EN1 - Raw materials used by weight or volume

The raw materials used only include non-renewable materials, i.e. materials which do not regenerate in the short term.
In Enel’s production activities, expendables are mainly used in thermal and nuclear power plants and in geothermal drilling activities.
Their uses testify the higher environmental awareness of Enel, which employs them in various treatment processes.
Among the main expendables and their most common uses, it is worth mentioning:

• ammonia (20,567 t): used to balance the pH of the thermal-cycle water, but above all as a reagent in the flue gas denitrification process;
• limestone (1,097,191 t): the reagent for the flue gas desulfurization process;
• lime (33,374 t): mainly used in waste water treatment, thanks to its neutralizing and/or flocculating properties;
• sulfuric acid, hydrochloric acid (15,111 t), and caustic soda (32,118 t): most commonly used in the regeneration of ion-exchange resin and in the clean-up of equipment, but also in waste water treatment. In geothermal activities, soda has various applications, including as an additive in the slurries used in the drilling of geothermal wells.

For the data on the main expendables, the reader is referred to page 183 at the end of this chapter.

Additionally, the following expendables are used in electricity generation:   

• resins (504 t): used to produce (via ion exchange) the high-purity water
  needed for the thermal cycle of steam-cycle power plants;
• hydrazine (83.4 t), carbohydrazide (296 t), and hydrogen peroxide (0.230 t): used for deoxygenation and pH balancing of thermal-cycle water and steam;
• magnesium oxide (326 t): injected into the flue gas circuits of thermal power plant boilers fed with vanadium-containing fuel, in order to prevent corrosion of heat-transfer surfaces due to the indirect action of vanadium;
• sodium hypochlorite (5,827 t), chlorine dioxide (0.514 t), ferrous sulfate (272 t), ferrous chloride (40.1 t), and trisodium phosphate (35.6 t): occasionally added to the cooling waters of steam-cycle power plants to prevent deposits and fouling or to protect condenser tube surfaces from corrosion.
• ferric chloride (1,239 t) and polyelectrolyte (120 t): mainly used in waste water treatment, thanks to their neutralizing and/or flocculating properties;
• bentonite (1,739 t): clay used as a slurry for the drilling of geothermal wells;
• barite (471 t): used in some cases to thicken bentonite slurries, thereby improving their effectiveness when drilling into mechanically unstable rock formations;
• geothermal cement (4,559 t): used for joining the steel walls of new wells and for permanent plugging of disused wells.
  Finally, expendables such as lubricating oil, dielectric oil, antifouling, defouling, deoxidizing, antifoam, detergent, and antifreezing agents, carbon dioxide, bottled hydrogen, etc., totaling 12,519 t, are used in all installations.

Fossil fuels

Almost all fuel (mostly of fossil origin) is used for thermal generation.

• The consumption of fuel oil is classified on the basis of its sulfur content (HS = high: >2.5%; MS = medium: >1.3% and ≤ 2.5%; LS = low: >0.5%, and ≤1.3%; VLS = very low: ≤ 0.5%).
• Coal and brown coal are used in power plants usually equipped with flue gas desulfurizers and denitrification systems.
• Gas-oil, a high-cost fuel, is used on an exceptional basis: i) in single-cycle gas-turbine power plants that are not connected to the natural gas grid (and as an emergency fuel in the other gas-turbine power plants); ii) in dieselengine power plants (supplying some small Italian islands); iii) in the start-up of steam-cycle power plants, auxiliary boilers, and emergency generating sets.
• The consumption of natural gas is broken down on the basis of its uses:
  non-technologically captive (when the use of gas is a corporate choice) and technologically captive (when gas feeds single-cycle, combined-cycle, or repowering gas turbines, for which it is the only practicable option).
• The contribution of non-fossil fuels consists of:
  • refuse-derived fuel (RDF), co-fired with coal;
  • solid biomass, used as the main fuel or co-fired with coal;
  • biodiesel, used in some gas-turbine units located on small Italian islands;
  • biogas, used in some small installations with alternative engines located in
    Spain.

Natural gas and start-up gas-oil feed the boilers which heat the fuel oil stored in the tanks of fuel oil storage areas. (Heating fluidifies fuel oil before its transfer to destination.)
Small quantities of gas-oil are also used for driving geothermal drilling equipment and in emergency generating sets, which are present in practically all of Enel’s installations.

Fuel consumption, obtained from data measured and certified in each installation, is expressed in metric units (thousand tons or million cubic meters).
To sum the various contributions, use is instead made of the corresponding energy potential (thousand tons of oil-equivalent, toe).

