Article pubs.acs.org/est
Reducing Nitrous Oxide Emissions to Mitigate Climate Change and Protect the Ozone Layer Li Li,† Jianhua Xu,*,‡ Jianxin Hu,† and Jiarui Han† †
State Key Joint Laboratory for Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China ‡ Department of Environmental Management, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China S Supporting Information *
ABSTRACT: Reducing nitrous oxide (N2O) emissions offers the combined benefits of mitigating climate change and protecting the ozone layer. This study estimates historical and future N2O emissions and explores the mitigation potential for China’s chemical industry. The results show that (1) from 1990 to 2012, industrial N2O emissions in China grew by some 37-fold from 5.07 to 174 Gg (N2O), with total accumulated emissions of 1.26 Tg, and (2) from 2012 to 2020, the projected emissions are expected to continue growing rapidly from 174 to 561 Gg under current policies and assuming no additional mitigation measures. The total accumulated mitigation potential for this forecast period is about 1.54 Tg, the equivalent of reducing all the 2011 greenhouse gases from Australia or halocarbon ozone-depleting substances from China. Adipic acid production, the major industrial emission source, contributes nearly 80% of the industrial N2O emissions, and represents about 96.2% of the industrial mitigation potential. However, the mitigation will not happen without implementing effective policies and regulatory programs.
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INTRODUCTION Global climate change efforts aim to build an inclusive universal binding agreement on emissions reductions under the United Nations Framework Convention on Climate Change (UNFCCC); this agreement requires the consent of all signatory parties. This results in the lowest of common denominators1 over the years which are inadequate to prevent the climate system from reaching a tipping point of no return. This situation has prompted climate change advocates to seek complementary approaches, which can be implemented either within nations or at the international level but covering fewer sectors and with less institutional complexities. Among them are reducing non-CO2 global warming agents, increasing biosequestration through forest protection, and implementing the much-debated geoengineering options. In particular, reducing non-CO2 global warming agents has garnered increasing attention in recent years.2−6 The “immediacy” of the climate benefit and/or the cobenefit of their reduction is expected to provide a strong incentive for nations to consider adopting policies and taking rapid responses.7,8 For instance, black carbon and tropospheric ozone are short-lived global warming agents;9 both have adverse human health effects, the control of which has positive climate and health benefits. There have been voices advocating their inclusion in successive post-Kyoto climate negotiations.10 © 2014 American Chemical Society
Nitrous oxide (N2O) has strong global warming potential (GWP) and is an ozone-depleting agent as well. Reducing N2O emissions offers the combined benefits of mitigating climate change and protecting the ozone layer. This has attracted attention in discussions of regulating ozone-depleting substances (ODSs) under the Montreal Protocol on Substances that Deplete the Ozone Layer.11,12 In this paper, estimates are made of industrial N2O emissions and their reduction potential in China to facilitate development of N2O phase-out strategies. Nitrous oxide is emitted from both natural and anthropogenic sources. The anthropogenic sources include agricultural activities, industrial chemical production, fuel and biomass combustion, and sewage treatment operations.11,13 Globally, the relative contributions from the various sources has remained steady over the years, with agricultural activities being the largest contributor (60%), followed by industrial processes and fuel combustion, each contributing about 10%.13 In China, agricultural activities are the largest (81%) sources as well, while the contributions of industrial processes and combustion were about 9.8% and 9.1% in 2007.14 Received: Revised: Accepted: Published: 5290
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China, few studies have been conducted to assess the mitigation potentials.26 A systematic investigation of historical and future emission trends, as well as the potential for mitigating N2O emissions, is needed. This study presents the annual industrial N2O emissions, historically from 1990 to 2012 and projected from 2013 to 2020, and potential for mitigation. Constraints and challenges for reducing N2O emissions are discussed.
In the climate regime, N2O is listed as one of the six greenhouse gases under the UNFCCC and the third largest radiative forcing contributor. Its 100-year global warming potential is 265 times that of CO2, and its contribution to net global radiative forcing is about 6% that of CO2.13 Although there has been a lot of academic and policy discussion about the benefit of reducing N2O, not much has been done to reduce N2O emissions other than in a few carbon markets, including the Clean Development Mechanism (CDM), the Joint Implementation, and the European Emissions Trading Scheme. In these markets, the abatement efforts are limited to industrial sector. Even there, not all N2O projects are viewed favorably due to suspected “carbon leakages”15 where some producers are suspected of operating new adipic acid plants solely for the profit generated from CDM credits.16 In the agricultural sector, policies specifically aiming at reducing N2O emissions are absent, and N2O emissions abatement so far is just the cobenefit of measures targeted at controlling nonpoint-source water pollution, such as enhancing fertilizer use efficiency and managing animal manure.11 These actions are mainly taken in developed nations. In most developing countries, there is not much progress on controlling agricultural N2O emissions due to poor agronomic management practices. Thus far in China, the only N2O mitigation measure has been abating industrial N2O from chemical production under CDM. In the ozone regime, N2O is the primary source of stratospheric NOx, a strong ozone depleting agent.11,17 The ozone depleting potential (ODP) of N2O is 0.017,17 which is comparable to that of HCFCs, the immediate substitutes of the first generation ODS, CFCs. In the past, efforts to protect the ozone layer have emphasized reducing CFCs and HCFCs. So far, 95% of the CFCs have been eliminated and the abatement of HCFCs is ongoing. With the progress on phasing out CFCs and HCFCs, N2O has become the largest anthropogenic source threatening the ozone layer for the remainder of the century.17 For this reason, regulating N2O under the Montreal Protocol is becoming a more important policy discussion.12 Estimating China’s industrial N2O emissions and the potential for reduction is important for two reasons. First, although agricultural activities are the largest contributor to N2O emissions, the most cost-effective control options lie in the industrial sectors.18 Second, China is becoming the leading global industrial N2O emitter with its emissions increasing by 6fold from 14.8 Gg in 199419 to 106 Gg in 200520 according to successive National Communications on Climate Change in response to UNFCCC; during the same period, emissions from all industrialized countries in Annex I to the UNFCCC decreased by almost half from 549 to 317 Gg.21 Left uncontrolled, the industrial N2O emissions are projected to continue growing at a rapid rate since a large proportion of the N2O-related industrial activities takes place in China. For instance, China has more than 50% and 22% of the global nitric and adipic acid16 production capacities, respectively. Several studies have estimated the overall N2O emissions in China for recent years, including the industrial contribution as a part.14,22−25 However, large discrepancies are noted in the industrial emission estimates from different studies. For example, there is almost a 100-fold difference between the two industrial N2O emission estimates by Olivier22 (48.6 Gg N2O−N) and Li and Lin24 (0.41−0.90 Gg N2O−N) for the year 1990. The discrepancy may be due to the difference in emission factors and/or the lack of reliable and consistent data on chemical production. In spite of the large N2O emissions in
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METHODOLOGY Methodology for Estimating N2O Emissions and Reductions. Industrial N2O emissions result primarily from the production of three chemicals: nitric acid, adipic acid, and caprolactam. N2O emissions are estimated following the 2006 IPCC Guidelines for National Greenhouse Gas Inventories;27 N2O reductions are estimated following the CDM Approved Consolidated Baseline and Monitoring Methodology and CDM Approved Baseline and Monitoring Methodology.28,29 Emissions (E) are estimated by multiplying activity data (AD) describing the intensity of N2O-generating industrial activities with emission factor (EF) quantifying N2O emissions per unit of the product (eq 1). E = AD × EF
(1)
Reductions (R) are estimated by multiplying emissions with the corresponding reduction rate (RR), a parameter indicating N2O removal efficiency (eq 2). R = E × RR
(2)
