Unreacted fly ash

In order to reduce the total time of synthesis ( 30 min), it has been reported to employ two stage synthesis processes, an initial microwave heating for around 30 min. followed by the conventional synthesis method [8, 40, 41]. Although, the purity of zeolite has been of much concern, the final yield used to contain unreacted (read residual) fly ash, which can affect the characteristics of synthesized zeolite. In fact, the presence of considerable quantity of unreacted fly ash residue in the final yield can be attributed to the factors, which are being discussed in the next chapter The mechanism of zeolite formation . 

Rayalu et al. [12] have estimated the crystallinity of fly ash zeolites-A using XRD and infrared (IR) spectroscopy for identification, quantification and their framework structure. Zeolite A has been synthesized by fusion of mixtures of fly ash and sodium hydroxide in 1 1.2 ratios at temperatures from 550 to 600 °C for 1-1.5 h of heating time. It has been reported that powder XRD analysis was employed to monitor the formation of zeolite-A. The infrared, IR, technique has been proposed for monitoring crystaUinityof end product after synthesis. Based on the characterization results, it has been opined that the unreacted fly ash associated with the zeohte-A as impurities in the final yield, have negligible influence on its application as an adsorbent or cation exchanger. As such, the crystallinity of end product as per interpretation of XRD peaks of commercial zeolite-A have been reported to match closely with the crystallinity interpreted from IR spectrum of the mineral. 

The throw-away processes with aqueous slurries of lime or limestone as the scrubbing media are the most extensively installed processes. These processes create a waste sludge containing calcium sulfite, calcium sulfate, fly ash, unreacted alkali, and other minor dissolved species in the free water contained in the sludge. Since flue gas contains oxygen, some of the dissolved sulfur dioxide is oxidized, and calcium sulfate is formed. 

Although many Ca-rich fly ashes are self-cementing, strength gain is usually too slow for most practical applications. Moreover, aluminosilicate rich ashes are essentially unreactive with water. The most important application of fly ash is as a partial replacement for Portland cement. In this application, Portland cement furnishes much of the early strength, up to 1 month, while the high alkalinity of cement chemically activates the ash such that its slow reaction with cement components and water contribute increasingly to... 

The DWSA installation can be divided into two main parts. The first part consists of an air preheater, fluidised bed reactor, solid fuel dosing vessel with on-line mass determination system and a hot gas cleaning section, containing a cyclone and a ceramic candle filter (Schumacher type). In the fluidised bed reactor the solid fuel is gasified with air to produce a low calorific value (LCV) gas that is cleaned of fly ash and unreacted solid carbonaceous material. Air and also additional nitrogen can be preheated and is introduced into the reactor by four nozzles just above the distributor plate. The reactor is electrically heated in order to maintain a constant temperature over bed as well as freeboard section. The solid fuel is fed into the bed section in the bottom part Just above the distributor by a screw feeder from beside. The hot gas cleaning section ensures a good gas-solid separation efficiency, with filter temperatures of about 500 C. 

The fly ash is assumed to pass through the calciner unreacted. Although MgSOj is easily calcined at 1600°F, MgSOij (formed in either the absorber or the ash purge section) requires higher temperatures or the presence of carbon for the decomposition reaction to occur. Since carbon in the form of coal is present in the calciner, the MgSOij is assumed to be calcined as shown in reaction 7. 

If coal or oil is the fuel source, the FGD control technologies result in the generation of solid wastes. Wet lime/limestone scrubbers produce a slurry of ash, unreacted lime, calcium sulfate, and calcium sulfite. Dry scrubber systems produce a mixture of unreacted sorbent (e.g., lime, limestone, sodium carbonates, and calcium carbonates), sulfur salts, and fly ash. 

Dry injection of limestone and hydrated lime particles in desulfurization processes is currently under active development (Stouffer et al, 1986 Weinstein, 1989 Yoon et al, 1986). Furnace dry sorbent injection for SO2 control consists of injecting a dry pulverized calcium based material directly into furnace cavity of a coal fired boiler. The product CaS04 and unreacted sorbent particles are removed from the furnace together with fly ash. Fan and Satija (1984), Milne et al (1990b) Krammer and Staudinger (1991) reported kinetic data and model expressions which are applicable to dry injection systems. 

In the combustor, added CaCOj undergoes a decomposition to CaO. In many cases, the reaction is incomplete and some unreacted CaCO remains in fly ash. 

In prior investigations concerning the behavior of iron in alkali silicate solution, iron was only added as a minor component. " " Jimenez et al." found that iron added as crystalline Fe203 powder was relatively unreactive in highly alkaline solution. However, iron located within the glassy fly-ash was reactive enough to be incorporated into the hydration product of the final geopolymer matrix. 

The byproduct removed from a lime spray dryei/particulate control system is a dry, flow-able powder containing calcium sulfite, calcium sulfate, fly ash, and unreacted absoibent. It is usually conveyed pneumatically to a silo Ah storage priw to disposal and is typically disposed of in a landfill. Water is often added for dust control This causes pozzolanic reactions to occur resulting in a final byproduct of low penneabili and desirable landfill characteristics (Liegois, 1983). Table 7-17 gives important properties of spray dryer byproduct. 

Salt transport in the process is entirely pneumatic. The dry sorbent is usually delivered by bulk carrier (truck or rail) and off-loaded pneumatically into covered storage silos. From the silos, it is pulverized to 10-20 microns mean mass diameter to optimize both the rate of decomposition and sorbent utilization (Bennett, 1992 Bennett and Nastri, 1990 Hooper, 1990). The pulverized sorbent is conveyed to a day silo, metered into the pneumatic conveying line, and distributed into the flue gas through a system of injection nozzles. The sorbent feed rate can be automatically controlled based on upstream and/or downstream sulfur dioxide gas stream concentrations (Bennett, 1992). The end product (a mixture of sodium sulfate, sodium chloride, sodium nitrate, fly ash, and unreacted soda) is a dry powder easily coUected by either a fabric filter or an ESP (Bennett and Nastri, 1990). 

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