Fuel-fired kilns

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. 

These inefficiencies, which are found in fuel-fired kilns, furnaces, boilers, ovens, and dryers, can be reduced through various process rearrangements, including recycling of waste energy and recovery of waste energy. 

Direct Firing. The firing of pottery or vitreous enamelware in a fuel-fired kiln or furnace without protection of the ware from the products of combustion. Directional Solidification. This is a technique for preparing lamellar or fibrous composites of ceramics and/or metals by growth from a molten eutectic... 

Kassel Kiln. An old type of intermittent, rectangular, fuel-fired kiln which diminished in cross-section towards the end leading to the chimney it originated in the Kassel district of Germany. Kavalier Glass. An early type of chemically resistant glass characterized by its high potash content it was first made... 

Incinerators Cement Kilns Aggregate Kilns Boilers3 Liquid Fuel-Fired Boilers Production Furnaces... 

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. 

As a fuel, the most common uses of petroleum coke are in firing cement kilns and steam generators. In the cement industry, petroleum coke is suitable as fuel in kilns because of its low ash content, high heating value and the process s high sulfur allowances. As much as 50% coke can be burned in combination with bituminous coal or 75% coke when burned in combination with Oil and/or gas. 

The thermal efficiency of ceramic kilns depends on a number of factors, in particular on the kiln type. The consumption of heat related to unit amount of ware is highest for periodic furnaces, and lower by about one half for tunnel or continuous multi-chamber kilns. The consumption of heat also depends on the kind of firing, a direct-fired kiln having a lower consumption than a muffle kiln where the ware is protected from direct contact with combustion gases. For example, a direct-fired tunnel kiln for glost firing of tiles has a fuel consumption 40% lower than a similar muffle tunnel kiln (Holmes, 1978),... 

Chemical Processes for Particulate Matter Formation Most chemical processes that produce particulates are combustion processes, as we have mentioned already. These processes include fossil fuel fired power plants, incinerators, home furnaces, fireplaces and stoves, cement kilns, internal combustion engines, forest, brush and grass fires, and active volcanoes. Particles from combustion sources tend to occur in a size < 1 fjtm. Such small particulates are particularly important because they are readily carried into the alveoli of lungs and are relatively enriched in more hazardous constituents, such as toxic heavy metals and arsenic. 

In the case of the cement rotary kiln, the fluid flow through the kiln freeboard comes from several sources such as secondary hot combustion air, combustion and decarbonation products and inleakage air. In direct firing kilns, the pulverized solid fuel is injected through the burner pipe nozzle with the external diameter in the range of 250 to 600 mm into the kiln with an internal diameter of 2.4 to 6.2 m. 

By limiting the proportion of fuel fired through the lower burners, the kiln can produce a reactive low carbonate product. Further details are available in the literature [16.18]. 

A feature of rotary kilns is the formation of rings . These consist of an accumulation of material on the refractory in a part of the kiln which has the appropriate temperature for a semi-liquid phase to form. Such rings can form from ash in coal-fired kilns and from calcium sulfate deposits. Alkalis (sodium and potassium oxides), clay and lime can contribute to the build-ups, which can be troublesome. In the case of coal-firing, fine grinding of the fuel can significantly reduce the rate of build-up. 

Sensible heat carried out of the furnace by the furnace gases (poc) is often the largest loss from high-temperature furnaces and kilns. It is evaluated by the available heat charts mentioned in section 5.1 100% — %available heat = %heat carried out through the flue. It can be reduced by careful air/fuel ratio control, use of oxy-fuel firing, and good furnace pressure control. 

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). 

Since is dependent on the fuel fired in the kiln, it will have to be calculated ... 

As in an oil-fired kiln, the shape of the gas flame can be modified by varying the ratio of the axial to the swirling flow rate of the fuel A factor to be taken into account in determining the capacity of various parts of the equipment, especially the exit gas fan, is that with gas firing the quantities of exit gas are larger than with coal or oil. 

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

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. 

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. 

The carbonization by-products are usually refined, within the coke plant, into commodity chemicals such as elemental sulfur (qv), ammonium sulfate, benzene, toluene, xylene, and naphthalene (qv) (see also Ammonium compounds BTX processing). Subsequent processing of these chemicals produces a host of other chemicals and materials. The COG is a valuable heating fuel used mainly within steel (qv) plants for such purposes as firing blast furnace stoves, soaking furnaces for semifinished steel, annealing furnaces, and lime kilns as well as heating the coke ovens themselves. 

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