comparing different biogas upgrading techniques

Comparing different biogasupgrading techniques

Final 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, July 3, 2008

Abstract

This report is the result of a multidisciplinary project at the technical uni-versity of Eindhoven commissioned by Dirkse Milieutechniek BV. The goalof the project was to research and compare the currently available techniquesto upgrade biogas. Upgrading of biogas comprises the removal of CO2, H2Sand other possible pollutants from biogas. This increases the concentrationof CH4 which gives the biogas a higher calorific value allowing for injectionin the gas grid or to use as a fuel. H2S has to be removed because of itscorrosiveness.

Five techniques have been investigated. Chemical absorption of H2S andCO2 into iron-chelated cq. amine solutions offers a highly efficient removalof H2S from a gaseous biogas stream. The catalyst solutions function asa pseudo-catalyst which can be regenerated. The H2S is removed almostcompletely and converted to elemental sulphur. The CO2 is removed and istreated as a waste stream.

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

Pressure swing adsorption separates certain gas species from a mixture ofbiogas under pressure, according to the species molecular characteristics andaffinity for an adsorption material. The adsorption material adsorbs H2Seither irreversibly or reversible. Therefore a complex H2S removal step orregeneration phase is needed for this process.

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 -100 ◦C and a pressure of 40bars 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.

Each technique is compared on financial feasibility, impact on the envi-ronment and ease of operating the process. Furthermore, each technique hasits own unique advantages and disadvantages. Table 1 gives an overview ofthe costs, yield and purity of each technique.

Table 1: Comparison of prices, yield and purity of the different techniques

Technique Price per Nm3 of biogas Yield Purity€ % %

Chemical Absorption 0.28 90 98High Pressure Water Scrubbing 0.15 94 98Pressure Swing Adsorption 0.26 91 98Cryogenic separation 0.40 98 91Membrane separation 0.22 78 89

Financial FeasibilityTable 1 shows that high pressure water scrubbing seems to be the cheap-

est technique to upgrade biogas. Also this technique gives quite high yieldand purity. Cryogenics is the most expensive way of upgrading biogas but itgives the highest possible yield.

Impact on the environmentChemical absorption has several waste streams, one containing CO2 and

two different streams containing amines or Fe/EDTA complexes.These arethe catalysts used in the absorption processes. All streams need to be dis-posed as chemical waste. High pressure water scrubbing has two wastestreams. The water waste stream contains such a low concentration of H2Sand CO2 that it does not need further treatment. The second waste stream isa gas stream which also contains H2S and CO2 but also some CH4. BecauseH2S is rather poisonous, this stream should be treated and the CH4 shouldbe burned. Pressure swing adsorption and membrane separation both haveone waste stream that mostly contains CH4 and has to be burned. Cryo-genics has also one waste stream containing mostly CO2 and some traces ofH2S and CH4. This waste stream needs treatment.

Ease of operationThe operation of the pressure swing adsorption and chemical absorption

process is quite simple. However, the plant needs to shut down several timesper year because the catalyst has to be replaced. Membrane separation andhigh pressure water scrubbing are the simplest processes to operate becausethey do not need special chemicals or equipment to run. Cryogenics is diffi-cult to operate because it works on high pressure and really low temperaturesand therefore need good checking of the insulation. But for scaling up cryo-genics seems to be the most suitable technique.

ConclusionIt can be concluded that high pressure water scrubbing is performing

the best. With the low cost price, high purity and yield it is a promisingupgrading technique. Though one waste stream needs treatment, it is acontinuous process which operates almost on it self.

Preface

This report presents the results of a multidisciplinary project executed at theEindhoven University of Technology commissioned by Dirkse MilieutechniekBV (DMT). The results are also presented on a poster and a website(http : //students.chem.tue.nl/ifp24/).

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.

DMT solves environmental problems with tailor made solutions and isalways seeking new possibilities to do so. DMT offers a wide range of prod-ucts and services varying from research, development, consultancy and de-sign to rental of equipment, installations service and maintenance. DMTsupplies equipment and systems for air treatment, odor abatement, (bio)gasdesulphurization, groundwater purification, soil remediation and waste watertreatment.

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 descriptionDMT has developed a biogas upgrading technology based on high pressurewater scrubbing. To get a leading position in the market, it is of mostimportance to know the advantages and disadvantages of all the differentprocesses 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 thecost price per cubic meter of upgraded biogas. Thereafter, a comparison ofthe advantages and disadvantages of the different techniques is given. Theseresults will help Dirkse Milieutechniek decide which option to upgrade biogasbest fits their customers demands.

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Contents

1 Introduction to Biogas 4

2 Upgrading techniques 82.1 Chemical absorption . . . . . . . . . . . . . . . . . . . . . . . 82.2 High pressure water scrubbing . . . . . . . . . . . . . . . . . . 152.3 Pressure swing adsorption . . . . . . . . . . . . . . . . . . . . 202.4 Cryogenic separation . . . . . . . . . . . . . . . . . . . . . . . 232.5 Membrane separation . . . . . . . . . . . . . . . . . . . . . . . 28

3 Comparison 32

4 Conclusions 35

Acknowledgement 37

Bibliography 38

A Alternate cost estimation PSA 41

B Cryogenic equipment 44

C CO2 footprint 46

D Visit to SMB Stortgas BV in Tilburg 49

E Visit to Carbiogas BV in Nuenen 51

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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 a renewable fuel, an energy source that can be applied in manydifferent settings. It is defined as a combustible gas mixture produced by theanaerobic fermentation of biomass by bacteria and takes only a relativelyshort time to form. In nature, the fermentation process occurs in placeswhere biological material is fermented in an oxygen deprived environmentsuch as swamps and waterbeds. The two main sources of biogas from humanactivities are domestic garbage landfills and fermentation of manure and rawsewage. The advantage of processing these waste products anaerobically,compared to aerobically, is the larger decrease in volume of waste product.For this reason, the industry nowadays prefers anaerobic fermentation toprocess waste streams.

Biogas mainly consists of combustible methane (CH4) and non-combustiblecarbon dioxide (CO2). Besides CH4 and CO2, biogas also contains smallamounts of hydrogen sulphide (H2S) and some other pollutants. The com-position of biogas strongly depends on its source. Table 1.1 [1] shows thecomposition of biogas from various sources. It can be seen that biogas froma garbage landfill also contains some nitrogen (N2).

CH4 combusts very cleanly with hardly any soot particles or other pol-lutants, making it a clean fuel. But CO2, the non-combustible part of thebiogas, lowers the calorific value of the biogas. Biogas containing 60% CH4

has a calorific value of 21.5 MJ/Nm3 while pure CH4 has a calorific value of35.8 MJ/Nm3.

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Table 1.1: Overview of compositions of biogas from different sources

Component Biogas factory Sewer factory Garbage landfillCH4 (%) 60-70 55-65 45-55CO2 (%) 30-40 35-45 30-40N2 (%) <1 <1 5-15H2S (ppm) 10-2000 10-40 50-300

Besides CO2, biogas also contains small amounts of H2S. H2S is poi-sonous when inhaled. Furthermore, when water is present, H2S forms sul-phuric acid (H2SO4), which is highly corrosive, resulting in extra costs formaintenance when using the biogas.

Depending on the source of the biogas, it can contain other pollutants.Common pollutants are water vapor, ammonia (NH3) and siloxanes. Watervapor in biogas forms, combined with NH3 or H2S, a corrosive solvent.Siloxanes are silicate compounds that have oxygen groups replaced by organicgroups like CH3. When this compound is burned, it will form SiO2 (sand)which can cause severe damage to equipment.

There are a number of uses for biogas. Currently, biogas which has beenstripped of H2S is mainly used in gas turbines to produce electricity. How-ever, most energy is lost as heat in this process, which results in a low overallefficiency. But biogas can also be used for injection in the gas grid or as acar fuel. The requirements for the end product depend on the final use of thebiogas. The average composition of gas in the gas grid for low calorific gas,used in The Netherlands, and high calorific gas, used for example in Canada,are shown in table 1.2 [2].

