There are basically two ways of reducing acid deposition. Emission control technologies can be attached to smokestacks at power plants and other industries, removing the acid gases before they are emitted into the atmosphere. In coal-fired power plants, sulfur emissions are removed with a ‘‘scrubber,’’ in which a limestone slurry is injected into the flue gas to react with the SO2. The resulting gypsum slurry can eventually be used in other industrial processes. The main problem with scrubbers is that they are expensive, and they decrease the overall operating efficiency of a power plant. The decreased efficiency results in increased emissions of carbon dioxide, a greenhouse gas.
The other alternative to reduce acid deposition is to burn less high-sulfur fossil fuel. This can be
accomplished by switching to alternative sources of energy or improving the efficiency of energy-consuming technologies. Coal-fired power plants can reduce SO2 emissions by burning coal with a lower sulfur content. Another alternative is for these power plants to switch to fuels with lower acid gas emissions, such as natural gas. Ultimately, the most effective ways to reduce acid rain are the use of renewable energy and improving energy efficiency. Such measures will decrease the use of fossil fuels and therewith reduce the emissions. Renewable energy technologies, such as solar and wind energy, can produce electricity without any emissions of SO2 or NOx. There are also many ways to decrease consumption of energy by improving the efficiency of end-use technologies. Both renewable energy and energy efficiency have the added benefit that they also result in reduced emissions of carbon dioxide, the greenhouse gas most responsible for global warming.
It is technically feasible and likely economically possible to further decrease NOx emissions from fossil fuel combustion to the point at which they become only a minor disturbance to the nitrogen cycle at all scales. Clean electric generation and transportation technologies are commercially available today, or will be commercially available within one or two decades, that have the potential to further decrease NOx emissions in industrialized countries and to decrease NOx emissions from projected business-as-usual levels in developing countries. In addition, technologies currently under development, such as renewable energy and hydrogen-based fuel cells, could operate with zero NOx emissions.
During the 1970s, major attention was given to energy conservation measures designed to avoid depletion of coal, crude oil, and gas reserves. Since then, the known reserves of natural gas have increased from 40,000 to 146,400 billion m3 in 1998. The current world coal, crude oil, and natural gas reserves are estimated to be 143,400, 500, and 131,800 Mtoe, respectively (Mtoe = million ton oil equivalents). Thus, there appears to be enough fossil fuel available for at least 50-100 years, and it is expected that this will increase as new technologies become available with which to discover and add new sources of fossil fuels. The current world concern about fossil fuel combustion is driven by the apparent necessity to decrease CO2 emissions. The increase in CO2 content of the atmosphere and the associated climate change will call for a drastic change in energy production and use. Focusing only on SO2 and NOx as pollutants from energy production and use will not lead to drastic changes in the energy systems. In order to abate these emissions from energy production, a multiple - pollutant approach should be used; that developed within the Goteborg Protocol appears to be very promising. Unless yet-to-be developed carbon sequestration technology emerges, decreasing CO2 emissions will automatically decrease NOx and SO2, although this has to be proved. On the contrary, NOx and SO2 abatement almost always leads to increased energy use and thus increased CO2 emissions. These technologies are focused on enhanced combustion, producing less NOx (lean burn), or using end of pipe technologies, such as SCR or scrubbers. Both technologies decrease energy efficiency, and the majority of SCR technologies require ammonia or urea as a reductor, which is produced from natural gas.