Biomass and Bioenergy Vol. 3. No. 6, Pp. 381-387, 1992 0961-9534/92 55.00 + 0.00 Printed in Great Britain. All rights reserved 0 1992 Pergamon F’ress Ltd

HARVESTING OPERATIONS AND ENERGETICS OF TALL GRASSES FOR BIOMASS ENERGY PRODUCTION: A

CASE STUDY

P. MISLEVY*$ and R. C. FLUCK~

*Agricultural Research and Education Center, University of Florida, Ona, FL 33865, U.S.A. tDepartment of Agricultural Engineering, University of Florida, Gainesville, FL 32611, U.S.A.

(Received 16 June 1992; revised received 9 July 1992; accepted 13 July 1992)

Abstract-The U.S.A. imports about 50% of its energy needs while Florida imports about 85%. Among the renewable energy sources available, biomass appears promising especially in the southeast which includes Florida because of a favorable environment for production and the available methods to convert biomass to energy. Optimal production of biomass requires the identification and management of high yielding persistent perennial cultivars. Elephantgrass (Pemtierum purpureum Schum.) and energycane (Sac&rum spontaneum L.) are two tall grasses that meet these requirements. To optimize the supply of convertible biomass, suitable methods of harvesting the crop must be available. The purpose of this research was to study the feasibility and energetics of harvesting, drying, and baling tall grasses with conventional farm machinery.

A Mathews rotary scythe and a New Holland 849 Auto Wrap large round baler were determined to provide a practical harvesting system for baled biomass averaging 15-27 Mg ha-‘. The rotary scythe can be used for harvesting and fluffing or turning a windrow over to expedite drying. This harvesting system requires about 3 kg diesel fuel Mg-’ dry biomass (DB), 25 min of time Mg-’ DB, and a cost of about $10 to 12 Mg-’ DB. Energy requirements of harvesting operations would be about 300-375 MJ Mg-’ DB, and primary energy requirements for production and harvesting are about 1100-1500 MJ Mg-’ DB. For each unit of fossil fuel invested in the total production and harvesting system, 12-15 units would be returned in biomass.

Keywords-Harvesting and baling biomass, energetics, tall grasses, fuel consumption.

1. INTRODUCTION

As world fossil fuel supply declines, it is impera- tive that renewable energy sources be utilized. One alternative is the use of biomass crops for energy. Tall, perennial, tropical grasses are some of the most efficient energy-storing species found in the plant kingdom.’

Fermentation, to convert biomass to energy, has many advantages, the most important of which is the utilization of wet and dry feed- stocks.2 This system allows the use of direct cut, ensiled, or dried biomass from storage. Direct cutting under tropical conditions has the advan- tage of biomass utilization when plants cannot be dried for storage because of frequent rainfall. Optimal production of energy from biomass requires the identification, production, harvest, and storage of high yielding perennial cultivars. These cultivars must tolerate periods of drought,

jcorresponding author. Florida Agricultural Experimental Station Journal Series

No. R-02547.

saturated soil conditions, limited fertility, ne- matodes, insects, diseases etc. During a 5 year period, tall growing grass and broadleaf herba- ceous species were tested for biomass herbage yield and persistence. 3*4 Elephantgrass or napier- grass (Pennisetum purpureum L. “PI 300,086”) and energycanes (Saccharum spontaneum L. “L79-1002”, Saccharum spp. “US 72- 1153”) were three entries selected for further testing due to their high DB yields, rapid growth rates, freedom from pests and persistence rather than sugar content. 5*6 All three entries are C4 tropi- cal, perennial, tall, strong-stemmed grasses that produce considerable DB per unit of fertilizer and require little or no supplemental water.

A successful biomass energy program re- quires a year-round supply of economical biomass material. Therefore, mechanical har- vesting, drying, and baling methods need to be researched for tall-grass biomass crops to be used for energy. This paper will address harvest- ing operation experiences (cutting, fluffing, rak- ing, and baling) and the fuel consumption, time requirements, and energetics associated with

381

382 P. MISLEVY and R. C. FLUCK

elephantgrass and energycane operations. Using these data, total production and harvesting costs will also be estimated.

