Renewable and Sustainable Energy Reviews 54(2016)857-865 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews ELSEVIER journal homepage:www.elsevier.com/locate/rser A review of methane production from agricultural residues in China CrossMark Kun Lia.b,Ronghou Liu .b.*,Chen Sun 3.b Biomass Energy Engineering Research Centre.School of Agriculture and Biology.Shanghai Jiao Tong University.800 Dongchuan Road.Shanghai 200240. PR China Key Laboratory of Urban Agriculture(South).Ministry of Agriculture,800 Dongchuan Road,Shanghai 200240.PR China ARTICLE INFO ABSTRACT Article history: Anaerobic digestion is an effective technology for resources recycling.The application of anaerobic Received 6 Octoher 2014 digestion has been a hotspot due to its capability of converting solid organie waste into methane.The Received in revised form metabolism of acetoclastic methanogens,anaerobic digestion features and strategies of three main 19 August 2015 Accepted 22 October 2015 agricultural residues and current situation of large,medium biogas plant built in China are summarized. Available online 11 November 2015 hoping to promote the application of this technology to deal with agricultural residues.Also,the current problems are presented and future research and development of biogas technology are proposed. Keywords: 2015 Elsevier Ltd.All rights reserved. China Anaerobic digestion Biogas Agricultural residues Contents 1. 857 2. Methanogens in anaerobic digestion....................................................................................858 3. The adaptability of substrate materials for methanogenic fermentation 858 4. Methanogenic fermentation of livestock manures.……… 859 4.1. Anaerobic digestion characteristics of livestock manures,...........,..++ 859 4.2 Development of biogas plant for manure treatment in China 859 5. Methanogenic fermentation of fruit and vegetable waste................................................................... 860 5.1. Anaerobic digestion characteristics of fruit and vegetable waste........................................................ 860 5.2. Development of biogas plant for fruit and vegetable waste treatment in China ........................................... 860 6. Methanogenic fermentation of lignocellulosic biomass...,,。。。… 860 6.1. Pretreatment of crop residue for enhanced methane yield............................................................. 860 62. Development of biogas plant for crop straw treatment in China ....................................................... 861 7. Biogas technology application and policies in China........................................................................ 861 8. 862 8.1. Conclusion... 862 8.2. Recommendations for future work.........................................................................862 Acknowledgments. 862 References.,,· 862 1.Introduction China has abundant biomass resource which refers to residues and by-products of agriculture,forestry and other related indus- .Corresponding author at:Biomass Energy Engineering Research Centre,School of Agriculture and Biology,Shanghai Jiao Tong University.800 Dongchuan Road. tries,as well as the biodegradable fraction of industrial and Shanghai 200240,PR China.Tel.:+86 21 34205744. municipal waste 1,2.21%of total biomass resource was occupied E-mail address:liurhou@sjtu.edu.cn (R.Liu). by agricultural residues which are associated with manure of http://dx.doi.org/10.1016/j.rser.2015.10.103 1364-0321/2015 Elsevier Ltd.All rights reserved
858 K.Li et al.Renewable and Sustainable Energy Reviews 54(2016)857-865 livestock,fruits and vegetable wastes produced during store, are the same with the ones of Methanosaeta except enzymes in the transportation and handling of vegetables,and lignocellulosic process of acetyl-CoA synthese.As a result,Methanosaeta has biomass such as rice straw,wheat straw,corn Stover,etc.[3-6]. lower minimum threshold acetate concentration than Methano- They are biodegradable,rich in organic matter and can be used for sarcina.While Methanosarcina species grow faster than Methano- methane production by anaerobic digestion 7.In 2012,846 saeta species when acetate concentrations are beyond 1 mM[19]. million tons crops residues and 3.21 billion tons livestock manure The methanogens population are not only affected by acetate were produced in China.If these wastes are utilized for anaerobic concentration,but also by temperature 20]or substrate 21. fermentation,4.23x 101 m3 biogas could be produced ]How- When temperature is increased within a certain range,methano- ever,these resources have not been used efficiently.72.5%of crop gens can grow and multiply.Generally.mesophilic anaerobic straw is combusted,lost or discarded,left only 0.5%for biogas digestion is more stable than thermophilic fermentation for lower production 9].Besides,the