Substitute SNG From Biomass

8/7/2019 Substitute SNG From Biomass

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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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Substitute SNG From Biomass

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