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Session: New applications: Process Integration
Session starts: Tuesday 08 October, 10:00
Presentation starts: 10:00
Room: Willem Burger Zaal

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Eduardo Pinto de Sousa (CMI Greenline)
Ludovic Ferrand (CMI Greenline)
Samer Maaraoui (MINES-Paristech)
Elias Boulawz Ksayer (MINES-Paristech)
Denis Clodic (EReIE)
Xueqin Pan (EReIE)

Abstract:
INTRODUCTION Steel galvanized products and derivates have seen a growing demand for the past years specially on the emergent markets like the BRIC and in developing countries. Galvanized steel is used for many applications such as building, roofs and walls up to electrical applications and car bodies. This massive use is due to its excellent properties of resistance and formability. Unfortunately, carbon steel suffers from oxidation and must be coated prior to any application. The process of galvanizing steel goes through different processing steps before getting the ideal mechanical properties. Initially, the strip is preheated on a direct-fired furnace to a temperature near 650°C under substoechiometric conditions before an additional heating taking the strip temperature up to the annealing temperature, typically 800°C, with radiant tubes or electric heating. In order to complete the thermal cycle, the steel strip is cooled on jet cooling sections generating the adequate metallurgical phases. Then, the steel strip is dipped into a zinc-pot at a temperature of about 460°C where the strip gets a thin zinc layer that will react with oxygen protecting the strip from oxidation. The jet cooling process is done in several cooling boxes and is one of the most important of the galvanizing process. Lately, recent developments have been concentrated on this part of the process. This is due to manufacture’s demand for specific metallurgic properties that must be accomplished using the adequate cooling rates. Most typical, cooling the steel strip is done by impinging nozzles or slots of an hydrogen-nitrogen content gas (HNx). HNx JET COOLING SYSTEM Typically, a jet cooling section is composed of a sequence of cooling boxes where a mixture of hydrogen-nitrogen gas is blown into the steel strip through nozzles or slots, extracting the steel strip available heat by forced convection at a speed up to 120 m/s. This gas mixture profits from the high thermal conductivity property of H2 to increase the cooling rate. The hot HNx gas coming out of the cooling section is then cooled down at a tube-finned heat exchanger with cold water in closed circuit coming from the aero-refrigerants, thus all the energy extracted from the strip is invariably lost. Another major point revolves with the fans feeding HNx to the nozzles or slots which electrical consumption is not negligible – the net electrical consumption will range between 0.5 to 2 MWe. GOALS OF THE R&D PROJECT Conscious of the potential for energy savings as well as the fact that energy is a major component for the competitiveness of the steel industry, CMI Greenline, Mines Paristech and EReIE have designed a system to take advantage of the energy extracted from the steel strip by the HNx gas in order to produce electricity through an Organic Rankine Cycle. System design and optimisation As the strip slides through the number of jet cooling sections, different levels of energy are available with different temperatures. Therefore, the first step of this project was an optimisation study focused on the process parameters that influence the strip cooling so that the available heat can be valorised and at the same time respecting the strip metallurgical constraints. Several configurations for the recuperation system such as a simple accumulator tank, multi-stages or stratified were drawn and their exergetic efficiencies compared. The Heat Transfer Fluid (HTF) choice to carry the heat up to the ORC power system followed four main criteria : thermodynamic performance ; the domain use of the HTF that establishes the minimum and maximum temperature admitted and thus the pressure-limit to the system ; exploiting limitations such as corrosion of the equipments, fouling or chemical decomposition ; techno-economics of the installed equipment and the fluid itself. Selecting the working fluid for this application comply respectively with thermo-physical, environmental and security criteria : condensing pressure superior to 100 kPa to avoid vacuum at the condenser, low volume ratio and high vapor density to limit the size of the turbine and pumps, avoid liquid droplets on the turbine blades ; non-flammable and non-toxic ; low GWP and zero ODP. Heat-exchanger design Secondly, the tube-finned heat exchangers were revisited and redesigned to assure the HNx delivers the maximum power to the HTF at the best efficiency possible. Optimisation criteria were assigned for the heat exchanger design as the heat transfer exchange between hot HNx and the HTF, the surface of exchange and pressure drop that ought to be limited. A set of laboratory experiments were conducted to simulate and validate the heat exchanger design, heat transfer coefficients, the suitable materials to apply to tubes and fins and the correspondent pressure drop. TECHNICAL ANALYSIS AND FUTURE DEVELOPMENTS A technical analysis points out that the high grade energy from jet cooling sections of a typical galvanizing/annealing processing line is capable of delivering 1 MWe on an adapted ORC cycle with one regenerator and an one-stage turbine. This innovation that enables steel manufacturers to produce electricity from the jet cooling sections targets the hundreds of worldwide existing processing lines of this kind for a total estimated installed electric capacity of more than 1 TWhe. A pilot demonstrator will be erected to demonstrate the feasibility of the concept as well as to test the interaction between the HNx blowing loops and the ORC electric production, the possibility to introduce a thermal oil as the heat transfer fluid rather than the water that is currently used, and the design of the tube-finned heat exchangers. BIBLIOGRAPHY Gray, D.L and Webb, R.L. (1986) ‘Heat transfer and friction correlations for plate fin-tube heat exchangers having plain fins”, Proc. of the 9th Int.Heat Transfer Conf. San Francisco, pp. 2745-1750 Wang, C. Hwangb, Y. and Lin, Y. (2002) ‘Empirical correlations for heat transfer and flow frictions characteristics of herringbone wavy fin-and-tube heat exchangers’. International Journal of Refrigeration, 25: 673-680 Zoghaib, M. (2010) ‘Etude et simulation de méthodes de refroidissement des bandes d’acier défilantes’. PhD thesis. Centre Energetique et Procédés, Ecole des Mines de Paris. Quoilin, S. (2007) Experimental study and modeling of a low temperature Rankine cycle for small scale cogeneration”. Master’s degree. University of Liège, Faculty of Applied Sciences, Aerospace and Mechanical Engineering, Department of Thermodynamics Laboratory ACKNOWLEDGMENTS The authors gratefully acknowledge partial financial support from ADEME – French Environment and Energy Management Agency according to the contract 11-81-C0069