Identifying Material and Device Targets for a Flare Gas Recovery

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Identifying material and device targets for a flare-gas recovery system utilizing electrochemical conversion of methane to methanol Patcharapit Promoppatum, and Venkatasubramanian Viswanathan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01714 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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ACS Sustainable Chemistry & Engineering

Identifying material and device targets for a flare-gas recovery system utilizing electrochemical conversion of methane to methanol Patcharapit Promoppatum and Venkatasubramanian Viswanathan∗ Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15213, USA E-mail: [email protected]

Abstract Natural gas flaring causes enormous damage to the environment, in addition to the wasted energy. The importance of this problem is highlighted by an initiative by the United Nations to end flaring by 2030. There exists an immediate need for identifying routes for converting flare into usable energy. In this study, we propose a scheme that at the core utilizes an electrochemical cell to convert methane into methanol, an easily transportable fuel. The electrochemical cell uses electricity provided by solar photovoltaics to power the electrochemical cell. We carry out a detailed techno-economic analysis of the entire system and analyze the merits and demerits of the proposed approach as compared with other flare gas recovery systems, gas-to-liquid (GTL), electricity generation with gas turbine, gas compression system, and the electricity generation by solid oxide fuel cell (SOFC). The developed model shows that the current ∗

To whom correspondence should be addressed

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state-of-the-art materials available for different system components, proton conductor, and electrocatalysts are inadequate to make the scheme practical. We outline the minimum performance metrics, i.e., input voltage at the cell level of ∼ 0.5 V which corresponds to an overpotential of ∼1 V and a current density of 0.5 A/cm2 , which requires a proton conductor that can conduct 10−1 − 10−2 S/cm at the temperature range, 100-250◦ C, required for the system to become financially competitive. Of note, improvements in the conductivity of proton conductors at intermediate temperatures and identification of active and selective electrocatalysts for the conversion of methane to methanol are the key parameters that determine the overall viability of the proposed scheme. We discuss the environmental impacts of the proposed scheme and provide an outlook on directions required in materials research that could meet the outlined performance metrics.

Introduction Associated petroleum gas (APG) is natural gas obtained in the production process of industrial plants such as oil refineries, chemicals plants, and natural gas processing. In regions that lack a gas transportation infrastructure, this type of gas is typically considered as waste and usually burnt off in gas flares. 1 Global flare gas represents about 21% of total natural gas consumption in the United States. 2 A recent study showed that every year, about 139 billion m3 of natural gas is flared, which eventually generates about 281 million tons of CO2 emission. 3 Flaring APG creates a threat to human health as well as increases greenhouse gas. Moreover, natural gas is a valuable energy source which can decrease the dependency on fossil fuels. 4 The global importance of this problem is highlighted by the initiative “Zero Routine Flaring by 2030” launched by the United Nations with endorsements from nine countries, ten major oil companies, and six development institutions. 5 The global consumption of a natural gas is on the rise, 6,7 and due to the benefits of APG, several flare gas recovery systems have been proposed to reduce flare gas and convert it into usable energy. 8,9 There are four major

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methods that have been proposed for flare gas recovery systems; (i) gas-to-liquid (GTL) system, (ii) gas turbine (GT) technology, (iii) gas compression system, and (iiii) electricity generation from solid oxide fuel cell (SOFC). 10,11 GTL system converts flare gas to a liquid fuel based on the Fischer-Tropsch process. 12,13 Gas turbine and SOFC aim to produce electricity from the flare gas while the gas compression system compresses the gas and sends it along a pipeline to the gas refineries. 14,15 The electricity generation systems suffer from the drawback that the generated electricity needs to be consumed instantaneously, which is not practical and even feasible for the grid in a remote location. Therefore, such a system often requires energy storage for storing the electricity and it has been shown that the total cost of the energy storage, for e.g. lithium-ion batteries, could be as high as the cost of electricity generation systems. 16 Gas compression system suffers from the issue that there is a large distance between the remote production site and the consumption regions. 17 As a result, among the four types of gas recovery systems, the GTL system is the most attractive methods for use in a remote area as storing the energy of flare gas in a liquid form allows it to be easily transported. 18 In addition, the GTL system does not need any additional energy storage and can leverage the already prevalent transmission network for liquid fuels. However, GTL systems suffer from an enormously high capital cost of investment. 11,19 Additionally, to reach the financial break-even point, the conventional GTL plant needs to be operated at a large scale, with the plants typically requiring a minimum operation of 10,000 bpd. 20,21 However, the total production of oil in the United States is about 1,600 million barrels, and more than 85% of the production is from wells with production rate less than 10,000 bpd as shown in Fig. 1. 22 Therefore, there exists a need to find small-scale GTL reactors, which would satisfy the economic payback as well as the size constraint of the system. As a result, in the last decade, researchers have devoted much attention into the development of the direct selective conversion of methane to methanol as the alternative of GTL technology. 23,24 Nevertheless, detailed system-level understanding and an economic evaluation are missing. In the present study, we propose a complete system that utilizes an electrochemical cell to convert methane,

