Comparing different biogasupgrading techniques

Interim report

J. de HulluJ.I.W. MaassenP.A. van Meel

S. ShazadJ.M.P. Vaessen

L. Bini, M.Sc. (tutor)dr. ir. J.C. Reijenga (coordinator)

Eindhoven University of Technology, April 3, 2008

Abstract

This interim report is the result of a multidisciplinary project at the Eind-hoven University of Technology commissioned by Dirkse Milieutechniek BV.The goal of the project was to research and compare the currently availabletechniques to upgrade biogas. Upgrading of biogas comprises the removal ofCO2, H2S and other possible pollutants from biogas. CO2 removal increasesthe concentration of CH4 which gives the biogas a higher calorific value al-lowing for injection in the gas grid. H2S has to be removed because of itscorrosiveness.

Five techniques have been investigated. Chemical absorption of H2Sinto iron-chelated solutions offers a highly efficient removal of H2S from agaseous biogas stream. The iron-chelated solutions function as a pseudo-catalyst which can be regenerated. The H2S is removed almost completelyand converted to elemental sulphur. After the absorption process a scrubberis needed to remove the CO2.

High pressure water scrubbing is based on the physical effect of dissolvinggases in liquids. In a scrubber, CO2 as well as the H2S, dissolves into thewater while CH4 does not because of their difference in solubility. This makesit a very simple process.

Pressure swing adsorption separates certain gas species from a mixtureof gases under pressure, according to the species molecular characteristicsand affinity for an adsorption material. The adsorption material adsorbsH2S irreversibly. Therefore a complex H2S removal step is needed for thisprocess.

The fourth process separates the components cryogenically. The differentchemicals in biogas liquefy at different temperature-pressure domains allow-ing for distillation. Typically a temperature of -170 C and a pressure of 80bar is used.

Finally, it is possible to separate CO2 and H2S from CH4 using a mem-brane. Because of selective permeation, CO2 and H2S will pass through acertain membrane while CH4 does not. This is also a very simple techniquesince only a compressor and a membrane are needed. The latter however isexpensive.

These techniques all have their unique advantages and disadvantages.When an estimate of the costs for each technique has been made, an objectivecomparison will be made.

Preface

This report presents the results of a multidisciplinary project executed at theTechnical University of Eindhoven commissioned by Dirkse MilieutechniekBV (DMT). The results are also presented on a poster and a website.

The aim of such a project is to teach students, by means of real problems,to combine and apply professional knowledge and skills and to integrate theseinto non-technical aspects of importance and new technical knowledge. Themain goals are learning to communicate with colleagues from various fields,and to gain experience in working as a team, executing a research project.

Dirkse Milieutechniek solves environmental problems with tailor madesolutions and is always seeking new possibilities to do so. DMT offers a widerange of products and services varying from research, development, consul-tancy and design to rental of equipment, installations service and mainte-nance. DMT supplies equipment and systems for air treatment, odour abate-ment, (bio)gas desulphurization, groundwater purification, soil remediationand waste water treatment.

This project was focused on the upgrading of biogas. Biogas is a resultof anaerobic digestion of organic material, resulting in methane and carbondioxide gas and some pollutants. The methane gas can be used as a greenenergy source by upgrading the biogas to natural gas and injecting it intothe existing gas grid. Upgrading of biogas signifies removal of the CO2 andpollutants such as H2S. Currently, several processes are available for theupgrading.

Project descriptionDirkse Milieutechniek is developing a biogas upgrading technology based onhigh pressure water scrubbing. To get a leading position in the market itis of most importance to know the advantages and disadvantages of all thedifferent processes available for upgrading biogas and their cost.

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A literature study was conducted to create a clear overview of the presentupgrading techniques allowing for an objective comparison. The comparisonof the different options was focused on:

chemical absorption

high pressure water scrubbing

pressure swing adsorption

cryogenic separation

membrane separation

Firstly, each technique is described shortly including a cost estimate of thefinal price per cubic meter of gas. Thereafter, a comparison of the advantagesand disadvantages of the different techniques is given. These results will helpDirkse Milieutechniek decide which option to upgrade biogas best fits theirdemands.