2. PROCEDURE

The experiment was conducted on a Smyrna fine sand (sandy, siliceous, hyperthermic, Aeric Haplaquod) located in south central Florida (27”25’N, 8 1 ‘SSW), at the University of Florida, Agricultural Research and Education Center, Ona, FL. In mid-April of 1984, 168 kg N, 20 kg P, and 72 kg K ha-’ were applied to encourage establishment. Elephantgrass and en- ergycane were each established on 0.4 ha in December of 1983. Stem pieces each containing two nodes were placed in the soil approximately 0.6m apart within rows, and averaged 0.9m between rows.

Annual fertilization thereafter was 168 kg N, 25 kg P, and 93 kg K ha-’ applied in mid- to late-March. The micronutrient mixture F 503 (containing the following elemental contents: Fe, 18%; Zn, 7.0%; Mn, 7.5%; Cu, 3.0%; and B, 3.0%) was applied annually at 20 kg ha-’ except in 1990 when 2.8 kg ha-’ of Cu, Zn, Fe, and Mn were applied in the sulfate form. Total S applied in 1989 and 1990 was 18 and 10 kg ha-‘, respectively. The biomass site was har- vested annually in late October, starting in 1985 and continued over a 6 year period.

Diesel fuel consumption and operating time were recorded for most harvesting operations of energycane and elephantgrass from 1985 through 1990. The tractors’ fuel tanks were care- fully topped to the same point both before and after each operation and the fuel consumption determined by weight differences of the refuel- ing container. Time was recorded by stopwatch.

The harvesting system was analyzed from a production cost basis. All costs for equipment, labor, and fuel were calculated based on operat- ing results and totalled for the proposed har- vesting system as configured in 1990 (Table 1). Assumptions’ included 10,000 h of use over 15 years for tractors, 2000 h of use over 10 years for other equipment, labor at $8.00 h-‘, 10% annual interest, diesel fuel cost SO.264 I-‘, total lifetime repair and maintenance cost of 80% of cost for equipment and 100% cost for tractors, and the sum of taxes, insurance, and housing at 2% initial cost yearly.

An energy analysis was performed on the harvesting and total production system. Costs of depreciation, interest, repairs, maintenance, taxes, insurance, and housing, as well as labor and diesel fuel consumption were each con- verted to their total primary energy require- ments using conventional methods.’ The harvesting and total production system energy requirements were compared to the energy con- tained in the harvested DB. Also, the energy contained in the harvested DB was compared with harvesting costs.

3. RESULTS

3.1. Harvesting operations

3. I. 1. Harvesting John Deere 1326 mower- conditioner. In 1985, a John Deere 1326 Impeller mower-conditioner (Deere and Co., Moline, Ill.) was purchased to harvest mature energycane and elephantgrass. This machine has oval disks that rotate horizontally. Each oval disk contains 2 knives, attached at opposite ends. These knives rotate at about 3060 rpm.

Table 1. Equipment used for harvesting operations of tall grasses

Operation and year Equipment Tractor

Cutting 1985-89 Mathews M-C 9E Rotary Scythe Ford 6600 1990 Mathews M-C 9E Rotary Scythe Ford 8350

Flusfng 1985-86 John Deere 1326 Impeller Mower-Cond Ford 6600 1987-90 Mathews M-C 9E Rotary Scythe Ford 6600

Raking 1986 John Deere Side Delivery Rake Deere 1250

Baling 1985 New Holland Auto Wrap 849 Round Baler IH 966 1986 New Holland Auto Wrap 849 Round Baler MF 265 1987-90 New Holland Auto Wrap 849 Round Baler Deere 2950

