1
Introduction
Aim of the work
Major industrialized countries have been engaged in a course of transformation
of the present energy technologies based on fossil fuels towards new
technologies founded on hydrogen. This action derives from the unbearableness
of the present industrial politics pivots on the economy of fossil fuels and oil in
particular, more and more in crisis for global extent problems, such as: the
limited quantity, and therefore duration, of ascertained fossil fuel reserves,
carbon dioxide accumulation in atmosphere caused by fossil fuel combustion
and incidental increment of greenhouse effect, uncertainty of supplies and
vulnerability of energy systems founded on oil. Time required to put into effect
this transition process will last at least 50 years. A first phase, of nearly 20
years, should be characterized by conspicuous public investments for research
and development activities, with the fulfilment of demonstrative and innovative
systems of hydrogen production and utilization, and building of initial
infrastructures.
If in a long-term perspective hydrogen will be produced prevalently from
water by means of thermal or electrolytic decomposition starting almost
exclusively from renewable and nuclear energy sources, in a short and middle-
term, the first phase of transition, it could be produced from renewable sources
only in a minimum amount, due to high costs related to the use of these sources.
Instead, it mostly will have to be produced from fossil fuels, through chemical
processes of reforming (particularly steam methane reforming), partial
oxidation and gasification (of coal, TAR, etc.), already available and mature at
industrial level for other applications in chemical, petrochemical and energy
sectors. These processes produce, even if with different modalities, a fuel gas
which can be turned after various treatments to a hydrogen and carbon dioxide
mixture, with subsequent final “sequestration” of the latter.
Hydrogen produced with current technologies still presents rather high
costs, that if are bearable for a use in process applications, they are not for
employment as an energy carrier, related to the cost of fuels nowadays
commonly used in civil, industrial and transport sectors. For this reason it is
necessary an improvement of those technologies permitting the fulfilment of
easier and more efficient plant solutions, in order to make hydrogen cost (in
energy terms) comparable with that of other fuels. On the basis of U.S.A. and
E.U. “outlooks” about the transition towards a hydrogen economy these
solutions should be industrially mature by 2015.
Introduction
2
The present work is set in this context and fits into the framework of the
research project funded by MIUR (Italian Ministry for Education, Universities
and Research) titled “Sviluppo di tecnologie per la produzione ed il trattamento
del syngas da carbone, mirato all’ottenimento e all’utilizzo di vettori energetici
di alta valenza ambientale e dell’idrogeno in particolare” (“Development of
technologies for production and treatment of coal syngas aimed to obtaining and
using high environmental valence energy carriers and hydrogen in particular”),
developed by Sotacarbo (Società Tecnologie Avanzate Carbone S.p.A.) in
collaboration with ARI (Ansaldo Ricerche Srl), DIMECA (Department of
Mechanical Engineering of the University of Cagliari) and ENEA (Ente per le
Nuove tecnologie, l’Energia e l’Ambiente, Italian National Agency for New
Technologies, Energy and the Environment). The project proposes the
development of technologies for production and treatment of raw syngas
produced by coal gasification to the purpose of hydrogen production, through
the following processes:
− Purification, with the removal of the solid and liquid particulate, sulphur
products and other unwanted polluting and contaminating species (TAR,
ammonia, acid gases, etc.). These processes allow to obtain a purified
syngas essentially made up of CO, CO
2
, H
2
, N
2
and H
2
O.
− Conversion of CO into CO
2
(CO-shift conversion), in which the syngas CO
content is converted, through a reaction with steam at medium-low
temperature, into CO
2
and H
2
, therefore with further hydrogen production.
At the end of the process a syngas made up of H
2
, CO
2
, N
2
, H
2
O and traces
of CO with concentrations depending on the modalities of the shift reaction
development is obtained.
− Hydrogen separation, in order to obtain high purity hydrogen. The residual
gas mixture is essentially made up of CO
2
and N
2
(apart from minimal
concentrations of H
2
and H
2
O), and it can be directly destined to
sequestration.
− Carbon dioxide separation to be destined to sequestration, with production
of a fuel mixture essentially constituted of H
2
and N
2
(apart from residual
concentrations of CO
2
and H
2
O). This operation is not very effective at
current technologies state for the scanty discrimination between CO
2
and
N
2
of the membranes commonly used at industrial level.
