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L’énergie dans le développement de la Nouvelle-Calédonie

Yves Le Bars
Elsa Faugère
Philippe Menanteau
et al.

Emissions and the reduction of greenhouse gas emissions in New Caledonia

Carbon dioxide capture and geological storage

Texte intégral

1Sustainable Carbon dioxide Capture and Storage, or CCS, can be achieved using geological means, an approach that differs in many ways from CO2 capture and storage in vegetation. Firstly, it differs because this latter approach enables CO2 to be stored only temporarily – for less than one year in annual plants or for several centuries in tree phytomass. Secondly, CO2 capture is associated with bioconversion of the sun’s energy which is then stored in biochemical form in the phytomass.

2As the trees grow, the carbon stores in their phytomass increase; this happens quickly when the trees and stores are young, but less quickly as they age. This means that vegetation’s role as a “sink” is restricted. Finally, when the phytomass decomposes, it again releases the CO2 it had taken from the atmosphere while growing.

3Another difference is that while the bioconversion of solar energy and CO2 by vegetation (or algae) fundamentally requires solar energy (which is renewable), the geological capture and storage of CO2 requires additional fossil energy. CO2 stored geologically can no longer provide energy whereas increasing carbon stores in the phytomass (or deriving glycerides using algae) results in plant matter which, when it decomposes thermally (including as a biofuel), or organically, again provides usable energy.

4These options are, however, not mutually exclusive; the capture (followed by geological sequestration) of part of the CO2 that comes from fossil fuels can be combined with the capture of CO2 from plant biomass which is also burnt in these boilers. However, storing biological CO2 geologically results in a net reduction in CO2 in the atmosphere, whereas that from all the CO2 from fossil fuels would result only in energy production without net CO2 emissions (as with the production of renewed plant biomass when not combined with the geological sequestration of the CO2 emitted during conversion into usable energy). For more details, please refer to the diagrams under “Emissions and the reduction of greenhouse gas emissions in New Caledonia” on the CD ROM. Geological storage does, however, prove to be a good option in developing energy vectors and industrial processes with a reduced carbon content. According to the IEA, there is potential of 920 Gt for exhausted gas or oil reserves, including between 400 and 10 000 Gt for deep saline reservoirs. The total potential should be compared with total CO2 emissions which are of the order of 2 000 Gt/annum according to an IPCC projection for 2000 to 2050.

5The feeling is that it would be possible to store captured CO2 at the exit of old oil and gas sinks in saline aquifers, or by mineralising the CO2 in rocks such as peridotite. This rock is found in many of the world’s basins, in particular those in Oman and New Caledonia (see below).

6Currently, CO2 capture and storage are planned only for major, local sources, mainly heat generating plants and heavy industry with discharge levels greater than 100 000 tCO2/annum. In other words, CO2 emissions from the transport sector and residential use cannot be captured. The quantities of CO2 discharged into the atmosphere by energy plants in New Caledonia are relatively low (Prony: 2*40 MWelectrical: 400 tCO2/day or ~550 000tCO2/anmum and Doniambo: 3*70 MWelectrical 4 000 t CO2/day or ~1.4Mt CO2/annum); they are, however, low compared with the discharge from power plants in the large industrialised nations. We therefore need to rely on the development – expected by 2020 – of technologies for capture from countries with the necessary R & D capabilities. Once developed, these technologies can be transferred to power plants in New Caledonia. Conversely, geological storage in peridotite will require local studies and development. None of these capture and storage techniques will become economically competitive before 2020, and only then under stringent cli-mate constraints designed to stabilise the climate by 2050, with a repurchase price per tonne of net CO2 emitted approaching €25/tCO2 and a storage price of between €60 and 80/tCO2!

7Given the growing interest in conducting studies into CCS, the French minister of research asked the national research agency to fund a programme on “capturing and storing CO2” between 2005 and 2008. Then, in 2009, a new programme on “the energy efficiency of industrial systems” was launched with the objectives of validating technological and geological solutions to enable CO2 to be handled sustainably at an acceptable cost and of monitoring underground stores.

