Scholarly article on topic 'How gas separation membrane competes with chemical absorption in postcombustion capture'

How gas separation membrane competes with chemical absorption in postcombustion capture Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Li Zhao, Ernst Riensche, Ludger Blum, Detlef Stolten

Abstract This paper describes an investigation for multi-stage systems used in coal-fired power plant. The whole work was divided into two steps: energetic and economic analyses. In the first step: on the basis of a serial concept, through varying the position of compressors and vacuum pumps, recycling the retentate of the 2nd membrane to the feed side of the 1st membrane, a cascade variant was developed and analysed. In the second step: an economic model was developed to calculate the capture cost of the cascade system. The total cost is composed of investment cost, operation and maintenance (O&M) cost and electricity cost. A correlation between the membrane parameters: selectivity & permeability and capture performance: energy consumption & capture cost was built up. Using Polyactive® membrane developed by GKSS with CO2 permeance of 3 Nm3/m2hbar and CO2/N2 selectivity of 50, under the separation target of 70% degree of CO2 separation and 95 mol% CO2 purity, adopting the cascade membrane system in the 600 MW NRW-reference power plant, the specific energy consumption including CO2 compression (110 bar, 30 °C) is 256 kWh/tseparatedCO2 with 6.4%-pts efficiency loss. The capture cost is 31 euro/tseparatedCO2 , which could be a promising solution as a retrofit for the existing power plants.

Academic research paper on topic "How gas separation membrane competes with chemical absorption in postcombustion capture"

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Energy Procedia 4 (2011) 629-636 :

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How gas separation membrane competes with chemical absorption in postcombustion capture

Li Zhao 1*, Emst Riensche, Ludger Blum, Detlef Stolten

Institute of Energy Research - Fuel Cells (IEF-3), Jülich Forschungszentrum GmbH

D-52425 Jülich, Germany

Abstract

This paper describes an investigation for multi-stage systems used in coal-fired power plant. The whole work was divided into two steps: energetic and economic analyses. In the first step: on the basis of a serial concept, through varying the position of compressors and vacuum pumps, recycling the retentate of the 2nd membrane to the feed side of the 1st membrane, a cascade variant was developed and analysed. In the second step: an economic model was developed to calculate the capture cost of the cascade system. The total cost is composed of investment cost, operation and maintenance (O&M) cost and electricity cost. A correlation between the membrane parameters: selectivity & permeability and capture performance: energy consumption & capture cost was built up. Using Polyactive® membrane developed by GKSS with CO2 permeance of 3 Nm3/m2hbar and CO2/N2 selectivity of 50, under the separation target of 70% degree of CO2 separation and 95 mol% CO2 purity, adopting the cascade membrane system in the 600 MW NRW-reference power plant, the specific energy consumption including CO2 compression (110 bar, 30°C) is 256 kWh/tseparated CO2 with 6.4%-pts efficiency loss. The capture cost is 31 euro/tseparated CO2, which could be a promising solution as a retrofit for the existing power plants.

© 2011 Published by Elsevier Ltd.

Keywords: post-combustion, gas separation membrane, multi-stage, energy consumption, economic analysis

1. Introduction

Although the chemical absorption method occupies a leading position in R&D on postcombustion with CCS [1], it has several inherent weaknesses: a) degradation of the solvent owing to the influence of the SO2 and NOx in flue gas and b) high energy consumption for the solvent regeneration process. As a technology competing with chemical absorption, the CO2/N2 gas separation membrane process for post-combustion capture is attracting more and more attention around the world. In comparison with the above mentioned weaknesses of chemical absorption, CO2 gas separation membranes possess the following advantages: a) less environmental impact; b)

* Corresponding author. Tel.: +49-2461-614064; fax: +49-2461-616695. E-mail address: l.zhao@fz-juelich.de.

ELSEVIER

doi:10.1016/j.egypro.2011.01.098

can be designed as turnkey CO2 separation equipment both for new-build power plants and as a retrofit for existing power plants. These are eminently important properties of the gas separation membrane process distinguishing it from the other post-combustion capture technologies.

