Development Trends of Non-thermal Plasma （ NTP ） Technology Used in Environmental Pollutants Treatment: A Patent-Bibiometric Analysis

. Non-thermal plasma Technology (NTP) has been widely used in various fields, especially in environmental pollutants treatment field. This paper provides an overview of the global NTP patent landscape by analyzing relative patents based on CNABS and DWPI, selected from 1987 to 2019, which covers patenting activity related to technologies in pollutant removal from the process in the above-mentioned areas. The most common technologies for each area were identified and their developments were analyzed over time. Information related to the main countries, key applicants, current activities and the technology strength were identified by synthesizing important findings from qualitative research and quantitative literature analysis.


Introduction
Non-thermal Plasma (NTP) technology has become a research focus in the field of pollutant control due to its advantages of high efficiency, low energy consumption, co-processing multiple pollutants, and simple operation [1][2][3][4] . Currently, NTP has been widely used in various fields, including environmental pollution control, metallurgical refining, etching and material surface treatment, ozone production, etc. [5][6][7] , among which flue gas and wastewater treatment are mostly studied. Its advantages are low energy consumption, low pollutant removal rate, simple reactor structure, wide application range, no secondary pollution, and can achieve coordinated control of pollutants such as NO, SO 2 , and Hg 0 under normal temperature and pressure [8] .
Non-thermal Plasma Technology generates highenergy electrons through a high-frequency pulse discharge. The strongest oxidants such as hydroxyl, free radicals and ozone are generated when the high-energy electrons inelastic collision bombard flue gas molecules [1] , which will occur complex physicochemical reactions in the discharge field to remove pollutants [4] . According to the discharge principle of non-thermal plasma, it can be divided into corona discharge, dielectric barrier discharge, gliding arc discharge, glow discharge and microwave discharge. Only dielectric barrier discharge and corona discharge can produce nonthermal plasma under normal temperature and pressure [2] . The patent-bibiometric analysis in a certain technical field can make a preliminary judgment on the technology research trend in this field, and provide trend prediction for future technology trends [19][20][21] . This study aims to explore the bibliometric overview on NTP technology patents to understand the development trend of the technology in the field of pollution control from 1987 to 2019, and to reveal the technology gaps between China and other countries. Information support can also provide a reference for future patent layout of relevant Chinese institutions in this technical field.

Data sources and search results
The patents in 2017-2019 related to the NTP technology in Chinese Patent Abstracts (CNABS) and Derwent World Patents Index (DWPI) were collected in April 2020. Based on patent bibliometrics, the NTP treatment technology was classified into two and three levels according to the technical content, application situation or other technical characteristics. 3,328 Chinese patents related to NTP technology and 2,173 foreign patents were obtained (See search strategies in Table 1). Abstracts of these 5501 patents were read one by one, and irrelevant patents were removed. After screening, 1,293 Chinese patents (1,278 after 2000) and 149 foreign patents were retained. In the actual retrieval process, since it is difficult to determine whether it is "nonthermal plasma", in order not to omit data, all documents involving the keyword "plasma" are retained. In the manual analysis of data, each patent was indexed and classified as NTP technology according to the technology used. If it can be further distinguished, the NTP technology is further indexed as corona, dielectric barrier or other discharge mode. If it is indistinguishable, it will be indexed as NTP technology with pollutants and discharge mode index "Not clear". TS=(((purify* or recycl* or remov*) and (Hg or mercury or mercur* )) or demercur*) #5 TS=(((purify* or recycl* or remov*) and (SOx or SO2 or SO3 or ((sulfur or sulphur) and oxide))) or desulfur*) #6 TS=(((purify* or recycl* or remov*) and (NOx or NO2 or (nitrogen and oxide))) or denitr*) #7 TS=((purify* or recycl* or remov*) and (dioxin or PCDD or PCDF or chlorobenz)) #8 TS=((purify* or recycl* or remov*) and (VOC* or tvoc or (volatile or organic or compound*)))

