how to reduce co2 emissions in the lng chain

8/12/2019 HOW TO REDUCE CO2 EMISSIONS IN THE LNG CHAIN

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Paper PS2-7

PS2-7.1

HOW TO REDUCE CO2EMISSIONS IN THE LNG CHAIN

Pierre Rabeau

Henri Paradowski

Jocelyne Launois

with the participation of Andr Le Gall and Joelle Castel

TechnipParis, France

ABSTRACT

LNG is a clean fuel and its use instead of other hydrocarbons reduces pollution andCO2 emissions. However the liquefaction of natural gas to produce LNG, thetransportation in LNG carriers, the vaporization of LNG to produce natural gas, and the

use of that gas for the generation of electric power and heat produce large quantities ofCO2.

Whereas previous studies have examined costly and unproductive techniques forcapture and sequestration of CO2 at LNG production facilities, in this paper the reductionof CO2 production and hence emissions at moderate cost are discussed at some levels ofthe LNG plant, including the production of electric power and heat.

Based on the results of LNG projects, the contribution of each step to the total CO2release in a typical LNG plant is analyzed.

The CO2 emissions are reduced when the energetic efficiency of the processes isincreased. Possibility to increase the efficiency is discussed on some process units:Condensates Stabilization, NGL Recovery, Liquefaction and LNG End Flash.

The efficiency of the generation of heat and power is of prime importance and theCO2 emissions of five different systems are compared.

The authors conclude that significant reductions of CO2 emissions can be obtained.Some of them are easy to implement and do not generate complexity or reduced

availability. The fuel savings are sufficient to justify most of the proposed solutions froman economic point of view. A CO2 tax could lead to the selection of more sophisticated

solutions less proven in the LNG industry.


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INTRODUCTION

Many authors have already discussed the subject of CO2 emissions in the LNG chainand it is not the purpose of this paper to challenge the authors or present very innovativesolutions.

On the opposite what we wish to demonstrate is that very simple techniques, provenon some projects, easy to use, can contribute to reduce significantly the CO2 emissions.

Natural gas is a clean fuel and its proper use produces limited amounts of CO2.

To reduce the CO2 emissions we will follow two routes, one on the process side, andanother one on the energy generation.

The possible process optimizations will be illustrated by few examples but manyother improvements are feasible.

The method used to determine CO2 emissions in each case study is rigorous. It takesinto account reduced efficiency of power generators when running at partial load. Thismodel is also considering the split between process units for all energy uses (steam,electricity, fuel gas).

LNG CHAIN CO2 EMISSIONS

CO2 Emissions from Natural Gas

Natural gas can be used to produce power or heat. It is a much better fuel than liquidhydrocarbons.

To produce power the emissions of CO2 depend on the technology that is used:

0.55 kg/kW.h for a simple cycle Industrial Gas Turbine,

0.39 kg/kW.h for a combined cycle.

To produce heat the emissions of CO2 depend on the temperature level and on thetechnology that is used:

To produce heat at 150C the emissions are the following:

0.23 kg/kW.h for direct fired heater,

0.13 kg/kW.h for recovery on Gas Turbines Exhaust Gases,

0.09 kg/kW.h for recovery on a Combined cycle.

To produce heat at 250C the emissions are the following:

0.25 kg/kW.h for direct fired heater,

0.16 kg/kW.h for recovery on Gas Turbines Exhaust Gases.

If we use Propane instead of Natural Gas the emissions are 15% higher and the use of

Fuel Oil increases the emissions by more than 50%. This is due to the ratio of hydrogento carbon that is much lower in heavy hydrocarbons than in methane.


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Product ion of Natural Gas by Means of the LNG Chain

Large reserves of natural gas are located overseas and to be used in the countries thatneed imports it is necessary to build an LNG chain:

Transportation of Natural gas to the LNG Plant, Liquefaction and storage and loading of LNG on an LNG carrier,

Transportation of LNG,

Regasification of LNG.

