Coupling Carbon Capture from a Power Plant with Semi-automated Open Raceway Ponds for Microalgae Cultivation

This semi-automatic raceway cultivation system controlled by a pH sensor can directly capture flue gas from power plans for microalgae cultivation and can monitor microalgae growth with real-time measurements. The reactor system allows the direct capture and usage of carbon from industrial flue gas to grow microalgae in on-site semi-automated open pond raceway systems. The system can be used to cultivate alternative algae species and to capture carbon from any power plant.

To set up an open raceway pond, attach a 0.95 centimeter fuel hose to capture the flue gas during the post-combustion process, a few meters before the flue gas enters the stack to be discharged into the atmosphere. Place a 20 liter water trap and an approximately 12 meter condenser between the stack and the compressor to remove water from the flue gas. To monitor algal growth, connect a real-time optical density sensor that measures the absorbance at 650 and 750 nanometers, a dissolved oxygen sensor, air and pond thermocouples, a pH sensor, and an electroconductivity sensor to a data logger.

To set up a pH control system, connect a compressor and control valve system to the data logger program to manage the flue gas injection and use a tube to direct the flue gas from the control valve through a stone diffuser to the bottom of the raceway pond, then set the system to inject flue gas when the pH value was greater than 8.05 and to stop the flue gas injection when the pH is less than eight. To set up an algal culture system, set the culture room to 25 degrees Celsius in a 12-hour light, 12-hour dark cycle, and use deionized water salts and macro and micronutrients to prepare BG-11 culture medium. Use a sterile loop to select a single algal colony from the culture plate and inoculate the algae in a 50 milliliter tube containing sterile growth medium in a clean biosafety cabinet.

Grow the small liquid culture on a shaker table at 120 revolutions per minute for one week. At the end of the incubation, transfer the entire volume of algal culture into 500 milliliters of liquid culture in a one liter flask and close the flask with a rubber stopper fitted with stainless steel tubing to provide aeration. Filter the air with 0.20 micron air sterilization filters and allow the culture to grow for another one to two weeks using a spectrophotometer to monitor the cell density daily.

At the end of the culture period, add 500 milliliters of the liquid algal culture to eight liters of non-sterile culture medium in a 10 liter carboy and inject a mixture of 5%carbon dioxide and 95%air into the carboy. Monitor the stock plate and liquid cultures under a light microscope at the 10 and 40X magnifications once a week to ensure growth of the strain of interest. To inoculate an outdoor open raceway pond with algae, first, thoroughly clean the reactor overnight with 30%bleach.

Rinse the reactor the next morning until all of the bleach has been removed and calibrate all of the sensors according to their corresponding calibration procedures. Fill the raceway pond up to 80%with water to dilute the concentrated medium and inoculate the pond with the 10 liter carboy of algae culture. Bring the pond to its final volume, then partially shade the raceway pond with wooden pallets for about three days as an adaptation strategy to avoid photo inhibition to allow the microalgae to acclimate to the culture system.

To perform a batch growth experiment, inspect and record any day-to-day variations including water evaporation, paddle wheel motor and sensor functionality, or anything else out of the ordinary. Drain and inspect the compressor and water trap daily to allow the removal of any excess water and to minimize flue gas corrosion. Configure the data logger to scan each sensor measurement every 10 seconds and to store the average sensor and air and reactor temperature data every 10 minutes.

Make sure that the water level remains constant at the reactor's final volume to avoid affecting the optical density measurement. After replenishing the water in the reactor, collect an algal biomass sample using a 250 milliliter bottle. Measure the optical density in the spectrophotometer.

Check the quality of the algae culture three times a week by light microscopy. When a culture is close to reaching stationary phase, harvest 75%of the total algae culture volume and use two to five liters of the culture suspension to perform biomass productivity analysis in the laboratory, then process and convert the rest of the algae into the desired algal products. Here, a comparison between the sensor and laboratory measurements can be observed.

Both breeding show similar trends with the data increasing as a function of time. Optical density values increase during the day, but decrease at night during respiration, indicating a change in biomass productivity, thus the integration of a real-time optical density sensor makes it possible to make effective management decisions about the overall algal production system. In this analysis, flue gas was injected from approximately 8:00 a.m.

to 6:00 p.m. but was not injected between 6:00 p.m. and 8:00 a.m.

This day/night cycle reflects the daylight sunlight exposure and the lack of light during the night, and consequently, the activation of photosynthesis or photorespiration respectively. As illustrated in this figure, as the algal grow rate increases, more flue gas is required, confirming that the on/off flue gas pulse injection system is effective at facilitating carbon capture and utilization through microalgae cultivation. Other physical chemical parameters can also be used to establish a correlation between the parameters and algal growth and productivity.

It is important to make sure that the pH system is properly set up and that all of the sensors are calibrated before inoculating the raceway ponds with microalgae. Other methods that can be performed with the raceway pond while producing microalgae biomass include lipid, carbohydrate, or pigment extraction, ash-free dry weight measurements, and PCR quality monitoring.