Increasing Biochar Yield in Pyrolysis by Adding Ash

Wood ash additives increase biochar yield in pyrolysis via catalysis by alkali and alkaline earth metals. However, it remains unclear how ash and biomass properties govern the magnitude of this catalytic effect. Here, we pyrolyzed pelleted and ash-amended woody and gramineous biomasses at 500-550 °C. At pilot scale, 5% (w/w) amendment of different ashes increased dry and ash-free biochar yield (ydaf) by 13-37% and carbon yield (yC) by 2-33% for woody feedstocks and decreased yields by 5-6% for grass or straw. Accordingly, mixing grass with softwood at increasing ratios reduced the effects of added ash. The yield increase depended on biomass type rather than on the intrinsic ash content of the biomass, as positive effects were observed even for woody feedstocks with a relatively high ash content, unlike gramineous biomass. Yield increases observed for different added ashes did not correlate with ash properties, pending further investigation. Toxic hexavalent chromium from ash additives was not detected in biochar, suggesting its reduction to nontoxic Cr(III) during biochar production. At industrial scale, ydaf increased by 11% and yC by 6% through adding 4% ash to landscaping wood, demonstrating the scalability of wood ash use. Adding ash to woody feedstock increased biochar production, pyrolysis economy, and carbon sink potential.