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the major composition of APG, into methanol. The proposed approach uses electricity provided by solar photovoltaics to power the electrochemical cell. We would like to point out that the proposed system and the analysis carried out could be applied for other sustainable sources of methane (for e.g. biogas, RNG). Through detailed techno-economic analysis, we identify the minimum performance metrics at the material and device level required for the system to become commercially competitive.

Figure 1: United States Total Distibution of Wells by Production Rate during 2009 (bpd/day)

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Technical concept The proposed technical concept attempts to marry the benefits of the GTL system which produces the transportable liquid fuel and a SOFC, which requires low capital investment. The fundamental challenges in developing a solid oxide fuel cell which is capable of converting methane into methanol are two-fold: (i) high temperature operation using methane as a fuel typically results in complete oxidation to CO2 , while at (ii) room temperatures, it is not possible to activate methane on catalyst surfaces. Hence, intermediate temperature fuel cells (ITFC) have been touted to be attractive candidates for enabling partial oxidation of methane to methanol. The electrochemical cycle used here borrows the core scheme from the recent experimental work of Hibino and co-workers. 25,26 A schematic of the cell concept is shown in Fig. 2. The anode is fed a stream of methane and steam, and the half reaction at the anode is given by

CH4 (g) + H2 O(g) → CH3 OH(g) + 2H+ + 2e− . Uo = 0.60 VRHE (at 500 K)

(1)

At the cathode, oxygen from air is reduced, given by 1 O2 (g) + 2H+ + 2e− → H2 O(g). Uo = 1.16 VRHE (at 500 K) 2

(2)

The protons generated at the anode are conducted using a proton conducting membrane to the cathode. The overall cell reaction is the oxidation of methane to methanol. A benefit of this scheme is that it uses readily available water vapor as the active oxygen source. The best-identified system thus far is based on V2 O5 /SnO2 as the catalyst/support for the anode and carbon supported platinum for the cathode with Sn0.9 In0.1 P2 O7 as the proton conductor, at a temperature of about 100-200◦ C. This system was able to achieve a selectivity of 61% with a continuous operation of 6 hours at this conversion rate. The system was run at a current density of ∼2 mA/cm2 with an overall single-pass product yield of about 0.3%. It

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O2

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H2 O Cathode H+

e− Proton Conductor

V e−

H+

O Anode

CH4 + H2 O

CH3 OH

Figure 2: Schematic of the electrochemical cell for the overall conversion of methane to methanol. The anode reaction is CH4 (g) + H2 O(g) → CH3 OH(g) + 2H+ + 2e− and the cathode reaction is 21 O2 (g) + 2H+ + 2e− → H2 O(g). is worth highlighting that the thermodynamics of methane to methanol yielded a powerproducing device. However, experiments show that a device only works in power-consuming mode requiring nearly >1 V of applied potential. The main aim of this work is to identify the technical targets at the materials and device level required for the proposed scheme to become economically feasible and the identified targets can be used to guide material discovery.

