Present and future trends in thermal desalination with possible solar application

transcript

Present and Future Trends in

Thermal Desalination with Possible

Solar Application

Mohamed Darwish

Qatar Environment and Energy Research

Institute, Doha, Qatar

www.qeeri.org.qa KAUST, 2013

Content

• Current large capacity Thermal Desalting systems

• MSF and TVC/ME Thermal Energy consumption

• Recent advances in MSF and TVC/ME

• Raising the TBT for MSF by Nano filtration

• Modifying TVC/ME by raising TBT and number of effects

• Using MVC in place of TVC

• Feasibility of solar applications

Large capacity thermal desalting systems

• Multi Stage Flash (MSF), most used in GCC.

• Reliable, mature, more than 50y experience in

design, operation, material selection, maintenance.

• Largest unit capacity 20 MIGD, (in Ras Al Khaiir)

• Thermal Vapor Compression (TVC) combined with

conventional Multi Effect (TVC/ME)

• Up to 8 MIGD capacity, TBT = 70 C

First MSF unit was one MIGD in Kuwait in 1960

Capacity reached 20 MIGD/unit, GR = D/S = 89,

Specific thermal energy, Q/D = 260 MJ/m3, TBT = 90 112C

Requires steam at 117oC saturated T.

Pumping energy 3.5-4 kWh/m3

One of 8 MSF units for the Ras Al Khair , SA , total cost $ 1.76 B

Capacity/unit = 91,000 t/d (20 MIGD), $11M/MIGD

123 m (l)x33.7 m (w), weighs 4,150 t

Distillate water

Seawater

D = 200.2 [kg/s]

B6 = 400.4 [kg/s]

D1 = 24.62 D2 = 23.59 D3 = 23

D4 = 21.48 D5 = 19.13 D6 = 17.21

Df = 10.15 [kg/s]

Ds = 12.3 [kg/s]

T1 = 62.8

T1 = 62.8 T2 = 59

T2 = 59 T3 = 55.2

T3 = 55.2

T4 = 51.4 T5 = 47.6 T6 = 43.8

F = 600.7 [kg/s]

Mc = 1031 [kg/s]

t1. = 55.2

t2. = 51.4

Ds = 12.3 [kg/s]

Ated = 333.7 [m2/kg/s]

GR = 8.139

MIGD = 3.803

Qd = 300.6 [kJ/kg]

Ad = 76.71 [kJ/kg]

Flow sheet diagram of Al-Taweelah A1 ME-TVC desalination plant

GR= D/S = 8-10

LP Steam Supply 2.5-3 Bar (started with boiler and at 20 bar)

Pumping power 2 KWh/m³, compared to 4 for MSF

26-Apr-15 10

• No MSF units built outside GCC for long time

• Shuaiba Barge, 52,000 m3/day (14 MGD) SWRO

• 2-pass, 5,656 (8” elements), output TDS <100 ppm

• Hamma SWRO in Algiers, 200,000-m3/d,

Recovery ratio 40% 44.5%.

• Perth SWRO in Australia, 143,000 m3/d, Wind

operated plant by 83 MW wind farm. (48 WT)

• 11 SWRO plants planned in California, 1.117

Mm3/d.

26-Apr-15 13

MSF and TVC/ME Consumed Energy

• In MSF and ME, steam supplied few T 7oC above TBT,

say at 117oC

• TVC can be at higher T as steam operate ejector

• Better generate steam at HP and T (as in PP), expands it in

ST, producing work before inlet to DS units at relatively LP

• This saves about 50% of fuel energy

• Required thermal energy 240-300 kJ/kg

• Expressed as Gain ratio D/S

• 4 kWh/m3 for MSF and 2 kWh/m3 for TVC/ME

B: Pressure, Bar

H: Enthalpy, kJ/kg

T: Temperature, oC

m: mass flow rate, kg/s

DrumDe-aerator

STEAM TURBINE GENERATOR(1 UNIT)

GAS TURBINE GENERATOR

(1 OF 3 UNITS)

GT Comp.

