Exergy Evaluation of the Arctic Snøhvit Liquefied Natural Gas

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Exergy Evaluation of the Arctic Snøhvit Liquefied Natural Gas Processing Plant in Northern NorwaySignificance of Ambient Temperature Anne Berit Rian and Ivar S. Ertesvåg* Department of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjørn Hejes v. 1b, NO-7491 Trondheim, Norway ABSTRACT: The first Arctic liquefied natural gas (LNG) plant was evaluated using the exergy method. The well stream is separated into flows of tradable products, substances for deposit or reinjection, pollutants for capture, and discharged products. Some natural gas is used for fuel in the on-site combined heat and power (CHP) plant. The exergy of the delivered products was 95.1% of the feed stream exergy, as most of the hydrocarbons were flowing through the plant without chemical change. The consumed exergy was the thermomechanical exergy of the feed stream, mainly as a result of its pressure, and the chemical exergy of the CHP fuel. The exergy efficiency taken as the ratio of the desired exergy change owing to separation, cooling, and compression to the consumed exergy was 23.2%. Here, separation accounted for 1.9%, while compressed CO2 accounted for 0.7%. The separation exergy was expressed as the total change of the mixing term of the chemical exergy across the processing plant. The exergy losses were 37%, 52%, and 11% in the processing plant, gas turbines, and heat recovery unit, respectively. To assess the effects of low ambient temperature, the actual plant at 4 °C was compared to a “twin” with the same material and energy flows and then to another with the same overall exergy efficiency. The former approach required an exergy efficiency of 25.6% at 20 °C and 28.1% at 36 °C, while the latter resulted in 10.9% and 19.9% less fuel consumption at 4 °C compared to 20 and 36 °C, respectively. These values represent the benefits of the cold climate compared to a similar plant in a tempered or a tropical climate.

1. INTRODUCTION The main processes of a liquefied natural gas (LNG) plant are separation, cooling, and liquefaction. As opposed to heating or power production, there are no obvious energy quantities that can be compared for performance assessment. Hence, an exergy analysis is required for an efficiency evaluation. A cold climate reduces the efforts necessary for cooling and liquefaction of natural gas (NG). The first arctic LNG processing plant has been built near Hammerfest, Norway.1,2 The gas and condensate are piped from subsea installations at the Snøhvit (Norwegian for Snow White) field located in the Barents Sea, at 71° north, about 140 km northwest of the plant. Although it was completed in 2008−09, operational problems required modifications to the processing plant, and the expected production at full capacity has been postponed several times. The products are LNG, liquefied petroleum gases (LPGs), and condensate. Furthermore, CO2 is separated from the natural gas and deposited in a geological structure near the gas field.3 The development of the Snøhvit field initiated political controversy in Norway, as this field was the first to be developed in the environmentally vulnerable area.1 In addition, the on-site gas power plant increased Norway’s total CO2 emissions by nearly 2%.1 Although the products are exported, the emissions are accounted for under the Norwegian obligations to the Kyoto protocol. According to the developer, new technology should reduce the specific energy consumption by 50−70% of the usual consumption at a processing plant at the time of design.1 This is, of course, a message reflecting the political debate: If the same LNG is to be produced somewhere else, the emissions © 2012 American Chemical Society

would be even larger. On the other hand, reduction in energy consumption is also caused by the cold environment. Exergy analysis and other second law analyses of liquefaction and separation were pioneered in the 1950s and 60s.4−7 The methods are now found in textbooks, for instance, refs 8 and 9. In spite of this, there seem to be only a modest number of studies on exergy and liquefaction in the archival journals. Examples of exergy analyses on liquefaction plants are the study by Kanoglu10 on multistage cascade refrigeration cycles, the studies of Remeljej and Hoadley11 and Cao et al.,12 who evaluated different processes of small-scale LNG production, and Aspelund et al.,13 who demonstrated a procedure for combined pinch and exergy analysis on NG liquefaction. The methods are applied to varying extents in industrial design, with the objective of optimizing subprocesses, sections, and complete plants, in combination with economic and other considerations. This can guide issues such as the choice of subprocesses, equipment, and process flows. Despite this, much of the literature on LNG processes mainly focuses on capacity, construction cost, and construction time, rather than thermodynamics and resource management. However, the choices of things such as refrigeration cycles, distillation levels, and compressor stages are of limited interest to the surrounding society. From this point of view, the interest is in the use of resources by installations like an LNG plant. An amount of fuel is used to separate, liquefy, and ship the natural gas. How well is Received: November 8, 2011 Revised: December 23, 2011 Published: January 2, 2012 1259

