Cyclone collection system

Kiln dusts (KD) are fine by-products of Portland cement and lime high-tem-perature rotary kiln production operations [88-98] that are captured in the air pollution control dust collection systems (cyclones, electrostatic precipitators, and baghouses). Different types of KD are discussed in Table 4. 

Collection system Cyclone, filter bags, and electrostatic precipitators... 

Inertial collectors for mists and droplets are widely used. They include cyclone collectors, baffle systems, and skimmers in ductwork. Inertial devices can be used as primary collection systems, precleaners for other... 

The separation step requires (1) application of a force that produces a differential motion of the particles relative to the gas, and (2) sufficient gas-retention time for the particles to migrate to the collecting surface. Most dust-collections systems are comprised of a pneumatic-conveying system and some device that separates suspended particulate matter from the conveyed air stream. The more common systems use either filter media (e.g., fabric bags) or cyclonic separators to separate the particulate matter from air. 

Within the range of their performance capabilities, cyclones are one of the least expensive dust-collection systems. Their major limitation is that, unless very small units are used, efficiency is low for particles smaller than five microns. Although cyclones may be used to collect particles larger than 200 microns, gravity-settling chambers or simple inertial separators are usually satisfactory and less subject to abrasion. 

The dry product is primarily collected in cyclone collectors (a few bag houses still remain), sieved, and finally packaged in moisture barrier containers. The exit air from the dryer often has to be treated to meet local pollution control laws. While many of the older dryers use gas incineration, as energy costs have increased these incineration systems have become quite costly to operate. New dryer installations use scrubbing systems (e.g., aqueous/chemical sprays) to remove entrained solids and gaseous volatile flavors. 

Because most of the ash is removed as molten slag, addition of a bottom grate is not necessary.7 However, small TDF is required, because much of the combustion must occur in suspension.7 TDF that is too large to combust completely can get carried over into the boiler or dust collection system, and cause blockage problems.9 Therefore, particle size may inversely determine the amount of TDF that can be used in a cyclone boiler.11 Three cyclone-fired boilers at utilities have burned 1M x 1" TDF in test operation, one at the 2 percent, one at the 5 percent, and one at up to a 10 percent level.3,9,12 One pulp and paper mill plans the use of TDF in a cyclone-fired hog-fuel boiler.13... 

Dust entrained in the exit-gas stream is customarily removed in cyclone collectors. This dust may be discharged back into the process or separately collected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube dryer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collectors in drying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the collection system. 

A schematic diagram of the entrained flow reactor is shown in Figure 1. At the top of the reactor, a screw feeder and semi-venturi system is used to entrain the ground coal particles in the cold primary gas stream. The coal is then injected into the reactor where it is entrained in, and heated by, the preheated secondary gas. The pyrolyzing coal particles fall in a thin stream through the reactor and are collected by a movable water-cooled collector probe. The time which the particles spend in the reactor is controlled by moving the collector probe up and down the reactor axis. The pyrolysis reactions are rapidly quenched in the collector probe, and the particles are separated from the gas stream by a cyclone in the collection system. 

Current industry trends are toward the installation of enclosed conveyors and the construction of enclosed areas with negative pressure aspiration for oilseed unloading and meal load-out. Highly efficient dust collection systems and control equipment (cyclones and bag houses) are used throughout the processes to control both fugitive and point source emissions. 

Two experiments (RSI, RS2) were performed with DC-II and Swedish wheat straw with particle size in the range of 1 to 2 mm. During these experiments the solids recovery system (cyclone and impinger, Figure 1) did not perform satisfoctorily with the very fragile straw char. The char, exposed to the very harsh envirorunent (fast moving sand particles) of the CFB reactor and atritted to very fine particles, almost submicTon powder, could not be collected by conventional solids recovery systems. Consequently, the char was either adhered with the liquids heavy fraction to the STHE inner tubes or readilly accumulated in the liquid recovery system. 

In a single-stage spray dryer the process is finished when most of the moisture in the pore space has dried. The agglomerates accumulate in the lower part of the spray-drying chamber and are removed by the suction of a fan driving a dust-collection system. The agglomerates are collected in a cyclone while dust is collected in a wet scrubber. Material-laden scrubber water may be recirculated and mixed with the liquid feed. 

A further technology for the production of instant (agglomerated) materials in a spray dryer uses the recirculation of dry, already pre-agglomerated particles into the spraying zone of the dryer. Recirculating flnes are particles captured in cyclones of the dust collection system and product screen undersize. Figs. 6.4-10 and 6.4-11... 

In each type of process the dust can be collected in two ways a portion of the dust can be separated and returned to the kiln from the dust collection system (like a cyclone) closest to the kUn, or the total quantity of dust produced can be recycled or discarded... 

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