Catalytic Mechanism of Inherent Potassium on the CharâNO Reaction

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Catalytic mechanism of inherent potassium on char–NO reaction XingYuan Wu, Qiang Song, HaiBo Zhao, and Qiang Yao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01550 • Publication Date (Web): 19 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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Energy & Fuels

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Catalytic mechanism of inherent potassium on

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char–NO reaction

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Xingyuan Wu, Qiang Song*, Haibo Zhao, Qiang Yao

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Key Laboratory of Thermal Science and Power Engineering of Ministry of Education,

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Department of Thermal Engineering, Tsinghua University, 100084 Beijing, China

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ABSTRACT:

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NO can be reduced by biomass char, which is catalyzed by the inherent potassium in

8

char. The isothermal reactions of char–NO and temperature-programmed desorption

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of reacted chars were studied in a fixed bed reactor with original biomass char and

10

acid-washed biomass char as samples. Experimental results showed that significant

11

formation of C(O) occurred during the isothermal reactions, and more C(O) was

12

formed on original biomass char than on acid-washed biomass char. Formation of

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C(N) on both chars was negligible, and even inherent C(N) was consumed. The main

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product of char desorption was CO, with a small amount of CO2. Both the thermal

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desorption spectra of two chars showed two peaks during CO production and one

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peak during CO2 production with the increase in temperature. But the production of

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CO or CO2 in original biomass char was much higher than that in acid-washed

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biomass char. A catalytic mechanism of inherent potassium on char–NO reaction was

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proposed, with non-dissociative chemisorption of NO on K catalytic active sites being

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the rate-controlling step.

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1. INTRODUCTION

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Thermal utilization of biomass as a renewable energy has drawn much attention.

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Biomass is considered a potential reburning fuel because of its high volatile matter

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and catalytic mineral (such as K) content. NO in flue gas can be reduced by about 50%

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to 66% through biomass reburning.1

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NO reduction by biomass reburning includes NO reduction by volatiles and NO

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reduction by char. Both of them have important contribution to NO reduction during

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biomass reburning.2, 3 Biomass char is reactive because of its large specific surface

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area and active mineral content. Lu et al.3 found that NO reduction by biomass char is

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about 59% to 68% in a drop tube furnace. Experimental results of Zhong et al.4

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showed that NO reduction by biomass char is about 40% at 1523 K. NO reduction by

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char not only depends on the reburning conditions, such as residence time and excess

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air ratio,3 but also on char properties, such as specific surface area and carbon

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structure.5, 6

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Yamashita et al.7 studied the char surface chemistry during NO reduction by char,

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and found that NO chemisorbed on the char surface formed C(O) and N2. C(O) is an

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important intermediate that can directly react with NO or desorb and form a new

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active carbon site. Suzuki et al.8 found that C(N) was formed during the char–NO

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reaction through element conservation at 600 °C–900 °C. Furthermore, Chambrion et

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al.9 showed that C(N) played an important role in the char–NO reaction, reacting with 2


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NO to form N2. Pevida et al.10, 11 deduced that NO reduction by synthetic coal chars at

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250 °C–750 °C was controlled by the desorption of C(O); at 750 °C–1000 °C,

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gasification by NO started to control the process, with NO first attacking on the char

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surface and forming C(N), which reacted with NO to produce N2.

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Previous studies found that adding mineral matters to char enhanced char

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reactivity12; the catalytic effect of mineral matter depends on mineral species,

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concentration, and form of chemical compound.13, 14 Zhong et al. showed that adding

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0.5% KOH or NaOH to acid-washed char enhanced char reactivity from 41% to 95%

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or 74%, respectively.4 The catalytic effect order of mineral matters was K > Fe > Ca.13,

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15

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increases with K concentration,16 and the catalytic effect of K2CO3 is better than that

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of KCl.17

Other studies showed that K is slightly better than Na.4 The catalytic effect of K

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K, Ca, and Mg are some of the common mineral matters in biomass, and K

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concentration is generally the highest among these minerals.18 A part of mineral

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matters is released to gas during devolatilization, and major mineral matters remain in

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char19, 20. These mineral matters will catalyze the char–NO reaction. Previous studies

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showed that reactivity of original biomass char was much higher than that of

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acid-washed biomass char.21, 22 Sorensen et al. showed that wheat straw char was 2.5

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times more reactive than acid-washed wheat straw char.22 Wu et al. found that the

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main mineral matter that catalyzes the rice straw char–NO reaction is K, and the

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transformation of K during this reaction significantly changed char reactivity23. 3


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As discussed above, both mineral matters added to char and inherent mineral

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matters in char catalyze the char–NO reaction. However, studies on the catalytic

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mechanism of mineral matters on the char–NO reaction are few. Bueno-Lopez et al.

