Rotary kilns efficiency

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. 

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. 

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. 

Two major types of lime caleiners are die rotary (Fig. I) and (he vertical kiln, In North America, rotary kilns are widely used for lime cnlcinuiion. whereas in Europe vertical kilns are most popular. Rotary kilns typically have higher output [up to 6 tons (545 metric tons)/da> and lower labor cost. Vertical kilns can be designed for higher fuel efficiency and lower capital investment. They handle down to about -in. (19-cnt) stone, hut... 

This paper presents the first experimental results for the solid motion inside a pilot-scale rotary kiln. Such data are useful to enable efficient pyrolysis reactions inside a rotary kiln to be carried out, through the prediction of residence time and material hold-up, and the evaluation of different surfaces and temperature profiles according to the operating conditions. In the first part, the pilot-scale rotary kiln and the principle of the experiments will be described. An original dynamic solid motion model will be presented in the second part, this dynamic model is derived from the original static model of Seaman [1], The static and dynamic experimental results are finally compared with the simulated results. 

In the rotary kiln process, coke is fed to a rotating cylindrical furnace sloped slightly toward the discharge end. Coke flows down the kiln countercurrent to the hot gas flow. Moisture is liberated from the coke in the feed zone, then the coke passes through the combustion zone where VCM is liberated. As coke leaves the kiln, it is discharged to a cooler where it is quenched with water and then cooled with ambient air. Recent designs have incorporated energy efficient features such as air preheat and steam... 

To improve energy efficiency, refractories with superior K factors are used in lining the kiln, thus reducing radiant heat losses. Moreover, kiln mounted blowers now inject combustion air into the kilns in the zone where the volatiles evolve from the coke, thus permitting utilization of the Btu content in these previously wasted gases. In modern calciners, most of the energy required is obtained by burning the coke volatiles and fine particulate matter in the kiln. In some instances, rotary kilns equipped with kiln mounted blowers actually operate without external fuel (except for start-up). When these units are also equipped with incinerators (to combust the unburned volatiles and emitted coke fines) and waste... 

Design Methods for Rotary Kilns In rotary kilns, the material is not showered through the air stream but is retained in the lower part of the cylinder. Gas-solids contacting is much less efficient than in flighted units. Heat transfer is by radiation and convection from the flowing gas to the kiln brick and exposed bed surface and by radiation from die brick to the bed. For units employing separate combustion chambers, it can be assumed that at high temperatures the wall-film... 

In rotary kilns the raw powder is preheated by the hot flue gases. The use of special preheaters (grate preheaters, cyclone preheaters) can make heat recovery even more efficient. 

Pyrolysis of agricultural residue was experimentally assessed as a fuel production process for farm applications. A rotary kiln (3.4 m by 0.165 m I.D.) was used due to its ease of operation, commercial availability, low operating costs and ease of start-up and shutdown. Ground oat straw and corn stover at less than 10% moisture were pyrolysed in an indirectly fired continuous-flow rotary kiln located at the University of Sherbrooke. The principle products were char and gas, less than 1% of the feed mass was converted to tar. Calorific values were about 17 MJ/kg for the feed, 26 MJ/kg for the char, and 12 MJ/m3 for the gas. Calculations indicate that the thermal efficiency of a self-sustaining process would be around 65%. 

Countercurrent operation of shaft and rotary kiln furnaces leads to extensive heat transfer between feedstock and outgoing products. The sensible heat of the products remains within the system which enhances its thermal yield. Cocurrent operation shows none of these advantages yet it is a simple and efficient method to avoid tar problems. Indeed the tar is extensively cracked while moving through the hearth zone. Also the problem of wastewater is somewhat simplified. Crosscurrent operation is only seldom used. 

The flow of the combustion gases in direct-fired systems may be concurrent (i.e., the gases flow in the same direction as the feedstock) or countercurrent (i.e., the gases flow opposite to the movement of the feedstock). The benefits of a concurrent system are higher treatment temperatures, longer residence times, and treatment of fines. Countercurrent systems offer more efficient heat transfer, and lower volumes and temperatures of off-gas. ° In large part due to the beneficial aspects of the off-gas and consequent simplified APC system, countercurrent systems are used more commonly in rotary kilns. 

Current disposal methods include liquid injection incineration, rotary kiln incineration, fluidized bed incineration, and air stripping (HSDB 1995), however, data on the efficiency of these methods are lacking. This information will be useful in identifying the media of concern for human exposure and populations at risk of adverse health effects from exposure to carbon disulfide. 

Mustard Mustard is immediately destroyed at temperatures above 500°C to S02, HQ, C02 and water vapor. The contractor operated a commercial, transportable 7 MM Btu/hr incinerator which could readily achieve an organic destruction efficiency of 99.9999% and which was used to destroy frozen bulk mustard introduced as solid waste and to thermally treat agent-contaminated metal scrap. The incinerator incorporated a two-stage combustion process, e.g., a rotary kiln (900°C, 1.5 tonnes/hr capacity) and a down-fired secondary combustion chamber (1200°C, 2.5s residence time), backed by a venturi wet flue gas scrubber. System components were erected on a prepared concrete foundation and housed in a prefabricated building to provide shelter for the equipment and operators. Natural gas from a nearby well provided a fuel source. 

Rotary kilns can also be fitted with a kiln-feed preheater see Figure 6.8, which enables a shorter kiln to be used, approximately one-half the length of a conventional rotary. The exhaust heat is used to preheat the burden entering the kiln thus increasing heat recovery and improving the thermal efficiency of the furnace. 

The wiped-film evaporator is a special type of evaporator that permits evaporation to a much higher concentration of solids than do other evaporators. Liquid is fed into a heated cylinder that contains rotating blades or wipers to reduce the liquid to a film, thereby improving the heat-transfer efficiency. Wiped-film evaporators can also be operated as dryers. Other equipment that can be used for drying and calcining non-high-level waste is the same as for HLW, e.g., spray calciners, fiuidized-bed calciners, and rotary kilns. 

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