Switchgrass Particles Research Articles

Dynamic biomass char porosity during gasification: Model compared with data

This investigation presents a new biomass char porosity model as a function of char conversion during gasification and compares it with entrained-flow data from poplar, corn stover and switchgrass particles reacting with H2O, CO2, and combinations of both at temperatures of 1050–1350 °C. The experimental data include particle temperature, mass, size, and shape combined with porosity, BET surface area, geometric specific surface area and SEM images of char vascular structure. The new theoretical model describes char porosity changes during char conversion and the model indicates that porosity increases monotonically, consistent with the data. The vascular char structure remains intact even at the highest conversions and this structure effectively transports reactants and products through the particle, with reactions occurring on the walls of these vascular vessels, referred to here as vascules. The investigation shows that char porosity most strongly depends on average vascule diameter which, in turn, increases with increasing conversion. This investigation extends current understanding of the mechanism of heterogeneous biomass char reactions (gasification and oxidation) under kinetically controlled conditions. This work concludes that (1) kinetically controlled biomass gasification involves reactions on the walls of vascules with small contributions from the nanopores or even mesopores; (2) char porosity increases monotonically with increasing char conversion in the kinetically and internal transport-controlled regimes; and (3) observed porosity behaviors correspond the predicted behaviors.

Terminal Velocity Determinations for Switchgrass Nodes and Internodes

HighlightsA vertical wind tunnel was fabricated to aerodynamically suspend switchgrass particles to measure terminal velocity.Overall terminal velocity estimates for node and internode were 8.04 and 5.68 m s-1, respectively.Terminal velocity prediction equation for spherical particles (Mohsenin, 1970) produced values almost identical to experimental values unlike an equation for infinite-length cylindrical particles.Hollow switchgrass particle behavior was complex with particle bouncing, rolling, tumbling, and other dynamic behaviors during aerodynamic equilibrium. Abstract. Terminal velocity is a fundamental aerodynamic property often used to separate plant anatomical components. A vertical wind tunnel was fabricated to investigate whether terminal velocities of switchgrass stem internodes and nodes were distinctly different. Wind tunnel features included a clear observation section for suspended particles and fine adjustment of airflow. Each sample terminal velocity was measured with 45 anemometer readings from nine locations and five repeated measures. The experiment evaluated two sample moisture contents (52% and 15% wet basis, hereafter referred to as w.b.), three particle lengths (0.64, 1.27, and 2.54 cm), node versus internode, and 12 replications. The 6,480 anemometer readings provided significant differences between factors and interactions. The greatest magnitude in mean terminal velocity difference was 4.55 m s-1 (i.e. 9.62 to 5.07) that resulted in a velocity ratio of 190% [(9.62/5.07) x 100]. The least magnitude of mean terminal velocity difference, while significant, was 0.36 m s-1 (i.e. 6.59 to 6.23). This offered a limited difference and may indicate that separations may need to be confined within groups of moisture category and/or particle length. Terminal velocity measurements were most similar to those calculated using the Mohsenin (1970) spherical particle equation yielding a mean (6.86 vs. 6.91 m s-1), standard deviation (1.41 vs. 1.42 m s-1), minimum (4.65 vs. 4.69 m s-1), and maximum (9.95 vs. 10.03 m s-1) for measurement and spherical particle equation, respectively. Keywords: Anatomical component, Biomass property, Physical experiment, Separation, Sorting, Switchgrass, Vertical wind tunnel.

Physical characteristics of ground switchgrass related to bulk solids flow

Switchgrass has potential as an energy crop, but problems with handling and flowability hinders its utilization. The primary objective of this study was to investigate the fundamental flow-related physical characteristics of switchgrass particles ground through three hammermill screen sizes of 6.4, 3.2 and 1.6 mm. Physical properties were determined through sieve analysis, density testing, image analysis, angle of repose and fractal analysis. The geometric mean diameter and geometric standard deviation of the ground material decreased with size reduction. Particle size significantly influenced particle and bulk density. Similar conclusions were made for angle of repose; values decreased with size reduction. Shape and surface parameters from image and fractal analyses were not significantly different among particle sizes. Non-spherical, needle-shaped particles were maintained through size reduction. Particle size reduction may slightly improve the flow of ground switchgrass bulk, but will always experience some degree of flow problems in regular storage silos and hoppers.

