Fuel 103 (2013) 1003–1009

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Biomethane production by anaerobic digestion of organic waste

A. Molino a,⇑, F. Nanna a, Y. Ding b,⇑, B. Bikson b, G. Braccio a

a ENEA, National Agency for New Technologies, Energy and Sustainable Economic Development, UTTRI S.S. 106 Ionica, km 419+500, 75026 Matera, Italyb PoroGen Corporation, 6C Gill Street, Woburn, MA 01778, USA

h i g h l i g h t s

" Production of biomethane from biogas produced from anaerobic digestion of organic matter." Possibility to use biomethane in the grid injection." Use of polymeric membrane for biogas upgrading." Cleaning biogas.

a r t i c l e i n f o

Article history:Received 10 May 2012Received in revised form 30 July 2012Accepted 31 July 2012Available online 23 August 2012

Keywords:BiomethaneAnaerobic digestionBiomassLandfill gasOrganic waste

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.07.070

⇑ Corresponding authors. Tel.: +39 (0)835 97473(A. Molino).

E-mail address: antonio.molino@enea.it (A. Molino

a b s t r a c t

Anaerobic Digestion (AD) is a biological process that takes place naturally when bacteria break downorganic matter in environments with or without oxygen. Controlled anaerobic digestion of organic wastein enclosed landfill will generate methane. Almost any organic material can be processed with AD, includ-ing waste paper and cardboard (of a grade that is too low to recycle because of food contamination), grassclippings, leftover food, industrial effluents, sewage and animal waste. AD produces biogas which is com-prised of around 60% methane (CH4) and 40% carbon dioxide (CO2). This biogas can be used to generateheat or electricity and/or can be used as a vehicular fuel. If the intended use is for power generation thebiogas must be scrubbed to remove a number of impurities. After conditioning the biogas can be used foronsite power generation, to heat homes or can be added to the national natural gas grid. In recent yearsseveral research groups have shown the possibility of upgrading the biogas for biomethane production[1]. This study will show the feasibility of integrating anaerobic digestion plant with onsite polymericmembrane purification system for conditioned biomethane production.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The anaerobic digestion is the technology that can convert theagro-industrial waste chain, the municipal solid waste and/orwastewater sludge into renewable energy. There are multiple char-acteristics that make this technology applicable to industrial en-ergy generation processes. Nevertheless, improvements in bothenvironmental characteristics and overall process economics arestill required to make the technology acceptable broad base. Animportant economic consideration is the fact that the biogas canbe produced at the biomass production site reducing transporta-tion costs. The AD plants can be scaled down that makes the pro-cess ideal for rural area development. The biogas can be used ina cogeneration system or used for biomethane production whichin turn can be sent to national natural gas network or used as abiocombustible fuel in the automotive sector.

ll rights reserved.

6; fax: +39 (0)835 974210

).

The process of upgrading biogas realizes a carbon negativechain because the biomethane substitutes the fossil natural gasand the carbon dioxide can be captured and used in industrialprocesses [1].

The objective of the paper is the feasibility of a biomethane pro-duction plant from anaerobic digestion of organic waste and theupgrading of biogas with polymeric membrane.

2. The process

Anaerobic digestion is a natural biological process when bacte-ria break down organic matter in environments with little or nooxygen. A controlled enclosed version of the anaerobic breakdownof organic waste is a landfill process which releases methane asone of end products. Several research groups have shown thatthe AD process can be split into three main stages: hydrolysis, aci-dogenesis and methanogenesis as show in Fig. 2.1 [2].

Anaerobic fermentation significantly reduces the total mass ofwaste, generates solid or liquid fertilizer and yields energy. It can

Fig. 2.1. Schematics of anaerobic digestion process.

