H2O atmospheres

Fuel Processing Technology 186 (2019) 8–17

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Research Article

Combustion characteristics of lignite char in a fluidized bed under O2/N2, O2/CO2 and O2/H2O atmospheres

T

Lin Lia, Lunbo Duana, , Shuai Tonga, Edward John Anthonyb ⁎

a b

Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China Centre for Combustion and CCS, School of Energy, Environment and Agrifood, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK

ARTICLE INFO

ABSTRACT

Keywords: O2/H2O combustion Lignite char Fluidized bed (FB) Gasification Activation energy

As a possible new focus of oxy-fuel work, O2/H2O combustion has many advantages over O2/CO2 combustion, and has gradually gained increasing attention. The unique physicochemical properties (thermal capacity, diffusivity, reactivity) of H2O significantly influence the char combustion characteristics. In the present work, the combustion and kinetics characteristics of lignite char particle were studied in a fluidized bed (FB) reactor under N2, CO2 and H2O atmospheres with different O2 concentrations (15%–27%) and bed temperatures (Tb, 837–937 °C). Results indicated that the average reaction rate (raverage) and the peak reaction rate (rpeak) of lignite char in H2O atmospheres were slower than those in CO2 atmospheres at low O2 concentrations. However, as the O2 concentration increases, the rpeak and raverage of lignite char in H2O atmospheres significantly improved and exceeded those under CO2 atmospheres. The calculation result for the activation energy based on the shrinkingcore model showed that the order of activation energy under different atmospheres is: O2/CO2 (28.96 kJ/ mol) > O2/H2O (26.11 kJ/mol) > O2/N2 (23.31 kJ/mol). Furthermore, gasification reactions play an important role in both O2/CO2 and O2/H2O combustion, and should not be ignored. As the Tb increased, the active sites occupied by gasification agent were significantly increased, while the active sites occupied by oxygen decreased correspondingly.

1. Introduction

results in low SOx and NOx emissions; and 4) the power consumption associated with pumps/fans is relatively low. Therefore, O2/H2O combustion can represent a promising next generation of oxy-fuel combustion despite the problems associated with the high latent heat of vaporization of water. When this technology is introduced in fluidized bed combustion, it also confers other advantages such as its potential for burning low-rank coals like lignite, despite their high inherent moisture content, and offers flexible temperature control and low gaseous pollutant emissions (such as SO2 and NOx) [9–11]. To date, relatively few studies focused on O2/H2O combustion have been published, and those that exist are mostly about pulverized coal (PC) furnaces [12–18]. Jin et al. [13] investigated the process characteristics of O2/H2O combustion power plants using a steady-state Aspen process model. It was found that the thermodynamic and economic performances of O2/H2O combustion were better than that of O2/CO2 combustion. Richards et al. [14] studied CO emissions and residence time of oxy-steam combustion by means of a combination of experiments and simulations, and indicated that the CO levels and residence time in the O2/CO2 environment were all higher than that in O2/H2O environment. Zou et al. [15–17] carried out a series of

Oxy-fuel combustion technology (O2/CO2 combustion) is recognized as a competitive technology for Carbon Capture and Storage (CCS), and has attracted extensive research interest [1]. Compared with conventional combustion, a mixture of pure O2 and recycled flue gas is utilized as the oxidizer rather than air, resulting in levels of > 90% of CO2 in the exhaust gas after condensation, which is advantageous for CO2 separation [2–4]. However, the low net efficiency and higher cost of CCS remains the greatest obstacle to commercialization of this technology [5,6]. Recently, the route of O2/H2O combustion, which used H2O to moderate the high furnace temperature was discussed by Salvador [7], and Seepana and Jayanti [8]. In O2/H2O combustion, the flue gas recycle system is not required, further reducing the energy consumption associated with this option. In addition, O2/H2O combustion has many advantages over O2/CO2 combustion [7,8], specifically: 1) the latent energy is easily recovered owing to the enriched water vapor content in flue gas; 2) the major equipment size of O2/H2O combustion is smaller than that for conventional oxy-fuel combustion; 3) O2/H2O combustion

⁎

Corresponding author. E-mail address: [email protected] (L. Duan).

https://doi.org/10.1016/j.fuproc.2018.12.007 Received 6 October 2018; Received in revised form 13 November 2018; Accepted 7 December 2018 0378-3820/ © 2018 Elsevier B.V. All rights reserved.

