Model for the Evolution of Pore Structure in a Lignite Particle during

Jul 12, 2017 - the CPD model.6−8 In the CPD model, coal is defined as a .... cal represent population of clusters and population of intact loops and...
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Model for the Evolution of Pore Structure in a Lignite Particle during Pyrolysis. 2. Influence of Cross-Linking Reactions, Molten Metaplast, and Molten Ash on Particle Surface Area He Yang,† Thomas H. Fletcher,‡ Sufen Li,§ Haoquan Hu,*,† Lijun Jin,† and Yang Li† †

State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering and §School of Energy and Power Engineering, Dalian University of Technology, Dalian 116024, China ‡ Chemical Engineering Department, Brigham Young University, Provo, Utah 84602, United States ABSTRACT: A model for the evolution of the pore structure in a lignite particle during pyrolysis was established previously based on the chemical percolation devolatilization (CPD) model, using coal polymer network parameters to calculate the surface area and porosity of the particle. In this paper, to get the accurate surface area of coal particle at high pyrolysis temperature, the previous model was improved by considering the effects of cross-linking reactions, molten Metaplast, and ash. The good agreement of the predicted surface area with experiments at temperature below 1200 K in the previous model is maintained, and model accuracy is improved at temperatures above 1200 K. A correlation between cross-links and cleaving of side chains was established to describe the increasing amount of cross-links during coal pyrolysis and was introduced to modify the amount of bridges calculated by the CPD model. Higher temperatures can provide more energy for cleaving of side chains, and therefore the cross-linking reactions have a greater influence on the change of the surface area in a lignite particle during pyrolysis at a higher temperature. The influence of Metaplast on the surface area of a lignite particle is limited. The molten ash reduces the particle surface area at high temperature, and this influence is larger on the CO2 surface area than on the N2 surface area. calculated by a first-order reaction and a second-order reaction, respectively; however, the amount of cross-links was not calculated. Molten Metaplast can influence the pore structure during coal pyrolysis, especially for high volatile bituminous coal. Molten Metaplast makes the particle become plastic and swell, which in turn affect the pore structure significantly.12,13 However, the amount of Metaplast in lignite during pyrolysis is small, and therefore the influence is limited, making the diameter of lignite particles change little during pyrolysis with no thermoplastic deformation,3,14 but the influence still exists to a small degree. The surface area of the ash phase inside the char plays a very significant role in char reactivity. Choi et al.15 measured the surface area of the ash particles prepared by burning out the fixed carbon and volatile matter at a temperature below the ash melting point, reporting surface areas 4.5−5.3 times higher than the raw coal particle. During pyrolysis, the weight fraction of the ash phase increases with release of volatiles, and therefore, the surface area of the pores in the molten ash may be a sizable part of the particle surface area at high temperature with large volatile yields. In a previous paper,16 based on the chemical percolation devolatilization (CPD) model, a model for the evolution of the pore structure in a lignite particle during pyrolysis was established to calculate the surface area and porosity of the particle using the coal polymer network parameters. The calculation results matched the porosity and surface area in the

1. INTRODUCTION Coal may be viewed as a polymer network consisting of aromatic clusters with aliphatic bridges, loops, and side chains.1 The change of the pore structure in a lignite particle during pyrolysis is mainly caused by the changing structure of the network. The expansion of pores is caused by increasing voids as bridges crack and tars release, and, on the other hand, the expansion can also be restrained by the reformed bridges during cross-linking reactions. Moreover, at a high temperature Metaplast and ash in char are molten, and some pores are closed by the molten Metaplast and ash, making the particle surface area and porosity reduced. Therefore, in addition to the bridge breaking and tar release process, the description of crosslinking reactions, molten Metaplast, and molten ash is essential in analyzing the characteristics of pore structure evolution in a lignite particle during pyrolysis. Cross-linking reactions are important in coal pyrolysis because they control the ultimate tar yield, the tar molecular weight distribution, and the char’s fluidity, molecular order, surface area, and reactivity.2 Cross-linking does not generate any new attachment sites and occurs only at existing sites that are initially occupied by a side chain.3,4 There are various types of side chains in coal, and the cross-linking reactions of different side chains are different. However, the models to describe cross-linking reactions are usually simplified. In the functionalgroup-depolymerization, vaporization, and cross-linking (FGDVC) model,5 cross-linking reactions were correlated with CO2 and CH4 production determined from the FG model; one cross-link was assumed to be formed for each CO2 or CH4 molecule formed. In the chemical percolation devolatilization (CPD) model6−8 and the FLASHCHAIN model,9−11 the amount of Metaplast recombined with coal matrix was © XXXX American Chemical Society

