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ENGINEERING FOR RURAL DEVELOPMENT Jelgava, 20.-22.05.2020.
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INFLUENCE OF MW PRETREATMENT ON MAIN GASIFICATION/COMBUSTION
CHARACTERISTICS OF DIFFERENT TYPES OF BIOMASS
Inesa Barmina, Raimonds Valdmanis, Maija Zake
Institute of Physics, University of Latvia, Latvia
inesa.barmina@lu.lv, raimonds.valdmanis@lu.lv, maija.zake@lu.lv
Abstract. Microwave (mw) pre-treatment of biomass pellets (wood, straw and peat) with 2.45 GHz frequency is
provided with the aim to break down the structure of lignocellulosic pellets, activate their thermal
decomposition, formation of combustible volatiles (CO, H
2
) and improve the gasification/combustion
characteristics. Gasification/combustion characteristics of mw-activated pellets are tested using a pilot device,
which combines a biomass gasifier and a combustor with a heat output up to 5 kW. The complex research of the
main gasification/combustion characteristics for raw and mw pre-treated biomass samples includes the time
dependent measurements of the weight loss of biomass pellets during thermal decomposition, flame temperature,
produced heat energy per mass of burned pellets, combustion efficiency and the composition of the products.
Kinetic study of the weight loss rate of pre-treated pellets and composition of volatiles at the outlet of the gasifier
confirm that mw pre-treatment of pellets activates the thermal decomposition of the main constituents of biomass
(hemicellulose, cellulose and lignin) intensifying the axial flow of combustible volatiles (CO, H
2
, C
x
H
y
) at the
flame base promoting their faster ignition and burnout, which depends on the duration of mw pre-treatment of
biomass pellets. The enhanced thermal decomposition of mw pre-treated pellets correlates with increase of the
flame temperature during the self-sustaining combustion of volatiles with correlating increase of the total amount
of produced heat energy per mass of burned pellets by 18-23 % and the produced heat output from the device by
~ 24-33 %, indicating improved combustion of volatiles. The enhanced burnout of activated pellets is confirmed
by a decrease of the air excess ratio in the flame reaction zone with a correlating increase of the average value of
the volume fraction of CO
2
in the products by about 7–12 %.
Keywords: biomass, pellets, microwave pre-treatment, gasification, combustion characteristics.
Introduction
In order to limit the Earth’s climate change from combustion of fossil fuels (natural gas, coal, oil),
more efficient use of carbon neutral renewable fuels (forestry and agricultural residues) for heat
production is becoming more relevant in Europe and the world [1; 2]. However, the use of renewable
sources, especially agricultural or herbaceous residues, for energy production is limited by their low
calorific value, high moisture, nitrogen and ash contents in biomass [3]. Combined processes of
biomass thermochemical conversion are developed to improve the main combustion characteristics
and the composition of the products by co-firing of problematic biomass fuels (agricultural or
herbaceous residues) with wood [3; 4] or fossil fuels (gaseous, coal or peat) [5-7]. Moreover, biomass
characteristics can be improved by washing, drying, granulation or conversion of biomass into biogas
or bioethanol. The results of previous studies suggest that efficient improvement of biomass
combustion characteristics can be achieved by microwave pre-processing of biomass residues [8-12],
based on the dielectric heating of dipolar molecules of lignocellulose biomass. Vibration of dipolar
molecules in electromagnetic field and their thermal heating leads to partial destruction of
hemicellulose, cellulose in biomass and promotes faster biomass thermal decomposition [10; 11],
increasing the yield of gaseous and solid products with enhanced char conversion, while limiting the
formation of liquid components [12]. The surface area of the produced char is larger by mw
pretreatment of biomass and almost half of biomass can be converted into combustible gases (H
2
, CH
4
,
CO, CO
2
), increasing the yield of the produced bioenergy [12]. The previous investigations of the
effect of mw pre-treatment of straw, wood or peat pellets and their mixtures on their thermal
decomposition [13; 14] confirm that mw pre-treatment of these pellets partially reduces the moisture
content of the biomass and breaks down the structure of the biomass constituents (hemicelluloses,
cellulose and lignin) determining enhanced formation of combustible volatiles in the gasification zone,
completing combustion of volatiles in the flame reaction zone and increasing the heat output from the
device. Based on the experience gained in the previous research, the main aim of this study is to assess
the effect of mw pre-treatment of wood, straw and peat pellets to achieve the optimal combustion
conditions in the flame reaction zone with improved combustion efficiency and the optimal heat
output from the device by varying the duration of microwave pre-treatment using the experimental
setup with heat output up to 5 kW.
