Oxy-fuel combustion characteristics and kinetic parameters of lignite coal from thermo-gravimetric data

Thermochimica Acta 553 (2013) 54–59

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Oxy-fuel combustion characteristics and kinetic parameters of lignite coal from thermo-gravimetric data Zhijun Zhou, Xin Hu, Zhuo You, Zhihua Wang ∗ , Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China

a r t i c l e

i n f o

Article history: Received 14 September 2012 Received in revised form 29 November 2012 Accepted 30 November 2012 Available online 19 December 2012 Keywords: Oxy-fuel combustion Thermo-gravimetric analysis Combustion characteristics Kinetic parameters Isoconversional method

a b s t r a c t Combustion characteristics of Yi Min lignite and Zhun Dong lignite in O2 /CO2 atmosphere were investigated by thermo-gravimetric analysis. The effects of the coal type and O2 concentration were studied. The results indicate that the increasing of O2 concentration can significantly improve the combustion performance especially when the O2 concentration is less than 60%. The activation energy corresponding to the YM lignite combustion in O2 /N2 and O2 /CO2 atmospheres was evaluated by two isoconversional methods. The calculation results showed that further research is required whether the activation energy calculation with isoconversional methods for complex reaction process can be used as a basis for judging the degree of the reaction difficulty. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Human-induced warming of the climate system has been accepted as one of the most important problems driving climate change and needs to be controlled. Most of the observed global warming over the last 50 years was likely caused by greenhouse gas forcing, with the dominant contributor being CO2 . Fossil fuel combustion systems such as coal-fired power plants are considered to contribute 33–40% of all anthropogenic emissions of carbon worldwide [1]. With the increasing growth in demand for electrical power, developing countries such as China have constructed new coalfired power plants to keep pace with economic development and demands. In China, coal is a much more abundant resource than other fossil fuels, such as oil and natural gas, so China chooses coal as the primary fuel for power production. Therefore, we must reduce the CO2 emission of coal-fired power plants and try to maintain the position of coal as a viable energy option in a carbon-constrained world. There are several potential strategies to achieve this goal: (1) improving the efficiency of power plants; (2) introducing of combined cycles-as-fired or IGCC, which can reach high thermal efficiencies; (3) replacing hydrocarbon fuels with renewable resources; (4) capturing and storing CO2 from conventional plants [2]. Currently, capture and storage of CO2 has

∗ Corresponding author. Tel.: +86 571 87953162; fax: +86 571 87951616. E-mail address: [email protected] (Z. Wang). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.11.030

attracted attention from the worldwide scientific community. Carbon capture technologies can be generally divided into three main categories: post-combustion capture, pre-combustion capture and oxy-fuel combustion capture. Compared to other technologies, oxy-fuel combustion is less expensive and more effective according to several techno-economic assessment studies [3,4]. In oxy-fuel combusiton technology N2 is replaced with CO2 in order to obtain a high CO2 concentration in the flue gas. In most concepts, oxyfuel combustion technology uses recycled flue gas to lower the flame temperature. To implement this technology, significant fundamental research should be done on various aspects of the system, including heat and mass transfer effects, the process and kinetics of combustion, pollutants emissions and ash deposition chemistry. Tan et al. [5] have performed oxy-fuel combustion experiments using a variety of coals. Their results showed that oxy-fuel combustion can be used to retrofit existing coal-fired power plants or to build new power plants with zero emission potential. Shaddix and Molina [6] conducted experiments in a combustion-driven laminar flow reactor that revealed that the use of oxy-fuel recycle combustion can produce ignition times and volatile flames similar to those of air–coal combustion. The successful research results mentioned above indicate that oxy-fuel combustion is possible and merits further investigation. Recently, several extremely large reserves of lignite have been discovered in the mid-west of China, it is foreseeable that many new lignite-fired power plants will be built in the near future. Therefore, it is meaningful to do the research of the oxy-fuel combustion with the lignite coal. This research may contribute to the CO2 emission reduction.

