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THERMAL CONVERSION OF BIOMASS AND WASTE : THE STATUS 

AV Bridgwater 

Bio-Energy Research Group, Aston University, Birmingham B4 7ET, UK 

Tel: +44 121 359 3611; Fax; +44 121 359 6814; a.v.bridgwater@aston.ac.uk

ABSTRACT 

Bioenergy is now accepted as having the potential to provide the major part of the projected 

renewable energy provisions of the future. There are three main routes to providing these biofuels – biological conversion, physical conversion and thermal conversion – all of which 

employ a range of chemical reactors configurations and designs. This review concentrates on 

thermal conversion processes and particularly the reactors that have been developed to 

provide the necessary conditions to optimise performance. A number of primary and 

secondary products can be derived as gas, liquid and solid fuels and electricity as well as a 

considerable number of chemicals. The basic conversion processes are summarised with their 

products and a brief comparison with biological conversion processes and products is 

included. The main technical and non-technical barriers to implementation are identified. 

INTRODUCTION 

Renewable energy is of growing importance in satisfying environmental concerns over fossil 

fuel usage. Wood and other forms of biomass including energy crops and agricultural and 

forestry wastes are some of the main renewable energy resources available. These can 

provide the only source of renewable liquid, gaseous and solid fuels. Biomass is considered 

the renewable energy source with the highest potential to contribute to the energy needs of 

modern society for both the developed and developing economies world-wide [1, 2]. Energy 

from biomass based on short rotation forestry and other energy crops can contribute 

significantly towards the objectives of the Kyoto Agreement in reducing the green house 

gases emissions and to the problems related to climate change [3]. 

Biomass fuels and residues can be converted to energy via thermal, biological and physical 

processes. Each process area is described with the greatest emphasis on the technologies that 

are attracting the most attention in the research and demonstration arenas. In the 

thermochemical conversion technologies, biomass gasification has attracted the highest 

interest as it offers higher efficiencies compared to combustion and fast pyrolysis is still at a 

relatively early stage of development. 

It is important to appreciate that biomass is a diffuse resource, arising over very large areas 

and thus requiring large lands areas with substantial collection and transport costs. Typically 

a sustainable crop of 10 dry t/ha/y of woody biomass can be produced in Northern Europe 

rising to perhaps 15 or maybe 20 dry t/ha/y for energy crops in Southern Europe. Thus an 

area of 1 sq km or 100 ha will produce 1000 dry t/y enough for around 150kW at low 

conversion efficiencies or 300 kW at high conversion efficiencies. It is therefore difficult to 

visualise power generation plants much bigger than around 30-40 MWe anywhere in Europe 

and even these will require a planted area of around 100 sq km. The current view is that even 

these plant sizes are limited in number with a more typical plant size of 5-15Mwe likely to 

dominate the market in the short term. Wastes and waste processing is very similar. 
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THERMAL CONVERSION PROCESSES 

There are three main thermal processes available for converting biomass to a more useful 

energy form – combustion, gasification and pyrolysis. Their relationship to each other and 

the oxygen requirements are summarised in Figure 1 and their products and applications are 

summarised in Figure 2. 

125% 

100% 

75% 

50% 

25% 

0%

Gasification by pyrolysis Pyrolysis for liquids Pyrolysis for charcoal

Gasification by 

partial oxidation

Starved air 

incineration

Oxygen, % stoichiometric

Close coupled 

combustion 

......... 

..... 

..

and 

gasification

Combustion

Figure 1 Thermal conversion processes 

Turbine 

Gasification 

Pyrolysis 

Combustion Heat 

Fuel gas 

Bio-oil 

Electricity

CHP 

Storage

Boiler

Engine 

Chemicals 

Figure 2 Products from thermal biomass conversion 

Combustion 

Combustion of biomass and related materials is widely practised commercially to provide 

heat and power. The technology is commercially available and presents minimum risk to 

investors. The product is heat, which must be used immediately for heat and/or power 

generation as storage is not a viable option. Overall efficiencies to power tend to be rather low 

at typically 15% for very small plants up to 35% for larger and newer plants. Costs are only 

currently competitive when wastes are used as feed material such as from pulp and paper, and 

agriculture. Emissions and ash handling remain technical problems. The technology is 

however, widely available commercially and there are many successful working examples 
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throughout North America and Europe, frequently utilising forestry, agricultural and 

industrial wastes. 

Gasification 

Fuel gas can be produced from biomass and related materials by either partial oxidation to 

give a mixture of carbon monoxide, carbon dioxide, hydrogen and methane or by steam or 

pyrolytic gasification as illustrated in Table 1. 

Table 1 Modes of thermal gasification 

Partial 

oxidation 

with air 

Main products are CO, CO2, H2, CH4, N2, tar 

This gives a low heating value gas of ~5MJ/m3. Utilisation problems can 

arise in combustion, particularly in gas turbines. 

Partial 

oxidation 

with oxygen 

The main products are CO, CO2, H2, CH4, tar [no N2]

This gives a medium heating value gas of ~10-12 MJ/m3

. The cost of 

providing and using oxygen is compensated by a better quality fuel gas. The 

trade-off is finely balanced. 

Steam 

[pyrolytic] 

gasification 

The main products are CO, CO2, H2, CH4, tar

This gives a medium heating value gas of ~15-20 MJ/m3

. The process has 

two stages with a primary reactor producing gas and char, and a second 

reactor for char combustion to reheat sand which is recirculated. The gas 

heating value is maximised due to a higher methane and higher hydrocarbon 

gas content, but at the expense of lower overall efficiency due to loss of 

carbon in the second reactor. 

A number of reactor configurations have been developed and tested, with advantages and 

disadvantages as summarised below. A recent survey of gasifier manufacturers found that 

75% of gasifiers offered commercially were downdraft, 20% were fluid beds [including 

circulating fluid beds], 2.5% were updraft and 2.5% were other types [4]. 

