eJournals International Colloquium Fuels 13/1

International Colloquium Fuels
icf
expert verlag Tübingen
101
2021
131

Influence of variable oxygen concentration on the accelerated ageing of liquid fuels

101
2021
Chandra Kanth Kosuru
Simon Eiden
Klaus Lucka
Temperature has always been the vital parameter in case of fuel ageing. In the accelerated ageing method, liquid fuels are subjected to higher temperatures with compressed gas as the atmosphere, to induce stress and accelerate the thermal or thermo-oxidative process leading to a fast-ageing process. In this method, the heating rate, cooling rate and the oxygen concentration (in the gas phase) have always been constant with variable temperature and ageing time; depending on the fuel quality. In the current topic, a middle distillate fuel will be subjected to accelerated ageing with variable oxygen concentration in the gas phase and keeping the temperature and ageing time constant. The impact of variable oxygen concentration on the auto-oxidation will be discussed with the help of some initial observations. Furthermore, the impact of this oxygen variations on the physio-chemical properties of the fuel and therefore the fuel stability itself will also be discussed.
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13th International Colloquium Fuels - September 2021 117 Influence of variable oxygen concentration on the accelerated ageing of liquid fuels Chandra Kanth Kosuru Tec4Fuels GmbH, Aachen, Germany Simon Eiden Tec4Fuels GmbH, Aachen, Germany Klaus Lucka Tec4Fuels GmbH, Aachen, Germany Summary Temperature has always been the vital parameter in case of fuel ageing. In the accelerated ageing method, liquid fuels are subjected to higher temperatures with compressed gas as the atmosphere, to induce stress and accelerate the thermal or thermo-oxidative process leading to a fast-ageing process. In this method, the heating rate, cooling rate and the oxygen concentration (in the gas phase) have always been constant with variable temperature and ageing time; depending on the fuel quality. In the current topic, a middle distillate fuel will be subjected to accelerated ageing with variable oxygen concentration in the gas phase and keeping the temperature and ageing time constant. The impact of variable oxygen concentration on the auto-oxidation will be discussed with the help of some initial observations. Furthermore, the impact of this oxygen variations on the physio-chemical properties of the fuel and therefore the fuel stability itself will also be discussed. 1. Introduction Ageing of liquid fuel has always been of prime importance in estimating their quality over extended periods of time. The degradation of liquid hydrocarbons due to oxidation with the oxygen, from atmosphere or from the interacted gas medium, is called as ageing. In general, these ageing reactions take places over the course of months to years depending on the fuel quality and the surrounding atmospheric or ambient conditions. The common oxidation reactions are very well described by Biernat [1] with radical chain reactions. The critical part of this oxidation lies on the initiation step where the separation of the allyl radical starts [1, 2]. This required the longer time depending on different factors, where the following steps are in general very fast and self-accelerating chain reactions [1, 3]. So, the initiation step of the reaction is considered as the time taking step, also known as induction time, required to start the ageing and this in turn represented as the stability time for estimating the quality of the fuel over longer periods of time [2]. Accelerated ageing methods have helped in this regard, to analyze the ageing behavior of fuel over longer periods in short amount of time [2, 4]. The principal idea behind these methods is to induce stress on the fuel and force the oxidation reactions [1, 2, 5]. For this the medium is provided with abundant oxygen and heated to higher temperatures to provide the required activation energy for the reaction to start. After the initiation step the reaction flow is self-sustaining and fast chain reactions [2, 4]. Thus, the fuel can be aged in a short amount of time, which in turn takes months, to analyze the long-term behavior. The application of such methods for the novel fuels such as E-fuels could help in estimating their long-term behavior in the existing infrastructure and also implement necessary changes. They can be on the fuel side, by adding Additives or from the infrastructure side such as sealing materials, etc [1, 3]. Current paper discusses one of the typical accelerated ageing methods and its applications. This paper covers the unknown territories of the method such as 1. The impact of variable heating and cooling rates on the ageing method and ageing phenomenon 2. The impact of variable oxygen content available for the ageing of liquid fuels. The idea behind the current investigation is to understand the detailed impact of different parameters on the fuel ageing and also implying towards the criticalities of the method itself. 