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INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2011; 35:835–862 Published online 3 January 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.1804

REVIEW

Bio-oil, solid and gaseous biofuels from biomass pyrolysis processes—An overview D. Vamvuka,y Department of Mineral Resources Engineering, Technical University of Crete, Hania, Greece

SUMMARY As the global demand for energy rapidly increases and fossil fuels will be soon exhausted, bio-energy has become one of the key options for shorter and medium term substitution for fossil fuels and the mitigation of greenhouse gas emissions. Biomass currently supplies 14% of the world’s energy needs. Biomass pyrolysis has a long history and substantial future potential—driven by increased interest in renewable energy. This article presents the stateof-the-art of biomass pyrolysis systems, which have been—or are expected to be—commercialized. Performance levels, technological status, market penetration of new technologies and the costs of modern forms of biomass energy are discussed. Advanced methods have been developed in the last two decades for the direct thermal conversion of biomass to liquid fuels, charcoals and various chemicals in higher yields than those obtained by traditional pyrolysis processes. The most important reactor configurations are fluidized beds, rotating cones, vacuum and ablative pyrolysis reactors. Fluidized beds and rotating cones are easier for scaling and possibly more cost effective. Slow pyrolysis is being used for the production of charcoal, which can also be gasified to obtain hydrogen-rich gas. The short residence time pyrolysis of biomass (flash pyrolysis), at moderate temperatures, is being used to obtain a high yield of liquid products (up to 70% wt), particularly interesting as energetic vectors. Bio-oil can substitute for fuel oil— or diesel fuel—in many static applications including boilers, furnaces, engines and turbines for electricity generation. While commercial biocrudes can easily substitute for heavy fuel oils, it is necessary to improve the quality in order to consider biocrudes as a replacement for light fuel oils. For transportation fuels, high severity chemical/catalytic processes are needed. An attractive future transportation fuel can be hydrogen, produced by steam reforming of the whole oil, or its carbohydrate-derived fraction. Pyrolysis gas—containing significant amount of carbon dioxide, along with methane—might be used as a fuel for industrial combustion. Presently, heat applications are most economically competitive, followed by combined heat and power applications; electric applications are generally not competitive. Copyright r 2011 John Wiley & Sons, Ltd. KEY WORDS biomass; pyrolysis; biofuels; bio-energy Correspondence *D. Vamvuka, Department of Mineral Resources Engineering, Technical University of Crete, Hania, Greece. y E-mail: [email protected] Received 24 March 2010; Revised 14 October 2010; Accepted 26 October 2010

1. INTRODUCTION Energy drives human life and it is crucial for continued human development. A booming global population demands increased energy. The world relies heavily on fossil fuels to meet its energy requirements. However, with a worldwide increase in consumption, these fuels—especially oil and natural gas—will be exhausted, probably before the end of this century. Furthermore, fossil fuels and nuclear energy consumption are closely linked to environmental degradation Copyright r 2011 John Wiley & Sons, Ltd.

that threatens human health, through climate changes and greenhouse gas emissions. Biomass currently supplies 14% of the world’s energy needs [1]. Technologies and processes exist today which—if used properly—make biomass-based fuels less harmful to the environment than fossil fuels. Applying these technologies and processes on a sitespecific basis, to minimize negative environmental impacts, is a prerequisite for the sustainable use of biomass energy in the future. Bio-energy is one of the key options for a short and medium term substitution

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for fossil fuels and the mitigation of greenhouse gas emissions. This is most evident in Europe, where a kaleidoscope of activities and programs are being executed to develop and stimulate bio-energy—both on the European and national levels. The European Union has targeted 2010 as the deadline by which 10% of the total energy supply should come from biomass. A recent directive on biofuels for the transport sector sets a goal of 5.75% for the use of biomass for transport fuels by December 2010. In the long term, the considerable progress made by substitute fuels might make it technically possible to replace 20% of the petrol and diesel fuel used for road transport, by the year 2020 [2,3]. Biomass pyrolysis has a long history and considerable future potential, driven by the increased interest in renewable energy—following the environmental commitments in the Kyoto Agreement and, recently, increased market prices for fossil fuels. Pyrolysis can be described as the direct thermal decomposition of the organic matrix in the absence of oxygen to obtain an array of solid (charcoal), liquid (bio-oil) and gas products. The pyrolysis method has been used for commercial production of a wide range of fuels, solvents, chemicals and other products from biomass feedstocks. Liquids are particularly interesting as energetic vectors, because they have, in comparison with gas and solid, a high energy density and offer advantages in transport, storage, flexibility of use and retrofitting. Another important point is the possibility of using these liquids in existing facilities, such as boilers, diesel engines or turbines. However, pyrolysis with high yield oil production in focus is a relatively new ‘re-discovery’. It was recognized, only in the 1980s, that fast pyrolysis is a good alternative to expensive hydrocracking technology. Knowledge of the effects of various independent parameters, such as biomass feedstock type and composition, reaction temperature and pressure, residence time and catalysts on reaction rates, product selectivities and product yields, has led to the development of advanced biomass pyrolysis processes. The accumulation of considerable experimental data on these parameters has resulted in advanced pyrolysis methods, for the direct thermal conversion of biomass to liquid fuels and various chemicals, in higher yields than those obtained by the traditional long residence time pyrolysis methods. Thermal conversion processes have also been developed, for producing high yields of charcoals from biomass. In this article, the modern reactor configurations and the state-of-the-art systems that have been—or are expected to be commercialized—are analyzed. The performance levels, their technological status and their economics are discussed. Examples of a variety of processes are given, which show how countries at different levels of development and with different endowments can find ways to benefit from local biomass 836

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resources, for both energy provision and sustainable development. Finally, market penetrations of the new technologies and the costs of modern forms of biomass energy are presented.

2. FUNDAMENTALS OF PYROLYSIS Conventional pyrolysis consists of the slow, irreversible, thermal decomposition of the organic components of biomass, most of which are lignocellulosic polymers. Slow pyrolysis (hrs-days) has traditionally been used for the production of charcoal. Short residence time pyrolysis (few seconds to a fraction of a second) of biomass (flash pyrolysis), at moderate temperatures (400–6501C), has generally been used to obtain high yield of liquid products (up to 70% wt). Fast pyrolysis is characterized by high heating rates and rapid quenching of the liquid products, to terminate the secondary conversion of the products. In comparison with coal, biomass pyrolysis starts earlier and the volatile matter content is higher. The fractional heat contribution by volatile matter is of the order of 70%, compared with 36% for coal; however, biomass char has more oxygen and its fractional heat contribution is of the order of 30%, compared with 70% for coal [4].

2.1. Mechanism and reaction modeling Each of the structural constituents of biomass (hemicelluloses, cellulose, lignin, extractives) pyrolyze at different rates and by different mechanisms and pathways. The rate and extent of degradation of each of these components depend on the process parameters of reactor type, temperature, particle size, heating rate and pressure. It is believed that as the reaction progresses the carbon becomes less reactive and forms stable chemical structures. Dehydration, cracking, isomerization, dehydrogenation, aromatization, coking and condensation reactions and rearrangements occur during pyrolysis. The products are water, carbon oxides, other gases, charcoal, organic compounds, tars and polymers [5,6]. The complexity of pyrolysis is illustrated in Figure 1. A novel technique [7], based on flash devolatilization of biomass and direct molecular-beam, mass-spectrometric analysis, has shown that levoglucosan is a primary product of the pyrolysis of pure cellulose. However, the yield of levoglucosan on pyrolysis of most biomass is low, even though the cellulose content is about 50% wt. From early work on the mechanisms and kinetics of biomass pyrolysis, a two-path mechanism was proposed (Figure 2). One involves dehydration and charring reactions via anhydrocellulose intermediates to form chars, tars, carbon oxides and water and the other involves depolymerization and volatilization via Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er


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Figure 1. Mechanisms of pyrolysis.

Figure 2. Two-path mechanism for cellulose pyrolysis.

levoglucosan intermediate, to form chars and combustible volatiles [8]. The first pathway would be expected to occur at lower temperatures, where dehydration reactions are dominant. The second pathway results in the formation of oligomeric species, as well as their degradation products, which immediately enter the vapor phase. If permitted to quickly escape the reactor, the vapors form condensed oils and tars. If held in contact with the solid biomass undergoing devolatilization within the reactor, the vapors degrade further to form chars, various gases and water. The two competitive pathways help to explain the effects of pyrolysis conditions on product Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

yields and distributions. Although these pathways may be dominant, there are undoubtedly many other pathways that are operative with actual biomass species. The pyrolysis of hemicellulose gives rise to analogous families of chemical compounds like cellulose [7]. Lignin decomposes over a wider temperature range compared with cellulose and hemicelluloses, which rapidly degrade over narrower temperature ranges; hence, the apparent thermal stability of lignin during pyrolysis. The kinetics of thermal decomposition of biomass materials is complicated, as it involves a large number of reactions in parallel and series. Several studies have pointed out the existence of slight interactions between cellulose, hemicellulose and lignin components during thermal decomposition [9,10]. One-component or multi-component mechanisms of primary pyrolysis have been proposed (Figure 3), based on the analysis of experimental data on biomass pyrolysis obtained for isothermal condition, or fast heating rate [11,12]. However, one-component behavior for composite fuels, such as biomass, produces inaccuracies in the details of the decomposition rates. Devolatilization mechanisms are also available, where additional reactions are introduced to describe volatile formation from minor components, or to take into account 837

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each study. Activation energy values for cellulose decomposition vary between 195 and 286 kJ mol1, whereas for hemicellulose and lignin decomposition they vary between 80 and 116 kJ mol1, and 18 and 65 kJ mol1, respectively [23].