Each of these steps produces CO2 emissions. Typical numbers for a Chain connecting

Nigeria to Europe [1] are as follows :

0.01 kg CO2/kg LNG for step 1




More than 75% of the CO2 emissions are due to the LNG plant.

Use of Natural gas

The production of electric power has been given a lot of consideration and veryefficient gas fired combined cycles are used. Research and development is on going andshould result in even better efficiencies.

The use of natural gas for domestic heating purposes is very inefficient from athermodynamic point of view. The development of the micro turbine technology is not

promoted as it should be.

LNG PRODUCTION PLANT

As the LNG plant is the main contributor to the CO2 emissions, we shall focus on thissubject.

There are two ways to increase the efficiency and decrease the emissions:

Improve the processes,

Improve the efficiency of the production of heat and power.

Of course there are many interactions between these two ways.

IMPROVE PROCESS TO REDUCE ENERGY REQUIREMENT

At first we have to analyze where the consumptions of energy and the emissions ofCO2 are located.

For a plant producing about 25 MTA of liquefied gases: Low Btu LNG, Propane and

Butane, the figures are summarized in Table 1 hereafter:


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Table 1 CO2 balance for LNG plant

Process units % Tons/h of CO2

Warm units 14.0 % 135

Cold units 82.0 % 790

Storage and loading 2.2 % 21Others 1.8 % 17

Total 100.0% 963

The cold units that are the NGL recovery and the LNG production are the maincontributors but the warm units that include the Condensates Stabilization, the Acid GasRemoval unit and the Dehydration should not be neglected.

Use of Heat Integration in Warm Pre-Treatment Units

The condensate stabilization unit represents 20 to 45% of the CO2 emissions of the

warm units depending on the heat power generation systems that are used and that willbe discussed later on. To reduce the energy consumption we do have two mainpossibilities:

Optimize the process scheme to obtain a better heat integration,

Optimize the operating parameters, mainly the pressure of the stabilizer.

We will show the improvements obtained on the stabilizer reboiler duty and on the offgas compressor power.

Condensates Stabilisation Unit Heat Integration. The simplest process scheme

used for the condensates stabilization is shown of figure 1a.

M

HP Gas

HP Feed

Stabilized

C5+

HP steam

A2

V1

V4

A1

V3

E1

E2

A3

55 bar

15C

25 bar

40C

K2 K1

V2

9 bar

Figure 1a Condensates Stabilization unit


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In this scheme n 1, the feed from the MP separator is split in two parts:

One is cold and fed on the first tray of the stabilizer,

The second is heated against the hot condensates from the bottom of the column.

In a second scheme we add a reflux to the stabilizer to decrease the power of the offgas compressor.

In a third scheme we add a side reboiler to scheme n2,

In a fourth scheme that is shown on figure 1b we add a second side reboiler.

M

HP Gas

HP Feed

Stabilized

C5+

HP steam

A2

V1

V4

A1

V3

E1

E4

E3

V5

E2

A3

55 bar

15C

25 bar

40C

K2 K1

A4

V2

8 bar

Figure 1b Condensates Stabilization Unit Heat Integration

For each scheme we optimize the pressure of the stabilizer to obtain the lowest CO2

emissions. The improvements obtained on the stabilizer reboiler duty and on the off gascompressor are shown on Table 2.

Table 2 Condensates stabilization heat integration results

Scheme 1 Scheme 2 Scheme 3 Scheme 4

Reboiler duty kW 53.6 54.5 42.2 38.4Off gas compressor power kW 11.2 10.9 11.4 11.3CO2 emissions T/h 24 24 20.6 19.4Stabilizer pressure Bar 9 8.5 8 8


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When CFD is used for design then it is possible to reduce the pressure drops atcompressor suction from the conventional 0.15 bars to 0.10 bars. By doing that we can

save 1600 kW per LNG train, that is 6400 kW for the LNG plant and 4 t/h of CO2emissions.

