EP2300132A1

EP2300132A1 - Treatment of gaseous effluents

Info

Publication number: EP2300132A1
Application number: EP09769322A
Authority: EP
Inventors: Johan Behen; Olivier Douxchamps; Sébastien MALCHAIRE
Original assignee: AGC Glass Europe SA
Current assignee: Behen Johan; AGC Glass Europe SA
Priority date: 2008-06-26
Filing date: 2009-06-25
Publication date: 2011-03-30
Also published as: WO2009156474A1

Abstract

The invention relates to a method of treating combustion flue gases by means of seaweed cultures suspended in an aqueous medium, the culture being produced in a reactor exposed to radiation resulting in the growth of seaweed by photosynthesis, in which method the flue gases are passed into the seaweed culture medium in order to dissolve at least some of the CO2 present in these flue gases, the pH of the solution being maintained between 6 and 8, characterized in that the treated flue gases contain at least 50% CO2 by volume.

Description

Treatment of gaseous effluents

The present invention relates to the treatment of gases, especially industrial containing a significant share of carbon dioxide.

Regulations aimed at limiting greenhouse gas emissions are increasing. The regulations in question are most often accompanied by penalties for those who do not comply with the requirements. Significantly, the requirements are more and more restrictive. Industries in particular, which constitute an important source of emission of these gases, are led to find solutions making it possible to reach the conditions satisfying these regulations.

By nature some industries that consume large quantities of fossil energy also produce large quantities of gaseous effluents containing these gases whose emission is regulated. All industries in whose productions the implementation of fusion calcination, heat treatment of any kind are concerned. Among these are thermal power plants, iron and steel, glass production, refining of petroleum products, cement plants ...

At present research has focused mainly on techniques to reduce gas emissions, and therefore on ways to limit the consumption of these fossil fuels. Another avenue of research has been the use of energy sources that do not generate gaseous effluents. These sources, however, are generally more expensive and often only shift emissions to upstream industries. This is the case of electrical energy when it is produced in thermal power plants.

Beyond the reduction of regulated effluent emissions, it is necessary to have means to limit their release into the atmosphere. Solutions consist in burying these effluents in the measure where this solution is industrially possible. This is practiced, for example, on certain sites producing natural gas or oil, or even mines. The gaseous effluents are, for example, reintroduced into the underground pockets from which the extracted fuels originate.

The removal of gases such as carbon dioxide can also be the subject of treatments that lead to fixing this gas in a chemical form without impacting the environment, for example in the form of calcium carbonate. The products of this transformation are not of significant value. The overall cost is therefore relatively large.

The invention therefore proposes to implement solutions which, beyond the reduction or elimination of these gases, in particular carbon dioxide, lead to their transformation into non-harmful and, if possible, upgraded products.

The natural cycle of carbon dioxide leads to a transformation by organisms to the formation of organic substances and possibly the formation of oxygen. The invention aims to develop a set of treatments of the type including a transformation of biological nature. More particularly, the invention relates to the conversion of carbon dioxide using algae leading to the development of what is called biomass.

All the techniques of treatment with algae have drawbacks and in particular that of the return on investment required. Moreover, the treatment often requires the implementation of operations that are themselves energy-consuming. For these reasons, the application of these techniques involving the development of biomass in practice has remained very limited despite the interest they arouse. The invention aims to implement the biomass culture from gaseous effluents produced under conditions that make this culture advantageous. The invention for this relates to gaseous effluents in which the carbon dioxide content is relatively high. This content is according to the invention at least equal to 50% by volume of these effluents and preferably at least equal to 60%.

The choice of CO ₂ -rich effluents has several advantages. Firstly, the dissolution in the algae culture medium is facilitated by high content in the treated gases. The passage of these gases into the solution does not require prolonged contact and / or extensive fractionation of these gases. The installation leading to this introduction may be limited accordingly to bubbling the gas in the solution.

In addition, the high concentration of carbon dioxide also makes the volume to be treated even smaller. Investments in installation are therefore lower. For comparison, combustion under ordinary conditions with air as an oxidant, even if the flue gases are not mixed with the ambient atmosphere, contain not more than 20% of carbon dioxide. For the same amount of CO _2, the volume to be treated is thus at least two times greater than that used according to the invention.

