The type of ignition that occurs is a self-ignition occurring on the material. Humans do not combust thanks to [thermal inertia (the human body is 2/3 water).][1]

Assuming the Jedi wear a basic cotton material, the self-ignition flashpoint (the material catching on fire) is [407 degrees C.][2]

The lava flowing is viscous, flowing quite rapidly, and appears to go for kilometers, suggesting a [rhyolite lava][3]. This type of lava is unusually hot in the greater than 950 degrees C range. 

[![The spice will flow][4]][4]

Using the [Sakimoto and Zuber: Flow and Convective Cooling in Lava Tubes Model][5] and Anakin's slow descent down the loosely graveled walls we see that Anakin finally comes within range to trigger the self-ignition point of 470. 

[![He's on fire!][6]][6]

  [1]: https://www.quora.com/At-what-temperature-does-the-human-body-instantly-incinerate
  [2]: http://www.tis-gdv.de/tis_e/ware/fasern/baumwoll/baumwoll.htm#temperatur
  [3]: https://en.wikipedia.org/wiki/Lava
  [4]: https://i.sstatic.net/FvhHI.jpg
  [5]: http://www-geodyn.mit.edu/zubersite/pdfs/Sakimoto_103JGR1998.pdf
  [6]: https://i.sstatic.net/FKXWJ.jpg