CO2 mixture in a drop-tube furnace: Experimental investigation and numerical simulation

Accepted Manuscript Combustion interactions of blended coals in an O2/CO2 mixture in a drop-tube furnace: Experimental investigation and numerical simulation Lun Ma, Anlong Guo, Qingyan Fang, Tingxu Wang, Cheng Zhang, Gang Chen PII: DOI: Reference:

S1359-4311(18)33454-9 https://doi.org/10.1016/j.applthermaleng.2018.09.033 ATE 12642

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

3 June 2018 15 August 2018 8 September 2018

Please cite this article as: L. Ma, A. Guo, Q. Fang, T. Wang, C. Zhang, G. Chen, Combustion interactions of blended coals in an O2/CO2 mixture in a drop-tube furnace: Experimental investigation and numerical simulation, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.09.033

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Combustion interactions of blended coals in an O2/CO2 mixture in a drop-tube furnace: Experimental investigation and numerical simulation

Lun Ma, Anlong Guo, Qingyan Fang※, Tingxu Wang, Cheng Zhang, Gang Chen

†

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, P R China

A manuscript submitted to

Applied Thermal Engineering 

to whom all correspondence should be addressed

Qingyan Fang Address: State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, P R China Tel.:+86 27-87542417; fax: +86-27-87540249; e-mail: [email protected].

1

ABSTRACT Interactions, known as the ignition promotion and burnout inhibition, exist during the combustion of blended coals in the air. The oxy-fuel conditions influence these interactions due to the change of the gas physical properties and the gasification reaction. However, little research has focused on the oxy-fuel combustion interactions of blended coals. To investigate the combustion interactions of blended coals in an O2/CO2 atmosphere, experimental and numerical studies of the mixed combustion of less- and more-volatile coals are performed in a drop tube furnace. The results show that the more-volatile coal combustion brings the promotive effect on the devolatilization of the less-volatile coal and the inhibitive effect on the char combustion of the less-volatile coal. Compared with that in the air, the promotive effect is weaker, but the inhibitive effect is stronger in the 21%O2/79%CO2 environment due to the different physical properties of the gases. In the O2/CO2, increasing the oxygen concentration increases the promotive effect but weakens the inhibitive effect owing to the change of the physical properties of gases and the reactivity improvement of the less-volatile coal char. In addition, the promotive and inhibitive effects achieve a level similar to that in the air when the oxygen concentration is increased to approximately 30-35%. Furthermore, the char-CO2 gasification reaction in the O2/CO2 has little impact on the promotive effect but weakens the inhibitive effect. Increasing the reactor wall temperature weakens both the promotive effect and the inhibitive effect.

Key words: Oxy-fuel combustion; blended coals; interactions.

2

Highlights: 1.

Weaker promotion and stronger inhibition are shown in O2/CO2 than in O2/N2.

2.

Increasing O2 increases promotion but weakens inhibition.

3.

Promotion and inhibition in 30-35% O2 achieve similar levels to that in air.

4.

Char-CO2 reaction slightly affects promotion but greatly weakens inhibition.

5.

Increasing the reactor wall temperature weakens promotion and inhibition.

3

1. Introduction Blended-coal combustion technology has been widely used in electric power plants to improve fuel flexibility, increase coal performance, reduce pollutant emissions, improve ash deposits and so on [1-5]. Interactions, known as the promotive effect on the ignition of the less-volatile coal and the inhibitive effect on the burnout of the less-volatile coal, exist during the combustion of blended coals and influence the combustion behavior [6-12]. The promotive effect is that volatile matter from the more-volatile coal easily releases and combusts to form a high gas temperature environment, which heats the less-volatile coal and promotes its devolatilization and ignition. The inhibitive effect is that oxygen is rapidly consumed by more-volatile coal, which delays the char combustion and burnout of less-volatile coal. The competition between the promotive and inhibitive effects is interactions that affects the

combustion

characteristics

of

blended

coals

and

the

combustion

additive/non-additive behavior of blended coals. Currently, oxy-fuel combustion is recognized as one of the most promising CO 2 reduction and capture technologies [13-17]. For coal-fired power stations, the combination of oxy-fuel and blended coals combustion can not only reduce the CO2 emissions but also improve both the energy utilization and conversion efficiency. Coal combustion in an O2/CO2 atmosphere differs from that in the air due to (i) the differences in the physical and thermal properties of N 2 and CO2 [16, 18, 19] and (ii) char-CO2 gasification [16, 20-22]. Oxy-fuel conditions influence these interactions by changing the physical properties of gas and the gasification reaction, which may 4

further influence the combustion characteristics and the additive/non-additive behavior of blended coal combustion. Therefore, it is necessary to investigate the combustion interactions of blended coals in the O2/CO2 atmosphere to provide useful information for the design and optimization of blended thermo-chemical conversion systems in the O2/CO2 atmosphere. Although many studies about the oxy blend combustion, especially oxy-coal–biomass combustion [23, 24], have been reported, the combustion characteristics between biomass and coal are different and few researches have focused on the interactions during the oxy-fuel combustion process of coal-coal blends (blended coals). In this study, to reveal the combustion interactions of blended coals in the O2/CO2 atmosphere, experimental studies were firstly performed in a drop tube furnace (DTF). Then, numerical simulations were carefully carried out to further elucidate the effects of physical properties of gas, oxygen concentration, char-CO2 gasification reaction, and reactor wall temperature on the combustion interactions of blended coals. 2. Experimental section 2.1 Samples Generally, the combustion interactions are relatively significant for blended coals with a large difference in ranks. Two different ranks of coals, Jincheng (JC) and Shenhua coal (SH), are used in this study. Table 1 summarizes their proximate analysis and ultimate analysis, obtained using Vario-EL-2 and TGA 2000 (Navas Instruments, Spain). Before each experiment, air-dried coal samples are ground and 5

sieved to 75-125 µm particle size and then dried at 105℃ for 24 h to remove the moisture. In addition, except for pure coals (JC and SH), blended coals consisting of 25, 50, and 75% SH coal are tested. Table 1. Proximate analysis and ultimate analysis of coal samples in this work. Sample

d

Ultimate analysis (d, wt%)

Proximate analysis (d, wt%)

Qnet,d

name

Volatile matter

Ash

Fixed carbon

C

H

Oa

N

S

(MJ/kg)

JC

10.89

14.46

74.65

70.29

4.08

9.98

0.90

0.29

27.3

SH

32.32

10.27

57.41

73.15

2.99

12.25

1.06

0.28

24.5

dry; a By difference.