All of the values mentioned in table 1.2 are averages, except for the Wobbeindex. The Wobbe index of the gas should always be in between the men-tioned boundaries. To reach the calorific value of Dutch natural gas themethane purity should be increased to a value of 88%. But if Canadian stan-dards must be achieved, as shown in table 1.2, the calorific value of biogasshould be increased above the calorific value of methane. This of course cannever be reached by increasing the purity of CH4. Therefore, the purity ofCH4 should be as high as possible and some higher alkanes are added to thegas to obtain the required calorific value. For injection of biogas into the gasgrid there are some additional requirements. These are shown in table 1.3[2].

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Table 1.2: Average compositions of gas used in the commercial gas grid in The Nether-lands and Canada

Dutch natural gas Canadian natural gasComponent (Source: Dutch Gas union) (Source: Uniongas Canada)

Volume % Volume %Methane 81.30 94.9Ethane 2.85 2.5Propane 0.37 0.2Butane 0.14 0.06Pentane 0.04 0.02Hexane 0.05 0.01Nitrogen 14.35 1.6Carbon Dioxide 0.89 0.7Oxygen 0.01 0.02Water vapor Unknown UnknownHydrogen Unknown TracesDensity (kg/m3) 0.833 0.7525Wobbe index (MJ/m3) 43.1-44.6 50.5-52.5Calorific value (MJ/m3) 31.669 37.8

The minimum amount of CH4 required as well as the maximum amountof N2 depends on the Wobbe index. The Wobbe index is defined as follows:

Wobbe index =calorific value(MJ/m3)√

relative density(1.1)

The Wobbe index is a measurement for the combustion behavior. If thisvalue is too high or too low, the combustion behavior will be disturbed. Thevalues may not deviate from the desired range.

Biogas can also be used as a car fuel. However, because of the low en-ergy per volume the biogas must be compressed up to 200 bars. Also, the

Table 1.3: Requirements for injection of biogas into the gas grid

Component RequirementCO2 < 8 vol %Water dewpoint < -10◦COxygen < 0.5 vol %H2S < 5 mg/Nm3

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calorific value of the biogas should be at least the value of low calorific gas.Furthermore, there may be no water or heavier alkanes than propane in thebiogas because it will form condensate at such a high pressure.

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 necessaryto examine different upgrading methods to see which method might be im-plemented in the industry. Calculating the so called CO2 footprint of eachtechnique is valuable to determine the durability [3] [4].

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Chapter 2

Upgrading techniques

In this chapter, the five investigated upgrading techniques are explained. Foreach technique, a short description including a process flow diagram (PFD)is given and the distinctive advantages and disadvantages of each techniqueare discussed. The environmental impact of the upgrading processes is animportant factor to compare the different techniques, so this is discussed foreach technique. In order to compare the different techniques, the cost priceof the produced upgraded gas must also be taken into account. The costprice per Nm3 biogas are calculated using the following formula, in whichthe interest rate on the investment is taken to be 6%:

Price per Nm3 =

investmentdepreciation period

+ investment · interest rate+ annual cost

Nm3 produced upgraded gas per year(2.1)

For each technique the input flow is taken to be 250 Nm3/h containing60 % of CH4. The output is calculated as follows:

Output = input ·% CH4 · yield (2.2)

The total running costs are determined by the operating costs, the elec-tricity and the water costs. The current electricity price is about € 0.10 perkWh [13]. The price of water is € 0.92 per m3 [14]. The service costs are€ 50,000 per year.

2.1 Chemical absorptionBoth the chemical absorption of CO2 and H2S were investigated.

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2.1.1 Chemical absorption of CO2

Multiple theories exist about the removal of CO2 in gas streams. However,these theories are often contradictive. In the following text, CO2 absorptionusing aqueous amino acid salt solutions will be discussed.

Absorption column

Regenerationcolumn

Heat exchanger

Cooler

Gasstream in

Biogas outCO2

Figure 2.1: Process flow diagram for chemical absorption of CO2

An amino acid dissolved in water exists as a zwitter ion. A zwitter ion canhave a positive and a negative charge depending on the pH of the solution.The amino group has to be deprotonated before it reacts with CO2. Thisdeprotonation is mostly done by addition of an equimolar amount of base,according to the following mechanism [5]:

HOOC −R1 −NH+3 OOC −R2 −NH+

3 OOC −R3 −NH2 (2.3)

These aqueous solutions react with CO2 to absorb this component. Inopen literature about chemical absorption of CO2, no reliable informationabout the reaction mechanism and kinetics is available. Therefore, the as-sumption is made that the reaction mechanism occurs according to the ex-perimental studies of Kumar, Hogendoorn, Feron and Versteegh, 2002. Themain reactions occurring during the absorption of CO2 are the following:

2 RNH2 + CO2 RNHCOO− +RNH+3 (2.4)

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CO2 +OH− HCO−3 (2.5)RNH+

3 RNH2 +H+ (2.6)H2O H+ +OH− (2.7)

In reaction 2.4, the reaction of CO2 with an amino acid can be seen. Thecontribution of reaction 2.5 to the conversion of CO2 is not significant, whilenot much OH− ions are present in the solution because the pH is very low.Since the OH− ions are in equilibrium with the amine molecules, reactions2.6 and 2.7 have to be taken into account.

This study also discusses the Membrane Gas Absorption (MGA) inves-tigated by TNO [6]. Research has been done by TNO at the membraneabsorption technique. According to TNO, this is a technique which makesuse of porous, water-repelling membranes for transport of components. Cur-rently, new absorption liquids, called CORAL, are developed, which show astable operation with cheap olefin membranes. According to P.S. Kumar etal. the MGA technique is economically not very attractive in comparison toconventional absorption processes, because of the limited availability of thefibres. The process flow diagram of the CO2 absorption process is shown infigure 2.1.

2.1.2 Chemical absorption of H2S

In the literature [7] [8] several processes are presented which discuss theremoval of H2S. Many of these processes remove this pollutant only fromthe gaseous stream, but do not convert H2S into a more stable or valuableproduct, or convert it into the elemental form sulphur (S). The conversion ofH2S into S or a valuable compound is an advantage of chemical absorptionwith respect to other methods.

The process of chemical absorption of H2S into iron-chelated solutionsoffers a highly efficient H2S-removal, a selective removal of H2S and a lowconsumption of chemicals, because the iron-chelated solutions function as apseudo-catalyst that can be regenerated. The overall reaction of this purifi-cation process is expressed as follows [9]

H2S +1

2O2(g)→ S +H2O (2.8)

In the reaction described above, H2S is first absorbed into water and thenundergoes the dissociation as follows:

H2S(g) +H2O H2S(aq) (2.9)

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H2S(aq) H+ +HS− (2.10)HS− H+ + S2− (2.11)

The formation of S occurs according to the reaction mechanism is describedhere:

S2− + 2Fe3+ S + 2Fe2+ (2.12)

By means of oxygenation the aqueous iron-chelated solution will be re-generated. This oxygenation is followed by conversion of the pseudo-catalystinto its active form Fe3+. This mechanism is shown in the following equa-tions:

1

2O2(g) +H2O(l) → 1

2O2(aq) (2.13)

1

2O2(aq) + 2Fe2+ → 2Fe3+ + 2OH− (2.14)

In this mechanism, several chelate agents can be used for the specificproposal of the overall reaction, with the EDTA being the most used commonchelate [10]. 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 at ambient temperature.

Figure 2.2: Process flow diagram for chemical absorption of H2S

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Figure 2.2 shows an overview of the units that are used to remove theH2S 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 bubblesat 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. Themechanism of this transformation can be seen in the equations in the formersection. In the particle separator, the small particles of S that have formedare separated from the product stream. After this separation, 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 itselemental form, so it can be sold to other companies. A big disadvantageis that after the absorption process a scrubber is still needed to remove theCO2. It is not possible with this absorption process to remove the CO2.

Waste streams

Chemical absorption of CO2

The only process stream next to biogas needed in the absorption process isa liquid water phase in which amines are dissolved. As can be seen in figure2.1 the biogas flows through a column filled with the amine solution. In thiscolumn, the CO2 is split from the biogas and the biogas leaves the absorptioncolumn. The amine solution including the captured CO2 leaves the columnand will be generated in the generation column. During this process, theCO2 is split off and is emitted in the atmosphere as a waste stream. Theamine solution will be regenerated and flows back into the column to captureCO2 again. This solution must be replaced a few times a year and then itbecomes a waste stream too. This solution can be separated into a waterphase and the amines using a membrane. The clean water phase can thenbe purged to a river. The only real waste streams are the CO2 stream andthe amines.