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were specified to balance the amounts of all species in a spreadsheet. The pressure and temperature of each flow were also defined. PRO/II version 7.0,23 with a mixture Soave−Redlich−Kwong (SRK) equation of state,24,25 provided the enthalpy and entropy differences of each flow. Then, the corresponding exergy values were calculated in the spreadsheet, while the energy and exergy losses were determined from the respective balances. The composition of the CHP exhaust was balanced with air and fuel in a spreadsheet assuming complete combustion. It was assumed to be an ideal gas, and differences of enthalpy and entropy were calculated with specific-heat temperature functions.26 The flow exergy can be split into a thermomechanical component and a chemical exergy component. The rate of chemical exergy of an ideal mixture is given by

this resource utilized? The societal perspective is taken when, for instance, a country or a region is treated in an energy or exergy analysis.14 Other examples of the societal perspective are found in the legislation of many countries regarding CHP15 and in the standardized testing (ISO, CEN) of domestic heat pumps. The CHPs and heat pumps are evaluated without considering the specific cycles, only the delivery and consumption. Furthermore, the analysis of the Algerian Arzew LNG plant16 can also be viewed in this perspective. The efficiency of LNG plants is often measured in terms of specific refrigeration power. That is, the required power for compression and refrigeration cycles, divided by the mass flow rate of LNG. Alternatively, the amount of consumed fuel energy per mass of LNG is used as a metric. The electric efficiency of a power plant provides a conversion between the two metrics. A review by Brendeng and Hetland17 showed a variation in required specific power, ranging from 0.610 kWh kg−1 at Skikda, Algeria, in 1974 to 0.296 kWh kg−1 at Bonny LNG, Nigeria, in 2003. Finn et al.18 regarded 0.330 kWh kg−1 as typical for a cascade cycle (the value was adopted by ref 10), and the fuel amount given by Kuz’menko et al.19 converts (with a 40% electric efficiency) to 0.27 kWh kg−1 of specific power for a mixed-refrigerant refrigeration loop. In comparison, Mosbergvik20 (representing Statoil, the developer) gave a specific power of 0.234 kWh kg−1 for the Snøhvit LNG plant. However, the specific fuel consumption or specific power of an LNG plant will vary with the composition of the raw gas, the degree of separation, exchange of other energy forms (e.g., electricity) with the surroundings, and, last but not least important, the ambient temperature. In particular, Dufresne et al.16 reported variations in specific compression power of 60% as a result of variation in feed gas composition, 28% as a result of the degree of LPG extraction, and 45% as a result of feed gas pressure. The aim of the study is to quantify the desired changes (separation, liquefaction, compression) to the stream of raw natural gas in terms of exergy and compare this to the exergy consumed by the plant. An interesting question in the Snøhvit case is how much the plant gains due to the low ambient temperature. Instead of comparing Snøhvit to other plants, a metric for performance can be obtained by comparing it to an ideal plant with the same products and the same environment. This is done in an exergy analysis by considering all the inflows and outflows of the plant. In this approach, information about the internal processes is not needed. Some preliminary results of this study, including energy analysis, were presented by Rian et al.21 Originally, the study was initiated as part of a well-to-consumer analysis22 of LNG. The results are still applicable in that context, although the present study is restricted to the LNG plant. In the following, the method used in the analysis of Snøhvit LNG plant is described in section 2, while the onshore LNG processing plant is outlined and further details of the assumptions are presented in section 3. Here, the installation is subdivided into a processing plant and a heat and power plant (CHP). Then, section 4 turns to the results and discussion of both subsystems and the overall plant, before the significance of ambient temperature is considered by two different approaches. Finally, the conclusions are presented in section 5.

Ekch = nk ε̅ ch k =

∑ ni , k ε̅0, i + RT̅ 0 ∑ ni , k ln xi , k i

i

(1)

where the indices i and k denote chemical species and the stream, respectively. The last term of eq 1 represents the reduced exergy due to mixing of the components. The assumption of an ideal mixture is reasonable at ambient temperature and pressure. Data for the chemical exergies of individual species were based on ref 8 and corrected for deviating ambient conditions.27,28 In particular, the change of chemical exergy for a subsystem due to separation, that is, the “separation exergy”, can be expressed as

ΔE ch =

∑ Ekch − Einch k

(2)

where k denotes the outflow streams and subscript in denotes the inflow stream. When eq 1 is introduced into eq 2 for each stream, the component exergy terms (involving ε0̅ ,i) will cancel for all inflowing substances that exit the system without chemical change. Consequently, the remaining terms can be rewritten as

ΔE ch = RT ̅ 0 ∑ { ∑ ni , k (ln xi , k−ln xi ,in)} k

i

(3)

or split into contributions for each of the streams flowing out of the system,

ΔEkch = RT ̅ 0 ∑ ni , k (ln xi , k−ln xi ,in) i

(4)

