3.4 Plant End Systems
At the end of the power plant facility, flue gases from the burning of fuel will come out of the stack. However, to meet mandated emission standards, there will be units to help reduce the "bad" emitters.
The primary combustion products come from carbon and hydrogen and are shown in the reaction equations below:
C + O2CO2
4H + O2 → 2H2O
Carbon dioxide and water are formed. But they are not the only products of combustion.
Coal also has sulfur, nitrogen, and minerals that go through the combustion process. Sulfur turns into sulfur dioxide and trioxide, also known as SOx. Nitrogen in coal can form NO, N2O, and NO2, also known as NOx (fuel NOx). NOx can also form from the nitrogen in air when the temperature in the boiler is high (thermal NOx). Minerals that go through combustion are called ash and are the oxygenated compounds of the minerals in coal. If you have ever burned wood in a fireplace or at a campsite, you have seen the ash that remains.
The constituents can be summarized in a pneumonic: NO CASH. Every product of combustion, other than water, has been implicated in an environmental problem of some sort. Table 3.1 shows a summary of NO CASH:
Table 3.1: Summary of NO CASH
Coal Components - Environmental Issues
One of the worst environmental consequences that can occur is when NOx and SOx are released in the atmosphere and eventually converted into the corresponding acids:
NOx + O2 + H2O → HNO3
SOx + O2 + H2O → H2SO4
Both nitric and sulfuric acids are very soluble in water. They will eventually fall to the earth either as acid precipitation (acid rain or snow) or as deposits.
In many parts of the US, rainfall is 10 times as acidic as rain falling in unpolluted areas. In some locations, or on some occasions, it can be 100 times more acidic. Numerous environmental and health problems are related to acid rain, including the following:
- Acid rainfall accumulates in streams and lakes, so fewer and fewer aquatic species can reproduce or survive. Water areas can become biologically "dead."
- Acid rain in the soil can leach key nutrients out of the soil.
- Acid rain can affect trees, especially on mountain tops. The type of rainfall that can be particularly damaging is a fine mist of acid rain.
- Whole forests can be wiped out if the damage is extensive enough, including entire ecosystems of plants and some animals.
- Acid rain or deposition can be corrosive. It can attack marble, limestone, etc. Historic buildings, monuments, and statues have been defaced by acid deposition.
- Human health can be affected by acid rain. Humans can inhale a mist of dilute acids, which can irritate the respiratory tract, which, in turn, exacerbates chronic respiratory illnesses. The elderly and infants are at greatest risk.
Degree of Acidity in an Aqueous Solution - pH Scale
Here are some key facts about pH:
- pH = 7 is perfectly neutral
- pH < 7 is acidic
- pH > 7 is basic (alkaline)
- smaller the number = more acidic the solution
- for each 1 unit change in pH, there is a ten-fold change in acidity
- a solution with pH=5 is 10 times more acidic than pH=6; pH=4 is 100 times more acidic than pH = 6
Natural rainfall is mildly acidic because carbon dioxide in the air (CO2) is moderately acidic and soluble in water.
CO2 + H2O = H2CO3
(carbonic acid, pH=5-6)
So, acid rain is defined as rainfall having a pH < 5.6.
When coal is burned in the absence of control equipment, smoke is generated. Smoke is a mixture of fly ash particles and unburned char. On a day of high humidity, the smoke particles act as points to condense moisture from the air. When coal has high sulfur content, you also have SOx emissions. Under these conditions, the dispersion of sulfuric acid droplets occur, and when associated with the particles of smoke:
SMOKE + FOG = SMOG
There have been sulfuric acid smog events that have killed people - in Donora, PA (1947), in New York City (1966), and in London (1952). In most industrialized nations, this is no longer a problem, as regulations have reduced the smoke and sulfur emissions at power plants and there is now little domestic use of coal.
There are several options for cleaning up the bad emissions:
- Do nothing. (Use a tall stack to disperse pollutants: the solution to pollution is dilution.)
- Remove or reduce sulfur and nitrogen in fuel feedstock before it is burned (precombustion). This includes sulfur, nitrogen, and minerals.
- Allow the SOx, NOx, and ROx to form in the boiler, but capture them before they can be emitted into the environment. These are called post-combustion strategies.
The "do nothing" strategy is illegal in the US. The Clean Air Act of 1977 and amendments to the Clean Air Act of 1990 have changed the air environment in the US. However, this is still a problem in the former Soviet Bloc, China, and third world nations.
Precombustion strategies can be approached in the following ways. One way is to switch to cleaner fuel, such as natural gas. In order to do so, however, extensive changes may need to be made to the burners and boilers. Another way is to switch to a cleaner form of coal. Most low sulfur coals are in the western US and have to be transported to the east. These coals tend to have a lower heating value, which leads to more expensive operating costs related to the need to purchase more coal. Finally, impurities can be removed from coal; this can be done by removing minerals that contain sulfur or nitrogen, such as pyrite (FeS). However, some S and N are chemically bonded to the organic portion of coal itself and cannot be removed. Petroleum and natural gas can also have sulfur associated with it. For petroleum products, as discussed in Lesson 2, hydrogen is used to react with sulfur to form hydrogen sulfide (H2S). H2S can be captured from natural gas as well; H2S can be converted into solid sulfur and sold to the chemical industry.
There are also post-combustion strategies for removing impurities. Most of the ash that forms during combustion drops to the bottom of a boiler (~80%) and can be removed for disposal back into the mine. However, up to 20% is carried out of the boiler through the flue gas and is known as fly ash (and can be called particulate matter). Fly ash can also cause health problems. A tiny particle of ash can get lodged in narrow air passages of the lungs. If the body cannot remove it by coating it with mucus and expelling it, then the body will try to seal it off with scar tissue. Solid particulate matter can be in handled in two ways: the fly ash can be caught in gigantic fabric filter bags (like a vacuum cleaner bag), which is called the baghouse (see Figure 3.12a and 3.12b). The particles can also be given an electric charge. At high electric potentials, the charged particles are attracted to the electrode of opposite charge; the device used to do this is called an electrostatic precipitator (ESP) (see Figure 3.13).
We can also remove SOx in the flue gas. The SOx can dissolve in water to form an acid, which can then be neutralized by reacting it with a base. The cheapest and most available base is lime or limestone, which reacts:
Ca(OH)2 + SOx → CaSO4 + H2O
Calcium sulfate (CaSO4) is an insoluble precipitate; the SOX wasn't destroyed; we just convert it from a gas to an easier to handle solid. The technology for the removal of SOx is called flue gas desulfurization (FGD). The hardware is called a scrubber (see Figure 3.14). The SOx scrubbers are effective, as they capture 97% of the emitted sulfur. The CaSO4 produced is called scrubber sludge and is either put back in the mine or sold as gypsum to make drywall.
The hardest pollutant to deal with is NOx. A scrubber does not work for NOx control because nitrate salts are water-insoluble. To limit the production of thermal NOx, low-temperature burners produce less NOx or they use staged combustion so that the temperatures will be low enough to allow the reverse reaction:
2NO → N2 + O2
Flue gas NOx can be treated with ammonia:
2NH3 + NO2 + NO → 2N2 + 3H2O
All of the technologies discussed work. All add costs to producing power (a scrubber will add ~33% to the capital cost of a plant as well as operating costs). Coal cleaning adds $2-3 per ton of coal to the coal cost. And hydrotreating diesel and heating oil add 5-7¢/gal to the cost of the fuels. And these costs are passed on to the consumer.