Flare gas recovery at an LNG plant GL1/Z -Arzew

doi:10.21203/rs.3.rs-3115972/v1

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Flare gas recovery at an LNG plant GL1/Z -Arzew

https://doi.org/10.21203/rs.3.rs-3115972/v1

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Natural gas is the fastest growing source of fossil energy since the 1970s. Due to its economic and environmental benefits, natural gas is becoming more attractive to many countries every day. The properties of this product, as for example, the low combustion range characterizing it, make it one of the most used reliable sources known to date. Currently, it is the second most used energy source after oil.

The GL1/Z plant -Arzew -SONATRACH Company is designed for the liquefaction of natural gas, which is an essential way of transporting gas over long distances. This liquefaction (at -160 ° C) is obtained by successive cooling through compressed refrigerants and then relaxed between two pressure stages. This liquefaction technique consumes a lot of gas considered as internal consumption. In addition to this loss, there is a certain amount of gas that is flared during startup which accounts for about 2/3 of the total amount of flared gas.

The reduction of flared gases is important to reduce carbon emissions and rationally use natural gas.

In this work we simulate by ASPEN-HYSYS software a three method of gas flare recovery such as power electric generation, LPG (Liquid Petroleum Gas ), and GTL technology (Gas to liquids) to evaluate natural gas resources of GL1/Z plant (SONATRACH Industry –Algeria).

Gas flares recovery

GTL technology

LPG

Electricity generation

Gas flared in Algeria

The problem of recovering flared gas has always been a major concern for SONATRACH, especially in large oil and gas fields such as Hassi Messaoud, where the slightest stoppage of a gas reinjection station could result in the loss of 10 million m³/day of flared gas.

This has led SONTRACH to implement a major programme to reduce greenhouse gas emissions through methane flaring at its facilities, with the aim of achieving between 0 and 1% maximum emissions by 2030.

In 2016, Sonatrach had already reduced the flaring rate to 60% compared to the 1980s. This reduction allowed the recovery of about 4 billion cubic meters of gas.

The efforts deployed by SONATRACH Company have made it possible to reduce flared gases by 500 million M³ during the year 2020–2021, he said, assuring that other projects will be undertaken to further reduce flared gases in collaboration with the Algerian Space Agency.

It should be noted that in 2022, 139 billion cubic meters of flared gas in the world, Algeria came in 4th place with 8.6 billion cubic meters of flared gas, according to data from the World Bank (2022). Figure 1 below shows the ranking of flaring countries.

In the literature, we find various studies for flare gas recovery. The studies that investigated the potential of adopting diverse technologies for flare gas recovery in countries such as Brazil, Nigeria, Algeria, Tunisia, Egypt, Italy, the United States and Iran.

The reduction of CO₂ emissions and the production of premium diesel by recovering flared gas in Brazil using an offshore gas to liquids (GTL) plant are studied by Branco et al. 2010). The recovery of flared gases resulting from the separation of Algerian crude oil into GTL was the subject of a study presented by (Djebri et al. 2009).

The potential for producing electricity from associated gas in Tunisia are studied by (Bauer et al. 2012). They concluded that electricity production is an appropriate option from an economic and environmental perspective. (Fallah et al. 2019) analysed the capability for producing electricity from associated gas produced through a Tunisian oil company. The payback period for the production unit is 3.2 years.

In the context of implementing the clean development mechanism, (Abdulrahman et al. 2015) conducted research on Egypt's first project to recover flare gas from a refinery project at Suez Oil Processing Company in Egypt.

The critical challenges of developing the GTL production cycle using flared gas have been evaluated by Shell in Nigeria (Rahimpour et al. 2011). Odumugbo (2010) provided a description of the state of the natural gas industry in Nigeria, as well as the demand for and supply of electricity. The mentioned study evaluated a number of technologies to save natural gas resources through flared gas recovery. (Adewale and Ogunrinde 2010) evaluate the financial and economic viability of gas flaring reduction options in a selected field in Nigeria. (Ojijiagwo et al. 2016) analysed electricity generation from flared gas taking into consideration the projected electricity power requirements and the feasibility of electricity generation from flared gas in Nigeria.

In the same context, development projects involving the recovery of flared gases and studies involving various technologies such as electricity production, methanol, fertilizers and liquid gas (GTL) have been presented by (Nwaoha and Wood. 2014).

