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Gray, A., C. Anderson, E. Koppelman, B. Bjornsen, K. Frank, and M. Siedell. 1996. Alfalfa stems: Potential biofuel for woodstoves. p. 260-262. In: J. Janick (ed.), Progress in new crops. ASHS Press, Alexandria, VA.

Alfalfa Stems: Potential Biofuel for Woodstoves

A. Gray, C. Anderson, E. Koppelman, B. Bjornsen, K. Frank, and M. Siedell


  1. METHODOLOGY
  2. RESULTS
    1. Combustion Characteristics of Alfalfa Plant Components
    2. Nutritional Quality of Alfalfa Separation Co-Products
  3. SUMMARY
  4. REFERENCES
  5. Table 1

Irrigated alfalfa (Medicago sativa L., Fabaceae) is a high-yielding, nitrogen-fixing perennial when produced on high pH soils in the arid Western United States. For most soil types in this region, phosphorus is the only soil amendment required for alfalfa production. Irrigated alfalfa is well adapted to the high elevation (1070-2200 m) growing conditions of Wyoming. At least three harvests are possible (Gray et al. 1992) during the growing season for areas located at elevations up to 1680 m. Livestock is the most important commodity in Wyoming's forage-based agriculture. Hay continues as Wyoming's most economically important agronomic crop, but unstable demand and price fluctuations have piqued interest in the use of hay as a value-added, renewable biofuel. This prompted an evaluation of alfalfa stems as an environmentally friendly, biofuel alternative for woodstoves.

METHODOLOGY

Samples from eleven randomly-selected harvest lots of alfalfa hay from each of first, second, and third cuttings were subjected to a leaf-stem separation process. Hay bales from each lot were sampled with a coring tool (50.8 mm diam) designed to sample baled wool. For each harvest lot, a hay sample was divided into two equal fractions with a riffle-splitter. One fraction was leaf-screened once with a Number 32 sieve (12.7 mm diam) and then twice again with a Number 12 (3.2 mm diam) to separate leaves from stems.

Leaf and stem separations, including whole hay (unseparated) samples, were analyzed for energy, fixed carbon, ash, volatile gases, and sulfur (American Society for Testing and Materials 1993). Hay samples were also evaluated for crude protein (CP), acid and neutral detergent fiber (ADF and NDF) , and relative feed value (RFV) to determine analytical characteristics (AOAC 1990; U.S. Alfalfa Hay Quality Committee 1986) important in ruminant nutrition. Differences among cuttings, leaves, stems, and whole samples were determined with a 3 by 3 factorial experimental design. An economic evaluation of leaf-stem separation processes or the market potentials of separation co-products is not addressed in this study.

The combustion characteristics of alfalfa co-products were compared with those for bituminous coal. Combustion reference values for coal from the Powder River Basin (PRB) of northeastern Wyoming and for coal from eastern U.S. appear in Table 1. The PRB coal is considerably lower in sulfur when compared to coal from eastern U.S. On the other hand, a major consideration in the cost of shipping is the moisture content of PBC at 30% compared to 3%-13% for coal from the eastern U.S. The moisture content of pelletized alfalfa might vary but would be less than 10%. All combustion and nutritional quality characteristics in this study are reported on a dry matter basis.

RESULTS

Combustion Characteristics of Alfalfa Plant Components

Ash is an approximate measure of the mineral content and other inorganic matter which remains as residue after the combustion of organic material. Ash presents a disposal problem, particularly in biofuels with a higher ash content. Stems had a lower (P = 0.01) ash content than leaves (Table 1). In general, the ash content of alfalfa components was quite similar to the ash content of PRB coal (Table 1).

The test for volatiles determines the percentage of gaseous products (exclusive of moisture) as a result of combustion. High values resulting from a volatile test indicate a higher potential to produce smoke. Volatile gases produced from alfalfa components were considerably greater than reference values for volatiles produced from coal (Table 1). Volatiles are sometimes used to rank or determine the market value of coal. However, in the case of an alfalfa biofuel product, volatiles might not be important because wood pelletstoves are practically smokeless. Significant differences in volatile gas contents were not detected among cuttings or plant parts.

The Environmental Protection Agency standard for sulfur allows 0.523 kg of sulfur dioxide emission per million British Thermal Units (BTU). Thus, the lower sulfur dioxide emission potential of alfalfa is attractive even with the lower BTU content (Table 1). Compared to stems, the sulfur dioxide and elemental sulfur content of leaves were considerably greater (P = 0.01) and probably relate to higher levels of crude protein (CP) in leaves.

