Flashchain Theory for Rapid Coal Devolatilization Kinetics. 7

Energy & Fuels 1996, 10, 173-187

173

Flashchain Theory for Rapid Coal Devolatilization Kinetics. 7. Predicting the Release of Oxygen Species from Various Coals Stephen Niksa† Molecular Physics Laboratory, SRI International, 333 Ravenswood Avenue, Menlo Park, California 94025 Received April 12, 1995. Revised Manuscript Received September 18, 1995X

The release of oxygen from any coal primarily involves only three mechanisms: the shuttling of oxygen as an element in tar molecules; the simultaneous release of CO2, H2O, and small amounts of CO when labile bridges are converted into char links; and the release of CO from the residual oxygen in nascent char links at high temperatures. This modeling study characterizes these processes for heating rates from 0.5 to 104 K/s, temperatures to 1550 K, and pressures from 0.1 to 1 MPa. Evaluations against a database compiled from the behavior of 27 coal samples representing ranks from lignite to anthracite demonstrate that CO2, H2O, and CO yields plus the oxygen contents of tar and char can be predicted within useful tolerances throughout this domain. With Flashchain, no additional parameters or rate expressions are required to quantitatively predict the contributions from tar shuttling from any coal at any operating conditions. And the release rates of CO2, H2O, and low-temperature CO release are set equal to the previously evaluated formation rates of char links. Only one reaction rate expression (but no hypothetical ultimate yield parameter) is required to predict the yields and evolution rates of CO at high temperatures. This modeling approach departs from multiple, independent conversion channels for each gas product and instead recognizes the conversion of labile bridges into refractory char links as the fundamental process underlying the release of most of the oxygen-bearing noncondensible gases. As consequences of this premise, (a) oxygen is shuttled away in tars and simultaneously released as CO2 and H2O at all heating rates; (b) oxygen gas yields, especially H2O and CO yields, diminish in tandem with enhanced tar yields for faster heating rates or lower pressures; and (c) the onset of oxygen gas release shifts to higher temperatures for coals of progressively higher rank, particularly for low volatility coals. Production rates of hightemperature CO accelerate rapidly after the end of tar evolution, overtaking the production of CO2 and H2O from all coal types. CO is released over narrower temperature ranges for coals of progressively higher rank.

Introduction The organic coal mass contains oxygen at levels that range from a few weight percent in anthracites to 30% in lignites. Devolatilization releases about two-thirds of it as CO2, H2O, and CO, regardless of rank. These products comprise up to 85% of all the noncondensible gases released during the devolatilization of lignites. Although this contribution falls for coals of progressively higher rank, it is still 50% for anthracites. Oxygen functionalities that decompose into CO2 and H2O also deposit refractory cross-links throughout coal’s macromolecular structure, which suppresses liquids production and alleviates caking tendencies in bituminous coals. Oxygen is also released as an element in tar molecules. Tars and oils contain substantially less oxygen than their parent coals, but still enough to raise their viscosities and make them more mutagenic. The little oxygen left in char after devolatilization enhances its oxidation rate. Oxygen is present in coal in both inorganic and organic components. Decades of analytical research have delivered a fairly complete picture of all the oxygen † X

FAX: (415) 859-6196. E-mail: [email protected]. Abstract published in Advance ACS Abstracts, November 1, 1995.

0887-0624/96/2510-0173$12.00/0

functional groups in coal, but not their quantitative distribution functions.1,2 In the organic coal mass, the fraction of total oxygen in phenolic hydroxyl groups rises from 40% in lignites to 60% in high-volatile bituminous coals, then holds steady through the higher ranks.3 The very substantial estimates for carbonyl oxygen made in the 1960s were later moderated to accommodate an abundance of ether linkages, although quantitative estimates for these two components are still subject to large uncertainties. One-quarter of the oxygen in lignites is in carboxylic acid groups, including a substantial fraction in salt forms involving alkaline earth metal cations, but this contribution diminishes in coals of higher rank until it vanishes in the bituminous ranks.3 Methoxyl groups are a minor constituent of lignites only. Very simple chemistry has been proposed to explain how the major functional groups are converted into gaseous products during pyrolysis.4 For example, water (1) van Krevelen, D. W. Coal; Coal Science and Technology 3; Elsevier: Amsterdam, 1981. (2) Given, P. H. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: Orlando, FL, 1984; Vol. 3, p 136. (3) Blom, L. Thesis, University of Delft (1960) [Data obtained from Dryden, I. G. C. In Chemistry of Coal Utilization; Lowry, H. H., Ed.; John Wiley & Sons: New York, 1963, p 262.]

© 1996 American Chemical Society

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Energy & Fuels, Vol. 10, No. 1, 1996

is released when two phenolic hydroxyl groups condense into an ether linkage, or when a phenolic hydroxyl and methyl substituent condense into a methylene bridge between aromatic nuclei. Carbon dioxide is the decomposition product of carboxylic acid groups. Carbon monoxide can come from carbonyl groups or ether linkages, although its very broad thermal evolution profile suggests a variety of chemical precursors. Such chemical reactions do not convey two additional phenomenological aspects of oxygen conversion during devolatilization, cross-linking, and inorganic precursors. Refractory cross-links are added into the macromolecular coal matrix whenever CO2 and H2O are expelled.5 These cross-links inhibit the subsequent disintegration of the matrix into fragments that are small enough to evaporate and escape as tar molecules. Conversely, the liquid yields from low-rank coals are enhanced when cross-linking is suppressed by converting their carboxylic acid salts with ion exchange.6 Also, the liquids yields from pyrolysis in reactive hydrocarbon vapors increase in tandem with reduced water yields,7 again corroborating the connections among cross-linking, the production of liquids, and release of CO2 and H2O. In addition to the organic functional groups, inorganics also release CO, CO2, and H2O. The phenolic hydroxyl groups in clays such as kaolin, montmorillonite, and illite decompose into H2O.8,9 CO2 is expelled from carbonates, siderite, and calcite, along with minor amounts of CO from the latter mineral. The pyrolysis model developed by Gavalas and coworkers in the 1980s4 is the first and only one to contain legitimate chemical mechanisms among the oxygen functional groups in coal. In the other early models,10,11 the evolution rates of water and carbon oxides were represented with 1-3 first-order global reactions. These global rate expressions do not predict the maximum yield of each product. Instead, measured ultimate yields are used to evaluate their hypothetical ultimate yield parameters, in lieu of any explicit connections to the concentrations of functional groups in the coal, and then the rate laws are integrated to describe the dynamic approaches to these prescribed ultimate values. The functional group model12 evaluated a few of the hypothetical ultimate yield parameters as functional group concentrations inferred from FTIR spectra but retained several independent, distributed-energy conversion channels for each product. In the most recent version of this model,13 one cross-link is put into the macromolecular structure for every molecule of CO2 and CH4 that escapes with the gaseous products, but the production rates are still based on multiple, independent global reactions. (4) Gavalas, G. R.; Cheong, P. H.-K.; Jain, R. Ind. Eng. Chem. Fundam. 1981, 20, 113. (5) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 1668. (6) Tyler, R. J.; Schafer, H. N. S. Fuel 1980, 59, 487. (7) Miurra, K.; Mae, K.; Asaoka, S.; Yoshimura, T.; Hashimoto, K. Energy Fuels 1991, 5, 340. (8) Given, P. H.; Yarzab, R. F. In Analytical Methods for Coal and Coal Products, Vol. II; Karr, C., Ed.; Academic Press: New York, 1978; p 3. (9) Solomon, P. R.; et al. Energy Fuels 1990, 4, 319. (10) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 37. (11) Juntgen, H.; van Heek, K. H. Fuel Process. Technol. 1979, 2, 261. (12) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Markham, J. R. Fuel 1986, 66, 627. (13) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405.