(Comodi et al. 2016) provided the advantages of a flare recovry device the use of the ejector and liquid ring compressor in an oil refinery. They applied the generation offered using a take a look at case from an Italian refinery.

In the USA (Beal et al. 2016) proposed using flared gas to produce heat, electricity and ammonia for algae cultivation in a combined heat and power plant. A plan to recover flare gas using an electrochemical cell to convert methane into methanol have been developed by Promoppatum and Viswanathan (2016).

(Zadakbar et al. 2018) developed methods for the recovery and reuse of flare gas from two refineries in Iran (Tabriz Petroleum refinery and Shahid Hashemi-Nejad natural gas refinery). The authors explored the design and financial aspects of flare gas recovery as combustible gas. (Rahimpour et al. 2012) proposed three methods of flare gas recovery from the Farashband gas refinery, including electricity generation using a gas turbine, compression and injection into refinery pipelines and GTL. The results of their economic evaluation revealed that electricity generation was the most financially advantageous technique. (Rahimpour and Jokar. 2012) examined the same three techniques to recover flare gas in the Asalooye refinery, this study revealed that the gas compression method is the best choice for Iran's Asalooye gas refinery due to a high efficiency rate and lower capital costs.

In another study (Ghorbani et al. 2013) focused on improving the GTL process based on flare gas recovery. A new GTL loop with a lower CO₂ emission rate and a higher hydrogen and C₅⁺ production rate was proposed. The study of (Heidari et al. 2016) examined two techniques for producing electricity from flared gas from a refinery and compared the energy and financial efficiency of these scenarios. (Zolfaghari et al. 2017) also compared the economics of three available flared gas recovery methods, that is, gas toliquid (GTL), gas to ethylene (GTE) and gas turbine generation of electricity at the Asalouyeh refinery in Iran. They concluded that using gas turbines to produce electricity gives the highest rate of profit and annual return.

Liquefied petroleum gas (LPG) production, liquefaction and three-stage compression procedures were tested by (Hajizadeh et al. 2018) as a substitute for flared gas recovery from a giant gas refinery in Iran. They found liquefaction and LPG production, as opposed to compression, to be the preferable strategies for the case study they examined. Saidi (2018) analysed the energy and environmental aspects of another flared gas recovery process, a catalytic membrane reactor producing pure hydrogen. This flared gas recovery approach increases the production of pure hydrogen and greenhouse gas emissions from the flare at the Asalouyeh gas processing plant in Iran can be significantly reduced. The recovery of hydrogen and carbon dioxide from a methanol plant as a flared gas recovery process has been studied by (Khanipour et al. 2020). In a study developed by (Hamidzadeh et al. 2020), they explored the best combinations of different flare recovery systems by maximising the objectives economic and environmental factors. (Nezhadfard and Khalili-Garakani 2020) examined the environmental and economic performance of four potential scenarios for producing electricity from flare gas including simple cycle, combined cycle, gas engine cycle, and solid oxide fuel cell. (Khalili-Garakani et al. 2021) studied the different flared gas recovery processes in Iran and concluded that there is a potential to save $5 billion per year by producing electricity from the quantity of flared gas currently used. They also illustrated the necessity and potential of applying flared gas recovery technologies, by compiling the capacity and technologies available for flared gas recovery and their decision factors.

Torch system at GL1/Z plant

Non-liquefied gas vapours are combusted in the flares. The flare system is designed to safely evacuate the gases released during start-up, perturbed mode (transitory) and normal mode (permanent), by using various valves and purges. Gases flow into the flare system to a main collector and then to the flare. Torch system at GL1/Z plant is composed of two torches: the cold torch and the hot torch.

Cold torch

Each of the six (06) LNG trains is equipped with a collector of 508 mm diameter and a collector of 609.6 mm diameter that join a principal collector. The collector joins the separator tank of the cold torch. The hydrocarbon vapours then pass to this torch, where they are combusted constantly after passing through a molecular joint.

Hot torch

Each of the six (06) LNG trains has is equipped with a collector measuring 914.4 mm in diameter that runs the length of the plant and ends at the hot flare separator tank. The hydrocarbon vapors then pass into the hot flare where they are fired by a pilot flame, where they are combusted constantly after passing through a molecular joint. A small purge pot is placed between the separator flask and the flare stack to vaporise liquid hydrocarbons that may condense on the way to the flare. A flame front generator is provided to separately ignite the four (04) pilot flames placed at the top of the flare stack.

