English

• Biomass is the fourth largest energy source after coal, oil and natural gas and it has the characteristics of renewable, transportable, storable, low pollution and high reactivity. Utilization of biomass energy in the world has attracted great attention for low carbon footprint^{[1, 2]}. Meanwhile, gasification is the indispensable way to achieve carbon cycle utilization of carbonaceous materials including biomass^{[3, 4]}. However, biomass gasification is restricted due to its disadvantages of low density, low calorific value and low thermal efficiency^{[5, 6]}. Co-gasification of biomass and coal overcomes various disadvantages in the process of biomass gasification, and also optimizes the utilization of fossil fuels^[7]. What's more, abundant alkali and alkaline earth metals (AAEMs) in biomass are the potential catalyst to enhance coal gasification reactivity. In hence, it is of great practical significance to investigate co-gasification of biomass and coal.

  Pyrolysis is regarded as the first step of gasification, and coal-biomass interaction during co-pyrolysis dominates the coal char structure. Coal char gasification is rate controlling step of co-gasification of biomass and coal due to the slower gasification rate compared to co-pyrolysis. The interaction between coal and biomass and the potential synergistic effect during gasification has been widely investigated in the last decades^[8]. The dominating factors affecting char gasification reactivity are physicochemical structure and inorganic minerals with catalytic action. In co-pyrolysis process, the physicochemical properties of coal char are changed by biomass composition, and further affects gasification reactivity of coal char^[9]. There are different conclusions about effect of biomass on microcrystalline structure of co-pyrolysis char. Cellulose promoted ordering of micro-scale structure and uniformity of both anthracite and bituminous char^[10], and then cellulose had negative synergetic effect during the gasification process. Adversely, wheat straw might enhance coal char reactivity by inhibiting ordering and uniformity of char structure^[11]. Additionally, pore structure was also influenced during co-pyrolysis due to the difference in volatile content and properties between biomass and coal. Wu et al^[11] showed the addition of wheat straw promoted the increase in number of mesopores within the range of 2-7 nm from co-pyrolysis char of coal and biomass, which improved the specific surface area and led to promotion effect on co-gasification reactivity. In contrast, most scholars pointed out that char gasification reactivity was closely associated with the content of AAEM in biomass ash. Li et al^[12] reported that the reactivity index of co-pyrolysis char of sawdust and bituminous coal reached maximum when the mass ratio of sawdust was 80%, demonstrating that the increasing mass ratios of biomass could promote content of AAEM and specific surface area of co-pyrolysis chars. Zhu et al^[13] mixed coal samples with raw and acid washed wheat straw, and then co-pyrolysis and co-gasification characteristics of the blend were systematically examined. The results showed that the gasification reactivity could be associated with catalysis of AAEM from biomass.

  Much attention has been paid on physicochemical structure and the subsequent gasification characteristics of co-pyrolysis char samples, and then the interaction mechanism between coal and additives was proposed. However, the coal char sample has not been separated from the mixed sample, which would confuse one with these conclusions. To make an in-depth and intuitive understanding of effect of biomass additive on coal char structure and gasification behavior during co-gasification, it is imperative to study the separated coal char sample.

  Notably, effect of volatile-char interactions on char structure and AAEMs in char during co-pyrolysis of biomass and coal has been widely studied. The H radicals formed from thermal cracking and reforming of volatiles are easily released by the volatile-char interactions. Meanwhile, these radicals could penetrate deeply into the char matrix, thus condensation reaction of aromatic rings is enhanced, leading to those small aromatic rings converted into large ones^[14]. Besides, the volatiles are passed through the fine char under the top frit during co-pyrolysis, leading to the AAEM species are deposited on external surface of the char^[15]. Also, the literature^[15] reported that there was a competition between escaping of volatiles from the carbon matrix and deposition of volatiles in the pore structure surface. The influence of volatile-char interactions on the separated coal char sample was remain unclear.

  In addition, pyrolysis conditions and raw material composition have a significant influence on the char structure. Liang et al^[16] found that increasing pyrolysis temperature could greatly decrease graphitization degree and slightly change crystal plane spacing when the temperature was below 1000 ℃. Moreover, many researchers claimed that the synergy effect between biomass and coal might be related to the mass ratio of biomass. Yuan et al^[17] carried out co-pyrolysis of bituminous coal and rice straw, and then showed that synergy effect promoting char conversion was reduced when the mass ratio of biomass was from 20% to 80%, while synergy effect promoting conversion of tar to gas was on the contrast. The reason was that the heating rate was changed by the increased mass ratio of rice straw and packing density was viewed as crucial factor on the heat transfer rate and heating rate. From scientific and engineering points of view, an in-depth knowledge of pyrolysis conditions and raw material composition could provide valuable reference for designing gasification reactors. Meanwhile, the quantitative correlation between structural parameters and reactivity of the char sample should be further explored to clarify the dominant factor affecting the co-gasification reactivity.

