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Show '20- 'CC 3C .u> "TJ s- O) ^> OJ Ol ra 53 u 50 - AC - 2C - \ • \. V \ 0 • Lmby. 10J K/s. 2 s hold, 1 atm '20- 10C- <_ 0>) _i "TO c CD o <u CT "a c D U tu 0. an 50 40 ?o Gedling, 10 K/s, 2s hold, r a Tm 120 '00'- •=: 30 L ro c CD 30 - O 40 V CD _! c 0) y 53 20 ' Pit 8, 10 K/s 2s hold, 1 atm 400 600 300 1000 1200 1400 1600 Temperature, C Figure 1 Evaluation of predicted char compositions against datasets of Cai (1995) for 3 hv bituminous coals, including ^actions of the coals' original levels of C(B), H(#), 0(dotted ;me), and N(A), plus the fractional char yields(O). Secondary Volatiles Pyrolysis C F D simulations of full-scale, coal-fired utility 'umaces are now being used for troubleshooting and design worldwide however, one recent literature survey (Niksa 1996) ^ctec universally poor predictions of gaseous emissions in the exhaust, with no instances of Quantitative agreement witnm jseful tolerances for C O and N O concentrations anywhere <n furnaces This important limitation was attributed to deficiencies in the submodel for combustion of voiatne matter Almost universally, the mecnanism that is utilized n current C F D simulations asserts combustion of voiatnes at their rate of mixing with air according to eddy creaKop or eddy dissipation mechanisms. Equilibrium combustion products are usually assigned pased on the assumption that volatiles have the same elemental compositions as their parent coals. The mixing-limited approach to equihbnum comcustion products is implausible for two reasons: (1) voiatiies have markedly different compositions than their parent cca.s because dunng rapid coal devolatilization. ail the coars oxygen and sulfur and 80 to 9 0 % of its hydrogen are released, whereas only about half the carbon and nitrogen are expelled into the gas phase. (2) Products of primary devolatilization are rapidly transformed by secondare volatiles pyrolysis once they have been expelled mto ~oi gases. Consequently, the very complex distributions of cnam and condensed polynuclear aromatic hydrocarbons, carbon oxides, water and hydrogen that are released initially - and •vould appear to burn very 'apidly - are converted mto much simpler mixtures of soot, carbon oxides, H 2 0 . and H 2 <vith small amounts of C H 4 and C2 H2 (Bruinsma. 1988. Neison 1986; Chen, 1992) The abundance of soot CO. and stable combustion products in the secondary pyrolysis proaucts ensures that they will burn relatively siowiy (Manow '992. Cho, 1995). indeed, soot and C O may not even compete effectively for the available oxygen with the esoecai'y reactive chars from coal ranks of subbituminous and be^c* (Niksa. 1998). The pnmary product distributions from FLASHCHAiN'" are converted to secondary pyrolysis products by applying phenomenological mechanisms based on the obsen/at:cns of Chen et al. (1992). The distnbutions are formulated in terms of an extent of secondary pyrolysis. so no rinite-rate kinetics are incorporated The extent of secondary pyrolysis is assigned as the soot fraction, which is the ratio of the yieid of soot to the sum of the yields of tar, oils, and soot The soot composition is based on a constant value of n/C for all soots from all coal types, and retention of a 'ixed fraction of the nitrogen in tar All heteroatoms m tar are expelled as noncondensibles Tar-0 is released as C O n exchange for incorporation of C2H2 into soot Tar-N s released as H C N and tar-S is released as H : S ^he buik ;f tar-H is released as H 2 Consequently, the sum of the yields of tar. soot, and oils remains nearly invanant '.hrcug-c_: secondary pyrolysis, even though the yields of C O C;n: -: and H C N surge throughout Non-condensibie hydrocarbons are converted into C H 4 and C2H2. although CH4 s a>sc converted into C2H2 during the last naif of secondary pyrolysis. Product distnbutions based on this phenomenoiogicai mechanism are evaluated against data in Fig. 2 The measurements were reported by Chen et al. (1992) 'or a ? t 8 hv bituminous coal. The predictions in Fig. 2 appear versus the soot fraction. Values range from zero, whicn denotes pnmary devolatilization, to unity, which denotes the complete conversion of tar into soot. In the :aoora:ony progressively higher soot fractions were achieved by raising the furnace temperature. The faster neatmg -ates associated with progressively higher furnace temperatures were accounted for in the simulations. Predicted weight ess |
