Integration of decentralized torrefaction with centralized catalytic pyrolysis to produce green aromatics from coffee grounds

Accepted Manuscript Integration of decentralized torrefaction with centralized catalytic pyrolysis to produce green aromatics from coffee grounds Li Chai, Christopher M. Saffron, Yi Yang, Zhongyu Zhang, Robert Munro, Robert M. Kriegel PII: DOI: Reference:

S0960-8524(15)01598-9 http://dx.doi.org/10.1016/j.biortech.2015.11.065 BITE 15798

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

30 September 2015 21 November 2015 24 November 2015

Please cite this article as: Chai, L., Saffron, C.M., Yang, Y., Zhang, Z., Munro, R., Kriegel, R.M., Integration of decentralized torrefaction with centralized catalytic pyrolysis to produce green aromatics from coffee grounds, Bioresource Technology (2015), doi: http://dx.doi.org/10.1016/j.biortech.2015.11.065

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Integration of decentralized torrefaction with centralized catalytic pyrolysis to produce green aromatics from coffee grounds Li Chai1, Christopher M. Saffron1,2,3*, Yi Yang4, Zhongyu Zhang1, Robert Munro1, Robert M. Kriegel5 1

Depatment of Biosystems and Agricultural Engineering, Michigan State University, MI,

USA 2

Department of Chemical Engineering & Materials Science, Michigan State University,

MI, USA 3

Department of Forestry, Michigan State University, MI, USA

4

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou,

China 5

The Coca-Cola Company, Atlanta, GA, USA

*Corresponding

author. E-mail: [email protected]

1

Abstract The aim of this work was to integrate decentralized torrefaction with centralized catalytic pyrolysis to convert coffee grounds into the green aromatic precursors of terephthalic acid, namely benzene, toluene, ethylbenzene, and xylenes (BTEX). An economic analysis of this bioproduct system was conducted to examine BTEX yields, biomass costs and their sensitivities. Model predictions were verified experimentally using pyrolysis GC/MS to quantify BTEX yields for raw and torrefied biomass. The production cost was minimized when the torrefier temperature and residence time were 239 °C and 34 minutes, respectively. This optimization study found conditions that justify torrefaction as a pretreatment for making BTEX, provided that starting feedstock costs are below $58 per tonne. Keywords: torrefaction; catalysis; pyrolysis; aromatics; biomass.

2

1.

Introduction Monoaromatic hydrocarbons, such as benzene, toluene, ethylbenzene, and xylenes

(BTEX), are widely used as additives to gasoline and precursors to polymers (GarciaPerez and Metcalf 2008). Green aromatics from renewable biomass, as a substitute for fossil aromatics from petroleum refining, are essential to reduce petroleum consumption and carbon dioxide emissions. Catalytic fast pyrolysis offers one green route for making aromatics from biomass by anoxically heating it to temperatures around 500 °C, for short residence times (less than 1 second), using a high heating rate (larger than 1000 K/s) (Garcia-Perez et al. 2008, Thangalazhy-Gopakumar et al. 2011, Jae 2012, ThangalazhyGopakumar et al. 2012, Kelkar et al. 2015). This technique has been applied to a variety of feedstocks, including woody plants, herbaceous plants and their processed residues. Coffee, an important agricultural product, is consumed worldwide with a global market of over 9 million tonnes per year (USDA 2012). U.S. food processing plants consume a large amount of coffee and produce 1.5 million tonnes of coffee waste every year (USDA 2012). Spent coffee waste is a form of lignocellulosic biomass with an oil content of around 15 wt% (Kondamudi et al. 2008). Previous research has demonstrated the conversion of spent coffee to green aromatics using HZSM5, a crystalline aluminosilicate, as heterogeneous catalyst ( Fischer et al. 2015, Kelkar et al. 2015). However, biomass’ high oxygen content is undesirable as it leads to high coke formation and low aromatic yields (Mihalcik et al. 2011). Further, hauling bound oxygen is expensive, leading to poor economics in the supply chain. Torrefaction is a pretreatment technology that upgrades the physicochemical properties of biomass (Li et al. 2012, Via et al. 2013, Chen et al. 2014, Klinger et al. 3

