My Compost Heap just Caught Fire

My Compost Heap just Caught Fire - why?

Happy Christmas by the way.

At the mo' the heap's about four and a half by three foot wide and three foot high. During the summer and Autumn it's been having grass cuttings interspersed with hay and food waste put on it with no turning. Recently I've been putting food waste and (definately cold) ashes on it. It's got perspex sitting on top of it and open sides. Certainly a little surprisng as we've had snow here which has been thawing over the last three days.

I was just going to have a cup of tea when my wife called from upstairs that the compost heap was on fire!

Here it is (after I put it out):


I went to have a look and found the smouldering was coming from old grass cuttings near the open edge of the heap. About four inches down and about five inches in from the open edge, above where the grass is just going gooey, I could see a large area of blackened grass and the smoke. The smouldering stopped when I turned the affected part over and exposed it.

So what's all that about? We're going away soon so I'd like to know so I don't have to worry while I'm away. It's well away from buildings but the wire fence it's next to is plastic coated and it would shame to have that ruined.

Any help much appreciated.

 

I'm amazed that anything's willing to burn after all the water that's fallen out of the sky recently.

 

Are you sure it was actually on fire and not just hot and giving out steam in the ambient air that was cold and that the blackened grass wasn't just grass that had decomposed to a black state?
Your heap seems to be very small for spontaneous combustion.

If you are really worried about leaving it a good drenching with water should keep things cool. All decomposition produces some heat.

 

Infernal combustion is the term the priests used ...........

 

I read somewhere that the microbes that do the decomposition thrive at about 80'C, but die at 90'C, so that the process is self regulating. That means that for spontaneous combustion to happen, there'd have to be something in there with a flash point cooler than 90'C. I'm not sure there's anything likely to be in a compost heap that would ignite at such low temperature.

 

Thanks. I notice yours is the opposite of mine: covered sides but open top: why is that? It also looks pretty full. How do you get the done compost out from the back - make a tunnel? Mine's actually smaller than it was: I've been stacking the compost to one side of the area I have so that when I come to use it, I can shift the top layer over to the other side I've kept free. As to the microbes: well, I've been pretty careful to even up green grass with the hay. I guess the food waste gives them a varied diet and they seem to like the occasional drop of decanted yellow stuff!

 

My understanding is that the bacteria take the temperature up high enough for the moulds to take over, and probably some bacteria that thrive in hot conditions, and then provided that Oxygen is present the temperature reached will then be sufficient for an oxidation reaction, which will raise temperatures even higher; the heat generated will often dry the outer layers of the heap, improving its insulation properties (ie.. making it like dry straw / hay in that regard) so the centre of the heap gets even hotter.

Although I agree that a metre-cube compost heap is unlikely to be big enough, so it si probably steam rather than smoke. A haystack, stored when the hay was damp, or a grain silo filled with grain that is not properly dried, can definitely get to combustion temperatures.

 

Thanks. Yes, it was smoking thick smoke like a chimney. It's pretty dry under that perspex - perhaps because the sides are exposed to some fierce winds. When I removed the perspex and turned over some quite old dry grass near the top edge, it was burning red. It was coming from two spots fairly close together.

Would leaving the top off while I'm away help?

 

That's every female, I think you'll find!

 

My

My wife was wondering if it could be the sudden rise in temperature. It was well below freezing and then suddenly got very warm. It is very dry in there, and the two layers of (pretty clean) perspex catch the sun....... Could it be that?

I wish I'd taken a photo of the burning, but putting it out was the first thing on my mind.

 

That makes much more sense. I'd missed the point about the perspex.

Yes if its dry, and the bacteria are getting it up to about 80 or 90'C, and that heat cant't easily escape, and the sun shines through the perspex heating the surface layers still further. That could do it.

 

Are you sure your ashes were cold?

 

My wife assures me they were.
Still, doesn't hurt to check them more thoroughly too. I guess it means leaving the top off while we're away which is a shame 'cos the compost heap's been dry all year.