Bottom-fired furnaces

Bottom-fired furnaces are not very common in modern ammonia plants. They have a rather constant heat flux profile along the tube with high metal temperatures on the outlet side. Examples are the Exxon reformer and the old Chemico round furnaces. 

The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel - the mud drum - which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the "hot" tubes moves upwards from the mud drum to the steam drum, and the water in the "cool" tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe-0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C. 

Steam stripping is not adequate for the bottoms purity required. More positive stripping is obtained by charging the tower bottom liquid to a heating unit known as a reboiler. In a typical reboiler, 50% of the feed is vaporized and returned to the tower below the bottom plate. A fractionating tower equipped with a steam heated reboiler is shown in Figure 4. The reboiler may also be heated by a hot oil stream, such as a pumparound reflux stream from the primary fractionator of a cracking unit, or by a fired furnace. 

The terraced-wall type, developed by Foster Wheeler may be regarded as an intermediate between the side-fired and bottom-fired tubes. The reformer has inclined walls with several terraces on which upward firing burners are installed. This unique burner positioning makes it possible to adjust the heat flux in each zone. Figure 41 is a schematic drawing of the Foster Wheeler terraced-wall furnace [426],... 

Exxon Chemical Process. The Exxon Chemical process [1092], [1093] was specifically designed for the company s own site in Canada and so far not built for third parties. It uses a proprietary bottom-fired primary reformer furnace and a proprietary hot potash carbon dioxide removal system with a sterically hindered amine activator. Synthesis loop and converter are licensed by Haldor Topsoe A/S. Synthesis is carried out at 140 bar in a Topsoe S-200 converter and total energy consumption is reported to be 29 GJ/t NH3. 

Coal bottom ash (BA) and boiler slag (BS) are the coarse, granular, incombustible by-products that are collected from the bottom of furnaces that burn coal for the generation of steam, the production of electric power, or both. The majority of these coal by-products are produced at coal-foed electric utiUty generating stations, although considerable BA and/or BS are also produced from many smaller industrial or institutional coal-fired boilers and from coal-burning independent power production facilities [50-57]. The type of by-product (BA or BS) produced depends on the type of finnace used to burn the coal. The main differences between coal bottom ash and boiler slag are summarized in Table 3. 

Fig. 1.6. Roller hearth furnace, top- and bottom-fired, multizone. Roller hearth furnaces fit In well with assembly lines, but a Y In the roller line at exit and entrance Is advised for flexibility, and to accommodate parking the loads outside the furnace In case of a production line delay. For lower temperature heat treating processes, and with Indirect (radiant tube) heating, plug fans through the furnace celling can provide added circulation for faster, more even heat transfer. Courtesy of Hal Roach Construction, Inc.

Many believe that for greatest uniformity of temperature in top- and bottom-fired continuous furnaces, it is desirable to favor almost constant temperature from furnace end to end plus a soak zone for the ultimate heat flow rate per unit of time. This is not true if reflecting scale forms in the charge or preheat zone at temperatures above 2320 F (1270 C). Such scale will reduce heat transfer so that the product will be colder and productivity will be lower than if the charge zone had been limited to between 2250 F and 2300 F (1232 C and 1260 C). Reflecting scale develops when scale softens and becomes very smooth and the steel temperature under the scale has relatively low conductivity, preventing the steel from absorbing heat from the scale. 

Heating capacity of furnaces with top and bottom firing is less than twice that of furnace with top heating only because (1) the required water-cooled supports reduce the loads exposed heat transfer area and (2) the cold supports also act as heat sinks, stealing heat from the load and from the hot furnace gases, and (3) bottom-zone heat transfer also is reduced by movement of the hot furnace gases from the bottom zone to the top zone. 

In furnaces with bottom-fired heat or preheat zones (firing below the work load), there is often greater resistance to poc gas flow in the bottom zones than in the top zones because the bottom zones usually contain conveying equipment, support... 

To understand this zone length problem, the reader should envision a 100 ft (30.5 m) long furnace, top and bottom fired for heating 8.5" to 10" (0.216 m to 0.254 m) thick load pieces. 

With the authors recommended six top heating zones and six bottom heating zones, the temperature measurement would control each small zone as the heating curve directs and would not get out of step as has been the case with very large zones. A furnace with the many zones recommended would probably be a roof-fired or side-fired furnace. Side firing would need ATP technology to control the loads temperatures evenly from end to end across the furnace width. 

