Development process and probable future transformations of rural biogas in China

Renewable and Sustainable Energy Reviews 55 (2016) 703–712

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Development process and probable future transformations of rural biogas in China Xiaojiao Wang a,n, Xingang Lu b, Gaihe Yang a, Yongzhong Feng a, Guangxin Ren a, Xinhui Han a a b

Northwest A&F University, Yangling, Shaanxi Province 712100, PR China Northwest University, Xi'an, Shaanxi Province 710069, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 11 June 2015 Received in revised form 14 September 2015 Accepted 26 September 2015

In order to provide a clear guidance for biogas development in rural China, the progress and current status of rural biogas, critical factors influencing biogas development in rural areas, and probable biogas transformations are discussed in this paper. Rural biogas in China experiences rapid development in the past fifteen years and subsidies and policies from Chinese government are two main driving forces for promoting this development. For the recent days, the growth rate of household biogas decreased and that of Medium-to-large biogas projects (MLBPs) obviously increased. Several surveys showed that the utilization rate of household biogas greatly decreased and more and more digesters were discontinued. The underlying reasons for these changes come from the development of large-scale livestock farms, agricultural modernization and urbanization, which deeply affect the agricultural and social environment in rural China. The development of these agricultural and social factors all create positive conditions for MLBPs, but they negatively affect household biogas projects in terms of biomass supply and their participation in modern agricultural processes. Ongoing transformations and developments in local agriculture and rural society significantly influence the patterns of biogas projects. For future biogas development, MLBPs should be encouraged and the reduction of household biogas projects should be allowed to continue. However, recommendations on probable biogas development transformations suggest that the decision for expanding or abandoning the household projects in certain regions should be consistent with the developments and trends of local agriculture and society. For MLBPs, suitable developing models and reasonable operating mechanisms should be fully considered for assuring their benefits and continuous operation. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Biogas Household Medium-to-large biogas projects Transformation

Contents 1. 2.

3. 4.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Progress and current status of rural biogas in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Driving forces for rapid biogas development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Significant benefits of rural biogas in China. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Current status and limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . How about the future of rural biogas in China—viewpoints from recent literature reviews . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of potential factors affecting biogas development in rural China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Development of large-scale livestock farms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Urbanization development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Agricultural modernization and land circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Probable future transformations of rural biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Sustainably developed biogas programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. Tel.: þ 86 15902916021; fax: þ 86 29 87092265. E-mail address: [email protected] (X. Wang).

http://dx.doi.org/10.1016/j.rser.2015.09.097 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

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5.2. Retarded household biogas development. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Encouraged development of medium and large biogas projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Recommendations on probable biogas development transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction It has been widely accepted that biogas production using agricultural residues plays an important role in energy supply, environmental protection and the promotion of ecological agricultural development for rural areas. As the largest developing country in the world, China highly depends on agriculture to support its people. As a result, about 809 million tons of crop straw and 1629 million tons of animal manure are collected as biomass resources every year, with the biogas potential of 33.5  1010 m3 [1]. However, according to the National Bureau of Statistics of China, a total of only 1.67  1010 m3 of biogas in 2012 was generated through household and medium-to-large biogas projects (MLBPs) [2]. This means that only 5% of total agricultural waste was utilized for biogas production in rural areas. To reduce the pollution from agricultural wastes and solve the shortage of energy consumption in rural areas, the Chinese government has made great progress in developing the biogas industry, especially since 2003. As a result, the Chinese biogas industry has experienced extremely rapid development over the past decade. However, many problems continue to hinder the further improvement of this industry. Studies related to biogas development in rural China mainly focus on the potentials, constraints, challenges and solutions of household biogas projects [3–6]. However, the construction of biogas projects should not only be consistent with the supply of biomass and/or the demand for bioenergy, but also should also accord with the agricultural and social environment. Previous research has not paid enough attention to the recent changes in rural areas and may therefore not sufficiently predict future trends of biogas development in rural China. Therefore, this study describes the progress and current status of rural biogas in China, and analyzes several factors influencing biogas development in rural areas as well as discussing probable biogas transformations, with the aim to draw a clear picture for future development of rural biogas.

