Bioenergy systems for the future 10 formation of hydrogen rich gas via conversion of lignocellulosic biomass and its decomposition products

Bioenergy systems for the future 10 formation of hydrogen rich gas via conversion of lignocellulosic biomass and its decomposition products Bioenergy systems for the future 10 formation of hydrogen rich gas via conversion of lignocellulosic biomass and its decomposition products Bioenergy systems for the future 10 formation of hydrogen rich gas via conversion of lignocellulosic biomass and its decomposition products Bioenergy systems for the future 10 formation of hydrogen rich gas via conversion of lignocellulosic biomass and its decomposition products

Formation of hydrogen-rich gas

via conversion of lignocellulosic

biomass and its decomposition

products

J Grams, A.M Ruppert

Lodz University of Technology, Lodz, Poland

10.1 Introduction

In recent years, the increased interest in the use of renewable energy sources has beenobserved It is related to the rapid depletion of fossil fuels and growing energydemand One of the most promising alternatives for traditional energy resources islignocellulosic biomass The advantages of such a feedstock are usually associatedwith global availability, relatively low price, and limited influence on the increase

in the greenhouse effect

On the other hand, hydrogen is considered one of the most environmentallyfriendly energy carriers Moreover, it can be used in a number of chemical processes,including the production and valorization of platform molecules originating from bio-mass Unfortunately, currently, its main production methods are steam reforming ofnatural gas or coal gasification It is widely known that such processes require the use

of traditional carbon resources and due to that affect considerably the quality of theenvironment

Taking that into account researchers began the studies focused on the development

of methods that allow for direct production of hydrogen from lignocellulosic stock The literature data shows that it can be formed by high temperature treatment

feed-or in milder conditions, that is, ffeed-ormic acid decomposition where arising H2can beused as a reducing agent in different industrial processes related to the valorization

of intermediates formed in lignocellulosic processing In the further part of this ter, both mentioned directions will be discussed in more detail

chap-10.2 High-temperature conversion of lignocellulosic

biomass towards hydrogen-rich gas

The production of hydrogen by high-temperature methods can be attractive from botheconomic and environmental points of view (Ni et al., 2006) However, the efficientconversion of lignocellulosic biomass is not an easy task due to the problems withobtaining high yield and selectivity of occurred reactions The literature data

Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00010-7

© 2017 Elsevier Ltd All rights reserved.

demonstrate that pyrolysis and gasification are the most popular methods of the temperature processing of lignocellulosic feedstock (Bridgwater, 2012; Saxena et al.,2008; Balat et al., 2009) In this case, the feedstock is heated in the presence of inertgas or gasifying agent (i.e., oxygen or steam), respectively It leads to the production

high-of permanent gases, bio-oil, and carbonaceous residue as demonstrated in Eq.(10.1):

Lignocellulosic biomass! H2+ CO + CO2+ CH4+ CnHm+ bio

However, it should be noted that thermal conversion of lignocellulosic biomass is avery complex process that proceeds in several steps (Wang et al., 2014) The first oneconsists of the decomposition of the feedstock in the temperature range of 300–500°Cthat results in the formation of thermally produced oxygenates In the further part ofthe process, the oxygenates are submitted to dehydration and cracking reactions.Moreover, decarboxylation, decarbonylation, and oligomerization processes canproceed (Ruddy et al., 2014) They lead to the arising of liquid fraction containingwater, hydrocarbons, and their derivatives (i.e., carboxylic acids, ketones, aldehydes,alcohols, esters, ethers, sugars, among others)

In the case of the formation of gaseous products, steam reforming, dry reforming,water-gas shift, and methanation play a major role (Zhao et al., 2009) It results in theproduction of gas mainly consisted of hydrogen, carbon oxide, carbon dioxide, meth-ane, and lower amount of light hydrocarbons The presence of hydrogen mixedtogether with other gaseous compounds makes it necessary to separate and purify thiscomponent before its use in industrial processes or as a fuel It can be performed bydifferent methods including membrane separation, absorption of carbon dioxide, ordrying (Ni et al., 2006)

