Science of the Total Emvironment 793(2021)148340Contents lists available at ScienceDirectScience of the Total EnvironmentELSEVIERjournal homepage:www.elsevier.com/locate/scitotenvBiogas feedstock potentials and related water footprints from residues inChina and the European UnionHIGHLIGHTSGRAPHICAL ABSTRACTBiogas from residues can substantiallycontribute to natural gas supply inChina.中国和欧盟的生物燃气原料供应潜力Biogas from residues can also play a rolein EU gas supply.China's dominantresidues for biogas aremaize,rice,wheat and cotton.Water footprints of residues show largedifferences among crops and countries.Biogas water footprints of sugarbeetresidues are favorable.Artide history:China encounters heavy air pollution caused by coal consumption.China and the EU aim to decrease greenhouseReceived 15 February 2021gas emissions.Shifting to biogas from residues contributes to solving both proble ms.This study assesses China'sReceived in revised fomm 30 April 2021biogas potentials and related water footprints (WFs)and compares results with potentials and WFs for the EU.Accepted 5 June 2021Starting from a literature review on EU biogas potentials,it analyzes information resulting in a calculation meth-Available online 10 June 2021odology,its validation and application to China.Finally,it estimates WFs and makes a comparative assessment ofEditor:Baoliang Chenbiogas potentials of the EU and China.In the EU,biogas from agricultural,forestry and other residues might con-tribute 8%(5300 PJ)to primary energy consumption,in China 10%(13,275 PJ.)In the EU,agriculture contributes41%,forestry 26%,other residues 23%,and manure 10%.The corresponding results for China are agriculture (67%).forestry (23%).manure (7%)and other residues(3%).In the EU,biogas might contribute 45%to total gas demand:EU and Chinese biogas potentialsin China more biogas can be produced than consumed in 2018(185%of demand).The EU results fall in the rangeof residue potentials from earlier studies.Maize,wheat,barley and rapeseed contribute 78%to the EU agr iculturalForestry residuesbiogas potential In China,dominant crops are maize (49%).rice (18%),wheat (12%)and seed cotton (6%).ForResidue availabilitywater,there are large differences among WFs of specific crop residues,but also between WFs for EU and ChineseBiogas water footprintComparative assessmentcrop residues.Most Chinese crop residues have larger WFs than the EU residues.Biogas from sugar beetresidueshas the smallest WFs,biogas from tobacco residues the largest.Although using residues for energy does notchange total national WFs,it reallocates WFs over main products and residues.The comparative assessment sup-ports better use of biogas potentials from residues with lower WFs and is also applicable for other regions andcountries.2021 The Author(s).Published by Elsevier B.V.This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Corresponding author.0048-9697A0 2021 The Author(s).Published by Elsevier B.V.This is an open access artide under the CC BY license (http://creativecommons.org/licenses/by/40/).Z.L Yuan and P.W.Gerbens-Leenes1.Introduction1990 and 2018(IEA,2021).However,natural gas from fossil sourcescontributes to greenhouse gas emissions,so biogas from residues is farGlobally,climate change is regarded asone of the main problemshu-more favorable.manity is facing today (IPCC,2019a:IPCC,2019b).Attempts to decreaseSince the EU is greatly concemed about climate change,it formu-greenhouse gas emissions include a shift away from the consumption oflated clear goals and ambitions to change the EU energy mix towardsfossil fuels,like coal,crude oil and natural gas,to renewable sources ofdifferent successive targets between now and 2050.It aims to achieveenergy,like solar,wind,or biomass energy.At present,biomass is stilla reduction of Green House Gas emissions (GHG)from all sources ofproviding a large share of renewable energy(IEA,2020).Energy from40%in the year 2030 compared with the emission level in 1990biomass includes traditional biomass,mainly fuelwood,and modem(European Commission,2014,2015,2019).China has identified the de-biomass that includes three different generations (Gerbens-Leenesvelopment of a "clean,low-carbon,safe and efficient energy system"et al.,2020).The first-generation energy from biomass is derived fromand is aiming to become carbon neutral by 2060(Mallapaty,2020)crops.Examples of first-generation energy carriers are ethanol fromThe demand for natural gas is booming because of the massive cam-sugar cane or biodiesel from rapeseed.These energy carriers competepaign to replace coal by gas (China National Renewable Energy Centrewith energy for human nutrition.To avoid competition with food,2018).China encounters heavy smog problems due to coal buming es-attempts are made to apply residues to generate so-called second-pecially in wintertime when coal is used for both electricity generationgeneration energy carriers.Examples of residues are straw fromand heating.Since 2013,Chinese policy has encouraged the substitutionagriculture,branches and leaves from forestry,or municipal wastesof coal-based centralized heating by a natural gas-based heating system(Panoutsou et al.,2009).These residues can in tur be converted into en-together with promoting gas-based electricity generation,since naturalergy (heat)or into other energy carriers,such as ethanol,pyrolysis oil,gas is much cleaner than coal.In this way,China hopes to diminish thecharcoal or biogas.Mathioudakis et al.