Torrefaction of biomass is a promising thermochemical pretreatment technique used to upgrade the properties of biomass to produce solid fuel with improved fuel properties. A comparative study of the effects of torrefaction temperatures (200, 250, and 300 °C) and residence times (0.5 and 1 h) on the quality of torrefied biomass samples derived from spent coffee grounds (SCG) and coffee husk (CH) were conducted. An increase in torrefaction temperature (200-300 °C) and residence time (0.5-1 h) for CH led to an improvement in the fixed carbon content (17.9-31.8 wt %), calorific value (18.3-25 MJ/kg), and carbon content (48.5-61.2 wt %). Similarly, the fixed carbon content, calorific value, and carbon content of SCG rose by 14.6-29 wt %, 22.3-30.3 MJ/kg, and 50-69.5 wt %, respectively, with increasing temperature and residence time. Moreover, torrefaction led to an improvement in the hydrophobicity and specific surface area of CH and SCG. The H/C and O/C atomic ratios for both CH- and SCG-derived torrefied biomass samples were in the range of 0.93-1.0 and 0.19-0.20, respectively. Moreover, a significant increase in volatile compound yield was observed at temperatures between 250 and 300 °C. Maximum volatile compound yields of 11.9 and 6.2 wt % were obtained for CH and SCG, respectively. A comprehensive torrefaction model for CH and SCG developed in Aspen Plus provided information on the mass and energy flows and the overall process energy efficiency. Based on the modeling results, it was observed that with increasing torrefaction temperature to 300 °C, the mass and energy yield values of the torrefied biomass samples declined remarkably (97.3% at 250 °C to 67.5% at 300 °C for CH and 96.7% at 250 °C to 75.1% at 300 °C for SCG). The SCG-derived torrefied biomass tested for CO2 adsorption at 25 °C had a comparatively higher adsorption capacity of 0.38 mmol/g owing to its better textural characteristics. SCG would need further thermal treatment or functionalization to tailor the surface properties to attract more CO2 molecules under a typical post-combustion scenario.
Torrefaction of biomass is a promising thermochemical pretreatment technique used to upgrade the properties of biomass to produce solid fuel with improved fuel properties. A comparative study of the effects of torrefaction temperatures (200, 250, and 300 °C) and residence times (0.5 and 1 h) on the quality of torrefied biomass samples derived from spent coffee grounds (SCG) and coffee husk (CH) were conducted. An increase in torrefaction temperature (200-300 °C) and residence time (0.5-1 h) for CH led to an improvement in the fixed carbon content (17.9-31.8 wt %), calorific value (18.3-25 MJ/kg), and carbon content (48.5-61.2 wt %). Similarly, the fixed carbon content, calorific value, and carbon content of SCG rose by 14.6-29 wt %, 22.3-30.3 MJ/kg, and 50-69.5 wt %, respectively, with increasing temperature and residence time. Moreover, torrefaction led to an improvement in the hydrophobicity and specific surface area of CH and SCG. The H/C and O/C atomic ratios for both CH- and SCG-derived torrefied biomass samples were in the range of 0.93-1.0 and 0.19-0.20, respectively. Moreover, a significant increase in volatile compound yield was observed at temperatures between 250 and 300 °C. Maximum volatile compound yields of 11.9 and 6.2 wt % were obtained for CH and SCG, respectively. A comprehensive torrefaction model for CH and SCG developed in Aspen Plus provided information on the mass and energy flows and the overall process energy efficiency. Based on the modeling results, it was observed that with increasing torrefaction temperature to 300 °C, the mass and energy yield values of the torrefied biomass samples declined remarkably (97.3% at 250 °C to 67.5% at 300 °C for CH and 96.7% at 250 °C to 75.1% at 300 °C for SCG). The SCG-derived torrefied biomass tested for CO2 adsorption at 25 °C had a comparatively higher adsorption capacity of 0.38 mmol/g owing to its better textural characteristics. SCG would need further thermal treatment or functionalization to tailor the surface properties to attract more CO2 molecules under a typical post-combustion scenario.
The
transition toward attaining a sustainable and renewable energy
economy from a petroleum-based economy has gained much attention in
recent times. The worldwide interest in sustainable energy resources
is mainly due to the escalating demand for energy resources, the gradual
depletion of petroleum resources, and the environmental concerns associated
with its worldwide utilization.[1] Moreover,
the consumption of non-renewable petroleum resources is presumed to
have a negative impact on the ecological system.In the past
few decades, the global CO2 emissions from
large-point stationary sources have increased significantly and are
the main contributor to climate change. It is predicted that by the
year 2035, the atmospheric concentration of CO2 could reach
about 550 ppm.[2] In addition, the global
temperature is also expected to rise by approximately 2 °C.[2] Therefore, researchers, the scientific community,
and policymakers face the challenges of developing sustainable and
environmentally benign energy resources as well as developing innovative
and cost-effective methods to minimize CO2 or other toxic
gas emissions.The post-combustion CO2 capture units
are considered
as one of the feasible solutions to reduce CO2 emissions.[3,4] The post-combustion method involves capturing CO2 from
the mixed flue gas stream after the complete combustion. Compared
to other existing CO2 capture technologies (pre-combustion
or oxy-fuel combustion), post-combustion capture can be retrofitted
to the existing industries without significant modifications.[5,6] In addition, the post-combustion method is easy to set up and is
also a cost-effective CO2 capture technology.Post-combustion
CO2 capture technology uses wet/dry
adsorbents and the principle of adsorption/desorption to trap CO2 molecules from the flue gas stream. Activated carbon (AC)
and biochar are widely studied as the carbon-based adsorbents for
post-combustion capture techniques due to the availability of a large
specific surface area, microporous structure, hydrophobicity, and
superior CO2 adsorption capacity.[4] Activated carbon can be produced from a single- or two-step pyrolysis
(thermally treating biomass/organic wastes in an inert environment)
and subsequent physical or chemical activation.The previous
study of Mukherjee et al.[5] showed that
biochar synthesized from spent coffee grounds (SCG)
could serve as a promising carbon-based adsorbent for CO2 capture via post-combustion technology. The SCG-derived biochar
produced from slow pyrolysis at 600 °C displayed a relatively
higher specific surface area of 539 m2/g, microporous structure,
and improved adsorption capacity of 2.8 mmol/g.[5] Although promising results were realized, several knowledge
gaps are prevailing. For instance, the biochar used in our previous
study was produced by slow pyrolysis. However, a few studies have
evaluated the post-combustion CO2 capture of carbonaceous
materials produced from other thermochemical conversion methods such
as torrefaction or hydrothermal carbonization. Furthermore, most reports
presented in this field are related to the experimental studies[7] or kinetics.[8,9] However, it
