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Bio-coal, torrefied lignocellulosic resources – Key properties for its use in co-firing with fossil coal – Their status
b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 0 7 e1 1 1
Available online at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Short communication
Bio-coal, torrefied lignocellulosic resources e Key properties for its use in co-firing with fossil coal e Their status D. Agar a,*, M. Wihersaari b a b
Department of Chemistry, University of Jyva¨skyla¨, P.O. Box 35, 40014 Jyva¨skyla¨, Finland Department of Biological and Environmental Science, P.O. Box 35, 40014 Jyva¨skyla¨, Finland
article info
abstract
Article history:
Bio-coal has received generous amounts of media attention because it potentially allows
Received 29 March 2012
greater biomass co-firing rates and net CO2 emission reductions in pulverised-coal power
Received in revised form
plants. However, little scientific research has been published on the feasibility of full-scale
6 April 2012
commercial production of bio-coal. Despite this, several companies and research organi-
Accepted 11 May 2012
sations worldwide have been developing patented bio-coal technologies. Are the expec-
Available online 7 June 2012
tations of bio-coal realistic and are they based on accepted scientific data? This paper examines strictly peer-reviewed scientific publications in order to find an answer. The
Keywords:
findings to date on three key properties of torrefied biomass are presented and reviewed.
Bio-coal
These properties are: the mass and energy balance of torrefaction, the friability of the
Torrefaction
product and the equilibrium moisture content of torrefied biomass. It is these properties
Co-firing
that will have a major influence on the feasibility of bio-coal production regardless of
Co-combustion
reactor technology employed in production. The presented results will be of use in
Grindability
modelling commercial production of bio-coal in terms of economics and green-house gas
Equilibrium moisture content
emission balance. ª 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years so-called bio-coal (or green coal ) has received at lot of attention in the energy sector. To be clear bio-coal, as the name implies, is a fossil coal substitute which is produced from renewable biomass resources. It is considered a coal substitute because it can be handled and combusted in the same way as fossil coal in pulverised-fuel power plants without the need of additional infrastructure. Coal-like physical properties would permit much greater co-firing rates in these plants enabling substantial net carbon dioxide emissions. Additionally, bio-coal is assumed to be in pellet or briquette form in order to achieve greater bulk energy density
(closer to that of lesser coal) for transport purposes. Herein when using the term bio-coal, it is this which is meant. Serious investigation into bio-coal production began at the Energy Centre of the Netherlands (ECN) and resulted in an extensive ECN report on the topic in 2005 which is wellcirculated and frequently cited [1]. The report focuses on the process at the heart of bio-coal production, torrefaction, which was demonstrated in France in the 1980s as a method of upgrading green wood as a fuel and reducing agent without the energy losses associated with traditional wood charcoal production [2]. Since ECN’s activities began, which includes investing in their own bio-coal production process (the TOP process [3])
* Corresponding author. E-mail addresses: [email protected], [email protected] (D. Agar). 0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2012.05.004
108
b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 0 7 e1 1 1
Nomenclature T t Dq LHV HHV daf d50 Eg
Torrefaction temperature ( C) Torrefaction time (min.) Relative heating value increase (%) Lower heating value (MJ kg1) Higher heating value (MJ kg1) Dry ash free The particle diameters obtained from the cumulative distribution data at 50% Specific energy consumption for grinding untreated material (kWh t1)
several companies and other research organisations worldwide have been developing their own patented technologies. Proponents of renewable energy would like to know if the expectations of bio-coal are realistic. This paper looks at some key findings. Recent peerreviewed scientific publications are examined in order to review the experimental data on three key properties of torrefied biomass which are often touted by those promoting the technology. These properties will have a major influence on the feasibility of bio-coal production regardless of reactor technology employed in production. Herein some familiarity with torrefaction and proposed bio-coal production is assumed. The purpose is to answer the question: How well do recent experimental findings support some of the more popular claims made of bio-coal? The results presented here will be of use to those wishing to realistically assess commercial production of bio-coal, with regards to economics and green-house gas emission balance.
2.
Materials and methods
Torrefaction is the thermal process, similar to coffee roasting, used to pre-treat biomass in bio-coal production. It has also been called incomplete pyrolysis but to be clear, torrefied biomass is not charcoal. Torrefaction is a distinct thermal regime using temperatures of 220e300 C and, as in charcoal production, a low-oxygen atmosphere so that combustion reactions are suppressed. The result is literally a toasted product containing most of the initial volatile matter content, not carbonised char. A more extensive description of the process can be found elsewhere [1,4]. Typically the biomass is pelletised (or briquetted) after roasting to achieve a higher bulk energy density. It is natural to compare bio-coal production with conventional wood pellet production so as to recognise the potential added value of torrefaction and determine feasibility. Bergman has done this using one possible economic production scenario [3]. Based on such comparisons, the expected benefits of torrefaction technology have been summarised qualitatively [5]. Some of these will depend strongly on process technology, location and feedstock making them a challenge to quantify without the hindsight of experience with demonstration plants. Nonetheless, physical properties which lead to three specific benefits can be evaluated
0
Eg DEg HGI mH 2 O mfuel EMC RH
Specific energy consumption for grinding torrefied material (kWh t1) Change in specific energy consumption for grinding (%) Hardgrove grindability index (unitless) Mass of water content in fuel (kg) Mass of dry matter in fuel (kg) Equilibrium moisture content, mH2O [(mH2O þ mfuel)]1 Relative humidity (%)
quantitatively from recent scientific literature. The properties and their expected benefits are summarised in Table 1. These properties are the heating value increase (mass-energy balance), grindability and water-uptake ability (hydrophobicity) of torrefied biomass.