Niksa

This modeling study expands Flashchain theory to predict the evolution rates and yields of all oxygenbearing species. The approach departs from multiple, independent conversion channels for each gas product and instead regards the formation of all refractory char linkages in coal macromolecules as the fundamental process underlying the release of most of the oxygenbearing noncondensible gases. In Flashchain,14 noncondensible gases have always been expelled when char links form. Here this connection is elaborated to resolve the proportions of CO2, H2O, and CO from different coal types. To avoid the multitude of adjustable parameters in multiple, independent reaction rate modeling, the release rates for CO2 and H2O are set equal to the char formation rates previously assigned to predict the evolution rates and yields of tar from any coal type.15,16 Only a single distributed-energy reaction rate law is added to account for CO production during the stage of devolatilization that follows tar evolution at high temperatures.17 This approach has the added advantage of avoiding all hypothetical ultimate yield parameters. As recently demonstrated with fuel-nitrogen release,18 a competition between tar shuttling and gas production determines how any particular element partitions among noncondensible gases, tar, and char at any stage of devolatilization. With oxygen, the interplay involves the release of oxygen as an element in the linkages in tar molecules, the simultaneous release of CO2, H2O, and CO when char links form, and the release of CO at high temperatures. Instead of evaluating hypothetical ultimate yield parameters from measured yields of the oxygenbearing gases, we evaluate their precursor concentrations from the coal’s ultimate analysis and then let the competing rate phenomena determine how the oxygen is apportioned among the various species at particular operating conditions. Shifts among these three competing rate processes explain the impact of variations in heating rate, temperature, pressure, and coal type. These extensions are evaluated against a database that represents heating rates from 0.5 to 104 K/s, temperatures to 1550 K, and noncondensible oxygen species distributions from 27 different coal samples representing ranks from lignite to anthracite. Extensions to the Theory As explained elsewhere in more detail,14 Flashchain represents coals’ cross-linked macromolecular structure as a mixture of chain fragments ranging in size from a monomer to the nominally infinite chain. The diverse assortment of structural components in real coals is rendered coarsely with four structural components: aromatic nuclei, labile bridges, char links, and peripheral groups. Aromatic nuclei are refractory units having the characteristics of the hypothetical aromatic cluster inferred from 13C NMR spectra. They also contain all the nitrogen in the coal. Except for HCN production from their nitrogen, nuclei are immutable. Nuclei are interconnected by two types of linkages: labile bridges or char links. Labile bridges represent groups of aliphatic, alicyclic, and heteroatomic functionalities, not (14) Niksa, S.; Kerstein, A. R. Energy Fuels 1991, 5, 647. (15) Niksa, S. Energy Fuels 1994, 8, 659. (16) Niksa, S. Energy Fuels 1994, 8, 671. (17) Chen, J. C.; Niksa, S. Energy Fuels 1992, 6, 254. (18) Niksa, S. Energy Fuels 1995, 9, 467.

Rapid Coal Devolatilization Kinetics

distinct chemical bonds. They contain all the oxygen and sulfur and the aliphatic carbon and hydrogen, but no aromatic components. Peripheral groups are the remnants of broken bridges on fragment ends that have the same composition. Being refractory, char links initially present in the coal are completely aromatic with no heteroatoms. But labile bridges that decompose into char links during devolatilization leave a fraction of their oxygen in the char link. This residual oxygen is released as CO at high temperatures. Connectedness among nuclei is an important aspect of coal rank independent of chemical constitution. The initial distribution of fragment sizes in this model is empirically related to extract yields in pyridine because, qualitatively, fragment distributions skewed toward smaller sizes correspond to coals with substantial amounts of readily extractable material. Throughout devolatilization, fragments in the condensed phase disintegrate as bridges break and reintegrate as char links form, so that the amount of tar precursors, called metaplast, passes through a maximum value that depends on coal rank. Fragment statistics incorporate the chemical kinetic rates of bridge scission and recombination to describe the changing fragment size distributions, including that for tar precursors. Tar production rates are related to metaplast concentrations with the flash distillation analogy,19 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. In the model’s four-step reaction mechanism, labile bridges are the key reaction centers. The population of labile bridges contains the pool of all aliphatic hydrocarbon elements and all oxygen, but none of the aromatic constituents. Consequently, their elemental compositions are radically different than values for whole coals.15 Values of atomic hydrogen-to-carbon ratios for bridges, denoted as (H/C)B, actually increase for coals of progressively higher rank, whereas wholecoal ratios diminish. The magnitudes of (H/C)B, (O/C)B, and (O/H)B are roughly twice the whole-coal ratios so that, for example, (O/C)B can be as large as 0.6 for lignites. Bridge-based ratios also show much more sample-to-sample variability than whole-coal ratios. Bridge conversions in Flashchain are complex chemical processes involving numerous steps and species, not unimolecular scissions. However, a single distributedenergy rate expression represents the temperature dependence of bridge conversion. This rate is increased in proportion to (O/C)B because oxygen is the most effective promoter of pyrolytic decompositions in this reaction system. Conversion of a bridge initiates two distinct reaction pathways, either to generate smaller fragments including precursors to tar or to form a new refractory char link accompanied by the immediate release of noncondensible gases. The pathway to char links depletes the bridge population without inducing fragmentation, thereby suppressing the production of tar precursors. The oxygen in bridges shifts the selectivity between scission and spontaneous char condensation toward the production of char links, consistent with the observed connections between cross-linking and the (19) Niksa, S. AIChE J. 1988, 34, 790.

Energy & Fuels, Vol. 10, No. 1, 1996 175

release of CO2 and H2O.5 To incorporate this tendency, the magnitude of the scission selectivity coefficient, νB, is increased in proportion to the value of (O/H)B. As an analog to cross-linking, additional char links and gases may also form by bimolecular recombination between the ends of metaplast fragments. This framework for primary devolatilization is adapted to describe the evolution of the oxygen in coal by expanding upon three processes: (1) the release of oxygen as an element in the bridges, peripheral groups, and char links in tar molecules; (2) the production of CO2, H2O, and CO from the oxygen in labile bridges whenever they are converted to char links; and (3) the release of CO from the residual oxygen in newly formed char links throughout the condensed phase. Only a simple accounting scheme without any new adjustable parameters or rate expressions is needed to account for the oxygen shuttled away in tar molecules. The formation of char links has always been accompanied by the release of noncondensible gases in Flashchain. Now the production of most of these gases is resolved into two separate stages: In the first stage, most of the oxygen in bridges is expelled as CO2 and H2O along with lesser amounts of CO. This conversion proceeds at the total rate of char link formation, which sums contributions from spontaneous condensation of bridges, bimolecular recombination of fragments that have bridge remnants on their ends, and unimolecular elimination of the bridge remnants. Since these three rates were defined previously, no new kinetic rate parameters are needed for stage 1, but three stoichiometric coefficients are introduced to apportion the oxygen in a labile bridge among the three gases and the residual oxygen in nascent char links. In the second stage, the residual oxygen in the newly formed char links is released as CO. The rate of this process is represented with a single distributed activation energy rate law, but no stoichiometric coefficients are involved. Oxygen species expelled from mineral matter are not resolved in the product distribution because the analytical data needed to specify the concentrations of their precursors is never available. As depicted in Figure 1, the mechanisms for oxygen release in Flashchain act on fragments of all three size classes in the condensed phase. Reactant fragments have sizes that extend to nominally infinite chain lengths. They may disintegrate into metaplast and intermediate fragments when their bridges break, carrying over the oxygen in their linkages, but no oxygen species are released by bridge scissions. CO2, H2O, and CO are released from reactant fragments only when their bridges and peripheral groups spontaneously condense into char links, because reactant chains are too large to evaporate as tar and have too few ends to recombine at an appreciable rate. Metaplast fragments are the smallest, comprising all potential tar molecules. Whenever metaplast fragments evaporate, they carry away the oxygen in their linkages and peripheral groups into the cumulative tar product. This is called tar shuttling. Their bridges and peripheral groups also decompose into char links, releasing CO2, H2O, and CO. The recombination mechanism also forms char links among metaplast fragments so, if peripheral groups are on either or both of the recombining ends, then CO2, H2O, and CO are released. CO is also eliminated from all the newly formed char links,

176


Niksa

Figure 1. Mechanisms for oxygen release in Flashchain.

but only at high temperatures. Since metaplast tends to vanish as the temperature is progressively increased, high-temperature CO production is less important for metaplast than for the larger fragments. Intermediate fragments have sizes between metaplast and reactant chains (but no special reactivity is implied by their name). They participate in the same oxygen release mechanisms as reactant fragments: CO2, H2O, and CO are released when their bridges and peripheral groups condense into char links, and CO is eliminated from their newly formed char links. Scissions in this lump do not expel oxygen species, and intermediate fragments do not evaporate or recombine. Collectively, the sum of the oxygen in the bridges, peripheral groups, and char links in all three fragment classes determines the oxygen content of char throughout devolatilization, which must match the coal’s oxygen content initially. An analogous sum determines the oxygen content of the cumulative tar product. Before we formulate the mathematical rate model for this mechanism, two aspects of the simultaneous release of CO2, H2O, and CO during bridge condensation should be clarified. First, the reason that CO2 and H2O release are associated with one common rate process is evident in the evolution rates of these gases from a Wyodak subbituminous coal while it is heated at 0.5 K/s at atmospheric pressure,9 in Figure 2. Note that all the relative maxima in the production rates of CO2 and H2O coincide (at 5, 11, and 18 min). The same thermogravimetric features were seen by Solomon et al. for the other Argonne coals9 and by Juntgen and van Heek for several German coals at similar conditions.11 Corroborating observations were also reported by Chen and Niksa,17 who saw very similar features in the transient yields of CO2 and H2O from four coals heated in excess of 104 K/s, and by Xu and Tomita,18 who reported the same temperature dependence for the yields of CO2 and H2O from eight coals at atmospheric pressure. These coinciding features strongly suggest that a single process, albeit a multistage one, is responsible for the release of both gases.

