FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 10

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FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 10. Extents of Conversion for Hydropyrolysis and Hydrogasification of Any Coal Stephen Niksa Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03064 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 10. Extents of Conversion for Hydropyrolysis and Hydrogasification of Any Coal Stephen Niksa Niksa Energy Associates LLC, 1745 Terrace Drive, Belmont, CA 94002 (650) 654 3182; [email protected] ABSTRACT This paper extends FLASHCHAIN theory with mechanisms for (i) hydrogenation of labile bridges in the condensed coal phase which enhance tar yields during primary devolatilization under some, but not all, conditions; and (ii) heterogeneous hydrogasification of char into CH4. Under the H2 pressures associated with entrained coal gasification technology, coal devolatilization at heating rates faster than 103 °C/s does not provide sufficient time for appreciable bridge hydrogenation so tar yields are not enhanced. Conversely, tar yields under slow heating conditions are strongly enhanced.

The mechanisms proposed for bridge

hydrogenation and its associated impact on fragment recombination in FLASHCHAIN® accurately depict the joint impact of heating rate and H2 pressure on tar yields. Whereas tar

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yields from rapid devolatilization diminish for progressively higher pressures, total weight loss remains steady or passes through a minimum at some H2 pressure around 1 MPa, because char hydrogasification counteracts the lower weight loss associated with diminished tar yields at progressively higher pressures. A single, half-order methanation reaction within the CBK/G framework gave results within the measurement uncertainties through 15 MPa H2.

In

combination, the mechanisms for hydropyrolysis and hydrogasification accurately interpreted a database representing coals of rank from lignite to anthracite; heating rates from 1 to 103 °C/s; temperatures from 550 to 1150 °C; reaction times to 180 s; and particle diameters to 1 mm. The assigned hydrogasification reactivities are far less variable than those for char gasification by steam and CO2, and show no consistent trend with rank.

Keywords:

Hydropyrolysis, Gasification, Reaction Mechanism, FLASHCHAIN


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Introduction Throughout the 1980s, coal hydropyrolysis was aggressively developed to produce chemical feedstocks from coal as a technological response to the OPEC oil shocks. Even though the interest in hydropyrolysis technology has waned, the hydropyrolysis database is directly relevant to coal gasification at moderate temperatures, particularly when gaseous hydrocarbons (GHCs) in the synthesis gas are generated to boost calorific values, and when coal is co-fired with natural gas. Such processes often sustain H2 pressures approaching 1 MPa, which is high enough, in principle, to enhance primary tar yields, hydrogenate tar into oils and GHCs, and convert char into CH4 via hydrogasification. Yet the enormous hydropyrolysis database has not been fully interpreted to extract accurate kinetics for hydropyrolysis, secondary tar hydrogenation, and char hydrogasification and, especially, to accurately describe the rank dependences. The earliest mechanisms proposed for hydropyrolysis were based on classical devolatilization theory1,2, whose essential scheme is secondary redeposition of volatiles into residues which remain in the char on a time scale set by the transport mechanism for volatiles escape. Hydrogen disrupts redeposition by stabilizing the escaping volatiles, and also enhances conversion via the hydrogasification reaction. Conversely, the longer transit times through larger particles are purported to lower yields.

Volatiles escape mechanisms such as continuum or Knudsen

diffusion, continuum diffusion of liquids through a melt, bulk flow through macropores, and bubble rupture and growth in a viscous melt have been analyzed.

Whereas classical

devolatilization theory is definitively contradicted by the absence of a particle size dependence in the yields for primary devolatilization under inert gases, some reported hydropyrolysis yields do diminish for progressively larger sizes and, consequently, classical theory remains viable for hydropyrolysis.

However, the analysis in this paper interprets the size dependence in


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hydropyrolysis yields without resorting to intraparticle redeposition of volatiles. Moreover, classical theory has no means to describe the behavior of different coals other than to re-specify the kinetic parameters. Among the network depolymerization mechanisms, both FLASHCHAIN®3 and CPD4 were expanded for hydropyrolysis applications. The base reaction mechanisms in FLASHCHAIN® were supplemented with finite-rate bridge hydrogenations that expand the population of tar precursors and thereby enhance tar yields. A transport analysis estimates the H2 pressure within devolatilizing particles, and Henrys Law relates the internal H2 pressure and the H2 concentration in the condensed coal phase, which factors into the rate of the hydrogenation reaction. This analysis accurately depicts the joint impacts of heating rate and H2 partial pressure on the total and tar yields, and accurately interpreted a database representing 32 coals of ranks across the entire rank spectrum.3 Similarly, Guan et al.4 substituted a bridge hydrogenation reaction for the scission reaction in CPD, and postulated that the selectivity ratio between hydrogenation and crosslinking was proportional to the ambient H2 pressure. They also modified correlations for the distribution of noncondensable products to skew the predictions toward CH4, H2O, and other light GHCs. The analysis accurately interpreted the aggregate yields of tar and gas and selected noncondensables from 15 coals subject to the same thermal history and H2 pressure, as well as datasets covering broad ranges of temperature and H2 pressures.4 However, this analysis does not include the char hydrogasification reaction and, consequently, the interpretations of the same datasets as the FLASHCHAIN® validations are fundamentally different. Problems associated with the omission of hydrogasification would become apparent in interpretations of the joint impacts of heating rate and H2 pressure variations, which Guan et al. did not evaluate. This paper presents the formal mathematical development of the expansion of FLASHCHAIN®


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for hydropyrolysis and hydrogasification applications, and updated data interpretations based on the latest parameter assignments. These data validations cover the yields of char, tar, and gas in aggregate, and thoroughly interpret the impacts of all the major operating conditions. But detailed distributions of the major product species are relegated to a future extension, because product distributions are radically altered by tar hydrogenation chemistry, which will be analyzed as a separate reaction mechanism. Following a brief review of the underlying process chemistry and the formal mathematical development, several datasets are quantitatively interpreted for variations in heating rate, temperature, reaction time, H2 pressure, particle size, and coal quality. Mathematical Analysis The following sections first describe the phenomenology and then, in series, the state variables that describe bridge hydrogenation, bimolecular recombination, and char hydrogasification; the associated conservation equations; and the data validation work. Phenomenology The most direct evidence for hydrogenation during primary coal devolatilization is that the yields of both tar and gas are enhanced under elevated H2 pressures, where the enhancements are much greater under slow heating conditions than during rapid coal devolatilization.5 This heating rate dependence could conceivably connect to the extensive rearrangements associated with the catalytic hydrotreatment of petroleum resids, which comprise the conversion of condensed aromatics into naphthene rings with subsequent scission into GHCs, especially longer aliphatics and olefins.

Only very slow heating provides sufficient reaction time for such complex

chemistry. For rapid heating conditions, two much less extensive roles for H2 are proposed: (1)


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Direct hydrogenation of labile bridges in coal without any transformations of aromatic nuclei; and (2) Suppression of recombinations among the ends of mobile fragments in the condensed phase that would otherwise introduce refractory char links into the nascent char phase, and thereby diminish the level of tar precursors. These reactions utilize H2 within the condensed phase, either as a dissolved species in the coal melt during the plastic stage of bituminous coal decomposition or as adsorbed species on the pore surfaces of solid reacting particles of coals of low- or very high-rank. In either case, the level of H2 within the condensed phase is directly related to the H2 pressure within the particle which, in turn, reflects the ambient H2 pressure through an analysis of the penetration of H2 against the outward flow of volatiles. Succeeding subsections analyze these processes in detail. Once in the condensed phase, dissolved H2 may react with the components of labile bridges in various ways, including rupture of hydroaromatics, saturation of olefins into aliphatics, and conversion of carboxylic acids into alcohols. The elimination of carboxylic acids is especially important because carboxylic acids are widely regarded as crosslinking agents that skew bridge conversions toward the spontaneous generation of refractory char links away from scission of longer fragments into smaller tar precursors. Accordingly, on a phenomenological level, bridge hydrogenation shifts bridge decomposition chemistry away from spontaneous condensation into char links toward scission. Once a bridge has been hydrogenated, it may also interfere with another charring process, bimolecular recombination of mobile fragments.

