Lecture slides for basic concepts of thermochemical processing of biomass to valuable products.

1. Lecture given in master course
Thermochemical conversion of biomass
Prepared by Chaudhary Awais Salman
Course: Biomass as energy source
Mälardalen University, Västerås, Sweden
Email: casalman@kth.se
ovaissalman@gmail.com

2. OUTLINE
● Thermochemical conversion processes
● Combustion
● Gasification
● Pyrolysis
● Brief introduction of other thermochemical processes
● Torrefaction
● Hydrothermal carbonization
● Hydrothermal Liquefaction
● Process control for thermochemical conversion processes
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3. A general representation of biomass
Thermochemical process
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Feedstock
Product
(energy,
biofuels etc)
Thermolytic
substrate
https://www.nap.edu/read/14683/chapter/5
Decomposition
Upgradation
Heat
Thermolytic substrate:
An intermediate biomass product
produced after the addition of heat
to feedstock

4. Basic Definitions
Combustion
● Thermal conversion of biomass with oxygen (usually excess amount of air) to produce primarily
carbon dioxide, nitrogen and water at high temperature.
Gasification
● Thermal conversion of biomass at elevated temperature and partial amount of oxygen to produce
primarily permanent gases (syngas), with char, water, and condensable as minor products.
Pyrolysis
● Thermal conversion of biomass in the absence of oxygen generally at lower temperature than
gasification and combustion. Main products produced from pyrolysis are bio oil, biochar and syngas.
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Combustion Gasification Pyrolysis
Excess oxygen Partial oxygen NO oxygen
Heat (Flue gases) Syngas (CO +
H2)
Bio oil, biochar, syngas

5. Combustion
● Combustion aims to release all the chemical energy stored in fuel.
● Proper combustion requires
● High temperatures for ignition
● Sufficient turbulence to mix all of the components with the oxygen (mainly air).
● Time to complete all of the oxidation reactions.
These requirements are also called the three Ts of combustion.
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6. Stages of combustion and main combustion reactions
1. Drying
● Physical process (No chemical reaction)
2. Devolatilization
CxHyOz ⇒ H2 + CH4 + C2H4+ CO+ CO2 + C
3. Flame Combustion:
H2 +1/2 O2 ⇒ H2O
CH4 + 1/2 O2 ⇒ CO2 +H2O
C2H4 + 2O2 ⇒ CO2 +H2O
4. Char combustion
C + O2 ⇒ CO2
C + 1/2O2 ⇒ CO
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7. Combustion of a match stick
The flaming match has all three thermochemical processes occurring. Lightning of match
produces gases, vapors and char. Char is produced from the pyrolysis and gases from
gasification. Gasification occurs due to limited availability of oxygen (air) present at the
bottom of matchstick. Gases are then combusted resulting in a flame. The combustion of
gases and vapors also provide the heat required for pyrolysis.
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8. Amount of air required for combustion
The real combustion takes place with air.
● Air main composition:
● O2 (21 % volume)
● N2 (79 % volume)
● For each mole of O2 we have 3.78 mol of N2
Overall combustion reaction
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9. Combustion application: Steam/Power production
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Biomass
1. Biomass is combusted in the boiler to produce hot gases
2. Heat from hot gases from combustion are recovered to
generate steam and run steam turbine
3. The whole steam cycle to produce electric
power is called Rankine cycle

10. Emissions from combustion
● Emissions from combustion must meet the strict
environment regulations imposed by governments.
● Emissions from a boiler can either be solid, thermal, or
air, and the different sources for various components:
● Particulates
● CO/CO2
● NOx
● SOx
● Volatile organic compounds
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11. Gasification
”Conversion of solid biomass into a gaseous product with a usable heating value in
the presence of partial amount of air”
Gasification is not Combustion!
Gasification produces a synthetic gas (Syngas) containing
CO, H2 , H2O, CO2 , N2 , NH2 , CH2 and small amount of H2S, and HCl.
In gasification, about 1.5–1.8 kg of air per kg of biomass is supplied, while in combustion, the
amount of air supplied is 6–7 kg of air per kg of biomass.
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12. History of Gasification
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Did you know that ?
• over one million vehicles in
Europe ran onboard gasifiers
during WWII
• fuel was made from wood
and charcoal, as gasoline and
diesel were rationed or
otherwise unavailable
Working principle of a wood gasifier car
https://www.youtube.com/watch?v=vGO5J9HMkGE

13. Fundamentals of gasification
● The gasification of a biomass occurs in four stages.
1. Drying
2. Pyrolysis (Devolatilization)
3. Combustion (gas phase reactions)
4. Reduction (gas-solid reactions)
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http://www.infiniteenergyindia.com/biomass-
gasifiers.html