Switchgrass as oil and water-spill sorbent: Effect of particle size, torrefaction, and regeneration methods

Switchgrass, both raw and torrefied, was tested for its ability to sorb water or oil. The cyclic performance was also examined, utilizing centrifugal extraction as the regeneration method. Both oil and water sorption capacity increase with the decreasing size of raw switchgrass particles. Results indicate that 3 mm raw switchgrass can sorb water at a capacity of about 6 times its mass and can sorb oil at a capacity of about 3 times its mass, which makes it a suitable biodegradable sorbent. Torrefaction at 220 °C for 30 min reduces water sorption capacity by an average of 55% but does not have a statistically significant impact on oil sorption. Sorption of liquid is negatively correlated to particle size. Centrifugation is able to partially desorb either liquid from the sorbent, and subsequent sorption cycles do not display lower sorption capacity than the first cycle when calculated on a dry mass basis.

Discrete element modeling of switchgrass particles under compression and rotational shear

Switchgrass is a perennial herbaceous plant regarded as a biomass energy crop in the United States for its high adaptability and yield potential. Processing and handling of switchgrass particles are challenging due to the erratic mechanical and flow behavior originating from their intrinsic particulate properties. In this work, we present a bonded-sphere discrete element model designed specifically for switchgrass particles. The model simultaneously captures three key particulate features, i.e., fibrous particle shapes, a wide range of particle sizes, and particle deformability. Realistic yet computationally efficient particle shape templates are created based on the image analysis data of switchgrass specimens. A fitting procedure is proposed to ensure both the particle width and length distributions are captured, a unique requirement for fibrous particles. Two full-scale numerical models, i.e., a uniaxial compression model and a Schulze ring shear model, are developed using information from physical experiments. The model is calibrated using experimental data of chopped-small switchgrass specimens, and then, is validated using data of chopped-large specimens in both compression and ring-shear tests. Numerical results show that the numerical models capture bulk densities accurately (with an error of 3%) while slightly underestimate the bulk friction angle. Furthermore, an extensive sensitivity analysis reveals that (1) switchgrass particles with rougher edges (due to different processing techniques) exhibit a higher shear strength and a lower flowability; (2) stiffer particles yield a lower bulk density (up to 21% lower) compared to more deformable particles, indicating particle deformability should be incorporated when modeling biomass flow in a preprocessing system.

MicroCT imaging to determine coordination number and contact area of biomass particles in densified assemblies

Coordination number, denoting the number of neighboring particles contacting one central particle, and the corresponding contact area can provide crucial information about the arrangement of particles for the formation of a stable assembly. Towards that goal, two temperatures (75 °C and 90 °C) and two pressures (60 MPa and 100 MPa) were selected to form densified assemblies of biomass particles. Lengths of selected needle-shaped particles, that were chosen as central particles to determine the coordination number, were from 2.2 to 3.0 mm (2.66 ± 0.19 mm). The range of coordination number for switchgrass particles was from 9 to 12 for Treatment A (75 °C and 60 MPa), 12 to 14 for Treatment B (75 °C and 100 MPa), 6 to 7 for Treatment C (90 °C and 60 MPa), and 8 to 10 for Treatment D (90 °C and 100 MPa). The irregular shape of the biomass particle makes the standard scale of coordination number inapplicable, where the value of 14 tends to imply a fully densified assembly. The associated contact area of the particles can provide crucial complementary information leading to a thorough understanding of the arrangement of neighboring biomass particles. The percentage of contact area for Treatments A, B, C, and D were 58.3 ± 8.8, 81.7 ± 4.5, 36.2 ± 4.2, and 40.3 ± 2.1, respectively. Both, coordination number and associated percentage contact area, were highest for Treatment B and lowest for Treatment C. It was observed that higher pressure during the densification process results in higher coordination number and contact area, which also correlates to higher density and strength of assemblies. Coordination number and percentage contact area of all specimens were found to be positively correlated (R2 = 0.872). In addition, the correlation between microscale properties, i.e., coordination number and associated percentage area, and macroscale properties, i.e., densities and strength of assemblies, were also studied (R2 > 0.85).