Chemical absorption of H2S

For theH2S absorption process only the removal ofH2S is taken into account.In figure 2.2 a scrubber is also shown, but since this process is discussed in

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another part of the report, we will focus only on the H2S removal. Figure2.2 shows the process flow diagram. The biogas stream can be seen andin the regeneration part also some other streams are added to the process.The biogas flows through the absorption column and the H2S is capturedin the liquid phase. The liquid phase consists of water in which Fe/EDTAis dissolved. The biogas leaves the column containing almost no H2S. TheFe/EDTA solution flows to the regeneration part in which the sulphur isseparated from the solution. After this step, the Fe is regenerated fromFe2+ to Fe3+. This aqueous solution is again used in the absorber columnto capture H2S. The separated elemental sulphur is collected and becauseit is pure it can be sold to other companies. However, the amount is smalland to sell a reasonable amount would take quite a long time to collect.Because of these circumstances, the sulphur is mostly treated as a wastestream and has to be put away as chemical waste. Another waste stream isthe Fe/EDTA solution. This solution has to be replaced a few times a year.The solution can be filtered using a membrane, to separate the water phaseand the Fe/EDTA complexes. These components are another waste streamof the absorption process and need to be disposed of as chemical waste.

The purity of the obtained biogas is approximately 98%. In both processesthe yield for CH4 is 90%. The CH4 waste stream is best handled by sendingthe stream to a flare. Burning CH4 is better for the atmosphere than emittingthe gas. Looking at the two absorption processes the absorption of CO2 seemsto have less waste streams than the absorption of H2S, at least less harmfulwaste streams.

Cost estimation for chemical absorption

For the absorption process, two cost prices of upgraded biogas are calculated,one for the absorption of CO2 and one for the absorption of H2S. The pricefor biogas using both methods at the same time is calculated in the end.

Cost estimation chemical absorption of CO2

· one time per year general inspection in- and outside· one time per year general inspection outside· if necessary, cleaning of internals· maintenance of recirculation pump· calibration of instrumentation

Output: 137 Nm3/h, 1,127,000 Nm3 per year90% CH4 yield, purity output: 98% CH4

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Investment costsAbsorber column2

€ 125,000Additional costs6 € 100,000Pump 3

€ 5,000Heat exchanger4 € 15,000Cooler1 € 18,000Regeneration column1

€ 90,000

Total investment costs € 353,000Depreciation period of equipment is 10 years.

Running costsEnergy costs6 € 30,000Catalyst costs5 € 50,000Operator € 50,000Maintenance6

€ 4,500

Total running costs € 134,500

Costs per Nm3 biogas without H2S removal: € 0.17

After the absorption of CO2, an amount of 3% H2S is still present in thebiogas. For excellent cleaning of biogas, also the H2S has to be removed,because the requirements are less than 5 mg/Nm3 biogas.

Cost estimation chemical absorption of H2S

· one time per year general inspection in- and outside· one time per year general inspection outside· if necessary, cleaning of internals· maintenance of recirculation pump· calibration of instrumentation

Investment costsAbsorber column2

€ 125,000Additional costs6 € 100,0002 Pumps3 € 10,000Regeneration column1

€ 90,000Particle separator1 € 100,000

Total investment costs € 516,000

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Depreciation period of equipment is 10 years.

Running costsEnergy costs6 € 30,000Catalyst costs5 € 15,000Operator € 50,000Maintenance6

€ 4,500


The costs per Nm3 produced are calculated according to formula 2.1.Costs per Nm3 biogas: € 0.16

When the price of the complete upgrading process, including both CO2

and H2S absorption, is calculated, we obtain a price of € 0.28 per Nm3

upgraded biogas. This price is based on the following values:

Investment costs € 869,000Running costs € 179,500

1 costs from Aspen Icarus Project Evaluator2 costs from offer of Rootselaar3 costs from offer of Grundfoss4 costs from calculation of Mauri5 costs from excursion to Cirmac6 costs from offer of E-kwadraat

2.2 High pressure water scrubbingWater scrubbing is a technique based on the physical effect of gases dissolvingin liquids. Water scrubbing can be used to remove CO2 and H2S frombiogas since these components are more soluble in water than in CH4. Thisabsorption process is a fully physical process. The main parts of the processare shown in figure 2.3. In high pressure water scrubbing, gas enters thescrubber at high pressure. This high pressure increases the dissolubility ofgases in water. Then, water is sprayed from the top of the column so thatit flows down counter-current to the gas. To ensure a high transfer surfacefor gas liquid contact, the column is usually filled with a packing material.

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Figure 2.3: PFD for high pressure water scrubbing

In the flash vessel the pressure is decreased and some traces of CH4 will beregenerated. In the stripper the washing water is regenerated. CO2 and H2Sare stripped by air in this vessel. After a drying step, the obtained CH4

purity can reach 98% using this process and yields can achieved up to 94%.There are two types of water scrubbing [2]:

Single pass scrubbingIn single pass scrubbing, the washing water is used only once. Theadvantage of this type of scrubbing is that no contamination in thewater occurs like 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 techniqueis only feasible when working near a sewer water cleaning plant fromwhich water 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, theregenerative absorption process from DMT that washes 330 Nm3/h biogas

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requires approximately 50 l/h 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 usage of such large amounts ofwater. Therefore, the main focus will be on regenerative absorption.

When working at 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 thus be a lot lower. Also, the washing water is oversaturated at atmo-spheric pressure so regenerating will be a lot faster. The driving force behindthe regenerating process is the concentration difference between the oversat-urated concentration and the equilibrium concentration. With this being ashigh as possible, the speed of the process will be highest.

For the design of a water scrubber it is rather important to know howmuch H2S and CO2 can be dissolved. The increasing dissolubility of H2Sand CO2 with increasing pressure is described by Henry’s Law:

Pi = H · Cmax (2.15)

Cmax Saturation concentration of the component [mol/m3]H Henry’s coefficient [Pa ·m3/mol]Pi Partial pressure of the component [Pa]

According to Dalton’s law, the total pressure is the sum of all partialpressures. So if the total pressure is increased, the partial pressure increasesby the same factor. This means the saturation concentration rises as well.

However, when higher pressures are reached, the dissolubility of the com-ponents will no longer linearly increase with the pressure. At higher pressuresthe increase of dissolubility becomes lower. Up to a pressure of 20 bars thedissolubility can be described according to Henry’s law [11]. These calcula-tions are based on the ideal situation so non idealities should be taken intoaccount in the design of a scrubber.

Another important factor for the dissolubility of the components in wateris the pH [2]. Furthermore, the pH depends on the amount of H2S and CO2

that has been dissolved into water. Water becomes more acid when moreH2Sand CO2 are absorbed. When the pH is decreased, CO2 will dissolve less andthe H2S will dissolve less. At a pH of 1, the dissolubility of H2S is only halfof the dissolubility at a pH of 7. Therefore, a low pH is not feasible becausethe H2S removal is important; the stripping process becomes more difficultand acid water damages equipment. Working at a high pH is unfeasible aswell because sulphur and carbonate ions will precipitate. It is best to workat a pH of 7.

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The mass transfer of components from the gas phase to the water phaseand vice versa is important to know. When it is known, the dimensions of thereactor 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. This model is shown in figure 2.4.

When two layers with different concentration profiles intersect, the fol-lowing equations are valid:

NAG = kG · a · (CAG − CAGi) (2.16)

NAL = kL · a · (CALi− CAL) (2.17)

Figure 2.4: Concentration profile in double film model

The mass transfer coefficients, kL and kG, are dependent on a lot ofparameters. It is difficult to get a precise measurement of these values. Buta rough estimate of these values suffices to design the dimensions of thescrubber.

