, Oel - Wärme - Institut gGmbH, Kaiserstr. 100, 52134 Herzogenrath, Germany ABSTRACT Modern mixture preparation systems for liquid fuels require a homogeneous fuel vapour air mixture. To make this possible a separation of the combustion- and mixing zone from the high temperature oxidation zone is necessary. Simultaneously the demand of a simple and inexpensive construction has to be fulfilled. The use of cool flame reactions offers the opportunity to establish a technical vaporizing system for various fuels. Through such systems pre-mixing technologies can be applied for a number of liquid hydrocarbons such as FAME, n- heptane, fatty acids and mixtures with IGO. The chemical reaction is investigated in a glass reactor that offers an optical approach and the possibility for temperature measurement in the reaction zone. Samples of the reaction gas can be taken and analysed by gas chromatography. INTRODUCTION In case of an oxidation of hydrocarbons the implementation of a premixing technology is favourable for different reasons. The quality of the fuel air mixture has a substantial influence on the development of products and pollutants. By means of separating the mixing zone from the oxidation process a selective creation of products can be achieved. In the application of liquid fuels a simple technical execution of such systems exists in the injection of fuels into a preheated air flow. For a number of fuels this can be combined with the occurrence of exothermal reactions before the actual ignition. A significant feature of a set of liquid hydrocarbons is a temperature range where the conversion due to these reactions is decreasing with increasing temperature. This area is characterized as the negative temperature coefficient NTC [Bernard]. So called cool flames appear. These are combined with a partial oxidation of the fuel and a partial consumption of oxygen. A number of complex chemical chain reactions appear within the cool flames with the involvement of free radicals and a multitude of different intermediate products that show different life times. Stimulated aldehydes emit a weak blue light within the reaction [Lewis]. The cool flame reaction can be determined for a number of liquid hydrocarbons, especially representative pure chemicals have been studied. Also technical fuels such as industrial gas oil (IGO) and diesel as well as fuels from renewable energy sources e.g. rapeseed methyl ester (RME) show cool flames [Lucka99]. It was found that the initial and final temperatures of the fuels used are virtually identical. Apart from the initial temperature of RME, which is raised due to the boiling range of 330 °C to 340 °C, all the temperatures lie within a narrow band. The dependency of the final temperature on the air ratio is negligible. The heat released within the reaction makes an autothermal and at the same time residue free evaporation of the liquid fuels possible. Autoignition of the fuel air mixture can be securely avoided by the chemically restricted cool flames. Thus the apparent contradiction with liquid fuels can be handled which is that the temperature necessary for the complete evaporation of the fuel exceeds the theoretical ignition temperature. The key to the production of cool flames lies in the lack of stability of the links formed within the chain reactions as a result of oxygen being absorbed. That means that by achieving gas temperatures of approximately 480 °C further reactions of the mixture are limited. For this reason a reduction in the preheating of the air has only a very small effect on the final temperature of the cool flame. The evaporation of fuels using cool flames enables the application of liquid fuels in the fields of application including the burner technology, fuel processors, the technique of turbines and engine technology. EXPERIMENTAL The investigations were carried out in a double-walled glass reactor. Thus an optical approach of the reaction was possible, which allowed a recording of the pale blue light of the reaction products. A photograph of the cool flame reaction could be taken with a ten minutes exposure time. The fuels RME, IGO a mixture of IGO and 5% RME and the representative fuel n-heptane were sprayed into a preheated air flow by means of a simplex nozzle. A geared pump provided the necessary fuel pressure. The temperature of the gas flow was measured along the reactor axis. At the end of the 0.7 m long reactor samples of gas could be taken from the reaction and analysed by gas chromatography.