2. Progress and current status of rural biogas in China

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rate has clearly decreased as low as 2.1% after the year 2010 and the annual biogas yield has remained stable at around 1.67  1010 m3. MLBPs are other sources for biomass utilization and biogas production (Fig. 1). Most projects of this kind are constructed based on medium or large livestock and poultry farms, which are a little far away from residential areas. So generally, biogas from MLBPs is used for cooking, heating and lighting within the farm regions, as well as for power generation. In some cases, MLBPs are built near the villages with centralized biogas supplements for local villagers as household energy consumption. Very few MLBPs existed throughout the country before 2000, but the number increased nearly 30 times from 2001 to 2010 (Fig. 1) [7,8]. By the end of 2012, more than 30 thousands MLBPs were built and 1.4  109 m3 biogas per year were generated by these projects. 2.2. Driving forces for rapid biogas development The Chinese government recently proposed specific demands on the distribution of biogas, in consideration of improving the sanitation situation both in households and the village, supporting rural development, ameliorating clean energy, protecting the forest and improving the ecological environment of rural communities. To achieve its multiple goals, Chinese government played a direct and important role in promoting the biogas development in rural areas. Based on a survey with 1227 households from Guangxi, Hubei, Shandong and Gansu provinces, results of a binary Probit Model show that the governmental promotion of biogas has a significant effect on households’ decision-making [9]. In this paper, the authors declared a fact that that knowing about biogas from the government will make farm households much more likely to construct a biogas digester, compared with other resources like friends, family, relatives or media. Form some previous literature reviews, sufficient evidence has also linked the rapid growth of rural biogas production to Chinese governmental promotion programs of biogas, including biogas policies, laws, subsidies, rules and regulations (Table 1) [5,10,11]. The important role of policy supports in rural biogas development

2.1. Progress China is a typical country carrying out the small-scale farming system in the world, which has witnessed household-based and scattered farming and operation. As such, household biogas projects are the main form of biomass energy generation in rural areas. Generally, biomass wastes produced by farming and livestock in each household are used for biogas production in 8–20 m3 digesters. After digestion, the biogas product is used for cooking and lighting, and the residues are used as green fertilizers for planting. Although China has a long history of biogas utilization, rapid developments in the biogas production industry took place after the year 2000 (Fig. 1) [7,8]. From 1990 to 2000, household biogas plants increased from 4 million to approximately 8 million. However, from 2001 to 2010, the total number increased sharply up to 40 million, with an average annual rate of 17.6%. The growth

Fig. 1. The development in the number of rural biogas plants from 1990 to 2012 [7,8]. The number of medium-to-large biogas plants from 2011 to 2013 was not recorded in China livestock statistical yearbook.

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Table 1 Major policies, laws, rules, regulations and standards for rural biogas development in China. Typically items

References

“Biologically Enrichment of the Countryside Project” in 2000, “Rural Household Biogas State Debt Project” in 2003, “National Rural Biogas Construction Plan (from 2006 to 2010)in 2006, “Project of Rural Biogas Project ” in 2007, “Act on the Development of Circular Economy” in 2008, The 12th Five-Year Plan (2011–2015) for bioenergy development in 2012 Laws Agricultural Law in 2002; Renewable Energy Law in 2005; Animal Husbandry Law in 2005; Energy Conservation Law in 2007; Act on the Development of Circular Economy in 2008; Renewable Energy Law in 2009 (amended) Rules and regulations “Administration Method of Rural Biogas Construction Project Fund”, “Rural Biogas Service System Development Scheme”, and “Farming Community and Associated Households Biogas Construction Plan” Standards (Part) GB/T 4750-2002 Collection of standard design drawings for household anaerobic digesters GB/T 4751-2002 Specification for check and acceptance of the quality for household anaerobic digesters GB/T 4752-2002 Operation rules for construction of household anaerobic digesters GB/T 26715-2011 Biogas valve NY/T 1639-2008 Technology criterion on rural biogas digester and three renovations NY/T 1700-2009 Determination of methane and carbon dioxide in biogas—gas chromatography NY/T 1704-2009 Biogas power generation technology criterion NY/T 1916-2010 Technical specifications for fixed discharge facilities for digested sludge and slurry NY/T 667-2011 Classification of scale for biogas engineering NY/T 2065-2011 Technical code for application of anaerobic digestion fertilizer