Water scrubbing is a simple process because it only requires water andan absorption column to upgrade the biogas. Scrubbers also have someadvantages [12] compared to other devices. Wet scrubbers are capable ofhandling high temperatures and moisture. The inlet gases are cooled so theoverall size of the equipment can be reduced. Wet scrubbers can remove bothgases 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 recirculated

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back 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 elemental sulfur which causes operational problems. Alsoat high levels of H2S the dissolubility is limited because of decreasing pH.

Waste streams

The water scrubbing process contains two main waste streams. The firstwaste stream is the exhaust of air which was used to strip the regeneratedwater. This stream mainly consists of air and a high percentage of CO2 butalso contains traces of H2S. Because H2S is rather poisonous this streamneeds to be treated. Also the stream contains small amounts of CH4. BecauseCH4 is far more damaging to the environmental than CO2 the CH4 in thisstream should be burned.

The second waste stream is a purge of water. To keep the dissolubilityas high as possible a part of the washed water is purged and replaced withclean water. In this way the concentration of CO2 and H2S in the waterstream to the scrubber will remain as low as possible and CO2 and H2S willnot accumulate. Because most of the CO2 and H2S will be absorbed in thegas phase in the stripper the purge stream does not have to be treated.

Cost estimation for high pressure water scrubbing


Compressor (10 bars, 250 Nm3/h biogas) € 110,000Columns /> € 140.000Heat exchangers € 5,000Pumps and blowers € 10,000Total investment costs € 265,000Depreciation period of equipment is 10 years.

Running costsEnergy costs € 60,000Operator € 50,000


Costs per Nm3 biogas: € 0.13

19

This cost price is in close accordance to the costs in Tilburg at the biogasupgrading plant, SMB Stortgas BV. At this upgrading plant the cost pricewas approximately € 0.11 to € 0.12 per Nm3.

2.3 Pressure swing adsorptionPressure swing adsorption (PSA) is another possible technique for the up-grading of biogas. PSA is a technology used to separate certain componentsfrom a mixture of gases under pressure according to the species’ molecularcharacteristics and affinity for an adsorption material. Figure 2.5 shows howthe adsorption material selects the different gas molecules. The adsorptionmaterial adsorbs H2S irreversibly and is thus poisoned by H2S [15]. For thisreason, an H2S removal 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 molecular sieves, 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(for instance activated carbon, silica gel, alumina and zeolite). The processthen swings to low pressure to desorb the adsorbent material [16]. Desorbingthe adsorbent material leads to a waste stream, containing concentrations ofimpurities.

The upgrading system consists of four adsorber vessels filled with ad-sorption material, as can be seen in figure 2.6. 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 adsorbent material surface. This can beseen in figure 2.5 where N2, O2, H2O, H2S and CO2 are adsorbed in theadsorber. The gas leaving the top of the adsorber vessel contains more than97% CH4. This methane-rich stream is substantially free from siloxane com-ponents, volatile organic compounds (VOCs), water and has a reduced levelof CO2. Before the adsorbent material is completely saturated with the ad-sorbed feed gas components, the adsorption phase is stopped and anotheradsorber vessel that has been regenerated is switched into adsorption modeto achieve continuous operation. Regeneration of the saturated adsorbentmaterial is performed by a stepwise depressurization of the adsorber vesselto atmospheric pressure and finally to near vacuum conditions. Initially, thepressure is reduced by a pressure balance with an already regenerated ad-sorber vessel. This is followed by a second depressurization step to almost

20

Figure 2.5: The principle of pressure swing adsorption, picture taken from [17]

atmospheric pressure. The gas leaving the vessel during this step containssignificant amounts of CH4 and is recycled to the gas inlet. These significantamounts of CH4 were trapped within the voids of the adsorbent particles.Before the adsorption phase starts again, the adsorber vessel is repressurizedstepwise to the final adsorption pressure. After a pressure balance with anadsorber that has been in adsorption mode before, the final pressure build-upis achieved with feed gas. A complete cycle is completed in approximately3-5 minutes [20]. The advantages of the PSA-process are the high CH4-enrichment of more than 97%, the low power demand and the low level ofemission. The waste stream of the PSA-plant consists of N2, O2, H2O, H2Sand CO2. The main disadvantage is the H2S-removal step. This is a complexstep in the process, which is necessary.

Waste streams

The PSA-plant has a final product stream, the upgraded biogas, which con-tains more than 97% CH4. Next to the product stream, a waste stream isproduced. The waste stream leaves the adsorber vessels at the bottom andcontains all the adsorbed material from the carbon molecular sieves. Also,some significant amounts of CH4 are found in this waste stream (among other

21

Figure 2.6: PFD for pressure swing adsorption [17]

things the remaining 3% CH4). CH4 is more damaging than CO2, so it is ofmost importance to make sure that CH4 is not emitted into the air. Burningthe CH4 is less harmful to the environment in comparison with emitting CH4

directly into the air. Therefore, the waste stream can be led to a gas enginelinked to a generator. Increasing the yield of CH4 in the product stream canbe achieved by recycling the waste stream. This has also a positive effect onthe amount of CH4 in the waste stream, which will decrease.

Cost estimation for PSA

Using the process flow diagram of the PSA-process, gives the following costestimation. The costs for the removal of H2S are included in the investmentcosts as well as in the running costs. The costs of the pressure swing ad-sorption depend on which type of adsorbent material is used in the columnsand the number of units used. The operational costs are influenced by theoperating pressure, which on its turn is dependent on the adsorbent material.The compressor needed in the beginning in order to compress the incomingbiogas is the last element which contributes to the cost of the whole plantsignificantly. Compression is expensive and in order to make it profitable,it is needed to recover the required pressure. The pressure recovery can beenabled by several pressure valves.

The type of adsorbent material used in the PSA is a carbon molecular

22

sieve. The choice for this adsorbent material can be explained by the ability ofremoving N2 and O2 from the biogas. The lifetime of the adsorbent materialis taken to be 3 to 4 years. Furthermore, there are four adsorber vesselsneeded in the plant. Figure 2.6 shows the overall scheme of the PSA-plant.The compressor, the four adsorber vessels, the vacuum pump and the H2S-removal step are included in the cost estimation. Appendix A shows analternate way of estimating the cost of a PSA-plant. The equations used, arefound in [18, 19]. In this chapter, the cost estimation is adjusted to the costestimations of the other techniques which are investigated.


Investment costsCO2 adsorber columns (4) 4

€ 500,000Additional costs6 € 70,000Pumps (2) 3

€ 10,000Compressors (2) € 100,000Total investment costs € 680,000Depreciation period of equipment is 10 years.

Running costsEnergy costs € 33,500Catalyst costs € 100,000Operator € 50,000Maintenance € 3,750


The costs per Nm3 produced are calculated as explained at the beginning ofthis chapter.


2.4 Cryogenic separationThe name cryogenic separation already reveals the fact that this techniquemakes use of low temperatures, close to -90 ◦C, and high pressure, approxi-mately 40 bars. Because CO2, CH4 and all other biogas contaminants liquefy

23

at different temperature-pressure domains, it is possible to obtain CH4 frombiogas by cooling and compressing the crude biogas to liquefy CO2 which isthen easily separated from the remaining gas.

Among the existing techniques for biogas upgrading, cryogenic separationof impurities from biogas is still in the early stages of research and develop-ment. In order to investigate the feasibility of this technique, in the firstdesigning steps, the focus has been only on the separation under low tem-perature and high pressure. When the desired purity of the upgraded gas isachieved, the designing of the cooling and compressing unit in this techniquecan be continued. Finally these two models, for compressing and separat-ing of biogas, is put together to achieve the final separation model which isshown in figure 2.9. Figure 2.7 shows this primary model for the cryogenicseparation of biogas. The calculations for this model are based on the crudeinlet biogas with an inlet gas flow of 250 Nm3/h. The inlet gas is assumedto be dried, under atmospheric pressure and has an ambient temperature.The composition of the inlet gas is given in table 2.1.

Distillation Column

1

2

3

Figure 2.7: A simple model of cryogenic separation of biogas. Streams 1, 2 en3 respectively are the crude biogas (inlet gas), the upgraded biogas(product) and the impurities.

