NOx Emissions from Regenerator of Calcium Looping Process

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NOx EMISSIONS FROM REGENERATOR OF CALCIUM LOOPING PROCESS Jaroslaw Krzywanski, Tomasz Czakiert, Tadaaki Shimizu, Izak MajchrzahKuceba, Yuuto Shimazaki, Anna Zylka, Karolina Grabowska, and Marcin Sosnowski Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00944 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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NOx EMISSIONS FROM REGENERATOR OF CALCIUM LOOPING PROCESS Jaroslaw Krzywanski*1, Tomasz Czakiert2, Tadaaki Shimizu, Izabela Majchrzak-Kuceba2, Yuuto Shimazaki3, Anna Zylka1, Karolina. Grabowska1, Marcin Sosnowski1 1

Jan Dlugosz University in Czestochowa, al. Armii Krajowej 13/15, Czestochowa, Poland

2

Czestochowa University of Technology, ul. Dabrowskiego 73, Czestochowa, Poland

3

Niigata University, 2-8050 Ikarashi, Niigata 950-2181, Japan

*Corresponding author: T/F: +48 34 36 15 970; e-mail: [email protected]

Keywords: Calcium looping, Nitric oxides, Dual fluidized bed, Fluidization, Modeling. Abstract The Calcium Looping (CaL) process usually employs a dual-fluidized bed (DFB) solid circulating unit. In the regenerator (calcinator or calciner), decomposition of CaCO3 proceeds. To supply the heat of decomposition, oxyfuel combustion of coal is conducted. However, since coal contains nitrogen, the NOx formation occurs during oxyfuel combustion. Because of the fact that NOx formation and destruction during combustion of solid fuels in fluidized bed is a complex process, a predictive approach of NOx emissions has not yet been sufficiently recognized, especially during oxyfuel combustion conditions in the CaL systems. The paper introduces a regression-based method for the prediction of NOx emissions from a CaL DFB experimental unit. Effects of fuel type, excess oxygen feed, and NO addition to

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primary or secondary feed gas, on NOx emissions in the regenerator were evaluated. The presented way constitutes a straight-forward method to run a complementary technique in relation to other methods of data handling, including the programmed computing approach and measurements. The developed model can be simply employed by scientists as well as engineers for optimization purposes. Nomenclature: C

carbon content, %

F.C.

fixed carbon, %

N

nitrogen content, %

NOx

NOx (i.e. NO + NO2) concentration in flue gas from absorber, ppm

O2

oxygen concentration in flue gas from regenerator, %

V.M.

volatile matter content, %

Superscripts d

desired (measured)

p

predicted (calculated)

Acronyms CaL

Calcium Looping

CFB

Circulating Fluidized Bed

CFBC

Circulating Fluidized Bed Combustor

DFB

Dual Fluidized Bed

FBC

Fluidized Bed Combustion

1. Introduction Apart from oxy-fuel combustion and chemical looping combustion, calcium looping (CaL) process has been developed as one of the promising post-combustion CO2 capture


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technologies suitable for coal-fired combustors [1 - 7]. Calcium looping is a dry absorption and desorption route, carried out in two reactors i.e. carbonator (absorber) and regenerator (calcinator or calciner) where a reversible reaction of CO2 absorption by solid CaO absorbent (Eq. (1)) and CaCO3 decomposition (Eq. (2)) occur [1]: CaO+CO2 → CaCO3

(1)

CaCO3 → CaO+CO2

(2)

