C. Igathinathane

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.

Combined effect of pelleting and pretreatment on enzymatic hydrolysis of switchgrass

Switchgrass was pelleted to evaluate the effect of densification on acidic and alkaline pretreatment efficacy. Bulk density and durability of pellets were 724 kg/m(3) and 95%, respectively. Ground switchgrass (D(90) = 21.7 mm) was further ground to a fine power (D(90) = 0.5mm) in the pellet mill prior to densification. This grinding increased enzymatic hydrolyzate glucose yields of non-pretreated materials by 210%. Pelleting had no adverse impact on dilute acid pretreatment efficacy. Grinding and pelleting increased hydrolyzate glucose yields of switchgrass pretreated by soaking in aqueous ammonia (SAA) by 37%. Xylose yields from SAA-pretreated switchgrass pellets were 42% higher than those from the original biomass. Increases in sugar yields from SAA-pretreated pelleted biomass are attributed to grinding and heating of biomass during the pelleting process. Potential transportation, storage, and handling benefits of biomass pelleting may be achieved without negatively affecting the downstream processing steps of pretreatment or enzymatic hydrolysis.

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.

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.

Moisture diffusion modelling of drying in parboiled paddy components. Part I: starchy endosperm

Abstract A non-dimensionalised isothermal liquid diffusion drying model in prolate spheroid geometry using prolate spheroidal co-ordinate system was developed, and applied to drying of parboiled polished rice. Using appropriate hydrothermal treatment the parboiled polished rice was prepared, and its drying characteristics were determined using fluidised bed drying at air temperatures varying from 50 to 100 ∘ C. A mathematical model simulating the drying was solved using the finite difference methodology. The liquid diffusion coefficients of starchy endosperm of parboiled polished rice while drying with various air temperatures were determined by minimising the sum of squared differences between experimentally observed and model predicted characteristics. The developed models showed good agreement with the experimentally observed data. Temperature dependence of liquid diffusion coefficients was expressed by an Arrhenius type of equation, which had a high correlation coefficient: D vs =1.079706×10 −7 exp (−2036.252438/T a ) [ r =0.9887]

COMBINATION SOAKING PROCEDURE FOR ROUGH RICE PARBOILING

Soaking of rough rice is the most time-consuming operation in the parboiling process. A combination soaking procedure for parboiling of rough rice was developed based on the gelatinization temperature of rice starch to achieve rapid completion of soaking. The objectives of this study were to determine the gelatinization temperature of rice starch, the soaking characteristics of the rough rice, and prescribe the operating parameters of the combination soaking procedure. Medium-grain rough rice (‘Pankaj’) was selected, and its starch gelatinization temperature was determined as 72°C using alkali degradation and hot stage microscopy methods. Soaking characteristics of rough rice were studied at 60°C, 70°C, and 80°C. Below the gelatinization temperature, soaking proceeded in the normal way; however, above the gelatinization temperature, excessive water absorption, husk splitting, actual cooking of rice kernels, and loss of quality due to soak water contamination were observed. The combination soaking procedure, involving 80°C water as the first stage until an intermediate moisture content of 35.0% d.b. (approx. 45 min) followed by 70°C as the second stage up to the saturation moisture content of 42.7% d.b. (approx. 3 h 15 min), resulted in a 67% time reduction when compared with single-stage soaking at 70°C. Rice from the combination procedure resembled that obtained from 70°C single-stage soaking in all respects. In milling analysis, after parboiling the rough rice from both soaking methods, the polished rice did not show any difference in terms of head rice yield, broken grains produced, and cracked grains produced.

Biomass moisture relations of an agricultural field residue: corn stover.

Moisture of corn stover was field monitored under southeast U.S. ambient conditions to aid biomass collection decisions. Timing to collect stover at low moisture depended on elapsed time on field, elapsed time after precipitation, time of day, contact with soil, and conditioning effect by combine header. Grain had been combine-harvested at kernel moistures of either 25% or 15% wet basis (w.b.). Stover moisture was determined by weighing large in-situ baskets for a month and with frequent grab samples. Experiment controls included stover dried under tent shelter and mower-cut stover for combine-conditioning effect. Stover moisture asymptotically declined over time from approximately 70% (w.b.) to an equilibrium of approximately 20% (w.b.) for 25% (w.b.) grain harvest. Moisture reduction was not constant due to daily diurnal variation of eight percentage points (w.b.), and light precipitation that re-hydrated the stover. Stover moisture was significantly greater in the morning compared to afternoon and was greater for stover in contact with soil. A combine corn stalk conditioning effect reduced mean moisture (approx. 10 percentage points) for high-moisture stover at early harvest, yet conditioning increased moisture for a period after light precipitation. Correlation of daily stover moisture with the corresponding day’s evapotranspiration factor was not as strong as correlations with other combinations of environmental factors. Stover moisture generally peaked two days after rain events, so correlations and regressive predictions used previous data (2-day delay) for rainfall, air relative humidity, and evapotranspiration data. In addition to mechanical harvest method (stalk conditioning effect), the strongest environmental/timing correlations to predict stover moisture on the field after grain harvest included the following daily-averaged factors: elapsed time (days) after sowing (collect later for reduced moisture), time of day (evening collection preferred over morning collection), soil moisture, 2-day previous rainfall amount, 2-day previous relative humidity, and 2-day previous evapotranspiration factor. Thus, increased elapsed time after sowing/harvest, evening harvest times, and the immediate (2-day) exposure history of corn stover to available moisture and drying potential are useful in determining strategies to collect corn stover with minimum moisture content.

Moisture Sorption Thermodynamic Properties of Corn Stover Fractions

Efficient processing, handling, and storage of corn stover, a major crop-based biomass, require an understanding of the moisture sorption thermodynamic properties of its fractions. Moisture sorption thermodynamic properties of the major corn stover fractions such as leaf, stalk skin, and stalk pith were determined, utilizing the static gravimetric sorption isotherms data in the temperature range of 10°C to 40°C. Brunauer-Emmet-Teller (BET) monolayer moistures of stover fractions, determined using standard and modified BET equations, decreased with an increase in temperature. Mean values of monolayer moisture contents were in the range from 3.8% to 4.9% d.b., and the whole range of associated water activity based on BET equations varied between 0.013 and 0.225. Net isosteric heat of sorption, evaluated using the Clausius-Claperon equation, and differential entropy values of stover fractions decreased exponentially with increased moisture and approached the latent heat of vaporization of pure water at around 20% d.b. The moisture sorption process was determined as enthalpy driven. Inequality in isokinetic and harmonic mean temperatures confirmed the enthalpy-entropy compensation theory. Among stover fractions, leaf had the greatest spreading pressure, followed by stalk skin and stalk pith. Spreading pressures increased with increase in water activity and decreased with temperature increase. Net integral enthalpy increased to a maximum and then decreased with increasing moisture content, whereas net integral entropy displayed an opposite trend. Mean values of the net integral enthalpy and entropy of stalk pith were the highest and progressively decreased for leaf and stalk skin.