1004 A. Molino et al. / Fuel 103 (2013) 1003–1009

be maintained at psychrophilic conditions (12–16 �C, e.g. in land-fills, swamps or sediments), mesophilic conditions (35–37 �C, e.g.in the rumen and in anaerobic digester) or thermophilic conditions(55–60 �C; e.g. in anaerobic digesters or geothermally heated eco-systems). Disadvantages of the thermophilic anaerobic fermenta-tion are the reduced process stability and reduced dewateringproperties of the fermented sludge and the requirement for largeamounts of energy for heating, whereas the thermal destructionof pathogenic bacteria at elevated temperatures is considered abig advantage [2]. The slightly higher rates of hydrolysis and fer-mentation under thermophilic conditions have not led to a highermethane yield. No significant change in the total methane yieldfrom the organic matter for fermentation temperatures rangingfrom 30 �C to 60 �C have been reported [3,4].

In the first stage of hydrolysis, or liquefaction, fermentative bac-teria convert the insoluble complex organic matter, such as cellu-lose, into soluble molecules such as sugars, amino acids and fattyacids. The complex polymeric matter is hydrolyzed to monomers,e.g. cellulose to sugars or alcohols and proteins to peptides or ami-no acids, by hydrolytic enzymes, (lipases, proteases, cellulases,amylases, etc.) secreted by microbes. The hydrolytic activity is ofa significant importance in the high organic content waste andmay become rate limiting. Some industrial operations overcomethis limitation using chemical reagents to enhance the perfor-mance of the hydrolysis process. The application of chemicals toenhance the first step has been found to result in a shorter diges-tion time and it provides a higher methane yield [5].

2.1. Hydrolysis/liquefaction reactions

Lipids ? Fatty AcidsPolysaccharides ? MonosaccharidesProtein ? Amino AcidsNucleic Acids ? Purines & Pyrimidines

Fig. 2.2. Process scheme f

In the second stage, acetogenic bacteria, also known as acidformers, convert the products of the first phase to simple organicacids, carbon dioxide and hydrogen.

The principal acids produced are acetic acid (CH3COOH), propi-onic acid (CH3CH2COOH), butyric acid (CH3CH2CH2COOH), and eth-anol (C2H5OH). The products formed during acetogenesis are dueto a number of different microbes, e.g. syntrophobacter wolinii, apropionate decomposer and sytrophomonos wolfei, a butyratedecomposer. Other acid formers are clostridium spp., peptococcusanerobus, lactobacillus, and actinomyces.

The acetogenesis reaction is shown below:

C6H12O6 ! 2C2H5OHþ 2CO2

Finally, in the third stage the methane is produced by bacteriacalled methane formers (also known as methanogens) in twoways: either by means of cleavage of acetic acid molecules togenerate carbon dioxide and methane, or by reduction of carbondioxide with hydrogen. Methane production is higher from reduc-tion of carbon dioxide but limited hydrogen concentration indigesters that results in acetate reaction as the primary producerof methane [6].

The methanogenic bacteria include methanobacterium, methan-obacillus, methanococcus and methanosarcina. Methanogens canalso be divided into two groups: acetate and H2/CO2 consumers.Methanosarcina spp. and methanothrix spp. (also, methanosaeta)are considered to be important in AD both as acetate and H2/CO2

consumers. The methanogenesis reactions can be expressed asfollows:

CH3COOHðacetic acidÞ

! CH4ðmethaneÞ

þ CO2ðcarbon dioxideÞ

2C2H5OHðethanolÞ

þCO2 ! CH4 þ 2CH3COOH

CO2 þ 4H2ðhydrogenÞ

! CH4 þ 2H2OðwaterÞ

The biogas products from the anaerobic digestion contain meth-ane, carbon dioxide, hydrogen, hydrogen sulfide, ammonia, silox-anes and other substances that may inhibit the anaerobicdigestion process or cause corrosion problems in pipelines ofplants or in the distribution network [7–9].

Several research groups have summarized technologies for bio-gas purification, in particular, for hydrogen sulfide, ammonia andsiloxane removal [10]. At the end of the purification process thebiogas still contains hydrogen, carbon dioxide and trace of sulphi-dric acid and ammonia (<100 ppm) that must be removed from thestream to produce biomethane.

or biogas upgrading.