uncontrolled decomposition of animal ratio of free ammonia to total ammonium ion,as well as greater manures have caused serious environmental problems.Obviously. diversity and evenness of bacterial communities [22].Methano- technologies for biomass utilization are urgently needed in China sarcinales is a common major methanogenic order in mesophilic either for energy or for environmental purpose. conditions 23.but in thermophilic reactor hydrgenotrophic Biomass can be converted to gaseous.liquid and solid biofuels methanogens dominated [241.The order of Methanosarcinales only by technologies like anaerobic digestion and gasification,pyrolysis occupied a percentage of 10%in methanogenic population at and carbonization [10,11.Compared with other bioenergy tech- temperatures of 55C,in which Methanosaeta was almost absent nologies,biogas technology is quite mature and has already been Additionally,reducing digester feeding frequency can increase at its industrialization stage.Furthermore,biogas project combines Methanosarcina predominance,and Methanosarcina-enriched together the ideas of ecological agriculture and recycle agriculture. reactor has better performance than Methanosaeta-enriched and develops several practical agricultural production patterns like reactor [25.In the industry application,Methanosaeta is important the "Pig-Biogas-Fruit"model in south China and the "Four in One for the upflow anaerobic sludge blanket (UASB)reactor,due to its model in north China [12,13].Extensive fundamental researches enhancing granulation [26]and attachment ability [27].Hydro- about biogas have also been conducted.For example,microbial genotrophic methanogens can tolerate the ammonium con- community shifts during thermophilic methane fermentation centration of 6000 mg/L which is sixfold higher than the threshold fermentation characteristics of crop straw pretreated by microbial ammonium concentration of Methanosaeta and twofold higher community or steam explosion [14J,and trace elements'stabiliz- than that of Methanosarcina (24,28.Therefore,the stable methane ing effects on long-term anaerobic digestion have been studied production when ammonia level increases may be explained by [15.It has also been found that digested liquid swine manure can the increasing activity of hydrogen-utilizing methanogens. control soybean cyst nematode 16,and biogas fertilizer can Overall,several species of methanogens participate in the improve the yield of cherry tomato 17. terminal steps of methane generation.They are high susceptible to However,understanding the process of methane fermentation the environmental conditions.As a result,anaerobic digestion is difficult because of various influence factors.The broad appli- process is fragile and highly sensitive to external influences.The cation of biogas technology is greatly limited by the disconnection prevalence of certain members of archaeal methanogens depends of practical operation from its mature research status.So,in order on physical and chemical conditions like temperature,pH,NH3-N to improve the practical efficiency of anaerobic digestion,to grasp and VFA etc.The shifting of this anaerobic microbe community the current study of anaerobic digestion for biogas production in might influence the biogas yield as well as the stability of China is necessary. biomethanation. This article reviews the metabolism of methanogens and the present situation of biogas technology dealing with three main agricultural residues including livestock manure.fruit and vege- 3.The adaptability of substrate materials for methanogenic table waste (FVW)and lignocellulosic biomass in China.The cur- fermentation rent status of large,medium biogas plant built in China are also summarized.In addition,problems and recommendations for The feedstock of anaerobic digestion is widely tolerant,but if future development are put forward based on current situation of the maximum biogas yield is expected to obtain,several factors biogas technology,with the hope of improving anaerobic digestion should be concerned,including C/N ratio,substrate concentration, efficiency and promoting the application of biogas technology physical and chemical property of feedstock. in China. In terms of C/N ratio,low C/N ratio may Icad to ammonia releasing [29].while methanogenesis inhibition also occurs when the ratio is high either due to nitrogen shortage or to organic acid 2.Methanogens in anaerobic digestion accumulation caused by excess carbon 30,31.As a result,various feedstock with various C/N should be matched properly to produce Methanogens are strictly anaerobic archaea and have limited a proper ratio range,generally proposed 20/1 to 30/1 [32,33. substrate range.Thus the bioconversion of organic waste to Substrate concentration refers to the proportion of total solids methane generally needs the