is challenging to scale up a process without implementing and comparing
modeling studies to the experimental results. For instance, it is
challenging to calculate the energy requirements of an entire process
with experimental results alone.[10] Also,
experimental results alone do not provide adequate information needed
for preliminary economic evaluation.Torrefaction is fundamentally
a mild thermochemical pretreatment
exploited mainly for upgrading biomass characteristics. The process
is conducted at a moderate temperature ranging from 200 to 300 °C
in an oxygen-deficient condition. Moreover, torrefaction is characterized
by lower heating rates and long reactor residence time under atmospheric
conditions.[11] The fundamental advantage
of torrefaction is that the biomass samples are converted into high-quality
fuels with lower atomic ratios and high energy density. The properties
of torrefied biomass are comparable to those of conventional fossil
fuels such as coal.[12] Hydrophobicity properties
in torrefied biomass make it easier for efficient storage, handling,
and long-distant transportation.Several researchers have studied
and reported the torrefaction
process as a biomass pretreatment technique for improving its fuel
characteristics.[13,14] In a recent study, Sarker et
al.[15] reported the improvement in biomass
characteristics after undergoing mild to severe torrefaction treatment
using barley straw (BS), canola hull (CH), and oat hull (OH). Chen
et al.[16] showed that torrefied biomasses
fall in the periphery of high-volatile bituminous coal when synthesized
at high temperatures. Despite many impressive studies related to biomass
torrefaction, a few studies have reported the effect of torrefaction
on the fuel properties of SCG and coffee husk (CH). To the best of
the author’s knowledge, no study has reported the possibility
of using torrefied SCH or CH as solid material for postcombustion
CO2 capture. In contrast, numerous studies have reported
the use of activated carbon for post-combustion CO2 capture.[8,17,18] Tiwari et al.[18] showed that almond shell-prepared activated carbon is effective
for CO2 removal. In another study, Dilokekunakul et al.[17] prepared activated carbon from bamboo waste
and studied the effect of N, O, and different functional groups on
CO2 capture. To fill the knowledge gaps, this study aims
to evaluate the impacts of temperature and residence time on the yield
and physicochemical properties of torrefied SCG and CH.Another
novelty of this study is the development of a comprehensive
biomass torrefaction model. The model is essential for process optimization
and techno-economic analyses. The torrefaction model can estimate
the product distribution and by-products of the process. Furthermore,
torrefaction models provide the information required to bridge the
gap between academia and industry research. SCG and CH were selected
as feedstock for this study due to their availability and the promising
results obtained from both feed materials in our previous studies.[5,19] Furthermore, SCG and CH have no significant applications as they
are mostly discharged into landfills, causing a detrimental impact
to the environment owing to the emission of toxic materials. Therefore,
their valorization would help minimize environmental pollutions originating
from their disposal.
Results and Discussion
Ultimate, Proximate, and pH Analyses of the
Precursors and Torrefied Biomass Samples
The ultimate, proximate,
and pH analyses reveal the modifications in chemical composition before
and after the exposure of the precursors (SCG and CH) to torrefaction,
and the findings are summarized in Table . The carbon, hydrogen, nitrogen, sulfur,
and oxygen compositions of SCG and CH were characteristics of typical
lignocellulosic (agricultural) biomass materials such as bamboo, rice
husk, and oil palm.[20] The carbon contents
of SCG (50 wt %) and CH (48.5 wt %) fall in the periphery of typical
lignocellulosic biomasses. On the contrary, SCG has superior hydrogen
(6.7 wt %) and sulfur content (0.9 wt %) compared with CH. It should
be emphasized that both coffee residues showed a low sulfur content
of less than 1 wt % and nitrogen content (<3 wt %). Low sulfur
and nitrogen contents are desirable for thermochemical conversion
processes, especially the reactions involving a catalyst. Moreover,
low sulfur and nitrogen contents mean that there will be fewer emissions
of nitrous and sulfides during thermochemical conversion processes.
Table 1
Ultimate, Proximate, and pH Analyses
of the Precursors and Torrefied Biomass Samples (Dried Basis)
ultimate analysis
proximate analysis
atomic ratios
precursor
torrefaction
conditions (temperature–time) (°C–h)
C (wt %)
H (wt %)
N (wt %)
S (wt %)
Oa (wt %)
volatile
matter (wt %)
ash (wt %)
moisture (wt %)
fixed carbonb (wt %)
H/C
O/C
pHc
coffee husk
(CH)
raw material
48.5
5.9
2.8
0.6
40.6
77.7
1.7
2.7
17.9
1.43
0.63
6.8
200–0.5
50.5
5.6
2.8
0.2
36
75.3
1.9
2.2
20.6
1.31
0.53
7.1
200–1
51.6
5.5
2.9
0.1
33
74.6
2.1
1.9
21.4
1.26
0.48
7.2
250–0.5
56.3
5.4
3.3
0.1
25.6
71.1
2.2
1.8
24.9
1.13
0.34
7.4
250–1
58.6
5.3
3.3
0.1
22.1
70.4
2.7
1.7
25.2
1.07
0.28
7.7
300–0.5
60.3
5.0
3.3
0.1
17.6
64.8
3.1
1.5
30.6
0.99
0.22
8.3
300–1
61.2
4.8
3.5
0.2
15.3
63.5
3.3
1.4
31.8
0.93
0.19
8.8
spent
coffee
grounds (SCG)
raw
material
50
6.7
2.3
0.9
39.0
81.2
0.9
3.3
14.6
1.6
0.60
5.5
200–0.5
52.8
6.6
2.2
0.06
37.3
79.7
1.07
3.5
15.7
1.5
0.53
5.6
200–1
54.4
6.5
2.8
0.1
35.1
78.3
1.1
2.5
18.1
1.45
0.48
5.9
250–0.5
56.3
6.3
2.4
0.03
33.5
73.2
1.3
2.2
23.3
1.37
0.45
6.1
250–1
59.4
6.3
2.7
0.04
30.1
72.6
1.6
2.1
23.7
1.3
0.38
6.3
300–0.5
67.8
6.2
3.0
0.03
21
68.3
1.8
1.8
28.1
1.13
0.23
6.4
300–1
69.5
6.0
3.2
0.03
19
67.8
2.0
1.2
29
1.0
0.20
6.7
lignited
61.9
4.3
0.9
16.4
8.5
0.80
0.20
Calculated by the difference: O
(wt %) = 100 – (C + H + N + S + ash) wt %.
Calculated by the difference: fixed
carbon content (wt %) = 100 – (volatile matter + ash + moisture)
wt %.
Standard deviations
for the pH measurements
of the tested samples were ±0.02.
Values for lignite are obtained
from Kim et al.[24]
Calculated by the difference: O
(wt %) = 100 – (C + H + N + S + ash) wt %.Calculated by the difference: fixed
carbon content (wt %) = 100 – (volatile matter + ash + moisture)
wt %.Standard deviations
for the pH measurements
of the tested samples were ±0.02.Values for lignite are obtained
from Kim et al.[24]The ultimate analysis of the torrefied biomass samples
reveals
a change in elemental composition for both precursors. The results
indicate that notable alterations in composition occur during torrefaction
regardless of the type and nature of the precursor. Moreover, an elevation
in the torrefaction temperature from 200 to 300 °C at 0.5 h led
to a significant improvement in C content for both the precursors.
For instance, the C content of SCG-derived torrefied solid was 52.8
wt % at 200 °C and 0.5 h residence time. However, a rise in the
torrefaction temperature to 300 °C and 1 h produced an elevation
in the C content to 69.5 wt %. On the contrary, with an increase in
torrefaction temperature and residence time, the torrefied biomass
samples’ oxygen, hydrogen, and sulfur contents reduced, irrespective
of the biomass. The reduction in oxygen fraction could be attributed
to the disruption of the polymeric structure mainly hemicellulose
followed by cellulose between 200 and 300 °C during torrefaction.