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Contents

1 Introduction to Biogas 4

2 Upgrading techniques 62.1 Chemical absorption . . . . . . . . . . . . . . . . . . . . . . . 62.2 High pressure water scrubbing . . . . . . . . . . . . . . . . . . 82.3 Pressure swing adsorption . . . . . . . . . . . . . . . . . . . . 112.4 Cryogenic separation . . . . . . . . . . . . . . . . . . . . . . . 142.5 Membrane separation . . . . . . . . . . . . . . . . . . . . . . . 15

3 Comparison 18

4 Conclusions 19

Acknowledgement 21

Bibliography 22

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Chapter 1

Introduction to Biogas

The current use of fossil fuels is rapidly depleting the natural reserves. Thenatural formation of coal and oil however is a very slow process which takesages. Therefore, a lot of research effort is put into finding renewable fuelsnowadays to replace fossil fuels. Renewable fuels are in balance with theenvironment and contribute to a far lesser extent to the greenhouse effect.

Biogas is such a renewable fuel, an energy source that can be applied inmany different settings. It is a combustible gas mixture produced by theanaerobic fermentation of biomass by bacteria and takes only a relativelyshort time to form. The gas mainly consists of combustible methane (CH4),and non-combustible carbon dioxide (CO2). CH4 combusts very cleanlywithout hardly any soot particles or other pollutants, making it a clean fuel.In nature the fermentation process occurs in places where biological materialis fermented in an oxygen deprived environment such as swamps and wa-terbeds. The two main sources of biogas from human activities are domesticgarbage landfills and fermentation of manure and raw sewage. The advantageof processing these waste products anaerobically, compared to aerobically, isthe larger decrease in volume of waste product. For this reason, the industrynowadays prefers anaerobic fermentation to process waste streams.

Besides CO2, biogas also contains small amounts of hydrogen sulphide(H2S). When water is present, H2S forms sulphuric acid (H2SO4), which ishighly corrosive, rendering the biogas unusable. Currently, biogas which hasbeen stripped of H2S is mainly used in gas turbines to produce electricity.However, most energy is lost as heat in this process which results in a lowoverall efficiency.

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The incombustible part of biogas, CO2, lowers its calorific value. Onaverage, the calorific value of biogas is 21.5 MJ/m3 whereas that of naturalgas is 35.8 MJ/m3. By removing CO2 from the biogas the calorific value isincreased. Stripping CO2 and H2S from biogas is the so called upgrading ofbiogas. By upgrading biogas to natural gas quality, containing approximately88% CH4, it is suitable for more advanced applications in which the heat isnot wasted, resulting in a higher efficiency. It is then applicable for use inthe gas grid and vehicles for instance.

Removing CO2 and H2S from the biogas is not easy. However, the up-grading technology is rapidly evolving, bringing biogas as a reliable energysource in sight. To produce large amounts of upgraded biogas, it is neces-sary to examine different upgrading methods to see which method might beimplemented in the industry. [1] [2]

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Chapter 2

Upgrading techniques

This chapter gives a short description of the different techniques available toupgrade biogas. For each technique, a short description including a diagramis given, followed by the distinctive advantages and disadvantages of eachtechnique. Finally a cost estimate is given.

2.1 Chemical absorptionIn the literature [3][4] several processes are presented that deal with removingof H2S. Many of them remove this pollutant only from the gaseous stream,but do not convert H2S into a more stable or valuable product, or convertit into the elemental form sulphur (S). The conversion of H2S into S or avaluable compound is an advantage of chemical absorption with respect toother methods.

The process of chemical absorption of H2S into iron-chelated solutionsoffers a high efficiency of H2S-removal, the selective removal of H2S and alow consumption of chemicals because the iron-chelated solutions functionas a pseudo-catalyst that can be regenerated. The overall reaction of thispurification process is expressed as follows [5]:

H2S +1

2O2(g) S +H2O (2.1)

In the reaction described above, H2S is first being absorbed into waterand then undergoing the dissociation as follows:

H2S(g) +H2O H2S(aq) (2.2)H2S(aq) H+ +HS (2.3)

HS H+ + S2 (2.4)

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The formation of S occurs according to the following reaction mechanism:

S2 + 2Fe3+ S + 2Fe2+ (2.5)By means of oxygenation, the aqueous iron-chelated solution will be re-

generated. This oxygenation will be followed by conversion of the pseudo-catalyst into its active form Fe3+. This mechanism is shown in the followingequations:

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2O2(g) +H2O(l) 1

2O2(aq) (2.6)

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2O2(aq) + 2Fe

2+ 2Fe3+ + 2OH (2.7)

In this mechanism, several chelate agents can be used for the specificproposal of the overall reaction, with the EDTA being the most used commonchelate [6]. In this process, the sulphur produced can be removed easily fromthe slurry by sedimentation or filtration operations. Next to that, the wholeprocess can be carried out by ambient temperature.