consortia of interacting micro- accounting for the reactor medium [34].Proper solid content is organisms including fermentative bacteria,H2-producing aceto- important for biogas production.Too much water in the fluid can genic bacteria and methanogens.and the final generation of cause low organic loading rate and too little can lead to organic methane by methanogens is the rate-limiting step. matter accumulation resulting in fermentation inhibition,both of Currently,two acetate-utilizing methanogens,Methanosarcina which will decrease biogas yield.The substrate concentration and Methanosaeta,are most explored among methanogens,partly capable of producing biogas is quite wide.TS content ranging from because 70%methane in nature derives from methyl group of 1%to 40%[35].even to 55%is feasible [36].In rural area of China. acetate 18.Methanosarcina have one times larger genomes than substrate concentration of 6-12%is generally adopted during obligate hydrogenotrophic methanogens,which reflects powerful anaerobic digestion [37]. metabolic capabilities.For example,it has the broadest substrate Physically,anaerobic digestion can be divided into liquid anae spectrum,acetate,methylated Ci compounds and H2/CO2.The key robic digestion (L-AD)and solid-state anaerobic digestion (SS-AD). enzymes for methane formation from acetate in Methanosarcina with solid concentrations between 0.5%and 15%,and higher than
K.Li et al.Renewable and Sustainable Energy Reviews 54(2016)857-865 859 15%,respectively [31.L-AD is a widely used technology to produce Spartina alterniflora.Similarly,co-digestion of goat manure with biogas and most commercial digesters in China are L-AD systems three crop residues,wheat straw,corn stalks and rice straw,obtain which can process municipal organic waste for energy production. 62.1%,83.02%and 111.28%higher biogas yield than corresponding SS-AD is hindered from wide application due to its low operational single crop residues,and 23.04%,54.44%and 51.31%higher than stability,although it is superior to L-AD for less heating energy,less single goat manure [61.Moreover,the co-digestion of dairy and leachate,higher volumetric loading capacity and less floating pro- chicken manure mixture with corn stalk was compared with single blems [35,38.Moreover,the particle size of straw or fruit vege- manure co-digestion with corn stalks,the former got better results table waste(FVW)like substrate also have certain requirements.For under both temperature of 15C and 35C [62].The apparent example,corn stover had better be cut into range of 0.25-1.0 mm synergistic effect in co-digestion can be attributed to a better buffer [39.When particle size is bigger than 1 mm,the cumulative yield capacity,balance nutrients,as well as less risk for the occurrence of of hydrogen,acetate and butyrate is decreased [40. ammonia/ammonium inhibition [63.Trace elements in manure When it comes to the chemical feature,it is generally required may be another reason [64].Yue et al.also considered that micro to be easily available for microbe.Straw,mainly composed of lig- bial community composition shifted with different feed composi- nin,cellulose and hemicelluloses,is difficult to be broken down. tion which has an effect on the hydrolysis of fiber [65]. Therefore.for the purpose of improving degradation efficiency. Still.the species.growth stages of animal as well as the fraction preprocessing straw firstly is necessary because straw composi- of manure can exert direct effects on methane production from tions can be partly degraded into easily utilized organics in this livestock manures [66].For example.the solid fraction of pig process. manure contains higher organic matter content than liquid frac- Clearly,different substrates have different features such as tion and produces more biogas than raw pig manure [36];the nitrogen-rich or carbon-rich which directly influence anaerobic manure excreted by gestating sow,post-weaned piglet and digestion.Thus,the optimal process methods for different feed- growing fattening pig have different components.This may be stock should be explored based on the characteristics of each kind caused by their different nutrient metabolic capacity and feed of biomass materials. formulas,additive at different growth stages (67]. In conclusion,manures characterized by rich in nitrogen likely lead to ammonia accumulation in reactor.Simultaneously treating 4.Methanogenic fermentation of livestock manures manures with other nutrient complementary feedstocks can improve the anaerobic process yields.Additionally,higher 4.1.Anaerobic digestion characteristics of livestock manures methane yield can also be obtained by manure pretreatment for accelerating hydrolysis. Animal manure is a kind of low C/N ratio feedstock and it is characterized by high organic matter and high moisture [41.as 