Biogas Systems in China

The biogas digestion has experienced rapid development for its unique characteristics for dealing with energy shortage and environmental pollution. In this chapter, we reviewed the development trend of biogas projects in China from 2000 to 2014. The significance of developing biogas in China was analyzed by accounting the energy supply and emission reduction. The details about the biogas industrialization were then probed into, with the conclusion that the smallholder farms are gradually transforming to larger-scale farms. The strategic development mode of biogas production industry was summarized. Then, the relationship among three types of biogas industries including biogas plant industry, biogas equipment industry, and biogas service industry was analyzed in detail. Finally, the development planning and development motion for biogas production as well as corresponding policies in China were reviewed. The policies, including energy policies, environmental policies, and economic policies, laws, rules and regulations, and standards for the household biogas projects were summarized, which may contribute to the rapid promotion of biogas project.

In order to have a better understanding of the different aspects (economic performance, environmental impacts, sustainability) of the biogas projects, ecological–economic methods are used to formulate the integrated assessment framework. First, economic method (cost–benefit analysis) was used to assess the economic feasibility of the biogas project. Moreover, DEA method was used to make an economic efficiency assessment, based on which the optimization suggestions could be provided. In terms of the environmental impact evaluation, life-cycle assessment model was established to evaluate the energy cost and environmental impact of biogas system, based on which the key sections for energy saving and emission reduction of biogas systems could be identified. As for the sustainability analysis, considering ecological and social inputs, emergy analysis and exergy analysis were employed to quantify the environmental pressure, renewability, economic efficiency, and sustainability of biogas systems. Possible pathways to achieve sustainable and low-carbon biogas project management were also analyzed based on the scenario analysis. Finally, analytic hierarchy process (AHP) method was adopted to incorporate categories of indicators to have a comprehensive performance analysis of the biogas system.

In this chapter, detailed descriptions of four typical systems are presented including “Six-in-One” biogas system (SIOBS), biogas-persimmon cultivation and processing system (BCPS), wastewater treatment plants (WWTPs), and the “Three-in-One” biogas project (TIOBS). The similarities and differences of how it is constructed, operated, and maintained are addressed. Besides, the unique characteristics are also depicted, containing digestate process in SIOBS, digestate reuse, and biogas utilization for persimmon cultivation in BCPS, biogas–sludge use in WWTPs, and comprehensive utilization in TIOBS.

One of the most serious environmental problems that household biogas project would cause is the wastewater discharges and greenhouse gas emissions. Environmental discharges to the water (COD, TN, and TP) and emissions to the air (CO₂, CH₄, NH₃, SO₂, CO, NO_x, and PM₁₀) were identified. We conducted a life-cycle impact assessment (LCIA) based on the well-established mid-point methodology, Centrum voor Milieukunde Leiden (CML). The following indicators were selected: global warming potential (GWP, in g CO₂ eq), acidification potential (AP, in g SO₂ eq), eutrophication potential (EP, in g PO ₄ ^3- eq), photochemical oxidation (POCP, in g C₂H₂ eq), human toxicity potential (HTP, in g 1,4-DCB eq), and abiotic depletion potential (ADP, in g Sb eq). Direct and indirect emissions are differentiated in the accounting process. The direct category includes fuel combustion in transportation, elimination of emissions from raw materials, and methane combustion as alternative household fuel, while the indirect category accounts for biogas infrastructure and equipment, digestate reuse as substitution for chemical fertilizers, machine adaptation, etc. Results show that the total greenhouse gas mitigation is 3614.157 kg CO₂-eq, to which feedstock supply stage, digestate processing stage, and biogas energy utilization stage contribute most.

Biogas plant construction has been boosted in rural China not only due to the immediate merit from biogas production but also due to the succeeding benefit from by-product utilization in agro-industry, both of which are significant strategies to address energy shortage and global warming issues. Combined heat and power and household utilization were prior options in achieving net energy production, provided that power conversion efficiency and biogas production remained at satisfactory levels. A life-cycle energy evaluation of a biogas–digestate utilization system is performed. Besides, energy inputs and outputs within the system boundary are calculated to address the possibility of net energy production by wastewater treatment combined with biogas–sludge use technologies based on input–output analysis. Taking all the life-cycle stages into account, the net energy gain of the entire system is 10,458.024 MJ. Different bioresource use alternatives had a bias toward the efficiency of energy production.

