The biomethane producing potential in China:A theoretical and practical estimation☆

Chang Liu *,Jun Wang ,Xiaoyan Ji,Hongliang Qian ,4,Liangliang Huang ,Xiaohua Lu

1 College of Chemistry and Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

2 School of Chemical,Biological&Materials Engineering,University of Oklahoma,Norman 73019,United States

3 Division of Energy Science,Luleå University of Technology,97187 Luleå,Sweden

4 School of Pharmacy,China Pharmaceutical University,Nanjing 210009,China

1.Introduction

Nowadays,the fossil fuels,especially coal,are still the main energy resources in China.In 2014,a total of 4.26 billion ton standard coal contributed 66%of the primary energy consumption.This heavy energy dependence on coal results in both serious environmental impacts[1].Natural gas as a clean energy is an ideal substitute of coal,which can reduce dramatically environmental damage.In 2014,the amount of natural gas consumed in China was 190 billion m3,which is only 5%of the total energy consumption.Another limitation is that,about 30%of the consumed natural gas was imported from other countries[2].According to the Chinese energy development strategy(2014–2020),the proportion of natural gas in primary energy sector needs to be increased 10%by 2020,which corresponds to a production of 400 billion m3natural gas in 2020[3].Itis ofgreatimportance to estimate the naturalgas production potential in China and explore different resources to produce natural gas.

Among different natural gas resources,biomethane is considered as a very promising one and has been developed rapidly in many countries.Germany began to develop biomethane from 2006,and after years of study,there had been 107 biogas plants engaged in the biomethane production by 2011[4].In Sweden,there had been more than 120 biomethane gas filling stations and 40000 biomethane cars were built up,and biomethane consumption had accounted for 10%of the national nature gas consumption by 2010[5].The number of biomethane gas filling stations was increased from 60 to 100 in Switzerland in 2005–2008,and Swiss Gas Industry Association had signed an agreement with Biogas Association,to make some cost discount for biomethane incorporating into natural gas pipe network[6].And the developmentofbiomethane production also hasreceived an increasing attention in China for the past few years[7].Biomethane is the fuel that obtained from biogas after removal of CO2and H2S,also called biological natural gas.The methane concentration in biomethane can reach up to 95%–97%and meet the Chinese natural gas standard(GB 17820-1999),so that biomethane can be used as vehicle fuel gas or incorporated into natural gas pipe network as the substitute natural gas[8,9].Biogas is obtained from the low-grade biomass anaerobic fermentation process,and the low-grade biomass is commonly used as substrates,including livestock manure[10],straw[11],municipal solid waste[12]and sewage sludge[13,14].Some studies already investigated the biogas potential in China.Tian[15]estimated the biogas production potentialbased on the amountoflivestock manure fromlarge-scale farms in China in 2009,they concluded a biogas production potential of 47.21 billion m3peryear.Zhang etal.[16]estimated thatthe biogasproduction potential was 119.844 billion m3in China in 2009 from all excrement,among which 24 billion m3was from large-and mediumsized livestock and poultry breeding farms.Chang et al.[17]evaluated the biogas potential of crop straw and livestock manure through timeseries and geographical approach,indicated the great regional differences in biogas potential between districts and provinces in China,and the most part of total biogas potential in China is allocated among Mid-South district,East district and South West district.And an international collaboration between China and Denmark was carried out to estimate the biogas production potential in 2011,the study considered resources such as livestock manure,municipal solid waste and sewage sludge.The biogas production potential was estimated to be 206.8 billion m3(124.08 billion m3biomethane)in 2009[18].However,in the above studies,the biogas potential was just estimated according to the current digestion technology in China,which was a more practical potential.When considering the development of technology,the efficiency of anaerobic digestion process can be increased,and the potential will be improved.On the other hand,many studies just considered the common low-grade biomass in China,the potential of energy crop has never been estimated.To increase the biomethane production,energy crops have been considered to be the better choice for its high biogas productivity and yield,and the broad marginalland resources are good place for energy crop planting,due to the large population and a very limited cultivated land in China[19–21].

In this work,based on the updated data collected from traditional biomethane production,the theoretical and practical biomethane potentials in China were estimated with appropriate methods(Buswell formula for the agriculture waste and energy crop,IPCC formula for the municipalsolid waste and sewage sludge).And differentwith previous work,two appropriate energy corps were chosen from the screening process designed,and assumed to be planted on the marginal land to get the biomethane potential of energy crop in China.We hold that agricultural waste should be the preferential development biomass,planting energy crops on marginal lands is the most promising way to enhance biomethane production,developing biomethane is a promising way for energy supplement in China.

