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A review on advances of torrefaction technologies for biomass processing

Article  in  Biomass Conversion and Biorefinery · December 2012

DOI: 10.1007/s13399-012-0058-y

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REVIEWARTICLE

A review on advances of torrefaction technologiesfor biomass processing

Bimal Acharya & Idris Sule & Animesh Dutta

Received: 17 April 2012 /Revised: 1 July 2012 /Accepted: 30 July 2012 /Published online: 20 September 2012# Springer-Verlag 2012

Abstract Torrefaction is a thermochemical pretreatmentprocess at 200–300 °C in an inert condition which trans-forms biomass into a relatively superior handling, milling,co-firing and clean renewable energy into solid biofuel. Thisincreases the energy density, water resistance and grindabil-ity of biomass and makes it safe from biological degradationwhich ultimately makes easy and economical on transpor-tation and storing of the torrefied products. Torrefied bio-mass is considered as improved version than the currentwood pellet products and an environmentally friendly futurealternative for coal. Torrefaction carries devolatilisation,depolymerization and carbonization of lignocellulose com-ponents and generates a brown to black solid biomass as aproductive output with water, organics, lipids, alkalis, SiO2,CO2, CO and CH4. During this process, 70 % of the mass isretained as a solid product, and retains 90 % of the initialenergy content. The torrefied product is then shaped intopellets or briquettes that pack much more energy densitythan regular wood pellets. These properties minimize on thedifference in combustion characteristics between biomassand coal that bring a huge possibility of direct firing ofbiomass in an existing coal-fired plant. Researchers aretrying to find a solution to fire/co-fire torrefied biomassinstead of coal in an existing coal-fired based boiler withminimum modifications and expenditures. Currently avail-able torrefied technologies are basically designed and testedfor woody biomass so further research is required to addresson utilization of the agricultural biomass with technicallyand economically viable. This review covers the torrefactiontechnologies, its’ applications, current status and future rec-ommendations for further study.

Keywords Torrefaction . Bioenergy . Coal-fired plant

NomenclatureBO2 Bio-dioxide (like carbon dioxide)CV Calorific valueGHG Green house gasLCA Life cycle analysisSCD Screw conveyors dryersTB Torrefied biomassVOC Volatile organic compounds

1 Introduction

Carbon-offset programs to limit the amount of GHG emis-sion have not only dominated the global warming discus-sions but also the continuous rise in world populations hasincreased the energy demand in a more unsustainable fash-ion. As a result, this has spearheaded the increasing demandfor clean and sustainable sources of energy. For instance,Europe established a cap-and-trade system in 2005 thatlimits CO2 emissions from about 50% of industry to reachits emission target as dictated by the Kyoto Protocol [1].Furthermore, fossil fuels like petroleum, natural gas or coal,which are the main sources of energy in most industrializednations, are major contributor to global warming through theGHG emissions, and their sources are depleting. For in-stance, coal-fired plants use most coal and produce mostof the fossil fuel air pollution, and for each ton of carbonburned, 3.67 tons of CO2 is generated. The emission is notonly damaging to the environment but also to the humanhealth. The global use of carbon causes emission of approx-imately 7 billiontons/year, and it is projected to reach 14 bil-liontons/year by 2050 [1]. These global challenges havetriggered an increase in the adoption of alternative sourcesof energy, including renewable sources.

B. Acharya : I. Sule :A. Dutta (*)School of Engineering, University of Guelph,Guelph, ON, Canadae-mail: adutta@uoguelph.ca

Biomass Conv. Bioref. (2012) 2:349–369DOI 10.1007/s13399-012-0058-y

Consequently, as one of the game changer, bio-energyhas been discovered to be one of the key renewable energyinitiatives to substantially reduce GHG emissions and con-tribute enormously to sustainable energy generation forelectricity and industrial applications. Renewable energy isderived from natural resources that may be replenishedunlike fossil fuels. Biomass energy products are referred toas bio-energy, which can be in the form of solid (bio-solids),liquid (bio-oil) or gas (bio-gas).

Despite the tremendous popularity gained by biomassenergy in the recent years, the fraction of its utilization inproducing energy remains insignificant in the overall sourceof energy production in industrialized nations. This can bedue to several factors, including the limitation associatedwith its properties [2]. The variations in biomass feedstockcause several challenges during the conversion process,including excess smoke during combustion and low com-bustion efficiency. Torrefaction, a biomass pretreatment pro-cess, has been found to improve biomass combustibleproperties [2, 3]. Torrefaction is partially an endothermicprocess that requires heat thermal decomposition andrequires approximately 0.6–1 MJ/kg based on scale and ener-gy balance of the overall process and product in terms ofhigher heating value [4]. Since the main changes in biomassdue to torrefaction include the decomposition of hemicellu-lose and partial depolymerization of lignin and cellulose,torrefied biomass (TB) has higher content of carbon, lowermass and higher calorific value (CV) than the raw biomass [4].The temperature and residence time of torrefaction processmust be precisely controlled to ensure higher energy efficien-cy of the biomass conversion process [5].

Hence, the main objectives of this paper is to provideupdates on the torrefaction research activities which mainlyinclude (a) issues with biomass and its components andcomponent analysis procedure; (b) torrefaction and itschemistry, reaction, kinetics, process integration, torrefiedfuel characteristics, technology used and recent develop-ment; (c) application of torrefaction technologies in pelleti-zation, combustion/co-firing, gasification and emission and(d) economics and further research potential for energyapplication.

2 Biomass fuel

According to Yoshida et al. [6], the word “biomass” origi-nally meant the total mass of living matter within a givenunit of environmental area, but more recently, it has alsobeen described as plant material, vegetation or agriculturalwaste used as an energy source. Tumuluru et al. [7] alsodefined biomass materials as a composite of carbohydratepolymers with a small amount of inorganic matter and lowmolecular weight with extractable organic constituents.

Generally, biomass is a biological or organic material,which can serve as source of renewable energy throughthermal or biochemical conversion processes. It can alsobe classified as carbon-based material, which composed ofmixture of organic molecules including hydrogen, oxygen,nitrogen and small quantities of atoms including alkali,alkaline, earth and heavy metals. Because biomass areorganic materials which encompasses all living matter,their energy contents are obtained from the sunlight andstored in form of chemical energy that is then convertedinto heat energy through thermal or biochemical process-es. A good illustration of biomass as one of the source ofrenewable energy is wood, which is obtained from trees.Trees absorb sunlight and CO2 from the atmosphere dur-ing photosynthesis to make cellulose from sugars; conse-quently, the cellulose, which contains stored chemicalenergy, releases this energy as heat when combusted andthe CO2 liberated as off-gas is approximately equivalent tothe amount absorbed during photosynthesis process.Hence, biomass can be greenhouse gas emission neutral[8]. Unlike fossil fuels, biomass is a renewable source ofenergy that can be replenished and add zero net green-house gas to the atmosphere.

2.1 Biomass challenges

Biomass materials have several limitations that limit theirutilization for energy generations. This can be due to manyfactors, including their physical and chemical properties [2].Some of these challenges include low heating value, highmoisture content, hygroscopicity, excess smoke during com-bustion, low energy density, higher alkali contents and lowcombustion efficiency [9].

These limitations greatly impact not only the combustionperformances but also the biomass-to-energy supply chainlogistics due to costly handling and transportation of bio-mass. As a result, biomass materials must be treated toovercome these challenges and make them suitable forenergy use.

2.2 Biomass components

The three main polymeric constituent of biomass are hemi-cellulose, cellulose and lignin, and generally, they cover,respectively, 20–40, 40–60 and 10–25 wt.% for a lignocel-lulosic biomass [10, 11]. Figure 1 shows the polymer struc-ture of a woody biomass.

2.2.1 Cellulose

Cellulose, a linear polymer that makes up about 45 % of thedry weight of wood, is composed of D-glucose subunits linkedtogether to form long chains (elemental fibrils), which are

350 Biomass Conv. Bioref. (2012) 2:349–369

further linked together by hydrogen bonds and Van der Waalsforces. The cellular fibre formed by several micro-fibrils com-ing together can either be crystalline or amorphous [12].

Furthermore, cellulose is a high molecular weight polymerthat makes up the fibres in lignocellulosic materials, and itsdegradation starts anywhere from 240 to 350 °C because ofhigh resistance of its crystalline structure to thermal depoly-merization owns to its strength [7]. The waters held in theamorphous regions of the cellulosic wall rupture the structurewhen converted into steam as a result of thermal treatment [7].