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System Model A simplified process flow diagram for the entire electrochemical GTL system is illustrated in Fig. 3. To complete the conversion, the balance of plant equipment which includes expander, heat exchanger, pressure vessel, insulation, anode blower, and heat exchanger are also included. The primary advantage of the proposed system is that it could, in principle, be operated regardless of the location. The process begins by feeding natural gas along with water vapor into the anode. The incoming gas must be heated to approximately 250 ◦ C for carrying out selective oxidation to methanol. As most remote plants generate electricity from gas turbine, the exhaust gas from the gas turbine exits around 550 ◦ C and can be used as the heat source for the proposed system. 27 The oxidation of methane at the anode produces the methanol and protons. The protons are transported through a proton conducting membrane and gets reacted with oxygen to produce steam at the cathode. The electricity required by the cells to carry out the desired reaction is supplied by a solar PV as shown in the process flow diagram in Fig. 3. In this work, the separation cost has not been included. An estimate of the separation cost can be made by comparing with the separation membrane of CO2 , which costs around 20 $/ton. 28 Therefore, we expect the cost of separation to contribute around 10% increase in the overall cost. It is also worth noting that there is a distinctive difference between electricity generating SOFC and the proposed scheme. For the effective production of methanol, the proposed scheme requires a better catalyst than that required for the electricity generating SOFC. According to manufacturing cost analysis, the catalyst contributes about 60% to the overall cost of the cells. 29 Therefore, the cell cost could be expected to increase significantly when a better catalyst is required. As a first approximation, in this study, we assume that the cost of catalyst is the same as that used for an electricity generating SOFC.

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cost of a typical solid oxide fuel cell. 29,30 The costing model is outlined in Table 2 and shows that the capital investment depends primarily on the area and the total number of cells. The required area of cells to recover the entire gas inlet can be calculated from the current density and the total current of the system. This study assumes conversion units with a stack design of hundred cells. As is typically assumed, the installation cost is equivalent to 42% of total stacks cost. 29 The installation is assumed to occur only once over the lifetime of the system. However, it is possible that the oil production rate might decline over time so that the flare gas recovery unit needs to be moved to a new facility. Therefore, the cost of re-installation should be accounted to the overall cost in the case that relocation is required. The high modularity of the proposed scheme provides a distinct advantage to address the challenges associated with relocation. The system could be designed as a compact unit such that it can be easily integrated and disjoint from the gas production facility. This flexibility and modularity is not present for the GTL system due to its large system level requirements. Table 2: Costing model of cells based on ref. 30 Components $cell Ntotal Nstack $stack $install

Value ($) Ac × 0.1442 Atotal /Ac Ntotal / 100 2.7 x ($cell × Ntotal + 2 × Nstack × Ac x 0.4625) 42% of Pstack

The balance of plant is calculated based on a power-law dependence on the cell area as shown in the following equation. 29

$bop = $bop,ref

Atotal Aref

n

(3)

$ref is the reference cost of the reference cell area, Aref , while n is the scaling factor. Our work assumes the scaling factor of 0.71. 31 The reference cost of the BOP is varied according to the annual production. The annual production typically ranges from a premature market producing 50 units to a mature market producing 10,000 units. To be conservative, this 10


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study uses the $bop as shown in Table 3 by assuming the market as premature. Table 3: Reference cost of the balance of plant based on ref. 29 Equipment Expander/Compressor Heat exchanger Pressure vessel Insulation Anode blower/Pump Desulfurization

Value ($) 25,337 75,852 145 1,800 15,840 5,168

Energy input price The fuel cell system requires energy input to carry out the electrochemical GTL conversion. As this system is being designed to be operated in a remote location, the energy source should be easily accessible. Hence, solar photovoltaic (PV) stands out as an attractive energy input for several reasons including the rapid reduction in the panel price, low maintenance cost, and the availability of solar energy near production facilities. 32 The system price of solar PVs have decreased by 12%-15% annually. 33 The cost of solar PV is often reported as the levelized cost of energy (LCOE), which is the assessment of the electricity generation cost for a particular system. It refers to the minimum price of electricity per kilowatt-hour ($/kWh) for the system to reach the break-even of the investment over its lifetime. The LCOE for solar PV in this study is 0.134 $/kWh, which is a reasonable estimate for various locations in the United States such as south-western part. 34 Even though the choice of using PVgenerated electricity is made to allow for relocation, from an economic perspective, if input electricity is available at a lower price and can be accessed more easily, it should be given priority over solar PV. Operations and Maintenance Most of operation and maintenance costs are available for the SOFC operating as an electricity generation device. This study calculates O&M cost by assuming as the power input to 11



