Hydrothermal Carbonization of Microalgae Biomass from Wastewater Treatment: Effects of Acid Pretreatment

Hydrothermal carbonization (HTC) of wastewater-grown microalgae biomass is an attractive route for producing hydrochar (a solid fuel). However, HTC optimizations are needed to enhance energy recovery from biomass. Among the possible strategies, biomass pretreatment through acid washing is often applied to promote increased carbon content. The effects of this pretreatment require additional research, as the biomass may undergo adverse changes and affect the characteristics of the hydrochar. Therefore, in this study, three biomass samples grown in swine wastewater were washed with HCl solution and then subjected to HTC at 170 °C for 10 min. The pretreatment removed ash content from 15.9-38.6% in the raw biomass to 9.8-10.7% in the acid-washed biomass. This reduction was accompanied by a twofold increase in protein content. During HTC, acid pretreatment contributed to the formation of semicrystalline structures. It resulted in a hydrochar with a higher heating value (up to 22.0 MJ/kg) and energy recovery (up to 68.2%) compared to hydrochar without pretreatment (up to 13.1 MJ/kg and 57.2%, respectively). Despite the positive aspects, the HTC of acid-pretreated biomass increased the N and S contents of the hydrochar (61.4-70.7 g·N/kg and 5.9-6.5 g·S/kg) when compared to the hydrochars derived from biomass without acid washing (33.7-46.5 g·N/kg and 4.5-4.6 g·S/kg), which poses a risk of greenhouse gas emissions if used as fuel. Thus, the beneficial and undesirable effects of acid washing as a pretreatment of algal biomass for biofuel production must be weighed for each case with an integrated analysis of the entire process. Life cycle and techno-economic analysesespecially when considering scale-upcan be used for this purpose by providing feasibility results in real microalgae bioenergy contexts.