Bioresource Technology 101 (2010) 6381–6388

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Bioresource Technology

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Comparative study of thermochemical processes for hydrogen productionfrom biomass fuels

Enrico Biagini a,*, Lorenzo Masoni a, Leonardo Tognotti b

a Consorzio Pisa Ricerche, Divisione Energia Ambiente, Lungarno Mediceo 40, Pisa, Italyb Università di Pisa, Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Via Diotisalvi 2, Pisa, Italy

a r t i c l e i n f o

Article history:Received 31 December 2009Received in revised form 8 March 2010Accepted 11 March 2010Available online 1 April 2010

Keywords:HydrogenBiomassRenewable energyGasificationElectrolysis

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.03.052

* Corresponding author. Tel.: +39 0502217850; faxE-mail address: e.biagini@cpr.it (E. Biagini).

a b s t r a c t

Different thermochemical configurations (gasification, combustion, electrolysis and syngas separation)are studied for producing hydrogen from biomass fuels. The aim is to provide data for the production unitand the following optimization of the ‘‘hydrogen chain” (from energy source selection to hydrogen utili-zation) in the frame of the Italian project ‘‘Filiera Idrogeno”. The project focuses on a regional scale (Tus-cany, Italy), renewable energies and automotive hydrogen. Decentred and small production plants arerequired to solve the logistic problems of biomass supply and meet the limited hydrogen infrastructures.Different options (gasification with air, oxygen or steam/oxygen mixtures, combustion, electrolysis) andconditions (varying the ratios of biomass and gas input) are studied by developing process models withuniform hypothesis to compare the results. Results obtained in this work concern the operating param-eters, process efficiencies, material and energetic needs and are fundamental to optimize the entirehydrogen chain.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

This work is a preliminary output of the ‘‘Filiera Idrogeno”(hydrogen chain) project. This is a multidisciplinary project joiningresearchers, engineers, economists, that study all steps and possi-ble synergetic effects in the hydrogen chain, from the energysource selection, to hydrogen production, storage, distribution, uti-lization, with safety, economic, social and environmental evalua-tions on a regional scale (Tuscany, Italy). General objects of theproject are the use of renewable energy sources and the utilizationof hydrogen as an automotive fuel.

Among renewable energies available in Tuscany (solar, wind,geothermal, biomasses), biomass fuels can be used for hydrogenproduction in different processes and conditions. Biomass fuelsrepresent a renewable energy source, can substitute fossil fuelsand reduce the CO2 emissions. The inter-linkages with other tech-nological (thermo-chemical conversion options, biotechnology,agronomic, etc.) and policy areas (climate, energy, agricultureand waste policy) and important sectors of the economy (agricul-ture, forestry, food processing, paper and pulp, building materials)give bio-energy many opportunities to generate multiple benefitsapart from energy generation (Faaij, 2006).

As major obstacles, biomasses have a low energy density, a rel-atively high cost and a limited (from a geographical and temporalpoint of view) availability. The production of hydrogen as an en-

ll rights reserved.

: +39 050931640.

ergy carrier may represent a successful option for converting theenergy content of biomasses into a more practical and clean gas-eous fuel. Many processes are studied for producing hydrogen frombiomasses of different origin (Ni et al., 2006; Navarro et al., 2007;Cantrell et al., 2008; Saxena et al., 2008). Among them, thermo-chemical processes (combustion, gasification, pyrolysis) are themost promising and applied solutions for ‘‘second generationfuels” (Chum and Overend, 2001; Ptasinski, 2008).

Comparative process studies are desired for providing data onthe hydrogen production unit to be inserted in the comprehensivestudy of the chain. In this work, different process configurationsand operating conditions are studied. Small size plants, locatednear the biomass production sites or where logistics (collection,transport, storage) are convenient, are recommended for a regionalcontext. Indeed, small-scale hydrogen generation systems fromvarious hydrocarbon feedstocks are required due to the limitedexisting hydrogen infrastructures, for automotive fuels as well asfor distributed cogeneration (in fuel cells, see for instance Seo etal., 2006, and internal combustion engines, White et al., 2006).

2. Description of processes

Different process configurations are studied for producinghydrogen. The main characteristics of each one are listed inTable 1. The schemes are shown in Fig. 1. In all cases the samebiomass (poplar wood) and its flow rate are fixed. In the firstprocess the biomass is gasified in air and the produced syngas,

Table 1Characteristics of thermochemical processes in this study.

