Technoeconomic and emissions evaluation of mobile in-woods biochar production

Abstract

In-woods torrefaction of forest residues to produce biochar can sequester CO2 in long-lived products at potentially lower-cost and complexity than centralized biochar production. Yet there are major uncertainties around operations, cost, and emissions associated with in-woods processing. We present a technoeconomic and emissions analysis for mobile in-woods biochar production systems employing oxidative torrefaction, associated with a timber harvest in Jackson Demonstration State Forest (JDSF) of California as case study. Most of the processing site area is required for the pile of grinded slash feed and drying operations. Breakeven production costs for transport, processing, and application together range between 567 and 573 USD/ton biochar-C (equivalent to 392–341 USD/ton biochar). The dryer, if employed, and the reactor constitute the major portion of capital costs. Labor constitute the largest portion of operating costs, followed by tub grinder rental and biochar application within the forest soil. The requirement of a separate dryer can be relaxed for a mobile in-woods facility, as oxidative torrefaction provides the opportunity to process feedstock with relatively high moisture content (~50%). The respective net GHG emissions for two systems with and without dryer are 5.43 and 7.94 ton CO2 eq/ton biochar-C. However, when the emissions from natural decay of slash (if left unprocessed) are accounted as avoided emissions, the systems with and without dryer result in the respective GHG emission benefits of 6.61 and 4.85 ton CO2 eq/ton biochar-C. The systems reach their break-even point when CO2 emission benefits are valued at a minimum of 85.7–118.1 USD/ton CO2.