Integration of Waste Heat Recovery in Process Sites

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Abstract

Exploitation of waste heat could achieve economic and environmental benefits, while at thesame time increase energy efficiency in process sites. Diverse commercialised technologiesexist to recover useful energy from waste heat. In addition, there are multiple on-site and offsiteend-uses of recovered energy. The challenge is to find the optimal mix of technologiesand end-uses of recovered energy taking into account the quantity and quality of waste heatsources, interactions with interconnected systems and constraints on capital investment.Explicit models for waste heat recovery technologies that are easily embedded withinappropriate process synthesis frameworks are proposed in this work. A novel screening toolis also proposed to guide selection of technology options. The screening tool considers thedeviation of the actual performance from the ideal performance of technologies, where theactual performance takes into account irreversibilities due to finite temperature heat transfer.Results from applying the screening tool show that better temperature matching between heatsources and technologies reduces the energy quality degradation during the conversionprocess. A ranking criterion is also proposed to evaluate end-uses of recovered energy.Applying the ranking criterion shows the use to which energy recovered from waste heat isput determines the economics and potential to reduce CO2 emissions when waste heatrecovery is integrated in process sites.This thesis also proposes a novel methodological framework based on graphical andoptimization techniques to integrate waste heat recovery into existing process sites. Thegraphical techniques are shown to provide useful insights into the features of a good solutionand assess the potential in industrial waste heat prior to detailed design. The optimizationmodel allows systematic selection and combination of waste heat source streams, selection oftechnology options, technology working fluids, and exploitation of interactions withinterconnected systems. The optimization problem is formulated as a Mixed Integer LinearProgram, solved using the branch-and-bound algorithm. The objective is to maximize theeconomic potential considering capital investment, maintenance costs and operating costs ofthe selected waste heat recovery technologies.The methodology is applied to industrial case studies. Results indicate that combining wasteheat recovery options yield additional increases in efficiency, reductions in CO2 emissionsand costs. The case study also demonstrates that significant benefits from waste heatutilization can be achieved when interactions with interconnected systems are consideredsimultaneously.The thesis shows that the methodology has potential to identify, screen, select and combinewaste heat recovery options for process sites. Results suggest that recovery of waste heat canimprove the energy security of process sites and global energy security through theconservation of fuel and reduction in CO2 emissions and costs. The methodologicalframework can inform integration of waste heat recovery in the process industries andformulation of public policies on industrial waste heat utilization.

Keyword(s)

Site utility system; Waste Heat Recovery; absorption chiller; absorption heat pump; absorption heat transformer; cogeneration; graphical tools; heat exchanger; mechanical heat pump; mixed integer linear program; organic Rankine cycle; process integration; waste heat to power

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