biomass characterization and analytical pyrolysis · 2019. 11. 8. · x emissions during...

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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 764089

Biomass characterization and analytical pyrolysis

ABC Salt Summer School – Aston University, 12-14 August ‘19

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Content

1. Recap of biomass composition

2. Basic physicochemical biomass characterization techniques

3. “Advanced” and thermal decomposition based characterization

4. Analytical pyrolysis

• Working principle

• Analytical pyrolysis of biomass

• As a “micro-scale” reactor: 2 case-studies

• Lignin pyrolysis

• Zeolite-catalyzed pyrolysis

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Recap of biomass composition

What is biomass composed of ? 4 main groups of compounds

1. Water

2. Inorganic compounds (called ash, as the content is determined by ashing): K, Ca, Mg, P, S, N, Si,...

3. Extractives: non-structural compounds which can be leached using water or solvents

• Sugars and starch

• Lipids (oils and fats), waxes and resins

• Proteins and peptides

4. Cell wall: structural compounds (allow the plant to grow upright), consisting of three kinds of

polymers:

• Hemicellulose

• Cellulose

• Lignin

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Plant

Glucose

eenheid

Lignine

Hemicellulose

Cellulose

microfibril

Plantaardige

cellen

celwand

(10 – 20 n

m)

Plant cellsPlant

Cell wall

Hemicellulose

Lignin

Glucose unit

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Cellulose

• Linear polymer chain of D-glucose, formula: (C6H10O5)n• DP ~ 10000 (degree of polymerization)

• Multiple cellulose chains are laid out in parallel, stabilized through H-bonds imparts (predominantly) a

crystalline structure

• The crystallinity also explains the rather high thermal stability, as well as the difficulty by which cellulose is

enzymatically hydrolyzed

• Not digestible by mammals, except ruminants

• In woody biomass: up to 40 to 50 wt.% (dry biomass) is cellulose

• Examples of ‘pure’ cellulose: paper, cotton

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Hemicellulose

• Branched polymer of different hexoses and pentoses (i.e. C5 and C6 sugars like xylose, glucose, mannose,

galactose en glucuronic acid), formula: (C5H8O4)n• DP ~ 100 to 200 (degree of polymerization), much smaller molecule than cellulose

• Low DP and amorphous, leading to low thermal stability and easy to hydrolyze with dilute acids or bases

• In woody biomasses: 20 to 30 wt.% (dry basis) is hemicellulose

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Lignin

• Complex amorphous polymer, with aromatic

functionalities, hydrophobic in nature

• Formula: (C31H34O11)n, remark that lignin

contains far less oxygen than sugar polymers

like cellulose and hemicellulose

• Is composed of three subunits (i.e.

monolignols), and are bound by ether and C-

C covalent bonds

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• Lignin functions as a ‘cement’ in between the cellulose and hemicellulose plant fibers.

• Different bond types different energies of dissociation complex thermal degradation pattern

• Softwoods (e.g. pine, spruce): mainly. G-units, while hardwood (e.g. oak): G and S; grasses: G + H + S

• Softwoods 25 – 35 wt.% lignin, hardwood 18 – 25 wt.% lignin content (on dry basis)

p-coumaryl alcohol (H-unit)

conipheryl alcohol (G-unit)

sinapyl alcohol (S-unit)

Lignin monomers Lignin bond types

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How to characterize biomass

Basic techniques to characterize biomass prior to thermochemical conversion:

• Physical

• Density (true, apparent and bulk)

• Thermal (conductivity and specific heat)

• Morphology (particle size distribution, sphericity)

• Chemical

• Proximate analysis

• Elemental (CHNS via combustion /chromatography; ICP-OES/MS)

• Bomb calorimetry

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Proximate analysis

= The distinction into ash, water, volatile matter and fixed carbon – purely based on weight loss after oven-

treatment

1. Water content (MC, moisture content)

• Important property in thermochemical conversion: al water has to evaporated (except in

hydrothermal processes) and requires latent heat !

• Distinction in freely available and bound water

• Freely available water = water available above equilibrium concentration (e.g. pine: 9 wt.%

equilibrium concentration water at Tair = 20°C and RHair = 0.5)

• Bound water = total moisture content – free water. Is the water bound to the cell wall constituents.

