The present invention relates to a method and apparatus for reducing the concentration of carbon dioxide from flue gas comprising carbon dioxide.

Background of the invention

The Intergovernmental Panel on climate Change (IPCC) 4th Assessment Report confirmed that global warming is unequivocal and those anthropogenic emissions of greenhouse gases are very likely the cause. The combustion of fossil fuels for electricity generation accounts for as much as 22% of anthropogenic greenhouse gas emissions. Carbon capture and sequestration (CCS) has been widely considered as one of the most viable ways of reducing carbon dioxide (CO ₂ ) emissions from power plants that combust fossil fuels. CCS is about capturing CO ₂ from the exhaust gases of power plants (also known as flue gases), compressing or liquefying the captured CO ₂ and then transporting it to a location where it can be stored permanently in appropriate geological features or under the sea (Page et al., 2009).

Carbon capture using chemical or physical absorption means is currently being applied in the petrochemical and petroleum industry, but only at a small scale in gas- and coal-fired boilers. Lessons gleaned indicate that, in most cases, carbon capture (including compression) is the largest cost component for CCS - typically accounting for 80% of the costs for CCS from power plants. It is estimated that a pulverize coal (PC) plant with CCS, with geological storage, will incur an avoidance cost of US$30-70 per ton of CO ₂ avoided (with the reference plant as a PC plant without CCS). However, the avoidance cost of CCS is highly variable and depends on the fuel prices, technology and market structure changes with time. In fact, the avoidance cost may range from US$0-270 per ton of CO ₂ avoided. Energy penalty is the energy requirement for the regeneration of solvents used for capturing the CO ₂ from flue gas. The exact energy penalty values depend on the types of capture technique used. When the partial pressure of CO ₂ in the flue gas is low (about 7-8vol%) - as in the case for PC and natural gas combined cycle (NGCC) power plants - chemical solvents are required ((usually monoethanolamine, MEA)). Page et al. (2009) conducted a thorough review of the energy penalty values computed from process simulations. For PC plants, the penalty ranges from 15-28%; for NGCC plants, these values are 15- 16%. Their own calculations, based on a 90% capture rate in a sub-critical PC plant, with compression to 11 MPa ₁ yielded an energy penalty of 37%. Real world experiments of CCS technology also showed that the regeneration energy of capture solvent is very high. CASTOR, the largest CO2-capture facility in Europe that is operating at the Esbjerg power plant in Denmark, is able to capture CO ₂ from 0.5% of the plant's flue gas. Measurements showed that as much as 3.7 GJ of regeneration energy was required for every ton of CO ₂ captured when MEA was used. Besides, MEA is highly corrosive on carbon and steel - common materials used for constructing reactors in power plants. However, MEA tends to have lower corrosive effect on stainless steel. These observations prompted the research team to create a design solvent - known only as CASTOR-2 - and experimented it at CASTOR as a possible replacement for MEA. Although this resulted in a small decrease in the regeneration energy, the research team found that more solvent was actually lost in the regeneration process compared to when MEA was utilized. CASTOR- 2 is also more expensive than MEA. Integrated coal gasification combined cycle (IGCC) plants enables capturing of CO ₂ from a synthesis gas at a higher partial pressure (>12vol%), thus permitting the use of physical absorption carbon capture methods that are generally more cost-effective and less energy intensive (requiring about 0.6-1.5GJ of energy per ton of CO2 captured). In its estimations of worldwide geological storage capacities, the IPCC (IPCC, 2005) deduced that all eligible oil and gas fields in the world can store 675-900 Gt of CO ₂ . The storage in un-minable coal seams and deep saline formations are expected to be 3-200 Gt CO ₂ and between 1000 Gt CO ₂ and an order of 10,000 Gt CO ₂ respectively. Concern over leakage from various means of storing captured CO ₂ is also significant. Using Delphi method, Gough (2008) collected experts' views on the perceived risk of CO ₂ leakage from geological storage reservoirs. Although a majority of the experts feel that the risk of leakage in CCS is much less than that in nuclear reactors, most feel that the probability of a leak from CCS is higher than from a nuclear plant.

The ocean is considered as another viable storage reservoir. It contains approximately 40,000 Gt of carbon. Over a century, it is estimated that around 80% of today's anthropogenic emissions of CO ₂ will be absorbed by the ocean. Direct injection of captured CO ₂ into the ocean is a way to reduce peak atmospheric CO ₂ concentration. However, 15-20% of the CO ₂ injected into the ocean will leach back into the atmosphere over hundreds of year. Besides, ocean injection and storage may modify the natural environment in the ocean. For example, dispersing about 200 years of CO ₂ , or 1 ,300 Gt of CO ₂ , into the ocean would decrease the average ocean pH by around 0.3, which will in turn decrease deep sea pH by as much as 0.5. This would principally affect organisms at depths of greater than 1000 m. A way of reducing such a change in pH is to improve the vertical dispersion of CO ₂ ; but this will in turn increase the leakage of the dissolved CO2 from the ocean.

Although carbon capture and sequestration (CCS) plays a critical role in controlling the atmospheric carbon dioxide (CO ₂ ) level, conventional methods using chemical and physical means are facing tremendous technical and economic challenges. Therefore, besides chemical and physical absorption means of capturing carbon, biological methods using photosynthetic agents as CCS media are also being considered. Predictions made by carbon-uptake modeling (Burgermeister, 2007) indicate that out of the annual 8 billion tons of anthropogenic carbon, about 3.2 billion tons remain in the atmosphere, 2.2 billion tons are stored in the oceans and 2.6 billion tons are sucked up by land- based carbon sinks, mainly forests. Herzog (2001) opined that the total sequestration capacity of terrestrial reservoirs is of the order of 10Gt of CO ₂ - two orders of magnitude lower than that of geological and ocean storage. Agroforestry and reforestry are two methods widely being studied as possible CCS options. It was estimated that on the average, a mature tree can remove 22 kg of CO ₂ in a year. However, this estimate is based on the situation in which trees are removing CO ₂ from an environment with a carbon dioxide concentration of 385 ppm. If the surrounding concentration of carbon dioxide is higher, then the rate of removal is likely to change.

Relying on forests to remove CO ₂ that is already released and dispersed into the atmosphere is a passive way of capturing carbon. Efforts have been made to capture CO ₂ in flue gas of power plant before the flue gas is released into the air. For example, the flue gas is channelled into photo-bioreactors in which photosynthetic bacteria or micro-algae fix and hence reduce the amount of CO ₂ in the gas stream. The bacteria or algae are then harvested for their oil, biomass or other by-products such as hydrogen. However, obtaining such bacteria or micro-algae which will effectively capture a significant amount of CO ₂ is expensive and economically not efficient.

