Jennifer Sinclair Curtis, PhD, University of Florida
Pratap Pullammanappallil, University of Florida
The purpose of this study was to investigate the effect of biomass compaction on the biomethane potential of sugar beet tailings by using different bulking/packing conditions.
On the experimental side, sugar beet tailings were shipped from American Crystal Sugar Company and then washed consecutively three times with slight squeezing before storing in a cold storage in order to remove excess surface sugars and mud. The sugar beet tailings were thawed before loading them into the anaerobic digester. For each digestion run, three kg of sugar beet tailings were layered with lava rocks. The inoculum was added to the digester to make the working volume up to 12 L. Buffer (sodium bicarbonate) of 5 g/L was added to the digester to maintain pH range of 7-8.5. The inoculum was re-circulated with a peristaltic pump at the rate of 100 ml/min to maintain uniform heating and a consistent temperature of 55 degrees C. The liquid circulation was done from bottom to top. The average size of the lava rocks (bulking agent) was measured as 1" in diameter and the distributed size includes a range of 0.75" the smallest and 1.5" as the biggest in diameter. Other bulking materials, chromium steel balls and wooden balls, were also tested.
The biomethane potential of sugar beet tailings was first performed with three layers of lava rocks as the bulking material and was recorded as liters of CH4@STP per kg volatile solids (VS). The methane yield was higher than in the case of no bulking material. When chromium steel and wooden balls were used as the bulking agent, the methane production was inhibited, even below the case with no bulking materials. Visual and odor inspection of the leachate indicated that some inhibitory substance was present in the bulking material. Other bulking materials that will be tested in the future for compatibility with anaerobic digestion are acrylic and polystyrene. The sugar beet tailings digestion was further studied with three and six layers of lava rocks as the bulking agent. This larger bulking in the digester indicated an increase in the overall biomethane potential from 165 L CH4@STP/kg VS to 319 L CH4@STP/kg VS but did not significantly increased the rate of methane production. Further studies are needed to be done to more thoroughly investigate this very interesting result.
On the computational side, the packing of the biomass was studied via discrete element method (DEM) simulations. The biomass was represented as an elongated cylindrical particle which is formed by overlapping identical spheres in a line or as a true cylindrical particle, which has the exact geometry of a cylinder. The effects of various particle and fill parameters, such as coefficient of friction, coefficient of restitution, drop height, fill height and particle surface roughness on packing density, were studied. Three methods of packing density analysis were also studied - (i) center of particle method, (ii) top grid analysis method, and (iii) side view image analysis – to most accurately assess the packing density of the biomass particle bed. The following results were obtained from these DEM simulations: (a) the packing density of the particles increases with increasing coefficient of restitution, increasing dropping height and decreasing surface roughness, and the packing density decreases with increasing coefficient of friction and Young's modulus; (b) the coordination number increases with increasing coefficient of restitution and increasing dropping height and decreases with increasing coefficient of friction; (c) the best agreement between particle packing density predictions via DEM simulations and actual experiments were obtained with the true cylindrical particles and the Hertzian columbic sliding friction contact model; and (d) the center of particle and top grid analysis methods are superior methods of packing density analysis. Through complementary (non-reactive) experimental studies of particle packing, it was also concluded that the packing method (fill height, effective drop height, radial position of the point from which the particles are dropped, and fill rate) plays a critically important role in final packing density. The adverse wall effects on packing density in smaller diameter cylindrical containers were observed. However, in larger diameter cylindrical containers, the adverse effect of the fixed radial position of the point of dropping particles dominates the inhibitory wall effect. When particles are dropped from the center in large diameter containers, they form a pile in the bed, thus leading to a less efficient packing scheme. In fact, it is possible for a lower packing density to occur in larger diameter cylindrical containers than in smaller diameter ones due to this "piling" effect. Future work will consider a distribution of sizes of cylindrical particles to more closely mimic the size distribution of the biomass particles. In addition, spheres will be added to the biomass mix to emulate the bulking agent. We are also now developing a flexible particle model which will even more closely mimic the biomass particles.