Review of energy consumption research for papermaking industry based on life cycle analysis☆

Yi Man ,Yulin Han ,Jigeng Li ,Mengna Hong

1 State Key Laboratory of Pulp and Paper Engineering,South China University of Technology,Guangzhou 510640,China

2 Guangdong Provincial Key Lab of Green Chemical Product Technology,South China University of Technology,Guangzhou 510640,China

ABSTRACT Papermaking industry is a high-energy-consuming industry with long supply chain.The growth of paper product demand further intensifies the need of energy consumption.Energy saving through the full supply chain has become a focal point for long-term sustainable development of the papermaking industry.This paper reviews the advances in life cycle analysis for the papermaking industry in recent years.All the stages from the full supply chain are involved to give a panoramic overview of the papermaking industry.The object of this paper is to provide scientific basis to industry and decision-makers with profound understanding of the energy consumption and energy saving potential in a life cycle perspective.

Keywords:Life cycle analysis Pulp Papermaking Energy consumption Energy saving

1.Introduction

Papermaking industry is one of the main pillars of the national economy.In recent years,with the increasing demand for paper products in China,the papermaking industry has got the rapid development.From 2006 to 2016,the average growth rate of China’s papermaking industry output is as high as 6%,as shown in Figs.1 and 2 [1].In 2016,the production of paper and paperboard in China reached 109 million tons,the output ranks the largest in the world[1].However,the paper consumption per capita in China is only 78 kg,compared with the 300 kg per capita consumption of developed countries,China’s papermaking industry has great potential for future development [2,3].

The papermaking industry is the fourth largest energyconsuming industrial sector in the world,which accounts for about 5%of the total industrial energy consumption[4].High energy consumption levels in the pulp and papermaking industry have also brought about the problem of high carbon emissions.The study shows that China is the country with the greatest potential for carbon reduction in the world pulp and papermaking industry,and the total emission reduction potential is approximately 55% [5].Meanwhile,the papermaking industry is also a high-waterconsuming industry,in 2014,the papermaking industry used 3.35 billion tons of fresh water,accounting for 8.7% of the total industrial fresh water consumption [6,7].The rapid development of the papermaking industry has brought certain pressures for China’s energy conservation and emission reduction targets.It is an important scientific issue to continuously reduce the production resources and energy consumption of the papermaking industry[8].

The papermaking industry has a long manufacturing supply chain,which spans the processes of resource extraction,logistics,manufacturing,and market distribution [9].If the study only focuses on energy saving and emission reduction in the paper production process itself,the problem of local optimization is likely to arise.For example,in the energy-saving transformation of paper production,the replacement of high-efficiency motors can significantly reduce the energy consumption of the enterprise,however,the energy saving of the motor during its life cycle cannot always offset the energy consumed in its manufacturing process [10].As for energy-saving and emission-reduction problems of the papermaking industry,the research needs to cover the entire supply chain based on the life cycle as a measure,so as to avoid wasting the resources caused by unreasonable industrial layout planning and prevent the rapid decline and elimination due to the lack of rational design [6].

Fig.1.Pulp consumption from 2000 to 2016 in China.

Fig.2.Paper production from 2000 to 2016 in China.

The paper products undergo a number of different production processes,however,the life cycle research involving the resources’ extraction,processing,transportation,and energy consumption,is not sufficient and is difficult to provide a scientific analysis basis for the papermaking industry’s industrial layout planning.This makes the government often create a sweeping approach situation in the formulation of industrial policies.Take Guangdong Province,China as an example,during the Twelfth Five-Year Plan period,Guangdong Province formulated relevant energy-saving and consumption-reduction standards for the papermaking industry,requiring paper enterprises to reduce energy consumption per ton of paper to 400 kg of standard coal[11].However,paper companies that use non-wood pulp (especially bagasse) as raw materials are unable to compete with paper-based companies that use wood pulp or waste pulp as raw materials due to process limitations,resulting in that all paper enterprises that use bagasse as raw materials ceased production and moved away.Since Guangdong Province is one of the main producing areas for sugarcane planting and processing,a large amount of bagasse needs to be shipped to Guangxi or other regions.After being processed,the paper products based on sugarcane are resold to Guangdong.From the life cycle perspective,this not only greatly increases the resources and energy consumption and carbon emissions of products during transportation,but also brings about social problems such as reduced employment and increased transportation burden.Therefore,the research needs to consider the energy saving and emission reduction issues from the life cycle scale integrating upstream and downstream processes.