3 HP EJECTORS

3 MSF UNITS

ST 215.7 MW

215.5 MW

Make up water

HEAT RECOVERY STEAN GENERATOR

(1 OF 3 UNITS)

CONDENSATE RETURNFROM DESAL PLANT

CONDENSATE

HEATERS

DESALINATION PLANTS

BLOWDOWN

CONDENSER

625.8 T

591.5 m

75 B 560 T

3550.7 H 293.58 m

6.8 B 142.3 T599.3 H 101.33 m

183.1 T

591.5 m

13 B 118 T496.7 H 293.58 m

IP PROCESS STEAM

LP PROCESS STEAM

30.3 B 449.3 T

3342.7 H 7.5 m2.8 B 158.8 T

2781.5 H 286.08 m

13 B 115.8 T486.7 H 98.25 m

87.2 B 142.3 T603.9 H 3.47 m

15 B 30 T127.1 H 3.08 m

13 B 60 T252.2 H 1.167 m

HRSG # 2

HRSG # 3

HRSG # 2

HRSG # 3

HRSG # 2

HRSG # 3

87.2 B 142.3 T603.9 H 10.41 m

2.8 B 137 T2734.6 H 2.91 m

2.5 B 135 T

2733.1 H 293.58 m

GTCC Steam extracted

Energy consumption calculations

• Steam from ST to DS at 2.8 bar, 158oC, 2781.5 h,

• de-superheated to enter DS at 2.5 bar, 135C, 2733 h

• S =293.6, D=2368 kg/s, D/S=8.06, 3 MSF (45

• Wde (lost work or equivalent) = ms (hMSF – h cond)

• = 293.7 (2781 -2345.5)/1000 = 127.8 MW

• 127.8 MW work (eq) to Q=657 MW supply to DS

• (14.6 MW Q/MIGD and 2.84 MW Weq/MIGD

• 54 kJ/kg D, 15 kWh/m3, adding 4 kWh/m3 pumping,

• Total consumed W(eq) 19 kWh/m3 for MSF

• Total consumed W(eq) 17 kWh/m3 for TVC/ME

• Wangnick, reported 4 kWh/m3 for pumping and 14 kWh/m3 for thermal, total 18 kWh/m3.

• Hamed, [8] of SWCC analysis shows MSF plants inherited exergy loss in range of 15.2 23.7 kWh/m3,

Desalted water fuel cost by work loss method

• Extracted steam to MSF can produce EE if not extracted,

• No cheap or wasted energy as claimed

• Coupling MSF with steam turbines reduces energy 50% compared with boiler operated MSF

• Still, much very high compared with SWRO

• MSF combined with steam turbine consume at least 20-kWh/m3 or 5 times that of SWRO.

• MSF widespread in GCC is due to low calculated fuel cost as compared with international fuel cost

Treatment

Brine Heater Heat Recovery SectionHeat Rejection

Section

Recirculaion Steam

Blow Down

Distillate

Cooling

Water Mc

Condensate

Figure 1 Recirculation Multi Stage Flash Desalting System

Thermal

Energy to

Pumping

Energy to

streams

Relation between

GR, n number of

stages and specific

heat transfer area.

Ras Al

Improving Prospects of MSF by its

combination with NF Pretreatment

• No doubt that MSF system is simple, highly reliable,

robust, and has higher capacity/unit than SWRO

• MSF can deal with worst seawater quality and produces

almost pure water.

• There are concerns on reliability of SWRO

• This is not excuse to avoid the SWRO use and

development, same way MSF developed with many

failures at the beginning

• New suggested MSF improvement:

• Pre-treat its feed water, fully or partially by NF

• NF as pretreatment for SWRO and MSF suggested and

extensively studied in S.A.

• Awerbuch [10] showed its benefits of removal of scale

elements from seawater, and suggested using NF

permeate for partial feed to thermal process.

• NF pre-treatment lowers significantly the concentration

of hard scale elements in seawater such as Ca2+, Mg2+,

SO4, and HCO3-

• This permits raising the TBT and recovery ratio of MSF

26-Apr-15 29

• The maximum (TBT), at which sulfate scale

begins to precipitate, is shifted to 120, 135 and

145oC when the NF-treated portion increased

from 10, 25 and 50%, respectively.

• Combination of the MSF unit with NF

pretreatment is not free.

26-Apr-15 30

• MSF D-output flashing range, (TBT– Tn).

• TBT Increase from 110C 135C gives 35% D

increase, i.e 7.29.72 MIGD,

• 2.52 MIGD increase.

• Minimum specific mechanical energy for the modified

case is 27.33 kWh/m3 (22.7 for heat and 4.625 for

pumping).