2.2 Drop tube furnace and experiment

(a)

(b)

Fig. 1. Schematic of DTF (a), and measured center-line gas temperature without coal feeding at a reactor temperature of 1323 K (b). In order to reveal the combustion interactions of blended coals in the O2/CO2 atmosphere, experimental studies are performed in a DTF with a high heating rate and short residence time. Generally, the particle heating rates in the DTF are close to those occurring in industrial coal combustors (104–105 K/s) [25, 26]. A schematic diagram of the DTF is shown in plane (a) of Fig. 1. The DTF has an internal diameter of 0.056 m and a length of 1.60 m, where the reaction zone is assumed to have a length up to 6

1.30 m. The heat required to maintain the designed temperature is supported by 36 electric-heated silicon carbide rods uniformly distributed around the tube and the temperature is measured by 4 thermocouples. The DTF is electrically heated and has a maximum temperature of 1300℃. A powder feeder with two-stage vibration is used to continuously introduce coal particles into the reactor tube, and a water-cooled injector is adopted to ensure that the temperature does not exceed 100℃ before entering the reactor tube. Further, a water-cooled collecting probe is inserted into the reaction chamber from below. In this work, three binary gas mixtures of O2 and CO2 (21% O2/79% CO2, 25% O2/75% CO2 and 30% O2/70% CO2) are used in the experiment, and the air (21% O2/79% N2) is used as the reference. The flow rates of N2, CO2 and O2 from the gas cylinders are controlled by mass flow controllers, and the total gas flow is adjusted to 5 L/min (cold). The excess oxygen coefficient is set at 1.2. The coal feeding rate of each case is determined based on the ultimate analysis data to ensure the total flow rate of 5 L/min (cold) corresponding to the same residence time. The primary inert gas (N2 or CO2) is used to carry pulverized-coal particles into the reactor tube, and the remaining mixture gas (O2/N2 or O2/CO2) is introduced via the secondary ports. The combustion experiments are carried out at a reactor temperature of 1323 K in this study. Plane (b) of Fig. 1 shows the measured centerline gas temperature without coal feeding at a reactor temperature of 1323 K under different atmospheres. The stable operation of the DTF is the basis for obtaining reliable experiment burnout, and this process requires the pulverized-coal feeding system to work well. 7

Typical variations (O2 and NOx) at the exit are monitored during the experiment to ensure the stability of the DTF operation, and some monitored results are displayed in Fig. 2. It can be seen that the concentration curves remain stable under each condition, the stable values under different conditions indicate that the coal feeding system is working well and the experimental data are reliable.

Fig. 2 Typical variations at the DTF exit during the experiment (25%SH+75%JC combustion in 21%O2/79%N2, excess oxygen coefficient is different for three conditions, condition 1: 1.0, condition 2: 1.2, condition 3: 1.4). In this study, the ash tracer method is adopted to determine the burnout of the tested coals. The obtained burnout value is calculated according to the ash balance as follows [26, 27]: B u r n o( u%t ) (1

m /a m) / ( 1a, 0m

a, 0

/ 100) 100

(1)

Where ma,0 is the ash content of the raw pure coal or the ash content of raw blending coal (which is calculated as the mass-weighted average of pure coals), ma is the ash content of the collected sample. 3. Simulation section The commercial CFD (computational fluid dynamics code) ANSYS FLUENT 16.0 is adopted to predict the coal combustion process in the DTF [28]. Fluid phase 8

and particle phase are modeled using the Eulerian and Eulerian–Lagrangian approaches, respectively. A steady-state RANS (Reynolds Averaged Navier Stokes) approach with realizable k-epsilon model is used to calculate the dynamic of the turbulence flow in the DTF, which has been used successfully to air-coal and oxy-coal combustion [29, 30]. The stochastic particle trajectory model is used to simulate the flow of the pulverized coal particles in the DTF. Fig. 3 shows the coal particles size distribution. The particle size is assumed to obey the Rosin-Rammler distribution from 75 µm to 125 µm (a mean diameter: 95 µm, a spread parameter: 1.5). In this simulation, 10 size groups are considered, and 21360 particles are tracked for the JC and SH coals, respectively. Due to the higher accuracy and smaller optical length of the DTF, the discrete ordinate model is adopted to solve the radiation heat transfer equation, and the scattering factor and emissivity of the coal particle are set at average constants of 0.6 and 0.9, respectively [31]. To simulate the radiative heat transfer of gases, the cell-based weighted-sum-of-gray-gas (WSGG) model proposed by Smith is inappropriate for oxy-fuel combustion because the molar ratio was considerably different from that in air-fired combustion due to the high concentrations of CO2 and H2O [32, 33]. In the present study, a refined WSGGM by HUST is applied to calculate the gas emissivity, more details could be found from these studies [28, 34]. The refined WSGGM is implemented in the CFD model via the user-defined function in ANSYS FLUENT. The devolatilization behavior is characterized via the chemical percolation devolatilization (CPD) model [35]. Each parameter is an input for the CPD sub-model, 9

as shown in Table 2.

Fig. 3 Coal particles size distribution Table 2 CPD input parameters for devolatilization Coal

Input parameter for devolatilization rate p0

σ+1

Mw,1

Mw,δ

JC

0.842

4.27

244.2

14.7

SH

0.637

5.2

303.6

37.0

p0: initial fraction of bridges in the coal lattice; σ+1: lattice coordination number; Mw,1: cluster molecular weight; Mw,δ: side chain molecular weight.