8Already, the world’s large industrial groups have built (or have plans to build) platforms, but no real technology has yet emerged. If this technology is to be deployed, the economic and energy context must be considered, not forgetting human safety and what people find acceptable. There also needs to be a systematic global approach which includes energy resources, the industrial process used for the chemical conversion of energy and effluents, particles, aerosols and noise management.


9The biggest cost within the CCS chain is of capturing CO2 which means separating it from other combustion products (nitrogen from fuel burnt in the air, unused oxygen, water and pollutants). One of the processes under consideration consists of combustion in pure oxygen (oxy-combustion) or combustion in air enriched with oxygen to obtain, post-combustion and after drying the steam, a high concentration of CO2 which is then easy to capture. The energy cost of production needs to include the cost of producing the oxygen. Using this process does, nevertheless, yield a dramatic reduction in the cost of capturing carbon dioxide.


10Of the steps involved in the capture, transport and storage of CO2, it is capture that will define the overall CCS cost since it accounts for around 75% of the total cost. Since it can also increase electricity production costs by 50%, the major focus is on reducing the cost of capture to make this an economically acceptable system. There are three CO2 capture systems currently under development.

Capture and post-combustion

11Capture consists of extracting the CO2 from the other flue gases by means of absorption (chemical and physical), adsorption, cryogenics or membranes.

Figure 7. Post-combustion capture technology
Coal combustion reactions: C + Air ® CO2 natural gas CH4 + Air ® CO2 + H2O.
Principle: to remove CO2 from flue gases.

12It is thus a case of attaching a CO2 capture unit to a conventional power plant (boiler or gas turbine).

13In the case of a CFB boiler, the fuel (lignite or biomass) is injected with air into the boiler’s hearth; the heat exchangers raise the temperature of the water vapour (700 °C) which is then released into a steam turbine to pro-duce electricity by means of an alternator which is attached. The cinders are recuperated from the lower part of the boiler, loose dirt is removed from the flue gases which are then desulphurised and re-cooled and the combustion water is then condensed. The CO2 is then captured (by absorption) using an amine, ammonia or other solvent. The solvent is next regenerated by means of heating and releases the CO2 which is then compressed before entering the transport network en route to the storage location.

14In the case of a gas turbine with a combined cycle, the principle is similar but a gas combustion turbine driven by natural gas and attached to an alternator is used. To increase the yield from the process, a steam turbine attached to a second alternator is used to increase the heat of the gas at the gas turbine outlet.

15This is the only process capable of being used immediately on an existing installation provided the effluents from the combustion process are purified sufficiently to comply with CO2 compression, liquefaction, transport and geological storage conditions. This will need to be verified if the Prony and Doniambo power plants and the pyrometallurgy plant are to be adapted to the capture of effluents. An initial study may require denitrification and flue gas desulphurisation units to be added before any CO2 is captured; because of the high level of investment necessary, this would increase the cost per tonne of CO2 avoided.

Capture and pre-combustion

16The purpose of this process is to capture the CO2 during pre-combustion with oxygen prior to combustion of the hydrogen produced in air. To do this, the fuel (coal, lignite, biomass) is gasified, generally at atmospheric pressure or at high pressure (15 to 40 bar). The gasification products, known as synthesis gases, or syngas, are primarily a mix of CO/H2. This report is adapted to the application; the procedure varies according to the fuel used and the volume of water used as steam in the gasifier. The CO is oxidised into CO2 (shift reaction) which is found in high concentrations (15 to 40%), and then separated from the hydrogen. The hydrogen is then combusted with the air in a gas turbine.

Figure 8. Pre-combustion CO2 capture technology
Reaction from the combustion of Coal C + O2 ® CO et CO + H2O ® CO2 + H2 natural gas CH4 + H2O ® CO2 + H2
Principle: combustion of synthesis gas without coal.

17The main advantage of this technique is the large-scale production of hydrogen. While the process still needs to be proven, it is already promising benefits. However, a unit is needed to separate the oxygen and nitrogen from the air.

18The gasification technique consists of partial (or controlled) oxidation of the fuel (coal, lignite, biomass). During this step, the fuel is pyrolysed then oxidised to form synthesis gases made up of a mixture of CO and H2. The yield from gasification is increased if done under pressure using pure oxygen.

