Chemical Engineering January 2018 - 28

Cover Story
Praxair
Combustion
Hot oxy-fuel
fluegas
~2,800°F (1,540°C)
Fluegas
recycle
Cooled fluegas (C02, H20) to stack,
~1,300°F (700°C)
FIGURE 7. The overall concept
for the Optimelt thermochemical
regenerator is shown here
Praxair: Oxygen-fired combustion with
thermochemical regenerators
Currently, most high-temperature furnaces
still operate at net energy efficiencies below
50%, despite the many advancements
made in heat-recovery technology for industrial
process furnaces. The main heat loss is
the sensible heat in the fluegas, due to the
large fluegas volume of fuel-air combustion.
Increasing efficiencies.
Oxy-fuel combustion eliminates
N2 that would be
present using combustion
air, and hence substantially
reduces the fluegas volume
and the sensible heat loss.
For example, oxy-fuel firing
reduces fuel consumption
by about 30% for steel reheat
furnaces equipped
with metallic recuperators to
preheat air. For glass melting,
in furnaces equipped
with efficient regenerators
for waste-heat recovery, up
to 10 to 15% fuel savings
Syngas (C0, H2rich)
~2,300°F
(1,260°C)
Cold
natural
gas
are achieved by oxy-fuel conversion and NOx
emissions are reduced by as much as 80%.
In the U.S., about 30% of container glass
furnaces have been converted to the " best
available technology " for NOx control. However,
less than 10% of the world's glass is
produced using oxy-fuel combustion.
Metallic recuperators were recently developed
to preheat both oxygen and fuel for recovering
waste heat from oxy-fuel fired glass
furnaces. Air is heated first by hot fluegas in
a radiant-type recuperator and then the preheated
air is used as the heat-transfer medium
to heat both fuel and O2 in separate
recuperators. The indirect heating design
addresses corrosion and fouling concerns
for the heat exchangers for O2 and fuel. The
maximum O2 preheating temperature is limited
to about 600°C due to material compatibility
with high-temperature O2. The maximum
natural gas preheating temperature is
limited to about 450°C due to cracking of hydrocarbons
and soot buildup. The maximum
heat recovery efficiency is only about 24% of
the sensible heat in the fluegas. Fuel savings
achieved with this heat-recovery system are
reported to be 8 to 9%. To date, there are
only a few commercial systems installed due
to the high capital cost of the system and its
relatively low heat recovery efficiency.
Optimelt™ TCR. Praxair has developed
a novel heat-recovery technology, the Optimelt
Thermochemical Regenerator (TCR)
28
that maximizes waste heat recovery by recovering
waste heat in fluegas from oxy-fuel
fired furnaces and returning the energy to
the furnace as hot synthesis gas (syngas).
The Optimelt TCR process (Figure 7) is the
first known commercial oxy-fuel fired glass
melting process utilizing endothermic chemical
reactions for waste heat recovery.
During the heating cycle, waste heat from the
glass furnace fluegas (about 1,540°C) is collected
and stored in a regenerator. During the
endothermic reforming cycle, this stored heat
is used to heat and reform a mixture of natural
gas and recycled fluegas to produce syngas at
about 1,260°C. No catalysts are required for the
reforming reactions due to the high regenerator
temperature. By using two regenerators, they
can alternate between heating and reforming
cycles, so that one is always storing heat while
the other is supplying preheated syngas to the
furnace. Water vapor and CO2 in the oxy-fuel
combustion fluegas are synergistically utilized as
reactants so the steam generation normally required
for reforming reactions is eliminated. The
syngas created from the reforming of natural
gas contains hydrogen, carbon monoxide and
a significant fraction of carbon (soot) particles.
Soot particles are advantageous in the combustion
process to produce a highly luminous
flame for efficient heat transfer.
The Optimelt regenerators are similar in design
to those used for conventional air heating
but only require one third of the checker volume
due to the reduced fluegas volume from oxy-fuel
combustion, making retrofit an economically attractive
option, especially when space is limited.
Commercialization. After verifying the technical
feasibility, a pilot plant was constructed at
the Praxair Technology center in Tonawanda,
N.Y., with testing starting in 2012. The pilot scale
TCR was about 1/40th of the expected size for
a typical 300-ton/d commercial glass-container
furnace, and utilized a natural gas flowrate for
the reforming reactions of about 30 Nm3/h.
The demonstration of the Optimelt TCR process
started in a 50-ton/d container-glass furnace
at Pavisa in Mexico in late 2014 (adopted
for commercial operation in mid-2015). Fuel and
O2 savings of 15 to 18% and low NOx emissions
were demonstrated. For a larger-scale
commercial furnace, expected fuel savings are
about 20% compared to oxy-fuel and about
30% compared to air-regenerator furnaces.
A larger commercial system was installed
for a tableware furnace at Libbey Glass in
Holland in late 2017. Application of the technology
to steel and other high-temperature
industrial furnaces are also being planned. n
Edited by Gerald Ondrey
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JANUARY 2018
Heating
Checker
Reforming
http://WWW.CHEMENGONLINE.COM
Chemical Engineering January 2018

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