4.2.Development of biogas plant for manure treatment in China well as fast conversion rate (42,43.Manure ferments well and can be used as mono-substrate if proper temperature,substrate con- In the past years,the promotion of household biogas project centration as well as inoculum [44-46]are given.For example. fed with manure was strategically coupled with the idea of ecology human excreta and chicken manure with a C/N ratio of 2.88:1 47] in China.During the development of rural household biogas, and 8.84:1 [48]respectively can digested alone [49]. several ecological models have been evolved.Two successful As sole substrate for methane fermentation,fresh pig manure models among them are the "Pig-Biogas-Fruit"model in south can rapidly generate biogas without adjusting C/N ratio 50.The China and"Four in One"model in north China.In "Pig-Biogas- CH4 content of biogas from pig manure anaerobic digestion is Fruit"model,pigpen and fruit trees are combined with biogas quite high,typically in the rang of 70-80%51.yielding biogas digester through manure and biogas residue 68.The "Four in 495 ml/g VS.while the CH4 content of biogas from cattle manure One"model is popular in cold area for it solves the problem of has a slightly lower CH4 content,in the rang of 55-75%,potentially biogas digester that can not operate under cold weather.In this giving 398 mL /g VS.Methane yield of 324 mL/g VS for chicken model,pigpen,toilet and a biogas digester are built in a green- manure and 294 mL/g VS for human feces were obtained [52]. house which can provide suitable temperature for the growth of Rabbit manure was degraded faster than duck and sheep manure, pig,vegetable and the production of biogas.In return,burning for 87.8%,67.6%and 65.0%of total biogas yield could respectively biogas increases the temperature of greenhouse and provide CO be obtained after 20 days of digestion at 35+1C [53].Great and light for vegetable.Meanwhile,biogas digestate can be used as differenccs of biogas yicld and biogas production ratc among organic fertilizer for vegctablc production [69]. various livestock manures can be explained by the different con- The biogas-plant industry has grown quickly in China.By 2010 tent of protein,carbohydrate,fat and lignin in substrate.Manures more than 40 million household biogas project have been built. with higher liquid content have high methane potential due to the and there were 27,410 medium-and large-scaled biogas plants highest energy density in fat.but longest retention time is dealing with manure [70.The fermentation material is mainly required for the poor bioavailability of fat.Conversely,carbohy- composed of swine manure,cattle manure and chicken manure drates and proteins can degraded rapidly 42,541. 71.In reality,the anaerobic digestion of single manure has low Currently,the study of manure digestion in China mainly con- performance caused by ammonia inhibition and nutrient imbal- cerned with the co-digestion of animal manure with other biosolid ance 72.In some biogas plants which were built in well devel- wastes.The advantages of co-digestion are summarized as: oped country like German,livestock manure was seldom used as adjusting the C/N ratio of low nitrogen material with manure to single substrate but mixed with organic residues 73.Presently. generate biogas stably:increasing methane yield and proportion in the development of biogas plants are far from maturity and still biogas [55].According to comparative studies in China.when low exist deficiencies in respects of electricity production [74].hea nitrogen agricultural residues like dried grass silage [56].maize preservation in winter [75].etc.However,some biogas plants have straw 57].vegetable stalks [58]were digested for methane pro- obtained great achievements.For example,a large-scale biogas duction.it is feasible to improve feedstock adaptability by adjusting plant located in Tongzhou District of Beijing treats 30t dairy C/N ratio with high nitrogen substrates like pig manure and urea manure and 30 t wastewater per day,and particularly adopts 59].For example,Chen et al.[60]reported that 7.09-44.26%higher combined heat and power(CHP)systems which has an installed methane yield was obtained by co-digestion of cow feces and electricity capacity of 160 kw.Waste heat from generator keeps