If a hydrogenated bridge later undergoes

scission, its remnants will remain attached to the newly formed ends of the two new fragments. A fragment with hydrogenated ends is regarded as capped by stable functionalities and therefore unable to produce a char link in combination with another fragment end.

According to


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FLASHCHAIN®, without hydrogenation the ends of mobile fragments can recombine regardless of whether or not the end contains the remnants of broken bridges.

Since hydrogenation

eliminates unsaturated components from bridges, and since such components readily condense further into aromatics, and since aromatics are essential to the refractory character of char links, hydrogenation interferes with bimolecular recombination.

This effect will be especially

pronounced with hv bituminous coals, which normally generate an abundance of mobile fragments that may either recombine into larger refractory fragments or depolymerize further into tar precursors. Mathematical Development In FLASHCHAIN6, coals' macromolecular structure is rendered as a mixture of chain molecules ranging in size from a monomer to a nominally infinite degree of polymerization. For species which are distributed in size, the concentration of chains of degree of polymerization j (i.e., j linked nuclei) is denoted by xj in the condensed phase and tj in the vapor phase. The diverse assortment of structural components in real coal is rendered coarsely with four structural components: aromatic nuclei (A), labile bridges (B), char links (C), and peripheral groups (S). Hydrogenated bridges and peripheral groups are denoted as B* and S*, respectively. The model is formulated in terms of scaled molar concentrations, in units of moles per volume in the coal phase. All species concentrations are scaled with respect to the initial concentration of aromatic nuclei in coal, A0. The proportions and elemental compositions of the structural components A, B, C, and S are specified from the ultimate analysis with the coal constitution submodel.7 Bridge Hydrogenation The bridge hydrogenation rate is developed in terms of the H2 concentration in the condensed


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phase, CH2, in moles/m3-condensed phase. Since the mole fraction of H2 within the particle void space is relatively small, Henry’s Law relates the molar concentration to the H2 partial pressure, according to

pH2 = H H 2 CH2

where

H H2 =

vN

(1a)

λH ρ L 2

where pH2 is the partial pressure of H2 vapor within the particle; HH2 is the Henrys Law coefficient in MPa-m3/mole; vN is the molar volume at standard conditions of 22.4 l/mole; ρL is the density of the condensed phase; and λH2 is the solubility coefficient in normal liters of H2/kgcondensed phase-MPa. For example, the solubility coefficient for H2 in hydrocarbon mixtures such as mineral oil was reported8 as

λH2 = −0.559729 − 0.42947x10−3 T + 3.07539x10−3

T 0.835783 + 1.94593x10−6 T 2 + ρ L (20C ) ρ L (20C ) 2

(1b)

where the temperature is in °C and ρL(20C) is the liquid density at 20°C. Since no such expressions are available for mixtures as complex as softened, devolatilizing coal melts, the Henry’s Law coefficient will necessarily be factored into other unknown rate parameters, as seen shortly. The bridge hydrogenation reaction is based on

B +ν HP H 2 → B *

(2a)

where B and B* are the molar concentrations of the original and hydrogenated labile bridges, respectively; and νHP is the stoichiometric coefficient for complete hydrogenation, which is specified below. In its simplest form, the global hydrogenation rate is given by


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n

' RHP = k HP BC HH2 2

' = k HP B (1 − ε G − ε A )

0 ρORG [PxH ρORG

]

nH2

2

(2b)

where k’HP is the product of the Henry’s Law constant and an Arrhenius rate constant; xH2 is the mole fraction of H2 in void space; εG and εA are the void fractions of vapor and ash in the fuel; ρORG0 and ρORG are the bulk densities of the organic material in the original coal and in the

devolatilizing particle; and nH2 is the reaction order with respect to H2 in the condensed phase. P is pressure within a particle, which is evaluated as the ambient pressure. The bridge hydrogenation rate is the source term for hydrogenated bridges. They are also destroyed by scission and spontaneous condensation, and shuttled out of the condensed phase as structural elements in tar. So the hydrogenated bridge concentration is determined from

ρ0 dB * ' = k HP B (1 − ε G − ε A ) ORG PxH 2 dt ρORG

[

*

]

nH 2

J*

ptlj

j =1

pt j

− k B B * −∑ ( j − 1)

Γj

(2c)

where kB is the distributed-energy rate constant for bridge conversion. In the tar shuttling rate, the ratio ptjl*/ptj gives the fraction of the j-1 intact linkages in a tar j-mer that have been hydrogenated; and Γj is the molar release rate of tar j-mers. It is easier to incorporate bridge hydrogenation into FLASHCHAIN® in terms of the original concentration of labile bridges, without hydrogenation, which is denoted by B0. Then at any moment during the devolatilization period, B0 = B+B*, so that B = B0-B*, and the rate equation becomes

ρ0 dB * ' = k HP ( B0 − B*)(1 − ε G − ε A ) ORG PxH 2 dt ρORG

[

]

nH2

*

J*

ptl j

j =1

pt j

− k B B * −∑ ( j − 1)

Γj

(2d)

The stoichiometric requirement for hydrogenation is based on complete hydrogenation of all carbon, oxygen, and sulfur in the original bridge.

FLASHCHAIN®’s coal constitution


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submodel7 already defines the average numbers of these three elements per bridge, CNB0, ONB0, and SNB0. The carbon is hydrogenated into a mixture of CH4 and C2H4, in proportions that maintain an H/C ratio of three. All oxygen becomes H2O and all sulfur becomes H2S. Hence, hydrogenation leaves only short methylene hydrocarbon chains capped by methyl groups and alcohol functional groups in the hydrogenated bridge, which is the theoretical maximum extent possible.

This limiting structural situation specifies the stoichiometric requirement for

hydrogenation as the difference between the H-numbers of hydrogenated and original bridges, according to

ν HP =

3C N B0 + 2(ON B0 + SN B0 )− H N B0 2

(3a)

where HNB0 is the number of H-atoms per original bridge. This value already accounts for the assortment of mostly unsaturated functional groups in the original bridges, including carboxylic acids. Since hydrogenation does not affect CNB0, ONB0 or SNB0, the mean H-number in bridges throughout hydropyrolysis is evaluated as a weighted average of the concentrations of original and hydrogenated bridges, as

H

 B* H 0  B − B * H 0  N B + 2ν HP + 0  N B N B =  B B 0  0  

[

]

(3b)

The value of HNB directly indicates how hydrogenation affects the scission selectivity during bridge conversion.

In FLASHCHAIN®, this selectivity is evaluated as a stoichiometric

coefficient, νB, that gives the probability that a bridge conversion results in scission rather than spontaneous condensation. Based largely on the crosslinking character of carboxylic acids, this coefficient is evaluated as a function of the atomic O/H ratio of bridges.7 As the O/H ratio falls below about 0.2, bridges mostly consist of methylene chains whose dominant conversion channel


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is scission. Consequently, the scission selectivity coefficient surges toward greater values for O/H ratios below about 0.15. Given the H-number in bridges throughout hydropyrolysis from eq. 3b, the value of νB is updated simply by evaluating the ratio ONB0/HNB, and applying the functional relation for νB in the coal constitution submodel. Hydrogenation increases HNB and thereby diminishes ONB/HNB, which shifts the bridge conversion selectivity toward scission at the expense of spontaneous charring. This is how bridge hydrogenation suppresses spontaneous charring in FLASHCHAIN®.