2.1.
Mass and energy balance (heating value increase)
The mass and energy balance of torrefaction is important because it determines the heating value of the solid product and the required volumes of raw materials. The rationale of torrefaction is that the energy yield is greater than the mass yield; thereby resulting in an increase in the material’s heating value. It has been shown that the mass yield is less because the most volatile compounds (which are the first to undergo thermal degradation) have relatively low energy content. These stem mainly from hemicellulose [6]. The gaseous products in torrefaction, so-called torgas, contain the energy extricated from the solid. Utilising the low heating value of these gases in the process is seen as a major aim in most bio-coal production schemes. Bergman has defined autothermal operation as the point where the energy needs for drying and torrefaction are met by the heating energy of the torgas [1]. This is assumed to be optimal operation for a given feedstock. Too little energy in the torgas necessitates an auxiliary fuel and too much means a loss of product. A commonly seen value of mass and energy yield (for woody biomass) is 70 and 90% respectively.
2.2.
Grindability of torrefied biomass
The grindability of torrefied biomass is an important benefit for two reasons. Firstly, during the production of pellets made
Table 1 e The expected benefits of three key properties of torrefied biomass, modified from [5]. Key property Mass and energy balance Improved grindability
Lower equilibrium moisture content (EMC)
Expected benefit þ Improved heating value þ Reduced electricity use for size reduction þ Enables displacement of fossil coal use þ Reduced storage infrastructure investment þ Higher received heating value
109
b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 0 7 e1 1 1
from torrefied material, easy grindability reduces electricity costs in size reduction before densification in addition to allowing investment savings of milling equipment. Secondly, for these same reasons, the ease of grinding permits bio-coal co-firing rates to be increased at fossil coal power plants using existing coal mills.
2.3.
Equilibrium moisture content (EMC)
The low moisture content of torrefied fuels is a major claim made of torrefied material. The moisture content of a fuel has a direct impact on its heating value. At power plants, fuel suppliers are paid according to the received heating value (not higher heating value) of their fuel which is often measured from samples taken on-site. The market price therefore depends directly on the moisture content of the fuel. It has often been stated that bio-coal is, at least partly, hydrophobic and consequently can not reabsorb moisture to the same extent as untreated feedstock such as conventional wood pellets. This has led to the speculation that bio-coal can then be stored in heaps outdoors much like fossil coal without the need for costly dedicated storage infrastructure.
the table. The number of significant digits in the table corresponds to those given in the publications. Experimental uncertainties were for the most part not given.
3.2.
Grindability data is shown in Table 3 and is taken from two available publications. Results with torrefaction temperatures corresponding to those in Table 2 were selected when possible. The number of significant digits in the table corresponds to those given in the publications. Experimental uncertainties are not considered. The data in the table is from studies which measured the grinding energy.
3.3.
Results
3.1.
Mass and energy balance (heating value increase)
Table 2 displays experimentally data on the physical properties of torrefied materials from a number of peer-reviewed journal publications. Some of these publications contained material data from several different process temperatures within the torrefaction regime. If results were not stated explicitly in the publication, they were either found graphically from figures or calculated as described in the footnotes to
Equilibrium moisture content
Limited publications currently exist on the equilibrium moisture content of torrefied biomass. Both publications used here are from the same institution and appear to refer to the same experimental data on EMC. Table 4 displays the measured EMC which was measured at 30 C ambient temperature.
4. 3.
Grindability of torrefied biomass
Discussion and conclusions
It is now time to return to the question: How well do recent experimental findings support some of the more popular claims made of bio-coal? Concerning the mass and energy balance of torrefaction, nowhere in peer-reviewed literature have experimentally determined yields been found to equal 70 and 90% respectively. The values of 70 and 90% given in ECN publications [1,3,7] refer to an online publication as a source [17]. Upon investigation however, the publication itself states a value between 80 and 90% for energy yield and appears to use as a reference
Table 2 e Selected experimental results on torrefaction of biomass raw materials from recent literature. Material Willow Beech Willow Willow Wheat straw Rice straw Rape stalk Loblolly pine Loblolly pine Miscanthus Eucalyptus Pine chips Southern yellow pine residues Willow
Mass yield (%)
Energy yield (%)
78.6b 73.8b 79.8 81.6 71.5 36.6 25.3 74.2 60.5 75.7 80 73 70 n/a
91.9c 88.1c 85.8 89.9 78.2 39.9c 29.1c 83.1 73.2 81.0 90 87 82 n/a 0
Heating value (MJ kg1) 17.7/20.7 17.0/20.3 20.0/21.4 19.8c/21.8 18.9/20.7 17.1/18.7 18.8/21.6 19.55/21.80 19.55/23.56 20.2c/21.6 19.4/22.2 18.5/21.8 18.8/22.0 17.6/21.0 0
(LHV) (LHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (LHV)
Dqa (%)
T ( C)
t (min.)
