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Separation and Purification Technology 74 (2010) 83–92

Contents lists available at ScienceDirect

Separation and Purification Technology

journa l homepage: www.e lsev ier .com/ locate /seppur

embrane biogas upgrading processes for the production of natural gasubstitute

. Makaruk, M. Miltner, M. Harasek ∗

ienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166, 1060 Vienna, Austria

r t i c l e i n f o

rticle history:eceived 12 March 2010eceived in revised form 12 May 2010ccepted 13 May 2010

eywords:embrane gas permeationatural gas substitute

a b s t r a c t

The biogas processing and production of natural gas substitute have gained importance in recent years.It is often considered to be superior to the production of electricity with internal combustion enginesmainly because of the better energy utilisation. The processed biogas in the form of natural gas substitutecan be supplied to the already developed natural gas grids and delivered to households and industry.Alternatively, it can be used as a fuel for CNG-vehicles.

The present work reviews the literature information that is available on the biogas upgrading pro-cesses, the biogas compositions and the permeation of biogas components through typical polymeric

iomethaneiogas upgradingystem integration

membranes. Subsequently, the membrane configurations for biogas upgrading are discussed and thor-oughly simulated using numerical modelling. The work proposes basic concepts for the integration ofmembrane biogas upgrading plants into biogas plants while taking into account the permeate utilisationand the heating requirements of biogas plants.

The membrane gas permeation systems provide enough flexibility for heat integration within biogasrgy re

plants. The expected ene

equal to around 0.3 kWh.

. Introduction

Biogas production and utilisation is an emerging alternativenergy technology. Biogas is obtained by a complex degradationrocess of organic matter that is performed by a set of bacteriander anaerobic conditions [1]. The resulting gas consists primar-

ly of methane and carbon dioxide together with smaller amountsf other gases and vapours. As shown by considerable number ofiogas plants around the world, if done properly, the biogas pro-uction and utilisation can result in additional available energyith reduced emission of carbon dioxide to the atmosphere and

he production of valuable fertiliser.According to some studies, biogas produced from anaerobic

igestion is one of the most favourable bioenergy forms mainlyecause of the high net energy yields per acreage [2] and the sub-trate flexibility [3]. Indeed, biogas is produced from a variety of

rganic materials in plants ranging from sewage treatment plants,o organic waste utilization in landfill sites, to farm biogas produc-ion that utilize pure energy crops.

∗ Corresponding author. Tel.: +43 1 58801 15925; fax: +43 1 58801 15999.E-mail addresses: aleksander.makaruk@tuwien.ac.at (A. Makaruk),

artin.miltner@tuwien.ac.at (M. Miltner), michael.harasek@tuwien.ac.atM. Harasek).

URL: http://www.thvt.at (M. Harasek).

383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2010.05.010

quirement for a single produced cubic meter of natural gas substitute is

© 2010 Elsevier B.V. All rights reserved.

For example in Europe, the yearly total biogas production in2006 was equal to around fourteen billion cubic meters and isexpected to grow at the rate of 10–20% yearly (estimated from [4]).

Several biogas utilisation methods can be applied. Presently themost popular approach is the production of Combined Heat andPower (CHP) by means of internal combustion engines. This is ajustified choice if heat consumers are in the vicinity of the plant.However, this requirement cannot be met in most of the cases. Asa result, the excess of the heat energy must be vented to the atmo-sphere, which lowers the overall energy production efficiency andrequires additional electrical power.

The biogas upgrading to the quality of natural gas avoids thisshortcoming. The upgraded biogas in the form of natural gassubstitute1 can be injected into existent natural gas grids or used asa vehicle fuel using natural gas infrastructure. The upgraded biogascan be distributed with low cost to consumers, who can utilise itwith a higher efficiency.

1.1. Advantages of membrane separation in biogas upgrading

As reported by Beil et al. [5], in 2008 there were about 80 biogasupgrading plants in operation in Europe. They delivered the power

1 A number of terms are used to describe the upgraded biogas: renewable naturalgas, biomethane, bio-substitute of natural gas.

8 urification Technology 74 (2010) 83–92

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f 200 MW (heating value related) which corresponded roughly to70 million standard cubic meters of natural gas substitute. Theotal capacity of the biogas processing plants seems still to be rela-ively small in comparison to CHP plants. This is supposed to changen the coming future because the upgrading allows a better energytilisation than CHP.

