Alvin R. Womac

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.

Bulk density and compaction behavior of knife mill chopped switchgrass, wheat straw, and corn stover.

Bulk density of comminuted biomass significantly increased by vibration during handling and transportation, and by normal pressure during storage. Compaction characteristics affecting the bulk density of switchgrass, wheat straw, and corn stover chopped in a knife mill at different operating conditions and using four different classifying screens were studied. Mean loose-filled bulk densities were 67.5+/-18.4 kg/m(3) for switchgrass, 36.1+/-8.6 kg/m(3) for wheat straw, and 52.1+/-10.8 kg/m(3) for corn stover. Mean tapped bulk densities were 81.8+/-26.2 kg/m(3) for switchgrass, 42.8+/-11.7 kg/m(3) for wheat straw, and 58.9+/-13.4 kg/m(3) for corn stover. Percentage changes in compressibility due to variation in particle size obtained from a knife mill ranged from 64.3 to 173.6 for chopped switchgrass, 22.2-51.5 for chopped wheat straw and 42.1-117.7 for chopped corn stover within the tested consolidation pressure range of 5-120 kPa. Pressure and volume relationship of chopped biomass during compression with application of normal pressure can be characterized by the Walker model and Kawakita and Ludde model. Parameter of Walker model was correlated to the compressibility with Pearson correlation coefficient greater than 0.9. Relationship between volume reduction in chopped biomass with respect to number of tappings studied using Sones model indicated that infinite compressibility was highest for chopped switchgrass followed by chopped wheat straw and corn stover. Degree of difficulty in packing measured using the parameters of Sones model indicated that the chopped wheat straw particles compacted very rapidly by tapping compared to chopped switchgrass and corn stover. These results are very useful for solving obstacles in handling bulk biomass supply logistics issues for a biorefinery.

Fast classification and compositional analysis of cornstover fractions using Fourier transform near-infrared techniques.

The objectives of this research were to determine the variation of chemical composition across botanical fractions of cornstover, and to probe the potential of Fourier transform near-infrared (FT-NIR) techniques in qualitatively classifying separated cornstover fractions and in quantitatively analyzing chemical compositions of cornstover by developing calibration models to predict chemical compositions of cornstover based on FT-NIR spectra. Large variations of cornstover chemical composition for wide calibration ranges, which is required by a reliable calibration model, were achieved by manually separating the cornstover samples into six botanical fractions, and their chemical compositions were determined by conventional wet chemical analyses, which proved that chemical composition varies significantly among different botanical fractions of cornstover. Different botanic fractions, having total saccharide content in descending order, are husk, sheath, pith, rind, leaf, and node. Based on FT-NIR spectra acquired on the biomass, classification by Soft Independent Modeling of Class Analogy (SIMCA) was employed to conduct qualitative classification of cornstover fractions, and partial least square (PLS) regression was used for quantitative chemical composition analysis. SIMCA was successfully demonstrated in classifying botanical fractions of cornstover. The developed PLS model yielded root mean square error of prediction (RMSEP %w/w) of 0.92, 1.03, 0.17, 0.27, 0.21, 1.12, and 0.57 for glucan, xylan, galactan, arabinan, mannan, lignin, and ash, respectively. The results showed the potential of FT-NIR techniques in combination with multivariate analysis to be utilized by biomass feedstock suppliers, bioethanol manufacturers, and bio-power producers in order to better manage bioenergy feedstocks and enhance bioconversion.

Direct measures of mechanical energy for knife mill size reduction of switchgrass, wheat straw, and corn stover.

Lengthy straw/stalk of biomass may not be directly fed into grinders such as hammer mills and disc refiners. Hence, biomass needs to be preprocessed using coarse grinders like a knife mill to allow for efficient feeding in refiner mills without bridging and choking. Size reduction mechanical energy was directly measured for switchgrass (Panicum virgatum L.), wheat straw (Triticum aestivum L.), and corn stover (Zea mays L.) in an instrumented knife mill. Direct power inputs were determined for different knife mill screen openings from 12.7 to 50.8 mm, rotor speeds between 250 and 500 rpm, and mass feed rates from 1 to 11 kg/min. Overall accuracy of power measurement was calculated to be +/-0.003 kW. Total specific energy (kWh/Mg) was defined as size reduction energy to operate mill with biomass. Effective specific energy was defined as the energy that can be assumed to reach the biomass. The difference is parasitic or no-load energy of mill. Total specific energy for switchgrass, wheat straw, and corn stover chopping increased with knife mill speed, whereas, effective specific energy decreased marginally for switchgrass and increased for wheat straw and corn stover. Total and effective specific energy decreased with an increase in screen size for all the crops studied. Total specific energy decreased with increase in mass feed rate, but effective specific energy increased for switchgrass and wheat straw, and decreased for corn stover at increased feed rate. For knife mill screen size of 25.4 mm and optimum speed of 250 rpm, optimum feed rates were 7.6, 5.8, and 4.5 kg/min for switchgrass, wheat straw, and corn stover, respectively, and the corresponding total specific energies were 7.57, 10.53, and 8.87 kWh/Mg and effective specific energies were 1.27, 1.50, and 0.24 kWh/Mg for switchgrass, wheat straw, and corn stover, respectively. Energy utilization ratios were calculated as 16.8%, 14.3%, and 2.8% for switchgrass, wheat straw, and corn stover, respectively. These data will be useful for preparing the feed material for subsequent fine grinding operations and designing new mills.

