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Production of Synthetic Fischer-Tropsch Diesel from renewables: Thermoeconomic and Environmental Analysis Mahrokh Samavati, Massimo Santarelli, Andrew Martin, and Vera Nemanova Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02465 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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Energy & Fuels

Production of Synthetic Fischer-Tropsch Diesel from renewables: Thermoeconomic and Environmental Analysis Mahrokh Samavati*‡,ǁ, Massimo Santarelli‡, ǁ, Andrew Martinǁ, Vera Nemanovaǁ

‡

Department of Energy, Polytechnic University of Turin (POLITO), Corso Duca degli Abruzzi 24, 10129 Turin, Italy

ǁ

Energy Department, Royal Institute of Technology (KTH), Brinellvägen 68, SE-100 44 Stockholm, Sweden

Abstract

In this study, a novel integrated system for production of advanced synthetic diesel is proposed and analyzed from thermodynamic, economic, and environmental perspectives. This system consists of a solid oxide electrolyser (SOEC), an entrained gasification (EG), a Fischer-Tropsch reactor (FT), and upgrading processes. Eleven different combinations of precursor syngas production through steam and CO2 co-electrolysis and biomass gasification are investigated. Results show that increasing share of

*

Royal Institute of Technology, Department of Energy Technology, Brinellvägen 68, SE-100 44 Stockholm, Phone: +46

8-7907481, E-mail: [email protected]

1

ACS Paragon Plus Environment

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produced syngas in the electrolyser unit results in higher system efficiencies, emission savings, and levelized cost of FT diesel. Moreover, different options of heat and mass flow recovery are considered. It is concluded that recovery of produced medium pressure steam in the system is highly beneficial and recommended. Besides, it is shown that while oxygen recovery is the best choice of mass recovery, hydrogen recovery for internal use has adverse effect on the system performance.

Keywords: Solid Oxide Electrolyser, Entrained Gasification, Fischer-Tropsch, Synthetic Fuel Production, emission, economic analysis

Nomenclature

Abbreviations

ASU

Air separation unite

BCC

Base capacity of component

ECC

Estimated capacity of component

EG

Entrained gasification

FT

Fischer-Tropsch process

FTS

Fischer-Tropsch Synthesis

HTE

High temperature electrolyser 2


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Energy & Fuels

LGHC

Light gaseous hydrocarbons

LHV

Lower heating value

LTFT

Low temperature Fischer-Tropsch

MP

Medium Pressure

OCV

Open circuit voltage

RES

Renewable energy systems

SOEC

Solid Oxide Electrolysis Cells

WGS

Water gas shift

Greek symbols

β

Biomass exergy constant

Ψbiomass

Exergy content of biomass, MW

ΨF,sys

Exergetic fuel, MW

ΨP,sys

Exergetic product, MW

ηs

Energy efficiency of the system 3


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Latin symbols

b

Exergy content of stream, MW

bph

Physical exergy content of stream, MW

bch

Chemical exergy content of stream, MW

CICP

Annualized cost of installed capital of plant, $/year

CO&M

Annualized cost of operation and maintenance, $/year

CU

Annualized cost of utilities, $/year

CEl.

Annualized cost of electricity, $/year

CFeedstock

Annualized cost of feedstocks, $/year

Cincome

Annualized income from selling the byproducts, $/year

Cp

Purchase cost of heat exchanger, $

CB

Heat exchanger base cost, $

CHX-Capital

Capital cost of heat exchanger, $

Cbase

Base cost of each component, $ 4


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Energy & Fuels

Ccomponent

Capital cost of component, $

Es

Exergy efficiency of the system

E

Total emissions from the use of the renewable fuel, gCO2eq/MJ

Ef

Emissions from the use of the fossil based fuel, gCO2eq/MJ

eec

Emissions from the extraction or cultivation of raw materials, gCO2eq/MJ

el

Annualized missions from carbon stock changes caused by land-use change, gCO2eq/MJ

ep

Emissions from processing, gCO2eq/MJ

etd

Emissions from transport and distribution, gCO2eq/MJ

eu

Emissions from the fuel in use, gCO2eq/MJ

esca

Emission saving from soil carbon accumulation via improved agricultural management, gCO2eq/MJ

eccs

Emission saving from carbon capture and geological storage, gCO2eq/MJ

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eccr

Emission saving from carbon capture and replacement, gCO2eq/MJ

eee

Emission saving from excess electricity from co-generation, gCO2eq/MJ

FP

Pressure factor of heat exchanger

FM

Material factor of heat exchanger

Qtot

Total heat input to the system, MW

Wel,tot

Total electrical power demand of system, MW

x

Cost scaling factor

1

Introduction

The global transportation sector is one of the major sources of greenhouse gas (GHG) emissions.1 According to a recent European Commission report, the EU transportation sector is responsible for 25% of Europe’s GHG emissions, of which 70% is linked to road transportation.2 As a result, replacing fossil-based transportation fuels with CO2-neutral bio-based or synthetic transportation fuels continues to gain attention. This approach not only results in reduction in GHG emissions but also increases energy security by eliminating dependency on imported fossil fuels. The European Union introduced a blending target of 10% renewable share in form of advanced (non-food based) biofuels into the transportation sector by 2020.3 6