Figure 2. Release rates of H2O (dashed curve), CO2 (solid curve), and CO (dotted curve) from a Wyodak subbituminous coal heated at 0.5 K/s at atmospheric pressure in a TGA, reported by Solomon et al.9

In contrast, the CO evolution rate in Figure 2 has only one maximum that coincides with the others (at 18 min), and its main maximum is at 30 min, long after CO2 and H2O production has finished. Such distinctive thermogravimetric peaks for CO evolution at high temperature were also reported by Juntgen and van Heek11 and Campbell.21 The release of CO at high temperatures must be modeled as a separate stage of primary devolatilization, along with the observed evolution of C1 and C2 hydrocarbons, HCN, H2, and small amounts of water after tar production has ended.17 The second aspect of gas production during spontaneous bridge condensation that needs clarification is its selectivity toward the three gaseous products. One interpretation for the distinct peaks in TGA curves such as Figure 2 is that different functional groups decompose in different temperature regimes, which sounds plausible, although most of the precursors have never (20) Xu, W.-C.; Tomita, A. Fuel, 1987, 66(5), 632. (21) Campbell, J. H. Fuel 1978, 57, 217.


Figure 3. Comparison between reported product yield ratios and oxygen functional group distributions. Product yield ratios are based on the coal-oxygen fractions in the oxygen gas yields reported by Xu and Tomita22 for atmospheric pyrolysis at 1037 K, for ratios of CO2 to H2O (b) or of CO to the sum of CO2 plus H2O (O). The ratios of oxygen fractions in oxygen functional groups are based on Blom’s functional group distributions,3 for twice the carboxylic acids to the phenolic hydroxyl groups (solid curve) or for carbonyl oxygen to the sum of carboxylic acids plus phenolic hydroxyl groups (dashed curve).

been explicitly identified.9 A more coherent interpretation is that the different formation mechanisms for char links are responsible for the complex thermal response, because spontaneous condensation, peripheral group elimination, and bimolecular recombination each has distinctive dependencies on temperature and coal composition. To implement either interpretation in a reaction model, product selectivity should be evaluated from the functional group distributions in the coal. Otherwise hypothetical ultimate yield parameters will need to be assigned from lab tests on every coal sample under consideration. Unfortunately, product yields are firmly related to precursor concentrations only for selected coal ranks and for certain products, as illustrated in Figure 3. The data points are based on the yields of CO2, H2O, and CO reported by Xu and Tomita22 for atmospheric pyrolysis of 16 coals at 1037 K after rapid heating. Each point is a ratio of the fractions of the coal’s oxygen in the indicated products, either for CO2 to H2O or for CO to the sum of CO2 and H2O. Note that these ratios of oxygen fractions are independent of tar production because (1) conditions were severe enough that tar evolution was complete and (2) tars would carry away a random sampling of the precursors to the gaseous products. The smooth curves in Figure 3 are based on the apportioning of coal oxygen among the three primary functional groups reported by Blom.3 The first comparison is between twice the ratio of oxygen fractions in carboxyl groups to those in phenolic hydroxyl groups vs the ratio of reported yields of CO2 to H2O. This comparison evaluates the hypothesis that CO2 comes directly from carboxyl groups, and water is released when two phenolic hydroxyl groups condense. These stoichiometries are consistent with the observed yields of these gases for lignites and subbituminous coals, but bituminous coals expel nearly half as much of their oxygen in CO2 as in H2O even though they have (22) Xu, W.-C.; Tomita, A. Fuel 1987, 66(5), 627.


no carboxyl groups. The larger-than-expected product ratio for bituminous coals is consistent with recent suggestions that CO2 can form from CO within coals’ porous structure,23 but no detailed chemical reaction mechanism has yet been proposed to explain this process. The second comparison attempts to evaluate the hypothesis that “carbonyl” oxygen is the precursor to CO by comparing the ratio of the oxygen fraction in CO to the sum of the fractions in CO2 and H2O vs the oxygen fraction in carbonyl to those of the precursors for CO2 and H2O. This hypothesis is not supported by the data. The gas product ratio is remarkably constant for all coal types, such that CO carries away about onethird the oxygen in CO2 and H2O from any coal. In contrast, the ratio based on carbonyl oxygen rises from unity for lignites to a value of 1.5 for high-volatile bituminous coals, so neither the magnitude nor the rank dependence is consistent with the hypothetical stoichiometry for CO production. The magnitudes of this product ratio would nearly double if it was evaluated from the ultimate yields of these gases. But they would still be less than half the values based on the functional group distributions and show no rank dependence. The findings in Figures 2 and 3 are at least consistent with a common underlying process for CO2 and H2O production and a separate process for CO release at high temperatures. However, it is also clear that neither the distributions of oxygen functional groups in coal nor the chemical mechanisms for CO2 and CO production can yet be evaluated within tolerable uncertainties for predictive rate modeling. We proceed by introducing global processes for oxygen release during spontaneous bridge condensation and peripheral group elimination and then develop regressions for the stoichiometric coefficients in terms of parameters that can be evaluated from the coal’s ultimate analysis. The global conversions are (1 - nB)kB

θBOB 98 a1CO + a2CO2 + a3H2O + θCOC νBkB

θBOB 98 2θSOS kg

θSOS 98

a1 a2 a3 θC CO + CO2 + H2O + OC 2 2 2 2

(1a) (1b) (1c)

where θB is the number of O atoms/labile bridge; θS ) θB/2, number of O atoms/peripheral group; θC ) θB a1 - 2a2 - a3, number of O atoms/char link; a1, a2, and a3 are the stoichiometric coefficients for oxygen partitioning; (1 - νB)kB is the spontaneous bridge condensation rate; νBkB is the bridge scission rate; and kG is the peripheral group elimination rate. The subscripts B, C, and S denote oxygen in bridges, char links, and peripheral groups, respectively. Initially and throughout devolatilization, each labile bridge contains a fixed amount of oxygen, denoted by θB, and all peripheral groups contain half that amount. But the oxygen number in char links is either a fixed amount, denoted by θC, or zero. Char links present in the coal initially have no oxygen, but the residual oxygen from (23) Chatterjee, K.; Balkrishna, B.; Stock, L. M.; Zabransky, R., F. Energy Fuels 1989, 3, 427.

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Niksa

spontaneous bridge condensation can be removed by the following process: kCO

OC 98 CO

(2)

Rate equations for CO2, H2O, and CO are formulated by relating the stoichiometries in the global processes to the rates of scission and spontaneous condensation of labile bridges. For the molar yield of CO2, the rate equation is

dYCO2 dt

) a2(1 - νB)kBB +

a2 k S + a2RREC ) a2RCHAR 2 G (3)

where RCHAR is the total formation rate of char links; B the total moles of labile bridges in the condensed phase; S the total moles of peripheral groups in the condensed phase; RREC the bimolecular recombination rate of metaplast given by

RREC )

pEM

kR J* ( mj)2 2 j)1

∑

where pEM is the probability that a metaplast fragment end contains a peripheral group, kR the recombination rate, and mj the moles of metaplast of degree of polymerization j. In these expressions all molar quantities are normalized by the initial moles of aromatic nuclei per unit volume of original coal, A0. According to eq 3, the total release rate for CO2 is a sum of contributions from spontaneous condensation, peripheral group elimination, and recombination of metaplast. These same mechanisms determine the H2O production rate, according to

dYH2O/dt ) a3RCHAR

(4)

The CO production rate also contains these same three contributions, plus an expression for its production at high temperature that accounts for the fact that some char links have residual oxygen while others do not:

dYCO/dt ) a2RCHAR + kCOΘC*C*

(5a)

where ΘC* is the instantaneous average value of the moles of O per mole of char links in the condensed phase and C* is the total moles of char links, accounting for peripheral groups that have already lost CO2 and H2O as one-half char link. kCO is a distributed activation energy rate for CO production given by

kCO ) -

d

∫0∞exp(-∫0tACOe-E/RT(t′) dt′)f(E) dE}

{ln

dt

(5b)

