(373r) Optimal Design and Operation of Flexible Energy Polygeneration Systems Using Decomposition Algorithms

Authors: 
Rammohan Subramanian, A. S., Norwegian University of Science and Technology
Adams, T. A. II, McMaster University
Gundersen, T., Norwegian University of Science and Technology
Barton, P. I., Massachusetts Institute of Technology
Energy polygeneration systems offer several economic and environmental advantages over single product systems as a result of tight integration of multiple processes into one plant [1]. Furthermore, using multiple feedstocks may enable the exploitation of certain synergies, for instance, by heat integration of exothermic and endothermic processing units or blending the different qualities of syngas generated to provide the correct ratio for downstream synthesis processes [2], [3]. In addition, utilizing solid waste as a sustainable feedstock together with a conventional fossil fuel (such as natural gas) has the added benefit of providing an efficient means of waste disposal with only a relatively small negative impact on economics [4]. In this paper, the optimal design and operation of an energy polygeneration system that converts a hybrid solid waste and natural gas feedstock to electricity, liquefied synthetic natural gas (LNG), dimethyl ether (DME) or olefins is investigated. In order to maintain competitiveness in the face of uncertainties (for instance in product prices or environmental policies), a flexible design solution that enables adjustment of operating conditions to maximize the production of the most valuable product(s) is proposed [5]. In this work, 100 % operational flexibility is assumed thus no limits are imposed on the turndown ratios of the product trains. Previous research on the optimal design and operation of flexible polygeneration systems suggests that under most scenarios the plant should produce only one fuel or chemical at a time in addition to electricity [6],[7]. Thus, the proposed process operates in either an LNG, DME or olefins mode, with electricity produced from off-gases in all three modes. In the LNG mode, the syngas generated from solid waste gasification is cleaned and sent to a methanation process to produce synthetic natural gas which is then blended with the natural gas feed prior to liquefaction. In the DME or olefins modes, the different qualities of syngas from solid waste gasification and natural gas reforming are blended in correct proportions and sent to their respective synthesis trains.

The above problem is formulated as a scenario-based two-stage stochastic MINLP to optimize simultaneously design and operational decision variables. This work uses the recently developed GOSSIP software framework that implements decomposition algorithms such as nonconvex generalized Benders decomposition (NGBD) ([8], [9]), Lagrangian relaxation, and modified Lagrangian relaxation for efficient and scalable solution of nonconvex stochastic MINLPs [10]. The results of the stochastic formulation are compared with the deterministic approach to demonstrate the improvement in economic performance as a result of taking uncertainty into consideration.

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