The carbonation reaction (Eq. (1)) proceeds in the carbonator whilst the decomposition of CaCO3 (Eq. (2)) happens in the regenerator. To recirculate the solid absorbent, solid transportation lines connect the calciner and the regenerator. The CaL process is dedicated for oxyfuel combustion of coal which is a practical fuel to supply necessary heat for endothermic calcination reaction (Eq. (2)). Fuel is burned in pure oxygen (diluted by recirculated CO2), so CO2 and H2O are the components of the flue gas [1]. However, fuels usually contain nitrogen and this becomes a source of NOx during combustion. Nitric oxide in CO2 is anticipated to form nitric acid in the back-pass where water vapor is removed by condensation at lower temperatures. Therefore, NOx emissions from the regenerator should be also taken into account when CaL is put into practice. The emission of NOx from fluidized bed combustion, including calciner of the CaL process, is a result of complex factors and is the result of competing formation/destruction processes [8]. The main factors are: volatile and nitrogen content, primary to secondary gas ratio, oxygen partial pressure in the combustion chamber, the local temperature, the existence of calcined limestone in the furnace and the gas residence time. The geometry of the system also has an impact on NOx emissions, as boiler size and exit effects influence mixing processes and solids distribution in the combustion chamber [8-26]. The effects of temperature, volatile matter content, oxygen partial pressure, and limestone feed on NOx formation were discussed by Zhao et al. [26]. Oxidation of volatiles and char-bound nitrogen


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take part in NOx formation. Reduction of NOx was favored by the presence of char and the lower oxygen partial pressures, whereas the NOx formation mechanism dominated for higher oxygen partial pressures and higher volatile release. These results are consistent with the opinion that low rank coals can yield more NOx than those coals of higher rank [15, 18]. Similar results are reported by Gungor and Eskin [16, 17]. As was confirmed, staged combustion is a useful method for lowering of NOx emission, and the concentration of NOx decreased with the secondary air ratio due to three reasons: 1) the atmosphere with limited oxygen concentration in the lower part of the furnace leads to converts of the volatiles – N to N2 not to NOx, 2) higher secondary to primary air ratio leads to the increase in char and CO concentrations in the lower part of the furnace intensifying the NO decomposition processes, 3) the residence time of gas in the bottom part of combustion chamber rises with the secondary air ratio. As the result, during staged combustion, the NOx decomposition becomes the more dominant mechanism over NOx formation. Since the measurements belong to the main techniques of data acquisition, most of the results reported in the literature are obtained via experimental procedures on the existing objects. However high costs and time-consumption often make the measurements an inefficient method for data acquisition [8, 27-29]. An alternative can be the mathematical modeling approach. Different in details and/or sophistication, models of solid fuels incineration in fluidized bed systems are reported in the open literature [12, 17, 30-36]. Some of them refer to oxy-fuel conditions. Several models of chemical looping combustion processes can also be found in literature. Some of them consider the whole CaL system (full-loop), others only the carbonator or calcinier [37-43]. However, it is questionable if the results of conventional and oxyfuel FBC can be directly applied for the analysis of the CaL process. One difference is


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that a part of char formed in the regenerator of CaL process is transported to the carbonator with recirculating solids, whereas nearly all char (except for carbon in fly ash) is burned in the combustion chamber of FBC [7]. Thus the amount of char in the combustion chamber may differ between the regenerator of CaL and FBC even when the same fuel is burned under the same condition using a combustor of the same geometry. This difference indeed affected the NOx emissions [7]. Since the character of industrial processes is usually non-linear and even highly complicated, models usually include some empirical parameters to provide necessary data in cases where up-to-date modeling is unsatisfactory [8, 27, 36]. It happens e.g. when adjusting parameters of the model are not defined straightaway, e.g. for changing operating parameters [8, 27]. Models are often time consuming. The time required to conduct a numerical test can be fairly long in order to acquire accurate predictions, in spite of the fact that they usually use some simplified assumptions to obtain a tractable solution to make the models simpler and easier [8, 27, 32]. The algorithms are convoluted and, as usual, basing on differential equations. The above listed main shortcomings of the programmed computing approach [27, 28] pointed out the another estimation procedure in engineering approach, i.e. the use of artificial intelligence (AI) techniques. Besides artificial neural networks, fuzzy-logic (FL) and evolutionary computations, constitutes one of the main and promising examples of the AI approach [8, 22, 27-29, 44-46]. The paper introduces a regression-based method for the prediction of NOx (i.e. NO + NO2) emissions from a CaL DFB system. The parameters in the model were obtained by experimental study using a bench-scale DFB system using different coals. The results of the model applications for the prediction of NOx emissions from CaL DFB are discussed in the paper.