Moreover, owing to the series of devolatilization and decomposition
reactions of the lignocellulosic components that take place during
torrefaction, the oxygen content was reduced sharply. The decline
in hydrogen content with elevating torrefaction temperatures to 300
°C could also be attributed to the release of hydrocarbons (CH4 and C2H6) during torrefaction.[15] It should also be highlighted that no clear
pattern was evident in the change in nitrogen composition in torrefied
biomass samples with variations in torrefaction temperatures or residence
time. Similar observations were reported in previous studies.[21,22]The proximate analysis presented in Table indicates that both SCG and CH showed a
high content in the volatile matter range of 77.7–81.2 wt %
and therefore could exhibit poor combustion efficiency and fuel characteristics.
However, torrefaction of the precursors led to a decline in the volatile
matter and moisture content with increasing process severity (Table ). Surprisingly, the
ash content of all the torrefied solids is greater than those of the
precursors. However, all the ash contents are still low and less than
lignite coal (8.5 wt % ash content). Decreasing ash content is favorable
for thermochemical conversion processes. Biomass ash contains inorganic
elemental composition that often creates operating challenges such
as slagging, fouling, or obstruction in the combustion units.[9] For that reason, a low ash content of the torrefied
solid residues is desirable. Significant changes in the proximate
analysis were not evident at a lower range of temperature (200 °C)
because only moisture and light volatile compounds were eliminated
from the biomass samples. The Van Krevelen diagram presented in Figure shows the extent
of reactivity and fuel characteristics of the precursors and the torrefied
biomass samples. Compared to the precursors, the torrefied biomass
samples showed lower atomic ratios (H/C and O/C) owing to low H and
O content. Moreover, comparatively, CH-derived torrefied biomass samples
under the most severe conditions (300 °C and 1 h) had the lowest
values of H/C (0.93) and O/C (0.19). Both the atomic ratios (H/C and
O/C) declined remarkably and were influenced by increasing torrefaction
temperature and longer residence time. The atomic ratios of the torrefied
biomass samples are within the periphery of lignite coal (Table ). Chen et al.[16] and Lu al.[23] have
reported a similar finding on declining atomic ratios using rice husk
and oil palm fiber as the starting materials, respectively, as shown
in the Van Kravelen plot. The findings on atomic ratios reveal that
torrefaction of both the precursors improved their fuel properties
for subsequent biological or thermochemical conversion processes.
Figure 1
Van Krevelen
Plot of the precursors and torrefied biomass samples.
Van Krevelen
Plot of the precursors and torrefied biomass samples.pH analysis was used to assess the acidity or basicity of
the torrefied
samples. The pH values of the tested samples are also summarized and
presented in Table . SCG and CH had pH values of 5.5 and 6.8, respectively. These values
are often regarded as very weak acidic values. However, torrefaction
of the precursors led to an increase in pH values beyond neutral values
to the very weak to mild basic range. Therefore, it can be inferred
that torrefaction of SCG and CH improved their pH values. An increase
in pH for both the precursors also indicates a decrease in acidic
functional groups in the torrefied solids and a similar finding was
observed from the FTIR analysis. Owing to the Lewis acid nature of
CO2 molecules, the improvement of pH for both the precursors
to the basic range could facilitate the CO2 capture process
owing to acid–base interactions.
Moisture-Sorption
Test of the Precursors and
Torrefied Biomass Samples
The improved hydrophobic characteristics
of the torrefied samples were evaluated through an equilibrium moisture
content test, and the results are presented in Figure a,b. The torrefaction temperature significantly
impacted the equilibrium moisture content compared with the influence
of residence times. Therefore, the impact of varying torrefaction
temperatures at a fixed residence time of 1 h is presented in Figure a,b. The figures
also depict the relationship between the percentages of moisture absorbed
with time at varying torrefaction temperatures for the individual
coffee residues. SCG and CH are hydrophilic (strong affinity for water)
due to the presence of increased polar moieties in hemicellulose (oxygenated)
and the ability to form a hydrogen bond.
Figure 2
Moisture-sorption test
of precursors and torrefied biomass samples:
(a) SCG and (b) CH.
Moisture-sorption test
of precursors and torrefied biomass samples:
(a) SCG and (b) CH.As presented in Figure a,b, the equilibrium
moisture contents of SCG and CH after
120 h were 30 and 29 wt %, respectively. However, the moisture uptake
of torrefied biomass samples derived from SCG and CH synthesized under
the severe conditions (300 °C for 1 h) reduced its level in the
range of 8.2–7.1 wt % compared with the precursors. The decline
in the equilibrium moisture content of the torrefied biomass samples
compared with the biomass samples indicates that the thermal treatment
transformed the physical structure and characteristics of the precursor
from its hydrophilic nature to more hydrophobic.The increased
hydrophobicity of SCG- and CH-derived torrefied biomass
samples could be due to the dissolution and disintegration of polar
functional groups such as O–H and C–O bonds present
in hemicellulose molecules of the biomass and the release of oxygenated
hydrophilic groups. In addition, the formation of an unsaturated polymer
structure in the precursors and the disintegration of hemicellulose
and lignin during torrefaction could also lead to the elimination
of hydrogen bonds in water, thereby improving their hydrophobicity.[12] Moreover, as the severity of torrefaction conditions
increases, the amorphous hemicellulose and small cellulose crystallite
fractions degrade, thereby limiting the adsorption of moisture in
the torrefied samples.[25] It is necessary
to highlight that the resistance of biomass samples to fungal attack
is proportional to the moisture uptake. Therefore, a decline in the
moisture uptake of the torrefied biomass implies that torrefaction
would enhance the resistance of biomass to fungal attack under humid
conditions.[26] Improved hydrophobicity is
favorable for long-time storage, transportation, and CO2 capture under typical postcombustion conditions.
BET Analysis of the Precursors and Torrefied
Biomass Samples
A summary of the textural properties of the
precursors and torrefied biomass samples is presented in Table . As evident irrespective
of the precursors, the torrefaction temperature significantly influenced
the specific surface area. The specific surface area for SCG-derived
torrefied biomass samples increased from 11 m2/g to 100
m2/g when the torrefaction temperature rose from 200 °C
for 0.5 h (mild) to 300 °C for 1 h (severe). A similar trend
in the improvement of the specific surface area was observed for CH-derived
torrefied biomass samples, as highlighted in Table . However, the values obtained are comparatively
less than the SCG-derived torrefied biomass. The specific surface
area of CH-derived torrefied biomass samples is within the range of
15–24 m2/g for mild–severe treatment conditions.
A more porous structure in SCG-derived torrefied samples could be
attributed to the removal of volatiles, tars, degradation of lignin,
and loss of oxygenated species from the pores, creating a network
of void spaces in the carbon matrix. Similar findings were reported
by Sarker et al.[15] They have reported that
with increasing severity of the torrefaction process conditions, the
biomass structures rupture to produce more porous structures and a
higher specific surface area.