Potentially, the technologic innovation on syngas purification and
treatment processes directed towards the hydrogen production concerns the new
generation catalyst sector, of interest for CO-shift processes, and the membrane
separation process sector, of interest for the separation of H
2
and CO
2
from
syngas finalized to hydrogen production of various purity and for CO
2
capture
and subsequent sequestration.
The project is developed using the experimental equipment partially built
in the Sotacarbo Coal Research Centre, based on two fixed-bed coal gasifiers in
pilot scale fit for operating with a coal feed of 700 kg/h and 35 kg/h
Introduction
3
respectively, fed with air or oxygen-enriched air. Of the two gasifiers, the
former will be used for the experimental campaign of the gasification process
while the latter will be used to produce a syngas sample used for the
experimentation of all the syngas treatment sections downstream of the gasifier.
More specifically, the work developed in the present PhD thesis involves
the syngas post-treatment for the production of a high-content hydrogen fuel, by
means of the water-gas shift conversion. This occurs in a laboratory-scale test-
rig, entirely designed and accomplished in the ambit of the doctorate and
located at the “Energy Technologies” laboratory at the Department of
Mechanical Engineering of the University of Cagliari. The test-rig is equipped
and instrumented for the experimentation on two-stage CO-shift reactors. It
receives at the inlet the five main components of a typical coal gasification
syngas, CO, CO
2
, H
2
, N
2
and H
2
O, coming from proper cylinders or generators,
and mixes them in the desired proportions and flow rates. The so-obtained
mixture is made pass through the two CO-shift reactors (of high- and low-
temperature), where the water-gas shift reactions occur by means of proper
catalysts. The analysis of the gas CO content at the outlet of the two reactors
makes it possible to carry out a valuation of the conversion rate.
The work can be considered articulated in the following four phases:
1. Basic study of the CO-shift conversion process and design of the plant
section aimed at a systematic experimental campaign;
2. Development of physical-chemical-mathematical models for the CO-shift
process simulation;
3. Construction and setting up of a laboratory scale test-rig for the CO-shift
process characterization;
4. Blank experimentation on the test-rig and technology valuation.
The test-rig has been designed in order to offer the highest flexibility, enabling
the study of both a two-stage system, with the two CO-shift rectors arranged in
series, and a single-stage system, with the use of only the high-temperature
reactor or the low-temperature reactor. The usable flow rates, as well as the
proportions between the gases, can considerably differ from the design ones,
and potentially further gases can be used in addition to the typical ones, or
different gases can also been used. This makes it possible to use the test-rig for
additional scopes apart from the CO-shift conversion study, such as CO-shift
integrated with innovative membrane systems for CO
2
or hydrogen separation,
applications on internal combustion engines (reciprocating and gas turbines)
and applications on fuel cells.
5
Chapter 1
Hydrogen production from coal
Hydrogen is an energy vector that is acquiring more and more importance
because of political, economical, strategic and environmental considerations. Its
use as main energy resource is the way the current research efforts are directed
towards. At present, hydrogen production comes prevalently from fossil fuels,
e.g. by reforming of hydrocarbons, but its entire generation from renewable
sources is the prospect for the next future. In the short term, new possibilities of
using the large coal sources, even when coal presents problems of high sulphur
content, have arisen. On this context, a gasification pilot plant is in construction
in Sardinia for research purposes. The plant will be able to gasify a mixture of
various types of coal in order to obtain a syngas that will be subjected to
purification and shift conversion processes. The final product will be a high
purity hydrogen gas.
1.1. Coal
Coal is a sedimentary rock formed by the accumulation and decay of
organic substances derived from plant tissues and exudates that have been
buried over periods of geological time along with various mineral inclusions
[1]. Coal represents the solid fuel for excellence. In the last years more than
24% of world-wide used energy has been produced from coal and, more
generally, 80% of it comes from fossil fuels (oil and natural gas, besides coal
itself) (Tab. 1).
Tab. 1.