The model in figure 2.7 has been created by using the Aspen Plus softwarepackage. In this model, the impurities from crude biogas are separated usinga distillation column which operates at a temperature of -90 ◦C and a pressureof 40 bars. The results of the modeling are summarized in table 2.2.

As can be seen in table 2.2 the product stream, upgraded biogas (stream2), has a CH4 purity of 91%. Again it should be mentioned here that thispurity is based on the model made in Aspen Plus. However, according to [31]it should be possible to upgrade biogas to a higher purity of CH4. Anotherdemand for the upgrading of biogas is the reduction of H2S quality with a

24

Table 2.1: The average biogas composition assumed for use in the model [21]

Biogas component Volume %CH4 60CO2 35CO 0.15N2 3H2 1.55H2S 0.3Oxygen, Siloxane traces

Table 2.2: The results of the modeling for the cryogenic separation

Stream 1 2 3Temperature (◦C) 25 -91 1.4Pressure (bar) 1 40 40Vapor Fraction 1 1 0Mole Flow (kmol/h) 10.11 6.48 3.63Mass Flow (kg/h) 263.42 105.63 157.78Volume Flow (m3/h) 250 1.26 0.17Enthalpy (MMkcal/h) -0.44 -0.11 -0.34Mass FractionsCH4 0.369 0.91 0.006CO2 0.591 0.00014 0.98CO 0.001 0.004 1.91E-09N2 0.032 0.08 2.29E-08H2 0.001 0.003 4.23E-15H2S 0.004 Trace 0.006

factor 1000 which is achieved as well. Knowing these demands are achieved,the second step in the process design will be designing of the cooling andcompressing units. Figure 2.8 shows these process units.

In these process units the crude inlet biogas goes through the first heatexchanger in which it is cooled down to -70 ◦C. This heat exchanger uses theproduct stream as a cooling medium, which has the advantage of preheatingthe upgraded biogas before leaving the plant as well as the energy efficiencybenefit of the process. The first cooling step is followed by a cascade ofcompressors and heat exchangers which cool the inlet gas down to -10 ◦C andcompress up to 40 bars before entering the distillation column. To defrost

25

Biogas 2 3

Cooler

1

Compressor Cooler Compressor

54

Cooler

Figure 2.8: Cooling and compressing units in cryogenic separation

frozen water each heat exchanger needs a parallel heat exchanger. Table 2.3shows the stream conditions through this process unit.

Table 2.3: Stream conditions through the cooling and compressing process units

Stream Inlet gas 1 2 3 4 5Temperature (◦C) 25 -70 207 -10 54 -10Pressure (bar) 1 1 21 20 40 40Vapor Fractions 1 1 1 1 1 1Mass Flow (kg/h) 177.70 177.70 177.70 177.70 177.70 177.70Volume Flow (m3/h) 168.64 114.21 12.89 6.81 4.27 3.07Enthalpy (MMkcal/h) -0.29 -0.30 0.28 -0.30 -0.29 -0.30

Figure 2.9 shows the complete PFD for the cryogenic separation process

Waste streams

The fact cryogenic separation uses no chemicals makes of this separationan environmental friendly technique. The only waste stream is stream 8shown in figure 2.9. (The same as stream 3 in figure 2.7) This stream mainlyconsists of a high percentage of CO2 but also contains traces of H2S andCH4. Because H2S is rather poisonous and CH4 is more damaging to theenvironment comparing CO2, this stream needs to be treated.

Cost estimation for cryogenic separation

The cost analysis for the final designed process is estimated using quotationsfrom DMT, the Matches process, a cost engineering website (appendix B)and Aspen Icarus process evaluator.


26

CoolerCompressor Cooler Compressor Cooler

Distillation Column

Product91 % CH4

8 % N21% other

Recirculation of product stream as cooling agent

Waste0.6 % CH498 % CO2

Biogas25 oC1 bar

37 % CH458 % CO25 % other

-70 oC1 bar

207 oC21 bar

-10 oC21 bar

54 oC40 bar

-10 oC40 bar -90 oC

40 bar

Figure 2.9: PFD of the cryogenic separation of biogas

Investment costsHeat Exchanger 1 € 10,300Heat Exchanger 2 € 26,500Heat Exchanger 3 € 21,700Compressor1 € 200,000Compressor2 € 250,000Separation train € 400,000


Running costsEnergy costs € 343,000Operator € 50,000Maintenance € 4,500Total running costs € 397,500


27

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 inpartial pressure between gases. The properties of this separation techniqueare highly dependent on the type of membrane used. Many different mem-branes are available each with its particular specifications [26]. The generalprinciple however is basically the same and is explained below on the basisof a membrane from the Natcogroup [22].

The Natcogroup use membrane gas separation modules which operate onthe basis of selective permeation [22]. The technology takes advantage ofthe fact that gases dissolve and diffuse into polymeric materials. If a pres-sure differential is set up on opposing sides of a polymeric film, a membrane,transport across the film (permeation) will occur. The rate of permeationis determined by the product of a solubility coefficient and a diffusion co-efficient. Very small molecules and highly soluble molecules (such as He,H2, CO2 and H2S), permeate faster than large molecules (such as N2, C1,C2 and heavier hydrocarbons including CH4). When a biogas stream con-taining CO2 is fed to a membrane, the CO2 will permeate the membrane ata faster rate than the natural gas components. Thus, the pressurized feedstream (coming from below in picture 2.10) is separated into a CO2 rich, lowpressure permeate stream on the right hand side and a CO2-depleted, highpressure CH4 gas stream.

Figure 2.10: 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-

28

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 abase membrane material [22]. Cellulose acetate is very inert and stable inCO2/hydrocarbon environments. Application of polyimide membranes hasalso been found [23]. For this type of membrane a single stage unit is suffi-cient to achieve 94% enrichment from gas with a common concentration ofCH4. Using a liquid as a membrane is also possible making it possible toreplace the membrane in situ by circulating the liquid [24].

The permeation of H2S depends on the choice of membrane. If H2Spermeates only partly both exit streams contain H2S. Either the inputstream or the output streams can be cleaned. Since the CO2 rich streamstill contains a relatively high concentration of CH4 ( 10-15%) this streamis best used in a gas engine to produce electricity or heat. For that, theH2S does not have to be removed. This will result in more wear of theengine but maintaining an engine is cheaper than the removal of H2S. Thecheapest option therefore is only cleaning the CH4 stream which constitutesa significantly smaller amount of gas than the input. A membrane whichfully removes the H2S from the biogas would be a great improvement. Theneed for other pre-treatment such as drying or heating is fully dependenton the membrane used. A higher pressure gives a higher gas flux throughthe membrane. However, the maximum pressure is determined again by themembrane. For this reason, high strength hollow fibre membranes have beendeveloped.

Overall, the efficiency of the entire process mainly depends on the mem-brane used. Its selectivity towards the gases having to be separated, mem-brane flux or permeability, lifetime, operational temperature and humidityrange, maintenance and replacement costs are all factors that determine theoverall performance of such a biogas upgrading technique. It is thereforedifficult to judge this technique in total. Some main characteristics can begiven; it is a proven technology. It has been applied for many years to extractnitrogen from ambient air. It has also already been used to upgrade biogas;experimentally [27] as well as commercially. Membranes, especially hollowfibre membranes, are very compact, light weight and allow for a modular de-sign making expansion and replacement very easy. However, well maintainedmembranes hardly need any maintenance and can last as long as 10 to 15years. Other equipment such as the compressor and pumps do need mainte-nance but this is also true for the other techniques. The total energy needsare very low since the membrane itself is passive. Because the membraneis passive the entire process is easy to operate and simple to understand.

29

Membranes however can be expensive and also very fragile. Certain solventsor fine colloidal solids such as graphite can permanently destroy or foul themembrane.

Waste streams

A major disadvantage of this technique is the low methane yield. The wastegas still contains CH4 which is highly polluting. Part of it can be fed backinto the inlet or, as mentioned above, the waste gas can be burnt in a gasengine linked to a generator. Using a multistage setup also increases theyield. Positive results have been found using an internally staged permeator[25], depicted in figure 2.11. Electrical costs are low since only a compressorhas to be powered. The generator can power the compressor which resultsin an even higher CH4 efficiency. The CO2 stream is then of no further use.If the waste stream is not burned in an engine it is very polluting since CH4

is far more harmful than just CO2.

