Coal and biomass char reactivities in gasification and combustion environments

Combustion and Flame xxx (2015) xxx–xxx

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Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e

Coal and biomass char reactivities in gasification and combustion environments Matthew B. Tilghman ⇑, Reginald E. Mitchell High Temperature Gasdynamics Laboratory, Mechanical Engineering Department, Stanford University, Stanford, CA 94305, United States

a r t i c l e

i n f o

Article history: Received 20 February 2015 Received in revised form 6 May 2015 Accepted 6 May 2015 Available online xxxx Keywords: Gasification Combustion Intrinsic reactivity Coal Biomass Char

a b s t r a c t Mass loss data obtained in gasification and combustion tests in a thermogravimetric analyzer were used to adjust parameters in an intrinsic chemical reactivity model developed to predict char conversion rates and off-gas compositions when pulverized coal and biomass char particles are exposed to reactive gases. Char reactivity tests were performed in H2O/H2/N2 environments in order to obtain data to determine kinetic parameters for char intrinsic reactivity to H2O, in CO2/CO/N2 environments in order to obtain data to determine kinetic parameters for char intrinsic reactivity to CO2, and in O2/N2 environments in order to obtain data to determine kinetic parameters for char intrinsic reactivity to O2. Wyodak coal and corn stover chars produced at high heating rates were used in the reactivity tests, which were performed under kinetics-controlled conditions. Values determined for the heats of formation and absolute entropies of species adsorbed onto the carbonaceous surfaces as well as the values determined for activation energies of reaction rate coefficients are in the expected ranges for the type reaction considered. The reaction mechanism correctly describes the impact of temperature, H2O, CO2 and O2 mole fractions, and total pressure on the conversion rates of coal and biomass chars and it captures the inhibiting effects of H2 and CO on char reactivities to H2O and CO2 over the ranges of temperature, pressure and gas composition relevant to coal and biomass gasifiers and combustors. The heterogeneous reaction mechanism developed accurately predicts the effects of heterogeneous reaction in gasification and combustion environments, including oxy-combustion environments. Ó 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction The use of coal continues to grow, as do concerns over pollutant emissions from coal-fired power plants and the impact of the emissions on climate change. Such concerns have accelerated research into clean coal technologies, like gasification and oxy-combustion. Coal gasification provides more options for gas cleanup than combustion, and involves coal conversion in environments at high temperatures and often, elevated pressures, with the carbonaceous char surrounded by high concentrations of the product gases CO and H2, species that inhibit char reactivity. Coal oxy-combustion yields a product stream with nearly capture-ready CO2, and involves coal conversion in environments with CO2 as the primary diluent (instead of N2, the traditional diluent) at temperatures high enough that the CO2 gasification reactions impact the char conversion process. Both coal gasification and coal oxy-combustion technologies make use of environments that correspond to a gap in our predictive capabilities.

⇑ Corresponding author.

Many previous studies have been undertaken to characterize the kinetic mechanisms associated with the steam and dry char gasification reactions, C + H2O M CO + H2 and C + CO2 M 2CO, respectively, as well as the char combustion reactions C + O2 M CO2 and C + 1/2O2 M CO. Of concern in the gasification studies have been the kinetics of key reaction paths at atmospheric [1–13] and elevated pressures [14–21], the inhibiting effects of H2 and CO on carbon conversion rates [15,22,23,9,24–29], the reductions in carbon conversion rates due to heat treatment [30], and the catalytic effects by the types of minerals present in coals [31–33]. Efforts to characterize the kinetic mechanism associated with the char combustion reaction have also been undertaken, with the temperature dependence of the key reaction pathways for CO and CO2 formation [34–45] and carbon deactivation [46–48] being of primary concern. To date, however, no char kinetic mechanism has been proposed that accurately predicts the combined effects of temperature, pressure, and high concentrations of CO2, CO, and H2 (i.e., environments typical of entrained flow gasifiers and oxy-combustion reactors), on char reactivity. Consequently, a study was undertaken to correct this deficiency in our predictive

http://dx.doi.org/10.1016/j.combustflame.2015.05.009 0010-2180/Ó 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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capability. In light of previously published results and based on the results of reactivity tests performed in our laboratory, a reaction mechanism was developed that accurately describes coal and biomass char conversion rates in environments containing reactive gases (O2, CO2, CO, H2O, and H2) at the temperatures and pressures expected in advanced energy conversion systems. 2. Experimental setup The coal selected for study was Wyodak coal, a low-ash, low-sulfur sub-bituminous coal from Wyoming. It is typical of the coals from the Powder River Basin that are currently being used for electric power generation in the United States. Corn stover, a common agricultural waste-product in areas where corn production is high, was selected as the primary biomass material for study. Corn stover contributes to the approximately billion tons of raw lignocellulosic biomass materials that could be used as fuels over the next 25–50 years. Proximate and ultimate analyses for the fuels, performed by Hazen Research, are shown in Supplement Tables 1 and 2, respectively, of the supplementary material provided online. The raw materials were ground, sieved and classified to obtain particles in the 75–106 lm size-range for testing. The size distributions were also measured, and in some cases deviate from the 75–106 lm sieve size, though in all cases are small enough to ensure the data obtained in the reactivity tests were not influenced by the transport of reactive gases to the outer surfaces of the particles or through the particles’ pores. The data reflect only the impact of chemical kinetic effects. The size distributions can be found in the supplementary information. 2.1. Char production and characterization The size-classified coal and biomass particles were converted into char particles in our flamelet-based, laminar entrained flow reactor. The flow rates of the reactor feed gases (CH4, H2, O2, and N2) were adjusted to provide a stable, rich flame having an adiabatic temperature near 1700 K. The reaction products consisted primarily of H2, CO, H2O, CO2, and N2, typical gasifier gases. The flow rates of the reactor input gases used when producing the chars for this study are presented in Table 1 along with the calculated composition of the post-flame gases, assuming a constant pressure/constant enthalpy combustion event. Note that the O2 and O concentrations are in the parts-per-billion range; char oxidation is negligible. Measured temperatures along the centerline of the reactor where particles were injected ranged from 1560 K near the start of biomass devolatilization to 1545 K near the end of coal devolatilization. Temperatures were lower than the calculated adiabatic flame temperature (1666 K) owing to the small flow of room-temperature nitrogen needed to advect the particles into the flow along the reactor centerline. Selected amounts of coal and biomass particles were injected at the base of the reactor, along its centerline. The particles passed through the flame and flowed upwards in the flow reactor, heating up and devolatilizing in the process. Particle heating rates were calculated to be greater than 104 K/s, similar to the heating rates devolatilizing coal particles experience in real commercial

entrained flow gasifiers. A more detailed description of the laminar flow reactor used in this work can be found in the thesis of Campbell [45], p. 49. Particles were extracted from the reactor using a water-cooled, helium-quenched solids sampling probe. The probe was positioned approximately 1.5 in. above the burner surface, a height at which devolatilization of the coal and biomass particles was essentially complete. This position in the reactor was determined by successively increasing the sampling probe height until the measured mass loss no longer changed significantly with probe position. Since char reactivity is quite slow compared to devolatilization rates in reducing environments, this method of determining the end of devolatilization is quite reasonable. Once the particles entered the solids sampling probe, they were promptly quenched with helium to prevent further reaction. The char particles were collected on glass-fiber filter paper, dried, weighed, and then stored in an oxygen-free environment until used in experiments. Results indicated that during devolatilization, 57% of the mass of the Wyodak coal was loss and 86% of the mass of the corn stover, a somewhat larger mass loss percentage for the corn stover than would be suggested by the proximate analysis. Since volatile yields increase with increasing devolatilization temperatures, it is not surprising that the volatile yields observed in the flow reactor are higher than the volatile matter content as determined in the standard proximate analysis. It appears that the higher oxygen level of the corn stover may have played a role in producing a higher quantity of tars and such light gases as CH3OH and HCOOH (species that coals do not produce), which escaped the particle before reattachment to the carbonaceous matrix via secondary reactions. It must be admitted that the error in the mass loss measurements during devolatilization of the corn stover are somewhat higher than with the Wyodak coal owing to the increased unaccounted for mass loss as sticky corn stover char particles (a consequence of tars condensing on particles during cooling in the sampling probe) adhered to the sampling probe walls and filter paper. Size distributions were also measured for the post-devolatilization chars. The mean size of the Wyodak coal char particles was 74 lm, about 14 lm smaller than that of the raw coal particles and the mean size of the corn stover char particles was 42 lm, about 16 lm less than the mean size of the raw corn stover particles. These data can be found in Supplement Figs. 1 and 2. 2.1.1. Char apparent density and specific surface area Apparent densities for the coal and biomass chars were measured using a tap density technique, a technique that involves placing a measured amount of the char in a small, vertically oriented graduated cylinder and noting the total volume occupied after repeatedly tapping the outside of the cylinder until the particles are well packed. Additional amounts of char are added to the cylinder and the total volume occupied noted after continued tapping. A plot of the data, mass versus volume occupied, yields the bulk density of the packed bed of particles. Employing a void volume fraction in the range 0.30–0.33 for spherical particles, [49], the apparent density of the char was calculated from the measured bulk density. An apparent density of 0.56 ± 0.012 g/cm3 was

