Slagging coal combustion

The emission of sulfur oxides leads to the corrosion of equipment and slagging of combustion or boiler equipment, as well as contributing to atmospheric pollution and environmental damage. Sulfur data are therefore necessary for the evaluation of coals to be used for combustion purposes. 

A compositional analysis of the ash in coal is often useful in the total description of the quality of the coal. Knowledge of ash composition is also useful in predicting the behavior of ashes and slags in combustion chambers. Utilization of the ash by-products of coal combustion sometimes depends on the chemical composition of the ash. In addition, concern over release of certain trace elements to the environment as a result of coal utilization has made determination of these elements an increasingly important aspect of coal analysis. 

Combustion of coal produces ash that can be transported through the air. Slagging and fouling problems can also be predicted from elemental analysis. Therefore, elemental analysis of both the coal as well as the ash are important. Procedures for dissolution and analysis of coal and combustion products of coal have been reported [334-336]. Laser ablation sampling has been successfully used for coal and combusted materials [337,338]. The direct introduction of slurries has also been used [339]. Comparison of ICP-MS and PIXE analysis of coal combustion aerosols showed that analysis errors can occur in ICP-MS if particle vaporization is incomplete in the ICP [340]. 

In comparison with coal, combustion of biomass fuels and fuel mixture of biomass and coal has advantages with lower sulphur emissions and less tendency for slag formation. 

Blander, M. Pelton, A. D. Computer Assisted Analyses of the Thermodynamic Properties of Slags in Coal Combustion Systems, ANL/FE-83-19, Argonne National Laboratory, Argonne, IL, 1983 Available from NTIS, U.S. Dept. of Commerce, Washington, D.C. 

The fly ash formed in coal combustion also represents a disposal problem (see also Chap. 14). Although there are some uses such as in concrete and bricks, soil stabilization, soil conditioner, and landfill cover, more need to be found.24 Additional uses in wallboard, concrete blocks, and other construction materials should be possible. Other ashes include bottom ash and boiler slag. Experiments have been run on the recovery of iron, aluminum, and other metals from the ashes, but the processes may not be economical at this time. This could reduce the need to mine for these other materials. Coal-fired power plants produce over 100 million tons of ash annually in the United States. Coal fly ash is routinely mixed with water and put into settling basins. This process extracts some arsenic, cadmium, mercury, selenium, and strontium into water, which can then cause abnormalities in amphibians.25... 

Addition of limestone has been used to reduce slag viscosity in wet bottom coal fired plant and pulverised coal combustion, to facilitate slag and deposit removal. 

C/C-SiC tubes for high temperature heat exchangers in coal combustion chambers could withstand highly aggressive environments including water vapour and liquid coal slags, especially coated with multilayer EBC based on B4C, SiC and Cordierite. 

Mineral matter in coal is one of the most important sources of problems in coal combustion, including fouling, slagging, corrosion, among others. Mineral matter transformation and slag formation are specific properties of coal that provide more information on the suitability for coal combustion or gasification. 

Essentially no particulates from coal combustion should be ejected into the atmosphere aU particulate streams should be collected and either returned to the combustors, where they melt and are removed as slag, or are removed as fly ash. Any eventual dispersion of the elements present depends on the possibility of leaching. The concern, therefore, is to identify elements that may be occurring in the gaseous state. 

M.C. Mayoral, M.T. Izquierdo, J.M. Andres, B. Rubio, Mechanism of interaction of pyrite with hematite as simulation of slagging and fireside tube wastage in coal combustion. Thermochim. Acta 2002, 390, 103-1 I I. 

Water-insoluble pozzolanic substances containing oxyanionic silicon such as SiO and usually reacted with calcium hydroxide are used for waste solidification. The most abundant and commonly used pozzolanic substance is fly ash from coal combustion. Other pozzolanic substances, which are usually waste products from various processes include flue dust and ground slag from blast furnaces. 

Cyclone boilers, installed by electric utilities until about 1975 in order to utilize slagging coals typically found in the Midwestern US, are distinct combustion systems that are highly favorable to cofiring petroleum coke. Coal crushed typically to 6.35 mm x 0 mm (V4 x 0 ) is introduced into a barrel typically 2.4 - 3.0 m (8 - 10 ft) diameter and 3.0 m - 3.7 m (10 - 12 ft) long where combustion occurs in a highly intense environment. Normally 2-14 cyclone barrels are mounted horizontally for a given furnace the largest cyclone boiler has 23 cyclone barrels. 

The sulfur removed during coal combustion generates gypsum (see below) and slag material. Slag has no commercial value. By contrast, the sulfur removed from oil and gas can often be recovered in the form of useful materials, including elemental sulfur. NO , is not usually removed, but a small proportion may be retained during FGD. 

Figure 4.6 XRD scans of common SCMs siliceous fly ash from coal combustion (FA), blast furnace slag from iron smelting (SL), thermally activated metakaolin (MK) and a volcanic tuff natural pozzolan (Po). The broad hump beneath the diffraction peaks of the crystalline phases indicates the presence of one (or more) amorphous phases. The main crystalline phases identified are mullite (M), quartz (Q), gehlenite (G), anatase (An) and albite (Al).

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