The composition of the gaseous products obtained in the thermal conversion of nocellulosic feedstock depends on the final temperature of the process, heating rate,residence time, and type of biomass, among others (Ni et al., 2006; Saxena et al., 2008;Czernik and Bridgwater, 2004; Huber et al., 2006) Therefore, the change of the con-ditions of lignocellulosic biomass treatment makes it possible to control the concen-tration of particular chemical compounds in the formed mixture However, due to thefact that described process consists of a high number of chemical reactions, bothdesired and undesired, this control is not satisfactory It also affects the profitability

lig-of the hydrogen production That is why the scientists undertook research aimed atdeveloping new methods of high-temperature conversion of lignocellulosic biomassthat involved the use of heterogeneous catalysts

The literature data demonstrates that an application of the catalyst leads to theincrease in the efficiency of hydrogen production (Fig 10.1) It can be produceddirectly from lignocellulosic feedstock or by the conversion of earlier formed bio-oil in steam reforming process There are numerous examples of the studies con-cerning an application of the catalysts to the steam reforming of model bio-oilcompounds, such as acetic acid, acetone, ethanol, ethyl acetate, benzene, xylene, phe-nol, glycerol, or glucose (Chen and He, 2011; Braga et al., 2016; Nabgan et al., 2016;Gao et al., 2016; Zou et al., 2015; Seung-hoon et al., 2014)

10.2.1 Effect of the type of catalyst

At first, the high-temperature treatment of lignocellulosic biomass was conductedwithout the use of catalysts However, it was observed that an addition of natural min-erals such as dolomite or olivine increased the conversion of tar formed during thedecomposition of biomass (Yoon et al., 2010) In spite of the fact that those materialsare cheap and thermally stable, their performance in the mentioned process was mod-erate In the further part of the studies, an influence of zeolites and various mesoporousmaterials was investigated (Adam et al., 2006; Jeon et al., 2013; Iliopoulou et al.,

2012) Although it was demonstrated that the efficiency of thermal conversion of mass can be increased when the supported catalysts containing the metallic phase areused, there are several examples of an application of noble metals as an active phase ofthe catalysts for fast pyrolysis of biomass (Kaewpengkrow et al., 2014; Lu et al.,

bio-2010) However, the literature data exhibit that due to the lower price and highperformance in described process nickel is the most commonly used metal forhigh-temperature conversion of lignocellulosic biomass (Melligan et al., 2012;Swierczynski et al., 2007; Wu et al., 2013) Obviously, it should be noted that an appli-cation of nickel is associated with several drawbacks A deactivation of the catalyst

by carbon deposit formation is the most important of them This process cannot befully eliminated during the conversion of lignocellulosic feedstock in the presence

of metallic catalyst However, it can be noticeably reduced by the choice of the able support for Ni The influence of the support on the activity and stability of the

suit-Intensification of gas

production, higher

selectivity to H2

Thermal decomposition, cracking, reforming, water-gas shift

Intensification of cracking, dehydration, decarboxylation, decarbonylation

Lignocellulosic biomass

Gaseous products (H2, CO, CO2, CH4, C2Hx)

Liquid phase (oxygenates) Tar and char

Fig 10.1 Influence of the catalyst on the efficiency of lignocellulosic biomass conversionprocess

catalysts in high-temperature conversion of biomass will be discussed in the furtherpart of this work.

The mechanism of catalytic conversion of lignocellulosic material is not fullyunderstood yet Although the performed studies suggest that intermediates formedduring initial decomposition of biomass are adsorbed on the surface of the catalystand undergo dehydrogenation reaction (Ruppert et al., 2014) Swierczynski et al.(2007)demonstrated that the presence of nickel catalyst facilitates the cleavage ofCdO and CdC bonds in the molecules of primary products of the decomposition

of lignocelluloses Subsequently, the smaller products formed in cracking reactionare easier dehydrogenated, and this way, larger amount of hydrogen can be obtained.Considering other components of gaseous phase formed in thermal treatment of lig-nocellulosic feedstock, it should be noted that the presence of catalyst promotesreforming reaction that leads to a decrease in the methane content comparing withthe process performed without catalyst Furthermore, water-gas shift reaction can pro-ceed in the presence of residual water originated from the mixture of primary products

of biomass decomposition

The investigations performed byWang et al (2014)suggested that intermediatescontaining oxygen formed in dehydration step were rather subjected todecarbonylation in the presence of H-ZSM5 material in comparison with the reactionperformed without catalyst In the latter case, decarboxylation path was more favored.Chen and He in their work (Chen and He, 2011) described the possible mechanism

