Fossil Fuel Electricity Generation
Introduction to electricity generation from fossil fuel sources in New Brunswick:
Electricity generated from burning fossil fuels (a name acknowledging that the carbon in the material came from ancient living organisms) accounts for most of the electricity generated globally. In the energy sector, fossil fuels refers to coal, oil, and natural gas. A great deal of energy is released from the combustion of these materials and this fact has made them immensely useful in advancing human progress.
Almost universally, fossil fuels are burned in a controlled burner and the hot gases produced (including large amounts of CO2) are used to heat water, turning it into steam. This hot, high energy steam is passed through a turbine where it yields its thermal energy into the motion of the turbine shaft, which once again turns the shaft of an electric generator. See Figure 1.
Figure 1. Schematic of a coal fired power plant. http://www.hk-phy.org/energy/power/print/elect_is_print_e.html
The facility described above is typical for an oil or coal burning facility; a power plant using natural gas functions slightly differently. Because the fuel is a gas, it is sent directly into a turbine (essentially the engine used on a jet airplane) and ignited. The hot gasses expand as they become hot and spin a turbine, they also spin a turbine as they cool down again. This rotational energy is used to spin the shaft in an electric generator. This type of generation facility is called a gas turbine.
Even after passing through the turbine, there is still some heat in the gases being released through the smoke stack. In the newest facilities, this heat is used to boil water and generate steam which is passed through a water-only turbine causing it to spin an electric generator. These facilities are more efficient than steam only or combustion turbine only facilities and they are called combined-cycle gas turbines.
Because facilities that burn fossil fuels are not constrained by local geography and come in a variety of price brackets, their sizes can be tailored to the application and use. Large, 1000 MW coal plants have their use on the grid, as do small, 100 MW gas fired turbines. Here are some typical sizes:
- Coal/oil fired steam generating facilities – 300 MW and higher
- Natural gas or diesel turbines – around 300 MW (varies)
- Combined cycle gas turbine – around 400 MW
- Upfront costs – Power plants of this nature are complex from an engineering standpoint, but can range in sizes (and therefore price). Here are some typical costs for fossil fuel plants:
- Coal/oil fired steam generating facilities – $3,500,000/MW
- Natural gas or diesel turbines – $800,000/MW
- Combined cycle gas turbine – $1,100,000/MW
- Fuel costs – The prices of fuel for fossil fuel generating plant often constitute the bulk of their overall cost. Unlike some other generating technologies, they can be cheaper to build but expensive to operate. Here are some typical fuel costs for fossil fuel generating stations:
- Coal/oil fired steam generating facilities – $1.77/GJ
- Natural gas or diesel turbines – $3.2/GJ
- Combined cycle gas turbine – $3.2/GJ (Note – the higher efficiency of this plant means that one GJ of raw energy will yield more electricity than the same GJ in a straight gas/diesel turbine)
- Flexibility– Perhaps the most useful aspect of fossil fuel generation is how flexible the technologies are in adapting to the needs of the grid. Once again, it is necessary to break down the characteristics of each generation technology separately.
- Coal/oil fired steam generating facilities – Slow start up and shut down times (~12 hours from cold). They can change their output at the rate of 1 MW/ minute.
- Natural gas or diesel turbines – Very fast start up and shut down times (minutes from cold), they can change their output at the rate of 5-10 MW/ minute or faster.
- Combined cycle gas turbine – Fairly fast start up and shut down times (minutes to hours from cold), they can change their output fairly quickly at a rate of MW/minute.
- Capacity Factor - Can be very high or low depending on the use of the facility. A base load power plant will only shut down for short periods for required maintenance, while a peaking power station may only operate for a total of weeks per year to supply power at peak demand periods.
- Environmental Impact – The combustion of organic material generates a lot of waste, including soot, ash, polluting gases like sulfur dioxide and nitrous oxides which can be greenhouse gases, cause acid rain, cause smog, among many other effects. For example, a 1000 MW coal fired power plant produces around 400,000 tonnes of ash per year. One of the principal drawbacks of fossil fuel generating technologies are the emissions of CO2 that arise from burning carbon based fuels. Different fuels have different emissions such that some are “cleaner” than others. A breakdown of these emissions are provided below:
- Coal/oil fired steam generating facilities – 800 grams / kWh
- Natural gas or diesel turbines – 500 grams / kWh
- Combined cycle gas turbine – 400 grams / kWh