I have a scenario where I used pelleted macroalage for pyrolysis. The heating rates were 20, 30 and 40 C/min. I had experience in using isoconversitional methods in the past were I have seen values as high as 350 kJ mol-1 for microalgae. But recently, with a type of macroalage, the activation energies were more than 1200 kJ mol-1 at few conversions and the average kinetic energy was around 600 kJ mol-1. Is this possible? If yes, what could be the possible reasons?
The macroalage is pelleted and stored for a long time before the experiments were done. I have used both integral and differential isoconversional (KAS and Friedman) methods and got the same range of activation energy.
Thanks for the valuable explanations.
What is the density of your pellets? I reckon the problem is that the volatiles cannot escape from your pellets and the tars are cracking internally and making char - your problem is that the heating rate is only a surface phenomena and not intraparticle. Variablility in density and algae will lead to variations in the activation energy. Use less material and not pelletised. You will get very different results.
What is the density of your pellets? I reckon the problem is that the volatiles cannot escape from your pellets and the tars are cracking internally and making char - your problem is that the heating rate is only a surface phenomena and not intraparticle. Variablility in density and algae will lead to variations in the activation energy. Use less material and not pelletised. You will get very different results.
Hi,
You need to reduced the density of the biomass (possibly powder ) which will help to removal of hot vapors and ultimately reduced the activation energy. Also, i was not aware of the composition of the biomass such as hemicellulose, cellulose etc, which have highly affect on the pyrolysis. On the other side what model you have adopted along with approximation may be other possible reason.
The pelletizing process can cause chemical modification in the particle and this could alter its activation energy. It may be the effect of temperature during pelletizing process of the particles.
Best Regards
Sample density and high heating rates could be responsible for the high values of Ea
Hi,
You should take the powdered sample of macroalgae for TGA experiments and runs at least 3 times at each heating rate. Friedman and KAS and other isoconversional models are assumptions based models. I suggest that you should calculate the activation energy by using Vyazovkin and Vyazovkin AIC models which most accurate.
Best Regards
Your heating rate range is far too small to derive reliable kinetic parameters. You have only a factor of 2, and 5 is the minimum I would consider, and I prefer 10-20 times different. For a small heating rate range, a small temperature measurement error has a large effect on the activation energy. You also need to check to make sure your sample is small and dispersed enough that you have no thermal gradients. This also enhances mass transfer. All this is described in my papers and book and other ICTAC papers.
I agree with the comments raised by Prof. Burnham. Before discussing the calculated value of Ea, we should seriously consider the reliablity of the experimentally resolved kinetic curves. The kinetic curves recorded using thermal analysis are always influenced by mass and heat transfer phenomena. Please visit the ICTAC commommendation concerning the kinetic data collection.
10.1016/j.tca.2014.05.036
Interesting.
If you could provide conversion curve, then it would be possible to comment on possible causes of abnormally high activation energy (1200 kjoule/mole)
From the point of kinetic analysis the problem can be in the data range (as Professors Burnham and Koga already mentioned) - the broader range is needed. Another important aspect is the method that is used to derive the activation energy - you mentioned the isoconversionsl technique, but there are few and they have different accuracy. I advise you to use the most precise among them - differential Friedman or integral advanced Vyazovkin. Another powerful approach is to use the model fitting analysis. All these kinetic methods are included in free software at www.thinks.chemphys.ru.
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