In system 4, the 450 °C flue gas from the generator is divided into two parts: one part is supplied to HE2 for heating the digester in priority and the other part is transported to the double-effect absorption chiller for producing chilled water. The cooling/heating outputs are regulated by distributing the flue gas according to the demands, thus the heating requirement of the digester could be guaranteed in priority. In cold days, the jacket hot water is firstly heated by the 200 °C exhaust gas from the chiller and then reheated by the 450 °C bypassed flue gas. In all, there are three heat exchangers in the primary water loop: HE1 is served to recover the exhaust heat in the flue gas; HE2 is to reheat the jacket water by the bypassed flue gas; HE3 is for heating the digester.