Coal rock stratum in-situ liquid nitrogen injection device and method

The invention relates to the technical field of permeability increase of low-permeability coal seams, and provides an in-situ liquid nitrogen injection device and method for a coal rock layer, aiming at solving the problems that the permeability increase effect of an existing liquid nitrogen well injection method on coal rock is limited, and the freezing effect of low-temperature liquid nitrogen cannot be fully utilized. Under the scouring action of high-pressure water flow, the coal body is broken to generate cracks, four times of liquid nitrogen freezing and thawing cycles and two times of alternate injection of high-pressure liquid nitrogen and low-pressure liquid nitrogen are carried out, the high-pressure liquid nitrogen is used for generating and prolonging the cracks, the low-pressure liquid nitrogen is used for generating low-temperature conditions, and a more remarkable cracking effect is generated in the process of multiple times of high-pressure and low-pressure freezing and thawing cycles of the c