Larger pond sizes would enable economically viable algal biomass production [ 23 ].
The operation cost for algae cultivation such as utilities, chemicals, labor, overheads, maintenance, insurance tax, etc. BG was assumed to be cultivated in seawater or brackish water. The only fertilizer used for BG cultivation is phosphorus since it uses dinitrogen in air as a nitrogen source, and seawater would supply rest of micronutrients. From laboratory experiments it was determined that the phosphorous requirement of BG is 8. These assumptions were also used for the analysis of subsequent biogas production, conversion and upgrading processes.
The production cost was calculated as follows:. The anaerobic digester was designed to treat the un-dewatered whole algae culture from the pond. The energy-intensive steps like algae harvesting and dewatering are avoided in this process which is different from most research [ 8 , 22 , 23 ]. The product biogas was analyzed for economic performance in two different applications: biogas purification or electricity production through combined heat and power.
The first step in modeling mass flow rate of reactor outputs and determining energy requirements is to establish the stoichiometry of reactions. The stoichiometry of methane fermentation of algae biomass was developed based on the following assumptions: 1 microbial cells cyanobacteria and bacteria can be represented by the empirical formula CH 1. The empirical formula for algae biomass was CH 1.
The stoichiometry for methane formation is written as follows:. This corresponds to 0.
The other stoichiometric coefficients can now be solved from elemental balances for C, H, O and N. The stoichiometry is. Different scenarios three anaerobic digester types were investigated to evaluate the economic and energetic performance. A schematic of biorefinery scenarios are shown in Figure 2.
Case 1. Above ground mesophilic anaerobic digester. Case 2. Above ground low-temperature anaerobic digester. Anaerobic digestion at low temperatures LTAD was applied to improve the energy balance. However, with the same flow rate, the digester volume is larger to achieve a higher HRT for LTAD than mesophilic and thermophilic anaerobic digestion. Case 3. Covered anaerobic lagoon.
Covered anaerobic lagoon CAL does not require additional energy for the biogas production because no heating or mixing processes are involved. Besides, it is economical to construct and operate. The CAL in this research was 6 meters deep and covers an area of 1.
Operating costs including utility usage are minimal. In all three cases above, the capital cost of anaerobic digester was estimated using vendor quotation or literature values. The operating cost was estimated by Aspen Process Economic Analyzer. Several biogas upgrading or purification methods are available such as high-pressure water scrubbing, membrane, pressure swing, gas permeation and chemical scrubbing.
High pressure water scrubbing and chemical scrubbing using amine solutions—MEA are two of the most commonly used processes. The amine process has two main steps, absorption and stripping [ 96 ]. The detailed MEA scrubbing process is shown in Figure 3. Raw biogas goes through a scrubbing column in which MEA is flowing counter-current to biogas.
At the top of scrubber, pressurized water is fed. Then the CO 2 -rich water goes through a CO 2 desorption process from the water stream by air [ 97 ]. The technical specification details are shown in Table 3. The table shows high pressure water scrubbing to be a more economical alternative and was chosen for the integrated process. While the raw biogas can be purified to obtain biomethane, another option is to use the raw biogas to produce heat and power. Steam and electricity can be generated by burning the raw biogas through a combined heat and power CHP system. The operating cost includes direct operating cost such as operating labor, supervised labor, maintenance and repairs, as well as indirect operating cost such as overhead, taxed, insurances.
The BG cultivation economics analysis details are shown in Table 4. Some of the estimates also account costs for dewatering of algae [ 22 , 27 ]. Thus, it is difficult to make a direct comparison between different studies. Besides, specific assumptions in each study could be based on different social-economic conditions, which makes comparisons more complicated [ 98 ].
Details of the production cost of renewable natural gas for the three anaerobic digestion scenarios are shown in Table 5. Case 2 contains two scenarios: The size of anaerobic digester in Case 2 a is two times of that in Case 1. This is because the hydraulic retention time is longer under lower temperature, the volume of digester needs to be larger to keep the same production scale the inflow rate. The size of anaerobic digester in Case 2 b is the same as Case 1.watch
algae biofuels chapter 2 the 8 best algae strains to start with and where to get them Manual
Keeping the digester volume same as Case 1, because the temperature is lower, the productivity will be lower as well. Thus Case 2 b has a lower production scale compared to other cases. The main contributor to the production cost of biogas is the biomass cost. The methane yield is 0. Process and economic assessment for purified biogas production through anaerobic digestion of Cyanothece BG biomass.
