Experimental Investigation and Process Simulation of Oxy-fuel Flue

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Experimental Investigation and Process Simulation of Oxyfuel Flue Gas Denitrification in CO2 Compression Process Xiaoshan Li, Liqi Zhang, Cong Luo, Zewu Zhang, Yongqing Xu, and Chuguang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02660 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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Experimental Investigation and Process Simulation of Oxy-fuel Flue Gas Denitrification in CO2 Compression Process Xiaoshan Li1, Liqi Zhang1,*, Cong Luo1, Zewu Zhang1, Yongqing Xu1, Chuguang Zheng1 1

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei Province P.R. China

* E-mail: [email protected] (Zhang L.) Tel: +86 27 87542417. Fax: +86 27 87545526.

ABSTRACT

In an oxy-combustion power plants, flue gas impurities such as NOx must be removed before CO2 recovery. Currently, sorbents or catalysts as well as flue gas treatment device were required for the application of the traditional NOx control technologies. To produce NOx-lean oxyfuel-derived CO2, an attractive option is using the existing CO2 Compression and Purification Units (CPU) as NO removal units instead of traditional Selective Catalytic Reduction (SCR) technology. The special condition in CO2 CPU system, high pressure, was one of the most crucial operating parameters for NOx removal in CO2 compression process. Appropriate pressure along with sufficient residence time of the pressurized denitrification system should be provided to achieve cost efficient NOx removal. In this work, to simulate the dynamic process of flue gas

1

ACS Paragon Plus Environment

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denitrification at elevated pressures, a two-staged compression process simulation model was built based on the experimental results and good agreement between experimental and simulated data was obtained. The optimization on the pressure of the compression process as well as residence time showed the feasibility of the system eliminating 94% of NOx with the emission concentration of 48 ppm (100 mg/m3) at the optimized pressure of 2.6MPa and residence time of 223s. This work provided a possibility for design and optimization of the pressurized denitrification in oxy-fuel CO2 compression system. Keywords: NO removal; Oxy-fuel combustion; CO2 compression; Operation parameters Nomenclature Cin[NO]

Initial concentration of NO in simulated flue gas (ppm)

Cout[NO]

Outlet concentration of NO in outlet flue gas stream (ppm)

Cout[NO2]

Outlet concentration of NO2 in outlet flue gas stream (ppm)

η[NO]

NO removal efficiency (%)

t-eq

Time to reach equilibrium (min)

[NOx]-eq

Outlet concentration of NOx at an equilibrium (ppm)

d[NOx]/dt

The differential of NOx outlet concentration with time

W

Electricity consumption of the compressor (kJ/h)

W/∆[NOx]

Ratio between the electricity consumption and the amount of NOx emission reduction

1. Introduction One of the main causes of global climate and eco-environment changes is the increasing emission of CO2, to which the anthropogenic activities contributes most

2


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due to the combustion of fossil fuel. Carbon Capture and Storage (CCS) technologies which could achieve a meaningful reduction in CO2 emissions have gained much attention.1,2 Among several CCS technologies, oxy-fuel combustion or O2/CO2 combustion is considered as a potential technology with the advantages of large-scale CO2 emission control and suitable for retrofit.3-5 In an oxy-fuel boiler, coal is combusted in the mixture of almost pure O2 and recycled CO2, instead of O2/N2, which produces highly enriched CO2 in exhausted flue gas. Since O2 is in place of O2/N2, the formed NOx is significantly reduced in oxy-fuel combustion plants.6-9 Although the amount of NOx emission per unit of energy is reduced, the concentration of the impurity remains high level due to the recycle of flue gas.10,11 The presence of NOx may cause a potential acidic corrosion risk to the pipelines and equipment.12,13 Consequently, it is essential to remove the impurity to produce NOx-lean oxyfuel-derived CO2 before the transportation, storage and utilization such as geological disposal and enhanced oil recovery. Currently, Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) are conventional technologies for post-combustion NOx control and several novel solid or liquid sorbents and catalysts were developed for NOx reduction.14-17 Assuming that the emission of NOx from oxy-fuel flue gas is controlled using traditional SCR or SNCR technology, this will add to the investment and operation costs because additional flue gas denitrification device was required in an oxy-fuel plant. There is possibility of using the existing CO2 compression and purification units (CPU) as a cleaning step for NOx removal instead of traditional flue gas denitrification device. Air Products and Chemicals, Inc.

