State of art review on conventional and advanced pyrolysis of macroalgae and microalgae for biochar, bio-oil and bio-syngas production

Highlights

• Macroalgae and microalgae are excellent pyrolysis feedstock.

• Chemical constituents of algae are important criteria as feedstock.

• Advanced pyrolysis derived products show higher energy content and better yield.

• Technological limitations associated with each technique have been detailed.

Abstract

Algal biomass including macroalgae and microalgae show great potential as pyrolysis feedstock in generating energy-dense and valuable pyrolytic products such as bio-oil, biochar and bio-syngas. The chemical constituents of macroalgae and microalgae show great variations, especially their lipid, carbohydrate and protein contents, which could affect the qualities of the pyrolytic products. From the established conventional pyrolysis, the products produced from both macroalgae and microalgae show moderate energy contents (<34 MJ/kg). The review identifies the issues associated with development of conventional pyrolysis such as flash and intermediate pyrolysis. To enhance the production of biofuels from algal biomass, advanced or non-conventional pyrolysis techniques have been employed. Catalytic pyrolysis on algal biomass could reduce the nitrogenates and oxygenates in the biofuels. On top of that, co-pyrolysis with suitable feedstock shows great enhancement on the bio-oil yield. As for hydropyrolysis of algal biomass, their generated biofuels can produce up to 48 MJ/kg with high yield of bio-oil up to 50 wt%, comparable to conventional fuels. Microwave-assisted pyrolysis of algal biomass greatly shortens the processing time through advanced heating; however, favours the formation of bio-syngas by improving the yield up to 84 wt% depending on the feedstock used. Therefore, formation of biofuel fraction suitable for energy generation highly depends on the selected pyrolysis technologies.

Introduction

Global energy consumption has increased considerably mainly because of the increase in world population as well as the rapid industrialisation over the years. To accommodate such development, generation of energy from conventional fuel combustion has created high level of greenhouse gases (GHG) emissions such as methane (CH₄), carbon dioxide (CO₂), carbon monoxide (CO), nitrogen oxide (NO_X), sulphur oxide (SO_X) and other hydrocarbons [1], [2]. Especially the emission of CO₂, about 18.6% of the CO₂ (approx. 2 billion tons of CO₂) generated from the power generation sector contributed greatly to the global warming issue [3], [4]. Furthermore, fossil fuels are finite resources which might be depleted in the near future with the current application and usage. Greener fuel type with low carbon emission is desirable, such as biofuels due to their renewable, biodegradable, and low GHG emission natures as well as comparable energy content with conventional fuels [5]. The derivation of biofuel from biomass is especially attractive because biomass are low-cost, abundant and can be replenished by themselves over time.

Biomass can be produced from both living and dead organisms as well as non-living materials which are mostly non-fossil in nature and can be in liquid, solid or semi-solid forms [6]. Structurally, biomass is a complex and heterogeneous biopolymer, which may consist of carbohydrates, cellulose, hemicellulose, proteins, lignin, lipids, extractives, water, ash and other trace components [7], [8], [9]. Therefore, depending on their chemical constituents, there are two main groups of biomass, namely the non-lignocellulosic and waste biomass, and lignocellulosic biomass, respectively, as summarised in Table 1 [10], [11]. Non-lignocellulosic and waste biomass refer to materials mainly comprise of proteins, lipids, polysaccharides or carbohydrates (holocellulose), inorganics, minerals and photosynthetic pigments along with a minor fraction of lignocellulosic contents [11], [12]. Due to the broad definition of non-lignocellulosic and waste biomass, most of the materials not categorised under lignocellulosic materials can be classified under this group which includes aquatic biomass, human and animal waste, industrial and municipal waste and contaminated biomass and semi-biomass waste. While, lignocellulosic biomass materials primarily consists of lignin, hemicellulose, cellulose, extractives and ash [11]. Emerging aquatic biomass such as azolla, wood, plant, herbaceous materials, agricultural crops, oils and residues are mostly considered to be lignocellulosic biomass based on their chemical constituents such as the presence of holocellulose and lignin (Table 1).

Among those biomass feedstock, algae as relatively new form of energy plant or crop, have shown great potential as thermochemical feedstock because algae grow faster and mature with abundant mass generation in matter of days when compared to terrestrial crops and plants which may take months to years [18], [19], [20]. By converting algal biomass into biofuel forms, these are regarded as 3rd generation biofuels. The main advantages of 3rd generation biofuels are non-potable water requirement, non-arable land cultivation, simple nutrient requirement such as CO₂ and sunlight, non-herbicidal/pesticidal cultivation, carbon-neutral feedstock usage and high quality production of pyrolytic materials [16], [18], [21], [22], [23]. Furthermore, the high photosynthetic efficiency of algae (6 – 8%) compared to terrestrial biomass (1.8 – 2.2%), as well as high CO₂ fixation by algae, make them excellent thermochemical feedstocks and natural environment remediators [4], [19], [24], [25]. In the long term, the generation of viable feedstock and reduction of GHG (e.g: CO₂) can be achieved through algae cultivation.