860 K.Li et al.Renewable and Sustainable Energy Reviews 54(2016)857-865 feed in the reactor fermented at 38C 76.Moreover,a solar- straw [57].The theoretical methane yield of crops straw is greatly powered water-heating system is also proved feasible for tem- higher than its practical methane yield.Previous studies in China perature keeping (77].Another case is concerned the Minghe showed that methane yield of cotton stalk was about 240 mL/g-VS Animal Husbandry Company's biogas plant with continuous stir- while its theoretical yield was 356.7 mL/g-VS [92].Similarly,the red tank reactor(CSTR)and CHP technology which disposes 500 t practical and theoretical yield of wheat straw were 297 mL/g-VS chicken manure and daily generates electricity 60,000 kW h [78]. and 426 mL/g-VS.respectively [93].This may be largely due to the The biogas power-generation project of the Hainan Luoniushan Pig fact that crop straw contains high content of lignocelluloses Farm generates 420,000 kW h per year,with an annual biogas compounds and tight structure between cellulose,hemicellulose output of 600,000 m79.This high efficiency of electricity gen- and lignin,which makes it difficult to degrade and results in low eration may mainly results from its biogas plus electricity pro- methane yield.Pretreatment of lignocellulosic materials can duction system. decompose hemicellulose and cellulose,and break down the linkage between polysaccharides and lignin,thus improving digestibility 94.Therefore,pretreatment is normally a necessary 5.Methanogenic fermentation of fruit and vegetable waste step in the anaerobic digestion processes for straw. 5.1.Anaerobic digestion characteristics of fruit and vegetable waste 6.1.Pretreatment of crop residue for enhanced methane yield The C/N ratios of majority FVW are ranged from 20:1 to 35:1 At present,the pretreatment methods are generally classified [6,80-82]and then FVW can be digested without additiona into chemical,physical,biological pretreatment methods or their nitrogen source [83].For those FVW with unsuitable C/N ratio. combination (95.Among them,pretreatment with alkali,steam mixing two or more sorts of them together to improve nutrients explosion and microbial agents are the most broadly implemented balance is a practical method. methods to treat lignocellulosic materials like straw in China FVW contains 75%easy biodegradable organic matter and are Each method is surely efective despite their limitations.But low in nitrogen concentration 6.When digested alone,redun- chemicals especially alkaline are superior to fungus96,bacter- dant volatile fatty acids will be produced in the process [84].But ium agents 97.enzyme 98.Alkaline can lead to the saponifi- when the FVW is co-digested with swine feces and urine which cation of uronic bonds between hemicelluloses and lignin,facil- contain high level of nitrogen,the amount of inhibit acids can be itating biological hydrolytic breakdowns 99.Study shows that decreased (85. rice straw pretreated by 4-10%NaOH can obtain 3.2-58.1%more In fact,the VFA from vegetable wastes acidification can loose biogas yield than untreated rice straw,corresponding to the lignocellulose structure in flower stems and helps its degradation reduction of 8.0-44.5%,35.2-54.2%and 14.2-16.4%in total lignin. 86.Also,adding corn straw to algae with a C/N ratio of 20/1 can hemicellulose,and cellulose contents,respectively 100.Hydro- obviously enhance the methane yield of algae from 201 mL/g VS to gen peroxide is also an excellent choice for pretreatment,since it 325mL/gVs[871. has strong oxidizability and hardly leaves residues as secondary For continuous stirred-tank test,the best ratio for the co- products in biomass.Improved biodegradability of lignin,cellu- digestion of fruit and vegetable waste with food waste was 1:1. lose,and hemicellulose has been reported when rice straw treated At this ratio and organic loading rate of 3 kg VS/(m.day).the with hydrogen peroxide,and the optimal conditions result in methane yield was 0.49 m2 CH4/kg VS compared with 0.3 m3 CH4 88.0%higher methane yield(290 mLg-1 VS)than untreated rice kg VS of FVW as single substrate.In addition,increasing the pro- straw 101.Approximately,biogas yield of 327.5 and 319.7 mL g- portion of FVW can lead to a higher degree of acidification in two- VS could be obtained from 3%and 4%H2O2 pretreated rice straw. phase AD system88.It seems that FVW can be well co-digested respectively.And pretreatment with H2O2 got higher performance with substrates rich in nitrogen.For carbohydrate-rich wastes. than pretreatment using ammonium hydroxide [102. alkali addition is another suitable measures in addition to mixing Physical pretreatment means using machine or heat etc.to substrates to maintain stable digestion. reduce particle size and crystallinity of fiber,thus increases the available surface for enzymatic attack [103,104].This method is 5.2.Development of biogas plant for fruit and vegetable waste energy consuming but quite efficient.Under high temperature, treatment in China high pressure and saturated steam conditions,steam explosion pretreatment can break down the links between amorphous cel- 130 million tons FVW arc annually produccd in China.Unfor- lulosc and crystallinc ccllulosc by abruptly releasing pressurc tunately,these wastes were usually discarded [88,89].Due to the [105].Guo et al.reported that the highest