For separation, the values of eqs 3 and 4 are positive, which means that exergy (such as work) is required. This expression for the separation exergy is essentially consistent with the models of Agrawal and Xu29 and Demirel.30 In contrast, Hinderink et al.31 proposed that the mixing term of the flow exergy is to be calculated at the actual temperature and pressure of the stream. Apparently, that approach has some practical benefits regarding the calculation. However, the mixture terms need to be calculated at the same temperature for all involved streams in order to express exergy differences due to mixing or separation processes. The exergy balance of the total plant is first expressed in terms of the total exergy flow rates as

E feed =

∑ Ek + IOA + Ecool + Eexhaust k

(5)

Here, the LHS term is the total exergy rate of the feed, while the RHS covers the exergy of the products, all irreversibilities and the exergy rejected to the environment with cooling and with the exhaust from combustion. Then, the total flow exergy for each stream can be split into thermomechanical exergy, component chemical exergy, and mixture chemical exergy. Consequently, the component chemical exergies of the products cancel from the balance, and the remaining terms can be written as

2. METHODOLOGY The analyses of the overall system and the subsystems were based on the steady-state, steady flow (SSSF) rate balances of mass, elements, energy, and exergy. The mass flow rates and compositions of all flows 1260

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sulfur compounds are separated from the hydrocarbons. MEG is added to the raw NG in the well in order to avoid the formation of hydrates. After separation, MEG is pumped back through a separate pipeline into the well for reuse. The CO2 content of the well stream corresponds to approximately 2%1 of the total Norwegian emissions. Before liquefaction of natural gas, the level of CO2 in the processed stream needs to be reduced to less than 50 ppm.1 Subsequently, the separated CO2 is compressed, liquefied, and pumped through a separate pipeline for deposit near the field. The CHP plant consists of five General Electric LM6000PD gas turbines (GT) with exhaust heat recovery units (HRU). GT temperatures were found in ref 37, and the fuel had the same composition as LNG. Thermal oil with no phase change was used as the energy carrier from the CHP to the processing plant. Within the CHP subsystem, the GT and the HRU could, to a large extent, be treated separately in the analysis. The term “products”, cf. eqs 5−7, refers to the eight streams leaving the total system directly from the PP. Thermomechanical exergy of nitrogen and water were not included, however, as these flows were released to the environment. Similarly, the contributions of nitrogen and water outflows were also left out from the output-to-input ratios. Table 1 shows key data for the flows of the analysis. Compositions of the well stream (raw NG) and the main product (LNG) are shown in Table 2. These assumed

∑ ni ,fuel( ε̅0, i + RT̅ 0 ln xi ,feed) i

=

∑ Ektm + ∑ ΔEkch + IOA + Ecool + Eexhaust k

k

(6)

The LHS of this balance now forms the consumption, that is, the thermomechancial flow exergy rate of the feed and the chemical exergy of those components of the feed that are used as fuel for combustion within the plant. The desired outcome of the plant is represented by the first two RHS terms: the rates of thermomechanical flow exergy of all products and the rates of separation exergy, eq 4, for these streams. In turn, the efficiency of the plant then can be described by the ratio of the desired outcome to the consumption as

ψOA =

∑k Ektm+ ∑k ΔEkch tm + ∑ n Efeed ̅ 0 ln xi ,feed) i i ,fuel ( ε̅0, i + RT

(7)

For systems delivering or receiving heat, work, or electricity, the associated exergy rates are then added to the product or consumption, respectively. This definition corresponds to, for example, the general exergy efficiency of Fratzscher et al.32 and the special efficiency formulated for separation by Kotas8 and Demirel.30 In 1969, Kostenko (cf. ref 32) defined the transit exergy as the exergy flow that remains constant during a process, and this concept was further developed by Brodyansky et al.33 Here, the component chemical exergies of the products represent the transit exergy. The purpose of the definition above and the concept of transit exergy is obvious when considering hydrocarbon systems, where the chemical exergy is, by far, the main contributor to the in- and outflows. When the component chemical exergy remains constant throughout the process, the ratio of outflow to inflow can be close to unity. Thus, using the outflow to inflow ratio as a performance indicator will be misleading because it gives the impression of a very good performance of the process (cf. ref 34.). Moreover, a large through-flow or transit exergy will reduce the visibility of improvements in the outflow-to-inflow ratio. For a power plant, with no transit exergy, the above definition reduces to the usual ratio of exergy rates of heat and power delivery to input fuel exergy.