If the dissolution of carbon dioxide is facilitated as just said, this dissolution is accompanied by a decrease in pH. The growth of algae requires a medium that is not too acidic. With the dissolution of carbon dioxide it is therefore necessary to adjust the pH to remain within the limits most favorable to the growth of biomass. These conditions depend in part on the varieties of algae used. In practice, useful algae grow favorably in environments with a pH between 6 and 8, and preferably between 7 and 7.8. According to the invention the growth medium is adjusted so as to permanently present these pH values. The combustion fumes before being used in the reactors where the biomass is developed, are systematically dedusted and cooled. The temperature of the growth medium is a factor that significantly influences this growth. A temperature above 40 ° C can destroy the crop. Conversely, a temperature of less than 10 ^° C. or even 15 ° C. considerably limits the growth rate. For these reasons, the culture medium is advantageously maintained between 20 and 35 ° C.

Immediately after combustion the effluents are at temperatures of several hundred degrees. Lowering the temperature by mixing with room temperature air is obviously undesirable since it dilutes the carbon dioxide and increases the volume to be treated. For these reasons, the cooling is advantageously carried out by carrying out a heat exchange in a closed exchanger. In practice, as indicated below, the energy taken from the fumes is advantageously used in the subsequent stages of the treatment.

The algae culture according to the invention is carried out in reactors in a medium comprising the nutrients necessary for growth. In addition to water and carbon dioxide, growth requires a small proportion of nitrogen products and phosphorus.

Even if the combustion is carried out in an atmosphere essentially free of nitrogen, it is almost inevitable to find a very low proportion in the smoke, for example of the order of 1%. These quantities should be sufficient to provide the necessary input. Conversely, phosphorus, which is not normally present in fumes, must be systematically added.

Apart from the constituents of the culture medium, growth requires an energy supply in the form of radiation specific to develop photosynthesis. The conditions under which this contribution is made must be optimized for growth to be rapid. In particular the medium in which the microalgae are dispersed should be exposed as homogeneously as possible.

The useful radiation does not penetrate very deeply the culture medium. Moreover, if the medium is left to rest, a seaweed veil develops rapidly on the face exposed to radiation that hinders the penetration deep. For these reasons, the medium placed on a limited thickness, and secondly this medium is advantageously maintained stirred so that the entire volume benefits from this contribution.

If the growth of algae must take place under radiation favoring photosynthesis, the good development also requires periods of rest during which the culture is not exposed. These periods are as brief as possible.

The growth of the biomass is accompanied by a release of oxygen in the culture medium. Beyond a certain threshold the presence of this oxygen is opposed to the development of algae. The residence of the algae in the reactor is therefore limited in time by the progressive increase of the oxygen content.

In order to obtain as high a yield as possible, the reactor in which the growth is carried out operates continuously. The culture medium progresses in the reactor from an end in which the appropriately prepared culture medium is introduced, to the opposite end where the suitably developed culture is recovered to extract the biomass. In known manner the reactor consists of elongated enclosures such as tubes. The latter, apart from the fact that their section must be limited for the reasons of exposure to radiation, have a length which is determined by the increase of the oxygen content in the growth medium. When the threshold of oxygen tolerated by the algae is reached it is necessary to proceed to the extraction of this one. This operation is advantageously carried out of the growth reactor, and is one of those which lead to the recovery of biomass previously developed, and the regeneration of the culture medium for recycling.

Advantageously, the culture medium circulates continuously in the growth reactor. At the outlet of the latter, the culture medium loaded with the biomass is separated from the latter. Different separation techniques are known. They constitute one of the most important elements of the reprocessing of the gases in the establishment of the economic balance sheet. For this reason a known separation mode that is centrifugation is not preferred because relatively expensive. The most economical current means are based on flocculation techniques such as that presented in US 6,524,486.

