Thermogravimetric Analysis of Huadian Oil Shale Combustion at

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Thermogravimetric analysis of Huadian oil shale combustion at different oxygen concentrations Feng-tian Bai, Youhong Sun, and Yumin Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02888 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on April 27, 2016

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Energy & Fuels

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Thermogravimetric analysis of Huadian oil shale combustion at different oxygen concentrations

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Fengtian Bai, Youhong Sun*, Yumin Liu*

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College of Construction Engineering, Jilin University, Changchun 130021, PR China

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Keywords: oil shale; combustion; thermogravimetric analysis; oxygen concentration; kinetics

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Abstract: As the simplest conversion route, combustion is extensively applied to oil shale

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utilization. To improve oil shale conversion techniques, we used non-isothermal thermogravimetric

8

analysis to explore the combustion reactivity and kinetics of Huadian oil shale at various oxygen

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concentrations (10, 20, 30, 50, 65, and 80 vol.%) and heating rates (5 °C min−1, 10 °C min−1, and 20

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°C min−1). With an increase in oxygen concentration, the combustion performances of oil shale

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could be significantly improved; the volatile-releasing temperature, ignition temperature, and

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burnout temperature decreased; the mass loss rate increased; and the integrated combustion

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characteristics of oil shale enhanced. These improvements were attenuated when the oxygen

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concentration exceeded 50 vol.%. When the oxygen concentration increased from 10 to 80 vol.%,

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the average activation energy in the second combustion stage increased from 46.85 kJ mol−1 to

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117.98 kJ mol−1 by the Kissinger–Akahira–Sunose method, from 46.85 kJ mol−1 to 117.98 kJ mol−1

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by the Starink method, from 59.08 kJ mol−1 to 129.17 kJ mol−1 by the Friedman method, and from

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36.34 kJ mol−1 to 57.58 kJ mol−1 by the Coats–Redfern method at a heating rate of 20 °C min−1.

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Results indicated that oxygen enrichments beyond which additional enrichment yields significantly

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less enhancement to the combustion process.

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ACS Paragon Plus Environment

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Nomenclature Abbreviation

(dw/dt)max

TG

thermogravimetric

Tmax

DTG

derivative thermogravimetric

Symbols A E

pre-exponent factor, s-1 activation energy, kJ mol-1

Ti Tb △T1/2 r R

f(α)

general expression of kinetic model function

R5

G(α)

integral form of f(α)

R50

T t

n R2

β

absolute temperature, K the time of conversion, s the extent of conversion, α = (m0-m)/(m0-m∞), m = sample mass at temperature T , m0= values of initial weight, m∞ = values of total weight heating rate, °C min-1, β = dT / dt

k

rate constant, k = A exp( − E / RT )

α

Rc dα/dt

RD D-R5 D-R50

-1

-1

universal gas constant, 8.314 J mol K conversion rate of reaction, % min–1

D-r RE

maximum mass loss rate, % min–1 maximum mass loss rate temperature, °C ignition temperature, °C burnout temperature, °C temperature range of half-peak, °C product release index combustion reactivity index combustion reactivities at both 5% conversion fraction combustion reactivities at both 50% conversion fraction reaction order coefficient of correlation differential curve of the fitting curve of mass loss rate differential curve of R5’s fitting curve differential curve of R50’s fitting curve differential curve of r’s fitting curve differential curve of E’s fitting curve

21 22

1. Introduction

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An unconventional low-calorific-value fossil fuel with combustible organic materials, oil shale can

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be converted into useful forms of energy mainly by pyrolysis or combustion, and approximately

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69% of the global oil shale production is used to generate electricity and heat by combustion,1–4

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particularly in Estonia.2–6 Accordingly, numerous studies have examined the characteristics,

27

properties, mechanisms, and modeling of oil shale combustion, particularly by thermogravimetric

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(TG)/differential thermogravimetric (DTG) analysis.7–17

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Kök and Iscan7 reported two distinct exothermic peaks demonstrating low-temperature oxidation