18-20

have proposed a

novel compression method for separating NOx from compressed oxyfuel-derived CO2. This process mainly involved three steps: (1) condensing the impure CO2 to remove

3


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water vapor and soluble gases; (2) CO2 compression in two stages for gas phase reactions; (3) sufficient contact time in a reactor for impurities removal as acidic liquids. The relevant reactions are similar to the Lead Chamber Process (LCP) for industrial acid production.21 NO can be oxidized and converted to nitric acid and nitrous acid at elevated pressure in the presence of O2 and water. In addition, SO2 present in flue gas could be almost completely removed as sulphuric acid and elemental mercury could also be removed through reaction with NO2.22-24 The approach has gained a substantial amount of attention since it presents significant potentials to be applied in the existing CO2 compression unit with significant costs reduction. Removing NOx impurities in CO2 compression process has been investigated at laboratory and pilot plant scale.23-27 The experimental work using actual flue gas via a sidestream at Doosan Babcock’s 160 kW coal-fired oxyfuel rig demonstrated that 90% of NOx was removed on the sour compression apparatus at the pressure of 1.4MPa.28 Previous studies have found that NO removal efficiency was dependent on the pressure, temperature, residence time and flue gas composition, which also significantly influenced the reaction pathway.

29

In general, oxy-fuel

derived CO2 compression experiments with removal of NOx were performed at moderate temperature or room temperature because low temperature is strongly in favor of NO oxidation and absorption. It was well accepted that the oxidation of NO to NO2 is the key step for NO removal and formation of acid in the whole pressurized denitrification process.30 The oxidation reaction is kinetically controlled with the slowest reaction rate that limits the conversion process.31 The previous experimental reports have demonstrated that the increase in pressure significantly enhanced NO oxidation since the oxidation rate is proportional to pressure to the third power.26 The oxidation of NO to NO2 would not be speeded up until the pressure has increased to at

4


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least 0.3MPa and the preferable pressure is in the range of 1.0MPa to 5.0MPa.21 Under a given pressure, NO removal efficiency is strongly affected by gas-liquid contacting. Residence time also played a leading role in determining the extent of the conversion of NOx to HNO3. It was suggested that the amount of conversion to acid could be optimized by controlling the residence time in the system.28 Previous reports have demonstrated that NOx removal could be achieved in the CO2 compression system in place of traditional flue gas denitrification device. High pressure in CO2 CPU system was one of the most crucial operating parameters for NOx removal in CO2 compression system. It was reported that the discharge pressure of multi-stage CO2 compressor in CO2 CPU system could be as high as 30 bar.31 Appropriate pressure along with sufficient residence time of the pressurized denitrification system should be provided to achieve cost efficient NOx removal. To the best of the authors’ knowledge, reports regarding the optimization of operating parameters in oxy-fuel CO2 compression process for flue gas denitrification were hardly available. In this work, a two-staged compression process simulation model was built to simulate the dynamic process of flue gas denitrification at elevated pressure. The operating conditions are optimized to balance the required NO removal efficiency and energy consumption for CO2 compression system. Firstly, NO removal performance was experimentally investigated on a pressurized reaction system at pressures up to 2.5MPa. Then, the pressurized denitrification processes was performed by process simulation based on the experimental results of the pressurized reaction system with an evaluation of NOx reduction. An optimization was focused on the pressure of the compression process as well as residence time since the two factors are extremely meaningful to practical application. The results would give some insights to the design and optimization of the pressurized denitrification in

