Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2000
Publication Date: N/A
Citation: Interpretive Summary: There is a critical need to diversify farming systems and strengthen the profitability of American agriculture. At the same time, the growing need for electrical energy in the USA is reason to explore new options for sustainable energy production. Trees and grasses have been used as biomass sources that are burned to generate electricity, but there are a number of problems with using these plant species in this way. One of the largest problems is that both trees and grasses require nitrogen fertilizer to stimulate their growth. Considerable amounts of energy are needed to manufacture nitrogen fertilizer, so the net energy yield from wood or grass biomass production is low. Alfalfa is a high yielding legume that can obtain most of its nitrogen supply from the air. Alfalfa has traditionally been used as a feed for livestock, but alfalfa stems are less nutritious to the animals than the leaves. A new, multi-agency project is investigating whether alfalfa stems might be a good fuel for electricity generation, while using the leaves for a high value animal feed. If alfalfa stems are burned for fuel, what is to be done with the leftover ash? In this research, we discovered that alfalfa stem ash is quite clean in contrast to wood or coal ash. It has high potassium content and could serve as a liming agent to 'sweeten' acid soils. Coincidentally, alfalfa requires high potassium supply and grows best in neutral pH (non-acid) soils. Thus, results of this research will provide benefits to alfalfa producers who will be able to use this valuable byproduct, energy producers who may be able to defray expenses by selling the alfalfa stem biomass ash, and the American public who will be served by a more sustainable electrical energy production system.
Technical Abstract: Electricity generation from biomass is an attractive option from an environmental perspective. Plans are underway in Minnesota to construct a power plant that will utilize alfalfa (Medicago sativa L.) stems as feedstock for energy generation via gasification. Byproducts will include more than 50,000 Mg of ash annually. Detailed information on chemical characteristics of the ash is required to develop environmentally and economically sound ash management strategies. Gasified alfalfa fly and bottom ashes were characterized with respect to chemical properties that are important in developing ash management practices with emphasis on beneficial utilization as a soil amendment. Mean concentrations of total C, K, Ca, and Cl were 424, 120, 85, and 26 g/kg, respectively, in fly ash. In bottom ash, the mean concentrations of C, K, and Ca, were 63, 61, 193 g/kg. Concentrations of total Pb, As, Cd, Co, and Se were below detection limits in both ash types. Naphthalene ranged from 6.2 to 74 mg/kg, but concentrations of many other polyaromatic hydrocarbons were low or below mg/kg detection limits. Mean CaCO3 equivalent value of fly ash was 400 g/kg, its electrical conductivity and pH were 127 dS/m and 11.5, respectively. These results suggest that when managed properly, gasified alfalfa ash could potentially be utilized as a beneficial soil amendment with few potential environmental concerns.