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Substitute natural gas (SNG)from Biomass
Dr. A K Gupta
Ex. Scientist ( Directors Scale)IIP, Dehradun,India
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What is SNG ?
SNG is subtitute natural gas derived from biomass/
organic wastes. Since it is obtained from renewable
resources it is also known as green gas
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SNG Production:
There are four options for producing SNG fro biomass/
waste organic matter (MSW):
1. SNG production by upgrading Biomass from AnaerobicDigestion
2. SNG production by combined biomass
gasification/methanation process.
3. SNG production by biomass hydrogasification process
4. Cogeneration as well as stand-alone production of F-Tliquids and SNG
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SNG production by upgrading Biomass from
Anaerobic Digestion
Anaerobic digestion is a biological process by which organic
wastes, (in absence of air ) is converted to biogas i.e. a mixture of
methane(40-75 mol%) and CO2.
Simplified stoichiometery for anaerobic digestion of biomass is:
C6H10O5 + H2O 3 CH4 + 3 CO2
The process is based on the breakdown of organic macro-molecules
of biomass by naturally occurring micro-organisms.
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SNG production
The process involves four steps:
HYDROLYSIS:
cellulose, starch, proteins and lipids are hydrolysed to solublecompounds such as sugars.
FERMENTATION:
soluble compounds are converted to different compounds such asamino acids, fatty acids, alcohols, CO2, H2, NH3 and H2S.
ACETOGENESIS:
fermentation products are converted to a mixture of H2, lowmolecular weight acids (primarily acetic acid) and CO2.
METHANOGENESIS:
products of acetogenesis step are reacted together to producemethane.
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SNG production..
First three steps in anaerobic digestion are relatively fast, while the
methanogenesis is slow and more sensitive process.
Compared to thermal processes, the residence time of biomassdigester is relatively long (seconds verses weeks).
Optimal condition for methanogene bacteria is essential for good
anaerobic conversion.
The activity of methanogene bacteria is highest within two
temperature ranges:30 40 oC (mesopillic condition) and 50 70 oC (thermophillic
condition).
Other important conditions are :
Neutral pH (6.6 8)
Low concentrations of ammonia and heavy metals
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SNG production..
Typically 30-60% of input biomass is converted to
biogas
Co-products consist of undigested residue (sludge) and
various water soluble substances.
The ratio of CH4/CO2 depends upon the composition of
feedstock.
Conversion of ethanol gives CH4/CO2 ratio of 3
Oxalic acid results in a CH4/CO2 ratio of 1/7.
Mixtures of biomasses is therefore used to get higher
biogas yields and higher methane fraction
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SNG production..
Development status:
Anaerobic digestion is a well established technology for waste
treatment, and generally available on commercial scale.
Millions of anaerobic digesters (commonly known as biogas
plants) have been built around the world, most of which are very
small, built in developing countries.
Large scale digesters have been built in France, Germany, and
Belgium for treating Municipal Solid waste (MSW) and farm based
digesters dotted around Europe. In UK use of land fill gas is
significant.
The technology is fully commercial.
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Digestion systems:
Digestion systems can be divided in wet and drysystems.
WET SYSTEMS:
In wet systems the process takes place in two separate
and independently controlled reactors.Average residence time is between 10 and 20 days.
Acid formation (hydrolysis, fermentation and
acetogenesis) occurs in first reactor.
After which the soluble products are led to secondreactor where biogas is produced.
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DRY SYSTEMS:
In dry systems the digestion is done in one reactor.
These systems can be operated in batch or continuous
mode.
Average residence time is between 2 and 4 weeks The dry matter (dm) content is between 20 to 25%. In
case of higher dm content of feedstock, water should be
added to achieve the desired dm% for digestion residue.
The removed water can be recycled or discharged to
sewer.
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Digestive Systems:
Digestion
Wet
Continuous
CSTR
Thermophillic Mesophillic
BTAVagron
PAQUES
BIOTHANE
DRY
Batch/continuous CSTR
VALORGAPlug-flow
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Biogas applications:
Besides conventional application of biogas for heat
production, it can also be used for combined heat and
power (CHP) application, or upgraded to natural gas
quality.
An important disadvantage biogas engines, compared to
application of fuel cell systems is high NOx emission of
gas engines. In order to satisfy the emission norms
additional costly flue gas clean-up (deNOx systems will
be necessary
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Pipeline quality gas from biogas:
To obtain pipeline quality gs biogas must pass through
two major processes:
1. Removal of trace components harmful to natural gas grid,
appliances or end-users.
2. An upgrading process in which calorific value, Wobbe-index
and other parameters are adjusted to meet pipeline
specifications.