Volatiles (%)
Fixed carbon (%)
Ref.
16.9 19.4 7.00 10.2 9.52 9.11 15.1 11.5 20.5 6.98 14.4 18.2 17.2 19.3
270 280 270 290 270 300 300 275 300 290 240 275 275 300
15 30 30 10 30 30 30 80 80 10 30 30 30 10
n/a n/a 79.3 72.4d 65.2 n/a n/a 83.0 82.3 63.8d 75.4 76.4 71.4 n/a
n/a n/a 18.6 23.3d 26.5 n/a n/a 16.4 17.0 32.6d 21.8e 23.3 26.7 n/a
[6] [6] [8] [9] [8] [10] [10] [11] [11] [9] [12] [13] [13] [14]
a Relative heating value increase Dq ¼ 100(X X)[X]1 where X is the LHV or HHV of the torrefied solid product and X is LHV or HHV of the untreated material. b Estimated from on-screen graphical data. c Calculated value using heating value (LHV or HHV) according to energy yield definition given in [8]. d As received (not daf). e Calculated from given data on ultimate analysis and (n/a) data not available.
110
b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 0 7 e1 1 1
Table 3 e Selected experimental results on the grindability of torrefied materials. T ( C) t (min.)
Material
Beech Spruce Pine Logging residues
280 280 275 275
5 5 30 30
Initial particle size 2e4 mm 20.94e70.59 15.08e39.70 1.88e4.94 mm
Final particle size d50 d50 d50 d50
¼ 140 mm ¼ 93 mm ¼ 270 mm ¼ 460 mm
0
Eg DEga Eg (kWh t1) (kWh t1) (%) 840 750 241c 242c
90 150 52.0 78.0
89 80 78 68
Equipment specification Retsch XMI mill, 500 mm sieve Retsch SM2000 mill, 1.5 mm sieve
Fraction of Ref. heating valueb (%) 2 2.8 1.0 1.5
(LHV) (HHV) (HHV) (HHV)
[15] [15] [13] [13]
0
a Change in grinding energy DEg ¼ 100(Eg e Eg) [Eg]1. b Used heating values (expressed as MJ kg1) were Spruce 19.0 [16], Pine 18.46, Logging residues 18.79 [13] and Beech 17.0 [15]. c Values calculated by fitted equation from reference at 25 C.
a conference paper by Burgeois and Doat [18]. However, the highest yield result for mass and energy stated by Bourgeois and Doat is 74 and 90% respectively for eucalyptus wood. From the results in Table 2, the mass and energy balance of torrefaction equate to a heating value increase in the range 7e21%. For the agro biomass results in Table 2, the corresponding increase is 7e15%. If the mass and energy yield would be 70 and 90% respectively, the increase in heating value would be 29%. For woody biomass the range of mass and energy yields are 61e82 and 73e92% respectively. The corresponding yields for agro biomass are 25e76 and 29e81%. Additionally, the maximum heating value increases (for both woody and agro biomass) were achieved using the lengthiest torrefaction times and highest temperatures; two parameters which will have significant influence on economy of reactor and process design [19]. In summary, 70 and 90% mass and energy yield (29% heating value increase) is not observed from experimental data on a given material. According to the results available, grindability of four biomass types is greatly improved through torrefaction. The used grinding energy (52e150 kWh/t, d50 ¼ 93 e 460 mm) was reduced by 68e89% compared to untreated biomass. This is a significant improvement but still short of the assumption that grindability is the same as coal whose value is found in the range 7e36 kWh/t [20]. Nonetheless, this does provide measured data of the energy requirements. Another recent study aimed to evaluate the grindability of torrefied biomass using the Hardgrove Grindability Index (HGI) without measuring the specific grinding energy [9]. It found that willow wood required high temperature (290 C) and long residence times (60 min) to obtain similar grinding behaviour as that of coal. From Table 4 the EMC of torrefied pine wood is indeed very low (2.2%) at low relative humidity (11.3%). At higher RH (83.6%) the EMC is greater (8.7%). From literature it can be seen that the EMC of biomass at constant relative humidity is
Table 4 e Equilibrium moisture content of torrefied Loblolly pine measured at 30 C from [11]. Material
Loblolly pine
T ( C)
275 300
t (min.)
w80 w80
Equilibrium moisture content, (0.005) RH ¼ 11.3%
RH ¼ 83.6%
0.022 0.022
0.087 0.087
inversely correlated to temperature [21]. The data in Table 4 was measured at 30 C which is well above the ambient temperature found at fuel yards across many northern European countries (especially Nordic countries) during the annual heating season. Although specific density of biomass (for example as pellets or briquettes) may influence its EMC [22], significantly higher EMC would be expected at lower temperatures for a given relative humidity. In summary, experimental results on three key properties of torrefied biomass from peer-reviewed publications have been presented. From recent results it can be seen that high energy yields on studied materials are achieved at the expense of other key properties; heating value increase, grindability and equilibrium moisture content. These trade-offs are determined by the physical nature of the materials themselves. That large-scale bio-coal production would achieve more commercially advantageous results than these, should not be assumed. The results can be used to realistically model bio-coal production scenarios in terms of economics and green-house gas emission balance.
