Note: Descriptions are shown in the official language in which they were submitted.
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Process for converting hydrogen into substitute natural gas
The invention relates to a process for producing methane-rich product
gas (SNG, Synthetic Natural Gas), which comprises feeding biomass and/or
fossil
5 fixels to a first reactor to form gaseous reaction products and feeding the
reaction
products from the first reactor to a methanation reactor in which the gaseous
reaction product fed thereinto are converted into the methane-rich product
gas.
Hydrogen will play an important part in the future sustainable supply of
energy. Transportation and storage of hydrogen in its free form (H2) is more
10 complicated and will probably require much more energy than transportation
and
storage of hydrogen chemically stored in the form of methane, for example. An
additional advantage of the indirect use of hydrogen as a source of energy is
that
the fixture (sustainable) supply of energy will still allow use to be made of
(parts of)
the existing large-scale energy infrastructure such as, for example, the
natural-gas
15 grid. One of the possible processes for storing hydrogen in chemically
bound form
is hydrogasification of carbon-containing compounds such as, for example,
biomass
and waste. Pyrolysis of these compounds in an H2 atmosphere allows green
natural
gas to be produced.
EP-A-0 699 651 discloses that biomass, organic waste or fossil fuels
20 can be converted in a hydrogasification reactor, with the addition of
hydrogen, into
a gas mixture having a high methane content and with small amounts of carbon
dioxide. In a second process step, the gas mixture is converted, in a steam
reformer,
into synthesis gas which, in a third process step, is converted into methanol
in the
presence of a catalyst known per se, based on Cu/Zn. The hydrogen remaining at
25 the final step, after removal of the methanol, is passed to the
hydrogasification
reactor. This process is suitable only for producing methanol.
Also known, from US-A-3,922,148, is a process according to the
preamble of Claim 1, where oil is converted, with the addition of steam and
oxygen,
into synthesis gas which is converted, in a three-stage methanation process,
into a
30 product gas containing 99 mol% of methane and 0.8 mol% of hydrogen. Owing
to
the high CO/C02 concentrations of the synthesis gas a relatively large number
of
methanation reactors are required in order to convert this synthesis gas into
methane. In addition, combustion of the oil takes place in the synthesis gas
reactor
to supply heat in order to form the synthesis gas. Owing to the relatively
large heat
35 loss in the methanation reactors, the efficiency of the known process is
low.
It is an object of the present invention to provide a process by means of
which hydrogen can be stored e~ciently and economically in chemically bound
form, with relatively little carbon monoxide being present in the synthesis
gas
formed, and a simple and relatively small methanation reactor being suiEcient
for
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the process. To this end, the process according to the invention is
characterized in
that the first reactor comprises a hydrogasification reactor which is fed with
hydrogen, said hydrogen coming from an external source, and in that the
product
gas {SNG) has a Wobbe index of between 40 and 45 MJlm3(s.t.p.), preferably
5 between 42 and 45 MJ/m3(s.t.p.), and having a methane molar percentage of at
least .
75%, preferably of at least 80%.
"External" source here refers to a source which is not formed by the
methanation reactor, but independently of the process for methane production
according to the present invention supplies hydrogen to the hydrogasification
10 reactor, such as hydrogen formed by electrolysis of water, steam reforming
of light
hydrocarbons, hydrogen formed by partial oxidation of heavy hydrocarbons such
as
oil or coal by means of steam, or hydrogen from industrial processes such as
the
production of chlorine by means of membrane or diaphragm cells, methanol
production, production of acetone, isopropanol or methyl ethyl ketone, or
hydrogen
15 from blast furnaces.
Feeding external hydrogen into the hydrogasification reactor proved to
make it possible to obtain a product gas having a Wobbe index, a CH4 molar
percentage and a calorific value which are very close to the Wobbe index, the
CH4
percentage and the calorific value of natural gas (for example Groningen
natural
20 gas), so that the SNG formed can be delivered without any problems to
consumers
via the existing gas grid and can be used in existing facilities. At the same
time the
process according to the invention can be managed with a very compact
methanation reactor having a small number of components, whilst a reduction in
the
amount of tar formed (compared with other gasification schemes) is also one of
the
25 options.
