States and challenges for high-value biohythane production from waste biomass by dark fermentation technology
Highlights
► Hythane (H2 + CH4) has attracted growing attention as clean vehicle fuel. ► We review the progress of bioH2 and bioCH4 fermentation in the past ten years. ► We discuss the feasibility of biohythane output from bioH2 and bioCH4. ► This review is intended to provide a state-of-the-art insight to biohythane.
Introduction
Hydrogen and methane are the two main gaseous energy carriers and also are widely used in the chemical industry. Each has independently attracted broad commercial interest and is highly valued. Hydrogen is regarded as the cleanest fuel. The hydrogen-based economy is described as a developing trend of future society with zero carbon emissions (Turner, 2004), as it is clean and sustainable compared with the current fossil fuel-based economy. However, the development of a hydrogen-based society has been restricted mainly by its cost-intensive processes and operations. Methane is being commonly used, not only in the chemical industry but also in transport as compressed natural gas (CNG), which has been regarded as the clean energy carrier in comparison to gasoline or diesel. Based on data from NGV (Global NGV, 2011), CNG-powered vehicles have been rapidly developed throughout the past decade, particularly in China. To satisfy the huge demand in east China, several projects, for instance the west-east natural gas transmission, have been established (Xinhua, 2004).
A mixture of hydrogen and methane is called hythane, which was trademarked by Eden (2010), HCNG or methagen (Ljunggren and Zacchi, 2010). Typically, the suggested hydrogen content in hythane is 10–25% by volume (Fulton et al., 2010). By combining the advantages of hydrogen and methane, hythane is considered one of the important fuels involved in achieving the transition of technical models from a fossil fuel-based society to a terminal hydrogen-based society (Bauer and Forest, 2001, Das et al., 2000, Fulton et al., 2010). Hythane has been used commercially as vehicle fuel in the USA and India (Das et al., 2000, Eden, 2010) and has also received much attention from many individual companies such as Volvo, Fiat, Airproducts and others. Currently, the industrial chain of hythane is mainly limited by the production and instrumentation associated with hydrogen since methane is a kind of primary energy carrier with relatively abundant sources of diverse origins, such as natural gas, landfill gas, biogas, etc. At present, hydrogen is mainly produced by physical/chemical methods, for instance, via the reforming of methane or during the production of syngas. These approaches are mostly not sustainable due to their heavy dependence on fossil based energy. The ability of providing sustainable and environmentally benign hythane is a key factor that will allow applications of hythane to attain its potential respected market position. One method is to add clean hydrogen to natural gas. Two separate gas channels with quantitative control of the gas flow rates would be required to produce hythane with the desired H2/CH4 ratio.
Another approach is to biologically produce hydrogen and methane concurrently in one system, which can be realized through a two-stage dark fermentation using biomass as the substrate (Banks et al., 2010, Liu et al., 2012, Ljunggren and Zacchi, 2010, Lu et al., 2009). If the system can reach an H2/CH4 ratio suitable for hythane, a sustainable route for the generation of so-called biohythane can be expected. In fact, the nature of the biological method guarantees the H2/CH4 ratio because it can be easily regulated by adjusting the conditions of the microbial fermentations. Therefore, biohythane production from biomass via two-stage biological fermentation can be a sustainable win–win solution, as it provides an output of renewable biological hythane and the efficient utilization of waste biomass. However, the application potential of biohythane has not yet received the attention it deserves. In addition, a large body of work focused on independent biohydrogen or biomethane production from biomass. This paper firstly summarizes the status of investigations and applications of hythane, then reviews the progress in the coproduction of biohydrogen and biomethane via dark fermentation in terms of substrate, microbial consortium, reactor configuration, product yield as well as H2/CH4 ratio by focusing on the feasibility of biohythane generation in the past ten years. This review also covers how microbial consortium control, process control and system integration influence the performance of the two-stage system. Challenges and perspectives on the development of biohythane will finally be addressed. This review is expected to provide a state-of-the-art technological insight to the study and applications of biohythane production by two-stage dark fermentation from waste biomass.
Section snippets
Hythane and its significance
Hythane has received extensive attention as a vehicle fuel since the 1980s (Bauer and Forest, 2001, Ma, 2008, Nagalingam et al., 1983). The characteristics of hydrogen and methane are given in Table 1 (Bauer and Forest, 2001). Methane (CNG) is considered to be a clean fuel for vehicle use compared to gasoline or diesel. It is, however, limited by its narrow flammability range, slow burning speed, and high ignition temperature (Table 1), which result in poor combustion efficiency and an
Process description
The end use of hythane is quite straightforward with a clear business model. The production of biohythane from waste biomass can be achieved via two-stage dark fermentation if the H2/CH4 ratio can be feasibly controlled. The traditional anaerobic methane fermentation normally consists of four steps (Gerardi, 2003): hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Microorganisms with different functions are incorporated together to establish a synthetic route for methane production
Key factors affecting the performance and application of two-stage fermentation for coproduction of hydrogen and methane
According to the published results (Table 2), the hydrogen content by volume (H2/(H2 + CH4), given that the gas mixtures are purified with the removal of CO2) via two-stage dark fermentation ranged from 0.01 to 0.75, depending on operating conditions and substrates used. The hydrogen content covered the suggested range (0.1–0.25) for hythane. A flexible and controllable H2/CH4 ratio afforded by two-stage fermentation is of great importance in making biohythane. This section will discuss the key
Challenges and possible solutions
Although there is an emerging potential technology for biohythane production from waste biomass, several challenges still need to be addressed:
Conclusions
This review linked the valorization of waste biomass and the generation of biohythane via two-stage dark fermentation and discussed the key issues affecting biohythane production. Based on its properties and the strong demand for hythane fuel, as well as the stringent requirement of treatment of biomass waste, the concept of simultaneous high-value biohythane production and valorization of waste biomass was proposed. The coproduction of hydrogen and methane from waste biomass would be a
Acknowledgements
This work was supported by the Project of the National Basic Research Program of China (973 Plan) (2011CB707404), NSFC-JST Cooperative Research Project (21161140328), NSFC project (21106080) and Chinese Universities Scientific Fund (2012RC030).
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