The process described herein converts biomass directly into a combination of hydrogen, methane and carbon dioxide. A portion of the gases are collected at pressures above the thermodynamic critical pressure for water, which is 3200 psi (pounds per square inch). Typical operating pressure at the point where the first portion of gas collected can range from 3200 psi to 6000 psi. Upon cooling, most of the CO.sub.2 condenses to a liquid. At this density and pressure, the CO.sub.2 can be injected into a deep well aquifer to sequester the carbon dioxide. The overall process is superior to carbon neutral processes, can be carbon negative, and possesses the potential to reverse atmospheric CO.sub.2 trends if implemented on a global scale.

A system and a method for preparing hydrogen-rich syngas by taking biogas as a composite gasification agent for the gasification of biogas residue are provided. The system includes an anaerobic fermenter for biomass gradient, a solid-liquid separation device, a drying reactor, a gasification reactor, a condensing tower, and a syngas purifier; the anaerobic fermenter for biomass gradient is connected to the drying reactor through the solid-liquid separation device, to transport separated biogas residue to the drying reactor, and the drying reactor and the anaerobic fermenter for biomass gradient are connected to the gasification reactor, respectively, to jointly input biogas and a biogas residue dried into the gasification reactor for reaction; and the gasification reactor is connected to the condensing tower, to transport reaction products to the condensing tower, the condensing tower is connected with the syngas purifier, performing purification treatment on a gas cooled, to obtain the hydrogen-rich syngas.