ReviewNitrogen and sulphur in algal biocrude: A review of the HTL process, upgrading, engine performance and emissions
Graphical abstract
Introduction
The rise in oil prices in the 1970s opened the gates to seek alternative fuels as fuel depletion and the control of fuel reserves by some countries became a national security concern. A model, which predicts the depletion date of fossil fuel resources, was developed by Shafiee et al. [1] and it was forecast that oil, coal and gas reserves will be exhausted in 35, 107 and 37 years, respectively. This view point is widely held by the research community. Mueller [2], however, disagrees with this depletion theory and argues that fossil fuel resources are abundant. Mueller estimates of conventional oil, gas and coal reserves are 1.35 trillion barrels, 6600 trillion cubic feet (tcf) and 861 billion tonnes respectively. Mueller [2] suggests that the increase in oil prices will reduce consumption and initiate investments in unconventional fossil fuel resources (e.g. oil shale, shale gas, sandstone natural gas and methane hydrates) which were previously perceived as being uneconomic. In addition, new extraction methods will open the gate for the discovery of new reserves and so resources which were discovered but were unreachable become viable.
Despite differing viewpoints on the availability of fossil fuels, their contribution to global warming remains a challenge. In the early 1990s, developed countries experienced a rapid economic boom due to extensive investment in their natural resources, however this came with associated higher levels of pollution [3]. Therefore, it is important to find alternatives to fossil fuel resources such as 2nd generation (e.g. bagasse and crop waste) and 3rd generation (e.g. algae) biofuels. Biofuels is a term referring to renewable or novel fuels and includes biodiesel and biocrude among others. Biodiesel is the product of the transesterification reaction between lipid and alcohol. Fatty acids can be sourced from a wide variety of feedstocks; animal fats, vegetable oils, waste cooking oils, algae and so on. Biocrude is produced by thermochemical conversion processes such as pyrolysis and hydrothermal liquefaction. Though these conversion routes require high energy input, they can breakdown feedstock of complex structure, such as plastic waste [4], tyres [5], bagasse [6] and algae [7] into valuable products.
Early investigations focused on food crops for novel fuel production, which were later classified as first generation biofuels (FGBs). This raised ethical, environmental and economic concerns. The production of biodiesel and bioethanol is usually derived from food crops. Large volumes of crops being taken away from the food market to be used in biofuel production, led to an increase in food prices [8]. The actual benefit of FGBs in reducing greenhouse gases (GHGs) and CO2 emissions is also debatable [9]. This led to second generation biofuels (SGBs) produced from lignocellulosic material and non-food crops to overcome issues manifested in FGBs [10]. These biomasses include cereal straw, forest residues, sugarcane bagasse, organic waste, food crop waste and specialised crops (e.g. jatropha). Third generation biofuels (TGBs) or “advanced biofuels” refers to biofuels mainly produced from algae [11] because, unlike 1st generation biomass, it induces minimal to no impact on food supply or land use.
Algae require light, nutrients, CO2 and water; they are microorganisms which self-replicate swiftly through undergoing photosynthesis. The cultivation medium can potentially be grey water, sewage, animal farm wastewater and brackish water [12]. It can also be cultivated on non-agricultural land in open ponds and photo-bioreactors. One of the most favoured characteristics is its high growth rate, algae can potentially produce 8–36 times the oil per acre per year than oil palm [13]. In contrast, when soybean is used as a feedstock for biodiesel production, it requires 100 times more land and affects food supply and prices [14]. Therefore, algae are of interest in the biofuels debate because they do not require agricultural land and pose no threat to conventional terrestrial food supplies.
The economic viability of biofuel algae is curbed by the cost of conventional cultivation systems (infrastructure, technology and operation), consumption of nutrients (mainly N and phosphorus), harvesting and dewatering. The high cost associated with algae dewatering could be moderated by using HTL for biocrude production [15]. However meeting the nutrients requirement of algae is a challenge not only in terms of cost but also in terms of availability and sustainability of nutrients. It is estimated that one tonne of algae requires 40–90 kg of N, 3–15 kg of P [16] and 1.8–2 tonnes of CO2 [17]. Thus, novel and economic sources of nutrients are required. One possible route to sourcing nutrients for algae growth at lower cost can be achieved by growing algae on wastewater which is rich in nutrients. For example, the Pacific Reef prawn farm in Ayr, North Queensland grows algae to treat dirty water [18]. However, wastewater alone cannot meet the high demands of N and P to produce enough algae which could potentially replace conventional fuel [19]. Another possible option to secure nutrients would be algal blooms caused by eutrophication (water pollution). In algal bloom events, the excess of nutrients, mainly P and N in the water medium causes a dramatic algae growth rate. Thus, algal blooms could serve as a source of algae without cultivation cost (just harvesting costs). Another explored route is recycling the nutrients of aqueous product from the HTL process [20], [21].
