enhancement of biogas plants efficiency by the improvement of hydraulic mixing
DESCRIPTION
ENHANCEMENT OF BIOGAS PLANTS EFFICIENCY BY THE IMPROVEMENT OF HYDRAULIC MIXING Mixing appears to be necessary for effective operation of biogas plants and consistency of feedstock fermentation inside the reactor. Mixing plays an important role as it leads to more close connection between microorganism and a substrate, even distribution of рН and temperature, and furthermore, prevents stratification of the sediment and crust formation and helps to release biogas bubbles. Within the scope of this work the study of hydraulic mixing for a tank with two input pipes was carried out (pic.1). The substrate stream from the side input pipe ensures sediment mixing at the bottom of the tank. The substrate stream from the upper input pipe prevents crust build up on the surface of the liquid. At a certain distance from the input pipe there is a reflecting partition to reduce the opportunity for crust formation.Assessment of mixing quality was carried out taking into account the intensity of biogas formation. Dependencies of mixing quality change on time and substrate input were received. Numerical investigations of organic substrate circulation in tanks with different location of input and output pipes show that the given location of the pipes provides better mixing (mixing rate increases by 3 times). Reflecting partition prevents from crust formation and influences mixing quality. In this case mixing improves with the increase of distance from upper input and of reflecting partition radius. Efficiency of anaerobic digestion process improves at the expense of homogeneous fermentable organic matter maintenance that is achieved by modification of the reactor components.TRANSCRIPT
The Branch of Research Center of Power Engineering Problems of Institution the Russian Academy of Sciences the Kazan Scientific
Centre RAS
ENHANCEMENT OF BIOGAS PLANTS EFFICIENCY BY THE IMPROVEMENT OF
HYDRAULIC MIXING
I.A. Trakhunova
Implementation of environmental protecting measures to reduce toxic emissions into air, water and soil;
Increasing farm profits by using biogas for heat and power production, as well as offering to the market organic fertilizers with stable properties (absence of smell or repeated rotting);
Improvement of reliability and safety in energy supply of cattle breeding and poultry farms by using its own renewable energy sources;
Improvement of soils fertility and fodder area crops by using fertilizers, as a source of biogenic elements and humus.
THE MODERN IMPORTANCE OF BIOGAS
Mixing appears to be necessary inside the reactor for:
ensuring efficient use of the total volume of digester;
preventing separation of sludge;
preventing crust formation;
equalizing the temperature field;
equalizing the concentration of metabolites producing during fermentation.
mechanical mixing
pneumatic mixing
hydraulic mixing
TYPES OF MIXING SYSTEMS
THE PURPOSE OF STUDY:
the intensification of an anaerobic fermentation under various types of hydraulic mixing
Tasks:1. The study of an influence of
the inlet and outlet pipes location on the quality of mixing.
2. The study of an influence of the a reflecting partition on the crust formation.
Anaerobic digestion plant
1 – organic wastes tank; 2 –milling device; 3 – mixing device for feedstock with sludge; 4 - heat exchanger; 5 – feeding system; 6,7 - digester feed pipes; 8 – reactor; 9- reflecting partition; 10 – biogas extraction system with a vacuum pipe; 11 – sludge removal system; 12 – effluent pipe with flange and valve.
-the hydrodynamic regime of the circulation of the substrate in digester can be considered laminar;
-consider a medium (the slurry) is expected homogeneous fluid with density and coefficient of effective viscosity depending on the concentration of the dispersed phase;
-the mixing is non-stationary and isothermal.
Assumptions :
6
Mathematical model
Mathematical model
Boundary conditions:
2, 3 – the most popular types of load 1 – type of load suggested in this research
Numerical investigations of organic substrate circulation in tanks with different location of input and output pipes
Type of load 1 Type of load 2 Type of load 3
Distribution of bulk concentration fields over time, [s]
Type of load 1:
Type of load 2:
Type of load 3:
t=0 t=100 t=300 t=600
To estimate the quality of mixing the following criterion is suggested :
1 - type of load 12 - type of load 23 - type of load 3
Q = 0.0003 m3/s
Location of the pipes influences mixing quality
Reflecting partition prevents from crust formation
Values of h and r0 used in the calculation
1- h=0.96, r0=0.1;
2 - h=0.9, r0=0.1;
3 - h=0.85, r0=0.1
4 - h=0.96, r0=0.2;
5 - h=0.96, r0=0.05;
6 – without reflecting partition.
The velocity distribution on the liquid surface:
h=0.85, r0=0.1
h=0.9, r0=0.1
h=0.96, r0=0.1
Dependencies of mixing quality change on time t [s] and substrate input Q [m3/s]
1- h=0.96, r0=0.1 Q=0.0002;
2 - h=0.9, r0 =0.1, Q=0.0002;
3 - h=0.85, r0 =0.1, Q=0.0002; 4 - h=0.96, r0 =0.05, Q=0.0002;
5 - h=0.96, r0 =0.2, Q= 0.0002; 6 - h=0.96, r0 =0.1, Q=0.0001; 7 - h=0.96, r0 =0.1, Q=0.0003.
Conclusion:
The numerical studies and comparative analysis of the circulation of the organic substrate and the corresponding distribution of the bulk concentration of dispersed phase in tanks with different locations of inlet and outlet pipes were carry out.
It is shown that the proposed pipe location provides better mixing and the reflecting partition prevents free surface of the mixture on the crust formation.