Table 1 Reactor inlet flow rates and equilibrium composition of the reactor gases. Burner input N2: 39.9 lpm

O2: 6.0 lpm

Equilibrium post-flame mole fractions XN2 = 7.46e01 XH2O = 1.63e01 XN = 2.38e05 XOH = 3.62e07

CH4: 2.2 lpm

XH2 = 5.04e02 XO2 = 2.03e09

XCO = 2.07e02 XO = 1.39e09

H2: 7.0 lpm

XCO2 = 2.04e02 XCH4 = 1.19e11

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determined for the Wyodak coal char particles and 0.26 ± 0.006 g/cm3 for the corn stover char particles. Tests were undertaken in our CahnÒ TGA-151 Pressurized ThermogravimetricAnalyzer (P-TGA, see Campbell [45] for more information) to determine the mass specific surface areas of the Wyodak coal and corn stover char particles. The method of Brunauer, Emmett and Teller [50] was employed, yielding what is referred to as BET surface areas. The gas adsorption experiments were carried out in the TGA at room temperature and a pressure of 10 atmospheres, using CO2 as the adsorption gas. The initial mass specific surface area (Sg,0) determined for the Wyodak coal char was 475 m2/g; a value of 453 m2/g was determined for Sg,0 for the corn stover char. The mass specific surface areas of particles vary with conversion. We employ the surface area model developed by Bhatia and Perlmutter [51] to follow such variations during the gasification process. The expression developed is presented in Eq. (1)

Sg ¼ Sg;0

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1  w lnð1  xc Þ

ð1Þ

Here, xc is the fractional conversion of the carbonaceous material and w is a char structural parameter that depends on its morphology (pore size distribution, porosity, tortuosity, etc.). A least-squares procedure was used to determine this structural parameter from specific surface area measurements made during the course of gasification. Shown in Fig. 1 are surface area data obtained during the course of gasification of the Wyodak coal and corn stover chars exposed to 100% CO2 at 800 °C in the TGA. As noted, the mass specific surface areas of the chars increase during char conversion. The values of w that yielded calculated mass specific surface areas via Eq. (1) that best agree with the measurements were found to be similar (w = 8 for the Wyodak coal char and 7 for the corn stover char), suggesting similar pore structures for the coal and biomass chars. The properties determined for the raw Wyodak coal and corn stover chars particles are summarized in Table 2. 2.2. Char reactivity studies The Wyodak coal char and the corn stover char produced from the entrained flow reactor were subjected to tests in the TGA in order to determine their reactivities in a range of conditions that rendered kinetics-limited conversion rates in environments containing mixtures of H2O, H2 and N2, mixtures of CO2, CO and N2, and mixtures of O2 and N2. The different mixtures provided data that permitted the inhibiting effects of CO and H2 on char reactivity to be quantified as well as provided targets for adjusting kinetic and thermochemical parameters for the char-H2O, char-CO2, and char-O2 reaction mechanisms, respectively.

Table 2 Wyodak coal char and corn stover initial char particle properties. Property

Wyodak coal char

Corn stover char

Average size (lm) Apparent density (g/cm3) Surface area (m2/g) Structural parameter, w

74 0.56 475 8

42 0.26 453 7

In the char gasification tests, room-temperature nitrogen was used to purge air from the test chamber after a measured amount (20 mg) of char had been placed in the TGA balance pan. The nitrogen was allowed to flow through the test chamber for 60 min, allowing steady state conditions to be established. After this dwell period, the chamber was heated to the desired reaction temperature, at a rate of 15 K/min. It was then held at this reaction temperature, with nitrogen still flowing, for two hours, allowing the chamber to reach steady state conditions once more. This procedure assured a clean char sample prior to testing as well as assured that observed mass loss at the desired reaction temperature was not influenced by the release of any residual volatiles in the char. After this second dwell period at the desired reaction temperature, the test-gas was admitted to the reaction chamber. During the entire procedure, weight loss was monitored at 5-s intervals over a period of four hours or until conversion was greater than about 90%. In order to input steam into the TGA, the nitrogen diluent stream that enters the TGA was diverted through a water reservoir, which was fed liquid water by a low flow-rate pump. The reservoir was heated with a furnace; the heating element of the furnace was tuned so that the observed exit temperature corresponded to the vapor saturation pressure that yielded the steam concentration desired to study. The steam concentrations studied in this work were approximately 10% and 20%, by volume (remainder nitrogen), corresponding to saturated streams at 47 °C and 60 °C, respectively. The maximum steam concentration for testing was limited in order to prevent condensation at the location where the room temperature purge gas mixes with the test gas. To minimize mass transport effects during the reactivity tests, besides using small particle sizes, particles were scattered over the balance pan of the TGA, rendering a thin bed through which gases must diffuse. Repeated tests with varying amounts of char indicated that in tests performed with less than about 20 mg of char placed in the pan, mass transport effects were insignificant. In all of the tests performed, 20 mg or less of sample was used. In order to account for drag and buoyancy forces that act on the TGA balance pan within the reaction chamber, for each test-gas

Fig. 1. Mass specific surface area variations during gasification of Wyodak coal char (left) and corn stover char (right) in 100% CO2 at 800 °C.

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mixture used, a test was run when no sample was placed in the balance pan. Subtraction of measurements from runs with and without the sample yields drag- and buoyancy-corrected weight loss measurements. The drag- and buoyancy-corrected thermograms (mass versus time profiles) were numerically differentiated, providing the mass loss rate, dm/dt, as a function of time. These profiles were manipulated to give dm/dt versus extent of conversion and finally, the specific mass loss rate, 1/m  dm/dt, versus extent of conversion in the environment established in the TGA reaction chamber. 3. Modeling studies 3.1. Heterogenous reaction mechanism Char reactivity is modeled by adsorption/desorption reaction mechanisms developed for the char-CO2, char-H2O and char-O2 reaction systems. The mechanism is based on the turnover concept most recently discussed by Haynes and co-workers [43,44], wherein the carbon atoms that desorb from the carbonaceous matrix (creating gas-phase species) expose underlying carbon atoms that become free carbon sites, carbon sites available for adsorption of gas-phase species. It is assumed that there is a distribution of binding energies holding adsorbed species onto carbonaceous surfaces. Our approach uses Arrhenius parameters to determine the reaction rate coefficients for the forward reactions, while the rate coefficients for the reverse reactions are determined via the enthalpies of formation and absolute entropies for the adsorbed species present in the mechanism. The enthalpies of formation and absolute entropies permit the evaluation of equilibrium constants for the reactions in the mechanisms, providing a path for the calculation of reaction rate coefficients for the reverse reactions. This method also permits that the enthalpies of formation and absolute entropies of the adsorbed species may be constrained to predict the correct thermodynamic equilibrium for both the C–CO2 and C–H2O systems. In our char reactivity model, one of the parameters that characterizes a particular char is its site density, Sd, the number of sites per unit surface area of char (#sites/m2). In our approach, a value

for Sd of 6.5  1019 sites/m2 was used for all chars, a value calculated from the interatomic spacing of graphite, and within the experimentally observed range of site densities for activated carbons [52]. Sd was taken to be the same for both chars while thermochemical and kinetic parameters were varied for each of the chars. It is assumed that there is only one chemisorbed species per site and that the concentration of chemisorbed species Xi ([Xi], in mol/m2) is given by

½X i  ¼ ðSd =NAV Þhi =ri

ð2Þ

where NAV is Avogadro’s number, hi is the site fraction (the fraction of the total sites occupied by chemisorbed species Xi), and ri is the number of sites that species Xi occupies. Since the site fractions sum to unity, the fraction of unoccupied carbon sites, i.e., free carbon sites, hf, is determined from hf = 1  hO  hO2  hCO  hH  hOH, where O, CO, H and OH species are chemisorbed onto carbonaceous surfaces. The site fraction hO2 denotes the fraction of oxygen atoms chemisorbed onto two adjacent carbon sites – not the fraction of O2 molecules chemisorbed onto a single carbon site. The reaction mechanism developed to describe char reactivity to H2O, CO2 and O2 is shown in Table 3, along with expressions b k , in mol/m2 s), in terms of site fracfor the net reaction rates ( R tions. In the mechanism, C(Xi) denotes an adsorbed species (i.e., a carbon site filled with an adsorbed species Xi), Cf denotes a free carbon site (i.e., a carbon site available for adsorption), and Cb denotes a bulk carbon site (an underlying site that will be exposed upon desorption of a carbon atom from the carbonaceous matrix). The activity of a bulk carbon site is taken to be unity. The adsorbed complex C2(O2) (see reactions R.15 and R.18) represents two adjacent adsorbed oxygen atoms (i.e., oxygen atoms adsorbed onto adjacent carbon sites). Whereas the adsorbed complex C(O) is representative of carbonyl- and ether-type complexes that desorb to yield CO (via reaction R.4), the adsorbed complex C2(O2) is representative of lactone- and acid anhydride-type complexes that desorb to yield CO2 (via reaction R.18). In the reaction rate expressions shown in Table 3, the concentrations of gas-phase species (e.g., [CO2], [H2O], [O2], and [H2]) are expressed in mol/m3, and the forward reaction rate coefficients (kif) are expressed in Arrhenius form: kif = Ai exp(Ei/RT).