The life cycle analysis methods are widely applied in the energy and chemical processes.They provide a good theoretical basis for the application in the papermaking industry.The study of the life cycle energy consumption is conducive to discovering the energy-saving potential of the whole papermaking industry chain[12,13].The life cycle analysis method has also been applied in the business analysis,for example,Procter &Gamble analyzed the life cycle process of its tissue paper products from raw material extraction to product consumption,and explored the potential for improvement in energy consumption,material consumption and emissions in the entire industry chain [14].The life cycle analysis is very conducive to guiding the healthy operation and development of the industry.

This paper focuses on energy-saving issues in the papermaking industry and reviews the development and trend of the life cycle research of the pulp and papermaking industry in recent years.The aim of this paper is to provide references for the energysaving issue of the entire supply chain,and provide scientific and evaluation basis for decision makers about the clean,efficient,and sustainable use of energy and resources in the papermaking industry.

2.Life Cycle Energy Consumption for Paper Industry

The energy consumed by paper production refers to the input of all direct and indirect energy sources throughout its life cycle,which is an important factor in determining the energy saving potential of paper.In addition to the energy consumption of the papermaking process,the resource consumptions in other life cycle stages are actually a transfer of industry and region involved in the final paper products.Taking primary energy consumption as an indicator of paper life cycle assessment,a large number of studies have been conducted at home and abroad:Xu and Becker[15]conducted field visits to plantations and pulp mills in South China and conducted life cycle assessments on the production of bleached sulfate eucalyptus pulp.The study found that although the pulping process was a major cause of global warming,due to the use of chemical fertilizers in the management of eucalyptus plantations,the forest system also contributed significantly to the environmental burden.González-García et al.[16]led the Spanish eucalyptus pulp production process:Life cycle assessment was conducted for the whole chlorine-free bleached sulfate eucalyptus wood pulp.The consumption of non-biological resources was almost entirely caused by the production of pulping chemicals.Ghose and Chinga-Carrasco[17]gave a life cycle inventory of thermomechanical pulp when studying the environmental impact of the Norwegian pulp and papermaking industry.Sun et al.[18]filled in the gap in the assessment of agricultural straw pulping environment,the study found that Alkali recovery could significantly reduce the consumption of fossil fuels,compared with wood pulp and waste pulp,straw pulping had a greater impact on the environment due to energy consumption and high demand for chemicals.Kiatkittipong et al.[19]studied four scenarios of bagasse waste management from the life cycle scale,including the production of bagasse pulp.González-García et al.[20]used life cycle assessment to quantify the use of non-renewable energy sources of hemp and flax pulp.Compared with primary fiber pulp,the production of waste pulp is relatively simple,so the assessment of its life cycle is often associated with paper.

Masternak and Rybaczewska[21]and Gemechu et al.[22]compared the environmental impact of the production of tissue paper using native wood pulp and waste pulp,the results indicated that the production process based on waste paper was relatively low in energy,plus the end use of tissue paper as a fiber,it was highly recommended to use recycled waste paper.Hong and Li [23]evaluated the energy consumption of printed writing paper in the same way and obtained similar conclusions.Cui et al.[24]conducted a life cycle assessment of coated white paperboard according to the specific raw material ratio,yielded 1 kg of paper,and the non-renewable energy consumption of approximately was 27 MJ.Gao et al.[25]compared the four uses of field straw,including the life cycle assessment of the production of corrugated paper.Liang [26]constructed an input-output life cycle assessment model and investigated the environmental impacts of straw,bagasse and waste paper making.Lopes et al.[27]compared the environmental impact of the use of fuel oil and natural gas in the Portuguese papermaking industry using life cycle assessment methods.Silva et al.[28]described the cradle-to-gate life cycle assessment of offset printing in Brazil,where the primary energy consumed by the industrial system accounted for 91% of the total demand.Ren[29]conducted a life cycle assessment of China’s copy paper and newsprint production,and established a life cycle inventory of thermal power generation and automobile road transport in China.Life cycle assessment helps identify hotspots in order to propose escalation options to reduce the impact on the environment[19].

