1. Upgrading upgrading techniques
biogas
Comparing different 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
Water
Treated
Biogas
CO2 absorption using aqueous amino
acid salt solutions:
Fe3+/EDTA
Biogas
Particle
Separation
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/
mdp1.st@tue.nl
Air
Cooler
Fe2+/EDTA
Main reaction:
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
(1)
CH4
C2H6
N2
CO2
Density
kg/m3
Wobbe index
MJ/m3
Calorific value
MJ/kg
(1)
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
+
RN HCOO− + RN H3
−
HCO3
RN H2 + H +
H + + OH −
(2)
(3)
(4)
(5)
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.
1
H2 S + O2 (g) → S + H2 O
2
S 2− + 2F e3+  S + 2F e2+
Upgrading biogas:
• Increasing calorific value
• removing H2S




Sulphur
H2 S(g) + H2 O  H2 S(aq)
H2 S(aq)  H + + HS −
HS −  H + + S 2−
Introduction to Biogas
2 RN H2 + CO2
CO2 + OH −
+
RN H3
H2 O
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
(6)
Gas
stream in
(7)
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
Investment
cost
ab-
High
pressure
water scrubbing
Pressure swing
adsorption
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-
(1)
(2)
(3)
(4)