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Decentralized biogas facilities have become an important component of agricultural sectors in Chinese rural area. This study is the first attempt to investigate how transition in Chinese rural areas affects the revolution of decentralized biogas facilities and its sustainability. The characteristics of the household biogas digesters (HBD), simple biogas plants (SBP), and well equipped biogas plants (WEBP) with higher level of technologies were clarified, and their practical feasibility was correspondingly investigated through a comprehensive survey of 15 selected villages. More effort was devoted to the adoption of feedstock availability, energy supply stability, and digestate distribution optimization for each facility. The results showed that feedstock collection and insufficient biogas production were the main restraining factors for HBD, while SBP found to be ineffective facilities for competitive energy production due to the un-guaranteed biogas supply. The survey results demonstrated that the average biogas expenditure of dwellers, who were connected to WEBP, accounted for 39.61% commercial energy cost. According to the one-year operation data collected from a 52-household club revealed that WEBP would effectively maintain stability and continuous biogas supply with the average feeding interval of 6.2 d. Accordingly, WEBP groups were found as the best option for regional development respected with the professional management unit. Overall, commercial biogas is the orientation of decentralized biogas facility construction. However, to form a robust biogas-centric industrial chain, future research should focus on how to establish relative policies and regulations supporting the comprehensive operation framework of a multi-participant collaboration leadership system. • Decentralized biogas facilities in Chinese rural area have been scrutinized. • Household digesters, simple, and equipped plants are prevalent biogas facilities. • Feedstock collection and low gas production were restraining factors in HBD. • WEBPs were found as the best option for regional development. [ABSTRACT FROM AUTHOR]

Abstract Fermentation material fluid mixing is an indispensable subsidiary technology for modern biogas engineering, which can increase fermentation efficiency substantially. Taking advantage of computational fluid dynamics (CFD), the flowing progress of biogas fermentation material fluid in the anaerobic digesters can be reappeared, the distribution curve of flow pattern can be obtained, the visualization of flow pattern can be achieved, the features and defect of flow configuration can be differentiated on the basis of the flow pattern visualization. Scholars quantified and evaluated advantages and disadvantages of fermentation material fluid mixing schemes by using CFD and setting the parameters of "poor mixing zone", "dead zone" and so on, which became the important basis of modern biogas engineering design. Meanwhile, the mixing form and running parameters can be excellently designed and thus no longer added at random. On the basis of the classic k-ε model, researchers have developed a variety of improved models, which are more suitable for complex and diverse conditions of biogas fermentation material fluid. In this present study, the applicable algorithm models of various working conditions are proposed after subdivision research. During the first decade of the 21st century, one research upsurge of simulating biogas fermentation material mixing by using the CFD simulation is growing up in the biogas field, which makes a great progress on the aspect of optimal design of digesters shape, mixing form, mixing running parameters and methods of fluid, promotes the research levels of biogas science and becomes one of the most important stage of the biogas science phylogeny. All of these represent the development direction of future, accordingly, the summary of progress related with above research aspects is given in this present study. Graphical abstract fx1 Highlights • The measured results of many flow field parameters can be fitted with simulation. • The methods of setting the mixing parameters by CFD are listed. • Successful examples of optimization of biogas engineering design by CFD are presented. • The applicable algorithm models of various working conditions are proposed. • The role of CFD in promoting the upgrading of biogas project is discussed. [ABSTRACT FROM AUTHOR]