Production and characterization of two-step condensation bio-oil from pyrolysis

The goal of this project is to investigate the potential of fuel production directly from slow pyrolysis of straw pellets without further refining. A two-step condensation was used to separate liquids from the volatiles. The thermal process chosen is slow pyrolysis at 500°C to couple the fuel production with carbon capture by maximizing the char production alongside bio-oils collection. Condensation parameters were tuned to improve the pyrolysis oil quality. To assess the oil quality some requirements on marine fuels, bio-oils and boiler fuels were taken as reference. Three condensation conditions were inspected: the best pyrolysis overall process was repeated three times for a total of five pyrolysis processes (lately referred to as runs). As expected, the bio-oil characteristic changed with the condensation temperature. As a result of lowering the bio-oil condensation temperature, the bio-oil results less viscous, less dense, with lower heating value and with higher water content. The final bio-oil produced was almost compliant to ISO 8217:2017 for residual marine fuels K 380, apart from water content and acid number, while it was almost compliant for the standard EN 16900:2017 for industrial boilers fuels, apart for the kinematic viscosity. It was assessed that the char is produced with an average yield of 27 w% on dry basis, while bio-oil has a yield between 5 and 10 w%. It is possible to recover an average of 72 % of the total energy contained in the straw in the valuable products, gas, char and bio-oil. The carbon captured with charcoal was around 49 w% of the total present in straw. The pyrolysis experiments were performed using an innovative semi-batch, packed-bed reactor with gas re-circulation and a new two-fraction condensation appliance that is DTU patented. The innovation is both in the reactor, where the gas is directly heated and flushed back into the pyrolyser, and in the condensation section, where the bio-oil is separated from the water fraction during the pyrolysis. The company Stiesdal SkyClean A/S showed interest in further development and scale up of this pyrolysis and condensation system.