The test was carried for bio-diesel by accelerated oxidation conditions with oxygen and heat at 40°C.
We observed an increase in unsaturation in the FAME composition, increase in peroxide value (found the induction period), kinematic viscosity, a slight increase in density, and an increase in acid value with respect to the base sample.
We can't justify why calorific value or higher heating value is decreasing.
Is there any relation or correlation between kinematic viscosity and heating value?
If the FAME composition changes on rapid oxidation conditions, that may itself cause changes in thermophysical properties including calorific value. Please compare the composition of the initial fuel and after the process is complete. It may also be helpful if flash points are measured.
Prof. Raghavan, thanks for your suggestion. Oleic acid variation showed 35% increase compared to base oil, but other fatty acids did not show too much variation. Flash and fire points was also measured during experiment. Flash point was observed with decreasing trend up to induction period and then increasing trend thereafter. flash point had 0.05% increase and fire point had 5% decrease as compared to the base oil. I hope flash point and HHV correlations are available to justify the result.
whether due to oxidation chances of water content increases, as the samples was cooled down to room temperature for HHV measurement..
As a chemist I have a chemical explanation. (1) Oxydation may create water, thats' true. (2) The oxidation products have intrinsically a lower calorific power because the carbon-oxygen bond in these products will produce less heat in burning than C-C or C-H bonds to give CO2 and H2O.
Hi Balu, It may be that the increase of high calorific values of biodiesel due to increased in unsaturation!!, but it is confusing that was observed an increase in unsaturated of biodiesel.
I understood that the calorific value decreased with oxidation (and unsaturation).
Oxidation may increases unsaturation by removing H2 from hydrocarbons:
alkane + 1/2 O2 --> alkene + H2O... etc
Hi Noureddin, kindly check the google book by Amit sarin (biodiesel production and properties) --page 129 (Prediction of HHV). It states "As fatty acid carbon chain increases in length (for a constant unsaturation level) the mass fraction of the oxygen decreases , so HHV increases. However this increase in HHV with chain length is not readily apparent "....
Using heat of formation of model compounds and increments for CH2, CH=CH, COOH, COOR, etc. it may be possible to make quantitative and accurate estimates of the heat of ,=combustion, but I suppose that the specialists already use this possibility. The reference you give uses (approximate) correlations between physical properties (viscisity, density...) and HHV. The problem is that the molecular composition should be known. Check "Prediction of Heating Value of Vegetable Oil-based ethyl esters..." (on Google ) J. Agric. Cult. Machinery Sci, 2011-7(4)-361-366, the heat of combustion is related to viscosity, density, etc.
I totally agree with the views expressed by Prof. Jean Francois Gal and Prof Raghavan regarding the reduction of calorific value of FAME with rapid oxidation condtions.
From; Method for simultaneous determination of thermal conductivity and kinematic viscosity
US 5452601 A
The relationship between kinematic viscosity of the fluid and a differential value between the temperature of the heating sensor and the temperature of the fluid is used to determine the kinematic viscosity of the fluid. ,
Additional supply of oxygen may change the positon of unsaturated fattty acids in the composition of biodiesel, and may cause changes in density, kinematic visocity and other properties. Initial temperature is not specified . Otherwise temperature effects may also to be considered.
In Chemistry, you can estimate HHV of a fuel from Dulong's formula: HHV = 14,544C + 62,028(H -O/8) + 4050S. where C=wt. fraction of carbon in the fuel; (H-O/8)= total wt. fraction of Hydrogen minus wt. fraction of Oxygen/8 and S = wt fraction of net Sulphur. Assuming no sulphur in your BD. Oxidation could be the removal of Hydrogen or addition of Oxygen from/to the fuel. Therefore the removal of hydrogen or addition of oxygen would contribute to the decrease of HHV according to Dulong's formula.
For the increase in saturation, this could be associated with the removal of hydrogen from the fuel.
Aldo made a nice addition to the discussion. Although the Dulong formula is probably an simplification of additivity rules and group increments to heat of combustion (from the heat of formation), it is applicable when the molecular composition is not known.
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