CH4 + CO2

mainly CH4 +small amount of CO2

mainly CO2 +small amount of CH4

Figure 2.11: Schematic representation of an internally staged membrane separa-tor

Cost estimation membrane separation

To prevent damage to the membrane, intensive pre-treatment might be nec-essary. This could be quite expensive. However, it is not taken into accountin this report.

· Without H2S removal· 150 hours of maintenance per year· Flare recommended (especially during start-up)

30

Output: 130 Nm3/h, 1,002,400 Nm3 per year78% CH4 yield, purity output: 89.5% CH4

Investment costsAdditional costs € 100,000Pumps (2) € 10,000Compressor (5-10 bar) € 100,000Membrane2

€ 23,000


Running costsEnergy costs (41 kWh) € 28,000Operator € 50,000Maintenance1

€ 3,750


The costs per Nm3 produced are calculated as explained at the beginning ofthis chapter.

Costs per Nm3 biogas without H2S removal: € 0.12

To remove H2S the process described in 2.1.2 is added to these costs.This results in a total cost price for upgraded biogas of € 0.22. This price iscalculated from the following values:

Investment costs € 749,000Running costs € 126,750

1 Estimate E-kwadraat2 Estimate from Cirmac

31

Chapter 3

Comparison

This chapter will compare the five different techniques which are investigatedfor biogas upgrading. The techniques will be compared on a couple of fac-tors. Of course, every technique has its own advantages and disadvantages,but there is more than that. The techniques will be compared on the priceper Nm3 upgraded biogas; how easy or hard the process runs looking atmaintenance and scale up; and the impact on the environment by examin-ing the waste streams. The cost estimate is used to calculate the price ofone Nm3 of upgraded biogas. Also, there are the costs of investment andthe operating costs. The consideration to be made is the best combinationof advantages and disadvantages, the cost for operating and investment andfinally, the price which has to be paid for the upgraded biogas, the wastestreams and maintenance. The table at the end of this chapter gives anoverview of this comparison. Furthermore, appendix ?? gives the advantagesand disadvantages of each technique in the current opinion of DMT [29].These are compared to the findings presented in this report.

FinancesLooking at the price of the upgraded biogas, it can be seen that high

pressure water scrubbing is the cheapest. This can be linked to the invest-ment costs which also are the lowest. Cryogenic separation sticks out of thelist with the highest investment cost and also the cost price is with € 0.44the highest. While the investment costs of pressure swing absorption are alsoquite high, the cost price is the average compared to the other four techniques.

Impact on the environmentLooking at the amount of waste streams, it can be easily seen that pres-

sure swing adsorption and membrane separation have only one waste stream,where chemical absorption and high pressure water scrubbing have two waste

32

streams. But it does not automatically mean that chemical absorption andhigh pressure water scrubbing are a bad technique. Not only the amount ofwaste streams has to be noticed, also the content of the waste streams have tobe determined. The waste stream produced with pressure swing absorptionand membrane separation both will be led to a gas engine linked to a gener-ator. This is the best solution, because CH4 is more harmful when emittedinto air compared to burning a waste stream containing CH4. Chemical ab-sorption has two real waste streams, namely a stream containing CO2 anda stream periodically catalyst stream. High pressure water scrubbing has awaste stream containing CO2 and some traces of H2S. The last component ispoisonous, which result in the fact that this waste stream needs waste treat-ment. The second waste stream is a water stream containing CO2 and H2S.Because the amount of CO2 and H2S is rather small, this stream does notneed any treatment. Cryogenic separation has one waste stream containing ahigh percentage of CO2 and some traces of H2S and CH4. This waste streamneeds treatment. High pressure water scrubbing and membrane separationare the only two techniques that don’t produce pure CO2.

Ease of operationNot each technique requires the same amount of maintenance, materials,

catalyst and operators. Therefore, a distinction between this has to be made.Looking at chemical absorption, an expensive catalyst is used in order toabsorb CO2 and H2S. This catalyst has to be changed twice a year, whichleads to a shut down. The same is partly true for pressure swing adsorption.High pressure water scrubbing however is a very simple process. The onlyseparation parameter is the pressure of the water scrubber and this can beeasily kept under the desired condition. Because of this easiness, there isstill an operator needed to check if everything goes well, which is there allthe time. Another advantage is the absence of using special chemicals ora catalyst, which makes that the process can run continuously without aperiodic shut down. Since membrane separation only needs few equipmentand makes no use of chemicals, no operator is needed which is constantly atthe plant. However, in order to check the running process, one operator isneeded. Another consequence is the simple process, which makes it an easyrunning process. At last, cryogenic separation is looked at. Because of thelarge amount of equipment needed, it is a complex process. Furthermore, thehigh pressure and very low temperature makes it a dangerous process, whichhas to be controlled. Operators are certainly needed therefore.

33

Tab

le3.

1:Anoverview

inorderto

compa

rethediffe

rent

techniqu

eson

ourcriteria,witho

utH

2S

removal.OnlyforPSA

,an

H2Sremoval

step

isneeded

aspre-treatm

ent.

Fortheothe

rprocesses,

H2Scanbe

removed

inad

vanc

eor

afterw

ards.

Technique

Investment

Running

Costprice

Max

imum

Max

imum

Advantages

Disad

vantages

cost

cost

upgrad

edachievable

achievable

biogas

yield

purity

€€

€/N

m3biog

as%

%Chemical

ab-

sorption

353,00

013

4,50

00.17

9098

·Alm

ost

completeH

2S

re-

moval

·Onlyremoval

ofon

ecompo

-nent

incolumn

·Exp

ensive

catalyst

High

pressure

water

scrubb

ing

265,00

011

0,00

00.13

9498

·Rem

oves

gasesan

dpa

rticu-

late

matter

·Lim

itationofH

2Sab

sorption

dueto

chan

ging

pH·H

ighpu

rity,g

oodyield

·H2S

damag

esequipm

ent

·Simple

techniqu

e,no

spe-

cial

chem

icalsor

equipm

entre-

quired

·Requiresalotof

water,e

ven

withtheregenerative

process

·Neutralization

ofcorrosive

gases

Pressure

swing

adsorption

680,00

018

7,25

00.25

9198

·Morethan

97%CH

4enrich-

ment

·Add

itiona

lcomplex

H2S

re-

moval

step

needed

·Low

power

deman

d·L

owlevelo

femission

s·A

dsorptionofN

2an

dO

2

Cryog

enic

sepa

-ration

908,50

039

7,50

00.44

9891

·Can

prod

ucelargequ

antities

withhigh

purity

·Aloto

fequ

ipmentisrequ

ired

·Easyscalingup

·Nochem

icalsused

inthepro-

cess

Mem

bran

e23

3,00

081

,750

0.12

7889

.5·C

ompa

ctan

dlig

htin

weigh

t·R

elativelylowCH

4yield

sepa

ration

·Low

maintenan

ce·H

2S

removal

step

needed

·Low

energy

requ

irem

ents

·Mem

bran

escanbe

expe

nsive

·Easyprocess

Chapter 4

Conclusions

Comparing the five techniques for the upgrading of biogas with the goal ofinjecting it into the commercial gas grid is done in the previous section. Fromthat, it can be concluded that looking only at the cost price, high pressurewater scrubbing is the best option. But there is more than only cost priceto make a process succeed or fail. Therefore, some other criteria are set up.The waste stream inventory gives another picture. Pressure swing adsorptionand membrane separation are the only two techniques which have only onewaste stream, which can be cleverly used by driving a generator. The othertechniques have waste streams which need some waste treatment, which alsohave to be taken into account looking from an environmental and economicalpoint of view.

Furthermore, the yield and purity is of great importance. The purity ofthe upgraded biogas is comparable for most of the techniques, but membraneseparation has the lowest purity of 89.5% CH4. The other techniques have apurity of 98% CH4. The yield of methane achieved with chemical absorptionis the highest with 98%. Pressure swing adsorption, chemical absorption andcryogenic separation are at average, where membrane separation is the lowestwith 78% CH4 yield. Membranes in series increase the yield, but this resultsalso in an increase in costs.

