PD042838IN-NP FORM2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION (See section 10, rule 13) 1. Title of the invention: GAS-LIQUID MIXING DEVICE AND EXHAUST GAS DESULFURIZATION DEVICE INCLUDING GAS-LIQUID MIXING DEVICE 2. Applicant(s) NAME NATIONALITY ADDRESS MITSUBISHI POWER, LTD. Japanese 3-1, Minatomirai 3-Chome, Nishiku, Yokohama-shi, Kanagawa 2208401, Japan 3. Preamble to the description COMPLETE SPECIFICATION The following specification particularly describes the invention and the manner in which it is to be performed. 1 GAS-LIQUID MIXING DEVICE AND EXHAUST GAS DESULFURIZATION DEVICE INCLUDING GAS-LIQUID MIXING DEVICE TECHNICAL FIELD 5 [0001] The present disclosure relates to a gas-liquid mixing device for injecting a mixed fluid (scrubbing liquid containing fine bubbles) to a liquid reservoir of an absorption tower to bring scrubbing liquid into contact with exhaust gas discharged from a combustion device, and an exhaust gas desulfurization device including the gas-liquid mixing device. 10 BACKGROUND [0002] For example, exhaust gas discharged from a combustion engine such as a boiler contains air pollutants such as SOx (sulfur oxide). As a method for reducing SOx contained in exhaust gas, there may be mentioned a wet desulfurization method in which substances such as SO2 are absorbed and removed by an absorption liquid such as an alkaline aqueous 15 solution or an absorption slurry. [0003] Some exhaust gas desulfurization devices used in the wet desulfurization method are provided with an absorption tower including a gas-liquid contact part configured to bring exhaust gas and scrubbing liquid into contact by spraying the scrubbing liquid to the exhaust gas flowing in the absorption tower, and a liquid reservoir, disposed below the gas-liquid 20 contact part, for storing the scrubbing liquid which has been sprayed (for example, see Patent Document 1). When the exhaust gas comes into contact with the scrubbing liquid, SO2 contained in the exhaust gas is absorbed in the scrubbing liquid. The scrubbing liquid that has absorbed SO2 is stored in the liquid reservoir. [0004] Since the scrubbing liquid stored in the liquid reservoir contains reaction products 25 such as sulfites produced from SO2 absorbed from the exhaust gas, in order to remove the reaction products, the reaction products may be oxidized by spreading a gas containing oxygen such as air through the scrubbing liquid stored in the liquid reservoir. [0005] Patent Document 1 discloses a gas-liquid mixing apparatus including an injection 2 nozzle configured to inject a mixed fluid of scrubbing liquid and the gas containing oxygen from a discharge port to the liquid reservoir. The injection nozzle has a contraction portion in the middle of a flow passage for the scrubbing liquid, and the contraction portion contracts the flow of the scrubbing liquid flowing through the flow passage to generate a negative 5 pressure region. The suction force generated in the negative pressure region sucks a gas supplied via a branch pipe to a portion of the flow passage on the downstream side of the contraction portion. Further, the injection nozzle shears and atomizes the gas sucked by the scrubbing liquid flowing in the flow passage of the scrubbing liquid to generate a mixed fluid (scrubbing liquid containing fine bubbles), and injects the mixed fluid from the discharge port. 10 Citation List Patent Literature [0006] Patent Document 1: JP5046755B 15 SUMMARY Problems to be Solved [0007] If the proportion of the gas (bubbles) contained in the mixed fluid discharged from the discharge port is too small, the amount of oxygen used to oxidize the scrubbing liquid is insufficient, which may result in insufficient oxidation of the scrubbing liquid by the mixed 20 fluid in the liquid reservoir. Further, if the proportion of the gas (bubbles) contained in the mixed fluid discharged from the discharge port is too large, the shearing of the gas by the scrubbing liquid may be insufficient, and the effect of atomizing the gas (bubbles) may decrease. When the effect of atomizing the gas (bubbles) decreases, and the size of bubbles increases, the contact area between the gas and the scrubbing liquid decreases. Further, 25 when the size of bubbles increases, the buoyancy of the bubbles increases, so that the period of time that the bubbles can stay in the scrubbing liquid stored in the liquid reservoir, i.e., the period that the bubbles can oxidize the scrubbing liquid, is shortened. This may result in insufficient oxidation of the scrubbing liquid by the mixed fluid in the liquid reservoir. 3 [0008] In view of the above, an object of at least one embodiment of the present invention is to provide a gas-liquid mixing device whereby it is possible to prevent insufficient oxidation of the scrubbing liquid by the mixed fluid in the liquid reservoir. 5 Solution to the Problems [0009] (1) A gas-liquid mixing device according to at least one embodiment of the present invention configured to inject a scrubbing liquid and a gas containing oxygen to a liquid reservoir for storing the scrubbing liquid in an absorption tower configured to bring the scrubbing liquid into gas-liquid contact with an exhaust gas discharged from a combustion 10 device comprises: a first cylindrical portion internally defining a first flow passage and having a scrubbing liquid introduction port for introducing the scrubbing liquid into the first flow passage, a gas introduction port for introducing the gas into the first flow passage along a direction perpendicular to a flow direction of the scrubbing liquid introduced from the scrubbing liquid introduction port and flowing through the first flow passage, and a discharge 15 port for discharging a mixed fluid of the scrubbing liquid introduced from the scrubbing liquid introduction port and the gas introduced from the gas introduction port; and a contraction portion disposed, in the flow direction of the scrubbing liquid, upstream of a merging portion at which the scrubbing liquid introduced from the scrubbing liquid introduction port merges with the gas introduced from the gas introduction port. The gas20 liquid mixing device satisfies 0.5