2015). When biomass is heated in the absence of oxygen at temperatures between 200 °C and 300 °C, the oxygen content is lowered (Bergman 2005, Klinger et al. 2014), which benefits the production of renewable aromatic hydrocarbons from torrefied biomass (Neupane et al. 2015). Lower transportation cost is an additional benefit of lower oxygen contents, as oxygen’s high molecular weight leads to expensive hauling. Torrefaction primarily degrades the hemicellulose in biomass, though the cellulose and lignin can undergo thermal cleavage and aromatization (Mani et al. 2009). It has been hypothesized that glycosidic bonds in cellulose are cleaved during torrefaction (Srinivasan et al. 2014). Such reactions provide small molecules for aromatization during catalytic pyrolysis. The effects of torrefaction on catalytic pyrolysis of lignin were studied by Adhikari et al., who found that torrefaction enhances aromatics production from lignin (Adhikari et al. 2014). However, severe torrefaction needs to be avoided because this may lead to crosslinking, reduce aromatic yields and increase coke formation (Zheng et al. 2013). Hilten et al. demonstrated the appropriate torrefaction temperature and residence time for reducing coke formation, which increased BTEX selectivity and yields on a torrefied biomass basis (Hilten et al. 2013). Counterbalancing enhanced BTEX yields on torrefied biomass is mass loss during torrefaction, which leads to lower overall yields (Hilten et al. 2013, Zheng et al. 2014). Further, capital and operating costs are increased because of the purchase and operation of the torrefaction reactor. Thus, we hypothesize that a trade-off exists between enhanced BTEX yields on torrefied biomass and the reduction of overall BTEX yields due to mass loss during torrefaction. Economic optimization studies of green aromatics from torrefied biomass are needed to obtain such metrics as the minimum BTEX production cost. This analysis is 4

needed to compare BTEX from torrefied biomass at optimized conditions to equally optimized BTEX directly from raw biomass pyrolysis and catalysis. To date, no such study exists that makes this comparison. In this research, the effects of torrefaction on BTEX yields and production costs were studied. Optimized torrefaction severity, defined by temperature and residence time, was obtained by balancing the cost addition due to torrefaction with cost reductions in transport and catalytic pyrolysis. The economics of making BTEX from torrefied biomass were then compared with raw biomass to demonstrate whether the concept of integrating torrefaction with BTEX production is economically justified.

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2.

Methods

2.1.

Process description Spent coffee grounds (SCG), a food processing plant waste, are converted to

green aromatics as per the process depicted in Figure 1. A centralized BTEX production facility, where catalytic pyrolysis is performed to produce BTEX, is located among several food processing plants that preprocess SCG using torrefaction. In the “torrefaction scenario,” rotary dryers located at food processing plants first dry SCG from 20 wt% to 5 wt% moisture content. After drying, moving bed torrefaction reactors are used to upgrade biomass at food processing plants, as this reactor type is advantaged by low maintenance and capital costs. The torrefaction off-gas, containing carbon oxides, molecular hydrogen and light hydrocarbons, is recycled and burned to heat the torrefier. The solid torrefied product is then transported by truck to the central conversion plant. In the “untreated scenario,” no torrefaction is required, so dried SCG are directly transported to the central conversion plant to make BTEX. At the centralized BTEX production facility, untreated or torrefied SCG are pyrolyzed in a fluidized bed reactor that is packed with solid catalysts to produce BTEXrich gas. HZSM5, an aluminosilicate zeolite catalyst, is effective for cracking, deoxygenating and aromatizing pyrolysis products (Pattiya et al. 2010, Carlson et al. 2011, Hilten et al.2011, Kelkar et al. 2015). Catalyst deactivation occurs as carbonaceous coke deposits on catalyst active sites, which is known to reduce product yields, lower reaction rates, increase costs, and decrease profitability. In this study, HZSM5 catalyst was assumed to remain active for 45 days, which is within the range of activity expected of actual fluidized catalytic cracking units used in petroleum operations. To account for 6

deactivation, catalyst efficiency was assumed to decline at a rate of 1% per day. Deactivated catalyst is auger conveyed from the fluidized-bed reactor to the catalyst regenerator, which reactivates the catalyst by combusting the coke. After reactivation, 20% of the catalyst, mainly fine material that has been abraded, is discharged and referred to as “spent catalyst.” This is a sensitive assumption that must be verified by pilot trials, as fresh catalyst feed can lead to significant additional cost. The heat needed by catalytic pyrolysis is provided by catalyst coke and by combustion of the non-condensable gas coproduct of pyrolysis. Next, BTEX-rich vapor is condensed and BTEX is separated and collected. Transportation of BTEX from the production facility to end users is not included in this study. Though torrefaction may lead to high selectivity and coke reduction (Hilten et al. 2013) during catalytic pyrolysis, such positive effects were not considered to simplify process optimization and economic analysis. 2.2.

