Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
1Recycling Technology of De-NOx Catalysts
for the Effective Use of Thermal Power Plant’s
Recycled Resources

DAISUKE MUKAI*1

HIROSHI KAKO*2

TAKAHIRO SUKENOBU*3 KATSUMI NOCHI*4
MASANAO YONEMURA*5 KAZUHIRO IWAMOTO*5

It is essential to efficiently utilize recycled resources for the establishment of a sustainablesociety. For thermal power plants, the reduction of the amount of wasted valuable and useful
resources is also required. Since De-NOx catalysts that detoxify nitrogen oxides (NOx) incombustion exhaust gas contain expensive metals such as titanium and tungsten, there is strong
desire for technology to reuse them instead of discarding them after use in power plants.
Therefore, Mitsubishi Power, Ltd. (Mitsubishi Power) has developed a technology for reusing
De-NOx catalysts used for coal and heavy oil-fired exhaust gas. The technology for reusing spent
catalysts not only reduces the amount of waste and the waste disposal costs of our customers, butalso contributes to securing rare recycled resources and preserving the global environment.

|1. Introduction
The development of dry De-NOx catalysts commenced in the 1960s, and in 1977, theworld’s first De-NOx device for thermal power generation was commercialized. Since then,De-NOx devices or De-NOx catalysts have been provided to more than 1700 commercial andindustrial boilers or gas turbines(1).
A De-NOx catalyst is one of the key components of a De-NOx device that detoxifies NOx inthe combustion exhaust gas. With the evolution of boilers and gas turbines, the development ofNOx catalysts has continued so that solutions that meet the needs of our customers can beprovided.
This report describes the catalyst improvement development that Mitsubishi Power has
implemented in recent years as a technology for reusing De-NOx catalysts contributing to thereduction of waste from thermal power plants.

|2. Technology for reusing De-NOx catalysts for coal-fired power
plantsWhen ash (fly ash) generated by the combustion of coal flows through the gas passage holeof the honeycomb catalyst together with the exhaust gas, components such as calcium in the fly ash
gradually adhere to the inner wall surface of the gas passage hole and the NOx removal reaction on
the catalyst surface is inhibited. In addition, the fly ash itself partially accumulates inside the gas
passage hole, which makes the exhaust gas flow difficult, and ultimately the gas passage holebecomes completely plugged and the NOx removal performance decreases. As a measure tomaintain the NOx removal function, it is necessary to replace the spent catalyst periodically with afresh one. However, De-NOx catalysts contain rare valuable resources such as titanium, tungsten
and molybdenum, so the fresh one is expensive. For this reason, there is a strong need to reduce
De-NOx catalyst renewal costs and waste (spent catalyst) treatment costs. Therefore, reusing spent*1Manager, AQCS DeNOx Catalyst Solution Department, Mitsubishi Power, Ltd.

*2

Chief Staff Manager, AQCS DeNOx Catalyst Solution Department, Mitsubishi Power, Ltd.

*3

AQCS DeNOx Catalyst Solution Department, Mitsubishi Power, Ltd.

*4

Research Manager, Chemical Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*5

Chemical Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
2

catalysts that were previously discarded not only contributes to the reduction of waste from power
plants, but also contributes greatly to the establishment of a sustainable society and theconservation of the global environment.
We have commercialized a regenerated catalyst that has NOx removal performance the sameas that of a fresh one by removing the substances that cause the deterioration of the NOx removal
reaction over time, and then applying an activation treatment of coating the surface with freshcatalyst components(1)(2). Two types of regenerated catalyst manufacturing methods have been
established: Ecotype I, which uses the spent catalyst removed from the De-NOx reactor as-is as ahoneycomb substrate , and Ecotype II, which crushes the spent catalyst to make a raw material andthen shapes it into a honeycomb shape for use as a substrate. Ecotype I uses the spent catalyst as asubstrate as-is, so if the spent catalyst has significant cracks or damage, it cannot be reused andneeds to be replaced with a fresh catalyst. On the other hand, Ecotype II crushes the spent catalyst
to make a raw material and then shapes it into a honeycomb shape for producing a substrate, so adamaged catalyst can be effectively regenerated. The coating layer of the fresh catalyst component
provided on the honeycomb catalyst by the activation treatment has been proven to have excellent
durability due to the bimodal coating technology using our unique two-particle-size catalyst shownin Figure 1 even in a coal-fired boiler exhaust gas environment where a large amount of fly ash ispresent. Both Ecotype I and Ecotype II have already been used in many plants.