On figures 4a and 4b the LP MR line connecting the MCHE to the LP MR suctiondrum is shown.

Figure 4a Model for LP MR Line from MCHE to Suction Drum

Figure 4b Pressure profile in LP MR Line from MCHE to Suction Drum


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Optimization of Suction Drums. Another area where CFD has become a design toolis the design of the suction drums.

With use of CFD it has become obvious that the feed distributors previously used,such as half open pipes, were not able to ensure a proper distribution of gas in large KO

drums.

The vane type distributor has proved to be much more efficient.

Many separation drums have been retrofitted with this type of distributor in capacityenhancement projects and the results have always been good.

For new projects the size of the suction drum will depend on the capacity of the misteliminator but also on nozzle diameters, distances between the distributor and the misteliminator and distance between the distributor and the liquid level.

On figure 5 we can see a KOD designed with the use of CFD.

Figure 5 Velocities in Knock Out Drum

Integration of NGL recovery and LNG units

The cold units that are the NGL recovery and the LNG production are the maincontributors to the consumption of fuel gas and therefore for the emissions of CO2.

The successful integration of the NGL unit with the LNG unit is very important.


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Two main parameters are to be considered:

The pressure of the recovery tower in the NGL unit,

The pressure of the gas sent to the liquefaction.

Pressure of Recovery Tower. A schematic of the NGL recovery unit is presented onfigure 6. The process selected ensures a propane recovery of more than 98%.

Dry feed gas

Treated gas

to compression

V1

C2

NGL

LP steam

Recovery tower

T1

Turbo-expander

Cold box

De-ethanizer T2

C3R

C3R

P1

Figure 6 NGL Recovery Unit

The dry feed gas is cooled to about 43 C and partly condensed in the cold box.Vapor and liquid are separated in the cold separator V1. The vapor is sent to the turbo-expander where it is cooled and partly condensed by means of an isentropic expansion.The resulting two-phase flow is sent to the Recovery Tower operating at 20.5 bars. Theliquid from the cold separator is directly sent to the bottom of the recovery tower. Theliquid from the bottom of the recovery tower is sent to the de-ethanizer after reheating in

the cold box.

The de-ethanizer is operated at a pressure slightly higher than the Recovery tower. Itproduces a C3+ cut that is sent to the fractionation, a C2 cut used for refrigerant make-upand a vapor distillate that is a methane-ethane mixture. The vapor distillate is condensedin the cold-box and sent to the recovery tower as reflux. The Vapor from the Recovery

Tower is reheated in the cold box and compressed in the compressor driven by theexpander to about 24 bars. The treated gas is compressed in a booster compressor to theliquefaction pressure. (Refer to figure 7).

Propane refrigerant from the liquefaction unit is used in the cold box to supplyrefrigeration required at about 30C.


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The recovery tower pressure has to be optimized. When the pressure is increased, thepower of the expander is reduced and more propane is required. The power of the booster

compressor is decreased but additional power is required from the propane cycle. Resultsare shown in Table 4.

Table 4 NGL Recovery Optimization results

Recovery

tower

pressure

Cold

separator

temperature

Propane

refrigerant

flow rate

Booster

compressor

power

Propane

compressor

power

Total

power

CO2

emissions

Bars C Kmoles/h MW MW MW T/h

20.5 -42.8 2030 40.3 3.4 43.7 26.221.5 -43.8 2350 38.7 3.9 42.6 25.622.5 -45 2620 37.4 5.2 42.6 25.623.5 -46.2 3000 36.0 6 42.1 25.324.5 -47.3 3370 34.8 6.8 41.6 25.

25.5 -48.4 3800 34.6 7.8 42.4 25.426.5 -49.3 4300 34.4 8.9 43.3 2627.5 -50.1 5600 34.2 10 44.2 26.528.5 -50.9 6600 33.9 11.7 45.6 27.4

A careful optimization of the recovery tower pressure can save about 2 MW of energy

per LNG train (i.e. 8 MW for the LNG plant) and 5% on CO2 emissions.

