Biogas Poster Web x5k5h

Upgrading biogas Comparing different biogas upgrading techniques Project group: Jos de Hullu, Jenny Maassen, Paul van Meel, Siamak Shazad and Jessica Vaessen Tutor: Laura Bini M.Sc. Coordinator: dr. ir. Jetse Reijenga

Introduction to Biogas Upgrading biogas: • Increasing calorific value • removing H2S

Component

The Wobbe index is a measurement for the combustion behaviour of a gas. If this value is too high or too low the combustion behaviour will be disturbed. calorif ic value W obbe index = √ relative density

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CH4 C2H6 N2 CO2 Density kg/m3 Wobbe index MJ/m3 Calorific value MJ/kg

Treated Biogas

CO2 absorption using aqueous amino acid salt solutions:

Fe3+/EDTA

Biogas

Particle Separation

Water

Air

Cooler

Fe2+/EDTA

Main reaction:

Dutch natural gas Volume % 81,3 2,85 14,3 0,89

Canadian natural gas Volume % 94,9 2,50 1,60 0,70

0,83

0,75

43,7

49,5

31,7

37,8

2 RN H2 + CO2 CO2 + OH − RN H3+ H2 O

   

RN HCOO− + RN H3+ HCO3− RN H2 + H + H + + OH −

Sulphur

1 H2 S + O2 (g) → S + H2 O 2

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H2 S(g) + H2 O  H2 S(aq) H2 S(aq)  H + + HS − HS −  H + + S 2−

(2) (3) (4)

S 2− + 2F e3+  S + 2F e2+

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Biogas out CO2

Absorption and dissocation of H2S:

Formation of S:

Table 1: requirements for injection in a gas grid. To reach the calorific value of Dutch natural gas the methane purity should be > 88%. But if Canadian standards must be achieved the calorific value of biogas should be increased above the calorific value of methane therefore some higher alkanes must be added to the gas.

H2S: biogas contains small amounts of H2S and some other pollutants. H2S is poisonous when inhaled. Furthermore, when water is present, H2S forms sulphuric acid (H2SO4), which is highly corrosive, rendering the biogas unusable.

Water

Regenerator

Biogas is a result of anaerobic digestion of organic material, resulting in CH4 and CO2 gas and some pollutants. The CH4 can be used as a green energy source by upgrading biogas to natural gas quality and injecting it into the existing gas grid or to using it as a fuel. Upgrading of biogas signifies removal of the CO2 and pollutants such as H2S.

To get a leading position in the market it is of most importance to know the advantages and disadvantages of all the different processes available for upgrading biogas and their cost. Therefore, a literature study was conducted to create a clear overview of the present upgrading techniques allowing for an objective comparison. Several techniques were investigated and compared.

Atmosphere

Absorber

Currently, several processes are available for the upgrading. Dirkse Milieutechniek is developing a biogas upgrading technology based on high pressure water scrubbing.

This poster presents the results of a multidisciplinary project executed at the Technical University of Eindhoven commissioned by Dirkse Milieutechniek BV (DMT) focused on the upgrading of biogas.

Chemical absorption Upgrading Techniques H2S CO2 Scrubber

Project

http://students.chem.tue.nl/ifp24/ [email protected]

Absorption column Regeneration column

Regeneration of the iron-chelated solution: 1 1 O2 (g) + H2 O(l) → O2 (aq) 2 2 1 O2 (aq) + 2F e2+ → 2F e3+ + 2OH − 2

Heat exchanger

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Gas stream in

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High pressure water scrubbing Based on the physical absorption of dissolving gases in a liquid. The dissolubility of CO2 and H2S is much larger compared to the dissolubility of CH4. Also, the dissolubility of all components increases when pressure is higher.

> 98%

1

1

Calorific value MJ/kg 21.5 38.0

This table shows the most important facts for each technique to allow for an easy and objective comparison. Technique

Chemical sorption

ab-

High pressure water scrubbing

Pressure swing adsorption

Investment cost

Running cost

 353,000

265,000

1

680,000

Cryogenic separation

908,500

Membrane separation

233,000

 134,500

Cost price upgraded biogas /N m3 biogas 0.17

Maximum achievable yield % 90

Maximum achievable purity % 98

110,000

0.13

94

98

187,250

397,500

81,750

0.25

91

0.44

98

0.12

78

98

91

89.5

Advantages

High Pressure Water Scrubbing + Removal of H2S, H2O, CO2 in one step This has to be done in two steps or the H2 has to be removed from the water afterwards + Dry gas at pressure. With use of silicates it will dry

· Removes gases and particulate matter · High purity, good yield · Simple technique, no special chemicals or equipment required · Neutralization of corrosive gases

· Can produce large quantities · A lot of equipment is required with high purity · Easy scaling up · No chemicals used in the process · · · ·

Cryogenic separation - Relative high investment costs

Compact and light in weight Low maintenance Low energy requirements Easy process

• Cryogenic separation works at very low temperatures and high pressures which requeirs an operator and safety restrictions have to be set making it and expensive technique. • Overall, high pressure water scrubbing performs the best for DMT: low cost price, high purity and yield, only one waste stream needs treatment and it is a continuous process.