2.Estimation Methods

2.1.Theoretical and practical biochemical methane potentials

In this work,the theoretical biochemical methane potential(TBMP)represented the maximum biomethane yield per volatile solid(VS).To get the TBMP,the organic carbon in biomass was assumed to be transferred into CH4and CO2completely during the anaerobic digestion process,which were the two main composition of biogas.In this respect,a formula was firstly proposed to simulate this extreme hypothesis by Buswell and Mueller based on the element compositions of different biomass[22],and this formula was also cited in some other studies.Møller et al.[23]used the Buswell formula to calculate the theoretical methane potential of different specific organic components in biomass,and Li etal.[24]studied the methane production ofthree organic wastes during a batch anaerobic digestion process,and the Buswell formula was used to calculate the theoretical methane yield of organic wastes.In the Buswell formula,the distribution ratio of organic carbon in CH4and CO2was determined via one mole of biomass with a known elementary composition,and the theoretical production of methane was calculated according to Eqs.(1)and(2):

The TBMP can be calculated by Eq.(2):

where Vmis the molar volume of methane at standard temperature and pressure.

When considering the actual digestion conditions,biomass can't be degraded completely by microorganism during the digestion process,so in order to get the practical biochemical methane potential(PBMP),an important factor needs to be taken into account,namely the biodegradability(BD)[25].During the research process,Labatut et al.[26]thought that the extent of such an effect tends to be correlated with the composition in biomass.Triolo etal.[27]studied the relationship between biomethane potentialand different compositions(lignin,neutral detergent fibers,acid detergent fiber,cellulose),they reported that the lignin content in volatile solid was the most important parameter for predicting biochemical methane potential of all kinds of biomass.And Li et al.[28]conducted a series of biomethane potential assay of many organic substrates(mesophilic digestion,VS concentration:3 g·L−1),to study the relationship between the lignin contentand the biodegradability.After a series of data analysis,they observed a good linear correlation between lignin content and the biodegradability(BD)for the lignocellulose and manure wastes,this could be used as a fast method to predict the biodegradability of fiber rich substrates.In this work,we used this linear correlation(Eq.(3))to estimate the BD of crop residue,livestock manure and energy crop:

where CLigninis the lignin content(wt%VS)of different biomass.

Then the PBMP can be calculated from TBMP and BD by Eq.(4):

2.2.Biomethane potential of crop residue

The biomethane potential of crop residue depends on their production and biochemical methane potential(BMP).The production of crop residue can be calculated based on the crop yield,straw grain ratio of different crop and the general crop residue collection rate.

In this work,the biomethane potential of crop residue was calculated by Eq.(5):

where Ybmcis the biomethane potentialofcrop residue,representing the theoretical(Ytbmc)or practical(Ypbmc)biomethane potential,billion m3;Ycis the yield of crop,104ton;SGR is the straw grain ratio;CR is the residue collection rate;and BMP can be the theoretical(TBMP)or practical(PBMP)biochemical methane potential,L CH4·(g VS)−1.

2.3.Biomethane potential of livestock manure

The biomethane potential of livestock manure was calculated based on the manure production and BMP.Different with crop residue,the production oflivestock manure depends on the yield oflivestock,excretion coefficient,livestock raising cycle and the general manure collection rate.

In this work,the biomethane potential of crop residue was calculated by Eq.(6):

where Ybmlis the biomethane potential of livestock manure and can be the theoretical(Ytbml)or practical(Ypbml)biomethane potential,billion m3;Ylis the yield of livestock,104capita;P is the livestock excretion coefficient,kg·capita−1·d−1;and T is the livestock raising cycle,d.

2.4.Biomethane potential of energy crop planted on marginal land

In order to calculate the biomethane potentialofenergy crop,we collected the data of different marginal land terrains and selected suitable energy crop species for planting.Depends on the general yield of different energy crop,their BMP and the marginal land area selected,the calculation of biomethane potential of energy crop is shown in Eq.(7):

where Ybmeis the biomethane potential of energy crop planted on the marginal lands and it can be the theoretical(Ytbme)or practical(Ypbme)biomethane potential,billion m3;Ydeis the general yield of energy crop in dry-basis on the marginalland,t·hm−2·a−1;S is the area ofeach marginal land,million hm2.

