Energie z biomasy VII. – odborný seminář
Brno 2007
NEW TRENDS IN HYDROGEN PRODUCTION FROM BIOMASS
Aleš Douček, Ondřej Prokeš, Daniel Tenkrát
The main objective of this paper is to summarize technologies for hydrogen production from biomass. Thus
following processes are described: catalytic steam reforming, biotechnological hydrogen production including
dark fermentation and photofermentation and solar thermochemical hydrogen production. Transformation
of biomass to hydrogen is discussed as the most efficient pathway to utilize it.
Key words: hydrogen, biomass, thermochemical processes, biotechnological processes, bioethanol, biogas
INTRODUCTION
Because of oncoming fossil fuel depletion, attention is nowadays paid to alternative resources, especially
renewables. Its major handicap is limited possibilities of utilization compared to current resources. Hydrogen,
owes great properties to easy and high-efficient conversion to electricity, is consequently regarded as a universal
energy carrier. On this account, R&D of new methods to produce hydrogen from renewable energy sources, its
transport, storage and utilization should be a priority. In this paper, transformation of biomass to hydrogen is
discussed as the most efficient pathway to utilize it.
POSSIBILITIES OF HYDROGEN PRODUCTION
THERMOCHEMICAL PROCESSES
Thermochemical processes are a group of technologies where temperature is higher than limit of chemical
stability. (cca. 300 – 2000 °C). According to chemical character of reaction, the processes could be further
subdivided to (i) oxidative, where oxidant content in reaction zone is equal or higher in respect of stoichiometry
(combustion) and (ii) reductive, where oxidant content is substoichiometric or even zero (pyrolysis, gasification).
It should be noted that not only oxygen can play a role of oxidant and in some processes it is substituted by
others e.g. CO2 or H2O [2]
Steam reforming of biomass
Steam reforming gasification consists of two processes. The first one is pyrolysis, where the biomass undergoes
reactions (1) and (2):
CH x O y → (1 − y )C + yCO + x / 2 H 2
(1)
CH x O y → (1 − y − x / 8)C + yCO + x / 4 H 2 + x / 8CH 4
(2)
After these reactions slower steam reforming of residual organic solids (reaction (3)) at 600-1000 °C follows in
combination with further increase of H2 yield by water-gas shift reaction (4) [3].
C + H 2 O → CO + H 2
0
ΔH 298
= +163kJ / mol
CO + H 2 O → CO2 + H 2
(3)
(4)
0
ΔH 298
= −41.2kJ / mol
Aleš Douček, Vysoká škola chemicko technologická v Praze, Technická 5, Praha, [email protected]
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Energie z biomasy VII. – odborný seminář
Brno 2007
Substrates which can be utilized by steam reforming cover broad spectrum from municipal solid waste, waste
from food industry [13], dry biomass, oil to further substances e.g. methanol and propane or LPG [14].
Hydrogen from biomass derivates
Hydrogen can be obtained also by reforming of biomass derivates, which are prepared from various biomasses
by biochemical treatment, e.g. biogas and bioethanol.
Catalytic steam reforming of biogas
During the steam reforming process, methane rich gas reacts with steam at high temperatures - typically
500 – 950 °C in the presence of catalyst (conventionally nickel). Following reactions describe the process:
CH 4 + H 2 O → CO + 3H 2
(5)
0
ΔH 298
= +206.2kJ / mol
CH 4 + 2 H 2 O → CO2 + 4 H 2
(6)
0
ΔH 298
= +165kJ / mol
The right side of both reactions is preferred by high temperatures and relatively low pressures. Practically, an
excess of steam about 300% related to stoichiometry is added, which move the equilibrium towards CO2
production and enable to use pressures about 4 MPa. The other reaction, water-gas shift reaction (7) is a
homogenous reaction preferred to the right side by lower temperature [3].
CO + H 2 O → CO2 + H 2
(7)
0
ΔH 298
= −41.2kJ / mol
Synthesis gas – the mixture of H2 and CO – can be used for chemical industry directly or has to be treated
(purified) before use in the fuel cells since CO is a catalytic poison for noble metals.
Beside catalytic steam reforming, there are further thermochemical reductive processes that can be used
for hydrogen production, i.e. (i) CO2 reforming, (ii) partial oxidation and (iii) thermal cracking. A short review of
these processes follows.
(i) CO2 reforming (8) – a part of steam or all steam is replaced by carbon dioxide leading to production
of synthesis gas with lower H2/CO ratio [3].
CH 4 + CO2 → 2CO + 2 H 2
(8)
0
ΔH 298
= +247kJ / mol
This process could be useful for certain applications e.g. the synthesis of oxygenated chemicals [4]. Main benefit
of this process is the utilization of two greenhouse gasses. For production of pure hydrogen for fuel cells this
technology is not very suitable regarding the necessary purification [4].
