Department of Food Science and Technology, Federal University of Santa Maria, Santa Maria, RS, Brazil
Received date: October 04, 2016; Accepted date: October 07, 2016; Published date: October 07, 2016
Citation: Severo IA, Deprá MC, Zepka LQ, Jacob-Lopes E (2016) Photobioreactors and Oxycombustion: A Mini-Review on the Process Integration. J Chem Eng Process Technol 7:310. doi: 10.4172/2157-7048.1000310
Copyright: © 2016 Severo IA, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Some strategies for reducing greenhouse gas emissions to the atmosphere, especially carbon dioxide, have been gaining growing attention by researchers with the aim of develop new technologies that are energy-efficient and economically viable in industrial processes. In this sense, the aim of this paper is to provide a mini review of industrial systems of oxycombustion integrated in microalgal photobioreactors. Divided into four distinct topics, the paper discusses issues related to biological carbon capture and utilization, the elements of oxycombustion technology, the reuse of exhaust gases from photobioreactor and the process integration, summarizing a range of useful strategies directed to the industrial sustainable development.
Biological carbon capture and utilization; Biooxycombustion; Oxyfuel; Microalgae
Concerns over global climate change lead efforts on developing technologies to reduce carbon dioxide emissions from anthropogenic activities. Technological solutions to this problem includes research and development systems and alternative techniques that are economical and energy-efficient in industrial processes, enabling mitigation of polluting compounds . The oxycombustion has proven to be one of the best forms of carbon capture from stationary sources, due to its ability to mitigate emissions and improve combustion performance. This technological route is based on the replacement of the air used in combustion processes by oxygen-rich atmospheres. As a consequence, there is a gain in thermal capacity, resulting in greater overall efficiency of equipment and reducing fossil fuel consumption . Despite the oxycombustion be an attractive technology, this approach has faced significant difficulties in implementation related to the high cost and the purity of the oxygen produced, unfeasible their application in industrial scale .
These issues can be circumvented through the photobioreactors, using photosynthetic microorganisms, as the microalgae, which have as a metabolic co-product the O2 supported by the water photolysis reactions. Additionally, the bioprocess mediated by microalgae produce in parallel numerous volatile organic compounds that have considerable energy content, besides releasing substantial concentrations of CO2 in exhaust gases, serving as N2 diluent [4,5].
Several technologies have been proposed for reducing CO2 emission focusing on the capture, storage and/or utilization of carbon . The biological carbon capture and utilization (BCCU) is a highly potential technology for industrial application, because it converts biologically carbon dioxide with their integration into a oxycombustion process . Therefore, the bio-oxycombustion technique is discussed in this mini review, including fuels derived from biological conversion processes of greenhouse gases through photosynthesis.
Global emissions of carbon dioxide reach more than 20 billion tons per year. In contrast, it is estimated that such emissions should be reduced by at least 50% to mitigate the increase in global temperature. In this scenario, the oxycombustion technology have been proposed as a viable option to capture approximately 90% CO2 in large point sources of emissions, consisting of compression stages, capture, transport and storage or utilization . Complementing the carbon capture and storage (CCS) and carbon capture and utilization (CCU) technologies, more recently the BCCU technology represents a potential strategy for CO2 capture from polluting sources. This technological route consists of the biological conversion of CO2 into biomass through photosynthesis, for the extraction of high value-added products. In this context, the microalgae-based processes, for example, present high conversion rates, which provide an attractive form of BCCU, recycling the CO2 from different industrial processes [7,9].
The use of pure oxygen as oxidant in combustion processes is a relatively new approach to the greenhouse gases mitigation. In oxycombustion, the fuel is burnt in an atmophere enriched with high purity oxygen and free nitrogen, differing from conventional processes that use air as oxidant. The flue gas are recycled, producing a stream containing mainly CO2 and H2O. After this step, CO2 is ready for capture after purification and compression . Due to the high flame temperature resulting from burns with practically pure O2, part of the gas from combustion, rich in CO2, returns to the system to refund the volume of gas lost when N2 is removed from the air and to prevent the excessive increasing temperature in the system . The main differences in the replacement of N2 for O2 are in relation to the temperature, the stability, the propagation velocity and delay in the flame ignition, the heat transfer and reduction of pollutants emissions, impacting positively in the overall heat capacity of the equipment. Many of these effects can be explained by differences in gas properties CO2 and N2, which influence both heat transfer and combustion reaction kinetics (density, heat capacity, diffusivity and radiative properties of the gases). The oxycombustion can be used with any fuel to thermal energy production . Despite the inherent advantages of this technology, one of the main drawbacks associated with oxycombustion refers to O2 production of high purity. In this case, a unit for separation and purification is necessary, which causes an additional cost for the plant and increased energy consumption. These problems are the main challenges to be overcome .