Table 3 Reaction mechanism for carbonaceous solids exposed to H2O, CO2 and O2. Reaction R.1

2Cf + H2O , C(OH) + C(H)

R.2

C(OH) + Cf , C(O) + C(H)

R.3

C(H) + C(H) , H2 + 2Cf

R.4

C(O) + Cb ? CO + Cf

R.5

C(OH) + Cb , HCO + Cf

R.6

Cb + Cf + C(H) + H2O , CH3 + C(O) + Cf

R.7

Cb + Cf + C(H) + H2 , CH3 + 2Cf

R.8

Cf + C(H) + CO ? HCO + 2Cf

R.9

C(H) + C(H) ? CH2 + Cf

R.10

CO2 + Cf , C(O) + CO

R.11

Cb + CO2 + C(O) ? 2CO + C(O)

R.12

CO + Cf , C(CO)

R.13

CO + C(CO) ? CO2 + Cf + Cb

R.14

2Cf + O2 ? C(O) + CO

R.15

2Cf + O2 ? C2(O2)

Reaction rate (mol/m2 s) n o b 1 ¼ ðS=N AV Þ2 k ½H2 Oh2  k1r hOH hH R 1f f   b 2 ¼ ðS=N AV Þ2 k2f hf hOH  k2r hO hH R n o b 3 ¼ ðS=N AV Þ2 k h2  k3r ½H2 h2 R 3f H f b 4 ¼ ðS=N AV Þk4f hO R   b 5 ¼ ðS=N AV Þ k5f hOH  k5r ½HCOhf R   2 b 6 ¼ ðS=N AV Þ k6f ½H2 Ohf hH  k6r ½CH3 hf hO R n o 2 2 b R 7 ¼ ðS=N AV Þ k7f ½H2 hf hH  k7r ½CH3 hf b 8 ¼ ðS=N AV Þ2 k8f ½COhf hH R b 9 ¼ ðS=N AV Þ2 k9f h2 R H   b R 10 ¼ ðS=N AV Þ k10f ½CO2 hf  k10r ½COhO b 11 ¼ ðS=N AV Þk ½CO2 hO R 11f   b 12 ¼ ðS=N AV Þ k12f ½COhf  k12r hCO R b R 13 ¼ ðS=N AV Þk13f ½COhCO

R.16

Cf + Cb + C(O) + O2 ? CO2 + C(O) + Cf

R.17

Cf + Cb + C(O) + O2 ? CO + 2C(O)

b 14 ¼ ðS=N AV Þ2 k14f ½O2 h2 R f n o b 15 ¼ ðS=N AV Þ ðS=N AV Þk15f ½O2 h2  k15r hO2 R f   b 16 ¼ ðS=N AV Þ2 k16f ½O2 hf hO  k16r ½CO2 hf hO R 2 b R 17 ¼ ðS=N AV Þ k17f ½O2 hf hO

R.18

Cb + C2(O2) ? CO2 + 2Cf

b 18 ¼ ðS=N AV Þk18f hO2 R

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Reactions R.1 through R.9 represent the reactivity of carbon to H2O. Dual site chemisorption of steam is assumed (reaction R.1), yielding adsorbed H and OH species on the carbonaceous surfaces. The adsorbed OH further dissociates on the surface yielding adsorbed O and H (reaction R.2). The primary products of steam gasification are H2 and CO. Hydrogen is produced via reactions R.3 and CO is produced via reaction R.4. These four reactions represent the vast majority of the carbon’s reactivity to H2O, and are the same reactions set forth by others (e.g., see Blackwood and McTaggart [53] and Laurendeau [54]). Reactions R.5–R.9 are of minor significance to the reactivity of carbon, yet their inclusion serves an important purpose toward a complete mechanism: they result in the production of methane. The carbon containing radicals released by these equations (CH2, CH3, and HCO) quickly form CH4 via gas phase kinetics. Reaction R.5 represents the desorption of C(OH) from the carbonaceous matrix and into gaseous HCO, analogous to the desorption of CO via reaction R.4. Reaction R.5 is expected to be much slower than reaction R.4, as the H atom bonded to the O atom means that the adsorbed O atom will not be able to erode as many carbon–carbon bonds in the underlying carbonaceous matrix. Furthermore, the C(OH) complex is expected to be relatively short-lived. The resultant HCO radical is also too short-lived to be observed in traditional temperature-programmed desorption techniques, and as such, reaction R.5 is often not included in many mechanisms. However, infrared spectroscopy does detect the presence (and stretching) of C(OH) during the gasification of carbon by H2O [55], with the species being relatively stable at temperatures as high as 680 °C. Thus, we choose not to ignore the possibility that a very small fraction of C(OH) may directly desorb into HCO. Reactions R.6 and R.7 are methods of CH3 production (and thus subsequently CH4 production) that were proposed by Laurendeau when dissociative adsorption of H2O dominates H2O reactivity, which our mechanism assumes [54]. Furthermore, a density functional theory (DFT) study by Espinal et. al. [56] purports that the major pathway for methane production from carbonaceous materials is by the desorption of a methyl (CH3) radical from the hydrogenated surface upon reaction with H2. While their reaction included several transition states – species not included in our reduced mechanism – it is summarized, in effect, by reaction R.7. While Espinal and Mondragon did not investigate the reaction of the hydrogenated surface with H2O, it should be possible by analogy, though perhaps less favorable energetically, if we assume the dissociative adsorption of H2O. For this reason we also included it in our mechanism, as reaction R.6. Reaction R.8 represents the reaction of gaseous CO with an adsorbed hydrogen atom to yield gaseous HCO, similar to how reaction R.10 reverse represents the reaction of gaseous CO and adsorbed oxygen to yield CO2. While the HCO product would be very short lived and therefore undetected in most studies, we have chosen to include reaction R.8 because of its analogy to R.10 reverse, a widely accepted reaction. When two adsorbed hydrogen atoms react, in addition to desorbing as gaseous hydrogen (reaction R.3), they may also bond to a single carbon atom. Many mechanisms include this step, the formation of a stable C(H2) surface species, as the precursor to methane formation. As we do not believe that the persistence of stable C(H2) or C(H3) surface species is either likely or necessary to predict adequate methane production, we have not included these surface species in our reduced mechanism. Nevertheless, while we do not believe a C(H2) species is stable enough to warrant inclusion in our mechanism (as it would re-dissociate long before further solid phase reactions), we cannot rule out the creation of such a species, however briefly, when two C(H) species react. And indeed, if the species briefly exists, we must also account for the chance that it desorbs and removes a carbon from the solid

5

phase. This is the reason for inclusion of reaction R.9 in the reaction mechanism. A five-step heterogeneous reaction mechanism (reactions R.4 and R.10 to R.13) based on previous studies [1– 3,5,15,18,21,28,57] was developed to describe carbon reactivity to CO2. The mechanism has been used to describe the mass loss rates in environments containing CO2 at atmospheric and elevated pressures, including environments that contain modest levels of CO, which inhibits carbon reactivity to carbon dioxide [21]. Single-site chemisorption of oxygen atoms is assumed when CO2 contacts a carbonaceous surface, leading to CO formation and an adsorbed oxygen atom. Reaction R.12 (complex-enhanced adsorption of CO2) was included based on the work of Montoya et al. [57], who studied the mechanism of CO2 chemisorption on carbon surfaces, and experimental results and molecular orbital calculations of Pan and Yang [58] and Chen et al. [28], who proposed CO2 reacting with off-plane O-atoms chemisorbed on C-atoms near semi-quinone and carbonyl type structures. The inhibition of the carbon gasification rate by CO is due, in part, to the reverse of reaction R.10, which removes an adsorbed oxygen atom from the carbonaceous surface, not permitting it to remove a carbon atom from the carbonaceous matrix via either reaction R.4 or R.11. Reactions R.12 and R.13 were included based on transient gasification studies using radioactive tracers that indicated that the carbon atom from gas phase CO can be incorporated into the carbonaceous matrix [59–61]. Espinal et al. [62] found that CO adsorbed at active carbon sites can interact with gas-phase CO to form such intermediates as cyclic ethers, carbonyls, lactones, ketones, carbonates, and semi-quinones, depending on the structure (e.g., armchair or zigzag) of the carbon site. Carbon dioxide is eventually liberated from these intermediates. A six-step heterogeneous reaction mechanism (reactions R.4 and R.14–R.18) was developed to describe carbon reactivity to O2. The mechanism was derived by considering previous studies [43–45,57] concerned with the carbon–oxygen reacting system. Dual site chemisorption of oxygen is assumed (reactions R.14 and R.15), yielding adsorbed oxygen species on the carbonaceous surfaces, some of which are on adjacent carbon sites (denoted by C2(O2) in the mechanism). Complex enhanced adsorption is also taken into account (reactions R.16 and R.17), leading to both CO2 and CO formation. The adsorbed oxygen atoms desorb, producing CO (via reaction R.4) and CO2 (via reaction R.18). Since CO2 is a reaction product when carbon is exposed to O2, reactions between carbon and CO2 come into play, however, under most conditions of interests, the carbon–O2 reactions dominate. It is possible that ash impurities within the carbon matrix play a role in influencing the reactivity of the char. For instance, the work of Hippo and Walker suggests that the presence of Mg and Ca tends to increase char reactivity to both O2 and CO2 [63]. On the other hand, the work of Sekine et al. suggests that the presence of Si and Al tends to reduce reactivity to H2O [64]. In this work, we do not attempt to correlate the kinetics of any reaction to the presence or absence of ash constituents. We assume that the ash in the char is inert and uniformly distributed throughout the carbonaceous matrix and that during reactivity tests in the TGA, no ash constituents leave the char (for example, as a consequence of vaporization or chemical reaction processes) during char conversion. Since our kinetic parameters were determined from char mass loss measurements, which inherently include any impacts of ash on char reactivity), the kinetic parameters for Wyodak coal char include the effects of the ash in the Wyodak coal and the kinetic parameters for the corn stover char include the effects of the ash in the corn stover. Based on reaction R.1–R.9 (the carbon–H2O reaction mechanism), the following expression for the intrinsic chemical reactivity of the carbon to H2O, RiC,H2O (in kg/m2 s) is obtained:

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n

bC R b4 þ R b5 þ R b6 þ R b7 þ R b9 RiC;H2 O ¼ M

o

ð3Þ

b C is the molecular weight of carbon. The specific mass loss Here, M rate in environments containing H2O, H2 and N2 (i.e., in environments free of CO2, CO and O2) is given by



1 dmC mC dt



¼ Sg RiC;H2 O

ð4Þ

n 1 dmtot b H OR b1 þ M bH R b3 þ M b CO R b4 þ M b HCO R b5 ¼ Sg  M 2 2 mtot dt b CH  M b H OÞR b 6 þ ðM b CH  M b H ÞR b7 þ ðM 3 2 3 2 o b HCO  M b CO Þ R b8 þ M b CH R b9 þ ðM 2

ð5Þ

Reactions R.4 and R.10 to R.13 (the carbon–CO2 reaction mechanism) yield the following expression for the intrinsic chemical reactivity of the carbonaceous material to CO2, RiC,CO2 (in kg/m2 s), the rate per unit surface area at which carbon is removed from the bulk material due to chemical reaction:

n o bC R b4 þ R b 11  R b 13 RiC;CO2 ¼ M

ð6Þ

The char specific mass loss rate in environments containing CO2, CO and N2 (i.e., in environments free of H2O, H2 and O2) is given by

1 dmc mC dt



¼ Sg RiC;CO2

ð7Þ

CO2

In such environments, the specific rate of change in the total weight due to adsorption and desorption reactions is given by

n 1 dmtot b CO  M b CO Þ R b 10 þ M b CO R b 4 þ ð2 M b CO  M b CO Þ R b 11 ¼ Sg ð M 2 2 mtot dt o b CO R b 12 þ ð M b CO  M b CO Þ R b 13 ð8Þ M 2

Based on reactions R.4 and R.14–R.18 (the carbon–O2 reaction mechanism), the following expression for the intrinsic chemical reactivity of the carbon to O2, RiC,O2 (in kg/m2 s) is obtained:

n o bC R b4 þ R b 14 þ R b 16 þ R b 17 þ R b 18 RiC;O2 ¼ M

ð9Þ

The specific mass loss rate in environments containing O2 and N2 (i.e., in environments free of CO2, CO, H2O and H2) is given by



1 dmC mC dt

! rffiffiffiffiffiffiffi 1 ðE  Emean Þ2 exp  f ðEÞ ¼ r 2p 2r 2 1

ð13Þ

H2 O

where mC is the mass of the particle’s carbonaceous material. In such environments, the specific rate of change in the total weight due to adsorption and desorption reactions is given by



Here, f(E) is the Gaussian distribution function used to describe the distribution of activation energies for the adsorbed species on the carbonaceous material. This distribution is defined by a mean energy and standard deviation, Emean and r, respectively, as follows:

 ¼ Sg RiC;O2

ð10Þ

O2

In environments containing O2 as the significant reactive gas, the specific rate of change in the total weight due to adsorption and desorption reactions is given by

n 1 dmtot b CO R b 4 þ ðM b CO  M b O ÞR b 14  M b CO R b 15 ¼ Sg M 2 2 mtot dt o b CO  M b O ÞR b 16 þ ð M b CO  M b O ÞR b 17 þ M b CO R b 18 þ ðM 2 2 2 2 ð11Þ 3.1.1. Activation energy distributions for desorption reactions For desorption reactions, account is made for the distribution of activation energies that must be overcome to release adsorbed species by employing the following expression for the desorption rate:

d½X i =dt ¼ ½X i 0

Z

1

A  expðE=RTðtÞÞ

0

 Z  exp A 0

t

 exp ðE=RTðtÞÞdt f ðEÞdE

ð12Þ

In this treatment, the rate coefficient for a desorption reaction will depend on time since the onset of reaction. At early times the desorption rate will be faster than at later times, as the weakly bound adsorbed complexes are easier to release. More information on the derivation and use of the above equations can be found in the thesis of Ma, [21], p. 44. The distribution of activation energies is measured via a technique called temperature programmed desorption (TPD), which involves loading a char sample with the adsorbed species of interest and monitoring the concentrations of the species that desorb as the temperature is increased. This gives the distribution of the adsorbed species on the char surface as a function of activation energy, from which A, Emean and r for the reaction can be determined. In this study, TPD was employed to determine these parameters for reactions R.4, R.12 (the reverse reaction) and R.18. In the experimental procedure, the carbon surface is loaded with complexes (using air if loading C(O) complexes and CO if loading C(CO) complexes), typically at 500 °C for about 30 min. The temperature in the chamber is then reduced to 250 °C, before being ramped from 250 °C to 1100 °C (in the presence of N2) over a time of 45 min. Gas concentrations of CO2 and CO are measured during the temperature ramp. If CO is observed during the ramp for the char loaded using air, it is attributed to reaction R.4, as no other reaction yields CO from C(O) and no gaseous reactants. If CO2 is observed, it is attributed to reaction R.18, as no other reaction yields CO2 from C(O) (C2(O2) being simply two adjacent adsorbed oxygen atoms) and no gaseous reactants (for instance, R.10r requires gaseous CO, whose concentration is negligible inside the reaction chamber). If CO is observed during the ramp for the char loaded using CO, it is attributed to R.12, as no other reaction yields gaseous CO from C(CO) and no gaseous reactants. Greater detail regarding the implementation of the TPD method in our pressurized TGA can be found in the thesis of Ma [21], p. 38. 3.2. Thermochemistry For each reaction that is considered reversible (see Table 3), the rate coefficient for the reverse reaction, kir, is determined from the forward rate coefficient and the equilibrium constant, KC, for the reaction, where KC is expressed in terms of the concentrations of the species involved in the reaction. The equilibrium constant KC is related to the equilibrium constant in terms of partial pressures, Kp, via the expression K C ¼ ðSD =N AV ÞDtS  ðP ref =RTÞDtg  K P , where Dts is the change in solid-phase moles upon reaction (free sites are included in the calculation of Dts), Dtg is the change in gas-phase moles upon reaction and Pref is the reference pressure, 1 atm. Thermodynamic properties are used to evaluate Kp: Kp = exp(-DG/RT), where DG is the Gibbs function change for the reaction, DG = DH  TDS. Evaluating DH and DS requires knowledge of the heats of formation and absolute entropies of the species involved in the reaction. These thermochemical properties for the adsorbed species are unknown. In our approach, values for these properties are determined that yield best fits to the measured mass loss data. The unknowns in our approach are the heats of formation and absolute entropies for adsorbed species (oxygen atoms, hydroxyl radicals, hydrogen atoms, and carbon monoxide) and the forward rate coefficients for all reactions except for the four desorption reactions (R.4, R.5, R.12, and R.18). Thermochemical

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properties of gas-phase species were taken from the JANAF Thermochemical Tables [65]. 3.3. Determination of parameters In order to determine the unknown parameters, a TGA simulator code was developed to simulate char conversion in the TGA reaction chamber. The chamber is modeled as a constant pressure, constant temperature control volume containing a specified mass of char that is exposed to a constant molar flow of a gas mixture of specified composition. In the model, the differential equation that governs the total mass on the TGA balance pan is solved along with the differential equations that govern the changes in the surface site fractions of adsorbed species on the char surfaces as a result of heterogeneous reactions and the changes in the gas-phase mole fractions as a result of homogeneous reactions. The source terms in the adsorbed species equations are determined from the heterogeneous reaction mechanisms, either the carbon– H2O reaction mechanism for carbon exposed to H2O/H2/N2 mixtures, or the carbon–CO2 reaction mechanism for carbon exposed to CO2/CO/N2 mixtures, or the carbon–O2 reaction mechanism for carbon exposed to O2/N2 mixtures. The source terms in the gas-phase species equations were determined using GRI-Mech [66], an established gas-phase reaction mechanism optimized for methane oxidation. The TGA simulator code is executed using Cantera [67], a suite of object-oriented software tools for problems involving chemical kinetics, thermodynamics and transport processes. In GRI-Mech 3.0, account is made for 53 gas phase species involved in 325 reactions. A Matlab code was written to implement the model. For specified Arrhenius parameters for the reaction rate coefficients and input conditions (TGA inlet flow rates and reaction chamber temperature), the governing equations were integrated simultaneously to yield as a function of time the mass on the TGA balance pan, the concentrations of the adsorbed species, and the mole fractions of gas phase species in the control volume containing the char particles. In order to fit the experimental data, a least-squares wrapper function was developed to alter values for the forward rate coefficients and heats of formation and absolute entropies for the adsorbed species until the difference is minimized between calculated and measured specific mass loss rate profiles. The forward reaction rate coefficients determined were fit in Arrhenius form via a least-squares procedure to yield the values for pre-exponential factor A and the activation energy E for each reaction (except reactions R.4, R.12, and R.18, whose values were kept fixed at the those determined from the TPD studies). Initial values are needed for the heats of formation (DHf ;CðX i Þ ) and absolute entropies (S0X i ) of the adsorbed species in order to implement the iterative process to determine the unknown parameters. The initial value used for the heat of formation for C(O) was based on the heat of adsorption of oxygen onto activated carbon measured by Menendez and co-workers [68] and that for C(CO) was based on the heat of adsorption of CO on palladium measured by Tracy and Palmberg [69]. Little information is available on the heats of formation of C(H) and C(OH). The initial value assumed for C(H) was based on the heat of physisorption of H2 on graphite [70–72] and the energies required to cleave a H–H bond and a C–C bond. The heat of formation for C(OH) was estimated from its formation reaction from reference species (C(s) + 1/2 H2 + 1/2 O2 ? C(OH)) and the bond energy model, which assumes that the total bond energy of a molecule equals the sum of the individual bond energies. The following correlation put forth by Campbell and Sellers [73] was used to obtain absolute entropies for the adsorbed species:

S0ad ðTÞ ¼ 0:7S0gas ðTÞ  3:3R

ð14Þ

7

where R is the universal gas constant. The 3.3R term, obtained from a fit to data, corresponds to the elimination of one degree of translational entropy upon adsorption. The 0.7Sgas0 term indicates that seventy percent of the entropy of the gas-phase species being adsorbed (less the one degree of translational entropy) is maintained by the adsorbed species formed. Since graphite was one of the substrates used to obtain the data for establishing Eq. (14), the values obtained from fits to the data should serve as a reasonable initial guesses. 4. Results and discussion 4.1. Kinetic and thermochemical parameters 4.1.1. Thermochemical parameters Values for kinetic and thermochemical parameters discussed here obtained using the experimental data shown in the subsequent section. This section preempts the experimental data section in order to facilitate discussion of the observations in that section within the context of the overall mechanism. Values determined for the heats of formation and absolute entropies of the adsorbed species are shown in Fig. 2 along with values used as initial values. As noted in the right panel of the figure, the values for the absolute entropies of the adsorbed species determined from the data fitting procedure (solid lines) are in line with the correlation given in Eq. (14) (dotted lines). It is also worth noting that the Wyodak coal and corn stover chars yielded quite similar results for the absolute entropies of the adsorbed species. The absolute entropies of the adsorbed species on the carbonaceous materials were found to differ slightly from the 70% value of gas-phase absolute entropies (less the one degree of translational entropy), and CO was the only adsorbed species that exhibited an absolute entropy that had a temperature dependence that differed significantly from that of the gas phase species. In our approach, the following correlation is used to describe the absolute entropies of the adsorbed species: 0

S0ad ðTÞ ¼ fSgas ðTÞ  3:3R

ð15Þ

For generality, the function f was correlated with temperature in the form f = a(1 + bT); values for a, and b are tabulated in Table 4 for each of the adsorbed species. It should be emphasized that employing the parameters given in Table 4 will yield values for the absolute entropies of the adsorbed species in the temperature range 1173–1573 K that agree with the ones determined in our fitting procedure quite well. The correlation is expected to yield reasonable estimates of the absolute entropies over the wider temperature range 300–2200 K, however no data exist to support this contention. The enthalpies of formation and absolute entropies for the adsorbed species shown in Fig. 2 were used to calculate the enthalpy and entropy changes for each reaction, and this information is presented in Supplement Tables 2 and 3. For each reaction, the values of these thermochemical properties are reasonable and supportable given the range of thermochemical properties expected for the adsorbed species. 4.1.2. Arrhenius parameters for forward reaction rate coefficients The forward reaction rate coefficients for the reactions given in Table 3 were adjusted until specific mass loss rates calculated employing the mechanism agreed with the measured normalized mass loss rates. The dashed lines shown in Figs. 3–12 were calculated employing the TGA simulator model. As noted, the calculations faithfully reflect the measurements over the range of conditions considered in the experiments, suggesting that the heterogeneous reaction mechanism presented in Table 3 with the

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Fig. 2. Enthalpy of formation (left) and absolute entropy (right) of adsorbed species on Wyodak coal char (solid lines) and corn stover char (dashed lines). Dotted lines represent initially assumed values based on data for adsorption on their substrates.

Table 4 Parameters for absolute entropies of adsorbed species Sad,i(T) = fi Sgas,i(T)  3.3R where fi = a(1 + bT), T in Kelvins. Species

C(O) C(H) C(OH) C(CO)

Wyodak coal char

Corn stover char

A

B

A

B

0.72 0.54 0.70 0.59

0 0 0 1.44e4

0.71 0.52 0.66 0.6

0 0 0 1.00e4

kinetic parameters determined for the forward reaction rate coefficients and enthalpies of formation and entropies presented in Fig. 2 are quite adequate to describe Wyodak coal char reactivity and corn stover char reactivity in gasification and oxidation environments when conversion rates are limited by the effects of chemical reactions. It should be emphasized that all reactivity tests with a particular reactive gas were considered together when determining the thermochemical properties and rate coefficients using a least squares scheme. When any one reactivity test was considered alone, much better agreement between measured and calculated profiles could be obtained for that particular test. The least-squares-determined values for the Arrhenius parameters that describe the forward reaction rate coefficients for the

Wyodak coal char and corn stover char in environments containing H2O, H2, CO2, CO, and O2 are shown in Table 5. The lone exception is reaction R.12, for which the stated Arrhenius parameters describe the reverse reaction (i.e. the desorption reaction). The activation energies are within the ranges expected for the type reactions considered, in that several are in line with values previously published for chars and activated carbons. For example, the activation energy of reaction R.10 is in agreement with other observations [57,74,75] and the activation energy for reaction R.14 is similar to other findings [76–78]. Also, the activation energies of the desorption reactions (R.4, R.12r and R.18) are in agreement with results of temperature-programmed desorption experiments performed by other researchers [21,43,45,79]. The activation energies of a few of the reactions, e.g. R.1 and R.3, are in agreement with those for similar reactions but on non-carbon substrates [80]. For those reactions for which no information on activation energies could be found in the literature on any type of substrate (e.g. reactions R.5–R.9), the activation energies were constrained to a lower bound of DHrxn, as determined by the gas phase species and the determined entropy of the adsorbed species. This typically yielded high values for the activation energies of those reactions that involved gas-phase radicals and consequently, those reactions were quite slow.

4.2. Experimental reactivity studies

Fig. 3. Measured and calculated and normalized mass loss rates Wyodak cal char in 20% steam and 80% N2, at 700 °C, 800 °C and 900 °C all at one atmosphere.

4.2.1. Wyodak char reactivity to steam (H2O) The temperatures for testing were selected to ensure that the conversion process is limited solely by the rates of chemical reactions and not influenced by any mass transport effects. To this end, steam gasification tests were performed at temperatures up to 900 °C (1173 K). Seven reaction environments were used in our investigation of the reactivity of Wyodak coal char to steam. Experiments performed in these environments capture the combined effects of temperature, concentration and the presence of hydrogen on reaction rates. The experiments include tests in 20% H2O and 80% N2 at three different temperatures (700 °C, 800 °C and 900 °C), tests at 800 °C in environments having two different H2O concentrations (20% and 10%), tests at 900 °C in environments having two different H2O concentrations (20% and 10%), as well as two tests with hydrogen addition (800 °C and 900 °C in an environment

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Fig. 4. Normalized mass loss rates for Wyodak coal char in three different gaseous environment at 800 °C and 900 °C and 1 atm. The lines represent calculations based on a char-steam reactivity model discussed in this paper.

Fig. 5. Normalized mass loss rates for corn stover char in seven different gaseous environments at selected temperatures from 750 °C to 950 °C and 1 atm. Steam concentrations are 10% (right panel) and 20% (left panel) in nitrogen with some environments containing 2.7% (by volume) H2. The lines represent based on a char-steam reactivity modal discussed previously in this paper.

Fig. 6. Comparisons of normalized mass loss rates for corn stover char and Wyodak coal char in environments containing 20% H2O (by volume, balance N2) at 800 °C (left panel) and 900 °C (middle panel) and in an environment containing 10% H2O and 2.7% H2 (by volume, the balance N2) at 900 °C (right panel).

containing 10% H2O, 87.3% N2 and 2.7% H2) to assess the impact of hydrogen inhibition. The measured thermograms indicate that at 800 °C, in the environment containing 10% H2O and 90% nitrogen, Wyodak coal char is almost completely gasified at a time of 6000 s. When as little as 3% of the nitrogen was replaced by hydrogen (yielding a hydrogen concentration of only 2.7%, by volume), the char sample (of nearly identical mass) had only reached a conversion of approximately 50% by the same time. This demonstrates inhibition by H2. At 900 °C, the effect of hydrogen inhibition is less,

but still quite strong. By the time the char that was not exposed to the H2 had reached nearly complete conversion, the char that was exposed to the H2 had only reached about 80% conversion. Specific surface area measurements were made during the course of gasification in an environment containing 20% H2O, by volume, at 800 °C. The gasification tests in steam were actually performed after the tests in CO2, and a new batch of Wyodak coal char had to be prepared for these experiments. The initial char properties had to be measured for this new batch of coal char.