2.2.2 Hemicelluloses

Hemicellulose is a complex carbohydrate polymer with alower molecular weight than cellulose and makes up 25–30 % of total dry weight of wood. It consists of D-xylose, D-mannose, D-galactose, D-glucose, L-arabinose, 4-O-methyl-glucuronic, D-galacturonic and D-glucuronic acids [12]. Theprincipal component of hardwood hemicellulose is glucuro-noxylan whereas glucomannan is predominant in softwood[12]. In contrast to cellulose, hemicelluloses are easily hydro-lysable polymers and do not form aggregates. It consists ofshorter polymer chains with 500–3,000 sugar units as com-pared to the 7,000–15,000 glucose molecules per polymer seenin cellulose [7]. Thermal degradation of hemicellulose occursbetween the temperature of 130–260 °C, with the majority ofweight loss occurring above 180 °C [13, 14]. Hemicelluloseproduces less tars and char due to its low degradation temper-ature range compared to that of the cellulose [7].

2.2.3 Lignin

Lignin along with cellulose is the most abundant polymer innature [12]. Lignin is an unstructured and highly branchedpolymer that fills the spaces in the cell wall between

cellulose, hemicellulose and pectin components [7]. It iscovalently bonded to hemicellulose and thereby exhibitsmechanical strength on the cell wall. It is relatively hydro-phobic and aromatic in nature and decomposes between 280and 500 °C when subjected to a thermal treatment [13, 14].Lignin is difficult to dehydrate and thus converts to morechar than cellulose or hemicelluloses [7].

3 Overview of torrefaction

Torrefaction is a method to improve biomass properties forenergy generation. In literature, it is defined as a thermaltreatment process through which biomass is heated betweentemperature of 200–300 °C in an inert condition and at arelatively low residence time. Historically, torrefaction princi-ple became known in relation to wood pretreatment in the1930s in France [25] when the production of torrefied wood(TW) was researched for use in gasifier, not until the 1980swhen there is an interest in substituting charcoal for TW inmetallurgic processing plant that first torrefaction demonstra-tion plant was built in France by a French company, Pechiney,to produce TW of 12,000 tons/acre [3]. During torrefaction,the biomass properties are changed to better fuel character-istics for combustion and gasification applications. The torre-fied products show relatively similar characteristics as coal[3]. Torrefaction combined with densification provides anenergy dense fuel of 20 to 25 GJ/ton [3].

Torrefied materials exhibit following characteristics:

1. Hydrophobic behaviour: TB has hydrophobic character-istics owning to the destruction of its O–H bond struc-ture, hence making it incapable to retain or absorbmoisture. Although no standardized test exists yet forvalidating hydrophobic properties of torrefied biomass,Bergman et al. [5] demonstrated hydrophobic test byimmersing torrefied fuel in water for 2 h, drained andmeasured weight changes.

2. Inhibiting biological decomposition: stopping biologi-cal decomposition like rotten

3. Improved grindability: Torrefied biomass has improvedgrindability. This leads to more efficient co-firing inexisting coal-fired power stations or entrained-flow gas-ification for the production of chemicals and transpor-tation fuels. TB is more brittle owing to its higher C/Hand C/O ratios, hence provides enhanced pulverizecharacteristics and requires far less energy for grindingcompared to that of raw biomass [3, 13, 14].

4. Higher heating value: Torrefaction increases the cal-orific value of biomass and as a result increasestheir energy density [5, 15]. Densification increasesthe bulk and volumetric density of biomass. Hence,a combination of torrefaction and pelletization

Fig. 1 Polymer structure of a woody biomass (source: [16])

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processes produce torrefied pellets, which pilot-scaleexperiments have shown to have better handlingthan biomass pellets due to its hydrophobicity[17–19]. Torrefaction process causes dehydrationthat initiates and propagates cracks in the lignocel-lulosic structure (e.g. wood), as a result inducesporosity and density changes [20]. Increased poros-ity, due to more particle voids, decreases particlesize but inevitably increases the particle density and bulkdensity [21]. Generally, density varies in a different waydepending on wood species during temperature treatment[20, 22], and the changes with respect to torrefactionmight not be very significant [23]. The particle densityof torrefied pine chips (TPC) and torrefied logging resi-dues (TLR) did not change compared to that of untreatedbiomass; while the bulk density of TPC particles de-creased until the torrefaction temperature of 250 °C thenincreased up to the torrefaction temperature of 300 °C, nosignificant change occurs in TLR compared to the un-treated sample [22, 24, 25].

5. Particle sizes and distribution of torrefied biomass: Pul-verized torrefied biomass exhibit more uniform andsmaller particle sizes compared to that of pulverizedraw biomass [22, 26].

3.1 What is torrefaction?

Although the definitions exhibit similarities in terms oftorrefaction processes, the operating temperature range dif-fers from studies to studies depending on the biomass typesthat were researched. [3, 16, 27, 28] defined the torrefactiontemperature range from 200 to 300 °C; Prins et al. [29] andPimchuai et al. [30] defined temperature between 230 and300 °C; meanwhile, Arias et al. [13] defined temperaturerange between 220 and 300 °C, and [31–33] defined tem-perature range between 225 and 300 °C. Studies have shownthat biomass exhibit different behaviour to thermal treat-ment owing to their types, origin and properties [2]; hence,the initiation of biomass decomposition depends on the typeof biomass. In order to develop a more general definition oftorrefaction, an experimental study on a range of biomasstypes will be required to determine the temperature at whicha biomass sample is torrefied. This may be exemplified byhydrophobicity, i.e. the operating temperature and residencetime when the torrefied biomass seizes to absorb water.Although the typical definition that mostly occur in pub-lished journals is “the thermal pretreatment method carriedout between the operating temperature of 200 °C and 300 °Cunder inert condition and relatively short reactor residencetime and slow heating rate less than 50 °C/min” [2]. It iscarried out under conditions of atmospheric pressure and inthe presence of a minimum amount of oxygen in order toavoid spontaneous combustion. Recently, a number of

researchers including present authors have carried out torre-faction research at different oxygen concentrations. P.Rousset et al. [27] in their study showed that the differentoxygen concentrations did not significantly affect the com-position of the solid by-product for low temperatures. Anoxygen concentration of 6 % apparently shows better char-acterisations on grindability and hydrophobicity tests oftorrefied biomass [34]. Therefore, torrefaction of biomasscan be defined as a thermochemical pre-treatment process inan oxygen reduced condition at a temperature range from 200to 300 °C for a shorter residence time that maximizes thesolids content and enhances its hydrophobic characteristics.

The torrefaction process involves the decomposition ofbiomass during which various types of volatiles are liberat-ed, and the final product is a solid fuel generally calledtorrefied biomass or torrefied fuel [2, 3, 5].

3.2 Torrefaction process

The pre-conversion of biomass using torrefaction involvesthree main steps: chopping, drying and torrefaction(roasting) [31, 32] as shown in Fig. 2. During torrefactionprocess, biomass is fed into a chopper to reduce them intofine or more uniform particles. The chopped biomass thengoes through the drying section to remove the moisture andthen fed into the torrefaction reactor [33, 35]. The moistureliberated during drying composed of both condensable andnon-condensable gases and volatiles as stated in Fig. 3 [29,36]. The higher the temperature of torrefaction, the higherthe combustion heat of the waste volatiles gas liberatedduring the process.

After a complete devolatilisation of the biomass, the finalsolid product that remains is often referred to as torrefiedbiomass or char [3, 37]. The improved combustible proper-ties of biomass after torrefaction result in an attractive solidfuel for combustion and gasification processes. Further-more, the improved grindability of torrefied biomass makesit advantageous for pelletization, which facilitates storage,transportation and co-combustion of biomass with coal [3,38]. During torrefaction process, biomass undergoes seriesof decomposition reactions that cause the liberation of gas-eous products including volatile organic compounds. Inparticular, the C, H, O compositions of the biomass becomealtered, and the H/C (or O/C) ratio decreases because it losesits hydrogen and oxygen in more proportion compared tocarbon [3, 5]. The decomposition of biomass polymer struc-ture during torrefaction causes the destruction of its hydrox-yl (OH) group and making it incapable to form hydrogenbond with water and hence loses its tendency to absorbwater [5, 17, 39]. As a result, torrefied biomass is non-polar molecular structure, which is practically hydrophobic[27]. During torrefaction process, biomass undergoes two-stage processes: drying and torrefaction. During drying,

352 Biomass Conv. Bioref. (2012) 2:349–369

biomass loses majority of its moisture at temperature around110 °C, and further increase of treatment temperature ini-tiates the decomposition of its polymeric structure, predom-inantly, hemicellulose. At torrefaction temperature between250 and 300 °C [3, 5, 17, 27], high significance of decom-position occurs in hemicellulose and relatively slight de-composition in lignin and cellulose. Consequently,majority of biomass weight loss is attributable to the de-composition of hemicellulose into volatile compound. Sinceonly slight devolatilisation occurs in lignin and cellulose,torrefied biomass retains majority of its energy content [5].According to Bergman et al., the typical process of torre-faction retains around 70 % of its mass which containsaround 90 % of its initial; hence, around 30 % of the masscontaining only 10 % of energy content of the biomass isconverted into torrefaction gases (i.e. the volatile organiccompounds released as flue gas). This is illustrated in Fig. 4.