The finite rate/eddy dissipation model is applied to simulate the gas phase reactions. A two-step global reaction mechanism is employed for gas-phase combustion of the released volatiles for both air-fuel and oxy-fuel combustion modeling. x z y m Cx H y Oz Sn N m  (  n  )O2  xCO  H 2O  nSO2  N 2 2 2 2 2

(2)

CO  0.5O2  CO2

(3)

Where the coefficients x, y, z, m and n are obtained from the proximate and ultimate analyses of coal. The multiple-surface-reaction model is employed to model the char reaction. In this study, coal samples are fully dried before each experiment. Meanwhile, the mass fraction of H2O is very little and significantly lower than that of CO2 in the dry 10

oxy-fuel combustion process. As a consequence, the effect of the H2O gasification reaction is ignored. Therefore, only the char oxidation and char gasification with CO2 are considered in this study. At high temperatures as in this DTF, CO is the dominant product in char oxidation [32]. C(s ) 0 . 5O 2 C O

(4)

C(s )  CO2  2CO

(5)

According to the difference of SH and JC reactivity characteristics, different coals adopt different kinetic parameters [13, 22, 32, 36, 37]. Table 3 lists the kinetic parameters for different reactions. The overall mass diffusion-limited constant is 5.32×10−12 kg/m2·s·Pa (air-fuel condition) and 4.13×10−12 kg/m2·s·Pa (oxy-fuel condition) for char oxidation reaction, 1.72×10−12 kg/m2·s·Pa (air-fuel condition) and 1.69×10−12 kg/m2·s·Pa (oxy-fuel condition) for CO2-char reaction [13]. Table 3 Kinetic parameters Reaction (2) (3) (4) (5)

Activation energy

Coal

Pre-exponential factor

JC

2.119E11

2.027E08

SH

2.119E11

1.800E08

JC/SH

2.239E12

1.700E08

JC

0.00170

8.370E08

SH

0.07400

5.700E07

JC

0.00635

1.620E08

SH

0.06500

1.650E08

(J/kmol)

In the simulation, mesh is critical to the accuracy of the calculation results [38-40]. In this paper, hexahedral mesh elements are adopted in the DTF. Fig. 4-(a) shows the CFD three-dimensional model and the cross-section meshing (near the primary inlet) of the DTF. As demonstrated in Fig. 4-(b) for the predicted burnout

11

under different grid cells, the grid of 350,000 cells yields almost the same results as the grids with 492,000 and 584,000 cells. Thus, the grid with 492,000 cells is selected and used in this study. The SIMPLE algorithm is used to consider the pressure and velocity coupling [41].

(a) CFD model and mesh

(b) mesh-independence test

Fig. 4 CFD model and mesh of DTF During the combustion process of blended coals, the more-volatile coal (SH) within blended coals presents preferential combustion to form a high gas temperature environment, promoting the devolatilization and ignition of the less-volatile coal (JC); but the more-volatile coal combustion consumes more oxygen, as a results, a depleted oxygen condition is formed around the less-volatile coal (JC) particle and the char combustion of the less-volatile coal (JC) is delayed and inhibited. Some parameters are introduced to evaluate the combustion interactions and the additive/non-additive behaviors of blended coals as the followings. The devolatilization of coal has a great effect on the coal ignition. In this paper, to evaluate the promotive effect on the devolatilization of JC coal, the duration ( t ) and the duration difference ( t ) of JC coal devolatilization are defined. The duration 12

( t ) is the time from start to finish of the coal devolatilization. When the duration of component JC coal in the blended coals is lower than that of pure JC coal, t  0 , demonstrating that there exists the promotive effect on the devolatilization of component JC coal in the blended coals. This is advantageous to the ignition of blended coals. The larger the value of t , the stronger the promotive effect on the devolatilization of the component JC coal, the better the ignition of the component JC coal and blended coals. The duration difference is calculated as follows: t =t pure JC  tcomponent  JC

(6)

where tpure-JC is the devolatilization duration of the pure JC coal, tcomponent-JC is the devolatilization duration of the component JC coal in the blended coals. In addition, to evaluate the inhibitive effect on the char combustion process and burnout of the component JC coal, the location (L) and the location difference ( L ) of the char peak burning rate of the component JC, as well as the relative difference of JC coal char burnout (RDB of JC char), are also introduced in this study. When the location of the peak burning rate of the component JC coal char is farther than that of the pure JC coal, L  0 , demonstrating that there exists the inhibitive effect on the char combustion process of component JC coal in the blended coals. As a consequence, the char burnout of the component JC coal in the blended coals is affected and lower than that of the pure JC coal, which is presented as: RDB of JC char <0. The larger the L and |RDB| of JC char, the stronger on the char combustion process and burnout of the component JC coal. L and RDB of JC char are calculated using the following equations: 13

L=Lcomponent  JC -char  Lpure  JC char

RDB of JC Char (%)=

Burnoutcomponent  JC char  Burnout pure JC char Burnout pure JC char

(7) 100

(8)

where Lcomponent-JC-char is the location of the peak char burning rate of the component JC coal in the blended coals and Lpure-JC-char is the location of the char peak burning rate of the pure JC coal. Burnoutcomponent-JC-char is the char burnout of the component JC coal in the blended coals, and Burnoutpure-JC-char is the char burnout of the pure JC coal. The competition between the promotive and inhibitive effects influences the combustion characteristics and the additive/non-additive behavior of blended coals. To evaluate the additive/non-additive behaviors of blended coal combustion, the relative difference of blended coal burnout (RDB of blended coals) is introduced in this paper. When RDB of blended coals >0, indicating that the promotive effect plays a more important role than the inhibitive effect during the total combustion process of blended coals, which results in that the experimental burnout of blended coals is higher than the linear burnout of blended coals. By contrast, when RDB of blended coals < 0, indicating that the inhibitive effect plays a more important role than the promotive effect during the total combustion process of blended coals, which results in that the experimental burnout of blended coals is lower than the linear burnout of blended coals. The larger the |RDB| of blended coals, the more obvious the non-additive behavior of blended coal burnout. The linear burnout of blended coals is calculated as the mass-weighted average of pure coal burnout, and RDB of blended coals is determined by the linear and experimental burnout. They are calculated as 14

follows: Burnoutcal .blended coals =(1-x)  Burnoutexp. pure JC  x  Burnoutexp. pure SH

RDB of blended coals (%)=

Burnoutexp.blended coals  Burnoutcal . blended coals Burnoutcal .blended coals

(9)

