Modified ceramsite packing useful for biomembrane trickling filter and a process for removing SO2 from flue gas using the trickling filter

Abstract

A process of removing SO.sub.2 from a flue gas with a trickling filter using modified ceramsite packing is described. The biological flue gas desulfurization process includes: feeding the flue gas containing sulfur dioxide through the column bottom into the biomembrane trickling filter at certain temperature, contacting with the modified ceramsite biomembrane packing and purifying, which purified flue gas is discharged via the column top; and spraying the nutrient fluid rich in high concentration of the desulfurization strain through the top to the modified ceramsite biomembrane packing, thereby the sulfur-bearing pollution source in the flue gas is degraded, so as to discharge a purified flue gas satisfying the environmental requirements.

Claims

1. A process of removing SO.sub.2 from flue gas comprising utilizing a biomembrane trickling filter comprising a packing, in which the biomembrane trickling filter has a principle section of a packed column and further comprises a liquid circulation system and a gas circulation system, wherein the packing is a modified ceramsite obtained from chemically modifying a ceramsite, wherein the modified ceramsite has performance parameters of: a density of about 2.1-about 2.5 g/cm.sup.3, a specific surface area of about 0.4-about 0.8 m.sup.2/g, a porosity of about 0.5-about 0.7, a pH at isoelectric point of about 7.5-about 9.0, and a surface pH of about 5.0-about 6.0.

2. The process according to claim 1, wherein a desulfurization strain is introduced into a nutrient fluid after the chemical modification of the packing, so as to carry out a biomembrane colonization for the packing in the column, and to form a biomembrane of the desulfurization strain on a packing surface.

3. The process according to claim 2, comprising a counter-current operation manner in the column, wherein the flue gas containing SO.sub.2 is fed through the column bottom into the biomembrane trickling filter, contacted with the modified ceramsite biomembrane packing and purified, which purified flue gas is discharged via the column top; and the nutrient fluid rich of the desulfurization strain is sprayed through the column top to the modified ceramsite biomembrane packing, passed through the packing layer and discharged via the column bottom, wherein a ratio of nutrient fluid:bacteria-containing fluid is 50-100:1.

4. The process according to claim 1, wherein the process for chemically modifying the ceramsite packing comprises: drying a cleaned ceramsite; impregnating with a solution of an acid, and washing with water to be neutral; ultrasonic impregnating the neutral ceramsite obtained into an aqueous solution of modifier and baking; calcinating at a temperature of 300 degrees C. to 700 degrees C., cooling to room temperature, washing with water, and baking, so as to obtain the modified ceramsite packing.

5. The process according to claim 1, comprising a membrane-forming step, wherein the membrane-forming step in the column comprises: introducing the desulfurization strain into the nutrient fluid, and spraying the nutrient fluid containing the desulfurization strain in the biomembrane trickling filter downward to the packing, so as to immobilize the desulfurization strain to the packing, and adding dominant bacteria periodically, wherein a ratio of nutrient fluid:bacteria-containing fluid is 50-100:1.

6. The process according to claim 1, wherein the nutrient fluid consists of an aqueous mixture of two or more of ammonium sulfate, potassium chloride, potassium hydrogen phosphate, magnesium sulfate heptahydrate, calcium nitrate, ammonium chloride, ammonium dihydrogen phosphate, calcium chloride and iron(II) sulfate heptahydrate.

7. The process according to claim 2, wherein the desulfurization strain is one or more selected from the group consisting of Thiobacillus ferroxidans, Thiobacillus thioparus, thiobacillus denitrificans, desulfovibrio, Leptospirillums ferrooxidans, thiobacillus thiooxidans and Thiothrix.

8. The process according to claim 1, wherein the biomembrane trickling filter is constructed as a multi-stage packing column.

9. The process according to claim 1, wherein the temperature during the desulfurization in the biomembrane trickling filter is controlled to be 20-40 degrees C.