The biomass extracted from the culture medium is then dried and is subject to various valuations which are discussed below. Drying is advantageously carried out using heat from the fumes themselves. The fumes pass through a heat exchanger, which allows to lower their temperature. The exchange leads to heating a coolant which is used to dry the biomass. As coolant is used for example air.

The culture medium recovered, the accumulated oxygen is extracted. This is advantageously conducted at the same time as the introduction of carbon dioxide. The gaseous effluents are, for example, conducted in a column containing the medium freed of biomass. The passage of gaseous effluents previously cooled in the medium is accompanied by the dissolution of a portion of the carbon dioxide present, the excess resulting in the dissolved oxygen. The oxygen-loaded gas, if its concentration is sufficiently high in this recovered mixture, can be separated, for example by liquefaction followed by distillation. For this operation to be economical, the concentration of oxygen must be at least 15% and preferably at least 20% of the recovered gas mixture.

The gaseous oxygen carbon dioxide mixture may also optionally replace air for burners when the oxygen level is sufficiently high, and in particular greater than 20%. A particular advantage in this case is the fact that nitrogen is not introduced into the flame and therefore the formation of NOx is not favored.

After recovery of the biomass the culture medium is readjusted before being recycled for a new operation. Its water content is supplemented to account for losses resulting from both biomass growth and the inevitable entrainment of water during biomass recovery. Optionally, the pH is adjusted as previously indicated if the dissolution of the carbon dioxide has led to lowering it outside the favorable limits. Finally we add the elements necessary for growth that do not bring the treated fumes.

Apart from the elimination of at least a portion of CO ₂ from the fumes, the reprocessing additionally provides the production of useful biomass. One of the most advantageous uses is the production of oils. The choice of micro-algae helps to promote this use. Under the best conditions, the oil content can represent up to 50% of the weight of the dried biomass.

The extraction of the oil is advantageously carried out by evaporation at elevated temperature. This operation is conducted in the absence of oxygen to prevent oxidation. The oil recovered for example by evaporation at 400 ⁰ C can then serve as a biofuel. A use It is particularly interesting to add this fuel to that used in the operation which leads to the production of treated fumes.

Biomass can still be the subject of various extractions of products intended in particular for pharmaceutical or other preparations. The useful principles are then generally extracted under more moderate conditions, in particular using solvents.

The exhausted biomass of these various constituents is still a valuable material, mainly as a meal for animal feed.

The combustion fumes used in the process according to the invention are in particular those produced in installations using burners operating in the oxy-combustion mode. These burners use an oxidizer consisting of substantially oxygen or a very rich oxygen mixture.

The choice of oxy-combustion in industrial plants is justified in several ways. For the same fuel consumption the recovered energy is more important. At the same time, the amount of carbon dioxide released is significantly lower. Moreover, the absence of nitrogen in the oxidant essentially avoids the formation of particularly harmful NOx oxides and for this reason must be eliminated costly before the discharge of fumes.

However, the use of oxy-combustion imposes new costs, in particular that of oxygen which replaces the air of traditional combustion.

An advantage of the flue gas treatment technique according to the invention is the possibility of recovering part of the oxygen necessary for feeding the burners. For that implementation of this recovery Oxygen is economically interesting, but the volumes treated must be sufficient.

The implementation of the techniques according to the invention in the context of glass melting furnace in which the volumes concerned are very important, fall into the category that justifies where appropriate the recovery of oxygen. Even if this recovery concerns only a limited fraction of the oxygen consumed in the furnace, it contributes usefully to the economy of the use of oxy-combustion.

In the absence of using oxygen alone, the CO ₂ oxygen mixture may advantageously be recycled as indicated above.

The technique according to the invention is shown schematically in the appended figures in which:

FIG. 1 is a schematic block diagram of the flue gas treatment process for supplying biomass production; FIG. 2 is a schematic view of the operation of the bioreactor.

In the remainder of the description reference is preferably made to the application of the invention to gaseous effluents from glass melting furnaces.

The overall scheme of the process shown in Figure 1 comprises a furnace 1 fueled with fossil fuel such as heavy fuel oil. The oxidant is either air or advantageously oxygen. The combustion gases from this furnace 1 are treated in 2 to remove some constituents and especially dust.