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Energy & Fuels

Production of liquid transportation fuels from biomass through Fischer-Tropsch (FT) process and upgrading technology is one of the suggested routes towards meeting and later exceeding this blending target.4 Precursor syngas for this approach can be provided from gasification of lignocellulosic biomass feedstocks in high temperature gasifiers such as entrained flow gasification (EG) reactors. The EG technology allows for production of high quality syngas with low or no tar production from wide range of biomass feedstocks.5–7 However, the production potential of advanced biofuels usually cannot cover all the transportation sector demand. To exemplify, the Italian transportation sector’s demand was 246 TWh diesel and 101 TWh gasoline in 2012.8 On the other hand, the annual potential for domestic production of lignocellulosic based biofuels was only in the range of 10-17 TWh for the same year.9

To close this gap between production and demand, another pathway for supply of precursor syngas from renewable sources can be included. For this purpose, high temperature solid oxide electrolysis cells (SOEC) can be considered as a viable option. SOECs are modular devices that have the unique ability of simultaneously electrolyzing carbon dioxide and steam, with lower electricity demand owing to their high operating temperature (600-900 oC) in comparison to the low temperature electrolysis technology.10–12 The required carbon dioxide can be provided from coal power plants, steel mills, or other sources towards the final target of direct recirculation of CO2 from atmosphere, resulting in reuse of CO2 emission as well as reduction of fossil fuel consumption for chemical and fuels production. Produced syngas then can be considered as sustainable and emission free provided that the required electricity for co-electrolysis process is generated from renewable resources. Such approach not only results in increase of renewable contributions in the transportation sector but also provides a solution

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for their unreliable intermittent nature. The latter can be presented as energy storage when the excess produced electricity that cannot be absorbed by grid will be used by SOEC instead.

Many published studies have considered production of FT liquid transportation fuels from a variety of biomass sources. In one study, Manganaro et al. presented an energy balance on the production chain of high quality liquid transportation fuel from harvesting of surplus biomass to production of FT diesel.13 They showed that the weight of produced FT diesel accounts for about 13% of initial biomass input, and recommended using produced char and non-condensable gases for drying and pyrolysis processes in order to increase the system efficiency. In another study, Baliban et al. introduced a process framework for the conversion of hardwood biomass to liquid transportation fuels.14 They performed 12 case studies to determine the effect of key operating parameters on the overall system cost. Their results showed that after reaching a certain price of hardwood biomass, the proposed refinery cannot economically compete with crude oil refinery. This threshold price depends on the refinery capacity and decreases with decrease in the refinery capacity. As an example, 70 $/tdry is the threshold purchase cost of hardwood biomass where refineries with production capacity of one thousand barrels per day would be economically competitive with crude oil refinery provided that crude oil price would be 105$/bbl. Niziolek et al. used an optimization-based process synthesis framework to investigate production of liquid transportation fuel from coal and biomass.15 This investigation considered 24 case studies in their work based on three different combinations of coal and biomass as feedstock. It was concluded that production of liquid transportation fuels from mixture of biomass and coal is economically competitive with conventional fuels with GHG emission reductions of 30-50%.

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Energy & Fuels

In contrast only a handful of studies have considered integration between electrolysis and FT processes. Becker et al. proposed a theoretical model for integration of FT to a high temperature solid oxide co-electrolyser unit.16 It was suggested that storing renewable energy in form of liquid synthetic fuel is a promising technology. In a more recent study, Stempien et al. used a thermodynamic model of a simplified integration between SOEC and FT systems and concluded that driving system compressors with the energy of recovery turbines will result in highest system efficiency of about 66%.17 In another study, Li et al. suggested a route for synthetic fuel production via integration of electrochemical conversion of CO2 and FT process.18 According to this study, the produced synthetic fuels cost would be in range of 3.80 to 9.20 $/gallon depending on the level of technology advancement. Chen et al. performed a numerical study on SOEC-FT for methane production at pressures 1-5 bar.19 In this pressure range, they suggested optimal pressure of 3 bar where methane production would be at its peak value. However, the main focus of their study was the reactor and reactions and they did not consider other elements that are required for the system.

