• Energy Research

The CO2 adsorption capacities (AC) of biochars obtained at 350, 550, and 750 °C from the main organic (cellulose, lignin, and protein) and inorganic (CaCO3) macro-components of biogenic waste, as well as from co-digested manure (CDM), have been determined for different CO2 concentrations (2–83 vol%) at 25 °C and atmospheric pressure. CO2 adsorption isotherms have been determined using two different experimental methodologies: thermogravimetric and fixed-bed dynamic adsorption tests, yielding similar results. The composition effect has been analyzed by comparing the adsorption performance of the chars derived from individual macro-components and the potential interactions occurring during their co-pyrolysis. Lignin and cellulose-derived chars showed higher CO2 retention (≈77 mg gbiochar−1) than those produced from protein (≈40 mg gbiochar−1). Pyrolyzed CaCO3 exhibited negligible CO2 adsorption. For surrogate_CDM chars, prepared at pyrolysis temperatures high enough to decompose CaCO3 in the organic matrix, experimental results showed a synergistic effect, with AC between 14 % and 47 % higher than theoretical predictions. This decomposition promoted the reverse Boudouard reaction and enhanced char microporosity. However, the improvement was insufficient to offset the dilution effect caused by the high CaCO3 content. AC results have been discussed based on the biochar textural and chemical properties, with ultramicroporosity being the key factor determining adsorption capacity. The AC of CDM-derived sorbents is similar to that of cellulose-derived, expressed per gram of waste (7–13 mg gwaste−1). Furthermore, the biochars retained at least 80 % of their initial AC after 3 adsorption-desorption cycles, indicating their potential for stable CO2 capture.

The yield and properties of char derived from the co-digested manure and its main macro-components, including organic (cellulose, lignin, and protein) components and an inorganic component (CaCO3), produced at different pyrolysis temperatures (350, 550, and 750 °C) have been studied. Experimental results obtained from a surrogate co-digested manure were compared with the theoretically calculated values to explore potential interactions between these macro-components. The char properties analyzed included elemental analysis, pH, FTIR, XPS, and specific surface area. The effect of pyrolysis temperature on many properties was similar, regardless of the precursor (macro-component). Increasing pyrolysis temperature led to higher C content (>90 wt% for cellulose char at 750 °C), pH (from (∼7 for cellulose at 350 °C to ∼13 for co-digested manure), and specific surface area, observing a marked development of ultramicroporosity and microporosity, especially at the highest pyrolysis temperature studied, 750 °C. An exception was observed for the char derived from proteins due to melting during pyrolysis. By far, the solids from the pyrolysis of cellulose and lignin exhibited the most microporosity development (SSDR ≥ 650 m2 g-1), reaching, at the highest temperature studied, values close to those of physically activated carbons. Pyrolysis of the surrogate co-digested manure revealed the occurrence of Maillard reactions and also showed an interesting interaction involving CaCO3. The CaCO3 thermal decomposition is promoted when it is embedded into the organic matrix, where the CO2 generated during decomposition favored the Boudouard reaction of C from the organic components. This results in a lower biochar yield, 32 wt% versus 37 wt% (expected value), and a higher development of microporosity in the char.