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.
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.
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.
|Components||Harvest||Ash (%)||Volatiles (%)||Fixed C (%)||MAFx BTU/kg||Sulfur (%)||SO2 kg/ MBTUx||CPx (%)||ADFx (%)||NDFx (%)||IVDMDx (%)||RFVx index|
|Reference Coal Values|
|Powder River Basin||7.4||47.7||45.1||5763||0.45||0.35|