Renewable energy technologies are usually more expensive than fossil fuel technologies. The reasons could be environmental costs associated with fossil fuels that are not paid by the rate payers, mechanical difficulty in bioenergy production, start-up issues and so on. European countries such as Germany and UK governments subsidize the production of renewable energy by introducing feed-in tariffs. These tariffs may be important to make bioenergy industry profitable. Nutrient mostly nitrogen and phosphorous recycling such as utilizing the digestate or wastewater for microalgae cultivation was highlighted in various studies [ 59 , 60 , 61 , 62 , 63 , , , , , , , , ].
Recycling the effluent from the anaerobic digester for algae cultivation could mitigate the costs associated with supplying nutrient for algal biomass growth and effluent treatment. The biomass production level is closer to the case in which conventional medium is used. One option to increase algae biomass productivity and its concentration in the culture is to enrich the air with CO 2. The increased productivity of algae biomass will reduce further the cost for biomass production.
The CO 2 released from the biogas upgrading process or waste gases from biogas combustion containing CO 2 could be recycled to the algae growth ponds for enriching the air. The economic analysis for this scenario was also performed assuming algae biomass concentration is 1. The estimated production cost for Cyanothece BG algae biomass is This was calculated by accounting for the following additional costs: 1 capital cost associated with pipes and pumps to take CO 2 from biogas purification system or biogas combustion output to the pond and 2 operating costs resulting from more nutrient addition to maintain higher cell density and power consumption of compressing CO 2 for sparging [ ].
Only biogas production from covered anaerobic lagoon as in Case 3 was considered here.
The estimated cost of renewable natural gas is now reduced to Upgrading biogas by fixation of the CO 2 in biogas via photosynthesis by microalgae has been investigated with respect to CO 2 removal capability, biomass productivity and O 2 desorption minimization [ 16 , 63 , 64 , 65 , 66 , 67 ]. The biomethane produced met specification for injection into natural gas grids. However, this technique requires closed photobioreactors. Hydrogen sulfide H 2 S is another contaminant to be removed from the biogas.
Hydrogen sulfide removal was realized by the oxidation of H 2 S to sulfate by sulfur oxidizing bacteria that used the oxygen produced photosynthetically in situ. In this case, the algae-bacteria symbiosis was employed in the photobioreactors [ 67 ]. Nutrient recycling and biogas upgrading provides not only the opportunity for AD of microalgal biomass to be cost-effective, but also the potential to reduce the environmental impacts.
To move industrial application of biogas production from microalgal biomass towards commercialization, additional assessment is required regarding large scale operations. These include 1 strain robustness, outdoor productivity, location and seasonal effects, yield from real production systems, and harvesting strategy for algae cultivation 2 for biomass to biogas conversion processes, the conceptual process design needs to take the following factors into consideration: costs associated with digester heating, land, and infrastructure as well as operational parameters such as maintaining pH, temperature, mixing, power consumption, and production of coproducts like fertilizer.
The uncertainty of large-scale algae cultivation is still a challenge which prevents commercialization; process modeling could provide useful information about the performance of microalgae cultivation systems by estimation and optimization of microalgae productivity under different conditions [ ]. Based on their study, there was a tradeoff between the accuracy of the model representation and real-world usability.
A future modeling framework should consider along with limiting nutrients, integration of light and temperature, and incorporation of species diversity. AD can be integrated to biorefineries which produce high value products from algae such as chemicals for cosmetics, nutraceuticals and pharmaceuticals. This requires diversified business strategies which benchmark the market potential for the total raw materials and alternative products.
In previous sections, the cost of electricity from microalgae derived biogas is comparable with market value while cost of the renewable natural gas from microalgae is much higher than the current market value of natural gas. This chapter reviewed the literature on TEA of biogas production from algae. The key drivers to the overall production cost were identified and possible process improvements to reduce cost were discussed.
Related Algae Biofuels 101: Chapter 2-The 8 best Algae Strains to Start With and Where To Get Them
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