Acknowledgements The Fortum Foundation is acknowledged for financial support during the writing of this manuscript.
references
[1] Bergman PCA, Boersma AR, Zwart RWR, Kiel JHA. Torrefaction for biomass co-firing in existing coal-fired power stations. Petten: Energy Research Centre of the Netherlands; 2005. Month. p. 61, Report No.: ECN-Ce;05e013. [2] Doat J. Un noveau produit e´nerge´tique pour les pays en de´velopment: le bois torre´fie´. Revue Bois et Foreˆts des Tropiques 1985;208(2e trimestre):57e67. [3] Bergman PCA. Combined torrefaction and pelletisation e the TOP process. Petten: Energy Research Centre of the Netherlands; 2005 July. p. 29, Report No.: ECN-Ce;05e073. [4] Ratte J, Fardet E, Mateos D, He´ry J- S. Mathematical modelling of a continuous biomass torrefaction reactor: TORSPYDTM column. Biomass Bioenergy 2011;35(8):3481e95. [5] Agar D, Wihersaari M. Torrefaction technology for solid fuel production e a move towards greater sustainability. Glob Change Biol Bioenerg, in press.
b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 0 7 e1 1 1
[6] Prins MJ, Ptasinski KJ, Janssen FJJG. Torrefaction of wood part 2. Analysis of products. J Anal Appl Pyrolysis 2006;77:35e40. [7] Kiel JHA, Verhoeff F, Gerhauser H, Meuleman B. BO2technology for biomass upgrading into solid fuel - pilot-scale testing and market implementation. Petten: Energy Research Centre of the Netherlands; 2008 June. p. 7, Report No.: ECNMe;08e036. [8] Bridgeman TG, Jones JM, Shield I, Williams PT. Torrefaction of reed canary grass, wheat straw and willow to enhance solid fuel qualities and combustion properties. Fuel 2008;87: 844e56. [9] Bridgeman TG, Jones JM, Williams A, Waldron DJ. An investigation of the grindability of two torrefied energy crops. Fuel 2010;89:3911e8. [10] Deng J, Wang J, Kuang J, Zhang Y, Luo Y. Pretreatment of agricultural residues for co- gasification via torrefaction. J Anal Appl Pyrolysis 2009;86:331e7. [11] Yan W, Acharjee T, Coronella C, Va´squez V. Thermal pretreatment of lignocellulosic biomass. Environ Prog Sust Energ 2009;28:435e40. [12] Arias B, Pevida C, Fermoso J, Plaza MG, Rubiera F, Pis JJ. Influence of torrefaction on the grindability and reactivity of woody biomass. Fuel Process Technol 2008;89:169e75. [13] Phanphanich M, Mani S. Impact of torrefaction on the grindability and fuel characteristics of forest biomass. Bioresour Technol 2011;102:1246e53.
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[14] Prins M, Ptasinski K, Janssen F. More efficient biomass gasification via torrefaction. Energy 2006;31:3458e70. [15] Repellin V, Govin A, Rolland M, Guyonnet R. Energy requirement for fine grinding of torrefied wood. Biomass Bioenerg 2010;34:923e30. [16] Nurmi J. Heating values of the above ground biomass of small-sized trees. Kannus: Finnish Forest Research Institute; 1993. p. 30, Report No.: Acta Forestalia Fennica 236. [17] Archive 1999e2006 [cited 2012 Mar 31] New process for torrefied wood manufacturing, vol. 2 No. 4 Bioenergy update [home page on the internet]. Florence, AL: General*Bioenergy. Available from: www.bioenergyupdate. com/magazine/security/NL0400/bioenergy_update_april_ 2000.htm; April 2000. [18] Bourgeois JP, Doat J. Torrefied wood from temperate and tropical species e advantages and prospects. Bioenergy 1984; 84:15e21. June; Go¨tenborg, Sweden. [19] Prins MJ, Ptasinski KJ, Janssen FJJG. Torrefaction of wood Part 1. Weight loss kinetics. J Anal Appl Pyrolysis 2006;77:28e34. [20] Esteban LS, Carrasco JE. Evaluation of different strategies for pulverization of forest biomasses. Powder Technol 2006;166: 139e51. [21] Mehta S, Singh A. Adsorption isotherms for red chilli (Capsicum annum L.). Eur Food Res Technol 2006;223:849e52. [22] Singh RN. Equilibrium moisture content of biomass briquettes. Biomass Bioenerg 2004;26:251e3.
