Coal under reducing atmosphere

Table 3.14 Ash composition and ash fusion behavior under reducing atmosphere from coals of different rank [4,9).

Figure 4.2 Coal slag viscosity behavior under reducing atmosphere [15].

Globally, volcanoes release about 17 150 metric tons (t) of arsenic per year into the atmosphere (It equals 1000 kg (Matschullat, 2000), 300). Other significant natural sources of gaseous arsenic emissions include geothermal vents, wind erosion of soils and sediments, forest and coal seam fires, and sea spray ((Cullen and Reimer, 1989), 740 (Nriagu, 1989) Chapter 3). Under reducing conditions in soils, fungi and... 

The physical differences between inherent and extraneous ash are important not only to those interested in cleaning coal but also to those concerned with the fireside behavior of coal ash. Inherent material is so intimately mixed with coal that its thermal history is linked to the combustion of the coal particle in which it is contained. Therefore, it will most likely reach a temperature in excess of the gas in the immediate surroundings. The close proximity of each species with every other species permits chemical reaction and physical changes to occur so rapidly that the subsequent ash particles formed will behave as a single material whose composition is defined by the mixture of minerals contained within the coal particle. The atmosphere under which the individual transformations take place will, no doubt, approach a reducing environment. Figure 2 illustrates a model of the coal and mineral matter as fed to the combustor and the fate of the minerals after combustion [13]. 

The samples used in this study were Witbank coal and Goonyella coal. To reduce the effect of residual water in the coal on the microimage, the coal samples were evacuated at about 10 2 Torr for approximately 24 h at room temperature and stored in a glovebox under a dry N2 atmosphere. The size of the bulk sample used was about 2.5 mm x 2.5 mm x 100 pm, which was cut artificially in order to reduce the measurement time for the 3D-SPI experiment. The specimen was placed into a 5 mm diameter high-temperature ESR tube with AFO-t powder. 

Plastic properties of coal, as determined by the Gieseler plastometer, appear to be sensitive to oxidation, which can have a maiked effect in decreasing the maximum fluidity. In fact, prolonged oxidation may completely destroy the fluidity of a coal. To reduce oxidation, samples should be tested soon after collection or, if delay is unavoidable, storage under water or in a nonoxidizing atmosphere such as nitrogen is advisable. 

The addition of H2O and CO2 to the fuel gas modifies the equilibrium gas composition so that the formation of CH4 is not favored. Carbon deposition can be reduced by increasing the partial pressure of H2O in the gas stream. The measurements (20) on 10 cm x 10 cm cells at 650°C using simulated gasified coal GF-1 (38% H2/56% CO/6% CO2) at 10 atm showed that only a small amount of CH4 is formed. At open circuit, 1.4 vol% CH4 (dry gas basis) was detected, and at fuel utilizations of 50 to 85%, 1.2 to 0.5% CH4 was measured. The experiments with a high CO fuel gas (GF-1) at 10 atmospheres and humidified at 163°C showed no indication of carbon deposition in a subscale MCFC. These studies indicated that CH4 formation and carbon deposition at the anodes in an MCFC operating on coal-derived fuels can be controlled, and under these conditions, the side reactions would have little influence on power plant efficiency. 

A process pioneered by the National Coal Board in England that has not reached the fully developed stage but that has considerable potential is supercritical gas extraction. In this process the coal is pyrolized at a relatively low temperature, around 400°C, in the presence of a compressed supercritical gas, that is, a gas whose temperature is above the critical temperature at which it can be liquefied. Suitable gases are, for example, a number of petroleum fractions. Under these conditions at high pressures, around 10 MPa, the gas density is like that of a liquid, and the gas acts like a strong solvent that causes the liquids to volatilize and be taken up by the vapor. By transferring the gas to a vessel at atmospheric pressure, the density of the solvent gas is reduced and the extracted... 

As stated in Preface, the oxidatively-heating behavior of a gas-permeable oxidatively-heating substance, such as sawdust, coal dust or oil-soaked lagging, is also of the TD type. Therefore, it is possible, in principle, to calculate the critical temperature for the spontaneous ignition or Tc for a heap of a gas-permeable oxidatively-heating substance such as sawdust of every wood species, having an arbitrary shape and an arbitrary size, placed in the atmosphere under isothermal conditions, by applying the reduced form of the F-K equation, i.e., Eq. (79) derived in Section 6.2, provided the value of for the shape of the substance is known. 

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