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Thermo-gravimetric (TG) study is a traditional combustion analysis method and has been widely used for the assessment of the combustion behavior of coal. TG study is already being used in research on oxy-fuel combustion. Niu et al. [7] studied three types of pulverized coal in the oxy-fuel atmosphere by thermogravimetric analysis and suggested that the O2 concentration should be less than 40% in the oxy-fuel combustion. The maximum O2 concentration Niu chose was 60%. Li et al. [8] finished TG analysis with one coal type and four different O2 concentrations. Their results showed that the burning process of pulverized coal in an O2 /CO2 environment is delayed when compared with that in an O2 /N2 environment at the same O2 concentration. When the O2 concentration increases up to a maximum of 80%, the combustion rate increases and the burnout time is shortened. In this study, two different kinds of lignite coal and five O2 concentrations ranging from 21% to 100% were chosen to perform TG experiments. The oxy-fuel combustion process can be investigated more comprehensively with the corresponding combustion characteristic parameters from the TG–DTG curves. Furthermore, kinetic analysis of combustion by a model-free isoconversional method was used to obtain data on the reaction kinetics of oxycoal combustion with Yi Min lignite in different O2 concentration atmospheres. 2. Experimental 2.1. Lignite coal sample Two kinds of representative Chinese coal were selected from two different lignite coal mines. These samples were denominated according to the location of the coal mines: YM lignite from the Yi Min coal mine; and ZD lignite coal from the Zhun Dong coal mine. The proximate and ultimate analyses data of these coal samples were listed in Table 1. The coal samples were first crushed and pulverized with a bench-scale mill in the laboratory, and then sieved through 200-mesh and 325-mesh screens to obtain a sample size between 45 ␮m and 75 ␮m. Finally, the samples were stored in sealed sample bags for TG experiments. 2.2. Apparatus and procedure TG experiments were performed in a TGA/SDTA851e thermogravimetric analyzer produced by Mettler-Toledo International Inc. The mass loss signal (TG) and mass loss rate signal (DTG) were recorded continuously under a stable heating rate. Before the formal tests started, we performed the preliminary experiments three times to examine the reproducibility of the experimental conditions. One coal type (YM) and one test condition (O2 /N2 21%) were selected for the preliminary experiments. The TG curves perfectly overlapped and the error was acceptable. In this paper, first we compared the two kinds of lignite coal with their oxy-fuel combustion characteristics data. Both the YM lignite and ZD lignite were used for the TG experiments. Approximately 10 mg of pulverized lignite coal sample was heated at a linear rate of 30 K/min. The temperature was raised from 298 K to 1073 K to ensure that the weight curve was leveled and stable. The combustion atmospheres included O2 /N2 and O2 /CO2 with an O2

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concentration of 21%, and O2 /CO2 with O2 concentrations of 40%, 60%, 80% and 100%. All gas sample purities were over 99.9%, and N2 purity was over 99.999%. Two mass flow meters were used to control the O2 concentration and the total gas flow was stable at 100 ml/min. After the comparison of the oxy-fuel combustion characteristics data with the two lignite coal, YM lignite was selected to do the kinetics analysis in different oxygen concentration atmospheres. According to the recommendations for performing kinetic computations on thermal analysis data from the ICTAC Kinetics Committee [9], three different heating rates were applied in each atmospheres (10 K/min, 30 K/min, 50 K/min). Furthermore, in order to demonstrate that there was no sample mass dependence, three runs were performed on samples of three markedly different masses (5, 10 and 15 mg) in 100% concentration N2 atmosphere. The three TG curves were identical within the experimental error.