14. Fundamentals of gasification: Drying
● Drying is the removal of the moisture in the biomass.
● Drying happens around 100°C.
● It is a physical process and no chemical reaction occurs in drying.
● The time required for drying is dependent on the particle size and
the ignition temperature of the biomass.
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15. ● After drying, the temperature of the biomass particle increases, and
pyrolysis reactions begin to occur.
● Pyrolysis is the thermal decomposition of organic materials around
500 oC, in the absence of air or oxygen.
● The major products of pyrolysis are hydrogen, carbon monoxide,
carbon dioxide, methane and other light hydrocarbons, and various
high molecular weight hydrocarbons also known as tar.
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Fundamentals of gasification: Pyrolysis
(Devolatilization)

16. ● At the end of pyrolysis, the volatile gases and the char formed continue to
react independently.
● Sufficient oxygen must be present to complete the combustion process.
● Combustion is the only net exothermic process in gasification (i.e., it releases
heat).
● All of the heat for drying, pyrolysis, and reduction comes directly or indirectly
from combustion in a gasifier.
● Combustion can be fueled by either the tar gasses or char from Pyrolysis.
Different reactor types use one or the other or both.
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Fundamentals of gasification: Combustion

17. ● Reduction is the removal of oxygen atoms off combustion products
of hydrocarbon molecules. (Reduction is opposite of combustion)
● In the reduction zone, the thermal energy which was generated in the
combustion stage is converted into chemical energy via following
reactions:
C + CO2 ⇒ 2CO
H 2 O + C ⇒ H 2 + CO
C + H 2 ⇒ CH4
● The reactions are endothermic and reduce the temperature of syngas
● Final composition of syngas produced in this stage
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Fundamentals of gasification: Reduction

18. Types of gasifiers
1 Fixed-bed gasifiers
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2 Fluidized bed gasifiers 3 Entrained flow gasifiers

19. Gasifier types: Advantages and disadvantages
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Fixed bed
1 Updraft
2 Down draft
Fluidized bed
Entrained flow
Feed size limitations
High tar
Less efficient for large scale applications
Less efficient for large scale applications
Moisture sensitive
Medium tar yield
High particles in syngas
High particles in syngas
Feed size has to be very small or slurry
type
Mature for small scale applications
Can handle high moisture
No carbon in ash
Small scale applications
Low particles in syngas
Low tar in syngas
Large scale applications
Can support direct/indirect mode
heat
Syngas composition can be
controlled
Very low tar
Large scale applications
DisadvantagesAdvantages
https://www.nrel.gov/docs/gen/fy04/36831e.pdf

20. Gasification efficiency
● Thermal efficiency - conversion of chemical energy of biomass
fuel to chemical energy and sensible heat of gaseous product
(Syngas: CO/H2)
● High temperature, high-pressure gasifiers (Entrained flow):
>95%
● Typical fluidized bed gasifiers: 70 - 90%
● Cold gas efficiency (CGE)– conversion of chemical energy of
solid fuel to chemical energy of gaseous product
● Typical biomass gasifiers: 50-75%
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https://www.nap.edu/read/14683/chapter/5

21. Routes for conversion of syngas
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Syngas can be converted to
biofuels, biochemicals or
power
Fischer-Tropsch (FT) fuels
can replace diesel and
gasoline
Methanol and DME can also
be used as an alternative
renewable fuel
How are the FT fuels
related to drop-in fuels?
https://www.nrel.gov/docs/gen/fy04/36831e.pdf
?

22. Upgradation of syngas
● Syngas contains impurities such as: particulates, tar, sulphur, nitrogen, chlorine, alkaline metals.
● These impurities has to be removed from syngas prior to upgradation to biofuels or power.
● The amount and type of impurities depends on the type and characteristics of feedstock.
● Conversion of syngas to biofuels (methane, DME, FT, methanol etc) require catalysts (see previous slides
● The conversion of syngas to biofuel also require a specific ration of H2/CO in syngas in different biofuels production. A water gas shift reactor
usually have to install prior to synthesis reactor to maintain the ratio.
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The figure below is the generalized flow diagram of biofuels production through
biomass gasification
https://www.nap.edu/read/14683/chapter/5

23. Gasification: Advantages/concerns
Positives
● Can handle all type of biomass (waste, wood, sludge)
● Syngas produced can be used in many ways
● Process can integrate in biorefineries
● Higher energy efficiencies
Concerns
● Cleaning of syngas from tar and other impurities is troublesome and the whole
process takes a toll.
● Hard to compete with cheaper options (coal, oil and combustion of biomass instead
gasification)
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24. Pyrolysis
● Require dry feedstock: <10%
● Small particle size of biomass requried : <3mm
● Pyrolysis occurs at moderate temperatures (400‐500
°C)
● Short residence times required :0.5‐2 sec
● Rapid quenching at the end of the process
● Typical yields
● Bio Oil: 60 ‐75%
● Bio Char:15 ‐25%
● Syngas:10 ‐25%
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“Pyrolysis is the conversion of a biomass into liquid (biofuel),
solid, and gaseous fractions by heating the biomass in the
absence of air”

