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Syngas production fromwood pellet using filtrationcombustion of lean natural gaseair mixtures

Karina Araus, Felipe Reyes, Mario Toledo*

Department of Mechanical Engineering, Universidad Tecnica Federico Santa Marıa, Av. Espana 1680,

Valparaıso, Chile

a r t i c l e i n f o

Article history:

Received 16 December 2013

Received in revised form

17 March 2014

Accepted 19 March 2014

Available online 16 April 2014

Keywords:

Hydrogen

Syngas production

Hybrid filtration combustion

Wood

Gasification

* Corresponding author.E-mail address: mario.toledo@usm.cl ( M

http://dx.doi.org/10.1016/j.ijhydene.2014.03.10360-3199/Copyright ª 2014, Hydrogen Ener

a b s t r a c t

A common method for the production of hydrogen and syngas is solid fuel gasification.

This paper discusses the experimental results obtained from the combustion of lean nat-

ural gaseair mixtures in a porous medium composed of aleatory alumina spheres and

wood pellets, called hybrid bed. Temperature, velocity, and chemical products (H2, CO, CO2,

CH4) of the combustion waves were recorded experimentally in an inert bed (baseline) and

hybrid bed (with a volume wood fraction of 50%), for equivalence ratios (4) from 0.3 to 1.0,

and a constant filtration velocity of 15 cm/s. Upstream, downstream and standing com-

bustion waves were observed for inert and hybrid bed. Themaximum hydrogen conversion

in hybrid filtration combustion is found to be w99% at 4 ¼ 0.3. Results demonstrate that

wood gasification process occurs with high temperature (1188 K) and oxygen available, and

the lean hybrid filtration process can be used to reform solid fuels into hydrogen and

syngas.

Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Currently hydrogen (H2) is considered the fuel of the future as

a possible replacement for hydrocarbons. It is a clean fuel,

primarily to its low level of contamination when is burned. It

more energy per unit mass than any other fuel and applica-

tions of H2 as an energy source include electricity via fuel cell

[1]. Also hydrogen has disadvantages as low energy content

per unit volume, stored as liquid or compressed forms re-

quires special and expensive infrastructure, and safety as-

pects. Hydrogen can be obtained from either renewable or

non-renewable sources. Reforming methane (CH4) with the

addition of water vapors is the industrial processmost utilized

and economically feasible for the H2 production [2,3].

ario Toledo).40gy Publications, LLC. Publ

However, carbon dioxide (CO2) is generated during its pro-

duction contributing to greenhouse gases and the subsequent

global warming. The main challenge competing energy

requirement and environmental protection is to find the bal-

ance between sustainable energy while reducing CO2 emis-

sions generated by fossil fuels. As such, biomass can be

considered an excellent alternative to the production of

energy.

Biomass is considered a renewable source of energy

with zero emissions of CO2 to the atmosphere. It is available

in a diverse array of forms and types; animal refuse, forestry

residues and agriculture waste. There are also a number

of technologies available for the production of H2, syngas,

electricity, and generation of heat using biomass as the

starting feedstock; these include combustion, gasification,

ished by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 8 1 9e7 8 2 57820

liquefaction, hydrolysis, super-critical conversion, and pyrol-

ysis [4e9]. However, some of these techniques are far from

being efficient. Lack of stable production of gases, limited

flexibility to operate a wide range of biomass, difficulties to

scale-up and low quality of obtained products are a few of the

disadvantages [5]. Recently, new technologies have been re-

ported to convert biomass into H2; for example, gasification

under super-critical conditions and catalytic partial oxidation,

techniques which are not commercially available [10e14].

For other hand, research has been devoted to the benefits

of combustion in packed reactors with a porousmatrix for the

production of H2, also known as filtration combustion.

Comparing this technique with conventional combustion in

an open flame, filtration combustion offers a wider power

range, higher efficiency, compact structure of higher energy

concentration per unit volume, stable combustion over a wide

range of equivalence ratios (4), due to the capacity of the

porous media to recirculate the heat within the reactor. This

method produces temperatures exceeding adiabatic values at

equilibrium, due to a large superficial area offered by the

porous media, which is responsible for heat transfer between

gaseous and inert solid phases [15e19]. Combining the bene-

fits of filtration combustion and addressing requirements of

sustainable energy and minimal environmental impact, an

excellent proposal is hybrid filtration by homogenous mixing

of inert porous media and a solid fuel [20]. Thus, converting

solid and gas fuels into energy or H2 and syngas simulta-

neously [21].