Single particle mechanical characterization of ground switchgrass in air dry and wet states using a microextensometer

To understand the mechanical process of pelletization, it is critical to study the particle level interactions that cause particles to bind together and form a pellet. A micromechanical extensometer device, inspired by the MEMS technology, was developed and used to perform tensile experiments to deduce the stress-strain response of single particles of ground biomass. The effect of moisture, which has a significant role in forming pellets, was examined based on the micromechanical characterization of moisture conditioned and unconditioned (control; air dried) switchgrass particles. Conditioned particles exhibited three phases sigmoidal shaped stress-strain response. The three phases include the first linear elastic zone, where the particle behaved linearly up to approximately 1.6% strain, the second linear elastic zone (strain ranging from 1.6 to 2.1%) with significantly increased elastic modulus (168.9–223.7%), and the third zone (strain beyond 2.1%), where elastic modulus declined sharply (down to 90.9% of the second zone). The modulus of elasticity up to 1.5% strain for unconditioned and conditioned switchgrass particles were 1.60±0.33GPa and 6.99±1.66GPa, respectively (p=0.00). The nominal fracture strength of unconditioned (6.2%, w.b.) and conditioned (17.5%, w.b.) switchgrass particles were determined as 35.77±14.99MPa and 130.42±87.56MPa, respectively (p=0.08). The nominal fracture strain of unconditioned and conditioned switchgrass particles were determined as 2.43±0.70% and 1.51±0.66%, respectively (p=0.06). Increase in the stiffness of switchgrass particles is contributed to the bundling of fibers promoted by the activation of binders due to increased moisture content.

Comprehensive rheological characterization of chopped and ground switchgrass

Background: In this work the impact of grinding and chopping on the rheological properties of four switchgrass materials (panicum virgatum) is investigated, comparing physical characterization to previously published feeding performance in bulk equipment in a high-tonnage facility, in which chopped materials exhibited dramatically better handling performance than corresponding ground materials. Results: Physical characterization included sieve analysis, automated digital image analysis of particle size and shape distributions, microscopy of particle microstructure, shear tests using two different sizes of ring shear testers, flow tests in a custom hopper with an adjustable outlet, and uniaxial compression and spring back tests. Shear tests failed to demonstrate consistent statistically significant differences between the shear strengths of the chopped and ground materials subjected to uniaxial compression. However, in laboratory-scale hopper flow tests, the chopped materials exhibited substantially better flow performance than the ground materials, in agreement with the previously published bulk handling tests. Conclusion: Chopped switchgrass materials exhibit substantially better flowability properties in large-scale bulk equipment and in laboratory-scale hopper feeding tests; however, shear tests were not able to reliably predict the flowability properties of bulk switchgrass particles, likely because uniaxial-compression shear tests do not capture the effects of compressibility and elasticity in multi-dimensional flow streams.

Modeling enzymatic hydrolysis of lignocellulosic substrates using fluorescent confocal microscopy II: Pretreated biomass

In this study, we extend imaging and modeling work that was done in Part I of this report for a pure cellulose substrate (filter paper) to more industrially relevant substrates (untreated and pretreated hardwood and switchgrass). Using confocal fluorescence microscopy, we are able to track both the structure of the biomass particle via its autofluorescence, and bound enzyme from a commercial cellulase cocktail supplemented with a small fraction of fluorescently labeled Trichoderma reseii Cel7A. Imaging was performed throughout hydrolysis at temperatures relevant to industrial processing (50°C). Enzyme bound predominantly to areas with low autofluorescence, where structure loss and lignin removal had occurred during pretreatment; this confirms the importance of these processes for successful hydrolysis. The overall shape of both untreated and pretreated hardwood and switchgrass particles showed little change during enzymatic hydrolysis beyond a drop in autofluorescence intensity. The permanence of shape along with a relatively constant bound enzyme signal throughout hydrolysis was similar to observations previously made for filter paper, and was consistent with a modeling geometry of a hollowing out cylinder with widening pores represented as infinite slits. Modeling estimates of available surface areas for pretreated biomass were consistent with previously reported experimental results.

