Upgrading biogas 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 hp://students.chem.tue.nl/ifp24/ [email protected] Comparing different biogas upgrading techniques Upgrading Techniques Membrane separation Cryogenic separation Pressure swing adsorption High pressure water scrubbing Chemical absorption Project Introduction to Biogas Component Dutch natural gas Volume % Canadian natural gas Volume % CH 4 81,3 94,9 C 2 H 6 2,85 2,50 N 2 14,3 1,60 CO 2 0,89 0,70 Density kg/m 3 0,83 0,75 Wobbe index MJ/m 3 43,7 49,5 Calorific value MJ/kg 31,7 37,8 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. Table 2: Biogas mainly consists of combustible CH 4 and non-combustible CO 2 . CH 4 combusts very cleanly with hardly any soot particles or other pollutants, making it a clean fuel. But CO 2 , the non-combustible part of the biogas, lowers its calorific value. CH 4 Volume % Calorific value MJ/kg Biogas 60 21.5 Methane 100 38.0 Cooler Compressor Cooler Compressor Cooler Distillation Column Product 91 % CH 4 8 % N 2 1% other Recirculation of product stream as cooling agent Waste 0.6 % CH 4 98 % CO 2 Biogas 25 o C 1 bar 37 % CH 4 58 % CO 2 5 % other -70 o C 1 bar 207 o C 21 bar -10 o C 21 bar 54 o C 40 bar -10 o C 40 bar -90 o C 40 bar Absorption column Regeneration column Heat exchanger Cooler Gas stream in Biogas out CO 2 Scrubber Absorber Water Atmosphere Water Treated Biogas Biogas Fe 2+ /EDTA Particle Separation Sulphur Regenerator Air Fe 3+ /EDTA H 2 S Removal Compressor Gas Conditioning Biogas Condensate Vacuum pump for regeneration Waste Gas CH 4 Production > 97 % CH 4 -rich gas Purge Gas Adsorption vessel Adsorption vessel Adsorption vessel H 2 S Removal Compressor Biogas Membrane separator Internally staged > 78 % CH 4 CO 2 (+ H 2 S) + 10 ~15 % CH 4 CO 2 (+ H 2 S) > 90 % CH 4 2 RNH 2 + CO 2 RNHCOO − + RNH + 3 CO 2 + OH − HCO − 3 RNH + 3 RNH 2 + H + H 2 O H + + OH − H 2 S + 1 2 O 2 (g ) → S + H 2 O H 2 S (g )+ H 2 O H 2 S (aq ) H 2 S (aq ) H + + HS − HS − H + + S 2− S 2− +2Fe 3+ S +2Fe 2+ 1 2 O 2 (g )+ H 2 O(l ) → 1 2 O 2 (aq ) 1 2 O 2 (aq )+2Fe 2+ → 2Fe 3+ +2OH − W obbe index = calorific value √ relative density is poster presents the results of a multidisci- plinary project executed at the Technical Uni- versity of Eindhoven commissioned by Dirkse Milieutechniek BV (DMT) focused on the up- grading of biogas. Biogas is a result of anaerobic digestion of or- ganic material, resulting in CH 4 and CO 2 gas and some pollutants. e CH 4 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 CO 2 and pollutants such as H 2 S. Currently, several processes are available for the upgrading. Dirkse Milieutechniek is devel- oping a biogas upgrading technology based on high pressure water scrubbing. 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. erefore, a lit- erature study was conducted to create a clear overview of the present upgrading techniques allowing for an objective com- parison. Several techniques were investigated and compared. e Wobbe index is a measurement for the combus- tion behaviour of a gas. If this value is too high or too low the combustion behaviour will be disturbed. H 2 S: biogas contains small amounts of H 2 S and some other pollutants. H 2 S is poisonous when in- haled. Furthermore, when water is present, H 2 S forms sulphuric acid (H 2 SO 4 ), which is high- ly corrosive, rendering the biogas unusable. e current use of fossil fu- els is rapidly depleting the natural reserves. e natu- ral formation of coal and oil however is a very slow process which takes ages. erefore, a lot of research effort is put into finding re- newable fuels nowadays to replace fossil fuels. Renew- able fuels are in balance with the environment and con- 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. e two main sources of bi- ogas are domestic garbage landfills and fermentation of manure and raw sewage. Upgrading biogas: Increasing calorific