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the SOFC is equivalent to that of the power output from a traditional electricity generating SOFC. In addition, during the operation, fuel cell stacks degrade over time and studies show that the degradation rate of stacks is about 0.25%/1,000 hours, and the stack has about 5 years life expectancy. 35 Therefore, in order to maintain a nearly constant performance of cells, the stack replacement needs to be accounted into the financial analysis. The replacement of stacks is usually included in the operation and maintenance cost. Based on the study by the U.S. Energy Information Administration, the operation and maintenance cost for an energy producing fuel cell is estimated to be 43 $/MWh, in which the majority of the cost contributes to the stack replacement of the system. 36 Financial parameters The effective tax rate is assumed to be 38.9%, which accounts for 35% of federal tax rate and 6% of state tax rate. The discount rate for the renewable technology typically range between 5%-12%. 37 Thus, we use a discount rate of 10%. The financial parameters are assumed to be constant over the lifetime of the system.

Revenue Model The NPV can be calculated as the difference between cash flow and capital investment with the cash flow appropriately discounted. This is given by,

NPV = −Inv +

15 X k=1

CF (1 + r)k

(4)

The cash flow represents the annual net income after tax. The net income is calculated from the revenue deducting the total expenditures, which include energy input cost, operation and maintenance, and the depreciation of the system. The depreciation refers to the reduction of the asset’s value over time and this study assumes a linear depreciation and thus, the annual

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depreciation cost remains constant with time.

CF = (Rev − $el − $om − $depn )(1 − Tax)

(5)

The revenue is assumed to come only from the direct sale of the produced methanol to the market without any further conversion into electricity or any higher value chemicals. In addition, none of the environmental benefits associated with the scheme have been accounted for. The price of methanol shows volatility based on many factors such as price of natural gas, demand and supply and the state of the economy. A study shows that the methanol’s price decreased down to 1.7 $/Gallon in 2009, which this price is sometimes referred as the floor price of methanol. 38 Then, the price continuously increased reaching 2.6 $/Gallon at the end of 2010. Therefore, this study will refer to these two values as the lower and upper bound for methanol’s price. The overall production rate of methanol depends on the methane-to-methanol conversion efficiency and the amount of incoming methane. Hence, the revenue can be calculated as,

Rev = (ηc )(NCH4 )($CH3 OH )

(6)

The input energy cost is calculated from the total consumption of electrical energy over the period of operation times. The LCOE of solar PV is assumed to be fixed in this study. Therefore, only route to reducing the cost of input electricity is to reduce the input power, i.e. making the electrochemical cell more efficient.

Ce = (Pinput )(t)($LCOEP V )

13


(7)


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The required input power depends on the voltage of cell and the total current. The total current is determined from the mole rate of incoming methane and the number of electrons involved in the anode reaction. Pinput = IVc

I = (2)

dNCH4 dt

(8)

(F)

(9)

The analysis and the model can be applied to various operating scenarios. We carry out the analysis for one refinery conditions to illustrate the trade-offs. The assumptions of flare gas conditions are including as the following. (i) Methane accounts for 0.85 mole fraction of APG. (ii) Molar flow rate is 17,760 kgmol/h. (iii) Pressure of inlet gas is 305 kPa. (iiii) Temperature of inlet gas is 34.2 ◦ C. 10 This choice is made so that we could provide a comprehensive comparison of the proposed scheme to other flare gas recovery schemes.

Results and Discussion Cost distribution over the lifetime Based on the developed system model, the system costs over the lifetime are broken down and shown in Fig. 5. From our analysis, we identify that the energy input price and the initial investment for cells are the major contributors, accounting for 40% and 26% of the total system cost, respectively. We have assumed that LCOE of solar PV is fixed and hence, the energy input cost depends on the voltage required for the electrochemical cell. The cell price depends on the current density, which determines the number of cells required.

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Figure 5: Cost distribution over the lifetime. The total costs include the input energy cost, operation and maintenance cost, the total cost of cells, and the total cost of balance of plant.