Salganskii et al. [20] modeled gasification in a fixed bed

formed by a mixture of coal with an inert component. The

results of the thermodynamic model of the exhaust-gas

composition should be interpreted as the upper estimate in

terms of the CO and H2 contents in the gaseous products. On

the other hand, Salgansky et al. [22] performed theoretical and

experimental studies on combustion of coal/inert mixtures

Fig. 1 e Schematic of the

with filtration of a gaseous oxidant. Experimental results

suggested that as the coal content in the bed exceeded 60%,

the volume concentration of CO and H2 were increased by

25%. The results of Salgansky et al. [22] demonstrated that

more studies are needed. Toledo et al. [21,23,24] have reported

a number of applications related to reforming gaseous and

solid fuels into H2 and syngas by using hybrid filtration com-

bustion technology. Particularly, for rich and ultra-rich com-

bustion of butane inside porous media composed of

homogeneous wood pellets and alumina spheres, they

observed that syngas yield in hybrid filtration combustion is

essentially higher than for butane filtration combustion in an

inert porous medium. Whereas rich and ultra-rich combus-

tion of natural gas in a porous medium composed of homo-

geneous coal particles and alumina spheres, they reported

that syngas yield in hybrid filtration combustion was found to

be higher than for the inert porous medium case. The

maximum H2 conversion was 55% for the hybrid coal and

alumina bed at a volumetric coal value of 75%. Wood and

Harris [25] have reviewed research on lean CH4 combustion in

porous burners, specifically on ultra-lean combustion, and the

advanced of premixed fueleair mixtures burning inside of an

inert bed. This technology is capable of burning low-calorific

value fuels and very lean fueleair mixtures that would not

normally be flammable, potentially allowing the exploitation

of what would otherwise be wasted energy resources. How-

ever, above researchers still indicates that further studies are

required for investigation on filtration combustion related to

lean combustion for gaseous and solid fuels.

In this work, the results on H2 and syngas production in

lean filtration combustion of natural gaseair mixtures in

packed beds formed by wood pellets and alumina spheres are

reported. Temperature profiles, combustion wave velocities,

and chemical products were measured experimentally for

beds with wood content of 50%. Particular interest is the

experimental setup.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 8 1 9e7 8 2 5 7821

concept and utilization of hybrid filtration combustion for

conversion of solid fuel to hydrogen and syngas.

Fig. 2 e Combustion temperature profiles depicted at

various propagation times at 4 [ 0.6. Two cases are

presented: A) upstream inert bed and B) upstream hybrid

bed.

Experimental apparatus and procedure

Fig. 1 shows the experimental apparatus, having three main

components: reactor, control system, and data logging. Lean

filtration combustion of natural gaseair mixtures was carried

out in a quartz tube 260 � 39 � 44 mm (length, ID, and OD,

respectively). The tube was packed with an equal volumetric

mixture of cylindrical wood pellets (5.6 � 5.6 mm, length � D)

and alumina spheres (Al2O3, 5.6 mm D). This hybrid bed has

40% porosity, occupying 80 mm of the length of the tube.

Wood pellets have a calorific value of 4406 kcal/kg, 0.6 g/ml of

apparent density, and 14.05% of moisture. Internal and

external surfaces of the tube were covered with thermal

insulation (Fiberfrax, 1.5 and 2.0 mm, respectively): thus,

allowing minimum heat loss and resulting to temperature

profiles almost uniformly, as well as protecting the tube from

excessive thermal expansion due to elevated temperatures.

Mass flow controllers (Aalborg, Orangeburg, USA) were

used to measure flow of natural gas (96% CH4) and air,

generated by an industrial air compressor (Qualitas, Florida,

USA). Reactants were mixed in a pre-reactor chamber, as-

suring a homogeneous gaseous mixture into the reactor.

Mixture is fed through the bottom of the burner and the

reactor is open to the atmosphere.

