Hydrothermal processing of microalgae using alkali and organic acids

Abstract

Aquatic organisms such as microalgae have been identified as a potential source of third generation biofuels due to their fast growth rate, ability to sequester CO₂ and their potential for producing lipids. Conversion by hydrothermal liquefaction is ideally suited to high moisture content feedstocks such as microalgae and involves the processing of biomass in hot compressed water with or without the presence of a catalyst. This study aims to investigate the conditions for producing high quality, low molecular weight bio-crude from microalgae and cyanobacteria containing low lipid contents including Chlorella vulgaris and Spirulina. Liquefaction experiments have been performed in a high pressure batch reactor at 300 °C and 350 °C. The influence of process variables such as temperature and catalyst type has been studied. Catalysts employed include the alkali, potassium hydroxide and sodium carbonate and the organic acids, acetic acid and formic acid. Liquefaction yields have been determined and the bio-crude has been analysed for CHNOS content and calorific value. The bio-crude has been analysed by GC/MS to examine composition and thermal gravimetric analysis (TGA) to estimate its boiling point range. The aqueous fraction has been analysed for typical cations and anions by ion exchange chromatography and for total organic carbon (TOC). The yields of bio-crude are higher using an organic acid catalyst, have a lower boiling point and improved flow properties. The bio-crude contains a carbon content of typically 70–75% and an oxygen content of 10–16%. The nitrogen content in the bio-crude typically ranges from 4% to 6%. The higher heating values (HHV) range from 33.4 to 39.9 MJ kg⁻¹. Analysis by GC/MS indicates that the bio-crude contains aromatic hydrocarbons, nitrogen heterocycles and long chain fatty acids and alcohols. A nitrogen balance indicates that a large proportion of the fuel nitrogen (up to 50%) is transferred to the aqueous phase in the form of ammonium. The remainder is distributed between the bio-crude and the gaseous phase the latter containing HCN, NH₃ and N₂O depending upon catalyst conditions. The addition of organic acids results in a reduction of nitrogen in the aqueous phase and a corresponding increase of NH₃ and HCN in the gas phase. The addition of organic acids has a beneficial effect on the yield and boiling point distribution of the bio-crude produced.

Introduction

There is a pressing need to develop alternative low carbon energy technologies which can replace petroleum derived transport fuels and tackle climate change. First generation biofuels such as bioethanol and biodiesel are well established, but at present cannot meet demand, and there is a question mark over their environmental impact and sustainability. It is generally agreed that the development of next generation liquid biofuels are required derived from biomass that do not compete with food supplies. Second generation biofuels are produced from lignocellulosics, while microalgae are seen as being a future source of third generation biofuels and chemicals, due to their fast growth rate and high lipid content. Microalgae can grow in either fresh water or saline environments, can be cultivated on a large scale and do not require the use of agriculturally productive or environmentally sensitive land.

Microalgae systems have been proposed as a CO₂ mitigation option with subsequent production of biofuels [1]. Recent interest has focussed on the production of biodiesel from microalgae [2], [3]. Many potential algae strains have been identified that contain high lipid content and a number of studies have investigated the subsequent extraction and trans-esterification of the lipids to make biodiesel [4], [5]. Alternative liquefaction schemes include fast pyrolysis [6], [7] and liquefaction [8], [9], [10], [11], [12]. The former requires a relatively dry biomass but the latter is tolerant to high moisture content and so is ideally suited to biomass from an aquatic origin.

Hydrothermal liquefaction involves the reaction of biomass in water at high temperature and pressure with or without the presence of a catalyst. The products include a bio-crude, an aqueous fraction and a gaseous fraction. The hydrothermal processing of biomass was investigated by Shell research in the 1980s [13] and is the basis of the HTU process [14]. Hydrothermal processing of lignocellulosic biomass has received extensive research over the last two decades for both the production of liquid fuels (subcritical conditions) and for gasification (supercritical conditions) and is extensively reviewed by Peterson et al. [15]. Recently, interest has also been focused on the hydrothermal gasification of microalgae [16]. Hydrothermal liquefaction of microalgae was first reported by Dote et al. for the high lipid forming Botryococcus braunii [8]. Yields of 57–65 wt.% oil were reported although the lipid content of B. braunii cells are already as high as 50%. Further developments by Minowa et al. reported yields of 37% for the lower lipid containing algae Dunaliella tertrolecta [9] (lipid content ∼20 wt.%). The catalyst used for both studies was the alkali, Na₂CO₃. The energy balance and CO₂ mitigating effect of hydrothermal liquefaction of microalgae were investigated by Sawayama et al. for the same microalgae and suggested that the higher lipid content microalgae performed more favourably than the lower lipid algae [10]. Yang et al. have investigated the energy conversion characteristics using Microcystis viridis in Na₂CO₃ and reported maximum yields of 33–40 wt.% [11]. Lower lipid containing cyanobacteria such as Spirulina have been liquefied under reducing conditions by Matsui et al. in organic solvents such as tetralin and toluene using a dispersed iron catalyst. High oil yields were reported (78%) despite the low lipid content. Under hydrothermal conditions, a surprisingly high yield of oil is also produced but with a high oxygen content [12].

Some of the most productive microalgae in terms of biomass production are lower in lipid and contain larger amounts of protein and carbohydrate. Growing these algae for biodiesel is unlikely to be economical and alternative processing routes would be advantageous. The conversion of low lipid content microalgae and cyanobacteria by hydrothermal processing is an alternative route to produce biofuels and chemicals from algae; involving the production and subsequent upgrading of the bio-crude. This bio-crude may be processed by a conventional refinery and potentially augmented with a petroleum crude. Alternatively, chemicals can be separated from the bio-crude such as nitrogen heterocycles and n-alkanes. There may also be advantages for utilising lower lipid containing microalgae in mitigation schemes for trapping of CO₂ and for extraction of high value chemicals; hydrothermal liquefaction is then capable of processing the residue. The water phase concentrates trace mineral matter and nitrogen, phosphorus, potassium (NPK) and may represent a route for recycling of nutrients.

This investigation is focused towards the production and nature of the bio-crude produced from hydrothermal liquefaction of a microalgae (Chlorella vulgaris) and cyanobacteria (Spirulina) containing relatively low lipid content and high protein. The influences of process variables such as temperature and catalyst type have been studied. The influences of potential in situ hydrogen donors or hydrogenating agents such as formic acid are compared to the conventional alkali catalysts. The bio-crudes produced have been analysed for proximate and ultimate analysis and by GC/MS and thermal gravimetric analysis. The aqueous fraction has been analysed by ion exchange chromatography (IEC) and for total organic carbon (TOC). The influence of process variables on the yield and quality of the bio-crude is discussed including the carbon balance and the nitrogen partitioning between the product phases.