The emission control of non-CO2 greenhouse gases is conducive to slowing down global warming. It is also helpful in controlling environmental pollution, and beneficial in improving the local health benefits. This paper aims at six kinds of non-CO2 greenhouse gases under United Nations Framework Convention on Climate Change, namely methane (CH4 ), nitrous oxide (N2 O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6 ), and nitrogen trifluoride (NF3 ). This paper analyzes the emission status and trend of China’s non-CO2 greenhouse gases, and provides some technology selections for non-CO2 emission reduction. Through strategic policy arrangements and appropriate technology choices, China can gain environmental protection and greenhouse gas control.
non-CO2 emission ; status ; trend ; technology selection
The Kyoto Protocol (KP) signed in 1997 clearly defined the remaining five kinds of greenhouse gases (GHGs) besides CO2 , namely methane (CH4 ), nitrous oxide (N2 O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6 ). During the 2011 Durban negotiations, the second commitment period of KP, nitrogen trifluoride (NF3) was added to the GHG index. Thus, under United Nations Framework Convention on Climate Change (UNFCCC), there are six kinds of non-CO2 GHG, which can be divided into three broad categories: CH4 , N2 O, and fluorinated gases (F-gases). CH4 and N2 O are natural gases which already exist, and increase due to human activities, while F-gases are entirely the product of human activities. In order to measure the global warming effect of these non-CO2 GHGs, the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) introduced the concept of CO2 equivalent (CO2 -eq). Through multiplying one gas’s emissions in tons and its global warming potential (GWP), we can obtain the CO2 -eq of this gas. By such a method we can standardize the warming effect of different GHG.
Due to the dual pressures caused by global warming and environmental pollution, the international community is beginning to focus on non-CO2 GHG emissions. The Nordic region (Denmark, Finland, Faroe Islands, Iceland, Norway, Sweden and the Aland Islands) signed the Svalbard Declaration, aiming at reducing the short-lived climate forcers (SLCFs) in March 2012, and released the report of Nordic Workshop on Action Related to Short-lived Climate Forcers on December 26, 2012. The report regarded that taking early actions on CH4 , black carbon (BC) and some F-gases could reduce the speed of global warming in short term (20–30 years). In 2010 the global non-CO2 GHG emissions were 11.702 Gt CO2 -eq [ EPA , 2012a ], accounting for 27.8% of the total GHG emissions. IPCC  projected that emissions of HFCs will triple in 2015, which are due to the growing use of HFCs in the refrigeration industry, and also the HFC-23 emissions as byproduct of increased production of HCFC-22 (chlorodifluoromethane). Nordic Air Quality Working Group [ NCCAQ , 2013 ] considered that, although the current proportion of HFCs is not high, its contribution to global warming will reach 19% in 2050. Rao and Riahi  argued that under the same reduction targets, reducing CO2 and non-CO2 simultaneously will cut down mitigation costs significantly in a short time. Lucas et al.  argued that CH4 related to the energy activity and F-gases have huge reduction potential (in 2100 will reduce by about 90% compared with the baseline① ), while the CH4 and N2 O related to land have difficulty in achieving emission reductions.
Domestic studies on the F-gases also began to take shape. It is pointed out [SNARCCWC , 2011] that China’s F-gases are mainly concentrated in economically developed urban agglomerations and industrial areas, including Beijing, Tianjin, Hebei, the Yangtze River Delta, Pearl River Delta, Hong Kong and Taiwan. Domestic scholars have done a lot of work on CH4 reduction. Huang  argued that from the view of national sustainable development, the advantage of levying tax on GHG emissions from rice paddies outweighs the disadvantage. Ma et al.  found that changing the fertilization strategy can effectively reduce CH4 emissions from rice paddies. Le et al.  have shown that China’s largest source of anthropogenic CH4 emissions comes from coal mining, followed by enteric fermentation and rice cultivation in the second and third place. Pan et al.  argued that the development of biochar technologies can not only solve the problem of GHG released from the combustion of straw, but also improve the soil and increase production, and will be a future potential technology option of climate change mitigation. In addition, there are also studies calculating GHG emission reduction potential of agricultural waste disposal at different costs [ Qin , 2012 ].
China emits much more non-CO2 GHG than any other countries. In 2010, China’s non-CO2 GHG emissions accounted for 13.6% of the global emissions. With rapid urbanization and industrialization moving forward, China will inevitably consume a lot of energy and resources, and produce more emissions. Facing global climate change problem issues, China can cut down non-CO2 GHG emissions to reduce international pressure on CO2 emissions, and to provide more flexible spaces and policy choices for China’s low-carbon economy. As survival time of some non-CO2 GHGs in the air is short, and there are many practical and effective reduction measures, the effect of non-CO2 GHG emission reduction will be seen in a short term. In addition, the reduction of non-CO2 GHG emissions will also bring co-benefits to health, economy and agricultural activity. Under the strategic policy arrangements and appropriate technology choices, China can gain environmental protection and GHG control.