Side view of a batch car-hearth furnace with bottom-firing, top-fiuing. 

Bottom firing (i.e., burners below the loads) delivers heat to the usually cooler hearth, making up for hearth losses that otherwise would be taken from the loads or from the gas blanket. (See fig. 7.3.) Bottom firing is sometimes used with roof vents, but roof flues can be undesirable because at low-firing rates, the gases may short-circuit direct to the roof flues (giving poor temperature uniformity and poor fuel economy). Roof vents also can cause negative or low furnace pressure therefore, oversize vents should be avoided, and furnace pressure should be controlled with a stack closure. Tall furnaces are especially susceptible to this problem. 

Q9. Why is the cycle time shorter when firing batch furnaces with both top and bottom firing ... 

A12. With top and bottom firing, the flue exits are normally installed in the furnace roof. If more than one flue is to be used, they should be placed to avoid gases from one zone moving through another zone. With three top and three bottom zones, two flues are necessary—on centerlines between zones. 

Of the various methods of conventional combustion techniques, pulverized coal-fired furnaces are preferred for power stations and larger industrial facilities. Combustion processes create very fine effluents of which 70%-90% is carried to the air fly ash and 10%-30% remains as bottom ash (Commission on Energy, 1981). Difficulties in disposing fly ash stem from the enormous quantities collected (Scanlon and Dugan, 1979). 

The sodium reduction of titanium tetrachloride was actually carried out as early as 1939 in Germany, and about 670 kg was produced by the Deutsche Gold and Silber Scheideanstalt, during the 1939-45 war. The process, now obsolete, involved reduction in a molten bath of 50 per cent sodium chloride and 50 per cent potassium chloride at 800°C in an atmos phere of hydrogen. The reactors consisted of expendable welded sheet-iron cylindrical vessels, 50 cm diameter by 70 cm deep and 2 mm thick. These rested loosely in a stout iron crucible, fitted into a gas-fired furnace. A portable stirrer was used to agitate the reactor contents. Approximately 20 kg batches of titanium were reduced by distilling 85 kg of titanium tetrachloride at a controlled rate into a melt of 15 kg sodium chloride and 15 kg of potassium chloride, covered with a layer of 46 kg of molten sodium. The titanium sank to the bottom of the molten salts, and at the end of the reaction was recovered from the crushed solidified melt by leaching with dilute hydrochloric acid, in a ceramic-lined vessel. It was finally washed in water and dried at a moderate temperature. The same plant was also used for the production of zirconium metal by the sodium reduction of potassium fluorozirconate (KaZrF ]. 

In top- and bottom-fired box type furnaces, tubes are arranged in a number of parallel rows with the burners between the tube rows either in the top or bottom. The flue gas is taken out in the bottom of the top-fired and in the top of the bottom-fired reformer. These t5 es have fewer burners. 

Schematic temperature and heat flux profiles for a top-fired and a sidewall-fired reformer for identical process outlet conditions are seen in Figure 3.5 below. The top-fired furnace has a high heat flux at the inlet, whereas the sidewall-fired furnace has a more equally distributed heat flux profile. The top-fired furnace has an almost flat tube temperature profile, whereas in a sidewall-fired furnace the tube-wall temperatures increase down the reformer. The terrace-wall fired reformer has profiles similar to the sidewall-fired reformer, whereas the bottom-fired reformer has a larger heat flux in the lower part of the reformer.

The temperature profile in Run A corresponds to an ordinary sidewall-fired furnace. The temperature profiles in Run B allow establishment of reforming equilibrium at the outlet due to the reduced firing in the bottom. The profile in Rim C simulates maximum heat flux at the very bottom similar to a bottom-fired reformer. It is evident that such changes can only be performed in a pilot plant. 

The crude will next go through a fired furnace to be heated to 650-700 F (343-37rC), and it will then go to the atmospheric distillation tower. Here it will be broken into four major fractions gas and naphtha, gas oil, kerosene, and bottoms (the heavier fraction that is removed from the bottom of the tower). 

The Foster Wheeler reformer furnace is a Terrace-Wall furnace. The unique feature of this side-fired design is the burner location (Fig. 27 and 28). The burners are directed at the walls of the furnace, which radiate heat to the tubes. As with the top-fired reformer, the process gas enters the top and passes to the bottom. Unlike many top-fired furnaces, the terrace-walled furnace tubes have... 

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