Upgrading Biogas to Biomethane Using Membrane Separation

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Upgrading biogas to biomethane using membrane separation Veronika Vrbová, and Karel Ciahotny Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00120 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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UPGRADING BIOGAS TO BIOMETHANE USING MEMBRANE SEPARATION AUTHOR NAMES: Veronika Vrbová*, Karel Ciahotný AUTHOR ADDRESS: Department of Gaseous and Solid Fuels and Air Protection, University of Chemistry and Technology Prague, 166 28 Prague 6, Czech Republic KEYWORDS: Membrane Separation, Biogas, Biomethane, CO2 Removal

ABSTRACT

Biogas contains carbon dioxide in the range 35 – 45 vol.%. Upgrading biogas to biomethane is primarily based on carbon dioxide removal. Biomethane is essentially purified biogas containing at least 95 vol.% methane and it can be either used as fuel for vehicles running on CNG (Compressed Nature Gas) or injected into the natural gas grid. Nowadays various techniques are used for CO2 removal from biogas. Among the most commonly used technologies are adsorption, absorption, cryogenic separation and membrane separation. Currently in the Czech Republic no unit for biogas upgrading to biomethane operates. Additionally during the summer months there is heat overproduction from the cogeneration units. In this work a suitable unit for carbon dioxide separation is proposed. Carbon dioxide separation is possible using membrane separation. Along carbon dioxide minor compounds present in biogas such as hydrogen sulfide

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and water are also separated. The implementation of this unit makes possible to obtain biomethane form biogas. Membrane separation was tested in a pilot scale using real biogas. All experimental tests took place at the Central Waste Water Treatment Plant in Prague. Experimental tests were carried out using different kinds of membrane. For comparison purposes the following membrane modules materials were chosen: polysulphone and polyimide fibre membranes. Separation of moisture and trace compounds present in biogas was tested for these two types membrane material. Other tests were carried out using polyimide membranes. Parallel connection of membrane modules was the most effective to remove carbon dioxide from biogas. Purified biomethane contained at least 95 vol. % of methane, as is required, even when the highest flow rate was applied of 7 m3·h-1 of biomethane (measuring conditions: 0.6 – 0.8 MPa). This small membrane separation unit is recommended for biogas units in waste water treatment plants. 1. INTRODUCTION The Czech Republic has a high biogas production potential, primarily biomethane, as a natural gas substitute. Biogas generated from the anaerobic digestion of agricultural products makes up for about 25 % of all bioenergy produced in the Czech Republic. Currently there are over then 500 biogas station with a total installed capacity of 360 MW. Biogas is usually burned in cogeneration units that are used for combined heat and power generation. During the summer is low heat consumption. For this reason, technologies for the biogas upgrading to the biomethan are being developed. Biogas composition depends on the organic substrate and digestion conditions. Depending on the production parameters landfill gas, biogas from agricultural biogas plants or biogas from sewage sludge is generated. Typical biogas is a mixture containing methane, carbon dioxide and a


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number of other components. Minor biogas components include water, hydrogen sulphide, nitrogen, oxygen and small amounts of ammonia. The calorific value of biogas is lower due to the presence of carbon dioxide. The presence of the listed minor components causes corrosion or clogging of the engines for biogas combustion. Therefore often biogas purification to biomethane is performed. One of the primary utilizations of biogas is combustion in cogeneration units, where combined production of heat and electricity takes place. During the summer months heat consumption decreases hence excess heat is released in the air. As a result it is convenient to convert the produced gas to biomethane. Methane content in biomethane is over 95 % making it a suitable biofuel for vehicles.1, 2, 3, 4, 5 The most common technological processes for biogas upgrading to biomethane are PSA (Pressure Swing Adsorption), absorption, membrane separation and cryogenic separation. Adsorption and absorption unit generally have high energy demands and require relatively large equipment. On the contrary CO2 membrane separation is among the technologies with lower energy demands. Additionally the equipment for membrane separation technologies is less voluminous. Recent research uses two PSA units connected in series. From the first PSA unit is obtained biomethane and gas rich in carbon dioxide. The gas stream rich in carbon dioxides is used as feed stream for the second PSA unit. Output stream from the second unit consists of very pure carbon dioxide (≥ 99 %). Carbon dioxide can be subsequently used as a carbon source in a variety of chemical and biochemical processes.6, 7, 8 Among the mentioned biogas upgrading methods membrane separation using polymer membranes is the most competitive one. However, it should be noted that polymer membranes are very susceptible to degradation caused by a series of compounds present in biogas. Among the impurities that have the most adverse effect are


