Numerical Simulation and Experimental Research on Drying Behavior

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Numerical simulation and experimental research on drying behavior of single lignite particle (SLP) under high temperature flue gas Hao Li, Shouyu Zhang, You Li, Chen Mu, Yifan Zhang, Fenghao Jiang, and Caiwei Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02364 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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Energy & Fuels

Numerical simulation and experimental research on drying behavior of single lignite particle (SLP) under high temperature flue gas

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Hao Lia, Shouyu Zhanga*, You Lia,b, Chen Mua, Yifan Zhanga, Fenghao Jianga, Caiwei Wanga,

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a

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Shanghai for Science and Technology, Shanghai, 200093, P.R. China

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b

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Department of Thermal Engineering, School of Energy and Power Engineering, University of

Suzhou Industrial Park, Huaneng Power generation limited liability company, Suzhou, 215424,

P.R. China

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*Corresponding author. Address: Department of Thermal Engineering, School of Energy and Power Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China

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Tel: +086 18321083616; Fax: +086 21 65711123.

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E-mail addresses: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT:

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The drying behavior of single lignite particle (SLP) under high temperature (600-900oC) flue

3

gas was addressed in this paper from two aspects: experiments and numerical calculation. A

4

self-designed horizontal fixed-bed reactor was employed for the high temperature drying

5

experiment. Based on the drying curves obtained from the experiments, no constant drying rate

6

stage was found and SLP drying process included increasing and decreasing drying rate stage. In

7

order to explore the drying process in depth, a mathematical model was developed by which SLP

8

was simplified into simple spherical model. Based on the dry-wet zone theory, the heat and mass

9

transfer equations were established to describe the drying process. The simulated results

10

calculated by MATLAB agreed well with the experimental data. The model can predict SLP

11

drying behaviors including the influence of drying time, the surface and internal temperature

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distribution of the lignite particle, the migration of evaporation interface inside the particle and

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so on. The predicted results indicated that the evaporation interface migration velocity and

14

temperature were linearly dependent and the high temperature of 700oC was the most suitable

15

temperature for lignite drying. Furthermore, the drying time can be predicted using the model

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according to practical application. Thus, the model can be used for the optimization of the drying

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processing parameters and offer guidance for the development of new drying technologies.

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Keywords: lignite; drying; single particle model; heat and mass transferring

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2


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Energy & Fuels

1. Introduction

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High-grade fossil energy consumption, including high-rank coal (bituminous coal and

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anthracite), oil and so on, brings the world great energy crisis. Thus, the efficient and

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environmental friendly utilization of low-rank coal has really come to the forefront. As a largest

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storage and most widely used low-rank coal1, lignite has become the energy mainstay of China2,

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especially for its abundance, easy availability, low exploitation difficulty and high reactivity3-9.

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However, the feature of the high moisture content10 may act as a dominating barrier hampering

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its wide application. To be specific, it may result in the reduction of calorific value and

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combustion efficiency11-14 and bring about higher costs and potential safety hazards during

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transportation and storage15-18. Besides, the high moisture content may lead to CO2 emission

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increase17, devolatilization delay during pyrolysis process7 and the degradation of the gas quality

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during gasification process8. Under such a circumstance, it is imperative for lignite to develop

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some rational dewatering technologies to improve the competitiveness for its further utilization.

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Various dewatering technologies for lignite upgrading have been developed all over the

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world, which are mainly classified into evaporative drying technology and non-evaporative

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drying technology with their own strengths and weaknesses12,

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non-evaporative drying process, the evaporative drying technology is widely used for lignite

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dewatering10, 14, and the heated air, flue gas and superheated steam are the most common heating

19

medium4, 13, 14. However, the usage of traditional heating medium has a lot of inadequacies12, 23.

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The drying process with hot air as heating medium is easy to cause explosion due to the high

21

oxygen content. For superheated steam used as heating medium, the requirement for water and 3


17, 19-22

. Compared with

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the rigorous working condition restrict the application and extension of the technology. To avoid

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these problems, high-temperature (>600oC) flue gas is proposed as the drying medium for lignite

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dewatering process, which drying rate is higher than that of the traditional low temperature

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(600oC) flue gas as drying agent, few studies have been published on

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modeling the drying process of SLP.

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The paper is concerned with the experimental and numerical investigation of the drying

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process of SLP under simulated high-temperature (600, 700, 800 and 900oC) flue gas. A

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mathematical model of the drying process of SLP under high-temperature flue gas is established.

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Focusing on the moisture removal and heat transfer processes, hundreds of experiments have

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been conducted to verify the result of simulation and some conclusions which are beneficial to

12

the development of new technology are summarized. Via the model, some useful information

13

about SLP drying process can be predicted and offer guidance for the engineering designs.

14

2. Experimental

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2.1 Materials

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Yimin (YM) lignite, a typical Chinese lignite obtained from the east of Inner Mongolia, was

17

chosen as the raw material for the research. The related description of the coal had been involved

18

in other literatures4, 37. The ultimate and proximate analyses of the raw coal were summarized in

19

Table 1, obtained by Institute of Coal Chemistry, Chinese Academy of Sciences.