The current study presents a novel integration between SOEC, EG and FT technologies as a pathway for advanced synthetic liquid hydrocarbon fuel production from renewable sources. As explained earlier, this integration increases production potential of advanced liquid hydrocarbon fuels and consequently decreases the gap between production rate and transportation demand. Also, an integrated system allows for near closed-loop operation since a byproduct of one subsystem can be considered as fuel for another one. For example, pure oxygen from SOEC can be used as gasification agent while produced carbon dioxide in the entrained gasifier can be sent to the electrolysis unit. In this study, the main output of the system is considered to be FT diesel while the input is biomass, carbon

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dioxide, water, and the excess electricity from renewable power plants such as solar and wind. Naphta, wax, light gaseous hydrocarbons (LGHC), and hydrogen which are produced during FT process and product upgrading are considered as byproducts. The system is designed in such way to be able to produce 30 m3/h of FT diesel. This system size is selected considering the limits that are suggested for FT plants in order to be economically sustainable.20 The proposed system is analyzed numerically considering thermodynamic, economic, and environmental aspects. Furthermore, different interlinks between subsystems are analyzed and suggested.

2

System Description

Figure 1 illustrates the baseline integration between SOEC, EG, and FT subsystems. As can be seen, the required precursor syngas for further production of 30 m3/h of FT diesel is produced from SOEC and/or EG. Eleven different combinations of SOEC and EG syngas production capacity are considered in this study. These combinations are defined based on the percentage of produced syngas in the SOEC subsystem using increments of 10%, thus yielding 11 cases. Table 1 presents the nominal capacity of syngas production in each combination as percentage of total required syngas to meet the target production of FT diesel.

10


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Figure 1. Schematic of SOEC-EG-FT integrated system (baseline integration)

Table 1. Nominal syngas capacity of SOEC and EG subsystems as percentage of final required syngas

1

2

3

4

5

6

7

8

9

10

11

SOEC

0

10

20

30

40

50

60

70

80

90

100

EG

100

90

80

70

60

50

40

30

20

10

0

(%)

production Capacity

Combination No.

Nominal Syngas

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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3

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Model Description

The system was modeled using ASPEN Plus software.21 A process flow sheet of each subsystem containing all of the necessary components is created and then the parameters for each component were introduced to the software. Standard components from ASPEN Plus library have been used to model components like heat exchangers, mixers and separators, pumps and compressors. The model parameters have been chosen such that to assure achievement of the desired operating temperature and pressure in each subsystem as well as hydrogen to carbon monoxide ratio for the FT diesel production process.

The final target of the system is to produce about 30 m3/h of FT diesel using the produced syngas in SOEC and/or EG subsystems. The selected capacity of FT diesel production is in line with medium scale FT systems. A constraint is introduced to the model to define the percentage of precursor syngas that should be produced in either SOEC or EG subsystem. To ensure maximum production of middle distillates in the FT reactor, an operation point of 25 bar and 240 oC over cobalt based catalyst was chosen. The FT operating condition is selected based on the reported operating condition for the Sasol chemical process plant in South Africa.22 Also, the ratio between hydrogen and carbon monoxide at the electrolysis and gasification system outlet was considered to be 2.1, a typical value for Fischer-Tropsch reactions using a cobalt catalyst.16,23,24 SOEC and EG subsystems operate at atmospheric pressure and operating temperatures of 800 and 1200 oC, respectively.

12


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Energy & Fuels

3.1

Solid Oxide Electrolysis Unit

Table 2 shows main reactions in solid oxide electrolyser system. In the SOEC unit, steam and carbon dioxide are reduced simultaneously at cathode according to the electrochemical reactions 1 and 2. The oxygen ions produced in these reactions further travel to the anode electrode of the cell through solid electrolyte and form oxygen via electrochemical reaction 3. Since hydrogen, steam, carbon dioxide and carbon monoxide are present on the same time in the cathode compartment, chemical reactions between these elements is quite expected. Therefore, as shown in Table 2, water gas shift reaction, reaction 4, also occurs in situ of SOEC cathode. This reaction governs the relative contributions of each element in the inlet and outlet streams. It is widely accepted that the major part of carbon monoxide content from the cathode outlet is produced from reversed water gas shift reaction rather than electrochemical reaction.10,25 Note that, since SOEC operates at atmospheric pressure, internal production of methane is negligible.26

Table 2. Main reactions in the solid oxide electrolyser

No

Reaction

Heat of Reaction (kJ/mol)