The combustion effluent gases used according to the invention must contain a high proportion of CO _2, the remainder being mainly water. The presence of nitrogen in the gases sent to the bioreactor 3 can provide the useful complement to the development of algae. The proportion of nitrogen required is nevertheless extremely limited compared to that of CO ₂ .

The amount of CO ₂ produced by the furnace is most often in excess of the absorption capacity of the crop. The intake of CO ₂ must be controlled to maintain the pH of the solution at values suitable for growth of algae. This pH is of the order of 6 to 8 and preferably of 7 to 7.8. The excess gases are evacuated at 6, while the toxic gases possibly present are removed beforehand in 5.

The diagram in Figure 1 does not show the use of fumes as a source of heat. In practice, the fumes exiting a glass melting furnace are at temperatures above 1400 ^° C. These fumes are systematically the object of energy recovery. The recovery is used for various purposes. Traditionally the main use is the heating of fuels and oxidants, and if necessary, also that of raw materials. These measures improve the overall energy balance. This is not shown in the diagram of Figure 1.

The recovery of heat as just indicated, does not exhaust the available energy. For this reason the surplus can be used for other operations. In particular, this energy can be used for drying the biomass in the treatment that it undergoes.

The CO ₂ -rich gas is passed into the reactor 3, which is also supplied with the necessary nutrient supplements which are not found in the gases themselves. It is mainly a phosphate feed.

The growth of algae is accompanied by the production of oxygen. Oxygen must be systematically evacuated 9 so as not to slow growth. The oxygen is optionally recovered for example by distillation in an air separation plant not shown in the diagram.

The suspended biomass is then separated and dried. After drying the biomass gives rise to one or more recovery operations in 4.

The choice of cultures determines the subsequent valuations. In all cases, these valuations concern on the one hand the production of oils used as biofuel which can be used in addition to the fuel 14 for heating the kiln. Mass may also be extracted from the "co-product" compounds 13, especially for pharmaceutical or cosmetic purposes. The dry matter after these extractions is usually used as a component of flours for animal feed.

Bioreactors are designed to promote the rapid growth of algae in a necessarily limited volume. In addition to supplemental nutrients and CO ₂ , growth imposes exposure to radiation essential for photosynthesis. The penetration of the radiation necessarily limits the depth of the suspension to a few centimeters. This depth does not normally exceed 30cm, and preferably does not exceed 20cm. The most frequent configuration of the reactor in the form of a battery of transparent tubes, as shown schematically in FIG. 2, is such that each tube has a diameter of the order of 10 cm. These tubes are fed with algae suspension at low concentration by a pump 16 which maintains a stirred circulation, so that the algae are as much as possible homogeneously exposed favoring rapid growth. This period of growth must also provide periods of rest which condition the good development.

The length of the tubes 15 is a function not only of the growth but also takes into account the speed of circulation and the presence oxygen. Efforts are made to ensure that the oxygen content remains lower than that for which photosynthesis would be inhibited. This limits the length of the tubes 15 and therefore the residence of the culture before the suspension is subjected to a treatment as schematized at 17 in a degassing column which allows the extraction of oxygen.

Previously the suspended mass is at least partly separated from the solution and directed to the upgrading treatment at 18. The biomass separation techniques are, for example, flocculation and filtration or by centrifugation. Flocculation is preferred as it is generally less expensive to implement.

The depleted solution which contains only a residual fraction of the developed algae is directed by the pipe 19 to the degassing chamber 17. It is preferably subject to a temperature adjustment to lead it to the most favorable conditions to growth by means of a control system represented in 20. The temperature should be neither too low nor too high. Generally the most suitable temperature is between 15 and 35 ° C and preferably between 20 and 30 ^° C. The temperature regulation is made necessary especially when exposure to radiation R is intense and results in a excessive rise in temperature in the tubes 15.

The oxygen extraction can be carried out under a partial vacuum or, as shown, by driving by bubbling the fumes admitted at 21 and discharged at 22.

