Thermo-chemical behaviour and chemical product formation from Polar seaweeds during intermediate pyrolysis
Introduction
Biomass is a sustainable and renewable resource that offers an alternative pathway to fulfil the growing world demand for energy. In addition, its photosynthetic activity offers significant greenhouse gas mitigation potential. To process biomass, thermo-chemical conversion technologies like pyrolysis are promising routes to generate gaseous, liquid and fossil fuels and also to generate chemicals to reduce dependency on petroleum derived products [1]. Wood and energy crops, waste materials from the wood industry and agriculture as well as municipal waste are viewed as potential biomass resources for pyrolysis [2], [3], [4]. However, recently aquatic biomass is seen an attractive alternative to terrestrial biomass, as it reduces the impact on land use management, food crop production and overcoming nutrient constraints [5].
At Aston University a new type of pyrolysis processing waste and algal derived biomass, termed intermediate pyrolysis, is patented [6]. Intermediate pyrolysis operates at moderate reaction temperatures up to 500 °C, residence times for feedstocks of 0.5–25 min and moderate vapour residence times of 2–4 s. The product distribution obtained by this process is 40–60% of pyrolysis liquids, 20–30% non-condensable vapours and 20–30% biochar, while liquids and vapours are further processed to electricity, heat and transportation fuels. The dry and tar-free biochar can be used as a soil amendment and fertilizer, while sequestering carbon in the soil [6], [7], [8].
Whereas the pyrolytic behaviour of many terrestrial biomass types has been studied intensively, only little work has been done on marine macroalgae. Even though, obvious differences regarding the overall chemical composition of macroalgae and terrestrial biomass exist. Terrestrial biomass is made of carbohydrates mainly and contains minor amounts of 1–5% of organic extractives (proteins, lipids) and inorganic materials [3], [9]. In contrast, macroalgae biomass contains ca. 40–60% carbohydrates, 5–20% proteins and 1–5% lipids [10], [11], [12]. In addition, the constituents of the carbohydrates vary substantially between aquatic and terrestrial biomass. Generally, terrestrial plants produce cell walls consisting of 60–90% of lignocellulosic material composed of cellulose microfibrils, hemicellulose (mainly made of xyloglucans and glucuronarabinoxylans) and lignin [9]. Cell walls of marine seaweeds contain a distinctive taxonomic-based polysaccharide composition, which is in addition to cellulose made of sugars, sugar acids and sulphonated pyrans (an acidic sulphate group replaces a hydroxyl group). Moreover, the lack of the phenolic macromolecule lignin in seaweeds is of importance for thermo-chemical processing applications [13], [14].
Characteristic for species of the division Chlorophyta (green algae) are sulphated heteropolysaccharides containing galactose, arabinose, xylose and rhamnose (glucuronoxylorhamnans, glucuronoxylorhamnogalactans, xyloarabinogalactans). In addition, Chlorophyta synthesises xylans forming a triple helix providing a strong fibrous structure and mannans, building a crystalline skeletal component, as structural polysaccharides [14]. The major polysaccharides of the Rhodophyta (red algae) cell walls are sulphated galactans, such as carrageenan and agar. Agars are mainly composed of repeating d-galactose and l-galactose units, whereas carrageenan consists only of d-galactose. In addition, carbohydrate residues such as xylose, glucose and uronic acids are present next to substituents such as methyl ethers [15], [16]. In the division Phaeophyta (brown algae) alginic acid is the primary polysaccharide, consisting of unbranched chains formed of β-1,4-linked d-mannuronic acid, adjacent to blocks of on average 20 units of α-1,4-linked l-glucuronic acids. This algal class also contains “fucans” (also known as fucoidin, fucoidan and sargassan), which are sulphated polysaccharides containing l-fucose and varying proportions of d-galactose, d-mannose, d-xylose, d-glucuronic acid and mannuronic acid [14], [17].
Overall it is noted, that the proportions of constituent sugars and acids vary within the divisions from genus to genus, among species and within the thallus of a single plant [15], [19]. Furthermore, additionally to cell wall polysaccharides macroalgae organisms contain energy storage carbohydrates. Green macroalgae contain starch (made up of amylose and amylopectin), red algae contain floridean starch, which lacks amylose and brown algae contain mannitol and laminaran (β-glucan) [13], [20].
In addition, to their different chemical constitution macroalgae species that grow in Polar regions are highly adapted to the prevailing harsh climates and strong seasonally changing environmental factors [20] and may offer new and so far unexplored opportunities in applications for bioenergy generation and chemical feedstocks. Characteristics are high photosynthetic efficiencies and hence lower light requirements [9] to compensate the 30–50% lower annual radiation compared to temperate and tropical regions. As well are the organisms adapted to low seawater temperatures between 0 °C and 10 °C and to variations in water levels, where algae growing in intertidal zones can be exposed to air during low tide and dehydrate for several hours or may even freeze if air temperatures falls below 0 °C [21], [22], [23], [24].
In summary, the divergent chemical composition of aquatic biomass and its adaptation to harsh environments makes it an interesting source for thermo-chemical processing and hence requires fundamental pyrolysis studies. Therefore, this study investigates the thermal decomposition behaviour of Polar macroalgae of the three main divisions under intermediate pyrolysis conditions via TGA and Py-GC/MS and characteristic degradation temperatures and chemical compounds formed are presented.
Section snippets
Algal material and analysis
The Arctic specimens (Prasiola crispa, Monostroma arcticum, Polysiphonia arctica, Devaleraea ramentacea, Odonthalia dentata, Phycodrys rubens, Sphacelaria plumosa) were collected in Kongsfjorden (Spitsbergen, Norway, 78°55.5′N; 11°56.0′E) within the period of June–August. The Antarctic specimens (Gigartina skottsbergii, Plocamium cartilagineum, Myriogramme manginii, Hymencladiopsis crustigena, Kallymenia antarctica) were collected in Potter Cove (King George Island, Antarctic Peninsula,
Characterisation of macroalgae
Table 1 presents the ultimate analysis, ash and calorific values (HHV) of the analysed Polar seaweeds. Ultimate analysis revealed elemental contents of carbon between 22% and 39%, hydrogen between ca. 4% and 6%, nitrogen in the range of 1–4% and oxygen within 15–31%. Whereas terrestrial biomass exhibits a nitrogen content of typically <1% [27], significant higher values have been obtained by the macroalgae biomass. This is caused by nitrogen being a substantial element of amino acids, proteins,
Conclusion
Biomass of marine Polar seaweeds collected in the Arctic region (Kongsfjorden) and Antarctic region (Potter Cove) were investigated during intermediate pyrolysis by TGA and Py-GC/MS. This work reveals the complexity of processes and products formed under conditions of intermediate pyrolysis. Ash values in the range of 27–45%, which is much higher than typical values of terrestrial biomass were obtained. Three degradation stages (ca. 100–180 °C, 180–650 °C and 650–900 °C) of seaweeds biomass could
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