Kilns efficiency

In practice, however, it is impractical and undesirable to condense the water vapour. Thus, from the viewpoint of determining kiln efficiency, the latent heat of condensation of the water should be subtracted from the gross calorific value. This gives the net calorific value. 

Carbon dioxide emissions have also become an important issue, especially since the process of calcination liberates 500 kgcoj/tonne of clinker. The fuel combustion-related emissions add between 150 and 400 kg/tonne depending on kiln efficiency and fuel used. Total per unit CO2 emissions will be reduced gradually over the next 20 years through plant efficiency improvements, the use of waste fuels otherwise landfilled, landfill gas as a secondary fuel, and the use of flyash in blended cements to reduce the need for clinker. 

Helping to propel capacities upward has been the advent of greatly improved preheaters, which partially calcine the stone and significantly improve thermal efficiency. Modem preheaters improve capacity by 15—20% and decrease fuel consumption a similar percentage. Other kiln appurtenances and accessories that enhance efficiency and lime quahty are the contact coolers, and such kiln internals as metal refractory trefoil systems that act as heat exchangers, dams, and lifters. 

The double-inclined kiln calcines even smaller si2ed stone of 1.88—3.75 cm and at reduced capacity with stone of only 0.63 cm minimum si2e. Most of these kilns operate using gaseous or oil fuels, including propane. An exception is the double-inclined kiln, which appears to operate at optimum efficiency with a mixture of fuel, ie, 60—75% natural gas or oil and 40—25% coke, although it can operate on 100% gas or oil. 

Another thermally efficient kiln is the modem mixed-feed vertical kiln ia which coke is admixed with 8.5—20 cm lump limestone and charged into the top of the vertical kiln by a mobile, overhead charging system. However, use of this kiln is waning since the quaUty of mixed-feed kiln lime does not equal that of the other three kilns described above, owing to ash contamination from the coke and poorer reactivity and to the higher cost of coke in most areas. 

Many kilns that formerly were direct coal-fired or producer-gas verticals were retrofitted to natural gas firing with center-burners and after World War II, dramatically improving lime quaUty, kiln capacity, and fuel efficiency. By the 1960s, this improved vertical kiln had lost favor to rotary and other special kilns because of the supply and cost problems of oil and gas in the United States and the spectacular improvement in rotary kiln performance. Many natural gas-fired center burners were permanently closed and dismanded because they could not be converted to coal. However, the reverse occurred in Europe where the extensive oil and gas discoveries heightened interest in the new, advanced vertical kilns. 

Miscellaneous Kilns. A U.S. kiln, the Fluo-Sohds, appears to be another vertical kiln type, but this is its only similarity. It operates on a different principle. It utilizes as kiln feed only a discrete granulation of 0.225—2.4 mm (65—8 mesh) sizes. DeHcately controlled by air and exhaust gas pressure, the kiln feed of granules is fluidized as a dense suspension. Because it is instmmented, this kiln can produce a very reactive lime at better than average thermal efficiency. The kiln, however, has limited utifity because the cost of obtaining the kiln feed with many hard, compact limestones is prohibitive. 

The Calcimatic is a patented kiln of Canadian origin that is radically different from other kiln types. It consists of a circular traveling hearth of variable speed, supported on two concentric tiers of rollers. Kiln feed of 12.7 mm is fed onto the hearth in a 2.5—10 cm bed from a preheater chamber. The kiln is usually fired with natural gas or fuel oil, although the option of using pulverized coal has also been developed. After great interest, resulting in sales of many units throughout the world, the popularity of the Calcimatic has ebbed because of disappointment in the unit s mediocre thermal efficiency. 

Significant waste heat may be recovered from the high (about 600°C) kiln off-gas. Pre-heating combustion air or feed ore improves the energy efficiency of the process. Reduction of barite in a fluid bed with CO and/or hydrogen has been performed on an experimental scale. 

Bricks are the oldest manufactured building material in use. Sun-dried bricks were manufactured as eady as 6000 BC, and fired bricks were used during the Middle Ages. Today s bricks differ very Htde except in the efficiency of manufacture they ate stiU made from clay or shale, a clay-based sedimentary rock that is kiln-fired. 

The sodium carboaate process is used ia a number of dry-ice plants ia the United States, although its operating efficiency is generaHy not as high as that of processes using other solutions. These plants obtain the carbon dioxide from flue gases as weH as lime-kiln gases. 

