THERMO-CHEMICAL
RECUPERATION
IMPROVES FURNACE THERMAL EFFICIENCY
Thermochemical
recuperation (TCR), if
successfully developed
and commercialized in the
near/medium term, will
provide increases in
furnace thermal efficiency,
reduce fuel consumption,
and significantly reduce
air-pollutant emissions.
Steve Sikirica,
Harry Kurek,
Dr. Alexsander Kozlov,
and Mark Khinkis
Gas Technology Institute,
End Use Solutions Section
Des Plaines, Ill.
T
he process of recuperating the
energy contained in exhaust
gases from high temperature
process furnaces, engines, etc.,
for hydrocarbon fuel reforming and
for oxidant preheat is called thermochemical recuperation (TCR). If successfully developed and commercialized in the near/medium term, TCR
will provide increases in furnace
thermal efficiency by 15 to 35% and
reduce hydrocarbon fuels consumption by 15 to 60% compared with conventional recuperation/regeneration
when usually only combustion air is
preheated. TCR will also significantly
reduce by 30 to 80% air pollutant
emissions (NOx, CO, THC, CO2, etc.).
TCR Advantages
The major advantage for TCR is
the opportunity to improve process
efficiency. TCR has been extensively
studied in Japan, U.S., Ukraine, and
Russia. For heating processes, efficiency increases of 20 to 50% have
been noted, and for processes using
thermal cycles (e.g., internal combustion engines, gas turbines) efficiency
increases of 8 to 15% have been
noted.
Figure 1 shows a general example
to illustrate the concept. At an 1100°F
air preheat temperature, and furnace
Kf
0.92
0.9
0.80
1
0.8
0.78
0.70
0.7
2
0.6
800
1000
1100
1200
T2, °F
Fig. 1 — Coefficient of exhaust gas heat
use (Kf) for furnaces at two levels of exhaust
gas temperatures (1700 and 2000°F, or 930
and 1095°C) versus temperature of preheated
air (dashed line) and temperature of preheated
air and reformed gas (solid line).
exhaust temperature of 2000°F, 71%
of the total heat in the exhaust is recovered. If a reformer is added to
make up a TCR system, and the reformed fuel is now at 1100°F also, the
amount of total heat recouped increases to 78% or an 8% increase,
which can be only achieved using an
air preheat temperature of 1450°F.
Recuperative reforming is a technique that recovers sensible heat in
the exhaust gas, and uses that heat to
transform the hydrocarbon fuel
source into a reformed fuel having a
higher calorific heat content. The reforming process uses the waste heat
plus steam (water vapor) and/or
carbon dioxide (CO2) to convert the
fuel into a combustible mixture of
1000°F
preheated air
1200°F
reformed fuel
Air heater
High-temp. 2000
furnace
°F
1200
°F
ID fan
535
°F
Flue gases
67%
33%
Recuperative
Ejector/
mixer
Natural
gas
Air
Fig. 2 — TCR system as applied to a high-temperature steel reheat furnace with natural
gas/flue gas reforming (62% efficiency).
28
HEAT TREATING PROGRESS • AUGUST 2007
hydrocarbons, hydrogen, and carbon
monoxide (CO). The calorific content
of the fuel can be increased by up to
28% with the TCR process if the original source fuel is natural gas. When
the reformed fuel is combusted in the
furnace, fuel economy is improved,
system efficiency is increased, and
emissions are reduced. In addition,
the fuel is preheated during the reforming process, adding sensible
heat to the fuel. Because both steam
and CO2 can be used in the reforming
process, it is advantageous for natural gas-fired systems because both
of these gases are major products of
combustion and, therefore, are
readily available in a preheated
state. Further, they can be used in
the same ratio as they exist in the
combustion products. A schematic
for application of a TCR system to
an industrial gas-fired furnace is
shown in Fig. 2.
Similar results were shown by
modeling of other metallurgical
furnaces including heat treating, carburizing, and sintering. Another example of a TCR application for a lowtemperature metallurgical melting
furnace using steam reforming is
shown in Fig. 3. An example of a TCR
application for an oxy-gas glassmelting furnace is represented both
in Table 1 and in the Sankey diagram
(Fig. 4) using flue gas as the reforming medium together with the
flow diagram in Fig. 5. The data in
Table 1 show there is a substantial reduction in both natural gas and
oxygen consumption.
Table 1 — Parameters of oxy-fired high-temperature glass
melter with natural gas/flue gas reforming
TCR Status
In process heating, GTI has focused on steel industry furnaces and
glass and aluminum melting furnaces. GTI designed, built, and tested
a bench-scale TCR test facility using
a modified radiant tube test rig for
both flue gas and steam reforming.
A schematic of one configuration for
testing and evaluating flue gas reforming is illustrated in Fig. 6, with
the physical unit shown in Fig. 7. Potential applications of a highly efficient radiant tube indirect heating
system may be evaluated in the near
future.
Figure 8 shows initial experimental
Fig. 4 — Sankey diagram for oxy-fired high-temperature glass melter with natural gas/flue
gas reforming.
Parameters
Without
reformer
(baseline)
With
reformer
Increase (+)
or
decrease (-)
26,000
59,000
85,000
26,000
21,754
45,824
67,578
44,280
-16%
-16%
-20%
+70%
25.5
13.3
5.0
7.2
—
52.0
21.3
13.3
5.0
2.7
0.3
62.2
-17%
—
—
-63%
nil
+10%
Flow rate, scfh
Natural gas consumption
Oxygen consumption
Flue gas
Fuel to burners
Heat balance components, MMBtu/hr
Heat input
Useful heat
Heat losses through walls
Heat losses with flue gas
Heat losses in reformer*
Thermal efficiency, %
* Assumed heat losses in fuel reformer = 5% of heat load
Reformed
fuel,
1000°F
Water (1,530 lb/h)
68°F
Melting
furnace
2050°F
Steam
boiler
1740°F
Preheated air,
1000°F
Reformer
230°F
1013°F
190°F
Natural gas
(907 lb/h)
Steam
Air
480°F
Recuperator
Combustion air
(15,400 lb/h)
Fig. 3 — TCR system applied to a low-temperature melter with natural gas/steam reforming
(51% efficiency).