In the long term, where hydrogen from electrolytical processes via
sustainable sources will become important, the process according to the
invention
forms a suitable approach to upgrading biomass and organic waste, using
hydrogen,
to form SNG. In the short term, however, hydrogen can be obtained from fossil
30 sources. A practical application of this is provided by the following
example.
According to an advantageous embodiment of the process according to
the present invention, the hydrogen is formed by means of pyrolysis in a
plasma
reactor, for example via a CB&H process as described in S. Lynum, R. Hildrum,
K.
Hox, J. Hugdahl: Kvaerner Based Technologies for Environmentally Friendly
35 Energy and Hydrogen Production, Proceedings of the 12th World Hydrogen
Energy Conference, vol. I, pp.637-645, 1998. Via the plasma process, hydrogen
and pure carbon are formed from natural gas.
The invention will be explained in more detail with reference to the
accompanying drawing, in which:
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Fig. 1 shows a schematic depiction of the process to form a methane-
rich product gas (SNG) according to the present invention, and
Fig. 2 shows a schematic depiction of a process according to the
invention, in which the hydrogen for hydrogasification is obtained from a
plasma
process.
Fig. 1 schematically shows the process stream for forming substitute
natural gas (SNG) according to the invention. Via a feeder 1, biomass is
passed to a
dryer 2. This biomass can include wood chips, vegetable waste or other organic
hydrocarbon sources. As well as biomass it is also possible to feed the
i 0 hydrogasification apparatus 3 with fossil fuels, a drying step not being
required in
that case. Via an injection line 4', COz is introduced into the biomass feed
line 4, in
order to inject the biomass at the prevailing operating pressure (for example
30 bar)
into the hydrogasification apparatus 3. Via a feed line 5, the
hydrogasification
apparatus is fed with hydrogen from an external hydrogen source. The hydrogen
15 source may comprise a water electrolysis process or be derived from
industrial
processes in which hydrogen is formed as a by-product.
At the outlet 6 of the hydrogasification apparatus 3, gaseous reaction
products are removed from the hydrogasification apparatus, the main
constituent
being CH4, with CO, Hz, COz and H20 also present. The gas mixture is fed, via
a
20 heat exchanger 9, to a high-temperature gas purification apparatus 7 to
remove
solid residue and gaseous impurities from the synthesis gas, for example, H2S,
HCI,
HF, NH3. The solid residue from the hydrogasification apparatus 3 is removed
via a
discharge line 8. Via line 10 and heat exchanger 11, the purified methane-rich
gas
mixture is fed to a methanation reactor 12, in which the methane-rich gas
mixture is
25 converted into substitute natural gas (SNG) which, via a heat exchanger 14
and a
water separator 15, is passed to a discharge line 16. Thence, substitute
natural gas
can be injected into the existing gas grid to be delivered to the end user.
The heat removed from the methane-rich gas mixture at outlet 6 and in
line 10, and the heat removed from the product gas at outlet 13 is supplied,
via the
30 heat exchangers 9,11 and 14, to a steam generator 19, the steam generated
by
which is fed to a steam turbine 20 which drives generator 17 to produce
electricity.
The condensed steam is recycled from the steam turbine 20 via a return line 22
to
the inlet of the steam generator 19. Part of the low-pressure steam from the
steam
turbine 20 heats the dryer 2 via a heat exchanger 18. The condensed low-
pressure
3 5 steam, having passed the heat exchanger 18, is supplied to the steam
generator 19.
The following reactions take place, inter alia, in the hydrogasification
apparatus 3:
C+2HzHCH4 (1)
CO + 3 Hz t~ CH4 + Hz0 (2)
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2COt-~C+C02 (3)
CO + H20 H C02 + H2 (4)
At a constant temperature (T=800°C), if no hydrogen were to be fed
in
at thermodynamic equilibrium, a pressure increase in the reactor would lead
to:
- a decrease in the CO and Hi concentration and an increase in the
concentrations of CH4, COZ and H20 in the synthesis gas discharged via the
5 discharge line 6;
- a decrease in the conversion of carbon from the biomass, and
- a decrease in the heat required in the reactor 3.