The Impact of Hydrogenation on Bimolecular Recombination Bimolecular recombination of coal fragments is restricted to the smallest size class size called metaplast. Presumably, only the smallest fragments are mobile enough to engage in bimolecular recombination reactions.6 These recombinations cannot occur if the participating fragment ends hold the remnants of a broken hydrogenated bridge, because recombining fragments will ultimately be connected by a refractory char link which, by definition, is completely aromatic. Since hydrogenation destroys aromaticity, it also eliminates the potential to create an aromatic link during recombination. The inhibition is implemented with the probability that an end contains a hydrogenated peripheral group, which is specified as follows: The concentration of hydrogenated peripheral groups within all fragment sizes is determined by (i) their production via hydrogenation of original peripheral groups (like bridge hydrogenation); (ii) their production via scission of hydrogenated bridges; (iii) their elimination via conversion to noncondensable gases; and (iv) vaporization of tar molecules that contain them, according to 0 ρ ORG dS * = k HP ( S 0 − S *)(1 − ε G − ε A ) Px H 2 dt ρ ORG

[

]

nH 2

J*

+ 2ν B k B B * − k G S * −2 p He ∑ Γ j

(4a)

j =1


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where kG is the Arrhenius rate constant for peripheral group elimination; and pHe is the probability that the tar fragment ends contain hydrogenated peripheral groups.

The same

hydrogenation kinetics are applied to the hydrogenation of bridges and peripheral groups because their compositions are the same. Once S* is evaluated as the solution of eq. 4a, the probability that a fragment of any size contains a hydrogenated peripheral group is evaluated from

p He =

S*

(4b)

∞

2∑ n j j =1

where the summation in the denominator gives the number of fragment ends in the entire condensed phase.

The tacit assumption is that an aggregate probability for hydrogenated

peripheral groups pertains specifically to metaplast fragments, which is an approximation because recombination affects metaplast but not the other fragment size classes. The ends of fragments that have lost their hydrogenated peripheral groups, I, is determined by J* dI = k G S * −2 p Ie ∑ Γ j dt j =1

(5a)

where pIe is the probability that the tar fragment end has lost a hydrogenated peripheral group. Once I is evaluated as the solution of eq. 5a, the probability that a fragment of any size no longer contains a hydrogenated peripheral group is evaluated as

p Ie =

I

(5b)

∞

2∑ n j j =1

Finally, the impact of hydrogenation on bimolecular recombination is expressed by reducing the recombination rate constant in proportion to the fraction of metaplast fragment ends that contain hydrogenated peripheral groups, according to


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k R → k R (1 − pHe )

(6)

where kR is the Arrhenius rate constant for recombination. If all the ends of metaplast fragments contained hydrogenated ends, recombination would cease; otherwise, the recombination rate is reduced in proportion to the fractions of ends that carry hydrogenated peripheral groups.

Hydrogen Pressures Within Devolatilizing Particles The transport of H2 against an outward flow of volatiles determines the H2 pressure within particles throughout hydropyrolysis. Hydrogen is also released as a primary devolatilization product but mostly during the latest stages after tar production has ended. The contribution of chemically formed H2 is omitted because (i) the H2 partial pressure is only important while labile bridges are present and essentially all bridges are consumed before the end of tar production; (ii) the absolute magnitude of the contribution compared to the ambient H2 pressure diminishes for progressively higher pressure, becoming negligible for pressures above a few megapascals; and (iii) bridge hydrogenation becomes negligible for low ambient H2 pressures where the contribution is appreciable. The system is a porous spherical particle of radius “a” whose temperature is spatially uniform but changes in time. The surroundings contain H2 at pressure pH2∞ but only an insignificant amount of volatiles. The analysis is conducted in terms of molar fluxes, Ni, where i = V, H2, and I denotes volatiles, H2, and inert diluents. The quasi-steady species conservation laws in one radial dimension, r, are 2 1 d (r N V ) = R DVOL (T (t ), t ); dr r2

1 d (r 2 N H 2 ) = − R HP (T (t ), t ); dr r2

N V ( 0) = 0 N H 2 (0) = 0

(7a)

(7b)


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1 d (r 2 N I ) = 0; dr r2

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N I ( 0) = 0

(7c)

where RDVOL is the molar production rate of volatiles per unit gas volume; and RHP is the H2 consumption rate for bridge hydrogenation per unit gas volume. These equations integrate to NI= 0 for all r; NV= rRDVOL/3; and r

NH 2 =

− r RHP 3

RHP =

;

∫r

2

RHP (r )dr

0

(7d)

r

∫ r dr 2

0

where is the mean hydrogenation rate throughout the particle.

Since NI = 0, effective mixture-based diffusivities for volatiles and H2, DVM and DH2M, are introduced into the flux definitions as

N V = − DVM C

dxV + xV ( N V + N H 2 ) dr

N H 2 = − DH 2 M C

dx H 2

dr

(7e)

+ xH 2 ( NV + N H 2 )

(7f)

where C is the aggregate molar gas concentration within particles. These flux definitions are combined with the ones in eq. 7d and rendered nondimensional to obtain the following equations for the concentration profiles:

dxV a2 RDVOL − 2rxV ∆R = −2rRDVOL where RDVOL = dr 6DVM C

and ∆R = RDVOL −

RHP

δ

(8a) dx H 2 − 2r x H 2δ∆R = 2r RHP where dr

RHP =

a 2 RHP and 6 DH 2 M C

δ=

DVM DH 2 M

(8b)


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where the radial coordinate has been scaled on the particle radius. Since RHP is proportional to the mole fraction of H2, as in RHP=k’HPxH2 for nH2 equal unity, the concentration profiles are found by integration to be

x H 2 = xH∞ 2 exp(−(1 − r 2 ) β ) xV =

where β = δ∆R + RHP

[

R DVOL 1 − exp(−(1 − r 2 )∆R ) ∆R

]

(8c) (8d)

and, by subtraction, the inerts concentration is

x I = 1 − xV − x H 2

(8e)

The quantity of greatest interest is the mean H2 concentration within the particle, which is 1

xH 2 =

∫r

2

x H 2 (r )dr

1

∫r

1

= 3x

0

2

dr

∞ H2

∫r

2

exp(− β (1 − r 2 ))dr

(8f)

0

0

which must be evaluated numerically. The reaction rates in these solutions are evaluated as follows:

RDVOL =

dW ρ b0 [1 − (M C − AC ) / 100]] daf MV εG dt

(8g)

where ρb0 is the bulk particle density of the original coal; MV is the mean molecular weight of volatiles; εG is the void fraction; MC and AC are the percentages of moisture and ash in the coal; and dWdaf/dt is the mass loss rate on a dry, ash-free basis. Both MV and dWdaf/dt are predicted by FLASHCHAIN®. The H2 consumption rate is given by

R HP =

ν HP k HP P( B0 − B*) x H 2 (1 − ε G − ε A ) εG

(8h)


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where the rate parameters for hydrogenation were defined previously. The void fractions of gas and ash in these expressions are evaluated with a submodel in FLASHCHAIN® that tracks the densities of organics and minerals throughout devolatilization, and adjusts the densities for the loss of moisture, volatiles, and sulfur, and for swelling based on a swelling factor correlation. Diffusivities are evaluated from the Stefan-Maxwell relations which, for this system, are evaluated from Chapman-Enskog theory based on a nominal molecular weight for volatiles of about 40. Inerts may be specified as N2, He, and Ar.

Incorporation of Ambient H2 Into Devolatilization Products Ambient H2 will be incorporated into tar and char, because these products contain hydrogenated bridges and peripheral groups, and into noncondensable gases as well, because some gases are the decomposition products of hydrogenated peripheral groups. Supporting material S1 presents the relations to track H2 addition to all three product classes.