In estimating the ozone protection benefit from N2O reduction, the reduction R is directly used. In estimating the climate benefit as a result of N2O reduction, the leakage issue is taken into account.28,29 Decomposing N2O in emission abatement facilities consumes electricity, particularly during adipic acid production process. Generating electricity produces CO2, especially in China where about 80% of the electricity is generated from fossil fuels.30 The emission leakage is estimated by multiplying annual electricity consumption for abating N2O with CO2 emissions per unit electricity generated, assuming the electricity consumption of the abatement facility is proportional to the total amount of adipic acid produced. It is noted that historical reductions (1990−2012) of N2O were extracted from the project design documents of existing CDM projects (i.e., the projects registered before December 31, 2012) available in the CDM project database.31 Data Sources. The activity data for nitric acid (converted to 100% concentrated nitric acid), adipic acid, and caprolactam come from the China Industry Economy Statistical Yearbook,32 China Petrochemical Corporation Yearbook,33 Yearbook of World Chemical Industry,34 and peer-reviewed literature,35,36 as shown in Table S1 in the Supporting Information (SI). These nationally compiled nitric acid production data underestimate the actual production since a portion of the nitric acid produced is used as an intermediate to produce other chemicals and not counted in the national nitric acid statistics. Thus, the activity data for nitric acid are adjusted by dividing by a conversion factor of 0.6 (0.5−0.7) following Bouwman et al.;37 this is consistent with the status in China over the years (see text section in SI for details). For estimating the leakage emissions, according to the available project design documents,38,39 electricity consumption per unit of adipic acid was estimated to be 70 kWh/t-adipic acid, and CO2 emission per unit electricity was estimated to be 1.2 × 10−3 t-CO2/kWh. 5291
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N2O reduction rates of 80%48,49 and 90%50 are reported for the facilities adopting the two N2O abatement technologies, respectively. Because it is not feasible to identify N2O abatement technologies for individual facilities at plant levels, a conservative emissions reduction rate of 80% is assumed with a 5% uncertainty.41 During adipic acid production, catalytic and thermaloxidative decomposition techniques are used to remove tailgas N2O from the downstream after NOx absorption.20 For both decomposition techniques, the reduction rate (RR) is calculated using eq 3:27
Emission Factors. During nitric acid production, N2O is generated as an unwanted exhaust gas from the catalytic oxidation of ammonia.27 The yield of N2O varies significantly with production conditions, such as catalytic temperature, operating pressure, and tail-gas abatement technologies.40 Therefore, different emission factors and uncertainty factors are adopted for the different process techniques (Table 1) Table 1. Emission Factors, Recommended Estimation Uncertainty, and Relative Capacity Shares of Four Currently Used Configurations in Nitric Acid Production in China configuration
emission factor (kgN2O/t-nitric acid)
uncertainty (%)
share of installed capacity (%)
dual-pressure high-pressure medium-pressure normal-pressure
9 9 7 5
±10 ±40 ±20 ±10
54 10 12 24
RR = DF × ASUF
(3)
where DF is the destruction factor (also referred to as removal efficiency) of the abatement technology representing the percentage of N2O decomposed in decomposition units. ASUF is the abatement system utilization factor describing the percentage of tail gas passing the N2O decomposition unit. It is characterized by the percentage of time the abatement system is operated through a year, namely, the duration that tail-gas line valve is open but the bypassing valve is closed. DF and ASUF are assumed to be 95% and 90%, respectively, representing the lowest values in plants in China. The reduction rate is calculated to be 85.5%. According to ref 41, the reduction rate uncertainty is estimated to be 10%. Currently, there are no programs or regulations in China to reduce emissions during caprolactam production. For the foreseeable future, this is unlikely to change unless abatement policies identifying this need are adopted. Uncertainty Characterization. The overall uncertainties of historical and projected emissions and reductions are estimated from the uncertainties in the parameters used, including adjustment factor for nitric acid output, emission factors, and reduction rates. They are estimated using Monte Carlo simulation, by assuming that emission factors and reduction rates follow normal distribution and the adjustment factor follows uniform distribution. In presenting the results, median values were used, and semi-interquartile range (intervals between the 75th and 25th quartiles) was used to characterize the uncertainties.
which are recommended by the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and Good Practice and Uncertainty Management in National Greenhouse Gas.27,41 To estimate overall emissions, a combined emission factor weighted by the capacities from the different process techniques42 is adopted, as the statistical data for nitric acid output is the lump sum of the products produced from many different technologies and the output from any one specific technology is not available. It is assumed that production capacity is proportional to the nitric acid output from each configuration. China’s weighted emission factor is calculated to be 7.8 kg-N2O/t-nitric acid, which is in agreement with the global average emission factors used in similar types of CDM projects (8.92 and 8.98).43 The emission factor uncertainty is estimated likewise on a production capacity-weighted basis, yielding ±15% in this study. During adipic acid production, N2O is formed when cyclohexanol and cyclohexanone are oxidized by nitric acid.44 The theoretical emission factor in manufacturing adipic acid is 300 kg-N2O/t-adipic acid. However, different measured values have been used, ranging from 260 to 350 kg-N2O/t-adipic acid in previous studies.19,20 In this study, an emission factor of 300 kg-N2O/t-adipic acid is used, with 260 and 350 representing the lower and upper bounds of uncertainty. During caprolactam production, N2O is generated during ammonia oxidation in the hydroxylamine intermediate production unit. In the 2006 IPCC Guidelines for National Greenhouse Gas Inventories,27 the best available engineering default emission factor of 9.0 kg-N2O/t-caprolactam with an uncertainty of ±40% is recommended. This default value is used here since no China-specific emission factor is available. Reduction Rates. In producing nitric acid, the secondary and tertiary N2O abatement technologies are the primary options used to reduce N2O emissions.45,46 Within the secondary technologies, N2O decomposition unit is installed inside the ammonia oxidation reactor, and N2O is removed immediately after its generation by ammonia oxidation. Within the tertiary technologies, the N2O decomposition unit is installed after the NOx absorption tower, and tail-gas N2O is removed from the downstream after NOx absorption.47 The secondary technologies dominate in China. In CDM database,31 41 out of all 45 existing nitric acid projects from China adopted the secondary abatement technologies, making up 83.4% of announced annual reductions in China. The average
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RESULTS AND DISCUSSION Historical Industrial N2O Emissions. To estimate industrial N2O emissions and evaluate the achievement of mitigation efforts taken in China, two emission scenarios are designed: (a) all generated N2O was released assuming the absence of current CDM projects (the no controls scenario), and (b) N2O was emitted following the implementation of the current CDM projects reflecting the actual emissions (the withcontrols scenario). Industrial N2O emissions from 1990 to 2012 under these two scenarios are shown in Figure 1, and the specific emissions from production processes of the three chemicals are tabulated in SI Table S2. In general, N2O emissions steadily increase during this period. The only exception occurs around 2007 when CDM projects were first introduced. A significant decline is observed from 87.2 (76.9− 97.5) Gg in 2006 to 49.7 (38.7−61.2) Gg in 2008, indicating that the reduction from the CDM projects is greater than the increase in emissions occurring from the normal growth in chemical production during this time. The comparison of the N2O emissions under the two scenarios shows the performance of the existing CDM projects, which annually reduce 70 Gg N2O emissions, a 30% reduction of total generated N2O. So far, 5292
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7 to 27 kg-N2O/t-nitric acid in different plants worldwide53 and different studies used different emission factors. For example, emission factors of 5.0 and around 27 were used in Chen and Zhang14 and Second National Communication,20 respectively, resulting in a 4-fold gap between the two estimates (10.5 and 46.8 Gg, respectively) even though the nitric acid productions were almost equal in the two studies. The estimated industrial N2O emissions in EDGAR v4.2 are much higher than ours. Emission factors used in EDGAR v4.2 inventory were those recommended in the 2006 IPCC Guidelines,54 and comparable with those used in our study. Thus, the large difference may lie in the activity data used. The data source for the EDGAR v4.2 inventory is heavily reliant on international data. Substantial differences have been observed between the international data that EDGAR used and national statistics from specific countries.22 Besides, in using international data, interpolations and extrapolations have to be made over a period of time and quite often a linear growth rate is assumed; in reality, the increases in nitric and adipic acid output in China are almost exponential after 2000. For example, China doubled its nitric acid production (824 to 1840 kt) and quadrupled its adipic acid production (70 to 280 kt) from 2000 to 2008. If the activity growth rates from 2000 to 2008 were used in the interpolations and extrapolations, overestimation of the activity level would have resulted for the period before 2000. To illustrate China’s growing contribution to the global N2O burden, industrial N2O emissions in China, other major countries,21 and the world55 are compared (Figure 2). The