Table 2
BET Analysis of the
Precursors and
Torrefied Biomass Samples
parameter
SCG
SCG-200 (0.5 h)
SCG-300 (1 h)
CH
CH-200 (0.5 h)
CH-300 (1 h)
BET surface area, m2/g
2.3
11
100
3.5
15
24
total pore volume, 10–3 cm3/g
1.2
7.2
10.4
2.4
3.7
5.5
mean pore size, nm
10.1
8.4
5.8
23.1
13.4
8.1
FTIR of the Precursors and Torrefied Biomass
Samples
To determine the impact of torrefaction conditions
on the chemical structure of the biomasses and torrefied biomass samples,
the Fourier transform infrared spectroscopy (FTIR) analysis was conducted,
as shown in Figure a,b. The FTIR spectra of the biomasses and torrefied samples heated
up to 250 °C are almost similar in shape owing to the absence
of significant changes in their chemical structure. However, the changes
in the vibration intensity of the FTIR spectra at 300 °C for
1 h (severe treatment conditions) are more pronounced than precursors
at 200 and 250 °C. The characteristic peak at around 3200–3400
cm–1 characterizes O–H vibration (stretching)
present mainly in the lignocellulosic component (cellulose). The peak
reduced drastically and disappeared with increasing torrefaction temperature
due to partial dehydration and carbohydrate decomposition with increasing
severity of torrefaction conditions.[27] The
inception of the peaks at around 2980–2850 cm–1 is attributed to the presence of vibrations (stretching) of asymmetric
and symmetric aliphatic groups (C–H), which narrowed and disappeared
in the torrefied samples.[28] The band at
around 1730 cm–1 in both precursors and torrefied
biomass samples could be attributed to the carbonyl stretching (C=O)
of acetyl, carboxylic acid, aldehyde, or ketone groups in hemicellulose.
It starts to disappear from 250 °C and with increasing temperature
progressively to 300 °C and 1 h duration. The peak at around
1730 cm–1 is eliminated owing to the complete decomposition
of the carbonyl groups in hemicellulose present in the torrefied biomass
samples. This demonstrates that a chemical change appears from the
decomposition of hemicellulose and the disintegration of long-chain
polysaccharides in the tested samples. The peak at 1620–1600
cm–1 represents the aromatic skeletal vibration
of C=O with no significant change in the vibrational intensity
of this peak observed at a lower range of temperatures.[7] The inception of the peak reveals the aromatization
of the torrefied samples. This characteristic reflects its stability
in the torrefied biomass samples and enrichment of lignin components.
Among all the build block components found in biomasses, hemicellulose
is the most reactive biopolymer owing to lack of crystallinity and
lower degree of polymerization.[29] Therefore,
hemicellulose goes through the most significant decomposition reactions
during torrefaction, as evident from the less intensified peaks at
300 °C for the respective torrefied samples. From the spectrum
presented in Figure a,b, it can be concluded that by increasing the temperature to mild
conditions (200 and 250 °C), the peaks are retained, but under
the severe torrefaction condition (300 °C–1 h), noticeable
changes in the spectra of the tested samples are observed. The changes
are attributed to the release of oxygenated species mainly from complete
destruction of hemicellulose and limited disintegration of cellulose.
Figure 3
FTIR analysis
of precursors and torrefied samples at different
temperatures: (a) SCG and (b) CH.
FTIR analysis
of precursors and torrefied samples at different
temperatures: (a) SCG and (b) CH.
Thermal Stability Analysis of the Torrefied
Biomass Samples
The TGA-DTG profiles for SCG and CH and the
torrefied biomass samples are presented in Figure S1 in the Supporting Information. Moreover, the devolatilization
profiles of the torrefied samples are shown in Figure a,b. The thermal decomposition pattern of
the torrefied biomass samples differs from that of the precursors,
most significantly under mild to severe torrefaction conditions (250
and 300 °C). From the DTG curve, the two prominent peaks observed
for the precursors overlapped for the torrefied biomass samples, and
the peaks represent the disintegration temperature of cellulose and
lignin. Owing to the loss of hemicellulose as the main lignocellulosic
component at around 320 °C, the lignin content increased in both
the tested samples. Additionally, the decline in the rate of mass
loss of the torrefied biomass samples and the peak shifting to a higher
temperature also indicated the attainment of thermal stability of
the torrefied biomass samples compared to the precursors. Irrespective
of the precursors, the thermal stability is attained under the severe
torrefaction conditions (300 °C and 1 h) and the CH-derived torrefied
biomass sample is thermally more stable than SCG.
Figure 4
TGA-DTG analysis of torrefied
biomass samples at different torrefaction
temperatures: (a) SCG and (b) CH.
TGA-DTG analysis of torrefied
biomass samples at different torrefaction
temperatures: (a) SCG and (b) CH.As can be seen (Figure a,b), the thermal decomposition process of the precursors
(Figure S1) and torrefied samples can be
separated into three distinct stages. The first stage that occurs
at a temperature of up to 200 °C corresponds to dehydration and
the removal of light volatile matter content from the precursors.
However, the mass loss associated with dehydration was insignificant
for the torrefied samples, confirming the tested samples’ hydrophobic
nature compared to the raw precursor. The second stage occurs at temperatures
ranging from 200 to 500 °C. This stage is characterized by pyrolytic
volatile combustion and is termed the active phase.[22] Decomposition of the basic lignocellulosic building blocks,
including hemicellulose (220–315 °C), cellulose (315–400
°C), and lignin (160–900 °C), occurred at this stage.
Depending on the torrefaction temperature, this stage could proceed
in one or two phases. The first phase corresponded to the combustion
of hemicellulose and cellulose components within the temperature range
of 190–400 °C. Two visible peaks with maximum mass loss
in DTG curves can be observed in this stage (Figure a,b). The first peak corresponds to the degradation
of hemicellulose at 322 °C with a maximum mass loss rate of 1.02%/°C,
and the second peak relates to cellulose degradation at 400 °C
with a maximum mass loss rate of 1.03%/°C. Moreover, the weight
loss at the first phase was approximately 70% for raw precursors,
which decreased to about 30% for the torrefied 300 °C–1
h samples, and corresponded to the maximum devolatilization process.
It should be observed that the decomposition of hemicellulose in the
first peak had the highest reactivity compared to cellulose and lignin
for both the precursors. This indicates the high reactivity of hemicellulose
as confirmed by the FTIR analysis. Moreover, the peak representing
the decomposition of hemicellulose did not appear for the torrefied
samples at 300 °C–1 h, which further highlights the loss
and disintegration of hemicellulose during torrefaction.The
third stage (500–800 °C) corresponded to char oxidation.
Residue fixed carbon is combusted in this stage, which has the lowest
reactivity, and the rate of mass loss declined. As seen from Figure a,b, no peak appeared
in the DTG curve during this stage. The decline in the rate of mass
loss of the torrefied biomass samples and the peak shifting to a higher
temperature also indicated the attainment of thermal stability of
the torrefied biomass samples compared to the precursors. Irrespective
of the precursors, the thermal stability is attained under the severe
torrefaction conditions (torrefaction temperature of 300 °C and
longer residence time of 1 h). The CH-derived torrefied biomass sample
is thermally more stable than SCG owing to a lesser mass loss rate
and peak shifting to a higher temperature.
XPS of
the Torrefied Biomass Samples
X-ray photoelectron spectroscopy
is carried out to understand the
impact of torrefaction temperatures and residence time on the elemental
composition and surface functional groups in the precursors and torrefied
biomass samples. Moreover, it also provides information on the qualitative
and quantitative analyses of the different amounts of elements in
the precursors and torrefied biomass samples. The findings of the
survey scan for the respective torrefied biomass samples are presented
in Table .
