Estimating carbon emissions from the pulp and paper industry: A case study

Applied Energy xxx (2016) xxx–xxx

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Estimating carbon emissions from the pulp and paper industry: A case study Yutao Wang a,b,⇑, Xuechun Yang a, Mingxing Sun a, Lei Ma c, Xiao Li c, Lei Shi c,⇑ a

Institute of Ecology and Biodiversity, School of Biological Sciences, Jinan 250100, Shandong University, PR China Department of Industrial System and Engineering, University of Tennessee, Knoxville, USA c State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China b

h i g h l i g h t s
This study calculated carbon emissions from China’s Pulp and Paper Industry (CPPI) from 2005 to 2012 with local inventory.
CO2 emissions from energy consumption contributed the largest share of total emissions.
Recovered biomass energy has great potential in reducing total carbon emissions.
CH4 generated from sewage treatment should be taken into account in order to achieve low carbon development.

a r t i c l e

i n f o

Article history: Received 1 March 2016 Received in revised form 28 April 2016 Accepted 3 May 2016 Available online xxxx Keywords: Carbon emissions Climate change Industrial process Low carbon development Pulp and paper

a b s t r a c t The pulp and paper industry is a high energy consuming and polluting sector, and carbon emissions emitted from this sector are worthy of attention. This article, based upon an analysis of China’s Pulp and Paper Industry (CPPI), provides estimates of each of the following: carbon emissions from energy consumption, pre-treatment sector, combustion of condensed black liquor, and methane emitted from incomplete aerobic digestion during sewage treatment of CPPI. During the study period (2005–2012), total CO2 emissions ranged from 126.0 Mt to 155.4 Mt. Energy consumption was estimated to be the largest source of carbon emissions, however, due to the application of the local emission inventory rather than the IPCC inventory, energy consumption decreased by 4.7%, a lower percentage than was calculated in a previous study. According to this study’s estimation, the emissions caused by the recovery of biomass energy contributed 26–29% of the total CO2 emissions. CH4 generated from sewage treatment accounted for 9– 11% of the total carbon emissions. The CO2 intensity dropped during the study period, which reflected the improvement of energy efficiency in the pulp and paper industry. The outcome of this study provides not only detailed information about CPPI’s carbon emissions, but also a calculation framework for studying carbon emissions from pulp and paper sector in the other regions. It suggests that the local carbon emissions inventory should be used for estimating carbon emissions and to reduce the fossil fuel energy, increase energy recovery from biomass, and that promoting cleaner production is essential to achieve a low carbon development of the pulp and paper industry. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Climate change is greatly challenging the sustainability of human society. Efforts have been made to more accurately evaluate the amount of human-induced carbon emissions. Emissions from combustion have been widely studied and reported. As indus⇑ Corresponding authors at: Institute of Ecology and Biodiversity, Jinan 250100, Shandong University, PR China (Y. Wang). School of Environment, Tsinghua University, Beijing 100084, PR China (L. Shi). E-mail addresses: [email protected], [email protected] (Y. Wang), [email protected] (L. Shi).

try is one of the major users of energy, carbon emissions caused by combustion from industrial sectors have also been widely studied and estimated. However, in addition to emissions caused by fossil fuel combustion, industrial processes and waste management can also generate carbon emissions. Compared to studies on carbon emissions from combustion, fewer studies focused on the emissions from industrial processes and waste management, especially in the context of developing countries [1]. China, as the largest developing country and carbon emitter, has been significantly developing its industry since the 1980s. A few recent studies have examined the GHG emissions from industrial processes in industrial sectors in China. For example, Hao et al. [2] estimated the

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GHG emissions from the primary aluminum production of China in 2014 accounted for 4% of China’s total GHG emissions. Liu [3] estimated carbon emissions of industrial processes from five high carbon intensity industrial sectors in China and found that the carbon emissions of these five sectors in 2013 were equivalent to the total quantities of CO2 emissions of Spain in the same year. Shan et al. [1] presented the first study on CO2 emissions from China’s lime industry and found that it contributed a significant amount of emissions annually. This study focused on the pulp and paper sector. The pulp and paper sector has received widespread attention due to its extensive energy demand [4] and high emissions generation [5]. Most of the studies conducted on the pulp and paper industry have centered upon pollutants and waste treatment [6] and cleaner production [7]. There have also been increasing researches on the energy consumption and carbon emissions of the pulp and paper industry worldwide. However, most studies were conducted in developed countries, while much less attention has been given to the pulp and paper industry of developing countries regarding its energy consumption and carbon emissions. Though developed countries dominated the total pulp and paper production in the world and the United States was the largest producer for a long period, China has become the largest paper products producer and the second largest pulp producer in recent years (Fig. 1). Compared to the situation in the developed countries, straw pulp rather than wood pulp contributes a large share of total raw material in CPPI. In addition, the lack of advanced technologies leads to higher energy demands and heavier pollutants generation than in developed countries [8]. Moreover, we found that very few studies in China and other countries estimated GHG emissions from the pulp and paper sector caused by paper mill waste management. Aerobic biological treatment is widely used for sewage disposal, and CO2 emitted from aerobic respiration is excluded from total carbon emissions due to its biogenic origin [9]. But incomplete aerobic digestion can produce methane emissions, which should be taken into account [10]. This paper is organized as follows. Section 2 presents a comprehensive literature review and identifies the research gaps. Section 3 gives an overview of the pulp and paper production process. Section 4 introduces the methodology and data sources. Section 5 presents the results, which are further discussed in Section 6. Section 7 is the conclusion.