2.2. Pyrolysis products Figure 3. One-component mechanism of primary wood pyrolysis and multi-component devolatilization mechanism (Ci is the volatile fraction of the jth pseudo-component) [11,12].

different steps in the volatile release from the main biomass components, or power law dependence on the mass fractions are assumed. Different schemes of reactions have been considered, i.e. single, series, consecutive, consecutive series and parallel independent reactions [9,13–19]. Nevertheless, a first-order rate expression has been commonly used for each reaction in the proposed scheme. Most authors have reported three independent parallel reaction models as the most realistic approach in the case of lignocellulosic materials [14,15,20,21]. According to this model, the decomposition of biomass is described by three independent parallel reactions, each corresponding to the decomposition of the constituent components hemicellulose, cellulose and lignin. The overall rate of conversion for N reactions can be expressed as dm X dai ¼ ; i ¼ 1; 2; . . . ;N ð1Þ ci dt dt i dai ¼ Ai expðEi =RTÞð1 ai Þ dt

ð2Þ

where dm/dt is the mass loss rate, i the component, ai the conversion rate, ci the contribution of the partial process to the overall mass loss, m0mchar, Ai the frequency factor, Ei the activation energy, R the gas constant and T the temperature. The DAEM model describes a complex reaction as a number of parallel first-order reactions, each occurring with its own rate coefficient. Usually, it is further assumed that all reactions share the same frequency factor. The n-order DAEM, a phenomenological model, combines a reaction order with an activation energy distribution. The reaction rate can be expressed as: dðVi =Vi Þ ¼ ki ð1 Vi =Vi Þn dt

ð3Þ

The released mass fraction at time t for the ith constituent is denoted by Vi (t) and the total released mass fraction for the ith constituent is Vi [22]. Substantial differences can be observed in the kinetic parameters reported in the literature, which can be due to several factors related to the experimental methods, operating conditions and data analysis, but also to the chemical composition of raw materials examined in 838

2.2.1. Liquids. The liquid fraction of biomass pyrolysis bio-oils is a complex mixture of water and organic chemicals. Water contents are typically in the range of 15–35% wt, although values outside this range have been reported. Organic components consist of acids, alcohols, aldehydes, ketones, esters, phenols, guaiacols, syringols, sugars, furans, alkenes, aromatics, nitrogen compounds and miscellaneous oxygenates. Their average molecular weight varies in the range of 300–1000 g mol1. The highest concentration compound is hydroxyacetaldehyde (up to 10% wt), followed by acetic and formic acids (5% and 3% wt, respectively) [24–29]. The physicochemical properties of bio-oil are well documented in the literature [24,25,28–30], so that a short summary will be presented in this article. The presence of water lowers the heating value and flame temperature; however, it reduces the viscosity and enhances the fluidity, which is good for bio-oil combustion in engines. The high oxygen content (35–40%) lowers the energy density and immiscibility with hydrocarbon fuels. Depending on the biomass feedstocks and pyrolytic processes, the viscosities of bio-oils vary in a large range (10–100 cP at 401C). Aging studies have shown that viscosity and molecular weight increase together with time. The addition of methanol reduces the viscosity and density and increases the stability, with the limitation of a lowered flash point in the blend. The carboxylic acids of bio-oil account for its acidic pH of 2–3. Acidity makes bio-oil very corrosive, requiring upgrading before use. The ash content of the pyrolysis oil has shown to be directly related to the char content of the oils. By hot-gas filtration, the ash content can be lowered to below 0.01% wt, which meets the requirement for even the best quality diesel fuel. In addition, its alkali content can be lowered to about 2 ppm, which is very close to that recommended for gas-turbine fuels. The lower heating value (LHV) of bio-oils is only 40–45% of that exhibited by hydrocarbon fuels. On a volume basis it is 60% of the heating value of hydrocarbon oils, because of the high oxygen content, the presence of water and the higher density. A typical heating value of biooil is 17 MJ kg1. The storage stability of pyrolysis oil is rather poor. Bio-oil ages after it is first recovered, which is observed as a viscosity increase. Some phase separation may also occur. The instability is believed to result from a breakdown in the stabilized microemulsion and to chemical reactions, which continue to proceed in the oil. However, it has been shown that Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

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Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

1.14 2.2 9.5 (371C) 31.4 9 1.23 2.7 175.6 (401C) 16.8 25

19.9 0.35 1.18 3.7 11 (501C) 17.0 36

26.4

1.24 2.7 105.7 (401C) 17.1

0.1 1.0 0.94 18.6

Softwood

Elephant grass/agro-industrial residues 2.2 Mapleoak

BTG Bioware Technologia Ensyn RTP Ensyn

Hardwood Eucalyptus

2.7 2.1 177.8 (401C) 15.0

23.44 10.19 5.13 3.95 0.88 0.22 1.30 0.29 45.40 0.38

1.2 3.2 40 (401C) 17.5

22.61 10.81 5.90 3.70 0.89 0.19 1.22 0.26 45.58 1.09

25.6

21.42 3.75 10.42 7.66 1.83 0.50 1.63 1.13 48.34 0.94

23.0

0.03 4.07 10.95 1.59 0.15 16.79 37.43

23.3 o0.1 1.2 2.3 73 (201C) 16.6

0.03 4.10 11.17 1.49 0.14 16.93 36.40

Water content (wt%) Solids content (wt%) Specific gravity pH Viscosity (cSt) HHV as receiv. (MJ kg1) Pour point (1C)

0.03 4.12 11.19 1.51 0.21 17.06 33.66

Hardwood

Spruce (wt%)

Wood

Pine (wt%)

Feed

Gases H2 CO CO2 CH4 C2 H 4 Subtotal Charcoal Pyroligneous oil water settled tar soluble tar volatile acids alcohols aldehydes esters ketones Subtotal Losses

Birch (wt%)

ENEL

Products

Unioń Ele`ctrica Fenosa

Table I. Product yields from thermal decomposition of birch, pine and spruce woods are heated over an 8-h period to final temperature of 4001C [31,32].

DynaMotive Techn. Corp.

2.2.3. Char. Char is the other major pyrolysis product. Depending on temperature, the char fraction contains inorganic materials ashed to varying degrees, any unconverted organic solids and carbonaceous residues, produced on thermal decomposition of the organic components, in particularly lignin. The small particle size and high volatility of char, made in fast pyrolysis, cause it to be very flammable (autoignition temperature between 200 and 2501C), similar to powdered coal; hence, hot char must be properly handled. The ash content of the char is about

Table II. Comparison of some properties of bio-oils, obtained from different sources, with heavy fuel oil [28,33,34].

2.2.2. Gases. The pyrolysis gas contains carbon dioxide, carbon monoxide, methane, hydrogen, ethane, ethylene, minor amounts of higher gaseous organics and water vapor. Primary gases of fast pyrolysis (less than 5% wt of the dry feed) contain about 53% wt CO2, 39% CO, 6.7% hydrocarbons (including methane) and 0.8% H2. In practice, a part of the organic vapors is cracked to secondary gases, containing 9% wt CO2, 63% CO, 27% HC and 1.4% H2. The LHV of these primary gases is 11 MJ m3 and that of pyrolysis gases formed, after severe secondary cracking of the organic vapors, is 20 MJ m3 [25,32].

Heavy Fuel Oil

pyrolysis oils, which had been hot-gas filtered to remove char fines, had a much better storage life in accelerated aging tests. A distribution of specific products on long-term pyrolysis of some woody biomass feedstocks is shown in Table I, while important physical properties of pyrolytic liquids are shown in Table II.

50 (501C) 40.0 18


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6–8 times greater than in the original feed and as its alkali content is high it may cause slagging, deposition and corrosion problems in combustion. The LHV of chars have been reported to be about 32 MJ kg1 and volatilities between 15 and 45% wt. High-pressure pyrolysis leads to a higher charcoal yield. This charcoal has a higher volatile content and a slightly higher heating value [35].

2.3. Effect of feedstock and operating conditions on yield and composition The yields of chars, gases and liquids depend particularly on the feed composition (Figure 4), dimensions of the feed particles, heating rate, temperature and reaction time. The major products of key pyrolysis processes are presented in Table III. The particle size has an important bearing on the ability to be heated quickly in a given heat flux environment [37]. With increasing particle size, the process becomes more and more controlled by heat and mass transfer effects. However, size reduction can be a significant cost of operating a pyrolysis unit. Higher temperatures and longer residence times promote gas production, whereas higher char yields are obtained at lower temperatures and slow heating rates, as Table III shows. The conditions for increasing the yields of organic liquid products would be expected to involve

Figure 4. Effect of feedstock on biomass pyrolysis products [36].

short heat-up and reaction times (few seconds to a fraction of a second), intermediate temperatures (400–6501C) and rapid removal and quenching of the organic volatiles, before they are carbonized [38–41].