BIOMETHANE

FEED BIOGAS

PERMEATE

Fig. 2.3. PEEK-SEP™ membrane for biogas upgrading.

A. Molino et al. / Fuel 103 (2013) 1003–1009 1005

A number of studies have shown the industrial feasibility ofupgrading biogas with polymeric membranes [11,12]. Membranetechnology was used to separate carbon dioxide from the biogasin order to obtain biomethane of suitable quality for placing intothe national distribution network. Most of the literature relatedto the use of polymeric membranes for carbon dioxide removal,however, is directed to natural gas purification.

State of the art polymeric membranes are economically com-petitive in separating CO2 and H2S from the biogas as comparedto conventional technologies in both capital and operating costs[13,14]. However, commercially available polymeric membranesare typically susceptible to degradation by a number of biogascomponents such as ammonia and thus require extensive feedgas pre-treatment to protect membranes from degradation whichincreases purification cost.

For several years ENEA Trisaia has been working on biogas pro-duction from various agro-industrial wastes or municipal solidwaste. Recently we have initiated a project for biogas upgradingwith polymeric membranes to increase caloric value of the gasand to purify the gas to natural gas pipeline specifications. ENEAhas selected membrane technology from PoroGen Corporation asa main component of biogas upgrading system. PoroGen’s mem-brane technology was selected because of the superior membranechemical durability (membranes do not require specialized pre-treatment to protect from aggressive biogas components that cancaused degradation of most commercial membrane systems), be-cause of the compact membrane module size and high membraneseparation efficiency. Initial process simulation has indicated thatto attain biogas product with methane concentration higher than95% (by volume) combined with a high methane recovery it wasnecessary to deploy a two stage membrane system as further de-scribed below. The schematic of the membrane separation systemis shown in Fig. 2.2. To generate driving force for membrane sepa-ration process the feed gas is compressed from the atmosphericpressure to 31 barg.

The separation system is comprised of the feed biogas compres-sor to upgrade biogas pressure derived from anaerobic digestionfrom atmospheric pressure to 31 barg; the compressed biogas isstored in a first tank to stabilize feed pressure and to regulatethe gas flow to first stage membrane module. The stabilized feedgas is directed into the first membrane module that removes car-bon dioxide, water vapor and some additional impurities (hydro-gen, hydrogen sulfide, oxygen, ammonia) by permeation andgenerates the non-permeate (retentate) product biogas comprisedprimarily of methane. The product gas is generated at the targetmethane purity, it is dried and is collected at about 30 barg pres-sure. To increase methane recovery the permeate gas from the firststage membrane module is processed through a second stagemembrane system.

The first membrane module splits the feed gas stream into twogas streams, the biomethane product non-permeate gas streamwith methane content higher than 95% by volume collected at ahigh pressure of about 30 barg and the permeate gas stream thatcontains majority of carbon dioxide, water vapor and additionalimpurities collected at a low pressure of 2 barg.

This one can be recirculated at the compression stage and afterthis it can be fed at the second stage membrane. With this secondmembrane module it is possible to recover additional productstream with methane concentration greater than 85% and a secondstage permeate with a low methane content can be used as a fuelas shown in Fig. 3.5. The permeate stream from the second stagecan be alternatively vented since it contains all impurities removedfrom the biogas, i.e. most of carbon dioxide, water vapor, hydrogen,ammonia, sulphidric acid, and some nitrogen.

Polymeric membrane modules utilized in the two stage processwere provided by PoroGen Corp., a US based company that special-izes in industrial separation process. PEEK-SEP™ hollow fibermembranes composed of poly (ether ether ketone) polymer wereused. The membranes are designed to remove acid gases and watervapors from raw natural gas or biogas to improve gas quality.

Table 3.1Load to the anaerobic digester.