2. Literature review The pulp and paper industry has been regarded as a very high energy-consuming sector [11,12], and thus many researchers have analyzed energy efficiency and energy saving potentials in the pulp and paper industry from both the technological improvement perspective and the energy policy perspective. Farla et al. [13] examined the energy efficiency development of the pulp and paper industry of eight OECD countries base on physical production data during 1973–1991 and found that the growth of primary energy consumption was significantly reduced due to energy efficiency improvements. However, the carbon emission during the study period was not given in their study. Lopes et al. [14] employed life cycle assessment to study the environmental impacts of two different fuels used in the pulp and paper industry of Portugal and showed that the substitution of heavy fuel oil by natural gas in the paper production process has less environmental impacts. Consonni et al. [15] argued that gasification-based pulp mill biorefineries should be promoted because of their great potential in offering energy benefits in the United States. Stenqvist [4] examined the energy performance of the Swedish pulp and paper industry with the decomposition method and found energy efficiency

improvement had limited the growth of primary energy use in this sector. With regards to carbon emissions, most researches focused on CO2 that was caused by energy use in the pulp and paper industry. Though fossil energy consumption contributes most of the CO2 in the pulp and paper industry [12], carbon emissions caused by biomass loss during the whole production process of the pulp and paper industry should not be ignored. Carbon emissions such as CH4 caused by the wastewater treatment process should also be taken into consideration [16]. Additionally, considering that the original resources of the pulp and paper industry are wood and straw, it is important to note that these resources, (wood in particular), play a crucial role in carbon sinks [17]. According to [18], as a unique industrial sector, biomass should be recycled for its high potential in both energy and carbon emissions reduction from a life-cycle perspective. Another two studies have also shown that pulp and paper mills can significantly reduce the net carbon emissions by using and recovering biomass based energy in several European country contexts [19,20], Black liquor from the pulp production process can be used for producing hydrogen which can potentially replace a large share of motor gas use in countries with a large size pulp and paper industry [21,22]. Except for the few studies reviewed above, the knowledge about biomass-induced carbon emissions in the pulp and paper sector has not been well addressed. Thus, it leads to the first research purpose of this study: 1. CO2 reduction potentials via biomass energy recovery and GHG emissions from the sewage treatment of the pulp and paper industry should be further addressed. There have been only a few studies on the pulp and paper industry’s energy consumption and carbon emissions in a developing country context and most of them were conducted in China. A study conducted by Kong et al. [23] estimated CO2 emission and mitigation potentials in CPPI by using conservation supply curve methods. This study found the cost-effective CO2 emissions reduction potential by applying energy efficiency methods to be approximately 17 Mt CO2 which is equivalent to 17% of total CO2 emissions of CPPI. Peng et al. [24] also analyzed energy efficiency and CO2 reduction and identified the energy saving and CO2 emission reduction potentials of CPPI. However, the estimated CO2 emissions of their studies only included those caused by energy consumption during production. Except for China, there have been only a few researches on the pulp and paper industry’s energy consumption and CO2 emission in other developing countries. For example, Poopak and Agamuthu [25] conducted a life-cycle assessment on a paper mill in Iran and found that using bagasse as the raw material and hydroelectricity as the energy source would have less global warming impact. However, Brazil, Indonesia, and Russia, as the other three main developing countries who produce pulp and paper, have rarely been mentioned in the literature concerning energy use and carbon emissions of their pulp and paper sector. The outcome of the previous studies conducted in China and other developing countries were similarly based on the data of energy use of production and IPCC emission inventory. CO2 emission can be overestimated with using IPCC inventory according to [26]. Moreover, due to the less advance technology and environmental policy, the pulp and paper industry usually has a poor environmental performance in most developing countries. With the increasing concerns of local environmental issues and also global climate impacts, the environmental standards have to be raised. However, there might be conflicts between local environmental issues and global climate change issues. To meet regional emission standards (much stricter environmental regulations than national standards, see [7]), local industry needs to invest in wastewater

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Fig. 1. The productions of pulp and paper in China and other countries during 1996–2012 (Data source: Almanac of China Paper Industry).

treatment and recycling systems which will certainly increase its total energy consumption which in turn may lead to the increase of total carbon emissions, while GHG emissions from the sewage treatment of the pulp and paper industry may also increase the total carbon emissions [7]. Thus, from the above literature review, another research gap has been identified: 2. Carbon emissions from the pulp and paper industry in developing countries have not been well studied. Thus, in order to fill the gaps identified above, CPPI, the largest paper producer in the world, was chosen as a case to study. The outcome of this study will provide not only detailed information about CPPI’s carbon emissions, but also a calculation framework for studying carbon emissions from the pulp and paper sector, especially when it comes to carbon emissions from biomass energy recovery and wastewater management.