Tall grasses for biomass energy 383

The machine appeared to have capability and durability to cut clusters of dense stems each 5-6 m tall and a stem diameter of 2-4 cm, with an average DB yield of 18-20 Mg ha-‘. Several hours of continued use of this machine in tall grasses resulted in the main drive belt (rotor belt) slipping and breaking. A new rotor belt was installed which also quickly failed. During the initial 1 to 2 h of use, several of the heavy duty knives broke and were thrown through the back curtain and some 35 m beyond the ma- chine. For additional safety to the tractor oper- ator, a steel cage consisting of a rear and right side panel was constructed, to prevent injury from flying plant parts or damaged blades. After several days of machine repair and frustration it was concluded that this rotary-conditioner was not designed to harvest the mature elephant- grass and energycane species tested in this study.

Advantages:

(1)

(2)

Machine can rapidly recut through har- vested plant material, if needed. Harvested plant material pass through high speed (850 rpm) flails, damaging stems and waxy cuticle for faster drying.

Disadvantages:

(1) Machine not designed to cut biomass yield- ing 20-25 Mg ha-‘.

(2) Main cutting mechanism is driven by belt. (3) Knives not sufficiently durable to cut high

yielding biomass. (4) Broken knives become a safety hazard.

Mathews rotary scythe. The second machine tested was a M-C Rotary Scythe Model 9 E (B. C. Mathews Co., 500 Industrial Ave, Crystal Lake, IL 60014). This flail shredder mower appeared to be designed to cut plant material yielding high biomass. The main cutting mech- anism (rotor assembly) is driven by a double chain from the heavy duty 100 h.p. gear box. The machine contains no belts.

To harvest 5-6 m tall grass plants at a smooth and consistent travel speed, a parting blade attachment needs to be constructed and at- tached to the right side of the rotor assembly. This blade would sever tall plants that have been cut at the base but have fallen to the right side, where they accumulate, before being drawn through the machine. As harvested plants pass through the machine they are chopped into shorter lengths (stems range from 0.6-3.0 m long) and are partially conditioned (damage to

stems, leaves, waxy cuticle) which aids in the drying process. Harvested plants can be directly placed into a swath for fast drying, or by adjustment of the windrower wings any size windrow can be formed.

Since the purchase of the M-C rotary scythe in 1985, few mechanical problems have devel- oped. It is not known how many acres can be successfully harvested before the machine develops major mechanical problems.

Advantages:

(1)

(2)

(3)

(4)

Main cutting mechanism driven by a heavy duty chain and gear box, making it quite durable. High speed cutting flails partially condition stems for more rapid drying. Adjustable windrower wings for placement of windrow. Can rapidly recut through harvested plant material.

Disadvantages:

(1) Needs parting blade to prevent build up of harvested biomass.

(2) Field capacity for harvesting is relatively low (0.29 ha h-‘).

The use of a sugarcane harvesting machine (not tested) may also have potential for cutting elephantgrass and energycane. These type of machines have the capability of cutting tall grass plants at a much faster speed (1.9-3.5 ha h-‘) when compared with the M-C rotary scythe. Sugarcane harvesting machines cut the plant material near the soil surface, allowing plants to remain intact or in various size pieces from 0.5-3.3 m, with no damage to the stems. There- fore, plant material cut by the sugarcane har- vester would need to be conditioned by another machine to expedite drying. The M-C rotary scythe has a low field capacity but does harvest and condition in one operation.

3.1.2. Drying. Once mature biomass consist- ing of 2-4 cm stem diameters is harvested and conditioned, approximately 7-10 days is re- quired before moisture content of plants drops to 15-20%. This is the general moisture content range needed for safe storage.

