Prediction model of biocrude yield and nitrogen heterocyclic compounds analysis by hydrothermal liquefaction of microalgae with model compounds

Highlights

• The possible mechanism of interaction among different components in algae.

• Developed HTL biocrude yield model invovled interaction among different components.

• The formation mechanism of N-heterocyclic compounds in biocrude.

• The effects of biochemical content on N-heterocyclic compounds content in biocrude.

Abstract

The model of biocrude yield and the nitrogen heterocyclic compounds in biocrude of microalgae hydrothermal liquefaction are two of the most concerned issues in this field at present. This study explored a hydrothermal liquefaction biocrude yield model involved in the interaction among biochemical compounds in microalgae and analysed nitrogen heterocyclic compounds in biocrude. The model compound (castor oil, soya protein and glucose) and Nanochloropsis were liquefied at 280 °C for 1 h. The products were analyzed by GC–MS, element analysis and FTIR. The results suggested that interactions among different components in microalgae enhanced biocrude yield. The biocrude yield prediction model involved cross-interactions performed more accurate than previous models.When the ratio of protein and carbohydrate around 3, the cross-interaction and nitrogen heterocyclic compounds in biocrude would both reach the highest extent.

Introduction

Compared with fossil energy, biomass energy not only has the advantages of easy combustion, relatively less pollution, renewable, etc., but also can achieve zero CO₂ emision since there exists a balance between the CO₂ absorption during biomass growth and the CO₂ emission during bioenergy using. This environmental-friendly characteristic is critically in accordance with the urgent need of reducing greenhouse gases release. Among numerous bio-resources, microalgae is perceived as considerable biomass feedstocks to produce energy-dense fuels for its advantages including high photosynthetic efficiency (Neveux et al., 2014), fast growth rate, unnecessity of arable land, high efficiency in CO₂-fixing (Khoo et al., 2011, Soratana and Landis, 2011), and ability to absorb nutrient elements like nitrogen and phosphorus (Brennan and Owende, 2010, Demirbas, 2010). Besides, microalgae mainly contains proteins, lipids, carbohydrates, and other organic components, whose unique chemical constitution and structure character render it excellent feedstock in biofuel production (Rawat et al., 2011, Teri et al., 2014). There are various methods capable of converting microalgae into solid, liquid, or gaseous products that can be used as or further transformed into biofuel (Brennan and Owende, 2010). Hydrothermal liquefaction (HTL) is one of the most potential technologies for the production of bio-oil from microalgae, it can directly convert wet biomass to biocrude directly at high-temperature (200–350 °C) with high-pressure (5–15 MPa) water (Brown et al., 2010), avoiding the energy-intensive drying steps of biomass (Valdez et al., 2012, Zou et al., 2010).

In the field of HTL of microalgae, the model of biocrude yield and the nitrogen-heterocyclic (N-heterocyclic) compounds in biocrude are two of the most concerned issues at present. There have been some prior works discussing about biocrude yield model. Initial predictive model developed by Biller and Ross (Biller and Ross, 2011) sought to estimate biocrude yield by linear summation of the yields obtained from HTL of individual model lipid, protein and carbohydrate compounds (termed here as component additivity). Their formula biocrude yield% = (protein yield% ∗ protein content%) + (carbohydrate yield% ∗ carbohydrate content%) + (lipid yield% ∗ lipid content yield%) has been revised by Leow et al. (Leow et al., 2015). But this model omitted the effect of interaction on biocrude yield. Teri et al. (Teri et al., 2014) proposed that interaction during hydrothermal treatment of the biomolecule mixtures influences the biocrude yield and provided a model with six coefficients based on the HTL results of different model compounds and their binary and ternary mixtures. But the accuracy of the model proposed by them was low. Li et al. (Li et al., 2017) considered the effect of ash on biocrude yield and established a four terms model, but it still involved no interaction. Many kinetic models (Hietala et al., 2016, Valdez et al., 2014, Vo et al., 2016) treated each biochemical component as if it reacts independently during HTL and used first-order rate laws. Sheehan & Savage (Sheehan and Savage, 2017) taken the interaction among different components into accountant and treated them as second-order reaction. Their kinetic model including interaction terms performed high accuracy.

Biocrude can be catalytically upgraded to a product similar to petroleum crude (Elliott et al., 2013). However, N-heterocyclic compounds in biocrude, such as quinoline and pyridine, are difficult to upgrade by catalytic conversion processes for the reason that high basicity of N-heterocyclic compounds may adhere to active acidic catalysts surface and poison the catalysts (Furimsky and Massoth, 2005, Vardon et al., 2012). In addition, the N-heterocyclic compounds in biocrude also affect smell, combustion, and various other properties. Thus, analyzing the formation mechanism of N-heterocyclic compounds and studying the effects of components content in feedstock on the N-heterocyclic compounds content in biocrude are critically needed, while there is little information available in the extant literature about these aspects.

In this study, three model compounds (castor oil (Teri et al., 2014), soya protein (Teri et al., 2014) and glucose (Zhang et al., 2016)) and Nanochloropsis (Brennan and Owende, 2010) were liquefied to comprehensively investigate the interaction mechanism among different components, and study the effect of biochemical content in microalgae on biocrude yield and interaction among different components. Based on these experiments and previous researches, the interaction mechanism among different components was summarized, and a biocrude yield model involved interaction among different components was setted, which can be used to predict the interaction products content in biocrude and give the highest biocrude yield for microalgae with different biochemical composition. In addition, the formation mechanism of N-heterocyclic compounds was also comprehensively analyzed, and the effect of biochemical content in microalgae on the content of N-heterocyclic compounds in biocrude was studied. It lays a foundation for the research of denitrogenation of biocrude produced by HTL and point out the direction for microalgae cultivation with reasonable composition.