Method for recycling tail gas fraction generated in liquid-nitrogen washing process of lurgi ammonia prepared from coal

The invention relates to a method for recycling a tail gas fraction generated in the liquid-nitrogen washing process of lurgi ammonia prepared from coal, which comprises the following steps of: using a part of tail gas of a liquid-nitrogen washing device as a circulated tail gas fraction; boosting the tail gas fraction to the pressure of 2.5 to 3.0MPa by a compressor; after carrying out cooling and temperature reduction on the tail gas fraction, returning the tail gas fraction to the middle part of a low-temperature methanol washing decarbonizing tower; and converging the tail gas fraction into low-temperature methanol washing decarbonization gas so as to meet the flow rate of each channel of a main heat exchanger and ensure that the temperature of the cold end of the main heat exchanger is equal to 190 DEG C below zero. The using amount of the circulated tail gas fraction depends on the amount of equivalent carbon monoxide in raw material gas and is determined by calculating according to the following formula that Q=Q0*(C1-C2)/(C3+C4), wherein Q represents the flow rate of the circulated tail gas fraction and the unit of Q is Nm /h; Q0 represents the flow rate of the raw material gas and the unit of Q0 is Nm /h; C1 represents the concentration of CO in the raw material gas and the unit of C1 is percent; C2 represents the concentration of CO in the transformed raw material gas and the unit of C2 is percent; C3 represents the concentration of CO in the circulated tail gas fraction and the unit of C3 is percent; and C4 represents the concentration of N2 in the circulated tail gas fraction and the unit of C4 is percent.