Hydrogen recycling - LOI Thermprocess GmbH

102xx-09 LOI Sonderdruck Heat Processing
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VULKAN-VERLAG · ESSEN
2
2009
H E AT
PROCESSING
Originally published in
Heat Processing
2/2009
INTERNATIONAL MAGAZINE FOR INDUSTRIAL FURNACES · HEAT TREATMENT PLANTS · EQUIPMENT
SPECIAL
PRINT
Hydrogen recycling – a way to improve
the efficiency of HPH®®–bell-type
annealing furnaces
LOI Thermprocess GmbH - Am Lichtbogen 29 - 45141 Essen / Germany
Tel. +49 (0)201 1891.1 - Fax +49 (0)201 1891.321 - [email protected] - www.loi-italimpianti.com
Hydrogen recycling – a way to
improve the efficiency of HPH®-belltype annealing furnaces
Peter Wendt, Udo Dengel
Hydrogen is used around the world as a protective gas in modern bell
annealing furnaces and hydrogen costs represent a significant proportion
of operating costs. This article describes a method for recycling of
hydrogen in bell annealing resulting in significantly reduced operating
costs. The integration of the hydrogen recycling system with the bell
annealing furnaces is presented and operating costs and environmental
benefits are assessed. The system can be retrofitted to existing bell
annealing installations and has potential for other applications where
hydrogen is used.
L
OI Thermprocess GmbH, as one of
the leading suppliers for bell annealing furnaces, has installed over 8,000
annealing bases worldwide, including
more than 3,000 bases with the socalled HPH®-technology (HPH = High
Performance Hydrogen), in which
hydrogen is used as a protective atmosphere and for improved energy transfer
purposes.
Among other measures to save energy
and increase efficiency, such as
process. Two reference installations are
already operating in the United States
and a third is currently under commissioning. H2Gen is seeking a reference
installation in Europe.
LOI Thermprocess, as supplier of bell
annealing furnaces, and H2Gen, as
manufacturer of the recycling system,
intend to cooperate on projects where
hydrogen recycling for bell annealing
installations can contribute to a more
efficient use of resources and lower
operating costs.
The MARS® system can also be retrofitted to existing bell annealing installations or other equipment that uses
hydrogen atmospheres.
Use of hydrogen in bell
annealing furnaces
For over 25 years bell annealing furnaces have been operated with hydrogen as a protective atmosphere, resulting in higher furnace performance at
significantly lower operating costs and
improved strip quality and cleanliness.
Figure 1 shows the schematic of a bell
annealing furnace [2]. Coils are stacked
on the annealing base. For better heat
transfer, convector plates are placed
between the coils. To prevent oxidation,
coils are annealed under a protective
atmosphere, which for modern bell
t Exhaust heat utilization through
higher air preheating,
t Stack heat recovery during cooling,
t Exhaust heat usage by preheating of
coils,
t Exhaust heat usage for hot water /
electricity generation
LOI Thermprocess has also addressed
the question of hydrogen recycling.
About three years ago US-based H2Gen
Innovations, Inc. developed a solution
for recycling of hydrogen in bell annealing applications with their MARS® system [1]. The MARS® system collects
waste hydrogen rich annealing atmosphere from the bell furnace vent stack
containing hydrocarbons, moisture and
other impurities. This mixed gas is then
purified to 99,999% pure hydrogen gas
which is re-used in the annealing
Fig. 1: Working principle of a HPH®-Bell-type annealing furnace
Fig. 2: Thermal conductivity of N2 and H2
Fig. 3: Density of N2 and H2
Table 1: Comparison of important parameters of HNx- and HPH®-bell-type annealing
furnaces
ments for the annealed steel, such an
installation consumes approximately 2
million m3 of hydrogen per year. This
represents approximately 12% of the
annual utility costs (Fig. 5), which is
roughly equal with the combined costs
for electricity, cooling water and nitrogen.
Working principle of hydrogen
recycling
annealing systems, consists of 100%
hydrogen.
To meet safety requirements and avoid
hydrogen leaks, the annealing chamber
is sealed with a gas-tight seal between
inner cover and annealing base. For
heating of the coils a heating hood is
used while cooling is done with a cooling hood [3].
potential. Despite all these advantages,
the high price of hydrogen is a significant cost factor for bell annealing operations.
Figure 4 shows a reference installation
with more than 60 bases. Depending on
the degree of coil fouling prior to
annealing and the cleanliness require-
The main advantages of using hydrogen
as opposed to nitrogen are its high thermal conductivity and low density. Both
values change with temperature. At the
operating temperature of 700°C the
thermal conductivity of hydrogen is 7
times that of nitrogen (Fig. 2), while its
density is approximately 14 times lower
(Fig. 3).
These characteristics allow a more than
twofold increase in productivity for
hydrogen annealing furnaces, as shown
in Table 1, which compares a traditional
HNX-furnace to a modern HPH®-installation [4].