ECO and ACO are fixed for all coal types, but σCO is diminished for coals of progressively higher rank as explained below. In the rate of CO production, the product ΘC*C* denotes the amount of oxygen in all char links in the condensed phase. Since C* is based on the total population of all chain fragments in the condensed phase, it can be evaluated from p, pL, and pE, the respective probabilities for intact linkages (of either kind), labile bridges, and peripheral groups on fragment ends. Recall that

p≡

B+C B ; pL ≡ ; pE ) ∞

S

∞

jxj ∑ j)1

jxj ∑ j)1

(6a)

∞

xj ∑ j)1

2

where xj is a fragment of degree of polymerization j. C* is defined as ∞

C* ) (p - pL)

∑ j)1

( )∑

jxj + 2

1 - pE

∞

2

j)1

xj

(6b)

which is brought into final form by applying the relation p + (∑xj/∑jxj) ) 1: ∞

jxj ∑ j)1

C* ) [(p - pL) + (p - pE)(1 - p)]

(6c)

We next formulate a rate equation for ΘC*C*. Initially, there is no oxygen in char links and all fragment ends contain peripheral groups, so ΘC*C* is zero. But during devolatilization residual oxygen accumulates in the newly formed char links, and ΘC*C* first increases and then diminishes as CO depletes the residual oxygen. This variable is evaluated by applying a conservation principle to the total amount of oxygen in all char links in the condensed phase throughout devolatilization, which yields

d(ΘC*C*) ) θCRCHAR - ΘC*RTAR C* - kCOΘC*C* dt where RTAR C* is the rate of char-O release via tar shuttling: j*

RTAR C*

)

( ) pbmj

1∑ p j)1

mj

Γj

(j - 1)Γj + 2(1 - pE)

2

b /pmj is the labile bridge fraction in a metaplast j-mer pm j and Γj the release rate of tar j-mers. According to eq 7, oxygen in char is created by spontaneous bridge condensation but depleted by tar shuttling and CO production. Once eq 6 is incorporated into eq 7, the derivative can be expanded and ultimately brought into the defining rate equation for ΘC*:

C* dΘC*/dt ) (θC - ΘC*)RCHAR - kCOΘC*

where

f(E) )

1

x2πσCO

exp(-(E - ECO)/2σCO2)

and the mean energy and standard deviation in the distribution of energies for CO production are denoted as ECO and σCO, respectively. The same frequency factor, ACO, is applied to all channels in the manifold.

(7)

(8)

The final rate equation describes the accumulation of oxygen in the cumulative tar product. A balance is 0 written in terms of fTAR , the fraction of the oxygen in the original coal that has been released into the cumulative tar sample, and a series of terms that track the oxygen release in the labile bridges, peripheral groups, char links, and uncapped ends in tar molecules,



as follows: b pm j ) θB(j - 1) Γj + dt pmj j)1

0 F0O%(0) dfTAR

A0MW0

J*

ΘC*

∑ j)1

J*

∑

[ ( ) (j - 1) 1 -

b pm j

pmj

] () Γj

Γj + 2(1 - pE)

2pE

+

2

θB 2

J*

Γj ∑ j)1

(9)

where F0 is the coal bulk density, O%(0) the coal oxygen content, wt %, daf and MW0 the molecular weight of oxygen. Once eqs 3, 4, 5, 8, and 9 are solved for the primary variables, the more familiar characteristics of oxygen release are defined as follows: The fractional mass yields, Wi, and coal oxygen fractions, f0i , for the three major gas products are

()

( )

A0 MW0i Wi 0 MWiYi; fi ) Wi ) F0 MWi O%(0)

(10)

where i ) CO2, H2O, CO and MW0i is the atomic or molecular weight of oxygen in product i. The oxygen content of the cumulative tar product is 0 O%TAR ) O%(0)fTAR /WTAR

(11)

The oxygen content and coal oxygen fraction in char are given by

O%CHAR )

(θB* + ΘC*C*)MW0 (F0/A0)MWA(0)WCHAR

(12a)

where B* is the sum of labile bridges and peripheral groups in the condensed phase, given by [pL + pE (1 ∞ p)]∑j)1 jxj 0 ) fCHAR

O%CHARWCHAR

(12b)

O%(0)

In the past, the molecular weight of char links was set equal to a fixed fraction of the bridge weight. Now this weight must be updated continuously to account for the depletion of oxygen by both bridge condensation and CO production, according to

MWC(t) )

ΘC* (MWC(0) - MWCOθ∞C) + ∞ θC ΘC* 1 - ∞ MWC(0) (13) θC

(

)

where θ∞C is an average number of oxygen atoms per char link that accounts for the absence of oxygen in the links in the original coal, given by [1 - p(0) + pL(0)]θC, and MWC(0) ) 0.45MWB. Model Parameters Flashchain’s extended submodel for coal constitution15 drastically reduces the input requirements by incorporating regressions among elemental compositions and

all other input data. The ultimate analysis now specifies initial values for all the structural parameters. Whereas most of the reactivity parameters are the same for all coal types, the handful that change for every sample are correlated with the elemental compositions of labile bridges, which are also evaluated from the ultimate analysis for the whole coal. None of the relations for either the structural or reactivity parameters have been adjusted to simulate oxygen release. The parameters introduced to predict oxygen release are θB, a1, a2, and a3, and ACO, ECO, and σCO. The number of oxygen atoms in a labile bridge is not an adjustable parameter. It is evaluated from the coal’s oxygen content as follows:

O%(0) )

A0MW0 θBB*(0) F0

(14a)

where B*(0) ) 1 - p(0)(1 - Fb(0)) ≡ 1 - β and Fb(0) is the fraction of labile bridges among all coal linkages. Equation 14 can be rearranged into the following definition of θB:

θB )

F0O%(0) A0MW0(1 - β)

(14b)

The most expedient way to evaluate the three stoichiometric coefficients is to reference the values of a1 and a3 to a2. The coefficient for the yield of CO from spontaneous condensation, a1, equals 0.629a2 for all coals. It is not an adjustable parameter. This relation is based an average value of 0.4 for the molar product ratio YCO/YCO2 developed from two datasets. One set of values is based on the product yields from the TGA/ FTIR experiment9 for the six Argonne coals of lowest rank (cf. Figure 2) up to point when the last rate maxima for these products coincide (at 20 min). The second set of values is based on the yields for heating eight coals to 773 and 863 K for 4 s20 because these temperatures are so cool that CO from the hightemperature stage is negligible. Certainly, tar yields had not reached their ultimate values at these conditions, so the second stage of primary devolatilization had not yet begun. The ratio a3/a2 determines the selectivity for H2O and CO2 production during char link formation. Considering the stoichiometric connection between these product yields and the relative amounts of carboxylic acids and phenolic hydroxyls in Figure 3, this value should be based on measured functional group distributions, at least for low-rank coals. Since this is currently impossible, two alternatives were devised. When both product yields have been measured for conditions severe enough to achieve their ultimate asymptotic valuesssuch as pyrolysis at 1050K for several secondssthe ratio of coefficients is evaluated as

( )

∞ MWCO2 WH a3 2O ) ∞ a2 MWH2O W CO2

(15a)

If measured ultimate yields are not available, then the following regression between a3/a2 and (O/C)B is used:

a3/a2 ) 1.73 + 19.11[0.59 - (O/C)B]1.132 (15b)

180


Niksa

Figure 4. Coalification diagram of the samples used in the data evaluations. The three primary subsets are based on laboratory studies reported by Xu and Tomita22 (b), Chen and Niksa17 (O), and Solomon et al.9 (3).