3. BIOMASS PYROLYSIS SYSTEMS Pyrolysis systems are as varied as combustion systems. The ancient process of making charcoal by the slow pyrolysis of a pile of wood covered with earth is still used today in some developing countries. Pyrolysis require several days and charcoal is the main product. Before fossil fuels became the preferred feedstocks for chemical production, in the early part of the twentieth century, biomass pyrolysis reactors in industrialized countries consisted of various types of ovens and horizontal and vertical steel retorts, essentially all of which were operated in the batch mode. Provision was made for charcoal recovery, pyroligneous acid refining, by-product recovery and gas recovery and usage. The older production methods for conversion of biomass to charcoal are slow processes and the yields are low. Several days are required to complete the process in earthen pits with seasoned wood and the yields are only about 10–15% of the dry wood weight, because most of the volatile organics leave the pyrolysis zone before carbonization occurs. In advanced charcoal processes, both the moisture in the biomass and the pressure can be manipulated, to maximize charcoal yields. The vapors emitted during the process are kept in contact with the solid biomass undergoing pyrolysis. These conditions result in increased char and low tar yields, at short reaction times. As research progressed on the conversion of biomass to liquid fuels, the optimum conditions for maximum liquid yields were found to be temperatures within the range of 400–6001C, vapor residence times within the range of 0.1–2 s, particle sizes less than 2 mm and a maximum of 5 mm for wood feeds and an oxygen-free gaseous atmosphere, such as recycled flue gas in the pyrolysis zone. Any reactor that can be operated under these conditions and that provides for biomass heating, so that the particle temperatures can exceed about

Table III. Typical biomass pyrolysis technologies, conditions and major products. Technology Conventional carbonization Pressurized carbonization Conventional pyrolysis Conventional pyrolysis Flash pyrolysis Flash pyrolysis Flash pyrolysis Vacuum pyrolysis Pressurized hydropyrolysis

840

Residence time

Heating rate

Hours–days 15 min–2 h Hours 5–30 min 0.1–2 s o1 s o1 s 2–30 s o10 s

Very low Medium Low Medium High High Very high Medium High

Temperature (1C) 300–500 450 400–600 700–900 400–650 650–900 1000–3000 350–450 o500

Major products Charcoal Charcoal Charcoal, liquids, gases Charcoal, gases Liquids Liquids, gases Gases Liquids Liquids



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Table IV. Operational pyrolysis units [43]. Property

Status ]

Bio-oil (wt%)

Fluid bed Demo 75 CFB Pilot 75 Entained 65 Rotating cone Pilot 65 Ablative Lab 75 Vacuum Demo 60 Grey cells show undesirable characteristics CFB: circulating fluid bed

Complexity Medium High High High High High

Feed size

Inert gas need

Specific size

Small High Medium Medium High Large Small High Large Very small Low Small Large Low Small Large Low Large ] Demo 5 demonstration (200–2000 kg h1) ] Pilot 5 pilot plant (20–200 kg h1) ] Lab 5 laboratory (1–20 kg h1)

Scale up Easy Easy Easy Hard Hard Hard

Fluid beds: 400 kg h1 Dynamotive, 250 kg h1 Wellman (UK), 20 kg h1 RTI, Many research units. CFB: 1000 kg h1 Red Arrow (Ensyn), 20 kg h1 VTT (Ensyn). Rotating cone: 120 kg h1 BTG (Netherlands). Vacuum: 3500 kg h1 Pyrovac. Others: 350 kg h1 (Fortum, Finland).

4501C before 10% weight loss occurs, can be used as a flash pyrolysis reactor [42]. The status and the characteristics of such pyrolysis units are summarized in Table IV. The design considerations for fast pyrolysis processes are indicated in Table V. In Europe, a wide network of active researchers in fast pyrolysis has been set up, known as PyNe-Pyrolysis Network. The overall objective is to establish a forum for the discussion and exchange of information on new scientific and technological developments on biomass pyrolysis and related technologies, for the production of fuels and chemicals around the world. Modern pyrolysis reactor configurations include fixed beds, moving beds, suspended beds, fluidized beds, entrained flow reactors, stationary vertical shaft reactors, horizontal shaft kilns, inclined rotating kilns, single and multihearth reactors and a host of other designs. The most important configurations are described below. Representative biomass pyrolysis processes (pilot plant or demo scale) that are near commercialization are described in this article, to illustrate some of the details of reactor designs and the operating results.

Table V. Key fast pyrolysis design features [44]. Preatreatment Feed drying Particle size Washing and additives Reactor Reactor configuration

Essential to 10% Small particles needed. Costly For chemicals production

Many configurations have been developed, but there is no best one Heat supply High heat transfer rate needed Heat transfer Gas-solid and/or solid-solid Heating rates Wood conductivity limits heating rate Reaction temperature 5001C maximizes liquids from wood Product conditioning and collection Vapour residence time Critical for chemicals, less for fuels Secondary cracking Reduces yields Char separation Difficult from vapour or liquid Ash separation More difficult than char separation Liquid collection Difficult. Quench and EP seem best

3.1. Fluidized bed reactor Fluid beds are the most popular configurations, due to their ease and reliability of operation and ease of scaling to commercial plant sizes. They provide good temperature control and very efficient heat transfer to biomass particles. Liquid yields are typically 75% wt on dry feed, with 10–15% char and gas making up the balance [45]. Secondary reaction of the volatiles in the biomass matrix and additional heat supply from the inbed heaters cause additional vapor cracking to char and non-condensable gases [46]. The residence time of the reactants can be controlled by the flow of fluidizing gas, while special attention must be given to the system separating coke from the reaction products. A typical bubbling fluid bed configuration is depicted in Figure 5, with utilization of the by-product gas and char to provide the process heat. The figure Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

Figure 5. Fluid bed pyrolysis process.

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includes the necessary steps of drying the feed to less than 10% water, to minimize the water in the product liquid oil and grinding the feed to around 2 mm, to give sufficiently small particles to ensure rapid reaction. In the circulating fluidized bed reactor, char particles have the same residence time with reaction products. The char is more attrited, because of higher gas velocities, which can lead to higher char contents in the condensed bio-oil. Liquid yields and quality are comparable to bubbling fluid beds [43]. Scale-up is potentially comparable to fluid beds and is generally preferred for the largest systems in the process industries. Although concerns have been expressed about the implications for the endothermic nature and requirements for fast pyrolysis of biomass, such operations are common place in oil refining. Efforts are being made to provide autothermal operation of the reactor, by incorporating an integral char combustor [47]. Bubbling fluid beds have been selected for further development by several companies, including Union Fenosa who has a 200-kg h1 pilot unit in Spain, Dynamotive which has a 80-kg h1 unit in Canada and Wellman which has commissioned a 250-kg h1 unit in the UK. More research and development systems have been built based on fluid beds, than any other process. Circulating fluid beds and transported bed reactors have been developed to commercial status and are used in the USA for food flavorings and related products, in several plants of 1–2 t h1. Ensyn has supplied a 650-kg h1 unit to ENEL in Italy for fuel production and a 20-kg h1 plant to VTT in Finland. Wellman Process Engineering, UK, designed and constructed an advanced fast pyrolysis reactor in Oldbury, to process 250 kg h1 of wood. Fast pyrolysis of 1–2 mm soft wood particles is performed under reducing conditions, with an anticipated liquid yield of 71%. The reactor is a bubbling fluidized sand bed. After the operating temperature has been reached, char and gas byproducts are burned to provide the heat necessary for the process. Product char is removed from the raw products by cyclones, whereas the liquid is condensed by direct contact with cooled circulating pyrolysis liquid. Aerosols are collected from the gas stream through two electrostatic precipitators, which are connected in series. The gas, after pressure boosting, is used as fluidizing gas for the fast pyrolysis reactor. The plant was the first to obtain Integrated Pollution and Control (IPC) authorization for the production of pyrolysis liquids from biomass in the UK. Initial hot commissioning of the char combustor was started in 2000, but stopped in 2003 when the IPC authorization expired, due to the implementation of European Directive on Integrated Pollution Prevention and Control legislation. Additional funding is being sought to continue the work [48,49]. DynaMotive, UK, constructed a 10-t d1 bio-oil plant in 2000, with a production capacity of 6000 l d1 of bio-oil. After full plant commissioning, the company has built a 25 t d1 commercial demonstration plant, 842


Table VI. Typical properties of bio-oil compared with diesel fuel, DynaMotive [50]. Parameter 1

Calorific value (MJ kg ) Kinematic viscosity (cSt) Acidity (pH) Water (wt%) Solids (wt%) Ash (wt%) Alkali (Na1K) (ppm)

Bio-oil

Diesel

15–20 3–9 at 801C 2.3–3.3 20–25 o0.1 o0.02 5–100

42 2–4 at 201C 5 0.05 v% (combined) 0.01 o1

which will serve as a springboard for design and construction of full-scale 100–400 t d1 commercial plants to be built in Europe, Canada, Brazil, Asia and other international markets. Feedstocks used include sugarcane bagasse and softwood bark. The tests demonstrated that bio-oil, used directly from this plant without any additional upgrading or refining, can both effectively power a gas turbine (GT) engine and simultaneously reduce NOx and SOx air emissions, compared with traditional fossil fuels. Some of its typical properties are compared with those of diesel fuel in Table VI [50,51]. In the Institute of Technical and Macromolecular Chemistry of the University of Hamburg, Germany, there are extensive laboratory and pilot plant units with throughputs of 100 g h1, 500 g h1, 2–3 kg h1 and 20–50 kg h1 for the pyrolysis of plastics, rubber, bio-polymers, sewage sludge and oil shale. The pyrolysis of waste polymers is of increasing importance, as land filling and combustion become more expensive and the acceptability of these methods is decreasing. The process flowsheet is shown in Figure 6. The key feature of the plant is a quartz sand bed reactor, with an inner diameter of 450 mm. The polymers are introduced into the reactor through a double flap gate or screw and are depolymerized at temperatures between 450 and 9001C. Recycled pyrolysis gas preheated to 300–4001C, or steam, or nitrogen is used to create the swirl in the fluidized bed. The process heat takes place indirectly into the bed through radiant heat tubes, which are heated by pyrolysis gas. The pyrolysis gas is cleaned of dust, quenched and condensed into oils, which are distilled. Up to 50% of the feed can be recovered as liquid, which is equivalent to a mixture of light gasoline and coal tar. About 95% of the oil consists of aromatics. Reactor residence times are short and in the range of a few seconds. The original aim of the process was to produce benzene, toluene and xylene, through pyrolysis of plastics, at temperatures in excess of 7001C. In recent years, research efforts have been widened to investigate feedstock and monomer recovery at lower temperatures of 450–6001C. The products are hydrocarbons, gases, oil and a solid residue containing fillers and impurities, such as heavy metals. The pyrolysis oil contains less than 10ppm of chloro-organic compounds, from Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er