Days of load Type of biomass Volumeloaded (l)

Total volumeinto reactor (l)

1 Pig manure 200 2004 Pig manure 200 4006 Fruit and vegetables 16 4167 Fruit and vegetables 16.5 432.58 Fruit and vegetables 22 454.5

11 Fruit and vegetables 22 476.512 Fruit and vegetables 22 498.513 Fruit and vegetables 24 522.514 Fruit and vegetables 25 547.515 Fruit and vegetables 26 573.518 Fruit and vegetables 27 600.519 Fruit and vegetables 28 628.520 Fruit and vegetables 29 657.521 Fruit and vegetables 30 398.522 Fruit and vegetables 31 718.525 Fruit and vegetables 32 750.526 Fruit and vegetables 33 783.527 Fruit and vegetables 34 817.528 Fruit and vegetables 36 853.529 Fruit and vegetables 38 891.532 Fruit and vegetables 40 931.533 Fruit and vegetables 42 973.534 Fruit and vegetables 45 1018.5

Fig. 2.4. Rendering of the upgrading plant.

Fig. 3.2. Experimental results for DA. Methane concentration and biogas produc-tion vs days of experimental test.

1006 A. Molino et al. / Fuel 103 (2013) 1003–1009

The membrane modules used for biogas upgrading are shows inFig. 2.3.

Fig. 2.3 shows the polymeric membrane used in this upgradingplant built by Porogen Corporation, that has a technology based onmelt extruded porous poly (ether ether ketone), PEEK, membranes.PoroGen products are made from VICTREX

�PEEK high performance

polymers and are used in the most demanding separation applica-tions. The VICTREX

�PEEK polymer was chosen for its outstanding

combination of high heat and chemical resistance. Membrane poresize and surface chemistry of each membrane product is tailored tomeet a specific separation application. For high precision separa-tion composite membranes are manufactured by depositing anadditional ultra-thin separation layer on top of the porous PEEKmembrane. Composite membrane technology platform enables ra-pid commercialization of new applications by tailoring separationlayer material characteristics towards the target application.

PEEK-SEP™ membranes offer the best overall property profile ofany polymeric membrane on the market today-allowing it to per-form in the most demanding environments. PEEK-SEP™ mem-branes can operate at temperatures as high as 200 �C and are notaffected by aggressive chemicals present in ‘‘real life’’ processstreams. PoroGen membranes are inexpensive, yet sufficientlydurable to be employed in industrial applications (high tempera-ture gas separations, natural gas treatment, and aggressive solventfiltration) under operating conditions in which other polymericmembranes cannot be used.

The ENEA’s biomethane plant was initially commissioned withonly one first stage membrane module unit. The plant configura-tion is shown in Fig. 2.4.

The plant is automated and is controlled by a PLC system thatprovides for data acquisition and valve control.

3. Experimental results

The ENEA Trisaia research center operates anaerobic digestionplant of plug flow technology type, PFR, with throughput of 70 kg

Fig. 3.1. Anaerobic digestion plant.

Fig. 3.3. Biomethane upgrading plant.

Upgrading Plant Control room Plug Flow Reactor for Anaerobic Digestion

Fig. 3.4. Integrated AD plant coupled with polymeric membrane upgrading.

Fig. 3.5. Two configurations for biogas upgrading with polymeric membrane system.

Table 3.2Wobbe index specifications for town gas, natural gas and LPG.

Family Type of gas Wobbe indexrange (MJ/Sm3)

Wobbe numberrange from [13]

1 Town gas/syngas 22.5–30 24–292 L Natural 39–452 H 45.5–55 48–533 LPG 73.5–87.5 72–87

Table 3.3Gas specifications for gas grid injection in Germany.