26-Apr-15 31

Figure 3: Influence of NF on sulfate scale potential in BR-MSF plant

26-Apr-15 32

26-Apr-15 33

Case 1: Using SWRO system to augment the

existing MSF

Treatment

Section

Recirculaion Steam

Blow Down

Distillate

Cooling

Water Mc

Condensate

Thermal

Energy to

Pumping

Energy to

streams

26-Apr-15 34

• The maximum (TBT), at which sulfate scale

begins to precipitate, is shifted to 120, 135 and

145oC when the NF-treated portion increased

from 10, 25 and 50%, respectively.

• Combination of the MSF unit with NF

pretreatment is not free.

Thermal vapor compression (TVC) and Multi

effect (ME), i.e. TVC/ME

Forward feed multi-effect desalting unit

with pre-feed heaters

Multi-effect thermal vapor desalting unit.

Ratio of

S/Dr for

different

expansio

n Ps/Pn

compres

ratios

ALBA ME-TVC

plant (

Schematic diagram similar to Al-Jubail (MARAFIQ) ME-

TVC unit, 6.5 MIGD.

The increase of unit

size capacity of ME-

TVC desalination

systems.

The increase in the gain output ratio of new ME- TVC projects

• Logic question, why thermal compressor is used in TVC/MED.

• Mechanical vapor compressor (MVC) is more efficient, energy wise

• This fact is illustrated first, and then the obstacles of using the MVC are studied.

15نيسان، 26 45

B= F - D

Compressed vapor D

evapora

Multi - flow heat

exchanger

Compres

Feed F

B at To

Feed F

B= F - D

Compressed vapor D

Figure 2: Single Effect Mechanical Vapor desalting unit

evapora

Multi - flow heat

exchanger

Compres

Feed F

B at To

Feed F

• Work loss due to steam extraction to TVC/MED is similar to that of MSF

• 54 kJ/kg D, 15 kWh/m3, adding 2 kWh/m3

pumping , total eq. energy is 17 kWh/m3

• Twice energy reported by leading MVC manufacturer of 8 kWh/m3 for units producing 3000 m3/d, and expected to be 7.5 kWh/m3 for 4000 and 5000 m3/d for newly designed units.

26-Apr-15 49

• MSF D-output flashing range, (TBT– Tn).

• TBT Increase from 110C 135C gives 35% D

increase, i.e 7.29.72 MIGD,

• 2.52 MIGD increase.

• Minimum specific mechanical energy for the modified

case is 27.33 kWh/m3 (22.7 for heat and 4.625 for

pumping).

26-Apr-15 50

Comparison between augmenting MSF

with SWRO, or modifying it with NF

26-Apr-15 51

Case 1: Using SWRO system to augment the

existing MSF

Treatment

Section

Recirculaion Steam

Blow Down

Distillate

Cooling

Water Mc

Condensate

Thermal

Energy to

Pumping

Energy to

streams

26-Apr-15 52

Unmodified 7.2 MIGD MSF+SWRO unit will produce:

• MSF output 10.752 Mm3/y (if CF = 0.9), and consumes

EE of: 216.12 GWh (20.1 kWh/m3).

• SWRO 2.52 MIGD (11.456×103 m3/d) produce 3.76

Mm3/y (CF = 0.9), and consumes:

• EE 15.05 GWh (based on 4 kWh/m3) EE .

• A total EE consumption of 231.65 GWh, .

• A total EE consumption of 231.65 GWh,

• fuel energy consumption of 2.3165 MGJ (0.38 Mbbl)

based on 10,000 kJ/kWh heat rate, and at a cost of $M

26.58.

• When the fuel energy represents 70% of the EE cost,

the EE cost is $M37.97.

• The capital cost of the SWRO system is in the range of

$750/(m3/d), and the 2.52 MIGD will cost $M8.592

Case 2

26-Apr-15 55

• Case 2:

• Modified MSF unit by raising its TBT from 110135oC to produce

• 9.72 MIGD or 14.515 Mm3/y, and consumes:

• 396.7 GWh (27.33 kWh/m3) EE.,

• fuel energy consumption of 3.967 MGJ

• 0.6503 Mbbl at a cost of $M 45.522, and

• EE cost of $M 65.032.

26-Apr-15 56

• The annual consumed electric energy in case 2 is more 165.05 GWh than case 1, and its electric energy cost is $M 27.062 more than case 1.