Sensitivity analysis In order to provide an intuition for the dominant variables affecting the NPV, we begin by carrying out a sensitivity analysis to study the impact of the variation in different parameters on the net present value (NPV). The key parameters that affect the NPV are the conver-

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sion efficiency (ηc ), the current density (i), the voltage of a cell (Vc ), and the methanol’s price ($CH3 OH ). The conversion efficiency and the methanol’s price play an important role in determining revenue while the current density and voltage of a cell have an influence on the expenses. Fig. 6 shows the sensitivity analysis by comparing the percentage change in each parameter with the percentage change in NPV. A key conclusion from the sensitivity analysis is that all four parameters are equally important as improvement or deterioration in each parameter has a substantial impact on the NPV. Increasing the conversion efficiency and methanol’s price result in a higher revenue while an increase in current density reduces the number of cells required. Decreasing the voltage required for the electrochemical cell lowers the input energy cost thereby enhancing the NPV. Fig. 7 shows the rate of methanol production based on the electricity input under different cell voltages and conversion efficiencies. It emphasizes the importance of the reduction in the required cell voltage. A higher required cell voltage leads to a higher electricity input to yield the same methanol production rate compared to that of cells with a lower voltage requirement. Consequently, as the input power increases, the total area of solar panels required to provide the electricity to operate the conversion unit is also larger. The large size of the solar panel might hinder the installation of the flare gas recovery unit on the oil production facility, which usually has limited space. In addition, it is worth noting that the annual global demand for methanol is estimated to be around 70 million metric tons, of which the U.S. market represents a share of 10%. 39 Even under an idealized scenario when all flare gas is converted, the total amount of methanol produced will be comparable to the size of the U.S. market. However, in practice, not all flare gas will be converted into methanol. Hence, the production of methanol from the proposed scheme will fall well within the national demand and will have a ready market for selling.

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Figure 6: Sensitivity analysis in term of percentage change in NPV. Conversion efficiency, current density, voltage of cells, and methanol prices are examined by 50% increasing and decreasing from its nominal value.

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50

Required Power Input (kW)

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c

40

c c

= 80% V = 0.5V c

= 60% V = 0.5V c = 80% V = 0.1V c

= 60% Vc = 0.1V c

30

20

10

0

0

1000

2000

3000

4000

Production of Methanol (bpd) Figure 7: The relation between the conversion efficiency, and the voltage of cell on required power input as a function of production rates of methanol.

Nevertheless, the assumed base case values lead to the overall system which has a negative NPV, i.e. the system is not economically competitive. Fig. 8 shows the sensitivity analysis of the system in term of the calculated NPV. It can be seen from Fig. 8 that, even though the sensitive parameters are determined, changing any of these parameters individually does not result in a positive NPV. This implies that even if one of the cell parameters is dramatically enhanced, practical adaptation of this scheme may be unlikely. Therefore, to make the system financially practicable, it is necessary to optimize these factors simultaneously.

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Table 4: Sensitivity parameters

ηc (%) i (A/cm2 ) Vc (V) $CH3 OH ($/Gallon)

Minimum value 40 0.07 0.7 1.19

Base value 70 0.1 1 1.7

Maximum value 100 0.13 1.3 2.21

Figure 8: Sensitivity analysis in term of net present value. The NPV of the system is shown when the voltage of cells, current density, conversion efficiency, and methanol price are changed by 30%.

Identifying the material and device targets The required operating parameters that can satisfy the zero NPV over the system’s lifetime are shown in Fig. 9. This shows that if the conversion efficiency is at 60% with the methanol’s price of 2.6 $/Gallon, and has no enhancement in current density from the assumed base case of 0.1 A/cm2 , the input voltage need to be reduced by ∼ 0.8 V for obtaining a system with zero NPV. On the other hand, if the overall current density can be improved 19



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four-fold, which requires dramatic improvements in conductivity of proton conductors, the input voltage needs to only be reduced by ∼ 0.3 V. It can also be seen that at the same current density, cells with higher conversion efficiency or with higher methanol’s price could have the higher voltage (overpotential) and still be financially competitive. This analysis provides a comprehensive understanding of the interplay between the different factors. Additionally, methanol price plays a significant role in determining the NPV and increase in methanol price can accelerate adoptation of GTL technology.

Figure 9: Minimum requirement of cell properties, which could satisfy zero NPV at 10% discount rate over the 15 years lifetime at different conversion efficiencies and methanol prices.

