Compost: spontaneous combustion

Overheating in Compost heaps has been identified  by some for as a possible cause of recent fires resulting from climate change It is true that  spontaneous combustion can occur in agricultural and horticultural situations  e.g., haystacks and large  compost piles. It can also  occur, very rarely, in domestic compost heaps.

In the UK most domestic composters use compost bins, rather than piles and the heat inside a compost bin can become hotter than in an open  compost pile as the bin will retain some of the heat generated by the composting process that will be lost to the atmosphere in an open pile. If the compost bin is stored in direct sunlight  the temperature will be increased.

If fresh “greens” e.g., grass,  are added to the bin the heat generated can be significant  and ignition may occur if dry material is added and not mixed or watered. The Mail online,  in July 2022, reported such a case where a bin fire damaged a house, and it was said by a fire officer that  grass cuttings had heated up during hot weather to such an extent that the compost  self-combusted.

Spontaneous Composting  in composting can occur at  temperatures of   150⁰C - 200⁰C which may result in  a  smouldering fire, this  can become to a traditional flaming fire if oxygen is introduced by aeration.

However, the risks are very small  in most domestic composting using  cold composting systems as the temperatures are normally at, or near, ambient.

 Higher temperatures are necessary to destroy pathogens and weed seeds and  “Hot composting” systems are designed to produce and hold the temperatures at  40-60⁰C with  temperatures of 76⁰C often reached for short periods.

“Smoke” can often  be seen rising from a compost heap,  but this is actually not smoke, but water vapor and the compost is  not smouldering. 

The measures necessary to reduce the risk are simply good management  and good composting. Keep the bin contents moist adding water when turning, adding additional materials and in hot weather.