• The saving of the energy cost when an SWRO is added to the unmodified MSF unit compared to modifying the MSF unit in one year is $M27.062, which is more than 3 times the cost of adding the SWRO unit.

Suggested ISCC using PTC and Desalination

• Co-generation Power Desalting Plants (CPDP)

using CC,

• and integrated with Multi Stage Flash (MSF) or

Multi Effect Distillation (MED)

• Examples are: Shuaiba North in Kuwait,

• Jabal Ali in United Arab Emirates (UAE), and

• Ras Girtas, and Mesaieed in Qatar.

Examples of the ISCC in operation or under

construction • Kureimat (Egypt), 140 MW, and 20 MW,

• Hassi R'Mel (Algiers), 130 MW, 25 MW,

• Ain Beni Mathar (Morocco), 472 MW, 20 MW,

• Yazd (Iran), 430 MW, 67 MW,

• Martin solar, Florida (USA), 480 MW, and 75 MW,

• Agua preta (Mexico), 480 MW, and 31 MW,

• Victorville, California (USA), 563 MW and 50 MW, and

• Palmdale, California (USA), 617 MW, and 62 MW.

• Suggested here;

Gas Turbine Generators

3 × 215.5 MW

HEAT RECOVERY

STEAM GENERATOR

Steam Turbine Generator

1 × 215.7 MW

DESALINATION PLANTS

Fuel3 HP EJECTORS

3 × 15 MIGD MSF UNITS

Shuaiaba CC3 GT×215.5 MW each + 3 HRSG + 1 BPST×215.7 MW + 3

MSF of 15 MIGD each

B: Pressure, Bar

H: Enthalpy, kJ/kg

T: Temperature, oC

m: mass flow rate, kg/s

DrumDe-aerator

STEAM TURBINE GENERATOR(1 UNIT)

GAS TURBINE GENERATOR

(1 OF 3 UNITS)

GT Comp.

3 HP EJECTORS

3 MSF UNITS

ST 215.7 MW

215.5 MW

Make up water

HEAT RECOVERY STEAN GENERATOR

(1 OF 3 UNITS)

CONDENSATE RETURNFROM DESAL PLANT

CONDENSATE

HEATERS

DESALINATION PLANTS

BLOWDOWN

CONDENSER

625.8 T

591.5 m

75 B 560 T

3550.7 H 293.58 m

6.8 B 142.3 T599.3 H 101.33 m

183.1 T

591.5 m

13 B 118 T496.7 H 293.58 m

IP PROCESS STEAM

LP PROCESS STEAM

30.3 B 449.3 T

3342.7 H 7.5 m2.8 B 158.8 T

2781.5 H 286.08 m

13 B 115.8 T486.7 H 98.25 m

87.2 B 142.3 T603.9 H 3.47 m

15 B 30 T127.1 H 3.08 m

13 B 60 T252.2 H 1.167 m

HRSG # 2

HRSG # 3

HRSG # 2

HRSG # 3

HRSG # 2

HRSG # 3

87.2 B 142.3 T603.9 H 10.41 m

2.8 B 137 T2734.6 H 2.91 m

2.5 B 135 T

2733.1 H 293.58 m

Mass and heat balance diagram of Shuaiba North GTCC Power- Desalination Plant.

Superheater Evaporator Economizer

CT o6254

steam 560

CT osat 7.290.

CT ostuck 183

CT ofeed 142

CT ogpp 7.310

CT opp 20

Gas and steam-water temperature profile of the HRSG

GE912FA Gas turbine, GT

3 No. of units

Natural Gas Type of fuel

215.5 Gross output, MW

50 Ambient temperature, oC

Natural

circulati

HRSG, Type

3 No. of HRSG

3 Integral de-aerator

1 blow down, %

BPST Steam Turbine, ST

1 No. of ST

215.7 Gross capacity, MW

MSF Desalination

3×15 MIGD #unitsx Capacity

Gas turbine combined cycle,CC of 862.2 MW Gross output,

Desalination units

• Steam flow rate to one MSF = 97.86 kg/s, 1/3 of

BPST discharged

• D = 15 MIGD (789 kg/s). This gives:

• GR = 789/97.86 = 8

• Turbine work loss = 42.57 MW equivalent to the heat = 218.68 MW supplied to the MSF unit.

