M. F. Digman

Optimizing on-farm pretreatment of perennial grasses for fuel ethanol production.

Switchgrass (Panicum virgatum L.) and reed canarygrass (Phalaris arundinacea L.) were pretreated under ambient temperature and pressure with sulfuric acid and calcium hydroxide in separate experiments. Chemical loadings from 0 to 100g (kg DM)(-1) and durations of anaerobic storage from 0 to 180days were investigated by way of a central composite design at two moisture contents (40% or 60% w.b.). Pretreated and untreated samples were fermented to ethanol by Saccharomyces cerevisiae D5A in the presence of a commercially available cellulase (Celluclast 1.5L) and beta-glucosidase (Novozyme 188). Xylose levels were also measured following fermentation because xylose is not metabolized by S. cerevisiae. After sulfuric acid pretreatment and anaerobic storage, conversion of cell wall glucose to ethanol for reed canarygrass ranged from 22% to 83% whereas switchgrass conversions ranged from 16% to 46%. Pretreatment duration had a positive effect on conversion but was mitigated with increased chemical loadings. Conversions after calcium hydroxide pretreatment and anaerobic storage ranged from 21% to 55% and 18% to 54% for reed canarygrass and switchgrass, respectively. The efficacy of lime pretreatment was found to be highly dependent on moisture content. Moreover, pretreatment duration was only found to be significant for reed canarygrass. Although significant levels of acetate and lactate were observed in the biomass after storage, S. cerevisiae was not found to be inhibited at a 10% solids loading.

Single-pass, split-stream harvest of corn grain and stover.

A grain combine was equipped with a whole-plant corn head and modified to produce single-pass, whole-plant corn harvesting with two crop streams: grain and stover. Capture of potential stover DM varied from 48% to 89% for leaves, from 49% to 92% for stalks, and was greater than 90% for husks and cobs, depending on corn head height. Stover aggregate moisture varied between 36% and 50% (w.b.), and area capacity ranged between 1.6 and 2.6 ha h-1, depending on corn head height. Whole-plant harvesting reduced area capacity by nearly 50% compared to harvesting with a conventional ear-snapping head. Single-pass stover had an average particle size of 69 mm and bulk densities of 51 and 110 kg DM m-3 in the wagon and bag silo, respectively. Estimated ethanol yield ranged between 2600 and 3945 L ha-1, depending on corn head height. Fermentation of single-pass stover in a bag silo was adequate, with average losses of 6% of total DM.

Pilot-scale on-farm pretreatment of perennial grasses with dilute acid and alkali for fuel ethanol production.

Switchgrass (Panicum virgatum L.) and reed canarygrass (Phalaris arundinacea L.) were pretreated during anaerobic wet storage by adding sulfuric acid or calcium hydroxide (50 g kg-1 DM). Experiments were conducted at both laboratory scale (250 g DM) and pilot scale (250 kg DM) for either 60 or 180 days. Pretreated and untreated samples were fermented to ethanol by Saccharomyces cerevisiae in the presence of a commercially available cellulase (Celluclast 1.5L) and s-glucosidase (Novozyme 188). With acid pretreatment, conversion of cellulose to ethanol was 35 and 12 percentage units higher than for untreated controls for reed canarygrass and switchgrass, respectively. Similarly, lime-pretreated reed canarygrass and switchgrass out-yielded controls by 3 and 13 percentage units, respectively. Cellulose and lignin contents were largely unaffected by pretreatment and anaerobic storage, but hemicelluloses were lower for both pretreatments.