DOI:10.22616/ERDev.2020.19.TF020
ENGINEERING FOR RURAL DEVELOPMENT Jelgava, 20.-22.05.2020.
89
Materials and methods of the research
The effect of mw pre-treatment on thermochemical conversion of biomass pellets (wood, peat,
wheat straw) was studied using the small-scale periodic operation experimental setup with a heat
output up to 5 kW and the previously developed methodology of experimental measurements [7]. The
main components of the setup: a biomass gasifier, two vertical and one T-shaped water-cooled
sections of the combustor with the inner diameter D = 88 mm. The total length of the setup
L = 1220 mm. The mw pre-treatment of wood, straw and peat pellets was carried out at a frequency
2.45 GHz using a microwave reactor with heat output 700 W. The duration of the mw pre-processing
of pellets was varied in a range from t = 120 to t = 300 s, by varying the total amount of mw energy
consumed during the mw pre-treatment of pellets in the range from 0.17 to 0.22 MJ·kg
-1
for wood, in
the range from 0.20 to 0.44 for straw and from 0.16 up to 0.37 for peat. The mw pre-processing of
biomass results in the weight loss of pellets depending on the duration of mw pre-processing and the
energy consumed during the mw pre-treatment. The measurements of the weight loss rate of pellets
suggest that for equal durations of mw pre-treatment of pellets (t = 220 s) the weight loss of pellets is
influenced by their elemental composition. The fastest and the highest weight loss (∆m, %) is observed
at mw pre-treatment of straw pellets (up to 25 %), which predominately refers to exothermal
decomposition of hemicellulose, developing at temperature about 570 K [15]. The weight loss for
wood pellets during their endothermal decomposition of hemicellulose at T ≈ 600 K reaches 12 %,
while for peat pellets with reduced contents of hemicellulose and cellulose the weight loss rate
decreases to 9 % (Fig. 1).
Fig. 1. Effect of mw pre-treatment duration on the mass loss of biomass pellets
To estimate the effect of mw pre-processing of pellets on their thermochemical conversion, the
untreated or mw pre-treated biomass pellets are filled into the gasifier, by limiting the height of the
biomass layer for all biomass samples to 120 mm. Because of different mass density of pellets, the
average mass of wood pellets filled in the gasifier was varied from 500 g for untreated pellets to 480-
420 g for mw pre-treated pellets. The total mass of straw pellets was varied from 450 g for untreated
pellets to 380-340 g for mw pre-treated pellets and for peat – from 570 g to 530-520 g.
To initiate the thermochemical conversion of pre-treated pellets, the propane flame with heat
output 0.89 kW was supplied into the upper part of the biomass layer and was switched off after
ignition of volatiles – t = 400 s for wood and straw pellets, and t = 500 s for peat pellets. To sustain the
biomass thermal decomposition (gasification) and formation of volatiles (CO, H
2
, C
x
H
y
), primary air
was supplied to the lower part of the gasifier at constant rate 30 l·min
-1
and average air excess α ≈ 0.3-
0.5. To sustain the thermochemical conversion of biomass pellets, the secondary swirling air at
constant rate 40 l· min
-1
was supplied into the outlet of the gasifier using 8 tangential nozzles with
d = 3.3 mm. The average air excess rate in the combustion zone achieved α ≈ 1.1-1.2.
All the sections of the combustor are equipped with special holes for introduction of diagnostic
tools into the flame and to provide on-line measurements of the main gasification/combustion
characteristics. The composition of volatiles (H
2
, CO) from gas samples (50 ml) was extracted at the
Δ
m, %
ENGINEERING FOR RURAL DEVELOPMENT Jelgava, 20.-22.05.2020.