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experiments on ignition and combustion behavior of pulverized coal (PC) in O2/N2 and O2/H2O environments. Their results showed that the ignition delay time of PC under O2/H2O conditions was shorter than that for O2/N2 environments under the same conditions. This can be attributed to the existence of the gasification reaction and the CO shift reaction in a H2O atmosphere. However, fluidized bed combustion technology is significantly different from PC combustion in terms of hydrodynamics, temperature and reaction kinetics and needs to be specifically investigated. Existing research for oxy-fuel fluidized bed combustion is mainly concerned with O2/CO2 rather than O2/H2O atmospheres. A 0.8 MWth circulating fluidized bed (CFB) boiler study carried out by Tan et al. [19] and this achieved smooth transition between oxy-fuel and air combustion. Under oxy-fuel combustion, CO2 concentrations can reach > 90% (on a dry basis), while the NOx and SOx emissions are simultaneously reduced. A similar conclusion was drawn by Lupion et al. [20] for a 30 MWth CFB boiler. Duan et al. [21] tested three types of fuel in a warm flue gas recycle oxy-fuel CFB combustor (50 kWth), and the results showed that an equivalent or higher carbon burnout can be realized with a higher O2 concentration in O2/CO2 condition. Scala et al. [22] investigated single char particle combustion performances in a fluidized bed reactor under high CO2 concentration and found that the C-CO2 gasification could not be ignored in oxy-fuel combustion. Additionally, a series of studies on combustion mechanism of single particles has also been performed in a FB under different atmospheres [23–26]. As noted above, there has been little or no work published on O2/ H2O combustion of coal in FB combustor, and the combustion mechanism of O2/H2O combustion is not fully understood. Consequently, the present work was aimed to investigate the combustion and kinetic characteristics of coal char in a FB reactor under O2/H2O atmosphere, and to quantitatively analyze the degree to which gasification also occurs in the overall combustion process.

heated in an evaporator. Then the water vapor was mixed with O2 or N2 as the fluidization gas and fed into the combustor. The FB reactor was made of quartz glass, and included a preheating section and a reaction section with inner diameter of 22 mm, which were heated by two electrical heaters, respectively. The bed temperature (Tb) was measured by a K-type thermocouple and continuously controlled by a proportional-integral-derivative (PID) controller. The flue gas (include O2, N2, CO2, H2O, CO, H2 and very little CH4) was rapidly cooled and dried in the outlet of the FB reactor, then the gas products were analyzed by an online multi-component FTIR gas analyzer (with a resolution of 1 ppm). The measurement ranges of FTIR for CO, CO2 and CH4 are 0–10,000 ppm, 0–250,000 ppm and 0–5000 ppm, respectively. In order to ensure that all of the gas products were in the optimum measuring range of the gas analyzer, a selected amount (1.2 L/min for O2/N2 atmosphere, 1.8 L/min for O2/CO2 atmosphere and 2.5 L/min for O2/H2O atmosphere) of N2 was used as a dilution gas before the flue gas entered the gas analyzer. 1-Deionized water; 2-Injection pump; 3-Steam generator; 4-Thermal insulation section; 5-Mass flowmeter; 6-Gas mixer; 7-Preheating section; 8-Gas distributor; 9-Thermocouple; 10-Temperature controller; 11-Feeding port; 12-Water removal; 13-Dilution gas (N2); 14-Gas analyzer. 2.2. Materials As lignite coal is a commonly used fuel for fluidized bed boilers, a lignite was selected as the test coal in the work. The proximate and ultimate analysis are given in Table 1. The particle size of coal in CFB boilers is generally 0–13 mm, which depends on the type of fuel [27]. However, the combustion process for large char particles are significantly affected by diffusion, and in addition too small char particles are easily carried out of the reactor by the fluidizing gas. Therefore, in order to evaluate the combustion and kinetic characteristics of lignite char in fluidized beds, a relatively small particle size range for the test coal (1.25–1.5 mm) was selected to minimize the influence of the diffusion control region and avoid excessive elutriation of char fines. Before the test, the lignite was devolatilized for 7 min in a FB reactor with N2 atmosphere at 900 °C, to produce char particles which were then sieved into the size range of 1–1.2 mm. Quartz sand (diameter: 0.3–0.35 mm) was chosen as the bed material with an unexpanded bed height of 150 mm in these experiments. The corresponding minimum fluidization velocity of the bed material was 0.042 m/s. The fluidizing velocity in this work was set to 0.126 m/ s, which corresponds to strongly bubbling bed condition.

