Biofuels blend measurements with portable mid-infrared analyzers
Globally, governments are putting more emphasis on sustainable energy sources and are creating energy policies that promote the production and use of biofuels. With the Renewable Fuels Standard-2 (RFS-2), the United States set biofuels blending volumes to 12.95 billion gallons (49.0 billion liters) for 2010, increasing to 36 billion gallons (136 billion liters) by 2022. With similar mandates or guidelines coming from other countries, the need to easily and accurately measure biofuels blends becomes more important.
BLENDING METHODS
The splash blend method is often used for blending biodiesel. With this method, the petrodiesel fuel and B100 (i.e., 100% biodiesel) are pumped separately into a delivery truck or storage tank, and it is assumed the blend will have been adequately mixed in the tank by the time the truck arrives at the delivery site. However, a demonstration test was recently conducted with the Wilks InfraCal Biodiesel Blend Analyzer (shown in Fig. 1) five minutes after filling the truck for B20. A sample taken from the top measured 11.9% biodiesel, while another from the bottom was 24.1% (c.f. inform 19:787–789, 2008). If the first delivery of fuel from a splash blend-loaded truck is only a few miles away on a smooth road, the chance for delivering an accurate blend/mix is far from guaranteed. And subsequent deliveries will also have an incorrect blend. In-line (injection) blending offers better blend consistency than splash blending and is typically used for ethanol blending at pipeline racks and terminals. It is also becoming more common for biodiesel blending. The biofuel is mixed as it is metered into the pipe with the diesel or gasoline. Additional mixing occurs as the fuels enter the receiving tank or truck. For biodiesel, density and viscosity changes require adjustments to the meters for an accurate blend. Although manufacturers of in-line blending systems claim indisputable accuracy, a quick check for the correct blend gives actual data to validate this claimed assumption.
Infrared Analysis of Biofuels
The approved methods for biodiesel measurement, EN 14078 and the ASTM Method D7371, both specify mid-infrared spectroscopy for the measurement of the biodiesel blend ratio. Infrared analysis works well for FAME (fatty acid methyl esters) because the biodiesel ester has a characteristic infrared absorption due to the carbonyl bond at 5.7 micrometers or 1745 cm–1. In other words, as the concentration of biodiesel goes up, the infrared absorbance at that wavelength increases. The infrared absorbance can be directly calibrated to read out in percent biodiesel. The same is true for ethanol, as it has an infrared absorbance band unique to gasoline at 9.6 micrometers (1042 cm–1). Again, the increase of infrared absorbance correlates with the increase in concentration of ethanol in gasoline.
Advantages of filter-based infrared analyzers
An infrared analyzer can be set up with a filter mounted on a detector that is specific to the analysis, in this case either 1745 cm–1 for biodiesel or 1042 cm–1 for ethanol. The advantages of a filter-based infrared analyzer are many: convenient size, lower cost, ruggedness, decreased power usage. Regulatory agencies often travel to a fuel station and check to ensure what is specified on the pump is correct. For their application, a six inch square box that will fit in their truck, operate off of a 12-volt car battery, function in a wide range of temperature and humidity, and is quite rugged with no moving parts would be ideal. These features are also important to blenders and terminal operators as they would like the advantage of checking a truck, railcar, or tank on-site after it is loaded rather than hold up a delivery while a sample is taken to a laboratory for testing. Another critical feature is ease of use. In the analyzer pictured in Figure 1, a fuel sample is placed directly on the exposed sample window, and in less than a minute the percent ethanol or biodiesel is displayed. The sample is cleaned with a wipe, and the analyzer is ready for the next measurement.
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