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ammonia, hydrogen sulphide and water. In order to prolong membrane service lifetime hydrogen sulphide removal and drying are recommended.9, 10 Currently in Czech Republic no biogas upgrading unit to biomethane is operating. Therefore the aim of this work consisted on proposing a suitable membrane separation unit that can be implemented in a small biogas station. Moreover the proposed unit should be financially feasible. During the past years membrane separation compared to other technologies has become very competitive. Membranes have a series of applications such as potable water production, waste water disposal, gas purification, etc. Membrane processes have very high separation efficiency and usually produce very high purity retentate.11, 12 Several published works describe biogas upgrading to biomethane via membrane separation. Scholz et. al13 state that the main advantage of membrane separation is its utilization under very severe operating conditions such as high pressure up to 2.5 MPa and the presence of aggressive compounds (hydrogen sulphide and water vapour). Baker et. al14 researched multistage separation i.e. the utilization of various membranes to obtain high purity biomethane.13, 14 This work was based on these two researches. In order to prolong membrane longevity a biogas precleaning step was proposed. Pre-cleaning was performed using silica gel and impregnated activated carbon sorbents. Silica gel served for water removal while impregnated activated carbon served for hydrogen sulphide removal. After membrane separation, the retentate was then compressed via a three-stage compressor to 25 MPa. The compressed biomethane was fed to the biomethane storage cylinder. Biomethane form the cylinder could be than directly used as automobile vehicle fuel.


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2.

THEORY

Membrane separation is a highly selective process based on the different transport rates of chemical species through the membrane interphase. The main drawback of membrane separation is low membrane resistance to aggressive compounds present in the separated medium. The key parameters of porous membranes are pore size, shape and transport flow through the membrane interphase depends on pore size and shape, layer porosity and the interaction between pore walls and the separated compounds.15 In Table 1 a comparison of the various carbon dioxide removal technological processes and subsequent biogas upgrading by Ryckebosch et al. and Wellinger A. et al. are depicted. From the summary emerges that apparatus simplicity, setup demands and reliability are the main advantage of CO2 membrane separation from biogas. The advantages of membrane separation are particularly evident in small biogas plants applications.16, 17, 18


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Table 1. A comparison of technologies for biogas upgrading Process Technology Advantages description Adsorption CO2, higher -high quality gas CxHy, H2S, Si-, F- - dry process , Cl- compounds - no chemicals and odorous - no waste water compounds - partial N2 and O2 removal - no bacterial contamination removed via activated in waste gas carbon/carbon - certified technology molecular sieves

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Disadvantages -needs H2S removal - 3 to 4 adsorption columns - CH4 quality is not stable - a complicated process - higher investment costs - high energy demands - large equipment

Chemical and physical absorption

CO2 a H2S are absorbed in the scrubbing medium (e.g. water, ammines, glycols, etc.)

- high quality gas - suitable investment costs - no need for gas purification prior to the process - a compact process - certified/tested technology - it is possible to reuse CO2

- waste water liquidation - high water consumption - higher investment costs - high energy demands - large equipment

Membrane separation

CO2 separation based on different compound permeability through the membrane CO2 is liquefied due to high pressure and low temperature

- dry process - no chemicals - low mechanical deterioration - a compact process

- gas pre-purification needed - higher methane losses - unstable long term behaviour

- high quality gas - no chemicals - no water - compact process - it is possible to reuse CO2

- gas pre-purification needed - very high energy consumption - high investment cots - a complicated process

Cryogenic separation

2.1.

Materials

The most common materials able to separate CO2 and CH4 are polymers or inorganic materials. In practical applications polymer membranes are the most common. The separation efficiency depends on the polymer molecular structure and the so called solubility-diffusion mechanism. As a result higher gas solubility and conductivity leads to higher permeability through the


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membrane interphase. Gas flow depends on the membrane effective surface area and driving permeability force i.e. concentration gradient across the membrane walls.19, 20 The most commonly used membranes are made of polyimide, polyamide and cellulose acetate. These membrane materials are very susceptible to moisture content thus biogas should always undergo drying prior to membrane separation.13, 21 Aromatic polyimides are formed by aromatic rings and functional groups, which deals like molecular sieves. Due to the high glass transition temperature and low solubility of the polymer it is possible to use it in a wide range of pressures and temperatures. Polyimide also has excellent chemical resistance.22 A different class of membranes is made of polysulfone, polyetheramide or polyethylenimide. Polysulfones contain a sulfonic group (O = S = O). The polymer chains are composed of aromatic rings which are linked by sulfol groups. Polysulfones are able to withstand water, weak acids and alkalis. Very promising appears to be ionic liquid embedding in the membrane pores. The main advantages of these membranes are the high flow permeability and high selectivity. It has been proved that ionic liquids are primarily selective for methane and carbon dioxide mixtures. However, currently this class of membranes is too expensive for industrial applications. Additionally due to membrane’s hygroscopic properties humid environments cause fast properties loss. Even though membranes have low chemical reactivity membrane fouling still occurs even for biogas containing only trace amounts of impurities. As a result ionic liquid membranes are not suitable for CO2 separation from biogas at an industrial scale.23