2. Experimental 2.1. Fluidized bed reactor A bench-scale bubbling FB reactor was used for these experiments (Fig. 1). The system consisted of five sub-systems: the gas supply line, the steam generation and supply system, the fluidized bed reactor, the temperature control system and the flue gas data acquisition system. The flow of O2, N2 and CO2 from cylinders were controlled by three digital mass flowmeter controllers. A high-precision injection pump was used to accurately control the rate of deionized water before it was

2.3. Experimental procedure After heating the FB combustor to the set temperature, water and fluidizing gas were introduced into the reactor. Approximate 200 mg char samples were fed from the two-stage hopper at the top of the reactor after the fluidization condition reached steady state. The tests were performed in O2/N2, O2/CO2, O2/H2O, N2/CO2 and N2/H2O atmospheres with different oxygen concentrations (0%, 15%, 21% and 27%) and bed temperatures (837 °C, 887 °C and 937 °C). The operating conditions are summarized in Table 2. The tests of N2/CO2 and N2/H2O atmospheres are aimed at determining the gasification rate under different gasification agent concentrations, and to provide support for quantitative analysis of the gasification reaction ratio in the oxy-fuel combustion. Before the experiments, a series of water balance tests were performed by comparing the volume of water entered the reactor (Vin) with the volume of condensate water in the outlet (Vout). The water balance ratios (Vout/Vin) under both O2/H2O and N2/H2O atmospheres are all > 98% within a 2 h period.

Fig. 1. The schematic of experimental system. 9

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Table 1 Proximate and ultimate analysis of the test coal. Fuel

Proximate analysis, wt% (ar) Moisture

Volatile

Fixed carbon

Ash

C

H

Oa

N

S

15.97

39.11

35.83

9.09

67.05

4.26

25.10

1.48

2.11

Lignite a

Ultimate analysis, wt% (daf)

By difference.

weight of carbon in CO and CO2 generated at the time interval of 0 to i, respectively, g; WC,∞ represents the weight of carbon generated over the entire reaction, g. The amount of carbon in CO and CO2 can be described as:

Table 2 Operating conditions used in tests. Fuel

Tb (°C)

Atmosphere

O2 or N2 (%)

Lignite char

837, 887, 937 887 837, 887, 937 887

O2/N2, O2/CO2, O2/H2O O2/N2, O2/CO2, O2/H2O N2/CO2, N2/H2O N2/CO2, N2/H2O

21 (O2) 15, 21, 27 (O2) 21 (N2) 15, 21, 27 (N2)

WCO,i = WCO2,i =

2.4. Data treatment

H=

394 kJ/mol

MC CCO Q/Vm dt i

0

MC (CCO2

(8)

CCO2,0 ) Q/Vm dt

(9)

where MC is the carbon molecular weight, g/mol; CCO2 and CCO are the concentration of CO2 and CO which were obtained from gas analyzer, respectively, μL/L; CCO2,0 is the CO2 concentration at the inlet of reactor, μL/L; Q is the volumetric flow rate of outlet gas, L; Vm represents the molar volume of ideal gas under standard conditions, 22.4 L/mol; t is the reaction time, s.