Environmental Pollution Control. The cement iadustry has had an iatensive program of capital expenditure to iastaH dust collection equipment on kilns and coolers siace the 1970s (60). Modem equipment collects dust at 99.8% efficiency. Many smaller dust collectors are iastaHed ia aew plants (61). 

Direct-flame afterburners are nearly 100% efficient when properly operated. They can be installed for approximately 350-700 per cubic meter of gas flow. Operating and maintenance costs are essentially those of the auxiliary gas fuel. On larger installations, the overall cost of the afterburner operation may be considerably reduced by using heat recovery equipment as shown in Fig. 29-16. In many industrial situations, boilers or kilns are used as entirely satisfactory afterburners for gases generated in other areas or processes. 

Control of particulate matter emissions from the kilns, dryers, grinders, etc. is by means of standard devices and systems (1) multiple cyclones (80% efficiency), (2) ESPs (95% -I- efficiency), (3) multiple cyclones followed by ESPs (97.5% efficiency), and (4) baghouses (99.8% efficiency). 

At the alumina plant, the bauxite ore is further crushed to the correct particle size for efficient extraction of the alumina through digestion by hot sodium hydroxide liquor. After removal of "red mud" (the insoluble part of the bauxite) and fine solids from the process liquor, aluminum trihydrate crystals are precipitated and calcined in rotary kilns or fluidized bed calciners to produce alumina (AljOj). Some alumina processes include a liquor purification step. 

The key to efficient destruction of liquid hazardous wastes lies in minimizing unevaporated droplets and unrcacted vapors. Just as for the rotary kiln, temperature, residence time, and turbulence may be optimized to increase destruction efficiencies. Typical combustion chamber residence time and temperature ranges arc 0.5-2 s and 1300-3000°F. Liquid injection incinerators vary in dimensions and have feed rates up to 1500 gal/h of organic wastes and 4000 gal/h of aqueous waste. 

FIG. 17-73 Normal (p erpendicular) rapping efficiency for various precipitated dust layers having about 0.03 g dust/cm2 (0.2 g dust/in2) as a function of maximum acceleration in multiples of g. Curve 1, fly ash, 200 or 300°F, power off. Curve 2, fly ash, 70°F, power off also 200 or 300°F, power on. Curve 3, fly ash, 70°F, power on. Curve 4, cement-kiln feed, 300°F, power off. Curve 5, cement dust, 300°F, power off. Curve 6, same as 5, except power on. Curve 7, cement-kiln feed, 300°F, power on. Curve 8, cement dust, 200°F, power off. Curve 9, same as 8, except power on. Curve 10, cement-kiln feed, 200°F, power off. Curve 11, same as 10, except at 70°F. Curve 12, cement-kiln feed, 200°F, power on. Curve 13, cement-kiln feed, 70°F, power on. °C = (°F — 32) x %. [SprouM, Air Pollut. Control Assoc. J., 15, 50 (1965). ]... 

Exhaust gases leaving the kiln pass through highly efficient air pollution control devices such as baghouse filters or electrostatic precipitators. The high temperatures required to make cement destroy 99.99% or more of the organic hazardous wastes. The content of hydrocarbons... 

The Pyretron thermal destruction technology is a burner system designed to be used in conjunction with any conventional transportable or fixed rotary kiln incinerator and is intended to increase the efficiency of conventional incineration. The commercially available technology controls the heat input during incineration by controlling excess oxygen available to oxidize hazardous waste. 

The Contamination Technologies, Inc. (CTI), low-temperature thermal absorber (LTA) is an ex situ rotary kiln thermal stripping technology. The use of aerospace ceramic material enhances the ability of the kiln to operate at the temperatures necessary for high efficiency while maintaining a high throughput. 

Soils with high moisture or clay contents may reduce the efficiency of the LTTD system. Hot spots in the influent soils may cause fluctuations in the dryer temperature. The vaporized hydrocarbon concentration in the kiln must be kept below the lower explosive limit (LEL). 

Calcium oxide dates from prehistoric times. It is produced by heating limestone to drive off carbon dioxide in a process called calcination CaCO , —-—> CaO, + CO ,At tem-peratures of several hundred degrees Celsius, the reaction is reversible and calcium oxide will react with atmospheric carbon dioxide to produce calcium carbonate. Efficient calcium oxide production is favored at temperatures in excess of 1,000°C. In prehistoric times limestone was heated in open fires to produce lime. Over time, lined pits and kilns were used to produce lime. Brick lime kilns were extensively built starting in the 17th century and the technology to produce lime has remained relatively constant since then. 

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