Table 3
Elemental Composition of Torrefied
Biomass Samples
sample
C1s
O1s
N1s
Si2p
SCG-200-0.5h
84.9
15.3
1.3
2.2
SCG-300-1h
90.8
8.7
1.6
0.5
CH-200-0.5h
83.4
14.2
1.2
1.1
CH-300-1h
85.9
10.9
2.4
0.7
The elemental composition obtained
from the wide survey scan of
the tested samples reveals a similar trend where the surface is dominated
by C followed by O, N, and a small amount of Si, irrespective of the
precursors. The XPS analysis findings complement the ultimate analysis
(bulk) findings, where SCG-derived torrefied biomass samples were
predominantly carbonaceous in nature, followed by CH-derived torrefied
biomass samples synthesized at 300 °C for 1 h (Table ). The C content increased to
90.8–85.9 wt %, and O reduced slightly to 8.7–10.9 wt
% for SCG- and CH-derived torrefied biomass samples synthesized at
300 °C. The increase in C content and a decline in O content
and O/C demonstrate the carbonization, cracking of bonds, aromatization,
and decarboxylation during biomass torrefaction.
Table 4
Solid Yield (%), HHV, Energy Density
Ratio, and Energy Yield (%) of the Precursors and the Torrefied Biomass
Samples
The deconvoluted
C1s and O1s spectra of the respective precursors
and the SCG- and CH-derived biochars at 300 °C for 1 h are shown
in Figure a,b, and
the variation in content obtained from the survey scan is presented
in Tables S1 and S2. Azargohar et al.[30] suggested that the C1s spectra contain the following
functional groups in the carbon matrix: peak (I) for aromatic/aliphatic
sp2 carbon (C–C/C=C/C–CH) observed at BE = 284.1–284.7 eV, peak (II)
for sp3-C and C–O bonding at B.E. = 285.5–285.9
eV, peak (III) for −C–OR for ether and hydroxyls/phenol
group (C–OH) observed at B.E. = 286.1−286.4 eV, and
peak (IV) for C=O for carboxylic acids or ester observed at
B.E. = 288.0–288.8 eV. Also, Azargohar et al.[30] suggested that the O1s spectra could be deconvoluted into
three peaks; peak (I) at 531.2–531.5 eV for carbonyl and ketone/lactone,
peak (II) at 532.04–532.3 eV for C–OH functional groups,
and peak (III) at 533.3 eV for ether oxygen atoms in anhydrides and
esters. As can be seen, both SCG and CH are dominated by C–C/C–H functional groups. Also, the relative proportion
of C–C/C=C/C–H functional
groups increased from 25.2–24.4 wt % to 34.3–31.0 wt
% for SCG- and CH-derived torrefied samples, respectively, when the
torrefaction conditions changed from mild to severe conditions.
Figure 5
Deconvoluted
spectra of SCG-300-1h and CH-300-1h: (a) C1s and (b)
O1s.
Deconvoluted
spectra of SCG-300-1h and CH-300-1h: (a) C1s and (b)
O1s.The trend of increasing −C–C/C=C/C–H confirmed the increment in aromatic content
and disintegration of aliphatic groups. On the contrary, the hydroxyl,
carbonyl, and ester functional groups reduced with torrefaction harshness.
These findings indicated the occurrence of series of dehydration and
decarboxylation reactions releasing CO2 and H2O during torrefaction and agree with the findings of the FTIR analysis.
Also, owing to the decomposition of the O–H group from the
surface of biomasses, the hydrophobic characteristics were improved
in the torrefied biomass samples as also evident from the moisture-sorption
test presented in Section .
SEM of the Precursors and Torrefied Biomass
Samples
To gain a deeper insight into the impact of torrefaction
on the morphology of SCG and CH, the scanning electron microscopy
(SEM) images of the respective precursors before and after torrefaction
at different temperatures (200 and 300 °C) and residence times
(0.5 h and 1 h) are presented in Figure . It should be mentioned that the images
have been amplified by a factor of 1000.
Figure 6
Scanning electron microscopy
of SCG- and CH-derived torrefied biochar
samples at different temperatures.
Scanning electron microscopy
of SCG- and CH-derived torrefied biochar
samples at different temperatures.The structural change is much noticeable at a higher range of temperatures
compared to the precursors. As the torrefaction conditions shifted
to 300 °C for 1 h, the surface degrades. An increasing number
of characteristic openings and voids are observed on the surface,
resulting in a much scrappier structure. During torrefaction, especially
under the most severe torrefaction condition (300 °C and 1 h),
cell-wall distortion is visible, and microapertures are created owing
to the disintegrated torrefied solid surface. This phenomenon could
be attributed to the release of volatile compounds and carbonization
under the severe torrefaction conditions that caused the formation
of porous and brittle torrefied biochar samples. The microstructure
can be observed on the surfaces where some microfibers are seen due
to biomass structure rupture.A similar observation on the change
in the morphology of the torrefied
biomass samples was reported by Sarker et al.[15] They have observed that more porous but disintegrated biochar structures
are formed with increasing severity of the torrefaction conditions.
Solid and Energy Yield of the Precursors and
the Torrefied Samples
The resultant mass yield, higher heating
value (HHV), energy yield of CH and SCG, and torrefied samples are
summarized in Table . The results show that biomass exposure to a higher temperature
had a negative impact on mass yield compared to the residence time.
For instance, at a constant residence time of 0.5 h, an increase in
temperature from 200 °C to 300 °C led to a decline in mass
yield from 93.2 wt % to 55.1 wt % for SCG. This behavior could be
because of the decomposition of volatile components into liquid and
gaseous products.[31] It could also be attributed
to the accelerated thermal degradation of the lignocellulosic components,
mainly hemicellulose, without any significant degradation on cellulose
or lignin in precursors.[21]Considerable
decreases in the mass yield of torrefied CH and SCG to 48.1 and 54.3%,
respectively, were observed at 300 °C and 1 h of residence time.
This finding implies that approximately 45.7–51.9 wt % coffee
residues degraded thermally as the temperature increased and with
prolonged duration. Moreover, due to the inherent difference in composition,
the SCG had a superior solid yield compared to CH.The energy
yield ranged between 70.6 and 67.1% for SCG- and CH-derived
torrefied samples. However, CH-derived torrefied samples have a superior
energy yield compared to SCG samples. It should be highlighted that
the energy yield of the torrefied samples at all temperatures was
below 100% due to the loss in energy during the torrefaction process.
Additionally, from Table , it is evident that the energy density ratio improved for
both the precursors with increasing severity of the torrefaction.
Fuel with high energy density is always desirable. It will be less
expensive in transportation and storage because it would occupy less
storage or unit energy for transportation. The HHV values of the torrefied
solids also increase with the severity of torrefaction. Furthermore,
when compared with the raw precursor, torrefied solids have higher
HHV values. The HHV values are widely represented in the Van Kravelen
diagram, where torrefied samples obtained at 300 °C and 1 h were
found at the lower end of the diagram, demonstrating an improved HHV.
