Organic Rankine cycle technology with direct heat exchanger for the waste heat recovery of energy intensive industries

Panel: 4. Technology, products and system optimisation

Authors:
Pedro Egizabal Luzuriaga, TECNALIA Research & Innovation, Spain
Paolo Belotti, Turboden SrL, Italy
Sabrina Santarossa, Turboden SrL, Italy
Mikel Merchán, Tecnalia Research & innovation, Spain

Abstract

Industrial plants of Energy Intensive Industries (EII) such as cement, glass, petrochemical and steelmaking, are at present dissipating huge amounts of heat into the atmosphere. Furthermore, these flue gases have to be cooled before being vented either with heat exchangers or by adding external air. Both solutions introduce additional energy consumption. Unfortunately, this waste heat is recovered for internal needs only in few cases.

The ORC technology is considered as a technology with a high real potential for the recovery of these waste heats. The ORCs (Organic Rankine Cycles) typically use a high molecular mass organic working fluid such as butane or pentane that have a lower boiling point, higher vapour pressure, higher molecular mass and higher mass flow compared to water operating in the steam Rankine Cycles. These features enable higher turbine efficiencies than those offered by a steam system. The ORC systems can be utilized for waste heat sources as low as 150 °C, whereas steam systems are limited to heat sources greater than 260 °C.

The ORC systems are typically designed with two heat transfer stages. The first stage transfers heat from the waste gases to an intermediate heat transfer fluid (e.g., thermal transfer oil). The second stage transfers heat from the intermediate heat transfer fluid to the organic working fluid. The work presents the design and development of a new generation of the Organic Rankine Cycle (ORC technology) to generate electricity based on the application of direct heat exchange solution. The main objective is to eliminate the intermediate heat carrier medium circuit that transfers the thermal energy from the first heat exchanger to ORC units in conventional Waste Heat Recovery systems.

Aspects related to the analysis of materials, control and monitoring and modelling of the gas flows within the heat carrier have been approached. Eventually an industrial scale demonstrator with a capacity of generating up to 2 MWe is being built that will be installed in a cement plant for the validation of the process. The design and details of this case study will be shown in detail.

The new ORC developed during the project presents a lot of innovative aspects if compared to a standard configuration with thermal oil. The first worth mentioning aspect is the use of a working fluid that is nor toxic, nor flammable and with low GWP for high temperature applications. Based on the existing literature it is the first time worldwide this specific low GWP refrigerant is used as working fluid for a real scale ORC. Design of heat exchangers and turbine blades have been changed according to thermodynamic properties of the new fluid. The WHRS design is able to efficiently recover heat from gases and maintain organic fluid temperature below decomposition limits. The WHRS materials are able to resist high temperatures and dust, but also cheap enough to be economically feasible.

This solution (ORC with Direct Heat Exchange) represents the first worldwide application in the cement sector and is furthermore suitable for cross sectorial applications. It should allow reducing the overall costs while increasing efficiency, considered as the key factors to cut existing barriers for a significant penetration of the ORC technology in the industrial energy efficiency market.

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