On-farm Pretreatment Technologies for Improving Enzymatic Degradability of Cellulose and Hemicellulose Present in Perennial Grass

This research investigated the ability of on-farm pretreatments with acid, alkali, ozone or novel enzymes to improve enzymatic degradability of cellulose and hemicelluloses in biomass at the biorefinery. Two perennial grasses, switchgrass (Panicum virgatum) and reed canarygrass (Phalaris arundinacea L.), were direct-cut harvested, pretreated, and stored anaerobically for 30 d. Pretreated and untreated samples were fermented to ethanol by Saccharomyces cerevisiae in the presence of commercial cellulase for 72 hr to ethanol. Xylose yields were also measured following fermentation because it is not metabolized by S. cerevisiae. Sulfuric acid and lime pretreatment technologies look promising considering the high conversion yields and ease of application. Efficiencies of nearly 80% for cellulose conversion to ethanol and hemicellulose to xylose were realized, albeit at high chemical loadings. Ozonolysis results demonstrated similar success, but integration of this technology into current storage systems will be challenging. Enzyme addition (xylanases or feruloyl esterase) at ensiling only marginally improved conversion efficiency.

Real-Time Moisture Measurement on a Forage Harvester Using Near-Infrared Reflectance Spectroscopy

A mobile, diode-array NIR spectrometer was integrated into the spout of a self-propelled forage harvester to measure crop moisture. Spectra and moisture reference samples were collected in 2004 and 2005 for the development of laboratory and field-based moisture calibrations. Moisture prediction models for whole-plant corn silage (WPCS) developed using laboratory data had a root mean standard error of cross-validation (RMSECV) of 1.1% using five principle components (PCs), while a calibration developed using field data had an RMSECV of 3.3% using four PCs. Alfalfa validation results produced RMSECVs of 2.5% using four PCs and 3.7% using three PCs for models using laboratory and field data, respectively. Field data were predicted with calibrations developed using laboratory data with similar error levels, but more spectral information was required. A laboratory-based alfalfa model predicted field data with a root mean standard error of prediction (RMSEP) of 3.4% using three PCs as compared to the field models RMSECV of 3.7% using three PCs. Similar trends were found with WPCS models. Predicting data independent of type of crop resulted in the utilization of more PCs but with higher RMSEPs than the cross-validation results of the predicted dataset. The sensor and associated calibrations were able to predict forage moisture adequately, although more diverse data and further calibration development are needed to improve sensor accuracy to the desired range of ±2.0 percentage units.

Harvest Fractionation of Alfalfa

Fractionation of alfalfa leaves and stems at harvest could allow ruminant rations to be tailored for optimum economic return or improve the viability of alfalfa as a biomass feedstock. Harvest fractionation was done by stripping the leaves from the stem at the time of harvest using a tined rotor. The stripped fraction consisted of about 90% leaf tissue, and 94% of the available leaf dry matter (DM) yield was removed in the stripped fraction. The standing fraction was either cut immediately after stripping or allowed to stand and regrow leaves for a period of 7 or 14 days. Leaf regrowth was evident in three to five days, but leaf yield was much less than that at initial stripping. The particle size of the stripped fraction was no different than chopped whole-plant alfalfa, so no further size reduction of the stripped fraction was needed before ensiling. The density of the stripped fraction was 11% greater than that of the chopped whole-plant in a drop hammer density test. The stripped fraction was successfully ensiled in mini-silos using ground corn grain as an amendment or formic acid as an additive. After cutting and windrowing, the drying rate of the standing fraction (mainly stems) was greater than that of whole-plant windrows of similar density. The standing fraction, consisting of 92% stems, dried to ensiling moisture typically within about 4 to 6 h after stripping and cutting but in as short as 1.5 h under very good drying conditions. Therefore, a single-day fractionated harvesting scheme is possible.

Improving ethanol production from alfalfa stems via ambient-temperature acid pretreatment and washing

The concept of co-production of liquid fuel (ethanol) along with animal feed on farm was proposed, and the strategy of using ambient-temperature acid pretreatment, ensiling and washing to improve ethanol production from alfalfa stems was investigated. Alfalfa stems were separated and pretreated with sulfuric acid at ambient-temperature after harvest, and following ensiling, after which the ensiled stems were subjected to simultaneous saccharification and fermentation (SSF) for ethanol production. Ethanol yield was improved by ambient-temperature sulfuric acid pretreatment before ensiling, and by washing before SSF. It was theorized that the acid pretreatment at ambient temperature partially degraded hemicellulose, and altered cell wall structure, resulted in improved cellulose accessibility, whereas washing removed soluble ash in substrates which could inhibit the SSF. The pH of stored alfalfa stems can be used to predict the ethanol yield, with a correlation coefficient of +0.83 for washed alfalfa stems.