Char Hydrogasification Char hydrogasification is described with a special version of the Carbon Burnout Kinetics Model called “CBK/G”.9 CBK/G predicts the rate of char conversion, the char particle temperature, and the changes in the particle diameter and bulk density as gasification proceeds, given a complete description of the gasification environment; namely, profiles of gas temperature and radiative exchange temperature, and the partial pressures of H2O, CO2, CO, and H2. Char reactivity is a dynamic function of heat treatment severity, based on a distributed activation energy model of thermal annealing. The theory also tracks the impact of pore diffusion based on an effectiveness factor defined by a Thiele analysis, and a random pore model to account for the loss of surface area with conversion. This standard model of the reaction/diffusion process within porous char


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particles, and a 5-step surface reaction model are used to predict gasification rates over a wide range of conditions, including Zone I (reaction control), Zone II (pore diffusion control), and Zone III (film diffusion control) and their transitional regimes. The code also includes a model of the effect of ash inhibition during the later stages of gasification of high-ash coals. The H2 gasification rate is several orders of magnitude slower than those for CO2 or steam.10 But out of necessity, the char hydrogasification kinetics in CBK/G are oversimplified because so few rate studies on this chemistry have been reported.11-13 Among the reported rate expressions, two11,12 validated first-order kinetics, based on respective activation energies of 126 kJ/mol and a range of 59 to 218 kJ/mol. Li and Sun13 proposed half-order kinetics with an activation energy of 197 kJ/mol. The present study uses a single nth-order rate law in the surface concentration of H2, as follows: RHG = k HG ( p H 2 , S ) n HG

(9)

where kHG is an Arrhenius rate constant; pH2,S is the H2 partial pressure on the char surface; and nHG is the reaction order for hydrogasification. In the rate constant, AHG and EHG were specified to quantitatively interpret reported extents of char conversion, as explained below. In CBK/G, the impact of pore transport is assessed from a conventional Thiele analysis in terms of the ratio of scales for chemical conversion to pore diffusion (Thiele modulus), and the effective diffusivity for pore transport is normally evaluated as the ratio of total porosity to a tortuosity factor. This simplification gives equivalent intraparticle and ambient H2 pressures (pH2,S = pH2∞) and was deemed unsuitable for char hydrogasification of softening coals, because it gave negligible resistance for even the largest char sizes in the validation database. It was replaced with the effective diffusivity from the random pore model developed by Wakao and


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Smith,14 with micropore and transitional pore sizes of 25 and 250 Angstroms, respectively, and micropore voidages that are six times larger than the voidage of transitional pores. These values are suitable for softening coals that lose their original pore systems during devolatilization, and were uniformly applied in all simulations with ranks from subbituminous through mv bituminous in this study. However, the much more open structures in chars from nonsoftening coals were assumed to provide full H2 accessibility to the internal surface area.

Results Parameter Assignments This analysis introduces seven adjustable parameters, three in the distributed-energy rate for bridge hydrogenation, kHP, plus one for the reaction order, nH2; and two in the Arrhenius rate constant for hydrogasification, kHG, plus the reaction order nHG. For all coals, the mean activation energy and std. dev. about EHP were fixed at 20.9 and 2.5 kJ/mol, respectively, and the order was unity. The pseudo-frequency factor was adjusted for each coal sample to fit the reported total and tar yields, and these values are reported in Fig. 8 below. However, most of the validation tests imposed heating rates of at least 103 °C/s, which is too fast for appreciable bridge hydrogenation, so the assigned parameters should be regarded as preliminary values because the enhancements to total and tar yields were usually small. In contrast, the validation data on char hydrogasification covered substantial extents of char hydrogasification and a broad domain of operating conditions, so the assigned activation energy of 81.5 kJ/mol and the order of one-half are more secure. Whereas these values were fixed for all coals, the pseudo-frequency factor for hydrogasification was tuned-in for each sample, and reported in Fig. 8 below. The same enthalpy of hydrogasification was applied to all coal types, based on the hydrogasification of


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pure carbon into methane.

Validation Database and Simulation Protocol The validation database comprises seven datasets compiled in electrically heated wire-mesh reactors (WMRs). Collectively, these datasets represent heating rates from 1 to 103 °C/s; temperatures from 530 to 1080 °C; IRPs from 2 through 185 s; pressures from 0.1 to 15 MPa; and 21 samples representing coal ranks from brown coals through mv bituminous. The measurement uncertainties on tar and total yields are ±2 daf wt. % for the best testing protocols,15 but no better than ±5 daf wt. % for the oldest systems, based on the reported yields for replicate operating conditions. With modern WMRs, thermal histories are unambiguous even under elevated H2 pressures because strictly uniform heating rates to isothermal stages are imposed for prescribed isothermal reaction periods (IRPs). But with older systems,16,17 none of these specifications were uniform or monitored with high accuracy. Notwithstanding, all tests were simulated with the reported heating rates, temperatures, and IRPs. Sample and support temperatures during the hydrogasification stage were equivalent, based on the following quasi-steady enthalpy balance:

(

0 = q HG ∆H HG − h (T p − TG ) − σε p T p − TW 4

4

)

where

h=

Nu λ G dP

(10)

where qHG is the mass consumption rate per unit external surface for the hydrogasification rate, RHG; ∆HHG is the specific hydrogasification enthalpy change; h is the convective heat transfer coefficient; εP is the hemispherical total emittance of the particle; Nu is Nusselt number; λG is the thermal conductivity of the gaseous atmosphere; and dP is particle diameter. The convective heat


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transfer coefficient is evaluated in the Stokes limit, for which Nu equals two for the small sizes in the validation database. For hydrogasification in WMRs the ambient gas and radiation temperatures, TG and TW, are specified as the reported mesh temperature throughout the IRP, assuming that hydrogasification starts at the end of the heating period. The same values for AHP and AHG were used for every simulation with a particular sample. The equation set comprises the original FLASHCHAIN® equations to determine B0 and S0 plus the equations for B* and S* and the associated probabilities pHe, pIe, and pl*/p. Each simulation required less than a second on an ordinary personal computer.

Validation Cases In Figures 1 – 7, the measured values appear as data points and the calculated values appear as curves or line segments. The most distinctive feature of coal hydropyrolysis is apparent in the joint dependence on heating rate and H2 pressure in Figure 1. The various test series swept through four heating rates to 700 °C with 10 s IRP under five H2 pressures with a hv bituminous coal.15 The FLASHCHAIN results correctly exhibit greater weight loss for progressively faster heating rates at the lowest pressure; near-neutrality at the next two intermediate pressures; and lower yields for progressively faster heating rates at both the highest pressures. They accurately depict tar enhancement for faster heating at both lower pressures; insensitivity to heating rate at 1 and 2 MPa; and suppression of tar production for progressively faster heating at the highest pressure. None of the discrepancies for tar yields are greater than 2 daf wt. %. Clearly, the proposed hydropyrolysis mechanisms accurately depict the joint influences of heating rate and pressure variations across the entire domain. The data in Figure 2 evaluate the joint impact of variations in temperature and H2 pressure for


