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In this paper, CiteSpace (6.1.R6) software was used to visualize and analyze the number of papers, terminology, countries, institutions, and collaborative networks of authors in the field of biogas upgrading in the years 2009 to 2023 to provide theoretical references for related researchers and departments. The progress of biogas upgrading research is divided into three phases: 2009 to 2011 (Phase I), 2012 to 2015 (Phase II), and 2016 to 2022 (Phase III). The first phase laid the theoretical foundation for the later phase, which included the fields of biomethanation and variable pressure adsorption. The physical/chemical biogas upgrading technology in the second phase gradually matured. The number of research papers published in the third phase increased rapidly, and with the rapid increase in the number of published research papers, which accounted for 73.2% of the total amount of research and statistical literature, this phase of biogas upgrading technology research is even more mature. In addition, the field of biogas upgrading has a small network of cooperation among researchers, and no close cooperation exists among the countries and research institutions involved. Under the trend of carbon emission reduction, “life cycle assessment” and “biomethanation” clusters have become the current research frontiers in the field of biogas upgrading.

Effects of different concentrations of biogas slurry (BS) were evaluated relative to vegetable cultivation. Five concentration levels of BS were used as organic fertilizer to grow Chinese cabbages (CA) by drip irrigation to study the effect of BS on the yield and quality. Each level was replicated three times and BS was used seven times. Results showed that the average plant height, fresh weight, dry weight, and soluble sugars and protein contents of Chinese cabbage under the T1 (BS 25%) treatment were optimal. Among them, dry weight per plant (3.53g), soluble sugar content (0.41%), and soluble protein content (0.0039%) were 1.74 times, 1.41 times, and 1.14 times more than under the CK treatment, respectively. In addition, the application of biogas slurry improved the physical and chemical properties of soil. Soil total nitrogen (TN) and available phosphorus (AP) content increased after the use of biogas slurry, and the soil total nitrogen and available phosphorus content increased with the increase in the concentration of the applied biogas slurry. The available potassium (AK) content of soil fluctuated with the concentration of biogas slurry. In addition, biogas slurry can properly improve soil pH. In conclusion, BS 25% was the best for the growth of CA and its quality, which provided scientific basis for biogas slurry as fertilizer.