Available online at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Short communication
Bio-coal, torrefied lignocellulosic resources e Key properties for its use in co-firing with fossil coal e Their status D. Agar a,*, M. Wihersaari b a b
Department of Chemistry, University of Jyva¨skyla¨, P.O. Box 35, 40014 Jyva¨skyla¨, Finland Department of Biological and Environmental Science, P.O. Box 35, 40014 Jyva¨skyla¨, Finland
article info
abstract
Article history:
Bio-coal has received generous amounts of media attention because it potentially allows
Received 29 March 2012
greater biomass co-firing rates and net CO2 emission reductions in pulverised-coal power
Received in revised form
plants. However, little scientific research has been published on the feasibility of full-scale
6 April 2012
commercial production of bio-coal. Despite this, several companies and research organi-
Accepted 11 May 2012
sations worldwide have been developing patented bio-coal technologies. Are the expec-
Available online 7 June 2012
tations of bio-coal realistic and are they based on accepted scientific data? This paper examines strictly peer-reviewed scientific publications in order to find an answer. The
Keywords:
findings to date on three key properties of torrefied biomass are presented and reviewed.
Bio-coal
These properties are: the mass and energy balance of torrefaction, the friability of the
Torrefaction
product and the equilibrium moisture content of torrefied biomass. It is these properties
Co-firing
that will have a major influence on the feasibility of bio-coal production regardless of
Co-combustion
reactor technology employed in production. The presented results will be of use in
Grindability
modelling commercial production of bio-coal in terms of economics and green-house gas
Equilibrium moisture content
emission balance. ª 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years so-called bio-coal (or green coal ) has received at lot of attention in the energy sector. To be clear bio-coal, as the name implies, is a fossil coal substitute which is produced from renewable biomass resources. It is considered a coal substitute because it can be handled and combusted in the same way as fossil coal in pulverised-fuel power plants without the need of additional infrastructure. Coal-like physical properties would permit much greater co-firing rates in these plants enabling substantial net carbon dioxide emissions. Additionally, bio-coal is assumed to be in pellet or briquette form in order to achieve greater bulk energy density
(closer to that of lesser coal) for transport purposes. Herein when using the term bio-coal, it is this which is meant. Serious investigation into bio-coal production began at the Energy Centre of the Netherlands (ECN) and resulted in an extensive ECN report on the topic in 2005 which is wellcirculated and frequently cited [1]. The report focuses on the process at the heart of bio-coal production, torrefaction, which was demonstrated in France in the 1980s as a method of upgrading green wood as a fuel and reducing agent without the energy losses associated with traditional wood charcoal production [2]. Since ECN’s activities began, which includes investing in their own bio-coal production process (the TOP process [3])
* Corresponding author. E-mail addresses: [email protected], [email protected] (D. Agar). 0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2012.05.004
108
b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 0 7 e1 1 1
Nomenclature T t Dq LHV HHV daf d50 Eg
Torrefaction temperature ( C) Torrefaction time (min.) Relative heating value increase (%) Lower heating value (MJ kg1) Higher heating value (MJ kg1) Dry ash free The particle diameters obtained from the cumulative distribution data at 50% Specific energy consumption for grinding untreated material (kWh t1)
several companies and other research organisations worldwide have been developing their own patented technologies. Proponents of renewable energy would like to know if the expectations of bio-coal are realistic. This paper looks at some key findings. Recent peerreviewed scientific publications are examined in order to review the experimental data on three key properties of torrefied biomass which are often touted by those promoting the technology. These properties will have a major influence on the feasibility of bio-coal production regardless of reactor technology employed in production. Herein some familiarity with torrefaction and proposed bio-coal production is assumed. The purpose is to answer the question: How well do recent experimental findings support some of the more popular claims made of bio-coal? The results presented here will be of use to those wishing to realistically assess commercial production of bio-coal, with regards to economics and green-house gas emission balance.
2.
Materials and methods
Torrefaction is the thermal process, similar to coffee roasting, used to pre-treat biomass in bio-coal production. It has also been called incomplete pyrolysis but to be clear, torrefied biomass is not charcoal. Torrefaction is a distinct thermal regime using temperatures of 220e300 C and, as in charcoal production, a low-oxygen atmosphere so that combustion reactions are suppressed. The result is literally a toasted product containing most of the initial volatile matter content, not carbonised char. A more extensive description of the process can be found elsewhere [1,4]. Typically the biomass is pelletised (or briquetted) after roasting to achieve a higher bulk energy density. It is natural to compare bio-coal production with conventional wood pellet production so as to recognise the potential added value of torrefaction and determine feasibility. Bergman has done this using one possible economic production scenario [3]. Based on such comparisons, the expected benefits of torrefaction technology have been summarised qualitatively [5]. Some of these will depend strongly on process technology, location and feedstock making them a challenge to quantify without the hindsight of experience with demonstration plants. Nonetheless, physical properties which lead to three specific benefits can be evaluated
0
Eg DEg HGI mH 2 O mfuel EMC RH
Specific energy consumption for grinding torrefied material (kWh t1) Change in specific energy consumption for grinding (%) Hardgrove grindability index (unitless) Mass of water content in fuel (kg) Mass of dry matter in fuel (kg) Equilibrium moisture content, mH2O [(mH2O þ mfuel)]1 Relative humidity (%)
quantitatively from recent scientific literature. The properties and their expected benefits are summarised in Table 1. These properties are the heating value increase (mass-energy balance), grindability and water-uptake ability (hydrophobicity) of torrefied biomass.