Figure 1 Schematic diagram of bimodal coating technology using two-particle-size catalyst
This figure indicates that a coating layer is provided on the honeycomb catalyst. The size of thecircles in this figure indicates the coating particle size. A stable coating layer is formed by ourunique bimodal coating technology.

Furthermore, in recent years, we have developed a chemical cleaning technology that
restores the NOx removal performance of spent catalysts by cleaning them with a unique chemicalin order to deal with various substances that cause deterioration over time. This technology canremove not only fly ash and the calcium component contained in fly ash, but also catalyst
poisoning components causing the deterioration of the catalyst that cannot be removed by cleaning
with water. Catalysts regenerated using this technology have a small amount of highly-insoluble
catalyst poisoning component residue, and the NOx removal performance is slightly lower than thatof a fresh catalyst, but we used our unique chemical to realize an optimal cleaning process that does
not require activation (Figure 2). As a result, significant recovery of the NOx-removal performance
without increasing the SO2 oxidation rate compared with the spent catalyst before cleaning was
made possible (Figure 3). Catalyst regeneration using chemical cleaning technology is applied toeach of the catalyst modules, so the process is simple (Figure 4). This technology can satisfy thedemands of our customers for short delivery times, while also being economical. With thistechnology, we succeeded in greatly expanding the after-sales service options for catalyst
replacement and have already successfully completed the catalyst regeneration work for coal-fired
power plants in Japan. We have also demonstrated that the catalysts regenerated using thistechnology have excellent durability.

Figure 2 Comparison of
De-NOx performance

Figure 3 Comparison of
SO2 oxidation rate

Figure 4 Photograph ofcatalyst cleaningMitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
3|3. Technology for reusing De-NOx catalysts for heavy oil-fired
plantsIn the case of a heavy oil-fired power plant, when dust generated by combustion flows
through the gas passage hole of the honeycomb catalyst together with the exhaust gas, components
such as vanadium contained in the fuel gradually adhere to the inner wall surface of the gas passage
hole and the NOx removal reaction on the catalyst surface is inhibited. Vanadium is known to be acomponent that increases the SO2 oxidation rate of De-NOx catalysts and when deposited on thecatalyst, not only decreases the NOx removal performance over time, but also increases the SO2oxidation rate. Since this vanadium compound contains an insoluble compound and is firmly
deposited on the catalyst, it was difficult to remove the insoluble compounds to restore NOxremoval performance while reducing the SO2 oxidation rate.
Therefore, in order to remove the firmly deposited insoluble vanadium compound, wedeveloped a cleaning and regeneration technology using our unique chemical. This technology can
selectively remove the insoluble vanadium compound, which is a component that inhibits catalyst
performance, while suppressing the elution of active vanadium components contained in thecatalyst in order to control the NOx removal performance. We realized an optimal cleaning processthat does not require activation by using our own chemicals and developing an efficient cleaning
method. As a result, the NOx removal performance was restored to almost the same level as thefresh catalyst and the SO2 oxidation rate could be significantly reduced compared to that beforecleaning (Figure 5 and Figure 6). Similar to the regeneration of catalysts for coal-fired plants, thiscatalyst regeneration using chemical cleaning technology is applied to each of the catalyst modules,
so the process is simple. This technology can satisfy the demands of our customers for shortdelivery times, while also being economical (Figure 7). We have successfully completed thecatalyst regeneration work for heavy oil-fired power plants in Japan using this technology and havealso demonstrated that the regenerated catalysts have excellent durability.

Figure 5 Comparison of
De-NOx performance

Figure 6 Comparison of
SO2 oxidation rate

Figure 7 Photograph ofcatalyst cleaning|4. Conclusion
In recent years, there has been demand for catalyst reuse technology that can contribute not
only to reducing waste from thermal power plants, but also to environmental conservation.
Therefore, Mitsubishi Power has developed (1) technology for reusing spent De-NOx catalysts forcoal-fired plants and (2) technology for reusing spent De-NOx removal for heavy oil-fired power
plants. Currently, both reuse technologies have been put into practical use. In the future, it isexpected that the effective utilization of recycled resources will be promoted toward theestablishment of a sustainable society, and that waste including valuable resources from power
plants will be reduced. We will continue to respond to customer requests and accelerate related
development in order to offer NOx removal device that can contribute to environmental
conservation and resource security, while also being economical.

References(1)(2)

Business & Strategic Planning Headquarter, Mitsubishi Hitachi Power Systems, Ltd., Mitsubishi Heavy
Industries Technical Review Vol. 52 No. 2 (2015), p.101~104
Tomotsugu Masuda，PETROTECH，Vol.37, No.11 (2014) p.847~852