  In this work, the char mixture after co-pyrolysis of anthracite and biomass was separated based on the different shape and size, and then structure and reactivity of the coal char were analyzed to reveal the mechanism of coal-biomass interaction. Corn straw (CS) was co-pyrolyzed with an thracite (AC) in a fixed bed reactor at 600 and 900 ℃. After pyrolysis, the biomass char and coal char were physically separated. Inductively coupled plasma-optical emission spectrometry (ICP-OES) system and X-ray diffraction (XRD) were employed to analyze AAEM concentration and microcrystalline structures of coal char samples, respectively. At the same time, thermogravimetric analyzer (TGA) was used for gasification reactivity with CO₂ at 900 ℃. Moreover, relationship between physicochemical structure and reactivity of the char sample was explored.

  1.   Experimental materials and methods

  1.1   Raw materials and preparation of coal char samples

  The typical anthracite (AC) was selected from Shanxi province, and local corn straw (CS) was collected for co-gasification. Raw AC and CS samples were dried at 60 ℃ for 24 h. For the subsequent separation of coal char from co-pyrolysis char sample, AC was grounded and sieved to less than 200 mesh, while the CS was sieved to 40-60 mesh. Table 1 lists the proximate and ultimate analyses of AC and CS samples. The ash chemical compositions (%) of AC and CS ash samples are shown in Table 2. The experimental temperature of the typical anthracite (AC) ash preparation was set at 815 ℃, and the specific procedures followed Chinese National Standards of GB/T 1574—2007. The corn straw (CS) ash was performed at 550 ℃ on the basis of Chinese National Standards of GB/T 30725—2014. The heating rate of both typical anthracite (AC) and corn straw (CS) was controlled at 5 ℃/min Under air atmosphere in the muffle.

  Table 1

  

  Table 1.  Proximate and ultimate analyses of raw materials

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  Sample	Proximate analysis w/%					Ultimate analysis wdaf/%				St, d
  	Mad	Ad	Vdaf	FCd		C	H	Oa	N
  AC	0.80	25.19	14.00	64.33		89.52	4.02	4.42	1.59	0.34
  CS	4.98	5.01	80.58	18.45		48.53	5.64	45.17	0.47	0.19
  ad: air dried basis, d: dry-basis, daf: dry ash-free basis, a: by difference

  Table 2

  

  Table 2.  Chemical compositions of AC ash and CS ash

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  Sample	Content w/%
  	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	TiO2	K2O	P2O5	Na2O
  AC ash	49.11	29.02	12.04	3.91	0.50	2.36	1.91	0.52	0.12	0.51
  CS ash	27.01	0.86	0.43	7.95	12.01	7.54	0.05	35.91	5.86	2.39

  Prior to pyrolysis experiments of the mixture, CS sample was sufficiently blended with AC sample using a mechanical method. The char samples, including AC char, CS char, and the mixture chars of AC with different CS mixing ratios, were obtained from pyrolysis in a fixed bed reactor. The sample was heated from ambient temperature to target temperature (600 and 900 ℃) under high-purity nitrogen. The heating rate was 10 ℃/min. It was kept for 30 min after the temperature reached target temperature, and then cooled down by high purity nitrogen. After co-pyrolysis, the biomass char and coal char were separated physically. The coal char samples separated from that the mixed char samples were named as "CS2-AC8", "CS5-AC5", and "CS8-AC2", indicating that the mass ratio of CS in the mixed sample was 20%, 50%, and 80%, respectively.

  1.2   Concentration of elements in char sample

  The catalytic activity of inorganic minerals was determined by the chemical forms and concentration of different elements in char samples. The measurement was carried out on the basis of Chinese National Standards of MT/T 1014—2006. The detailed procedures were as follows: the coal char sample was heated up to a specific temperature in a muffle furnace. The corresponding ash sample was decomposed with hydrofluoric acid-perchloric acid. Then inductive-coupled plasma optical emission spectroscopy (ICP-OES) was used to determine the contents of K, Na, Ca, Mg, Al, Fe, Si, P, Ti and S. Afterward, the results were given as the form of oxides.