10. The process according to claim 4, wherein the modifier is an aqueous solution of iron chloride, ferric nitrate, aluminum sulfate or aluminum chloride.

11. The process according to claim 9, wherein the temperature during the desulfurization in the biomembrane trickling filter is controlled to be 25-35 degrees C.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a flow chart showing the process according to the Examples of the present invention.

(2) In the FIG. 1, 1 denotes an raw material gas inlet, 2 denotes an nutrient fluid outlet, 3-1, 3-2, 3-3 denote respectively the nutrient fluid spraying inlets 1, 2 and 3, 4 denotes an purified gas outlet, 5 denotes a gas distributor, 6 denotes a demister, 7 denotes a circulating liquid store tank, 8 denotes a circulating liquid pump, 9 denotes a nutrient fluid inlet, 10 denotes an effluent outlet, 11 denotes the modified ceramsite packing, and 12 denotes the biomembrane trickling filter.

(3) FIG. 2 shows the influence by the height of the packing on the desulfurization efficiency from the Examples.

EMBODIMENTS

(4) The present invention will be illustrated in more detail with Examples and referring to the figures below.

(5) As shown in FIG. 1, the present invention is achieved by using a biomembrane trickling filter, operated in the form of a gas-liquid phase counter-current operation. A flue gas containing sulfur dioxide is fed via the raw material gas inlet 1, and introduced into the biomembrane trickling filter 12 through the gas distributor 5, then the purified flue gas is discharged via the purified gas outlet 4 at the column top through demister 6. The nutrient fluid from the circulating liquid store tank 7 is fed via the nutrient fluid spraying inlet 3-1 (at 300 mm height for a one-stage packing layer) or 3-2 (at 600 mm height for a two-stage packing layer) or 3-3 (at 900 mm height for a three-stage packing layer) through the circulating liquid pump 8, flowed downward through the modified ceramsite packing 11, and returned finally into the circulating liquid store tank 7 via the nutrient fluid outlet at the column bottom. In addition, a part of the nutrient fluid effluent is discharged periodically via the effluent outlet 10, and fresh nutrient fluid is supplied to the circulating liquid store tank 7 via the nutrient fluid inlet 9.

EXAMPLES

Materials

(6) The conventional ceramsite: from Ping Xiang Feiyun Ceramics Industries Co., Ltd., Jiangxi, China, which was in the form of a sphere in a particle diameter of Φ3-5 mm, having performance parameters of: a density of 2.041 g/cm.sup.3, a specific surface area of 0.35 m.sup.2/g, a porosity of 0.55, a pH value at isoelectric point of 1.50, and a surface pH value of 6.96.

(7) The flue gas containing sulfur dioxide to be treated: the flue gas containing sulfur dioxide being a simulating flue gas formulated with SO.sub.2, N.sub.2 and air, wherein the concentration of SO.sub.2 was controlled to be 1000-10000 mg/m.sup.3, and the feeding flow of the flue gas was 30 L/h-300 L/h.

(8) The nutrient fluid rich in high concentration of the desulfurization strain:nutrient fluid:bacteria-containing fluid=50-100:1, in which the nutrient fluid consisted of an aqueous mixture of two or more of ammonium sulfate, potassium chloride, potassium hydrogen phosphate, MgSO.sub.4.7H.sub.2O, calcium nitrate, ammonium chloride, ammonium dihydrogen phosphate, calcium chloride and FeSO.sub.4.7H.sub.2O. The desulfurization strain was one or more selected from the group consisting of Thiobacillus ferroxidans, Thiobacillus thioparus, thiobacillus denitrificans, desulfovibrio, Leptospirillums ferrooxidans, thiobacillus thiooxidans or Thiothrix.