Sorption equilibrium moisture characteristics of selected corn stover components.

Corn stover equilibrium moisture isotherms were developed to aide biomass engineering of consistent, uniform-quality feedstock supplies for renewable bioenergy and bioproducts. Equilibrium moisture content (EMC) and equilibrium relative humidity (ERH) sorption data of corn leaf, stalk skin, and stalk pith were experimentally determined using the static gravimetric method at six temperatures ranging from 10°C to 40°C and at ten ERH values ranging from 0.11 to 0.98. The greatest EMC values for corn leaf and stalk pith generally corresponded with ERH below and above 0.90, respectively, at all temperatures. Only at some intermediate ERH range at 20°C to 40°C was stalk skin EMC greater than stalk pith EMC. Corn stover components followed a type II isotherm typically observed among food materials. EMC of all components was proportional to ERH and inversely proportional to temperature. Observed EMC ranges were 3.9% to 56.4%, 3.1% to 41.1%, and 2.7% to 71.5% dry basis (d.b.) for corn leaf, stalk skin, and stalk pith, respectively. Calculated whole-stalk EMC values ranged from 3.1% to 49.2% d.b. Isotherm data were fitted with the EMC model of Henderson, and modified versions of Henderson, Chung-Pfost, Halsey, Oswin, and Guggenheim-Anderson-deBoer. The modified Oswin model (R2 > 0.98; F > 2085) followed by the modified Halsey model (R2 > 0.97; F > 1758) produced the best fit for corn stover components studied. The Henderson, modified Henderson, and modified Chung-Pfost models were not suitable since these models did not produce randomized residuals. The modified Oswin model (R2 = 0.99; F = 6274) best described the stalk EMC. Results have practical applications in corn stover collection method and timing; process handling, grinding, and drying requirements; transportation efficiency of dry matter; and necessary storage environment, shelf life, and potential microorganism safety hazards. For example, results indicated that higher EMC values for corn stover leaf may result in greater propensity for the onset of mold growth and may determine minimal storage requirements or potential advantages in separating leaf from stalk fractions.

MEASUREMENT VARIATIONS IN REFERENCE SPRAYS FOR NOZZLE CLASSIFICATION

Variations in spray droplet measurements were identified for reference nozzles serving as category thresholds defined for the standardized classification of agricultural spray nozzles. Spraying Systems, Delavan, and Lurmark manufactured brands of reference nozzles produced mean volume median diameters (Dv0.5) that varied from 0.5 to 34 µm within a given nozzle size, as measured with a laser diffraction instrument. Similarly, mean Dv0.1 and Dv0.9 values differed from 0.1 to 14 µm and from 0.9 to 74.2 µm, respectively. Coefficients of variation (CV) in Dv0.1, Dv0.5, or Dv0.9 within a nozzle brand and size ranged from 0.19 to 3.62% across the test. Two additional laser instruments that were tested included an imaging probe and a phase Doppler instrument. Relative droplet size differences between nozzle brands were noted when using different types of laser instruments compared with number density weighted values. Similarly, droplet size differences were observed between instruments compared with number flux weighted values. Results indicate that dedicated reference nozzle sets may be preferred to increase the overall uniformity of classification thresholds; any of the tested brands of nozzles would be suitable; and laser instrument differences may contribute to relative shifts among thresholds for nozzle classification, thereby reducing the precision of uniform nozzle classification.

Biomass Pre-Processing Size Reduction with Instrumented Mills

Rotary size reduction equipment were instrumented to identify important operating parameters with the aim of minimizing energy consumption while reducing the size of lignocellulosic biomass. Size reduction equipment included a knife mill, hammer mill, and disc mill. Monitored parameters included input power and particle size distributions.. Operating parameters included angular velocity, mass rate, screen size, and biomass selection. Results indicated optimum operating conditions with minimal energy input. Geometric mean dimensions of biomass particles varied from 5x for particle length to 0.3x for particle width when comparing actual to ASABE sieve results – highlighting the need for better consensus standards on particle size reporting.