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Figure 6. Fluid bed process flowsheet in Hamburg, Germany [52].

pyrolysis of mixed plastic fractions from household waste separation, with a low PVC content. Research is continuing [52] to optimize the amounts of monomers and oil from different polymers and to construct an industrial pilot plant. At the Technical University of Denmark, DKTEKNIK Energy and Environment and the Department of Energy Engineering are carrying out pyrolysis tests in a fluid bed facility [53]. Their aim is to convert straw into a gas, as a pre-treatment step for co-combustion with coal. During this process, most of the potassium, sodium and chlorine in the straw are left in the char. A cyclone is used to separate the char from the gaseous products. At temperatures lower than 5001C, the char contains all of the potassium and sodium and about 80% of the chlorine, as well as about 45% of energy in the straw. A part of this cleansed gas is used for fluidization of the pyrolysis reactor, by recirculation with a blower and another part is driven to a conventional power plant boiler. The gas contains most of the original energy in the straw, whereas alkali metals and other residues remain in the char. These residues can be utilized, as the biomass ash is not contaminated with fossil fuel ash. The largest fast pyrolysis plant in Europe, based on fluid bed technology, has been erected at the ENEL thermoelectric power plant of Bastardo, in Italy and was commissioned in 1998. It is sized at 3.3 MW and at Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

Table VII. Characteristics of typical RTP bio-oil, ENEL [54]. Bio-fuel properties Moisture (%) Specific gravity HHV (MJ kg1) LHV (MJ kg1) Viscosity at 401C (cSt) Acidity (pH)

Range of values

Typical values

15–31 1.15–1.25 15–18

23 1.2 17.5 16.2 40 3.2

35–53 2.8–3.8

the nominal feed of 625 kg h1 of hardwood sawdust it produces about 400 kg h1 of bio-oil, with a water content of about 25%. A typical composition of bio-oil produced is shown in Table VII. The trend is to directly utilize crude bio-oil, or to improve its characteristics through modifications at the pyrolysis process level, for future use in medium-size boilers designed for light fuel-oil. The plant uses the Rapid Thermal Pyrolysis process, a trademark of the Canadian company Ensyn (Figure 7). The major achievements of the project can be summarized as follows. A second cyclone has been installed for better separation of the entrained solids and constant quality oil with almost constant content of solids and water has been produced at PDU scale. Preliminary combustion tests with pyrolysis liquid, at PDU scale, have 843

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Figure 7. ENEL fast pyrolysis flowsheet, Italy [55].

Figure 8. BioThermTM flowsheet, Canada [58].

been carried out. Modifications to the liquid recovery stage have been initiated. The work program of the combustion tests of bio-oil in the boilers will be carried out in parallel in Italy and in Finland (VTT). While ENEL will overhaul the pilot plant and improve its operability, VTT will conceptualize and will test, at PDU scale, methods for quality improvements to be integrated in the plant. Boilers designed for light and heavy fuel oils will be used, after having modified the feeding system and the burners [54,55]. Unioń Ele`ctrica Fenosa, S.A. is a utility that produces and supplies electrical power to approximately 16% of Spain, providing 20 000 million kWh annually. Related to fast pyrolysis of biomass, Unioń Ele`ctrica Fenosa has developed, built and operated a 844

pilot plant, based on fluid bed technology, of semiindustrial scale, 200 kg h1 of dry biomass, in Galicia, Spain. Within several European projects and inside the own R&D framework of the group, the bio-oil produced has been analyzed and tested, in order to evaluate the possibilities and opportunities, from an economic point of view, of this bio-oil, as a fuel mainly for power generation. Oil yields of up to 70% wt on a dry feed have been reported. Eucalyptus, pine and oak have been tested as feeds [56,57]. DynaMotive Technologies Corporation, in Canada, is currently commercializing its biomass refinery family of clean energy products and technologies. The core of the refinery is the BioThermTH process (Figure 8)—a fluidized bed fast pyrolyzer based on a previous design Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er


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by Resource Transforms International Ltd (RTI), which began operation in 1997. The process has been demonstrated at RTI, at 15 kg h1 capacity, using various materials such as agriwaste, wood waste, sawdust or the lignocellulosic portion of steam-processed municipal solid waste (MSW). A pyrolysis facility having a capacity of 25 t d1 has been designed. The feed is comminuted and dried before use, so that its moisture content is at most 15%. By-products such as pyrolysis gas, char or other Biomass Refinery products usually provide the heat necessary for operating the reactor. Pyrolysis gases and mists are separated from the solids, to form a gaseous product called BioSol. This can be condensed for making bio-oil, or can be used as fuel in nearby appliances. From char and ash, which are separated from the BioSol stream, heat can be recovered, or value-added products, such as activated carbon, can be produced. Exit gas is utilized via a recycle blower as fluidizing gas to the pyrolyzer. Alternatively, it can be burned to provide heat to the reactor. Bio-oil can be used as a fuel, or upgraded to a variety of chemical products. Typical yields are bio-oil 65–72%, char 15–20% and non condensable gases 12–18%. These vary with feedstock types. A fourth product, Bio-oil Plus, can be produced by adding back to the separated char into the bio-oil in a finely ground form of 8 mm in size. A comparison of DynaMotive’s biofuels and fossil fuels is made in Table VIII. A commercial plant, located at the Eric Flooring and Wood Products in West Lorne, Ontario, started operation in early 2005, with a design capacity of 100 t d1 of waste sawdust. Biomass throughput has been increased to 130 t d1. In 2007, a 200 t d1 unit was installed at Guelph, Ontario, to produce bio-oil from waste wood building materials. The plant will process, once in full operation, 66 000 dry tons of biomass a year and have an energy output equivalent to 130 000 barrels of oil [51,58,59]. Ensyn’s RTPTH technology, in USA, has represented a true breakthrough in biomass thermal conversion. It is the world’s first and only proven commercial biomass fast pyrolysis process (since 1989),

Table VIII. Comparison of DynaMotive’s biofuels and fossil fuels [51].

Fuels

Specific gravity

Energy (MJ kg1)

High heating values (Btu lb1)

Light fuel oil Heavy fuel oil Diesel Natural gas Coal Bio-oil Char Bio-oil Plus Non-condensable gas

0.82 0.99 0.84 NA NA 1.2 NA 1.22–1.3 NA

46 42 46 38.1 34.8 15–20 23–32 18–20 7.7–10.2

19 800 18 050 19 800 1020 Btu ft3 15 000 6500–8500 10 000–13 800 7750–8500 3300–4400


based on fluid bed technology. Based on the biomass refining concept, wood and other biomass are converted to high yields of bio-oil, up to 83%. Typical characteristics are listed in Table IX. Value-added chemical products, such as products for food industry, polymers and adhesives, are first recovered from biooil and the remaining liquid is then used for fuel and power applications. In addition, the char (13% wt) is a high quality consistent product, which may be consumed for energy in the process itself, sold as is, or activated for higher value applications. Ensyn has designed, built and commissioned seven commercial RTPTH plants in the US and Canada. The largest, located in Renfrew Ontario, has the capacity to process 100 t of dry residual wood per day. Projects now underway will result in plants 5–10 times the size of the Renfrew plant. Development work [62] includes hot vapor filtration to reduce ash and char, liquid filtration to reduce char, supply of oil for engine and turbine testing in Canada and Europe, combuston testing, upgrading and product characterization. A fluidized bed reactor was added to the NREL Thermochemical Users Facility, in Colorado. It is equipped with a perforated gas distribution plate and an internal cyclone, to retain entrained bed media. The reactor is heated electrically and can operate at temperatures up to 7001C, at a throughput of 15–20 kg h1 of biomass. Superheated steam is used as the carrier gas and pyrolysis vapors, before condensation, are thermally cracked in the secondary tubular reactor at 800–8501C. The char particles are then separated from the gas stream using two cyclones and occasionally hot baghouse filter. The tars and steam are removed in a scrubber system. All units have capabilities to analyze products on line, over a wide spectrum of chemical compositions, using dedicated analytical instruments. Clean dry gas of HHV, in the range of 17–19 MJ kg1, is used as a fuel for a 15-kW commercial engine, devised for combustion of natural gas. A catalytic fluid bed steam reformer was also coupled to the system [63,64], to produce hydrogen from the pyrolysis gas. Bioware Tecnologia has grown out of and is scientifically supported by the University of Campinas, Brazil. Its main aim is to develop and produce high added-value products from forest and agro-industrial waste, using state of the arts and environmentally friendly technologies. Its research intends to develop biomass fast pyrolysis technology, in a continuous atmospheric bubbling fluidized bed reactor, in order to produce bio-oil and charcoal. The basic raw materials used are: elephant grass, cane trash and bagasse, which have already been processed with a measure of success. The bed temperature in the reactor fluctuates from 480 to 5001C and the gas/feedstock mass ratio is 0.4 (dry basis). When the reactor operates under these conditions, the wet scrubber will recover an average of 40% wt bio-oil on dry feed. The pilot facility, which is fully automated, has a nominal capacity of 300 kg h1 and is 845

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Table IX. Wood derived pyrolysis oils from Ensyn [60,61]. Physical property Moisture content PH Specific gravity Elemental analysis (mf) C H O N S Ash C/H molar ratio C/O molar ratio HHV (mf) HHV (as produced) Viscosity (401C) Kinematic viscosity (401C) ASTM vacuum distillation 160 193 219 Distillate Flash point Pour point Solubility Hexane insoluble Toluene insoluble Acetone/acetic acid insoluble

Typical value 22% 2.5 1.18 56.4% 6.2% 37.1% 0.2% o0.01% 0.1% 0.76 2.02 23.1 MJ kg1 17 MJ kg1 45 cP 134 cSt 10% 20% 40% 501C 551C 251C

Property Specific gravity, 20/201C Kinematic viscosity at 371C Kinematic viscosity at 651C Higher heating value Copper corrosion, 3 h at 1001C Pour point PH Total number of acids Ash Moisture content (Karl Fischer method) Elemental analysis (%)

99% 84% 0.14%

3.2. Entrained flow reactor Entrained flow fast pyrolysis was developed at Georgia Tech Research Institute and scaled up by Egemin.