100

(10)

where Burnoutcal.-blended-coals is the linear burnout of blended coals, Burnoutexp-pure-JC. and Burnoutexp-pure-SH. indicate the experimental burnout of pure JC and SH coals, x is the blending ratio of SH coal in the blended coals, Burnoutexp-blended-coals. is the experimental burnout of the blended coals. 4. Results and discussion 4.1 Validation of the simulated results To validate the simulated results, the experimental and simulated values are compared. Fig. 5-(a) shows the comparison of the experimental and simulated burnouts under different atmospheres. The simulated burnouts are in agreement with the experimental, with a maximum error of 2.62% at the SH blending ratio of 75% in the 30%O2/70%CO2. Fig. 5-(b) shows the RDB of blended coals under different atmospheres, and it can be seen that the simulated RDB features are in accordant with the experimental, with a maximum error of 2.57% at the SH blending ratio of 75% in the 21%O2/79%N2. In both the 21%O2/79%N2 and the 21%O2/79%CO2, temperatures measured in different ports are compared, as shown in Figs. 5-(c)(d), and these predicted values are consistent with the measured, with a maximum error of 15.3℃ (port 2#) for the pure JC in the 21%O2/79%CO2. Especially, the temperature of port-1# in the 21%O2/79%N2 is not measured. This is because that in the 21%O2/79%N2, the combustion near the port-1# is greatly intense and lots of ash is 15

settling onto the thermocouple; as a results, the measured values of this port show significant fluctuation. These agreements confirm that the mesh and models adopted in this work are reasonable for describing the combustion of coal blends in the DTF.

Fig. 5 Comparison of the experimental (Exp.) and simulated (Sim.) results;(a) burnout and (b) RDB of blended coals under different atmospheres, (c) Temperature in the 21%O2/79%N2, (d) Temperature in the 21%O2/79%CO2.

Fig. 6 The burnout of blended coals and component coals in the blended coals under different SH blending ratios 16

4.2 Analysis of interaction mechanism during blended coal combustion A typical combustion environment (21%O2/79%CO2) is chosen to analyze the interaction mechanism during the combustion process of the blended coals. The experimental and simulated burnouts and RDB of the SH+JC blended coals under different SH coal blending ratios are shown in Fig. 6. Both the experimental and simulated results show that as the SH proportion increases, the blended coals burnout gradually increases, but it also shows a non-additive behavior. In order to get more information to reveal the interaction mechanism, the simulated results are further analyzed in details as the followings. With increasing the SH proportion in the blended coals, the simulated burnout of the component SH coal changes slightly, but the simulated burnout of the component JC coal gradually decreases and the |RDB| of the component JC coal gradually increases. This can be explained by the existence of two effects during the combustion process of the blended coals. One is the promotive effect that the component SH coal combustion produces an initial high gas temperature field to heat the component JC coal, thereby both heating the component JC coal particle and promoting its devolatilization and ignition. Another is the inhibitive effect that a strong oxygen-deficient environment forms initially because the intense combustion of the component SH coal consumes a lot of oxygen, which then delays the char combustion process of the component JC coal and inhibits the char burnout of the component JC coal. Similar phenomena are also observed in the air [8, 9]. The promotive and inhibitive effects are further analyzed as follows.

17

(a) Temperature distribution (K)

(b) Average temperature along the axis

Fig. 7 Simulated temperature distribution under different blending ratios of SH coal Fig. 7-(a) shows the simulated temperature distribution under different blending ratios of SH coal. There is a similar temperature trend for all cases. After the fuel/gas mixtures enter the DTF at a short distance, they are quickly heated to a higher temperature by the high-temperature environment. A significant release and ignition of volatiles in the pulverized coal is achieved at a certain distance, and the intense combustion of volatiles in a relatively small volume releases a large amount of heat to form a high temperature zone. Compared with the pure JC coal, the high-temperature zone of the pure SH coal is significantly closer to the primary inlet, and its temperature level is higher because SH coal releases heat more centrally and intensely. As the SH coal proportion increases from 0% to 25%, the high-temperature zone of the blended coals moves significantly closer to the primary inlet. With further increases in the SH coal proportion from 25% to 100%, the high-temperature zone of the blended coals moves gradually to the primary inlet. Fig. 7-(b) presents the simulated average temperature along the axis. Along the axis, the stream temperature range from a low temperature to over 1000 K at approximately 0.1 m. The pure SH and JC coals have only one temperature peak, but 18

for the blended coals, two temperature peaks occur along the axis, which corresponds to the intense combustion of the component SH and JC coals, respectively. In the region from 0 m to 0.20 m, the average temperature increases quickly with the increasing the SH coal proportion, which promotes the devolatilization and ignition of the component JC coal.

Fig. 8 Particle mass histories and heating rate curves for JC particle (dinitial=90 µm) (a), t and t (b) under different blending ratios of SH coal (Simulated) To clarify the promotive effect on the devolatilization of the component JC coal, mass histories and particle heating rates for particles with initial particle diameter of 90 µm (dinitial=90 µm) are shown in Fig. 8. The particle heating rate is calculated by the first derivative of the particle temperature versus time (dT/dt). Based on the particle mass, different periods are defined: preheating, devolatilization and char combustion [42]. It is clear that as increasing SH coal proportion, the particle heating rate difference between the component JC coal particle and the pure JC coal is more obvious, the mass loss rate of the component JC coal at the devolatilization period is larger compared with that of the pure JC coal. Consequently, the duration of the devolatilization (t) of the component JC coal is shorter and the duration difference ( t ) of devolatilization decreases. Therefore, increasing the SH coal proportion 19

promotes the devolatilization of the component JC coal in the blended coals. Fig. 9 shows the simulated JC- and SH-volatile reaction rate distributions under different SH blending ratios. As the SH coal proportion increases, the location of the peak volatile reaction rate of the component SH coal changes slightly, but that of the component JC coal obviously advances owing to the promotive effect on the devolatilization of the component JC coal. That is to say, the increase of SH coal in the blended coals improves the promotive effect on the devolatilization and the volatile reaction of the component JC coal, which is advantageous to improving the ignition of the component JC coal and the flame stability of the blended coals.