5


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oxy-fuel CO2 compression system. 2. Methods 2.1. Experimental section NO removal from flue gas in CO2 compression process was experimentally investigated on a pressurized reaction system, as shown in Figure 1. Table 1 presents the experimental condition for the pressurized reaction system. Gas composition of the investigated atmosphere consisted of 800ppm NO/3% O2/CO2. The simulated initial gas concentration was prepared using three mass flow controllers and verified through a flue gas analyzer (Multilyzer STe M60, AFRISO). The investigations on removal of NO in water were performed on the high pressure reactor loaded with 200mL deionized water. After the pressure increased to the given value, the gas stream flowed through the reactor where NOx was absorbed in deionized water. During the removal process, the temperature of the reactor was maintained at 25 ºC which was controlled by the external thermometer. The effect of pressure on NO removal performance was studied over a wide pressure range of 0.1-2.5MPa, which was monitored using the back pressure regulator. After contacting with water, the outlet concentration of NO and NO2 through the reactor was measured using NOx analyzer (Multilyzer STe M60, AFRISO). NO removal efficiency was used to evaluate the removal performance in the pressurized reaction system, which is calculated by Eq.(1). η [ NO ] =

Cin [ NO ] − (Cout [ NO ] + Cout [ NO2 ]) × 100% Cin [ NO ]

(1)

NOx was removed as nitrate and nitrite acid when absorbed in water at elevated pressures. The determination of N-containing ions by ion chromatography (ICS-90) was to obtain N mass balance before and after NOx removal. Since nitrous acid (HNO2) was unstable and easy to decompose to NO and NO2, the resulted solution 6


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was treated by purging with O2 of 50ml/min for 1h or mixed with 30% H2O2 of 2mL, in order to oxidize nitrite ion to stable nitrate ion. The total concentration of nitrate in the resulted solution was detected through ion chromatography which could reflect the conversion rate of N species.

Figure 1 The experimental apparatus of the pressurized reaction system. (1) O2; (2) CO2; (3) NO/N2; (4) Pressure reducing valves; (5) Mass flow controllers; (6) High pressure reactor; (7) Deionized water; (8) Cut-off valves; (9) Back-pressure valve; (10) NOx analyzer. Table 1 Experimental condition for the pressurized reaction system. Pressure/MPa

Temperature/ºC

Residence time

Atmosphere

0.1~2.5 MPa

25 ºC

5min

800ppm NO/3% O2/CO2

2.2. Process simulation Process simulation software ASPEN Plus was used to simulate the dynamic process of flue gas denitrification at elevated pressure. According to the experimental apparatus of the pressurized reaction system, the simulation model of flue gas denitrification process during compression was established as presented in Figure 2. The simulation model consists of three batch reactors (RBATCH), a flue gas mixer (MIXER), a water-gas flash (FLASH2) and a separator (SEP2). The feeding gases containing NO, O2 and CO2 were fed continuously into the system and the 7


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concentration of each gas was in consistent with the experimental values. Water of 200 mL was loaded through batch feeding. High pressure was achieved using three RBATCH model in series. The whole compression process was divided into two phase, pressure-increasing phase and constant pressure phase. In pressure-increasing phase, the pressure of feeding gases was compressed to a given value in RBATCH1 and RBATCH2 in 50s. Then the pressure remained constant in RBATCH3, where NOx removal reactions occurred with the residence time of 5min. Outlet gas from RBATCH3 entered in to a flash for gas-liquid separation. A separator was then used for dewatering the mixed gas. The concentration of NOx in the final outlet gas stream was real-time monitored.

Figure 2. The simulation model of flue gas denitrification process during compression.