90
gasifier outlet (L/D = -0.9 from secondary air supply) and analyzed by a Testo 350 gas analyzer with
an accuracy ± 5 %. The flame axial (u) and tangential (w) velocity profiles were measured with the
Pitot tube and a Testo 435 flow meter at the distance L/D = 1.6 from secondary air supply with
accuracy ± 1 %. The temperature of the flame was measured using the Pt/Pt-Rh thermocouple at the
distance L/D = 3.2 with an accuracy ± 1 %. The composition of emissions (O
2
, CO
2
, NO
x
, CO, H
2
,),
temperature, air excess and the combustion efficiency were measured at the combustor exit (L/D = 11)
using a gas analyser Testo 350. The measurements of time-dependent variations of the biomass height
(dL/dt) in the gasifier with estimation of the biomass weight loss rate (dm/dt) were controlled by a
moving rod supplied with a pointer (accuracy ± 2 %). The heat production rates from the gasifier
(Pgas), from the vertical primary section of the combustor (P-1) and from the vertical secondary and
T-shaped sections of the combustor (P-2) were measured from the calorimetric measurements of
cooling water mass flows from each section and the temperatures, which were measured by AD 590
thermo-sensors (accuracy ± 2.5 %) with data registrations by Data Translation DT 9805 data
acquisition module.
Experimental results
As mentioned above, mw pre-treatment of biomass pellets promotes the breakdown of
hemicellulose and cellulose structures with different contents for wheat straw, wood and peat biomass
pellets. The higher average content of hemicelluloses is observed for wheat straw samples (21÷28 %),
while reduced content of hemicelluloses is observed for wood (23÷25 %) and peat (10÷25 %).
Moreover, the wood biomass has higher content of cellulose (41÷43 %), with reduced content of
cellulose for straw (24÷35 %) and peat (0÷20 %), while the highest lignin content is observed for peat
biomass (up to 40 %) in comparison with wood (28÷29 %) and straw (16÷20 %) [11]. Therefore, as
follows form Figure 1, higher mass loss during mw pre-processing is observed for straw and wood
pellets. The differences in the chemical and elemental composition of the biomass pellets significantly
influence the thermal decomposition of pellets.
The comparative analysis of the weight loss rate (dm/dt) for untreated (mw = 0) wood, straw and
peat samples has shown that during the primary stage of thermal decomposition t ≈ 300-800 s (min-
max), when mainly endothermic release of light volatiles and water occurs, the highest average weight
loss rate is observed for wood pellets from 0.07 to 0.31 g·s
-1
and for straw pellets from 0.04 to
0.28 g·s
-1
, the peat pellets with smaller content of hemicellulose and cellulose have less weight loss
rate from 0.02 to 0.18 g· s
-1
(Fig. 2, a). During the self-sustaining combustion for all type pellets the
average weight loss rate reaches 0.17-0.22 g·s
-1
(max-end). The peat biomass with higher content of
lignin is more resistant to thermal decomposition of pellets with the maximum value of weight loss
rate (up to 0.39 g·s
-1
) during the char conversion stage (t ≈ 2000-2750 s). The char conversion stage
for straw and wood pellets occurred at t ≈ 1900-2300 s with maximum of the weight loss rate for wood
up to 0.36 g·s
-1
and for straw – up to 0.30 g·s
-1
(Fig. 2, a).
The mw pre-treatment of pellets with partial destruction of biomass structure leads to faster
ignition and decomposition of biomass with increase of the weight loss rate and correlating decrease
of the air excess ratio in the flow during primary (min-max) and self-sustaining combustion (max-end)
stages (Fig. 2, a, c-d). As can be seen from the Fig. 2, c-d, the dependence of the mw pre-treatment
duration of pellets on the weight loss and air excess rates for straw, wood and peat is different. For
wheat straw pellets the minimum of air excess rate α ≈ 0.97 and maximum average weight loss rate
dm/dt = 0.27 g·s
-1
is observed at mw pre-treated duration 140 s, for wood pellets the minimum of air
excess rate α ≈ 0.65 and maximum average weight loss rate 0.4 g· s
-1
is observed at duration of mw
pre-treatment 240 s, accordingly for peat pellets minimum of air excess α ≈ 0.8 and maximum average
weight loss rate 0.28 g·s
-1
is observed at t = 240÷300 s. Besides, increasing of the mass loss rate
during thermal decomposition of mw pre-treated pellets is accompanied by intensive formation of
volatiles (CO, H
2
) at the outlet of the gasifier (Fig. 2, b).
