My attention went to such grass fires and cotton waste fires. Earlier, in this forum, when I wrote on the same topic, I found not much information. However, a search in books like Kirk's fire investigation (John D.DeHaan) and a hand book of fire technology (R.S.Gupta) and in internet revealed lot of information. It is observed that self ignition of cotton waste that got some moisture and drying oil is possible within few hours (5-24), at temperatures above 15 deg C and optimum ventilation and oil load (0.1-0.3 lit/square meter surface of cotton).
The drying of oils produce heat. May be bacterial action inside also produce heat. This heat, if not dissipated i.e more than enough to heat other oil / cotton or not vented out and there is sufficient porosity to permit entry of oxygen (in air) but not enough for generated heat to dissipate, there will be heat accumulation and can lead to so called spontaneous combustion.
This is possible with slightly wet grass also, or if laundered and pressed clothes are kept in piles before allowing to cool.
What to do to avoid such conditions? The solutions suggested are, to dry such cotton waste / grass by spreading sufficiently so that the moisture in it can be dried. Cotton waste can be disposed to authorized dealers or can be burnt regularly (preferably on daily basis), or land burial. It it is to be stored, then after drying, such cotton waste can be kept in air tight metallic drums and sealed. The oxygen initially present in the drums after sealing will be used for oxidation but as there are no fresh supplies, further oxidation process will stop. Or, keep such cotton waste in drums with sufficient water above so that insufficient moisture conditions do not arise.
Few years ago, I saw fire involving electrical cable reel, covered under tarpaulin in front of a telephone office, that also on a summer day. May be the heat accumulated inside might have caused this fire.
Many such summer fires involving material under tarpaulin were reported.
A spontaneous heating parameter Z is given in an article (SINTEF Report NBL A05145), to know about the topic. More can be read from the link given above.
Few years ago, I had read about bursting of drums filled with used oil. The reasons mentioned were bacterial action causing generation of gases like methane, hydrogen sulfide, carbon dioxide, etc leading to pressurization and bursting ultimately. It is suggested that as soon as somebody observed any bulging of the top cover of such drums, one should release the pressure from the drum by slowly opening the lid with gaps between uncorking. I feel this pressurization in such drums may increase in summer.
