Indirect fired kilns

Blezard. Reflections of the History of the Chemistry of Cement. Society of Chemical Industry (SCI) Lecture Series, UK, 1998. 

Expanded Shale, Clay, and Slate Institute, ESCSI, 2225 Murray Holladay Road, Salt Lake City, http //www.escsi.org/. 

The Rotary Cement Kiln. Chemical Publishing Inc., New York, 1986. 

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Indirect fired kilns are those heated externally. They are usually designed for applications where direct contact between the material and the gas providing the heat source is undesirable. In this case, the heat source is external to the kiln (Figure 1.9). Any internally flowing gas that is in the freeboard is used for purging any volatile or gas that arise from the bed as a result of chemical/physical reactions. Because of their low thermal efficiency, externally heated kilns are small, typically up to 1.3 m (50 in.) diameter and are used for niche applications such as calcining of specialty materials. 

The majority of the cyanuric acid produced commercially is made via pyrolysis of urea [57-13-6] (mp 135°C) primarily employing either directiy or indirectly fired stainless steel rotary kilns. Small amounts of CA are produced by pyrolysis of urea in stirred batch or continuous reactors, over molten tin, or in sulfolane. The feed to the kilns can be either urea soHd, melt, or aqueous solution. Since conversion of urea to CA is endothermic and goes through a plastic stage, heat and mass transport are important process considerations. The kiln operates under slight vacuum. Air is drawn into the kiln to avoid explosive concentrations of ammonia (15—27 mol %). 

Direct-fired combustion equipment is that in which the flamt of combustion are used to achieve the desired result by r and convection. Common examples include rotary kilns and open-hearth furnaces. Indirect-fired combustion equipment is that in wh e... 

It has been shown in these studies that the principal, and probably only significant source of NDMA, is malt which had been dried by direct-fired drying (21, 73). It is well known that malts kilned by indirect firing have either low or non-detectable levels of NDMA (74). Consequently, changes in malting procedures have been implemented in both the U.S. and Canada which have resulted in marked reductions in N-nitrosamine levels in both malts and beer (70,74). For example, sulfur dioxide or products of sulfur combustion are now used routinely by all maltsters in the U.S. to minimize N-nitrosamine formation (70). The Canadian malting industry, on the other hand, has... 

The SoilTech ATP system is a type of indirectly fired rotary kiln. The system treats soils, sediments, and sludges contaminated with compounds that vaporize at temperatures up to 1100°F (590°C), resulting in treated solids that are free of organics and suitable for on-site backfill. The treatment system uses filtration, oxidation, and adsorption operations to remove contaminants from aqueous condensate. 

Other less common methods of calcining petroleum coke are also or have been practiced. In electric calcining, practiced where an abundant supply of cheap electrical power is available, the required temperature is achieved by electrical arcing and resistance heating such calciners employ either batch or continuous vertical shaft kilns. Indirect-fired vertical shaft kilns, by-product coking... 

In the pilot plant the coke-KOH mixture was charged with a screwfeeder to a precalciner which was an indirectly-fired rotary kiln. The reaction proceeded in the precalciner at internal temperatures of 400° to 500°C. The product, called precalcinate, was cooled and temporarily stored in sealed 30-gallon drums. 

The low-temperature carburization LTA process developed by VTA meets the worldwide need for a process which economically reduces petrochemical and hydrocarbon residues into recyclable products, feedstocks or clean fuels. The indirectly fired rotary kiln system can be operated up to 850°C. The throughput varies in between 800 and 2000 kg/h. 

Specify type of incinerator (e.g., rotary kiln, fluid bed, infrared, indirect fired desorber). 

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%. 

Indirect firing systems All systems in which the pulverized coal is stored after the grinding and metered to the rotary kiln from an intermediate surge bin. 