Received: April 24, 2017 Revised: July 12, 2017 Published: July 12, 2017 A

DOI: 10.1021/acs.energyfuels.7b01163 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels lignite particle during pyrolysis measured in experiments at temperature below 1200 K. The rapid increase of the surface area when the mass release reached 30%3,17,18 was explained by the change in the quantity of pores which can adsorb adsorbate in the particle during pyrolysis due to the cracking of side chains. However, the model agreement with surface area data was poorer at temperatures above 1200 K because the effects of cross-linking reaction, molten Metaplast, and molten ash on the pore structure were neglected completely. In the previous paper,16 neglecting the cross-linking reactions made some cross-links be treated as cleaved side chains in the calculation, and the amount of pores in the particle which could adsorb adsorbate was overestimated. Neglecting the effects of molten Metaplast and molten ash made some closed pores be treated as open pores and also caused overestimation of the surface area. This paper describes improvements to the previous model to provide more accurate particle surface areas at temperature above 1200 K. The improvements included a correlation for estimating cross-links and a method for estimating the surface area by molten Metaplast and ash. The correlation for estimating cross-links was used to modify the remaining cleaved side chains, and the method for estimating the closed surface area was used to modify the overestimation of open pores. In addition, the influences of cross-linking reactions, molten Metaplast, and molten ash on the particle surface area were discussed.

sparticle =

t0 adsorption pore SaNANclust Nclust ‐ coal + Sparticle

[(1 − ftar − fgas )/(1 − Adry ) + Adry ]mp0

(1)

pore Nadsorption clust

represents the quantity of pores consisting of two g1 spaces which can adsorb adsorbate, per aromatic cluster, and it is calculated by eq 2. adsorption pore = Nclust

g12(σ + 1) 4(δ + g1)

(2)

Nclust‑coal represents the quantity of the aromatic clusters in the particle, and it is described in eq 3 ⎛ 1 ⎞ Nclust ‐ coal = (1 − Adry )⎜ − Nclust ‐ tar ⎟mp0 ⎝ Mclust ⎠

(3)

where 1/Mclust represents the initial population of clusters per mass of dry ash free coal; m0p represents the initial mass of the dry coal particle; and Nclust‑tar represents the population of clusters removed out of the particle by evaporation of tar per mass of dry ash free coal

Nclust ‐ tar = dt

∑ vii

(4)

where vi represents the generation rate of tar whose molecule consists of i aromatic clusters. 0 Stparticle is the balance of the initial surface area in the coal particle during pyrolysis, and it is described in eq 5

2. MODELING

t0 0 Sparticle = sparticle [(Nclust ‐ coalMclust) + Adry ]mp0

2.1. Review of the Previous Surface Area Model. (1) CPD Model: In this paper, pyrolysis reactions are described by the CPD model.6−8 In the CPD model, coal is defined as a macromolecular array whose building blocks are clusters of fused aromatic rings of various sizes and types, and 13C-nuclear magnetic resonance (NMR) structural parameters are used as inputs. According to a kinetic scheme for breaking labile bonds and side chains, the time-dependent value of coal polymer network parameters (p, fraction of attachments that are intact loops or bridges; c, fraction of the charred bridges; δ, side chains) and yield of light gases (g1, generated from cracking of side chains; g2, generated along with the formation of char bridge) can be calculated. Through percolation statistics based on a Bethe lattice, the number of aromatic clusters detached from the infinite lattice network (Metaplast) can be calculated. A vapor−liquid equilibrium treatment of Metaplast vs tar is used to calculate the yield of tar. A cross-linking mechanism is used to calculate the mass of Metaplast reattaching to the solid char. However, the number of the cross-links is not calculated in the CPD model. (2) Surface Area Model: The surface area in the particle can be estimated by the quantity of the adsorbate (N2 or CO2) molecules in a monolayer molecular adsorption in the particle. In the previous paper,16 it was assumed that when both chains cleaved from one bridge (twin chains) are transformed to g1, the pore consisting of the two g1 spaces is big enough for one adsorbent molecule to move in. The surface area in a char particle during pyrolysis is described by eq 1, where Sa represents the adsorption cross section of the adsorbing species, NA represents Avogadro’s number, fgas and f tar represent the yield of gas and tar on a dry ash free basis, respectively, and Adry represents the ash content of dry basis.

