Feasibility study for the application of a high-temperature heat pump in the pulp and paper industry: an Italian case study

The application of high-temperature heat pump (HTHP) in the pulp and paper industry can promote a more efficient and sustainable use of waste energy. In particular, the paper aims to perform a preliminary feasibility study for the upgrade of heat extracted from the wastewater aiming to increase the circularity of pulp and paper production through a combination of technological innovation. In the analysed case study a 1.3 MW th tailored cascade HTHP is designed to supply heat at 120°C at the thermal dryer while cooling the wastewater from 42°C to 28°C. The proposed solution generates several benefits: i) reduction of sludge mass associated with higher heating value for use as secondary fuel; ii) energy saving (i.e. the heating demand of the thermal dryer is not covered by the steam produced by the existing CHP plant) and consequent recovery of waste heat and CO2 and pollutants emission reduction; iii) wastewater cooling without cooling tower operation, with a consequent reduction of fan power consumption up to 44 kW; iv) reduction of oxygen demand during the wastewater treatment due to temperature decreasing before aerobic stage, with a further power consumption reduction of the air fan. The preliminary techno-economic analysis shows a negative NPV at year 20 for both HTHP integration strategies analysed, that is completely fed by the power produced by the existing natural gas CHP unit or partially fed by a 0.54 MW el peak PV plant. Despite the contribution to emissions reduction, that is 1,989 ton/year of equivalent CO2 with PV plant and 1,613 ton/year of equivalent CO2 without PV, the economic sustainability cannot be guaranteed in the conditions examined. The preliminary sensitive analysis highlights a relevant impact of incentives intensity and duration and of the COP of HTHP.