Combustion temperature data was collected by five S-type

(platinum/rhodium) thermocouples (T1eT5, see Fig. 1)

(OMEGA Engineering, Inc., Stamford, USA), shielded inside a

multi-bore ceramic tube running axially in the center of the

reactor, which provided temperature values in close prox-

imity to the solid phase. Voltages measured by the thermo-

couples were recorded by an OMB DAQ 54 acquisition module

and converted by the Personal DaqView Software (OMEGA

Engineering, Inc., Stamford, USA). The thermocouple junc-

tions were equally spaced at 4 cm interval along the length of

the shell, leaving 5 cm at each end of the reactor. Propagation

rates were obtained from thermocouple tracing. The experi-

mental error in the temperature measurements was esti-

mated at 50 K. Wave velocity measurements were based on

displacement of thermal profile along the reactor length,

having an error less than 10%.

Combustion products were sampled at the reactor’s exit by

using a ceramic tube, which is inserted 5 cm inside the packed

reactor (discrete sampled). The tube was connected to a vac-

uum line used as a sample probe. The exhausted gas was

filtered in a gas wash bottle and finally stored in Tedlar� bags

for quantitation. The sample gas was taken when the flame

front was positioned in the middle length of the reactor.

Concentrations of H2, CO, CH4, and CO2 were quantified

directly by gas chromatography (GC), using a slightly modified

method reported by Pedersen-Mjaanes et al. [26]. Briefly, a

Clarus 500 GC (PerkineElmer, Massachusetts, USA) fitted with

two packed S/S columns (HayeSep Q, 3 m � 1/8 in OD and

Molecular Sieve 5A, 1 m � 1/8 in OD, Supelco, New Jersey,

USA), one detector (TCD), and helium (Linde Gas Chile, S.A.) as

carrier gas. Errors in GC analysis were estimated to be 15%.

Before studying the combustionwave in the hybrid reactor,

a combustion wave in porous media with alumina spheres

(inert bed) only was studied, establishing validation and

baseline for the experimental procedure.

Using wood pellets in the packed reactor at 0 and 50%,

with 4 from 0.5 to 1.0, direction of the combustion wave

propagation was upstream (from T1 to T5). Ignition was

initiated at the reactor’s exit and propagation was recorded.

As the wave reached the reactor bottom, the flame was

turned off. However, using 4 of 0.4 and 0.3, in the presence/

absence of wood pellets, direction of the combustion wave

was downstream in propagation (from T5 to T1). Ignition

was initiated at position T5 (50 mm of inert packing, Fig. 1).

Once combustion progressed to position T4, reactor was

packed with hybrid bed until position T2 (solid particle and

alumina spheres were mixed aleatory before filtrate in the

reactor), and finally inert bed from T2 to the exit of the

reactor. The downstream propagation was recorded. As the

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 8 1 9e7 8 2 57822

wave reached the reactor exit, the flame was turned off. In

both cases (upstream and downstream propagation) the

alumina spheres were slowly shifted downwards occupying

the space of the wood pellets consumed by the moving

combustion wave. It was found that in both cases the wood

pellets were completely consumed through oxidation and

gasification. The operation of the reactor is a batch process.

Results and discussion

Experimental results are presented in terms of combustion

temperature, propagation rate, composition of combustion

products, and degree of conversion to H2 for mixtures of

natural gaseair in a packed reactor with a porousmedia either

inert (no wood pellets) or hybrid (50% wood pellets), within

equivalence ratio (4) range from 0.3 to 1.0, with a constant

velocity of filtration of 15 cm/s. Comparative data was

Fig. 3 e Combustion temperature profiles depicted at

various propagation times at 4 [ 0.4. Two cases are

presented: A) downstream inert bed and B) downstream

hybrid bed.

considered between measurements T2 to T4 in the packed

reactor.

Combustion temperature and propagation rate

Profiles obtained showed the potential use of leanmixtures to

produce simultaneously conversion of solid and gaseous fuel

to energy. Figs. 2 and 3 present the profile along the reactor (T1

to T5) for mixtures of natural gaseair with presence/absence

of wood pellets in the bed at various times of propagation

waves. The direction of propagation rate in the pack bed (inert

or hybrid), was upstream at 4¼ 0.6 and downstream at 4¼ 0.4.

