Biochar is a high-carbon, fine-grained residue that is currently produced through modern pyrolysis processes; it is the direct thermal decomposition of biomass in the absence of oxygen (preventing combustion), which produces a mixture of solids (the biochar proper), liquid (bio-oil), and gas (syngas) products. The specific yield from pyrolysis is dependent on process condition, such as temperature, residence time, and heating rate. These parameters can be optimized to produce either energy or biochar. Temperatures of produce more char, whereas temperatures above favor the yield of liquid and gas fuel components. Pyrolysis occurs more quickly at the higher temperatures, typically requiring seconds rather than hours. The increasing heating rate will also lead to a decrease of pyrolysis biochar yield, while the temperature is in the range of . Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis can produce substantially more char (≈35%); this contributes to the observed soil fertility of terra preta. Once initialized, both processes produce net energy. For typical inputs, the energy required to run a "fast" pyrolyzer is approximately 15% of the energy that it outputs. Modern pyrolysis plants can use the syngas created by the pyrolysis process and output 3–9 times the amount of energy required to run.