In my previous blog article on the topic of the role of elemental analysis in making energy greener, I covered lithium-ion batteries. In this second of three articles, I’ll explore the realm of biofuels and discuss the part that elemental analysis plays in ensuring the quality of these materials.
The role of biofuels in the drive toward a more sustainable energy infrastructure
Biofuels, by definition, are fuels produced from biomass sources, such as plants, or industrial/domestic waste (of biological origin), via microbial processes, such as fermentation, or chemical processes, such as photosynthesis or transesterification. The term biofuel is usually applied to liquid or gaseous fuels used for transportation purposes. The two most common types of biofuel are bioethanol and biodiesel. Bioethanol is most often produced by fermentation of carbohydrates in sugar or starch crops, such as corn or sugarcane, but use of cellulose-based biomass (trees and grasses) is also being explored for ethanol production. Ethanol can be used in its pure form (i.e., E100) for powering vehicles, but it’s usually used as an additive to fossil fuels to improve the fuel octane rating and reduce vehicle emissions. This year, the EU and the UK introduced E10 petrol (i.e., petrol containing 10% bioethanol) at all fuel stations, bringing them into line with the U.S. and Australia, where this fuel blend has been commercially available for several years.
Biodiesel, in contrast, is produced from oils or fats using a transesterification process. It is currently the most common biofuel used in Europe. Like bioethanol, biodiesel can be used as a fuel in its pure form (B100), but it’s usually used as a blend with conventional diesel. The most common biodiesel blend is B20 (up to 20% biodiesel mixed with petroleum diesel), but B5 (5% biodiesel, 95% diesel) is also commonly used in fleet vehicles. Biodiesel increases the cetane number of diesel (a number related to the ignitability of the fuel) and reduces the level of particulates, carbon monoxide, and hydrocarbons emitted from diesel-powered vehicles.
Biofuel, as well as being a key component in the push toward a net zero carbon economy, is big business. In 2019, worldwide biofuel production reached 161 billion liters (an increase of 6% over 20181) and biofuels provided 3% of the world's road transport fuel. The International Energy Agency has stated that they want biofuels to meet more than a quarter of world demand for transportation by 2050, but the world is currently not on track to meet this goal. To meet the IEA’s target, from 2020 to 2030 global biofuel output needs to increase by 10% each year, but just 3% growth annually is anticipated over the next five years.1
Which elements are important in biofuels and how are they measured?
The elements of particular concern in biofuels are sulfur, phosphorus and copper. Analysis of sulfur in bioethanol is required to ensure that the level of sulfur dioxide emissions produced when the fuel is burned comply with environmental regulations. Sulfur dioxide leads to acid rain, which has serious, well-documented environmental consequences1, so minimizing the concentration of sulfur in biofuels — as well as fuels generally — is critically important. The concentrations of copper and phosphorus need to be monitored and controlled because these two elements can adversely affect engine operation.
Copper is an efficient catalyst for low-temperature oxidation of hydrocarbons. At Cu concentrations above as little as 0.012 mg/kg, rapid increases in the rate of oxidation lead to formation of a gum that deposits in engine components such as fuel injectors, causing blockages and engine failures. Phosphorus poisons catalysts used in engine exhaust systems, leading to increased emissions of environmentally harmful gases.
The importance of measuring and controlling sulfur, phosphorus and copper has led to the development of various corresponding standards. The current ASTM standard for the copper and sulfur content of denatured fuel ethanol, ASTM D4806-17, gives the permitted concentration of Cu as 0.1 mg/kg and S as 30 mg/kg in most locations, but 10 mg/kg in California. In addition, the International Organization of Standardization (ISO) have produced the ISO EN 15837:2009 standard for analysis of ethanol for copper, phosphorus and sulfur, with copper having a permitted range of 0.05 to 0.3 mg/kg, phosphorus 0.13 to 1.9 mg/kg and sulfur 2 to 15 mg/kg. As well as S, P and Cu, monitoring the concentration of toxic elements such as arsenic, cadmium, mercury and lead in biofuels is important, as combustion of the fuel releases these elements into the environment and also potentially into the food chain.
What trace elemental analysis techniques are suitable for biofuel analysis?