The feed gas components in simulation model were defined exactly the same as the experimental simulated flue gases. The final products in liquid phase include nitric acid and nitrous acid. Since there were electrolyte components in the reaction system, the electrolyte ELECNRTL model was used as the property method. Four The physical properties were calculated using the package Aspen Properties.33 Four electrolyte equilibrium reactions were considered in the ELECNRTL model, as described in (2)~(5). CO2 + 2 H 2O ↔ H 3O + + HCO3−

(2)

8


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HCO3− + H 2O ↔ H 3O + + CO32 −

(3)

HNO3 + H 2O ↔ H 3O + + NO3−

(4)

HNO2 + H 2O ↔ H 3O + + NO2−

(5)

The activity coefficients were calculated by the ELECNRTL model for multicomponent mixtures. For the simulation of the gas-liquid equilibrium of NOx in water, Henry's law was used to describe the dissolved gas or supercritical components which consist of NO, NO2, O2, CO2, N2, HNO2 and HNO3. Henry constant parameters of NO, O2, CO2 and N2 were taken from the default values in ASPEN Plus and those of NO2, HNO2 and HNO3 referred to the compilation of Henry constants for inorganic N species by Sander.34 ASPEN Plus provides a properties database for thousands of chemical components, but there is little information for HNO2. Properties Estimation was used to obtain the critical properties of HNO2, as seen in Table 2. Table 2 The critical properties of HNO2 used for the denitrification system. Properties

units

HNO2

Properties

CHARGE

0

DVBLNC

CHI

0

HCOM

units

HNO2 1

J/KMOL

-41400000

DGFORM

J/KMOL

-46000000

HCTYPE

0

DGSFRM

J/KMOL

0

MW

47.01348

DHAQRM

J/KMOL

-119200000

OMEGA

0.70619174

DHFORM

J/KMOL

-79500000

PC

N/SQM

5640960.44

DHSFRM

J/KMOL

0

RHOM

KG/CUM

0

DHVLB

J/KMOL

41792092.3

RKTZRA

1

S025E

J/KMOL-K

366291.5

DLWC

0.22512829

TB

K

295.98

TC

K

444.308557

TREFHS

K

298.15

VB

CUM/KMOL

0.04354805

VC

CUM/KMOL

0.1315176

VCRKT

CUM/KMOL

0.1315176

9


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VLSTD

CUM/KMOL

0.03908184

ZC

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0.2518514

3. Results and discussion 3.1. Experimental investigation on NO removal at elevated pressures Since NO was insoluble and unreactive, the conversion of NO to NO2 in gas phase plays a major role in the flue gas denitrification process. Firstly, the gas phase reaction of NO and O2 at elevated pressures was studied in dry condition. The outlet concentration of NO and NO2 after the simulated gas flows through the reactor without any water over a wide range of pressures was presented in Figure 3. Most of the outlet gas stream was NO at atmospheric pressure due to the slow NO oxidation rate at ambient conditions. It is known that the oxidation reaction of NO is kinetically controlled and the reaction rate is proportional to pressure to the 3rd power.26 High pressures are favorable to the oxidation of NO and also increase the residence time. As the pressure increased, the conversion of NO to NO2 was significantly promoted. Then, the absorption of NO in wet condition was investigated in the reactor with 200mL water pre-loaded over a wide range of pressures from 0.1 to 2.5MPa. As presented in Figure 3 in wet condition, NO outlet concentration decreased with the increase of pressure while NO2 outlet concentration almost remained unchanged at each pressure. NOx removal efficiency in CO2 compression system was highly sensitive to the pressure. It was found that 94% of reduction in NOx outlet concentration after contact with water was achieved at pressures up to 2.5MPa. Compared to NO2 in dry condition, NO2 outlet concentration in wet condition significantly decreased and was completely removed at pressures up to 2.5MPa. The decrease in NO2 was due to the absorption of NO2 in water. Given that the absorbed NOx were completely converted into HNO3, the theoretical concentration of NO3- in the acid solution was calculated based on the difference between the inlet and outlet 10


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concentration. The actual concentrations of NO3- in three solution were measured through ion chromatography. Figure 4 shows the comparison between the calculated theoretical concentration of NO3- in the acid solution and the experimentally measured value. The measured nitrate concentration in original solution without any treatment was far below the calculated theoretical value, indicating that the resulted solution contained not only HNO3. The concentration of HNO3 in the original solution is 40%~50%. Then the original solution was purged with O2 of 50ml/min for 1h or mixed with 30% H2O2 of 2mL, in an attempt to oxidize other species (such as nitrite) to stable nitrate. Results of ion chromatography showed that the measured nitrate concentration increased significantly, especially in the solution after H2O2 treatment. The concentration of nitrate in the solution after H2O2 treatment reached around 90%, in which the oxidation of nitrite contributed almost a half. The dissolved NO2 in water is readily transformed into HNO2 and HNO3, while the formed HNO2 can be easily decomposed into HNO3 and NO. In addition, the measurement through ion chromatography was performed under atmospheric pressure and NOx in solution could be released back into the gas stream during the decompression process. The release of NOx made the mass balance of nitrogen species complicated.