19One initial route is to produce purified synthesis gases using the Fischer Tropsch process (for an explanation of this process, please see the “New energy production and storage technology”.

20A second route is to convert the synthesis gases into a mix of CO2/H2 using shift reaction and injecting H2O in the presence of a catalyser; the CO2 is then separated from the hydrogen under pressure using a process of chemical or physical absorption; it then needs to be extracted using a sol-vent scrubbing or other technique (adsorption, membrane) suitable for treating pressurised gas.. The CO2 is the packaged for transportation to a storage location, and the hydrogen is purified.

21The hydrogen is combusted either in a boiler or in a gas turbine operating with or without a combined cycle (combination of gas and steam turbines); combustion instabilities always remain a restriction to overcome prior to deploying this technique.

22In comparison with capturing CO2 from a power plant’s flue gases under atmospheric pressure, this high pressure process, known as IGCC, (Integrated Gasification Combined Cycle) enables use of a smaller gasifier and restricts yield losses. The gas to be burnt in the combustion turbine is thus very rich in hydrogen, and the CO2 captured will be very clean (85%). This is the only technique which opens up a route for power plants creating electricity, heat and hydrogen with low CO2 emissions.

23However, it can be deployed only at new installations and for large, centralised production capacities go-generating electricity and heat from natural gas, coal, heavy oils or biomass. In addition, research needs to continue to improve the availability of IGCC power plants, reduce energy costs and consumption for the production of each unit of oxygen, develop better catalysers for the shift conversion of CO into CO2, develop higher performance solvents for physical absorption and manage new high performance methods of CO2/H2 separation.

24Is IGCC technology, currently the most expensive but also the one that delivers very clean combustion, suitable for deployment in New Caledonia? Certainly not considering that the Territory has no outlets for a hydrogen market and that operating an IGCC process using biomass can be envisaged only with imported wood since local plant resources are insufficient.

Oxy-combustion processes

25The objective of this process is to obtain a highly concentrated flow of CO2 at the combustion chamber outlet so that the nitrogen does not need to be separated following combustion. To achieve this, the nitrogen is extracted from the system before or during the combustion/conversion stages. The process will consist of producing a gaseous flow with a very high oxygen content by separating the air into oxygen and nitrogen before inserting it into the hearth.

Figure 9. Oxy-combustion CO2 capture technology
Combustion reactions: Coal C + O2 ® CO2 natural gas CH4 + O2 ® CO2 + H2O.
Principle: production of a flow of pure CO2.

26The technology of combustion using oxygen has been used successfully for several years to improve performance in industrial procedures (glass furnaces in particular). This is also one of the main ways of reducing CO2 emissions. In fact, the combustion products are essentially composed of water vapour and CO2 separated at the outset by means of condensation. More precisely, the CO2/nitrogen separation step in flue gases downstream of the process is replaced by an air separation step upstream of the process.

27This highly attractive process further reduces the size of the furnaces or hearths, increases the flame/target radiative transfer (heat exchangers) and reduces flow noise. Conversely, oxy-flames are very hot (Tflame>2 800 K) which presents material handling problems and produces very high NOx levels if there is the slightest trace of nitrogen in the furnace. Complex tiered combustion techniques, or techniques which dilute reactives in combustion products (CO2-water) prior to combustion therefore need to be used. Another benefit is that all the impurities are captured at the same time as the CO2 at the chamber outlet since the process is not linked to the combustion products.

28Oxy-combustion does not require any major modifications to existing installations, and has proven its technical feasibility. There are still some specific problems to resolve such as the formation of soot, 2-phase oxy-combustion, the oxidation of walls in a high temperature oxidising environment, the filtration of effluents, thermo-acoustic couplings and the management of combustion instabilities in the gas turbines.

Oxy-combustion of pulverised coal (PC)

29The fuel is burned in a pulverised coal (PC) boiler which operates using a mix of oxygen (around 25% by volume) and recirculated flue gases (primarily CO2), and a retrofit is possible without changing the geometry of the boiler. The process enables easy adaptation to current PC technology with air; significant energy gains can be achieved in flue gas post-treatment systems through amine scrubbing and the technological building blocks are in place to separating the air and package the CO2.