2.5.Biomethane potential of municipal solid waste

Differentfromcrop residue and manure,the composition ofmunicipal solid waste is closely related to people's living condition and changed greatly with season and region,which means that the elemental composition in a certain region can't represent the nation-wide elemental composition.However,very few reports are involved with the precise data of different elemental compositions in national municipal solid waste,so in this work,we didn'tuse the BMP method to estimate the biomethane potential of municipal solid waste across the country.Instead,an approach to estimate the amount of nation-wide methane emissions from land fill to atmosphere was established by intergovernmental panel on climate change(IPCC),and the formula is shown in Eq.(8)[29]:

where Q isthe amountofmethane emissions frommunicipalsolid waste,billion m3;MSWTis the municipal solid waste production,104t;MSWFis the municipalsolid waste processing ratio;MCF is the methane correction factor;DOC is the biodegradable organic carbon content in municipal solid waste,wt%;DOCFis the decomposition proportion of biodegradable organic carbon;F is general methane content in land fill gas;16/12 is the coefficient of carbon transfer into CH4,which is denoted by M in this work;R is the amount of methane already recovered,billion m3;and OX is the methane oxidation factor.

In the approach proposed by IPCC,we can observe that italso means the organic carbon in municipal solid waste completely transfers into the land fill gas during digestion process,and the carbon distribution ratio is determined by the general content of methane in land fill gas.Moreover,the key parameter needed in this formula is the carbon contentofnation-wide municipalsolid waste,which have reported in some studies.In this work,this formula was made some modification,and used to calculate the biomethane potential of municipal solid waste in China.In the modified formula,YMSWwas the amount of municipal solid waste clean-up,the parameter MCF which depends on the digestion condition was changed to correspond to the condition in anaerobic fermentation reactor,so it could be applied to biogas,and degradable organic carbon(DOC)was used to indicate biodegradability of biomass which correspond to the practical biomethane potential.Meanwhile,the total organic carbon content(TOC)in municipal solid waste could be used to calculate the theoretical biomethane potential in municipal solid waste.R and OXwere externalfactor,we removed it.The modified IPCC approach used in this work is given by Eqs.(9)and(10):

where Ybmmis biomethane potential of municipal solid waste and it can be the theoretical(Ytbmm)or practical(Ypbmm)biomethane potential,billion m3;YMSWis the amount of municipal solid waste,104t;MCF is related to the land fill management condition and equal to one when the waste is managed well in a complete anaerobic condition[29];F is the methane contentofbiogas due to biomethane is upgraded from biogas;M is the coefficient of carbon transfer into CH4,equal to 16/12.

2.6.Biomethane potential of sewage sludge

Similar to municipal solid waste,the composition of sewage sludge also changes greatly with the season and region and IPCC[29]has developed a method to estimate the amount of nation-wide methane emissions from sewage sludge,as given in Eq.(11):

where Q is the amount of methane emissions from waste water,billion m3;TOW is the total organic material in waste water,(kg COD)·a−1;S is the organic component removed from sludge,(kg COD)·a−1;Bois the maximum methane production capacity of sewage recommended by IPCC experts,kg CH4·(kg COD)−1;MCF is the methane correction factor;and R is the amount of methane already recovered,billion m3.

In the IPCC formula,chemical oxygen demand(COD)was used to represent the organic matter content in sewage sludge,and Bo(kg CH4·(kg COD)−1)was used as the key parameter.In our work,Bowas used to represent the theoretical methane production capacity and expressed as Bto.Meanwhile,Bpowas used to represent the practical methane production capacity of sewage sludge in China collected from literature.MCF was changed to correspond to the condition in anaerobic digestion reactor.R is external factor.The modified equation is shown as Eqs.(12)and(13):

where Ybmsis the biomethane potential of sewage sludge and it can be either the theoretical(Ytbms)or the practical(Ypbms)biomethane potential,billion m3;YCODis the total COD emission from sewage sludge,ton COD;MCF is related to the sewage sludge treatment situation and equal to 0.8 when sewage sludge was treated in an anaerobic reactor[29].

