The role of drying, grinding, torrefaction and flash 

pyrolysis in the production of liquids and chemicals 

from biomass via gasification

Ilkka Hannula &

Kari Vanhatalo

Introduction

• Thermochemical processes are sensitive 

to feedstock quality

• Biomass feedstocks are very 

heterogeneous

Æ Need for pretreatment technologies

Introduction

• Economies of scale exist

• Biomass is rarely available abundantly

Æ Logistics are crucial
Æ Role of energy densification?

Thermochemical routes

Figure 1. A schematic diagram of different thermal process routes for biomass conversion

Gasification in large scale

FEEDSTOCK

AIR

ASH

GAS

Feedstock size <10 mm

Tars

Tars

Gasification in large scale

GAS

SLAG

FEEDSTOCK

OXYGEN & STEAM

Feedstock size <0.1 mm

No tars

No tars

Pretreatment for gasification

Fluidised-bed

Entrained flow

Gasification

Pre-treatment

Drying

Grinding

Pyrolysis

Torrefaction

Commercial

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Require R&D   

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Drying

• Biomass moisture content could be 40 – 60%

• For synthesis gas production purposes, 

feedstock moisture content must usually be 
dried down to 15-30 %.

• Choosing of right drying technology is a 

question of operational and economic factors 
of the specific application

Drying

• Important issues in drying are energy 

efficiency, emissions, heat integration and 
dryer capacity

• Heat integration: air, flue gas, or steam as 

drying medium

• Because of organic raw material, dust 

emissions can cause fire or explosion risk

Biomass dryer types

• Commonly used dryer types 

for biomass

• Flash dryer

• Belt  dryer 

• Drum dryer

Grinding

• Biomass is a very heterogeneous raw material 

and requires size reduction

• Feedstock particle size to gasification processes 

have to be <10 mm

Crushing, 

shredding, 

cutting

10- 100 mm

Grinding

Synthesis gas 

production

”Pre-pre”-treatment methods

< 10 mm

Drying

Grinding

• Feedstock hardness, moisture content and 

shape have an effect to processing efficiency 
and equipment choosing

• Energy consumption of grinding is between 30-

300 kwh/t

Grinding

• Commonly used equipment  for biomass 

to achieve required particle size (< 10 mm)

– Hammer mill (Wide feedstock scale)

– Knife mill (Suitable for annual plants)

Torrefaction

• Thermal treatment

• T range of 200 to 300 °C

• Atmospheric pressure, absence of O2

Torrefaction

• Low particle heating rates (< 50 °C/min) 

• Residence time from 10 to 30 minutes

Æ Particle size < 4 cm

• Loss of mass and chemical energy

Torrefied wood

• Slightly higher heating value (21–22 MJ/kg, 

LHV)

• Improved storing properties (is hydrophobic, 

limited biological degradation)

• Brittle in character leading to lower energy 

consumption in grinding

Effect of torrefaction to grinding

• Torrefaction change energy consumption of 

grinding. 

• 80 % less energy is required comparing dry 

wood chips and torrefied chips.

Flash pyrolysis

• Thermal decomposition phenomenon

• Absence of oxygen

• First step in combustion and gasification

Flash pyrolysis

• Process conditions that maximise liquid 

production

– moderate temperature 
– rapid heat up rate

Æ small particle size

– short vapour residence time 

• In addition

– dry feestock to prevent high water content in 

the product

Flash pyrolysis

Flash pyrolysis

• Very high heating and heat transfer rates

• Carefully controlled pyrolysis reaction 

temperature of around 500 ºC and vapour 
phase temperature of 400-450 ºC,

• Short vapour residence times of typically less 

than 2 seconds, and rapid cooling of the 
pyrolysis vapours to give the bio-oil product

Pretreatment for gasification

Fluidised-bed

Entrained flow

Gasification

Pre-treatment

Drying

Grinding

Pulverising

Pretreatment for gasification

Fluidised-bed

Entrained flow

Gasification

Pre-treatment

Torrefaction

Grinding

Pulverising

Pretreatment for gasification

Fluidised-bed

Entrained flow

Gasification

Pre-treatment

Pyrolysis

Grinding

Grinding

Drying

Pretreatment for gasification

Fluidised-bed

Entrained flow

Gasification

Pre-treatment

Pyrolysis

Grinding

Torrefaction

Discussion

• Two commercial pre-treatment methods exist for 

larger capacities than 50 t/h, namely drying and 
grinding. 

• Two solutions exist for large scale gasification, 

namely fluidised-bed and entrained flow reactors

– Both require drying to around 15 - 30 wt-%. 

– Fluidised-bed reactors require feedstock sizing to 

around 10 - 50 mm 

– Entrained-flow reactors require additional 

pulverisation to achieve <0.2 mm. 

Discussion

ÆPre-treatment requirements more challenging for 

entrained-flow than for fluidised-bed. 

ÆDifferences (economic) could be leveled by 

constructing larger plants.

ÆUsing torrefaction or pyrolysis as an energy 

densification method.

Discussion

Two interesting options exist for lowering the energy 

consumption of biomass pulverisation, namely 
torrefaction and pyrolysis. 
ÆFast pyrolysis of biomass makes pulverization step 
unnecessary by turning the solid feedstock into liquid 
form. 
ÆTorrefaction requires additional pulverization step, but 
significantly lowers the energy consumption of this step.

Discussion

• The final choice of pre-treatment/gasification 

configuration should be a result of overall evaluation

• Operational and economic factors specific to the 

application. 

• Economies of scale usually dictate that the larger the 

operational scale, the more expensive treatment 
systems are available to choose from. 

• Successful integration of the system is always of 

importance