(5)

s0particle

where represents the initial specific surface area in the coal particle. 2.2. Modification of the Model. 2.2.1. Effect of CrossLinking Reactions. The change of the amount of cross-links during coal pyrolysis cannot be measured directly. The CPD model is connected with the change of bridges and loops per cluster measured by Fletcher et al.3 to calculate the change of the amount of cross-links per cluster during coal pyrolysis, as shown in eq 6 ⎛ 1 ⎞ cross cal cal = bex ⎜ − Nclust bclust ‐ tar ⎟Mclust − (σ + 1)pcal + 2btar Mclust ⎝ Mclust ⎠ (6)

where bex represents the bridges and loops per cluster measured in experiments; pcal represents intact loops or bridges; Ncal clust‑tar and bcal tar represent population of clusters and population of intact loops and bridges removed out of the particle by evaporation of tar per mass of dry ash free coal, respectively; cal and pcal, Ncal clust‑tar, and btar are calculated by the CPD model. The change of bex of four kinds of coals (North Dakota Beulah Zap lignite, Illinois #6 hvb Bituminous, Pittsburgh #8 hvb Bituminous, New Mexico Blue #1) during pyrolysis are calculated by eq 6. The ultimate analysis, proximate analysis, and chemical structure parameters of these coals reported in the literature3,18−20 are shown in Table 1 and Table 2. (c0 is calculated from a correlation21 developed for use with the CPD model.) The amount of cross-links increases with increasing g1 as shown in Figure 1, indicating that the amount of cross-links increases with an increasing amount of cleaved side chains. This result agrees with the experimental phenomenon that crosslinking occurs only at existing sites occupied by a side chain.3 It B

DOI: 10.1021/acs.energyfuels.7b01163 Energy Fuels XXXX, XXX, XXX−XXX

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3. Some side chains, whose twin chains have not been cleaved, can cross-link with their twin chains and generates g1 gases, and the new bridges transformed from the twin chains can also be further cleaved to generate g1 with the cross-links retained as shown in reaction path C. In this paper, the pyrolysis dynamic parameters in the CPD model for the generation of g1 are also used to describe the cleaving of side chains, and in addition, a correlation is established to estimate the amount of cross-links formed with the cleaving of side chains. Oxygen is important in cross-linking reactions. The small yield of tars from coals having a high oxygen content is mainly because of the phenolic and carboxylic condensation.5,23,24 To represent the effect of oxygen in cross-linking reactions, it is assumed that all the aromatic carbons with O attachments can be linked to other clusters after their attachments cleaved and that means side chains connected to these kinds of aromatic carbons (Ar−O−δ) can only be consumed through reaction paths B and C. The distance between a pair of twin chains is smaller than that between a δ and a δtb, and it seems logical that the possibility of cross-linking between a pair of twin chains is larger. Therefore, it is assumed that except for Ar−O−δ, other side chains can only cross-linked through reaction path C. Because of cross-linking reactions, the amount of g1 spaces (spg1) is not equal to the amount of g1 gases, and therefore spg1 is calculated by eq 7