Heat losses
through walls
5.0 MMBtu/h
Heat input –
21.3 MMBtu/h
13.3 MMBtu/h
Useful heat
2.7 MMBtu/h
Fuel reformer
4.1 MBtu/h
0.3 MMBtu/h
Reformed fuel
composition
(mole fraction)
CH4 – 0.131
CO2 – 0.073
H2O – 0.077
CO – 0.222
N2 – 0.007
H2 – 0.490
Oxygen
Heat losses with
flue gas
Heat losses in
fuel reformer
Flue gas
composition
(mole fraction)
CO2 – 0.327
H2O – 0.636
N2 – 0.006
O2 – 0.031
1200°F
Melter
2284°F
475°F
45,824 scfh
Flue gas
67,578 scfh
271°F
25%
Natural gas
21,754 scfh
Fig. 5 — TCR system applied to an oxy-fired high-temperature glass melter with natural
gas/flue gas reforming (62% efficiency).
HEAT TREATING PROGRESS • AUGUST 2007
29
Fig. 6 — Flow diagram of lab-scale TCR
test unit at GTI Combustion Laboratory.
Exhaust
Damper
Orifice
plate
TC and
analyzer port
4
Mixer
Thermocouple (TC)
Pressure
difference
2
3
TC and
analyzer
port
1
5
TC
TC
Flow
meter
Furnace
6
TC and
analyzer
port
8
Natural
gas
TC and
analyzer port
TC and
analyzer port 9
Reformer
7 TC
and Burner
#1
analyzer
port
Flow meter
Flow meter
Combustion air
Reformed gas
Flow meter
Flow meter
Combustion air
Burner
#2
Exhaust
results by GTI regarding TCR chemical efficiencies (increase in heating
value not including increase in
thermal energy) for varying reforming temperatures and flue
gas/natural gas ratios. At a reforming temperature of 1150°F
(620°C) and a flue gas/natural gas
ratio of 2.5, the reformed fuel heating
value is about 10% higher than that
of natural gas.
GTI is also currently developing
reciprocating engine applications,
with initial work funded by the Utilization Technology Development
(NFP), a consortium of natural gas
distribution companies (NYSERDA)
and the California Energy Comminssion (CEC). In the project, a benchscale recuperative reformer will be
built and tested for operating on a 50
kWe research engine.
The design basis will be established
for scale-up of the technology. Various
arrangements will be studied for applying TCR to lean-burn and stoichio-
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HEAT TREATING PROGRESS • AUGUST 2007
metric versions of a Cummins
QSK19 gas engine including a
demonstration for 500 hours on a
331 kWe engine at Cummins Technical Center in Columbus, Ind.
Future Plans
GTI intends to continue collaborative efforts with industry and
government. More work is
needed to gain a more complete
understanding of TCR operational
parameters and control and to design practical and cost-effective
TCR Systems. Dave Rue, R&D
Manager – Process Heating states:
“We expect TCR to become a significant option for industry to consider in the future when addressing energy efficiency, and
that GTI will play a major role in
helping bring the technology to the market.”
Fig. 7 — Actual lab-scale TCR test unit at GTI
Combustion Laboratory using a modified heat-treat
furnace having one burner simulating a furnace
process and a second burner simulating a reformed
fuel burner.
Literature
N. Maruoka, et. al., Feasibility Study
for Recovering Waste Heat in the Steelmaking Industry Using a Chemical Recuperator, ISIJ International, Vol 44, No.
2, p 257-262, 2004.
Alternative Heat Recovery Method
Based on Methane Reforming, C&EN,
July, p 26-27, 1985.
Demonstration Industrial Heating Furnace with ThermoChemical Recuperation, Institute of Engineering Thermophysics of the National Academy of
Sciences of Ukraine.
Catalytic Reforming of the Natural Gas
in the Industrial Furnace Heat Recovery
System, translated from the book Cat-
alytic Reforming of the Hydrocarbons
(Kataliticheskaja Konversija Uglevodorodov), Kiev, USSR, Naukova Dumka, Issue
3, p 81-86, 1978.
New Power-Saving Technology of
Fuel Utilization, Journal of the Institute of
Engineering Thermophysics of the National
Academy of Science of Ukraine.
V.G. Nosach, Fuel Energy, Kiev, USSR,
Naukova Dumka, 1989.
For more information: Steve Sikirica is
Institute Engineer, Gas Technology Institute, End Use Solutions Section, 1700
South Mount Prospect Rd., Des Plaines,
IL 60018; tel: 847-768-0859; fax: 847-7680600; e-mail: steve.sikirica@gastechnology.
org; Web site: www.gastechnology.org.
25
Flue gas/natural gas (FG/NG) mole ratio ~2.5
Flue gas/natural gas (FG/NG) mole ratio ~0.8
20
Theoretical, FG/NG = 2.5
DHHV/HHV, %
Theoretical, FG/NG = 0.8
15
10
5
0
800
850
900
950
1000
1050 1100
Temperature, °F
1150
1200
1250
1300
Fig. 8 — Chemical efficiency of natural gas reforming with flue gases: DHHV (fuel higher
heating value increase due to reforming); HHV (natural gas higher heating value).
HEAT TREATING PROGRESS • AUGUST 2007
31