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Fig. 7. Normalized mass loss rates for Wyodak coal char in 100% CO2 at 1 atm and selected temperatures (left) and in selected environments at 900 °C and 1 atm (right). The dashed lines represent calculations made using the model discussed in previously in this paper.

Fig. 8. Normalized mass loss rates for Wyodak coal char in 50%/50% CO2/CO gas environment at 1 atm and selected temperatures (left) and in char in selected environments containing CO2 at 1000 °C and 1 atm (right). The dashed lines represent calculations made using the model discussed previously in this paper.

Fig. 9. Normalized mass loss rates for corn stover char in 100% CO2 at 1 atm and selected temperatures (left) and in selected environments at 900 °C and 1 atm (right). The dashed lines represent calculations made using the model discussed previously in this paper.

Although efforts were made to ensure identical devolatilization conditions in the flow reactor, the value measured for the initial char specific surface area, Sg,0, was 299 m2/g, a value lower than the 475 m2/g determined for the initial batch of coal char. The value determined for the structural parameter w of this new batch of char was in agreement with that determined for previous samples of the Wyodak coal char as was the value of the initial apparent density. In addition, gasification tests in CO2 with this new batch of coal char yielded similar gasification rates as the old batch of coal char. Normalized mass loss rates (1/m  dm/dt) were determined from the measured thermograms and plotted as a function of

conversion. Shown in Fig. 3 as data points are profiles obtained for Wyodak coal char exposed to 20% steam and 80% nitrogen at 700 °C, 800 °C, and 900 °C at atmospheric pressure. [The dashed lines in the figure represent calculations made using the reactivity model previously discussed. Error bars are shown at early (25%), middle (50%), and late (80%) stages of conversion. Uncertainty grows toward the later stages of conversion, due to the lower masses involved, and hence higher fractional error.] The normalized mass loss rates indicate increased char reactivity to H2O with increasing temperature. For comparison, at 900 °C, Wyodak coal char conversion rates in 20% H2O are about twice as fast as they are in 100% CO2.

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Fig. 10. Normalized mass loss rates for corn stover char in 50%/50% CO2/CO gas environment at 1 atm and selected temperatures (left) and in selected environments containing CO2 at 1000 °C and 1 atm (right). The dashed lines represent calculations made using the model discussed previously in this paper.

Fig. 11. Normalized mass loss rates for Wyodak coal char in 6% oxygen, by volume, at 1 atm and selected temperatures (left panel), and at 6% and 12% oxygen at 500 °C (right panel). The dashed lines represent calculations made using the model discussed previously.

Fig. 12. Normalized mass loss rates for corn stover char in 6% oxygen, by volume, at 1 atm and selected temperatures (left panel), and at 6% and 12% oxygen at 450 °C (right panel). The dashed lines represent calculations made using the model discussed previously in this paper.

The profiles shown in Fig. 4 indicate that char reactivity to steam increases with increasing steam concentration, and that the addition of H2 to the gas phase has the effect of decreasing the conversion rates. The inhibiting effect seems to be stronger at the lower temperature. 4.2.2. Corn stover char reactivity to steam (H2O) Seven experiments were performed in the investigation of the reactivity of corn stover char to steam. As with the Wyodak coal char, these experiments capture the combined effects of temperature, concentration and the presence of hydrogen. The experiments include tests in 20% H2O and 80% N2 at three different

temperatures (750 °C, 800 °C and 900 °C), tests at 800 °C in environments having two different H2O concentrations (20% and 10%), tests at 900 °C in environments having two different H2O concentrations (20% and 10%), as well as two tests with hydrogen addition (900 °C and 950 °C in an environment containing 10% H2O, 87.3% N2 and 2.7% H2) to assess the impact of hydrogen inhibition. The thermograms measured in the gasification tests were analyzed in the same manner discussed above to determine normalized mass loss rates for the chars in the environments established in the reaction chamber of the TGA. The normalized mass loss rates determined for the seven tests are shown in

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Table 5 Kinetic parameters for the reaction rate coefficients for Wyodak coal and corn stover char particles.

R.1 R.2 R.3 R.4 R.5 R.6 R.7 R.8 R.9 R.10 R.11 R.12r R.13 R.14 R.15 R.16 R.17 R.18

Pre-exponential A Coal

Biomass

Coal

Biomass

Coal

Biomass

2Cf + H2O , C(OH) + C(H) C(OH) + Cf , C(O) + C(H) C(H) + C(H) , H2 + 2Cf C(O) + Cb ? CO + Cf C(OH) + Cb , HCO + Cf Cb + Cf + C(H) + H2O , CH3 + C(O) + Cf Cb + Cf + C(H) + H2 , CH3 + 2Cf Cf + C(H) + CO ? HCO + 2Cf C(H) + C(H) ? CH2 + Cf CO2 + Cf , C(O) + CO Cb + CO2 + C(O) ? 2CO + Cf C(CO) , CO + Cf CO + C(CO) ? CO2 + 2Cf 2Cf + O2 ? C(O) + CO 2Cf + O2 ? C2(O2) Cf + Cb + C(O) + O2 ? CO2 + C(O) + Cf Cf + Cb + C(O) + O2 ? CO + 2C(O) Cb + C2(O2) ? CO2 + 2Cf

2.1  106 4.1  1011 1.4  1011 1.0  1013 1.0  1013 1.0  1013 1.0  1013 1.0  1013 3.0  1011 3.7  103 1.26  108 1.0  1013 9.8  106 5.0  1010 4.0  107 1.5  107 2.1  107 1.0  1013

7.3  107 1.5  1012 1.0  1012 1.0  1013 1.0  1013 1.0  1013 1.0  1013 1.0  1013 3.0  1011 8.6  104 3.26  1012 1.0  1013 3.36  106 7.0  1010 3.0  108 1.5  107 2.1  107 1.0  1013

105 80 67 353 393 300 300 300 426 161 276 455 270 150 93 78 103 304

106 150 100 353 393 300 300 300 426 188 367 455 266 150 103 78 103 304

– – – 28 28 – – – – – – 53 – – – – – 33

– – – 28 28 – – – – – – 53 – – – – – 33

Fig. 5. As expected, for a fixed steam concentration, char reactivity increases with increase in temperature. Also as expected, the data support an increase in char reactivity to steam with increase in steam concentration at fixed temperature. The addition of hydrogen decreases the reactivity, as noted when 2.7% hydrogen is substituted for a portion of the nitrogen in the reactant mixture containing 10% steam. When compared to the reactivity of the Wyodak coal char to steam, as shown in Fig. 6, the corn stover char reactivity is quite lower – up to an order of magnitude lower for some tests. This is quite unexpected, as biomass char reactivity is often higher than coal char reactivity, and will be discussed further. Kinetic parameters in the carbon–H2O reaction mechanism were adjusted until the specific rate of change in weight calculated via Eq. (5) agreed, in the least squares sense, with the measured normalized mass loss rates in all of the environments containing only H2O, H2 and N2 in which measurements were obtained. The dashed lines shown in Figs. 3–5 were calculated using the kinetic parameters for the rate coefficients determined for the Wyodak coal and corn stover chars. The agreement is deemed to be reasonably good. As noted, the heterogeneous reaction mechanism and associated rate parameters and thermochemistry yield accurate predictions of char specific mass loss rates over a range of temperatures and H2O concentrations, and also capture the effect of H2 inhibition. Although adequate, agreement with the corn stover char is not as good as with the Wyodak coal char. The corn stover char exhibits a slight decrease in reactivity with conversion that the chemical model does not quite capture. This may be due to deactivation of carbon sites, a phenomenon not currently included in our mechanism, however, deactivation via sintering is not expected to be significant at the relatively low temperatures at which the reactivity tests were performed. Hydrogen inhibition is a consequence of the reverse of reaction R.3, which becomes effective with even modest levels of hydrogen in the gas phase (see supplemental information for reaction pathways). Once on the surface, the C(H) complexes inhibit reactivity by decreasing the concentration of free carbon sites (and hence decreasing the rate of reaction R.1), as well as by increasing the reverse rate of R3 (and hence reducing the number of C(O) complexes thereby decreasing the rate of reaction R4). It should be pointed out that this mechanism also describes char reactivity to hydrogen. In an environment consisting of H2 as the sole reactive gas, reaction R.3r will result in adsorbed

Activation energy E (kJ/mol)

Std. dev. r (kJ/mol)