The market of the biogas upgrading technologies is rather inhe stage of development. The majority of the biogas upgrad-ng technologies derive from other gas separation technologies,

ainly from the natural gas treatment. The important marketlayers include systems that are based on pressurised water scrub-ing, pressure swing adsorption, amine scrubbing, scrubbing withrganic solvents and membrane permeation.

There is a series of advantages that can favour membrane gasermeation systems in the field of biogas upgrading. The most

mportant include safety and simplicity of operation, easy main-enance and operation without hazardous chemicals [6–8]. Gasermeation plants can be operated unattended. These factors are

mportant in biogas upgrading since biogas production plants usu-lly are not operated by a specially trained personnel.

Furthermore, the membrane gas separation is a process thatcales down very well. As reported by Babcock et al. [9] and moreecently by Baker and Lokhandwala [7], from the economic pointf view, the membrane gas separation is advantageous if the gasolume flow is relatively low and inlet carbon dioxide content iselatively high. These parameters fit very well to a typical biogaspgrading task; the average biogas volume flow of biogas plants inurope equals 500m3

stp/h [5], and the carbon dioxide content usu-lly exceeds 40% (v/v). If the upgraded biogas is to be supplied tohe gas grid under pressure, the gas permeation technology shouldain advantage over upgrading technologies that operate at atmo-pheric pressure since it uses the compression for both upgradingnd grid injection.

As far as natural gas upgrading is concerned, it is difficult forembrane permeation systems to compete with the amine scrub-

ing technology in terms of the methane recovery. Higher methaneosses generated by membrane systems may contribute to higherosts of the gas processing [6,10]. However, the inherent methaneoss of membrane systems based on polymeric membranes is not aisadvantage in the field of the biogas upgrading because anaero-ic digestion typically requires certain amounts of heat [11,12] thatan be covered through the combustion of the permeate stream.

.2. Biogas upgrading with membranes in literature

Biogas upgrading deals primarily with the separation of car-on dioxide and methane. Actually, the same task is performedhen upgrading natural gas. Therefore, research results published

n natural gas upgrading with membranes can be applied to bio-as upgrading. However a few studies focused specifically on thespects connected with biogas upgrading.

The potential of using membranes for biogas upgrading wasoticed more than thirty years ago. Schell and Houston [13]eported on a process for biogas treatment that used commercialellulose acetate spiral-wound membranes. The permeation testsave been performed for 18 months. The plant was able to processpproximately 60 m3

stp/h of raw biogas at pressures of 17–30 bar.hree different process configurations were suggested for the pro-uction of natural gas substitute with the methane content of 96.5%v/v). Although the authors reported that no significant differencesere seen between the permeation of landfill gas with and with-

ut pretreatment, they suggested that the gas pretreatment couldrolong the service lifetime of membranes.

Roehr and Wimmerstedt [14] tested two commercial membraneodules for the permeation of biogas from a sludge digester. The

nvestigated modules contained two different membrane mate-

Fig. 1. Interior of a membrane biogas upgrading plant.

rials: cellulose acetate and silicon coated polysulfone. Only adehumidification step was used for the gas pretreatment. The pilotplant was operated with the gas volume flow of 15 m3

stp/h and atpressures of 10–35 bar.

Rautenbach and Welsch [15] reported on a two-year-long oper-ation of a landfill gas upgrading plant with the feed volume flowof 200 m3

stp/h. In that work the importance of gas pretreatmentwas underlined and a two-stage membrane configuration was sug-gested for higher product recoveries.

Stern et al. [16] performed short term and long term (1000 h)tests on biogas sourced from a wastewater treatment plant. Thepilot plant was able to process approximately 3.4 m3

stp/h of rawbiogas and operated at pressures of up to 55 bar. During the tests,it has been discovered that some of the secondary components ofbiogas could damage the hollow fibers, which led to a significantdeterioration of separation performance.

More recently Harasek et al. [17] and Makaruk et al. [18] havereported on a membrane gas permeation plant for biogas upgrad-ing and grid supply of 100 m3

stp/h of natural gas substitute. Miltneret al. [19] have reported on an island concept for the local produc-tion of 35 m3

stp/h of CNG-fuel from biogas. The interior of the biogas

upgrading plant that has the production capacity of 100 m3stp/h is

presented in Fig. 1; the machinery part together with the gas injec-tion, the control room and the gas analysis are installed in a 9 mlong container.

1.3. Objectives of the work

Although the gas permeation technology has been presented asa viable technique for biogas upgrading, there were no thoroughinvestigations on the process integration with biogas plants. Thiswork first reviews the information that is available concerning thebiogas compositions and the permeation of the biogas componentsthrough polymeric membranes. Subsequently, possible membraneconfigurations for biogas upgrading are discussed.

