Coke and Char

The degree of graphitization of carbon precursors such as those shown in Table 4.1 varies considerably, depending on whether a coke or a char is formed, as will be shown in the following sections. 

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Processes and equipment for the gasification of inferior coke and chars from low-temperature carbonization of subbituminous and lignitic coals were developed in Germany (13), where the relatively large yield of low-temperature tar was welcomed as raw material for hydrogenation processes. The market for low-temperature tar in the United States has been too small and the selling price too low to encourage the use of low-temperature chars for gas production. 

The burning rate of chars at a pressure of 1 atm for petroleum coke and the different coal rank has been compared by Sergeant and Smith [27]. The collected experimental data show that the char burning rates expressed in (kg m- s ) is slightly lower for the same particle temperature, for petroleum coke against anthracite however the behavior of both coke residue (petroleum coke) and char (anthracite) is similar. The burning profile technique is a method for predicting the relative combustion characteristics of fuels. 

Line B Ratios for steam-only batch fluidized-bed gasification of a wide range of cokes and chars (including high temperature coke) at atmospheric pressure. 

Catalytic pyrolysis of Hgnocellulose from beech wood at 500 °C over hierarchical P-zeolite [188]. This apphcation has been less successful, as hierarchical fi-zeolite yields less Hquid bio-oil and more coke and char than Al-MCM-41, leading to increased production of aromatics and polyaromatic hydrocarbon (PAH) because of the stronger acidity of their acid sites. 

Blayden HE, Gibson J, Riley HL. An X-ray study of the structure of coals, cokes and chars. 

Several methods can be employed to convert coal mto liquids, with or without the addition of a solvent or vehicle. Those methods which rely on simple pyrolysis or carbonization produce some liquids, but the main product is coke or char Extraction yields can be dramatically increased by heating the coal over 350°C in heavy solvents such as anthracene or coal-tar oils, sometimes with applied hydrogen pressure, or the addition of a catalyst Solvent components which are especially beneficial to the dissolution and stability of the products contain saturated aromatic structures, for example, as found in 1,2,3,4 tetrahydronaphthalene Ilydroaromatic compounds are known to transfer hydrogen atoms to the coal molecules and, thus, prevent polymerization... 

Deoxygenation reactions are catalyzed by acids and the most studied are solid acids such as zeolites and days. Atutxa et al. [61] used a conical spouted bed reactor containing HZSM-5 and Lapas et al. [62] used ZSM-5 and USY zeolites in a circulating fluid bed to study catalytic pyrolysis (400-500 °C). They both observed excessive coke formation on the catalyst, and, compared with non-catalytic pyrolysis, a substantial increase in gaseous products (mainly C02 and CO) and water and a corresponding decrease in the organic liquid and char yield. The obtained liquid product was less corrosive and more stable than pyrolysis oil. 

Figure 2 shows the results of the pyrolysis experiments conducted with the Spanish lignite at 750-960°C at residence times of 0.52-0.72 sec. It is seen that under the pyrolysis conditions used, 60 - 70% of the sulfur in this coal appears in the gaseous products as H2S, COS, and CS2. As in the previous sulfur study (1), the principal sulfur gaseous product at all temperatures is H2S, with some CS2 formed at T >840°C. The CS2 is apparently formed at the expense of the H2S, by any of several reactions H2S may react with the carbon of the coal and/or the methane evolved in the pyrolysis of the coal to form CS2- A small amount of COS is detected at all temperatures trace amounts of SO2 are also detected. Moreover, the total sulfur yield appears to reach a maximum about 900°C. The decrease in sulfur volatilization as pyrolysis temperature is increased above 900°C is attributed to sulfur retention in the char due to the reaction of H2S with coke or char to form more stable thiophenic structures (2). GC/MS analysis of the tars (diluted to 10 ml) from the pyrolysis at 750 and 850°C did not reveal any sulfur-containing structures. Tars from the pyrolysis at 900 and 950°C, however, contain dibenzothiophene. 

Combined Gas, Soot, and Particulate Emission In a mixture of emitting species, the emission of each constituent is attenuated on its way to the system boundary by absorption by all other constituents. The transmissivity of a mixture is the product of the transmissivities of its component parts. This statement is a corollary of Beer s law. For present purposes, the transmissivity of species k is defined as xk = 1 — Et. For a mixture of combustion products consisting of carbon dioxide, water vapor, soot, and oil coke or char particles, the total emissivity eT at any wavelength can therefore be obtained from... 

In Eq. (5-150), T has units of kelvins and L is measured in meters. Since coke or char emissivities are gray, their addition to those of the C02, H20, and soot follows simply from Eq. (5-148) as... 

Sprensen, L.H., Gjemes, E., Jessen, T. Fjellcrup, J. (1996). Determination of reactivity parameters of model carbons, cokes and flame-chars. Fuel, Vol, 75, No. l,pp. 31-38. 

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