In 2010, China’s non-CO2 GHG emissions were 1.587 Gt CO2 -eq [ EPA , 2012b ], accounting for 18% of the total GHG emissions. Among them CH4 accounted for 58.3%, N2 O accounted for 26.2%, and F-gases accounted for 15.5%.
The GWP of CH4 is 25 times as that of CO2 (in 100 years), and its lifetime is 12 years. About 40% of atmospheric CH4 emissions are from natural sources, including wetlands, alpine meadow plants, termites and oceans. The main sources of anthropogenic CH4 are ruminant feeding, petroleum systems, landfills, rice cultivation, coal mining, wastewater treatment, and biomass burning. In China’s oil and gas industry, 39% of CH4 emissions is from the production of crude oil, 30% is from natural gas production, and 16% comes from natural gas transportation [ Yang and Chen , 2012 ]. On the landfill aspect, China’s current annual production of landfill CH4 is more than 5 billion m3 , which can be transformed into about 20 billion kW h electrical energy. In those areas with more dense populations and higher economical development, the landfill gas production is higher, showing the geographical decreasing trend from east to west. If full use of landfill gases is achieved, China’s annual consumption of alternative natural gas for livelihood will be about 30% [ Bu and Zhang , 2012 ]. Paddy fields are one of the most important sources of atmospheric CH4 emissions. According to the IPCC Fourth Assessment Report [ IPCC , 2007 ], the annual CH4 emissions from paddy fields are 31–112 Mt, accounting for 5%–19% of the total global CH4 emissions. China is the world’s largest producer of rice, and the rice paddies fields account for 22% of the world’s total, so the quantity of CH4 emissions from rice paddies is larger.
The main factor leading to the atmospheric N2 O increase is the excessive use of agricultural nitrogen fertilizer, which causes an increase in emissions from agricultural soils. The GWP of N2 O is 296 times that of CO2 (in 100 years), and its lifetime is 120 years. China’s N2 O emissions from industrial production process have three main sources: the nitric acid production process, adipic acid production process, and caprolactam production process. Sewage treatment process is another important source of N2 O release. China has the largest sewage treatment capacity in the world. The N2 O emissions of landfill account for 3% of the total N2 O emissions. Studies have shown that landfill sites and leachate treatment plants are the main sources of N2 O from municipal solid waste landfill. The analysis and estimation of China’s regional N2 O source show that the natural source and anthropogenic source accounts for 71% and 29%, respectively [ Wang and Su , 1993 ].
Although currently the F-gases’ proportion of the total GHG is not high, these gases generally have a high GWP level, which is hundreds or even tens of thousands times that of CO2 . In 2005, China’s F-gases emissions during industrial process were 93–282 Mt CO2 -eq. With the help from an ideal technical and financial support, the relative reduction potential of China’s industrial process F-gases by 2020 will be 96–346 Mt CO2 -eq [ Huang , 2012 ]. In the 1990s, HFCs were first produced and applied in developed countries, and the United States was the largest output country at that time. After 2007 HFCs entered the fast growth phase in developing countries, especially since the financial crisis, a number of developed countries have started to transfer HFCs production capacity to China, and as a result China’s HFCs production has surpassed that of the United States. With the rapid development of the domestic auto industry, the policy implementation of electrical home appliances to the countryside as well as the real estate development, China’s HFCs are experiencing rapid expansion of production capacity.
PFCs have excellent stability, low surface tension, water and oil repellency, and their production has over 50 years history. PFCs are widely used in chemical, textile and other fields. PFCs can be accumulated in blood, liver, kidneys and other organs of animals and humans. With long-distance transmission capacity, PFCs thus have a wide range of contamination. China has big production and use of PFCs organic compounds, and some studies indicate that there are PFCs contamination in water, biological and other media.
Although the impact of SF6 accounts for only 0.1% currently, its GHG effect is 25,000 times that of CO2 , and it has a very long lifetime about 3,400 years. Foundries use SF6 as a protective gas to prevent the high melting magnesium from being oxidized. Usually all SF6 used as a protective gas, will eventually be released into the atmosphere. According to SNARCCWC , China will emit 3–33 Mt CO2 -eq SF6 during magnesium production process in 2020.
As an excellent plasma etching gas, NF3 is widely used in chip manufacturing and high-energy lasers. In recent years it has been widely used for liquid crystal displays and photovoltaic fields. Most NF3 is destroyed in the production process, but there is some slip escape into the atmosphere, which can retain in the atmosphere for 740 years. For 2005–2011, the actual average annual growth rate of global NF3 production was 19.4% [ CCN , 2009 ]. China has large capacity in the production and use of products from the semiconductor industry, photovoltaic industry and display industry. During the development of these emerging industries, there will be inevitably large NF3 emissions.