Parameter Unit Value

Wobbe index MJ/Nm3 46.1–56.5 H37.8–46.8 L

Relative density – 0.55–0.75Dust – Technically freeWater dew point �C <t2 (where t is the earth temperature)CO2 vol.% <6O2 vol.% <3 (in dry distribution grids)S mg/Nm3 <30

A. Molino et al. / Fuel 103 (2013) 1003–1009 1007

per day. The plant has been in operation for several years. The plugflow reactor is comprised of a stainless steel cylinder 70 cm indiameter and 350 cm long with an internal volume of about of1.3 m3; the reactor is inclined at a 20� angle and contains a40 dm3 gasometer at the elevated section of the reactor. The ADplant in show in Fig. 3.1.

The Anaerobic Digestion plant at ENEA Trisaia Research Centre,Fig. 3.1, is arranged to enable material sampling from three differ-ent reactor zones. The reactor is heated to 35–45 �C under meso-philic conditions. The thermal heat is provided through electricresistances distributed within the reactor in order to ensure ingreater load near the feed.

A number of different mixtures of fermentable organic sub-stances were tested including animal waste and agroindustrial res-idue The composition of fermentable sources evaluated are shownin Table 3.1.

The biogas produced was a gas mixture comprised mostly ofmethane and carbon dioxide with additional gas componentshydrogen sulfide, nitrogen, hydrogen, mercaptans and oxygen.

At regime the experimental results showed that the methanecontained in the biogas is in the range of 50–60% by volume [15](see Fig. 3.2).

As can be seen, after the first 5 days of operation, the concentra-tion of methane reaches concentration of about 55–60% byvolume. The rest of the biogas was composed by 40–50 vol.%in Carbon dioxide, 1–3 vol.% in nitrogen and the same forHydrogen.

4. Future developments

Recently the biogas production system was upgraded and thereactor was coupled with membrane gas separation system. Initialprocess simulation shows that the gas generated by the reactor

1008 A. Molino et al. / Fuel 103 (2013) 1003–1009

can be efficiently processed by membrane system to generatebiomethane 96vol.%. The feed gas is compressed to 40 barg andtreated by membrane system to generate biomethane productstream (retentate) at about 38 barg and permeate waste streamat about 2 barg. The product biomethane is dry and essentiallyhydrogen sulfide free. The permeate contains carbon dioxide,hydrogen sulfide, water vapor, most hydrogen and some nitrogenwith balance methane. The biogas upgrading plant comprised ofbioreactor coupled with membrane separation system is shownin Figs. 3.3 and 3.4.

As noted previously, process simulation has shown that to ob-tain a high product purity combined with a high methane recoveryit is necessary to deploy a two stage membrane system. In fact witha single stage there is a methane content about of 28 vol.% For eachone double stage configuration was fixed the purity in methane to95% in volume.

Methane recovery utilizing two stage system will depend on thespecific system configuration. In this paper, two different configu-ration have been analyzed – a two stage tandem design and a twostage cascade design, as you shown in Fig. 3.5.

In the first case, the biogas produced by AD is combined/mixedwith the retentate gas stream generated in the second stage mem-brane module, after an intermediate compression stage. With thetandem configuration, fixed the biogas in inlet to the upgradingplant, the energy consumption is about of 0.32 kW/kg h of biome-thane at 31 barg.

The two stage cascade design is composed by two membrane inseries with the recycling after compression of the permeate streamin output at the second stage. For this configuration is necessary apower consumption about of 0.14 kW/kg h of biomethane at thesame pressure to respect the previously configuration.

At the end you can see that the cascade configuration is morecompetitive than the tandem configuration because the compres-sion work of this one is greater than the cascade configuration.

Another configuration was simulated in order to verify the ef-fect of the pressure on the tandem design.

Started from a biogas with a pressure about of 17 barg, the com-pression work was of 0.11 kW/kg h but, at the other hand, thebiomethane pressure was lower than the other configurations.Fixed the biomethane purify and the mass flow rate in inlet tothe biogas upgrading plant, the pressure influence on the outletpressure, compression work and methane content in the permeateflow, infact in the low pressure cascade configuration the methanecontent in the permeate flow is about of 24% in volume, higherthan the same configuration at high pressure that is 20% in volume.

