Biomass gasification

1. BIOMASS GASIFICATION
S K Singh
Centre for Energy Studies
IIT Delhi

2. OVERVIEW:
• What is biomass ?
• Introduction to Gasification.
• Basic chemistry.
• Feed-Stock properties.
• Feed-Stock treatment.
• Gasification – Types
• Syngas conditioning
• Biological Gasification
• Advantages
• Disadvantages

3. WHAT IS BIOMASS ?
• Biomass is organic material that comes from plants and animals.
• Biomass includes wood, agricultural waste and cow dung.
• Biomass is probably our oldest source of energy after the sun. For thousands of years,
people have burned wood to heat their homes and cook their food.
• Biomass contains stored chemical energy from sun.

4. INTRODUCTION TO GASIFICATION
• Basic process of gasification is conversion of biomass into combustible gas mixture.
• It is done by partial oxidation of biomass at very high temperatures in a gasification
medium.
• Gasification medium could be air, oxygen, steam…etc.
• It is not a single step process like combustion.
• Biomass Gasification occurs through a sequence of complex thermo-chemical reactions.

5. BASIC CHEMISTRY
• Partial Oxidation : C + 0.5 O2 CO ( dH = -268 MJ/Kg Mole )
• Complete Oxidation : C + O2 CO2 ( dH = -406 MJ/Kg Mole )
• Water gas reaction : C + H2O CO + H2 ( dH = +118 MJ/Kg Mole )
• Water gas Mixture Shift Reaction : CO + H2O CO2 + H2 ( dH = -42 MJ/Kg Mole )
• Methane Formation : CO + 3H2 CH4 + H2O ( dH = -88 MJ/Kg Mole )
• All above reactions are in equilibrium, can proceed in either direction depending on temperature,
pressure, concentration of reactant.
• Product gas from gasification is a mixture of CO, CO2, CH4, H2, H2O(g)..

6. BASIC CHEMISTRY
• Three Product gas qualities can be obtained by varying a) Gasifying Agent b) Method of
operation c) Operation Conditions.
Low CV 4 – 6 MJ/N m^3 Air /Steam + Air
Medium CV 12 – 18 MJ/N m^3 O2 + Steam
High CV 40 MJ/N m^3 O2+ Steam, H2

7. FEED STOCK PROPERTIES
• Moisture Content : More moisture content affects the gasifier performance, makes
ignition difficult and results in low CV value of product gas mixture.
• More moisture content can lower temperature in oxidation zone resulting in the
incomplete cracking of the hydrocarbons releases from pyrolysis zone.
• Higher level of moisture content and the presence of CO produces H2 by the “ Water gas
shift reaction” and in turn the rise in the content of H2 leads to more methane formation
by direct hydrogenation.

8. FEEDSTOCK PROPERTIES
• Ash Content : Higher mineral matter poses a great challenge.
• Oxidation temperature is often above the melting point of biomass ash, leading to
clinkering / slagging problem and subsequent feed blockages.
• Clinker occurs when ash content is above 5% which essentially leads to formation of
oxides.

9. FEED STOCK PROPERTIES
• Volatile Compounds : Most of the volatiles that are formed here are not healthy for
humans.
• They produce lot of irritants, cause lot of skin rashes, lungs related issues.
• It is very harmful from environment point of view. We should be able to destroy or
remove tars that are formed from terpenoids or convert to some user friendly products. It
is very important from green chemistry point of view.
• So, Choosing a gasifier that can destroy tars or the heavy hydrocarbons is very important.

10. FEEDSTOCK PROPERTIES
• Particle Size : Depends on the size/dimension of hearth.
• Larger the particles, lesser the reaction as the exposed area for reaction is very less.
• Very Smaller particles also poses problems like clogging in some parts and may shut
down.
• So it is important to choose a optimum value of particle size.