30There is one remaining restriction regarding large-scale oxygen production at low cost. Preliminary technical and economic analyses have now been conducted for units of 450 MWelectricity as well as for the design of a pulverised coal installation with supercritical oxy-combustion of 1 200 MWelectricity.

31Once commercially available, this technique will be able to be adapted to the Prony pulverised lignite power plant taking into account the small unit sizes, but without the boiler requiring major modification; only one air separation unit will need to be added. The flue gases will, if necessary, need to be purified prior to packaging and transportation to the storage location. This process is expected to be commercially available in around 10 years.

Oxy-combustion in a circulating fluidised bed (CFB) boiler

32The area of application for this technology remains the same, i.e. electricity generation with in situ CO2 capture for enhanced recovery and storage. The fuel is burned in a compact circulating fluidised bed operating on a mix of oxygen (70% vol.) and recycled flue gases (primarily CO2). Only simply adjustment of current air-based compact circulating fluidised bed technology by means of extrapolating the external beds is needed.

33Significant cost savings can be achieved given the major decrease in the size of the boiler block compared with an air-based CFB and because of the system of post-treating the flue gases through amine scrubbing. Initial technical and economic analyses have already been conducted for the 40 and 250 MWelectrical units and there are ongoing developments to design a supercritical oxy-combustion CFB for 400 MWelectricity and 600 MWelectricity..

34In summary, there would be no technical obstacle to adapting industrial installations in New Caledonia to an oxy-combustion system since the process can be applied equally well to a pulverised coal boiler or circulating fluidised bed as to a gas turbine. There would, however, need to be modifications to the chamber’s internal geometry, in particular to optimise internal or external recirculation to enable the reactives to be diluted in the combustion products. The oxy-combustion option could be introduced to the Doniambo and Prony electrical power plants if an economic and energy preliminary study were to confirm this is of interest.


CO2 separation by amine absorption

35This process consists of capturing the CO2 using a physical or chemical solvent in an absorption phase. To optimise the process, the amine (mol CO2/mol amine) is selected on the basis of its concentration in the aqueous solution, the reaction and gasificiation heat, the chemical kinetics, the regeneration temperature and the level of corrosion… The solvent is then regenerated by applying energy, and the CO2 is released during regeneration and then compressed and transferred to the storage location. It should be noted that some research has been conducted on amines in Australia (CSIRO).

36If the desire is to use this method of capture by amines, technologies will need to be improved to significantly reduce the energy used for regeneration, reduce the considerable size and cost of capture installations, restrict solvent losses and, finally, resolve the problems of corrosion caused by the presence of oxygen and CO2.

CO2 separation by membranes

37Here, separation takes place by means of membrane permeation. The membranes are made up of polymer, metal or zeolithic materials in thicknesses varying from a few nanometres to a few millimetres. This technology is difficult to apply to CO2 separation in atmospheric flue gases because the partial pressure of CO2 is too low. To be competitive, this system would have to be used on flue gases rich in CO2 corresponding, for example, to enriched air combustion.

38Any restrictions are still associated with the development of new membranes which are more permeable to CO2, are better priced and have improved mechanical, thermal and chemical properties. This process will become competitive when the differences in pressure and concentration between the sides of the membrane have increased.

CO2 separation using a cryogenic process (amine frosting/defrosting process)

39This process consists of frosting the CO2 under atmospheric pressure at a low temperature (-120°C) after condensing the water vapour found in the flue gases. To be economically viable, this process needs to be used on flue gases with a high CO2 content. The benefit of this process is that it can be used on existing combustion installations across all industrial sectors. It ensures good purity of the CO2 and enables solid CO2 to be converted immediately into liquid CO2 which can the be fed into the pipelines – a major benefit from an energy perspective.

CO2 separation by adsorption on a solid

40This technique adsorbs the CO2 found in flue gases on a solid either continuously or non-continuously. The solid is thus regenerated by introducing energy which increases the temperature or reduces the pressure. The CO2 released during regeneration is directed to its storage location. The solid which is regenerated is then re-used in adsorption. The main limitation to this process relates to the selectivity and capacity of the absorbants as well as to reducing the adsorbant surfaces so as to bring down the size of the installations.