2.1.
Mass and energy balance (heating value increase)
The mass and energy balance of torrefaction is important because it determines the heating value of the solid product and the required volumes of raw materials. The rationale of torrefaction is that the energy yield is greater than the mass yield; thereby resulting in an increase in the material’s heating value. It has been shown that the mass yield is less because the most volatile compounds (which are the first to undergo thermal degradation) have relatively low energy content. These stem mainly from hemicellulose [6]. The gaseous products in torrefaction, so-called torgas, contain the energy extricated from the solid. Utilising the low heating value of these gases in the process is seen as a major aim in most bio-coal production schemes. Bergman has defined autothermal operation as the point where the energy needs for drying and torrefaction are met by the heating energy of the torgas [1]. This is assumed to be optimal operation for a given feedstock. Too little energy in the torgas necessitates an auxiliary fuel and too much means a loss of product. A commonly seen value of mass and energy yield (for woody biomass) is 70 and 90% respectively.
2.2.
Grindability of torrefied biomass
The grindability of torrefied biomass is an important benefit for two reasons. Firstly, during the production of pellets made
Table 1 e The expected benefits of three key properties of torrefied biomass, modified from [5]. Key property Mass and energy balance Improved grindability
Lower equilibrium moisture content (EMC)
Expected benefit þ Improved heating value þ Reduced electricity use for size reduction þ Enables displacement of fossil coal use þ Reduced storage infrastructure investment þ Higher received heating value
109
b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 0 7 e1 1 1
from torrefied material, easy grindability reduces electricity costs in size reduction before densification in addition to allowing investment savings of milling equipment. Secondly, for these same reasons, the ease of grinding permits bio-coal co-firing rates to be increased at fossil coal power plants using existing coal mills.
2.3.
Equilibrium moisture content (EMC)
The low moisture content of torrefied fuels is a major claim made of torrefied material. The moisture content of a fuel has a direct impact on its heating value. At power plants, fuel suppliers are paid according to the received heating value (not higher heating value) of their fuel which is often measured from samples taken on-site. The market price therefore depends directly on the moisture content of the fuel. It has often been stated that bio-coal is, at least partly, hydrophobic and consequently can not reabsorb moisture to the same extent as untreated feedstock such as conventional wood pellets. This has led to the speculation that bio-coal can then be stored in heaps outdoors much like fossil coal without the need for costly dedicated storage infrastructure.
the table. The number of significant digits in the table corresponds to those given in the publications. Experimental uncertainties were for the most part not given.
3.2.
Grindability data is shown in Table 3 and is taken from two available publications. Results with torrefaction temperatures corresponding to those in Table 2 were selected when possible. The number of significant digits in the table corresponds to those given in the publications. Experimental uncertainties are not considered. The data in the table is from studies which measured the grinding energy.
3.3.
Results
3.1.
Mass and energy balance (heating value increase)
Table 2 displays experimentally data on the physical properties of torrefied materials from a number of peer-reviewed journal publications. Some of these publications contained material data from several different process temperatures within the torrefaction regime. If results were not stated explicitly in the publication, they were either found graphically from figures or calculated as described in the footnotes to
Equilibrium moisture content
Limited publications currently exist on the equilibrium moisture content of torrefied biomass. Both publications used here are from the same institution and appear to refer to the same experimental data on EMC. Table 4 displays the measured EMC which was measured at 30 C ambient temperature.
4. 3.
Grindability of torrefied biomass
Discussion and conclusions
It is now time to return to the question: How well do recent experimental findings support some of the more popular claims made of bio-coal? Concerning the mass and energy balance of torrefaction, nowhere in peer-reviewed literature have experimentally determined yields been found to equal 70 and 90% respectively. The values of 70 and 90% given in ECN publications [1,3,7] refer to an online publication as a source [17]. Upon investigation however, the publication itself states a value between 80 and 90% for energy yield and appears to use as a reference
Table 2 e Selected experimental results on torrefaction of biomass raw materials from recent literature. Material Willow Beech Willow Willow Wheat straw Rice straw Rape stalk Loblolly pine Loblolly pine Miscanthus Eucalyptus Pine chips Southern yellow pine residues Willow
Mass yield (%)
Energy yield (%)
78.6b 73.8b 79.8 81.6 71.5 36.6 25.3 74.2 60.5 75.7 80 73 70 n/a
91.9c 88.1c 85.8 89.9 78.2 39.9c 29.1c 83.1 73.2 81.0 90 87 82 n/a 0
Heating value (MJ kg1) 17.7/20.7 17.0/20.3 20.0/21.4 19.8c/21.8 18.9/20.7 17.1/18.7 18.8/21.6 19.55/21.80 19.55/23.56 20.2c/21.6 19.4/22.2 18.5/21.8 18.8/22.0 17.6/21.0 0
(LHV) (LHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (HHV) (LHV)
Dqa (%)
T ( C)
t (min.)
