The latest research progress of biogas production from energy grass fermentation

With the growing depletion of conventional energy, the development and utilization of new energy is undoubtedly the way out. Energy plants are a renewable biomass resource in which the energy grass is large in biomass and rich in lignocellulose, and the conversion of lignocellulosic materials into high calorific value by anaerobic fermentation is the most developed bioenergy One of the ways of foreground.

01 Energy grass resources collection and cultivation

Looking for a herbaceous energy plant suitable for anaerobic fermentation to produce biogas, it is necessary to do a lot of research work and to improve the conversion rate of biomass energy and improve the quality of the transformed products by breeding and biotechnology.

In the 1980s, the United States and Europe had systematic screening and research of perennial herbs as energy plants, and cultivated special varieties of energy grasses, and realized large-scale cultivation and development and utilization. In 1984, the United States launched the "Energy Grass Research Program", focused on 35 kinds of herbs were screened, access to 18 kinds of energy and development potential. In Europe, about 20 species of perennial herbs were studied. Finally, four kinds of energy grasses of Miscanthus sinensis, Phalaris arundinacea, Panicum virgatum and Arundo donax were selected for further study.

China has a wide range of plants, rich and diverse plants, widely distributed, a wide range of herbal energy plants in the energy grass germplasm collection and screening has carried out a lot of research work, and achieved important research results.

Institute of Livestock and Veterinary Medicine, Lanzhou Institute of Animal Science and Veterinary Medicine, Chinese Academy of Agricultural Sciences began to collect, identify and acclimatize cultivars of Pennisetum alopecuroicles germplasm resources during the "Eighth Five-Year Plan" period. A total of seven 47 materials were collected. For nearly 10 years, the Beijing Grass Industry and Environmental Research and Development Center has collected 208 kinds of energy and grass resources including switchgrass, miscanthus, arundinaceae, Achnatherum splendens and Pennisetum hybrid. (Institute of Plantation, Chinese Academy of Sciences, Beijing 100083, China) Based on the collection of Saccharum arundinaceum and its biological traits in Minjiang River Basin, Qingyi River Basin and Tuojiang River Basin in Sichuan Province, it is suggested that maize be developed and utilized as an energy plant.

If the energy plant cell wall contains high lignin, it will affect its biomass energy conversion efficiency. Chang Rina and other clones have obtained the key enzymes of the Miscanthus floridulus lignin synthesis, CCoAOMT and 4CL, which will help to further improve the energy plants. Mango energy grass is transformed into biomass energy is relatively new type of industry, requires breeding and biotechnology support. For switchgrass, the future work is to increase the number of varieties of high-yielding hybrids and the use of transgenic technology to increase yield and cellulose content.

Raw material grass planting and harvesting

Energy grass raw materials is a major factor affecting the development of the industry, many countries have begun to plant a large number of energy grass. More than 90% of the land for agricultural production in Ireland is planted with energy grass. The United States plans to 2030, perennial energy plant biomass generated by all bio-renewable energy will account for 35.2%.

After the harvest of energy plants in different periods, the amount of biogas produced by anaerobic fermentation is different, mainly because the chemical composition of plants changes with the growth time. Lehtomki et al. Studied the effects of the harvest period on the biogas production of Helianthus tuberosus, Phleum pratense-Trifolium pratense, and A variety of energy plants, and found that as the energy plants mature , Most plants produce increased biogas production per ton of wet weight. And Massé et al. Studied the yield of switchgrass and silage in the middle and late summer and late autumn, and the biogas produced by the silage in the anaerobic fermentation was the highest, Delayed harvests reduce biogas production. What factors are needed to affect their biogas production throughout the growth cycle of energy grasses requires more in-depth research.

Pretreatment of raw materials

As the lignocellulosic feedstock has a high degree of crystallinity and degree of polymerization, the raw material is pre-treated prior to conversion to increase the yield of the product. The role of pretreatment is to change the structure of natural fibers, reduce the degree of polymerization of cellulose and crystallinity, destruction of lignin, hemicellulose combination layer, off the lignin. Pretreatment methods are physical, chemical and biological methods.

In recent years, the research on the pretreatment of energy grass fermentation is more. Zuo Xingxing and other Spartina alterniflora were steam blasting before anaerobic fermentation. The results showed that with the increase of steam explosion pressure, the cumulative gas production rate decreased. Jackowiak et al. Studied the effect of microwave pretreatment temperature and treatment time on the anaerobic fermentation rate of switchgrass and found that only the temperature had a significant effect on it. Frigon et al. Studied the biogas production after the pretreatment of temperature, sound wave degradation, alkalization and high pressure in winter and summer. The final conclusion was temperature and sonic degradation, and high pressure had no significant effect on the biogas production of switchgrass , But can improve the summer harvest of switchgrass fermentation production of biogas. Li Lianhua studied the effects of steam heating, ultrasonic and freeze-thaw on the anaerobic fermentation performance of Pennisetum purpureum × P. americanum in South China. In contrast, steam heating can significantly reduce the crystallinity of Wang Cao and improve biogas Gas production rate. Li et al. Were pretreated with anaerobic fermentation of Pennisetum ammodendron by heat treatment and microwave. The results showed that the heat treatment increased the biogas production of anaerobic fermentation, while the microwave treatment played the opposite role. Xiao Zheng and other use of biogas slurry on the giant grass (Pennisetum sinese Roxb) for retting treatment, 15 days cumulative gas production of 406ml / TS.

