CO₂ reduction by chars obtained by pyrolysis of real wastes: Low temperature adsorption and high temperature CO₂ capture

In this work, the carbon dioxide capture of waste derived chars was investigated. The char samples were obtained by pyrolysis of municipal plastic waste, biomass and sewage sludge from agriculture at 400, 600 and 900 °C in nitrogen atmosphere. For further experiments, chars with a grain size between 0.315 mm and 1.50 mm were investigated. The CO₂ uptake capacity of samples was tested at 40 °C through a 10 adsorption-desorption cycles using a mixture of 70 % nitrogen and 30 % carbon dioxide. The CO₂ uptake capacity of the reference activated carbon was 3.71–3.90 mmol CO₂/g, while that of the waste derived char samples varied between 0.76–2.33 mmol CO₂/g depending on the pyrolysis temperature and the raw materials. Chars with large specific surface area obtained at 900 °C had the highest CO₂ uptake capacity. Char obtained from municipal plastic waste at a pyrolysis temperature of 900 °C has the largest specific surface area. The biomass and sewage sludge derived chars contained alkali metals and earth metals in oxide form, therefore the possibility of their application for carbonization-calcination cycles was also investigated. In case of the high-temperature tests, the CO₂ uptake took place at 750 °C, while the release at 900 °C. During the 10 cycles test, significant decrease in capacity up to the 5th cycle was found. The capacity of char obtained from agricultural sewage sludge was 18.68 mmol CO₂/g in the first cycle, which decreased drastically to 2.88–2.96 mmol CO₂/g after the 5th cycle.