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and high-temperature oxidation during the combustion of three Turkish oil shale samples, whose

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activation energies vary from 13.1 kJ mol−1 to 408.4 kJ mol−1, as determined by differential

32

methods. Kök and Senguler8 studied the fundamental geochemical and combustion characteristics

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of new oil shale in the Eskisehir region in Turkey and determined that the activation energies varied 2


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with the methods. Han et al.9,10 presented important combustion information and established a

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mechanism model for Huadian oil shale. Sun et al.11 analyzed the effects of various parameters

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(e.g., particle size and heating rate) on the combustion process and introduced a multistage parallel

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reaction model and bi-Gaussian distribution function to analyze the combustion kinetics of Huadian

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oil shale. They found that coarser particles were more difficult to combust than finer particles, and

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Huadian oil shale combustion was controlled by multiple reaction mechanisms. Kaljuvee et al.12

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and Bai et al.13 indicated that the differences in the organic and mineral compositions of oil shale

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have considerable effects on its combustion behavior and kinetic characteristics. However, most of

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these studies were conducted in a normal air atmosphere, that is, high oxygen concentration during

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the combustion process was not considered.

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In a real combustion process, ambient oxygen concentration varies with time, resulting in

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inadequate air supply to diffuse into the solids and consequently affecting the combustion reactions

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of solids. The mechanisms and modeling of combustion may be affected by oxygen concentration.

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Therefore, the combustion kinetics of solid fuels at various oxygen concentrations are of interest in

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fuel research.18 In addition, with the current development of low-cost oxygen-production

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technology, high-oxygen-concentration combustion technology is gradually developing as its

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application range continuously expands.19 Many TG studies have been conducted to explore the

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combustion behaviors of solid fuels in atmospheres with various oxygen concentrations.18–23 Loo et

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al.14 found that an increased oxygen ratio during combustion increased the overall combustion rate

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of Estonian oil shale, but they did not investigate the combustion kinetics of this oil shale at high

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oxygen concentrations. To date, Huadian oil shale combustion in the presence of high oxygen

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concentrations has not yet been thoroughly explored. The literature concerning the combustion

3


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kinetics of oil shale in high-oxygen-concentration atmosphere is still insufficient. A new and

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in-depth investigation of the complex combustion process of Huadian oil shale in a

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high-oxygen-concentration atmosphere is required.

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In the current paper, TG analysis was used to investigate the combustion characteristics and kinetics

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of oil shale at six oxygen concentrations and three heating rates. The combustion reactivity index

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and product release index were introduced to analyze the complexity of the combustion reaction.

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Coats–Redfern, Kissinger–Akahira–Sunose (KAS), Starink, and Friedman methods were utilized to

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establish the kinetic parameters.

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2. Experimental

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2.1 Material

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The oil shale used in this study was from Huadian in Northeastern China. Raw oil shale samples

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were obtained, crushed, and sieved to a grain size of 0–0.088 mm and dried at 45 °C–50 °C to a

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constant mass according to the ASTM D2013-07 (USA) and GB 474-2008 (China) standards. The

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results of the proximate, elemental, and Fischer assay analyses of the samples are summarized in

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Table 1.

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Table 1. Physical properties of Huadian oil shale Proximate analysis (wt.%, ad) Ultimate analysis (wt.%, ad) Volatiles 39.34 C 29.23 Fixed carbon 3.75 H 4.28 Ash 56.91 N 0.61 Moisture (as received) 3.26 S 4.92 Calorific value (MJ kg−1) 13.07

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Note: ad = air-dried

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2.2 TG analysis

4


Fischer assay analysis (wt.%, ad) Shale oil 19.69 Gas 6.38 Water 4.98 Residue 68.95

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TG analysis was performed using a Netzsch STA 449C thermal analyzer system (Germany) at

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heating rates of 5 °C min−1, 10 °C min−1, and 20 °C min−1 with temperatures ranging from ambient