of hydrogen formation in reforming of oxygenates originating from biomass position in the presence of metallic catalyst In the investigations, ethylene glycol waschosen as a model compound This molecule can be adsorbed on the surface of thecatalyst via two carbon atoms or one carbon atom and one oxygen atom, which results

decom-in the formation of two bonds with an active phase A desired pathway of genation of oxygenates consists of the formation of hydrogen and carbon oxide viathe cleavage of CdC bond In this case, the subsequent conversion of carbon oxide

dehydro-to carbon dioxide and the production of additional amount of hydrogen via water-gasshift reaction are also observed

The second possibility is associated with the cleavage of CdO bond and the mation of alcohol In the following step, the CdC or CdO breaking can proceed inthe alcohol molecule It results in the formation of hydrogen and carbon dioxide orlight alkanes, respectively The next path can lead to the production of acids thatcan be transformed into alkanes, carbon oxides, hydrogen, and water Taking that intoaccount, it seems that hydrogen production can be considerably enhanced by the initialdehydrogenation of primary products of biomass decomposition and subsequentcleavage of CdC bonds while their dehydration and successive breaking of CdObonds does not affect H2formation so positively

for-In spite of that the authors of the mentioned publication presented also the results oftheoretical calculation concerning the ability of different metals to the cleavage ofCdC and CdO bonds It was noted that catalytic properties of particular elementsstrictly depend on numerous factors such as the composition of the feedstock, type

of the precursor of the catalyst, its preparation method, type of the support, metal ing, and conditions of thermal treatment of biomass Therefore, it is difficult to referthese results to the real reaction systems

load-Considering that an issue of hydrogen production from lignocellulosic biomass hasbeen already raised in several reviews mentioned earlier, the following part of thischapter will be focused on the presentation of the examples of the latest developmentsrelated to the use of heterogeneous catalysts in this process (Fig 10.2).

perovskie-type catalyst

Li et al (2014)investigated an influence of the addition of copper to the catalyst pared from hydrotalcite-like compounds containing nickel, magnesium, and alumi-num on its performance in steam reforming of lignocellulosic biomass tar derivedfrom pyrolysis of cedar wood The obtained results revealed that Ni-Cu/Mg/Al cata-lyst possessed the higher activity than monometallic systems The best catalytic per-formance was observed for the sample with copper-to-nickel ratio¼0.25 The authorsconcluded that the most important factor for the enhancement of the catalytic activitywas the formation of Ni-Cu alloy that led to higher dispersion of nickel, increase in theamount of the active sites on the catalyst surface, and its higher affinity to the oxygen

pre-Efficiency improvement

of high temperature conversion of biomass

Bimetallic catalysts based

During the steam reforming process, the large molecules existing in tar undergodissociation and fragmentation on the surface of an active phase Simultaneously,steam dissociates on the metal particles giving hydrogen and oxygen atoms thatcan interact with hydrocarbon fragments and form the next portion of hydrogenand carbon monoxide molecules Due to the presence of Ni-Cu alloy, the smallernickel species are formed on the catalyst surface It increases an ability of the catalyst

to the creation of nickel active sites (better metal dispersion) and adsorption of gen Thus, a dissociation of tar takes place more efficiently Furthermore, the smallerbimetallic particles demonstrate higher affinity to oxygen in comparison with biggernickel species It promotes the decomposition of tar fragments adsorbed on the cata-lyst surface and limits the formation of carbon deposit The coke resistance of bime-tallic catalyst can be also enhanced due to the presence of smaller nickel crystallitesbecause coke formation is stimulated by larger Ni particles than steam reforming reac-tion The stability of Ni-Cu/Mg/Al catalyst was also linked with no aggregation ofbimetallic species during the reaction

hydro-The similar investigations of the steam and dry reforming of model pyrolysis gaswith the use of Ni/Fe/Ce/Al2O3catalyst were described byXu et al (2015) In thiscase, the role of the second metal (iron) consisted of the promoting of cracking ofpyrolysis intermediates and deposited carbon, which results in the increase in hydro-gen production It was suggested that iron oxide can react with water and hydrocarbonmolecules according to the reactions(10.2), (10.3):