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observed that K catalyst plays a major role in the chemisorption of NOx. The

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chemisorption process can be split into two steps:

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NO X + ∗ → ∗ − O +

1 N2 2

(1)

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C( ) + ∗ − O → ∗ + (CO) #

(2)

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where ∗ is the catalytic active site of K, (CO) # is the oxygen complex, and C( )

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is the nascent site of char. Chambrion et al.9, 24 revealed that the nitrogen complex

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C(N) was an important intermediate in the char–NO reaction; the reaction between

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C(N) and NO was the main route of N2 formation. Matsuoka et al.

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catalytic mechanism of Pt on the char–NO reaction through an isotopically labelled

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method. The process of chemisorption of NOx on Pt is as follows:

25

studied the

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NO+Pt( ) → Pt (N)+Pt(O)

(3)

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Pt (N)+C( ) → C(N) + Pt( )

(4)

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Step response experiment revealed that the main N2 formation path in the absence of

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O2 was nitrogen coupling on the Pt catalyst surface through reaction (5).

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Pt (N)+Pt (N) → N 2 + Pt( )

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The catalytic effect of Pt is different from those of common mineral matters in char;

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the catalytic mechanism of Pt cannot be directly applied on the biomass char–NO

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reaction. 4


(5)

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Previous studies showed that K is the most reactive among the non-expensive

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mineral matters added to catalyze char reaction; simultaneously, K also is the main

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mineral in biomass char. Research on the catalytic mechanism of K on the char–NO

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reaction is important for improving NO reduction by biomass char with the addition

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of K to char or presence of inherent K in char. In the present study, the isothermal

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reactions of char–NO and temperature-programmed desorption of reacted chars were

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developed in a fixed bed reactor with original biomass char and acid-washed biomass

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char as samples. The influence of K on the formation and desorption of C(N) and C(O)

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during the char–NO reaction was investigated by comparing the experimental results

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of both original and acid-washed rice straw chars. The catalytic mechanism of K on

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the char–NO reaction was proposed.

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2. EXPERIMENTAL

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2.1. Fixed bed reactor. The char–NO reaction was studied in a fixed bed reactor, as

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shown in Figure 1. The quartz reactor was fixed in a quartz tube heated using an

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electrical furnace. The diameters of the reactor and the quartz tube were 1.7 and

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2.7 cm, respectively. Biomass char (400 mg) was packed in the quartz reactor, the

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reactant gas was Ar + 1000 ppm NO at a gas flow rate of 2.16 NL/min. The purity of

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Ar is higher than 99.999%. After reacting with char, the gases were analyzed by

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Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, USA) and mass

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spectroscopy (MS, QC200, USA). Since the concentration of NO was very low, the 5


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heat release from the char-NO was small, its influence on the char sample bed

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temperature was negligible. The deviation of char sample bed temperature measured

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by a thermocouple during the reaction was smaller than 1 °C.

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Figure 1. Schematic of fixed bed reactor system

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2.2 Samples. Rice straw from Hunan Province, China, was used in the study. The

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particle size distribution of the rice straw was in the range of 98–125 µm.

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Acid-washed biomass was prepared by washing original rice straw with 1 mol/L HCl

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for 10 h and then with distilled water until no Cl− was found, followed by drying at

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105 °C for 10 h. Biomass char samples were prepared by pyrolysis at 1000 °C for

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15 min in a fixed bed reactor under Ar atmosphere, and the yield of char obtained

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from the raw rice straw was 31.4%. The ultimate analysis of two rice straw char

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samples is shown in Table 1. The mineral contents of two char samples are shown in 6


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Table 2. As shown in Table 2, the mineral concentration in acid-washed rice straw

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char was much lower than that in original rice straw char, nearly all the K in char was

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removed by acid washing.

