Ash stream

During MSW combustion in a modern mass burn combustor with reciprocating grates, 11 of MSW is converted to heat gases such as C02, NOx, SOx, and H20 and about 350 kg of ash residuals partitioned into the various ash streams. For every tonne of MSW combusted, about 5 kg of grate siftings, 295 kg of grate ash, 5 kg of boiler/economizer ash, 20 kg of ESP ash, and 12 kg of dry scrubber solids are produced (Chandler et al. 1997). 

Table 16.5 lists the surrogate (simulated) waste streams that were part of this study. The ash stream represents radioactive waste from the inventory of US Department of Energy (DOE) facilities. The Delphi DETOX streams are secondary waste generated during destruction of organics from similar waste streams [57]. The soil represents the waste from Argonne National Laboratory s inventory that was included in a site treatment plan for actual treatment. 

Only a small stream leaves the combustor as solid i.e. the ashes corresponding to their weight fraction of the biomass (i.e. 1% for wood). This means that 25 kg/hr ash or max. 5.5 kg/hr unbumed char (= 2 % of the fed char) is lost in the ash stream thus stressing that this loss should not be accounted for in the heat balance and associated calculations. 

A design strategy for a CFB combustor for biomass was presented and illustrated for a 2SOO kg/hr combustor. The main operating parameters are the solids flux and gas velocity in the riser of the CFB. Isothermal conditions will be guaranteed by circulating inert sand particles of 300 pm at a solids flux of 20 kg/m s. The gas velocity in the riser is determined at 7.9 m/s, exceeding the transport velocity (-6.6 m/s) of the inert sand bed by 20%. About 2% of the diar will be lost in the ash stream. The net combustion capacity of the riser is equal to 4.1 M Wih per m of the riser, comparable with a coal CFB combustor. 

If the MgO and ash particles actually have a significant difference in density/particle size, the ash purge section could be redesigned to include a separation device such as a thickener that could be used to split the solids into an enriched fly ash stream and one enriched in MgO. Since the recycle MgO must be reslurried before entering the spray dryer, wetting this stream would not present any significant problems to the process. The fly ash-enriched stream could then be sent to a smaller ash purge section. 

Municipal solid waste (MSW) combustor ash is the by-product that is produced during the combustion of municipal solid waste in solid waste combustor facilities. In most modern mass burn sohd waste combustors, several individual ash streams are produced. They include grate ash, siftings,boiler ash, scrubber ash and precipitator or baghouse ash [113-118]. 

At the present time in the United States, all of the ash streams are typically combined. This combined stream is referred to as combined ash. The term bottom ash is commonly used to refer to the grate ash, siftings and, in some cases, the boiler ash stream. The term fly ash is also used and refers to the ash collected in the air pollution control system, which includes the scrubber ash and precipitator or baghouse ash. In Europe, most facilities separate and separately manage the bottom ash and fly ash streams. Table 6 summarized the different types of MSW combustor ash [113-128]. 

Bottom ash (BA) - Approximately 90% of the bottom ash stream consists of grate ash, which is the ash fraction that remains on the stoker or grate at the completion of the combustion cycle... 

The grate ash stream consists primarily of glass, ceramics, ferrous and nonferrous metals, and minerals... 

Boiler ash and fly ash - The haghouse or precipitator ash comprises approximately 10-15% of the total combined ash stream... 

Case a The costs associated with streams 3 and 4 are calculated from F equations Each exeigy unit in the ash (stream 3) and in the flue gas (stream 4) is supplied by coal and oxygen at the same average cost. 

Looking at the consumption and performance of the gasification process, the carbon conversion Xc in percent (Equation (4.16)) describes the amoimt of carbon converted to a gaseous product by relating the mass flow of carbon entering the process in the feedstock wc,in in kg/s to the unconverted mass flow of residual carbon me,res, which typically leaves the process in dust or ash streams. 

The gas, along with entrained ash and char particles, which are subjected to further gasification in the large space above the fluid bed, exit the gasifier at 954—1010°C. The hot gas is passed through a waste-heat boiler to recover the sensible heat, and then through a dry cyclone. SoHd particles are removed in both units. The gas is further cooled and cleaned by wet scmbbing, and if required, an electrostatic precipitator is included in the gas-treatment stream. 

The reactor effluent is separated by conventional distillation into recycle solvent, light gases, to 537°C bp distillate, and a heavy vacuum bottoms stream containing unconverted coal and ash. The recycle solvent is hydrogenated in a separate reactor and sent back to the Hquefaction reactor. 

The heavy vacuum bottoms stream is fed to a Flexicoking unit. This is a commercial (125,126) petroleum process that employs circulating fluidized beds at low (0.3 MPa (50 psi)) pressures and intermediate temperatures, ie, 480—650°C in the coker and 815—980°C in the gasifier, to produce high yields of hquids or gases from organic material present in the feed. Residual carbon is rejected with the ash from the gasifier fluidized bed. The total Hquid product is a blend of streams from Hquefaction and the Flexicoker. 

The water—carbon slurry formed in the quench vessel is separated from the gas stream and flows to the carbon recovery system needed for environmental reasons and for better thermal efficiency. The recovered carbon is recycled to the reactor dispersed in the feedstock. If the fresh feed does not have too high an ash content, 100% of the carbon formed can be recycled to extinction. 

A development in the 1960s was that of on-line elemental analysis of slurries using x-ray fluorescence. These have become the industry standard. Both in-stream probes and centralized analyzers are available. The latter is used in large-scale operations. The success of the analyzer depends on how representative the sample is and how accurate the caUbration standards are. Neutron activation analyzers are also available (45,51). These are especially suitable for light element analysis. On-stream analyzers are used extensively in base metal flotation plants as well as in coal plants for ash analysis. Although elemental analysis provides important data, it does not provide information on mineral composition which is most cmcial for all separation processes. Devices that can give mineral composition are under development. 

PermeOx is also used to improve the bioremediation of soils contaminated with creosote or kerosene (see Bioremediation (Supplement)), to deodori2e sewage sludges and wastewater (see Odormodification), and to dechloriaate wastewater and effluents. A special formulation of calcium peroxide, made by FMC and sold ia the United States under the trademark Trap2ene, is used for removing metal ions from acidic waste streams such as coal ash leachate and acid mine drainage (see Wastes, industrial). 

The product stream from the kilns is collected in storage bins. Black ash from the bins is fine-ground in a ball mill and fed to a leacher circuit, which is a system of stirred tanks, where it is dissolved in water and the muds are separated by countercurrent decantation. The solution from the decantation is passed through filter presses the muds are washed, centrifuged, and discarded. The filtered product, a saturated solution containing 12—13 wt % strontium sulfide, is sent to an agitation tank where soda ash is added to cause precipitation of strontium carbonate crystals ... 

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