Waste recycling into carbon adsorbents is a way to solve two ecologically important problems simultaneously: waste destruction and creation of materials for trapping ecotoxicants from water and air environments. One of the large-tonnage wastes is high-ash lignin formed by sprinkling lignin with sand to prevent spontaneous combustion in storage areas. The purpose of this work is to study the conversion of high-ash lignin into nanoporous carbons (NCs) during alkaline thermochemolysis and to study their porous structure. NCs were obtained by heating lignin with KOH at 800 °C for 1 h, cooling, washing, and drying. The specific surface areas and pore volumes of NCs were calculated from low-temperature nitrogen adsorption–desorption isotherms using the 2D-NLDFT-NS method. The dependences of the NC yield and porous structure parameters on the KOH/lignin weight ratio were established. Alkaline thermochemolysis was shown to transform lignin into materials with a pore volume of ~1.02 cm3/g and a specific surface area of ~1400 m2/g. With increasing the amount of alkali, the total pore volume and the micropore volume change extremely through maxima. The volume of ultramicropores decreases, and the volume of supermicropores increases, i.e. alkali transforms ultramicropores into supermicropores due to the removal of C-atoms from the ultramicropore walls. Thermally initiated reactions of lignin with KOH preferentially form a surface of pores with a diameter of ≤5 nm. The dominant contribution to the specific surface area is made by the micropore surface: its portion depends little on the alkali amount and varies within 90–94%.