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extended IRPs.18

The predictions for three temperatures in the left panel are generally within

measurement uncertainties, except for the longest time at 750 °C and for intermediate times at 900 °C. These discrepancies reflect a fit to the activation energy for char hydrogasification, in so far as greater energies improve the fit for 900 °C at the expense of the 530 °C case and vice versa. They could be eliminated by incorporating a distribution of activation energies for char hydrogasification, although the dataset in Figure 2 is one of very few with the necessary time resolution. The discrepancies in the right panel are comparable for the longest times for the highest two pressures at 750 °C, yet the bulk of these datasets are accurately described through 100 s IRP. These discrepancies probably do carry theoretical implications in so far as the proposed nth-order hydrogasification rate cannot shift toward zero-order for progressively greater pressures, as would many legitimate surface reaction mechanisms. Note also that at 750 °C, primary devolatilization is finished within 1 s IRP19 and, consequently, nearly all the growth in weight loss for longer IRPs in Figure 2 is due to char hydrogasification. Hydropyrolysis and hydrogasification must be resolved as distinct conversion mechanisms because they act on grossly different time scales. The case in Figure 3 also resolves independent contributions from hydropyrolysis and hydrogasification because the tar yields are only affected by hydropyrolysis, whereas weight loss includes greater contributions from hydrogasification for progressively higher H2 pressures. Due to the rapid heating rate in these tests, the tar yields convey the continuous decay toward a saturation limit for progressively greater pressures that is expected for primary devolatilization under inert atmospheres. The predicted tar yields are within measurement uncertainties across the entire pressure range; indeed, the predicted tar enhancements from hydropyrolysis are negligible throughout, and less than 1 daf wt. % at the highest H2 pressure. In contrast, the


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enhancements to weight loss due to hydrogasification are roughly as large as the absolute tar yields at the highest H2 pressures. Weight loss passes through a minimum near 1 MPa, then increases in direct proportion to the H2 pressure. The predictions accurately depict the minimum and remain within measurement uncertainties of the measured values throughout. The minimum reflects the acute sensitivity to pressure in the tar yields for the lowest test pressures and the positive reaction order for H2 pressure in the hydrogasification kinetics; once the extent of hydrogasification, XHY, exceeds 25 % at 5 MPa, hydrogasification is responsible for all further increases in weight loss. The variable contributions from tar and char hydrogasification products to total weight loss are resolved in a different way in Figure 4. One test series was conducted under pure H2, so that total and H2 pressures were the same across the entire pressure range; in the other, only the partial pressure of H2 was varied in a H2/He mixture at a fixed pressure of 6.9 MPa.16 IRPvalues were only specified within the range of 5 – 20 s, so all simulations used 20 s IRP. At low H2 pressures, the weight loss under pure H2 is 10 daf wt. % greater than that under the H2/He mixtures at 6.9 MPa, then the difference diminishes for progressively higher pressures until it vanishes at 5 MPa. The predictions accurately depict this tendency, even though the predicted extents of hydrogasification are the same for both test series. However, the predicted difference under the H2/He mixtures is about 5 wt. % smaller than the measured difference. The difference in the weight loss values at low H2 pressures directly reflects the suppression of tar yields in the H2/He mixtures at 6.9 MPa, since 6.9 MPa is much greater than the pressure at which the ultimate yield for pyrolysis first reaches its saturation value for progressively higher pressures. For this coal type, the ultimate tar yield at atmospheric pressure would be diminished by 25 – 40 % (relative) in the saturation limit, 20 which is compatible with the 10 daf wt. % measured


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reduction and the 12 % predicted reduction at low pressures in Figure 4. The temperature dependences in the weight loss from a brown coal (LY) and an hv bituminous (MN) are compared in Figure 5 for rapid heating with a 2 s IRP at 7 MPa.21 The only appreciable discrepancy in the predictions is for the lowest temperature with the brown coal, which is due to an excessive value for the std. dev. about the mean energy for bridge decomposition in the primary devolatilization mechanism (as illustrated in Fig. 5 of ref. 7), rather than a problem in the hydropyrolysis chemistry. Otherwise the predictions are fairly accurate for both coals. The most interesting feature is that the predicted extents of char hydrogasification are quite comparable, which is the first indication that the very strong coal quality impact on gasification with steam and CO2 does not pertain to hydrogasification. The joint influences of temperature and reaction time are evaluated in Figure 6 for the same sample in Figure 4. Since the IRPs were substantially different at different test temperatures, neither the data nor the predictions exhibit any monotonic tendency for increasing temperature. Each test ought to be viewed as an isolated trial. The predicted weight loss is within a measurement uncertainty of ±5 daf wt.% in all but three cases, and the discrepancies in these cases are as large as 8 %. All the predictions in this paper are based on the same activation energies for both bridge hydrogenation and hydrogasification for all coal samples, yet they are accurate throughout the entire temperature range in commercial applications. However, the agreement was not nearly as satisfactory for a test series analogous to Figure 6 with a lignite, because there were large discrepancies for three of the ten temperatures in the series. Primary devolatilization under inert gases is unaffected by variations in particle size provided the size is smaller than a few millimeters. In contrast, reported hydrogasification yields diminish for


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progressively larger sizes, as seen in Figure 7 and in another dataset with the same coal.22 This behavior cannot be attributed to any aspect of hydropyrolysis, because the heating rates in these tests were too fast for appreciable hydrogenation in the coal phase. The predictions are within the measurement uncertainties across the entire range based on CBK/G’s transport submodels. CBK/G includes three explicit size dependences in the char conversion analysis: The film diffusion rate; the transport rate through the internal pore system; and an empirical expression that describes how size and density vary throughout a gasification history. Whereas the first and third factors are negligible for the slow hydrogasification kinetics, pore transport resistances do explain the apparent size dependence, provided that the effective diffusivity is evaluated for micro- and transitional pores, as explained above. The interpretation of the coal quality impacts is based on the data from Strugnell and Patrick,21,23 who monitored weight loss, total liquids (as the sum of tar and H2O), and noncondensable gas yields from 15 diverse coals for hydropyrolysis applications. For all 15 coals, weight loss was monitored after heating at 1000 °C/s to 1000 °C with a 2 s IRP under 7 MPa H2. Six of these coals were also tested under 7 MPa He, and three were tested at temperatures from 550 to 1000 °C with IRPs of 2 and 5 s.21 The tar yields from pyrolysis were first estimated from the total liquids yields using the coal-O distributions in CO and CO2, and assuming that the tar-O fraction was 0.30, as seen in measured product distributions for numerous coals.20 Then the increase in the liquids yields during hydropyrolysis was attributed to tar alone to estimate the tar yields for hydropyrolysis. Parameters in FLASHCHAIN were adjusted to fit the reported tar and total yields for the pyrolysis tests for the six coals with both pyrolysis and hydropyrolysis test data. Then the rate


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constant for tar hydrogenation was assigned to match the hydropyrolysis tar yield, and the rate constant for char hydrogasification was adjusted to match the measured total weight loss. These six cases determined a functional form for the coal quality dependence in the frequency factor for bridge hydrogenation which could be applied to the other nine coals. Finally, the frequency factor for char hydrogasification was assigned to match the predicted and measured total weight loss for the other nine coals. Since all total and tar yields from the simulations were fit to the measured values and none of the discrepancies were greater than 1 daf wt. %, they will not be shown. But the assigned parameter values are certainly important, as seen in Figure 8 for all tests in the validation work that gave an extent of hydrogasification of at least 10 %. The frequency factors for bridge hydrogenation fall within a narrow band for ranks from lignite through hv bituminous. For higher ranks, they surge and reach up to four times the mean of the band for lower ranks. This abrupt transition coincides with the near-elimination of oxygen from the labile bridges in coal, which leaves primarily aliphatic, hydroaromatic (naphthenic), and olefinic functional groups.

Indeed, these purely

hydrocarbon linking structures would be more easily hydrogenated than links that contain carboxylic acids, ethers, and esters as well. In contrast, the assigned frequency factors for hydrogasification show no consistent tendency with rank. The most remarkable feature is that the values for nearly all coals lie within 50 % of a mean value. In contrast, the frequency factors for gasification in steam and CO2 vary by as much as two orders of magnitude for coals of the same rank,9 and those for char oxidation vary by up to an order of magnitude.20 Both rates increase for coals of progressively lower rank. However, all the values in Figure 8 express a variation of only a factor of four, and show no consistent trend with rank. This fundamentally different rank independence probably indicates that the


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hydrogasification surface chemistry is not coupled with the surface chemistry for gasification by steam and CO2.