• Moisture content determined gravimetrically, by drying up to 105°C

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2. ash content

• The inorganic remnant after combustion

• Woody biomass < 1 wt.%; SRWC < 2 wt.%; energy grasses ~ 7 to 10 wt.%. Residues like digestate, sewage

sludge > 10 wt.%

• K, Ca (and other alkali and alkaline earth metals): can act catalytically at higher temperatures.

• K (+Si): ‘Low’ melting point, results in ash melting and ‘fouling’

• Cl, S: Yield HCl and H2SO4 and are corrosive.

3. Volatile matter (VM)

• The organic part that volatilizes at temperatures of 950 °C (volatilization does not require O2 !).

4. Fixed carbon (fC)

• The organic part that does not volatilize at a temperature of 950°C, or fC = 100 – Ash – MC – VM.

• Char = high fC content. The majority of the volatiles have been driven off in the pyrolysis.

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Elemental composition (ultimate analysis)

• Ultimate analysis is the determination of weight fractions of H, S, O, N, C in the organic matter fraction in

the biomass (+ additionally metals, trace elements)

• C & H contribute to the HHV

• S undesired: results in SOx emissions during combustion. Biomass: typically < 0.1 wt.% however fossil fuels

> 1 wt.% ! Combustion of fossil fuels requires active removal of SOx from the flue gases by e.g. injection of

lime which happens in coal combustion (Ca(OH)2 which reacts with SOx to CaSO4).

• N undesired: yields NOx emissions during combustion. Usually, N content is low in fossil fuels but can be

high (> 1 wt.%) in certain biomass feedstocks due to the presence of proteins. Requires active removal

from flue gases by selective catalytic or non-catalytic reduction using NH3 as reductans.

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Elemental composition (ultimate analysis)

• The atomic ratios of O/C en H/C determine the fuel

quality of biomass, biomass-derived products and

fossil fuels

• If O/C ↑ then HHV ↓. If H/C ↑ then HHV ↑.

• O/C and H/C ratios on the so-called Van Krevelen

diagram

Dehydratatie

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Heating value

• Is the heat released during combustion (reaction with oxygen) per mass

unit of fuel/biomass.

• Measured by means of a bomb calorimeter

• Distinction between higher heating value (HHV) and lower heating

value (LHV)

• HHV = LHV + latent heat (heat of evaporation at 25°C) of water vapor in

the combustion gases

• At room temperature: latent heat of water is 2.5 MJ/kg

• HHV can be estimated based on the known elemental composition

(C,H,N,O,S and ash in wt.%) of the biomass (the so-called Dulong

formula),

Warmte

Roerder OntstekingThermometer

Thermisch geïsoleerd vat

O2-gas

Bom

Staalhouder met biomassa-monster

Water

Stirrer Ignition

Thermally insulated water vessel

Bomb

Sample holder

Heat

ashNOSHCkgkJHHV 1.211.154.1035.1003.11781.349/

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Basic techniques to characterize biomass prior to thermochemical conversion:

• Physical

• Density (true, apparent and bulk)

• Thermal (conductivity and specific heat)

• Morphology (particle size distribution, sphericity)

• Chemical

• Proximate analysis

• Elemental (CHNS via combustion /chromatography; ICP-OES/MS)

• Bomb calorimetry

Gives substantial compositional information, but doesn’t tell quite as much as to what to expect in pyrolysis

• “Advanced” methods which are based on

thermal decomposition

• Thermogravimetry (TGA, TGA-MS, TGA-FTIR)

• Analytical pyrolysis

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Thermogravimetric analysis

• Measuring weight loss evolution when heating a sample

• Low heating rates (i.e. 5, 10 °C/min)

• In inert (He, N2) atmosphere pyrolysis, or oxidative (O2)

atmosphere combustion

• Can be used to predict optimum pyrolysis temperature ranges

and expected char yields

• Can be used to perform proximate analysis

• TGA-MS, TG-FTIR: combined TGA and evolved gas analysis

through mass spectrometry or FTIR

Source: Setaram

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Thermogravimetric analysis

Cellulose

HemicelluloseLignine

Ge

wic

ht

(w%

)

Sn

elh

eid

ge

wic

hts

ve

rlie

s

(w%

/°C

)

Temperatuur (°C)

0 200 400 600 800

0

20

40

60

80

1003.0

2.5

2.0

1.5

1.0

0.5

0.0

Mass (

%)

Mass loss r

ate

(%

per

°C)

Temperature (°C)