Figure 2.1 shows an overview of the units that are used to test the H2Sremoval from the biogas stream. The complete system consists of an absorbercolumn, a particle separator, or filter, and a regeneration column. Undercontinuous operating conditions, the biogas is introduced as small bubbles

Figure 2.1: Process flow diagram for chemical absorption of H2S

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at the bottom of the absorber of the column. These bubbles pass throughthe Fe/EDTA solution flowing downwards to the particle separator. Inthe absorber column, the H2S will be absorbed and transformed into S.The mechanism of this transformation can be seen in the equations on theformer page. The small particles of S that are formed, are separated from theproduct stream in the particle separator. After this operation, the outgoingproduct stream is regenerated from Fe2+/EDTA into Fe3+/EDTA in abubbling air column. The last step in this purification is washing the treatedbiogas with water in a packed column to remove residual traces of H2S.

The advantages of this absorption process are the almost complete re-moval of H2S from the biogas. The removed H2S is also converted into itselementary form, so it can be sold to other companies. A big disadvantageis that after the absorption process still a scrubber is needed to remove theCO2. It is not possible with this absorption process to get rid of the CO2.

Currently, research is done on an absorption process with active coal as acatalyst. This is however still in a beginning state, and will not be includedin this report.

Cost estimateWork in progress

2.2 High pressure water scrubbingWater scrubbing is a technique based on the physical effect of dissolving gasesin liquids. Water scrubbing can be used to remove CO2 and H2S from biogassince these components are more soluble in water than in methane. This ab-sorption process is a pure physical process. In high pressure water scrubbing,gas enters the scrubber under high pressure. Then, water is sprayed fromthe top of the column so that it flows down counter-current to the gas. Toensure a high transfer surface for gas liquid contact, the column can be filledwith packing material. After a drying step, the obtained methane purity canreach 98% using this process. Figure 2.2 shows the flow diagram for highpressure water scrubbing.

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Figure 2.2: Flow diagram for high pressure water scrubbing

There are two types of water scrubbing [7]:

Single pass scrubbingIn single pass scrubbing the washing water is used only once. Theadvantage of this type of scrubbing is that there is no contaminationof the water with traces of H2S and CO2. This gives that the totalamount of CO2 and H2S is at its maximum. The disadvantage of thistechnique is that it requires a large amount of water. This technique isonly feasible when working near a sewer water cleaning plant of whichwater can be used.

Regenerative absorptionIn regenerative absorption, the washing water is regenerated after wash-ing the biogas. The main advantage of this technique is that the totalamount of water required is much lower compared to single pass scrub-bing.

Water scrubbing requires a large amount of water. For example, the re-generative absorption process from DMT that washes 330 Nm3/hr biogasrequires around 1500 L/hr of water. So single pass scrubbing is practi-cally impossible in The Netherlands because water is too expensive and thegovernment will have objections against the use of such amounts of water.Therefore, the main focus will be on regenerative absorption.

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When working at a high pressure, there are two advantages compared toworking at atmospheric pressure. The main advantage is that the dissolu-bility increases when the pressure is higher. This results in a lower requiredamount of water per amount of biogas. The total amount of water requiredwill be a lot lower. Also, the washing water is over-saturated at atmosphericpressure so regenerating will be a lot faster. The driving force behind the re-generating process is the concentration difference between the over-saturatedconcentration and the equilibrium concentration. With this being as high aspossible the speed of the process will be highest.

How much H2S and CO2 can be dissolved is rather important for thedesign of a water scrubber. The dissolubility of H2S and CO2 increases withincreasing pressure. This relation can be described by Henrys Law:

Pi = H Cmax (2.8)Cmax Saturation concentration of the component [mol/m3]H Henrys coefficient [Pa m3/mol]Pi Partial pressure of the component [Pa]

According to Daltons law, the total pressure is the sum of all partialpressures. So, if the total pressure is increased, the partial pressure increasesthe same factor. This gives that the saturation concentration rises as well.

However, when higher pressures are reached the dissolubility of the com-ponents will not linearly increase with the pressure. At higher pressures, theincrease of dissolubility becomes lower. Until a pressure of 20 bar the dissol-ubility can be described according to Henrys law [8]. These calculations arebased on the ideal situation so non-idealities should be taken into accountwhen designing the scrubber.