It should be noted that only a small amount of CH4 (< 40 ppm) was measured in O2/H2O atmosphere, so only the CO and CO2 are considered as parameters in the carbon balance calculation. The concentration curves of CO and CO2 for the combustion test under different conditions are shown in Fig. 2. The lignite char combustion process in O2/N2 and O2/CO2 atmospheres can be described by three global reactions [22,26]:

C + O2 = CO2

i 0

2.5. Carbon balance ratio

(1)

C + CO2 = 2CO H = + 171 kJ/mol

(2)

In present work, the carbon balance ratio was used to examine the reliability of the measurements, which can be calculated by:

2CO + O2 = 2CO2

(3)

Carbon balance ratio =

H=

283 kJ/mol

Char combustion in O2/H2O atmospheres can be represented additionally by the following reactions [16,26]:

C + H2 O = H2 + CO

H = +130 kJ/mol

CO + H2 O = H2 + CO2 2H2 + O2 = 2H2 O

H=

H=

40 kJ/mol

242 kJ/mol

× 100%

(10)

(6)

where WCO,∞ and WCO2,∞ are the weight of carbon in CO and CO2, respectively, g; WC,0 is the total amount of carbon of the test sample, g. The carbon balance ratios under different test conditions are shown in Fig. 3 and for all tests were in the range of 91.8% - 106.5%, which demonstrates that the measurements obtained are accurate and reliable.

(7)

3. Data processing method

(4) (5)

The carbon conversion of the test char is expressed as

X = (WCO,i + WCO2,i)/WC,

WCO, + WCO2, WC,0

where X represent the carbon conversion; WCO,i and WCO2,i are the

3.1. Kinetic model The combustion reaction rate was described as:

Fig. 2. The concentration profiles of CO and CO2 during the combustion tests (Tb = 937 °C, 21% O2): (a) O2/N2 combustion; (b) O2/CO2 combustion; (c) O2/ H2O combustion.

Fig. 3. Carbon balance ratio under different conditions. 10

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(11)

dX/dt = k f(X)

If gasification and oxidation do not affect each other, and both gasification and oxidation are in the kinetic control region, then the simulated carbon consumption rate (vcal) can be calculated by adding the carbon consumption rate of the oxidation and gasification reactions. It needs to be pointed out that the amount of quartz sand was much greater than the test char, and the fluidization state of the bed was vigorous, so good heat transfer between char and bed material ensured that their temperatures are effectively the same. This hypothesis was also supported by Roy and Bhattacharya [26], who carried out experiments and simulation researches on single char particle combustion with different particle sizes (≥1 mm) in a FB reactor, and found that when the size of char was < 1 mm, the error caused by the assumption of equal particle and bed temperatures was negligible. The vcal can be expressed as:

where f(X) is the reaction model; k represents the reaction rate constant, which could be calculated by: (12)

k = Aexp( E/RT)

where A is the pre-exponential factor; and E is the activation energy, J/ mol; T is the reaction temperature, K; and R is the gas constant, J/ (mol·K). Rearranging Eq. (11),

1 dX = kdt f(X)

(13)

The gasification and combustion reaction kinetic models for coal are both complex and numerous. In the present work, a shrinking core model was adopted to calculate kinetic parameters of lignite char combustion, as this has been demonstrated by many researchers to be a suitable model for coal combustion and gasification research [28–30]. The model is described by:

G(X ) = 1

(1

vcal = voxi + vgasi

where voxi and vgasi are the reaction rates for oxidation and gasification, respectively, g/s. The voxi and vgasi were obtained by testing the char reaction rate in O2/N2, N2/CO2 and N2/H2O atmospheres at the same concentration of reactant. When X < 0.85, the char oxidation under the bed temperature is almost completely under kinetic control region. Therefore, the theoretical value of oxidation reaction rate in O2/CO2 and O2/H2O atmospheres can be expressed by

(14)

X )1/3

where G(X) is the mechanism function model. Combining Eq. (13) with Eq. (14),

G(X) = 1

(1

X

1

X )3 =

0

1 dX = f(X )

t 0

kdt = kt

(15)