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temperature to 850 °C. The combustion experiments were performed in 10% O2/90% N2, 20%

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O2/80% N2, 30% O2/70% N2, 50% O2/50% N2, 65% O2/35% N2, and 80% O2/20% N2 atmospheres

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with a flow rate of 60 mL min−1. Small samples (approximately 5 mg) were used in each

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experiment to eliminate the effects caused by mass- and heat-transfer limitations. Two repetitive TG

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curves were obtained to assure the reproducibility of the results. Prior to the experimental work, the

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instrument was subjected to mass scale calibration, following the procedure described by the ASTM

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E 2040-08 standard, and temperature calibration based on the Curie point measurements of three

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standards, namely, alumel, nickel, and iron. Mass loss and temperature errors were less than ± 0.5

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wt.% and ±1 °C, respectively.

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2.3 Characteristic parameters and combustion reactivity

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Ignition temperature (Ti), maximum mass loss rate temperature (Tmax), and burnout temperature (Tb)

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were used to describe the thermal behavior of oil shale during combustion.

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The combustion reactivity index, R, is defined as24

R=−

1 dm 1 dα = m − m∞ dt 1− α dt

(1)

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where α = (m0-m)/(m0-m∞) is the conversion fraction obtained from the TG/DTG curves and m0,

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m∞, and m represent the initial, final, and instantaneous mass of the sample, respectively. The

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combustion reactivity is dependent on the solid-fuel conversion. In this study, the combustion

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reactivities at both 5% conversion fraction, R5, and 50% conversion fraction, R50, were selected for

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the evaluation of the combustion performance of oil shale. 5


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The product release index, r25, was used to reflect the intensity of the product release during

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combustion and is defined as

r=

(dw/ dt)max Tmax ⋅ Ti ⋅ ∆T1/ 2

(2)

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where (dw / dt ) max is the maximum mass loss rate (% min−1) and △T1/2 is the temperature range of

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half-peak (°C). Generally, the products were intensively and intensely released when the product

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release index r was high.

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2.4 Kinetic method

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In solid-fuel kinetic analysis, the rate of the kinetic process is usually expressed as dα = k f (α ) dt dα / dt

or

dα A E = exp( − ) f (α ) dT β RcT

(3)

is the conversion rate of the reaction, k is the rate constant, f (α ) is the kinetic model

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where

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function, A is the pre-exponent factor (s−1), and E is the activation energy (kJ mol−1), β is the

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heating rate (°C min−1), and Rc is the universal gas constant (8.314 J mol−1 K−1).

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Coats–Redfern, KAS, Starink, and Friedman methods were used to obtain the activation energies of

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the combustion reactions. The Coats–Redfern equation26 is given as   1 − (1 − α)1− n  AR E 1 − ⋅ , n ≠ 1 ln −  = ln 2 βE R T   T (1 − n )  c   ln − ln(1 − α)  = ln AR − E ⋅ 1 , n = 1    βE Rc T T2   

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Plotting the left-hand side of Eq. (4) against 1 / T yields a straight line with a slope

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intercept ln AR . The values of E and A can be derived using the TG data.

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The KAS equation27 is given as

βE

6


(4)

−E / R

and an

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ln(

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 AR  β E  − ) = ln  RcT T2  EG (α ) 

(5)

The Starink equation28 is as follows: E β ) = Const − 1.0008 R cT T 1.92

ln( 110

(6)

The Friedman equation29 is defined as

ln( β

E dα ) = ln[Af (α )] − RT dT

(7)

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The activation energies for different conversion values can be determined by the slope of the

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regression lines of ln(β / T 2 ) vs. 1 T for the KAS method,

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method, and ln( β

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3. Results and discussion

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3.1 TG study

ln( β T 1.92 )

vs. 1 T for the Starink

dα ) vs. 1 T for the Friedman method. dT

116 117

Figure 1. TG and DTG curves of Huadian oil shale combustion at different oxygen and nitrogen

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concentrations.

