The paper presents an improved well steam generator design and the mathematical model for calculating the operating parameters. Well steam generator increases efficiency of underground gasifier by recovery of the flue gas heat and converting it to the superheated steam. Calculations revealed that the heat recovery efficiency of the well design increases with coil diameter and does not strongly depend on fossil coal type and composition of gas mixtures of oxygen and air supplied as oxidizing agents. (C) 2017 Elsevier Ltd. All rights reserved.