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With the depletion of conventional energy sources, the development and utilization of new energy is undoubtedly the only way out. Energy plants are a kind of renewable biomass resources. Among them, energy grasses have large biomass and are rich in lignocellulose. The conversion of lignocellulosic materials into biogas with high calorific value through anaerobic fermentation is currently the most developed bioenergy. One of the prospects.
1 Energy Grass Collection and Cultivation
To find a suitable herbaceous energy plant suitable for anaerobic fermentation and biogas production, a large amount of research work needs to be done. It is also necessary to use breeding and biotechnology to improve the target plant in order to increase the conversion rate of biomass energy and improve the quality of transformed products.
In the 1980s, the United States and Europe have systematically screened and studied perennial herbaceous plants as energy plants, cultivated special-purpose energy grass varieties, and realized large-scale planting and development and utilization. In 1984, the United States launched the "Energy Grass Research Program," which focused on screening 35 species of herbaceous plants and obtained 18 kinds of energy grasses with potential for development and utilization. In Europe, about 20 species of perennial herbs were studied. Finally, Miscanthus sinensis, Phalaris arundinacea, Panicum virgatum and Arundo donax were selected for further research.
China has a vast area, rich and diverse plants, and a wide range of herbaceous energy plants. A large number of researches have been carried out on the collection and selection of energy grass germplasm resources, and important research results have been achieved.
The Lanzhou Institute of Animal Husbandry and Veterinary Medicine of the Chinese Academy of Agricultural Sciences began collecting, identifying and domesticating cultivation of Pennisetum alopecuroicles germplasm resources during the Eighth Five-Year Plan period. A total of seventy-seven materials were collected. In the past 10 years, the Beijing Prairie and Environment Research and Development Center has collected 208 energy grass resources including switchgrass, miscanthus, arundo, Achnatherum splendens and Pennisetum hybrid. Yan Jiajun et al. proposed to use swamp as an energy plant through the collection of Saccharum arundinaceum in the Lancang River Basin, the Qingyi River Basin and the Lancang River Basin in Sichuan, and observation of its biological characteristics.
If the energy plant cell wall contains higher lignin, it will affect the conversion efficiency of its biomass energy. Chang Ruina and other clones obtained the key enzyme genes CCoAOMT and 4CL for Miscanthus floridulus lignin synthesis, which will help to further improve energy plants. Mangana is a relatively new type of energy conversion to biomass energy, which requires the support of breeding and biotechnology. For switchgrass, the work to be done in the future is to increase the number of high-yield hybrids and use transgenic technology to increase yield and cellulose content.
2 Raw grass planting and harvesting
Energy and grass raw materials are a major factor influencing the development of the industry. At present, many countries have begun to plant energy grass in large quantities. More than 90% of the land for agricultural production in Ireland has been planted with energy grass. The United States plans that by 2030, biomass energy produced by perennial energy plants will account for 35.2% of all bio-renewable energy sources.
After energy plants are harvested at different times, 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 harvest time on the biogas production of various energy plants such as Helianthus tuberosus, Phleum pratense-Trifolium pratense, and grass reeds. Most plants produce more methane per ton of wet weight. Massé et al. studied the changes in the amount of methane produced by silage and anaerobic fermentation of switchgrass and grass reeds during mid-summer, late summer and early autumn. Delaying harvesting will reduce biogas production. In the entire growth cycle of energy grass, which factors affect its biogas production still needs more in-depth research.
3 raw material pretreatment
Due to the high crystallinity and degree of polymerization of the lignocellulosic feedstock, pretreatment of the feedstock prior to conversion is required to increase product yield. The main effect of pretreatment is to change the structure of natural fibers, reduce the degree of polymerization and crystallinity of cellulose, destroy the binding layer of lignin, hemicellulose, and remove lignin. Pretreatment methods include physical methods, chemical methods, and biological methods.
In recent years, there have been many studies on the pretreatment of energy grass fermentation. Zou Xingxing et al. performed steam explosion treatment on Spartina alterniflora before anaerobic fermentation. The results of fermentation experiments showed that with the increase of steam explosion pressure, the cumulative gas production rate showed a downward trend. 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 conditions of the production of biogas after pretreatment of temperature, sonic degradation, alkalinization, and high pressure in switchgrass harvested in winter and summer. The final conclusion is that temperature, sonic degradation, and high pressure have no significant effect on the biomass of switchgrass harvested in winter. However, it can increase the amount of biogas produced by switchgrass during summer harvest. Li Lianhua et al studied the effects of steam heating, ultrasonic waves, and freezing and thawing on the anaerobic fermentation performance of perennial king grass (Pennisetum purpureum × P.americanum) in South China. In contrast, steam heating can significantly reduce the crystallinity of Wangcao and improve biogas production. Gas production rate. Li et al. used heat treatment and microwave to perform anaerobic fermentation pretreatment on Pennisetum japonicum. The results showed that heat treatment increased the anaerobic fermentation of biogas production, while microwave treatment played an opposite role. Xiao Zheng et al. used biogas slurry to process the Pennisetum sinese Roxb. The cumulative gas production over 15 days was 406 ml/TS.