3. An overview of the pulp and paper production process Generally, pulp and paper production includes two processes: pulping and paper making. Depending on the type of raw materials, paper pulp can be classified into 3 main types: wood pulp, non-wood pulp, and waste paper pulp. The sulfate process is mostly used for the pulping process of wood pulp and non-wood pulp in China, and the mechanical method is normally utilized for waste paper pulp. The sulfate pulping process is comprised of the pre-treatment sector, boiling sector, washing and sifting sector, bleaching sector, alkali recycling, and sewage treatment (Fig. 2). Wood material and non-wood material must be pre-treated. Wood pre-treatment typically includes peeling, sawing, chopping, and the selection process, which lead to biomass loss in the form of organic solid waste [27]. This part of solid waste will later be delivered to the auxiliary boiler to produce steam for the purpose of energy recycling which then acts as one source of carbon emissions in the pulp and paper making process. The purpose of the boiling sector is to add an alkane chemical in order to remove lignin contained in raw materials by decomposing the lignin macromolecule. During boiling process, 20–50% of plant cellulose is dissolved in the boiling liquor, which turns into sulfate pulp waste water, the black liquor. The main ingredients of black

liquor are lignin, carbohydrates, and boiling chemicals. Black liquor is condensed in the alkali recovery boiler and condensed black liquor is burned to recover energy. The process of black liquor combustion also contributes to carbon emissions in the pulp and paper making process [27,28]. The washing and sifting sector is necessary since soluble organic and inorganic matter must be removed to minimize the effects on bleaching and the process of paper making. The wastewater coming from the washing and sifting sector contains organic ingredients similar to black liquor. These organic ingredients are the main causes of the Chemical Oxygen Demand (COD) in pulp waste water. The bleaching process, which enhances pulp whiteness and whiteness stability as well as improving the physical and chemical properties of pulp, is the last sector of the pulping process. This sector generates pulp bleaching waste water which contributes to the total discharge of the COD. The paper making process generates less pollution when compared to the pulping process, which mainly comes from the paper machine. Waste water from all sectors finally goes into the sewage treatment plant to remove most of the COD before discharge into the environment. During the whole production process, biomass loss mainly occurs in 3 forms: pre-treatment organic solid waste, black liquor, and wastewater. During the pre-treatment of the raw material, bark, branch knots, and other useless organic segments are removed, which result in organic solid waste. Black liquor is mainly derived from the steaming, washing, and sifting process. The dominant organic components of black liquor are lignin and carbohydrates, which are caused by biomass loss. Wastewater is generated during the whole process of pulp and paper making. The COD contained in wastewater also mainly arises from biomass loss during the production process.

4. Methodology and data collection 4.1. System boundary Fig. 3 presents the system boundary of this study. The system boundary was limited in order to estimate carbon emissions the from manufacturing process of pulp and paper production and emissions generated from raw material collection, transportation, and utilization while wastepaper disposal was not taken into account (Fig. 3). The total carbon emission calculation of the pulp

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Fig. 2. The pulp and paper production process.

Fig. 3. System boundary carbon emissions from pulp and paper production.

and paper industry on a national scale involves all types of pulp and various production technologies. On the basis of the above paper industry process analysis, carbon emissions primarily arise from the following parts in a paper mill: (1) energy consumption including in-boundary fossil fuel combustion and electricity imported from out-boundary, (2) combustion of useless biomass from the pre-treatment sector, (3) combustion of condensed black liquor; and (4) methane emitted from incomplete aerobic digestion during sewage treatment. Carbon emissions from biogenic origins are excluded from total carbon emissions according to the Intergovernmental Panel on Climate Change (IPCC) guideline [9]. As aerobic biological treatment is widely used for sewage disposal, CO2 emitted from aerobic respiration is excluded from total carbon emissions due to its biogenic origin. However, incomplete aerobic digestion can cause methane emissions, which should be taken into account. Carbon emissions from combustion of useless biomass in the pre-treatment sector and combustion of condensed black liquor were also excluded in

previous studies due to their biogenic origin. The energy generated from these two processes can also help to reduce using more fossil fuel as well as to prevent generating more CO2. Except for the above four carbon emission sources, during the pulp and paper production process, calcium carbonate decomposition also contributes CO2 emissions. Calcium carbonate is mainly used for alkali recovery, desulfurization, and papermaking filler. Nevertheless, only calcinations of calcium carbonate during the desulfurization process contribute net CO2. During the boiling sector of the alkali pulping process, sodium hydroxide (NaOH) is added to decompose and remove the lignin macromolecule. In black liquor, sodium ions (Na+) coexist with various anions. After condensed black liquor burning, sodium ions unite with carbonate anions (CO2 3 ), which arise from CO2 absorption, to form sodium carbonate (Na2CO3). In order to recover the alkali, calcium hydroxide (Ca(OH)2) reacts with Na2CO3, and thus, CaCO3 and NaOH can be produced. The generated CaCO3 can be used for different industrial purposes, and the recycled NaOH can be added in the boiling

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sector. Alkali recovery involves both CO2 absorptions and CO2 emissions, and therefore the process can be considered carbon neutral. When calcium carbonate is used as papermaking filler, the process is still carbon neutral, and thus no net CO2 generated by CaCO3 is emitted. In CPPI, not all the enterprises choose CaCO3 for desulfurization, and moreover, the relevant data are not available. Hence, this part of CO2 emissions was excluded from this study. 4.2. Calculation method Eqs. (1)–(4) present the calculation methods of carbon emissions from different sources of pulp and paper production.