Downdraft - Fixed bed reactor. [Figure 3] 

• Solid moves slowly down a vertical shaft and air is introduced and reacts at a throat that 

supports the gasifying biomass. 

• Solid and product gas move downward in co-current mode. 

• The technology is simple, reliable and proven for fuels that are relatively uniform in size 

and have a low content of fines [below 5mm] 

• A relatively clean gas is produced with low tar and usually with high carbon conversion 

• There is limited scale-up potential to about 500 kg/h federate 

• There is a maximum feed moisture content of around 35% wet basis 

Examples: 

Biomass Engineering [5], Rural Energy [6], BTG&KARA [7], Fluidyne [8], Johanssen [9] 

Status 

Atmospheric downdraft gasifiers are attractive for small scale applications up to about 1.5 

MWth as there is a very big market in both developed and developing economies [10]. 

However, the problem of efficient tar removal is still a major problem and a higher level of 

automation is needed especially for small scale industrial applications. Nevertheless, recent 
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progress in catalytic conversion of tar gives more credible options and these systems can 

therefore be considered of average technical strength. 

Biomass 

Product 

gas 

Air 

Ash 

Updraft 

(Counter-current)

Fixed bed 

of 

biomass 

Grate

Throat

Product gas,

Ash

Downdraft 

(Co-current)

Biomass 

Fixed bed 

of 

biomass 

Air 

Char 

BIOMASS 

Fluid 

bed 

reactor

FUEL GAS

Filter 

(optional) 

Cooler 

(optional) 

Air 

Figure 3 Fixed bed gasifiers Figure 4 Fluid bed gasifier

Updraft - Fixed bed reactor. [Figure 3]

• Solid moves down a vertical shaft and contacts a counter-current upward moving product 

gas stream, 

• The technology is simple, reliable and proven for fuels that are relatively uniform in size 

and have a low content of fines [below 5mm], 

• The product gas is very dirty with high levels of tars, although tar crackers have been 

developed, 

• Scale up limited to around 4 dry t/h feed rate, 

• There is high thermal efficiency and high carbon conversion, 

• Intolerant of high proportion of fines in feed, 

• The gas exit temperature is low, 

• Good turn down capability. 

Examples: 

Wellman [11], Volund [12], Bioneer [13] 

Status 

Atmospheric updraft gasifiers seem to have little market attractiveness for power applications. 

While this may be due to the high tar levels in the fuel gas, recent developments in tar 

cracking have shown that very low levels can be achieved from dedicated thermal/catalytic 

cracking reactors downstream of the gasifier [11]. Another possible reason is that the upper 

size of a single unit is around 2.5 MWe so larger plant capacities require multiple units. 

Bubbling fluid bed [Figure 4]

• Good temperature control & high reaction rates, 

• Higher particulates in the product gas and moderate tar levels in product gas, 

• Good scale-up potential to 10-15 dry t/h with high specific capacity and easily started and 

stopped, 
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• Greater tolerance to particle size range, 

• Good temperature control, 

• Tar cracking catalyst can be added to bed, 

• Limited turndown capability, 

• There is some carbon loss with ash. 

Examples: 

EPI [14], Carbona [15], Dinamec [16], 

Status 

Atmospheric bubbling fluidized bed gasifiers have proven to be reliable with a variety of 

feedstocks at pilot scale and commercial applications in the small to medium scale up to about 

25 MWth. They are limited in their capacity size range as they have not been scaled up 

significantly and the gasifier diameter is significantly larger than that of circulating fluid beds 

for the same feedstock capacity. On the other hand they are more economic for small to 

medium range capacities. Their market attractiveness is thus relative high as well as their 

technology strength. 

Circulating Fluid Bed [Figure 5]

• All the features of bubbling beds PLUS 

• Large minimum size for viability, above around 15 t/h dry feed rate. 

• High cost at low capacity 

• In-bed catalytic processing not easy 

Examples: 

Technical University of Vienna [development] [17], TPS [18], Lurgi [19], Foster Wheeler 

[20], Sydkraft (Varnamo) pressurised [21]. 

Status 

Atmospheric circulating fluidised bed gasifiers have proven very reliable with a variety of 

feedstocks and are relative easy to scale up from a few MWth up to 100 MWth. Even for 

capacities above 100 MWth, there is confidence that the industry would be able to provide 

reliable gasifiers. This appears to be the preferred system for large-scale applications and is 

used by most industrial companies and these systems therefore have high market 

attractiveness and are technically well proven. 
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BIOMASS

Gasifier

FUEL GAS

Air 

Filter 

(optional) 

Cooler 

(optional) 

BIOMASS Flue gas 

Sand + 

Char 

Gasifier 

Air 

Hot 

sand 

Combustor 

Bubbling fluid bed 

or 

Circulating fluid bed 

Air Ash 

Filter (optional) 

FUEL GAS

Cooler 

(optional) 

Figure 5 Circulating fluid bed 

gasifier 

Figure 6 Twin fluid bed gasifier with char 

combustor as a CFB or bubbling bed 

Entrained flow

• Inherently simple reactor design, but only potentially viable above around 20 dry t/h feed 

rate and with good scale-up potential 

• Costly feed preparation needed for woody biomass 

• Carbon loss with ash 

• Little experience with biomass available 

Examples: 

Texaco R&D 

Twin fluid bed [Figure 6]

• Complex process with two close coupled reactors with difficult scale-up and high cost, 

• The gasifier is usually a circulating fluid bed while the char combustor can be either a 

bubbling bed or a second circulating fluid bed, 

• Complexity requires capacities of > 10 t/h for viability, 

• MHV gas produced with air and without requiring oxygen, 

• Low carbon conversion to gas as carbon in char is lost to reheat sand for recycling, 

• High tar levels in gas, 

• Tar cracking catalyst can be added to bed. 