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Figure 1 indicates the effect of oxygen concentration on the combustion behavior of oil shale at a

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heating rate of 20 °C min−1. Figure 2 depicts the TG and DTG curves of oil shale combustion at

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heating rates of 5 °C min−1, 10 °C min−1, and 20 °C min−1 and an oxygen concentration of 10 vol.%.

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Notably, three individual stages are distinguishable in the combustion process of oil shale. The first

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stage (< 200 °C) corresponds to moisture loss. The second stage, from 200 °C to 493 °C–685 °C

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(depending on the oxygen concentration and heating rate), is characterized by a major mass loss and

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principally corresponds to the release and combustion of organic components, such as volatile

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matter and fixed carbon. As the temperature increases to ~250 °C, a certain amount of dissolved

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bitumen in oil shale and light oil and gas from the asphalt pyrolyzed by the volatiles of kerogen are

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first combusted; this mechanism corresponds to the DTG curve’s first peak, which is usually called

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the low-temperature oxidation peak.8,30–32 The second obvious peak (high-temperature oxidation

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peak) in the DTG curve corresponds to the combustion of macromolecular non-volatiles in kerogen

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and the restructuring of certain compounds.11 The combustion of the most difficult organic matter,

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particularly fixed carbon, occurs at the end of the second stage.32 The third stage is from the end of

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the second stage to 850 °C; it corresponds to inorganic matter decomposition. The mass loss and

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mass loss rate in the second stage are notably higher than those in the other two stages. Therefore,

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the second combustion stage forms the core of the discussion in this paper.

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Figure 2. TG and DTG curves of Huadian oil shale combustion at different heating rates in 10%

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O2/90% N2 atmosphere.

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3.2 Effect of oxygen concentration on the combustion process of oil shale

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Similar patterns can be observed in the TG and DTG curves of the first stage at different oxygen

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concentrations (Figure 1), indicating that water evaporation rate is mainly related to temperature but

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uninfluenced by oxygen concentration.

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The oxygen concentrations in the studied range significantly influence the second combustion stage.

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An increase in oxygen concentration allows the combustion of organic matter to shift to a

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low-temperature region over a narrow temperature range, particularly below 50 vol.% oxygen

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concentration (Figure 1). The combustion reaction proceeds for a short time in a

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high-oxygen-concentration atmosphere. The DTG curves in Figure 1 show that the demarcation

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point between 250 °C and 450 °C for the two peaks is more evident at high oxygen concentrations

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(e.g., 50, 65, and 80 vol.%) than at low oxygen concentrations. This result can be attributed to the

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enhancement of the volatile compound burning rate at high oxygen concentrations. However, a fast 9


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combustion reaction is insufficient to ignite the residual volatile compounds and fixed carbon at 350

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°C–370 °C, resulting in the reaction to proceed for a long time.

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Table 2 shows the characteristic parameters representing the second combustion stage of oil shale at

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various oxygen concentrations. Notably, the characteristic temperatures obtained from the TG/DTG

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curves decrease with an increase in oxygen concentration, particularly for Tb and Tmax. This result

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indicates that increasing oxygen concentration can intensify the combustion reaction of Huadian oil

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shale.

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Table 2. Characteristic parameters of oil shale in the second combustion stage at different oxygen

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concentrations Atmosphere

β, °C min–1

(dw/dt)max, % min–1

Tmax, °C

Ti, °C

Tb, °C

△T1/2, °C

R5

R50

r(×10−7)

