A problem that is sometimes encountered with biodiesel blends is inaccurate blend ratios. This article describes a quick analytical method to assess blend ratios and ensure fuel performance.
According to a study1 done by the National Renewable Energy Laboratory, the chief problem found with biodiesel blends was inaccurate blend ratios. Retail customers and fleet managers do not typically worry about whether the blend ratio is correct until a problem occurs. For fleet managers, this can mean a total shutdown of operations. With the image of biofuels vacillating in the public eye because of food versus fuel issues and the environmental advantages of biofuels, it is important to keep quality standards high in order to further biofuels’ acceptance. Analytical testing equipment is essential to this goal.
Portable mid-infrared analysers offer a quick analytical method to assess the blend ratio of either biodiesel in diesel or ethanol in gasoline and can be a valuable asset not only for distributors and terminal managers who blend the fuel, but also for finished product testing by fleet operators and regulatory agencies.
There are basically two methods of blending biodiesel: splash blending (either in a tank or in the delivery truck) and in-line blending. The fact that biodiesel is typically denser than petroleum diesel, and that the cold flow properties can change with different biodiesel feedstocks, adds challenges to getting an adequate mix during blending.
Ambient temperature is another issue. Blending fuels in cold weather with non-heated biodiesel further reduces the chance of an adequate blend prior to delivery. Each feedstock has a different temperate at which it will begin to solidify or gel, called the cloud point. To account for cold flow properties of each feedstock, B100 should ideally be kept at least 10º above the cloud point for blending and storage. If unheated biodiesel is loaded into an empty delivery truck on a cold day, there could be little or no mixing.
Cold flow properties can also vary according to the feedstock used. In colder temperatures crystals can form and could plug a filter. A reliable distributor may want to change blend ratios according to climate conditions to ensure that customers will have a reliable fuel. All of these mixing challenges suggest the need for quick on-site blend measurements.
In-line (injection) blending: While typically used for ethanol blending at pipeline terminals and racks, some biodiesel distributors are just beginning to consider using in-line blending. With in-line blending, the biofuel is metered into the diesel or gasoline as it travels through a pipe. There are different blending system configurations and equipment. Sequential blending loads the biofuel and petroleum fuel through the same meter sequentially. Ratio blending loads the individual products through common, and least accurate, blending method used for biodiesel is splash blending. Diesel fuel and B100 are pumped by the distributor separately into the delivery truck at the time it is loaded. The hope is that the blend will be adequately mixed by the time the truck gets to the delivery site. Splash blenders are not necessarily open to on-site blend testing. As one blender who preferred to remain anonymous for obvious reasons stated, “What is in it for us except making our blending techniques look bad?”
At a demonstration of a portable fixed-filter infrared analyzer that measures per cent biodiesel (Photo 1), a sample was taken from a delivery truck destined to deliver B20, but the analyser indicated that the blend was only 8.7 per cent biodiesel. The initial reaction was that something must be wrong with the analyzer, which was then checked with a B20 standard and the reading was 20.1 per cent. After some investigation, it was discovered that they had run out of B100 and the truck had been topped off with diesel.
Splash blending also has the risk of inaccurate blend ratios if the delivery truck has not traveled far enough to provide adequate mixing. In another demonstration test, five minutes after filling the truck for B20, a sample taken from the top was 11.9 per cent and another from the bottom was 24.1 per cent.