According to Babrauskas in the Ignition Handbook: Self-heating and thermal runaway is "possible" with sawdust and sawn wood, but the parameters where this can occur are difficult to achieve. Pile size can be critical. Small piles are less likely to self-heat enough to achieve combustion.

See the Ignition Handbook Chapter 9 and pp.95-9 in Chapter 14.

Pat Kennedy, CFEI, CFPS, MIFireE
Fire and Explosion Analyst
Sarasota, Florida
[www.kennedy-fire.com]