A method for cogenerating gas-steam based on gasification and methanation of biomass. The method includes: 1) mixing oxygen and water vapor with biomass, transporting the resulting mixture via a nozzle to a gasifier, gasifying the biomass to yield crude gasified gas, and transporting superheated steam having a pressure of 5-6 MPa resulting from sensible heat recovery to a steam turbine; 2) adjusting the hydrogen/carbon ratio of the crude gasified gas generated from step 1) to 3:1, and eluting the crude gasified gas whereby yield purified syngas; 3) introducing the purified syngas from step 2) to a methanation unit and transporting intermediate pressure superheated steam generated in the methanation unit to the steam turbine; and 4) concentrating methane of synthetic natural gas containing trace nitrogen and water vapor obtained from step 3) through pressure swing adsorption.

A carbonaceous fuel gasification system for all-steam gasification with carbon capture includes a micronized char preparation system comprising a devolatilizer that receives solid carbonaceous fuel, hydrogen, oxygen, and fluidizing steam and produces micronized char, steam, volatiles, hydrogen, and volatiles at outlets. An indirect gasifier includes a vessel comprising a gasification chamber that receives the micronized char, a conveying fluid, and steam. The gasification chamber produces syngas, ash, and steam at one or more outlets. A combustion chamber receives a mixture of hydrogen and oxidant and burns the mixture of hydrogen and oxidant to provide heat for gasification and for heating incoming flows, thereby generating steam and nitrogen. The heat for gasification is transferred from the combustion chamber to the gasification chamber by circulating refractory sand. The system of the present teaching produces nitrogen free high hydrogen syngas for applications such as IGCC with CCS, CTL, and Polygeneration plants.

A method for converting a feedstock comprising solid hydrocarbons to a sweet synthesis gas, involving the steps a. gasifying said feedstock in the presence of steam, an oxygen rich gas and an amount of sour process gas to form a raw synthesis gas optionally comprising tar, b. optionally conditioning said raw synthesis gas to a sour shift feed gas, c. contacting said sour shift feed gas with a sulfided material catalytically active in the water gas shift process for providing a sour hydrogen enriched synthesis gas, d. separating H.sub.2S and CO.sub.2 from said sour hydrogen enriched synthesis gas, for providing said sour recycle gas and a sweet hydrogen enriched synthesis gas.