Model and experimental yields of BTEX from biomass

2.2.1. Experiment design Both BTEX and torrefaction mass yields are functions of torrefaction severity. In order to formulate such functions, a central composite design (CCD) with two factors (torrefaction temperature and residence time) at five levels and three replicates at the center point, was employed to conduct experiments. A CCD is a fractional factorial design that is supplemented with axial points to estimate curvature. Torrefaction temperature and residence time were allowed to range from 200 °C to 300 °C, and from 16 minutes to 50 minutes, respectively. A design of 11 groups of torrefied SCG with three replicates in each group results in a total of 33 torrefied SCG samples. Another three untreated SCG samples were also prepared as the control group. A surface response 7

model consisting of a quadratic polynomial regression equation (1) was used to perform all predictions. 2

2

2

y = β0 + ∑ βi xi + ∑ βii xi + ∑∑ βij xi x j 2

i =1

i =1

(1)

i =1 j >i

Where y indicates the response factor (torrefaction mass yield or BTEX yields), the independent variables xi and xj are the torrefaction temperature and residence time. β0, βi, βii, and βij are the intercept, linear, quadratic and interaction coefficients, respectively. 2.2.2. Biomass preparation and torrefaction Fresh SCG were frozen and shipped from The Coca-Cola Company in Atlanta, Georgia. SCG were further ground and screened through a 60 mesh sieve tray. Sieved SCG were dried in the oven at 60 °C overnight and then stored in desiccators to maintain a constant moisture content before the experiment. Torrefied SCG were made in a torrefaction furnace using nitrogen as a purge gas at a flow rate of 54 ml/min. 2.2.3. Catalyst preparation ZSM5 catalyst with a silica–alumina ratio of 23, in the ammonium cation form, was obtained from Zeolyst Co. (Conshohocken, PA). This catalyst was prepared by calcination in air at 550 °C for 4 hours to obtain acidic HZSM5. As reported by Zeolyst, the catalyst has a pore size of 0.6 nm, a pore volume of 0.14 m3/g and a surface area of 400 to 425 m2/g. 2.2.4. Catalytic pyrolysis Torrefied SCG (or untreated SCG) and the catalyst were mixed in a 1:5 weight ratio. Approximately 3 mg of the mixture were then packed in a quartz tube where quartz 8

wool and a quartz filler rod were placed above and below the feedstock. A CDS Pyroprobe 5250 was employed to perform the catalytic pyrolysis. Samples in the pyroprobe were heated in the absence of oxygen at a temperature of 550 °C with a heating rate of 999 °C/s. The BTEX-rich vapors produced in the pyroprobe were transferred by helium at flow rate of 1 ml/min to a Shimadzu QP-5050A gas chromatograph/mass spectrometer (GC/MS) (Kelkar et al. 2015). The transfer line was heated to a temperature of 300 °C to avoid condensation of aromatics vapors. The sample was subjected to a 1:100 split before entering the capillary column. A Restek rtx-1701 capillary GC column (Restek, Bellefonte, PA) was used to separate the BTEX-rich vapors using a previously described method (Kelkar et al. 2014). BTEX compounds were quantified using an external standardization method. 2.3.

Economic analysis

2.3.1. Feedstock cost SCG were assumed to be collected at the food processing plant for a cost of $15 per dry tonne. Biomass cost per tonne of torrefied SCG is a function of torrefaction severity as expressed in equation (2).
 =
 /  (  ,  )

(2)

Where BCtor is the biomass cost per tonne of torrefied SCG, BCSCG is the biomass cost per tonne of untreated SCG and is equal to $15 per dry tonne. Ytor is the torrefaction mass yield predicted using the surface response regression model in equation (1). Ytor is a function of torrefaction temperature (Ttor) and residence time (timetor).