Reaction

hydrogen species, which desorb via reactions R.6, R.7 and R.9 producing gas-phase CH2 and CH3. Methane is rapidly produced via the following sequence of gas-phase reactions: CH2 + H2 ? CH3 + H and CH3 + H2 ? CH4 + H. Since the CH2 and CH3 formation reactions are slow, the overall C + 2H2 ? CH4 reaction is slow. This is in line with the observations of many others, e.g. Schmal et al. [81]. As noted, corn stover char reactivity to H2O is much lower than Wyodak coal char reactivity to H2O. One possible explanation for the slower steam gasification process for the corn stover char is due simply to the intrinsic differences between the carbonaceous surfaces of the corn stover and Wyodak coal chars, which may cause each char to have a different propensity for adsorption of different gases. For instance, this could be caused by the presence of different ash impurities within the carbonaceous matrix that may impact the reaction rates of certain steam reactions [64]. Another possible explanation is that the carbonaceous material of corn stover char is more readily adsorbed onto by all gases, both the primary reactive gases (O2, CO2 and H2O) and the inhibitory gases (CO and H2), and inhibition by H2 is significantly stronger for the corn stover char, thereby reducing the reactivity more. This possibility is supported by the reaction rates predicted by the mechanism. In 20% H2O at 700 °C, reaction R1f is over a magnitude greater for the corn stover char than it is for the Wyodak char, meaning that indeed the steam is more reactive to the corn stover char. However, reaction R1r is also far greater for the corn stover char, such that the net reaction rate of reaction R1 (R1f–R1r) is greater for the Wyodak char. At these conditions, the desorption rate of H2 via reaction R3 is slower for the corn stover char than for the Wyodak char by about a factor of two, meaning that a greater amount of the C(H) resulting from the H2O adsorption re-reacts with the C(OH) and leaves as H2O instead of desorbing as H2. This reduces the amount of C(OH) that subsequently forms C(O), which promptly desorbs as gaseous CO, thereby reducing reactivity. Another possibility is raised by Keown et al. [82], who observed a drastic change in the reactivity (to air) of biomass (cane waste) char after it had been exposed to steam at high temperatures. Their FT-Raman spectroscopy studies indicated that immersion in steam significantly changed the char structure, namely, fusing carbon rings into larger networks of rings. These authors hypothesize that H-atoms penetrate deep into the char matrix to induce ring condensation. They did not compare biomass char to coal char, but rather biomass char exposed to steam and biomass char not

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exposed to steam. The biomass char exposed to steam showed a much reduced reactivity to air than the char not exposed to steam, presumably owing to a change in char structure. It is quite possible that such a change in char structure occurred in the biomass char studied in this work, but to a lesser extent in the coal char, if at all. 4.2.3. Wyodak char reactivity to CO2 The Wyodak coal char was exposed to selected environments containing CO2 (100% CO2, 50% CO2/50% N2 and 50% CO2/50% CO) at 1 atm and selected temperatures between 800 °C and 1000 °C in our TGA in order to obtain mass loss data for analysis. As in the environments containing steam, the measured thermograms were manipulated to obtain the specific mass loss rate, 1/m  dm/dt, versus extent of conversion. Results are shown in Fig. 7 as the data points. As noted, the specific mass loss rate decreases with decreasing temperature and with decreasing CO2 concentration. Comparison of the normalized mass loss rate profiles in the equimolar CO2/N2 mixture with the profile in 100% CO2 demonstrates the reduction in the conversion rate with a decrease in the CO2 concentration and comparison of the mass loss rate profiles in the equimolar CO2/CO mixture with the equimolar CO2/N2 mixture demonstrates the reduction in the conversion rate owing to CO inhibition. The results shown in the left panel of Fig. 8 indicate that the inhibiting effect of CO depends on temperature. When compared to the results in Fig. 7, the results in the right panel of Fig. 8 indicate that while the presence of CO slows down reactivity at all temperatures, the effect is greater at 900 °C (1173 K) than at 1000 °C (1273 K). 4.2.4. Corn stover char reactivity to CO2 The reactivity of pure corn stover char to carbon dioxide was analyzed in the same manner as the Wyodak coal char. The corn stover char was exposed to selected environments containing CO2 (pure 100% CO2, 50% CO2/50% N2 and 50% CO2/50% CO) at 1 atm and selected temperatures between 800 °C and 1000 °C in the TGA in order to obtain mass loss data for analysis. The normalized mass loss rates determined from the thermograms obtained with the corn stover char particles are shown as the data points in Figs. 9 and 10. The trends are the same as those noted with the Wyodak coal char: conversion rates increase with increasing temperature and with increasing CO2 concentration. Upon CO addition to the gaseous environment, the mass loss rate is lowered, CO inhibiting the heterogeneous reactions. Kinetic parameters in the carbon–CO2 reaction mechanism were adjusted in a similar manner to the carbon–H2O mechanism. The dashed lines shown in Figs. 7–10 were calculated using the kinetic parameters for the rate coefficients determined for the Wyodak coal and corn stover chars. The agreement is deemed to be quite good. As noted, the heterogeneous reaction mechanism and associated rate parameters and thermochemistry yield accurate predictions of char specific mass loss rates over a range of temperatures and CO2 concentrations, and also capture the effect of CO inhibition. Analysis of the calculated rates of the reactions as char conversion progresses indicates that reaction R.10 reverse is the key reaction that controls the impact of CO on char reactivity to CO2. In the absence of CO in the ambient gas, owing to the low CO concentration the rate of reaction R.10 reverse is insignificant compared to the forward rate and hence, the impact of reaction R.10 reverse on the adsorbed oxygen atom surface concentration is negligible. As the CO concentration in the ambient gas increases, the reverse rate of reaction R.10 increases and a larger portion of adsorbed oxygen atoms will react with CO before they have a chance to desorb, removing a carbon atom from the carbonaceous surface. Reaction R.12 has the potential to retard reactivity because as the CO concentration increases, a larger number of CO molecules

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can occupy carbon sites that may have otherwise been occupied by oxygen atoms. However, reaction R.12 is not as important as the reverse of R.10 at the temperatures of interest. Note that reactions R.12 and R.13 do not result in removal of a carbon atom from the carbonaceous surface. The source of the carbon atom in the CO2 formed via reaction R.13 is a gas-phase CO molecule, not a carbon atom from the solid, carbonaceous material.

4.2.5. Wyodak coal and corn stover char reactivities to O2 The Wyodak coal and corn stover chars were exposed to environments containing 6 and 12 mol% O2 in nitrogen at 1 atm and selected temperatures in our TGA in order to obtain mass loss data for analysis. For both chars, temperatures ranged from 400 °C to 550 °C. These conditions were selected to ensure kinetics-limited burning. Normalized mass loss rates determined from the measured thermograms (mass versus time curves) are shown in Figs. 11 and 12. As evidenced, char reactivity to oxygen increases with increasing temperature. At 400 °C, the reactivity of the corn stover char is roughly 15% higher than the reactivity of the Wyodak coal char in 6% O2 (by volume) and at 500 °C, the reactivity of the corn stover char is about 20% higher than the reactivity of the coal char. As expected, char reactivity to oxygen increases with increasing oxygen concentration. Kinetic parameters in the carbon–O2 reaction mechanism were in a similar manner to those for the carbon–CO2 and carbon–H2O mechanisms. The dashed lines shown in Figs. 11 and 12 were calculated using the kinetic parameters for the rate coefficients determined for the Wyodak coal and corn stover chars. The agreement is deemed to be quite good. As evidenced, the heterogeneous reaction mechanism and associated rate parameters yield accurate predictions of char specific mass loss rates over a range of temperatures and O2 concentrations. As noted, char reactivity to oxygen increases with increasing temperature. As observed, for the same ambient conditions, the reactivity of the corn stover char in general is slightly higher than that of the Wyodak coal char, a consequence of the slightly faster rate of the C2(O2) loading reaction, reaction R.15 (one of the most important pathways in this temperature range) for the corn stover char. At the temperatures employed in the oxidation tests, the oxide loadings on the char surfaces were relatively high; the oxidation rates were limited by the desorption rates of CO and CO2, and to a lesser extent, the loading of C2(O2). Since the activation energy of reaction R.15 is larger for the corn stover char than for the Wyodak coal char, as the temperature increases, the biomass char is oxidized at a progressively faster rate than the coal char. The product distribution of the carbon–oxygen reaction depends on temperature: the higher the temperature, the more CO in the product mixture, however, there is controversy as to the actual temperature dependence. The CO/CO2 product ratios predicted for the coal and biomass chars are shown in Fig. 13 along with 50% and 95% confidence intervals determined by Campbell [45] for the CO/CO2 ratio as a function of temperature. These confidence intervals were based on the data of several investigators [34–42] who studied the carbon–oxygen reacting system. The solid line in each figure is the calculated CO/CO2 ratio employing the kinetic parameters presented in Table 5 to determine mass loss rates when the coal and biomass chars are exposed to 6% oxygen, by volume. The ratios determined from the model calculations lie within the region of highest confidence (i.e., within the 50% confidence interval) over a wide temperature range for each char. Since these are intrinsic chemical reactivities, they are expected to apply even in conditions when the overall burning rates of particles are limited by the combined effects of chemistry and mass transport.

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M.B. Tilghman, R.E. Mitchell / Combustion and Flame xxx (2015) xxx–xxx

Fig. 13. Simulated CO/CO2 product ratio for Wyodak coal char (left) and corn stover char (right) in oxygen at 1 atm, overlaid on confidence range compiled by Campbell [45] containing research from others [34-42].