118 13th International Colloquium Fuels - September 2021 Influence of variable oxygen concentration on the accelerated ageing of liquid fuels 2. Test setup Current section explains the test setup that is developed at Tec4Fuels. Figure 1 shows the P&ID diagram of the test setup. Figure 1: P&ID of the accelerated ageing test setup The test setup consists of an oil bath, in which the reactors can be placed for heating. The bath can accommodate 8 reactors simultaneously. The bath is equipped with a heating coil heater which is used to heat up the bath to the required temperature. The Stirrers at each end of the bath and the heating coil pump in the middle of the bath helps to maintain the bath temperature through out test period. The fuel sample will be placed in a clean reactor and placed in the bath. The reactor is a 1.5 litre steel vessel to which a thermocouple, pressure sensor and a magnetic valve are attached. The pressure inside the reactor is limited to 10 bar due to the sensitivity of the pressure sensor. The reactors are compatible with Diesel, Gasoline and similar type of fuels and further cooling fluids were also tested. These reactors are filled with the required amount of fuel, 500 ml to 1 litre and then filled with required gas medium (Air, Nitrogen or Oxygen). The reactors can be placed inside the bath and the test times are flexible depending upon the requirement or fluid quality. The temperature settings are also flexible from 60 °C to 150 °C, depending on the fluid type. As the bath can accommodate 8 reactors, the fluids that can be used are flexible from 8 different type of fluids to same fluid for reproducibility tests. After the experiment, the reactors are removed from the bath, cooled down and de-pressurised before opening the cap and collecting the sample. As the temperature conditions, reactor volumes are the same, the experiments are highly reproducible. The pressure curve determines the stability of the fluid sample. The consumption of oxygen by the fluid for the oxidation reaction is reciprocated by a pressure drop due to the reduction of oxygen partial pressure. Thus, the time required for the pressure drop to occur indicate the stability of the fluid sample also known as the Induction time. The comparison between the induction times of different fluid samples indicates the comparison of their stability. The fluid samples are also analysed before and after the ageing experiment, in order to determine their degradation over time. Figure 2 indicate a sample pressure curve from the ageing of standard heating oil. Figure 2: Sample pressure curve of a typical BigOxy test. The pressure drop is observed between phases 2 and 3. 3. Test Matrix In the current investigation, a standard heating oil was used according to DIN 51603-1. The current test matrix consists of two different types of experiments a) with variable heat up rates and b) varying the oxygen concentration available for the ageing method. In the first case, the heating rate can be varied by placing the reactor in the bath at different times. In this case, one reactor was placed before the heat up of the oil bath and heated up together, while the second method is to place the reactor after the bath has arrive the required temperature. The former method is called as ramp heating while the second method is known as fast heating. The pressure and temperature of both the reactors were kept constant at 5 bar and 105 °C. Second type of experiments involves varying the oxygen concentration. This was achieved by changing the gas pressure inside the reactor. The gaseous atmosphere in the current experiments was air. With varying the air pressure inside the reactor, the partial pressure of oxygen is also varied. Thus, different reactors are filled with different pressure from 1.2 bar, 2 bar, 3 bar and 5 bar. The temperature of the fuel is kemp content in both the test methods at 105 °C. this temperature was determined to be the idle temperature setting for Diesel like fuels by Koch, in his investigations. 