A further advantage of hydrogen compared to nitrogen is its high reduction
Fig. 4:
HPH®-Bell-type
annealing furnace
installation with
more than 60 bases
In the first stage of the recycling
process, hydrocarbon removal, oils
(hydrocarbons) and heavy impurities are
removed from the hydrogen rich process
atmosphere that is collected from the
furnaces and fed to the MARS® system.
This feedgas passes through several
stages of compression and subsequent
cooling. Each stage further purifies the
gas. This process is achieved by using a
combination of a variable speed blower
and a multistage reciprocating compres-
Fig. 5:
Utility costs for a
bell-type annealing
furnace
rate the remaining components (CO,
CO2, H2O, CH4, N2) from the atmosphere, recovering high purity hydrogen
for re-use in the annealing process.
The gas separation
The Pressure Swing Adsorption (PSA)
unit consists of several vessels. Each vessel has a layer of Silica, activated carbon
and Zeolite. The Silica layer will remove
water while the activated carbon will
mainly remove CO2 and CH4, with CO
and N2 being removed by the Zeolite
layer.
sor, taking the feedgas from the furnace
at a few mbar up to 14 bar.
Using this approach, all the heavy impurities are removed from the atmosphere
which prepares the gas for the next
stage, a gas separation process by PSA
(Pressure Swing Adsorption). At a pressure of up to 14 bar all other gases are
removed from the atmosphere, leaving
only high purity hydrogen, ready for reuse in the annealing process.
From an oil tank with a volume of
approximately 20 liters, small amounts
of solvent oil are added to the feedgas
stream during compression. The solvent
oil improves the efficiency of the hydrocarbon removal steps. A full tank lasts
for several weeks of operation. A separate pump is used for solvent oil dosing.
After the hydrocarbon removal, the
feedgas is sent through a PSA to sepa-
Once a PSA vessel reaches operating
pressure of up to 14 bar, it produces
hydrogen which is fed into a buffer tank
on the skid. While one or more PSA vessels are in production, other vessels
undergo equalization and purge to
regenerate the adsorbent. After regeneration the PSA cycles starts again. The
modular design of the PSA allows easy
expansion of the PSA capacity to accommodate different capacity ranges.
Figure 6 shows the working principle
(flow diagram) of a hydrogen recycling
system while Figure 7 shows an
installed MARS® unit with a capacity to
process 160 Nm3/h of feedgas.
Hydrocarbon removal
Feedgas from the furnace is fed into a
water cooled suction vessel of the
MARS® system. A pressure transmitter
at the buffer vessel is used to control
blower and reciprocating compressor
suction. The minimum inlet pressure is
set below 10 mbar.
If feedgas flow is low and feedgas pressure drops below the set level, the gas is
re-circulated in the MARS® system to
keep the rotating equipment operating
and to avoid shutdowns. As the feedgas
pressure and flow rise again, the speed
of the blower increases and feedgas is
again sent for hydrocarbon removal and
subsequent gas separation.
With this set up, the MARS® system can
accommodate feedgas flow rate fluctuations and does not interfere with the
operation of the bell annealing bases,
thus no change in the operating regime
of the annealing facility is required.
After each compression stage the
feedgas is cooled in water-cooled reflux
condensers to remove hydrocarbons.
Fig. 6: Process flow diagram of a hydrogen recycling system
Fig. 7:
MARS® system
with capacity to
process 160 Nm3/hr
of feedgas
Fig. 8:
PSA vessels of a
MARS® system
Figure 9 shows the results of a simulation of the recycling rate as a function of
the purity of recycled hydrogen. For
example, at a 99.99% hydrogen purity
the calculated recycling rate is over
82%, which can increase to 83% if a
purity of 99.98% is acceptable.
Figure 10 shows the levels of CO, CH4
and N2 in the purified hydrogen. These
impurities are in the low ppm range and
are well within the acceptable range for
bell annealing applications. Dew points
of -60°C are easily achieved. The calculations in Figures 9 and 10 are based on
feedgas with the following composition:
95.5% H2, 0.1% CO2, 0.4% CO, 3%
CH4 and 1% N2.
To avoid contamination of the PSA
adsorbent it is important that no contaminants such as heavy hydrocarbons
(C2 and heavier) or other impurities
reach the PSA, which can result in lower
performance and ultimately failure of
the PSA to separate other gases from
hydrogen.
Figure 8 shows a PSA unit consisting of
two PSA modules with each module
consisting of 8 individual PSA vessels.
Recycling rate and purity of
hydrogen
The recycling rate of hydrogen achieved
by the MARS® is typically above 75%,
based on the quantity and purity of
hydrogen that reaches the system. The
first reference installation at Stripco Inc
in Indiana, US, achieves a recycling rate
above 80%.
The recycling rate depends on several
factors. The two most important factors
are the level of impurities in the feedgas,
notably the amount of hydrocarbons in
the feedgas, and the desired purity of
the recycled hydrogen. At the reference
plant above, which achieves recycling
rates above 80%, the purity of the recycled hydrogen is 99.999% with feedgas
that has average levels of impurities. At
higher impurity levels, the recycling rate
drops but will still be above 70%.