Examples: 

Ferco, Vermont USA [22], 
- 7 - 

Other reactors

• Moving bed with mechanical transport of solid; usually lower temperature processes. 

Includes: Multiple hearth; Horizontal moving bed; Sloping hearth; Screw / auger kiln 

• Rotary kiln: good gas-solid contact; careful design needed to avoid solid carry over, 

• Multi-stage reactors with pyrolysis and gasification separated for improved process control 

and better quality gas 

• Cyclonic and vortex reactors: high particle velocities give high reaction rates 

Examples: 

Rotary kiln – Waste Gas, Siemens, [23], 

Two stage pyrolysis + gasification – Thermoselect [23], Compact Power [23] 

Status 

Rotary kilns are popular for waste processing due to good solids control and good mixing. 

Cyclonic gasifiers have only recently been tested for biomass feedstocks and although they 

have medium market attractiveness due to their simplicity, they are still unproven. Entrained 

bed gasifiers are still at an early stage of development and since they require feedstock of a 

very small particle size, their market attractiveness is very low. No company is known to be 

developing pressurised systems for downdraft, updraft, cyclonic or entrained bed gasifiers for 

biomass feedstocks and it is difficult to imagine that such a technology could ever be 

developed into a commercial product due to the inherent problems of scale, tar removal and 

cost. 

Use of oxygen

• Gives better quality gas, 

• High cost of providing oxygen and high cost of meeting extra process requirements, 

• No evidence that benefits exceed costs. 

Examples: 

There are no known current or recent examples of oxygen fuelled gasifiers 

High pressure gasification

• Significant efficiency and cost advantage in IGCC applications, but large sizes are needed 

• Significant additional cost for pressure with smaller savings from reduced vessel and 

piping sizes, 

• Avoidance of gas compression for IGCC operation. 

Examples: 

The most recent example is at Varnamo [Foster Wheeler and Sydkraft] which finished 

operation in 2000, but is planned to be re-started [21], Carbona [15] 

Status 

Pressurised fluidized bed systems either circulating or bubbling are considered of more 

limited market attractiveness due to the more complex operation of the installation and the 

additional costs related to the construction of pressurised vessels. However, pressurized 
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fluidized bed systems have the advantage in integrated combined cycle applications as the 

need to compress the fuel gas prior its utilisation in the combustion chamber of the gas turbine 

is avoided. 

All biomass fuelled gasifiers

• Feeding can give problems 

• Ash slagging and clinkering potential 

Fuel gas quality

The fuel gas quality requirements, for turbines in particular, are very high. Tar is a particular 

problem and remains the most significant technical barrier. There are two basic ways of 

destroying tars [24], both of which have been and continue to be extensively studied: 

• by catalytic cracking using, for example, dolomite or nickel, 

• by thermal cracking, for example by partial oxidation or direct contact. 

The gas is very costly to store or transport so it has to be used immediately. Hot gas 

efficiencies for the gasifier [total energy in raw product gas divided by energy in feed] can be 

as high as 95-97% for close-coupled turbine and boiler applications, and up to 85% for cold 

gas efficiencies. In power generation, using combined cycle operation, efficiencies of up to 

50% for the largest installations have been proposed which reduces to 35% for smaller 

applications. A number of comprehensive reviews have been published such as [25, 26]. 

Status

There is still very little information on costs, emissions, efficiencies, turn-down ratios and 

actual operational experience. In particular, no manufacturer is willing to give full guarantees 

for technical performance of their gasification technology. This confirms the limited 

operating experience and the limited confidence in the technology. Figure 7 suggests a 

relationship between gasification technologies in terms of their strength and their market 

attractiveness for power generation [derived from 25] 

TECHNOLOGY STRENGTH 

 Strong Average Weak 

High

MARKET

ATTRACTIVENESS

Low

Atmospheric CFB 

Atmospheric BFB 

Pressurised CFB 

Pressurised BFB 

Updraft 

Downdraft 

Cyclonic 

Entrained Bed 

Rotary kiln 
- 9 - 

Figure 7 Technology status of biomass gasification [derived from 25] 

In conclusion, for large scale applications (e.g. above 25-50 MWe) the preferred and most 

reliable system is the circulating fluidized bed gasifier while for the small scale applications 

(e.g. up to 0.5 MWe) the downdraft gasifiers are the most extensively studied. Bubbling 

fluidized bed gasifiers can be competitive in medium scale applications. Large scale fluidized 

bed systems have become commercial due to the successful co-firing projects [see below] 

while moving bed gasifiers are still trying to achieve this. 

Applications for gas

Figure 8 summarises the range of fuel, electricity and chemical products that can be derived 

from the product gas. Medium heating value gas from steam or pyrolytic gasification, or 

from oxygen gasification, is better suited to synthesis of transport fuels and commodity 

chemicals due to the absence of diluent nitrogen, which would pass through unchanged but 

reduce process efficiency and increase costs. The exception is ammonia synthesis when the 

nitrogen content derived from air gasification can be utilised in the ammonia synthesis 

process. In electricity generation, there is no evidence that the benefits of producing higher 

heating value gas with oxygen gasification justifies the cost of providing and using oxygen, 

which explains the low level of interest in oxygen gasification. The technology for synthesis 

of commodity chemicals is commercially available but requires a very high gas quality, which 

is still elusive as well as a very large scale of operation, which for biomass systems is difficult 

to locate. 

The major interest currently is in electricity generation due to the ease of distributing the 

product and absence of product quality requirements concerning compatibility in the market 

place, which remains a significant problem with many fuel and chemicals products. This 

attraction is enhanced by the widespread incentives for electricity generation from renewable 

resources through out Europe. 