Eenergy ¼

X 44 ADi  NCV i  EF i  OF i  12 i

Ebiomass ¼ Pi  Ri  Loss ratei  Eblack

liquor

¼ Pi  ai 

22 15

22 15

Esewage ¼ CODinout  B0  MFC  GWP CH4

ð1Þ

ð2Þ ð3Þ ð4Þ

where Eenergy, Ebiomass, Eblack liquor, and Esewage refer to carbon emissions from energy consumption, combustion of useless biomass from the pre-treatment sector, combustion of condensed black liquor, and methane emitted from incomplete aerobic digestion during sewage treatment respectively. In Eq. (1), ADi is the activity data of energy i (unit: t, m3, kW h), NCVi is the average caloric value of energy i (unit: TJ/t, TJ/m3, TJ/ kW h), EFi is the carbon emission factor of energy i (unit: t carbon/TJ), and OFi is the oxygenation efficiency of energy I. In Eq. (2), Pi is pulp yield of pulp i (unit: t), Ri is raw materials per ton air-dry pulp i [29,30], Loss ratei is biomass loss rate of pulp is the conversion factor between i during pre-treatment [31], and 22 15 lignose and CO2, which is deduced from the chemical reaction of lignin combustion. In Eq. (3), Pi is pulp yield of pulp i (unit: t), ai is raw organic matter (t) contained in black liquor per ton pulp I, The value range of ai is 1.0–1.5 t, and an average value of 1.25 t as ai was recommended according to [27]. In Eq. (4), CODinout is the COD removal during sewage treatment, B0 is maximum capacity of CH4, here it is recommended to use the value suggested by the National Development and Reform Commission (NDRC) of China in 2015 [32], which is 0.25 kg, MFC is the modifying factor of CH4, which according to NDRC [32], MFC = 0.125 kgCH4/kg COD, and GWPCH4 refers to the global warming potential of CH4, which is 25, according to IPCC. 4.3. Carbon emission factors and data collection 4.3.1. Carbon emission factors (1) The EFi were derived from NDRC [32], however, the data of raw coal and other petroleum products that NDRC [32] failed to provide were substituted by those in the Guidelines for Provincial Greenhouse Gas Inventory provided by NDRC [33]. In addition, the EFi of ‘‘lubrication oil” and ‘‘paraffin” were derived from IPCC since local data were not available. Approximate substitution was used when the data sources mentioned above were not proper. (2) The value of OFi also followed the principle in item (1). (3) The emission factor of heat was provided by NDRC [11], which was 0.11 tCO2/GJ.

5

(4) The national average electricity emission factor was used for the carbon emission of electricity. (5) The NCVi of ‘‘carbon coke” was substituted for that of ‘‘other coke products”, the qi of ‘‘lubricating oil,” ‘‘paraffin,” and ‘‘solvent oil” were substituted by that of ‘‘gasoline”, and the qi of ‘‘crude oil” was utilized as the missing data qi of ‘‘other petroleum products.” 4.3.2. Data collection The data was mainly collected from official published statistics, official documents, cleaner production audit reports, and the literature. The China Energy Statistical Yearbook provided the total energy consumption of the pulp and paper industry according to diverse energy types each year in China, and the carbon emissions from energy consumption of the paper making process and pulping process of different pulp types and technologies were aggregated. Biomass loss occurs in the course of wood and non-wood material pre-treatment, and waste paper pulp making generates negligible biomass solid waste. This study aggregated the total carbon emissions from useless biomass of wood and non-wood pulp making. Black liquor originates from the chemical pulping process instead of mechanical pulping or semi-chemical pulping. Due to a lack of data on mechanical and semi-chemical methods as well as chemical pulping being the dominant technology needed for pulp making in China, this study supposed that the chemical method was applied to all of the wood and non-wood pulp making processes in China. Total COD removal of waste water treatment in China, which involves all types of pulp sewage treatment, was provided in [8], so that methane emissions of incomplete aerobic digestion could also be calculated.1 4.3.3. Carbon intensity In order to better understand the current status of carbon emissions from CPPI, the CO2 intensity was also calculated as follows:

CO2 intensity ¼

CO2 emission Paper product yield

ð5Þ

The CO2 intensity (t CO2 eq/t paper) of year i was the ration between CO2 emission of year i and paper product yield in year i. 5. Results 5.1. Carbon emissions from different sources of CPPI during 2005–2012 5.1.1. Eenergy Energy consumption acts as the greatest emission source when compared with other emissions. CO2 emitted from energy consumption accounts for more than 60% of the total carbon emissions of CPPI. From 2005 to 2012, the carbon emissions revealed a general trend of a slow increase, of which the average annual growth rate was 2.3%. In 2011 the CO2 emission reached the highest value: 98.5 Mt CO2. Energy consumption emissions from 2009 to 2011 demonstrated a noticeable increase, with the average annual growth rate being 6.4%. Energy consumption emissions decreased slightly in 2012 compared with 2011, but the proportion of them in total carbon emissions remained stable. Energy consumption includes 5 main types: raw coal and coal products, crude oil and crude oil products, natural gas, heating power, and electricity. Raw coal and coal products refer to raw coal, fine washed coal, other washed coal, coke, coke oven gas, 1 Assumptions: (1) The pulp-making process of waste paper pulp produces negligible organic solid waste, the main carbon emissions of waste paper pulp are from energy consumption and sewage treatment. (2) The papermaking process of all kinds of pulp mainly produces wastewater, so the carbon emissions of the papermaking process mostly arise from energy consumption and sewage treatment.

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and coke products. Crude oil and crude oil products include crude oil, gasoline, paraffin, diesel, fuel oil, lubrication oil, wax, solvent oil, petroleum gas, and other petroleum products. Natural gas and liquefied natural gas are classified as ‘‘natural gas” type. The contribution of 5-type energy consumption to carbon emission can be seen in Fig. 4. Within the limits of energy consumption emissions, electricity kept the most emission contribution with an annual average emission of 46.78 Mt CO2 during the study period. The second contributor was raw coal and coal products, of which the annual average emission was 40.48 Mt CO2. CO2 resulted from crude oil, crude oil products, and natural gas, while heating power contributed approximately 2.5% of total CO2 emitted by energy consumption. From 2005 to 2009, electricity carbon emission showed a moderate increasing trend during fluctuations. From 2009 to 2012, the CO2 emitted by electricity consumption increased 14.5 Mt CO2 at an average annual growth rate of 10.53%. In 2009, coal-related CO2 emission reached its highest level of coal emission: 43.39 Mt, a value even higher than that of electricity in 2009 by 2.1 Mt CO2. Since 2009, coal-related CO2 emissions began to be reduced at an average annual rate of 5.9%. The consumption of raw coal and coal products, crude oil, crude oil products and natural gas, and heating power remained basically stable during the study period, and thus, the relative carbon emissions showed a subtle change. 5.1.2. Ebiomass In the process of pulp making, biomass combustion contributes to total carbon emission, though as a means of energy recovery. During the study period, Ebiomass varied from 3.4 Mt CO2 to 4.1 Mt CO2, and the annual average rate of increase was 1.4%. Ebiomass was 3% of the total CO2 emission, and the statistics from year 2005 to 2012 reflected a slight fluctuation. Ebiomass is mainly generated from the wood and straw pulp making process, and therefore, this part of CO2 emissions is associated with the production of wood and straw pulp. From 2006 to 2008, the total yield of wood and straw pulp increased, while in 2009 the yield fell dramatically. From 2009 to 2011, wood and straw pulp yield increased, but in 2012 dropped again [8]. Ebiomass from 2005 to 2012 presented a similar trend, with the peak appearing in 2011. 5.1.3. Eblack liquor Black liquor is generally disposed of by combustion in most pulp and paper mills for the purpose of energy recovery. The boiling, washing, and sifting process of wood and straw pulp all lead to black liquor. Provided that the boiling, washing, and sifting technologies are much the same in the pulp mills of China, the amount of black liquor is directly related to the quantity of raw materials consumed. On the basis of wood and straw pulp yield, the corresponding raw material quantity can be obtained. Black liquor is one of the main CO2 emission sources, and it contributed 23–26% of total carbon emissions according to our estimation. As with the variation of wood and straw pulp production, CO2 caused by black liquor had two increases and two decreases. The minimum and the second-lowest values were 29.9 Mt CO2 in 2005 and 31.8 Mt CO2 in 2009 respectively, and the maximum was 37.8 Mt CO2 in 2011. From 2005 to 2008, a continuous increase was observed, of which the average annual increase rate was 6.3%. Another rise was from 2009 to 2011, and the mean annual rate of increase was 9.0%. 5.1.4. Esewage The COD contained in pulp and paper effluent is generally removed by aerobic biological treatment. The remaining COD is discharged into the environment according to local environmental