World primary energy supply and electricity production (2003; Sources: [2–7])
World total primary
energy supply
World electricity
generation
Fuel
[Mtoe] [%] [TWh] [%]
Fossil fuels 08,463 080.0 11,063 066.4
Coal 02,581 024.4 06,681 040.1
Gas 02,243 021.2 03,232 019.4
Oil 03,639 034.4 01,150 006.9
Renewable sources 01,428 013.5 02,966 017.8
Nuclear 00,688 006.5 02,632 015.8
Total 10,579 100.0 16,661 100.0
Hydrogen production from coal
6
It has been estimated [8, 9] that around 2010 oil will reach the “production
peak” (corresponding as a general rule to the exploitation of half the currently
available supplies) with a subsequent gradual reduction of availability and a
rather marked rise of price. After about further ten years it is moreover expected
the reaching of the natural gas production peak. On the other hand the
development of technologies for the renewable source utilization is not ready
for a diffusion on a large scale yet, while nuclear energy still involves
considerable problems of economic and politic nature. Coal happens instead to
be rather abundant and uniformly distributed at world level (as shown in Tab.
2), therefore it is likely to become the principal source of energy in the
transition period between traditional technologies and energy production solely
from renewable sources. For this reason coal will take on more and more
interest as strategic fuel.
Tab. 2.
Distribution of proved reserves
1
of fossil fuels (2004; Source: [10])
Fuel
North
America
So.&Cent.
America
Europe&
Eurasia
Middle
East
Africa
Asia
Pacific
World
2
R/P ratio
3
Oil
05.1 8.5 11.7 61.7 9.4 03.5 100.0 040.5
Gas
04.1 4.0 35.7 40.6 7.8 07.9 100.0 066.7
Coal
28.0 2.2 31.6 00.0 5.6 32.7 100.0 164.0
Notes:
1
Proved reserves: Generally taken to be those quantities that geological and engineering information
indicates with reasonable certainty can be recovered in the future from known deposits under existing
economic and operating conditions.
2
World: Sum can not equal 100% due to cumulative rounding.
3
Reserves-to-production (R/P) ratio: If the reserves remaining at the end of the year are divided by the
production rate in that year, the result is the length of time that those remaining reserves would last for if
production were to continue at that level.
1.1.1. Coal classification
Coal is mainly classified by type and rank. Coal type classifies coal by the
plant sources from which it was derived. Coal rank classifies coal by its degree
of metamorphosis from the original plant sources and is therefore a measure of
the age of the coal. The process of metamorphosis or aging is termed
“coalification”, hence rank is a measure of the degree of coalification of coal.
Coalification describes the process which the buried organic matter goes
through to become coal.
Coal rank is the most important property of coal, since is the one which
initiates the classification of coal for use. The most widespread rank
classification scheme is that defined by the American Society for Testing and
Materials (ASTM), that has become the standard classification. In this scheme,
the properties of gross calorific value and fixed carbon or volatile matter content
are used to classify a coal by rank. Gross calorific value is a measure of the
energy content of the coal and is usually expressed in units of energy per unit
Chapter 1
7
mass. Calorific value increases as the coal proceeds through coalification. Fixed
carbon content is a measure of the mass remaining after heating a dry coal
sample under conditions specified by the ASTM.
According to ASTM, the rank of coal proceeds from brown coal or lignite,
the “youngest” coal, through sub-bituminous, bituminous, and semi-bituminous,
to anthracite, the “oldest” coal. Each rank is then subdivided in various sub-
ranks, depending on volatile matter content and heating value.
The composition of a coal is typically reported in terms of its proximate
analysis and its ultimate analysis. The proximate analysis of coal is made up of
four constituents: volatile matter content, fixed carbon content, moisture
content, and ash content, all of which are reported on a weight percent basis.
The measurement of these four properties of a coal must be carried out
according to strict specifications codified by the ASTM. The ultimate analysis
of a coal reports the composition of the organic fraction of coal on an elemental
basis. Like the proximate analysis, the ultimate analysis can be reported on a
moist or dry basis and on an ash-containing or ash-free basis. The moisture and
ash reported in the ultimate analysis are found from the corresponding
proximate analysis [1]. Besides these valuations, the complete analysis of the
characteristics of a coal involves the determination of some physical parameters
and specific mechanical properties, such as the apparent density, particle size
distribution, grindability, heating value and ash melting temperature. The results
of all the analysis are reported with reference to different valuation bases, the
mostly used of which in the various classification systems are: as-received
basis, that represents the weight percentage of each constituent in the sample as
received in the laboratory; dry (moisture-free) basis (db); dry, ash-free basis
(daf), with data referred to coal free of ashes and moisture; dry, mineral matter-
free (dmmf) with data referred to coal free of ashes, moisture and mineral
elements, with reference to only organic fraction; moist, ash-free (maf) with
data referred to coal free of ashes but still containing moisture; moist, mineral
matter-free (mmmf) with data referred to coal free of ashes and mineral
elements but still containing moisture.