3.Results and Discussions

3.1.Theoretical and practical biochemical methane potentials

Based on Buswell formula,the TBMP and PBMP of different biomass was calculated,and the calculation results of the biomass other than municipal solid waste and sewage sludge are listed in Table 1 and illustrated Fig.1.In calculation,the data was taken from Phyllis2(biomass database designed by energy research center of the Netherlands)[30]and the literature[28,31,34].Meanwhile,the data of livestock manure was taken from the literature[28,32,33].

Based on the calculation,it can be seen that the horse manure had the maximum TBMP.This is mainly due to its high carbon content and low oxygen content.However,the PBMP of horse manure was low because of the high lignin content.In contrast,the sugar beet top was the most promising biomass with a maximum PBMP value of 0.3338(L CH4·(g VS)−1.This is because the main composition in sugar beet top is saccharide and the lignin content is very low.

Table 1 TBMP and PBMP of different biomass

3.2.Biomethane potential of agricultural waste

Both crop residue and livestock manure can be classified as the agricultural waste.The biomethane potentials of main crop residue and livestock manure in China were listed in Tables2 and 3.Forthe crop residue,the total collection quantity in 2010 was 6.484×108ton,and the estimated theoretical biomethane potential of crop residue was 237.67 billion m3.Among the studied crop residue,the corn stalk represented the biggest contribution with a proportion of 28.02%,and it was followed by the rice straw(21.67%)and wheatstraw(16.74%).The total practical biomethane potential was 110.82 billion m3,where the corn stalk shared the biggest contribution with a proportion of 30.88%and it was followed by the rice straw(28.77%)and wheat straw(19.28%).Furthermore,the proportions of corn stalk,rice straw and wheat straw in practical biomethane potential were higher than those in theoretical biomethane potential because of their low lignin contents.The average biodegradability of crop residue in China was estimated to be 46.63%

Fig.1.Histogram of TBMP and PBMP for different biomass.

Table 2 Biomethane potential of crop residues in China

For livestock manure,the total collection quantity in 2010 was 1.195×109ton.The average biodegradability of livestock manure in China was estimated to be 31.53%.The estimated theoretical biomethane potential was 147.36 billion m3where the cattle manure shared the biggestcontribution with a proportion of57.70%and was followed by the pig manure(29.52%).The estimated practical biomethane potential for livestock manure was 46.46 billion m3where the pig manure was the biggest contribution with a proportion of64.33%while the cattle manure was the second biggest contribution(18.49%).This is mainly because of the high lignin content in the cattle manure.

3.3.Biomethane potential of energy crops planted on marginal lands

Considering the actual situation in China,we assumed that the energy crops would be planted on the marginal lands to increase the biomethane production.During the calculation,the areas of marginal lands and the selection of energy crops are two important factors.

Marginal lands refer to the unexploited lands which are unsuitable for food butcan be used for energy plants[43].Kou etal.[44]considered five kinds of marginal lands that are suitable for energy crops in China and calculated the total area of the marginal lands with a value of 26.80 million hm2.Furthermore,Shi[45]thought that the barren hill and sandy land can also be used to plant energy crops,and their total area is 57.04 million hm2.However,they only used the land slope as the key factor to select the availabilities ofmarginal land,and the classification was unclear.Recently,Jiang et al.[46]studied the change of the marginal lands that are suitable for energy crops during 1990–2010 in China based on the GIS technology,and in their research,limiting factor(land slope,temperature,moisture,soil character,energy plant characteristics,policy limiting factor and land use data in China)were used to screen the appropriate marginal lands for energy crops planting.They reported the spatial–temporaldistribution and area ofdifferentmarginal land terrains suitable for energy crops in China.In this work,the data reported by Jiang was used to estimate the biomethane potential of energy crops in the marginallands.The spatialdistribution ofthe marginal lands was depicted in Fig.2.It should be mentioned that data of the shrub land as well as dense and moderate-dense grasslands was not used because the exploitation of these marginal lands can't cause ecoenvironmental problem[47].The areas of different marginal land terrains suitable for planting energy crops in China are listed in Table 4.

Because the marginal land suitable for energy crops in Jiang's work indicated that this land could meet the minimum limits of necessary constraints for energy plant growth,so for marginal land,the appropriate growing land type of different energy crops became the main marginal land limiting factor.In terms of energy crop,the kind of energy crops planted on marginal land is another key factor that should to be considered for the biomethane potential calculation.In this work,six kinds of promising energy crops(switch grass,miscanthus,sweet sorghum,alfalfa,napier grass,giant reed)and three limiting factors(Yde,TBMP,types of marginal lands)were considered in selecting suitable energy crops to achieve the largest biomethane potential.The theoretical and practical biomethane potentials of different energy crops were calculated based on Eq.(7),and the calculated results are listed in Table 5.