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Biogas cleaning:
Gas cleaning includes removal of H2S, water, halogenatedhydrocarbons, NH3, O2, organic silicon compounds.
water removal:
drying methods include..
refrigeration, adsorption on silica gel, aluminium oxide, magnesiumoxide, zeolites.
absorption with glycol or triethylene glycol Halogenated hydrocarbons:
with activated carbon
Ammonia:
with activated charcoal, adsorption and water scrubbing
Oxygen removal:
membrane separation , PSA
Organic Silicon compounds:
Absorption in a liquid media
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Hydrogen sulphide removal:
In situ reduction in digester: either with metal ion to
form insoluble metal sulphide or oxidised to elemental
sulphur ( levels of 100-150 ppm)
Removal with metal oxides or hydroxides: Ironoxide/hydroxide and Zinc oxide.
Removal by oxidation with air: Addition of 5-10% air to
biogas results in biological conversion of H2S to
sulphur.(levels 50-100 ppm)
Removal by adsorption on activated carbon: Carbon
impregnated with KI or H2SO4 at ambient temperature.
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Upgrading:
The upgradation process is basically a separation of
methane and CO2 of biogas to obtain pipeline quality
with respect to calorific value, Wobbe-index, relative
density etc. Possible upgrading processes are:
1. Membrane separation
2. Pressure swing adsorption (PSA)
3. Absorption without chemical reaction water wash with or
without regeneration, Selexol process.
4. Absorption with chemical reaction alkali, amines5. Cryogenic removal of CO2
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Wobbe-index:
Wobbe index(W) is defined as ratio of gross calorific
value (HHV) to the square root of relative density.
W =HHV
dg - dair
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SNG production by combined biomass
gasification/methanation process:
Gasification
Gas
Clean-up
Methanation
Gas conditioning
SNG
Biomass
Steam(O2)
SNG production by combined biomass gasification/ methanation process
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Gasification:
Gasification is a thermochemical process that converts
soild fuel such as biomass into a gaseous fuel consisting
mainly of H2 and CO and commonly known as Syn gas.
The produced gas can be used as: Fuel for heating/power generation,
As feed stock for chemical industries,
To produce liquid fuels( FT-process)
To produce H2 (after reforming and shift reaction)
Upgraded to SNG (after additional methanation/gas conditioningsteps)
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Gasification Technologies:
Suitability of different gasification technologies is
dependent on:
Type of fuel
Scale of installation
Fuel gas application
Most important characteristics are:
the way heat is supplied to the gasifier,
Operating pressure Reactor type
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Heat supply to the gasifier:
Direct or indiret
In autothermal or direct gasification the required heat
can be supplied by combustion of part of feedstock
within the gasifier. Air, enriched air or O2 can be used asgasifying agent.
In case of external heat source, indirect gasification
steam is used as gasifying agent.
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Operating pressure of gasifier:
Most gasifiers are operated at atm. pressure. In some
systems pressurised gasification is used with gas
application in gas turbines.
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Gasification Reactor types:
Fixed-bed
Fluidized-bed
Entrained flow reactor
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Fixed-bed reactors:
In fixed-bed reactors the feedstock is fed at the top and
moves down slowly as a result of conversion in the lower
layers.
Fixed-bed reactors are suitable for small scale operationup to about 10 MW.
Fixed bed reactors are not suitable for large scale SNG
production by biomass gasification.
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Fluidized-bed gasifiers:
In fluidized bed reactors the crushed feedstock particles
are fluidized by gasification agent which is fed at the
bottom of the reactor.
In bubbling fluidized bed gasifier the feedstock alongwith inert bed material ( mostly sand) is fluidized with the
help of gasifying agent fed at the bottom.
In circulating fluidized bed gasifier the velocity of
gasifying agent is so high that the feedstock particles
and bed material are circulated in a system consisting ofreactor vessel, cyclone, and feedback pipe.
The product gas has high methane and C2 content.
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Entrained flow gasifiers (EFG):
In EFG pulverized feedstock is fed to the reactor
pneumatically.
O2 is used as gasifying agent instead of air to achieve
high conversion. Gasification takes place at temperatures above melting
point of ash which leaves the reactor as molten slag.
Due to high temperature (>1300 oC) no tar is formed.
The product gas has high concentration of CO andH2and no methane or C2 fractions. Thus EFG is not
suitable option for SNG production
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Methanation:
In methanation step CO and CO2 are converted to
methane:
CO + 3 H2 CH4 + H2O
CO2 + 4 H2 CH4 + 2 H2O Depending upon the reaction conditions either forward or
backward water-gas shift reaction takes place:
CO + H2O CO2 + H2
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Side reactions:
At the inlet of methanator thermodynamically carbon canform
2CO CO2 + C
CO + H2 H2O + C
Also cracking reactions of alkenes and aromatics canlead to carbon formation.
