Multiple flues

Having discussed the specifics of a single breeching unit, we will now turn to the overall arrangement of multiple flue entries into a single shaft. In this area, the factors to consider are those related to structural design and gas mixing. 

Modern practice tends to use a single large flue instead of multiple small flues because of the difficulty in balancing multiple flues for even heating. Undersized flues may be very difficult to enlarge, but oversized flues can be partially reduced in size quite easily. 

Multiple flues were once popular as a means of distributing the gas flows along the furnace length. That idea works only if there is a near-equal 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... 

Using a long shaft to operate many dampers in parallel at the tops of in-wall stacks presents a balancing-problem nightmare. Air dampers (sec. 6.6.3) also may be difficult to balance with multiple flues. A better way to protect personnel is to simply erect open-bottomed stacks as barometric dampers at each flue, positioned to shield anyone from the hot flues. 

With multiple flues, if anything (scale, refractory crumbs, misplaced loads) partially blocks one or more of the hearth-level flues, that flue s low flow will cause it to cool and other hotter flues will carry more flue gas load, causing them to get hotter. This results in irregular heating of the loads in the furnace, and may eventually cause runaway overheating of the hotter flues. This same sort of unbalance of flue loads can be caused by different firing rates in adjacent zones or by burner locations that create localized positive or negative pressure on one flue entrance more than on another. 

Minor and potential new uses include flue-gas desulfurization (44,45), silver-cleaning formulations (46), thermal-energy storage (47), cyanide antidote (48), cement additive (49), aluminum-etching solutions (50), removal of nitrogen dioxide from flue gas (51), concrete-set accelerator (52), stabilizer for acrylamide polymers (53), extreme pressure additives for lubricants (54), multiple-use heating pads (55), in soap and shampoo compositions (56), and as a flame retardant in polycarbonate compositions (57). Moreover, precious metals can be recovered from difficult ores using thiosulfates (58). Use of thiosulfates avoids the environmentally hazardous cyanides. 

Gas-Fired water heaters are also made more efficient by a variety of designs that increase the recov-ei y efficiency. These can be better flue baffles multiple, smaller-diameter flues submerged combustion chambers and improved combustion chamber geometry. All of these methods increase the heat transfer from the flame and flue gases to the water in the tank. Because natural draft systems rely on the buoyancy of combustion products, there is a limit to the recovery efficiency. If too much heat is removed from the flue gases, the water heater won t vent properly. Another problem, if the flue gases are too cool, is that the water vapor in the combustion products will condense in the venting system. This will lead to corrosion in the chimney and possible safety problems. 

Chang et al. conducted pilot scale tests in a corona radical shower system with ammonia injection for simultaneous removal of NO and S02 from coal boiler flue gases [61]. The corona radical shower system used had dimensions of 2.1 x 1.8 x 2m3 and consisted of 20 parallel flow channels with five multiple nozzle corona electrodes per... 

SO2 absorption and particulate removal begins at the quench section and continues as the flue gas rises up through the main spray tower where the gas is again contacted with high-density water curtains produced by additional spray nozzles. The spray tower itself is an open tower with multiple levels of the BELCO spray... 

The usual temperature of flue gas entering the shield section is 1300-1650°F and should be 200-300°F above the process temperature at this point. The proportions of heat transferred in the radiant and convection zones can be regulated by recirculation of hot flue gases into the radiant zone, as sketched on Figure 8.19(b). Such an operation is desirable in the thermal cracking of hydrocarbons, for instance, to maintain a proper temperature profile a negative gradient may cause condensation of polymeric products that make coke on the tubes. Multiple chambers as in... 

Multiple measurements of flue gas composition can further improve boiler efficiency. 

Today, Low-Btu Gas (LBG) is expected to be preferred in small demand applications for single users located outside of downtown metropolitan areas. Specific industries in which LBG is expected to be most competitive include primary metals, iron ore beneficia-tion, metal finishing, lime brick refractory, and food industries. Another potential market for Low-Btu Gas is in combined cycle power generation. Cleaned Low-Btu Gas may be particularly advantageous when a plant has many separate combustors which, because of the anticipated new environmental standards, would require either multiple scrubbers or a flue gas collection system. Cleaned Low-Btu Gas is also one of the few options available to a user planning a plant expansion in a non-attainment area. 

In multiple hearth furnaces, the waste is fed into the top and moves down through the hearths by the rotation of rabbling arm. Air is injected at the bottom and at different levels so that the flue gas mixed with the pyrolitic gases flow counter-currently upstream, heating the waste. Ash is extracted at the bottom of the furnace. A Nesa furnace has been installed to produce fuel gas at the Origny cement kiln plant (Rochefort-sur Nenon, France). 

The Problem. The problem is to automatically and continually optimise combustion in a multiple burner furnace or boiler plant by altering the air inlet valve to each of the burners depending upon the carbon monoxide and oxygen readings taken from their common flue. The optimum air/fuel mixture will be different for each burner, because of their varying type, age and condition, and it can be achieved for a given supply of fuel by individually adjusting the air inlet valve to that burner. 

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