3.System Boundary and Pathways

3.1.System boundary

Fig.3.Different life cycle boundaries of papermaking industry.

Paper’s life cycle assessment has no given boundary,some researches take the cradle-to-gate (from raw material acquisition to paper completion)as system boundary,some researches expand the boundary:(1)The origins are dated back to the cultivation and planting of raw materials,this type of research divides the paper life cycle system into two subsystems:forestry and industry[15,16,28].(2) Extend the gate to the grave,that is,from the use of the paper to its final disposal,including landfill,incineration,composting,and recycling[25,26,30-32].Gaudreault et al.[32]discussed the choice of paper life system boundaries.The departed research on the paper life cycle system boundary is shown in Fig.3.

The research started from the acquisition phase of the forestry subsystem neglected the stages of forest tree cultivation and planting,on the one hand,the primary energy demand in the forest growth process will vary with the changes in the environment,on the other hand,wood chips used in paper production are usually by-products of sawmills or scraps from the wood processing industry [32,33],the corresponding paper of agricultural straw uses by-products of crops and even wastes.Therefore,the choice of energy distribution methods will have a great impact on the energy consumption of the forestry subsystem[32,33].Gemechu et al.[22]makes an explanation on the setting of the end point to the gate.First,there is a lot of uncertainty regarding the after-use stage of papers because it is not easy to determine whether they are sent to landfill,incinerated or disposed of by any other means.The second reason is that even if we do know the end-of-life,it may be assumed that the GHG emissions associated with the after-use stages of both products would be the same regardless of the origins of the material used to make them.

The life cycle energy consumption of industrial subsystems includes a variety of indirect energy consumption in addition to the direct energy consumption purchased at paper sites and offsite:first is the direct consumption in the additional energy supply during its life cycle,such as the power input for coal mining and gasoline refining,second is the production consumption of pulping chemicals and paper fillers,the energy consumption of waste water and solid wastes is included in the energy consumption of paper sites,the energy consumption of raw materials and energy transportation depends on the transportation way and distance.

The word water-energy nexus covers every aspect of interaction of water and energy,water is an integral part of energy production (cooling,hydroelectric power generation,extraction of some mineral raw materials,etc.),and energy is used in many water supply,treatment and use processes [34],the water sector process involving energy use is shown in Fig.4.Man et al.[6]recently analyze the life cycle of water consumption of China’s papermaking industry,including indirect water consumption caused by energy consumption.The paper making industry,as the user of large water consumption,has not been assessed the indirect energy consumption of its water intake and water use processes in previous life cycle assessments.Although it is assumed that water consumption is not a threat to energy security,such inferences are being challenged [35].

GHG emissions in the papermaking industry have always been the focus of research,and one of the goals of energy conservation is to reduce GHG emissions of paper production,almost all paper life cycle assessments have analyzed its global warming potential.Leon et al.[36]adopt advanced paper structure design and fiber modification methods and quantize the GHG emission reduction potential of magazine paper production,Tahara et al.[37]compared the life cycle GHG emissions of e-books and paper books,when the average time for the use of electronic devices is set,paper books have higher GHG emissions,but considering the repetition use of paper books,the opposite result will be achieved.Tsinghua University proposes a calculation method for GHG emissions in paper companies[38].Wang et al.[39]use this method to calculate the total GHG emissions in China’s papermaking industry,including energy consumption,residual biomass and black liquor combustion,and waste water treatment processes.

The papermaking industry’s raw material types and product structures are complex,different types of pulp and paper have their unique life cycle paths,this paper refers to the division of Chinese paper species by the China Paper Association [1]and establishes the life cycle boundaries of China’s main paper production,as shown in Fig.5.The boundary of Fig.5 is determined from the acquisition of raw materials to the completion of the paper,showing the input of major materials and the emission of some greenhouse gases,although the process of material acquisition and transportation is not marked in the figure,but it should also be within the boundaries of the system,the output of paper types accounts for more than 95% of the total national production in 2016.