Table 1. Ultimate and Proximate Analyses of Coal ultimate analysis (wt % daf)

proximate analysis (wt %)

coal

C

H

N

O

Adry

Vdaf

Beulah Zap Illinois #6 Pittsburgh #8 Blue #1

66.56 74.1 84.23 74.97

4.26 4.96 5.54 5.26

1.12 1.45 1.65 1.32

25.16 13.18 7.56 17.33

12.49 11.3 3.73 4.60

46.59 44.7 39.88 48.17

Table 2. Chemical Structure Parameters of the Parent Coal coal

p0

c0

Mclust (g/mol)

Mδ (g/mol)

σ+1

Beulah Zap Illinois #6 Pittsburgh #8 Blue #1

0.59 0.56 0.45 0.42

0.15 0.00 0.00 0.07

410 270 356 410

51 34 34 47

5.0 4.1 5.0 5.0

spg = g1 − bcross ‐ link 1 − 2bcross ‐ link 2 − g1cross ‐ linkedδ 1

(7)

where bcross‑link 1 and bcross‑link 2 represent the amount of crosslinks forming through reaction paths C and B, respectively. bcross‑link 1 is calculated by eq 8 dbcross ‐ link1 δ uncross ‐ linked 1 = kgδ uncross ‐ linked uncross ‐ linked dt 2 δ + spg

Figure 1. Calculated change of the amount of cross-links per cluster with an increasing amount of cleaved side chains during coal pyrolysis.

1

⎧ f aS faP ⎫ ⎪ ⎪ 1 ⎬ ×⎨ + S P Cal S P ⎪1 + f a + fa ⎪ R f a + fa ⎩ ⎭ σ+1

seems that when side chains are cleaved, a fraction of the side chain sites can be linked together to be transformed to crosslinks. There could be three paths for the cleaving of side chains as shown in Figure 2.

(8)

where kg represents the reaction rate for g1 release in the CPD model; fsa and fpa represent the amount of aromatic carbon with alkyl and O attachment per cluster respectively; Cal represents the amount of aliphatic carbon per cluster; R represents the average ability of each aliphatic carbon to donate hydrogen to make radicals stable and is set to 0.35 in this paper; and δuncross‑linked represents the amount of side chains that is uncrosslinked, which is calculated by eq 9 δ uncross ‐ linked = δ − δ cross ‐ linked

(9)

where δ represents side chains that cross-link with their twin chains but have not been cleaved as calculated by eq 10

Figure 2. Reaction scheme of side chain cleaving.

cross‑linked

δ cross ‐ linked = bcross ‐ link1 − g1cross ‐ linkedδ

1. Some side chains are cleaved to generate g1 gases directly with no cross-links formed as shown in reaction path A. Some functional groups in the cleaved side chain are not released and form tightly bound side chains (δtb) connected to aromatic carbons.22 2. Some side chains, whose twin chains have been cleaved, can cross-link with the δtb left by their twin chains and generates g1 gases as shown in reaction path B.

(10)

guncross‑linkedδ 1

where represents g1 generating from the cleaving of the cross-linked side chains as calculated by eq 11: dg1cross ‐ linkedδ dt

= kgδ cross ‐ linked

(11)

bcross‑link 2 is calculated by eq 12: C

DOI: 10.1021/acs.energyfuels.7b01163 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels spg faP dbcross ‐ link 2 uncross ‐ linked 1 = k gδ dt δ uncross ‐ linked + spg f aS + faP

connected with the outside, the surface area of this pore region theoretically can be measured. Moreover the amount of Metaplast in a lignite particle during pyrolysis is small. Therefore, only completely closed pores are considered. Eq 3 is therefore modified into eq 15

1

(12)

Then the amount of bridges per cluster bclust is calculated by eq 13.

