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[JayAlchemAPL]

We have been studying phase diagrams in my thermodynamics class which has been really interesting (we have midterms this week!)

During one of the lectures last week, Dr. Tobin showed a phase diagram of glass. Apparently, there is a solid-solid phase transition that occurs with glass at 800C (1 atm) which reminded me of the problematic clinker formations sometimes found in gasifiers.

At 800C, the pure amorphous structure undergoes a phase transition to equilibrate at a crystalline structure, Tridymite (SiO2). However, clinker formation seems to happen at higher temperatures, but this is probably attributed to the fact that clinkers are not exactly pure silicon dioxide as it forms in an environment of carbon and ash, so it seems to make since that it would take more energy for this phase transition to occur.
(It turns out opal is silica & nH20. I've seen clinkers come out with some beautiful shimmering colors, curious if this is the structure created here? I'm not sure but, just a thought).

According to Wikipedia, Tridymite was first discovered in volcanic rocks, which clinkers have some resemblance to with respect to the porous surfaces created by hot gas jets during burning.

So if clinkers are the problem, what is the solution?
While clinkers happen at higher temperatures, these temperature ranges are still considered normal to high operating temperatures of gasification needed to crack tars.

Dr. Tobin had mentioned as well as a reference here by Modern Inorganic Chemistry pg 772 " that the structure of fused silica is broken up by alkalies. Which is kind of convenient in a way because there is already alkalies in the ash content of wood. Maybe during clinker formation there is not enough alkalies locally to prevent the structure from forming which could be one possibility.
Maybe the solution to clinker formation is to find a mixture of say two feed stocks. One of high silica content with enough of a second feed stock with non-silica ash content to prevent clinker formation of the first.

Has anyone studied clinker formation in gasification that could give some insight?



(This is an article that has some phase equilibrium charts that might be of interest.
Anthropogenic Polymetamorphism)

[HarryN]

Hi, I need to seek out the reference again, but IIRC, limestone and / or lime can be used to increase the melt temperature and suppress clinker formation.