John Deere 650 side-delivery rake. Even with no rainfall for a 7-10 days period, some type of windrow movement is required to allow drying of plant parts in direct contact with the soil. The most desirable method would be to turn the windrow over with a side-delivery rake. This

384 P. MISLEVY and R. C. FLUCK

would allow biomass on the soil surface to be exposed to the sun, for more uniform drying. The John Deere 650 side-delivery rake (Deere and Co., Moline, Ill.) tested in this study was not capable of rolling heavy windrows of biomass yielding 20 Mg ha-’ DB (Table 2). When plant material in the windrow was 50-75% dry and evenly distributed, the John Deere model 650 side-delivery rake would roll the windrow.

Any time a harvested crop requires more than 2-3 days of drying time, the probability in- creases of that crop getting wet from rain. About 65% of the time the biomass harvested at this experimental site was exposed to rainfall. During some years the windrows of biomass were exposed to several days of rain. Once rainfall ceases and the upper surface of the windrow dries, some type of windrow move- ment is essential to prevent fungal infection on the wet biomass. As indicated earlier, the side- delivery rake tested was not capable of rolling 0.5 by 1.0 m wet windrows.

Advantages:

(1) Will turn windrows of biomass with low (9 Mg ha-‘) DB yields.

Disadvantages:

(1) Not designed to turn windrows of biomass containing 20 Mg ha-’ DB yields.

John Deere 1326 mower-conditioner. The John Deere 1326 impeller mower-conditioner was subsequently evaluated for turning the windrow. Due to the high speed (3060 rpm) of the rotating disks and knives, the cutting plat- form of the machine can pass under a windrow at a height of about 2.5-5.0 cm above the soil surface. As the machine moves down the windrow, the harvested biomass passes through

Table 2. Dry biomass yields of elephantgrass and energycane’ calculated from bale weights

over a 6 year period

Year Elephantgrass Energycane

Mg ha-’ 1985 16.8 19.3 1986 17.1 15.2 1987 23.3 18.8 1988 27.1 21.4 1989 19.5 18.6 1990 17.0

Average 20. I 18.7

*Energycane consists of L 79-1002 and US 72-1153.

the rotating impellers, fluffing and stirring the material.

Advantages:

(1)

(2)

(3)

Machine will fluff or move harvested biomass windrows rapidly. Machine will easily pass through the same windrow several times. Impellers break stems and damage waxy cuticle for faster drying.

Disadvantages:

(1) The machine may pass over about 2.5-5.0 cm of biomass in direct contact with the soil, especially after a heavy rain, which forces plant material in direct soil contact.

(2) Windrower wings are stationary, limiting windrow movement.

Mathews rotary scythe. The same machine used to harvest biomass can also be passed down the windrow to aid in drying by recutting and slashing stems. With proper adjustment of the windrower wings, biomass discharged from the machine can be turned over. This allows the wet material to rest on top of the windrow, being exposed to wind and sun for rapid drying. Since the plant material in the windrow is already cut and conditioned at least once, both this machine and the John Deere mower-condi- tioner can stir and rotate the windrow at a rapid speed (8-10 km h-‘). This procedure can be repeated once or twice without serious mulching of harvested material.

Advantages:

(1)

(2)

(3) (4)

(5)

Can fluff and rotate harvested and con- ditioned biomass windrows rapidly. Will pick up biomass in direct contact with soil. Adjustable wings aid in rotating windrows. Tends to fluff windrow higher than other machines tested. Flails tend to recut long stems for faster drying and easier baling.

3.1.3. Baling. New Holland 849 round baler. Once the moisture content of the biomass dropped to 35% or lower, the material was easily baled with a New Holland Auto Wrap 849 round baler. The only baling problem en- countered during the 6 year period of this study was a feeding problem which happened infre- quently, due to a large pile of biomass in the windrow. One year the authors attempted to bale freshly cut biomass; the baler experienced

Tall grasses for biomass energy 385

difficulty in discharging the high moisture bales especially if the baler was manually triggered to discharge a bale before completion.

Table 4. Time requirements for harvesting operations of tall grasses

Operation Grass mm ha-’

Advantages:

(1) Biomass easily baled when moisture dropped to 3540% or lower.