(a) SH-volatile reaction rate

(c) JC-volatile reaction rate

(b) Average SH-volatile reaction rate near the burner exit

(d) Average JC-volatile reaction rate near the burner exit

Fig. 9 Simulated JC and SH-volatile reaction rate distributions under different blending ratios of SH coal

20

(a) O2 distribution

(b) Average O2 concentration along the axis

Fig. 10 Simulated O2 concentration distribution under different SH blending ratios The simulated O2 distribution under different blending ratios of SH coal is shown in Fig. 10-a. SH coal particles with fast devolatilization and high combustion reactivity, rapidly consume more oxygen and lead to a depleted oxygen condition, which will have the inhibitive effect on the char combustion of the component JC coal. Fig. 10-b shows the simulated average O2 concentration along the axis. For pure JC coal, the average O2 concentration declines gradually along the axis, whereas for pure SH coal, it decreases sharply to a low value (about vol. 4.0%) in a region that is not far from the primary inlet, approximately 0.2 m. Here, taking the oxygen concentration, volatile and char combustion into comprehensive consideration under different SH coal blending ratios, the region from approximately 0.2-1.6 m is defined as a relatively oxygen-deficient region where the O2 curves of blended coal combustion are lower than that of pure JC coal combustion. With increasing SH coal proportion, the O2 concentration near the primary inlet and in the oxygen-deficient environment region becomes lower. This finding indicates that a worse oxygen-deficient environment is initially formed with increasing SH coal proportion.

21

(a) SH-char burning rate (kg/s)

(c) JC-char burning rate (kg/s)

(b) Average SH-char burning rate along the axis

(d) Average JC-char burning rate along the axis

Fig. 11 Simulated char burning rates under different blending ratios of SH coal Fig. 11 shows the simulated char burning rates under different blending ratios. For all cases, the most intense char combustion of SH coal occurs in the region from 0-0.2 m, and with increasing SH coal proportion, the location of the peak char burning rate (L) for SH coal changes only slightly. The oxygen-deficient environment has little effect on SH-char combustion. However, for the JC char, although the initial burning rate increases slightly when SH coal is mixed, the effect on the overall char combustion process of the JC coal is relatively slight, because the JC char combustion occurs mainly in a relatively oxygen-deficient environment. As the SH coal proportion increases, the location of JC-char peak burning rate is significantly later owning to the worse oxygen-deficient condition, indicating that an increase in SH 22

coal delays JC char combustion process.

Fig. 12 Location of JC-char peak burning rate, char burnout and RDB of the component JC under different blending ratios of SH coal (Simulated) Furthermore, the inhibition on the JC char combustion process influences the char burnout of JC coal. As shown in Fig. 12, with an increase in the SH coal proportion, the char burnout of the component JC coal decreases, and the |RDB| of JC-char increases. This result demonstrates that an increased SH coal proportion increases the inhibitive effect on the combustion process, resulting in the decrease of the JC char burnout in the blended coals. The above analysis reveals that the promotive effect on JC coal combustion mainly includes (i) the promotive effect of JC coal devolatilization and JC volatile reaction, and (ii) the promotive effect of JC char burning at the initial stage of JC char reaction. However, the promotive effect of char combustion at the initial stage of JC char reation is much smaller compared with the inhibitive effect on JC char combustion at the intense burning stage. Therefore, to discuss and analyze the effects of different conditions on the interactions, the devolatilization of the component JC coal (the promotive effect) and the char combustion of the component JC coal (the inhibitive effect) are studied as the following sections. 23

4.3 Effect of atmosphere on the combustion interactions of the blended coals

Fig. 13 Particle mass histories and heating rate curves for JC particles (dinitial=90 µm), t and t under different atmospheres (Simulated). Fig. 13 displays the particle mass histories and heating rate curves (dinitial=90 µm) under different atmospheres. Different atmospheres show similar features: the heating rate of the component JC coal particle is enhanced gradually with increasing the SH coal proportion; as a consequence, the mass loss during the devolatilization becomes 24

faster and the duration of the devolatilization (t) is shorter. However, as the SH coal proportion increases, the change degree of mass loss rate, t and t at the period of the devolatilization are clearly different under different atmospheres. Due to the difference of the gas physical properties (specific heat, diffusivity and kinematic viscosity, et al) [19, 43, 44], compared to the 21%O2/79%N2, the 21%O2/79%CO2 lowers the particle heating rate difference between the component JC coal particle and the pure JC coal at the same SH blending ratio. Consequently, compared with that in the 21%O2/79%N2, the enhancement of the mass loss rate is much lower, t and t are larger in the 21%O2/79%CO2 at the same SH coal proportion,. This result demonstrates that the promotive effect on the devolatilization of the component JC coal is weaker in the 21%O2/79%CO2 mixture than in the 21%O2/79%N2 mixture. As the oxygen concentration increases in the O2/CO2 mixtures, the increase in the mass loss rate at the period of the devolatilization gradually increases, t and t gradually decrease at the same SH blending ratio. This occurs because increasing the oxygen percentage in the O2/CO2 can improve the physical properties (specific heat, diffusivity and kinematic viscosity, et al) of the O2/CO2 mixture. As a consequence, the heating rate of the component JC coal particle during the devolatilization increase gradually and the particle heating rate difference between the component JC coal particle and the pure JC coal becomes more obvious. When the O2 concentration is increased to the value of 30-35%, the enhancement of the mass loss rate during the devolatilization, t and t are similar to those obtained in the 21%O2/79%N2 mixture. This phenomenon may be attributed to the fact that the physical properties (specific 25

heat, diffusivity and kinematic viscosity, et al) and combustion temperature with approximately 30%-35% O2 in the O2/CO2 mixture are similar to that in the 21%O2/79%N2 mixture. These results demonstrate that (i) the promotive effect on the devolatilization of the component JC coal is improved with an increase in the O2 concentration in the O2/CO2 mixtures, and (ii) the promotive effect on the devolatilization of the component JC coal achieves a similar level to that in the air with approximately 30%-35% O2 in the O2/CO2 mixture. The JC-char burning rates under different atmospheres are displayed in Fig. 14. Similar trends can be found under different atmospheres: the location of the peak JC-char burning rate moves farther away as increasing the SH coal blending ratio. This indicates that there exists an inhibitive effect on the char combustion process of the component JC coal under different atmospheres. However, the degree of the inhibitive effect under different atmospheres also displays different features. Compared with that in the 21%O2/79%N2 mixture, the peak JC-char burning rate location is farther along the axis, and L is larger in the 21%O2/79%CO2 mixture at the same SH blending ratio. These results demonstrate that the inhibitive effect on the JC-char combustion process is lower in the 21%O2/79%CO2 mixture than in the 21%O2/79%N2 mixture. This is attributed to the different physical properties of CO2 and N2. On the one hand, the higher specific heat of the O2/CO2 mixture leads to a change in the gas heat transfer process, which causes comparatively lower gas temperatures and therefore reduces the particle temperature in the 21%O2/79%CO2 compared to that in the 21%O2/79%N2 [15, 45]. The lower oxygen mass diffusivity in 26

the O2/CO2 atmosphere weakens O2 transport to the particle surface, especially under the oxygen-deficient condition, which significantly reduces the JC-char burning rate in the 21%O2/79%CO2 environment. Affected by these comprehensive effects, the char burning rate of the component JC coal lowers, and the char combustion process is inhibited and delayed [32].