10% O2/90%N2

5 10

−1.55 −2.15

415.0 454.1

275.7 283.4

474.7 518.2

138.0 187.4

0.001263 0.001295

0.01242 0.00972

0.98167 0.89149

20 20

−3.27 −3.93

467.4 459.8

306.9 296.8

650.8 540.6

310.5 214.4

0.000799 0.000968

0.00548 0.00888

0.73418 1.34318

20 20 20 20

−4.95 −6.05 −6.27 −6.49

436.9 421.2 418.5 417.6

284.7 277.9 273.2 269.0

501.7 477.2 471.7 459.0

178.4 162.0 152.9 154.6

0.000928 0.001189 0.001547 0.001642

0.00980 0.01066 0.00976 0.00928

2.23070 3.19053 3.58661 3.73700

20% O2/80%N2 30% O2/70%N2 50% O2/50%N2 65% O2/35%N2 80% O2/20%N2 160

161

10


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Figure 3. Fitting curves and differential curves of the (a) maximum mass loss rate and (b) average

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mass loss rate in the second combustion stage of the Huadian oil shale at different oxygen

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concentrations.

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The maximum mass loss rates are 3.27, 3.93, 4.96, 6.06, 6.27, and 6.49 % min−1 at oxygen

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concentrations of 10, 20, 30, 50, 65, and 80 vol.%, respectively. A similar increasing trend can also

167

be observed in the calculated average mass loss rate of oil shale in the second combustion stage.

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This result reveals that the thermal decomposition of oil shale is accelerated by the increase of

169

oxygen concentration. Figure 3 illustrates the fitting curves and differential curves (RD) of the

170

maximum and average mass loss rates in the second combustion stage. The maximum and average

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mass loss rates gradually increase as the oxygen concentration increases; the models of these two

172

fitting curves are logistic. The fitting curve of the maximum mass loss rate is

173

y=6.58723−3.43828/(1+(x/29.29282)3.13446), and its corresponding R2 is 0.9984. The fitting curve of

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the average mass loss rate is y=18.40778−36.45968/(1+(x/0.58932)0.06394), and its corresponding R2

175

is 0.9646. This progressive increases in both the maximum and average mass loss rates result from

176

the fast ignition of the oil shale and the enhanced burning rate in the high-oxygen-concentration

177

atmosphere. Considerable heat is also released in a short time, accelerating the decomposition of

178

volatile compounds and resulting in a better combustion performance of oil shale. Moreover, the

179

heat-transfer process is favored in the high-oxygen-concentration environment, increasing the

180

driving force of the heat exchange. The amount of inert nitrogen contained in the flue gases is lower

181

in higher oxygen concentrations; therefore, the amount of heat loss through the furnace exhaust is

182

reduced, and higher efficiency is achieved.19,21 Figure 3 shows that the RD of the maximum mass

183

loss rate initially sharply increases to the maximum value at an oxygen concentration of 24 vol.%

184

and subsequently decreases, whereas the RD of the average mass loss rate constantly decreases with 11


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increased oxygen concentration. This result indicates that beyond an oxygen concentration of 24

186

vol.%, additional oxygen enrichment may yield less combustion benefits.

187

188

Figure 4. Fitting curves and differential curves of (a) R5, (b) R50, and (c) r in the second combustion

189

stage at different oxygen concentrations.

190

Figure 4 shows the fitting curves and differential curves of the combustion reactivity indices R5 and

191

R50 and the product release index r in the second stage at various oxygen concentrations. As shown

192

in Figure 4a, R5 increases as the oxygen concentration increases, and D−R5 initially significantly

193

increases to the maximum value at an oxygen concentration of 53 vol.% and subsequently

194

decreases. R50 (Figure 4b) initially sharply increases and subsequently decreases, reaching its

195

maximum value at an oxygen concentration of 42 vol.%. The product release index r (Figure 4c)

196

also increases as the oxygen concentration increases, indicating that the products are released

197

rapidly and intensively at high oxygen concentrations, and the oxygen concentration corresponding

198

to the maximum value of D−r is 24 vol.%. Similar combustion characteristics can also be observed

199

in other solid fuels.21,33,34 Therefore, an increase in oxygen concentration can significantly improve

200

the combustion performance of oil shale. However, the combustion benefits from further oxygen

12


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enrichment reduce sharply above the 50 vol.% concentration; therefore, the additional cost for this

202

enrichment may not be justified.