Before returning the solution in the tubes 15 through the pipe 24, it is subject to an adjustment at 23, usually comprising the addition of water to compensate in particular the entrained water, additional nutrients and where appropriate a certain amount of algae to adjust the initial content. The reconditioned solution is sent into the tubes via the pump 16. It should be noted the importance of agitation in the tubes 15 to prevent a curtain of algae is formed on the side exposed to radiation at the expense of development in the entire available volume.

Exposure to radiation is advantageously natural.

It can be arranged by placing panels located near the tubes 15, reflecting the light towards them. In particular, a reflective surface is advantageously formed under the tubes 15.

Additionally it is also possible to have means developing the adequate radiation even in the absence of natural radiation, especially in the night time. Obviously natural radiation is preferred for reasons of economy of the overall process. However, especially when the main purpose of the implementation of these techniques is the elimination of a portion of the CO ₂ , the continuous operation of the furnace also leads to the preference for a permanent elimination and therefore the uninterrupted development of the biomass requiring continuous radiation. , subject to the necessary rest periods.

For reasons of economy, the artificial radiation is advantageously produced by means of light-emitting diodes which are known to provide a higher light output than other possible sources. In addition, the LEDs can be arranged as close to the biomass as possible for the greatest possible use of the radiation. They can in particular be placed all around the tubes in a regular distribution and with each diode a reflector directing all the radiation to the inside of the tubes. Their small dimensions still make it possible to envisage placing them inside the tubes themselves, for example in a tube concentric with the tubes containing the culture medium, and with a very small diameter in order to allow a maximum of volume to the suspension. LEDs in Because of their sufficiently small dimensions, they can still be inserted into the material constituting the walls constituting the bioreactors.

It is possible to evaluate biomass production according to the amount of CO ₂ absorbed. Even if the CO ₂ conversion used in the bioreactor is not total, this conversion can reach and even exceed 90%. In the most suitable facilities this conversion can be 95%. For a dry biomass unit the amount of CO ₂ absorbed is of the order of 1.6 to 2 mass units and on average of the order of 1.8 mass units.

The main difficulty in recovering carbon dioxide from flue gases is the required surface area and available solar energy on this surface. In temperate areas of Europe, the biomass production yield can be estimated at 200-300 tonnes per hectare per year. This production in regions with more abundant and regular sunshine can reach 400 tonnes per hectare per year.

On the basis of an average conversion of 1 tonne of biomass per 1.8 tonnes of CO ₂ , the conversion of the latter is therefore between 3600 and 7200 tonnes per hectare. These values are to be compared with the quantities of CO ₂ produced in large glass furnaces. As an indication for a kiln producing 650 tons of glass per day, CO ₂ production amounts to about 140,000 tons per year for a furnace operating in aero-combustion and about 120,000 tons in oxy-combustion. A complete treatment would consequently require a considerable area of the order of 15 to 40 ha according to the conditions. Such an area is rarely available in the immediate vicinity of the ovens in question. But the implementation of these techniques is advantageous even if it concerns only a part of the fumes produced. CO ₂ conversion and savings on emission penalties are not the only benefits of biomass production. Another factor that, depending on the circumstances, may be decisive, is the use of biomass for the production of oils that can be used as biofuel.

While different methods can be used to extract the most valuable components of microalgae, such as solvent extraction for the most fragile products, these techniques are too expensive for oil recovery. An effective technique for recovering oil for the preparation of biofuels is pyrolysis. To preserve the oil it is necessary to operate in the absence of oxygen and at a limited temperature for a relatively short time.

Pyrolysis conducted under controlled conditions leads to the evaporation of the oil. The temperature is advantageously between 400 and 500 ° C. After evaporation the oil is cooled rapidly to avoid any degradation.

The energy balance to be the best possible, the heat necessary for evaporation is preferably that recovered directly or not on the fumes of the oven.

The proportion of oil extracted from dry biomass depends on the nature of the algae and certain growth conditions. The study of growth conditions shows an increased proportion of oil, for example when the crop is conducted in deficit of nitrogen or silica in the growth medium. Nevertheless it is important to carry out an overall assessment, the enrichment in oil under these conditions may accompany a reduced growth rate.

