Brewery waste pyrolysis: the effect of technological parameters on the fuel properties of the prepared chars

The aim of the study was to evaluate the fuel properties of chars obtained during low-temperature pyrolysis (final temperatures: 350, 450, and 550 °C; heating rates: 5 and 30 °C min –1 ) of brewery waste. Three types of waste were selected for the study: barley spent grain, mixed spent grain (wheat-barley), and hops. Subsequently, the raw materials and the resulting chars were subjected to detailed characterization. The results confirmed that the highest biochar yield was obtained during pyrolysis at 350 °C. Interestingly, proximate and ultimate analysis indicated that the best-quality fuels were chars obtained at 350 °C (especially derived from hops), as they were characterized by the highest HHV value and energy yields, the highest content of volatile matter, and the lowest content of ash. Further studies indicated that spent grain chars contained significant amounts of acidic elements, while hop chars contained a significant amount of basic elements, with their amounts increasing with pyrolysis temperature. Therefore, spent grain chars seem more suitable for combustion and hop chars for gasification. Surface properties indicated that, regardless of the pyrolysis temperature and raw material type, the surface of the resulting chars was poorly developed. Finally, assessment of environmental impact (especially PAHs content and toxicity) indicated that the use of chars as feedstock for thermal conversion (especially gasification) is the most optimal, and hops chars can be considered an environmentally friendly fuel. In summary, chars obtained at 350 °C (regardless of the heating rate), particularly those based on hops, have the greatest potential for use as high-quality fuel.