⎛ 1 ⎞ Nclust ‐ coal = (1 − Adry )⎜ − Nclust ‐ tar − Nclust ‐ met⎟mp0 ⎝ Mclust ⎠

cal ⎧ (p + b ⎫ cross ‐ link 1 + bcross ‐ link 2)(σ + 1) − 2btar Mclust ⎬ bclust = ⎨ 1 − MclustNclust ‐ tar ⎭ ⎩ (13) ⎪

⎪

⎪

⎪

(15)

where Nclust‑met represents the amount of clusters in the Metaplast and is calculated by eq 16

The trend of the results calculated by eq 13 agrees with the experimental results (Error estimate is ±0.3.)3 as shown in Figure 3. The trend of this correlation can be introduced to the

Nclust ‐ met =

∑ li × i

(16)

where li represents the amount of clusters consisting of i aromatic clusters in the Metaplast. 2.2.3. Effect of Molten Ash. The surface area of char particles consists of two parts, the surface area of the organic structures and the surface area of the ash, and the properties of these two parts during pyrolysis are different. Eq 5 is modified into eq 17 t0 0 Sparticle = [(Nclust ‐ coalMclust)(1 − Adry )sorganic + Adry sash]mp0

(17)

s0organic

where represents the initial specific surface area of the organic structures, and sash represents the specific surface area of ash. During pyrolysis, with the release of organic material between mineral grains in the lignite, the specific surface area of ash increases and will be reduced when the ash is molten and the pores in ash are closed. It is assumed that the initial specific surface area of ash is equal to the initial specific surface area of organic structures in the raw coal particle, s0particle = s0ash = s0organic; and it is assumed that the specific surface area of ash in the char has a linear relationship with the release of organic material during pyrolysis and gasification. In addition, it is assumed that ash is melted above 1300 K, and after melting the specific surface area of ash is set to 0. The specific surface area of ash in char during pyrolysis can then be calculated by eq 18

Figure 3. Comparison of the predicted change of the amount of bridges per cluster with data from the Fletcher experiment.3,4

surface area model to reflect the increasing amount of bridges caused by cross-linking reactions, although the value calculated by the correlation does not match the experimental data very well. Not all the spaces left by volatiles can increase the surface area, and only the spaces on the surface of the solid and big enough for adsorbates to come in can be measured in the monolayer molecular adsorption. However, the spaces left by g1 and occupied by cross-links are not big enough to adsorb one adsorbate molecule. The g1 and δ in eq 2 actually represent the amount of g1 spaces and the amount of their twin side chains, respectively, and without considering the effect of cross-linking reactions they are equal to g1 and δ, respectively. However, after considering the effect of cross-linking reactions eq 2 becomes incorrect and must be modified into eq 14. adsorption pore Nclust

=

sash

(18)

where B is set to 4.88, which is the average ratio of the specific surface of ash to that of the raw coal particle, as measured by Choi et al.,15 and the ash was prepared by burning out the fixed carbon and volatile matter from coal at temperatures below the ash melting point.

3. RESULTS AND DISCUSSION The results of model calculations at various conditions (as shown in Table 3) are compared with data to show the improvement of this model and analyze the influence of crosslinking reactions, molten Metaplast, and molten ash on the particle surface area. 3.1. Improvement of this Model. After considering the effect of cross-linking reactions and considering the effect of molten Metaplast and ash (CCCM), the predicted change of the final surface area in Zap lignite chars with increasing maximum particle temperature (Tmax) is compared with experimental data18 in Figure 4 (c). The results calculated in

spg 2(σ + 1) 1

4(δ uncross ‐ linked + spg ) 1

0 ⎧s0 ⎪ particle + (ftar + fgas )(B − 1)sparticle Tp ≤ 1300 K =⎨ ⎪0 Tp > 1300 K ⎩

(14)

2.2.2. Effect of Molten Metaplast. The effect of molten Metaplast on the pore structure could be divided into two parts: (a) the pores of the molten Metaplast are closed completely; and (b) the molten Metaplast can close some pores in the solid char matrix. The pores in char are usually continuous, and therefore it is hard for some partially molten Metaplast to close a region of pores in the solid char matrix completely. This means if a region of pores has one path D