41Intensive research into solids for CO2 capture is still underway in both Japan and the United States. The French Petroleum Institute (Institut français du pétrole = IFP) also has a wealth of experience in this type of solids in applications other than CO2 capture. However, as before, research is essential to minimise the energy cost of regeneration and the cost of the adsorber, and to reduce installation size.

Oxygen production technologies

42Cryogenic distillation today appears to the only method suitable for application with commercial oxy-combustion units through to 2015. Indeed, the very high outputs required (in the order of 1 000 tO2 per day for a 50 MWelectricity, coal power plant) make the use of the other processes unthinkable at the moment. Existing cryogenic technologies achieve 5 000 tO2/day, and ongoing design studies have established that volumes in the order of 7 000 tO2/day will be achieved in the years ahead with no major departure from current technology. Access to oxygen production in New Caledonia can therefore be envisaged.

43It is essential to master the oxygen production technology used to generate energy, but other aspects are equally important and must be integrated within the CO2 capture plan: these are the reliability, availability and management of operations, management of the risks associated with the use of oxygen and the possibility of small or large-scale integration of the oxygen production unit within the overall energy production plan.


44All these methods of capturing CO2 (membranes, cryogenic, amine absorption or adsorption on a solid) are known, but none has achieved a sufficient degree of maturity to be adopted by an industrial unit. If they are to be validated, platforms will need to be developed and demonstrations and research results provided. Suppliers will need to put forward solutions for 2015 by providing Capture Ready CO2 heat generating plants.

Some innovative means of capturing CO2

CO2 capture using the calcium cycle

45This process consists of carrying out a calcium cycle which will absorb the CO2 contained in the flue gases in order to form calcium carbonate. This calcium carbonate is then regenerated as lime having first been cracked in an oxy-combustion circulating fluidised bed reactor. A simple process of condensing the water results in a flow of virtually pure CO2 which can then be compressed and transported to the storage location. Bellite-type cement can also be manufactured from the calcium that is lost. The benefits of this technique are the low cost of limestone, well-proven CFB technology and the low volume of oxygen required. Adaptation for an existing installation is simple.

46The process is still at the research stage and there are some restriction associated with the lifespan of the limestone compound, controlling porosity during the cycles and the flow of solids in the reactors.

The chemical cycle (circulating fluidised bed)

47This technology finds application in electricity generation with in situ capture and storage of CO2. The process involves indirect combustion of the coal (or natural gas) using a chemical cycle. Solid vectors of oxygen (oxides of nickel, iron or manganese, for example) fully oxidise the fuel in a circulating fluidised bed in a gaseous mix made up primarily of CO2 and H2O. A simple process of condensing the water results in a flow of virtually pure CO2 which can then be packaged and transported to the storage location. Solids low in oxygen undergo compete oxidation in a second air-operated CFB before they are reinjected into the combustion reactor. The nickel could serve as the oxygen vector.

48CFB CLC (Chemical Looping Combustion) technology is very closely related to current CFB technology. The oxygen is created without the energy penalty associated with an air-operated separation unit, and there is no cost for investment in an oxygen production unit. The main limitations identified are in developing a coal barrier between the two CFBs, the lifespan of the oxides and the method of producing these oxides on a large scale. Adapting to coal marks an important step in development, and in interest in this route. The process is in its initial stages, with commercial application for heavy residues/coal envisaged by around 2015.


49Adding CO2 capture units to the sites at Prony and Doniambo will be costly given the power plants’ low power, even taking into account the decisions that have been taken and the investments committed.

What change can be made to current installations?

  • At the Prony pulverised coal power plant, despite the use of lowsulphur lignite, capture using amine or ammoniac absorption would result in pollution and rapid degradation of the solvent unless a denitrification of desulphuration unit is installed. The same difficulties in regenerating would be encountered using membranes.

