Intensification of production of biogas by exposure of electromagnetic field and ultrasound

Abstract

The growing shortage of fuel resources and the increase in the cost of traditional fuels (coal, petroleum products, natural gas, etc.), highlights the urgent need to find alternative sources of biogas - a mixture of 65% methane, 30% carbon dioxide, 1% hydrogen sulfide and minor impurities of nitrogen, oxygen, hydrogen and carbon monoxide. In 1 m3 of biogas there is an energy equivalent to 0.6 m3 of natural gas, or 0.74 and 0.66 liters of oil or diesel fuel, respectively.

In the production of methane, a significant amount of energy is consumed to support the fermentation process, namely: compliance with the required temperature regime inside the bioreactor and mixing the substrate without which the process efficiency is significantly reduced. The average consumption of energy produced to ensure the process in the bioreactor itself in the latitudes of Ukraine is: 15-30% of thermal energy, and 6-9% of additional electrical energy. At the same time, after clearing biogas from non-combustible and harmful impurities, its cost approximates to the cost of natural, which may be economically inexpedient.

 It is known that the formation of biogas occurs at temperatures from 0 ° C to 97 ° C, and in this interval, conditionally, three temperature regimes  are distinguished: psychophilic (up to 20-25 ° C), mesophilic (25-40 ° C), and thermophilic (over 40 ° C). The first one is observed in unheated installations in which there is no temperature control, and the most significant gas evolution occurs at 23 ° C. The second and third, for which the optimal are 34-37 ° С and 52-54 ° С, respectively, are inherent to biogas plants working on mixed raw materials of animal origin. In this case, the intensity of methane emissions increases with increasing temperature and is limited to the formation of free fermentation of ammonia, which slows down the process. Therefore, in practice, the most recent regimes have become widespread, the advantage of which is the increased rate of decomposition of raw materials and a higher yield of biogas, as well as virtually complete destruction of pathogenic bacteria contained in the raw material, which allows the use of the residues of the substrate as biological fertilizers.

The duration of the technological cycle of biomass processing in terms of energy is a determining factor in the cost of production of biogas. Depending on the selected temperature regime and the composition of the raw material, the full fermentation time may be at the following intervals, days : psychophilic (30-40 and more), mesophilic (10-20) and thermophilic (5-10). In this case, the heating time of the substrate to the maximum temperature, as a rule, ranges from 46 to 68 hours, and the amount of energy consumed during this period exceeds 50% of the total need for the cycle.

The aim of the study. To systematize and generalize the information received by domestic and foreign scientists on the research of intensification of biomethanogenesis and increase of efficiency of biogas plants due to electromagnetic and ultrasonic influence on the substrate.

 In order to achieve high efficiency of bioreactors and obtain the maximum amount of biogas per unit volume of biomass, it is necessary to create optimal technological parameters in the bioreactor . The intensity of the process of digestion and, consequently, the formation of biogas is influenced by four groups of factors: biological, physical, chemical and organizational-technological

The biological factors include: the composition of fermented biomass (the content of proteins, fats, carbohydrates, lignins); composition of microflora (number and group of microorganisms of the corresponding stage of decomposition); conditions of vital activity of microorganisms (content of harmful impurities). Physical factors include: the temperature of digestion; bioreactor pressure; hydraulic mode. Chemical factors are determined by the acidity of the medium (pH value); the content of volatile fatty acids in the digestible mass; the volume and composition of the resulting biogas. Organizational-technological factors include: the dose of daily loading of new portions of the digestible mass; load of ashless substance; the content of non-biodegradable substances.

Approximately, the volume of expenses for own needs for a particular installation can be estimated by the technical characteristics of the manufacturer of the specified equipment, presented in the documentation, but in their absence or in order to clarify for a certain locality the effectiveness of non-transparent bioreactor can be established by the method of calculation experiment . As an example, the methodology determined the thermal balance for methane tanks, in which the methane formation cycle lasts for 19 days, subject to compliance with the thermophilic regime and typical climatic conditions of the northern regions of Ukraine during the winter period.

The use of electromagnetic microwave radiation to maintain optimal thermal regimes during biomethane can achieve much higher technological effects compared to conventional heating systems .

Research results. After processing various plant substrates to fermentation in the fermentation modeling chambers, the best results on the efficiency of biogas production and biogas quality were recorded in a series of corn silage and grass silage. In the first case, the amount of methane produced in the technological system, stimulated by microwave radiation, amounted to 15.26%, while in the second part of the substrate 12.62% more methane was produced in reactors without processing .

When processing the substrate (waste water) by the ultrasonic field, the biogas output increased by 20-24% compared to the untreated, with a maximum yield of biogas of 0.92 dm3 was observed on the 9th day of fermentation. The content of volatile fatty acids decreased to 139 mg CH3COOH dm-3 for 20 days. To date, the results show that the effectiveness of using ultrasound depends on the timing of the synchronization, type, as well as power and frequency

Experimental studies have shown that a significant effect on the process

methane fermentation has a magnetic field with induction of 0,38 T. A positive effect was achieved at the lowest magnetic flux value of 173 × 10-3 [mVb], and the most effective option, with a magnetic flux of 3,016 × 10-3 [mV],. At the same time, the biogas output increased by 14% compared to the untreated substrate

Electrokinetic decay is one of the high-voltage electrical methods. In the electric field, cell walls are deformed, making their contents readily available to bacteria . The voltage value ranges from 10 to 100 kV. the value of the generating current does not exceed 250 mA. The destruction of the biomass structure in the process of electrokinetic decay results in a reduction in the storage time of the substrate in the reservoir and the acceleration of biogas production, with the largest production of biogas accounting for 18%. Secondary but very significant for the energy balance effect is a noticeable decrease in energy consumption by mixers in methane tanks. This is due to a decrease in the viscosity of biomass after decomposition, and can result in energy savings of up to 20-30% of the previously mentioned consumption.

The effect is from the pre-treatment of the substrate (bird droppings) with microwave radiation and ultrasound. Four reactors with a volume of 2 liters were used for the study. Reactors were operated with a hydraulic holder for 30 days and mixed twice daily. Studies have shown that treatment of substrates with microwave radiation and ultrasound caused an increase in biogas production by about 10% -12%

Conclusions and perspectives. As a result of the analysis of scientific research carried out on different substrates with the use of electric, microwave and ultrasonic effects, it has been established that there is a direct dependence of biological effects in them on the characteristics of the indicated fields: intensity, frequency, pulse shape, type of modulation, and duration. It has been established that correctly selected parameters of the latter can stimulate the process of production of biogas and increase the efficiency of biogas plants in general. However, due to the unsystematic and selective nature of the experiments, as well as their obviously accidental nature and not optimality in terms of the energy criterion, there is a need for further clarification and generalization of the parameters of the intensification of biomethanogenesis at each of its stages, which will, in the long run, create a tool for dynamic control of this process on more energy efficient level.