2n + m

ð ÞFexOy+ CnH2 m! 2n + mð ÞFexOy1+nCO2+mH2O (10.3)Moreover, it was demonstrated that the presence of iron can enhance the conversion ofmethane and selectivity to hydrogen and carbon dioxide in both steam reforming andwater-gas shift reactions

Lang et al (2015)developed iron catalyst for water-gas shift reaction for hydrogenenrichment in a gas from steam gasification of biomass They used ceramic foamsimpregnated by ceria The presence of ceria allowed for the increase in the surfacearea of the support and formation of relatively well-dispersed crystallites of iron.However, the submission of the synthesized catalyst to the activity test resulted inthe increase in iron oxide species from about 30 to 45 nm

Hydrogen-rich gas was also produced by catalytic steam reforming of bio-oil orbioslurry (containing both bio-oil and biochar with the ratio of 9:1) with the use ofperovskite-type materials (La1xKxMnO3or LaCo1xCuxO3) (Chen et al., 2016a; Yao

et al., 2016) The advantage of perovskites is their stability at high temperatures that vents agglomeration of metal atoms in the structure of the catalyst submitted to thermaltreatment The smaller metal species suppress the formation of carbon deposit thatenhances the activity of the catalysts Moreover, the substitution of lanthanum present

pre-in the structure of the perovskite by potassium can lead to the enhancement of the oxygenmobility and surface area of the prepared material Both tested systems allowed for theproduction of hydrogen with the yield up to 70%–75% of the stoichiometric yield

The mixture of KAl and NiAl catalysts was applied in the steam gasification of wheatstraw (Lv et al., 2014) that was conducted in double-bed reactor In the first step, themixture of catalysts and biomass (gasification bed) was fluidized Subsequently, inter-mediates were directed to the reforming bed for further conversion The obtained resultsshowed that owing to the use of such reaction system it is possible to obtain about 97%efficiency of carbon conversion Moreover, it was suggested that KAl promotes decom-position and cracking of biomass and primary products, NiAl favors reforming of tarand light hydrocarbons, while both catalysts enhance water-gas shift reaction.The examples of the application of bimetallic containing nonnoble metals andperovskite-type catalyst to the high-temperature conversion of biomass are also pres-ented inTable 10.1.

As mentioned earlier, Ni-based systems are the most popular groups of catalysts used

in the high-temperature conversion of lignocellulosic biomass In recent years, themain attention of the researchers focused on the investigation of the influence of

Table 10.1 Application of bimetallic containing nonnoble metals and perovskite-type catalyst to the high-temperature conversion

of biomass

Process, products,and remarksregarding theinfluence of

Xu et al.(2015)

Lang et al.(2015)

4 La1 xKxMnO3 Bio-oil from

fast pyrolysis ofpinewoodsawdust

Steam reforming

of bio-oil

Chen et al.(2016a)

in double-bedreactor

Lv et al (2014)

support on the catalytic performance of such materials including catalyst activity, bility in high-temperature range, and susceptibility to deactivation (Table 10.2andFig 10.3).

sta-The studies performed in our group (Matras et al., 2012) revealed that ZrO2was themost promising support of the nickel catalyst applied to the cellulose pyrolysis processconducted in a stirred batch reactor at the temperature range up to 700°C The Ni/ZrO2

sample appeared the most active among the investigated materials (Ni/Al2O3,Ni/SiO2, Ni/CeO2, Ni/TiO2, and Ni/MgO) However, in this case, nickel was intro-duced on the surface of commercial oxides The commercial ZrO2 possessed low

Table 10.2 Modification of support of Ni catalyst used in

high-temperature conversion of biomass

Process, products,and remarksregarding theinfluence of catalyst Reference

Ruppert

et al (2014)

3 Ni/CeO2-ZrO2 Cellulose Pyrolysis

CeO2was introducedinto ZrO2structure byimpregnation,precipitation, andsol-gel method

Grams et al.(2016a)

Chen et al.(2016b)

surface area equal about 5 m2/g (considerably lower than SiO2or Al2O3) and clinic phase It indicated the high potential of the use of zirconium oxide as a catalystsupport in the described process Therefore, in the next step of our studies, the inves-tigations were focused on the choice of the optimal method of ZrO2synthesis ensuringthe formation of the material having the best physicochemical properties, whichwould allow for the achievement of the highest hydrogen yield in the high-temperature treatment of lignocellulosic biomass.