amount of organic acid high moisture and biodegradability of FVW.anaerobic digestion is (2.99 g/l)was obtained from corn stalk pretreated by steam more suitable for treating FVW in industrial scale rather than explosion after 3 days fermentation at 50C,and the amount of combustions or landfill deposition.Limited by shortages,biogas 2.74.1.96.1.45.and 2.21 g/l organic acid were respectively plants seldom use FVW as single substrate,but mix FVW with obtained from other pretreatments,including sodium hydroxide. manure,food waste and straw for co-digestion.In this way,not sulfuric acid,acetic acid,and aqueous ammonia 106. only nutritional balance can be maintained in large-scale biogas Biological methods use lignocellulose-degrading microorgan- plant,but also reduce the effect of acidification caused by high isms or enzymes to decompose substrate into simpler and more sugar content in FVW [90].In these biogas technologies,CSTR are biodegradable molecules like sugar,starch and pectin.Thus widely applied in China [70].In regard of FVW's easily acidifica- shorter digestion time,improved total solid reduction and biogas tion.anaerobic digesters for FVW had better adopt two-phase conversion rate could be achieved during this process 107,108]. digestion technology [91]. The key step of this technology is to look for the high lignocellulose-degrading strains.For example,Zhong et al.treated corn straw with 0.01%dose of a new complex microbial agents 6.Methanogenic fermentation of lignocellulosic biomass resulting in 75.57%increased CHa yield and 34.6%digestion time reduction for 15 days at 20C 109. The biogas yield of wheat straw is the highest among agri Additionally,there are also some other effective pretreatment cultural residue straws,followed by rice,corn,peanut and soybean methods.For example,biogas yield of rice straw pretreated by
K.Li et al.Renewable and Sustainable Energy Reviews 54(2016)857-865 861 ionic liquid at 140C could increase by 137.8%110];biogas yield of Household-scale biogas digesters can be used by scattered cassava residues pretreated by thermal-dilute sulfuric acid could farmers.In 2007.0.9 billion people were lived in rural area of increase by 56.96%111]:biogas yield of corn stalk stacked with China [1221.To meet energy demand of rural and to improve cow dung could increase by 19.6%[112]. household hygiene,household-scale biogas digesters were built In brief,hydrolysis is an important step in methane production throughout the country.Currently,China has the most household and is assumed as rate-limiting step especially for materials like biogas plants in the world.And based on the concept of devel- straw.To this end,the process of methane production can be oping eco-cycle-agriculture,Chinese household-scale biogas enhanced by favoring the hydrolysis of substrate.Generally. digesters integrated livestock breeding.biogas and vegetable treating lignocellulosic materials with physical,chemical and planting which is well-known in the world.However,these biological methods prior to anaerobic fermentation can accelerate household biogas digesters were not work well.Only 60%were hydrolysis rate and increase biogas yield. operated normally in 2007 [123.This fact largely decreased the efficiency of biogas utilization.The reason accounting for this is 6.2.Development of biogas plant for crop straw treatment in China poor maintenance and shortage of technical support for the biogas digester. Currently.the biogas plants for straw are in the initial phase of For the maintenance of biogas digesters.USA has good development in China.At the end of 2010,273 straw biogas plants experiences.The Natural Resources Conservation Service(NRCS)of including 47 medium-and large-scale biogas plants had been built U.S.Department of Agriculture (USDA)provides technical assis- in China [113].The straw biogas plants have many technological tance accompanied with financial assistance to farmers and ran- difficulties due to the character of straw.In response to these chers.Considering that Chinese farmers generally have little problems,three kinds of anaerobic digesters are highly proposed. knowledge and training about biogas operation,such kind of namely CSTR,membrane covered trough(MCT)and vertical plug technical assistance for the long-term running of biogas digesters flow anaerobic reactor (VPF)[114].The Green Harbor straw biogas is very necessary. plant located in Chongming,Shanghal,China employs CSTR tech- In addition,blogas electricity generation is an emclent way to nique as well as biogas-heat-power-fertilizer cogeneration mode use biogas.However,only 2.53%biogas was used to generate and annually generates electricity 584,000 kW [115J.Biogas plant electricity in China.far from the 98.5%percentage of German applied MCT technique,like Daxing plant in Beijing,has a high [124].In 2012.Germany