Journal of Sustainable Bioenergy Systems, 2013, 3, 234-241 Published Online September 2013 (
Co-Gasification of Mesquite and Coal Blend in an Updraft
Fixed Bed Gasifier
Wei Chen1, Siva Sankar Thanapal1, Kal ya n Anna malai1, Robert James Ansley2, Mustafa Mirik2
1Department of Mechanical Engineering, Texas A&M University, College Station, USA
2Texas AgriLife Research, Vernon, USA
Received July 10, 2013; revised August 11, 2013; accepted August 30, 2013
Copyright © 2013 Wei Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In order to reduce the emission of NOx, SOx, and CO2 and mitigate the dependen ce on the fossil fuel, the use of renew-
able energy, especially the biomass energy, has been explored. Since most biomass fuels are hard to grind to the very
fine size, gasification is the preferred technology of energy conversion. Updraft fixed bed gasification using partial oxi-
dation process is adopted where air less than the stoichiometric quantity is admitted to oxidize the char to CO. The
temperature profile within th e b ed reveals a characteristic temperature peak. The results reveal a correlation between the
higher heating value (HHV) of producer gas and the peak gasification temperature (Tpeak). Coal with higher char content
(~45%) is blended with biomass of low char content (~20%) in order to produce high quality gas. In this study, the
Texas-based mesquite fuel was blended with Wyoming Power River Basin (PRB) coal with mass ratios of 100:0, 90:10,
and 80:20 and fired downward into the gasifier. It was found that at a given mesquite to the coal mass ratio, the peak
gasification temperature decreased with the increase in ER. With the increase of the coal ratio in the mesquite: coal
blend and the peak temperature increased significantly; more combustible gases such as CO, CH4 were generated at the
end of product gas, and the HHV of the product gas increased by 10% - 20%.
Keywords: Gasification; Bioenergy; Coal; Heating Value; Sustainability
1. Introduction
The utilization of wastes as a renewab le energy source in
a thermo-chemical process to generate electricity or heat
has been widely used. The US independent biomass en-
ergy industry today provides for the disposal of ap-
proximately 22 million tons/yr of solid biomass waste [1].
Brian et al. [2] reported that approximately 60% of bio-
mass energy consumption occurs in the forest products
industry in US. The forest produ cts industry produces its
own sources of biomass such as bark, sawdust, wood
scraps/shavings, and waste water treatment sludge. Nor-
mally, bio-chemical and thermo-chemical are the two
main ways to convert biomass into energy. Gasification
is a thermo-chemical process where a solid fuel was
converted into gaseous species through a series of
chemical reactions and physical transformation. Air,
steam, and pure oxygen are the three main gasifying me-
dia, although other agents like CO2 or H2 are also being
The gasifiers can generally be classified into two dif-
ferent types: fixed bed and fluidized bed gasifier. For
fixed bed gasifier, the flow velocity is low; there is a
grate at the bottom of the gasifier and ash was disposed
through the grate while the flow velocity is high and
there is no grate for the fluidized bed gasifier. The fixed
bed gasifier can be classified as updraft, downdraft, and
crossdraft. The updraft fixed bed gasifier is a counterflow
reactor in which fuel is fed into the top and the air or the
steam is supplied at the bottom. The ash was removed
through the grate. In a downdraft fixed bed gasifier, fuel
and gases both flow in the same direction. Fixed bed
gasifiers are well suited for small-scale applications
(Power < 10 MW) [3]. Fluidized bed gasifier usually has
a large scale size and is used for industrial applications.
For mesquite and coal co-gasification, the updraft fixed
bed gasifier is used since it is easy to construct and oper-
ate. The temperature of gas coming out from the updraft
gasifier is less than 200˚C [4].
2. Literature Review and Objective
Extensive studies have been carried out on the biomass
gasification using air, steam, or air-steam mixture as
opyright © 2013 SciRes. JSBS
W. CHEN ET AL. 235
gasification media. Kumabe et al. [5] carried out the co-
gasification experiments using Japanese cedar and Mu-
lias coal in a downdraft gasifier by using air and steam as
gasification media. It was found that with an increase in
the biomass ratio in the mixture, the H2 % decreased and
the CO2% increased while the CO % was independent of
the biomass ratio. A low biomass ratio led to the produc-
tion of a gas favorable for methanol and hydrocarbon
fuel synthesis, and a high biomass ratio led to the pro-
duction of a gas favorable for Dimethyl Ether (DME)
synthesis. The cold gas efficiency of the co-gasification
ranged from 65% to 85%.