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A series of studies have been carried in the frame of membrane development technologies to assess options for membrane fouling reduction. However, the proposed measures reduce gas flow rate through the membrane walls. Polyethersulphone is a possible membrane material to obtain membranes that are chemically resistant, thermally stable and with high mechanical strength. Moreover carbon based membranes have been tested and showed higher selectivity compared to polymer materials.24, 25, 26 Polydimethylsiloxan (PDMS) is a commonly used membrane material. PDMS is a stable, dense and nonporous material. PDMS based membranes prevent bubble formation, at slightly elevated pressure, and pore contamination. Both phenomenons are key disadvantages of other membrane materials that are commonly used for biogas upgrading.27 Membrane fibre materials used for gas separation technologies can be divided in three main categories: polymer, inorganic and mixed matrix membranes. Polymer membranes are the most commonly used membranes for gas separation due to their low cost, simple production and high stability when operating at higher pressure. The most common polymer materials are cellulose acetate, polyimide, polysulfone and polysiloxane. Inorganic membrane materials, such as ceramics and metal membranes, are still in the research stages. Mixed matrix membranes are membranes that consist of a combination of polymer matrix and inorganic additives. Polymer membranes used in most commercial applications work based on the dilution-diffusion mechanism. Depending on the polymer material permeability and selectivity of membranes differs. In Table 2 the selectivity of various membrane materials based on the size of gas molecules is depicted. Gas permeability is highly influenced by dilution which depends on the ability of molecules to condense. The choice of the most suitable membrane material for a given


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gas separation process depends on the membrane costs, selectivity, permeability, chemical and mechanical stability, material availability, glass transition temperature and thermal stability.28 Table 2: Permeability and selectivity of polymeric membranes for gas separation28 Permeability at 30 °C/Barrer Selectivity Polymer H2 N2 O2 CH4 CO2 CO2/CH4 cellulose acetate (CA) ethyl cellulose (EC) polycarbonate (PC) polydimethylsiloxane (PDMS) polyimide (PI) polymethylpenten (PMP) polyphenyloxide (PPO) polysulfone (PSf)

Tg [°C]

2,63

0,21

0,59

0,21

6,30

30,0

80,0

87,0

8,40

26,5

19,0

26,5

1,39

43,0

-

0,18

1,36

0,13

4,23

32,5

150

550

250

500

800

2700

3,38

-123

28,1

0,32

2,13

0,25

10,7

42,8

317

125

6,70

27,0

14,9

84,6

5,75

30,0

113

3,81

16,8

11,0

75,8

6,89

210

14,0

0,25

1,40

0,25

5,60

22,4

190

1 Barrer = 10-10 cm3 (STP) cm cm-2s-1·cmHg-1

2.2.

Membrane modules

Membranes are divided into symmetric (homogeneous), asymmetric (heterogeneous) and composite. Symmetric membranes consist of a single material. Membrane thickness varies from a few tenths of a millimetre to millimetre. Asymmetric membranes consist of a series of very thin active layers. Each layer has a thickness ranging from a few tenths to tens of micrometre. The active layer is placed on a thicker porous support layer of the same material. The porous layer can be up to e few millimetres thick. When using asymmetric membranes separation takes place only in the active layer. The thicker porous layer serves only to improve the mechanical properties of the membrane. Composite membranes are made from several types of materials.


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When using composite membranes separation also takes place in the active layer. Several interlayers can be placed between the support and active layer.29 Membrane modules consist of a membrane placed into a capsule. The modules are divided in planar and tubular. Planar modules include hollow fibre module and spiral wound configurations. Spiral wound membranes contain several membrane envelopes and feed spacers wrapped around a central collection pipe. Each pair of membranes is attached to the permeate tube and glued together. Gas feed injection is in the direction of the central axis of the tube. Permeate passes across the membrane surface perpendicular to the centre of the tube from where subsequently exits.27 Tubular modules involve a series of modules consisting of various size tubules such as tubular (4 – 20 mm diameter), capillary (1.5 – 4 mm diameter) and hollow-fibre (diameter smaller than 1.5 mm). 2.3.

Membranes of UBE Group

UBE membrane modules are composed of polyimide membranes which are produced by condensation polymerization of biphenyl tetracarboxylic dianhydride and aromatic diamines. Polyimide membranes are a type of hollow fiber module with an asymmetric structure where the separation layer serves to separate the gas mixture and is designed to be very thin to achieve a practical permeation range. The microporous structure of the membrane only enhances the surface density of the layer and practically does not affect the separation of the gas. There is space between the molecules along which the gases pass. The gases are moved from the side with higher partial pressure to the lower side due to the pressure difference.30


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3. 3.1.