Prior to torrefaction process, chopping of biomass feed-stock into uniform sizes may be required depending on thefeedstock type and properties. Particle size has significanteffect in torrefaction reactions according to Ciolkosz andWallace [40], especially when large biomass feedstock isbeing processed. Although no definite sizes are recommen-ded for biomass in torrefaction process by most studies, thesizes can be based on processing equipment and biomassproperties. In the torrefaction experiments conducted byPrins et al. [41] on deciduous wood (beech and willow),coniferous wood (larch) and straw, the particle sizes usedwere in the range of 0.7 to 2.0 mm in all cases, except forstraw where it was less than 5 mm. Furthermore, accordingto Ciolkosz and Wallace [40], most studies to date onlyexamined torrefaction of ground material (or pellets) andhave not studied the complicating factors that the torrefac-tion of larger material may introduce. Most studies agreethat temperature parameter has more significant effect inenhancing the combustible properties of biomass than resi-dence time [2, 5, 16]. According to Bergman and Kiel [3],the torrefaction products are classified based on their state at

room temperature. The products in the solid phase are darkbrown-coloured carbon-rich char with traces of ash; those ingas phase are referred to as non-condensable or permanentgases.

3.3 Torrefaction kinetics

Prins et al. [41] explored the weight loss kinetic of torrefiedwood and concluded that the kinetics of torrefaction occursin two steps reactions: hemicellulose decomposition andcellulose decomposition. And since hemicellulose decom-position occurs faster than the cellulose decomposition, itcontributes significantly towards the overall mass yield oftorrefied wood. Due to these different fractions, biomass candecompose in different way under various conditions. Bio-mass undergoes four stages during torrefaction process:moisture evaporation, hemicellulose decomposition, lignindecomposition and cellulose decomposition [42].

3.4 Torrefaction mechanism

During torrefaction process, the thermal decomposition ofbiomass causes numerous reactions to occur through theirpolymer/cell structure. The decomposition process was welldocumented in Bridgeman et al. [2] as seen in Fig. 5. At lowtorrefaction temperatures, decomposition occurs in thehemicellulose structure by means of a limited devolatilisa-tion and carbonization; meanwhile, in the lignin and cellu-lose structure, a minor decomposition occurred. Figure 5shows that hemicellulose undergoes extensive thermaldecomposition between 200 and 300 °C while only limiteddevolatilisation and carbonization occurred in the lignin andcellulose structure.

It can also be noted that the transition from one decom-position regime occurs at narrow temperature range forhemicellulose while the transitions for lignin and celluloseoccur over at wide temperature range. Hence, it can beconcluded that hemicellulose is the most reactive polymer

Fig. 2 Basic principle conceptfor directly heated, two stagetorrefaction with gas recycling[5, 9]

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constituent of biomass, and it is attributed to the significantmass loss in biomass during torrefaction [3, 16, 27].

Temperature has a significant effect in the degree ofdecomposition of biomass. In the process of heating ligno-cellulosic materials, the decomposition of the polymer struc-ture of the material undergoes stages of decompositionregimes as seen in Fig. 6 due to increasing temperature.

At temperature between 200 and 280 °C, only hemicellu-lose has undergone depolymerization with limited devolatili-sation reactions while in lignin and cellulose, these reactionsare still occurring. After majority of the moisture has beenremoved at temperature between 100 and 120 °C, the signif-icant weight loss of biomass is attributable to depolymeriza-tion and limited devolatilisation of the hemicellulose duringtorrefaction process (between temperature of 200 and 300 °C).Hence, since only slight depolymerization and devolatilisationreaction occur in lignin and cellulose during torrefaction,majority of the energy content remains in the torrefied

products. Furthermore, in a study on the torrefaction impacton lignocellulosic structure of biomass, Rousset et al. [27]concluded that the slight weight loss that occurred in biomassat temperature of 230 °C was attributed to slight decomposi-tion of hemicellulose, and at temperature around 260 °C,severe decomposition of hemicellulose and slight of lignincontributed to massive biomass weight loss. These conclu-sions were similar to those from [3, 16, 17, 27, 43]. Rousset etal. [27] went further to categorize the temperature range forthermal decomposition of hemicellulose as 150 to 350 °C,cellulose as 275 to 350 °C and lignin as 250 to 500 °C. Duringdecomposition of lignocellulosic polymer structure, other im-portant parameter is the residence time, which accounts for thetransition periods that exist from a decomposition regime toanother. For instance, the transition period from the depoly-merization regime to devolatilisation regime is shorter forhemicellulose due to its high reactivity and lower temperaturerange [3, 16] than that of the lignin and cellulose. Thisexplains why during torrefaction process, increase in resi-dence time decreases the mass yield of biomass [2, 3, 5]because of more devolatilisation that occurs at specified oper-ating temperature for a span of time. However, temperatureeffect is more significant to weight loss of biomass comparedto that of the residence time [5].

3.5 Effect of temperature and residence time on productcharacteristics

Torrefaction treatment improves the combustible (physicaland chemical) properties of biomass, and the characteristicsof torrefied products depend on the biomass properties andthe operating temperature and residence time used in thetreatment. The main characteristics of torrefied products areas listed in Section 3. Generally, biomass density varies in a

Fig. 3 Products formed duringtorrefaction process (source: [5])

Torrefaction (200-300°C)

Torrefied Gas as Loss 30%M +10%E

Biomass Feed Stock input 100%M +100%E

Torrefied Biomass 70%M +90%E

Fig. 4 Mass and energy and energy balance of a typical torrefactionprocess (M = mass and E = energy) [5]

354 Biomass Conv. Bioref. (2012) 2:349–369

different way depending on wood species and temperaturetreatment [23, 44]. According to Bourgeois et al. [44], theparticle density of TPC and TLR did not change comparedto that of untreated biomass, while the bulk density of TPCparticles decreased until the torrefaction temperature of250 °C but then rose after 250 °C to the torrefaction

temperature of 300 °C; however, no significant changeoccurred in TLR compared to the untreated sample [24,25, 44]. Table 1 below summarizes the comparison in fuelproperties and handling characteristics of raw wood, woodpellets, torrefied wood pellets, coal and charcoal.

Bergman et al. [3, 5] further examined the CV of thetorrefaction gas experimentally, while mass and energybalance thermal process efficiency, auto-thermal operationand combustibility of the torrefaction gas were investi-gated by means of process simulations. In their studies,the yield of reaction water varied between 5 and 15 %weight, resulting in a concentration of 50–80 wt.% in thetorrefaction gas (excluding free water from the feedstock). It is found that the major difference betweencharcoal and torrefied wood is the volatile content. Vol-atiles are lost during charcoal production, which alsomeans a possible loss of energy. On the other hand,during torrefaction, most of the volatiles are retained. Itis also recommended that every form of carbonization beavoided during torrefaction. From the data, torrefied pel-lets have product characteristics, like handling, millingand transport requirements, similar to coal. Torrefiedpellets allow for higher co-firing percentages up to40 % due to matching fuel properties with coal, and theycan use the existing equipment setup for coal.

The reaction water yield increased with residence timeand temperature, while its concentration decreased. Conse-quently, the relative contribution of combustible productsincreases with increased temperature and residence time as

Fig. 6 Stages in the heating of moist biomass as translation of energy requirement [3]

Fig. 5 Decomposition regimes of lignocellulosic material during ther-mal treatment [17]

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does the CV, which ranges from 5.3 to 16.2 MJ/Nm3.Despite the high water content of the torrefaction gas, theCV value is relatively high. It can be compared to producegas from air blown biomass gasification (4–7 MJ/Nm3) andsyngas from an indirectly heated gasification process (15–20 MJ/Nm3). Based on this comparison, the torrefaction gasshould be combustible and can play an important role in thetorrefaction process [3, 24, 44]. Typical experimental resultsfor torrefaction mass and energy yields and gas-phase com-position for willow are given in Fig. 7.