Policies

Fig. 2. Financial support on biogas development from Chinese government from 2002 to 2012 [3,13].

has been recently reviewed by Feng et al. [11]. Major policy events included the “Biological Enrichment of the Countryside Project” in 2000, the “Rural Household Biogas State Debt Project” in 2003 and the “Project of Rural Biogas Project” in 2007 [5,11]. According to compare the release date of these policies with the developing data of rural biogas in Fig. 1, it is clearly concluded that the first policy resulted in the beginning of rural biogas project and the last two policies achieved the dramatic increase in the number of household biogas plants. The “Renewable Energy Law” of the Peoples’ Republic of China, issued in 2005 and amended in 2009, is considered a milestone in China's development of a rural renewable energy policy [12]. The Law aims to boost China's renewable energy capacity to 15% by the year 2020 and outlines a commitment to invest $180 billion in renewable energy over this period. In conjunction with these policies and laws, large amounts of subsidies were invested in biogas projects. In the survey by Qu et al. [9], government programs played an important role in promoting household biogas development in rural China, through both direct subsidies and indirect tools, such as biogas technology training. Between 2001 and 2012, 34.6 billion Yuan was provided from central government funds for biogas development, of which 25.6 billion Yuan (74% of the total subsidies) was directed at rural households (Fig. 2) [3,13]. The average cost of each household biogas digester was 2000 Yuan, approximately 53.2% and 43.7% of which was paid for by household investments and government subsidies, respectively. Since 2009, there has been a three-fold increase in subsidies for household biogas production compared

[1,3,4,8,10,11]

[3,8,11]

[3] [1,3]

Fig. 3. Subsidies on each household biogas digesters in different regions of China in different years [14–16].

with figures in 2002 (Fig. 3) [14–16]. The research by Sun et al. [17] suggested that a 10% increase in the subsidy-cost ratio resulted in a 2.2% increase in digester installations. The amount of the subsidies to support one MLBP is regulated by two principles: first, the governmental subsidies compensate around 50% of the construction cost and the livestock farm pays the rest and second, the total subsidies from central government cannot exceed 2 million Yuan for one MLBP [10]. Due to the stimulation of subsides, the quantities of MLBPs increased by 670% from 2006 to 2010 (Fig. 1). Besides policies and laws mentioned above, a series of rules and regulations were also promulgated, including “Administration Method of Rural Biogas Construction Project Fund”, “Rural Biogas Service System Development Scheme”, and “Farming Community and Associated Households Biogas Construction Plan” [3]. As a consequence, the projects of county-level service centers and rural service outlets have been launched, improving government follow-up services to maintain the normal operation of biogas projects and providing stable support for the sustained and healthy development of rural biogas. The standard system for rural biogas was another guarantee for promoting biogas industrialization. The standards that have been promulgated and implemented include 35 criteria and 4 categories (basic standards, product standards, technical specifications, and construction specifications), involving the design, construction, operation, and facility production of biogas plants (Table 1).

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Fig. 4. The overall benefits of household biogas in rural China [5,18–20].

2.3. Significant benefits of rural biogas in China The development of biogas programs in rural China has led to great economic, environmental, ecological and social benefits (Fig. 4). The use of biogas could decrease the annual consumption of coal used for heating and cooking by 10.2–59.6% in different investigated regions [18,19]. Accordingly, due to the application of biogas dregs, slurry, and marsh liquid to the agricultural crops, rural households with biogas digesters could save 11.4%–50.5% of payments on commercial fertilizers than those without biogas digesters [18,19]. Biogas energy utilization also cut down the consumption of firewood, thus helping to protect forests and reduce carbon dioxide emissions. A study based on the survey from 2000 to 2008 showed that if 5% of consumed firewood was replaced by biogas, an average of 9.14 million tons of firewood and 1.07 million hm2 forest land every year could be saved. As a result, 10.98 million tons of carbon dioxide emission could be avoided [20]. Besides of the direct benefits of biogas on economy, society and environment, more obvious advantage of biogas lies in its role as a link of various agricultural items. As a consequence, various ecoagricultural models are built, including “Three in One”, “Four in One”, and “Five in One” models [5]. The “Three in One” ecoagricultural model, which combines the biogas digester with a pigpen and toilet, is popular in southern China. The “Four in One” eco-agricultural model, which combines the biogas digester, pigpen, solar greenhouse, and toilet, has been proposed for northern China. The “Five in One” eco-agricultural orchard model, which combines the biogas digester with solar-powered barns, watersaving irrigation system, water cellar, and toilet is more suitable for northwest China. Consequently, under this type of circular agriculture system, agricultural wastes are fully utilized by bioconversion and the input of chemical fertilizer and pesticide into planting are significantly reduced, bringing about food production with high quality and safety. 2.4. Current status and limitations Although household biogas programs experienced rapid development and achieved remarkable benefits, the current widespread reduction in the use of digesters may hinder further progress. A survey in Hebei Province by Wang et al. [21] indicated that 18.5% of

