Co-pyrolytic mechanisms, kinetics, emissions and products of biomass and sewage sludge in N-2, CO2 and mixed atmospheres

The co-pyrolysis technology of the second-generation feedstocks has both engineering and environmental advantages towards resource recovery, waste stream reduction, and energy generation. However, there exists a large knowledge gap about the co-pyrolytic mechanisms, kinetics, emissions and products of biomass wastes. This study aimed to quantify the co-pyrolytic interactions between the five (N-2, CO2, and three mixed) atmospheres and the two feedstocks of sewage sludge (SS) and coffee grounds (CG) as well as their emissions and products. Thermogravimetric-Fourier transform infrared spectrometry, two-dimensional correlation spectroscopy and pyrolysis-gas chromatography/mass spectrometry analyses were combined. An eight-parallel distributed activation energy model was adopted to elucidate the dynamic reaction mechanisms in the co-pyrolytic atmospheres. The co-pyrolytic interaction changed the maximum weight loss rate of the first peak by -2.5 to 38.6% and -1.9 to 36.9% in the N-2 and CO2 atmospheres, respectively. The mass loss rate peak in the first stage was higher in the N-2 than CO2 and mixed atmospheres, while the peak temperature of the maximum mass loss rate in the second stage declined with the elevated CO2 concentration. The replacement of N-2 with the different CO2 concentrations significantly increased the activation energies of the 5th and 7th pseudo-components. The temperature dependency of evolved gases was of the following order: ethers/esters -> acids/ketones/aldehydes/CO2 -> hydrocarbons in the N-2 atmosphere, and acids/ketones/aldehydes -> esters/ethers -> hydrocarbons in the CO2 atmosphere. The co-pyrolysis improved the yields of the hydrocarbon and phenol-type compounds and reduced the formations of the acid and nitrogenous compounds. Our results yielded valuable insights into a cleaner co-pyrolysis process.