The intensive formation of volatilies in the gasifier during thermal decomposition of mw pre-
treated pellets shows the influence on the flow dynamics. Increasing the axial flow of volatiles at
thermal decomposition of pretreated pellets promotes a decrease of the swirl intensity near the walls of
the combustor (Fig. 3, a). This suggests intensive mixing of the axial flow of volatilies with swirling
airflow completing combustion of volatiles and increasing the efficiency of energy production. As a
ENGINEERING FOR RURAL DEVELOPMENT Jelgava, 20.-22.05.2020.
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result, for all types of mw pre-treated pellets the total heat output from the device increases: at
thermochemical conversion of pretreated straw pellets the heat output increases by about 33 %, for
wood by 26 % and for peat by 24 % (Fig. 3, b). Moreover, the produced heat energy per mass of mw
pre-treated straw pellets increases by 23 %, wood pellets – by 18 % and peat pellets – by 23 % (Fig. 3,
b). In addition, the results of the previous research have shown [11, 12] that mw pre-treatment of
lignocellulosic biomass promotes increase of the heating values (LHV, HHV) and carbon content in
the biomass. As a result, increased carbon content in mw pre-treated biomass, faster and higher
thermal decomposition of pretreated pellets with intensive formation of volatiles are the main factors
promoting enhanced formation of the main product (CO
2
) during primary and self-sustaining
combustion stages (Fig. 3, c).
Fig. 2. Effect of straw, wood and peat pellet mw pre-treatment duration on the weight loss rates
(a, c, d), air excess rates (c, d) and formation of volatiles (b) during thermal
decomposition of pellets
The average value of CO
2
emission in products is growing by 12 % for mw pre-treated straw
pellets, by 9 % and 7 % respectivelly for wood and peat pellets. However, as a negative result should
be noticed an increase of the mass fraction of CO in the products (Fig. 3, d), which is mainly the result
of intensive thermal decomposition of pre-treated pellets, decreasing the air excess in the flame
reaction zone and determining the formation of fuel-rich conditions with α = 0.7-0.8 for wood and peat
pellets (Fig. 2, c-d). With the aim to improve the combustion of pre-treated pellets and reduction of
CO emission in the products, it is necessary to increase the secondary swirling air supply, which is
confirmed by the preliminary results of the experimental study, when increasing the secondary air
suplly up to 60 l· min
-1
allows to reduce the mass fraction of CO emissions with correlating increase of
c
d
dm/dt, g·s
-
1
CO, g·m
-
3
dm/dt, g·s
-
1
dm/dt, g·s
-
1
α
sum
α
sum
ENGINEERING FOR RURAL DEVELOPMENT Jelgava, 20.-22.05.2020.
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the air excess ratio in the flame reaction zone and the CO
2
volume fraction in the products. It should
be noticed that mw-pretreatment of pellets contributes to decrease of NO
x
emissions in the products by
about 35 %. Currently the reson for the NO
x
decrease is unclear and will be futher investigated.
Fig. 3. Effect of mw pre-treatment duration of pellets on the tangential flow velocity (a),
produced heat output from the setup and produced heat per mass of burned pellets (b); on
formation of volume fraction of CO
2
(c) and mass fraction of CO (d) in the products during
thermal decomposition of straw, wood and peat pellets
Conclusions
From the experimental results presented above it follows that mw pre-treatment of straw, wood
and peat pellets at different duration results in:
1. Partial destruction of the biomass structure promoting faster decomposition of pre-treated pellets
with enhanced formation, ignition and burnout of combustible volatiles (CO, H
2
).
2. The enhanced formation and burnout of volatiles contributes to noticeable increase of the total
heat output from the device and the produced heat energy per mass of burned biomass (for straw
by 23 %, for wood by 18 % and for peat by 23 %). Besides, the average value of CO
2
emission in
products increases by 12 % for straw pellets, by 9 % for wood pellets and 7 % for peat pellets.
Moreover, during combustion of mw pre-treated straw pellets NOx emissions in the products
decrease up to 35 %.
b
b
w, m·s
-
1
Power, kW
Heat energy, MJ·kg
-
1
r/R
ENGINEERING FOR RURAL DEVELOPMENT Jelgava, 20.-22.05.2020.
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3. To reduce the mass fraction of CO emission in the products at thermochemical conversion of pre-
treated pellets, it is necessary to control the air supply in the combustion zone.
4. The mw pre-treatment of biomass is an effective tool for cleaner and effective energy production.
Acknowledgements
The authors would like to express their gratitude for financial support from the European
Regional Funding for Project SAM 1.1.1.1. /19/A/010.
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