Performance Modeling of Integrated Chemical Looping Air Separation

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Performance modeling of integrated chemical looping air separation and IGCC with CO2 capture Yang Cao, Boshu He, Guangchao Ding, Liangbin Su, and Zhipeng Duan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01894 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 13, 2016

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Performance modeling of integrated chemical looping air separation and IGCC with CO2 capture Yang Caoa, ∗, Boshu Hea, b, c, Guangchao Dinga, Liangbin Sua, Zhipeng Duana, ∗ a

Institute of Combustion and Thermal Systems, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China b Beijing Key Laboratory of Powertrain for New Energy Vehicle, Beijing Jiaotong University, Beijing 100044, China c Department of Mechanical and Electrical Engineering, Haibin College of Beijing Jiaotong University, Huanghua 061199, Hebei Province, China

Abstract: A new power cycle system composed of chemical looping air separation (CLAS) and integrated gasification combined cycle (IGCC) with CO2 capture and storage (CCS) is presented and analyzed. Key issues of the CLAS-IGCC-CCS system and corresponding solutions are proposed and explained in this work. A conceptual CLAS-IGCC-CCS system is set up and its performance is modeled and validated. In the CLAS-IGCC-CCS system, the CLAS sub-system is used to supply O2 for the coal gasification in the IGCC sub-system and Mn2O3/Mn3O4 is selected as the suitable oxygen carrier. The temperature of the reduction reactor (TRR) should be kept at 850 o

C to obtain high cold gas efficiency (CGE) and high system efficiency (SE). The

oxygen to coal mass ratio (OTCR) and the water to coal mass ratio (WTCR) should be kept around 0.7 and 0.06, respectively, to achieve high CGE and SE. The clean syngas from coal gasification process then burns with air in the gas turbine combustor. The heat recovery boiler with three-pressure reheat is chosen to recover the waste heat from the gas turbine. Chemisorption using monoethanolamine (MEA) is used for CO2 capture in the CCS sub-system. With the optimized parameters, the CGE of 82.55% and the SE of 46.22% can be reached for the CLAS-IGCC-CCS system. Keywords: Chemical looping air separation (CLAS); Integrated gasification combined cycle (IGCC); CO2 capture and storage (CCS); Coal gasification; Gas turbine; Heat recovery steam generator (HRSG) ∗

Corresponding authors. Email: [email protected] (Y. Cao) Tel.: +86-10-5168-8542;

Email: [email protected] (Z.P. Duan) Tel.: +86-10-5168-8542; Fax: +86-10-5168-8404. 1

ACS Paragon Plus Environment

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1. INTRODUCTION Coal is the most abundant fossil fuel in the world.1,2 Direct combustion of coal leads to massive emissions of NOX, SOX, CO2, CO and dust, not only resulting in serious harm to the environment, but also causing enormous waste of resources.3 The integrated gasification combined cycle (IGCC) is generally believed to be a promising and efficient solution for these issues, because it is an advanced and environmentally friendly coal power generation system, combining gasification and gas-steam combined cycle to achieve the environmental benefits. In such a system, coal is converted to clean syngas mainly composed of H2 and CO, and the clean syngas is used in a topping Brayton cycle gas turbine and a bottoming Rankine cycle steam turbine.4-6 In the conventional IGCC process, oxygen is commonly produced through cryogenic air separation unit (CASU), adsorption and membrane air separation technologies. However, CASU is well known for its high energy consumption and large investment cost. Approximately 3-4% more energy penalty is caused for oxygen production by using CASU for the oxy-fuel process and the capital cost of CASU-based systems accounts for about 14% of the total oxy-fuel plant.7-12 Although the cost of adsorption and membrane air separation may be decreased by about 10-25%, fabrication, integration and maintenance will lead to high cost of these two methods. Moreover, compared with the cryogenic based system, the energy footprints of adsorption and membrane based air separation methods are not too low.9-14 In order to achieve low-cost and energy-extensive oxygen production, chemical looping air separation (CLAS), as a novel oxygen generation technique, has been proposed by Moghtaderi in recent years.15 The CLAS is expected to offset 1-3% of the energy penalty compared to CASU.16 This method separates the oxygen from the air using oxygen carriers through redox reactions. In the oxidation reactor, the reduced oxygen carriers capture oxygen from the air, as shown by reaction R1:

MexOy-2 + O2 (g) → MexOy

(R1) 2


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In the reduction reactor, the oxidized oxygen carriers can release oxygen under certain conditions, as shown by reaction R2：

MexOy → MexOy-2 + O2 (g)

(R2)

Steam is selected as the inert medium because it can promote the O2 releasing process by reducing the equilibrium partial pressure (EPP) of O2 and the mixture of steam and oxygen exiting from the reduction reactor can be separated easily. As for another issue of IGCC, CO2 capture and storage (CCS) is an effective way to reduce CO2 emissions and has been considered as an important strategy for stabilizing CO2 concentration in the atmosphere.17-20 Among the alternatives for CO2 capture, chemical absorption with amine aqueous solution is demonstrated as one of the most mature and less expensive technology to be applied to power plants. Chemisorption using monoethanolamine (MEA) is an available technology for removing CO2 from flue gas stream with low concentration of CO2. The main advantages of using MEA/water solution are its high CO2 reactivity, high limit load (0.5 mole of CO2 per mole MEA) and low molecular weight. Its main drawback is the stability of the carbamate ion that makes the regeneration more heat demanding. The main reactions that take place in aqueous system of amine and CO2 are:

2H 2 O ↔ H 3O + +OH -

(R3)

CO 2 + 2H 2 O ↔ H 3O + + HCO 3−

(R4)

HCO 3− +H 2 O ↔ H 3O + + CO 32-

(R5)

2MEA + CO 2 ↔ MEAH + + MEACOO −

(R6)

MEA + + H 2 O ↔ MEA + H 3 O +

(R7)

MEACOO − + H 2 O ↔ MEA + HCO 3−

(R8)

In light of the advantages of CLAS and IGCC, a new integration of CLAS and IGCC with CO2 inherent separation by application of MEA sorption enhanced technology is presented and the characteristics of CLAS-IGCC-CCS system are analyzed. In this work, key issues of CLAS-IGCC-CCS system are explained firstly 3


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and proper solutions are proposed. Then, a detailed integrated CLAS-IGCC-CCS system is set up with Aspen Plus.21 With the optimized operation conditions and parameters determined, the sensitivity analyses for the whole CLAS-IGCC-CCS system are investigated.

2. THERMODYNAMIC MODELING 2.1. Overview of CLAS-IGCC-CCS system 2.1.1. Processes in CLAS-IGCC-CCS system The layout of the newly developed CLAS-IGCC-CCS system is shown schematically in Figure 1. The main processes proceeded in the CLAS-IGCC-CCS system include: (1) Oxygen carrier is oxidized by air in the oxidation reactor; (2) The oxidized oxygen carrier releases O2 in the reduction reactor under the promotion action of steam; (3) Separation by condensation method is used to obtain pure oxygen and then the pure oxygen is pressurized in order to enter the coal gasifier; (4) Coal is gasified in the gasifier with O2 and a small amount of water vapor to generate raw syngas rich in H2 and CO; (5) The raw syngas is cleaned in a deduster to remove dust; (6) The syngas then enters desulfurizer to remove H2S and COS; (7) The cleaned syngas is sent to the combustion chamber and reacts with air; (8) The air is compressed via a compressor, and about 83% of the air enters the combustion chamber, while the rest is the cooling air; (9) The product from the combustion chamber mixes with one part of the cooling air and then enters the turbine to do work; (10) The turbine exhaust is discharged after mixing with the other part of the cooling air; (11) The turbine exhaust enters the heat recovery steam generator (HRSG) and the feedwater is heated; (12) The exhausted steam from the steam turbine finally enters the condenser; (13) The flue gas from HRSG is then absorbed and compressed for CO2 capture. The reaction taking place in the reduction reactor is R9: 4


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6 Mn 2O3 = 4 Mn 3O4 + O2

(R9)

The reaction taking place in the oxidation reactor is R10:

4 Mn3O4 + O2 = 6 Mn 2O3

(R10)

The dominated reactions taking place in the coal gasifier are R11-R15:

2 C + O2 = 2 CO,

∆H0298 = 110.4 kJ/mol

C + O2 = CO2 , ∆H0298 = 394.1 kJ/mol C + CO2 = 2 CO,

∆H0298 = −173.3 kJ/mol

CO + H2O = CO2 + H2 , C + H2O = CO + H2 ,

(R11) (R12) (R13)

∆H0298 = 38.4 kJ/mol

(R14)

∆H0298 = −135.0 kJ/mol

(R15)

Figure 1. Schematic of CLAS-IGCC-CCS for process simulation.