Sources of flare gas

Various sections of a liquefaction train are concerned by the flaring gas at the torchs (Fig. 2).

Figure 3 presents the evolution of flared gas at the GL1Z plant. There is a slight increase for the year 2022. Figure 4 presents the ratio of flared gas to the treated gas, with a considerable decrease in recent years

Fig. 4. Ratio of flared gas and treated gas.

Flared gas analysis

The analysis of the composition of the flared gas is carried out by using gas chromatography (G.C.). In order to obtain the best result that can give us the composition of the flared gas, the selection of the samples was based on :

At the level of the collector tank top of the hot torch.
At the level of the collector tank top of the cold torch.

Following the analysis of the different gas samples, we obtained the results reported in Table 1.

The operating conditions of the two flares during normal train operation are summarized in Table 2.

Table 1

Composition of flared gas.
Components	Hot torch (% molar)	Cold torch (% molar)
CO₂	0.102	0.980
Nitrogen	1.221	4.000
Methane	77.080	60.230
Ethane	9.621	16.140
Propane	5.450	8.810
n-Butane	2.930	4.430
i-Butane	2.335	4.110
n-Pentane	0.195	0.255
i-Pentane	0.960	0.329
n-Hexane	0.106	0.725

Table 2

Operating conditions of the two flares.
	Hot torch	Cold torch
Flow rate Kg/h	15954.15	23798.4
Pressure Bar	1	1
Temperature oC	25	-70

Simulation of flare gas recovery system

The simulation of flare gas recovery methods is carried out using ASPEN-HYSYS V12 software. This software is extremely powerful for steady state simulation. In the present research, the Peng-Robinson thermodynamic model is used for the simulations, which is the most widely used model in the field of oil and gas (hydrocarbon). In this study we have simulated three methods of flared gas recovery at the GL1Z complex, namely electricity, LPG and GTL production (Fig. 5).

Production of electricity

Flared gases are used to generate electricity. This production is used for distribution or for internal consumption, generally using a turbine. The Brayton cycle is one of the most efficient cycles for converting gaseous fuels into mechanical energy and electricity using a turbine.

The role of the combustion chamber is to combust a mixture of air and flared gas coming from the compressor, where the pressure is increased from atmospheric pressure to 23 bar. The gases produced by combustion, where the potential chemical energy contained in the fuel is transformed into heat energy, are directed towards the turbine. The turbines of turbo engines are the site of an adiabatic detent which transforms the energy available in the active fluid into mechanical energy, which is converted to electricity using the generator. The exhausted gases then leave the gas turbine. Fig. 6 shows the Brayton cycle simulated in this study.

Figure 6. Production of electricity (Rahimpour et al. 2012).

Using the ASPEN HYSYS v12.0 simulator, a gas turbine cannot be simulated directly, but a system representing a gas turbine can be simulated. Figure 7 shows the simulation of the transformation of flared gases to produce electricity.

The combustion reactions that take place in the combustion chamber are grouped together in the following Table 3:

Table 3

Combustion reactions.
Reactions	ΔH^o₂₉₈ (*10^+ 6 KJ/Kmol)
CH4 + 2O2 → CO2 + 2H2O	-0.8
C2H6 + 3.5O2→ 2CO2 + 3H2O	-1.4
C3H8 + 5O2→ 3CO2 + 4H2O	-2.0
C₄H₁₀ + 6.5O₂→ 4CO₂ + 5H₂O	-2.6 for iC4 -2.7 for nC4
C5H12 + 8O₂ → 5CO2 + 6H2O	-3.3
C6H14 + 9.5O2→ 6CO2 + 7H2O	-3.9

To ensure the complete combustion of the entire gas charge, a sufficient quantity of oxygen must be given for the complete combustion of each component of the charge. In the ASPEN-HYSYS simulator we have the "Set" function to establish a relationship between the gas molar flow rate Q_Gas and the oxygen molar flow rate Q _O2 :

𝑄_O2 = 𝛼 ∙ 𝑄_𝐺𝑎s (1)

To calculate the α coefficient, we need the stoichiometric coefficients Ki of each reaction, then multiplying by the molar fraction Xi of the components between the fuels, eliminating the non-combustible components (CO₂ and N₂). To do this we need to recalculate the X'i of New.