The regression is a least-squares fit of values that closely match the yields from 16 coals at 1037 K reported by Xu and Tomita.22 Similarly, there are two ways to evaluate a2. If WCO2 has been monitored, then a2 can be evaluated by matching the model results for the experimental operating conditions. Once its value is assigned, this parameter can then used to predict the yields from the same coal at any other operating conditions. Otherwise, the following regression was developed from the 16 yields in the Xu and Tomita dataset:

a2 ) 0.18 + 5.37[(O/C)B - 0.3]1.58 a2 ) 0.18

if (O/C)B > 0.3

otherwise

(16)

Model results based on both the empirical approach and the regressions are presented in the Results section. Parameters in the rate expression for high-temperature CO production were assigned by fitting the CO yields over a range of temperatures for three coals in the Xu and Tomita dataset, and by fitting the transient CO yields from two coals in the Chen and Niksa dataset.17 The pseudo-frequency factor and mean activation energy are fixed for all coals at respective values of 1012 s-1 and 230 kJ/mol, and the standard deviation about the mean energy varies with (O/C)B according to

σCO(kJ/mol) ) 143.8(O/C)B - 11.76

(17)

General Guidelines for the Data Evaluations The coalification diagram in Figure 4 is based on all the coal samples in this evaluation, showing complete coverage of the rank spectrum. However, some general limitations in the available data should be recognized at the outset. No one dataset includes the yields of all the major oxygen species; in fact, there are no data in the literature on the oxygen contents of char and tar pairs for well-defined pyrolysis conditions, so oxygen balances have never been closed in any laboratory study. Another limitation is that, in most cases, reported water yields are scattered by a factor of 3 or more, making them useless for model evaluations. Many of the available results for a range of pressures also happen to be badly scattered. The broadest coverage of coal rank effects is apparent in the wiregrid study of atmospheric pyrolysis of 17 coals

Figure 5. Consistency check based on the oxygen fractions in reported CO2, H2O, and CO yields using the symbol key defined in Figure 4.

by Xu and Tomita.20,22 The thermal histories in these experiments are unassailable because they used a Curie point induction heating scheme that circumvents the need for transient thermometry. Ultimate grid temperatures are known very well and regarded as the actual sample temperature. Although their reported heating rate of 3000 K/s is only an estimate, each run also included a 4 s isothermal reaction period. Such extended heating mitigates the impact of ambiguities in the heating characteristics and also in the cooling characteristics of this heater. Chen and Niksa used their radiant coal flow reactor operated in its mode that eliminates secondary pyrolysis of the volatiles to monitor transient yields of the three major oxygen gases throughout primary devolatilization from four coals.17 This entrained flow furnace heats coal suspensions with radiant fluxes as large as 60 W/cm2 at atmospheric pressure for which calculated particle heating rates exceed 104 K/s. Simulations of these data are based on uniform heating at 1.5 × 104 K/s to 1550 K with no isothermal reaction period, and quenching at 13 300 K/s. Ultimate yields of CO2 and CO from the TGA/FTIR experiments of Solomon et al.9 are used to assess the predicted effects of heating rate variations. These data were acquired with a uniform heating rate of 0.5 K/s to ultimate temperatures as hot as 1400 K at atmospheric pressure with the eight Argonne coals. Solomon et al. also reported water yields. However, as seen in Figure 5, the cumulative oxygen contents of their ultimate yields of CO2, H2O, and CO exceed the coals’ oxygen contents in seven of the eight cases. This inconsistency is probably due to erroneous water yields, which are almost double the values in the other datasets for comparable coals (as seen below in Figure 9b). This consistency check is satisfied for the Xu and Tomita dataset, for which the median coal oxygen release in the noncondensibles is 69%. It is breached for only the South Beulah lignite in the Xu and Tomita dataset, and for the low-volatility sample in Chen and Niksa’s dataset. Results for both these coals are not considered further. A wire-grid heater was used by Griffin et al.24 to monitor CO and CO2 yields from a single hvA bituminous coal over the same thermal history at pressures (24) Griffin, T. P.; Howard, J. B.; Peters, W. A. Fuel 1994, 73, 491.


Figure 6. An evaluation of Flashchain predictions against the yields of H2O (1), CO2 (b), and CO (O) reported by Xu and Tomita20 from a hv bituminous coal (XTLD) throughout primary devolatilization for heatup at 3000 K/s to 1037 K with a 4s isothermal reaction period at 0.1 MPa. Model predictions appear as curves.

from 0.1 to 1 MPa. These tests imposed a nominal heating rate of 1000 K/s to reaction temperatures from 800 to 1250 K, followed by immediate cooling. Representative thermal history records were not reported, so the simulations are based on uniform heating at 1000 K/s to the stated reaction temperatures with no decomposition after the ultimate temperature was achieved, as appropriate for cooling rates in excess of roughly -500 K/s. Since water yields were not determined, the oxygen fraction released as noncondensibles for these experiments is unknown. In the forthcoming Flashchain simulations, the operating conditions of temperature, heating rate, time, and/or pressure were varied to match those in the experiments. The ultimate analysis of each coal sample specifies all the input to the simulations, except in the cases where measured ultimate yields of CO2 and H2O are used to evaluate the stoichiometric coefficients a2 and a3. Aside from the rate parameters for hightemperature CO production discussed above, all other structural and reactivity parameters were evaluated from the regressions presented in parts 114 and 4.15 A simulation of each thermal history usually required less than 20 s on a 100 MHz, Pentium-based personal microcomputer. Results In this section, the first several evaluations consider gas product yields resolved either in temperature or in reaction time, to illustrate aspects of the reaction dynamics. Then additional cases characterize the impact of variations in coal rank, pressure, and heating rate. The distribution of all three oxygen gases throughout atmospheric pyrolysis in Figure 6 illustrates many of the general attributes of oxygen release during devolatilization. This case is based on the behavior of a highvolatile bituminous coal heated at 3000 K/s to 1037 K and then held at temperature for 4 s. The computed values incorporate the measured yields of CO2 and H2O at 1037 K only, via eq 15a and preliminary trials to assign a value for a2. CO2 and H2O are released on the same time scales, in rough mass proportions of 1:2 for this particular coal.


Figure 7. An evaluation of Flashchain predictions against the sum of the yields of H2O, CO2, and CO reported by Xu and Tomita20 from a lignite (XTMW); subbituminous (XTWD); three hv bituminous coals (XTHV, XTLD, XTNV); and two low volatility coals (XTKS, XTHG) throughout primary devolatilization for heatup at 3000 K/s to 1037 K with a 4 s isothermal reaction period at 0.1 MPa. Model predictions appear as solid curves, except for the dotted curve for the XTHV coal.

Their asymptotic ultimate yields are achieved by 900 K, at which point tar evolution is also exhausted. The computed values are within experimental uncertainty throughout primary devolatilization, which is noteworthy because the kinetics that determine the temperature dependence for CO2 and H2O release are based on the rate parameters for char link formation. None of these parameters were adjusted from their former values that predict tar yields and weight loss for diverse coal types and operating conditions, so the close agreement in Figure 6 strongly supports the premise that char link formation is the underlying process for CO2 and H2O release during devolatilization. CO is released on considerably longer times scales than the other gases. Somewhat less than one-fourth of its ultimate yield is expelled along with CO2 and H2O when bridges and peripheral groups decompose into char links. But after tar evolution, its release from the nascent char links during the second stage of primary devolatilization accelerates rapidly with increasing temperature. The predicted values are within experimental uncertainty throughout, except at the highest measurement temperature. However, this discrepancy is probably a fault of the experimental configuration, not the model. The fractional oxygen release as noncondensibles based on the data at 1193 K is 93%, which is too high to accommodate typical levels of tar-O plus the O in oils plus residual char-O. The fractional release of 80% based on the predicted gas yields is more plausible. This discrepancy may be an indication that tar recirculated onto the wire grid at 1193 K, releasing some of its oxygen as CO via a firmly established conversion channel for secondary volatiles pyrolysis.25 The temperature dependencies for oxygen gas release are evaluated in Figure 7 for a broad range of coal type. Xu and Tomita reported only the sum of the yields of CO2, H2O, and CO for these cases, so individual species yields cannot be considered. The predictions correctly display the tendencies for coals of progressively higher rank for (1) diminishing oxygen gas yields and (2) higher (25) Chen, J. C.; Castagnoli, C.; Niksa, S. Energy Fuels 1992, 6, 264.