Itcan be seen thatsweetsorghumand napiergrass were screened to be the two most promising energy crops planted on the marginal lands,due to the maximum theoretical biomethane potential on their appropriate growing land type.Then the practical biomethane potential was calculated based on the two energy crop selected.Sweet sorghum was suitable to plant on the alkaline and bare lands,and in China the corresponding available area could be 4.213 million hm2.Meanwhile,the shoal/bottom land as well as the sparse-forest and sparse-grass lands were suitable for planting napier grass,and the total available areawas 41.432 million hm2.Based on this,the theoretical and practical biomethane potentials of energy crops on the marginal lands in China were 473.65 and 145.72 billion m3,respectively.

Table 3 Biomethane potential of livestock manure in China

Fig.2.Spatial distribution of the marginal lands suitable for energy crops in China in 2010[46].

Table 4 Areas of differentmarginal land terrains suitable for energy crops in China in 2010[46,47]

3.4.Biomethane potential of municipal solid wastes

The theoretical and practical biomethane potentials of municipal solid waste in China were calculated by Eqs.(9)and(10).The calculation results were listed in Table 6.The total organic carbon(TOC)and the biodegradable organic carbon(DOC)were used for the calculation of the theoretical and practical biomethane potentials,respectively.Because very few reports including the TOC content of nation-wide municipal solid wastes,we used the average organic material(OM)content and the empirical constant(TOC/OM)to calculate the TOC content.MCF was set to be 1 for municipal solid waste due to the complete anaerobic condition of biomethane production.The general methane content(F)in biogas was set to be 0.5.Therefore,the theoretical and practical biomethane potentials of municipal solid wastes in China were estimated to be 20.22 and 9.58 billion m3,respectively,and their average biodegradability was 47.38%.

3.5.Biomethane potential of sewage sludge

The biomethane potentialofsewage sludge(industrialand domestic sewage sludge)was calculated based on Eqs.(12)and(13).The calculation results are listed in Table 7.MCF from IPCC was set to be 0.8 for sewage sludge because of the complete anaerobic condition of biomethane production.Btoand Bpowere used for calculating the theoretical and practical biomethane potential,respectively.The theoretical and practical biomethane potentials of sewage in China were estimated to be 9.88 and 3.72 billion m3,respectively,and their average biodegradability was 37.65%.Compared with the domestic sewage sludge,industrial sewage sludge have higher practical biomethane potential,which is due to the high organic matter content in industrial sewage sludge.

Table 5 Biomethane potential of promoting energy crops planted on marginal lands in China

Table 6 Biomethane potential of municipal solid wastes in China

Table 7 Biomethane potential of sewage sludge in China

3.6.Biomethane distribution and development prospect in China

Based on the above calculation,the biomethane potential distribution of different kinds of biomass in China was indicated in Table 8.It can be seen that the energy crop shared the largest biomethane potential with a theoretical proportion of 53.29%and practical proportion(46.07%)respectively,this is probably due to its high biomass production and large potential planting area.And it showed that planting energy crop can be the most promising way to enhance biomethane production.For the agricultural wastes including crop residue and livestock manure,their total biomethane potential was slightly lower than that of energy crops.However,the huge quantity of agricultural wastes cause serious environmental pollution each year,converting the agricultural wastes into biomethane,in addition to get large number of bioenergy,but also reduce pollution.Agricultural waste should be the preferential development biomass for biomethane production in China.Municipal solid waste and sewage sludge only had a small contribution to the total biomethane potential,which is probably due to the high inorganic matter content in it.Finally,the total theoretical and practical biomethane potentials in China were estimated to be 888.78 and 316.30 billion m3respectively,and the general ef ficiency of the current anaerobic digestion process was calculated to be 35.59%.