Process Reactor of conversion Gas feed to the reactor Product of thermo-chemical process Product use Hydrogen production method

1 Gasifier Air Syngas Combustion in electric engine Electrolysis2 Gasifier Oxygen/steam Syngas Combustion in electric engine Electrolysis3 Gasifier Oxygen/steam Syngas Catalytic conversion Separation4 Combustor Air Heat Steam cycle for power generation Electrolysis

Electrolyzer

electrolyte

waterDryer high purity

hydrogen

electricity

oxygen

oxygen/steam (sch.2)

Gasifier

ash

biomass

syngas

waste water

water (basic)

Scrubber

air (sch.1)

Filter Engine

stack

electrical net

(schemes 1 and 2)

off-gas

high purityhydrogen

oxygen

Gasifier

ash

biomass

syngas

waste water

water (basic)

Scrubber

steam

HT WGSRLT WGSR

Compressor

Membranes

Filter

steam

(scheme 3)

Electrolyzer

water

electrolyte

Dryer

high purityhydrogen

oxygen

air

biomass

electricity

Scrubber

water (solution)

heat carrier

exhaust

ash

Steamturbine

Condenser

waste waters

(scheme 4)

Filter

Combustor

Fig. 1. Schemes of processes in this study.

6382 E. Biagini et al. / Bioresource Technology 101 (2010) 6381–6388

after purification, is burnt in an engine for giving electricity. Thislatter is used to produce hydrogen via electrolysis. The secondprocess is identical but an oxygen/steam mixture is used inthe gasifier instead of air. The syngas produced in process 3(via gasification with oxygen/steam mixtures) is converted in awater–gas-shift unit and then separated to give hydrogen di-rectly. In process 4 the biomass is burnt with air in a combustorto generate heat, produce steam for a steam cycle and electricityin a turbine. Finally, hydrogen is produced via electrolysis.

Pretreatments (drying, grinding) of the biomass are not in-cluded in this study. This is of course a fundamental step in bio-mass thermochemical processes and consumes importantamounts of energy, because lignin-cellulosic materials may containmore than 50% of moisture. A classical drying unit is assumed toreduce the biomass moisture to the same value (14 wt%) for allprocesses. The comparison of the results is valuable this way, eventhough the needs of these steps will have to be quantified for ancomplete energetic evaluation.

E. Biagini et al. / Bioresource Technology 101 (2010) 6381–6388 6383

The option of small size plants implies limited efficiencies.However, many advantages should be evaluated in a complete sce-nario: logistics are less onerous, plant management is easier (espe-cially for decentred sites or integration with agricultural activities),the plant can operate at variable loads (due to the heterogeneousnature of biomass), safety and regulation requirements are lessstringent.

Commercial or almost-ready technologies are considered in theprocess study and described in the next sub-sections. In all cases,the produced hydrogen is released at room pressure and tempera-ture, so that also the following steps of compression, storage or dis-tribution will need to be evaluated.

2.1. Description of process 1

A scheme of the downdraft gasifier used in process 1 is reportedin Fig. 2. It is a versatile and proven solution in the range 50–500 kWth (Bridgwater, 1995, gasifier producers: Caema-ITA, Guas-cor-ESP, Xylowatt-BEL, Entimos-FIN, AHT Pyrogas-DEU). Mostreactors are operated with sub-stoichiometric air in commercialunits and for scientific investigation (Dogru et al., 2002; Zainal etal., 2002; Wander et al., 2004; Gribik et al., 2007). The internal peaktemperature is around 1200 �C.

The installation of a heat exchanger to recover heat from thesyngas at the exit of the gasifier is problematic for slagging andfouling problems (due to the presence of tar, alkali aerosol, dustand particulate, which adhere to the surfaces). The risks are erosionof metal surfaces, high costs of maintenance, plugging of lines, andcould be conveniently overcome in larger scale plants. The solutionadopted here is to wash directly the syngas in a scrubber to cooldown the temperature, condensate tar and alkali compounds,and remove solid residues (Hasler and Nussbaumer, 1999). Thisis the briskest solution to avoid practical problems, but obviouslywill reduce the process efficiency, because sensible heat are lostand some organic matter is not converted in syngas.

