Kilns cement manufacture

Portland Cement Manufacture. The second greatest use of limestone is as raw material in the manufacture of Pordand cement (10). The average limestone factor per ton of Pordand cement is 1.0—1.1 t of pulverized limestone. The limestone, calcined to lime in the cement rotary kiln, combines with siUca and alumina to form tricalcium siUcate and tricalcium aluminate and other lesser cementing compounds (see Cement). Most cement companies operate captive limestone quarries. 

In 1991, Goodyear began working with Cadence Environmental Energy (Indiana) to market a whole tine feed system to supplement fuel for cement kilns. The system is used by several cement manufacturers. In 1992, Goodyear furnished tines for a Tennessee Valley Authority (TVA) test bum at a Memphis power plant. The electric utiUty used tine-derived fuel (TDE) to supplement coal fuel in a cyclone boiler. These tests were successflil. 

Particulate matter emissions are the primary concern with cement manufacture. Fugitive emissions and uncontrolled kiln emissions are shown in Table 30-23. 

Nodulizing is another process of size enlargement by fusion. This employs a rotary kiln like those used for cement manufacture. The product is uniform, about 0.5 in. dia, and more dense than sinter. 

The chief examples are smelting for the recovery of metals from ores, cement manufacture, and lime burning. The converters, roasters, and kilns for these purposes are huge special devices, not usually adaptable to other chemical applications. Shale oil is recovered from crushed rock in a vertical kiln on a batch or continuous basis—moving bed in the latter case—sometimes in a hydrogen-rich atmosphere for simultaneous denitrification and desulfurization. The capacity of ore roasters is of the order of 300-700 tons/(day)(m3 of reactor volume). Rotary kilns for cement have capacities of 0.4-1.ltons/(day)(m3) for other purposes the range is 0.1-2. 

Rotary kilns (Chapter 2, Fig. 2.4) have advantages where the solid particles tend to stick together as in cement manufacture, and in the reduction and carbonylation steps in the purification of nickel. In rotary kilns, the flow of gas may be co-current or countercurrent to the solids. 

Fly ash, as it is a large-volume industrial waste, is both cheap and abundant, so that there is an economic incentive to use fly-ash-modified cements. In addition, C02 is also produced as a waste by-product of industrial processes (power generation, cement manufacture, etc.), and its permanent sequestration into cement is an added environmental benefit. A fully carbonated Portland cement permanently sequesters about 130 L of C02 per kilogram of cement. Figure 15.8 shows the structural and chemical modifications produced in cemented fly ash microspheres as a result of the supercritical C02 treatment. As is the case with fly ash, kiln dusts are primarily siliceous, so that the same benefits can be derived from their use as modifiers in immobilization and S/S matrices. 

Cement manufacture is a power-intensive process, which allows cement companies to buy fuel in bulk and obtain the fuel at a somewhat lower price. Also, kiln feed mechanisms are easily modified, to accept alternate fuels. Further, because temperatures in a kiln reach 2700 F, kilns can bum poorer quality coal than pulp and paper mills or even utilities, and can easily tolerate a wide variety of waste products.10 In addition, kilns can accommodate the lower priced TDF (wire-in TDF and even whole tires). These factors make the economics of supplying TDF to cement manufacturers less favorable than for pulp and paper mills.10 One cement manufacturer is paying approximately 30/ton for TDF.11... 

A cement manufacturer using tires to supplement its primary fuel in a wet process cement kiln... 

Controlled burning of tires or TDF for fuel value occurs most frequently in tvo types of process units - kilns and boilers. This chapter will describe the general process operation of cement kilns and boilers. The various types of boiler configurations will be described with attention to the implications for burning tires or TDF. Kilns in two industries have burned tires or TDF supplementally - lime manufacturing and, more commonly, cement manufacturing. 

When whole tires are used as supplemental fuel in cement manufacture, they generally enter the process at the upper feed end of the kiln. Depending on the specific process flow at a facility, TDF can be added at the feed end, at the lower (firing) end, or in a raw feed preheater/precalciner that is located before the raw feed entrance. These options are described in more detail in Chapter 4, Tire and TDF Use in Portland Cement Kilns. 

Many industry-wide changes over the last decade have dramatically affected fuel use and efficiency in the cement industry. First, a trend toward more prevalent use of the dry process of cement manufacture rather than the wet process continues. New technology in conjunction with fuel savings provided by the dry process have made it the process of choice. In fact, no new wet process kilns have been built in over 15 years.2 Second, over the last decade, many plants have converted their kilns to coal firing because of coal s cost effectiveness in comparison to oil and gas. Although both of these trends have had a considerable effect on fuel efficiency and cost in the industry, use of supplemental fuels, such as waste tires, continues to be of high interest to the industry. All fuels are purchased, however, based on regional prices. 

In the portland cement manufacturing process, three steps occur. First, raw materials are crushed and mixed. The raw materials are powdered limestone, alumina, iron, and silica. Second, the raw materials are fed to an inclined rotary kiln in which they are heated to at least 2700 F. A rock-like substance called clinker is formed, which exits the kiln and is cooled. Third, the cooled clinker is finely crushed, and about 5 percent gypsum is added to produce finished cement. Details of the process are explained below. 

Note that data from a boiler burning TDF at a silicon manufacturing facility, Dow Coming in Midland, Michigan, are reported in Chapter 5 with waste wood boilers, because the primary fuel for this boiler is wood chips. Further, data on TDF use at Boise Cascade, an "other" manufacturing facility, are included in Chapter 4 with cement manufacturing, because the rotary kiln used to manufacture lime is similar to the rotary cement kilns. 

Cement manufacturing companies use tires and TDF to supplement their primary fuel for firing cement kilns. Some of the companies were using tire or TDF directly in the kiln, some were using tires or TDF in the precalciner (prior to the kiln proper), and one company was using tires or TDF in both processes. 

OEC has been used to reduce NOx in a fixed-based resource recovery process.20 Giant Resource Recovery (GRR) is a subsidiary of the Giant Group, Ltd., which is involved with cement manufacture and the use of waste materials as fuel and raw materials supplements. GRR processes creosote-contaminated soil through countercurrent rotary kilns. By a patented process, the decontaminated soil is then used as a raw material for cement production, thus replacing a certain portion of the traditional feed material stream. The combustion products are ducted into the cement kilns. 

One of the most satisfactory uses of waste lubricants is as a fuel extender in cement production, subject to control over metal content. The advantages of this disposal route are the low-cost, or free, calorific value of the waste as a fuel to the cement manufacturer and the wastes which might otherwise be vented to the atmosphere with the flue gases are retained within the cement product with no adverse environmental consequences. There are, of course, limits to the cement production capacity which can use this waste as a fuel, particularly as cement kilns are seen as the ideal disposal route for other hazardous wastes such as contaminated solvents. The capacity of cement plants to dispose of combustible wastes is also dependent upon the level of demand for cement, which reduces when the building industry is in recession. 

Manufacturing processes.—Large quantities of COj are added to the air in the vicinity of lime- and brick-kilns, cement-works, etc. 

Kiln dusts (KD) - 64% of the cement kiln dusts (CKD) produced is reused within the cement plant - CKD is used as a stabilizing agent for wastes, where its absorptive capacity and alkaline properties can reduce the moisture content, increase the bearing capacity, and provide an alkaline environment for waste materials - At the present time, approximately 80% of the surplus CKD remaining after reuse in cement manufacturing is stockpiled or land filled - Most of the LKD generated in the United States is currently disposed of in stockpiles or landfills 88-98... 

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