To avoid carbon formation higher H2/CO ratios and
steam is used
C + 2H2 CH4
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Catalysts for methanation:
Nickel is most commonly used catalyst
Other metals catalyse this reaction include metals of
Group VIII, Molybdenum (Group VI) and Silver (Group I).
Ni is cheap, very active and selective to CH4. Ni is poisoned by Sulphur, requiring thorough gas clean
up.
The commercial catalyst is NiO supported on oxides
such as alumina, silica gel, MgO, together with calciumaluminates, and reduced to Ni for use.
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Methanation reactors:
Methanation reactions of both CO and CO2 are highly
exothermic.
High release of heat strongly affects the reactor type
and design. To avoid high temperatures in the reactor which leads
to catalyst deactivation, several types of reactors are
proposed to be used.
1. Equilibrium limited reactor
2. Through -wall cooled reactor
3. Steam moderated reactor
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Methanation reactors.
In Equilibrium limited reactorthe temperature is controlled by recycling
large amount of gas ( max. temperature aprox 450 oC). Operated
adiabatically with preheated feed; multi-stages are used.Most commercial methanation plants have been built on this concept
(example: Lurgi SNG process)
The temperature in through wall cooled reactor is controlled by
absorbing heat in boiling water outside the catalyst filled tube bundle
and recycling the cold gas.Complete methanation can be achieved in one reactor;
Scaleup is simple.( example: IGT cold gas recycle process)
Steam moderated reactorinvolves the combinatin of gas shift and
methanation reaction with steam in one reactorNeed for recycle gas is eliminated
(Example: ICI process, Krupp-Koppers process)
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Gas clean-up:
The product gas from gasifier has to be cleaned beforeentering into the methanator.
1. Particles:
Particles can deposite on the catalyst.
Cyclone separators, Ceramic candle filters, Electro-staticprecipators (ESP), bag-house filters and scrubbers.
2. Hydrocarbons/Tars:
May affect metanation activity of the catalyst if present insufficient amounts
Cracking of tar using dolomite at around 1000oC, filters,scrubbing, activated carbon/zeolite filters
3. Nitrogen compounds:
scrubbing at low pH4. H2S and Halogen compounds:
Scrubbing at high pH, With high H2S content desulfurisationprocesses as used inrefinery or coal gasification are used.
Selexol process,MDEA process, Sulfinol process etc.
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SNG production by biomass hydrogasification
process:
Hydrogen and biomass are fed to the reactor operating
at 30 bar and 800-850oC.
Due to exothermic reactions this reactor can be operated
autothermally. ( Example Deutshe Montan Tehnologies(DMT)
Hydro-
gasification
Gas
Clean-up
Gas
conditioning
Biomass
H2 (rich) gas
Final
Methanation
SNG
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SNG co-production by conversion of biomass
through Fischer-Tropsch/metanation:
In this process syn gas produced in the gasifier is
converted to liquid fuel by once through F-T process.
The off gas is from F-T process which containsunconverted CO and H2 and methane produced in
gasification and C1 C4 hydrocarbons produced in F-T
synthesis is upgraded by methanation and CO2 removal.
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Cogeneration as well as stand-alone production of
F-T liquids and SNG
GasificationF-T
SynthesisMethanation
CO2
removal
SNG
CO2
F-T liquids
Biomass
Gasification
Gasification
Biomass
F-T
synthesis
Methanation CO2removal
SNG
F-T liquids
CO2
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F-T reactions:
CO + 2 H2 - (CH2)- + H2O
Catalysts are CO and Fe based
Typical conditions for producing long chain hydrocarbonproducts:
Temperature 200-250 oC
Pressure 25-60 bar
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Conclusions:
Substitute natural gas (SNG) can play an important role inrealisation of climate and energy security.
The produced SNG can most suitably be used for heat
and power production in domestic and industrial sectors.
It should also be possible to use SNG as a substitute for
natural gas in transport sector.
Anaerobic digestion is a proven technology being applied
for small scale centralised conversion of wet biomass at
its origin.
Over all energetic efficiency of F-T SNG co-prodution ispractically equal to the stand alone SNG option.
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Conclusions.
The energetic efficiency of SNG production by biomass
hydrogasification is higher.
Capital investment in F-T SNG option is higher than instand alone SNG option.
Production cost in Hydrogasification option are lower
than in case of SNG production by biomassgasification/methanation
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