CHAPTER 4: CONCEPTUAL PROCESS DESIGN AND SIMULATION OF MICROALGAE OIL
4.1 INTRODUCTION
4.1.1 Background, jet fuel and biodiesel
The current need for sustainable energy is focusing on the development of ground-breaking and alternative types of renewable energy. This trend aims to fill the gap created by an increasing demand from the energy industry.[1] Therefore, options related to the production of biofuels such as ethanol, biodiesel and jet fuel from biomass are currently highly considered.
Ethanol derived from starch, sugar or any other type of biomass can be used as an alternative or additive to gasoline.[2,3,4,5] Furthermore, in case the cellulosic biomass is successfully used to produce bioethanol at affordable costs, there will consequently be a competition with petroleum-based fuels. Regarding the biodiesel alternative, some edible and non-edible oils can be a substitute for petroleum diesel provided that the production costs are reduced.[6,7]
Similarly, it is important to stress the fact that the use of these oils should not be a threat to food production, lubrication and many other industrial applications. Unfortunately, this kind of biofuel cannot be directly used in aircraft engines as substitutes to jet fuel for which high energy density and low-temperature are highly required and very critical parameters. They are among key parameters that define the performance of a jet fuel.
The energy density per unit volume for ethanol is not sufficient to run an aircraft because it is half of the total energy produced by a conventional jet fuel. However, the energy density per unit volume in biodiesel represents almost 80% of the one contained in jet fuel.[8]
The major weakness of biodiesel is that at lower temperatures, with an aircraft flying at higher altitudes, there will be fuel solidification taking place.[8,9] This is due to the fact that biodiesel freezing point is much higher than that of conventional jet fuel. Studies to find an appropriate flow improver will be needed for biodiesel.[9] It is a stringent requirement for any aviation fuel to have a very low freezing point to avoid disastrous consequences due to its solidification during the flying time. Therefore, biodiesel cannot be a suitable fuel for aircraft.
4.1.2 Market expectations, current state of technology and requirements to produce algae bio-oil
It is essential to produce algae-based jet fuel with similar physico-chemical properties to conventional jet fuel; it has to be cost-effective and sustainable and have low carbon emissions.
A designing and simulating process including modelling and optimizing some parameters could be a key milestone in the production of sustainable jet fuel from microalgae oil. Many studies are currently underway but not many of their outputs are in the public domain.
76
The field of alternative aviation fuels, and more especially work on microalgae-based jet fuels, is becoming particularly competitive. Demand and market expectations are very high with regard to compliance and costs. ‘Drop-in’ fuels are the most needed but they are not sustainable as yet; however, the blending of current fossil fuels with algae-derived fuels is considered to be a sustainable option. Due to strong similarity between petroleum crude oil and algae bio-oil qualitatively and quantitatively (depending on the species/strain), it is possible to use a process that is similar to that used in refineries for conventional jet fuel in order to produce algae-based jet fuel [10]. However, the process has to be improved economically for the algae-based jet to be competitive and finally become commercialized. There is currently no jet fuel on the market produced on a larger scale from algae bio-oil. Many projects are still at trial and pilot scale.
Algae-based jet fuel is only blended with conventional jet fuel on 50/50 ratio as allowed by ASTM certification.
For microalgae to produce oil, biomass cultivation must take place under defined conditions of temperature and pH. The addition of nutrients and carbon dioxide to allow effective growth should also be part of the cultivation process. Harvesting takes place after the cultivation period and thereafter bio-oil extraction is undertaken to produce bio-oil for the conversion process. In some situations, microalgae grow best in saline water / seawater or wastewater more generally, and in domestic sewage because of the presence of nutrients such as nitrates and phosphates.
However, this is not always possible for all species or strains; some will grow easily in any water or wastewater streams and other not, and it also depends on the species or strain’s nature or type. Being photosynthetic organisms, microalgae cells require sufficient light to guarantee an effective growth in order to produce enough biomass. Species can therefore multiply in a very short period of time if all growth conditions are gathered. Sunlight can be used, in this regard, as a cheap option for culture illumination; it is an energy-efficient way to cultivate algae, although there could be daily and seasonal variations in terms of light intensity [11,12]
conventional jet fuel on 50/50 ratio as allowed by ASTM certification.