Unit

Bio-oil

Mg KOH g1 %wt %wt

1.1493 9500 1100 31.41 1b 9.0 2.2 30.4 0.55 2.21

Carbon Hydrogen Oxygen Nitrogen Sulphur

70 7.1 21.05 1.7 0.15

cSt cSt MJ kg1 1C

Table XI. Properties of Bioware charcoal and elephant grass raw material [65].

Property

designed to produce bio-oil samples, which will be tested in laboratory and industrial applications. References, concerning the physico-chemical properties of bio-oil and charcoal thus produced, are reported in Tables X and XI, respectively. There is a scale-up project and a 500–1000 kg h1 facility was planned to be built in 2005. The bio-oil could be used as an emulsifying agent for heavy petroleum, an additive for cellular concrete, a substitute of phenol in PF resin formulations and fuel for the generation of energy. Moreover, the charcoal produced could be used in boiler’s ovens as fuel, as a pre-reducer for iron ore pellets, activated carbon and catalytic substrate. The supply of cheap raw materials to be used in the Bioware process is plentiful and practically limitless. Specifically, great support and interest have been shown by the Brazilian sugarcane industry regarding this technology. In addition, the pulp and paper sawmill and rice industries have expressed interest in becoming active partners. The company already cooperates with many national groups and companies [65], while some international connections are under way with firms in Cuba and Spain.

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Table X. Properties of Bioware bio-oil [65].

Bulk density (kg m3) Volatiles (%) (d.b.) Fixed carbon (%) (d.b.) Ash (%) (d.b.) Moisture (%) (d.b.) Higher heating value, MJ kg1 (d.b.) Particle average diameter (mm)

Elephant grass

Charcoal fines

76 73.5 20.2 6.4 10.1 17.03

140 7.4 61.9 30.7 2.7 22

2.24

75

However, probably because of the difficulties that have been encountered in achieving good heat transfer from a gaseous heat carrier to solid biomass, the Egemin process is no longer operational or being further developed. Liquid yields are lower than other fast pyrolysis processes, with a maximum of around 60% wt on dry feed and there are significant design constraints, as heat transfer is completely reliant on gas to solid limitations, which is unique to this particular configuration. Very small particles are therefore necessary to achieve the required heat transfer rates [57,66–68]. One flash pyrolysis process operating at atmospheric pressure was developed in USA, which affords liquid yields in excess of 60%, on a dry basis, from hardwoods. Yields as high as 70% were projected for commercial plants. The GTEFP process was developed in bench-scale studies and a large-scale PDU, in which the particulate feed is entrained in hot combustion gases and pyrolyzed at millisecond residence times and Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er


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Figure 9. Rotating cone reactor process [70].

temperatures in the range of 5001C. Typical higher heating values of the product oils were 22 MJ kg1 [69].

3.3. Rotating cone reactor The rotating cone reactor invented at the University of Twente and being developed by BTG is a recent development and effectively operates as a transported bed reactor, but with transport affected by centrifugal forces rather than gas. A 200-kg h1 unit is now operational, which includes integrated heat recovery with a secondary char combustor. In the high intensity reactor, (Figure 9) biomass of ambient temperature is mixed with hot sand. Upon mixing with the hot sand of 5501C, biomass decomposes into 70% wt condensable vapors, 15% wt noncondensable gases and 15% wt char. An important characteristic of this reactor type is the absence of carrier gas, since it is the rotating action of the cone, which propels the solids from the reactor entrance to its exit. Because of the absence of carrier gas, the vapor products are not diluted and their flow is minimal. An undiluted and concentrated product flow from the reactor leads to small downstream equipment with related minimal investment costs. In mechanical terms, the reactor technology is remarkably simple and robust. The rotational speed of the cone is only 300 rpm and after more than 1000 h of operation signs of abrasion or wear are absent. Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

Scaling-up of the reactor is possible by increasing its diameter. For capacities, which require a cone diameter larger than 2 m, stacking of multiple cones on a single axis leads to the lowest investment costs. This conventional approach is also encountered in centrifugal disk separators, or rotating disk contactors [57,66,71]. With these options, all pilot plant capacities between 2 and 100 t h1 can be served. At the Shenyang Agricultural University in China, BTG and Royal Schelde have delivered a biomass unit with a capacity of 50 kg h1, which was scaled up, under an EC contract, to a biomass feed rate of 260 kg h1 dry basis. In this unit, hot sand is introduced from the char combustor, which is located above the reactor, while the biomass feed is introduced into the reactor through a lock hopper. The char exiting the unit, containing sand, is recycled into the char combustor. All gaseous pyrolysis products are cooled in a condenser, producing heat, which is recovered from the biofuel oil through a water circuit. The bio-oils produced have been tested for their combustion properties in a flame tunnel and turbine combustor. The major accomplishments of this system are longest test run 80 h, total bio-oil production 15 t, maximum bio-oil yield 75% wt on dry feed, various woods and other feedstocks, such as rice husks, wheat straw and organic waste processed and particle sizes up to 6 mm, including fines. For future plants larger than 5 t h1, the price of the bio-oil is estimated to be 4 h GJ1 [71–74]. 847

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Table XII. Product distributions of different biomass feedstocks in mass-%, Lurgi-Ruhrgas [75–77].

Material

Oil1 Ash Moisture Char1ash moisture Gas L/S

Beech wood 0.8 Wheat straw 6.0 Rice straw 18.8 Hay 6.5 Wheat bran 6.0

Figure 10. Principle and construction of the twin screw (LR) reactor with low axial and fast radial mixing in a shallow hot sand/biomass particle bed [75].

At the Karlsruhe research centre, in Germany, tests have been carried out on a twin screw or LR (LurgiRuhrgas) mixer reactor for fast pyrolysis. In 2003, a PDU for 10 kg h1 biomass throughput was designed, built and put into operation. The central part of the unit consists of a 5001C hot sand loop with a bucket elevator, a sand heater, a controlled screw feeder for sand and the LR-reactor. As a follow up, studies and research are now done on pneumatic sand transport. Cold dry straw particles are fed into the circulating hot sand stream in the LR-reactor by means of a screw feeder, with a sand-biomass ratio between 5 and 20. In the sand heater, which is a vertical shell and tube-type vessel, flows the sand of the whole circuit down the tubes, and is heated through a fluidized sand bed on the shell side. It is heated and stirred with hot flue gas from the combustion of pyrolysis gas or methane. The LR-mixer is actually mechanically fluidized, while the distinguishing features are two parallel screws, revolving a few times per second in the same direction, cleaning each other with intertwining flights. This is clearly shown in Figure 10. Removal of current-flowing vapors from a shallow sand bed is concurrent with the transport of slight axial and full radial mixing. The fine char particles, carried along with pyrolysis gas and vapors, are removed by a hot cyclone. Lower down the cyclone, two condensates, one mostly organic and one mostly aqueous, are recovered by two sequentially installed condensers operating at decreasing temperatures. A maximum process capacity of 200 kg h1 sand mixed with 10 kg h1 of fine straw particles has been achieved, at screw speeds of several revolutions per second. Total condensate output equals half of the input weight. Most of the char could be removed from the hot cyclone in powder form and only a small fraction of the char is then left over in the quench condensates. There is still on-going research for the improvement of its design and long-term operating conditions in particular [75–77]. The now available industrial experience will contribute to the construction of a commercial unit of a throughput capacity 848

12.4 11.5 9.5 9.8 8.4

17.9 25.5 35.1 24.3 28.5

67.2 49.8 42.4 43.5 59.2

14.9 24.7 22.5 32.2 12.3

3.8 2.0 1.2 1.8 2.1

of 10 t h1, soon to be demonstrated. In the near future, the pyrolysis products will be converted in a large pressurized entrained flow gasifier to a tar-free syngas with low methane content. Product distributions of the fast pyrolysis (Table XII) showed that in case of wood and wheat straw/wheat bran, sufficient liquids were produced, to mix the emerging char to a pumpable slurry feed suited for the following gasification step. An innovative technology was developed in the University of Perugia, in Italy. It utilizes a regenerated microturbine, coupled to a rotary-kiln pyrolyzer, for energy conversion of biomass on the microscale (below 500 kW). A 70-kWe-integrated Pyrolysis Regenerated Plant (IPRP) has been built at the Terni facility (Figure 11). The IPRP demonstrative unit pyrolyzes ligneous biomass, to produce a medium LHV gas, which fuels the microturbine. Waste heat from the microturbine is returned to the pyrolyzer, to provide part of the energy required for thermal degradation of the biomass. The eventual residual energy is provided by char or tar combustion, which are by-products of the pyrolysis process. The plant will produce 400 MWh y1, with an efficiency of around 16% and will avoid the production of around 280 t y1 of CO2. IPRP is an easily scaleable technology, designed on the basis of commercially available engines—from a min 100 kW size up to tens of MW [78,79], which would be suitable to implement a combined gas–steam cycle.