Xi: molar fraction of each compound in the gas charge.

Ki: stoichiometric coefficient of oxygen in the reaction with component i.

X’i: molar fraction of compound in combustible compounds.

$${X}_{i}^{{\prime }}=\frac{{X}_{i}}{{\sum }_{C1}^{C6}Xi}$$ 2

αi : partial coefficient of flow regulation.

𝛼_𝑖 = 𝐾_𝑖 ∙ 𝑋′_𝑖 (3)

α : flow regulation coefficient.

$$\alpha =\sum _{1}^{6}{\alpha }_{i}$$ 4

⟹𝛼 ≥ 2.6

The relation (1) becomes :

𝑄_𝑂2 = 2.6 𝑄_g𝑎s (5)

The molar composition of air is 78.05% nitrogen N₂, 20.95% oxygen O₂ and 1% other gases.

The relation (5) becomes

Q _air = 12.41 Q_gas (6)

- For the compressor « gas comp » :

Consumed power E₁ = 4297 kWH.

- For the compressor « Air comp » :

Consumed power E₂ = 9.527 10⁴ kWH.

- For the turbine « Elecrical Power » :

Power generated E₃ = 2.590 10⁵ kWH

Production of electricity E = E₃-E₂-E₁ = 1.13 10⁵ kWH

Production per year = 1.13 10⁵*24*365 = 9.910⁸ kWH.

Simulation of LPG unit

One of the methods used to recover flared gas using a treatment plant is to produce:

Fuel gas.
LPG.
Condensate.

Recovery of the LPG contained in the flared gas is a method of increasing the quantity of LPG produced (Fig. 8).

The system for transforming flared gases into LPG, condensate and Fuel gas comprises four steps:

- Demethanisation: recovery of methane gas.

- Deethanisation: recovery of ethane.

- Depropanisation: recovery of propane.

- Debutanisation: recovery of butane.

The mixture of gases at the outlet of the demethaniser and dethaniser is the Fuel gas. The LPG is the product of the mixture of gases at the top of the depropaniser and debutaniser. The condensate is recovered at the bottom of the debutaniser column. Figure 9 shows the simulation of the flared gas recovery unit in the form of LPG, condensate and Fuel gas.

Simulation of a process to recover 39752.55 Kg/h of flared gas results in :

27663.79 Kg/h Fuel gas.
10900.01 Kg/h LPG.
1188.75 Kg/h condensate.

The composition of the Fuel gas, LPG and condensate is given in Table 4.

Table 4
Compositions of finished products.
Components	Fuel gas (%molar)	LPG (%molar)	Condensate (%molar)
CO₂	3.211	0.000	0.000
Nitrogen	0.686	0.000	0.000
Methane	78.323	0.000	0.000
Ethane	15.289	0.431	0.000
Propane	2.152	53.116	0.000
n-Butane	0.133	28.729	0.999
i-Butane	0.201	14.858	0.071
n-Pentane	0.002	0.555	17.055
i-Pentane	0.003	2.294	34.091
n-Hexane	0.001	0.015	47.784

The molar composition of the LPG produced by the simulation is compared with the composition of the LPG produced at the GP1Z¹ plant. This comparison is reported in Table 5.

Table 5
Comparison of LPG composition.
Components	LPG (GP1Z) % molar	LPG (simulation) % molar
Methane	0.23	0.0
Ethane	1.59	0.431
Propane	59.30	59.118
i-Butane	14.10	14.858
n-Butane	24.54	24.729
i-Pentane	0.20	0.294
n-Pentane	0.03	0.555
n-Hexane	/	0.015
H₂O	0.002	/

Simulation of GTL unit

GTL (Gas to Liquids), a technology that enables natural gas to be converted directly into various liquid synthetic petroleum products, has been the subject of growing interest in recent years from oil companies, governments, industry, universities and research and technology development centres. The aim is to use natural gas, which is made up of small molecules, to produce products with much larger molecules. The main component of natural gas is methane, whose molecule comprises 4 hydrogen atoms solidly arranged around a carbon atom, and is characterised by particular stability.