was the leading EU country regarding energy efficiency [114J.Another plant with VPF reactor in Xinjin of electricity generation from biogas,with 19,426 GW h generated Sichuan processes 1.3 t of straw each day and produces 400- from modern 7000 operating biogas plants [125.The technology 600 m biogas [116J. of combined generation of heat and power (CHP),employing However,there still exist problems of pretreatment.For internal combustion engines coupled with electricity generators,is example,the large amount of chemical solution used to soak the most common way for biogas utilization in Germany.The substrates has potential environmental pollution and seriously installed electric capacity has three level:150 kWel.500 kWel,and corrodes equipment [117,118].While biological method requires 1 MWel [12G].The waste heat generated by CHP plants was strict reaction environment and its treatment cost becomes very managed in four ways:no heat usage,local heat network,mobile expensive if a high biodegradability rate is going to be obtained heat storage and Organic Ranking Cycle(ORC)process.The waste 119.These problems limited large-scale application of present heat can be converted into electricity by ORC process.On the other pretreatment methods.As pretreatment is a key technology in hand,the financial policies in German have been proved to be straw biogas digestion,therefore,more research work should successful in promoting biogas electricity generation.The Feed-in focus on straw pretreatment. tariffs guaranteed fixed prices for 20 years [127].Moreover,addi- tional premiums for use of heat further promote the energy con- version efficiency of biogas. 7.Biogas technology application and policies in China Actually.China has almost reached the advanced level of Ger- man in respect of technology and equipment for biogas electricity The Chinese government has paid a great attention to the generation.The electricity conversion rate of Chinese electricity development of a sustainable biogas industry in recent 20 years. generator was only 7%lower than the same type German gen- From 2003 to the end of 2014,36.4 billion Yuan was invested by erator.But the financial policies are imperfect and the private central government to support development of biogas industry.By investors have not realized the potential of biogas systems.While the cnd of 2014,there were 41.50 million houschold biogas the China's financial subsidics for biogas technology mainly sup- digesters that annually produced 13.67 billion m3 biogas.The port the construction of biogas plants,and then the lower financia number of various biogas plants reached 99,957 in which large- return from biogas products dampened investors'enthusiasm and scale biogas plants,medium-scale biogas plants and small-scale constrained the commercialization of biogas industry. biogas plants were 6160.10.285 and 83.512.respectively.2.103 The widely application of biogas in China was strongly sup- billion m3 biogas was produced annually which supplies 1.7157 ported by the central government and a series of energy policies. million household for biogas uses and generates 0.433 billion subsidies and tax incentives continuously became effective in kW h electricity per year [120.Encouraged by the benefits recent years.In February 28,2005,the "Renewable Energy Law" brought by incentive policies,more than 1300 domestic enter- was issued which emphasizes the exploitation and use of renew- prises were involved in biogas production and biogas equipment able energy [128].In 2006.the National Development and Reform manufacture 121.Efficient and professional subsequent service Committee promulgated two regulations:"Regulations Related to system has been established and improved over the years.How- Renewable Energy Power Generation"and "Renewable Energy ever.there still existed problem of low profitability of biogas plant Price and Cost-sharing Management Trial Procedures"in which caused by unstable price of raw material,inefficiency technical management and price of power generated by renewable energy support,and low level of commercialization,etc.The government was stated in details [129,130].In 2007,China's government pub- of China trusts that the biogas technology has economic,energy lished the "Medium-and Long-term Development Program for and environmental benefit which is an important form of resource Renewable Energy".and on April 1,2010,the revised "Renewable recirculation uses.By the end of 2014.China was qualified to Energy Law"was passed which stipulated a renewable energy develop biogas comprehensively. development fund to support renewable energy power-generating