1

H2O + 2e- → H2 + O2-

+241.8

2

CO 2 + 2e- → CO + O2-

+393.5

3

O2-→ 1/2O2 + 2e-

---

4

CO + H2O ↔ H2 + CO 2

-41

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A model of the SOEC subsystem has been developed in ASPEN Plus software. In order to maximize contents of hydrogen and carbon monoxide at the subsystem outlet, the system operating pressure is set to atmospheric conditions.26 Standard components from ASPEN library are used to model components such as compressors, heat exchangers, separators, and stream divider. However, no standard component is yet available in ASPEN library to model SOEC unit. Hence, a SOEC unit model is developed using available reactors in combination with custom-developed calculator blocks. Figure 2 shows the SOEC model that is developed in this study. It is normally assumed that the input gaseous mixture comes to equilibrium upon entering SOEC cathode compartment with respect to water-gas shift reaction before going through electrochemical reactions. Similarly, cathode products are also come to equilibrium with respect to water-gas shift reaction before leaving the SOEC unit.27,28 Hence, three separate reactors from standard library of ASPEN are used to model (in practice these stages occur simultaneously). In the first reactor, PRE-WGSR, cathode inlet stream (S13) first reaches equilibrium according to reaction 4. The equilibrium mixture (S23) then enters the second reactor, SOEC, where electrochemical reactions occur. Here, steam and carbon dioxide are dissociated to form hydrogen, carbon monoxide, and oxygen. The outlet stream (S24) then is a mixture of these elements while in reality oxygen and syngas are produced in separate compartments. Therefore, a separator block, ELECTROD, is included in model to separate oxygen content of stream (S26) from syngas (S25). Syngas stream (S25) finally enters the last reactor, POST-WGS, and reaches equilibrium based on water-gas shift reaction.12,29 The Gibbs reactor (RGibbs) is selected for first and third reactors whereas stoichiometry reactor (RStoic) is used to model second reactor.

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Energy & Fuels

Figure 2. SOEC model in ASPEN

More information regarding the SOEC subsystem ASPEN Plus model, especially model of SOEC unit itself and its validation, can be found in a related publication.26

3.2

Entrained Gasification System Model

A steady state model of entrained gasification is developed in ASPEN Plus. As with the SOEC subsystem, standard components from ASPEN Plus library are used to model most of components, with the addition of custom model blocks. For example, a calculator block is developed and added to the model to compensate for the fact that methane content of syngas at the gasification outlet in reality is higher than what is estimated in the equilibrium model. This block is developed based on the empirical expressions presented by Wu et al..30 However, in that study mixture of high temperature air and steam has been used as oxidant, while in the present study oxygen is considered as the oxidizing agent which results in lower percentage of methane in comparison with steam as oxidant.4 Therefore, equations presented by Wu et al. are modified by modifying pre exponential factor to fit the methane content reported in the experiments.5 Complete and comprehensive explanation of the EG model built in ASPEN Plus and its validation can be found in Samavati et al.31

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3.3

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Fischer-Tropsch and Upgrading System Model

A slurry phase reactor is selected to model FT reactor based on the selected operating conditions. This reactor type features efficient heat management, which is favorable considering exothermic FT synthetic (FTS) reactions. RCSTR reactor is used from ASPEN Plus library which models continuous stirred tank reactors such as slurry phase reactors.21 The carbide mechanism is used to represent the kinetic model of FTS. The main feature of this mechanism is the formation of hydrocarbon chains by successive addition of building units of one carbon atom and no oxygen.32 In this study, FTS kinetic model and reaction system are modeled according to the approach presented by Todic et al. 33, while hydrocracking process of FT wax and its product distribution was modeled based on the approach described by Pellegrini et al..34 The distillation towers in the upgrading system are modeled using PetroFrac columns from ASPEN Plus standard library. The first distillation tower consists of 54 stages with a lateral stripper of 12 stages, reflux ratio of 1.8 between every two consecutive trays and the bottom re-boiler. The second distillation tower, on the other hand, is smaller and consists of 15 stages. More information regarding the modelling procedure can be found in Selvatico et al.35

3.4

Thermodynamic Model

The efficiency of the integrated system, ηs, is defined as the useful output from the system to the energy content of input streams,

ηs =

∑ m& .LHV i

(1)

i

m& biomass .LHV biomass + Wel , tot + Qtot

16


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Energy & Fuels

where m and LHV represent the mass flow rate and lower heating value of each component, Wel,tot is the total electrical consumption of the system and Qtot is the total input heat to the system. In this equation, i, represents different products of FT, namely: diesel, naphta, wax, LGHC, and hydrogen.

Similarly, integrated system exergy efficiency, Es, is defined as the ratio of outlet streams exergy content to the exergy content of all the inlet streams to the system (equation 2).

Es =

ψ P , sys ψ F , sys

(2)

Exergy content of each stream is calculated by estimating share of its physical and chemical exergy according to equations 3-5.36

b T , P = b ph

b ph

T ,P

b ch

T 0 , P0

T ,P

(

+ bch

(3)

T0 , P0

) (

= h T , P − h T , P − T0 s T , P − s T , P

=

∑

0

0

0

0

)

(4)

y i .b ch ,i + R T 0 ∑ y i . Ln ( y i )

(5)

M

However, the exergy content of input biomass stream is calculated using the equations presented by Wu et al.:30

ψ biomass = m& biomass .β .LHV biomass

(6)

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H   O   H  1.0414 + 0.0177  − 0.3328  1 + 0.0537  C C      C  β= O 1 − 0.4021  C 

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(7)

where, [C], [H] and [O] are the molar fractions of carbon, hydrogen and oxygen in wood pellets, respectively.