Outlet concentration (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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800

800

600

600

400

200

NO_dry NO2_dry

NOx_dry NOx_wet

NO_wet NO2_wet

400

200

0

0

0.0 0.5 1.0 1.5 2.0 2.50.0 0.5 1.0 1.5 2.0 2.5

Pressure (MPa)

Pressure (MPa)

Figure 3 The outlet concentration of NO, NO2 and total NOx after the simulated gas with 800ppm NO/3% O2/CO2 flows through high pressure reactor under dry and wet condition at pressures up to 11


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2.5MPa and 25 ºC. Original solution Solution after O2 treatment

140 Experimental nitrate (mg/L)

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120

Solution after H2O2 treatment

100 1:1 line

80 60 40 20 0 0

20

40

60

80

100

120

140

Theoretical nitrate (mg/L)

Figure 4 Comparison between the calculated theoretical concentration of nitrate in the acid solution and the experimentally measured nitrate concentration in the solution through ion chromatography (■ Original solution without any treatment; ● Solution after O2 treatment for 1h; ▲Solution after 2mL 30% H2O2 treatment).

Separating NO from flue gas was mainly achieved by the oxidation of NO to water-soluble NO2 as well as the transformation of NO2 into liquid acid. NOx removal reactions in wet condition were fairly complex, including three terms: gas phase oxidation, gas-liquid absorption and liquid phase reactions. The pathway of NOx removal at elevated pressure was presented in Figure 5. The gas phase oxidation of NO by O2 played a significant role in the whole reactions. The major species in gas phase include NO, NO2, N2O4 and N2O3. It was suggested that N2O3 could be excluded in the gas phase regarding the thermal dynamic equilibrium while N2O4 slightly affected the NOx absorption.35 In gas-liquid reactions, the NO2 and N2O4 absorption contributed most to the formation of acid liquid. The dissolved NO2 and N2O4 is readily converted into nitric acid and nitrous acid. In the final produced acid liquid solution, HNO3 could form a stable nitrate while HNO2 was mainly present in its molecular form and is sensitive to pressure change. These reactions gave a pathway for NOx removal as acid liquid at elevated pressure. Since the oxidation of 12


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NO and gas-liquid reactions are kinetically controlled, it is essential to provide a series of optimized operation parameters such as the pressure, residence time and flue gas composition to achieve high-efficient removal.

Figure 5 The pathway of NOx removal at elevated pressure.

3.2. Model validation The validation of the built model in Aspen plus was performed through comparing the simulation results with the experimental data. The experimental data was obtained from the measured NOx concentration in wet condition on the pressurized reaction system. Figure 6 shows the comparison between experiment and simulation value of NO removal efficiency at elevated pressures. It can be seen that the experimental curves and simulation curves fit well at pressures up to 1.0MPa. The simulated value of NO removal efficiency at 1.0MPa was 92.2%, which agreed well with the measured value of 92.1%. The deviation of the two value was less than 1% at pressures above 1.0MPa. Therefore, the suggested model presented an accurate description on pressurized denitrification process and enabled appropriate design and optimization of this system. The effects of flue gas compositions on NO removal were investigated and the optimization of pressure and residence time in pressurized denitrification process was discussed based on the simulation model. 13


Energy & Fuels

100

NO removal efficiency (%)

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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80 Experiment value Simulation value

60 40 20 0 0.0

0.5

1.0

1.5

2.0

Pressure (MPa)

Figure 6 The comparison between experiment and simulation value of NO removal efficiency at elevated pressures.