Table 1. Mass Flow Rates, Temperatures, and Pressures of the Flows into and out of the Processing Plant (PP) and Combined Heat and Power Plant (CHP)

3. CASE DESCRIPTION AND ASSUMPTIONS The data for this study are compiled from a variety of sources (refs 1, 2, 20, 22, 35, 36.). Together, the data set comprises what we regard as a realistic case. The operational data are expected to have day-to-day variation, seasonal variation, and field-lifetime variation. Moreover, the design of a complex plant is divided into subtasks that do not always have consistent data sets. Finally, modifications are already being made to the plant, introducing deviations from the original design. The system of analysis is shown in Figure 1. At the processing plant, water, nitrogen, monoethylene glycol (MEG), carbon dioxide, and polluting components such as mercury and

stream

mass flow rate (kgs−1)

temp. (°C)

pressure (bar)

feed (raw NG) LNG LPG condensate fuel to CHP CO2 for deposit MEG for reuse H2S captured nitrogen for release water for release air to GT exhaust (from GT) exhaust (to stack) cooling water in cooling water out

227.44 151.17 7.95 24.67 10.72 22.36 3.22 0.0017 5.40 1.96 481.74 492.46 492.46 11.94 × 103 11.94 × 103

0.0 −162.2 −41.0 4.0 11.0 52.0 0.1 52.0 17.0 17.0 4.0 452.0 165.0 4.0 12.6

70.00 1.01 1.01 1.01 66.25 180.00 70.00 180.00 1.01 1.01 1.01 1.01 1.01 1.01 1.01

compositions are possible and realistic, although not available in open documents. The steady-state, steady-flow assumption is reasonable for the CHP and for the separation and liquefaction processes with delivery to the intermediate storage tanks. However, it implies that the intermittent bulk delivery and the intermediate storage were not a part of the analysis. The cooling water and atmospheric air temperatures were both assumed to 4 °C. In the exergy calculations, the air was assumed to have a relative humidity (RH) of 70% and the dryair composition was taken from ref 8. The ratio of the chemical exergy to the LHV was 1.04 and 1.05 for LNG and raw NG, respectively. Cooling of the process plant is conducted with seawater at a flow rate of 43 000 m3 h−1, which can be heated to 15 °C.35 The power plant has no cooling.

Figure 1. Overall system and subsystems with streams of matter and energy: processing plant and combined heat and power plant (CHP). 1261

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were followed: First, mass flow rates, compositions, and states of the flows were maintained, except from the feed temperature, which was set to 4 K below ambient for all cases. The supplies of electricity and heat from the CHP were also kept constant. Second, the analysis was repeated for ambient temperatures of 20 and 36 °C, given constant overall exergy efficiency. This was achieved by adjusting the amount of fuel (and hence the amount of feed), while the mass of the main product (LNG) was maintained at the same level.

Table 2. Compositions of the Feed NG (Raw Natural Gas) and LNG substance

feed NG (%)

LNG (%)

methane ethane propane n-butane i-butane n-pentane i-pentane n-hexane n-heptane C8+a arenesb hydrogen sulfide water carbon dioxide nitrogen MEG sum

80.92 4.77 2.42 0.596 0.380 0.226 0.269 0.317 0.353 0.785 0.194 0.0005 1.037 4.84 2.40 0.494 100.00

91.88 5.32 1.93 0.101 0.124 0.0001 0.0004

4. RESULTS AND DISCUSSION 4.1. Combined Heat and Power (CHP) Plant. This subsystem may be treated in two separated parts: gas turbines (GTs), where all the fuel is burned and all the electrical energy is produced, and the heat-recovery unit (HRU), where the heat of the exhaust is utilized. Thus, the CHP delivers all the heat and electricity needed to operate the processing plant. The irreversibility due to the heat transfer from the exhaust to the hot oil in the HRU was included in the CHP analysis, whereas the irreversibility due to heat transfer from the hot oil to the process was included in the analysis of the processing plant (section 4.2). The net power delivery from the plant was set to 184 MW, while the heat delivery was 142.45 MW. The exergy associated with the delivered heat was 56.79 MW. Consequently, the total exergy delivered from the CHP amounted to 240.8 MW, which gave a CHP exergy efficiency of 43.5%. The rate of total exergy loss of the CHP amounted to 313.1 MW, which was 56.5% of the fuel exergy, whereas the GT irreversibility rate was estimated to 258.2 MW or 46.6% of the fuel exergy. Normally, the main contributor to GT irreversibility is combustion, which typically comes to one-third of the fuel exergy. The irreversibility of the HRU was 23.0 MW or 4.2% of the fuel exergy, mainly as a result of heat transfer over large temperature differences between the exhaust and the hotoil system. Moreover, the rejected exhaust still carried 5.8% of the fuel exergy, as a result of a relatively high exhaust temperature. 4.2. Processing Plant. The exergy values for the material flows of the processing plant are shown in Table 3. The total exergy rates of the tradable products (LNG, LPG, and condensate), the fuel to the CHP and the MEG for reuse were 10 032.7 MW, which amounted to 98.1% of the total exergy inflow with the feed stream, the heat, and the electricity.