Model Validation
Experimental data
for mass yields and HHV values from SCG and CH torrefaction at different
torrefaction temperatures and residence times were used to validate
the accuracy of the torrefaction model in this regard. The model validation
plots for mass yields from the respective biomasses are presented
in Figure a,b. It
can be seen from Figure a,b that the experimental mass yield correlates with the model predictions
at a lower range of torrefaction temperature (200 °C). However,
the model predictions are higher than the experimental results for
both biomasses at mild-higher torrefaction temperatures (<200 °C).
On the other hand, Figure a,b compares the HHV values from experimental and model yields.
From Figure a,b,
it can be seen that the HHV values from experimental data are close
to the model values with a deviation of less than 7% observed at 300
°C.
Figure 7
Model validation for mass yields during torrefaction of coffee
residues for (a) SCG-derived torrefied solids and (b) CH-derived torrefied
solids.
Figure 8
Model validation for HHV during torrefaction
of coffee residues
for (a) SCG-derived torrefied solids and (b) CH-derived torrefied
solids.
Figure 10
Effect
of torrefaction temperatures on the decomposition of volatile
compounds.
Model validation for mass yields during torrefaction of coffee
residues for (a) SCG-derived torrefied solids and (b) CH-derived torrefied
solids.Model validation for HHV during torrefaction
of coffee residues
for (a) SCG-derived torrefied solids and (b) CH-derived torrefied
solids.Some deviations exist between
the experimental results and model
values due to several reasons such as the reactor type, type of precursors,
and heating rate. However, the trends for both HHV values and mass
yields are similar. Moreover, the most significant deviations for
both biomasses are less than 10%. These deviations are relatively
small and permissible for engineering applications in the industries.
Therefore, the model could be used to further to explain different
phenomena occurring during the torrefaction process.
Parametric Studies
The impacts
of torrefaction temperatures on the mass yield and HHV values of the
torrefied biomass samples are studied and presented in Figure a,b. The simulation was performed
at a temperature range of 200–300 °C. It should be noted
that the effect of time was less pronounced when compared with torrefaction
temperature; therefore, the residence time was kept constant at 1
h. It can be observed from Figure a,b that the mass yield remains almost the same at
a temperature range between 200 and 250 °C. However, when the
temperature rose above 250 °C, a significant decline in mass
yield was observed for both precursors. For example, the mass yield
of SCG solid at 250 °C was reported as 96.7%. Moreover, a rise
in temperature to 300 °C led to a significant decline in the
mass yield to 67.5%. Similarly, the mass yields of CH solids decreased
from 97.3% at 250 °C to 75.1% at 300 °C.
Figure 9
Effect of torrefaction
temperatures on the mass yield and HHV values
of (a) SCG-derived torrefied biomass samples and (b) CH-derived torrefied
biomass samples.
Effect of torrefaction
temperatures on the mass yield and HHV values
of (a) SCG-derived torrefied biomass samples and (b) CH-derived torrefied
biomass samples.Regarding the HHV values,
it was observed that there is a consistent
increase in the HHV values of the torrefied biomass samples with temperature,
irrespective of the precursors. For instance, the HHV values of SCG
solids rose from 22 MJ/kg at 200 °C to 23.7 MJ/kg at 300 °C.
In the same way, CH solid HHV values increased from 20. 9 MJ/kg to
22.2 MJ/kg. The increase in HHV values and the decline in mass yields
with temperature confirm the analytical characterization results
reported in the previous sections. To understand why there is a significant
decline in mass yields with temperature and an elevation in HHV values,
the yields of volatile components were also simulated.The change
in volatile compound yields with torrefaction temperature
is shown in Figure . As shown in Figure , at temperatures between 200 and 250 °C
for both biomasses, the amount of volatiles decomposed is almost negligible.
This explains why there is an increase in mass yield at this temperature
range. On the contrary, beyond 250 °C, a significant increase
in volatile compounds yield was observed. Maximum volatile compound
yields of 11.9 and 6.2 wt % were obtained for CH and SCG, respectively.Effect
of torrefaction temperatures on the decomposition of volatile
compounds.The simulations result of volatile
yield decomposition aligns with
the TGA findings reported in the previous section. Additionally, the
increased HHV values of the torrefied samples could be attributed
to the change in their elemental compositions (CHNSO values) compared
to the raw precursors, as reported in Table . The C content of the torrefied biomass
samples increases when compared with the biomasses, although the oxygen
and hydrogen content is reduced. These changes in ultimate composition
promote the elevation in HHV values. These findings are in agreement
with the previously reported literature.[10,27,30]
CO2 Capture Performance of Torrefied
Samples from SCG and CH
The CO2 adsorption performance
of SCG- and CH-derived torrefied biomass samples at 300 °C and
1 h was executed in a fixed-bed reactor at 25 °C and in the presence
of 30 vol % CO2 (balanced by N2). The breakthrough
capture performance of the tested samples is shown in Figure . As observed from Figure , the adsorption
of gases proceeds continuously until the point of saturation is attained
by the bed of torrefied biomass samples.
Figure 11
Breakthrough CO2 capture balanced by N2.
Breakthrough CO2 capture balanced by N2.In general, under the similar capture scenario (25 °C and
30 vol % CO2 balanced by N2), the torrefied
sample derived from SCG presents a higher adsorption capacity (0.38
mmol/g) at equilibrium than that derived from CH (0.23 mmol/g). The
physiochemical transformation of SCG-derived torrefied samples under
the severe conditions (300 °C and 1 h) could probably account
for a higher equilibrium adsorption capacity under a similar capture
scenario. It presents a comparatively porous structure, higher specific
surface area, and well-developed functional species (basic-oxygenated)
on the surface of the SCG-derived torrefied biomass sample. It had
a significant impact on the capture performances. However, to improve
the CO2 capture performance, the biomass needs further
thermal treatment or chemical functionalization to attract more CO2 molecules under the post-combustion scenario.
Conclusions
The impact of torrefaction temperature
and residence time on the
yield and physicochemical properties of torrefied biomass samples
derived from SCG and CH was studied. Furthermore, the performance
of the torrefied biomass for post-combustion CO2 capture
was evaluated. An increase in torrefaction of SCG and CH led to a
rise in the carbon content of the torrefied solids. On the contrary,
the torrefied solids’ hydrogen, sulfur, and oxygen contents
decrease with an increase in torrefaction temperature for both precursors.
The decline in hydrogen content with elevating torrefaction temperature
could be attributed to the release of lighter hydrocarbons during
torrefaction. An equilibrium moisture-sorption content test confirms
a decrease in the torrefied samples’ moisture uptake compared
with the raw precursors. The observation confirms an improvement in
the hydrophobicity of the torrefied samples. The increased hydrophobicity
of torrefied solids could be attributed to the dissolution and disintegration
of the polar groups, such as hydroxyl (O–H bonds) present in
hemicellulose molecules of the precursors.The experimental
mass yields were compared with model results obtained
from Aspen Plus simulation. The experimental mass yield correlates
with the model predictions at 200 °C. However, the model predictions
are slightly higher than the experimental results for both precursors
at temperatures above 200 °C. Overall, this study shows that
torrefaction influences the fuel properties of biomass by increasing
the heating value, decreasing the moisture content and moisture uptake,
and reducing fouling tendency. XPS and FTIR results proved that the
conversion from “–C–O/—C=O”
to “aromatic −C–C/=C” was the key
point for improving phenol and aromatic content. The development of
basic functionalities and developed pore structure facilitated the
CO2 capture for SCG, but further thermal treatment under
more severe conditions is necessary to develop the porous structure.
