Submitted to: Fuel
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 28, 2006
Publication Date: May 30, 2006
Citation: Boateng, A.A., Jung, H.G., Adler, P.R. 2006. Pyrolysis of energy crops including alfalfa stems, reed canarygrass, and eastern gamagrass. 2006. Fuel 85,p.2450-2457. Interpretive Summary: When combined with the appropriate conversion technologies, perennial grasses can provide the needed feedstock that can make biomass refining a competitive alternative to fossil fuel refining in the United States. One such technology is by thermally converting the biomass into intermediate forms of energy such as burnable gas, bio-oils and solid charcoal. We conducted experiments including heating alfalfa stems, a legume, reed canary grass, a cool-season grass and eastern gamagrass, a warm-season grass in the absence of air to determine the yields of the burnable gas. Also tested was the effect on the plant’s maturity of such yields. We found that the gas yield from the alfalfa stems was greater than from the grasses. With respect to maturity, the more matured alfalfa produced more burnable gas than the immature alfalfa. The maturity effect on reed canarygrass was opposite. The optimum temperature at which the burnable gas yield is maximized was 900 oC for all biomass samples studied. This is also the temperature at which the gas produced had the highest energy content. We conclude that eastern gamagrass can be a competitive energy crop when thermally converted and deserves more of a look as a bioenergy crop than it has previously received. The information obtained will benefit policy makers on the important role thermal conversion of energy crops plays in achieving energy self-sufficiency and which crops can be competitive. It will also benefit the farmer by advising him/her on the alternative fuel potentials of the energy crops we studied and which are also identified under the biomass initiative program.
Technical Abstract: Production of energy from renewable biomass resources would reduce atmospheric CO2 increase associated with fossil fuel use. The objective of this study was to evaluate the energy potential of a thermochemical conversion platform of three herbaceous biomass crops. The biomass crops tested were stems of alfalfa, a legume, and whole herbage of reed canarygrass and eastern gamagrass, cool- and warm-season grasses, respectively. Two stages of physiological development were included for the alfalfa and reed canarygrass; bud and full flower stages for the alfalfa, and vegetative and ripe seed stages for the reed canarygrass. The eastern gamagrass was fully mature, senescent material. Fast pyrolysis products at 600 to 1050 oC were characterized for gas and char yields, and non-condensable gas product composition. Gas yields for alfalfa were greater than from the grasses at all temperatures. For the alfalfa stems, the more mature sample yielded more gas. For reed canarygrass, higher gas yield was obtained for the sample harvested at the vegetative stage than the more mature flowering stage. Char was greater for mature reed canarygrass than eastern gamagrass at temperatures of 900 oC or less, with the other biomass samples being intermediate. Maturity effect on alfalfa char yield was not significant. However, all samples had similarly low char residues at 1050 oC. With regard to the composition of non-condensable gas produced, the grasses had higher heating values than alfalfa; however, at 900 oC where the gas heat of combustion is maximized the calorific value of the gas was similar for all samples at about 3250 kcal/kg. This compares to about 4400 kcal/kg of the parent biomass i.e., about 75%. The activation energy for thermal decomposition, estimated from first order reaction kinetic models, did not exhibit a consistent trend with maturity, but mean activation energy was lower for alfalfa (2837 J/mol) than the grasses (3427 and 3419 J/mol for reed canarygrass and eastern gamagrass, respectively). The effect of maturity on the pyrolysis response was more pronounced for alfalfa than for reed canarygrass. This information aids evaluation and comparison of alternative conversion platforms identified under the US National Biomass Initiative.