We are doing conventional pyrolysis of sewage sludge. The other products showed the common traits, while char yield decreases first until 850 °C and started to increase at later temperatures. I could not find a logical explanation behind it. The other test analysis showed the secondary decomposition reactions for char. In that case, the yield should have gone down.
TIA
Dear A. Siddiqui,
At temperatures above 800 deg C, the composition of mineral compounds in biomaterials (e.g biomass, sewage sludge) changes drastically. In particular, the composition of these alkali and alkali earth metals (AAEMs) which catalyse thermochemical degradation is diminished.
As such their influence on the process decreases significantly thereby resulting higher char yield. This view is corroborated by studies which have shown that processes such as washing and dimeralisation have increased yields of biomasses after pyrolysis.
Hope this helps.
Best regards,
BBN
Generally, char production for a single particle during a single experiment monotonically decreases, so I understand your confusion. I do not know the details of your question, but if you are observing a single sample during a single experiment, it may be because of how you are measuring char yield. An ash tracer technique estimates char yield, where char yield (with ash basis) is
yc = x0/x
where x is some typically inorganic mass fraction of the char. Some biomass inorganic components begin the evolve at about this temperature, and others decompose - alkali halides, sulfates, and carbonates are examples. This may play a role.
If you are comparing different samples conducted under different conditions, the result could be because secondary reactions convert tars to char. At low temperatures, the tars are able to escape the particle without further reacting, but at high temperatures the tars themselves produce char that is trapped in the particle.
Larry
How were the tests performed; TGA or gravimetrical?
Try some runs using TGA in inert atmosphere and see if You get the same results.
we have performed the experiment three four times. Out of these four times, we got very high yield three times. We carried out the pyrolysis in a fluidized bed reactor. Until 850, the char yield decreased, as the temperature ascended further, the yield escalated. Normally, char yield decreases at higher temperatures, yielding more non-condensables due to secondary reactions. But in our case, the gas yield increased marginally only, yielding a little higher fraction of char. This triggered the confusion.
Bemgba B. Nyakuma and Larry L. Baxter
Sir,
Thank you very much. The reasons you gave are very logical. I will look into these more deeply. Thank you very much.
Sometimes, due to the presence of silica and high quantity of fluxes like CaO, Na2O etc.., glassy phase is formed when the sewage sludge is heat treated at high temperature. The glassy phase has the ability to hinder the oxidation of char and trap the evolved gases, hence, the char yield is increased.
The formation of the glassy phase is normally accompanying with noticeable decrease in the size of the sewage sludge and increase in its strength.
The higher production of char at high temperature may be due to two main reason. 1: the secondary reaction which convert the tar into char and the 2 one is the use of microporous catalysts with low acidity.
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