NOVELTY - Device for preparing natural gas by virtue of double-fluidization bed gasification of biomass comprises a combustion carrier heating unit, a pyrolysis gasification unit, a biological carbon/carrier separation and conveying unit and a methane synthesis and tar returning unit, where the pyrolysis gasification unit is connected with the combustion carrier heating unit, the biological carbon/carrier separation and conveying unit and the methane synthesis and tar returning unit. The combustion carrier heating unit comprises a fluidized bed combustion furnace and a flue gas cyclone.    USE - The device is useful for preparing natural gas by virtue of double-fluidization bed gasification of biomass (claimed).    ADVANTAGE - The device utilizes different pyrolysis products of biomass, achieves the treatment of different biomass.    DETAILED DESCRIPTION - Device for preparing natural gas by virtue of double-fluidization bed gasification of biomass comprises a combustion carrier heating unit, a pyrolysis gasification unit, a biological carbon/carrier separation and conveying unit and a methane synthesis and tar returning unit, where the pyrolysis gasification unit is connected with the combustion carrier heating unit, the biological carbon/carrier separation and conveying unit and the methane synthesis and tar returning unit. The combustion carrier heating unit comprises a fluidized bed combustion furnace, a flue gas cyclone, a first refeeder, a steam generator, an air preheater and a blower. The pyrolysis gasification unit comprises a pyrolysis fluidized bed gasifier, a gas cyclone, a biomass feeding device and a second refeeder. The biological carbon/carrier separation and conveying unit comprises a biological carbon separation cooling furnace, a biological carbon whirlwind, a carrier discharge pipe and a carrier transport device. The methane synthesis and tar returning unit comprises a gas purification unit, a methane device and a tar pump. An INDEPENDENT CLAIM is also included for preparing natural gas by virtue of double-fluidization bed gasification of biomass using the device, comprising: (i) passing the biomass feedstock into pyrolysis fluidized bed gasification furnace for pyrolysis reaction to generate tar-containing high-temperature pyrolysis gas and biochar, passing the biochar into gas-solid separation cyclone, then passing the pyrolysis gas into gas purification unit, conveying biochar into refeeder and biochar cooling separation oven; (ii) passing the tar-containing high-temperature pyrolysis gas into gas purification unit for removing moisture, dust and carbon dioxide in tar and to obtain purified synthetic gas, where the synthetic gas includes carbon monoxide, hydrogen, methane hydrocarbons and polymer, passing the synthetic gas into synthetic device to synthesize methane natural gas; (iii) gas purification unit to obtain purification after the tar delivery pump into fluidized bed combustion furnace combustion efficiency, main heat generated heat carrier for heating, high temperature flue gas generated by carrying into the high-temperature heat carrier, flue gas cyclone, the flue gas solid separation through the steam generator and the air preheater after discharge, high-temperature heat carrier entered through refeeder, pyrolysis fluidized bed gasifier; (iv) steam into the steam evaporator to generate gas a fluidized bed gasifier as pyrolysis, fluidizing media and participate in the gasification reaction, air passes into air preheater as fluidized bed combustion furnace tar combustion air and to achieve a heat carrier fast fluidization; (v) passing the obtained purified gas and carbon dioxide into biochar separate cooling furnace for cooling to obtain biochar, the use of density difference to achieve the separation of the heat carrier and biochar, biochar cooled by carbon dioxide gas and passed into the biochar whirlwind for the gas-solid separation to obtain biochar; and (vi) the heat carrier through the discharge pipe is passed into the carrier transport device, in the carbon dioxide gas for pneumatic conveying and passing into fluidized bed combustion furnace.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of the device for preparing natural gas by virtue of double-fluidization bed gasification of biomass (Drawing includes non-English language text).