EXPERIMENTAL STUDY OF BIOGAS OUTPUT PARAMETERS IN THE BATCH ANAEROBIC DIGESTION OF STRAW
Given the scale of the formation and prevalence, as well as taking into account compliance with the set of criteria for sustainable development in the production of energy from renewable sources, the production of biogas from straw looks promising in Ukraine.
The effective production of biogas from straw in CSTR reactors requires its pretreatment. The main purposes of this pretreatment are to increase bioavailability of cellulose as a result of destruction of ligno-cellulosic complexes, increase of the specific surface and porosity of the material, decrease of crystallinity of cellulose. Such pretreatment should lead to an increase in the rate of hydrolysis of organic compounds of straw, the bioavailable biogas yield potential, and the reduction of hydrophobic properties. The promissing method in regard to biogas production is the straw extrusion (straw pellets production), which combines fine mechanical grinding and thermal effect.
In this study the biogas yield potential from wheat straw samples with a linear particle size L=10–150 mm, mechanically milled wheat straw with a linear particle size L<2 mm and wheat straw processed by extrusion (in the form of pellets with a diameter D=6,4 mm) was experimentally investigated in batch reactors at a temperature of 36±1°С. Also samples pellets of wheat straw (75%) and soybean (25%), as well as rape straw pellets were studied. All samples were inoculated at the straw VS to inoculum VS ratio 0,50–0,55 and initial concentration of the samples 25,1±0,4 gVS∙kg-1.
It was found that reducing the linear sizes of wheat straw fractions leads to an increase in the first order rate constant of the biogas from 0,060±0,007 day-1 to 0,101±0,006 day-1, but practically does not affect the amount of bioavailable potential of the CH4 yield which was estimated at 191 LNCH4∙kg-1VS.
Wheat straw extrusion allows to increase the specific CH4 yield by 10–16%, extrusion of mixture of wheat straw (75%) and soybean (25%) by 25–33%. By the first order rate constant, the most effective was the sample of extruded mixture of wheat straw and soybean k=0,131 day-1, and for samples of wheat straw and rape straw, constant k was estimated at the same level 0,121–0,122 day-1.
Thus, reducing the linear size of straw fractions can improve the kinetics, but does not affect the amount of bioavailable biogas yield potential. Pre treatment of straw by extrusion allows to improve the kinetics, even in comparison with fine mechanical shredding, and at the same time allows to significantly increase the bioavailable potential of biogas yield.
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