Thread: Biodiesel Cetane Number

No 6 in the series

We often hear about cetane number mentioned in our circles in regards to biodiesel quality
Very rarely if ever we heard or even seen the term used for dino diesel
When it comes to petrol just about all who drive a motor car wll have come accros High octane or low octane at least the word octane is known
We hear and often see petrol with a label indicating this Octane rating on the fuel bowser, we feel we know the term and also understand it as applied to petrol
We never see a cetane number at the pump for diesel fuel anywhere, do we?
Most of us do not really know what an octane no is or what it stands for in hard reality
We instinctively know that the 100 octane stuff will be better then 94 or a lesser no, the price also tells us the higher no, has to be better
Some consider the higher the octane rating the better the fuel will burn, so the engine will perform better which is about right
Since we have a fair idea of Octane we can bet and say, well Cetane will be for diesel as Octane is for petrol
Now when it comes to diesel fuel the situation is interesting, at the bowser we may see it labelled as diesel or distillate, however there is only one version of it anywhere
No choice on diesel, even though there are a few choices with a variety of octane ratings in petrol
Why is that? would a higher cetane no give me better performance like a high octane no petrol and if so why I cannot buy the better stuff?
What does cetane no mean, how is it measured and how does it come about?
Well one can draw a paralel and say; Cetane number for diesel fuel is analogous to the octane rating of Petrol
Octane and Cetane numbers are a measure of how easily the fuel will ignite in the appropriate engine
We got to keep in mind it is only an analogy, a fair analogy, it is a measure of ignition, but if we think about it carefully it really works the other way with diesel fuel
So in another sense cetane no is the opposite of octane no
Why is that?
Cetane no refers to how a fuel will auto ignite which is the exact opposite to petrol, here is the catch they are both a measure of fuel ignition
The point been that we do not want the petrol in an engine to auto ignite
It will be the cause of pre ignition resulting in the engine to "knock"
The spark plug is of course the agent of ignition in a petrol engine, where, the temperature increase of the extremely high compression of the air intake in a diesel engine is the method of igniting the fuel
I hope the above explantion is sufficient, one needs to think about it for a bit
To get the Cetane no of any diesel fuel a test is carried out in a standardised engine under laboratory conditions which is compared to a reference in the very same engine
The name Cetane is the common description or trivial name of Haxadecane, an alkane which has a straight chain with 16 carbons
It has been given the number 100 as the highest standard,
The lowest standard is at number at 15,
This number has been given to a HMN, another alkane with 9 carbons and a highly branched structure
The proper name for HMN is Heptamethylnonane
We have now got the cetane scale, lowest sitting at 15, the highest at 100
We can then measure the Cetane number of any diesel fuel within the range from the highest standard to the lowest standard
Obviously the ideal fuel would be a straight carbon chain type substance such as Cetane, the worst will be HMN, this highly branched type of material
Well fellows this is where we get very lucky, as biodiesel makers as against dino diesel producers
Nature has smiled upon us, what do we have in Fatty acids or oils and fats?
Just that, Straight chains of carbons
May be it was mean to be, like good old Rudolf said many years ago, he may have had a chat with her, who knows
We make biodiesel from these fatty acids, known as methyl esters, the straight chain carbons are there before and after we have processed the fat or oil to make biodiesel of them
Dino diesel manufacturers produce a fuel which may have the two above mentioned straight chain hydrocarbons, as well as other straight chain components
The fuel will contain a very broad mixture of hydrocarbons other the straight chain ones, they are all derived from crude oil
Naphthalene Toluene benzene, which are not straight chained hydrocarbons will also be included, besides the straight chain carbons
These benzene ring structured compounds are known as Aromatics will be included
Poly aromatics, other hydrocarbons will also form part of the profile of the dino fuel as well Napthalene Toluene etc benzene ring type of hydrocarbons do not have a high Cetane no at all, but they are tightly packed,
Their high density elevates the punch or energy contained in a litre of dino fuel
Since dino diesel manufacturers have been at it for many years the composition of the fuel they produce is known well in advance by knowing the composition of the crude oil they are distilling at the time
Further, the distillation process in itself as well as the down processing carried out in the refinery will determine what short of Distillates will be obtained, so it is fair to say they will be fairly close to the standard
Suffice to say the standard has been established as what is possible within the constrains of the crude oil refining process
Well then what has all this to do with biodiesel?
If we look at any standard pertaining to Cetane number it will most likely say it has to be near or more than 47
Since just about all of the esters found in natural fats and oils have a cetane number near or above 47 and since we only use oil or fat to make biodiesel it stands to reason that the fuel will be at the required standard in so far as cetane rating is concerned
This is quite the opposite of dino diesel where there are mixtures of hydrocarbons all with different properties
Dino Diesel even though predictable, will contain a whole variety of hydrocarbons necessitating a test
Assuming the fuel does not meet the cetane no rating additives as well as adjustment of the fuel takes place before it is offered for sale so as to meet the required standard
This is something the biodiesel producer does not have to be concerned with for the reasons mentioned above
As a final blessing for biodiesel producers, if the composition of the oil is known the fatty acid profile can be easily established
Knowing the fatty acids that make up the oil he can then estimate the cetane number within an acceptable range
Suffice to say Biodiesel made out of natural fats and oils, will most likely meet the required cetane rating specified in the standard even if it fails every other test included there
It is in the nature of the raw material we use to make Biodiesel
Corrections comments or additions welcome