Professor Universidad de Santiago de Chile Santiago, Region Metropolitana, Chile
The current energy production based on fossil fuels, mainly in developing countries, has caused severe environmental problems such as high pollution levels. In fact, according to WHO, Chile has 9 of the 10 cities with the worst environmental pollution in the whole American continent. Firewood and pellets manufactured from raw sawdust have been designated as the main source of particulate matter and polycyclic aromatic hydrocarbons emissions. Consequently, the biomass conversion by Waste to Energy (WtE) technologies has been evaluated as a real alternative for the replacement of this type of fuel. It can also respond to the challenges associated with the final disposal of organic waste from industries in developing countries, and it would allow the circular economy implementation at a local and regional scale. Among the available processes used to treat organic waste, hydrothermal carbonization (HTC) converts raw biomass into a solid product (called hydrochar) with a higher energetic value and hydrophobic properties. This study proposes the HTC process optimization with two Chilean biomass: sawdust (Pinus radiata) and rapeseed (Brassica napus). The mass yield (MY), high heating value (HHV), and energy yield (EY) were studied by the application of a Design of Experiments (DoE) with 3 factors and 2 levels (23). The variables used and their levels were temperature (T): 190 and 250°C; time (t): 60 and 120 min, and biomass/water ratio (B/W); 8 and 14% for the rapeseed, and 10 and 16% for the sawdust. The selected time and temperature values were based on previous work. The B/W ratio was different in both biomasses due to the initial humidity of the biomass: sawdust (56%) and raps (6.8%). For the sawdust, the increase in temperature from 190 to 250°C produced an increment of 23% in the HHV, and a decrease in MY and EY of 21 and 18%, respectively. The rapeseed HHV showed an increase of 23%, while the MY and EY decreased 11 and 14%, respectively. In the statistical analysis, the most influential variables for both biomasses were temperature and the B/W ratio. Besides, using the Response Surface Methodology and the equations obtained from the DoE (R2 above 0.95), new operating points were determined to maximize energy efficiency. For the sawdust, EY of 77% with an HHV of 28.6 MJ/kg (+42.2%) were obtained with a temperature of 280°C, 100 min, and a B/W of 13%. On the other hand, EY of 49.2% and HHV of 29.1 MJ/kg (+36.4%) were achieved for the rapeseed with 280°C, 90 min, and B/W ratio of 10%. Finally, the determined ash content in the hydrochar met the Chilean standards for biofuels.
