WO2005028084A1 - Process and plant for the purification of high-temperature gaseous flows - Google Patents

Abstract

The gaseous flow at a temperature higher than the water-saturated ambient temperature, in particular at 55-60°C, forming the effluent of a primary purification plant, is brought into contact with thermophile microorganisms in a biofiltration bed formed by inert and preferably porous media, on the surface of which and in the pores of which said microorganisms form colonies, for a period of time sufficient for metabolisation of the polluting components of the gaseous flow into gaseous inert compounds, said biofiltration bed being sprayed with moistening water and substances with a nutrient effect for said microorganisms being added to said gaseous flow.

Description

Title

"Process and plant for the purification of high- temperature gaseous flows" Description The present invention relates to the biological purification of gaseous flows polluted by foul-smelling compounds and volatile organic substances (VOS) of an organic and inorganic nature, in particular high- temperature gaseous flows . Examples of gaseous flows of the type in question are the fumes generated by various industries such as : chemical and petrochemical processing (downstream of gas washing systems and wet precipitators) ; - food processing; - generation of electric power; and - thermal destruction of both urban and industrial waste. A specific example of industrial production to which the present invention is applicable is that of chipboard panel production using recycled wood. In the remainder of this description, reference will be made to the fumes of this latter type, it being understood that this must not be regarded as a limitation of the scope and extent of the invention. In the manufacture of chipboard panels, where both virgin wood and waste material is used as raw material, the following operations are envisaged: - grinding of the wood and drying of the wood particles using hot air; - spreading over the particles, after selection by means of vibration screening, a thermosetting resin of a synthetic nature so as to obtain at least partial impregnation of the particles; - formation of a layer (so-called "mattress") of adhesive-coated particles and hot-pressing so as to produce hardening of the adhesive. The fumes resulting from the different stages of the production process (in particular the stages of drying, coating with adhesive and impregnation, pressing) are polluted by various substances and the composition of the pollutants may vary substantially, especially if waste products - which may contain remnants from previous processing operations (glues, metallic materials, etc.) - are used at least partly as raw materials. Generally, plants such as "multicyclones" or dry or wet "precipitators" are used for purification of the fumes, said plants providing satisfactory results as regards the separation of the dusts suspended in the fumes, but not achieving complete or at least satisfactory elimination of the substances - especially organic substances - which are present in the fumes and vary greatly as regards both type and quantity. In the case of fumes from plants for the production of chipboard panels, the organic substances present in the fumes consist mainly of volatile organic substances (generally indicated by the abbreviation VOS) and formaldehyde. In general the VOS are produced by the hydroly ic decomposition of the wood and are released during the drying stage when the particles of wood are subject to a notable degree of heating. The fumes also contain terpenes, long-chain fatty acids, resins, ammonia, metals and inorganic compounds mainly of an acidic nature (such as hydrochloric acid and hydrofluoric acid) . By way of example, m the gaseous flow emerging from a precipitator for treating fumes of a chipboard panel manufacturing plant, the following pollutants are present :