0.0051 0.638 100.00

a

C8+ includes 0.287% n-octane, 0.122% n-nonane, 0.1213% n-decane, 0.0542% n-undecane, 0.0534% n-dodecane, 0.0422% n-tridecane, 0.0284% n-tetradecane, 0.0215% n-pentadecane, 0.0129% n-heksadecane, 0.0029% n-heptadecane, 0.0086% n-oktadecane, 0.0060% nnonadecane, and 0.0146% n-eicosane. bArenes include 0.065% benzene, 0.077% toluene, and 0.052% m-xylene.

The exergy required for compression of the fluid was estimated on the basis of the results from Ertesvåg et al.,38 as the full specifications for the subprocess of CO2 separation and compression were not available. Their work described a threestep intercooled compression from 1 atm to 60 bar, condensing, and subsequent pumping to 200 bar. Similar to the total plant, cf. eq 6, the PP can be described by a balance where the consumption is the rate of thermomechancial flow exergy of the feed and rates of exergy in heat and power. Accordingly, the desired outcome is the rates of thermomechancial flow exergy of the products and the CHP fuel and separation exergy of the outflows. Thus, the exergy efficiency of the PP can be expressed as the ratio of the desired outcome to the consumption. The analysis was repeated for ambient temperatures of 20 and 36 °C, while the ambient RH and pressure were maintained at 70% and 1 atm, respectively. Two approaches Table 3. Exergy of Material Flows specific flow exergies −1

feed (raw NG) LNG LPG condensate fuel for CHP CO2 for deposit MEG for reuse H2S captured nitrogen for release water for release exhaust (from GT) exhaust (to stack) cooling water out

rates of flow exergy −1

chemical (kJ kg )

thermomechanical (kJ kg )

chemical (MW)

thermomechanical (MW)

total (MW)

43 497 51 159 48 625 47 169 51 159 425.23 19 436 23 937 23.89 45.72 28.49 28.49

419.45 875.55 54.02 0.00 527.62 202.86 7.33 186.70 0.31 1.53 198.17 36.16 0.55

9891.63 7733.63 386.41 1163.46 548.20 9.51 62.57 0.04 0.13 0.09 14.03 14.03

95.40 132.36 0.43 0.00 5.65 4.54 0.02 0.00 0.00 0.00 97.59 17.81 6.55

9987.03 7865.98 386.84 1163.46 553.85 14.04 62.60 0.04 0.13 0.09 111.62 31.84 6.55

1262

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Here, the thermomechanical exergy of the products only contributed to 1.4% of the total exergy, while the chemical exergy covered the remaining. Furthermore, the separated and compressed CO2 and H2S for deposit and storage added another 14.1 MW to the outflow, or 0.1% of the total feed exergy. When decomposing the stream exergies, the component chemical exergies cancels from the balance of the PP, as shown in eq 6. The feed component chemical exergy rates summarized to 9914.7 MW, which was the transit exergy of the PP (see section 2). Obviously, this value dominates the balance when it is included on both sides. The rate of exergy consumed by the PP amounted to 336.2 MW, where 95.4 MW was due to the thermomechanical exergy of the feed and the remaining part was due to heat and electricity received from the CHP. Still, according to eq 6, the useful output of the PP is the rate of thermomechanical exergy of the products and fuel (143.0 MW, Table 3) and the separation exergy of all outflows (12.4 MW, Table 4). Hence, the exergy efficiency defined similar to eq 7 was 46.2%. Thus, the thermomechanical exergy of LNG contributed the most to the exergy efficiency. Regarding the

