Commercial biogas plants: Review on operational parameters and guide for performance optimization

1. Introduction 
The demand for climate change mitigation and natural environ-
mental protection has imposed a constant need for society to develop 
sustainable alternative energy sources to reduce dependency on fossil 
fuels. Biogas technology, which is based on the decomposition and 
stabilization of organic materials from various sources by anaerobic 
digestion (AD), plays a positive role in optimizing energy structure and 
enhancing energy security worldwide 
[1,2]
. The World Biogas Associ-
ation confirmed in a recent report that AD has the potential to reduce 
global greenhouse gas (GHG) emissions by between 3,290 and 4,360 Mt 
CO
2 
eq., which is equivalent to 10–13% of the world’s current emissions 
[3]
. The application of AD can reduce organic waste deposition into 
landfills, carbon emissions and the production of hazardous materials, 
and, most importantly, produce the clean energy source known as biogas 
[4]
. Therefore, as the demand for sustainable development has become 
more urgent in recent years, biogas production has exhibited steady 
progress towards industrialization and commercialization 
[5]
. As shown 
in 
Fig. 1
, there were 18,943 medium- and large scale biogas plants 
operating in Europe at the end of 2019, which was more than three times 
the number of plants operating in 2009 
[6]
. In China, the number of 
biogas plants reached 32,624 by the end of 2016, which represented an 
increase of 46% in comparison with the number of plants operating in 
2009 
[7]
. 
AD is a complex multi-stage biochemical process during which 
organic material is converted into biogas by various groups of anaerobic 
microorganisms in four stages: hydrolysis, acidogenesis, acetogenesis, 
and methanogenesis. The efficient conversion of organic matter into 
methane depends on mutual and syntrophic interactions among the 
functionally distinct anaerobic microorganisms involved in each stage, 
and process stability of AD is achieved by maintaining the delicate 
balance between the production and consumption of intermediate 
products 
[8]
. Although AD is a mature technology that is well- 
established in many parts of the world, poor system stability and low 
efficiency of methane production are commonly encountered main 
problems during the operation of conventional commercial biogas 
plants 
[9,10]
. 
* Corresponding author at No. 174, Shapingba Zhengjie Street, Chongqing, 400045, China. 
E-mail address: 
[e-mail] 
(L. Li).
Contents lists available at 
ScienceDirect 
Fuel 
journal homepage: 
www.elsevier.com/locate/fuel 
https://doi.org/10.1016/j.fuel.2021.121282 
Received 11 March 2021; Received in revised form 10 June 2021; Accepted 14 June 2021

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Fuel 303 (2021) 121282
2
Process instability is a commonly reported issue in anaerobic di-
gesters in commercial biogas plants that results in decreased biogas 
production, acidification, foaming or even a total crash of the entire AD 
system. For example, process instability lasting several weeks to months 
occurs frequently in Danish centralized biogas plants, with a loss of 
approximately 20–30% of biogas production 
[11]
. Moreover, foam 
formation was found to be common in 80% of the biogas plants inves-
tigated in studies conducted in Germany, Denmark, and America, which 
showed that foam reduced biogas yield of 30–50% during foaming pe-
riods, and in some cases led to total process failure 
[12–14]
. Further-
more, the operational efficiency of existing commercial biogas plants 
running under stable conditions is generally far from the full capacity. 
For example, the conversion rate of AD systems treating food waste (FW) 
generally ranges from 40% to 70% 
[15]
. The lack of profit associated 
with low biogas yield is often reported as the primary reason that biogas 
plants cease operation, especially for smaller biogas plants (15 ~ 
99KWel) 
[16]
. Therefore, research is focused on boosting the opera-
tional efficiency, productivity, and sustainability of biogas production 
systems by optimizing their upstream (substrate pretreatment), 
mainstream (biogas production) and downstream (biogas upgrading) 
[17,18] 
(
Table 1
). 
For substrate pretreatment and biogas production, enhanced process 
stability and operational efficiency can be achieved by improving sub-
strate biodegradability, balancing nutrition, and optimizing microbial 
physiology 
[19,20]
. The purpose of biogas upgrading, on the other hand, 
is to obtain high quality biomethane by increasing the methane content 
of raw biogas, with the goal of improving the economy viability of 
biogas production of AD 
[21,22]
. To date, many studies have focused on 
the theory and applications of upstream and downstream processes, and 
laboratory approaches for stabilizing and enhancing biogas production 
are under vigorous development. However, few studies have presented a 
comprehensive review or analysis focusing on the influence of opera-
tional parameters on process stability and operational efficiency from 
the perspective of commercial operation. 
Operational parameters, including organic loading rate (OLR), hy-
draulic retention time (HRT) and temperature, are key factors that 
determine the operational efficiency of biogas production and the pro-
cess stability of AD 
[23]
. Optimal configuration and manipulation of the