TABLE 1

It is therefore obvious that the VOS, formaldehyde and other polluting components must be eliminated by means of a specific treatment of the fumes already partly purified in a precipitator or in a multicyclone . This treatment has hitherto consisted in carrying out a regenerative thermal combustion process : in this way, even though a high degree of elimination of the VOS, total organic carbon (TOC) and formaldehyde is ensured, other not insignificant problems arise. In fact, in addition to the high industrial cost of this treatment, the fumes thus purified have a very high content of so-called "greenhouse gases" (C0₂ and N0₂) and have a high temperature when leaving the combustion chamber. With regard to solely elimination of the formaldehyde, treatments based on the use of so-called "mesophile" bacteria have also been applied within the context of biofiltration technology. These bacteria are active at moderate temperatures which can not exceed the limit value of 38°C, so that they are unsuitable for the treatment of gaseous flows which usually have temperatures of 55-60°C. Obviously it would be possible to use them only if prior cooling of the hot gaseous flow were to be performed, but this would result in an industrially unacceptable increase in the treatment costs . In addition it must also be remembered that the gaseous flow leaving a wet precipitator normally has a water content close to the saturation value at the temperature of the flow (at 55°C, for example, the water content at saturation is 104.5 g of water per kg of dry air) so that cooling of the gaseous flow, necessary before the specific treatment with mesophile bacteria, would result in addition in the production of huge quantities of water condensate. The main object of the _ present invention is that of providing a process for the treatment of the gaseous flow leaving a primary purification plant, in particular a wet precipitator, in which the problems and drawbacks briefly described above are substantially eliminated and at the same time purification of the fumes which is satisfactory and advantageous from an industrial point of view is achieved. A more specific object of the present invention is that of providing a treatment of the abovementioned type by means of which it is possible to achieve substantial elimination of the VOS and formaldehyde from the fumes . These and other objects are achieved with the process and the plant according to the present invention, the main characteristic feature of which consists in the fact that the gaseous flow to be purified, as resulting from primary purification, is brought into contact with thermophile microorganisms, namely microorganisms which are capable of reproduction in high temperature conditions, in particular in temperature conditions normally associated with the fumes to be purified, for a sufficient period of time to allow the microorganisms to metabolise the polluting compounds, transforming them into iner,t compounds, in particular inert inorganic compounds such as carbon dioxide and water. In the preferred embodiment of the present invention said contact between the fumes to be purified and the thermophile microorganisms takes place in at least one filtration bed formed by porous inert elements, on the surface of which and in the pores of which colonies of the abovementioned microorganisms have been cultivated. Therefore, the present invention envisages on the one hand a process for the purification of gaseous flows, which^' are at a temperature higher than the room temperature, in particular at temperatures in the region of 55-60°C, which process consists in bringing into contact for a sufficient amount of time, preferably in the region of 20-30 seconds, the gaseous flow to be purified and therefore the polluting components contained therein with thermophile microorganisms capable of reproduction in high temperature conditions and capable of metabolising the VOS and the formaldehyde converting them into carbon dioxide and water. On the other hand the present invention envisages a plant for the purification treatment of high- temperature gaseous flows, which is characterized in that it comprises at least one biofiltration bed formed by at least partially porous inert ^' media, on the surface and pores of which thermophile microorganisms are adsorbed able to metabolise the polluting components of the gaseous flow to be purified, in particular the volatile organic substances and the formaldehyde, said bed being sprayed in a controlled manner with moistening water so that the spraying water and the gaseous flow to be purified move in the same direction of flow. In the preferred embodiment of the plant according to the present invention, the plant comprises two filter beds which operate in series and are passed through in succession by the gaseous flow to be purified which is made to flow from the top downwards and therefore in the same direction 'as the spraying water, said spraying being performed only on the first bed. As mentioned, the present invention is based on the action of thermophile microorganisms, such as: - Bacillus smithii; _.