203

3.3 Effect of heating rate on the low-oxygen-concentration combustion process of oil shale

204

As shown in Figure 2 and Table 2, the characteristic temperatures in the second combustion stage

205

increase as the heating rate increases at an oxygen concentration of 10 vol.%; this mechanism is

206

similar to that in conventional combustion.11,13 However, r and R50 exhibit different variations. This

207

result is related to mass-transfer resistance and thermal hysteresis.11,13 An increase in the heating

208

rate strengthens the inertial effect of devolatilization for large temperature gradients between the

209

surface and center of the particles. This increase in heating rate also weakens the products’

210

out-diffusion effect, which is necessary for faster production speeds.11,13,35

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3.4 Analysis of kinetic parameters

13


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212 213

Figure 5. Combustion activation energies (E) of Huadian oil shale at different oxygen

214

concentrations (α) by KAS, Starink, and Friedman methods.

215

The activation energies calculated by the KAS and Starink methods are much closer to but lower

216

than that determined by the Friedman method, as shown in Figure 5 and Table S1. This deviation

217

may be ascribed to the different equation parameters, imprecision of numerical differentiation, or 14


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the assumptions on which the models are based.36 The activation energy values fluctuate

219

remarkably. The initial decrease in activation energy indicates the significant complexity of the

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second combustion stage and the difficulty of the reaction to proceed in the beginning period. After

221

breaking through the threshold energy, the product turns into a reactant and burns, and the reaction

222

subsequently proceeds more easily. The fluctuating activation energies, particularly at high oxygen

223

concentrations, are mainly related to the two DTG peaks;11 different components are combusted at

224

low-temperature oxidation and high-temperature oxidation.32 The more inert components are

225

combusted at high temperatures, degrading the mass transfer and combustion reactivity and

226

consequently increasing the activation energy. The transition between these two oxidation processes

227

is gentle at low oxygen concentrations; thus, the increases in the activation energies at α=0.4 are not

228

significant. When the conversion rate exceeds 0.9, a higher energy is required for the decomposition

229

of inorganic matter. Remarkably, similar values and change tendencies can be found in the

230

activation energies at oxygen concentrations above 50 vol.%.

231 232

Figure 6. Fitting curves and differential curves of the average activation energies in the second

233

combustion stage at different oxygen concentrations. 15


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Table 3. Kinetic parameters in the second combustion stage of Huadian oil shale at different oxygen

235

concentrations by the Coats–Redfern method 10% O2

20% O2

30% O2

50% O2

65% O2

80% O2

β

5

10

20

20

20

20

20

20

E

48.42

38.13

36.34

40.15

48.06

54.57

56.10

57.58

A

257.269

34.582

29.272

91.919

673.594

3859.7

6033.1

9376.4

n

1.1

0.8

1.2

0.9

1.2

1.5

1.6

1.7

0.9985

0.9992

0.9980

0.9981

0.9979

0.9944

0.9923

0.9881

R

2

236 237

The second-combustion-stage average activation energies obtained by the KAS, Starink, and

238

Friedman methods are also shown in Table S1, in which the values are notably larger than those

239

calculated by the Coats–Redfern method (Table 3). These differences can be explained either with

240

the equations used or with the assumptions considered.8 Figure 6 shows the fitting curves of the

241

average activation energies in the second combustion stage at various oxygen concentrations. The

242

average activation energy in the second combustion stage increases with an increase in oxygen

243

concentration; this result is consistent with the observations on other solid fuels.21,34,37–39 The

244

differential curves (RE) of the fitting curves of the average activation energies in Figure 6 each

245

initially increase up to 2.3967 (22.7 vol.%) and to 0.7424 (24.2 vol.%) for the KAS, Starink,

246

Friedman, and Coats–Redfern methods and subsequently sharply decrease, indicating that the

247

activation energy increases rapidly at oxygen concentrations between 10 vol.% and ~23.5 vol.% and

248

then the rate of increase decelerates, particularly above 50 vol.%. Oil shale combustion is generally