DOI: 10.1021/acs.energyfuels.7b01163 Energy Fuels XXXX, XXX, XXX−XXX

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at temperatures above 1200 K. The predicted CO2 surface areas are 30%−40% higher than the measured values, and the predicted N2 surface areas are 60%−250% higher. In this paper, the effect of molten Metaplast and ash closing pores in the solid char matrix is still neglected, but this effect exists. In the adsorption measurement, the CO2 molecule can go through some very small pores and can diffuse into the areas that N2 cannot reach. The closing effect of molten Metaplast and ash on pores in the solid char matrix for the N2 surface area is larger than that for CO2 surface areas, and therefore the predicted CO2 surface areas are closer to measurements than N2 surface areas. Because of the closing effect of molten Metaplast and ash on pores in the solid char, the accuracy of the model in this paper is still not adequate, and this closing effect is a subject of ongoing research. For the CCNM case, the predicted surface areas are obviously smaller than the NCNM case. In the CCNM case, the predicted N2 surface areas are 44% smaller, and the predicted CO2 surface areas are 14% smaller than the NCNM case at a temperature of 1600 K. This difference is caused by cross-linking reactions. The influence of cross-linking reactions will be discussed in Section 3.2. The difference between the CCCM and CCNM cases is small for the N2 surface area (sN2) at all temperatures and for sCO2 at lower temperatures. The amount of Metaplast formed during lignite pyrolysis is small, so the influence of molten Metaplast is not obvious. The fraction of the ash surface area in the total particle CO2 surface area can become sizable with

Table 3. Calculation Conditions case

condition

CCCM

The effect of cross-linking reactions and the effect of molten Metaplast and ash are both considered. The effect of cross-linking reactions is considered, and the effect of molten Metaplast and ash is neglected. The effect of cross-linking reactions is neglected, and the effect of molten Metaplast and ash is considered. The effect of cross-linking reactions and the effect of molten Metaplast and ash are both neglected.

CCNM NCCM NCNM

the NCNM case (previous model) and the results calculated in the CCNM case are also shown in Figure 4. The filled symbols are the experimental data, the open symbols are the predicted data, and the lines represent the trend of the prediction. The temperature histories used in the calculations are the actual temperature histories reported in the experiments.18 Note that the predicted points and the lines for the trend of prediction in Figure 4 (c) are the predicted points and their trend lines for CCCM in Figure 4 (a) and Figure 4 (b). In the previous model,16 the predicted surface area was much higher than measured at temperatures above 1200 K. The good agreement between measured and predicted surface areas at temperatures below 1200 K is maintained in the CCCM case, and the accuracy is improved at temperatures above 1200 K. The accuracy of predicted N2 surface areas is improved by 53%−65%, and that of predicted CO2 surface areas is improved by 63%−74% at temperatures above 1200 K. However, the predicted values are still much higher than the measured values

Figure 4. Comparison of the predicted change of the Zap lignite particle surface area during pyrolysis with the increase of Tmax to experimental data.18 E

DOI: 10.1021/acs.energyfuels.7b01163 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels increasing ash weight fraction. Because of the melting of ash the difference in predicted sCO2 between these two cases becomes significant at temperatures above 1300 K. The molten ash influences the sCO2 more than sN2; the influence of molten Metaplast and ash will be discussed in Section 3.3 and Section 3.4, respectively. 3.2. Influence of Cross-Linking Reactions. The N2 surface areas of a Zap lignite particle during pyrolysis at various Tmax for the CCCM and NCCM cases at an ambient pressure of 0.1 MPa were calculated, and the results are compared to analyze the influence of cross-linking reactions. Calculation conditions are as follows: a heating rate of 1 × 104 K/s from 300 K to various values of Tmax was used, and then the particle temperatures were held at that temperature until 600 ms had elapsed. The calculated surface areas for the CCCM and NCCM cases with increasing Tmax are shown in Figure 5. The CCCM

Figure 6. Predicted change of 1/2g1, 1/2 bcross‑link 1, bcross‑link 2, 1/ 2g1cross‑linkedδ, spg1, 1/2δ, 1/2δcross‑linked, and 1/2δuncross‑linked during Zap lignite pyrolysis. (Zap lignite, calculations were performed at 0.1 MPa, 1 × 104 K/s, and final temperature of 1200 K, holding at the final temperature until 600 ms elapsed time.)