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2. Experimental procedure and methods The data of NOx emissions from a regenerator were acquired from experiments carried out using a CaL DFB facility at the Department of Chemistry and Chemical Engineering, Niigata University in Japan. A schematic diagram of dual-fluidized bed system employed for the present work is illustrated in Figure 1. The details are given elsewhere [6, 7]. to NOx and O 2 analyzers

Regenerator flue gas Cyclone Regenerator (Fast fluidized bed)

Carbonator flue gas Carbonator (Bubbling bed)

Secondary gas Air NO/N 2

Carbonator fluidizing gas Air

Coal and pneumatic transportation gas (Air)

Loopseal gas (Air)

Bottom fluidizing gas NO/N 2 O2 Air

Figure 1. Experimental apparatus

As the regenerator, a fast fluidized bed reactor was employed. The inner diameter and the height of the fast fluidized bed reactor were 2.2 cm and 1.93 m, respectively. The bed material was inert silica sand of average grain size 0.15 mm. The particles transported to the top of the regenerator were separated from the flue gas by a cyclone, then the particles were introduced into the carbonator. As the carbonator, a bubbling fluidized bed reactor of 9.3 cm inner diameter was employed. The fluidizing bed height was fixed at 30 cm by an overflow tube. The particles drained from the overflow tube were conveyed to the bottom of the regenerator through a loopseal.


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The operating conditions are summarized in Table 1. The condition of the regenerator gas feed , gas feed staging, and oxygen enrichment to primary/secondary gas, was chosen to attain long residence time of solids in the regenerator and to reduce NOx emissions from the regenerator, as described elsewhere [6].

Table 1. Operating conditions

Regenerator Temperature: 1223 K Total gas feed rate: 0.00023 Nm3/s (superficial gas velocity = 2.75 m/s) Overall oxygen concentration in regenerator feed gas: 30 % (Primary gas)/(Total gas) ratio of regenerator feed gas: 0.5 Oxygen concentration in regenerator flue gas: 3 – 6 % (controlled by coal feed rate) NO/N2 ratio in feed gas: 0 – 400ppm Carbonator Temperature: 873 K Total gas feed rate: 0.00033 Nm3/s (superficial gas velocity = 0.16 m/s) The regenerator was fluidized by oxygen-enriched air. The primary gas consisted of bottom fluidizing gas, loopseal gas, and gas for pneumatic transportation of fuel. The ratio of primary gas to the total gas feed rate was fixed at 0.5 so that the residence time of solid could be as long as about 40 s [6]. The overall O2 concentration in the whole feed gas to the regenerator was 30% by volume. Oxygen was enriched only to the bottom fluidizing gas. For the secondary gas, loopseal gas, and fuel pneumatic conveying gas, air was used. Though O2enrichment to the secondary gas was carried out, NOx emissions were found to be higher than the present O2-enrichment operation [6]. The regenerator was heated by use of electric heaters. The temperature in the regenerator was measured at 0.33 m (below the secondary gas inlet) and 1.53 m (above the secondary gas inlet) above the distributor. The temperature was maintained at 1223 K. This temperature was determined by the lower limit of the calcination reaction of CaCO3 under high CO2 partial pressure conditions and by the upper limit to avoid ash fusion problem. The


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superficial gas velocity above the secondary gas inlet was fixed at 2.75 m/s at this temperature. Due to the vigorous solids mixing the use of fast fluidized bed for the regenerator is suitable for oxyfuel combustion, as it suppress hot-spot formation. The diameter of the regenerator (calciner) was only 2.2 cm and the particles were vigorously mixed by highvelocity gas stream. Thus it was unlikely that hot spot was formed in the horizontal direction. To the vertical direction, temperature was measured at two different locations, below the secondary inlet and above the secondary air inlet. The temperature was found to be uniform as shown below. The uniform temperature was attributable to the transportation of heat by solids which was vigorously mixed throughout the regenerator. Fuel Medium volatile bitumnous coal