Figure 1. Historical industrial N2O emission (in Gg) in China as estimated by different studies.14,19,20,22,24,25,52
a total of 338 Gg N2O emissions has been abated by the CDM projects in China, which is equivalent to 89.6 Tg of CO2 eq and 5.74 Gg of ODP. From 1990 to 2012, the actual cumulative emissions (the with-controls scenario) of N2O for the production of adipic acid, nitric acid, and caprolactam accounted for 76.8%, 20.2%, and 3.0% of the total industrial emissions, respectively. The share of N2O emissions from producing adipic acid has increased significantly from 18% in 1990 to 92% in 2012. This sharp increase mainly occurred after 2000. A nearly 9-fold increase in adipic acid output occurred from 2000 to 2010.35 As the key feedstock of nylon-66, a widely used industrial chemical being increasingly demanded at an average annual rate of 6.8%, adipic acid had been in short supply in China during this decade with 70% of the demand being met by import.51 The burgeoning market stimulated the growth of adipic acid production. In China, the total production capacity of adipic acid was 126 kt in 200036 with only two plants manufacturing 70 kt; by 2010, the national production capacity has grown to 700 kt, with an annual output of 660 kt following construction of several large plants.35,36 The study results here are compared with the results from other studies. The results are slightly different but comparable with those from the Initial and Second National Communication on Climate Change of the People’s Republic of China,19,20 International Energy Agency,52 Chen and Zhang,14 Li and Lin,24 and EDGAR (The Emissions Database for Global Atmospheric Research, an open-access database of anthropogenic emissions of greenhouse gases and other air pollutants) Fast Track 2010,25 but differ significantly from the emission data in EDGAR at its version 4.223 (Figure 1). Further investigation shows that estimated N2O emissions from producing adipic acid and caprolactam are in good agreement with those from other studies (all but EDGAR v4.2), and the slight discrepancies between ours and the others are largely due to the difference in estimated N2O emissions from producing nitric acid. Two reasons are given. First, the other studies did not include the N2O emissions from producing the part of the nitric acid which was used as an intermediate. Second, measured emission factors were reported ranging from
Figure 2. Comparison of industrial N2O emission in China with global emissions and emissions in major groups.
trends show that N2O emissions globally and in major countries or regions are decreasing, whereas China has been experiencing a sharp increase. From 1990 to 2010, global industrial N2O emissions decreased by 41% from 643 to 379 Gg55 and the total industrial N2O emissions from Annex I countries decreased by 71% from 584 to 171 Gg.21 During the same period, China’s emissions increased 34 times to 160 (135−185) Gg, representing a mass load close to all Annex I countries. By 2009, the emissions from China surpassed those from the European Union (27 members) and United States for the first time, making China the world’s largest industrial N2O emitter. 5293
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Projected N2O Emissions. To project N2O emissions from 2013 to 2020, two scenarios are designed which are the existing trends scenario and the mitigation scenario. The existing trends scenario assumes that all current mitigation options are effective, i.e., registered CDM projects are in operation and acquired credits from N2O abatement projects are tradable, however, no more CDM projects are assumed and no additional regulatory programs are implemented to further curb N2O emissions. The mitigation scenario assumes that the proportion of domestic production capacity equipped with N2O abatement technologies increases linearly from the current level to 100% in 2020 under increasingly stringent mitigation policies without impeding the development of N2O-relevant chemical industry. Under the existing trends scenario, it is assumed that consumption demand is the only driving force for the increased production of nitric acid, adipic acid, and caprolactam, and the production of the chemicals in the present year is estimated by multiplying their production in the preceding year by an annual growth factor. The annual growth factor is set to be the weighted expected increase rate of respective chemical demand in those sectors consuming the chemicals. The weights are the market share of the chemicals. For instance, in China, concentrated nitric acid is mainly used in chemical, metallurgical, medical, and other industries with their market shares being 69%, 14%, 5.5%, and 11%, respectively, and the annual increase rates for nitric acid demand in these sectors are 8%, 14%, 1%, and 1%, respectively.56 Therefore, a market-shareweighted growth rate of 7.6% is derived as the annual production increase rate during the period 2013−2020. Based on the production of the chemicals as estimated in each year, eq 1 is used to estimate the potential N2O emissions from the production of the chemicals. The actual N2O emissions are derived by subtracting the N2O reductions by installing abatement technologies under CDM from the potential N2O emissions. By the end of 2012, there were 47 CDM-registered N2O abatement projects in China, and 338 Gg N2O had been mitigated and 20.3 million certified emissions reductions (equivalent to 64.6 Gg N2O) had been issued.31 Normally, the operational life of CDM projects is 20−25 years. In the first 7 years (the crediting period), the emissions from the projects are monitored and reductions from the projects are verified annually. For most of the projects, only the reduction data for the first 7 years are listed in the project design document and included in the CDM project database.31 It is assumed that the annual reductions for the rest of the operational life are the same as those in the last year of the crediting period. Under the mitigation scenario, it is assumed that the proportion of domestic production capacity equipped with N2O abatement technologies will increase from 45% (543031 out of the 12 000 kt national total57) for nitric acid and 21% (19631 out of the 920 kt national total35) for adipic acid in 2012 to 100% in 2020 in China. It is chemical output, instead of production capacity, needed for estimating N2O emissions. Thus, chemical outputs are assumed to increase at the same rate as the capacities. Under the mitigation scenario, the climate benefit can be quantified from the reduction of industrial N2O emissions at a reasonable pace. Figure 3 shows the projected industrial N2O emissions under both scenarios. In the existing trends scenario, the emissions are projected to increase from 174 (148−201) Gg in 2012 to 561 (490−632) Gg in 2020. With the current abatement strategies, 75 Gg of N2O (equivalent to 19.9 Tg CO2 eq) are eliminated
Figure 3. Projected industrial N2O emission in China from 2013 to 2020 under the existing trends scenario and mitigation scenario. The blue bars at the right side indicate mitigation potential from producing adipic and nitric acid.
annually, which is comparable to the total annual industrial N2O emissions from Italy (71.5 Gg in 2011) or Japan (91.5 Gg in 2011).23 In the mitigation scenario, the annual emissions are estimated to decrease by 37% from 174 (148−201) Gg in 2012 to 108 (94−122) Gg in 2020, a level comparable to that in 2009. Clearly, the mitigation scenario represents a reasonable but ideal case, in which favorable policies and incentives are assumed to be readily available to make the abatement happen. To illustrate the situation confronting China in 2020, the emissions in China are compared with those of other major emitters. By then, China is projected to be the largest industrial N2O emitter, with annual N2O emissions reaching 561 Gg under the existing trends scenario, which is significantly higher than those of major emitters, e.g., United States (projected to be 103 Gg N2O58) and United Kingdom (projected to be 0.3 Gg N2O59). The industrial N2O emissions from China are likely to triple from 2013 to 2020, during which emissions from United States will only grow by about 10%, from 93.5 to 103 Gg.58 Additional N2O Mitigation Potential in China. The mitigation potential is defined as the cumulative difference in emissions between the existing trends scenario and the mitigation scenario. From 2013 to 2020, the cumulative mitigation potential is 1.54 (1.31−1.77) Tg N2O. If effective control measures designed to reduce N2O emissions from the industrial sector in China from 2013 to 2020 are carried out, the climate benefit (408 Tg CO2 eq) is comparable to reducing all greenhouse gas emissions from Australia in 2011 (552 Tg CO2 eq),21 and the ozone protection benefit (26.2 Gg ODP) is greater than reducing all halocarbon ozone-depleting substances from China in 2011 (22.3 Gg ODP).60 The largest mitigation potential exists from adipic acid production. From 2013 to 2020, the mitigation potential from producing adipic and nitric acid accounts for 96.2% and 3.8% of the total, respectively (blue bars in Figure 3). So far, there are only two registered adipic acid CDM projects in China covering 196 kt of production capacity,31 accounting for 21% of the total industrial production capacity (920 kt in 2012).35 This means that about 218 Gg N2O (equivalent to 58 Tg CO2 eq) generated from the remaining 79% capacity was directly emitted into the atmosphere if all 5294
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The industrial emissions are only part of the total N2O emissions. The emphasis here on industrial emissions is based on it being easier to start from phase-out of N2O, feasibilitywise in the short run. In the long run, attention is also needed to reduce N2O emissions in the agricultural sector where N2O emissions are also very significant.
