The problem of sulfur oxides has challenged combustion plant operators and regulators since there became an awareness of the harmful effects of acid rain. Sulfur oxides are formed during the combustion of sulfur-containing carbonaceous fuels and are referred to generally as SOx while comprising sulfur dioxide (SO2) and sulfur trioxide (SO3). The vast majority of SOx is present as SO2. The SO3 (as H2SO4) can, however, add to particulates emitted and can cause cold end corrosion. Accordingly, an effective system must address both SO2 and SO3. Ideally, the process should also address the problem of hydrochloric acid (HCl).
The art has provided a wide range of technologies. As a group, they can be called flue gas desulfurization technologies, FGD. See, for example, Srivastava, Ravi K.; Controlling SO2 Emissions: A Review of Technologies; EPA/600/R-00/093, November 2000. These include both wet and dry technologies and can employ existing equipment, such as duct work, or provide separate reactors.
According to Srivastava, FGD technologies fall into two main categories: (1) once-through and (2) regenerable. In the former, the sorbent is discarded after use; and in the latter, the sorbent is regenerated after it has sorbed SO2.
In once-through processes, sorbed SO2 is bound by the sorbent and the sorbent is considered spent. The spent sorbents can be disposed of or recovered as a useful by-product, like gypsum, depending on quality and market factors.
Technologies considered regenerable can treat the sorbents to release the SO2 and obtain useful products. After regeneration, the sorbent can be recycled for additional SO2 scrubbing.
Each of the once-through and regenerable technologies can be further broken down as wet or dry. Wet processes produce a wet slurry waste or by-product, and scrubbed flue gas is saturated with water. The dry processes produce dry waste material, and scrubbed flue gas is not saturated.
The reader is referred to Srivastava, supra, for a closer view of the various technologies, where the authors group major FGD technologies into three major categories: (1) Wet FGD (composed of once-through wet FGD), (2) Dry FGD (composed of once-through dry FGD) and (3) Regenerable FGD (composed of wet and dry regenerable FGD)
The wet FGD processes can employ wet scrubbers, which typically employ large towers that cause contact between combustion flue gases and a slurry of calcium carbonate or the like that is sprayed countercurrently to the flue gas flow. Suitable chemical slurries can include calcium carbonate (limestone), lime (CaO in slurry as Ca (OH)2), trona (sodium sesquicarbonate), sodium bicarbonate, dolomite, and the like, or blends of these materials. In limestone-based scrubbers, the SOx is captured to form CaSO3, which is naturally oxidized in part or overtly oxidized to form gypsum (CaSO4), which can be used commercially. Reaction between the SOx and the sorbent occurs in the liquid phase in a stirred tank over considerable time periods. Fuels high in chlorides will alter the chemical equilibrium in the liquid and can adversely affect scrubber efficiency. Quality and market conditions will dictate the value and fate of the spent sorbent. These wet scrubbers are expensive to install and operate and cannot be easily adapted to all plants.
The dry processes can introduce these same type of chemicals, either dry or as slurries that rapidly dry, into a flue gas stream in the furnace, a separate reactor or a duct or other passage carrying the flue gas, wherein the SOx is captured to some extent and can be disposed of in dry particulate form.
In one group of dry processes, a slurry is sprayed into a separate reactor—adapted from industrial spray driers—to cause intimate contact with the flue gases for moderate reaction times, e.g., ten seconds or more. These processes are quite effective, while not as effective as the wet scrubbers. They, however, are also capital intensive but cannot provide the high quality gypsum achievable by wet scrubbers.
In in-furnace sorbent injection, a dry sorbent is injected directly into the furnace in the optimum temperature region above the flame. As a result of the high temperature (e.g., on the order of 2000° F.), sorbent particles (e.g., often calcium hydroxide or calcium carbonate) decompose and become porous solids with high surface systems. Residence time is very short, on the order of a few seconds, and the sorbent particles are easily fouled before the chemical is fully utilized.
In-duct sorbent injection, like in-furnace sorbent injection, involves direct injection of sorbent into SOx-containing gases. In these processes, the sorbent is introduced into a flue gas duct, but in contrast to spray drying, contact is made without the advantage of a large reaction vessel as used in spray dryers, and suffers from greatly diminished contact times, e.g., often only a few seconds. In-duct injection, typically uses an alkali metal or alkaline earth oxide or hydroxide, like trona, sodium carbonate, calcium hydroxide, magnesium hydroxide, dolomite, or the like, as outlined by Srivastava, supra, and U.S. Pat. No. 5,658,547 to Michalak, et al. U.S. Pat. No. 5,492,685 to Moran describes a hydrated lime having high surface area and small particle size prepared by hydrating lime with an aqueous hydration solution of an organic solvent, and preferably washing the resulting hydrate with an aqueous solution of an organic solvent prior to drying. The high surface area hydrates (e.g., up to 85 m2/g) are sorbents for SO2 removal from gas streams.