Pan et al. [6] mixed the pine chips with black coal and
Sabero coal, in the ratio range of 0/100 - 100/0, respec-
tively. Experimental tests were carried out using air and
steam as gasification agent at gasification temperatures
of 840˚C - 910˚C and superficial fluidized gas velocities
of 0.7 - 1 .4 m/s using fludized bed gasifier. It was found
that the dry product gas heating value increases with in-
creasing blend ratio from 3700 to 4560 kJ/Nm3 for pine
chips/black coal, and from 40 00 to 4750 kJ/Nm3 for pine
chips/Sabero. Dry product gas yield raised with the in-
crease of the blend ratio from 1.80 to 3.20 Nm3/kg (pine
chips/black coal), and from 0.75 to1.75 Nm3/kg (pine
chips/Sabero coal), respectively. About 50% co-gasifi-
cation processes overall thermal efficiency can be achieved
for the two types of blend.
Lu et al. [7] studied the effect of the equivalence ratio
on the co-gasification of pine sawdust and bituminous
coal in a bubbling fluidized bed. It was found that when
blending fuel ratio is 50% - 50%, with ER increasing
from 0.2 to 0.28 the volume concentration of H2 rose
from 14.1% to 26.9%, and CO% decreased from 28.9%
to 21.8%. The CO2% showed an increasing tendency in
the range of ER, while those of CH4 and CnHm kept de-
creasing. The maximum of the lower heating value
(LHV), is about 7180 kJ/m3 when ER is 0.25. The gasi-
fication efficiency ranged from 44% - 53% and the car-
bon con- version rate was between74% to 76%.
Chen et al. [8] used the mesquite wood chips as feed-
stock for a fixed bed gasification experiment. It was
found that the HHV of the gas produced from the mes-
quite fuel decreased when equivalence ratio (ER) in-
creased from 2.7 to 4.2 and the HHV was in a range of
2400 kJ/Nm3 to 3500 kJ/Nm3.
Gerado et al. [9] used a mixture of dairy biomass (DB)
and Wyoming sub-bituminous coal (WYC) with a ratio
of 90:10 for co-gasification study in a 10 kW updraft
gasifier using air-steam as gasification media. Due to the
presence of higher amount of fixed carbon in the WYC,
the peak gasification temperature and the % of CO in the
end produced gas increased and the HHV of the producer
gas increased correspondingly. The HHV of the gases
varied from 3649 to 4793 kJ/ Nm3.
In these studies, the variation of HHV of the gases
produced and the gasification efficiency with ER and
mesquite: coal r atios were investig ated. It was also found
that the HHV of the product gas increased as coal % was
increased in the blends. In the current study, the Texas
based mesquite was blended with PRB coal for air gasifi-
cation in order to produce higher quality gas (i.e. in-
creased HV gas) and convert more volatile matter into
combustible gases (e.g. reduce the tar content in the
product gas due to higher Tpeak). The effect of the ER and
coal% in mesquite: coal blend (MCB) on the gasification
temperature, gas compositions, and HHV were investi-
gated. The main objective of this study was to use the
Texas based Mesquite and PRB coal blended fuel to pro-
duce higher quality gas (i.e. increased HV gas) and con-
vert more volatile matter into combustible gases (e.g.
reduce the tar content in the product gas due to higher
Tpeak) in an air gasification process. The effect of the ER
and coal percentage in mesquite: coal blend (MCB) on
the gasification temperature, gas compositions, and HHV
were investigated.