capacity were operated at full loads. The remaining capacity is from the production lines installed and operated after 2005 and ineligible for generating CDM credits since credits are only allowed to be generated from existing capacity operated by the end of the year 2004 according to the accounting rules.29 Under CDM, the exclusion of new capacity was designed to minimize possible greenhouse gas emission leakage,16 the phenomenon of operating new adipic acid plants solely for the attractive profit generated from CDM credits and resulting in additional but unnecessary emissions. These efforts to avoid emission leakage are legitimate as 17%−22% of certified emissions reductions issued from global N2O abatement projects have already been associated with emission leakage to some extent.16 Moreover, the use of CDM credits from adipic projects has been strictly limited during the phase-III trading period of the European Emissions Trading Scheme since 2013, according to the European Union Commission Regulation 550/2011.15 Discussion. As the economy grows, inevitably more nitric acid, adipic acid, and caprolactam will be produced, and as a byproduct, more N2O is expected to be generated in China. CDM had played an important role in abating N2O in China in the past. However, suspected carbon leakage made the new adipic acid production capacity built after 2005 not eligible for registration under CDM, while N2O emissions for producing adipic acid accounts for the lion’s share in industrial N2O emissions in China. Alternative mechanisms are required to facilitate the abatement of N2O. Domestic regulation of N2O emission is a viable option. From a policy perspective, air pollution control and CO2 abatement have always been given higher priority than N2O mitigation for understandable political and psychological reasons.61,62 Nowadays, however, throughout China, haze pollution besieges almost all major cities and attracts much public, media, and political attention. Curbing CO2 emissions by expanding renewable energy production, improving energy efficiency, and promoting energy-saving behavior have all been given more attention in policy discussions domestically and internationally. In comparison, the internal pressure on regulating N2O is simply lacking. Regulating N2O emissions under the Montreal Protocol might be a feasible strategy,12 since reducing N2O emissions has the cobenefit of protecting the ozone layer, besides its climate benefit. After all, N2O is projected to be the largest ozone-depleting contributor for the remainder of the century, and the Montreal Protocol is viewed by many to be the most successful international treaty for improving the environment.63 The ODSs regulated under the Montreal Protocol cover only a few sectors and the substitutes for those ODSs are readily available, which makes its implementation less institutionally complicated. As a byproduct of economic activities, nitrous oxide does not need a substitute, but technologies for abating it are readily available.11 The Montreal Protocol adopts a staged implementation mechanism which enhances the feasibility of regulating new ODS. The design of our “mitigation scenario” in estimating N2O reduction potentials actually resembles the phasing-out pattern of ODS under the Montreal Protocol. Of course, regulating N2O under the Montreal Protocol will not make building a universal binding agreement under UNFCCC to abate climate change agents any easier, but having an agreement should never be the ultimate goal of this framework. Abating climate change agents should be the goal. Abating global warming agents out of the UNFCCC should be encouraged.
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ASSOCIATED CONTENT
S Supporting Information *
Conversion factor used in this study for adjusting nationally compiled nitric acid production to actual nitric acid production; the annual outputs of nitric acid, adipic acid, and caprolactam in China from 1990 to 2020; and the estimated historical N2O emissions from producing nitric acid, adipic acid, and caprolactam from 1990 to 2012. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86 10 62752436; fax: 86 10 62751480; e-mail: jianhua. [email protected]; address: 319 Laodixue Building, 5 Yiheyuan Road, Haidian District, Beijing, 100871, PR China. Notes
The authors declare no competing financial interest.
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REFERENCES
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DB/PC4EBQSJUB9IV2FS9TMQV8DFM3X6MZ (accessed May 20, 2013). (30) Editorial Board of China Electric Power Yearbook. China Electric Power Yearbook 2012; China Electric Power Press: Beijing, 2013. (31) United Nations Framework Convention on Climate Change (UNFCCC). CDM Project Search. http://cdm.unfccc.int/Projects/ projsearch.html (accessed May 4, 2013). (32) Department of Industry and Transport Statistics of National Bureau of Statistics of China. China Industry Economy Statistical Yearbook (1990−2012); China Statistics Press: Beijing, 1991−2013. (33) China Petrochemical Corporation. China Petrochemical Corporation Yearbook (1996−2012); China Petrochemical Press: Beijing, 1997−2013. (34) China National Chemical Infomation Centre (CNCIC). Yearbook of World Chemical Industry(1990−1993); Insitute of Science and Technology Information of Ministry of Chemical Industry: Beijing, 1991−1994. (35) Cui, X. Market status and development prospect of adipic acid at home and abroad. Fine Spec. Chem. 2013, 21 (1), 6−16 in Chinese with English abstract. (36) Wang, J. Production status and development prospect of adipic acid at home and abroad. China Pet. Chem. Ind. 2010, 17 (6), 42−45 in Chinese. (37) Bouwman, A.; Van der Hoek, K.; Olivier, J. Uncertainties in the global source distribution of nitrous oxide. J. Geophys. Res. 1995, 100 (D2), 2785−2800. (38) United Nations Framework Convention on Climate Change (UNFCCC). Project 1083: N2O Decomposition Project of Henan Shenma Nylon Chemical Co., Ltd. http://cdm.unfccc.int/Projects/ registered.html (accessed May 9, 2013). (39) United Nations Framework Convention on Climate Change (UNFCCC). Project 1238: N2O Decomposition Project of PetroChina Company Limited Liaoyang Petrochemical Company. http://cdm.unfccc. int/Projects/registered.html (accessed May 9, 2013). (40) Pérez-Ramırez, J.; Kapteijn, F.; Schöffel, K.; Moulijn, J. A. Formation and control of N2O in nitric acid production: Where do we stand today? Appl. Catal., B 2003, 44 (2), 117−151. (41) Mainhardt, H.; Kruger, D. Good Practice and Uncertainty Management in National Greenhouse Gas Inventories; Vol. 3: Industrial Process and Product Use; Intergovernmental Panel on Climate Change (IPCC): Geneva, 2000. (42) Tang, W. New progress in production technology in nitric acid industry in China. Chem. Fert. Ind. 2008, 35 (5), 14−18 in Chinese with English abstract. (43) Kollmuss, A.; Lazarus, M. Industrial N2O Projects Under the CDM: The Case of Nitric Acid Production; SEI Working Paper WP-US1007; Environment Institute: Stockholm, 2010. (44) Shimizu, A.; Tanaka, K.; Fujimori, M. Abatement technologies for N2O emissions in the adipic acid industry. Chemosphere: Global Change Sci. 2000, 2 (3), 425−434. (45) United Nations Framework Convention on Climate Change (UNFCCC). Approved baseline and monitoring methodology AM0034 “Catalytic reduction of N2O inside the ammonia burner of nitric acid plants”, version 5.1.1. http://cdm.unfccc.int/methodologies/DB/ 993RRDBB2WJI9TAD2XCKPK5YATQXY6 (accessed May 20, 2013). (46) United Nations Framework Convention on Climate Change (UNFCCC). Approved baseline and monitoring methodology AM0028 “Catalytic N2O destruction in the tail gas of Nitric Acid or Caprolactam Production Plants”, version 5.1. http://cdm.unfccc.int/methodologies/ DB/Y0S50SAZFK4FJOMZH2T7EN1I3HI8T0 (accessed May 20, 2013). (47) Lee, S. J.; Ryu, I. S.; Kim, B. M.; Moon, S. H. A review of the current application of N2O emission reduction in CDM projects. Int. J. Greenhouse Gas Control 2011, 5 (1), 167−176. (48) United Nations Framework Convention on Climate Change (UNFCCC). Project 1606: Huayang Dier Line 2 N2O Abatement 5296