  1.3   Microcrystalline structure analysis of the char sample

  The coal char samples separated from char mixture was characterized by a PANalytical X'Pert³ Powder diffractometer. The sample was scanned at a speed of 4(°)/min with a step length of 0.02° between 10°-80°.

  According to the references^{[18, 19]} carbon structure has three different forms, including poor quality (broad amorphous carbon), a better setting degree (relatively broad turbostratic carbon) and graphite-like (graphitic carbon) carbon structure. In order to obtain the quantitative information about graphitization degree of the char samples, 002 peak was divided into two parts, which were relatively poorly-defined microcrystalline structure (P band) and good microcrystalline structure (G band). The curve-fitted results of peak analysis from XRD pattern of the different char samples are shown in Figure 2. The microcrystalline structure parameters of separated parts could be calculated according to the following conventional Bragg's law and Scherrer equations^[20].

  Figure 2

  

  Figure 2.  Curve-fitting XRD spectrum for 002 peak of various resulting chars

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  $ {d_{{\rm{002, P}}}} = \frac{\lambda }{{2\sin \left( {{\theta _{002, {\rm{P}}}}} \right)}} $

  (1)

  $ {d_{{\rm{002, G}}}} = \frac{\lambda }{{2\sin \left( {{\theta _{002, {\rm{G}}}}} \right)}} $

  (2)

  $ {L_{{\rm{C, P}}}} = \frac{{0.94\lambda }}{{{\beta _{002, {\rm{P}}}}\cos \left( {{\theta _{002, {\rm{P}}}}} \right)}} $

  (3)

  $ {L_{{\rm{C, G}}}} = \frac{{0.94\lambda }}{{{\beta _{002, {\rm{G}}}}\cos \left( {{\theta _{002, {\rm{G}}}}} \right)}} $

  (4)

  where λ is the wavelength of X-radiation, 2 θ_{002, P} and 2 θ_{002, G} mean the scattering angles of P and G peak respectively, d_{002, P} and d_{002, G} represent the corresponding interplanar spacings of the P and G peak, L_{C, P} and L_{C, G} are the crystallite thickness of the P and G peak, β_{002, P} and β_{002, G} are the full widths at half maximum of the P and G peak. The final results of the crystalline structure parameters of char samples could be calculated by following equations:

  $ {d_{{\rm{002, a}}}} = {X_{\rm{P}}}{d_{{\rm{002, p}}}} + {X_{\rm{G}}}{d_{{\rm{002, G}}}} $

  (5)

  $ {L_{{\rm{c, a}}}} = {X_{\rm{P}}}{L_{{\rm{C, P}}}} + {X_{\rm{G}}}{L_{{\rm{C, G}}}} $

  (6)

  $ {X_{\rm{P}}} = \frac{{{S_{\rm{P}}}}}{{{S_{\rm{P}}} + {S_{\rm{G}}}}} $

  (7)

  $ {X_{\rm{G}}} = \frac{{{S_{\rm{G}}}}}{{{S_{\rm{P}}} + {S_{\rm{G}}}}} $

  (8)

  where d_{002, a} and L_{c, a} represent the final values of interplanar spacing and the stacking heights of char samples; X_P and X_G is the percentages. In addition, S_P and S_G is the band areas of the P and G part, respectively^[18].

  1.4   Char gasification

  The gasification reactivity of char samples after pyrolysis were performed via a SETSYS thermogravimetric analyzer (TGA). About 10 mg of the char was used in each experiment. TGA temperature was raised from room temperature to 900 ℃ at 30 ℃/min under inert atmosphere (Ar, 140 mL/min). When the temperature reached 900 ℃, Ar was cut off and CO₂ was immediately injected to start isothermal gasification until the sample mass did not change. The experimental was optimized to eliminate influence of internal and external diffusion.

  $ {x_{\rm{C}}} = \frac{{{m_{\rm{0}}} - {m_t}}}{{{m_{\rm{0}}} - {m_{\rm{a}}}}} \times 100\% $

  (9)

  where x_C is carbon conversion (%); m₀ is initial quality of char sample (mg); m_t is the char quality at time t (mg) and m_a is mass of the char at the end of reaction (mg)^[21].

