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Faisabilité, conception et ingénierie

Microalgues

Valorisation énergétique

Méthanisation

BioVALG, écosystème industriel

Abstract

The production of biofuel is now focused on the agricultural sector to produce oil from rapeseed, sunflower, beet or cane sugar. The environmental balance of such an agriculture in particular is far from being positive mainly due to indirect effects of these agricultural cultures. It is therefore necessary to find more satisfying ways of producing fuel in the respect of our environment. Biodiesel production from micro-algae is part of the most promising green technologies. The yield per hectare of these organisms is far better than sunflower or rapeseed. The algae could become the oil of tomorrow.

 

Introduction

Our project brings together various recycling and agricultural production technologies which needs are complementary and interrelated. The aim is to obtain from industrial and agricultural waste the raw material necessary for the creation of algal oil, electricity and heat.


Figure 1. Working diagram of the eco-system

 

 

Description

Combustion of polyethylene

The combustion of polyethylene waste produces steam and CO2 that are used for the cultivation of algae. The steam will produce electricity and heat (today polyethylene waste is burned mixed with household waste without any enrichment).

Today more than 134.000.000 tons of plastic are produced per year, of which only 430.000 tons are recycled. The result of this combustion does not present chemical pollution. Polyethylene does not have a harmful action on the ground and the ground water, as its combustion releases only steam and carbonic gas. The calorific value of polyethylene is higher than fuel, which means that one can save the equivalence in fuel by using its combustion. It is entirely recyclable. However, it is very often soiled, which makes it uneasily usable in second life.

Our production line includes several work steps, of which two are specialized in the preliminary wash and the washing of the PE with water treatment systems to avoid any pollution. For economic reasons, we will create a final purification plant which will enable us to re-use waste water in our process (saving 90%). These water treatments will produce mud resulting from washing and preliminary wash. To limit its volume, the production lines will be equipped with press filters. The mud at the exit of filters will be sent in the digester (see “2.2. Anaerobic digestion”).

To be able to use plastic like fuel, it is necessary to acquire an adapted boiler. The technology of this boiler is particular : it requires a special burner and a boiler for the steam production. The steam is used as driving force in an electricity and heat producing generator (more commonly called system of cogeneration).

 


Figure 2. Working diagram of the PE burner system

 

The burner adapted to the combustion of the PE is already developed ; the existing prototype could be placed at the disposal of the project at the time of the pilot phase.

 


Table 1 : Balance table of the PE combustion

 

At the end of this transformation process, the mud is sent to the digester and the CO2 is partly used for the growth of the algae.

 

Anaerobic digestion

The digestion of farm wastes produces biogas (CO2 + CH4) and a by-product rich in nitrogen, phosphorus and potassium in liquid and solid forms.

We will use the last generation digester using intensive mixing at 55°C. The engine is a cylinder laid out vertically, which diameter equals its height.

 


Figure 3. Working diagram of the digester

 

Compressed Biogas is injected into the central tube and creates a flow due to the lower density of materials in the tube. A horizontal circulation is created by a pump placed apart from the engine. Thus, no parts are moving inside this engine. This ensures a continuous operation.

 


Figure 4. Example of intensive mixing digester

 

This digester will be supplied by breeding wastes which will represent the majority of treated volume. However at the end of the bio diesel extraction process will remain a mass of fermentable products (between 20 and 50%). This by-product will be also used in the digester.

By the cogeneration of Biogas, we will obtain a green electricity which selling price at EDF will be approximately 0,13 €/kWh insofar as the valorisation of the heat produced by cogeneration is high (need for the digester, heating of the greenhouse and the basins).

 


Table 2 : Balance table of the digester process

 

Nearly 37% of the waste treated per year will result from the residues of the algae production, and of mud purification from the PE cleaning process. At the end of the digestion process, the 2000 tons of liquid digestat will be used like nutrient for the algae.

 

Algae production

Algae are grown in greenhouses using CO2 and heat produced by the combustion of PE and the liquid digestat from the digester.

The choice of the microalgae is based on the fact that many natural species are rich in triacylglycerols (the association of a molecule of alcohol, glycerol, and of three fatty-acids). Once refined by cross esterification, we can obtain a fuel close to the diesel which can be used directly in vehicles. By cultivating algae in large basins under greenhouse, we could produce sufficient biomass to extract oils.

These micro-organisms can be particularly prolific: some are able to accumulate 50 to 80% of their dry weight in lipids.

Moreover, the majority of these organisations have properties usable in human and animal consumption, in the cosmetic industry sector including the natural health products. Antioxydant carotenoid pigments, unsaturated fatty-acids poly of omega-3 type proteins, carbohydrates and lipids are principal components of the cultivated algae.

