Scientific and technical recommendations on intensification of the anaerobic frog process in reactors of biogas plants

Abstract

Relevance of research. In any biotechnological process, biological agent - microorganisms, its nature and physiological and technological properties plays a major role. In this case, the temperature of the fermented mass is very important factor in the effective process of fermentation. Methane fermentation begins at 6 ° C. At lower temperatures, methane emissions are stopped. At the same time as the temperature increases, the gas is rapidly increased. Thus, at a temperature of 30 ° C biogas is 12 times faster than at a temperature of 10 ° С. For the stable development and life of bacteria, the deviation of temperature in the reactor biogas plant from the nominal should not exceed 2.8 ° C. The thermostabilization of the biogas plant reactor is provided by various heat exchange devices and by the appropriate thermal insulation, and the methane tanks themselves must have a minimum surface area or an underground or semi-underground location [1].

Analysis of recent research and publications. Research of the biogas output at straw digestion with the addition of manure of cattle was conducted by prof. Shvurov S.A. and associate professor Polischuk V.M. within the framework of the initiative research "To substantiate the parameters of the substrate for the improvement of the efficiency of biogas technologies" (DR №0116U008083) and the research project "Development of innovative high-efficiency technologies for the collection and processing of energy crops for biogas plants" (DR № 0117U001254) commissioned by the management of biogas the factory (group of companies "Silhospprodukt") in willage Rokitne of the Kyiv region, which plans to implement the technology of preparation and use of organic fertilizers from biogas plant planned capacity of 2.38 MW.

The research objective is to increase the efficiency of anaerobic digestion processes by transitioning from the thermotolerant temperature regime of the reactor to the thermophilic by using the excess heat energy of the cogeneration unit in the summer.

Materials and methods of research. The efficiency of biogas production was determined depending on the type of raw material, the presence of osobrastrates, the temperature regime of the biogas plant, the presence or absence of mixing of the substrate. Experimental researches were carried out in parallel on two biogas plants in the educational and scientific laboratory of bioconversions in the APC of the NULES of Ukraine. The biogas installation consists of a 30-liter methane tank containing a mixer and heating device, and a gas cylinder.

Influence of temperature regime. The degree of influence of the temperature regime of methane tanks on the efficiency of biogas production was studied on the example of methane fermentation of cattle manure with a humidity of 93.4% at temperatures of 55, 50, 45 and 40 ° С. From the results of the study it can be seen that with increasing methane tanks the biogas output increases. Thus, the average yield of biogas at 55 ° C is 7,103 cm³ / day, at 50 ° C. - 5226 cm³ / day, at 45 ° C. - 4893 cm³ / day, at 40 ° C. - 2041 cm³ / day.

Analysis of data tests shows that the heating time of the substrate to a temperature of 35 ° C is - 46 hours, and to a temperature of 54 ° C - 68 hours. Daily consumption of solid fuel (barn) is - 31 kg / day, efficiency of a fuel boiler – 78.5%. Manure productivity is 0.5 - 0.7 t / day. According to biogas - 6.5 ... 11.5 m³ / day. The output of biogas in the mesophilic mode is 6.5 m³ / day., In the thermophile - 11.5 m³ / day.

As a result, the output of biogas during the transition to the thermophilic fermentation regime, with the same volume of the reactor, will increase at least 1.5 - 2 times, which additionally leads to a decrease in the cost of 1 kW of installed power , which implies that if the main purpose of waste digestion is to obtain biogas, the thermophile regime is more rational.

Effect of thermal insulation effectiveness. Investigation of the instability of the temperature regime of the reactor caused by the change in ambient temperature on the biotechnological process was carried out on the example of the biogas plant with an average time temperature in the reactor T = 35 ° С (deviation within the limits - 1.5 ° C.

A higher level of thermal stability of the reactor is provided by stabilizing not only the average temperature of the substrate in the reactor, but also the temperature of the inner wall of the reactor. That is, simultaneous compliance with two conditions is necessary: -1,5≤ ΔT≤1,5; -1,5≤t_vn≤1,5. The performed numerical studies showed that in the case of BSU operation in the same temperature mode when applying the recommended isolation, the temperature of the inner wall of the reactor goes beyond -1.5≤t_vn≤1.5 during the transition period of the year. Therefore, in order to provide complete thermal stability, it is proposed in the warm period of the year to change the thermal regime of the biogas plant from the mesophilic (T = 32 ... 35 ° C) or thermotolerant (T = 39 ... 42 ° C) thermophilic (T = 52 ... 54 ° C) or, subject to increasing the thickness of the insulation layer, switch to permanent use. The energy opportunity and feasibility of such a transition is described below.