3.5

Economic Model

Levelized cost of the produced FT diesel is estimated for each integrated system as shown in equation 8.

Levelized Cost FT Diesel =

CICP + CO&M + CEl. + CFeedstock + CU − Cincome Annual FT Diesel Production × Capacity Factor

(8)

where, CICP, CO&M, CU, CEl., CFeedstock, and Cincome are the cost of installed capital of plant, operation and maintenance, utility, electricity, feedstock, and the income that is gained by selling the produced byproducts, respectively. Information regarding estimating each one of these costs is provided in the supporting information.

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Energy & Fuels

3.6

Emission Model

The GHG emissions of produced FT diesel is calculated based on the guidance provided by the European Commission in EU directive 2009/28/EC on the promotion of the use of energy from renewable sources. According to this directive, the GHG impacts of production and consumption of biofuels can be estimated according to equation 9.37

E = e ec + e l + e p + e td + e u − e sca − e ccs − e ccr − e ee

where,

E: total emissions from the use of FT diesel,

eec: emissions from the extraction or cultivation of raw materials,

el: annualized emissions from carbon stock changes caused by land-use change,

ep: emissions from processing,

etd: emissions from transport and distribution of fuel,

eu: emissions from the fuel in use,

esca: emission saving from soil carbon accumulation via improved agricultural management,

eccs: emission saving from carbon capture and geological storage,

19


(9)

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eccr: emission saving from carbon capture and replacement,

eee: emission saving from excess electricity from co-generation.

However, only emissions from extraction of raw materials, processing, transport and distribution, fuel in use, and carbon capture and replacement are relevant here. Hence, equation 9 will be reduced to

E = eec + e p + etd + eu − eccs − eccr

(10)

Emissions from extraction and cultivation for farmed wood and waste wood are set to 4 and 1 gCO2eq/MJ, respectively which are the values that are suggested in the mentioned directive.37 Similarly, emissions from transport and distribution are also set to the proposed values of 2 gCO2eq/MJ for farmed wood and 3 gCO2eq/MJ for waste wood. 37 The emission from processing is equal to the carbon dioxide that leaves the integrated system in the process of FT diesel production (e.g., emission associated with heating utilities). Note that since FT diesel is produced using renewable sources, emissions from its usage, eu, would be equal to zero.37 The values for eccs and eccr are extracted from the simulation model and are equal to the CO2 that is provided either from external sources or internal recovery. According to the same directive, in case of existence of byproducts in the system the real emissions would then be the estimated by multiplying the total emissions by the allocation factor, as presented in equation 11.

Allocation factor =

Energy content Energy content

FT Diesel

(11)

FT Diesel

+ Energy content

byproducts

20


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Energy & Fuels

Emission savings due to use of FT diesel instead of fossil based fuel are calculated for each integrated system combination as shown in equation 12. Ef is the emission from using fossil based fuels and its value is set to 83.8 gCO2,eq/MJ which is the recommended value by European Commission.37 Emission savings =

4

Ef − E

(12)

Ef

Results and Discussion

Figure 3 illustrates efficiency of each integrated system based on the percentage of syngas that would be produced in the SOEC subsystem. Since SOEC produces syngas with higher efficiency than the EG subsystem, both energy and exergy efficiency of the integrated system increases. Results showed that energy efficiency increases from 48% for the integration of EG and FT to 65% when EG subsystem is completely replaced with SOEC, while exergy efficiency exhibits a rise from 45% to 63%. In other words, from the thermodynamic point of view it is beneficial to produce a higher share of precursor syngas through co-electrolysis process rather than gasification of biomass. On the other hand, increasing capacity of SOEC results in drastic increase of electrical power consumption of the integrated system. So, as shown in Figure 4, produced FT diesel has higher levelized cost. However, in case of low prices of renewable electricity, levelized cost of FT diesel in case 11 is in the same range of cost in case 7 (2.07 $/liter in case seven vs. 2.06$/liter in case eleven). Hence it seems that including gasification unit and its supporting components in the integrated system with the nominal syngas production capacity of less than 40% of required syngas is not economically a viable choice in case of access to low price electricity. Such a conclusion is not true when renewable electricity prices are high.

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On the whole, it can be concluded that an increase of SOEC syngas production capacity has an adverse effect on the levelized cost of produced FT diesel and therefore contradicts the benefits that it has from thermodynamic point of view.