- Clostridium thermocellum; - Rhizomucor pusillus; - Chaetomium thermophilum; - Myceliophthora thermophila; - Thermoascus aurantiacus; - Bacillus schlegelii; - Geobacillus thermoglucosidasius ; - Methanoculleus thermophilus; - Scytalidium thermophilum - Thermithiobacillus tepidarius - Methanothrix thermophila - Bacillus thermocloacae - Desulfotomaculum thermoacetoxidans - Desulfotomaculum thermobenzoicum - Brevibacillus brevis - Brevibacillus borstelensis - Brevibacillus agri - Hydrogenobacter thermophilus - Aquifex pyrophilus - Clostridium thermocellum - Alicyclobacillus sp - Chlorobium tepidum - Thermocrinis ruber - Thermacetogenium phaeum - Caloramator coolhaasii - Thermotoga lettingae - Sphaerobacter thermophilus - Clostridium sphenoides • - Methylobacterium extorquens - Pseudomonas - Methanosarcina thermophila DSM 1825 - Pseudomonas rhodos - Methanosarcina thermophila - Thermotoga lettingae - Pseudomonas fluorescens - Pseudomonas putida - Acidomonas methanolica - Staphylococcus sp - Coliform Enteritidis; ^' - Coliform Freundi ; - enterobacteria such as Extorquens, Aspergillus Fumigatus . As mentioned above in the filter bed according to the invention the microorganisms form colonies on the external surfaces of media having a controlled porosity, so that the microorganisms also nest in the pores . In brief, the activity of the microorganisms may be summarised as consisting of the following basic stages : ,; 1. Diffusion of the polluting compounds from the mass of the gas to the surface of the porous and inert medium; 2. Adsorption onto the activated surface of the said medium; 3. Diffusion, towards the cellular wall, of the microbe colonies confined on the porous medium in a position adjacent to the surface; 4. Metabolic degradation reactions inside the microorganisms ; 5. Counter-diffusion and expulsion externally of the gas-phase products (C0₂, H₂0, NH₃) . The bio-oxidation reactions are exothermic and greatly displaced to the right, even at high temperatures. The accumulation of the reaction products in the vicinity of the microorganism does not cause stoppage of the process, but any drop in the pH below 6 may inhibit the microbic growth and the metabolic activity of the bacteria and thus reduce the efficiency of the process . It is therefore appropriate to keep the pH close to the neutral values required, by removing the H⁺ ions produced from the system. This and other important functions- are performed by the water for spraying and moistening the filter bed,' which is able to: - maintain optimum growth conditions within the material; - keep the pH value in the filter bed neutral; - remove the dissolved salts and heavy metals; - purge the metabolites and by-products from the system. The moistening water must be bacteriologically controlled and its flowrate must be kept within well- defined limits. In fact, in an environment which is too wet with excess water, the microorganisms could grow in the spaces between particles, causing an increase in the flow resistance or pressure drop of the biofiltration bed and an undesirable production of sludge. From an operating point of view, the spraying water is continually recirculated in a closed circuit and every so often, by means of timed opening of the motor operated valves, is conveyed onto the biofiltration bed in order to keep the moistness of the material as constant as possible. On the basis of the results from experimental pilot tests, the preferred quantity of spraying water per square metre of filter bed is about 5 litres/hour. As already mentioned, the plant according to the present invention in the preferred embodiment is formed mainly by two biofiltration beds which operate in series. The gaseous flow passes in succession through the two beds of filtering material axially, from the top downwards, in the same direction of flow as the spraying water, which however is distributed solely over the first filtration bed. The two beds, which may be designed with different dimensions, operate in synergy so as to ensure efficient removal and a high speed of degradation of the pollutants. The first bed, in fact, since it is acted on by the flow from the precipitator, retains most of the pollutants which prevent microbic growth (hydrogen chloride, hydrogen fluoride, heavy metals) , thus allowing the second bed to process pollutants which are non-toxic for the microorganisms . Basically, the bed which is first acted on by the polluted flow not only performs initial biodegradation of the VOS and the formaldehyde, but ^• in particular neutralises the acid compounds owing to its deliberately alkaline nature and retains the heavy