In Fig. 3.5 are reported the different configurations for biome-thane production fuelled by biogas.

The parameter typically used to determine the combustiblequality of the gas is the Wobbe index.

The Wobbe Index (WI) or Wobbe number is an indicator of theinterchangeability of fuel gases such as natural gas, liquefied petro-leum gas (LPG), and town gas and is frequently defined in the spec-ifications of gas by customers and utilities.

If Vc is the higher heating value, or calorific value, and Gs is thespecific gravity, the Wobbe Index, Iw is defined as:

Iw ¼ Vc=ðGsÞ0:5

The Wobbe Index is used to compare the combustion energyoutput of fuel gas of different quality for use in an appliance (fire,cooker, etc.). If two fuels have identical Wobbe Indices then for gi-ven pressure and valve settings the energy output will also beidentical. Typically variations of up to 5% are allowed as this wouldnot be noticeable to the consumer.

The Wobbe Index is a critical factor to minimize the impact ofthe changeover when analyzing the use of substitute natural gas(SNG) fuels such as propane–air mixtures.

There are three ranges or ‘‘families’’ of fuel gases that have beeninternationally agreed upon based on the Wobbe index. Family 1covers manufactured gases, family 2 covers natural gases (withhigh and low ranges) and family 3 covers liquefied petroleum gas(LPG). Combustion equipment is typically designed to burn a fuelgas within a particular family: hydrogen-rich town gas, naturalgas or LPG.

The simulation results show that the biomethane produced bycascade configuration has the Wobbe index in the range of 46–51 corresponding to the family 2H, i.e. similar to the natural gas.It is thus possible to use this biomethane in the natural gas grid.

Currently there are no unified, European technical standardswhich regulate the conditions for injecting biogas into the naturalgas grid [14].

The European Commission is currently working on developingsuch standards and determining quality specifications for thebiomethane (see Table 3.2). The regulations allows for injectioninto natural gas grid of two types of biogas: Type ‘‘H’’ (High), agas having a high calorific value and type ‘‘L’’ (Low) – having alow heating value.

In some European Union countries, such as Germany, the qual-ity specification for the biogas (biomethane) is based on naturalgas specifications. Table 3.3 shows German biogas specificationsfor gas to be injected into gas grid.

As one can see the biomethane produced by AD process [16,17]has the prerequisite quality for the gas grid injection, in fact theonly problem can be caused by the sulfur content in the biogas, be-cause typically the sulphidric acid content in the biogas is 0.01–0.2% by volume depending on the organic matter processed. Withthe POROGEN’s PEEK-Sep™ membrane is also possible to removeH2S from the biomethane and obtain a product stream with suphi-dric acid content lower than 30 mg/Nm3.

5. Conclusions

Biomethane produced from biogas generated by anaerobicdigestion of organic matter is an alternative gas source to that ofthe natural gas. The European Directive 2003/55 has authorizedconnection to the natural gas grid. Of particular interest is the pos-sibility to inject biomethane, refined biogas with quality compara-ble to that of natural gas (CH4 concentration greater than 95%),which can be used in place of fossil fuels in all its network applica-tions, and in transportation. To produce pipeline quality biome-thane starting from the biogas generated by AD process it isnecessary to remove water, sulfur compounds, halogenated organ-ic molecules, carbon dioxide, oxygen and metals.

Several research groups have shown that the biogas can be pro-duced at about 8–10€ cent per cubic meter of biogas, depending onthe organic matter source, with the methane content of 55–60% (byvolume) while the upgrading cost is about of 7–8€ cent consideringthe cost of kWh of about 20€ cent the total process cost is about20–22€ cent for cubic meter of biomethane compressed to gasgrid at 30 bar. Italy’s market price of natural gas is fixed by thenational authority for the electrical energy and gas use and is equalto € 40.09 cent/Nm3 referred to January 2010, will justify theindustrial feasibility of this process.

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