Is it really possible to volunteer abroad for free?

no, at the very least you will have to pay for travel and boarding expenses. The cheapest one I have found is www.kidsworldwide.org. I don't know what they do in the was of conservation, but they are extremely cheap. There is no cost for the programs, but they are more 'do it yourself' than other programs....and you must still pay for travel. You might also want to try www.vayama.com for volunteer flights. They are generally cheaper than other carriers. good luck.