862 K.Li et al.Renewable and Sustainable Energy Reviews 54(2016)857-865 plants [131.The polices for the development of biogas is normally 8.2.Recommendations for future work general guideline however more detail measures are needed. The development of biogas industry should consider the fol- The benefits of biogas technology have already been remark- lowing respects.At first,financial support for the constructions of ably known in China,such as energy capture from organic waste. super-large scale biogas plants for bio-natural gas production environment protection.less dependency on fossil fuels,reducing should be emphasized by the government.According to National greenhouse gas emissions,etc.,while its development is full of Biogas Project Plan in 2015,in 2015,2 billion RMB Yuan will be challenges.The problems are mainly related with technology. invested by Chinese government to support super-large scale More specifically,there is a shortage of technical support for high- biogas plants as a priory.In addition,the functions of biogas plants efficiency gas production.In addition,the extension and policies in balancing development between farm breeding and planting for biogas technology are also proposed.To accelerate the devel- should be improved in order to develop eco-cycle-agriculture. opment of biogas technology,a great attention should be paid to Further more,standard and management information platform of the following aspects in future: biogas plant should be further established.Finally.the policy for biogas technology should be easily operated and be long term (1)Biogas plants with diversified feedstock materials should be effective in order to promote the development of biogas promoted.In regards of the nutrient imbalance in mono technology. substrate anaerobic digestion,optimum C/N ratio and reduced relevant toxicity can be obtained by adding alternative mate- rials to single substrate 8.Conclusions and recommendations for future work (2)To improve the hydrolysis rate of straw materials,studies about finding suitable disintegration methods need to carry 8.1.Conclusion out in respect of combined pretreatment methods which are reported as effective,economical and eco-friendly.In addition, The anaerobic methane fermentation is an effective technology harvest time of biogas crops should also be considered since for the energy conversion and resource recovery of agricultural lignin content increased during their progressive maturity. residues.Due to its economical and renewable advantages,biogas (3)More appropriate biogas process technologies and reactors as a suitable substitution for fossil fuels,has attracted the attention need to be explored according to anaerobic digestion char- of the world.Researches demonstrated that biogas yield of bio- acteristics of substrate materials such as the crust forming and mass materials could be improved by adjusting C/N ratio of feed- outputting problems of straw,and the fast acidification pro- stock,controlling fermentation conditions and using various pre- blem of FVW. treatment methods.etc.Conducting such researches are very (4)Financial support for the constructions of super-large scale biogas plants for bio-natural gas production should be empha- important to provide theoretical basis for the operation of biogas sized by the government. plants.In addition,the industrialization of biogas needs to be (5)The development of biogas electricity generation and vehicle developed further.In brief,main conclusions are as follows: biogas should be emphasized.Only in this way could the utilization of biogas be efficient and diverse. (1)Effects of temperature,ammonium etc.on methanogens were (6)To promote the commercialization of biogas technology,the compared.The ammonium/acetate sensitive Methanosaeta financial subsidies for biogas should be given not only to were almost absent at 55-60C,which may suggest an biogas project constructions but also to biogas products and effective way to improve the process stability by digestion at follow-up services.Meanwhile,standards and regulations for 55-60C to exclude more stress susceptible acetotrophic quality control,inspection and evaluations need to be carried methanogens. out strictly. (2)To improve the biogas yield,pretreatment of feedstock materials like C/N ratio adjusting,disintegration and grinding are necessary in order to make substrate nutrition balance and Acknowledgments degraded easily. (3)The co-digestion of livestock manure with low nitrogen Financial support from National Natural Science Foundation of biomass can generate higher methane yield than the mono- digestion of cither manurc or bio-solid wastc like straw,grass China through contract(Grant no.51376121)is greatly acknowledged. silage and FVW. (4)Methanogenesis is the rate limiting step in the anaerobic digestion of cellulose-poor wastes like FVW due to the rapid References acidification of these waste.Therefore,other feed stocks or alkaline buffer should be added to FVW. 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