## 1 Introduction  
Biogas production is a uniquely flexible form of energy generation. It can be used to generate baseload electricity, meet high demands, provide low-carbon heat, or be upgraded for use as a transport fuel [1]. Since the introduction of a fixed feed-in tariff (FiT) for electricity from biogas plants (BGPs) in 1991, Germany has been continuously developing its biogas technology [2]. The implementation of the German Renewable Energy Sources Act (Act on Granting Priority to Renewable Energy Sources, known by its German acronym \"EEG\") since 1 April 2000, in which the original legislation guarantees a grid connection for electricity from renewable sources and a governement-set feed-in-tariff for 20 years, has made a great contribution to the biogas sector development. With multiple amendments of the EEG, a tender system in response to market trends was established in the current EEG version from 2017, which will allow the German biogas sector to focus on market-driven electricity production [3; 4; 5]. In the case of biomass power plants, the average tenderingprice reached 14.37 \(\mathsf{\mathsf{\mathsf{\mathsf{C}ct}}}\)/kWh in 2018 for existing plants [6]. Plants smaller than 75 kW can receive a fixed price up to 23.14 \(\mathsf{\mathsf{\mathsf{\mathsf{C}ct}}}\)/kWh (biogas from at least 80% manure) under the current EEG 2017. Up to the end of 2019, there were about 9527 BGPs in Germany, with a total installed electricity capacity of 5.0 GW [7]. In general, the performance-based FiT scheme has led the German BGP operators to focus heavily on the plant operation efficiency to maximize their income and revenue. Renewable resources are widely used as the main substrates for biogas production, accounting for 48.9% of the total substrate input in BGPs in 2016 [8; 9]. Livestock excrements (slurry and manure) took second place (44.5%), municipal biowaste ranked third (4.2%), followed by residues from industry, trade, and agriculture (2.4%) [9]. Meanwhile, three nation-wide biogas measurement programs had been conducted to identify the technical, biological, and financial operation efficiency. The first German Biogas Measurement Program (BMP I), conducted from 2001 to 2004, examined operations in each of the 60 BGPs over a one-year period [10]. BMP II was performed from 2006 to 2008 for 61 BGPs put into operation according to the EEG 2004 [11]. The most recent BMP III placed an additional focus on the repowering measures and possibilities of flexible biogas production for a demand-driven energy supply, apart from the operation efficiency of 60 BGPs and the impact of the EEG 2009. As one of the largest agricultural countries, China has made efforts to develop its biogas sector by implementing a variety of programs, initially as household-based, and gradually switching supports to farm-based BGPs [12; 13]. By the end of 2015, investment by the central and local governments supported the completion of 41.6 million rural household biogas digesters and 110,975 biogas projects of various types [14]. During 2000-2017, the central government invested 42 billion Chinese Yuan (CNY) (equivalent to \(\mathsf{\mathsf{\mathsf{\mathsf{C}}5.5}}\) billion) in the biogas sector. Furthermore, multilateral and bilateral international development organizations also played a key role in promoting the biogas sector development in China, either through grants or long-term loans [15; 16]. In China, most agricultural BGPs used livestock manure as the main substrates. In 2015, there were about 1.06 billion tons of livestock manure available for biogas production, accounting for 75.5% of the total substrates, followed by agricultural straws (12.8%). However, NDRC and MOA (2017) pointed out that nearly 180 million tons of crop straws were not used properly but were burned directly in the field, causing severe air pollution.