3. Sustainabilityof Mesquite
The sustainability of any energy source must satisfy the
following requirements: Abundance of energy sources,
maintaining integrity of environment including air, land
(soil) and water, renewability and affordability (i.e. low
cost)[10]. Most biomass fuels satisfy the requirement
including mesquite. The Mesquite (Prosopis glandulosa)
is a deciduous wood which can reach a height of 6 to 9 m
(20 to 30 ft), grows rapidly and furnish shade and wild-
life habitat where other trees will not grow [11]. It is an
extremely hardy, drought-tolerant plant growing on semi-
arid non-cultivated land s because it can draw water from
the water table through its long taproot and thus it can be
harvested nearly year round [11,12]. Depending upon
availability, mesquite can also use water in the upper part
of the ground. Mesquite trees have very strong regrowth
after top-kill damage [12]. Like many members of the
Legume Family, it fixes nitrogen in the soil where it
grows and therefore satisfies most of its nutrient needs
[13]. It is estimated that of the 21 M total h a of mesquite
in Texas alone [14,15] , about 20%, or 4.2 M ha, could
be harvested for bioenergy needs. At an average of 18
dry Mg/ha [12], this could amount to over 75 teragrams
(Tg) of total mass available. There is no planting, culti-
vation, irrigation and fertilization costs for this naturally
occurring, nitrogen-fixing species [12]. This species can
be used as feedstock to produce syngas and bio-oil in
small scale gasification units [4]. Since coal has higher
amount of char compared to mesquite, then the heat
value of gas produced could be enhanced by blending
small amount of coal with mesquite; such a process in-
creases the usage of gas produced from gasification of
Copyright © 2013 SciRes. JSBS
Copyright © 2013 SciRes. JSBS
mesquite, reduces the transportation cost of gas per GJ
and makes it more affordable.
4. Preparation of Solid Fuel
Mesquite trees used in this gasification study were 3 - 4
m tall and had multiple basal stems. Basal stem diameter
ranged from 5 - 15 cm. Tree ring counts indicated that
above ground portions of these trees were 15 to 35 years
old. Tree branches (5 - 10 cm diameter) were chain sawed
down and then passed through a Vermeer wood chipper.
Leaf and small twigs were removed from branches be-
fore chipping [8]. Chipped material was then passed
through a motorized sieve system to separate into differ-
ent particle sizes. No attempt was made to separate heart-
wood, sapwood and bark in either species. In this study,
the mesquite particles with size of 2 - 6 mm were se-
lected for gasification. At the time of harvest, the mois-
ture content of fresh cut wood was between 30% - 45%
[Jim Ansley unpublished data]. After chipping and siev-
ing process, the moisture content of the fuel declined to
10% - 20%. The moisture content of the mesquite fuel in
this study was in a range of 10% - 12%.
5. Experimental Facility and Procedure
The gasifier (72 cm tall) was divided into four sections
which are joined by using ring type flanges of 12.7 ×
35. 6 × 50 .8 mm (Figure 1). The gasifier was constructed
of castable alumina refractory tube. The inner and outer
diameters of tube are 13.9 cm and 24.5 cm, respectively.
The tube was surrounded by 4.45 cm insulating blanket
in order to minimize heat losses. The layer was then sur-
rounded by a steel outer tube with an inner diameter of
34.3 cm. An ash disposal system is installed to maintain
quasi-steady operation. A conical gyratory cast iron grate
drilled with large number of holes with diameter of 6.4
mm is coupled to a pneumatic vibrator of variable fre-
quency that vibrates the grate in order to dispose the ash
continuously from the bed. The rate of ash removal can
be controlled by changing the vibration frequency in the
vibrator [16].
At the beginning of the experiment, the empty bed was
preheated to 600˚C using a propane torch. After the tem-
perature reached 600˚C, the torch was turned off and
biomass samples were gradually added to the gasifier.
This addition continued until the bed height of the gasi-
fier reached 22 cm (8.5 in). Afterwards, the fuel port was
closed and air or the mixture of air and steam was sent
into this system at the desired rate. Because mesquite has
low ash content (<3%), the vibrator operated for <1
minute to dispose the ash from the plenum before it
reached steady state. Afterwards, the grate was vibrated
over a short period of 5 to 10 s to dispose of the ash,
maintain a constant bed height, and obtain a steady tem-
perature profile within the reactor. Air was used as the
source of oxygen for gasification. The desired ER can be
Figure 1. Gasification facility. Adapted from [16].