Fig. 9: Recycling rate versus purity of recovered hydrogen
Install conditions
The footprint of a MARS® system with a
capacity to process 200 Nm3 per hour of
feedgas is approximately 2.4 x 8 m. The
unit is skid build at the H2Gen factory
and only requires a concrete foundation
for installation. An indoor installation in
proximity to the bell annealing furnaces
is recommended but outdoor installation is also possible.
Integration with the bell
annealing furnace
Operation of the MARS® is fully automatic, PLC controlled, and does not
interfere with the operating regime of
Fig. 10: Impurity levels in recovered hydrogen
the furnaces. Operation is controlled
over a touch screen interface that is easy
to use and which can be integrated
with the controls of the annealing facility or displayed on any network computer.
The post-annealing, “dirty” hydrogen is
no longer burned as fuel in the furnace
but instead it is collected and fed to the
MARS® system. Integration of the
MARS® system requires significant
changes in instrumentation and piping
on the bell annealing plant. For example, the piping set up to collect the
feedgas for the MARS® has to direct
oxygen containing process gas, present
during purge cycles, to vent. At the
same time the piping has to allow a
seamless switch back to the initial mode
of operation of the furnace during a
shut down of the MARS®.
The recycled, cleaned up hydrogen has a
pressure of 14 bar and can be fed
directly into the hydrogen supply system
for re-use in the furnace.
Environmental benefits and
CO2 balance
In addition to reducing operating costs,
hydrogen recycling will result in lower
CO2 emissions due to lower overall
hydrogen consumption.
Assuming a traditional supply scheme
for gaseous hydrogen via tube trailer
and use of the post-annealing hydrogen
as fuel, each kg of hydrogen generates
approximately 17 kg of CO2. In comparison, each kg of recycled hydrogen generates approximately 400 g of CO2
emissions, assuming the post-annealing
hydrogen is flared. Accounting for CO2
emissions to replace the fuel value of
post-annealing hydrogen with natural
gas, and for CO2 emissions to generate
the make up hydrogen that cannot be
recycled, the overall CO2 balance for the
recycling case is still 20% lower compared to a traditional hydrogen supply
scheme.
For a reference installation with 60
annealing bases the CO2 savings
achieved with hydrogen recycling add
up to about 600 tons of CO2 per year.
Economic benefits
For the economics calculation current
average energy costs for Germany and a
recycling rate of 75% have been
assumed.
Fig. 11: Operating cost savings through hydrogen recycling
Operating costs of the MARS® system,
consisting of the necessary utilities and
maintenance costs, translate into variable cost of approximately 0.06 á/m3
for recycled hydrogen compared to
0.50 á/m3 for conventionally supplied
hydrogen. Figure 11 shows operating
cost savings through hydrogen recycling
as a function of annual hydrogen consumption.
For a modern HPH®-bell-type annealing
facility with 60 bases and an annual
consumption of approximately 2 million
m3, operating cost savings amount to
approximately 660,000 Euros per year.
For a smaller facility with 12 bases and a
hydrogen consumption of about
400,000 m3, the annual operating cost
savings are still 132,000 Euros.
The payback period for large facilities
(> 30 bases) is in the range of 24
months. For large users such as the
Thyssen Group, with over 150 HPH®
bases at 6 sites the largest operator of
HPH® bases in Germany, implementing
hydrogen recycling could generate cost
savings of 1.5 million Euros per year and
reduce CO2 emissions by approximately
1,500 tons per year.
Conclusion
It can be concluded that hydrogen recycling works and can be integrated with
the bell annealing furnaces without
affecting the operating regime and
safety of the furnace facility. The
MARS® system can adapt automatically
and is able to handle variations in
feedgas flow that are typical for annealing facilities. The system reacts to minimal changes in flow and pressure of the
feedgas from the furnaces, which in
turn drives the operation of the system.
The installation of a hydrogen recycling
plant is beneficial from an economical
and an environmental perspective.
Literature
[1] Lomax, F.; Todd, R.; Heinrichs, Chr.; Shaffer, J.; Dengel, U.: Atmosphere Gas Recycling, Heat Treating Process, Volume 7,
Number 6, September/October 2007
[2] Scheuermann, W.; Maschler, F.; Wendt,
P.: Aspects of HPH®-Hydrogen Batch
Annealing, Vortrag zur “LOI International
Customer Convention on Heat Treatment of Steel Strip and Wire”, Bonn
2004
[3] Wendt, P; Maschler, F.; Wittler, P.: Sophisticated Cooling Philosophies in HPH®Bell-type Annealing, LOI Thermprocess
GmbH, February 2002
[4] Wendt, P.; Gasse, W.: Benefits of Converting HN Batch Annealing to Hydrogen, Steel Times International, 9/2002 Dr.-Ing. Peter Wendt
LOI Thermprocess GmbH
Director of Sales,
HPH®-Bell-type Annealing
Furnaces & Heat Treatment
Furnaces for Steel Strip
Tel. +49 (0)201 1891 236
[email protected]
Dipl.-Ing. Udo Dengel
H2Gen Innovations Inc.
International Sales Manager
Tel. +1 (703) 778-3113
[email protected]