Gas separation membranes used for post-combustion capture have been investigated by several groups independently [2-8]. In the present paper, multi-stage membrane processes are investigated in two steps: a) energy consumption and b) capture cost analyses. In the first step, by varying the position of compressors and vacuum pumps recycling the flue gas to the feed side, a cascade variant was developed and analyzed in detail. The cascade system was integrated with the 600 MW North Rhine-Westphalia reference power plant and compared with the chemical absorption process. In the second step, an economic analysis process was explored for a cascade membrane system for use in coal-fired power plant. A cost model was developed to make a further analysis of the cascade variant in view of the correlation between membrane parameters (selectivity, permeability) and system performance (energy consumption, capture cost).

In view of the R&D situation of the CO2/N2 gas separation membrane, the properties of Polyactive® polymer membranes with CO2 permeance of 3 Nm3/m2hbar and CO2/N2 selectivity of 50 developed by GKSS, Germany [9], are used here. The PRO/II software (Simulation Science Inc.) was used for the simulation.

2. Investigation strategy

The investigation strategy is illustrated in Fig. 1. The whole simulation process is divided into two steps. In the first step, the influence of membrane parameters on membrane area to achieve the specified target is analyzed. This is combined with the evaluation of various process parameters, components and membrane arrangements. Membrane area and energy consumption are the two outputs of the analyses presented in this paper. In the second step, the effect of the variation of these parameters on the capture cost will be analyzed, especially the correlation between the membrane parameters (selectivity and permeability) and the capture performance (energy consumption and capture cost).

Step 1 Step 2

Fig. 1

Outputs

Investigation strategy for multi-stage gas separation membrane systems

3. Reference power plant and simulation method

In the present work, a reference power plant termed the Reference Power Plant North Rhine-Westphalia (RKW-NRW) [10] was chosen for the analyses. The multi-stage polymer membranes should be installed after the SCR-DeNOx, dust removal (E-filter) and desulphurization (FGD) processes and prior to emissions passing through the cooling tower, analogous to amine stripping processes [11, 12].

The hard coal grade "Klein Kopje" was used to simulate the flow rate and the components of the flue gas for the multi-stage membrane calculation. The element analysis data of Klein Kopje coal are: C 65.5%, H 3.5 %, O 7.4%, N 1.5%, S 0.6%, ash 14.2%, moisture 7.3%; and the heat value is 25 MJ/kg. The coefficient of air excess (air-to-fuel ratio) was assumed to be 1.15. The basic data of RKW-NRW and the simulation results of the flue gas are listed in Table 1. The residue of the pollutant in the flue gas consists of approximately 50 vppm SO2 and approximately 200 ppm NO2.

The PRO/II (Simulation Science Inc.) software was used for the simulation. Different thermodynamic models for the energy balance calculation are available in PRO/II; for the case described here the Soave-Redlich-Kwong equation of state was chosen. The adiabatic efficiency of the compressors, expanders and vacuum pumps is assumed to be 85%. A detailed description of the membrane module in the PRO/II software was given in our previous paper [12]. A binary flue gas system - 14 mol% CO2 and 86 mol% N2 was simulated.

Table 1 RKW-NRW power plant basic data [10] and simulation results of the flue gas conditions after removal of the pollutants using Klein Kopje hard coal