Fig. 14 Comparison of JC-char burning rates, L and L under different atmospheres 27

As the oxygen concentration in the O2/CO2 increases, the peak JC-char burning rate location advances and L decreases (as shown in plane-f of Fig. 14). In particular, L and L with approximately 30%-35% O2 in the O2/CO2 mixture are similar to those obtained in the 21%O2/79%N2 mixture. These results demonstrate that increasing the oxygen in the O2/CO2 mixtures weakens the inhibitive effect on the JC-char combustion process, and when the oxygen concentration is increased to approximately 30%-35%, the inhibitive effect level is similar to that in the 21%O2/79%N2 mixture. This occurs because the physical properties (specific heat, diffusivity and kinematic viscosity, et al) of O2/CO2 and combustion temperature clearly change with the increasing O2 concentration, which improves the char reactivity of the component JC coal [16].

Fig. 15 Burnout and RDB of JC char under different atmospheres (Simulated) The change of the inhibitive effect on JC-char combustion process greatly affects the JC-char burnout. The char burnout and RDB of JC coal under different atmospheres are shown in Fig. 15. Compared with that in the air, the pure SH-char burnout in the 21%O2/79%CO2 changes slightly, but the pure JC-char burnout decreases greatly. That is to say, the atmosphere has a greater effect on the JC-char 28

burnout than the SH-char burnout. At the same SH blending ratio, the char burnout of JC coal in the 21%O2/79%CO2 is much lower than that in the 21%O2/79%N2. As the oxygen concentration in the O2/CO2 mixtures increases, the char burnout of the component JC gradually increases due to the weaker inhibition of the component JC coal combustion process. When the oxygen concentration in the O2/CO2 mixtures is increased to a value of 30%, the char burnout of JC coal is similar to that in the air. Regardless of blending ratios, compared with that in the 21%O2/79%N2, |RDB| of JC char in the 21%O2/79%CO2 is higher. This result demonstrates that the inhibitive effect on the JC-char burnout is stronger in the 21%O2/79%CO2 mixture than in the 21%O2/79%N2 mixture. As the oxygen concentration in the O2/CO2 mixture is increased, |RDB| of JC char gradually decreases due to a weaker inhibition on the JC-char combustion process, indicating that the inhibitive effect on the JC-char burnout becomes weaker. When the oxygen concentration in the O2/CO2 mixtures is increased to the approximate 30%-35%, |RDB| of JC char is similar to that in the air. The conclusion can be obtained that affected by the inhibitive effect on the JC char combustion process, an enhancement of oxygen in the O2/CO2 mixtures weakens the inhibitive effect on the JC-char burnout, and when the oxygen concentration increases to the approximately 30%-35% in the O2/CO2 mixtures, the inhibitive effect on the JC-char burnout is similar to that in the air.

29

Fig. 16 Total burnout of the blended coals and the burnout of the component coals in the blended coals (a), DRB of the blended coals and the component JC coal (b) under different atmospheres. (Simulated) Fig. 16 shows the burnout and DRB of the component JC coal and the blended coals under different atmospheres. In these figures, "JC+SH" indicates the blended coals, ""SH" and "JC" indicate the component coal in the blended coals. During the combustion process of blended coals, the competition between the promotive and inhibitive effects, influences the combustion characteristics of blended coals and the combustion additive/non-additive behavior of blended coals. For all cases under different atmospheres, the burnout change trends between the component JC char and JC coal are similar: their burnouts decrease as increasing the SH coal proportion. This indicates that the inhibitive effect plays a more important role on the component JC coal in the competition between the promotive and inhibitive effects during the overall combustion process of the component JC coal. In addition, it can also be seen from plane-(a) of Fig. 16 that as the blending ratio is varied, the burnout for component SH coal within the blended coals changes slightly, but the component JC coal burnout within the blended coals decreases remarkably. As a consequence, regardless of the atmosphere, the blended coals burnout shows obvious non-additive behaviors as the SH coal blending ratio increases under different atmospheres (this 30

simulated feature of the burnout of the blended coals burnout is in accordance with the experimental, as mentioned in section 4.1, Fig. 5-a). That is to say, the non-additive behavior of blended coal combustion mainly is affected by the inhibitive effect on the component JC char burnout. As shown in plane-(b) of Fig. 16, the |RDB| of the component JC coal in the 21%O2/79%CO2 mixture is more notable than that in the 21%O2/79%N2 mixture at the same blending ratio. This is because that as previously mentioned, compared with that in the 21%O2/79%N2 mixture, the promotive effect on the devolatilization of the component JC coal is lower and the inhibitive effect on the char combustion process of the component JC coal is stronger in the 21%O2/79%CO2 mixture; as a result, the overall burnout of the component JC coal becomes worse in the 21%O2/79%CO2 mixture. The above results indicate that the non-additive behavior in the 21%O2/79%CO2 mixture is more dramatic than that in the 21%O2/79%N2 mixture. Consequently, the |RDB| of the blended coals in the 21%O2/79%CO2 mixture is more notable than that in the 21%O2/79%N2 mixture. As oxygen concentration in the O2/CO2 mixture is increased, the |RDB| of the component JC coal gradually decreases owing to the increase in the inhibitive effect and the decreases in the promotive effect, thus improving the overall combustion process and burnout of the component JC coal. These result in the lower |RDB| and non-additive behavior for the blended coals with the increase of oxygen concentration in the O2/CO2 mixture (this simulated feature is in coordination with the experimental, as mentioned in section 4.1, Fig. 5-b). Furthermore, when the oxygen concentration 31

increases to approximately 30%-35% in the O2/CO2 mixtures, the |RDB| of the component JC coal and the blended coals are similar to that in air. Therefore, it can be concluded that (i) the non-additive behavior becomes weaker as increasing O2 concentration in the O2/CO2 mixtures, and (ii) the non-additive behavior achieves a similar level to that in the air when the oxygen concentration increases to approximately 30%-35% in the O2/CO2 mixtures. 4.4 Effect of char-CO2 gasification reaction on the combustion interactions of the blended coals