households used biogas intermittently and 8.2% of households discontinued their digesters. In the southeast of Guizhou Province, 62.03% of household biogas projects continued to operate and 36.72% were discontinued [22]. According to the data from National Bureau of Statistics in 2014, the ratio of biogas digesters under normal operation in Shannxi rural area were only 30–70% and the usage rate declined in the region from far to near the city [23]. From the data of developing process in this study or previous review reports, we just conclude the growth rate is slowed down in recent years. However, because the number of discontinued plants is not cut down from the total statistical number, it is no doubt that the actual problem in household biogas operation is much more serious than that just reflected by reduced increasing rate. As early as 2008, Han et al. [24] pointed out that small-scale bioenergy projects in rural China did not achieved a satisfying performance and the failure was attributed to a complex of shortcomings in institutional structure, technical level, financial support and social factors. From previous literature reviews, the main factors restricted household biogas development are summarized as follows: (1) Technological aspects Besides animal manure, crop straws from agricultural activities are also important sources as fermentation materials for biogas production. However, due to high contents of cellulose, hemicellulose and lignin, straws under anaerobic digestion have very poor degradability and low efficiency for biogas production, thus leading to unstable and insufficient biogas supply for household daily life [5]. Although many researches make progress in pretreatment of straw by physical, chemical and biological methods to increase the utilization of straw, these pretreatment technologies are still immature for practical application as a consequence of high cost or secondary pollution [25,26]. In addition, most biogas plants in rural China are just comprised by cement or metal tanks set up underground, without any device for keeping or raising temperature. These kind of plants operate normally in southern China, but show low efficiency in northern and northwest China, covering about 75% of land area and 45% of the population. Thus, technological shortage for anaerobic digestion under cold region result in the insufficient supply of biogas for annual consumption.

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(2) Social aspects The follow-up services and management of biogas digesters is the key factor to ensure the sustainable development of rural biogas industry. Although “Rural Biogas Service System Development Scheme” was implemented in 2007, most countylevel rural energy offices only have three to seven staffs, which cannot meet the rapidly growing demand in the follow-up services of rural household biogas digesters [4]. Additionally, the finical support for the construction of digesters from the government is a one-off activity. During the operation of biogas digesters, the maintaining of these tanks and the exchanges of accessories, as well as the service from technical staffs, all give rise to operating costs, which significantly affects the farmers’ commitment to persistently use biogas. (3) Household aspects The rapid development of large-scale breeding industry drive more and more single households give up breeding livestock by themselves due to high costs, the difficulty of disease prevention, the instability of the market, and a falling profit margin [27]. This phenomenon directly results in the shortage of raw materials for anaerobic digestion because of most household digesters relying on manure as the single substrate. For obtaining high incomes, more and more young adults gave up planting and transferred to the cities for other jobs, leaving children and seniors in the village without physical and technical abilities to operate biogas plants [8]. In addition, with the increase of household incomes in rural region, more families tend to use electricity or coal as the main sources of energy consumption [28]. For MLBPs, although previous surveys have shown that they give more benefits to the user and society than household biogas digesters [29], many problems remain that restrict such a development. From a technical aspect, many bottleneck problems in areas such as pretreatment, material transportation and mixing, solid–liquid separation, biogas purification and equipment for power generation have yet to be resolved [3]. For biogas project management, the lack of a reasonable pricing system discourages cooperation between the owners of biogas plants, residents, enterprises and urban sanitation authorities, leading to low economic benefits [8]. Most MLBPs are located far away from local villages, so biogas is difficult to transport to households. Without developing and implementing specific policies for MLBPs, the electricity generated by most MLBPs cannot connect to the national power grid. Therefore, most MLBPs are operated to meet the energy demands of individual farms only. Additionally, most livestock farms with MLBPs cannot establish the circular agriculture model without the support of their own planting; therefore, biogas slurry and residues cause secondary pollution.