The mass transfer of components from the gas phase to the water phaseand vice versa is important to know. When this is known, the dimensions ofthe reactor can be calculated. Mass transfer occurs when a high concentrationdifference between two phases is realized. The mass transfer can be describedusing the double film model.

When two layers with different concentration profiles intersect, the fol-lowing equations can be written:

NAG = kG a (CAG CAGi) (2.9)NAL = kL a (CALi CAL) (2.10)

The mass transfer coefficients, kL and kG, are dependent on a lot ofparameters. It is difficult to get a precise measurement of these values. But

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Figure 2.3: Concentration profile in double film model

when there is a general idea of this value, the dimensions of the scrubber canbe designed.

When looking at the five techniques investigated, water scrubbing is thesimplest way to upgrade biogas. It is this simple because it only requireswater and an absorption column to upgrade the biogas. Scrubbers also havesome advantages [9] compared to other devices. Wet scrubbers are capableof handling high temperatures and moisture. The inlet gases are cooled, sothe overall size of the equipment can be reduced. Wet scrubbers can removeboth gases and particulate matter and can neutralize corrosive gases.

Furthermore, water scrubbing can be used for selective removal of H2Sbecause this is more soluble in water than CO2. The water which exits thecolumn with the absorbed components can be regenerated and recirculatedback to the absorption column. This regeneration can be done by depressur-izing or by stripping with air in a similar column. When levels of H2S arehigh, it is not recommended to strip with air because the water can becomecontaminated with elementary sulfur which causes operational problems.

Cost estimateWork in progress

2.3 Pressure swing adsorptionPressure swing adsorption (PSA) is another possible technique for upgrad-ing of the biogas. PSA is a technology used to separate certain components

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from a mixture of gases under pressure according to the species molecularcharacteristics and affinity for an adsorption material. Figure 2.4 shows howthe adsorption material selects the different gas molecules. The adsorptionmaterial adsorbs H2S irreversibly and is thus poisoned by H2S [10]. For thisreason, a H2S removing step is often included in the PSA-process. Distur-bances have been caused by dust from the adsorption material getting stuckin the valves. Special adsorption materials are used as a molecular sieve, pref-erentially adsorbing the target gas species at high pressure. Aside from theirability to discriminate between different gases, adsorbents for PSA-systemsare usually very porous materials chosen because of their large surface areas.(activated carbon, silica gel, alumina and zeolite). The process then swings tolow pressure to desorb the adsorbent material [11]. Desorbing the adsorbentmaterial leads to a waste stream, containing concentrations of impurities.

Figure 2.4: The principle of pressure swing adsorption, taken from [12]

The upgrading system consists of four adsorber vessels filled with ad-sorption material, as can be seen in figure 2.5. During normal operation,each adsorber operates in an alternating cycle of adsorption, regenerationand pressure build-up. During the adsorption phase, biogas enters from thebottom into one of the adsorbers. When passing the adsorber vessel, CO2,O2 and N2 are adsorbed on the adsorption material surface. The gas leavingthe top of the adsorber vessel contains more than 97% CH4. This methane-rich stream is substantially free from siloxane components, VOCs, water

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Figure 2.5: Flow diagram for pressure swing adsorption [12]

and has a reduced level of CO2. Before the adsorption material is completelysaturated with the adsorbed feed gas components, the adsorption phase isstopped and another adsorber vessel that has been regenerated is switchedinto adsorption mode to achieve continuous operation. Regeneration of thesaturated adsorption material is performed by a stepwise depressurizationof the adsorber vessel to atmospheric pressure and finally to near vacuumconditions. Initially, the pressure is reduced by a pressure balance with analready regenerated adsorber vessel. This is followed by a second depres-surization step to almost atmospheric pressure. The gas leaving the vesselduring this step contains significant amounts of CH4 and is recycled to thegas inlet. These significant amounts of CH4 were trapped within the voids ofthe adsorbent particles. Before the adsorption phase starts again, the adsor-ber vessel is repressurized stepwise to the final adsorption pressure. After apressure balance with an adsorber that has been in adsorption mode before,the final pressure build-up is achieved with feed gas.

The main advantages of PSA are:

A high CH4-enrichment Low power demand Low level of emission

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The main disadvantage of PSA is:

H2S needs to be removed prior to PSACost estimate

Work in progress

2.4 Cryogenic separationCryogenic separation is a distillation process that demands cryogenic tem-perature, which means low temperatures close to -170 C, and high pressure,approximately 80 bar. Because CO2, CH4 and all other biogas contaminantsliquefy at different temperature-pressure domains, it is possible to producepure CH4 from biogas. This is done by cooling and compressing the crudebiogas to liquefy CO2 which is then easily separated from the remaining gas.The extracted CO2 can also be used as a solvent to remove impurities fromthe gas.