Results of the research and their discussion. One of the ways to improve the energy efficiency of the BSU is to reduce the cost of energy for ensuring its technological processes. In accordance with the proposed design and technological scheme of the energy-saving biogas installation, the energy costs for thermostabilization of anaerobic fermentation of biomass and heat loss coatings may be compensated by the excess heat energy of the cogeneration unit in the summer.

Taking into account the fact that all four reactors of the customer, the total volume of biomass in which 14445.3 m³, operate in thermotolerant mode, as well as the cycle of loading of reactors 2x100 t / day at an interval of 12 hours, the thermal power of the installation for maintenance and warming a new portion of biomass in 1 hour will be:

1. During the cold season (at an average temperature of the outside air -5 ° C) [6]: for thermotolerant mode (40 ° С) - 6.5 MWh, thermophilic (53 ° С) - 8.38 MW • year

2. In the summer period of the year (at an average temperature of the outside air +17 ° C) [6]: for thermotolerant mode of operation - 3.32 MWh, thermophile - 5.2 MWh.

The energy used to heat the reactor for 12 hours, in all other equal conditions, during its operation in the thermophilic mode should be: in the first version 0.83 MWh, in the other 0.52 MWh.

Taking into account the difference in the consumed thermal power of the first and second variants, as well as the nominal thermal power of the cogeneration unit, the operating time of the latter, with the operation of reactors biogas plant in thermophilic mode, should be (subject to the heating of a new portion of biomass):

- for 1 hour - both in the first and in the second variant - in 0,8 hours;

- 12 hours: 0.35 hours in the first version, and 0.22 hours in the second one.

That is, the nominal thermal power of the cogeneration unit is sufficient to cover the heat load and the transition of the biogas plant reactors from thermotolerant to the thermophilic regime even during the cold period of the year. Such a step does not require significant investment, and the economic effect of its implementation will increase the output of biogas, at least 1.5 - 2 times, and hence, to reduce the cost of 1 kW of installed capacity.

However, preliminary calculations show that an additional increase in the thickness of the thermal insulation layer by 50 mm helps to reduce heat losses through the surface of the reservoir by 15 ... 20 %, which in turn leads to an additional reduction of energy consumption, which, for example, the mesophilic mode of operation, is 3.6 % or 21 kWh / m³. The payback period of the event - less than 6 months.

As a result, the analysis of the obtained results shows that the energy expenditures on the intensification of the processes of anaerobic fermentation of biomass for the climatic conditions of the Kyiv region can reach up to 250 kWh / year per 1 m³ of mesophilic regime installations and almost 400 kWh / year per 1 m³ - for the thermophilic regime of the biogas plant. These costs can be offset by using excess heat energy of the cogeneration unit in accordance with the proposed energy saving scheme of the biogas plant and the design of the reactor with improved thermal insulation.

Conclusions and perspectives:

1. Increasing the temperature regime of methane fermentation leads to an increase in the output of biogas, but also increases the cost of heating the substrate.
2. In the summer and autumn transition to the thermophilic fermentation regime, with the same volume of methane, the biogas output will increase by at least 1.5-2 times, which additionally leads to a decrease in the cost of 1 kW of installed capacity.
3. It was determined that when the thermostabilization of the temperature regime of the reactor BSU (T = 32 ... 35 ° С) is maintained, with the required condition -1,5≤ΔT≤1,5, the insulation with thermal resistance with Ris = 2 ... 3 (m² · K) / W.
4. The energy consumed for heating the reactor within 12 hours, in all other equal conditions, during its operation in the thermophilic mode shall be: in the winter period, 0,83 MWh, in the summer - 0,52 MWh.
5. It was established that an increase in the thickness of the thermal insulation layer by 50 mm contributes to a decrease in thermal losses through the surface of the reservoir by 15 ... 20 %, which in turn leads to a decrease in energy consumption, which, according to the mesophilic operating mode, is 3.6% or 21 kWh / m³. The payback period of the event - less than 6 months.
6. The operating time of the cogeneration unit, which should be (provided that the new portion of the biomass is warmed up) is calculated: for 1 hour - both in the first and in the second variant - 0.8 hours; in 12 hours: 0.35 hours in the first version, and 0.22 hours in the second one.
7. It is shown that the nominal thermal power of the cogeneration unit is sufficient to cover the heat load and carry out the transition of the reactors of the biogas plant from thermotolerant to the thermophilic regime, even in the cold period of the year.
8. It was found that energy expenditures on the intensification of processes of anaerobic fermentation of biomass for the climatic conditions of the Kiev region can reach up to 250 kWh / year for 1 m³ of installations for the mesophilic regime and almost 400 kWh / year for 1 m³ - for the thermophilic mode of operation biogas plant.