249

a complex phenomenon because numerous reactive processes simultaneously occur. Therefore,

250

different products are continuously generated, and they serve as new reactants. The activation

251

energy is affected by the activated molecule concentration decrease, diffusion limitation, and

252

organic impurities during the combustion process of the samples.18, 21, 37 The increase in the oxygen 16


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concentration can accelerate the oxidation of macromolecular non-volatiles in kerogen, enhance the

254

decomposition of inorganic matter, and promote the concentration and intensity of the reactions,14

255

thereby increasing the activation energy. The minimal difference among the activation energies at

256

oxygen concentrations between 50 and 80 vol.% indicates that similar combustion processes occur;

257

thus, the increased oxygen concentration may not cause an earlier decomposition of the

258

macromolecular and inorganic matter in a high-oxygen atmosphere. The higher the activation

259

energy is, the more difficult the reaction is. Therefore, simply increasing the oxygen concentration

260

is not necessarily favorable for the combustion of oil shale. More elaborate schemes should be

261

implemented to better predict the optimal oxygen concentration for oil shale combustion.

262

In addition, the kinetic parameters in the second combustion stage of oil shale decrease as the

263

heating rate increases (Table 3). This result is consistent with that obtained by Wang et al.40 and Cui

264

et al.41 and may be related to the complex physical structure and chemical composition of oil shale.

265

As the heating rate increases, the fast-rising temperature of oil shale particles leads to the rapid

266

combustion reaction and short reaction time; as a result, volatiles are released, the combustion

267

reactivity is enhanced, and the activation energy is reduced. However, Han et al.10 reported that the

268

combustion activation energy of Huadian oil shale increases as the heating rate increases. Sun et

269

al.11 and Kök42 indicated no clear relationship between activation energy and heating rate. These

270

discrepancies in the activation energy for oil shale are not surprising. The thermal and kinetic

271

characteristics of oil shale remarkably vary because of its complex composition and origin and the

272

adoption of different equipment, experimental parameters, and laboratory operations.7,11,13,42

273

4. Conclusion

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

274

In this paper, the combustion characteristics of Huadian oil shale at different oxygen concentrations

275

were studied by TG analysis. The experimental results indicated that the increase in oxygen

276

concentration could significantly improve the combustion performance of Huadian oil shale,

277

decrease the burning time by oxygen enrichment, and improve the combustion reactivity of oil

278

shale. With increasing oxygen concentration, the volatile-releasing temperature, ignition

279

temperature, and burnout temperature decreased; the mass loss rate increased; and the integrated

280

combustion characteristics of oil shale were enhanced. This improvement was weakened when the

281

oxygen concentration exceeded 50 vol.%. The average activation energies of the oil shale in the

282

second combustion stage increased as the oxygen concentration increased. These results indicate

283

that, at certain oxygen concentration levels, additional oxygen enrichment contributes less to the

284

combustion process. However, more elaborate schemes should be implemented to better predict the

285

optimal oxygen concentration for oil shale combustion.

286

At an oxygen concentration of 10 vol.%, the ignition temperature, burnout temperature, and

287

maximum mass loss rate increased as the heating rate increased, whereas the combustion reactivity

288

indices and product release index decreased. In addition, the activation energy calculated by the

289

Coats–Redfern method decreased as the heating rate increased.

290

Corresponding Author

291

*Tel./Fax: +86 431 88502066. E-mail address: [email protected]; [email protected]

292

Notes

293

The authors declare no competing financial interest.

294

Acknowledgments 18


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This work was supported by the National Cooperative Innovation Project on Chinese Potential Oil

296

and Gas Resources (Grant No. OSR-06), the Strategic Emerging Industry Development Projects of

297

Jilin Province, China (Grant No. 2013Z050), the Science and Technology Project of the Department

298

of Jilin Province, China (Grant No. 20130302030SF), the Science and Technology Development

299

Project of Jilin Province, China (Grant No. 20150520073JH).

300

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