Bottom flues

Bottom flueing is preferred, but in-the-wall vertical flues have been found too costly, and they pull a harmful negative pressure at the hearth level. With top firing, the best arrangement is hearth-level flues with automatic furnace pressure (damper) control. If fired with top and bottom burners, use of a roof flue with automatic furnace pressure control is suggested. The flue location should be determined to enhance the design circulation pattern. (See chap. 7.)... 

With any kind of individual vertical flue controls, a flue that happens to carry more hot gas will get hotter and natural convection will create more draft or pull, causing that flue to get even hotter—a true snowball in hell. If scale or refractory crumbs accumulate unevenly on the floor near multiple bottom flues, this same sort of acceleration will happen in the least-plugged flue. These sorts of problems have led many engineers to favor one flue per zone, or per furnace, and to use wise engineering in burner placement, and best control of furnace circulation. (See chap. 7.) This is more easily accomplished in continuous furnaces where the pieces march through several zones and past a number of burners. 

Multiple flues are difficult to balance, whether individual dampers are used for every flue or a single damper is positioned beyond where they merge into a single stack. The idea of downdrafting (flues at furnace bottom) is good for furnace circulation and efficient use of fuel. It has sometimes been done with a row of flues at hearth level. However, designers have often connected bottom flues to refractory stacks within thick furnace walls to protect persons around the furnace from burns by hearth-level... 

Heavy fuel oil usually contains residuum that is mixed (cut back) to a specified viscosity with gas oils and fractionator bottoms. For some industrial purposes in which flames or flue gases contact the product (eg, ceramics, glass, heat treating, and open hearth furnaces), fuel oils must be blended to low sulfur specifications low sulfur residues are preferable for these fuels. 

A variation of the n on regen erabi e absorption is the spray dry process. Time slurry is sprayed through an atomizing nozzle into a tower where it countercurtendy contacts the flue gas. The sulfur dioxide is absorbed and water in the slurry evaporated as calcium sulfite-sulfate collects as a powder at the bottom of the tower. The process requires less capital investment, but is less efficient than regular scmbbing operations. 

Recovered catalyst and blowdown gas (- 3% of the flue gas) exit from the bottom of the separator to an electrostatic precipitator or to a small, fourth-stage cyclone for further concentration of catalyst fines. The flue gas, with 70—90% of the catalyst particles removed, passes from the separator into the power expander. 

In ECS s 1986 repowefing project Babcock and Wilcox (B W) constmcted a bubbling-bed section to ECS s existing 125 MWe pulverized-coal furnace to produce 31.3 t/h of lime, usiag cmshed coal as the source of heat to calciae limestone ia the fluidized bed. A portion of the lime is drawn from the bed as bottom ash and a portion is collected as fly ash. Both portions are transferred to a cement (qv) plant adjacent to the boiler. The hot flue gas from the EBC flows iato the existing main pulverized-coal furnace, ia which a B W LIMB system was also iastaHed to absorb sulfur dioxide dufing those times when the EBC is not operating. 

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. 

The unit operating philosophy and its apparent operating limits often dictate unit constraints. For example, limitations on the main column bottoms temperature, the flue gas excess oxygen, and the slide valve delta P often constrain the unit feed rate and/or conversion. Unfortunately, some of these limits may no longer be applicable and should be reexamined. Some of them may have resulted from one bad experience and should not have become part of the operating procedure. 

A forced-draft, gas-fired burner is seated at the top of the inner tube and produces a spinning cyclonic flame that reaches down to the bottom of the furnace tube. The hot combustible gases return over the boiler shell, which is provided with heat convection fins to extract more heat before the upward flowing gases exit the boiler. The furnace tube is fitted with a top and bottom, cast-steel flame retainer. These design features act to increase flue gas residency time and provide improved structural integrity to the pressure vessel. 

Figure 5. Top Adsorption isotherms of C02 for 1-en at the indicated temperatures. Bottom Adsorption-desorption cycling of C02 for 1-en showing reversible uptake from (a) simulated air (0.39 mbar C02 and 21% 02 balanced with N2) and from (b) simulated flue gas (0.15 bar C02 balanced with N2). (c) time-dependent C02 adsorption for porous materials (A = 1-en, B = mmen-Mg2(dobpdc), C = 1, D = Mg-MOF-74, E = Zeolite 13X, F = MOF-5). (d) C02 adsorption ratio of 1-en in flue gas (after 6 min exposure to 100% RH at 21 °C) to 1-en in flue gas (Adapted from [192]).