metals, preserving the biological activity of the second bed and its duration over time. During operation of the plant, in order to remove the accumulated pollutants from the first biofiltration bed, the bed itself must be ideally sprayed more frequently. Before reaching the two filtration beds, during normal operating conditions, the gaseous flow is treated with metered quantities of nutrient solutions of an organic and/or inorganic nature in order to ensure continued stimulation of the microorganisms present in the inert media and responsible for purification. These solutions are periodically enriched with chemical additives based on phosphorus, potassium and trace elements. This operation is particularly important for the first bed which is more greatly affected by all those compounds which tend to inhibit the biological activity of the microorganisms present in the biofiltration bed. The nutrient solution is supplied in atomised form in the gas supply pipes leading to the filtration bed. Supplying is ideally performed at a certain distance from the filter bed so as to allow the solution time to mix thoroughly with the gaseous flow and its components, ,in particular the water vapour. The nutrient solutions are moreover very important during start-up of the plant and reactivation after long stoppages, when the colonies of useful microorganisms are limited or when they have to become accustomed to the support medium and adapt or readapt to the process conditions. In a bio-purification plant based on the use of thermophile microorganisms, rapid lowering of the operating temperature must always be avoided. During stoppages in the production process upstream of the purification system for a period of time greater than 16 hours, for example in order to carry out maintenance, the two filter beds are kept at the required temperature by causing a "dummy" air flow to pass through them, said air flow being kept hot and moist by the addition of direct steam at a moderate pressure (for example a pressure of 2 Bar gauge) . The dummy air flow, before being directed to the filter bed, is enriched with a suitable quantity of pollutant solutions adjusted to the right degree of moisture and temperature by supplying of live steam produced by a steam generator of suitable size. The attached drawing shows in schematic form an example of embodiment of a plant according to the present invention. The reference number 10 indicates a wet precipitator, fed through the inlet 11 with the gaseous flow to be purified and provided with a top shut-off valve 12 downstream of which the final flue 14 for expelling the purified gases into the atmosphere is arranged. The outlet 16 for drawing off the gaseous flow ,; already treated in the precipitator is arranged upstream of the shut-off valve. The outlet 16 is connected by a pipe to the first filtration bed 20 to which the flow is supplied so as ^■ to pass through the filter from the top towards the bottom. From the bottom of the first filtration bed, the gaseous flow, which has already passed through the first purification stage, is conveyed by means of the pipe 22 to the second filtration bed 24 which is also passed through from the top towards the bottom. The purified gaseous flow is removed at the outlet of the filtration bed 24 by means of the pipe 26 connected to the suction side of a fan 28 by which the gaseous flow is drawn from the outlet 16 of the precipitator 10 and conveyed through the two filter beds 20 and 24. The delivery side of the fan 28 is connected, by means of a pipe 30, to an inlet 32 of the flue 14 downstream of the shut-off valve 12. As illustrated above, before reaching the two filtration beds, during normal operating conditions, the gaseous flow is treated with metered quantities of nutrient solutions of an organic and/or inorganic nature, this operation being particularly important for the bed 20 which is acted on most by all those compounds which tend to inhibit the biological activity of the microorganisms present in the biofiltration bed. The nutrient solution is contained in the storage tank 34 from which a metering pump 36 supplies the solution, after atomisation, inside the pipe 18 for conveying the gas to the first filter bed. The pump 36 is operated in accordance with a suitable timing program which is managed by a timed control system. The spraying water of the first filtration bed is drawn from the tank 38 and conveyed to the supply nozzles 40 by means of the electric pumps 42 and 44. The chemical additives, based on phosphorus and trace elements, with which as already mentioned the water present in the tank 38 is periodically enriched, are drawn from a tank 46 by means of a metering pump 48. The percolation water coming out in particular from the first filtration bed 20 is conveyed into a collection tank 50 and from here transferred into the tank 38 by means of a