Temperature peaks reached at T3 thermocouple in the

absence and presence of wood pellets were 1249 K and 1183 K

(Fig. 2, upstream propagation), and 1245 K and 1179 K (Fig. 3,

downstream propagation), respectively.

Experimental combustion temperature in a packed reactor

in either inert or hybrid media for natural gaseair mixtures

are presented in Fig. 4. Combustion temperature in the inert

bed reached a relatively constant value of 1275 K throughout

the tested 4 region. Temperatures obtained are basically in-

dependent from 4 or CH4 content, effect that is similar to

observed by Kennedy et al. [27]. However, temperatures

reached by hybrid bed were about 1188 K for 4 less than 0.8,

whereas temperatures diminished drastically for values (4)

above 0.8 (around 244 K). Drop in combustion temperature

suggest a change of kinetic mechanism due to reduce of ox-

ygen available.

Propagation velocities of combustion are presented in

Fig. 5. Downstream and upstream propagation velocities were

observed, primarily depending on equivalence ratio. Close to

extinction limit, flame propagation is noticed as downstream.

Starting at 4¼ 0.3, wave velocity decreaseswith an increase in

CH4, approaching zero at 4 ¼ 0.45 for inert bed and 4 ¼ 0.40 for

hybrid bed. A standing combustion wave is formed under

these experimental conditions. With further increase of the

CH4 amount, the regime of propagation changes to upstream.

The absolute value of velocity grows with the increase of 4 in

Fig. 4 e Combustion temperatures for inert and hybrid bed

as a function of equivalence ratios.

Fig. 5 e Combustion wave velocity for inert and hybrid bed

as a function of equivalence ratios.Fig. 7 e Carbon monoxide concentration of lean natural

gaseair mixtures for inert and hybrid bed.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 8 1 9e7 8 2 5 7823

the leanmixtures, reaching themaximumat 4¼ 1.0, using the

inert and hybrid bed.

Combustion products

The importance of this study is to evaluate the potential of

lean hybrid filtration combustion for the production of H2 and

syngas. Concentration of H2, CO, CH4, and CO2 measured

during the filtration combustion of natural gaseair with and

without wood pellets are presented as a function of 4 in Figs.

6e9, respectively. Specifically, for the inert bed and starting

from equivalence ratios below 1.0, complete combustion can

be achieved due to oxygen content in themixtures. Therefore,

in this regime the concentrations of products such as H2 and

CO appear in insignificant concentrations in the exhaust

gases, as natural gas does not through partial oxidation under

these experimental conditions. As an example, H2 concen-

tration remains null and independent from 4, for the range

Fig. 6 e Hydrogen concentration of lean natural gaseair

mixtures for inert and hybrid bed.

4 ¼ 0.3e4 ¼ 0.9, for natural gaseair mixtures in inert bed.

Increasing 4 ¼ 1.0, concentration of H2 reached 2.9% (Fig. 6),

similarly to reported by Toledo et al. [24]. For hybrid bed, H2

concentrations were independent from 4 for the range

0.3 < 4 < 0.8 reaching a constant value about 5.9%. Increasing

4 from 0.8 to 1.0, H2 concentration decreased from 6.8% to

1.5%, respectively (Fig. 6). The result shows that low com-

bustion temperatures difficulties H2 production in hybrid

filtration combustion.

Fig. 7 shows the CO concentrations as a function of 4

for natural gaseair mixtures in the inert and hybrid bed. For

inert bed the concentration of CO measured remains

almost constant (w0.2%) and independent from 4 tested at the

range 0.3 < 4 < 0.8. While incrementing 4 from 0.8 to 1.0, CO

concentration increased from 0.2 to 3.4%, respectively. In the

hybrid bed, the concentrations of CO measured experimen-

tally were slightly dependent of 4 and themaximumwas 1.4%

reached at 4 ¼ 1.0.

Fig. 8 e Methane concentration of lean natural gaseair

mixtures for inert and hybrid bed.