20

2.2 Sample preparation

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The study is based on the spherical model (details are shown below). In order to be consistent 5


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with the assumption condition of numerical model, the drying experiment was conducted with

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the spherical lignite samples, which preparing process was given as follows. The raw coal was

3

first ground and sieved to provide the samples with the particle size of 0.5-1.0mm. Then the pure

4

water was added into the powder and the mixture was whisked until smooth and creamy.

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According to the experiment requirement, the mixture was shaped into sphere particles of 10, 15,

6

20, 25 mm with the diameter error of 5% by means of a self-made mold respectively. At last, the

7

samples were dried in a moderate way until the moisture was equal to that in the receive base and

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the spherical samples with the diameter change lower than 5% were selected as the experimental

9

samples. The specific experimental process had been involved in previous studies37, and all

10

particle sizes were measured by a vernier caliper to ensure dimensional accuracy.

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2.3 Apparatus and method

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The drying experiments were performed in a self-designed horizontal fixed-bed dryer, whose

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schematic diagram was shown in Fig.1. In the figure, it can be seen that the apparatus mainly

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consists of a preheater, a heater and a water-cooling room. The preheater and the heater are both

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electric furnaces, which temperature can be auto-controlled at the desired value. The gas

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temperature in the corresponding reaction chambers was measured by several K-type

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thermocouples (1 mm dia.) inserted.

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In order to simulate the actual process, the flue gas (well-mixed 96%N2 and 4%O2) was used 38

19

as the drying medium

. Based on the pre-experiments, 40 seconds was selected as the drying

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time to ensure that no volatile emission occurred during the drying process. The flue gas with a

21

flow rate of 60L/h was first purged into the preheater and heater, which temperature was set at 6


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600, 700, 800 and 900oC, respectively. The flue gas flow was kept for 30min to confirm that

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there was no original air in the outlet of the apparatus. The detailed information about the high

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temperature drying process on the horizontal fixed-bed dryer has been given in our previous

4

research 37. During the experiment, a high-precision electronic balance was used to determine the

5

mass changes of the coal samples before and after drying treatment.

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3

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3.1 Analysis of particle drying progress

SLP drying model

8

Because lignite is an unsaturated multi-phase porous material with a large number of

9

oxygen-containing functional groups on the inner surface, its dehydration is a complicated

10

process involving the heat transfer and mass transfer (moisture removal). During the drying

11

process, the moisture absorb the heat transferred from the high-temperature flue gas into the

12

interior region of the particle. The moisture cannot transfer in the form of liquid during the

13

drying process owing to the coal-water interaction39-42. Thus, liquid water evaporates into vapor

14

and then spread out of the particles. Based on the spherical model, the drying process is shown in

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Fig.2.

16

Because of the convection of high temperature flue gas, the temperature difference between

17

the flue gas and lignite particle is so large that nearly all the water on the outside surface of the

18

particle is removed in a relatively short time. Thus, an interface in the sphere particle is formed

19

between the gaseous vapor and the liquid water, which is defined as evaporation interface. At the

20

beginning of drying process, the heat, which is equal to the latent heat of water evaporation, is

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transferred to the coal sample 43 and the evaporation interface is exactly the outside surface of the 7


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particle. With the drying process going on, the pressure, temperature and concentration gradient

2

increase and the evaporation interface begins to move inward the particle because the liquid

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water in the particles diffusing toward the surface is not enough to sustain evaporation on the

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particle surface36. Therefore, an approximate dry area containing nearly no liquid water is formed

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between the evaporation interface and is defined as dry region. The rest area to the center of the

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sphere, which still contains liquid water, is defined as wet region. Fig.2 also shows schematic

7

diagram of volume element in dry region and wet region. At the end of the drying process, the

8

evaporation interface moves to the center of lignite particles and the wet region completely

9

disappears. In this paper, the interface moving rate is an important index to the drying rate

10

description.

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In addition to mass transfer caused by the moisture removal, heat transfer works throughout

12

the entire drying process, as shown in Fig.2. In lignite particle, being different from the gaseous

13

vapor, the liquid water has little fluidity because of the hydrogen bond formed between water

14

and oxygen functional groups on the inner surface of SLP42 and the moisture evaporation and

15

vapor diffusion requires more heat. Therefore, heat transfer and mass transfer are both involved

16

to describe the drying process in dry region and only heat transfer is considered in wet region.

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3.2 Model hypothesis

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Based on the above analysis of the drying process, some basic simplification and

19

assumptions on the model of SLP drying process are listed as follows2, 44：(1) SLP is isotropic

20

spheres so that conservation equation can be established to describe the heat and mass transfer

21

processes in one-dimensional spherical coordinate system. (2) The deformation of lignite 8


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Energy & Fuels

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particles are ignored during the drying process and the particles size are deemed as constant. (3)

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Water species existing in coal samples are not distinguished. Moisture distribute evenly in the

3

interior of the particles. (4) The flue gas does not react with the lignite particle. (5) The gaseous

4

phase can be treated as all ideal gas. (6) The lignite particles are in local thermodynamic

5

equilibrium. (7) The thermal resistance of liquid water in the SLP is ignored.