pump 52. The steam generator 54, of suitable dimensions, is envisaged for stoppages of the plant upstream of the precipitator or inactivity of the filter bed, during which as already mentioned a dummy air flow is fed to the filter bed. The process and the plant according to the present invention have been the subject of experimental tests, the main and most important one of which is illustrated hereinbelow. A purification plant based on thermophile microorganisms was designed to purify a gaseous flow with a nominal flowrate of 400,000 Nm3/h supplied by a double-stage wet precipitator. The plant is intended to operate in a continuous manner for 8000 hours per year. The solution adopted consists of eight modules or units, each provided with two filtration beds 20 and 24 operating in series. The eight units are connected in parallel by means of two symmetrical headers, one of which is arranged on the right and the other on the left of the terminal section of the precipitator. Owing to its modular design, it is possible to perform all the routine and extraordinary maintenance operations without stopping the entire plant, but only the apparatus in question. The main gaseous emission conveyed to the filtration beds has characteristics typical of a gas emitted from a precipitator both in terms of temperature and moisture and in terms of type of pollutants and their associated concentration levels, as for example indicated in Table 1 above. Said emission is derived, using a suitable shut- off valve, from the gaseous flow emitted from the precipitator and, via the two main headers which are suitably insulated, is distributed to the modules. The flow is divided into eight identical fractions and conveyed to the eight first filtration beds. At the outlet of each of the second biofiltration beds, the purified flows are combined in two headers from which four fans, which operate in parallel and which are provided with a device for varying the number of revolutions, convey them back to the flue of the precipitator, downstream of the shut-off valve, merging them with the cooling gases of the apparatus so as not to cause immediate condensation of the water vapour at the flue outlet. The filtration beds were designed with dimensions so as to obtain an overall contact time of at least 20 seconds and preferably about 30 seconds (7-8 seconds on the first bed and 23-22 seconds on the .'.second bed); moreover, by dividing up the flow into eight filtration units, the specific speed of flow through the beds is kept at levels which are not too high, thus avoiding major pressure drops and consequently high levels of energy consumption. The specific speed of flow through the beds is equal to about 277 m³/m²h and the overall pressure drop in each filtration bed is equal to about 250 mm H₂0. It is calculated that this filtration plant, in maximum output conditions and during the winter period, produces a quantity of water condensate equivalent to about 5 m³/h; this condensate percolates through the beds and is collected in a tank of suitable size. A pump conveys the condensate to another recirculation tank and from here to the filtration beds with the aid of recirculating pumps. The excess water is conveyed, with control of the level, to the water treatment system of the precipitator and/or^" directly into the sewerage system. External environmental factors may influence the temperature of the inlet gases and the gases inside the pair of filtration beds. By means of suitable measures and adequate insulation, however, it is possible to avoid a drop in the temperature of the gases, maintaining ideal working conditions in the biofiltration modules and reducing to a minimum the _* quantity of water condensate. The use of a filtration plant based on thermophile microorganisms downstream of the precipitator constitutes a valid solution to the problems of polluting gaseous emissions from chipboard panel production lines. The plant ensures efficient removal of the pollutants in all operating conditions, proving to be an optimum purification system in situations where the polluting loads are extremely variable from both a qualitative and quantitative point of view, as is the case precisely downstream of a chipboard panel production line which uses waste wood as raw material. In this particular case, the combined treatment system "precipitator/thermophile biofilter" therefore allows control of the levels of emission, into the atmosphere, of dusts, VOS, formaldehyde, terpenes and ammonia . It should be pointed out moreover that the purification process performed with the thermophile biofilter is not accompanied by any production of secondary liquid or solid effluent and that the energy consumption is limited. The following Table indicates the- removal efficiency, for each individual pollutant, determined during tests carried out with the plant described aboveς