3.2.Pathways for different types of paper

The type of pulp used for paper is affected by various factors such as raw material acquisition,cost,and paper use,and some paper types need to be produced according to a specific pulp ratio.Under the premise of guaranteeing the quality of paper,raw materials of paper can be selected according to the following principles[40]:

(1) Make up for the lack of quality of main raw materials,for example,to improve the opacity of newsprint,it is necessary to add 2%to 32%of grindstone pulp[41],in order to increase the tear strength to meet the needs of high-speed rotary printing,it is necessary to increase the length of fiber slurry.

Fig.4.A conceptual model of water-sector processes involving energy use.(Modified from ref.[34].)

Fig.5.Life cycle boundaries for different paper products.

(2) Add some low cost and easily available slurry to reduce costs and replace some scarce slurry.

(3) Add some slurry to improve the operation of the paper machine.

The selection of pulp makes a great difference on the consumption structure of pulp in China over the years,for example,in 2000,the proportion of rice straw was 31%,and that of waste pulp was 41%,in 2016,the proportion of straw was reduced to 2%,the proportion of waste pulp raises up to 65% [1],the proportion of raw materials used to produce with these two will naturally change accordingly.The distribution of raw materials in various provinces in China will also affect the raw materials used in paper,such as the utilization of bagasse in Guangxi and the mass production of bamboo pulp in Sichuan.

The selection of slurry results in that almost every paper has multiple production pathways,according to the use of each paper type and the current status of China’s papermaking industry,the author summarizes the production pathways of major paper types in China,as shown in Fig.6.

The pulp ratio of newsprint production using a Fourdrinier machine:15%-20% bleached sulfate wood pulp,20%-32% milled wood pulp,30%-45% deinked pulp,and thermomechanical pulp 10%-30%[41].Household paper mainly uses primary fibers for production according to its use.The main raw material of CPP and UPP is wood;the addition of waste pulp will reduce the cost of paper.Boxboard and corrugated medium paper do not have the requirement of paper whiteness,and both use the original color paste and unbleached waste paper pulp,rice straw pulp accounts for about 40% in the early production of boxboard [42].WB (white board) and WP (wrapping paper) will use primary fibers correspondingly according to the production purpose and the grade of the paper.

4.Process-level Energy Consumption Estimates

This section assesses the energy consumption of each stage within the boundary of the papermaking industry system,namely,the life cycle energy consumption of related energy and materials in the industrial chain.

4.1.Energy supply

Fig.6.The production pathway for different paper products.

In 2015,the sum of energy consumption of natural gas and biomass in the papermaking industry in Europe and the United States exceeded 60% [43,44],coal consumption accounted for 32.2% in China’s papermaking industry[45],during the 12th Five-Year Plan period,the proportion of biomass energy accounts for 20% in the papermaking industry [46],other sources of energy are mainly purchased steam and electricity.China’s large and medium-sized paper enterprises mainly use coal-fired power for cogeneration to supply energy,and natural gas and other petroleum products are used less in China’s papermaking industry because of resources and costs.The energy consumption for biomass acquisition is detailed in Section 4.2,and the other sources of energy resources are listed below.

Most studies of the life cycle energy consumption are related to GHG emissions.Ou et al.[47-49]studied vehicle fuels as the research object,and discussed the life cycle energy consumption and GHG emissions of fossil fuels in China,Li et al.[50]and Chang et al.[51]gave energy consumption for mining,washing,and transportation of coal during the life cycle evaluation of different power generation technologies.Burnham et al.[52]compared the life cycle GHG emissions of conventional fossil fuels with shale gas,and Venkatesh and Jaramillo [53]analyzed the uncertainty of life cycle GHG emissions from gasoline and diesel.

The life cycle energy consumption of power generation mainly depends on the type of energy,boiler efficiency and power generation technology.In 2016,the average power coal consumption of China’s thermal power units is 312 kgce·(kW·h)-1(kgce is the kg standard coal equivalent,1 kgce=29307 kJ) [54],the operating efficiency of coal-fired boilers is 70% [55],and the total efficiency of power generation and power station heating is 44.2% [55].Domestic and international research on the life cycle assessment of various power generation technologies mainly focuses on GHG emissions,Chang et al.[51]compared the life cycle GHG between China’s coal power generation and shale gas power generation.Spath et al.[56]evaluated six environmental indicators included life cycle energy consumption and GHG emissions of three coal power generation technologies in the United States.Hondo [57]established the life cycle boundaries of nine different types of power generation in Japan and calculated their GHG emissions.IPCC [58]and Feng [59]summarized the previous studies on the life cycle assessment of electricity and obtained GHG emission ranges of various power generation technologies.