Figure 5. Predicted change of sN2 (CCCM), sN2 (NCCM), and adsorption pore ΔNcluster at various Tmax. (Zap lignite, calculations were performed at 0.1 MPa and 1 × 104 K/s, holding at the final temperature until 600 ms elapsed time.)

case, which includes the effect of cross-linking, has a similar N2 surface area at lower temperatures to the NCCM case but shows a much lower N2 surface area than the NCCM case at higher temperatures. This is the main reason that the improved model (CCCM) can not only maintain a good agreement at temperatures below 1200 K just as the previous model (NCNM) but also have a much better accuracy at temperatures above 1200 K as shown in Figure 4. The predicted changes of 1/2g1, 1/2bcross‑link 1, bcross‑link 2, 1/ 2gcross‑linkedδ , 1/2spg1, 1/2δ, 1/2δcross‑linked, and 1/2δuncross‑linked are 1 shown in Figure 6. bcross‑link 1, bcross‑link 2, and gcross‑linkedδ occupy 1 some spaces left by g1, making spg1 quite different from g1, and some δ are cross-linked, making the difference between δuncross‑linked and δ also obvious. A higher temperature can provide more energy for the cleaving of side chains to generate g1. Along with increasing g1 the amount of cross-links per cluster (Ncross‑link ) increases as clust pore shown in Figure 7, and therefore the difference of Nadsorption clust adsorption pore between the CCCM and NCCM cases (ΔNclust ) increases with an increasing volatile yield as shown in Figure 5. 3.3. Influence of Molten Metaplast. The surface area change of a Zap lignite particle during pyrolysis at an ambient

Figure 7. Predicted change of the amount of g1 generated from per aromatic cluster (solid line) and the amount of cross-links per aromatic cluster (dash line) with increasing volatile yield. (Zap lignite, calculations were performed at 0.1 MPa, 1 × 104 K/s, and final temperature of 1200 K, holding at the final temperature until 600 ms elapsed time.)

pressure of 0.1 MPa for the CCCM and CCNM cases was calculated, and the results are compared to analyze the influence of molten Metaplast. A heating rate of 1 × 104 K/s from 300 to 1200 K was used, and then the particle temperatures were held constant until 600 ms had elapsed. Note that it was assumed that the ash melts above 1300 K in the ash specific surface area model in this paper (eq 18). The difference in the predicted surface areas between the CCCM and CCNM cases (ΔsN2 and ΔsCO2) is very small, as shown in Figure 8. The amount of Metaplast influences the change of the surface area during coal pyrolysis. The difference in calculated surface areas between the CCCM and CCNM cases is proportional to the change of the Metaplast content, as shown in Figure 9. The small amount of Metaplast formed F

DOI: 10.1021/acs.energyfuels.7b01163 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 8. Predicted change of sN2 and sCO2 for the CCCM and CCNM cases and predicted change of ΔsN2 and ΔsCO2 between the CCCM and CCNM cases during Zap lignite pyrolysis. (Calculations were performed at 0.1 MPa, 1 × 104 K/s, and final temperature of 1200 K, holding at the final temperature until 600 ms elapsed time.)

Figure 9. Predicted change of the surface area difference between the CCCM and CCNM cases and predicted change of the Metaplast content during Zap lignite and Pittsburgh #8 bituminous pyrolysis. (Calculations were performed at 0.1 MPa, 1 × 104 K/s, and final temperature of 1200 K, holding at the final temperature until 600 ms elapsed time.)

Figure 10. Predicted changes in sCO2 for the CCCM, CCNM, and NCNM cases during Pittsburgh #8 bituminous pyrolysis. Calculations were performed at 0.1 MPa, 1 × 104 K/s, and final temperature of 1200 K, holding at the final temperature until 600 ms elapsed time. (Data are from Gale et al.18)

during lignite pyrolysis makes a limited change in the surface area. In contrast, the amount of Metaplast in bituminous during pyrolysis is larger, and therefore the influence of molten Metaplast is much more obvious as shown in Figure 10. The comparison of predicted CO2 surface areas vs the corresponding Pittsburgh #8 coal data from Gale et al.18 is shown in Figure 10 for the same condition used for the Zap lignite in Figure 8. Calculations of sCO2 for the CCCM case for Pittsburgh #8 bituminous coal agree with the measured CO2 surface areas in general, except at the highest values of mass release. The difference between the predictions and the experimental data at the region with high mass release may be caused by the molten ash. The experimental data at high mass release region are