3. How about the future of rural biogas in China—viewpoints from recent literature reviews In view of mentioned limitations above, previous reports indeed proposed some recommendations or presented developing prospects with the aim to promote rural biogas development. Recommendations by Chen et al. [5] in 2010 involved advancing straw fermentation and cold fermentation technologies, raising the skills of biogas customers and improving the follow-up services and management of biogas plants. Research by Jiang et al. [10] in 2011 proposed several recommendations, including improving biogas production efficiency by economic temperature control method, building appropriate pricing systems for the input raw materials and output biogas and organic fertilizer, expanding financial subsidies from supporting the construction of biogas

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project to the end use of biogas products and technical services, and boosting service systems. After reviewed the progress and problems of rural biogas production, Chen et al. [3] in 2012 pointed out the developing prospects of biogas in China from four aspects. First, raw material for biogas plants will diversify; second, the construction site of biogas plants will be extended from villages to small towns; third, there will be greater focus on efficient, high-value, and comprehensive utilization of biogas products; fourth, the operation and management model of biogas plants will be specialized. By evaluating and comparing the dis-/advantages of decentralized and centralized bio-energy systems, He et al. [12] in 2013 formulated four recommendations referring to design innovated, streamlined and context-dependent investment mechanisms, establish a stronger relation between long-term environmental and social benefits, set up financial incentives for well performed biogas system and develop criterion that building biogas system based on the local circumstances. In the review of status and prospects of rural biogas development in China in 2014, the author put forward the biogas development tendency comprising of producing commercial fertilizer from biogas residue and slurry and popularizing electricity generation using biogas [4]. From the typical reviews in the past five years, the recommendations mainly focus on the technical properties and operating aspects of rural biogas. However, when facing the above mentioned problems of rural biogas, we could try to address them and to create suitable conditions for its further development; or we could reevaluate the suitability of the discussed biogas development patterns with respect to the current situation of Chinese rural society. Additionally, the choice of whether to prioritize household biogas projects or MLBPs in the future requires deep consideration. Today, Chinese agricultural methods and organizational structures are undergoing major changes, which may deeply affect and potentially transform biogas development. Due to leave out the impact of rural situation, previous researches did not give accurate answers how to develop biogas programs further in rural china. Thus, potential factors affecting the further development of biogas in rural China are analyzed in the following sections, aiming to draw a clear picture for future development of rural biogas.

4. Analysis of potential factors affecting biogas development in rural China 4.1. Development of large-scale livestock farms Household biogas programs were initially carried out to meet energy demands, improve the rural environment and establish the circular agriculture model with combinations of planting and breeding. Animal manure and crop straw were the most common raw materials used for biogas production. However, in household biogas digesters, animal manure was preferred because of its high degradability and stability during biomethanation. Thus the operation of household biogas projects was largely dependent on the extent of livestock breeding in each household. The production rates of the smallest farms in 2013 declined by 55%, 15.5%, 21.97% and 64.10% for swine, cattle, sheep and chicken, respectively, compared with the rates in 2004 (Fig. 5) [30]. These declining rates are the direct reason for discontinuation of the household biogas digesters. For larger-scale livestock farms, production rates increased significantly in the past decade. Thus, livestock breeding operations are gradually shifting from small-scale to large-scale farms. In 2010, the Ministry of Agriculture described their stance on accelerating the development of large-scale breeding of livestock and poultry [31]. From 2008 to 2013, Chinese government invested 13.5 billion Yuan to promote the construction of largescale breeding farms. In terms of improving the production

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Fig. 5. Changes of the ratios of different scales of animal farms [30]. The data in x-axis represent the number of swine, cattle, sheep and chicken of each farm.