For the most advantageous varieties of algae in oil production, the latter may represent between 30 and 50% by weight dried biomass. This oil in terms of energy source usually delivers between 0.7 and 1 times the energy of a same mass of heavy fuel oil and can be mixed with such fuel to fuel the combustion of the furnace. In particular, the viscosity qualities do not favor the use of this oil as a substitute for heavy fuel oil. But this oil can possibly be a complement when it is introduced in small proportion or be used as fuel in other applications.

The economic balance is possibly further improved if the oxygen recovery is added for furnaces operating in oxy-combustion.

For a glass smelting furnace operating in oxy-combustion and producing 650 tonnes of glass per day, for each hectare dedicated to the production of biomass under the conditions described above, the CO ₂ emission reduction can thus reach 0 , 5 to 1% of the total emission. Under the same conditions, the production of oil can lead to a reduction in fuel consumption of 0.5 to 1% per hectare and an oxygen production of between 0.5 and 1.5% of the consumption of the furnace.

All these elements, regardless of the subsequent use of dry materials, show the interest that can be attached to the implementation of smoke treatment techniques by the biomass culture.

Claims

1. Process for treating combustion fumes using algae cultures suspended in an aqueous medium, the culture being carried out in a reactor exposed to radiation leading to the development of algae by photosynthesis, a process in which the fumes are passed through the algae culture medium to dissolve at least part of the CO ₂ present in these fumes, the pH of the solution being maintained between 6 and 8, characterized in that the treated fumes consist of at least 50% by volume of CO ₂ .

2. Method according to claim 1 wherein the CO ₂ content of the treated fumes is at least 60% by volume.

3. Method according to one of the preceding claims wherein before passage into the culture medium the fumes undergo dusting.

4. Method according to one of the preceding claims wherein the pH of the solution is maintained between 7 and 7.8.

5. Method according to one of the preceding claims wherein the culture medium circulates between the reactor in which the photosynthesis is developed and plant elements in which the biomass of the algae is recovered and the culture medium is adjusted before to be reused in the reactor.

The method of claim 5 wherein the biomass is recovered from the culture by a flocculation technique.

7. The method of claim 5 or claim 6 wherein the volume of the solution recovered after extraction of the biomass, is restored and the previously cooled fumes are passed into this solution, the gas recovered after this passage subject to a treatment to extract the oxygen produced by the development of biomass.

8. The method of claim 7 wherein the gases after passage into the culture solution, are liquefied and distilled to recover the oxygen they contain.

9. The method of claim 7 or claim 8 wherein the solution before recycling is adjusted to pH and additional nutrient content, including phosphorus.

10. Method according to one of the preceding claims wherein the bomass recovered is dried and subjected to heat treatment to extract the oils that contains this mass.

11. The method of claim 10 wherein the drying and / or the heat treatment is performed by means of the energy recovered from the flue gases.

The method of claim 10 wherein the dried biomass is evaporated from the oils at a temperature not exceeding 50 ° C, and in a substantially oxygen-free atmosphere.

13. Method according to one of the preceding claims wherein the development of the biomass is carried out in substantially permeable tubes to radiation leading to photosynthesis.

14. The method of claim 13 wherein the culture medium circulates in these tubes and is agitated so that the entire medium can develop the biomass by exposing it to radiation causing photosynthesis.

15. Method according to the preceding claim wherein the radiation is predominantly solar radiation.

16. The method of claim 15 wherein the contents of the tubes may be subjected to artificial radiation especially for periods when solar radiation is lacking or is insufficient.

17. The method of claim 16 wherein the artificial radiation is provided by means of light emitting diodes disposed near the culture tubes.

18. Method according to one of the preceding claims wherein the fumes come from the combustion in a glass melting furnace.

19. The method of claim 18 wherein the combustion is oxy-combustion type.

20. The method of claim 19 wherein the oxygen recovered from the culture is used on the one hand for the oxidant used in the oxy-combustion of the melting furnace.

21. Method according to one of claims 18 to 20 wherein the oil produced from the biomass is used in low proportion with heavy fuel oil as fuel in the melting furnace.