1.1.2. Sulcis coal
The gasification pilot plant in construction at the Sotacarbo Research
Centre in Sardinia is designed to burn any kind of coal. An important aspect
connected with this characteristic of the plant concerns the possibility of using a
coal mixture made up also of the only Italian (Sulcis) coal source, which has a
high sulphur content.
The Sulcis coalfield is located in the south-west of Sardinia and extends in
an overall area of about 200 km
2
, included in the municipal land of Carbonia,
Gonnesa, Postoscuso and San Giovanni Suergiu, delimited to the north by the
creek of Funtanamare, to the south by the gulf of Palmas, to the east by the
emergences of Palaeozoic basement, to the west by the coastline. The coalfield
Hydrogen production from coal
8
was formed during Cainozoic era, Eocene epoch, and has an age of about 50
million years.
The Sulcis coalfield is the sole coal seam in Italy. Sulcis coal is perfectly
characterized from a physical-chemical point of view and is univocally
classifiable with the most recurring classifications (International Classification,
ASTM, DIN). According to International Classification, which privileges heat
value, carbon content and incident moisture, it is placed between Class 7 and 8,
from Group 0 to Group 1 and from Subgroup 0 to Subgroup 1. According to
ASTM and DIN classifications, which privilege vitrinite reflectance and volatile
matter, ash and moisture contents, Sulcis coal is classified respectively as Sub-
bituminous A-B (ASTM) and Glanzbraunkohle (DIN), almost lignite. Its
constitutive characteristics vary from layer to layer and from zone to zone,
especially concerning sulphur and ash content.
The extention of the coalfield, as well as its conformation, is ascertained by
mining and numerous borings made in the course of its exploitation. The
coalfield is made up of a number of seams formed by several layers of coal
alternated with barren sedimentaries of various nature (sandstones, argillites,
limestones, marls, siltstones). The checked formation of coal-barren layers (the
“Productive”) extends on a surface of over 100 km
2
, to elevation between 100
meters above sea level in the coalfield eastern side (zone of Barbusi) and more
than 750 meters under sea level in the coalfield western side (zone of
Portovesme). Only for 50 km
2
research and exploration data are owned which
allow a valuation with several approximation levels of potential productive
supplies. Of these, about the half was object of prospecting, boring and heading
mining works that permitted to find out coal sources. The area of the coalfield is
covered for 59.4 km
2
by the mining concession “Monte Sinni”, granted to
Carbosulcis S.p.A. by the Sardinian Regional Assessorship of Industry with the
decree n. 241/1982 for the exploitation of the fossil fuel layer for a duration of
30 years. The supplies, if related to the area where the available information
density is adequate to their valuation, decrease to about 375 million potential
tonnes, of which the research, exploration and heading works executed by
previous and present pit concessionaries have brought to light nearly 50
millions still to mine.
In Tab. 3 and 4 are provided the Sulcis coal chemical analysis on samples
representative of a wide pit mining. They are therefore distinctive of the tout-
venant minable from the various productive layers and related coal subjected to
enrichment process. Tab. 3 quotes the average constitutive characteristics
(proximate and ultimate analysis on a dry basis) of a washed Sulcis coal sample
of about 20 tonnes mined in 1988 and gasified in the plant of Texaco
(Monbello, California) and used as a reference sample for an IGCC project.
Organic sulphur, practically uniformly distributed in coal, appears of about
65÷75% of total, while the remaining 25÷35% is essentially made up of pyritic
sulphur, that is finely scattered as well, and of very few sulphatic sulphur (<1%)
Chapter 1
9
[11]. Tab. 4 reports the composition of the Sulcis coal which will be used in the
gasification pilot plant of Sotacarbo Research Centre, for the 35 kg/h section.