Co-firing is a particularly attractive option since most biofuels including gases, liquids and 

solids can be readily introduced into conventional power stations and this takes advantage of 

the economies of scale, contributes comparable fossil fuel savings and reduces risks and 

uncertainties. There has been little commercial activity in this area. 
- 10 - 

MHV gas 

Turbine 

Engine 

Boiler 

Gasification 

LHV gas Electricity 

Steam or 

oxygen 

Air 

Conversion 

Transport 

fuels etc 

Fuel cell 

Heat 

Synthesis 

Ammonia & 

Fertilisers 

Chemicals

Figure 8 Applications for gas from biomass gasification 

Summary

Although biomass gasification technologies have been successfully demonstrated at large 

scale and several demonstration projects are in operation or at an advanced stage of 

construction [27, 28], they are still relatively expensive compared to fossil based energy and 

thus face economic and other non-technical barriers when trying to penetrate the energy 

markets [29, 30, 31]. 

Pyrolysis 

Pyrolysis is thermal decomposition occurring in the absence of oxygen. It is always also the 

first step in combustion and gasification processes where it is followed by total or partial 

oxidation of the primary products. Lower process temperature and longer vapour residence 

times favour the production of charcoal. High temperature and longer residence time increase 

the biomass conversion to gas and moderate temperature and short vapour residence time are 

optimum for producing liquids. Table 2 indicates the product distribution obtained from 

different modes of pyrolysis process. Fast pyrolysis for liquids production is of particular 

interest currently. 

Fast pyrolysis occurs in a time of few seconds or less. Therefore, not only chemical reaction 

kinetics but also heat and mass transfer processes, as well as phase transition phenomena, play 

important roles. The critical issue is to bring the reacting biomass particle to the optimum 

process temperature and minimize its exposure to the intermediate [lower] temperatures that 

favour formation of charcoal. One way this objective can be achieved is by using small 

particles, for example in the fluidised bed processes that are described later. Another 

possibility is to transfer heat very fast only to the particle surface that contacts the heat source 

[this second method is applied in ablative processes that are described later]. 

The process of pyrolysis is endothermic, requiring a substantial heat input to raise the biomass 

to reaction temperature, although the heat of reaction is insignificant. Heat transfer in 

commercial reactors is a significant design feature. Figure 9 summarises the main methods of 

providing the necessary heat. 
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Wood feed 

Vapour product 

3 Hot fluidising gas

2 Recycled

hot sand

5 Hot tubes 

1 Hot wall

4 Air addition 

Char + air 

Figure 9 Methods of heat transfer to a pyrolysis reactor 

Table 2. Typical product yields [dry wood basis] obtained by different modes of 

pyrolysis of wood 

 Liquid Char Gas 

Fast pyrolysis moderate temperature, short residence time 

particularly vapour 

75% 12% 13% 

Carbonisation low temperature, very long residence time 30% 35% 35% 

Gasification high temperature, long residence times 5% 10% 85% 

In order to illustrate the science and technology of thermal conversion in sufficient detail to 

appreciate the potential, fast pyrolysis is described at length. 

Principles

In fast pyrolysis biomass decomposes to generate mostly vapours and aerosols and some 

charcoal. After cooling and condensation, a dark brown mobile liquid is formed which has a 

heating value about half that of conventional fuel oil. While it is related to the traditional 

pyrolysis processes for making charcoal, fast pyrolysis is an advanced process, with carefully 

controlled parameters to give high yields of liquid. The essential features of a fast pyrolysis 

process for producing liquids are: 

• very high heating and heat transfer rates at the reaction interface, which usually requires 

a finely ground biomass feed 

• carefully controlled pyrolysis reaction temperature of around 500oC and vapour phase 

temperature of 400-450oC, 

• short vapour residence times of typically less than 2 seconds 

• rapid cooling of the pyrolysis vapours to give the bio-oil product. 

The main product, bio-oil, is obtained in yields of up to 75% wt on dry feed basis, together 

with by-product char and gas which are used within the process so there are no waste streams 

other than flue gas and ash. 

A fast pyrolysis process includes drying the feed to typically less than 10% water in order to 

minimise the water in the product liquid oil [although up to 15% can be acceptable], grinding 

the feed [to around 2 mm in the case of fluid bed reactors] to give sufficiently small particles 

to ensure rapid reaction, pyrolysis reaction, separation of solids [char], and collection of the 

liquid product [bio-oil]. Any form of biomass can be considered for fast pyrolysis. 
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At the heart of a fast pyrolysis process is the reactor. Although it probably represents at most 

only about 10-15% of the total capital cost of an integrated system, almost all research and 

development has focused on the reactor. The rest of the process consists of biomass reception, 

storage and handling, biomass drying and grinding, product collection, storage and, when 

relevant, upgrading. The key aspects of these peripheral steps are described later. A 

comprehensive survey of fast pyrolysis processes has been published that describes all the 

pyrolysis processes for liquids production that have been built and tested in the last 10-15 

years [32]. 

Bubbling fluid beds [Figure 10]

• simple construction and operation 

• good temperature control 

• very efficient heat transfer to biomass particles due to high solids density 

• easy scaling 

• well understood technology 

• good and consistent performance with high liquid yields: of typically 70-75%wt. from 

wood on a dry feed basis 

• heating can be achieved in a variety of ways as shown in figure 3 

• residence time of solids and vapours is controlled by the fluidising gas flow rate and is 

higher for char than for vapours 

• char acts as an effective vapour cracking catalyst at fast pyrolysis reaction temperatures so 

rapid and effective char separation/elutriation is important 

• small biomass particle sizes are needed to achieve high biomass heating rates of less than 

2-3 mm 

• good char separation is important - usually achieved by ejection and entrainment followed 

by separation in one or more cyclones 

• heat transfer to bed at large scale has to be considered carefully due to scale-up limitations. 