  The gasification activity index (R_0.5) of the char sample is defined as follows:

  $ {R_{{\rm{0}}{\rm{.5}}}} = \frac{{0.5}}{{{\tau _{{\rm{0}}{\rm{.5}}}}}} $

  (10)

  where τ_0.5 refers to the required time when the carbon conversion reaches 50%.

  The carbon conversion rate (x_C) was differentiated with the corresponding gasification reaction time (t), and the gasification rate (dXc/dt) was obtained.

  $ r = -\frac{1}{{{m_0} - {m_{{\rm{ash}}}}}}\frac{{{\rm{d}}m}}{{{\rm{d}}t}} = \frac{{{\rm{d}}Xc}}{{{\rm{d}}t}} $

  (11)

  2.   Results and discussion

  2.1   Effect of biomass on physicochemical structure of the coal char samples during co-pyrolysis

  2.1.1   Evolution of active AAEM concentration in char samples

  The active AAEM concentration of char samples with various CS mixing ratio at different pyrolysis temperatures are shown in Table 3. Water-soluble and ion-exchanged AAEMs are considered as active AAEMs due to their favorable catalytic effects on coal pyrolysis and gasification. Water-soluble AAEMs mainly contain KCl, K₂CO₃, CaCl₂, etc., while ion-exchanged ones chiefly consist of AAEMs with oxygen-containing functional groups, such as COOK, COK, (COO)₂Ca, etc.^[22-24]. As literature reported, active AAEM concentration was abundant in biomass sample^[13]. It was obviously observed that the active AAEM concentration (especially K and Mg) of the separated coal char samples increased with the CS addition, which could be attributed to the active mineral transformation during co-pyrolysis. Meanwhile, effect of biomass addition on the active AAEM content of coal char was closely associated with the blending ratio of biomass. With the increase of the CS blending ratio, active K and Mg concentration of coal char samples gradually increased. It could be inferred that K and Mg were easily released from biomass during co-pyrolysis, and then usually deposited within the coal char matrix. Taken the volatile-char interactions on char structure into consideration, deposition of AAEM species in the coal char structure was more than its escaping from the carbon matrix during co-pyrolysis process.

  Table 3

  

  Table 3.  AAEM concentration in different char samples

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  Sample	Content w/%
  	SiO2	Al2O3	Fe2O3	CaO	MgO	TiO2	SO3	K2O	Na2O	P2O5
  600AC	50.42	31.19	9.05	3.52	0.43	2.03	2.29	0.46	0.45	0.16
  600CS2-AC8	56.62	27.41	3.69	3.59	0.94	1.24	2.21	3.2	0.81	0.29
  600CS5-AC5	54.65	25.46	3.66	3.52	1.06	1.14	2.37	7.00	0.82	0.32
  600CS8-AC2	54.41	20.92	3.62	3.70	1.78	0.85	2.32	10.93	0.96	0.51
  600CS	18.07	0.74	0.64	10.17	15.14	0.03	7.02	39.73	4.59	3.87
  900AC	51.67	32.76	8.17	3.04	0.66	2.15	0.49	0.43	0.45	0.18
  900CS2-AC8	56.63	28.42	3.96	3.47	1.13	1.28	0.66	3.41	0.74	0.3
  900CS5-AC5	54.42	26.38	3.45	3.64	1.66	1.19	1.42	6.88	0.6	0.36
  900CS8-AC2	52.45	21.98	2.94	4.16	2.89	0.96	2.5	10.9	0.58	0.64
  900CS	26.1	0.81	0.56	10.78	15.62	0.04	4.78	34.08	3.16	4.07

  In addition, pyrolysis temperature had a slightly negative influence on K and Na contents of the char samples. In contrast, Ca and Mg contents slightly increased with the rise of temperature. It may be attributed to the slight inactivation and volatilization of active AAEM content at higher pyrolysis temperature. Also, volatilization of K and Na through volatile-char interaction was more remarkable compared with Ca and Mg.

  2.1.2   Evolution of microcrystalline structure of char samples

  As shown in Figure 1, two obvious peaks at 2 θ angle range of 14°-37.5° and 38°-50° existed in XRD patterns of char samples, which corresponded to diffraction planes (002) and (100), respectively. With the increase of CS proportion, the 002 peak intensity of coal char samples at 600 ℃ decreased from 6049 to 3672, while that at 900 ℃ reduced from 5787 to 4781. Besides, the shape of 002 peak gradually became asymmetrical and broad. It could be rationally inferred that addition of CS could hinder the order degree of AC char, and inhibition effect on the graphitization process was gradually promoted with increasing CS proportion. Also, pyrolysis temperature played a significant role in the variations of the microcrystalline structure of the co-pyrolysis char samples.