The lime-bnrning plant consists of 7 rotary kilns each 8 feet in diameter by 125 feet long from which the calcined lime is fed to 7 rotary coolers each 5 feet in diameter by 50 feet long. The I ated capacity of these Is 700 tons of calcined lime per day (24 hours) which is stored in four similar concrete silos of 250 tons capacity. Fuel for the time kilns is prepared in two Fuller indirect-fired rotary driers, 42 inches by 42 feet, and four Fuller-Lehigh mills. This plant has a rated capacity of 336 tons of dried and pulverized coal per day. 

Indirect firing or the use of low NO emission burners for direct-fired kilns. Even using low NOx burners low levels of NDMA were not always obtained in high kilned malts. 

Oil-contaminated soil can also be treated by passing through a rotary kiln where it is allowed to remain for sufficient duration to achieve a soil temperature sufficient (between 371 and 482°C) to volatilize the contaminants (HTTP). The combustion gas in the HTTD system remains either in direct contact with contaminated soil (direct-fired high-temperature thermal desorption) or is separated from the vaporized petroleum hydrocarbons or volatilized residue (indirect-fired HTTP). The treated soil is then cooled and passed through the rehydration unit before being screened and stockpiled. The HTTD-treated soil is black in color, completely sterile, and lacks soil structure [15]. It is also expensive compared to bioremediation. 

Recycl Blacks. The pyrolysis of carbon black containing rubber goods has been promoted as a solution to the accumulation of waste tires. In the processes in question, tires are pyrolyzed in the absence of oxygen, usually in indirect fired rotary kiln-type units. The mbber and extender oils are cracked to hydrocarbons which are collected and sold as fuels or petrochemical feedstocks. The gaseous pyrolysis products are burned as fuel for the process. Steel tire cord is removed magnetically and the remainder of the residue is milled into a pyrolysis black. This contains the carbon black, silica, and other metal oxides from the rubber and some newly created char. Typically these materials have 8-10% ash, and contain... 

Indirect gas-fired Direct or indirect fired Gas suspension type kilns or flash calciners... 

Figure 1.9 Indirect-fired small rotary kiln used for niche applications. (Courtesy of Harper International, Lancaster, NY.)...

A distinction can be drawn between direct and indirect firing systems for rotary kilns, with semi-direct firing as an intermediate solution. 

Direct firing operates in conjunction with an additional primary air blower, enabling the pressure with which the air is blown into the kiln to be adjusted to the desired value of 120-150mbar. With the indirect firing system all the exhaust air from the mill is dedusted and then discharged into the atmosphere (Figs. 2, 3 and 4). 

The tube mill or ball mill is especially suitable for the indirect firing system, i.e., where there is no direct connection between mill and kiln and where the pu Iverized and dried coal is stored in an intermediate bin of ample capacity, Thus the mill can be operated economically at a constant optimum rate of throughput, independently of the demands of the burners fed by it. 

By direct or semi-indirect firing in kiln and/or calcinator the recuperated hot cooler air is replaced by relatively cold and humid air from the coal mill. The additional cost for the air will be almost as mentioned above for the kiln hood, but due to the high heat capacity of water vapour, the smoke loss increases considerably. 

Thermal procedures for soil treatment include (1) fluidized>bed furnaces (2) chain-grate furnaces (3) direct-fired drum furnaces (rotary kilns) with afterburners (4) indirect-fired drum furnaces (pyrolysis furnaces) with afterburners and (5) plasma high-temperature technology. 

Indirect fired Direct-fired Kilns kilns) 370-590 1,280-2,050 installation type of fuel maybe detrimental installation, CO, THC mey increase... 

Operating experience and design wiU likely improve the concepts. It should be noted that while normal kiln coal burners receive their primary combustion air directly with the fuel, low-NO burners require an independent air supply (indirect firing) largely fed from the chnker cooler. On an existing kiln, this represents a significant cost compared to that of the new burner. 

Pulverized fuel can be delivered and fired in kilns either directly (direct system) or indirectly (indirect system). The simplest and most commonly used system for pulverized fuel firing is the direct-fired system (Figure 6.7). 

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