(2017),for example,calculatedsmog problem(Chinese National Council,2013)by increasing the gasthe most efficient conversion technologies from a water perspective,supply annually (Zou et al.,2018).In 2020,Sinopec reported a 5.3%in-concluding that heat and biogas are the energy carriers with the smallestcrease in gas supply during the winter season,compared to 2019,andwater footprints.Third generation energy carriers include energy fromPetro China reported an increase of 11.3(Xu,2020).The use of naturalalgae.gas in China can be expected to increase even further.Domestic biogasNot all crop residues can be used to serve as an energy source,sinceproduction can also become a significant component of the total gaspart of it needs to beretumed to the soil.Siedt et al.(2020),for example,supply (Gao et al.,2016).So,this study aims to explore the residue avail-have shown that carbon-rich organic amendments,such as straw fromability for second-generation biogas in both the EU and China and to es-cereals,compost or biochar,improve soil quality.Especially biochartimate the biogas potentials from residues allowing for a comparativeserves to immobilize pollutants like toxic heavy metals that could other-assessment for both regions.wise end up in plants posing environmental and health risks (ZubairOrganic residues serving as a feedstock for biogas originate from ag-et al,2021).For example,biochar is effective in immobilizing lead inriculture and forestry,as well as a large part of the share of other resi-lead-polluted soils (Naeem et al.,2021).In Pakistan,it was shown thatdues from household or industrial wastes.This means that naturalbiochar addition to soils prevents crops from taking up too much leadresources,such as freshwater were needed to produce the crops andfrom soils contaminated by irrigation water containing lead from batte-residues.Globally,agriculture is the largest consumer of freshwaterries (Khan et al.,2020).Biochar also immobilizes other heavy metalswith an overall share of 92%of human consumption (Hoekstra andlike nickel (Turan,2019)and cadmium (Zubair et al.,2021).ThisMekonnen,2012).This means that energy derived from agriculturemeans that estimating the theoretically available energy from all resi-also requires much water.The study of water footprints(WFs)of energydues by subtracting the residues from the total biomass yield using a(Gerbens-Leenes et al.,2020)has provided an overview of the WFs pervalue forthe harvest index,the ratio of the yield and the sum of the res-energy type indicating the large WFs of some types,e.g.WFs of first-idues and yield,overestimates actual residue availability.It is also rele-generation biofuels or hydropower.When residues are applied to pro-vant to indicate whether collecting residues is economically viable,duce an energy carrier,e.g.biogas,the total WF for agricultural produc-since costs play a role too,as well as technical feasibility.Based on thetion remains the same,because water consumption is allocated over thecriteria mentioned above Niclas and Caus (2010)defined four levelsmain product and the residue.However,some crops have relativelyof biomass for energy potentials:(i)theoretical:(ii)technical:(iii)eco-large WFs or small collectable residue fractions,increasing the WF ofnomic:and (iv)sustainable potentials in which the theoretical potentialthe biogas.We therefore included an assessment of the WFs of biogasis the maximum energy potential attainable.The other potentials de-per crop type per country.The method was adopted from Hoekstrapend on the criteria chosen and specific system constraints.et al.(2011)that identified green (precipitation),blue (surface andSince it is important to find otherways of generating energy with lim-groundwater)and grey (pollution related)WFs.This gave rise to the fol-ited greenhouse gas emissions,several studies aimed at calculating en-lowing research questions:What is the availability of residues forergy potentials from residues for the European Union (EU).Examplessecond-generation biogas in the EU and China and what are the currentare the Baltic Renewable Energy Centre (2004).Camia et al.(2018).sustainable biogas potentials?What is the green,blue and grey WF re-Elbersen et al.(2012),Intelligent Energy Europe(2011),Monforti et allated to biogas production per crop per country and how large is the(2015).Panoutsou et al.(2009).Ronzon and Piotrowski (2017).ScarlatWF related to the EU and Chinese potentials?et al.(2019)and Searle and Malins (2016).However,all studies arrivedFirst,we reviewed the relevant scientific literature,and comparedat different levels of biomass potentials,because they included differenttheir methods system boundaries and estimations of EU potentials.residue types,applied other system boundaries,and also expressed po-Next,we developed a calculation method to assess potentials and vali-tentials in different units While there are EU-related estimates of energydated this with the results of earlier studies.Finally,we applied thepotentials from residues,no such estimate is published yet for China.method to estimate the biogas potential of China and calculated WFs.InThe IEA Bioenergy (2019)has shown that it is technically possible tothis way,the comparative assessment leads to options to decrease green-gasify biomass,e.g.residues and organic waste to supply natural gas inan attempt to decrease greenhouse gas emissions.For China,biogascould function as an additional energy source.China still relies heavily2.Review ofexistingstudies on biomass energy potentials fromres-on coal for its energy supply.In 2018,China derived 8.5 times more en-idues in the EUergy from coal than from natural gas,although consumption of naturalgas increased eighteen fold between 1990 and 2018 (IEA,2021).InSeveral studies made an inventory of energy potentials from resi-Europe,natural gas consumption increased by a factor of 1.3 betweendues for the EU.We performed a literature review into studies on