Biofuels - Diesel

08/06/2009 by David Slade.

The different flavors of renewable diesel

Introduction

Distillate fuel oils, a category of fuels which includes petroleum diesel and home heating oil, account for almost 30% of global petroleum consumption. As fossil fuel reserves deplete, sustainable alternatives to petroleum-based products are needed. Biologically based distillate fuel oils have a long history, as peanut oil and whale oil were used as lubricants and energy sources long before they were displaced by petroleum products.

Renewable diesel can be produced via several different technologies and from a wide variety of starting materials, including edible oils such as soybean oil, cottonseed oil, animal fats, and waste cooking grease – or non-edible oils such as jatropha oil or algal oils.

Straight Vegetable Oil

Unmodified vegetable-derived triglycerides, commonly known as vegetable oil, may be used to fuel a diesel engine. In 1900 Rudolf Diesel demonstrated the use of peanut oil as fuel for one of his early diesel engines. Modern diesel engines are also capable of running on straight (unmodified) vegetable oil (SVO) or waste grease. Numerous
engine performance and emission tests have been conducted with SVO derived from many various sources, either as a standalone fuel or as a mixture with petroleum diesel.

The advantage of SVO as fuel is that a minimal amount of processing is required, which lowers the production costs of the fuel. The energy return for SVO, defined as energy output over the energy required to produce the fuel, will also be higher due to the avoidance of energy intensive refining steps.

There are several disadvantages of using SVO as fuel. The first is that researchers have found that engine performance suffers, and that hydrocarbon and carbon monoxide emissions increase relative to petroleum diesel. Particulate emissions were also observed to be higher with SVO. However, the same studies found that nitrogen oxide (NOx) emissions were lower for SVO. On longer-term tests, carbon deposits have been found in the combustion chamber, and sticky gum deposits have occurred in the fuel lines. SVO also has a very high viscosity relative to most diesel fuels. This reduces its ability to flow, especially in cold weather. This characteristic may be compensated for by heating up the SVO, or by blending it with larger volumes of lower viscosity diesel fuels.

Biodiesel

Biodiesel is defined as the mono-alkyl ester product derived from lipid feedstock like SVO or animal fats. The chemical structure is distinctly different from petroleum diesel, and as a result biodiesel has somewhat different physical and chemical properties from petroleum diesel.

Biodiesel is normally produced by reacting triglycerides (long-chain fatty acids contained in the lipids) with an alcohol in a base-catalyzed reaction. Methanol, ethanol, or even longer chain alcohols may be used as the alcohol, although lower-cost methanol is typically preferred. The primary products of the reaction are the alkyl
ester (e.g., methyl ester if methanol is used) and glycerol. The key advantage over straight vegetable oil (SVO) is that the viscosity is greatly reduced, albeit at the cost of additional processing and a glycerol byproduct.

An EPA study published in 2002 showed that the impact of biodiesel on exhaust emissions was generally favorable. Compared to petroleum diesel, a pure blend of biodiesel was estimated to increase the emission of NOx by 10%, but reduce emissions of carbon monoxide and particulate matter by almost 50%. Hydrocarbon emissions from biodiesel
were reduced by almost 70% relative to petroleum diesel.

Biodiesel does have characteristics that make it problematic in cold weather conditions. The cloud and pour points of biodiesel can be 20° C or higher than for petroleum diesel. This is a disadvantage for the usage of biodiesel in cold climates, and limits the blending percentage with petroleum diesel in cold weather.

Green Diesel

Another form of renewable diesel is ‘green diesel.’ Green diesel is chemically the same as petroleum diesel, but it is made from recently living biomass. Unlike biodiesel, green diesel is composed of long-chain hydrocarbons, and can be mixed with petroleum diesel in any proportion for use as transportation fuel. Green diesel technology is frequently referred to as second-generation renewable diesel technology.

There are two methods of making green diesel. One is to hydroprocess vegetable oil or animal fats. Hydroprocessing has long been used in the petroleum industry to ‘crack’, or convert very large organic molecules into smaller organic molecules, ranging from those suitable for liquid petroleum gas (LPG) applications through those suitable for use as
distillate fuels. In addition to producing a petroleum diesel equivalent, the by-product of hydroprocessing is propane instead of glycerol.

The second method of making green diesel involves partially combusting (gasifying) a biomass source to produce carbon monoxide and hydrogen – syngas – and then utilizing the Fischer-Tropsch reaction to produce complex hydrocarbons. This process is commonly called the biomass-to-liquids, or BTL process. Renewable diesel produced via BTL technology has one substantial advantage over biodiesel and hydrocracking technologies: Any source of biomass may be converted via BTL. Biodiesel and hydrocracking processes are limited to lipids. This restricts their application to a feedstock that is very small in the context of the world’s available biomass.

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