Booster Compressor Discharge Pressure. The discharge pressure of the BoosterCompressor can be selected so as to minimize the power consumption and the CO2emissions. When the gas to be liquefied is available at the MCHE inlet at high pressure itis much easier to liquefy. The MR can then contain more propane and less methane.

The results of a detailed study are shown on Table 5 here below. High pressure givesa significant benefit: 13 MW per train are saved when the gas is liquefied at 67.8 Barsinstead of 47.8. This reduces the CO2 emissions by 31 T/h for the LNG plant.

Table 5 Booster Compressor Discharge Pressure Optimisation

NG

Pressure at

MCHE inlet

NG Booster

Power

MR

compressor

Power

Propane

compressor

Power

Total Power Total Power

Bars MW MW MW MW %

42.8 17.2 153.9 88.0 259.1 107.647.8 21.5 145.2 87.0 253.7 105.452.8 25.3 139.6 84.2 249.1 103.457.8 28.8 133.4 83.0 245.2 101.862.8 32.2 128.9 81.7 242.8 100.867.8 35.3 124.5 81.0 240.8 100.0


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Use of LNG Deep flash

At the outlet of the MCHE the LNG is often sent to an End Flash unit.

The use of End Flash has many advantages:

Reduced size of the MCHE,

Reduced power and volume flow rate of the MR compressor,

Produces high quality Fuel Gas,

Eliminates from LNG light components such as Nitrogen, Oxygen, and Helium.

Prevents high LNG flash at LNG tank inlet

With the line up that is considered in this paper and that is shown on figure 7, onequestion arises: would it be beneficial to produce more end flash gas than necessary forthe fuel and recycle the excess Fuel Gas to the suction of the Booster Compressor ?

M

NGL Recovery

Fuel gas

Liquefaction

M

Dry gas

50 bar

NGL

Gas

compression 70 barLNG EFG

Compression

30 bar

LNG

EFG recycle

Figure 7 End Flash Gas Unit

A study was conducted with variation of the temperature of the LNG at the outlet of

the MCHE. The results are presented on Table 6 here after for a constant LNGproduction.


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Table 6 Deep End Flash Study Results

Temperature of LNG at

outlet of MCHE

C -136.25 -141.25 -146.25 -151.25 -156.25

End Flash gas

compressor power

MW 32.1 24.8 17.9 11.2 5.6

MR compressor power MW 103.9 110.2 118.4 126.8 141.6C3R compressor power MW 82.7 82.5 82.3 80.7 83

NG Booster compressorpower

MW 39.2 37.7 36.3 34.9 33.7

Total power ofcompressors

MW 257.9 255.2 253.9 253.6 253.9

Total power ofcompressors

% 101.7 100.6 100.1 100 100.1

The total power is fairly constant in a large range of temperature. The split of thepower is different. Increasing the End Flash leads to a decrease of power of the MRcompressor and increases the power of End Flash Gas compressor and the power of the

NG Booster compressor. The choice can then be dictated by the energetic scheme and theselection of the driver for the End Flash Gas compressor.

BETTER ENERGY INTEGRATION TO REDUCE CO2 EMISSION

A rigorous model linked to all the process units and reflecting the reduced efficiencydue to running N+1 power generators at a partial load has been considered. This modelallows to determine CO2 emissions in a multicase study. This model is identical to the

ones used on the large LNG projects.

Base Case

A common practice in existing LNG plant is to use steam as heating medium and toproduce it in package boilers, to use gas turbines as refrigerant compressor drivers and toproduce electricity with another set of gas turbines in a dedicated power generation unitas shown on figure 8.

By allocating shares of steam and electricity to the consuming process units, a CO2balance per process units has been established and is presented in Table 7 here below as

the base case. The CO2 contained in the feed gas and rejected to the atmosphere from theacid gas removal unit is not included in this balance because capture and reinjection of

CO2 is not considered in this paper.