2.1.2. Key issues of CLAS-IGCC-CCS system One key issue that needs to be studied is the large-scale production of oxygen for 5


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CLAS sub-system. One solution is the selection of suitable oxygen carriers. Candidate oxygen carriers are selected according to the criteria such as the melting point, the heat capacity, the mechanical strength, the chemical stability, the price, the environmental impact, and the standard reaction Gibbs energy change.22-24 The other solution is the design of stable and safe reactors. The reactors arranged in chemical looping combustion, CLC, which have been investigated a lot, can be transformed into CLAS reactors easily. The interconnected fluidized bed reactor is proposed for CLC sub-system, which is the basis for the study of CLAS sub-system, and is found to be highly stable and efficient.25-27 Another key issue is that the operation cost of IGCC-CCS sub-system should be reduced. Using alternative feed stock rather than coal could reduce the cost of IGCC-CCS plants.28,29 A higher percentage of low cost raw materials, such as coke, biomass, and so on, is provided to further reduce the cost of the IGCC-CCS system. Furthermore, a structured exergy analysis is significant to analyze where the large exergy losses occurred in the IGCC-CCS and to which extent they can be prevented.30 Hence, the optimizations of units with larger exergy losses will have more significance effect on improving the overall IGCC-CCS plant performance.

2.2. Model validation for the main parts of CLAS-IGCC-CCS system The CLAS-IGCC-CCS system mainly consists of six components, or six sub-systems: a CLAS, a gasifier, a cleaning unit including one deduster and one desulfurizer, a gas turbine, a HRSG and steam turbine (ST) sub-system, and a CCS unit. Therefore, the operation conditions and parameters of gasifier, gas turbine, and HRSG-ST are important to the whole system. Thus, model validations and sensitivity analyses should be done for these sub-systems prior to the integration research on the CLAS-IGCC-CCS system. 6


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2.2.1. Thermodynamics analysis The Gibbs energy change is an important parameter in thermodynamics, which is used as a criterion for a chemical reaction at a specified temperature and pressure. A chemical reaction at a specified temperature and pressure proceeds in such a direction that tends to decrease the Gibbs energy. The chemical equilibrium is established when the Gibbs energy reaches the minimum value. The Gibbs energy change commonly can be expressed by ∆G. For an independent reaction system, the ∆G can be calculated with Eq. (1).

∆G = ∑ ( ni ∆Gi,f )product − ∑ ( ni ∆Gi,f )reactant

(1)

Equation (1) is a measure of the thermodynamic driving force that makes the reaction proceed.31

∑ ( n ∆G ) i

i,f product

and

∑ ( n ∆G ) i

i,f reactant

are the total Gibbs

energies of the product and the reactant, respectively. For reaction R2, the decomposition condition of the oxygen carrier can be expressed by Eq. (2).

∆G ≤ 0

(2)

EPP of O2, PO2 , is an important factor that can determine the reduction reaction direction at a certain temperature. According to Le Chatelier’s principle, if the equilibrium of a reaction is destroyed by the change of EPP, the system will move to a new equilibrium. EPP of O2 can be calculated with Eqs. (3) and (4).

 −∆G  K p = exp    RT 

(3)

PO2 = Kp

(4)

where, Kp denotes the ideal gas chemical equilibrium constant; and R denotes the ideal gas constant; T denotes the absolute temperature. 7


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2.2.2. Validation for the coal gasifier model The coal gasifier model based on the Aspen Plus platform is shown in Figure 2. Table 1 shows the analytical parameters of coal used in the simulation and boundary conditions are shown in Table 2. Detailed operation conditions can be found in the literature.32 Comparisons of the simulation results and the experiment data are shown in Table 3. The simulation results are found to agree well with the experiment data. The justifiable reason for the larger error of H2O was explained by the previous article.33 The reasons can be explained as follows：

The gasifier model is supposed to be continuously stirred tank reactors with perfect mixing, which indicates that the temperature in the gasifier is homogeneous and steam condensation cannot be modeled in the cold section of the gasifier. However, the parameters in a practical gasifier are not homogeneous.

H2O is treated as ideal gas, which is not true for the practical operation.