The membrane configurations are evaluated in detail usingnumerical modelling for several membrane selectivities. Specifi-cally, the evaluation concerns most important upgrading param-eters like the energy consumption and the total membrane arearequirement. The work suggests also basic concepts for the inte-

gration of membrane biogas upgrading plants into biogas plantsfocusing primarily on the permeate utilisation and on the heatingrequirement of biogas plants.

urification Technology 74 (2010) 83–92 85

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Table 1Exemplary limits on the secondary components in natural gas (an extract from theAustrian standard ÖVGW G31/G33).

Gas component Limit Unit

Oxygen ≤0.5 % m/m

A. Makaruk et al. / Separation and P

. Membrane permeation of biogas

In the first instance the biogas processing to natural gas qual-ty deals with the separation of methane and carbon dioxide. Largemount of scientific information is available that prove that glassyolymers, mainly polyimides and cellulose acetate, can provideigh selectivities and are a good choice for this kind of separa-ion [20–22]. However, besides carbon dioxide and methane a greatariety of other components can be found in biogases. This makeshe biogas permeation a complex problem; the biogas componentsan have adverse effects on the separation of carbon dioxide andn the membrane stability.

Typically biogas is described as a gas mixture contain-ng methane (50–70%, v/v), hydrogen sulphide (up to around000 ppm), sometimes ammonia (in ppm range) with carbon diox-

de being the balance. Biogas is usually saturated with water vapournd sometimes may contain air components if air is used for the bio-ogical desulphurisation or in case of an uncontrolled air intrusion.he permeation properties of the aforementioned gases are wellnown for typical glassy polymers [23,20]. The order of the per-eabilities of the basic biogas components in the polyimides is as

ollows: water, carbon dioxide, hydrogen sulphide, oxygen, nitro-en, methane with the longer alkanes being the least permeating.he ideal carbon dioxide/methane selectivities of polyimides cane as high as 60 [20].

Biogas can contain also a large number of Trace Organic Com-onents (TOC) that in total usually do not exceed 1% (v/v). Theseomponents can be divided into several groups: alkanes andycloalkanes, halogenated hydrocarbons, ketones, aromatic com-ounds, terpenes, thiols (mercaptans), siloxanes, alkyl sulphides,olatile fatty acids and alcohols [1,24–27]. The concentrations ofOC depend on the substrate used for the anaerobic digestion. Theargest variety of these compounds can be met mainly in biogasesrginating from landfill sites. However, farm biogas plants that userimarily biomass as substrate for the digestion may also produceiogas that contains some amounts of undesired components likeiloxanes, alkyl sulphides and traces of heavier hydrocarbons [26].he consideration of TOC existence should not be neglected in theembrane permeation of biogas, since they can interact with mem-

rane material lowering its stability or gas permeability. If presentn the produced gas, they can be corrosive or may have adverseffects on the human health.

Only a certain part of TOC present in biogases has been investi-ated in membrane gas permeation. The research focused mainlyn the permeation of aromatic components (benzene, toluene,ylene), ketones (mainly acetone) and halogenated hydrocarbonshrough rubbery polymers, mainly polydimethylsiloxane [28–34],or the emission control. Some information is also available onhe permeation of these components in glassy polymers [35–37].rganic vapor/air selectivities in the range of 20 and 500 were

eported for a polyimide material which was investigated in [35].his suggests that during biogas processing with membranes theoncentration of TOC can be lowered in the produced gas.

However, the TOC itself can affect the membrane material.ind et al. [22] performed permeation tests on polyimide mem-

ranes with gas containing methane, carbon dioxide, C2–C4 alkanesnd toluene. A significant decrease of carbon dioxide permeabilitynd slight decrease in the carbon dioxide/methane selectivity wasbserved when 300 ppm of toluene were present in the permeat-ng gas. In a further study Al-Juaied and Koros [38] suggested thathe separation of natural gas can be adversely affected by four pos-

ible phenomena: competition effects between permeating gases,lasticisation, swelling and compaction of the membrane material.series of experiments were run in order to understand the com-

lex mechanisms that can occur during the permeation of carbonioxide, methane, heptane and toluene.