  Figure 1

  

  Figure 1.  XRD patterns of different char samples at a pyrolysis temperature of (a) 600 ℃ and (b) 900 ℃

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  To quantitatively determine graphitization degree of the char samples, the 002 peak was well curve-fitted in Figure 2, which could provide reliable structure parameters. The microcrystalline structure parameters of the char samples are listed in Table 4. At a pyrolysis temperature of 600 ℃, L_{c, a} of coal char decreased from 1.974 to 1.215 nm with increasing CS proportion, and N value reduced from 5.453 to 3.338. Similarly, L_{c, a} of coal char samples at 900 ℃ decreased from 1.633 to 0.989 nm, and N value reduced from 4.425 to 2.680. It could be concluded that the stacking heights L_{c, a} and the stacking layer number N of coal char samples at the same pyrolysis temperature gradually decreased with increasing CS proportion, while the interplanar spacing d_{002, a} showed negligible changes. Also, at the same mixing ratio, the L_{c, a} and N of coal char samples significantly decreased with the increase of pyrolysis temperature, indicating that a more disordered structure in the char samples formed with the treatment of higher temperature. Combined the qualitative and quantitative results, it was conspicuous that the additive CS promoted the disorder of coal char, and then reduced the degree of graphitization. Meanwhile, the inhibition effect on the graphitization process was enhanced at a higher pyrolysis temperature. Liang et al^[16] obtained the similar conclusions that the band area ratios I_D/I_G increased with increasing pyrolysis temperature from 300 to 900 ℃, indicating that pyrolysis temperature had a negative influence on the graphitization process below 900 ℃. These phenomena could be attributed to inhibition effect of AAEM species on the graphitization process and volatile-char interaction. As mentioned above, the deposited AAEM species on the char matrix might be fixed on adjacent basic structural units, resulting in destruction of parallelism of the layers. Accordingly, the literature supported that the AAEM species could obviously hinder graphitization process of coal sample at a relatively low temperature^{[25, 26]}. Although the volatile-char interaction could enhance condensation reaction of aromatic rings^[14], the inhibition effect of AAEM species on the graphitization process played a decisive role in the microcrystalline structure of coal char instead of the promotion effect of volatile-char interaction. Therefore, the more disordered structure formed with increasing CS proportion.

  Table 4

  

  Table 4.  Microcrystalline parameters of studied char samples

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  Sample	d002, P/nm	LC, P/nm	d002, G/nm	LC, G/nm	XP	XG	d002, a/nm	Lc, a/nm	N(Lc, a/ d002, a)
  600AC	0.408	2.063	0.347	1.945	24.09	75.91	0.362	1.974	5.453
  600CS2-AC8	0.402	2.014	0.348	1.939	24.25	75.75	0.361	1.957	5.421
  600CS5-AC5	0.411	2.308	0.351	1.747	12.64	87.36	0.358	1.818	5.078
  600CS8-AC2	0.408	1.725	0.351	1.561	13.53	86.47	0.359	1.583	4.409
  600CS	0.413	2.020	0.356	1.087	13.77	86.23	0.364	1.215	3.338
  900AC	0.397	1.528	0.343	1.732	48.40	51.60	0.369	1.633	4.425
  900CS2-AC8	0.399	1.574	0.349	1.562	36.58	63.42	0.367	1.567	4.270
  900CS5-AC5	0.400	1.417	0.357	1.351	18.48	81.52	0.365	1.363	3.734
  900CS8-AC2	0.391	1.273	0.352	1.253	33.05	66.95	0.365	1.259	3.449
  900CS	0.402	1.231	0.356	0.892	28.55	71.45	0.369	0.989	2.680

  The decreasing extent of graphitization degree was controlled by the pyrolysis temperature, which could be resulted from two aspects. During pyrolysis the macromolecular network structure of coal sample will undergo polycondensation, and then form condensed aromatic structural units. Meanwhile, the chemical bonds in the macromolecular compounds break to form small molecular compounds. Accordingly, some small compounds are deposited on the surface of char sample, forming a large number of defects and amorphous structures. On the other hand, coal char sample is still in carbonization stage at low pyrolysis temperature, and the graphitization process has not yet begun^[16].