W. CHEN ET AL. 237
reached by varying the air flow rate. Fuel was fed at the
ted at 2 cm, 4 cm,
the composi-
ow the mesquite and PRB coal
dry, ash free basis, and thus the HHV of the PRB coal is
top of the gasifier while air was supplied from the bottom.
As fuels gasified in the reactor chamber, negative pres-
sure was maintained using vacuum fan in order to ex-
haust the gases from the gasifier. It took approximately
60 minutes for experiments to reach steady state, this
being when Tpeak varied lesser than 10˚C over the period
of 15 minutes and the location of the Tpeak remained at
the same position. Once the steady state condition was
achieved the gas analysis was started and the gas compo-
sitions were recorded for 8 - 10 minutes.
5.1. Temperature Measurement
Eight K type thermocouples were loca
7 cm, 10 cm, 13 cm, 20 cm, 24 cm, and 28 cm along the
gasifier axis to measure the temperature in the gasifica-
tion chamber during gasification process. The tempera-
ture was recorded every 60 seconds.
5.2. Gas Compositions Measurem
A mass spectrometer was used to measure
tion of the produced gases such as CO, CO2, N2, CH4,
C2H6, and H2. After the steady state condition was
reached within the reactor, the producer gas was ana-
lyzed for its composition. A small amount of gas was
supplied to the mass spectrometer by using a vacuum
pump. The gas first passed through a condenser to re-
move tar and condensable vapors, and then was passed
through a series of filters to capture particulates sus-
pended in the gas. Afterwards, a small amount of gas was
sent into the gas analyzer. The gas analyzer was pre cali-
brated using a standard mixture of gas (N2, CO, CO2, H2,
C2H6, and CH4) and an inert gas (Helium) once in every
three days in order to get accurate measurements [8].
6. Results and Discussion
6.1. Fuel Properties
Figures 2(a) and (b) sh
used for the gasification study. Table 1 presents the proxi-
mate and ultimate analyses of the mesquite and PRB
coal.The ultimate and proximate analysis of mesquite
which was used for the present study is shown i n Table 1.
It can be found that the mesquite fuel had very high
VM content (>80%) while volatile matter of the PRB
coal was less than 50% under DAF basis, which means
less gas would be liberated from PRB coal during the
gasification process. However, the FC (DAF basis) for
the Wyoming coal is significantly higher than that of
mesquite. Higher C element implies more C is available
to form the gas such as CO2, CO and CH4. It can be
found from Table 1 that the C/O atom ratio for the PRB
coal is 4.12 while it was only 1.48 for mesquite fuel on
much higher than that of mesquite. On a dry ash free
(DAF) basis the higher heating value (HHV) of mesquite
is 19,902 kJ/kg and the PRB coal has a HHV of 29,593
kJ/kg. Generally the HHV of the biomass is roughly 2/3
of HHV of coal.
(a) (b)
Figure 2. (a) Mesquite chips; (b) PRB coal.
Table 1. Mesqal proximate
and ultimate an
As received PRB Mesquite
uite fuel and Wyoming PRB co
alysis [17].
Moisture 12.2 10.86
Ash 7.39 3.19
Volatile M
Fixed car43.
HHV (kJ/kg) 23802019
Dry, Ash Fee (DAF)
Moisture 0
Ash 0 0
Volatile Matter 82.17
Fixed Carbon 53.17.
Car 55.
Empirula CH0.70N8S0.002 CH1.3582O0.5 .0122S0.0003
HHV of VM (kJ/kg)25921693
Carbon 60.
Oxygen 14.
Hydrogen 3.57 5.17
ogen 0.
Sulfur 0.
51 bon 75.
Oxygen 18.36 37.54
drogen 4.
ogen 1.
Sulfur 0.
ical form
0.02 0.1
77 0
HHV (kJ/kg) 29593 2
+lation HHVF = FCDAF × HVMDAF ×
HEstimated using the re
HVDAF [18]. DA FC + V
Copyright © 2013 SciRes. JSBS
6.2ture Prf the Mesqud
5 present the temperature profiles for the
peratsame trend. Along the axis of
. Temperaofile oite an
PRB Coal Blends
Figures 3-
mesquite and coal mixtures with ratio of 100:0, 90:1
and 80:20 at different ER. It was found that all the tem
ure profile share the
the gasifer, the temperature increased first and reached a
maximum value (above 3 - 5 cm from the grate) due to
the accumulation of the ash at the bottom, and then de-
creased. In addition, as ER increased the gasification
temperature decreased due to less air being supplied into
the gasifier.