25. Pyrolysis products
● Syngas: non‐condensable gases like carbon dioxide,
carbon monoxide, hydrogen, methane.
● Biochar: mixture of carbonaceous (charcoal) and
inorganic compounds (ash).
● Biooil: mixture of water, oxygenated organic
compounds , and polymers.
Out of above three products mainly biooil is the desired
product from pyrolysis process.
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Biooil
http://task34.ieabioenergy.com/bio-oil/

26. Bio-oil characteristics
Advantages
● Bio-oil has characteristics that may require it to be upgraded or treated before it
can be used for many applications.
● Bio oil can be upgraded to hydrocarbon fuels.
● Liquid nature of bio oil makes it easier to transport to other facilities for
upgradation
Disadvantages
● Low pH: Corrosion
● High viscosity: Difficult to transport in pipes
● Water content: Low homogeneity and complex viscosity
● High oxygen content: Low stability and low heat value
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27. Slow pyrolysis
● Another variation of pyrolysis is slow pyrolysis
● Slow pyrolysis has been used for thousands of years for the production of
charcoal.
● Biomass is heated to about 500 oC for 5–30 min, and the main product from slow
pyrolysis is biochar or charcoal.
Burning of a bread in a toaster to make a charcoal like toast is an example of slow
pyrolysis.
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28. Biochar
● Biochar is the carbonaceous material obtained from the
pyrolysis of biomass/waste.
● It has fine porous structure
● High organic content
● Can be used in several applications
● As an energy source
● For soil amendment application
● Carbon sequestration agent
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29. Another thermochemical process 1: Torrefaction
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Torrefied BiomassBiomass
• Torrefaction converts the biomass into coal like product by heating the biomass in
the absence of oxygen to a temperature of typically 250 to 350°C.
• A torrefied biomass can ships like coal, stores like coal, have as much energy as
coal and behaves like coal in a coal power plant.
• Torrefied biomass combusts cleaner, gasifies easier and cleaner than raw biomass.
• Heating value for torrefied biomass is higher amongst other solid biofuels.

30. Another thermochemical process 2: Hydrothermal carbonization
● Biomass is pressurized at a temperature of 180-200 °C and 20-
25 bar to form a coal like material.
● Reaction time varies from 1-72 h (depending in feedstock)
● The process is more feasible with waste having high moisture
such as sludge.
● The product obtained is called hydro char
● The heating value of final product is much higher.
30https://www.sciencedirect.com/topics/engineering/hydrothermal-
carbonization

31. Another thermochemical process 3: Hydrothermal Liqufaction
● Effective technology to treat wet biomass feedstock (lignocellulose,
algae, sludge)
● Require high pressure (100-350 bar) and temperature 250 – 450°C
● Suited for wet feed, hence need for drying is alleviated.
● Main products are biocrude (oil), biochar and small amount of product
gases.
● The biocrude from hydrothermal liquefaction is an energy-dense liquid
and a renewable oxygenated fuel equivalent to fossil petroleum.
● Biocrude can be further processed into transport fuels and research is
undergoing to make the process feasible in terms of technical and
economic aspects.
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https://res.mdpi.com/d_attachment/energies/energies-11-03165/article_deploy/energies-11-03165.pdf
https://vbn.aau.dk/files/233637033/PHD_Thomas_Helmer_Pedersen_E_pdf.pdf

32. Process control
● Process control in defined as ”Continuous maintaining of desired outputs and
process conditions in thermochemical processes by adjusting the selected
variables in the process.”
● Without adequate and reliable process controls, an unexpected process
occurrence cannot be monitored, controlled, and eliminated.
● Process controls can range from simple manual actions to computer logic
controllers, remote from the required action point, with
supplemental instrumentation.
“A good example of automatic control is electric heating of a house in a cold
environment, a process where a single variable is controlled. A thermostat (sensor)
monitors the temperature. When the temperature drops below the setpoint, a
switch is closed and the heater comes on. When the temperature rises above the
setpoint, the switch opens and the heater is turned off.”
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https://www.sciencedirect.com/science/article/pii/B9780128142387000246

33. Process control in thermochemical process
● Some of the most important variables in thermochemical processes that need to be controlled to get the
desired output are
● Combustion
● Biomass characteristics
● Amount of excess air
● Temperature in the boiler
● Demand of power and heat at consumer side
● Gasification
● Equivalence ratio (Amount of oxygen enter in the gasifier)
● Steam to biomass ratio
● Temperature of gasifier
● Pressure of gasifier
● Elemental analysis of biomass (C, H, O)
● Moisture of biomass
● Pyrolysis
● Size of biomass
● Characteristics and moisture of biomass
● Temperature of Pyrolysis reactor
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