Kajiwara et al. (1997) found that Synechococcus requires darkness and influx of CO ₂ to produce starch and hydrogen. This strand recorded a maximum CO ₂ uptake rate of 0.025 g/L/h, or, 0.6 g/L/day, at a cell mass concentration of 0.286 g/L. If scaling up is possible, then theoretically a photo-bioreactor of a capacity of 4,000 m ³ can achieve an average CO ₂ fixation rate of 1 ton CO ₂ /h from the flue gas. Using the strand Chlorella sp. UK001 , Hirata et al. (1996) found a mean rate of CO ₂ fixation of 0.0318 g CO ₂ /L/day, with an energy-to-biomass efficiency of about 4.3%. In all these studies, the average CO ₂ fixation rate is dependent on factors such as the light delivery arrangements, configurations of the bioreactors and temperature of the substrate in which the strand is cultivated. Therefore, it can be seen that there remain many barriers to large scale commercialization of these bioreactors, with the main drawback being the high cost of technology and concomitant long payback periods. One reason behind such high costs is the need for pure, and even genetically-modified, strains of bacteria and/or algae that have high yield for useful by-products of CO ₂ fixation.

There is therefore a need in the art for an improved method and apparatus for reducing the concentration of CO ₂ from flue gases before releasing the flue gases into the atmosphere.

Summary of the invention

The present invention seeks to address the problems above by providing an improved method and apparatus for reducing carbon dioxide concentration. According to a first aspect, the present invention provides a method for reducing concentration of carbon dioxide from flue gas comprising carbon dioxide, comprising the step of:

(a) contacting flue gas comprising carbon dioxide with at least one photosynthesising agent, wherein the contacting of step (a) is for a pre-determined period of time.

The flue gas may be flue gas comprising carbon dioxide at elevated concentrations. The flue gas may be from any source which emits carbon dioxide. In particular, the flue gas may be from any source which emits carbon dioxide in elevated concentrations. For example, the flue gas may be from a power plant, particularly from the combustion, steamer or boiler unit of a power plant. The flue gas may be produced by industrial processes. According to a particular aspect, the contacting of step (a) is non-continuous. In particular, the pre-determined period of time may be less than 24 hours. For example, the pre-determined period of time may be about 1.5 -20 hours, 2-12 hours, 4-10 hours, 5-8 hours, 6-7 hours. In particular, the pre-determined period of time may be about 2-4 hours. According to a particular aspect of the invention, the pre-determined period of time is about 12 hours. Even more in particular, the pre-determined period of time may be during daylight. According to another particular aspect of the invention, for example, when the concentration of carbon dioxide in the flue gas is about 8000 ppm, the predetermined period of time may be about 2-4 hours. The method according to any aspect of the invention, wherein after step (a), may further comprise the steps: (b) removing the flue gas from contacting the photosynthesising agent; and (c) repeating a step of contacting the photosynthesising agent with flue gas comprising carbon dioxide for a pre-determined period of time. The flue gas of step (b), which is removed from contacting the photosynthesising agent, may have been treated (i.e. having a reduced carbon dioxide concentration). The flue gas of step (c) may be distinct from the treated flue gas which is being removed in step (b). In particular, the flue gas of step (c) may be untreated. The flue gas of step (c) may be flue gas comprising carbon dioxide at elevated concentrations. In particular, the flue gas of step (c) may be from the same source as the flue gas of step (a). The pre-determined period of time of step (c) may be the same as the pre-determined period of time of step (a).

According to a particular aspect, the removing of step (b) and the contacting of step (c) are performed repeatedly.

According to a particular aspect, the pre-determined period of time for each of steps (a) and (c) is the same.

According to a particular aspect, the pre-determined period of time for each of steps (a) and (c) is different. According to another particular aspect of the present invention, the step (b) of removing and the step (a) or (c) of contacting are separated by a step of non- contacting for a pre-determined period of time. The step of non-contacting is when the photosynthesis agent is not in contact with flue gas comprising carbon dioxide at elevated concentrations. The pre-determined period of time for the step of non-contacting may be about 5 minutes-24 hours, 10 minutes-18 hours, 30 minutes-12 hours, 2-6 hours, 1-4 hours. In particular, the pre-determined period of time for the step of non-contacting is about 12 hours. Even more in particular, the pre-determined period of time may be during night time, without any daylight.

The at least one photosynthesising agent may be any photosynthesising agent suitable for the purposes of the present application. The at least one photosynthesising agent uses the carbon dioxide from the flue gas for photosynthesis. According to a particular aspect, the at least one photosynthesising agent may be at least one plant, plant cell, or part thereof. The plant, plant cell, or part thereof may comprise at least one leaf capable of carrying out photosynthesis. For example, the plant may be Vigna Radiata, water hyacinth, or tapioca. Any other suitable plant may also be used for the purposes of the present invention. If a plurality of photosynthesising agents are used, each of the photosynthesising agents may be the same or different from one another.

According to a particular aspect of the present invention, the at least one photosynthesising agent may be comprised in a housing into which the flue gas is directed for the contacting of step (a). The housing may be made from any suitable material. For example, the housing may be transparent to allow light to pass through the housing. The light passing through the transparent housing will provide light energy to the at least one photosynthesising agent for photosynthesis. The method may further comprise a step of mixing the flue gas with air prior to the contacting of step (a). The step of mixing may be carried out within the housing. The step of mixing may be carried out for a suitable period of time. In particular, the step of mixing may be carried out to reduce the total concentration of carbon dioxide in the mixture of air and flue gas to a predetermined concentration.

The pre-determined concentration may be less than 150000 ppm, 100000 ppm, 80000 ppm, 70000 ppm, 65000 ppm, 50000ppm, 40000 ppm, 38000 ppm, 28000 ppm, 20000 ppm, 18000 ppm, 10000 ppm, 8000 ppm, 5000 ppm. In particular, the pre-determined concentration may be 8000-150000 ppm, 9000- 100000 ppm, 10000-90000 ppm, 15000-70000 ppm, 20000-60000 ppm, 30000- 40000 ppm. Even more in particular, the pre-determined concentration is 7500- 9000 ppm.

The method according to any aspect of the present invention may further comprise a step of reducing the temperature of the flue gas prior to the contacting of step (a). Any suitable method of reducing the temperature of the flue gas may be used. For example, the flue gas may be passed through a heat exchanger to reduce the temperature of the flue gas. According to a particular aspect, the temperature may be reduced to 20-50°C, 25-47°C, 27-46°C, 30- 45°C, 32-42°C, 35-37°C. In particular, the temperature is reduced to 30 ⁰ C.