Parallel fixed bed filters (generally with wood chips) reduceresidual tar and particulate of the syngas below the limit requiredby the engine (Hasler and Nussbaumer, 1999; Dogru et al., 2002;Wang et al., 2008; Stassen and Knoef, 2005) in a practical way.

The clean syngas is fed to an internal combustion engine to gen-erate electric power. No heat recovery from this unit is considered.Generally the efficiency of engines with biogas or syngas is quitelow (30–32% for 100–300 kWel, 36–38% for 1000 kWel [engine

OXIDATIONZONE

PYROLYSISZONE

DRYINGZONE

air inlet

periodically removed solid

syngas

biomassinlet

fixed bed over grateGASIFICATION

ZONE

Fig. 2. Sketch of the downdraft gasifier and

producers: Caterpillar, MAN engines, Deuts Energy, GE Jenbacher]),but the reliability is proven, the investment cost is relatively lowand variable loads (due to the heterogeneous nature of biomassfuels, for instance) are tolerated.

The electricity can be used to produce high purity hydrogen viaelectrolysis. Electrolytic cells (connected in series or parallel) dis-sociate the water molecules of an electrolytic solution in hydrogenand oxygen. The product purity may be greater than 99.99%. Theefficiency of the cells is around 85%, but decreases to 60–70% ifthe entire system (pumps, valves, power supply) is considered.On the whole, an energetic cost of 4.5–6 kWh/Nm3 of producedhydrogen is estimated (electrolyzer producers: Avalence LLC,Hydrogenics, Proton Energy, Piel).

2.2. Description of process 2

Process 2 coincides with process 1 but for the substitution ofpure oxygen in the gasifier instead of air. Also steam can be addedto enhance the gasification reactions. This solution is more onerousdue to the need of steam and oxygen supplying (higher cost, com-plex handling and safety issues). The produced syngas contains nonitrogen and thus has a higher heating value. Furthermore, a lowerproduction of tar is observed with respect to the gasification in airat comparable temperatures (Baker et al., 1984; Lv et al., 2007).

2.3. Description of process 3

In this case, the gasifier and the scrubber are the same as in pro-cess 2. Active carbon filters instead of wood chips filters (implyinghigher costs and more complex operations for maintenance anddisposal) are required to meet more stringent limits for the conver-sion/separation units. The pure syngas is fed to the CO-shift unit. Itis assumed that the conventional solution of two reactors in seriescan be adopted in a small size plant. The first one operates at rel-atively High Temperature (HT), with a catalytic bed of Fe/Cr activeat temperature 600–850 K; the second reactor operates at LowerTemperatures (LT), with a catalytic bed of Cu/Zn active in the range420–550 K (Keiski et al., 1993).

The separation of hydrogen is performed in metallic (Pd) thinmembranes, which assure high purity and easy operation.Although this solution is not completely established, it is here pre-ferred to others (e.g., Pressure Swing Adsorption) which arethought to be complex and unfeasible in a small size plant. Such

DECOMP

MIX1

H-IN

EQUILIB

H-REC

SEPASH

MIXHEAT

air inlet(or O2/steam)

ash

syngas

biomass inlet

H-DEVO

H-IN

H-REACT

H-REC

DISP

BLOCK GASIFIER

model scheme of the gasification unit.

6384 E. Biagini et al. / Bioresource Technology 101 (2010) 6381–6388

membranes work at temperatures higher than 500 K and need adriving force (pressure difference of 15–30 bar, depending on theconditions, see Adhikari and Fernando, 2006) to separate hydrogenin the permeate stream and reduce its lost (on the order of 10%with respect to the feeding) in the retentate stream.

2.4. Description of process 4

Biomass combustors are commonly used in small size plants(fixed beds, grate furnaces). More recent configurations assurelimited investment costs and high heat efficiency (>85%), eventhough the efficiency for power production is quite low and de-pends on the heat cycle conditions, the technology and heatrecovery for cogeneration (see for instance Prasad, 1995). In thisstudy all heat is used to produce electricity and small size steamturbines with high efficiency (Badami and Mura, 2009) are con-sidered. Electrolysis units are used to produce hydrogen like inprocesses 1 and 2.

