Combustion of solid fuels

Sulphur attack on nickel-chromium alloys and nickel-chromium-iron alloys can arise from contamination by deposits resulting from the combustion of solid fuels, notably high-sulphur coals and peat. This type of corrosion, which has been observed on components of aircraft, marine and industrial gas turbines and air heaters, has been associated with the presence of metal-sulphate and particularly sodium sulphate arising directly from the fuel or perhaps by reaction between sodium chloride from the environment with sulphur in the fuel. Since such fuels are burned with an excess of air, corrosion occurs under conditions that are nominally oxidising although the deposits themselves may produce locally reducing conditions. 

Gas-solid where the solid may take part in the reaction or act as a catalyst. The reduction of iron ores in blast furnaces and the combustion of solid fuels are examples where the solid is a reactant. 

Toftegaard, Maja Bog et al. Oxy-fuel combustion of solid fuels. Progress in Energy and Combustion Science. 2010, 36(5). 581-622. 

The Bed Material. Only catalytic processes are relevant with respect to modifying the attrition resistance of the bed material. In other processes, e g., drying, the bed material is the product and cannot be changed. In the combustion of solid fuels, the particle degradation due to attrition enlarges the reacting surface and thus increases the reactivity of the fuel. On the other hand, the lack of attrition resistance is often a major obstacle that hinders the commercialization of fluidized bed catalytic processes. 

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. 

Particles of char are produced as a normal intermediate product in the combustion of solid fuels. Following initial particle heating and devolatilization, the remaining solid particle is termed char. Char oxidation requires considerably longer periods (ranging from 30 ms to over 1 s, depending on particle size and temperatur than the other phases of solid fuel combustion. The fraction of char remaining after the combustion zone depends on the combustion conditions as well as the char reactivity. 

Tillman D.A., The Combustion of Solid Fuels and Wastes, Academic Press, Inc, San Diego, (1991). 

In addition to rotary and vertical kilns, hearth furnaces or fluidized-bed reactors may be used. These high-temperature reactors convert minerals for easier separation from gangue or for easier recovery of metal. Fluidized beds are used for the combustion of solid fuels, and some 30 installations are listed in Encyclopedia of Chemical Technology (vol. 10, Wiley, 1980, p. 550). The roasting of iron sulfide in fluidized beds at 650 to 1100°C (1202 to 2012°F) is analogous. The pellets have 10-mm (0.39-in) diameter. There are numerous plants, but they are threatened with obsolescence because cheaper sources of sulfur are available for making sulfuric acid. 

In recent years increasing interest has been shown in the mechanism of reactions between gases at solid interfaces and between gases and solids at elevated temperatures. These reactions include many of interest to catalysis, the protection of metals and the combustion of solid fuels. From the practical viewpoint these are some of the most useful reactions to mankind in our present state of industrial development. These reactions have been studied extensively in the past but owing to lack of precise experimental techniques and the lack of adequate theoretical interpretation, progress in the understanding of the mechanism of the reactions has been slow. 

The vacuum microbalance can be applied effectively to the study of the mechanism of combustion of solid fuels. Since the interpretation of the weight change in combustion may involve several reactions unless pure materials are chosen for study, this section will present the results of a systematic study of the rate of oxidation of pure artificial graphite with oxygen as a function of time, temperature, pressure, and pretreatment. 

The second method that particles may be emitted from the combustion system is through the production of particles in the combustion process. For example, in the combustion of solid fuels, like coal, for example, ash is normally produced. The airborne portion of the ash, usually referred to as fly ash, may be carried out of the combustor by the exhaust gases. The use of OEC should reduce fly ash emissions because of more-complete combustion of the fuel compared with an air/fuel system. 

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