Examples: 

Waterloo [basic research] [33]; Union Fenosa [34]; Dynamotive [35]; Wellman [36] 
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BIOMASS

BIO-OIL 

CHAR 

For heat 

or export 

Gas recycle

GAS 

For export

Fluid 

bed 

reactor 

Quench 

cooler 

Electrostatic 

precipitator 

Recycle gas heater and/or oxidiser

BIO-OIL

BIOMASS 

Flue 

gas 

Gas recycle 

GAS 

For export

Sand + 

Char

Combustor 

Pyrolyser

Air 

Hot 

sand

Figure 10 Bubbling fluid bed reactor Figure 11 Circulating fluid bed reactor 

Circulating fluid beds and transported bed [Figure 11]

• good temperature control can be achieved in reactor, 

• residence time for the char is almost the same as for vapours and gas, 

• CFBs are suitable for very large throughputs, 

• well understood technology, 

• hydrodynamics more complex, 

• char is more attrited due to higher gas velocities; char separation is by cyclone, 

• closely integrated char combustion in a second reactor requires careful control, 

• heat transfer at large scale has to be proven. 

Examples: 

Ensyn [37]; CRES [38] 

Ablative pyrolysis [Figure 12]

• high pressure of particle on hot reactor wall, achieved due to centrifugal force [NREL] or 

mechanically [Aston] 

• high relative motion between particle and reactor wall, 

• reactor wall temperature should be less than 600°C. 

• large feed sizes can be used, 

• inert gas is not required, so the processing equipment is smaller, [in case of mechanically 

applied pressure] 

• the reaction system is more intensive, 

• reaction rates are limited by heat transfer to the reactor, not to the biomass, 

• the process is surface area controlled so scaling is more costly, 

• the process is mechanically driven so the reactor is more complex. 
- 14 - 

Examples 

CNRS Nancy [basic research] [39]; NREL [40]; Aston University [41] 

Solids 

 recycle 

 loop 

Steam or 

nitrogen 

at 700°C Ejector 

BIOMASS

GAS 

Cylindrical 

wall heaters 

BIO-OIL 

Liquid 

collection 

Vortex 

reactor 

Particle trajectory 

 Heated 

 rotating 

 cone

Particle 

Figure 12 NREL Vortex ablative reactor Figure 13 Principle of 

rotating cone pyrolysis 

reactor 

Entrained flow

• Simple technology, 

• Poor heat transfer, 

• High gas flows give large plant and cause difficult liquid collection, 

• Good scale-up, 

• Lower liquid yields. 

Examples 

GTRI [42]; Egemin [43] 

Rotating cone [Figure 13]

• centrifugation [ at around 10Hz] drives hot sand and biomass up a rotating heated cone, 

• vapours are collected and processed conventionally, 

• Char and sand drop into a fluid bed surrounding the cone from where they are lifted to a 

separate fluid bed combustor where char is burned to heat the sand which is then dropped 

back into the rotating cone, 

• char is burned in a secondary bubbling fluid bed combustor. The hot sand is recirculated to 

the pyrolyser 

• carrier gas requirements in the pyrolysis reactor are much less than for fluid bed and 

transported bed systems, however, gas is needed for char burn off and for sand transport, 

• complex integrated operation of three subsystems is required: rotating cone pyrolyser, riser 

for sand recycling, and bubbling bed char combustor, 

• liquid yields of 60-70% on dry feed are typically obtained. 
- 15 - 

Examples 

Twente University [44]; BTG [45] 

Vacuum pyrolysis

• not a true fast pyrolysis process as solids residence time is very high, 

• it can process larger particles than most fast pyrolysis reactors 

• there is less char in the liquid product due to lower gas velocities 

• there is no requirement for a carrier gas 

• liquid yields of 35-50% on dry feed are typically obtained with higher char yields than fast 

pyrolysis systems; conversely, the liquid yields are higher than in slow pyrolysis 

technologies because of fast removal of vapours from the reaction zone 

• the process is relatively complicated mechanically. 

Example 

Pyrovac [46] 

Char removal

Char acts as a vapour cracking catalyst so rapid and effective separation from the pyrolysis 

product vapours is essential. Cyclones are the usual method of char removal however, some 

fines always pass through the cyclones and collect in the liquid product where they accelerate 

aging and exacerbate the instability problem, which is described below. Hot vapour filtration, 

analogous to hot gas filtration in gasification processes, gives a high quality char free product 

[47], however the liquid yield is reduced by about 10-20% due to the char accumulating on 

the filter surface that cracks the vapours. 

Pressure filtration of the liquid is very difficult due to the complex interaction of the char and 

pyrolytic lignin, which appears to form a gel-like phase that rapidly blocks the filter. 

Modification of the liquid micro-structure by addition of solvents such as methanol or ethanol 

that solubilise the less soluble constituents will improve this problem and also contribute to 

improvements in liquid stability as described below. 

Liquid collection

The gaseous products from fast pyrolysis consist of aerosols, true vapours and noncondensable gases. These require rapid cooling to minimise secondary reactions and to 

condense the true vapours, while the aerosols require coalescence or agglomeration. Simple 

heat exchange can cause preferential deposition of lignin derived components leading to 

liquid fractionation and eventually blockage. Quenching in product oil or in an immiscible 

hydrocarbon solvent is widely practised. Orthodox aerosol capture devices such as demisters 

and other commonly used impingement devices are not very effective and electrostatic 

precipitation is currently the preferred method at smaller scales up to pilot plant. The vapour 

product from fluid bed and transported bed reactors has a low partial pressure of collectible 

products due to the large volumes of fluidising gas, and this is an important design 

consideration in liquid collection. 
- 16 - 

Pyrolysis Liquid - Bio-oil 

Pyrolysis liquid is referred to by many names including pyrolysis oil, bio-oil, bio-crude-oil, 

bio-fuel-oil, wood liquids, wood oil, liquid smoke, wood distillates, pyroligneous tar, 

pyroligneous acid, and liquid wood. The crude pyrolysis liquid is dark brown and 

approximates to biomass in elemental composition. It is composed of a very complex mixture 

of oxygenated hydrocarbons with an appreciable proportion of water from both the original 

moisture and reaction product. Solid char and dissolved alkali metals from ash [48] may also 

be present. 