3.3. Dynamic simulation on pressurized denitrification system The validated model could be used to evaluate the dynamic pressurized denitrification process at an equilibrium. Two equilibrium parameters, time to reach equilibrium (t-eq) and NOx outlet concentration at an equilibrium ([NOx]-eq), were used, among which t-eq is related to the reaction rate and [NOx]-eq is related to the removal efficiency and reflects the extent of NO removal. Adequate time for NOx removal reactions was ensured to achieve an equilibrium during the dynamic simulation. It is assumed that the pressurized denitrification process reached equilibrium at the time when the differential of NOx outlet concentration with time (d[NOx]/dt) is less than 0.5%.

3.3.1 The effects of initial NO and O2 concentrations Firstly, the simulation was carried out with a range of different compositions of flue gas to identify the influence of the flue gas components (NO and O2). According to the kinetic model of NOx removal reactions, the reaction rate is highly sensitive to NO concentration. Effect of initial NO concentration on NO removal was presented in Figure 7a. It should be noted that the pressure line means that the pressure change in the

pressurized

denitrification

process

was

divided

14


into

two

phases,

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pressure-increasing phase and constant pressure phase. The negative time (-50s~0s) corresponds to time to reach the required conditions before NOx reactions. At the first 50s, the pressure increased to 1.0 MPa, at the same time, the initial NO and O2 concentration could reach to a given value. Then the pressure remained constant during the reaction time of 5min. In the pressure-increasing phase, initial gases of 165ppm~950ppm NO and 5% O2 were introduced into the RBACTH. NOx concentration reached the maximum in 50s with the pressure increasing to 1.0MPa. In constant pressure phase, there was a significant drop and then a slow decrease in NOx outlet concentration with the increasing reaction time. The degree of decline was noticeable with the largest reduction of NOx emission when the initial gas contains NO with high concentration of 950ppm. High NO concentration would enhance the conversion to NO2 and increase the reaction rate, leading to a high removal efficiency. 1000

Pressure

(a)

NOx (ppm)

800

165ppm NO 280ppm NO 420ppm NO 560ppm NO 700ppm NO 835ppm NO 950ppm NO

600 400 200 0 -1

0

1

2

3

4

5

time (min) 40

[NOx]-eq (ppm)

12

Equilibrium NOx Time to reach equilibrium

(b)

38

10 36 8

34

6

32

4

30 150

300

450

600

750

900

Initial NO concentration (ppm) 15


time-eq (min)

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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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

Figure 7 The effect of initial NO concentration on NOx removal: (a) NOx outlet concentration vs. reaction time, (b) equilibrium parameters vs. initial NO concentration.

It is worth noting that as the reaction time increased, the outlet NOx seemed to reach equilibrium concentration at the same level of 10 ppm for the gases with various initial NO in Figure 7b. At each atmosphere with different initial NO concentration, NOx outlet concentration at an equilibrium remained almost unchanged as well as the equilibrium time. It seemed that the effect of initial NO concentration was negligible on NOx emission with sufficient residence time. Currently, most of studies regarding NO removal performance has focused on NO removal efficiency. It is believed that NOx emission concentration is a more critical indicator for evaluating NOx control technology since the emission standard for power plants stipulates NOx emission concentration rather than removal efficiency. It can be deduced from Figure 7b that even though NO removal efficiency at the atmosphere with small initial NO concentration was relatively low in comparison to high initial NO concentration, NOx emission concentration was at the same level. Effect of initial O2 concentration on NO removal was also investigated when the initial gases contain 835ppm NO and O2 of 1% to 10% at 1.0 MPa, as presented in Figure 8. Similarly, NOx concentration decreased with the reaction time in steady pressure phase. The emission of NOx was at the lowest stage when high O2 concentration of 10% was present in initial gases. The presence of O2 is crucial to the oxidation of NO. By increasing the reaction time of the process, the effect of initial oxygen concentration on two equilibrium parameters was studied as shown in Figure 8(b). Initial oxygen concentration played a role in NOx removal equilibrium. When the initial oxygen content was 1% to 3%, t-eq and NOx-eq decreased gradually with the increase in initial oxygen concentration. For the initial oxygen concentration ranging from 4% to 10%, the effect of initial oxygen concentration was relatively 16