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Table 1. Ultimate analysis of char sample (wt %, dry and ash-free basis) C

N

H

S

acid-washed rice straw char

58.78

1.54

0.74

0.10

original rice straw char

54.60

1.29

1.05

0.18

Table 2. Concentrations of main mineral matters in rice straw chars (mg/g) K

Ca

Mg

Na

Fe

acid-washed rice straw char

0.2

2.0

0.1

0.2

1.1

original rice straw char

39.9

16.2

5.2

2.0

1.8

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2.3. Methods. 2.3.1. Isothermal reaction. Char samples were first heat treated at

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1000 °C for 15 min under Ar to remove the oxygen complex on the char surface. The

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temperature was then decreased to 700 °C, and the reactant gas was switched to

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Ar + 1000 ppm NO. The reaction time was 30 min, so that C(N) and C(O) on the char

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surface were stable.

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2.3.2. Temperature-programmed desorption experiment. After isothermal reaction,

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the reactant gas was switched to Ar, and the temperature was raised to 1000 °C at a

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rate of 10 °C/min. When temperature reached 1000 °C, the temperature was fixed for

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10 min.

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During both experiments, the outlet gases were analyzed by FTIR and MS. CO, NO,

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NO2, N2O, HCN and NO were measured by FTIR; O2, CO2, N2 were measured by MS

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on line. The relative measurement error of FTIR was less than 2%, whereas that of 7


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MS was less than 5%.

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3. RESULTS AND DISCUSSION

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The isothermal reactions of char–NO and temperature-programmed desorption of

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reacted chars were developed with original biomass char and acid-washed biomass

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char as samples. The difference in experimental results of two char samples was

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caused by the difference in the properties of both chars. The major difference between

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the two chars was the mineral content. As discussed in our previous studies23, the

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main inherent mineral that catalyzes the rice straw char–NO reaction is K. Hence, the

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difference between the two char was attributed to K.

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3.1. Formation of the nitrogen and oxygen complex. The outlet gas of isothermal

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reaction was analyzed by FTIR and MS. The outlet gases containing oxygen were NO,

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CO, and CO2, and the outlet gases containing nitrogen were NO and N2; the

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concentrations of HCN, N2O, and NO2 in the outlet gas were lower than 2 ppm, thus

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they were negligible. The formation of the nitrogen and oxygen complexes on char

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was calculated by element conservation of N and O between the inlet and outlet gases.

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3.1.1. Formation of oxygen complex. The oxygen concentration in reactant gas

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absorbed by the char surface was computed by CNOin -CNOout . The oxygen

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concentration in reactant gas desorbed from the char surface was computed by

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CCO +2 ⋅ CCO2 . Thus, the net concentration change of oxygen in reactant gas was

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computed by CNOin -CNOout − (CCO +2 ⋅ CCO2 ) . Isothermal reaction results of both 8


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original and acid-washed rice straw chars are shown in Figure 2. As shown in Figure

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2(a), the amount of oxygen absorbed by the acid-washed rice straw char surface was

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much higher than that desorbed from the char surface at the initial stage, which

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indicated that the amount of oxygen absorbed by the char surface increased at the

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initial stage. When the reaction time reached 12 min, the amount of oxygen absorbed

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by the char surface was equal to that desorbed from char surface, indicating that the

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oxygen complex formed on the char surface was stable. The amount of oxygen

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enriched on the char surface during the reaction was approximately 0.20% of char

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mass. Thus, the formation of the oxygen complex was significant during the char–NO

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reaction. The results of experiments carried out over original rice straw char are

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shown in Figure 2(b). The amounts of oxygen absorbed by and desorbed from the

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char surface of original rice straw were much higher than those of acid-washed rice

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straw. The amount of oxygen absorbed by the char surface continuously decreased

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during the reaction, but the amount of oxygen desorbed from the char surface slowly

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increased. The amount of oxygen absorbed by the rice straw char surface was much

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higher than that desorbed from the char surface at the initial stage. When the reaction

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time reached 24 min, the amounts of oxygen absorbed by and desorbed from the char

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surface were equal. The amount of oxygen enriched on original rice straw char during

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the reaction was approximately 0.57% of char mass, which was much higher than that