Figure 2.6 shows the process flow diagram (PFD) of a cryogenic separa-tion unit. This process flow diagram is made using the AspenTech program.In this process the crude biogas is compressed to approximately 80 bar. Thepre-cooled compressed gas is dried to avoid freezing during the cooling fol-lowing on the compression. The next step is the further cooling of the biogasin chillers and series of linear heat exchangers to -45 C. Condensed CO2 isremoved in a separator. This CO2 is processed further to recover dissolvedCH4 which is recycled to the gas inlet. The gas is cooled further to ap-proximately -55 C by heat exchangers. The cold gas is expanded througha Joule-Thomson nozzle into an expansion vessel. The pressure in the vesselis 8-10 bar and the temperature is approximately -110 C. In the expansionvessel a gas-solid phase equilibrium is established. The solid phase is frozenCO2. The gas phase, which consists of more than 97% CH4, is heated beforeleaving the plant.

The main advantages of cryogenic separation are:

Cryogenics can produce large quantities with high purity of the prod-ucts. Expansion or reduction of product quantity by cryogenics processes,i.e. scaling up, does not need new equipments in the process. The process makes no use of chemicals.

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Figure 2.6: Flow diagram for the cryogenic separation process

The main disadvantage of cryogenic separation is:

Cryogenic processes require the use of numerous equipments and de-vices, namely: compressors, turbines, heat exchangers, insulators, anddistillation columns. The need to maintain these equipments makesfrom this separation technique a process with large capital costs. Con-sequently, this technique becomes only economically feasible, if the sep-aration of a large amount of biogas is needed.

Cost estimateWork in progress

2.5 Membrane separationCH4 and CO2 can also be separated using a membrane. Because of the dif-ference in particle size or affinity, certain molecules pass through a membranewhilst others do not. The driving force behind this process is a difference inpressure between gases.

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Membrane gas separation modules can operate on the basis of selectivepermeation [13]. The technology takes advantage of the fact that gases dis-solve and diffuse into polymeric materials. If a pressure differential is setup on opposing sides of a polymeric film, a membrane, transport across thefilm (permeation) will occur. The rate of permeation is determined by theproduct of a solubility coefficient and a diffusion coefficient. Very smallmolecules and highly soluble molecules (such as He, H2, CO2 and H2S),permeate faster than large molecules (such as N2, C1, C2 and heavier hydro-carbons including CH4). When a biogas stream containing CO2 is fed to amembrane, the CO2 will permeate the membrane at a faster rate than thenatural gas components. Thus, the pressurized feed stream (A+B, depictedbelow) is separated into a CO2 rich, low pressure permeate stream (B) and aCO2-depleted, high pressure natural gas stream (A). Depending on the mem-brane, it is possible to also remove H2S from the biogas making this processfavorable for upgrading biogas since no pre- or after-treatment is needed.

Figure 2.7: Schematic representation of membrane separation

Any polymeric material will separate gases to some extent. Proper selec-tion of the polymeric material comprising the membrane is extremely impor-tant. It determines the ultimate performance of the gas separation module.Membranes made of polymers and copolymers in the form of a flat film or ahollow fibre have been used for gas separation. Several different membraneshave been found in literature. The Natcogroup use cellulose acetate as a basemembrane material [13]. Cellulose acetate is very inert and stable in CO2hydrocarbon environments. Application of polyimide membranes has alsobeen found [14]. For this type of membrane, a single stage unit is sufficientto achieve 94% enrichment from gas with a common concentration of CH4.

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A higher pressure gives a higher gas flux through the membrane. How-ever, the maximum pressure is determined by the membrane. For this reason,high strength hollow fibre membranes have been developed. The efficiency ofthe entire process mainly depends on the membrane. Its selectivity towardsthe gases having to be separated, membrane flux or permeability, lifetime ofthe membrane, maintenance and replacement costs are all factors that de-termine the overall performance of such a biogas upgrading technique. Anoverview of the main advantages and disadvantages is given below.

