Gasification and production of biochar from wastewater grown algae

This paper presents the results of some preliminary experiments on gasification of algal biomass harvested from wastewater treatment HRAP, the production of biochar from pyrolysis and some beneficial properties of the resulting biochar. A laboratory-scale gasifier was constructed to mimic the conditions in a High Temperature Downdraft Gasifier (HTDG), HTDG requires a biomass with specific properties including: uniform size; structural integrity; low moisture content and; low nitrogen content to reduce NH3 and NOx emissions. Since harvested algal biomass has a very high moisture content (typically ~96%) a drying step is required before gasification. Wastewater treatment HRAP algal biomass was able to be air-dried to 12% moisture in 24 hours during NZ summer conditions using a simple non-woven geotextile drying rack. Air-dried algal flakes were gasified in the updraft gasifier with 20.6% of the total energy content of the algae captured as water heating energy. Potential problems with nitrogenous gas emissions due to the high nitrogen content of algal biomass could be eleveated by using various adsorption and scrubbing technologies. Pyrolysis was used to produce a high quaility biochar from air-dried algal flakes with a 28.1% conversion efficiency. For every kg of algal biomass that was converted into biochar, 0.44 kg of CO2 was sequestered as biochar carbon. Pyrolysis of wastewater treatment HRAP algal biomass to biochar could sequester ~130 kg CO2 per ML of wastewater treated in the HRAP. Powdered activated algal biochar had nitrate absorption capacity of 0.8 mg N/g which is similar to that found for other biochars e.g. wood. Powdered activated algal biochar had nitrate absorption capacity of only 0.8 mg N/g which is similar to that found for other biochars e.g. wood, but would be of little value for use in a final treatment polishing. A basic energy analysis indicates that there could be sufficient energy within algal biomass to dry freshly harvested algae for pyrolysis to sequester CO2 as biochar.