Temp erature (
Distance above t he g rate
ER=2.7 ER=3.2
ER=3.7 ER=4.2
Figure 3. Temperature profile for mesquite:coal (100:0)
0510 15 20 25 30
T emperature (
Distance above the g rate (cm)
Figure 4. Temperature profile for mesquite:coal (90:10)
T emperature (
Distance abo ve the g rate (cm )
ER=2.7 ER=3.2
ER=3.7 ER=4.2
Figure 5. Temperature profile for mesquite:coal (80:20)
Figure 6 shows the Tpeak for the mesquite: coal blend
with ratios of 100: 0, 90:10, and 80:20. It was found that
Tpeak increased with the increase of the coal percentage in
the blend. The temperature increased from 989˚C to
1200˚C when PRB coal percentage increased from 0 to
20% at ER = 2.7. Since coal is a higher quality fuel, more
char is in the coal compared to woody biomass. In Table
1, the C/O ratio for the PRB coal is 4.12 while only 1.48
for mesquite fuel. Temperature is expected to be higher
for mesquite and coal blend due to higher amount of-
treleased from coal.
ause high gasification temperature
favors the formation of CO and H2 [9]. Lower ER im-
pplied into the gasifier which
6.3. Gas
The main gas compositions in the end product are CO,
CO2, CH4, H2, and N2. Figure 7 gives the gas composi-
tion of the producer gas as a function of ER for mesquite
gasification. Temperature is expected to be higher for
mesquite and coal blend due to higher amount of heat
released from coal. It can be seen that the concentration
of CO and H2 decreases with the increase of ER, while
CO2 content increases with the increase of ER. And the
amount of change in CH4 and C2H6 at different ER is
negligible. This is bec
plies that more air was su
promotes the oxidation of the carbon which results in
high temperature.
2.5 3.0 3.5 4.0 4.5
Tpeak (oC )
Mesquite :Coal (100:0)
Mesquite :Coal (90:10)
Mesquite :Coal (80:20)
Figure 6. Tpeak for the coal and mesquite blend ratio.
2.5 33.5 44.5
D ry b asi s m o le c o m p os i ti on
CH4 C2H6
Figure 7. Mesquite gas compositions (dry basis) vs. ER for
air. Adapted from [8].
Copyright © 2013 SciRes. JSBS
W. CHEN ET AL. 239
Figures 8 and 9 present the CO2 and CO concentration
(dry basis) under different coal: mesquite ratio. It was
found that the CO2 concentration decreased while CO
concentration increased when the PRB wt % increased in
the blend. This is because higher gasification temperature
favors the formation of the CO and decreases the per-
centage of CO2 [16]. The CO2 concentration was between
6% - 18% and the CO percentage was between 14% -
23% when ER decreased from 4.2 to 2.7.
Figure 10 presents the H2 and CH4 concentration with
blend ratio as a parameter. The solid and dotted lines
represents the H and CH percentage, respectively
2 4
was found that H2 mole faction was in a range of 2.5% -
4% and the CH mole fract
. It
4ion was between 0.7% - 2.5%.
Increase of the coal ratio in the mixture resulted in slight
increases of the CH4 concentration because more char
was available to react with H2 to form CH4.
6.4. HHV of Gas
When producer gas is used as a fuel in internal combus-
tion engines or other applications, the optimal gasifica-
tion conditions are those that yield the highest HHV and
have a high thermal efficiency. Figure11 gives HHV of
the gas from coal and mesquite mixture. It was found that
HHV increased with the increase of the coal:mesquite
ratio. From Figure 12, it was seen that HHV of gas pro-
duced from mesquite gasification is between 2000 - 3000
kJ/Nm3;It increased to 2900 - 3600 kJ/Nm3(20% to 45%
C O2 m ol e f rac ti o
basis %)
n (D ry
me squite:c oal(100:0)
Me squite:coal(90 : 10)
mesquite: coa l (80:2 0)
Figure 8. CO2 concentration for different mesquite and coal
2.5 33.5 44.5
CO m ole fr acti o n (d ry
mesquite:coa l (90 :1 0)
mesquite :co al (80:20 )
Figure 9. CO concentration for different mesquite and coal
CH4 and H2 Mole percenta ge
ER .5
me squite:coal (0:100)-CH4me squite:coal (90:10)-CH4
me squite:coal (80:20)-CH4me squite:coal (100:0)-H2
me squite:coal (90:10)-H2me squite:coal (80:20)-H2
Figure 10. H2 and CH4 concentration for different mesquite
and coal blend.