Power plant RKW-NRW

Output gross 600 MW

Output net 555 MW

Net efficiency 45.9 %

Steam parameters 285 bar/ 600°C / 620°C

Operation time 6000 h/year

Fuel input 1.0 Mt/year*

Investment costs 517.1 million euro

O & M costs 7.8 million euro/year

Fuel costs 41 euro/t

Electricity price 3.37 cent/kWh

Flue gas conditions after removal of the pollutants

Pressure 1.05 bar

Temperature 50 °C

Flow rate 1.6 million m3/h*

Main components

CO2 13.5 mol%*

N2 70.1 mol%*

o2 3.7 mol%*

H2O 11.9 mol%*

Ar 0.8 mol%*

* simulated by PRO/II

4. Energetic analysis

4.1. Variations with compressor and vacuum pump

On the basis of the concepts of enricher and stripper, a matrix plan was developed for the probable arrangements of compressor and vacuum pump [13]. It is known that the enricher concept contributes to a higher CO2 purity with a common separation degree than the stripper concept. In this paper, four variants V1-V4 (V1-V2: 1st membrane using compressor, V3-V4, 1st membrane using vacuum pump) of enricher are shown in Fig. 2. The relevant simulation results of the energy consumption and membrane area are listed in Table 2. In the simulation the flow rate of the feed gas is 100 Nm3/h. The compressors are driven at 8 bar and the vacuum pumps at 30 mbar. The degree of CO2 separation of each variant is defined as 70%, CO2 purity as 90 mol%. Variant V3 provides a promising potential for fulfilling these requirements. This can be explained logically: using a vacuum pump for the 1st membrane to achieve a certain degree of CO2 separation, then using a compressor for the 2nd membrane to obtain the desired CO2 purity, this variant has an energy

advantage in comparison with those using a compressor for the feed flue gas, by means of which a considerable amount of energy is applied for N2 compression.

permeate

Fig. 2 Variations with compressor and vacuum pump for enricher concept

Table 3 Comparison of energy consumption of the 4 variants shown in Fig. 2, compressor: C = C1 = C2 = 8 bar, vacuum pump: 30 mbar

Variants Separation degree [%] CO2 purity [mol%] Area = (1st + 2nd) [m2] Specific energy [kWh/t: separated CO2]

V1 70 90 27 272

V2 70 90 42 296

V3 70 90 52 164

V4 70 90 71 204

4.2. Recirculation of flue gas

Another measure investigated here is recirculating the retentate of the 2nd membrane back to the feed side of the 1st membrane, on the basis of the simulation results of the single-stage membrane system [12], i.e. higher CO2 composition in the feed gas enables higher CO2 purity to be achieved after separation with the same membrane parameters and under the same operating conditions. Then a further retrofit V3-I was performed as shown in Fig. 3. Here the recirculation rate is defined as the ratio between the flow rate of the retentate of the 2nd membrane and the total feed flow rate. By recycling the retentate of the 2nd membrane to the feed side of the 1st membrane, 95 mol% CO2 purity can be reached. A detailed parametric study can be found in [13]. In order to explain how retentate recycling works within the V3-I cascade, an example is illustrated in Fig. 3, in which the flow rate and composition of different streams are labeled. Variant V3-I has most of the properties of the V3 cascade; a higher CO2 purity can be achieved by recycling the flue gas within the system; it logically leads to higher energy consumption and a larger membrane area.

4.3. Comparison with MEA absorption

Variant V3-I was applied for 600 MW NRW-RKW including the CO2 compression process (110 bar, 30°C). On the basis of the above simulation results, the following operating conditions were adopted: the vacuum pressure level of the 1st membrane was kept 100 mbar, and the feed pressure of the 2nd membrane remains at 4 bar. One aspect to be highlighted here is the question of the feasibility of a vacuum pump with a 30 mbar pressure level. Even large vacuum pumps will probably have a suction pressure of 50 mbar in the future, so the pressure drop within the module channels and connecting tubing should be considered additionally, which leads to reasonable vacuum pressure level of 100 mbar. The different degrees of CO2 separation 50%, 70% and 90% are simulated with a uniform CO2 purity of 95 mol%. The specific energy both for membrane capture and CO2 compression process, so as to the efficiency loss are listed in Table 4.

« = 0.4564

Xco2 = 28 Xm = 72

H = 4.0013 Xn2 = 95

retentate

^ feed

h= 4.4615 h = 4.9178

Xco? = 14 Xooz = 15

Xn2 = 86 Xw = 85

n= 0.9165

Xcoz ~ 62

permeate

Fig- 3

: - 0.4601 Xaii • 95

An example ot variant V J-i (/u% degree ot (J(J2 separation and ys moi%> cû2 purity), nfeed = 100 Nm 3 • h-1 = 4.4615 kmol • h-1

The energy penalty of the current MEA technologies ranges from about 8~14 percentage points for different types of power plants [14, 15]. Updated simulation results show that the capture process of MEA absorption consumes almost 10 percentage points of efficiency by adopting a similar power plant type [16, 17]. The efficiency losses of MEA absorption with 50%, 70% and 90% degrees of CO2 separation including the CO2 compression process (110 bar, 30°C) are listed in Table 5.