(2) Machine appeared to be designed to handle large tonnage of biomass, encountering no serious problems over the 6 year period of study.

Cutting

Fluffing

Baling

Totals?

Energycane Elephantgrass Energycane Elephantgrass Energycane Elephantgrass Energycane Elephantgrass

212.3 203.4 69.9 47.8

137.8 160.4 420.0 411.6

min Mg-’ DB*

12.8 10.1 6.0 2.6

10.4 9.0

29.3 21.7

*DB = oven dry biomass. tTotals represent time requirements for cutting, fluffing,

and baling. Disadvantages:

(1) Machine had difficulty discharging freshly cut biomass bales, when triggered to dis- charge prematurely.

with about 40% of the cost ha-’ in baling. This would indicate about 10% of the total harvest- ing costs are involved in each fluffing.

3.2. Fuel consumption, time requirements and energetics

Equipment used and the operations per- formed, varied over time, as experience was gained and equipment availability changed (Table 1). Raking was performed only in 1986, then eliminated because it could not be per- formed satisfactorily with the John Deere side delivery rake used.

Total production costs including establish- ment, harvesting, fertilizer, and lime totalled $614 ha-’ for both grass species and ranged from $28.~$34.7 Mg-’ DB for elephantgrass and energycane, respectively (Table 6).

Energycane harvesting operations required slightly less fuel per hectare and more fuel per unit DB harvested than elephantgrass harvest- ing operations (Table 3). Dry biomass yields of elephantgrass were slightly higher (20.1 Mg ha-‘) than energycane (18.7 Mg ha-‘) (Table 2).

The amount of biomass energy harvested per dollar of total harvesting system cost was 1232 MJ for energycane and 1504 MJ for ele- phantgrass. The estimated amount of energy per dollar of total production cost, planting through harvesting, was 509 and 620 MJ for energycane and elephantgrass, respectively (Table 6).

Energycane required slightly more total time for the various harvesting operations than ele- phantgrass on both per unit area and per unit DB basis (Table 4).

Total harvesting costs were $236 ha-’ and $11.8 Mg-’ of DB for energycane and $236 ha-’ and $9.6 Mg-’ of DB for elephant- grass (Table 5). These data indicate approxi- mately 50% of the total harvesting operation cost/ha are in the actual harvesting procedure,

Taking the energy content of the harvested DB at 17.6 MJ kg-‘, the energy ratio of the energycane harvesting was calculated at 49 and that of the elephantgrass at 58 (Table 7). The energy ratio of the harvesting operations is the energy content of the harvested biomass divided by the total primary energy required for the complete harvesting system. This means that for every unit of fossil fuel energy invested in harvesting operations, 49-58 units of biomass energy were harvested.

In addition to harvesting operations, the other major energy input for tall grass pro- duction is fertilizer. Energy requirements for the

Table 3. Average diesel fuel consumption over 6years for harvesting tall grasses

Operation Grass kg ha-’ kg Mg-’ DB*

Cutting Energycane 25.0 1.46 Elephantgrass 28.3 1.41

Fluffing Energycane 5.2 0.31 Elephantgrass 7.4 0.35

Baling Energycane 22.8 1.41 Elephantgrass 19.2 1.09

Totalst Energycane 53.0 3.18 Elephantgrass 54.9 2.85

*DB = oven dry biomass. tTotals represent energy consumed for cutting, fluffing,

and baling.

Table 5. Calculated costs for harvesting operations of tall grasses

Operation Grass % ha-’ % Mg-’ DB*

Cutting Energycane 117 5.8 Elephantgrass 114 4.7

Fluffing Energycane 30 1.5 Elephantgrass 21 0.9

Baling Energycane 89 4.4 Elephantgrass 101 4.1

Totals? Energycane 236 11.8 Elenhantgrass 236 9.6

*DB = oven dry biomass. tTotals represent costs of all harvesting operations

(cutting, fluffing, and baling).