182


onset temperatures for oxygen release. The shift to higher temperatures is especially pronounced for lowvolatility coals. The model predictions are within experimental uncertainty throughout devolatilization for most coals. However, the oxygen gas yields at 1193 K in Figure 7 are consistently underpredicted, especially for the Wandoan subbituminous coal (WD). But the fractional oxygen release in this case is 105% based on the computed CO2 and H2O yields plus an amount of CO that matches the sum of the oxygen species yields to the observed value, which is impossible. Since the predicted O-species yields are in close agreement with the observed values at low and moderate temperatures, it is very unlikely that the estimate is biased by the predicted CO2 and H2O yields. Rather, the consistent underpredictions at 1193 K for almost all coals in this dataset appear to be evidence of CO release from tar cracking. This artifact can be eliminated by carrying away the volatiles in an inert sweep gas. The second systematic discrepancy in Figure 7 is for one of the three hv bituminous samples. Predicted yields for the Hunter Valley coal (HV) are significantly larger than those for both others, whereas the observed yields for all three coals are indistinguishable. Here too, the model predictions may not be erroneous because the reported oxygen content of HV is 12.2% and of the others, 8.4 and 8.9%. It is impossible to explain why one of these three coals violates the clear tendency in the rest of this dataset for diminishing oxygen species yields for lower coal-O contents, unless the coal oxygen content is erroneous. An evaluation that resolves the three oxygen gas species for very rapid heating conditions appears in Figure 8. The product yields are plotted against the extent of primary devolatilization, evaluated as observed weight loss for progressively longer reaction times expressed as a fraction of the observed ultimate yield for this heating rate and pressure. The respective ultimate yields in order of increasing rank are 52.5 wt % daf for the subbituminous coal, 55.9 wt % daf for the Illinois No. 6, and 58.4 wt % daf for the Pittsburgh No. 8. This abscissa circumvents the need to relate the measured residence times to thermal histories, which were not measured. Generally speaking, these cases further corroborate Flashchain’s premises that char link formation expels CO2 and H2O on the shortest time scales, and that CO production is slow initially but that it accelerates very rapidly after the end of tar formation, eventually overtaking the production of CO2 and H2O. The predicted CO2 and H2O yields are within experimental uncertainty throughout primary devolatilization for all three coals, and the model correctly predicts that the proportions of CO2 and H2O approach 1:1 for the lowest ranks, but fall to less than 1:2 for high-rank bituminous samples. The predicted CO yields are also within experimental uncertainty for all coals except the Illinois No. 6 at intermediate extents of devolatilization. We turn away from the dynamics to evaluations of the ultimate yields of the oxygen gas species for a broad range of coal rank. The cases in Figure 9a are for the 16 coals in Xu and Tomita’s dataset for atmospheric pyrolysis at 1037 K, which cover nominal ranks from lignite to semianthracite. The calculations for these cases incorporate the measured yields of CO2 and H2O via eq 15a and preliminary trials to assign a value for

Niksa

Figure 8. An evaluation of Flashchain predictions against the CO (O), H2O (1), and CO2 (b) yields reported by Chen and Niksa17 from subbituminous (1488), Illinois No. 6 (1493), and Pittsburgh No. 8 (1451) coal samples during primary devolatilization during heat-up at more than 104 K/s. The abscissa indicates the percentage of the ultimate weight loss for each of the observed residence times. Model predictions appear as curves.

a2. The format of Figure 9 resolves the sample-tosample variability of the CO2, H2O, and CO yields and compares the predicted and observed values point by point. These results should be viewed sequentially from left to right, noting if the observed perturbations to the yields are depicted by the theory for every incremental change in carbon content. For the CO2 yields, they are correctly predicted in all cases, and the computed values are within an experimental uncertainty of 0.5 wt % in all but three cases. For the water yields, the perturbations are correctly predicted in all but four cases, and the model values are within an uncertainty of 1 wt % in all but two cases. Perturbations in the CO yields are correctly predicted in all but one case, although an uncertainty of 1 wt % is exceeded in six of the 16 cases. Since there are no hypothetical ultimate yields for the oxygen species in Flashchain and the precursor concentrations for CO are affected by the release of CO2, H2O, and tar-O, the predicted CO yields carry the cumulative errors and are therefore the largest. Notwithstanding, Flashchain predictions that incorporate measured yields of CO2 and H2O at a single operating condition faithfully represent the sample-to-sample variability of all the oxygen species yields for any coal type. The evaluation of ultimate oxygen gas yields in Figure 9b contains the entire Xu and Tomita dataset considered in Figure 9a as well as ultimate yields from TGA experiments for the Argonne coals reported by Solomon et al.9 However, all the computed values in Figure 9b



Figure 9. (a) An evaluation of the yields of CO2 (top), H2O (middle), and CO (bottom) for atmospheric devolatilization at 1037 K reported by Xu and Tomita.22 For all three products, measured values (b) are plotted with the corresponding Flashchain predictions (O and solid curves). These model results incorporate the measured yields of CO2 and H2O via eq 15a and preliminary trials to evaluate a2. (b) An evaluation of the yields of CO2 (top), H2O (middle), and CO (bottom) for atmospheric devolatilization at 1037 K reported by Xu and Tomita22 (b) and at roughly 1400 K reported by Solomon et al.9 (9). For all three products, measured values are plotted as filled symbols and the corresponding Flashchain predictions appear as the same unfilled symbols connected by solid curves, for Xu and Tomita’s data, and by dashed curves, for Solomon et al.’s data. These model results are based on the regression values for a2 and a3 in eqs 15b and 16 and do not incorporate any measured product yields.

are based on the regression values of the stoichiometric coefficients from eqs 15b and 16, to illustrate the performance for situations when no measured CO2 and H2O yields are available. Considering first such predicted values for the Xu and Tomita dataset, we see that the perturbations in the CO2 yields are correctly predicted in all but three cases and the predictions are accurate to within 1 wt % in all but four cases. Perturbations in the H2O yields are correct in all but four cases but a tolerance of 1 wt % is satisfied for just over half of the coal samples. The predicted CO yields are just slightly less accurate than the H2O yields. For ultimate yields from the TGA for a heating rate of only 0.5 K/s, the predicted CO2 yields are within 1 wt % of the observed values in all but three cases, but this dataset is too sparse to evaluate sample-to-sample variability. As illustrated below, variations in heating rate do not affect CO2 yields, so the overall similarities among the TGA and wire grid values and the predictions is not coincidental. The measured H2O yields from all eight coals are significantly greater than both the predictions and the observed values in the other dataset for the same nominal coal ranks. Note also that the magnitudes of the discrepancies between the observed and predicted H2O yields in Figure 9b correspond to the magnitudes of the discrepancies in the consistency check for these coals in Figure 5, except for the sample with 91% C. Since the predicted CO yields are not biased either high or low, the reported H2O yields appear to

be erroneous and responsible for the breached consistency check in Figure 5. Whereas the predicted CO yields abide by the tendencies with rank in the data, the quantitative discrepancies are usually more than 2 wt %. Note also that CO yields from the TGA experiment are significantly higher than those from the rapidly heated wire grid for comparable coal types. The predicted values also exhibit this tendency (except for the coal with 91% C which breaches the consistency check in Figure 5 by an inordinate amount), because of the following two reasons: First, only the heating conditions in the TGA were severe enough to release all the residual char-O as CO (because the wire-grid reaction temperature of 1037 K is too cool to achieve ultimate CO yields, as evident from the additional incremental CO yields at 1193 K in Figures 6 and 7). Second, as seen below, the TGA’s slower heating rate would enhance its CO yield by up to 1 wt % over the yield at 3000 K/s from the wire grid. So the evaluations in Figure 9b support Flashchain’s performance for different heating conditions, albeit within fairly loose tolerances. An evaluation of the impact of pressure on the oxygen gas species appears in Figure 10. The data were collected with the same Pittsburgh No. 8 sample used by Chen and Niksa, so all parameter values for this evaluation were specified without regard for the data in Figure 10. The observed CO2 yields are independent of pressure, except perhaps at the highest experimental

184


Figure 10. An evaluation of Flashchain predictions against the CO (filled symbols) and CO2 (unfilled symbols) yields reported by Griffin et al.24 from a Pittsburgh No. 8 coal heated at 1000 K/s to the indicated temperatures at 0.1 (b), 0.2 (9), 0.5 (2), and 1 MPa (1). Model predictions appear as solid curves for 0.1 MPa, and as broken curves for 1 MPa.

temperatures. Similarly, predicted CO2 yields increase by only about 0.1 wt % as the pressure is increased from 0.1 to 1 MPa. Whereas the observed CO yields are more variable than the CO2 yields, the measured yields confirm a monotonic pressure effect only for temperatures above 1100 K, where the yields at 1 MPa are about 0.5 wt % greater than the values for 0.1 MPa. Similarly, the predictions for these pressures are nearly indistinguishable at temperatures below 1100 K, but differ by 0.6 wt % at 1200 K. Whereas accurate H2O yields for a range of pressures are not available in the literature, the predicted values are independent of pressure at low temperatures. Like the CO yields, ultimate H2O yields are enhanced as the pressure is raised from 0.1 to 1 MPa, by 0.4 wt % in this case. To this point, the data evaluations have been based on measured CO2, H2O, and CO yields because no tar-O or char-O values have been reported in the literature. Since the predicted oxygen gas yields are accurate for most coals at most conditions, the predicted oxygen contents of condensed phase species cannot be far off and are worth looking at. Complete oxygen distributions appear in Figure 11 for a lignite (XTMW) and an hv bituminous coal (XTLD) heated at 103 K/s to 1550 K at atmospheric pressure. As in Figure 8, the abscissa indicates the fractional ultimate yield as a scale for the extent of primary devolatilization. The most striking feature in both distributions is the near-linear relation between the residual char-O level and the extent of devolatilization. If they were not subject to such large experimental uncertainties, char oxygen values would be excellent measures for the extent of devolatilization wherever product measurements are impractical. It is also worth noting that fully devolatilized chars contain little, if any, oxygen. Tars are significantly depleted of oxygen with respect to their parent coals. Whereas the tar yield for the lignite is 28 wt %, tars contain only 20% of the oxygen. For the bituminous coal, the respective depletion factors are virtually the same. Tars are depleted of oxygen because char links form before metaplast begins to accumulate, releasing CO2, H2O, and small amounts of CO from the macromolecules before they become small enough to evaporate and escape as tar. For this reason,

