Lab- and pilot-scale biochar production from cotton gin waste

As much as 2.32 × 10 6 metric tons of cotton gin waste (CGW) is generated in the U.S. annually. Technologies for utilization of CGW, including improved nutrient cycling via conversion to biochar, are of high interest to the agroindustry seeking answers regarding technoeconomic feasibility. Yet there is generally a gap between what is understood about CGW and similar crop residue biochar made in the lab as opposed to that which might be made at larger scales. The purpose of this work was to examine the quality and quantity of biochar made from CGW, at lab- and pilot-scale, taken directly from a working gin. We considered scaling up of CGW biochar production at 350-600 °C. Specifically, we compared the reactor operation and the quality of biochar produced with a research-grade lab-scale muffle furnace to a less controlled pilot-scale rotary kiln. The average biochar yields were 37.1% ± 6.39% (mean ± sd) for the muffle furnace and 25.8% ± 8.99% for the rotary kiln, and these differences were significant (p < 0.05). We attribute many of the differences in yield and quality to unintended air entry into the rotary kiln. Porosity of the biochar was higher for the muffle furnace (0.929) compared to the rotary kiln (0.747). The estimated stability of all biochar was generally high (O/C < 0.2; >1,000-year half-life). The biochar nutrients most extractable by water were S, Mg, B, and K, with extraction fractions ranging from 6-59%. The technoeconomic potential of CGW biochar produced in the pilot-scale reactor was approximately $3,000/t; costs are expected to decrease substantially for a full-scale industrial process if a conservative economy of scale can be harnessed. More studies on increasing biochar production from lab to pilot scale are needed if biochar is to reach its potential for large-scale production and applications. Specifically, studies should focus on the unit cost of production and the consistency of biochar quality, since much of biochar’s ultimate value depends on the demonstration of specific properties for each application. • Lab scale yields averaged 37.1%, while pilot continuous rotary kiln was lower at 25.8% • Stability of biochar high regardless of scale with projected half-lives > 1,000 years • Even heating was achieved in pilot rotary kiln, but an inert gas needed to avoid excessive ash • Cotton gin waste biochar showed good water extraction fraction for nutrients S, Mg, B, K • Feedstock will, in practice, likely require pre-processing for process upscaling • Pilot biochar cost ∼$3,000/t with lower costs achievable at industrial scale