Gas collector and removal systems

This section presents an overview of collector design and materials, followed by a discussion of the three parts of a liquid management system the LCRS above the primary liner, the secondary leak detection, collection, and removal (LDCR) system between the primary and secondary liners, and the surface water collection system above the closure of the completed facility. The section concludes with a discussion of gas-collector and removal systems. 

Gas generation occurs over a period of 70-90 years, so gas-collector and removal systems must work for at least that long to avoid gas pressure on the underside of the cover. 

Conversion of the RDF into a low calorific value,fuel gas is accomplished in a fluidized bed realtor. Both air and solid fuel are fed into the reactor under positive pressure. The RDF is immediately decomposed into gas, char and small amounts of tar. The char is subsequently oxidized by air to provide the heat necessary to sustain reaction conditions. Ash from the system is removed by cyclone collectors for subsequent disposal. The cleaned off-gas is then burned in a tangentially fired combustor. 

A well-designed wet stack system has minimum droplet carry-over from the mist eliminators, uses sloped duct floors, has properly located and amply sized liquid collectors and drains in the outlet ductwork and stack, limits the gas velocity particularly in the stack, and has smooth outlet ductwork and stack walls to allow entrained liquid collected on the walls to flow to the drains. Most of the entrained liquid is removed in the outlet ductwork. Usually, reduced-scale flow model studies are performed to evaluate the best locations and sizes for liquid collectors and drains. EPRI s Entrainment in Wet Stacks (Maioti and Doie, 1982) provide some guidelines which aid in evaluating and solving wet stack entraimnent problems. 

Lurgi CFB Process. In this SO2 removal process, hydrated lime is injected into a circulating fluid bed (CFB) reactor located ahead of the particulate removal system. Water is also injected into the reactor to cool and humidify the gas and increase the SO2 removal efficiency. A mixture of fly ash and reacted and unieacted sorbent particulate in the gas from the CFB reactor is collected in a special downstream mechanical dust collector curtain followed by an ESP. Most of the collected particulate is recycled to the CFB reactor (Keetii et al., 1991B). In 1993, eight systems of this type were operating or were under construction in Europe, and two were under construction in the U.S. (Moore, 1993B). 

Dust entrained in the exit-gas stream is customarily removed in cyclone cohectors. This dust may be discharged back into the process or separately cohected. 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 diyer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collec tors in diying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the cohection system. 

Ferric hydroxide coprecipitation techniques are lengthy, two days being needed for a complete precipitation. To speed up this analysis, Tzeng and Zeitlin [595] studied the applicability of an intrinsically rapid technique, namely adsorption colloid flotation. This separation procedure uses a surfactant-collector-inert gas system, in which a charged surface-inactive species is adsorbed on a hydrophobic colloid collector of opposite charge. The colloid with the adsorbed species is floated to the surface with a suitable surfactant and inert gas, and the foam layer is removed manually for analysis by a methylene blue spectrometric procedure. The advantages of the method include a rapid separation, simple equipment, and excellent recoveries. Tzeng and Zeitlin [595] used the floation unit that was devised by Kim and Zeitlin [517]. 

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. 

As a further stage to ensure the removal of any small particles of dust, flue gas can be directed to a Venturi-type dust collector. At both the Met and the outlet of this type of collector, dust is covered by condensate through adiabatic expansion and collected by sprayed water that is circulated via tanks installed at the inlet and the outlet, respectively. This system is also a useful preparation for the removal of traces of sulfur dioxide by passing the gases through an absorber tower where the sulfur oxides are removed by contact with circulating water that is sprayed into the tower. 

The proper operation of a spray dryer-scrubber also requires that a dry product be formed and subsequently removed from the gas stream. Pilot tests have shown that the product salts will be dry and collectable if the gas temperature at the dryer outlet is maintained about 20 °F above its dewpoint. This also tends to minimize plume formation. The cyclone collectors used in the pilot tests removed 89-99% of the product. Although this was excellent performance by mechanical collectors, particulate emission standards will require either replacement of the cyclones or additional collection devices in series with the cyclones. The system design presently favored involves using cyclones to remove the bulk of the product and adding a small electrostatic precipitator for final particulate removal. The sodium salts produced in the spray dryer-scrubber... 

Many types of particulate collection devices are available commercially (see Table 53.2). Each operates on a different principle for accomplishing removal of particulates from the gas stream. Four basic types are common in drying systems (1) the drying vessel itself (in the case of vessel dryers), (2) cyclones, (3) bag filters, and (4) wet scrubbers. Electrostatic precipitators (ESPs) are not used widely in drying installations in spite of their low-pressure drop and high collection performance. The initial cost of purchase and construction is high. For this reason, the emphasis in Section 53.3 will be on the three most widely used devices, e.g., cyclones, fabric filters, and wet scrubbers. For a concise discussion of various types of solid-gas separation equipment and guidelines for selection of dust collectors,... 

The CSP plant consists of a solar collector field, a receiver, a heat transfer fluid loop and a heat storage system. The mirrors of the solar field concentrate the direct solar radiation on the solar receiver set at the focal line. The heat transfer fluid (e.g., molten salts) removes the high temperature solar heat from the receiver that is afterwards collected into an insulated heat storage tank to be pumped, on demand, to the heat users (steam generators, endothermic reactors, etc.), where its sensible heat is released. Finally, the heat carrier fluid is stored in a lower temperature tank ready to restart the solar heat collection loop. The idea to match the CSP plant with natural gas steam-reforming Pd-based MR derives from the thermal level reached by molten salt stream (550°C), which meets the thermal requirements of MR (preferred operating at around 500°C). 

The mixture of flue gas and dry particles (including fly ash) flows out of the top of the reactor into a cyclone separator. Most (about 90%) of the entrained particles are removed in the cyclone. A major portion of the collected material is recycled to the reactor, while the balance is discharged as byproduct. Gas from the cyclone next passes through a high efficiency particulate collector system then to the stack. Material from diis collector is added to the cyclone discharge to form the total byproduct stream. The process has been used in several waste incineration units in Europe, and a 10 MW demonstration plant was started up at TVA s National Center for Emissions Research in November 1992 (Aiipol, 1993). 

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