Opinion ID: 314135
Heading Depth: 2
Heading Rank: 1

Heading: Present types of Emission Control in the Manufacture of Portland Cement

Text: In the manufacturing process for portland cement, [56] the principal ingredients, limestone and clay, are combined, after having been reduced to a powdery fineness, to make a substance known as raw feed. The powdered limestone and clay are mixed by either the wet process or the dry process. In the wet process, water is added to the limestone and clay to make a slurry, which is then introduced into a kiln. In the dry process, the two substances are mixed mechanically and by use of air before the mix is introduced into a kiln. Raw feed is introduced to the kiln at ambient air temperature and is then heated to a temperature of about 2700° Fahrenheit, produced within the kiln by the use of various fuels. The emission standards under challenge here relate solely to the control of particulate matter produced by the kiln operation. The kiln operation involves the chemical process known as calcining limestone; carbon dioxide is driven from the limestone, converting calcium carbonate (CaCO3) into calcium oxide (CaO), (CaCO3 yields CO2 + CaO). The calcium oxide later combines with the clay to form a substance known as clinker, the basic component of cement. The calcination process produces gases and dust as by-products. The particulate matter is suspended in the hot exhaust gas and the various types of emission control devices remove this matter from the gas, before it is emitted into the atmosphere through a stack. The two types of equipment principally used in removing particulate matter from the exhaust gas are electrostatic precipitators and glass fabric bags, impregnated with graphite, located in a bag house. When the precipitator is used, dust particles are charged and pass through an electrical field of the opposite charge, thus causing the dust to be precipitated out of the exhaust gas and thereafter collected by the device. When glass fabric bags are used, the exhaust gas is cooled, sometimes by a water spray, so that the bags will operate without damage from excessive heat. The bag filters out the particulate dust, though sometimes the coolant combines with the dust to form a gummy substance as residue in the bags, which must be continuously cleaned out in order to avoid impairing the permeability of the bag. It is the ability of control devices such as precipitators and bags to separate out a sufficient amount of particulate from the exhaust  in accord with the proposed standards  which is under challenge by the manufacturers. The standard requires that the particulate matter emitted from portland cement plants not be in excess of 0.30 lb. per ton of feed to the kiln . . . maximum 2-hour average.