Figure 5 shows GHG emission savings that can be achieved from usage of produced FT diesel in each integrated system combination. As can be seen here, regardless of whether biomass is produced from waste wood or farmed wood resources, higher emission savings are obtained by reducing the EG syngas production capacity. In other words, the GHG emission saving increased from 96% in Case 1 to about 116% in Case 11. This trend occurs due to not only a decrease in the amount of emissions attributed to preparation and processing of biomass, but also to an increase in CO2 demand of SOEC. GHG emission savings greater than 100% means that the SOEC CO2 demand cancels out all other emission sources in the entire chain; hence, the GHG emission rate from the use of FT diesel is negative. Since the source of biomass (waste wood or farmed wood) does not have prominent impact on the final GHG emission savings, average values between the two biomass feedstocks are used subsequently for determining GHG savings.

All in all, it can be concluded that increase in SOEC syngas production capacity has contradicting effects on the integrated subsystem depending on the study perspective. Although it seems beneficial to reduce EG syngas production capacity from thermodynamic and environmental point of view, the levelized cost of produced FT diesel would be higher. Therefore, the best combination should be selected based on the user priority as well as considering renewable resources potential in a given region.

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70 Energy Efficiency 60 Exergy Efficiency

Efficiency (%)

50 40 30 20 10 0 0

10

20

30 40 50 60 70 SOEC Syngas Production Capacity (%)

80

90

100

90

100

Figure 3. Effect of SOEC subsystem size on total efficiency of the system

7 0.075 $/kWh

FT Diesel Levelized Cost ($/liter)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

6 0.23 $/kWh 5 4 3 2 1 0 0

10

20


80

Figure 4. Effect of SOEC subsystem size on the levelized cost of produced FT diesel

23


Energy & Fuels

140 Waste wood

Farmed wood

120

GHG Saving (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 60 40 20 0 0

10

20


80

90

100

Figure 5. Effect of SOEC subsystem size GHG saving

4.1

Internal Thermal Integration

As shown in Figure 1, several heating and cooling utilities are required. Figure 6 illustrates type and amount of each heating and cooling utility that is used in each subsystem for each case. As can be seen, high temperature (HT) combustion exhaust and medium pressure (MP) steam are selected as heat sources. On the other hand, MP steam generation, water cooling, and air cooling are considered to provide a heat sink for the integrated system. Although all subsystems require some type of cooling utility, only SOEC subsystem requires integration of heating utility. In general, there are two ways of integrating heat flows in the system, one by burning the produced LGHC and second by considering the generated MP steam as a byproduct. This section investigates the effects that latter can have on the system performance from different perspectives. The impact of former case is presented in section 4 of the supporting information. 24


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700 1

2

3

4

5

6

7

8

9

10

11

600

500

Heat (MW th)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

400

300

200

100

0 SOEC EG

FT SOEC EG

FT SOEC EG

FT SOEC EG

HT Combustion Exhaust

FT SOEC EG

MP steam

FT SOEC EG

Cooling Air

FT SOEC EG

FT SOEC EG

MP steam Generation

FT SOEC EG

FT SOEC EG

FT SOEC EG

FT

Cooling Water

Figure 6. Heating and cooling utility requirements of each subsystem in each case

4.1.1

MP steam recovery

One possible integration of heat flow between subsystems is to include the generated MP steam in the calculations. As it is shown in Figure 1, MP steam generation is used for the cooling requirements of the subsystems, while MP steam provides a part of the heating requirements for the SOEC subsystem. The difference between this integration and the baseline is that the MP steam is completely eliminated from the heating utility and the required heat flow would be provided from the generated MP steam in the integrated system. Any remaining MP steam is considered as a useful byproduct that can be sold to the district heating network. Energy and exergy outputs of the integrated system in all combinations increase owing to this integration, while the energy and exergy inputs decrease due to elimination of MP steam heating utility. Therefore, as also can be seen in Figure 7, both energy and exergy efficiencies are enhanced. However, since exergy content of heat flow is lower than its energy content the increase in exergy efficiency is smaller. For example, in case 6, the exergy efficiency 25


Energy & Fuels

experiences an increase of 12 percentage points while energy efficiency exhibits an increase of about 40 percentage points.

Figure 8 shows the effect of recovering generated MP steam on the FT diesel levelized costs. As can be seen, in this case FT diesel has lower levelized costs than in the baseline design. Two factors are responsible for this effect: the heating utility operation cost is lower due to elimination of MP steam utility; and income would be higher due to addition of the remaining MP steam for sale as a byproduct.

Another advantage of generated steam recovery would be the higher GHG emissions savings from the use of the FT diesel (Figure 9). In this case, the emissions linked to the heating utility are lower since no MP steam is supplied externally. As a result, the total allocated GHG emissions to the FT diesel are lower than the baseline design, and consequently use of FT diesel results in higher emission savings.

