The invention aims to provide a method for preparing
methane-rich gas through cyclic utilization of alkali / alkaline earth metals contained in
biomass for autocatalytic
pyrolysis and autocatalytic pressurized gasification of the
biomass. According to the method, water
quenching is carried out on
biomass ash so as to cyclically extract alkali / alkaline earth metals in the
biomass ash, and the alkali / alkaline earth metals are sprinkled on and dipped in biomass; the biomass containing the alkali / alkaline earth metals undergoes autocatalytic
pyrolysis at a low temperature of 280 to 320 DEG C at first, then a
pyrolysis gas product enters into a
combustion reactor and burns together with
solid particles separated from a gasification reactor so as to supply heat for the process of pyrolysis, a pyrolysis
solid product enters into the gasification reactor and undergoes autocatalytic pressurized gasification in a steam /
oxygen atmosphere,
hydrogen-rich gas is separated from crude synthetic gas produced in gasification and is returned to the pressurized gasification reactor, and the pyrolysis
solid product is subjected to hydrogasification at the same time. With the method provide by the invention, biomass can be used for preparing
methane-rich gas with low
tar content, and high gasification efficiency is obtained. The method has the advantages of simple operation, less
water consumption, strong technological adaptability and easy operation.