1000

5.0

3.0

900 2.0 850 temp. 1.53m

1.0

temp. 0.33m

riser ⊿Pr

bottom ⊿Pb

800 0

2000

4000

6000

8000

10000 time[s]

12000

14000

16000

18000

0.0 20000

Fig.2. Typical result of temperature in the bottom part (0.33 m) and that above the secondary air inlet (1.53 m), and pressure drop in the bottom part and total pressure drop across the regenerator. Nitric oxide (NO) was also added to the regenerator fluidizing gas to simulate the gas recirculation employed as a diluent of O2 for oxyfuel combustion. For this reason, the same NO/N2 ratio in feed gas was adopted for the primary and secondary gases. The particles in the carbonator were fluidized by air. The carbonator was heated by an electric heater. The bed temperature was fixed at 873 K, which is a typical temperature of CaL process [1]. The superficial gas velocity was set at 0.16 m/s at this temperature. It should


Pres. Drop[kPa]

4.0

950

Temp.[℃]

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

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be noted that a part of char formed in the regenerator was transported to the carbonator with the recirculating bed material and consumed by oxygen there. Therefore, the amount of char combustion in the regenerator of CaL DFB system differs from those in oxyfuel and air-fired CFBs with single combustion chamber in which all char is consumed. The detail of char transportation and combustion of char in the carbonator is described elsewhere [6, 7]. In this study, two kinds of high-volatile bituminous coals (HVB1 and HVB2), mediumvolatile bituminous coal (MVB), and semi-anthracite (SA) were employed, as shown in Tab. 2. The particle size was 297–1000 µm. Fuel was fed into the bottom of the regenerator, conveyed pneumatically by air. The O2 concentration of the regenerator flue gas was controlled at typically 3-6% by controlling the fuel feed rate. Flue gas sampling and analysis were conducted after a steady state was attained. The flue gas from the regenerator was analyzed for O2 and NOx (NO+NO2) using a magnetic oxygen analyzer and a chemical luminescence NOx analyzer, respectively. Table 2. Analyses of fuels (high-volatile bituminous coals (HVB1 and HVB2), medium volatile bituminous coal (MVB), and semi-anthracite (SA)) Proximate analysis (Air-dried) Ultimate analysis (daf.) Coal V.M. F.C. Ash Moisture C H N O S HVB1 39.2 41.2 14.3 5.3 78.1 6.3 1.3 13.4 1.0 HVB2 39.7 39.7 14.1 6.5 80.1 6.5 1.7 11.1 0.6 MVB 26.3 56.2 15.0 2.5 85.9 4.9 1.7 7.0 0.5 SA 15.5 70.7 10.9 2.8 89.7 4.1 1.9 3.6 0.7

3. Results and Discussion 3.1. Experimental results of dual fluidized bed experiments Figure 3 shows typical results of the relationship between the regenerator flue gas oxygen concentration and NOx emissions from the regenerator, with and without NO feed to the regenerator. With increased flue gas oxygen concentration i.e. with rising excess air ratio, NOx emissions increased. Since the scattering of the data was observed, typical correlation between NOx and O2 concentration in flue gas was obtained by straight-line approximation. ACS Paragon Plus Environment

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The parameters (slope and intercept) of the approximated relationship were determined by least-square method. NOx emission at a given flue gas O2 concentration was determined by this straight-line relationship. Figure 4 depicts the influence of NO addition to feed gas on the NOx concentration in flue gas from the regenerator at O2 content of 4%. Such relationship between NOx emissions and NO concentration in the diluent could be expressed by straight lines. The slopes of these lines were found to be less than unity i.e. a part of the fed NO with fluidizing gas was decomposed in the regenerator. The intersection points of the tie line and the experimental results shown in Figure 4 refer to the steady NOx emissions when recirculating flue gas is used as diluent of the oxygen i.e. the NOx content is equal to the concentration of NOx in the diluent. The influence of fuel type on steady NOx emissions is depicted in Fig. 5. With increasing fixed carbon content, steady NOx emissions decreased. 300