Finally, as this technology is in a nascent stage, technological uncertainty must be taken into account. One way to account for technological risk is to identify the minimum performance requirement under different discount rate. 40 A higher discount rate is desired when 20


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Figure 10: Minimum requirement of cell properties, which could satisfy zero NPV at different conversion efficiencies and methanol prices over the 15 years lifetime at different discount rates. the system is viewed as very risky, while a system that has a low risk of failure, a smaller discount rate is sufficient. The minimum performance was again calculated under discount rate ranging from 10 - 20%, as shown in Fig. 10. It was seen that the cell required much better minimum performance to satisfy the zero NPV under the higher discount rate. For example, at the lower bound of the methanol’s price, if the conversion efficiency and voltage of cells were fixed at 80% and 0.35 V, the current density of cells must be about 0.18, 0.25, 0.32 A/cm2 so that the system could have the zero NPV over its lifetime under the discount rate of 10, 15, and 20%, respectively. Hence, almost a two-fold increase in current density is required between discount rate of 10% compared to 20%.

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Comparison with other flare gas recovery systems As introduced earlier, there are several flare gas recovery methods including gas turbine technology, gas-to-liquid (GTL) system, SOFC for electricity generation, and gas compression systems. 10 Each system has its own advantages and disadvantages as mentioned earlier. This study emphasized on the conversion of methane-to-methanol, as no energy storage and electricity transmission system are required, which are attractive characteristics for remote plants. Therefore, this proposed scheme should directly compete with the GTL system because both systems essentially carry out the same function. The capital investment of different flare recovery systems are shown in Fig. 11. The calculations are performed under the same flare gas condition as reported by Rahimpour et al. 10 The capital investment of the proposed system is varied according to the current density. This analysis highlights that the minimum threshold current density required for the proposed scheme to be competitive with GTL is at ∼ 0.5 A/cm2 . Any further improvement in current density makes the proposed system even more attractive.

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4500

Capital investment (million $)

0.1 A/cm 2 0.5 A/cm 2 1 A/cm 2

3600

2700

1800

900

io n

TL co m

pr es s

G as

T

Te ch no lo gy

FC SO

G

G

op os ed

sy

st

em

0

Pr

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Figure 11: Comparison of the captial investment with other flare gas recovery systems based on ref. 10. The capital investment of the proposed system depends on the current density, which is varied between 0.1 and 1 A/cm2 .

The overall profitability of the system can be reported in terms of Return on Revenue (ROR) and IRR. The ROR compares the annualized profit with the capital investment. The analysis from our model shows that ROR increases with improvement in the voltage of cells and the current density as shown in Fig. 12. Even though the ROR of the proposed system under some specified conditions is still under that of the GT technology, GTL, and Gas compression, as shown in Fig. 13, it is worth reiterating that the practicality of different systems depends primarily on various factors. GTL system requires oil production rates in excess of 10,000 bpd, while GT technology and gas compression system are not suitable for remotely located plants.

23



80 V c = 0.5 V $CH

Return on Revenue (%)

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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V c = 0.1 V $CH

60

V c = 0.5 V $CH V c = 0.1 V $CH

3

3

3

3

OH OH OH OH

= 1.7 $/Gal = 1.7 $/Gal = 2.6 $/Gal = 2.6 $/Gal

40

20

0

0

0.2

0.4

0.6

0.8

1

Current density (A/cm2 ) Figure 12: Return on Revenue (ROR) of the proposed system. The ROR depends on the current density varied from 0 - 1 A/cm2 , the voltage of cells varied between 0.1 and 0.5 V, and the methanol prices varied between 1.7 and 2.6 $/Gallon.

24


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200

Return on Revenue (%)

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


150

100

50

0

GT Technology

GTL

Gas Compression

Figure 13: Return of Revenue (ROR) of the different flare gas recovery systems. The financial determination is performed under the same flare gas conditions based on ref. 10.

In order to demonstrate the scalability of the proposed scheme, we show the IRR of the system under different oil production capability, cell conditions, and methanol’s prices in Fig. 14. It can be immediately seen that the IRR of the system only weakly depends on the inlet condition. The IRR saturates out at oil production rate beyond 500 bpd. Thus, the proposed scheme offers comparable financial outcome beyond a production rate of about 500 bpd. This is about 20 times smaller than the threshold limit for GTL systems. Therefore, this technology could be very suitable for recovering flare gas in facilities, where the average daily production rate is not adequate for GTL system.

25



90

Internal Rate of Return (%)

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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60

30

0.25 V, 1 A/cm2 , 1.7 $/Gal

0

60

2

0.5 V, 1 A/cm , 1.7 $/Gal 0.25 V, 1 A/cm2 , 2.6 $/Gal

40 20

2

0.5 V, 1 A/cm , 2.6 $/Gal

0 0

-30

0

2000

4000

6000

1000

8000

2000

10000

Oil Production Capacity (bpd) Figure 14: Internal Rate of Return of the proposed system. The IRR is plotted under several cell conditions with oil production rate from 0-10000 bpd.