The k under different conditions can be calculated by the slope of Eq. (15). Then by taking the logarithm of Eq. (12),

ln k = ln A

E / RT

(17)

voxi,O2/CO2 = vO2/N2

(18)

voxi,O2/H2O = vO2/N2

(19)

where vO2/N2, voxi,O2/CO2, and voxi,O2/H2O represent the oxidation reaction rates in different atmospheres, respectively, g/s; The char gasification rate in the bed temperature is slow, and the concentration of gasification agent (CO2 or H2O) in the environment is completely excessive, so the char gasification is mostly like in the kinetic control region [31,32].The theoretical value of gasification reaction rate in O2/ CO2 and O2/H2O atmospheres are given by

(16)

the E and A were calculated by the slope (−E/R) and the intercept (lnA), respectively, of Eq. (16). In order to minimize the influence of char heating, char mixing and the diffusion control region on reaction kinetics analysis in the combustion process, carbon conversion in the range of 0.1–0.85 was chosen to analyze reaction kinetics in this work. 3.2. Calculation of carbon consumption rate Generally, the differences in char combustion in the three different atmospheres are strongly influenced by: (1) Gasification, the gasification reaction of char (such as Eq. (2) and Eq. (4)) cannot be neglected in O2/CO2 and O2/H2O atmospheres; (2) Diffusivity of oxygen, the diffusivity of O2, CO2 and H2O in binary gaseous mixtures are shown in Fig. 4. The diffusivity of O2 in H2O atmosphere is over 20% higher than that in N2 atmospheres, and the diffusivity of O2 in CO2 atmosphere is by far the lowest; and (3). There is also a possible competitive relationship between gasification and oxidation.

vgasi,O2/CO2 = v N2/CO2

(20)

vgasi,O2/H2O = v N2/H2O

(21)

where vN2/H2O, vN2/CO2, vgasi,O2/CO2 and vgasi,O2/H2O represent the gasification reaction rates in the different atmospheres, respectively, g/s. However, in the oxy-fuel combustion process for char, the gasification and oxidation are not independent of each other. In this work, in order to analyze the interaction mechanism between gasification and oxidation, the summation of carbon consumption rates of oxidation and gasification was used to compare with the comprehensive carbon conversion rate from experimental conditions (vexp) of O2/CO2 and O2/H2O atmospheres. The relationship between char gasification and oxidation in different combustion conditions can be categorized into four categories: (I) (II) (III) (IV)

Mutual promotion: vcal < vexp; Shared active site: vcal = vexp; Partial shared active site: voxi < vexp < vcal; Mutual competition: vexp ≤ voxi.

4. Results and discussion 4.1. The carbon consumption rate profiles The reaction rate and cumulative carbon conversion rate of lignite char with various oxygen inlet concentrations under O2/N2, O2/CO2 and O2/H2O atmospheres at different bed temperatures are plotted in Fig. 5 and Fig. 6. The general char combustion behavior in O2/N2 atmosphere is as follows: after being feed into the reactor, the char sample ignited immediately and the reaction rate reached its peak value rapidly; as the char was continuously consumed, the reaction rate gradually decreased until the reaction stopped, the whole combustion

Fig. 4. Diffusion rate in different binary gas atmospheres. 11

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Fig. 5. Reaction rate of char under different conditions.

process typically took about 369–770 s. Interestingly, there were two peaks in the reaction rate curves under CO2 and H2O atmospheres, while there was only one peak under N2 atmosphere, and the reaction rate peaks under H2O atmosphere were larger than those under CO2 atmosphere at the same condition. Furthermore, the second reaction rate peak increased with the steam concentration under O2/H2O atmosphere, as shown in Fig. 5c. This may be caused by the gasification in CO2 and H2O environments. With the advance of the reaction, the pore structure and the specific surface area of char are significantly increased, further promoting the gasification and oxidation reaction. The strengthened gasification results in a higher peak value of CO2 and H2O environments than N2 environment. Furthermore, due to the faster rate of the C-H2O reaction than the C-CO2 reaction at the same concentration and temperature [33], the reaction rate peaks under H2O atmosphere were higher than those under CO2 atmosphere. As the bed temperature and O2 concentration increased, the peak reaction rate of char improved, and burnout time of char decreased. At the same bed temperature and O2 concentration, the burnout time of char in O2/CO2 and O2/H2O atmospheres were all longer than that in O2/N2 atmosphere. The quantitative analysis and discussion of these combustion characteristic parameters are demonstrated in Section 4.2.