The GHG emission factor of power grids in Chinese provinces can be regarded as a weighted average of the GHG emissions caused by different power generation technologies according to their power structure.Actually,there is power transmission and trade among provinces,which means that it will be very inaccurate to calculate the indirect GHG emissions from companies purchasing electricity with GHG emission factors using the province’s power generation.Qu et al.[60]solved this problem using a network approach and obtained GHG emission factors for purchasing electricity in Chinese provinces,the author combines the yearly power generation of each province[45]and finds that the average GHG emission factor in China’s power grid is 683.1 g CO2eq·(kW·h)-1,however,this data only represents the GHG emissions caused by the power generation process and has not been analyzed from the perspective of the life cycle,from the above review of GHG emissions from various power life cycles,the life cycle GHG emission factor for China Power in 2016 was 764.5 g CO2eq·(kW·h)-1.

The energy sector is the main user of freshwater resources[61].Water consumption in this context is water withdrawn from a freshwater resource and not returned either because of evaporation,transpiration,deep injection,or major quality degradation[62].The water requirement is defined as the life cycle water consumption associated with individual fuel supply chain process.Zhang and Anadon [61]investigated the life cycle water intake,energy consumption,and environmental impacts associated with water consumption in China’s energy sector.Cai et al.[63]calculated the life cycle water intake of the energy sector in China’s provinces and forecast future water in-taking based on different scenarios.Lampert et al.[62]calculated the life cycle water consumption of US transportation fuels and defined the water resources within its life cycle.Multiple studies on the life cycle water consumption of China’s coal mining and washing [64-67]made statistics.The cooling of power production requires the extraction of large amounts of fresh water [64,65,67].Although the actual water consumption per unit of electricity production is much lower than the water withdrawal,the huge amount of electricity consumed by the power generation is quite considerable.Feng et al.[59]summarizes the life cycle water consumption of electricity production.

From the above-mentioned literature,the life cycle GHG emissions and water resource requirements for electricity production are shown in Fig.7.With the improvement of energy efficiency and power generation technology,there will be positive changes in electricity emissions and water consumption.

4.2.Raw material collection

The State Forestry Administration [68]has stipulated the energy consumption of wood production in forest areas,the acquisition of forest trees requires the preliminary processing and storage of chainsaw logging,rope way or tractor skids,winch loading,and wood storage yards.The Forestry Administration points out each part’s energy consumption and energy consumption correction factors and differentiates between the North and South’s energy consumption of wood production.Abbas and Handler [69]collected raw data from loggers in Tennessee,and obtained average fossil energy demand and GHG emissions for the whole state log supply chain,which are 524 MJ·t-1wood and 36.5 kg CO2eq·t-1wood respectively,the result of the study shows that chainsaw-based timber collection is less energy-intensive than feller buncher-based;if it is assumed that the wood transport distance is 150 km there and back,the environmental impact of transport is almost the same as that of timber harvesting.Similar research was conducted by Handler et al.[70],compared with the carbon content of the supplied wood and the contained energy,the environmental impact of the log acquisition stage is quite low.Zhang et al.[71]calculated the cost,energy consumption,and GHG emissions of different types of wood in different collection scenarios.The research on the energy consumption of agricultural straw collection is concentrated in China,Xing et al.[72]and Xu et al.[73]established a mathematical model for straw collection,storage,and transportation,and calculated the energy consumption and cost,Xing’s research showed that although the energy consumption in the process of straw compression bales is high,it lowers energy consumption per unit mass of straw in the transport process.The average energy consumption of wood and straw in China is shown in Fig.8.Laurijssen et al.[74]studied the energy consumption of waste paper recycling,and proposed that with the increase of the recovery rate and the amount of recycled fiber,the life cycle energy of paper decreased.Merrild et al.[75]found that due to the difference in mechanization degree,the global warming factor of waste paper recycling ranged from 4 kg CO2eq·t-1waste paper to 38 kg CO2eq·t-1waste paper,which was far less than the GHG emission produced by waste paper making.

Fig.7.The total life-cycle GHG emission and water requirements of eight electricity generation technologies.