value • removing H • 2 S Carbon molecular sieve Gas molecules CH 4 N 2 /O 2 H 2 O/H 2 S CO 2 > 98% Four adsorber vessels operate in an alternating cycle to allow for continuous operation: 1. adsorption 2. regeneration 3. pressure build-up Chemical Absorption + Removal of H 2 S, H 2 O and CO 2 in one step H 2 and CO 2 are separated using dif- ferent absorption columns - Active carbon required for CO 2 recycling Other absorbents also possible High Pressure Water Scrubbing + Removal of H 2 S, H 2 O, CO 2 in one step is has to be done in two steps or the H 2 has to be removed om the water aſterwards + Dry gas at pressure. With use of silicates it will dry So, like chemical absorption a dry- ing step is still required - Large Water scrubbing can be imple- mented quite compactly. For large flows however, the scrubbing col- umn becomes increasingly larger. Pressure swing adsorption + Removal of H 2 S, H 2 O, CO 2 in one step For H 2 S removal an extra step is required because it poisons the adsorbent material Cryogenic separation - Relative high investment costs Investment costs are exceptionally high compared to the other techniques Membrane separation - Not a proven technique Cirmac already built a plant using mem- brane separation proving this technology - Chemicals required e membranes investigated do not require additional chemicals - Regeneration is energy expensive No regeneration is necessary. Mem- branes have a long lifetime - Relative high investment costs e costs of investment are lowest of the five techniques investigated High pressure water scrubbing is the cheapest option. • PSA and membrane separation waste streams are eas- • ily dealt with. e other techniques have waste streams which need more advanced waste treatment. Membrane separation and high pressure wa- • ter scrubbing are the easiest processes to oper- ate. No catalysts or chemicals are needed. Cryogenic separation works at very low temperatures and • high pressures which requeirs an operator and safety restric- tions have to be set making it and expensive technique. Overall, high pressure water scrubbing per- • forms the best for DMT: low cost price, high pu- rity and yield, only one waste stream needs treatment and it is a continuous process. is list compares the (dis)advantages of the techniques in the current opinion of DMT to the findings of the project group. is table shows the most important facts for each technique to allow for an easy and objective comparison. H 2 S removal: with separate removel step • also posible with certain membranes • CH 4 purity and yield highly de- pendent on choise of membrane Internally staged membrane increases purity is picture shows the results of a model made in the Aspen Plus soſtware package. Based on the physical absorption of dissolving gases in a liquid. e dissolubility of CO2 and H2S is much larger compared to the dissolubility of CH4. Also, the dissolubility of all compo- nents increases when pressure is higher. Regeneration of the iron-chelated solution: Formation of S: Absorption and dissocation of H 2 S: CO2 absorption using aqueous amino acid salt solutions: Main reaction: H 2 S CO 2 Technique Investment Running Cost price Maximum Maximum Advantages Disadvantages cost cost upgraded achievable achievable biogas yield purity /Nm 3 biogas % % Chemical ab- sorption 353,000 134,500 0.17 90 98 · Almost complete H 2 S re- moval · Only removal of one compo- nent in column · Expensive catalyst High pressure water scrubbing 265,000 110,000 0.13 94 98 · Removes gases and particu- late matter · Limitation of H 2 S absorption due to changing pH · High purity, good yield · H 2 S damages equipment · Simple technique, no spe- cial chemicals or equipment re- quired · Requires a lot of water, even with the regenerative process · Neutralization of corrosive gases Pressure swing adsorption 680,000 187,250 0.25 91 98 · More than 97% CH 4 enrich- ment · Additional complex H 2 S re- moval step needed · Low power demand · Low level of emissions · Adsorption of N 2 and O 2 Cryogenic sepa- ration 908,500 397,500 0.44 98 91 · Can produce large quantities with high purity · A lot of equipment is required · Easy scaling up · No chemicals used in the pro- cess Membrane 233,000 81,750 0.12 78 89.5 · Compact and light in weight · Relatively low CH 4 yield separation · Low maintenance · H 2 S removal step needed · Low energy requirements · Membranes can be expensive · Easy process