(ii) Partial oxidation (9) –this process is able to convert even heavy hydrocarbons either with use of catalyst
(methane about 600 °C) or without (methane to heavy oil and coal at 1100 – 1500 °C) [3].
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Energie z biomasy VII. – odborný seminář
Brno 2007
CH 4 + 0.5O2 → CO + 2 H 2
(9)
0
ΔH 298
= −71kJ / mol
In spite of lower efficiency compared to steam reforming, exothermicity and grater selectivity for synthesis gas
production yields to lower overall cost [4],[5]. If steam is added to the fuel and the oxidant, it is possible to bring
the exothermic partial oxidation reaction (9) with the endothermic reforming reactions (5) and (6) into equilibrium
[3]. The reaction is then said to be autothermal, meaning that no external heat source is required what raises the
efficiency [3].
(iii) Thermal cracking – another alternative to steam reforming of methane is single-step thermocatalytic
decomposition. Methane or higher hydrocarbons decompose between 700 and 980 °C when no air is present,
forming hydrogen and carbon [3]:
CH 4 → C + 2 H 2
(10)
0
ΔH 298
= +75kJ / mol
CH4 – H2 mixture is generated with a wide range of concentrations from 30 to 98 % vol. [6].
Catalytic steam reforming of bioethanol
Ethanol seems to be a promising substrate for hydrogen production because of many advantages: (i) it can be
produced from biomass and consequently became renewable, (ii) it is easy to handle (non-toxic biodegradable
liquid), (iii) it readily decomposes in presence of water and catalyst at relatively mild temperatures, (iv) it is
catalyst poison (e.g. sulfur) free [7], [8], [9].
Chemical base of the process
Stoichiometrically, the overall steam reforming reaction of C2H5OH could be represented as follows.
C 2 H 5 OH + 3H 2 O → 2CO2 + 6 H 2
(11)
0
ΔH 298
= +347.4kJ / mol
Technically, the typical process consists of three major steps: (i) steam reforming, (ii) WGS (water-gas shift) and
(iii) methanization or purification in order to remove residue CO which is poisonous to the noble-metal catalysts
mostly used in fuel cells (Figure 1).
The first step (i) of the H2 production process – steam reforming occurs in presence of a catalyst at a temperature
about 750-800 °C. In this stage, C2H5OH is introduced into a reformer or a reactor, where the liquid is
thermochemically broken down into shorter-chained carbonaceous species. These compounds would react with
steam over the catalyst to produce a mixture of H2 and other compounds, such as carbon monoxide (CO) and
CO2, C2H4O, C2H4, or CH3COCH3. Conversion of the C2H5OH to H2 may occur through the reactions depicted
below (reactions (12) and (13)) [10].
C 2 H 5 OH + H 2 O → 2CH 4 + CO2 + 2 H 2
0
ΔH 298
= +51.3kJ / mol
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(12)
Energie z biomasy VII. – odborný seminář
Brno 2007
CH 4 + H 2 O → CO + 3H 2
(13)
0
ΔH 298
= +206.2kJ / mol
Since almost all catalysts used for the steam reforming of C2H5OH produce CO [11], the water-gas shift (WGS reaction (4)) is an important step in the reforming process. During the WGS, CO is converted to CO2 and H2
through a reaction with steam. Undesirable CO content is typically reduced by performing the reaction in excess
steam. At the end of the WGS reaction, the CO concentration is between 0.5 mol % and 1 mol % [1]. The
chemical reaction for WGS is shown as a reaction (4). The reaction is reversible, and therefore, reaction
equilibrium shifts to the right and favors the formation of the H2 and CO2 as products at lower temperatures.
Although equilibrium favors formation of products at lower temperatures, reaction kinetics is faster at elevated
temperatures. For this reason, the catalytic WGS reaction is usually performed in two steps: high-temperature
shift (HTS) and low-temperature shift (LTS). Thereby, about 90% of the CO is converted to H2 in the first HTS
reactor and 90% of the remaining CO is converted in the LTS reactor [1].
Further reduction in the amount of CO in reformate can be achieved by catalytic methanation (iii). Methanation
reactor converts any residual carbon oxides back to CH4, so CO concentration becomes lower than 10 ppm [1]. H2
would be consumed there for the process as is shown by the chemical reactions below.
CO + 3H 2 → CH 4 + H 2 O
(14)
0
ΔH 298
= −251kJ / mol
CO2 + 4 H 2 → CH 4 + 2 H 2 O
(15)
0
ΔH 298
= −253kJ / mol
In addition to the methanation, other methods could be used to purify H2, such as pressure swing adsorption,
cryogenic distillation, or membrane technology in which ~99.9% purity of H2 can be reached, so the methanation
is no longer needed [12]. The three processes, namely steam reforming, WGS, and methanation may occur
simultaneously in a single steam reforming reactor (reformer), depending on the type of used catalysts. Different
catalysts lead to different reaction pathways and different effluent compositions [1]. Haryanto et al. [1] give
comprehensive information about using of catalysts in each step. Thus for steam reforming Co/ZnO, ZnO,
Rh/Al2O3 etc. and for WGS Ru/ZrO2, Pt/CeO2 and more cost-effective Cu/ZnO and Fe/Cr2O3 are reported as the
most prospective ones .
