Closing the loop between food, energy, and waste: A review of energy conversion configurations to support Hawaiʻi’s transitions to a circular agro-economy

Abstract

Soil amendments and fertilizers for agricultural production of food crops can be enhanced by anaerobic digestion (AD; biological degradation of natural materials such as plant biomass or manure into digestate) and pyrolysis (PY; a process of thermophilic decomposition into biochar). Digestate and biochar can improve soil organic matter, modulate pH levels, increase nutrient mineralization and availability, and establish optimal conditions for soil microbial populations. Soil amendments produced by AD and PY contribute to a circular agro-economy that maximizes resource utilization, recovery, and regeneration by directly and indirectly displacing fossil fuel production/consumption, thus contributing to climate change mitigation. A literature review was conducted to determine if AD and PY produced separately (AD/PY) or cogenerated (AD+PY, where the digestate serves as the feedstock for pyrolysis producing pyrochar) would provide positive outcomes within a circular agro-economy. PY may increase efficiency properties of independent AD systems. The application of pyrochar improved fertilizer ratios (1.5:1.5:5.9%), water holding capacity (10-15%), and cation exchange capacity (4-17%), thus improving soil structure, tilth, and nutrient uptake in plants. During the processing of biochar, ammonia toxicity can be prevented by approximately 71% and increase energy output, removes contaminants in soil, and sequesters atmospheric carbon. The catalytic cogeneration of digestate, biochar, and pyrochar uses less energy compared to intensive, high input agriculture and can produce up to a net 42% increase in electricity production. Optimized agronomic use of digestate and biochars should consider direct impact assessments to crop yields and offsets for well-informed management strategies.