Fig. 7. The rpeak, tb and raverage with 21%O2 under N2, CO2 and H2O atmospheres at different bed temperatures.

gasification and oxidation reactions are enhanced at higher bed temperatures. At the same O2 concentration and Tb, the burnout time sequence was: O2/N2 < O2/H2O < O2/CO2, the tb of oxy-fuel combustion is higher than that of air combustion. This may be caused by competition between gasification and oxidation in oxy-fuel combustion. In addition, at 837 °C, the diffusion of O2 in steam (2.68 m2/s) is 1.63 times larger than that in a CO2 atmosphere (1.64 m2/s), which also promotes the oxidation of char particles, so the Tb in O2/H2O atmosphere is lower than that in O2/CO2 conditions. The variation of rpeak, tb and raverage vs. O2 concentrations under N2, CO2 and H2O atmospheres at 887 °C, are plotted in Fig. 8. It is clearly

4.2. Char combustion characteristics Fig. 7 shows the peak reaction rate (rpeak), burnout time (tb) and average reaction rate (raverage) with 21% oxygen concentration under N2, CO2 and H2O atmospheres at different bed temperatures. It should be pointed out that the tb refers to the time during which carbon conversion rate ranges from 0 to 90% in this work. When the carbon conversion rate reaches 90%, the char reaction rate turns low (as shown in Fig. 2), which may lead to large measuring and statistical errors. It can be observed that with the increase of bed temperature, tb decreased, and rpeak and raverage increased. This result can be expected, because the

Fig. 6. Cumulative carbon conversion rate under different conditions. 12

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Fig. 10. Fitting results between reaction rate and temperature. (The solid points are the experimental data).

Fig. 8. The rpeak, tb and raverage vs. O2 concentrations under different atmospheres at a bed temperature of 887 °C.

Table 3 Kinetics parameters of lignite char.

that the higher O2 concentration is beneficial to char combustion. At low O2 concentration, the tb of char particles in H2O atmosphere was longer than that in CO2 and N2 atmospheres. Nevertheless, as the O2 concentration increases, the tb in H2O atmosphere significantly improved and exceeded that in O2/CO2 atmosphere. There are two effects of steam on the char combustion in H2O atmospheres namely on: 1) the steam-char gasification reaction (Eq. (4)); and 2) the diffusivity of O2. At low O2 concentrations, higher partial pressures of steam or CO2 result in a faster gasification reaction, which will significantly reduce the temperature and reactivity of char particles, and the gasification activity of steam is higher than that of CO2. So the tb of char particles in H2O atmosphere is the longest. As the O2 concentration increases, the oxidation reaction is strengthened, while the gasification reaction weakens as the concentration of the gasification agent decreases. Furthermore, the diffusion rate of O2 in O2/H2O atmosphere is larger than that in O2/CO2 atmosphere. Therefore, with the increase of O2 concentration, the reaction rate of char particles under O2/H2O atmosphere increased more rapidly than that under O2/CO2 atmosphere. Similar conclusions were also obtained by Zou et al. [16], who investigated the ignition characteristics of PC in a DTF reactor under O2/N2 and O2/H2O atmospheres.

Atmosphere 21% O2/79% N2 21% O2/79% CO2 21% O2/79% H2O

E (kJ/mol)

A (s−1)

R2

23.31 28.96 26.11

0.0332 0.0366 0.0313

0.992 0.999 0.985

and A of lignite char were obtained by linear fitting and the results are shown in Fig. 10. In the three environments, it can be expected that a greater slope can be observed at higher O2 concentration and Tb. The calculation results of kinetics parameters are given in Table 3. The results illustrate that the order of apparent activation energy under different atmospheres is: O2/CO2 (28.96 kJ/mol) > O2/H2O (26.11 kJ/ mol) > O2/N2 (23.31 kJ/mol). It is widely accepted that a larger apparent activation energy means a lower overall reaction rate. So the lignite char had the highest reaction rate in N2 atmosphere and the lowest reaction rate in CO2 environment. It is evident that the char oxidized and gasified simultaneously under CO2 and H2O environments, whereas only the oxidation reaction occurred under N2 atmosphere. The heat absorption of gasification reduces the char particle temperature, and reduces the activity of the char. In addition, it is possible that the gasification and oxidation reaction have the same activity site and compete with each other.