300

Coal: HVB2 NOx conc. in Reg. flue gas [ppm]

NOx conc. in Reg. flue gas [ppm]

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

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200

100 NO feed 400ppm Without NO feed Approx. line NO 400ppm Approx. line without NO feed 0

Coal: SA NO feed 400ppm Without NO feed Approx. line NO 400ppm Approx. line without NO feed

200

100

0 0

1

2 3 4 5 6 O2 in Reg. flue gas [%]

7

8

0

1

2 3 4 5 6 O2 in Reg. flue gas [%]

7

8

Figure 3. Relationship between oxygen concentration in regenerator flue gas and NOx emissions from regenerator, with and without NO feed


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NOx in Reg. flue gas [ppm]

500

HVB1 HVB2 MVB SA

400 300 200 100 0 0

100 200 300 400 NO/N2 in Reg. feed gas [ppm]

500

Figure 4. Effect of NO concentration in diluent (N2) of regenerator gas on NOx concentration in flue gas from regenerator at O2 concentration of 4%. (intersection points marked by arrows: steady NOx emissions at which NO concentration in diluent is equal to NOx content in the flue gas)

The method to determine the unreacted fraction of fed NO in the regenerator is given in Figure 6. Because inert N2 was used as a diluent in this work, N2 can also be used as a tracer gas to determine the flow rate of each gas component. By plotting the NO/N2 ratio in the flue gas against NO/N2 in the feed gas, a straight-line relationship resulted. The slope of the straight line refers to the ratio of fed NO that reached the regenerator exit i.e. the unreacted fraction of fed NO. Figure 7 illustrates the influence of fuel type on the unreacted portion of NO in the regenerator. For the increase in fixed carbon content, the unreacted fraction of NO decreased. 300

Steady NOx emissions [ppm]

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

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HVB1 HVB2 MVB SA

200

100

0 0

2

4 FC/VM [-]


6

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Figure 5. Influence of FC/VM ratio on steady NOx emissions at which NOx concentration in diluent is equal to NOx content in the flue gas (O2 concentration in Reg. flue gas = 4%) 400 NO/N2 in Reg. flue gas [ppm]

HVB1 HVB2 MVB SA

300

200

Slope: Unreacted fraction of NO fed to Reg.

100 0

100 200 300 400 NO/N2 in Reg. feed gas [ppm]

500

Figure 6. Determination unreacted fraction of fed NO in the regenerator 0.3

Unreated fration of fed NO into Reg. [-]

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

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HVB1 HVB2 MVB SA

0.2

0.1

0.0 0

2

4

6

FC/VM [-]

Figure 7. Influence of FC/VM ratio on unreacted fraction of fed NO in the regenerator 3.2. Regression analysis of the experimental results The acquired experimental results were used to derive and validate model parameters. For the purpose of this work, the regression analysis-based method was applied to predict the NOx emissions from the considered CaL DFB system. The RegLINP statistical function of EXCEL was also applied in the study. The following three input parameters are assumed for operations without NO feed: fixed carbon FC (daf.), the ratio of molar nitrogen to carbon


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content in fuel N/C, and the O2 concentration in the flue gas from the regenerator, whereas the NOx emission constitutes the output parameter (Table 3). Table 3. The input parameters used in the study Input Parameter Fixed carbon content FCdaf. wt. %

Value 50 – 82

N/C molar ratio,

0.014 – 0.020

NO/N2 ratio in feed gas, ppm

0 – 400

Oxygen concentration in flue gas, vol. %

0–8

The emissions of NOx without NO feed were assumed to be given by Equation (3-a) below with four parameters a, b, c, and d. Since the increase in NOx content with NO feed was proportional to the added NO concentration and the slope was affected by the fuel type, the increase in NOx concentration was assumed to be given by a factor η which is a function of FC content of fuel (Figure 8). NOx = −a ⋅ FC − b ⋅ N / C + η (FC)NO / N 2 + c ⋅ O2 + d

(3-a)

This approach allowed conducting the regression analysis and obtaining correlations describing the NOx emissions versus main input parameters i.e. FC, N/C, NO/N2 and O2. The determined equation is as follows: NOx = −1.17763⋅ FC − 15245⋅ N / C + η (FC) ⋅ NO / N 2 + 15.165791⋅ O2 + 396.84982

(3-b)

where

η(FC) = −0.0024FC + 0.2852.