The temperature and residence time have effect on theproperties of torrefied biomass. From the data analysis, it isfound that percentage of mass yield decreases with theincrease in the temperature. Similarly, with the increase inthe residence time, the percentage of mass yield decreasesslightly. Hence, the net effect of temperature rise has signif-icant effect on the percentage of mass yield rather than theresidence time. It is observed that raw biomass has thehighest properties of moisture retaining capacity while thetorrefied biomass at the highest temperature has the leasthygroscopic behaviours [45, 46]. From the literature [7], ifwe compare the conversion of agricultural residues of ricestraw and rape stalk with woody biomass, the solid to liquidconversion of the former is much higher than that of thelatter under the same temperature and residence time. This isbecause of the higher volatile matter contents in the agricul-tural residues and hemicellulose decomposition temperaturerange. Bridgeman et al. also concluded similar findingswhere mass yield in dry ash free was 55.1, 61.5 and72.0 % for wheat straw, reed canary grass and willow,respectively, at 290 °C for 30 min residence time. Thecalorific value of TB increases with increase in treatmenttemperature and residence time [5, 16], and this can beexplained by the fact that TB has lost its moisture contentand its oxygen–carbon or hydrogen–carbon ratio reduceswith increasing temperature

Torrefied biomass produces more uniform and smoothparticle sizes compared to untreated biomass because oftheir brittleness, which is similar to that of coal, and thisbehaviour is supported by their lower energy consumptionduring grinding [20, 44]. In their experiment to examine theparticle size and particle size distribution of a torrefied pinechips and logging residues, Phanphanich and Mani [22]found out that the mean particle size of ground torrefiedbiomass decreased with increase in torrefaction temperature.Consequently, torrefaction of biomass not only decreasedthe specific energy required for grinding but also decreasedthe average particle size of ground biomass. Furthermore,they concluded that the particle size distribution curves oftorrefied biomass produces smaller particles than that ofuntreated biomass, and their results were comparable tothe studies by Mani [21]. Cumulative percent passing curvealso showed the similar behaviour for torrefied biomass.

3.6 Technology

Torrefaction is based on thermal drying principle; there aremany established and patented potential methods for carry-ing out torrefaction of biomass, which are majorly based ondifferent drying equipment. However, there exist severalchallenges which have made it hitherto difficult to run a fullcommercial scale torrefaction plant; one of these challengesis the complex characteristics of biomass and ability tocontrol operating conditions that will improve the qualityof torrefied products at low costs. There are two principlesof heat contact during a drying process: directly heateddrying and indirectly heated drying. In the directly heateddriers, biomass is brought in contact with the heat carrier,which can either be hot steam or hot air. However, inindirectly heated dryer, biomass is not in direct contact withheat carrier [3, 47, 48]. Many drying technology can bemodified to meet the specifications of a torrefaction reactor.

Table 1 Summary of torrefied pellets properties versus coal (source: [64])

Parameters Wood Wood pellet Torrefied pellets Coal

Moisture content (wt.%) 30–40 7–10 1–5 10–15

Calorific value (MJ/kg) 9–12 15–16 20–24 23–28

Volatiles (% db) 70–75 70–75 55–65 15–30

Fixed carbon (% db) 20–25 20–25 28–35 50–55

Bulk density (kg/m3) 200–250 550–750 750–850 800–850

Volumetric energy density (GJ/m3) 2.0–3.0 7.5–10.4 15.0–18.7 18.4–23.8

Dust explosibility Average Limited Limited Limited

Hydroscopic properties Hydrophilic Hydrophilic Hydrophobic Hydrophobic

Biological degradation Yes Yes No No

Milling requirements Special Special Classic Classic

Handling properties Special Easy Easy Easy

Transport cost High Average Low Low

356 Biomass Conv. Bioref. (2012) 2:349–369

These include rotary drum dryer, fluidized bed dryer, beltdryer, conveyor dryer, screw (auger) dryer, microwave dryerand multiple hearth furnace dryer (or turbo dryer):

(a) Rotary drum reactor consists of a rotating drum, whichrotates about a fixed point via a rotating shaft and caneither be configured in an inclined or vertical position.Most widely used type is the directly heated single passin which hot gas (or steam) is contacted with biomassin a rotating drum. The rotating drum causes the bio-mass particles to tumble through hot gas to promoteheat and mass transfer [49]. In addition, hot steam canbe used as heat carrier in a rotary drum dryer. Thefeedstock (biomass) normally flows co-currently withthe hot carrier through the reactor to facilitate drying.Moreover, if contamination is not a concern in thereactor, hot flue gas can be fed into reactor to supple-ment for energy source for the operation.

(b) Fluidization is one of the most commonly used techni-ques and found to have widespread applications for dry-ing of solid particulates. The techniques require high-velocity hot gas stream that creates a “fluid bed” withspecial hydrodynamics and heat and mass transfer char-acteristics [50]. Fluidized bed drying offers many advan-tages, including fast drying and high thermal efficiencywith uniform and closely controllable bed temperature[51]. It offers good mixing and ease of combining severalprocesses [51]. However, its fast drying advantage is notideal for torrefaction because torrefaction requires a slowand controllable drying rate (i.e. slow pyrolysis). Thedisadvantages, however, include high-pressure drop,abrasion of the solids causing erosive surfaces, bed heightcontrol to accommodate the height for fluidization and

the height allowed by the pressure drop, and restriction inparticle sizes and size distribution [50].

(c) A moving bed chemical reactor is characterized by themovement of both solid and fluid phase during chemicalreaction and the operation may be countercurrent, co-current or cross flow depending upon the relative direc-tions of fluid and solid [52, 53]. The moving bed tech-nique, especially on its application in agricultural dryers,has become popular owing to its lower investment, lowerenergy consumption, less mechanical damage to theseeds [54], high heat transfer rate, good hold time fortemperature, fast drying [3], low pressure drop [52] andgood plug flow. The design can be compact, highlyefficient and flexible to combine with other reactors(e.g. fluidized bed) to optimize their applications.

(d) A screw conveyor consists of a helical flight fastenedaround a pipe or solid shaft that is mounted within atubular or U-shaped trough; hence, when the screwrotates, material heaps up in front of the advancingflight and is pushed through the trough [55]. Varioustypes of screw configurations have been reported tohandle variety of materials and flow rate requirements[55]. The screw conveyor dryer consists of a jacketedconveyor in which material is simultaneously heatedand dried through heating medium such as hot steam ora high-temperature heat transfer medium such as potoil and fused salt [55]. The heat carrier may be througha hollow flight and shaft (indirect contact) to providegreater heat transfer area with minimum space require-ments [55]. Screw conveyors dryers have utilities inmany industrial applications, including agricultural,food, chemical, pharmaceutical and pyrolytic processof coal [55–57]. Some of the advantages are theirapplication for drying wide range of solid particlesranging from fine powder to lumpy, sticky and fibrousmaterials [44, 58]. Waje et al. [55] found their averagevalue of heat transfer rates to be between 42 and105 Wm−2°C−1. Some of the disadvantages are highcost of maintenance due to several moving parts, lowheat transfer rate [3] and not recommended for materi-als that have tendency to cause fouling [55].

(e) Microwave heating is very attractive for various chem-ical processes as it produces efficient internal heatingfor chemical reactions, even under exothermic condi-tions [48], and has become a widely accepted non-conventional energy source for performing organicsynthesis [59]. In addition, microwave heating pro-vides shorter residence time, prevents undesirable sec-ondary reactions that lead to formation of impuritiesand provides volumetric heating with good penetrationdepth [48, 60]. Two most common frequencies allocat-ed for material heating are 915 and 2,450 MHz forindustrial, scientific and medical applications [61].

Torrefaction (32 minutes at 260°C)

Gas Phase ComponentsCO=0.1% CO2=3.3% H2O=89.3% Acetic Acid=4.8% Furfural=0.2% Methanol=1.2% Formic Acid=0.1% Remainder=1.0%

Feed: Willow Size: 10-30mm LHV=14.8MJ/kg MC =14.4% (wb) Fixed Carbon=16.8%

Torrefied Willow Size: 10-30mm LHV=18.5MJ/kg MC =1.9% (wb)

Mass yield=75.3% Energy Yield

Fig. 7 Experimental results of torrefaction of willow [6]

Biomass Conv. Bioref. (2012) 2:349–369 357

Several advantage of microwave drying comes fromvolumetric heating rather than surface heating andsince the electromagnetic energy is dissipated directlyin the dried material, heat losses are considerably re-duced [50]. However, some of the drawbacks of mi-crowave heating technology are inability to processfines and allow scale up of operation [26] and inabilityto provide uniform heating.

(f) Multiple hearth furnace (MHF) is a verticalrefractory-lined cylindrical steel shell reactor, whichcontains circular hearths that rotate in horizontalplane about a centre shaft installed with rabble armsthat moves in spiral path across each hearth [62].The materials that enter the top hearth pass througha drop hole to the hearth below. The retention timeof the materials in the multiple hearths can be from0.5 to 3 h depending on the shaft speed and on thenumber of hearths [62]. In some operations, com-bustion of charged-elements supplies the heat, whilein other cases, it is furnished with combustion ofauxiliary fuel by direct or indirect firing [63].According to Dangtran et al. [62], a multiple hearthfurnace is divided into three zones: The upper zones(or the drying zone) is where raw materials undergodrying to remove moisture; the middle hearth zone(or the combustion zone) is where the dried materi-als are exposed to the combustible reactions at hightemperatures; hence, the residence time is usuallyshort; and the lower hearths (or the cooling zone)where the products are cooled and its heat is trans-ferred to the incoming combustion air/steam. Somebenefits of MHF are their capacity to: allow widerange of processing conditions including mode ofheat transfer (co-current, counter-current or crossflow), control temperature and residence time, pro-vide high heat and mass transfer and ensure goodmixing [62, 63]. MHF drawbacks, however, aretheir sensitivity to change in feed characteristics,sealing issues and high cost of maintenance duemultiple moving parts.