Carbon dioxide ≤2.0 % m/mNitrogen ≤5.0 % m/mHydrogen sulphide ≤5 mg/m3

Total silicon ≤10 mg/m3

It can be expected that biogas that contains substances withhigh boiling temperatures together with a number of other variousgas components may lower the separation performance of a mem-brane material. The large number of the components that appearin biogases has not been tested yet in membrane gas permeationexperiments. Further scientific investigations are required in orderto examine the behaviour of the biogas components on the stabilityof membrane polymers. In practice, a gas pretreatment is requiredbefore the gas is contacted with the membrane in order to prolongthe service life-time of membranes.

2.1. Quality requirements

Typically, natural gas qualities are defined through naturalgas standards. There are certain differences in the gas qualitiesbetween regions. In some cases the natural gas quality is regu-lated between the vendor and customer only. Probably the mostimportant factor that describes a natural gas quality is its calorificvalue that can be expressed in terms of Lower Heating Value(LHV), Higher Heating Value (HHV) or Wobbe index. As informedby the European standard ISO 13686 the local standards usu-ally put the upper and lower limits on the calorific values andWobbe indices. Additionally the secondary gas components arelimited. Typically limited are: oxygen, nitrogen, carbon dioxide,hydrogen sulphide, ammonia, total sulphur, water, condensabil-ity of alkanes and carbon monoxide. However, the great partof the components that can be present in biogas, mainly theTOC, is typically not mentioned in natural gas standards. Exem-plary limits from a national natural gas standard are presented inTable 1.

The main component of biogas is methane, other combustiblehydrocarbons of biogas are rather in the ppm-range and do notcontribute much to the calorific value of the gas. The upper heatingvalue of pure methane for the reference temperature of 0 ◦C andatmospheric pressure equals to 11.07 kWh/m3

stp (according to ISO6776). Therefore only natural gases with the heating values thatare below this value can be substituted for the upgraded biogas.A considerable number of natural gases contain however longerhydrocarbons and as a result higher energy contents. For this typeof natural gases the substitution is possible if longer hydrocarbonsare dosed to the upgraded biogas from a tank. This must be done ifthe agreement with the standard is desired although it considerablylowers the economics and the sustainability of the process.

2.2. Membrane configurations for biogas upgrading

A large amount of studies has been published on the topic ofmembrane configurations for the separation of carbon dioxide andmethane [10,13,15,39–46]. For example [40,41] searched for opti-mal membrane configurations for the upgrading of natural gaswith 5–40 mol% of carbon dioxide. The processes could be opti-

mised through the variation of the operating pressure and theflows in recycles to find the optimal balance between the operatingcosts and the investment costs. It was suggested that a three-stage cascade with one additional compressor would be the mostadvantageous for the separation of carbon dioxide and methane

86 A. Makaruk et al. / Separation and Purification Technology 74 (2010) 83–92

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layer, ˘ is the permeance, pF is the absolute pressure in the feed

Fig. 2. Two-stage low-pressure-feed cascade.

n natural gas processing. Similar conclusions were drawn also byther authors [44].

However, this design approach may be not applicable to biogaspgrading. Although biogas upgrading deals with the separationf carbon dioxide and methane, similarly as natural gas upgradingoes, the framework for the design of membrane permeator sys-ems for biogas upgrading is rather different. As regards natural gaspgrading, the cost of the gas processing can be strongly influencedy the amounts of methane gas lost in permeate. Therefore, in theesign of natural gas upgrading processes, the product recoveriesre maximised, which leads to a higher number of compressors andtages.

The design of biogas systems can be based upon the followinguidelines. Firstly, biogas is delivered from digesters at a nearlytmospheric pressure. Therefore, at least one compressor is neces-ary for the operation. Secondly, at biogas plants there exists thepplication for the permeate usage. Hence, the upgrading plant con-guration does not have to be optimised for the maximisation ofhe product recovery. It should be rather adjusted for the optimalermeate use and heat integration with a biogas plant. Althoughhe configurations with multiple compressors can provide bet-er product recoveries, they are more complex and require highernvestment and operational costs [13]. These costs can be avoided ifne-compressor configurations are used. In this work it is assumedhat the one-compressor low-pressure-feed configurations shouldrovide enough flexibility to meet the product recoveries adjustedo the heating requirement of biogas plants. The two-compressor or

ulti-compressor configurations are not considered in this work.From the comprehensive collection of membrane cascades for

he gas separation presented in [42] one can distinguish follow-ng single compressor low-pressure-feed configurations: the singletage, the two-stage configuration with recycle presented in Fig. 2,

he two-stage configuration with sweep as presented in Fig. 3 andhe three-stage configuration with sweep presented in Fig. 4.

Fig. 3. Two-stage low-pressure-feed cascade with sweep.

