Dr. Johannes Ewers & Georg Wiechers, RWE Power, Germany
Dark green is the colour of the turbid liquid in the V-shaped plastic hoses. Two rows of these transparent containers so-called photobioreactors form a futuristic espalier in a greenhouse. As extraordinary as the setting is the idea behind it. At the Niederaussem power plant site, RWE Power has erected a pilot plant for the production of microalgae.
RWE Power’s pilot project feeds desulphurized flue gas to a greenhouse, where microalgae binds the CO2
Here, the microalgae binds the CO2 that is released during the combustion of lignite, turning it to good use and sparing the climate. To do so, Germany’s biggest power generator makes use of the most important biogeochemical process on earth: photosynthesis. RWE has studied in detail various options for climate-beneficial recycling and trapping CO2 in order to identify potentials and obtain recommendations for action. One result of these investigations is the project for binding CO2 using microalgae.
The nature of microalgae
Microalgae are unicellular, plant-like organisms that absorb CO2 by photosynthesis in order to grow. One crucial advantage is that they have much higher growth rates than any land-based plant, enabling them to convert CO2 into biomass faster than plants on land. Moreover, marine microalgae grow in saltwater, which significantly increases the stability of production compared with freshwater algae.
In German latitudes, microalgae produce 60 to 100 t/ (ha x a) dry substance, with 120 to 200 t/(ha x a) of CO2 being bound. The growth rate of microalgae is much higher than that of fast-growing energy crops, like willows or poplars from short-rotation plantations with 12 t/(ha x a) or Miscanthus with 15 t/ (ha x a) dry substance.
Marine microalgae can be produced in closed systems, and use can be made of locations, which, owing to their soil characteristics, are not suitable for growing plants. This avoids site competition with the cultivation of food and feed. The aim is to use the specific capabilities of microalgae to bind CO2 from flue gases at lignite fired power plants.
Before commercial-scale deployment becomes feasible however, numerous points still have to be resolved. Besides technical issues associated with the development of this technology, it will have to be shown above all that the total energy balance from algae production to conversion is positive and that a net CO2 reduction is obtained.
It is with a view to investigating these points systematically and to developing solutions that RWE together with noted partners has launched this project: flue gases from the Niederaussem power station are fed into an algae production plant in the vicinity of the station to convert the CO2 from the flue gas into algae biomass. On the basis of the algae biomass produced, a further aim is to investigate different conversion routes for the algae involving energetic and material use, e.g. for chemicals or fuels.
In the process, flue gas is withdrawn from a power plant unit and transported through pipes to the microalgae production plant. The CO2 contained in the flue gas is dissolved in the algae suspension and absorbed by the algae for growth. The algae are removed (harvested) and further investigated for conversion into chemically or energetically usable products. The flue gas to provide the algae with the CO2 is withdrawn from a conventional lignite based power plant unit.
The amount of flue gas needed is diverted downstream of the flue gas desulphurization (FGD) system, i.e. in a state in which it is normally released into the environment. The flue gas downstream of the FGD contains a high percentage of water vapour however, so the gas is dried before propelled with the aid of a fan through a pipe to the greenhouse.
The pipe is made of polyethylene. This plastic was selected to avoid corrosion from the condensation of residual amounts of water vapour. The greenhouse in which the algae production system is built stands on a site adjacent to the power plant, and the flue gas pipe is 750 m in length.
The Greenhouse Effect
The flue gas pipe ends in front of the greenhouse in which the algae production plant is located. The flue gases are fed into a so-called bubble reactor outside the greenhouse using a process from Novagreen Projektmanagement GmbH. The container has an algae suspension consisting of saltwater and the microalgae. The flue gases mix with the suspension, which absorbs the CO2 from the flue gas up to saturation. The gas exiting the suspension at the top is reduced by the corresponding amount of CO2 and is released into the environment via a chimney.
Thanks to the bubble-reactor process only the CO2 is dissolved in the algae suspension and fed into the greenhouse. The CO2-enriched algae suspension is conducted into the greenhouse and fed into the photobioreactors developed by Novagreen Projektmanagement GmbH. These consist of clear plastic hoses that are fixed in V shape to supports.
To optimize growing conditions, the bioreactors are located in a greenhouse. There, relatively constant temperatures as well as optimal nutrition and lighting conditions, can be set for the algae’s high growth rates. The required heat is made available by the neighbouring power plant from unused cooling tower waste heat (Hortitherm).
The photobioreactors are currently erected on an area covering 600 m2, with up to 1000 m2 available for future expnasion. The first extension stage to 600 m2 contains approximately 52 m3 algae suspension.
The suspension is fed in by a small hose from above, and drained off at the bottom, again by a hose. In the transparent photobioreactor, the algae comes in contact with light and grows. The CO2 needed for photosynthesis is dissolved by the microalgae from the suspension and absorbed. So that all microalgae receive the same amount of light, the photoreactors are stirred by blowing air bubbles into the reactor from below that pearl through it.
The suspension’s pH indicates the CO2 content and is used to control the feed-in of CO2-enriched suspension from the bubble reactor. The drained-off suspension is fed into the bubble reactor again. The algae amount is established by measuring the suspension’s optical density. If this is sufficiently high, the algae suspension is conducted to a harvest container, instead of the bubble reactor.
The algae are then separated from the saltwater. The separated microalgae have a paste-like consistency and are then available for further processing. The saltwater is reintroduced into the cycle.
The pilot system can produce up to 6000 kg algae (dry substance) per year, and this binds 12 000 kg of CO2. Thus, the system for algae production with flue gases erected by RWE Power at its Niederaussem power plant offers unique opportunities for studying this option of CO2 binding in a project involving noted partners.
The main objective of the study is the long-term energetic, ecological and economic balance of the entire algae production process and subsequent conversion and use options. In the initial phase, the algae growth rates are to be increased by using different algae cultures, optimized growth conditions and improved greenhouse technology. Also, the entire process management is to be optimized with the aim of ensuring the lowest possible energy is required to achieve the greatest possible biomass production and net CO2 reduction.
Another focus is on the development of concepts for using the algae. A process for converting the algae biomass into energetically usable products is already under investigation. Some of the algae is also to be used in a biogas plant. Further promising conversion routes are being established for a follow-up project phase.
For future applications, overall concepts with suitable locations are to be developed and assessed using cost, benefit and potentials analyses.
Using the algae biomass produced, investigations include hydrothermal carbonization, in which biomass is heated under pressure, with water being added and oxygen cut off to obtain different hydrocarbon products. Basic research establishes and optimizes the process parameters for the algae input.
Analysis shows which properties of the algae biomass favour its conversion by hydrothermal carbonization in order to optimize algae breeding using this feedback. The fermentability of algae as monosubstrate is to be investigated within the scope of a study. Here, maximum gas yield, the influence of the salt content and possible co-substrates and pretreatment are to be clarified. The experimental work uses the algae produced in Niederaussem. Planning also calls for research into adding algae biomass to a biogas system.
With the pilot algae plant, RWE Power has created excellent prerequisites for studying the energetic, ecological and economical issues of the use of CO2 for the production of microalgae and subsequent conversion steps. Even if this technology does not have the potential to replace carbon storage on a large-scale, it is yet another element of power generation in a carbon-constrained world.