Torrefaction is still an evolving technology, and manytechnologies, which are based on the drying techniquesadopted for industrial processes such as in the agricul-tural and mining industries, have been proposed bymany research institutes and technologies developersacross the Europe and North America. Although fewcompanies have claimed to develop torrefaction technol-ogies that can be operated commercially, no provencommercial application exists yet. Overview of varioustorrefaction reactor technologies has been documentedin torrefaction review papers and conference presenta-tions [26, 64] and these reviews include the lists of

companies, their reactor technologies and the principaldevelopers. Consequently, to compare the aforemen-tioned reactor technologies as potential candidates fortorrefaction, the reactor technology must be proven andversatile enough to accommodate all the operating con-ditions, including the capacity to: control temperatureand residence time, accommodate wide range of feedstocks, accommodate the heat integration system to takeadvantage of energy recirculation to supplement theprocess heat, accommodate scale-up of operations, en-hance mixing, provide uniform heating, provide highheating rate, enhance mass and heat transfer and processlarge and small particles. Ranking these different reactortechnologies will be based on the above criteria viadecision matrix. Table 2 below shows the total ratingof each potential reactor technology for torrefactionoperation based on decision matrix principles. The tech-nology that scored the highest is the fluidized bedfollowing by the multiple hearth furnace. These ratingsare slightly different from those from Ferro et al. [46]due to the consideration of moving parts. Moving partsmay lead to high cost of maintenance or unnecessaryinterruptions of plant operations.

3.7 Recent development

According to Kleinschmidt [64], torrefaction technology isin the process of commercialization even though the tech-nology and quality are still surrounded by many maturitiesand uncertainties. EU is leading on the execution of thetorrefaction in the world. Energy Center of the Netherlandis one of the first to recognize the potential of torrefactionfor biomass to energy purposes. Initial small scale researchwas started in 2002–2003. Based on the small-scale re-search, 25 tons of torrefied material was produced in 2008from poplar chips, softwood/hardwood mixture and agricul-tural residues at 220–280 °C. European utilities EssentB.Vm DELTA N.V. had taken the risk to produce torrefiedbio-product and supply to the other utilities RWE Innogy forlong-term basis. This brings new rays of hopes on thecommercialization of torrefaction technology [65, 66]. It isexpected that developmental stages in Europe will lead togear the momentum of commercialization of torrefaction inthe North America and other world.

There are more than 50 development projects under wayin European Union out of which more than ten projects weretargeted to be in production before end of 2011, but none ofthe literatures confirms these claims. One of the projects ofCanada was from The Centre for Energy Advancementthrough Technological Innovation (CEATI) program [67].CEATI evaluated most promising torrefaction/carboniza-tion/steam explosion/microwave technologies and providedcritical assessment of the leading sources/vendors that offer

358 Biomass Conv. Bioref. (2012) 2:349–369

the best short- and long-term technology/project potential todetermine the economic viability of beneficiated fuels and todetermine the technical viability of beneficiated fuels basedon actual testing, comprehensive lab analysis, 1 MW pilottest burns, 150 MW full scale and 100 % beneficiatedbiomass test burns. The tentative project timeline was toinitiate in March 2010 and Energy Research Center of theNetherland (ECN) was selected for lab test.

According to Dana et al. [68], several manufacturers andresearchers are developing torrefaction units for commercialuse. Integro Earth Fuels, LLC reports that their torrefactionprocess reduces 20–30 % of the mass while retaining 90 %of its energy. Their torrefaction process operates in thetemperature range of 240–270 °C. The company anticipatesproducing 4,000 tons of torrefied biomass each month in thepilot plant. Knowledge gained from the pilot plant wasintended to develop a full-sized torrefaction facility [69].Heating values of the final product range from 9,500 to11,000 Btu/lb. Southern pine species have an energy valueof approximately 8,500 Btu/Lb (dry weight). Under thistorrefaction process, the energy value from a dry ton ofwood would be reduced from 8,500 to 7,650 Btu/lb (a10 % loss); however, there are mass losses associated withthe process. If the mass reduction from the process is 20 %,the final product has an increased energy value of 9,563 Btu/lb or a 12.5 % increase in energy value. Thermya, a Frenchengineering company, has developed a continuous torrefac-tion process called TORSPYD. In April 2010, World Bio-energy News reported that Thermya was the only Europeancompany to offer an industrially proven, fully operational,continuous biomass torrefaction process [70]. The system isreported to operate in the lower range of temperaturesreported for torrefaction. TORSPYD processing operates

in temperatures ≤240 °C, a soft thermal treatment. Unitcapacities can range from 100 to 5,000 kg/h. The finalproduct is called bio-coal and is marketed as a coal substi-tute to be co-fired with coal or used in industrial boilers forproducing electricity. The bio-coal can also be used in pelletmanufacture and eliminates the need for sawdust [70]. Agri-Tech Producers, LLC, a company based in South Carolina,is reported to be nearing the completion of a commercial-grade torrefaction machine. Using technology developed atNorth Carolina State University, their process operates in alow-oxygen environment at temperatures ranging from 300to 400 °C. The first built plant was named as the Torre-Tech5.0. The production rate of this machine was 5 tons oftorrefied wood/h. Researchers in the Netherlands are con-tinuing to research on a torrefaction process that began inthe 1980s by a French aluminium company. Originally, theprocess was used to produce metal from metal oxides.Today, the current process is called TOP for torrefaction andpelletization. Early results in 2005 (Bergman and Kiel) indi-cated that a commercial scale plant could produce 60–100 greenktons/year (approximately 66,000–110,000 greentons/year) of high-energy torrefied pellets. Researchers indi-cate that TOP pellets could be delivered to power plants at alower cost/Btu as compared to standard wood pellets. Theyattribute some of the cost savings to the pelletization process,but the majority of the savings is attributed to transportationlogistics from transporting an energy dense product.

In 2009, Natural Fuels Industries, Inc. of Calgary, AB,Canada announced plans to build biomass processing plantsin Georgia (USA) and Brazil. The company planned to pro-duce bio-coal briquettes using torrefaction technology. Thebriquettes could be shipped to European markets. In theirinitial announcement [67], they stated that there is a

Table 2 Comparison of potential torrefaction technologies [9, 69–72]

Torrefierstechnology

Mode ofheating

Status criteria

Rotary drumreactor

Direct Proven technology, minimum heat transfer, high heating rate, medium temperature control, good residencetime control, excellent heating integration, enhanced mixing, large size tolerance, high moving parts, goodfouling,, little scaling problem

Fluidized bedreactor

Direct Proven technology, enhanced heat transfer, high heating rate, medium temperature control, medium residencetime control, excellent scalability, excellent heating integration, excellent uniform heating materials,enhanced mixing

Moving bedreactor

Direct Under development, enhanced heat and transfer, high heating rate, medium temperature control, goodresidence time control, excellent heating integration, enhanced mixing, good fouling

Screw conveyor Direct Indirect Proven technology, enhanced heat and transfer, high heating rate, medium temperature control, good residencetime control, excellent heating integration, enhanced mixing, large size tolerance, high moving parts, bestfouling and scaling

Microwave Direct Indirect Under R&D, enhanced heat and transfer, high heating rate, good temperature control, good residence timecontrol

Multiple hearthfurnace

Direct Proven technology, enhanced heat and transfer, high heating rate, medium temperature control, good residencetime control, excellent heating integration, enhanced mixing, large size tolerance, high moving parts, perfectscaling and best scalability

Biomass Conv. Bioref. (2012) 2:349–369 359

tremendous demand from European and American pulverizedcoal plants for bio-coal to meet cap and trade regulations andrenewable portfolio standards for power generation. Recentproject around the world is stated in the “Appendix”.

Kiel et al. [19] developed BO2 technology under theumbrella of ECN for biomass upgrading into commodityfuel, a technology that combines torrefaction and pelletiza-tion processes to produce products called torrefied pellets(BO2 pellets™). BO2 pellets™ possess the benefits of bothprocess but with higher bulk density (1.5–2 times conven-tional pellets) and calorific values and can be produced froma broad range of biomass streams, such as woodchips,agricultural residues and various residues from the foodand feed processing industry [6, 19].

The BO2 technology consists of three main process steps:drying, torrefaction and pelletization. The drying and pelleti-zation components are conventional technologies that arecommercially available. The innovative part in the BO2 tech-nology is the torrefaction step. The central element in this stepis a directly heated moving bed torrefaction reactor in whichbiomass is heated using recycled torrefaction gases which hasbeen re-pressurized to compensate for the pressure drop in therecycle loop and of the heating of the recycle gas to deliver therequired heat demand in the torrefaction reactor [19].