Liquid product characteristics 

The liquid is formed by rapidly quenching and thus ‘freezing’ the intermediate products of 

flash degradation of hemicellulose, cellulose and lignin. The liquid thus contains many 

reactive species, which contribute to its unusual attributes. Bio-oil can be considered a microemulsion in which the continuous phase is an aqueous solution of holocellulose 

decomposition products, that stabilizes the discontinuous phase of pyrolytic lignin macromolecules through mechanisms such as hydrogen bonding. Aging or instability is believed to 

result from a breakdown in this emulsion. In some ways it is analogous to asphaltenes found 

in petroleum. 

Fast pyrolysis liquid has a higher heating value of about 17 MJ/kg as produced with about 

25% wt. water that cannot readily be separated. The liquid is often referred to as ‘oil’ or ‘biooil’ or ‘bio-crude’ although it will not mix with any hydrocarbon liquids. It is composed of a 

complex mixture of oxygenated compounds that provide both the potential and challenge for 

utilisation. There are some important characteristics of this liquid that are summarised in 

Table 3 and discussed briefly in Table 4. 

Table 3 Typical properties of wood derived crude bio-oil 

Physical property Typical value Characteristics 

Moisture content 15-30% 

pH 2.5 

Specific gravity 1.20 

Elemental analysis C 55-58% 

 H 5.5-7.0% 

 O 35-40% 

 N 0-0.2% 

 Ash 0-0.2% 

HHV as produced 16-19 MJ/kg 

Viscosity [at 40°C and 25% water] 40-100 cp 

Solids [char] 1% 

Vacuum distillation residue up to 50% 

• Liquid fuel 

• Ready substitution for conventional 

fuels in many static applications 

such as boilers, engines, turbines 

• Heating value of 17 MJ/kg at 25% 

wt. water, is about 40% that of fuel 

oil / diesel 

• Does not mix with hydrocarbon 

fuels 

• Not as stable as fossil fuels 

• Quality needs definition for each 

application, 

Upgrading pyrolysis liquid 

The most important properties that adversely affect bio-oil fuel quality are incompatibility 

with conventional fuels, solids content, high viscosity, and chemical instability. The field of 

chemical and physical upgrading of bio-oil has been thoroughly reviewed [49]. Hot-gas 

filtration can reduce the ash content of the oil to less than 0.01% and the alkali content to less 

than 10 ppm - much lower than reported for biomass oils produced in systems using only 

cyclones. 
- 17 - 

Table 4 Typical properties and characteristics of wood derived crude bio-oil 

Appearance Pyrolysis oil typically is a dark brown free flowing liquid. Depending upon 

the initial feedstock and the mode of fast pyrolysis, the colour can be almost 

black through dark red-brown to dark green, being influenced by the 

presence of micro-carbon in the liquid and by the chemical composition. Hot 

vapour filtration gives a more translucent red-brown appearance due to the 

absence of char. High nitrogen contents in the liquid can give it a dark green 

tinge. 

Odour The liquid has a distinctive odour - an acrid smoky smell, which can irritate 

the eyes if exposed for a prolonged period to the liquids. The cause of this 

smell is due to the low2 molecular weight aldehydes and acids. The liquid 

contains several hundred different chemicals in widely varying proportions, 

ranging from formaldehyde and acetic acid to complex high molecular 

weight phenols, anhydrosugars and other oligosaccharides. 

Miscibility The liquid contains varying quantities of water which forms a stable single 

phase mixture, ranging from about 15 wt% to an upper limit of about 30-

50wt% water, depending on how it was produced and subsequently 

collected. Pyrolysis liquids can tolerate the addition of some water, but there 

is a limit to the amount of water, which can be added to the liquid before 

phase separation occurs, in other words the liquid cannot be dissolved in 

water. It is miscible with polar solvents such as methanol, acetone, etc. but 

totally immiscible with petroleum-derived fuels. 

Density The density of the liquid is very high at around 1.2 kg/litre compared to light 

fuel oil at around 0.85 kg/litre. This means that the liquid has about 42% of 

the energy content of fuel oil on a weight basis, but 61% on a volumetric 

basis. This has implications on the design and specification of equipment 

such as pumps. 

Viscosity The viscosity of the bio-oil as produced can vary from as low as 25 cSt to as 

high as 1000 cSt [measured at 40°C] or more depending on the feedstock, 

the water content of the oil, the amount of light ends that have been 

collected and the extent to which the oil has aged. Viscosity is important in 

many fuel applications [50]. 

Distillation Pyrolysis liquids cannot be completely vaporised once they have been 

recovered from the vapour phase. If the liquid is heated to 100oC or more to 

try to remove water or distil off lighter fractions, it rapidly reacts and 

eventually produces a solid residue of around 50wt% of the original liquid 

and some distillate containing volatile organic compounds and water. The 

liquid is, therefore, chemically unstable, and the instability increases with 

heating, so it is preferable to store the liquid at room temperature. These 

changes do also occur at room temperature, but much more slowly and can 

be accommodated in a commercial application. 

Aging of 

pyrolysis 

liquid 

The complexity and nature of bio-oil causes some unusual behaviour, 

specifically that the following properties tend to change with time: 

• viscosity increases 

• volatility decreases 

• phase separation and deposition of gums can occur. 

A process for producing stable micro-emulsions with 5-30% of bio-oil in diesel has been 

developed at CANMET [51] and the University of Florence, Italy, has been working 
- 18 - 

emulsions of 5 to 95% bio-oil in diesel [52]. The addition of polar solvents, especially 

methanol, gave a significant positive effect on the oil stability [53]. 