After the validation, the sensitivity analysis is done to investigate the effects of reaction temperature T, pressure p, oxygen to coal mass ratio (OTCR) and water to coal mass ratio (WTCR) on the coal gasification performance. The objective of parameter optimization for the coal gasification is that the mole fractions of H2 and CO should be as high as possible. Thus, in order to achieve this goal, the optimal operating conditions, (T, p, OTCR, WTCR)≈(1315 oC, 2.4 MPa, 0.75, 0.06), are suggested based on the sensitivity analysis.

Q Q

Q-DECOMP EX

WET -COAL

IN-DRY

DRY-COAL

IN-GASIF

PRODUCTS

N2 DRY-REAC

DRY-FLSH

DECOMP

GASIF IER

O2

Figure 2. The flowchart for coal gasification process. 8


H2O

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Table 1. Proximate and ultimate analyses of the coal used in the simulation Approximate analysis (%)

Ultimate analysis (%)

Mad

Vad

Cad

Aad

C

H

O

N

S

12

8.8

42.85

36.35

61.2

4.7

8.8

1.1

3.4

Table 2. Simulation conditions for coal gasifier Parameter

value

O2/coal（kg / kg） H2O/coal（kg / kg） N2/coal（kg / kg） Pressure（kPa）

0.80 0.08 0.13 2431

Table 3. Comparisons of simulation results and experiment data for a coal gasifier Gas composition (mol%)

CO

CO2

H2

N2

H2O

COS

CH4

H2S

This work

60.69

0.99

29.63

5.44

1.94

0.11

0.01

0.19

61.5

1.6

30.6

4.7

-

0.1

0.0

1.2

32

Experimental data

2.2.3. Validation for the gas turbine model The gas turbine model set up with Aspen Plus is shown in Figure 3. This gas turbine works under standard conditions and the atmospheric parameters with temperature of 15 oC, pressure of 101.325 kPa, and relative humidity of 60%, respectively. The methane and air flow rates required for combustion and the heat loss of combustion chamber need to be determined. Also, the expansion ratio and isentropic efficiency of turbine will be computed for Aspen Plus inputting. Detailed operational formulas can be found in the literature.34 Comparisons of simulation values and design values for a gas turbine using methane as the fuel are shown in Table 4. The simulated unit net power of the gas turbine and the turbine inlet temperature agree quite well with the designed parameters, indicating that the calculation method and the established model are reasonable. In this work, using clean syngas as fuel, based on the operating parameters with methane, the detailed simulation and analysis are done. Because the change of turbine inlet temperature has a great influence on the unit power and the efficiency, the turbine inlet temperature and the rotation speed are maintained the same when using 9


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the clean syngas as the fuel. Then, the gas turbine exhaust enters the HRSG to heat the water. Although the model of the HRSG-ST is not compared with the existing power plant, the efficiency of HRSG-ST is 36.31%, showing that the model is reasonable.

W W

Q

MIXER2

QLOSS T URB IN-AIR

WORKTURB

WORKCOMP

EXHAUST COMBUST FUEL

COMP OUT-TURB IN-TURB COMB-OUT AIR1 OUT-AIR

MIXER1

FSPLIT COOLER AIR2

AIR2-OUT AIR3

Figure 3. The gas turbine flowchart. Table 4. Comparisons of simulation values and design values for a gas turbine using methane Parameter Rotation speed (r/min) Fuel heating value (kJ/kg) Fuel flow rate (kg/s) Air flow rate (kg/s) Compressor pressure ratio Compressor isentropic efficiency (%) Compressor input power (MW) Cooling air flow rate (kg/s) Efficiency of the combustion chamber (%) Combustion heat loss (MW) Heat balance temperature (oC) Turbine output power (MW) Gas turbine inlet temperature (oC) Turbine expansion ratio Turbine isentropic efficiency (%) Turbine exhaust temperature (oC) Unit net power (MW) Unit efficiency (%)

Simulation value

Design value

3000.00 50030.00 14.286 651.00 17.00 89.00 267.61 104.16 99.00 7.15 1245 539.14 1401.64 16.33 91.00 584.24 271.53 37.99

3000.00 651.00 17.00 89.00 1245 1400.00 586.00 270.30 38.20

2.3. Models and methods

In this work, the models of CLAS-IGCC-CCS system are set up using the Aspen 10


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Plus software. The most important unit operations represented by Aspen Plus models are shown in Table 5. Then, the process integration and sensitivity analysis are applied in order to improve the system efficiency (SE). The cold gas efficiency (CGE) is the performance of the gasification section, expressed by Eq. (5). However, the system efficiency is an indicator of efficiency of the overall process, presented in Eq. (6).