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3.3 Mathematical equation model

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For the porous media material, the radiation heat usually can be ignored when the internal

8

temperature of particles is below 600K in the drying process

9

previous study

37

45

. It has been reported in the

that the internal temperature of the coal samples are lower than 570K.

10

Meanwhile, the residence time of the lignite particle in the high temperature environment is very

11

short. Moreover, the main emphasis of our research is the heat and mass transfers inside the

12

particle. Therefore, the convection and conduction heat transfers are considered, while radiation

13

heat transport is neglected. Based on the above assumptions of the isotropic sphere, the

14

conservation equations in one-dimensional spherical coordinates can be established. The

15

mass conservation equation can be expressed as:

∂ρ ∂ ( ρu ) + = m& v ∂τ ∂r

(1)

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where ρ is the density of the lignite particle, τ is time, u is the macroscopic velocity of the water

17

vapor, r is the radius of the spherical particle and ṁv is the mass source term in the unit volume

18

of the lignite particle.

19

The momentum conservation equation can be written as: 9


Energy & Fuels

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∂ ( ρu ) ∂u ∂p N + ρu = − + ∑ ρk f k M k ∂τ ∂r ∂r k =1

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(2)

1

where p is the pressure, ρk is the density of the k-th component, N is the number of components,

2

fk is the volume force of the k-th component, Mk is the mass fraction of the k-th component.

3

The energy conservation equation can be given as:

ρCp

∂T ∂p 1 ∂  2 ∂T  N ∂T & = + 2  λr + WT  − ∑ ρk uCpk ∂τ ∂τ r ∂r  ∂r  k =1 ∂r

(3)

4

where Cp is the specific heat capacity of the spherical particles, T is thermodynamic temperature,

5

λ is the thermal conductivity, Cpk is the specific heat capacity of the k-th component and ẆT is the

6

total reactive heat and phase change heat of all components.

7

According to the analysis of the model mentioned in 3.1, the heat transfer and mass transfer

8

are considered in the dry region of the spherical particles, while only heat transfer is considered

9

in the wet region.

10 11

Combined with the above assumptions, the heat transfer equilibrium equation in the dry region is obtained as follows: ρg C g

 2 ∂ Tp ∂ 2Tp = ( λ g + λs )  +  r ∂r ∂τ ∂r 2 

∂ Tg

  + rf m& 

(4)

12

where ρg is the density of the high-temperature flue gas, Cg is the specific heat of the

13

high-temperature flue gas, Tg is the temperature of the high-temperature flue gas, Tp is the

14

temperature of the lignite particle surface, λg and λs are the thermal conductivity of the gaseous

15

substances and solid material in the lignite particles, respectively, rf is the latent heat of

16

vaporization of water, m& is the evaporation rate of water in unit time. 10


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Energy & Fuels

Similarly, we have the mass transfer equilibrium equation in the dry region as follows:

 2 ∂M p ∂ 2 M p  m& + ug = Dg  + + 2   r ∂r  ρ ∂τ ∂r ∂ r   g

∂M p

∂M p

(5)

2

where Mp is the moisture content on the surface of the particles, ug is the velocity of the gaseous

3

substance in the lignite particles, Dg is the mass transfer coefficient of the gas in lignite particles.

4

In the wet region, the heat transfer can be written as:

∂T ∂  2 ∂T  ερlCpl + (1− ε ) ρpCps  p = ( λl + λc )  + p  ∂τ ∂r  r ∂r 

(6)

5

where ρl is the density of liquid water, Cpl is the specific heat capacity of liquid water, ρp is the

6

density of lignite particles (solid phase), Cps is the specific heat capacity of solid phase of lignite

7

particles, λl is the thermal conductivity coefficient of liquid water.

8

3.4 Initial conditions and boundary conditions

9 10

Here it is assumed that the moisture distribute evenly inside the particles and SLP is an isotropic sphere. Thus, the initial condition of the drying process can be expressed as:

τ = 0,0 ≤ r ≤ R, M ( r， 0) = M 0

(7)

τ = 0,Tg = T0 ,Tp = T1

(8)

11

where M0 is the initial moisture content of the lignite particles, T0 is the initial flue gas

12

temperature, Tl is the initial particle temperature.

13

Also, the boundary conditions can be obtained as follows:

∂T

∂Mp

∂r

∂r

τ >0，r =0， p = 0，

=0

(9)

11


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r = R， − Dg

∂Mp ∂r

= hm ( Mp − Mg )

r = R， − ( λg + λc )

∂Tp ∂r

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(10)

= hs (Tp − Tg )

(11)

1

where Mg is the moisture content of the flue gas, Mp is the moisture content on the surface of the

2

particles, hm is the convective mass transfer coefficient, hs is the convective heat transfer

3

coefficient.

