Lignosulfonate-modified calcium hydroxide for SO2 control during furnace injection

Abstract: A method is provided for removing sulphur-containing gases from fossil fuel-fired combustors comprising the step of introducing sorbent particles consisting of calciumhydroxide or calcium-magnesiumhydroxide modified with a ionic surfactant like calcium lignosulfonate into the exhaust gases. It is also discribed the method of preparing those particles

Description: The present invention is directed to a method of reducing agglomeration and enhancing the handling characteristics of calcium hydroxide or calcium magnesium hydroxide particles which are suitable for use in removing sulphur-containing gases from the exhaust gases of a fossil fuel-fired combustor. The method of the invention produces lignosulfonate-modified calcium hydroxide particles. In particular, the invention provides for removing sulphur-containing gases by introducing these particles downstream of the flame zone of the combustor. Relatively costly methods exist for control of sulphur oxide pollution from fossil fuel-fired combustors, but they are most commonly applied to new facilities where the expense and control of the technology is justifiable due to the long remaining depreciable lifetime of the facility. There is a large population of older combustors in existence for which the cost-effective methods of reducing sulphur oxide emissions are not generally available due to the substantial additional capital expense which would need to be expended on an old facility. There is a need, therefore, to develop cost-efficient, retrofittable technology to control sulphur oxides and nitrogen oxides in this population of older plants. In other methods, inorganic promoters have been added to each of the calcium-based sorbents in an effort to increase their reactivity. Promoters can increase reactivity significantly but each group of effective promoters has problems which preclude it from practical application. Transition metals such as chromium are effective in various forms but they are relatively expensive and may produce environmental side effects of their own. Alkali metal additives are aggressive promoters during natural gas-firing, but are condensed on the ash which is produced during coal-firing and are unavailable to promote SO₂ capture by the sorbent. There remains a need in the art, therefore, for a sorbent with an improved capability of capturing SO₂ that will not be depleted by ash interactions in a coal-fired combustor, and which does not introduce detrimental environmental effects. We have now developed a method for removing sulphur-containing gases, particularly sulphur oxides, from the exhausts of fossil fuel-fired combustors and a method of producing an improved sorbent for removing sulphur oxides from the exhaust gases of fossil fuel-fired combustors which sorbent is adapted to be injected into, or downstream from the combustor. The improved sorbent has advantageous handling characteristics. In accordance with one aspect of the present invention there is provided a method of reducing agglomeration and enhancing the handling characteristics of calcium hydroxide or calcium magnesium hydroxide particles comprising the step of hydrating calcium oxide or calcium magnesium oxide respectively with water containing a sufficient amount of an alkaline earth metal lignosulfonate to result in the production of the respective hydroxide dry particles containing up to 5% by weight of the alkaline earth metal lignosulfonate. The method of the invention provides lignosulfonate-modified calcium hydroxide sorbent particles containing up to 5% by weight calcium lignosulfonate and having a finer particle size distribution than a corresponding non-surfactant modified calcium hydroxide sorbent. Sorbent particles comprising a lignosulphonate and calcium hydroxide and/or calcium magnesium hydroxide form the subject matter of a divisional European Patent application no 93202494.6. In accordance with a second aspect of the invention provides for use of the above-described particles in a method for removing sulphur-containing gases from exhaust gases of fossil fuel-fired combustors by introducing into the exhaust gases the sorbent particles which particles are capable of reacting with sulphur oxides. The sorbent particles preferably comprise 1 to 2 percent by weight calcium lignosulfonate and are characterized by a significantly finer particle size distribution than the equivalent sorbent without lignosulfonate. In the practice of the method according to the first aspect of the invention lignosulfonate-modified sorbent particles result. The surfactant is an alkaline earth metal lignosulfonate. The sorbent is selected from the group consisting of calcium magnesium hydroxide [Ca(OH)₂·Mg(OH)₂] and calcium hydroxide [Ca(OH)₂]. Initially, limestone (CaCO₃) or dolomite [CaMg(CO₃)₂] is calcined to its respective oxide (CaO or CaO·MgO), usually in the temperature range of 900 to 1100°C (lime temperature) but preferably at around 1000°C. This produces a soft-burned lime characteristic of that available at commercial lime plants. The resulting oxide is then hydrated at a water to oxide ratio ranging from 2 to 3 but preferably around 2.6, with water containing the desired amount of calcium lignosulfonate. The amount of calcium lignosulfonate utilized will be that sufficient to produce a resulting calcium hydroxide/calcium lignosulfonate product containing up to about 5 percent by weight calcium lignosulfonate. Preferably, the resulting product will contain about 1 to 2 percent calcium lignosulfonate. The calcium lignosulfonate-modified calcium hydroxide sorbent particles made in accordance with the present invention result in a higher capture of SO₂ than an unmodified calcium hydroxide when injected into a fossil fuel-fired combustor or combustor flue system. Typically the modified sorbent particles will be injected into the flue where the exhaust gases are at a temperature in the range of about 65 to 1230°C. Since the improved performance is the result of a physical rather than a chemical change, the tendency of coal ash to remove the enhancement is avoided. The lignosulfonate-modified sorbent particles are also improved in that they have improved handling characteristics. Commercial hydrates frequently have the tendencies to cake and agglomerate during storage and handling which cause problems at their point of use. The lignosulfonate-modified sorbent particles made in accordance with the present invention are characterized by a reduced amount of caking during storage and reduced amount of agglomeration during handling, thus facilitating handling and controllability of fluidization during feeding to the reactor or combustor. While not intending to be bound to any particular theory of operation of the present invention, it is believed that the lignosulfonate-modified sorbents made in accordance with the present invention exhibit these improved properties, at least in part, because of formation of smaller calcium hydroxide particles. Smaller particles are formed in two ways: (1) during the hydration reaction, surfactant (lignosulfonate) decreases the surface energy of the nucleus/solution interface (surface tension) and thereby increases the nucleation rate. This results in a large number of small crystals rather than a small number of large crystals; (2) the tendency of small calcium hydroxide crystals to form larger particles through agglomeration is reduced by eliminating the layer of adsorbed water which surrounds such crystals. This is accomplished by introducing a hydrophobic surfactant layer around the crystals. Thus, both an increase in nucleation rate during the hydration reaction, and a decrease in adsorbed water by enhanced hydrophobicity of crystal surfaces are effected by a small quantity of lignosulfonate. The present invention is described with reference to the following Examples and the accompanying drawings, in which: Fig. 1 is a graph of the amount of increase in calcium utilization (over unmodified control sorbent) in the sorbent utilized in accordance with the present invention versus the amount of lignosulfonate concentration in the sorbent. Fig. 2 is a graph showing the comparison in the amounts of calcium utilization versus the calcium/sulphur ratio in two sorbents, with and without the improvement according to the present invention. EXAMPLE 1 A commercially available limestone, Presque Isle, was calcined to calcium oxide at 1000°C for 16 hours. A chemical analysis of the resulting lime showed no significant quantities of known inorganic promoters. The resuslting calcium oxide was then hydrated at a water to calcium oxide ratio of 2.6 with water containing varying amounts of calcium lignosulfonate. The amounts of the calcium lignosulfonate surfactant in the water were calculated to yield 0, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, and 5.0% of calcium lignosulfonate in the product calcium hydroxide. The modified calcium hydroxides were then tested for reactivity with sulphur dioxide in a laboratory scale isothermal flow reactor in which the reactor temperature was held at 1000°C, the residence time for the reaction was 1 second, and the sulphur dioxide concentration was 3000 ppm. These are conditions within the range of those which would be encountered during downstream injection of a sorbent for SO₂ capture in a coal-fired boiler. The increased calcium utilization by the sorbent-containing calcium lignosulfonate over the control sorbent (containing no surfactant) are shown in Fig. 1. At the optimum calcium lignosulfonate concentrations in the hydroxide (1.5 to 2.0 weight percent), calcium utilization increases of approximately 5% absolute (20 to 25% relative) is achieved. With calcium to sulphur ratio of 2 an SO₂ capture of over 60% is calculated. EXAMPLE 2 Two sorbents were produced for testing in a pilot-scale, coal-fired combustor. The control sorbent was commercial Longview calcium hydroxide. The modified sorbent was made from commercial Longview calcium oxide, experimentally hydrated with water containing calcium lignosulfonate to produce a hydroxide containing 1% calcium lignosulfonate. The sorbent was then injected into a 10⁶ Btu, coal-fired furnace at 1250°C. The resulting calcium utilizations of the control and modified sorbents are shown in Fig. 2. A 20 to 25% relative increase in calcium utilization by the modified sorbent is shown.