CGE =

SE =

Qsyngas

(5)

mcoal,ar × LHVar

Wnet + Qrecoverable mcoal,ar × LHVar

(6)

where, Qsyngas denotes the clean syngas low heating value; mcoal,ar denotes the feed coal mass flow rate; LHVar denotes the coal low heating value; Wnet denotes the power output of whole CLAS-IGCC-CCS system; and Qrecoverable denotes the recovered sensible heat.

Table 5. Representative unit operations used in the simulation of the CLAS-IGCC-CCS system Unit operation

Aspen Plus model

Comments/specifications

Redox of oxygen carriers

RGibbs

Coal drying

RStoic

Coal gasification

RGibbs

Cleaner

SSplit, Sep

Syngas combustor

RGibbs

Gas and steam turbines HRSG

Compr MHeatX

CO2 capture

RadFrac

Rigorous reaction and multiphase equilibrium based on Gibbs free energy minimization Stoichiometric reactor based on known fractional conversions or extents of reaction Calculation based on phase equilibrium and chemical equilibrium Simplified simulation of gas/solid and gas/gas separation by fixed split fraction specification All components may appear in the product stream Calculate power produced Simulation of the steam cycle with heat recovery of the gas turbine exhaust Rigorous simulation of CO2 absorption and regeneration

11


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2.4. Integration of the CLAS-IGCC-CCS system According to the schematic shown in Figure 1, the flowchart for the newly developed CLAS-IGCC-CCS system is proposed and established as shown in Figure 4 based on the validated models with the assistance of Aspen Plus commercial software. The CLAS-IGCC-CCS system consists of six components, or six sub-systems, including a CLAS, a gasifier, a cleaning unit, a gas turbine, a HRSG-ST, and a CCS unit. The main design assumptions used in the modeling and simulation are presented in Table 6. In the CLAS sub-system, the heat generated during oxidation reaction is assumed to be equal to that required during reduction. The cold fresh air (F-AIR stream) is preheated to 175 oC by depleted air (AR-AIR stream) before entering the oxidation reactor (AR module), where the oxygen carrier is oxidized by the O2 in the air and the process is adiabatic. The oxidized oxygen carrier is separated from the depleted air by a separator (SPLIT module) and sent to the reduction reactor (RR module). In the same way, the water is heated by the gaseous products of reduction reaction to 300 oC before entering the reduction reactor. In order to produce more O2, the oxidized oxygen carrier releases O2 in reduction reactor under the condition of 850 oC and 1 bar. The pure oxygen is obtained after the steam is condensed and separated. The water required by the gasifier is heated to 150 oC by part of the heat absorbed from the COOLER1. In this work, Mn2O3/Mn3O4 is used as the oxygen carrier, exhibiting a high reaction rate under mild conditions.31 Through the study of the sensitivity analysis of CLAS, the results show that when the mole flow rate of oxygen carrier is 5 kmol/s, the O2 production is 0.83 kmol/s, and the required mole flow rate of fresh air is 11.11 kmol/s. In the gasifier sub-system, an operation pressure of 2.4 MPa is chosen for the gasifier.. In order to inject O2 to the gasifier, the pressure of O2-IN must be a little higher than 2.4 MPa and a value of 2.5 MPa is chosen. A heat loss of 2.6% of the heating value of the coal is assumed. The raw syngas from the gasifier to the shock chamber (MIXER1 module) is cooled to 900 oC by mixing with raw syngas 12


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(MIXER-IN stream) after cooling and dedusting. The stream GASES is cooled to 340 o

C and 35% of its sensible heat is recovered. After dedusting, the syngas is divided

into two streams, one returns (SYNGAS2 stream) to the shock chamber and the other (SYNGAS1 stream) enters the desulfurization process after cooled to 150 oC. Similarly, 35% of the sensible heat is recovered. The sulfur recovery after the desulfurization is not simulated in this work. In the gas turbine sub-system, the clean gas burns with air in the gas turbine combustor (COMBUST module). The combustion efficiency is assumed to be 98%. The gas turbine exhaust enters HRSG (MX module) to heat the water. The three-pressure reheat type for HRSG is chosen according to the gas turbine exhaust temperature.34 The steam inlet temperatures and pressures for the high pressure steam turbine (HTURB module), the intermediate pressure steam turbine (MTURB module) and the low pressure steam turbine (LTURB module) are set as 540 oC /11 MPa, 540 o