Its transformation into a liquid first requires the chemical bonds between atoms to be destroyed by a major external input of energy, in the form of heat and at high pressures. Carefully selected catalysts are involved in the chemical reaction, without actually being altered or consumed. The conventional "indirect" approach used to break these bonds is based on pure power; the chemical bonds of the methane molecule are ruptured using vapour, heat and a catalyst, resulting in the formation of a mixture of CO and hydrogen, generally known as "SynGas" or synthetic gas.

The next step is to convert the syngas into various liquid fuels using the Fisher Tropsch process, invented in 1923. Catalysts are essential to this process. The liquid fuels formed are refined to obtain a variety of products using conventional methods (Fig. 10).

The objective is to transform flared gases containing methane and ethane of a majority composition into components with longer molecular chains. The GTL process is very complex compared with other methods.

The GTL process is composed of different units, some of which are principal, such as the synthesis gas production unit, the FT (Fisher Tropsch) synthesis unit and the product valorization unit.

✓ Syngas production unit: the reaction section comprises two reactors. The Pre- reformer is feed by a charge with a flow ratio of H₂O/CH₄ = 2.3.

All hydrocarbons heavier than methane react completely in the first reformer. As a result, the feed stream to the secondary steam reformer contains no hydrocarbons heavier than methane.

The next Table 6 shows the various reactions:

Table 6

Pre-reformer reactions.

Type of reactions

Reactions

ΔH₂₉₈ ⁰ KJ/mol

Reactions of conversion

C₂H₆ + 2H₂O→ 2CO + 5H₂

+ 350

C₃H₈ + 3H₂O → 3CO + 7H₂

+ 500

iC₄H₁₀ + 4H₂O→ 4CO + 9H₂

+ 660

nC₄H₁₀ + 4H₂O→ 4CO + 9H₂

+ 650

iC₅H₁₂ + 5H₂O→ 5CO + 11H₂

+ 810

nC₅H₁₂ + 5H₂O→ 5CO + 11H₂

+ 800

nC₆H₁₄ + 6H₂O→ 6CO + 13H₂

+ 950

Equilibrium reactions

CO + 3H₂ → CH₄ + H₂O

-210

CO + H₂O → CO₂ + H₂

-41

A third Auto Thermal Reformer Reactor (ATR) fed by a charge with an O₂/CH₄ = 0.5 ratio given by Otaraku and Vincent (2015). Table 7 shows the equations reactions and their corresponding reaction enthalpies.

Table 7
Auto Thermal Reformer Reactor (ATR) reactions.
Reactions	ΔH₂₉₈ ⁰ KJ/mol
CH₄ + 1.5O₂ → CO + 2H₂O	−519
CH₄ + H₂O → CO + 3H₂	210
CO + H2O → CO₂ + H₂	-41

✓ The Fisher -Tropsch (FT) synthesis unit: the reaction section is charged with H2/CO = 2.1. The FT reactions are given in Table 8.

The stoichiometric coefficients for the FT reactions are based on the ASF distribution determined by (Kerron et al. 2014) and Otaraku & Vincent (2015) :

Xn = (1-ά)2άn-1 ά = 0.9 (7)

ά : chain growth probability, which is a direct measure of the probability of an FT catalyst to catalyse a chain.

Xn : mole fraction of each carbon number (n)

Table 8
Fisher -Tropsch reactions Otaraku and Vincent (2015).
Reactions	ΔH₂₉₈ ⁰ KJ/mol
$CO+2.1{H_2} \to ~\sum\limits_{{n=1}}^{{20}} {{{(ASFcoefficient)}_n}{C_n}{H_{2n+2}}+{{(ASFcoefficient)}_{30}}{C_{30}}{H_{62}}+{H_2}O}$	-160
CO + 3H₂→CH₄ + H₂O	-210

The catalyst used is Cobalt, and the following reaction given by Otaraku and Vincent (2015) gives a better representation for the H2/CO = 2.1 ratio.