Volatiles (%)
Fixed carbon (%)
Ref.
16.9 19.4 7.00 10.2 9.52 9.11 15.1 11.5 20.5 6.98 14.4 18.2 17.2 19.3
270 280 270 290 270 300 300 275 300 290 240 275 275 300
15 30 30 10 30 30 30 80 80 10 30 30 30 10
n/a n/a 79.3 72.4d 65.2 n/a n/a 83.0 82.3 63.8d 75.4 76.4 71.4 n/a
n/a n/a 18.6 23.3d 26.5 n/a n/a 16.4 17.0 32.6d 21.8e 23.3 26.7 n/a
[6] [6] [8] [9] [8] [10] [10] [11] [11] [9] [12] [13] [13] [14]
a Relative heating value increase Dq ¼ 100(X X)[X]1 where X is the LHV or HHV of the torrefied solid product and X is LHV or HHV of the untreated material. b Estimated from on-screen graphical data. c Calculated value using heating value (LHV or HHV) according to energy yield definition given in [8]. d As received (not daf). e Calculated from given data on ultimate analysis and (n/a) data not available.
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b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 0 7 e1 1 1
Table 3 e Selected experimental results on the grindability of torrefied materials. T ( C) t (min.)
Material
Beech Spruce Pine Logging residues
280 280 275 275
5 5 30 30
Initial particle size 2e4 mm 20.94e70.59 15.08e39.70 1.88e4.94 mm
Final particle size d50 d50 d50 d50
¼ 140 mm ¼ 93 mm ¼ 270 mm ¼ 460 mm
0
Eg DEga Eg (kWh t1) (kWh t1) (%) 840 750 241c 242c
90 150 52.0 78.0
89 80 78 68
Equipment specification Retsch XMI mill, 500 mm sieve Retsch SM2000 mill, 1.5 mm sieve
Fraction of Ref. heating valueb (%) 2 2.8 1.0 1.5
(LHV) (HHV) (HHV) (HHV)
[15] [15] [13] [13]
0
a Change in grinding energy DEg ¼ 100(Eg e Eg) [Eg]1. b Used heating values (expressed as MJ kg1) were Spruce 19.0 [16], Pine 18.46, Logging residues 18.79 [13] and Beech 17.0 [15]. c Values calculated by fitted equation from reference at 25 C.
a conference paper by Burgeois and Doat [18]. However, the highest yield result for mass and energy stated by Bourgeois and Doat is 74 and 90% respectively for eucalyptus wood. From the results in Table 2, the mass and energy balance of torrefaction equate to a heating value increase in the range 7e21%. For the agro biomass results in Table 2, the corresponding increase is 7e15%. If the mass and energy yield would be 70 and 90% respectively, the increase in heating value would be 29%. For woody biomass the range of mass and energy yields are 61e82 and 73e92% respectively. The corresponding yields for agro biomass are 25e76 and 29e81%. Additionally, the maximum heating value increases (for both woody and agro biomass) were achieved using the lengthiest torrefaction times and highest temperatures; two parameters which will have significant influence on economy of reactor and process design [19]. In summary, 70 and 90% mass and energy yield (29% heating value increase) is not observed from experimental data on a given material. According to the results available, grindability of four biomass types is greatly improved through torrefaction. The used grinding energy (52e150 kWh/t, d50 ¼ 93 e 460 mm) was reduced by 68e89% compared to untreated biomass. This is a significant improvement but still short of the assumption that grindability is the same as coal whose value is found in the range 7e36 kWh/t [20]. Nonetheless, this does provide measured data of the energy requirements. Another recent study aimed to evaluate the grindability of torrefied biomass using the Hardgrove Grindability Index (HGI) without measuring the specific grinding energy [9]. It found that willow wood required high temperature (290 C) and long residence times (60 min) to obtain similar grinding behaviour as that of coal. From Table 4 the EMC of torrefied pine wood is indeed very low (2.2%) at low relative humidity (11.3%). At higher RH (83.6%) the EMC is greater (8.7%). From literature it can be seen that the EMC of biomass at constant relative humidity is
Table 4 e Equilibrium moisture content of torrefied Loblolly pine measured at 30 C from [11]. Material
Loblolly pine
T ( C)
275 300
t (min.)
w80 w80
Equilibrium moisture content, (0.005) RH ¼ 11.3%
RH ¼ 83.6%
0.022 0.022
0.087 0.087
inversely correlated to temperature [21]. The data in Table 4 was measured at 30 C which is well above the ambient temperature found at fuel yards across many northern European countries (especially Nordic countries) during the annual heating season. Although specific density of biomass (for example as pellets or briquettes) may influence its EMC [22], significantly higher EMC would be expected at lower temperatures for a given relative humidity. In summary, experimental results on three key properties of torrefied biomass from peer-reviewed publications have been presented. From recent results it can be seen that high energy yields on studied materials are achieved at the expense of other key properties; heating value increase, grindability and equilibrium moisture content. These trade-offs are determined by the physical nature of the materials themselves. That large-scale bio-coal production would achieve more commercially advantageous results than these, should not be assumed. The results can be used to realistically model bio-coal production scenarios in terms of economics and green-house gas emission balance.