(9) The biomembrane trickling filter: a counter-flow type reactor, made from plexiglass, consisting of a packing column, a gas circulation system and a liquid circulation system, plus with a circulating water sheath outside for the temperature control. The bio-trickling filter had an inner diameter of 80 mm, a height of 1200 mm being divided into three layers with a packing height of 300 mm for each layer, loaded with a unmodified or modified ceramsite packing. In addition to the gas inlet, gas outlet, liquid inlet and liquid outlet, a liquid distributor was placed between every two layers of packing, a liquid spraying device was placed above every packing layer, and a demister was further placed at the column top.

Example 1

(10) A conventional ceramsite was chemically modified as follows: (1) the conventional ceramsite was rinsed repeatedly with distilled water to be clean, and baked for 2 hours; (2) impregnated with a 10% solution of H.sub.2SO.sub.4, and washed with water to be neutral; (3) placed in an oven at a temperature of 105 degrees C. for 2 hours of drying; (4) the baked haydite was introduced into the modifier before cooling, for 20 hours of ultrasonic impregnation; (5) placed in an oven at a temperature of 105 degrees C. for 2 hours of drying; (6) the ultrasonicaly impregnated and baked ceramsite was placed immediately into a muffle furnace, calcinated at a temperature of 300 degrees C. for 1 hour, and cooled to room temperature; (7) rinsed with distilled water to be clean, and baked for 2 hours, to obtain the modified ceramsite packing. The modified ceramsite had performance parameters of: a density of 2.1 g/cm.sup.3, a specific surface area of 0.4 m.sup.2/g, a porosity of 0.5, a pH value at isoelectric point of 7.5, and a surface pH value of 6.0.

(11) In order to make a qualitative analysis with the modified ceramsite, an X-ray powder method was used to analyze the coating composition of the modified ceramsite. The testing result showed that the principal ingredient of the oxidized film on the modified ceramsite surface was an oxide of iron, namely a hematite, and alpha-Fe.sub.2O.sub.3 with favorable adsorbability was present on the surface of the surface-modified ceramsite. The result by scanning electron microscope (SEM) showed that there were spheric deposits on the ceramsite surface, which were aligned homogeneously and thickly, representing the successful adhesion of the iron compound on the surface.

(12) As the amount of the surface coating would affect the desulfurizing ability of the modified ceramsite, unmodified ceramsites with an average weight of about 10.0000 g were weighed accurately with an analytical balance, resulting in coating-modified ceramsites with an average weight of 10.4146 g, such that the coating amount of the ceramsite was 0.4146 g, and the coating amount per unit weight was 41.46 mg/g.

(13) The modified ceramsite was used as the packing to prepare for a biomembrane with membrane-forming process in the column. Nutrient fluid rich in high concentration of desulfurization strain (nutrient fluid:bacteria-containing fluid=50:1) were injected respectively into the bio-trickling filter. The biomembrane colonization of the modified ceramsite was carried out for 12 days, resulting in a maximal biomass of 1.457 mg/g.

(14) The desulfurization rate was measured at 97.8%, using the modified ceramsite as the packing, having a temperature of 30 degrees C., a gas flow of 180 L/h, a SO.sub.2 concentration of 2000 mg/m.sup.3, a packing height of 900 mm, a Fe.sup.3+ concentration of 0.8 g/L, an initial pH value of 1.8 and a liquid spraying rate of 6 L/h in the biomembrane trickling filter.

Example 2

(15) A conventional ceramsite was chemically modified as follows: (1) the conventional ceramsite was rinsed repeatedly with distilled water to be clean, and baked for 4 hours; (2) impregnated with a 30% solution of H.sub.2SO.sub.4, and washed with water to be neutral; (3) placed in an oven at a temperature of 105 degrees C. for 4 hours of drying; (4) the baked haydite was introduced into the modifier comprising a 1.5 mol/L of FeCl.sub.3 aqueous solution before cooling, for 40 hours of ultrasonic impregnation; (5) placed in an oven at a temperature of 105 degrees C. for 4 hours of drying; (6) the ultrasonicaly impregnated and baked ceramsite was placed immediately into a muffle furnace, calcinated at a temperature of 700 degrees C. for 5 hours, and cooled to room temperature; (7) rinsed with distilled water to be clean, and baked for 4 hours, to obtain the modified ceramsite packing.