Niksa

Figure 11. Cumulative distributions of coal-oxygen fractions among the major oxygenated products based on Flashchain predictions for atmospheric devolatilization during heatup at 1000 K/s to 1550 K for a lignite (XTMW, top) and hv bituminous coal (XTLD, bottom). Table 1. Oxygen Species Distributions (in wt % daf) for Various Heating Rates to 1037 Ka coal type lignite (MW w/26.8% O) hv bituminous (LD w/8.4% O

Q, K/s WG 102 103 104 102 103 104

32.0 30.9 29.5 17.1 16.6 16.1

WT

Otar Ochar WCO2 WH2O WCO

21.5 21.0 26.1 20.8 31 20.8 18.2 7.3 22.0 7.0 25.5 6.9

5.3 5.3 5.3 1.1 1.1 1.1

11.2 10.8 10.4 1.7 1.6 1.6

7.5 7.2 6.9 4.0 3.9 3.8

8.4 8.0 7.6 2.8 2.7 2.6

most of the oxygen in the cumulative ultimate tar sample is released during the earliest stages of tar evolution, before most labile bridges have been converted. Although char links continue to form after the end of tar formation, the main release mechanism in this stage is CO production from the char links formed at earlier times. Evidently, this same process acts on the oxygen in nascent char links in tar molecules during secondary volatiles pyrolysis, converting all the tar-O into CO.25 Additional predictions in Table 1 show the major product yields and oxygen contents of char and tar for various heating rates for the same two coals. As heating rates are increased, tar yields are enhanced so more coal-O is shuttled away and the yields of the three oxygen gases must diminish in tandem. The oxygen contents of char are independent of heating rate because CO production will always scavenge away any and all residual oxygen, so the tar-O contents are also virtually independent of heating rate. Discussion Only a handful of different oxygen functional groups are present in any coal type, and only a few simple elementary reactions seem to explain the major product formation channels, at least for CO2 and H2O. Yet formidable impediments must be overcome before elementary reaction models will be used to predict the release of the major oxygen gases during coal devolatilization. Two gaps in our understanding of the conversion mechanisms are especially conspicuous.


First, how does CO2 form from bituminous coals that contain no carboxylic acids? Direct elimination of carboxylic acids and condensation of phenolic hydroxyls appear to be the major conversion channels in lignites and subbituminous coals but, clearly, other pathways must come into play with all the bituminous coal ranks. Any process proposed to explain the unexpected CO2 must also explain the constant ratios of the fractional oxygen contents in the yields of CO2 to H2O and in the yields of CO over the sums of those for CO2 and H2O (in Figure 3). The second gap is the mechanism for simultaneous CO, CO2, and H2O production during the initial stage of primary devolatilization, particularly the identity of the functional group precursors for low-temperature CO. The first attempts to predict the database in this paper were based on only the independent, high-temperature pathway for CO release. Even with extremely broad activation energy distributions, the simulated CO evolution histories for this scenario could not represent the way that CO is released throughout all stages of primary devolatilization. The premise that residual oxygen in nascent char links is the precursor to hightemperature CO quantitatively explains the production of about three-fourths of the total CO yields from all coal types. But the rest is expelled at low temperatures along with CO2 and H2O, and its precursors have not yet been identified. Until these issues are resolved, phenomenological mechanisms will be used to simulate the release of oxygen species during devolatilization. The long-standing phenomenological approach grew out of the twocomponent hypothesis,26 which proposed that the organic material in coal contains a distinct lump of precursors to volatile matter and a separate lump of inert, graphitic material. The purest manifestation of this view is in multiple, independent, first-order reaction models that associate specific groups of volatile compounds with increments in a single distribution of activation energies. This one-to-one correspondence develops the cumulative yield curve from incremental contributions from distinct volatile products, as illustrated in Figure 12. An activation energy distribution for all volatiles would be proportional to the derivative of the yield curve. At the time that Figure 12 was presented three hypothetical precursors each were proposed for CO and CO2 but only 1 for H2O. Actually, these precursors were never identified as functional groups because the reaction model included hypothetical ultimate yield parameters that were evaluated from measured ultimate product yields. In simulations, these parameters fix the ultimate yields of the oxygen gases, and the reaction model describes the dynamic approaches to these asymptotic values for different operating conditions. Except the model developed by Gavalas et al.,4 all subsequent reaction models for oxygen gas production (26) Essenhigh, R. H.; Howard, J. B. In Combustion Phenomena in Coal Dusts and the Two-Component Hypothesis of Coal Constitution; The Pennsylvania State University Studies, Monograph 31; The Pennsylvania State University Press: University Park, PA, 1971. (27) Solomon, P. R.; Colket, M. B. Symp. (Int.) Combust., [Proc.] 17 1979, 131. (28) Solomon, P. R.; Hamblen, D. G. In Chemistry of Coal Conversion; Schlosberg, R. H., Ed.; Plenum: New York, 1985; Chapter 5, p 121. (29) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405.


Figure 12. Cumulative yield of all volatile products indicated at and to the left of a given point for lignite pyrolysis at roughly 1000 K/s and the associated activation energy distribution reported by Suuberg et al.10 Table 2. Number of Parameters for Oxygen Gas Production in FG or FG/DVC at Different Stages of Development

period

hypothetical ultimate yield parameters for CO2/H2O/CO

sum of all kinetic rate parameters

total parameters

1979, ref 27 1985, ref 28 1988, ref 29 1993, ref 30

1/1/2 3/2/2 3/2/3 3/3/3

8 21 24 27

12 28 32 36

retained multiple independent production channels for the oxygen gases. They feature additional channels and more potent statistical tools to represent the kinetics of individual channels but nevertheless retain the twocomponent phenomenological hypothesis. As an illustration, the number of channels for each of the three oxygen gases and the number of associated hypothetical ultimate yield parameters and kinetic rate parameters for the FG model and its current incarnation, FG-DVC, at various stages of development are collected in Table 2. When it was introduced, this model had two separate channels for CO production, and one each for CO2 and H2O, and the kinetics for each channel were represented with a single first-order reaction. So there were four hypothetical ultimate yield parameters, and four pseudofrequency factors plus four apparent activation energies. Several years later, there were three channels for CO2 plus two each for CO2 and H2O, and the kinetics for each channel were represented with distinct activation energy distributions. In total, the 1985 version used 28 adjustable parameters, because now each rate constant has a pseudo-frequency factor, mean activation energy, and a standard deviation. The latest version has three distributed-energy channels each for CO2, H2O, and CO, for a total of 36 parameters to predict the yields of only these three gases. All but the pseudo-frequency factors are adjusted to fit product data for different coals. For a particular coal sample, these parameters are assigned by fitting the model predictions to data for heating rates of 3, 30, and 100 K/min from TGA experiments with FTIR product detection as explained separately.30 (30) Solomon, P. R.; Hamblen, D. G.; Serio, M. A.; Yu, Z.-Z.; Charpenay, S. Fuel 1993, 72, 469. (31) Manion, J. A.; McMillan, D. F.; Malhotra, R. Energy Fuels, submitted for publication. (32) Kaeding, W. K. J. Org. Chem. 1964, 29, 2556.

186


Niksa

Table 3. Nominal Decomposition Rates of Various Carboxylic Acids at 400 °Ca acid

k400, s-1

ref

COOH

8.5 × 10-6

31

CH2COOH

1.4 × 10-4

31

COOH

>1.3 × 10-3

32

OH COOH

1.6 × 101

32

OH

OH a

These estimates are extrapolated values based on reported activation energies and a frequency factor of 1012 s-1.

