NOVELTY - A gas-steam cogeneration technique based on biomass gasification and methanation includes biomass gasification; purifying; carrying out methanation; and adsorbing and concentrating methane from the synthetic natural gas.    USE - Gas-steam cogeneration technique based on biomass gasification and methanation.    ADVANTAGE - The method does not only convert the biomass energy into clean natural gas high in heat value and convenient to use, but can also recycle large amounts of heat produced by the biomass gasification and methanation by a way of high grade steam recycle mode.    DETAILED DESCRIPTION - A gas-steam cogeneration technique based on biomass gasification and methanation comprises:    (A) using air separation plant for producing oxygen and water vapor mixed biomass through the nozzle at high temperature and high pressure biomass gasification pressure in the furnace at 1-3 MPa, controlling the gasification furnace at 1500-1800 degrees C to obtain the thick gasification gas, where the generated pressure is 5-6 MPa of the sensible heat recovery steam feeding steam turbine;    (B) purifying the generated according to the requirement of methanation reaction thick gasification gas by using transformation reaction adjusting gas hydrogen-carbon ratio of 1, and obtaining purified synthesis gas via low temperature methanol sulfur removing;    (C) adjusting the purifying of the hydrogen-carbon ratio synthesis gas methanation unit, synthesis gas methanation process of methanation stage and complete methanation stage, stage is methanation reactor in series, each stage has two shunt-wound reactors, the second stage of methanation, a part of the process gas is recycled back to the first stage of methanation reactor inlet, mixing with fresh raw material gas of the first methanation reactor to reduce large amount of methanation inlet reactant concentration on the other hand circulation technology process gas as the inert medium to heat to control the temperature of the catalyst bed layer, most methanation reaction of the methanation step after methanation of synthesis gas enter into complete methanation reaction, methanation stage comprises two complete methanation reactors are connected in series, and the majority of carbon dioxide (CO2) into a small amount of unreacted for methane (CH4) methanation phase to generate medium-pressure over-heat steam to the steam turbine; and    (D) adsorbing and concentrating methane from the synthetic natural gas obtained in step (C) with high molar concentration doped with a small amount of nitrogen gas and water vapor through changing the pressure, where the molar concentration of the methane is 96%, the heat value of the synthetic natural gas is 8256 kcal/Nm3, which meets the current national standard.    An INDEPENDENT CLAIM is included for a system applied to the efficient gas-steam cogeneration technique based on biomass gasification and methanation.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic diagram of gas-steam cogeneration technique based on biomass gasification and methanation (Drawing includes non-English language text).