(16) The modified ceramsite had performance parameters of: a density of 2.5 g/cm.sup.3, a specific surface area of 0.8 m.sup.2/g, a porosity of 0.7, a pH value at isoelectric point of 9.0, and a surface pH value of 5.0.

(17) In order to make a qualitative analysis on the modified ceramsite, an X-ray powder method was used to analyze the coating composition of the modified ceramsite. The testing result showed that the principal ingredient of the oxidized film on the modified ceramsite surface was an oxide of iron, namely a hematite, and alpha-Fe.sub.2O.sub.3 with favorable adsorbability was present on the surface of the surface-modified ceramsite. The result by scanning electron microscope (SEM) showed that there were spheric deposits on the ceramsite surface, which were aligned homogeneously and thickly, representing the successful adhesion of the iron compound on the surface.

(18) As the amount of the surface coating would affect the desulfurizing ability of the modified ceramsite, unmodified ceramsites with an average weight of about 10.0000 g were weighed accurately with an analytical balance, resulting in coating modified ceramsites with an average weight of 10.5832 g, such that the coating amount of the ceramsite was 0.5832 g, and the coating amount per unit weight was 58.32 mg/g.

(19) The modified ceramsite was used as the packing to prepare a biomembrane with biomembrane-colonizing process within the column. The biomembrane colonization of the modified ceramsite was carried out for 8 days, resulting in a maximal biomass of 1.587 mg/g.

(20) The desulfurization rate was measured at 100%, using the modified ceramsite as the packing, having a temperature of 30 degrees C., a gas flow of 180 L/h, a SO.sub.2 concentration of 2000 mg/m.sup.3, a packing height of 900 mm, a Fe.sup.3+ concentration of 0.8 g/L, an initial pH value of 1.8 and a liquid spraying rate of 6 L/h in the biomembrane trickling filtration column.

Example 3

(21) A toconventional ceramsite was chemically modified to have performance parameters of: a density of 2.238 g/cm.sup.3, a specific surface area of 0.72 m.sup.2/g, a porosity of 0.60, a pH at isoelectric point of 8.50, and a surface pH of 5.63.

(22) The modified ceramsite was used as the packing to prepare a biomembrane with biomembrane colonization process in the column. The biomembrane colonization of the modified ceramsite was carried out for 10 days, resulting in a maximal biomass of 1.543 mg/g.

(23) The desulfurization rate was measured at 98.4%, using the modified ceramsite as the packing, having a temperature of 28 degrees C., a gas flow of 180 L/h, a SO.sub.2 concentration of 2000 mg/m.sup.3, a packing height of 900 mm, a Fe.sup.3+ concentration of 0.8 g/L, an initial pH value of 1.8 and a liquid spraying rate of 6 L/h in the biomembrane trickling filtration column.

Comparative Example 1

(24) An unmodified ceramsite was used as the packing, other operation conditions being same as example 3.

(25) The original ceramsite had performance parameters of: a density of 2.041 g/cm.sup.3, a specific surface area of 0.35 m.sup.2/g, a porosity of 0.55, a pH at isoelectric point of 1.50, and a surface pH of 6.96.

(26) The unmodified ceramsite was used as the packing to prepare a biomembrane with biomembrane colonization process in the column. The biomembrane colonization of the modified ceramsite was carried out for 16 days, resulting in a maximal biomass of 1.263 mg/g. Therefore, compared with the biomembrane colonization using the modified ceramsite as packing, it was found that the modified ceramsite packing had the advantages of a shorter duration for biomembrane colonization, more bio-loading and the like.

(27) The desulfurization rate was measured at 75.2%, using the unmodified ceramsite as the packing in the biomembrane trickling filter.