EXPERIMENTAL SECTION Methods

During membrane material selection for biogas upgrading attention should be payed not only to carbon dioxide and methane content but also to hydrogen sulphide and water vapour content. Hydrogen sulphide and water vapour can cause problems during biogas separation. Therefore the membrane material should be chemically stable and resistant to the presence of these two compounds. Moreover the selected material must meet certain requirements such as resistance to higher pressure and temperature. It should be noted that operational temperature can sometimes exceed 50 °C.13 In the Czech Republic quality requirements for biogas utilization as vehicle fuel are established by the standard ČSN 65 6514. This standard is applicable for biomethane sellers. In Table 3 biomethane quality requirements are depicted. Additionally in Table 3 a comparison of the gas parameters as specified by the standard and the actual values obtained from a biogas upgrading unit are depicted.31


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Table 3. Biomethane quality requirements for vehicle fuel utilization according to standard ČSN 65 651431 Required biomethane Real biomethane Requirements Notes value (ČSN 65 6514) value min. 95.0 % mol.

Safety margin in the number of membranes

≤ 10 mg·m-3

≤ 10 mg·m-3

Unless gas is pretreated, necessary to remove prior to membrane access via adsorbents

max. 5 % mol.

max. 5 % mol.

max. 5 % mol.

(max. 2.5 % mol.)

max. 3 % mol.

max. 3 % mol.

- Content N2

max. 1 % mol.

max. 1 % mol.

- Content O2

max. 2 % mol.

max. 2 % mol.

max. 32 mg·m-3

Unless gas is pretreated, necessary to dry prior to membrane access

Methane

Hydrogen sulphide

min. 95.0 % mol.

Content CO2 + N2 + O2 -

Content CO2

Content H2O

1) 2)

max. 32 mg·m-3

Temperature at which under operating pressure of 4 MPa water condenses form gaseous to liquid phase. Temperature at which under operating pressure hydrocarbons condense from gaseous to liquid phase.

Tests were performed at the Central Waste Water Treatment Plant in Prague where biogas is produce by the anaerobic fermentation of sewage sludge. For comparison purposes the following membrane modules materials were chosen: polysulphone fibre membranes with trademark name PRISM® from the company Air Products and polyimide fibre membranes model CO-C07FH from the Japanese company UBE. The membranes were tested using real biogas to obtain biomethane. Based on the acquired from tests both membrane materials proved to be suitable. With both membranes was possible to reach a methane content of over 95 vol.% in the retentate. The influence of pressure and biogas flow on the membrane modules carbon dioxide separation


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efficiency was tested. Furthermore humidity was determined in the biogas and the separated streams exiting each membrane module. Biomethane exiting the polyimide membrane module had lower moisture content. Hence polyimide membrane modules were subject to further testing.30 Methane, higher hydrocarbon, carbon dioxide, nitrogen and oxygen content in the real biogas samples before membrane testing was determined via a gas chromatograph GC HP 5890. Hydrogen sulphide and total sulphur content was determined via a GC Agilent HP 7890A coupled with a sulphur chemiluminescence detector Agilent SCD 355. Retentate and permeate gas composition from the membrane modules was determined using the same chromatographic methods. Online methane, carbon dioxide, oxygen and sulphur content in the retentate and permeate was determined via the portable analyser Sewerin Multitech 540. The analyser is equipped with an electrochemical sensor to determine hydrogen sulphide and oxygen content and with an infrared sensor to determine methane and carbon dioxide content. Gas relative humidity was determined using Testo 400 Probe. Table 4. Biogas composition Input content of the determined compounds*)

Unit

Methane

61 - 63

vol.%

Carbon dioxide

37 - 39

vol.%

0

vol.%

Hydrogen sulphide

70 - 100

mg·m-3

Relative humidity

40 - 50

%

Gas temperatures

20 - 24

°C

Requirements

Oxygen

*)

All values were determined at pressure 0.005 MPa and operating temperature.


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Carbon dioxide separation from real biogas was tested with two different membrane modules. The first, a membrane module from the company Air Products with a trademark name PRISM®, had polysulphone fibre membranes. The second, a membrane module from the Japanese company UBE, had polyimide membrane fibres.30 The separation properties of the modules were tested in the apparatus depicted in Figure 1. In Table 4 feed biogas composition was depicted. Firstly was tested the influence of pressure on the membrane’s carbon dioxide separation efficiency. The results were depicted in Table 5. In order to assess the optimal degree of biogas purification depending on the apparatus power a series of experimental tests applying different types of connections were performed. It was observed that at constant pressure increasing flow decreased membrane CO2 separation ability. It should be noted that based on biomethane quality requirements at least 95 % CH4 content should be reached. Taking into consideration practical applications it was possible to establish the best biogas to biomethane production ratios. The acquired data showed that it was possible to produce 6 m3·h-1 biomethane of the required quality from biogas. 3.2.

