A system for producing aqueous sulphuric acid from a sulphur dioxide containing gas of a liquor circulation of a pulp mill, the system comprising - the pulp mill, configured to produce at least some gas containing sulphur dioxide, - a pipeline configured to convey at least some of the gas containing sulphur dioxide to a converter, the converter comprising • an inlet arrangement for receiving some gas containing at least some sulphur dioxide and some oxygen or some gas containing at least some sulphur dioxide and some other gas comprising oxygen and • catalyst for converting at least some of the sulphur dioxide and the oxygen to sulphur trioxide, - a pipeline configured to convey gas containing sulphur trioxide from the converter to an inlet of a pre-cooling unit, and - a first heat exchanger configured to cool water and/or aqueous sulphuric acid for producing cooled water and/or cooled aqueous sulphuric acid, wherein - the pre-cooling unit is configured to pre-cool some gas containing sulphur trioxide, the pre-cooling unit comprising • the inlet or inlets (212, 214) for receiving [i] the gas containing sulphur trioxide and [ii] the cooled water and/or the cooled aqueous sulphuric acid, • an outlet for letting out aqueous sulphuric acid and the gas containing sulphur trioxide, and • a first nozzle for spraying the cooled water and/or the cooled aqueous sulphuric acid onto the gas containing sulphur trioxide to cool the gas containing sulphur trioxide and to react some of the sulphur trioxide with some of the cooled water and/or aqueous sulphuric acid, thereby strengthening or producing some aqueous sulphuric acid, the system comprising - a pipeline configured to convey the cooled aqueous sulphuric acid and/or the cooled water from the first heat exchanger to the pre-cooling unit, - a condensation tower comprising • a first inlet for receiving the cooled gas containing sulphur trioxide and aqueous sulphuric acid from the pre-cooling unit and • means for circulating the aqueous sulphuric acid within the condensation tower by spraying in order to strengthen the aqueous sulphuric acid, and - a channel configured to convey the aqueous sulphuric acid and the gas comprising sulphur trioxide from the outlet of the pre-cooling unit to the condensation tower. - the pulp mill, configured to produce at least some gas containing sulphur dioxide, - a pipeline configured to convey at least some of the gas containing sulphur dioxide to a converter, the converter comprising • an inlet arrangement for receiving some gas containing at least some sulphur dioxide and some oxygen or some gas containing at least some sulphur dioxide and some other gas comprising oxygen and • catalyst for converting at least some of the sulphur dioxide and the oxygen to sulphur trioxide, • an inlet arrangement for receiving some gas containing at least some sulphur dioxide and some oxygen or some gas containing at least some sulphur dioxide and some other gas comprising oxygen and • catalyst for converting at least some of the sulphur dioxide and the oxygen to sulphur trioxide, - a pipeline configured to convey gas containing sulphur trioxide from the converter to an inlet of a pre-cooling unit, and - a first heat exchanger configured to cool water and/or aqueous sulphuric acid for producing cooled water and/or cooled aqueous sulphuric acid, wherein - the pre-cooling unit is configured to pre-cool some gas containing sulphur trioxide, the pre-cooling unit comprising • the inlet or inlets (212, 214) for receiving [i] the gas containing sulphur trioxide and [ii] the cooled water and/or the cooled aqueous sulphuric acid, • an outlet for letting out aqueous sulphuric acid and the gas containing sulphur trioxide, and • a first nozzle for spraying the cooled water and/or the cooled aqueous sulphuric acid onto the gas containing sulphur trioxide to cool the gas containing sulphur trioxide and to react some of the sulphur trioxide with some of the cooled water and/or aqueous sulphuric acid, thereby strengthening or producing some aqueous sulphuric acid, the system comprising • the inlet or inlets (212, 214) for receiving [i] the gas containing sulphur trioxide and [ii] the cooled water and/or the cooled aqueous sulphuric acid, • an outlet for letting out aqueous sulphuric acid and the gas containing sulphur trioxide, and • a first nozzle for spraying the cooled water and/or the cooled aqueous sulphuric acid onto the gas containing sulphur trioxide to cool the gas containing sulphur trioxide and to react some of the sulphur trioxide with some of the cooled water and/or aqueous sulphuric acid, thereby strengthening or producing some aqueous sulphuric acid, the system comprising - a pipeline configured to convey the cooled aqueous sulphuric acid and/or the cooled water from the first heat exchanger to the pre-cooling unit, - a condensation tower comprising • a first inlet for receiving the cooled gas containing sulphur trioxide and aqueous sulphuric acid from the pre-cooling unit and • means for circulating the aqueous sulphuric acid within the condensation tower by spraying in order to strengthen the aqueous sulphuric acid, and • a first inlet for receiving the cooled gas containing sulphur trioxide and aqueous sulphuric acid from the pre-cooling unit and • means for circulating the aqueous sulphuric acid within the condensation tower by spraying in order to strengthen the aqueous sulphuric acid, and - a channel configured to convey the aqueous sulphuric acid and the gas comprising sulphur trioxide from the outlet of the pre-cooling unit to the condensation tower.