Fig.8.Energy consumption of wood and straw collection.

4.3.Chemical production

The main role of adding chemicals to primary pulp production is cooking and bleaching;deinking pulp needs to add the deinking agent.The preparation of the sulfate cooking liquid can be directly dissolved or diluted with NaOH and Na2S,or can be supplemented with an appropriate amount of chemicals such as Na2SO4in the process of alkali recycling to obtain a rejuvenating liquid that satisfies cooking requirements.The main components of the deinking agent are saponification agents,including NaOH and Na2SiO3,H2O2,in addition to having a certain saponification,can also bleach waste paper containing mechanical wood pulp[76].Xu and Becker[15]and González-García et al.[16]gave a list of chemicals for the production of bleached sulfate wood pulp,and Sun et al.[18]and Ren [29]provided chemical types and dosage level for bleaching chemical pulp and deinked pulp respectively,the amount of chemicals used in the production of thermomechanical pulp is about 60%of the chemical pulp[17],the chemical list of unbleached pulp can be simply viewed as the corresponding bleach pulp removes decolorizer.Zhou [40]gave the range of main chemical dosage used by most pulp types in China.

Paper filler is the general term for sizing and filling processes,sizing can make the paper more resistant to water,the main component of traditional sizing agent is rosin plus alum,at present,AKD and ASA are most widely used.The role of paper filler is to increase the opacity of paper;the main components are kaolin,talc and other minerals[76].The types and amounts of fillers for different papers can be obtained from the paper life cycle evaluation[22,24]and pulp and paper manual [40,77].

Energy consumption,GHG emissions,and water consumption of chemical production can be obtained from life cycle databases such as Gabi and Ecoinvent,the study on environmental impacts of single chemical production [78]is few,if it needs to obtain the relevant data from the literature,the author proposes to conduct inquiries based on agricultural chemicals [79,80],the energy consumption and GHG emissions from China’s major production of pulp and paper chemicals are shown in Figs.9 and 10.

4.4.Pulping and papermaking

Energy-saving research in the papermaking industry itself can be achieved through improving processes,improving energy efficiency,and energy transformation,Kong et al.[3]identified 23 energy-saving technologies applicable to the paper process in China,the conservation supply curve was applied to analyze the energy-saving potential of technology and cost,by improving the paper process,38% of the fuel usage amount can be saved.Fleiter et al.[81]established the energy-saving model of the German papermaking industry,analyzed 17 specific process technologies from the bottom up,and predicted the energy saving capacity of the German papermaking industry due to efficiency improvement by 2035.Bajpai [82]proposed energy saving methods in various pulp and paper production processes in the book Pulp and Papermaking Industry:Energy Conservation.Peng et al.[5]pointed out that new technologies and energy structure adjustment are longterm strategies for energy conservation and development through calculating the vertical energy efficiency of China’s papermaking industry and the differences in horizontally comparing with foreign advanced levels,and the adjustment of papermaking industry scale will provide chances for energy efficiency improvements in the short term.Hong et al.[83]calculated the energy flow in the papermaking industry,focusing on how to capture the lost energy in the process and identify areas with potential for energy savings,this analysis can be served as a benchmark for the current pulp and paper business,and its energy flow is shown in Fig.11.Lin and Moubarak[84]established a mathematical model of China’s papermaking industry’s energy intensity (energy consumption per unit of value added in China),and made energy forecast (business as usual(BAU),intermediate and active)for the three paper industries in the future,in 2025,China’s papermaking industry can reduce its energy intensity by 90%,after that,Lin and Zheng [85]evaluated the energy issue by applying the DEA(Data envelopment analysis)method and the econometric model based on the input-output data of the papermaking industry in various provinces in China.The DEA method was also applied in the energy conservation assessment of the Swedish papermaking industry [86].

The formulation of policies can accelerate energy conservation in the papermaking industry [81],in 2015,the Chinese government issued a clean production evaluation index system for the pulp and papermaking industry[87],this indicator divides the environmental factors such as energy consumption,water consumption,alkali recycling rate and waste water output into three benchmarks,and proposed a process method to achieve the corresponding benchmarks.The government subsequently proposed the mandatory standards for the papermaking industry[88],and imposed limits on the unit energy consumption of the existing and new paper industries.Local governments have also stipulated the energy consumption of paper,such as Guangdong and Shandong,the paper outputs of both are at the forefront in the country.The National Development and Reform Commission and other departments will calculate and plan the energy consumption of paper pulp and paper for the recent five years and the next five years in the Five-Year Plan[89]of the papermaking industry,as shown in Fig.12.