Equipment

In Figure 1 was depicted the experimental apparatus. Gas from the biogas pipe was situated after the gas heater was fed to the apparatus via an oil-free rotary screw compressor (1). Feed gas was compressed to the desired pressure ranging from 0.6 to 0.8 MPa. Then, the compressed biogas was fed via a pressure equalizing cylinder (4) to the membrane modules (5). The separation performance was tested for both series and parallel connected membrane modules. In Figure 1 parallel-connected membrane modules were depicted. Retentates from the membrane modules were combined into a single stream. The retentate stream was conducted to the diaphragm gas


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meter (6) via a needle valve. The diaphragm gas meter was determined the retentate volume while gas flow was set with the needle valve. Retentate flow was adjusted so that output gas methane concentration would not fall below 95 %. Permeate and retentate methane, carbon dioxide, oxygen and hydrogen sulphide content were monitored via Sewerin gas analyser (7). Simultaneously were also monitored moisture and temperature via the Testo probe (8). The produced biomethane was subsequently compressed to 25 MPa via a three-stage oil-free compressor. Permeate from the membrane modules was also combined into a single stream directed to the diaphragm gas meter (6) in order to determine gas volume. Methane, carbon dioxide, oxygen and hydrogen sulphide content was determined with a second Sewerin gas analyser (7). Permeate moisture and temperature were monitored via the Testo probe (8). Permeate gas flow containing 25 % methane was returned to the biogas pipe where after being mixed with raw biomethane was used as fuel in the co-generation units. The same experimental procedure was applied also for series-connected membrane modules.


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Figure 1. Biogas ugrading to biomethane apparatus scheme for parallel-connected membrane modules. Legend: 1 – oil-free compressor, 2 - manometers, 3 - needle valve, 4 - biogas storage and pressure equilizing cylinder, 5 - membrane modules, 6 - gas meter, 7 - gas analyser, 8 - moisture and temperature probe, 9 - oil free compressor, 10 - storage biomethane cylinder Measurement was performed at the temperature range 15 – 25 °C and pressure 0.6 – 0.8 MPa.

4.

RESULTS AND DISCUSSION

The aim of the work was designed equipment that would be available for biogas stations with heat power around 1 000 kW. The separation of the hydrogen sulphide by membrane modules was also determined. In order to evaluate the carbon dioxide removal efficiency by membrane separation, the concentrations of the components of methane, carbon dioxide, oxygen and sulphate in permeate and retentate were measured. The relative humidity and temperature in the biogas were monitored to influence the separation throughout the measurement period. In previous testing of UBE membrane modules, literature indicated that a moderate increase in methane concentration


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on the permeate side was observed at the temperature from 10 to 60 °C, but the concentration of methane unchanged on the retentate side.30 The experiments were carried out at the temperature range from 10 to 25 °C. Therefore it was not necessary to solve its minimal influence. The acquired data from the experimental tests was indicated that both membrane modules can be utilized for biogas upgrading to biomethane. In both cases methane content in the retentate was at least 95 vol.%. In Table 5 can be observed that between both membrane modules are no substantial differences. As a result, for all following experimental tests was used the membrane module from the company UBE. Initially the influence of pressure on carbon dioxide separation was established. Based on the experimental results pressure of 0.7 MPa was applied for the subsequent tests. At 0.7 MPa was possible to achieve methane content in the retentate above 95 vol.%.32 Table 5: A comparison concentration of methane between UBE and AIR Product modules at different pressures Pressure [MPa] Modules 0.3 0.5 0.7 AIR Product

93.29

94.37

94.75

UBE

95.00

94.89

95.50

During the next experimental step was examined whether three membrane modules exhibited the best gas flow to methane content in the retentate (above 95 vol.%) ratio when they were seriesconnected or parallel-connected. In Figure 2 and 5 the results obtained for each module connection and retentate flow were depicted. Retentate flow was gradually regulated until reaching the maximum possible value of 7 m3·h-1. For this work, it was important to determine


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the parameters for equipment that produces a high amount of biomethane with concentration of methane 95 vol.%. Similar results were reported in the work of Harasimowicz et. al22. They tested the same membrane material but used as inlet a model gas mixture. The focus of their work was the influence of gas flow on the final methane content in the retentate. Separation was performed the pressure under 0.58 MPa. In our work separation was performed in the pressure range 0.6 - 0.8 MPa using real biogas. Therefore it was crucial to determine the dependency of outlet methane content in the reatentate on retantate gas flow. Another difference between the two works is that real biogas contains also other minor compounds such as hydrogen sulphide and water. The presence of minor compounds can cause slight differences on the final results. The inlet content of each individual compound is depicted in Table 4. Moreover the influence of parallel and series membrane module connection was tested. The results are depicted in Fig. 2 and 4. In Fig 3 and 5 changes in hydrogen sulphide content with increasing flow of retentate gas can be observed. This effect to date has not been much tested in available works.22 In Figure 2 and 5, for better visibility of methane content, the y-axis value scale is from 90 to 100 vol.%. In Figure 2 can be observed that when using parallel-connected membrane modules methane content wasn´t decreased bellow 95 vol.% even at the highest flow rate.