According to the United States Environmental Protection Agency (EPA) forecast data (Fig. 1 ), regardless of the total emission perspective or the emission growth rate perspective, the amount of China’s non-CO2 emissions takes the first place in the world, and the two indices are both far above those of other countries. Such a situation is really worrying, because some non-CO2 GHGs exist for a long time with high GWP, and have extremely negative impacts on the global environment. In the future, China will face huge international pressure to reduce emissions, and the domestic environmental conditions will deteriorate, possibly resulting in the economy and society stepping into a period of extremely unhealthy development.
The global non-CO2 emissions before 2030 [ EPA , 2012b ]
Seen from Figure 2 , before 2030 among China’s non-CO2 GHG emissions, CH4 will rank first for a long time, while N2 O will increase slowly. However, F-gases will show a sharply accelerated upward trend, and in 2030 it is likely to exceed CH4 . Since F-gases are mainly emitted during the industrial processes, with the rapid development of China’s automobile industry and new energy industry, there will surely be much more F-gases used in the manufacturing process. Therefore, finding a way to effectively control F-gases emissions, reducing its escape as well as leakage, and disposing the end gas safely, will become the future top priority of non-CO2 GHG reduction strategy.
China’s non-CO2 emissions before 2030 [ EPA , 2012b ]
Most of CH4 emissions are from agricultural activities, hence there is a certain degree of difficulty in achieving rapid reduction. Comparing with continuous submergence during rice growing period, baking the field, intermittent irrigation and wetting irrigation can reduce CH4 emissions from rice paddies by 45%, 59% and 83%② , respectively. Comparing with stable manure, green manure and straw manure, no fertilizer or chemical fertilizer can reduce CH4 emissions by 50%–66%, while compost or bio-gas residues can reduce CH4 emissions by 45%–62%. By changing the planting and fertilizing method in the agricultural production process, China can reduce CH4 emissions. During the coal mining process, China needs to quickly develop gas production technologies and processes which are suitable for the storage conditions of coal bed gas to reduce CH4 escape. In terms of waste disposal, the main artificial controlling measures of CH4 emissions include: 1) the classification of waste and landfill, taking full advantage of CH4 generated from organic waste as fuel or electricity; 2) improving landfill facilities, and reducing landfill humidity to suppress the generation of CH4 ; 3) screening CH4 -oxidizing bacteria with good environmental adaptability, and inoculating it in landfills, promoting the oxidation of CH4 ; 4) changing the landfill construction from oxygen-weary type to semi-aerobic type or leachate recirculation semi-aerobic type.
N2 O emissions from agricultural soil accounts for a large proportion. Although its fast reduction cannot be achieved in a short term, we are still able to reduce N2 O emissions by artificial means, such as using precision fertilizer, improving efficiency of nitrogen fertilizer and avoiding excessive fertilization. In China the fertilizer utilization rate of rice and wheat is only 28%–41%, hence choosing the appropriate nitrogen fertilizers, applying deep application of fertilizer or mixed fertilizer and using nitrification inhibitors can significantly lower emissions. During the process of farmland cultivation, the future mainstream strategy will be to vigorously promote formula fertilization by soil testing. If the formula fertilization by soil testing can become the nationwide standard, the fertilizer utilization rate is expected to increase by more than 3%, and can reduce farmland N2 O emissions by around 3% [ Qin , 2012 ]. According to the statistics of UNFCCC③ , the current abatement cost of N2 O in the CDM (clean development mechanism) projects is about $9 per ton CO2 -eq.
Most of F-gases will generate toxic gases and powders after decomposition, which result in strong irritation and corrosion for the upper respiratory tract of humans as well as animals. There should be a requirement for the allowable concentrations of SF6 and its toxic decomposition. It is suggested that in the workplace where SF6 is produced and used, the maximum concentration of SF6 should not exceed 6,000 mg m–3 . Besides, it is suggested that measures be taken to improve gas recycling of SF6 and its composition, reduce leakage rates and adopt other substitute gases. As a byproduct and escape gas, technically speaking, more than 90% of HFC-23 can be captured and eliminated from the production of HCFC-22. However, from now until 2015, the anticipated production of HCFC-22 will increase, and the HFC-23 emissions may increase by 60%. A very effective mitigation option to capture and eliminate HFC-23 is thermal oxidation, and the clearance rate can reach 99% or more, but the impact of thermal oxidation equipment downtime on emissions must be taken into consideration. Assuming the lifetime of HFC-23 is 15 years, the abatement cost is less than $0.2 per ton CO2 -eq.
This work was supported by the Policy Study on China’s Low-carbon Industrial Development (No. 201312).
①. The baseline refers to emissions without any abatement measures
②. The data are obtained by collecting and analyzing CH4 emission data of more than 70 domestic papers and 350 different water management tests