Throwaway systems

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). 

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. 

Method A packed-bed throwaway system to remove organic pollutants from oily waste stream. System component A contactor system, and a pump station designed for a contact time of 30 min and hydraulic loading of 162 L/min/m2 (4gpm/ft2). 

A related area of research would include efforts to reduce the water requirements of flue gas treatment systems. This would be especially important to the wet throwaway systems most commonly used today. Research to improve the dewatering properties of scrubber sludge could be combined with the development of techniques to separate spent and unused sorbent, thus reducing both water and sorbent consumption while also reducing the volume and mass of solid waste to be handled. The use of waste or recycled water in flue gas treatment systems would also reduce water needs. Improved sludge stabilization methods that also remove water could also lower costs associated with both waste disposal and water consumption. 

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. 

Recovery systems in which sulfur dioxide or elemental sulfur is removed from the spent sorbing material, which is recycled, are much more desirable from an environmental and sustainability viewpoint than are throwaway systems. Many kinds of recovery processes have been investigated, including those that involve scrubbing with magnesium oxide slurry, sodium sulfite solution, ammonia solution, or sodium citrate solution. One type of recovery system uses a solution of sodium sulfite to react with sulfur dioxide in the flue gas... 

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). 

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). 

Figure 11-69 describes a typical central system. Either water or direct-expansion refrigerant coils or air washers may be used for cooling. Steam or hot-water coils are available for heating. Humidification may be provided by target-type water nozzles, pan humidifiers, air washers, or sprayed coils. Air cleaning is usually provided by cleanable or throwaway filters. Central-station air-conditioning units in capacities up to about 50,000 cu ft/min are available in prefabricated units. 

There are three major approaches to enzymatic hydrolysis (1) single-use or throwaway lactase systems (2) lactase recovery systems based on membranes to retain the lactase for reuse and (3) immobilized systems in which the enzyme is physically or chemically bound to a solid matrix. 

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. 

Flue gas desulfurization systems are classifled in two general categories. One group involves throwaway product systems where sulfur product (untreated or treated) is disposed of as a landfill. The other group produces saleable products such as sulfuric acid, elemental sulfur or, as in Japan, gypsum for wallboard and sodium sulfite for paper mills. 

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. 

Clayton, T., Radcliffe, N., 1996. Sustainabdity A Systems Approach. Earthscan, London. Cooper, C., 2010. Longer Lasting Products Alternatives to the Throwaway Society. Gower, Famham. Creative Skillset, Industry and Sector Information. Available at http //fashionandtextiles-gta. 

One such model is the prototyping approach. The first delivery to the customer is a prototype of the envisaged system. The purpose of the prototype is to assess the feasibility of the product and to verify that the requirements of the customer have been understood by the developer and will be met by the system. The prototype is then thrown away (in fact, it is sometimes called a throwaway prototype), and development starts on the real product based on the now firmly established requirements. The prototyping approach addresses the difficulty of understanding the real requirements but it does not eliminate the time gap between the definition of requirements and delivery of the application. 

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