CH4 Production

Gas Conditioning

Waste Gas Vacuum pump for regeneration

Cryogenic separation Biogas 25 oC 1 bar 37 % CH4 58 % CO2 5 % other

This picture shows the results of a model made in the Aspen Plus software package. Recirculation of product stream as cooling agent

-70 oC 1 bar Cooler

207 oC 21 bar

Compressor

-10 oC 21 bar

Cooler

54 oC 40 bar

Compressor

-10 oC 40 bar

-90 oC 40 bar

Cooler Distillation Column

Investment costs are exceptionally high compared to the other techniques Membrane separation - Not a proven technique Cirmac already built a plant using membrane separation proving this technology - Chemicals required The membranes investigated do not require additional chemicals - Regeneration is energy expensive No regeneration is necessary. Membranes have a long lifetime - Relative high investment costs The costs of investment are lowest of the five techniques investigated

Purge Gas

Condensate

· Relatively low CH4 yield · H2 S removal step needed · Membranes can be expensive

Conclusions

• Membrane separation and high pressure water scrubbing are the easiest processes to operate. No catalysts or chemicals are needed.

Carbon molecular sieve

· Limitation of H2 S absorption due to changing pH · H2 S damages equipment · Requires a lot of water, even with the regenerative process

· More than 97% CH4 enrich- · Additional complex H2 S rement moval step needed · Low power demand · Low level of emissions · Adsorption of N2 and O2

Pressure swing adsorption + Removal of H2S, H2O, CO2 in one step For H2S removal an extra step is required because it poisons the adsorbent material

• PSA and membrane separation waste streams are easily dealt with. The other techniques have waste streams which need more advanced waste treatment.

Biogas

> 97 % CH4-rich gas

Gas molecules CH4 N2/O2 H2O/H2S CO2

· Almost complete H2 S re- · Only removal of one compomoval nent in column · Expensive catalyst

So, like chemical absorption a drying step is still required - Large Water scrubbing can be implemented quite compactly. For large flows however, the scrubbing column becomes increasingly larger.

• High pressure water scrubbing is the cheapest option.

Compressor

Disadvantages

This list compares the (dis)advantages of the techniques in the current opinion of DMT to the findings of the project group. Chemical Absorption + Removal of H2S, H2O and CO2 in one step H2 and CO2 are separated using different absorption columns - Active carbon required for CO2 recycling Other absorbents also possible

Four adsorber vessels operate in an alternating cycle to allow for continuous operation: 1. adsorption 2. regeneration 3. pressure build-up

Waste 0.6 % CH4 98 % CO2

Product 91 % CH4 8 % N2 1% other

Membrane separation H2S removal: • with separate removel step • also posible with certain membranes

CO2 (+ H2S) + 10 ~15 % CH4

CH4 purity and yield highly dependent on choise of membrane Internally staged membrane increases purity Biogas

Compressor

> 78 % CH4 Membrane separator CO2 (+ H2S) > 90 % CH4

Internally staged

H2S Removal

Comparison

Pressure swing adsorption

Adsorption vessel

Table 2: Biogas mainly consists of combustible CH4 and non-combustible CO2. CH4 combusts very cleanly with hardly any soot particles or other pollutants, making it a clean fuel. But CO2, the non-combustible part of the biogas, lowers its calorific value.

Adsorption vessel

Biogas Methane

CH4 Volume % 60 100

Adsorption vessel

tribute to a far lesser extent to the greenhouse effect. Biogas is such a renewable fuel. It is a combustible gas mixture produced by the anaerobic fermentation of biomass by bacteria and takes a short time to form. The two main sources of biogas are domestic garbage landfills and fermentation of manure and raw sewage.

H2S Removal

The current use of fossil fuels is rapidly depleting the natural reserves. The natural formation of coal and oil however is a very slow process which takes ages. Therefore, a lot of research effort is put into finding renewable fuels nowadays to replace fossil fuels. Renewable fuels are in balance with the environment and con-

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