CO + 2.1H₂ → 0.01CH₄ + 0.009C₂H₆ + 0.008C₃H₈ + 0.007C₄H₁₀ + 0.007C₅H₁₂ + 0.006C₆H₁₄ + 0.005C₇H₁₆ + 0.005C₈H₁₈ + 0.004C₉H₂₀ + 0.004C₁₀H₂₂ + 0.003C₁₁H₂₄ + 0.003C₁₂H₂₆ + 0.003C₁₃H₂₈ + 0.003C₁₄H₃₀ + 0.002C₁₅H₃₂ + 0.002C₁₆H₃₄ + 0.002C₁₇H₃₆ + 0.002C₁₈H₃₈ + 0.002C₁₉H₄₀ + 0.001C₂₀H₄₂ + 0.012C₃₀H₆₂ + H₂O. (8)

Fig .11 below shows the simulation of flared gas recovery using the GTL process

The simulation of a flared gas recovery process enables :

45.54 Kg/h fuel gas
1.87 10⁴ Kg/h de condensate.

To help the Sonatrach company make an investment decision. The comparison on annual revenues and return on investment between the three alternative flare gas recovery processes such as electricity production, LPG production and GTL technology is carried out. After simulating the three above-mentioned processes using steady-state simulation, an economic evaluation of each process is carried out. Figure 12 shows the annual revenues from the various flare gas recovery.

The investment estimates for the construction of the various units are shown in Table 9. The estimates were calculated using the economic evaluation of ASPEN-HYSYS V.12 and the estimates of recent projects carried out by the SONATRACH Company. Figure 13 shows the total capital investment for various simulated electricity, LPG and GTL production units. The Fig. 14 shows the payback periods for the three simulated units

Table 9

Estimated capital investment
	Electricity	LPG	GTL
	[M$ (million dollars ]
Purchased equipment cost	8.55	15.21	36.72
Piping & installation of equipment	1.80	3.59	6.25
Building including services	1.66	1.84	3.36
Total direct cost	1.72	2.00	4.28
Engineering	1.97	2.88	5.83
Contractor’s fee	1.97	2.16	3.56
Total indirect cost	1.49	1.60	3.1
Total fixed cost	1.31	1.84	2.98
Working cost	0.53	1.38	3.19
Total capital investment	20.99	32.50	69.27

A comparison of total investment in the three units is shown in Fig. 13. It can be seen that the electricity production unit has the lowest capital investment. Figure 12 shows that the annual GTL profit is higher than that of the LPG unit and electricity production.

However, the return on investment years of the electricity production unit is the lowest compared to the LPG and GTL unit (Fig. 14).

There are many methods for reducing gas flaring in oil and gas plants. In order to identify the best method for recovering flared gas from the GNL1-Arzew plant, we carried out simulations of the three methods of recovering and exploiting flared gas, namely electricity production, LPG production and the GTL process.

Based on a comparison of the results of the economic evaluations, the electricity production method shows a lower return on investment than the two other methods, but with lower annual profits. Although flare gas reforming based on GTL technology is more expensive than electricity generation, the annual profit of this method compensated for this deficiency. Given that Algeria does not have a deficit in the production of electricity and LPG, and given that the GTL process has not been installed, we recommend recovering flared gas using the GTL process to acquire expertise in this technology.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. On behalf of all authors, the corresponding author states that there is no conflict of interest.

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GP1Z-Arzew liquefied petroleum gas plant.

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Reviewers agreed at journal
18 Sep, 2023
Reviewers invited by journal
18 Sep, 2023
Editor invited by journal
10 Jul, 2023
Editor assigned by journal
03 Jul, 2023
First submitted to journal
27 Jun, 2023

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Flare gas recovery at an LNG plant GL1/Z -Arzew

Status:

Version 1

Abstract

Figures

Introduction

Review of literature

Torch system at GL1/Z plant

Cold torch

Hot torch

Sources of flare gas

Methodology

Flared gas analysis

Simulation of flare gas recovery system

Production of electricity

Simulation of LPG unit

Simulation of GTL unit

Results and discussion

Conclusion

Declarations

References

Footnotes

Status:

Version 1

Reactions	ΔH₂₉₈ ⁰ KJ/mol
\(CO+2.1{H_2} \to ~\sum\limits_{{n=1}}^{{20}} {{{(ASFcoefficient)}_n}{C_n}{H_{2n+2}}+{{(ASFcoefficient)}_{30}}{C_{30}}{H_{62}}+{H_2}O}\)	-160
CO + 3H₂→CH₄ + H₂O	-210