Fossil Fuels Used to Generate Electricity
Coal
Coal is the most widely used fossil fuel for electricity generation; its composition is a complex combination of organic compounds and inorganic, mineral matter. Coal is fossilized plant material preserved by burial in sediments and changed by geological forces that compact and condense it into a carbon-rich rock. It has been suggested that coal formation may date back to Precambrian times but most of the coal was originally laid down as organic material during the carboniferous period, 286 million to 360 million years ago, when the earth’s climate was warmer and wetter. Coal is considered a non-renewable energy source since it takes so long to form [16, 47, 48].
Coal is classified by type based on its stage of formation. This classification consists of five categories: peat, lignite, sub-bituminous, bituminous and anthracite (see Figure 3.2). Younger coals, such as lignite and sub-bituminous coals, are easier to burn because they contain a larger amount of volatile compounds that evolve as gases when the coal is heated. In contrast, older coals are more difficult to burn as they are made almost entirely of solid carbon. However, anthracite was preferred in the past instead of bituminous because it burns cleaner, producing less smoke and leaving less ash—and is more efficient in terms of units of heat produced per unit of weight. Emissions from coal combustion strongly depend on the rank and composition of the coal. Pollutants emitted from coal combustion include greenhouse gases (mainly CO2), particulate matter (including ash and unburned carbon resulting from incomplete combustion), nitrogen oxides and sulfur oxides. Other emissions from coal power stations are carbon monoxide, unburned hydrocarbon compounds, some carcinogens like dioxins and furans, and trace metals (e.g., lead and mercury).
Fuel Oil
Fuel oil is derived from crude petroleum and is the most widely used liquid fuel for power generation. Distillate and residual fuel oils are the two major categories of this type of fuel. Fuel oils are classified by grade numbers, with No. 1 and No. 2 being distillate fuel oils and No. 5 and No. 6 being residual (heavy) fuel oils (such as are preheated and fired in power plants). The Mexican oil, called “combustóleo” has similar characteristics to residual fuel oil No.6 [46–48]. Distillate fuel oils contain less than 0.3 % sulfur (by weight), are more volatile and less viscous than residual oils and have negligible nitrogen and ash contents. Distillate fuel oils are commonly used in domestic and small commercial applications and include kerosene and diesel. On the other hand, residual fuel oils are highly viscous and may need heating for easy handling and proper use in combustion. Residual fuel oils contain important amounts of ash, sulfur and nitrogen and are mainly used in industrial and large commercial applications. Due to differences in composition and combustion characteristics, distillate and residual oils result in different emissions upon combustion. For example, particulate matter emissions from distillate oils are lower than from residual fuel oils. On the other hand, No. 6 fuel oil generally has a higher sulfur content, and because the emissions of sulfur oxides are directly related to the sulfur content of the oil, emissions from No. 6 fuel oil are more polluting than those from distillate oils. Other pollutants generated during combustion of fuel oils are nitrogen oxides, carbon monoxide, greenhouse gases, volatile compounds (like unburned hydrocarbons) and toxic trace metals.
Natural Gas
After coal and oil, natural gas is the third type of fuel most widely used for electricity generation. The main component of natural gas is methane (85–90%); it also contains propane, ethane, butane, some inert gases such as nitrogen, helium and carbon dioxide, and trace amounts of other gases. Natural gas is the most rapidly growing energy source in the world, and is considered to be the cleanest fossil fuel. Most of the air pollutant emissions from natural gas combustion processes are nitrogen oxides and greenhouse gases (mainly CO2). Other emissions are small amounts of particulate matter, sulfur oxides, and trace amounts of metals. When natural gas is burned, it produces only half as much CO2 as an equivalent amount of coal (in thermal energy terms); therefore, if this cleaner fuel were substituted for coal, a reduction in emissions could be achieved. However, this is sometimes economically and/or strategically infeasible [45, 46, 48].