• E.g. lignocellulosic biomass

constituents

• TGA-dTG

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Analytical pyrolysis

• Originated out of the idea to

analyze solid (non-volatile) samples

in gas chromatography

• Uses pyrolysis to thermally

crack/decompose the sample into

volatile, GC-detectable compounds

• Decomposition pattern is unique to

the composition of the sample

tested

GCGCMSMS

Carrier gasPyrolysis oven

Chromatographic

column

Mass detector

Biomass sample

Sample cup

time

Working principle

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Pyrolysis methods

1. Microfurnace

• Preheated tubular furnace (quartz or steanless

steel lined), flushed with carrier gas

• Small volume of furnace to reduce dead volume

• Sample introduction by dropping sample into the

furnace (by gravity), held in a open-ended holder

(cup)

• Liquid samples can be injected directly

• Sample (+holder) heating by convection

• Thermal inertia of the furnace (in EGA, evolved

gas analysis or gradient pyrolysis slow heating)

• E.g. Frontier lab

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Pyrolysis methods

2. Filament pyrolyzer

• Sample placed in a quartz tube held within a Pt coil (on

a probe tip) or directly coated on a Pt strip (liquids)

• Coil is heated by electrical current (resistive heating) –

temperature is not measured but assumed from

resistivity and current

• The probe tip is place in a preheated cavity (

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Pyrolysis methods

3. Curie point pyrolyzer

• Curie point temperature: the temperature at which a

ferromagnetic metal loses its magnetic properties

• Metal alloys are heated inductively to exactly the Curie

point

• The sample is in direct contact with the metal alloy (be

aware of potential catalytic effect)

• Only some select temperatures available, each alloy has

different Curie point temperature

• Can only heat rapidly to the Curie point temperature

• Sample size is limited

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Pyrolysis methods

3. Curie point pyrolyzer

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Pyrolysis methods

4. Laser pyrolyzer

• Heating of sample by means of laser light

• Fast heating, but exact pyrolysis temperature is not

known, also not exactly known how much sample is

pyrolyzed problems in reproducibility

• Only suited for materials that are opaque (i.e. light

absorbed), not for transparent materials (or absorbents

need to be added…)

• The beam can be directed to specific parts in the

biomass (i.e. different parts in plant tissues)

• Not common, not commercially available

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Important parameters in analytical pyrolysis

• Heating rate (i.e. rate of heating before reaching pyrolysis temperature,

HRp):

• too low heating rate slow release of volatiles peak

broadening/poor separation on the GC

• Extra focus step may be required if HRp is low

• Too low heating rate secondary vapor-phase pyrolysis reactions

• Pyrolysis time (Dtp)

• Pyrolysis temperature (Tp,f)

• Sample size

• Too large sample’s heat transfer limitations

• Too small Low accuracy, catalytic wall effects may be dominant

• Interface temperature (Ti,f)

• Sufficiently high to avoid condensation of pyrolysis vapors

Run time

Te

mp

era

ture

T0

HRi

Ti,f

Tp,f

HRp

T0Dti

Dtp

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• Extra focus step

• Directly on the column (cryofocus)

• Separate trap (tenax), trough gas

switching valve (also allows to have a

different carrier gas than the one in

which was being pyrolyzed)GCGCMSMS

Carrier gas

Pyrolysis oven

Chromatographic

column

Mass detector

Biomass sample

Sample cup

Cold or liquid N2

Pyrolysis oven

Cooling N2

GC column

Cold spot (trapping in

column)

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• Extra focus step

• Directly on the column (cryofocus)

• Separate trap (tenax), trough gas switching

valve (also allows to have a different carrier

gas than the one in which was being

pyrolyzed)

Valve oven

Interface

Pt-filament coil

Quartz tube

Sample

Quartz wool

GC carrier gas

(helium)

Heated transfer line

GC/MS or

GC/FIDRotary

valve

Trap

Outlet

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Analytical pyrolysis: biomass

What to expect ?

Example: ~300 µg, sugarcane bagasse, 500°C, microfurnace pyrolyzer

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Cellulose

Zheng et al., 2016

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Hemicellulose

Patwardhan et al., 2011

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Lignin

Kosa et al., 2011

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Analytical pyrolysis: case examples

1. Pyrolysis of mutant Poplar with deficient lignin biosynthesis

• Vercruysse et al. (2016)

• Lignin biosynthesis pathway

• The HCT-enzyme (hydroxycinnamoyl-CoA:shikimate

hydroxycinnamoyl transferase 1) forms the

branching point between H and G/S bioynthesis

• HCT-low or deficient mutants: accumulation of H-

units in lignin, also shown to reduce MW of lignin

polymers

• Hypothesis: in fast pyrolysis do these HCT-low

mutants,

• produce more monophenolic species ?