Considering the ease of operation of each process, membrane separationand high pressure water scrubbing are the easiest processes to operate. Nocatalysts or chemicals are needed. Cryogenic separation has the problem ofthe need to work at very low temperatures and high pressures. Therefore,it needs to be controlled by an operator and some safety restrictions haveto be set, because of the high possibility of explosion. Chemical absorptionand pressure swing adsorption both need a catalyst in order to upgrade thebiogas. This catalyst has to be changed twice a year which leads to a shutdown.

35

From this all, it can be concluded that high pressure water scrubbingperforms the best. With the low cost price, high purity and yield it is apromising upgrading technique. Though one waste stream needs treatment,it is a continuous process which operates almost on it self.

RecommendationsFor further investigation we recommend the following subjects:

• The waste stream treatment is not considered in our investigation andcan influence the price of the upgraded biogas.

• For a better cost investigation more quotations should be acquired andmass balances should be made. Then, a more precise estimation of thecost per Nm3 biogas can be made.

• The CO2 footprint is mentioned in the report but not calculated. Whenmass balances are made the CO2 footprint can also be calculated.

• For chemical absorption it could be useful to look for more types ofcatalyst.

• The performance of the membrane separation is highly dependent onthe type of membrane used. An investigation of more types of mem-branes can be useful.

• Cryogenics is only investigated at one pressure and temperature. Wedo not know if this is the optimal condition and therefore the cryo-genics process should be investigated on a range of temperatures andpressures.

36

Acknowledgement

During our Multi Disciplinary Project we received help from may people.Without their help we would not have been able to successfully finish thisproject. Therefore our gratitude goes out to the following people.

First of all we want to thank Laura, our tutor. We greatly appreciatedLaura’s presence during our weekly meetings. Her input was always insight-ful. Although she was rather quiet, the things she said were well worthlistening to.

We would like to thank the people at Dirkse Milieutechnology, in particu-lar Robert Lems, Déborah Felisoni and Pieter-Durk van Jaarsveld for makingthis project possible and receiving us at DMT in Joure. We gathered lotsof useful information and had some good fun during our overnight stay inJoure.

Furthermore we thank René van den Kieboom for making the excursionto Tilburg possible and thank Maarten van der Heuvel and Olivier Kuijerfor arranging the excursion to Nuenen. Both excursions were a real additionto our project and gave us a good understanding of the reality of upgradingbiogas.

Also, we would like to thank Jetse Reijenga, our project coordinator, foralways being attentive to our work and his interest in our progress. Theshort introduction into building websites was quite helpful and resulted inan awesome website for our project.

37

Bibliography

[1] R. Lems, Biogasopwaardering: Het DMT-TS-PWS systeem, februari 2006

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

[3] Wahyudin, W., Biogas upgrading installation unit, 2007.

[4] Information collected from DMT

[5] TNO Environment, Energy and Process Innovation, CO2-recovery usingmembrane gas absorption, brochure

[6] P.S. Kumar, J.A. Hogendoorn, P.H.M. Feron, G.F. Versteegh, New ab-sorption liquids for the removal of CO2 from dilute gas streams usingmembrane contactors, Chem. Eng. 57, 2002, 1639 - 1651

[7] 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 Maringá, 2001

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

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

[10] 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

[11] Perry, R.H. en D. Green, Perry’s chemical engineers handbook, McGraw-Hill Book Company, USA, 6th print.

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

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[13] Eneco, http : //mkb.eneco.nl/productenentarieven/tarieven/tarieven.asp

[14] WMD, http : //www.wmd.nl/MijnWMD/TariefNota/TarievenGrootverbruik.html

[15] http://www.biotech-ind.co.uk/Methane-RGP-Process.htm, visited atthe 24th of February 2008

[16] O. Jönsson, M. Persson, Biogas as transportation fuel, Swedish Gas Cen-tre, 2003

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

[18] O. Smith, A. Westerberg, The optimal design of pressure swing adsorp-tion systems, Chemical engineering science, Vol. 45, No. 12, pp. 2967 -2976, 1991

[19] P. Cruz, J. Santos, F. Magalhães, A. Mendes, Cyclic adsorption sepa-ration processes : analysis strategy and optimization procedure, Chemicalengineering science, 58 (2003) 3143 - 3158

[20] Information from excursion to Cirmac in Nuenen

[21] http://www.kolumbus.fi/suomen.biokaasukeskus/en/enperus.html, vis-ited at the 8th of May 2008

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

[23] 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.

[24] Asim K. Guha, Sudipto Majumdar and Kamalesh K. Sirkar, A larger-scale study of gas separation by hollow-fiber-contained liquid membranepermeator, Journal of Membrane Science 62 (1991) 293-307

[25] K. Li and W.K. Teo, Use of an internally staged permeator in the en-richment of methane from biogas, Journal of Membrane Science 78 (1993)181-190

[26] Danial L. Ellig, Joseph B. Althouse and F.P. McCandless, Concentra-tion of methane from mixtures with carbon dioxide by permeation throughpolymeric films, Journal of Membrane Science 6 (1980) 259-263

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[27] S.A. Stern, B. Krishnakumar, S.G. Charati, W.S. Amato, A.A. Fried-man, D.J. Fuess, Performance of a bench-scale membrane pilot plant forthe upgrading of biogas in a wastewater treatment plant, Journal of Mem-brane Science 151 (1998) 63-74

[28] Carbon Trust, Carbon footprint measurement methodology, version 1.3,march 2007

[29] R. Lems, Upgrading biogas, 2008

[30] Brochure Biogas CHP, The use of biogas in Tilburg The Netherlands,2000

[31] Myken A., Jensen J., Dahli A., Final report, Adding Gas from Biomassto the Gas Grid, Contract No: XVII/4.1030/Z/99-412; Danish Gas Tech-nology centre a/s, Swedish Gas Center

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Appendix A

Alternate cost estimation PSA

The costs of the pressure swing adsorption depend on which type of adsor-bent material is used in the columns and the number of units used. Theoperational costs are influenced by the operating pressure, which on its turnis dependent on the adsorbent material. The compressor needed in the be-ginning in order to compress the incoming biogas is the last element whichcontributes to the cost of the whole plant significantly. Compression is ex-pensive and in order to make it profitable, it is needed to recover the requiredpressure. The pressure recovery can be enabled by several pressure valves.

Now, some assumptions are made to be able to make a cost estimation.The type of adsorbent material used in the PSA is a carbon molecular sieve.The choice for this adsorbent material can be explained by the ability ofremoving N2 and O2 from the biogas. The lifetime of the adsorbent materialis taken to be 3 to 4 years. Furthermore, there are four adsorber vesselsneeded in the plant. Figure 2.6 shows the overall scheme of the PSA-plant.The compressor, the four adsorber vessels, the vacuum pump and the H2S-removal step are included in the cost estimation. The several pressure valveswhich are required are included in the equations by the modular factor in it.

The costs of the PSA-plant are divided in two parts: operational costs andcapital costs. First, the operational costs will be calculated. The operationalcosts depend on the operating pressure, the flow rate, and the dimensions ofthe adsorber vessels. Therefore, first those parameters are determined. Theoperating pressure of the PSA-plant is 1 bar at the inlet and the productstream is at 5 bar. The bed length is set at 6 m with a diameter of 1,5 massuming cyclic steady state. The inlet flow is assumed to be 250 Nm3/h.

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The following equation from [19] determines the operational costs:

OC = Qin ·W · EC (A.1)

Where:

OC Operating costs [E/year]Qin Average inlet flow [Nm3/year]W Theoretical work [J/m3]EC Energy costs [E/J ]

The energy input amounts to 1,875 kWh. The energy price per kWhamounts to € 0.10. The operational costs become € 35,721-. The other part,the capital costs, can now be calculated. These are divided in three parts.The bed metal shell which includes the adsorber vessels, abbreviated withCShell; the costs of the compressor, CComp; and finally, the costs of the driverfor the compressor, CDriver. The following equations are taken from [18].

CShell = P 0.584 ·(4.93 · d · l + 3.74 · d2 + 739

)(A.2)

Where:

P Pressure [Pa]d Bed diameter [m]l Bed length [m]

The costs for the shell become € 425.500,-

CComp = 14.020 ·Q0.435in (A.3)

Qin Volumetric flow rate at the inlet [ft3/min]

The costs for the compressors become € 79,423-.