One of the key concerns of biogas plants is the disposal of comparatively large amounts of digestates in an economically and environmentally sustainable manner. This work analyzes the economic performance (both economic feasibility and economic efficiency) of anaerobic digestion of a given household biogas project (Gongcheng project) based on cost–benefit analysis and DEA method. Firstly, the economic profitability and operation risk were analyzed based on economic indicators. Then, DEA approach was used to measure the relatively economic efficiency of eight household biogas projects in different part of China, based on which the optimization suggestions for economic efficiency can be obtained. Results show that as for the economic feasibility analysis, the net present value (NPV) index as a reflection of the economic feasibility of Gongcheng project is 4754.13 Yuan, showing it has good economic profitability. The payback period (PB) is 2.2 years for the biogas project, which is only 1/5 of the lifetime of the household biogas project (10 years). It can be seen that all the economic investment to support the operation of biogas project could be returned. According to the DEA analysis, the values of technical efficiencies for five biogas projects are smaller than 1 (technical inefficient), indicating that most biogas projects involved are economic inefficient. For further optimization, the input variables (“construction investment” and “maintenance cost”) of eight biogas projects should reduce 159.43 (4.55% reduction) and 9.07(5.03% reduction) units (unit: 10⁴ yuan), respectively, to increase the economic efficiency. With regard to the output variables, “biogas benefits” variable should increase 33.59 (unit: 10⁴ yuan) in total.

This chapter provides an overview of the economic and environmental performance of biogas-linked agricultural system (BLAS) in China based on emergy. A set of emergy indices are incorporated to describe the energy and materials transformation within the system, and an emergy-based CO₂ emission indicator (EmCO₂) is proposed to achieve low-carbon optimization of the whole system. Emergy synthesis and emergetic ternary diagram are then utilized to evaluate the BLAS and its subsystems, with scenario analysis performed to identify a more sustainable development pathway for the BLAS. Finally, a framework is developed to track dynamical behaviors of the whole system (Level I), transforming process (Level II), and resource component (Level III) simultaneously, and two new indicators, emergy contribution rate (ECR) and emergy supply efficiency (ESE) are proposed to address the contribution and efficiency of resource components within each process. The results showed that BLAS made a favorable contribution to carbon mitigation and was more environment-friendly than the traditional agricultural systems. Scenario analysis demonstrated that continual biogas construction and effective technological revolution were preferable routes to further improve the whole system’s performance. It can be concluded that breeding and biogas subsystems were economic input-dependent. Electricity and diesels were the most efficient components in supplying all the processes in BLAS. The relatively high transformities and the constant descent of sustainability within all processes are the key problem that hinders the promotion of BLAS.

To evaluate the effects of biogas on agro-ecosystem from a systematic perspective, we discussed the current situation of household biogas and identified its main factors that may have impacts on agro-ecosystem. An indicator framework covering social, environmental, and economic aspects was set up. A case study of Gongcheng was then conducted to evaluate the combined impact of biogas project through the proposed indicator framework. In addition, to integrate the socioeconomic-environmental relationships, we innovatively introduced extended exergy analysis to the sustainability evaluation of biogas project. Besides, new extended exergy-based sustainability evaluation indicators that unified greenhouse economic performance, gas emissions, and resource depletion were first proposed to identify biomass conversion pathway planning and as goals for possible system sustainability optimization. The sustainability of a “Three-in-One” biogas production system in southern China was then evaluated based on the proposed framework. Results showed that there was a significant positive effect resulted from the application of biogas, and the integrated benefits have been improved by 60.36%, implying that biogas as a substitute energy source can promote the sustainable level of rural areas. According to the extended exergy analysis, the biogas project has a higher renewability (0.925) and economic return on investment ratio (6.82) and a lower GHG emission intensity (0.012) compared with other renewable energy conversion systems.