It should be noted that, from the 184 MW power supplied to the plant, only 127 MW20 is used for the refrigeration cycle. This value corresponds to a specific power of 0.234 kWh kg−1, which was a low value compared to those of other plants, as described in section 1. However, the distinction between refrigeration and other-purpose power was not clear. Yet another Statoil document2 specified 144 MW (out of 184 MW total) for the liquefaction compressors. These values illustrate that the specific power has limitations as an indicator for LNG plant performance. Without doing a detailed process analysis, it can be assumed that most of the electric power (176 MW, when the CO2 compression is subtracted) and the thermomechanical exergy of the feed (95 MW due to pressure) are used for refrigeration/ liquefaction and most of the heating (57 MW exergy) is used for separation. When these values are compared to the results of liquefaction (132 MW thermomechanical exergy) and separation (12 MW chemical exergy difference), it seems apparent that the liquefaction processes are considerably more efficient than the separation processes. 4.3. Overall Plant. The exergy analysis of the overall plant gave an exergy loss of 494.1 MW, which amounted to 4.9% of the incoming flow (feed). The exergy of the products was 95.1% of the exergy of the feed. This ratio is high because the output is hydrocarbons, which do not change chemically during this processthey are just separated and cooled. The transit exergy, that is the component chemical exergy of the products, came to 9913.5 MW, which amounted to 93.8% of the feed exergy. The exergy efficiency, as defined in eq 7, of the total plant made 23.2%. The exergy loss rates summarized to 462.0 MW. The total lost exergy, comprising the irreversibilities of the plant and flow exergy of the rejected flows, were distributed as 36.6% in the processing plant, 52.3% in the gas turbine, and 11.1% in the heat recovery unit, including 6.4% in the rejected exhaust. It should be noted that once the plant is built, efficiency is rather hard to improve. Redesign may require costly outages, and construction work within the plant imposes a risk of unplanned shutdowns. Hence, the economy of such thermodynamical improvements will always be regarded doubtful by the operator of the plant. Therefore, effective solutions have to be encouraged and implemented in the first place, that is, when designing the plant. 4.4. Accuracy. There were no iteration errors in composition, temperature, and pressure, because the amounts of species were specified to balance and the temperatures and pressures of the flows were given. Similarly, the energy and exergy losses were found from the balances. The uncertainties39 of the analysis emerged from errors in the calculation of specific enthalpies and exergies of the flows. Some of these contributions either canceled from the balances or had minute effects. Some uncertainties of chemical exergy and lower heating value could be found from data.40 No error data were available for the thermal components of enthalpy and entropy, as calculated by means of PRO/II. Therefore, an estimate was based on single-component comparisons of PRO/II versus a multiparameter curve-fit of experimental data41,42 of the most important components. The latter data were assumed to be more accurate than the Soave−Redlich−Kwong equation used by PRO/II and the deviations to indicate the accuracies of the present calculations.

Table 4. Component Chemical Exergy and Separation Exergy for the Streams from the Process Plant stream

comp. chemical exergy rate

separation exergy rate

ratio

molar separation exergy

k

∑ini,kε0̅ ,i (MW)

ΔEkch (MW)

ΔEkch/ ∑ini,kε0̅ ,i

ΔEkch /nk (kJ kmol−1)

LNG LPG condensate fuel for CHP CO2 for deposit MEG for reuse H2S captured nitrogen for release water for release

7741 386.9 1165 548.7 9.51

2.11 1.17 1.99 0.15 3.54

0.00027 0.0030 0.0017 0.00027 0.37

62.57

0.63

0.010

12 236.8

0.04

0.0014

0.035

28 210.4

0.13

1.66

12.84

8595.1

0.09

1.15

12.78

10 528.7

244.6 7354.3 8541.7 244.6 6977.2

separation exergy, the major contributions in terms of component chemical exergies were associated with the species that were concentrated to pure substances, such as CO2, N2, and H2O, see Table 4. When considering the molar separation exergy for each product, H2S and MEG gave the most significant contributions, while those of the hydrocarbon flows were small, LNG in particular. The remaining 180.8 MW of exergy was lost as irreversibilities (174.2 MW) of the PP or rejected with cooling water (6.6 MW). Because CO2 is always removed from natural gas before liquefaction, the separation cost is not particular for the plant of the present analysis. However, the CO2 content of the well stream changes from field to field, and hence, the exergy required to separate it also differs. Moreover, while other plants simply release it to the atmosphere, the Snøhvit plant deposits the CO2. The results of ref 38 implied a specific work for compression of 366 kJ kg−1 CO2 or 8.2 MW in total, and a specific thermomechanical exergy of 203.0 kJ kg−1 of or 4.54 MW in total for the present case. The compression power obtained here was consistent with Statoil’s value of 8 MW.2 1263

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thermomechanical exergy increased, the total exergy losses decreased, the exergy transferred with heat decreased, and the other exergy flows showed only minor changes. As the process is more demanding at high ambient temperatures, keeping the fuel consumption constant means that the efficiency had to increase with ambient temperature. Table 5 shows the efficiencies for the cases with ambient temperature of 4, 20, and 36 °C, while Figure 2 gives the distribution of the useful

