Mineral Nutrient Recovery in Co-Products of Pyrolysis for Varied Biomass Feedstocks. II.

Plant biomass can be pyrolyzed under oxygen-free conditions and high temperature to produce fuel, bio-oil and synthesis gas, leaving behind a charcoal byproduct commonly termed “bio-char.” Sustainable bioenergy systems will require recycling of bio-char to replenish soil nutrients on original fields used to produce biomass for pyrolysis. However, little research is available illustrating the relationship between feedstock properties, bio-char yield, and conservation of plant nutrients under varying pyrolysis temperatures and inert gas flow-rates. Some research suggests limited recovery of key plant nutrients in bio-char indicates that these nutrients are present in the bio-oil or synthesis gas. This research studies the relationship between classic fiber analysis of forages and bio-char yield. In addition, mass balance of plant nutrients was quantified in the bio-char, bio-oil and synthesis gas derived from pyrolysis of three biomass feedstocks (high-energy sorghum, corn stover, and switchgrass) at controlled temperature and gas flow rate. Significant differences in bio-char yield were observed among inert-gas flow rates and feedstocks (p<0.05), but bio-char yields were not correlated with feedstock fiber or ash contents. Analysis of bio-char samples indicated poor mass-balance recovery of potassium, phosphorus and other nutrients in the bio-char. For sorghum, less than 60% of pre-pyrolysis biomass potassium and phosphorus was recovered in bio-char. Low recoveries in bio-char were due, in part, to potassium and phosphorus recovery in remaining co-products, including condensed tar and bio-oil and non-condensable synthesis gas. Bio-oil and tar fractions revealed recovery of nearly 15% of biomass potassium, but recovery of less than 1% of biomass phosphorus. The synthesis gas acid trap captured less than 4% of biomass phosphorus and slightly over 35% of the biomass potassium. Filter paper was used remove residue from the acid trap and contained nutrient levels similar to the bio-oil and tar fractions. Low nutrient recovery in recycled bio-char could limit nutrient return to production fields and sustainability of feedstock production.