The method according to any aspect of the present invention may further comprise a step of removing other contaminants from the flue gas prior to the contacting of step (a). The contaminants may include NO _x and/or SO _x components. Any suitable method known in the art may be used for removing the contaminants. For example, the flue gas may be scrubbed to remove NO _x and/or SO _x components. According to another aspect, the present invention provides an apparatus for reducing concentration of carbon dioxide from flue gas comprising carbon dioxide, the apparatus comprising at least one photosynthesising agent. The apparatus is capable of receiving flue gas comprising carbon dioxide. In particular, the apparatus is capable of receiving flue gas comprising elevated concentration of carbon dioxide.

The at least one photosynthesising agent may be any suitable photosynthesising agent. In use, the at least one photosynthesising agent comprised in the apparatus may use the carbon dioxide from flue gas for photosynthesis. According to a particular aspect, the at least one photosynthesising agent may be at least one plant, plant cell, or part thereof. The plant, plant cell, or part thereof may comprise at least one leaf capable of carrying out photosynthesis. For example, the plant may be Vigna Radiata, water hyacinth, or tapioca. Any suitable plant may be used for the purposes of the present invention. If a plurality of photosynthesising agents are comprised in the apparatus, each of the photosynthesising agents may be the same or different from one another.

According to a particular aspect, the apparatus may be transparent. In particular, at least a part of a surface of the apparatus may be transparent. The apparatus may be made from any suitable material. For example, the apparatus may be transparent to allow light to pass through the apparatus. The light passing through the transparent apparatus will provide light energy to the at least one photosynthesising agent for photosynthesis.

According to another aspect, the present invention provides an assembly for reducing concentration of carbon dioxide from flue gas comprising carbon dioxide, the assembly comprising at least one apparatus according to any aspect of the present invention. Brief description of the figures

Figure 1 shows the decrease in CO ₂ concentration with time for various concentration starting values.

Figure 2 shows the 24-hour reduction of CO ₂ by dosed and control specimen.

Figure 3 shows the mass of CO ₂ removed after 24 hours by dosed and control specimen.

Figure 4 shows a schematic view of an assembly of four photosynthesis units.

Figure 5 shows the decrease in CO ₂ concentration with time, using Vigna Radiata (plant specimen) after a first cycle of the method of the present invention on the first day of experiment. The experiment is repeated using three sets of specimens at each time and each set of specimens is represented by curves A, B and C, respectively.

Figure 6 shows the decrease in CO ₂ concentration with time, using Vigna Radiata (plant specimen) after a second cycle of the method of the present invention on the first day of experiment. The experiment is repeated using three sets of specimens at each time and each set of specimens is represented by curves A, B and C, respectively.

Figure 7 shows the decrease in CO ₂ concentration with time, using Vigna Radiata (plant specimen) after a first cycle of the method of the present invention on the second day of experiment. The experiment is repeated using three sets of specimens at each time and each set of specimens is represented by curves A, B and C, respectively.

Figure 8 shows the decrease in CO ₂ concentration with time, using Vigna Radiata (plant specimen) after a second cycle of the method of the present invention on the second day of experiment. The experiment is repeated using three sets of specimens at each time and each set of specimens is represented by curves A ₁ B and C, respectively.

Figure 9 shoes the decrease in CO ₂ concentration with time, using Vigna Radiata. (plant specimen) after a first cycle of the method of the present invention on the third day of experiment. The experiment is repeated using three sets of specimens at each time and each set of specimens is represented by curves A, B and C, respectively.

Detailed description of the invention

Bibliographic references mentioned in the present specification are for convenience listed in the form of a list of references and added at the end of the examples. The whole content of such bibliographic references is herein incorporated by reference.

Eamus and Jarvis (1989) were among the first researchers to study the effects of elevated carbon dioxide concentration on plants. They found that increase in the concentration of carbon dioxide increases the net photosynthetic rate of "C3" plants. This was also the general conclusions by Blom et al. (2002), who found that plants could grow up to 50% faster in carbon dioxide concentrations of 1000 ppm when compared with atmospheric conditions. Morison (1998) observed stomatal closure in response to elevated concentrations of carbon dioxide. These results were verified by Tognetti et al. (2001 ) who found that olive trees (Olea europaea L.) cultivars exposed to elevated concentrations of carbon dioxide in free-air CO2 enrichment facility recorded higher net photosynthetic rate even though stomatal conductance decreased. Furthermore, Populus euphratica increased in water use efficiency when it was subjected to high concentrations of carbon dioxide; however, the rate of increase in this efficiency was limited by the degree of water stress, as measured by the groundwater depth (Zhou et al., 2009). These results corresponded with earlier works by Liang et al. (1995). Croonenborghs et al. (2009) were more specific in quantifying the results of increased photosynthesis due to elevated concentrations of carbon dioxide. They found that three species of ornamental bromeliads (including Guzmania 'Hilda') increased leaf area by average 34% and leaf thickness by average 11%, with the surplus biomass gained in only the first 22 weeks of the experiment, before acclimation occurred. However, Allen and Vu (2009) discovered that such an increase in net photosynthetic rate was actually regulated by the availability of water and surrounding temperature (which in turn determined the vapour pressure deficit). They observed this from measuring the growth of young sour orange trees grown under mid-latitude desert conditions, and comparing them to the growth outcomes under humid subtropical climate.

The above studies were all conducted at an upper limit of carbon dioxide concentration of 720 ppm. Recent concerns over the possible ill effects of global warming on plants' growth and health prompted studies of plants' reaction at much higher concentrations of carbon dioxide. When wheat seedlings were subjected to carbon dioxide concentrations of 1500 ppm and 10000 ppm, it was found that they exhibited metabolite profile characteristics of older plants under atmospheric carbon dioxide concentration (Levine et al., 2008). At 10000 ppm, the specimens had higher transient starch content, although only plants exposed to 1500 ppm showed an increase in initial growth rate. However, both types of specimens recorded 25% increase in biomass over the control plants after 4 weeks of experiments. Concerns over how climate change will change the concentration of carbon dioxide in the deep ocean and how that will in turn affect the survival of aquatic plants underlined the effort by Bemhard et al. (2009) to study the response of Allogromia laticollaris to ultra high carbon dioxide concentrations. The team subjected specimens to concentrations of 15000, 30000, 60000, 90000 and 200000 ppm by incubating them in an enclosure with the intended carbon dioxide concentration over 10-14 days. The temperature was maintained at 23°C. They found that substantial populations of the species could actually survive 200000 ppm, although the rate of survival is statistically lower than that under atmospheric conditions.