Development of NTP technology in related patents
NTP technology has undergone a major transformation from space plasma exploration in the early 1960s to materials-oriented research in the 1980s and 1990s. With the rapid development of microelectronics science, environmental science, energy and material science, and physical technology, etc, has brought new opportunities to NTP technology. Since the 1980s and 1990s, global researchers began to use discharge plasma to oxidize Hg0. In recent decades, research on plasma technology has been very active, providing a new technology, new method and a new process for the synthesis of new substances, new materials and environmental pollution control.NTP technology degradation of pollutants is to use these high-energy electrons, free radicals and other active particles and pollutants in the exhaust gas to decompose pollutant molecules in a very short time, and various subsequent reactions occur to achieve the degradation of pollutants. The development trend of related patents from 1987 to 2019 is shown in Figure 1.  Fig.1). As shown in Fig. 2, China ranked No.1 with 1,237 applications after 2000, and the number of applications has increased significantly (from 73.9% to 86.56%) since 2017. The United States and Japan followed closely, accounting for 3.15%, and South Korea ranked No.4 with 2.94%. It can be seen that the patent applications of China, the United States, Japan, and South Korea account for nearly 90% of the total NTP technology applications. And the large number of patent applications in China is mainly due to its much more attention to NTP technology patenting activities in environmental protection and pollution control.

Thematic analysis on NTP technology patents on target pollutants
The NTP technology patent applications for different target pollutants from 2000 to 2019 are shown in Fig. 3. It can be seen that the NTP technology from 2000 to 2019 is mainly used for the removal of conventional pollutants such as denitrification, desulfurization, and removal of VOCs, which was affected by the general requirements of the air pollutant emission industry for these pollutants. In recent years, with the gradual tightening of national pollution emission standards, the application and patent applications of NTP in removing mercury, dioxins and other unconventional pollutants have gradually increased. As showed in Fig. 4   In 213 NTP technology patents from 2000 to 2019, there are mainly four discharge methods: corona discharge, dielectric barrier discharge, microwave discharge and gliding arc discharge. And according to the number of patents, corona discharge is a relatively mature NTP technology (13.41%). As shown in Fig. 6, dielectric barrier discharge accounted for the main part of the patents in each country. China has applied for more patents on dielectric barrier discharge (11.2%) and corona discharge (8.5%). And microwave discharge and dielectric barrier discharge, which were used more frequently in the United States, accounting for 11.1% and 8.9% in the number of related patents, respectively. The technology adopted in Japan was mainly corona discharge technology, accounting for 17.8%, and dielectric barrier discharge and microwave discharge (8.9%). Among these technologies, South Korea, applications for corona discharge and dielectric barrier discharge technology is equivalent, each accounting for 9.5%. In general, the different technology preference between China, South Korea (corona discharge technology) and the United States (microwave discharge technology) may be related to the positioning of different pollutant removal targets of the patented technology.  Table 2)

Conclusions and prospects
As the control of environmental pollution has been paid more and more attention, NTP technology is developing rapidly in recent years. And the number of Chinese patent applications has increased more quickly than the global applications, due to its increasing investment and development of NTP technology, which also shows that Chinese market's large demand for NTP technology, as well as the enterprises and R&D institutions' attention to the value of NTP technology. From a global perspective, the number of NTP patent applications in China currently ranks first, and countries and regions such as the United States, Japan, Europe and South Korea still maintain a strong momentum in NTP technology patent applications. Although these countries and regions have a lower proportion of patent applications than China, due to their large number of technical research and development personnel and strong technical strength, the development trend in subsequent NTP technology research and development and patent applications still cannot be ignored. Moreover, in order to continue to enhance core competitiveness in this field in the future, it is necessary to continue to increase investment in research and development of NTP technology on the basis of maintaining a rapid increase in the number of applications. In addition, as NTP technology has a wide range of applicability, major applicants also need to pay attention to the main research areas of advanced companies, establish their own research and development directions in the field, learn to write highquality patent application documents, and formulate corresponding intellectual property strategies.