The abovementioned reaction number (4), the water gas shift
equilibrium, is independent of the pressure, whereas the other reactions are
shifted
10 to the right with increasing pressure and are all exothermal in that
direction.
At higher operating temperatures, at a pressure P=30 bar, if no
hydrogen is fed in and at thermodynamic equilibrium, the above mentioned four
equilibrium reactions are shifted to the left, which results in:
- an increase in the CO and H2 concentration and a decrease in the
15 concentrations of CH4, C02 and H20 in the synthesis gas discharged via the
discharge line 6;
- an increase in the conversion of carbon from the biomass, and
- an increase in the heat demand of the reactor.
Only at temperatures below 550°C will the process become
20 autothermal.
Feeding in hydrogen at T=800°C, P=30 bar and at thermodynamic
equilibrium gives rise to the following effects in the hydrogasification
process in the
hydrogasification apparatus 3:
- an increase in the methane concentration and a decrease in the heat
25 demand according to reactions ( 1 ) and (2),
- a decrease of the CO concentration according to reaction (2), and
- an increase in the carbon conversion according to reaction (1).
If hydrogen is fed in via the feed line 5 to an amount of 75 mol/kg of
biomass (moisture-free), the synthesis gas formed in the hydrogasification
apparatus
30 3 comprises 29 vol% of methane and 7 vol% of CO, with a carbon conversion
of
the biomass of 78% and a heat demand of 1.2 MW~,,/kg of biomass (moisture-
free).
Wherever biomass is referred to hereinafter, this relates to moisture-free
biomass.
An increase in the operating pressure T=800°C, with a hydrogen feed of
75 mol/kg
of biomass, will result in an increase in the carbon conversion, since
reaction (1)
35 becomes dominant in that case.
There follows a more detailed description of the process parameters in
the hydrogasification apparatus 3, the high-temperature gas purification
apparatus
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7, and the methanation reactor 12, these parameters having formed the basis
for
calculating the composition of the substitute natural gas (SNG) discharged via
the
discharge line 16. The calculation was based on biomass in the form of poplar
sawdust having the composition as shown in Table 1:
5
Table 1 Specification of the biomass
Units Value
Composition C wt% 51.32
H wt% 6.16
N wt% 1.18
S wt% 0.13
O wt% 34.57
Ash wt% 6.64
Total wt% 100.00
Low heating value (LHV)",o;.a~.~ MJ/kg 21.57
Low heating value (LHV), 30 wt% moisture MJ/kg 14.53
In the computational model, the hydrogasification apparatus 3 was
operated at a temperature of 800°C and a pressure of 30 bar. At this
setting it is
possible, given a specific deviation from the thermodynamic equilibrium, to
obtain a
10 carbon conversion of the biomass of 89%, with a hydrogen feed of 75 moUkg
of
biomass, the process being autothermai. Since, however, the biomass fed in is
not
free from moisture, and the hydrogasification apparatus 3 is fed with
additional
COa, the hydrogen feed in the model was increased from 75 to 100 moUkg of
biomass to render the process autothermal. At this setting, the predicted
conversion
15 of carbon from the biomass is 83%.
The gaseous products from the hydrogasification reactor 3 are cooled
in two steps, via heat exchangers 9 and 11, from 850°C to the inlet
temperature of
the first methanation reactor at 400°C. In this temperature range, a
high-
temperature gas purification apparatus 7 can be used to remove solid residue
and
20 gaseous contaminants such as H2S, HCI, HF, NH3 from the synthesis gas.
The methanation reactor 12 is based on the ICI high-temperature
single-pass process as described in the Catalyst Handbook, second Edition,
edited
by M.V. Twigg, ISBN 1874545359, 1996. This makes use of a series of reactors
operating at successively lower outlet temperatures.