Rhee and lamchaturapatr (2009) subjected specimens of aquatic plants, including Cyperus alternifolius and Iris setosa, to input carbon dioxide concentration of 500-2500 ppm at a constant temperature of 25°C and measured the amount of carbon dioxide concentration reduction for each input of carbon dioxide concentration. They found that increasing input carbon dioxide concentration increased the rate of removal - for example, at 2.2 g CO ₂ /m ² , the rate of removal is about 40-60% higher than that at atmospheric level. More specifically, they found that these plants reduce the input carbon dioxide concentration of 2500 ppm to less than 200 ppm when the retention time of CO ₂ in the glass enclosure was longer than 5 hours.

Conventional methods of channelling industrial flue gases (containing, amongst other gases, carbon dioxide (CO ₂ )) into closed systems (also known as photo- bioreactors, in which carbon sequesters are kept and cultivated) can be described as the "through-flow" method. In particular, the input flue gas is continuously passed into and through a feeding vessel (or de-gassing column) in which the input CO ₂ is absorbed away from the flue gases by a liquid medium (such as water) and the CO ₂ -rich medium is subsequently circulated within the photo-bioreactors. The input flue gas comprising CO ₂ is continuously passed through the medium. This is not a very efficient way of removing CO ₂ since experiments have been conducted to indicate that water can only absorb about 8% of the CO ₂ at a concentration level of about 8000 ppm. The present invention provides a method for reducing concentration of carbon dioxide from flue gas comprising carbon dioxide, comprising a step of:

(a) contacting the flue gas with at least one photosynthesising agent, wherein the contacting of step (a) is for a pre-determined period of time. For the purposes of the present invention, "reducing" in relation to the concentration of carbon dioxide means lowering the concentration of carbon dioxide in the flue gas to an environmentally acceptable level. For example, the environmentally acceptable level may be set by the environmental agency of individual countries. According to a particular aspect, "reducing" in relation to the concentration of carbon dioxide is defined as lowering the concentration of carbon dioxide in the flue gas to the concentration of carbon dioxide in ambient air. For example, "reducing" is defined as lowering the concentration of carbon dioxide to about 390 ppm or less than 390 ppm. In particular, "reducing" may be defined as lowering the concentration of carbon dioxide to a concentration of to about 360-390 ppm. According to another particular aspect, "reducing" in relation to the concentration of carbon dioxide is defined as lowering the concentration of carbon dioxide in the flue gas to almost 0 ppm.

The flue gas may be flue gas comprising an elevated concentration of carbon dioxide. The flue gas may be from any source which emits carbon dioxide. In particular, the flue gas may be from any source which emits carbon dioxide in elevated concentrations. For example, the flue gas may be from an incinerator, furnace or a power plant, particularly from the combustion, steamer or boiler unit of a power plant. The flue gas may be produced by industrial processes. For the purposes of the present invention, flue gas is considered as having an elevated concentration of carbon dioxide when the concentration of carbon dioxide in the flue gas is higher than the concentration of carbon dioxide in the ambient air. For example, flue gas comprising an elevated concentration of carbon dioxide is flue gas comprising more than about 390 ppm of carbon dioxide. In particular, the concentration of carbon dioxide in flue gas may be about 30000-150000 ppm (i.e. 3-15 volume %).

The at least one photosynthesising agent may be any photosynthesising agent suitable for the purposes of the present application. The at least one photosynthesising agent may use the carbon dioxide from the flue gas for photosynthesis. If a plurality of photosynthesising agents are used, each of the photosynthesising agents may be the same or different from one another. For the purposes of the present invention, the term "photosynthesising agent" includes plants, plant parts thereof or plant cells capable of carrying out photosynthesis. According to a particular aspect, the at least one photosynthesising agent may be a plant, plant cell, or part thereof, or a combination thereof. For example, the plant, plant cell, or part thereof may comprise at least one leaf capable of carrying out photosynthesis. The plant or plant cell may be Vigna Radiata, water hyacinth, or tapioca. Any other plant suitable for the purposes of the present invention may be used. For the purposes of the present invention, plants and plant cells do not include fungi, bacteria and algae capable of carrying out photosynthesis.

The plants and/or plant cells may be naturally occurring, modified artificially or by gene manipulation. While certain plants and/or plant cells disclosed in the context of the present application are particularly suited for the method of the present invention, and while in the examples below, the results are shown in the context of the utilization of Vigna Radiata as the photosynthesising agent, it would be understood by a skilled person that in other embodiments, other photosynthesising agents may be utilised in place of or in addition to Vigna Radiata. According to a particular aspect, naturally occurring plants or plant cells are used for the purposes of the present invention as these can be easily and more conveniently obtained as compared to genetically modified strains of bacteria and/or algae.

According to a particular aspect, the pre-determined period of time may be less than 24 hours. The contacting of step (a) is not carried out continuously, i.e. the contacting of step (a) is non-continuous. The contacting of step (a) is carried out in batches of a pre-determined period of time. For example, the pre-determined period of time may be about 1.5-20 hours, 2-12 hours, 4-10 hours, 5-8 hours, 6- 7 hours. In particular, the pre-determined period of time may be about 2-4 hours. According to a particular aspect, the pre-determined period of time is about 12 hours. Even more in particular, the pre-determined period of time may be during daylight. According to another particular aspect, for example when the concentration of carbon dioxide in flue gas is about 8000 ppm, the predetermined period of time may be about 2-4 hours.

The method according to any aspect of the invention, wherein after step (a), may further comprise the steps of: (b) removing the flue gas from contacting the photosynthesising agent; and (c) repeating a step of contacting the photosynthesising agent with flue gas comprising carbon dioxide for a predetermined period of time. The flue gas of step (b), which is removed from contacting the photosynthesising agent, may have been treated (i.e. having a reduced carbon dioxide concentration). The flue gas of step (c) may be distinct from the treated flue gas which is being removed in step (b). In particular, the flue gas of step (c) may be untreated. The flue gas of step (c) may be flue gas comprising carbon dioxide at elevated concentrations. In particular, the flue gas of step (c) may be from the same source as the flue gas of step (a). The predetermined period of time of step (c) may be the same as the pre-determined period of time of step (a).

According to a particular aspect, the removing of step (b) and the contacting of step (c) are performed repeatedly.

According to a particular aspect, the pre-determined period of time for each of steps (a) and (c) is the same.

According to a particular aspect, the pre-determined period of time for each of steps (a) and (c) is different.