50The only feasible solution would be oxy-combustion in which case it would be necessary to add an air separation unit upstream of the PC burners and redesign the boiler’s hearth to promote internal and external recirculation of the combustion products in order to dilute the reactives and eliminate zones that are too hot. The result would be a stabilised system of diluted combustion. The nitrogen effluents removed could, after packaging (compression, purification), be sent directly to their storage location.

  • In the case of the pyrometallurgy plant in Doniambo, it would appear difficult to capture effluent from the rotary furnace; on the other hand, fuel-operated reheating systems could be converted to electrical processes provided the electricity generation plant has CO2 capture facilities.

Developing a chemical cycle for the planned power plant in Doniambo with zero CO2 emissions

51The planned pulverised coal power plant in Doniambo presents good opportunity to integrate CO2 capture. The hearth is a circulating fluidised bed hearth with the fuel and oxidiser (air) fed into the combustion chamber where the walls are manufactured from heat resistant materials. The coal is thus gasified and combustion takes place between these coal degradation products and the ambient air at a temperature in the order of 920°C. The hot effluents are then transferred to a boiler to raise the temperature of the steam which will be used to operate the steam turbine. It should be noted that this form of combustion is particularly well suited to an island power plant (optimal size for a CFB, flexible in terms of the fuel used), even though the yield is slightly lower than that of a PC flame because the thermodynamic cycle requires a slightly lower temperature.

52The technique proposed is that of the chemical cycle or chemical looping as presented above. The principle of the thermochemical cycle is to use metal oxide (typically a nickel oxide) to oxidise a gasified fuel in the combustion reactor. Since the fuel is oxidised without air, the combustion products are, principally, air and water vapour which means that the effluents can be transferred to the storage location following packaging. The metal oxide is thus reduced to metal (the nickel) which is transferred to an oxidation reactor where the metal oxidises with the high temperature air before being transferred back to the combustion reactor which completes the cycle.

53The planned Doniambo power plant also has the benefit of already having a CFB which could be turned into the oxidation reactor in the future; the second similar, but smaller, CFB and a metal transport system would simply need to be added to it. As for the metal itself, the nickel is of course available on site.

54One of the barriers to this technique is degradation of the oxidation sites and the reduction on the metal which means lower transport yields. Doniambo can easily lift this restriction by periodically incorporating nickel regeneration in the furnaces into the loop.

55The planned SLN power plant thus has some major points in favour of being transformed into a chemical cycle and these are summarised below:

  • no reappraisal of the options already reviewed since this would require a rebuild;

  • the presence of nickel with the option of regeneration on-site;

  • no CO2 emissions;

  • Eramet’s involvement in developing a CO2 capture technique;

  • the offer of a new, international outlet for the regeneration of nickel polluted by combustion products;

  • the low energy and economic cost compared with other methods of capture;

  • additional equipment at a reasonable size and cost.

56If there was a global decision to launch a strong campaign again climate change, and New Caledonia effectively planned to reduce its CO2 emissions, it would be possible to commit to actions in Prony as well as in Doniambo. Support initiatives from Europe, the State and different Caledonian governments could, when the time comes, contribute to adapting this innovative technique, validating it in pilots, assessing its economic feasibility and extending it to the Territory.

Transporting CO2

57Given the volumes to be transported, the only feasible solutions are pipelines and maritime transportation in the case of off shore storage or storage at a site off Grande Terre (in Australian aquifers for example).

58The process of constructing pipelines is now under full control, but technical specifications for the transport of CO2 need to be taken into account. If the CO2 is dry and purified, standard carbon steel is sufficient; on the other hand, stainless steel may be required in specific conditions in order to avoid corrosion. The operating pressure for transportation will be between 100 and 150 bar which enables technologies used for natural gas (90 to 100 bar) to be utilised. Ground or air control accompanied by pressure measures will enable the gas pipeline to be monitored. The CO2 is transported either in its high pressure dense phase or as a liquid which requires thermal insulation and intermediate recompression. Such technologies are already used in North America and Norway to transport CO2 over long distances.

59When transported by sea, CO2 is generally in the form of a moderate pressure, low temperature liquid as in tankers. The transfer of heat with the environment though the walls of the tank will promote evaporation and increase the pressure making permanent cooling a necessity. Recompression may be required for transport to the geological storage location.