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Project. http://cdm.unfccc.int/Projects/registered.html (accessed May 9, 2013). (49) United Nations Framework Convention on Climate Change (UNFCCC). Project 1618: Wulashan Line 2 N2O Abatement Project. http://cdm.unfccc.int/Projects/registered.html (accessed May 9, 2013). (50) United Nations Framework Convention on Climate Change (UNFCCC). Project 0837: Kaifeng Jinkai N2O Abatement Project. http://cdm.unfccc.int/Projects/registered.html (accessed May 9, 2013). (51) Hua, Y.; Liu, Z.; Liu, Q.; Zhang, L.; Zhang, W. Present situation and development suggestion of nylon-66. China Elastomerics 2010, 20 (6), 78−82 in Chinese with English abstract. (52) International Energy Agency (IEA). IEA Statistics. http://www. iea.org/stats/index.asp (accessed May 6, 2013). (53) Kroeze, C. Nitrous Oxide (N2O). Emission Inventory and Options for Control in the Netherlands; National Institute for Public Health and the Environment: Bilthoven, the Netherlands, 1994. (54) European Commission’s Joint Research Centre (JRC) Netherlands Environmental Assessment Agency (PBL). Emission Database for Global Atmospheric Research (EDGAR) Factsheet Industrial processes (2), solvent and other product use (3). http:// edgar.jrc.ec.europa.eu/factsheet_2-3.php (accessed May 7, 2013). (55) U.S. Environmental Protection Agency. Global Anthropogenic Non-CO2 Greenhouse Gas Emissions: 1990−2030; EPA 430-R-12-006; U.S. EPA: Washington, DC, 2012. (56) China National Chemical Infomation Centre (CNCIC). China Chemical Industry Yearbook (1993−2011); China Statistics Press: Beijing, 1994−2012. (57) Zhang, Z. Present situation and market development trend of nitric acid in China. Modern Chem. Ind. 2006, 26 (11), 64−66 in Chinese with English abstract. (58) U.S. Department of State. U.S. Climate Action Report 2010 (Fifth National Communication of the United States of America Under the United Nations Framework Convention on Climate Change). http:// unfccc.int/resource/docs/natc/usa_nc5.pdf (accessed May 13, 2013). (59) U.K. Department of Energy and Climate Change. Projected Emissions of Methane, Nitrous Oxide and F-Gases; Department of Energy and Climate Change: London, UK, 2013. (60) Wan, D.; Xu, J.; Zhang, J.; Tong, X.; Hu, J. Historical and projected emissions of major halocarbons in China. Atmos. Environ. 2009, 43 (36), 5822−5829. (61) Lo, A. China’s response to climate change. Environ. Sci. Technol. 2010, 44 (15), 5689−5690. (62) Wiener, J. B. Climate change policy and policy change in China. UCLA L. Rev. 2007, 55, 1805. (63) United Nations Environment Programme (UNEP). The Montreal Protocol on Substances that Deplete the Ozone Layer: A Success in the Making; United Nations Environment Programme: Nairobi, Kenya, 2007.
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Reducing Nitrous Oxide Emissions to Mitigate Climate Change and Protect the Ozone Layer Li Li,† Jianhua Xu,*,‡ Jianxin Hu,† and Jiarui Han† †
State Key Joint Laboratory for Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China ‡ Department of Environmental Management, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China S Supporting Information *
ABSTRACT: Reducing nitrous oxide (N2O) emissions offers the combined benefits of mitigating climate change and protecting the ozone layer. This study estimates historical and future N2O emissions and explores the mitigation potential for China’s chemical industry. The results show that (1) from 1990 to 2012, industrial N2O emissions in China grew by some 37-fold from 5.07 to 174 Gg (N2O), with total accumulated emissions of 1.26 Tg, and (2) from 2012 to 2020, the projected emissions are expected to continue growing rapidly from 174 to 561 Gg under current policies and assuming no additional mitigation measures. The total accumulated mitigation potential for this forecast period is about 1.54 Tg, the equivalent of reducing all the 2011 greenhouse gases from Australia or halocarbon ozone-depleting substances from China. Adipic acid production, the major industrial emission source, contributes nearly 80% of the industrial N2O emissions, and represents about 96.2% of the industrial mitigation potential. However, the mitigation will not happen without implementing effective policies and regulatory programs.
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INTRODUCTION Global climate change efforts aim to build an inclusive universal binding agreement on emissions reductions under the United Nations Framework Convention on Climate Change (UNFCCC); this agreement requires the consent of all signatory parties. This results in the lowest of common denominators1 over the years which are inadequate to prevent the climate system from reaching a tipping point of no return. This situation has prompted climate change advocates to seek complementary approaches, which can be implemented either within nations or at the international level but covering fewer sectors and with less institutional complexities. Among them are reducing non-CO2 global warming agents, increasing biosequestration through forest protection, and implementing the much-debated geoengineering options. In particular, reducing non-CO2 global warming agents has garnered increasing attention in recent years.2−6 The “immediacy” of the climate benefit and/or the cobenefit of their reduction is expected to provide a strong incentive for nations to consider adopting policies and taking rapid responses.7,8 For instance, black carbon and tropospheric ozone are short-lived global warming agents;9 both have adverse human health effects, the control of which has positive climate and health benefits. There have been voices advocating their inclusion in successive post-Kyoto climate negotiations.10 © 2014 American Chemical Society
Nitrous oxide (N2O) has strong global warming potential (GWP) and is an ozone-depleting agent as well. Reducing N2O emissions offers the combined benefits of mitigating climate change and protecting the ozone layer. This has attracted attention in discussions of regulating ozone-depleting substances (ODSs) under the Montreal Protocol on Substances that Deplete the Ozone Layer.11,12 In this paper, estimates are made of industrial N2O emissions and their reduction potential in China to facilitate development of N2O phase-out strategies. Nitrous oxide is emitted from both natural and anthropogenic sources. The anthropogenic sources include agricultural activities, industrial chemical production, fuel and biomass combustion, and sewage treatment operations.11,13 Globally, the relative contributions from the various sources has remained steady over the years, with agricultural activities being the largest contributor (60%), followed by industrial processes and fuel combustion, each contributing about 10%.13 In China, agricultural activities are the largest (81%) sources as well, while the contributions of industrial processes and combustion were about 9.8% and 9.1% in 2007.14 Received: Revised: Accepted: Published: 5290
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China, few studies have been conducted to assess the mitigation potentials.26 A systematic investigation of historical and future emission trends, as well as the potential for mitigating N2O emissions, is needed. This study presents the annual industrial N2O emissions, historically from 1990 to 2012 and projected from 2013 to 2020, and potential for mitigation. Constraints and challenges for reducing N2O emissions are discussed.