25 The steam generator 19 generates superheated steam at a pressure of
40 bar. The heat derived from the methanation reactor 12 and from the cooling
of
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the methane-rich synthesis gas via heat exchangers 9 and 11 was used in the
model
to form steam, while the remainder of the heat released during cooling of the
methane-rich gas mixture in lines 6 and 10 was used to superheat steam. The
steam
formed was expanded to 0.038 bar in two steps (from 40 to 10 bar in the first
step,
5 and from 10 to 0.038 bar in the second step).
Based on the abovementioned process settings, the mass balance and
energy balance of the system according to Figure 1 were calculated using the
ASPEN PLUS process simulation program. Table 2 shows the properties of
Groningen natural gas (NG) and of the synthetic natural gas (SNG) formed in
the
10 hydrogasification process according to Figure 1. Very importantly, it can
be seen
that the heating value in MJ/kg and the Wobbe index of the synthetic natural
gas are
virtually identical to those of natural gas. This allows the product gas
formed in the
hydrogasification process according to Figure 1 to be injected directly into
the
natural-gas grid and to be burnt using existing facilities. '
15
Table 2 Pro erties of as (SNG) and en natural NG~
the roduct Groning gas (
Composition NG SNG
mol% 81.30 81.55
H2 mol% 0.00 8.70
C02 mol% 0.89 8.54
C2+ mol% 3.49 <1
N2 mol% 14.31 0.77
mol% 0.01 0.00
Molecular weight kg/kmol 18.64 17.33
Low heating value (LHV) MJ/kg 38.00 39.00
Low heating value (LHV) MJ/kmol 708.32 676.08
Wobbe index MJ/m3(s.t.p.) 44.20 43.87
The Wobbe index, based on cubic meters at standard temperature and
pressure (m3[s.t.p.]) at 0°C and 1 atmosphere (MJ/m3[S.T.P]), is the
ratio of the
high calorific value and the square root of the relative density of the gas.
The
20 Wobbe index is defined according to the following formula:
HHV
W=
~Pg ~Pair~
where HHV is the high heating value in M3/m3(s.t.p.), and pg and p,;~
are the densities of gas and air, respectively, in kg/m3(s.t.p.). The Wobbe
index is
the measure of the amount of energy which is delivered to a burner via an
injection.
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Two gases having a different composition but the same Wobbe index provide the
same amount of energy, given a predetermined injection direction at the same
injection pressure.
Figure 2 shows an embodiment of a process in accordance with the
5 present arrangement, in which the hydrogen is formed via a CB&H process as
described in R.A. Wijbrans, J.M. van Zutphen, D.H. Recter: "Adding New
Hydrogen to the Existing Gas Infrastructure in the Netherlands, Using the
Carbon
Black & Hydrogen Process, Proceedings of the 12th World Hydrogen Energy
Conference", vol. II, pp. 963-968, 1998. Here, natural gas is fed, via a feed
line 23,
10 to a plasma reactor 24 in which a plasma is generated by electrical energy
being
supplied, and in which hydrogen and carbon are formed. Having passed a heat
exchanger 25 and a separator 26, the carbon in the known CB&H process is
discharged to be pelleted and packaged and the hydrogen is passed to a
compression and injection apparatus 27 in order then to be injected into the
natural-
15 gas grid. According to the invention, the hydrogen is passed not to
compression
and injection apparatus 27, but to the hydrogasification process, via the feed
line 5.
The use of the high-temperature plasma process to produce hydrogen from
natural
gas in combination with the hydrogasification process has the advantage that
the
CB&H process yields pure carbon, the reduction in calorific value as a result
of the
20 conversion of natural gas into hydrogen being compensated for, by more than
100%, by back-reaction of the hydrogen to form SNG. This therefore ensures
that
the fossil carbon disappears from the chain, whereas sustainable carbon is fed
from
the biomass, with a net gain in energy, owing to the introduction of biomass
to an
amount of roughly 60%.