Coal is marketed on the basis of moisture ash free (MAF) BTU content. On a dry matter basis, coal has a considerably higher MAF BTU content than alfalfa hay, but the PRB coal also has a moisture content of about 30%. The cost of removing moisture or transporting a high moisture product can be considerable, but the cost of milling and pelleting hay with an air dry moisture content of 7 to 13% can also be significant. In this study, alfalfa leaves had a slightly greater (P = 0.01) MAF BTU content than stems.

Fixed carbon, an indication of the ratio of combustible to incombustible constituents, was greater (P = 0.01) for stems than leaves. Levels of fixed carbon in alfalfa are considerably lower than levels in coal, as reflected by the higher BTU content of coal.

Nutritional Quality of Alfalfa Separation Co-Products

Compared to first and second cutting alfalfa hay, the nutritional quality of third cutting was considerably better (Table 1). For all cuttings, stems were lower (P = 0.01) in crude protein and relative feed value and higher (P = 0.01) in both acid and neutral detergent fiber than were leaves. Both high and low quality hay are feasible biofuels, but the value-added potentials of high quality hay, third cutting in particular, are more likely to be realized by addressing dairy, horse, or other speciality markets that pay cash bonuses for prime quality hay (Gray et al. 1992; Gray and Hill 1994).

Depending on the cost of milling and pelleting hay, we conclude that weathered or other low quality hay might be an appropriate biofuel in an oversupplied, price-depressed hay market. Alfalfa leaves are high in nutritional quality, regardless of hay quality, and would be less likely than stems to be utilized as a biofuel feedstock. When compared to stems, alfalfa leaves have more value-added potential in high quality specialty rations for livestock. And finally, a future study should evaluate the economics of milling, separating, and densifying alfalfa co-products as well as the competitiveness of these potential biofuels with conventional heat-producing alternatives such as coal, wood, natural gas or electricity.

SUMMARY

Alfalfa plant components were evaluated as an environmentally friendly, biofuel alternative for woodstoves. The ash content of alfalfa plant components and PRB coal was similar, but stems had a slightly lower ash content than leaves. Volatile gases produced from alfalfa components were considerably greater than reference values from coal. However, in the case of an alfalfa biofuel product, volatiles might not be too important because wood pelletstoves are practically smokeless. The sulfur dioxide emission potential and the BTU content of alfalfa were lower than coal. Hay is a feasible biofuel, but the value-added potential of high quality hay is more likely to be realized by addressing dairy, horse, or other speciality markets that pay cash bonuses for prime quality hay. Depending on the cost of milling and pelleting hay, weathered or other low quality hay might be an appropriate biofuel in an oversupplied, price-depressed hay market.

REFERENCES


Table 1. Combustion and forage quality variables of alfalfa componentsz

Components Harvest Ash (%) Volatiles (%) Fixed C (%) MAFx BTU/kg Sulfur (%) SO2 kg/ MBTUx CPx (%) ADFx (%) NDFx (%) IVDMDx (%) RFVx index
Whole plant 1 7.5 77.8 14.5 3848 0.24 0.28 18.6 30.1 39.3 63.0 158
2 7.5 77.5 15.0 3882 0.26 0.29 18.3 31.9 41.7 61.0 145
3 8.4 77.7 13.8 3972 0.29 0.32 20.4 23.7 31.5 68.3 212
Stems 1 6.4 77.5 16.5 3831 0.16 0.19 15.2 36.9 47.6 57.1 121
2 6.2 77.4 16.4 3798 0.16 0.19 14.5 39.5 50.2 57.5 110
3 7.1 77.7 15.2 3904 0.19 0.21 17.2 29.1 37.3 65.3 171
Leaves 1 9.1 77.9 13.0 3997 0.34 0.38 23.6 20.4 28.1 69.1 245
2 9.4 78.1 12.5 3969 0.32 0.37 24.4 19.9 27.0 68.8 257
3 10.1 77.4 12.5 4031 0.36 0.41 25.2 15.8 21.8 72.3 328
LSDy 5% 1.0 0.8 1.2 83 0.06 0.07 1.9 3.6 3.9 4.0 27
LSDy 1% 1.3 1.1 1.6 110 0.08 0.09 2.5 4.7 5.2 5.3 36
Reference Coal Values
Powder River Basin 7.4 47.7 45.1 5763 0.45 0.35
Eastern U.S. 9.8 37.8 52.4 6574 2.16 1.54
zAll variables reported on a dry matter basis.
yVariables within columns may be significantly different at 1 or 5% probability levels: least significant difference (LSD).
xAbbreviations: MAF BTU, moisture ash free British Thermal Units; SO2 lbs/million British Thermal Units; CP, crude protein; ADF, acid detergent fiber; NDF, neutral detergent fiber; IVDMD, invitro digestible dry matter; RFV, relative feed value.


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