In the climate regime, N2O is listed as one of the six greenhouse gases under the UNFCCC and the third largest radiative forcing contributor. Its 100-year global warming potential is 265 times that of CO2, and its contribution to net global radiative forcing is about 6% that of CO2.13 Although there has been a lot of academic and policy discussion about the benefit of reducing N2O, not much has been done to reduce N2O emissions other than in a few carbon markets, including the Clean Development Mechanism (CDM), the Joint Implementation, and the European Emissions Trading Scheme. In these markets, the abatement efforts are limited to industrial sector. Even there, not all N2O projects are viewed favorably due to suspected “carbon leakages”15 where some producers are suspected of operating new adipic acid plants solely for the profit generated from CDM credits.16 In the agricultural sector, policies specifically aiming at reducing N2O emissions are absent, and N2O emissions abatement so far is just the cobenefit of measures targeted at controlling nonpoint-source water pollution, such as enhancing fertilizer use efficiency and managing animal manure.11 These actions are mainly taken in developed nations. In most developing countries, there is not much progress on controlling agricultural N2O emissions due to poor agronomic management practices. Thus far in China, the only N2O mitigation measure has been abating industrial N2O from chemical production under CDM. In the ozone regime, N2O is the primary source of stratospheric NOx, a strong ozone depleting agent.11,17 The ozone depleting potential (ODP) of N2O is 0.017,17 which is comparable to that of HCFCs, the immediate substitutes of the first generation ODS, CFCs. In the past, efforts to protect the ozone layer have emphasized reducing CFCs and HCFCs. So far, 95% of the CFCs have been eliminated and the abatement of HCFCs is ongoing. With the progress on phasing out CFCs and HCFCs, N2O has become the largest anthropogenic source threatening the ozone layer for the remainder of the century.17 For this reason, regulating N2O under the Montreal Protocol is becoming a more important policy discussion.12 Estimating China’s industrial N2O emissions and the potential for reduction is important for two reasons. First, although agricultural activities are the largest contributor to N2O emissions, the most cost-effective control options lie in the industrial sectors.18 Second, China is becoming the leading global industrial N2O emitter with its emissions increasing by 6fold from 14.8 Gg in 199419 to 106 Gg in 200520 according to successive National Communications on Climate Change in response to UNFCCC; during the same period, emissions from all industrialized countries in Annex I to the UNFCCC decreased by almost half from 549 to 317 Gg.21 Left uncontrolled, the industrial N2O emissions are projected to continue growing at a rapid rate since a large proportion of the N2O-related industrial activities takes place in China. For instance, China has more than 50% and 22% of the global nitric and adipic acid16 production capacities, respectively. Several studies have estimated the overall N2O emissions in China for recent years, including the industrial contribution as a part.14,22−25 However, large discrepancies are noted in the industrial emission estimates from different studies. For example, there is almost a 100-fold difference between the two industrial N2O emission estimates by Olivier22 (48.6 Gg N2O−N) and Li and Lin24 (0.41−0.90 Gg N2O−N) for the year 1990. The discrepancy may be due to the difference in emission factors and/or the lack of reliable and consistent data on chemical production. In spite of the large N2O emissions in
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METHODOLOGY Methodology for Estimating N2O Emissions and Reductions. Industrial N2O emissions result primarily from the production of three chemicals: nitric acid, adipic acid, and caprolactam. N2O emissions are estimated following the 2006 IPCC Guidelines for National Greenhouse Gas Inventories;27 N2O reductions are estimated following the CDM Approved Consolidated Baseline and Monitoring Methodology and CDM Approved Baseline and Monitoring Methodology.28,29 Emissions (E) are estimated by multiplying activity data (AD) describing the intensity of N2O-generating industrial activities with emission factor (EF) quantifying N2O emissions per unit of the product (eq 1). E = AD × EF
(1)
Reductions (R) are estimated by multiplying emissions with the corresponding reduction rate (RR), a parameter indicating N2O removal efficiency (eq 2). R = E × RR
(2)
In estimating the ozone protection benefit from N2O reduction, the reduction R is directly used. In estimating the climate benefit as a result of N2O reduction, the leakage issue is taken into account.28,29 Decomposing N2O in emission abatement facilities consumes electricity, particularly during adipic acid production process. Generating electricity produces CO2, especially in China where about 80% of the electricity is generated from fossil fuels.30 The emission leakage is estimated by multiplying annual electricity consumption for abating N2O with CO2 emissions per unit electricity generated, assuming the electricity consumption of the abatement facility is proportional to the total amount of adipic acid produced. It is noted that historical reductions (1990−2012) of N2O were extracted from the project design documents of existing CDM projects (i.e., the projects registered before December 31, 2012) available in the CDM project database.31 Data Sources. The activity data for nitric acid (converted to 100% concentrated nitric acid), adipic acid, and caprolactam come from the China Industry Economy Statistical Yearbook,32 China Petrochemical Corporation Yearbook,33 Yearbook of World Chemical Industry,34 and peer-reviewed literature,35,36 as shown in Table S1 in the Supporting Information (SI). These nationally compiled nitric acid production data underestimate the actual production since a portion of the nitric acid produced is used as an intermediate to produce other chemicals and not counted in the national nitric acid statistics. Thus, the activity data for nitric acid are adjusted by dividing by a conversion factor of 0.6 (0.5−0.7) following Bouwman et al.;37 this is consistent with the status in China over the years (see text section in SI for details). For estimating the leakage emissions, according to the available project design documents,38,39 electricity consumption per unit of adipic acid was estimated to be 70 kWh/t-adipic acid, and CO2 emission per unit electricity was estimated to be 1.2 × 10−3 t-CO2/kWh. 5291
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N2O reduction rates of 80%48,49 and 90%50 are reported for the facilities adopting the two N2O abatement technologies, respectively. Because it is not feasible to identify N2O abatement technologies for individual facilities at plant levels, a conservative emissions reduction rate of 80% is assumed with a 5% uncertainty.41 During adipic acid production, catalytic and thermaloxidative decomposition techniques are used to remove tailgas N2O from the downstream after NOx absorption.20 For both decomposition techniques, the reduction rate (RR) is calculated using eq 3:27
Emission Factors. During nitric acid production, N2O is generated as an unwanted exhaust gas from the catalytic oxidation of ammonia.27 The yield of N2O varies significantly with production conditions, such as catalytic temperature, operating pressure, and tail-gas abatement technologies.40 Therefore, different emission factors and uncertainty factors are adopted for the different process techniques (Table 1) Table 1. Emission Factors, Recommended Estimation Uncertainty, and Relative Capacity Shares of Four Currently Used Configurations in Nitric Acid Production in China configuration
emission factor (kgN2O/t-nitric acid)
uncertainty (%)
share of installed capacity (%)
dual-pressure high-pressure medium-pressure normal-pressure
9 9 7 5
±10 ±40 ±20 ±10
54 10 12 24
RR = DF × ASUF
(3)
where DF is the destruction factor (also referred to as removal efficiency) of the abatement technology representing the percentage of N2O decomposed in decomposition units. ASUF is the abatement system utilization factor describing the percentage of tail gas passing the N2O decomposition unit. It is characterized by the percentage of time the abatement system is operated through a year, namely, the duration that tail-gas line valve is open but the bypassing valve is closed. DF and ASUF are assumed to be 95% and 90%, respectively, representing the lowest values in plants in China. The reduction rate is calculated to be 85.5%. According to ref 41, the reduction rate uncertainty is estimated to be 10%. Currently, there are no programs or regulations in China to reduce emissions during caprolactam production. For the foreseeable future, this is unlikely to change unless abatement policies identifying this need are adopted. Uncertainty Characterization. The overall uncertainties of historical and projected emissions and reductions are estimated from the uncertainties in the parameters used, including adjustment factor for nitric acid output, emission factors, and reduction rates. They are estimated using Monte Carlo simulation, by assuming that emission factors and reduction rates follow normal distribution and the adjustment factor follows uniform distribution. In presenting the results, median values were used, and semi-interquartile range (intervals between the 75th and 25th quartiles) was used to characterize the uncertainties.