9

2.3.2. Torrefaction cost A longer residence time means that a larger torrefier is needed to produce the same amount of torrefied SCG. Thus, torrefier capital investment was assumed to be a power function of residence time, as expressed in equation (3).  ( ) =  ∗ 

."

(3)

Where, CItor is the torrefier capital investment at the required time. CIbase is the torrefier capital investment at the base time of 30 minutes and timetor is the torrefaction residence time in minutes. A scale factor of 0.6 is assumed in this calculation, in accordance with the classical sixth-tenths rule (Peters et al. 1968). 2.3.3. Transportation cost Semi-trucks having trailer cargo capacities of 100 m3 and 20 tonnes are used to transport SCG from the food processing plant to the central conversion plant. All semitrucks were assumed to be purchased for $100,000 and employed for 10 years. A salvage value of $20,000 was assumed to estimate the depreciation cost. The diesel consumption rate for trucking was assumed to be 57 liters/100 km with full load and 35 liters/100 km with an empty load (Nylund and Erkkilä 2005). Diesel was assumed to be purchased at a price of $1 per liter, which is the U.S. average retail price in 2014. The actual cargo capacities of trucks are limited by either weight or volume depending on the bulk density of SCG. When the bulk density of torrefied SCG was larger than 200 kg/m3, the actual cargo capacity was determined to be 20 tonnes; otherwise, 100 m3 was used to calculate the cargo load. The bulk density of dried SCG was assumed to be 400 kg/m3, while the bulk density of torrefied SCG was calculated according to the mass loss during 10

torrefaction. It was assumed that torrefaction has no impact on the biomass volume in this calculation. Transport cost mainly depends on the total travel distance and time (Sokhansanj et al. 2009). The average one-way travel distance from food processing plants to the central conversion plant was set at 160 km. Total transport time includes the round-trip driving time, loading time and unloading time (Sokhansanj et al. 2006). The loading and unloading time for dry bulk material can be roughly estimated to be one minute for each cubic meter of volume (Ranta and Rinne 2006). Driving time was calculated assuming an average travel speed of 80 km/ hour. The driver was assumed to be paid at an hourly wage of $20. 2.3.4. Economics at the centralized BTEX production facility For a given amount of SCG, a higher mass loss during torrefaction equates to smaller capacities required of a centralized BTEX production facility (CBPF). In the untreated scenario, an amount of 100 thousand tonnes of SCG per year was assumed to be available to a CBPF. In the torrefaction scenario, the capacity of the CBPF was calculated by subtracting the mass loss during the torrefaction from the total available SCG in food processing plants, which results in a smaller CBPF. The main equipment costs at the CBPF were sized and inflated to 2014 USD using the chemical engineering plant index. After computing capital investment, operating costs were tabulated for raw materials, utilities, labor and depreciation. The labor cost at the CBPF, a function of its capacity, was scaled as a power function using an exponent of 0.25, as described by 11

Peters et al. and Leboreiro et al. (Peters et al. 1991, Leboreiro and Hilaly 2011). Electricity was assumed to be purchased at a price of 7.32 cents/kWh, which is the U.S. average industrial electricity price in 2014. The on-line time was set at 90%, which means that 10% of the time is used for maintenance and repairs. The straight-line method was adopted in this study to estimate depreciation cost. Total BTEX production cost at the CBPF gate, denoted as CCBPF, was calculated by equation (4). #$%& = ' + )

(4)

FC is the fixed cost of the untreated SCG scenario in which the CBPF processes one hundred thousand tonnes of SCG per year, which includes costs of depreciation, equipment maintenance and labor. VC is the variable cost per tonne of feedstock, which includes costs of electricity, cooling water, and fresh catalyst. 2.3.5. Total BTEX production cost The total production cost (TPC), as shown by equation (5), involves the biomass cost, drying cost, torrefaction cost, transportation cost, and the conversion cost at the CBPF. By dividing by YBTEX, which denotes the BTEX mass per mass of torrefied SCG, TPC is expressed in dollars per tonne of BTEX. During optimization, ten-thousand values of TPC at different torrefaction temperatures and residence times were calculated, from which the minimum TPC was obtained. The corresponding temperature and residence time were determined to be the optimized torrefaction severity. * = (
 + +, +  + -./0 + 
*2 )/
34

12

(5)

3.