According to another particular aspect, the step (b) of removing and the step (a) or (c) of contacting are separated by a step of non-contacting for a predetermined period of time. The step of non-contacting is when the photosynthesis agent is not in contact with flue gas comprising carbon dioxide at elevated concentrations.

The pre-determined period of time for the step of non-contacting may be about 5 minutes-24 hours, 10 minutes-18 hours, 30 minutes-12 hours, 2-6 hours, 1-4 hours. In particular, the pre-determined period of time is about 12 hours. Even more in particular, the pre-determined period of time may be during night time, without any daylight.

According to a particular aspect, the at least one photosynthesising agent may be comprised in a housing into which the flue gas is directed for the contacting of step (a). The housing may comprise a flue gas inlet from which flue gas enters the housing and a flue gas outlet from which flue gas having a reduced concentration of carbon dioxide after the contacting of step (a) exits the housing. The flue gas exiting the housing may be vented to the atmosphere or may be passed to further treatment options.

The flue gas inlet of the housing may be in fluid communication with the outlet from which flue gas is released. A forced draft fan may be used to facilitate the transfer of the flue gas and/or to push the flue gas into the flue gas inlet of the housing. An induced-draft fan may be used to pull the flue gas through the housing, or a forced-draft fan may be used upstream of the housing instead of or in addition to the induced-draft fan. By using an induced-draft fan, the housing may be maintained at negative pressure, thereby reducing the risk of unintentional venting of untreated flue gas to the atmosphere. Any suitable induced-draft fan may be used for the purposes of the present invention. For example, the induced-draft fan may be a blower.

The housing may have at least one surface at least a portion of which is partially transparent to light of a wavelength capable of driving photosynthesis, such as light of a wavelength of 400-700 nm. In particular, all the surfaces of the housing may be transparent to allow maximum amount of light of a wavelength capable of driving photosynthesis to enter the housing.

The housing may optionally comprise at least one artificial light source providing light at a wavelength able to drive photosynthesis. The at least one artificial light source may be utilised in supplement to or instead of natural sunlight.

According to a particular aspect, the method of the present invention further comprises a step of mixing the flue gas comprising carbon dioxide with air prior to the contacting step of (a). In particular, the step of mixing is carried out until the concentration of carbon dioxide in the mixture of air and flue gas is reduced to a pre-determined concentration.

The pre-determined concentration may be less than about 150000 ppm, 100000 ppm, 80000 ppm, 70000 ppm, 65000 ppm, 50000ppm, 40000 ppm, 38000 ppm, 28000 ppm, 20000 ppm, 18000 ppm, 10000 ppm, 8000 ppm, 5000 ppm. In particular, the pre-determined concentration may be about 8000 ppm. In particular, the pre-determined concentration may be about 8000-150000 ppm, 9000-100000 ppm, 10000-90000 ppm, 15000-70000 ppm, 20000-60000 ppm, 30000-40000 ppm. Even more in particular, the pre-determined concentration is 7500-9000 ppm.

Accordingly, the housing according to any aspect of the present invention may further comprise an inlet to allow air to enter the housing to mix with the flue gas comprising carbon dioxide until the concentration of the carbon dioxide in the mixture of air and flue gas is reduced to the pre-determined concentration. In allowing the concentration of the carbon dioxide in the mixture of air and flue gas to be reduced to a pre-determined concentration before the contacting of step (a), the time required for the contacting of step (a) is reduced. Accordingly, the method of the present invention results in a more efficient method of reducing the concentration of carbon dioxide in flue gases. This is exemplified in Example 1 below. Upon reaching the pre-determined concentration, the mixture of air and flue gas is allowed to contact with the at least one photosynthesising agent for a predetermined period of time. The pre-determined period of time may be as described above. After the pre-determined period of time, the concentration of the carbon dioxide in the mixture of flue gas and air reduces further as the carbon dioxide is utilised by the at least one photosynthesising agent for photosynthesis. The mixture of flue gas and air is then released from the housing through the flue gas outlet.

The method according to any aspect of the present invention may further comprise a step of reducing the temperature of the flue gas prior to the contacting of step (a). Any suitable method of reducing the temperature of the flue gas may be used. For example, the flue gas may be passed through a heat exchanger to reduce the temperature of the flue gas. According to a particular aspect, the temperature may be reduced to about 20-50°C, 25-47°C, 27-46°C, 30-45°C, 32-42°C, 35-37°C. In particular, the temperature is reduced to about 30 ⁰ C.

The method according to any aspect of the present invention may further comprise a step of removing other contaminants from the flue gas prior to the contacting of step (a). The contaminants may include NO _x and/or SO _x components. Any suitable method known in the art may be used for removing the contaminants. For example, the flue gas may be scrubbed to remove NO _x and/or SO _x components.

According to a particular aspect, the method of the present invention can be used as part of an integrated method for treating waste gases. The waste gases may comprise carbon dioxide. In particular, the waste gases may comprise an elevated concentration of carbon dioxide. The waste gases may be produced by industrial processes. According to another aspect, the present invention provides a method for reducing concentration of carbon dioxide from flue gas comprising carbon dioxide, comprising the steps of:

(a) mixing the flue gas with air to reduce the concentration of carbon dioxide in the mixture of air and flue gas to a pre-determined concentration; and

(b) contacting the mixture of air and flue gas with at least one photosynthesising agent, wherein the contacting of step (b) is for a pre-determined period of time.

The flue gas may be flue gas as described above. The pre-determined concentration may be as described above. The pre-determined period of time may be as described above. The at least one photosynthesising agent may be as described above.

According to another aspect, the present invention provides an apparatus for reducing concentration of carbon dioxide from flue gas comprising carbon dioxide, the apparatus comprising at least one photosynthesising agent. The at least one photosynthesising agent may be as described above. In particular, the present invention provides an apparatus for reducing concentration of carbon dioxide from flue gas comprising carbon dioxide, the apparatus comprising at least one plant, plant cell, or part thereof.

According to a particular aspect, the apparatus may be capable of receiving flue gas. The apparatus may comprise a flue gas inlet from which flue gas can enter the apparatus. The apparatus may comprise a flue gas outlet from which flue gas having a reduced concentration of carbon dioxide exits the apparatus. The flue gas exiting the housing may be vented to the atmosphere or may be passed to further treatment options. The apparatus may have at least one surface, at least a portion of which is partially transparent to light of a wavelength capable of driving photosynthesis. For example, the surface may be transparent to light of a wavelength of 400- 700 nm. In particular, all the surfaces of the apparatus may be transparent to allow maximum amount of light of a wavelength capable of driving photosynthesis to enter the apparatus.