Flow performance of ground biomass in a commercial auger

The flow performance of preprocessed biomass plays an important role in biomass transportation and handling. The research as presented here investigated how the Angle of Repose (AOR) of miscanthus and switchgrass is related to flow performance of biomass particles in an auger that was originally designed to convey corn and soybeans. The flow performance metrics were the specific energy consumption (SEC), energy efficiency (EE), volumetric efficiency (VE), volumetric flow rate (VFR) and mass flow rate (MFR).The results showed that the EE and MFR while conveying miscanthus and switchgrass particles ground through 6.35-, 9.53-, 12.7- and 25.4-mm milling screens were much lower than those of corn. However, the differences in VFR between corn and biomass were much smaller than that in SEC and mass flow rate (MFR). This result implies that the low bulk density of biomass feedstock is a more pronounced limiting factor in biomass handling than the conveying mechanism used.The AOR of miscanthus and switchgrass particles was found proportional to particle size and moisture content. While AOR is an indicator of the material's internal friction, in this study, the AOR of miscanthus and switchgrass was not significantly related to the energy/volumetric efficiency of the auger. By comparing the measured auger power consumption to predictions from empirical equations developed for corn and soybean, it became evident that these equations do not perform well for biomass feedstock.

팝핑전처리한 스위치그라스로부터 바이오에탄올 생산

본 연구에서는 스위치그라스를 바이오에탄올 생산용 바이오매스로 선정하여 팝핑 전처리, 효소가수분해 및 발효 과정을 거쳐 바이오 에탄올의 생산 가능성을 조사하였다. 팝핑 전처리 된 스위치그라스에 효소를 처리하여 가수분해 한 결과 95% 이상의 효소 가수분해 효율을 보였고, 가수분해 후 생성된 당화액을 효모(Saccharomyces cerevisiae)로 발효하였을 때 생산된 바이오에탄올의 수율은 89.6%에 이르렀다. 팝핑 전처리 후 바이오매스의 글루코스와 자일로스의 함량은 각각 40.8%와 20.3%로 나타나 전처리 후 글루코스 함량에는 큰 변화가 없었으나, 자일로스의 함량이 4.7% 감소하는 것으로 보아 헤미셀룰로스 영역이 전처리 과정에서 제거된 것으로 보인다. 또한, 주사형 전자현미경 결과에 의하면 전처리 전에는 스위치그라스 표면이 비교적 매끈하고, 입방체 모양을 이루고 있었으나, 전처리 후에는 섬유가 각각 분리되어 있었으며 표면에 많은 미세공극이 생겨난 것을 관찰할 수 있었다. 그러므로 팝핑 전처리는 스위치그라스 시료의 셀룰로스 노출면적을 넓혀주는 역할을 하여 셀룰레이즈 효소의 접근성을 높여 효소 당화 효율을 증대시키는 것으로 사료된다. Switchgrass was selected as a promising biomass resource for bioethanol production through popping pretreatment, enzymatic saccharification and fermentation using commercial cellulase and xylanase, and fermenting yeast. The reducing sugar yields of popping pretreated switchgrass after enzymatic saccharification were above 95% and the glucose in thesaccharificaiton solution to ethanol conversion rate after fermentation with <TEX>$Saccharomyces$</TEX> <TEX>$cerevisiae$</TEX> was reached to 89.6%. Chemical compositions after popping pretreatment developed in our laboratory were 40.8% glucose and 20.3% xylose, with much of glucose remaining and only xylose decreased to 4.75%. This means that the hemicelluloses area broke off during popping pretreatment. FE-SEMexamination of substrate particles after popping pretreatment was showed fiber separation, and tearing and presence of numerous micro pores. These changes help explain, enhanced enzymatic penetration resulting in improved hydrolysis of switchgrass particles after popping pretreatment.

Energy requirement for comminution of biomass in relation to particle physical properties

The energy requirement for biomass comminution and the resulting particle physical properties are important factors to study logistic components, select equipment, and assess the overall efficiency of feedstock supply–conversion chain. In this study, mechanical size reduction of Miscanthus (Miscanthus giganteus), switchgrass (Panicum virgatum), willow (Salix babylonica), and energy cane (Saccharum spp.) was carried out using a commercial-scale hammer mill, a bench-scale Retsch SM2000 knife mill and a Retsch SK100 hammer mill. The results showed that the specific energy consumption of biomass comminution and the aperture sizes of the milling screens were related in power-law forms. Biomass moisture significantly influenced comminution energy consumption, especially for finer size reduction. Given a specific milling screen, the Retsch SK100 hammer mill was found more energy efficient than the SM2000 knife mill. This was mainly attributed to the higher motor speed and axial feeding mechanism of the hammer mill. The particle sizes after comminution were found inversely proportional to the bulk densities of all four energy crops used in experiments. In addition, the comminution ratio, being the ratio of the final mean particle size and the original mean particle size, was proportional to energy consumption for all four energy crops. The bulk densities for 4-mm and smaller Miscanthus and switchgrass particles were higher than those of the original bale. Particle size and surface area estimates using commonly used ANSI/ASAE Standards S424.1 and 319.4 were highly sensitive to particle size distributions and shapes. Further studies on standardization of particle size and surface area estimates are needed.