which are recommended by the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and Good Practice and Uncertainty Management in National Greenhouse Gas.27,41 To estimate overall emissions, a combined emission factor weighted by the capacities from the different process techniques42 is adopted, as the statistical data for nitric acid output is the lump sum of the products produced from many different technologies and the output from any one specific technology is not available. It is assumed that production capacity is proportional to the nitric acid output from each configuration. China’s weighted emission factor is calculated to be 7.8 kg-N2O/t-nitric acid, which is in agreement with the global average emission factors used in similar types of CDM projects (8.92 and 8.98).43 The emission factor uncertainty is estimated likewise on a production capacity-weighted basis, yielding ±15% in this study. During adipic acid production, N2O is formed when cyclohexanol and cyclohexanone are oxidized by nitric acid.44 The theoretical emission factor in manufacturing adipic acid is 300 kg-N2O/t-adipic acid. However, different measured values have been used, ranging from 260 to 350 kg-N2O/t-adipic acid in previous studies.19,20 In this study, an emission factor of 300 kg-N2O/t-adipic acid is used, with 260 and 350 representing the lower and upper bounds of uncertainty. During caprolactam production, N2O is generated during ammonia oxidation in the hydroxylamine intermediate production unit. In the 2006 IPCC Guidelines for National Greenhouse Gas Inventories,27 the best available engineering default emission factor of 9.0 kg-N2O/t-caprolactam with an uncertainty of ±40% is recommended. This default value is used here since no China-specific emission factor is available. Reduction Rates. In producing nitric acid, the secondary and tertiary N2O abatement technologies are the primary options used to reduce N2O emissions.45,46 Within the secondary technologies, N2O decomposition unit is installed inside the ammonia oxidation reactor, and N2O is removed immediately after its generation by ammonia oxidation. Within the tertiary technologies, the N2O decomposition unit is installed after the NOx absorption tower, and tail-gas N2O is removed from the downstream after NOx absorption.47 The secondary technologies dominate in China. In CDM database,31 41 out of all 45 existing nitric acid projects from China adopted the secondary abatement technologies, making up 83.4% of announced annual reductions in China. The average
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RESULTS AND DISCUSSION Historical Industrial N2O Emissions. To estimate industrial N2O emissions and evaluate the achievement of mitigation efforts taken in China, two emission scenarios are designed: (a) all generated N2O was released assuming the absence of current CDM projects (the no controls scenario), and (b) N2O was emitted following the implementation of the current CDM projects reflecting the actual emissions (the withcontrols scenario). Industrial N2O emissions from 1990 to 2012 under these two scenarios are shown in Figure 1, and the specific emissions from production processes of the three chemicals are tabulated in SI Table S2. In general, N2O emissions steadily increase during this period. The only exception occurs around 2007 when CDM projects were first introduced. A significant decline is observed from 87.2 (76.9− 97.5) Gg in 2006 to 49.7 (38.7−61.2) Gg in 2008, indicating that the reduction from the CDM projects is greater than the increase in emissions occurring from the normal growth in chemical production during this time. The comparison of the N2O emissions under the two scenarios shows the performance of the existing CDM projects, which annually reduce 70 Gg N2O emissions, a 30% reduction of total generated N2O. So far, 5292
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7 to 27 kg-N2O/t-nitric acid in different plants worldwide53 and different studies used different emission factors. For example, emission factors of 5.0 and around 27 were used in Chen and Zhang14 and Second National Communication,20 respectively, resulting in a 4-fold gap between the two estimates (10.5 and 46.8 Gg, respectively) even though the nitric acid productions were almost equal in the two studies. The estimated industrial N2O emissions in EDGAR v4.2 are much higher than ours. Emission factors used in EDGAR v4.2 inventory were those recommended in the 2006 IPCC Guidelines,54 and comparable with those used in our study. Thus, the large difference may lie in the activity data used. The data source for the EDGAR v4.2 inventory is heavily reliant on international data. Substantial differences have been observed between the international data that EDGAR used and national statistics from specific countries.22 Besides, in using international data, interpolations and extrapolations have to be made over a period of time and quite often a linear growth rate is assumed; in reality, the increases in nitric and adipic acid output in China are almost exponential after 2000. For example, China doubled its nitric acid production (824 to 1840 kt) and quadrupled its adipic acid production (70 to 280 kt) from 2000 to 2008. If the activity growth rates from 2000 to 2008 were used in the interpolations and extrapolations, overestimation of the activity level would have resulted for the period before 2000. To illustrate China’s growing contribution to the global N2O burden, industrial N2O emissions in China, other major countries,21 and the world55 are compared (Figure 2). The
Figure 1. Historical industrial N2O emission (in Gg) in China as estimated by different studies.14,19,20,22,24,25,52
a total of 338 Gg N2O emissions has been abated by the CDM projects in China, which is equivalent to 89.6 Tg of CO2 eq and 5.74 Gg of ODP. From 1990 to 2012, the actual cumulative emissions (the with-controls scenario) of N2O for the production of adipic acid, nitric acid, and caprolactam accounted for 76.8%, 20.2%, and 3.0% of the total industrial emissions, respectively. The share of N2O emissions from producing adipic acid has increased significantly from 18% in 1990 to 92% in 2012. This sharp increase mainly occurred after 2000. A nearly 9-fold increase in adipic acid output occurred from 2000 to 2010.35 As the key feedstock of nylon-66, a widely used industrial chemical being increasingly demanded at an average annual rate of 6.8%, adipic acid had been in short supply in China during this decade with 70% of the demand being met by import.51 The burgeoning market stimulated the growth of adipic acid production. In China, the total production capacity of adipic acid was 126 kt in 200036 with only two plants manufacturing 70 kt; by 2010, the national production capacity has grown to 700 kt, with an annual output of 660 kt following construction of several large plants.35,36 The study results here are compared with the results from other studies. The results are slightly different but comparable with those from the Initial and Second National Communication on Climate Change of the People’s Republic of China,19,20 International Energy Agency,52 Chen and Zhang,14 Li and Lin,24 and EDGAR (The Emissions Database for Global Atmospheric Research, an open-access database of anthropogenic emissions of greenhouse gases and other air pollutants) Fast Track 2010,25 but differ significantly from the emission data in EDGAR at its version 4.223 (Figure 1). Further investigation shows that estimated N2O emissions from producing adipic acid and caprolactam are in good agreement with those from other studies (all but EDGAR v4.2), and the slight discrepancies between ours and the others are largely due to the difference in estimated N2O emissions from producing nitric acid. Two reasons are given. First, the other studies did not include the N2O emissions from producing the part of the nitric acid which was used as an intermediate. Second, measured emission factors were reported ranging from
Figure 2. Comparison of industrial N2O emission in China with global emissions and emissions in major groups.