4.3. Reaction mechanism pathways and sensitivity A full analysis of the reaction pathways and the mechanism’s sensitivity to each kinetic parameter can be found in the supplementary material. In H2O environments (see Supplement Fig. 6), one key result is the nearly equal net rates of reactions R.1, R.2, and R.4. This suggests that the dominant pathway for carbon conversion is the creation of a C(OH) complex by dissociative adsorption of gaseous H2O, followed by the splitting of C(OH) into C(H) and C(O) complexes, followed by the desorption of C(O) into gaseous CO. Additionally, the absolute reaction rate of the reverse reaction for reaction R.1 is roughly equal to that of the forward one, which suggests that the net progress of reaction R.1 is significantly slowed by the reverse reaction: C(H) and C(OH) recombining to form gaseous H2O. This is the primary mechanism for inhibition by H2: the addition of C(H) species increases the reverse rate of R.1, reducing the number of C(O) species capable of consuming carbon by desorption. Creation of H2 occurs primarily by reaction R.3, which also has the same net forward rate as those of reactions R.1, R.2, and R.4, since C + H2O ? CO + H2. This reaction also has significant reverse rates, indicating that some H2 immediately re-adsorbs onto the carbon. In CO2 environments (see Supplement Fig. 7), the trends exhibited in the reaction pathway analysis are very similar to those observed in H2O environments. The primary pathway for carbon consumption is the creation of a C(O) complex via loading by CO2 (reaction R.10), followed by the desorption of C(O) into gaseous CO (reaction R.4). At 800 °C with no CO in the ambient gas to which the char is exposed, the reverse rate of reaction R.10 is slow, a consequence of the low CO concentration. This reverse rate increases significantly when there is CO in the ambient, thus significantly decreasing the fraction of C(O) that desorbs into CO, and hence decreasing carbon reactivity. In O2 environments (see Supplement Fig. 8), at low temperatures, the chemical pathways for the C–O2 reaction mechanism indicate that the main product is CO2, which is produced primarily by the desorption of adjacent adsorbed oxygen atoms (reaction R.18). As temperature increases, the major product shifts to CO. At 800 °C, CO is primarily produced via the loading reaction (R.14) and the C(O) desorption reaction (R.4). The CO2 production pathway is still somewhat active at this temperature. At even higher temperatures (1200 °C), the C(O) desorption is so fast that the char conversion rate is limited by the loading step, reaction R.14. This renders the concentrations of C(O) always low enough that the complex enhanced adsorption reactions are all negligible.

At temperatures greater than 1200 °C, CO far outweighs CO2 as a reaction product. 4.4. Validation of the reaction mechanism Our mechanism was applied to a wide variety of char reactivity data from the studies of others in environments containing CO2, CO, H2O, and H2 over a wide range of temperatures and pressures. The dry gasification data obtained in the atmospheric pressure experiments by Koenig et al. [24] with activated coconut charcoal were considered as were the high-pressure experiments by Blackwood and Ingeme [14] with coconut shell char and the high-pressure experiments by Tsai [29] with a coal char. Also considered were the high-pressure steam gasification experiments of Schmal et al. [81] and Mühlen et al. [15]. In addition, the gasification experiments performed with high levels of CO and H2 by Gadsby et al. [83], Long and Sykes [84], Hedden and Löwe [85], and Bjerle et al. [9] were considered. The full analyses of these data are available in the supplementary material. Kinetic parameters were determined for the chars of these studies in a manner similar to this study. A major result is that the mechanism succeeds at predicting the reactivities at a wide range of reactant concentrations and pressures: total pressures as high as 40 bar, and up to 85% CO in the reactant gas mixture. Another key result is that while the kinetic parameters differed significantly for each char, the thermochemical parameters of the adsorbed species did not differ significantly, suggesting that the values presented here may potentially be used as an initial guess for a wide range of carbonaceous chars. 5. Conclusions In this work, the intrinsic chemical reactivities of the chars of a Wyodak coal and corn stover were measured in tests performed in a TGA. Both chars have similar reactivities in environments containing CO2 as the reactive gas, and the reactivities of both chars are reduced when CO is added to the ambient gas, the Wyodak coal char being slightly more inhibited (at 1000 °C). In environments with oxygen as the key reactive gas, the corn stover char has slightly higher reactivity to O2 than does the Wyodak coal char. In 6% O2 (remainder N2, 1 atm) at 400 °C, the corn stover char is roughly 15% more reactive than the Wyodak char and at 500 °C, the corn stover char is roughly 20% more reactive than the Wyodak char. The corn stover char has a lower reactivity to H2O, under the conditions tested in this study. At 800 °C in 20% H2O

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(remainder N2), the reactivity of the Wyodak coal char is roughly 3 times higher than that of the corn stover char. Raising the temperature widens the gap in reactivity for the two chars, with the Wyodak coal char reactivity being roughly 4 times higher than that of the corn stover char at 900 °C. Addition of H2 to the ambient exacerbates this difference in char reactivities further still: at 900 °C in 10% H2O and 2.7% H2 (remainder N2), the Wyodak char is nearly an order of magnitude more reactive than the corn stover char. The reaction mechanism developed to describe char intrinsic chemical reactivity in gasification and combustion environments can be used to accurately predict the effects of heterogeneous reaction over wide ranges of temperatures, pressures, and reactive gas concentrations. Values for the Arrhenius parameters for the reaction rate coefficients for the reactions in the mechanism as well as heats of formation and absolute entropies of adsorbed species (O, H, OH, and CO) on the carbonaceous surfaces for the Wyodak coal and corn stover chars are reasonable and in the expected ranges. The overall fit to the entire suite of data is quite good. In general, deviation between the observed reactivity and that predicted by the model tends to increase toward the later stages of conversion. Several possible explanations for this could be the buildup of an ash film, the deactivation of carbon sites due to thermal annealing (not expected to be a major factor at these temperatures), or the altering of the char structure somehow by the reactive gases. Other researchers have indeed recently linked the exposure of char to hot steam with a subsequent decrease in reactivity [82]. This possibility also would explain why the deviation between observed and predicted reactivity happens more frequently in the environments containing H2 and H2O. Also, it can be concluded from our data analysis that differences in char reactivity are not due solely to differences in the surface site densities of the chars (i.e., to differences in Sd) but also due to differences in the rates of key reaction pathways (i.e., to differences in kinetic parameters that describe the reaction rate coefficients). In this study we employ the same site density for all coal and biomass chars (Sd = 6.5  1019 sites/m2) with variations in the reaction rate coefficients reflecting variations in the reactivities of the different carbonaceous materials. Attempts to determine a single set of reaction rate parameters with differences in the surface site densities for the coal and biomass chars were unsuccessful, not leading to calculated reactivities that adequately described the reactivities of both the coal and biomass chars in all the environments considered. It may be asked whether the determined values for kinetic and thermochemical parameters extend to other types of coal char. While sub-bituminous, bituminous, lignite, and anthracite coals can indeed all differ significantly in their reactivity, so too does coal versus biomass. However, only slight differences are observed in values determined for the heats of formation of the adsorbed species on Wyodak coal and corn stover chars (hypothesized to be a consequence of the differences in properties that impact the electronic structure and distributions of dangling bonds on cleaved, exposed carbon surfaces). Given that the thermochemical parameters did not differ much between the coal and biomass of this study, and indeed were also used in the fitting of the data of others in the supplemental section (which included many different types of coals), we are hopeful that they can extend to a range of coals and biomasses, and that average values for the heats of formation of adsorbed species may be appropriate for all chars: DHf,C(O) = 145 kJ/mol, DHf,C(H) = 20 kJ/mol, DHf,C(OH) = 190 kJ/mol, and DHf,C(CO) = 215 kJ/mol. The absolute entropies determined for the adsorbed species are also similar (see Table 4) for the Wyodak coal and corn stover chars, suggesting that average values for the absolute entropies may be appropriate for all chars as well. Many of the reactivity trends and inhibition trends can also likely

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be extended to other coals and biomass as well, since the two fuels of study showed similar trends, as did the fuels studied by others in the supplemental section. The reaction mechanism accurately describes data and trends from this work as well as work of several other researchers. Such trends include: (a) the increase in reaction rates with increasing temperature, reactive gas concentration, and pressure; (b) the progressive decrease in the enhancement in reaction rates as pressure is increased; (c) the increase in the inhibiting effects of CO and H2 with increasing pressure and the decrease in the inhibiting effects with increasing temperature; and (d) the progressive increase in the CO-to-CO2 product ratio as temperature increases in oxidizing environments. The reaction mechanism can be used to predict coal and biomass char mass loss rates and off-gas compositions in the type environments that exist in commercial coal-fired and biomass-fired gasifiers and combustors. In such high-temperature environments, char particles are not totally penetrated by the reactive gases. As such, account must be made for the concentration gradients established inside particles in order to get accurate predictions of overall particle mass loss rates. This requires either solving the differential equations that govern the transport of reactive gases through the particle’s pores taking into account the combined effects of diffusion and chemical reaction or using the effectiveness factor-Thiele modulus approach of Thiele [86]. In either case, the reaction mechanism developed in this work can be used to describe the effects of chemical reaction. Acknowledgments MBT and REM would like to acknowledge the support of the U.S.D.O.E., managed through N.E.T.L. (DE-FC26-10FE0005372, Arun Bose, Project Manager). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.combustflame. 2015.05.009. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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