2.5 33.5 44.5
HHV (kJ/Nm
mesquit e : c oa l (10 0: 0)
mesquite: coal (90:10)
mesquite: coal (80:20)
Figure 11. HHV for the PRB and mesquite mixture gas.
H HV = 10. 7Tpe ak -7804 . 3
H HV = 3. 9Tpea k -5 12 . 1 7
HHV= 2.1Tpea k + 136 5 .8
9009501000 1050 1100 1150 1200
HHV (kJ/Nm3)
mesquite: coal (100 :0)
mesquite: coal (90:1 0)
mesquite: coal (80:2 0)
Figure12. HHV vsTpeak for mesquite andcoal blend gasifica-
d) with 10% PRB coal in the mixture;HHV of
gas can increase up to 4000 kJ/m3at ER = 2.7 with 20%
PRB coal in the mixture. The low HHV of gas is due to a
high % of N2 originating from air.
As discussed before, with an increase in the coal % in
the blend, Tpeak and gas HHV increased.In order to obtain
the correlation between the Tpeak and HHV, Tpeak vs.
HHV was plotted in Figure 12. It was seen that higher
Tpeak would result in increase of HHV of the producer gas.
It is because more combustible gases such as CH4 and
CO were produced under th e high gasification peak tem-
Copyright © 2013 SciRes. JSBS
Copyright © 2013 SciRes. JSBS
this study, PRB coal was blended with mesquite for air
product gases. When the coal wt. % increa
end product gases contained 1
% CO, 2.5% - 4% H, and 0.7% -
ing released
omass and Coal-Dairy
eam as Oxidizer,” PhD Disser-
mples,” Energy, Vol. 41, No. 1, 2012, pp. 454-
461. doi:10.1016/
7. Conclusions
gasification. The gasification temperature, gas composi-
tions, and gas HHV were regarded as functions of mes-
quite, and co al ratios were investigated. The results of the
experiments are summarized as foll
1) When PRB coal was mixed with mesquite fuels at
ratios of 10:90 and 20:80 for gasification experiment,
Tpeak increased significantly due to the higher HHV of the
PRB coal. The Tpeak of coal and mesquite blend was
1200˚C at ER = 2.7 and blend rati o 2 0:80.
2) For Co-gasification, Tpeak increased with the in-
crease of the PRB coal percentage, and CO2 concentra-
tion decreased while CO and CH percentage increased at
the end of sed
4% -
from 0 to 20%, the
23% CO, 6% - 182 2
2.5% CH4.
3) The HHV of gases from the mesquite and PRB coal
blend increased with the coal% in the mixture due to the
higher peak temperature and carbon content in the coal
which resulted in more combustible gas be.
e higher the gasification temperature is, the higher will
be the HHV of the gases. The HHV increased around
20% when the coal percentage was 10% in the blend.
8. Acknowledgements
The authors wish to acknowledge the financial support
from Texas AgriLife Research State Bioenergy Initiative
Funding and US Department of Energy-NREL, Golden,
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W. CHEN ET AL. 241
CO: Carbon Monoxide
HHV: Higher Heating Val ue
C2: Carbon Dioxide
H4: Methane
2H6: Ethane
AF: Dry, Ash Free Basis
ME: Dimethyl Ether
R: Equivalence Ratio
C: Fixed Carbon
LHV: Lower Heating Value
MCB: Mesquite to Coal Blend
N2: Nitrogen
O2: Oxygen
PRB: Powder River Bas
Tpeak: Peak Gasification
VM: Volatile Matte
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