120 100 Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20 0 1

2

3

4

5

6 7 Case Number

8

9

10

Energy Efficiency- Baseline

Energy Efficiency- With Heat Recovery

Exergy Efficiency- Baseline

Exergy Efficiency- With Heat Recovery

Figure 7. Effect of recovering generated MP steam on total efficiency of the system

26


11

7 6 5 4 3 2 1 0 1

2

3

4

5

6 7 Case Number

8

9

10

0.075 $/kWh- Baseline

0.075 $/kWh- With Heat Recovery


0.23 $/kWh- With Heat Recovery

11

Figure 8. Effect of recovering generated MP steam on FT diesel levelized cost

140 120

GHG Saving (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 60 40 20 0 1

2

3

4

5 6 7 Case Number

Baseline

8

9

With Heat Recovery

Figure 9. Effect of recovering generated MP steam on GHG saving

27


10

11

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4.2

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Mass Recovery

Besides heat, there is a possibility to use some mass flows that are produced in one subsystem as input of another subsystem. In general, there are four different possibilities for internal mass recovery inside the integrated system:

•

Oxygen recovery from the SOEC subsystem to the EG subsystem

•

Carbon dioxide recovery from the EG subsystem to the SOEC subsystem

•

Hydrogen recovery from the FT subsystem to the SOEC subsystem

•

Water recovery from FT subsystem to the SOEC subsystem

Figure 10 illustrates the amount of specific element that is available to be recycled to another subsystem (availability) versus consumption rate of that element. To exemplify, the values shown for oxygen represents the amount of oxygen produced during co-electrolysis divided by oxygen consumption rate inside entrained gasifier. In other words, available oxygen to be recovered in cases 6 and 9 is about 1.2 and 5 times of its consumption rate in the EG subsystem, respectively. In the following sections the effects of oxygen and hydrogen recovery from thermodynamic, economic, and environmental point of view are explained. The possible impacts of carbon dioxide and water recovery are presented in supporting information.

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30 Oxygen

Carbon Dioxide

Hydrogen

Water

25

Availability / Consumption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

20

15

10

5

0 1

2

3

4

5

6 Case Number

7

8

9

10

11

Figure 10. Availability versus consumption of each recycled stream

4.2.1

Oxygen

Another product of co-electrolysis is the pure oxygen, which is produced in the anode compartment. In the baseline design, this produced oxygen is diluted using sweep-air and is then released to the environment. However, it is also possible to use it internally as an oxidizing agent for gasification process. In this case, no sweep air will be used on the anode side of electrolysis unit, since pure oxygen is required by gasifier, and consequently air compressors and anode heat exchangers from SOEC subsystem are completely removed. When less than 50% of syngas is produced through co-electrolysis process, the produced oxygen would not be enough to cover all the required oxidant of EG subsystem (Figure 10). Hence, it is assumed that for the cases that SOEC covers less than 50% of precursor syngas, cases 2-5, the air separation unit (ASU) is smaller than the baseline, while for the cases of 50% and more, cases 6-10, the ASU is completely eliminated. The elimination of air compressor and

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reduction of ASU capacity results in lower electricity consumption of the integrated system. Besides, the required heat from heating utility is reduced owing to the elimination of required heat exchangers on the anode side of the SOEC subsystem. Therefore, both energy and exergy efficiencies of the integrated system increase. Nevertheless, since exergy content of the excluded heat is less than its energy content, increase rate of exergy efficiency would be smaller as shown in Figure 11. By including oxygen recovery in the system integration, not only the capital cost of system is reduced but also annualized cost of electricity and heating utility are decreased. Consequently, as can be seen in Figure 12, levelized costs of FT diesel drop compared to the baseline design. Moreover, decrease in emission from heating utility owing to elimination of heating requirements results in slight increase of emission savings (Figure 13). As an example, in case 10, where ninety percent of syngas is produced from co-electrolysis, the emission saving value increases by 0.7 percentage point.

According to results shown in Figure 11-Figure 13, it can be concluded that internal recovery of oxygen from SOEC subsystem to be used in EG subsystem is beneficial from thermodynamic, and economic perspective. It also can be considered as beneficial from environmental point of view although the increase in the emission savings are minuscule. Note that, since there is no oxygen production in the first combination (EG+FT) and no internal oxygen usage in the eleventh combination (SOEC+FT), internal oxygen recovery would not be possible. Therefore, no change is shown in the presented figures for the first and last combinations.

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70 Partial Coverage of Internal Demand

Complete Coverage of Internal Demand

Efficiency (%)