4 Microbial Inoculants Category
Since microorganisms play a vital role in the anaerobic fermentation process, and the amount of microbial flora attached to the energy grass itself is small, a large amount of inoculum must be prepared when conducting energy anaerobic fermentation to produce biogas.
Methanogens are widely distributed in nature. For example, fresh animal excrement, sewage treatment plant sludge, and spoiled river mud can meet the requirements of energy grass fermentation biogas production. Song Li compared the potential of anaerobic fermentation of sheep manure, duck manure and rabbit manure to produce biogas. It was found that duck dung was the best, followed by sheep manure, and rabbit manure was the worst. Liu Dejiang et al. set three cow manure fermentation concentration gradients (6%, 8%, and 10% total solids content) to study the effect of anaerobic fermentation on methane and hydrogen sulfide levels in biogas production. The results showed that 8% For the best concentration of fermentation. Xie et al set a 1:0, 3:1, 1:1, 1:3, and 0:1 mix ratio of swine manure and silage to study the effect of manure ratio on anaerobic fermentation production of biogas. The results showed that The 1:1 methane content in biogas is the highest.
5 fermentation conditions control
Anaerobic fermentation system temperature, initial pH value and the concentration of raw materials in the system and other factors have been the research field of anaerobic fermentation of biogas production. Under normal circumstances, the anaerobic fermentation reaction can be carried out faster at higher temperatures because the metabolism of microorganisms is faster, but the stability of the reaction system is poor at high temperatures.
Liu Ronghou et al. used swine manure as a raw material to study the effects of room temperature, medium temperature (37°C), and high temperature (52°C) on anaerobic fermentation production of biogas. The results showed that in the initial and middle stages of fermentation, room temperature and high temperature experimental groups of microorganisms The activity was affected and the methanation reaction was inhibited. The daily gas production in the high-temperature experimental group was significantly higher than that in the other two groups. Zhu Hongguang et al. set the temperature group (35±2)°C and the room temperature group to 15～33°C to study the biogas production of Spartina alterniflora and found that Spartina alterniflora was suitable as the raw material for biogas production. The average daily gas production rate of medium temperature group was 4.58ml /(g・d) The average daily gas production rate in the normal temperature group was 2.54ml/(g・d), and the difference was very significant. Zhao Hong et al. set seven pH gradients (5.5, 6.0, 6.5, 7.0, 7.5, 8.0, and 8.5) and analyzed the effect of pH on the gas production and gas production characteristics of anaerobic fermentation of fresh swine manure. The value of 6.5 group is the fastest, the total gas production of pH 7.0 group and pH 6.5 group is the highest, and the methane content of pH 7.0 group is the highest. It is concluded that the pH value of the fermentation system can promote the start of anaerobic fermentation when the pH value is 6.5-7.0. To improve the quality of biogas. Wang Xiaoman used Poa annua L., stems of Sechium edule and leaves of tomato (Solanum lycopersicum) as raw materials for fermentation to study the gas production potential of the three types of raw materials. The order of main factors affecting gas production was inoculation volume>fermentation concentration>carbon-nitrogen ratio. The main factor affecting methane content was inoculation quantity>carbon nitrogen ratio>fermentation concentration.
6 gas composition analysis
The content of methane and carbon dioxide in the biogas is an important parameter that reflects the operational status of the anaerobic fermentation process. In order to maximize the production efficiency of the anaerobic fermentation process, the entire production process must be under optimized operating parameters and environmental conditions. At present, the main methods for the detection of biogas components include austenitic gas analysis methods, gas chromatography GC analysis methods, thermal catalytic element detection methods, and infrared detection methods.
In the measurement of methane range, the thermo-catalytic element detection method is 0 to 5%, and the remaining three kinds of measurement ranges are 0 to 100%. When analyzing the gas composition, the method of austenitic gas analysis and gas chromatography GC analysis method can also determine carbon dioxide and Oxygen content, infrared detection method in addition to the determination of carbon dioxide and oxygen content, but also the determination of hydrogen sulfide content, and the thermal catalytic element detection method can only determine the methane content; 4 analysis methods of gas analysis time were 1h, 30min, 30s, 5s; Overall, infrared detection methods have obvious advantages in all aspects. For rough estimation, the methane content in the gas can be determined by observing the flame color of the biogas combustion.
Outlook
The world’s energy problems have become increasingly prominent, forcing countries to develop and use new energy sources to ease the shortage of domestic energy. China's research on energy conversion is also underway, but it is still in its infancy. It still requires the continued efforts of the research workers, as well as relying on national policies to promote the planting of energy grasses, and the industrialization of energy grass conversion to contribute to the national energy issue.
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