Environmental Assessment An additional issue is regarding the environmental impact of gas flaring and the benefit of the proposed scheme in reducing the green house gas (GHG) emissions. The GHGs from combustion include CO2 , CH4 , and N2 O, of which CO2 accounts for over 99% of the GHG emissions. 41 Therefore, our analysis will focus primarily on the generation of CO2 . The CO2 emission from burning the natural gas is determined by the complete combustion reaction, where burning 1 kmol of CH4 would create 44 kg of CO2 . The combustion efficiency is assumed to be 98%. 42 Therefore, the uncombusted natural gas, which is released to the atmosphere, is an additional source of GHG emission. It is converted to CO2 equivalent by

26


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the Global Warming Potential (GWP) of 25. The GWP is obtained from the ability of a gas to trap heat compared to CO2 over 25 years. 43 The following equation calculates the equivalent mass of CO2 .

m ˙ CO2 ,

equiv

= GW P · m ˙ CH4 + m ˙ CO2

(10)

The present study compares the CO2 emission from three different scenarios: (i) the natural gas is burnt by flaring without the direct conversion to the methanol, (ii) the natural gas is injected to the proposed scheme for the methanol conversion but unconverted natural gas is released to the atmosphere without flaring and (iii) after the natural gas goes through the methane to methanol conversion process, the unconverted methane is flared to prevent the release of methane to the atmosphere. Fig. 15 displays the normalized CO2 equivalent based on the methane-to-methanol conversion efficiency. The normalized CO2 equivalent from the first scenario is always one as it is independent of the conversion efficiency. The second and third scenarios generate less emissions at higher conversion efficiency. However, it is worth pointing out that unless the conversion efficiency is very high, ∼90%, using the proposed scheme without further flaring will create higher CO2 emissions than that of the only flaring scheme. This is because even though the proposed scheme does not create the CO2 from combustion, it releases the large amount of methane, which has a higher GWP. Hence, improvements in conversion efficiency are crucial in lowering the emissions. The third scenario which is the most likely operating situation demonstrates substantial reduction of CO2 emissions. For example, at 70% conversion efficiency, the reduction of CO2 emission is over 70% compared to that of the only flaring scheme.

Not only does the proposed scheme reduce the environmental impact from the flaring the natural gas, but it also has an additional economic benefit if carbon tax is taken into account. The carbon tax rate is rather difficult to estimate as it varies greatly based on

27



local climate change as well as the technological improvement. 44 This study shows that an estimated tax rate ranges between $3 to $95 per metric ton of CO2 . However, by using the mean tax rate of $12 per metric ton of CO2 , the reduction of carbon tax could be as high as 10% of the total revenue, and this percentage could be higher if a greater tax rate is applied.

3.5 Only flaring Scheme without flaring Scheme with flaring

3

Normalized CO 2 Equivalent

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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2.5 2 1.5 1 0.5 0 0.6

0.7

0.8

0.9

1

c

Figure 15: Normalized CO2 equivalent based on three different scenarios; (i) only flaring, (ii) scheme without flaring, (iii) scheme with flaring. A scheme with flaring will always have less normalized CO2 emission than that of the only flaring. However, a scheme without flaring will create less emission if the conversion efficiency is higher than 87%.

Outlook This study outlines the material targets required for the economic feasibility of the proposed electrochemical GTL scheme. In this section, we provide an outlook for possible directions of 28


