From Low-Grade Biomass to Clean Energy: Biomethane

English translation of the original Chinese article. Publication dates and the extent of recovered text are preserved. Figures retain their original labels. Read the Chinese original.

In recent years, the international community has faced energy shortages while environmental pollution has become increasingly serious. Converting low-grade biomass, such as crop straw and poultry manure, into renewable energy has become an international priority in meeting future demand for clean energy. China has abundant crop-straw resources as a traditionally agricultural country. Incomplete statistics indicate that crop-straw production exceeded 700 million tonnes in each of 2015 and 2016 [1]. Yet much of this straw was burned where it lay, wasting biomass energy and causing air pollution and other environmental problems.

Producing Biomethane

Through anaerobic microbial fermentation, low-grade biomass such as crop straw and poultry manure can be converted into biogas. This can then be purified into high-purity biomethane for use as vehicle fuel, providing clean energy while reducing pollutant emissions.

Although many breakthroughs have been made in China and abroad in producing biogas from crop straw through anaerobic fermentation, large-scale biogas projects still face numerous challenges. These include effective physical, chemical or biological pretreatment of crop straw to improve microbial utilization; selecting and engineering microbial strains that use straw efficiently; designing anaerobic bioreactors and developing fermentation processes; and precise monitoring and control during microbial anaerobic fermentation.

Meanwhile, biogas from microbial anaerobic fermentation typically contains 50%–65% methane and 35%–50% carbon dioxide, with very small amounts of water and hydrogen sulfide. Carbon dioxide lowers the heating value of biogas and can corrode gas-transport pipelines. To meet market demand, biogas must therefore be purified into high-purity biomethane with a carbon dioxide content below 2%. Purification approaches include high-pressure water scrubbing, chemical adsorption, membrane separation and pressure-swing adsorption [2]. Low-cost purification that also effectively controls carbon dioxide emissions has become a research focus.

(Image sourced from the internet)

Relevant Measures in China

Article 77 of the Law of the People's Republic of China on the Prevention and Control of Atmospheric Pollution, revised in 2015 and effective from 2016, states that provincial, autonomous-region and municipal governments must designate areas where open burning of straw, fallen leaves and other substances producing smoke and dust pollution is prohibited. The General Office of the State Council issued Opinions on Accelerating the Comprehensive Utilization of Crop Straw. The National Development and Reform Commission and Ministry of Agriculture jointly issued a notice requiring provincial, municipal and county governments to complete plans for comprehensive straw utilization. These plans explicitly call for supportive policy frameworks, an industrial layout for straw utilization, and key projects. Although many problems remain, we are confident that governments, farmers, universities and research institutes with advanced technologies can work together to overcome difficulties, solve problems and make effective use of low-grade biomass.

References

[1] L.K.; L.R.. A review of methane production from agricultural. Renew.Sust.Energ.Rev.2016, 54:857-865.

[2] Z.J.; C.X.. Effects of various factors on biogas purification and nano-CaCO3 synthesis in a membrane reactor. Energy.2010, 53(4):1702-1706.

Additional References

• Official full text of the law, Article 77
• Ministry of Ecology and Environment explanation of authority to designate no-burning areas