Chemical/catalytic upgrading processes to produce hydrocarbon fuels that can be 

conventionally processed are more complex and costly than physical methods, but offer 

significant improvements ranging from simple stabilization to high-quality fuel products[54]. 

Full deoxygenation to high-grade products such as transportation fuels can be accomplished 

by two main routes: hydrotreating and catalytic vapour cracking over zeolites, both of which 

have been reviewed [55, 56]. 

Applications for Bio-oil

Bio-oil can substitute for fuel oil or diesel in many static applications including boilers, 

furnaces, engines and turbines for electricity generation. The possibilities are summarised in 

Figure 10. There is also a range of chemicals that can be extracted or derived including food 

flavourings, specialities, resins, agri-chemicals, fertilisers, and emissions control agents. 

Upgrading bio-oil to transportation fuels is feasible but currently not economic. At least 400 

hours’ operation has been achieved on a 250 kWe specially modified dual fuel engine and 

limited experience has been gained on a modified 2.5 MWe gas turbine. 

TRANSPORT FUEL ELECTRICITY and/or HEAT CHEMICALS

Stabilise or 

upgrade

Combustion 

Turbine, Engine, Boiler 

Fast pyrolysis to 

liquids

Extract and/or 

react

PREPARED BIOMASS 

Dried and controlled particle size

Figure 14 Applications for Bio-oil. 

A range of chemicals can also be produced from specialities such as levoglucosan to 

commodities such as resins and fertilisers as summarised in Table 5. Food flavourings are 

commercially produced from wood pyrolysis products in many countries. All chemicals are 

attractive possibilities due to their much higher added value compared to fuels and energy 

products, and lead to the possibility of a bio-refinery concept in which the optimum 

combinations of fuels and chemicals are produced. 

Table 5 Chemicals from fast pyrolysis 

Acetic acid Adhesives Calcium enriched bio-oil Food flavourings 

Hydrogen Hydroxyaceladehyde Levoglucosan Levoglucosenone 

Preservatives Resins Slow release fertilisers Sugars 
- 19 - 

Summary

The liquid bio-oil product from fast pyrolysis has the considerable advantage of being 

storable and transportable as well as the potential to supply a number of valuable chemicals, 

but there are many challenges facing fast pyrolysis that relate to technology, product and 

applications. The problems facing the sector include the following: 

• Cost of bio-oil, which is 10% to 100% more than fossil fuel 

• Availability: there are limited supplies for testing 

• There is a lack of standards for use and distribution of bio-oil and inconsistent quality 

inhibits wider usage; considerable work is required to characterise and standardise these 

liquids and develop a wider range of energy applications. 

• Bio-oil is incompatible with conventional fuels 

• Users are unfamiliar with this material 

• Dedicated fuel handling systems are needed 

• Pyrolysis as a technology does not enjoy a good image. 

The most important issues that need to be addressed seem to be: 

• Scale-up, 

• Cost reduction, 

• Improving product quality including setting norms and standards for producers and 

users, 

• Environment health and safety issues in handling, transport and usage, 

• Encouragement for developers to implement processes; and users to implement 

applications. 

• Information dissemination. 

COMPARISON WITH BIOLOGICAL CONVERSION PROCESSES 

There are some key fundamental differences between thermal and biological conversion: 

Biological conversion 

• gives single or specific products such as ethanol or methane, 

• slow process – hours, days, weeks or years 

Thermal conversion 

• gives multiple and often complex products, and catalysis is often used to improve 

the product quality or spectrum, 

• fast process – seconds or minutes. 

In order to put these basic differences into context, a brief summary of the status of biological 

conversion is given below. 

Fermentation 

The cellulose and hemicellulose in lignocelluloses need to be hydrolysed to sugars prior to 

fermentation and both enzyme and acid hydrolysis is employed for this purpose. 

Carbohydrates such as starch also require hydrolysis. Only the cellulose in biomass is 

converted to ethanol, although recent developments now convert some hemicellulose in 

integrated processes. The lignin is a residue that can be used for process heat, particularly for 

ethanol concentration. Recent developments include SSF – Simultaneous Saccharification 

and Fermentation – to reduce costs and improve conversion efficiency. 
- 20 - 

Efficiencies overall are rather low due to the need to concentrate the dilute ethanol solution 

and there is intensive development work underway to improve overall efficiencies and reduce 

costs. Fermentation is particularly suitable for materials with a high moisture content as 

drying is not required. The technology is commercial and tends to be high cost unless 

subsidies are applied and low efficiency unless credits are applied to the by-products. 

Another recent development is fermentation of synthesis gas from thermal gasification. The 

carbon monoxide, carbon dioxide and hydrogen are biologically converted to ethanol in high 

yield and at very high reaction rates (57). The process is still at an early stage of 

development. 

Fermentation for alcohol production is readily available commercially, but without incentives 

is expensive and efficiencies are low. 

Digestion 

Anaerobic digestion is microbial conversion of organic materials to methane and carbon 

dioxide with typically around 60% methane, although higher levels have been reported. This 

can take place in landfill sites or purpose built digesters. Digestion is particularly suitable for 

materials with a high moisture content as drying is not required. The product gas can be used 

for heat with minimal processing or for power generation in engines, turbines and fuel cells 

which require increasingly stringent quality specifications. 

Landfill sites tend to be very large and typically produce gas over a 20 to 25 year life. Gas is 

collected through a system of wells, which pipe the gas to the user. The gas is wet and 

contains acid components, which require careful management to avoid or minimise problems 

in using the gas for heat or power production. As landfill gas collection is required in many 

countries to comply with legislation to minimise hazards, the additional costs of distributing 

gas to users is very small, so landfill gas is an economically attractive renewable energy 

resource. However as pressure grows to reduce landfilling, this resource is expected to 

reduce. 

