Pilot-scale slow pyrolysis of residual and invasive biomass produces feedstock-dependent biochar properties

Slow pyrolysis offers a potential route for valorising residual and invasive biomass within circular bioeconomy systems. This exploratory study assessed five feedstocks sourced in Portugal—beach-cast seaweed, water hyacinth, pampas grass, apple prunings, and vine prunings—under three pilot-scale, semi-controlled batch conversion regimes. The regimes combined heating rates of 0.44–8.5 °C min⁻¹ , maximum recorded external temperatures of 642–706 °C, and residence times of 40–360 min. Recovered solid yield was calculated on an as-received basis, while moisture, ash, volatile matter, fixed carbon, and pH were determined from single homogenised biochar samples. Clear feedstock-dependent property patterns were observed. Apple- and vine-pruning biochars contained 70.5–84.8% fixed carbon and 6.8–10.3% ash, whereas beach-cast seaweed and water hyacinth produced more ash-rich materials, with ash contents of 41.4–81.3%. Their operational as-received yields ranged from 42% to 79%, although these values were not corrected for initial feedstock moisture. Pampas grass produced a comparatively carbon-rich material under Test 1 conditions, whereas the product obtained under Test 2 showed high volatile matter, indicating incomplete carbonisation; its yield could not be validated because of an invalid recorded mass balance. The results suggest that contrasting biomass feedstocks can produce substantially different biochar properties under nominally similar external processing conditions. Because heating rate, temperature, and residence time varied simultaneously, their individual contributions could not be isolated. The findings should therefore be interpreted as exploratory pilot-scale evidence supporting feedstock-specific process development rather than as a mechanistic or statistically validated comparison.