3. Description of models

A process model is developed for each scheme described in theprevious section. The commercial software Aspen Plus� (by AspenTech) is used. Some units are common for all schemes and uniformhypothesis (Table 2) are adopted for a more realistic comparison ofdifferent solutions. A synthetic description of the process models isgiven in this section for each unit (if not described, conventionalblocks of the software are adopted). Poplar wood chips are consid-ered in input in all simulations. The biomass (with a reference mois-ture content of 14 wt%) is classified as a non conventional compoundin the code by inserting the specific properties (Proximate analysis:Volatile Matter 85.1, Fixed Carbon 13.0, Ash 1.9 wt% on a dry basis;Ultimate analysis: C 50.7, H 6.35, N 0.25, S 0.05, Cl 0.012 wt% on adry basis; LHV 18.1 MJ/kg dry). The other conventional compoundsare: H2O, CO, CO2, H2, CH4, O2, N2, NH3, NO, H2S, SO2, HCl, Cl2. Charis assumed as C (solid), while tar is represented as levoglucosan(C6H10O5, which is the monomer of cellulose).

The gasification unit (schemes 1–3) is modelled according tothe scheme of Fig. 2. A first reactor block (DECOMP) decomposesthe biomass in moisture and chemical elements, balancing the

Table 2Hypotheses of the process.

Schem

Char conversion in the reactora (%) 95Tar produced in the reactora (g tar/kg dry biomass) 12

Thermal dispersion in the reactora (% of the total thermal input) 2

Difference of syngas temperature at the reactor exit with respect to thepeak temperature (K)

�200

Reactora maximum temperature (K) Frombalanc

Efficiency of ash removal in the reactora (%) 97Efficiency of solid and tar removal (scrubber e filters) (%) 100Syngas temperature at the scrubber exit (K) 318

Oxygen concentration in combustion (from combustor or engine) exhaustgas (vol%)

6

Hydrogen purity (%) 100Hydrogen loss in retentate (%) n.a.Global engine efficiency (%) 30Electrolyzer consumption (kWh/Nm3 H2) 5Pressure of produced steam (bar) n.a.Temperature of produced steam (K) n.a.Turbine efficiency (isentropic) (%) n.a.

n.a. not applicable.a Gasifier or combustor.

proximate and ultimate compositions. A first heater (H-IN) quanti-fies the heat necessary to preheat the feed to the reaction temper-ature (to be defined). A Gibbs reactor (EQUILIB) calculates thespecies at the equilibrium in the actual conditions. A separator(SEPASH) separates the solid residue from the syngas (accordingto the efficiency value reported in Table 2) and a cooler (H-REC)simulates the heat transfer between the syngas and the reactor be-fore the exit. All heat streams (positive or negative) are added inthe mixer (MIXHEAT), which calculates the sum (stream DISP). Adesign specification is used to iteratively define the reaction tem-perature, so that the value of the dispersion converges to the as-sumed value (see Table 2).

Equilibrium reactors are indeed used for simulating the gasifi-cation unit, because this approach is the simplest solution and syn-gas composition is generally near the equilibrium conditions.Models with independent operating parameters (for instance reac-tor temperature and fuel-to-oxygen ratio) can be found in litera-ture works (Kovacik et al., 1990; Ni and Williams, 1995; Yuehonget al., 2006; Xu et al., 2007; Yoshida et al., 2008), but are hardlysuitable for optimization studies. The solution developed in thisstudy couples the equilibrium reactor to a comprehensive heat bal-ance and thus allows to relate the operating conditions of the reac-tor in a more realistic approach. Moreover, char and tar aregenerally not predicted in equilibrium conditions (see for instanceShen et al., 2008), even though their quantification is essential topredict the efficiency of the gasifier (they represent a lost of organ-ic matter, which is not converted in syngas). Hence a calculatorfunction is added in the code to compute the quantity of char andtar exiting the reactor block in accordance to the experimental data(see Table 2). These values are fixed, though operating conditions(temperature and oxygen concentration in the reactor, above all)are known to influence tar and char formation (Kinoshita et al.,1994; Corella and Sanz, 2005), and this should be borne in mindwhen low temperatures and oxygen concentrations are studied,as the management of the reactor could be problematic (for slag-ging and fouling).

The model of the engine (schemes 1 and 2) is inserted in a userblock: it allows the heat generated by the combustion of syngas tobe converted in electric power according to the assumed efficiency(see Table 2).

e 1 Scheme 2 Scheme 3 Scheme4

Reference

95 95 95 Dogru et al. (2002)3.4 3.4 0 Dogru et al. (2002), Lv et al.

