In steel industry, reheating furnaces are used in hot rolling mills to heat the steel stock (billets, blooms or slabs) to temperature of around 1200°C which is suitable for plastic deformation of steel and hence for rolling in the mill. The heating process in a reheating furnace (see Figure 1) is a continuous process where the steel stock is charged at the furnace entrance, heated in the furnace and discharge at the furnace exit. In addition to many other feature design of the furnace such as type of burners, furnace dimensions, number of furnace zones, type of wall, roof insulation and skid design, a recovery heat exchanger is usually installed for preheating combustion air by hot flue gases coming out from furnace exit to improve energy efficiency of the process.
Figure 1. Reheating furnace .
In general, a reheating furnace consumes in term of heat from natural gas combustion of about 350 kWh per ton steel product, and from 25% to 35% of this energy input is still wasted via gaseous effluents in spite of aforementioned measures for energy efficiency improvement of reheating furnace. An additional recovery system, e.g. waste heat to power system, is expected to recover 50% of this energy loss corresponding to more than 43 kWth per ton steel product for every hour. This will improve the energy efficiency of reheating furnace. Furthermore, the Waste Heat to Power (WHTP) system produces the electricity without additional GHG emissions.
The project ORCAL (stands for waste heat recovery by means of ORC connected to gravity-assisted heat pipes or two-phase closed thermosyphons) aims at recovering heat from exhaust stream of a rolling mill reheating furnace using an ORC system connected to two-phase closed thermosyphons as shown in Figure 2. The project is carried out within the collaboration between the University of Liège and C.M.I. (Cockerill Maintenance & Ingénierie) group.
In practice, an intermediate heat carrier loop (e.g. pressurized water, saturated steam or thermal oil loop)  is often used to transfer heat from exhaust stream to ORC system. However, using a heat carrier loop increases the complexity and the investment cost as well as the operating cost of waste heat to power plant. A direct exchange between hot exhaust stream and ORC working fluid may be used to improve the cycle efficiency and cost by eliminating the pumps, heat exchangers and other cost of the additional heat carrier loop. However the installation of the ORC evaporator directly in the hot gas paths poses concerns for the decomposition of the organic working fluid at high temperature as well as the safety due to the flammability and/or toxicity of organic compound [2, 3]. Using an ORC system connected to two-phase closed thermosyphons, the cycle efficiency and cost will be improved by eliminating the pump and additional cost of the heat transfer loop while the organic working fluid is external to the hot gas path.
Figure 2. Schematic of waste heat recovery system using ORC connected to two-phase closed thermosyphons 
 Delabroy O, Gouefflec GL, Lebrun C, Barbotin A, Cervi R. Performances enhancement of reheating furnaces using oxycombustion. AISE Annual Convention. Chicago, USA2000.
 Turboden. Turboden Waste to Energy Solutions. 2016.
 Booth CM, Swanson LW, Taylor RW. Heat pipes for transferring heat to an organic rankine cycle evaporator. Google Patents; 2011.
 Le VL, Declaye S, Dumas X, Ferrand L, Lemort V. Performance evaluation of an Organic Rankine Cycle (ORC) connected to two-phase closed thermosyphons. The 29th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems. Portorož, Slovenia2016.