The biomethane potential was compared with the latest Chinese natural gas consumption in Table 9,which shows that natural gas consumption only accounts for 48.15%of the practical biomethane potential and 17.14%of the theoretical biomethane potential in China.Considering the huge amount of CO2emissions from natural gas and other fossil fuels in China,as well as the clean and renewable bene fit from biomethane,it will be very wise to develop biomethane production rapidly in China.And according to the Chinese energy development strategy,in 2020,400 billion m3natural gas will be needed in China[3].Moreover,the total practical biomethane potential in China is slightly lower than the demand of natural gas set for 2020,but the maximum biomethane potential is more than twice of the natural gas demand.Therefore,with the developmentofanaerobic digestion process,the demand of natural gas set for in 2020 can be met.Developing biomethane is a promising way for energy supplement.

4.Conclusions

During the calculation of energy crop biomethane potential,sweet sorghum and napier grass were selected to be the two most promising energy crops planted on marginal land for biomethane production in China from the screening process designed.And five kinds of marginal land terrains were chosen for the energy crop planting.The theoretical and practical biomethane potential of energy crops on the marginal lands in China were 473.65 and 145.72 billion m3,respectively.And planting energy crops on marginal lands can be the most promising way to enhance biomethane production in China.

The total theoretical and practical biomethane potential of China were further estimated to be 888.78 and 316.30 billion m3,and the general efficiency of the current anaerobic fermentation process was calculated to be 35.59%.Energy crop shared both the largest theoretical and practical biomethane potentials,and sweet sorghum and napiergrass were screened to be the two most promising energy crop with a marginal land planting area 4.213 and 41.432 million hm2respectively.Compared with energy crop,agricultural wastes have a slightly lower biomethane potential with theoretical proportion(43.32%)and practical proportion(49.73%)respectively.Municipal solid waste and sewage sludge only had a minor contribution to the total biomethane potential.

Table 8 The distribution of theoretical and practical biomethane potentials in China

Table 9 Biomethane potential compared with natural gas consumption in China

When compared with other fossil energy,Chinese natural gas consumption in 2013 only accounts for 48.15%of the practical biomethane potential and 17.14%of the theoretical biomethane potential.And the demand for natural gas in 2020 could be met with the development of anaerobic digestion technology,developing biomethane is a promising way for energy supplement in China.

Nomenclature

BD biodegradability,%

BMP biochemical methane potential,L CH4·(g VS)−1

Bpopractical methane production capacity of sewage sludge,kg CH4·(kg COD)−1

Btotheoretical methane production capacity of sewage sludge,

kg CH4·(kg COD)−1

CR collection rate

CLigninlignin content,wt%VS

DOC biodegradable organic carbon content in municipal solid waste,wt%,AR

F general methane content in biogas

M coefficient of carbon transfer into CH4

MCF methane correction factor

OM average organic material content of municipal solid waste,wt%,dry

OX methane oxidation factor

P livestock excretion coefficient,kg·capita−1·d−1

PBMP practical biochemical methane potential,L CH4·(g VS)−1

R the amount of methane recovered,billion m3

S area of each marginal land,million hm2

SGR straw grain ratio

T livestock raising cycle,d

TBMP theoretical biochemical methane potential,L CH4·(g VS)−1

TOC totalorganic carbon contentin municipalsolid waste,wt%,AR

VS volatile solid,wt%,AR,dry

Vmthe molar volume of methane at standard temperature and

pressure,m3·mol−1

Ybmcbiomethane potential of crop residue,billion m3

Ybmebiomethane potential of energy crop,billion m3

Ybmlbiomethane potential of livestock,billion m3

Ybmmbiomethane potential of municipal solid wastes,billion m3

Ybmsbiomethane potential of sewage sludge,billion m3

Ycthe yield of crop,104t

YCODthe total chemical oxygen demand emission from sewage sludge,t

Ydegeneralyield ofenergy crop in dry-basis on the marginalland,t·hm−2·a−1

Ylthe yield of livestock,104capita

YMSWthe amount of municipal solid waste,104t

Ypbmcpractical biomethane potential of crop residue,billion m3

Ypbmepractical biomethane potential of energy crop,billion m3

Ypbmlpractical biomethane potential of livestock,billion m3

Ypbmmpractical biomethane potential of municipal solid wastes,

billion m3

Ypbmspractical biomethane potential of sewage sludge,billion m3

Ytbmctheoretical biomethane potential of crop residue,billion m3

Ytbmetheoretical biomethane potential of energy crop,billion m3

Ytbmltheoretical biomethane potential of livestock,billion m3

Ytbmmtheoretical biomethane potential of municipal solid wastes,billion m3

Ytbmstheoretical biomethane potential of sewage sludge,billion m3

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