You need to create high pressure at high temperatures for any hydrothermal system.
From my knowledge, I don't think its possible to achieve the required high pressures for hydrothermal reactions without using an autoclave.
Why are you interested to know this, is it because of the high costs of autoclave or what?
Dear Dr Osama , I checked the references you suggested but couldn't get the answer, it would be nice if you can elaborate and help us to understand the alternative to autoclaves?