U.S. Pat. No. 5,658,547 to Michalak, et al., describes removing SOx and particulates from the combustion gases of a large boiler. In a primary treatment zone, a slurry comprising an alkaline SOx-reducing composition and preferably a nitrogen-containing composition effective to reduce NOx, is introduced into combustion gases at a temperature of from about 900° to about 1300° C. (about 165° to about 2375° F.). The gases are cooled by initial contact with steam-generating means, and then by contact with a gas-to-gas heat exchanger. Cooled gases are then subjected to a secondary treatment in which they are first humidified and further cooled by introduction of a water spray or aerosol to reduce the temperature to 100° C. (212° F.) or below. Contact between the SOx-reducing composition and the humidified gas is maintained for a reaction period of at least two seconds. Particulate solids are then separated from the gases with a fabric filter. The cleaned gases are reheated by the gas-to-gas heat exchanger prior to discharge to the atmosphere.
These processes require feeding large quantities of these SOx-reducing reagents, whether to the furnace or to back end duct work, and add significant solids to ash capture equipment and in some cases can degrade performance and cause operating and handling problems under certain conditions. There remains a need for a dry scrubbing process that can increase the sorbent utilization and removal efficiencies.
Other dry processes can include fluidized beds that provide longer reaction times. These processes are typically engineered to recirulate the sorbent for multiple passes with the combustion gases to enhance economy by increasing utilization of the sorbent. The sorbents for these processes are intended for recycling and are, therefore, more expensive to make and handle.
An example of these latter types of processes is seen from U.S. Pat. No. 4,755,499 to Neal, et al., which describes sorbents that are intended to be resistant to normal physical degradation which results from recurring adsorption and regeneration for use in a fluidized bed absorber. The sorbent is constructed of (a) an alumina substrate having a specified pore volume and (b) an alkali or alkaline earth component in defined amount relative to the substrate. Minor amounts of other metallic oxides can also be employed. The sorbents are manufactured to be regenerable and attrition resistant. They can be regenerated by heating in an inert atmosphere at temperatures up to about 350° C. and then reused.
In a related disclosure, U.S. Pat. No. 6,281,164, Demmel, et al., teach that the useful life of SOx additives having a SO2 to SO3 oxidation catalyst component and a SO3 absorption component can be extended by employing each of these components as separate and distinct physical particles or pellets. The particles are prepared by spray drying or desiccation followed by calcination to produce microspheroidal particles having a range of sizes such that essentially all such particles will be retained by a Standard U.S. 200 mesh screen and essentially all particles will be passed by a Standard U.S. 60 mesh screen. Processing to reduce SOx entails capturing the SOx on the particles and then regenerating the particles for reuse. These particles are too expensive for once-through processes and are, in fact, too large to achieve good utilization in those processes.
Another example of regenerable sorbents is found in U.S. Pat. No. 5,114,898 to Pinnavaia, et al., which describes processes for removing noxious sulfur oxides from gas streams, particularly from flue gases of coal-burning power plants, using heated layered double hydroxide (LDH) sorbents. The sorbent compositions contain metal components, incorporated into the sorbents either by isomorphous replacement of all or part of M11 and/or M111 ions (the patent defining M11 as a divalent metal and M111 as a trivalent metal) in layers of LDH structures or by impregnation as a metal salt, to promote the oxidation of sulfur dioxide.
In another related teaching, U.S. Pat. No. 5,520,898 to Pinnavaia, et al., describes the use of base/clay composite materials as sorbents for the removal of SOx from flue gas streams. The composite contains a smectite clay and a sorbent component, such as alkaline earth metal hydroxides and carbonates, and a metal oxide or metal oxide precursor, preferably selected from transition metal ions. The smectite-type clays are said to serve as supports for the reactive base and as a dispersing agent for improved reactivities. The swelling properties of smectite clays are said to be responsible for higher reactivity of the sorbents. The injection of the sorbents into these, particularly to the boiler (700°-1000° C.), along with coal was considered.
There is a present need for technology that can improve on the capture of SO2 and/or HCl in high percentages and in an economical manner in terms of material, equipment and disposal.