The terms biodiesel and green diesel are easy to confuse and often are. Both of these products are refined from vegetable oil (and occasionally animal fat). What sets these two diesel fuels apart are the processes used to produce them and the end product that results.
Biodiesel is produced by reacting triglycerides with alcohol to produce fatty acid esters as well as glycerol, which is a byproduct. This chemical reaction is referred to as transesterification. It produces biodiesel, which is about 9% less energy dense than petro diesel. Biodiesel requires that engines be modified so that damage to rings, fuel lines, and other rubber components.
Green diesel is produced through a refining process, rather than through a chemical reaction. The result is that green diesel is chemically identical to petro diesel, except that it does not contain sulfur. Green diesel is nothing more than renewable diesel fuel. It is not necessarily green in the sense that it protects against global warming. It is "greener" than standard diesel though because it reduces particulate emissions as well as odor. Green diesel will run in any engine without modification.
Production of Biodiesel
The production of green diesel is, in many ways, no different from the production of petrol diesel. Both go through a refining process to yield a fuel at the end. In other words, producing green or petrol diesel is more about separating the components of interest from the rest of the material than about changing the fundamental properties of the feedstock. Biodiesel, on the hand, is all about chemically altering the feedstock.
There are actually several processes for producing biodiesel that include batch processing, supercritical processing, ultrasonic processing, and microprocessing. Though vastly different in application, each of these procedures has the same outcome; the transesterification of triglycerides.
Triglycerides are a type of biological molecule common in plants and animals. They are made up of three atoms, oxygen, carbon, and hydrogen. They look something like this.
The part in red is called glycerol and the three black chains are each fatty acids. So, a triglyceride molecule is made up of one glycerol and three fatty acid molecules. The fatty acid chains are very similar to hydrocarbons, so scientists use processes to break off the glycerol and release the fatty acids. This process is called transesterification and it produces an ester (actually three, one for each fatty acid). An ester looks like this:
The red part is referred to as the "R group" and can be any length from one carbon up. Biodiesel also contains fatty acid alcohols, which look like the following.
The process of transesterification also produces glycerol, a type of alcohol, as a byproduct. For every metric ton (tone) of biodiesel that is produced, 100 kilograms of is produced. Glycerol looks like the following.
Biodiesel Properties and Cetane Rating
The esters in biodiesel are superior to petrol diesel in centane rating, which is similar to octane rating for gasoline. The cetane rating is a measure of how well a fuel combusts under compression ignition. This is different from octane, which measure combustion under spark combustion. What is the difference?
In spark combustion, an electric spark (from a spark plug), is used to ignite a fuel. This is similar to putting a flame to a can of gasoline. Compression ignition works differently. In this process, a gas is compressed to well above atmospheric pressure. Compressing a gas causes its temperature to increase because pressure and temperature are related by one of the three gas laws as follows:
This law tells us that pressure is inversely related to temperature. In other words, if we raise the pressure, then we also raise the temperature so that the equation P/T remains a constant. In diesel engines, the pressure is increased to the point that temperature rises above the combustion point of the fuel, causing ignition. That is to say, no spark is needed in a diesel engine. A standard petrol engine usually has a compression ratio of about 10:1. A diesel engine will have a compression ratio that ranges from 14:1 to 18:1.
So, cetane rating tells us how well a fuel burns under the conditions found in a diesel engine and can basically be thought of as a quality rating. The rating is based off of an alkane called cetane, which has the formula C16H24. With this fuel, the time between fuel being injected into the cylinder and ignition is 2.407 milliseconds, which gets a cetane rating of 100. The longer ignition takes, the lower the cetane number will be.
Cetane number is not the only property of biodiesel that is of interest. Biodiesel is liquid, usually yellow to dark brown, and has a high boiling point. Of most importance, biodiesel has no sulfur content.
Of note, diesel engines do not use lubricant. Instead, they rely on the fuel, which is more oil-like than gasoline, to lubricate the pistons and reduce friction with the cylinder wall. Biodiesel is a better lubricant than low-sulfur diesel, though not quite as good as standard diesel (which is being phased out due to pollution).
Gelling and Low Temperatures
One of the things that determines the temperature at which a molecule freezes is polarity. The more polar a molecule is, the easier it is for it to form a repeating crystal structure and become solid. Biodiesel, because it contains oxygen, is more polar than standard diesel and thus more prone to freezing. Though the fuel usually does not freeze solid, it does develop small crystals. This is referred to as gelling.
Biodiesel will begin to gel at temperatures as high as 16 degrees Celsius depending on the feedstock used in its production. The best feedstock in terms of gelling is canola oil, which produces biodiesel that gels at -10 degrees Celsius. Petro diesel usually gels at -15 to -19 degrees Celsius.
Like other biofuels, biodiesel cannot be used in standard diesel engines. The alcohol content and different structure of biodiesel means that it can react with the rubbers used in standard engines and cause them to dry and crack. This leads to leaks and engine failure. The solution is to use different rubbers, which is the case with newer diesel engines.
Fuel Economy, Greenhouse Gases, and Acid Rain
Diesel engines are "lean burn" engines, which means they have more air in the combustion chamber than needed to complete the reaction. This means they use less fuel to go the same distance than a gasoline engine. Biodiesel usually offers about 9% less energy density than standard diesel, which reduces the distance traveled on a given quantity of fuel. However, these engines are still more efficient than gasoline engines and provide better distance for a given quantity of fuel.
By burning less fuel per distance, diesel engines also produce less greenhouse gas emissions. Even though a burning the same quantity of diesel fuel will yield MORE CO2 than an equivalent amount of gasoline, the increased fuel economy more than offsets this. A diesel engine will produce 10-20% less greenhouse gas than the same sized gasoline engine. Overall, the EPA estimates that biofuels reduce greenhouse gas emissions by 57% compared to diesel.
The down side to diesel is that it produces more nitrogen compounds, which means more acid rain. This is actually made worse by using biodiesel because the inevitable contamination with biologic material means there is more nitrogen present in biodiesel. Counterbalancing the production of acid rain from nitrogen compounds is the lack of sulfur in biodiesel. This means there is less sulfuric acid produced by biodiesel.
As usual, feedstock and where it is grown can make or break biodiesel. Though most biodiesel is currently made from waste vegetable oil or processed corn, there is research into algae, pongamia, Jatropha, Fungi, and used coffee grounds. Algae presents the best situation as it would not result in land-use changes, would not replace or impact a food crop, and is economical. What is even more appealing is that it may be possible to grow algae on ponds in wastewater treatment facilities, making it possible to improve water quality while also producing fuel.