• 2.5 MW is added for steam supply to ejector

• Total 45.1 MW, 45100/789=57.16 kJ/kg = 15.9 kWh/m3

• Shuaiba CC equivalent power output for:

• 3 GT (3×215.5 MW = 645.5 MW) +

• 1 BPST of 215.7 MW +

• 3 MSF producing 45 MIGD

• Equivalent to

• CC of 3 GT (3×215.5 MW) + 1 ST of 350.7 MW = 997.2 MW

• So Qatar needs at least similar units by 2020

Final combination of the solar field with the CC

• ISCC consists of CC PP connected to Solar Field.

• Heat to HRSG and solar field to generate steam operating one ST

• Solar field produce heat to produce steam.

• Solar collectors heat exchangers (called SSG)

• Solar steam from SSG integrated either at high, or medium, or low T

section of HRSG to oversized ST

• This ST (designed): larger than ST of original CC, but same GTs

• Operating with no solar steam, ST operate at part load

• With solar steam, ST operate at full load.

• For 350 MW ST as in Shuaiba CC, and adding 50 MW

capacity by solar steam,

• ST Load range 350 MW(87.5%) to 400 MW (100%)

• Usually, ST have almost at 85% to 100% of nominal

• If solar Capacity limited to 15%, part load negative

effect is negligible.

Sizing solar collectors

• (Carnot) =1 – (320/539.2)= 0.4065; (Cycle)

=0.5x0.9=0.36

• (collector)=heat gained by SSG/incident solar

energy)0.5

• Solar capacity = 54 MW equivalent work (Power+

Desalting)

• Heat input by solar steam = 54/0.18 = 150 MWt

• 54 MW equivalent work is assumed, to be checked later

after sizing solar field delivering 150 MW to the SSG.

• (SPP) Electricity to solar efficiency equal 18%.

• Required incident solar energy 300 MWt,

• [300,000/(0.95)]x1.2 = 378,947 m2, or 7,017 m2/MW,

Compared to Hassi R'mel ISCC in Algiers, (25 MW and

183,860 m2, 7,354 m2/MW

• Solar field parallel solar collector assemblies (SCA).

• Each 4 SCA form single circuit (loop)

• SCA example: 150 m length and 5.45 m aperture width,

area = 817.5 m2/SCA,

• Required SCA # = 378,947/817.5 = 463.5. The number

of SCA can be taken as 364, for 4 SCA per loop, the #

loops 116 loops

Solar collector assembly (SCA),

loop width = 4x aperture 36 m, length=2x 150 m, + 10 m

from each side or 330 m, area= 11,880 m2.

total loops area (solar field) = 11,880×116 =1,378,080 m2

Final combination of the solar field with the CC

Final ISCC with Desalination

• Q(SSG) = 150×1000 = MS× (2765 – 593), MS=

69.06 kg/s.

• Ms (solar) delivered/HRSG = 23.02 kg/s each HRSG.

• new steam flow rate to steam turbine is: 3×91.73 +

69.06 = 344.25 kg/s

• Original mass flow rate of 293.58 kg/s to the steam

turbine,

• Steam turbine power output 17% or from 215.7 MW

252.4 MW, or 36.67 MW increase.

• Increase of desalination output is 7.65 MW.

• Now the new CC plant consisting of 3GT×215.5 MW + 1 ST of 252.4 MW + MSF units producing 51.75 MIGD is equivalent to:

• 3GT×215.5 MW + 1 ST of 252.4 MW + 51.75 ×3 (MW/MIGD), or

• 3GT×215.5 MW + 1 ST of 407.65; total equivalent output of 1054.15 MW.

• Compared with original CC of total 997.2 MW before adding the solar field, or

• 56.95 MW equivalent power output increase

• 36.7 MW for ST and

• 6.75MIGD ×3=20.25 MW for desalting plant

Conclusion

• Kuwait and the GCC should stop installing MSF units

• They consume at least 4 times energy than SWRO

• No such thing as waste or cheap heat source

• MSF system drains country energy resources.

• EE cost by MSF in 1 year reached $M2233

• It would have been $M 361.6 if SWRO was used.

• MSF Improvements by NF to raise its capacity 35%

lead to increase of energy cost more than 3- times cost

of adding simple SWRO system to produce the same

amount of water obtained by modifying the MSF unit.

Present and future trends in thermal desalination with possible solar application

Engineering