Materials and Methods
Biomass Collection and
Pretreatment
The collection and pretreatment of the biomasses
(SCG and CH) were
reported in our previous study.[19] SCG and
CH were collected from a local coffee café located at the University
of Saskatchewan, Saskatoon campus, and Road Coffee Inc. (Saskatoon).
The biomasses without any modification were referred to as the “as-received
samples”. The as-received samples were thoroughly washed with
water (distilled) to eliminate any impurities and oven-dried at 105
± 5 °C for 12 ± 3 h. The dried precursors were collected
and stored in air-tight glass containers for further thermal treatment
to avoid any contamination.
Torrefaction Experiment
A schematic
representation of the fixed-bed reactor for torrefaction is presented
in Figure . The
torrefaction of SCG and CH was performed in a 1 in. fixed-bed Inconel
tubular reactor. The overall details of the torrefaction reactor system
have been meticulously explained in a previous study.[15] The dimensions of the tubular reactor are 22 mm (inner
diameter), 25.4 mm (outer diameter), and 870 mm (reactor length).
A standard temperature controller was used to monitor the temperature
change and ensure that the desired reaction temperature was attained.
A K-type thermocouple was inserted inside the reactor bed to monitor
the temperature of the bed. The average geometric mean particle size
of dried biomasses (SCG and CH) was already small and in the range
of 0.52–0.61 mm, so they were fed directly into the reactor.
During torrefaction, the temperature was raised from 25 °C to
the desired peak temperatures (200, 250, and 300 °C). In addition,
a constant heating rate of 10 °C/min was maintained throughout
the reaction. Like torrefaction temperature and residence time, the
heating rate influences the properties in torrefied biomass samples.
However, the influence is minimal compared to the other parameters.
Usually, the range of heating rate studied for torrefaction is between
10 and 50 °C/min. The heating rate of 10 °C/min was selected
because a lower heating rate favored the generation of higher torrefied
biomass (solid) yield and improved hydrophobic characteristics. As
the torrefied biomass is the main product of consideration in this
study, the heating rate was kept at the lower level and fixed at 10
°C/min. Nitrogen gas was added at 100 mL (STP)/min to maintain
an inert atmosphere and avoid undesirable reactions (oxidation or
ignition). The flow rate of gas was continuously monitored using a
mass-flow controller. Once the desired torrefaction temperature was
reached, a known amount of biomass (10 ± 0.5 g) was loaded into
the reactor and then tightly sealed to ensure that the inertness and
temperature inside the reactor were well maintained. It should be
emphasized that two sets of residence times were considered in this
study (0.5 and 1 h).
Figure 12
Fixed-bed torrefaction reactor setup.
Fixed-bed torrefaction reactor setup.Once the experiment was completed, the reactor was cooled down
to room temperature (25 ± 5 °C) in the presence of N2 at 100 mL (STP)/min. The final products (torrefied biomass
and liquid samples) were collected for further mass balance. The torrefied
biomass samples were stored in a glass container inside a desiccator
at room temperature until other chemical analyses and adsorption performance
studies were performed. The corresponding torrefied biomass samples
were labeled according to the precursor-torrefaction temperature–residence
time. For instance, SCG-200-0.5h implies a torrefied biomass sample
derived from SCG at 200 °C and 0.5 h hold time, or CH-300-1h
indicates a torrefied biomass sample derived from CH at 300 °C
and 1 h hold time.Temperature and residence time ranges of
200–300 °C
and 0.5–1 h were defined in this study for the following reasons:The devolatilization and depolymerization
of lignocellulosic
components (hemicellulose and cellulose) occur within the temperature
range (200–300 °C).[32]The objective of the present study is to
improve the
solid fuel properties and yield for potential CO2 capture.
An increase in the temperature beyond 300 °C could lead to a
drastic decline in solid yield. Therefore, 300 °C was selected
as the maximum temperature.The torrefaction
temperature range of 200–300
°C was used to prevent excessive mass loss in the precursors.The residence time range of 0.5–1
h was selected
for this study because previous studies reported a decline in solid
yield with a residence time above 1 h.[32−34] Furthermore, a residence
time below 0.5 h does not provide enough duration for intermediate
reactions such as depolymerization, dehydration, and deoxygenation
to occur.
Chemical
Analysis
The analytical
characterizations of the tested samples were carried out on a dried
basis. The proximate analysis was conducted based on standard ASTM
procedures as stated by Sarker et al.[15] The proximate analysis showed the fraction of moisture, ash, and
volatile matter contained in the biomasses and torrefied biomass samples.
Further, by mass balance, the fixed carbon content was determined.
To evaluate the composition of carbon, hydrogen, sulfur, and nitrogen
in the biomasses and torrefied biomass samples, the ultimate analysis
was performed with the aid of a PerkinElmer CHNS analyzer.[35] The mass balance was used to determine the oxygen
content. The pH values of the biomasses and torrefied samples in aqueous
solutions were analyzed using a pH meter according to the standard
method reported by Patra et al.[36]A PARR 6400 calorimeter was used to determine the calorific value
based on thermal energy generation, as stated by Sarker et al.[15] The moisture uptake of SCG, CH, and torrefied
biomass samples was tested using a climate chamber (SH-641). Before
the sorption tests, the biomasses and torrefied samples were dried
in a vacuum oven at 65 ± 5 °C to eliminate excess moisture
from the samples. Then, the samples were loaded into a glass dish
and placed into a climate chamber. The climate chamber was operated
at a relative humidity (RH) of 90% and a temperature of 30 °C
for 120 h. The equilibrium moisture content was determined after 120
h, and the weight of the samples was monitored and recorded at 4 h
of interval. In the end, the samples’ weight was measured,
and the relative weight gain by the samples compared to the initial
sample weight represents the moisture uptake of the samples.The Brunauer–Emmett–Teller (BET) analysis of the
coffee residues and the torrefied samples was determined using a Micrometrics
ASAP-2020 instrument. Before the test, the samples were degassed at
300 °C for 4 h followed by N2 adsorption and desorption
studies at −196 °C.[35]The Fourier transform infrared (FTIR) spectroscopy of SCG, CH,
and the torrefied samples was performed to identify different functional
groups present in the tested samples. This analysis was performed
with a Bruker Vertex 70 FTIR spectrometer according to the method
stated by Mukherjee et al.[5] The spectrum
was set in the range of 4500–400 cm–1.To elucidate the devolatilization behavior of the biomasses and
torrefied biomass, the TGA-DTG analysis was executed. About 10–20
mg of the tested samples was heated at a temperature range of 20–800
°C at a constant heating rate of 10 °C/min in the presence
of N2 to maintain the reduced environment.[5]The X-ray photoelectron spectroscopy (XPS) analysis
was used to
observe the functional moieties present in the sample. Details of
the procedure have been meticulously explained in our previous publication.[5] The SEM analysis was performed according to the
method specified by Patra et al.[36] The
SEM analysis was used to investigate the change in the morphology
of the biomasses and torrefied biomass samples after torrefaction,
CO2 Capture Setup
The
information of the CO2 capture setup and the schematic
has been meticulously described in our previous report.[5] Before each adsorption experiment, the setup
was loaded with 2 ± 0.2 g of torrefied biomass samples and then
preheated to 160 ± 5 °C for 2 h in the presence of N2 gas at 50 ± 0.5 mL/min. N2 gas was used to
maintain the inertness and to remove excess moisture inside the reactor.After dehydration, the reactor was cooled down to 25 °C, after
which pure CO2 (30 vol %) balanced by N2 was
purged through the bed of torrefied adsorbents. A mass-flow controller
was used to monitor the gas-flow rate throughout the adsorption process
continuously. The adsorption capacity was evaluated by using eq .[5]where q , m, Q, Co, C, and t represent
the CO2 uptake of the torrefied biomass samples at time t (mmol/g), the mass of the torrefied biomass samples (g),
the flow rate of the gas (mL/min), and inlet CO2 and outlet
CO2 concentrations in the mixed gas streams (volume %)
and time (min), respectively.