• and/or different spectrum of phenolic species ?

NH2

OHO

phenylalanine

OHO

cinnamic acid

OHO

OH

p-coumaric acid

O

OH

SCoA

p-coumaroyl-CoA

O

OH

SR

p-coumaroyl

shikimic/quinic

acid

O

OH

SR

OH

caffeoyl-shikimic/

quinic acid

O

OH

S

OH

CoA

caffeoyl-CoA

O

OH

SCoA

OMe

feruloyl-CoA

p-coumarylaldehyde

p-coumarylalcohol

O

OH

H

OH

OH

O

OH

OMe

HO

OH

OMe

H

HO

O

OH

OMe

H

MeO

OH

OMeMeO

OH

OH

OMe

OH

sinapaldehyde 5-hydroxy

coniferaldehyde

coniferaldehyde

sinapyl alcohol coniferyl alcohol

C4H PAL

HCT C3H HCT CCoA-OMT

COMT F5H

4CL

CCRCCR

CAD CAD CAD

H-lignin S-lignin G-lignin

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• Py-GC/MS, 500°C of homozygous (D73/D73), heterozygous (D73/+) mutants and wild type (+/+) poplar

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• Py-GC/MS, 500°C of homozygous (D73/D73), heterozygous (D73/+) mutants and wild type (+/+) poplar

• Detail of phenolics (according to degree of methoxylation) in py-GC/MS:

% H % G % S

WT 0.39 ± 0.09 35.23 ± 3.17 63.20 ± 3.18

Heterozygous 0.56 ± 0.10 35.03 ± 2.86 64.41 ± 2.93

Homozygous 6.98 ± 0.95 26.98 ± 0.42 66.04 ± 1.37

Compositional data from NMR:

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2. Testing catalysts in catalytic pyrolysis

• Analytical pyrolysis has become more than just an

‘analytical’ technique, it also allows to perform

‘microscale’ pyrolysis experiments

• For instance, catalysts can be tested with respect

to activity/selectivity both ‘in-situ’ or ‘ex-situ’

(with secondary reactor – independently

temperature-controlled)

• Also, reactive gases may be supplied to test

hydropyrolysis or catalytic hydropyrolysis

• Case example: catalyst testing in CFP (Yildiz et al.,

2016)

GCGCMSMS

Carrier gas

Pyrolysis oven

Ex-situ catalytic reactor

Catalytic reactor oven

Reactant gas

Chromatographic

column

Mass detector

Biomass sample

Sample cup

time

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• In-situ testing (500°C)

• Feedstock: pine (300 µg)

• HZSM-5 (A) and metal-modified HZSM-5

based catalysts (A-M) in low (L) and high (H)

loading

• Pyrogram peak areas cumulated according

to chemical functionality

• Clearly, the aromatization activity of the

zeolite catalyst is seen

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• In-situ testing (500°C)

• Feedstock: pine (300 µg)

• HZSM-5 (A) and metal-modified HZSM-5

based catalysts (A-M) in low (L) and high (H)

loading

• Pyrogram peak areas cumulated according

to chemical functionality

• Clearly, the aromatization activity of the

zeolite catalyst is seen

• Comparison with a bench-scale reactor and

its subsequent pyrolysis liquid analysis

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Analytical pyrolysis: pitfalls, how to avoid them

• Difficult result comparison between different types of instruments, different protocols, etc…

• Most results are reported in relative abundance (good for analytical purposes but not if you want to

use the technique for ‘microscale’ pyrolysis) calibrate & report in absolute quantities/yields

• Be aware you’re only quantifying a certain part of the pyrolysis products (i.e. GC-detectable

volatiles)

• Statistical data processing is required

• Py-GC/MS results can not be 100% scaled to larger scale pyrolysis

• No vapor condensation

• Differences in heating rate/vapor residence time/feedstock particle morphology/…

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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 764089

Prof. dr. ir. Frederik Ronsse

Contact:

Dept. of Green Chemistry and Technology

Ghent University

Coupure Links 653

B-9000 Ghent

Belgium

biomass characterization and analytical pyrolysis · 2019. 11. 8. · x emissions during...

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