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Methane content [vol.%]

100

95

90 0

1

2

3

4

Retentate flow

5

6

7

8

[m3·h-1]

Figure 2. Dependence of methane content [vol.%] on retentate flow for parallel-connected membrane modules. Another monitored parameter was hydrogen sulphide content in the retentate stream. Input biogas hydrogen sulphide content was in the range from 70 to 100 mg·m-3. In Figure 3 can be observed that when using polyimide membrane modules partial hydrogen sulphide separation took place. Hydrogen sulphide content in the retentate at the highest flow was around 25 mg·m-3.

60

Hydrogen sulphide content [mg·m-3]

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

input concentration

30

retentate concentration

20 10 0 0

1

2

3

4

5

6

7

8

Retentate flow [m3·h-1] Figure 3. Hydrogen sulphide content in the retentate for parallel-connected membrane modules.


19

Energy & Fuels

When flow was set at the highest value retentate and permeate were sampled in Tedlar gas sampling bags. Samples were analysed via GC coupled with TCD and FID detection. In Table 6 the results from GC-FID/TCD analyses are depicted. Table 6. The composition of each individual gas stream at retentate flow 7 m3·h-1 and parallel-connected membrane modules Retentate Permeate Requirements Input content*) composition**) composition CH4

61.82 vol.%

96.37 vol.%

25.16 vol.%

CO2

37.90 vol.%

2.21 vol.%

74.92 vol.%

N2

0.23 vol.%

0 vol.%

0 vol.%

O2

0 vol.%

0.34 vol.%

0.39 vol.%

H2S

100 mg·m-3

21.25 mg·m-3

72.86 mg·m-3

*)

All values were measured at pressure 0.005 MPa and operational temperature. All values were measured at pressure in the range 0.6 – 0.8 MPa and operational temperature.

**)

When three membrane modules were series-connected a decrease of methane content with increasing flow was observed. At retentate flow 6.5 m3·h-1 methane content decreased below 95 vol.%. In Figure 4 methane content changes depending on retentate flow are depicted. 100

Methane content [vol.%]

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

Page 20 of 28

95

90 0

1

2

3

4

5

6

7

Retentate flow [m3·h-1]

Figure 4. Dependence of methane content [vol.%] on retentate flow for series-connected membrane modules.


20

Page 21 of 28

Hydrogen sulphide content in the retentate was simultaneously monitored. Also for seriesconnected membrane modules was confirmed partial hydrogen sulphide separation. When applying series-connected membrane modules was reached an even greater hydrogen sulphide decrease compared to parallel-connected membrane modules. In Figure 5 changes on hydrogen sulphide content for different retentate flow are depicted.

40

Hydrogen sulphide content [mg·m-3]

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

35 30

input concentration

25 20 15

retentate concentration

10 5 0 0

1

2

3

4

5

6

7

8

Retentate flow [m3·h-1] Figure 5. Hydrogen sulphide content in the retentate for series-connected membrane modules. When flow was set at the highest value retentate and permeate were sampled in Tedlar gas sampling bags. In Table 7 the results from GC-FID/TCD analyses are depicted.


21

Energy & Fuels

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Table 7. The composition of each individual gas stream at retentate flow 6.5 m3·h-1 and series-connected membrane modules Retentate Permeate Requirements Feed composition*) **) composition**) compostion CH4

61.82 vol.%

96.37 vol.%

25.16 vol.%

CO2

37.90 vol.%

2.21 vol.%

74.92 vol.%

N2

0.23 vol.%

0 vol.%

0 vol.%

O2

0 vol.%

0.34 vol.%

0.39 vol.%

H2S

36.43 mg·m-3

9.11 mg·m-3

21.25 mg·m-3

*)

All values were measured at pressure 0.005 MPa and operational temperature. All values were measured at pressure in the range 0.6 – 0.8 MPa and operational temperature.

**)

The final experimental step consisted on compressing the purified biomethane to 25 MPa in order to determine whether the gas composition was in agreement with the norm ČSN 65 6514 requirements. After being compressed in a cylinder biomethane was sampled and analysed as described in the previous experiments. In Table 8 a comparison between biomethane composition and the norm requirements was depicted. Table 8. Compressed purified biomethane composition ready for utilization as vehicle fuel Required value - biomethane Requirements Real values - biomethane*) (ČSN 65 6514) Methane Hydrogen sulphide Content CO2 + N2 + O2 - Content CO2

*)

min. 95,0 % mol.

96,72 % mol.

≤ 10 mg·m-3

n. d. mg·m-3

max. 5 % mol.

3,27 % mol.

(max. 2,5 % mol.)

2,8 % mol.

- Content N2

0,4 % mol.

- Content O2

0,07 % mol.

All values were measured at pressure in the range 25 MPa and operational temperature


22

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Energy & Fuels

5.