Discussion Elevated H2 pressures can affect both primary devolatilization and char conversion, depending on the processing conditions. However, since hydrogenation chemistry is relatively slow, its impact during devolatilization is governed by the time scale for primary devolatilization during particle heating. Rapid devolatilization at heating rates faster than 103 °C/s does not provide enough time for appreciable bridge hydrogenation so tar yields are not appreciably enhanced under the H2 pressures associated with entrained-flow coal gasification technology. Conversely, tar yields under slow heating conditions are enhanced at elevated H2 pressures. The proposed mechanism for bridge hydrogenation and its associated impact on fragment recombination in FLASHCHAIN depicts these tendencies within measurement uncertainties for H2 pressures to 15 MPa. The predicted yield enhancements due to bridge hydrogenation were modest at even the highest test pressures for rapid heating, but became much more substantial at slower heating rates. Consequently, the proposed reaction mechanisms correctly depict the inversion of the heating rate dependence for the greatest H2 pressures of interest. During primary hydropyrolysis, three independent mechanisms come into play: (1) Inward H2 penetration against the outward flux of volatiles; (2) The direct promotion of bridge scission by bridge hydrogenation; and (3) The suppression of bimolecular recombination by remnants of hydrogenated bridges on fragment ends.

Parametric sensitivity studies resolve the relative

impact of each of these three mechanisms based on the interpretations of the validation data in Figures 1 through 7. Hydrogen fully penetrated the coal particles to achieve the same internal H2


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pressure as the ambient value throughout the entire operating domain in the validations. The only exceptions were for 0.1 and 0.12 MPa H2 for 103 °C/s with hv bituminous coals, for which the worst case was an internal H2 pressure of 0.096 MPa vs. 0.1 MPa in the free stream. All these calculations were made for a particle size of 70µm.

Appreciably lower internal H2

pressures were determined for 104 °C/s over a broad range of H2 pressure. But the lower internal H2 pressures did not affect tar production, because such rapid heating does not provide sufficient contact time for bridge hydrogenation anyway. Long contact times are essential for bridge hydrogenation and, for the assigned kinetics, heating rates faster than about 1000 °C/s do not provide them. Moreover, heating rates of 1000 °C/s or slower do not generate a volatiles flux that is strong enough to exclude H2 from the particle interiors. Whereas hindered H2 penetration does not inhibit bridge hydrogenation under conditions that provide sufficient time for the hydrogenation to occur, it does give lower internal H2 pressures for heating rates faster than about 104 °C/s. Consequently, it is not necessary to accurately describe the evolution of char structure to simulate hydropyrolysis. However, char morphology does appear to factor into the char hydrogasification rate, so char morphology at the end of hydropyrolysis is probably important. The remaining two mechanisms were evaluated from the results for the tests with hv bituminous under 7 MPa H2 for heating rates from 1 to 103 °C/s (cf. Figure 1), with a heating period extended to bring the sample to 900 °C. As seen in Figure 9a, the ultimate tar yields were achieved before the ends of the heating period for all rates except 103 °C/s, and that rate achieved 84 % of the ultimate tar yield.

The dynamic tar yields were examined to determine if

hydrogenation occurred during the interval for maximum tar production.

Direct bridge


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hydrogenation is apparent in Fig. 9b as increases in the average number of H-atoms per bridge which, in turn, increases the scission selectivity coefficients in Fig. 9c and thereby promotes bridge scission and the associated accumulation of tar precursors.

The indirect impact on

bimolecular recombination is apparent in Fig. 9d as the reduction in the bimolecular recombination rate compared to the rate during primary devolatilization without H2 for the same thermal history and pressure. Both the direct and indirect impacts of bridge hydrogenation are strong for 1°C/s, and both are synchronized with tar production. The average H/bridge in Fig. 9b increased from 13 to 25, which increased the fraction of bridge decompositions that break bridges in Fig. 9c from the initial value for these coal properties, 0.41, to the maximum value of 0.71 for this coal. In other words, hydrogenation skewed bridge decomposition as far as possible toward scission, and the hydrogenation kinetics did not limit the impact. Similarly, the recombination rate in Fig. 9d was reduced to 10 % of the original value by interference from hydrogenated fragment ends, and this minimum arose immediately after the maximum in the tar production rate. Conversely, both these impacts nearly vanished for 103 °C/s, because the hydrogenation kinetics are too slow to keep pace with this thermal time scale. The average H/bridge increased to only 13.5, which hardly perturbed the selectivity toward bridge scission from its coal-based initial value. And the recombination rate remained at its original value throughout tar production as well. Both of the other heating rates sustained hydrogenation effects that were between the two extremes, as were the associated tar yields. Calculations without any effect of hydrogenation on the recombination rate were run to isolate the impact on the selectivity for bridge scission. Without the diminished recombination rate, the


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tar yield decreased from 21.4 daf wt. % to 18.9 %. This change accounts for 24 % of the total enhancement due to hydrogenation, which increased the tar yield from 11 to 21.4 %. Hence, both the impacts from hydrogenation are substantial under the heating conditions that provide sufficient contact time to appreciably enhance the tar yields. Generally speaking, the theory exposes the term “flash hydropyrolysis” as the oxymoron that it is. Rapid or “flash” heating is incompatible with appreciable enhancements of primary tar yields during hydropyrolysis because the slow hydrogenation chemistry cannot keep pace with the relatively very fast time scale for primary devolatilization. Since nominal rates for primary devolatilization increase in rough proportion to increases in heating rate,20 and since primary devolatilization chemistry is spontaneous, hydrogenation can only affect the distribution of primary products under slow heating conditions, because only slow heating enables bridge hydrogenation to occur before the bridges decompose through the spontaneous channels. At moderate-to-fast heating rates, tar yields diminish for progressively higher H2 pressures, while weight loss either remains steady or passes through a minimum at some H2 pressure around 1 MPa. Higher H2 pressures promote more extensive char hydrogasification, which counteracts the lower weight loss associated with lower tar yields.

A single, one-half-order

hydrogasification reaction within CBK/G accurately interpreted a database representing ranks from lignite through anthracite; pressures to 15 MPa; heating rates from 1 to 103 °C/s; temperatures from 550 to 1150 °C; reaction times to 180 s; and particle diameters to 1 mm.

Conclusions 1) Coal hydropyrolysis occurs through three main conversion channels: (i) Hydrogenation


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of labile bridges and peripheral groups, which skews bridge conversion toward scission and away from spontaneous conversion into char links; (ii) Suppression of bimolecular recombination by hydrogenated peripheral groups, which indirectly enhances the amounts of tar precursors; and (iii) a one-half order char hydrogasification reaction. The first two channels can only occur before bridges are converted by the normal, spontaneous channels in the primary devolatilization mechanism. The third occurs on much longer time scales and, consequently, char hydrogasification must be explicitly resolved from the bridge hydrogenation chemistry. 2) The time scale for spontaneous primary devolatilization determines whether the hydrogenation chemistry affects tar yields during hydropyrolysis. Since nominal devolatilization rates increase in rough proportion to increases in heating rate, hydropyrolysis tar yields are only enhanced under slow heating conditions, and tar and total yields diminish for progressively faster heating rates under the highest H2 pressures. 3) Assigned frequency factors for bridge hydrogenation during devolatilization were uniform for ranks from lignite through hv bituminous, then surged for low volatility coals. This abrupt transition coincides with the near-elimination of oxygen from the labile bridges in low volatility coals, which leaves them almost entirely composed of aliphatic, hydroaromatic (naphthenic), and olefinic functional groups. 4) A single one-half order hydrogasification reaction within the CBK/G framework accurately depicts the influences of temperature, contact time, H2 pressure, and particle size on total weight loss across the entire domain of operating conditions.