C /2.2 MPa and 280 oC /0.6 MPa, respectively. In this work, the simulation result of

the gas turbine exhaust temperature is 610.36 oC and that of the HRSG exhaust temperature is 131.73 oC. In the CCS sub-system, the flue gas is scrubbed with MEA, an amine-based solvent in an absorption column (ABSOR module). The MEA solvent is then regenerated in a distillation column (REGEN module), thereby, releasing a high purity CO2 product (CO2-GAS stream). The 30% by weight of MEA in water is set and the CO2 removal system is assumed to separate 90% of the CO2 contained in the flue gas. A four stage intercooled compression is used to obtain liquefied CO2.

13


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AR-AIR

F-AIR

HeatX

AIR-IN RGibbs

HEATX1 HOT-OUT

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STEAM-IN SSplit

AR-OUT

Mn2O3

SSplit

RR-OUT

RGibbs

SPLIT1

AR

Q-LOSS

RRGAS1

HeatX

SPLIT2

RR

RRGAS2

Gasifier

SEP1

Mn3O4

Q-RYIELD

CLAS

H2O-IN Compr

O2-IN

IN-DRY

O2 COMPR-O2

EX

RStoic

Sep

COOLER1

W1

WET-COAL N2

RRGAS3

HEATX2

Flash2

DRY-COAL

INGASIFIER

RYield

RGibbs

DRY-REAC DRY-FLSH

DECOMP

Mixer

PRODUCTS

SSplit

IN-SEP

MIXER1

GASIFIER

Q-LOSS

GASES

FSplit

SYNGAS

SPLIT3 SOLID

COOLER2

FSPLIT1

Q-LOSS Q-LOSS

SYNGAS3

Sep

CLEAN GAS

SYNGAS2

SEP2

W2

RE-S

Compr SYNGAS1

Deduster

COOLER3

COMPR-GAS

MIXER-IN

Desulfurizer COMB-OUT IN-AIR FSplit

OUT-AIR

Compr

RGibbs

AIR1

AIR2-OUT COMBUST

FSPLIT2

COMPR-AIR W3

Mixer

COOLER4

AIR2

IN-TURB

OUT-TURB

Mixer MIXER3

TURB

Q-LOSS W4

Gas turbine

AIR3 Q-LOSS

FSplit

L-IN

WATER

FSPLIT3

L-Turb

L-OUT

M-OUT

EXHAUST

M-Turb

LTURB W7

COOLER5 L-WATER

MHeatX

M-IN

MTURB W6

H-IN

H-Turb

MX H-OUT

HTURB W5

FLUE GAS

HRSG-ST

H-WATER H-MEA

F- GAS

Q T-GAS

MAKEUP H-LMEA

HeatX

L-LMEA

Flash2

W8

P1 FLASH-1

Mixer

Compr

LMEAH

P1C

H2O-P1

R-MEA

HEATER P-MEA

REGEN

W10 Q3

Q2

P1H Flash2

P2

Compr

FLASH-2 COOLER-1

P2C

COMP-2

P2H COOLER-2

Flash2

P3 FLASH-3

H2O-P3

H2O-P2

Q4 Flash2

P4

Compr

P4C

FLASH-4

COMP-4

H2O-P4

W11

COOLER6 Q-LOSS

PUMP

W9

Q1

COMP-1

LMEA

MIXER4

HEATX3

ABSOR CO2-GAS

CCS

Turb

MIXER2

CO2-LIQ COOLER-4

Figure 4. The CLAS-IGCC-CCS system flowchart.