mono-The zirconium oxides were prepared by precipitation with organic template, cipitation with sodium oxide, and calcination of zirconium salt (Ruppert et al., 2014).The obtained results exhibited that the highest hydrogen yield was achieved in thepresence of the catalyst where nickel was introduced on the support synthesized fromZrOCl2by precipitation with NaOH, which was followed by calcination at 700°C inair Such system contained tetragonal zirconia phase, relatively small crystallites ofnickel oxide, and retained surface area in the reaction conditions In contrast, the cat-alyst containing ZrO2prepared with the use of organic template showed the highestsurface area, but it was not stable during the reaction and decreased of about 40% afterthe reaction Moreover, in the last case, zirconium oxide was amorphous It is believedthat these differences were the reasons of low activity of that material

pre-Another crucial aspect was related to the ability of migration of zirconium ions onthe surface of the active phase The XPS results revealed that the highest migrationtendency was characteristic for tetragonal phase of zirconia (followed by monoclinicand amorphous) It was responsible for the closer contact between support and metal-lic phase during the reaction that can be associated with higher stability of the catalystand the enhancement of the catalytic activity

Nickel catalyst

Type of support

Different metal oxide supports

Application of

mesoporous silicas

Modification of ZrO2 by cerium and various alkali and alkaline earth metals

Development of optimal method

of ZrO2 synthesis (precipitation, calcination of Zr salts, use of organic

template)

Fig 10.3 Routes of the modification of the support of Ni catalysts for high-temperatureconversion of lignocellulosic biomass

The next step of the studies was devoted to the investigation of the effect of theaddition of various dopants to the structure of zirconium oxide In the first case,ZrO2 was doped by CeO2(Grams et al., 2016a) The supports containing 15 and

50 wt% of cerium oxide were prepared by three different methods such as tion, precipitation, and sol-gel As in the previous cases, nickel (20 wt%) was intro-duced onto the support surface by impregnation method, and the activity of thecatalysts was tested in high-temperature conversion of cellulose in the atmosphere

impregna-of inert gas It was demonstrated that an addition impregna-of cerium oxide to the zirconiumoxide support considerably increased the amount of the formed hydrogen comparingwith the Ni/ZrO2catalyst The production of the highest amount of H2was observed inthe case of the materials containing supports prepared by sol-gel and impregnationmethods

It was suggested that zirconium oxide promotes the activity of nickel in thereforming reaction Support containing ZrO2can accumulate H2O molecules (whichare present in the reaction mixture) and produce hydroxyl groups participating in thehydrogen formation process On the other hand, literature data show that ceriumoxide, due to its high oxygen storage/release capacity and thermal stability, can limitthe coke formation and increase an efficiency of carbon deposit removal (Ebiad et al.,2012; Shao et al., 2014) It was also observed in the described measurements whereNi/ZrO2catalyst loses its activity in high-temperature conversion of cellulose notice-ably faster than the material consisted of Ni supported on CeO2-ZrO2

The performance of Ni/ZrO2catalyst in the hydrogen production from biomass can

be also enhanced by the modification of zirconia support by alkali and alkaline earthmetals (Ryczkowski et al., 2016) As in the previous case, it is related to the inhibition

of carbon formation process (Nichele et al., 2014) An addition of alkali metals leads

to the formation of oxygen vacancies that can be responsible for the arising of OH and

O radicals able to stop the accumulation of carbon species on the catalyst surface.Moreover, dopants take part in the formation of basic sites responsible for carbondioxide chemisorption and facilitation of the gasification of accumulated coke (Liu

et al., 2008) The adsorbed CO2can also shift an equilibrium of the reactions that takeplace in the high-temperature biomass conversion and additionally increase the hydro-gen production The obtained results revealed that in spite of the substantial decrease

in the surface area the presence of calcium resulted in the production of higher amount

of hydrogen in comparison with the catalysts containing sodium or potassium Furtherstudies, performed byChen et al (2016b), showed also that an addition of Ca can beresponsible for the increase in CO selectivity and simultaneous drop in CO2produc-tion in pyrolysis-steam reforming of wood sawdust