Fig. 4. Three-stage low-pressure-feed cascade with sweep.

3. Modelling

Within this work membrane biogas upgrading processes areinvestigated for the raw biogas flow of 1000 m3

stp/h and for themethane content of 60% (v/v). The desired methane content inthe produced gas is 98% (v/v). These process boundary conditionsare typical for a biogas upgrading task. Moreover it is assumedthat the plant operating pressure equals 10 bar. The plant is givena two-stage compressor. The electric power that is required forbiogas upgrading is calculated from the isentropic compressionwork taking into account the compression efficiency and mechan-ical efficiency as suggested in [47]. The data that was requiredfor the modelling of the permeation is given in Table 2. Althoughthe boundary conditions of other biogas upgrading tasks can dif-fer from those presented here, the qualitative interpretation andthe main conclusions should remain the same. For the variation ofthe membrane ideal selectivity the carbon dioxide permeance iskept constant. This corresponds to the movement parallel to theordinate axis (selectivity) in the Robeson’s diagram [48].

3.1. Numerical algorithm and model

The modelling results of this paper were worked out usingthe recently developed numerical procedure for the calculation ofmembrane gas permeation systems. The employed algorithm sim-plifies hollow-fibre membrane modules to the one-dimensionalspace. The length-specific transmembrane gas flow is calculatedusing the solution–diffusion equation

Q ′ = ˘(xpF − ypP)s�d, (1)

where s is the total number of fibres, d is the diameter of the active

channel, pP is the absolute pressure in the permeate channel and xand y are the volume fractions in the feed channel and in the per-meate channel respectively. Subsequently, the gas conservation foreach of the permeating gas components is formulated for the feed

Table 2The boundary conditions and data for the modelling of biogas upgrading systems.

Parameter Value Unit

Biogas methane content 60 % v/vProduced gas methane content 98 % v/vBiogas flow 1000 m3

stp/h

Compressor inlet absolute pressure 1 barCompressor interstage absolute pressure 5 barPlant operating absolute pressure 10 barCompression efficiency 80 %Compressor mechanical efficiency 90 %CHP electrical efficiency 35 %Thermal efficiency (CHP and burner) 80 %Carbon dioxide permeance 100 GPU

urification Technology 74 (2010) 83–92 87

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ide and for the permeate side of the membrane. This results in theormation of a system of differential equations. After the discreti-ation with the first order upwind finite difference method, the gasonservation in a discrete point can be represented by

n+(1/2)j

= Fnj−1 − �l˘n

j (xnj pn

F,j − ynj pn

P,j)s�d, (2)

here F is the flow of a single gas component in the feed channel,l is the distance between discrete points, j is the discrete point

ndex and n is the iteration index. Similar equation is formed for theermeate side of the membrane. The full iteration step is expressedy the following extrapolation

n+1j

= Fnj + �(Fn+(1/2)

j− Fn

j ), (3)

here � is the relaxation factor. The algorithm iterates throughhe discrete points until the convergence criterion is met:

Fn+(1/2)j

− Fnj

∣∣∣ ≤ ı, (4)

here ı is an arbitrary number that is several orders of magni-ude smaller than the gas flow. The algorithm and the modellingpproach were validated against permeation experiments withgas mixture consisting of carbon dioxide, methane and oxy-

en. Good agreement between the experimental results and theodelling was demonstrated during the validation. More detailed

nformation on the algorithm and the modelling approach can beound elsewhere [49].

In this work, it is assumed that the transmembrane flow of theas components is governed by the solution–diffusion mechanism.o flux-coupling between gas components and no effects on theembrane itself are considered. The pressure losses in the feed

hannel and in the permeate channel are neglected. The consideredembrane systems are equipped with hollow-fiber modules in the

ounter-current configuration.In the modelling cases of this work, the total membrane area and

he area ratio are varied to provide required values of the gas purity98%, v/v) and the methane recovery, which is a variable parametern the analysis of the results. The other process boundary conditionsre kept constant as provided in Table 2. The adjustment of the totalembrane area and the area ratio to the required results is per-

ormed by the Levenberg–Marquardt algorithm [50,51], which is aumerical iterative method for the optimisation of nonlinear multi-ariable problems. The Levenberg–Marquardt implementation wasaken from the MATLAB®-environment. The flow diagram repre-enting the functioning of the membrane gas permeation solvernd the Levenberg–Marquardt algorithm is given in Fig. 5.