The consumption of fossil fuel in the Group went up from about 33.1 Mtoe in 2008 to about 37.4 Mtoe in 2009 as a result of the full consolidation of Endesa (February 2009) and of the different weight of OGK-5, whose data are reported for the first time for the entire year.
In the overall mix of fuels, the share of coal was up, the one of gas-oil was slightly up, and the ones of brown coal, natural gas, and fuel oil were down.
With respect to 2008, the consumption of oil products with different sulfur content was as follows: high-sulfur oil was close to zero (0.2%); medium-sulfur oil passed from 12% to 10%; low-sulfur oil rose from 59% to 68%; and very low-sulfur oil decreased from 29% to 22%.

Geothermal fluid

Geothermal fluid, in the form of steam at adequate pressure and temperature, is the energy source for geothermal power generation.
If the extracted fluid has thermodynamic properties unsuitable for geothermal generation, it may be employed: i) for the same purpose, in an indirect way, through binary cycles (as in North America, where the geothermal resource consists of a low-salinity brine at a temperature of 135°C to 165°C) or ii) in non-electric uses. In the case of Enel, these uses are now limited to the supply of heat (especially for greenhousing and district heating, but also as process heat in the food industry). For the supply of heat, use is also made of the fluid which becomes available after expansion in Enel’s only geothermal unit equipped with an atmospheric-exhaust turbine.
The capability of geothermal fields is chiefly sustained by the reinjection of fluids into geothermal reservoirs. These fluids consist of: i) water entrained by steam and separated from it at the well outlet; ii) steam condensed after its expansion in the turbines; iii) fluid remaining after being used in the primary circuit of binary cycles; and iv) fluid remaining after non-electric uses.
Reinjection and extraction of fluids into/from the deep subsoil does not jeopardize shallow aquifers which, among other things, are isolated from the wells by metal pipings cemented to the soil and to each other.

The difference between the total fluid extracted and the fluids reinjected is due to: i) the incondensibility of the gases contained in geothermal steam; ii) vaporization and entrainment of condensates in cooling towers (by far the largest contribution); and iii) inevitable losses.
With respect to 2008, the amount of fluid used sharply rose in connection with the opening of the new wells in Stillwater and Salt Wells (Nevada - USA).

Nuclear fuel

Enriched natural uranium, improperly called “fuel”, is the energy source for nuclear power generation.
The uranium found in nature is practically composed of two isotopes: uranium 238 (about 99.3%) and uranium 235 (only 0.7%), as uranium 234 only accounts for 0.056%.
Uranium enrichment – usually obtained by diffusion or centrifugation of a gaseous uranium compound (hexafluoride, UF6) – raises the U235 content to values lying in the typical range of 3-5%. U235 is the only fissile isotope. When the nucleus of a U235 atom is hit by a slow neutron, it splits up into two smaller nuclei (fission), releasing energy and other (fast) neutrons. These neutrons are slowed down by the water that is contained in light-water reactors and acts as a moderator” (the water also carries the heat produced by the fission process), and they hit other nuclei, inducing a chain reaction.
Nuclear fuel can generate 50,000 times as much energy as that released upon combustion of an equal mass of fuel oil.
In a nuclear power plant, nuclear fuel management consists of three stages:

• procurement of fresh fuel;
• transport of fresh fuel to the power plant site (dry storage containers in the reactor building or fresh fuel pond), preparation of reload, reload, start-up tests, monitoring of operation, unloading from the reactor, and storage in the reactor pools (prior to transfer to temporary storage pools);
• organization of the transfer of the spent fuel to the pools of the temporary storage facility (if it exists, the storage facility may be on-site or off-site) or to reprocessing facilities. The spent fuel must be transferred to a temporary storage facility or to reprocessing facilities after a given number of years of operation of the plant, in order to avoid saturation of the storage capacity of reactor pools.

Reload is needed when, after being utilized in the reactor for a few years, the fuel loses its efficiency (i.e. its U235 content diminishes) owing to the fission process. Reload is usually carried out on a 12-, 18- or 24-month basis, but only replacing a fraction of the core. Fuel is loaded into the core, shuffling the remaining assemblies that have not been unloaded so as to optimize fuel utilization and the overall efficiency of the plant. The content of fission products (high-activity and long-lived radioactive waste) in the spent fuel is only about 3%. The remaining components are unused uranium (96%), which is recovered via reprocessing and may be used for generating new fuel, and plutonium (about 1%), which is a by-product arising from nuclear reactions and  radioactive decays of U238. The plutonium isotopes (Pu239 and Pu241) are fissile. Plutonium may be recycled as mixed oxide fuel (MOX, i.e. UO₂ + PuO₂), which combines normal fuel with fissile substances (plutonium).
MOX, consisting of 7-9% plutonium mixed with depleted uranium, is equivalent to uranium oxide fuel enriched by 4.5% in U235.