Assessment of Hydrotreatment for Hydrothermal Liquefaction Biocrudes from Sewage Sludge, Microalgae, and Pine Feedstocks

Bulk property measurement, simulated distillation, gas chromatography mass spectrometry (GC-MS), and ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) are utilized for direct description and comparison of the chemical composition of raw and hydrotreated biocrude samples from pine, microalgae (Chlorella sp.), and sewage sludge. With hydrotreatment, the nitrogen, oxygen, and sulfur content as well as viscosity, density, and moisture content of all biocrudes decreased to yield a more desirable product. For upgraded biocrudes, simulated distillation and GC-MS data reveal that the microalgae and sewage sludge products comprise a high proportion of <i>n</i>-alkanes, which distill between 260 and 350 °C, whereas the pine hydrotreated biocrude product has a lower concentration of <i>n</i>-alkanes and is more compositionally diverse with an abundance of saturated cyclic compounds. FT-ICR MS analysis of the raw biocrudes showed predominantly O_<i>x</i> species, whereas raw microalgae and sewage sludge biocrudes comprise primarily N_<i>x</i>O_<i>y</i> species. After hydrotreatment, FT-ICR mass spectra of all three biocrudes revealed a significant reduction in mass spectral complexity (observed as the loss of O_<i>x</i>, N_<i>x</i>, and N_<i>x</i>O_<i>y</i> species) and the formation of hydrocarbon compounds, as expected. The hydrodeoxygenation and hydrodenitrogenation reactions of hydrotreatment convert higher (>2) heteroatom-containing species to a variety of hydrocarbon and lower heteroatom-containing species