regulations. For the purposes of this study, this part of the COD was excluded from the system boundary, however, the amount of COD removal in sewage treatment was considered. CH4 emission is generated during waste water disposal because of incomplete aerobic digestion. The quantity of removed COD of each studied year was obtained from the Almanac of China Paper Industry, as well as the amount of COD generation,2 COD discharge, and COD removal. Under the condition that the 2011 replacement value was applied, the emission of CH4 from 2005 to 2012 varied from 0.48 Mt to 0.63 Mt (Fig. 5). The CH4 in 2005 amounted to approximately 0.50 Mt, and in 2012 the value increased by 26.5% which totaled 0.63 Mt. In 2006, the quantity of CH4 was the lowest (0.48 Mt), most likely due to the corresponding COD removal totaling less than those in other years. Moreover, the CH4 emission increased from 2006 to 2012, and the average annual growth rate maintained a level of 4.9%. The sewage-associated CH4 was translated into CO2 emission, with the maximum and minimum 15.9 Mt CO2 eq and 11.9 Mt CO2 eq respectively. 5.2. Total Carbon emissions and carbon intensity of CPPI during 2005– 2012 The total CO2 eq emissions of CPPI during the study period (2005–2012) varied from 126.0 Mt to 155.4 Mt. A trend of volatile growth can be observed. In 2005 the total CO2 eq emissions were approximately 126.0 Mt, and in 2012 the total emission was nearly 148.0 Mt. Between the first year and the last year studied, CO2 eq emissions increased over 17%, with a yearly rate of increase rate of 2.3%. This study’s results ranked energy consumption first out of 4 emission sources, and the contribution level of the other analyzed emission sources from high to low includes: black liquor, sewage treatment, and organic solid waste (Fig. 6). Fig. 7 shows total CO2 intensity of CPPI from 2005 to 2012. Based on the results, a general rise of total CO2 eq emissions can be observed, which stands in direct contrast to the decline of CO2 intensity from the first year to the last year. In 2005, CPPI had a carbon intensity of 2.2 t CO2 eq/t paper, while in 2012, CO2 intensity diminished by 35.8%, specifically 0.8 t CO2 eq/t paper lower than that of the number determined in 2005. The noticeable reduction of CO2 intensity reflected improvements due to low carbon development in the pulp and paper industry. 6. Discussions and implications 6.1. Uncertainty analysis In previous sections, we estimated the carbon emission of CPPI during 2005–2012. Due to the various sources used in the calculation, it is essential to discuss the uncertainty. (1) Sensitivity analysis of energy consumption The elasticity value is employed as an effective tool to quantify the sensitivity of different energy consumptions, and we tested the difference that energy amount changes can make to the total CO2 emissions. The calculation was based on the method proposed by [34]:

2 The primary data mentioned above were examined and calculations were made. However, the original COD generation datum in 2011 showed a lack of consistency compared with those in the other years. The study found that the data of COD generation in 2011 was approximately 8 times lower than the others with their difference being an order of magnitude. As a consequence, the involved datum was invalid, and the study used the median value between 2010 and 2012 COD removal as a substitute for 2011 COD removal quantity.

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Fig. 4. The contribution of 5-type energy consumption to carbon emissions.

Fig. 5. COD removal and CH4 emission from sewage treatment from 2005 to 2012.

ei ¼

DC=C 0 DEi=Ei0

ð6Þ

where ei refers to the elasticity value of energy type i (unit: t, m3, kW h), DC represents the variation of total CO2 emissions, C0 represents the original total CO2 emissions, DEi is the variation of consumption of energy type i, and Ei0 is the original consumption of energy type i. The amount of energy type i is considered to be a sensitive factor when its elasticity value exceeds the threshold of 0.1 [34]. Table 1 shows the elasticity of main energy consumption and indicates that the consumption of raw coal and electricity are two factors that have significant correlations to the total CO2 emissions. For instance, from 2005 to 2012, the elasticity values of raw coal varied from 0.24 to 0.31, which means that a 1% change of raw coal consumption can cause a 0.24–0.31% change of total CO2 emissions. (2) Uncertainty of parameters used in the inventory

In this paper, the average caloric values of lubrication oil, wax, solvent oil, coke products, and other petroleum products were alternatives due to missing data. In addition, the carbon emission factors of coke products and solvent oil, the oxygenation efficiencies of lubrication oil, wax, and solvent oil were also substituted. However, according to the above sensitivity analysis, the changes of these parameters had negligible impact on total carbon emissions. An important parameter related to energy use is the oxidation rate. Previous study has shown that the change of the oxidation rate of raw coal over time can significantly influence the estimation of carbon emissions [26]. Thus, the sensitivity of the oxidation rate was investigated. We only studied correlations between the oxidation rate of raw coal use and carbon emissions because raw coal and electricity are the only two significant energy sources in our above estimations. We found that a 1% change of the raw coal oxidation rate leads to a 0.3% change of the total carbon emissions. Due to raw coal oxidation rate discrepancies among different countries, CO2 emissions caused by raw coal use from the pulp and

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Fig. 6. CO2

eq

emissions from different sources of CPPI from 2005 to 2012.

Fig. 7. Total CO2

eq

emissions of CPPI and CO2 intensity from 2005 to 2012.

