Atmosphere high-sulfur coal

Coal ash corrosion is a widespread problem for superheater and reheater tubes in coal fired power plants that bum high-sulfur coals. The accelerated corrosion is caused by liquid sulfates on the surface of the metal beneath an over-lying ash deposit. Coal ash corrosion is very severe between 540 and 740°C (1000°F and 1364°F) because of the formation of molten alkali iron-trisulfate. Considerable work has been done to predict corrosion rates based on the nature of the coal (its sulfur and ash content). This was accomplished by the exposure of various alloys to synthetic ash mixtures and synthetic flue gases. The corrosion rates of various alloys were repotted in the form of iso-corrosion curves for various sulfur dioxide, alkali sulfiite, and temperature combinations. An equation was developed to predict corrosion rates for selected alloys from details of the nature of ash by analyzing deposits removed from steam generator tubes and from test probes installed in a boiler [33]. Then laboratory tests were conducted using coupons of various tdloys coated with synthetic coal ash that was exposed to simulated combustion gas atmospheres. 

Sulfuric acid. Sulfur dioxide (SO2), formed mostly by the burning of high-sulfur coal, forms sulfurous acid (H2SO3) in contact with water. The atmospheric pollutants hydrogen peroxide (H2O2) and ozone (O3) dissolve in the water in clouds and oxidize the sulfurous acid to sulfuric acid ... 

Properties. The properties of char products from two possible coal feeds, a low sulfur Western coal, and a high sulfur Midwestern coal, are shown in Table 11. The char derived from the low sulfur Western coal may be direcdy suitable as plant fuel, with only minor addition of clean process gas to stabilize its combustion. Elue gas desulfurization may not be required. Elue gas from the combustion of the char derived from the high sulfur Illinois coal, however, requires desulfurization before it may be discharged into the atmosphere. 

Sulfurous acid (H SOj) can be produced by burning sulfur to form sulfur dioxide (SOj) gas and by then dissolving the gas in water to form sulfurous acid. This is the acid produced by burning coal that has a high sulfur content the gaseous sulfur dioxide by-product of combustion then combines with atmospheric water to form acid rain. ... 

Acid rain. Lakes in some areas of the world are now registering very low pH s because of excess acidity in rain. This was first noticed in Scandinavia and is now prevalent in eastern Canada and the northeastern U.S. Normal rainfall is 5.6 (because of CO2 in the air forming H2CO3). However, excessive use of fossil fuels (especially coal) with high sulfur and nitrogen content cause sulfuric and nitric acids in the atmosphere from the sulfur dioxide and nitrogen oxide products of combustion. Some rain in the Adirondack Mountains of upper New York State has been measured with a pH of 3.0. This problem is not specific to the chemical industry but should be of concern to all of us. 

Materials problems in newer coal gasification processes accrue generally from operating temperatures of 1500 -2800°F and pressures of 150-1200 psi. Added to these conditions are low oxygen activity and high sulfur activity in the product gas atmosphere. Finally, coal ash and sulfur sorbents present in the system can cause materials failure by corrosion and/or erosion-corrosion. Current metallic alloys that were developed... 

Environmental Impact - Sulfur dioxide and acid mist released to the atmosphere contribute to local and regional acidification of the atiriosphere and may contribute to acid rain. However, SO emissions from sulfuric acid plants are small compared with large-scale power stations burning coal and high-sulfur fuel oil. 

Industrial smog consists primarily of particles (ash and smoke), S02(g), and H2SO4 mist. A variety of industrial operations produce significant quantities of S02(g), but the main contributors to atmospheric releases of S(32(g) are power plants burning coal or high-sulfur fuel oils. SO2 can oxidize to SO3, especially when the reaction is catalyzed on the surfaces of airborne particles or through reaction with NO2 ... 

---