Elsevier

Fuel

Volume 240, 15 March 2019, Pages 169-178
Fuel

Full Length Article
Elucidation of reaction pathways of nitrogenous species by hydrothermal liquefaction process of model compounds

https://doi.org/10.1016/j.fuel.2018.11.136Get rights and content

Highlights

  • The nitrogen pathway in HTL process was clarified using simple model compounds.

  • A relocation of water soluble products into oil phase by acylation was disclosed.

  • The lipid effect includes: higher bio-oil yield and lower N-content of bio-oil.

  • The amount of N species that are recalcitrant to hydrotreatment was reduced.

  • The design of waste biomass blends can be used to produce high quality bio-oils.

Abstract

The reaction pathway of nitrogen containing compounds under hydrothermal liquefaction (HTL) conditions was investigated by using amino acids as protein model compounds. The effect of organic acids and alkaline catalysts was also investigated by determining the structure and the partition of nitrogen containing species between the resulting solid, aqueous and bio-oil phases.

Representative results showed that operating in a water-acetic acid (95/5% v/v) binary solvent system resulted in a dramatic improvement in carbon recovery in the oil phase due to the transformation of water soluble hydrophilic products into oil soluble derivatives by acylation of the amino moiety.

The conclusions of this study provide a useful tool to improve the HTL process applied to nitrogen rich biomass, such as the organic fraction of urban waste, sewage sludge, and aquatic biomass (microalgae).

Introduction

The growing concerns over the availability of fossil fuel resources have increased an interest in searching new renewable alternatives. The waste organic biomass types, including sorted domestic organic waste and sewage sludge, are highly perishable wet materials with a typical moisture content ranging from 50 to 80% [1]. These feedstocks often cause serious environmental problems if not properly disposed. Due to the absence of the competition with the food and feed use, they are considered as sustainable feedstocks for energy production. They are also readily available: around 800 million tons of organic wastes are annually produced worldwide [2].

However, from a technological point of view, the high water content makes this feedstock not suitable for most thermochemical processes, such as fast pyrolysis and gasification, which require feedstock with lower moisture content, generally less than 30% [3]. Conversely, they are the typical feeds for the hydrothermal processes, such as hydrothermal liquefaction (HTL) [4] and hydrothermal carbonization (HTC) [5], since they do not need to dry the biomass before the treatment. Targeting the production of liquid biofuels, HTL has to be preferred to HTC that produces a solid hydrochar.

Generally, HTL process is carried out at 250–350 °C temperature range under 50–200 bar autogenous pressure. It is a non-selective process due to the complex composition of the waste biomass, containing polysaccharide, lipid and protein fractions.

The liquefaction mechanism comprises the following consecutive steps: (i) depolymerization of the biomass to form water soluble monomers, (ii) degradation of monomer by dehydration, deamination and decarboxylation reactions, (iii) recombination of the reactive fragments to form the bio-oil, (iv) further polymerization at prolonged reaction time to form char.

The produced bio-oil shows significantly higher oxygen and nitrogen contents, typically 10–20% and 1–8%, respectively, compared to the conventional crude oil (both elements <1%) [6]. Therefore, the crude bio-oil is not directly suitable to fuel conventional engines and has to be upgraded by hydrotrating or cracking processes in order to produce liquid transportation fuels. The high heteroatom content, especially nitrogen, significantly reduces the efficiency of the typical refinery processes by poisoning the conventional catalysts, even in the case of a co-feeding of bio-oil with the current fossil fuels, hence creating a major technological challenge. For this reason the selection of an appropriate upgrading strategy needs a considerable effort to improve the bio-oil chemical composition and specifications that can be achieved only by a deep comprehension of the HTL reaction mechanism.

Although a number of studies [7], [8], [9], [10], [11] has been published on the decomposition of amino acids as representative model compounds of proteins, the liquefaction mechanism of the high-protein biomass is still unclear. Most of these studies are typically limited to the behavior of individual compounds and only a few works have explored binary and ternary mixtures with the carbohydrates [12], [13] and lipid components [14], [15]. Moreover, none of the studies on the model compounds has been focused on the role of catalysts, while it is known that alkali hydroxides, carbonates, bicarbonates and organic acids as homogeneous catalysts can affect the HTL product yields [16].

Therefore, the main goal of the present study is the investigation of the HTL reaction mechanism, focusing the attention on the nitrogen containing species pathways, with the goal to increase the energy yields and reduce the nitrogen content in the produced bio-oil. Due to the complex features of both waste feedstock and the reaction products, phenylalanine, leucine, glucose and tripalmitin were selected to simulate the interaction behavior between the main biomass components. Additionally, the effect of acid and alkaline homogeneous catalysts was also investigated.

Section snippets

Materials

All chemicals were purchased from Sigma Aldrich and Alfa Aesar with the purity of 98–99% and used as received.

HTL experimental setup

The HTL experiments were performed in a Parr 2L batch reactor (4520 series) up to 300 °C at a heating rate of 2.5 °C/min under 80–85 bar of autogenous pressure and at residence times of 60 min. In a typical experiment, 300 g of water and 7 g of starting feedstock were loaded into the reactor. In case of binary mixtures, equal masses of each component (4 g) were used. After the reactor

HTL of phenylalanine and leucine

The effect of alkali and acid catalysts on the bio-oil yield and nitrogen content is presented in Table 1. The bio-oil yields produced from phenylalanine and leucine HTL in the reference water medium were 7.9 and 6%, respectively. A significant increase in bio-oil yield up to 30.2 and 28.6%, was observed for both the model substrates of phenylalanine and leucine, respectively, when water-acetic acid medium was employed. The bio-oil composition was also affected by an increase in oxygen and

Conclusion

This paper has presented the modeling study on HTL focusing on the mechanism of nitrogen species formation. The choice of monomeric substrates, representative of protein, polysaccharide and lipid biomass components, has made possible to simplify the spectrum of the products, thus allowing the identification of the main reaction pathways. In particular, the key role of the acetylation reaction of nitrogen containing compounds was highlighted in the case of biomass feedstock containing protein

Acknowledgements

We gratefully acknowledge the financial support received from SINCHEM Grant to carry out this work. SINCHEM is a Joint Doctorate program selected under the Erasmus Mundus Action 1 (framework agreement N° 2013-0037) of the European Union.

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