The other group of the investigations was devoted to the application of mesoporoussilicas as supports for nickel catalyst used in the cellulose conversion to hydrogen-richgas (Grams et al., 2016b) The samples containing SBA-15, SBA-16, KIT-6, andMCM-41 were tested The obtained results revealed that an introduction of Ni onthe surface of mesoporous support can increase H2yield in comparison with the cat-alyst supported on commercial SiO2 The highest amount of hydrogen was produced

in the presence of 20% Ni/SBA-15 and 20% Ni/KIT-6 samples It was demonstratedthat catalytic performance of the studied materials depended not only on surface

acidity but also on pore size and pore volume, stability of their structure, and sibility of Ni on the surface and inside the pores of the catalyst The high surface areaand bigger pore diameter favored the penetration of the structure of the catalysts bylarger intermediates formed in the initial step of biomass pyrolysis process, whichfavors the hydrogen production However, the comparison of the activity of the cat-alysts supported on mesoporous materials with the catalytic performance of the sys-tems based on zirconium oxide exhibited that the latter are more efficient in theformation of H2in high-temperature treatment of lignocellulosic biomass.

In spite of that nickel catalysts are the most widely used in high-temperature biomassconversion processes the researchers continue also the studies on the systems con-taining noble metals (Table 10.3) Czernik and French (2014) investigated thehydrogen production by autothermal reforming of fast pyrolysis bio-oil originatedfrom oak, poplar, and pine They applied commercial Pt/Al2O3catalyst containing0.5 wt% of platinum that favors the reforming of bio-oil vapors The reaction systemconsisted of reformer working at 850°C and WGS reactor operating at 350°C (in thewater-gas shift reaction step iron/chromium system was used) The reaction mixtureformed in the first step of the process besides hydrogen also contained nitrogen,

Table 10.3 Application of catalysts containing platinum or nickel

to the novel processes of high-temperature conversion of biomass

Process, products,and remarks regardingthe influence of catalyst Reference

1 Pt/Al2O3

(reforming)

Fe/Cr (WGS)

Bio-oil fromfast pyrolysis

of oak, poplar,and pine

Autothermal reforming

of fast pyrolysis bio-oil

Czernik andFrench (2014)

Pt/TiO2, and

Pt/Al2O3

Wheat strawhydrolysatesand glucosesolution

Aqueous-phasereforming

Irmak et al.(2015)

gasification of pyrolysisgases and char inentrained flow-bedreactor

Chen et al.(2015)

nanoparticles

Pinewood,wheat straw

Subcritical andsupercritical watergasification of biomass

Nanda et al.(2016)

5 Ni/Al2O3

doped with

Ca

Pinewoodsawdust

Continuous fastpyrolysis and in-linesteam reforming

Arregi et al.(2016)

carbon oxides, and traces of methane (CH4content was much lower than that obtained

in the process with the use of nickel catalyst (Czernik et al., 2002)) Additional amount

of H2was produced by the conversion of steam and carbon oxide in the second reactorfor water-gas shift reaction Of course, the steam reforming process contributes to theproduction of the highest hydrogen yield, but it is endothermic and requires the deliv-ery of the external heat This can be provided by exothermic partial oxidation reactionthat also occurred in autothermal reforming

One of the drawbacks of the described method is the presence of heavier molecules(i.e., oligomeric lignin or carbohydrates) resulted in incomplete volatility of theprocessed bio-oil It leads to the decrease in the efficiency of the whole process More-over, the material that cannot be evaporated contributes in the formation of carbondeposit on the walls of the reactors or on the surface of the catalyst, which results

in the further drop in the H2production The catalyst may also undergo deactivationdue to the presence of various inorganic impurities in the feedstock Usually, the con-version of carbon remains at the level of 70%–90%, and nonvolatile residues aretrapped in the filter before their introduction to the reforming unit

Not only Czernik and French but alsoIrmak et al (2015)studied the performance

of platinum catalysts in the conversion of biomass to hydrogen-rich gas In this case,the investigations were focused on the influence of the pretreatment method on theactivity of Pt supported on active carbon, titanium oxide, and aluminum oxide inaqueous-phase reforming of wheat straw hydrolysates and glucose solution conducted

at 250°C in the stainless steel microbench reactor This time, the prepared catalystscontained higher amount of noble metal equal 8 wt% The obtained results confirmedthat the best distribution of metal particles can be obtained on the surface of activecarbon (about 10 nm), while in the case of other supports the control of the size of