Photobioreactors may be defined as systems utilized for the development of photosynthetic reactions. These equipments present different configurations, although the most common designs for the cultivation of microalgae are open or closed systems. Besides, some process parameters should be taken into account to achieve good yields, such as nutrient availability, efficient supply of CO2, temperature control, agitation, mass transfer, ease of control of the reaction conditions and ease of scale-up . Regardless of these questions, growth of microalgae in photobioreactors occurs through the photosynthetic mechanism. This in turn, consists in a chemical process that converts carbon dioxide into biomass using light energy and producing oxygen molecules . Photosynthesis is characterized by two interdependent phases. Firstly, carbon dioxide is incorporated into ribulose 1,5-biphosphate (rubisco) energy being required during the catalytic reaction of the enzyme rubisco carboxylase. The reaction product is broken into three-carbon molecules, 3-phosphoglyceraldehyde (PGA). In the presence of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), the PGA is reduced to glyceraldehyde 3-phosphate (G3P), leads to the production of sugars and subsequently glucose. This sequence of metabolic transformations is known as the Calvin-Benson cycle. Therefore, the light energy is retained in two photoreactions prepared by two pigment-protein complexes, called photosystem I and II, exciting the chlorophyll a molecule and their electrons are transferred to a molecule electron acceptor. Part of the energy liberated is incorporated into ATP in the phosphorylation process . The ultimate source of electrons for photosynthesis is H2O, which yields in the process of photolysis, or Hill’s reaction, hydrogen atoms, electrons and free oxygen. Thus, this reaction is an important product obtained from photosynthesis microalgae, because the O2 in the process is generated exclusively from the water photolysis .
Despite the existence of many studies considering the fraction of non-volatile metabolites, it is also observed that the yield in biomass does not completely fulfill the total carbon balance of the system. One should consider that other products are involved in the photosynthetic conversion of the CO2, including the secretion of biopolymers into the culture media, and of particular interest, the release of volatile organic compounds (VOCs) . The VOCs are unexplored biomolecules, which are influenced by the culture parameters. This way, microalgae are capable of synthesizing and release specific compounds which indicate a great commercial interest in biomolecules from renewable sources, such as alcohol, esters, hydrocarbons, terpenes, ketones, carboxylic acids and sulfurized compounds . Some cyanobacteria are known to synthesize alkanes or alkenes that have desirable properties for combustion. These VOCs can be used for the biofuels production, representing sustainable solutions for the energy sector . The occurrence of VOCs in photosynthetic microorganisms is a consequence of their metabolism, and their biosynthesis, will depend exclusively on the availability of carbon, nitrogen and energy from the primary metabolism. The possible metabolic pathways CO2 conversion for the synthesis of VOCs, were previously described in other studies [4,20,21]. These compounds could, therefore, be considered a source of useful chemical products with potentiality for commercial applications.
The process integration is a relatively current concept and has been implemented in most industrial production processes. This technological route is defined as a series techniques and methods in integrated production systems that combine several unit operations inside an industrial plant . Thus, the main objectives this strategy is to reduce the operational costs of the plant and the minimization of large equipment. As result, there is a lower energy consumption and increased overall efficiency in the system .
Alternatively, industrial processes which using fossil inputs have been gradually replaced by biobased processes. Microalgaebased processes, for example, are a potential way to a sustainable and economically viable production of consumables. The integration of microalgae into industrial processes is a suitable and innovative strategy to comply with the requirements of green engineering. For this technological route, is possible lowered its overall energy consumption of the process, which implies the reduction of fossil raw materials, potable water consumption and minimizing emissions [24,25]. Several integrated processes have been developed and some possibilities are considered through the reuse, the recovery and the possibly recycling of surpluses mass, effluents, water and energy for industrial application . These strategies are highlighted for being economically viable and for improving the thermal performance of industrial combustion systems, effectively contributing to sustainable development.
The different strategies of integration of the bio-based processes are considered important approaches towards sustainable development. For this reason, the use of oxygen biologically generated in photobioreactors and their integration in processes that require the enrichment of this compound as oxidizer, such as bio-oxycombustion process, are considered potential examples for industrial application. These technological routes can significantly contribute to the bioeconomy of the future.