(4)

The measured and calculated by the model results are given in Figure 9. The data shown in Figure 9 takes into account the four kinds of coal used and corresponds to various input


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data, including different oxygen content in the regenerator flue gas. The achieved correlation coefficient was equal to 0.925 and the maximum relative errors are lower than ± 20 %. This methodology was carried out as the comparison of measured and predicted by the model results is considered as the highly demanding type of checking way [27, 28]. In order to validate the model, the outputs were also compared with some independent experimental data which has not been previously used to derive the parameters of the developed model. This comparison is shown in Figure 10. The calculated results are spread within a range of ± 10 % when compared to the measured date. Thus the developed model can be used to run a non-iterative procedure for studying the effects of the various process parameters on the NOx emissions in the regenerator of CaL DFB unit.

(Increase in NOx)/(NO/N2 of feed) η [-]

0.3

0.2

0.1 y = -0.0024x + 0.2852

0.0 40

50

60 70 FC [%]

80

90

Figure 8. Influence of FC content on (increase in NOx during NO feed)/(NO/N2 ratio of feed) 325 HVB1 275

HVB2 MVB

225 NOxp [ppm]

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

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SA 175 125 75 25 25

75

125

175 225 NOxd [ppm]


275

325

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Figure 9. Comparison of the NOx content measured and calculated from the model

The flow chart of the model application is given in Figure 11. The calculation procedure using the performed model is a quick and simple way of NOx emissions prediction from a CaL DFB unit. One only needs to enter the necessary data considered by the model; input parameters (FC, N/C, NO/N2, and flue gas O2) to obtain the NOx emissions from the CaL DFB system. To determine study the influence of a selected input value on the NOx concentration in flue gas in the regenerator of CaL DFB unit, other inputs should be stable as the relationship can be defined only under fixed conditions. Such procedure was undertaken in this paper. 220 200 180 NOxp [ppm]

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

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160 140 120 100 100

120

140

160 NOxd [ppm]

180

200

220

Figure 10. Comparison of the measured and calculated NOx emissions for the new, previously unused set of input data

START

Input parameters

Call the model

NOx

STOP

Figure 11. Application of the model for the evaluation of NOx concentration if flue gas


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3.2.1. Influence of O2 content in flue gas from the regenerator The effect of oxygen concentration in the flue gas leaving the regenerator on the NOx emissions is illustrated in Figure 12. The calculations are performed for all coals considered in the paper. As expected, the nitrogen to NOx conversion growths with the rise in oxygen content in the flue gas leading to higher NOx emissions. Such results are coinciding with other findings, as described in [6, 16, 17]. Higher O2 contents in the reactor make the combustion of volatile gases and the chare more enhanced, leading to the increase in NOx emissions. 350 300 250 NOx [ppm]

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

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200

HVB2

150

MVB

100

HVB1 50

SA

0 0

2

4

6

8

10

O2 [%]

Figure 12. The influence of flue gas O2 concentration on the NOx emissions from the CaL DFB unit

3.2.2. Influence of N/C molar ratio, FC and VM content The NOx emissions are the result of complex reactions which occur in the system where the two competing mechanisms occur i.e. formation/reduction of NOx. Since the fuel-bound nitrogen constitutes the main source of NOx in fluidized bed combustors and char is a both a main agent in NO reduction process as well as it acts as a catalyst of NO reduction by reducing gases, including CO, the increase in N/C molar ratio should result in an increase of the conversion rate of fuel-N to NOx inside the regenerator [6, 14, 16, 17]. However, as the inlet gas consists of an NO and N2 mixture, the balance between the two competing mechanisms shifts toward the destruction processes of nitrogen oxides. As a result, the