MASS AND MOISTURE DISTRIBUTION IN ABOVEGROUND COMPONENTS OF STANDING CORN PLANTS

Corn stover mass and moisture properties were identified to aid decisions regarding collection of standing corn stover dry matter with least moisture, and to aid development of moisture prediction tools for applications including harvest, transport, size reduction, and storage. Vertical distributions of mass and moisture in standing stalks and aboveground components such as leaf, husk, and ear in standing corn plants were evaluated over time. Stalks were cut into 254 mm long sections to facilitate analysis, and to correspond with billet length collected for cane-type crops. Stalks had the greatest wet mass (72.6%) followed by leaf (20.7%) and husk (6.8%) during the normal harvest period. Moisture profile in aboveground plant components exhibited two separate, sequential linear relationships when plotted with time. The first zone was rapid moisture reduction prior to the normal harvest period. The second zone, corresponding with but not the result of grain harvest, had slow, gradual moisture reduction and stabilization. Geometrical analyses of stalk cross-sectional area, volume, and lateral surface area to volume ratio documented physical properties for future moisture prediction tools. Considering stalks only, the bottom 1 to 4 stalk sections had 60.6% of total dry matter, although that was the last portion of the stalk to dry, thereby increasing the liability of added moisture on transportation and storage. An apparent drying front moved downward through the plant over time and may be explained by reduced the lateral surface area to volume ratio from stalk top to stalk bottom. Dry matter and moisture content of stalks were not significantly influenced by the observed soil and environmental parameters, including rainfall. It was hypothesized that standing stalks readily shed rainfall and allowed less opportunity for moisture penetration. Mass and moisture content of discrete stalk sections were normalized using plant height to facilitate multiple regression equations applicable to other crop heights. Results pertain to assessment of mass and moisture status of standing stalks in the field before and after grain harvest, identity of moisture factors related to the supply of uniform-quality feedstock, and discovery of relevant biomass properties needed for design and management of efficient biomass processes and equipment.

EVALUATION OF SAMPLERS FOR SPRAY DRIFT

Collection of airborne spray drift of malathion released from a ground-driven boom sprayer was investigated using six types of samplers: (1) horizontal alpha cellulose fallout sheets; (2) high-volume air samplers; (3) sampling trains of rotary disk impactor and two bubblers (RDI); (4) rotating rod samplers; (5) vertical string samplers; and (6) polyurethane foam plugs (PUF). Spray deposit was determined with malathion residue collection on horizontal alpha cellulose sheets spaced at 3 m intervals in the spray swath and at 6-m intervals along a line perpendicular and downwind from the spray swath. Spray drift residues were collected by the samplers at four stations along a 90 m sampler line located 30 m downwind and parallel to the spray swath. Gas chromatographic analysis was used to quantify the concentration of malathion. Results indicated that fallout deposits (1) in the spray swath, (2) at 6, 12, 18, or 24 m from the spray swath edge, and (3) at 30 m downwind from the spray swath edge were approximately (1) 47, (2) 0.7, and (3) 0.09% of the total spray application rate, respectively. The low in-swath deposit was partially attributed to (1) a 1.2 m boom height—to ensure that samplers of the evaluation were challenged with a uniform cloud of spray droplets, and (2) the use of a single swath width. No differences were observed in residue collections from high-volume air samplers (P>0.4), rotating rod samplers (P>0.3), or vertical strings (P>0.7) at the four sampling stations. The collection from a high volume-PUF air sampler was 1108 ng/m3, with 728 ng/m3 from the filter and 380 ng/m3 from the PUF. Malathion residues were not detected in the RDI under the selected test conditions. A low airflow rate of 1.2 L/min combined with the short duration of exposure to the moving spray cloud provided little opportunity for the RDI to collect a detectable level of malathion.

Characteristics of Air-assisted and Drop-nozzle Sprays in Cotton

The effects of air-assisted, drop-nozzle, and over-the-top spray applications were characterized with a wide array of targets and chemicals in mature cotton foliage. Spray rates of 47 and 94 L/ha were applied at droplet volume median diameters (Dv.5) ranging from 114 to 192 µm. Increased spray rate predominantly increased deposition and chemical efficiency under most conditions. Sprays assisted with air (velocities up to 16 m/s) increased fluorescent tracer deposits on the canopy middle up to 92% of that from the canopy top, and resulted in a significant increase in bifenthrin on leaves and squares located within the canopy. Use of an upward-trajectory thiodicarb spray from drop-nozzle applications tended to control beet armyworm (Spodoptera exiqua [Hubner]) on the leaf underside more efficiently than the other application methods. Orienting the air-assisted sprayer air stream 30° back tended to increase deposits on the leaf underside. Use of air-assisted applications at 47 and 94 L/ha and an over-the-top application at 94 L/ha for a butifos and ethephon spray mixture provided the highest mean defoliation ratings that ranged from 68.1 to 78.4%.