11. FEED STOCK TREATMENT
• Degree of treatment of the biomass feedstock is depending on Gasification Technologies.
• Some critical parameters : Drying, Particle Size, Fractionation, Leaching.
• Drying : Moisture level is maintained below 10 to 15%. It can be done through sun
drying or extra energy input.
• Moist feedstock + Heat Dry feedstock + H2O(g)

12. FEED STOCK TREATMENT
• Particle Size : Optimum feed stock particle size is 20 to 30 mm.
• Maximum area of particle should come into contact in order to take place a reaction.
• There are many processes for reducing particle size.
• Chopping, Grinding are some techniques for reducing particle size.

13. FEED STOCK TREATMENT
• Fractionation: Seperation process in which certain quantity of mixture is divided into a
number of smaller quantities in which composition varies according to gradient.
• N2, alkali content are very critical as they are partially carried over into gas stream.
• Small particles tend to contain some N2 and alkali. So fractionation into fine and course
particles helps to produce product gas with fewer impurities.

14. FEED STOCK TREATMENT
• Leaching : N2 and alkali removal with water prior to gasification.
• Washing with water is found to be very effective in removing water soluble impurities like
K, Mg, Ca, Cl .
• After leaching, the moisture content from stock needs to be removed.
• This can be done by sun drying reducing to 20%, but 20% needs to be taken down to 10
by external heat input.

15. GASIFIER TYPES
• Design of gasifier depends on fuel availability, shape and size, moisture content, ash
content and end user applications.
• Depending on requirement, gasifiers are classified into 2 types. They are fixed bed type
gasifier and fluidized bed type gasifiers.
• Depending on the way of interaction of air/steam/O2 with biomass, fixed bed type
gasifiers are again classified into downdraft, updraft and crossdraft gasifiers.

16. FIXED BED GASIFIERS
• Fixed bed type gasifiers simply consisting of cylindrical reactor in which solid biomass
fuel gasifying and produced gas move either upward or downward.
• These types of gasifiers simple in construction.
• Generally Operates with high carbon conversion, long residence time, low gas velocity
and low ash carry over.
• Several types of gasifiers are operating worldwide and further these can be classified
according to the way in which primary air to gasify the biomass enters into gasifier.

17. FIXED BED TYPE GASIFIERS
•
Fixed bed
type
Gasifiers
Crossdraft
Gasifier
Downdraft
Gasifier
Updraft
Gasifier

18. UPDRAFT GASIFIER
• Biomass feed moves downwards and gasifying agents move upward counter current to
biomass through fixed bed of biomass.
• A metallic grate is provided at bottom of reactor which supports the reaction bed.
• It has well defined drying, pyrolysis, reduction and combustion zones.
• It has higher thermal efficiency.
• Ash is removed either as dry ash or slag.

19. UPDRAFT GASIFIER

20. DOWNDRAFT GASIFIER
• In Downdraft gasifier, feedstock is introduced at the top and gasifying agent is introduced
through a set of nozzles located on sides of reactor.
• Reaction zones are similar to updraft gasifier.
• The most important difference is that pyrolysis products in downdraft type are allowed to
pass through high temperature oxidation zone.
• Tar levels are lower in product gas.
• Size and shape and low moisture content of biomass particles must be controlled within
close limits.

21. DOWNDRAFT GASIFIER

22. CROSSDRAFT GASIFIER
• Air or Steam mixtures are introduced in the side of the gasifier near the bottom while the
product gas is drawn off on the opposite side.
• Crossdraft gasifiers exhibit many operating characteristics of the downdraft units.
• The combustion and reduction zones are both concentrated to a small volume around the
sides of unit.
• They are simpler to construct and more suitable for running engines than other type of
fixed bed type gasifiers.

23. CROSSDRAFT GASIFIER

24. FLUIDIZED BED GASIFIER
• Air is blown through a bed of solid particles at a sufficient velocity to keep these in a state
of suspension.
• The bed is originally externally heated and the feedstock is introduced as soon as a
sufficiently high temperature is reached.
• The fuel particles are introduced at the bottom of the reactor and almost instantaneously
heated up to the bed temperature.
• As a result of this treatment the fuel is pyrolysed very fast, resulting in a component mix
with a relatively large amount of gaseous material.