(2007)2 2 2 Prasad (1995), Collot

(2002)�200 �200 n.a. Dogru et al. (2002)

heate

From heatbalance

From heatbalance

1500 Assumption

97 97 97 Producers100 100 100 Assumption318 318 318 Hasler and Nussbaumer

(1999)6 n.a. 6 Assumption

100 100 100 Assumptionn.a. 10 n.a. Preliminary calculations30 n.a. n.a. Producers5 n.a. 5 Producersn.a. n.a. 100 Badami and Mura (2009)n.a. n.a. 723 Badami and Mura (2009)n.a. n.a. 67.5 Badami and Mura (2009)

0

5

10

15

20

ER

H2

prod

uctio

n (g

H2/

kg b

iom

)

1000

1200

1400

1600

1800

2000 T peak gasifier (K)

proc.1proc.2 St/Biom 0proc.2 St/Biom 0.50

(a)

02468

10121416

0.1 0.2 0.3 0.4 0.5

0.1 0.2 0.3 0.4 0.5

ER

LHV

syng

as

(MJ/

Nm

3)

0

1

2

3

4 Syngas production (N

m3/kg biom

)

proc.1 proc.2 St/Biom 0.50 proc.2 St/Biom 0(b)

0

0.2

0.4

0.6

0.8

1

0.1 0.2 0.3 0.4 0.5 0.6ER

effic

ienc

y

syngas eff.

electricity eff.

hydrogen eff.

(c)

Fig. 3. Results of simulations (processes 1 and 2): (a) comparison of gasifiertemperature and hydrogen production from the processes; (b) comparison ofamount and heating value of produced syngas; (c) efficiencies of process 1, asfunctions of the equivalence ratio.

E. Biagini et al. / Bioresource Technology 101 (2010) 6381–6388 6385

The model of the electrolyzer (schemes 1, 2 and 4) is inserted ina user block, which is fed by water and power streams to give purehydrogen and oxygen according to the assumed values ofefficiency.

CO-shift reactors (scheme 3) are modelled as equilibrium reac-tors (for the exothermic reaction CO + H2O = CO2 + H2) in adiabaticconditions. Temperatures of outlet streams are higher than thoseof inlet streams, so two heaters are also included to regulate the in-let temperature of both reactors. Steam is added to the input of theHT reactor to guarantee the same H2O:CO ratio in all cases.

The model of membranes (scheme 3) is inserted in a user block:the syngas stream (at high pressure) is converted in a permeatestream (pure hydrogen at low pressure) and a retentate stream(off-gases at high pressure), according to the separation efficiency(Table 2).

The combustor (scheme 4) is modelled as a Gibbs reactor oper-ating at a fixed temperature (see Table 2) to calculate the equilib-rium species. Unconverted carbon is taken into account in acalculator function. The generated heat is transferred to the steamcycle. Steam conditions are adopted from literature (Badami andMura, 2009) as well as the efficiencies of the turbine.

4. Results and discussion

All simulations are performed maintaining the same biomassflow rate (250 kg/h) with the reference value of moisture (14%)and varying the Equivalence Ratio ER (defined as the ratio of oxy-gen entering the process, as pure gas or in the air, and that requiredto completely oxidize the fuel). The operating range of ER is 0.2–0.45 for gasification (Zainal et al., 2002; Corella and Sanz, 2005;Wang et al., 2008), while values around 1 is used for combustion.All results are normalized on the hydrogen production or the bio-mass input (on a wet basis). According to the process schemes, theefficiencies are defined as follows:

gsyngas ¼FsyngasHVsyngas

FbiomassHVbiomassðsyngas efficiencyÞ

gelectricity ¼Wnet

FbiomassHVbiomass

¼ Wgen �Wneed

FbiomassHVbiomassðelectrical efficiencyÞ

ghydrogen ¼FH2prodHVH2

FbiomassHVbiomassðhydrogen efficiencyÞ

where F is the flow rate (of syngas, biomass or produced hydrogen),HV is the low heating value (of syngas, biomass or hydrogen), W isthe power (net, generated by the engine and the turbine, or neededby compressors, pumps and auxiliaries). The efficiencies representthe fraction of input energy (as biomass) which is transformed inenergy associated to syngas, electricity or hydrogen, respectively.