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PROCESS HEAT

EXCHANGERS

MOTOR

FG

FG

MOTOR

BFW

BFW

GE9

GE9LP MR

LP C3

MP/HP MR

HP C3

LS

PACKAGE

BOILER

BFW

Figure 8 Base Case Energy Scheme

The process units have been grouped in four different entities.

Warm units are the inlet facilities, acid gas removal, dehydration and mercury

removal units.

Cold units are the NGL recovery, Fractionation, Liquefaction and End flash units.

The storage and loading are for LNG, LPG and Condensates storage and loading.

Others are for Excess steam air coolers and Fuel gas heater, water and air utilityunits.

The main contributors of the inlet facilities and of the acid gas removal units are thesteam consumptions. The main contributors of the NGL recovery, liquefaction and endflash units are the refrigerant compressor drivers and the refrigeration air coolers. Themain contributors of the storage and loading units are the loading pumps and compressor

drivers.

Table 7 CO2 Balance for base case

Process units % Tons/h of CO2

Warm units 14.0 % 135

Cold units 82.0 % 790

Storage and loading 2.2 % 21

Others 1.8 % 17

Total 100.0 % 963


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Use of Heat Recovery Steam Generation

The idea of reducing CO2 emissions by applying better energy integration at thesources led us to consider the well-known and mature technology of heat recovery steamgeneration (HRSG).

In the first case, the steam generation though HRSG has been adjusted to the steam

demand (figure 9) In this configuration, only one gas turbine needs to be equipped with aHRSG system. Conventional steam pressure level has been selected and at the same timea back pressure steam turbine has been added to replace the electric motor driving the end

flash gas compressor.

PROCESS HEAT

EXCHANGERS

FG

MOTOR

FGHRSG

MOTOR BFW

BFW

EFG

BFW

GE9

GE9LP MR

LP C3

MP/HP MR

HP C3

LS

Figure 9 Heat Recovery on One Gas Turbine

In the second case, the two gas turbines have been equipped with HSRG and theexcess steam is used to produce electricity within the LNG trains through condensing

steam turbines generators (figure 10).


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PROCESS HEAT

EXCHANGERS

G

FGHRSG

MOTOR BFW

FGHRSG

MOTOR BFW

BFW

EFG

BFW

GE9

GE9LP MR

LP C3

MP/HP MR

HP C3

LS

Figure 10 Heat Recovery on Two Gas Turbine and Electricity Generation

The CO2 balance showing the emissions reduction is shown in Table 8.

Table 8 CO2 balance for one HRSG per train and two HRSG per train

Number of HRSG One per train Two per train

Process units %Tons/h of

CO2 %Tons/h of

CO2

Warm units 7.4% 61 7.5% 51

Cold units 88.2% 725 90.0% 614

Storage and loading 2.8% 22 1.4% 10

Others 1.6% 13 1.1% 7

Total 100.0% 821 100.0% 682

It can be observed that for one HRSG per train, the main benefit on the reduction ofCO2 emissions is within the warm units (mainly inlet facilities and amine unit) becauseof the steam generation package boilers deletion. With two HRSG per train, the reduction

is observed everywhere because of the reduction of CO2 emission in the powergeneration unit. Compared to the base case, CO2 emissions have been reduced by about15% by using one HRSG per train and by about 30% by using two HRSG per train.

Use of Combined Cycle in Power Generation Unit

The next technique that is available and can be applied in the power generation unit isto use aero-derivative gas turbines known for their better efficiency than the widely used

heavy-duty gas turbines. The comparison has been done on the basis of the GE LM6000


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aero-derivative gas turbine but many other possibilities exist as described by Peterson [2],Avidan [3] and Yates [4].

Finally, this idea can be extended by using combined cycle power generation insteadof open cycles. The new case with combined cycle has been done on the basis of the GE

PG9171 and same level of steam as base case but many other possibilities exist asdescribed by Kikkawa [5, 6].