Both numerical procedures form an optimisation tool thatllows the multivariable adjustment of the gas permeation param-ters (pressure, area, area ratio etc.) to the required results (gasontent in product gas, recovery, compressor power etc.) withouttrial-and-error search for these parameters.

. Results

The two-stage cascades with and without sweep (Figs. 2 and 3)s well as the three-stage cascade with sweep (Fig. 4) were mod-lled using the aforementioned numerical tools. The first resultshowed that the two-stage cascade without sweep was able torovide high methane purities in the produced gas for the studiediogas upgrading process. On the other hand the two-stage cas-ade with sweep was not able to reach the methane content of 98%

v/v) for typical operating pressures and membrane selectivities.he sweeping of the second permeate with raw gas destroys theffect of the counter-current flow that allows reaching high puri-ies. Similar conclusion for this configuration was also drawn in42].

Fig. 5. Solution procedure.

When performing the optimisation of the area ratios for thethree-stage cascade with sweep (Fig. 4), for the minimal total mem-brane area and for the minimal compression power, the algorithmstended invariably to reduce the area of the middle stage C to zero.This suggests that the investigated three-stage configuration can-not supersede the two-stage configuration with recycle in termsof the energy and the membrane area requirement. This was alsoconfirmed in [42].

Thus the only cascade to be considered further in this sectionis the two-stage cascade with recycle presented in Fig. 2 and thesingle stage. These two configurations provide enough flexibilityfor the integration into biogas plants. The two-stage cascade with

recycle allows broad range of product recoveries; changing arearatios, i.e. the area of stage B in comparison to stage A, the methanerecovery is expected to increase at the expense of the compressionenergy consumption.

88 A. Makaruk et al. / Separation and Purification Technology 74 (2010) 83–92

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methane content in the permeate can vary from 40% (v/v) down tovalues of below 1% (v/v) depending on the membrane selectivityand area ratio.

Fig. 10 shows an exemplary behaviour of the concentration ofa secondary component in the produced natural gas substitute for

ig. 6. Specific compression power versus methane recovery. Membrane selectivi-ies are given in ©.

In the evaluation of the results the term membrane area ratio issed which is defined as the ratio of the area of stage B over the areaf stage A. The compression power over the produced gas standardolume flow is named shortly specific compression power.

.1. Variation of the area ratio

In Fig. 6 the dependence of the specific compression power onhe methane recovery can be observed for several membrane selec-ivities. It can be seen that the growth of the specific power is slowor the increasing recovery if the value of recovery is moderate.owever, for every selectivity there exists a recovery value beyondhich the specific compression power increases rapidly to infinity.

rom the point of view of the energy consumption, the methaneecovery of 98% seems to be plausible for the selectivites of 35 or0. As it can be expected, lower selectivities require more com-ression power to reach the same methane recovery. For exampleor the selectivity of 10 the methane recovery of 98% would not beffordable any more. Observing the trend lines for the selectivitiesf 35 and 50 in Fig. 6 only little difference in the energy consump-ion is noted between these selectivity values. This indicates thatn the range of relatively high selectivities, the selectivity increaseeads to a small decrease in the energy consumption.

The dependence of the total membrane area requirement onhe methane recovery is visualised in Fig. 7. By increasing the mem-rane area ratio, higher recoveries can be achieved. In the same timehe total membrane area needs to be increased in order to guaran-ee the gas purity. Similarly as for the specific compression power,he increase of the total membrane area requirement is moderateor lower methane recoveries. However in order to reach very highecoveries that are close to 100% considerably more membrane areas necessary.

Fig. 8 visualises how the variation of the area ratio of the cascadenfluences the methane recovery and specific compression power.t can be noted that the increase of the area of the stage B results inhe increase of the methane recovery. However, this is not associ-ted with the growth of the specific compression power for lower

rea ratios. For the increasing area ratio the plant produces moreetentate flow but the compression energy requirement for a singleubic meter of produced natural gas substitute remains the same.his holds till the area ratio of approximately 1. Beyond this point,

Fig. 7. Total membrane area versus methane recovery. Membrane selectivities aregiven in ©.

the specific compression power requirement grows parabolicallyand the increase of the methane recovery flattens out.

Methane is a greenhouse gas that is much more potent thancarbon dioxide. Since biogas plants are supposed to representan environmentally friendly technology that prevents the globalwarming, the venting of the permeate, which contains significantamount of methane, should be avoided. Methane in the permeatecan be utilised through the combustion for the production of heat.Several combustion technologies like typical gas burners, porousburners, catalytic burners or even CHP-plants are available for thisassignment and the choice will depend on the methane content inthe permeate gas. Fig. 9 gives a dependence of the methane contentin first permeate on the membrane area ratio. As it can be noted the

Fig. 8. Methane recovery and specific compression power for different area ratios.Membrane selectivities are given in ©.