efficiency and the level of food safety, and reducing the threat to ecological environment, the Chinese government would continue to implement new policies and increase the subsidies to encourage large-scale breeding. Therefore, an increasing number of households ceased breeding in the following years, resulting in the shortage of fermentation materials for household biogas projects. Conversely, the development of large-scale breeding created opportunities for the expansion of MLBPs. 4.2. Urbanization development The rate of urbanization in China increased following the initiation of the reform and opening policy. By the end of 2013, 53.7% of the total population lived in urban areas, a rate that rose from 36.2% in 2000 (Fig. 6) [32]. In 2014, Chinese government released “National New-type Urbanization Plan”, in which the government calls for more than 100 million people to move to cities by 2020, pushing China's urban population to 60% [33]. Urbanization has influenced biogas development from two aspects. First, an increasing number of rural families moved to live in cities, directly reducing the number of households with a biogas energy supply. Urbanization created many job opportunities and prompted a large workforce transfer from the country to the city. In 2013, more than 200 million Chinese migrant labforces worked in the city where they were not registered and are thus legally excluded from some local public services, and consequently a large amount of children and old ages had to live in their rural area [34]. The resulting reduction of the rural workforce restricted the normal operation of household biogas projects. Second, urbanization is geared toward the development of small cities and towns, further increasing the population density, which means that an increasing number of people live in apartment buildings. Therefore traditional courtyards would gradually disappear; because household biogas digesters were commonly built in such yards, their disappearance means no space for the construction of biogas projects.

Fig. 6. The urbanization level in China from 1978 to 2020. Data from China Statistical Year book 2013 published by National Bureau of Statistics and China's Newtype Urbanization Plan since 2014 [32,33].

4.3. Agricultural modernization and land circulation To ensure a sufficient and sustainable food supply for its people, the Chinese government had to modernize agricultural methods and practices. In the No.1 Document in 2007, the Chinese government first outlined a set of instructions on how to develop modern agriculture [35]. According to the No. 1 Document in 2015, China's top priorities are rural reform, developing modern agriculture and maintaining agriculture as the foundation of the economy [36]. Therefore, agricultural modernization is expected to be a long-term trend. The key characteristics of Chinese modern agriculture are to improve the levels of mechanization and informatization, the efficiency of land output, resource utilization, labor productivity, the capacities of risk resistance, market competitive and sustainable development [36]. However, according to the

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Fig. 7. The underlying and direct reasons for influencing the development of household biogas in rural China.

experiences of developed countries, the prerequisite to achieving these goals lies in highly intensive and large-scale land management [37]. Therefore, Chinese agriculture is in the process of being transformed from individual, small, family-based farmland to cooperative-based large-scale farmland. Land circulation was the key policy to promote this progress and was greatly supported by the Chinese government. Land circulation means the transfer of contracted management rights of farmland between farmers with a focus on sub-contracting, renting, entrusting, exchanging and sharing. In this manner, land was circulated to large-scale farmers, agricultural cooperatives or to companies for large-scale management. According to the data from the Department of Agriculture in 2014, the use rights of 25.3 million hm2 contracted farmland were transferred, accounting for 28.8% of the total contracted farmland of the whole country [38]. The direct influence of land circulation was the growth of large-scale farms. For example, by the end of 2012, there were 877 thousand family-based large-scale farms with an average land area of 13.3 hm2, extremely higher than the average area of 0.5 hm2 for each household [39]. The development of family-based large-scale farms was greatly supported by the government, especially after the release of the No. 1 Document in 2013. As a result, more families in rural areas were left out of such modern agriculture processes, leading to the reduction of raw materials for biogas production and insufficient conditions for developing circular agriculture based on biogas utilization. However, the growth of large-scale farms, especially in terms of planting, make the cooperation with livestock farms much easier when building centralized biogas projects and establishing circular agriculture through a combination of planting and breeding.