4.1. Results of process 1

The results of process 1 are reported in Fig. 3. The gasifier tem-perature (the peak value) increases with ER and reaches the typicalvalues of commercial downdraft reactors (1500 K) for ER between0.39 and 0.43. In this range the production of syngas is around2.7 Nm3/kg biomass and the low heating value is 4 MJ/Nm3, typicalvalues of these conditions (Zainal et al., 2002; Wander et al., 2004;Wang et al., 2008). For lower values of ER the HV increases, theproduction of syngas decreases and more tar production, compro-mising the quality of the syngas, is expected in practical applica-

tions. For high values of ER the temperature is generallyunacceptable and the HV is too low. All efficiencies of process 1 de-creases with ER and this is due to the increase of the oxidation levelof the gasification. Ptasinski (2008) found that the exergetic effi-ciency of gasification is reduced for fuels with higher O/C ratios,such as wood, that are over-oxidized in the gasifier in order toreach the required gasification temperature. A further consider-ation can be drawn from Fig. 3: the reductions of efficiency dueto the energy transformations of the process (chemical from bio-mass to syngas, thermal from syngas to heat and electricity, electri-cal from electricity to hydrogen) are costly and, all contributionsconsidered, the production of hydrogen is relatively low and de-pends on ER (it is approximately 15 g/kg biomass for ER 0.42, giv-ing T 1500 K).

4.2. Results of process 2

The heating value of the syngas produced in process 1 can beincreased (from 4 to 10 MJ/Nm3) if pure oxygen is fed to the gas-ifier instead of air (comparison of processes 1 and 2 in Fig. 3). Atthe same time, the syngas production decreases (from 2.6 to1.3 Nm3/kg biomass) and the temperature increases significantly(the reactor achieves 1500 K for low values of ER, around 0.28).Steam can be added to increase the gasification conversion andcontrol the temperature (in order to increase ER to higher valuesand limit the tar formation). The results are reported on the mas-sive Steam-to-Biomass ratio St/Biom, normalized on the amountof biomass input (wet basis). For St/Biom 0.50 the temperature inthe reactor is acceptable (it reaches 1500 K for ER 0.36), but the en-

0

0.05

0.1

0.15

0.2

0.25

0.8 1 1.2 1.4 1.6 1.8

ER

effic

ienc

y

0

0.02

0.04

0.06

0.08

0.1

O2 m

ole fraction (exhaust gases)

electrical efficiency

hydrogen efficiency

O2 in exhaust gases

Fig. 5. Results of simulations (process 4): process efficiencies and oxygen concen-tration in the exhaust gases, as functions of the equivalence ratio.

6386 E. Biagini et al. / Bioresource Technology 101 (2010) 6381–6388

ergy content of the syngas (product of flow rate and HV) practicallyremains the same as in process 1. The confirmation of this is givenby the values obtained for the final hydrogen production (or corre-spondingly for the hydrogen efficiency), which are slightly higherthan previous results. Considering the same gasifier temperature(1500 K), process 2 produces 17 gH2/kg biomass. This small in-crease of efficiency with respect to process 1 can hardly justifythe high costs of pure oxygen and steam used. For a plant of250 kg/h of biomass, 112 kg/h (79 Nm3/h) of oxygen and 125 kg/h (156 Nm3/h) of steam should be provided in this case. Althoughno economical evaluations are discussed here, process 2 is thoughtto be unprofitable for hydrogen production. It is more interestingwhen considering the high hydrogen content of the syngas, the ab-sence of nitrogen and, thus, the possibility to separate hydrogen di-rectly. This option leads to the study of process 3 in the next sub-section.

4.3. Results of process 3

Gasifier temperature and syngas parameters (flow rate, heatingvalue, composition) of process 2 and 3 coincide, at least before theCO-conversion section, because they are obtained in the same con-ditions. The CO-shift reactors and the separation in the membranesincrease significantly the hydrogen production: the hydrogenamount in the pure syngas before the CO-conversion and that atthe exit of the process are compared in Fig. 4. A maximum of70 gH2/kg biomass can be reached at the exit of the process forthe reference case of wood moisture, against 37 gH2/kg biomassbefore the CO-conversion unit. The results for wood moisture of8% and 20% are also compared in the same figure: the lower themoisture, the higher the hydrogen production (see also Ptasinski,2008).