4. Analysis Figure 3 indicates the pressure curves of the variable heating rate experiment. The experiment was repeated several times to observe the repeatability of the test. From the pressure curves it is evident that the change of heating rate has no impact on the induction time. This was also proved by comparing the analytical data of the fuel sample. This means that the fuel quality was the same in the heat up cases after the ageing for 72 hours. If the curves are observed closely the ramp heating suggests that it has observed the heat almost 30 minutes longer than 13th International Colloquium Fuels - September 2021 119 Influence of variable oxygen concentration on the accelerated ageing of liquid fuels the fast-heating case. But it has no impact on the overall stability or the ageing of the fuel. This phenomenon can be explained by supporting the argument by Koch in his ageing method investigations. “The temperature has a big impact on the ageing of the fuels. Also, until a certain temperature limit, the ageing is, in general, slow due to the lack of required activation energy and the rate might increase rapidly after certain temperature limit”. Since in the ramp heating case, the fuel has observed lower temperature for almost 15 minutes and only a few minutes of higher temperature relative to fast heating, the impact on the ageing is negligible. Thus, the heating rate in general can be neglected in the ageing method unless the heating rate is too slow and takes much longer time to reach the maximum temperature point. So, fast heating method is generally recommended to keep the method highly repeatable and avoid any additional impacts. Figure 3: Pressure curves with different heat up rates Figure 4 shows the pressure curves at different initial gas pressure in the atmosphere. The first indication is that the induction times have been the same in all the cases. This can be explained by implying that the activation energy required in all the cases is similar irrespective of the oxygen concentration. So, the initiation step has started at the same point in all the four cases. But a close observation has shown that the pressure drop is different in different cases. Figure 4: Pressure curves with variable oxygen concentration In case of 1.2 bar pressure reactor the pressure drop is 20 % imply that all the oxygen from the reactor gas volume is absorbed, while in case of 5 bar the absorption is limited. In terms of quantity that has been absorbed, in 5 bar pressurised reactor, more oxygen is consumed compared to the 1.2 bar reactor. In the following table the absorbed oxygen quantities were mentioned. Thus, the variable oxygen consumption has shown a direct correlation with the acid content and water content of the fuel sample after ageing. The higher the amount of oxygen consumed the higher the water and acid contents. Thus, implies that by controlling oxygen content in the gas phase the ageing method can be controlled. Implying that the atmosphere where the fuel is stored in the real time is also very important along with the temperatures it is exposed to. Table 1: Comparison of Acid content and water content with the rate of oxygen consumption (% of oxygen consumed from the available gas phase) 5. Conclusion and outlook In the current paper two variable that impact the ageing methodology are briefly discussed. The heating rate has low to no impact on the complete ageing method until the heating rate is not too slow. Also, the oxygen concentration available for the ageing method has a direct impact on the fuel quality after the ageing method. These analyses show a respectable argument on the above-mentioned statements, but a further investigation is still due considering different additional variable that may impact the degradation process. For example, the presence of different oxygen concentration in the gas phase but with similar gas pressure inside the reactor. This could lead to different diffusion rates and can lead to more oxygen consumption and different ageing products. These cases are still under investigation. Similarly, the variable cooling method, and the impact of thermal ageing alone after the pressure drop are also few important variables that must be investigated. 120 13th International Colloquium Fuels - September 2021 Influence of variable oxygen concentration on the accelerated ageing of liquid fuels References [1] Biernat K (Ed.). Storage Stability of Fuels. InTech (2015). [2] Chandra Kanth Kosuru, Natalia Eurich, Simon Eiden (Ed.). Development of a Test method for Accelerated ageing of Gasoline fuels (2019). [3] Owczuk M, Kołodziejczyk K. Liquid Fuel Ageing Processes in Long-term Storage Conditions. In: Biernat K (Ed.). Storage Stability of Fuels. InTech (2015). [4] Koch W, Eiden S, Feldhoff S, Diarra D. Entwicklung Einer Neuen Prüfmethode Zur Bewertung Der Stabilität Von Heizölen Mit Biogenen Anteilen: Winfried Koch, Simon Eiden, Sebastian Feldhoff, David Diarra. Deutsche Wissenschaftliche Gesellschaft für Erdöl, Hamburg (2017). [5] Black S, Ferrell JR. Accelerated aging of fast pyrolysis bio-oil: a new method based on carbonyl titration. RSC Adv. 10(17), 10046-10054 (2020).