04 Microbial Inoculum Category

Since microbes play an important role in the process of anaerobic fermentation, and the number of microbial flora attached to the energy grass itself is small, it is necessary to prepare a large number of inoculants when producing anaerobic fermentation of energy plants.

Methanogens are widely distributed in nature, such as fresh animal feces, sludge from sewage treatment plants, and corrupt mud and mud to meet the requirements of energy biogas fermentation. Song Li and so on the sheep manure, duck manure and rabbit manure anaerobic fermentation of biogas potential, the best duck dung, sheep feces second, rabbit feces the worst. Liu Dejiang and other three cow dung fermentation concentration gradient (total solid content of 6%, 8% and 10%) to study its anaerobic fermentation of methane and hydrogen sulfide content of the impact of the results show that 8% For the best concentration of fermentation. Xie and so on set 1: 0, 3: 1, 1: 1, 1: 3 and 0: 1 dairy manure and silage mix ratio, to study the fecal comparison of anaerobic fermentation of biogas production, the results show that 1: 1 methane in the biogas content of the highest.

05 Fermentation conditions control

The temperature of the anaerobic fermentation system, the initial pH and the concentration of raw materials in the system have been the field of research on anaerobic fermentation of biogas production. Under normal circumstances, anaerobic fermentation reaction at a higher temperature can be carried out faster, because at this time the rapid metabolism of microorganisms, but the high temperature reaction system stability is poor.

The effects of room temperature, medium temperature (37 ℃) and high temperature (52 ℃) on the biogas production from anaerobic fermentation were studied. The results showed that the microbial activity of the microbes in the initial and middle stage, Activity was inhibited and the methanation reaction was inhibited. The daily production of the experimental group was significantly higher than that of the other two groups. (35 ± 2) ℃ and 15 ~ 33 ℃ at room temperature were used to study the biogas production of Spartina alterniflora and found that Spartina alterniflora was suitable for the production of biogas. The average daily gas production rate in the medium temperature group was 4.58ml / (G? D), the average daily gas production rate is 2.54ml / (g? D), the difference is very obvious. Zhao Hong et al. Set seven pH gradients (5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5). The effects of pH on the gas production and gas production characteristics of fresh pig manure were analyzed. The results showed that the total methane production was the highest, the pH value was 7.0, the pH value of 7.0 was the highest, and the pH value of 7.0 was the highest, and the pH value of the fermentation system was 6.5 ~ 7.0, which promoted the start of anaerobic fermentation , To improve the quality of biogas. The yield potential of three kinds of raw materials was studied by using the stem and leaf of Poa annua L., Sechium edule and the stem and leaf of Solanum lycopersicum. The results showed that the accumulation rate of Kentucky bluegrass was the highest, The main factors influencing the production of gas were the inoculation amount> fermentation concentration> C / N ratio, and the main factors affecting the methane content were inoculation amount> C / N ratio> fermentation concentration.

Analysis of Gas Composition

The methane and carbon dioxide content in the biogas is an important parameter that reflects the operation of the anaerobic fermentation process. In order to maximize the production efficiency of the anaerobic fermentation process, the entire production process must be in the optimal operating parameters and environmental conditions. At present, the main methods of biogas composition testing are Achorage gas analysis method, gas chromatography GC analysis method, thermal catalytic element detection method and infrared detection method.

In the measurement of methane range, the thermal catalytic element detection method is 0 ~ 5%, the other three kinds of measurement range is 0 ~ 100%; gas composition analysis, the Austrian gas analysis method and gas chromatography GC analysis method can also determine the carbon dioxide and Oxygen content, infrared detection method in addition to the determination of carbon dioxide and oxygen content, but also can determine the content of hydrogen sulfide, and thermal catalytic element detection method can only measure the methane content; four kinds of analytical methods of gas analysis time were 1h, 30min, 30s, 5s; Overall, the infrared detection method has obvious advantages in all aspects. A rough estimate can determine the methane content of the gas by observing the flame color of the biogas combustion.

07 Outlook

The world's energy problems have become increasingly prominent, forcing countries to develop and use new energy to alleviate the shortage of domestic energy. China's energy grass transformation research work is also ongoing, but still in the initial stage, still need to study the continued efforts of workers, and rely on national policies to promote the cultivation of energy grass, to achieve the transformation of energy grass industrialization, to contribute to national energy issues.