Kiel et al. [6, 19] provides the summary of the test results(Table 3) showing the comparison of “BO2 Pellets™” prop-erties against those from raw wood chips, wood pellets andtorrefied woods. Moreover, there is high expectation ofstrong growth in pelleting equipment and will continue toproject through the future; also the use of briquetting densi-fication will continue, although on a smaller scale thanpelleting. Overall, with any densification process, reliablecontrol of process variables and feedstock properties isessential to good results.

AMANDUS KAHL is a German-based company and oneof the leading manufacturers of pellet equipment from smallto industrial scale. KAHL pelleting plants have been appliedsuccessfully for compacting organic products of differentparticle sizes, moisture contents and bulk densities. Theirpelleting presses are designed for array of feedstock charac-teristics as seen in Fig. 8. Available pelleting presses consist

of a drive power of 3 to 500 kW and a throughput between0.3 and 8 tons/h. KAHL recently developed pellet pressequipment with 15 to 20 tons/h capacity.

4 Application

The high fuel quality of torrefied biomass makes it veryattractive for combustion and gasification applications whichare summarized from [3, 9–74] (http://www.ecotechenergy-group.com/index.php/alternative-energy). Due to high calorif-ic values, the thermal energies of the combustion andgasification system can be improved significantly [5, 19]. Theother applications include (a) biomass solid fuel (acting as coal)for thermal power plant to generate heat and electricity; (b) co-firing in pulverized boilers; (c) co-gasification in entrained-flowgasifier (biofuels production); (d) good-quality fuels for domes-tic and commercial use; (e) pellets, briquettes used as fuels; (f)small-scale pellet boilers/stoves and (g) high-quality fuel foradvanced bioenergy application [74].

4.1 Pelletization

Kumar et al. [75] conducted a detail study in westernCanada on the cost to produce biomass power by directcombustion; they concluded that transportation was thesecond-most factor that influence the net cost of operation.One of the techniques that can address these limitations isto densify biomass materials into pellets, briquettes orcubes [76]. Methodology of simple pelletization processis given in Fig. 9.

Densification increases the bulk density of biomass froman initial bulk density (including baled density) between 40and 200 kg/m3 to approximately bulk density of 600 to800 kg/m3 [44, 77, 78]. Hence, densification of biomassmaterials could reduce the costs of transportation, handlingand storage. Because of uniform shape and sizes, densifiedproducts can be easily handled using the standard handlingand storage equipment and can be easily adopted in directcombustion or co-firing with coal, gasification, pyrolysisand in other biomass-based conversion processes [76].

Table 3 Comparison of BO2

pellet properties [19] Properties (typical values) Wood chips Torrefied wood Wood pellets BO2 pellet

Moisture wt.%) 35 0 10 3

LHV (kJ/kg)

Dry 17.7 20.4 17.7 20.4

As received 10.5 20.4 15.6 19.9

Bulk density

kg/m3 475 230 650 750

MJ/m3 5.0 4.7 10.1 14.9

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Table 4 below shows the summary of commercial scalepellet mill specifications from four different manufacturers.With pelletization being the most popular densification pro-cess, integrating it with torrefaction process can even in-crease their properties, including their volumetric densities.Although there have been many advancements made todensification equipment to improve their throughputs andtheir performances, the technology still remains the same.More recently, research has discovered a biomass treatmentprocess that combines the densification (pelletization) andtorrefaction to increase the bulk density and the calorificvalue of biomass.

4.2 Combustion and co-firing

The most important application of biomass is in the co-firingof pulverized coal boilers. In this application, biomass has tobe fed to the reactor as a powder, which is costly andachievable only at very low capacity in classical coal mills.Due to this limitation, wood pellets are currently the state-of-the-art for co-firing, as they consist of sufficiently smallparticles. But wood pellets also have some limitations interms of energy content and moisture content which createproblems during storage and transportation [79]. Torrefiedbiomass, because it is energy dense and hydrophobic in

nature, can be a good replacement for wood pellets in co-firing and gasification plants. The high fuel quality of torre-fied biomass makes it very attractive for combustion andgasification applications. Due to high calorific values, thethermal energies of the combustion and gasification systemcan be improved significantly. However, data are lacking onmilling, handling, storing, transporting and combusting.Almost complete combustion is possible with torrefied bio-mass for heat generation which ultimately can lead for elec-tricity generation, centralized heating system etc. [7, 80].

4.3 Gasification

The main application of torrefied biomass (wood) is as arenewable fuel for combustion or gasification. Prins et al.[72] studied the possibility of more efficient biomassgasification via torrefaction in different systems: air-blown circulating fluidized bed gasification of wood,wood torrefaction and circulating fluidized bed gasifica-tion of torrefied wood and wood torrefaction integratedwith entrained flow gasification of torrefied wood. Gasi-fication is a process that converts biomass into carbonmonoxide, hydrogen and carbon dioxide. This isachieved by reacting the material at high temperatures(>700 °C), without combustion, with a controlled amountof oxygen and/or steam. The resulting gas mixture iscalled syngas (from synthesis gas or synthetic gas) orproducer gas and is itself a fuel. The power derived fromgasification of biomass and combustion of the resultantgas is considered to be a source of renewable energy; thegasification of fossil fuel-derived materials such as plasticis not considered to be renewable energy.

The advantage of gasification is that using the syngasis potentially more efficient than direct combustion of theoriginal biomass because it can be combusted at highertemperatures or even in fuel cells, so that the thermody-namic upper limit to the efficiency defined by Carnot’srule is higher or not applicable. Syngas may be burneddirectly in gas engines, used to produce methanol andhydrogen, or converted via the Fischer–Tropsch processinto synthetic fuel. Gasification can also begin with ma-terial which would otherwise have been disposed of suchas biodegradable waste. In addition, the high-temperatureprocess refines out corrosive ash elements such as chlo-ride and potassium, allowing clean gas production fromotherwise problematic fuels. Gasification of fossil fuels iscurrently widely used on industrial scales to generateelectricity.

Gasification of biomass that in many ways is a moreefficient use of the feedstock is nowadays an interestingalternative to combustion for many industries but is stilllimited. Tar production is a major drawback of woodygasification in any convention gasifier which is leading

Fig. 8 Pictures of raw and pelletized materials (source: http://www.akahl.de/akahl/files/Prospekte/Prospekte_englisch/1322_Strohpell_10e.pdf)

Drying Torrefaction

Cooling

Densification

TOP pellets

Fig. 9 Methodology for torrefaction and palatalization process [5]

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towards the application of torrefied woody biomass. Otherdisadvantages are the relatively low energy content and itshydroscopic character. Additionally, Prins et al. have shownthat higher gasification efficiency can be achieved by fuelswith lower O/C ratio by thermochemical process. Torrefac-tion is a process that effectively lowering the O/C ratio ofbiomass in a simple way and lowers the power cost duringmilling and transportation cost. The output product in theform of powder greatly enhances the feeding properties.Although extensive studies have been made on the solidproduct and its application of gasification, limited publica-tions has been made on the utilization of torrefied product inexisting thermochemical process [8, 79].

5 Emission

Biomass could reduce pollutants emitted in power pro-duction. Burning biomass is generally carbon neutral;net carbon emissions would be zero and that wouldhelp control global warming. This is one of the majorconcerns of the industrialized nation. Many countriesare planning to replace coal-fired plant by biomass tominimize the greenhouse gas effect. Torrefied biofuelwill be much safer and environmentally friendly thanthe present fossil fuels.

However, from the torrefaction process, the outputproduct contains gaseous, volatiles, organic acids andprimary tars. This needs to be minimized by capturinggaseous and liquid products of the process, and the

remaining emissions consists only of CO2, H2O, NOx

and Sox. NOx emissions can be negligible due to lowtemperature, and SOx emissions can be considered aszero due to least sulphur contents of the lignocellulosicbiomass. Condensed tars are a major concerned on theapplication of torrefied biomass. As the temperatureincreases during torrefaction, the tar formation also in-creased exponentially. This issue needs to be addressedvery carefully. According to Kleinschmidt [64], testresults have shown that even after combustion, the fluegas contains some organic compounds like hydrogenfluorides, sulphides and nitrates that need to be removedbefore emitting the flue gas. This needs additional careon flue gas. Bag filters and ceramic filters with anabsorbent are suggested to minimize the emissions.The emissions of biomass torrefaction are not expectedto be a major technical challenge, but reduction on theash, chlorine, sulphur and alkaline production should beminimized.