obtained at a temperature above 1300 K, at which temperature the ash may be molten. However, the predicted curves in Figure 10 were calculated at 1200 K, only considering the influence of molten Metaplast; there was no effect of molten ash. The calculated particle surface area first increased slightly, then decreased, and finally increased again with increasing mass release, which agreed with a similar trend of changes in the surface area with increasing temperature observed by Qiu et al.25 3.4. Influence of Molten Ash. Before melting, the surface area of the ash is maintained as a part of the particle surface area. After melting, the pores in the ash are closed completely, and the particle surface area is reduced. The fraction of ash G

DOI: 10.1021/acs.energyfuels.7b01163 Energy Fuels XXXX, XXX, XXX−XXX

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A correlation between cross-links and cleaving of side chains was established to describe the increasing amount of cross-links during coal pyrolysis. The trend predicted by the correlation agrees with experimentally observed trends reported in the literature for the amount of bridges per cluster with increasing mass release. During the cleaving of side chains, a fraction of the side chain sites is linked together to be transformed to cross-links, and higher temperatures can provide more energy for the cleaving of side chains; and therefore the cross-linking reactions influence the change of the surface area in a lignite particle during pyrolysis more significantly at a higher temperature. The amount of Metaplast in the particle during lignite pyrolysis is small, and therefore its influence on the change of the surface area is limited. However, in coals that generate more Metaplast during pyrolysis, the molten Metaplast may have a greater influence on the change of the surface area during coal pyrolysis. The molten ash reduces the particle surface area at high temperatures. This influence is larger for the CO2 surface area than N2, because before ash melting the fraction of ash contributing to the particle CO2 surface area is much larger than that in the N2 surface area.

contributing to the particle surface area before ash melting determines the specific surface area reduced during ash melting. During lignite pyrolysis, the weight fraction of ash ( fash) increases with increasing mass release, which increases the fraction of ash contributing to the particle surface area. Meanwhile, the pore structure of organic matter in char expands with increasing mass release, which can decrease the fraction of ash contributing to the particle surface area. The decreasing effect is bigger than the increasing effect, and therefore the fraction of ash contributing to the particle surface area decreases with increasing mass release as shown in Figure 11.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID Figure 11. Predicted change of fs

N2 ‐ash

, fs

CO2 ‐ash

He Yang: 0000-0001-7604-3750 Thomas H. Fletcher: 0000-0002-9999-4492 Haoquan Hu: 0000-0002-5288-2186

, and fash during Zap

lignite pyrolysis. (Calculations were performed at 0.1 MPa, 1 × 104 K/ s, and final temperature of 1200 K, holding at the final temperature until 600 ms elapsed time.)

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported in part by the National Key Research and Development Program of China (2016YFB0600301), the China Postdoctoral Science Foundation (2017M61227), and the Fundamental Research Funds for the Central Universities No. DUT16RC(3)048.

The fraction of ash contributing to the particle CO2 surface area ( fs ‐ash ) decreases with increasing mass release more CO2

slowly than the particle N2 surface area ( fs

N2 ‐ash

). The initial

CO2 surface area is much larger than the N2 surface area, making the effect of the increasing ash weight fraction on fs ‐ash larger than fs ‐ash . The final fs ‐ash in the particle is CO2

N2

about 35%; however, fs

■

CO2

N2 ‐ash

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4. CONCLUSIONS An improved model for the change of the pore structure in a lignite particle during pyrolysis was developed and evaluated. After considering the effect of cross-linking reactions and considering the effect of molten Metaplast and ash, good agreement between calculated and measured surface areas in lignite particles at temperatures lower than 1200 K shown in a previous model was maintained, and the accuracy with the new model was improved at temperatures above 1200 K. H

DOI: 10.1021/acs.energyfuels.7b01163 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b01163 Energy Fuels XXXX, XXX, XXX−XXX