Fig. 9 e Carbon dioxide concentration of lean natural

gaseair mixtures for inert and hybrid bed.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 8 1 9e7 8 2 57824

Concentrations of CH4 are shown in Fig. 8 as a function of

equivalence ratio. Unburned CH4 is practically not measured

in the products starting from 4 ¼ 0.3 to 1.0 for natural gaseair

mixtures using the inert bed. Contrary, CH4 is slightly detected

using natural gaseair mixtures with the hybrid bed. Its con-

centration grows with increasing 4, achieving w2.6% for

4 ¼ 1.0. In fact, the filtration combustion of lean natural

gaseair mixtures under the experimental conditions tested is

capable of inducing chemical transformation of wood in the

mixtures with excess oxygen concentration, but this content

of oxygen is not sufficient to transformer all the CH4 content

in the mixture (unburned CH4 or incomplete combustion). In

comparison for the hybrid bed case the CH4 concentrations

increase for equivalence ratio of 0.9 and 1.0 (CH4 unreacted)

that suggests that CO concentrations is not higher than the

inert bed case.

Fig. 10 e Conversion degree of fuels to hydrogen for inert

and hybrid bed.

For natural gaseair mixtures using the inert bed the con-

centrations of CO2 increased with equivalence ratio (Fig. 9).

The maximum concentration of CO2 measured was w7.9% at

4 ¼ 0.9. For hybrid bed the maximum concentrations of CO2

were 7.8% and 8.5% which were reached at 4 ¼ 0.3 and 4 ¼ 0.8,

respectively.

Fig. 10 shows the degree of conversion of solid fuel to H2 for

natural gaseair mixtures in the inert and hybrid bed, as

function of the equivalence ratio. The yield is calculated using

the initial H2 content in the natural gas and wood pellets for

the case of the hybrid bed. The maximum yield recorded for

the hybrid bed was w99% at 4 ¼ 0.3. In fact, a lean wave is

capable of inducing chemical transformation of CH4 in mix-

tures with very high oxygen concentration, using inert bed,

whereas, in packed bed with wood and inert medium and

using leanwave prevailsmore the chemical transformation of

the wood than CH4 in the mixtures with very high oxygen

concentration. Thus, the results show that lean hybrid filtra-

tion can be used to reform solid fuels into H2 and syngas.

Furthermore, the hybrid filtration of lean burn offers advan-

tages where the energy content of the fuel is extremely low,

because the porous matrix can potentially provide high heat

recirculation.

Conclusions

Hydrogen and syngas production in lean filtration combustion

were studied experimentally for natural gaseair mixtures

using a packed bed with an inert medium and/or hybrid. The

hybrid bed was randomly arranged of wood pellets plus inert

medium, at volumetric wood content of 50%. The experi-

mental conditions testedwere at equivalence ratio (4) from 0.3

to 1.0, for a filtration velocity of 15 cm/s. The focus of research

was to analyze on combustion temperatures, combustion

wave velocities, the chemical products, and conversion of

solid fuel (wood) to H2 and syngas.

The combustion temperatures recorded for the inert bed

was practically independent of equivalence ratio (1275 K). For

hybrid bed the combustion temperature (1188 K) decreases at

4 > 0.8. Drop in combustion temperature suggest a change of

kinetic mechanism due to reduce of oxygen available.

Downstream and upstream wave propagation was

observed for lean natural gaseair mixtures. Close to the

extinction limit, downstream propagation was observed. The

velocity of the wave decreases with an increase of the CH4

concentration, approaching zero at 4 ¼ 0.45 for inert beds and

4 ¼ 0.40 for hybrid beds. A standing combustion wave is

formed under these experimental conditions. With further

increase of the CH4 amount, the regime of propagation

changes to upstream. The absolute value of velocity grows

with the increase of equivalence ratio in the lean mixtures,

reaching the maximum at 4 ¼ 1.0, using the reactor with and

without wood pellets.

The maximum degree of conversion of fuels to H2 for the

hybrid bed was w99% at 4 ¼ 0.3. A lean filtration wave is

capable of inducing chemical transformation of wood in

mixtures with very high oxygen concentration. The results

show that lean hybrid filtration combustion can be used to

reform wood into H2 and syngas.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 7 8 1 9e7 8 2 5 7825

Acknowledgments

The authors wish to acknowledge the support by the CON-

ICYT-Chile (FONDECYT 1121188).

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