CONCLUSIONS

The utilization of membrane separation for biogas upgrading to biomethane appears to be very interesting. Compared to other technologies that are currently used for biogas upgrading membrane separation possess the advantage of being relatively simple. Membrane separation does not require high operational demands. However, the biggest disadvantage is higher investment costs. Biogas production, in order to be accessible for agricultural biogas plants, should be first of all cheap and simple. The required energy to upgrade biogas to biomethane is a key parameter when choosing the most suitable technology. Biogas membrane separation is characterized by the production of very high purity biomethane. The produced biomethane when applying certain operation modes can have methane content up to 99 vol.%. In this work by regulating retentate flow was achieve methane content in the produced biomethane above 95 vol.%. Methane content above 95 vol% was achieved even at retentate flow as high as 7 m3·h-1. The produced biomethane met the requirements established by ČSN 65 6514. Additional produced biomethane can be compressed at 25 MPa. The compressed biomethane can be used as automobile fuel for vehicles running on CNG or can be injected into the natural gas grid. The residual permeate contained up to 25 vol.% CH4. Therefore it was possible to return it to the biogas pipe where was mixed back with raw biogas and used for electricity and heat production in the co-generation units. The experimental tests were performed using real biogas that contained 70 - 100 mg·m-3 H2S and 40 to 50 % relative humidity. In order to prolong membrane longevity it is recommended to place an adsorption unit for hydrogen sulphide and water vapour removal. Unfortunately


23

Energy & Fuels

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currently available research works focus primarily on carbon dioxide membrane separation from biogas. Therefore the results presented in this work are of great importance for the design of future biogas membrane separation units.

AUTHOR INFORMATION Corresponding Author E-mail: *[email protected]

ACKNOWLEDGMENT The results achieved in this paper were financially supported by TAČR project TA03020421 “Technological unit for limited local production of biomethane replacing fossil fuels in transportation and agriculture”. The authors of this paper would like to thank TAČR for the financial support as well as Central Waste Water Treatment Plant in Prague for creating the possibility to measure with real biogas.

REFERENCES (1)

Rasi, S.; Veijanen, A.; Rintala, J. Trace compounds of biogas from different biogas

production plants. Energy 2007, 32 (8), 1375–1380. (2)

Starr K., Villalba G., Gabarrell X. Upgraded biogas from municipal solid waste for

natural gas substitution and CO2 reduction – A case study of Austria, Italy, and Spain. Waste Management 2015, 38, 105-116.


24

Page 25 of 28

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

(3)

Balat, M., Balat H. Biogas as a renewable energy source—a review. Energy Sources

2009, 1280-1293. (4)

Molino, A., Nanna F. Iovane P. Low pressure biomethane production by anaerobic

digestion (AD) for the smart grid injection. Fuel 2015, 154, 319-325. (5)

Iovane, P., et al. Experimental test with polymeric membrane for the biogas purification

from CO2 and H2S. Fuel 2014, 135, 352-358. (6)

Augelletti, R. Pressure swing adsorption for biogas upgrading. A new process

configuration

for

the

separation

of

biomethane

and

carbon

dioxide. J.

Clean.

Prod. 2017, 140 (3), 1390–1398. (7)

Yajing X.; et al. Biogas upgrading technologies: Energetic analysis and environmental

impact assessment. Chin. J. Chem. Eng. 2015, 23 (1), 247–254. (8)

Grande C. A., Blom R. Utilization of dual-PSA technology for natural gas upgrading and

integrated CO2 capture. Energy Procedia 2012, 26, 2-14 (9)

Molino, A.; Migliori, M.; Ding, Y.; Bikson, B.; Giordano, G.; Braccio, G. Biogas

upgrading via membrane process: Modelling of pilot plant scale and the end uses for the grid injection. Fuel 2013, 107, 585–592. (10)

Molino, A. Iovane, P., Migliori, M. Biomethane production by biogas with polymeric

membrane module. Membrane Technologies for Biorefining. 2016, 465-482. (11)

Holloway R. W.; et al. Life-cycle assessment of two potable water reuse technologies:

MF/RO/UV–AOP treatment and hybrid osmotic membrane bioreactors. J. Membr. Sci. 2016, 507, 165-178. (12)

Alibardi L.; et al. Anaerobic dynamic membrane bioreactor for wastewater treatment at

ambient temperature. Chem. Eng. J. 2016, 284, 130-138.


25

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

(13)

Page 26 of 28

Scholz, M.; Melin, T.; Wessling, M. Transforming biogas into biomethane using

membrane technology. Renew. Sustain. Energy Rev. 2013, 17, 199–212. (14)

Baker, R. W. Membrane Technology and Applications, 3rd ed.; John Wiley & Sons, Ltd,

2012. (15)

Scholes, C. A., Stevens, G. W., Kentish, S. E. Membrane gas separation applica-tions in

natural gas processing. Fuel 2012, 96, 15–28. (16)

Ryckebosch, E.; Drouillon, M.; Vervaeren, H. Techniques for transformation of biogas to

biomethane. Biomass Bioenerg. 2011, 35 (5), 1633–1645. (17)

Wellinger, A.; Lindberg, A. Biogas Upgrading and Utilisation. IEA Bioenergy 2005.

