Co-Gasification of Mesquite and Coal Blend in an Updraft Fixed Bed Gasifier

doi:10.4236/jsbs.2013.33032

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Journal of Sustainable Bioenergy Systems, 2013, 3, 234-241

http://dx.doi.org/10.4236/jsbs.2013.33032 Published Online September 2013 (http://www.scirp.org/journal/jsbs)

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

Email: [email protected]

Received July 10, 2013; revised August 11, 2013; accepted August 30, 2013

permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

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

studied.

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

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

W. CHEN ET AL.

236

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,

ent

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.

Nitr

HHV (kJ/kg) 23802019

Dry, Ash Fee (DAF)

Moisture 0

Ash 0 0

Volatile Matter 82.17

Fixed Carbon 53.17.

Car 55.

Nitr

Empirula CH0.70N8S0.002 CH1.3582O0.5 .0122S0.0003

HHV of VM (kJ/kg)25921693

1990

atter

bon

37.7

70.63

15.32

Carbon 60.

Oxygen 14.

47.7

32.28

Hydrogen 3.57 5.17

ogen 0.

Sulfur 0.

0.72

0.08

46.34

51 bon 75.

Oxygen 18.36 37.54

drogen 4.

ogen 1.

6.02

0.84

Sulfur 0.

ical form

0.09

0.02 0.1

77 0

HHV (kJ/kg) 29593 2

+lation HHVF = FCDAF × HVMDAF ×

HEstimated using the re

HVDAF [18]. DA FC + V

W. CHEN ET AL.

238

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.

200

400

600

800

1000

1200

0102030

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)

mixture.

200

400

600

800

1000

1200

0510 15 20 25 30

T emperature (

Distance above the g rate (cm)

ER=4.2ER=3.7

ER=3.2ER=2.7

Figure 4. Temperature profile for mesquite:coal (90:10)

mixture.

200

400

600

800

1000

1200

0102030

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)

mixture.

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.

Composition

ause high gasification temperature

favors the formation of CO and H2 [9]. Lower ER im-

pplied into the gasifier which

hea

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.

800

900

1000

1100

1200

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

(%)

CO2 CO H2

CH4 C2H6

Figure 7. Mesquite gas compositions (dry basis) vs. ER for

air. Adapted from [8].

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%

2.533.544.5

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

blend.

2.5 33.5 44.5

CO m ole fr acti o n (d ry

basis%)

mesquite:coal(100:0)

mesquite:coa l (90 :1 0)

mesquite :co al (80:20 )

Figure 9. CO concentration for different mesquite and coal

blend.

0.8

1.8

2.8

3.8

4.8

2.533.544

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

(100:0)

Figure 10. H2 and CH4 concentration for different mesquite

and coal blend.

2000

3000

4000

5000

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

1500

2000

2500

3000

3500

4000

4500

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-

perature.

on.

increase

W. CHEN ET AL.

240

this study, PRB coal was blended with mesquite for air

ows:

product gases. When the coal wt. % increa

end product gases contained 1

% CO, 2.5% - 4% H, and 0.7% -

ing released

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7. Conclusions

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tions, and gas HHV were regarded as functions of mes-

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1) When PRB coal was mixed with mesquite fuels at

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8. Acknowledgements

The authors wish to acknowledge the financial support

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W. CHEN ET AL. 241

Nomenclature

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

Temperature

LHV: Lower Heating Value

MCB: Mesquite to Coal Blend

N2: Nitrogen

O2: Oxygen

PRB: Powder River Bas

Tpeak: Peak Gasification

VM: Volatile Matte