Performance and potential appraisal of various microalgae as direct combustion fuel
Introduction
Climate change caused mainly by the emissions of anthropogenic CO2 is the most severe problem threatening the survival and prosperity of humanity. To counter the problem, diverse CO2 capture, utilization and storage (CCUS) technologies have been suggested to decrease atmospheric CO2 levels. Of the various candidates, the biological conversion of CO2 to create biofuels has received considerable attention because this also offers a promising solution to overcoming current reliance on fossil fuels, doubly combatting the climate problem. Microalgae are considered the strongest candidate for use in biological systems for CCUS owing to their rapid photosynthetic CO2 fixation and high productivities (Chisti, 2007). In addition, unlike other crops, microalgae are not classified as edible crops and do not encroach on arable sites for the cultivation, thus being free from the so-called ‘food versus fuel’ debate. They also can be sustainably harvested throughout the year (Ruiz et al., 2016). These advantages over terrestrial plants strengthen the practicability of using biomass from microalgae as a raw material for sustainable fuels. Especially, microalgae have garnered substantial interest as a promising biodiesel feedstock owing to their oleaginous property (Chisti, 2007).
Meanwhile, a controversy has recently surfaced surrounding the carbon neutrality of biomass-derived liquid fuels because of the impacts of complex downstream stages like harvesting, processing and transporting that generate CO2 emissions (Clarens et al., 2011). Similarly, biodiesel production from microalgae requires lipid extraction, conversion and separation stages, all of which are energy intensive. Emissions from these stages can negate part of biofuel’s decrease in net CO2 emissions and thereby draw microalgae into debates over the energy system’s ability to deliver negative emissions. In this context, direct combustion of microalgae is worth reconsidering as a propitious alternative. This is because for the following: (i) The preparation of the biofuel for direct combustion does not involve diverse carbon-releasing downstream stages, thus enhancing the energy system’s life-cycle CO2 mitigation. (ii) The fuel systems can maximize the energy yield by using all of the biomass as an energy source. (iii) Microalgal solid fuel is compatible with conventional coal boilers and can thus be practicably used to generate useful energy in existing combustion systems with no need for extensive modifications. (iv) Microalgae are renewable energy sources with a sufficiently high energy density to substitute the coal usage without lowering thermal efficiency of the boilers. (Giostri et al., 2016). (v) The direct combustion strategy can be applied as a technological element of bio-energy with carbon capture and storage (BECCS), the most effective technology to deliver negative emissions (Fajardy and Mac Dowell, 2017), thus being expected to substantially decrease the anthropogenic carbon footprint. Indeed, a life-cycle assessment (LCA) showed CO2 emissions can be decreased in conventional coal-firing power plant when even 1% of coal consumption is substituted by dried microalgae (Giostri et al., 2016).
Various performance and potential appraisals should precede in order for the biomass to be practically exploited as a direct combustion fuel. However, so far, previous studies have mainly focused on the calorific analysis regarding a limited number of species (Illman et al., 2000, Kumar et al., 2017). Furthermore, they did not include a productivity and environmental assessment, thereby presenting difficulties for holistically evaluating the solid biofuel’s potential. Given this gap, herein, the potential of microalgae as a direct combustion fuel for a variety of species, including several green algae, diatom and cyanobacterium, all of which have been intensively studied for their biofuel-production potential was comprehensively appraised from the cultivation to the combustion. To evaluate the realistic potential, for example, the productivities of the algae were assessed with a photobioreactor (PBR) cultivation to simulate large-scale production and the combustion performance was investigated in the manner of coal-biomass co-firing which is the most common way to get energy from solid biomass recently. On the basis of the various analyses, new opportunities for a strategy that involves the direct combustion of raw microalgae, exploiting the biomass’ impressive calorific productivity and CO2 fixation ability were explored. Alongside these, the competitiveness of the solid fuel compared to currently commercialized woody biomass was demonstrated by providing its notable fuel properties, including areal productivity, grindability and combustibility.
Section snippets
Microalgal strains and biomass production
Chlamydomonas reinhardtii CC-125 was obtained from Chlamydomonas Resource Center at University of Minnesota while Chlorella sorokiniana UTEX 2714 (Bashan et al., 2016), Chlorella protothecoides UTEX 256 and Neochloris oleoabundans UTEX 1185 were acquired from UTEX Culture Collection of Algae at the University of Texas at Austin. Haematococcus pluvialis NIES-144 was purchased from the National Institute of Environmental Studies, Tsukuba, Japan. Chlorella vulgaris UTEX 395, Nannochloropsis oculata
Biomass productivity, heating value and cellular composition of microalgae samples
The heating value (also known as the heat of combustion or the calorific value) is defined as the total amount of heat energy released during the combustion reaction and is one of the most important factors for the design and operation of bioenergy systems (Sheng and Azevedo, 2005). The productivity of biomass is equally important for the appraisal of the practicability of biofuels. In this regard, 10 microalgae species were cultivated in readily scalable polymer-based PBRs designed to
Conclusion
The holistic analyses provide quantitative evidence to justify how microalgal solid fuel utilization doubly outperforms conventional lipid-targeted fuel system and extensively exploited woody biomass. The direct combustion strategy is expected to amplify negative emissions of microalgal energy system through high energy yield and CO2 fixation which were confirmed by BP and CFP, respectively. Excellent grindability of microalgae seems to increase the usability. Co-combustion study reveals
Acknowledgements
The authors are grateful to Prof. EonSeon Jun and Assistant Prof. Han Min Woo for kindly providing the microalgal strains. The authors also thank Dr. Won Yang, Dr. Sun Young Choi and Jongrae Kim for thoughtful technical advice. The authors gratefully acknowledge the support of the Korea CCS R&D Center (Korea CCS 2020 Project) which was funded by the Korean Government (the Ministry of Science and ICT) in 2018 (Grant No. KCRC-2014M1A8A1049278), the National Research Foundation of Korea (NRF)
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