A. Makaruk et al. / Separation and Purification Technology 74 (2010) 83–92 89

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ig. 9. Methane content in the first-stage permeate versus area ratio. Membraneelectivities are given in ©.

ifferent area ratios. The selectivity of the secondary component vsethane equals 10 which could correspond to the permeation of

uch biogas components like hydrogen sulphide or oxygen throughpolyimide membrane. The raw biogas concentration of this com-onent equals 1000 ppmv. It can be seen that generally the systemill tend to lower the content of the secondary component in theroduced gas except for high area ratios for which the producedas will be enriched with this component.

.2. Single stage with CHP

The combination of a biogas upgrading plant with a CHP plant

s presented in Fig. 11 could provide energy in the form of natu-al gas substitute, electricity and heat. The function of CHP woulde also the combustion of the permeate gas containing methane

n order to prevent methane emissions. The ability of internal

ig. 10. Content of the secondary gas component in produced natural gas substitute.he carbon dioxide/methane selectivity equals 35 and the secondary compo-ent/methane selectivity equals 10. The secondary component concentration in rawiogas equals 1000 ppmv.

Fig. 11. Simplified flowsheet for the single stage with a CHP unit for the productionof natural gas substitute, heat and electricity

combustion engines to burn the methane–carbon dioxide mix-tures depends largely on the methane content. According to Karimand Wierzba [52] the lower methane limit for the combustion ofmethane–carbon dioxide mixtures at the optimal conditions, atroom temperature and at atmospheric pressure equals approxi-mately 21% (v/v). This limit is however influenced by a number offactors like the premixing level, the Reynolds number, the turbu-lence intensity and the stochiometry. In real internal combustionengines the optimal combustion conditions cannot be achieved. Asa result engines require more than 30% (v/v) of methane in thecarbon dioxide–methane fuel mixture for the operation [52].

In the optimal case the amount of waste heat from the CHP-plant should be adjusted to cover the heating requirements of thebiogas plant. Only little information is available about the heatingrequirements of biogas plants. As suggested in [11], the heatingrequirement of the anaerobic digestion can be equal in average toaround 10% of the total produced biogas heating value. This num-ber can be influenced by several factors like the water contentin the substrate and the ambient temperature. According to theauthors’ simple investigation on an existing biogas plant, the heat-ing requirement of the biogas plant related to the produced biogasheating value can range from 5% during warmer days to 30% forlower ambient temperatures and if an intensive pasteurisation isperformed. From a study presented in [12] it can be estimated thatthe heating requirement for the sludge digestion in a waste water

treatment plant can equal from a few % up to 30% depending on theplant dimensions and the retention time.

Fig. 12 presents the results of the modelling for the variablebiogas consumptions. As can be seen, if all produced biogas is con-

Fig. 12. Share of the biogas heating energy related to the raw biogas LHV availablefor the heating purposes and the CHP inlet methane content for the configurationwith the single stage and CHP. Membrane selectivities are given in ©.

90 A. Makaruk et al. / Separation and Purification Technology 74 (2010) 83–92

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system to the single stage system.For the presented two cases of the heating requirements of

5% and 15% the corresponding methane content in the permeateequals 8.8% (v/v) and 22.4% (v/v) respectively. These values will

ig. 13. Simplified flowsheet for the two-stage configuration with a permeateurner for the production of natural gas substitute and heat

umed by upgrading (100% on the abscissa), the methane content inhe permeate is too low for a stable operation of the internal com-ustion engine except for the low selectivities of around 10. Also

ittle energy is available for the heating purposes at the biogas plant.his can be avoided by mixing a part of the raw biogas with the per-eate stream for the CHP fuel. For instance, for membranes with

he selectivity of 35 and for the consumption of 70% of raw biogas inhe upgrading, the permeate–raw biogas mixture contains around0% (v/v) of methane, which is enough for the stable operation ofCHP plant [52]. Furthermore the waste heat from the internal

ombustion engine is equal to around 17% of the produced biogasotal heating energy, which suffices to cover the heating needs oftypical biogas plant. For the plant discussed in this work with

he heating power of the produced biogas of 6 MW (1000 m3stp/h

nd 60% of methane content), the plant in this configuration deliv-rs around 3.4 MW power in the form of natural gas substitute and.0 MW in the form of electricity. Additional 1.0 MW is available forhe heating purposes of the biogas plant.

