Solid coal combustion

Table 7. Concentrations (in mg/kg) of minor elements in solid coal combustion residues and natural sediments...

S02 and NOx in flue gas from coal combustion contribute to smog and acid rain. Methods to remove these pollutants include alkaline wet scrubber systems that fix S02 to solid CaS04, and selective catalytic reduction by metal/metal oxide systems of NO/NOz to N2 and steam in the presence of ammonia. Particulate active carbons have also been used in flue gas decontamination, especially as they avoid costly scrubber processes and can operate at lower temperatures. The potential of active carbon fibers in this application has been explored by a... 

The types of reactions involving fluids and solids include combustion of solid fuel, coal gasification and liquefaction, calcination in a lime kiln, ore processing, iron production in a blast furnace, and regeneration of spent catalysts. Some examples are given in Sections 8.6.5 and 9.1.1. 

Pulverized-coal firing solid fuel combustion, 7 463 465 furnaces for, 7 401—400 Pumice... 

With the importance of the devolatilization process to solid particle combustion and the complexity of the chemical and physical processes involved in devolatilization, a wide variety of models have been developed to describe this process. The simplest models use a single or multiple Arrhenius rates to describe the rate of evolution of volatiles from coal. The single Arrhenius rate model assumes that the devolatilization rate is first-order with respect to the volatile matter remaining in the char [40] ... 

The description of the workings of the control technologies is beyond the scope of this article. However, it is worth noting that many of these technologies create substantial amounts of solid or liquid waste that needs to be disposed of properly. For example, in the USA the total amount of fly ash produced from coal combustion alone is about 57 Mt/y (Kalyoncu 2000). About one-third is utilized as secondary raw material (e.g., for aggregate and asphalt), but the rest is usually disposed of in landfills. The wet and dry scrubbers for S02 control produce a sludge or dry waste that finds little secondary use, and a large amount is disposed in landfills. 

As a result of the well-documented environmental concerns posed by coal combustion, and the disposal of CCPs, a large body of research has focused on characterizing the mechanisms of mobilization and attenuation of trace elements in coal and its ash. Based on their reported distribution in the solid phases of both source coals and coal ash, knowledge of the thermal transformations that occur to major mineral constituents during coal combustion, and a limited number of studies that have identified discrete solid phases of trace elements, a conceptual model of the chemical and mineralogical characteristics of trace elements in coal ash has been developed. 

As was previously mentioned, trace elements that sublime at temperatures below those attained during coal combustion (e.g., As, Se, Hg, Zn), and are associated with thermally unstable solid phases (in particular organic matter and sulphide minerals), are subject to vaporization into furnace gases. Once these gases, and fly ash particles entrained in the gases, are vented from the combustion furnace they quickly cool, leading to the condensation of volatilized elements onto the... 

Several studies investigating the environmental effects of controlled tyre combustion have been conducted. It is evident that atmospheric emissions can be greatly reduced if proper air pollution control systems are installed. Laboratory and field data provide evidence indicating that concentrations of some environmental pollutants, especially NOx, may decrease due to tyre combustion, whereas others increase compared to pure coal combustion. Zinc is an example of an element that increases in both solid combustion products and atmospheric emissions. The geochemical impact of higher Zn contents in fly and bottom ash on leaching processes in disposal sites remains to be tested. 

The speciation of trace elements in solid phases determines their mobility and toxicity. Spectroscopic techniques such as XANES and XAFS, can be used to determine directly the oxidation and structural state of elements in coal combustion byproducts. For example, Huggins et al. (2000) used these synchrotron techniques to determine that Cr and As occur predominately in the less toxic oxidation states Cr(IIl) and As(V) in CCBs. In addition, they found As, Cd, Cr, Ni, and Zn were present primarily as oxidized species (i.e., as oxides, sulphates, arsenates, etc.) in unweathered CCBs. 

Fuel. Any material which produces heat on combustion is known as fuel. Fuels may be divided into solid (coal, lignin, coke, wood,... 

Aside from coal-powered steam locomotives and seagoing ships, which essentially were retired from most regions of the world over the past several decades, solid coal is quite unsuited for transportation energy. The energy density of raw coal means that a significant portion of the energy obtained from combusting it is required to move it (as part of a transportation vehicle). This is further amplified by the equipment required to hum coal—massive, heavy furnaces and boilers—which also have to he moved with the vehicle. 

Coal is a solid, brittle, combustible, carbonaceous rock formed by the decomposition and alteration of vegetation by compaction, temperature, and pressure. It varies in color from brown to black and is usually stratified. The source of the vegetation is often moss and other low plant forms, but some coals contain significant amounts of materials that originated from woody precursors. 

Conceptually speaking, the technology for combustors is similar to that used for coal combustion (see Chapter 9) if solid biomass fuels are used. Typically used configurations for biomass combustion include [41] (1) pile-burned, (2) stoker-fired, (3) suspension-fired, and (4) fluidized-bed combustors. 

Aluminum is the third most abundant element in the soil, constituting about 8% of the minerals (Rosseland et al. 1990). This element can be released naturally by the weathering of aluminum-containing rocks. Aluminum is also released to soil as a major constituent of many mining wastes and is also contained in solid wastes from coal combustion and aluminum reduction and other metal processing operations (Gablerand Stroll 1983 Krishnaswamy 1984). 

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