(84a) Hydrothermal Liquefaction of Wet Waste for the Economical Production of Sustainable Liquid Fuels
AIChE Spring Meeting and Global Congress on Process Safety
Tuesday, March 28, 2017 - 10:15am to 10:45am
The cost of biomass feedstock is a primary contributor to the overall cost of producing renewable liquid transportation fuels. Biomass costs typically contribute between 25-50% of the total fuel cost. Waste streams represent low cost and, in many cases, carbon rich resources which could serve as economical feedstocks for the sustainable production of liquid fuels. Furthermore, many waste streams such as municipal solid waste already have collection and sorting infrastructures in place. This talk will focus on the conversion of municipal wastewater primary sludge via hydrothermal liquefaction (HTL) and hydroprocessing to liquid transportation fuels. Primary sludge contains about 50% cellulose and 50% bio-solids. Of the bio-solids, a large fraction consists of fats, oils and greases. Thus, the primary sludge is similar to algal feedstocks (i.e., rich in lipids) but without the high production costs. Indeed, costs are often incurred to dispose of primary solids in landfills, so an economic driver to produce value-added products from this wet waste is present. HTL is a wet conversion process well suited for wet feedstocks such as primary sludge. Feeds are typically 10-20% solids with the balance water. The process employs high temperature water (350°C) at sufficiently high pressure to maintain the feed in the condensed phase. Hence, no water is vaporized, avoiding significant energy costs. The HTL process produces a biocrude with a gasoline boiling range from cellulose conversion, which includes branched and cyclic compounds as well as aromatics. The diesel fraction is rich in free fatty acids obtained from lipid conversion. Hydroprocessing of the biocrude to increase the H/C ratio and decrease the O and N heteroatom content results in a naphthenic-rich gasoline fraction and n-alkane rich diesel fraction. About 77% of the hydroprocessed material distills in the diesel range; the cetane of the diesel range material has been measured in the high 50s. Economic modelling suggests that the conversion of primary sludge to biocrude could be achieved for $3.80/gal with upgrading and fractionation to diesel costing a total of $4.90/gal. Lifecycle analysis modeling suggests that a 54-74% reduction in greenhouse gas emissions are possible from converting primary solids to liquid fuels.