According to another aspect, the present invention provides an assembly for reducing concentration of carbon dioxide from flue gas comprising carbon dioxide, the assembly comprising at least one apparatus according to any aspect of the present invention.

According to another aspect, the present invention provides an assembly for treating flue gas comprising carbon dioxide to reduce the carbon dioxide concentration before discharging into the atmosphere, the assembly comprising at least one photosynthesis unit for receiving flue gas, wherein the at least one photosynthesis unit comprises at least one photosynthesising agent. The photosynthesising agent is capable of using the carbon dioxide from the flue gas for photosynthesis. According to a particular aspect, the at least one photosynthesising agent is at least one plant, plant cell, or part thereof. The plant and plant cell may be as described above.

The flue gas comprising carbon dioxide may be as described above. The photosynthesis unit may be the apparatus as described above.

The assembly may comprise a plurality of photosynthesis units. The plurality of photosynthesis units may be utilised simultaneously or sequentially. Each of the photosynthesis units may be interconnected to each other. Each of the photosynthesis units may comprise a flue gas inlet and a flue gas outlet. Each of the photosynthesis units may also comprise an inlet to allow air to enter the photosynthesis unit to mix with the flue gas within the photosynthesis unit. The assembly according to any aspect of the present invention may be part of an integrated assembly for treating waste gases. The waste gases may contain an elevated concentration of carbon dioxide. The waste gases may be produced by industrial processes.

Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting of the present invention.

Examples Example 1

This example investigates: a) the CO ₂ removal rates at different starting carbon dioxide concentration ([CO ₂ ]) ranging 0.8-5vol% (or 8000-50000 ppm) over a period of 24 hours, under controlled temperature of about 30 ⁰ C in an experimental photosynthesis unit; b) quantitatively assess the effects of high [CO ₂ ] on photosynthesising agents; c) identify any changes in the CO ₂ removal ability of the photosynthesising agents after they are being exposed to high [CO ₂ ]; and d) theoretically deduce the likely CO ₂ removal rate of an industrial scale photosynthesis unit, by extrapolating from the experimental results.

Photosynthesising agent (plant specimen):

The photosynthesising agent chosen for this example was the common mung bean - Vigna Radiata. It was chosen because of its fast growth rate and ability to grow under a range of environmental conditions. Above all, it is easily available and economical. However, Vigna Radiata is used as a model plant for the purposes of this example only and is by no means limiting to the present invention.

Experimental Set-up for Photosynthesis Unit

a) Carbon dioxide level

The specimens were grown in soil for approximately two weeks. Since the respiration and photosynthesis rates of the specimens were to be measured, the CO ₂ emission from micro-organisms and decay of organic materials in the soil must be omitted. Hence, for subsequent determination of results, hydroponic agriculture method was adopted. The soil was washed off from the roots with tap water.

The total area of the leaves was quantified by carefully tracing the outline of the individual leaves onto grid graph papers. Sufficient plants were gathered to yield a total leaf area of 500 cm ² .

The value of 500cm ² was chosen because this is a value that could be contained in the desiccator without having leaves overlapping and covering one another (which would decrease the amount of light reaching the surfaces of the leaves).

The plants were subsequently divided arbitrarily and placed in two separate transparent bottles containing a total of 650 ml_ of water with a pH of 7.0. Only the roots of the plants were immersed in the water, leaving the leaves fully exposed to the air above. The two containers of specimens were then placed in a glass desiccator of volume 5.5 L. The desiccator was rendered air tight by applying a layer of vacuum grease along the interface between the body and lid of the desiccator.

At the beginning of each experiment, laboratory-grade CO ₂ gas (purity >99.9%) was introduced into the desiccator containing the plant specimens via a gas regulator until the [CO ₂ ] within the desiccator reached the desired level. The supply of CO ₂ to the desiccator was then terminated, and the desiccator was sealed off by replacing the rubber stopper. When the portable infrared gas analyzer was used, the air intake and return tubes were positioned at the highest and lowest points in the desiccator respectively, to encourage mixing of the CO ₂ .

The [CO ₂ ] level in the desiccator was monitored using two types of CO ₂ sensors. For [CO ₂ ] below 10,000ppm, a Telaire® 7001 D hand-held CO ₂ monitor was employed. The Telaire® CO ₂ sensor is equipped with a data-logger which has been programmed to record the [CO ₂ ] in the desiccator at every 3 minutes interval. Above 10,000ppm, the CO ₂ level in the desiccator was ascertained using a Fuji® (Model: ZSV) portable infrared gas analyzer. The readings from the Fuji® analyzer were recorded at regular 30-second time intervals using a data-logger. b) Illumination conditions

To simulate the effects of daylight, the desiccator was illuminated using two 23 W cool daylight bulbs with colour temperature 6500 K to simulate daylight as much as possible. Outside the desiccator, the illuminance levels recorded was between 4,300 and 26,000 Ix and the average illuminance inside the desiccator was 2,000 Ix. To ensure that the specimens were more evenly illuminated by the bulbs, the desiccator was surrounded with a reflective surface made with aluminium foil. The desiccator was covered with a layer of black paper so that the plant specimens within were only illuminated by the lamps.

Experimental Procedure:

Each set of experiment lasted for 24 hours, and the desiccator was kept illuminated for the entire course of the experiment. This differs from the common 12-hour light-12-hour darkness experimental approach, because the objective is to measure the maximum CO ₂ removal rates of the plants and to understand whether the rate of CO ₂ removal is constant over 24 hours of illumination. The CO ₂ level within the desiccator was monitored regularly. The temperatures and pressures in the desiccator were recorded throughout the experiments. The average temperature within the desiccator was 3O ⁰ C and did not vary by more than 2 ⁰ C. The evaporation of water from the containers was also found to be negligible by comparing the initial and final water volumes after 24 hours.

It was also imperative to determine the CO ₂ leakage rate from the desiccator, and the rate of CO ₂ absorption by the water in the two containers holding the plant specimens. To quantify the CO ₂ leakage rate from the desiccator, the CO ₂ level in the desiccator was monitored without the water and plant specimens within the desiccator. In order to ascertain the CO ₂ absorption rate by the water, the CO ₂ level in the desiccator was subsequently measured with the two water containers in the desiccator, but without any plant specimens.