3.4. Vacuum pyrolysis reactor Vacuum pyrolysis is unique in that the rate of heating is very low compared with the other systems described above, but the effect (in terms of liquid product yield and quality) of fast pyrolysis is achieved by removing the vapors, as soon as they are formed, by operating under a vacuum. Vacuum pyrolysis of biomass is generally conducted at a temperature of 4501C and at a total pressure of 15 kPa. A 3.5 t h1 unit is currently operating in Canada. The liquid yield is typically believed to be 55–60% wt on dry feed, with a commensurately higher char yield. The rapid volatilization of fragmented products under vacuum minimizes the extent of decomposition reactions, so that the chemical structure of the Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er


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Figure 11. The IPRP process at Terni, Italy [78].

pyrolysis products resembles that of the original organic material [28]. The liquid is also significantly different to other fast pyrolysis processes including water content, a higher heating value and other physical properties. Groupe PyroVac Inc., from Canada and Ecosun bv, from The Netherlands, have built the first PyrocyclingTM industrial scale plant in Jonquie`re, Province of Que´bec, Canada, based on vacuum pyrolysis technology. The plant, which has a throughput capacity 3.5 t h1 of air-dry feedstock, has been constructed to demonstrate and further improve the technology and produce sizeable amounts of pyrolysis oils and wood charcoal. It will serve as cornerstone for the start-up of the Pyrochem-Saguenary plant, which will transform softwood bark residues into pyrolytic oil and wood charcoal. The pyrolytic oil will be used in the manufacture of phenol formaldehyde resins. The wood charcoal will be sold as a feedstock in the metallurgical and mineral industries. This process requires that the biomass be pneumatically carried over to the vacuum feeding equipment and then inserted into the reactor (Figure 12). The pyrolysis reactor, 13 m in length and 2.2 m in diameter, operates under vacuum in a continuous feed mode. Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

It works with a transport and stirring system using a feedstock moving bed, which has been patented. The heating source of the reactor is gas originating from the process, as well as natural gas. As a heat carrier, the unit uses molten salts, thus ensuring efficient heat transfer within the reactor. Two cooling columns, connected to a storage tank, achieve the recovery of the pyrolytic oils. The charcoal then is cooled at the reactor exit point and sent to storage. In order to maintain the whole system under a total pressure of 10–15 kPa, the reactor is equipped with a vacuum pump. In order to stop air leaking into the chamber, the reactor uses a nitrogen injection system, thus guaranteeing safety. Vacuum pyrolysis of bark residues of 15% moisture content gives out nearly 29.5% pyrolytic oils, 29% wood charcoal, 10% gas and 31.5% water (Table XIII). The plant will also enable the demonstration of a new electricity generation system. Groupe Pyrovac and Ecosun have developed the Integrated Pyrolysis Combined Cycle System, in collaboration with Orenda Aerospace Corporation of Canada, for a total electricity production of 14 MWe [80,81]. Orenda’s GT 2500 GT is the first of its kind in the world, to be fed with a bio-oil feedstock. 849

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Figure 12. PyrocyclingTM process flowsheet, Canada [80,81].

Table XIII. Typical yields of vacuum pyrolysis products [44]. Typical yields (wt%, dry wood basis) Pyrolysis oils Pyrolytic water Wood charcoal Gas Pyrolysis oil elemental composition (wt%, at 23%moist) C H O N S Ash Density (kg m3) HHV (MJ kg1) Water (wt%) Viscosity at 501C (cSt) Flash point (1C) Wood charcoal HHV (MJ kg1) Volatile matter Ash Fixed carbon Gas composition (vol%) Hydrogen Methane Carbon monoxide Carbon dioxide Others HHV (MJ kg1)

850

Fir/Spruce bark 35 20 34 11

55.4 8.4 35.3 0.6 o0.01 0.3 1140 23.0 23.0 5.6 495 30.4 20.3 7.6 72.1 6.6 10.0 32.0 41.5 9.9 10.9

3.5. Ablative pyrolysis reactor In this system, biomass particles are entrained tangentially into a vortex tube with a jet of carrier gas, at velocities over 100 m s1. This causes the solid particles to be centrifuged to the hot wall of the vortex reactor, where very rapid heat transfer occurs to the surface of the particles. Ablative or surface pyrolysis takes place at 550–6001C and high rates, essentially independent of the feedstock particle size. The biomass particles take a helical path through the reactor. Partially pyrolyzed feed and large char particles are removed tangentially and recycled to the eductor. The vortex tube acts as a particle classifier, with char fines exiting the axial outlet of the reactor, reentrained with the product vapors and gases (Figure 13). This type of conversion process favors chain-cleavage reactions to form oxygenated, organic vapors, rather than chars and gases and is expected to make it possible to design small reactors having high throughput rates. With temperatures of 6251C on the reactor walls, 60–70% wt of the primary vapor products are composed of oxygenated organic compounds and polymer fragments. They condense to form acidic, water-soluble liquids, which have nearly the same elemental composition as the feedstock. For softwood feeds, the vortex reactor produces about 58–67% wt of the dry feedstock as primary pyrolysis oils, 10–12% wt as char, 10–14% wt as gases and 13–16% wt as water [82]. As compared with other fast pyrolysis processes, liquids have a lower viscosity from a more cracked product and gas yields are lower. The vapor conversion rate can be increased, by increasing the gas phase space time [83]. Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er


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combusted within 10 h of continuous operation [85,86]. The promising results obtained so far led to the order for a 48 t d1 plant. The advantages and disadvantages of the various reactors are presented in Table XIV. Owing to the many different processes and status of development, an economic comparison is difficult at the moment. However, it seems that the fluidized bed and rotary cone reactors will be more cost effective at large scale. Figure 13. Original vortex reactor design.

Ablative pyrolysis is interesting, as much larger particle sizes can be employed than in other systems and the process is limited by the rate of heat supply to the reactor, rather than the rate of heat absorption by the pyrolyzing biomass. The key advantages are the ability to process much larger particles, as the mechanism of heat transfer is different and the absence of a fluidizing or transport gas, both of which contribute to a more compact and intensive reaction system. A disadvantage is that scaling is based on surface area, rather than volume, as is usual in chemical reactors and the economy of scale is thus more limited. Much of the pioneering work on ablative pyrolysis reactors has been carried out by NREL in their vortex reactor (abandoned in 1997, due to erosion and wear problems and uncertainties about scalability) and by CNRS at Nancy [44,84]. More recent developments have been carried out at Aston University, UK, with a prototype and second-generation reactor [57,66]. In Germany, PYTEC Thermochemische Aulagen GmSH, founded in 2002, has designed, built and now operates a new ablative pyrolyzer, with a nominal capacity of 15 kg h1. The pyrolyzer consists basically of a rotating, vertically orientated, electrically heated disk. The feed, solid wood boards, having dimensions of 10 47 mm in cross-sectional area and approximately 35 mm in length, is pressed against the disk by a piston. Four supply towers with automated feeding ensure the simultaneous introduction of four boards into the reactor chamber. The temperature of the disk is around 7001C, while the pressure varies between 30 and 50 bar. Ablation rates between 2 and 5.5 mm s1 have been achieved so far. Standard techniques such as cyclone, spray tower and electrostatic precipitator are used for the cleaning and condensation of the gas. Oil outputs, which vary between 55 and 70% on a dry feed basis, are measured through weight sensors on the collection vessel. Based on these encouraging results, an ablative fast pyrolysis pilot plant and a diesel-based combined heat and power (CHP) plant were built and operated in 2006 on a site of a sawmill. The plant has a design capacity of 6 t d1 dry biomass processing wood chips, while the CHP plant has a power output of approximately 300 kW h1. The CHP plant was modified and 1000 kg of bio-oil (with 4% wt of diesel) were Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

3.6. Other pyrolysis reactors There is a long history regarding the forest waste kiln, as well as the charcoal kiln in Norway. Their capacities are usually in the range of 30–35 m3 of wood and the yield of tar in the range of 30–35 kg m3 of wood. The plants in use today are mostly small-scale ones. The largest among them, operating only in the summer, has two retorts of 3.5 m3 and a yearly output of 15 000 l of tar. The retort unit is a batch process. Biomass is fed into the retort at the top and the charcoal remaining after the process is extracted at the bottom end. A heat exchanger reheats the gas circulating in the system, thus heating the retort. The gas exchanges heat with flue gas from the burning of the biomass and/or the pyrolysis gas. The temperature of the gas entering the retort ranges between 500 and 6001C. After tar and water are removed from the pyrolysis gas in the condenser, the gas is cooled down to about 201C. Before entering the heat exchanger, the gas is driven into a scrubber in order to minimize fouling problems. The output of tar for this particular process is around 60–80 kg m3 of wood, depending on the quality of the input. The tar is used mostly for the insulation of wood in buildings and boats, while the charcoal is locally marketed [87]. The British firm, Compact Power, has developed a process combining pyrolysis and gasification, which claims to offer a new and economical solution for the disposal of waste. The Compact Power process presents an innovatively designed and built unit, which has the ability to treat a wide variety of wastes, such as: MSW, clinical waste, animal and biomass wastes, sewage sludge, industrial wastes and tire wastes. The plants are defined by the number of pyrolysis ducts used, and range from one unit of the ‘MT2’ type with two ducts, capable of processing 6000 t a1 clinical waste, to an ‘MT8’ unit encompassing eight ducts and capable of treating 30 000 t a1 of MSW. Combinations of these units allow for the treatment of various wastes available, which is highly advantageous, as they can be up-sized or down-sized as disposal and recycling practices change. The ‘MT16’ can accept 60 000 tpa of waste and can export up to 7 MWe and 25 MWth of lower grade heat. The plant consists of two identical parallel ‘MT8’ lines. The schematic representation of the process (Figure 14) shows the various stages of this thermochemical conversion technology. Waste is 851

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Table XIV. Advantages and disadvantages of biomass pyrolysis systems. Reactor type Fluid bed

