Reaction mechanisms of hydrothermal liquefaction of model compounds and biowaste feedstocks

It has been demonstrated that hydrothermal liquefaction (HTL) could be an efficient technology that can not only reduce COD and pollutants in the bio-wastes but also generate valuable renewable bio-crude oil from many biomass sources. Previously, swine manure has been successfully converted into bio-crude oil through HTL with yield as high as 70% of the volatile solids at the University of Illinois at Urbana-Champaign. To further understand the fundamental mechanisms of HTL and to provide information for further developments of this technology, experiments with both model compounds and typical biomass (e.g. swine manure) as feedstock were conducted, the product distribution and the properties were analyzed. Some possible reaction pathways of specific compounds were proposed.;HTL tests were conducted for swine manure collected from nursery, grower-finisher and sow pigs. The testing conditions were: reaction temperature 305°C, initial solid content 20% wt, retention time 30 min, initial N2 gas pressure 0.65 MPa, and without catalysts or additives. Comparison between the bio-crude oil obtained from HTL of nursery, grower-finisher and sow pigs showed no significant differences. Length of manure storage time in a shallow pit did not profoundly affect the formation of bio-crude oil through HTL, although a slight decrease of refined oil yield from 42% to 35% was observed.;HTL tests of swine manure at transient temperatures (180-240°C) and retention times of 0-60 min showed that the release of fatty acids, the hydrolysis of hemicellulose and proteins, and the Maillard reactions were the major reactions happened at temperatures lower than 240°C. Maillard reaction started to occur at as low temperatures as 180°C. The formed melanoidins further decomposed to gas, oil, char and aqueous products. Fatty acids were the major components in the bio-crude oil at temperatures lower than 240°C, with some decomposition products from amino acids and melanoidins. At 200°C, between 0-15 min, a rapid and intensive hydrolysis of hemicellulose occurred and caused the decrease of pH in the aqueous phase. The decreased pH probably enhanced the decomposition of melanoidins.;A refined oil yield of 20% was obtained at 300°C from the HTL of egg albumin. Tests conducted between 200-300°C showed that degradation of large molecules and re-combination of small molecules generated in the degradation process were probably the two major pathways for biocrude oil formation from protein. HTL of carbohydrates generated large amount of solid residue (more than 40%), with the refined oil yield lower than 25%. However, addition of base catalysts (e.g. Na2CO3), lipid and protein into carbohydrates could significantly decrease the solid residue yield. Addition of protein into lipid caused the increase of refined oil yield and the decrease of aqueous products with the main interaction probably being the formation of amides.;Fifteen tests were conducted for regression between the refined oil yield and the lipid, protein and carbohydrates content in the feedstocks. A positive linear relationship was observed between the refined oil yield and the lipid content, as well as between the refined oil yield and the protein content. A negative linear relationship was found between the refined oil yield and the carbohydrates content in the volatile solids in the feedstocks. Based on the regression results, a series of formulae were obtained which could be used to predict the refined oil yield from HTL of biomass. The errors for two predictions for examining purpose were -1.68% and 14.74% respectively.;Based on the results and from both model compounds and real biomass, a scheme of the reaction pathways during the HTL of biomass was proposed. The scheme included the reactions which could possibly happen to lipid, protein and carbohydrates.