- 1 - 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 projectedrenewable 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.- 2 - 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% Pyrolysis for charcoal Pyrolysis for liquids Gasification by pyrolysis 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- 3 - 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, CO 2 , H 2 , CH 4 , N 2 , tar This gives a low heating value gas of ~5MJ/m 3 . Utilisation problems can arise in combustion, particularly in gas turbines. Partial oxidation with oxygen The main products are CO, CO 2 , H 2 , CH 4 , tar [no N 2 ] This gives a medium heating value gas of ~10-12 MJ/m 3 . 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, CO 2 , H 2 , CH 4 , tar This gives a medium heating value gas of ~15-20 MJ/m 3 . 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- 4 - 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 reacto r

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,- 5 - • 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.- 6 - BIOMASS Gasifie r

FUEL GAS

Air Filter (optional)

Cooler (optional) BIOMASS Flue gas

Sand +

Char Gasifier

A ir Hot sand

Combustor Bubbling fluid bed

or Circulating fluid bed A ir A sh 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- 8 - 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

A mmonia & 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.- 11 - 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.- 12 - 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]- 13 - BIOMASS

BIO-OIL CHAR For heat or export

Gas recycle GAS For export

Fluid bed
reactor Quench cooler

Electrostatic precipitator

R ecycle gas heater and/or oxidise r 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 manyreactive species, which contribute to its unusual attributes. Bio-oil can be considered a microemulsion in which the continuous phase is an aqueous solution of holocellulosedecomposition 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 about25% 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.

ELECTRICITY and/or HEAT TRANSPORT FUEL 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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