school energy&environment

The production of electric power

Thanks to its several economic and environmental benefits, in the latest years natural gas has converted into the main fossil fuel for the production of electricity. During the 70s and 80s, energy production was oriented towards coal and nuclear plants, but a series of economic, environmental and technological factors has provoked a shift towards the gas.
Vapour plants
Natural gas can be used as a fuel in vapour electric plants in order to produce vapour that, at a high pressure, activates a turbine, which makes the alternator turn. In order to create high- pressure vapour, water is superheated in a boiler: by hemertically closing the container, the vapour pressure increases and then violently gets out towards the turbine. With reference to the performance of these plants, approximately 40% of the energy contained in the fuel is transformed into electricity. The remaining 60% is lost during energy conversion from chemical to thermal, mechanical and electric.
Turbo-gas plants
Natural gas can also be used in turbo-gas electric plants. These thermoelectric plants directly exploit the energy produced during methane (or gas oil) combustion and work without boiler, in order to transform the water into vapour and without condenser in order to reconvert the vapour into water. A turbo-gas plant consists of:

• A compressor: it sucks air up from the atmosphere, compresses it and sends it to the combustion chamber.
• Combustion chamber: this is where the combustion between the air and the fuel occurs (methane or gas oil).
• Gas turbine: the air and gas mixture, at high temperature, gets into a turbine where the expansion of combusted gases makes the rotor blades rotate and subsequently activates the alternator therefore generating electricity.
  The advantages of turbo-gas plants are: the low costs of the plant, they start rapidity even if there is no energy in the network and the fact that these plants do not need cooling water. It is possible to build them in any place, even far from the rivers and the sea. The disadvantage is the very low output (around 30%) and therefore the very high energy cost.

Combined-cycle plants
Combined-cycle and co-generation systems are the most efficient technologies to produce electricity from natural gas. Both use the heat that would normally be lost. Combined-cycle plants exploit the heat they generate to produce electricity. These systems associate a turbo-gas plant with a vapour group: the residual heat of fumes going out from the turbo-gas group is used to produce vapour, increasing the performance by 56%. Moreover combined-cycle plants have lower building and maintenance costs, and a higher functioning reliability.
Combined cycle plants represent the most sustainable solution for thermoelectric power generation, guaranteeing a low overall impact on natural resources. In particular, the use of natural gas has a very limited environmental impact in terms of local atmospheric pollution. Respect to coal and oil, in fact, using natural gas eliminates sulphur dioxide and dust emissions and reduces nitrogen oxide emissions thanks to the introduction of state-of-the-art combustion technology.  In terms of global atmospheric pollution, the combination natural gas/combined cycle represents one of the most efficient alternatives to reduce greenhouse gas emissions. Natural gas is the fossil fuel whose combustion to produce a unit amount of energy generates the smallest amount of carbon dioxide: the “emission factor” amounts to 2.35 tCO₂/tep, around 26% lower that of petroleum-based products and around 41% lower than coal.