Tab. 3.
Characteristics of the washed Sulcis coal, mean values (Source: [11])
Coal composition [%] Ashes composition [%]
Moisture 10.8 SiO
2
06.0÷ 40
Volatile matter 43.3 Al
2
O
3
08.0÷ 18
Ashes 18.5 Fe
2
O
3
15.0÷ 39
Fixed carbon 38.2 TiO
2
00.3÷ 00.8
C 58.3 CaO 06.0÷ 20
H 04.5 MgO 02.0÷ 11
N 01.3 SO
3
05.0÷ 25
S (total) 07.3 K
2
O 00.4÷ 01.2
O (by difference) 10.1 Na
2
O 00.2÷ 01.3
LHV [MJ/kg] 23.6 P
2
O
5
00.2÷ 00.6
HHV [MJ/kg] 24.6
Ash melting temp.
[°C]
Oxidizing
atmosphere
Reducing
atmosphere
Softening 1200 ÷ 1250 1060 ÷ 1100
Fluidification 1230 ÷ 1320 1140 ÷ 1230
Tab. 4.
Ultimate analysis of the Sulcis coal in use for the 35 kg/h Sotacarbo gasification
pilot plant (Source: [12])
Content [kg/h] [% wt.]
Carbon 18.62 53.21
Hydrogen 01.36 03.89
Nitrogen 00.45 01.29
Sulphur 02.10 05.99
Chlorine 00.00 00.01
Oxygen 02.37 06.76
Ashes 06.07 17.33
Moisture 04.03 11.52
LHV [MJ/kg] 20.83
1.1.3. Coal gasification
At present, the principal use for coal in an energy system is to powder it
into dust for its direct combustion. As prospect, technologies of coal conversion
in a gaseous fuel, for favouring an easier purification from polluting substances
and a subsequent conversion in a gas with high hydrogen content, are in
growing development. The conversion of coal in a gaseous fuel occurs in an
energy system called gasifier, and following treatment sections are required for
the gas purification. Because of its features, this method is particularly suitable
for obtaining as final product a clean gaseous fuel, which can be further treated
in order to increase its hydrogen content with possible hydrogen separation.
Hydrogen production from coal
10
The gasification process consists in the non-catalytic partial oxidation of a
solid, liquid or gaseous substance, with the final aim of producing a gaseous
fuel (called syngas) principally made up of hydrogen, carbon monoxide, carbon
dioxide and light hydrocarbons such as methane, plus various impurities like
TAR (Topping Atmospheric Residue, heavy hydrocarbons liquid mixture),
sulphur compounds, ammonia and sulphur. This fuel, once purified from
polluting substances, can be used in high efficiency power plants, and
particularly in gas-steam combined plants and in fuel cell systems, or it can be
further changed for obtaining some synthetic substances (e.g. hydrogen,
methanol, petrol, etc.). Such technique is used to exploit either not valuable
fuels, such as TAR, biomasses, wastes, peat, which are generally not directly
burnt in power plants, or low quality fuels, like some types of coal abounding
with sulphur or some oil processing wastes, converting them in a cleaner
synthesis gas. These processes allow to obtain basically clean fuels, since the
main polluting substances are easily removable, and have rather high energy
efficiencies.
The gasifier is a reactor with essentially three inlet and two outlet flows: at
the inlet there are the primary fuel to be gasified (arranged opportunely), the
oxidant (generally high purity oxygen, more rarely air) and finally water (in
liquid or vapour state); at the outlet, besides the produced syngas, there are the
ashes, that is the solid residue of the various reaction.
In case of coal gasification, coal initially passes through a pyrolysis
process, that brings to formation of gas, TAR and “char” (i.e. coal after the
release of its contents in volatiles and water). The pyrolysis gas is prevailingly
made up of low molecular mass hydrocarbons (generally referred as C
n
H
m
), that
vaporize in connection with relatively low temperatures. The TAR is made up
of molecules of the same kind, but characterized by higher molecular mass.