Also the steam-to-biomass ratio influences the efficiency of theprocess (Fig. 4b): the higher this ratio, the lower the temperaturein the gasifier, even though similar values of hydrogen productioncan be achieved. If hydrogen production have to be maximized, theconstraint on temperature (imposed by the thermal resistance ofreactor materials) should be considered. Fig. 4c and d shows thefields of temperature and hydrogen production, respectively, asfunctions of both equivalence and steam-to-biomass ratios. The

0102030405060708090

0.2 0.3 0.4 0.5 0.6ER

H2

prod

uctio

n(g

H2/

kg b

iom

)

moist 8%moist 14%moist 20%

(St/Biom 0.50)

(at the exit of gasifier)

(at the exit of process)(a)

0.25 0.3 0.35 0.4 0.45 0.5 0.550

0.25

0.5

0.75

ER

St/biom

Gasifier temperature (K)

1200-1400 1400-1600 1600-1800 1800-2000

(c)

Fig. 4. Results of simulations (process 3): (a) hydrogen production as function of the efunction of gasifier temperature at different steam-to-biomass ratios; (c) gasifier temperato-biomass ratios.

maximization can be reached respecting the constraint of the reac-tor temperature. Obviously, the optimal conditions will be definedfrom a global evaluation of the hydrogen chain, accounting also forthe cost of steam and oxygen.

4.4. Results of process 4

Results of process 4 are shown in Fig. 5. The efficiencies reach amaximum for ER 1.0. For lower values some fuel remains uncon-verted, for higher values the slow decrease in the efficiency de-pends on the heat absorbed by the excess air fed to the process.For a practical and complete combustion, conditions for assuringthe concentration of 6 vol% in the exhaust gases are considered.The electrical efficiency in this case is 15% (comparable to commer-cial units of similar size (Prasad, 1995; Badami and Mura, 2009;McIlveen-Wright et al., 2001), and the hydrogen efficiency reducesto 7.9%. This means 10.2 gH2/kg biomass. Albeit many configura-tions can be studied (as for the furnace, the steam cycle, the tur-bine), the results obtained are thought to be realistic andrepresentative of small size combustion processes.

4.5. Comparison of results

Hydrogen production of all processes is compared in Fig. 6 asfunction of the equivalence ratio. Main operating parameters, pro-cess efficiencies and needs are listed in Table 3 for the same value

01020304050607080

1200 1300 1400 1500 1600 1700 1800 1900 2000T peak gasifier (K)

H2

prod

uctio

n(g

H2/

kg b

iom

)

St/biom 0St/biom 0.25St/biom 0.50St/biom 0.75

(b)

0.25 0.3 0.35 0.4 0.45 0.5 0.550

0.25

0.5

0.75

ER

St/biom

H2 production (g H2/kg biomass)

40-45 45-50 50-55 55-60 60-65 65-70 70-75

(d)

quivalence ratio at different wood moisture contents; (b) hydrogen production asture and (d) hydrogen production fields, as functions of the equivalence and steam-

01020304050607080

0 0.1 0.2 0.3 0.4 0.5 0.6ER (gasification)

H2

prod

uctio

n(g

H2/

kg b

iom

)

0 0.5 1 1.5 2

ER (combustion)

proc.1proc.2proc.3proc.4

proc.3

proc.1proc.2

proc.4

Fig. 6. Results of simulations (processes 1–4): comparison of hydrogen productionof all processes as function of the equivalence ratio (St/Biom 0.50 for processes 2and 3).

E. Biagini et al. / Bioresource Technology 101 (2010) 6381–6388 6387

of the maximum reactor temperature. As discussed in the previoussub-sections, the energy transformations of processes 1, 2 and 4 re-duce the hydrogen efficiency to low values. Processes 1 and 4 havethe benefit of a less onerous management, do not need oxygen andsteam and are quite established solutions on a small scale. Process2 requires oxygen and steam and the resulting higher costs do notcorrespond to significantly higher hydrogen production. Process 3gives higher values of hydrogen production, but the complexscheme, the high investment and operating costs make this solu-tion arguable on a small scale. Economic and environmental eval-uations should complete the analysis and will be the object offuture activities of the project.