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material research that could meet the said targets. The current density should be increased to at least ∼0.5 A/cm2 so that the the capital investment cost of the proposed system would be comparable to that of the current GTL system. In addition, the input voltage of cells should be reduced to ∼0.5 V so that the proposed scheme can break even at the methanol floor price. We believe that while these material targets are ambitious, they are achievable. In order to meet the ambitious targets of increasing the current density and the conversion efficiency, a better understanding of the anode reaction mechanism is required. The active reaction site must be capable of generating a reactive oxygen species while at the same time activating methane. The mechanistic investigations by the work of Hibino and co-workers 26 lead to the following important conclusions, (i) there was no significant dependence of the methanol concentration on the inlet methane concentration and (ii) water vapor concentration had a drastic effect on the overall rate. These results indicate that water vapor plays a crucial role in the formation of active oxygen responsible for the partial oxidation. Further, this suggests that the overall rate is controlled more by active oxygen generation as compared to methane activation on SnO2 /V2 O5 . It has been argued by Hibino and co-workers that chemisorption of methane likely requires twin vanadium-oxygen species. 26 However, the role of SnO2 is still largely unclear and it is believed that it plays a crucial role in generating the active oxygen species. A key aspect to discern the role of SnO2 is to identify the active oxygen species. A detailed mechanistic understanding based on experiments with well-defined electrodes coupled with density functional theory calculations is necessary to exactly map out the reaction landscape. This forms the key in meeting the technical targets identified of lowering the kinetic overpotentials by ∼0.50.9 V and increasing conversion efficiency. Another important area of materials research required is to enhance the proton conductivity at the intermediate temperature range. The conductivity of the proton conductor, Sn0.9 In0.1 P2 O7 , is still about an order of magnitude less than the targets outlined in this work. Metal-Organic-Frameworks are emerging as an attractive class of materials with their crystallinity, chemically functionalizable pores

29



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allowing operation over a wide variety of temperatures. 45 Solid acids are materials which present a chemistry between an acid and a salt and make potential candidates. These compounds undergo a structural change which leads to a highly conductive superprotonic phase at temperatures above 140◦ C. However, the stability poses a challenge and identifying stable materials with good proton conductivity (> 10−2 S/cm) in the range of 150-300◦ C is desirable. Another approach is to find polymer electrolytes that could be operated at these intermediate temperatures. However, there is dearth of well performing materials that can operate at intermediate temperatures. Acid doped membranes offer great promise with their ability to completely scrap any humidification system and high tolerance to impure fuel. However, these membranes also have a host of other issues, acid leaching, long start up times. 46 Hence, finding a novel proton conducting electrolyte with good conductivity, and long term stability is crucial for the viability of electrochemical GTL systems.

Conclusion We propose a direct electrochemical conversion of methane from APG into methanol as a method for flare gas recovery. The overall proposed system can be operated in remote areas without the need of the energy storage, electricity transmission system, and gas transporting pipelines. Based on a detailed techno-economic analysis of the entire system, we outline the minimum performance metrics at the material and device level required for the system to become commercially competitive. The minimum performance metrics, input voltage at the cell level of ∼ 0.5 V which corresponds to an overpotential of ∼1 V and a current density of 0.5 A/cm2 , which requires a proton conductor that can conduct 10−1 − 10−2 Scm−1 at the temperature range, 100-250◦ C, need to be met for this system to become financially competitive. The key scientific challenges involve identification of the active oxygen species which holds the key to identifying active and selective electrocatalysts and the development of novel intermediate temperature electrolytes which provide good conductivity while

30


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maintaining stability at intermediate temperatures.

Acknowledgement Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund and the Royal Thai Government for partial support of this research. This work is also supported in part by the Pennsylvania Infrastructure Technology Alliance, a partnership of Carnegie Mellon, Lehigh University and the Commonwealth of Pennsylvanias Department of Community and Economic Development (DCED).

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Nomenclature $bop,ref Cost of the balance of plant at the reference cell area $bop

Cost of the balance of plant at the specific cell area

ηc

Conversion efficiency of methane to methanol

dNCH4 dt

Molar flow rate of methane

Aref

Reference cell area

$c

Manufacturing price of a cell

$CH3 OH Price of methanol $depn Depreciation of the system $el

Cost of input eletricity

$i

Installation cost

$om

Cost of operation and maintenance

$stack Manufacturing price of a stack $LCOEP V Levelized cost of the energy of Solar Photovoltaics Ac

Specific area of a cell

bpd

Barrels per day

CF

Cash flow

F

Faraday constant

I

Total current of the system

i

Current density 37



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

Inv

Total investment of the system

IRR

Internal rate of return

k

year

NCH4 Total amount of methane Nstack Total number of stacks in the system Ntotal Total number of cells in the system NPV Net present value Pinput Total input energy of the system r

Discount rate

Rev

Revenue

t

Operational time

Vc

Voltage of a cell

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Identifying Material and Device Targets for a Flare Gas Recovery

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