Nitrogen transformation during pyrolysis of oilfield sludge with high polymer content

Oilfield sludge is recognized as a hazardous solid waste in China because of its considerable hydrocarbon content and other toxic components. In this study, the nitrogen transformation during pyrolysis of typical oil sludge samples produced in the Daqing oilfield (the largest oilfield in China), was experimentally investigated. X-ray photoelectron spectroscopy (XPS) and thermos gravimetric (TG) were used in laboratory-scale experiments and batch pyrolysis experiments were conducted at 200-800 °C in a tubular reactor to obtain quantitative information on the change of nitrogen conversion in the char, tar, and gas fractions. The thermal decomposition of amine-N compounds contributed to NH₃ release below 250 °C. However, the amine-N compounds in the char were converted to amine-N and imine-N compounds in the tar, and no obvious nitrogenous gases were found when the temperature increased from 250 to 500 °C. Between 500 and 800 °C, a large amount of NH₃ and a small portion of HCN were found owing to the secondary cracking of tar and ring opening of heterocyclic-N compounds in the char. Therefore, NH₃, HCN, and NO emissions can be reduced by accurate control of the pyrolysis temperature. A temperature of 400 °C was determined to be the optimal temperature for oilfield sludge pyrolysis, with 24.5% residual char and 43.0% tar produced. The results indicated that a suitable oilfield sludge pyrolysis temperature is helpful for reducing nitrogenous pollutant emissions, volume reduction, and energy recovery, which are beneficial for environmentally friendly production practices.