Advantages Good solids mixing High heat transfer rates Good temperature control Ease of scaling

Entrained flow

Rotating cone

Vacuum reactor

Ablative reactor

Good solids mixing No carrier gas required Ease of scaling Small investment cost No carrier gas required Lower temperature required Can process larger particles Heat transfer gas not required Lower temperature required Can process larger particles Compact design and intensive system

Disadvantages Heat transfer to bed must be proven at large scale Max particle sizes up to 6mm If circulating increased complexity of system, char attrition and reactor wear Low heat transfer rates Limited gas/solid mixing Small particle sizes required Heat transfer to bed must be proven at large scale Small particle sizes required

Low heat transfer rates Solids residence time high Liquid yield rather low Reaction rates limited by heat transfer to the reactor Char abrasion Scaling is costly

Figure 14. Compact Power-MSW pyrolyser plant, UK [88].

inserted into the pyrolysis chamber through ducts, constituting the basic unit of the system. The use of the Compact Power process and compact fuel ‘slugs’ enables a plant to operate on a continuous basis, which makes it ideal for CHP and several other applications, where high degree uninterrupted heat provision is required (such as district teleheating). Hydrocarbon gases are produced when the fuel material is pyrolyzed at about 7001C and are directly carried to a thermal 852

oxidation reactor operating at 12501C. This prevents both the cooling of the gases and the creation of noxious by-products. Then the carbon char in the solid residue is gasified at 10001C to produce a carbon monoxide and hydrogen off-gas, leaving an inert nonleachable ash suitable for various applications. The high temperature gases are oxidized in the thermal reactor at 12501C, for at least 2 s, time long enough to secure the complete destruction of any pollutant gases Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er


and particles. The off-gases are driven to the pyrolysis tubes so that they can produce enough energy to make the process self-sustaining. Then, the gases go through a steam boiler, in order to be cooled. This boiler has been designed to bring the temperature down, so that the reformation of dioxins or furans is successfully avoided. The exhaust gases go through a final ‘cleansing’ process before they are let out through a low chimney. This ensures that these gaseous emissions comply with the highest environmental standards. Modular design means low investment capital, while multiple tube design means low production and maintenance costs. According to Compact Power estimates, the expenditure of these units could be 60% lower than that of the latest conventional incineration units of the same capacity. The company has successfully completed experimental trials on a 400-kg h1 pilot plant and a 6000-t a1 MSW and clinical ‘Energy from Waste’ plant, located at Avonmouth, near Bristol, UK, is operating since 2001. It is a commercial plant, operating on clinical and pharmaceutical waste. It produces around 2 MWth with over 85% of this used in the sterilizer plant. Ethos Energy has designed a ‘MT16’ plant [88,89]. Completion was scheduled for 2009. PKA, a German company, has applied pyrolysis to find a solution for waste management problems and recover an energy product of great value. The use of a variety of feed materials, such as sewage sludge, old tires, car shredder residues, packaging materials and

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hazardous waste, has led to extensive and valuable experience. A three-step waste preparation, pyrolysis and clean gas production constitute the basic concept. There are two stages in order to prepare residues and wastes for processing. First, grinding and metal separation and second drying, if the moisture content is too high. The materials are then pyrolyzed at a temperature of 500–5501C, resulting in two end products: pyrolysis gas, which is thermally cracked and char, which is gasified and then mixed with the pyrolysis gas. This gas mixture is cleansed, providing a clean fuel gas capable of generating both heat and energy. The process is shown in Figure 15. The pyrolysis gas is taken through a hot zone in the gas cracking unit, at more than 10001C, where it remains for up to 2 s. Then, any organic pollutants, including tars and oils are thermally destroyed and the end result is of a consistent gas quality. For further utilization, the pyrolysis residue (char) is prepared by grinding and metal separation in a gasifier. The gas is cooled and driven through a wet scrubber, where any inorganic acid pollutants are washed away. Then follows drying and further processing to remove any organic pollutants, as well as mercury and hydrogen sulfide. This now clean gas has a heating value of approximately 4000 kJ m3. It can be immediately sent to various consumers, or converted into electricity and heat on the spot. In this system, the modules can be so arranged that the upgrading of the unit can be achieved in a rather flexible way. PKA is

Figure 15. PKA pyrolysis process flowsheet, Germany [90].


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now operating an experimental plant with a 400-kg h1 throughput. Moreover, a unit for household waste of 24 000 t a1 capacity has been built at Aalen and has been operating since 2001 [90,91]. The energy produced is driven to an electricity generating plant and heat is directly sent to a nearby hospital and school. Chemviron Carbon, in Germany, uses the Reichert process to produce around 24 000 t of charcoal from 70 000 t of beech wood annually. Seven retorts are in semi-continuous operation, in order to recover 500 t per year of highly pure acetic acid, to be sold to the electronic industry. This process consists of 19 separate stages. In addition, there is a production of smoke flavors, which are sold to the food industry. The main unit is a large steel vertical retort, with a charge capacity of about 100 m3. Its top and bottom are conical, but only the bottom one is lined with bricks. Feeding is accomplished through a conveyer belt at its top. When charging is over, the main heating pipe is opened, to allow hot gases in. During the 16–20 h cycle, the carbonization zone moves slowly and descends to the bottom of the retort. On the way out, pyrolysis vapors follow a counter-feed flow, pass through the un-carbonized feed, shedding any moisture present. On exiting the retort, the condensable part is removed in coolers and scrubbers. The non-condensable part is driven into a heat exchanger, where it is treated at the suitable carbonization temperatures (450–5501C). Left over gas is burned and used to heat up the heat exchanger and to pre-dry the feedstock. Charcoal is extracted from the bottom of the exchanger falling into air-tight storage containers to be cooled. The raw

material is wood of maximum dimensions 30 cm in length and 10 cm in thickness [92,93]. Another charcoal production system, heated with recirculated gas, the Lambiotte CISR retort, developed in Belgium, has a capacity to process 7000 t of dry wood per year, producing 2500 t of charcoal. There are several CISR carbonization plants operating in Europe. A similar concept has been used by Lurgi, in the construction of the charcoal plant, which forms part of the Silicon Metal Complex in Bunbury, Western Australia. About 27 000 t of charcoal are produced annually from local Jawah hardwood, in two retorts [93,94]. A schematic diagram of the process is shown in Figure 16. The retort is divided into an upper carbonization zone and a lower heating zone, each with its own re-circulating gas stream. Gas from the carbonizing zone is distributed across the middle of the retort and flows upwards. Pyroligneous vapors leave the retort at the top and are delivered to a specially designed incinerator for staged combustion. In the first stage, the retort gas is burnt at near stoichiometric conditions. In the second stage, more air is added to ensure complete combustion before the exhaust gases are released to the atmosphere. About one-third of the combustion gases are withdrawn and conditioned (550–7001C) to serve as rinse gas for the carbonization. Excess combustion gases from the second stage are conveyed via a stack to the atmosphere; however, waste heat recovery can be installed. The gas circulation loop, in the lower zone of the retort, is designed to sufficiently cool the charcoal descending from the upper zone and flow counter-current to the charcoal.

Figure 16. A schematic drawing of Lurgi’s carbonization process, Germany [93].

854



The hot cooling gas is re-cooled directly, by water injection in the scrubber, before it enters the retort in the bottom. In Senegal, about 360 000 charcoal tons are consumed every year for household heating, requiring over 2 million tons of wood as feed. NOVASEN, a private oil mill constructed in 1999, developed a project to substitute lump charcoal from Senegalese forests, from the processing of groundnut, using the groundnut shells as a raw material for charcoal briquettes. The process is carried out in three down draft retorts (Figure 17). The off-gases provide enough electricity to supply the mill, by fueling two 850-kW electric generators, while the pyrolysis oil from the retorts is burned in the heat boiler of the process. The prefeasibility study shows that the factory can achieve energy autonomy with 15 000 t of groundnut shells per year, while producing 4200 t of biocoal briquettes. In Senegal, as elsewhere in West Africa, the production of groundnut oil, for export, uses a huge amount of energy. If the ‘biocoal from groundnut shell’ strategy were introduced into all oil mills of Senegal, approximately 12 000 ha of forest would not be fueled every year, whereas the country could cover about 25% of its charcoal needs [95]. As proof of concept, BEST Energies Australia has a fully operational 300 kg h1 slow pyrolysis demonstration plant, for the production of green energy in rural and remote communities (Figure 18). This unit includes fully integrated energy utilization, proprietary gas clean-up system and an operational internal combustion engine generator running on syngas produced from the pyrolysis kiln. A significant amount of experience and process data has been gained operating this unit on a range of feedstocks including: paper sludge, cow and poultry manures, rice hulls, greenwaste and wood-waste. From these data, the design has been scaled up into a 48 and 96 t d1 (dry feed basis) commercial modular units [97]. These modular units can be designed with an engine component for

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electricity production or to interface with thermal energy processes, such as steam boilers.