Results and discussions

3.1.

Prediction of torrefaction and BTEX yields The torrefaction mass yields and BTEX yields at different torrefaction severities

were collected upon experiment and are shown in Table 1, with the aim of fitting the statistical prediction model. BTEX yields in the table are on a weight basis of torrefied biomass. Untreated SCG, the experimental control group which was not torrefied (Table 1), results in a BTEX yield of 8.37 wt.%. The BTEX yield from torrefied SCG is higher than the untreated SCG and grows with increasing torrefaction severity until a temperature of 285 °C. Above this temperature, crosslinking in biomass may occur and cause a reduction in BTEX yields (Mihalcik et al. 2011). The highest BTEX yields were observed around 250 °C and 50 minutes. Torrefaction and BTEX yield predictions were made by fitting a surface response regression model, as shown in equation (1), to the collected data using Minitab software; the model equations are shown in equation (6) and equation (7), respectively. Regression analysis shows high accuracy (p<0.05, R2=98.73%) for predicting torrefaction mass yield and acceptable accuracy (p<0.05, R2=90.13) for predicting BTEX yields. The prediction error for the BTEX yield regression model arises from measurement error during pyGC/MS.

 (%) = (6778 + 78. 9 + :; − 8. =7>: + 8. 7:?: − 8. >:@ ∗ ) ∗ =8A6

(6)


34 (%) = (−:@=; + >:. 8; − 8. 9> − 8. 87?;: − 8. 8@: + 8. 8>: ∗ ) ∗ =8A6 (7)

13

3.2.

Optimizing BTEX production cost The torrefaction temperature and residence time were optimized to obtain the

minimum BTEX production cost. Previous experimental research has demonstrated that torrefaction temperatures ranging from 240 °C to 275 °C positively affect BTEX production when using corncobs and loblolly pine as feedstocks (Hilten et al. 2013, Zheng et al. 2014). Economically, milder torrefaction severity is desired to avoid significant mass loss and the high biomass cost that results. As shown in Figure 2, at a temperature of 239 °C and a residence time of 34 minutes, BTEX production costs from torrefied SCG are minimized at $1,271/tonne. For comparison, the untreated SCG scenario has a total production cost of $1,423/tonne. This demonstrates that torrefaction is economically superior to untreated SCG, even though enhanced BTEX selectivity and catalyst coke reduction were not considered. Conversely, high torrefaction temperature and long residence time leads to severe mass loss and high biomass cost. As shown in Figure 2, when the torrefaction temperature is above 270 °C, BTEX production cost increases steeply because of severe mass loss during torrefaction. Furthermore, a long residence time decreases the throughput of torrefiers and therefore increases their capital cost. In the area of low torrefaction temperature and short residence time, which represents insufficienct torrefaction severity, BTEX production is not economical because BTEX yields (per torrefied biomass) are not enhanced enough to offset the increased costs of biomass and torrefaction. Figure 3 (a) shows the effect of residence time on the BTEX production cost at the optimized torrefaction temperature of 239 °C. At this temperature, a residence time ranging from 16 to 50 minutes demonstrates lower cost than the untreated scenario. 14

Figure 3 (b) shows the effect of torrefaction temperature on the BTEX production cost at the optimized residence time of 34 minutes. At this residence time, a torrefaction temperature below 275 °C has a lower total production cost than the untreated scenario. Overall, BTEX production cost is more sensitive to torrefaction temperature than residence time, so torrefaction temperature is a more important variable for minimizing production cost. 3.3.

Model verification The regression models (equations 6 and 7) were verified by running the

torrefaction furnace and py-GC/MS at the optimized torrefaction conditions (239 °C, 34 minutes). As shown in Table 2, the torrefaction mass yield at the optimal conditions was found to be 85.1%, which is 0.8% higher than the predicted value; the BTEX yield was 9.65%, which is 0.09% higher than the predicted value. The BTEX production cost calculated by experimental yields is $1,246 per tonne of BTEX, and is $25 lower than the prediction value by model. These results verify the regression models’ ability to predict torrefaction yields and BTEX yields with reasonable accuracy. 3.4.

