Hydrothermal liquefaction (HTL) is a promising thermochemical pathway developed to convert wet biomass or waste biomass into valuable bio-oil and biochemical products in hot pressurized water medium. Over the past few years, some studies have initiated to investigate the corrosion challenges under the HTL processes for the industrial-scale deployment of this conversion technology. Among them, the impact of organic acids generated during HTL processes on the corrosion of refinery reactor alloys remains as a puzzle. In this work, static autoclave tests under organic acid-containing HTL conditions were performed on candidate stainless alloys. The formed corrosion products after each exposure were characterized using advanced microscopic characterizations techniques to obtain the related corrosion modes and extents of the alloys. This paper presents the preliminary findings on this aspect.


Hydrothermal liquefaction (HTL) is a thermochemical process in which hot pressurized water is used as the reaction medium for one-step conversion of wet biomass and waste streams (e.g., municipal waste, black liquor, and hog fuel) into bio-fuels.1-9 To-date, most of the HTL studies focused on the impacts of biomass feedstocks, residence durations and operating temperatures on conversion efficiency and products chemical properties. 10-15 Little attention and effort has been employed to address impacts of organic corrodents produced by various biomass feedstocks, along with conversion catalysts, intermediates and final bio-products on the corrosion of HTL reactors. In fact, the produced crude bio-oils contain a range of corrosive inorganic and organic compounds, such as organic acids, aggressive sulfur and/or chlorinated compounds which have the potential to introduce serious corrosion damage on the HTL core equipment, especially conversion reactors, in the hot pressurized water.16-18

The corrosivity of organic acids produced from the HTL coherently relates to the biomass feedstocks and processing conditions. For instance, saturated short-chain organic acids, such as formic acid (HCOOH), acetic acid (CH3COOH) and butyric acid (CH3CH2CH2CO2H) are likely to be present during the conversion of conventional forest and agricultural biomass streams.19-24 Acetic acid, a weak reducing acid, is one of the most common acids produced from aforementioned biomass feedstocks and with the typical concentration of 8 - 10 wt.% in the final products.20 The corrosivity of acetic acid in water at room temperature is negligible to most alloys. However, acetic acid could become quite corrosive under HTL conditions as indicated by previous studies.25-28 For instance, Cheng et al. reported the structure of the passivation film formed on UNS S31603 would be decreased significantly at temperatures above 55°C when exposed to 60 wt% acetic acid solution. Sekine et al. also found that the corrosion rates of UNS S31600 increased with prolonged immersion time in 5-80 wt% acetic acid solutions at both room temperature and boiling temperature (117.9°C).26 It seems that certain conventional stainless steels would not be able to provide good corrosion resistance under organic-acid containing conditions and thus alternatives are needed.

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