Co-pyrolysis behavior and pyrolysis characteristics of anaerobic digestion residues and low-rank long-flame coal

The rational disposal of waste biogas residue can be contributed to the resource utilization rate of low-rank coal. In this study, a co-pyrolysis investigation was performed on the long-flame coal and biogas residue that mixed in equal proportions, in order to clarify the effect of temperature on the properties of co-pyrolysis products. A series of experiments were also carried out to improve the energy efficiency of waste biogas residue. The parameters were then measured using the gas chromatography-mass spectrometry (GC-MS) and GC. A product analysis was further made to evaluate the properties of the pyrolysis products. The experimental results showed that an outstanding synergistic effect was found in the co-pyrolysis of long-flame coal and biogas residue, according to the actual and calculated thermogravimetric curves. Interestingly, a synergistic effect was also found in the overlapping range for the pyrolysis temperature of the biogas residue and long-flame coal. The optimal range of pyrolysis temperature varied greatly to dominate the subsequent pyrolysis behavior and the thermogravimetric curve. Moreover, the presence of lignin in the biogas residue was promoted the formation of tar during co-pyrolysis. The formation of gaseous products was inhibited to determine the proportion of oil and gas in the co-pyrolysis products. As such, the yield of pyrolysis oil increased first and then decreased with the increase of temperature. Specifically, the oil yield rose from 9.23% to 12.12%, and then decreased to 9.30% at 700 ℃, as the pyrolysis temperature increased from 400 to 500 ℃. The water yield increased from 3.71% at 400 ℃ to 5.28% at 600 ℃, and then decreased to 4.81% at 700 ℃with the increase of temperature. The char yield gradually decreased with the increase of temperature, whereas, the gas yield increased moderately. The GC-MS data showed that the content of ketones decreased first and then increased, as the temperature increased. Nevertheless, the synergistic effect was inhibited the ketones that produced by the coal pyrolysis at high temperature. There was the highest relative content of mono- and bi-cyclic aromatic hydrocarbons in the pyrolysis oil at 600 ℃, but the oxygen content was less. It infers that the synergistic effect was significantly improved the compounds in the co-pyrolysis oil. The gas analysis showed that the yields of H2 and CO first decreased and then increased with the increase of temperature, while the yield of CH4 increased first and then decreased. Furthermore, the yield of H2 decreased from 10.82% to 8.23% at 500 ℃, and then increased to 37.68% at 700 ℃, while the yield of CH4 increased from 400 ℃ 9.69% of C increased to 18.28% of 500 ℃, and finally decreased to 16.58% of 700 ℃, as the temperature increased from 400 to 700 ℃. The high heating value of pyrolysis gas first increased, and then decreased with the increase of temperature, indicating the maximum of 15.33 MJ/m3 at 600 ℃. Consequently, the co-pyrolysis of biogas residue and long-flame coal can be expected as the optimal synergistic effect for the high yield and quality of pyrolysis products.