Some general comments are remarked:

– the final use of hydrogen should be defined in order to com-pare the purity of hydrogen obtained via electrolysis (pro-cesses 1, 2 and 4) and that of hydrogen separated inmembranes (assumed comparable in this work). In case offuel cell applications, high purity hydrogen is required(and thus an improved purification system could be

Table 3Comparison of simulation results for all processes.

Scheme 1

Biomass feed (14%moisture)

kg/h 250

Maximum reactortemperature

K 1500

Air to the reactor kg/h 585Oxygen to the reactor kg/h –Steam to the reactor kg/h –Water to the scrubber kg/h 14,200Syngas after

purificationkg/h 783Nm3/h 682MJ/Nm3 3.9H2 mole frac 0.11CO mole frac 0.18

Steam to CO-shift unit kg/h –CO-conversion in

CO-shift unit% –

Air to the engine kg/h 1300Steam to the turbine kg/h –Net electricity

productionkW 209

Power need forcompression

kW –

Exhaust gas Nm3/h 1600MJ/Nm3 0

Hydrogen production Nm3/h 42Syngas efficiency % 68.2Electrical efficiency % 19.3Hydrogen efficiency % 11.5

a Hydrogen production maximized optimizing the flow rates of oxygen and steam.b Case for 6 vol% oxygen concentration in exhaust gas.

included in process 3 with possible reduction in the effi-ciency), while less stringent limits are needed in internalcombustion engines or for hydrogen/methane mixtures;

– the produced hydrogen should be likely compressed forstorage and/or distribution: this step was not included inthis study and energetic needs should be added; it is worthnoting that electrolysis may be conducted in pressurizedcells (3–25 bar, according to the electrolyzer producers)and this will reduce the subsequent power need of processes1, 2 and 4;

– biomasses of different origin (species from energy crops, res-idues from agricultural and food activities and forestrymaintenance) can be considered to assure a continuousavailability of the fuel; the methodology developed anddescribed in this work can be applied to other biomasses(also in mixtures) available in Tuscany (e.g., olive cake andother residues);

– decentred biomass plants should be self-sufficient in termsof energy and material needs: the hydrogen production ofprocesses 1, 2 and 4 was based on the net electricity pro-duction (that generated less the auxiliary needs), whilethe main energetic need of process 3 (syngas compressionfor the separation) is only quantified and should bededucted (as well as auxiliaries consumptions); also oxy-gen supply (processes 2 and 3) should be taken intoaccount (Jackow, 2000; Anheden et al., 2005 suggested thatoxygen is conveniently stored as liquid for small scaleapplications); steam may be produced in an evaporator (fir-ing part of biomass or syngas); also heat recovery (not con-sidered in this study) can be evaluated for producingsteam, improving the efficiency or for side-unit applica-tions (e.g., biomass drying);

– processes 1, 2 and 4 produce electricity as an intermediate: asolution with parallel outputs (selling electricity to thenational net or using it to store energy in form of hydrogen)

Scheme 2 Scheme 3a Scheme 4b

250 250 250

1500 1500 1500

– – 1900132 132 –250 250 –21,270 21,270 –337 337 –336 336 –7.8 7.8 –0.34 0.34 –0.26 0.26 –– 120 –– 95.7 –

1160 – –– – 1190215 – 144

– 95 –

1140 308 15600 2.0 043 177 2968.8 68.9 –21.4 – 13.312.7 48.7 7.9

6388 E. Biagini et al. / Bioresource Technology 101 (2010) 6381–6388

is an intriguing option and the convenience should be eval-uated in a more comprehensive analysis.

5. Conclusions

Common hypothesis and realistic assumptions were made tocompare different thermochemical processes for producing hydro-gen from biomasses. The constraint of the reactor temperature wasconsidered in a comprehensive thermal balance. Equivalence ratio,steam addition and biomass moisture played a crucial role in theprocess efficiency. The hydrogen production was maximized forthe gasification/separation process (70 gH2/kg biomass). A lowerefficiency is reached in the gasification/electrolysis processes, thatin turn resulted more efficient than the combustion/electrolysisone. Obtained data on hydrogen production, operating parametersand process needs are valuable in itself and will be elaborated inthe optimization of the ‘‘hydrogen chain”.

Acknowledgement

The work collects part of the results obtained in the Italian pro-ject ‘‘Filiera Idrogeno”.

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