Fig. 17 Particle mass histories and heating rates for particles (dintial=90 µm), and t and t with or w/o char-CO2 gasification reaction (Simulated) In oxy-fuel combustion, it has been clarified that the char-CO2 gasification reaction has a great effect on the overall char consumption rate [46, 47]. The contribution of a carbon dioxide bath gas to the reactivity can be summarized in two aspects. On the one hand, the char-CO2 gasification under the CO2 atmosphere can play a positive role in enhancing the reactivity. On the other hand, the lower O2 diffusion rate through the CO2-rich boundary layer and the endothermic effect of the char-CO2 gasification reaction under a CO2 atmosphere can play a negative role in enhancing the reactivity. The competition between these two effects determines the 32

overall char consumption rate. Fig. 17 shows the particle mass histories and heating rates for particles (dintial=90µm), and t and t with or without (w/o) char-CO2 gasification reaction. At the devolatilization stage, whether there exists char-CO2 gasification reaction or not, the particle heating rate difference is very slight at the same SH blending ratio. More important, at the same SH blending ratio, the particle heating rate difference between the component JC coal particle and the pure JC coal is the same level under the reaction with or without (w/o) char-CO2 gasification. Consequently, the mass histories and particle heating rate curves at the period of the devolatilization, the duration of JC devolatilization (t) and the duration differences ( t ) with or without char-CO2 gasification reaction are similar. These results indicate that the char-CO2 gasification reaction has a relatively little effect on the promotive effect on the devolatilization of the component JC coal.

Fig. 18 Comparison of JC-char burning rate, L and L with or w/o char-CO2 gasification reaction (Simulated) Fig. 18-(a) displays the comparison of the average JC-char burning rate curves with or w/o char-CO2 gasification reaction. For the component JC coal at the same blending ratio, the burning rate with char-CO2 gasification reaction is slightly higher 33

than that without char-CO2 gasification reaction. This occurs because the positive role of the char-CO2 gasification in enhancing the char reactivity is higher than the negative role under the CO2 atmosphere; as a result, char-CO2 gasification reaction can improve the overall char consumption and combustion rate [22, 46]. As shown in Fig. 18-(b), regardless of the blending ratio, L and L with the char-CO2 gasification reaction are smaller than that without the char-CO2 gasification reaction. These demonstrate that the char-CO2 gasification reaction not only enhances the char burning rate, but also weakens the inhibitive effect on the char combustion process of the component JC owning to the more intense char consumption.

Fig. 19 Burnout (a-i) and RDB (a-ii) of JC char, burnout (b-i) and RDB (b-ii) of blended coals (Simulated) The char combustion process of the component JC determines the JC char burnout. As shown in Fig. 19-(a)-i, regardless of whether the char-CO2 gasification reaction exists, the component JC-char burnout decreases with increasing SH coal blending ratios due to the inhibitive effect on the JC-char combustion process. However, there are some differences. At the same SH blending ratio, a notable decrease in the char burnout of the component JC can be seen when the char-CO2 gasification reaction is not considered. Compared with that without the char-CO2 34

gasification reaction, the char |RDB| of the component JC with the char-CO2 gasification reaction is lower, indicating that char-CO2 gasification reaction weakens the inhibitive effect on the char burnout of the component JC owning to the weaker inhibition on the char combustion process of the component JC. As mentioned in section 4.3, the non-additive behaviors of blended coal burnout are mainly affected by the burnout of the component JC coal, and the competition between the promotive and inhibitive effects influences the burnout of the component JC coal. It can be seen from Fig. 19-(b) that the burnout of the component JC coal is higher and the |RDB| of the component JC coal is lower with the char-CO2 gasification reaction compared with that without char-CO2 gasification reaction. This occurs because the promotive effect changes slightly but the inhibitive effect decreases with the char-CO2 gasification reaction; as a result, the overall combustion process and burnout of the component JC coal improve. Furthermore, the burnout of the blended coals without the char-CO2 gasification reaction is higher than that with the char-CO2 gasification reaction, but the |RDB| without the char-CO2 gasification reaction is higher than that with the char-CO2 gasification reaction. This indicates that the char-CO2 gasification reaction weakens the non-additive behavior of blended coals burnout owning to the weaker inhibition. 4.5 Effect of reactor wall temperature on the combustion interactions of the blended coals The particle mass histories and heating rate curves are shown in Fig. 20. With increasing reactor wall temperatures, the particle heating rate difference between the 35

component JC coal particle and the pure JC coal becomes smaller at the same SH blending ratio. Consequently, as increasing reactor wall temperatures, the change degree of the mass loss at the period of the devolatilization becomes weaker with increasing SH coal content, t and t gradually decrease at the same SH blending ratio. This result indicates that increasing the reactor wall temperature weakens the promotive effect on the devolatilization of the component JC coal.

Fig. 20 Particle mass histories and heating rates for particles (dintial=90 µm), and t and t under different reactor wall temperatures (Simulated) The comparisons of the JC-char burning rate, L and L are shown in Fig. 21. With increasing reactor wall temperature, the average char burning rate of the component JC gradually increases because the higher reactor wall temperature enhances the intensity of the char-O2 and char-CO2 reaction rates. In addition, for the 36

component JC coal at the same SH blending ratio, L and L gradually decrease as the reactor wall temperature increases. These demonstrate that the higher reactor wall temperature weakens the inhibitive effect on the char combustion process of the component JC owning to the more intense char-O2 and char-CO2 reaction rates.

Fig. 21 Comparison of the JC-char burning rate, L and L under different reactor wall temperatures (Simulated)

Fig. 22 Char burnout and RDB of the component JC under different reactor wall temperatures (Simulated) 37

The inhibitive effect on the char combustion process of the component JC influences the char burnout of the component JC. Fig. 22 shows the char burnout and RDB of the component JC under different reactor wall temperatures. It can be found that the higher reactor wall temperature corresponds to the higher burnout and lower |RDB| of JC char at the same blending ratio, owning to the weaker inhibition on the char combustion process of the component JC. In conclusion, increasing the reactor wall temperature not only increases the component JC char burning rate, but also weakens the inhibitive effect on the char combustion process and burnout of the component JC.

