Study On The Impact Of Grinding Pretreatment On Scum Formation During Straw Biogas Fermentation

Cellulose feedstock including crop straw, low in density and hard to decompose, may float andcrust on biogas slurry, and limit its utilization efficiency in biogas production. This article employs theexisting fermentation process of straw biogas project and uses the straw of three major crops in Chinasuch as rice, wheat and maize as feedstock. By mechanical pretreatment method, straw is cut into 5cmand 2cm and the size through 10 mesh screen for mesophilic batch biogas fermentation experiment in a22 L digester made of Plexiglas(Fermentation temperature: 35±1℃, TS concentration: 4.2%, HRT: 60d). The purposes of the research include the research on biogas production performance, physicalproperty and crust breaking strength, and analysis on energy consumption for crust breaking in strawbiogas project, so as to find the maximal granularity of the three kinds of crops for the enhancement ofproject operation efficiency. The main conclusions of the test are as follows.(1)In terms of biogas production performance, the experiment on the three kinds of straw ofdifferent grain sizes each showed that the smaller the size, the higher the biogas production. For thesizes of 5cm, 2cm and the size through 10 mesh screen, biogas yield of rice straw were 382.42 ml/g TS,413.34 ml/g TS and 483.35ml/g TS, and the average methane content were 2.68%, 53.45% and 54.88%respectively; biogas yield of wheat straw were 377.25 ml/g TS, 398.14 ml/g TS and 431.04ml/g TS, andthe average methane content were 58.00%, 58.10% and59.53% respectively; biogas yield of cornstraw were 416.28 ml/g TS, 509.53ml/g TS and 529.10ml/g TS, and the average methane content were58.40%, 58.34% and 58.42% respectively.(2)In terms of crust layer features,the smaller the granularity of the straw, the higher the density ofthe crust while the smaller the thickness in the crust layer and the filtration layer, and consequently thesmaller the breaking strength would be. For the size of 5cm, 2cm and the size through 10 mesh screen,the ultima density of rice straw crust layer were 54.84g/L, 62.67g/L and 99.71g/L, while its ultimathickness were 20 cm, 17.5cm and 11 cm, and the filtration layer in the crust were 10 cm, 6cm and 1cm,then consequently, the crust breaking strength were 3.14 kPa, 2.10 kPa and 1.24 kPa respectively; theultima density of wheat straw crust layer were 51.40g/L,61.39g/L and 105.25g/L respectively, while theultima thickness of crust layer were 21.5cm,18 cm and 10.5cm, and the filtration layer in crust layerwere 10 cm, 6cm and 1cm respectively, consequently, the crust breaking strength were 3.20 kPa, 2.17 kPaand 1.27 kPa respectively; the ultima density of corn straw crust layer were 59.43g/L,75.27g/L and112.91g/L, while the ultima thickness of crust layer were 19 cm,15cm and 10 cm, and the filtration layerin crust layer were 10 cm,8cm and 2.5cm, then consequently, the crust breaking strength were1.90 kPa,1.08 kPa and 0.81 kPa respectively.(3)With scanning electron microscope(SEM), at the lowest position of the crust layer, thewaterline of slurry and the top of crust layer of the straw with different grain diameters, the maximaldecomposition degree and the fullest anaerobic digestion can be found at the bottom of the scum formedby the three crops, but the top is rather weak to decompose and decomposition hardly occurs, while thedecomposition level at the central part of the scum was between those at the bottom and the top. It canbe indicated that the formation of crust layer might directly cause dehydration in the fermentationmaterial and affect decomposition of straw.(4) For example, a full scale anaerobic digester with a working volume of 500 m3 loaded with rice,wheat and corn straw of 5cm, 2cm, and a size through 10 mesh screen respectively was operated at aSRT of 30 d. According to biogas production rate of raw material, crust breaking strength in theexperiment and the energy consumption of equipment during mechanical pretreatment where straw ofdifferent kinds was prepared into different sizes, the net energy production from straw of different sizeswere analyzed. As a result, the highest net energy outputs were obtained from rice, wheat and corn strawwith a size through 10 mesh screen, which were 24,352.97kW?h, 23,304.79kW?h and 27,816.60kW?hrespectively.