New applications: Process Integration
Chair: Dr. Riccardo Vescovo
ORC FOR JET COOLING SECTIONS OF GALVANIZING AND ANNEALING STEEL PROCESSING LINES
Eduardo Pinto de Sousa, Ludovic Ferrand, Samer Maaraoui, Elias Boulawz Ksayer, Denis Clodic, Xueqin Pan
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.
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.
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
The authors gratefully acknowledge partial financial support from ADEME – French Environment and Energy Management Agency according to the contract 11-81-C0069
COMBINING A THERMALLY SUPPORTED GROUND SOURCE HEAT PUMP WITH AN ORC PROCESS
Stefan Schimpf, Roland Span
Abstract: Stefan Schimpf*, Roland Span
Faculty of Mechanical Engineering
Chair of Thermodynamics
Universitätsstraße 150, 44801 Bochum, Germany
e-mail: firstname.lastname@example.org, http://www.thermo.rub.de/
The coupling of a ground source heat pump and solar thermal collectors in a domestic solar combisystem providing both space heating and domestic hot water is an established technology. As there is no demand for space heating during summer the area of the collector array is overdimensioned and the collectors come to a standstill whenever the maximum temperature of the storage is reached. This stagnation can be circumvented by the application of an ORC harnessing the excess heat. In the ORC the working fluid of the heat pump is evaporated by solar heat and is expanded through a scroll expander before it is condensed in the ground heat exchanger recharging the ground. The additional investments of the combined system only comprise a scroll expander, a pump and advanced controls and could therefore be greatly reduced if the already existing scroll compressor of the heat pump could also be used as expander.
Simulation runs are performed to discuss the energetic benefit of the ORC. The feasibility of two analytical solutions for the ground heat exchanger is evaluated. The first one is Eskilson’s g-function approach , which can be regarded as state-of-the-art for the calculation of the long term response of a ground heat exchanger. This approach is compared to a short-term solution developed by Javed et al. . Aspects of the regeneration of the soil are discussed.
The simulation results for the location Denver show that the reduction of the total electricity demand of the combined system resulting from the addition of an ORC is 51 kWh using the Javed-model, respectively 66 kWh using the Eskilson-model. Thus the overall savings are rather low, which indicates that the addition of the ORC can only become competitive in the future assuming rising energy costs. The results however indicate that for the simulation of systems in which the condensation of an ORC is carried out in a ground heat exchanger the application of long term response models is not sufficient. More advanced short-term solutions are required for this purpose.
 P. Eskilson, “Thermal analysis of heat extraction boreholes”, Dissertation, 1987. University of Lund, Department of Mathematical Physics, Lund, Sweden.
 S. Javed and J. Claesson, “New analytical and numerical solutions for the short-term analysis of vertical ground heat excahngers”, ASHRAE Transactions, Vol. 117, pp. 3-12, (2011).
LOW GRADE WASTE HEAT ORC OPTIMIZATIOM IN NORTHERN CLIMATES
Doris Weiss, Hank Leibowitz
Abstract: There is increasing global interest in the recovery of energy from low temperature streams from which low-grade waste heat (<100°C) is captured in order to increase the energy efficiency and reduce emissions of various oil/gas and energy facilities. While many sources of this waste heat exist, the challenge to utilize the heat economically still remains, especially in northern climates (above the 49th parallel) where water cooling is not an option.
Currently the Organic Rankine Cycle (ORC) technology is the most promising and much emphasis is placed on developing efficient expanders to operate with new organic refrigerants i.e. R134a and R245fa. This is necessary to allow for larger units that are more economically viable.
However, even the most efficient ORC expander will encounter operational challenges if certain aspects of the process are overlooked and not taken into consideration. Of prime importance in Canadian oil sands operations is the effect of ambient temperatures on air cooler/condenser design as well as the process control of the unit, both of which differ from traditional processes that use cooling water. Another challenge, for larger (>1MW) ORC units is the large quantity of waste heat fluid required and thus the larger waste heat exchanger in comparison to smaller low grade heat ORC units as well as those using high grade waste heat. Again, non-traditional heat exchangers should be evaluated and considered instead of the traditional shell and tube heat exchangers which are very large, costly and take up a large area.
