TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Apparatus: In the apparatus, the mixture TAR comprises tars, vegetable oil and/or biodiesel. The cooling means (50) comprise, downstream of the first scrubber (20), a second scrubber (30) suitable for treating the flow of gas (G) output from the first scrubber (20), the second scrubber (30) comprising: second means (31) for spraying a liquid (LQ) into the flow of gas (G); a second basin (32) suitable for storing in the second scrubber (30) a quantity of liquid (LQ) in the condensed state; a second recirculation circuit (33) suitable for removing the liquid (LQ) from the second basin (32) and supplying it to the second spraying means (31); a second bleeder pipe (34) suitable for removing the condensed pollutants from the bottom of the second basin (32) and conveying them to the gasifier (12); a discharge pipe (36) suitable for removing the excess liquid (LQ) from the second basin (32) and conveying it externally; and a heat exchanger (35) located along the recirculation circuit (33) of the second scrubber (30) and suitable to cool the liquid (LQ) along its path to the spraying means (31). The liquid (LQ) is selected from: water, lime milk, oil, vegetable oil and gas oil. The cooling means (50) comprise a heat exchanger (43) situated along the line which conveys the gas (G) from the first scrubber (20) to the electrostatic precipitator (40). The heat exchanger (25) is designed to produce vapor in the auxiliary circuit (26). The output line (18) comprises heating means (70) suitable for heating again the gas (G) so as to reduce its relative humidity. Preferred Plant: In the plant, the gasifier (12) is of the downdraft type. The gasifier (9) is of the open-core type, where the gasifier (12) comprises means for injecting oxygen into the vicinity of the lop mouth in order to enrich the air drawn in. Preferred Method: In cleaning the gas (G) produced by the gasifier (12), the sprayed mixture TAR comprises tars, vegetable oil and/or biodiesel. The step of removing a further quantity of heat from the gas (G) inside the cooling means (50) comprises the sub-steps of: conveying the flow of gas (G) from the first scrubber (20) to a second scrubber (30); spraying a liquid (LQ) into the flow of gas (G); storing in a second basin (32) of the second scrubber (30) a quantity of liquid (LQ) in the condensed slate; recirculating the liquid (LQ) by removing it from the second basin (32) and supplying it to the second spraying means (31); removing the condensed pollutants from the bottom of the second basin (32) and conveying them to the gasifier (12); removing the excess liquid (LQ) from the second basin (32) and conveying it externally; cooling the liquid (LQ) along its path from the second basin (32) to the spraying means (31); and conveying the flow of gas (G) from the second scrubber (30) to the wet electrostatic precipitator (40). The step of removing a further quantity of heat from the gas (G) inside the cooling means (50) comprises the sub-step of cooling the gas (G) in a heat exchanger (43) situated along the line which conveys the gas (G) from the first scrubber (20) to the electrostatic precipitator (40). The step of exchanging high-temperature heat between the mixture TAR and a service fluid in an auxiliary circuit (26) comprises the sub-step of obtaining vapor in the auxiliary circuit (26). The step of conveying the flow of clean gas (G) to an external user unit comprises the sub-step of heating again the gas (G) so as to reduce its relative humidity. EXAMPLE - No suitable example given.