6 Storage behaviour

Solid biofuels usually have porous moisture and areprone to off-gassing and self-heating caused by chemi-cal oxidation and microbiological activity. During stor-age, chemical–microbial reactions take place because ofthe presence of moisture on it. Tumuluru et al. [7]concluded that high storage temperatures of 50 °C canresult in high CO and CO2 emissions, and the

Table 4 Summary specifications of four different wood/sawdust pellet [6, 19]

Company La Meccanica NOVA Pellet Kerry Die Amandus Kahl

Model CLM 800 P LG N-Plus B-Mass 800 60–1250

Roller quantity 2 Unknown 6 4–5

Drive power (KW) Up to 280 160 450 3–500

Energy consumption Unknown Unknown Unknown 40–60 kWh/t

Capacity (T/H) 2.3–3 Up to 2.5 10 15–20

Operation mode Continuous Continuous Continuous Continuous

Weight (kg) 10,800 7,500 Unknown 9,370

Roll diameter (mm) Unknown 245 250 450

Motor speed (rpm) 750 Unknown 1,490 Unknown

Roller speed (m/s) 6.5–7.5 Variable Variable 2.5

Die diameter (mm) Unknown 580 840 175–1,250

Input density Unknown Unknown Unknown 150

Output density (kg/m3) Unknown Unknown Unknown 550–650

Feedstock moisture Unknown 8–12 % Unknown 12–15 wt.%

Feedstock size Unknown 0.5–1.5 mm Unknown 4 mm

Pellet moisture 9–12 wt.% Unknown Unknown 12 wt.%

Pellet diameter (mm) 6 6 8 2–30

362 Biomass Conv. Bioref. (2012) 2:349–369

concentrations of these off-gases can reach up to 1.7and 6 % for 60 days of storage period. These emissionswere also found sensitive to relative humidity and prod-uct moisture content [81]. Torrefied biomass or pelletsare superior to the regular raw pellets as they arehydrophobic in nature, and moisture uptake is almostnegligible even under severe storage conditions. Thestorage issues like off-gassing and self-heating may bevery low in torrefied biomass as most of the solid,liquid and gaseous products, which are chemically andmicrobiologically active, are removed during the torre-faction process. Some studies conducted by researchersat the University of British Columbia, Vancouver, Can-ada on off-gassing from torrefied wood chips indicatedthat CO and CO2 emissions were very low, nearly onethird of the emissions from regular wood chips. Costanalysis shipping, trucking, storage and others areshown in Fig. 10 [82].

7 Economic potential

To analyse the details of net profit of torrefaction, theimpact of the process on the all steps of the value chainis to be discussed. The segments of benefits are trans-port, storage, carbon neutral and production. Higher en-ergy density, condensation, pelletization and dried massof the torrefied products make economic benefit on thetransportation. Hydrophobic behaviour of torrefied bio-mass can be successfully stored outdoors, thus obviatingthe need for an enclosed storage bin or building butfurther studied is required on this issue. However, itshould be noted that, in dry climates, wood chips havebeen successfully stored in large outdoor piles. The rel-ative fuel losses (shrinkage) during storage are not well-known but can be expected to be higher for outdoorstorage. Comparisons of shrinkage losses of torrefiedversus raw biomass are needed for different storage con-ditions and climates. Utilization benefits are related to thehigher energy content, lower oxygen content and(probable) lower moisture content, relative to unprocessedbiomass. Torrefied biomass is expected to perform aswell or better than raw biomass for many bioenergyapplications, including combustion, gasification and fuelproduction applications [83]. Enhanced conversion andutilization, when compared to the other steps in thesupply chain, probably provide the most significant op-portunity for cost savings (followed by transport costs).Torrefied biomass is believed to be a superior solid fuelfor combustion, especially when co-fired with coal due toits higher energy density and coal-like handling proper-ties. Torrefied biomass is also expected to provide advan-tages as a fuel for thermochemical processing, due to the

removal of acids and oxygen. Gasification using torrefiedbiomass allows for improved flow properties of the feed-stock, increased levels of H2 and CO in the resultingsyngas and improved overall process efficiencies [66,83]. Torrefaction combined with pelletization provides alower cost fuel for power or fuel production when com-pared to pelletizing alone, with cost savings ranging from4 to 16 %, depending on the end use of the biomass.Figure 10 shows supply chain costs for several scalesand processing options for biomass, indicating that pel-letizing of torrefied biomass significantly reduces costs,that larger-scale operations are more cost efficient andthat integrated torrefaction and pelletizing is less costlythan pelletizing alone. Zwart et al. conclude that, whiletorrefaction is one of the most cost-effective options forsupply of overseas biomass, modifications to the supplychain, such as the centralized processing of raw feed-stock, can result in similar reductions in overall costs.

According to Van der Stelt et al. [42], the torrefactionstep represents an additional unit operation in the bio-mass utilization chain. The attendant capital and operat-ing costs, as well as conversion losses, are, however,offset by savings elsewhere. Recent cost estimates forthe ECN torrefaction technology indicate that the totalcapital investment of a standalone 75 ktons/year plantwill be in the range 6.1 to 7.3 MV. The assumed feed-stock is wet softwood chips. The plant consists of aconventional rotary drum for drying the biomass, ECNtorrefaction technology and conventional grinding equip-ment and pellet mill. No feedstock preparation (e.g.chipping) before drying was included. At 75 ktons/yearproduction rate (design), the total production costs arecalculated at 37 V/ton product (2.0 V/GJ), produced froma feedstock with 35 % moisture content. At 50 and 25 %moisture content, this is 50 V/ton (2.6 V/GJ) and 34 V/ton (1.9 V/GJ) of product, respectively. The moisture

Fig. 10 Delivery costs of pelletized biomass (numbers indicate nom-inal capacity of system (dry kilotons of raw biomass feedstock per year[28]))

Biomass Conv. Bioref. (2012) 2:349–369 363

content is one of the most influential parameters of thetorrefaction process as it predominantly determines theenergy input of the process. These data represent theadded cost for the torrefaction process without pre-processing of pre-drying process of biomass.

8 Research gaps

There exists several gaps in the development of torrefactiontechnologies and its maturities, and there is need for con-tinued research and development to characterize and opti-mize this promising option for bioenergy feedstockprocessing for the application of next generation fuel priorto the depletion of fossil fuels. Governments, privateparties and universities are investing a lot in the field ofbiomass applications. Several achievements are still underthe scope of laboratory. The most challenging is to see thelaboratory experiment in the commercial applications. Forthis, in-depth study on chemical reactions and its network,composition and application of tar, char and ash has yet tobe established, in part due to the complex chemical natureof the feedstock. The health and safety issues on torrefac-tion process and its product applications is another area offurther study [28]. Environmental effect, storage behaviourof torrefied biomass, energy analysis of the torrefied prod-ucts, temperature effect, heating values due to differenttemperature, practical reactors, residue management, syn-gas management, molecular level analysis and effectivetransportation possibilities are few areas of research onthe torrefaction process.

According to Chew and Doshi [8], torrefaction ofbiomass as a thermochemical treatment has the potentialto contribute to energy demand of the world. In therecent years, torrefaction studies on various agriculturalproducts associated with fuel properties have shownpromising result. However, due to the complexity andvariety of agricultural residues, all the process parame-ters have yet to be derived. Torrefaction process outputsolely depends on the polymeric structure of biomass.Detailed investigation is still required on polymericstructure. Future work can look into the possibility ofderiving indicative parameter to define process parame-ter for torrefaction based on the polymeric structure ofthe feedstock. Further research should focus on thepossibility of utilizing the by-products to improve theoverall efficiency of torrefaction. Another area of studycould be the kinetic analysis for torrefaction. Futurework should concentrate on different kinetic analysisapproaches to validate reliability and consistency ofthe kinetic information.

A primary goal of torrefaction of biomass is to in-crease its energy density such that biomass transporta-tion cost can be minimized. In terms of this attribute oftorrefaction, appropriate reactor selection is importantparameters. Energy yield is important when torrefactionis carried out at the point of use or at the end of itsmajor transportation. Identifying reactor for specific pur-poses with specific properties could be another area ofstudy. Similarly, the emission effect from the torrefiedbiomass is still under the further study.