The design as well as operation of low grade waste heat ORC units, in cold climates using air cooling, are very challenging and pose unique problems compared to water cooled operation. The authors are familiar with small units in the 50kW range that operate with wet cooling where only small amounts of water are required. However, for plants in excess of one megawatt air cooling becomes the only option due to the combination of sub-freezing ambient temperatures and scarcity of water. Where the source temperature and corresponding ORC thermal efficiency are low the air coolers represent approximately 30-40 %1 of the cost of the unit as the air exchanger requires a much larger surface area than one utilizing cooling water. The design of air cooled condensers (ACC) is more crucial where source temperatures are low and ACC fan power consumption represents a much larger proportion of generator output.
Because of this the ACC design becomes more critical in achieving favorable economics. Greater emphasis must be placed on reducing surface area and reducing fan power consumption. However, the design of the ACC must be done with due regard to ambient temperature and power output. An ACC designed for winter ambient will likely be substantially too small for summer ambient. Thus, the designer must conduct a trade study to reach an optimum solution. For example, if the maximum surface area is used for peak power production at the maximum anticipated ambient temperature (usually 30-35°C), then the unit becomes too expensive and uneconomical. On the other hand, if the cooler/condenser is kept smaller by designing for a lower ambient temperature (0-10°C in Northern climates), then the power production will drop off considerably during the spring, summer and fall months where temperatures can reach 15 – 30 ° for many weeks/months.
Common practice has shown that transient behaviour of air cooled condensers are often ignored in pursuit of optimizing the turbine design and cycle performance for steady state operation. Frequently, these result in operational problems (forced outages) that require expensive design modifications and/or repair of ORC units installed in northern climates. For example, temperature swings on the order of 15-20 °C per day are frequently encountered. In the morning it is common to have temperatures on the order of 0-5°C and then a peak temperature of 20 -25°C 6-8 hours later. If the condenser is allowed to float on pressure, then there can be a point where complete condensation is not achieved causing insufficient suction head to the pump resulting in plant shut down. . Thus, in order for the condenser to operate within these temperature swings, a condensing pressure should be chosen (usually at the higher pressure) and this becomes an explicit control parameter. (Refrigeration or Dew point control units operate in this manner in temperate climates and those above the 49th parallel in order to insure complete condensation of the working fluid or refrigerant).
Further, the heat exchanger design itself can also be challenge at very low ambient temperature. This will be explored in detail to show how fluid velocity inside the tubes increase with higher volume flow at lower temperatures. High fluid velocity increases pressure drop and potential for tube erosion. This is an important factor as these occur where plant output reaches its peak. There is a lack of discussion of these constraints in current literature for larger units in high grade waste heat recovery. Figure 1 shows the limitations that can be expected for a two bay air cooler/condenser for a nominal 1 MW ORC unit. The ambient design temperature is 5°C. As can be seen, the tube erosional velocity is exceeded at -5°C. Thus the expected increase in power production is limited also by the air cooler design. As previously mentioned the air cooler is one of the largest cost items and increasing its surface area can impede the success or implementation of the project as the payback period is high and more than the life of some projects.
Another issue is the potential to increase power production at low ambient temperatures. While this can be done, it is challenging to design a system that is optimized at both high and low ambient temperatures as the working fluid can create problems within the expander if the temperature is too cold. Very low ambient results in excessive volume flow, choking the expander nozzle, and over expansion causing lower expander efficiency, both combined to limit ORC output. This paper will explore the effects of low ambient temperature on the expander performance in conjunction with the waste heat and ACC exchangers.
Figure 1: Graph of Erosional Velocity vs.Outlet Expander Pressure and Power Production
This paper will discuss the various aspects and challenges of the design of low-grade ORC units in order to provide mechanical and process engineers with design considerations to produce workable ORC units in challenging environments.
In addition, this paper serves to provide realistic options that seek to reduce not only the cost of the ORC unit, but also reduce its footprint which is especially necessary in existing facilities.
 Franco,A. Villani, M., Optimal design of binary cycle power plants for water dominated, medium-temperature geothermal fields, Geothermics 38(2009) 379-391.
 G. Holdman, “The Chena Hot Springs 400 kW Geothermal Plant: Experience Gained During the First Year of Operation”, Chena Hot Springs/Chena Power. Fairbanks, Alaska, 2007.
 W. Gu and Y. Weng et al., “Theoretical and Experimental Investigation of an organic Rankine Cycle for Waste Heat Recovery System,” Proc IMechE., Vol.223, pp.523-531, (2008).
 M.Hettiarachchi and H.D. Golubovic et al., “ Optimum design criteria for an organic Rankine cycle using low-temperature geothermal heat sources,” Energy , Vol 9:32(9), pp.1698-706, (2007).