4

3.5 Model parameters

5

Based on the single particle spherical model and combined with the above constraints, the

6

relevant model parameters, mainly including physical parameters and heat and mass transfer

7

coefficient, are solved using MATLAB code. In order to accelerate the convergence and ensure

8

the stability of numerical solutions, Crank-Nicolson scheme is adopted46,

9

parameters of the equation are listed in table 2.

10

4

11

4.1 Experiment results and discussion

47

. The related

Results and discussion

12

In order to get closer to the actual working condition, the drying experiments in the research

13

were conducted under constant temperature. The previous studies have indicated that drying

14

temperature and particle size are the two main factors affecting the drying process10, 25, 48. By

15

means of variable-controlling approach, we have conducted plentiful experiments and obtained

16

some conclusions. A set of the representative drying curves under different drying temperatures

17

are showed in Fig.3.

18

Unlike the general drying process, it can be known from Fig.3a that there are only two 12


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Energy & Fuels

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stages involved in the high-temperature flue gas drying process of SLP, namely increasing drying

2

rate stage and decreasing drying rate stage. No constant drying rate stage in which evaporation

3

rate is equal to the internal water diffusion rate

4

studied the drying process of lignite under low temperature(＜150oC) conditions and obtained a

5

similar drying curve. Therefore, it can be deduced that there is no constant drying rate stage

6

involved in the high and low temperature drying process of lignite and the drying process is

7

unstable.

9, 14

is found. Likewise, Saban Pusat et al49-51

8

Fig.3b shows the drying rate curves corresponding to the drying curve in Fig.3a. From the

9

figure, it can be observed that the higher drying temperature results in the faster drying process.

10

The results also indicates that the evaporation interface steps inward SLP constantly during the

11

drying process because the high temperature of the drying medium significantly promoted the

12

heat transfer toward the evaporation interface and no enough liquid water in the particles diffuses

13

toward the surface to sustain evaporation on the particle surface. Moreover, the higher the drying

14

medium temperature are, the higher the heat transfer rate is.

15

The effect of the particle sizes of SLP is exhibited in Fig.4. Fig.4b shows the drying rate

16

curves corresponding to the drying behavior shown in Fig.4a. It can be found from Fig.4 that the

17

smaller particle sizes results in faster drying rate. And the maximum drying rate increases with

18

the decreasing particle size due to the high heat and mass transfer rate. The phenomenon may be

19

also attributed to the transform of internal pore structure. As the particles become smaller, some

20

holes are broken into smaller ones and some original non-connected holes are broken into

21

connected ones leading to larger specific surface area of the particle32, 52. The increase of specific 13


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surface area maybe benefits the enhancement of heat and mass transfer.

2

4.2 Comparison between the simulated and experimental results

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Fig.5 compares the experimental and calculated results of SLP drying curves. It is observed

4

that the experimental value is slightly larger than that of the corresponding numerical value. This

5

may be attributable to the incomplete insulation in the experiment resulting in partial heat loss32.

6

For further study, the error of the two sets of the data are analyzed. The results show that the

7

error between experimental value and calculated value is less than 10% within the allowable

8

range and the error increases gradually with the increase of drying temperature. As discussed

9

above, it may be deduced that SLP drying process is also affected by high temperature flue gas

10

radiation, which is ignored in the research. The effect of flue gas radiant heat will be considered

11

in our future research.

12

On the whole, the numerical simulation results reflect the regularity of the drying curve

13

obtained from the experiment and the model has a good agreement with the actual process.

14

Therefore, some useful predictions can be made from the model for the drying process of the

15

lignite of different particle sizes and in the flue gas of different temperatures. The model is of

16

great significance to the engineering design and can be used as a reference for practical

17

application.

18

4.3 Simulation results and discussion

19

Using the drying model established above, the drying process of the particles under different

20

temperature conditions can be simulated. Fig.6 exhibits the drying degree of SLP from the macro

21

and micro perspective, which contributes to a comprehensive analysis of the drying process. 14


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Fig.6a predicts the change of the moisture content during the drying process and provide a

2

theoretical basis for the selection of drying time. Fig.6b explores the migration of the internal

3

evaporation interface. It can be found that in the high temperature drying process, with the

4

increase of the drying time, the changes of the radius of evaporation interface show a certain

5

linear relationship, in which case the migration of evaporation interface can be considered as a

6

constant velocity process.

7

In combination with figure 6, relative parameters predicted by model are listed in Table 3.In

8

Table 3, it can be known that the higher the initial flue gas temperature is, the larger the average

9

migration velocity is. The corresponding linear relationship between the migration of

10

evaporation interface and temperature is also found, and the linear relationship between the

11

average migration velocity and temperature is proved by linear fitting with the determination

12

coefficient (R2) of 0.999.