The emerging thermochemical conversion techniques have generated great attention and interest from the science and research communities in recent years due to the technological advances and economical approaches [26], [27]. Thermochemical conversion techniques produce higher yield of energy products compared to biochemical technologies (fermentation, anaerobic digestion and others) [28], [29]. The establishment of these technologies such as combustion and co-firing, gasification, torrefaction and liquefaction has been well-reviewed on algal biomass as feedstock [10], [13], [19], [24], [30], [31], [32], [33], [34]. The main advantages of thermochemical conversion techniques are versatile with wet or dry biomass (moisture content >50%), fast processing time (from second to minute) and generation of different products (liquid, solid and gas phase products) [13], [30].

Algal biomass including macroalgae and microalgae are emerging as one of the most studied and most potential biomass to date as the 3rd generation biofuel feedstock. This work investigates the chemical constituents including moisture, volatile matter, carbon content, ash content lipid, protein and carbohydrate of both microalgae and macroalgae as pyrolysis feedstock, which is lacking in the current literature. Numerous reviews are available and focus on mainly microalgae as the pyrolysis feedstock [10], [31]. However, as part of algal family, macroalgae or seaweed as pyrolysis feedstock have been neglected over the years. Thus, a critical review which assesses and compares both macroalgae and microalgae as pyrolysis feedstock is important to identify the research gap in the current community. This is especially important because despite being in the algae family, the pyrolysis on both macroalgae and microalgae generates very different pyrolytic products distribution in term of yields, and product characteristics in term of physicochemical properties, as highlighted in this review.

In thermochemical conversion, algal biomass is subjected to decomposition via thermal treatments to produce different energy products such as heat, steam, syngas, bio-liquid and biochar products, as shown in Fig. 1. Combustion technology is an established technique which can be applied directly on algal biomass under high temperature (800 – 1000 °C) with the presence of excessive air [35], [36]. However, direct combustion of algal biomass for energy generation might not be applicable to all algal species even though this practice can reduce CO₂ emission as feedstock with high moisture content (>50%) would create fumes of white smoke [37]. Nonetheless, to date, microalgae such as Enteromorpha and Chlorella [36] and macroalgae such as Ulva lactuca [38] both show great energy content; however, their high ash, alkaline salts and moisture content affect their direct usage. Thus, usually, they can be used as co-feedstock in combustion (co-firing) without any modification due to their compatibility with the existing conventional coal boiler [31], [39].

Gasification is a well-known thermochemical technique which applies O₂, CO₂, steam or supercritical water treatments on algal biomass at an elevated temperature between 800 and 1000 °C [40], [41]. When the algal biomass reacts with the O₂/CO₂/steam system/supercritical water, the generation of syngas occurs and its composition mainly consists of H₂, CO, CO₂, N₂, water vapour, CH₄ and tar by-products [40], [42], [43]. The main issue with gasification is the formation of tar or hydrocarbon condensate affecting the syngas combustion quality but these issues can now be overcome by the use of Fe-based catalysts [42]. Dry and wet torrefaction, as emerging techniques, exert mild thermal treatment (160 – 300 °C) on algal biomass under the inert condition to produce energy dense solid fuels [21], [44]. While dry torrefaction resembles pyrolysis technology, wet torrefaction shares similar experimental settings as hydrothermal carbonisation [11], [45]. The resulted product known as torrefied biomass overcomes the disadvantages of direct usage of biomass as energy solid fuels such as high heterogeneity, short storage life and relatively higher moisture content [46], [47]. Subcritical water liquefaction can handle high moisture or wet biomass without energy-intensive pre-treatment such as drying, into liquid fuel (bio-crude) at moderate temperature (280 – 370 °C) and at applied pressure between 10 and 25 MPa [48], [49]. In addition to pure water solvolysis, different solvents such as tetralin, toluene, acetone and alcohols (ethanol, methanol, propanol, butanol and pentanol) can be used as solvolysis media [49]. Besides bio-oil, biochar produced from the liquefaction can be used as soil conditioner [50], electrodes for supercapacitor [51] and lithium batteries [52], while the gaseous phase may be used as syngas fuel directly [50]. Generally, the yield of bio-oil generated from seaweed macroalgae is lower compared with microalgae feedstock as their high AC and carbohydrate contents do not favour the formation of bio-oil [53].

Among those thermochemical conversion techniques, pyrolysis has been regarded as one of the most developed and robust methods in transforming biomass into energy products. Pyrolysis is a form of thermolysis or carbonisation that uses intense heat under low or absence of oxygen (O₂) environment to thermally decompose a variety of biomass to different pyrolytic products such as solid products (biochar), liquid products (bio-oil) and gas products (bio-syngas) suitable for applications such as energy generation and environmental remediation [28], [35]. As shown in Fig. 2, conventional pyrolysis techniques such as slow, intermediate, fast and flash pyrolysis are regarded as feasible and effective synthesis routes which can be applied impartially on different biomass materials [28]. For instance, non-lignocellulosic and waste materials [14], [55], [56], [57], [58] and lignocellulosic biomass [59], [60], [61] have since been utilised and studied as pyrolysis feedstock in great details. To enhance the pyrolysis process, advanced modification can be performed on conventional pyrolysis, resulting in progressive techniques such as catalytic pyrolysis, co-pyrolysis, hydropyrolysis and microwave-assisted pyrolysis [10], [30]. Pyrolysis technologies are therefore considered to be highly flexible and not limited to certain types of biomass, as long as the pyrolysis feedstock fits the criteria such as renewable, sustainable and abundant in supply sufficiently to support the continuous synthesis of pyrolytic products [55].