The same activation energy was

implemented for all operating conditions and coal types. The assigned frequency factors for hydrogasification are far less variable than those for gasification by steam and CO2, and show no


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consistent trend with rank.

Nomenclature a

Particle radius, m

A

Scaled molar concentration of aromatic nuclei in coal

AC

Ash content of coal, wt. %

AHG

Pseudo-frequency factor for char hydrogasification, s-1

Ai

Pseudo-frequency factor in a reaction involving structural component i, s-1

A0

Molar concentration of aromatic nuclei in coal, moles/cm3-coal

B

Scaled molar concentration of labile bridges in coal

B*

Scaled molar concentration of hydrogenated bridges in coal

B0

Scaled molar concentration of labile bridges during coal pyrolysis

C

Scaled molar concentration of char links in coal

CH2

Molar concentration of H2 in the condensed coal phase, moles/m3

DIM

Mass diffusivity of volatiles and H2 for I = V, H2 in a gas mixture, m2/s

dWdaf

dt

Mass loss rate on a dry, ash free basis, daf wt. %/s

EHG

Activation energy for char hydrogasification, kJ/mol

Ei

Activation energy in a reaction involving structural component i, kJ/mol

h

Convective heat transfer coefficient, cal/m2-s

HH2

Henry’s Law constant for H2 in the condensed coal phase

∆HHG Exotherm for char hydrogasification, J/g I

Scaled molar concentration of fragments previously capped by a hydrogenated peripheral group

j

Index for the degree of polymerization in coal macromolecules and tar

J*

Maximum extent of depolymerization for metaplast and primary tar molecules

kHG

Rate constant for char hydrogasification, s-1MPa-1


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kHP’

Rate constant for bridge hydrogenation which incorporates HH2, mole/MPa-s

ki

Rate constant for a reaction involving structural component i, s-1

kR

Rate constant for bimolecular recombination, s-1

MC

Moisture content of coal, wt. %

MV

Mean molecular weight of volatiles, g/mol

Page 32 of 45

MWi Molecular weight of species or component i, g/mol nHG

Reaction order for char hydrogasification

nH2

Reaction order for bridge hydrogenation

NI

Molar flux of volatiles, H2, and inerts for I = V, H2, I, moles/m2-s

e

Number of atoms of element e in structural component i

e

Ni0

Number of atoms of element e in structural component i in a parent coal

nj

Scaled molar concentration of fragments in the condensed coal phase

Nu

Nusselt number for heat transfer to a coal particle

p

Probability for any type of connection among nuclei in coal fragments

P

Ambient pressure, MPa

pHe

Probability that a fragment is capped by a hydrogenated peripheral group

pH2

Partial pressure of H2 within reacting coal particles, MPa

pH2,S

Surface pressure of H2 for char hydrogasification, MPa

ptk

Probability for any type of connection among nuclei in tar k-mers

pl*

Probability for an hydrogenated bridge among nuclei in coal fragments

ptkl*

Probability for an intact hydrogenated bridge among nuclei in tar k-mers

pHe

Probability for an intact hydrogenated peripheral group on the end of fragments

qHG

Char gasification rate per unit external surface, kg/m2-s

r

Radial position within a particle, m

Ni

RDVOL Volatiles release rate, moles/s RHP

Molar bridge hydrogenation rate, moles/s

r

Nondimensional radial position


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RDVOL Nondimensional volatiles release rate ∆R

Nondimensional net gas generation rate

S

Scaled molar concentration of peripheral groups in coal

S*

Scaled molar concentration of hydrogenated peripheral groups in coal

S0

Scaled molar concentration of peripheral groups during coal pyrolysis

TI

Temperature of a coal sample, ambient gas, and radiation source for I = P, G, W, in K

t

Time, s

tj

Tar fragment with j monomer units

xH2∞

Mole fraction of H2 in the ambient gases

xI

Mole fraction of volatiles, H2, and inerts for I = V, H2, I, moles/m2-s

Greek Symbols δ

Ratio of diffusivities for volatiles to H2

εA

Volume fraction of mineral matter in coal

εG

Void volume fraction in coal

εp

Hemispherical total emittance of coal

Γj

Molar release rate of tar j-mers

λG

Mean thermal conductivity of the ambient gas, cal/m-s-K

λH2

Solubility coefficient for H2 in a coal phase, Nl-H2/kg-MPa

νB

Scission selectivity coefficient for bridge conversion in coal

νHP

Stoichiometric coefficient for H2 consumption during bridge hydrogenation

ρb

Bulk particle density of coal, kg/m3

ρL

Hydrocarbon liquid density, kg/m3

ρORG

Density of condensed coal phase, kg/m3

ρORG0 Initial density of condensed coal phase, kg/m3 ρ0

Bulk coal density, kg/m3

σ

Stefan-Boltzmann constant, cal/m2-s-K


33

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 45

Subscripts B

Labile bridges

HG

Char hydrogasification

H2

Hydrogenation

I

Inerts in the ambient gas atmosphere

P

Coal particle

S

Peripheral groups

tj

Tar j-mer

References (1)

Howard, J. B. Fundamentals of coal pyrolysis and hydropyrolysis, In Chemistry of Coal

Utilization, 2nd Suppl. Vol.; Elliot, M. A., Ed.; Wiley-Interscience: New York 1981; Ch. 12. (2)

Suuberg, E. M. Mass transfer effects in pyrolysis of coals: A review of experimental

evidence and models, In Chemistry of Coal Conversion; Schlosberg, R. H., Ed.; Plenum: New York, 1985; Ch. 4. (3)

Niksa, S. Interpreting coal conversion under elevated H2 pressures with FLASHCHAIN

and CBK, Proc. 2011 Int. Conf. on Coal Science and Technol., IEA, Oviedo, Spain, 2011. (4)

Guan, Q., Dapeng, B., Xuan, W., Zhang, J. Kinetic model of hydropyrolysis based on the

CPD model, Fuel 2015, 152, 74-79. (5)

Eklund, H., Wanzl, W. Pyrolysis and hydropyrolysis of solid fuels at high heating rates

using Curie point technique, Int. Conf. Coal Sci., IEA, 1981, p. 701-707.


34

Page 35 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(6)

Niksa, S., Kerstein, A. R. Flashchain theory for rapid coal devolatilization kinetics. 1.

Formulation. Energy Fuels 1991, 5(5), 647-64. (7)

Niksa, S. Flashchain theory for rapid coal devolatilization kinetics. 4. Predicting ultimate

yields from ultimate analyses alone. Energy Fuels 1994, 8, 659-70. (8)

Korsten, H., Hoffman, U. Three phase reactor model for hydrotreating in pilot trickle-bed

reactors, AIChE J. 1996, 42(5), 1350-60. (9)

Liu, G.-S., Niksa S. Coal conversion submodels for design applications at elevated

pressures. Part II. Char gasification, Prog. Energy Combust. Sci. 2004, 30(6), 697-717. (10) Harris, D. J., Patterson, J. H. Use of Australian bituminous coals in IGCC power generation technologies, Aust. Inst. Energy J. 1995, 13, 22. (11) Blackwood, J. D. The reaction of carbon with hydrogen at high pressure, Aust. J. Chem. 1959, 12, 14. (12) Tomita, A., Mahajan, O. P., Walker, P. L., Jr. Reactivity of heat treated coals in hydrogen, Fuel 1977, 56, 490. (13) Li, S., Sun, R. Kinetic studies of a lignite char pressurized gasification with CO2, H2, and steam, Fuel 1994, 73, 413. (14) Wakao, N., Smith, J. M. Diffusion and reaction in porous catalysts, Ind. Eng. Chem. Fundam. 1964, 3, 123.