Advantages:

Compactness and light in weight Low labor intensity Modular design permitting easy expansion or operation at partial ca-pacity Low maintenance (no moving parts) Low energy requirements and low cost especially so for small sizes It requires no specific chemical knowledge Complex instrumentation is not required The basic concept is simple to understand

Disadvantages:

Membranes are expensive Certain solvents can quickly and permanently destroy the membrane Certain colloidal solids, especially graphite and residues from vibratorydeburring operations, can permanently foul the membrane surface The energy costs are higher than chemical treatment, although lessthan evaporation

Cost estimateWork in progress

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Chapter 3

Comparison

This chapter will contain the comparison of the different techniques.

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Chapter 4

Conclusions

The explanation of the different techniques has yielded an idea of the possibleresults of further research on these techniques. The costs of all differentprocesses will be estimated, which allows us to make a grounded and objectivecomparison between these processes. The chemical absorption technique willbe extended with a part about absorption with active coal as a catalyst.

Currently, we suspect the chemical absorption process being a good optionfor removal of H2S. But an important disadvantage is the need of a scrubberbehind this process to get rid of the CO2, because it is not possible to removeCO2 with chemical absorption using an Fe/EDTA catalyst.

The high pressure water scrubbing is found to be a relatively simpleprocess, compared to the other techniques. It can remove both H2S and CO2using a water stream, and can handle different temperatures and moisturecontent. However, the amount of water that has to be used for this processcan become very large. This is an aspect we have to look at carefully duringthe continuation of this research.

Pressure swing adsorption is already used in many upgrading processes.It is a technique which results in a high CH4-enrichment, without having alot of emissions while the needed power is relatively low. But, on the otherhand, when using this process, an extra step is needed to remove the H2S.

The next process that is described is the cryogenic separation. This pro-cess needs very low temperatures and high pressures and might therefore bevery expensive. Cryogenic separation is a technique that might be feasiblewhen a very large quantity of biogas has to be upgraded.

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Also membrane separation has been involved in our research. This tech-nique does not require much instrumentation apart from the membrane and acompressor. Both however, can be very expensive depending on the pressuresneeded. Next to that, at this moment we do not know what the lifetime ofa membrane is in an upgrading process we are investigating. The feasibilitywill therefore depend strongly on the costs of the membrane.

Comparing these techniques we can only speak about the theoretical pro-cess information. To obtain more information for a grounded comparison wewill estimate the costs for each process.

RecommendationsRecommendations for future research will be given in the final report.

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Acknowledgement

Thanking people for their support.

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Bibliography

[1] Wahyudin, W., Biogas upgrading installation unit (2007).

[2] Information collected from DMT

[3] Horikawa, M.S., Rossi, F., Gimenes, M.L., Costa C.M.M., Da Silva,M.G.C., Chemical absorption of H2S for biogas purification, Universi-dade Estaldual de Maring, 2001

[4] Astarita, G., Gioia, F., Hydrogen sulphide chemical absorption, ChemicalEngineering Science, 1964, vol. 19, pp. 963 - 971

[5] OBrien, M., Catalytic Oxidation of Sulfides in Biogas, Ventilation Air andWastewater Streams from Anaerobic Digesters, Proceedings 1991 FoodIndustry Environmental Conference, USA, 1991

[6] Wubs, H.J.and Beenackers, A.A.C.M., Kinetics of the Oxidation of Fer-rous Chelates of EDTA and HEDTA into Aqueous Solutions, Ind. Eng.Chem. Res., 1993, vol.32, pp2580 - 2594

[7] Harry Benning, Opwerken van biogas naar aardgas kwaliteit, maart 2005

[8] Perry, R.H. en D. Green, Perrys chemical engineers handbook, McGraw-Hill Book Company, USA, 6e druk.

[9] Wikipedia, http : //en.wikipedia.org/wiki/Wet_scrubber

[10] http://www.biotech-ind.co.uk/Methane-RGP-Process.htm, visited atthe 24th of February

[11] O. Jnsson, M. Persson, Biogas as transportation fuel, Swedish GasCentre, 2003

[12] Dr. Alfons Schulte-Schulze Berndt, Intelligent Utilization of Biogas -Upgrading and Adding to the Grid, Jonkping, May 2006

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[13] Natcogroup, Acid Gas (CO2) Separation Systems with Cynara Mem-branes, July 2007.

[14] M. Harasimowicz, P. Orluk, G. Zakrzewska-Trznadel, A.G. Chmielewski,Application of polyimide membranes for biogas purification and enrich-ment, Journal of Hazardous Materials 144 (2007) 698-702.

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Introduction to BiogasUpgrading techniquesChemical absorptionHigh pressure water scrubbingPressure swing adsorptionCryogenic separationMembrane separation

ComparisonConclusionsAcknowledgementBibliography