5. Probable future transformations of rural biogas 5.1. Sustainably developed biogas programs Sufficient reasons indicate that the Chinese biogas programs will develop sustainably. First, the situation that China suffers great pressures from energy shortage and agricultural waste pollution becomes more serve in recent years. At present, no alternative method could be equal with biogas program, due to its

great benefits, in solving these two problems. Second, the estimated data show that the theoretical total biogas potential from agricultural wastes in China was (3350.58 7 669.28)  108 m3 in 2012, equal to (239.22 7 47.79) million tons of equivalent standard coal [1]. However, according to the National Bureau of Statistics of China, a total of only 1.67  1010 m3 of biogas in 2012 was generated; only 4.98% of the biogas potential was utilized. So, the great biogas potential suggests related biogas programs are destined to go forward. Third, the great benefits of biogas were also proved in most developed countries. Nonetheless, on the whole, there is still a large gap when compared to developed countries, particularly when taking account of resource utilization, reasonable price structures, a comprehensive market and a timely service system [40]. Great efforts are still needed to achieve a well-operated biogas industry. In short, the process of biogas development in rural China should be allowed to continue without obstruction. However, the pattern of biogas development is likely to change and should be consistent with the transformation of the agricultural and rural social environment and other factors affecting the construction and maintenance of biogas digesters. The likely future transformations of biogas in rural China are discussed in the following sections. 5.2. Retarded household biogas development According to previous research, household biogas development was influenced by the supply of agricultural wastes, the number of laborers, the appropriateness of living spaces, household incomes and willingness of the residents directly affected by the economy and convenience of biogas utilization and digester maintenance [6,9]. Thus, agricultural and social factors, such as large-scale livestock farming, urbanization, agricultural modernization and land circulation, greatly weakened the development of household biogas in rural regions. This weakened development was also caused by reduction of available fermentation materials in each household, the transfer of labor forces from the countryside to the city, increased uptake of alternative energy in response to increased household incomes and reduced construction space as a result of intensive living (Fig. 7). Many researchers also pointed out the problems during household biogas development, such as

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the popularization of the use of commercial energy, poor followup services and management of biogas digesters and poor economic benefits resulting from the low integrative utilization rate of biogas production [1,6,13]. These authors also made some suggestions for solving these problems to promote household biogas development. However, these reports just found the direct reasons for the poor running of household biogas (Fig. 7). The underlying reasons came from the transformations of rural social and agricultural environment. That urbanization, agricultural modernization or the developing trends of large-scale planting and breeding are all the inevitable, irreversible products of Chinese social development is undebatable. Therefore, the Chinese government should allow for a slowdown in the development process of household biogas. 5.3. Encouraged development of medium and large biogas projects The construction and operation of MLBPs can benefit from the use of advanced biogas production technologies, such as heat extraction or power generation, which increase their financial returns. Wang et al. [41] also indicated that MLBPs produced more benefits to the user and society than did household biogas digesters. Recent surveys by Song et al. [8] have shown that household biogas projects and MLBPs both have their relative strengths and weaknesses, and the choice of a biogas generation system depends on the local circumstances. The authors also pointed out that MLBPs have a higher energy efficiency and better social effect, and are best suited to developed regions where people live close together [8]. Agricultural modernization promotes transfer of farmland from small households to family-based large-scale farms, agricultural cooperatives or companies. Also, the Chinese government has proposed the “National New-type Urbanization Plan” to speed up the migration of rural labor to cities. Rural China is presently undergoing a clear development trend from a small scattered economy to a large-scale intensive agricultural economy. These measures and changes all create favorable conditions for the development of MLBPs. Additionally, the newly developed large-scale farms usually cause increasingly serious environmental pollution caused by the substantial production of agricultural wastes that are hard to completely decompose and that tend to concentrate in a certain area [42]. Therefore, it is of practical significance to incorporate the concept of eco-agriculture into large-scale intensive farming in search of an energy production method that is both economically effective and environmentally friendly. According to the successful implementation of household biogas production in circular agriculture, MLBPs should play an important role in the development of eco-agriculture based on large-scale farming. In conclusion, because of their benefits and central role in developing modern agriculture, the development of MLBPs should be encouraged.