TABLE 2

The invention has been described in relation to a preferred embodiment, but it is understood that conceptually equivalent modifications and variants are possible and may be envisaged without departing from the scope thereof . For example, in place of the porous inert support media it is also possible to use natural or artificial non-porous media made of plastic or other material, provided that, obviously, the thermophile microorganisms are provided with a sufficient area to •form colonies having a number and extension such as to allow purification of substantial volumes of polluted gaseous flows in time periods which are reasonable from an industrial point of view.

Claims

Claims 1. Process for the treatment of a gaseous flow emitted from a primary purification plant, in which said gaseous flow is at a temperature higher than the water-saturated ambient temperature, characterized by bringing into contact for a sufficient period of time the gaseous flow to be purified and the polluting components contained therein with thermophile microorganisms capable of reproduction in high temperature conditions and capable of metabolising said polluting components, converting them into inert and non-polluting compounds. 2. Process for the treatment of a gaseous flow according to Claim 1, characterized in that said contact time is in the region of 20-30 seconds. 3. Process for the treatment of a gaseous flow according to Claim 1, characterized in that said thermophile microorganisms are chosen from among: Bacillus smithii; Clostridium thermocellum; Rhizomucor pusillus; Chaetomium thermophilum; Myceliophthora thermophila; Thermoascus aurantiacus ; Bacillus schlegelii; Geobacillus thermoglucosidasius; Methanoculleus thermophilus; Scytalidium thermophilum; Thermithiobacillus tepidarius; Methanothrix thermophila; Bacillus thermocloacae; Desulfotomaculum thermoacetoxidans; Desulfotomaculum thermobenzoicum; Brevibacillus brevis; Brevibacillus borstelensis;Brevibacillus agri ; Hydrogenobacter thermophilus; Pseudomonas sp; Aquifex pyrophilus; Clostridium thermocellum; Alicyclobacillus sp; Chlorobium tepidum; Thermocrinis ruber; Thermacetogenium phaeum; Caloramator coolhaasii; Thermotoga lettingae; Sphaerobacter thermophilus; Clostridium sphenoides; Methylobacterium extorquens; Pseudomonas; Methanosarcina thermophila DSM 1825*; Pseudomonas rhodos; Methanosarcina thermophila; Thermotoga lettingae; Pseudomonas fluorescens; Pseudomonas putida; Acidomonas methanolica; Staphylococcus sp; Coliform Enteritidis; Coliform Freundi ; enterobacteria such as Extorquens; Aspergillus Fumigatus . 4. Process for the treatment of a gaseous flow according to Claim 1, characterized in that said thermophile microorganisms are supported on at least partially porous media, on the surface of which and in the pores of which said microorganisms form colonies, the process envisaging also keeping the pH of the filtration bed at a value close to 8 and expulsion of the products resulting from metabolisation of the polluting components by the thermophile microorganisms. 5. Process for the treatment of a gaseous flow according to Claim 4, characterized in that said filtration bed is sprayed, at predetermined time intervals, with moistening water, supplying of the gaseous flow to be purified being performed so that it passes through the filtration bed in the same direction of flow as the water for spraying the filter; bed. 6. Process for the treatment of a gaseous flow according to Claim 1, characterized in that organic substances in solution form with a nutrient effect for the microorganisms present in the filtration bed are added to said gaseous flow before At reaches the filtration bed. 7. Process for the treatment of a gaseous flow according to Claim 6, characterized in that phosphorus and/or potassium and/or trace^' elements are added to said nutrient organic substances. 8. Process for the treatment of a gaseous flow according to Claim 1, characterized in that, during periods in which the purification process is stopped for time periods greater than a predetermined time limit, the filtration bed is supplied with a dummy air flow having a predefined degree of moisture and heated to a predetermined temperature, in order to avoid a deterioration in the purification properties of the colonies of microorganisms present in the bed. 9. Process for the treatment of a gaseous flow according to Claim 8, characterized in that said dummy air flow is supplied when stoppage of the purification process lasts for more than 16 hours. 10. Process for the treatment of a gaseous flow according to Claim 8, characterized in that predetermined and selected polluting components are added to said dummy air flow. 11. Plant for the purification treatment of gaseous flows at a high water-saturated temperature emitted from a primary purification stage (10) , characterized in that it comprises at least one biofiltration bed (20, 24) formed by at least partially porous inert media, the surface and pores of which adsorb thermophile microorganisms able to metabolise the polluting components of the gaseous flow to be purified, said -bed being sprayed in a controlled manner with moistening water so that the spraying water and the gaseous flow to be purified move in the same direction of flow. 12. Plant for the purification treatment of gaseous flows according to Claim 11, characterized in that said thermophile microorganisms are chosen from among: Bacillus smithii; Clostridium thermocellum Rhizomucor pusillus; Chaetomium thermophilum Myceliophthora thermophila; Thermoascus aurantiacus Bacillus schlegelii; Geobacillus thermoglucosidasius Methanoculleus thermophilus; Scytalidium thermophilum Thermithiobacillus tepidarius; _# Methanothrix thermophila; Bacillus thermocloacae; Desulfotomaculum thermoacetoxidans; Desulfotomaculum thermobenzoicum Brevibacillus brevis; Brevibacillus borstelensis Brevibacillus agri ; Hydrogenobacter thermophilus Pseudomonas sp; Aquifex pyrophilus; Clostridium thermocellum; Alicyclobacillus sp; Chlorobium tepidum Thermocrinis ruber; Thermacetogenium phaeum Caloramator coolhaasii; Thermotoga lettingae Sphaerobacter thermophilus; Clostridium sphenoides Methylobacterium extorquens ; Pseudomonas Methanosarcina thermophila DSM 1825; Pseudomonas rhodos; Methanosarcina thermophila; Thermotoga lettingae; Pseudomonas fluorescens; Pseudomonas putida; Acidomonas methanolica; Staphylococcus sp; Coliform Enteritidis; Coliform Freundi ; enterobacteria such as Extorquens; Aspergillus Fumigatus . 13. Plant for the purification treatment of gaseous flows according to Claim 11, characterized in that it comprises two filter beds (20, 24) operating in series and passed through in succession by the gaseous flow which is made to flow in the same direction as the said spraying water, the latter being supplied only to the first bed. 14. Plant for the purification treatment of gaseous flows according to Claim 11, characterized in that said spraying water is supplied to said first filtration bed at a rate of 5 litres/hour per square metre of filter bed. 15. Plant for the purification treatment of gaseous flows according to Claim 11, characterized in that it comprises means for supplying in atomised form to said gaseous flow to be purified, before it reaches said first filter bed, substances in solution form with a nutrient effect for the thermophile microorganisms forming said colonies . 16. Plant for the purification treatment of gaseous flows according to Claim 15, characterized in that phosphorus and/or potassium and/or trace elements are added to said nutrient substances .