Automatic tar circulating system with purification function

The invention discloses an automatic tar circulation system with a purification function, which comprises a purification water tank, a container, a grating ruler and a tar circulation unit, and is characterized in that the purification water tank receives fuel gas produced by a gasification furnace, purifies the fuel gas and discharges the purified fuel gas; the bottom of the container is communicated with the bottom of the purified water tank through a communicating pipe; the grating ruler comprises a laser source, a grating scale and a photoelectric coupler, the laser source emits light beams to irradiate liquid in the container, and the photoelectric coupler reads the liquid level height of tar in the purified water tank according to the light beams received by the grating scale; and the tar circulating unit pumps tar into the gasification furnace according to the liquid level height of the tar. Compared with an existing mode of judging the tar content in the purification water tank through a manual regula