SOLAR DECARBONIZATION OF HYDROCARBONS
Solar thermochemical process can be used to provide a splitting of hydrocarbons. It brings energy benefit and
thus more efficient utilization of either fossil or renewable fuels. Three solar thermochemical processes can be
used for this purpose: cracking, reforming and gasification [15]. Net gain ratio varies from 1.7 to 1.8 [15].
Figure 1 Scheme of steam reforming of ethanol. Figure 2 Outline of the bioprocess for production
(WGS, water-gas shift; HTS, highof hydrogen from biomass in a 2 stage
temperature shift; LTS, low temperature shift)
fermentation
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Energie z biomasy VII. – odborný seminář
Brno 2007
BIOTECHNOLOGICAL HYDROGEN PRODUCTION
Even though dry biomass is suitable to be converted by thermochemical processes, wet biomass utilization is not
economically feasible because transport and drying requires considerable amount of energy [16]. In the case of
wet biomass biotechnological hydrogen production may be useful, as it is catalyzed by microorganisms in
aqueous solution at low temperatures and pressure. Two basal ways of biotechnological hydrogen production can
be distinguished: the dark fermentation (i) and photobiological hydrogen production (ii).
Dark hydrogen fermentation
Dark hydrogen fermentation is a natural phenomenon that occurs under anoxic or anaerobic conditions. Organic
compounds are used both as hydrogen provider and energy source in this technology. Variety of bacteria uses
the reduction of protons to hydrogen to dispose electrons from oxidation of organic compounds in absence of
oxygen. However many organic compounds enable hydrogen production during dark fermentation, estimations of
potential are mostly based on hexose conversions [16]. Theoretical yield per mole of glucose is described in
following reaction [17], [18]:
C 6 H 12 O6 + 4 H 2 O → 2CH 3COO − + 4 H + + 4 H 2
ΔG 0 = −206kJ .mol −1
(16)
Maximum of 4 moles of hydrogen can be obtained by this process in simultaneously release of 206 kJ of energy
which is used for microbial growth. In addition, 8 moles of hydrogen, which thermodynamically cannot be
released by this action, are deposited in acetate (or different organic acid based compounds). To make the
process economically competitive, effluent from dark hydrogen fermentation should be utilize to hydrogen by
another technique e.g. photofermentation of organic acids [16]. The realization of a bioprocess for hydrogen
requires two consecutive steps for complete utilization of the chemical energy in the substrate. Scheme of the
process is showed on Figure 2. In the first step hydrogen is produced by dark hydrogen fermentation. In the
second step the effluent is converted to either hydrogen or methane for complete conversion of sugars in
presence of light. Intermediate products from the first step are good substrate for metabolization to methane or
conversion to hydrogen by photofermentation. In addition, the energy demand of the bioprocess should be
covered by utilization of the non-fermentable residual biomass to make process more economically feasible [16].
Photofermentation
Photofermentations are processes in which organic compounds, like acetic acid, are converted into hydrogen and
CO2 with sunlight by bacteria [16]. The process takes place under anaerobic conditions and can be combined
with the dark hydrogen fermentation described above. In the dark hydrogen fermentation acetic acid is one of the
end products. A photofermentation can be employed as the second stage in a two-stage biohydrogen production
process, where the organic substrate is completely converted into hydrogen and CO2. Purple bacteria are one of
the suitable organisms for photofermentation. Although their photosystem is not powerful enough to split water
under anaerobic circumstances, these bacteria are able to use simple organic acids, like acetic acid, or even
dihydrogensulfide as electron donor [3].
Figure 3 The design of the flat-plate photobioreactor; Figure 4 Photofermentation by purple bacteria
prototype of a sunlight collector [19]
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Energie z biomasy VII. – odborný seminář
Brno 2007
The electrons that are liberated from the organic carbon (e.g. acetate) or H2S are pumped around a large number
of electron carriers. Detail pathway of photofermentation is described in Figure 4 [16]. Although many types of
photobioreactors have been designed, there is currently only one type of photobioreactor implemented in
common use that could be used for biological hydrogen production (Figure 3) [19]. In practice, yields, production
rates and efficiencies are up to 0.8 mol/mol of substrate, 7.9 l/m2.h and 9.2 % respectively for this process with
use of photoheterotrophic bacteria [16].
CONCLUSION
Although the world economy is not prepared for transfer to hydrogen economy nowadays, several possibilities for
hydrogen production from biomass are known. If the processes are developed, they will be able to harvest energy
from biomass more efficiently than currently by sequence containing hydrogen production and then electricity
generation in fuel cells.
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