Finally the char is made up of coal organic compounds remaining at solid state
after the separation from volatile compounds and TAR; it can be referred as
almost pure carbon. The char afterwards undergoes combustion and gasification
processes, producing CO, CO
2
, H
2
and CH
4
. TAR and gas instead pass through
gasification and chemical bond breaking (the so-called “cracking”) processes,
that bring to the formation of CH
4
, H
2
and CO. During gasification, besides,
further numerous reactions develop contributing to modify the produced syngas
composition. Between them, the most important are [13, 14]:
Organic compounds → TAR + char + gas pyrolysis
Mineral compounds → ashes decomposition
C + O
2
↔ CO
2
+ 394 (kJ/mol) total combustion
C + ½ O
2
↔ CO + 110 (kJ/mol) partial combustion
CO + ½ O
2
↔ CO
2
+ 283 (kJ/mol) combustion
H
2
+ ½ O
2
↔ H
2
O + 242 (kJ/mol) combustion
C + 2 H
2
O ↔ CO
2
+ 2 H
2
– 90 (kJ/mol) gasification
Chapter 1
11
C + H
2
O ↔ CO + H
2
– 131 (kJ/mol) steam-carbon gasification
C + CO
2
↔ 2 CO – 172 (kJ/mol) Boudouard reaction
C + 2 H
2
↔ CH
4
+ 75 (kJ/mol) hydrogasification
CO + H
2
O ↔ H
2
+ CO
2
+ 41 (kJ/mol) shift conversion
CO + 3 H
2
↔ CH
4
+ H
2
O + 206 (kJ/mol) methanation
C
n
H
m
+ (2n–m)/2 H
2
↔ n CH
4
+ ΔH hydrocracking
C
n
H
m
+ n H
2
O ↔ n CO + (n+m)/2 H
2
– ΔH gasification
The real gasification reactions, endothermic, are obviously those more
important in the whole process. The shift conversion reaction is of considerable
interest for the determination of the ratio between hydrogen and carbon
monoxide contained in the syngas.
Depending on the particular composition of the fuel to be gasified
numerous secondary reactions occur, the products of which are found in the
synthesis gas. Particularly, sulphur in an oxygen-poor atmosphere turns into
sulphurated hydrogen (or hydrogen sulphide, H
2
S) and partly into carbonyl
sulphide (COS); nitrogen is almost completely found in molecular form (N
2
),
but traces of ammonia (NH
3
) and hydrocyanic acid (HCN) are present. Other
polluting substances frequently present are for example halogen compounds
(like HCl), TAR, alkali (sodium and potassium salts), phenols, particulate. The
presence of such substances in the synthesis gas makes necessary, immediately
downstream of the gasifier, a syngas cleaning and conditioning system.
The chemical composition of the outlet gas depends therefore on numerous
factors: the coal granulometry and its composition (ash content and ash melting
temperature, reactivity, heating value, moisture content), the processes adopted
for its preparation, the use of either pure oxygen or air as oxidant agent, the
proportion between primary fuel, steam and oxygen, the thermodynamic
conditions (pressure and temperature), the permanence time inside the gasifier,
the configuration of the gasifier (fixed, fluidized or entrained bed).
Gasification plants cannot be intended as stand-alone plants, but have to be
integrated with other processes in order to fully exploit their potentiality. The
synthesis gas produced from a gasification process, after that it has been
adequately treated and purified from undesired substances, is prevalently made
up of H
2
, CO and CO
2
. It can be used as fuel in power plants or as raw matter
for the production of synthetic fuels (such as hydrogen, methanol,
dimethylether, synthetic benzines and so on) or other industrial products (the so-
called chemicals, that are ammonia, ethylene, paraffin, plastic materials and so
on). Furthermore, since gasification processes produce a synthesis gas at high
temperature, it is generally possible to recover the produced heat, for example
with recuperative heat exchangers producing steam, usable for power generation
or other productive processes. From this it results clear that gasification
processes can be easily integrated with numerous industrial processes, in order
to minimize the energy losses.
Hydrogen production from coal
12
At present, the gasification technologies find application especially in the
field of power generation. In particular, the main application concerns
integration with Integrated Gasification Combined Cycles (IGCC). Apart from
IGCC plants, the syngas produced by means of gasification processes can be
used for power generation with non-conventional gas turbine plants and in fuel
cell systems, or as raw matter for the production of synthetic fuels.
