Gas-fired kilns

The high point, before the introduction of modem electrical and oil- or gas-fired kilns, was the development in China of the dragon kiln, a tunnel kiln built against a hillside, with a draft which resulted in the temperatures needed for the firing of porcelain ware. 

Since anthracite must be calcined at higher temperatures than can be reasonably attained in conventional gas-fired kilns, an electrically heated shaft kiln is used to calcine coal at temperatures up to 2000°C (8). 

Mined clay with 22 percent moisture is broken up into pieces of less than 5 cm (2 in) in a rotary impact mill without screen, and fed to a rotary gas-fired kiln for drying (see Fig. 20-58). The moisture content is then 8 to 10 percent, and this material is fed to a mill, usually a Raymond ring-roll mill with an internal whizzer classifier or a pan mill. Hot gases introduced to the mill complete the drying while the material is being pulverized to the required fineness. 

At compositions of higher alumina content, the products cannot be sintered in conventional gas-fired kilns at sufficient temperatures to have good density and PLC (reheat) properties. While 99% AI2O3 class bricks exist, they are primarily used for low-temperature applications such as in chemical processes. 

The decomposition of limestone, CaC03(s), into quicklime, CaO(s), and C02(g) is carried out in a gas-fired kiln. Use data from Appendix D to determine how much heat is required to decompose 1.35 X 10 kg CaC03(s). (Assume that heats of reaction are the same as at 25 °C and 1 bar.)... 

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. 

Refined calcined alumina is commonly used in combination with high purity limestone [1317-65-3] to produce high purity calcium aluminate cement (CAC). The manufacture, properties, and appHcations of CAC from bauxite limestone, as weU as high purity CAC, has been described (104). High purity CAC sinters readily in gas-fired rotary kiln calcinations at 1600 —1700 K. CAC reactions are considered practically complete when content of free CaO is less than 0.15% andloss on ignition is less than 0.5% at 1373 K. 

Oil- or gas-fired boilers >45 metric tons/day Kiln and clinker cooler Fugitive emission points Process equipment Process equipment Dryers, etc/... 

In addition to the materials costs associated with a given fabrication technique, the cost of the equipment needed to fabricate the cell and the operating costs of the equipment—electricity, natural gas for firing kilns, etc.—and other process costs of the... 

The ground mixture is heated to about 750 °C under reducing conditions, normally in a batch process. This can be done in directly fired kilns with the blend in lidded crucibles of controlled porosity, or muffle kilns. The heating medium can be solid fuel, oil, or gas. The sodium carbonate reacts with the sulfur and reducing agent at 300 °C to form sodium polysulfide. At higher temperatures the clay lattice reforms into a three-dimensional framework, which at 700 °C is transformed to the sodalite structure, with entrapped sodium and polysulfide ions. 

Because of obvious (and not-so-obvious) dangers involved, it is the teacher who does the firing in the activities in Chapter 4. In schools, electric kilns are usually chosen for firing because fuel-fired kilns require wood, natural gas, oil, coke, coal, or charcoal as fuel, as well as chimney ventilation systems. Electric kilns also require ventilation systems, such as negative pressure systems and a canopy hood, but the placement is more flexible than room placement for a fuel-fired kiln with a chimney ventilation system. 

Table 4-4. Effect of Burning 15 Percent TDF in a Gas-fired Rotary Line Kiln Boise Cascade, Wallula, WA18,a...

Figure 4-11. Percent change in emissions when burning 15% TDF in a gas-fired rotary lime kiln controlled by a venturi scrubber.18...

The so-called green petroleum coke produced by delayed coking still contains volatile components, which escape at high temperatures, leading to excessive porosity and cracking. In addition form stability cannot be ensured. To avoid this it is first subjected to calcination in gas fired rotary kilns or rotary hearth kilns at 1300 to 1400°C. This is also necessary with anthracite, which can be calcined at even higher temperatures. 

The development of the rotary kiln, now predominantly used for the production of Portland cement, started around the late 1870s but the technology was not patented until 1885 (Ransome [B.69]). A rotary kiln (Fig. 6.7-35) is a long refractory-lined steel cylinder that is inclined at about 3-6° to the horizontal. At the lower end is a burner (coal, oil, or gas fired) and the material to be calcined enters on the other end. While passing down the kiln, chemical and physical reactions take place in the oxidizing atmosphere. Hot clinker emerges at the burner end and must be suitably cooled before being milled to yield cement. The rotary kiln process may be carried out with wet (slurry), semi-dry, or dry feed. 

Natural gas (calorific value about 8,600 kcal/Nm ) is introduced via ports in the walls of the kiln. It burns with a longer, lazier flame than oil or coal and more readily produces medium reactivity lime. As it contains insignificant amounts of sulfur and produces no ash, the resulting lime is not contaminated by the fuel in any way. Moreover, natural gas does not readily produce smoke, so that the exhaust gases have a low opacity. The heat usages of gas-fired shaft kilns are generally about 1,150 kcal/kg. 

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