  2.2   Effect of biomass on the gasification characteristic

  2.2.1   Effect of biomass blending ratio

  The gasification characteristic curves of different char samples are shown in Figure 3 and Figure 4. As illustrated in Figure 3, carbon conversion and reaction time curves showed that the gasification reactivity was significantly influenced by the different CS blending ratios. It could be obviously observed that the reaction time for char samples prepared at 600 ℃ to reach the same carbon conversion gradually reduced with the increase of CS proportion. Given the whole gasification process, the gasification reactivity in the sequence of CS > CS8-AC2 > CS5-AC5 > CS2-AC8 > AC char was obvious. The sequence of gasification reactivity for different char samples prepared at 900 ℃ was consistent with that of 600 ℃. The results demonstrated that the CS additive was beneficial for improving gasification reactivity of the coal char samples prepared at different pyrolysis temperatures. In addition, the carbon conversion and reaction rate curves of coal char samples in Figure 4 showed that the reaction rate increased first and then decreased, indicating that the reaction rate varied with carbon conversion. The evolution of char structure could be responsible for the dynamic reaction rate during the whole gasification process. Therefore, an acceptable indicator should be proposed to quantitatively reflect the overall gasification reactivity.

  Figure 3

  

  Figure 3.  Carbon conversion versus reaction time of char samples at a pyrolysis temperature of (a) 600 ℃ and (b) 900 ℃

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  Figure 4

  

  Figure 4.  Carbon conversion and reaction rate of char samples at a pyrolysis temperature of (a) 600 ℃ and (b) 900 ℃

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  Reactivity index R_0.5 has been widely used for evaluating gasification characteristics. As shown in Figure 5(a), R_0.5 values of coal char samples prepared at 600 ℃ increased from 0.2215 to 1.5517 h^-1, while the one at 900 ℃ increased from 0.2389 to 1.2802 h^-1 with CS proportion increased from 0 to 80%. Moreover, the specific reactivity index SR_0.5 should be proposed to quantitively evaluate influence of CS additive on the gasification reactivity during co-pyrolysis in this work. The definition of SR_0.5 is described as follows^[27]:

  Figure 5

  

  Figure 5.  (a) Gasification reactivity index and (b) specific reactivity of char samples

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  $ S{R_{0.5}} = \frac{{{R_{{\rm{0}}{\rm{.5, char{\kern 1pt} {\kern 1pt}sample{\kern 1pt} {\kern 1pt}with{\kern 1pt} {\kern 1pt}CS{\kern 1pt} {\kern 1pt}additive}}}}}}{{{R_{{\rm{0}}{\rm{.5, char{\kern 1pt} {\kern 1pt}sample{\kern 1pt} {\kern 1pt}without{\kern 1pt} {\kern 1pt}CS{\kern 1pt} {\kern 1pt}additive}}}}}} $

  (12)

  SR_0.5 values of coal char samples at different mass ratios and pyrolysis temperature are presented in Figure 5(b). As the mass ratio of biomass increased from 20% to 80%, SR_0.5 values of coal char prepared at 600 ℃ monotonously increased from 1.38 to 7.01, and the one at 900 ℃ was from 1.18 to 5.36. It has been generally accepted that a higher SR_0.5 represented a more significant promotion effect of CS additive on gasification reactivity of coal char sample. Therefore, larger biomass blending ratio could enhance the promotion effect.

  2.2.2   Effect of pyrolysis temperature

  As shown in Figure 5(a), it was worth noticing that gasification reactivity index of coal char at the same CS proportion reduced with the rise of pyrolysis temperature, indicating that gasification reactivity of char sample was more significant at the lower pyrolysis temperature. Meanwhile, SR_0.5 values distinctly decreased as the pyrolysis temperature increased from 600 to 900 ℃. Consequently, low pyrolysis temperature could improve the promotion effect of CS additive on gasification reactivity of coal char sample. Therefore, large biomass blending ratio could improve the promotion effect. Interestingly, the influence of pyrolysis temperature on gasification reactivity was more significant as blending ratios increased from 20% to 80%. Many literatures have proposed that gasification reactivity is closely related to the physicochemical structure of char. A reasonable correlation between char structure and gasification reactivity should be explored to elucidate the phenomenon described above.