Effects of transport distance and biomass cost Two factors, namely the average transport distance from food processing plants to

the centralized BTEX production facility and the biomass cost at the food processing plant gate, were selected for sensitivity analysis on BTEX production costs. Parameters were varied with a range of 0 to 300 km for distance, and a range of 0 to $70 per dry tonne for biomass cost.

15

According to Figure 4 (a), the effect of transport distance is especially significant on the untreated scenario because the high oxygen content in raw biomass lowers the transport efficiency. If transport distance equals zero, which means BTEX is produced at the food processing plant, the torrefied SCG and untreated SCG scenarios have the lowest and very similar BTEX production costs of around $1,000/tonne. In reality, a large BTEX production facility, which is served by multiple food processing plants, benefits from “economics of scale.” At longer transport distances, the torrefaction scenario exhibits cost savings versus the untreated scenario. Thus, when long distance transportation of SCG is needed (e.g. oversea shipping), torrefaction greatly reduces the total BTEX production cost by reducing oxygen content and thus enhancing transport efficiency. According to Figure 4 (b), when biomass cost rises, the torrefaction scenario’s BTEX cost increases more steeply than untreated scenario because mass loss during torrefaction increases the total production cost. The intersection point of these two lines occurs at a biomass cost of $58 per dry tonne. Thus, when the biomass cost goes above $58/tonne, the untreated scenario (i.e. no torrefaction) should be chosen for making BTEX.

16

4.

Conclusions The torrefaction severity was optimized to reveal a minimum production cost of

$1,271/tonne at 239 °C and 34 minutes of residence time. Comparatively, untreated SCG results in a BTEX cost of $1,423/tonne, so the use of torrefaction can be justified under certain conditions. Additionally, the effects of transport distance and biomass cost on BTEX production cost were studied. With long hauling distances, torrefaction is strongly recommended to reduce oxygen content and hence, transport cost. Overall, torrefaction does not economically produce BTEX when biomass costs greater than $58/tonne, because mass loss negates its benefit.

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Acknowledgements: The authors thank The Coca-Cola Company for providing financial support. The corresponding author thanks AgBioResearch at Michigan State University for supporting his research appointment on this project.

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Food processing plants

Centralized BTEX production facility Fresh catalyst

SCGs 100 Drying

Untreated scenario

Catalyst regeneration

heat

0.5 Transportation

Catalytic pyrolysis 48

84

100

Condensation Torrefaction scenario

Torrefaction

84

38

16

heat

10

Non-condensable gas

Off-gas Separation & fractionation

Combustion

Combustion heat

BTEX

Figure 1. Generalized process flow diagram for converting SCG to BTEX. Numbers provide mass balance information for the optimized torrefaction scenario in units of thousand tonnes of dry matter per year.

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Figure 2. Contour plot of torrefaction severity vs. BTEX production cost.

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Figure 3. Plots of (a) residence time vs. BTEX production cost at the optimized torrefaction temperature (239°C); and (b) torrefaction temperature vs. BTEX production cost at the optimized residence time (34 mins).

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Figure 4. Effects of variable parameters on BTEX production costs: a) average distance from food processing plants to the centralized BTEX production facility, and b) the biomass cost at the food processing plant gate.

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Table 1. Torrefaction mass yields and BTEX yields for SCG according to torrefaction severity. BTEX yield is expressed per weight of torrefied SCG. Sample number Control 1 2 3 4 5 6 7 8 9 10 11

Torrefaction temperature (°C) untreated 200 215 215 250 250 250 250 250 285 285 300

Residence time (min) 30 16 44 10 30 30 30 50 16 44 30

Torrefaction mass yield (wt %) 100 96.30 95.57 94.26 92.19 84.96 84.98 85.07 83.02 77.25 69.49 65.73

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BTEX yield (wt %) 8.37 8.57 8.71 8.87 8.85 9.51 9.48 9.53 9.63 7.61 8.39 7.20

Table 2. Verification of regression model

Prediction Experiment

Torrefaction mass yield (wt%) 84.32 85.10

BTEX yield (wt%) 9.56 9.65

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BTEX production cost ($/tonne) 1,271 1,246

Highlights: •

BTEX costs can be reduced using biomass pre-processing methods such as torrefaction

•

Long distance biomass supply is benefitted when using torrefaction to pre-process biomass

•

When biomass cost exceeds $58/tonne, the mass loss that occurs during torrefaction negates its benefit

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