1.2. Hydrogen
Hydrogen is found in nature prevalently combined in chemical compounds
and particularly in water. At present hydrogen is mostly produced by means of
steam methane reforming (SMR) processes or reforming of other substances
containing it, hydrocarbon partial oxidation (POX), fractionated liquefaction
from coke gas, biomass pyrolysis or water hydrolysis. Hydrogen production
systems from fossil fuels and from synthesis gas coming from coal, TAR and
biomass gasification processes are of recent development. Hydrogen is
principally used as reducer in various chemical processes and to make synthetic
compounds, but several possibilities of application as fuel are being proposed
(mainly for its high heating value and nearly zero emissions) in distributed
power generation and road traction.
Steam methane reforming is currently the cheapest hydrogen production
method, and therefore is the most widespread one. It is estimated [16] that about
the 50% of world hydrogen production is made by means of this process. It is
obtained by the development, generally into two distinct reactors, of the
following reactions [17]:
CH
4
+ H
2
O ↔ CO + 3H
2
CO + H
2
O ↔ CO
2
+ H
2
The first one is the real reforming reaction between methane and water,
endothermic, operating at pressures between 30 and 250 bar and temperatures of
about 700÷850 °C. The second is instead a shift conversion reaction,
exothermic, taking place at the same pressures of the previous one and at
temperatures between 200 and 600 °C. The product of such reactions is a gas
made up of about 77% of hydrogen and containing CO (1%), CO
2
(19%), H
2
O
and CH
4
(3% in all).
1.2.1. Hydrogen separation from syngas
Downstream of the conditioning and purifying sections of a gasifier, the
syngas is nothing but a gas mixture prevalently made up of hydrogen, carbon
monoxide and carbon dioxide, with possible traces of methane and other
substances of secondary importance, the concentrations of which depend on the
particular gasification process that is used. Subjecting the syngas to shift
Chapter 1
13
conversion reaction, and in case combining it with a steam reforming reaction if
significant quantities of methane are present, it is possible to modify
considerably the gas composition which will be finally made up prevalently of
H
2
, CO
2
and possible steam. In order to obtain a high purity hydrogen gas, a
hydrogen separation system can be adopted.
The separation systems of hydrogen from syngas suitable for industrial
field are several. Some of them are the followings: membrane separation,
physical adsorption separation, chemical adsorption separation,
chemical/physical adsorption separation, cryogenic separation. At the present
time, the more used systems in industrial field are the membranes (prevalently
for high H
2
concentrations and low CO
2
concentrations) and the chemical or
physical CO
2
adsorption systems (when the concentrations of such component
are high, as it occurs downstream of the shift conversion reactors); in particular
the last ones operate analogously to the syngas desulphuration systems (in some
cases the same process is used for a combined removal of sulphur compounds
and of CO
2
).
Generally speaking, the membrane systems for CO
2
or H
2
separation from
a gaseous mixture can be classified according to the kind of membrane or their
position in the power plant. Membranes can be classified [18, 19] in organic
(made of polymeric materials) and inorganic (metallic and ceramic), while
concerning the plant aspects they can operate on burnt gases (therefore at the
plant outlet) or on the fuel, that is upstream of the combustion process, as it
occurs in the hydrogen production processes from gasification. A separation
membrane is in general a subtle layer of a specific material which permits a
selective flow of some components of a mixture. Selectivity is mainly due to
different solubility of the membrane constitutive material with respect to the
various gas components. The separation mechanism consists on solution-
diffusion: the gas dissolves in the membrane and successively flows through it
by diffusion.
At present, the attention is mostly focused on the development of inorganic
membranes, which can separate gases at high temperatures. There are
essentially three kinds of inorganic membranes, depending on the membrane
material and its texture and structure: dense Pd-based membranes for H
2
separation, microporous membranes for H
2
and CO
2
separation, dense
electrolytes and mixed conducting (ionic and electronic) membranes for O
2
or
H
2
separation. These membranes have different operating temperature regimes
and properties. Most relevant membranes have an asymmetric structure that
provides the mechanical strength. In these cases the selective properties are
determined by the top layer, that is the real membrane, while the support is built
up by a porous structure with layers of decreasing pore size in order to provide
an adequate surface for the top layer. Anyway, since the support is an integrated
part of the membrane structure, its properties have a decisive impact on the
membrane performance [18–22].
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