4.1.3 Cultivation, lipid production and lipid content
Cultivation takes place in photobioreactors or open ponds. Various microalgae species store energy in the form of hydrocarbon, which can produce lipids or bio-oil. This is obviously happening when nutrient depletion takes place during the cultivation period. This period is characterized by the species growth, which is regarded as the biomass production time.[13,14]
The cells’ buoyancy is also regulated by the lipids present in them. However, the lipid content for many species is generally low. By manipulating the microalgae cell genetics and the growth
77
conditions it is possible to increase bio-oil output. Genetic or physiological modification of microalgae cells will stimulate lipid increase. This is needed for conversion processes.
The genetic modification of an algal cell is all about stressing the species under adverse conditions to stimulate the metabolism of the microalgae cell. As a result, stressing microalgae cells under defined conditions will allow the production of more lipids.
4.1.4 the need for effective conversion processes and Overview of conversion processes
Algae-based fuels in general and jet fuel particularly require cost-effective processes or technologies that will: first, allow effective cultivation and harvesting of biomass, second, generate a high output of crude oil and finally, assist in optimising conversion processes or technologies to produce compliant jet fuel.[10,15] Process design and simulation including optimisation of parameters is therefore very important to produce a compliant and cost- competitive jet fuel as mentioned earlier. The design of conversion processes should thus take into consideration operating conditions, energy consumption, the cost of additives and the expectations of the aviation sector and the market.[10]
Many approaches can be explored to convert microalgae oil into liquid fuels. Gasification of algae biomass using Fischer–Tropsch (F–T) synthesis is known to be a successful process.[16–20] However, gasification is energy-intensive because it requires higher temperatures to reach an adequate gasification stage. The F–T synthesis is also characterized by low selectivity for liquid fuels – especially for fractions in the range between C6 to C22; there are lighter fractions, such as methane and ethane, and there are middle and heavy fractions.[21]
Transesterification is another approach that can be used to convert microalgae oil into aviation fuel; this approach is known as the biodiesel route, although it is not energy intensive; however, it involves catalytic processes such as decarboxylation, deoxygenation, and isomerization, which can be costly compared to the ones used for the production of conventional jet fuel.[10,22–26] Hydroprocessing oil from microalgae can also generate a jet-fuel type using hydrogen as a catalyst during cracking.[27,28] The hydrogen removes oxygen from algae crude bio-oil during the process. Despite the challenge related to low lipid content for many species, these processes are technically achievable. Another challenge is the costs related to the feasibility or economics of the processes. These costs are higher compared to those related to conventional jet fuel production.
78
4.1.5 Process design and the aim of the study
When designing an efficient process, it is important to take into consideration the type of species, growth conditions, growth time, the rate of biomass production, and the possibility of producing a large amount of lipids. The current study based the process design on laboratory experiment. Most parameters for design were collected from the experimental laboratory work.
Designing and simulating a process for algae-based aviation fuel should focus on developing a sustainable technology. This technology must be developed over time so that its escalation will need few modifications for the fuel to be ready for use. The maturity of the technology will be a major characteristic that will reflect on its readiness. This comes after successful trials at laboratory and pilot scale. It is followed by implementation on a larger scale.
The aim of this study is to establish a conceptual design and simulate the entire process by involving all the steps from cultivation to production of jet fuel using the Aspen HYSIS V8.8 package, which was the only version available for the study. The design can be used as a tool for a larger scale plant in the future once studies of the costs and economics have been completed. The strain used during experimental work to generate the biomass and the crude oil was Nannochloropsis sp., a marine species. The steps and operating conditions used in the laboratory constitute the basis for process design, as mentioned earlier. This approach aims to mimic a process close to that used for conventional jet fuel production. The specific aim in simulating the process is to generate a microalgae-based jet fuel that can comply with stringent aviation regulations. This is to some extent possible because of the similarity between algae crude bio-oil and crude petroleum oil as mentioned before.[10] Cost analysis is not the focus of this study; it can be addressed on its own in another study. It is a very interesting aspect of design but it needs to be undertaken after successful design and simulation. This study focuses on conceptual design and simulation based on data generated from a laboratory experiment.
Future studies can be undertaken on a very detailed design involving equipment modelling and sensitivity analysis needed for all variables before undertaking cost studies.