CDriver = 11.68 · hp1.61Comp + 2.470 · hp0.32

Comp (A.4)

hpComp Horse power of compressor [hp]; assumed to be 5 hp.

The costs for the driver of the compressor become € 2,771-.

In order to calculate the cost of the PSA-plant, also the investment costsfor the H2S-removal part has to be taken into account, therefore, the costsused for H2S-absorption is included which amount to € 516.000,-.

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The total capital costs are the cost of the bed metal shell summed up withthe costs of the compressor, the costs of the driver of the compressor and thecosts of the H2S-removing part. The total capital costs become € 1.023.694,-

The total running costs of the PSA can also be calculated by:

CAnnual =CCap

τpb

+ (1− tax) · Cop + dr · tax · CCap (A.5)

CCap Capital cost [E]τpb Pay back time [s]tax Tax rate [s]Cop Operating cost [E]dr Depreciation rate [−]

The pay back time is set at 3 years, which is equal to 94.608.000 seconds.The tax rate is assumed to be 0,6 and the depreciation rate 0,125. The totalrunning costs of the PSA-plant become € 282.616,-.

The final cost price per Nm3 biogas become: € 0,23.

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Appendix B

Cryogenic equipment

Compressor 1Manufacturer: Vilter (http : //www.vilter.com)Compressor type: oil flooded single screw compressorMotor power: 180KwPrice for complete package ready to work: €200,000

Compressor 2Manufacturer: Vilter (http : //www.vilter.com)Compressor type: oil flooded single screw compressorMotor power: 200KwPrice for complete package ready to work: € 250,000

Heat exchanger 1Price is calculated by http : //www.matche.com/EquipCost/Exchanger.htmHeat exchanger type: Condenser, vertical tubeArea: 70 ft2Internal pressure: 150 psiMaterial: Carbon Steel


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Appendix C

CO2 footprint

The carbon footprint is a method to measure the effect of a certain processon the environment in terms of the amount of green house gases producedduring the entire process. The process encompasses the whole life cycle of aproduct, thus from the production of the raw material to disposal of the finalproduct. This makes it a very extensive method. In order to produce sucha carbon footprint, a certain path needs to be followed. This methodologywill be explained in the following text. Because of the method being thatextensive, the carbon footprint is not calculated for every biogas upgradingtechnique separately.

Making a carbon footprint of a process will follow five major steps inorder to calculate the green house gases produced during the supply chain.Table C.1 gives a schematic overview of the methodology.

Step 1 Analyze internal product dataStep 2 Build supply chain process mapStep 3 Define boundary conditions and identify data requirementsStep 4 Collect primary and secondary dataStep 5 Calculate carbon emissions by supply chain process steps

Table C.1: The overview of the five major steps in order to calculate the carbonfootprint

These steps always have to be followed one by one and boundaries haveto be set. For instance, the carbon footprint can cover all the supply chainsteps from raw material to disposal, but this can be adjusted. Step 3 in themethodology takes care of that.

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Figure C.1: The steps used in order to produce the whole supply chain process map,picture taken from [28]

Step 1: Analyze internal product dataThe main goal of this first step is to develop a deeper understanding of theproduct. This implies determining what raw materials the product is madeof and which actions or process are needed to convert the raw material intothe desired final product. Next to that, the waste streams and the producedco-products have to be known. It is necessary carefully evaluate each step inthe entire process.

Step 2: Build supply chain process mapThe objective of the second step is to produce the whole supply chain processmap, which can be visualized using figure C.1.

Step 3: Define boundary conditions and identify data requirements Thethird step has two sub-objectives. First, the boundaries need to be set whichhave to be followed for the product. After that, the required data is neededin order to set up the mass balances and the carbon footprint.

Step 4: Collect primary and secondary dataFrom the data collected in the third step of the methodology, the requireddata is found. This data can be used in order to develop the mass balance

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and also to calculate the GreenHouse Gas emissions (GHG emissions) foreach step in the process.

Step 5: Calculate GHG emissions by supply chain process stepsNow all the required data is collected, a model can be designed to actuallycalculate the mass balance and the GHG emissions of each step in the process.

After the five steps, the carbon footprint is ready. It gives insight in theGHG emissions produced in the process. Then it is necessary to take a criticallook at the environmental performance of your process. When the emissionsare too large or harmful, the carbon footprint can help to design a solutionin order to reduce and control the produced greenhouse gas emissions.

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Appendix D

Visit to SMB Stortgas BV inTilburg

Since 1987, DMT has grown to be a multidisciplinary international and lead-ing company with important reference projects within the environmentalsector. DMT is expert in: air treatment and odor abatement systems, desul-phurization unit (both biological and chemical), ground water purificationand soil remediation plants, water treatment plants and aeration systemsand water management. At this moment DMT is developing its biogas up-grading technology. For SMB stortgas BV in Tilburg, a high pressure waterscrubbing plant is installed.

In Tilburg, a municipal association initiated a complex, including a land-fill gas installation, a biogas plant, and an upgrading plant which has beenrunning since 1994. The upgraded gas, which has natural gas quality, isinjected into the natural gas network. An association has been created in-volving 9 municipalities, of which Tilburg is the largest. The name is SMB(Samenwerkingsverband Midden Brabant) and the objective is to solve thewaste problem in the cities. In total, the 9 municipalities have 480,000 inhab-itants, who yearly produce 40,000 tons of organic waste. As a landfill biogastreatment plant was already present in Tilburg, SMB chose anaerobic diges-tion of the organic waste, which means Vegetable, Fruit and Garden waste,(VFG) [30]. In order to obtain more detailed information about the upgrad-ing processes of biogas and also to get answers to our questions regardingthis upgrading technique, we visited the SMB high pressure water scrubbingplant in Tilburg. Our visit took place on the 9th of April 2008. We arrangeda meeting with René van den Kieboom. The visit started with a generalpresentation about the upgrading plant in Tilburg, which was followed witha detailed explanation about each separation unit. After this presentationthere was the opportunity for asking our questions. Since this excursion was

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Figure D.1: The HPWS plant in Tilburg

planned in the middle of our project we had many questions, both about thetechnique of biogas upgrading as well as the treating method of the wastestreams and a cost estimation according to this technique. The questionswere extensively answered. Finally we went to the site to have a closer lookat different units of the plant, where the given presentation was coupled tomore detailed information about each separation unit of the plant. We talkedfurther about the different theoretical features of the process and how theyturn out to behave during operation. The picture shows the plant we visited.

AddressVloeiveldweg 105048 TD TilburgTelephone: 013-4556163

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Appendix E

Visit to Carbiogas BV in Nuenen

Cirmac International BV is a world-wide operating company, specialized ingas treatment systems for the petrochemical and chemical industry, refineriesand other industries. Cirmac is part of the Rosscor Group of companies.

In order to obtain more detailed information about the upgrading pro-cesses of biogas, we visited an installation build by Cirmac on the 20th ofMay 2008 in Tilburg at a site of Carbiogas BV. We arranged a meeting withIng. Olivier Kuijer and Ing. Maarten van den Heuvel in Nuenen.

Figure E.1: The VPSA plant in Nuenen

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The visit started with a presentation about the plant in Nuenen, in whichwe got insight in the processes of pressure swing adsorption and membraneseparation. At that moment we still missed some information about thecost estimation of a few upgrading processes. Our questions were answeredextensively, which gave us the opportunity to fill the gaps in our theories.

After the presentation, we went outside to take a look at the upgradinginstallation. The installation we viewed can be seen in the picture, it is avacuum pressure swing adsorption (VPSA) installation. We discussed thedifferent theoretical features of the process and how these turn out to behaveduring operation. Furthermore, we spoke about the waste streams, how theyare kept as low as possible and how they are disposed.

Finally, we walked to the top of a landfill, to have a look at the biogaswells and to see how a landfill is operated to obtain a large amount biogaswith the right conditions.

AddressGulberg 75674 TE NuenenTelephone: 040-839683

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comparing different biogas upgrading techniques

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