Biocrude Production from Switchgrass Using Subcritical Water

Subcritical water is an environmentally attractive solvent for organic matters and can be used to liquefy biomass to biocrude, which is a mixture of oxygenated hydrocarbons of varying molecular weights. Liquefaction of switchgrass in subcritical water is studied using a semicontinuous reactor in the temperature range of 235−260 °C. Subcritical water is pumped through a tubular reactor packed with switchgrass particles of 40−60 mesh size. The effects of reaction temperature and catalysis by K2CO3 are examined. Potassium carbonate significantly enhances the hydrolysis of macromoleculer components of switchgrass into water-soluble products. More than 50 wt % of the organic carbon available in switchgrass was converted to biocrude after 20 min of steady operation at 235 °C in the presence of 0.15 wt % of K2CO3. At the high temperature (260 °C), dehydration of biomass was favored over hydrolysis reactions. On the basis of chromatography and mass spectrometry analyses, biocrude contains lignin derived products, sugars, and its decomposition products. On the basis of the infrared spectroscopy and electron microscopy of residue solid, the subcritical water treatment causes complete breakdown of lignocellulosic structure of switchgrass. In fact, the residue solid mainly contained lignin fractions.

Bulk Density of Wet and Dry Wheat Straw and Switchgrass Particles

Bulk density is a major physical property in designing the logistic system for biomass handling. The size, shape, moisture content, individual particle density, and surface characteristics are few factors affecting the bulk density. This research investigates the effects of true particle lengths ranging from 6 to 50 mm and moisture contents ranging from 8% to 60% wet basis (wb) on the bulk density of wheat straw and switchgrass. Three types of particle densities of straw and switchgrass measured were: a hollow particle density assuming a hollow cylindrical geometry, a solid particle density assuming a solid cylindrical geometry, and a particle density measured using a gas pycnometer at a gas pressure of 40 kPa. The bulk density of both loose-fill and packed-fill biomass samples was examined. The calculated wet and dry bulk density ranged from 24 to 111 kg m-3 for straw and from 49 to 266 kg m-3 for switchgrass. The corresponding tapped bulk density ranged from 34 to 130 kg m-3 for straw and 68 to 323 kg m-3 for switchgrass. The increase in bulk density due to tapping the container was from 10% for short 6-mm particles to more than 50% for long 50-mm particles. An equation relating the bulk density of stems as a function of moisture content, dry bulk density, and particle size was developed. After the validation of this bulk density equation, the relationship would be highly useful in designing the logistics system for large-scale transport of biomass to a biorefinery. The bulk density and particle density data of uniform particles would be important, if straw and switchgrass is used for pulping and paper making.

Image analysis and nonlinear modeling to determine dimensions of wet-sieved, masticated forage particles2

A procedure to estimate dimensions of digesta particles was developed and evaluated. After wet sieving, particles of masticated switchgrass and bermudagrass hays retained on sieves with apertures of 1.0 mm2 or larger were placed on moist white filter paper, separated manually, and photographed. Photographs were scanned with a flat-bed scanner, scaled to size, and digitized. Projected area, perimeter, length, and width of particles were determined by a microcomputer system. Variables derived for each particle included the form factor, which was defined as (4 x pi x area)/perimeter2, and each particle's length: width ratio. Ninety-three percent of data from two trials with steers had significant lack of fit (P less than .05 to P less than .0001) to normal, lognormal, Weibull, or gamma distributions. As an alternative, individual particle measurements were summed on a cumulative percentage basis for each variable and were fit to an inverted Gompertz function to estimate median, mode, and mean dimensions of scanned particles. Estimates from the equation fit the distribution well; asymptotic SE averaged 3.5 and 1.4% of parameter means for dimensions of masticated switchgrass and bermudagrass hay particles, respectively. No analytic solution exists for the mean, which must be estimated numerically, but analytic solutions are available for the median and mode. Use of this equation will prevent bias caused by lack of fit to a particular distribution and yield more accurate estimates of mean particle dimensions than arithmetic means.