On-farm or industrial digesters are becoming more widely used for smaller scale applications 

using farm wastes, industrial food wastes and household wastes. The water industry in 

particular has successfully and effectively used digestion for in-plant power generation for 

many years. Dedicated digesters tend to be costly and have low efficiency as well as 

requiring disposal of a significant amount of solid residues. Thus small scale digestion is not 

often cost effective. 

In all types of digestion, the carbon dioxide can be reduced or removed to give higher quality 

gas up to natural gas qualities. There is a range of technologies available according to scale 

of operation including membrane processes and conventional scrubbing processes. All are 

costly and careful assessment is needed that considers the added value of the resultant higher 

quality gas. 

Although there is widespread interest in anaerobic digestion for both waste disposal and 

energy generation, economic opportunities are limited to niche applications. 
- 21 - 

Physical Processes 

Biodiesel is the ester formed by reacting vegetable oils or animal fats with methanol or 

ethanol. Vegetable oil can be recovered from oil seeds such as rape (canola or colza), or other 

crops such as linseed, sunflower etc. The product in its raw form is unsuitable for many 

applications due to high viscosity and other deleterious properties so the methyl or ethyl ester 

is formed by esterification. The most common product is Rape Methyl Ester or RME. 

The raw oil is recovered by pressing, usually accompanied by solvent extraction to improve 

yields. This raw oil is then subjected to esterification with methanol or ethanol over a catalyst 

giving a lower viscosity and more stable product with glycerine as a by-product. The ester is 

entirely compatible with diesel in use and applications and biodiesel is thus an attractive 

renewable transport fuel. The yield of vegetable oil per unit land area is, however, very low 

at around 1 t/ha/y and is accompanied by around 10t/ha/y of solid residues – rape straw for 

example. The product cost is therefore high. The technology is simple and proven. Animal 

fats and waste cooking oils can also be processed in an analogous way. 

PRODUCTS 

Table 6 summarises some of the main products that are derivable by processing biomass. 

Particular points to note in comparing different conversion processes are that biological 

conversion generally gives single or specific products, and is a slow process typically taking 

hours, days or weeks. 

Thermal conversion, however, gives multiple and often complex products with catalysis often 

being used to improve the product spectrum and/or quality and is a fast process with reaction 

times of seconds or minutes. 

Table 6 Some products from conversion of biomass 

 Biological 

processing 

Physical 

processing 

Thermal 

processing 

FUELS Additives √ √ √

 Alcohols √ √

 Charcoal √

 Diesel √ √

 Electricity √

 Fuel oil √

 Gas √ √

 Gasoline √

 Hydrogen √ √

CHEMICALS Acetone √ 

 Butanol √

 Ethanol √ √

 Fertilisers √ √

 Food additives √ √

 Hydrogen √ √

 Methane √ √

 Methanol √

 Resins √

 Specialities √ √ √
- 22 - 

BARRIERS 

The technologies have to overcome a number of technical and non-technical barriers before 

industry will implement their commercialisation. The technical barriers have been described 

above, while some of the more important non-technical barriers are summarised below. 

Economics 

All bio-fuels have to compete with fossil fuels. For those countries who have made 

commitments to reduce fossil fuel derived carbon emissions, the current disincentive to 

implementation of bio-energy on simple cost grounds will have to be overcome - no company 

is going to invest in ventures that are guaranteed to lose money, regardless of the 

environmental benefits that may accrue. Industry will only invest in technology that has an 

acceptable return at an acceptable risk [58]. Acceptable returns will come from incentives to 

capital expenditure of product purchase, or to dis-incentives to orthodox options by, for 

example taxation on fossil fuels or legislation, etc. This is one of the major roles that 

governments have to play. 

Perception 

There is widespread approval of the interest in, and move towards, renewal energy and bioenergy – as long as it is not near me! Increasing attention will need to be placed on “selling” 

the idea to the populace where the plant is to be built, and this problem could be exacerbated 

by the need to build smaller plants and thus many more plants than conventional power 

stations and refineries. Early plants will have the curiosity factor and may enjoy popularity as 

an attraction by itself or as part of an attraction of a “green” site. 

Politics 

For industry to implement renewable energy technologies in order that commitments made to 

mitigate greenhouse gases can be met, investment has to be attractive. Without some fiscal 

incentives [or disincentives against fossil fuels], companies will only invest in those projects 

that are sufficiently profitable and most of these will be in niche markets and special 

opportunities. As commented above, only governments can create the necessary instruments. 

Scale 

Economies of scale are a vital feature of the development of industry and technology in which 

the larger a process can be built, the cheaper it becomes. This is particularly important in the 

energy and process industries that will shoulder the responsibility for technology development 

and implementation. However in the bio-energy industry, biomass is a diffuse resource which 

has to be harvested over large areas. A modest 10 MWe power station operating at a modest 

efficiency of 35% will require about 40000 t/year of wood on a dry basis which will require 

about 4000 ha of land or 40 square km. This could reduce to 2000 ha or 20 square km if the 

promise of high yielding short rotation forestry is realised. Neither figure makes any 

allowance for non-productive land. There are therefore finite sizes that bio-energy processes 

can be built in considering the costs and logistics of transporting biomass to a processing 

plant. The maximum size that has been suggested in Europe ranges from 30 to 80 MWe in 

the short to medium term, and 100 to 150 MWe in North America. This places a practical 

upper limit on the benefits of scale. 
- 23 - 

Risk 

Investors are generally risk averse and always prefer low risk investments, but if risks have to 

be accepted, then an appropriately higher return is expected. Technology developers can do 

much to minimise technical risk and this topic has been thoroughly described and discussed 

[58]. 

Vested interests 

The established energy suppliers and providers have considerable investments in orthodox 

energy systems and will always seek to maximise their returns and maintain their competitive 

edge. Most major energy companies have their own programmes of supporting renewable 

energy, but there have always been concerns over the extent to which they will seek to protect 

their interests. 

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