3. Results and discussion 3.1. Effects of O2 /CO2 atmosphere compared to O2 /N2 using TG analysis Molina and Shaddix [10] conducted ignition experiments in a laminar flow reactor using pulverized, highly volatile bituminous coal. Their data showed that the exchange of CO2 for N2 did not significantly affect the devolatilization process. Kiga and colleagues [11,12] found that the high heat capacity of CO2 contributed to the delayed flame ignition in oxy-fuel combustion. Rathnam et al. [13] measured the reactivity of four pulverized Australian coals under simulated air and oxy-fuel atmospheres with a drop tube furnace. Their results showed that all kinds of coal had higher apparent volatile yields in CO2 than in N2 . However, burnout characteristics in the two atmospheres were different based on the coal types used. To explore and verify the effect of CO2 on coal combustion, thermo-gravimetric experiments were performed in a 21% O2 /79% CO2 environment and compared with experiments performed in a 21% O2 /79% N2 environment with YM and ZD coal. The TG and DTG curves are shown in Figs. 1 and 2. It can be seen from the curves that there are obvious differences between the two combustion processes in the two different atmospheres and for the two different lignites. The DTG curves of both the YM coal and ZD coal in an O2 /N2 atmosphere are higher and sharper when compared with an O2 /CO2 atmosphere. The maximum mass-loss rates of both the two lignites in an O2 /N2 atmosphere are obviously higher than that in an O2 /CO2 atmosphere. These phenomena indicate that because the heat capacity and transport property of the two atmospheres are different, the combusiton behavior in 21% O2 /CO2 changed compared with 21% O2 /N2 atmosphere. As a result, the ignition and burnout temperature in an O2 /CO2 atmosphere are higher than that in an O2 /N2 atmosphere. When the experiment atmosphere changed from 21% O2 /N2 to 21% O2 /CO2 , the maximum mass-loss temperature of YM lignite increased 8.1 K and the maximum mass-loss rates decrease 0.042%/s. On the other hand, the maximum mass-loss temperature of ZD only increased 3.9 K and the maximum mass-loss rates decrease 0.027%/s. Because the highly volatile YM coal sample will generate more pores and be filled with the ambient gas during the

Table 1 Proximate and ultimate analysis of the tested coal samples. Sample

YM ZD

Proximate analysis

Qbad (J/g)

Mad %

Aad %

Vad %

FCad %

13.88 23.45

11.01 2.94

31.95 24

43.16 49.61

22,319 21,572

Elementary analysis Cad %

Had %

Nad %

Stad %

Oad %

58.28 57.91

4.69 2.73

1.1 1.04

0.18 0.32

10.86 11.61

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Z. Zhou et al. / Thermochimica Acta 553 (2013) 54–59

Fig. 1. TG and DTG curves of YM coal combustion in two different atmospheres.

devolatilization process, the influence of CO2 is more effective on the combustion process of highly volatile YM coal than that of ZD coal.

Fig. 3. TG curves of YM coal combustion in different O2 concentrations.

3.2. Effects of O2 concentration on combustion characteristics in O2 /CO2 atmosphere First, all TG and DTG curves of the two coal types in different O2 concentration atmospheres are shown in Figs. 3–6. According to the TG and DTG curves shown above, it is clear that the coal combustion process is strongly affected by the O2 concentration. Lower O2 concentrations lead to more obvious effects on the TG curves. When the O2 concentration increased, the TG and DTG curves were obviously shifted to the lower temperature zone. The mass loss rate of the coal sample in high O2 concentrations was always higher than that in low O2 concentration. So O2 concentration increasing can significantly improve the combustion process of lignite. However, when O2 concentrations are above 60%, the shape of the TG curves is different from the regular TG curve. There is an inflexion point on the TG curve where the sample temperature slightly decreases. This phenomenon may be caused by the intense burning of lignite in high O2 concentration atmosphere. As a result, the sample’s temperature is provisionally above the heating program’s temperature. After a short time, the sample’s temperature

Fig. 2. TG and DTG curves of ZD coal combustion in two different atmospheres.

Fig. 4. DTG curves of YM coal combustion in different O2 concentrations.

Fig. 5. TG curves of ZD coal combustion in different O2 concentrations.