The uncertainties in chemical exergy and lower heating value had minor effects on the efficiencies, as the thermomechanical components dominated these ratios. The most significant uncertainties were found in the PP exergy efficiency, which was less than 0.4 %-points. This was caused mainly by the uncertainties of the LNG, which had relative uncertainties for thermal enthalpy and entropy differences of 5% and 1.5%, respectively. Other uncertainties either were relatively less or had less impact on the results. The effects of correcting the chemical exergy for mismatches between the phase of the tabulated pure substances and the substances in the actual mixtures at ambient pressure and temperature were examined. It was found that the deviation was only 0.03% in the chemical exergy for the condensate and less for the feed. Hence, this was negligible, as it was less than the uncertainty of the tabulated chemical exergies. The chemical exergies corrected for ambient temperature and RH27 exceeded the tabulated values with the temperature correction by 0.30% (feed) to 0.44% (LNG) when using the temperature correction of Kotas.8 These values were above the uncertainties of the tabulated values. 4.5. Effects of the Ambient Temperature. The benefits of low versus high ambient temperature for a power plant are well-known. For an LNG plant, a cold climate lowers the starting point for the liquefaction of gases, and the separation exergy decreases with decreasing ambient temperature. The significance of low ambient temperature for an LNG plant may be illustrated with the following estimate: when liquefying pure methane at −162 °C (saturation at 1 atm), the heat removed is independent of the ambient temperature. However, the required minimum exergy has increased 16% at an ambient temperature of 30 °C compared to 4 °C. If a substance with, for instance, a constant specific heating capacity and no phase change is cooled from ambient temperature to −162 °C, the heat removed has increased 16% at an ambient temperature of 30 °C compared to that removed at an ambient temperature of 4 °C. In comparison, the required exergy corresponds to a substantial 29% increase. A remaining question was how to compare performances of plants located in different climates. As mentioned in section 1, comparing the fuel consumption (mass or LHV) per unit of delivered LNG or other products is an option. This metric seems reasonable in the sense that a unit of consumed fuel is a unit of degraded natural recourse. However, it does not take the natural constraints into consideration. Even if the cold-climate plant spends less fuel for its operation, the warm-climate plant will still have a better thermodynamic performance if the amount of avoidable resource degradation is lower. A higher ambient temperature decreases the chemical exergy of hydrocarbons (at constant RH) and increases the thermomechanical exergy of substances cooled to a certain temperature. The comparison, therefore, leads to two options. The first is to compare the actual plant with a hypothetical “twin” plant, with the same mass flows, at different ambient temperatures. As the LHV varies very little with temperature, the “twin” will then, with a very close approximation, have the same energy flows as well. The exergy efficiency, eq 7, will, however, be different. The other option is to maintain the overall exergy efficiency and the main product (LNG) flow rate. The other mass flows, including the amounts of CHP fuel, will then vary with ambient temperature. First, the analysis was repeated for ambient temperatures of 20 and 36 °C with maintained mass flow rates. The product

Table 5. Performance in Terms of Exergy Ratios at Increasing Ambient Temperatures

ambient temperature (T0) overall exergy efficiency (%) process plant exergy efficiency (%) CHP exergy efficiency (%) overall products-to-input ratio (%) process plant products-to-input ratio (%)

overall exergy efficiency maintained

base case

all mass flows maintained

4 °C

20 °C

23.2 46.2

25.6 51.0

28.1 55.8

23.2 47.1

23.2 48.0

43.5 95.1

42.7 95.2

41.9 95.3

42.7 94.5

41.9 94.0

98.2

98.4

98.5

98.1

98.0

36 °C 20 °C 36 °C

exergy from the PP for the three ambient temperatures. The thermomechanical exergy of LNG is by far the main outcome,

Figure 2. Useful exergy rates of the processing plant (PP) at increased ambient temperature when all flows are maintained: thermomechanical exergy of LNG, other products, and CHP fuel; separation exergy for products and fuel.

which is increasing with the ambient temperature. In fact, LNG holds 99.7% of the thermomechanical exergy of the sale products. The separation exergy, eq 4, for the outflows of the PP is presented in Figure 3. Regarding H2S, the contribution was too small to be visible in this scale. The graph shows that the largest contribution to separation exergy came from substances completely separated, such as CO2 and N2. These quantities increased nearly linearly with the ambient temperature, as expected from eq 4. The exergy utilization of the total plant, distributed as thermomechanical and separation exergy, is given in Figure 4. In fact, the thermomechanical exergy of the sale products (mainly LNG) covers 88.8% of the overall utilized exergy, while the separation exergy amounts to 9.2% of the contribution of the sale products. Next, the analysis was repeated for a constant overall exergy efficiency of 23.2% at ambient temperatures of 4, 20, and 36 °C. The flow rates of feed and fuel were then changed, while the 1264

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Figure 3. Separation exergy rates of the streams from the process plant (PP) at increased ambient temperatures when all flows are maintained.

Figure 5. Useful and lost exergy of the overall plant at increased ambient temperatures at constant overall exergy efficiency: thermomechanical exergy of sale products and other products; separation exergy for all outflows; irreversibilities of the process plant (PP), the gas turbine (GT), heat-recovery unit (HRU), and flow exergy in exhaust rejected to stack.

