The production cycles of many manufacturing process are characterized by considerable waste of thermal energy, contained in the process fumes. The Organic Rankine Cycles (ORC) are well suited to the recovery that energy that would be otherwise lost: this is due to superior performance compared to traditional steam cycles in the presence of low temperature and / or low thermal sources, in addition to ability to operate with highly variable thermal inputs and low management difficulties ( they are completely automated systems).
By adopting an ORC system it is possible to recover energy from thermal sources both at high and medium or low temperature through appropriate choices of the fluid and the system components. In addition to the great goal of efficiency, the use of these systems for WHR (Waste Heat Recovery) introduces other advantages, even compared to some main renewable sources: power generation is distributed, therefore with reduced transmission or distribution losses, and it can often be totally self-consumed by limiting the unbalancing of the electrical system.
How does the recovery process work?
The availability of usable heat in thermal recovery typically consists of a flow of hot gases, less frequently in the form of liquids. Heat exchange can take place directly between the thermal source and the working fluid, or indirectly. In the case of heat recovery from gaseous effluents, the primary thermal source is generally not directly coupled with the working fluid: the heat recovered is in fact usually transferred to the cycle by means of a thermal vector (such as diathermic oil, pressurized water or steam ). In the case of heat recovery from liquid effluents, similar to geothermal applications, there is a direct exchange between the primary heat source and the working fluid of the ORC cycle.
Considering the primary importance of the production processes to which heat recovery is applied, the system layout generally requires the insertion of the recuperator in “bypass”. If there are any faults that make the recovery system unavailable, with this scheme it is possible to by-pass the recuperator and the ORC module using the original fume line for the evacuation of the hot gases to the chimney. The ORC cycle and the recuperator are designed to respond automatically to any variation in the flow rate and temperature of the heat source and, if necessary, to disconnect from the electricity grid and shut down the entire system in complete safety.
Applications
Thermal recovery for electricity production can have an important impact in many energy-intensive sectors, contributing significantly to reducing consumption and increasing the efficiency of the entire production process.
The most significant applications are industrial ones, in particular cement factories, iron and steel complexes and energy-consuming processes in general, where fumes are typically made available at temperatures between 200 and 600 ° C. It is also possible to recover heat from natural gas recompression stations, medium and small size gas turbines or internal combustion engines to produce electric power.
Here some examples:
Cement factories
The cement production process is characterized by a considerable availability of medium / low temperature waste heat which, despite all the plant solutions used, cannot be completely used. There are generally two heat sources available:Gas di combustione del forno (a valle del preriscaldamento delle materie prime), con temperature nell’ordine dei 250-400 °C
Furnace combustion gas, with temperatures in the order of 250-400 ° C
Clinker cooling air, at lower temperatures (<300 ° C).
Cement production requires an enormous amount of energy: the clinker firing furnace reaches temperatures of 1.500 ° C. Although a modern plant reuses most of the waste heat from the process for drying the material, a considerable amount of low temperature gas is still dispersed in the atmosphere. Air or exhaust gases at temperatures starting at only 150 ° C can be economically used to generate power from 500 kW up to a few megawatts.
Technical problems due to the dustiness of the gases have been resolved for years: consider how heat recovery from the cement production process is a widespread application, since the 1980s, throughout the Far East (in Japan, China, Indo-China and India there are over 1,000 MWe installed). The production processes of these countries, however, are characterized, compared to the Italian case, by low efficiency of the production process, that is by a greater availability of recoverable heat and therefore by larger plant sizes: therefore, besides being a more convenient solution from the point of view economic, the most widespread technology is the traditional one of steam cycles. In Italy it is estimated that on average clinker production requires about 1.15 MWht of thermal energy and about 0.15 MWh of electricity per ton of clinker produced. The adoption of ORC-based recovery systems would allow the use of the portion of heat discharged at low temperature (200-300 ° C).
Steel industry
Steel and ironproduction presents good possibilities of intervention in terms of heat recovery. Thermal energy can be obtained from:
“Clean” process fumes: result of the combustion of natural gas in furnaces or heat treatments, available at medium / low temperature
“Dirty” steelworks / foundry fumes: originated from the melting of the metal, available at medium / high temperature.
Production processes are less standardized than in the cement industry; reliable recovery solutions (made “ad hoc”) are used in the most varied processes (rolling mills, heat treatments etc). This generally occurs for clean fumes, at medium temperature (> 400 ° C) and when not in conflict with other potential interventions due to the increase in process efficiency (for example pre-heating combustion air). It may be more difficult to operate on dirty fumes, from a steel plant or foundry, where greater potential (but also required technological complexity) due to the high temperature are countered by problems related to the dust content, to the considerable variations in temperature and flow rate of the fumes , to environmental constraints on emissions.
The estimate of energy consumption per single ton of ferrous materials produced or processed is 1.25 MWht of thermal energy and 0.25 MWh of electricity. The italian national production of ferrous materials can be estimated at 36 Mt / year as far as the primary steel industry is concerned, to which must be added 25 Mt / year hot rolled and 8 Mt / year cold rolled. The recoverable energy is quantified in the order of 30-50 kWh per ton of material produced / processed. With a potential of 3 TWh / year of electricity production, even assuming a very conservative rate of heat recovery penetration, the impact at national level would be comparable (if not higher) to that in the cement sector.
Glass industry
Glass production is certainly another potential candidate for the application of heat recovery systems. From the technical point of view, the availability of gas from the melting of glass at high temperatures (400-600 ° C) can guarantee high electrical efficiencies, in the order of 25%. It is estimated that the average energy (thermal and electrical) required for the production of one ton of glass (for industrial plants) is around 1-1.5 MWh / t. Of the total energy supplied, on average about 20% is the fraction that is lost in the exhaust gases. It is estimated that the recoverable electricity is in the order of 30-45 kWh per ton of glass produced. The national production of glass is estimated at 1 Mt / year of flat glass, 3.8 Mt / year of hollow glass and about 0.5 Mt / year of other products (yarns, crystals, tubes, etc.). With a potential of 200 GWh / year of electricity production, even the energy recovery from this industrial sector could lead to significant results at national level.
Other application fields
Is possible to mention other industrial sectors in which heat recovery can be interesting: petrol-chemicals, production of non-ferrous materials, ceramics, incineration. In general, ORC technology allows the recovery of heat from any industrial process where the available thermal waste power is greater than 3/5 MWt, translating into an annual consumption of 20 MSm3 of natural gas (or alternatively 15 Mt of coal).
Source: Applicazione di Cicli ORC a Recuperi Termici da Processi Industriali Nicola Palestra, Riccardo Vescovo