14


Compr

P3C

COMP-3

COOLER-3

P3H

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Energy & Fuels

Table 6. Main design assumptions Unit

Parameter

Unit

Value

Chemical looping air separation (CLAS)

Steam/Mn2O3 ratio Operation pressure Oxidizing/reduction temperature Coal drying: 2% of input coal LHV Oxygen/solid fuel ratio Steam/solid fuel ratio Gasification pressure Heat loss: 2.6% of input coal LHV Carbon conversion Pressure drop Syngas temperature after quench Overall H2S and COS removal yield Ambient temperature & pressure Compressor pressure ratio Mechanical efficiency Combustion chamber efficiency Gas turbine inlet temperature Inlet pressure loss coefficient Combustion chamber pressure loss coefficient Exhaust pipe pressure loss coefficient Three pressure levels (HP/MP/LP) MP steam reheat Reheat temperature Steam turbine isoentropic efficiency Condensation pressure CO2 capture efficiency CO2 product final pressure

kg/kg bar o C

0.0097 1 830-880

kg/kg kg/kg bar

0.60-0.90 0.03-0.09 24

% bar o C % o C, kPa

99.9 1.5 340 99.9 15, 101.3 17 98.5 98 1400 1.0101 1.0204 1.0101 110/22/6

Gasifier

Cleaner Gas turbine

Heat recovery steam generator and steam turbine (HRSG-ST)

CO2 capture and storage (CCS)

% % o C

bar o

C % bar % bar

540 90 0.05 90 100

3. RESULTS AND DISCUSSIONS With the integrated CLAS-IGCC-CCS model and the determined parameters, a sensitivity analysis on the system performance is implemented. The effects of OTCR and WTCR on the cold gas efficiency and the system efficiency are analyzed. The temperature of the reduction reactor (TRR) has impacts on the oxygen production and further on the system performance. Therefore, the effects of TRR on CGE and SE are studied subsequently. 15


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

Effects of OTCR on the raw syngas mole fraction, the CGE and the SE are shown in Figures 5 and 6, respectively. The mole fraction of H2+CO is found the largest, from Figure 5, when OTCR is at 0.75. When OTCR is larger than 0.75, the mole fraction of H2+CO decreases as OTCR increases. This is because the combustion reactions proceed strongly in oxygen-rich atmosphere. The temperature in the coal gasifier rises rapidly, which results in declining the mole fraction of H2+CO and decreasing dramatically that of CGE. Different from the mole fraction of H2+CO, the

CGE has the maximum value when OTCR is at 0.7 as shown in Figure 6. The reason is that the mole fraction of CH4 significantly decreases and the mole heating value of CH4 is higher than that of H2. The trend of CGE is consistent with that of SE. The decrease of CGE indicates the decrease of LHV of the produced syngas and the reduction of the net work output from the gas turbine, which leads to the decrease of SE of the CLAS-IGCC-CCS system. When OTCR is less than 0.7, the CGE and SE values increase with the increase of OTCR. However, when OTCR is larger than 0.7, inverse changing trends of CGE and SE are detected. On the one hand, the gas turbine net power decreases with the CGE decrease. On the other hand, more O2 will be needed in the gasifier with the OTCR increment and then the amount of O2 will be produced from the CLAS sub-system. But, O2 will be pressurized via the compressor before being sent to the gasifier, causing the power consumption to increase. Therefore, the SE of the CLAS-IGCC-CCS system decreases when OTCR is over 0.7, as shown in Figure 6.

16


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Page 17 of 25

1.0

0.8

Mole fraction

0.6

H2 CO CO2 CH4 H2O H2+CO

0.4

0.2

0.0 0.60

0.65

0.70

0.75

0.80

0.85

0.90

OTCR Figure 5. Effects of OTCR on raw syngas mole fraction.

1.0

0.50

0.8

0.48

0.6

0.46

SE

CGE

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

0.4

0.44 CGE SE

0.2

0.42

0.0

0.40 0.60

0.65

0.70

0.75

0.80

0.85

0.90

OTCR Figure 6. Effects of OTCR on CGE and SE of CLAS-IGCC-CCS system.

Effects of WTCR on the raw syngas mole fraction, the CGE and the SE are shown in Figures 7 and 8, respectively. When WTCR is small, the water gas shift 17


Energy & Fuels

reaction R15 is inhibited due to the high temperature of gasifier, leading to the higher mole fraction of CO. The mole fraction of H2 increases while that of CO decreases slightly as the WTCR increases. This is because the forward water gas shift reaction is enhanced. The mole fraction of H2+CO has a maximum value when WTCR is at 0.06 as shown in Figure 7. The CGE and the SE increase at the beginning and then decrease as the WTCR increases, although the variation is slight as shown in Figure 8. This is because the carbon is not completely converted when WTCR is under 0.06. More steam will reduce the temperature of gasifier and lead to the decrease of the

CGE. At the same time, the decrease of the CGE indicates that the syngas with lower LHV will be produced. Thus, the gas turbine net power decreases with the decrease of LHV of syngas, leading to the SE slight decrease of the CLAS-IGCC-CCS system.

