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Show 1 0 03 fN * . .• • 'i_CHAR 04 0 2H 00 fNTAR^ <0 03 0 0 ; QQO | & Q; 25 H 20 <D 15 - 10- 5^ 0- 4 h Weight Loss -• \ \ 6 \ -ooo o "-8 86 88 90 92 94 96 98 100 Carbon Content, daf wt. % Figure 3 Predicted devolatilization behavior of 17 petroleum cokes for heating at 104 K/s to 1600 K at 0.1 MPa. The nitrogen partitioning (top) resolves char-N (•) and tar-N (O), and the mass partitioning (bottom) resolves weight loss (•) and tar yields(O). BIOMASS DEVOLATILIZATION Modeling Approach Previous attempts to adapt coal-derived depolymenzation models to biomass have so far met with limited success (Chen. 1998) primarily for the following three reasons: (1) The multitude of relevant biomass fuels is usually characterized m terms of the cellulose fraction plus contnbutions from several poorly defined components, including lignin, hemi-celluloses, and xylans; (2) The macromolecular structure and compositions of biomass components contain few, if any, condensed polynuclear aromatic compounds; and (3) ash catalysis significantly affects both yields and product release rates, yet ash compositions are almost never reported. in addressing these difficulties to adapt F L A S H C H A I N ™ for biomass applications, w e were led to develop a model with far more differences than similarities to the basic reaction mechanism that is effective across the spectrum of coal and petroleum coke properties. The new mechanism, called bio- F L A S H C H A I N ™ (bio-FC) is based on the following premises; (1) All biomass properties can be represented in terms of two sets of component properties, one set for cellulose and one set for a lignin-like component whose composition and m a s s fraction are assigned from the ultimate analysis of the whole biomass. Xylans and hemi-celluloses are ignored. (2) FLASHCHAIN's straight-chain macromolecular configuration model is applied intact to biomass. Chains are composed of two kinds of labile bridges, the original and dehydrogenated forms, plus refractory char links But the chains contain no aromatic nuclei or otherwise immutable units, so the connections between the linkages are massless. (3) Ash catalyzes the conversion of bridges into char links An abundance of ash tends to promote faster devolatilization rates but suppresses tar production. while the total volatiles yields remain approximately constant. However, the extent of catalysis cannot be correlated with total ash levels, rather, it must be inferred from a measured yield, such as the proximate volatile matter content. The only mechanism in FLASHCHAINT U that was unaltered for bio-FC is the flash distillation analogy for volatiles escape, in which a phase equilibrium relates the instantaneous mole fractions of like fragments in the tar vapor and condensed phase. No finite-rate mass transport phenomena are involved because all volatiles are presumed to escape in a convective flow that is initiated by the chemical production of noncondensible gases. The first premise is illustrated with the biomass properties m Table 2. These values were obtained by applying the submodel in bio-FC for fuel structure and composition to a database of 26 samples, including hardwoods and softwoods, various grasses and agncultural residues, and one paper. The table reports the proximate volatile matter (PVM) and ash contents, both in daf wt. %. Also shown are the number fraction of cellulose units and the number of H and O atoms per unit for the lignin-like component in all cases, the lignin-like component has 9 carbon atoms, and the cellulose composition is fixed by its molecular formula For biomass whose cellulose fraction is unity, the cellulose composition was adjusted slightly to match the reported ultimate analysis. Table 2. Assigned compositions and cellulose number fractions for of a lignin-like component vanous forms of biomass Form Wood Wood Wood Wood Wood Wood Wood Grass Grass Grass Grass Grass Grass Grass Paper Ag. Res. Ag. Res. Ag. Res. Ag. Res. Ag. Res. Ag. Res. Ag. Res. Ag. Res. Ag. Res. Ag. Res. Ag. Res. PVM 86.1 91.3 87.1 86.1 86.7 83.6 87.3 81.5 82.4 82.3 81.4 80.2 85.6 83.9 89.3 89.0 842 82.9 80.2 73.3 85.4 80.2 88.0 83.0 80.7 81.6 fcEL 0.703 0.755 0.530 0.437 0.564 0.682 0.640 0.778 0.668 0.000 0.798 1.000 0.626 0.564 1.000 0636 0.934 0.782 0.622 0.535 0.292 1.000 1.000 0.655 1.000 0.778 HllG 8.7 9.1 9.3 9.5 108 9.6 10.6 8.8 10.3 9.3 10.7 0.0 96 10.0 0.0 10.4 8.5 8.4 9.1 9.0 10.2 0.0 0.0 10.1 0.0 12.5 Ouc 30 3.0 30 40 30 30 30 4 0 3.0 5.3 2.0 0.0 5.0 3.0 0.0 6.0 5.0 4.0 40 6.0 6.0 0.0 0.0 3.0 0.0 1 0 Ash 0.4 0.3 0.9 6 1 0 3 09 1 5 52 56 4.5 5.8 198 09 11 2 80 39 29 09 0 7 7 1 2.8 6.8 54 59 18.1 23 5 |