  2.3   Correlation between char gasification reactivity and structural parameters

  The critical factors on gasification reactivity of the char sample have been widely studied, including volatile content, content and composition of ash, specific surface area and microstructure^[28-30]. Many researchers pointed out that gasification reactivity of char was inversely proportional to the graphitization degree^[31-33]. However, at the same CS proportion, the stacking heights L_{c, a} and the stacking layer number N of coal char samples prepared at 600 ℃ were larger than that of 900 ℃, while R_0.5 values of the coal char samples at 600 ℃ were higher than that of 900 ℃. Also, it could be observed in Figure 6 that there was no good relationship between the microcrystalline structural parameters and the reactivity index. Although an outlier was removed, the linear correlation coefficient between L_{c, a} and R_0.5 was only 0.5375 in Figure 6(c). That is to say, microcrystalline structure cannot be the crucial factor for gasification reactivity in this work.

  Figure 6

  

  Figure 6.  Correlation between R_0.5 and microcrystalline parameters of char samples

  (a): R_0.5 and L_{c, a} of all char samples; (b): R_0.5 and N of all char samples; (c): R_0.5 and L_{c, a} of chars where the abnormal sample was removed; (d): R_0.5 and N of chars where the abnormal sample was removed

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  As we know, most of the volatile matter has been removed during pyrolysis, indicating that volatile content may not be the primary factor determining the gasification reactivity. Accordingly, coal char consists of carbon matrix and ash minerals, implying that influence of the ash content on gasification characteristic of coal char should be explored. Moreover, many literatures have reported that K₂O, Na₂O, CaO, MgO and Fe₂O₃ in char sample could play an important role in catalytic effect during gasification. In contrast, SiO₂ and Al₂O₃ could play an inhibition effect on gasification reactivity of the char sample. Therefore, the concept of alkali index (AI) was proposed to quantify the catalytic activity of ash minerals^{[34, 35]}. The calculation formula is described as follows:

  $ AI = {A^{\rm{a}}} \times \frac{{{\rm{F}}{{\rm{e}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}{\rm{ + CaO + MgO + N}}{{\rm{a}}_{\rm{2}}}{\rm{O + }}{{\rm{K}}_{\rm{2}}}{\rm{O}}}}{{{\rm{Si}}{{\rm{O}}_{\rm{2}}}{\rm{ + A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}}} $

  (13)

  where A^a is the ash content in coal char samples, which could be obtained from the result of TG curve.

  As shown in Figure 7, a satisfactory linear correlation between alkali index AI and R_0.5 was established. Results showed that AI performed better than the microcrystalline structural parameters when they were employed for predicting gasification reactivity of the char samples prepared at different CS blending ratio under various pyrolysis temperatures. It was undeniable that the microcrystalline structure also had some influence on gasification process. However, effect of AAEMs and char structure evolution on char reactivity was not quantified in this work. More efforts will be paid on the biomass-char interaction to quantify its effect on gasification.

  Figure 7

  

  Figure 7.  Correlation between R_0.5 and AI of char samples

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  3.   Conclusions

  In this work, coal-biomass interaction during co-pyrolysis by char separation was investigated. The deposition of AAEM species in the coal char and volatile-char interactions demonstrated coal-biomass interaction on the coal char structure during co-pyrolysis process. Meanwhile, the dominant factor on gasification reactivity of char sample was explored. The main conclusions were summarized as follows:

  With the increase of CS blending ratio, concentration of active K and Mg in coal char samples gradually increased. The deposition of AAEM species in the coal char structure by volatile-char interactions was more than that escaping from biomass during co-pyrolysis process. Meanwhile, higher pyrolysis temperature led to the inactivation and volatilization of K and Na.

  Inhibition effect of AAEM species on graphitization process played a decisive role in microcrystalline structure of the coal char instead of promotion effect of volatile-char interaction, resulting in formation of more disordered carbon structure in the separated coal char samples as the CS proportion increased from 0 to 80%. Also, graphitization degree of the char samples prepared at 900 ℃ was slightly lower than that of 600 ℃.

  Both large CS proportion and low pyrolysis temperature could improve the promotion effect of CS additive on gasification reactivity of coal char sample. Also, influence of pyrolysis temperature on promotion effect was more significant as blending ratios increased from 20% to 80%.

  An acceptable linear correlation (R²=0.9009) between alkali index AI and R_0.5 of the char samples was established. AAEMs played a more significant role in evaluating the gasification reactivity compared with microcrystalline structure during char gasification with CO₂.