Table 1 Elasticity values of main energy consumption (2005–2012). Energy type

Raw coal Fine washed coal Other washed coal Gasoline Diesel Fuel oil Petroleum gas Electricity

Elasticity value (e) 2005

2006

2007

2008

2009

2010

2011

2012

Average

0.30 0.0056 0.0048 0.0019 0.0059 0.0067 0.0021 0.31

0.29 0.0046 0.0045 0.0020 0.0056 0.0067 0.0015 0.32

0.28 0.0052 0.0044 0.0021 0.0058 0.0062 0.0013 0.31

0.29 0.0053 0.0052 0.0024 0.0077 0.0049 0.0008 0.32

0.31 0.0042 0.0044 0.0028 0.0070 0.0044 0.0006 0.30

0.27 0.0039 0.0039 0.0022 0.0057 0.0038 0.0006 0.34

0.25 0.0042 0.0034 0.0017 0.0044 0.0025 0.0003 0.36

0.24 0.0024 0.0016 0.0018 0.0042 0.0015 0.0002 0.38

0.28 0.00 0.00 0.00 0.01 0.00 0.00 0.33

paper industry are likely to have deviations if local data are not used. In addition, the distinct raw coal oxidation rate is supposed to be applied in order to obtain more precise results in different years. We have also tested the parameters with range value used in our study. During pretreatment, the wood raw material loss rate

and non-wood raw material loss rate is 4.38–6.54% and 3–9% respectively [31], and we chose the median values, 5.46% and 6%, as loss rates of wood and non-wood materials for calculation due to a lack of specific information. The substitution of the wood raw material loss rate with the maximum and minimum value led to variations of 0.10–0.19% of total carbon emissions during

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the study period, while the variations, which arose from non-wood raw material loss rate replacement with the range of values was 0.80–1.10% during the study period. The organic content of black liquor generated by one ton wood or non-wood pulp is around 1–1.5 t [27], and in this paper the median value of 1.25 t was used for calculation. We analyzed the change of total CO2 emissions when replacing the median with maximum and minimum. Variations of 1.43–2.01% can be observed if the organic content of black liquor from wood pulp is substituted with the range of values. In terms of replacement of the organic content in non-wood black liquor, the variations of the rate turned out to be 2.66–3.67%. 6.2. Implications from this study (1) Compared with previous studies, the overall estimation of carbon emission from energy combustion is lower. For example, Kong et al. [23] studied CO2 emissions concerning energy consumption of CPPI from 2000 to 2010 in China. Kong’s results showed that in 2005, energy carbon emission reached approximately 85 Mt CO2, while in this study the value turned out to be 80 Mt CO2. The CO2 emission in 2010 that Kong determined was approximately 100 Mt, 4 Mt CO2 more than the number calculated in the present study. Moreover, in Kong’s calculation, CO2 emitted by energy consumption in 2006, 2007, 2008, and 2009 was around 90 Mt, 85 Mt, 93 Mt, and 95 Mt, respectively; the counterparts in this study were 85 Mt, 83 Mt, 91 Mt, and 87 Mt. Thus, the energy related CO2 emissions that this study calculated in the common studied years were all less than those determined in Kong’s research by an average of 4.7%. Since the original data of the two studies originated from the China Energy Statistical Yearbook, the disparity was very likely a result of emission factors and carbon oxygenation rates used by the two studies. Kong applied emission factors from the 2006 IPCC Guidelines for National Greenhouse Gas Inventories in order to conduct the calculation while this study utilized the latest local emission factors from NRDC in 2015 [32]. Currently, emission factors of coal are lower than those outlined in the 2006 IPCC Guidelines. In addition, the local carbon oxygenation rate of coal and coal products is lower than the IPCC default value. Liu’s [26] conclusion that CO2 emission can be overestimated in China if only the IPCC default value is used also supports the outcome of our study. Thus, the local inventory should be encouraged to be used in the estimation of carbon emissions of the pulp and paper industry for more precise results, not only in China, but for the other countries, especially for the developing countries who have also become obligatory in carbon emissions reductions according to recently signed the Paris Agreement. (2) Carbon emissions from combustion of useless biomass from the pre-treatment sector, combustion of condensed black liquor, and methane emitted from incomplete aerobic digestion during the sewage treatment of CPPI were estimated in this study. Previous studies did not provide estimations of carbon emissions for these processes in CPPI. In total, carbon emissions from these processes accounted for nearly 40% of total emissions, a figure that should not be ignored when it comes to carbon emission estimation and promotion of carbon reduction. However, it is necessary to understand the differences between Ebiomass, Eblack liquor, and Esewage though they are all biogenic emissions. Ebiomass and Eblack liquor were for the purpose of energy recovery which should be considered as carbon neutral. Fig. 8 shows the carbon emissions trend if equivalent energy (using raw coal for the calcula-