9 Conclusions

In major universities and green energy industries, inten-sive research on torrefaction of biomass materials is inprogress. Almost all countries have expressed concernedon the global warming and shifted towards the optimumutilization of GHG energy [84]. Torrefaction improvesthe physical, chemical and theological characteristics ofbiomass materials. Torrefied biomass is a group ofproducts resulting from the partially controlled and iso-thermal pyrolysis of biomass occurring at the 200–300 °C temperature range. The most common torrefactionreactions include devolatilisation and carbonization ofhemicelluloses in first steps and depolymerization anddevolatilisation of lignin and cellulose in other step.Torrefaction of the biomass helps in developing a uni-form feedstock with minimum moisture content and lessaffected by atmospheric environment. Torrefaction ofbiomass improves energy density, homogeneity, grind-ability and pelletability performance. Similarly ultimateand proximate analysis gives moisture contents; ashcontents; volatile matters; carbon content; oxygen, hy-drogen, nitrogen and sulphur contents; CV content andbiochemical composition. Lignin helps for better bind-ing in process of pelletization. During torrefaction, thebiomass loses most of the low energy content of thematerial which includes water, organics and lipids andgases, H2, CO, CO2 and CH4, CxHy, toluene and ben-zene. Torrefaction can keep the biomass for a long timewithout biological degradation due to the chemical re-arrangement of structures. Torrefied biomass can beused as an upgraded solid fuel in electric power plantsand gasification plants. Torrefied biomass provides al-ternative source of coal in the future for all coal basedplants by replacing carbon neutral energy dense torre-fied pallets. Torrefied biomass can directly mill and co-fire with coals. The typical calorific value of torrefiedbiomass is in the range of 18–22 MJ/kg. The product isbrittle and easily breaks down in small particles. Also, it

364 Biomass Conv. Bioref. (2012) 2:349–369

is less sensitive to degradation due to hydrophobicnature. The volumetric energy density of torrefied pel-lets is nearly 16 GJ/Nm3 compared to nearly 10 GJ/m3

of wood pallets. But torrefaction alone cannot signifi-cantly reduce sulphur, chlorine and alkali concentrationsof the biomass.

The present technologies mainly concentrate on process-ing of wood chips for narrow bandwidth of particle size.Agricultural residues are still a challenge because it igniteseasily, has a low bulk density and has long fibres. Till thisdate, only results from pilot plants are available. It will be achallenge for developers to develop a full commercial tor-refaction plant, which incorporates the necessary design andprocess modification for good commercial performance.Although some experience has been gained with pilot test-ing, real operational data will reveal the performance of thetorrefaction process. The trade-off between energy yield,product quality and production cost is important. The prod-uct needs to be validated by large co-firing trials. This canbe seen only after the commercial application. Most torre-faction developers are small companies with a limited fi-nancial base. Convincing investors are needed to finance thenecessary R&D, and an up-scaling effort is a real challenge.A dominant torrefaction concept will emerge out of a largevariety of technologies and initiatives to commercially pro-vide biomass according to the specifications. Product stan-dardization is needed to make the market more transparentand reliable. The urgency and quality of demand is signifi-cantly higher than the supply. Torrefaction suppliers arefacing the challenge to scale-up their first commercial dem-onstration plant in a rapid pace.

The combination of torrefaction and densificationoffers the opportunity to produce high-quality second-generation fuel pellets from a wide range of biomassfeed stocks. Due to their high energy density, hygro-scopic nature and easy grindability, BO2 pellets have thepotential to become a major commodity fuel with ex-cellent properties for co-firing applications, for biofuelsproduction via high-temperature gasification and forsmall-scale combustion applications. Moreover, the hy-groscopic nature makes the pellets highly resistant tobiological degradation and spontaneous heating, whichleads to large advantages in transport, handling andstorage. Significant cost savings can be achievedthroughout the biomass-to-energy chain when comparedto state-of-the-art wood pellets. Econcern and Chemfojointly have set up the first commercial plant at a scaleof approximately 70 ktonnes/year BO2 pellets in theNetherland. After the production from this plant, it canbe expected that torrefaction will have new dimensionand applications in the commercial market [85, 86].

Torrefied wood is being used in different proposesfrom the long time. The hydrophobic and brittle prop-erties of torrefied wood make it compatible with coal oras a coal replacement. In order for torrefied wood tocompete in the coal market, the cost of producingtorrefied wood, from the stump to the delivery point,must not exceed the price of coal deliveries. Otherpotential uses of torrefied wood include industrial boil-ers, residential heating, co-firing of thermal plants andfor backyard grilling. From the perspective of the log-ging and timber industry, literature indicates that rawmaterial can vary in size and can include thin and thickchips and even larger wood chunks. Depending on theequipment design and considering characteristics such aspre-drying, processing temperature and reaction time, itappears that feed stocks for the torrefaction processcould be produced by utilizing different types of woodprocessing equipment which are available in the presentcommercial or residential applications.

From the above study, the following recommendationsare made for further exploration on the commercializationof the biomass energy applications using torrefaction pro-cess: (1) further analysis of heating value and residencetime for particular biomass during torrefaction processes;(2) energy analysis during the process of torrefaction anddensifications processes; (3) in-depth study on the calcu-lation of activation energy required during the degradationof different chemical components of biomass; (4) molec-ular level analysis on the torrefaction; (5) study of theseverity of the torrefaction process based on colourchanges using the different types of colorimeter; (6) stud-ies on cost-effective transportation of torrefied biomassfuels; (7) effect of temperature on the different chemicalbonds of biomass structure; (8) commercial viability onthe integrated processes of torrefaction and densifications;(9) study of storage condition of different torrefied bio-mass at different environmental conditions; (10) study offinding out of suitable reactor for yielding the highestenergy and the best qualify biofuel from the torrefaction;(11) studies on the proper management and handling ofresidues from the torrefaction; (12) designing of an effi-cient, robust, fuel flexible, scalable and cost-effective tor-re fac t ion demonst ra t ion plant for commerc ia lapplication; (13) identify the extent if any of risk forself-ignition and biological degradation of torrefiedbiomass while stored; (14) assess the potential forslagging/agglomeration of fluidized bed and corrosionand fouling of super heater/economizer tubes and (15)LCA analysis of torrefied biomass and its applicationto power generation and/or making ethanol/biodiesel[2, 7, 87–91].

Biomass Conv. Bioref. (2012) 2:349–369 365

Appendix: Overview of torrefaction projects(source: [64])

Table 5 Overview of torrefaction projects (source: [64])

Company Demotechnology

Supplier Location (s) Prod.capacity(tons/year)

ESD ofoperation

Comments

3RAgrocarbon(Hungary)

Rotary kiln(3R pyrolysisbiochar)

Unknown Unknown Unknown Unknown Pyrolysis unit that can alsoperform torrefaction

4Energy Invest.(Belgium)

Unknown StramproyGreen Tech.(the Netherlands)

Amel (Belgium) 40,000 Q4 2010 Contract with StramproyGreen Tech Terminated inJune 2010; Renogen SA,to take full control ofproject

Agri-Tech ProducersLLC (USA/SC)

Belt conveyor Kuster ZimaCorporation(USA/SC)

Unknown Unknown 2010

Andritz(Austria)

Unknown Unknown Unknown ~50,000 Unknown Torrefaction processfor biomass: developprocess for medium sizedplant (~50,000 tons/year)pilot plant underconstruction

Atmosclear(Switzerland)

Rotary drum CDS (UK) Latvia, New Zealand,USA

50,000 Q4 2010

BioEnergyDevelopment(Sweden)

Rotary drum Unknown Ö-vik (Sweden) 25,000–30,000

2011/2012

Biogreen Energy(France)

Screw conveyor ETIA (France) Unknown Unknown Unknown No recirculation of thenon-condensable fractionof the Tor-Gas, hence thesystem could be energyconsuming

Biolake BV(the Netherlands)

Screw conveyor Unknown Eastern Europe 5,000–10,000

Q4 2010

CDS (UK) Rotary kiln Unknown Unknown Unknown Unknown

CMI (NESA) Multiple heathfurnace

EBES AG (Austria) Rotary drum Andritz (T) Frohnleiten(Australia)

10,000 2011

ECN (the Netherlands) Moving bed Unknown Unknown Unknown Unknown

FoxCoal B.V. (theNetherlands)

Screw conveyor Unknown Winschoten(the Netherlands)

35,000 2012

Integro Earth Fuels,LLC (USA/NC)

TurboDryer Wyssmont(USA/NC)

Roxboro, NC 50,000 2010

New Earth RenewableEnergy Fuels, Inc.(US/WA)

Fixed bed/pyrovac

Pyrovac Group(Canada/QU)

Unknown Unknown Unknown

Rotawave Ltd. (UK) Microwaveheating

Group’s Vikoma Terrace, B.C, Canada 110,000 Q4 2011

Stramproy GreenInvestment B.V.(the Netherlands)

Oscillating beltconveyor

StramproyGreen Tech.(the Netherlands)

Sreenwijk(the Netherlands)

45,000 Q3 2010

Thermya (France) Moving bed Lantec Group (SP) San Sebastian (SP) 20, 000 2011

Topell Energy B.V.(the Netherlands)

Torbed Torftech Inc (UK) Duiven(the Netherlands)

60,000 Q4 2010

Torr-Coal B.V. Rotary drum Unknown Dilsen-Stokkem(Belgium)

35,000 Q3 2010

366 Biomass Conv. Bioref. (2012) 2:349–369

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ESD ofoperation

Comments

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Unknown Bepex International(USA/MN)

Unknown Unknown 2013

Vattenfall (Sweden) Moving bed Unknown Unknown Unknown Unknown

WPAC (Canada) Unknown Unknown Unknown 35, 000 2011

Zilkha BiomassEnergy (USA)

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ESD estimated starting date

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