Pyrolysis of energy crops including alfalfa stems, reed canarygrass, and eastern gamagrass

Abstract

Production of energy from renewable biomass resources would reduce atmospheric CO 2 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. Pyrolysis products at 600–1050 °C were characterized for gas and char yields, and non-condensable gas product composition. Gas yields ranged between 15 and 36 Nm 3 kg −1 while char yield ranged between 5.4 and 26.7 wt% of dry initial biomass. Gas yields for alfalfa were greater than from the grasses at all temperatures ranging from 25 to 36 Nm 3 kg −1 compared to the grasses in the combined range of 15–27 Nm 3 kg −1 biomass. 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 °C 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 °C. With regard to the composition of non-condensable gas produced, the grasses had higher heating values than alfalfa; however, at 900 °C where the gas heat of combustion is maximized the calorific value of the gas was similar for all samples at about 13.6 MJ kg −1. This compares to about 18.4 MJ kg −1 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 kJ mol −1) than the grasses (3427 and 3419 kJ mol −1 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.