Table 4 Integration with NRW reference power plant using variant V3-I, separated CO2 compressed to 110 _bar, 30°C

Pressure Separation degree [%] CO2 purity [mol%] Membrane area x 106 [m2] Specific energy for capture [kWh/tco2] Specific energy for compression [kWh/tco2] Efficiency loss [%-pts.]

1st mbar 2nd bar

1st 2nd

100 4 50 95 1.13 0.04 124 105 4.1

4 70 95 2.39 0.06 151 105 6.4

4 90 95 6.37 0.08 244 105 11.4

Table 5 Efficiency loss of MEA absorption with 50%, 70% and 90% degree of CO2 separation, separated CO2 compressed to 110 bar, 30°C [16, 17]___

Separation degree [%] CO2 purity [mol%] Specific energy for capture [kWh/tcO2] Specific energy for compression [kWh/tco2] Efficiency loss [%-pts.]

50 99 220 100 5.8

70 99 220 100 8.2

90 99 220 100 10.5

It can be observed that the analyzed cases of variant V3-I have an energetic advantage in comparison with MEA absorption at 50% and 70% degree of CO2 separation. This leads to a potential tendency that the gas separation membrane could be an important capture option as a retrofit for existing power plants, considering the above mentioned degrees of CO2 separation.

5. Economic analysis

Applying a gas separation membrane system for post-combustion, the following cost factors should be considered: a.) capital cost (including membrane, frame, compression equipment and heat exchanger); b.) O&M cost; and c.) energy cost. An investigation of the literature [5, 18, 19] showed that the capture cost for MEA absorption is in the range of 30~50 euro/t , co2.

separated CO2

Referring to work by a Dutch group [5, 6], in the present paper a similar simulation method was used to calculate the capture cost using the 600 MW NRW reference power plant. Table 6 lists 12 equations applied to determine the total capture cost and CO2 specific separation cost. The relative cost and process parameters are shown in Table 7. Here the membrane cost is set to 50 euro/m2 and the membrane frame, e.g. casing, valve, tubing, is calculated using equation (2) in Table 6. The nomenclature can be referred in the paper [13].

Table 6 Equations applied to determine specific CO2 separation cost [5, 6]

Estimated investments I (components)

Im = A ■ Km (1) Membrane cost

If = (A/2000 ■ Kmf (2) Permanent membrane frame cost

Ic = Kcl ■ Fh + Kc2 ■ Fh (3) Compressor cost

I = K ■ F vp Kvp Fh (4) Vacuum pump cost

I = P ■ K ■ F ex ex ex h (5) Expander cost

hI e = CC e (6) Heat exchangers and cooling facilities

Energy consumption of compression equipment P

Ptot Z. Pc + Z. Pvp Z. Pex (7) Total energy consumption

Annual costs C

Ccap = (Z Ic + Z Ivp + Z Iex + Z Ihe + Imf ) ■ « + Im ■ «m (8) Capital cost

c0&m = 0.036 ■ (Z ic + Z ivp + Z iex + Z ihe ) + 0.01 ■ (im + imf ) (9) O&M cost

C =t P ■ K ^ en 1 op 1 tot ^ el (10) Energy cost per year

C = C + C + C Ctot Ccap + Cen + C0&M (11) Total cost

Specific CO2 separation cost CC

c = c / m CC02 Ctot' m C02,ann,separated (12)

Table 7 Assumptions for cost and process parameters [5, 6, 10, 20]