Z. Zhou et al. / Thermochimica Acta 553 (2013) 54–59

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Fig. 7. Ignition temperature (Ti ) defined by the TG–DTG tangent method. Fig. 6. DTG curves of ZD coal combustion in different O2 concentrations.

returns to the program’s temperature and the heating process continues. Hence, the combustion process under high O2 concentration should only be used to judge the trend of the lignite combustion changing, and cannot be used as the data for kinetics analysis. To evaluate the combustibility of the tested coal samples from these figures, characteristic parameters were obtained, including the ignition temperature (Ti ); the burnout temperature (Tb ); the maximum mass loss temperature (Tmax ); and the maximum mass loss rate. In this paper, the ignition temperature (Ti ) was defined by the TG–DTG tangent method as shown in Fig. 7 [14]. First, vertical line 1 was made through DTG peak point A. Line 1 meets the TG curves at point B. We then made a tangent line 2 to the TG curve at point B. Line 2 intersect the extended TG initial level line 3 at point C. Third, we made another vertical line 4 downwards through point C, which met the TG curve at point D. The ignition temperature was defined as the corresponding temperature at point D. The burnout temperature (Tb ) was identified as the corresponding temperature of no mass loss in TG curves [14]. The maximum mass loss temperature (Tmax ) and the maximum mass loss rate were the corresponding temperature and mass loss rate of the DTG peak point. All of the combustion characteristic parameters mentioned above were summarized in Table 2. From the combustion characteristic parameters listed in Table 2, it can be seen that combustion performance can be significantly improved by increasing O2 concentration. The tendency with the influence of O2 concentration increasing on the ZD lignite and YM lignite are nearly the same except the burnout temperature. The burnout temperature decreased more quickly for the YM lignite with higher O2 concentration. A higher O2 concentration depresses all Ti , Tb and Tmax , especially the burnout temperature (Tb ) and

the maximum mass loss temperature (Tmax ). It should be noted that when O2 concentration is over 60%, the Ti and Tmax enhanced level decrease. Meanwhile, the maximum mass loss rate increased in higher O2 concentration atmosphere. And the increased level was linear correlated to the O2 concentration increasing. This phenomenon may because the O2 concentration increasing is more effective on the volatile matter combustion process, and this influence is directly related to the maximum mass loss rate. However, the devolatilization temperature was not significantly decreased by increasing O2 concentration. Thus, the temperature range of the volatile matter combustion process could not be depressed in a higher O2 concentration atmosphere. As a result, the Ti and Tmax enhanced level decreased when the O2 concentration was over 60%. 3.3. Kinetics analysis of YM lignite The reaction of pulverized coal during thermo-gravimetric analysis is a heterogeneous solid-state reaction and can be described by the following equation [15,16]: d˛ = f (˛)k(T ) dt

(1)

where ˛ is the degree of reaction, t is time, k(T) is the temperaturedependent constant and f(˛) is a function called the reaction model that describes the dependence of the reaction rate on the reaction’s extent. The degree of reaction ˛ can be represented as: ˛=

M0 − MT M0 − Mf

(2)

where M0 is the beginning mass of the sample, MT is the mass of the sample at temperature T and Mf is the final mass of the sample.

Table 2 Combustion characteristic parameters in different O2 concentration atmospheres. Coal

YM

ZD

Characteristic parameters

21% O2 /CO2

40% O2 /CO2

60% O2 /CO2

80% O2 /CO2

100% O2 /CO2

Ti (◦ C) Tb (◦ C) Tmax (◦ C) Maximum mass loss rate (%/s) Ti (◦ C) Tb (◦ C) Tmax (◦ C) Maximum mass loss rate (%/s)

374.8 557.8 437 −0.217 407 618.6 509.9 −0.227

360.4 460.6 400.7 −0.425 394.4 559.5 454.4 −0.438

344.4 414.6 346.9 −0.756 364.6 540.1 390.8 −0.738

320.8 391.3 341.4 −1.042 360.5 526.3 387.3 −1.016

321.1 364.8 318.8 −1.246 354.3 509.7 379.4 −1.387

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According to the Arrhenius equation, the rate of a solid-state reaction can be described as follows:



d˛ E = A exp − RT dt



f (˛)