25. FLUIDIZED BED GASIFIER
• Ash particles are also carried over the top of the reactor and have to be removed from the
gas stream if the gas is used in engine applications. To remove ash particles cyclone and
candle filter are used
• The major advantage of fluidized bed is processing of feedstock. This type of gasifier are
mostly used for high ash coal and biomass.
• Since temperature is below the ash softening temperature so handling of ash is relatively
simple.
• Drawback of fluidized bed gasifiers is - high tar content in the produced syn gas.

26. FLUIDIZED BED GASIFIER

27. SYNGAS CONDITIONING
• The gas that is produces cannot be used as such in end used applications.
• It must be cooled, cleaned properly for smooth and efficient operations.
• Different types of cooling devices such as gas coolers and water coolers can be used in
order to bring the gas into ambient temperature conditions.
• There are two major cleaning options for producer gas. They are physical and chemical
process.

28. PHYSICAL GAS CLEANING PROCESS
• One of the simplest cleaning methods comprised of either filtration or wet scrubbing.
• Gas cleaning using simple filtration may be carried out at high or low temperature
conditions where as wet scrubbing is operated at low or ambient temperature conditions.
• There is a problem of fouling of tar or solid particles in simple filtration. This problem can
be shorted out by wet scrubbing.
• Quality of gas cleaning is dependent on water flow rate.

29. THERMAL DECOMPOSITION
• Particle deposition can be controlled by thermal degradation of heavy aromatic tar
compounds into small or lighter species.
• Some of tar compounds such as heterocyclic aromatic and heavy poly aromatic which
dominating tar composition and are difficult to decompose can be condensed to low
concentration.
• A tar limit above 50 to 100 mg/Nm^3 was reported critical for end use applications and
suggested producer gas conditioning using thermal, catalytic cracking and physical tar
removing techniques.

30. CATALYTIC CONVERSION
• Catalytic process operates at same temperature as the existing gas.
• The catalytic process is also capable in trapping particulate matter and ammonia contents
in the end product.
• Different types of catalysts like dolomites, olivine and FCC have been found to be best
solutions for tar cracking.
• Rhodium has been reported for very fast catalytic activity.

31. BIOLOGICAL GASIFICATION
• Here instead of using high temperatures we utilize two technologies .
• They are fermentation technology and anerobic digestion.
• Microbes like methanogenic bacteria in absence of oxygen act on biomass and they
produce gas.

32. ADVANTAGES
• Provides Sustainable & Affordable alternative to fossil fuel based power plants at low power levels.
• Efficiency of the system is very high when compared to other Renewable energy systems such as
Wind and Solar.
• Gasification is Eco-Friendly as it is "CO2 Neutral", generates very little SO2 & Nitrogen Oxides
compared to conventional fossil fuel based power plants.
• It is a cost effective solution as it combines 'Low Unit Capital Cost' with 'Low Unit Cost of
Production'. Cost of installation per kWe is about the same as for large power levels.
• Economic, Social and Environmental benefits associated with Biomass make it as an attractive
renewable energy option.

33. DISADVANTAGES
• Gasification is a complex and sensitive process.
• Gasifiers require at least half an hour or more to start the process.
• Getting the producer gas is not difficult, but obtaining in the proper state is the
challenging task. The physical and chemical properties of producer gas such as energy
content, gas composition and impurities vary time to time.
• Some amount of tar is released in the gas.
• All the gasifiers have fairly strict requirements for fuel size, moisture and ash content.
Inadequate fuel preparation is an important cause of technical problems with gasifiers.

34. FUTURE DIRECTIONS
• A significant improvement in the gasifier reactor, gas cooling and cleaning system could
lead to better energy efficient systems.
• In recent trends some investigations have been made in this direction and still of
considerable interest such as incorporation of heat recovery devices, improved tar
cracking methods, reuse of biochar as feedstock, transformation of ash and tar contents
into value added products, steam gasification for hydrogen yield.

35. THANK YOU