Claims: A method of reducing agglomeration and enhancing the handling characteristics of calcium hydroxide or calcium magnesium hydroxide particles comprising the step of hydrating calcium oxide or calcium magnesium oxide respectively with water containing a sufficient amount of an alkaline earth metal lignosulfonate to result in the production of the respective hydroxide dry particles containing up to 5% by weight of the alkaline earth metal lignosulfonate. A method as claimed in claim 1 wherein the particles comprise calcium hydroxide and the alkaline earth metal consists of calcium lignosulfonate. A method as claimed in claim 2 wherein in the step of hydrating the calcium oxide the water to calcium oxide ratio is in the range of from 2 to 3 to produce lignosulfonate-modified calcium hydroxide sorbent particles. A method as claimed in claims 2 and 3 wherein the dry hydroxide particles contain from 1 to 2% by weight calcium lignosulfonate. Use of the calcium hydroxide sorbent particles produced according to the method of claim 3 in a method for removing sulphur containing gases from the exhaust gases of a fossil fuel-fired combustor said method comprising the step of introducing into the exhaust gases said calcium hydroxide sorbent particles. Use as claimed in claim 5 wherein the particles are introduced into the exhaust gases at a temperature in the range of from 65 to 1230 °C.

Inventors: KIRCHGESSNER DAVID A, LORRAIN JEFFREY M, KIRCHGESSNER, DAVID A., LORRAIN, JEFFREY M.
Applicants: US ENVIRONMENT, UNITED STATES ENVIRONMENTAL PROTECTION AGENCY AN AGENCY OF THE UNITED STATES GOVERNMENT
Application year: 1987
Main classification: B01D53
IPC codes: B01D53, B01J20