13

In order to quantify the drying process further, the time for the decrease of moisture content

14

from 36% to 12% is defined as drying time. The drying time of the drying process under

15

different flue gas temperature and the corresponding particle surface temperatures are listed in

16

Table 3. It is found that when the ambient temperature is higher than 800°C, the temperature of

17

the particle surface rises above 300°C exceeding the finite temperature conditions established by

18

the above model. In order to understand the volatile emission properties of the lignite during the

19

drying process, the thermogravimetric experiment (10°C/min) of the lignite pyrolysis were

20

conducted and the pyrolysis behavior of the lignite is shown in Fig.7. The pyrolysis behavior

21

indicates that YM lignite begins to undergo a drastic pyrolysis at the temperature of about 280oC. 15


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As shown in Table 3, when SLP are completely dried at 800oC and 900oC, the surface

2

temperature of the particles is higher than 280oC. Therefore, the two drying temperature is not

3

suitable for the coal drying process. According to the data listed in Table 3, the drying process

4

under the temperature of 600oC needs more time. Thus, 700oC may be considered as more

5

suitable drying temperature for YM lignite with the particle size of 20mm.

6

At the optimum drying temperature of 700oC, the temperature distribution inside the lignite

7

particle during the drying process can be simulated by the model and the result is shown in Fig.8.

8

From the figure, as the drying time increases, the internal temperature of the particles increases

9

irregularly from surface to inside. During the drying process, the different particle radius all

10

undergo a rapid heating process within 75~200oC and then the temperature rise gradually slows

11

down, which is greatly different from the migration rule of the evaporation interface. Based on

12

the figure, the drying time can be selected according to the practical requirement to avoid the

13

excessive drying and lignite pyrolysis.

14

In the actual industrial process, the feed coal usually has a wide particle size distribution. In

15

order to ensure the consistency of drying effect, the drying conditions (temperature and time) of

16

the particles with different sizes should be selected seriously. Therefore, it is of great significance

17

to analyze the effect of particle size on the drying process. The dehydration process of SLP with

18

different particle sizes was investigated in the flue gas with the temperature of 700oC, which is

19

considered as the optimum drying temperature in the research and the results are shown in Fig.9.

20

It can be known that SLP with larger size requires longer drying time for complete drying.

21

5

Conclusion 16


Page 17 of 34

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

1

Energy & Fuels

The drying process of SLP in high-temperature flue gas is investigated experimentally and

2

numerically in the work. The main conclusions are listed as follows:

3

(1) SLP drying process in high temperature flue gas consists of two stages, namely the

4

increasing drying rate stage and the decreasing drying rate stage. There is no constant drying

5

rate stage involved in the lignite drying process. High drying temperature and small particle

6

sizes are beneficial to the heat and mass transfer during the lignite drying process.

7

(2) The dry and wet region numerical model for SLP drying process in high-temperature flue gas

8

was developed. The numerical simulation results from the model agree well with the

9

experimental data. Using the model, some drying characteristics of the lignite particles can

10

be obtained including drying time, temperature distribution inside lignite particle and average

11

migration velocity of evaporation interface.

12

(3) In the high temperature drying process, the migration of evaporation interface can be

13

considered as a constant velocity process and the average migration velocity of the

14

evaporative interface is linearly dependent on the drying temperature.

15

(4) According to the internal temperature distribution and moisture variation of the lignite

16

particles, the suitable drying conditions (temperature and time) can be predicted using the

17

model. For the high temperature drying environment (≥600oC) in the research, 700oC may be

18

the most suitable drying temperature compared with 600, 800, 900oC, under which the

19

changes in the basic structure of the lignite, especially devolatilization reaction, do not occur.

20

For the actual drying process, the appropriate drying time and particle size may be given

21

using the model for reference. 17


Energy & Fuels

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

1

18


Page 18 of 34

Page 19 of 34

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

1

Energy & Fuels

Nomenclature specific heat of the high-temperature flue gas (J·kg-1·K-1) specific heat capacity of the spherical Cp particles (J·kg-1·K-1) specific heat capacity of the k-th Cp k component (J·kg-1·K-1) specific heat capacity of liquid water Cpl (J·kg-1·K-1) specific heat capacity of solid phase of Cps lignite particles (J·kg-1·K-1) mass transfer coefficient of the gas in Dg lignite particles (m2·s-1) dimensionless moisture content DMR (kg·kg-1) volume force of the k-th component fk (N) convective mass transfer coefficient hm (m·s-1) convective heat transfer coefficient hs (W·m-2·K-1) evaporation rate of water in unit time m& (kg·m-3·s-1) initial moisture content of the lignite M0 particles (%) Mg moisture content of the flue gas (%) mass fraction of the k-th component Mk (%) moisture content on the surface of the Mp particles (%) mass source term in the unit volume ṁv (kg·m-3·s-1) Cg

rf latent heat of vaporization of water (kJ·kg-1) T thermodynamic temperature (K) T0 initial flue gas temperature (K) Tl initial particle temperature (K) Tg Tp

temperature of the high-temperature flue gas (K) temperature of the lignite particle surface (K)

τ

time (s)

u

velocity (m·s-1)

ug ẆT ρ

velocity of the gaseous substance in the lignite particles (m·s-1) total reactive heat and phase change heat of all components (W·m-3) density (kg·m-3)

ρk

density of the high-temperature flue gas (kg·m-3) density of the k-th component (kg·m-3)

ρl

density of liquid water (kg·m-3)

ρp

density of lignite particles (solid phase) (kg·m-3)

λ

thermal conductivity (W·m-1·K-1)

ρg

N

number of components

λg

p

pressure (N·m-3)

λl

r

radius (m)

λs

thermal conductivity of the gaseous substances in the lignite particles (W·m-1·K-1) thermal conductivity coefficient of liquid water (W·m-1·K-1) thermal conductivity of the Solid material of the lignite particles (W·m-1·K-1)

19


Energy & Fuels

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

1

Acknowledgments This work was financially supported by the National Program on Key Basic Research

2 3

Project (973 Program) of China (No. 2012CB214900).