(3)

where A is the pre-exponential Arrhenius factor, E is the activation energy, T is the temperature and R is the gas constant. When the heating rate ˇ remains constant, the above rate expression can be transformed into the following equation with a constant ˇ inserted:



d˛ 1 E = A exp − RT dT ˇ

˛

g(˛) ≡ 0

d˛ =A f (˛)

f (˛)

T exp



t

exp 0

 −E  RT

 −E  RT

dt

(5)

dT

(6)

0

Because Eq. (6) does not have an analytical solution, many approximations of the temperatures integral in Eq. (6) have been developed. The Kissinger–Akahira–Sunose method is one of the integral isoconversional methods and gives rise to the equation:



ln

ˇi



2 T˛,i

= const −

E˛ RT˛

(7)

2 ) The value of E˛ is determined from the slope of the plot ln(ˇ/T˛,i vs. 1/T˛ . Moreover, the kinetic parameters were only evaluated in the combustion temperature sections from the ignition temperature to the burnout temperature in order to avoid the interference that may cause by the drying process. The slopes corresponding to linear fittings, the corresponding correlation coefficients (R2 ) and the activation energy calculated by the Kissinger–Akahira–Sunose method are shown in Table 3. According to the recommendations for performing kinetic computations on thermal analysis data from the ICTAC Kinetics Committee [9], the concurrent use of two or more integral isoconversional methods similar to the Kissinger–Akahira–Sunose method should be avoid. Thus, to get contrasted results, the Friedman differential isoconversional method was used to calculate the activation energy of the combustion process in O2 /N2 and O2 /CO2 atmosphere for YM lignite coal [9]. The method is based on the Eq. (8):

ln

 d˛  dt

˛,i

O2 /N2

(4)

Integration of Eq. (3) gives rise to Eq. (5), where g(˛) is the integral form of the reaction model. Then for constant heating rate conditions, the integral with respect to time is replaced with the integral with respect to temperature. This replacement of Eq. (5) leads to: A g(˛) = ˇ

Atmosphere



Kinetics analysis is used to produce an adequate kinetic description of a process in terms of the ‘kinetic triplet’ (f(˛), E and A). Many methods exist for kinetics analysis. According to the recommendations for performing kinetic computations on thermal analysis data from the ICTAC Kinetics Committee [9], the methods that use multiple heating rate programs are recommended for computation of reliable kinetic parameters, and methods that use a single heating rate program should be avoided. In this paper, the Kissinger–Akahira–Sunose method [9,17], as an integral isoconversional method, was selected to determine the activation energy of YM lignite in different O2 concentration atmospheres. Integral isoconversional methods originate from the application of the isoconversional principle to Eq. (5):



Table 3 Slopes and correlation coefficients (R2 ) corresponding to linear fittings together with the resultant activation energy values using Kissinger–Akahira–Sunose method.

= ln[f (˛)A˛ ] −

E˛ RT˛,i

(8)

O2 /CO2

Conversion (%)

Slope

R2

E (kJ/mol)

20 30 40 50 60 70 80 20 30 40 50 60 70 80

−23.426 −15.866 −11.907 −9.276 −7.566 −6.256 −5.260 −16.570 −12.174 −8.996 −7.226 −5.891 −4.889 −4.048

0.9565 0.9794 0.9817 0.9790 0.9769 0.9764 0.9763 0.9913 0.9793 0.9865 0.9847 0.9837 0.9846 0.9842

194.8 131.9 98.9 77.1 62.9 52.0 43.7 137.8 101.2 74.8 60.1 48.9 40.6 33.7

For linear nonisothermal programs, Eq. (8) is usually used in the following form:

    d˛

ln ˇi

dT

˛,i

= ln[f (˛)A˛ ] −

E˛ RT˛,i

(9)