4. MARKET PENETRATION In general, there are no fundamental impediments that would act as barriers to utilizing biocrudes in industrial-scale combustion systems. Biocrudes can be burned using standard atomization techniques, with emissions controlled to acceptable levels. However, the burner must be set up [98] to accommodate the unique characteristics of the liquid biomass and combustion parameters must be appropriate to and optimized for the biocrude. The liquid bio-oil product from fast pyrolysis has the considerable advantage of being a storable and transportable fuel, as well as a potential source of a number of valuable chemicals, resins, binders, preservatives, etc., that offer the attraction of much higher added value than fuels. All products that currently result from the processing of petrochemicals can be produced from biomass feedstocks. These include lubricants, polymers, high matrix composites, textiles, biodegradable plastics, paints, adhesives, thickeners, stabilizers and a range of cellulosics. Recovery of pure compounds from bio-oil is presently economically unattractive. Advanced biomass conversion processes, which provide an opportunity to supply commodity chemicals at costs that are potentially competitive with the costs of the same chemicals from fossil feedstocks, are being developed [99,100]. Bio-oil can substitute for fuel oil or diesel in many static applications including boilers, furnaces, engines and turbines for electricity generation [29,101]. For example, bio-oil is routinely used as a boiler fuel by Red Arrow in the USA [102]. At least 500 h operation has been achieved in the last few years on various engines from laboratory test units to 1.4 MWe modified dual fuel diesel engines, such as the 250 kWe engine modified by Ormrad Diesels, in UK. However, most of these experimental activities reported problems in injector and

Figure 17. Process schema for the Geneltec/BASA pyrolysis plant, as planned for NOVASEN, Senegal [95].


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Figure 18. The BEST pyrolysis technology flow diagram, Australia [96].

pump components. Emulsions and blending with highcetane oxygenated compounds was found to improve the ignition properties of the fuel [103,104]. Unfortunately, the use of emulsions seemed to accelerate erosion– corrosion phenomena. Recent promising developments have been achieved using pyrolysis liquids-diesel oil mixtures in Diesel engines [105]. A 2.5 MWe GT has also been modified by Orenda, in Canada and successfully run on fast pyrolysis bio-oil, from DynaMotive and Ensyn, achieving low pollutant emissions. 15 t were co-combusted without problems in a 350-MWe gas-fired power station in Harculo, The Netherlands [106–110]. It was suggested that by the addition of water or alcohol to the collected oil, a biomass pyrolysis oil was produced, which was optimized for fuel handling and combustion characteristics [111,112]. Research is still needed for the demonstration of long-term performance and reliability of the modified GT system. The future use of pyrolysis liquids in Micro GT for decentralized power generation could probably extend the possibility for cogeneration and market penetration. Bio-oil has some properties that affect negatively fuel quality, such as high oxygen content, low heating value, incompatibility with conventional fuels, solids content, high viscosity, incomplete volatility and chemical instability. Some bio-oils and their properties 856

obtained from the different processes previously described are summarized in Table II and compared with heavy fuel oil. Owing to process diversity and feed composition, which influence the nature of the final product, a direct comparison cannot be made. Nevertheless, it can be observed that pyrolysis liquids are more acidic and viscous than heavy fuel oil and contain a high amount of chemically dissolved water. Their heating value varies with water and additives and is much lower than that of conventional fuels. For the production of electricity and heat supply, physical upgrading or mild catalytic processes are enough, according to quality requirements. For the production of transportation fuels, high severity chemical/ catalytic processes are needed [113], since the oils must be deoxygenated. An attractive future transportation fuel can be hydrogen, produced by steam reforming of the whole oil, or its carbohydrate-derived fraction. In addition, the char resulting from the pyrolysis processes can be used for the gasification, to obtain hydrogen-rich gas by thermal cracking. Furthermore, it can be used as a solid fuel in boilers, or for the production of activated carbon. Finally, pyrolysis gas containing significant amount of carbon dioxide along with methane, might be used as a fuel for industrial combustion purposes. Int. J. Energy Res. 2011; 35:835–862 r 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er


Biomass-derived transportation fuels currently represent a modest 0.5 EJ (or less than 1%) of total bioenergy use worldwide. But, it is especially in this field that global interest is growing in Europe (biofuel consumption reached 5.4 Mtoe in 2006 [114], Brazil, North America and Asia (most notably Japan, China and India). Four main drivers explain this growing interest: (a) the transport sector is particularly difficult to tackle in terms of GHG emission reductions; biomass is the only option for supplying carbon neutral hydrocarbons (b) the strategic importance of reducing the dependency on oil, imported from a declining number of exporting countries that experience political instability, is growing, as is concern that global oil production may peak sooner than previously expected; transport fuels are the by far the most important product produced from mineral oil (c) technological developments offer clear perspectives of competitive and efficient production of biofuels from biomass [1] (d) in the medium term, biomass use for transport fuels may prove to become a more effective way to reduce GHG emissions, than using biomass for power generation; this can be explained by the partly observed and partly expected reduction in carbon intensity of power generation, due to large-scale penetration of wind energy, increased use of highly efficient natural gas fired combined cycles and deployment of CO2 capture and storage. The electricity production cost from a pyrolysis process is compared with three other biomasses to electricity systems in Figure 19. It can be seen that none of the novel systems produce lower cost electricity than the established combustion system. The reasons are the lower capital amortization costs, the lower overhead and maintenance changes and the lower labor costs of the combustion system. The poor

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competitiveness of the fast pyrolysis and gasificationbased systems is largely due to the lack of experience. The opportunities for bio-oil from fast pyrolysis, in the heat and power markets of Europe, were evaluated by a quantitative assessment of the economic competitiveness of standard applications in 14 European countries [116]. Location-specific data were collected and combined with technology-specific data. A competitiveness factor was derived, which represents the total annual cost of a conventional alternative relative to a bio-oil application. It has been assumed that a single bio-oil production facility is located in a region of high biomass availability and takes all available biomass from an area of radius 100 km, up to a maximum of 200 000 dt a1 (the size of area has been limited to reflect the high transportation costs of raw biomass and the maximum plant size has been capped to reflect current technology expectations). Bio-oil is then produced and stored with an associated cost of production. A typical fluidized bed pyrolysis process is assumed. Biomass feedstock prices varied widely within Europe, ranging from 1.4 h MWh1 (Italy, furniture industry by-product) to 15.7 h MWh1 (Denmark, hardwood residues from forestry). Most of the data were in the approximate range of 8–10 h MWh1. Bio-oil cost ranged from 26 to 42 h MWh1 (thistle and forestry residues, application heat boilers). A wide variation was found across Europe in the levels of competitiveness and in the ranking of the different applications. In a total of six countries, at least one of the standard bio-oil applications is shown to be economically competitive. These are Italy, The Netherlands, Denmark, Greece, Austria and Spain. In Belgium, Finland, France, Germany, Ireland, Norway, Portugal and the United Kingdom, none of the standard bio-oil applications is shown to be competitive.

Figure 19. Comparison of electricity production cost for four biomass power systems [115].


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Table XV. Comparison of fuel prices for renewable and non-renewable liquid fuels [117]. Fuel Bio-oil Rape methyl ester Biodiesel Bio-ethanol Methanol No.2 fuel oil No.4 fuel oil Diesel Petrol Beef tallow

LHV (MJ kg1)

Density (kg m3)

LHV (GJ m3)

Cost (h GJ1) low

Cost (h GJ1) high

16 37.2 37.2 27.2 19.9 41.1 40.8 42.9 43.2 40

1.21 0.85 0.88 0.79 0.80 0.93 0.95 0.84 0.82 0.92

13.2 43.8 42.3 34.3 25.1 44.2 42.9 51.1 53.0 43.5

4.0 5.5 0.9 20.8 7.5 2.4 3.7 2.8 1.8 7.5

43.8 28.5 16.1 22.0 12.5 3.0 4.4 — — 10.0

Heat applications are the most economically competitive, followed by CHP applications, with electricity applications generally uncompetitive. Within a given technology, the larger the scale, the better the economic competitiveness. The boiler for heat applications showed on average the best economic competitiveness across Europe, followed by the IC engine for CHP and the Rankine cycle for CHP. Prices for some renewable and non-renewable liquid fuels are given in Table XV. Bio-oil may have the potential to compete with the domestic heating fuel market, as taxes and other additional costs vary from country to country [2,118]. For the production of electricity, this would appear to be viable only in site-specific situations, where there is no nearby low cost grid connection. If costs can be reduced to acceptable levels, they could become very attractive options for future transport fuels, given their high conversion efficiencies and very low well-to-wheels greenhouse gas emissions. For EU, where energy consumption per capita increases, along with the rate of global oil consumption and the degree of import dependency in energy [119], it is obvious that a robust energy policy has to address this unsatisfactory situation and envisage reversing this trend, for reasons of security of energy supply.

5. CONCLUSIONS Advanced biomass pyrolysis methods have been developed the last two decades for the direct thermal conversion of biomass to liquid fuels, charcoals and various chemicals in higher yields than those obtained by traditional pyrolysis processes. Moderate but optimized temperatures (400–6501C) are used at short residence times (0.1–2 s) to maximize liquid yields, whereas low temperature and long residence times (h) are used to maximize char yields. The most important reactor configurations are fluidized beds, rotating cones, vacuum and ablative pyrolysis reactors. Fluidized beds and rotating cones are easier for scaling and possibly more cost effective. Several plants have already been operating at a scale of over 200 kg h1 of 858

feedstock, such as those of DynaMotive, UK and Canada; ENEL, Italy; Ensyn, USA; Bioware Technologia, Brazil (fluidized beds); BTG, The Netherlands (rotating cone); PyroVac Inc., Canada and Ecosun by The Netherlands (vacuum reactor); PYTEC, Germany (ablative reactor); Compact Power, UK; PKA, Germany; Chemviron Carbon, Germany; Lambiotte, Belgium; Novasen, Senegal; BEST Energies, Australia (other reactors). Bio-oil produced from these processes is a source of valuable chemicals and can substitute for fuel oil—or diesel fuel—in many static applications including boilers, furnaces, engines and turbines for electricity generation. While commercial biocrudes can easily substitute for heavy fuel oils, it is necessary to improve the quality in order to consider biocrudes as a replacement for light fuel oils. Pyrolysis gas—containing significant amount of carbon dioxide, along with methane— might be used as a fuel for industrial combustion. Presently, heat applications are most economically competitive, followed by CHP applications electric applications are generally not competitive.

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