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on acid-washed rice straw char. These results revealed that inherent K promoted the

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formation of the oxygen complex during the char–NO reaction. 9


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(a) Acid-washed rice straw char

176 177

(b) Original rice straw char

178 179

Figure 2. Concentration of reacted O in reactant gas as a function of time during char-NO reaction

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3.1.2. Formation of nitrogen complex. The nitrogen concentration in reactant gas

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absorbed by the char surface was computed by CNOin -CNOout , whereas the nitrogen

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concentration in reactant gas desorbed from the char surface was 2 ⋅ CN2 . Thus, the 10


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net concentration change of nitrogen in reactant gas was computed by

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CNOin -CNOout − 2 ⋅ CN2 . The isothermal reaction results of both original and acid-washed

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rice straw char are shown in Figure 3. As shown in Figure 3(a), both the nitrogen

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amounts absorbed by and desorbed from the char surface decreased during the

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reaction. The amount of nitrogen absorbed by the char surface was lower than that

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desorbed from the char surface from 3 min to 18 min; when the reaction time reached

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18 min, the amounts were equal. The results showed that the amount of nitrogen in

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char decreased to approximately 0.14% of the char mass during the reaction. The

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results of experiments carried over original rice straw char are shown in Figure 3(b).

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The amounts of nitrogen absorbed by and desorbed from the char surface of original

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rice straw were much higher than those of acid-washed rice straw. The amount of

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nitrogen absorbed by the original rice straw char surface was lower than that desorbed

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from the char surface from 3 min to 10 min; when the reaction time reached 10 min,

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the amounts were equal. The amount of nitrogen in original rice straw char also

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decreased during the reaction. Thus, the amount of nitrogen complex enriched during

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the char–NO reaction was negligible; the inherent nitrogen complex in char reacted

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with NO to form N2. This result was different from that of Chambrion et al.,9, 24 which

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showed that the nitrogen complex concentration increased during the char–NO

201

reaction. The difference could be attributed to the samples, as in the case of

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Chambrion et al., in which samples were free of nitrogen; however, the nitrogen

203

complex already exists in biomass char. Bueno-Lopez et al.26 also showed that the 11


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formation of the nitrogen complex is insignificant during the activated carbon–NO

205

reaction.

206 207


208 209


210 211

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Figure 3. Concentration of reacted N in reactant gas as a function of time during char-NO reaction

3.2. Desorption of nitrogen and oxygen complex. After isothermal reaction, the 12


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reactant gas was switched to Ar and the temperature-programmed desorption

214

experiments of both rice straw chars were developed. The outlet gas was analyzed by

215

FTIR and MS. The results showed that the outlet gases containing oxygen were CO

216

and CO2. Only a little N2 (≤4 ppm) was detected during the process, which was

217

consistent with the results during the rice straw char–NO reaction in which the

218

nitrogen complex was not enriched.

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The main oxygen-containing product was CO for both chars, with a small amount

220

of CO2. The changes in concentrations of CO and CO2 during the process are shown

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in Figure 4. For both chars, two peaks were detected in the temperature desorption

222

spectra of CO production with increasing temperature: the first peak was at 884 °C,

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and the second peak was at the temperature higher than the studied temperature range.

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When the temperature stabilized at 1000 °C, the concentration of CO continuously

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decreased. The peak temperature was almost the same for both chars, indicating that

226

the chemical form of the oxygen complex was the same for both chars. The CO

227

concentration in original rice straw char was much higher than that in acid-washed

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rice straw char, as indicated by the first peak for acid-washed rice straw char and

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original rice straw char at 82 and 194 ppm, respectively. The concentration of CO2

230

during this process was very low; only one peak existed with increasing temperature

231

for both chars. The comparison between original and acid-washed rice straw char

232

showed that K promoted the formation of the oxygen complex but did not change the

233

chemical form of the complex. 13


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234 235


236 237


238

Figure 4. Concentration of CO, CO2 as a function of time

239

3.3. Catalytic mechanism of K on the char–NO reaction. As a typical gas–solid

240

reaction, the first step of the catalytic mechanism of K on the char–NO reaction was

241

the chemisorption of NO at K active sites. The concentration of generated N2 as a