60There are certain constraints inherent with this type of transport, including metal corrosion by the CO2, which happens on a very large scale where there is water present, or toxicity of the CO2 if it escapes and its concentration exceeds 5% in the air.

61Costs would be around ?0.50 and 10/100 km/tCO2 for transport by pipeline; the cost of transport by boat over long distances is lower, but in this case, intermediate storage is necessary.

62In conclusion, if CO2 produced by heat generation plants in New Caledonia is captured, and taking into account the low annual tonnages emitted, thought should be given to grouping emissions and centralising them at a single location in the region. From there, they could be carried by pipeline to storage locations in New Caledonia (peridotite sites) and/or to a gas terminal (to a storage location off the island).

63Packaging and land or sea transport technologies do exist, but will necessarily be very troublesome to put in place. Additionally, the scale is not beneficial for these low CO2 emission volumes. There can thus be no plans for carbon dioxide capture and storage (CCS) in New Caledonia without a global commitment to achieve a massive reduction in greenhouse gases.

Bioconversion of CO2 using algae

64We should also highlight the possibility of recycling CO2 emissions from power plants via bioconversion using algae. This reduces CO2 capture costs and eliminates the immediate need for geological storage of the CO2 emitted by the power plants. If the yield from photosynthesis is to be optimised, the algae in the basins or bioreactors require sufficient solar radiation and large quantities of CO2. Gaseous emissions from traditional heat generating plants producing electricity (coal-fired power stations for example) contain an average of 13% CO2 compared with less than 0.4 ,‰ in the atmosphere. This CO2 is left to bubble in basins or reactors and then assimilated by the algae. The algae are thus able to produce more biomass per unit of ground surface area. The enables waste water to be treated at the same time. The biomass or molecules produced by the algae can then be used to reduce consumption of fossil energy and, indirectly, net CO2 emissions. This does not, however, enable CO2 emissions to be reduced directly. This is not a means of eliminating CO2 in the same way as the geological storage of CO2, but rather a way of bioconverting solar energy similar to the way in which plants on the land are cultivated then harvested. Algae metabolism can thus be directed in order to optimise the production of molecules which may then be used directly as a biofuel. If only pure carbon dioxide were present at power plant outlet, the same reactors could be used to produce high value-added molecules or food. Such methods have been explored since the time of the second petrol shock. The triglycerides which the algae are capable of producing can, in theory, be converted into crude vegetable oil or biodiesel. The yield from bioconversion per hectare is, however, greater than for full-field superior vegetation, but also assumes greater investment. To-date, it has not been possible to implement such methods on an industrial scale for energy production since they have not become economically competitive with traditional plant biomass production. However, several industrialised countries have recently shown renewed interest in such methods which focus on producing biofuels. A new bioreactor which also allows part NOx emissions to be captured in algae has also been developed (cf. “New energy production and storage technology”). Various types of algae have been selected for their ability to capture CO2 and also for their resistance to toxins which may be contained in the combustion effluents from the power plants. Technical improvements will, however, be needed to get close to photosynthesis yields and improve product conversion. If we need to resort to transgenesis to increase yields, the possible risk of these organisms being diffused into the environment will also need to be considered. The various steps required to develop this technology are still in the initial stages of basic research. Much more progress needs to be made before this method can be applied. However a high price per tonne of CO2 not emitted could make this method competitive more quickly.

Table des illustrations

Légende Figure 7. Post-combustion capture technologyCoal combustion reactions: C + Air ® CO2 natural gas CH4 + Air ® CO2 + H2O.Principle: to remove CO2 from flue gases.
Fichier image/jpeg, 132k
Légende Figure 8. Pre-combustion CO2 capture technologyReaction from the combustion of Coal C + O2 ® CO et CO + H2O ® CO2 + H2 natural gas CH4 + H2O ® CO2 + H2Principle: combustion of synthesis gas without coal.
Fichier image/jpeg, 148k
Légende Figure 9. Oxy-combustion CO2 capture technologyCombustion reactions: Coal C + O2 ® CO2 natural gas CH4 + O2 ® CO2 + H2O.Principle: production of a flow of pure CO2.
Fichier image/jpeg, 133k

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