60 50 40 30 20 10 0 1

2

3

4

5


6 7 8 9 10 11 Case Number Energy Efficiency- With Oxygen Recovery


Exergy Efficiency- With Oxygen Recovery

Figure 11. Effect of oxygen recovery on the total efficiency of the system


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

7

Partial Coverage of Internal Demand Complete Coverage of Internal Demand

6 5 4 3 2 1 0 1

2

3

4

5

6 7 Case Number

8

9

10


0.075 $/kWh- With Oxygen Recovery


0.23 $/kWh- With Oxygen Recovery

Figure 12. Effect of oxygen recovery on the levelized cost of FT diesel

31


11

Energy & Fuels

140 Partial Coverage of Internal Demand Complete Coverage of Internal Demand 120

GHG Saving (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 60 40 20 0 1

2

3

4

5 6 7 Case Number

Baseline

8

9

10

11

With Oxygen Recovery

Figure 13.Effect of oxygen recovery on the GHG saving

4.2.2

Hydrogen

Hydrogen is one of the byproducts of the integrated system. In the baseline integration, one part of produced hydrogen covers the requirements for the hydrocracking process, and the remainder is assumed to be sold based on the common prices in the market. It is also possible to redirect a small part of this hydrogen towards entrance of cathode compartment of electrolysis unit. Therefore, the requirement for recycling a fraction of produced syngas to the cathode inlet of electrolysis unit and consequently recirculating compressor in the SOEC subsystem are eliminated (refer to supporting information). Although electrical power demand of the system will decrease, the output energy and exergy flow from the system also decrease due to lower hydrogen output. The recirculating compressor electricity demand is negligible compared to total electricity demand of the integrated system and consequently cannot counterbalance the decrease in energy and exergy output from the integrated 32


Page 33 of 42

system. Therefore, system energy and exergy efficiencies drop in all integrated system combinations that include the SOEC subsystem (Figure 14). Moreover, due to lower quantity of available hydrogen that to be sold to the market, annualized income of the system suffers as well. Consequently, as also shown in Figure 15, FT diesel has higher levelized costs.

According to equation 11, lower rate of hydrogen output as byproduct results in higher emission allocation factor of FT diesel and in turn lower emission savings (Figure 16). Hence, emission savings has higher values than the base case integration. The effect of internal hydrogen recovery is more prominent when SOEC contribution in precursor syngas production increases. At any rate, it seems that hydrogen recovery is not a viable option and cannot be justified from thermodynamic or economic perspectives.

70 60

Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

50 40 30 20 10 0 1

2

3

4

5


6 7 8 9 10 11 Case Number Energy Efficiency- With Hydrogen Recovery


Exergy Efficiency- With Hydrogen Recovery

Figure 14. Effect of hydrogen recovery on total efficiency of the system

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7 6 5 4 3 2 1 0 1

2

3

4

5

6 7 Case Number

8

9

10

11


0.075 $/kWh- With Hydrogen Recovery


0.23 $/kWh- With Hydrogen Recovery

Figure 15. Effect of hydrogen recovery on the levelized cost of FT diesel

140 120

GHG Saving (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy & Fuels

100 80 60 40 20 0 1

2

3

4

5 6 7 Case Number

Baseline

8

9

With Hydrogen Recovery

Figure 16. Effect of hydrogen recovery on the GHG saving

34


10

11

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5

Energy & Fuels

Conclusions

A novel integrated system for production of FT diesel from different renewable sources was proposed. Two pathways for production of required precursor syngas, through gasification and coelectrolysis process, were introduced. Eleven different combinations were analyzed from different perspectives based on the amount of syngas that is produced in the SOEC subsystem. Results showed that energy and exergy efficiencies favor the supply of syngas via the SOEC over the entrained gasifier. On the other hand, produced FT diesel exhibited higher levelized costs as the share of SOEC-supplied syngas increased. Nevertheless, it was shown from economic perspective that in case of low cost renewable electricity availability, it would be a better choice to eliminate the entrained gasification subsystem when its contribution to syngas production is less than 40%. However, it should be kept in mind that it may not be possible to have continuous production of FT diesel when system relies solely on renewable electricity. Although the final choice of subsystem capacity relies on the user priorities in a given application, it seems that considering equal share of syngas production in both SOEC and EG would be a good compromise among efficiency of the integrated system, final cost of produced FT diesel, and achieved emissions savings. Recovery of produced medium-pressure steam to cover the internal demand is recommended since it was shown to have positive effects on the integrated system from thermodynamic, economic, and environmental perspectives. Moreover, between different mass recovery options in the system, it can be concluded that oxygen recovery and hydrogen recovery were the best and worst options, respectively.

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Page 36 of 42

The present study considered steady state operation, which has limitations in terms of supplydemand matching to renewable electricity. Future studies should consider transient aspects in order to explore system performance in terms of capacity factors. Although GHG emission savings considered in this study are based on the guideline provided by the European Commission, it only includes emissions from production chain and consumption of the produced FT diesel. For a better grasp of such integrated systems’ impacts on the environment, a life cycle assessment should be considered in future publications.

6

Supporting Information

•

Subsystem Description

•

Detailed Economic Model

•

Heating and Cooling Utilities

•

Combustion of LGHC

•

Carbon Dioxide Recovery

•

Water Recovery

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7

Energy & Fuels

Acknowledgement

This research has been done in collaboration with Politecnico di Torino and Royal Institute of Technology, funded through Erasmus Mundus Joint Doctoral Programme SELECT+, the support of which is gratefully acknowledged.

8

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