The study on GHG emissions from the papermaking industry combined with life cycle assessments,Leon et al.[36]adopted advanced paper structure design and fiber modification methods to quantify the GHG emission reduction potential of magazine paper production,Tahara et al.[37]made a comparison between the lifetime GHG emissions from e-books and paper books,when the average time for the use of electronic devices was set,paper books had higher GHG emissions,but considering the repetition use of paper books,it will get the opposite result.Wang et al.[39]used this method to calculate the total GHG emissions from the papermaking industry in China.There was relatively few research on water consumption in the papermaking industry,and the research of Man et al.[6]filled this gap.

Fig.9.Energy consumption and GHG emission of the chemicals for major pulp products (ADt is the ton air-dried pulp).

Fig.10.Energy consumption and GHG emission of fillers for major paper products.

Fig.11.Energy flow profile of pulp and paper industry (tce is the ton standard coal equivalent,1 tce=29307 MJ).(Modified from ref.[83].)

Fig.12.Estimation of energy consumption for pulp and paper production processes from 2000 to 2020 in China.

4.5.Transportation

Domestic raw material transport distance is relatively short,and the environmental impact on the entire life cycle is not obvious [6],however,China’s major coal-producing bases and largescale pulp and paper enterprises are adversely distributed,the use of higher-efficient railway transportation will greatly reduce energy consumption and GHG emissions from long-distance transport.Judl et al.[90]compared life-cycle GHG emissions in the context of paper in pulp production in Finland and imported pulp from South America,the result shows that though the overseas production of pulp will reduce the environmental pressure in the local areas,GHG emissions from transportation from South America to Northern Europe still account for 27% of the entire life cycle process,even with energy-efficient ship transportation.

4.6.Other processes

Other processes within the system boundary,such as alkali recycling and waste water treatment,have a large impact on energy and the environment,and the related study has been made.Zhang[91]and Liu[92]analyzed the energy balance of the typical alkaline recovery system for China’s wood pulp and wheat straw pulp,and calculated the energy consumption and water consumption of black liquor evaporation,combustion,and causticization sections to obtain that the alkaline recycling of wood pulp and wheat straw pulp can get more than 60% energy of pulp heat energy.Stoica et al.[93]established a mathematical model for paper waste water treatment,focusing on energy use and recycling in waste water and sludge treatment.Ashrafi et al.[94]summarized various waste water treatment technologies in the pulp and papermaking industry and compared the efficiency of pollutant removal and GHG emissions.

5.Conclusions and Outlook

The energy saving issue in the papermaking industry has become one of the challenges the modern papermaking industry is facing.The energy-saving issue not only needs to focus on the pulp and paper production process itself,but also needs to be extended to the entire industry chain of the papermaking industry.For the various links involved in the entire industry chain of the papermaking industry,issues of energy consumption and energy saving potential have made more progress,which can provide data foundation for further life cycle analysis,however,energy saving issues based on the entire industry chain have only just begun,many aspects need further study.

(1) In the papermaking industry energy-saving technology innovation,with the help of process modeling,simulation,and industrial big data analysis methods,focusing on the whole life cycle analysis of the process system and the entire industry chain,it is one of the important research fronts of the papermaking industry.

(2) Due to a long industrial chain,the papermaking industry expands the life cycle analysis method and the ecological input-output analysis method,and integrates with the process simulation technology,it discusses the link system between direct energy consumption and indirect energy consumption under the whole life cycle scale,which is conducive to discovering new energy-saving potentials and realizing the coordinated optimization of energy-saving issues from the perspective of the entire industry chain.

(3) The study on energy-saving issues based on the life cycle scale in the papermaking industry is still at the exploratory stage.With the rapid popularization of paper enterprises’informatization in recent years,based on industrial big data’s analysis and optimization of papermaking industry,it is of great significance to realize energy-saving and emission reduction in the entire industry chain of papermaking industry and achieve the sustainable development of the papermaking industry.