Throwaway processes

Flue gas desulfurization may be carried out using either of two basic FGD systems regenerable and throwaway. Both methods may include wet or dry processes. Currently, more than 90% of utility FGD systems use a wet throwaway system process. Throwaway systems use inexpensive scrubbing mediums that are less costly to replace than to regenerate. 

In 1983 there were 116 flue-gas desulfurization (FGD) systems in service, representing 47 gigawatts-electric of power generation capacity (66). As of 1992, more than 150 coal-fired boilers in the United States operated with FGD systems. The total electrical generating capacity of these plants has risen to 72 gigawatts (67). FGD processes are classified into (/) wet-throwaway, (2) dry-throwaway, (J) wet-regenerative, and (4) dry-regenerative processes (68). 

Wet-Throwaway Processes. By 1978, three wet-throwaway systems were in commercial operation lime scmbbing, limestone slurry scmbbing, and dual alkah (1). Time/limestone wet scmbbing (Fig. 11) remains the most common post-combustion control technique appHed to utiHty boilers (67). The waste product from the scmbbers can either be sent to a landfill or be upgraded by oxidation to become saleable gypsum. 

Dry-Throwaway Processes. Dry-throwaway systems were the precursor of processes that removed SO2 iu the ductwork, eg, the BCZ and IDS processes. Here, however, the device is a spray chamber similar to the wet scmbbers such as the three modules of the Colstrip iastallation (Fig. 12). Into the upper portion of the chamber a slurry or clear solution containing sorbent is sprayed. Water evaporates from the droplets, the sorbent reacts with SO2 both before and after drying, and the dry product is removed ia a downstream baghouse or ESP (72). Unfortunately, dry scmbbiag is much less efficient than wet scmbbiag and lime, iastead of the much less expensive limestone, is required to remove SO2 effectively. Consequentiy, a search has been conducted for more reactive sorbents (72—75). 

When the Clean Air Act of 1990 was signed into law, electric utiUties were requited to estabUsh plans and initiate projects to comply with that Act s Tide IV. Each utihty had to evaluate how the various commercial and emerging clean coal systems fit into the utiUty s technical and business environment resulting in strategies to utilize fuel switching and wet throwaway FGD processes almost exclusively (38,85,86). 

The shaking and continuous filters are regenerative, but there is a third group usually associated with ventilation work rather than dust and fume. These are throwaway filters, which, as the name implies, means that when they become too caked with dust to operate correctly the filters are removed and replaced with new ones. They will only handle low incoming dust burdens, but their efficiencies are the highest of any filter. Typical applications are fresh air input plants, clean-room filtration and nuclear processes. 

Process Alternatives. Sulfur dioxide removal processes can be categorized as throwaway or recovery. Throwaway processes produce a liquid or solid waste that requires disposal. Recovery processes convert the sulfur dioxide to elemental sulfur or sulfuric acid. Throwaway processes have been used in most utility applications, but there could be greater incentives for using the recovery processes in industry. 

Throwaway processes generally remove sulfur dioxide by absorption into a lime or limestone slurry or a clear solution. Figures 3-5 show general diagrams for these processes (22,23,24, 25). 

Land availability could dictate the choice between throwaway and recovery processes. A considerable quantity of land is required for disposal of waste solids. 

All of the disadvantages of throwaway flue gas treatment systems can be lessened considerably by minimizing the amount of waste material produced. This can be accomplished by the choice of an effective sorbent material and the use of sorbent recycle to increase utilization. In turn, sorbent recycle can be optimized by the use of a separation process to remove the spent sorbent from unreacted sorbent. As water usually contributes significantly to the mass of the waste produced, the dewatering characteristics of the waste material are important. Efficient dewatering will not only minimize water losses but also reduce the disposal space required. 

There are numerous commercial processes available to remove the sulfur content from syngas, ranging from throwaway regenerable adsorbent-type to regen-erable solvent-type processes. Solvent-type processes are of primary interest for sulfur removal from syngas. The solvent-type processes can be divided into three types ... 

Variables affecting this process have recently been reviewed [44]. With either reagent, however, a throwaway product is obtained. Land has to be allocated for lagoon disposal of the spent scrubber slurries, or other systems have to be set up to handle the waste solid. Recent variations of this approach are to employ a zeolite prepared from fly ash [45] or the alkalinity of fly ash itself in water slurry as means to capture sulfur dioxide. 

The process is simple in concept, and to operate, but the stoichiometry of the gas-solid reaction is only about 20 to 60% of theory so that an excess of limestone is required for moderately efficient collection. This procedure also imposes a heavier solids handling load on the precipitators, and it yields a throwaway product. It does, however, produce a dry plume, which may be an advantage in some situations. 

The first step in achieving longer wear is to replace throwaway items by reusable ones. Even the reusable ones will wear out eventually. If the process is understood, it may be possible to intervene by altering the design or manner of use so that a longer service life is obtained. 

The capital investment for the spray dryer MgO process is approximately 14K higher than that for the comparable limestone scrubbing process. This is not an unexpected result since the MgO process is a regenerable system while the limestone scrubbing process is a throwaway system. 

There is still active development of alternatives to throwaway slurry scrubbing. Advanced throwaway processes such as dual alkali and dry scrubbing offer improved economics, efficiency, and/or reliability. Re-generable processes such as MgO scrubbing and absorption/stripping produce marketable products and thereby minimize waste production. However, each of these new processes includes many of the same physical and chemical processes as in a throwaway slurry scrubbing. In addition,... 

Four chapters address alternatives to throwaway slurry scrubbing. The development of the limestone dual alkali process is reviewed. Two chapters present results related to dry scrubbing with nahcolite or lime. A conceptual design and economics are given for MgO scrubbing using a spray dryer. 

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