.3. Two-stage with gas burner

In certain cases the application of CHP is not desired becausef high operational and investment costs, low electricity feed-inariffs or if there is no connection to an electrical grid. A biogaslant with the biogas upgrading for the delivery of energy in theorm of natural gas substitute still can be designed. A simplifiedrocess flow sheet for this task is presented in Fig. 13. In ordero achieve higher methane recoveries, the two-stage configurationith recycle is proposed.

The heating value of the first stage permeate flow can be influ-nced through variation of the area ratio. As a result the membraneonfiguration can be precisely adjusted to the heating needs of aiogas production plant. In this section, the membrane area rationd the total membrane area are adjusted to satisfy the bound-ry conditions specified in Table 2 and to the biogas plant heatingequirements of 5% and 15% (related to the heating value of theotal produced biogas). For the burner thermal efficiency of 80%hese values correspond to the methane recoveries of 93.8% and1.2%, respectively.

Fig. 14 gives the dependence of the membrane area ratio onhe membrane selectivity to meet the biogas plant heating require-

ents. As expected, for higher selectivities, less area in the secondtage is required and less methane recycling. For higher membraneelectivities and when relatively more heat is consumed by the bio-as plant the area ratio tends to zero which suggests that in thisituation the single stage should be sufficient. If membranes with

ery high selectivities are applied, a partial feed bypassing directlyo the gas burner is necessary.

The dependence of the specific compression power on the mem-rane selectivity for two heating requirement values is visualised

n Fig. 15. The specific compression power decreases with the

Fig. 14. Membrane area ratio versus membrane selectivity for an optimal supplyof heat energy for two different heating requirements. The plant heating energyrequirement related to the total biogas heating value is given in ©.

increasing membrane selectivity. However for higher membraneselectivities of 30 and more the reduction of the specific power isdefinitely less pronounced.

As regards the total membrane area requirement for the process,an optimal membrane selectivity exists for a given methane recov-ery. For instance, as presented in Fig. 16 for the heating requirementof 5% the optimal membrane selectivity for minimal membranearea equals around 22. For higher energy requirement of 15% theoptimal membrane selectivity may be as low as 12. It can be notedthat for the heating requirement of 15%, a reduction of the arearequirement is associated with the transition from the two-stage

Fig. 15. Required compression power versus membrane selectivity for an optimalsupply of heat energy for two different heating requirements. The heating energyrequirement related to the total biogas heating value is given in ©.

A. Makaruk et al. / Separation and Purific

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ig. 16. Required total membrane area versus membrane selectivity for an optimalupply of heat energy for two different heating requirements. The heating energyequirement related to the total biogas heating value is given in ©.

ot allow the stable combustion in a typical gas burner since theethane concentration is very close or below the lower methane

imit for the carbon dioxide–methane mixtures. However, tech-ologies are available that permit the thermal use of low-methaneixtures like porous burners [53] or catalytic burners [54].

. Conclusions

The membrane permeation is a viable process for biogas upgrad-ng to natural gas quality. The single compressor configurations:he two-stage cascade with recycle and the single stage provideood flexibility for the optimised integration into biogas plants.he calorific value of the permeate flow can be precisely adjustedo cover the heating requirement of biogas plants. Moreover, the

odular design of the membrane systems makes it possible to eas-ly modify the membrane area ratio in order to meet the variableeating requirement if it changes during operation of a plant.

Membrane systems for biogas upgrading require gas pretreat-ent to protect membranes. This is most important at sewage

reatment plants and at landfill sites that produce biogases withigh number of malicious gas components.

For state-of-the-art membranes the specific electrical energyonsumption for biogas upgrading related to the produced gas isqual to around 0.3 kWh/m3. This values is close to the values thatere reported for an operating membrane biogas upgrading plant

n [55]. The modelling results of this work form a basis for thestimation of operating and investment costs of membrane biogaspgrading systems.

As far as membrane biogas upgrading systems are concerned,he future membrane material developments should focus on themproved compatibility with the high variety of biogas componentsather that on the achievement of very high selectivities.

cknowledgements

The authors of this publication would like to thank Johanneszivacz from Axiom Angewandte Prozesstechnik for his practicalnput in the development of membrane biogas upgrading systems.he authors would like to thank Christian Jordan and Andras Hor-

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ation Technology 74 (2010) 83–92 91

wath from the TVT-group at Vienna University of Technology fortheir useful suggestions.

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