4.3. Combustion reaction kinetics

4.4. Quantitative analysis of gasification and oxidation reactions

According to Eq. (15), the reaction model G(X) should be proportional to the reaction time in theory. Fig. 9 displays the modeling and experimental results for char combustion. According to Eq. (16), the E

The detailed results of the char gasification experiment in N2/CO2

Fig. 9. Fitting results of char combustion using shrinking core model under different conditions. (The solid points are the experimental data, and the solid lines are from the model). 13

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Fig. 11. The variation of average carbon conversion rates vs. Tb and O2 concentration under O2/CO2 atmosphere.

Fig. 12. The variation of average carbon conversion rates vs. Tb and O2 concentration under O2/H2O atmosphere.

and N2/H2O atmospheres are summarized in the Appendix A. The variation of average carbon reaction rate vs. Tb and O2 concentration in CO2 and H2O atmospheres are displayed in Fig. 11 and Fig. 12. It is obvious that the calculated total carbon conversion rates were significantly larger than those from the experimental results, and the experimental carbon conversion rates were even lower than the calculated carbon conversion rates of oxidation. According to the competition/ promotion mechanism model introduced in Section 3.2, it can be concluded that competition exists between the gasification and oxidation reactions in the char combustion under CO2 and H2O atmospheres. Fig. 11 also shows that the difference between calculated results and experimental results increases with the bed temperature. This was because the reaction rate of gasification and oxidation increased significantly with increasing bed temperature, which causes more intense competition between these two processes. Otherwise, the partial pressure of CO2 and H2O decreased with increasing O2 concentration, the gasification reaction rate slightly decreased at high O2 concentration and the effect of gasification on the combustion progress was reduced. In order to quantitatively analyze the proportions of gasification and oxidation reaction in O2/CO2 and O2/H2O combustion, the equilibrium equation can be listed as follows:

vO2/CO2 = a ·v N2/CO2 + b ·vO2/N2

(22)

vO2/H2O = a ·v N2/H2O + b ·vO2/N2

(23)

a+b=1

(24)

where vO2/CO2 and vO2/H2O represent average reaction rate of char combustion in O2/CO2 and O2/H2O atmospheres, respectively, g/s; a and b represent the active sites occupied by gasification agent (CO2 or H2O) and oxidant (O2) on the char surface in the combustion progress, respectively. The variation of a and b vs. bed temperature and oxygen concentrations under different conditions are summarized in Table 4, and the corresponding reaction rates are displayed in Table 5. It was clear that the reaction rates of gasification and oxidation both increased with increasing Tb; this is attributed to the higher reactivity of char at high temperature. At the same oxygen concentration, the active sites occupied by gasification agent significantly increased with bed temperature, and the active sites occupied by oxygen decreased correspondingly. The phenomenon indicated that when the char was burned under CO2 or H2O atmosphere, the inhibitory effect of the gasification reaction on the oxidation reaction increased with an increase of Tb. Table 4 Values of the a and b parameters.

O2/CO2 a b O2/H2O a b

14

837 °C

887 °C

887 °C

887 °C

937 °C

21% O2 0.389 0.611 21% O2 0.286 0.714

15% O2 0.352 0.647 15% O2 0.443 0.556

21% O2 0.404 0.596 21% O2 0.385 0.615

27% O2 0.480 0.520 27% O2 0.316 0.684

21% O2 0.509 0.491 21% O2 0.404 0.596

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5. Conclusions

Table 5 Gasification and oxidation reaction rates during char combustion.