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minor. 1000

Pressure

(a)

1% O2

NOx (ppm)

800

2% O2 3% O2

600

4% O2 5% O2

400

7% O2 10% O2

200 0 -1

0

1

2

3

4

5

time (min) 60 (b)

Equilibrium NOx Time to reach equilibrium

40

50 40

30 30 20 20 10

time-eq (min)

50

[NOx-eq] (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

Energy & Fuels

10 0

0 0

2

4

6

8

10

Initial O2 concentration (%)

Figure 8 The effect of initial O2 concentration ranging from 1% to 10% on NOx removal: (a) NOx outlet concentration vs. reaction time, (b) equilibrium parameters vs. initial O2 concentration.

3.3.2 The influence of pressure Since NOx removal reactions are kinetically controlled, the influence of the pressure in pressurized denitrification system was performed at the atmosphere of 850ppm NO/4% O2/CO2. NOx removal equilibrium was studied over a wide range of pressure from 0.4 to 3.0Mpa when sufficient reaction time was provided. The dependence of t-eq and NOx-eq upon the pressure was shown in Figure 9. It can been seen that t-eq and NOx-eq decreased as the pressure increased, indicating that the increase in pressure can reduce both the equilibrium time and NOx emissions. This also confirmed that high pressure could increase the reaction rate and achieve a high removal efficiency. However, when the pressure increases to extremely high level, it 17


Energy & Fuels

will increase the energy consumption in the compression process and investment of equipment. At the pressure above 1.5MPa, the two equilibrium parameters decreased less obviously. Therefore, appropriate pressure should be provided to balance the efficiency and the cost. 60

50 Equilibrium NOx Time to reach equilibrium

40

50 40

30 30 20 20 10 0 0.0

time (min)

NOx concentration (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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10 0 0.5

1.0

1.5

2.0

2.5

3.0

Pressure (MPa)

Figure 9 The NOx outlet concentration at an equilibrium and time to reach equilibrium at pressures up to 3.0MPa.

3.3.3 The optimization on the pressure and residence time In a typical CO2 compression process, the oxy-fuel flue gas should be pressurized using compressors. The compressor is considered as one of the most important components in CO2 compression and purification units due to the demand of the greatest amount of energy and investment.36,37 The electricity consumption of the compressor W is positively related to the pressure, as presented in Eq.(6). W=

ρVK RT ln( P2 / P1 ) 3600ηTηM

(6)

where ρ (kg/m3) is the density of flue gas under standard conditions; Vk (m3/h) is the exhaust capacity of air compressor; R (kJ/kg·K) is gas constant; T (K) is ambient temperature; P1 and P2 (MPa) refer to the inlet pressure and operation pressure; ηT and ηM are isothermal efficiency and mechanical efficiency of air compressor. Thus, the compression power consumption is proportional to lnP2. High pressure enhances the NOx emission reduction but unfavors the economy 18


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of the system. In order to integrate the emission and economic factor, the ratio between the electricity consumption and the amount of NOx emission reduction (W/∆[NOx]) was used as an indicator for feasibility of pressurized denitrification system. Figure 10 shows this ratio of W/∆[NOx] at pressures up to 3.0MPa. This value increased gradually as the pressure increased, indicating that the electricity consumption increased faster than the amount of NOx emission reduction with the increasing pressure. It shoule be noted that there was a an inflection point at 2.6MPa. At the pressures below 2.6MPa, the increase rate of the ratio was lower than that at pressures above 2.6MPa. The ratio of W/∆[NOx] dramatically increased at pressure above 2.6MPa. Therefore, we consider the pressure of 2.6MPa as the optimized pressure. NOx outlet concentration and removal efficiency at an equilibrium were then studied at the optimized pressure, in an attempt to determine the residence time. 700 600 500 W/[NO]

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

Energy & Fuels

400 300 200 100 0 0.5

1.0

1.5

2.0

2.5

3.0

Pressure (MPa)

Figure 10 The electricity consumption per unit NOx emission reduction (W/[NOx]) at pressures up to 3.0MPa.