  
  
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• 

  Figure 2  Curve-fitting XRD spectrum for 002 peak of various resulting chars

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  Figure 1  XRD patterns of different char samples at a pyrolysis temperature of (a) 600 ℃ and (b) 900 ℃

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  Figure 3  Carbon conversion versus reaction time of char samples at a pyrolysis temperature of (a) 600 ℃ and (b) 900 ℃

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  Figure 4  Carbon conversion and reaction rate of char samples at a pyrolysis temperature of (a) 600 ℃ and (b) 900 ℃

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  Figure 5  (a) Gasification reactivity index and (b) specific reactivity of char samples

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  Figure 6  Correlation between R_0.5 and microcrystalline parameters of char samples

  (a): R_0.5 and L_{c, a} of all char samples; (b): R_0.5 and N of all char samples; (c): R_0.5 and L_{c, a} of chars where the abnormal sample was removed; (d): R_0.5 and N of chars where the abnormal sample was removed

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  Figure 7  Correlation between R_0.5 and AI of char samples

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  Table 1.  Proximate and ultimate analyses of raw materials

  Sample	Proximate analysis w/%					Ultimate analysis wdaf/%				St, d
  	Mad	Ad	Vdaf	FCd		C	H	Oa	N
  AC	0.80	25.19	14.00	64.33		89.52	4.02	4.42	1.59	0.34
  CS	4.98	5.01	80.58	18.45		48.53	5.64	45.17	0.47	0.19
  ad: air dried basis, d: dry-basis, daf: dry ash-free basis, a: by difference

  下载: 导出CSV

  Table 2.  Chemical compositions of AC ash and CS ash

  Sample	Content w/%
  	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	TiO2	K2O	P2O5	Na2O
  AC ash	49.11	29.02	12.04	3.91	0.50	2.36	1.91	0.52	0.12	0.51
  CS ash	27.01	0.86	0.43	7.95	12.01	7.54	0.05	35.91	5.86	2.39

  下载: 导出CSV

  Table 3.  AAEM concentration in different char samples

  Sample	Content w/%
  	SiO2	Al2O3	Fe2O3	CaO	MgO	TiO2	SO3	K2O	Na2O	P2O5
  600AC	50.42	31.19	9.05	3.52	0.43	2.03	2.29	0.46	0.45	0.16
  600CS2-AC8	56.62	27.41	3.69	3.59	0.94	1.24	2.21	3.2	0.81	0.29
  600CS5-AC5	54.65	25.46	3.66	3.52	1.06	1.14	2.37	7.00	0.82	0.32
  600CS8-AC2	54.41	20.92	3.62	3.70	1.78	0.85	2.32	10.93	0.96	0.51
  600CS	18.07	0.74	0.64	10.17	15.14	0.03	7.02	39.73	4.59	3.87
  900AC	51.67	32.76	8.17	3.04	0.66	2.15	0.49	0.43	0.45	0.18
  900CS2-AC8	56.63	28.42	3.96	3.47	1.13	1.28	0.66	3.41	0.74	0.3
  900CS5-AC5	54.42	26.38	3.45	3.64	1.66	1.19	1.42	6.88	0.6	0.36
  900CS8-AC2	52.45	21.98	2.94	4.16	2.89	0.96	2.5	10.9	0.58	0.64
  900CS	26.1	0.81	0.56	10.78	15.62	0.04	4.78	34.08	3.16	4.07

  下载: 导出CSV

  Table 4.  Microcrystalline parameters of studied char samples

  Sample	d002, P/nm	LC, P/nm	d002, G/nm	LC, G/nm	XP	XG	d002, a/nm	Lc, a/nm	N(Lc, a/ d002, a)
  600AC	0.408	2.063	0.347	1.945	24.09	75.91	0.362	1.974	5.453
  600CS2-AC8	0.402	2.014	0.348	1.939	24.25	75.75	0.361	1.957	5.421
  600CS5-AC5	0.411	2.308	0.351	1.747	12.64	87.36	0.358	1.818	5.078
  600CS8-AC2	0.408	1.725	0.351	1.561	13.53	86.47	0.359	1.583	4.409
  600CS	0.413	2.020	0.356	1.087	13.77	86.23	0.364	1.215	3.338
  900AC	0.397	1.528	0.343	1.732	48.40	51.60	0.369	1.633	4.425
  900CS2-AC8	0.399	1.574	0.349	1.562	36.58	63.42	0.367	1.567	4.270
  900CS5-AC5	0.400	1.417	0.357	1.351	18.48	81.52	0.365	1.363	3.734
  900CS8-AC2	0.391	1.273	0.352	1.253	33.05	66.95	0.365	1.259	3.449
  900CS	0.402	1.231	0.356	0.892	28.55	71.45	0.369	0.989	2.680

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