4

References

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Skodras, G.; Sakellaropoulos, G. P., Mineral matter effects in lignite gasification. Fuel Process Technol 2002, 77,

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drying of a Chinese lignite. Asia-Pac J Chem Eng 2013, 8, (6), 793-803. 10. Pusat, S.; Akkoyunlu, M. T.; Erdem, H. H., Evaporative Drying of Low-Rank Coal. 2016. 11. Akkoyunlu, M. T.; Erdem, H. H.; Pusat, S., Determination of economic upper limit of drying processes in coal-fired power plants. Dry Technol 2016, 34, (4), null. 12. Rao, Z. H.; Zhao, Y. M.; Huang, C. L.; Duan, C. L.; He, J. F., Recent developments in drying and dewatering for low rank coals. Prog Energ Combust 2015, 46, 1-11. 13. Song, Z. L.; Jing, C. M.; Yao, L. S.; Zhao, X. Q.; Sun, J.; Wang, W. L.; Mao, Y. P.; Ma, C. Y., Coal slime hot air/microwave combined drying characteristics and energy analysis. Fuel Process Technol 2017, 156, 491-499. 14. Komatsu, Y.; Sciazko, A.; Zakrzewski, M.; Kimijima, S.; Hashimoto, A.; Kaneko, S.; Szmyd, J. S., An experimental investigation on the drying kinetics of a single coarse particle of Belchatow lignite in an atmospheric superheated steam condition. Fuel Process Technol 2015, 131, 356-369. 15. Wang, Y.; Zhang, Y. F.; Qiao, X. X.; Liu, J.; Li, T.; Xu, Y., Effect of moisture on dehydration and heat transfer 20


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T.;

Zhu,

H.

C.;

Datta,

A.

K.;

Huang,

K.,

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electromagnetics-multiphase transport modeling with experimentation. Part II: Model validation and simulation results. Food Bioprod Process 2015, 96, 326-337. 31. Fu, B. A.; Chen, M. Q.; Song, J. J., Investigation on the microwave drying kinetics and pumping phenomenon of lignite spheres. Appl Therm Eng 2017, 124, 371-380. 32. Zhang, K.; You, C. F., Experimental and Numerical Investigation of Convective Drying of Single Coarse Lignite Particles. Energ Fuel 2010, 24, 6428-6436. 33. Shigeru, M.; Pei, D. C. T., A mathematical analysis of pneumatic drying of grains—I. Constant drying rate. International Journal of Heat & Mass Transfer 1984, 27, (6), 843-849. 34. Chen, Z.; Wu, W.; Agarwal, P. K., Steam-drying of coal. Part 1. Modeling the behavior of a single particle. Fuel 2000, 79, (8), 961-973. 35. Pelegrina, A. H.; Crapiste, G. H., Modelling the pneumatic drying of food particles. J Food Eng 2001, 48, (4), 21