5. CONCLUSIONS The effectiveness of the first arctic LNG processing plant was assessed by an exergy evaluation. For the overall system, the ratio of exergy of the products to exergy of the feed was 95.1%. This value was, as expected, high because of the dominating, nearly unchanged chemical exergy. Therefore, the exergy efficiency was taken as the desired change in the exergy of the products as a result of separation, cooling, and compression divided by the exergy consumed to achieve this. The exergy efficiency was a modest 23.2%. The analysis showed that 37% of the overall exergy losses occurred in the processing plant, whereas the gas turbines, including the combustors, caused 52% of the exergy losses, and the heat recovery unit and exhaust gas to the stack accounted for 11% of the overall exergy loss. The exergy efficiency of the combined heat and power plant (CHP) was only 43.5%. The heat recovery unit is the single process that appears to be the first candidate for improvement, as irreversibilities in combustion hardly can be avoided and the processing plant consists of numerous processes. This includes minimizing the temperature differences, as well as decreasing the exhaust temperatures (to stack). In addition, estimates indicate that the separation processes are less efficient than compression and refrigeration processes. The main part of the desired exergy change was attributed to the cooling of the LNG. Although the exergy change due to separation processes and compression of CO2 accounted for 1.9 and 0.7%, respectively, of the consumed exergy. Other contributions to the desired exergy change were, however, minute. The effects of the ambient temperature were also investigated. The low ambient temperature in Northern Norway, assumed to be 4 °C, had a considerable, positive impact on the fuel consumption. Reducing the ambient temperature from 36 to 4 °C implied a reduction in exergy consumption by 19.9%, while a reduction from 20 to 4 °C gave a reduction of 10.9%. This means that a reduction beyond these values can be attributed solely to improved technology.

Figure 4. Exergy utilization of the overall plant at increased ambient temperatures when all flows are maintained: thermomechanical exergy of sales products (LNG, LPG, and condensate) and other products (MEG, CO2, and H2S); separation exergy for all outflows (sale products, other products, nitrogen, and water).

product LNG stream was maintained. The efficiencies are shown in the two last columns of Table 5. The ratio of CHP fuel LHV to sales-product LHV (LNG, LPG and condensate) was 5.9% at 4 °C, while it increased to 6.6% at 20 °C and 7.4% at 36 °C. The Snøhvit plant should basically, because of the cold climate (4 °C), consume 80.1% of the exergy or amount of fuel, as compared to a similar plant in a warm climate (36 °C), or 89.1%, as compared to one in a tempered climate (20 °C). This means that a reduction beyond these values can be attributed to an improved technology. Thus, if the 30−50% reduction in fuel consumption expected by the developer1 holds, the new plant will represent a substantial improvement. In these computations, the separation exergy and the useful exergy were close to those seen in Figures 2 and 3, when all flows are maintained. The rates of useful and lost exergy are illustrated in Figure 5. It is seen that the consumption, given by the height of the columns in the graph, increases considerably, corresponding to the increase of the product exergies at increased ambient temperature. As emphasized in section 1, the comparability of values for different plants can be uncertain. However, if a specific power consumption of 0.3 kWh kg−1 has been achieved for a tropical plant, as indicated by ref 17, the reduced consumption at Snøhvit LNG is, to a large extent, due to the low ambient temperature. A further investigation of plants in tempered and tropical climates, using the method applied here with a “twin” in an arctic climate, would give a better basis for the comparison. 1265

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +47 73593839. Fax: +47 73593580. E-mail: Ivar.S. [email protected].

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ACKNOWLEDGMENTS Major data collection and some preliminary analysis were conducted by Helene Lie as part of her master’s thesis. The authors are grateful for discussions with and input from Dr. Jostein Pettersen and Dr. Geir Owren at Statoil.

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NOMENCLATURE Latin Symbols E = rate of flow exergy (W). I = rate of irreversibility (i.e., exergy destruction) (W). n = molar flow rate (mol s−1). R̅ = universal gas constant. T0 = ambient temperature (K). xi = molar fraction of species i. Greek Symbols

ΔE = difference in rate of flow exergy (W). ε̅ = molar flow exergy (J mol−1). ε0̅ ,i = molar chemical exergy of component i (J mol−1). ψ = exergy efficiency.

Subscripts and Superscripts

ch = chemical (exergy). i = index for chemical species. k = index for stream. OA = overall plant. tm = thermomechanical (exergy). Abbreviations

CHP = combined heat and power. HRU = heat recovery unit. GT = gas turbine. LHS = left-hand side. LHV = lower heating value. LNG = liquefied natural gas. LPG = liquefied petroleum gases. MEG = monoethylene glycol. NG = natural gas. PP = processing plant. RH = relative humidity. RHS = right-hand side.

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