FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION (See section 10, rule 13) 1. Title of the invention: SEPARATION AND RECOVERY SYSTEM AND SEPARATION AND RECOVERY METHOD TECHNICAL FIELD [0001] The present disclosure relates to a separation and recovery system including a plurality of solid-liquid separation devices configured to separate and recover from a scrubbing liquid a product produced by reaction of exhaust gas discharged from a combustion device with the scrubbing liquid for scrubbing the exhaust gas, and a separation and recovery method using the separation and recovery system. BACKGROUND [0002] Exhaust gas discharged from combustion facilities 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 (sulfur dioxide) are removed by a scrubbing liquid such as an alkaline aqueous solution or slurry containing alkaline components. [0003] As a desulfurization device used in the wet desulfurization method, an absorption tower is known which is configured to cause SOx in exhaust gas to be absorbed by the scrubbing liquid by bringing the exhaust gas introduced into the absorption tower and limestone slurry injected by a spray nozzle installed in the absorption tower into contact (see for example, Patent Document 1). [0004] Patent Document 1 discloses that SOx in exhaust gas removed in the absorption tower by the injected scrubbing liquid reacts with calcium in the scrubbing liquid to form calcium sulfite as an intermediate product, which is then oxidized to gypsum by air supplied to the absorption tower to form a final product (gypsum). Patent Document 1 also discloses that the scrubbing liquid extracted to the outside of the absorption tower by an extraction pump is delivered to a gypsum separator, where the final product is recovered as powdered gypsum. [0005] Generally, combustion facilities with one circuit are equipped with one absorption tower, one regular gypsum separator, and one spare gypsum separator. Combustion facilities with multiple circuits are equipped with one absorption tower, one regular gypsum separator, and one spare gypsum separator for each circuit. The absorption tower has a processing capacity capable of cleaning the exhaust gas emitted by the combustion facilities at full design load. Each of the regular gypsum separator and the spare gypsum separator has a processing capacity capable of treating the product (gypsum) produced by reaction between the exhaust gas and the scrubbing liquid when the combustion facilities are at full design load. Citation List Patent Literature [0006] Patent Document 1: JP2005-334770A SUMMARY Problems to be Solved [0007] For example, when a fuel with lower sulfur content than the design condition is used, or when the load of the combustion device is lower than the design condition, the desulfurization load of the absorption tower is low, so that the amount of product produced in the absorption tower is small, and the amount of product processed in the gypsum separator is small. In other words, the regular gypsum separator may have an excessive processing capacity for the amount of product introduced. If the processing capacity of the gypsum separator is excessive, the gypsum separator may be large and expensive, and the entire system including the gypsum separator may be large and expensive. [0008] In view of the above, an object of at least one embodiment of the present invention is to provide a separation and recovery system that can prevent the system from becoming large and expensive. Solution to the Problems [0009] (1) A separation and recovery system according at least one embodiment of the present invention comprises a plurality of solid-liquid separation devices configured to separate and recover from a scrubbing liquid a product produced by reaction of exhaust gas discharged from a combustion device with the scrubbing liquid for scrubbing the exhaust gas. The combustion device includes a first combustion device and a second combustion device different from the first combustion device. The separation and recovery system comprises: a first absorption tower configured to bring exhaust gas discharged from the first combustion device into gas-liquid contact with the scrubbing liquid; a second absorption tower different from the first absorption tower and configured to bring exhaust gas discharged from the second combustion device into gas-liquid contact with the scrubbing liquid; a first solid-liquid separation device configured to separate and recover the product from the scrubbing liquid; a second solid-liquid separation device different from the first solid-liquid separation device and configured to separate and recover the product from the scrubbing liquid; a third solid-liquid separation device different from the first solid-liquid separation device and the second solid-liquid separation device and configured to separate and recover the product from the scrubbing liquid; a first scrubbing liquid introduction line configured to deliver the scrubbing liquid from the first absorption tower to the first solid-liquid separation device; a second scrubbing liquid introduction line configured to deliver the scrubbing liquid from the second absorption tower to the second solid-liquid separation device; and a third scrubbing liquid introduction line configured to deliver the scrubbing liquid from each of the first absorption tower and the second absorption tower to the third solid-liquid separation device. [0010] With the above configuration (1), the separation and recovery system includes the third scrubbing liquid introduction line configured to deliver the scrubbing liquid from each of the first absorption tower and the second absorption tower to the third solid-liquid separation device. Accordingly, the third solid-liquid separation device can separate and recover the product from the scrubbing liquid in the first circuit, which is the scrubbing liquid after gas-liquid separation in the first absorption tower with the exhaust gas discharged from the first combustion device. Further, the third solid-liquid separation device can separate and recover the product from the scrubbing liquid in the second circuit, which is the scrubbing liquid after gas-liquid separation in the second absorption tower with the exhaust gas discharged from the second combustion device. Thus, with the above configuration, since the third solid-liquid separation device can be used as a spare unit for both the first solid-liquid separation device and the second solid-liquid separation device, the number of solid-liquid separation devices in the separation and recovery system can be reduced compared to the case where a dedicated spare unit is installed for each of the first solid-liquid separation device and the second solid-liquid separation device, which in turn prevents the separation and recovery system from becoming large and expensive. [0011] (2) In some embodiments, in the separation and recovery system described in the above (1), the separation and recovery system is configured to satisfy the following conditional expression (A): N1+N2