Over the past several years, the two-component hypothesis has been abandoned as a framework for devolatilization modeling, primarily because tar formation is now recognized as a statistical competition among chemical processes that form fragments vs those that reintegrate macromolecules into the coal matrix. Now the time has come to realize that the two-component hypothesis is also an unsuitable premise for modeling the release of individual volatile products, particularly the oxygen gases. The extensions to Flashchain in this paper demonstrate that the competing pathways for scission and char link formation in bridge decomposition chemistry also govern the production of CO2 and H2O from any coal type. Even the same values of the rate parameters previously assigned to predict tar evolution also describe the release of CO2 and H2O throughout primary devolatilization. No additional reaction rate parameters or conversion channels are required. More specifically, the three mechanisms that form char links in Flashchain are also responsible for the production of all the CO2 and H2O, and roughly onefourth of the CO from any coal type. Each of these mechanisms invokes free-radical chain chemistry, not unimolecular decomposition of functional groups. As radical pools accumulate, many different functional groups become dissociated, often through important interactions among different species. For example, as seen in Table 3, the presence of one or two phenolic hydroxyl groups dramatically accelerates the dissociation of a carboxylic acid on the same ring, whereas the loss of the carboxylic acid group activates the phenolic hydroxyl for its condensation reaction. Qualitatively, such interactions explain why CO2 and H2O have identical evolution histories even though their functional group precursors and respective formation mechanisms are different. Quantitatively, it is not necessary (or even possible) to represent them with elementary reaction mechanisms once their relation to the rates of char link formation are recognized. From a practical standpoint, the only modeling issue regarding the three early oxygen gas products is how to apportion the oxygen in labile bridges among gases and the residual oxygen in nascent char links. In total, Flashchain’s mechanism for CO2 and H2O production incorporates three stoichiometric coefficients, one of

which is the same for all coal types and operating conditions. To capture the sample-to-sample variability in gas product yields, the stoichiometric coefficients must be evaluated with measured ultimate yields of CO2 and H2O. Until determinations of the oxygen functional group distributions and mineral precursors become routine, and the uncertainties in coal oxygen analyses are substantially diminished, a priori predictions with very high accuracy will be impossible. But with Flashchain, only the ultimate yields of CO2 and H2O at the end of tar formation for any one well-defined thermal history are needed to evaluate all the stoichiometric coefficients. For situations where such yield determinations are unavailable, the coefficients can be evaluated from the regressions in eqs 15b and 16. This approach delivers reasonable comparative values to assess product compositions over broad ranges of temperature, heating rate, and pressure in surveys of coal rank effects. The attributes of high-temperature CO production are also inconsistent with the two-component hypothesis, even though this process is not related to bridge decomposition chemistry. The main reason is that, as the last of the oxygen species to be released, hightemperature CO has no predetermined precursor concentration in the coal. Its precursors are formed along with char links when, for example, an ether linkage forms from condensing phenolic hydroxyls. The concentrations of such precursors cannot possibly be represented by hypothetical ultimate yield parameters for diverse operating conditions, because they are determined by the competition between loss of char links via tar shuttling vs retention of char links in the char matrix. For this reason, CO yields are affected by variations in heating rate and pressure. Provided that this coupling to the other product formation channels is accounted for, only a single distributed-energy process is required to represent CO release at higher temperatures. This process occurs over broader temperature ranges in coals of lower rank, mimicking the same tendency in the bridge decomposition rates that has already been associated with the diversity of bridge compositions in low-rank coals.16 Finally, it is unfortunate that about half the studies with oxygen product distributions in the literature had to be ignored because of serious breaches in their partial coal-oxygen balances. Water determinations seem to be subject to the greatest uncertainties, especially when all sampling lines are not heated, and FTIR monitoring schemes are not immune from difficulties. Of course, legitimate total oxygen balances also require accurate determinations of the oxygen contents of matched chartar pairs throughout primary devolatilization. In addition to their mechanistic implications, these measurements will probably also define the most reliable tracers for extents of devolatilization for the typical situations where it is not possible to monitor the yields of all the major products. Conclusions 1. The release of oxygen from any coal primarily involves only three mechanisms, tar shuttling, the simultaneous release of CO2, H2O, and small amounts of CO when labile bridges are converted into char links, and the release of CO from the residual oxygen in nascent char links at high temperatures.


2. Bridge conversion is the fundamental process underlying both tar formation and the first stage of oxygen gas production, which is manifest in three attributes: (a) Oxygen is shuttled away in tars and simultaneously released as CO2 and H2O at all heating rates. (b) Oxygen gas yields, especially H2O and CO yields, diminish in tandem with enhanced tar yields for faster heating rates or lower pressures. (c) The onset of oxygen gas release shifts to higher temperatures for coals of progressively higher rank, particularly for low volatility coals. 3. Production rates of high-temperature CO accelerate rapidly after the end of tar evolution, overtaking the production of CO2 and H2O from all coal types. CO is released over narrower temperature ranges for coals of progressively higher rank. 4. With Flashchain, no additional parameters or rate expressions are required to quantitatively predict the contributions from tar shuttling from any coal at any operating conditions. And the rates of CO2, H2O, and low-temperature CO release are equal to the previously evaluated rates of char link formation by spontaneous condensation, peripheral group elimination, and bimolecular recombination. Only one reaction rate expression (but no hypothetical ultimate yield parameter) is required to predict the yields and evolution rates of CO after the end of tar evolution. Acknowledgment. Several helpful discussions with Dr. Don McMillen of the Molecular Physics Laboratory at SRI International clarified the basis in chemistry for the proposed global mechanisms for oxygen gas release in FLASHCHAIN. Nomenclature a1 a2 a3 A0 ACO B B* C C*

ECO f0CHAR

stoichiometric coefficient for CO release during char link formation stoichiometric coefficient for CO2 release during char link formation stoichiometric coefficient for H2O release during char link formation molar concentration of aromatic nuclei in original coal pseudo-frequency factor in the distributed-energy rate of high-temperature CO production total moles of labile bridges in the condensed phase sum of moles of labile bridges and peripheral groups in the condensed phase total moles of char links in the condensed phase total moles of charred bridges in the condensed phase, counting peripheral groups that have already lost CO2 and H2O as one-half char link mean energy in the Gaussian energy distribution for high-temperature CO production, f(E) coal-oxygen fraction retained in char

Energy & Fuels, Vol. 10, No. 1, 1996 187 0 fCO 2 0 fH2O 0 fTAR 0 fCO J*

kCO

kB kG kR mj MWA MWB MWC(t) MWMON MW0 O%(0) O%CHAR O%TAR p pL pE pEM RCHAR RREC RTAR C* S WCHAR WCO2 WH2O WCO WTAR xj YCO2 YH2O YCO

coal-oxygen fraction released as CO2 coal-oxygen fraction released as H2O coal-oxygen fraction released as tar coal-oxygen fraction released as CO maximum degree of polymerization of metaplast and tar equivalent first-order rate constant for the distributed energy rate of high-temperature CO release defined in eq 5b nominal bridge conversion rate constant peripheral group elimination rate constant recombination rate constant moles of metaplast of degree of polymerization j molecular weight of an aromatic nucleus in original coal molecular weight of a labile bridge molecular weight of a char link instantaneous molecular weight of a monomer molecular weight of oxygen daf wt % oxygen in original coal daf wt % oxygen in char daf wt % oxygen in the cumulative tar sample instantaneous fraction of intact links among all chains in the condensed phase instantaneous fraction of labile bridges among the chains in the condensed phase instantaneous fraction of ends having peripheral groups among all chains in the condensed phase probability that a metaplast fragment end contains a peripheral group total formation rate of char links bimolecular recombination rate of metaplast rate of char-O release via tar shuttling total moles of peripheral groups in the condensed phase char yield, daf wt % CO2 yield, daf wt % H2O yield, daf wt % CO yield, daf wt % tar yield, daf wt % moles of any condensed phase j-mer normalized by A0 molar CO2 yield normalized by A0 molar H2O yield normalized by A0 molar CO yield normalized by A0

Greek Symbols Γj νB F0 σCO θB θS θC ΘC*

molar release rate of tar j-mers scission selectivity coefficient bulk density of the original coal std dev about ECO in f(E) for high-temperature CO production oxygen atoms per labile bridge in the coal oxygen atoms per peripheral group in the coal oxygen atoms per newly formed char link, θBa1-2a2-a3 instantaneous average value of the moles of oxygen per mole of charred bridges in the condensed phase EF950067L

Flashchain Theory for Rapid Coal Devolatilization Kinetics. 7

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