Figure 12. Top, carbon dioxide and carbon monoxide emitted in flue gases from batchwise commercial heat treatment of Asplund board at 165 C versus time. In some plants the emission decreased more with time than here. Bottom, laboratory scale measurements at two temperatures. Data of emitted CO and total acids as weight % on dry hardboard. All data according to Nordenskjold and Ostman (3). (Reproduced with permission from ref. 10. Copyright 1989 De Gruyter.)...

The burners at the reformer s top are in an enclosure called a penthouse. The flue gas is collected at the bottom in horizontal fire-brick ducts called tunnels. Flue gas exits horizontally into a waste heat recovery (WHR) unit. Combustion gas is drawn through the WHR unit by an induced-draft fan and then discharged to the atmosphere through a stack. 

The flue gas tunnels are rectangular fire-brick structures at the reformer s bottom. They act as horizontal ducts for flue gas removal. The flue gas exits at 1,800°F to 1,900°F. A heat recovery unit is provided to recover heat from this gas. This unit contains a reformer feed preheat coil, steam superheat coil, steam generation coil and boiler feed water preheat coil. 

There are 720 coal-fired power plants in the USA. When coal is burned in these power plants, two types of ash are produced coal fly ash and bottom ash. Coal fly ash is the very fine particulate matter carried in the flue gas bottom ash (or slag) is the larger, heavier particles that fall to the bottom of the hopper after combustion [261-264]. The physical and chemical characteristics of these ashes vary depending on the type of coal burned. These ashes are characterized by the following ... 

More than 90% of the coal used by electric utilities is burned in pulverized coal boilers. In these boilers, 65-80% of the ash produced is in the form of fly ash. This fly ash is carried out of the combustion chamber in the flue gases and is separated from these gases by electrostatic precipitators and/or mechanical collectors. The remainder of the ash drops to the bottom of the furnace as bottom ash. While most of the fly ash is collected, a small quantity may pass through the collectors and be discharged to the atmosphere. The vapor is that part of the coal material that is volatilized during combustion. Some of these vapors are discharged into the atmosphere others are condensed onto the surface of fly ash particles and may be collected in one of the fly ash collectors. 

The flue gas passes through a number of small diameter high-efhciency cyclonic elements arranged in parallel and contained with the separator vessel. The UOP design uses an axial flow cyclone. After the catalyst particles are removed, the clean flue gas leaves the separator. A small stream of gas, called the underflow, exits the separator through the bottom of the TSS. In an environmental application, the underflow is diverted to a fourth stage separator (FSS) that is typically a barrier filter. The underflow rate is typically 2-5% of the total flue gas rate and is set by use of a critical flow nozzle. 

In wet scrubbers, upward-flowing flue gases have contact with a stream of water flowing down from the top of the scrubber chamber. The water absorbs particles in the flue gas and carries them to the bottom of the chamber, where they can be removed in the form of sludge. 

Figure 1. Flow schematic of a spray-tower adsorber. The scrubbing solution is contacted with hot flue gas, collected in the bottom, and continuously recycled and contacted. Suspended solids and pH of liquid in the recycle loop of Plant D spray towers ranged from 5.2 to 8.7%, and from 5.2 to 6.80%, respectively.

The chemical composition of CCPs varies with coal origin and rank however, the major elemental constituents of all coal ash residues are O, Si, Al, Fe, and Ca, along with lesser amounts of Mg, S, and C. The relative abundance of constituents that typically make up more than 1 % of the total mass of fly ash and bottom ash are summarized in Table 4. These elements are found in the ash because of their lower volatility and the short time the particles actually remain in the furnace during combustion (Helmuth 1987). Both crystalline and non-crystalline compounds form on the surface of fly ash particles when elements react with oxygen in the flue gases, and through... 

For the purpose of this discussion, coal combustion ash consists of two distinct products bottom ash and fly ash. The distinction between bottom ash and fly ash is how they form and exit from the boiler. Bottom ash is removed from the bottom of the boiler, whereas fly ash exits in the flue gas, where it is subsequently collected by a variety of devices. This distinction is important because the beneficiation options available for either of these products, as well as potential end uses, are different. 

The ash generated per tonne raw feedstock shale during combustion is different in terms of both amount and composition depending on where in the flue-line it accumulates. For example, Hasanen et al. (1997) distinguished between bottom ash - 39 wt%, cyclone ash - 32 wt%,... 

Fig. 6. Sketch of boiler (K-3A) system at the Eesti power plant, showing locations of ash sampling. Samples from locations 10 and 11 were taken from the Balti power plant (modified from Paat Traksmaa 2002, fig. 1, p. 374). Furnace bottom (1) gas duct superheater (2), economizer (3), cyclone (4), electrostatic precipitators (ESP) prechamber (5), field I (6), field II (7), field III (8), field IV (9), flue cyclone (10) and cloth filter (11). ( " Oil Shale.)...

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