242

function of the NO concentration in reactant gas at 700 °C–900 °C is shown in Figure 14


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5. As shown in Figure 5, the N2 concentration in the outlet gas was almost linear to

244

the NO concentration in the inlet gas for both chars. These results indicated that

245

chemisorption of NO at K active sites could not directly form N2, as shown in reaction

246

(1); otherwise, N2 in the outlet gas should form from the second-order reaction of NO

247

in the inlet gas. And the reaction between two nitrogen complexes is insignificant, as

248

the experiment results showed that formation of nitrogen complex on char is

249

negligible. NO chemisorbed at K active sites should form a nitrogen complex that will

250

react with NO to form N2. Previous studies23 showed that the converted NO

251

concentration is almost linear with the acid-soluble K concentration in char, indicating

252

that chemisorption of NO at K active sites is non-dissociative. Hence, chemisorption

253

of NO at K active sites can be expressed as:

254

C-K+NO → C-K(ON)

(6)

255

where C-K is the K active site on the char surface. C-K(ON) then directly reacts

256

with NO as shown in reaction (7)

257

C-K(ON)+NO → 2C-K(O)+N 2

258

or dissociates into C-K(N) and C-K(O) before C-K(N) reacts with NO as

259

shown in reaction (8).

260 261

C-K(N)+NO → C-K(O)+N 2 However, the present study cannot determine which route is important.

15


(7)

(8)

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262 263


264 265


266

Figure 5. Concentration of N2 as a function of NO concentration

267

Experiments showed that K could improve the formation of the oxygen complex

268

but could not change the chemical form of the complex, which suggested that oxygen

269

in C-K(O) quickly passed to carbon active sites; this process can be expressed as:

270

C-K(O) + C( ) → C-K+C(O) 16


(10)

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and then C(O) desorbs to form CO or CO2 as shown in reaction (11).

C(O) → CO X

272

(11)

273

As discussed above, the catalytic mechanism of K on the char–NO reaction can be

274

summarized as follows: NO first undergoes non-dissociative chemisorption at K

275

active sites, forming C-K(ON); C-K(ON) then directly reacts with NO or dissociates

276

into C-K(N), which reacts with NO to form N2 and C-K(O); O in C-K(O) quickly

277

passes to carbon active sites and forms C(O) which desorbs to form CO or CO2. The

278

N2 concentration in the outlet gas was almost linear to the NO concentration in the

279

inlet gas for both chars, and the reaction between the nitrogen complex and NO was

280

very quick, indicating that chemisorption of NO on the char surface is the

281

rate-controlling step of the char-NO reaction, no matter catalyzed or not.

282

4. CONCLUSION

283

Enrichment of the oxygen complex was significant for both chars during the

284

isothermal char–NO reaction at 700 oC; K promoted the formation of the oxygen

285

complex during the reaction. Nitrogen complex was not enriched during the reaction

286

for both chars.

287

The main desorption product of both chars after isothermal char–NO reaction was

288

CO. The amount of CO and CO2 desorbed from original rice straw char were much

289

higher than those from acid-washed rice straw char. Both the thermal desorption

290

spectra of two chars showed two peaks during CO production and one peak during 17


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291

CO2 production with the increase in temperature. The peak temperatures of both chars

292

were the same; K did not change the chemical form of the oxygen complex formed on

293

char surface.

294

The catalytic mechanism of K on the char–NO reaction was proposed. NO first

295

undergoes non-dissociative chemisorption at K active sites, forming C-K(ON).

296

C-K(ON) then directly reacts with NO or dissociates into C-K(N), which reacts with

297

NO to form N2 and C-K(O). O in C-K(O) quickly passes to carbon active sites and

298

forms C(O), which desorbs to form CO or CO2. Chemisorption of NO on the char

299

surface is the rate-controlling step.

300

AUTHOR INFORMATION

301

Corresponding author

302

*E-mail address: [email protected]

303

Notes

304

The authors declare no competing financial interest

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ACKNOWLEDGMENTS

306

This work was sponsored by the National Natural Science Funds of China (grant

307

number 51076072) and the National Basic Research Program of China (973 Program)

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(grant number 2013CB228500)

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REFERENCES:

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Catalytic Mechanism of Inherent Potassium on the CharâNO Reaction

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