1.0

0.8

Mole fraction

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

Page 18 of 25

0.6 H2 CO CO2

0.4

CH4 H2O H2+CO

0.2

0.0 0.03

0.04

0.05

0.06

0.07

0.08

WTCR Figure 7. Effects of WTCR on raw syngas mole fraction.

18


0.09

Page 19 of 25

1.0

0.50

0.48 0.8 0.46 0.6

SE

CGE

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

0.44 CGE SE

0.4

0.42

0.2

0.40 0.03

0.04

0.05

0.06

0.07

0.08

0.09

WTCR Figure 8. Effects of WTCR on CGE and SE of CLAS-IGCC-CCS system.

Effects of TRR on the raw syngas mole fraction, the CGE and the SE are shown in Figures 9 and 10, respectively, which indicate that the optimum TRR is about 850 o

C. This is because less oxygen yield causes incomplete conversion of coal when TRR

is lower than 850 oC. However, when TRR is higher than 850 oC, the oxygen produced via CLAS is increased. The carbon in coal is almost thoroughly converted. Therefore, increasing TRR does not contribute to the increase of CGE and the gas turbine net power. Thus, the CGE and the SE remain unchanged when TRR is over 850 oC.

19


Energy & Fuels

1.0

Mole fraction

0.8

0.6

H2 CO CO2 CH4 H2O H2+CO

0.4

0.2

0.0 830

840

850

860

870

880

TRR (°C) Figure 9. Effects of TRR on raw syngas mole fraction.

1.0

0.5

0.8 0.4

SE

0.6

CGE

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

Page 20 of 25

CGE

0.4

SE

0.3 0.2

0.0 820

830

840

850

860

870

880

0.2 890

TRR (°C) Figure 10. Effects of TRR on CGE and SE of CLAS-IGCC-CCS system.

20


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Energy & Fuels

4. CONCLUSION For the integrated CLAS-IGCC-CCS system, the key issues of the system and the corresponding solutions are presented and analyzed. In addition, a detailed CLAS-IGCC-CCS system is set up and modeled and sensitivity analysis is done in this work. The conclusions drawn from this research can be summarized as follows: 1) The selection of suitable oxygen carriers and the design of stable and safe rectors are proposed to serve as CLAS for large-scale oxygen production. The interconnected fluidized bed reactor is suggested for the best arrangement. Exergy analysis method is recommended as the basis for the IGCC-CCS retrofit. 2) Based on the Gibbs energy minimization principle, the CLAS is simulated thermodynamically. Mn2O3/Mn3O4 is selected as the suitable oxygen carrier. Steam is chosen as the inert medium to promote the process of O2 releasing through reducing the equilibrium partial pressure of O2. When TRR is 850 oC, the mole flow rate of oxygen carrier, Mn2O3, is 5 kmol/s, the mole flow rate of O2 production is 0.83 kmol/s, while those of the required fresh air is 11.11 kmol/s and the steam is 0.43 kmol/s, respectively. 3) In order to get high carbon conversion in the coal gasifier and high mole fraction of H2+CO, the gasification temperature and pressure, the OTCR and the

WTCR should be kept at around 1315 oC, 2.4 MPa, 0.75 and 0.06, respectively. 4) For the integration of the whole CLAS-IGCC-CCS system, the effects of key operation parameters on the system characteristics are assessed. The OTCR and the

WTCR should be kept at 0.7 and 0.06, respectively, to achieve high cold gas efficiency and high system efficiency. The TRR of reduction reactor should be kept at 850 oC so as to supply enough O2 to the gasifier and get high cold gas efficiency and high system efficiency. With these parameters, 82.55% of CGE and 46.22% of

SE of the CLAS-IGCC-CCS system can be reached.

21


Energy & Fuels

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AUTHOR INFORMATION

Corresponding Authors * E-mail: [email protected] (Z.P. Duan). Telephone: +86-10-5168-8542. Fax: +86-10-5168-8404. E-mail: [email protected] (Y. Cao). Telephone: +86-10-5168-8542.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (NSFC, 51576014 and 51576013).

REFERENCES

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Performance Modeling of Integrated Chemical Looping Air Separation

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