O2/CO2 Vgas voxi O2/H2O Vgas voxi

837 °C

887 °C

887 °C

887 °C

937 °C

21% O2 0.038 0.491 21% O2 0.047 0.574

15% O2 0.078 0.390 15% O2 0.108 0.335

21% O2 0.086 0.512 21% O2 0.086 0.529

27% O2 0.099 0.572 27% O2 0.066 0.752

21% O2 0.154 0.493 21% O2 0.126 0.598

Experimental investigation of combustion characteristics of lignite char under three different atmospheres were carried out in a FB combustor. The main conclusions are drawn as follows: (1) With the increase of bed temperature and O2 concentration, lignite char displayed similar combustion characteristics under H2O, CO2 and N2 atmospheres (the burnout time decreased, while the peak and average reaction rate increased). (2) At low oxygen concentration, the combustion performances of lignite char in H2O atmosphere were worse than those in N2 and CO2 atmospheres; however, as the oxygen concentration increased, the combustion performances in H2O atmosphere were significantly improved, and became better than those observed in CO2 atmosphere. (3) The calculation results showed that the relationship between apparent activation energy under different atmospheres is: O2/CO2 (28.96 kJ/mol) > O2/H2O (26.11 kJ/mol) > O2/N2 (23.31 kJ/ mol). (4) Char gasification in oxy-fuel combustion cannot be neglected, especially in the O2/H2O atmosphere. The active sites occupied by gasification agent significantly increased as the bed temperature increased, and the competitive effect of gasification on oxidation is enhanced.

Moreover, it can be seen that the proportion of gasification reaction in H2O atmospheres were larger than that in CO2 atmospheres. This is due to the fact that the activity of steam gasification is better than that of CO2 gasification. Interestingly, the voxi and (voxi + vgas) under H2O atmosphere were all less than those under CO2 atmosphere at low oxygen concentration. However, with the increase of O2 concentration, the voxi and (voxi + vgas) values under H2O atmosphere increased remarkably and exceeded those under CO2 atmosphere. At low oxygen concentration, the gasification reactivity of char in H2O atmosphere was higher than that in CO2 atmospheres, which means it has stronger inhibitory effect on oxidation. However, as the increase of oxygen concentration, on one hand, the oxidation reaction in H2O atmosphere was more prominently enhanced as compared to those in CO2 atmosphere due to the higher diffusion coefficient of oxygen in H2O atmosphere; on the other hand, the concentration of gasification agent decreased, and the gasification reaction gradually weakens. Therefore, the char combustion rate under H2O atmosphere was more significantly enhanced at high oxygen concentrations than that under CO2 atmosphere.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51776039).

Appendix A. Experimental results of char gasification The concentration curves of CO and CO2 for the gasification tests under different atmospheres and temperatures are shown in Fig. A1. Following the data processing method described in Section 2.4, the carbon balance ratios in different test conditions are shown in Fig. A2 and for all tests were in the range of 92.5%–104.8%. The corresponding reaction rate and cumulative carbon conversion rate of lignite char with various CO2 or H2O inlet concentrations under N2/CO2 and N2/H2O atmospheres at different bed temperatures are plotted in Fig. A3 and Fig. A4. The average gasification reaction rates in conditions are summarized in Table A1.

a

N2/CO2 atmosphere

(b) N2/H2O atmosphere

Fig. A1. The concentration profiles of CO and CO2 during the gasification tests. 1: 21%N2, 837 °C; 2: 15%N2, 887 °C; 3: 21%N2, 887 °C; 4: 27%N2, 887 °C; 5: 21%N2, 937 °C.

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Fig. A2. The carbon balance ratios in different test conditions.

a

N2/CO2

(b) N2/H2O

Fig. A3. Reaction rate in different gasification conditions.

(a) N2/CO2

(b) N2/H2O

Fig. A4. Cumulative carbon conversion rate under different conditions.

Table A1

Gasification reaction rates in different test conditions.

N2/CO2 vN2/CO2 N2/H2O vN2/H2O

837 °C

887 °C

887 °C

887 °C

937 °C

21% N2 0.099 21% N2 0.167

15% N2 0.220 15% N2 0.243

21% N2 0.212 21% N2 0.223

27% N2 0.206 27% N2 0.210

21% N2 0.303 21% N2 0.312

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