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301-310. 36. Looi, A. Y.; Golonka, K.; Rhodes, M., Drying kinetics of single porous particles in superheated steam under pressure. Chem Eng J 2002, 87, (3), 329-338. 37. Zheng, H. J.; Zhang, S. Y.; Guo, X.; Lu, J. F.; Dong, A. X.; Deng, W. X.; Tang, W. J.; Zhao, M. H.; Jin, T., An experimental study on the drying kinetics of lignite in high temperature nitrogen atmosphere. Fuel Process Technol 2014, 126, 259-265. 38. Ma, Y. F.; Zhang, H.; Yuan, Y. C.; Wang, Z. Y., Optimization of a lignite-fired open pulverizing system boiler process based on variations in the drying agent composition. Energy 2015, 81, 304-316. 39. Murata, S.; Hosokawa, M.; Kidena, K.; Nomura, M., Analysis of oxygen-functional groups in brown coals. Fuel Process Technol 2000, 67, (3), 231-243. 40. Nishino, J., Adsorption of water Vapor and carbon dioxide at carboxylic functional groups on the surface of coal. Fuel 2001, 80, (5), 757-764. 41. Nan, Q.; LeVan, M. D., Adsorption equilibrium modeling for water on activated carbons. Carbon 2005, 43, (11), 2258-2263. 42. Yu, J. L.; Tahmasebi, A.; Han, Y. N.; Yin, F. K.; Li, X. C., A review on water in low rank coals: The existence, interaction with coal structure and effects on coal utilization. Fuel Process Technol 2013, 106, 9-20. 43. Zhu, J. F.; Liu, J. Z.; Wu, J. H.; Cheng, J.; Zhou, J. H.; Cen, K. F., Thin-layer drying characteristics and modeling of Ximeng lignite under microwave irradiation. Fuel Process Technol 2015, 130, 62-70. 44. Zhang, K.; You, C. F., Numerical simulation of lignite drying in a packed moving bed dryer. Fuel Process Technol 2013, 110, 122-132. 45. Hutter, C.; Buchi, D.; Zuber, V.; von Rohr, P. R., Heat transfer in metal foams and designed porous media. Chem Eng Sci 2011, 66, (17), 3806-3814. 46. Massman, W. J., A non-equilibrium model for soil heating and moisture transport during extreme surface heating: the soil (heat-moisture-vapor) HMV-Model Version 1. Geosci Model Dev 2015, 8, (11), 3659-3680. 47. Pini, G.; Gambolati, G., Arnoldi and Crank-Nicolson methods for integration in time of the transport equation. Int J Numer Meth Fl 2001, 35, (1), 25-38. 48. Pusat, S.; Akkoyunlu, M. T.; Pekel, E.; Akkoyunlu, M. C.; Özkan, C.; Kara, S. S., Estimation of coal moisture content in convective drying process using ANFIS. Fuel Process Technol 2016, 147, 12-17. 49. Pusat, S.; Akkoyunlu, M. T.; Erdem, H. H.; Teke, I., Effects of Bed Height and Particle Size on Drying of a Turkish Lignite. International Journal of Coal Preparation & Utilization 2015, 35, (4), 196-205. 50. Pusat, S.; Akkoyunlu, M. T.; Erdem, H. H.; Dağdaş, A., Drying kinetics of coarse lignite particles in a fixed bed. Fuel Process Technol 2015, 130, (130), 208-213. 51. Pusat, S.; Erdem, H. H., Drying Characteristics of Coarse Low-Rank-Coal Particles in a Fixed-Bed Dryer. International Journal of Coal Preparation & Utilization 2016. 52. Zhao, Y. Y.; Zhao, G. B.; Sun, R.; Liu, H.; Wang, Z. Z.; Sihyun, L.; Kong, M., Effect of the COMBDry Dewatering Process on Combustion Reactivity and Oxygen-Containing Functional Groups of Dried Lignite. Energ Fuel 2017, 31, (4), 4488-4498.

39

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Page 23 of 34

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Energy & Fuels

1

Table Captions

2

Table 1 Proximate and ultimate analyses of the sample.

3

Table 2 Parameters used in the numerical simulation.

4

Table 3 Relative parameters calculated using SLP drying model under different

5

temperatures.

6 7

Figure Captions

8

Figure 1 Schematic diagram of the self-designed horizontal fixed-bed dryer.

9

Figure 2 Schematic diagram of the lignite particle drying process.

10

Figure 3 Drying process of the lignite particles (20 mm in diameter) in the flue gas of

11

different temperature. (a) Dimensionless moisture ratio (DMR) vs. residence

12

time (t). (b) (-dDMR/dt) vs. DMR.

13

Figure 4 Drying process of the lignite particles of different diameters in 800oC flue gas.

14

(a) Dimensionless moisture ratio (DMR) vs. residence time (t). (b) (-dDMR/dt)

15

vs. DMR.

16

Figure 5 (a) Comparison between the experimental results and SLP model simulated

17

results (SV means simulated value, MV means measured value). (b) Errors

18

between the experimental results and the simulated results.

19

Figure 6 Drying process simulation for the lignite particles (20 mm in diameter) in the

20

flue gas with different temperatures (600, 700, 800 and 900 oC). (a) Drying

21

curves. (b) Evaporation interface migration. 23


Energy & Fuels

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

1

Figure 7 TG and DTG curves of YM lignite. (a) TG curve. (b) DTG curve.

2

Figure 8 Simulating the temperature distribution inside the lignite particles (20 mm in

3 4 5

diameter) during the drying process in 700oC flue gas. Figure 9 Drying process simulation for the lignite particles (10, 15, 20 and 25mm in diameter) in 700 oC flue gas.

24


Page 24 of 34

Page 25 of 34

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

Energy & Fuels

1

Table 1

2

Proximate and ultimate analyses of the sample. Proximate analysis

Ultimate analysis (ad)

Mar

Mad

Aad

Vad

FCad

C

H

O

N

S

36.7

7.08

13.50

36.18

43.24

56.96

3.61

17.31

0.95

0.59

3

Mar is the moisture content (as received basis); Mad is the moisture content (air dried

4

basis); Aad is the ash (air dried basis); Vad is the volatile matter (air dried basis); FCad is

5

the fixed carbon (air dried basis).