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Upgrading biogasProject group: Jos de Hullu, Jenny Maassen, Paul van Meel, Siamak Shazad and Jessica VaessenTutor: Laura Bini M.Sc. Coordinator: dr. ir. Jetse Reijenga
http://students.chem.tue.nl/ifp24/[email protected]
Comparing different biogas upgrading techniques
Upgrading Techniques
Conclusions
Comparison
Membrane separation
Cryogenic separation
Pressure swing adsorption
High pressure water scrubbing
Chemical absorptionProject
Introduction to Biogas
Component Dutchnatural gas
Volume %
Canadian natural gas
Volume %CH4 81,3 94,9C2H6 2,85 2,50N2 14,3 1,60CO2 0,89 0,70Densitykg/m3
0,83 0,75
Wobbe indexMJ/m3
43,7 49,5
Calorific value MJ/kg
31,7 37,8
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.
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.
CH4
Volume %Calorific value
MJ/kgBiogas 60 21.5Methane 100 38.0
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
Absorption column
Regenerationcolumn
Heat exchanger
Cooler
Gasstream in
Biogas out
CO2
Scru
bber
Abso
rber
Water
Atmosphere WaterTreatedBiogas
Biogas
Fe2+/EDTA
ParticleSeparation
Sulphur
Reg
ener
ator
Air
Fe3+/EDTA
H2S
Rem
ovalCompressor
GasConditioning
Biogas
CondensateVacuum pumpfor regeneration
WasteGas
CH4Production
> 97 % CH4-rich gas
PurgeGas
Ads
orpt
ion
vess
el
Ads
orpt
ion
vess
el
Ads
orpt
ion
vess
el
H2S
Rem
oval
Compressor
Biogas
Membrane separator
Internally staged
> 78 % CH4
CO2 (+ H2S)+ 10 ~15 % CH4
CO2 (+ H2S)
> 90 % CH4
2 RNH2 + CO2 RNHCOO− +RNH+3 (1)
CO2 +OH− HCO−3 (2)
RNH+3 RNH2 +H
+ (3)H2O H+ +OH− (4)
1
H2S +1
2O2(g)→ S +H2O (1)
H2S(g) +H2O H2S(aq) (2)H2S(aq) H+ +HS− (3)
HS− H+ + S2− (4)
S2− + 2Fe3+ S + 2Fe2+ (5)
1
2O2(g) +H2O(l) → 1
2O2(aq) (6)
1
2O2(aq) + 2Fe2+ → 2Fe3+ + 2OH− (7)
1
Wobbe index =calorific value√relative density
(1)
1
This poster presents the results of a multidisci-plinary project executed at the Technical Uni-versity of Eindhoven commissioned by Dirkse Milieutechniek BV (DMT) focused on the up-grading of biogas.
Biogas is a result of anaerobic digestion of or-ganic 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.
Currently, several processes are available for the upgrading. Dirkse Milieutechniek is devel-oping a biogas upgrading technology based on high pressure water scrubbing.
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 lit-erature study was conducted to create a clear overview of the present upgrading techniques allowing for an objective com-parison. Several techniques were investigated and compared.
The Wobbe index is a measurement for the combus-tion behaviour of a gas. If this value is too high or too low the combustion behaviour will be disturbed.
H2S: biogas contains small amounts of H2S and some other pollutants. H2S is poisonous when in-haled. Furthermore, when water is present, H2S forms sulphuric acid (H2SO4), which is high-ly corrosive, rendering the biogas unusable.
The current use of fossil fu-els is rapidly depleting the natural reserves. The natu-ral formation of coal and oil however is a very slow process which takes ages. Therefore, a lot of research effort is put into finding re-newable fuels nowadays to replace fossil fuels. Renew-able fuels are in balance with the environment and con-
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 bi-ogas are domestic garbage landfills and fermentation of manure and raw sewage.