386 P. MISLEVY and R. C. FLLJCK

Table 6. Total biomass production cost estimates and biomass energy harvested per dollar of expense for energycane and elephantgrass

Cultural practice

Establishment* Fertilizer and lime7 Harvesting

Production cost estimates

Energycane Elephantgrass Energycane Elephantgrass

% ha-’ 8 Mg-’ DB 1333 133 S.l§ 6.6 245 245 14.8 12.2 236 236 11.8 9.6

Energy harvested/dollar expense

Energycane Elephantgrass

MJ %-’

Total 614 614 34.7 28.4 509 620

*Establishment costs prorated over a 6 year priod. Value used was calculated from sugarcane results.g tLime + Dolomite spread @ 560 kg ha-’ year-‘. ICost estimates rounded to nearest dollar. @Cost estimates rounded to nearest tenth of a dollar.

annual fertilization and one-sixth the starter fertilization are about 16,500 MJ ha-’ annually. Estimating planting and annual fertilizing appli- cation energy requirements and adding these to fertilizer and total harvesting energy brings the total annual energy requirements for tall grass to approximately 24,000 MJ ha-‘. Based on these inputs and our yields, we would predict that a complete system including land prep- aration and planting (performed only once every 6-8 years) would have an energy ratio of about 13-15.

diesel fuel ha-‘. Time requirements for all har- vesting operations, based on our yields, would be about 420min ha-’ or 25 min Mg-’ DB. Harvesting costs should be about $236 ha-’ and about $10 to 12 Mg-’ of DB. Harvesting oper- ations energy requirements would be about 6000 MJ ha-’ and 300-375 MJ Mg-’ DB. Total primary energy requirements for production and harvesting would be about 24,000 MJ ha-’ and 1200-1300 MJ Mg-’ of DB. Each unit of fossil fuel invested in the total production and harvesting system would return 13-15 units in the form of biomass.

4. SUMMARY AND RECOMMENDATIONS Research Needs:

Based on our experience, a practical harvest- ing system could be developed with the Mathews M-C 9E rotary scythe or similar rotary mower for cutting as well as for fluffing, followed by the New Holland 849 Auto Wrap or similar large round baler, each powered by a suitable tractor. The impeller mower-condi- tioner can also be used to fluff a harvested windrow. With such a harvesting system, total fuel consumption for all harvesting operations (cutting, fluffing and baling) would be about 3 kg of diesel fuel Mg-’ of DB and 54 kg of

(1)

(2)

Various types of sugarcane harvesting ma- chines need to be tested on elephantgrass and energycane for satisfactory operation and adequate field capacity and compare data with results obtained from the M-C rotary scythe. The M-C rotary scythe needs to be tested as a conditioner of plant material, harvested and windrowed by the sugarcane harvester, which presently does not condition plant material.

Table 7. Total primary energy requirements for harvesting operations of tall grasses

Operation Grass MJ ha” MJ Mg-’

Cutting Energycane 2889 175.0 Elephantgrass 3014 149.7

FhrIiing Energycane 658 39.8 Elephantgrass 667 33.1

Baling Energycane 2409 145.9 Elephantgrass 2391 118.8

Totals* Energycane 5956 360.7 Elephantgrass 6072 301.6

*Totals represent cutting, fluffing and baling energy requirements.

(3)

(4)

(5)

(6)

Better methods of sampling are needed to determine bale storage losses over time. Additional research regarding mechanical planting of biomass stem pieces is needed. Influence of stubble height and physiologi- cal stage of harvest on persistence of ele- phantgrass and energycane entries. Comparison of biomass stored as silage (40% DB) and large bales dried (80% DB) regarding costs and energetics.

Acknowledgements-This research was funded in part through an agreement between the Institute of Food and Agricultural Sciences (IFAS), University of Florida and the Gas Research Institute (GRI), Chicago, IL.

Tall grasses for biomass energy 387

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