Comparative studies of thermochemical liquefaction characteristics of microalgae, lignocellulosic biomass and sewage sludge
Introduction
Renewable energy has generated much interest due to the energy crisis, the rising oil prices, the green house effect, and political factors [1]. Biomass is one of the most abundant sources of renewable energy, and will be an important part of a more sustainable future energy system [2]. Biomass resources include wood and wood waste, energy crops, aquatic plants, agricultural crops, and animal wastes. Municipal and milling activities generate huge quantities of mixed biomass like sawdust, manure, sewage sludge and cooking wastes [3]. Biomass is a broad definition and includes a wide range of materials with varying compositions. The main biomass components are: carbohydrates, lignin, protein and lipids [2].
Conversion of biomass to energy is undertaken using two main process technologies: thermo-chemical and bio-chemical/biological [2]. Within thermo-chemical conversion, four process options are available: direct combustion, pyrolysis, gasification and liquefaction. Liquefaction is a low temperature and high pressure thermochemical process during which biomass is broken down into fragments of small molecules in water or another suitable solvent. These light fragments, which are unstable and reactive, can then re-polymerize into oily compounds with various ranges of molecular weights [4]. In recent years, liquefaction has been demonstrated with or without the presence of catalysts for a range of biomasses including lignocellulosic biomass [5], [6], [7], microalgae [8], [9], [10], sewage sludge [11], [12], [13], [14] and so on.
Lignocellulosic biomass materials are the most widely used types of biomass for bio-oil production through liquefaction [4]. The main components of lignocellulosic biomass are cellulose, hemicellulose, and lignin [15]. Microalgal biomass has been recognized as attractive feedstock for the third generation biofuel [9]. Microalgae, a ubiquitous eukaryotic microorganism, can be rich in proteins or rich in lipids or have a balanced composition of lipids, sugars and proteins [16], [17]. Sewage sludge produced in municipal wastewater treatment plants is composed mainly of bacterial constituents (nucleic acids, proteins, carbohydrates and lipids) and their decay products, undigested organic material (cellulose) and inorganic material [18]. In a word, above three biomasses have distinct compositions and structures to each other.
Biomass liquefaction product is determined by various factors, including substrate type, heating conditions, solvent type, reactor configuration and catalyst [19]. In particular, the type of substrate has remarkable effect on the liquefaction reaction. Specifically, the yields and chemical characteristics of bio-oils can be influenced by the ratio of protein, lipid, and carbohydrate fractions in the initial biomass feedstock [12]. Many papers have been published on the liquefaction of lignocellulosic biomass, microalgae and sewage sludge [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. However, these investigations cannot be compared due to the difference in the separation of products and definition of the liquid product. To the best of our knowledge, there are no studies carried out at identical conditions to understand the effect of distinct compositions and structures among lignocellulosic biomass, microalgae and sewage sludge on the distributions and properties of liquefaction products.
To enhance the bio-oil yield with lower oxygen content, organic solvents, such as ethanol, methanol, acetone, etc., have been utilized as the reaction medium instead of water in recent years [9], [10], [20]. Among these organic solvents, ethanol may be the most promising solvent for biomass liquefaction from the viewpoint of efficiency, environmental friendliness and reproducible ability. Ethanol has several advantages: First, the critical temperature and critical pressure of ethanol (516.2 K, 6.38 MPa) are far below those of water, so much milder reaction conditions can be obtained. Second, ethanol can provide active hydrogen as a hydrogen-donor in the liquefaction process. Third, ethanol can react with acidic components in the bio-oil by esterification reaction to obtain fatty acid ethyl esters similar to biodiesel. Finally, due to its relatively lower dielectric constant, ethanol can readily dissolve relatively high-molecular weight products derived from biomass [21], [22].
In the present work, rice straw (lignocellulosic biomass), Spirulina (microalgae) and sewage sludge were chosen as the liquefaction feedstocks and ethanol was adopted as the liquefaction solvent. All the liquefaction experiments were conducted in a same autoclave at identical conditions. The distributions of liquefaction products and bio-oil compositions were comparatively studied.
Section snippets
Materials
Dewatered sewage sludge (SS) was obtained from a sewage treatment plant in Changsha City, Hunan Province. Microalgae cells of Spirulina (SP) were provided by Xigema Biological Technology Co., Ltd. (Fujian, China). Rice straw (RS) was collected from a farm in the suburbs of Changsha City. Sun dried samples, separated from physical impurities, were ground in a rotary cutting mill and were screened into fractions of particle diameter (dp) between 0.2 and 0.9 mm. Then the powder was dried in an
Feedstock characterization
The physicochemical and compositional characteristics of the feedstock samples are given in Tables 1 and 2. The N concentration of the algal feedstock was higher in comparison to lignocellulosic biomass and sewage sludge, most likely due to higher protein content in algae. Hence, presence of nitrogenous compounds (formed due to thermal degradation of proteins) could be expected in the liquefaction products of algal biomass [26]. For all the samples, the content of oxygen ranged from 39.39% to
Conclusions
Three different biomasses (rice straw, sewage sludge and Spirulina) were liquefied at identical conditions. And their thermochemical liquefaction characteristics were systematically compared. The conclusions are as follows:
- (1)
The conversion rates of the three biomasses were as follows: Spirulina (79.7 ± 1.02%) > rice straw (74.6 ± 0.74%) > sewage sludge (54.3 ± 1.37%). However, sewage sludge produced the highest bio-oil yield (39.5 ± 1.16%). And the caloric value of bio-oil from sewage sludge
Acknowledgments
The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 21276069 and No. 51009063) and the Hunan Province Innovation Foundation for Postgraduate (No. CX2012B139).
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