Impact of feed injection and batch processing methods in hydrothermal liquefaction

Highlights

• Comparison between batch and feedstock injection for hydrothermal liquefaction.

• Gelatinization of starch facilitates feedstock injection using an HPLC pump.

• Solid hydrochar yield increases with the feedstock injection speed.

• HTL oil formation pathway established through oil phase fractioning.

• Tailoring starch HTL using batch and feed injection for target products.

Abstract

Starch hydrothermal liquefaction (HTL) using batch and feed injection methods was studied at 300 °C and 10, 30, 60 min residence times with 2% feedstock concentration. The authors found much higher solid yield in batch mode compared to feed injection mode and that the solid yield increased linearly with feedstock injection rate. The HTL oil formation pathway was established that light water-soluble oil (ethyl acetate extracted liquid phase oil) forms first, followed by heavy water-soluble oil (dichloromethane extracted liquid phase oil), and then heavy acetone-soluble oil (dichloromethane extracted acetone wash phase oil). Solid hydrochar was formed mainly through heavy acetone-soluble oil repolymerization as the reaction residence time increased. The oil compositions and HPLC analysis of the aqueous phase indicated that at an injection rate of 20 ml/min, the reaction followed the alkaline pathway while at higher injection rates of 50 and 100 ml/min, the reaction shifted to the acidic pathway.

Introduction

The global population is projected to reach 9.7 billion by 2050 [1], which puts increasing stress on the environment in terms of food, energy, and water demand. This stress is exacerbated by organic waste production. For example, in 2015, food waste accounted for approximately 30–40 percent of the total food supply in the U.S., with approximately 40 million tons wasted [2]. Other wastes, including sewage sludge, dairy waste, and anaerobic digestate from biogas production are also generated across the globe. Such wastes contain high amounts of biodegradable organics that can be recovered and used as a sustainable energy source. Valorizing organic wastes also reduces the need for waste treatment technologies such as landfilling and composting, reducing extraneous greenhouse gas (GHG) emissions and water pollution [3].

The high moisture content (typically > 80%) in these organic wastes limits the economic feasibility of combustion and pyrolysis due to the high enthalpy of vaporization of water [4]. Hydrothermal liquefaction (HTL) utilizes sub- and supercritical water to facilitate depolymerization reactions such as hydrolysis, decarboxylation, and decarbonylation of the organic compounds in the waste feedstocks and convert them into oil through repolymerization [4]. The produced oil usually has a higher heating value (HHV) of 20–40 MJ/kg and can be further upgraded through traditional hydrogenation processes [5], catalytic cracking with hydrogen and commercially available noble metal catalysts [6] or combine with supercritical fluids [7], catalytic esterification [8], etc.

Currently, Most of the published work on HTL uses a “batch-processing” method [[9], [10], [11], [12], [13], [14]] where feedstocks are prepared and sealed in an autoclave (or tubular) reactor having volumes ranging from 1 to 2000 ml [15]. The reactor is then heated to the target temperature and held for the desired residence time. However, due to the different sizes (in terms of length to diameter ratio and volume), heating rates, and target temperatures of the reactor, the heating time can range from less than one minute [16] to several hours [17,18] before the set temperature is reached. This slow heating process could result in unwanted side reactions and decrease oil yield and quality.

The semi-continuous or continuous method is less commonly utilized [19,20] due to the complexity in system configuration and higher operational costs compared to batch. However, continuous feed injection and product extraction are more energy-efficient and economical than batch processing when operating an HTL process on a larger scale. In this method, the reactor with water is preheated to the set temperature, and the feedstock is then injected into the reactor through a high-pressure pump. This method avoids the slow temperature ramping process, therefore, reducing unwanted side reactions [21] and maximizing oil production. Feed injection is often carried out for the HTL process of algae [[22], [23], [24]] due to the high slurry flowability of algal feedstocks. Feed injection is also used in tubular HTL reactor systems when studying water-soluble model compounds such as carbohydrates [25,26] and proteins [27,28] in the supercritical water gasification (SCWG) process for the production of H₂ and CH₄ [[29], [30], [31]].

However, no systematic study has been performed, to date, on differences between the two methods – batch processing and feed injection. In this study, a comprehensive comparison of batch processing and feed injection was performed using starch, a model compound for carbohydrates that are commonly seen in food wastes [32]. The oil yield, oil composition, and carbon distribution among the oil, aqueous, and solid phases were analyzed. Different feedstock injection rates were also employed to study its influence on the HTL reaction pathway.