Economic trade-off between char for active coal and oil for energy from fast pyrolysis of willow cultivated for phytoremediation

Abstract

Economic trade-offs exist between the production of biochar and bio-oil from pyrolysis of biomass. The yield of the typical fast pyrolysis products is dependent on the process temperature: maximum oil yields are obtained at temperatures between 500 and 520 °C, while the char yield decreases with temperature. Several valorization opportunities exist for these pyrolysis products: the char can be applied as a source of energy, a soil amendment or a resource for active coal production, whereas pyrolysis oil can be used for the production of heat and/or electricity and chemicals. These valorization routes influence the potential sales prices of the pyrolysis products. As a consequence, careful selection of the process temperature is required to optimize the total incoming revenues and hence the profitability of a fast pyrolysis plant. In this paper the optimal process temperature for a fast pyrolysis plant has been calculated, by elaborating Yoder’s model when biochar is used as a resource for active coal production and the pyrolysis oil is burnt in a combustion engine for the combined production of heat and power (CHP). It has been applied to the quantities of oil and char that can be expected from willow cultivation for phytoremediation in the Belgian Campine. The results indicate that the net revenues from the pyrolysis oil combustion are rather constant and hence independent of temperature, whereas the net revenues from char activation clearly decrease as temperature increases. Under current assumptions, the production of active coal apparently is more profitable than the production of oil and hence the optimal temperature equals 250 °C. However, one needs to take account of important uncertainties such as the gross revenue of active coal and the cost price of fume gas treatment. Besides, other opportunities might exist for the valorisation of the pyrolysis oil, resulting in differing optimal process temperatures for fast pyrolysis.