Alcohols/Ethers as Oxygenates in Diesel Fuel: Properties of Blended Fuels and Evaluation of Practiacl Experiences

Oxygenates blended into diesel fuel can serve at least two purposes. Components based on renewable feedstocks make it possible to introduce a renewable component into diesel fuel. Secondly, oxygenates blended into diesel fuel might help to reduce emissions. A number of different oxygenates have been considered as components for diesel fuel. These oxygenates include various alcohols, ethers, esters and carbonates. Of the oxygenates, ethanol is the most common and almost all practical experiences have been generated from the use of diesel/ethanol blends (E-diesel). Biodiesel was not included in this study. Adding ethanol to diesel will reduce cetane, and therefore, both cetane improver and lubricity additives might be needed. Diesel/ethanol emulsions obtained with emulsifiers or without additives are 'milky' mixtures. Micro-emulsions of ethanol and diesel can be obtained using additives containing surfactants or co-solvents. The microemulsions are chemically and thermodynamically stable, they are clear and bright blends, unlike the emulsions. Storage and handling regulations for fuels are based on the flash point. The problem with, e.g., ethanol into diesel is that ethanol lowers the flash point of the blend significantly even at low concentrations. Regarding safety, diesel-ethanol blends fall into the same category as gasoline. Higher alcohols are more suitable for diesel blending than ethanol. Currently, various standards and specifications set rather tight limits for diesel fuel composition and properties. It should be noted that, e.g., E-diesel does not fulfil any current diesel specification and it cannot, thus, be sold as general diesel fuel. Some blends have already received approvals for special applications. The critical factors of the potential commercial use of these blends include blend properties such as stability, viscosity and lubricity, safety and materials compatibility. The effect of the fuel on engine performance, durability and emissions is also of importance. So far, no engine manufacturers have indicated they will extend warranty coverage to their equipment when operating with E-diesel. They believe there are simply too many unanswered questions as well as the potential for liability exposure due to the increased flammability range of E-diesel. The reports on field tests with oxygenated diesel fuels are rather scarce, especially reports on recent tests. There are, however, some reports available on engine tests and tests with trucks, buses and even off-road equipment. Most of the available test results identified fuel economy and cost as the only appreciable differences between E-diesel and conventional diesel fuel. Most emissions tests with heavy-duty engines confirm the effect of a substantial reduction in PM when running with E-diesel. The typical range for PM reduction is 20 -- 40 %. Most studies also report reduced NOx emissions. Earlier, there were a lot of activities with E-diesel in Sweden. For the time being, California and Brazil are leading the development of E-diesel.