Wet Fractionation for Improved Utilization of Alfalfa Leaves

Utilization of alfalfa could be greatly improved if the protein-rich leaves were efficiently separated and preserved from the fibrous stems. This work envisions a new harvest scheme combining three processes: mechanical leaf separation, pressing, and anaerobic storage. To quantify the effectiveness of leaf dewatering, experiments were conducted in which leaves were pressed in a replicated factorial design, including maceration and four levels of backpressure. The amount of press filtrate extracted varied proportionally with press backpressure from 211 to 612 L Mg-1 fresh leaves and was composed of about 90% water. The resulting partially dewatered leaves were successfully ensiled and were found to be chemically similar to high-quality, whole-plant alfalfa silages. Additionally, we demonstrated that nutritionally valuable components in the press filtrate could be conserved by anaerobic storage. Based on our work, protein and lactic acid could be obtained from the ensiled press filtrate in quantities as high as 300 and 143 kg ha-1, assuming an average annual leaf yield of 10 Mg ha-1 and optimal process conditions. However, more work is necessary to determine these values.

Single-Pass, Split-Stream of Corn Grain and Stover: Characteristic Performance of Three Harvester Configurations

A grain combine was modified to produce single-pass, whole-plant corn harvesting with two crop streams, grain and stover. Three corn heads were used: ear-snapper, stalk-gathering and whole-plant. Capture of potential stover DM was 30, 67 and 90% of DM for a combine harvester configured with an ear-snapper, stalk-gathering and whole-plant heads, respectively. Stover aggregate moisture was 51.0 and 52.5% (w.b.) for the whole-plant and stalk-gathering heads (front wagon only), respectively. Aggregate moisture of stover from the ear-snapper and stalk-gathering heads (rear wagon only) was 38.5% (w.b.). When the stalk-gathering or whole-plant heads were used, greater stover feedrate limited ground speed, so area capacity was 3.4, 2.2, and 2.0 ha/h, for the ear-snapper, stalk-gathering and whole-plant heads, respectively. Wet and dry bulk density was 163 and 100; 147 and 70; and 80 and 38 kg/m3 for the ear-snapper, stalk-gathering and whole-plant heads, respectively. Fermentation of single-pass stover in a bag silo was very good with DM losses after eight months of storage of 4.1 and 6.7% for the material harvested with the whole-plant and stalk-gathering heads, respectively.

Characteristic Performance and Yields using a Single-Pass, Split-Stream Maize Grain and Stover Harvester

A grain combine was modified to produce single-pass, whole-plant corn harvesting with two crop streams, grain and stover. Capture of potential stover DM varied from 48 to 89% for leaves, 49 to 92% for stalks, and greater than 90% for husks and cobs, depending upon corn head height. Stover aggregate moisture was 50.2, 43.1 and 36.4% (w.b.) when the corn head height was 10, 44 and 63% of ear height, respectively. Greater MOG feedrate limited ground speed due to power availability, so area capacity was 2.3, 2.8 and 3.4 ha/h when corn head height was 10, 44 and 63% of ear height, respectively. Whole-plant harvesting reduced area capacity by nearly 61% compared to harvesting with a conventional snapping-roll head. Single-pass stover had an average particle size of 69 mm and bulk density of 51 and 110 kg DM/m3 in the wagon and bag silo, respectively. Based on polymeric sugar content, estimated ethanol yield was 3,945, 3,230, and 2,600 L/ha when the corn head height was 10, 44 and 63% of ear height, respectively. Fermentation of single-pass stover in a bag silo was adequate with average losses of 6% of total DM.