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increase in N/C molar ratio in the considered conditions leads to the decrease in NOx emissions in such conditions (Figure 13). The crucial role of fixed carbon in NOx emissions can be observed in Fig. 14. The results are given as a function of the FC/VM ratio since it is a property of each fuel. The increase in char content will lead to a decrease in NOx emissions for all considered fuels. As it was mentioned, char acts both as a reducing factor of NO and a catalyst in NO reduction process, by the reducing components, e.g. CO, via the reactions [47-49]: NO+Char-C → 1/2N2+CO

(5)

NO+CO → 1/2N2+CO2

(6)

Since the total sum of VM (daf.) and FC (daf.) is 100%, the N to NOx conversion decreases with the decrease volatile-matter content in the fuel irrespective of the nitrogen content in coal. This reported trend confirms the opinion that lower ranked coals can yield more NOx than higher rank ones [18]. 250

200 NOx [ppm]

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Energy & Fuels

150

100

50 0.013

FC/VM=1.10 FC/VM=1.00 FM/VC=2.14 FM/VC=4.56 0.015

0.017

0.019

0.021

N/C [-]

Figure 13. The influence of N/C molar ratio on the NOx concentration in flue gas from the CaL DFB unit (flue gas O2 = 3.5 %)


Energy & Fuels

250

200 NOx [ppm]

150

N/C=0.014 N/C=0.019 N/C=0.017 N/C=0.018

100

50 0

1

2

3 4 FC/VM [%]

5

6

Figure 14. The effect of the FC/VM ratio in coal on the NOx emissions from the CaL DF unit (flue gas O2 = 3.5 %) 250 200 NOx [ppm]

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150 100 50 0 HVB1

HVB2

MVB

Fuel type

SA

Figure 15. The influence of fuel type on NOx concentration from the CaL DFB unit (flue gas O2 = 3.5 %) Although the char of high-volatile coal is porous and highly reactive as well, the intrinsic oxidation rate is also greater than the one of low-volatile coals – the former fuel yields less char than the latter one (Figure 15). Therefore the effect of volatile matter is more enhanced by the simultaneous decrease in fixed carbon content in the considered kinds of coal. Similar observations were discussed by other researchers [10, 11, 14-16, 18, 22].


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Energy & Fuels

4. Conclusions The effects of flue gas O2 concentration, fuel properties (N/C ratio and FC content), and NO feed to the fluidizing gas, on NOx emissions were experimentally evaluated using a dual fluidized bed system operated under CaL process conditions using inert bed material. When O2 and NO content in gas as well as the N/C molar ratio increase, the NOx emissions also increase whilst the increase in fixed carbon content lead to the decrease the NOx concentrations in flue gas. The results of the regression analysis-based modeling technique for the prediction of NOx emissions in the regenerator from a CaL DFB system are discussed. The NOx emissions from the regenerator, predicted using the developed model, are in good coincidence with the experimental results. The achieved correlation coefficient was equal to 0.925 and the calculated data located within the range of ± 20 % relative error compared to the measured data. Moreover, the comparison of outputs with some independent experimental data, not previously used to derive the parameters of the developed model revealed, that the predicted by the model results are spread within a range of ± 10 % when compared to the measured data. It is shown that the method gives fast and correct results for various inputs. The developed model makes a simple in use and convenient optimization tool and can be applied as a sub-model in a larger predictive toolkit, capable to determine the NOx content in the gas from a regenerator of a CaL DFB system.

Acknowledgments This scientific work is funded from the project: “Long-term research activities in the area of advanced CO2 Capture Technologies for Clean Coal Energy Generation”: the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant agreement n° PIRSES GA-2013-612699. The support is


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gratefully acknowledged. Tadaaki Shimizu expresses his thanks to The Iwatani Naoji Foundation and Sasaki Foundation for financial support for the experiments.

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NOx Emissions from Regenerator of Calcium Looping Process

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