Diesel Fuel Additives

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Abstract: The characteristics of diesel fuel can be improved through the use of additives, which are added at the refinery, at the distribution terminal, or by the end user. The many different additives can be categorized in different ways based on the chemistry, purpose, etc. One convenient way of categorizing them is to group them as additives used to aid handling and distribution, to improve fuel stability, to protect engines and fuel systems and additives to influence the combustion process. Some additives may influence more than one category and of course additives can be combined to produce multi-functional additive packages.

Introduction

Until the latter part of the twentieth century there was little or no use of diesel fuel additives. Due to versatility and robustness of the diesel engine, suitable diesel fuel could be produced from a blend of straight-run atmospheric distillation components. Where a refiner had a necessity to bias production towards gasoline then the diesel pool could often be supplemented with cracked gas oils from the gasoline refining process. As fuel sulfur levels were gradually reduced then additional processing could be required depending on the crude oil source. With the increasing fuel demand, changing demand mix and tightening specifications the refining processes have changed and with it the use of diesel fuel additives. Although there is no rigorous definition of what constitutes an additive, as opposed to a blending component, it is generally accepted that an additive is something added at less than 1% w/w (i.e. 10,000 mg/kg or 10,000 ppm). Because of this low treat rate of additives the physical properties of the fuel, such as density, viscosity, and volatility are not changed significantly.

To increase the yield of diesel fuel the refiner must cut deeper into the crude feedstock; necessitating the use of flow improvers to restore the low temperature performance of the fuel. With increasing demand for improved ignition quality and increasing cetane number specifications the use of ignition improver additives has also risen. As legislation specifying ultra-lower fuel sulfur levels has spread, the ability of the diesel fuel to lubricate the fuel injection equipment has diminished; this has necessitated the use of lubricity additives. The additives discussed in the following sections can be categorized as follows:

The more widespread inclusion of biodiesel as part of the diesel fuel blend will also necessitate the use of fuel additives. However, these additives will usually be included in the biodiesel itself in order to ensure that the biodiesel meets the relevant specification. This is discussed in greater detail under Biodiesel—Mono Alkyl Esters. Therefore, blending diesel fuel with on-specification biodiesel should not require additional additization.

Additives may be added to diesel fuel at three different stages: (1) at the refinery, (2) in the fuel distribution system, and (3) after the fuel has left the control of the producer. Additives of the latter group, when added by the end user or a reseller, are called aftermarket additives. One notable exception to this latter point is the use of Fuel Borne Catalysts (FBC) that are added to the fuel on the vehicle and form part of the vehicle manufacturers emissions control strategy. These additives are discussed in Filters Using Fuel Borne Catalysts.

Refinery Additization. Fuel refiners must ensure that their products meet the relevant specifications for the location and time of year, and are suitable for the intended use. This can be achieved through such means as the choice of crude oil, refinery processing, blending, or the use of additives. The final choice of methods is driven by economics. The extent to which a particular refinery will rely on additives is therefore dependent on many factors and the exact extent of additive usage remains unclear.

Distribution System Additization. Pipeline operators sometimes inject drag-reducing additives (to increase the pipeline capacity) and/or corrosion inhibitors. To help control costs it is common practice in many countries for oil refiners to produce a fuel meeting the basic legislated specification and to sell or exchange this fuel to other fuel marketing companies. However, there is a growing awareness of the need for marketplace product differentiation; this applies to fuel as much as any other product. It is thus becoming common in many countries for additive packages to be included at the refineries distribution terminal to support the marketing company’s quality claims or standards; for example to produce “regular” or “premium” grade diesel fuels or simply to try and differentiate one company from another. This practice has been widely adopted in Europe and other parts of the world. Additives can also be added at the retail pump, allowing fuel retailers to market more than one grade of diesel fuel at a retail site without the need for separate storage tanks.

Aftermarket Additives. Some users will treat their fuel with additives to meet their particular needs, for instance cold climate operation, or because they believe they need a higher quality fuel. A wide range of aftermarket additives are available from a number of suppliers. Some of these additives may have legitimate uses. For instance, the use of de-icers may be warranted under cold weather conditions and/or when problems with fuel system icing are encountered. In many cases, however, aftermarket additives packages consist of compounds such as detergents, lubricity improvers and cetane enhancers that would normally be added at the refinery or fuel terminal by the fuel marketer.

Users should be cautious when considering the use of any aftermarket additives. Some aftermarket additives are aggressively marketed, with performance claims that are often too good to be true. Yet, in most cases, they are not needed and should be avoided; this is especially true of modern high technology diesel engines. Good quality commercial fuels from reputable marketers contain all the additives that a fuel needs and have been extensively tested to minimize the possibility of adverse interactions between different additive and/or fuel components.

If the user still feels that additives are needed, they should be chosen based on careful research, and used in accordance with the recommendations of the additive supplier and the engine manufacturer. Inappropriate use of additives may have adverse effects on the engine, and may affect engine warranties (for example, some engine makers require that alcohol-based de-icers not be used).