Poster

P-17

Heat recovery from low-temperature off-gas using micro-channel heatexchanger
Kazuaki KOBAYASHI1*, Yuki KUWAUCHI1 2, Yuji OGAWA11 3

research group based on economic rationality. However CO2 amount recovered with utilizing off-gas
specific heat is expected to around 2.4 million ton-CO2/year. Therefore, heat recovery will amountabout 3.9 PJ/year. Note that 115 oC saturated steam is required to heat the amine aqueous solution
from the viewpoint of process design.

1Process Technology Div., Process Research Laboratories, NIPPON STEEL & SUMITOMO METALCORPORATION, 20-1 Shintomi, Futtsu, Chiba, 293-0021 Japan.
2
Now at NIPPON STEEL & SUMITOMO METAL U.S.A., INC., 1251 Avenue of the Americas, Suite2320, New York, N.Y. 10020, U.S.A.
3
Now at Steel Foundation for Environmental Protection Technology, 3-2-10 Nihombashikayabacho,Chuo-ku, Tokyo, 103-0025 Japan.
Abstract: In the COURSE50 project, a CO2 separation and recovery system using an amine aqueous
solution has been considered. In the system, the aqueous solution should be heated in a recovering
process for the desorption of CO2.The recovering temperature is less than 100 oC; thus, exhaust gasesof steel plants can be utilized for heating. In this research, we first appraised the quantity of heatrequired in the project to recover 20 % of the CO2 emitted from a representative steel works, designed
a heat-recovering network to collect the heat from exhaust gas sources across the steel works, andfound that pressured water was appropriate as the heat recovering medium. Second, we estimated therequired efficiency of heat exchangers and realized that a heat exchanger with micro water vessels wasappropriate because the flow rate of pressured water is much less than the gases. We designed andmanufactured trial one and measured its efficiency in our laboratory using air and water. Finally, weexperimentally measured the variation of its efficiency with time using real exhaust gas of a furnace ofa steel plant.
1. INTRODUCTION
In the COURSE50 project, 30 % reduction of CO2 emitted from all steel mills in Japan by 2050 istargeted. 10 % of the reduction will be achieved by reducing coke usage in blast furnaces, and 20 %will be carried out by capturing CO2 emitted from blast furnace gas.
Chemical absorption process and physical adsorption process have been considered in the project
as the CO2 separation and recovery method. In the chemical process, the gas with rich CO2 isdissolved into low-temperature amine aqueous solution, the aqueous solution including CO2 is carried
to recovery process, and is heated to separate and capture CO2 gas from the solution.
As one of the accomplishments of the COURSE50 project, recovering temperature of the amineaqueous solution has been lowered to 95 oC and desorption calorie will be reduced to 1.6 GJ/ton-CO2.
For such low-temperature heating, exhaust gases from steel plants can be utilized as the heat sources.
Thus a technology to recover and transport the heat of unutilized low-temperature off-gas fromsteel mills is studied to provide the heat to the recover process in our research group.
In this article, we first appraised the quantity of heat required in the project to recover 20 % of theCO2 emitted from a representative steel works. Second, we discussed the thermal medium which isappropriate for this heat recovering. Finally, we designed and manufactured a heat exchanger for heatrecovering of low-temperature off-gas, and measured the variation of its efficiency.
2. REQUITED QUANTITY OF HEAT
Considering an integrated steel mill with 8 million ton/year of steel production and 1.9
ton-CO2/ton-steel of CO2 emission intensity, the CO2 emission amounts to about 15 million ton/year.
The CO2 reduction target is 20 % of the emission; therefore it is nearly 3 million ton-CO2/year.
The volume and temperature of off-gas emitted from a virtual integrated steel mill in Japan are
shown in Fig. 1, and the layout of the mill is shown in Fig. 2.
In the COURSE50 project, both of chemical absorption process and physical adsorption process
are availed to achieve the total amount of CO2 capture, but the recovery share is studied in other*

Corresponding author. E-mail: kobayashi.6cq.kazuaki@jp.nssmc.com, telephone: + 81-80-4602-1501.

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Figure 1.

Off-gas volume and temperature of a model integrated steel mill.

Figure 2.

Plant layout of a model integrated steel mill.