6 7

Table 2

8

Parameters used in the numerical simulation. parameter

dimension

Cg

value

parameter

0.9×103

λc

dimension

value 1.8

w/(m·K) 3

λl

Cps

1.13×103

ε

cm3/kg

0.65

ρg

0.39

M0

/

36%

1.0×103

r

kJ/kg

2.3×103

1.3×103

Tp

K

298

Cpl

ρl ρp

J/(kg·K)

kg/m3

4.18×10

9

25


0.615

Energy & Fuels

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

N2&O2 Outlet

Page 26 of 34

Heater

N2 Outlet

TC

TC Operating lever

Insulation

Flow meters

Cooling water

TC Valve

N2

N2&O2

1 2 3 4

Fig. 1. Schematic diagram of the self-designed horizontal fixed-bed dryer.

5 6

Fig. 2. Schematic diagram of the lignite particle drying process.

26


Page 27 of 34

1.0

Dimensionless moisture ratio

a 0.9

0.8 Drying temperature o

600 C o 700 C o 800 C o 900 C

0.7

0.6

0

4

8

12

16

20

24

28

32

36

40

Residence time (s)

1 1.5 1.4

o

b

900 C o 800 C o 700 C o 600 C

1.3 1.2 1.1 -2

-dDMR/dt (10 )

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

Energy & Fuels

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

DMR (%)

2 3

Fig. 3. Drying process of the lignite particles (20 mm in diameter) in the flue gas of different

4

temperature. (a) Dimensionless moisture ratio (DMR) vs. residence time (t). (b) (-dDMR/dt) vs.

5

DMR.

27


Energy & Fuels

Dimensionless Moisture Ratio

1.0

a

0.9

0.8

0.7 Particle diameters 25mm 20mm 15mm 10mm

0.6

0.5

0

4

8

12

16

20

24

28

32

36

40

Residence time (s)

1 1.6

b

10 mm 15 mm 20 mm 25 mm

1.4 1.2 -2

-dDMR/dt (10 )

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

Page 28 of 34

1.0 0.8 0.6 0.4 0.2 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00

2

DMR (%)

3

Fig. 4. Drying process of the lignite particle of different diameters in 800oC flue gas. (a)

4

Dimensionless moisture ratio (DMR) vs. residence time (t). (b) (-dDMR/dt) vs. DMR.

5

28


Page 29 of 34

1.0

Dimensionless moisture ratio

a 0.9

o

600 C SV o 700 C SV o 800 C SV o 900 C SV o 600 C MV o 700 C MV o 800 C MV o 900 C MV

0.8

0.7

0.6

0

4

8

12

16

20

24

28

32

36

40

Time (s)

1

7

600oC 700oC 800oC 900oC

b

6 5

Errors (%)

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

Energy & Fuels

4 3 2 1 0

0

4

8

12

16

20

24

28

32

36

40

Time (s)

2 3

Fig. 5. (a) Comparison between the experimental results and SLP model simulated results (SV

4

means simulated value, MV means measured value). (b) Errors between the experimental results

5

and the simulated results.

29


Energy & Fuels

0.40

o

Moisture content of particles (%)

600 C o 700 C o 800 C o 900 C

a

0.36 0.30

0.20

0.12 0.10

0.00

0

200

400

600

800

1000

1200

Time (s)

1 0.010

Radius of water evaporation interface (m)

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

Page 30 of 34

o

600 C o 700 C o 800 C o 900 C

b 0.008

0.006

0.004

0.002

0.000

0

200

400

600

800

1000

1200

Time (s)

2 3

Fig. 6. Drying process simulation for the lignite particles (20 mm in diameter) in the flue gas

4

with different temperatures (600, 700, 800 and 900 oC). (a) Drying curves. (b) Evaporation

5

interface migration.

30


Page 31 of 34

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

Energy & Fuels

1

Table 3

2

Relative parameters calculated using SLP drying model under different temperatures Parameters Flue-gas temperature/ oC

Drying time/s

Temperature of

Average migration o

particle surface/ C

velocity/10-5m·s-1

600

282

186

0.83

700

183

247

1.49

800

132

315

2.13

900

102

388

2.78

3

31


Energy & Fuels

a

100 95

TG (%)

90 85 80 75 70 65

0

100

200

280300

400

500

Temperature (oC)

1 3.0

b

2.5

DTG (% / min)

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

Page 32 of 34

2.0 1.5 1.0 0.5 0.0

0

100

200

280 300

400

500

o

2 3

Temperature ( C)

Fig. 7. TG and DTG curves of YM lignite. (a) TG curve. (b) DTG curve.

4

32


Page 33 of 34

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Energy & Fuels

1 2

Fig. 8. Simulating the temperature distribution inside the lignite particles (20 mm in diameter)

3

during the drying process in 700oC flue gas.

4

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Energy & Fuels

0.40

10mm 15mm 20mm 25mm

0.35

Moisture content of particles (%)

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

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0.30 0.25 0.20 0.15 0.10 0.05 0.00

0

100

200

300

400

500

600

700

800

900

1000

Time (s)

1 2

Fig. 9. Drying process simulation for the lignite particles (10, 15, 20 and 25mm in diameter)

3

in 700 oC flue gas.

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Numerical Simulation and Experimental Research on Drying Behavior

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