(18)

Valenti, G.; Arcidiacono, A.; Ruiz, J. Assessment of membrane plants for biogas

upgrading to biomethane at zero methane emission. Biomass and Bioenerg. 2016, 85, 35-47. (19)

Côté, P.; Bersillon, J.; Huyard, A.; Faup, G.; Pierre, C. Bubble – free aeroration using

membranes: mass transfer analysis. J. Membr. Sci. 1989, 47 (1-2), 91–106. (20)

Ahmed, T.; Semmens, M. J. The use of independently sealed microporous hollow fiber

membranes for oxygenation of water: model development. J. Membr. Sci. 1992, 69 (1-2), 11–20. (21)

Baker, R. W.; Lokhandwala, K. Natural gas processing with membranes: an over-

view. Ind. Eng. Chem. Res.2008, 47 (7), 2109–2121. (22)

Harasimowicz, M.; et al. Application of polyimide membranes for biogas purification

and enrichment. Journal of Hazardous Materials 2007, 144 (3), 698–702. (23)

Poloncarzova, M.; Vejrazka, J.; Vesely, V.; Izak, P. Effective Purification of Biogas by

a Condensing Liquid Membrane. Angew. Chem. 2011, 123 (3), 695–697.


26

Page 27 of 28

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

(24)

Jing, G.; et al. Preparation and characterization of polyethersulfone mixed matrix

membranes embedded with Ti- or Zr-incorporated SBA – 15 materials. J. Ind. Eng. Chem. 2017, 45 (25), 257–265. (25)

Li – Jing, Z.; et al. Hydrophilic and anti-fouling polyethersulfone ultrafiltration

membranes

with

poly(2-hydroxyethyl

methacrylate)

grafted

silica

nanoparticles

as

additive. J. Membr. Sci. 2014, 451, 157–168. (26)

Haider, S.; Lindbråthen, A.; Hägg, M. Techno – economical evaluation of membrane

based biogas upgrading system; a comparison between polymeric membrane and carbon membrane technology. Green Energy & Environment 2016. (27)

Kavousi, F.; et al. Hydrodynamics and gas transfer performance of confined hollow

fibre membrane modules with aid of computational fluid dynamics. J. Membr. Sci. 2016, 513 (1), 117–128. (28)

Basu, S.; Khan, A.; Cano-Odena, A.; Liu, C.; Vankelecom, I. Membrane-based

technologies for biogas separations. Chem. Soc. Rev. 2009, 39 (2), 750–768. (29)

Palatý, Z. Membránové procesy, 1st ed.; VŠCHT Praha: Praha, 2012.

(30)

UBE Group, separation membrane UBE, 2012. CO2 separation membrane.

http://www.ube.co.th/en/product-details.php?id=6 (accessed March 05, 2017). (31)

ČSN 65 6514. Automotive fuels - Biogas for positive (spark) ignition engines

requirements and test methods. Hradec Králové: Český normalizační institut, 2007. 10 p. (32)

Vrbová, V.; Ciahotný, K.; Hádková, K. Separation of CO2 from biogas by membrane

modules. PALIVA 2014, 6 (3), 78–82.


27

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LIST OF PICTURES Figure 1. Biogas ugrading to biomethane apparatus scheme for parallel-connected membrane modules. ........................................................................................................................................ 16 Figure 2. Dependence of methane content [vol.%] on retentate flow for parallel-connected membrane modules. ...................................................................................................................... 19 Figure 3. Hydrogen sulphide content in the retentate for parallel-connected membrane modules. ....................................................................................................................................................... 19 Figure 4. Dependency of methane content [vol.%] on retentate flow for series-connected membrane modules. ...................................................................................................................... 20 Figure 5. Hydrogen sulphide content in the retentate for series-connected membrane modules. 21

LIST OF TABLES Table 1. A comparison of technologies for biogas upgrading ........................................................ 6 Table 2: Permeability and selectivity of polymeric membranes for gas separation 26.................... 9 Table 3. Biomethane quality requirements for vehicle fuel utilization according to standard ČSN 65 6514 .................................................................................................................. 12 Table 4. Biogas composition......................................................................................................... 13 Table 5: A comparison concentration of methane between UBE and AIR Product modules at different pressures ......................................................................................................................... 17 Table 6. The composition of each individual gas stream at retentate flow 7 m3·h-1 and parallelconnected membrane modules ...................................................................................................... 20 Table 7. The composition of each individual gas stream at retentate flow 6.5 m3·h-1 and seriesconnected membrane modules ...................................................................................................... 22 Table 8. Compressed purified biomethane composition ready for utilization as vehicle fuel ...... 22


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Upgrading Biogas to Biomethane Using Membrane Separation

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