Acknowledgements The Fortum Foundation is acknowledged for financial support during the writing of this manuscript.
references
[1] Bergman PCA, Boersma AR, Zwart RWR, Kiel JHA. Torrefaction for biomass co-firing in existing coal-fired power stations. Petten: Energy Research Centre of the Netherlands; 2005. Month. p. 61, Report No.: ECN-Ce;05e013. [2] Doat J. Un noveau produit e´nerge´tique pour les pays en de´velopment: le bois torre´fie´. Revue Bois et Foreˆts des Tropiques 1985;208(2e trimestre):57e67. [3] Bergman PCA. Combined torrefaction and pelletisation e the TOP process. Petten: Energy Research Centre of the Netherlands; 2005 July. p. 29, Report No.: ECN-Ce;05e073. [4] Ratte J, Fardet E, Mateos D, He´ry J- S. Mathematical modelling of a continuous biomass torrefaction reactor: TORSPYDTM column. Biomass Bioenergy 2011;35(8):3481e95. [5] Agar D, Wihersaari M. Torrefaction technology for solid fuel production e a move towards greater sustainability. Glob Change Biol Bioenerg, in press.
b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 0 7 e1 1 1
[6] Prins MJ, Ptasinski KJ, Janssen FJJG. Torrefaction of wood part 2. Analysis of products. J Anal Appl Pyrolysis 2006;77:35e40. [7] Kiel JHA, Verhoeff F, Gerhauser H, Meuleman B. BO2technology for biomass upgrading into solid fuel - pilot-scale testing and market implementation. Petten: Energy Research Centre of the Netherlands; 2008 June. p. 7, Report No.: ECNMe;08e036. [8] Bridgeman TG, Jones JM, Shield I, Williams PT. Torrefaction of reed canary grass, wheat straw and willow to enhance solid fuel qualities and combustion properties. Fuel 2008;87: 844e56. [9] Bridgeman TG, Jones JM, Williams A, Waldron DJ. An investigation of the grindability of two torrefied energy crops. Fuel 2010;89:3911e8. [10] Deng J, Wang J, Kuang J, Zhang Y, Luo Y. Pretreatment of agricultural residues for co- gasification via torrefaction. J Anal Appl Pyrolysis 2009;86:331e7. [11] Yan W, Acharjee T, Coronella C, Va´squez V. Thermal pretreatment of lignocellulosic biomass. Environ Prog Sust Energ 2009;28:435e40. [12] Arias B, Pevida C, Fermoso J, Plaza MG, Rubiera F, Pis JJ. Influence of torrefaction on the grindability and reactivity of woody biomass. Fuel Process Technol 2008;89:169e75. [13] Phanphanich M, Mani S. Impact of torrefaction on the grindability and fuel characteristics of forest biomass. Bioresour Technol 2011;102:1246e53.
111
[14] Prins M, Ptasinski K, Janssen F. More efficient biomass gasification via torrefaction. Energy 2006;31:3458e70. [15] Repellin V, Govin A, Rolland M, Guyonnet R. Energy requirement for fine grinding of torrefied wood. Biomass Bioenerg 2010;34:923e30. [16] Nurmi J. Heating values of the above ground biomass of small-sized trees. Kannus: Finnish Forest Research Institute; 1993. p. 30, Report No.: Acta Forestalia Fennica 236. [17] Archive 1999e2006 [cited 2012 Mar 31] New process for torrefied wood manufacturing, vol. 2 No. 4 Bioenergy update [home page on the internet]. Florence, AL: General*Bioenergy. Available from: www.bioenergyupdate. com/magazine/security/NL0400/bioenergy_update_april_ 2000.htm; April 2000. [18] Bourgeois JP, Doat J. Torrefied wood from temperate and tropical species e advantages and prospects. Bioenergy 1984; 84:15e21. June; Go¨tenborg, Sweden. [19] Prins MJ, Ptasinski KJ, Janssen FJJG. Torrefaction of wood Part 1. Weight loss kinetics. J Anal Appl Pyrolysis 2006;77:28e34. [20] Esteban LS, Carrasco JE. Evaluation of different strategies for pulverization of forest biomasses. Powder Technol 2006;166: 139e51. [21] Mehta S, Singh A. Adsorption isotherms for red chilli (Capsicum annum L.). Eur Food Res Technol 2006;223:849e52. [22] Singh RN. Equilibrium moisture content of biomass briquettes. Biomass Bioenerg 2004;26:251e3.