From a fossil fuel standpoint, coal is a "young" material (see the OWOE topic: What is coal?
), and the younger the coal, the less carbon content there is to be burned and the more non-combustable content that remains after burning. In fact, burning lignite coal, the youngest, has been described by some as "burning dirt". The leftovers from the coal combustion process that are heavy and fall to the bottom of the furnace are called "bottom ash". Bottom ash is typically managed as a solid; however, that has its own set of issues. More challenging are the lighter components, including both gasses and small particulate matter, which are carried by the combustion air (flue gas), and, without some form of pollution controls, would get emitted into the atmosphere.
The most serious air emissions from coal burning power plants and how they are managed in modern power plants are:
- nitrous oxide (NOx): NOx is not only an air pollutant, but it reacts with the atmosphere to create ozone and is captured by moisture in the air to form acid rain, which became a serious environmental issue in the 1970s. Abatement strategies include reducing the combustion temperature, fuel reburning, and using a chemical reaction such as selective catalytic reduction (SCR), which typically injects ammonia into the flue gas to generate nitrogen gas and water (see Fig. 2), and selective non-catalytic reduction (SNCR) which, as the name suggests, avoids use of an expensive catalyst. SCR and SNCR systems can remove 80-90% of the produced NOx.
- sulfur dioxide (SO2): gaseous SO2 emissions result in breathing problems and can harm the respiratory system. It also reacts with other compounds in the air to create small particles and result in what is called a "yellow haze", which is exactly as it sounds. SO2 also contributes to acid rain. A process called flue-gas desulfurization (FGD) is used to remove SO2. There are a number of different approaches to FGD. Wet scrubbers use limestone or hydrated lime which chemically reacts with the SO2 and binds the sulfur into a solid (see Fig. 3). Dry scrubbers inject a calcium or sodium alkaline reagent directly into the furnace or ductwork. Modern FGD systems can remove 80-95% of the produced SO2, with wet scrubbers typically more efficient.
- carbon dioxide (CO2): CO2 is the primary greenhouse gas that is the cause of global warming. At this point in time, there is no cost-effective way to prevent CO2 from being released to the atmosphere. See the detailed discussion on how CO2 can theoretically be managed in power plants in the OWOE topic: What is clean coal? The lack of a way to prevent CO2 emissions is one of the issues contributing to the demise of coal as an energy source.
- particulate matter: this is the smoke, soot and ash that is released from the coal when the carbon is liberated through the burning process. The softer the coal, the more non-carbon material, and the more particulate matter that is released. Particulate matter creates a haze that reduces visibility, and the finer particles can get into lungs, causing breathing issues and asthma. There are two primary systems to remove particulate matter from the flue gas: baghouses, which are collections of large vacuum-cleaner-bag-like filters (see Fig. 4), and electrostatic precipitators (ESPs) that use oppositely charged plates to ionize the air and attract particles (see Fig. 5). ESP efficiency depends on the amount of power used, but is typically in excess of 80%, while baghouses can achieve close to 99% particle removal.
- heavy metals: these include such elements as cadmium, arsenic, selenium, lead, zinc and mercury. Although emission levels are low, they get deposited to the soil, and ultimately they can accumulate in food-chains where they can have significant health impacts. Baghouses are effective at capturing heavy metals along with the other particulate matter. However, mercury, in particular, requires additional control measures. Activated Carbon Injection (ACI) upstream of a baghouse appears to be the most effective means to remove 90% or more of the mercury in the flue gas.
The good news is that most of the components carried by the flue gas can be managed with modern pollution control technology. The bad news is that there is no cost-effective way to manage carbon dioxide (See OWOE topics What is clean coal?
and What is carbon capture and storage (CCS)?
), and these pollution control technologies are very expensive to implement, especially as a retrofit to an existing coal plant.