Example 4-1

(28) A conventional ceramsite was chemically modified to have performance parameters of: a density of 2.238 g/cm.sup.3, a specific surface area of 0.72 m.sup.2/g, a porosity of 0.60, a pH value at isoelectric point of 8.50, and a surface pH value of 5.63.

(29) The modified ceramsite was used as the packing to prepare a biomembrane with membrane-forming process in the column. The biomembrane colonization of the modified ceramsite was carried out for 10 days, resulting in a maximal biomass of 1.543 mg/g.

(30) The desulfurization rate was measured at 82.6%, using the modified ceramsite as the packing, having a temperature of 20 degrees C., a gas flow of 180 L/h, a SO.sub.2 concentration of 2000 mg/m.sup.3, a packing height of 900 mm, a Fe.sup.3+ concentration of 0.8 g/L, an initial pH value of 1.8 and a liquid spraying rate of 6 L/h in the biomembrane trickling filtration column.

Example 4-2

(31) The process according to example 4-1 was repeated to carry out a desulfurization test with the biomembrane trickling filter under same operation conditions except for a temperature of 25 degrees C. The desulfurization rate measured was 97.3%.

Example 4-3

(32) The process according to example 4-1 was repeated to carry out a desulfurization test with the biomembrane trickling filter under same operation conditions except for a temperature of 35 degrees C. The desulfurization rate measured was 94.4%.

Example 4-4

(33) The process according to example 4-1 was repeated to carry out a desulfurization test with the biomembrane trickling filter under same operation conditions except for a temperature of 40 degrees C. The desulfurization rate measured was 81.9%.

(34) TABLE-US-00001 TABLE 1 the relationship between temperature and the desulfurization rate Example Temperature/degrees C. Desulfurization rate/% 4-1 20 82.6 4-2 25 97.3 4-3 35 94.4 4-4 40 81.9

Example 5

(35) The desulfurization rates resulted from various packing heights were measured by varying the packing heights, using the modified ceramsite as the packing, with a temperature of 28 degrees C., a gas flow of 180 L/h, a feeding concentration of SO.sub.2 ranging from 500 to 3000 mg/m.sup.3, a Fe.sup.3+ concentration of 0.8 g/L, an initial pH value of 1.8 and a liquid spraying rate of 6 L/h in the biomembrane trickling filter, and the results were showed in FIG. 2.

(36) The testing results from FIG. 2 showed: the source of the flue gas and the desulfurization rate affected the selection on the packing height. The packing height affected directly the energy consumption. When the concentration of the feeding gas was 500 mg/m.sup.3, one stage of packing (with a packing height of 300 mm) resulted in a satisfactory desulfurization effect, such that the nutrient fluid was fed at the liquid inlet 3-1; when the concentration of the feeding gas was 1000 mg/m.sup.3, two stages of packing (with a packing height of 600 mm) resulted in a satisfactory desulfurization effect, such that the nutrient fluid was fed at the liquid inlet 3-2; and when the concentration of the feeding gas was 2000 mg/m.sup.3, three stages of packing (with a packing height of 900 mm) resulted in a satisfactory desulfurization effect, such that the nutrient fluid was fed at the liquid inlet 3-3.

(37) It could be seen by comparing the results from the examples and the comparative examples that, the modified ceramsite as packing increased the specific surface area and hydrophility thereof, so as to enhance the bio-capacity per unit volume of packing; and increased the porosity thereof, so as to enlarge the interface between the gas and the organism, to improve the mass transfer rate of contaminants in the biomembrane trickling filter, and to enhance the desulfurization efficiency. The present invention had the advantages including green environmental protection, high purification performance, low investment cost, low operation cost, high operability, and the like. Therefore, the present invention can be used to remove sulfur dioxide from coal-fired flue gas with high efficiency and low consumption, and thus has favorable economic benefit and social efficiency.