Pouring sand on biochar pit in arid climates

Is this a viable way to quench the fire once we’re ready to stop the burning? Water being scarce and all …

Yes, this is a viable method, though somewhat messy and labour intensive.

It takes a fair amount of sand to make a good seal. The char will stay hot for a long time, and will start burning again if any oxygen leaks through.

How big is your pit? I have a lid from an old portable firepit that I sometimes use. That way I only have to seal the edges. A large steel plate would also work.

Yes this works but like Douglas already mentioned is quite laborious and somewhat risky.
If you do go ahead with this make sure to screen your sand beforehand so you can also screen it out of your biochar afterwards.

And make sure everything has cooled down or it will reignite!

I've done pit burns where we used the digger to push the excavated dirt over the completed burn. That works well. I also use a thin piece of stainless steel to cover the top of my big kontiki when I want to make a batch of dry biochar. I have to seal the edges with clay.

In either case, be patient and don't uncover until you know it's cooled down. This will take longer than you expect. I've uncovered a batch after four days and had it reignite, even though the kontiki wall temperature was the same as the air around it. Dry biochar is a really good insulator and the interior of a big pile can take weeks to cool down sufficiently.

If the biochar is destined for soil application, then having sand or dirt mixed in is no big deal, and clay is actually a worthwhile additive for biochar inoculation. Have a look at the just-released Farmer's Guide from ANZBIG in the chapter on inoculating...page 121 talks about clay:

ANZBIG Farmer's Guide

Phil-
Thanks for the specifics on avoiding reigniting and for the great ebook on Biochar!

John S
PDX OR

Adding a very small amount of water to put the fire out helps to partially activate the charcoal.  When real activated charcoal is made they inject high temperature steam into glowing hot charcoal.  The steam, heat and low oxygen levels is what activates it.

When you add a small amount of water to the hot charcoal it instantly turns to steam, gets super heated, displaces and oxygen in the fire and partially activates the charcoal(most likely by adding water instead of host steam the coals are cooled too quickly to fully activate the coal)

Thanks for the link, Phil! That's an excellent, readable resource. Free pdf download -- woo!