Water detection in alcohol blended fuelsAudio version
Publication: PetrolPlaza Technology Corner
Issued: November 2010
Author: Schrittenlacher Wolfgang, Dr., FAFNIR GmbH
The global necessity of the reduction of CO2 emissions has led to ordinances in many countries for the use of so called biofuels. The most propagated ones are ethanol and biodiesel (RME) that are produced from plants having absorbed an equivalent amount of CO2 from the atmosphere during growth that is emitted again after the combustion. These biofuels are mainly used as an admixture to conventional gasoline and diesel for two reasons. One reason is its limited availability. The second reason is that most vehicles to date are not capable to use higher percentage of admixture of the biofuel. On the vehicle side there are problems with the resistance of elastomers to this fuels as well as the difference in the combustion properties /DGMK Report 645/. In the last years more and more flexible fuel vehicles (FFV) were put on the road equipped with automatic fuel quality sensor being capable to use gasoline from 0 % to 100 % ethanol content. New cars are equipped with sealings that can withstand moderate percentages of biofuel admixture. In order to gather experiences many countries have started with an admixture of 5 % ethanol to the gasoline (E5) and in the meantime several countries have increased the limit by ordinance to 10 % ethanol (E10). But there are also higher percentages in the market like E25, E50, E85 up to E100. Partially similar problems are encountered by the admixture of methanol to gasoline. The stimulus to use methanol is not the CO2 reduction goal but the limited availability of crude oil and foreign exchange in some countries. A substantial amount of methanol is produced from coal and admixed up to 25 % (M25). One further serious problem on the vehicle side due to the alcohol admixture is the increased wear in the engine by corrosion. Formic acid is formed by the combustion of blends and attacks cylinder and piston but also the engine oil content and therefore the bearings. Special coatings help to protect these components in modern vehicles. The acid formation is even stronger for the methanol admixtures.
There are problems not only on the vehicle side but also significant challenges on the side of the fuel production and in the distribution logistic. The problems in the distribution will be the main topic of the following discussion. The fuel changes vapour pressure and octane number if ethanol is added. Therefore the basic gasoline must be changed in its composition in order to compensate for these effects. The next problem is the much higher solubility of the ethanol blends to sludge and water that might be present in a storage tank. This causes many problems after a product change in the tank from a conventional gasoline to a gasoline blended with alcohol. The main problem is the frequent clogging of the fuel filters in the dispenser. Therefore many oil companies decided to clean the tanks before storing a blend. But also the attack to elastomers and increased corrosion of metals that have been used for the pipe work has been observed.
Automatic Tank Gauges
For the operation of storage tanks automatic tank gauges (ATG) are used since about 25 years. They eliminate the need of the use of the dip stick and are much more accurate and allow for many new features in the application as for example the optimisation of the logistics by using the level data from a remote computer system. The performance requirements for such ATG’s are given in the European standard /EN 13352/. It states the necessary accuracy for the product level determination, the temperature measurement and the water level detection capability. The latter feature is mandatory because without the detection of a water contamination there is a risk that the vehicles refuelled will stop running.
The water detection process is dependent of the type of ATG. For capacitive probes the water is detected by the difference in dielectric constant, for ultrasonic probes in the difference in acoustic impedance and in floater systems by the density. The floater systems on the basis of the magnetostrictive principle have by far the biggest market share and the corresponding challenges are therefore considered in more detail.