The series of experiments involving the plant specimens started with 65,000ppm of CO ₂ in the air mixture within the desiccator. That is, 65,000ppm was the starting [CO ₂ ]. After a 24-hour time interval, the [CO ₂ ] within the desiccator was noted and the experiment was terminated. The original batch of plant specimens was removed from the desiccator and left in a protected outdoor nursery, under natural environmental conditions for another 24 hours. This was done to prevent exposing the specimens to a high [CO ₂ ] environment for a prolonged period of time. This was intended to increase the chance of survival of the plant specimens too. The same specimens were then returned into the desiccator (but fresh water samples were used) and subjected to a starting [CO ₂ ] corresponding to the [CO ₂ ] in the desiccator at the end of the previous day's experiment. This sequence of experimentation was repeated until the final [CO ₂ ] after 24 hours dropped below atmospheric level. Accordingly, the specimens were subjected to descending starting [CO ₂ ] in the experiment, 24 hours at a time. Throughout all experiments, the plant specimens were kept in water. These specimens are referred to as 'dosed specimens' in the following examples.

Experimental results

The state of the plant specimens that were subjected to the starting [CO ₂ ] of 65000 ppm for 24 hours was noted. About 20% of the leaves withered and another 10% of the remaining leaves had brown spots and decolorized patches on the leaves. Since the emphasis was to use the same specimens for the remaining experiments, we must ensure that the approximate total leaf area of the specimens remained at 500cm ² . These specimens were discarded and a new batch was used and subjected to a starting [CO ₂ ] of 50000 ppm instead and the protocol described above was followed.

As shown in Figure 1 , the rates of CO ₂ removal by the dosed specimens are constant over 24 hours of illumination for [CO ₂ ] equal and higher than 18000 ppm. This proves that the specimens can be induced to undergo extended periods of photosynthesis under the high [CO ₂ ] considered, as long as light is provided over 24 hours.

After 24 hours, the plants did not show any visible sign of withering although there were signs of brown spots on some of the leaves. The [CO ₂ ] dropped to a final average value of about 38000 ppm. With the starting [CO ₂ ] of 38000 ppm, the final [CO ₂ ] was about 28000 ppm. When the starting [CO ₂ ] was 28000 ppm, the [CO ₂ ] decreased to about 18000 ppm after 24 hours. With the starting [CO ₂ ] of 18000ppm, the final [CO ₂ ] was about 8000 ppm. Finally, starting with a [CO ₂ ] of δOOOppm, the specimens reduced the [CO ₂ ] to about 180 ppm in 24 hours.

The average reductions in [CO ₂ ] (corrected for CO ₂ leakage from the desiccator and dissolution in water) were plotted in Figures 2 and 3 (indicated by the symbol "•"). Statistical analyses show that there is no significant difference amongst the results for the starting [CO ₂ ] values of 50000, 38000, 28000 and 18000 ppm. However, the dosed specimens removed the most amount of CO2 in 24 hours at the starting [CO ₂ ] of about 8000 ppm.

The above experimental results show that mung beans exposed to a [CO ₂ ] 23 to 143 times the atmospheric level can remove between 68 and 285 g CO ₂ /m ² /L/day. Prior dosage with CO ₂ has resulted in statistically significant increases in the CO ₂ removal ability at some but not all starting [CO ₂ ] within the range of values considered in this study.

Theoretical rate of CO ₂ removal in an industrial-scale photosynthesis unit

One can consider an industrial scale photosynthesis unit in the form of a large 4,000 m ³ (20m-by-20m-by-10m) housing of the same volume as the algae pond considered by Kajiwara et al. (1997). This photosynthesis unit may be considered to compose of units of 0.0055 m ³ modules equivalent to the desiccator used in the experiment, and it may be connected to a 6,000 MWe (megawatts electricity, MWe) pulverized coal (PC) power plant. The flue gas emitted from the plant may typically have a temperature of 47°C and a flow rate of 616 kg/s. The [CO ₂ ] may be about 13.3 vol%. Given that the molar mass of the flue gas is about 28 g and that the molar mass of CO ₂ within the flue gas is computed to be 5.85 g, in 1 second, 129 kg of CO ₂ will be released into the photosynthesis unit. To minimize energy consumption of the photosynthesis unit, the plant specimens are to be exposed for only 12 hours of illumination (preferably daylighting). The 12-hour reductions in [CO ₂ ] for the different starting [CO ₂ ] can be read off from Figure 1. The following assumptions are made:

• The flue gas is passed through a heat exchanger that reduces the temperature from 47 to 30 ⁰ C (or, 303K);

• The lighting condition in the photosynthesis unit is uniform and identical to that in the experiment above; and • The distribution of the input CO ₂ is uniform throughout the photosynthesis unit.

The flue gas is first scrubbed to remove SO _x and NO _x components from the flue gas before being introduced into (and through) the photosynthesis unit. The flue gas is allowed to mix with air until the [CO ₂ ] within the unit drops to about 8000 ppm. This starting [CO2] is chosen because the largest percentage and absolute reductions in [CO ₂ ] occurred as shown in Figure 1. The photosynthesis unit is then closed (and rendered as air-tight as possible) and the inflow of flue gas blocked. The Ideal Gas equation can be used to calculate the total mass of CO ₂ in the photosynthesis unit (Mo):

M ₀ = PV/(RT) = {8000X101325/(1 X10 ⁶ )}X4000/{189X303} = 56.6 kg CO ₂ where P is the partial pressure of CO ₂ , V the volume of the photosynthesis unit, R the gas constant for CO ₂ and T the temperature of the gas in the photosynthesis unit.

After the first 12 hours of daylight, applying the Ideal Gas equation again, the mass of CO ₂ removed by the plants is found to be M1 =49.8 kg CO ₂ . That is, the plant specimens remove about 88% of the input CO ₂ after one day. During the night, the photosynthesis unit may be opened and the plants may be taken out and the water within the photosynthesis unit may be replaced. On the next day, the previously-dosed plant specimens may be placed in the photosynthesis unit again, because the experiment results showed that prior dosage enhances their CO ₂ removal ability. Flue gas is again released. into the photosynthesis unit as before and mixed with air until the internal [CO ₂ ] reaches around 8000 ppm again.

If this 1-day cycle is repeated 364 times throughout a year, a maximum of about 18 tons of CO ₂ can theoretically be removed from the power plant annually. This will require replacements of mung beans that wither in the process. It is said that a mature tree can remove about 22 kg CO ₂ per year (including the absorption capacity of the soil). Therefore, annually a photosynthesis unit can function like 826 mature trees (occupying about 11 soccer fields worth of land area, assuming that each tree requires 500 ft ² of open space for optimum growth) absorbing CO ₂ from the atmosphere.