Upgrading biogas:Increasing calorific value•removing H• 2S
Carbonmolecularsieve
Gas moleculesCH4N2/O2H2O/H2SCO2
> 98%
Four adsorber vessels operate in an alternating cycle to allow for continuous operation:1. adsorption2. regeneration3. pressure build-up
Chemical Absorption+ Removal of H2S, H2O and CO2 in one stepH2 and CO2 are separated using dif-ferent absorption columns- Active carbon required for CO2 recyclingOther absorbents also possible
High Pressure Water Scrubbing+ Removal of H2S, H2O, CO2 in one stepThis 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
So, like chemical absorption a dry-ing step is still required- LargeWater scrubbing can be imple-mented quite compactly. For large flows however, the scrubbing col-umn becomes increasingly larger.
Pressure swing adsorption+ Removal of H2S, H2O, CO2 in one stepFor H2S removal an extra step is required because it poisons the adsorbent material
Cryogenic separation- Relative high investment costs
Investment costs are exceptionally high compared to the other techniques
Membrane separation- Not a proven techniqueCirmac already built a plant using mem-brane separation proving this technology- Chemicals requiredThe membranes investigated do not require additional chemicals- Regeneration is energy expensiveNo regeneration is necessary. Mem-branes have a long lifetime- Relative high investment costsThe costs of investment are lowest of the five techniques investigated
High pressure water scrubbing is the cheapest option.•
PSA and membrane separation waste streams are eas-•ily dealt with. The other techniques have waste streams which need more advanced waste treatment.
Membrane separation and high pressure wa-•ter scrubbing are the easiest processes to oper-ate. No catalysts or chemicals are needed.
Cryogenic separation works at very low temperatures and •high pressures which requeirs an operator and safety restric-tions have to be set making it and expensive technique.
Overall, high pressure water scrubbing per-•forms the best for DMT: low cost price, high pu-rity and yield, only one waste stream needs treatment and it is a continuous process.
This list compares the (dis)advantages of the techniques in the current opinion of DMT to the findings of the project group.
This table shows the most important facts for each technique to allow for an easy and objective comparison.
H2S removal:with separate removel step•
also posible with certain membranes•
CH4 purity and yield highly de-pendent on choise of membrane
Internally staged membrane increases purity
This picture shows the results of a model made in the Aspen Plus software package.
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 compo-nents increases when pressure is higher.
Regeneration of the iron-chelated solution:
Formation of S:
Absorption and dissocation of H2S:
CO2 absorption using aqueous amino acid salt solutions:
Main reaction:
H2S CO
2
Technique Investment Running Cost price Maximum Maximum Advantages Disadvantagescost cost upgraded achievable achievable
biogas yield purity /Nm3 biogas % %
Chemical ab-sorption
353,000 134,500 0.17 90 98 · Almost complete H2S re-moval
· Only removal of one compo-nent in column· Expensive catalyst
High pressurewater scrubbing
265,000 110,000 0.13 94 98 · Removes gases and particu-late matter
· Limitation of H2S absorptiondue to changing pH
· High purity, good yield · H2S damages equipment· Simple technique, no spe-cial chemicals or equipment re-quired
· Requires a lot of water, evenwith the regenerative process
· Neutralization of corrosivegases
Pressure swingadsorption
680,000 187,250 0.25 91 98 · More than 97% CH4 enrich-ment
· Additional complex H2S re-moval step needed
· Low power demand· Low level of emissions· Adsorption of N2 and O2
Cryogenic sepa-ration
908,500 397,500 0.44 98 91 · Can produce large quantitieswith high purity
· A lot of equipment is required
· Easy scaling up· No chemicals used in the pro-cess
Membrane 233,000 81,750 0.12 78 89.5 · Compact and light in weight · Relatively low CH4 yieldseparation · Low maintenance · H2S removal step needed
· Low energy requirements · Membranes can be expensive· Easy process
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