In addition to pressure, sufficient residence time of pressurized denitrification system should be also provided to achieve the emission standard of NOx. Figure 11 shows NOx outlet concentration and removal efficiency vs reaction time at the optimized pressure of 2.6MPa. At steady pressure phase, NOx outlet concentration significantly dropped along with the increasing removal efficiency. In China, the

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Energy & Fuels

Emission Standard of Air Pollutants for Boiler (GB 13274-2014) stipulates NOx emission limit of 300 mg/m3 for the newly-built coal-fired boiler. In developed countries, the requirements of NOx emission limit are relatively strict at 100~200mg/m3. To reach the NOx emission limit of 100 mg/m3, this pressurized denitrification system should achieve 94% of NOx reduction. Thus, when the reaction time increased to 223s, the process shows a feasibility of eliminating 94% of NOx with the emission concentration of 48 ppm (100 mg/m3) at the optimized pressure of 2.6MPa, which can meet the strict emission limit requirement.

223s, 94%

90

600

80 NOx concentration at 2.6 MPa NOx removal efficiency

400 200

223s, 48ppm (100mg/m3)

0

70 60

NOx removal efficiency (%)

100

800

NOx concentration (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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50 0

200

400

600

800 1000 1200 1400 1600

time/s

Figure 11 NOx outlet concentration and removal efficiency vs reaction time at the optimized pressure of 2.6MPa.

It is well known that traditional SCR technology for NOx removal requires high-efficient catalysts as well as flue gas treatment device. This work has shown that NOx impurities could be effectively removed in the existing CPU system of oxy-combustion plants. The condensers and compressors in CPU system offer high pressure and low temperature, which is in favor of NOx removal. Compared to SCR technology, the pressurized denitrification process in CPU provides the potential to reduce the total cost because gas cleaning units for denitrification are not required in oxy-fuel plants. In CO2 compression system, NOx removal efficiency was highly sensitive to the pressure. Although the increase in pressure promoted the formation of 20


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Energy & Fuels

soluble NOx species, it would contribute to economic cost and operating challenge on oxy-combustion power plants because high pressure increased the electricity consumption of CO2 compression system. Thus, optimal operating conditions are desirable to balance the required NO removal efficiency for CCS system and energy consumption. The optimization in this work provide an implication for the future design and operation of cost effective CO2 compression system in oxy-combustion plants.

4. Conclusions Investigations on oxy-fuel flue gas denitrification at elevated pressures were carried out through experimental methods and dynamic simulation. The separation of NOx impurities could be achieved in CO2 compression system at elevated pressures. It was found that 94% of reduction in NOx outlet concentration after contact with water was achieved at pressures up to 2.5MPa. The relevant reactions gave a pathway for NOx removal as nitric acid and nitrous acid. Based on the experimental data from the pressurized reaction system, two-staged compression process simulation model was established and well validated. Two equilibrium parameters, time to reach equilibrium and NOx outlet concentration at equilibrium were used to evaluate the influence of operation conditions. It was found that the two equilibrium parameters was highly sensitive to the pressure. There was a conflict that high pressure enhances the NOx emission reduction but unfavors the economy of the system. Taking account of efficiency and economic factor, an optimization of NOx removal in the investigated system was found at the pressure of 2.6MPa, with 94% of NOx removal at the residence time of 223s. These results would give some insights to further design and optimization of CO2 compression and purification units in oxy-fuel plants.

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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Zhang L.) Tel: +86 27 87542417. Fax: +86 27 87545526.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful for the financial support by the National Key R&D Program of China (2018YFB0605302), and the China Postdoctoral Science Foundation Funded Project (2018M632850). The authors also acknowledge the extended help from the Analytical and Testing Center of Huazhong University of Science and Technology.

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Experimental Investigation and Process Simulation of Oxy-fuel Flue

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