Figure 1 indicates that at the starting [CO ₂ ] of about 8000 ppm, the [CO ₂ ] reached the lowest level after only 5 hours. This means that after 5 hours, more flue gas can be released into the photosynthesis unit to elevate the [CO ₂ ] to about 8000 ppm, and another 49.8 kg CO ₂ can be removed after another 5 hours. Hence, within a 1-day cycle, there is a theoretical possibility to remove 99.6 kg CO ₂ .

In operation, the actual amount of CO ₂ removed is likely to be lower due to the modules of plant specimens at higher levels shading over those at lower levels. This is especially the case when daylight provides the illumination. Therefore, the photosynthesis unit may be designed so as to allow maximum light penetration into the interior and throughout the different levels of the photosynthesis unit. Furthermore, these experimental results are based on an average illuminance of 2000 Ix and the actual illuminance may be lower on overcast days. A "holding tank" could also serve as an installation that keeps the flue gas before it is released into the photosynthesis unit.

Further, the results obtained for Figure 1 were based on hydroponic horticulture in tap water throughout the durations of the experiments. However, more nutrient-rich fluids can be used to increase the rate of CO ₂ removal.

Example 2

This example compares the response for carbon dioxide removal of a photosynthesis unit comprising dosed plant specimens as compared to a photosynthesis unit comprising new batches of plant specimens. Control 1 (C1) experiments involved using new batches of Vigna Radiata plant specimens with approximately the same total leaf area (500 cm ² ) for every experiment and the protocol of Example 1 was followed.

The values for C1 were plotted in Figures 2 and 3 for comparison. C1 specimens (C1(A) in Figures 2 and 3) removed about 6% less CO ₂ than the dosed specimens at the starting [CO ₂ ] of about 8000 ppm, statistical analysis shows that this difference is significant. At approximately 18000 and 38000 ppm, C1 specimens (C1(B) and CI(D)) recorded different values from the dosed specimens, statistical analyses show that these differences are not significant. However, the higher CO ₂ removal rate of the dosed specimens at 28000 ppm is statistically significant.

In conclusion, prior dosed plant specimens result in significantly higher rate of CO ₂ removal at [CO ₂ ] of about 28000 and 8000 ppm.

Example 3

This example illustrates the comparison between the CO ₂ removal rate of Vigna Radiata plant specimens at indoor [CO ₂ ] (500 ppm) and the plant specimens used in Example 1. Control (C2) experiments measuring the CO ₂ removal rate of specimens at indoor [CO ₂ ] (500 ppm) were conducted. The Vigna Radiata plant specimen were put in an airtight glass container and allowed to remove the CO ₂ inside the container over 24 hours. The decrease in CO ₂ levels was then recorded after 24 hours. 3 readings were taken for each experiment. Statistically significant differences between readings were tested using the t- test, at a level of significance of 5% (a=0.05).

It was found that when placed under indoor [CO ₂ ], C2 plant specimens recorded an average reduction of 58 ppm in 24 hours. On average, this is two orders of magnitude lower than what was recorded for the elevated starting [CO ₂ ] values under Example 1. Example 4

This example illustrates use of an assembly for reducing carbon dioxide concentration from flue gas, wherein the assembly comprises four photosynthesis units (i.e. a multi-stage arrangement). A schematic arrangement of the assembly is shown in Figure 4.

Each of the photosynthesis units was in fluid communication with an output of a diesel generator as shown in Figure 4. The output emitted flue gas comprising CO ₂ . The flue gas comprising CO ₂ from the diesel generator was channelled to tanks A, B, C and D sequentially. Each photosynthesis unit comprised Vigna Radiata having a total surface area of 2000 cm ² . Each photosynthesis unit was isolated from the other photosynthesis units.

First, the flue gas from the diesel generator was passed into tank A. The flue gas in tank A was allowed to mix with ambient air until the concentration of CO ₂ reduced to about 8000 ppm. A Telaire® 7001 D hand-held CO ₂ monitor was used for monitoring the concentration of CO ₂ in each of the tanks. When the concentration of CO ₂ in the mixture of flue gas and air reached 8000 ppm, tank A was closed and flue gas from the diesel generator was then channelled into tank B. Like in tank A, the flue gas in tank B was allowed to mix with ambient air until the concentration of CO ₂ gas in tank B reduced to 8000 ppm. Once this concentration was reached, tank B was closed and the flue gas was channelled into tank C and subsequently into tank D by repeating the steps above.

The Vigna Radiata in the tanks was allowed to photosynthesise as soon as the tanks were sealed. Illuminence was provided by sunlight. Once the concentration of CO ₂ in each of the tanks reduced to less than 390 ppm, the flue gas from that tank was released into the atmosphere. This was considered to be one cycle. The cycles were repeated until the end of day light. Three sets of assemblies were set up as above to obtain three sets of results. Figures 5 to 9 show the results obtained over three days during daylight for three experiments. For each day, two cycles were performed. For the third day, the experiment was terminated after the first cycle as the Vigna Radiata in each of the tanks needed to be replaced as the leaves had withered. The average CO ₂ concentration for each of the four tanks was taken to plot the graphs of Figures 5 to 9.

It can be seen from Figures 5 to 9 that it takes the Vigna Radiata in the tanks only from about 2 hours to 4 hours to reduce the CO ₂ concentration from the initial pre-determined concentration of about 8000 ppm to atmospheric level. This example also shows that the same plant specimen can be used in the same way for three days. However, the plants used (Vigna Radiata) withered off or turned yellowish in colour by the third day.

Example 5

This example illustrates the different ranges of operating temperature within a photosynthesis unit for the reduction of CO ₂ concentration for the method of the present invention.

Flue gas from a diesel generator was channelled into a tank comprising Vigna Radiata having a surface area of 2000 cm ² . The flue gas in the tank was allowed to mix with ambient air until the concentration of CO ₂ reduced to 8000 ppm. Once this concentration was reached, the tank was sealed and the Vigna Radiata was allowed to photosynthesise in daylight. The concentration of CO ₂ was monitored using a Telaire® 7001 D hand-held CO ₂ monitor. Once the concentration of CO ₂ in the tank dropped to below 390 ppm, the flue gas was released from the tank. This was considered to be one cycle. The cycle was repeated until the end of daylight. Accordingly, two complete cycles were performed and the third cycle was terminated as there was not enough sunlight. The temperature, illuminance and concentration of carbon dioxide were recorded during the three cycles of the experiment. The results are provided in Table 1.

As shown in Table 1 , the temperatures at which the plants remove and utilize the input CO ₂ range between 27°C and 43°C. The data covers altogether 3 cycles over a period of one day. This experimental data indicates that the plant specimens can work within this temperature range. However, this example does not restrict the operating temperature of the photosynthesis unit to only this temperature range.

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