To develop and reach an optimal output, these microalgae have to be nourished with CO2 and organic waste (nitrogenizes, phosphates, nitrates…). Moreover these algae need heat to grow; certain species require water with 37°C. All these needs can be satisfied by association with the system of combustion of PE and the digester. Indeed the CO2 produced by the combustion of ethylene will be used by the algae, the heat will be used for the heating of the water of the basins, and the liquid fraction of the digester rich in nitrogen will be used to nourish the algae.

This process does not require disproportionate culture areas compared with the agricultural areas intended for human consumption. One hectare of algae can produce from 30 to 120 times more oil than one hectare of colza or sunflower.

Production method : we want to carry out our culture under greenhouse in closed basins of “Raceway” type. These systems arise as closed basins, 0,3m depth, forming loops. A system of propeller ensures the mixing of the culture volume while two connections ensure the arrival of fresh culture medium and the taking away of harvest. The interest of the greenhouse is obvious: powerful sensor of solar energy, it avoids the contaminations and allows maintaining a high CO2 concentration (13000ppm).

When subjected to a strong luminous intensity, the microalgae manage to recycle more than 80% of CO2 by photosynthesis, and approximately 50% in the event of less luminosity. Lightening the basins should allow a maximum absorption of CO2 and thus a maximum growth. The luminous intensity necessary for such species is of 3500 with 5000lux.

We consider a culture on several levels, lightening the lower basins. The basins being at very low height overall 0.5m, we can overlay four basins. The upstream basin will receive the natural light and the lowest ones part the artificial light using special lamps for growth of the plants (work INRA of Versailles). Tests will be carried out before validating the total installation. It should be remembered that we produce our electricity.

 

Figure 5. Algae production (industrial phase)

 

After pressing of the microalgae and extraction of oil, the residue will be sent back to the digester in order to create biogas, and close the cycle of valorisation. It is estimated that 50% of the production of biomass will return in the digester (approximately 2500 tons/year).

 

 

Table 3 : Balance table of the oil production

 


Table 4 : Balance table of the spiruline production

 

Energy producing greenhouse

The energy producing greenhouse captures solar energy, using heat exchangers. This heat is re-used to heat greenhouses or other applications.

 


Figure 6. Energy producing greenhouse

 

The basins contained in the greenhouse will have to be heated at temperatures between 20 and 40°C according to the type of cultivated algae. If the ground is appropriate, we can store in an aquifer the surplus of solar energy collected in the greenhouse and the unused heat produced by PE burner (Aquifer Thermal Energy Storage). This free heat could be proposed as a heating solution to residences located in the neighbourhood or other agricultural greenhouses.

 


Figure 7. Heat exchanger (FIWIHEX) used in energy producing greenhouse

 

The consumption of electricity in the greenhouse, limited to the consumption of the ventilators and the pumps, will be extremely low. The heat from the PE burner will be used to put the water of the basins in temperature. The heat loss of the basins will naturally heat the greenhouse. The temperature in the greenhouse will be between 26° and 30°C during at daytime in summer, and between 18 and 20°C during the coldest nights in winter.

 


Table 5 : Balance table of ATES

 

CO2 impact

We will achieve CO2 storage of almost 9500 tons per year. This volume corresponds to the carbon stock available in 47 ha of mature forest (100 years) in replacement of arable lands. The energy consumed is self produced (heat + electricity), the waste is reused in the digester. Thanks to the combination of these technologies we can ultimately produce an economically competitive algal bio fuel while destroying waste (polyethylene and manure) and achieving an efficient storage of CO2.

 

Economic Viability

Culture of microalgae with industrial scale for the production of Biodiesel still requires thorough studies but figures well among tomorrow’s green production technologies. The Shamash project in France, launched two year ago and leaded by INRIA, brings together eight French teams and companies for a total budget of 2,8 million Euros over three years.

The economic viability of algal biodiesel is still limited. Today, the results obtained in the laboratory showed we can produce up to 130,000 litres of oil per hectare, when the best production on an industrial scale do not exceed 50,000 litres. In order to maximize our production tool, the upper pond receiving more natural light will be dedicated to a culture of high added-value algae like spirulina sp. or chlorella. Both species have large nutritional qualities and have multiple applications in sectors such as pharmaceuticals, nutrition and aquaculture.

 

Conclusion

The main innovation of this project comes from the possibility to produce biodiesel algae all through the year in optimal conditions (light, heat, nutrients, CO2) at a very low cost.

Our project is complementary to the project SHAMASH and will bring exploitation solutions to the results of this one.

ESETA is searching for partners to start the experimental pilot step: financial partners, scientific partners specialised in algae culture and biodiesel production and a project holder.

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