N is a vital nutrient for algal growth and its biochemical composition. Many studies have investigated the effect of N starvation to promote algae rich in lipids or carbohydrates and supressing protein content [22]. S is another important nutrient responsible for sulfolipids, proteins and polysaccharides [23]. These nutrients not only affect the biochemical composition of algae but also can affect the likely choice of the processing route of algae to a renewable fuel. For example, algae with high carbohydrate content could likely be processed using anaerobic digestion to produce biogas, while algae with high lipid content would likely be converted to biodiesel by transesterification and algae with high protein content would be converted by hydrothermal liquefaction to biocrude. Hydrothermal liquefaction is particularly well suited for microalgae [6], [7] because it tolerates complicated structure (e.g. protein) and high moisture feedstock thus mitigating the cost of algae drying required by other processes. High protein algae HTL produces biocrude of high N (5–8 wt%) and S (0.5–1.5 wt%) content generating lower quality biofuel and requiring upgrading [24]. While S content in fuel is subject to increasingly stringent regulations [25], there is no limitation on N content. However N is problematic due to possible gum or sediment formation and low thermal and storage stability of the fuel [26].
The presence of N and S in biofuel has very different roles in in-cylinder combustion and subsequent emissions. N in biofuel will disassociate under combustion and undergo chemical transformation to form either N2 or NO and NO2 (NOx). N2 is not considered a pollutant since it is the major constituent of air. Other oxides of N are also produced in very small quantities including the greenhouse gas N2O. The most favourable pathway to either N2 or NOx is highly variable and depends significantly on engine operating conditions [27]. In contrast to N, S in the biofuel will always produce emissions that are considered to be harmful. The major S emission product is SO2 and SO3; although SO3 is not as abundant but causes engine corrosion and parts deterioration. S compounds have a strong tendency either to form particles (by nucleation) or to attach to existing particles [28], [29], [30]. In general, at least 20 wt% of S will be present in the exhaust emissions in aerosol (particle) form [31].
The existing literature reviews on the topic of microalgae biodiesel have thoroughly covered most of the stages involved in the algae biodiesel production [32]. Because HTL is highly suited to microalgae to make biocrudes, the potential presence of N, in particular, and S (to a lesser extent) in biofuels is a new area, so a review of their potential impact is needed. This work therefore investigates N and S in terms of algae composition (covering conventional cultivation and novel sources of N-S nutrients), fuel production (covering conversion processes and N-S distribution in the end products), and fuel properties. N and S removal from algae biocrude, the suitability of algae biofuels as a fuel for diesel engine and their emissions are also covered.
Section snippets
Cultivation and harvesting techniques
In the process of biofuel production from algae, cultivation and harvesting techniques account for a large part of the processing cost. Ongoing development of cost-effective cultivation and harvesting techniques reduces price and improves biomass yield, its lipid yield and lipid profile. To this end, cultivation and harvesting technologies have been developed for microalgae. Similar technologies could apply to macro-algae; however in the case of microalgae, a controlled cultivation is necessary
Conversion processes
Algae have gained attention as a viable source of oil as lipid composition ranges from 2 to 75 wt% of their dry weight. Biological conversion routes (anaerobic digestion [66] and fermentation), chemical conversion (transesterification [67]) and thermochemical conversion routes (liquefaction [68], pyrolysis [69] and gasification [70]) have been studied for making biofuels. Different algae species [71], process conditions, catalysts and solvents [72] were tested for their effect on product yield,
Biocrude upgrading
Biocrude oil contains high N content (5–7 wt%) and investigation on denitrogenation of biocrude is presented, in this section, through two-stage HTL and catalytic upgrading. Most of the research carried out on algae HTL did not explore S content in the biocrude and S-containing compounds were generally not reported. Although researchers suggested that N removal from biocrude is substantial as it will give rise to NOx emissions during combustion, the authors could not find any studies to support
Engine performance and emissions with microalgae biodiesel inferences for biocrude
Numerous studies have investigated algae biodiesel and blends in diesel engine and investigated the associated emissions in comparison with diesel [123], [124], [125], [126]. However, to the knowledge of the authors, there is a lack of research on biocrude engine performance and emissions, although there has been a study investigating HTL biocrude using a surrogate fuel in a diesel engine [127]. This study focussed on replicating the physical properties of algal HTL biocrude and so N and S
Conclusion
There is a perpetual need to find novel fuel, a fuel that is sourced from an abundant renewable biomass, could be produced at a cheaper price than conventional fuel and adheres to the current emission regulations and its prospects. Algae biofuel conforms to some of these requirements, algae being a renewable feedstock that could be widely available if integrated with wastewater treatment facilities and if we make use of algal blooms. The literature allows a good understanding of the
Acknowledgement
The authors appreciatively acknowledge the financial support provided by QUT, a PhD scholarship from the School of Chemistry, Physics and Mechanical Engineering and an ECARD grant.
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