What thermochemical process converts complete organic solids in the absence of added oxygen?

The thermochemical process that converts complete organic solids in the absence of added oxygen is pyrolysis. During pyrolysis, organic materials such as biomass are heated in a controlled environment without oxygen, which prevents combustion. Instead, this heat causes the organic materials to decompose chemically, resulting in the production of biochar, bio-oil, and gases like syngas.

This process is distinct from other methods like composting, which relies on aerobic decomposition by microorganisms, and incineration, which involves burning with oxygen to reduce materials to ash. Wet oxidation uses water and oxygen to oxidize waste at high temperatures, differing from pyrolysis due to the presence of oxygen, which again leads to combustion rather than thermochemical decomposition. Thus, pyrolysis is the unique method that specifically involves the thermal breakdown of organic solids without oxygen, making it the correct answer.