Parameter Value Unit Parameter Value Unit

Km 50 euro/m2 Kmf 0.25 million euro

Kc1 3 million euro Kex 0.3 euro/watt

K c2 30 million euro Kvp 4Kc1 million euro

Che 3.5 million euro Fh 1.8 -

a 0.064 - a m 0.225 -

top 6000 hour Kei 3.37 cent/kWh

The depreciation time for the components of compressor, expander, vacuum pump, heat exchanger and membrane module is 25 years, and the lifetime of the membrane is 5 years; the O&M cost of the components of compressor, expander, vacuum pump and heat exchanger is assumed to be 3.6% of their capital cost, and for the membrane and membrane frame the O&M cost is taken as 1% of their capital cost. Here the compressor cost is composed of two parts: for capture and for CO2 compression. The compressor for capture is related to the flue gas of 2~8 bar and a vacuum pump of 100 mbar. It is assumed that the vacuum pump costs 4 times as much as the compressor (Kvp = 4Kc1). The electricity price here is 3.37 cent/kWh. One aspect to be emphasized

is that this price is the current power cost. The capture cost calculated here shows the CO2 separation expense using the existing infrastructure. The capture cost was calculated for the variant V3-I, 70% degree of CO2 separation, listed in Table 8. It is obvious that the capital cost, mainly from the membrane cost, dominates the total capture cost.

Fig. 4 shows the results of a parametric study of membrane selectivity, when the CO2 permeance

3 2 1 1

is defined as 3 and 5 Nm m hbar. It can be observed that by increasing membrane CO2/N2 selectivity from 20 to 40, the energy consumption is effectively decreased, actually more than halved. However, from 40 to 80, this decreasing tendency is obviously slowed down: lines of open triangle for specific energy and open squares for capture cost. It should be mentioned here that with a CO2/N2 selectivity of 20, the recirculation rate is quite high at 30%, so that a large membrane area (1st membrane) is required to reach the required degree of CO2 separation. When the selectivity is decreased to 40 and 60, the recirculation rate is accordingly reduced to 13.5% and 8.5%, respectively.

Table 8 Capture costs for the V3-I under the conditions: 1st membrane permeate pressure 100 mbar, 2nd membrane feed pressure 4 bar, 70% degree of CO2 separation with 95 mol% CO2 purity

Cost Unit Value

Specific CO2 separation cost euro/t CO2/year 31

Total cost million euro/year 55.8

Capital cost 35.3

Membrane million euro 122.2

Membrane frame 36.2

Compressor 59.4

Vacuum pump 21.6

O&M cost million euro/year 4.7

Energy cost 15.8

■ . 1

„ P = 5

-PC0, = 3

SL 100S

20 40 60 80

Membrane selectivity C02/N2

Fig. 4 Influence of membrane parameters (permeability & selectivity) on energy consumption, capture cost

From the simulation results, it is known that the Polyactive® membrane with a CO2/N2 selectivity of 50 and a CO2 permeance of 3 Nm3m -2h-1bar-1 is attractive for a future gas separation membrane capture process. To realize the separation target of 95 mol% CO2 purity and 70% degree of CO2 separation, variant V3-I consumes 256 kWh/tseparated CO2 specific energy, with an efficiency loss of 6.4 percentage points and 31 euro/ tseparated co2 capture cost for NRW-RKW.

6. Conclusions

Gas separation membrane capture used for post-combustion, as a competing technology, possesses the advantages of end-of-pipe application, and of less environmental impact than the chemical absorption method. The compact and modular structure makes it flexible in use and could be a promising option for a retrofit. The cascade concept developed in this paper is driven by electrical energy, which can be used as turnkey equipment for the application.

Process investigation provides us with the following knowledge of the system: • A cascade arrangement makes it possible to reach high CO2 purity;

• Owing to the feasibility of large-scale vacuum pumps and the reality of the pressure drop within the system, a 50% or 70% degree of CO2 separation of the investigated cascade variant is attractive considering both energy consumption and capture cost;

• Membrane selectivity and permeability decide the CO2 purity and the degree of CO2 separation in a single-stage membrane, respectively, and strongly influence the energy consumption (electricity used to drive the compression machines) and total membrane area for a multi-stage membrane system, concerning the energy cost and capital cost, respectively. There is a trade-off balance between these pairwise parameters.

Acknowledgements

Financial support from the Helmholtz Association of German Research Centres (Initiative and

Networking Fund) through the Helmholtz Alliance MEM-BRAIN is gratefully acknowledged.

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