The activation energy is determined from the slope of a plot of ln[ˇi (d˛/dT )˛,i ] against 1/T˛,i . The slopes corresponding to linear fittings, the corresponding correlation coefficients, together with the activation energy obtained by the Friedman method, are shown in Table 4. According to Tables 3 and 4, there are significant variations of activation energy with conversion percentages. The phenomenon indicates that both the combustion process in O2 /CO2 and O2 /N2 atmospheres are kinetically complex, and should be considered as multi-step processes. Furthermore, the activation energy calculated by the differential isoconversional method is higher than the result obtained by the integral isoconversional method. And the activation energy corresponding to the YM lignite combustion in O2 /N2 atmosphere is always higher than combustion in O2 /CO2 atmosphere with both calculation methods. According to the reaction kinetic theory, higher activation energy means it is more difficult to react. And the thermogravimetric data showed that the combustion process of YM lignite was more intense in O2 /N2 atmosphere than in O2 /CO2 atmosphere with the same O2 concentration. As a result, the activation energy of YM lignite combustion in O2 /N2 atmosphere should be higher. But the calculation results by both the integral and differential isoconversional methods were opposite of the reaction kinetic theory. This condition was also found in other papers with isoconversional methods calculations [15,18]. Hence, further research is required

Table 4 Slopes and correlation coefficients (R2 ) corresponding to linear fittings together with the resultant activation energy values using Friedman method. Atmosphere

O2 /N2

O2 /CO2

Conversion (%)

Slope

R2

E (kJ/mol)

20 30 40 50 60 70 80 20 30 40 50 60 70 80

−41.237 −22.399 −16.425 −12.631 −10.669 −9.222 −9.425 −31.756 −15.583 −11.850 −9.471 −8.033 −7.154 −7.431

0.9643 0.9795 0.9806 0.9781 0.9790 0.9757 0.9684 0.9787 0.9901 0.9909 0.9887 0.9898 0.9879 0.9788

342.8 186.2 136.6 105.0 88.7 76.7 78.4 264.0 129.6 98.5 78.74 66.8 59.5 61.8

Z. Zhou et al. / Thermochimica Acta 553 (2013) 54–59

whether the activation energy of complex reaction process calculation with isoconversional methods can be used as a basis for judging the degree of the reaction difficulty. 4. Conclusions In this paper, two typical Chinese lignite coal and five different O2 concentrations ranging from 21% to 100% were chosen to perform thermo-gravimetric analysis. With the experimental data, the combustion characteristics and kinetic parameters were studied, and the following conclusions were obtained: (a) CO2 did have a delay influence on the combustion process, especially the maximum mass-loss rate and the burnout temperature. Furthermore, the influence of CO2 is more effective on the combustion process of highly volatile YM coal than that of ZD coal. (b) Increasing the O2 concentration can improve combustion performance. Moreover, when O2 concentrations are above 60%, the shape of the TG and DTG curves of lignite is abnormal, and cannot be used as the data for kinetics analysis. According to the coal reactivity index analysis, the ignition temperature of YM lignite decreases 14.4 K and the burnout temperature is 97.2 K lower when the O2 concentration increasing from 21% to 40%. The maximum mass-loss temperature also decreases 26.3 K. (c) Kinetic analysis with two isoconversional methods showed that both the combustion process in O2 /CO2 and O2 /N2 atmospheres should be considered as multi-step processes. Because of the activation energy calculation results of YM lignite combustion in O2 /N2 atmosphere was higher than in O2 /CO2 atmosphere, and it is inconsistent with the reaction kinetic theory, further research is required whether the activation energy calculation with isoconversional methods for complex reaction process can be used as a basis for judging the degree of the reaction difficulty. Acknowledgements The authors would like to acknowledge financial support from the National Basic Research Program of China (2012CB214906), and the Program of Introducing Talents of Discipline to University (B08026).

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