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tion) is used to substitute the biomass related energy in CPPI. It is evident that recovery of the energy from biomass loss will help to reduce the net carbon emissions from the pulp and paper industry, and it is essential that this action be encouraged throughout the process in the pulp and paper industry. For example, the promotion of a bio-refinery system, which includes the development of black liquor gasification, will contribute to energy efficiency improvement as well as carbon reduction [21]. Other studies recommend an increase in the use of recycled paper as a raw material, which will decrease energy use and carbon emissions from the pulp and paper industry [18]. (3) This study’s results indicated that carbon emissions (CH4) from sewage treatment accounted for 9–11% of total pulp and paper emissions. However, in most published articles, fossil energy consumption emissions were considered as total carbon emissions, and sewage emissions were overlooked. Although the results suggest that carbon emissions from sewage treatment in the pulp and paper industry should be taken into consideration when it comes to carbon reduction, there is still a policy trade-off for the pulp and paper industry. The pulp and paper industry in China and other developing countries is facing stricter environmental standards especially when it comes to wastewater discharge due to its high concentration of the COD. The industry has to reduce the concentration of the COD in order to meet local environmental standards. However, due to the limitations of current technologies, most of the pulp and paper companies have shifted to using wood instead of straw as the raw material. Previous studies have also suggested that CPPI should increase the rate of wood pulp because of its lower carbon footprint compared with that of straw pulp based on current technology [35]. Though using wood pulp may help to reduce COD discharge and CH4 generated from sewage treatment, CPPI is also facing the lack of a sufficient wood supply and increasing international pressures when it comes to importing wood products from abroad. Although IPCC does not include bio-original numbers in its carbon emission estimation, a forest is an important carbon sink which is important for combating CO2 increase, and the global forest is facing serious deforestation at the very same time. The pulp and paper industry has received more and more attention when it comes to sufficient wood supply [36] and illegal deforestation [37]. As such, CPPI should pay more attentions to working with different stakeholders in order to improve the sustainability of forest management as well as further promote projects such as forestry-paper integration [38] which will contribute to the CO2 mitigation target. More importantly, promoting cleaner production for preventing or minimizing waste generation in the production process of the pulp and paper industry should be encouraged from both the local environmental protection perspective as well as the global GHG emission perspective. For example, there are more than two dozen emerging energy-efficient technologies in the pulp and paper industry that will improve energy efficiency and reduce carbon emissions [39]. Moreover, to foster environmental friendly technology and innovations in using straw pulp more is also very much important for pulp and paper industry, especially in many developing countries where agricultural straw utilization is a serious challenge to the local environment. 7. Conclusions In this paper, we estimated the carbon emissions of CPPI from 2005 to 2012. During the study period, total CO2 eq emissions

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Fig. 8. Carbon emissions from energy combustions and equivalent biomass energy (substituted by raw coal) in CPPI (2005–2012).

ranged from 126.0 Mt to 155.4 Mt. The CO2 emissions from energy consumption ranged from 80.1 Mt to 98.5 Mt, which is over 60% of the total emissions and ranked first out of 4 different emission sources. Energy consumption moderately increased in the studied years with an average annual growth rate of 2.3%. Within the limits of energy consumption emissions, electricity kept the most contribution, followed by raw coal, and coal products. However, compared with previous studies, the CO2 emissions from the energy consumption of CPPI estimated in this study was 4.7% lower due to adopting the local emission inventory rather than IPCC inventory. Carbon emissions from the pre-treatment sector, combustion of condensed black liquor, and methane emitted from incomplete aerobic digestion during sewage treatment of CPPI accounted for nearly 40% of total emissions. Raw material residues, biomass loss during pre-treatment for the purpose of this article, acted as a means of energy recovery, and contributed to approximately 3% of total carbon emissions. CO2 from black liquor combustion accounted for 23–26% of total carbon emissions according to this study’s estimation. However, those emissions were for the purpose of biomass energy recovery which should be considered as carbon neutral. Recovering energy from biomass has saved a large amount of fossil fuel and has also reduced the release of net carbon emissions into the atmosphere. This study’s results also indicated that GHG emissions (CH4) from sewage treatment accounted for 9– 11% of total pulp and paper emissions. The CO2 intensity decreased from 2.2 to 1.4 (unit: tCO2 eq/ton paper) throughout the study period. This decrease includes the carbon emissions from biomass combustion, which reflected improvement of energy efficiency in CPPI. A few remarks from this study: (1) This study provides a framework for studying carbon emissions reduction potentials via biomass energy recovery and carbon emissions from sewage treatment of the pulp and paper industry for other countries. It also suggests that recovery of energy from biomass loss throughout the industrial process will significantly help to reduce the net carbon emissions from the pulp and paper industry. (2) This study provides a more comprehensive and precise understanding of the current situation of carbon emissions from CPPI compared with previous studies rather than only focus on energy related CO2 emissions. One suggestion for

other countries especially developing countries when it comes to estimating the carbon emissions from the pulp and paper sector is that if available the local inventory should be developed and used. (3) For the future carbon reductions in the pulp and paper industry, first, it is essential to reduce fossil fuel energy consumption as well as to improve energy efficiency. The government should promote the use of renewable energy in the industry sector. Second, recovering the energy from biomass during the production process should be encouraged. Third, cleaner production for preventing or minimizing waste generation in the production process of the pulp and paper industry should be promoted. The revised China’s Cleaner Production Promotion Law has been released in 2012 which has clearly stated it is mandatory to implement cleaner production audit in energy-extensive and highpollution industrial sector. Fourth, the fostering of environmental friendly technology and innovations in using straw pulp is necessary.

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