trends show that N2O emissions globally and in major countries or regions are decreasing, whereas China has been experiencing a sharp increase. From 1990 to 2010, global industrial N2O emissions decreased by 41% from 643 to 379 Gg55 and the total industrial N2O emissions from Annex I countries decreased by 71% from 584 to 171 Gg.21 During the same period, China’s emissions increased 34 times to 160 (135−185) Gg, representing a mass load close to all Annex I countries. By 2009, the emissions from China surpassed those from the European Union (27 members) and United States for the first time, making China the world’s largest industrial N2O emitter. 5293
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Projected N2O Emissions. To project N2O emissions from 2013 to 2020, two scenarios are designed which are the existing trends scenario and the mitigation scenario. The existing trends scenario assumes that all current mitigation options are effective, i.e., registered CDM projects are in operation and acquired credits from N2O abatement projects are tradable, however, no more CDM projects are assumed and no additional regulatory programs are implemented to further curb N2O emissions. The mitigation scenario assumes that the proportion of domestic production capacity equipped with N2O abatement technologies increases linearly from the current level to 100% in 2020 under increasingly stringent mitigation policies without impeding the development of N2O-relevant chemical industry. Under the existing trends scenario, it is assumed that consumption demand is the only driving force for the increased production of nitric acid, adipic acid, and caprolactam, and the production of the chemicals in the present year is estimated by multiplying their production in the preceding year by an annual growth factor. The annual growth factor is set to be the weighted expected increase rate of respective chemical demand in those sectors consuming the chemicals. The weights are the market share of the chemicals. For instance, in China, concentrated nitric acid is mainly used in chemical, metallurgical, medical, and other industries with their market shares being 69%, 14%, 5.5%, and 11%, respectively, and the annual increase rates for nitric acid demand in these sectors are 8%, 14%, 1%, and 1%, respectively.56 Therefore, a market-shareweighted growth rate of 7.6% is derived as the annual production increase rate during the period 2013−2020. Based on the production of the chemicals as estimated in each year, eq 1 is used to estimate the potential N2O emissions from the production of the chemicals. The actual N2O emissions are derived by subtracting the N2O reductions by installing abatement technologies under CDM from the potential N2O emissions. By the end of 2012, there were 47 CDM-registered N2O abatement projects in China, and 338 Gg N2O had been mitigated and 20.3 million certified emissions reductions (equivalent to 64.6 Gg N2O) had been issued.31 Normally, the operational life of CDM projects is 20−25 years. In the first 7 years (the crediting period), the emissions from the projects are monitored and reductions from the projects are verified annually. For most of the projects, only the reduction data for the first 7 years are listed in the project design document and included in the CDM project database.31 It is assumed that the annual reductions for the rest of the operational life are the same as those in the last year of the crediting period. Under the mitigation scenario, it is assumed that the proportion of domestic production capacity equipped with N2O abatement technologies will increase from 45% (543031 out of the 12 000 kt national total57) for nitric acid and 21% (19631 out of the 920 kt national total35) for adipic acid in 2012 to 100% in 2020 in China. It is chemical output, instead of production capacity, needed for estimating N2O emissions. Thus, chemical outputs are assumed to increase at the same rate as the capacities. Under the mitigation scenario, the climate benefit can be quantified from the reduction of industrial N2O emissions at a reasonable pace. Figure 3 shows the projected industrial N2O emissions under both scenarios. In the existing trends scenario, the emissions are projected to increase from 174 (148−201) Gg in 2012 to 561 (490−632) Gg in 2020. With the current abatement strategies, 75 Gg of N2O (equivalent to 19.9 Tg CO2 eq) are eliminated
Figure 3. Projected industrial N2O emission in China from 2013 to 2020 under the existing trends scenario and mitigation scenario. The blue bars at the right side indicate mitigation potential from producing adipic and nitric acid.
annually, which is comparable to the total annual industrial N2O emissions from Italy (71.5 Gg in 2011) or Japan (91.5 Gg in 2011).23 In the mitigation scenario, the annual emissions are estimated to decrease by 37% from 174 (148−201) Gg in 2012 to 108 (94−122) Gg in 2020, a level comparable to that in 2009. Clearly, the mitigation scenario represents a reasonable but ideal case, in which favorable policies and incentives are assumed to be readily available to make the abatement happen. To illustrate the situation confronting China in 2020, the emissions in China are compared with those of other major emitters. By then, China is projected to be the largest industrial N2O emitter, with annual N2O emissions reaching 561 Gg under the existing trends scenario, which is significantly higher than those of major emitters, e.g., United States (projected to be 103 Gg N2O58) and United Kingdom (projected to be 0.3 Gg N2O59). The industrial N2O emissions from China are likely to triple from 2013 to 2020, during which emissions from United States will only grow by about 10%, from 93.5 to 103 Gg.58 Additional N2O Mitigation Potential in China. The mitigation potential is defined as the cumulative difference in emissions between the existing trends scenario and the mitigation scenario. From 2013 to 2020, the cumulative mitigation potential is 1.54 (1.31−1.77) Tg N2O. If effective control measures designed to reduce N2O emissions from the industrial sector in China from 2013 to 2020 are carried out, the climate benefit (408 Tg CO2 eq) is comparable to reducing all greenhouse gas emissions from Australia in 2011 (552 Tg CO2 eq),21 and the ozone protection benefit (26.2 Gg ODP) is greater than reducing all halocarbon ozone-depleting substances from China in 2011 (22.3 Gg ODP).60 The largest mitigation potential exists from adipic acid production. From 2013 to 2020, the mitigation potential from producing adipic and nitric acid accounts for 96.2% and 3.8% of the total, respectively (blue bars in Figure 3). So far, there are only two registered adipic acid CDM projects in China covering 196 kt of production capacity,31 accounting for 21% of the total industrial production capacity (920 kt in 2012).35 This means that about 218 Gg N2O (equivalent to 58 Tg CO2 eq) generated from the remaining 79% capacity was directly emitted into the atmosphere if all 5294
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The industrial emissions are only part of the total N2O emissions. The emphasis here on industrial emissions is based on it being easier to start from phase-out of N2O, feasibilitywise in the short run. In the long run, attention is also needed to reduce N2O emissions in the agricultural sector where N2O emissions are also very significant.
capacity were operated at full loads. The remaining capacity is from the production lines installed and operated after 2005 and ineligible for generating CDM credits since credits are only allowed to be generated from existing capacity operated by the end of the year 2004 according to the accounting rules.29 Under CDM, the exclusion of new capacity was designed to minimize possible greenhouse gas emission leakage,16 the phenomenon of operating new adipic acid plants solely for the attractive profit generated from CDM credits and resulting in additional but unnecessary emissions. These efforts to avoid emission leakage are legitimate as 17%−22% of certified emissions reductions issued from global N2O abatement projects have already been associated with emission leakage to some extent.16 Moreover, the use of CDM credits from adipic projects has been strictly limited during the phase-III trading period of the European Emissions Trading Scheme since 2013, according to the European Union Commission Regulation 550/2011.15 Discussion. As the economy grows, inevitably more nitric acid, adipic acid, and caprolactam will be produced, and as a byproduct, more N2O is expected to be generated in China. CDM had played an important role in abating N2O in China in the past. However, suspected carbon leakage made the new adipic acid production capacity built after 2005 not eligible for registration under CDM, while N2O emissions for producing adipic acid accounts for the lion’s share in industrial N2O emissions in China. Alternative mechanisms are required to facilitate the abatement of N2O. Domestic regulation of N2O emission is a viable option. From a policy perspective, air pollution control and CO2 abatement have always been given higher priority than N2O mitigation for understandable political and psychological reasons.61,62 Nowadays, however, throughout China, haze pollution besieges almost all major cities and attracts much public, media, and political attention. Curbing CO2 emissions by expanding renewable energy production, improving energy efficiency, and promoting energy-saving behavior have all been given more attention in policy discussions domestically and internationally. In comparison, the internal pressure on regulating N2O is simply lacking. Regulating N2O emissions under the Montreal Protocol might be a feasible strategy,12 since reducing N2O emissions has the cobenefit of protecting the ozone layer, besides its climate benefit. After all, N2O is projected to be the largest ozone-depleting contributor for the remainder of the century, and the Montreal Protocol is viewed by many to be the most successful international treaty for improving the environment.63 The ODSs regulated under the Montreal Protocol cover only a few sectors and the substitutes for those ODSs are readily available, which makes its implementation less institutionally complicated. As a byproduct of economic activities, nitrous oxide does not need a substitute, but technologies for abating it are readily available.11 The Montreal Protocol adopts a staged implementation mechanism which enhances the feasibility of regulating new ODS. The design of our “mitigation scenario” in estimating N2O reduction potentials actually resembles the phasing-out pattern of ODS under the Montreal Protocol. Of course, regulating N2O under the Montreal Protocol will not make building a universal binding agreement under UNFCCC to abate climate change agents any easier, but having an agreement should never be the ultimate goal of this framework. Abating climate change agents should be the goal. Abating global warming agents out of the UNFCCC should be encouraged.
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ASSOCIATED CONTENT
S Supporting Information *
Conversion factor used in this study for adjusting nationally compiled nitric acid production to actual nitric acid production; the annual outputs of nitric acid, adipic acid, and caprolactam in China from 1990 to 2020; and the estimated historical N2O emissions from producing nitric acid, adipic acid, and caprolactam from 1990 to 2012. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86 10 62752436; fax: 86 10 62751480; e-mail: jianhua. [email protected]; address: 319 Laodixue Building, 5 Yiheyuan Road, Haidian District, Beijing, 100871, PR China. Notes
The authors declare no competing financial interest.
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REFERENCES
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