Pt crystallites was hindered due to the agglomeration of the metal species on theirsurface It was indicated that the size of metal particles is strictly connected with cat-alytic activity of the tested materials The catalyst with smaller size of Pt crystallitesenhanced the efficiency of aqueous-phase reforming process in a higher degree thanthose with larger metal particles The catalyst activity was additionally increased

by the choice of the optimal reduction of the investigated samples It was strated that the best effect was achieved when the catalyst was initially submitted

demon-to the chemical reduction with the use of NaBH4and subsequent thermal treatment

in the nitrogen atmosphere The chemical reduction influenced not only the size of

Pt particles but also the structure of active carbon The IR spectra revealed a icant decrease in the intensity of the band corresponding to the presence of CdOgroups, among others

signif-The independent studies have been devoted to the application of bimetallic Ni-Ptand Ni-Rh catalyst in the low-temperature gasification process (Dı´az-Rey et al.,

2015) In spite of the fact that algae biomass was used as a feedstock, it is worth tolook at the obtained results that showed that the presence of noble metals inhibitedthe migration of nickel atoms on the surface of the catalyst, stabilized the Ni species,and promoted their reducibility Owing to that, the bimetallic catalysts were moreresistant to deactivation related to not only carbon deposition but also poisoning by

sulfur present in the feedstock Moreover, the formation of Ni-Pt species can beresponsible for the further improvement of the efficiency of secondary reactions(i.e., cracking or reforming of biomass volatiles and tars produced in the initial step

of the process), which allows the enhancement of hydrogen production It is also ciated with the increase in the selectivity of the process toward hydrogen and carbonoxide rather than to the formation of coke

biomass conversion

Literature data demonstrate that not only new but also commercially available lysts can be used for the production of hydrogen-rich gas via high-temperature treat-ment of lignocellulosic feedstock (Table 10.3) However, in this case, the studies arerather focused on the development of new methods of lignocelluloses conversion.Chen et al (2015)investigated simultaneous gasification of gas and char formed inthe pyrolysis of cotton stalks in entrained flow-bed reactor Apart from the application

cata-of commercial Ni/MgO catalyst that was responsible for the increase in the efficiency

of steam reforming, the authors mentioned about the catalytic activity of char It wasdemonstrated that simultaneous gasification of char and pyrolysis gas allowed for theincrease in the total conversion of carbon from 79% (observed in the case of two-stagepyrolysis and steam reforming process) to about 92% The presence of char influencedthe composition of tar by the promotion of transformation of polycyclic aromatic com-pounds to low-ring or even low-chain compounds The described changes, resultingfrom the interactions between pyrolysis gas and char, led also to considerable decrease

in the carbon deposit formation on the surface of nickel catalyst and almost twofoldgrowth in the yield of the produced hydrogen

Nanda et al (2016)performed studies on subcritical and supercritical water fication of pinewood and wheat straw That process has already been conducted pre-viously, but in the present case, biomass submitted to gasification was additionallyimpregnated by nickel salt It was showed that the efficiency of the incorporation

gasi-of Ni into the structure gasi-of lignocelluloses depended on the amount gasi-of lignin that ders the penetration of the material by dopants This is why pinewood containing morelignin accumulated lower amount of nickel in comparison with wheat straw It is worthnoting that wheat straw possessed also higher content of alkali metals such as sodium,potassium, calcium, and magnesium The presence of the catalyst resulted in substan-tial increase in the total gas yield (about 60%) and the amount of hydrogen (even100%) with simultaneous growth in the gasification efficiency of carbon (about65%) The highest hydrogen yield was achieved at 500°C with biomass-to-water ratio1:10 and longer residence time that enhanced cracking reaction The higher temper-ature of supercritical water promoted the production of hydrogen, carbon dioxide, andmethane due to the increase in the efficiency of water-gas shift and methanationprocesses

hin-The efficiency of the hydrogen production can be also increased by the use of theintegrated process consisted of continuous fast pyrolysis of pinewood sawdust