Process
Modeling
Aspen Plus Model Description and Assumptions
Biomass torrefaction is a highly complex process with a series
of intermediate reactions. Therefore, it is challenging to model such
systems in Aspen Plus. This is because biomass contains several complex
components, including lignocellulosic compositions (hemicellulose,
cellulose, and lignin).[29] The thermal degradation
of these lignocellulosic components comprises of several intermediate
and complex reactions that could possibly yield various intermediate
products. Therefore, the modeling and identification of these products
are complicated and equally challenging. Consequently, the entire
torrefaction process was simulated by considering different unit operations.Figure shows
the flow diagram of the torrefaction model. The flowsheet, together
with the unit operations arrangement, was designed to be as simple
as possible. That way, it is easier for future adjustments and scale-up.
The model consists of a dryer, a series of heaters and compressors,
two yield reactors, and flash separators. The description of each
unit operation and the assumptions are summarized and presented in Table . It should be emphasized
that Aspen Plus does not contain a predefined feedstock for SCG and
CH. Therefore, a nonconventional stream was used to define the heterogeneous
solid feedstock based on their proximate and ultimate analyses together
with a calculator block. A detailed description of the nonconventional
stream modeling and the assumptions can be found in our previous report.[37]
Figure 13
Flow diagram of the coffee residue (SCG and CH) torrefaction
model
in Aspen Plus.
Table 5
List of Assumptions
and Description
of the Processing Blocks Used for the Torrefaction Model
default block
ID in Aspen Plus
unit operations
used in the flowsheet
descriptions
and assumptions
dryer
DRYER
This convective air-drying
unit was used to simulate the predrying steps.
A continuous operation mode
was assumed with a plug flow operation.
compressor
COMPSR
The compressor helps increase
the pressure of the incoming air that is fed to the dryer.
Isentropic operation
mode
was assumed.
heater
HEATER1
Heats air-dried feedstock
to the desired preheating temperature.
Rstoic
DRY-REC
Simulate the moisture removal
step in the torrefaction process.
Operates at atmospheric
pressure.
Sep
SEP
Separates liquid products
from other reaction products.
Sep
SEP2
Separates
volatile products
from solid products.
yield
RYIELD1
This unit helps simulate
the decomposition of coffee residues into volatiles and intermediate
products.
Operates at atmospheric
pressure.
yield
RYIELD2
The unit helps simulate
different solid intermediate decompositions into volatiles and torrefied
biomass.
Operates at atmospheric
pressure.
Flow diagram of the coffee residue (SCG and CH) torrefaction
model
in Aspen Plus.Moreover,
it should be noted that a steady-state system was assumed
in all calculations, while the Redlich–Kwong–Soave equation
was used to simulate the fluid properties. Based on the flowsheet
in Figure , the
coffee residues (SCG and CH) are fed into a convective air dryer (DRYER)
operating for 12 h. The dryer was designed to mimic the feedstock
pre-drying steps and reduce the feedstock moisture content. Air at
ambient temperature (ARN) is compressed and sent to the dryer for
feedstock drying. The dried feedstock exiting the dryer is named as
DRIEDSCG. The dried coffee residues enter the heater (HEATER1), where
the temperature is elevated to a preheating temperature of 200 °C
before entering the torrefaction reactor.The torrefied unit
was modeled with unit operations comprising
the stoichiometric reactor, two RGibbs reactors, and two separator
blocks. The entire torrefaction system was modeled in three sequential
steps: initial heating and moisture removal, biomass decomposition,
and intermediate decomposition. The first step in the torrefaction
module is the moisture removal step. This step was modeled using the
stoichiometric reactor (DRY-REC) and solid/gas phase separator.[38] The mass balance calculations to ensure that
the final moisture content is the same as the torrefied solid were
solved with a calculator block with an embedded FORTRAN code. The
dried solid stream (S3) leaving the stoichiometric reactor is fed
to the solid separator, where the inbound moisture is separated from
the solids, after which the hot stream containing dried solids (DRYSCG)
is fed to the Ryield reactor (RYIED1). The two yield reactors were
used to model the intermediate steps during coffee residue torrefaction
based on eq . The two
yield blocks were selected for several reasons. The torrefaction kinetic
models assume that the reaction occurs in two-step processes, including
biomass decomposition into intermediates and volatiles. They are followed
by intermediate degradation to form the torrefied solids.[10] In addition, a similar approach was employed
in previous studies related to the development of biomass torrefaction
models.[39] The first yield reactor represents
the decomposition of coffee residues into volatile compounds and solid
intermediates, while the second reactor models the intermediate decomposition
into other volatile compounds and torrefied biomass samples. It is
vital to note that the density and enthalpy of the nonconventional
solid feedstock were determined by the specific property methods of
DCOALIGT and HCOALGEN, respectively.[37]It is essential
to mention that the developed torrefaction models
in this study apply to other types of coffee residues and might not
be helpful for lignocellulosic biomass or agricultural residues due
to the varying chemical compositions. The disparity in chemical compositions
could lead to different product distributions and torrefaction behavior.
Torrefaction Yield and Energy Efficiency
Calculations
The main parameters that are exploited to determine
the efficacy of the torrefaction process are the mass and energy yields.
They indicate how much dry mass is removed or lost during the torrefaction
process and the amount of energy retained in the torrefied biomass
samples (solid residues). The mass yield is calculated from eq as follows:where M, MTORR, and MRAW represent the mass yield, the mass of the
torrefied
biomass sample (solid residue), and the mass of raw feedstock (biomass),
respectively.The energy yield was determined from the expression
in eq as follows:where E, HHVTORR, and HHVRAW represent the
energy yield and higher heating values of torrefied biomass and raw
feedstock (biomass), respectively.The overall energy efficiency
of the process is defined as the
amount of energy imposed on the process through utilities. On the
contrary, the torrefaction energy yield determines the quantity of
energy present in the raw feedstock transferred to the torrefied solid
after torrefaction.[7] The process energy
efficiency (ηp) was calculated from the lower heating
value of the feedstock (LHVRAW) and torrefied biomass (LHVTORR) on a dry basis (eq ).[40]FRTORR and FRRAW are the mass flow rates
of the torrefied biomass samples and raw feedstock (biomass), respectively. E represents the overall energy required to sustain the
entire process.
Figure 1
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Figure 13