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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

(15) Guell, A. J., Kandiyoti, R. Development of a gas-sweep facility for the direct capture of pyrolysis tars in a variable heating rate high-pressure wire mesh reactor, Energy Fuels 1993, 7, 943. (16) Anthony, D. B., Howard, J. B., Hottel, H. C., Meissner, M. P. Rapid devolatilization of bituminous coal, Fuel 1976, 55, 121-28. (17) Suuberg, E. M., Peters, W. A., Howard, J. B. Product composition and kinetics of lignite pyrolysis, Ind. Eng. Chem. Process Des. Develop. 1978, 17, 37. (18) Heyd, L. E. Weight loss behavior of coal during rapid pyrolysis and hydropyrolysis, M. S. thesis, Princeton Univ., Princeton, NJ, 1982. (19) Niksa, S., Russel, W. B., Saville, D. A. Time-resolved weight loss kinetics for the rapid devolatilization of a bituminous coal, Proc. Combust. Inst. 1982, 19, 1151-57. (20) Niksa, S., Liu, G.-S., Hurt, R. H. Coal conversion submodels for design applications at elevated pressures. Part I. Devolatilization and char oxidation, Prog. Energy Combust. Sci. 2003, 29(5), 425-477. (21) Strugnell, B., Patrick, J. W. Rapid hydropyrolysis studies on coal and maceral concentrates, Fuel 1996, 75, 300-06. (22) Suuberg, E. M. Rapid pyrolysis and hydropyrolysis of coal, Sc. D. thesis, MIT, Cambridge, MS, 1977. (23) Strugnell, B., Patrick, J. W. Hydropyrolysis yields in relation to coal properties, Fuel 1995, 74, 481-86.


36

Page 37 of 45

30

Tar Yield, daf wt. %

7.0

60

W eight Loss, daf wt.%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2.0 50

1.0 0.25

Linby HV Bit. 10s @ 700°C

0.1 MPa

0.25 25

1.0 20

2.0

40

7.0 15

0.1 MPa Linby HV Bit. 10s @ 700°C 30 1

10

100

1000

10 1

Heating Rate,°C/s

10

100

1000

Heating Rate,°C/s

Figure 1. Evaluation of (Left) weight loss and (Right) tar yields for various heating rates to 700 C with 10 s IRP under () 0.1 MPa He or H2 pressures of () 0.25, () 1, () 2, and () 7 MPa.14


Energy & Fuels

70

900°C

70

6.80 MPa

750°C

W eight Loss, daf wt. %

60

W eight Loss, daf wt.%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

50

40

530°C 30

60

3.40 MPa

50

1.10 MPa H2 40

Ill.#6, 127m 3.40 MPa H2, 1000°C/s

Ill.#6, 127m 1000°C/s, 750°C

20

30 0

25

50

75

100

125

150

175

200

0

25

Time After Heatup, s

50

75

100

125

150

175

Time After Heatup, s

Figure 2. Evaluation of weight loss from hv bituminous coal (Left) under 3.40 MPa H2 for variable IRPs after heating at 103 C/s to 900 (&dotted curve), 750 (&solid curve), and 530 C (&dashed curve) and (Right) under 6.80 MPa H2 ( & dashed curve), 3.40 ( & solid curve), and 1.10 ( & dotted curve) for heating at 103 C/s to 750 C with variable IRPs.17


200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Yields, daf wt.% & Char Conversion, %

Page 39 of 45

Pit. #8 hv bit. 3 10 °C/s to 700°C w/10s IRP

60

Weight Loss

40

XHY 20

Tar Yield

0 0

2

4

6

8

10

12

14

16

H2 Pressure, MPa

Figure 3. Evaluation of weight loss and tar yields from hv bituminous coal for heating at 103 C/s to 700 C with 10 s IRP under variable H2 pressures.15 The predicted extents of char hydrogasification appear as the dashed curve.


Energy & Fuels

50

75 70

60

Pure H2

30

55

H2/He 6.9 MPa

50 20 45 40

Pit.#8, 70 m 750°C/s, 1000°C, 5-20 s IRP

35

Char Conversion, %

40

65

Weight Loss, daf wt.%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

10

0

30 0

1

2

3

4

5

6

7

H2 Partial Pressure, MPa

Figure 4. Evaluation of weight loss and tar yields from hv bituminous coal for heating at 750 C/s to 1000 C with 5-20 s IRP under variable H2 pressures.16 One series (&solid curves) had equal total and H2 pressures, whereas the other (&dashed curves) had variable H2 pressures under 6.9 MPa total pressure in He/H2 mixtures. The predicted extents of char hydrogasification appear on the right ordinate.


Page 41 of 45

50 70

7 MPa H2, 1000°C/s, 2s IRP 40

LY 30 50

MN 20 40

LY

20 500

10

MN

30

Char Conversion, %

60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0 600

700

800

900

1000

Temperature,°C

Figure 5. Evaluation of weight loss vs. temperature from a brown coal (LY) and hv bituminous (MN) for heating at 103 C/s to various temperatures with 2 s IRP under 7.0 MPa H2.21 Extents of char hydrogasification appear on the right ordinate.


Energy & Fuels

85

80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

Pit.#8, 70m 6.8 MPa H2, 1000°C/s, 6-20 s IRP

75

70

65

60

55 800

850

900

950

1000

1050

1100

Temperature,°C

Figure 6. Evaluation of weight loss from hv bituminous for heating at 1000 C/s to various temperatures with 6 - 20 s IRP under 6.8 MPa H2.22


Page 43 of 45

70

Pit.#8, 6.9 MPa H2 750°C/s, 1000°C, 5-20 s IRP

65


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60

55

50

45 0

200

400

600

800

1000

Particle Diameter, m

Figure 7. Evaluation of weight loss from different size cuts of hv bituminous under 6.90 MPa H2 for heating at 750 C/s to 1000 C with IRPs from 5 – 20 s.16


Energy & Fuels

0.40 0.35

15000

7 MPa H2, 1000°C/s, 1000°C, 2s IRP 12500

0.30 10000

AHG, atm s

-0.5 -1

0.25

A HY, s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 45

0.20 0.15

7500

5000

0.10 2500

0.05 0.00 65

70

75

80

85

90

0 65

70

Carbon Content, daf wt. %

75

80

85

90

Carbon Content, daf wt. %

Figure 8. Assigned pseudo-frequency factors for (Left) bridge hydrogenation and (Right) char hydrogasification for () 15 diverse coals heated at 1000 C/s to 1000 C with 2 s IRP under 7 MPa H2,23 and () for the six coals in the validations in Figures 1 – 7.


(a)

(b)

20

25

20

1°C/s 10

15

15

100 1000

10

10

5

5

0 1.0

0 1.0

Pit. #8, 7 MPa H2

(c)

0.9

0.8

0.8 0.6 0.7 0.4 0.6

0.2

Scission Selectivity

Tar Yield, daf wt.%

25

kR/kR,0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

H-No. in Bridges

Page 45 of 45

0.5

(d)

0.4

0.0 0

100

200

300

400

500

600

700

800

900

0

100

200

Temperature,°C

300

400

500

600

700

800

900

Temperature,°C

Figure 9. In clockwise order from upper left, (a) tar yields; (b) number of H-atoms per bridge; (c) scission selectivity coefficient; and (d) ratio of modified to original rate constant for recombination for hv bituminous coal heated at 4 heating rates to 900 C under 7 MPa H2. The x-axes show temperature during heatup.


FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 10

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