3. THERMAL MEDIAIn this section, we discuss which thermal media can be applied for heat recover from off-gases andheat transporting from heat emission sources to the recovering process.
Solid, liquid, gas and electricity are considered as thermal media. Here, solid is generally difficult
to carry, so it is excluded from consideration. Moreover, the efficiency of power generation using
low-temperature gas is around 10 % of input energy, so it is also excluded from consideration.
Off-gas itself, air, steam, oil and compressed water are discussed as thermal media, and amount ofrecovered thermal energy available at recovery process is calculated. The calculation condition isshown in Fig. 3.
A case with oil used as the heat recovery carrier is explained below as an example. First, off-gas
calorie is recovered using oil. In this heat recovering procedure, the temperature deference of pinch
point for heat exchange is assumed to be 20 oC. However, maximum temperature of oil is confined to
be 300 oC. Secondly, the oil is transported through a pipe line to the recovery process. Here, 2 m/s of
economic transport velocity is adopted as the transport rate of oil, and 80 mm thickness of heatinsulator is wrapped around the line pipe. Next, steam is generated in a boiler using the thermal energyof the oil. The pinch point is also assumed to be 20 oC, and steam generation loss is assumed to be
10 %, thus 90 % of energy transferred from the oil to boiled water is actively used. Finally, the oil istransported back to the heat source.

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Poster

In addition to the above description, maximum temperature of compressed water is limited to 180
C and steam is generated in a flash tank with its efficiency being also 90 % in the compressed water
case, and economic transport velocity for gas and steam is assumed to be 20 m/s.
oFigure 3.

channels to improve efficiency, and corrugate fins were selected for gas channels to reduce dust
adhesion.
An experimental result of thermal efficiency of the developed micro heat exchanger in ourlaboratory is show in Fig. 7. Hot air and cold water were used in the experiment as high andlow-temperature fluid. We can see that the performance of the developed exceeds the target thermal
efficiency of 0.66 (66 %) under wide water equivalent ratio. In Fig. 7, the efficiency of a conventional
fin-tube type heat exchanger is also shown. Comparing both results, we can notice that theperformance of the developed heat exchanger is better than that of the conventional one.
We will experiment the developed micro heat exchanger in a steel plant to make long-term
evaluation.

Heat recovering carriers and heat loss source.

Steam calorie available at the recovery process is calculated under the above assumption in Fig. 4.
In Fig. 4, we can see that heat dissipation loss at transporting in off-gas and air cases are large. We
can also see that heat exchanging loss in air and oil cases are sizable because heat exchange is done
twice. Therefore, steam and compressed water are seemed preferable for the project as the heat carrier
because both of transporting and heat exchanging loss are small. Furthermore, the loss of compressedwater is slightly smaller than that of steam, thus compressed water is adopted as the heat carrying
medium.

Figure 4.

Utilizable heat quantity andheat loss of off-gas.

Figure 5. Relation of CO2 recovery amount andthermal efficiency.

Figure 6. Schematic diagram of thedeveloped micro heat exchanger for
low-temperature off-gas and water.

Experimental result of thermal
Figure 7.
efficiency of the developed micro heat exchanger.

4. HEAT EXCHANGER FOR LOW-TEMPERATURE OFF-GAS HEAT RECOVERY
Thermal efficiency of heat exchangers to recover the required quantity of heat is discussed in thissection. Thermal efficiency of high-temperature fluidand water equivalent ratio
are used toexpress the performance of heat exchangers.
(1)(2): specific heat,: fluid temperature,: mass flow rate,
h: high-temperature side, c: low-temperature side, i: inlet side, o: outlet side.
The relation between amounts of CO2 separated using recovered heat and thermal efficiency of heatexchangers are shown in Fig. 5. We can see the target thermal efficiency is 66 %.
Note that water equivalent ratio is within a certain range. First, water equivalent ratio should be set
small so that the compressed water temperature does not exceed 180 oC for the off-gas heat source
above 180 oC. Secondly, the ratio should be set to 1.0 in order to obtain the water as high as possible
for the heat source less than 180 oC. 1.0 of water equivalent ratio means that volume ratio of gas andwater is over 1,000, therefore it is necessary to adopt micro channels as water vessels in order to makethe heat exchanger compact.
A schematic diagram of the micro heat exchanger developed in this project for low-temperature
off-gas and water heat recovery is shown in Fig. 6. S-shape fins [1] were adopted for water micro

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5. CONCLUSION
Heat recovery technology for low-temperature off-gas from steel plants was studied in order tosupply the heat for recovery plants of chemical CO2 absorption process. The target of the heatrecovery quantity was specifically discussed and compressed water was selected as the thermal
carrying medium. A micro heat exchanger with S-shape fins for water channels and corrugate fins forgas channels was developed for the heat recovery. The performance of the developed micro heatexchanger was experimentally measured and it exceeded the objective thermal efficiency of 0.66.
Acknowledgements: This article is based on results obtained from "Development of technologies forenvironmentally harmonized steelmaking process, 'COURSE50'" project commissioned by the New
Energy and Industrial Technology Development Organization (NEDO).
REFERENCES[1] Tsuzuki, N., Kato, Y., Nikitin, K., and Ishizuka, T., J. Nucl. Sci. Technol., 46 (2009), 403–412.

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