The CO2 balance showing the emissions reduction is shown in Table 9. It can beobserved that the CO2 emissions are reduced in the cold units because they have the

highest power demand. Compared to the base case, CO2 emissions have been reduced bymore than 30% by simply applying available techniques.

Table 9 CO2 balance for improved efficiency in power generation unit

Power generation type LM 6000 gas turbines Combined Cycle

Process units % Tons/h of CO2 % Tons/h of CO2Warm units 7.5% 50 7.5% 48

Cold units 90.2% 602 90.6% 581

Storage and loading 1.3% 9 1.0% 6

Others 1.0% 7 0.9% 6

Total 100.0% 668 100.0% 641

CONCLUSION

In this study we have quantified some improvements that can be implemented in an

LNG plant to reduce the CO2 emissions by increasing the efficiency of processes andenergy generation systems.

In the following Table 10 and Table 11 a summary of the savings is shown togetherwith the fuel savings. The admissible CAPEX increase is calculated on the basis of thefuel savings only for a financed project and 20 years of operation. The figures are based

on fuel cost of 1.5 $/Mbtu and on a CO2 tax of 10 $/t. One day of production loss gives a20.5 M$ penalty.

Table 10 Summary of possible reductions of CO2 for process units

CO2 emissionsReduction

Fuelconsumption

reduction

AdmissibleCAPEX

increase

CO2 taxreduction for

20 years

T/h T/h M$ M$

Condensatesstabilization

5 2 8 7

NGL recovery 5 2 9 8Liquefaction

pressure31 11 56 50

Liquefaction

unit

8 3 15 13

Total 48 18 88 78


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All the proposed options for the optimization of the process units are economicallyjustified.

Table 11 Summary of possible reductions of CO2 for generation of energy

Option CO2emissions

Reduction

Fuelconsumption

reduction

AdmissibleCAPEX

increase

CO2 taxreduction for

20 years

T/h T/h M$ M$

1 One HRSG per LNGtrain instead ofconventional boiler

142 52 258 229

2 Two HRSG per LNGtrain instead of one

139 51 259 224

3 Aero derivative GTsinstead of heavy duty

GTs for electricitygeneration

14 6 56 26

4 Combined cycle insteadof aero derivative GTsfor electricitygeneration

27 10 15 44

Total 322 119 588 523

The savings in this field are very important. The use of HRSG on the exhaust gases ofthe process GTs brings a lot of advantages and option 1 does not lead to any loss of

availability and production.

For option 2, it is more difficult because the steam generated by the second HRSG isused for electricity generation. If the system is not correctly engineered the loss ofavailability for the LNG plant may exceed 1% and the loss of production may exceed1500 M$ over a 20 years period.

In regard of possible loss of availability options 3 and 4 are very dependent on thedesign basis and project strategy.

REFERENCES

1. How to reduce CO2 emissions from the LNG chain, H. Paradowski, J. Launois, GPAtechnical meeting - Bergen Norway, May 2002

2. Higher efficiency, lower emissions, N. Peterson, D. Messersmith, B. Woodard, K.Anderson , Hydrocarbon Processing, December 2001

3. LNG liquefaction technologies move toward greater efficiencies, lower emissions,A. Avidan, D. Messersmith, B. Martinez, Oil and Gas Journal, August 19, 2002

4. The DARWIN LNG Project, D.E. Yates, C. Schuppert, LNG14 - Doha - Qatar,

March 2004


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5. Zero CO2emission for LNG power chain ? , Y. Kikkawa, Y.N .Liu, LNG 13 - Seoul- Korea, May 2001

6. How to optimize the power system of baseload LNG plant with minimizing CO2emission, Y. Kikkawa, M. Ohishi , AICHE Spring meeting - New Orleans - 30/03/2003

how to reduce co2 emissions in the lng chain

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