Fig. 23 Total burnout of the blended coals and the burnout of the component coals in the blended coals (a), DRB of the blended coals and the component JC coal (b) under different reactor wall temperatures (Simulated) Affected by the competition between the promotive and inhibitive effects, the combustion behavior of blended coals also show different non-additive behaviors under different reactor wall temperatures. As shown in Fig. 23, at the same SH blending ratio, the burnout of the component JC coal increases and the |RDB| of the component JC coal decreases as increasing the reactor wall temperature. As a consequence, for the blended coals, the burnout improves and the |RDB| lowers. These results indicate that increasing the reactor wall temperature weakens the 38

non-additive behavior of blended coals burnout due to the change of the promotive and inhibitive effects on the component JC coal. 5. Conclusions In this paper, to reveal the combustion interactions of blended coals in the O2/CO2 atmosphere, experimental numerical studies and simulations were performed in a drop tube furnace. These studied results can provide useful information for understanding the interactions during the oxy-fuel combustion process of blended coals. Some conclusions can be drawn. (1) Compared with that in the air, the promotive effect on the devolatilization of the less-volatile coal is weaker but the inhibitive effect on the char combustion process and burnout of the less-volatile coal is stronger in the 21%O2/79%CO2 environment, which is attributed to the different physical properties of the gases. (2) In the O2/CO2, increasing oxygen concentration increases the promotive effect on the devolatilization of the less-volatile coal but weakens the inhibitive effect on the char combustion process and burnout of the less-volatile coal, owing to the changes in the physical properties of gases (specific heat, diffusivity and kinematic viscosity, et al) and the improvement of the less-volatile coal char reactivity. In particular, when the oxygen concentration in the O2/CO2 is increased to the values of 30-35%, the promotive and inhibitive effects achieve a similar level to that in the air. (3) The char-CO2 gasification reaction in the O2/CO2 has little effect on the promotive effect on the devolatilization of the less-volatile coal but weakens the inhibitive effect on the char combustion process and burnout of the less-volatile coal. (4) Increasing the reactor wall temperature decreases the promotive effect on the devolatilization of the less-volatile coal and weakens the inhibitive effect on the char combustion process and burnout of the less-volatile coal. 39

(5) In the future, the effect of blending method on the interactions will be carried out and the flame characteristic will be obtained using flame monitoring techniques during the oxy-fuel combustion process of blended coals.

Notes The authors declare no competing financial interests. Acknowledgements This work was sponsored by the National Natural Science Foundation of China (NO.51676076) and the Foundation of State Key Laboratory of Coal Combustion (No. FSKLCC1805). The technical support from the Analytical and Testing Center at the Huazhong University of Science and Technology is greatly appreciated.

Nomenclature t= the duration of devolatilization

t = the duration difference of devolatilization L=the location of the char peak burning rate L = the location difference of the char peak burning rate

RDB=the relative difference of burnout

40

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Table and Figure Captions Table 1. Proximate analysis and ultimate analysis of coal samples in this work. Table 2. CPD input parameters for devolatilization Table 3 Kinetic parameters Fig. 1

Schematic of DTF (a), and measured center-line gas temperature without coal feeding at a reactor temperature of 1323 K (b).

Fig. 2

Typical variations at the DTF exit during the experiment (25%SH+75%JC combustion in 21%O2/79%N2, excess oxygen coefficient is different for three conditions, condition 1: 1.0, condition 2: 1.2, condition 3: 1.4).

Fig. 3

Coal particles size distribution

Fig. 4

CFD model and mesh of DTF

Fig. 5

Comparison of the experimental (Exp.) and simulated (Sim.) results;(a) burnout and (b) RDB of blended coals under different atmospheres, (c) Temperature in the 21%O2/79%N2, (d) Temperature in the 21%O2/79%CO2.

Fig. 6

The burnout of blended coals and component coals in the blended coals under different SH blending ratios

Fig. 7

Simulated temperature distribution under different blending ratios of SH coal

Fig. 8

Particle mass histories and heating rate curves for JC particle (dinitial=90 µm) (a), t and t (b) under different blending ratios of SH coal (Simulated)

Fig. 9

Simulated JC and SH-volatile reaction rate distributions under different SH blending ratios

Fig. 10 Simulated O2 concentration distribution under different SH blending ratios Fig. 11 Simulated JC and SH-char burning rates under different SH blending ratios Fig. 12 Location of JC-char peak burning rate, char burnout and RDB of the component JC under different blending ratios (Simulated) Fig. 13 Particle mass histories and heating rate curves for JC particles (dinitial=90 µm), t and t under different atmospheres (Simulated). Fig. 14 Comparison of JC-char burning rates, L and atmospheres

L

under different

Fig. 15 Burnout and RDB of JC char under different atmospheres (Simulated) Fig. 16 Total burnout of the blended coals and the burnout of the component coals in the blended coals (a), DRB of the blended coals and the component JC coal (b) under different atmospheres. (Simulated) Fig. 17 Particle mass histories and heating rates for particles (dintial=90 µm), and t and t with or w/o char-CO2 gasification reaction (Simulated) Fig. 18 Comparison of JC-char burning rate, L and L with or w/o char-CO2 gasification reaction (Simulated) 46

Fig. 19 Burnout (a-i) and RDB (a-ii) of JC char, burnout (b-i) and RDB (b-ii) of blended coals (Simulated) Fig. 20 Particle mass histories and heating rates for particles (dintial=90 µm), and t and t under different reactor wall temperatures (Simulated) Fig. 21 Comparison of the JC-char burning rate, L and L under different reactor wall temperatures (Simulated) Fig. 22 Char burnout and RDB of the component JC under different reactor wall temperatures (Simulated) Fig. 23 Total burnout of the blended coals and the burnout of the component coals in the blended coals (a), DRB of the blended coals and the component JC coal (b) under different reactor wall temperatures (Simulated)

47

Highlights: 6.

Weaker promotion and stronger inhibition are shown in O2/CO2 than in O2/N2.

7.

Increasing O2 increases promotion but weakens inhibition.

8.

Promotion and inhibition in 30-35% O2 achieve similar levels to that in air.

9.

Char-CO2 reaction slightly affects promotion but greatly weakens inhibition.

10. Increasing the reactor wall temperature weakens promotion and inhibition.

48