As it relates to biochar or in general?
The thing is that biochar is produced by burning plant materials in an reductive (oxygen-free) atmosphere. The process drives off the volatile gases and leaves —carbon—. Essentially, charcoal plus some bacterial and fungal amendment—hence the bio- in biochar.
Percentage captured? I'm thinking all of it, but the answer you seek is probably in here:
https://scholar.google.com/scholar?&...ar.google.com/