6. Recommendations on probable biogas development transformations The declining trend of household biogas utilization does not mean that all household biogas projects will be discontinued. In Sichuan Province, especially in mountainous districts, household biogas projects continue to be used. Declining household biogas development rather refers to the slowing down of its progress. In any case, the wider transformations influencing the biogas development conditions are already in progress. Thus, the decision for expanding or abandoning the household projects in certain regions should be consistent with the developments and trends of local agriculture and society. When household biogas run into the period of rapid growth, many researches were conducted for

evaluation of regional suitability for biogas construction, by means of creating classification standard based on local properties in terms of climate, bioresources and economy [43,44]. Related evaluation system was helpful for developing biogas projects in that time, but at present, new regulations must be formulated to determine whether the biogas plants in certain regions should be expanded, upgraded or scraped. The author recommends one principle to make this decision is that the biogas development should be consistent with the process of agricultural reform and urbanization; if these process are slow and do not significantly influence the household and agricultural structure, household biogas plants can be newly built or invested more for upgrading, but if these process are positively going forward, household biogas development should be restricted. For all recommendations and prospects on technical properties and operating aspects of rural biogas in previous reviews (in Section 3), the author proposes positive attitude and also suggests to strength the related research. Until recently, most original papers paid attention to the technologies during anaerobic digestion, and some research with the topic of biogas development in China has focused on the economic and environmental evaluation on household biogas and its role in developing circular agriculture. However, very few researchers have focused on how to develop MLBPs. Delzeit and Kellner [45] analyzed the impact of plant size and location on the profitability of biogas plants in Germany. Pantaleo et al. [46] reported an investment decision methodology for the assessment of optimal size and feedstock mix of biogas power plants fed by cattle manure and energy crops in Italy. Although Deng et al. [40] created a hierarchy process model based on the characteristics of different biogas plants and geographic regions to obtain development strategies for regional distribution plans and the scale of biogas system construction, the research overlooked the effect of agricultural transformation on the supply of fermentation sources. In short, under the current rural situation in China, guidance on developing MLBPs is very limited. To address this issue and to further rural biogas development, two aspects need to be considered. First, a scientific evaluation model should be produced to determine how to develop a biogas project suited to local agricultural and social environments (Fig. 8). Several factors in developing a biogas project should be considered, including the biomass sources, plant size and location and its product utilization. The structure and scale of agriculture during its modernization strongly affects the methods used when treating the agricultural wastes. The total amount of biomass from local planting and breeding is no longer the only factor influencing biogas potential in certain regions, but the distribution, affiliation and availability of these biomass sources also need to be considered. Based on biomass availability, much more attention should be paid to determining the suitable size and location of biogas projects to maximize their benefits for ensuring the longterm running of the project. Plant size depends on the amount of biomass wastes and the local demand for its product. Thus, the utilization patterns of biogas and its related products and should also be considered before building a biogas project. Second, the continuous development of rural biogas, especially for MLBPs, also relies on reasonable operating mechanisms (Fig. 8). When compared with small scattered farms, the extended scale of arable agriculture as a result of land circulation creates more opportunities for its cooperation with large-scale livestock and poultry farms in establishing centralized co-fermentation biogas projects. This type of biogas project is clearly effective as a linkage in circular agriculture to achieve complete waste treatment of direct agricultural biomass or biogas byproducts. The government may play a role by developing polices to promote cooperation among different agriculture participants, and by helping to

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Fig. 8. Suitable models and reasonable operating mechanisms needed for sustainable development of rural biogas in China.

establish a reasonable pricing system to better balance the benefits among project owners, source suppliers and product users. In this case, the government may transfer subsidies supporting project construction to stabilizing the product cost and uptake. Overall, MLBPs are suited to the current trends in rural agriculture and their development should be encouraged, but strategies for assuring their benefits and continuous operation also should be fully considered.

7. Conclusion Based on its extremely high biomass potential, low utilization rate and significant benefits for rural communities in China, biogas should be promoted and further developed. However, ongoing transformations and developments in local agriculture and rural society significantly influence the patterns of biogas projects. Both agricultural modernization and urbanization create positive conditions for MLBPs, but they negatively affect household biogas projects in terms of biomass supply and their participation in modern agricultural processes. For future biogas development, MLBPs should be encouraged and the reduction of household biogas projects should be allowed to continue. Strategies and mechanisms for developing well operated MLBPs also need considerable improvement.

Acknowledgments This study is supported by the National Natural Science Foundation of China (51508467) and Chinese Universities Scientific Fund (Z109021511).The authors also thank the reviewers for their comments and suggestions on improving this paper.

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